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


• A Lab-on-a-chip device for marine pH quantification by colorimetry was developed.
• It integrates a microfluidic system and a coupling optical-electronics readout in a single device.
• It uses only 9.6 µL of meta-Cresol Purple for the marine pH quantification.
• The marine pH quantification was similar to commercial equipment with a maximum standard deviation of 0.00208 pH units.
• It can be used as a portable device for in-situ real-time monitoring of seawater pH, helping in the analysis of species’ distribution in aquatic habitats.


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 to 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.

Pinto V. C., Araújo C. F., Sousa P. J., Gonçalves L. M. & Minas G., in press. A low-cost lab-on-a-chip device for marine pH quantification by colorimetry. Sensors and Actuators B: Chemical. Article (subscription required).

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