A low-cost lab-on-a-chip device for marine pH quantification by colorimetry

Abstract

This paper presents a novel lab-on-a-chip for seawater pH measurement that features in-situ, autonomous, long-term and low-cost measurements. It comprises a poly(dimethylsiloxane) microfluidic system and an integrated low-cost optical-electronic system that uses light emitting diodes as light source and photodiodes as the detectors. The pH measurement is based on the colorimetric method, by optical absorption, using the meta-Cresol Purple (mCP) as indicator dye. Only 9.6 mu L of this mCP is needed to perform the pH measurement. Under the developed optimized conditions presented here, e.g., microfluidic channels geometry, indicator dye and seawater volumes, the device has a resolution of 0.002 pH units for the 7.500-8.200 seawater pH units range, with a significantly reduction of the optical path length to 5 mm and a total detection time of about 8 min. The lab-on-a-chip features high sensitivity, linearity and reproducibility. This approach will allow, in the future, the lab-on-a-chip to be moored in a submerged base, located in the depth of the ocean due to its inherent pressure independent operability, in opposition to conventional electrochemical methodologies.