1. Introduction

Regarding the metallic nanoparticles (NPs), their outstanding properties have caused the development of different methodologies for their synthesis, where gold (Au) and silver (Ag) NPs prepared from plant extracts are of great interest for the researchers in their attempt to develop suitable antimicrobial agents for agriculture [17,18,19,20]. Besides, these initiatives are considered as low-cost processes that allow avoiding toxic-generating products and benefit the agricultural activity. It is estimated that the preparation of one kg of silver nanoparticles (AgNPs) would cost about USD 4 million, while one kg of raw Ag costs around USD 14,000 [21,22].

In 2009, Raveendran et al. published one of the first green synthesis methods of metal NPs. They employed an aqueous starch solution subjected to heating, silver nitrate (AgNO₃), and glucose as the green reducing agent [23]. After that, researchers like Iravani and Kumar et al. presented high-quality review papers regarding the synthesis of metallic NPs using plant extracts as a green chemistry approach [24,25]. Since then, the synthesis of metal NPs has been performed by different research groups based on a variety of plants and their structures. Logeswari et al. developed an eco-friendly synthesis of AgNPs from plant powders of Solanum tricobatum, Syzygium cumini, Centella asiatica, and Citrus sinensis, while Yang et al. biosynthesized gold nanoparticles (AuNPs) using an agricultural waste mango peel extract [26,27]. Verma et al. and Bagherzade et al. showed the antimicrobial activity of metal NP obtained through green synthesis using Azadirachta indica leaves and Crocus sativus L. extracts, respectively [28,29].

Due to the nanotechnological boom, unusual physical, chemical, and biological methods have been developed for the synthesis and production of metal NPs [30,31,32,33,34,35].

2. AuNPs and AgNPs Applications in Agriculture

In general, the synthesis of NPs is of great interest because of their unique properties that can be incorporated into composite fibers, biosensor materials, cryogenic super-conducting materials, cosmetic products, and electronic components [88]. However, due to climate change and the depletion of natural resources, the synthesis of AuNPs and AgNPs from plant extracts, and even more by agricultural wastes, is a major topic for encouraging sustainable development in agro-industrial labors. Since plants are the basis of this green synthesis, the created NPs can be used in many agroindustry-related processes, from the application in the soil to the food chain, due to their low toxicity [89,90,91].

Nanotechnological food and agricultural applications were proclaimed in June 2009 in a joint venture of the Food and Agricultural Organization (FAO) and World Health Organization (WHO), with the inclusion of wide-ranging fields, such as nanostructured ingredients, nanosized biofortification, food packaging, nanocoating, and nanofiltration [92]. NPs may also act as “magic bullets”, containing nutrients or other substances, such as beneficial genes, and organic compounds, which are targeted to specific plant areas or structures to enhance their productivity. Thus, NPs represent smart nano-delivery systems for agricultural administration, specifically on crop nutrition [93].

Regarding direct applications of AuNPs and AgNPs in agriculture, many pieces of researches on this field have been focused on seed germination, root elongation, and plant responses towards the presence of metal NPs, like cellular oxidative stress or cytotoxicity [89,94]. In addition to that, metal NPs can be employed for nano-fertilizers and nano-pesticides development [95]. Indirect applications based on the antimicrobial activity of the NPs are mostly related to food packaging [93]. The mentioned applications have been widely addressed in the agro-industries in a great variety of products containing NPs of these metals with a particle size that ranges from 100–250 nm, making them more soluble in water, and increasing their activity [96].

2.1. Antimicrobial Properties

Metal NPs from green synthesis can be used as antioxidants, biosensors, and for heavy metal detection as well [97,98,99]. In general terms, their unique physicochemical properties, such as their ability to bind biomolecules, large surface area to volume ratio, high surface reactivity, easy to synthesize and characterize, reduced cytotoxicity, and their capacity of enhancing gene expression for redox processes, allow using as antimicrobial agents against plant disease pathogens and others that can cause foodborne diseases [100,101,102,103]. A great variety of plant extracts used for generating AuNPs and AgNPs have been processed and reported their potential antimicrobial activity against bacterial and fungal plant pathogens (Table 1) [104,105].

Table 1. Plant extract-mediated green synthesis of AuNPs and AgNPs with antimicrobial activity.

2. Water Treatment

Nanotechnology can take an important part of a sustainable productivity system, ensuring its quality and purity. The most attractive and cost-effective nanomaterials for environmental protection and water remediation processes are derived from noble metals. In this application, the high surface area to volume ratio, chemical stability, and enhanced catalytic activity properties from plant extract-mediated green synthesized AuNPs and AgNPs can be employed for water monitoring, purification, drinking water treatment, and agriculture wastewater treatment [147,148].

