Spectroelectrochemical (SEC) measurements are widely applied in analytical chemistry. Transparent or semitransparent electrodes are typically used for optical analysis of the electrochemical (EC) processes, where mainly EC readout indicates the state of the electrode while changes in transmitted optical spectrum help to identify products of the EC reactions. In this work, we propose SEC measurements enhanced by additional optical monitoring of the electrode. In this setup, an optical fiber sensor or rather a section of the polymer-clad silica multimode fiber core coated with a thin conductive oxide, i.e., indium tin oxide (ITO), acts also as the electrode. The properties of the film were appropriately tailored to give rise to the lossy-mode resonance (LMR) phenomenon and therefore, except for the SEC readouts, optical monitoring of the electrode was simultaneously possible. The LMR response depends on the properties of the ITO and the optical properties of the surrounding medium. Thus, in the proposed setup, three interrogation readouts may be received, i.e., EC, optical spectrum corresponding to the volume of the analyte as for standard SEC, and the LMR spectrum dependent on the state of the sensor/electrode surface. It must be noted that the optical measurements are performed in two, separate, perpendicularly arranged, spectrophotometric paths with the EC activation (S2EC). In this work, the three readouts have been correlated and compared to the application of other working electrodes applied in standard SEC, such as platinum mesh and ITO-coated glass slide. First, every interrogation path was investigated individually during the cyclic voltammetry (CV) with two metal ion oxidation-reduction reaction (redox) probes, such as potassium ferricyanide and methylene blue. Then simultaneous measurements during chronoamperometry (CA) were conducted with the sensor and cross-correlation between the readouts had been discussed. By combining the sensing techniques an enhancement in the amount of gathered information about the analyte has been achieved.
