Fig. 3 Optimization parameters for efficient sensing with variation in pH (A), temperature (B), R12-AgNPs concentration (C) and time (D).
Determination of immediate antimicrobial effect
Standard Plate Count (SPC) is the most widely accepted and frequently utilized approach for estimating the number of viable bacteria. In this study, the antibacterial activity of both R12-AgNPs and C-AgNPs (0-10 μg/mL) was evaluated against Gram-positive and Gram-negative bacteria (S. aureus and E. coli ) with a microdilution method (Fig. 4). The results indicated that the inhibitory effect of AgNPs on both types of bacteria was enhanced with the increase of AgNPs, signifying a positive correlation between the concentration of AgNPs and their antibacterial activity. According to Fig. 4 (A, B), almost 100% the inhibition of E. coli and S. aureus were achieved when the concentration of R12-AgNPs was 5 μg/mL and 10 μg/mL, respectively. However, the antibacterial effect of C-AgNPs was notably weaker at all concentrations of AgNPs (Fig. 4). Especially at lower concentrations, C-AgNPs exhibited negligible antibacterial activity. This difference may be attributed to the fact that C-AgNPs have a lower Ag+ release, which was confirmed by the Ag+ release test (Fig. 1D). Overall, R12-AgNPs demonstrated higher inhibitory effects against all tested bacteria, both low and high concentrations, in compared to C-AgNPs. This finding was consistent with previous results, highlighting that biosynthesized AgNPs, stabilized by biomolecules, possess a greater tendency to exhibit enhanced antibacterial activity when compared to chemically synthesized AgNPs [44, 45].