From Egg to Nanomaterials: Egg-Derived Precursors for Green Nanotechnology

Abstract

The increasing demand for sustainable and environmentally synthesis routes has driven significant interest in biogenic precursors for nanomaterial fabrication. Among these, egg-derived materials—including eggshell, eggshell membrane (ESM), egg white, and egg yolk—have emerged as versatile, low-cost, and multifunctional resources for green nanotechnology. This review provides a comprehensive and critical analysis of the physicochemical properties and functional roles of egg-derived components in nanomaterial synthesis. A comparative evaluation of egg-derived and conventional synthesis methods is presented, highlighting the trade-off between environmental sustainability and control over physicochemical parameters. Egg-derived approaches offer reduced toxicity, lower energy consumption, and intrinsic functionalization, but remain limited by compositional variability, reduced reproducibility, and challenges in process scalability. Furthermore, an application-oriented framework is proposed for selecting appropriate egg-derived precursors based on material type, targeted functionality, and processing constraints. The review also identifies key limitations, including mechanistic uncertainties, organic residue formation, and regulatory considerations, and outlines future research directions focused on process standardization, in situ characterization, and hybrid synthesis strategies.

1. Introduction

Structurally, the egg consists of approximately 59% egg white (albumen) and 31% egg yolk, both enclosed by a protective shell representing about 10% of the total mass. The edible fraction contains around 74.4% water, with proteins (12.3%) and lipids (11.6%) as the main nutritional components [1].

The egg represents a complex biological system composed of distinct components—eggshell, eggshell membrane (ESM), egg white, and egg yolk—each characterized by specific chemical compositions and functional properties relevant for nanomaterial synthesis [2]. Due to the presence of inorganic minerals, proteins, lipids, and various bioactive molecules, these components can act as precursors, reducing and stabilizing agents, templates, or support matrices for the formation of nanostructured materials. Figure 1 illustrates the main components of the egg.

The eggshell, primarily composed of calcium carbonate (CaCO₃) (94–97%), with minor amounts of calcium phosphate, magnesium carbonate, and organic matter, exhibits a naturally porous structure of calcite crystals embedded in an organic matrix. This structure makes it a suitable precursor for calcium-based nanomaterials, such as calcium oxide and hydroxyapatite, while facilitating nanostructure formation during thermal or chemical processing. Beneath the shell, the eggshell membrane forms a fibrous three-dimensional biopolymer network composed mainly of collagen, keratin, glycoproteins, and polysaccharides [3,4,5,6].

The high surface area and abundance of functional groups in ESM enable efficient metal ion binding, supporting its use as a biotemplate, catalyst support, adsorbent, or substrate for biosensors, as well as directing the growth of nanostructures such as nanowires and nanotubes [3].

Egg white (albumen) is a viscous aqueous medium containing approximately 90% water and 10% proteins, including ovalbumin, ovotransferrin, ovomucoid, and lysozyme [7,8]. These proteins contain reactive functional groups (–NH₂, –COOH, –SH) that facilitate interactions with metal ions, enabling their role as reducing and stabilizing agents, as well as structure-directing matrices. Consequently, egg white is widely employed in green synthesis routes for metal and metal oxide nanoparticles [9,10,11,12,13,14,15,16].

In contrast, egg yolk is rich in lipids and phospholipids, including lecithin, cholesterol, and triglycerides, along with proteins and carotenoids [17]. Lecithin, an amphiphilic molecule, functions as a natural surfactant and emulsifier, promoting nanoparticle stabilization, dispersion, and self-assembly, making egg yolk particularly suitable for nanoemulsions and lipid-based nanocarriers.

Despite the growing number of studies investigating egg-derived materials in nanotechnology, the existing literature remains largely fragmented and predominantly descriptive. Most studies focus on individual components or specific nanomaterials, without providing a systematic comparison of the roles, mechanisms, and performance of different egg-derived precursors. Furthermore, limited attention has been given to establishing clear structure–property relationships or to developing application-oriented selection criteria for these biogenic systems. In addition, important challenges such as reproducibility, compositional variability, and scalability are often addressed only superficially, leaving a gap between laboratory-scale demonstrations and practical implementation.

To situate this contribution relative to existing literature, it is useful to distinguish it explicitly from several representative prior works, each of which addresses a narrower slice of the egg-derived nanomaterials space. Reviews of eggshell-derived nanoparticles, such as that of Nada et al. [18], focus specifically on eggshell waste valorization routes (CaCO₃/CaO nanoparticle synthesis, characterization, and applications) without extending to the other three egg fractions considered here. Egg white-assisted synthesis has been reviewed on its own terms by Liu et al. [7], who cataloged the range of inorganic materials accessible via the egg-white precursor method, but without a quantitative benchmark against non-egg-derived green-synthesis platforms or a mechanistic treatment of the underlying protein chemistry. Eggshell membrane-supported nanomaterials have likewise been addressed in isolation, for example by Mallampati and Valiyaveettil [19], who focused specifically on ESM-supported noble-metal catalytic nanoparticles for organic transformations—a functional-application perspective rather than a synthesis-mechanism one. Eggshell-derived hydroxyapatite has received particularly extensive independent review coverage, from earlier overviews such as Abdulrahman et al. [20] to more recent, more specialized treatments such as Kareem and Eyiler [21], reflecting the relative maturity of that single sub-topic compared with the broader egg-derived materials space. Finally, single-material, single-precursor experimental studies—such as the egg-white-mediated ZnO synthesis of Vijan et al. [15] or the eggshell–copper nanotechnology synergy explored by Sharma et al. [22]—continue to expand the empirical literature but, individually, do not attempt cross-component or cross-platform comparison.

Collectively, these works illustrate that the literature on egg-derived nanomaterials, while extensive, remains organized around individual egg fractions, individual target materials, or individual applications, rather than around the egg as an integrated, multi-component precursor system. The present review differs from this body of work in four specific respects: (i) it considers all four egg-derived components—eggshell, egg-shell membrane, egg white, and egg yolk—within a single comparative framework (Section 3, Table 3), rather than treating any one fraction in isolation; (ii) it benchmarks egg-white-mediated synthesis quantitatively against plant-extract-, microbial-, and polysaccharide-based green-synthesis platforms for the same target nanomaterials (Table 2), a cross-platform comparison largely absent from the single-precursor reviews cited above; (iii) it provides an explicit mechanistic treatment of metal–protein coordination chemistry, nucleation pathways, and reduction chemistry (Section 4), going beyond the phenomenological, parameter-level descriptions ordinarily found in application-focused reviews; and (iv) it translates this comparative and mechanistic analysis into a practical, application-oriented precursor-selection framework (Section 6) and a critical, quantified assessment of the scalability, standardization, and techno-economic barriers that remain (Section 5).

In this context, the present review aims to provide a comprehensive and critical analysis of egg-derived precursors for nanomaterial synthesis. Specifically, the objectives of this work are: (i) to elucidate the fundamental mechanisms governing nanoparticle formation using eggshell, ESM, egg white, and egg yolk; (ii) to establish correlations between precursor composition, synthesis conditions, and resulting material properties; (iii) to compare egg-derived approaches with conventional synthesis methods; and (iv) to propose practical guidelines for selecting appropriate egg-derived precursors based on targeted applications. By addressing these aspects, this review seeks to contribute to the rational design and optimization of sustainable nanomaterial synthesis strategies.

2. Egg-Derived Components as Functional Precursors

To organize the mechanistically distinct roles that the four egg-derived fractions play in nanomaterial synthesis, this review adopts a precursor–function mapping framework that assigns each component a primary structural function within the synthesis process: eggshell is treated as a mineral precursor, supplying the calcium-based inorganic phase for calcium-containing nanomaterials (Section 2.1); egg white is treated as a chelating, reducing, and stabilizing medium, whose protein and thiol/amine functional groups coordinate metal ions, drive their reduction to the zero-valent state, and cap the resulting nuclei against uncontrolled growth (Section 2.2); egg yolk is treated as a lipidic soft template, whose phospholipid and lipoprotein assemblies direct particle morphology and size through micellar and interfacial confinement (Section 2.3); and eggshell membrane is treated as a fibrous biotemplate and carbon precursor, whose collagenous fiber network provides a structural scaffold for templated growth and, upon pyrolysis, a nitrogen-doped carbon source (Section 2.4). This mapping is applied consistently throughout Section 2, Section 3 and Section 4 and underlies the comparative structure–property–application analyses presented in Section 3 (Tables 3 and 6).

Table 1 summarizes, for each of the four functional roles defined above, representative material classes together with their synthesis route, dominant mechanism, advantages, limitations, and best-fit applications, providing a roadmap for the detailed discussion that follows in Section 2.1, Section 2.2, Section 2.3 and Section 2.4.

2.1. Eggshell as an Inorganic Precursor

In the context of the development of green nanotechnologies and the circular economy, the valorization of eggshells to obtain advanced materials has become a field intensively studied in recent years [23,24,25,26,27,28]. Worldwide, the food industry generates significant amounts of waste from egg processing annually, which are traditionally considered residues of no economic value. However, recent research has shown that this waste is a rich natural source of calcium carbonate, containing between 93 and 97% CaCO₃ in crystalline calcite form, along with low amounts of magnesium carbonate, calcium phosphates, water and organic compounds.

Through different synthesis methods, eggshell can be used to obtain a wide range of nanomaterials, such as calcium oxide (CaO) nanoparticles, calcium carbonate nanoparticles (CaCO₃), hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂), calcium phosphates, composite materials, porous materials and carbon-based materials [18,23,24,25,26,27,28,29,30,31,32,33,34,35,36].

In the case of CaCO₃ nanoparticle synthesis, the eggshell is usually dissolved in acidic solutions to obtain calcium ions, followed by controlled precipitation to form the nanoparticles. This method allows the control of particle size, morphology and crystalline phase (calcite, aragonite or vaterite), essential parameters for industrial and biomedical applications.

Sacia et al. synthesized CaO nanoparticles obtained from eggshells with a high specific surface area and high chemical reactivity, being used as heterogeneous catalysts, heavy metal adsorbents, antibacterial materials and carbon dioxide capture materials [34].

Table 1. Precursor–function mapping: representative egg-derived nanomaterials organized by dominant precursor role, with synthesis route, mechanism, advantages, limitations, and best-fit applications.

Material Type	Synthesis Route	Mechanism	Advantages	Limitations	Best-Fit Applications
Eggshell—Mineral Precursor
CaO nanoparticles	Washing → grinding → thermal calcination (600–1000 °C); or sol–gel route (HCl dissolution → sol → gel → calcination 900 °C, 1 h)	Thermal decomposition of CaCO3 to CaO; sol–gel route achieves homogeneous solution-phase nucleation prior to calcination	Low-cost bio-waste valorization; strong basic character; high specific surface area/reactivity	Direct calcination yields broader particle-size distribution	Heterogeneous catalysis (biodiesel), heavy-metal adsorption, CO2 capture
Hydroxyapatite (HA)	Calcination of CaCO3 to CaO → hydration to Ca(OH)2 → reaction with phosphate precursor (wet precipitation/hydrothermal)	Mineral-precursor phase transformation; Ca–P lattice substitution	Bone-like composition; high biocompatibility; antioxidant/anti-inflammatory activity reported	Incomplete phase conversion possible depending on conditions; extra purification needed for biomedical grade	Bone regeneration, dental/orthopedic implants, antibiotic-pollutant adsorption
Egg White—Chelating, Reducing, and Stabilizing Medium
Metal oxides (ZnO, TiO2, CeO2, Fe2O3, NiO, Al2O3, MgO)	Metal salt + egg white → gel formation → calcination (400–1100 °C, oxide-dependent)	Ovalbumin chelation/templating of metal ions (–NH2/–COOH/–OH) followed by oxidative calcination	Lower calcination temperatures than conventional solid-state routes; good dispersion, reduced agglomeration	Reproducibility sensitive to protein-batch and feedstock variability; scalability untested beyond lab scale	Photocatalysis (TiO2, CeO2), antibacterial/biomedical composites, gas sensing
Zero-valent metals (Ag, Au, Cu)	Metal salt (e.g., AgNO3) reduced in egg-white medium at controlled pH	Thiol/amine-mediated reduction to the zero-valent state; protein capping arrests post-nucleation growth	Aqueous, reductant-free green route; sub-20 nm Ag particles; strong antibacterial activity	Cu requires inert-atmosphere control to prevent oxidation; narrow processing window	Antibacterial coatings, nanomedicine (drug delivery, photothermal therapy)
Perovskites (LaMnO3, BaTiO3, BiFeO3)	Multi-cation gel formation in egg-white matrix → calcination (500–1000 °C)	Ovalbumin coordinates multiple A/B-site cations, suppressing selective precipitation	Phase-pure crystalline perovskites at temperatures 400–500 °C below conventional ceramic routes	Cross-study reproducibility not systematically documented; composite systems need 800–1000 °C	CO oxidation catalysis, piezoelectric/capacitor materials, multiferroic/photocatalytic materials
Egg Yolk—Lipidic Soft Template
Oxide nanoparticles via lipid templating (TiO2, ZnO)	Nanoemulsion-assisted synthesis using yolk lipoproteins/phospholipids as soft template, followed by calcination	Amphiphilic self-assembly of lecithin/lipoproteins into micelles/vesicles confines nucleation and growth	Smaller, more monodisperse particles (5–50 nm, PDI < 0.3) than many egg-white oxide routes	Post-calcination purity/porosity less predictable than in protein-gel systems	Catalytic and adsorption applications requiring controlled particle size
Lipid-based nanocarriers (liposomes, SLNs)	Self-assembly of yolk phospholipids/lipoproteins under controlled pH and ionic strength	Lecithin-driven amphiphilic self-assembly forming bilayer/micellar nanocarriers	High biocompatibility; 60–90% encapsulation efficiency for hydrophobic drugs; sustained release 24–72 h	Batch-to-batch compositional variability affecting particle size and morphology	Drug delivery, controlled-release systems, bioimaging
Eggshell Membrane (ESM)—Fibrous Biotemplate and Carbon Precursor
Metal/metal-oxide nanoparticles on ESM fibers (Ag, Fe3O4, Pd-PdO, ZnO nanorods)	In situ ion binding on ESM fibers (electrostatic/coordination) → reduction or thermal transformation	ESM functional groups (–NH2/–COOH/–OH/–SH) bind and immobilize metal ions, directing anisotropic templated growth	High surface area (200–400 m2/g); well-dispersed, low-aggregation particles; 1D anisotropic growth achievable	Composition/structure variability (species/diet-dependent) affects reproducibility	Antibacterial materials, heavy-metal removal, photocatalytic degradation, sensing
Porous doped carbon	Pyrolysis of ESM (600–900 °C)	Thermal conversion of the collagenous/keratinous fiber network into N/S-heteroatom-doped porous carbon	High surface area (300–800 m2/g); intrinsic N/S doping enhances conductivity/catalytic activity; standardizable process	Reported surface-area values vary with pyrolysis protocol and heating rate	Supercapacitor electrodes, oxygen-reduction electrocatalysis

Table 1. Precursor–function mapping: representative egg-derived nanomaterials organized by dominant precursor role, with synthesis route, mechanism, advantages, limitations, and best-fit applications.

