Comparison of the Nonspecific Binding of DNA-Conjugated Gold Nanoparticles between Polymeric and Monomeric Self-Assembled Monolayers

The nonspecific binding of DNA-conjugated gold nanoparticles (AuNPs) to solid surfaces is more difficult to control than that of DNA molecules due to the more attractive interactions from the large number of DNA molecules per AuNP. This paper reports that the polymeric self-assembled monolayers (SAMs) formed on indium-tin oxide (ITO) electrodes significantly inhibit the nonspecific binding of DNA-conjugated AuNPs. The random copolymers used to prepare the polymeric SAMs consist of three functional parts: an ITO-reactive silane group, a DNA-blocking poly(ethylene glycol) (PEG) group, and an amine-reactive N-acryloxysuccinimide group. In order to compare the polymeric SAMs with various monomeric SAMs, the relative nonspecific binding of the DNA-conjugated AuNPs to the ITO electrodes modified with (3-aminopropyl)triethoxysilane (APTES), 3-aminopropylphosphonic acid, 3-phosphonopropionic acid, or 11-phosphonoundecanoic acid is examined by measuring the electrocatalytic anodic current of hydrazine caused by the nonspecifically absorbed AuNPs and by counting the AuNPs adsorbed onto modified ITO electrodes. Carboxylic-acid-terminated and amine-terminated monomeric SAMs cause high levels of nonspecific binding of DNA-conjugated AuNPs. The monomeric SAM modified with the carboxylic-acid-terminated poly(amidoamine) dendrimer shows low levels of nonspecific binding (2.0% nonspecific binding relative to APTES) due to the high surface density of the negative charge. The simply prepared polymeric SAM produces the lowest level of nonspecific binding (0.8% nonspecific binding relative to APTES), resulting from the combined effect of (i) DNA-blocking PEG and carboxylic acid groups and (ii) dense polymeric SAMs. Therefore, thin and dense polymeric SAMs may be effective in electrochemical detection and easy DNA immobilization along with low levels of nonspecific binding.