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

       3.3.2 Nanoparticles

SSB derivatives are also designed to form NPs, which will allow the fluorescent material much more stable with multifunctionalities applicable for material science, bioimaging, and PDT. Two common design principles for SSB NPs are generally used. One way is growing a transparent shell such as silica out of the SSB fluorogens' aggregate core. As Figure 3.31a and b shows, SSB fluorescent aggregates encapsulated in silica nanoparticles (SiNPs) were reported according to the Stober standard method [77]. Three SSB‐based AIEgens 73, 74, and photoresponsive compound 75 were noncovalently embedded into SiNPs during the polymerization of silicate ester monomers to prepare AIE luminogen‐embedded fluorescent SiNPs (AIE‐FSNPs‐1–3). Compared with the conventional ACQ fluorophore fluorescein embedded in SiNPs prepared in the same way, AIE‐FSNPs exhibit an ~10‐fold fluorescent intensity; therefore, they show much higher sensitivity in further analytical application. Additionally, AIE‐FSNPs display satisfactory stability to external environmental variations. After experiencing multiple washings or under different pH buffers, the fluorescent spectra of AIE‐FSNPs show no obvious change. By covalently modifying AIE‐FSNPs with DNA aptamer AS1411, Apt‐AIE‐FSNPs were prepared and showed specific binding to nucleolin overexpressed on the surface of various cancer cells (MCF‐7, HeLa, etc.), thereby distinguishing cancer cells from normal cells in cell imaging. As shown in Figure 3.31c, Apt‐AIE‐FSNPs‐1 emitted a strong green fluorescence under a 405‐nm laser excitation after incubating with human breast cancer cells MCF‐7 but exhibited no obvious fluorescent signal with normal cells MCF‐10A, indicating their perfect performance in specific cancer cell recognition. More interestingly, as shown in Figure 3.31d, since the fluorescence of 75 was caged by photolabile group o‐nitrobenzyl, 75‐encapsulated Apt‐AIE‐FSNPs‐3 initially emitted no fluorescence incubated with MCF‐7. After irradiation with a 365‐nm UV light, the o‐nitrobenzyl group was left to recover the orange fluorescent signal. Such photoactivatable characteristics give Apt‐AIE‐FSNPs‐3 unique advantages in the selective imaging of target cells at a specific location of interest by controlling the site of UV irradiation at the desired time.

Another way to produce SSB‐based AIE NPs is to initiate self‐assembly of the monomer. By means of designing SSB fluorophores into the main chain of a polymer [78] or mixing with other small molecules [57], SSB dyes self‐assembled into substable NPs with fascinating functions in imaging or cancer therapy. As Figure 3.32a illustrates, Tang's group reported a dual‐organelle‐targeted NPs with synergistic chemo‐PDT functions [57]. Via self‐assembly of AlPcSNa₄ and AIE‐Mito‐TPP(39) through electrostatic, hydrophobic, and π–π interactions, the formed AIE‐Mito‐TPP/AlPcSNa₄ NPs almost did not show fluorescence due to the fluorescence resonance energy transfer (FRET) process between 39 and AlPcSNa₄ and the self‐quenching of π‐planar AlPcSNa₄ in the aggregation state. After uptake by cancer cells through endocytosis, NPs decompose rapidly in acid lysosomes, which releases 39 and AlPcSNa₄ in cytoplasm and subsequently light up mitochondria and lysosome in green and red fluorescence, respectively (Figure 3.32b). After decomposition, 39 effectively destroys mitochondria by reducing the mitochondrial membrane potential and inhibiting ATP synthesis, while AlPcSNa₄ effectively destroys lysosomes through ROS production under white light irradiation. As a result, the in vivo tumor growth could be efficiently inhibited (Figure 3.32c), which indicates AIE‐Mito‐TPP/AlPcSNa₄ NPs to be promising candidates for dual‐organelle‐targeted chemo‐PDT synergistic therapeutic strategy for cancer cells and tumor treatment.

Figure 3.31 (a) Chemical structures of 73, 74, and the reaction mechanism of photoactivatable fluorescence property of 75. (b) Synthesis of AIE‐FSNPs‐1 and the subsequent conjugation of DNA aptamers on the silica nanoparticles. (c) Fluorescence images of MCF‐7 and MCF‐10A cells incubated with Apt‐AIE‐FSNPs‐1. (d) Fluorescence images of MCF‐7 cells incubated with Apt‐AIE‐FSNPs‐3 before and after UV irradiation.

      Source: Reprinted from Ref. [77] (Copyright 2016 American Chemical Society).

Figure 3.32 (a) Schematic illustration of dual‐organelle‐targeted NPs with synergistic chemo‐photodynamic therapy functions through self‐assembly of mitochondria‐targeted chemotherapeutic agent AIE‐Mito‐TPP(39) and lysosome‐targeted photosensitizer AlPcSNa₄. (b) The time‐dependent CLSM images of A375 cells treated with the AIE‐Mito‐TPP(39)/AlPcSNa₄ NPs. (c) The tumor volume index of nude mice under control and treatment with AIE‐Mito‐TPP(39), AIE‐Mito‐TPP (+L), AlPcSNa₄, AlPcSNa₄ (+L), AIE‐Mito‐TPP/AlPcSNa₄ NPs, and AIE‐Mito‐TPP/AlPcSNa₄ NPs (+L).

      Source: Reprinted from Ref. [57] (Copyright 2018 John Wiley and Sons).

3.4 Summary and Perspectives

      In summary, as a typical class of AIEgens, SSB has been widely designed as fluorescent probes as well as materials. Thanks to the ESIPT characteristics, SSB fluorophores have large Stokes shift without self‐absorption, thus avoiding the interference from excitation light and performing satisfactory resistance to photobleaching. The spatial position of the oxygen atom on the hydroxyl group and the nitrogen atom on the imine in the specific molecular structure of SSB analogues allows it to strongly coordinate with metal ions such as copper(II) and zinc(II). Such a unique molecular structure endows SSB as an ideal candidate for designing metal ion fluorescent probes with high sensitivity and selectivity. Meanwhile, using the coordination properties of SSB with metal ions, SSB–metal complexes are utilized as nonmetal ion probes or stimuli‐responsive materials. Through the protection and deprotection of hydroxyl groups, SSB derivatives are modified with various recognition groups for the design of fluorescent bioprobes and functional materials. In addition, the hypsochromic shift of most SSB analogues after deprotonation attributes to their ratiometric fluorescence, which significantly reduces background interference during real sample detection. The luminescent advantage and ease of derivatization afforded by the unique molecular structure of SSB allow its widespread application in high‐resolution metal ion detection, environmental and biosensing, as well as functionalized optical materials.

On the other hand, the limitation and challenges in the development and application of SSBs have also achieved increasing attention and research interests in recent years. Although SSB fluorophore can be synthesized and purified very facilely by aldol condensation and recrystallization, the active hydroxyl group, the easily decomposed imine structure, and the too small solubility product (K_sp) all make SSB molecules limited for subsequent functionalized modifications and detection in complex environments. In terms of photophysical properties, the excessive Stokes shift is due to a large internal loss in the emission process, which inevitably leads to a decrease in the fluorescence quantum yield of the compounds themselves. In view of developing full‐spectrum‐emissive AIEgens, it is still difficult to achieve an emission below