A Tunable Loading of Single-Stranded DNA on Gold Nanorods through the Displacement of Polyvinylpyrrolidone

A quantitative and tunable loading of single-stranded (ss-DNA) molecules onto gold nanorods was achieved through a new method of surfactant exchange. This new method involves the exchange of cetyltrimethylammonium bromide surfactants for an intermediate stabilizing layer of polyvinylpyrrolidone and sodium dodecylsulfate. The intermediate layer of surfactants on the anisotropic gold particles was easily displaced by thiolated ss-DNA, forming a tunable density of single-stranded DNA molecules on the surfaces of the gold nanorods. The success of this ligand exchange process was monitored in part through the combination of extinction, X-ray photoelectron, and infrared absorption spectroscopies. The number of ss-DNA molecules per nanorod for nanorods with a high density of ss-DNA molecules was quantified through a combination of fluorescence measurements and elemental analysis, and the functionality of the nanorods capped with dense monolayers of DNA was assessed using a hybridization assay. Core–satellite assemblies were successfully prepared from spherical particles containing a probe DNA molecule and a nanorod core capped with complementary ss-DNA molecules. The methods demonstrated herein for quantitatively fine tuning and maximizing, or otherwise optimizing, the loading of ss-DNA in monolayers on gold nanorods could be a useful methodology for decorating gold nanoparticles with multiple types of biofunctional molecules.