Fluorescence Chemosensory Ultrathin Films for Cd2+ Based on the Assembly of Benzothiazole and Layered Double Hydroxide

Fluorescence indicator/layered double hydroxide (LDH) ultrathin films (UTFs) have been fabricated by alternate assembly of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) and Zn-Al LDH nanosheets on quartz substrates using the layer-by-layer (LBL) deposition technique and demonstrated to be good fluorescence chemosensors for Cd2+. The stepwise and regular growth of the ABTS/LDH UTFs with increasing deposition cycle was monitored by UV-vis absorption and fluorescence emission spectroscopy; a periodic layered structure perpendicular to the substrates with a thickness of 2.40-2.58 nm per bilayer was observed by X-ray diffraction, atomic force microscopy, and scanning electron microscopy. The fluorescence anisotropy of the UTFs was also observed by the polarization fluorescence technique. In addition, a fluorescence chemosensory UTF (20 bilayers) for Cd2+ exhibited a low detection limit (9.5 nM), good repeatability (relative standard deviation less than 3% in 20 consecutive measurements), high photostability and storage stability (with similar to 94.1% of the initial fluorescence intensity remaining after 1 month), and excellent selectivity toward Cd2+ from Zn2+ in the physiological pH range (6.5-7.5). The regeneration of the fluorescence chemosensor can be achieved by the use of ethylenediamine tetraacetic acid (EDTA). The mechanism is based on the competitive complexation of Cd2+ between EDTA in solution and ABTS in the UTF, as confirmed by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and fluorescence anisotropy measurements. Therefore, this work provides a facile approach for the fabrication of chemosensors based on the incorporation of a fluorescence indicator into an inorganic two-dimensional matrix that can potentially be applied for the detection and measurement of Cd2+.