Polysaccharides from Agro-Industrial Waste and By-Products: An Overview on Green Synthesis of Metallic Nanoparticles—An Ecofriendly Approach

Abstract

This review explores the eco-friendly synthesis of metallic nanoparticles derived from polysaccharides obtained from agricultural and food industry waste. Initially, it outlines the problem of agri-food waste, highlighting its abundance and the potential to extract valuable polysaccharides such as cellulose, hemicellulose, lignin, and pectin. The focus is on green synthesis methods that use these polysaccharides to produce metallic nanoparticles, emphasizing the environmental benefits compared to conventional methods. The article reviews the physicochemical properties of key polysaccharides and details their extraction processes from various agricultural waste. The synthesis of diverse types of metallic nanoparticles, including monometallic (e.g., gold, silver, and platinum), bimetallic (e.g., gold–silver and gold–zinc), and oxide nanoparticles (e.g., zinc oxide and iron oxide), is extensively covered. Additionally, mechanisms of nanoparticle synthesis, such as nucleation, growth, stabilization, and capping, are examined, alongside examples from existing research. The article highlights the applications of these nanoparticles in diverse fields, including food safety, healthcare, agriculture, and environmental protection. It concludes by underscoring the potential of green synthesis to reduce waste and promote sustainable industrial practices and calls for further research to optimize these methods.

1. Introduction

Agro-industry can be defined as the combination of industrial and agricultural processes aimed at producing and transforming raw materials destined for the market for purchase and sale, currently representing a fundamental economic activity [1]. Annually, it is estimated that 155 billion tons of organic matter are produced; however, only a small portion is utilized by humans and animals. As a result, a large amount of this material becomes non-edible waste, contributing to significant environmental, social, and economic impacts [2]. The concept of agro-industrial waste encompasses any material that has not been fully used and is recognized as the surplus of agro-food production; today, agro-industrial wastes derived from industrial, forestry, and agricultural activities are one of the major sources of pollution. Examples of this waste include crop residues, manure, pruning waste, feces, fruits, vegetables, whey, and others. Most of this material is mainly characterized by its polysaccharide content, such as lignin, cellulose, and hemicellulose [3,4].

Lignocellulosic residues are those in which the chemical composition of the plant cell wall is dominated by the presence of three major constituents, hemicellulose, lignin, and cellulose. These substances are commonly obtained from residues in the agri-food industry and are characterized by high potential for recycling and the creation of different materials [5]. Certain types of agro-industrial residues, such as lignocellulose and lipids, present challenges for degradation in some cases; however, the reduction to simpler molecules is possible through biological, physical, and chemical processes. In 2015, the World Bank estimated and reported for Latin America a gross domestic product (GDP) of approximately 21%, highlighting the value of the agro-food industry in the area [6].

Currently, nanotechnology is an innovative science that has gained the attention of the scientific community; biological or green synthesis aims to advance in process innovation, resulting in the reduction of environmental damage and generation of chemical waste, all at an economic cost. Metal nanoparticles possess unique physical, chemical, optical, electrical, and magnetic characteristics [7]. The application of biological methods through aqueous extracts of plants is a means of avoiding the use of chemical and physical methods. By using plants in the synthesis of nanoparticles, promising results have been obtained due to the phytocompounds present, which provide reducing capacity and stabilizing nanoparticles [8].

The use of the green synthesis method with polysaccharides is an environmentally friendly option for the synthesis of nanoparticles. It is a highly viable method for implementation and relatively easy to carry out [9]. The different carbohydrates such as monosaccharides, disaccharides, oligosaccharides, and polysaccharides from agro-industrial waste can function as natural reagents. Furthermore, nanoparticles exhibit stability and are biocompatible and non-toxic. As a result, they are feasible for biological and catalytic uses and applications [10].

The objective of this review is to demonstrate the importance and benefits of the synthesis of metallic nanoparticles through the green synthesis method using polysaccharides from agro-industrial waste from an eco-friendly perspective.

2. Description and Physicochemical Properties of Polysaccharides

2.1. Cellulose

Cellulose can be considered a polyol due to its abundant hydroxyl groups. This complex carbohydrate is a linear homopolymer composed exclusively of D-glucopyranose subunits through β-(1,4) bonds (Figure 1). In addition to hydroxyl groups, cellulose also has aldehyde and acetal groups, enabling it to undergo different types of reactions such as substitution, replacement, redox, depolymerization, graft addition, base exchange, and hydrolysis [11]. Its molecular structure provides various properties such as chirality, hydrophilicity, degradability, and chemical variability due to the high reactivity of the donor group (OH) [12,13]. The abundant hydroxyl groups allow it to react as either an ester, alcohol, or ether [14]. Using cellulose for the synthesis of nanoparticles provides several benefits, as this biopolymer is a source of renewable and biodegradable matter. Additionally, during synthesis, cellulose provides stability and protection, while enhancing the mechanical and optical properties of the nanoparticles [15].

2.2. Hemicellulose

This polysaccharide contains xylans, galactomannans, glucomannans, and xyloglucans. When hydrolyzed, hemicelluloses are easily fragmented into their pyranose and furanose sugar units (Figure 2). Moreover, there are structural and compositional differences in hemicelluloses depending on the source of the biomass [16].

Table 1. Classifications of hemicelluloses. Source: Zhou et al. [17].

Hemicellulose Groups	Characteristics
1	Xyloglucans that have a structure that is a glucose residue where xylose, fucose, or galactose residues are coupled.
2	Xylans include glucuronoxylans, which have a structure of xylose residues to which glucuronic acid residues are attached.
3	Mannans that have a structure of mannose residues and glucomannans. They have a backbone of mannose and glucose residues.
4	Mixed link glucans are unbranched chains of D-glucose residues joined by β-(1,3) or β-(1,4) bonds.

shows the classification of hemicellulose into different groups. Hemicellulose is a set of fibers whose structure is made up of D-pyranose residues linked by a β-(1,4) bonds. It contains in its chains carbohydrates with five and six carbon atoms, such as glucose, xylose, mannose, and arabinose, among others. Likewise, in their original state, the xylans and galactoglucomannans of hemicellulose are acetylated, and it has been shown that the acetyl groups can be joined through ester bonds at the C2 and/or C3 positions of the xylose residues [17]. Due to the structure and multiple hydroxyl groups (OH) of hemicellulose, metal particles can be encapsulated [18].

2.3. Lignin

This phenolic polysaccharide has a highly varied structure due to the different types of bonds and monomers that compose it. It is an abundant biopolymer on our planet, generally made up of three different monomers: p-coumaryl alcohol, coniferyl alcohol, and sinapil alcohol, also known as monolignols (Figure 3) [20]. This biopolymer contains several polar functional groups, which provide it with various uses within the industry. The following characteristics are helpful in the identification of lignins:

• They are vegetable polymers built on the basis of phenylpropanoid units.
• They are characterized by the predominance of methoxyl groups contained in the wood.
• They are resistant to acid hydrolysis, easily oxidizable, soluble in bisulfite or hot alkalis, and easily condensable with phenols or thiols.
• When reacting with nitrobenzene in a hot alkaline solution, they primarily produce vanillin, syringaldehyde, and p-hydroxybenzaldehyde, depending on their origin.
• When boiled in an ethanolic solution of hydrochloric acid, lignins form monomers of the Hibbert ketone type (aromatic ketones resulting from the breaking of the main ether bonds (β–O–4) between lignin units) [21].

The phenylpropane units of lignin are joined by a carbon–carbon bond and an ether bond [22]. This highly complex polysaccharide undergoes a purely chemical lignification or polymerization process, during which chiral centers are consecutively created with monolignols through coupling in their β positions [23].

Figure 3. Chemical structure of monolignols. Source: Akman [24].

Figure 3. Chemical structure of monolignols. Source: Akman [24].

2.4. Pectin

Pectin is a natural biopolymer present in the walls of non-woody plant cells, whose primary function is to trap water and create gels. The composition of its chemical structure consists of a network composed of different sections, each formed by galacturonic acid, with rhamnose and certain branches added. Pectin is characterized by having a good gelling capacity due to the formation of intermolecular bonds between two or more homogalacturonic chains with water [25]. Pectin is a complex carbohydrate that can be classified depending on the degree of esterification of its carboxyl groups with methyl groups, a process known as methoxylation. If pectin has 50% of its carboxyl groups esterified, it is known as high methoxyl; if it is below that, it is called low methoxyl. It is also considered a soluble fiber in the human diet. The structure of pectin is extremely varied as a result of the different sources and methods of obtaining it, but it can be classified into three different groups: homogalacturonans, rhamnogalacturonans-I, and rhamnogalacturonans-II. [26].

Homogalacturonans are based on linear polymers, composed primarily of d-galacturonic acid units (at least 65%) linked in chain by α-(1-4) glycosidic linkages. Rhamnogalacturonans I are made up of repeated units of galacturonic acid with an α-D-(1-2) bond and rhamnose with an α-L-(1-4) bond, alternating to form a linear and continuous structure. Rhamnogalacturonans II are an exceptionally complex polysaccharide, having homogalacturonans as a backbone and four side chains made up of different monosaccharides. Due to its structure, RG-II has the ability to form borate ester dimers [27,28,29].

