Figure 2. DLS and TEM analyses of particle sizes . A)DLS indicated a narrow size distribution for AgNPs-PVP, with an average diameter of 9.23 ± 2.03 nm and a PDI of 0.149; whereas unprotected AgNPs demonstrated an average diameter of 31.55 ± 7.9 nm with PDI of 0.336.B) TEM images of unprotected AgNPs and AgNPs-PVP, showing aggregates for the former and regular spherical morphology for the latter. C) Particle size distributions of unprotected AgNPs (25.82 ± 11.98 nm) and AgNPs-PVP (10.54 ± 3.11 nm) obtained by analyzing TEM images (n = 3 for each case) using ImageJ. The inset demonstrates the detailed size distribution for unprotected AgNPs with characteristic lengths > 20 nm. Data are mean ± SD.
Upon dispersion in water at the concentration of 0.25 mM, AgNPs-PVP formed a stable colloidal solution (Figure 3B), which remained unchanged after 7 days of storage under ambient conditions. The solution exhibited a maximum absorption at 404 nm, as characterized using UV-vis spectrophotometry (Figure 3B, corresponding to the bright yellow color), which also remained unchanged after the 7-day storage. Although a freshly made aqueous solution of unprotected AgNPs (0.25 mM) exhibited a similar color as that of AgNPs-PVP, the absorption peaks at 375 nm and 425 nm hinted at the polydispersed particle sizes due to aggregation. After 7 days of storage, the solution demonstrated a dark grey color, with no discernable UV-vis absorption (Figure 3A), indicating instability of the solution. The superior stability of AgNPs-PVP solutions likely led to their greater antimicrobial efficacy against OM pathogens, as described below.