Fabrication of Silica-Coated Gold Nanorods Functionalized with DNA for Enhanced Surface Plasmon Resonance Imaging Biosensing Applications

Abstract

A novel method for preparing gold nanorods that are first coated with a thin silica film and then functionalized with single stranded DNA (ssDNA) is presented. Coating the nanorods with 3-5 nm of silica improves their solubility and stability. Amine-modified ssDNA is attached to the silicacoated gold nanorods via a reductive amination reaction with an aldehyde trimethoxysilane monolayer. The nanorods exhibit an intense absorption band at 780 nm, and are used to enhance the sensitivity of surface plasmon resonance imaging (SPRI) measurements on DNA microarrays.Metallic nanorods are nanoscale materials that possess unique optical and electronic properties which make them extremely useful when incorporated into schemes for the detection of biomolecules such as DNA, RNA and proteins. To successfully integrate these materials into bioaffinity detection assays, the nanoscale surfaces must first be functionalized with biomolecules without altering their stability in solution. For example, thiol-modified singlestranded DNA (ssDNA) can be immobilized onto the surface of gold nanoparticles (NPs) in a single-step displacement reaction of electrostatically absorbed citrate anions. These DNAmodified NPs, first reported by Mirkin et al. 1 and Alivisatos et al. 2 have been used extensively for the detection and identification of oligonucleotides. The straightforward thiol attachment chemistry is made possible by the anionic character of the nanoparticle surface due to the presence of the citrate. In contrast, gold nanorods produced by the methods developed by either Murphy 3 or El Sayed 4 have a net positive surface charge due to the presence of an adsorbed monolayer of the surfactant, hexadecyltrimethyl-ammonium bromide (CTAB), on the nanorod surface. Thus, the thiol chemistry used to modify gold NPs is very difficult when employed for the attachment of ssDNA to surfactant-coated gold nanorods. The reasons for this are that the high density of the surfactant monolayer decreases the access of the thiol-modified ssDNA to the nanorod surface and the negatively-charged phosphate backbone of the ssDNA interacts with the positively charged CTAB molecules; the net result is typically a rapid aggregation and precipitation of the gold nanorods from solution. This letter describes an alternative strategy for preparing ssDNA-functionalized gold nanorods based on a multi-step process in which the gold nanorods are first modified with a thin silica film and then the ssDNA is attached to the silica shell via an aldehyde coupling reaction. We further demonstrate that these DNAfunctionalized silica-coated gold nanorods can be used to greatly enhance the sensitivity of surface plasmon resonance imaging (SPRI) measurements of DNA hybridization adsorption onto DNA microarrays.The preparation of DNA-functionalized silica-coated gold nanorods requires a sequential surface modification process that is shown schematically in Figure 1. The functionalized gold nanorod synthesis can be divided into three main steps: NIH Public Access Author Ma...

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