Dye-Assisted pH Sensing Using a Smartphone

We report the working of a smartphone-based optical sensor for the measurement of pH level of colorless aqueous media. By integrating readily available laboratory optical components to the camera of the smartphone, we convert the smartphone into a visible spectrophotometer with spectral resolution of 0.345 nm/pixel. Evanescent light signal from the designed optical sensing setup is allowed to interact with a pH sensitive dye sample. The transmitted light signal from the sensing region is then captured by the camera of the smartphone in the form of modulated visible image spectrum. The designed sensor can detect change in pH level of medium with a resolution of 0.12 pH unit within range of 6-8 pH unit. We evaluate the repeatability of our sensor for eight consecutive trials and obtained a standard deviation $sim 0.015$ in the transmission intensity within range of 6-8 pH units. We envision that our designed sensing technique could emerge as a low cost, portable, and robust pH sensor that has the ability to measure pH level of colorless aqueous medium with good accuracy and repeatability.