Digital pH Test Strips for In-Field pH Monitoring Using Iridium Oxide-Reduced Graphene Oxide Hybrid Thin Films

While pH test strips are not decimally accurate and only semiquantitative, the more accurate and automated laboratory-based pH meters lack disposability and portability. Benefiting from advantages of both sides and filling the gap of unmet needs in between, we integrated a miniaturized paper-fluidic electrochemical pH platform coupled with a portable pH device powered by consumer batteries to make digital pH test strips for truly in-field measurements. To overcome the inability to form the smooth and uniform pH-sensitive functionality on screen-printed carbon, i.e., anodically electrodeposited iridium oxide thin films (AEIROFs), reduced graphene oxide was used as substrate for synthesis of homogeneous metal oxide-nanocarbon hybrid thin films, IrO2-reduced graphene oxide (IrO2-rGO), as the sensing moiety with strong substrate adhesion. A hydrophobic barrier patterned paper micropad (mu PAD) with programmed flow rates can locally accelerate linear wicking speed to boost delivery and response time. IrO2-rGO shows slightly super-Nernstian linear responses from pH 2 to 12 with small hysteresis, fast response time, reproducible performances, and low sensitivities to interfering ionic species and dissolved oxygen. A miniaturized portable pH device made of high-impedance buffer amplifier and off-the-shelf digital multimeter provides results consistent with those of a commercial pH meter equipped with a glass electrode. Our system combines the accuracy of pH meters with the low-cost and convenience of pH strips, making it ideal for resource limited settings.