Handbook of Aggregation-Induced Emission, Volume 2. Группа авторов

with an increasing concentration of F⁻. This change was explained by the disruption of the existing six‐membered hydrogen bonding, involving the hydroxyl group and imine nitrogen, which allow the ESIPT process through deprotonation of the phenolic hydroxyl group. Excellent experimental results of detection limits and selectivity over other anions were also obtained in real sample detection (Figure 3.12f, g).

Chen et al. reported a fluorescence turn‐on SSB probe 25 for uranyl ion (UO₂²⁺) detection with high efficiency [32]. As one of the radioactive metal elements, uranium is an important raw material for the nuclear industry. At present, uranium‐based nuclear power facilities have gradually increased their proportion in power generation facilities in various countries. Metal uranium is extremely radioactive and chemically toxic, with a half‐life of hundreds of millions of years, which can cause lasting disturbances and damage to the immune, reproduction, and hematopoietic systems of the organism. Uranyl ion (UO₂²⁺) is the most commonly existing formation of uranium in natural water; therefore, the detection of UO₂²⁺ in water is of great significance for the assessment of water pollution. As illustrated in Figure 3.13a, 25 undergoes a complex interaction and forms aggregates with the addition of UO₂²⁺, exhibiting a fluorescence enhancement at 540 nm, which was linearly related to the concentration of UO₂²⁺ in the range of 1–25 ppb. The limit of detection was achieved as low as 0.2 ppb with a relative standard deviation (RSD) of 1.3% (Figure 3.13b, c). Such a detection method was successfully utilized in quantifying UO₂²⁺ in fuel processing wastewaters. Other fluorescent probes based on salicylaldehyde azine derivatives for the detection of sulfur(38) (S²⁻), hypochlorite(37) (ClO⁻), and so on were also developed by the modification of a metal complex of SSB.

In addition to necessary metal ions and nonmetal ions, a number of biologically small molecules are also important components to maintain the normal metabolic activities of complex biological organisms. For instance, pyrophosphate ion (PPi) is an important biologically related inorganic species, which plays an important role in the synthesis of DNA, RNA, and proteins and in life activities such as signal transduction. Studies have shown that the occurrence of arteriosclerosis and osteoarthritis is closely related to abnormal pyrophosphate levels in human body. In recent years, the analysis and detection of pyrophosphate have attracted increasing attention of scientists. Pyrophosphate has a strong coordination with Cu²⁺. Most fluorescent detection methods for pyrophosphate are based on this principle. Due to the superior coordination ability of SSB and Cu²⁺, SSB–copper(II) complex is therefore very suitable for the design and synthesis of pyrophosphate probes. As Figure 3.14a shows, Tong and coworkers developed a facile PPi fluorescence turn‐on probe 26 of copper(II) complex [39]. The AIE fluorescence of 26 was completely quenched due to the coordination of Cu²⁺. The complexation of PPi with Cu²⁺ resulted in the release of free 26, which reaggregated in the solution and recovered the orange fluorescence. Figure 3.14b reveals good selectivity of the probes with other common anions. The fluorescent signal enhancement showed excellent linear relationship with a PPi concentration range of 0–15 μM, and the detection limit was obtained as 0.064 μM.

Figure 3.12 (a) Chemical structures of cyanide probes 20 and 21. (b) Proposed sensing mechanism of sensor 20 for the detection of cyanide. (c) Fluorescence spectra (λ_ex = 347 nm) and photographs of fluorimetric (excitation at 365 nm) responses of 20 (50 μM) before and after the addition of various anions (100 equiv.) in water with cetyltrimethylammonium bromide (CTAB).

      Source: Panels (b) and (c) are reprinted from Ref. [34] (Copyright 2017 Royal Society of Chemistry).

      (d) Chemical structures of fluoride probes 22, 23, and 24. (e) The possible processes and mechanisms involved in the SSB probe with fluorine ions. (f) Linear relationship of 22 with the concentration of F⁻ ions. (g) Column diagrams of the fluorescence intensity of 22 with tetrabutylammonium (TBA) salts at λ_max 486 nm; red bars represent the addition of various anions to the blank solution, and black bars represent the subsequent addition of F⁻ (2 equiv.) to those respective solutions (22 + A⁻ + F⁻).

      Source: Reprinted from Ref. [31] (Copyright 2016 Elsevier B.V.).

Figure 3.13 (a) Design rationale of the fluorescence turn‐on detection of UO₂²⁺ based on AIE characteristics of 25. (b) Fluorescence spectra (λ_ex = 370 nm) of 25 (30 mM at pH 10.3) in the presence of different amounts of UO₂²⁺. (c) Linear relationship of 25 with the addition of different amounts of UO₂²⁺.

      Source: Adapted with permission from Ref. [32] (Copyright 2014 Elsevier B.V.).

Figure 3.14 (a) Schematic illustration of the PPi detection mechanism of 26 copper(II) complex. (b) Fluorescent intensity response at 570 nm of 26 copper(II) complex with different anions in a 20% DMSO aqueous solution.

      Source: Reprinted from Ref. [39] (Copyright 2015 Royal Society of Chemistry).

Cysteine (Cys) as one of the abundantly existing amino acids plays an indispensable role in physiological activities such as metabolism in complex biological organisms. However, how to improve the selectivity of the probe for cysteine and effectively distinguish it from homocysteine and glutathione in actual detection is a challenge for the fluorescence detection of Cys. Tong et al. reported an SSB‐based fluorescence turn‐on probe 27 for the detection of cysteine over homocysteine and glutathione [40]. As represented in Figure 3.15a, the cyclization reaction between Cys and the acryloyl ester on 27 results in the following hydrolyzation of 27 to produce SSB fluorophore with both AIE and ESIPT. Due to the selectivity of the cyclization reaction efficiency, this method is more selective for cysteine than for homocysteine and glutathione. The linear range of cysteine detection in the buffer is 0.5–30 μM, and the detection limit is 0.46 μM. This method was also applied for the effective quantitation of Cys in FBS.

Figure 3.15 (a) Cyclization reaction of 27 with Cys followed by hydrolysis to give the final SSB fluorophore. (b) Fluorescence spectra of 27 in the presence of different amounts of