Effect of phosphate buffer on aggregation kinetics of citrate-coated silver nanoparticles induced by monovalent and divalent electrolytes.

The attachment efficiency (α) is an important parameter that can be used to characterize nanoparticle (NPs) aggregation behavior and has been a topic of discussion of several papers in the past few years. The importance of α is because it is one of the key parameters that can be used to model NP environmental fate and behavior. This study uses UV-vis and laser Doppler electrophoresis to monitor the aggregation behavior of citrate-coated silver nanoparticles (cit-AgNPs) induced by Na+ and Ca2+ as counter ions in the presence and absence of Suwannee River fulvic acid (SRFA) as a surrogate of natural organic matter and different concentrations of phosphate buffer (0-1mM). Results demonstrate that phosphate buffer, which serves to maintain pH nearly constant over the course of a reaction, is an important determinant of NP aggregation behavior. Increasing phosphate buffer concentration results in a decrease in the critical coagulation concentrations (CCC) of cit-AgNPs to lower counter ion concentration and an increase of α at the same counter ion concentration, both in the absence and presence of SRFA. SRFA stabilizes AgNPs and increases the CCC to higher counter ion concentrations. The outcome of this study can be used to rationalize the variation in α and CCC values reported in the literature for NPs with similar physicochemical properties, where different α and CCC values are reported when different types of buffers and buffer concentrations are used in different studies.