Investigations of the mechanism of gold nanoparticle stability and surface functionalization in capillary electrophoresis

Abstract

Covalently functionalized gold nanoparticles influence capillary electrophoresis separations of neurotransmitters in a concentration- and surface-chemistry-dependent manner. Gold nanoparticles with either primarily covalently functionalized carboxylic acid (Au@COOH) or amine (Au@NH 2) surface groups are characterized using extinction spectroscopy, transmission electron microscopy, and zeta potential measurements. The impact of the presence of nanoparticles and their surface chemistry is investigated, and at least three nanoparticle-specific mechanisms are found to effect separations. First, the degree of nanopartiele-nanopartiele interactions is quantified using a new parameter termed the critical nanopartiele concentration (CNC). CNC is defined as the lowest concentration of nanoparticles that induces predominant nanopartiele aggregation under specific buffer conditions and is determined using dual-wavelength photodiode array detection. Once the CNC has been exceeded, reproducible separations are no longer observed. Second, nanopartiele - analyte interactions are dictated by electrostatic interactions which depend on the pk a of the analyte and surface charge of the nanopartiele. Finally, nanopartiele - capillary interactions occur in a surface-chemistry- dependent manner. Run buffer viscosity is influenced by the formation of a nanopartiele steady-state pseudostationary phase along the capillary wall. Despite differences in buffer viscosity leading to changes in neurotransmitter mobilities, no significant changes in electroosmotic flow were observed. As a result of these three nanoparticle-specific interactions, Au@NH 2 nanoparticles increase the mobility of the neurotransmitters while a smaller opposite effect is observed for Au@COOH nanoparticles. Understanding nanopartiele behavior in the presence of an electric field will have significant impacts in separation science where nanoparticles can serve to improve either the mobility or detection sensitivity of target molecules. © 2009 American Chemical Society.