Material Type	Synthesis Route	Mechanism	Advantages	Limitations	Best-Fit Applications
Eggshell—Mineral Precursor
CaO nanoparticles	Washing → grinding → thermal calcination (600–1000 °C); or sol–gel route (HCl dissolution → sol → gel → calcination 900 °C, 1 h)	Thermal decomposition of CaCO3 to CaO; sol–gel route achieves homogeneous solution-phase nucleation prior to calcination	Low-cost bio-waste valorization; strong basic character; high specific surface area/reactivity	Direct calcination yields broader particle-size distribution	Heterogeneous catalysis (biodiesel), heavy-metal adsorption, CO2 capture
Hydroxyapatite (HA)	Calcination of CaCO3 to CaO → hydration to Ca(OH)2 → reaction with phosphate precursor (wet precipitation/hydrothermal)	Mineral-precursor phase transformation; Ca–P lattice substitution	Bone-like composition; high biocompatibility; antioxidant/anti-inflammatory activity reported	Incomplete phase conversion possible depending on conditions; extra purification needed for biomedical grade	Bone regeneration, dental/orthopedic implants, antibiotic-pollutant adsorption
Egg White—Chelating, Reducing, and Stabilizing Medium
Metal oxides (ZnO, TiO2, CeO2, Fe2O3, NiO, Al2O3, MgO)	Metal salt + egg white → gel formation → calcination (400–1100 °C, oxide-dependent)	Ovalbumin chelation/templating of metal ions (–NH2/–COOH/–OH) followed by oxidative calcination	Lower calcination temperatures than conventional solid-state routes; good dispersion, reduced agglomeration	Reproducibility sensitive to protein-batch and feedstock variability; scalability untested beyond lab scale	Photocatalysis (TiO2, CeO2), antibacterial/biomedical composites, gas sensing
Zero-valent metals (Ag, Au, Cu)	Metal salt (e.g., AgNO3) reduced in egg-white medium at controlled pH	Thiol/amine-mediated reduction to the zero-valent state; protein capping arrests post-nucleation growth	Aqueous, reductant-free green route; sub-20 nm Ag particles; strong antibacterial activity	Cu requires inert-atmosphere control to prevent oxidation; narrow processing window	Antibacterial coatings, nanomedicine (drug delivery, photothermal therapy)
Perovskites (LaMnO3, BaTiO3, BiFeO3)	Multi-cation gel formation in egg-white matrix → calcination (500–1000 °C)	Ovalbumin coordinates multiple A/B-site cations, suppressing selective precipitation	Phase-pure crystalline perovskites at temperatures 400–500 °C below conventional ceramic routes	Cross-study reproducibility not systematically documented; composite systems need 800–1000 °C	CO oxidation catalysis, piezoelectric/capacitor materials, multiferroic/photocatalytic materials
Egg Yolk—Lipidic Soft Template
Oxide nanoparticles via lipid templating (TiO2, ZnO)	Nanoemulsion-assisted synthesis using yolk lipoproteins/phospholipids as soft template, followed by calcination	Amphiphilic self-assembly of lecithin/lipoproteins into micelles/vesicles confines nucleation and growth	Smaller, more monodisperse particles (5–50 nm, PDI < 0.3) than many egg-white oxide routes	Post-calcination purity/porosity less predictable than in protein-gel systems	Catalytic and adsorption applications requiring controlled particle size
Lipid-based nanocarriers (liposomes, SLNs)	Self-assembly of yolk phospholipids/lipoproteins under controlled pH and ionic strength	Lecithin-driven amphiphilic self-assembly forming bilayer/micellar nanocarriers	High biocompatibility; 60–90% encapsulation efficiency for hydrophobic drugs; sustained release 24–72 h	Batch-to-batch compositional variability affecting particle size and morphology	Drug delivery, controlled-release systems, bioimaging
Eggshell Membrane (ESM)—Fibrous Biotemplate and Carbon Precursor
Metal/metal-oxide nanoparticles on ESM fibers (Ag, Fe3O4, Pd-PdO, ZnO nanorods)	In situ ion binding on ESM fibers (electrostatic/coordination) → reduction or thermal transformation	ESM functional groups (–NH2/–COOH/–OH/–SH) bind and immobilize metal ions, directing anisotropic templated growth	High surface area (200–400 m2/g); well-dispersed, low-aggregation particles; 1D anisotropic growth achievable	Composition/structure variability (species/diet-dependent) affects reproducibility	Antibacterial materials, heavy-metal removal, photocatalytic degradation, sensing
Porous doped carbon	Pyrolysis of ESM (600–900 °C)	Thermal conversion of the collagenous/keratinous fiber network into N/S-heteroatom-doped porous carbon	High surface area (300–800 m2/g); intrinsic N/S doping enhances conductivity/catalytic activity; standardizable process	Reported surface-area values vary with pyrolysis protocol and heating rate	Supercapacitor electrodes, oxygen-reduction electrocatalysis

The preparation procedure of CaO involves several sequential steps. First, the eggshells are thoroughly washed with water to remove impurities [37]. After drying, the eggshells are ground into a fine powder and subjected to thermal treatment in a muffle furnace at temperatures between 600 and 1000 °C for several hours [38]. During this calcination process, calcium carbonate (CaCO₃) is thermally decomposed into calcium oxide (CaO) and carbon dioxide (CO₂):

$${CaCO}_3\to CaO+{CO}_2$$

(1)

The dried eggshells were subsequently crushed using a mortar and pestle in order to reduce the particle size to the micrometer range. Further size reduction to the nanoscale was achieved through continued grinding combined with sonication, which enabled the conversion of both duck [39] and hen [2] eggshells into nano-sized calcium particles.

Another useful method is sol–gel method using CaCO₃ from eggshells. The steps are:

Preparation of homogeneous solution:

Solid CaCO₃ is dissolved in a dilute HCl solution, resulting in the formation of a homogeneous CaCl₂ solution.

$${CaCO}_3\left(s\right)+2HCl\left(l\right)\to {CaCl}_2\left(l\right)+H_{2}O\left(l\right)+{CO}_2\left(g\right)$$

(2)

Formation of the sol:

A hydrolysis agent is slowly added to the CaCl₂ solution, leading to the formation of a sol, which represents a stable dispersion of colloidal precursor particles in a solvent formed through a hydrolysis reaction.

Formation of the gel:

The obtained sol suffered a condensation process, resulting in the formation of a highly crystalline Ca(OH)₂ gel. The gel was then aged and subsequently filtered to remove impurities.

Drying and calcination:

The gel is dried at 60–80 °C for 24 h to remove the solvent, after which it is calcined at 900 °C for 1 h, resulting in the formation of CaO nanoparticles.

CaO derived from eggshells has been widely used as a catalyst in the production of biodiesel, due to its strong basic character and low production cost [24,40,41].

To obtain CaCO₃ with a high degree of purity, it is necessary to remove this membrane, since the presence of organic compounds can negatively influence the physicochemical properties of the final product. The technological process of obtaining CaCO₃ from eggshells begins with the collection and selection of the raw material. The shells are taken from food processing facilities, restaurants, laboratories or households and undergo a washing operation to remove impurities, albumin residues and organic contaminants. After the washing stage, the ESM is separated by mechanical, thermal or chemical methods [42,43,44]. Mechanical methods involve manual ESM removal, while thermal methods use treatments at temperatures between 80 and 120 °C for degradation and detachment of organic components. In the case of chemical processes, dilute alkaline solutions, such as sodium hydroxide, are used, which facilitate the dissolution of proteins and the efficient separation of the membrane. The next step is to dry the material at temperatures between 100 and 110 °C for 12–24 h to completely remove moisture and prevent the development of microorganisms. After drying, the shells are subjected to a grinding process using ball mills, planetary mills or vibrating mills. Depending on the final application, particles ranging in size from a few micrometers to several hundred micrometers can be obtained. The resulting powders are then screened to obtain a uniform particle size distribution, using standardized sieves with mesh sizes ranging from 45 to 200 μm [43,44].

In its simplest form, the process described results in a natural calcium carbonate with a high degree of purity, suitable for use as a filler in composite materials, a food additive or a precursor for the synthesis of other calcium-based materials. For applications that require higher purities, such as biomedical or pharmaceutical, additional purification steps are required [45].

Another important compound, hydroxyapatite (HA), obtained from eggshells, is of particular interest in the biomedical field, due to its composition like that of human bone and high biocompatibility, being used in bone regeneration applications, implants and dental materials [46,47]. The synthesis of HA from eggshells typically involves thermal calcination of CaCO₃ to CaO, followed by hydration to Ca(OH)₂ and reaction with phosphate precursors such as phosphoric acid via wet chemical precipitation or hydrothermal methods [48]. Rivera et al. demonstrated that high-temperature treatment and subsequent phosphatization yield porous HA structures, although incomplete phase conversion may occur depending on temperature and reaction conditions [32]. More recent studies have advanced these methodologies by employing controlled wet chemical precipitation, sol–gel, and hydrothermal processes to enhance phase purity, crystallinity, and particle size control. Researchers developed an eco-friendly precipitation-based synthesis route using eggshell-derived calcium precursors, achieving highly crystalline and phase-pure nano-hydroxyapatite [23,36,49].

Hemocompatibility assessments indicate minimal hemolysis at lower concentrations, particularly when protein corona formation mitigates nanoparticle–cell interactions. Additionally, the material shows antioxidant and anti-inflammatory properties, including free radical scavenging and suppression of pro-inflammatory cytokines (e.g., IL-1β, IL-18), further enhancing its therapeutic potential [36].

Eggshell-derived HA has demonstrated significant potential for bone regeneration. In vitro studies indicate enhanced osteoblast adhesion, proliferation, and spreading on HA surfaces, confirming its osteoconductive properties [3,50,51].

Beyond biomedical uses, HA derived from eggshell waste has also been explored for environmental remediation. Alhasan et al. investigated its application as an adsorbent for removing cephalexin, a commonly detected antibiotic pollutant, from aqueous solutions [52]. The study demonstrated that HA nanoparticles achieve removal efficiencies of up to 70.7% under optimal conditions, with adsorption behavior following pseudo-second-order kinetics and exhibiting spontaneous, endothermic thermodynamics. These findings highlight the versatility of HA as both a biomaterial and an environmental adsorbent.

A major advantage of using eggshells as a precursor for nanomaterials is the sustainability of the process, as abundant bio-waste is recycled, reducing environmental impact and production costs. In addition, the materials obtained often have a natural porous structure and mineral impurities that can act as natural dopants, improving the catalytic, adsorbent or antibacterial properties of the final materials.

Comparing the two calcination routes described above reveals a trade-off between simplicity and control. Direct thermal decomposition of ground eggshell (600–1000 °C, several hours) is operationally straightforward but yields CaO with broader particle size distributions, as nucleation occurs heterogeneously across the solid powder. In contrast, the sol–gel route—via dissolution, controlled hydrolysis, and calcination of the resulting Ca(OH)₂ gel at 900 °C for only 1 h—achieves more homogeneous nucleation in solution prior to thermal treatment, typically producing smaller, more uniform particles at a shorter calcination time. This suggests that the choice between routes should be dictated by the target application: direct calcination is adequate for bulk uses (e.g., biodiesel catalysis, soil amendment), whereas the sol–gel route is preferable when narrow size distribution and higher purity are required, as in biomedical-grade CaO or hydroxyapatite precursors.

2.2. Egg White as a Biogenic Precursor in Nanomaterial Synthesis

The use of egg white (albumen) as a biogenic precursor in nanomaterial synthesis represents a well-established approach within green nanotechnology, owing to its multifunctional role, low cost, and environmental compatibility [53].

The range of nanomaterials synthesized with egg white is presented in Figure 2.

Egg white is not only a reaction medium, but performs three critical functions:

• Chelating agent by proteins (ovalbumin) which contain functional groups such as -NH₂, -OH and -COOH, coordinate and immobilize metal ions within the protein matrix, ensuring a uniform distribution in the protein matrix.
• Templating agent through the complex structure of proteins who directs the growth of crystals, controlling the size and shape of nanoparticles.
• Stabilizer because prevents particle agglomeration of particles, resulting in a fine and homogeneous product.

