Rational Functional Design of Carbon Quantum Dots for Food Safety and Preservation: A Critical Review

Abstract

Carbon quantum dots (CQDs) have attracted considerable attention as versatile fluorescent nanomaterials in the domains of food safety and preservation, primarily due to their tunable photoluminescence, high aqueous dispersibility, and favorable biocompatibility. Although numerous reviews have documented the synthesis and extensive applications of CQDs, a focused critical assessment specifically addressing how rational surface functionalization and heteroatom doping impact their performance within complex food matrices remains absent. This review provides a targeted analysis of the interplay between the functional design of CQDs, including both surface group engineering and elemental doping, and their practical efficacy in food-related applications. Initially, a concise overview of the fundamental aspects of CQDs relevant to their functionality is presented, emphasizing the origin and role of surface chemical groups and pivotal photophysical sensing mechanisms. Subsequently, the core of the review critically evaluates recent advancements (particularly those from 2022 onward) in the use of functionalized CQDs for detecting food contaminants (such as heavy metals, pesticide residues, antibiotic residues, pathogens, and additives) and in food preservation techniques, including active packaging, antioxidative and antimicrobial coatings, and photodynamic inactivation. Through a systematic comparison of analytical figures of merit and the effects of various matrices across different design approaches, we delineate both the established capabilities and the current limitations of CQD-based technologies in realistic food systems. The review concludes by identifying ongoing challenges, specifically, batch-to-batch consistency, the long-term safety profile of CQDs in food-contact applications, and the translation gap from laboratory innovation to industrial practice, and outlines prospective research directions. The overarching aim of this work is to provide a structured framework for understanding how deliberate functional design can lead to improved performance, thereby guiding the rational development of next-generation CQD-based materials for ensuring food quality and public health.

1. Introduction

Ensuring food safety and minimizing post-harvest losses represent significant challenges in global public health and sustainable food supply chains. The Food and Agriculture Organization (FAO) reports that approximately 1.31 billion tonnes of food, equivalent to 13.3% of total global production, are lost annually from harvest to retail. Fruits and vegetables exhibit the highest loss rates, at 25.4% [1]. Concurrently, foodborne diseases impose a considerable economic burden. Recent estimates suggest that foodborne illnesses cost the Australian economy around $3 billion annually [2]. In the United States, updated assessments of these costs reflect a significant and enduring economic impact due to food contamination [3]. The contamination of food with heavy metals, pesticide and veterinary drug residues, foodborne pathogens, and excessive additives continually threatens human well-being. Additionally, microbial spoilage and oxidative deterioration lead to substantial economic losses throughout storage and distribution phases [4,5,6]. In response to these issues, considerable research efforts focus on developing rapid, sensitive, and cost-effective analytical methods for hazard detection, along with innovative preservation technologies aimed at extending shelf life without compromising food quality or environmental sustainability [7,8].

Within this context, carbon quantum dots (CQDs) have emerged as a versatile class of fluorescent nanomaterials. Initially discovered during the electrophoretic purification of carbon nanotubes [9] and subsequently formally designated [10], CQDs have garnered sustained attention due to their unique combination of properties: tunable photoluminescence, excellent aqueous dispersibility, straightforward synthesis from abundant precursors, favorable biocompatibility, and remarkable photostability [11,12,13]. These characteristics have established CQDs as promising candidates for diverse applications, including bioimaging, drug delivery, photocatalysis, and, most relevant to the current discussion, food safety monitoring and preservation [7,14,15,16,17].

The rapid expansion of research on CQDs has been matched by an increasing number of review articles that catalog synthesis methods, structural characteristics, optical properties, and general areas of application. Recent reviews have systematically summarized CQD definitions, synthesis pathways, and applications across various fields [18,19,20,21]. In contrast, other reviews have focused on specific topics such as antimicrobial properties for food safety [22], applications in the food industry [23], the integration of CQDs with metal-organic frameworks for sensing [24], and environmentally friendly synthesis from biomass for food packaging [25,26,27,28]. These contributions have significantly enhanced the foundational knowledge base; however, many maintain a broad, encyclopedic perspective. Consequently, there remains a notable absence of literature that explicitly and critically examines the causal relationship between intentional functionalization of CQDs and their practical performance in complex food matrices. The term “functionalized design” is often used in titles and abstracts, but its practical meaning, how specific surface chemical groups and heteroatom doping affect sensing selectivity, sensitivity, antioxidant capacity, or antimicrobial effectiveness in actual food environments, is rarely explored in depth and with systematic precision. This oversight is particularly significant in the food sector, where the complexity of matrices, regulatory safety limits, and scalability requirements present unique challenges that differ significantly from those in idealized aqueous solutions or biomedical imaging scenarios [29,30].

