A review of screen-printed silver/silver chloride (Ag/AgCl) reference electrodes potentially suitable for environmental potentiometric sensors

Abstract

The screen-printed (SP), reference electrode (RE) has been shown to be a crucial element of potentiometric sensors but it is also the stumbling block for reliable and accurate SP sensors. The easiest, most common and most environmentally friendly, type of RE is the Silver/Silver Chloride (Ag/AgCl) RE. Unfortunately, until now the only reliable RE of this kind is the conventional, liquid or gel-filled type. However, for most environmental and soil applications the use of the conventional RE is not an option, which raises the demand for a robust, rugged and low-cost version to replace the conventional RE. This paper presents a review of the various attempts to produce reliable, SP, Ag/AgCl REs, and explain why almost all of them never reach the commercialisation stage. The paper provides an overview of the main challenges that need to be overcome, details of the electrode's construction, an analytical comparison of their performance in terms of chloride susceptibility, cross-sensitivity and lifetime, and their suitability in different applications depending on their performance characteristics. (Sophocleous M.) Conventional Ag/AgCl reference electrode Construction of the conventional Ag/AgCl reference electrodeThe most common type of silver-silver halide electrode is the Ag/AgCl due to the availability of silver chloride and the use of potassium chloride as the electrolyte. The electrode is constructed using a silver wire, electroplated with silver chloride and then immersed into a 3.5 M or saturated potassium chloride (KCl) solution. The electrolyte is generally contained in a glass tube having a porous glass frit at the tip to allow ion exchange at the liquid-liquid junction. The conventional silver/silver chloride reference electrode has a relatively simple construction. A typical construction of such an electrode is shown in Figure 1.

Flexible Printed Reference Electrodes for Electrochemical Applications

“…[13] Among these, the Ag|AgCl-based REs are common and more attractive because of easy fabrication, stable electrode potential, and their nontoxicity. For this reason, Ag|AgCl-based thin [17,18] and thick film [19][20][21][22] REs have also been explored recently. [8,10,16] However, as discussed earlier, due to rigidity of glass, it is difficult to use GREs in emerging application such as wearables' systems.…”

“…[10,14,15] Conventional REs are fabricated on glass using AgCl coated Ag metal wire in a saturated KCl or enriched Cl − ion solution. [20] The unstable potential due to the decomposition of AgCl layer over a period of time is detrimental to the long-term operation of thin film REs. For this reason, Ag|AgCl-based thin [17,18] and thick film [19][20][21][22] REs have also been explored recently.…”

“…For this reason, Ag|AgCl-based thin [17,18] and thick film [19][20][21][22] REs have also been explored recently. [20,21] A common issue with thick films REs is their short lifetime due to rapid decay of KCl layer. For example, being a quasi-RE (i.e., absence of KCl layer), the majority of thin film REs exhibit fluctuations in the open circuit potential (OCP) while reacting with Cl − ions.…”

“…Although both these types of REs have been shown to have good potential for wearable and disposable devices, they still have several limitations. [20,21,24] Further, the polymer-based KCl layer in these REs require long setting-up time (more than 2 h) prior to use. [20] The unstable potential due to the decomposition of AgCl layer over a period of time is detrimental to the long-term operation of thin film REs.…”

“…[8,10,16] However, as discussed earlier, due to rigidity of glass, it is difficult to use GREs in emerging application such as wearables' systems. For this reason, Ag|AgCl-based thin [17,18] and thick film [19][20][21][22] REs have also been explored recently. Although both these types of REs have been shown to have good potential for wearable and disposable devices, they still have several limitations.…”