A colorimetric method with the use of BDJ detector for seawater pH measurement

Abstract By using a recently developed, wavelength-sensitive BDJ (buried double p-n junction) detector, a reliable and inexpensive method for colorimetric determination of seawater pH is suggested. A BDJ detector is a silicon photodetector providing two photocurrents proportional to the incident radiation, and the ratio of photocurrents is wavelength-dependent. In the visible range, the ratio of the deep junction photocurrent to the shallow junction one is monotonic increasing with the wavelength. This characteristic allows the BDJ detector to be applied to the detection of colours or spectral changes. For colorimetric seawater pH measurement with the use of tymol blue (TB) as pH-indicator dye, we propose a simple and reliable method based on the concept of the photocurrent ratio. By determining the concentration of the acidic form and that of the alkaline form in a dye solution, the pH can be deduced. The presented method has experimentally been verified, with a more stable dye, bromo phenol blue (BPB), which has very similar absorption spectra to those of the TB dye. As the design and fabrication of the BDJ detector requires only a standard CMOS process, a miniaturised pH-measuring system can be developed, thanks to the potential of on-chip integration of BDJ detector and electronic circuitry.