Direct colorimetric detection of water pollutants with non-water soluble dyes

ABSTRACT In this study, we develop a high-resolution optical absorbance instrument for detecting waterborne pollutants using insoluble water dyes. We demonstrate the instrument using hydrogen sulfide, H2S, and mono-ethyl amine, Et-NH2, as target analytes/pollutants. We use colorimetric squaryllium dye ‘SQ1’ for H2S and isoindolene dye ‘ID1’ for Et-NH2. Dyes associate with target pollutants, and bleach absorption bands. Then, we select LEDs with peak emission that match the dyes' responsive bands, coat LEDs with dye, and measure the absorbance by immersing in a sampling cuvette, and detect the intensity of transmitted light. LED drive and light intensity detection employ a Lock-In amplifier, and a high signal-to-noise ratio allows detecting changes in absorbance <10–3. We calibrate by titrating target pollutants and find limits-of-detection far below the pollutants' limiting values. Bleaching at the dye film/water interface alone is sufficient for colorimetric detection of target analytes without an auxiliary solvent or a phase transfer membrane. We believe our approach can be readily extended beyond the examples shown here.