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 μL 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.