An instantly usable paper-based screen-printed solid-state KCl/Ag/AgCl reference electrode with long-term stabilityby Isao Shitanda, Masato Komoda, Yoshinao Hoshi, Masayuki Itagaki

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Komoda, Y. Hoshi and I. Masayuki, Analyst, 2015, DOI: 10.1039/C5AN00617A.

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Instantly Usable Paper-Based Screen-Printed Solid- State KCl/Ag/AgCl

Reference Electrode with High Long-Term Stability

Isao Shitanda,*a,b Masato Komoda,a Yoshinao Hoshia,b and Masayuki Itagakia,b

Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX

DOI: 10.1039/b000000x5

An instantly usable screen-printed paper-based Ag/AgCl electrode was fabricated for use as a cost-effective disposable reference electrode. The potential stability of the reference electrode was approximately 75 h. The setup time, which was less than 1 min, was much shorter than those for similar 10 previously reported electrodes.

Introduction

Ag/AgCl reference electrodes are the most practical and effective reference electrodes. The fabrication of a cost-effective and disposable Ag/AgCl reference electrode has attracted a great deal 15 of interest over the past decade 2-23. Recently, solid-state reference electrodes that have a gel or solid liquid junction have been reported 14,18,19,24,25.

Screen printing is used for fabricating cost-effective and disposable solid-state Ag/AgCl reference electrodes, since this 20 technique allows for precise control over the product dimensions, is highly reproducible, and can be used for mass production4, 9, 10, 14, 21.

In most of the screen-printed solid-state Ag/AgCl reference electrode, a paste containing KCl powder is printed on a Ag/AgCl 25 electrode surface as an inner electrolyte layer. In addition, a polymer liquid junction, which prevents serious leakage of KCl from the inner electrolyte layer into a solution, can be formed on a substrate. The screen-printed solid-state Ag/AgCl reference electrodes previously reported exhibit a good long-term stability4, 30 9, 10, 14. For example, Suzuki et al. fabricated a solid-state reference electrode that have a photocurable hydrophilic polymer liquid junction and a screen-printed electrolyte layer composed of a mixture of KCl powder, poly(vinylpyrrolidone) and 2-propanol9.

Tymecki et al. developed a fully screen printed solid-state 35

Ag/AgCl reference electrode10. Electrolyte layer was formed by printing a UV-cured polymer paste contained KCl powder, and chloroprene rubber was printed as a liquid junction10. Recently, we fabricated a screen-printed solid-state Ag/AgCl reference electrode in which a poly(dimethyl siloxane) emulsion was used to form the 40 inner electrolyte layer and the liquid junction14. The screen-printed solid-state Ag/AgCl reference electrode exhibited potentials accurately for more than 2 months during continuous measurements in a phosphate buffer solution14.

However, one problem encountered with previous solid-state 45

Ag/AgCl reference electrodes4,9,10,14 is that they require a long setup time prior to use (approximately 1–2 h).

When the reference electrode is immersed in the solution to be tested, the solution penetrates the liquid junction, coming into contact with both the electrode surface and the electrolyte layer. 50

As a result, the KCl powder contained in the inner electrolyte layer dissolves in the solution. Consequently, the Cl- concentration near the Ag/AgCl electrode surface reaches the saturation level, and the potential of the reference becomes stable. As mentioned above, the polymer liquid junction is very useful to prevent leakage of KCl 55 from the inner electrolyte layer. On the other hand, the speed of the penetration of the test solution is slow in the polymer liquid junction. Thus, the setup time becomes long.

If the reference electrode is to be used for in situ monitoring instantaneously, a solid-type reference electrode having a low 60 setup time would be more desirable. The test solution must be supplied quickly to the electrolyte layer, in order to reduce the setup time. A balance between a short setup time and the ability to accurately measure the potential for a long period is very important, if solid-state reference electrodes are to find use in field-65 monitoring pH sensors and biosensors.

We focused on paper-based devices. A paper has good potential to act as a liquid junction. The test solution can be supplied rapidly into an inner electrolyte layer by the phenomenon of selfpumping25-28, leading to the short setup time since the paper has 70 hydrophilic surface and porous structure. In addition, a large amount of KCl powder can be included in the paper layer. Thus, we consider that a screen-printed paper- electrode may have a long stability with a short set-up time.

Figure 1 shows a schematic diagram of the paper-based screen-75 printed reference electrode. We used a piece of filter paper as the substrate. The reference electrode consisted of a paper liquid junction and a paper-based electrolyte layer containing KCl. The