Strategy to Design a Flexible and Macromolecular Sensor to Bind Cd2+ Ions: A Complete Photophysical Analysis and Bio-Imaging Study

A novel triazole-bridged coumarin–benzimidazole-conjugated fluorescence sensor (4) has been developed for selective detection of Cd2+ over other competitive metal ions. The sensor exhibited quick “turn-on” responses upon interaction with a very low level of Cd2+ (14 nM). The photophysical changes in the complexation of Cd2+ with sensor 4 have been explained through the excited-state intramolecular proton transfer mechanism. The involvement of benzimidazole and triazole moieties in Cd2+ binding was confirmed by different spectroscopic techniques such as UV–vis, Fourier transform infrared, nuclear magnetic resonance, and ESI mass. The diameter of the circular shape of the sensor decreased upon complexation with Cd2+, which was confirmed by field-emission scanning electron microscopy. Furthermore, the quantum chemical (density functional theory) calculation supported the mechanism of interactions and the mode of binding of 4 toward Cd2+. The sensor was more effective for finding Cd2+ in two living cells, C6 (rat glial cell) and Hep G2 (human liver cell).

本研究开发了一种新型三唑桥联香豆素-苯并咪唑共轭荧光传感器（4），可在多种竞争金属离子存在下选择性识别镉离子（Cd²+）。该传感器与极低浓度（14 nM）的镉离子作用时，可快速呈现荧光“点亮”响应。传感器4与镉离子形成配合物过程中的光物理变化，可通过激发态分子内质子转移（excited-state intramolecular proton transfer, ESIPT）机制予以阐释。苯并咪唑与三唑基团参与镉离子配位的结论，可通过紫外-可见光谱、傅里叶变换红外光谱（Fourier transform infrared, FT-IR）、核磁共振波谱（nuclear magnetic resonance, NMR）及电喷雾电离质谱（ESI mass, ESI-MS）等多种光谱技术得以证实。场发射扫描电子显微镜（field-emission scanning electron microscopy, FE-SEM）表征结果显示，传感器4与镉离子形成配合物后，其圆形形貌的直径有所减小。此外，量子化学密度泛函理论（density functional theory, DFT）计算结果验证了传感器4与镉离子的相互作用机制及配位模式。该传感器可有效在两种活细胞——C6（大鼠神经胶质细胞）与Hep G2（人类肝细胞）中实现镉离子检测。