Towards the detection limit of electrochemistry: Studying anodic processes with a fluorogenic reporting reaction

Recently, shot noise has been shown to be an inherent part of all charge transfer processes, leading to a practical limit of quantification of 2100 electrons (~0.34 fC) (Curr. Opin. Electrochem. 2020, 22, 170177). Attainable limits of quantification are made much larger by greater background currents and insufficient instrumentation, which restricts progress in sensing and single-entity applications. This limitation can be overcome by converting electrochemical charges into photons, which can be detected with much greater sensitivity, even down to a single-photon level. In this work, we demonstrate the use of fluorescence, induced through a closed-bipolar set-up, to monitor charge transfer processes below the detection limit of electrochemical workstations. During this process, the oxidation of ferrocenemethanol (FcMeOH) in one cell is used to concurrently drive the oxidation of Amplex Red (AR), a fluorogenic redox molecule, in another cell. The spectroelectrochemistry of AR is investigated and new insights on the commonplace practise of using deprotonated glucose to limit AR photooxidation are presented. The closed-bipolar set-up was used to produce fluorescent signals corresponding to the steady-state voltammetry of FcMeOH on a microelectrode. Chronopotentiometry is then used to show a linear relationship between the charge passed through FcMeOH oxidation and the integrated AR fluorescence signal. The sensitivity of the measurements obtained at different timescales varies between 2200 - 500 electrons per detected photon. The electrochemical detection limit is approached using a diluted FcMeOH solution in which no current signal is observed. Nevertheless, a fluorescence signal corresponding to FcMeOH oxidation is still seen, and detection of charges down to 300 fC is demonstrated.