Fluorescence technique has been widely applied in the research of fundamental biology, biomaterial development, clinical test and even FDA-approved imaging guided surgery, due to the high sensitivity, real-time responsive and non-invasive characteristics. Molecules that emit fluorescence are called fluorophores or luminogens. Organic fluorophores have been widely used in biological applications. However, most organic fluorophores are not soluble in water and they tend to form aggregates in water or biological fluids. Unfortunately, traditional planar organic fluorophores suffer from aggregation-caused quenching (ACQ) effect, from which their fluorescence intensity decreased dramatically or even quenched. Regarding this situation, the ACQ fluorophores are often used in dilute sol...[ Read more ]

Fluorescence technique has been widely applied in the research of fundamental biology, biomaterial development, clinical test and even FDA-approved imaging guided surgery, due to the high sensitivity, real-time responsive and non-invasive characteristics. Molecules that emit fluorescence are called fluorophores or luminogens. Organic fluorophores have been widely used in biological applications. However, most organic fluorophores are not soluble in water and they tend to form aggregates in water or biological fluids. Unfortunately, traditional planar organic fluorophores suffer from aggregation-caused quenching (ACQ) effect, from which their fluorescence intensity decreased dramatically or even quenched. Regarding this situation, the ACQ fluorophores are often used in dilute solution, limiting the usage in various situations. In 2001, Tang et. al. discovered an opposite phenomenon to the ACQ. A group of novel fluorophores that are not emissive in dilute solution as separated molecule while become highly emissive after forming aggregates. This characteristic phenomenon is termed as Aggregation-induced emission (AIE). The most widely accepted mechanism of this phenomenon is the restriction of intramolecular motion (RIM). Guided by the RIM mechanism, many AIE luminogens (AIEgens) have been designed to show great potential in the application of biosensing, bioimaging and image-guided therapy

Owing to the AIE effect, the aggregates of AIEgens used in biological physiological fluid show intensive fluorescence and photosensitization. Thus, AIE aggregates can be used in some biomedical applications with better performance than planar ACQ molecules. In this thesis, AIEgens are used as highly emissive luminogenic aggregates in various applications. Organelle-specific AIEgens are investigated by easily tuning functional groups. By changing the charges of AIEgens, endoplasmic reticulum (ER) and mitochondria-specific imaging are realized.

Apart from the investigation of intracellular organelles, the cell membrane probe is also important since cell membrane and its substructures play a critical role in cell-cell recognition and substance transfer. Water-soluble near-infrared (NIR) AIEgen is designed for cell membrane and membrane nanotubes imaging. The generation of reactive oxygen species (ROS) by organic photosensitizer is also influenced by the aggregate formation. Therefore, AIE photosensitizer also shows high efficiency of ROS generation in the aggregate state. Photodynamic therapy (PDT) is to use photosensitizer to generate large amount of ROS that is used in cancer treatment, especially skin cancer. The fluorescence-guided cancer cell and tumor spheroid ablation were achieved by rational design of cell membrane and membrane nanotube targeting and near-infrared emission AIE photosensitizer.

In addition to pure organic AIEgen systems, phosphorescent metal complexes with AIE characteristics are constructed and their working mechanism has been investigated. Due to the restriction of intramolecular rotation (RIR), iridium(III) complexes with different number of rotors are developed and they show the luminescence phenomena transfer from aggregation-enhanced emission (AEE) to typical AIE while rotors increased. Taking advantage of AIE and transition metal, one of the iridium(III) complexes, Ir-1, has been applied for intracellular oxygen sensing. The other two complexes exhibited high quantum yield and photostability in aggregate state after encapsulation by DSPE-PEG₂₀₀₀. They were promising long-term tracking candidates for lysosomes/endosomes.

In addition to cell imaging, rapid and reliable microbial detection and sensing system is highly demanded. A system comprised of two AIEgens is successfully developed for microbial imaging and metabolic status sensing. The two AIEgens (DCQA and TPE-2BA) bear positively-charged groups or boric acid groups, providing universal microbial staining ability and specific affinity to dead microbes, respectively. On the basis of the distinctive fluorescence response produced by the diverse interaction of AIEgens with live or dead microbes, this dual-AIEgen system can detect universal microbes and identify their viabilities. Furthermore, the morphology and the metabolic status of a sessile biofilm can also be imaged and monitored. The system exhibits rapid labelling, suitable for various microbes, and good biocompatibility.

Apart from fluorescence, the thermal conversion by organic photothermal agents is also influenced by the aggregate formation. Based on intramolecular motion-induced photothermy (iMIPT) mechanism, NIR-absorption photothermal organic molecule has been investigated. Under the irradiation of 808 nm laser, the aggregates of AIE molecules in water suspension could generate heat with a high thermal conversion efficiency. Photothermal therapy (PTT) effect of 2TPE-2NDTA-02 NPs towards mature bacterial biofilms has been investigated. S. aureus biofilms are destroyed by this photothermal AIE aggregate nanoparticle system efficiently upon NIR irradiation. Additionally, the working mechanism has been proposed to be the disruption of the extracellular matrix of biofilm and the killing of embedded bacterial cells.