2.5. Others

2.5.1. Starch

Starch is a polysaccharide of the α-glucan family composed of amylose, a linear biopolymer of D-glucopyranoses, with a high molecular weight and the ability to form helical inclusion complexes with acids, organic alcohols, and iodine. Amylopectin is another high molecular weight biopolymer that constitutes starch, characterized by having branches of glucose monomers with α-(1,6) bonds throughout its structure. This complex carbohydrate can be made up of three different chains (A, B, and C). Type A or external chains are linked by α-(1,6) bonds, are free of side chains, and are the shortest. Type B chains function as a structural support for other type B and A chains. The C chain is characterized by having the only reducing end [30]. The characteristic granules of starch are formed due to the combination of amylose and amylopectin. These granules are semicrystalline and are formed with alternating semicrystalline and amorphous layers, referred to as growth rings. Amorphous lamellae are created between these rings, built from amylopectin branches, while semi-crystalline lamellae are built by side chains of clusters through the formation of double helices [31]. The most common functional group in starches is hydroxyl groups, and their physicochemical properties include retrogradation, syneresis, gelatinization, pasting, and degradation, although these depend on their origin [32,33].

2.5.2. Gums

The term ¨gum¨ is used to describe certain types of complex long-chain carbohydrates, which may have minimal, maximum, or no branching [34]. They are used for industrial purposes due to their ability to form viscous solutions, gels, or stabilizers. Vegetable gums can originate from different parts of the plant, such as cell walls, seeds, exudates, roots, and others [35]. It is important to note the different polysaccharides that make up gums can have a negative, positive, or neutral charge depending on the functional groups present in the monomeric units that compose them, resulting in differences in compatibility, solubility, synergy, gelling capacity, thickening, and emulsifying properties [36]. Water-soluble gums are called “hydrocolloids”, which are divided into “natural” (extracted from seeds), “semi-synthetic” (derived from modified starches or cellulose), and those obtained by “bacterial fermentation”. Additionally, gums can be classified depending on their origin as tree gums, polysaccharides of bacterial origin, algae, or botanical products [37]. Regarding the chemical structure and physicochemical properties of the gums, these will depend on their source of extraction. One of the oldest gums is Arabic gum, which has been commercialized for five thousand years. Its chemical structure is composed of (1,3) linked β-D-galactopyranosyl units linked to the main chain through (1,6) bonds. The most common monomeric units that compose it are D-galactose, L-arabinose, L-rhamnose, D-glucuronic acid, and 4-O-methyl-D-glucuronic acid, endowing it with thickening, gelling, and stabilizing properties [38].

3. Green Synthesis of Metallic Nanoparticles

3.1. Fundamental

Green synthesis is currently a fundamental branch of nanotechnology, using biologically derived entities such as plants, plant extracts, or metabolites, microorganisms, or even plant biomass for the synthesis of nanoparticles. The biological entities or their extracts are used for the green synthesis of metallic nanoparticles through the bio-reduction of metallic particles, leading to the synthesis of nanoparticles [39]. The green synthesis of nanomaterials refers to the synthesis of different metallic nanoparticles while avoiding the use of harmful chemicals and using bioactive agents with important reducing characteristics, such as plant materials, microorganisms, and different biological wastes, including vegetable waste, fruit peel waste, eggshells, agricultural waste, and others [40].

A wide range of molecules, ranging from proteins to various low molecular weight compounds such as terpenoids, alkaloids, amino acids, alcoholic compounds, polyphenols, glutathione, polysaccharides, antioxidants, organic acids, quinones, and others, have been reported to play an important role in the green synthesis of NPs. These molecules participate in the reduction of metal ions to NPs and in maintaining their stability. The mixing of these compounds with precursors of metal salts initiates the synthesis of nanoparticles, acting as reducing agents and capping agents for the synthesis of nanoparticles [41].

The use of naturally occurring polysaccharides for the green synthesis of metallic nanoparticles (MNPs) constitutes a promising area for nanotechnology, employing them as reducing and stabilizing agents [42]. Parameters such as the stability, size, and shape of MNPs are influenced by the polysaccharides employed, making them excellent candidates for controlling the synthesis process. These polymers can easily adhere to the metal surface of MNPs because they present numerous binding sites. As a result, these biomolecules can effectively create an organic–inorganic network of MNPs and confer significant protection against aggregation and chemical modifications [43].

3.2. Characteristics

Nanoparticles (NPs) are naturally occurring or engineered particles of extremely minute size, in the range of 1 to 100 nm [44]. The development of metallic nanoparticles using biological materials through an environmentally friendly approach has attracted attention. Due to their high surface area-to-volume ratio, nanoparticles exhibit increased reactivity, mobility, dissolution properties, and strength. Nanostructures have different applications attributable to their novel or enhanced features, depending on their size, distribution, and morphology [41,45]. At the nanoscale, the particles exhibit better catalytic, magnetic, electrical, mechanical, optical, chemical, and biological properties [44].

There are two general strategies for the synthesis of nanomaterials. The top-down approach describes the synthesis of nanoparticles from larger materials into smaller units. The top-down strategy is based on physical methods, requiring a large tube furnace to break down the material into smaller parts. In contrast, the bottom-up strategy suggests that NPs are synthesized from smaller molecules. In this technique, chemical methods are used, specifically oxidation–reduction reactions to reduce the metal ion, sometimes combined with a coating agent to stabilize the NPs. The latter synthesis strategy also includes biological methods, in which biomolecules are incorporated with metallic substances for the production of nanoscale materials [46].

The use of toxic solvents has been a problem in the synthesis of NPs; however, green synthesis eliminates these solvents by using natural compounds in the process, such as polysaccharides. This technique is a reliable, clean, biologically sound, and environmentally friendly approach. The use of toxic chemicals, in addition to generating volumes of hazardous substances such as waste, could pose potential hazards such as carcinogenicity, toxicity, and environmental toxicity. This constitutes a limitation for the use of nanoparticles in various clinical and biomedical applications [45].

The synthesis of nanomaterials using polysaccharide-based metallic NPs with specific magnetic, optoelectronic, and physiochemical properties, as well as their applications in biomedical and biochemical sciences, is an increasingly important area in green chemistry [43]. Biological resources (plants, bacteria, fungi, etc.) are documented as sources of different polysaccharides (starch, cellulose, xanthan, pectin, dextrose, glucan, etc.) that act in the formation and stabilization of MNPs produced by green synthesis. Additionally, polysaccharides could also increase the compatibility of MNPs with biological systems or their assembly with different molecules on the surface of MNPs, such as drugs [47].

Natural polysaccharides stand out for their biocompatible and biodegradable properties, as well as their diversity of sizes and structures, making them suitable for the reduction and stabilization of MNPs [42]. Heparin, hyaluronic acid, chitosan, cellulose, starch, alginate, and dextrans are the frequently used polysaccharides for nanoparticle stabilization and synthesis [48].

3.3. Advantages and Disadvantages Metallic Nanoparticles

3.3.1. Advantages

High Surface Area-to-Volume Ratio

Metallic nanoparticles possess a remarkably high surface area-to-volume ratio, significantly enhancing their reactivity compared to bulk materials. This property makes them extremely effective in various applications such as catalysis, drug delivery, and sensor technology. In catalysis, for example, the increased surface area allows for more active sites, leading to a faster and more efficient chemical reaction [49]. In drug delivery, the high surface area enables a higher load of therapeutic agents, improving the effectiveness of treatments [50]. Similarly, in sensor technology, the increased reactivity enhances sensitivity and accuracy, making metallic nanoparticles valuable in detecting and measuring trace amounts of substances [51].

Unique Optical Properties

Metallic nanoparticles exhibit unique optical properties, particularly localized surface plasmon resonance (LSPR), which occurs when conduction electrons on the nanoparticle’s surface oscillate in response to incident light. This phenomenon results in a strong absorption and scattering of light at specific wavelengths, which can be tuned by altering the size, shape, and composition of the nanoparticles. These properties are utilized in various applications, such as medical imaging, where nanoparticles enhance contrast; diagnostics, where they improve assay sensitivity; and photothermal therapies, where they convert light into heat to target and destroy cancer cells [52,53].

Enhanced Mechanical Properties

The incorporation of metallic nanoparticles into materials can significantly enhance their mechanical properties. For instance, adding nanoparticles to polymers or metals can increase the strength, hardness, and durability of the composite material. This makes metallic nanoparticles valuable in developing advanced coatings, construction materials, and various industrial applications where improved mechanical performance is crucial. The enhanced properties result from the nanoparticles’ ability to inhibit the movement of dislocations within the material’s structure, thereby increasing resistance to deformation and wear [54].

