V. Matylitskaya, L. Gajdosova, E. Kostal, S. Kasemann, C. Dincer, S. Partel
Vorarlberg University of Applied Sciences,
Keywords: lab-on-a-chip, nanogap-IDA, cyclic voltammetry, chronoamperometry, ionic strength
Summary:The interdigitated electrode arrays (IDAs) are widely used for Lab-on-a-Chip applications. We developed a nanogap-IDA (nIDA) sensor based on reversible redox processes for chronoamperometric detection of biomolecules.[1,2] With this configuration the main advantages are a current amplification as well as an increase of the signal-to-noise ratio. Whereas, the current flow increases at a small gap distance between the electrodes. However, at nanogap-IDAs the concentration of the supporting electrolyte (SE) affects the system essentially. In this work we examine the cyclic voltammetric (CV) response of two commonly employed redox species at nanogap-IDAs depending on the ionic strength of the SE. Ferrocene methanol (FcMeOH) and hexamine ruthenium (III) chloride (RuHex) were selected. The concentration of the SE was varied from 0.1 M to 1 fM in KCl or phosphate buffered saline (PBS) solution. CV measurements were performed to identify the oxidation and reduction potentials of both FcMeOH and RuHex. Furthermore, chronoamperometric measurements were utilized to determine the current amplification and the collection efficiency. The cyclic voltammetric experiments with 1 mM FcMeOH show different oxidation and reduction potentials depending on the concentration of the SE in the solution (Fig. 1). With 0.1 M KCl as SE, the difference between the anodic and cathodic peak potentials is 70 mV. This correlates with the theoretical value of 60 mV for the reversible one-electron redox process. In 1 fM KCl the peak separation is considerably larger and is about 440 mV, which is an indication of a quasi-reversible redox process. The potential window for chronoamperometric measurements was optimized according to the data obtained from the CV measurements. The experiments show that the achievable redox current using a low ionic strength SE is significantly higher than using 0.1 M KCl (Fig. 2a). For 1 mM RuHex a potential range from -0.4 V to -0.1 V (vs. Ag/AgCl) was defined as an optimal scan range regardless of the ionic strength of the SE. The chronoamperometric measurements showed that the achievable redox current correlates with the ionic strength of the SE. The higher the concentration of SE, the lower the current flow (Fig. 2b). For both utilized redox couples, the increase of the current at lower SE concentration can be explained by the enlargement of the electrical double layer size and the influence of the electrode potential on the ion transport inside the nanogap. These factors contribute to the diffusion of the tested redox species towards electrodes. Therefore, a positive charged FcMeOH+ (FcMeOH - 1ē ↔ FcMeOH+) experiences repulsive forces from an anode and attractive forces from a negatively charged cathode. However, the oxidized and reduced forms of the RuHex are charged positively ([Ru(NH3)6]3+ + 1ē ↔ [Ru(NH3)6]2+) and attract to both electrodes. In this work, we demonstrated that the electrochemical response of redox species depends on the ionic strength of the supporting electrolyte when using nanogap-IDAs. The resulting increase in current at lower SE concentrations can be explained by the enlargement of the electrical double layer size and the influence of the electrode potential on the ion transport inside the nanogap.