2.2.1. Metal Oxides Synthesis

The method, often referred to as the “egg white precursor method”, has been successfully applied to the synthesis of a wide range of metal oxides, including ZnO, TiO₂, Fe₂O₃, CeO₂, NiO, and Al₂O₃. The resulting nanomaterials typically exhibit particle sizes in the range of 10–100 nm, with good dispersion and reduced agglomeration due to the stabilizing effect of the protein matrix. In many cases, high crystallinity can be achieved at low temperatures (400–600 °C), significantly lower than those required in conventional solid-state synthesis routes. In

Table 2. Example of metal oxides synthesized with egg white.

Synthesized Oxide	Metal Salt	Calcination Temperature	Characteristics Obtained	Reference
ZnO	Zn(NO3)2	400–600 °C	Egg white improves to obtain various morphologies (nanoflowers, stems).	[15,16,52,53,54]
MgO	Mg(NO3)2	550 °C	The method prevents severe agglomeration, resulting in fine cubic nanopowders.	[53]
NiO	Ni(NO3)2	300–700 °C	Usually spherical, uniformly formed, and well-dispersed particles.	[9,54]
CuO	Cu(CH3COOH)2	550 °C	Irregular polyhedral particles.	[55]
TiO2	Ti{OCH(CH3)2}4/TiCl4	400–550 °C	Good control between anatase and rutile phases. Egg white can act as residual nitrogen doping (N-doping), increasing photocatalytic activity. TiO2 nanocomposites and hydrogels with enhanced antimicrobial and photocatalytic properties.	[15,56]
CeO2	Ce(NO3)3	500–600 °C	Very small spherical particles. Egg white helps maintain good dispersion, crucial for redox properties (oxygen storage).	[57,58,59,60,61]
Fe2O3	Fe(NO3)3	400–600 °C	Synthesis of Fe2O3, α phase. The process is fast. The egg white influences the magnetic properties by controlling the size of the crystallites. Doping with Co stabilizes the phase γ.	[62]
Al2O3	Al(NO3)3	800–1100 °C	Requires higher temperatures for phase α. Egg white helps to obtain porous alumina with high thermal stability.	[63]

the most synthesized metal oxide using egg white precursor is presented.

One of the most popular studies is aimed at using egg whites to control the size of ZnO particles [52,53,64,65,66,67]. The proteins in the egg white prevent clumping, resulting in different morphologies of particles, ideal for antibacterial and biomedical applications [13,16,53,55].

Ovalbumin acts as a matrix to chelate Mg(II) ions, controlling nucleation, and preventing particle agglomeration, resulting in stable, small-sized MgO nanoparticles (ranging from roughly 7.9 to 13.5 nm) [53].

Studies have demonstrated that ovalbumin dictates the porosity of the final material, creating a very large specific surface area. Egg white acts as a natural binder, foaming agent, and templates to synthesize Al₂O₃ nanoparticles, preventing agglomeration. It is used to produce porous alumina ceramics (150–240 μm pores [63,67].

Egg white helps stabilize the TiO₂ anatase phase, which is the most chemically active, and can be used to create high-surface-area, mesoporous particles by the sol–gel method [68]. Studies of Kadam et al. demonstrated expired egg whites are used to produce N–S–C tri-doped TiO₂ nanoparticles [69].

Also, waste eggs nearing their expiration date demonstrated to be a good precursor for TiO₂ hydrogel for removal of organic dyes and inactivating microorganisms from industrial wastewater [55].

An excessive amount of egg white can induce an uncontrolled and highly exothermic combustion process, leading to localized overheating and subsequent particle agglomeration. In contrast, maintaining an optimal precursor-to-egg white ratio (typically in the range of 1:1 to 1:2, volume/weight) promotes uniform nucleation and a more homogeneous particle size distribution. Furthermore, the use of egg white as a biogenic agent facilitates the formation of well-defined crystalline phases at significantly lower synthesis temperatures compared to conventional methods. For example, ZnO and MgO can be obtained with excellent crystallinity even at 550 °C, because proteins distribute metal ions at the molecular level before burning. The gases released during the decomposition of proteins (CO₂, N₂) create a porous structure (“foamy architecture”) [52]. This is particularly useful for TiO₂ and CeO₂, where a large surface area is essential for catalysis [68,70].

2.2.2. Metal Nanostructures Synthesis

The synthesis of nanostructured pure metals (in the elemental state) using egg white is a different technical challenge. Unlike oxides, where calcination occurs in the presence of oxygen, obtaining metals requires a reducing medium or strict control of the atmosphere to prevent instantaneous oxidation.

In this context, egg white plays a dual role: it is both a capping agent and a reducing agent due to the presence of sulfur amino acids (such as cysteine). The key differences are presented in Figure 3.

In oxide formation, proteins act mainly as chelating and templating agents, enabling controlled crystallization during thermal treatment. In contrast, in metallic nanoparticle synthesis, proteins promote ion reduction and preventing aggregation [71].

Silver (Ag) nanoparticles are the most studied by this method, due to their antibacterial potential and the ease with which Ag⁺ ions can be reduced. The synthesis typically employs silver nitrate (AgNO₃) as a precursor. Functional groups present in proteins, such as –NH₂ and –OH, facilitate the reduction of Ag⁺ ions to metallic silver (Ag⁰), while simultaneously acting as stabilizing agents that inhibit excessive crystal growth, thereby maintaining nanoparticle sizes below 20 nm [72]. The synthesis of colloidal gold using egg white as a biotemplate represents a promising approach for nanomedicine applications, including drug delivery, photothermal therapy and cancer therapy [72,73]. Under controlled pH conditions, egg white proteins undergo conformational changes, enabling them to encapsulate gold ions and direct the formation of nanostructures with well-defined geometries, such as spheres, triangles, or plates.

Copper nanoparticles, although highly attractive due to their excellent electrical conductivity, are challenging to synthesize because of their rapid oxidation in ambient conditions. The use of egg white mitigates this issue by generating a protective carbonaceous layer during controlled thermal treatment under an inert atmosphere (e.g., nitrogen or argon), which effectively prevents the formation of copper oxide (CuO) [54].

2.2.3. Perovskite-Type Materials

The synthesis of perovskite-type materials (ABO₃) using egg white as a biogenic precursor represents an effective approach for achieving high chemical homogeneity at the atomic level. Perovskites are complex materials whose electrical, magnetic, and catalytic properties are highly dependent on precise compositional uniformity [74,75,76,77]. The protein matrix of egg white provides a controlled environment that facilitates uniform distribution of multiple cations, thereby promoting the formation of well-defined perovskite structures. Unlike simple oxides such as ZnO, perovskite structures (e.g., LaMnO₃) require the simultaneous incorporation of multiple cations into a single crystalline framework. In this context, the functional groups of ovalbumin can coordinate different cations (A and B sites in the ABO₃ structure), thereby preventing phase segregation. In conventional chemical synthesis, differences in reactivity may lead to the premature precipitation of one metal species over another. In contrast, within the egg white-derived gel matrix, all ions are homogeneously immobilized in a common organic network, effectively suppressing selective precipitation and ensuring compositional uniformity.

Lanthanum-based perovskites (LaMnO₃, LaFeO₃, LaCoO₃) have been extensively investigated for their magnetic and catalytic properties, particularly in applications such as carbon monoxide oxidation in the automotive sector. It has been demonstrated that the use of egg white as a precursor enables the formation of phase-pure crystalline materials at relatively low temperatures (500–600 °C), significantly lower than the ~1000 °C typically required in conventional ceramic methods [77].

Similarly, barium titanate (BaTiO₃), widely employed in capacitors and piezoelectric sensors, was synthesized via green routes using egg white, yielding nanostructured materials with enhanced piezoelectric properties by [76].

In addition, LaFeO₃–NiFe₂O₄ nanocomposites with high dielectric constants have been successfully obtained at temperatures of 800–1000 °C using this method [77].

Multiferroic perovskites such as BiFeO₃, which exhibit both ferromagnetic and ferroelectric properties, are particularly challenging to synthesize without secondary phases. The egg white-assisted method has proven to be an effective route for obtaining high-purity BiFeO₃ and has also been applied to the synthesis of materials with photocatalytic activity [78,79]. The stages of the experimental process of developing egg white perovskites are presented in Figure 4.

2.2.4. Other Inorganic Materials

Spinel Structures (Aluminates and Complex Ferrites)

Unlike perovskites (ABO₃), spinel-type materials are characterized by the general formula AB₂O₄ and play a significant role in applications such as coatings, magnetic materials, and catalysis.

Magnesium aluminate (MgAl₂O₄), widely used as a refractory material and catalyst support, was synthesized by egg white-assisted synthesis which enables the formation of nanostructures with high specific surface area [80,81]. Zinc ferrite (ZnFe₂O₄), on the other hand, has attracted considerable attention due to its photocatalytic activity under visible light, particularly for the degradation of organic pollutants in aqueous systems [82,83]. Al-Senani et al. demonstrated the fabrication of nanocrystalline NiFe₂O₄ at a relatively low temperature (300 °C) and short time (15 min) at different egg white concentration [84].

The synthesis of ferrites by conventional methods requires enormous temperatures and a long time. Using egg white as a precursor, it has been demonstrated to achieve pure magnetic phases at lower temperatures (approx. 500–600 °C). For example, NiFe₂O₄ and La-doped nanoparticles were synthesized by a simple and cost-effective method using Ni and Fe nitrates and freshly extracted egg white (ovalbumin) in an aqueous medium [83,85,86].

Phosphors (Luminescent Materials)

Phosphors are luminescent materials typically doped with rare-earth ions (e.g., Eu³⁺ or Tb³⁺), which emit light upon optical or electrical excitation. Doped aluminates, such as MgAl₂O₄:Eu³⁺, was synthesized using egg white-assisted methods, where the protein matrix facilitates a homogeneous distribution of rare-earth ions within the host lattice, thereby enhancing luminescence intensity [87,88].

Silicates and Biocompatible Glasses

Although silica (SiO₂) is commonly synthesized via the Stöber method, the use of egg white enables the formation of nanostructured Co/SiO₂ through protein-mediated processes for catalysts [89,90].

Zirconium silicate (ZrSiO₄) is widely utilized as a thermally stable ceramic pigment, while bioactive glass can be synthesized with the assistance of egg white, which promotes the development of porous structures resembling the architecture of natural bone. This feature makes such materials particularly suitable for applications in bone regeneration and tissue engineering.

Nanocomposites (Metal–Oxide Hybrids)

The egg white-assisted method enables the simultaneous formation of multiple phases, resulting in composite materials with synergistic properties [91,92,93,94,95,96,97,98,99,100,101,102].

For example, Ag/TiO₂ nanocomposites consist of silver nanoparticles deposited on titanium dioxide, where egg white acts as a binding and stabilizing matrix between the two phases, leading to highly efficient antibacterial photocatalysts [6,11,72]. Similarly, Fe₃O₄/carbon composites can be obtained through controlled or incomplete calcination, during which egg white proteins are transformed into a carbonaceous matrix that encapsulates the magnetic core, enhancing stability and providing protection against oxidation. Successfully, ZnO-TiO₂ nanocomposites were synthesized using an egg-white-mediated co-precipitation technique coupled with microwave irradiation for photocatalytic activity [14].

Metal Sulfides

Although less commonly reported, the egg white method can be adapted for the synthesis of metal sulfides (e.g., ZnS or CdS) by introducing a sulfur source, such as thioacetamide, into the egg white-derived gel prior to thermal treatment [92]. These materials are typically obtained as quantum dots, exhibiting size-dependent optical properties and are widely used in bioimaging applications.

Relative to other green-synthesis platforms, egg white occupies a distinct niche defined by its protein-gel confinement mechanism rather than by reducing-agent chemistry alone [103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118]. Plant-extract-mediated synthesis typically achieves comparable or faster reaction times but relies on phytochemical profiles (polyphenols, flavonoids) that vary with plant species, harvest season, and extraction protocol, generally resulting in broader and less reproducible particle size distributions than the ovalbumin-gel-confined systems described here [118,119,120]. Microbial-mediated synthesis (bacterial, fungal, or yeast-based) offers enzymatic specificity and can yield highly monodisperse particles but requires longer incubation times (24–72 h) and careful maintenance of living cultures, which limits throughput and scalability compared to the single-step thermal protocols typical of egg-white synthesis [119,121]. Polysaccharide-assisted approaches (e.g., chitosan-, starch-, or alginate-based) provide tunable viscosity and are particularly effective for antibacterial coatings but generally offer lower thermal stability of the templating matrix than the protein network of ovalbumin, restricting their use in high-temperature calcination routes such as those required for perovskite or spinel formation [122,123,124,125]. In this context, the ability of egg white to lower crystallization temperatures by 400–500 °C relative to conventional ceramic routes (e.g., 500–600 °C versus ~1000 °C for LaMnO₃-type perovskites [77,80,81,83,84,85,86]) represents a comparative advantage specific to protein-mediated systems that is not consistently reported for plant-, microbial-, or polysaccharide-based methods.

To synthesize these observations into a critical, cross-cutting comparison rather than a class-by-class listing,

Table 3. Comparative structure–property–application analysis of the main classes of egg-white-mediated nanomaterials.