Antimicrobial Properties

Certain metallic nanoparticles, such as silver nanoparticles, have demonstrated strong antimicrobial properties. These nanoparticles can effectively kill or inhibit the growth of bacteria, fungi, and viruses, making them useful in a wide range of applications. In medical devices, coatings incorporating silver nanoparticles can prevent infections by reducing microbial colonization. In packaging materials, these nanoparticles can extend the shelf life of food products by preventing spoilage. Moreover, in textiles, the incorporation of antimicrobial nanoparticles can provide long-lasting protection against odor-causing bacteria [55,56,57].

Catalytic Efficiency

Due to their high reactivity and surface area, metallic nanoparticles serve as highly efficient catalysts in various chemical reactions. Their catalytic efficiency can lead to faster reaction rates and lower energy consumption in industrial processes, contributing to more sustainable and cost-effective production methods. For example, in the automotive industry, catalytic converters use platinum nanoparticles to reduce harmful emissions from vehicle exhaust. In the chemical industry, gold nanoparticles are used to catalyze oxidation and reduction reactions, improving yields and reducing waste [58,59].

3.3.2. Disadvantages

Potential Toxicity

The small size and high reactivity of metallic nanoparticles can pose potential toxicity risks to humans and the environment. Their ability to penetrate biological membranes and interact with cellular components can lead to cytotoxic effects, oxidative stress, genotoxic effects, and inflammatory responses. In animals, they can enter the body through ingestion, inhalation, or dermal contact, and their high permeability allows rapid entry and access to multiple organs, raising concerns about their biological fate and possible accumulation. The lack of comprehensive regulation governing the use of metal nanoparticles in agricultural, food, and health products poses a risk to consumer safety. This raises significant concerns about their safe use, especially in medical and consumer products. Furthermore, the long-term health effects of nanoparticle exposure remain unclear, underscoring the need for comprehensive toxicological studies and safety guidelines [60,61,62].

Aggregation

Metallic nanoparticles tend to aggregate or clump together over time, which can reduce their effectiveness in various applications. Aggregation can decrease the surface area available for reactions, diminish their unique properties, and complicate their handling and dispersion in solutions or matrices. This presents a significant challenge in maintaining the stability and performance of nanoparticle-based products. Researchers are actively exploring methods to prevent aggregation, such as surface modification, the use of stabilizing agents, and optimizing synthesis processes [63,64].

Stability Issues

Metallic nanoparticles can be chemically unstable, prone to oxidation and other reactions that can degrade their properties and functionality over time. For instance, silver nanoparticles can oxidize to form less active silver oxide, reducing their antimicrobial effectiveness. Similarly, gold nanoparticles can undergo surface changes that affect their optical properties. Ensuring the stability of nanoparticles during storage, handling, and use is crucial for their practical applications. Strategies to enhance stability include coating nanoparticles with protective layers, optimizing storage conditions, and developing robust synthesis methods [65,66,67,68].

High Production Costs

The synthesis of metallic nanoparticles, particularly with precise control over their size, shape, and composition, can be costly and require sophisticated equipment and processes. High production costs can limit the scalability and commercial viability of nanoparticle-based technologies. Research efforts are focused on developing cost-effective and scalable synthesis methods, such as green synthesis using biological materials, to reduce production costs and make these technologies more accessible for widespread use [69,70].

Environmental Impact

The long-term environmental impact of metallic nanoparticles is still not fully understood, and their release into ecosystems could have unforeseen consequences. Nanoparticles can accumulate in soil and water, potentially affecting microbial communities, plants, and aquatic life. Their small size and persistence in the environment raise concerns about bioaccumulation and potential entry into the food chain. Although green synthesis is more sustainable, the long-term environmental effects of nanoparticles constitute a risk. Their possible interactions in the different environmental compartments, biogeochemical cycles, climate change, and ecosystems need to be studied in detail to scale up this process to an industrial level and guarantee their safe use. Ecological methods of environmental remediation are another potentially viable area of opportunity that favors the use of nanoparticles [62,71,72,73].

4. Polysaccharides from Waste and By-Products

4.1. Wheat Straw

Agro-industrial waste is mostly lignocellulosic material, which, when subjected to biological, physical, or chemical processes, can be degraded and later reused as raw material. If not properly managed, these residues become an environmental problem [6]. Wheat straw is an agro-industrial residue generally composed of bran, leaves, straw, and ash [74]. Annually, it is estimated that 529 million tons of wheat straw are generated worldwide, with the United Kingdom alone producing 5 to 7 million tons each year, of which only 1% is marketed. Additionally, wheat straw is one of the most abundant raw materials globally. It is composed of different types of lignocellulosic materials linked by covalent bonds within its cell wall, which consist of lignin (8–15%), cellulose (35–45%), and hemicelluloses (20–30%) [75].

4.2. Lignocellulosic Material

4.2.1. Sodium Carboxylmetyl Cellulose

Different derivatives can be obtained from cellulose, one of which is sodium carboxymethylcellulose. Unlike cellulose, this compound is water soluble. Its synthesis occurs through a chemical reaction between acetic acid, chlorine, and cellulose water with alkali. This polymer has several applications in industries such as textile, pharmaceutical, food, biomedical, and cosmetic. It is highly stable and has a low cost [76].

4.2.2. Cellulose Nanocrystal/Zinc Oxide

Zinc oxide–cellulose nanocrystals are recognized for their various applications in different disciplines, including use for their antimicrobial properties, use as food containers, and use as photocatalysts. The nanocrystals are eco-friendly, non-toxic, and biodegradable materials [77].

4.2.3. Silver–Lignin Nanoparticles

Lignin is one of the lignocellulosic residues characterized by its high number of branches, low miscibility, solubility, non-toxicity, and aromatic rings. This biopolymer is common in the cell walls of plants, where it fulfills essential functions. Annually, the paper and pulp industry alone generates 50 to 70 million tons of agro-industrial waste. Lignin is an important material used in the production of paints, certain chemicals, cosmetics, concrete additives, and more. Silver–lignin nanoparticles are distinguished by their conductivity, stability, high surface area, and catalytic activity. Additionally, these nanoparticles exhibit good antimicrobial, antidiabetic, and antioxidant properties [78].

4.2.4. Shiitake (Lentinus edodes)

Lantin is a complex carbohydrate with a structure similar to that of a β-glucan. Due to this similarity, lantin has immunoregulatory, antioxidant, and anticarcinogenic properties. It also stands out for its nutritional and pharmaceutical potential. The interaction of the hydroxyl groups of this polysaccharide with noble metals can give rise to platinum nanoclusters, which exhibit fluorescence, magnetism, and catalysis properties [79].

4.3. Fruits and Vegetables

4.3.1. Guava (Psidium guajava L.)

Guava is a durable fruit primarily produced in subtropical and tropical countries, with the largest producers being Brazil, India, Pakistan, Mexico, China, Thailand, and Indonesia. This fruit is considered to have a high nutritional value due to its content of carotenoids, alpha-tocopherol, ascorbic acid, phenols, and dietary fiber. Consequently, it is attributed with high antioxidant and anticancer properties [80]. The complex carbohydrates found in the leaves of this plant can be used directly to synthesize silver nanoparticles, which are stable, non-toxic, and uniform. These nanoparticles, with sizes between 25 to 35 nm, exhibit antioxidant and antimicrobial capacities and are extremely useful for the preparation of hospital materials, particularly in pathologies caused by pathogens and free radicals [81].

4.3.2. Okra (Abelmoschus esculentus)

This vegetable is characterized by its structure, which contains different complex carbohydrates such as alpha cellulose, lignin, hemicellulose, and pectin. Its nutritional value is highlighted by the amount of water, minerals, and water-soluble and fat-soluble vitamins. It also contains compounds with antioxidant, anti-fatigue, anti-diabetic, neuroprotective, and anti-hyperlipidemic properties. For the synthesis of nanoparticles, both the aqueous extract and the cellulose of this vegetable are commonly used, achieving a green synthesis of gold, silver, and zinc oxide nanoparticles, with antibacterial activity being a particularly notable property [82].

4.3.3. Apple (Malus domestica)

Pectin from apple peel is a polysaccharide that has not been as extensively investigated as others. This biopolymer is a reliable source for producing inorganic nanoparticles because it is a plant heteropolysaccharide, and its anionic features can be extracted from apple and citrus peel. The wide applicability of pectin is due to its biocompatibility, biodegradability, and non-toxicity [83].

4.3.4. Durian (Durio zibethinus)

This fruit is a rich source of polysaccharides, including cellulose, lignin, hemicellulose, and pectin, which can be found in its rind. Pectin is an essential element for the biological synthesis of zinc oxide nanoparticles (ZnO). Durian has high nutritional value due to its content of macronutrients, polyphenols, macro minerals, trace elements, and water-soluble vitamins, contributing medicinal properties such as antioxidant, antimicrobial, hypoglycemic, and hypolipidemic activities [84].

