Application of Aggregation-induced Red Emission Materials in Bioimaging

Recently, fluorescent bioimaging technology has developed rapidly in the field of biomedical research, providing a direct visualization method for life science research and disease diagnosis. Organic fluorescent dyes are commonly used fluorescent reagents with the merits of great diversity, tunable structures and emission colors. However, organic fluorescent dyes mostly possess extended π-conjugation and rigid planar conformations, they also emit strong fluorescence in good solvent. By virtue of the strong hydrophobic property, they are prone to form aggregates in the living body, resulting in a sharp decrease in fluorescence intensity and weak fluorescence signals. This phenomenon is termed as "aggregation-caused quenching (ACQ)", which severely limits the practical application of organic dyes in fluorescence imaging technology. Aggregation-induced emission (AIE) materials show faint emission or even non-emission when dissolved in good solvents, while exhibit intensive fluorescence emission in the aggregate and/or solid states. Aggregation is the nature of hydrophobic group, the fluorescence intensity of AIE materials increases significantly after aggregation, which fundamentally overcomes the defects of ACQ materials. Due to the unique fluorescence property, AIE materials have attracted great attention from researchers worldwide since they were discovered. A large number of AIE molecules have been synthesized, and they have shown great application potential in the field of organic light-emitting devices (OLEDs), fluorescent chemosensors, bioassays and bioimaging. The majority of AIE materials are blue, green or yellow emitters, only very few are red ones. However, red emitting AIE materials with red fluorescence and AIE features are indispensible in both fundamental research and applied research, which have a variety of advantages in the field of bioimaging. On the one hand, red fluorescence possesses strong penetration ability, low excitation energy and faint background fluorescence interference. On the other hand, red emissive AIE molecules aggregate to emit strong fluorescence, therefore, they can be applied in bioimaging to enhance fluorescence intensity. Besides, nanoparticles (NPs) with strong photobleaching resistance, low toxicity and controllabilty can be prepared via red emissive AIE molecules, which are expected to be the substitution of inorganic quantum dots for applications in biological assays and medical imaging. Therefore, red emissive AIE materials have a wide application prospect in bioimaging. In this paper, a series of representative red emissive AIE materials are demonstrated, red emissive AIE small molecules, nanoparticles formed by physical cladding red emissive AIE molecules, covalently bonded red emissive AIE molecules and copolymer, and the application of silica na-noparticles based on the red emissive AIE molecules in bioimaging such as cell imaging and in vivo imaging are highlighted. Finally, a brief outlook on the design of red emissive AIE materials and the application prospect of the red AIE materials in bioimaging are proposed, aiming at providing a reference for the further investigation and application. © 2019, Materials Review Magazine. All right reserved.