Detection of transition metal ions, odorous compounds and wastewater in river water using a reaction-based optical fingerprinting strategy.

Although water quality monitoring using optical sensors has been extensively studied, it remains a frontline research area owing to its simplicity and adaptability. In this study, we continue to develop a kinetic-based sensing strategy based on multiple optical measurements during an indicator reaction. The effect on the reaction rate makes it possible to observe the effect of sample components that do not otherwise manifest themselves in photometry and fluorimetry. The oxidation of calcein (fluorescein-methyleneiminodiacetic acid) with hypochlorite or hydrogen peroxide and riboflavin with H2O2 in the presence of Cu(II) or Fe(II) were used as indicator reactions. Fluorescence intensity and absorbance/reflectance of reaction mixtures were measured by periodic photography of 96-well plates. Optical data were processed using supervised chemometric methods: linear discriminant analysis (LDA), logistic regression (LR) and others. To demonstrate the sensitivity of the indicator systems to compounds of various natures, six transition metal ions (Cu(II), Ni(II), Mn(II), Co(II), Fe(II), and Cr(III)) and five odor-causing organic compounds (trimethylamine, phenol, 2-chlorophenol, o-cresol, and mercaptoethanol), selected as model pollutants, were discriminated based on their nature and concentration with an accuracy of up to 95-100%. Eleven water samples were differentiated by sample and by water type (spring, well, borehole, pond, and tap) using the same indicator reactions. Municipal sewage water diluted up to 100 000 times was discriminated from river water, enabling the identification of unexpected cases of water source contamination. The obtained results surpass the benchmarks of previous studies and should serve as an incentive for further research aimed at the application of kinetic optical fingerprinting in water quality monitoring.