(Invited) Characterization of an Optically Transparent Thin-Film Electrode Chip for Spectroelectrochemical Sensors
Authors: Branch Shirmir D. , Lynch John , Bello Job M. , Heineman William R. , Bryan Samuel A. 
Published: February 2020

In this work, an optically transparent thin-film electrode chip is investigated for applications in electrochemical and spectroelectrochemical sensing. Compared to other electrode chips that are commercially available, this chip is designed for use with a miniature, portable system to take quick, accurate point of care measurements as well as long-term usage, such as environmental monitoring.
The electrode chip is fabricated from standard photolithography and thin-film deposition methods over 1.1 mm thick boro-aluminosilicate glass substrates. The use of glass as a substrate minimizes fluorescence background interferences that would be seen with plastic substrates. The chip has three electrodes: an indium tin oxide (ITO) working electrode (WE, 200 nm), a platinum quasi-reference electrode (RE, 200 nm), and a platinum auxiliary electrode (AE, 200 nm), as shown in Fig. 1a. The optically transparent electrode (OTE) ITO allows for spectroscopic detection using both absorption and fluorescence. The surface of the electrode area is defined by applying a protective photoresist over the chip, leaving the three-electrode area and the electrode leads exposed. The dimensions of the electrode area are 8 mm × 40 mm. The surface area of the working electrode is 7 mm2; the reference is 1 µm2; and the auxiliary is 39 mm2.
To improve the stability of the quasi-reference electrode, the platinum reference surface was coated with a planar Ag/AgCl layer. This was done by electroplating silver over the platinum reference electrode using 0.3 M AgNO3 in 1 M NH3­. The electroplated surface was then covered with a solution of 50 mM FeCl3 to chemically oxidize AgCl on top of the silver. Open circuit measurements against a commercial Ag/AgCl reference indicate that the potential of the planar Ag/AgCl reference on the electrode chip varies a maximum 1 mV within the same time span for measurements of the platinum reference electrode to vary nearly 1 V. Cyclic voltammetry measurements of 0.1 mM [Fe(CN)6]3- (0.1 M KCl) using the electrode chip with the planar Ag/AgCl reference shows stable voltammograms over 180 cycles. The formal reduction potential (Eo′) of [Fe(CN)6]3- using the planar reference is Eo′ = 102 mV (0.1 M Cl-). This is 83 mV less than the potential using the commercial reference (Eo′ = 185 mV, 3 M Cl-), as is expected due the difference in chloride ion concentration that the Ag/AgCl electrode is exposed to.
To demonstrate that the electrode chip is comparable with a standard electrochemical cell (ITO slide WE, liquid junction Ag/AgCl RE, Pt wire AE), Randles-Sevcik analysis of 10 mM [Fe(CN)6]3- (0.1 M KCl) was executed using both electrochemical systems. The diffusion coefficient of [Fe(CN)6]3- using the electrode chip and standard cell was 1.59 × 10-6 cm2/s and 2.38 × 10-6 cm2/s, respectively. Due to the values being in the same order of magnitude, the electrode chip with the planar Ag/AgCl reference electrode is able to function similarly to the standard three-electrode cell.
To demonstrate the spectroelectrochemical applications of the electrode chip, the chip is used in an optically transparent thin-layer electrode (OTTLE) cell for the modulation of [Ru(bpy)3]2+ (1 M KCl) fluorescence. The OTTLE cell was constructed by laying a 15 × 8 × 1 mm quartz slide over the electrode area of the chip. The chip and the quartz slide are separated by 177.9 nm thick silicone spacers. The spectroscopic signal of [Ru(bpy)3]2+ is modulated using potentiostatic techniques. A potential of 0.8 V is applied to hold the analyte in the emissive Ru2+ oxidation state. The potential was then stepped to 1.3 V to convert the analyte to the non-emissive Ru3+ oxidation state. The result of the spectroelectrochemical modulation of 2.0 µM [Ru(bpy)3]2+ (1 M KCl) is shown in Fig. 1b. For calibration, the Δemission is plotted against the concentration of [Ru(bpy)3]2+, from 0 – 5 µM, to yield a detection limit of 36 nM.
In conclusion, the electrochemical applications of the electrode chip are determined to be comparable to a standard electrochemical cell. The performance of the chip is improved laying a planar Ag/AgCl electrode over the platinum quasi-reference. The OTE electrode on the chip allows for spectroelectrochemical detection of target species. This low-cost, yet robust electrode chip is an ideal candidate for short-term measurements and extended monitoring.

Acknowledgments

This work was supported by the Department of Energy STTR Grant DE-SC0009718.

Figure 1