Enhanced spectroelectrochemistry with lossy-mode resonance optical fiber sensor

Abstract

Spectroelectrochemical (SEC) measurements play a crucial role in analytical chemistry, utilizing transparent or semitransparent electrodes for optical analysis of electrochemical (EC) processes. The EC readout provides information about the electrode's state, while changes in the transmitted optical spectrum help identify the products of EC reactions. To enhance SEC measurements, this study proposes the addition of optical monitoring of the electrode. The setup involves using a polymer-clad silica multimode fiber core coated with indium tin oxide (ITO), which serves as both the electrode and an optical fiber sensor. The ITO film is specifically tailored to exhibit the lossy-mode resonance (LMR) phenomenon, allowing for simultaneous optical monitoring alongside EC readouts. The LMR response depends on the properties of the ITO and the surrounding medium's optical properties. As a result, the setup offers three types of interrogation readouts: EC measurements, optical spectrum analysis corresponding to the volume of the analyte (similar to standard SEC), and LMR spectrum analysis reflecting the state of the sensor/electrode surface. In each interrogation path, cyclic voltammetry (CV) experiments were conducted individually with two oxidation-reduction reaction (redox) probes: potassium ferricyanide and methylene blue. Subsequently, simultaneous measurements were performed during chronoamperometry (CA) with the sensor, and the cross-correlation between the readouts was examined. Overall, this study presents a novel and enhanced SEC measurement approach that incorporates optical monitoring of the electrode. It provides a comprehensive understanding of EC processes and enables greater insights into the characteristics of the analyte.

Figure 1

A schematic representation of a S²EC (LMR sensor a WE). The elements are not to scale.

Figure 2

(A) Representative CV scans for Pt mesh in 1 mM FC and ITO glass in 0.125 mM MB identifying redox current peak position. (B) Absorbance spectra of the electrolytes containing FC and MB probes at potentials inducing their oxidation and reduction. Wavelengths at which maximum changes in the absorbance spectrum take place are marked for each of the solutions with black arrows.

Figure 3

(A) CV scans for LMR sensor in MB and FC. (B) Evolution of LMR spectrum with E applied to induce oxidation and reduction of the redox probes. (C) A chronoamperometric response showing repeatable response of the LMR sensor in the MB solution.

Figure 4

Results of the measurements in S²EC configuration for MSA, supported by LMR sensor, where (A) shows the EC current response, (B) absorbance at λ = 420 nm measured across the cell in Ch1, (C) LMR wavelength and transmission at 838 nm measured in Ch2.

Figure 5

Results of the measurements in S²EC configuration for MSA, supported by the LMR sensor, where (A) shows results of EC current response, (B) A at λ = 606 nm measured across the cell, i.e., in Ch1, and (C) LMR wavelength and transmission at λ = 838 nm measured in Ch2.