Sijie Chen

A Photostable AIE Luminogen for Specific Mitochondrial Imaging and Tracking

Tracking the dynamics of mitochondrial morphology has attracted much research interest because of its involvement in early stage apoptosis and degenerative conditions. To follow this process, highly specific and photostable fluorescent probes are in demand. Commercially available mitochondria trackers, however, suffer from poor photostability. To overcome this limitation, we have designed and synthesized a fluorescent agent, tetraphenylethene-triphenylphosphonium (TPE-TPP), for mitochondrial imaging. Inherent from the mitochondrial-targeting ability of TPP groups and the aggregation-induced emission (AIE) characteristics of the TPE core, TPE-TPP possesses high specificity to mitochondria, superior photostability, and appreciable tolerance to environmental change, allowing imaging and tracking of the mitochondrial morphological changes in a long period of time.

Full-Range Intracellular pH Sensing by an Aggregation-Induced Emission-Active Two-Channel Ratiometric Fluorogen

Intracellular pH (pHi) is an important parameter associated with cellular behaviors and pathological conditions. Sensing pHi and monitoring its changes in live cells are essential but challenging due to the lack of effective probes. We herein report a pH-sensitive fluorogen for pHi sensing and tracking. The dye is a tetraphenylethene-cyanine adduct (TPE-Cy). It is biocompatible and cell-permeable. Upon diffusing into cells, it responds sensitively to pHi in the entire physiological range, visualizing the acidic and basic compartments with intense red and blue emissions, respectively. The ratiometric signal of the red and blue channels can thus serve as an indicator for local proton concentration. The utility of TPE-Cy in pHi imaging and monitoring is demonstrated with the use of confocal microscopy, ratiometric analysis, and flow cytometry.

Cytophilic Fluorescent Bioprobes for Long-Term Cell Tracking

Fluorescence (FL) bioprobes have made important contributions to advancing our knowledge in life science, due to their unrivaled ability to image and monitor biological structures and processes in the living systems. [ 1,2 ] Typical materials used as biosensors include natural polymers, inorganic nanoparticles, and organic dyes. Green fl uorescent protein (GFP), for example, has been used as a reporter of expression for morphological differentiation. [ 1 ] The biosensing process, however, is realized through complicated yet time-consuming transfection procedures, which can lead to unexpected morphologies and undesired abnormality in the target cells. Inorganic nanoparticles, such as quantum dots (QDs), are highly luminescent and resistant to photobleaching but limited in variety and inherently toxic to living cells because QDs are commonly made of heavy metals and chalcogens (e.g., CdSe and PbS). [ 1,2 ]

Monitoring and Inhibition of Insulin Fibrillation by a Small Organic Fluorogen with Aggregation-Induced Emission Characteristics

Amyloid fibrillation of proteins is associated with a great variety of pathologic conditions. Development of new molecules that can monitor amyloidosis kinetics and inhibit fibril formation is of great diagnostic and therapeutic value. In this work, we have developed a biocompatible molecule that functions as an ex situ monitor and an in situ inhibitor for protein fibrillation, using insulin as a model protein. 1,2-Bis[4-(3-sulfonatopropoxyl)phenyl]-1,2-diphenylethene salt (BSPOTPE) is nonemissive when it is dissolved with native insulin in an incubation buffer but starts to fluoresce when it is mixed with preformed insulin fibril, enabling ex situ monitoring of amyloidogenesis kinetics and high-contrast fluorescence imaging of protein fibrils. Premixing BSPOTPE with insulin, on the other hand, inhibits the nucleation process and impedes the protofibril formation. Increasing the dose of BSPOTPE boosts its inhibitory potency. Theoretical modeling using molecular dynamics simulations and docking reveals that BSPOTPE is prone to binding to partially unfolded insulin through hydrophobic interaction of the phenyl rings of BSPOTPE with the exposed hydrophobic residues of insulin. Such binding is assumed to have stabilized the partially unfolded insulin and obstructed the formation of the critical oligomeric species in the protein fibrillogenesis process.

Benzothiazolium-functionalized tetraphenylethene: an AIE luminogen with tunable solid-state emission.

Melding a benzothiazolium unit with tetraphenylethene generates a new hemicyanine luminogen with aggregation-induced emission characteristics; the luminogen exhibits crystochromism and its solid-state emission can be repeatedly tuned from yellow or orange to red by grinding-fuming or grinding-heating processes due to the transformation from the crystalline to the amorphous state and vice versa.

