Quercetin is a biologically active and widely used dietary flavonoid with robust antioxidant and chelating effects. However, due to its lesser solubility, it has poor bioavailability, which results in limited medicinal effects. This study synthesized quercetin–metal complexes, which enhanced the medicinal properties and bioavailability of quercetin. Quercetin–copper (Cu) complexes were fabricated by using copper sulfate and copper chloride in the Quercetin/Cu ratios of 1:2 and 1:1, respectively. Furthermore, Cu complexes with quercetin extracted from onion peels or standard quercetin were obtained. The current research also sought to assess the bioavailability as well as antibacterial and antioxidant properties. Spectrophotometry revealed that better complexes were prepared with a ratio of 1:2 as compared to 1:1. Copper chloride and copper sulfate showed no significant (p < 0.05) effects on complex formation. These metal complexes had a considerable impact on Bacillus subtilis (Gram⁺) as compared to Escherichia coli (Gram^–). The pharmacokinetic parameters indicated that the oral bioavailability of quercetin–Cu(II) complexes was significantly enhanced as compared to pure quercetin. The results of this study suggested the successful formation of quercetin–metal complexes, which markedly increased the bio-availability of quercetin.

The green synthesis of metal oxide nanoparticles (MONPs) using Azadirachta indica (neem) as a biogenic reducing and capping agent is discussed in this review. Neem eliminates the need for harmful chemicals and solvents that are typically used in standard synthesis procedures of MONPs. This review discusses the many ways, such as coprecipitation, hydrothermal, and microemulsion approaches, that neem leaf extract can be used to produce oxide NPs. This review focused on obtaining data from recent advances in synthesis, characterization and applications of biological synthesis techniques. The relative abundance of A. indica (neem) means this can be widely used for creating reductant and stabilizing agents needed for MONPs formation. Using plant biomolecules in MONPs increases their toxicity toward microorganisms, enabling them to resist degradation and prevent ecological pollution. The review explores the size, shape, and crystal structure of neem-based NPs, which have potential uses in areas including water purification, biological imaging, and catalysis. Overall, this review finds that A. indica is a promising substitute for conventional synthetic techniques in the green synthesis of metallic oxide NPs.