Blue-green emitting cationic thiazole surfactants-based paper devices for highly sensitive and selective fluorescence detection of chromium oxyanions in wastewater

Abstract

Two cationic Schiff base sensors; N-(3-(((4E)-2-(2-((2-hydroxynaphthalen-1-yl)methylene) hydrazineyl)thiazol-4(5H)-ylidene)amino)propyl)-N,N-dimethyloctan-1-aminiumbromide (N2Q) and N-(3- (((E)-2-(2-((E)-(2-chloro-7-ethoxyquinolin-3-yl)methylene)hydrazineyl)thiazol-4(5H)-ylidene)amino)propyl)-N,N-dimethyloctan-1-aminium bromide (N3Q) were synthesized and characterized to detect Cr (VI) in the wastewater quite quickly, economically, and accurately. The sensors showed a superior selectivity to Cr (Ⅵ) oxyanions among other anions and cations and the quenching efficiency reached 98% at 125 µM. The fluorescence of the sensors is quenched by Cr (VI) oxyanions via electron transfer, electrostatic, and hydrogen bonding mechanisms as confirmed by optical spectra, FT-IR, Zeta potential, and DFT calculations. The N2Q sensor could lower the detection limits to 34 nM for CrO42- and 13 nM for Cr2O72-, with linear detection ranges of 0.0005–1025 nM. At the same time, the N3Q sensor had a good linear range of 5–125 µM and 0.0005–60 µM with detection limits of 18 and 11 nM for CrO42- and Cr2O72-, respectively. Furthermore, Cr (VI) oxyanions are detected in real samples, with recoveries ranging from 85.56 to 101%. More interestingly, portable, low-cost, and convenient-to-use paper devices are successfully used to visually detect Cr (VI) oxyanions at levels as low as 5 nM. The feasibility of using a smartphone readout device to determine Cr (VI) oxyanions has been established.