An "on-off-on" fluorescent sensor based on Sm:ZnO-NH₂ QDs for hexavalent chromium detection

The problems of poor water solubility, poor stability, and poor selectivity encountered in the determination of hexavalent chromium (Cr(VI)) in water using ZnO QDs need to be addressed. In this study, we successfully prepared Sm-doped, -NH₂-modified Sm:ZnO-NH₂ QDs via the sol-gel method. Sm doping was used to enhance the fluorescence intensity of ZnO QDs, while 3-aminopropyltrietoxysilane (APTEs) capping improved their water solubility and fluorescence stability. The fluorescence of Sm:ZnO-NH₂ quantum dots was quenched after the addition of Cr(VI) due to the internal filtration effect(IFE), and was restored after the addition of ascorbic acid due to the redox reaction between ascorbic acid (AA) and Cr(VI). Leveraging the fluorescence response patterns of the Sm:ZnO-NH₂ QDs system when exposed to Cr(VI) and AA, we developed an ''on-off-on'' fluorescent sensor that can specifically detect Cr(VI) and AA without interference from Cu2⁺ ions. The "on-off-on" fluorescent sensor exhibited a linear response to Cr(VI) concentrations ranging from 0.05 to 1.5 μg/mL, with a limit of detection (LOD) of 6.15 ng/mL. It exhibited excellent selectivity and repeatability. Furthermore, the Sm:ZnO-NH₂ QDs fluorescent sensor was effectively utilized for detecting Cr(VI) in tap water, offering a new method for heavy metal detection via an "on-off-on" fluorescence switching mechanism.