4.3.5. Mango (Mangifera indica L.)

Mango is a highly produced fruit in various countries worldwide. For example, in the Philippines alone, production reached 737.44 metric tons in 2019. Although fruit is the primary focus of cultivation, other parts of the tree, such as its bark and leaves, have unexplored potential. The leaves, in particular, are a rich source of anti-inflammatory, antioxidant, and anticancer compounds [85]. Specifically, mango peel is composed of different carbohydrates such as lignin, hemicellulose, flavonoids, and pectin, which can be used for the synthesis of silver nanoparticles characterized by their antioxidant activity [86].

4.3.6. Banana (Musa paradisiaca)

Approximately 105.95 million tons of bananas are produced annually, making it one of the most important fruits in the agri-food industry. The peel is the most utilized agro-industrial waste due to its high content of various polysaccharides. Applications of banana peel include its use as a substrate and for the adsorption of heavy metals in water [9]. Another significant agro-industrial waste is the banana bract, with approximately 300 kg of bract gathered per hectare of banana trees. The pectin found in the bract contains abundant carboxyl and hydroxyl groups, which can be used with calcium cations to synthesize hydroxyapatite nanoparticles, promising for biomedical applications [87].

4.3.7. Mangrove Fruit (Ceriops decandra)

This fruit is characterized by its contraceptive, antimicrobial, antioxidant, and antidiabetic properties. It is commonly used in traditional medicine for pathologies such as diarrhea, hepatitis, dysentery, diabetes, and angina. The polysaccharide of this fruit can be used for the synthesis of silver nanoparticles, providing a stable synthesis that results in nanoparticles with high antimicrobial activity against both Gram-positive and Gram-negative pathogens, reduced production cost, and eco-friendly properties [10].

4.3.8. Plant Waste

The most abundant structural polysaccharide in the plant kingdom is cellulose [88]. Plants are considered more suitable than microbes for the green synthesis of nanoparticles due to their non-pathogenic nature and the extensive research on various pathways [45].

For examples, silver nanoparticles were synthesized using geranium (Pelargonium hortorum) leaf extract, resulting in particles sized 25–150 nm [89]. Similarly, the synthesis of silver nanoparticles (20–480 nm) using sulfated polysaccharide extract from Sargassum siliquosum was investigated for its toxicity and hepatoprotective potential [90]. Additionally, silver nanoparticles (16–40 nm) were synthesized using the leaf extract of Datura metel, which contained biomolecules such as alkaloids, proteins, enzymes, amino acids, alcoholic compounds, and polysaccharides responsible for the reduction of the silver ions to nanoparticles [91].

Wang et al. [92] demonstrated the green synthesis of hexagon-shaped AuNPs (10–20 nm) and polygon-shaped AgNPs (100–150 nm) using Dendropanax morbifera leaf extract, where polysaccharides and other phytochemicals were thought to be responsible for the synthesis and stabilization of the nanoparticles. On the other hand, a rapid preparation process of Au NPs (10–30 nm) was investigated using lignin nanoparticles (LNPs) at room temperature without chemical additives. The LNPs acted as a reducing agent, stabilizing agent, and template for the preparation of LNPs/AuNPs [93].

4.4. Agro-Waste

Residues from vegetable processing, such as seeds, hulls, skins, husks, exhausted pulps, and immature or damaged fruits, constitute agro-industrial by-products rich in structural polysaccharides (cellulose, hemicellulose, or pectins) or other types of glycans and dietary fibers, potentially useful for the green synthesis of MNPs [88]. The large volumes of by-products generated pose a major challenge for the agri-food industry, requiring an integrated approach for recycling, reuse, and recovery. Although composting agricultural residues for manure and biofuels is already practiced, these residues can also be used for the synthesis of valuable nanoparticles [94].

Recently, efforts have been made to replace plant parts with agro-industrial wastes, such as mango and citrus peels, in the development of green methods to produce NPs [95,96]. Additionally, green synthesis of gold and silver nanoparticles has been achieved using leaf extract of the Capsicum chinense plant [97].

Metals and metal oxide nanoparticles can be synthesized and enhanced in their properties using different wastes. Ahmed et al. [98], synthesized Fe₃O₄ nanocomposites using papaya leaves as lignocellulosic agricultural waste through a thermal decomposition method.

Various weeds or shrubs, often considered unwanted in agricultural fields, are removed and burned. However, many of these weeds have significant pharmaceutical properties and great potential as bioreactors for the synthesis of nanomaterials. Their use for nanoparticle synthesis is increasingly being investigated, offering the additional advantage of avoiding the felling of large trees and promoting environmentally sustainable nanoparticles [94].

4.5. Advantages of Using Polysaccharides from Agro-Industrial By-Products for Nanoparticle Synthesis

Although the environmental impact of metal nanoparticles remains a controversial topic among researchers and institutions, green synthesis offers an alternative for the use and revaluation of agro-food by-products rich in carbohydrates and other reducing compounds. These polysaccharides exhibit properties such as biocompatibility, biodegradability, reductive character, solubility in non-toxic solvents, abundance, stabilizing capacity, encapsulation and coating capacity, cost-effectiveness, versatility, and compound diversity, making them particularly suitable for the synthesis of nanoparticles. Their biocompatibility allows their use in sustainable agriculture, medical sciences, and food preservation, exploiting their non-toxic properties and affinity with biological systems. While the nature of the nanoparticle itself may be a controversial factor, the polymeric matrix is not. Green synthesis reduces production costs and eliminates potentially polluting organic matter for terrestrial and marine ecosystems, driving a circular economy by valorizing waste materials [99,100,101].

5. Types of Metallic Nanoparticles That Can Be Synthesized from Polysaccharides

Metallic nanoparticles offer specific properties that are not available on bulk materials or isolated molecules, which has led to a rising interest in research [102]. Due to their electronic, magnetic, catalytic, and antimicrobial activities, metallic nanoparticles can be applied in areas such as electronics, cosmetics, coatings, packaging, biotechnology, and biomedicine [103]. Monometallic, bimetallic, and oxide nanoparticles obtained from polysaccharides are reviewed in this article.

5.1. Monometallic Nanoparticles

The green synthesis of monometallic nanoparticles from plants has gained interest due to their availability, simplicity, low-cost, reproducibility, stable nanoparticles, use of non-toxic chemicals, and reduced environmental impact [102]. Bioactive compounds in extracts from different parts of the plants, such as leaf, root, latex, fruit pericarp, fruit juice, seed, and stems, have been used for the synthesis of monometallic NPs [104]. In literature, silver, gold, platinum, and palladium monometallic nanoparticles have been reported as a result of green synthesis using polysaccharides.

5.1.1. Silver Nanoparticles

Silver nanoparticles (AgNPs) have been synthesized by the reduction of AgNO₃ with sugarcane bagasse as the reducing and capping agent; the nanoparticles had a spherical shape and a size of approximately 22 nm. Additionally, the nanoparticles showed cysteine detection capacity in human serum [105]. In another study, AgNPs were synthesized from peach gum polysaccharide though a photoinduction method induced by natural sunlight. The obtained nanoparticles presented a predominantly spherical shape and an average size of 23.56 ± 7.87 nm; moreover, they showed sensing activity against H₂O₂ [106].

Banana peel (Musa paradisiaca) extract was used by Bankar et al. [107] for the synthesis of AgNPs by reducing AgNO₃; synthesis parameters such as pH, banana peel extract content, AgNO₃ concentration, and temperature were varied to study their effects. The exact dimensions of the nanoparticles were not identified, but it was observed that they coalesced into nano-clusters, and when the reaction time was longer, some nanoparticles aggregated into microstructures. The obtained nanoparticles displayed antifungal activity against Candida albicans and Candida lipolytica, and antibacterial activity against Citrobacter kosari, Enterobacter aerogenes, Escherichia coli, Klebsiella sp., Proteus valgaris, and Pseudomonas aeruginosa. Similarly, Ibrahim [108] synthesized AgNPs using the same method and varying the same parameters. The synthesized nanoparticles were spherical and monodisperse in size, with an average diameter of 23.7 nm. The obtained nanoparticles showed antimicrobial activity against B. subtilis, S. aureus, P. aeruginosa, and E. coli; in addition, this characteristic was enhanced by combining the nanoparticles with the antibiotic levofloxacin.

Harish et al. [109] extracted xylan from wheat bran for the subsequent synthesis of AgNPs. These nanoparticles were spherical in shape and ranged in size from 20 to 45 nm. They were then used to evaluate their effect on preformed blood clots, showing a potential use for the treatment of thromboembolism. Similarly, starch was isolated from cashew nut shell, which acted as a capping and stabilizing agent in the synthesis of AgNPs with circular and oval shapes and sizes from 10 to 50 nm [110].

Mango (Mangifera indica Linn) peel extract was used to synthesize AgNPs from AgNO₃ as a source of silver. Reaction parameters, including pH, AgNO₃ concentration, extract content, and time, were varied for synthesis evaluation, resulting in quasi-spherical AgNPs with sizes of 7–27 nm. The obtained AgNPs were loaded into nonwoven fabrics, which exhibited antibacterial activity against E. coli, S. aureus, and B. subtilis [111]. Another study used durian (Durio zibethinus) rind for the photo-induced synthesis of AgNPs by varying the effect of light intensity, pH, and exposure time. The resulting nanoparticles were small in size (11.4 ± 3.2 nm), which can be related to their strong antibacterial activity [112].

