S2EC - enhanced spectroelectrochemistry with lossy-mode resonance
optical fiber sensor
Abstract
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.