The current review seeks to bridge this gap by offering a focused and critical analysis of the relationship between the rational functional design of CQDs and their effectiveness in food safety and preservation. Rather than providing an exhaustive summary, we intentionally focus on two main strategies to enhance functionality, surface group engineering and heteroatom doping, and examine how each affects the performance of CQDs in detection and preservation applications. A key goal is to progress beyond mere descriptive summaries to evaluate the extent to which reported systems have been validated in realistic food matrices, taking into account analytical figures of merit, matrix interferences, and practical applicability.

This review is structured as follows: Section 2 delivers a comprehensive overview of the foundational principles of CQDs, focusing on key aspects most pertinent to functional design. This includes discussions on the origin and reactivity of surface moieties and photophysical sensing mechanisms. Section 3 examines two principal strategies for functionalization: surface modification and elemental doping, elucidating their impact on the properties of CQDs. Section 4 forms the core analytical segment, critically analyzing recent advancements in the use of functionalized CQDs for detecting various food contaminants and systematically comparing performance metrics. Section 5 explores applications in food preservation, covering topics such as coatings, active packaging, photodynamic inactivation (PDI), and intelligent freshness indicators, while considering practical limitations. Section 6 synthesizes the identified advantages and ongoing challenges, proposing future directions for research. Section 7 presents concluding remarks that highlight the significance of rational design principles in advancing CQD-based solutions toward industrial and regulatory acceptance.

By establishing a clear conceptual framework that links functional design to application-specific performance, this review aims to serve as a valuable resource for researchers endeavoring to engineer next-generation CQDs with enhanced functionalities for ensuring food quality and safety.

2. Fundamentals of CQDs Relevant to Functional Design and Food-Related Applications

Before delving into functionalization strategies and their impacts on food safety, it is crucial to develop a clear understanding of the properties of CQDs that are most relevant to these concerns. This section selectively focuses on synthesis approaches, surface chemistry, and key photophysical mechanisms that influence how CQDs can be tailored for specific applications in sensing and preservation.

2.1. Brief Overview of Synthesis Approaches

Methods for preparing CQDs are broadly categorized into “top-down” and “bottom-up” strategies [18,20,21,25]. Top-down approaches involve the cleavage of bulk carbon materials into nanoscale fragments using methods such as laser ablation, arc discharge, or electrochemical exfoliation [21]. Conversely, bottom-up strategies construct CQDs from molecular precursors through controlled carbonization and polymerization [20]. Typically, top-down approaches yield CQDs with high crystallinity, while bottom-up methods produce CQDs with amorphous cores, abundant doping sites, and rich surface functional groups [31].

Among the various techniques, hydrothermal/solvothermal synthesis and microwave-assisted pyrolysis are the most widely adopted for food safety applications due to their operational simplicity, use of green solvents, and in situ generation of surface functional groups [32,33,34]. Hydrothermal synthesis is particularly notable for its high reactant reactivity, ease of solution control, and minimal environmental impact [32]. Recent advances have increasingly utilized biomass-derived precursors, such as fruit peels, agricultural residues, and food waste, aligning with sustainability goals while conferring unique surface functionalities [27,35,36].

Table 1. Comparison of CQDs synthesis methods and their relevance to food safety applications.

Synthesis Method	Typical Precursors	Key Advantages	Limitations	Relevance to Food Applications	Ref.
Hydrothermal	Citric acid, glucose, biomass	High quantum yield; abundant surface groups; green solvent	Batch-to-batch variation; requires autoclave	Most widely used for food sensors and preservation coatings	[32,33,34]
Microwave-assisted	Citric acid, urea, amino acids	Rapid synthesis; energy-efficient; good dispersibility	Non-uniform particle size	Rapid screening of functionalized CQDs	[21,27]
Pyrolytic carbonization	Biomass waste (peels, shells, whey)	Sustainable, low-cost; waste valorization	Complex purification; lower quantum yield	Green synthesis for active food packaging	[27]
Electrochemical	Graphite, carbon nanotubes	Strong fluorescence stability; controllable size	Low quantum yield; requires post-treatment	Limited food applications	[21]
Laser ablation	Graphite powder, carbon targets	Abundant raw materials; no chemical additives	Expensive equipment; low yield	Rarely used in food-related research	[21]

compares representative synthesis methods and their relevance to food safety applications.

