A dual colorimetric and fluorescent probe for Cu²⁺ and Sn⁴⁺ by a novel spirooxazine chemosensor with high selectivity and sensitivity

A novel spriooxazine derivative, 1-benzyl-3,3-dimethylspiro[indoline-2,3 '-naphtho[2,1-b][1,4]oxazine] (BnSPO), was synthesized and its molecular structure was confirmed by H-1 NMR, C-13 NMR, mass spectrometry and single crystal X-ray diffraction analyses. The sensing performance of BnSPO was investigated. The results showed that BnSPO possesses the fluorescent "turn-on" activity against Cu2+ and the photo-absorptivity for both Cu2+ and Sn4+, all in the mixed solution of H2O and MeCN. The results revealed that BnSPO displayed high fluorescent selectivity toward Cu2+ with high sensitivity at the micromolar concentration. In case of Sn4+, the sensitivity of the probe towards this metal ion was also in the micromolar level. The limit of detections (LOD) of the probe are 21.4 nM and 140 nM for Cu2+ (fluorescent) and Sn4+ (colorimetric), respectively. Based on the results obtained from Job's plot, H-1 NMR titration, and mass spectrometry, the sensing mechanism of BnSPO for the detection of Cu2+ and Sn4+ was proposed through spirooxazine ring-opening followed by a 1:1 stoichiometric complexation of the resulting open-form merocyanine and the metal ions. The association constant (Ka) was calculated to be 2.31 x 10(4) M-1 and 7.28 x 10(1) M-1 for Cu2+ (fluorescent) and Sn4+ (colorimetric) detection. In addition, this solution-based detection was demonstrated for its applicability to the detection of metal ions in both drinking water and sea water. Moreover, the BnSPO-based test strips were also fabricated for rapid and selective detection of both Cu2+ and Sn4+ qualitatively, by the naked eye under both ambient and UV light.