Abstract
In this study, we describe a novel solid-state thin-film Ag/AgCl reference electrode (SSRE) that was coated with a protective layer of graphene oxide (GO). This layer was prepared by drop casting a solution of GO on the Ag/AgCl thin film. The potential differences exhibited by the SSRE were less than 2 mV for 26 days. The cyclic voltammograms of the SSRE were almost similar to those of a commercial reference electrode, while the diffusion coefficient of Fe(CN)63− as calculated from the cathodic peaks of the SSRE was 6.48 × 10−6 cm2/s. The SSRE was used in conjunction with a laboratory-made working electrode to determine its suitability for practical use. The average pH sensitivity of this combined sensor was 58.5 mV/pH in the acid-to-base direction; the correlation coefficient was greater than 0.99. In addition, an integrated pH sensor that included the SSRE was packaged in a secure digital (SD) card and tested. The average sensitivity of the chip was 56.8 mV/pH, with the correlation coefficient being greater than 0.99. In addition, a pH sensing test was also performed by using a laboratory-made potentiometer, which showed a sensitivity of 55.4 mV/pH, with the correlation coefficient being greater than 0.99.
Highlights
Electrochemical sensors fabricated using microfabrication techniques have been receiving much attention because they are generally simple to make, portable, and cost effective [1]
During the cyclic voltammetry (CV) measurements, the solid-state reference electrode (SSRE) or the Orion Ag/AgCl reference electrode (ORE) was used as the reference electrode, a thin-film gold electrode 2 mm in width and 3 mm in length was employed as the working electrode, and a commercially obtained Pt wire (CH Instruments, Austin, TX, USA) was used as the counter electrode
The results of CV measurements and pH sensing tests, performed with laboratory-made sensors, suggested that the proposed SSRE could be used as a reference electrode in electrochemical applications
Summary
Electrochemical sensors fabricated using microfabrication techniques have been receiving much attention because they are generally simple to make, portable, and cost effective [1]. There are several methods of fabricating electrodes, such as electrochemical deposition [5,12], screen-printing [9,13,14], and sputter coating [4,6]. When the electrode is dipped in the test solution, the AgCl present on the thin Ag film gradually dissolves at high chloride-ion concentrations [5,21,22] This can lead to the potential of the reference electrode becoming unstable and cause the working electrode to give erroneous readings. We employ graphene oxide (GO) as an alternative protective layer to coat a solid-state thin-film Ag/AgCl reference electrode. We manufactured a laboratory-made potentiometer based on the integrated pH sensor in order to verify the applicability of the sensing system for practical use; the sensor and the potentiometer showed satisfactory pH sensing performance, as the results showed that the performance of the sensor is comparable to that of a commercial one
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