Selenium nanostructure: Progress towards green synthesis and functionalization for biomedicine

Selenium is a pivotal micronutrient required for the proper functioning of the body. Deficiency of Se leads to various diseases whereas a high dosage of Se possesses toxicity. To reduce the toxic effects of elemental selenium and to maximize its efficacy, synthesis of selenium at the nanoscale level is important. Selenium nanoparticles (SeNPs) have received considerable attention in the past few decades. Synthesis of nano selenium with unique properties is necessary for its biomedical applications. Synthesis steps involve physical, chemical and biological methods. The biological or green synthesis method is often more advantageous than other available methods because of the major drawbacks involved in those methods. SeNPs synthesised through green route, such as microbes and plants, are summarized in this review. Synthesised SeNPs that is functionalized with different biological moieties are discussed and listed. Green synthesis involves the use of eco-friendly plants and microbes as the route for the synthesis. SeNPs are non-toxic and stable due to the natural capping agent that prevents protein aggregation and maintains the stability of the nanoparticles. To achieve targeted drug delivery of the synthesized nanomaterials and to increase the efficacy of the SeNPs with minimal toxicity, functionalization is an essential tool in the field of nanomedicine. Functionalized SeNPs with various biological moieties offer enhanced cytotoxicity against various cancer cell lines, good stability and targeted drug delivery without non-specific interaction compared to nanoparticle alone. The present review focuses on detailed report and progress towards the green approach for SeNPs synthesis and engineering selenium nanomaterials with different moieties possibly used for various biomedical applications.