pH-Induced Concentration and Preservation of Zwitterion-Modified Gold Nanoparticles

In this work, the stability and reversible aggregation properties of zwitterion-modified Au nanoparticles (Au NPs) were studied under acidic and alkaline conditions. The UV-Vis spectra of the Au colloids were measured under different conditions, and the stability and reversible aggregation properties of the Au colloids were revealed through the spectral changes. The results showed that the modification with zwitterionic ligands largely improved the stability of the Au NPs under both acidic and alkaline conditions. Under strongly acidic conditions, the modified Au NP colloids lost stability and aggregated, but the aggregated colloids could be redispersed once the pH of the colloids was returned to an appropriate value. By exploiting their pH-dependent reversible aggregation properties, dilute Au NP colloids were concentrated to give concentrated Au NP colloids or solid Au NP aggregates; these systems could be preserved for long periods of time, and if needed, could be recovered to give well-dispersed dilute dispersions simply by adding water.