2.2. Surface Functional Groups: Origin and Roles

The efficacy of CQDs in complex food matrices predominantly depends on their surface chemistry. The bottom-up synthesis method typically produces a passivating layer enriched with oxygen- and nitrogen-containing functional groups, which include carboxyl (−COOH), hydroxyl (−OH), carbonyl (−C=O), and amine (−NH₂) moieties [37]. These groups fulfill several pivotal roles:

(i) Colloidal Stability. Hydrophilic groups confer excellent aqueous dispersibility, which prevents aggregation and maintains stable fluorescence signals, critical for reproducible sensing in food extracts [38].

(ii) Reactive Anchors for Functionalization. Surface functional groups act as adaptable anchors for post-synthetic modifications. For example, carboxyl groups can be activated to covalently bind with specific recognition elements such as antibodies and aptamers, effectively transforming a general fluorophore into a selective probe [39,40].

(iii) Inherent Recognition Sites. Some functional groups can directly interact with analytes via non-covalent interactions. For instance, the lone pair electrons on amine groups can coordinate with heavy metal ions like Hg²⁺ and Cu²⁺, eliciting an intrinsic fluorescence response [41].

Table 2. Functional roles of major surface groups on CQDs in food safety and preservation applications.

Functional Group	Property Conferred	Role in Sensing	Role in Preservation	Ref.
Carboxyl (−COOH)	Hydrophilicity; negative charge	Conjugation anchor; electrostatic attraction of cations	Enhances aqueous dispersibility	[37,39]
Hydroxyl (−OH)	Hydrophilicity; hydrogen bonding	Stabilizes CQDs in aqueous media; H-bonding recognition	Radical scavenging (antioxidant)	[37,38]
Amine (-NH2)	Positive charge (acidic pH); electron donor	Metal ion coordination; PET-based sensing	Antimicrobial activity via membrane disruption	[37,41]
Carbonyl (−C=O)	Electron-withdrawing	Modulates band gap and emission	Minor contribution to antioxidant activity	[37]
Thiol (-SH)	Strong metal affinity; redox activity	Selective binding of heavy metals (Hg2+, Pb2+)	Potential antioxidant effects	[41]

summarizes the functional roles of these prominent surface groups.

2.3. Photophysical Mechanisms Underpinning Fluorescence Sensing

CQD-based sensors function through various mechanisms, including static quenching, dynamic quenching, Förster Resonance Energy Transfer (FRET), the inner filter effect (IFE), and photoinduced electron transfer (PET) [41,42]:

(i) PET. Upon excitement, a CQD is quenched due to an electron transfer from an electron-rich receptor. The diminishing electron-donating ability of the analyte upon binding restores fluorescence, yielding a “turn-on” response [43].

(ii) FRET and IFE. FRET involves a non-radiative energy transfer from an excited CQD to a proximal acceptor, necessitating spectral overlap [44]. Conversely, IFE occurs when a chromophore in solution attenuates the excitation light or emission; IFE-based sensors are less complex as they do not require close proximity between donor and acceptor [45].

(iii) Aggregation-Induced Emission Quenching (AIEQ). The aggregation of CQDs, induced by analytes, leads to fluorescence quenching through π-π stacking interactions [11].

2.4. Biocompatibility and Safety Considerations for Food Contact

CQDs are generally recognized as having low cytotoxicity, especially when compared to quantum dots that contain heavy metals [11]. Recent safety assessments using citrus-derived CQDs in zebrafish exhibited no adverse effects at normal dietary exposure levels [46]. Equally, a study demonstrated the low toxicity of CQDs synthesized from paneer whey [47]. Nonetheless, comprehensive long-term toxicological data under realistic dietary conditions are lacking, which constitutes a significant barrier for their industrial application [48]. This topic is further explored in Section 6.

3. Strategies for Functional Design: Surface Engineering and Heteroatom Doping

The innate properties of pristine CQDs often do not suffice to meet the stringent requirements for food safety. This section assesses two main strategies: engineering of surface groups and heteroatom doping.

