Evaluating the surface density and heterogeneity of a dithiobis (succinimidylpropionate) self-assembled monolayer on gold and its coupling with DNA embedded within a matrix

The homogeneity of a self-assembled monolayer (SAM) on a surface is an important parameter which affects the ability of a SAM to fulfill its intended function. As an example, SAMs formed from octanethiols can form an impermeable surface, while SAMs based on a bifunctional coupling reagent can form a surface with uniform reactivity. Exposure of gold nanoparticles or gold surfaces to solutions of dithiobis (succinimidylpropionate) (DSP) gives rise to a surface which can react with DNA. Atomic force microscopy, UV–vis and gel electrophoresis experiments indicate that a self-assembled monolayer of DSP on gold nanoparticles can attenuate aggregation, inhibit the “lying down” of covalently-bound single-stranded (ss) DNA and promote more efficient hybridization. The determination of the point of aggregation after reacting DSP with colloidal gold yields 2.86 × 10−10 mol/cm2 or 42% of the value determined from molecular modeling. Cyclic voltammetry experiments validate that DSP on a gold quartz crystal (6.3 × 10−10 mol/cm2) forms a fairly uniform SAM that is within 94% of maximum coverage when compared with results obtained from molecular modeling (6.67 × 10−10 mol/cm2). Surface plasmon resonance experiments indicate that the reaction of a DSP coated gold surface with (ss) DNA yields 2.4 × 10−12 mol/cm2 or reaction with about 1% of the available surface area. Subsequent reactions of the DSP surface with the filler, n-boc-1,4-phenylene diamine (n-boc), yield a total surface coverage of 1.8 × 10−11 mol/cm2. The surrounded (ss) DNA yields a surface with 97% hybridization efficiency toward the complement.