Material Class	Synthesis Mechanism	Particle-Size Control	Reproducibility	Scalability	Purity/Residues	Environmental Impact	Best-Demonstrated Performance
Metal oxides (ZnO, TiO2, Fe2O3, CeO2, NiO, Al2O3, MgO)	Chelation/templating by ovalbumin –NH2/–COOH/–OH, followed by oxidative calcination (400–1100 °C)	Protein-gel mesh confinement; optimal precursor:egg-white ratio ~1:1–1:2 (v/w)	Moderate—10–100 nm reported across studies/oxides, sensitive to feedstock and protein-batch variability	Lab-scale; calcination energy cost scales with the oxide (400 °C for ZnO/CeO2 vs. 800–1100 °C for Al2O3)	Requires complete organic burn-off; incomplete combustion can leave carbonaceous residues	Aqueous, no toxic reductants; but CO2/N2/H2O off-gassing and calcination energy input	Strong for photocatalysis (TiO2, CeO2) and magnetic oxides (Fe2O3)
Zero-valent metals (Ag, Au, Cu)	Thiol oxidation + Maillard-derived reduction, with –NH2/–OH as capping/stabilizing ligands	Protein capping arrests growth post-nucleation (Ag typically <20 nm)	Ag well-reproduced across studies; Cu markedly less so (narrow processing window, inert atmosphere required)	Ag straightforward near RT; Cu needs inert-atmosphere control, adding engineering complexity	Cu requires a protective carbonaceous layer to block oxidation—a functional but purity-relevant trade-off	Avoids toxic chemical reductants (e.g., NaBH4, hydrazine) used in conventional routes—a genuine green-chemistry gain	Strong antibacterial activity (Ag); promising nanomedicine applications (Au)
Perovskites (LaMnO3, LaFeO3, BaTiO3, BiFeO3)	Multi-cation (A/B-site) coordination by ovalbumin, suppressing selective precipitation	Not explicitly quantified in the surveyed literature—a genuine gap	Compositional homogeneity depends on the protein–ion pre-coordination step; cross-study reproducibility untested	Reaches phase purity at 500–600 °C vs. ~1000 °C conventional—advantage erodes for composites requiring 800–1000 °C	Multi-cation systems more prone to secondary-phase formation without adequate gel confinement	Same aqueous-route benefits as oxides, plus meaningful calcination-energy savings for most compositions	High-purity BiFeO3 without secondary phases; enhanced piezoelectric/catalytic behavior
Spinels, ferrites, phosphors, silicates & composites	Chelation/templating logic extended to ternary/quaternary and doped/composite systems	Reported only qualitatively (e.g., porous MgAl2O4)—mechanism not established	Each system typically reported by a single or few groups; reproducibility largely untested across the class	No unified process window (15 min at 300 °C for NiFe2O4 vs. 800–1000 °C for composites)	Sulfide routes require an added sulfur precursor (thioacetamide), introducing an extra residue/purity variable	Retains aqueous green-synthesis benefits, partly offset by added dopant/precursor chemicals	Application-specific gains shown (luminescence, photocatalysis, quantum-dot optics) but rarely benchmarked head-to-head against conventional routes

presents a structure–property–application analysis of the four main classes of egg-white-mediated nanomaterials discussed above, contrasting their dominant synthesis mechanism, particle-size-control strategy, reproducibility, scalability, purity/residue considerations, environmental impact, and best-demonstrated performance. Several of these dimensions—most notably reproducibility and scalability for perovskites and spinel-type systems—are not yet reported quantitatively in the surveyed literature; this is stated explicitly in the table as a gap rather than presented as an unsupported claim.

2.3. Egg Yolk as a Functional Component in Nanomaterial Synthesis

Egg yolk represents a highly complex biological matrix rich in lipids (approximately 32–35 wt%), proteins (16–18 wt%), and phospholipids (≈10 wt%), along with minor fractions of cholesterol, carotenoids, and bioactive compounds, which collectively confer unique physicochemical properties relevant for nanomaterial synthesis [90,91,92,93].

In contrast to egg white, which is predominantly protein-based, egg yolk contains a significant proportion of amphiphilic molecules—primarily phospholipids such as phosphatidylcholine (lecithin)—as well as lipoproteins (low-density lipoproteins, LDL, and high-density lipoproteins, HDL) [17]. At the molecular level, lecithin-driven self-assembly leads to the formation of micelles and vesicular structures with typical diameters ranging from 20 to 200 nm, depending on pH and ionic strength. This compositional diversity enables egg yolk to function simultaneously as a reducing agent, stabilizing agent, soft template, and emulsifying medium in the synthesis of nanostructured materials [94].

The functionality of egg yolk in nanotechnology is largely governed by the amphiphilic nature of its phospholipids and lipoproteins. Lecithin, one of the major components, consists of a hydrophilic phosphate head group and hydrophobic fatty acid tails, enabling the spontaneous formation of supramolecular assemblies such as micelles, vesicles, and bilayers in aqueous media. These structures act as nanoscale reactors or confinement domains, facilitating controlled nucleation and growth of nanoparticles. Experimental studies have shown that such lipid-based assemblies can limit particle growth, leading to nanoparticles with sizes typically in the range of 5–50 nm and reduced polydispersity (polydispersity index < 0.3) compared to conventional chemical synthesis routes [95].

Beyond metallic systems, egg yolk has also been explored as a precursor for the synthesis of metal oxide nanomaterials and hybrid nanostructures. The emulsifying properties of yolk constituents enable the formation of nanoemulsions that can serve as templates for oxide formation. For example, nanoemulsion-assisted synthesis routes have been shown to produce oxide nanoparticles (e.g., TiO₂, ZnO) with controlled sizes below 100 nm and enhanced surface areas exceeding 50–150 m²/g after calcination [91,93]. Following thermal treatment, the removal of organic components can generate porous or hierarchical structures, which are advantageous for catalytic and adsorption applications. Moreover, residual carbon species originating from lipid decomposition can lead to in situ carbon doping, improving electrical conductivity and photocatalytic performance, particularly in TiO₂-based systems [93].

A particularly important application of egg yolk-derived systems lies in the development of lipid-based nanocarriers for biomedical applications. Due to their intrinsic biocompatibility and structural similarity to biological membranes, yolk-derived phospholipids are widely used for the fabrication of liposomes, solid lipid nanoparticles (SLNs), and nanoemulsions. Lecithin-based liposomes and solid lipid nanoparticles (SLNs) prepared from yolk phospholipids typically exhibit particle sizes between 80 and 200 nm, with encapsulation efficiencies ranging from 60% to 90% for hydrophobic drugs. These systems can encapsulate both hydrophilic and hydrophobic therapeutic agents, enhancing drug solubility, stability, and controlled release behavior [96,97]. Also, nanocarriers show improved drug stability and controlled release behavior, with sustained release profiles extending over 24–72 h depending on formulation parameters. Additionally, their biocompatibility and low cytotoxicity make them suitable for in vivo applications, including drug delivery and imaging [98].

The self-assembly behavior of egg yolk components also plays a critical role in directing nanostructure formation. Under controlled conditions of pH (pH 4–8), moderate ionic strength, and temperature, phospholipids and lipoproteins undergo conformational and phase transitions, leading to the formation of ordered supramolecular architectures. These structures act as soft templates for nanoparticle growth, enabling bottom-up fabrication of complex nanostructures with controlled morphology and hierarchical organization. Such biomimetic approaches offer advantages over conventional synthesis methods, particularly in achieving structural complexity and functional integration at the nanoscale [99].

Despite these advantages, several challenges limit the widespread application of egg yolk in nanomaterial synthesis. One of the primary limitations is the compositional variability of egg yolk, which depends on factors such as species, diet, and environmental conditions. This variability can lead to inconsistencies in nanoparticle size (±10–30 nm) and surface properties., morphology, and physicochemical properties, thereby affecting reproducibility [17]. Additionally, the high lipid content may necessitate additional purification or post-synthesis treatments to remove residual organic matter (300–600 °C), particularly in applications requiring high material purity. During thermal processing, incomplete decomposition of lipids can result in carbonaceous residues or undesired phase impurities. Therefore, careful optimization of synthesis parameters—such as precursor concentration, temperature, and atmosphere—is essential to ensure controlled material formation and reproducibility. The role of yolk in green nanotechnology is presented in Figure 5.

Overall, egg yolk represents a versatile and sustainable resource for green nanotechnology, offering unique functionalities that complement those of other egg-derived components. Its role as a natural emulsifier, stabilizer, and soft template enables the development of advanced nanomaterials with controlled properties, while maintaining low environmental impact. The integration of egg yolk-based approaches with modern synthesis techniques holds significant potential for the design of multifunctional nanostructures in biomedical, catalytic, and environmental applications.

Compared to egg white, egg yolk offers a fundamentally different control mechanism—amphiphilic self-assembly rather than protein-gel confinement—which explains several performance differences reported in the literature. Yolk-derived lipid assemblies typically yield smaller and more monodisperse particles (5–50 nm, PDI < 0.3 [95]) than many egg-white-mediated oxide syntheses, but this advantage is largely confined to soft, low-temperature templating; once thermal treatment is required to remove residual lipids (300–600 °C), the resulting porosity and phase purity are less predictable than in protein-gel systems, where organic decomposition is comparatively better characterized. This reconciles the apparently contradictory reports in the literature that describe egg yolk as both a superior size-control agent and a source of variable, batch-dependent outcomes [17]: the discrepancy reflects the specific metric being optimized (initial particle size versus post-calcination reproducibility) rather than an inherent inconsistency in the precursor itself.

2.4. Eggshell Membrane as a Biotemplate and Carbon Precursor

The eggshell membrane (ESM) is a naturally occurring fibrous biopolymeric structure located between the eggshell and the egg white, characterized by a highly organized three-dimensional network of interwoven protein fibers. This unique architecture, combined with its rich biochemical composition, makes ESM a biological template (biotemplate) for the synthesis of nanostructured materials within the framework of green nanotechnology.

Structurally, the eggshell membrane consists predominantly of fibrous proteins such as collagen, keratin, elastin, and glycoproteins, along with minor amounts of polysaccharides and other organic compounds. These biomolecules contain a variety of functional groups, including amino (–NH₂), carboxyl (–COOH), hydroxyl (–OH), and thiol (–SH) groups, which provide numerous active sites for the adsorption and coordination of metal ions [33]. The presence of these functional groups enables strong interactions between the membrane and precursor species, facilitating the nucleation and growth of nanomaterials directly on or within the membrane matrix. For instance, studies have shown that ESM can effectively bind metal ions such as Ag⁺, Cu²⁺, and Fe³⁺ through electrostatic interactions and coordination bonding, enabling their immobilization prior to reduction or thermal transformation [97].

One of the defining features of ESM is its hierarchical porous structure, consisting of interwoven fibers with diameters typically ranging from 1 to 5 µm and nanoscale substructures within the protein network. This morphology results in a high specific surface area (reported values up to 200–400 m²/g after carbonization) and interconnected pore channels that facilitate efficient mass transport and uniform precursor distribution [101,102]. Consequently, ESM promotes homogeneous nucleation and controlled growth of nanomaterials, leading to well-dispersed nanoparticles with reduced aggregation. For example, Ag nanoparticles synthesized on ESM fibers via in situ reduction methods typically exhibit particle sizes in the range of 10–40 nm and demonstrate strong antibacterial activity, with inhibition zones of 12–20 mm against E. coli [6]. Similarly, Fe₃O₄ nanoparticles anchored onto ESM have been reported with sizes between 15 and 50 nm, showing enhanced magnetic properties and efficient removal of heavy metal ions from aqueous solutions [42]. In the case of Pd-PdO nanoparticles anchored on surface of TiO₂ nanotubes, ESM-templated synthesis has yielded nanocomposite with improved dispersion and photocatalytic degradation efficiencies exceeding 90% for organic dyes such as rhodamine B under UV irradiation [101].

The templating effect of ESM is particularly advantageous for the synthesis of one-dimensional and hierarchical nanostructures. The fibrous morphology of the membrane can guide anisotropic growth, resulting in nanowires, nanotubes, or continuous nanoparticle coatings that replicate the biological architecture. For instance, ZnO nanorod arrays grown on ESM substrates have been reported with lengths of several micrometers and diameters below 100 nm, exhibiting enhanced photocatalytic and sensing performance due to their high aspect ratio and surface area [102,103].

In addition to serving as a template, ESM can also act as a precursor for carbon-based materials. Through pyrolysis at temperatures between 600 and 900 °C, the proteinaceous structure is converted into porous carbon materials with hierarchical porosity and high surface areas (typically 300–800 m²/g) [104]. The intrinsic N and S content of the proteins can lead to heteroatom doping in the resulting carbon matrix, enhancing its electrical conductivity and catalytic activity. Such materials have demonstrated promising performance in supercapacitors, with specific capacitances exceeding 200 F/g, as well as in electrocatalysis for oxygen reduction reactions (ORR) [103]. From a sustainability perspective, the use of ESM aligns with circular economy principles, as it represents an abundant by-product of the food industry. Its valorization reduces biological waste and eliminates the need for synthetic templates or surfactants. Furthermore, ESM-based synthesis routes often operate under mild conditions, reducing energy consumption and environmental impact compared to conventional methods [2]. Despite these advantages, several limitations must be considered. The variability in ESM composition, influenced by factors such as species, diet, and environmental conditions, can affect reproducibility and consistency. Additionally, precise control over nanoparticle size and crystallinity may require careful optimization of synthesis parameters, including pH, temperature, precursor concentration, and treatment time. Post-synthesis treatments may also be necessary to remove residual organic components, particularly for applications requiring high purity.

In conclusion, the eggshell membrane represents a highly versatile and efficient biotemplate for nanomaterial synthesis, combining structural complexity, chemical functionality, and sustainability. Its ability to direct the formation of nanostructures with controlled morphology and enhanced performance makes it a valuable resource for applications in catalysis, environmental remediation, energy storage, and advanced functional materials.

The specific surface area values reported for ESM-derived materials vary considerably across studies (200–400 m²/g after simple carbonization [101,103] versus 300–800 m²/g for optimized pyrolysis protocols [104]), a discrepancy that is attributable primarily to differences in pyrolysis temperature and heating rate rather than to intrinsic variability of the membrane itself, since both ranges were obtained within the same 600–900 °C window. This indicates that, unlike the compositional variability discussed for egg yolk, the reproducibility of ESM-derived carbon materials is governed mainly by process parameters that are, in principle, standardizable—an important distinction when positioning ESM against egg white and egg yolk as the most process-controllable of the three organic egg fractions.

