Figure 1. Synthesis and FTIR Characterization of AgNPs-PVP. A)Aqueous solutions of AgNO3 (1 mM; 7.5 mL) and PVP (1 mM; 3.75 mL) were combined and kept at 0ºC for 30 minutes to ensure the mixture was homogeneous and temperature was cool enough for addition of NaBH4 (NaBH4 reacted violently and quickly with water at room temperature) and prevention of aggregation (the nanoparticles were grown more rapidly at room temperature), where addition of an aqueous solution of NaBH4 to the final concentration of 0.175 mM initiated formation of AgNPs-PVP. B)FTIR spectra of AgNPs, PVP, and AgNPs-PVP, demonstrating AgNPs-PVP retained peaks from both of AgNPs (O-H at around 1060 cm-) and PVP (C=O at around 1640 cm-1).
Sizes of the AgNPs-PVP and unprotected AgNPs were characterized using DLS and TEM. For an aqueous solution of AgNPs-PVP (0.25 mM; AgNPs:PVP=2:1) (Figure 2A), DLS indicated a narrow range of size distribution, with an average diameter of 9.23 ± 2.03 nm and polydispersity index (PDI) of 0.149. In contrast, DLS captured a much broader size distribution for an aqueous solution of unprotected AgNPs (0.25 mM) due to aggregation, with an average diameter of 31.55 ± 7.9 nm and PDI of 0.336. TEM images further provided direct evidence of the effect of PVP on preventing the aggregation of AgNPs (Figure 2B). While AgNPs-PVP demonstrated a spherical morphology with an average diameter of 10.54±3.11 nm (Figure 2B and 2D), substantial aggregation was captured for unprotected AgNPs (Figure 2B). The characteristic length of the aggregates was calculated to be 25.82 ± 11.98 nm (Figure 2C), obtained by processing TEM images using ImageJ. The fact that DLS measured an average diameter greater than that captured by TEM is well documented in the literature60,61. It has been attributed to the hydrodynamic diameter (as measured by DLS) being greater than the projected area diameter (as captured by TEM), due to a solvent layer surrounding a colloid that are subject to the particle Brownian motion.