3.1. Surface Functionalization: Tailoring Surface Chemistry for Recognition and Reactivity

Surface functionalization entails the deliberate introduction or modification of functional groups to imbue specific properties [39]. This process typically involves covalent modifications, such as the formation of amide bonds between the carboxyl groups on oxidized CQDs and amine-containing recognition elements. Alternatively, noncovalent strategies, including electrostatic adsorption and π-π stacking, provide simpler methods [40]. For example, CQDs functionalized with polymyxin B, ampicillin, or gentamicin exhibit differentiated binding affinities towards various foodborne pathogens, thus facilitating discrimination within complex matrices [49].

3.2. Heteroatom Doping: Modulating Electronic Structure and Optical Properties

Heteroatom doping entails the incorporation of non-carbon atoms into the carbon lattice, fundamentally altering its electronic structure and optical properties [50,51]. This synthesis commonly includes the introduction of elements such as N, S, P, B, Si, Fe, Gd, Mn, Zn, Cu, Cl, and F, either individually or in combinations [51].

Nitrogen doping, extensively studied, enhances the quantum yield and facilitates electron transfer [52]. Doping with sulfur creates thiol-related states advantageous for the detection of heavy metals [53], while phosphorus doping broadens the bandgap and improves photocatalytic activity [54]. Co-doping strategies, such as N/S and N/P, achieve synergistic enhancements [55,56].

Table 3. Summary of heteroatom dopants for CQDs.

Dopant	Typical Precursors	Imparted Properties	Representative Food Safety Application	Ref.
Nitrogen (N)	Urea, ethylenediamine	Enhanced QY; n-type doping	Detection of antibiotics, pesticides, heavy metals	[52]
Sulfur (S)	Sodium sulfide, cysteine	Thiol-related states; affinity for Hg2+, Pb2+	Selective heavy metal detection in seafood	[53]
Phosphorus (P)	Phosphoric acid	Expanded bandgap; enhanced ROS generation	Photodynamic inactivation; antioxidant packaging	[54]
Boron (B)	Boric acid	p-type doping; enhanced sensing	Detection of phenolic contaminants	[27]
Silicon (Si)	APTES, APTMS	Up-conversion fluorescence; enhanced photostability	Dual-readout detection of alkaloids	[57]
N/S co-doping	Urea + thiourea	Synergistic QY enhancement; dual active sites	Simultaneous multi-metal detection	[53,58]
N/P co-doping	Ethylenediamine + phosphoric acid	Enhanced ROS generation	Photodynamic antibacterial coatings	[56]

summarizes commonly employed dopant elements and their applications in food safety.

3.3. Metal and Lanthanide Doping: Expanding Functionality

Doping with metal ions, including Cu, Mn, Fe, Zn, and Mg, imparts catalytic, magnetic, or enhanced optical properties [50]. Lanthanide elements, like Eu, Tb, Ce, and Nd, contribute sharp, long-lived emission peaks that enable ratiometric sensing with built-in calibration [17,50].

Table 4. Critical comparison of functional design strategies for CQDs.

Design Strategy	Key Advantages	Limitations	Best Suited for	Ref.
Surface functionalization	Direct control over surface chemistry; covalent attachment of biorecognition elements	May reduce QY; biomolecule stability in food matrices may be limited	Selective pathogen detection; aptamer/antibody-based sensing	[39,40]
Single-element doping	Significant QY enhancement; simple one-step synthesis	Limited tunability; predominantly blue-green emission	General fluorescence sensing; metal ion detection	[52,53]
Co-doping	Synergistic property enhancement; multifunctional capabilities	Complex optimization; reproducibility challenges	Ratiometric sensing; photodynamic inactivation	[55,56]
Metal/Lanthanide doping	Catalytic/redox activity; ratiometric sensing	Potential toxicity concerns; expensive precursors	Electrochemical sensors; high-precision ratiometric sensing	[17,50]

offers a critical comparison of functional design strategies.

3.4. Practical Comparison of Doping Strategies for Food-Relevant Applications

Among the three doping routes, single-element N-doping appears to be the most reproducible and presents the lowest toxicity concerns. This is evidenced by consistent quantum yields reported across different laboratories and its widespread use in food-matrix validated sensors [52,59,60,61]. Co-doping may enhance multifunctionality but introduces additional synthetic variables that compromise batch consistency [50]. In contrast, metal and lanthanide-doped CQDs offer unique catalytic or ratiometric signals but are associated with significant uncertainties regarding metal leaching under acidic food conditions and have yet to be thoroughly tested in real food products [50]. Therefore, N-doping and N/S co-doping currently stand out as the most practical approaches for near-term food safety applications. Nevertheless, metal/lanthanide doping should be supported by rigorous migration and toxicity data before being considered safe for contact with food.

