The determination of pH is ubiquitous from food and health care to environmental monitoring. Two most common ways to determine pH are pH meters and pH paper. pH paper measurement is only semi-quantitative by comparing colors to a chart and comparatively subjective. pH meter is large and lab-based and needs frequent calibrations and stringent storage conditions. A disposable paper-fluidic electrochemical pH sensor is thus promising as it combines advantages of both pH meters and strips and can be used in resource-limited conditions and point-of-care applications. Paper can serve as a filter to remove unwanted large particulates from samples while making low-volume measurements available and eliminating the need of external power supplies for sample delivering and handling by capillary pressures.
Graphene nanocomposites with metal oxides can have synergistic effects and are promising candidates as pH-responsive materials. Current production of graphene from graphene oxide and synthesis of nanocomposites from salt precursor solutions require reducing reagents such as hydrazine, NaBH4 and urea which are toxic, corrosive or even explosive, and thus there are serious safety and environmental issues. The process with reducing reagents is also undesirable due to their potential damage on electronic properties. Therefore a greener and more effective synthetic route of graphene nanocomposites is an unmet need.
Here we present a facile, controllable and inexpensive electrochemical synthesis of IrO2-graphene nanocomposites and fabricate an easy-to-use integrated paper microfluidic electrochemical pH sensing platform in resource-limited settings, composed of PDMS-patterned paper strips with hydrophobic barriers, screen-printed carbon electrodes modified with IrO2-graphene films and molded acrylonitrile butadiene styrene plastic holders. Repetitive cathodic potential cycling was employed for GO reduction which can completely remove unfavorable electrochemically unstable oxygenated groups and generate a 2D defect-free homogeneous graphene film with excellent stability and electronic properties. A uniform and smooth IrO2 film in nanoscaled grain size is anodically electrodeposited onto the graphene film, without any observable cracks. The resulting IrO2-graphene electrode showed a slightly super-Nernstian response from pH 2-12 in Britton-Robinson (B-R) buffer with linear responses, small hysteresis widths, fast response time and low sensitivities to different ionic species and dissolved oxygen. A digital portable pH meter was fabricated based on a multimeter. pH values of lake water at different locations of four major lakes in Madison, WI are measured both by a lab-based commercial pH meter with a glass electrode and our portable electrochemical paper-microfluidic pH sensor and the results are statistically consistent (R2=0.999).