Outer-Sphere to Inner-Sphere Redox Cycling for Ultrasensitive Immunosensors

This paper reports chemical-chemical (CC) and electrochemical-chemical-chemical (ECC) redox cycling, for use in ultrasensitive biosensor applications. A triple chemical amplification approach using an enzymatic reaction, CC redox cycling, and ECC redox cycling is applied toward electrochemical immunosensors of cardiac troponin I. An enzymatic reaction, in which alkaline phosphatase converts 4-aminophenyl phosphate to 4-amino-phenol (AP), triggers CC redox cycling in the presence of an oxidant and a reductant, and electrochemical signals are measured with ECC redox cycling after an incubation period of time in an air-saturated solution. To obtain high, selective, and reproducible redox cycling without using redox enzymes, two redox reactions [the reaction between AP and the oxidant and the reaction between the oxidized form of AP (4-quinone imine, QI) and the reductant] should be fast, but an unwanted reaction between the oxidant and reductant should be very slow. Because species that undergo outer-sphere reactions (OSR-philic species) react slowly with species that undergo inner-sphere reactions (ISR-philic species), highly OSR-philic Ru(NH3)(6)(3+) and highly ISR-philic tris(2-carboxyethyl)phosphine (TCEP) are chosen as the oxidant and reductant, respectively. The OSR- and ISR-philic QI/AP couple allows fast redox reactions with both the OSR-philic Ru(NH3)(6)(3+) and the ISR-philic TCEP. Highly OSR-philic indium tin oxide (ITO) electrodes minimize unwanted electrochemical reactions with highly ISR-philic species. Although the formal potential of the Ru(NH3)(6)(3+)/Ru(NH3)(6)(2+) couple is lower than that of the QI/AP couple, the endergonic reaction between Ru(NH3)(6)(3+) and AP is driven by the highly exergonic reaction between TCEP and QI (via a coupled reaction mechanism). Overall, the outer-sphere to inner-sphere redox cycling in the order of highly OSR-philic ITO, highly OSR-philic Ru(NH3)(6)(3+)/Ru(NH3)(6)(2+) couple, OSR- and ISR-philic QI/AP couple, and highly ISR-philic TCEP allows high, selective, and reproducible signal amplification. The electrochemical data obtained by chronocoulometry permit a lower detection limits than those obtained by cyclic voltammetry. The detection limit of an immunosensor for troponin I in serum, calculated from the anodic charges in chronocoulometry, is ca. 10 fg/mL.