Phosphonium dicyanamide ionogel incorporating bromophenol blue dye as a versatile platform for monitoring pH in solution

Online monitoring of pH levels in different environments such as bio-engineering and chemistry is vital for effective control of many critical industrial processes. The most common chemical parameter monitored is pH, and there is an increasing interest in the fabrication of robust, cheap and versatile pH sensing materials that can be easily integrated within existing industrial technologies. Ideally these materials present low fouling and do not require calibration, thus minimising manual attention over long operational intervals. In this work we present an innovative material (ionogel) that integrates pH-sensing capabilities for continuously measuring pH during chemical or biological processes. The ionogel is a solid, flexible and easily to pattern material generated using tetrabutylphosponium dicyanamide ionic liquid, hydrogel polymer (N-isopropylacrylamide and N,N-methylene-bis(acrylamide)) and a pH sensitive dye (Bromophenol Blue). Figure 1 shows the UV spectra of the ionogel-dye in an acidic and a basic pH environment as well as the pictures of the ionogels. A substantial colour variation is observed as the pH changes that can be monitored visually or optically. We incorporated the photoresponsive dye during photo-polymerisation of the monomer to improve stability, for example, by preventing leaching of the dye from the ionogel into the sample phase. This strategy was not found to inhibit the sensitivity of the optical response