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

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СКАЧАТЬ ratio fluorescent probe 17 for Zn²⁺ based on rhodamine B salicylaldehyde fluorene derivative [28]. In general, salicylaldehyde hydrazone structure can generate a strong fluorescence enhancement response with zinc ions, but the fluorescence intensity is low in the absence of metal ions. In order to achieve the effect of the dual‐wavelength ratiometric response, the probe needs to have a similar quantum yield before and after the reaction with the metal ions. Rhodamine dyes have a longer excitation wavelength, higher quantum yield, and suitable water solubility. Combining 4‐N,N‐diethylamine salicylaldehyde hydrazone structure with rhodamine dyes, ratiometric zinc ion probe 17 was designed and synthesized. By exciting the fluorescence at the iso-absorption point at 400 nm, the maximum emission wavelength is red-shifted by 17 nm before and after the probe 17 binding with zinc ions, and the iso-emission point appears at 500 nm (Figure 3.10b, c). Under the UV lamp, a clear fluorescent color change of the solution from dark blue-green to yellow-green was achieved and observed by naked eye. The fluorescence intensity ratio at 530 and 475 nm is linear in the Zn²⁺ concentration range of 0–10 μM and is independent of other interfering ions. The minimum detection limit is 0.05 μM. Other probes containing rhodamine and salicylaldehyde hydrazone for Zn²⁺ detection were also synthesized, and the salicylaldehyde structure is found to be indispensable for ratiometric detection of zinc ions due to the different emissions of the probe and probe–zinc complex, as shown in Figure 3.10a.

      

Figure 3.8 (A) Illustration of the light‐up detection of Ca²⁺ with probe 16. (B) Photoluminescence (PL) spectra of 16 treated with CaCl₂ at different concentrations in phosphate‐buffered saline (PBS) buffer solution (pH = 7.4). (C) PL spectra of 16 in PBS buffer upon the addition of various metal ions and biological molecules. (D) Confocal laser scanning microscopy (CLSM) images of bovine bone microcracks by staining with calcein and 16: (a–c) whole‐projection images (z stack) and (d–f) 3D images of the microcrack stained with calcein; (g–i) whole‐projection images (z stack) and (j–l) 3D images of the microcrack stained with 16. (E) CLSM images of CaCl₂ embedded in calcium deposits in psammomatous meningioma slice: (d) the bright‐field images; (e) the fluorescence images; and (f) the merged images.

      Source: Reprinted from Ref. [17] (Copyright 2018 American Chemical Society).

A ratiometric fluorescent probe 18 using 3‐hydroxyflavones and salicylaldehyde hydrazone Al³⁺ ion was reported [26]. The fluorescence color of the probe 18 solution changed from yellow to white after the addition of aluminum ions due to the emission intensity at 461 nm being increased but the intensity at 537 nm decreased as shown in Figure 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.

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