Electrochemically derived redox molecular architecture: a novel electrochemical interface for voltammetric sensing

Redox-active molecular architectures are electrochemically derived on the electrode surface by Michael addition reaction of o-quinone with surface adsorbed nucleophiles. Electrogenerated o-quinone undergoes facile Michael addition reaction with nucleophile mercaptotriazole (MTz) and mercaptoimidazole (MIm) preassembled on Au electrode. The Michael addition reaction yields redox molecular architectures of 4-(3-mercapto-[1,2,4]triazol-1-yl)-benzene-1,2-diol (MTBD) and 4-(2-mercapto-imidazol-1-yl)-benzene-1,2-diol (MIBD). Solution pH controls the Michael addition reaction; the reaction of o-quinone with MTz nucleophile is more favorable in neutral pH whereas it is favorable in pH >or=9 with MIm. Michael addition of electrogenerated o-quinone with the nucleophile is quantitatively followed in real time using electrochemical quartz crystal microbalance (EQCM). The redox molecular architecture on the electrode surface is characterized by attenuated total reflection (ATR) spectral and electrochemical measurements. ATR spectral measurement confirms the Michael addition with the nucleophile. The redox molecular architecture displays reversible voltammetric response at 0.2 V corresponding to the redox reaction surface confined catechol moiety. The surface coverage of MTBD and MIBD on the electrode surface at pH 7.2 is estimated to be (5.4 +/- 0.2) x 10(-10) and (2.0 +/- 0.2) x 10(-10) mol/cm(2), respectively. Both redox molecular assemblies efficiently mediate the oxidation of reduced nicotinamide adenine dinucleotide (NADH) at a favorable potential. A large decrease in the overpotential associated with an enhancement in the voltammetric peak current with respect to the unmodified electrode is observed. Flow injection amperometric sensing of NADH is performed at the potential of 230 mV. These modified electrodes could detect NADH at micromolar level. Mixed molecular architecture of cysteamine (CYST) and MTz/MIm are developed for the interference free voltammetric sensing of NADH.