5.1.2. Gold Nanoparticles

The green synthesis of gold nanoparticles (AuNPs) from polysaccharides has been reported in the literature. Passion fruit (Passiflora edulis) and pineapple (Ananas comosus) peel extract were used for the synthesis of spherical AuNPs with sizes of 18–20 nm, which did not show cytotoxicity in Vero and MCF-7 cells [113]. In another work, pineapple (Ananas comosus (L.) Merril) gum was also used for the synthesis of AuNPs (nearly spherical shape; 10.3 ± 1.6 nm) with sensing properties for the antihistamine drug promethazine hydrochloride [114]. AuNPs were developed from palm oil mill effluent (POME) by sonication-assisted synthesis. The synthesized nanoparticles presented predominantly spherical shapes with an average size of 18.75 ± 5.96 nm and have the potential for mercury removal in water purification [115].

Bankar et al. [116] used banana peel extract for the development of obtained AuNPs; due to a coffee ring phenomenon, the nanoparticles formed microcubes and networks. However, the nanoparticles showed antibacterial activity against Shigella sp., C. kosari, E. coli, P. valgaris, and E. aerogenes. Moreover, pectin extracted from banana peel developed AuNPs with anticancer potential against mammary adenocarcinoma cells [117].

5.1.3. Platinum Nanoparticles and Palladium Nanoparticles

Few are the studies available regarding the green synthesis of palladium nanoparticles (PdNPs) and platinum nanoparticles (PtNPs) from agro-waste and by-products. An aqueous extract of watermelon rind was used to obtain PdNPs with a controlled shape and average size of 96 nm, which can be used in catalytic applications in the industry [118]. In two other studies conducted by Ishak et al. [119,120], PtNPs were synthesized from sugarcane bagasse by reducing H₂PtCl₆₆H₂O. The nanoparticles obtained in both works showed greater electrochemical activity than commercial Pt black catalyst, giving them the potential for application as catalysts in direct methanol fuel cells. There were slight differences in the sizes and shapes of the PtNPs obtained in each study; shapes varied from quasi-spherical to spherical, while sizes averaged from 4 nm to 5 nm.

5.2. Bimetallic Nanoparticles

Bimetallic nanoparticles (BMNPs) are formed by two different metals. Compared to monometallic nanoparticles, BMNPs present characteristics that generally confer more significant properties, contributing to their applicability for antimicrobial, anticancer, biosensor, and catalysis effects, among others [121,122]. Bimetallic nanoparticles from agro-waste and by-products are not extensively reported in literature; iron–copper, iron–zinc, and silver–gold nanoparticles from polysaccharides are discussed next.

5.2.1. Iron–Copper Nanoparticles

Iron–copper nanoparticles (CuFeNP) were obtained by the milling of orange peels with Cu(NO₃)₂·3H₂O and Fe(NO₃)₃·9H₂O; subsequently, a calcination process with variable temperature was conducted to complete the synthesis. Variations in temperature affected the surface area, crystal structure, and particle size of the nanoparticles; however, it also influenced their catalytic performance with nanoparticles developed under 200 °C exhibiting favorable catalytic activity [123].

5.2.2. Iron–Zinc Nanoparticles

In a study conducted by Oruç [124], lemon (Citrus limon L.) pomace and leaves from juice production were used to produce activated carbon and iron–zinc (FeZnNPs) bimetallic nanoparticles. Activated carbon was obtained from the pomace, which was then used for the synthesis of the FeZnNPs using the lemon leaves as the reducing agent. Spherical, agglomerated particles with an average size of 126 nm was obtained. The synthesized nanoparticles showed Fenton-like catalyst activities against the azo-dye Reactive Red 2; this characteristic positions FeZnNPs as a potential catalyst for wastewater treatment.

5.2.3. Silver–Gold

Silver–gold nanoparticles (Ag-AuNPs) were successfully synthesized using waste tea leaves as a reducing agent [125]. Catalyst activity was then evaluated with Congo red dye molecules and 4-nitrophenol, resulting in rapid degradation due to the surface area and irregular facets present in the Ag-AuNPs. Wastewater purification and treatment could benefit from the rapid degradation process provided by waste tea leaves. In another study, agro-waste-derived lignin was analyzed as a source of Ag-AuNPs; however, the specific agro-waste was not mentioned [126]. The size of the nanocomplex was 25 nm, which was larger than nanoparticles synthesized individually. Antioxidant and antimicrobial activity were also higher in Ag-AuNPs than in AgNPs and AuNPs alone.

5.3. Metallic Oxide Nanoparticles

The green synthesis of oxide nanoparticles has gained interest due to their catalyst, sensing, (opto)electronic, antibacterial, and environmental remediation activities [127,128]. Similar to the metallic and bimetallic nanoparticles, metallic oxide nanoparticles have been synthesized from plant and plant extracts [129]. Metallic oxide nanoparticles such as zinc oxide, iron oxide, copper oxide, titanium dioxide, and manganese oxide have been reported to be synthesized from polysaccharides.

5.3.1. Zinc Oxide

Limited information on zinc oxide nanoparticles (ZnO) synthesized from polysaccharides derived from agro-waste and by-products was found in the literature. Date pulp (Phoenix dactylifera) waste was used to develop ZnONPs through ultrasonic-assisted synthesis. The characterization of the nanoparticles revealed a spherical shape with an average size of 31.6 nm. Additionally, the nanoparticles exhibited dye degradation of eosin yellow and methylene blue dyes due to their photolytic activity [130]. Durian (Durio zibethinus) rinds are rich in pectin, and ZnONPs were obtained from this source through green synthesis; sizes ranged from 280 and 283 nm with spherical shape. The nanoparticles showed antibacterial, antioxidant, and photocatalytic activities; furthermore, ZnONPs exhibited potential cytotoxic activity [84].

5.3.2. Iron Oxide

Iron oxide nanoparticles (IONPs) synthesized from various polysaccharide sources can be observed in

Table 2. Iron oxide nanoparticles obtained by green synthesis from agro-waste and by-products.

Agro-Waste or By-Product	Properties of the Obtained Nanoparticles	Shape and Size of Synthesized Nanoparticles	Reference
Orange peel	Cr (VI) removal	Nanoparticles clustered in nanorods in the range of 20 to 40 nm in diameter.	[131]
Tea waste	Cr (VI) removal	Form not specified. Size approximately 20 nm.	[133]
Coconut (Cocos nucifera L.) husk	Absorption of Ca and Cd	Not specified.	[132]
Papaya leaves	Dye sequestration	Nanoparticles with distorted spherical shape and diameter of 10.39 nm.	[98]
Paddy and wheat straw	Magnetic and antioxidant	Spherical nanoparticles with sizes 20–32 nm.	[134]
Watermelon (Citrullus lanatus) rinds	Catalyst in the synthesis of 2-oxo-1,2,3,4-tetrahydropyrimidine derivatives	Spherical nanoparticles with sizes 2–20 nm.	[135]
Peel extracts of C. limon, V. vinifera, and C. sativus	Removal of antibiotics	Spherical and polyhedral agglomerated nanoparticles. Diameters from 8 to 12 nm.	[136]

. Orange peel was used to develop iron oxide nanorods ranging 20–40 nm through the reducing action of the cellulose, hemicellulose, and lignin present in the peel. Additionally, Cr(VI) removal was evaluated, showing greater adsorption capacity than orange peel alone; this can be attributed to the surface interaction with the nanorods [131]. In another study, coconut (Cocos nucifera L.) husk was used for the rapid synthesis of IONPs by reducing ferric chloride. The resulting nanoparticles had absorption properties against heavy metals present in water, such as Ca and Cd; as a result, IONPs increased iron nutrition in Oryza sativa L. plants [132].

Papaya leaves, a lignocellulosic waste biomass material, were first used by Ahmed and Ahmaruzzaman [98] to stabilize IONPs, forming a nanocomposite. The study reported dye sequestration activity in wastewater from the nanocomposite. In a study by Periakaruppan et al. [134], lignin extracted from paddy and wheat straw was used to synthesize superparamagnetic iron oxide nanoparticles (SPIONs) by the reduction of FeSO₄; the nanoparticles showed good magnetic and antioxidant properties that can be applied in biomedicine.

5.3.3. Copper Oxide, Titanium Dioxide, and Manganese Oxide

In addition to ZnONPs and IONPs, other oxide nanoparticles have been reported in literature. Yadav et al. [137] synthesized cuprous oxide nanoparticles (Cu₂O-NPs) from sugarcane bagasse, which is rich in hemicellulose, cellulose, and lignin; these nanoparticles exhibited degradation activity against toxic organic dyes. Similarly, sugarcane bagasse was used for the synthesis of titanium dioxide nanoparticles (TiO₂-NPs) by a sol-gel method; the resulting nanoparticles degraded 95% of methyl orange dye due to their photocatalytic activity [138]. In another study, manganese oxide nanoparticles (Mn₃O₄-NPs) were developed from banana peel extract, resulting in spheroid morphology, and sizes ranging from 20 to 50 nm. The pseudocapacitive behavior of the nanoparticles was evaluated, exhibiting a 97.5% capacitance retention [139].

