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

Figure 3.24 (A) Chemical structures of 41 and 42. (B) Confocal images of HeLa cells stained with 10 μM 42 and containing with 50 nM LysoTracker Red. (C) Fluorescence images of 42‐stained HeLa cells before and after rapamycin treatment for different periods of time.

      Source: Panels (b) and (c) are adapted with permission from Ref. [66] (Copyright 2016 John Wiley and Sons).

      (D) Chemical structure of 43 and 44. (E) CLSM images of A549 cells incubated with 43 and 44, respectively.

      Source: Reprinted from Ref. [67] (Copyright 2016 American Chemical Society).

      (F) Chemical structure of 45. (G) Bright‐field and fluorescent images of E‐coli incubated with 45.

      Source: Reprinted from Ref. [65] (Copyright 2016 John Wiley and Sons).

A few numbers of SSB probes were also applied in bacterial imaging. For example, 45 is an amphiphilic SSB designed for light‐up detection of anionic surfactants. Due to the positive polarity of the quaternary ammonium salt, the probe can also be used for wash‐free imaging of bacteria enveloped by a negatively charged outer membrane. As Figure 3.24G shows, 45 performs high affinity to Escherichia coli and imaging with excellent contrast ratio.

Liu's group [63] also reported another SSB probe with “AIE + ESIPT” characteristic, which is based on a zinc‐coordinated salicylaldehyde hydrazine backbone for apoptotic cell membranes, LDs, and bacterial imaging. As Figure 3.25b shows, lipophilic 46 displays favorable affinity to LDs in both live and apoptotic HeLa cells. However, after complexation reaction with zinc perchlorate hexahydrate (Figure 3.25a), 47 becomes strongly hydrophilic even with exceptional water solubility, thus exhibiting a weak emission in water and high emission in water–THF mixtures with THF fractions higher than 80%. 46 and 47 displayed large Stokes shifts of 215 and 190 nm, respectively, with no overlap between the absorption and emission spectra. As illustrated in Figure 3.25a, since early apoptosis is characterized by partially exposed phosphatidylserine (PS) on the cell surface, positively charged probes can bind to PS to enhance fluorescence emission. For late‐stage apoptotic cells, the membrane integrity is completely compromised; 47 was found to light up nuclei specifically. As Figure 3.25c shows, 47 only stained the cytoplasm membrane of cells in early‐stage apoptosis induced by 1 μM staurosporine, while no obvious fluorescence signal was observed for healthy cells. The simple manipulation of the presence of metal ions can bring great changes to the properties of the probe and therefore lead to the alteration of the probe function for different applications. Another research also studied the photosensitization property of 47 and found its attractive performance in selective imaging and photodynamic extirpating of both Gram‐positive and Gram‐negative bacteria over mammalian cells [64]. As Figure 3.25d and e displays, due to the electrostatic interaction between the positively charged probe and the bacterial membrane, which has a more negative potential, 47 only emitted a strong green fluorescence upon incubating with bacteria (gram‐positive Bacillus subtilis or gram‐negative E. coli), yet showed nearly no signal with mammalian cells Jurkat T or K562. The probe was found to kill Gram‐positive bacteria due to depolarization of the bacterial membrane even in the dark. For Gram‐negative bacteria, 47 could generate ROS after white light irradiation for selective photodynamic killing.

3.3 Fluorescent Materials

       3.3.1 Solid Fluorescence Emitting and Stimuli‐Responsive Materials

      Organic solid fluorescent materials apply widely in organic light‐emitting diodes (OLEDs), photovoltaic devices, organic semiconductor lasers, fluorescent sensors, data storage, security printing, and anticounterfeit materials. Most conventional fluorescent molecules undergo fluorescence quenching in their aggregated state, and improvement of emission quantum yield and brightness are limited when designing for solid fluorescent materials. In contrast, the fluorescence enhancement of AIE molecules in the aggregated state has promoted their development in the field of solid fluorescent materials.

      SSB molecules exhibit the characteristics of ESIPT. On the one hand, their large Stokes shift weakens the self‐quenching effect and results in high quantum yields [68]. On the other hand, the ESIPT process can occur rapidly even at low temperature [69], which shows powerful advantages of these SSB molecules as solid fluorescent materials. Furthermore, SSB molecules usually show dual‐color emission and the fluorescence is susceptible to the foreign stimuli factors such as light, heat, mechanical forces, and organic vapor fumigation due to the variation of stacking mode and molecular arrangement in the solid sates, so it has great potential as stimulus‐responsive fluorescence sensing materials.

Figure 3.25 (a) Chemical structures of 46 and 47, and a schematic illustration of 46 for intracellular LD staining in healthy cell and 47 for the detection and membrane staining in apoptotic cells. (b) Confocal images of HeLa cells stained with 20 μM 46 and costaining with 0.5 μM Nile Red. c Confocal images of early‐stage apoptotic HeLa cells induced by 1 μM staurosporine for two hours, followed by incubation with 20 μM 47 for 30 minutes at 37 °C and stained with propidium iodide.

      Source: Panels (a–c) are adapted with permission from Ref. [63] (Copyright 2015 American Chemical Society).

      (d) Schematic illustration of 47 for selective targeting, imaging, and killing of bacteria over mammalian cells. (e) CLSM images of cells and bacteria incubated with 20 μM 47.

      Source: Adapted with permission from Ref. [64] (Copyright 2015 John Wiley and Sons).

A series of p‐carboxyl‐N‐salicylideneaniline