3. Comparative Analysis of Egg-Derived Precursors

3.1. Comparison Between Conventional and Egg-Derived Synthesis

The synthesis of nanomaterials has traditionally relied on well-established chemical and physical methods, such as sol–gel processing [105,106,107,108], hydrothermal synthesis [109], chemical vapor deposition (CVD) [110], thermal decomposition of inorganic precursors [109,110] and co-precipitation [111,112]. While these approaches offer high reproducibility, precise control over composition, and scalability, they are often associated with significant environmental and economic costs. In contrast, the use of egg-derived precursors represents an emerging green alternative, aligned with the principles of sustainable chemistry and circular economy [26,113,114,115]. A comparative analysis between these two approaches highlights both the advantages and limitations of biogenic synthesis routes is presented in Figure 6.

Conventional synthesis methods are typically characterized by a high degree of control over reaction parameters and material properties. For instance, the sol–gel method enables precise tuning of particle size, morphology, and chemical composition through controlled hydrolysis and condensation reactions of metal alkoxides or salts [116].

Here is a breakdown of the two methods, structured to highlight their fundamental differences. Sol–gel is based on the hydrolysis and condensation of metal alkoxides (e.g., TEOS) or inorganic salts. It forms a three-dimensional network of oxides through chemical bonds M-O-M.

Egg white is based on the thermal denaturation of proteins. It does not create an oxide network from the beginning, but an organic matrix (protein gel) that sequesters the metal ions and forces them to react only at the combustion stage [117].

Similarly, hydrothermal allows the formation of highly crystalline nanostructures under controlled temperature and pressure conditions, often resulting in uniform particle distributions and well-defined phases [109]. These methods are particularly advantageous in applications where strict control over structural and functional properties is required, such as in electronics, photonics, or advanced catalysis.

However, these conventional techniques often involve the use of toxic solvents, hazardous reagents, and energy-intensive processes. Metal alkoxides, organic solvents, and stabilizing agents commonly used in sol–gel or chemical reduction methods can pose environmental and health risks. Additionally, high-temperature treatments, long reaction times, and specialized equipment contribute to increased energy consumption and operational costs. These factors have driven the search for more sustainable alternatives, particularly in the context of green nanotechnology.

Egg-derived synthesis methods, on the other hand, utilize naturally occurring biomolecules and inorganic components as multifunctional agents in nanomaterial formation. Eggshell provides a renewable source of calcium carbonate for the synthesis of calcium-based nanomaterials, while egg white and egg yolk contain proteins and lipids that can act as reducing, stabilizing, and templating agents. The eggshell membrane further contributes as a natural biotemplate with a hierarchical porous structure. These materials enable the synthesis of nanoparticles under milder conditions, often in aqueous media, without the need for toxic chemicals or synthetic surfactants.

One of the key advantages of egg-derived approaches is their reduced environmental impact. The use of biological waste as a raw material contributes to waste valorization and reduces dependence on non-renewable resources such as mined limestone or chemically synthesized precursors. Furthermore, many biogenic synthesis processes occur at relatively low temperatures and ambient pressures, leading to lower energy consumption compared to conventional methods. This makes egg-derived synthesis particularly attractive for applications where sustainability and eco-efficiency are prioritized.

In terms of material properties, egg-derived methods can produce nanomaterials with unique structural features that are difficult to achieve through conventional routes. The presence of organic matrices and biomolecular templates often leads to the formation of porous structures, hierarchical morphologies, and surface-functionalized nanoparticles. These characteristics can enhance performance in applications such as catalysis, adsorption, and biomedical engineering. Additionally, natural impurities present in egg-derived precursors may act as intrinsic dopants, modifying the electronic or catalytic behavior of the materials.

Nevertheless, compared to conventional synthesis techniques, egg-derived methods generally offer lower precision in controlling particle size, crystallinity, and phase purity. The complex and variable nature of biological materials introduces uncertainties in reaction mechanisms and outcomes. While conventional methods allow fine tuning of synthesis conditions through well-defined chemical pathways, biogenic approaches often rely on less predictable interactions between biomolecules and inorganic precursors. This can result in broader size distributions, mixed phases, or residual organic content, which may limit their applicability in high-performance or highly standardized technologies.

Scalability represents another important point of comparison. Conventional industrial processes for nanomaterial synthesis are already optimized for large-scale production, with established infrastructure and quality control protocols. In contrast, egg-derived methods are still largely confined to laboratory-scale studies, and their translation to industrial production requires further development. Challenges related to raw material variability, preprocessing, and process standardization must be addressed to ensure consistent product quality [110].

From an economic perspective, egg-derived synthesis offers potential cost advantages due to the low price and widespread availability of egg waste. However, the overall cost-effectiveness depends on the efficiency of processing steps such as cleaning, separation, drying, and thermal treatment. In some cases, these additional steps may offset the savings associated with the use of inexpensive raw materials [111].

To move beyond the qualitative comparison above and directly address calls for quantitative substantiation,

Table 4. Quantitative comparison of egg-derived nanomaterial synthesis with conventional sol–gel, hydrothermal, precipitation, and combustion synthesis routes.

Parameter	Egg-Derived Synthesis (This Review)	Sol–Gel [105,107,108]	Hydrothermal [109]	Precipitation/Co-Precipitation [110,111,112]	Combustion Synthesis [75]
Synthesis temperature	400–1100 °C (oxide-dependent; e.g., 400–600 °C for egg-white ZnO/CeO2, 600–1000 °C for eggshell-derived CaO, 600–900 °C for ESM pyrolysis)	Typically 400–800 °C for post-gel calcination, occasionally up to ~1000 °C for full crystallization	120–250 °C under autoclave pressure	Near ambient to ~90 °C	Self-sustained exothermic reaction reaching transient temperatures of 500–1500 °C within seconds; external ignition may require only 300–500 °C
Processing time	Minutes to hours for gel/protein-matrix formation, plus 1 to several hours of calcination	Hours to days (gelation/aging 12–48 h, plus calcination 2–6 h)	Several hours to ~24 h	Minutes to a few hours	Minutes (very fast, self-propagating reaction)
Reported yield	Rarely quantified—reported in only 1 of 12 representative studies compiled in Table 5 of this review	Generally high, though often reported qualitatively rather than as a numeric percentage	Often reported as high (>80%) under optimized conditions	Typically high (>90%)	High but variable; strongly dependent on fuel-to-oxidizer ratio
Particle-size distribution	Moderate; 5–200 nm depending on precursor (metals <20 nm; oxides 10–100 nm; eggshell-derived 20–200 nm)	Narrow, well-controlled (often 5–50 nm) via controlled hydrolysis/condensation	Narrow and highly uniform; excellent morphology control	Broader; agglomeration common without capping/stabilizing agents	Broad; nanocrystalline but often agglomerated due to the exothermic burst
Crystallinity	High after calcination, though the initial protein/lipid-matrix stage offers less precise control than solution-phase conventional routes	High, tunable via calcination temperature/time	Very high; often near-single-crystal	Variable; frequently requires post-annealing to improve crystallinity	High, achieved rapidly, though secondary/impurity phases are more common
Specific surface area (BET)	Wide range, 10–800 m2/g (eggshell-derived CaO/HA ~10–150 m2/g; ESM-derived carbon up to 800 m2/g)	Moderate–high, ~50–150 m2/g	Moderate, ~20–100 m2/g depending on morphology	Variable, ~30–120 m2/g	Variable, often lower (~10–60 m2/g) due to sintering during the exotherm
Waste generation	Minimal; aqueous/biological byproducts (CO2, N2, H2O from protein/lipid decomposition); no toxic solvents or heavy-metal salts required	Organic solvent and metal-alkoxide waste; some toxic byproducts (e.g., alcohols)	Comparatively low chemical waste (aqueous system), but energy-intensive due to pressurized/heated vessels	Significant aqueous salt/base waste (e.g., NaOH, ammonia); requires neutralization and washing steps	Gaseous combustion byproducts (e.g., NOx, CO2); minimal liquid waste
Scalability	Still largely confined to laboratory scale; feedstock variability and lack of standardized protocols limit industrial translation (Section 5)	Well-established, mature industrial technology	Proven at pilot/industrial scale, though autoclave capacity constrains batch size	Excellent; one of the most industrially scalable routes (low cost, simple equipment)	Good; single-step process, though exotherm control becomes more challenging at larger scale

benchmarks egg-derived nanomaterial synthesis against the principal conventional routes named above—sol–gel, hydrothermal, precipitation/co-precipitation, and combustion synthesis—across eight parameters: synthesis temperature, processing time, reported yield, particle-size distribution, crystallinity, specific surface area, waste generation, and scalability. Because few individual conventional-synthesis studies report all eight parameters for a single material system, the values given for sol–gel, hydrothermal, precipitation, and combustion routes reflect broad, representative ranges commonly reported across the conventional nanomaterial-synthesis literature, illustrated by the specific studies already cited above (sol–gel [105,107,108]; hydrothermal [109]; precipitation/co-precipitation [110,111,112]; combustion [75]), rather than values extracted with page-level precision from a single study; egg-derived values are drawn directly from the data presented in Section 2.1, Section 2.2, Section 2.3 and Section 2.4.

While the qualitative comparison above outlines general trends between conventional and egg-derived synthesis, a direct quantitative benchmarking against other biogenic (“green”) synthesis platforms is necessary to substantiate claims of relative performance. Table 3 compiles representative literature data for the three nanomaterials most extensively documented across the egg-derived synthesis literature ZnO, Ag, and Fe₂O₃/Fe₃O₄—comparing egg-white-mediated routes against plant-extract-, microbial-, and polysaccharide-assisted syntheses of the same materials, in terms of reported particle size, reaction conditions, yield, and ecological/cost indicators.

Several cross-cutting patterns emerge from this comparison. First, egg-white- and polysaccharide-assisted routes to metal oxides (ZnO, Fe₂O₃) generally require an additional thermal decomposition or calcination step (400–600 °C) to achieve crystalline oxide phases, whereas several plant-extract-mediated syntheses of metallic Ag nanoparticles proceed at or near room temperature within minutes (e.g., 15 min for neem-leaf-mediated Ag synthesis [118]). This reflects a mechanistic distinction rather than a general superiority of one platform: egg-white and starch matrices act primarily as chelating/templating agents for subsequent oxide crystallization, while plant polyphenols more directly reduce noble-metal ions in solution.

Second, microbial routes achieved some of the finest and most tunable particle sizes identified in this comparison (2–10 nm for bacterially synthesized Fe₃O₄ [119]), but at the cost of substantially longer processing times—up to one week of incubation, compared to hours for egg-white, plant-extract, and polysaccharide-assisted routes. This time-scale disadvantage is a critical, and frequently under-discussed, scalability trade-off when microbial platforms are proposed as viable alternatives to egg-derived precursors.

Third, and most notably, quantitative yield was explicitly reported in only one of the twelve representative studies compiled in

Table 5. Quantitative benchmarking of egg-white-mediated synthesis against plant-extract-, microbial-, and polysaccharide-assisted green synthesis of ZnO, Ag, and Fe₂O₃/Fe₃O₄ nanoparticles.

Nanomaterial	Method	Biogenic Agent	Particle Size/Distribution	Reaction Conditions (Temp./Time)	Green/ Cost Indicator	Ref.
ZnO	Egg white	Fresh hen egg white (ovalbumin), Zn(NO3)2	23.6–52.2 nm (XRD crystallite); 0.1–3.0 µm agglomerates (SEM)	RT mixing; 120 °C/4 h drying; 550 °C/2 h calcination	Aqueous; low-cost, renewable, biodegradable agent	[15]
	Plant extract	Thyme (Thymus vulgaris) leaf extract	39.4–51.86 nm (FESEM); 35.2–243.3 nm (XRD, calcination-dependent)	80 °C/3 h synthesis; 150–450 °C/2 h calcination; pH 8	Aqueous plant extract replacing toxic reductants	[120]
	Microbial	Fusarium chlamydosporum (endophytic fungus), extracellular filtrate	~19.3 nm (TEM), spherical	Not specified (extracellular, ambient culture conditions)	“eco-friendly biosynthetic technique” (qualitative)	[121]
	Polysaccharide	Bitter yam starch (native and pregelatinized)	Not quantified in nm (finer for fully green-synthesized particles, per authors)	Not fully reported—verify in full text	Starch as reducing/capping agent, aqueous medium	[122]
Ag	Egg white	Fresh hen egg white, AgNO3	~20 nm average (TEM), spherical, protein-coated	Aqueous-phase reduction, near room temperature (exact time not reported)	Simple, cost-effective, environmentally friendly	[11]
	Plant extract	Azadirachta indica (neem) aqueous leaf extract	SPR peak 436–446 nm (UV-Vis); discrete nm size not captured—verify TEM data in full text	Room temperature, 15 min	Simple, one-step, eco-friendly, non-toxic	[123]
	Microbial	Aspergillus niger, fungal filtrate	20–55 nm, spherical	Not fully specified in accessible source	Fungal extracellular route	[124]
	Polysaccharide	Carboxymethyl chitosan	6–20 nm range; mean 12.22 ± 2.57 nm (optimal formulation)	Aqueous, microwave irradiation, 10 min	Green chemistry; non-toxic; energy-efficient (microwave vs. thermal)	[125]
α-Fe2O3	Egg white	Fresh hen egg white (ovalbumin), Fe/Co chlorides	~21 nm average crystallite (pure); ~43 nm (Co-doped)	80 °C/2 h stirring (500 rpm); 550 °C/2 h calcination	Simple, green, and cost-efficient process	[62]
Fe3O4	Plant extract	Chlorella-K01 microalgal extract *	76.5 nm avg. (TEM 50–100 nm; DLS 20–200 nm), spheroidal, monodisperse	65 °C	Environmentally friendly; no toxic chemicals	[126]
	Microbial	Indigenous Fe(III)-reducing bacteria	2–10 nm (optimal); 8–12 nm at 25 °C; 6–25 nm (less uniform) at 35 °C	25 °C, 1-week incubation, pH 8.5	Clean, nontoxic, environmentally acceptable; noted as cost-efficient for scale-up	[127]
	Polysaccharide	Sodium alginate coating, co-precipitation	13.70 nm	Conditions reported pertain to the adsorption step—verify synthesis-specific temp/time	Alginate as natural biopolymer coating agent	[128]

* Chlorella is a microalga; included as the closest verified quantitative match for a non-egg biogenic route to Fe₃O₄ and flagged here as borderline between the “plant-extract” and “microbial” categories—a strictly terrestrial-plant alternative.

