Electrochemistry has a limit of detection imposed by the inherent shot-noise present in electrical currents [1]. As a result, the direct measurement of single-electron charge transfer processes is not feasible using electrochemical methods. However, luminescence can be measured with single photon resolution and the development of a method to efficiently convert an electrochemical signal to an optical one is expected to be a key step in overcoming the detection limit of electrochemistry [2].

An optical conversion technique based on closed bipolar electrochemistry has recently been demonstrated using large glassy carbon electrodes [3,4]. In this technique, which is illustrated in Fig. 1, any electrochemical process of interest occurring in one cell (the detection cell) can be used to induce either fluorescence or chemiluminescence in another cell (the reporting cell). Fluorogenic reporting reactions have the advantage that several photons can be emitted during the lifetime of each fluorophore, leading to signals with a greater intensity. However, the facile photooxidation of some fluorophores introduces the need for chemical reductants, which complicates the electrochemistry [5]. Conversely, the constant electron-to-photon relationship of chemiluminescence makes it an easier process for calculating the number of electrons passed in the detection cell, but also leads to a reduced signal intensity [6]. There is still much work needed to understand which reporting mechanism is more suited to sub-detection limit electrochemistry and how to reach the maximum sensitivity of single-electron charge transfer processes.

In this work, we present our recent progress towards the measurement of sub-detection limit electrochemical processes via luminogenic reporting reactions. We compare the conversion mechanisms of reporting reactions which use either fluorescence or chemiluminescence, and we study the factors which can affect the sensitivity of the conversion, such as the presence of chemical reductants. Finally, we test the temporal resolution and sensitivity of optical conversion techniques in the measurement of single entity electrochemical processes. The results shown herein are of importance to electrochemists wishing to use optical conversion to probe below the detection limit of electrochemistry.

Literature

[1] R. Gao, M. A. Edwards, J. M. Harris and H. S. White, Curr. Opin. Electrochem., 2020, 22, 170–177.

[2] Y. Wu, S. Jamali, R. D. Tilley and J. J. Gooding, Faraday Discuss., 2022, 233, 10–32.

[3] J. P. Guerrette, S. J. Percival and B. Zhang, J. Am. Chem. Soc., 2013, 135, 855–861.

[4] J. S. Stefano, F. Conzuelo, J. Masa, R. A. A. Munoz and W. Schuhmann, J. Electroanal. Chem., 2020, 872, 113921.

[5] S. M. Oja, J. P. Guerrette, M. R. David and B. Zhang, Anal. Chem., 2014, 86, 6040–6048.

[6] P. A. Defnet and B. Zhang, ChemElectroChem, 2020, 7, 252–259.