Biogenic silica nanoparticles from agro-waste: Properties, mechanism of extraction and applications in environmental sustainability

Silica nanoparticles (SiNPs), have recently emerged as important nanomaterials with immense applications in various fields. Owing to their variable pore size, high surface area, surface reactivity, structural flexibility, stability, and low toxicity SiNPs are appealing to be employed in numerous fields including biomedicine and environment remediation. Though several methods are being applied for the synthesis of SiNPs from various sources; the key inclination in this regard is the concept of environment safety and "green" technologies. Numerous low-cost and renewable materials are being investigated as a source of precursor materials for the synthesis of SiNPs and agricultural waste holds great potential in this regard. Due to the abundance of silica in diverse agricultural residues, synthesis of nano-silica from them, leads to new possibilities for agro-waste management and valorization. For extraction of bonded silica from agrowastes different conventional methods are in place, however extraction, mediated by microbes can make the process further eco-friendly and hence holds great research potential. The biological extraction of these nanoparticles from agricultural waste and the mechanism behind it is not much explored and reviewed. This review highlights several agricultural wastes which can be applied as precursor for generation of SiNPs and elaborates the mechanism behind biological extraction of SiNPs from agro-waste in detail. We have also discussed various applications of SiNPs, emphasizing their role in environment monitoring, detection and bioremediation. Finally, we have discussed the challenges in producing nano-sized silica from agro-waste, along with research priorities in this field that will help researchers for further investigations in this field.