, nevertheless of the biogenic platform used. This systemic reporting gap, rather than being specific to egg-derived synthesis, appears to be a field-wide limitation of the green-nanomaterial-synthesis literature, and directly compounds the techno-economic uncertainty discussed further in Section 5.

Finally, this exercise also exposes genuine gaps in the egg-derived literature itself: combinations such as egg-white-mediated Fe₂O₃/Fe₃O₄ synthesis remain comparatively under-represented relative to ZnO and Ag, with fewer independent studies reporting complete quantitative characterization. We flag this explicitly in Table 3 rather than presenting an artificially complete picture, consistent with the more critical and evidence-anchored treatment requested for this review.

A SWOT analysis of egg white method is presented in Figure 7.

The SWOT analysis presented in Figure 7 should be interpreted as a qualitative, literature-based synthesis. The identified strengths and weaknesses were derived from the reported characteristics of egg-white-assisted synthesis, including its biogenic origin, protein-mediated control of nucleation and particle growth, reduced processing requirements in several synthesis routes, as well as limitations associated with precursor variability, reproducibility, and process control. Opportunities and threats were similarly identified from the technological perspectives and unresolved challenges reported in the literature, particularly those concerning process standardization, scalability, purification, and translation from laboratory-scale synthesis to reproducible large-scale production [55,92,93,94,95,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128].

In conclusion, the comparison between egg-derived and conventional synthesis methods reveals a clear trade-off between sustainability and precision. Conventional methods remain superior in terms of control, reproducibility, and scalability, making them suitable for advanced technological applications requiring strict material specifications. Conversely, egg-derived approaches provide an environmentally friendly and resource-efficient alternative, capable of generating functional nanomaterials with unique properties. Future research should focus on integrating the advantages of both approaches, for example by combining biogenic precursors with controlled synthesis techniques, to develop hybrid strategies that balance performance, cost, and sustainability.

Reproducibility remains one of the most significant challenges associated with egg-derived synthesis routes when compared to conventional nanomaterial fabrication methods. Unlike chemically defined precursors, egg-derived materials exhibit inherent compositional variability resulting from differences in species, breed, age, diet, environmental conditions, and storage history. Such variations can influence the concentration and distribution of proteins, lipids, minerals, and bioactive compounds that govern nucleation, crystal growth, and nanoparticle stabilization processes. Therefore, measurable differences in particle size, morphology, crystallinity, surface chemistry, and functional performance may occur between synthesis batches, even when nominally identical experimental conditions are applied. Studies have reported particle size variations on the order of ±10–30 nm in egg-derived mediated syntheses, highlighting the practical impact of precursor heterogeneity on process reproducibility [79,80]. Furthermore, the absence of standardized protocols for raw material selection, pretreatment, storage, and compositional characterization complicates direct comparison between studies and limits the transferability of laboratory-scale results. To address these limitations, future research should prioritize the implementation of rigorous quality-control procedures, including physicochemical characterization of biological precursors, statistical assessment of batch-to-batch variability, and the establishment of standardized synthesis protocols. Such measures are essential for improving process reliability and facilitating the scale-up and industrial adoption of egg-derived nanomaterial synthesis technologies.

3.2. Comparative Analysis of Egg-Derived Components

Although each egg-derived component has been individually investigated for nanomaterial synthesis, a direct comparative evaluation is essential to understand their relative advantages, limitations, and suitability for specific applications. In this context,

Table 6. Systematic comparison of eggshell, eggshell membrane (ESM), egg white, and egg yolk as biogenic precursors.

Parameter	Eggshell	Eggshell Membrane (ESM)	Egg White (Albumen)	Egg Yolk
Main composition	~94–97% CaCO3 (calcite), minor MgCO3, Ca3(PO4)2	Fibrous proteins (collagen, keratin), polysaccharides	~90% water, ~10% proteins (ovalbumin, lysozyme)	~32–35% lipids, ~16–18% proteins, ~8–12% phospholipids
Dominant functional role	Inorganic precursor (Ca source)	Biotemplate, adsorbent, carbon precursor	Chelating agent, reducing agent, stabilizer	Emulsifier, soft template, stabilizer
Key mechanism	Thermal decomposition (CaCO3 → CaO) and precipitation	Ion adsorption and templated growth on fibrous network	Protein denaturation generates gel and uniform ion distribution	Phospholipid self-assembly generates micelles/vesicles and confined nucleation
Typical synthesis routes	Calcination (600–1000 °C), precipitation, sol–gel	Adsorption, reduction, pyrolysis (carbonization)	Sol–gel-like, combustion, co-precipitation	Nanoemulsion-assisted synthesis, lipid-mediated reduction
Typical nanomaterials obtained	CaO, CaCO3, hydroxyapatite (HA), Ca phosphates	Metal NPs, metal oxides, carbon materials	ZnO, TiO2, Fe2O3, CeO2, NiO, perovskites, ferites	Ag, Au nanoparticles, nanoemulsions, hybrid oxides
Nanoparticle size range	20–200 nm (CaO, HA)	5–100 nm (metal NPs, carbon dots)	10–100 nm (oxides), <20 nm (metals)	5–50 nm (metals), 80–200 nm (liposomes)
Morphology control	Depends on precipitation conditions	Guided by fibrous structure	Protein matrix controls growth	Micelle/vesicle confinement)
Specific surface area (BET)	10–80 m2/g (CaO), up to ~150 m2/g (HA)	High: 200–800 m2/g (carbonized ESM)	High: 50–200 m2/g (porous oxides)	Moderate: 50–150 m2/g
Crystallinity	High after calcination	Moderate (depends on treatment)	High at relatively low temperatures (400–600 °C)	Moderate; may require post-treatment
Porosity	Moderate (inherited from shell structure)	Very high (hierarchical porous network)	High (foamy structure after protein decomposition)	Moderate (depends on emulsion template)
Thermal behavior	CaCO3 decomposition, CO2 release	Carbonization generates porous carbon andheteroatom doping	Exothermic protein decomposition gas release (CO2, N2, H2O)	Lipid decomposition possible carbon residues
Metal nanoparticle synthesis	Limited	Very effective (surface functional groups bind ions)	Possible but requires controlled atmosphere	Highly effective for noble metals (Ag, Au)
Metal oxide synthesis	Excellent (Ca-based materials)	Good (template-assisted oxides)	Excellent (ZnO, TiO2, Fe2O3, etc.)	Moderate (via nanoemulsions)
Carbon-based materials	Possible (residual carbon structures)	Excellent (activated carbon)	Possible (residual carbon structures)	Possible (carbon doping from lipids)
Antibacterial activity	CaO, HA: moderate (pH effect, ROS)	High (metal NP-loaded ESM systems)	High (ZnO, Ag systems; inhibition zones 10–25 mm)	High (Ag NPs; inhibition zones 12–20 mm; MIC 10–50 µg/mL)
Photocatalytic performance	Moderate (HA-based systems)	Moderate–high (hybrid systems)	High (ZnO, TiO2 with high surface area)	Moderate (enhanced via carbon doping)
Biomedical applications	Excellent (bone regeneration, implants, HA)	Good (scaffolds, tissue engineering)	Limited (indirect via oxides)	Excellent (liposomes, drug delivery, SLNs)
Reproducibility	High (inorganic composition stable)	Moderate (biological variability)	Good (protein composition relatively stable)	Lower (lipid composition varies)
Scalability	High (abundant waste material)	Moderate	Moderate–high	Moderate
Major advantages	Low cost, high Ca content, biomedical relevance	Unique 3D template, high surface area	Excellent control over size, morphology, crystallinity	Ideal for nanoemulsions and biomedical systems
Main limitations	Limited to Ca-based materials	Variability, structural heterogeneity	Risk of agglomeration if poorly controlled	Lipid residues, reproducibility issues
Best suited applications	Bioceramics, catalysis, CO2 capture	Adsorption, catalysis, energy storage	Photocatalysis, sensors, oxide nanomaterials	Drug delivery, nanomedicine, noble metal NPs

. provides a systematic comparison of eggshell, eggshell membrane (ESM), egg white, and egg yolk as biogenic precursors. The comparison is based on critical parameters such as composition, synthesis mechanism, control over particle size and morphology, surface properties (e.g., specific surface area), and functional performance (e.g., antibacterial and photocatalytic activity). This integrative analysis enables the identification of structure–function relationships and offers practical guidelines for selecting appropriate bio-precursors in green nanotechnology.

As shown in Table 6, significant differences can be observed among the four egg-derived components. Eggshell is particularly suitable for the synthesis of calcium-based materials and biomedical applications due to its high CaCO₃ content and structural similarity to bone minerals. In contrast, eggshell membrane provides a unique fibrous architecture that enables templated growth and the formation of hierarchical nanostructures, especially for carbon-based materials and supported catalysts.

Egg white offers superior control over nanoparticle size, morphology, and crystallinity, primarily due to its protein-mediated gel network, making it highly effective for the synthesis of metal oxides and functional ceramics [109]. Conversely, egg yolk is more suitable for soft-templating strategies and nanoemulsion-based systems, particularly in the synthesis of noble metal nanoparticles and lipid-based nanocarriers for biomedical applications [110].

Therefore, the choice of egg-derived precursor should be guided by the desired material properties and application requirements, balancing factors such as structural control, scalability, and functional performance.

4. Determinants of Synthesis Success in Egg-Assisted Nanomaterial Preparation: Mechanistic Insights

The successful synthesis of nanomaterials using egg-derived precursors, particularly egg white (ovalbumin), is governed by a complex interplay of physicochemical parameters that control nucleation, growth, and final material properties. Unlike conventional chemical routes, where reaction pathways are well-defined and highly controllable, protein-assisted synthesis relies on biomolecular interactions that introduce additional variables. This section addresses this gap by examining, in turn, the coordination chemistry between metal ions and the functional side chains of egg-white proteins (Section 4.1), the nucleation and growth pathways that emerge from protein-gel confinement (Section 4.2), the specific reduction chemistry responsible for zero-valent metal nanoparticle formation (Section 4.3), and, finally, how the macroscopic parameters of protein concentration, solution pH, and thermal treatment modulate these underlying molecular mechanisms in practice (Section 4.4).

4.1. Metal–Protein Interaction Mechanisms

Egg white is not a single protein, but a mixture dominated by ovalbumin (~54% of total protein), together with ovotransferrin (~12%), ovomucoid (~11%), and lysozyme (~3.5%), among minor globulins [7,8]. Each of these proteins presents a distinct constellation of coordinating side chains to solution-phase metal ions, and it is the sum of these interactions—rather than any single functional group—that accounts for the chelating behavior attributed to “egg white” throughout this review.

Four side-chain donor groups are principally responsible for metal-ion coordination: the carboxylate oxygens of aspartate and glutamate residues, which act as hard O-donors well suited to alkaline-earth and early transition-metal ions (e.g., Mg²⁺, Ca²⁺, Zn²⁺); the imidazole nitrogen of histidine, a moderately soft N-donor with strong affinity for transition-metal ions such as Cu²⁺, Ni²⁺, and Fe³⁺; the ε-amine nitrogen of lysine, which becomes an effective ligand once deprotonated at alkaline pH and is likely the dominant contributor to the –NH₂-mediated chelation described in Section 2.2 for Zn²⁺ and Mg²⁺; and the thiol sulfur of cysteine, a soft, highly polarizable S-donor with the highest affinity of the four for soft metal centers such as Ag⁺ and Cu⁺, and the group principally responsible for the redox chemistry discussed in Section 4.3.

The distribution of these groups is not uniform across the egg-white proteome. Ovotransferrin is the most structurally specialized of the four proteins with respect to metal binding, consistent with its established role as the principal iron-binding protein of hen egg white [7,8]. Like other members of the transferrin protein family, it is generally understood to possess two homologous lobes, each capable of coordinating a single Fe³⁺ (or, less selectively, Cu²⁺, Zn²⁺, or Al³⁺) ion together with a synergistic (bi)carbonate anion. This 1:1 (metal:lobe) stoichiometry is markedly more defined than the comparatively diffuse, multivalent binding reported for ovalbumin, where multiple surface Asp/Glu/Lys residues from one or several unfolded chains can coordinate a single divalent ion simultaneously, giving apparent metal: protein stoichiometries that are concentration- and pH-dependent rather than fixed [8,49]. Lysozyme, by contrast, contributes comparatively little redox-active or thiol-based coordination capacity: being a compact, highly cross-linked globular protein whose cysteines are predominantly engaged in intramolecular disulfide bonds, it presents markedly fewer free thiol groups than ovalbumin, leaving surface Asp/Glu/His residues as its main coordination sites unless the protein is reduced or denatured [7,8]. Ovalbumin itself occupies an intermediate position: of its six cysteine residues, one pair forms an intramolecular disulfide bond while four remain as free sulfhydryl groups in the native, folded protein, but these are substantially buried within the hydrophobic core and are only partially accessible to solution-phase metal ions under native conditions [8].

