(Invited) Spectroelectrochemical Sensors: The Quest for Selectivity
Authors: Heineman William R. , Branch Shirmir D. , Lines Amanda M. , Bryan Samuel A. 
Published: May 2020

Introduction

Exceptional selectivity is needed for sensors to accurately measure analytes in complex samples such as nuclear waste and natural water that contain many potentially interfering substances. Spectroelectrochemistry offers a unique means for providing selectivity by electrochemically modulating the optical signal used for quantitation. The modulated signal can be distinguished from the constant signals of potential interferences. An additional level of selectivity can be provided by adding a very thin film to the transparent electrode to first preconcentrated the analyte while rejecting interferences. Sensors using both visible absorption and fluorescence modes of detection have been developed for detection of organic and inorganic species including ferrocyanide [1] and technetium [2] complexes in nuclear waste and natural water. We have recently extended this research to the spectroelectrochemical investigation of molten salt systems, [3] including lanthanide and actinide measurements related to pyroprocessing [4] and molten salt reactor applications.

Thin layer fluorescence-based spectroelectrochemical sensor

A planar optically transparent electrode chip was fabricated and characterized for multimode sensing of target species. The chip was comprised of an indium tin oxide (ITO) working electrode, a platinum auxiliary electrode, and a Ag/AgCl reference electrode (Figure 1A,B). Fluorescence was used for detection because limits of detection are typically several magnitudes lower than for absorption spectroscopy and selectivity improved by having adjustable wavelengths for both excitation and emission. A fluorescence spectroelectrochemical measurement using this chip incorporated into an optically transparent thin layer electrochemical (OTTLE) cell was demonstrated using the modulated emission of [Ru(bpy)3]2+.

Molten salt absorbance-based spectroelectrochemistry

Molten salt systems are of growing interest for a variety of applications, including meeting future energy needs such as molten salt reactors (MSRs). These reactors are of interest for several factors, including the ability to be operated at atmospheric pressures even at extremely high temperatures. The molten salts serve several functions, including heat transfer fluid, reactor coolant, and ability to contain high concentrations of dissolved fuel (plutonium, uranium, thorium). While the benefits of molten salt reactors have been identified, the chemistry of the molten salts is still not understood. Spectroelectrochemistry can provide the tools to characterize, monitor, and control the chemical behavior of fuel species (U, Pu), fission, and corrosion products throughout the molten salt reactor loop. Several of these species are electroactive and have a unique spectroscopic fingerprint that can be optically detected and modulated using electrochemistry. Spectroelectrochemical characterization can provide fundamental information such as concentration, oxidation state, and speciation at any point in the molten salt reactor system. Absorbance-based spectroelectrochemical modulation of uranium was demonstrated in the molten LiCl-KCl eutectic. The unique spectroscopic signatures for U(III) and U(IV) were identified and monitored as it was electrochemically modulated between the U(III)/U(IV) redox states. Based on variable scan-rate cyclic voltammetry, the diffusion coefficients of U(III) and U(IV) were measured. The spectroelectrochemistry of lanthanides and other actinides in chloride melts is being explored for sensor development.

References

[1] Stegemiller, M.L., W.R. Heineman, C.J. Seliskar, T.H. Ridgway, S.A. Bryan, T. Hubler, and R.L. Sell, Spectroelectrochemical sensing based on multimode selectivity simultaneously achievable in a single device. 11. Design and evaluation of a small portable sensor for the determination of ferrocyanide in hanford waste samples. Environmental Science & Technology. 37 (2003) 123-130.
[2] Chatterjee, S., A.S. Del Negro, M.K. Edwards, S.A. Bryan, N. Kaval, N. Pantelic, L.K. Morris, W.R. Heineman, and C.J. Seliskar, Luminescence-Based Spectroelectrochemical Sensor for [Tc(dmpe)3]2+/+ (dmpe=1,2-bis(dimethylphosphino)ethane) within a Charge-Selective Polymer Film. Analytical Chemistry. 83 (2011) 1766-1772.
[3] Schroll, C.A., A.M. Lines, W.R. Heineman, and S.A. Bryan, Absorption spectroscopy for the quantitative prediction of lanthanide concentrations in the 3LiCl-2CsCl eutectic at 723 K. Analytical Methods. 8 (2016) 7731-7738.
[4] Schroll, C.A., S. Chatterjee, T.G. Levitskaia, W.R. Heineman, and S.A. Bryan, Electrochemistry and Spectroelectrochemistry of Europium(III) Chloride in 3LiCl-2KCl from 643 to 1123 K. Analytical Chemistry. 85 (2013) 9924-9931.

Figure 1. Spectroelctrochemistry and cyclic voltammetry of 1 wt% UCl4 in LiCl-KCl eutectic at 500°C. (A) Spectroscopic response of electrochemical modulation between U(III)/U(IV) redox states; (B) variable scan-rate cyclic voltammetry.

Figure 1