Functionalised monolayer for nucleic acid immobilisation on gold surfaces and metal complex binding studies
Adsorption of 4-thiopyridine on gold electrode surfaces followed by methylation of the pyridine moiety produces a cationic monolayer on which DNA adsorbs from neutral, buffered aqueous solution. The metal complex binding properties of the immobilised DNA can be studied using both cyclic voltammetry (CV) and quartz crystal microgravimetry (QCM). Electroactive metal complexes bound to the immobilised DNA can be quantified by CV after transfer to blank electrolyte. The binding of hexaammineruthenium(III) to immobilised DNA could be fitted by a Langmuir isotherm. The maximum surface coverage of hexaammineruthenium(III) was 0.32 nmol cm–2 and a binding constant of 2.0 × 104 l mol–1 was obtained. Metal complex binding could also be detected using DNA immobilised on a gold-coated quartz crystal oscillator. Crystal admittance measurements were consistent with rigid layer behaviour for the adsorbed DNA; however, the apparent mass of adsorbed hexaammineruthenium(III) determined by QCM was considerably larger (1.6 nmol cm–2) than that detected by CV. The difference between the two analytical techniques is attributed to desorption of the metal complex following the transfer step in CV and the effect of liquid density and incorporation of water in the adsorbed films on the QCM measurement.