Spirooxazine-Based Dual-Sensing Probe for Colorimetric Detection of Cu2+ and Fe3+ and Its Application in Drinking Water and Rice Quality Monitoring

A spirooxazine derivative, PheSPO (3,3-dimethyl-1-phenethylspiro-indoline-2,3'-naphtho[2,1-b][1,4]oxazineD, as a dual-sensing probe for Cu2+ and Fe3+ was synthesized, and its structure was confirmed by H NMR, C-13 NMR, HRMS, and single-crystal X-ray diffraction. The results reveal that the PheSPO probe is selective to both Cu2+ and Fe3+ through distinct colorimetric responses in acetonitrile. The sensing performance of PheSPO toward Cu2+ was investigated, and upon addition of Cu2+, an instant change in color from colorless to bright yellow with a strong absorption band at 467 nm was observed. Due to a dual-sensing behavior, PheSPO also exhibits a unique response toward Fe3+ that can be discovered from a color change from colorless to red at an absorption wavelength of 514 nm. Based on spectroscopic analyses and density functional theory calculations, the 1:1 stoichiometric complexation of PheSPO with the targeted metal ions was proposed and the binding constants of 1.95 x 10(3) M-1 for Cu2+ and 1.29 x 10(3) M-1 for Fe3+ were obtained. In addition, the detection limits of PheSPO for Cu2+ and Fe3+ were 0.94 and 2.01 mu M, respectively. To verify its applicability in real samples, PheSPO was further explored for quantitative determination of both Cu2+ and Fe3+ in spiked drinking water. The results showed that the recoveries of Cu2+ and Fe3+ examined using the PheSPO probe were found comparable to those obtained from atomic absorption spectroscopy. Moreover, the PheSPO strip test was developed, and its utilization for qualitative detection of Fe3+ in real rice samples was demonstrated.