Handbook of Aggregation-Induced Emission, Volume 2. Группа авторов
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Название: Handbook of Aggregation-Induced Emission, Volume 2
Автор: Группа авторов
Издательство: John Wiley & Sons Limited
Жанр: Химия
isbn: 9781119642961
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The fluorescence turn‐on metal ion probe emits no fluorescence or weak fluorescence and fluoresces strongly after interacting with metal ions. Compared with fluorescence turn‐off probes, the background fluorescence is weaker and thus higher signal‐to‐noise ratio and sensitivity, which is more preferable for the detection of metal ions in biological environments [17]. Most SSB turn‐on metal ion probes are mainly reported for Zn2+ detection. The specific molecular structures are summarized in Figure 3.6, as shown below. Probes 8–13 are zinc ion detection probes, and probes 14–16 are used for the detection of Al3+, Cu2+, and Ca2+, respectively [8,17–25].
Figure 3.5 (A) Proposed mechanism for AIE and self‐assembly of 5 or 6 by Cu2+. (B) Fluorescence spectra of probe 5 upon the addition of 0–10 equiv. Cu2+. Inset: Changes of intensity at 534 nm with [Cu2+]/[5]. (C) Human esophageal squamous KYSE510: (a) bright‐field image of cells incubated with 10 μM 5 for 30 minutes, (b) fluorescence image of 5, (c) fluorescence image of 5 in the presence 100 μM Cu2+ for 15 minutes, (d) bright‐field image of cells incubated with 10 μM 6 for 30 minutes, (e) fluorescence image of 6, (f) fluorescence image of 6 in the presence of 100 μM Cu2+ for 15 minutes at 37 °C.
Source: Reprinted from Ref. [14] (Copyright 2017 Elsevier B.V.).
Tong et al. reported a Zn2+ fluorescence turn‐on probe 8 based on SSB [24] (Figure 3.7). In a 99% water/DMSO mixed solvent, according to the gradual increase of the Zn2+ concentration, the absorption peaks at 310 and 346 nm in the UV absorption spectrum gradually decreased and the newly generated absorption peaks at 333 and 383 nm gradually increased. In the fluorescence titration performed under the same conditions, the initial fluorescence of the solution was very weak. With the addition of zinc ions, a gradually increasing fluorescence peak appeared at 460 nm, and its saturated fluorescence intensity reached 22‐folds compared with the initial intensity. A bright blue fluorescence was observed under UV light. Job's plot and ESI–MS results gave the binding ratio of the probe to the metal ion as 1 : 1, and the binding constant was calculated as 5 × 104 l/mol. The effects of different substituents of salicylaldehyde derivatives were studied by synthesizing various analogues of probe 8. At neutral pH, all substituted derivatives other than 4‐N,N‐diethylamine salicylaldehyde containing a strong electron‐donating group showed fluorescence turn‐on with zinc ions after condensation reaction with 2‐hydrazinopyridine. Among them, probe 8 exhibits the highest fluorescence enhancement and longer fluorescence emission wavelength. Under pH = 7, the detection linear range is 0.1–1 μM, and the detection limit is 30 nM. Immunity experiments show that only paramagnetic Cu2+ and Co2+ ions cause fluorescence quenching and affect zinc ion detection. Intracellular imaging of zinc ions was performed in HeLa cells, and significant intracellular fluorescence enhancement was observed under a confocal microscope.
Figure 3.6 Chemical structures of typical turn‐on SSB metal ion probes 8–16.
Figure 3.7 (A) Chemical structures of 8 analogues: 8a–f. (B) Fluorescence spectra of 10 mol/l 8 upon the addition of 0–15 mol/l Zn2+. Inset: the fluorescence intensity at 460 nm as a function of the Zn2+ concentration; excitation was at 364 nm. (C) Fluorescence images of probe 8 before and after adding zinc ion under UV light. The photographs on the right side show 8 in the absence and presence of 8 equiv. Zn2+ in a glass cuvette excited by sunlight and UV light (365 nm). (D) Fluorescence images of live HeLa cells. From left to right are bright‐field images, fluorescence images, and overlay images. Top (a–c): cells were incubated with 8 for 20 minutes and washed with TBS twice. Bottom (d–f): cells were incubated with 8 and then with Zn2+ (10 mol/l) and pyrithione (10 mol/l) for 20 minutes and washed with TBS twice. Emission was collected at 430–490 nm upon excitation at 405 nm.
Source: Reprinted from Ref. [24] (Copyright 2013 Elsevier B.V.).
The detection range of common metal ion fluorescent probes is generally at the nanomolar and micromolar levels. Few fluorescent probes detect metal ions at the millimolar level. This is because when the metal ion concentration is too high, an aggregation‐caused quenching (ACQ) effect may occur and thus result in an emitting annihilation of classical fluorescent probes. However, the Ca2+ concentration related to human diseases is generally in the millimolar range, which begets great challenges for the development of practically applied fluorescent probes for measuring the Ca2+ concentration in the human body. Tang's group has developed a calcium ion fluorescent probe 16 with a detection range of 0.6–3.0 mM, which is suitable for the detection of abnormally high blood calcium concentration (Figure 3.8A) [17]. The fluorescence quantum yield of the probe in the pure tetrahydrofuran (THF) solution was 0.23%, and it increased to 10.6% in the solid state. With the increase of Ca2+ concentration, the emission of probe 16 at 560 nm was significantly enhanced (Figure 3.8B). Some common metal ions and biological molecules (such as bovine serum albumin [BSA], porcine hemoglobin [PHB], and fetal bovine serum [FBS]) exhibit no interference with the fluorescence spectrum of the probe (Figure 3.8C). UV–vis absorption and high‐resolution mass spectrometry (HRMS) and isothermal titration calorimetry (ITC) determined that the coordination number between probe 16 and calcium ions was 1 : 1. Transmission electron microscopy (TEM) showed that probe 16 formed fibrous aggregates in the presence of calcium ions and irregular aggregates in the absence of calcium ions. The formation of regular aggregates induced by Ca2+ significantly enhanced fluorescence. Biological imaging of calcium deposits was performed in the soft tissue of human squamous meningioma and the cracks on the surface of the bovine bone to test the practical applicability of probe 16, and the results were satisfactory (Figure 3.8D, E). Compared with the commonly used calcium ion probe calcein, probe 16 shows advantages of a weak background signal, wash‐free imaging, and higher sensitivity.
Most metal ion probes are tested based on the fluorescence “on–off” mechanism, and the detection result is only related to the fluorescence intensity at a single wavelength, while the ratiometric fluorescence probe takes the advantage of the ratio change of the fluorescence intensity at two wavelengths for analyte detection, which can minimize errors caused by differences in experimental conditions and self‐emission of samples. Figure 3.9 shows a summary of ratiometric metal ion fluorescent probe structures based on SSB. Ratiometric fluorescent probes generally contain a stable fluorescent emission moiety and an active site that can react with metal ions. After reacting with metal ions, a fluorescent emission at another wavelength is generated, and the original emission peak intensity is unchanged or decreased, thereby realizing ratiometric detection [26–29].
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