Synthesis and Characterization of AgNPs-PVP and Unprotected AgNPs
AgNPs were synthesized via the commonly adopted approach of reducing silver ions (Ag+) in the presence of a polymeric stabilizer52. Sodium borohydride (BH4-) was used as the reducing agent53 because of its rapid reaction with Ag+ and room-temperature reaction conditions. Ag+ and BH4- quickly reacted to produce AgNPs following the equation below46:
Ag+ + BH4-+3H2O → Ag + H3BO3+7/2H2 (1)
As-synthesized AgNPs are prone to aggregation, driven by changes in pair potential as a result of the spontaneous hydrolysis of BH4- and production of sodium hydroxide and orthoborate ([BO3]3-)46,54. To prevent the aggregation of AgNPs and control their sizes, PVP was used due to its strong interaction with Ag+, which slowed down the growth of silver grains. And the distance between the resulting silver particles is larger that of the silver particles without PVP55). Compared to other stabilizers, such as polyvinyl alcohol (PVA)56, PVP demonstrated greater size-regulating capability, likely due to its stronger affinity to Ag+ (e.g., the hydroxyl groups in PVA led to weaker interaction than that of Ag+-PVP57and cellulose58 merely acted as a matrix for controlled diffusion of Ag+59). The synthesis followed an established protocol46, where 3.75 mL of an aqueous solution of PVP (1 mM) was added to 7.5 mL of an aqueous solution of AgNO3 (1 mM); and 18.75 mL of an aqueous solution of NaBH4 (0.2 mM) was added dropwise to obtain AgNPs-PVP with the final concentration of 0.25 mM. The Ag:PVP:NaBH4 ratio of 2:1:1.4 was chose based on the good stability of as-synthesized AgNPs-PVP in previous reports46. FTIR spectra of PVP and AgNPs-PVP both showed a pronounced peak at 1640 cm-1, indicating C=O stretching and thus presence of PVP. Furthermore, the FTIR spectra of unprotected AgNPs and AgNPs-PVP both showed a peak at 1060 cm-1, representing O-H vibration and presence of AgNPs. Therefore, the spectrum of AgNPs-PVP indicated that the characterizations of both AgNPs and PVP were fully retained in AgNPs-PVP.