This last point is mechanistically important, because it links directly to the pH- and temperature-dependent behavior discussed in Section 4.4. Thermal or pH-induced denaturation unfolds the native globular fold of ovalbumin and, to a lesser extent, the other egg-white proteins, exposing buried cysteine thiols, hydrophobic patches, and additional carboxylate and amine groups that are inaccessible in the compact native state. This denaturation-induced exposure of reactive groups—rather than a simple in-crease in the total number of binding sites—is the molecular basis for the improved ion-chelation efficiency observed at moderately acidic or basic pH and elevated temperature, and it directly enables the reduction chemistry discussed in Section 4.3, since newly exposed free thiols are the principal electron donors available for Ag⁺ and Cu²⁺ reduction.

4.2. Nucleation and Growth Pathways

The classical description of nucleation from solution, treats nanoparticle formation as a two-stage process—a rapid, homogeneous “burst” nucleation event once a critical supersaturation of monomeric ionic or molecular species is exceeded, followed by diffusion-limited growth of the resulting nuclei as the supersaturation relaxes toward equilibrium. This classical, burst-nucleation framework provides a reasonable first approximation for dilute, well-mixed aqueous syntheses in which reactive species are free to diffuse, but it assumes a homogeneous reaction medium that does not directly apply to protein-gel-confined systems such as those described throughout this review.

In egg-white-mediated synthesis, metal ions are not free in solution but are pre-organized within a three-dimensional protein network before thermal treatment begins (Section 3.1). Under these conditions, nucleation and growth are increasingly understood, by analogy with other biotemplated and protein-confined mineralization systems, to proceed through non-classical, aggregation-mediated pathways rather than through classical monomer-by-monomer burst nucleation. In such pathways, small primary clusters or amorphous precursor particles first form locally at coordination sites within the protein matrix and subsequently coalesce, via processes such as oriented attachment or particle-mediated aggregation and recrystallization, into the final crystalline nanostructure [4]. Direct evidence for such a pathway in an egg-derived, protein-confined system has been reported for eggshell-membrane-assisted growth of ZnO, where single-crystalline structures were shown to form via oriented attachment of smaller primary particles rather than by classical ion-by-ion growth [4]—a mechanism that is plausibly generalizable, at least in part, to the egg-white-confined oxide and metal syntheses discussed in Section 2.2.1, Section 2.2.2, Section 2.2.3 and Section 2.2.4, though this has not yet been directly verified by in situ techniques for egg white itself.

The protein network additionally imposes a physical, rather than purely chemical, constraint on growth. The mesh size and porosity of the protein gel—governed by protein concentration and the degree of thermally induced cross-linking (Section 4.4)—define a finite confinement volume around each nucleation site. This confinement limits the diffusion of additional metal ions or growing clusters toward a given nucleus, effectively capping particle size well before the gel is destroyed during calcination, and simultaneously suppresses long-range particle migration and coalescence, which is the principal cause of agglomeration in unconfined systems. This provides a mechanistic explanation, rather than a purely empirical one, for why particle sizes across egg-white-mediated syntheses cluster so consistently within the 10–100 nm range for oxides and below 20 nm for metals (Section 2.2.1 and Section 2.2.2): growth is not limited primarily by thermodynamic driving force, as in classical nucleation theory, but by the finite transport capacity of the surrounding protein-gel network. As the gel decomposes during calcination and its mesh structure collapses, this confinement is progressively lost, which is also why excessive heating rates—by destroying the confining network abruptly rather than gradually—tend to promote uncontrolled agglomeration, as discussed further in Section 4.4.

4.3. Reduction Mechanisms for Zero-Valent Metal Nanoparticle Formation

As illustrated schematically in Figure 3, egg-white-mediated synthesis diverges mechanistically depending on whether the target product is a metal oxide or a zero-valent metal. In oxide formation, egg-white proteins act principally as chelating and templating agents: metal ions remain in an oxidized state throughout processing and are converted to the corresponding oxide during oxidative calcination in air (Section 2.2.1). In metal nanoparticle formation, by contrast, the protein matrix must additionally supply electrons to reduce the metal ion to its zero-valent state before or during thermal treatment, while simultaneously acting as a capping agent that arrests particle growth (Section 2.2.2). Although Figure 3 depicts this contrast schematically, the electron-donating chemistry that enables the reduction step has not, until now, been discussed explicitly in the text.

Two complementary electron-donating pathways account for this reduction chemistry. The first is direct oxidation of protein thiols. Free cysteine sulfhydryl groups—exposed upon thermal denaturation, as discussed in Section 4.1—are readily oxidized to disulfides (2 Cys–SH → Cys–S–S–Cys + 2H⁺ + 2e⁻), and the electrons released in this reaction are available to reduce soft, thiophilic metal-ion centers, most notably Ag⁺ (E°(Ag⁺/Ag) = +0.80 V) and, under appropriate conditions, Cu²⁺ (via a stepwise Cu²⁺ → Cu⁺ → Cu⁰ pathway), consistent with the well-documented use of free cysteine and other thiol-bearing biomolecules as reducing agents in protein-mediated metal nanoparticle synthesis [71]. Because Ag⁺ is thermodynamically easier to reduce than Cu²⁺, this thiol-mediated pathway is consistent with the comparative ease of egg-white-mediated Ag⁰ formation described in Section 2.2.2 and suggests that the –NH₂/–OH groups invoked there are more accurately understood as stabilizing/capping ligands that limit crystal growth once Ag⁰ nuclei have formed, rather than as the primary reducing species [72].

The second pathway is Maillard-type chemistry. Egg white contains a small but significant pool of free reducing sugars (primarily glucose, at approximately 0.4–0.5 wt% [1]), which, upon thermal denaturation of the protein matrix, can react with the ε-amine groups of lysine residues (and the N-terminal amine) via the classical Maillard reaction. The resulting rearrangement products and their downstream degradation intermediates (reductones, dicarbonyls, and related enediol species) are themselves electron-donating and can act as auxiliary reducing agents, particularly at the higher processing temperatures used for less easily reduced ions such as Cu²⁺. This offers a mechanistic explanation for why copper nanoparticle formation in this system (Section 2.2.2) is reported to require controlled thermal treatment under an inert atmosphere rather than proceeding efficiently at or near room temperature as observed for Ag⁺:Cu²⁺ reduction likely relies more heavily on this thermally activated Maillard-derived pathway (and, at higher temperature, on carbothermal reduction by the protein-derived carbonaceous residue itself) than on the thiol-mediated pathway that dominates for Ag⁺.

Together, these two pathways reconcile the “dual role” of egg white described in Section 2.2.2—simultaneously a reducing agent and a capping/stabilizing agent—by assigning distinct chemical functions to distinct classes of protein functional groups: thiol- and Maillard-derived species as the principal electron donors, and amine/hydroxyl/carboxylate groups as the principal surface-stabilizing ligands that constrain post-nucleation growth. This distinction, made explicit here for the first time in the text accompanying Figure 3, also clarifies why oxide- and metal-forming systems—despite using the same protein precursor—diverge so markedly in their processing requirements (oxidative calcination in air versus reducing/inert-atmosphere thermal treatment), a point that the figure conveys visually but that had not previously been unpacked mechanistically in the main text.

4.4. Process Parameters Governing Synthesis Outcomes

The molecular mechanisms described in Section 4.1, Section 4.2 and Section 4.3 are, in practice, controlled experimentally through a small number of macroscopic processing parameters. The optimization of these parameters—often referred to as a “critical balance” or functional synthesis window—is essential for achieving reproducible and high-quality nanostructures, and each can be understood as a lever that tunes one or more of the underlying mechanisms discussed above.

• Protein Concentration

Protein concentration plays a fundamental role in determining the morphology, dispersion, and crystallinity of the synthesized nanomaterials. Egg white proteins, primarily ovalbumin, act as chelating, templating, and stabilizing agents due to the presence of functional groups such as –NH₂, –COOH, and –OH. At low protein concentrations, the number of available binding sites for metal ions is insufficient, leading to incomplete chelation and uneven ion distribution. This results in uncontrolled nucleation and significant particle agglomeration during subsequent thermal treatment.

Conversely, excessive protein content introduces a high organic load into the system. During calcination, the decomposition of this organic matrix generates large volumes of gaseous by-products (e.g., CO₂, H₂O, N₂), which may induce localized overheating, structural collapse, or the formation of residual carbonaceous phases. Additionally, higher temperatures are required to ensure complete removal of organic residues, which may negatively affect energy efficiency and phase stability. Therefore, an optimal protein-to-precursor ratio is required to balance ion distribution and thermal decomposition behavior.

• Solution pH

The pH of the reaction medium critically influences protein conformation, ion-binding capacity, and overall reaction kinetics. Proteins are highly sensitive to pH variations, which affect their tertiary structure and the ionization state of functional groups. At specific pH values, particularly near the isoelectric point, proteins exhibit minimal net charge and reduced solubility, leading to aggregation and decreased interaction with metal ions.

At moderately acidic or basic pH values, proteins unfold or partially denature, exposing reactive functional groups that enhance their ability to chelate metal ions. This facilitates homogeneous ion distribution within the protein matrix and promotes controlled nucleation during thermal treatment. Therefore, careful pH adjustment is essential to maximize the efficiency of biomolecule–ion interactions and to ensure uniform nanoparticle formation.

• Heating rate and thermal treatment

The thermal profile applied during synthesis has a direct impact on crystallization, porosity, and phase evolution. A slow and controlled heating rate allows gradual decomposition of the protein matrix, facilitating uniform nucleation and growth of crystalline phases. This typically results in well-defined, highly crystalline nanoparticles with reduced defect density.

In contrast, rapid heating or combustion-like processes lead to abrupt decomposition of the organic matrix, generating high internal pressures and rapid gas release. This phenomenon often produces highly porous, foam-like structures with increased surface area but reduced crystallinity and structural uniformity. While such morphologies may be advantageous for catalytic or adsorption applications, they may be unsuitable for applications requiring precise structural control.

Thus, the heating rate must be tailored according to the intended application, balancing crystallinity and porosity.

5. Challenges and Limitations of Egg-Derived Precursors in Nanomaterial Synthesis

Despite the significant advantages associated with the use of egg-derived materials in green nanotechnology, including sustainability, low cost, and multifunctionality, several challenges and limitations remain that hinder their widespread implementation and industrial scalability. A critical evaluation of these aspects is essential to provide a balanced perspective and to guide future research directions in this field. Key issues include compositional variability, limited control over synthesis parameters, presence of organic residues, scalability constraints, mechanistic uncertainties, trace impurities, and regulatory concerns. These are represented in Figure 8.

An additional challenge associated with the use of egg-derived precursors in nanomaterial synthesis concerns their potential application in biomedical fields, where stringent safety and regulatory requirements must be satisfied. Although thermal treatments, calcination processes, and chemical purification steps generally reduce or eliminate most biological contaminants, the possible presence of residual allergenic proteins, such as ovalbumin, ovomucoid, ovotransferrin, and lysozyme, cannot be entirely excluded without dedicated purification and analytical verification. These proteins may trigger immunogenic or allergic responses in susceptible individuals, particularly when nanomaterials are intended for direct contact with biological tissues or systemic administration. Furthermore, biological raw materials may contain microbial contaminants, endotoxins, or pathogen-derived residues originating from handling, storage, or processing conditions.

Importantly, the biogenic origin of these materials should not be considered evidence of inherent biological safety. Nanotoxicological evaluation should address the properties of the final nanomaterial, including its composition, particle size and morphology, surface chemistry, impurities, degradation behavior, dose, and route of exposure. Depending on the intended biomedical application, appropriate assessment of cytotoxicity, inflammatory responses, hemocompatibility, and other relevant biological endpoints is therefore required [26,30,41,45,103]. Furthermore, biological raw materials may contain microbial contaminants, endotoxins, or pathogen-derived residues originating from handling, storage, or processing conditions. While high-temperature synthesis routes typically ensure effective microbial inactivation, low-temperature or biomolecule-preserving synthesis approaches may require additional sterilization procedures. Consequently, comprehensive post-synthesis purification, endotoxin testing, sterility assessment, and biocompatibility evaluation are essential prior to biomedical implementation. Standard sterilization methods, including autoclaving, gamma irradiation, ethylene oxide treatment, or sterile filtration, should be carefully selected according to the physicochemical stability of the nanomaterial.

From a translational perspective, reproducible precursor composition, standardized synthesis and purification procedures, batch-to-batch quality control, comprehensive physicochemical characterization, and validated biological-safety testing remain essential requirements for progressing from laboratory-scale studies toward biomedical-grade materials. Future research should therefore focus on establishing standardized protocols for contaminant removal, biological safety assessment, and regulatory compliance to facilitate the translation of egg-derived nanomaterials from laboratory-scale investigations to clinical and biomedical applications.

Future research should focus on establishing standardized protocols for contaminant removal, biological safety assessment, and regulatory compliance to facilitate the translation of egg-derived nanomaterials from laboratory-scale investigations to clinical and biomedical applications.

One of the primary limitations is the inherent variability of biological precursors. Egg-derived materials such as eggshell, eggshell membrane, egg white, and egg yolk exhibit compositional differences depending on factors such as species, diet, age, and environmental conditions. These variations can influence the concentration of calcium carbonate, protein content, lipid composition, and trace elements, leading to inconsistencies in synthesis outcomes. Consequently, reproducibility becomes a significant challenge, particularly when aiming to obtain nanomaterials with well-defined physicochemical properties, such as particle size, morphology, crystallinity, and surface functionality.

Another important issue is the limited control over synthesis parameters compared to conventional chemical methods. While biological molecules act as natural reducing, stabilizing, and templating agents, their behavior is often complex and difficult to predict. The interactions between biomolecules and metal ions depend on multiple variables, including pH, ionic strength, temperature, and precursor concentration. This complexity can result in heterogeneous nucleation processes and broader particle size distributions, which may not be suitable for applications requiring high precision and uniformity, such as nanoelectronics or advanced photonic systems.

