Tailoring the surface potential of gold nanoparticles with self-assembled monolayers with mixed functional groups

Self-assembled monolayer (SAM)-modified gold nanoparticles can be used to immobilize and transport molecules including DNA and proteins. However, these molecules are usually covalently bound to the surface and chemical reactions are required to cleave and release them. Therefore, immobilizing molecules using electrostatic interactions might be beneficial. In this work, Au nanoparticles modified by SAMs with mixed carboxylic acid and amine functional groups are presented. The surface potential and the iso-electric point (IEP) of the nanoparticles can be tailored by the ratio of these functional groups and arbitrary IEPs between 3.2 and 7.3 can be achieved. As a result, based on electrostatic interactions, molecules could be triggered to adsorb/desorb by changing the environmental pH around this tunable IEP. These engineered nanoparticles were synthesized in a single-phase system based on the reduction of HAuCl4 by NaBH4 in ethanol with a mixture of 16-mercaptohexadecanoic acid and 8-amino-1-octanethiol that forms the SAM on the synthesized nanoparticles. Transmission electron microscopy, X-ray photoelectron spectroscopy, and electrophoresis light scattering revealed the particle size, ratio of the functional groups, and zeta-potential of the particles as a function of pH, respectively.