6. Mechanistic Aspects of Metallic Nanoparticles from Polysaccharides

6.1. Mechanistic Aspects Using Lignocellulosic Material

Lignocellulosic materials are very abundant in nature and are key components of agro-industrial by-products and wastes, such as straw. The main structure of lignocellulosic material is shown in Figure 4. Rice is one of the main consumable crops in many countries and is also a rich source of cellulosic polymer, which can release sugar and act as a natural reducing compound for the development of nanomaterials. Rice straw (~45% of the total amount of rice plants) is an available by-product potentially useful for nanomaterial synthesis due to its richness in lignin, sugars, proteins, and other bioactive reducing compounds [140].

In a study conducted by Areeshi [142], iron oxide nanoparticles (IONs) were synthesized using rice straw. A 0.1 M (volume 100 mL) ferric nitrate solution was firstly prepared, and then rice straw extract and an aqueous solution of ferric sulphate were mixed in a 2:1 ratio (volumetric ratio) for 30 min. Then, the nanoparticles were washed, centrifuged, and turned into a fine powder. X-ray diffractograms corresponded to the rhombohedral crystal structure of the hematite (α-Fe₂O₃) phase. Additionally, a few peaks were recorded, suggesting the possible presence of other crystalline structures/phases of IONPs as the minor phase. The FT-IR spectrum showed bands at 3440 cm⁻¹, 1660 cm⁻¹, and 1386 cm⁻¹, which could be correlated to the O–H group of phenolic compounds, C=C stretching, and C–O aromatic mode, respectively. Two bands recorded at 568 cm⁻¹ and 460 cm⁻¹ were attributed to the –Fe–O bonds. The specific mechanism attributed to the synthesis of the nanoparticles is the reduction of iron ions from lignocellulose and other phytochemicals in rice straw, the same compounds that contribute to the stabilization of the nanoparticles. Musa et al. [140] synthesized copper nanoparticles (CuNPs) using rice straw (RS). The study indicates that RS/CuNPs could be obtained by the chemical reduction of CuSO₄ 5H₂O salt using hydrazine, rice straw, and NaOH solution as the reducing agent, biotemplate support, and pH moderator, respectively. The mechanistic aspects of synthesis are shown in Figure 5.

Lai et al. [143] demonstrated the synthesis and characterization of a sulfonated (sulfonic acid supported) silica-magnetic nanoparticle composite as a catalyst in the hydrolysis of lignocellulosic biomass. These nanocomposites consist of sulfonated mesoporous silica as a promising hydrolysis catalyst, modified with iron oxide nanoparticles with magnetic properties. The mechanistic aspect is shown in Figure 6. This mechanism demonstrates the efficiency of using sulfonic acid as a nanoparticle functionalizer. The acid acts as a hydrolytic compound for the lignocellulosic biomass, helping to expose reducing ends that contribute to the formation of nanoparticles.

Lignocellulose is composed of cellulose, hemicellulose, and lignin [5]. Hemicellulose, in the case of nanoparticle synthesis, has the advantage of preventing the aggregation of nanomaterials, resulting in better dispersion and stability. In addition, its ability to form complexes with nanoparticles favors encapsulation [19,144]. When comparing hemicellulose with cellulose, cellulose exhibits superior mechanical properties to hemicellulosic materials, but hemicellulose confers greater flexibility, stability, and redispersion power. Furthermore, due to the structural difference between these molecules, hemicellulose presents greater heterogeneity in its hydroxyl groups, making them more available and, therefore, providing greater reducing power than cellulose, a favorable characteristic for the green synthesis of metallic nanoparticles [19,145]. Additionally, due to the structural difference between these molecules, lignocellulose presents greater reducing power because it includes lignin in its composition, a complex polymer with an aromatic structure containing methoxyl and phenolic hydroxyl groups. This reducing power is a favorable characteristic for the green synthesis of metal nanoparticles. Lignin also presents benefits as a potential coating matrix and stabilizer in the green synthesis of AgNPs, making the process more cost-effective [19,146]. Kraft lignin is reported to serve as an effective synthesis and stabilizing agent for gold, silica, and silver nanoparticles [22,93,147].

Saratale et al. [78] extracted lignin from wheat straw for use in the green synthesis of silver nanoparticles. They mention that the identified lignin constituents are carbohydrates, phenolic hydroxyl group, carboxyl groups, and total phenolic content. In their study, they report that the formation was completed by modifying the pH, concentration of salt, extract, and synthesis time, impacting the size and shape of AgNPs and their yield. The surface plasmon resonance (SRP) of AgNPs is influenced by the NP agglomeration, size, shape, and surrounding dielectric medium, where pH 8 resulted in the largest SRP. Additionally, the experimental results suggest that a 1:10 mixing ratio of lignin to AgNO₃ was identified as optimal, wherein a sharp and higher SRP spectrum was recorded. They conclude that the mechanism of Li-AgNP synthesis involves the lignin biochemical constituents—phenolic, aliphatic hydroxyl, and carboxylic groups—acting as capping, reducing, and stabilizing agents for AgNPs. The formation mechanism is shown in Figure 7.

Another study by Hu and Hsieh [148] synthesized AgNPs using 2 mmol/L AgNO₃ and 0.4 wt% alkali lignin at 85 °C via two procedures: (1) the direct mixing of AgNO₃ and alkali lignin at pH 10.01 for 60 min and (2) mixing AgNO₃ with neutral alkali lignin solution for 30 min, then adding NaOH for another 60 min. Mechanistic studies on AgNPs evidenced higher reaction speed and more uniformly sized AgNPs under increasingly basic conditions, where Ag₂O formation and the adsorbed AgNPs on the Ag₂O surfaces catalyzed faster Ag⁺ reduction. The elucidated mechanism involves the ionic binding of Ag⁺ with hydroxyl and phenolic sulfonate groups of lignin under neutral conditions, giving rise to closely associated complexes, which would be unaffected by the addition of NaOH, precluding the formation of Ag₂O. These silver–lignin complexes would have a shape and size congruent with the lignin, heterogeneous and irregular, a shape that they would retain when converted to zero-covalent nanoparticles by reduction, preserving their polydisperse size. Meanwhile, the Ag⁺ not associated with lignin could undergo a nucleation process after the addition of NaOH, changing Ag₂O instantaneously and subsequently to AgNPs by autocatalytic reduction.

6.2. Mechanistic Aspects Using Pectin

Dmochowska et al. [149] prepared zinc oxide (ZnO) nanoparticles using pectin-rich banana peel extract (BPE). They suggested that the compounds adsorbed on the ZnO surface were pectin. The FT-IR study showed mechanical aspects of the synthesis, where the strong intensity of the bands associated with the COO⁻ group could indicate their strong affinity to the surface of ZnO. Pectin and the adsorbed nanoparticles interact through hydrogen bonds, forming a structure that can be cross-linked or branched. The increase in the amount of extract used is related to the intensity of the peaks associated with organic compounds, directly correlating with the amount of substances adsorbed on the zinc oxide surface. The growth mechanism could be similar to that of the synthesis, although the higher concentration of extract might have strengthened the interactions between the organic compounds and the crystallites, sterically blocking recrystallization (Figure 8).

Another study by Wang et al. [150] synthesizes silver nanoparticles using pectin extracted from citrus peel. Silver nanoparticles coated by pectin macromolecules were developed by the chemical reduction of Ag⁺ cations under ultrasonic conditions. Pectin acted as both a reducing and stabilizing agent. In an alkaline medium, the reducing properties of pectin are determined by its free hydroxy hemiacetal groups; in addition, arabinans and galactans contain hydroxyl groups capable of exhibiting the reducing properties. Pallavicini et al. [151] also synthesized silver nanoparticles using pectin from citrus peel (Figure 9a). The results showed an abundant formation of AgNP-pectin, with an orange-brown slurry formed during the synthesis of Ag nanoparticles from Ag⁺, with pectin acting both as a reductant and coating agent. Approximately 100% Ag⁺ to Ag⁰ one-pot conversion was obtained, yielding p-AgNP, i.e., an aqueous solution of pectin-coated spherical Ag nanoparticles. AgNP synthesis was efficient even at higher Ag⁺ concentrations, namely 0.01 M Ag⁺, with a slightly higher absorption wavelength maximum (415–420 nm) obtained with preparations made with 0.5–2% pectin at 20 and 60 °C. This result was related to the oxidation of the diol moiety of galacturonic acid to dialdehyde, reducing Ag⁺ to Ag (Figure 9).