4. Applications of Functionalized CQDs in Food Safety Detection

This section critically examines the recent advances in the application of functionalized CQDs for detecting food contaminants. Figure 1 provides a schematic overview of the CQD-based fluorescence sensing platforms [16].

4.1. Detection of Heavy Metal Ions

Recent studies have demonstrated the practicality of sensors validated in real food matrices. Systematic reviews have highlighted functionalized carbon dots for the detection of heavy metal ions such as Hg²⁺, Pb²⁺, and Cu²⁺ in environmental protection and food safety [29]. For instance, Li et al. developed Ca, N-co-doped carbon dots for dual-mode detection of Hg²⁺ and Ag⁺, with smartphone integration enabling LODs of 56.2 and 90.96 nM, respectively, and recoveries ranging from 96.3% to 105.7% in real food samples [59]. Ali et al. devised a ratiometric sensor for Al³⁺ and Co²⁺ in canned tomato sauce and tuna, achieving LODs of 0.06 and 0.012 μM and recoveries between 97.0% and 100.7% [60]. Moreover, N-CQDs facilitated Hg²⁺ detection in seafood and rice via IFE, with an LOD of 42.4 nmol/L [61]. CQDs derived from avocado seeds have been used for analyzing Cr(VI) using enhanced absorption and fluorescence quenching techniques [62].

Table 5. Representative functionalized CQDs for heavy metal ion detection in food matrices.

CQD System	Functional Design	Target	Food Matrix	LOD	Recovery (%)	Ref.
E-CDs (Ca,N-co-doped)	Ca/N co-doping; dual-mode	Hg2+, Ag+	Real food samples	11.28 nM; 128.23 nM	96.3–105.7	[59]
N,S-doped CQDs	Dual-emission; ratiometric	Al3+, Co2+	Canned tomato sauce, tuna	0.06 μM; 0.012 μM	97.0–100.7	[60]
N-CQDs	N-doping; IFE	Hg2+	Shrimp, crab, carp, rice	42.4 nmol/L	Satisfactory	[61]
Avocado seed-derived CQDs	Biomass-derived	Cr(VI)	Water/food samples	Enhanced absorption/fluorescence	—	[62]

summarizes representative systems.

4.2. Detection of Pesticide and Veterinary Drug Residues

Pesticide detection. Ali et al. reported on azide-functionalized carbon dots utilized for the detection of propargite in citrus and tea, achieving a LOD of 0.35 ng/mL and recovery rates between 95.2% and 97.6% [63]. N,S-co-doped CQDs have been developed for the detection of carbendazim employing a turn-off-on approach, with an LOD of 27.84 ng/mL and recoveries ranging from 96.9% to 99.36% [58]. Mango peels-derived CQDs have been applied in optical sensing of diazinon using an SPR-based sensor, achieving an LOD as low as 0.01 nM [64]. Reviews on CQD-based nanocomposites have pinpointed their potential in targeted detection of food toxins, including pesticides [65]. Various functional CQDs as fluorescent probes for detecting pesticide residues in food have been comprehensively discussed [66].

Antibiotic detection. Self-nitrogen-doped CQDs derived from shrimp shell waste were used for detecting tetracycline in milk and eggs, with an LOD of 96 nM and recovery rates ranging from 98.2% to 109.7% [67]. Ce-doped CQDs facilitated the simultaneous detection of tetracycline and carmine in meat, boasting LODs of 0.037 μM and 0.035 μM, respectively, and recovery rates from 92.0% to 115% [68]. A dual-functional fluorescent sensor based on soybean-derived N-doped carbon dots enabled discrimination and quantification of tetracyclines in milk and chicken, with an LOD of 0.12 μM for TC and recoveries between 97.2% and 110.4% [69]. P-doped CQDs from Caesalpinia sappan L. were employed for the detection of ciprofloxacin in milk, achieving an LOD of 2.06 nM [70].

Table 6. Representative functionalized CQDs for pesticide and veterinary drug detection.

