Inorganic–Organic Hybrid 3D Redox Nanoarchitecture for the Electrocatalytic Sensing of Thiols

Abstract

We demonstrate the development of a novel redox active three-dimensional (3D) functional hybrid nanoarchitecture based on reduced graphene oxide (rGO) and sol–gel derived silicate network for the mediated electrocatalytic sensing of biological thiols. First, the thiol functionalized hybrid material was synthesized by the cross condensation reaction between graphene oxide (GO) and the in situ generated silanol derived from 3-(mercaptopropyl)trimethoxysilane (MPTS). The chemisorption of the hybrid material on Au electrode and the subsequent NaBH₄ treatment yield the thiol-terminated 3D hybrid inorganic–organic self-assembly. The 3D hybrid assembly was further tailored with a redox active 4-methyl catechol (MCA) or catechol (CA) moiety by taking advantage of the bias-driven Michael addition reaction between the surface-confined thiol-terminated self-assembly and electrogenerated quinone. The Michael addition of quinone was quantitatively monitored by electrochemical quartz crystal microbalance (EQCM). The redox-tailored architecture displays reversible voltammetric response corresponding to the redox reaction of catechol/quinone redox couple with standard heterogeneous rate constant of 75.74 s^–1 (for CA) and 71.43 s^–1 (for MCA). The MCA-tailored 3D hybrid architecture efficiently mediates the oxidation of biological thiols (cysteine, homocysteine, and glutathione). However, the CA-tailored hybrid architecture favors the Michael addition of biological thiols, as evidenced by EQCM studies. The kinetics for the mediated electrocatalytic oxidation of biological thiols on MCA-tailored assembly was studied using a rotating disk electrode. The rate constant for the oxidation of cysteine is higher than those of the other two thiols. The MCA-tailored hybrid assembly is highly sensitive, and we could achieve a very low detection limit of 0.3 nM in flow injection analysis at the potential of 270 mV. The electrode is highly stable, and only 10% decrease in the initial amperometric current was observed after 7 days.