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

summarizes representative fluorescent quenching metal ion probes [10,13–16]. Among them, probe 3 is special, the fluorescence of which is first enhanced by combining with zinc ions and then quenched by adding cobalt(II) ions, achieving in detection of both ions in sequence.

Tong et al. prepared a Schiff base‐immobilized hybrid mesoporous silica membrane that can be used for the detection of Cu²⁺ in real‐water samples by immobilizing the Schiff base‐immobilized hybrid mesoporous membrane (SB‐HMM) on the pore surface of mesoporous silica (pore size 3.1 nm) [16]. Fluorescent probe 1 was grafted to the mesoporous silica surface embedded in the porous alumina membrane channels to form SB‐HMM (Figure 3.4a). Probe 1 is not emissive in the homogeneous solution, but SB‐HMM emits strongly due to the aggregation of SSB groups with ESIPT and AIE on the surface of the pores, which enhances fluorescence intensity. The high quantum yield of probe 1 on the surface of SB‐HMM can be used as a fluorescence sensor for Cu²⁺ in aqueous solution and has good sensitivity, selectivity, and reproducibility. SB‐HMM showed significant fluorescence decrease after Cu²⁺ (0–60 μM) was added, which showed good selectivity from other common metal ions (Figure 3.4b). Under the optimal conditions, the detection limit of SB‐HMM is 0.8 μM. In addition, SB‐HMM can be reused for Cu²⁺ sensing after regeneration from an acidic solution, resulting in a reusable dye‐doped fluorescent solid sensor that can be applied for Cu²⁺ sensing in aqueous solutions and other real Cu²⁺‐containing samples (Figure 3.4c).

Liu et al. synthesized SSB AIE Cu²⁺ probes 5 and 6. Compared to the dispersed state, the fluorescence enhancement factors of the two probes in poor solvents are about 300‐ and 34‐folds, respectively. After adding Cu²⁺, the emission intensities of 5 and 6 were significantly reduced [14]. The fluorescence of the solution changed from green to colorless under a 365‐nm UV lamp, and the detection limits for copper ions were 200 ± 23 and 10 ± 0.3 nM. Through dynamic light scattering (DLS) analysis, scanning electron microscopy (SEM), proton nuclear magnetic resonance (¹H‐NMR) titration, electrospray ionization‐mass spectrometry (ESI‐MS), and other analytical methods, it was found that after adding Cu²⁺, the imine bonds in 5 and 6 coordinate with the copper ions, under the strong chelation‐enhanced fluorescence quenching (CHEQ); the rigid probe aggregates are bent and rearranged to form a fluorescent‐quenched complex (Figure 3.5A). After adding excess EDTA, the fluorescence of the system could not be recovered, indicating the irreversibility of the process. Subsequently, 5 and 6 (10 μM) were applied to intracellular Cu²⁺ detection. The probe showed a strong green fluorescence under confocal microscope, and an obvious turn‐off was observed after Cu²⁺ was added, indicating the good cell membrane permeability and intracellular Cu²⁺ detection ability of the probe (Figure 3.5C).

Figure 3.3 Chemical structures of typical turn‐off metal ion probes 1–7.

Figure 3.4 (a) Scheme for immobilization of 4‐chloro‐2‐[(propylimino) methyl]‐phenol (4Cl‐PMP) groups on the surface of mesoporous silica in hybrid mesoporous membranes (HMM, (3‐aminopropyl)triethoxysilane (APTES)‐HMM, SB‐HMM). (b) Fluorescence spectra of SB‐HMM upon the addition of Cu²⁺ (0, 5, 10, 20, 30, 40, 50, and 60 M). The inset shows the changes in the fluorescence intensities of SB‐HMM with and without Cu²⁺ and other metal ions (80 (M): 1, K⁺; 2, Ca²⁺; 3, Fe²⁺; 4, Fe3⁺; 5, Zn²⁺; 6, Ni²⁺; 7, Cd²⁺; 8, Pb²⁺; and 9, Cu²⁺. I₀ and I are the excitation peak intensities of SB‐HMM without and with metal ions, respectively. (c) Regeneration/reuse cycles for SB‐HMM upon the addition of 80 M Cu²⁺.

      Source: Reprinted from Ref. [16] (Copyright 2011 Elsevier B.V.).

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.