Photocatalytic and biological properties of silver nanoparticles synthesized using Callistemon lanceolatus leaf extract

Silver nanoparticles (AgNPs) have been extensively explored due to their diverse applications in catalysis, medicine, drug delivery, etc. Physical and chemical methods of synthesizing AgNPs pose a threat to the environment due to the use of toxic and non-biodegradable substances. Green synthesis of AgNPs due to its eco-friendly nature is considered a novel approach as it can be easily scaled up and avoid the use of toxic chemicals. The aqueous extract of leaves from Callistemon lanceolatus was used in the present study for the biosynthesis of AgNPs using the Box-Behnken design (BBD) of response surface methodology (RSM). The study involves the determination of antioxidant, antibacterial, and α-amylase inhibitory activities of biosynthesized AgNPs. In addition, the photocatalytic activity of the synthesized AgNPs was assessed through the degradation of methylene blue dye. The AgNPs showed a characteristic plasmon resonance absorption band around 424 nm. Furthermore, biosynthesized AgNPs were face-centered cubic (fcc) with an average diameter of 41.93 nm as determined by XRD and TEM analysis respectively. The chemical nature of biogenic AgNPs was ascertained using energy-dispersive X-ray (EDX) analysis. Similarly, biosynthesized AgNPs were also characterized using FTIR, SEM, and DLS analysis. The AgNPs showed potential DPPH radical scavenging (IC₅₀ of 40.86 ± 1.8 µg/mL) and α-amylase inhibitory (85.3 % at 100 µg/mL) activities. They also possessed appreciable antibacterial activity with MIC and MBC values ranging from 40 to 80 µg/mL and 60–150 µg/mL respectively. The photocatalytic degradation of methylene blue dye by AgNPs showed a rate constant of 0.009 min⁻¹. Also, AgNPs were effective in degrading the MB dye up to 91 % in 300 min with a concentration of 0.1 mg/mL. Additionally, the reusability of AgNPs as nano-catalyst showed a small decrease in their photo-catalytic activity after every consecutive cycle up to five cycles. It may be concluded that AgNPs produced from C. lanceolatus leaf extract may have biomedical uses and applications in the purification of water contaminated with organic dyes.