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

3.11A. I₄₆₁/I₅₃₇ has a linear relationship with the concentration of aluminum ions (Figure 3.11B). The lowest detection limit measured under the experimental conditions is 0.29 μM (7.8 ppb), and the fluorescence response is stable in the pH range of 3.0–10.0. DLS showed that the average diameter of aggregates decreased significantly before and after the interaction with aluminum ions, indicating that aluminum ions and probe 18 could form a water‐soluble complex, but there were still aggregates of probe 18 in the solution, thus showing that a white color fluorescence originated from the mixed color of the probe aggregate and the complex. The coordination ratio between probe 18 and the aluminum ion is 1 : 1 based on Job's plot; NMR titration indicates that both hydroxyl groups in the probe molecule participate in the coordination with the aluminum ion. Selectivity experiments with other metal ions show that a significant increase in I₄₆₁/I₅₃₇ was only found in the presence of Al³⁺ (Figure 3.11C). HeLa cells were selected for intracellular experiments, and the fluorescence signal was only observed in the yellow light channel. With the addition of aluminum ions, the yellow light signal was basically unchanged under the confocal microscope, and the fluorescence of the blue light channel gradually increased, showing that probe 18 can successfully perform ratio imaging of aluminum ion in cells (Figure 3.11D).

Figure 3.9 (a) Chemical structures of ratiometric fluorescent probes 17–19 based on SSB. (b) The proposed 1 : 2 metal‐to‐ligand ratio binding model of probe 19 with Zn²⁺. (c) Absorption and ratiometric fluorescence spectrum response of probe 19 with Zn²⁺.

      Source: Reprinted from Ref. [27] (Copyright 2009 Elsevier B.V.).

       3.2.2 Biologically and Environmentally Related Molecular Detection and Imaging

Except large amounts of reports for metal ion detection, SSB derivatives were also designed for inorganic species, small organic molecules, as well as macromolecules in biologically and environmentally related analytes. Many reports concern for charged species detection; the probes are usually designed by ionizing AIEgens to increase their solubility in water and rendering them nonfluorescent or weakly fluorescent. In the presence of target molecules with opposite charges, molecular aggregates formed and fluorescence‐enhanced signal was achieved. A series of fluorescent probes with excellent detection properties have been developed by the functionalization of the salicylaldehyde moieties [30] and were successfully applied to the detection of inorganic species [31–38], environmental pollutants [7, 37], biological inorganic molecules [38, 39], amino acids [40, 41], proteins [42–44], enzymes [45–47], and so forth. Herein this section is divided into three aspects: (i) inorganic species, (ii) small biomolecules, and (iii) biomacromolecules to introduce the design and applications for SSB‐based probes in chemical and biological sensing.

Figure 3.10 (a) Chemical structure of probe 17 and binding mode with zinc ion verified by ¹H‐NMR and mass spectra. (b) Fluorescence intensity ratio (F₅₃₀/F₄₇₅) of 17 (10 μM) and 17 with Zn²⁺ (10 μM) in the absence and presence of different metal ions. (c) Absorption spectra of 17 (10 μM) in the presence of 0–50 μM Zn²⁺ in 90% (v/v) ethanol/water (10 mM (4‐(2‐hydroxyethyl)‐1‐piperazineethanesulfonic acid) (HEPES), pH 7.0). Inset: Job plot analysis of 17 and Zn²⁺ at a total concentration of 33 μM, indicating the formation of a 1 : 1 metal‐to‐ligand complex. (d) Fluorescence excitation (a) and emission (b) spectra of 17 (10 μM) in the presence of different concentrations of Zn²⁺ in 90% (v/v) ethanol/water (10 mM HEPES, pH 7.0). Inset: fluorescence intensity at 530 and 475 nm and the ratio of F₅₃₀ : F₄₇₅ as a function of Zn²⁺ concentration added to 17.

      Source: Reprinted from Ref. [28] (Copyright 2010 John Wiley & Sons, Ltd.).

Figure 3.11 (A) Fluorescence spectra of 18 (20 mM) upon the addition of different concentrations of Al³⁺ (0–500 mM) in 10 mM HEPES buffer at pH 7.0 (containing 0.2% DMSO). Excitation wavelength is set at 370 nm. (B) Calibration curve based on the ratio of fluorescence intensities (I₄₆₁/I₅₃₇) as a function of Al³⁺ concentrations; error bar represents three repeated experiments. (C) Fluorescence intensity ratio (I₄₆₁/I₅₃₇) of 18 (20 mM) upon the addition of 500 mM metal ions in 10 mM HEPES buffer at pH 7.0 (containing 0.2% DMSO). Ions from 1 to 18: blank, Li⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺, Ba²⁺, Sr²⁺, Fe²⁺, Fe³⁺, Co²⁺, Cu²⁺, Hg²⁺, Ag⁺, Cd²⁺, Mn²⁺, Ga³⁺, and Al³⁺. (D) Confocal fluorescence images of live HeLa cells. (a–d) The cells were incubated with 18 (5.0 mM) for 30 minutes; (e–h) the above cells upon the addition of 200 mM Al³⁺ were then incubated for another 20 minutes; (a) and (e) bright‐field transmission images; (b) and (f) blue channel images at 429–469 nm; (c) and (g) yellow channel images at 511–611 nm; (d) and (h) ratio images generated from (b) and (c) and (f) and (g), respectively. The excitation was set at 405 nm.

      Source: Reprinted from Ref. [26] (Copyright 2014 Elsevier B.V.).

The detection of inorganic species such as CN⁻, F⁻, UO₂²⁺, S²⁻, and ClO⁻ is of great significance in environment, biology, and industry. Cyanide (CN⁻) is one of the most powerful poisons. It affects vascular, vision, cardiac, endocrine, and metabolic functions and causes fatal damage to the nervous system. The fluoride ion (F⁻) is very useful in the treatment for osteoporosis and orthodontics. However, excess F⁻ leads to fluorosis, which results in the increment in the bone density. Taking the advantage of hydrogen bonding with the hydroxyl group of SSB, the detection of such basicity anions like CN⁻ and F⁻ is facile by SSB fluorescent probes with high sensitivity and satisfactory selectivity. As shown in Figure 3.12a and b, after adding CN⁻, probe 20 undergoes deprotonation due to the basicity of CN⁻. Cyclization reaction occurred under the catalysis of CN⁻, and the corresponding benzoxazole formed gradually in this process, therefore lighting up the fluorescence [34]. The detection limit is 5.92 × 10⁻⁷ M, lower than the WHO guideline of CN⁻ in drinking water (1.9 μM). The competitive experiments reveal high sensing selectivity and sensitivity of 20 for CN⁻ over other anions (Figure 3.12c). Test papers were also prepared for the practical application of cyanide detection. By influencing hydrogen bonding, SSB‐based fluorine ion fluorescent probes were also reported. Figure 3.12d lists some SSB probes for F⁻ detection. The possible processes and mechanisms are proposed, as demonstrated in Figure 3.12e. The addition of F⁻ resulted in a blue‐shifted