Handbook of Aggregation-Induced Emission, Volume 2

Lu et al. developed a fluorescence‐amplified AIE probe by incorporating a hydrophobic AIE fluorophore 2‐1into biocompatible F127‐FA nanoparticles. A significant FRET effect took place through encapsulating donor 2‐1and acceptor bis(4‐( N ‐(2‐naphthyl)phenylamino)‐phenyl)fumaronitrile (NPAPF) simultaneously into the nanoparticles, which resulted in the notable increase of acceptor emission. Furthermore, 2‐1and NPAPF coloaded F127‐FA nanoparticles showed a large Stokes shift, spherical morphology, and low cytotoxicity [75].

Stable AIEgen‐based nanoparticles of different shapes were prepared by assembling the copolymer and DSA and applied in noninvasive long‐term imaging. The nanoparticles have excellent physical and photostability under physiological conditions. The in vitro experimental results verified that these AIE‐active organic nanoparticles are biocompatible and internalized through various pathways of cellular uptake. The long‐term imaging ability was verified by the in vitro and in vivo experiments. It was confirmed that the rod‐like nanoparticles were significantly more internalized than the spherical particles, and they exhibited a better imaging effect [76].

Tian et al. prepared fluorescent nanoparticles by constructing a coassembly of human papillomavirus (HPV) capsid protein L1 and a complex consisting of DNA and an AIE molecule 3‐5. The virus‐like particles (VLPs) of HPV encapsulate the complex via electrostatic interaction. The coassembled nanoparticles, 3‐5‐DNA@VLPs, demonstrated a homogeneous size of ∼53 nm and enhanced fluorescence. Notably, the strong cell‐entry ability of VLPs endowed the AIE–DNA complex to enter the cell easily, which led to efficient brighter imaging in HeLa and normal 293T cell lines. Therefore, this work supplied a powerful approach to deliver the AIEgen into cells, which not only provided a simple, fast, biocompatible, and highly efficient fluorescence tool for in vivo cell imaging but also largely extended the bioapplications of AIEgens [77].

AIEgen‐based poly(l‐lactic‐ co ‐glycolic acid) (PLGA) magnetic nanoparticles to localize cytokine vascular endothelial growth factor (VEGF) were developed. The nanoparticles, as a novel theranostic system, could be utilized for simultaneous photothermal therapy and magnetic resonance imaging, which was applicable to early diagnostics and treatment of cancer cells. The system was constructed by loading the oleic acid‐Fe 3O 4and an AIEgen, triphenylamine‐divinylanthracene‐dicyano, inside the PLGA shell, which was then modified with anti‐VEGF‐A antibodies. By adjusting the proportion of AIEgens and Fe 3O 4, this system showed a strong red emission desirable for early cancer cell diagnosis, as well as magnetic properties suitable for application in magnetic resonance imaging [78].

Besides the AIE small molecules, there are also some AIE macromolecules applied in the fluorescent bioimaging. A series of random copolymers (polymer 3‐6in Figure 2.8) based on poly[ N ‐(2‐hydroxypropyl)methacrylamide] (PHPMA) and hydrophobic AIE fluorophores DSA were prepared. It indicated that enhancing the molar fractions of the hydrophobic AIE fluorophores can lead to increasing the quantum efficiencies of the copolymers. These polymers were almost nonemissive in the solvent of dimethyl sulfoxide (DMSO) but emitted strong fluorescence in the mixed solvent of DMSO/water. It was confirmed that these polymers formed small micelles with an average diameter of about 10 nm in the aqueous solutions. Although the DSA fluorophore is water‐insoluble, when it chemically conjugates with the biocompatible polymer PHPMA, the prepared final copolymers, which are noncytotoxic to living cells, can be applied in biological condition for bioimaging [23]. In addition, by using PHPMA and DSA fluorophores, they prepared a new series of random copolymers (polymer 3‐7in Figure 2.8). Similarly, the fluorescence quantum yields of the AIE copolymers in aqueous solutions increase with increasing the molar fractions of the hydrophobic AIE fluorophores and/or the trifluoromethyl moieties. It was the first report that the AIE fluorophores were integrated with fluorine‐containing polymers to manipulate the quantum yields of the AIE fluorophores [79].

Tian et al. prepared new polymer dots (AIE Pdots) through a self‐assembly process by using an AIE‐conjugated block copolymer 3‐8. The copolymer contains an AIE fluorophore DSA, hydrophobic poly(ɛ‐caprolactone) segments, hydrophilic poly(ethylene glycol) segments, and folate groups. The AIE Pdots have an average diameter of 15 nm and exhibited a good monodispersity in H 2O. They possess a high solid‐state fluorescence quantum efficiency of Φ = 27.0%. Moreover, the AIE Pdots presented good stability and low toxicity to living cells. The biological imaging results showed that the folic acid‐functionalized AIE Pdots can be applied in targeted HeLa intracellular imaging [80].

Tian et al. successfully designed and synthesized an AIE macromolecule 3‐9by combining the red‐emissive fluorophores with propeller‐shaped AIE fluorophores based on DSA. Then, the ultrabright red‐emissive AIE dot 3‐9@PS‐PVP ( 3‐9in poly(styrene)‐poly(4‐vinylpyridine nanoparticles)) was fabricated by using 3‐9as the core and biocompatible PS‐PVP as the encapsulation matrix. The prepared AIE dots presented a spherical morphology and uniform small size. In addition, they have good stability in water and little cytotoxicity to living cell. Furthermore, the dots can stain both the cytoplasm and the nuclei and emit a strong red fluorescence signal. Figure 2.10a–c shows the confocal laser scanning microscopy images of HeLa cells after incubation with the AIE dots for 16 hours at 37 °C. Figure 2.10d illustrates a possible mechanism of the uptake of the AIE dots by the nuclei [81]. The pyridine salt on the outer layer of the nanoparticles carries positive charges, which was obtained through protonating the PVP chains by the hydrochloric acid. This pyridine salt is like the quaternary ammonium salt, which has a high affinity with negatively charged DNA (binding constant of ∼10 5M −1) [82]. Therefore, the surface of the dots could absorb DNA or RNA. Benefiting from their small size simultaneously, the dots could go along with DNA or RNA into the nuclei when the genetic substance delivery process occurs [83–85].

Figure 2.10 Confocal laser scanning microscopy images of HeLa cells after incubation with 3‐9@PS‐PVP ( 3‐9in poly(styrene)‐poly(4‐vinylpyridine nanoparticles) (with a fluorogen loading of 10%) for 16 hours at 37 °C. (a) Fluorescence image; (b) bright‐field image; and (c) overlay of (a) and (b). Concentration of AIE dots: 0.15 mg/ml. (d) Illustration of the possible mechanism of the uptake of the AIE dots by the nuclei [81].

Source : Reproduced from Ref. [81] with permission from the Royal Society of Chemistry.

Wei et al. utilized phospholipid monomer firstly to fabricate cross‐linked fluorescent polymeric nanoparticles by using the AIE dye 3‐10based on DSA fluorophore [86]. The prepared nanoparticles exhibited strong fluorescence and stable dispersibility in physiological solution below the critical micellar concentration. Moreover, the nanoparticles also have excellent biocompatibility, which makes them have potential applications in cell imaging.