CQD System	Functional Design	Target	Food Matrix	LOD	Recovery (%)	Ref.
Az-CDs	Azide surface functionalization	Propargite	Citrus, tea	0.35 ng mL−1	95.2–97.6	[63]
N-S@CQDs	N,S-co-doping; turn-off-on	Carbendazim	Real food samples	27.84 ng/mL	96.9–99.36	[58]
Mango peel-derived CQDs	Biomass-derived; SPR sensing	Diazinon	Environmental/food	0.01 nM	—	[64]
N-CDs (shrimp shell)	Self-N-doping	Tetracycline	Milk, egg	96 nM	98.2–109.7	[67]
Ce-CDs	Ce-doping	Tetracycline, Carmine	Meat	0.037; 0.035 μM	92.0–115	[68]
N-BCDs	Soybean-derived N-doping	Tetracyclines	Milk, chicken	0.12 μM	97.2–110.4	[69]
P-CQDs	P-doping; C. sappan L.	Ciprofloxacin	Milk	2.06 nM	—	[70]

provides a comparative summary of these findings.

4.3. Detection of Foodborne Pathogens

Li et al. developed mannose-grafted carbon dots for the detection of S. typhimurium, achieving an LOD of 117 CFU/mL [71]. Xie et al. developed pH-responsive CQDs leveraging bacterial metabolism for the detection of S. aureus and E. coli, with LODs ranging from 3 to 6 CFU/mL [72]. Hassan et al. created N-doped CQD composite films possessing antimicrobial activity and fluorescence-based pathogen detection capabilities, which extended the shelf life of tomatoes from 4 to 10 days [73]. A full-color CQDs fluorescence sensing array, combined with machine learning, facilitated rapid bacterial detection and classification of five common pathogenic bacteria [74]. Onion-peel-derived CQDs demonstrated antimicrobial efficacy and biofilm control on food contact surfaces against key foodborne pathogens [75]. Nd-doped CQDs have been developed for ratiometric detection with applications in food analysis [17].

Table 7. Representative functionalized CQDs for foodborne pathogen detection.

CQD System	Functional Design	Target	Food Matrix	LOD	Performance Notes	Ref.
g-CDs-M/GO-PBA	Mannose grafting	S. typhimurium	Buffer	117 CFU·mL−1	Linear range: 102–107 CFU·mL−1	[71]
pH-CDs	pH-responsive	S. aureus, E. coli	Water	3–6 CFU·mL−1	Species differentiation	[72]
N-CDs-CS-CMC film	N-doping; composite	E. coli, S. aureus	Tomato	—	Shelf life: 4 to 10 days	[73]
Full-color CQDs array	ML-assisted; multichannel	5 pathogenic bacteria	Pork matrix	>93% accuracy	Machine learning integration	[74]
Onion-peel CQDs	Biomass-derived	S. typhimurium, L. monocytogenes	Food contact surfaces	MIC: 1200–2200 μg/mL	Biofilm reduction: 74–91%	[75]

summarizes representative systems.

4.4. Detection of Food Additives and Nutritional Components

Detection of food additives. Safaei et al. developed a ratiometric sensor for tartrazine with a LOD of 64 nM and recovery rates ranging from 97.2% to 104.4% [76]. Gao et al. created a ratiometric probe for Brilliant Blue, which includes a smartphone-assisted readout feature, and achieved a LOD of 67.78 nM [77]. Researchers have designed surface-decorated, Zn-doped CQDs for detecting tartrazine in real food samples by utilizing the IFE mechanism [78]. Furthermore, N-CQDs have been employed for the direct detection of food additives such as STPP and Al³⁺, with a LOD of 0.024 μM for STPP [79]. Additionally, green-synthesized amino-functionalized CQDs derived from asparagus peel facilitated the detection of Sunset Yellow with a LOD of 0.92 μM [80].

Detection of nutritional components. Zhu et al. synthesized iron-doped CQDs from coffee grounds to detect ascorbic acid in various beverages [81]. N-doped carbon dots have been used for the sensitive detection of curcumin, employing smartphone-integrated, 3D-printed platforms [82]. A dual-emission CQD-based ratiometric fluorescence sensor has also been developed for hydrocortisone detection in food, with a LOD of 0.029 μM and recovery rates ranging between 93.37% and 103.87% [83]. Additionally, a ratiometric fluorescent sensor utilizing N-GQDs@AuNCs enabled Al³⁺ and Cu²⁺ detection in fried dough twists and shellfish [84].

A critical distinction must be established between sensors tested in actual food samples and those evaluated only in simplified media such as buffers, water, or films. For example, the mannose-grafted probe exhibited a LOD of 117 CFU·mL⁻¹ in a buffer solution [85], but its performance in complex food matrices was not explored. In another instance, a pH-responsive system demonstrated a LOD of 3–6 CFU·mL⁻¹ in water [86]; however, factors such as protein content, fat, and autofluorescence in real matrices could significantly affect detection performance. Conversely, several heavy metal sensors have been successfully validated in canned tomato sauce, tuna, shrimp, rice, and chicken, with recovery rates ranging from 92.0 to 115% [59,60,69,73]. This confirms the superior matrix tolerance of ratiometric or dual-mode designs. Future studies should quantify matrix effects under actual food conditions to provide a meaningful comparison across different sensor platforms.

