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

Чтение книги онлайн.

СКАЧАТЬ SSB or SSB–metal complexes can be performed.

      Since it was first reported in 2009 [7], the AIE properties of SSB molecules have been widely applied in designing fluorescent probes and fluorescent functional materials in chemistry, biology, and environmental science. This chapter summarizes the design and application of SSB as AIEgens of fluorescent probes and materials, for detection and imaging of metal ions, for biologically and environmentally related molecules, and as stimuli‐responsive materials and nanoparticles (NPs).

3.2 Fluorescent Probes

       3.2.1 Metal Ion Detection and Imaging

      Metal elements exist widely in nature and have applications in various fields of human daily life. Many metal ions support normal life processes and play an irreplaceable role in the organism. For example, as the second messenger in cells, calcium ions are of great importance in the process of signal transmission. Another example is iron ions, which are converted to each other in the form of ferrous and iron in human body. Inadequate intake of iron ions can cause diseases such as anemia and dysplasia, while excessive intake of iron ions can cause oxidization to damage the body, thereby endangering the human heart and circulatory system. In contrast, some metal ions, even when present in trace amounts in the environment, can cause great harm to living organisms. For example, even traces of chromium(VI) ions enter the human body; it will cause serious damage to human skin, respiratory system, kidneys and other tissues, and even cancer. Therefore, the development of simple and practical metal ion detection methods has always attracted great interests of researchers.

Schiff base compounds have lone‐pair electrons on the nitrogen atom in the structure, so that they can complex with metal ions, thereby changing the overall structure and properties of the compound. Due to the simple synthesis of Schiff base compounds, high yields, rich sources of raw materials, and wide choices, especially their good coordination properties, they have been widely used in the preparation of metal ion complexes. SSB fluorophores are therefore with unique advantages in AIE probes for metal ion detection. Additionally, nitrogen in the imine bond and hydroxyl oxygen atoms contained in SSB have excellent metal coordination ability, and the formed metal ion complex has a large stability constant, which is helpful to reduce the detection limit [8]. Because the coordination ability is influenced dramatically by the hydroxyl oxygen atom, which will experience deprotonation at high pH conditions, by adjusting the pH value, the probe can be recycled, and “logic gate” systems for the detection of both metal ions and pH can be designed accordingly [9–11]. In this section, SSB‐based metal ion probes are classified according to different response mechanisms, and three different types of turn‐off, turn‐on, and ratiometric are included.

Turn‐off metal ion probes have strong fluorescence in their aggregate state and fluorescence quenching after binding with target metal ions. Strong fluorescence comes from the AIE property of the probe, and after binding with metal ions, charge transfer between the metal ion and the probe molecule, i.e. metal‐to‐ligand charge transfer (MLCT), occurs, thus resulting in chelated fluorescence quenching. Most reported typical turn‐off metal ion probes are for Cu²⁺, which is due to the special electronic structure of divalent copper ions. The d9 valence electron layer configuration, which has a single electron, is prone to fluorescence quenching due to MLCT process [12]. Figure 3.3 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: СКАЧАТЬ