Moreover, research groups have taken advantage of the different properties exhibited by these NPs, including their high reactivity for identifying toxic substances, such as pesticides and heavy metals (e.g., lead, mercury and cadmium), by incorporating them into sensors for the rapid detection of these chemicals [149,150]. Water and wastewater detoxification can be achieved by adsorption, photocatalytic degradation, and nanofiltration techniques using NPs as well [151]. Different authors have described the process of pesticide mineralization in water using AuNPs and AgNPs, such as chlorpyrifos, malathion, and atrazine [152]. Pesticides extraction is achieved by their adsorption onto NPs, which retain them on their surface, interacting for long periods until the complex precipitates. Therefore, these NPs represent a suitable, convenient, and cost-effect means of removing pesticides for either drinking water or irrigation labors [152,153]. Moreover, AuNPs and AgNPs are considered an interesting approach for heavy metals elimination in water due to their high adsorption capacity [154].

In addition to that, water pollution with bacterial pathogens represents a high risk for water-borne, food-borne, and plant diseases. The antimicrobial properties of metal NPs have been reported to be effective in this type of water purification [155]. Francis et al. synthesized AuNPs and AgNPs from the leaf extract of Mussaenda glabrata and evaluated their capacity to inhibit pathogenic microorganisms. The NPs showed outstanding antimicrobial activity against P. aeruginosa, E. coli, A. niger, and Penicillium chrysogenum [156]. Besides, their catalytic capacities make them suitable for dye degradation, such as reported by Veisi et al. in their research, where green synthesized AgNPs from the leaf extract of Thymbra spicata decreased different dyes, such as nitrophenol, rhodamin, and methylene blue [157].

2.3. Development and Delivery of Nanofertilizers, Nanopesticides, and Nanoherbicides

AuNPs and AgNPs-based nanofertilizers have been developed to synchronize nutrient release with plant uptake. This system reduces nutrient loss, soil and groundwater contamination, and chemical reactions with water, soil, and microorganisms that transform them into unuseful or toxic substances for plants, helping to maintain the soil’s fertility [163,164,165]. Kang et al. applied 5 mg/L AgNPs fertilizer suspension to red ginseng shoot three times per day at 14-day intervals. After harvesting, they reported that the nanofertilizer had enhanced the ginsenoside content [163].

Nanoencapsulated pesticides and herbicides show enhanced properties in terms of solubility, specificity, permeability, and stability because the nanostructure protects the active substance from early degradation and provides pest control for longer periods [166]. Moreover, control of plant disease-causing phytopathogens, such as bacteria and fungi, can also be achieved by spraying a NPs solution directly on grains, seed, or foliage to inhibit the invasion of plant pathogens [167]. AgNPs green synthesis from Fusarium solani was done by El-Aziz et al. to evaluate their impact on grain borne fungi. The outcome of this research was that sprayed NPs solutions of 4% caused a 0% frequency of fungal pathogens [168]. Gnanadesigan et al. biosynthesized AgNPs using the leaf extract of Rhizophora mucronata to evaluate its potential larvicidal activity against Aedes aegypti and Culex quinquefasciatus, two vectors that affect workers in agricultural fields, causing dengue and filariasis [169].

The adoption of high-tech agricultural systems can reduce or even eliminate the negative environmental influence of modern agriculture, as well as enhancing the quality and quantity of crop production [170]. Although NPs for these applications provide a lot of benefits, their different properties give them different toxicities, which need further research [171,172,173].

3. Conclusions

Green chemistry is an innovative and growing resource in the search for more environmentally friendly processes. Using plant extracts for the synthesis of metal NPs is a recently growing area of interest due to its benefit in comparison to the traditional physicochemical methods. AuNPs and AgNPs generated by green synthesis have potential applications in agriculture and agroindustry, especially as antimicrobial agents of certain microorganisms for which their efficacy has been scientifically proven. Although recent studies suggest that environmental concentrations of AuNPs and AgNPs affect microbial biomass with low impact on their diversity, further research needs to be addressed in order to determine the effects they could produce to the soil, plants, and the environment, in general, due to long-term exposure. Therefore, local and national regulatory institutions must establish guidelines and monitoring methods for better use of these nanotechnological advances.

This entry is adapted from the peer-reviewed paper 10.3390/nano10091763