Aggregation-induced red-NIR emission organic nanoparticles as effective and photostable fluorescent probes for bioimaging

Organic fluorescent probes are widely used in bioimaging and bioassays, but the notorious photobleaching hampers their applications. Encapsulation of organic dyes into nanoparticles (NPs) is an effective strategy to minimize photobleaching, but classical organic dye molecules tend to have their fluorescence quenched in aggregate states, which is termed aggregation-caused quenching (ACQ). Here we demonstrate our attempt to tackle this problem through the aggregation-induced emission (AIE) strategy. 3,4:9,10-Tetracarboxylic perylene bisimide (PBI) is a well-known organic dye with a serious ACQ problem. By attaching two tetraphenylethene (TPE) moieties to the 1,7-positions, the ACQ-characteristic PBI-derivative was converted to an AIE-characteristic molecule. The obtained PBI derivative (BTPEPBI) exhibits several advantages over classical PBI derivatives, including pronounced fluorescence enhancement in aggregate state, red to near infrared emission, and facile fabrication into uniform NPs. Studies on the staining of MCF-7 breast cancer cells and in vivo imaging of a tumor-bearing mouse model with BTPEPBI-containing NPs reveal that they are effective fluorescent probes for cancer cell and in vivo tumor diagnosis with high specificity, high photostability and good fluorescence contrast.

Fluorescent pH sensor constructed from a heteroatom-containing luminogen with tunable AIE and ICT characteristics

A heteroatom-containing organic fluorophore 1-(4-pyridinyl)-1-phenyl-2-(9-carbazolyl)ethene (CP3E) was designed and synthesized. CP3E exhibits the effect of intramolecular charge transfer (ICT) caused by the donor–acceptor interaction between its carbazole and pyridine units. Whereas it emits faintly in solution, it becomes a strong emitter in the aggregated state, demonstrating the phenomenon of aggregation-induced emission (AIE). Its emission can be reversibly switched between blue and dark states by repeated protonation and deprotonation. Such behaviour enables it to work as a fluorescent pH sensor in both solution and the solid state and as a chemosensor for detecting acidic and basic organic vapors. Analyses by NMR spectroscopy, single-crystal X-ray diffraction and computational calculations suggest that the change in electron affinity of the pyridinyl unit and molecular conformation of CP3E upon protonation and deprotonation is responsible for such sensing processes.

An AIE-active hemicyanine fluorogen with stimuli-responsive red/blue emission: extending the pH sensing range by “switch + knob” effect

In this work, a red-emissive zwitterionic hemicyanine dye, named TPE–Cy, containing tetraphenylethene (TPE) and N-alkylated indolium is designed and synthesized. TPE–Cy inherits the aggregation-induced emission (AIE) feature of TPE and displays a large Stokes shift (>185 nm), overcoming the limitations of the concentration-quenching effect and small Stokes shift (from a few to 20 nm) encountered by conventional cyanine dyes. By taking advantage of the photophysical AIE property and chemical reactivity towards OH−/H+, TPE–Cy is able to sense pH in a broad range (the broadest to date) by showing different emission colors and intensities: strong to moderate red emission at pH 5–7, weak to no emission at pH 7–10, and no emission to strong blue emission at pH 10–14. The acid/base-switched red/blue emission transition is reversible and can be repeated for many cycles. By means of NMR and HRMS analyses, we have drawn a mechanistic picture at molecular level to illustrate how this dye works as a pH-sensitive fluorescent probe.

Biotin-decorated fluorescent silica nanoparticles with aggregation-induced emission characteristics: fabrication, cytotoxicity and biological applications

Biotin-decorated fluorescent silica nanoparticles (FSNPs) were successfully fabricated by a sol-gel reaction of silole-functionalized siloxane followed by a sequential reaction with tetraethoxysilane, (3-aminopropyl)triethoxysilane and biotin. The FSNPs were uniformly sized, spherical in shape and monodispersed. While their silole precursor was non-emissive in solution, the suspension of the FSNPs emitted strong green light upon photoexcitation due to the aggregation-induced emission characteristics of the silole aggregates in the hybrid nanoparticles. Morphology study and cell viability, trypan blue exclusion, Annexin V-FITC/PI apoptosis and ROS generation assays showed that the FSNPs showed low toxicity to living cells. The FSNPs worked as fluorescent visualizers for selective imaging of the cytoplasm of tumor cells with over-expressed biotin receptors. The fluorescent nanoparticles were lastingly retained inside the living cells, thus enabling long-term tumor cell tracking over multiple passages and quantitative analysis of tumor cell migration.

Defect-sensitive crystals based on diaminomaleonitrile-functionalized Schiff base with aggregation-enhanced emission

In this work, we report the synthesis and photophysical studies of a new luminogen, A3MN, a diaminomaleonitrile-functionalized Schiff base. A3MN is aggregation-enhanced emission (AEE)-active: the emission of A3MN is enhanced with the aggregate formation. A3MN also possesses twisted intramolecular charge transfer (TICT) properties, showing noticeable solvatofluorochromism. Interestingly, the crystals of A3MN are nonemissive; the defect areas of the crystal, however, are highly emissive, as confirmed by spectroscopic methods and confocal microscopy. By taking advantage of this defect sensitive feature, a “turn-on” type of mechanofluorochromic material is developed, the emission of which is significantly enhanced under pressure or shear force. The detection limit reaches 0.1 Newton owing to its “turn-on” nature. Such defect-induced emission also renders A3MN sensitive to various kinds of mechanical actions, including hitting, friction, sculpture, and ultrasonic vibration.