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.