5. Applications of Functionalized CQDs in Food Preservation

Functionalized CQDs have displayed significant versatility as active components in food preservation systems. Notably, carbon dots have been extensively reviewed regarding their sustainable antioxidant effects in active food packaging, with proven efficacy across fresh produce, meat, seafood, dairy, and bakery products.

5.1. Antioxidative and Antimicrobial Coatings and Films

Niu et al. developed CQD/chitosan-PVA composite films derived from peanut shells for strawberry preservation, demonstrating 80.19% DPPH scavenging activity and more than 94% antibacterial efficiency [85]. ZnO/CQD nanocomposite agar films facilitated greater than three-log CFU/g reduction of E. coli, Salmonella, and Listeria on chicken breast under blue light illumination [86]. Jafarzadeh et al. incorporated N-CDs into sago starch bioplastic films, which reduced UV transmittance by 56% to 74% and enhanced hydrophobicity [87]. Further, lignin-derived CQDs/PVA composite films exhibited antioxidant efficiency and UV blocking of more than 90%, with a 49.6% increase in tensile strength [88]. CQDs have been recognized as green and efficient materials for the preservation of fruits and vegetables [89].

Table 8. Representative CQD-based coatings and films for food preservation.

CQD System	Functional Design	Matrix	Target Food	Key Performance	Ref.
Peanut shell-CDs	Biomass-derived; photocatalytic ROS	CS/PVA	Strawberry	80.19% DPPH scavenging; >94% antibacterial	[85]
ZnO/CQD	CQDs improve ZnO dispersion	Agar	Chicken breast	>3-log CFU/g reduction	[85]
N-CDs	N-doping	Sago starch	Model system	56–74% UV reduction; improved hydrophobicity	[87]
LCQDs/PVA	Lignin-derived	PVA	Active packaging	>90% antioxidant/UV blocking; 49.6% tensile strength increase	[88]
Bread waste-CQDs	Waste valorization	Starch film	Smart packaging	79.13% DPPH scavenging; pH-responsive	[90]

summarizes various representative preservation systems.

5.2. CQD-Mediated Photodynamic Inactivation (PDI)

CQD-mediated PDI employs light-activated ROS generation for microbial inactivation [91]. The field of carbon dots represents a rapidly advancing area with significant potential for food safety applications, especially considering their benign and nontoxic properties compared to heavy metal-containing semiconductor QDs [22]. Du et al. utilized CQDs as photosensitizers for fresh Goji berries, extending their storage life by nine days and enhancing antioxidant enzyme activities [92]. PVA/CA-CD films have been shown to extend the shelf life of chilled mutton by six days [93]. Additionally, the use of edible coatings incorporating CQDs for fresh produce preservation has been critically reviewed from safety perspectives [94].

The efficacy of PDI is constrained by the shallow penetration of light (less than 1–2 mm for blue light), necessitating that the products be thin or transparent. Achieving uniform illumination on irregular surfaces remains technically challenging, and shadowed areas may remain untreated. The ROS generated are nonspecific and may oxidize lipids, vitamins, and pigments, possibly leading to off-flavors or nutrient degradation. Although comprehensive sensory data are still lacking, current studies are limited to the laboratory scale; scaling up to meet industrial throughput would require specialized equipment and validated treatment protocols. Additionally, the safety regarding the ingestion of residual CQDs and their photodegradation products on food surfaces has not yet been systematically assessed, which is a prerequisite for regulatory acceptance.

5.3. Intelligent Packaging: Freshness Indicators

CQDs, responsive to pH changes and biogenic amines, enable real-time freshness monitoring [95]. N, S, and P-doped carbon dots have been developed for intelligent paper sensors, enabling vapor-phase biogenic amine detection and freshness evaluation in shrimp, broccoli, and mushrooms with a LOD of 6.73 nM for spermine and 6.57 nM for spermidine [96]. Qazanfarzadeh et al. converted bread waste into CQD-starch smart films with UV-barrier, antioxidant, and antimicrobial properties [90]. CQDs synthesized from lemon extract in ethyl acetate have enhanced the performance of PLA films, achieving 82% UV-blocking and greater than 90% antimicrobial activity [97]. Furthermore, N, S-co-doped fluorescent carbon dots have been integrated into PVA composite films for dual functionalities: chlortetracycline detection and UV shielding, providing over 99.5% protection against UV-C and UV-B [98]. CQDs have also been integrated into stimulus-responsive hydrogels, advancing smart food packaging with capabilities in sensing, preservation, and AI-enabled predictive quality management [99].

