A LED-based fiber-optic sensor integrated with lab-on-valve manifold for colorimetric determination of free chlorine in water

In this work, a colorimetric methodology was developed for free chlorine determination by constructing a fiber-optic sensor (FOS) which was designed by using a small LED as light source and two silica fibers as light transmission elements. Then the FOS was integrated with the sequential injection lab-on-valve (SI-LOV) manifold to fabricate a FOS-SI-LOV system for free chlorine detection. The colorimetric determination was based on the reaction between N , N -diethyl- p -phenylenediamine (DPD) reagent and free chlorine, which would produce a chromophore showing maximum absorbance at 511 and 551   nm and detected by a micro spectrometer detector. By choosing a green LED with emission wavelength at 520   nm as light source, the chlorine concentration could be simply evaluated according to Beer's law. The developed FOS-SI-LOV system showed improved performances of wide linear range (10–400   μg   L −1 ), low detection limit (3.5   μg   L −1 , 3σ), fast analysis (6.85   s) and excellent repeatability (0.0083 RSD, n=8) for chlorine determination. The proposed FOS-SI-LOV system has been applied to on-line and real-time chlorine determination in different water samples with excellent selectivity. Furthermore, this FOS-SI-LOV methodology provided a colorimetric and fluorescence detection platform for other hazardous species monitoring in water environment based on Beer's law and Stern-Volmer relationship, respectively.