In addition, the presence of organic residues represents a critical challenge, especially in processes involving thermal treatment. During calcination or pyrolysis, incomplete decomposition of proteins, lipids, or polysaccharides may lead to the formation of carbonaceous impurities or residual functional groups on the surface of the nanomaterials. While in some cases these residues can be beneficial, contributing to surface functionalization or heteroatom doping, in other situations they may negatively affect material purity, stability, or performance. Therefore, additional purification steps or optimized thermal protocols are often required to ensure the desired material characteristics.

Scalability and process standardization constitute one of the most consequential, yet still only partially addressed, limitations for the industrial application of egg-derived nanomaterials. Most protocols reported in the literature are optimized empirically at laboratory scale—often for a single batch of egg white or eggshell from a specific supplier—with process parameters (protein-to-precursor ratio, pH, heating rate, calcination temperature) reported as point values rather than validated operating windows. This is illustrated by the wide range of calcination conditions reported for even a single oxide across independent studies (e.g., 300–700 °C for NiO and 400–600 °C for ZnO, Table 1), a spread that reflects the absence of standardized synthesis protocols and quality-control benchmarks comparable to those routinely applied in conventional ceramic powder production [107,108]. Without such standardization, batch-to-batch reproducibility cannot be guaranteed even within a single laboratory, let alone across different production sites, which is a prerequisite for any regulatory or industrial qualification pathway.

A second, closely related challenge is feedstock variability. Unlike synthetic precursors accompanied by a certificate of analysis, hen eggs are a biological product whose physical and compositional attributes vary measurably with hen breed, age, diet, laying-cycle stage, and storage conditions [2]. Because egg white protein content and eggshell mineral composition are not independently certified inputs, this variability propagates directly into the nanomaterial synthesis step, contributing to the ±10–30 nm particle-size variation already noted in Section 3.1 for nominally identical experimental protocols. Proteomic and compositional atlases of egg white and yolk [16] provide a starting point for characterizing this variability, but, to date, none of the egg-derived nanomaterial synthesis studies reviewed here has incorporated feedstock characterization (e.g., protein content assay, compositional fingerprinting) as a controlled input variable, leaving the relative contribution of feedstock variability versus process variability unresolved.

Techno-economic factors remain the least rigorously documented aspect of egg-derived nanomaterial synthesis. As already noted in the discussion of Table 2 (Section 3.1), quantitative yield was reported in only one of the twelve representative studies compiled across egg-white-, plant-extract-, microbial-, and polysaccharide-mediated green-synthesis routes, and none of the surveyed studies reported a capital or operating cost estimate, an energy balance, or a formal life-cycle assessment (LCA) for the egg-derived process itself. Broader circular-economy analyses of egg by-products indicate that the economic viability of valorizing egg-processing waste depends strongly on the scale and proximity of collection, cleaning, and preprocessing infrastructure relative to the waste-generating facility, and that these logistical costs can be comparable in magnitude to the value of the recovered material [110,111]. In the absence of nanomaterial-specific techno-economic assessments, these broader circular-economy studies of egg by-product valorization [110,111] represent the closest available benchmark, and their cost structures (collection, cleaning, drying, and preprocessing overhead) are directly transferable to the synthesis routes discussed in this review. A dedicated techno-economic analysis—comparing the fully loaded cost per gram of egg-derived nanomaterial (including feedstock collection, protein extraction or eggshell processing, and calcination energy) against conventional sol–gel or hydrothermal routes [112]—is, to our knowledge, entirely absent from the literature and represents a priority for future work.

Finally, translating egg-derived synthesis from laboratory batches to continuous or semi-continuous industrial production introduces engineering challenges that are qualitatively different from those encountered at the bench. Egg white and eggshell are perishable, microbiologically active biological materials that require refrigerated or rapid processing after collection to avoid spoilage-driven compositional drift, in contrast to the shelf-stable synthetic precursors (metal alkoxides, nitrates) used in conventional routes. Adapting existing industrial food-waste-processing infrastructure—such as that developed for hydrothermal treatment of other food-industry by-products [114]—may offer a more realistic scale-up pathway than de novo process design, since it would leverage established collection, cleaning, and thermal-processing logistics rather than requiring entirely new infrastructure. Nonetheless, the batch-to-batch feedstock heterogeneity discussed above is likely to remain a rate-limiting factor for continuous-flow implementation until standardized feedstock characterization and pre-processing protocols are established.

The influence of feedstock variability is component-specific and may propagate through different stages of nanomaterial formation. Variations in eggshell mineral composition and residual organic matter can affect the effective calcium content, thermal decomposition behavior, phase purity, and reactivity of CaCO₃/CaO-derived materials. In egg white, differences in total protein content and relative protein composition may modify metal-ion coordination, gel formation, nucleation, and particle stabilization, whereas variations in yolk lipid and phospholipid composition may influence emulsification, interfacial organization, and soft-templating behavior. For ESM, changes in membrane composition, thickness, fiber organization, and surface functional groups may affect ion binding and template-directed nanostructure growth. Consequently, biological variability can translate into differences in particle size and morphology, crystallinity, phase composition, surface chemistry, and ultimately functional performance.

Future translation therefore requires egg-derived feedstocks to be treated as controlled raw materials rather than as compositionally invariant biological waste. Practical standardization should include traceable sourcing, defined species or breed where relevant, controlled storage time and temperature, and fraction-specific washing, separation, drying, grinding, and thermal-processing protocols. Batch-level quality control should additionally quantify critical attributes appropriate to each fraction, such as CaCO₃ and major mineral content for eggshell, total protein content for egg white, lipid/phospholipid composition for egg yolk, and compositional and structural characteristics of ESM. Establishing acceptable compositional ranges before synthesis, together with pooling or homogenization of characterized feedstock batches and standardized synthesis operating windows, would help distinguish feedstock-induced variability from process-induced variability and improve batch-to-batch reproducibility. Such precursor specifications should ultimately be linked to critical quality attributes of the resulting nanomaterials, including particle-size distribution, phase purity, morphology, surface chemistry, and application-specific performance.

From a mechanistic perspective, there is still a limited understanding of the fundamental processes governing nanomaterial formation in biogenic systems. In many cases, the proposed mechanisms, such as the involvement of reactive oxygen species (ROS), pH-induced antibacterial effects, or specific biomolecule–ion interactions—remain partially speculative due to insufficient experimental validation. The absence of in situ characterization techniques, kinetic studies, or quantitative analyses (e.g., ROS detection, pH monitoring, or binding studies) reduces the ability to establish clear structure–property relationships. This lack of mechanistic insight limits the rational design and optimization of synthesis processes.

Another limitation relates to the potential presence of impurities originating from the biological source. Egg-derived materials may contain trace elements, contaminants, or residues associated with animal feed or environmental exposure. These impurities can act as unintended dopants, influencing the structural, electronic, or catalytic properties of the resulting nanomaterials. While in some cases such effects may be advantageous, they can also lead to variability and unpredictability, particularly in applications requiring high purity and strict compositional control.

Finally, as discussed above, economic and logistical aspects—feedstock collection, cleaning, drying, and thermal-treatment overheads—remain insufficiently quantified for egg-derived routes. A comprehensive, nanomaterial-specific life-cycle assessment, benchmarked against the circular-economy cost structures reported for other egg by-product valorization pathways [110,111], is needed to evaluate the true environmental and economic benefits of these green synthesis approaches relative to conventional methods.

In conclusion, although egg-derived precursors offer promising opportunities for sustainable nanomaterial synthesis, several critical challenges related to variability, reproducibility, process control, mechanistic understanding, scalability, and regulatory compliance must be addressed. Overcoming these limitations will require interdisciplinary efforts focused on process optimization, advanced characterization techniques, and the development of standardized protocols, ultimately enabling the transition from laboratory-scale demonstrations to industrially viable technologies.

6. Guidelines for Selecting Egg-Derived Precursors in Nanomaterial Synthesis

The selection of appropriate egg-derived precursors, namely eggshell, eggshell membrane, egg white, and egg yolk represents a critical step in the rational design of nanomaterials within the framework of green nanotechnology. Each component exhibits distinct physicochemical characteristics that directly influence nucleation mechanisms, particle growth, morphology, and final material performance. Therefore, a systematic selection strategy based on the targeted application and desired material properties is essential.

6.1. Selection Based on Target Material Composition

The chemical nature of the desired nanomaterial is a primary determinant in precursor selection. Eggshell, composed predominantly of calcium carbonate (CaCO₃), is particularly suitable for the synthesis of calcium-based materials such as calcium oxide (CaO), hydroxyapatite (HA), and other calcium phosphates. These materials are widely used in biomedical applications, including bone regeneration and implant coatings, where compositional similarity to natural bone is advantageous.

In contrast, egg white is more appropriate for the synthesis of transition metal oxides (e.g., ZnO, TiO₂, Fe₂O₃, CeO₂), owing to its high protein content and strong chelating capability. The protein matrix ensures homogeneous distribution of metal ions at the molecular level, facilitating the formation of highly crystalline phases. Egg yolk, due to its lipid-rich composition, is particularly suitable for the synthesis of noble metal nanoparticles (e.g., Ag, Au) and hybrid nanostructures, where soft-templating and stabilization are required.

Eggshell membrane (ESM) occupies an intermediate position, serving as both a template and a precursor for carbon-based materials and supported nanostructures. Its fibrous architecture enables the synthesis of hierarchical nanomaterials, including metal-decorated fibers and porous carbon matrices.

6.2. Selection Based on Structural and Morphological Control

When precise control over nanoparticle size, morphology, and dispersion is required, egg white represents the most effective precursor. The denaturation and subsequent decomposition of proteins generate a three-dimensional gel network that confines metal ions and promotes uniform nucleation. This often results in nanoparticles with sizes in the range of 10–100 nm and reduced agglomeration. Key issues include compositional variability, limited control over synthesis parameters, presence of organic residues, scalability constraints, mechanistic uncertainties, trace impurities, and regulatory concerns. These factors influence reproducibility, material performance, and industrial applicability.

Egg yolk, by contrast, provides control through self-assembled lipid structures such as micelles and vesicles. While this approach enables the formation of relatively uniform nanoparticles (typically 5–50 nm for noble metals), the degree of morphological control is generally lower compared to protein-mediated systems. However, yolk-derived systems are particularly advantageous for producing nanoemulsions and soft nanostructures.

Eggshell membrane offers unique advantages for the fabrication of one-dimensional or hierarchical structures, such as nanofibers, nanotubes, and coated networks. This makes it particularly suitable for applications requiring high surface area and structural complexity, such as catalysis and adsorption.

6.3. Selection Based on Functional Performance

The intended application of the nanomaterial should guide the selection of the egg-derived precursor. For photocatalytic and catalytic applications, egg white-derived materials often exhibit superior performance due to their high specific surface area (typically 50–200 m²/g) and enhanced crystallinity. Additionally, residual nitrogen species from protein decomposition may lead to in situ doping, further improving catalytic activity.

For antibacterial applications, both egg white- and egg yolk-derived systems are effective, particularly when used for the synthesis of silver or zinc oxide nanoparticles. Reported inhibition zones typically range from 10 to 25 mm, depending on particle size and concentration, while minimum inhibitory concentrations (MIC) can fall within the range of 10–50 µg/mL.

In biomedical applications, egg yolk is the preferred precursor due to its lipid composition and inherent biocompatibility. Lecithin-based nanocarriers, such as liposomes and solid lipid nanoparticles, exhibit particle sizes between 80 and 200 nm and encapsulation efficiencies up to 90%, making them suitable for drug delivery and imaging applications.

Eggshell-derived hydroxyapatite is particularly advantageous in bone tissue engineering, where its osteoconductive properties and chemical similarity to biological apatite promote cell adhesion and proliferation.

6.4. Selection Based on Process Conditions and Scalability

From a processing perspective, eggshell-based synthesis typically requires high-temperature calcination (600–1000 °C), making it more energy-intensive but highly scalable due to the abundance and stability of the raw material. Egg white-based methods often operate at lower temperatures and can produce crystalline materials at 400–600 °C, offering a balance between energy efficiency and material quality.

Egg yolk-based synthesis, while advantageous for soft-templating and biomedical applications, may require additional steps to remove residual lipids, particularly in high-purity applications. Furthermore, the variability in yolk composition can affect reproducibility.

Eggshell membrane-based methods are generally limited by scalability and variability but offer unique advantages in applications requiring hierarchical structures and carbon-based materials.

6.5. Integrated Selection Strategy

Based on the above considerations, the selection of egg-derived precursors can be summarized as follows:

• Eggshell best suited for calcium-based materials and large-scale applications;
• Eggshell membrane (ESM) is ideal for templated synthesis and carbon-based nanomaterials;
• Egg white is optimal for controlled synthesis of metal oxides and catalytic materials;
• Egg yolk is preferred for noble metal nanoparticles and biomedical nanocarriers.

In practice, hybrid approaches that combine multiple egg-derived components may offer synergistic advantages. For example, combining eggshell-derived calcium sources with egg white-mediated templating can enhance both compositional control and structural uniformity. Such integrated strategies represent a promising direction for future research.

7. Conclusions

Egg-derived precursors represent a versatile and sustainable platform for nanomaterial synthesis, offering unique advantages through their intrinsic multifunctionality. The distinct roles of eggshell, eggshell membrane, egg white, and egg yolk enable the fabrication of a wide range of nanomaterials with tunable structural and functional properties.

However, challenges related to reproducibility, compositional variability, and process scalability remain significant barriers to industrial implementation. Addressing these limitations requires the development of standardized protocols, improved mechanistic understanding, and advanced characterization techniques.

Future research should focus on integrating egg-derived systems with conventional synthesis methods, enabling hybrid approaches that combine sustainability with precise control over material properties. Such strategies are expected to enhance the applicability of egg-derived nanomaterials in emerging fields including catalysis, biomedicine, and energy technologies.