An Electrochemical Study with Three Redox Substances on a Carbon Based Nanogap Electrode Array
Authors: Matylitskaya Volha , Kasemann Stephan , Kostal Elisabeth , Partel Stefan 
Published: May 2020

The interdigitated electrode arrays (IDAs) are widely applied for sensor applications. In the last decade, carbon has been increasingly used as an electrode material and competes with materials such as gold and platinum. A wide electrochemical window, nonspecific adsorption of biomolecules, chemical inertness, excellent biocompatibility, and low market price make carbon an attractive candidate for electrochemical biosensor applications.[1] We developed a carbon nanogap-IDA (nIDA) sensor based on reversible redox processes for electrochemical detection of biomolecules.[2] The advantages of an IDA configuration as such and a small gap size are signal amplification and a high signal to noise ratio. Bard et al.[3 and Aoki et al.[4] showed that signal amplification can be achieved using chronoamperometry due to a reversible redox process. The gap size between finger electrodes of the IDA is the crucial factor for the attainable gain. Our nIDA transducer is deployed for a lab-on-chip application based on enzyme-linked bioassay protocol where p-aminophenol is used as redox couple. Additionally, the fabricated nIDA transducer is used for the detection of the neurotransmitter dopamine. For a conclusive comparison of nIDAs with different electrode materials, ferrocene methanol was utilized.
In this work, we report current results for the electrochemical characterization of the fabricated carbon nanogap IDAs. The three redox substances ferrocene methanol (FcMeOH), p-aminophenol (pAP) and dopamine (DA) were applied with 0.1 M KCl as supporting electrolyte. The information on the electron transfer during the reversible redox process was obtained by cyclic voltammetric measurements in generator/collector mode. In this mode, one working electrode (generator) is scanned while the second working electrode (collector) is fixed at a constant potential. Furthermore, chronoamperometric measurements were performed to determine the signal amplification and to obtain information on the sensitivity of the generated carbon nIDAs.
A concentration range of 10 pM to 1 mM for all three redox pairs was investigated to analyse the sensitivity of the carbon nIDAs. The largest linear characteristic was achieved with ferrocene methanol in the concentration range of 5 nM to 1 mM during chronoamperometry. In this case the oxidation potential of +0.3 V and the reduction potential of -0.05 V relative to the Ag/AgCl reference electrode were applied to the working electrode 1 and the working electrode 2, respectively. Compared to a single-electrode configuration, a current amplification of up to 117 was achieved for 10 μM FcMeOH. To the best of our knowledge, this is the highest amplification achieved by CA measurements in bulk solutions using carbon electrodes (the sensor was dipped into the solution) reported so far. This value is comparable to previous obtained results when measuring with platinum nIDAs.[5]

For pAP the oxidation potential of +0.4 V and the reduction potential of -0.2 V versus the Ag/AgCl reference electrode were determined. As expected, a polymer formation during the redox cycling of pAP was observed. A decrease in peak current was detected with each subsequent CV scan. This can be explained by an inhibited electron transfer process during CV measurements in the normal mode, as well as in the generator/collector mode. Inhibited electron transfer leads to a very low amplification factor of 5 at chronoamperometric measurements. These results are in good agreement with our experimental data obtained for platinum nIDA[5] as well as with the experimental data of other research groups.[6,7] Nevertheless, a linear correlation between current and concentration in the range of 50 nM to 100 μM was observed in chronoamperometric measurements with pAP.
For the electrochemical measurements with dopamine the oxidation potential of +0.75 V and the reduction potential of -0.2 V were selected as the optimum. The cyclic voltammetric measurements in the generator/collector mode indicate a kinetically limited electron transfer. An amplification factor of 38 when measuring with 1 mM DA was achieved. Further, a linear correlation between current and concentration was observed in the range of 500 nM to 1 mM.
In summary, we present the characterization of the fabricated carbon nanogap IDAs with three different redox substances. The linear correlation between current and concentration is shown as well as their corresponding range. Furthermore, we achieved a very high current amplification of 117 for the carbon nIDA transducer with FcMeOH.
[1] J.A. Lee, S. Hwang, J. Kwak, S.I. Park, S.S. Lee, K.-C. Lee, Sens. Actuators B Chem. 129 (2008) 372–379.
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[3] A.J. Bard, J.A. Crayston, G.P. Kittlesen, T. Varco Shea, M.S. Wrighton, Digital simulation of the measured electrochemical response of reversible redox couples at microelectrode arrays: consequences arising from closely spaced ultramicroelectrodes, Anal. Chem. 58 (1986) 2321–2331.
[4] K. Aoki, M. Morita, O. Niwa, H. Tabei, Quantitative analysis of reversible diffusion-controlled currents of redox soluble species at interdigitated array electrodes under steady-state conditions, J. Electroanal. Chem. Interfacial Electrochem. 256 (1988) 269–282.
[5] V. Matylitskaya, S. Kasemann, G. Urban, C. Dincer, S. Partel, Electrochemical Characterization of Nanogap Interdigitated Electrode Arrays for Lab-on-a-Chip Applications, J. Electrochem. Soc. 165 (2018) B127–B134.
[6] Arjomandi, J. & Kakaei, Z. Electrosynthesis and In Situ Spectroelectrochemistry of Conducting o -aminophenol- p -aminophenol Copolymers in Aqueous Solution. J. Electrochem. Soc. 161, (2014) E53–E60.
[7] Menezes, H. A. & Maia, G. Films formed by the electrooxidation of p-aminophenol (p-APh) in aqueous medium: What do they look like? J. Electroanal. Chem. 586, (2006) 39–48.

Figure 1