A flexible and disposable paper-based pH sensor fabricated with a pencil-drawn working electrode and a Ag/AgCl paste reference electrode is demonstrated for the first time to show pH response by the potentiometric principle. The sensor substrate is made of chromatography paper with a wax-printed hydrophobic area, and various types of carbon pencils are tested as working electrodes. The pH sensitivities of the electrodes drawn by carbon pencils with different hardnesses range from 16.5 to 26.9 mV/pH. The proposed sensor is expected to be more robust against shape change in electrodes on a flexible substrate than other types of chemiresistive/amperometric pH sensors.
This paper explains the ion-sensing mechanism and selectivity evaluation of the pencil-drawn carbon electrode (PCE) employed as a pH electrode in the authors’ previous report. The elemental composition of the PCE was found to be C, O, Si, Al, and Na by energy-dispersive X-ray spectroscopy (EDX). The carbon is regarded as the graphite performing electrical conduction, while the other elements formed clay as an ion exchanger. Furthermore, the potential responses to K+ and Na+ were measured, and the selectivity is discussed quantitatively under the Eisenman equation which describes the ion exchanger potential with cation interference. The selectivity coefficients were estimated to be 10−4.10 to 10−3.87 and 10−5.00 to 10−4.89 for K+ and Na+ with n of 4 to 5, respectively. The EDX analysis and selectivity measurement show that the PCE might have ion exchange properties similar to those of a glass electrode and it sufficiently behaves as a pH electrode in wide targets having only slightly interfering cations.
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