A Turn-On Fluorescent Chemosensor for Cyanide Ion Detection in Real Water Samples

Abstract

We have designed and synthesized a novel simple colorimetric fluorescent probe with aggregation-induced emission (AIE) properties. Probe 5-(4-(diphenylamine)phenyl) thiophen-2-formaldehyde W exhibited a turn-on fluorescent response to cyanide ion (CN^-), which induces distinct visual color changes. Probe W exhibited a highly selective and sensitive ratiometric fluorescence response for the detection of CN^- over a wide pH range (4-11) and in the presence of common interferents. The linear detection of CN^- over the concentration range of 4.00-38.00 µM (R ² = 0.9916, RSD = 0.02) was monitored by UV-Vis absorption spectrometry (UV-Vis) with the limit of detection determined to be 0.48 µM. The linear detection of CN^- over the concentration range of 8.00-38.00 µM was examined by fluorescence spectrophotometry (R ² = 0.99086, RSD = 0.031), and the detection limit was found to be 68.00 nM. The sensing mechanisms were confirmed by ¹H NMR spectroscopic titrations, X-ray crystallographic analysis, and HRMS. Importantly, probe W was found to show rapid response, high selectivity, and sensitivity for cyanide anions in real water samples, over the range of 100.17∼100.86% in artificial lake water and 100.54∼101.64% in running water by UV-Vis absorption spectrometry, and over the range of 99.42∼100.71% in artificial lake water and 100.59∼101.17% in running water by fluorescence spectrophotometry. Importantly, this work provides a simple and effective approach which uses an economically cheap and uncomplicated synthetic route for the selective, sensitive, and quantitative detection of CN^- ions in systems relevant to the environment and health.

Keywords: crystal structure; cyanide ion; fluorescent probe; real sample detection; synthesis.

SCHEME 1

Synthetic route to probe W.

FIGURE 1

(A) Fluorescence spectra of W (20 μM) in EtOH/water mixtures with different water fractions (λex/λem = 378 nm/724 nm, slit: 5/5 nm, and voltage: 800 V). (B) Plots of fluorescence intensity at 724 nm. Photographs in EtOH/water mixtures with different water fractions taken under (C) natural light and (D) 365 nm UV irradiation.

FIGURE 2

(A) The UV-Vis and (B) fluorescence spectra of fluorescence probe W (20 μM) in EtOH/water (VEtOH/VH2O = 3/2) at different pH values (λex/λem = 378 nm/479 nm, slit: 5/5 nm, and voltage: 500 v). Inset: (A) effect of different pH values on the absorbance of probe W at 382 and 576 nm. (B) Influence of pH values on fluorescence probe W at 478 nm.

FIGURE 3

(A) UV-Vis and (B) fluorescence spectra of fluorescence probe W (20 μM) in EtOH/water (VEtOH/VH2O = 3/2) with the addition of CN⁻ (40 μM) at different pH values (λex/λem = 378 nm/478 nm, slit: 5/5 nm, and voltage: 500 v). Inset: (A) effect of different pH values on the absorbance of probe W with the addition of CN⁻ at 382 and 576 nm. (B) Influence of pH values on fluorescence probe W with the addition of CN⁻ at 478 nm.

FIGURE 4

(A) UV-Vis and (B) the fluorescence spectra (λ ex/λ em = 378/478 nm) of fluorescence probe W interacting with different anions and amino-containing small molecules (slit: 5/5 nm, voltage: 500 V).

FIGURE 5

Bar diagram of the competitive experiments of various anions and amino-containing small molecules on the absorbance (A) and fluorescence intensity (B) of the probe/CN⁻ complex in buffer solution (λex/λem = 378 nm/478 nm, slit: 5/5 nm, and voltage: 500 V).

FIGURE 6

Bar diagram of the competitive experiments of various metal cations on the absorbance (A) and fluorescence intensity (B) of the probe/CN⁻ complex in buffer solution (λex/λem = 378 nm/478 nm, slit: 5/5 nm, and voltage: 500 V).

FIGURE 7

(A) UV-Vis absorption spectra on the addition of CN⁻ to the probe; (B) linear relationship between the ratio of absorbance at 382–576 nm and the concentrations of CN⁻ (4∼38 μM). Inset: the change curve of the ratio of absorbance at 382–576 nm with different concentrations of CN⁻; photograph of the solutions under illumination with sunlight showing the change of the solution after the titration is complete.

FIGURE 8

(A) Fluorescence spectra on the addition of CN⁻ to probe W (20 μM); (B) linear curve of fluorescence intensity of probe solution at λmax em = 478 nm and concentration of CN⁻ (8∼38 μM). Inset: curve of fluorescence intensity at λmax em = 478 nm with different concentrations of CN⁻; photograph of the fluorescence change under the irradiation of 365 nm UV lamp after the titration is complete.

FIGURE 9

Single crystal X-ray diffraction image of probe W.

FIGURE 10

Proposed sensing mechanism of probe W for CN⁻.

FIGURE 11

DFT calculation of W and W–CN⁻.

FIGURE 12

High-resolution mass spectra (HRMS) of the reaction product of probe W upon the addition of CN⁻.

FIGURE 13

NMR spectroscopic titration of the reaction product of probe W upon the addition of CN⁻.