6.3. Mechanistic Aspects Using Starch

Starch, like other polysaccharides, is capable of reducing nanoparticle precursor salts due to its hydroxyl groups and a hemiacetal reducing end. Polymer chains form complexes with metal ions in solution due to their considerable concentration of hydroxyl groups, while supramolecular nanostructures exert a template effect for growth processes. Khan [152] synthesized AgNPs using the following methodology. A 5.0 mL 0.01 mol/L AgNO₃ solution and 5.0 mL 5% (w/v starch were mixed in a two-necked reaction vessel containing 35 mL water. A glucose solution (0.01 mol/L) was added to the reaction mixture, and the temperature was increased to 50 °C. After 1 h, the reaction mixture changed from colorless to dark brown due to the reduction of silver ions to metallic silver. The effects of silver ions and glucose concentrations were also evaluated at a fixed starch concentration of 4.0% (w/v) and room temperature. The effect of glucose concentration was not significant on the SRP position of the starch-coated silver nanoparticles. However, the SRP intensity and λmax increased and redshifted with increasing starch concentration, elucidating that the nucleation and growth of nanoparticles are highly dependent on starch concentration, increasing the number of nanoparticles formed. Amylose, the water-soluble component of starch, is the most available structure for rendition. Amylose consists of α-D-glucose subunits, which have primary and secondary –OH groups, with the oxidation of the primary –OH group being easier than that of the secondary –OH. On the other hand, the anomeric –OH C1 and –OH C4 are not available for the reaction because they form the glycoside bond. Therefore, the oxidation–reduction mechanism between Ag⁺ and the glucose unit of amylose is as shown in Figure 10.

6.4. Mechanistic Aspects Using Gums

Gums act as reducing agents for gold ions and as capping agents after the formation of AuNPs. The most widely recognized mechanism for the development of AuNPs consists of two steps: formation and the polymerization of the atom. In the first step (formation), the temperature of the gum solution is raised using one of the commonly recognized technologies (autoclaving, microwave irradiation, normal heating, or sonochemical treatment). The thermal expansion of the biopolymer makes the functional groups more accessible to interact with the gold ions. Gums contain monomers such as rhamnose, galactose, and uranic acid, with functional groups such as hydroxyl, carbonyl, and carboxyl groups. Compared to other polysaccharides, gum has a higher anionic charge, with a higher charge density [153]. This negative charge contributes to the attraction of positively charged gold ions by the polymer chains. The gold ions oxidize the hydroxyl and carbonyl groups to carboxyl groups, and thus the ions are reduced. In addition to this inherent oxidation, the dissolved air also causes an oxidation of the existing hydroxyl groups to carbonyl (-CHO and -COOH) groups. In turn, these powerful reducing aldehyde groups, along with the other abundantly present carbonyl groups, reduce more gold ions to gold atoms. In the second step, the coating and stabilization of the AuNPs by the polysaccharides present in the gum occurs. The stabilization of the AuNPs occurs due to a strong association between the surface of the AuNPs and the “O” atoms of the hydroxyl and carbonyl functional groups of the natural gums. The resulting stabilized AuNPs avoid coalescing with the neighboring particles due to the electrostatic repulsion and steric effect [153].

Taher et al. [154] obtained a well-stabilized AgNP solution using polysaccharides extracted from L. leucocephala seed residues as reducing agent. The color change in the solution to brownish shades evidenced the occurrence of the synthesis reaction, resulting from the excitation of the surface plasmonic vibration of the AgNPs. FT-IR confirmed the formation and coating of the AgNPs, with a peak at 3422 cm⁻¹ demonstrating the stretching of the hydroxyl oxygen-hydrogen bond in the AgNPs, serving as evidence for the presence of the galactomannan gum coating.

On the other hand, Tagad et al. [155] used locust bean gum to synthesize and stabilize AuNPs. The AuNPs produced are spherical in shape, with a crystal structure and SRP band at 537 nm. It was concluded that the active reaction centers were the hydroxyl groups and the reducing hemiacetal ends of locust bean gum, enabling the reduction of Au³⁺ to Au⁰. FT-IR analysis showed that the band at 1728 cm⁻¹ disappeared after the addition of Au³⁺ ions in the reaction mixture, resulting from the carbonyl stretching at the reducing end of the gum. However, this band disappeared as the intensity of the bands at 1642 cm⁻¹ and 1554 cm⁻¹ increased. This is due to the oxygenation of the carbonyl groups, shifting to carboxyl groups, which are responsible for the increase in the intensity of the aforementioned bands. This indicates that the reduction of Au³⁺ is coupled with the oxidation of the hemiacetal and aldehyde groups, resulting in locust bean gum-stabilized AuNPs. The mechanism is shown graphically in Figure 11. The numerous hydroxyl groups and the hemiacetal reducing ends act as active centers to favor the reduction of Au³⁺ to Au⁰, keeping the nanoparticles embedded and stabilized within the polymeric matrix.

7. Applications of Nanoparticles Synthesized from Polysaccharides

7.1. Food

In the area of food, the use of nanotechnology to help combat food spoilage reactions is of significant interest, with nanoparticles being an effective tool to eliminate the spoilage and pathogenic microbial load, as well as helping to prevent the oxidation of nutrients and other components of the food matrix. Rodríguez-Félix et al. [156], evaluated AgNPs in a spoilage bacterium and a foodborne pathogenic bacterium. AgNPs were synthesized via a green method using aqueous extract from safflower (Carthamus tinctorius L.) waste. The AgNPs exhibited a spherical shape with an average diameter of 8.67 ± 4.7 nm, favoring their interaction with bacterial membranes. Antibacterial activity was assessed against Staphylococcus aureus (Gram-positive) and Pseudomonas fluorescens (Gram-negative), showing minimum inhibitory concentrations (MIC) of 1.9 μg/mL and 7.8 μg/mL, respectively. The nanoparticles effectively inhibited bacterial growth at low concentrations, with higher sensitivity observed in Gram-positive strains. These findings highlight the potential of green-synthesized AgNPs as antimicro-bial agents for food and medical applications. Similarly, Yu et al. [157] evaluated the antioxidant activity of gold NPs (CH/P-AuNP) developed with a mixture of chitosan and pectin, with diameters of 20 to 30 nm, against the same free radical, obtaining an IC50 at a concentration of 139 µg/mL. Additionally, Sathiyaseelan et al. [158] prepared spherical tellurium nanoparticles (CH-TeNPs) with an average size of 37.48 nm using chitosan. They evaluated the antioxidant activity against DPPH· and ABTS·⁺ radicals, reaching IC50 values at concentrations of 250 μg/mL and 125 μg/mL, respectively.

Within the area of food, a problem of great interest is the appearance of various contaminants from soil, water, or utensils, such as metals and toxins. To avoid the consumption of food with high levels of contaminants, various methods have been developed to detect these contaminants. An example of the use of metallic NPs for this purpose is the research by Park et al. [159], where they developed gold NPs (AX-AuNP) using pectin-rich extracts through green synthesis to detect aluminum contaminants in water and food. In their study, they were able to detect aluminum contaminants through a color change in the AX-AuNPs in an easy and fast manner (1 min), demonstrating improved sensitivity due to pectin. For the same purpose, Chrouda et al. [160] developed an amperometric aptasensor with a glassy carbon electrode modified with gold NPs stabilized with pectin on graphene oxide for the detection of aflatoxin M1. The sensor presented a detection rate of 0.2 ng/L, and a wide linear range from 10 to 1000 ng/L, successfully used in milk.

It should be noted that the promising effects of metallic NPs often impact several sectors simultaneously. For example, the antibacterial and antifungal effects discussed later for pathogenic or phytopathogenic species also have degradative effects on various food products, with NPs having a positive impact on this sector. Additionally, the antioxidant activity proposed in this section has beneficial effects on health and even intervenes in other applications of NPs. It is challenging to fragment and analyze the applications of metallic NPs.

7.2. Health

Several studies confirm that nanotechnology and health are two fields that, although distant, can achieved significant beneficial results through the use of NPs. Nanoparticles have been employed to combat various pathogenic microorganisms, achieving relevant results [156,161,162]. Pal et al. [162] also evaluated the effect of silver NPs against three pathogenic bacteria (Pseudomonas aeruginosa, Staphylococcus aureus, and Escherichia coli), but in this case, the NPs were developed by reducing silver with guar gum and polyvinyl alcohol. In their study, they successfully synthesized silver NPs (GG/PVA-AgNP) with diameters ranging from 2 to 18 nm, which were effective in inhibiting the growth of the three bacteria, being particularly effective against E. coli.

However, bacteria are not the only pathogenic species affecting humans; various fungi can develop in different areas of the human body and cause different conditions. In this sense, research has also been conducted with metallic NPs obtained from carbohydrates. Sathiyaseelan et al. [158] evaluated the effect of CH-TeNP against Candida albicans and obtained inhibition results very similar to the drug fluconazole. Meanwhile, El-Batal et al. [161] evaluated zinc NPs (Alg-ZnNP) made with sodium alginate and irradiation. At a concentration of 65 µg/mL, Alg-ZnNP achieved inhibition diameters of 10 mm, 11mm, and 10 mm for C. albicans, C. glabrata, and C. tropicalis, respectively.

