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

Abstract

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 Ca 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.

Effect of nanomaterial and media physicochemical properties on nanomaterial aggregation kinetics

“…The stability of hematite particles decrease in the presence of phosphate anions 89 . Furthermore, the CCC values for electrostatically stabilized Ag NMs decrease with the increase in carbonate buffer concentration 81 and phosphate buffer 83 . This behavior was attributed to the formation of Ag 2 CO 3 , or Ag 3 PO 4 layer on the surface of Ag NMs, altering NM surface charge 83 .…”

“…Furthermore, the CCC values for electrostatically stabilized Ag NMs decrease with the increase in carbonate buffer concentration 81 and phosphate buffer 83 . This behavior was attributed to the formation of Ag 2 CO 3 , or Ag 3 PO 4 layer on the surface of Ag NMs, altering NM surface charge 83 . For a constant Ag NM concentration (5.4 mg L -1 ), zeta potential increased from -42 mV to -29 mV with the increase in phosphate buffer concentration from (0 to 2.5 mM potassium phosphate monobasic) 83 .…”

“…Citrate was the most investigated capping agent among others such as SDS, alginate, PVP, casein, dextrin, tween, branched polyethyleneimine (BPEI), and Gum Arabic (Table S1 and S2). Parameters such as NM size, buffer concentration have rarely been investigated 17,83 . Below we rationalize the differences in the CCC values based on the variability in the experimental conditions, which includes NM and medium physicochemical properties.…”

“…This behavior was attributed to the formation of Ag 2 CO 3 , or Ag 3 PO 4 layer on the surface of Ag NMs, altering NM surface charge 83 . For a constant Ag NM concentration (5.4 mg L -1 ), zeta potential increased from -42 mV to -29 mV with the increase in phosphate buffer concentration from (0 to 2.5 mM potassium phosphate monobasic) 83 . Another study demonstrated that, at pH 7, the zeta potential of Ag NMs increases (become less negative) with the decrease in NM concentration (1 to 100 µM) in 10 mM carbonate concentration 90 .…”

“…However, emerging studies have reported the importance of chemical interactions in addition to physical interactions in controlling NM aggregation kinetics [118][119][120][121][122][123][124][125] . For instance, the CCC for citrate-Ag NMs decreases with the increased concentration of carbonate and phosphate anions in the medium, which was ascribed to the sorption of phosphate and carbonate anions on the surface of citrate-Ag NMs, and the concurrent reduction in the magnitude of Ag NM zeta potential 81,83 . Addition of cystine to citrate-Ag NMs results in NM concentration-dependent shift in the CCC, where the CCC of cystine decreases with the decrease in Ag NM concentration 77 .…”