Sensitive and selective ratiometric nanosensors for visual detection of Cu2+ based on ions promoted oxidation reaction

Copper is a highly toxic environmental pollutant in the environmental, biological and chemical fields. Therefore, sensitive, selective and visual detection of Cu 2+ is very important for human health and practical applications. Herein, a novel hybrid ratiometric fluorescent nanosensor was developed for rapid and on-site visualization of Cu 2+ on the basis of integrating benzothiazole fluorophore (MDBT) and red-emissive quantum dots (QDs) embedded in silica nanospheres. Such hybrid ratiometric fluorescent nanosensor exhibits dual emissions at 420 and 640   nm under a single excitation wavelength. Due to the ions promoted oxidation reaction of Cu 2+ towards MDBT, the fluorescence of MDBT can be selectively enhanced. Although the embedded QDs are insensitive to the analyte, variations of the dual emission intensity ratios display continuous colour changes from red to blue. The ratiometric fluorescent approach shows high sensitivity and selectivity towards Cu 2+ against other interfering metal ions. The detection limit of Cu 2+ for the hybrid ratiometric fluorescent nanosensor is determined to be 41.71   nM, which is much lower than the allowable level of Cu 2+ (∼20   μM) in drinking water set by U.S. Environmental Protection Agency. Furthermore, this kind of novel nanosensor can also be applied for Cu 2+ detection in actual water samples and exhibits excellent detection ability. This hybrid ratiometric fluorescent nanosensor is simple, fully self-contained and thus potentially attractive for visual identification without the need of elaborate equipments.