Size-dependent antibacterial efficacy of silver nanoparticles from a green synthesis method: Effects of extract quantity and origin

Background: Silver nanoparticles (AgNPs) are known for their antibacterial characteristics. The green synthesis method, in which plant extracts are used for AgNP preparation, has attracted attention recently since it provides an eco-friendly approach. However, there are many operating conditions for this green synthesis method, and a correlation between the antibacterial efficiency and AgNP characteristics is needed to maximize the bactericidal efficacy. Methods: AgNPs were synthesized via green synthesis methods from green tea leaf (GT) and Cassia seed (CS) extracts at 9:1, 1:1, and 1:9 precursor-to-extract v/v ratios. The resulting AgNPs were tested for antibacterial activity against S. aureus and the antioxidant level. Significant findings: This study revealed a robust quantitative correlation between antibacterial activity and AgNP size, with an R² value of 0.92. Green-synthesized AgNPs with an average particle size of 25 nm (GT) and 12 nm (CS) had a minimum inhibitory concentration (MIC) of 25 µg mL⁻¹ and 5 µg mL⁻¹, respectively. The enhanced antibacterial activity of AgCS is attributed to its smaller particle size. Additionally, we propose the use of a silver precursor to extract-dry-weight ratio (in mole g⁻¹) to represent an operating parameter for the green synthesis method. The resulting AgNP sizes showed a weak negative relationship with this ratio.