Emerging investigator series: Quantifying silver nanoparticle aggregation kinetics in real-time using particle impact voltammetry coupled with UV-vis spectroscopy

The aggregation of silver nanoparticles (AgNPs) as they encounter biological and environmental systems can dictate their fate and transport. Here, we present a rapid, affordable, and robust analytical method for quantifying AgNP aggregation that combines a single particle electrochemistry technique called particle impact voltammetry (PIV) with the strengths of UV-vis spectroscopy. This orthogonal technique, designated PIV/UV-vis, enables the quantitative evaluation of aggregation kinetics by simultaneously measuring changes in the redox behavior of individual AgNPs and spectroscopic changes in the bulk AgNP colloidal solution. We demonstrate that the frequency of AgNP collisions measured by PIV is correlated to the concentration of monodisperse AgNPs in solution. In this way, aggregation can be quantified by the disappearance of AgNP collisions, much like in UV-vis where aggregation is quantified by the rate of disappearance of the localized surface plasmon resonance band of monodisperse AgNPs. The PIV/UV-vis technique was validated by determining the critical coagulation concentration (CCC) of 40 nm AgNPs in the presence of monovalent and divalent cations. The CCC values determined by PIV and UV-vis were in excellent agreement with one another and were determined as 43 ± 4 and 43 ± 3 mM Na+ and 3.0 ± 0.3 and 3.0 ± 0.1 mM Mg2+, respectively. Using dynamic light scattering, aggregation was confirmed by monitoring changes in AgNP hydrodynamic diameter and results show a clear distinction in aggregation behavior above the CCC. Further, zeta potential measurements were used to monitor changes in AgNP surface charge as another measure of colloidal stability. Overall, PIV/UV-vis is a powerful technique to measure AgNP aggregation due to its speed, affordability, reproducibility, and potential broad applicability.