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].