Application in Agri-Food Industry

Silica has been utilized for many decades in the agri-food industry. No environmental risks have been associated with silica according to safety, toxicity, and physio-chemical and epidemiology data (Fruijtier-Polloth 2012). Further. Peters et al.(2012) have shown that nanosilica is a food additive during in vitro digestion of foods (Peters et al. 2012). In a study, they explained that higher amounts (5-40%) of silica were present in the saliva digestion stage, while it disappeared after successive gastric digestion. They also noted that in the intestine where pH comes to neutral, nanosilica particlesreappeared in higher amounts than present at the saliva stage. Hence, this study clearly shows that human intestines are majorly exposed to nanosilica. Applications of silica in the food sector include catalysis, packaging, and sensing.Initially, Diaz et al. (2000) and Marquez-Alvarez et al.

(2004) showedthe application of silica in food (Diaz et al. 2000; Marquez-Alvarez et al. 2004). They used silica nanoparticles for catalysis of fatty acids. The product of catalysis, and the monoesters of glycerol, are utilized as anemulsifier in the food and cosmetic sectors. The use of open-structured nanoporous solids are reported by Thomas and Raja (2006) to transform organic compounds (Thomas and Raja 2006). They described that these materials have better centers and regioselectivity. One of the products (nylon-6) synthesized by a green and clean method is in demand for the production of textiles, plastics,and films for food packaging. Moelans et al. (2005) has demonstrated a new area of immobilization of molecules to porous materials (Moelans et al. 2005). Initially, the use of nanoparticles to immobilize enzymes was reported (Wang 2009), and later, functionalized and modified nanostructures have beenpreferred to catalyze reactions. Xylitol dehydrogenase (XDH) can be used to produce sugars enzymatically. Hence, Zhang et al. (2011a) havereported the use of silica nanoparticles to immobilize recombinant rhizobium etli CFN42 xylitol dehydrogenase (ReXDH) to produce 1-xylulose sugar that is used in the diagnosis and treatment of hepatitis (Zhang et al. 2011a). Besides that, silica materials are useful in the synthesis of nutritional compounds. Kisler et al. (2003) firstly described this possibility to be utilized as molecular sieves (Kisler et al. 2003). Mesoporous silica nanostructures have promising applications for the separation of larger molecules like proteins, which have importance in the food industries. However, mesoporous structures have limited stability in aqueous solutions. To overcome this, hexamethyldisilazane was used to functionalize silica particles that provide hydrophobicity and stability to nanostructures in aqueous nanostructures.

Application in Sensing

The emergence of nanotechnology has opened new horizons for the development of a sensitive platform for sensing different biological, environmental, targets. Silica-based nanoparticles provide the advantage of optical transparency over other developed sensors. It allows for the surface functionalization of key biomarkers for the detection of substrates; higher surface area provides a better sensitivity and stability of the assays. Utilizing this property, a number of sensors are developed which detect a variety of target molecules. (Yang et al. 2003, 2004; Rossi et al. 2006; Wang et al. 2006; Dyba and Hell 2003; Kneuer et al. 2000; Ashtari et al. 2005). Silica nanoparticles are also currently used in a wide range of applications as sensors in the food industry. Sudan I (diazo-conjugate dye with the chemical formula of 1-phenylazo- 2-naphthol) is carcinogenic and its use in the food industry was banned,therefore an electrochemical method was developed to sense Sudan I (Yang and He 2010). This rapid and sensitive technique is based on the property of mesoporous silica. A sensitive oxidation of Sudan I is observed and the peak current goes higher at a mesoporous silica-coated electrode. This method was successfully applied to sense and quantify the presence of Sudan I in juices and hot chili powder. Liu et al. (2011) also developed an immunosensor by using nanogold-assembled mesopo- rous silica (GMSNs) to determine streptomycin residues (STR) in food (Liu et al. 2011). The developed method was validated by using STR containing milk, honey, and kidney. Further, Zhao et al (2012) developed a chemiluminescence-molecular imprinting (CL-MI) sensor with the use of mesoporous silica to detect fen- propathrin, an insecticide (Zhao et al. 2012). This sensor is also applied to determine fenpropathrin in the food samples. Further, in agriculture, mesoporous nanosilica bound to photosystem II was reported to increase photosynthetic oxygen evolution (Noji et al. 2011). Yao et al. (2009) used antibody-functionalized fluorescent silica nanoparticles in the detection of microorganisms to determine plant disease (Yao 2009). Further, porous silica nanoparticles were used to deliver pesticides. Liu et al. (2006) demonstrated the delivery of validamycin (pesticide) by using mesoporous silica nanoparticles (Liu et al. 2006). They show this as an efficient nanocarrier system for the controlled release of water-soluble pesticides. Barik et al. (2016) also reviewed the application of nanosilica as a nano-insecticide (Barik et al. 2016). Insects generally developed resistance to insecticide by the presence of various cuticular lipids to protect their bodies. But nanosilica has the ability to be adsorbed at cuticular lipids and that makes them effective against insect pests. A review study has shown that functionalized hydrophobic silica nanoparticles are efficient to be implemented to control various ectoparasites of agricultural and animal pests (Sekhon 2014). Nanosilica was used to increase salinity tolerance in plants by Mushtaq et al. (2018). They synthesized chitosan and sodium alginate-coated nanosilica and showed a slow release rate of nutrients to control salinity during drought or high saline conditions. Suriyaprabha et al. (2014) also applied silica nanoparticle to study the growth of maize plants and showed that the phytochemical response of nanosilica is better than bulk silica (Suriyaprabha et al. 2014). Thus, bio-compatibility, better catalytic activity, high surface functionalization, and rapid sensitivity make silica nanoparticles highly important materials to be utilized in the environmental and agri-food sectors. Nanosilica helps to manage an eco- friendly environment and agri-food products.

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