On site determination of free chlorine in water samples by a smartphone-based colorimetric device with improved 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 with absorbance (total optical path 20 cm) and fluorescence cuvettes. The reaction mixtures of free chlorine and 3,3′,5,5′-tetramethylbenzidine (pH = 1.8) were added into the absorbance cuvette and a methionine stabilized Au nanocluster (Met-Au NC) solution was added into 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 the absorbance cuvette. The fluorescence images of the Met-Au NC solution in the reference and measuring areas were captured by the camera in a smartphone. The absorbance was obtained by using a ratiometric fluorescence 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 by the proposed device obeys the 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 of the long-path colorimetric device was one order of magnitude higher than that of a spectrophotometer using a 1 cm cuvette. This simple colorimetric device offers the advantages of portability, sensitivity and reliability for field analysis.