An online pH detection system based on a microfluidic chip.

An online pH detection system is very critical in monitoring the sudden change of pH, especially in strongly acidic and alkaline conditions. We developed a pH sensing chip which works in the range of 5 M [H]-pH 3.0 and pH 6.0-2 M [OH] with response time of 90 s. The sensing chip was formed by coating a pH sensing membrane onto the wall of a microfluidic chamber. The pH sensing membrane was prepared by chemically immobilizing m-Cresol purple in polyvinyl alcohol (PVA). The pH detection system consisted of light source, pH sensing chip and photodiode (PD). Once the pH of fluid flowing in the sensing chip changed, the intensity of transmitted light changed. The intensity of transmitted light was converted to voltage, which was the function of pH value, by the PD. A feed-forward artificial neural network (ANN) with error back-propagation training algorithm was employed to model the behavior of the pH sensor and read out pH values of unknown solutions. The pH detection system shows high stability with increasing the ionic strength. It also possesses properties of repeatability, reversibility and long life-time. These advantages make the proposed pH detection system a promising solution for online detection of pH values in harsh conditions.