On site determination of free chlorine in water samples by a smartphone-based colorimetric device with improvement sensitivity and reliability
Chlorine is the most common chemical compound used for treatment of water, but the concentration of free chlorine must be strictly controlled. For fast and on-site detection of free chlorine, cost-effective and miniaturized devices are required. Herein, we reported a smartphone-based colorimetric device with improved sensitivity and reliability, equipped an absorbance (total optical path 20 cm) and a fluorescence cuvettes. The reaction mixture of free chlorine and 3,3’,5,5’-tetramethylbenzidine (pH=1.8) were added in the absorbance cuvette and methionine stabilized Au nanoclusters (Met-Au NCs) solution was added in the fluorescence cuvette.A 450 nm laser beam was passed through the fluorescence and absorbance cuvettes and reflected back by a triangular mirror at the end of absorbance cuvette. The fluorescence images of Met-Au NCs solution in the reference and measuring areas were captured by the camera in a smartphone. The absorbance was obtained by using a ratiometric fluorescent strategy, which is found to be independent of the variations in light source intensity, concentration of fluorescent luminophore, and exposure parameters of the camera. The absorbance measured in the proposed device obeys Lambert-Beer law in the range of 0~1.The triangular mirror is useful to double the absorption optical path and reduce the distance between the reference and measuring fluorescence areas. The sensitivity in long-path colorimetric device was one order of magnitude higher than that in spectrophotometer using 1 cm cuvette. This simple colorimetric device offers the advantages of portability, sensitivity and reliability for field analysis.