6. Current Advantages, Key Challenges, and Future Perspectives

6.1. Core Advantages

(i) Sustainable and Cost-Effective Synthesis. CQDs are synthesized from abundant renewable precursors, including food waste, reflecting principles of a circular economy [27,35].

(ii) Favorable Biocompatibility. CQDs demonstrate minimal cytotoxicity at concentrations relevant to the food industry [46,47].

(iii) Tunable Functionality. Techniques such as surface engineering and heteroatom doping facilitate precise control over the optical properties and selectivity of CQDs [51,52,53,54,55,56].

(iv) Multifunctional Integration. Functionalized CQDs can concurrently serve as fluorescent probes, antimicrobial agents, antioxidants, and freshness indicators [85,87].

(v) Compatibility with Digital Technologies. CQD sensors are compatible with smartphone readouts, machine learning, and IoT infrastructure [59,60,74,82].

6.2. Key Challenges

(i) Scalability and Reproducibility. Scaling up laboratory protocols often presents engineering challenges; furthermore, batch-to-batch variability necessitates systematic quality control [23,48].

(ii) Matrix Interference in Real Foods. Sensor effectiveness often diminishes in complex food matrices due to issues like autofluorescence and competitive binding [48].

(iii) Regulatory and Safety Uncertainty. Long-term toxicological data pertinent to chronic dietary exposure are scarce, and specific regulatory frameworks for CQDs have yet to be established [30,94].

(iv) Quantum Yield and Photostability Limitations. Biomass-derived CQDs typically exhibit moderate QY between 10 and 40 percent; their operational stability remains poorly characterized [48].

(v) Manufacturing Integration. Effective integration into polymer matrices and adhesion to various food surfaces require customized solutions.

6.3. Future Directions

(i) Standardization and Quality Control. The establishment of reference materials and standardized characterization protocols is crucial [100].

(ii) Long-Term Toxicological Assessment. There is an urgent need for chronic feeding studies and migration testing under realistic food-contact conditions [30].

(iii) Machine Learning-Assisted Design. ML-assisted design can significantly expedite the development of functional CQDs, with recent advances achieving 50–100 fold improvements in detection limits [74,101].

(iv) Hybrid Material Systems. The integration of metal-organic frameworks, stimuli-responsive hydrogels, and edible biopolymers can create versatile platforms [24,99,102].

(v) IoT and Supply Chain Integration. The deployment of wireless CQD sensor tags and cloud-connected monitoring networks can facilitate real-time quality tracking [103].

(vi) Green Manufacturing and Life Cycle Assessment. The adoption of continuous-flow synthesis along with comprehensive LCA studies should inform sustainable manufacturing practices [27,35].

7. Concluding Remarks

This review critically examines the relationship between the rational functional design of CQDs, through surface group engineering and heteroatom doping, and their practical performance in the detection and preservation of food contaminants. Several conclusions can be drawn. First, functionalized CQDs enable the sensitive and selective detection of a wide range of food-related analytes across various matrices, with enhancements such as ratiometric sensing and smartphone integration contributing to their robustness. Second, CQDs act as effective multifunctional components in preservation systems, providing antimicrobial activity, antioxidant protection, UV-barrier properties, and freshness indication. The conversion of waste into functional CQDs exemplifies a commitment to the principles of a circular economy. Third, the integration of CQDs with machine learning and IoT technologies holds transformative potential for real-time, predictive quality management.

However, several persistent challenges must be acknowledged: scalable synthesis with reproducible properties, effective performance in unprocessed food matrices, comprehensive long-term safety data, and well-defined regulatory pathways are yet to be fully addressed. These challenges, while significant, are surmountable and will require a sustained interdisciplinary effort. Looking forward, advances will depend on standardized quality control, systematic toxicological assessments under food-relevant exposure scenarios, machine learning-accelerated functional design, and the development of hybrid material systems. With focused investment in overcoming translational hurdles, functionalized CQDs are poised to become indispensable tools for ensuring food safety, reducing waste, and protecting public health.