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.