Another widely studied positive health effect of NPs is their potential anticancer effect, causing cytotoxicity in cancer cell lines [163]. Martinez-Torres et al. [164] also identified that the main effect of chitosan-coated gold NPs (CH-AuNPs), with dimensions of 3 to 10 nm, is to induce cell death through the production of reactive oxygen species and oxidative stress in hematological cancer cells, including chronic myelogenous leukemia, K562 (ATCC, CCL-243™), and T acute lymphoid leukemia, CEM (ATCC, CCL-119™), without causing significant effects on healthy cell lines of peripheral blood mononuclear cells (PBMC) and bone marrow (BM) cells. Yu et al. [157] evaluated the cytotoxicity of CH/P-AuNP in the gastrointestinal cancer cell lines AGS and KATO-III, in which they caused a decrease in viability and resulted in cytotoxicity for both cell lines, with IC50 values of 225.3 and 341.5 µg/mL for AGS and KATO-III, respectively. Furthermore, multiple NPs developed in different carbohydrate matrices and with different metals have been evaluated, showing similar results to those referenced above, making these NPs potential anticancer treatments [158,161].

In addition to measuring direct health effects, metallic NPs have also been used as vehicles for different substances for medical purposes, ensuring greater availability and accessibility of drugs due to benefits such as controlled release effects and dimensions that allow easier access to sites of action. An example of this is the gold NPs coated with hyaluronic acid (HA-AuNP) prepared by Kim et al. [165], which were incorporated with the hydrophobic drug sulfasalazine. The dimensions of the NPs depended on the irradiation dose of the hyaluronic acid, ranging from 12.70 to 24.42 nm, prior to the preparation. They achieved encapsulation efficiency and drug loading capacity values of up to 96 and 70%, respectively. While studying the release, they identified very low values in deionized water and simulated gastric solution; however, in BPS buffer and simulated intestinal solution, the release results were high, approximately 80 and 95%, respectively. Additionally, Kim et al. [165] evaluated the cytotoxicity of HA-AuNPs and HA-AuNPs loaded with the drug in C2C12 myoblast cell lines, identifying that HA-AuNPs were not cytotoxic, maintaining cell viability above 90% in all cases, while HA-AuNPs loaded with sulfasalazine were cytotoxic at concentrations above 100 µg/mL due to the presence and release of the drug, not the effect of HA-AuNPs, which were biocompatible.

El-Deeb et al. [166] developed silver nanoparticles (Af-AgNPs) using Arthrospira fusiformis cyanobacteria polysaccharide solution and evaluated their immunomodulatory and antibacterial effect against resistant Pseudomonas aeruginosa in infected rats. In their study, they identified that the topical use of Af-AgNPs did not cause any visual alteration; however, they helped combat bacterial infection. The infection decreased serum and tissue levels of alanine transaminase and aspartate transaminase, but treatment with Af-AgNPs increased these values during treatment. Additionally, urea and albumin concentrations remained at normal levels, although the increase in urea concentration may have favored healing [167]. Furthermore, El-Deeb et al. [166] documented an increase in cyclooxygenase-2 concentration, which was counteracted by the application of At-AgNPs, causing cyclooxygenase-2 levels to decrease, with no alterations in cyclooxygenase 1 due to infection or treatment, exerting an anti-inflammatory effect through the negative regulation of cyclooxygenases [168]. Regarding immunomodulatory action, At-AgNPs exhibited the ability to regulate TNF-α, IKaB, IL-1α, and IL-1β expression, helping to lower TNF-α expression levels and slightly increase IKaB, IL-1α, and IL-1β expression levels in both At-AgNP-treated infected rats and At-AgNP-treated uninfected rats.

7.3. Agriculture

In the areas of agricultural development, factors that negatively affect crops, such as pests and nutrient shortage in arable soils, are increasingly common. For this reason, several studies have developed NPs for agricultural purposes, achieving excellent results against phytopathogenic species and plant development. For example, AgNPs have multiple applications in agriculture due to their unique physi-cochemical and antimicrobial properties. They are used as nano-fertilizers to enhance nu-trient uptake and plant growth, and as nano-pesticides to control pests and diseases in an eco-friendly manner. AgNPs also improve seed germination, boost photosynthetic effi-ciency, and increase plant resistance to biotic and abiotic stress. Additionally, they are in-corporated into active food packaging to extend the shelf life of agricultural products. However, their effectiveness depends on proper dosage and application methods, as high concentrations can be toxic to plants [169].

In addition to the study of metallic NPs to combat agricultural pests, multiple minerals essential for plant development have been used as nanofertilizers. An example of this is the research conducted by Prabha et al. [170], where they developed bimetallic copper and silver NPs (AG-Cu/AgNP) using gum arabic. They identified that at concentrations below 200 mg/L these nanoparticles stimulated the germination and seedling development of Cicer arietinum L., achieving 85% germination for treated seeds compared to only 22% for the seeds treated with distilled water. Additionally, the total length of the seedlings at 14 days was 39.4 cm for seeds with AG-Cu/AgNPs, compared to 18.8 cm for those treated with distilled water. Similarly, Asanova et al. [171] evaluated the effect of silver NPs synthesized with glucose on seed germination and the growth of wheat seedlings, identifying that at NP concentrations of 0.06 to 0.5 mg/L they did not affect germination but significantly affected seedling development, increasing shoot and root mass, as well as root length, making them an ecological alternative to wheat fertilization.

7.4. Environmental

Environmental research has reached a peak in the last century due to society’s growing awareness of the importance of environmental care during development. As a result, the number of studies focused on environmental protection has multiplied. Mani et al. [172] developed a new method for detecting residues of the toxic herbicide amitrole in water, using gold NPs stabilized with calcium-cross-linked pectin (Pr-AuNP) as an ultrasensitive amitrole electrode. The increase in the concentration of the contaminant caused a decrease in the reduction peak of the gold NPs.

Within the area of environmental interest, the use of renewable energy stands out. In this sense, Flores-Gómez et al. [173] produced titanium oxide nanoparticles (P.TiO₂NPs) using pectin as a reducing agent and used these NPs to develop solar cells, along with pectin paste as a binder, sulfur and cadmium (CdS) and sulfur and zinc (ZnS) quantum dots, and a copper sulfide counter electrode. They achieved a voltage of 6.5 mV and an output current of 640 mA/cm², making this a viable alternative for solar energy collection and generation.

Additionally, bioremediation has recently gained significant attention as a fundamental environmental aspect. Metallic NPs have been developed to remove pollutants from water and soils. For example, Apriceno et al. [174] developed iron and manganese oxide NPs (MnFe₂O₄) coated with chitosan, on which the enzyme laccase was immobilized, for the bioremediation of the drug diclofenac in water. They achieved a drug removal efficiency of 78%, 8% higher than the efficiency of the enzyme without immobilization. Nguyen et al. [175] evaluated zinc oxide NPs on chitosan beads for the bioremediation of chromium (VI) in water, achieving a significant adsorption capacity of 130.38 mg/g and a removal efficiency of 97% for chromium (VI) ions. Meanwhile, Aziz et al. [176] developed zinc sulfide and chitosan NPs (CH-ZnSNP) and evaluated their photocatalytic degradation effect against the azo dyes Acid Brown 98 and Acid Black 234, achieving degradations of 92.6 and 96.7%, at 165 and 100 min, respectively.

8. Conclusions

The eco-friendly synthesis of metallic nanoparticles from polysaccharides extracted from agricultural and food industry waste offers a sustainable approach with significant environmental benefits. Researchers have developed innovative methods for synthesizing metallic nanoparticles using waste materials such as agro-wastes and food waste extracts. These methods aim to reduce the environmental impact associated with conventional manufacturing processes and add value to agricultural waste by converting it into high-value nanoparticles. The utilization of agri-food waste extracts as reducing agents in the synthesis of metallic nanoparticles contributes to sustainable practices through green synthesis methods. This approach helps minimize the use of toxic solvents and chemicals, thereby reducing human health risks and environmental degradation. Moreover, synthesizing nanoparticles from biodegradable waste extracts provides a cost-effective and environmentally friendly alternative to traditional methods.

The eco-friendly synthesis of metallic nanoparticles from polysaccharides extracted from agricultural and food industry waste not only addresses waste management challenges but also contributes to the development of sustainable nanotechnologies. By harnessing the potential of agricultural waste materials, researchers are paving the way for innovative applications in various industries while promoting environmental sustainability. The areas of application of these nanoparticles are diverse, with current research primarily focused on medicine, agriculture, environment, and food. Encouraging results have been obtained regarding the effect of metallic nanoparticles synthesized from polysaccharides in cancer cell lines, against pathogenic and deteriorative microorganisms, for the detection of environmental pollutants, in the exploitation of renewable resources such as sunlight, and as fertilizers and pesticides, among others.