Yuning Hong

Aggregation-induced emission: phenomenon, mechanism and applications

It is textbook knowledge that chromophore aggregation generally quenches light emission. In this feature article, we give an account on how we observed an opposite phenomenon termed aggregation-induced emission (AIE) and identified the restriction of intramolecular rotation as a main cause for the AIE effect. Based on the mechanistic understanding, we developed a series of new fluorescent and phosphorescent AIE systems with emission colours covering the entire visible spectral region and luminescence quantum yields up to unity. We explored high-tech applications of the AIE luminogens as, for example, fluorescence sensors (for explosive, ion, pH, temperature, viscosity, pressure, etc.), biological probes (for protein, DNA, RNA, sugar, phospholipid, etc.), immunoassay markers, PAGE visualization agents, polarized light emitters, monitors for layer-by-layer assembly, reporters for micelle formation, multistimuli-responsive nanomaterials, and active layers in the fabrication of organic light-emitting diodes.

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.

Fluorescence enhancements of benzene-cored luminophors by restricted intramolecular rotations: AIE and AIEE effects

Photoluminescence of simple arylbenzenes with ready synthetic accessibility is enhanced by two orders of magnitude through aggregate formation; viscosity and temperature effects indicate that the emission enhancement is due to the restriction of their intramolecular rotations in the solid state.

Label-free fluorescent probing of G-quadruplex formation and real-time monitoring of DNA folding by a quaternized tetraphenylethene salt with aggregation-induced emission characteristics.

Biosensing processes such as molecular beacons require non-trivial effort to covalently label or mark biomolecules. We report here a label-free DNA assay system with a simple dye with aggregation-induced emission (AIE) characteristics as the fluorescent bioprobe. 1,1,2,2-Tetrakis[4-(2-bromoethoxy)phenyl]ethene is nonemissive in solution but becomes highly emissive when aggregated. This AIE effect is caused by restriction of intramolecular rotation, as verified by a large increase in the emission intensity by increasing viscosity and decreasing temperature of the aqueous buffer solution of 1,1,2,2-tetrakis[4-(2-triethylammonioethoxy)phenyl]ethene tetrabromide (TTAPE). When TTAPE is bound to a guanine-rich DNA strand (G1) via electrostatic attraction, its intramolecular rotation is restricted and its emission is turned on. When a competitive cation is added to the G1 solution, TTAPE is detached and its emission is turned off. TTAPE works as a sensitive poststaining agent for poly(acrylamide) gel electrophoresis (PAGE) visualization of G1. The dye is highly affinitive to a secondary structure of G1 called the G-quadruplex. The bathochromic shift involved in the G1 folding process allows spectral discrimination of the G-quadruplex from other DNA structures. The strong affinity of TTAPE dye to the G-quadruplex structure is associated with a geometric fit aided by the electrostatic attraction. The distinct AIE feature of TTAPE enables real-time monitoring of folding process of G1 in the absence of any pre-attached fluorogenic labels on the DNA strand. TTAPE can be used as a K+ ion biosensor because of its specificity to K+-induced and -stabilized quadruplex structure.

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.

A superamplification effect in the detection of explosives by a fluorescent hyperbranched poly(silylenephenylene) with aggregation-enhanced emission characteristics

Light emission of a hyperbranched poly(silylenephenylene) is quenched exponentially by picric acid, with quenching constant up to ∼1.5 × 105 L mol−1. This superamplification effect makes the polymer a highly sensitive chemosensor for explosive detection.

Fluorescent Bioprobes: Structural Matching in the Docking Processes of Aggregation-Induced Emission Fluorogens on DNA Surfaces

Whereas most conventional DNA probes are flat disklike aromatic molecules, we explored the possibility of developing quadruplex sensors with nonplanar conformations, in particular, the propeller-shaped tetraphenylethene (TPE) salts with aggregation-induced emission (AIE) characteristics. 1,1,2,2-Tetrakis[4-(2-triethylammonioethoxy)phenyl]ethene tetrabromide (TPE-1) was found to show a specific affinity to a particular quadruplex structure formed by a human telomeric DNA strand in the presence of K(+) ions, as indicated by the enhanced and bathochromically shifted emission of the AIE fluorogen. Steady-state and time-resolved spectral analyses revealed that the specific binding stems from a structural matching between the AIE fluorogen and the DNA strand in the folding process. Computational modeling suggests that the AIE molecule docks on the grooves of the quadruplex surface with the aid of electrostatic attraction. The binding preference of TPE-1 enables it to serve as a bioprobe for direct monitoring of cation-driven conformational transitions between the quadruplexes of various conformations, a job unachievable by the traditional G-quadruplex biosensors. Methyl thiazolyl tetrazolium (MTT) assays reveal that TPE-1 is cytocompatible, posing no toxicity to living cells.

Quantitation, visualization, and monitoring of conformational transitions of human serum albumin by a tetraphenylethene derivative with aggregation-induced emission characteristics.

Human serum albumin (HSA) is a major protein component of blood plasma, and its assay is of obvious value to biological research. We, herein, present a readily accessible fluorescent bioprobe for HSA detection and quantitation. A nonemissive tetraphenylethene derivative named sodium 1,2-bis[4-(3-sulfonatopropoxyl)phenyl]-1,2-diphenylethene (BSPOTPE) is induced to emit by HSA, showing a novel phenomenon of aggregation-induced emission (AIE). The AIE bioprobe enjoys a broad working range (0-100 nM), a low detection limit (down to 1 nM), and a superior selectivity to albumins. The fluorescent bioassay is unperturbed by the miscellaneous bioelectrolytes in the artificial urine. The AIE luminogen can also be used as a rapid and sensitive protein stain in gel electrophoresis for HSA visualization. Utilizing the AIE feature of BSPOTPE and the Forster resonance energy transfer from HSA to BSPOTPE, the unfolding process of HSA induced by guanidine hydrochloride is monitored, which reveals a multistep transition with the involvement of molten globule intermediates. Computational modeling suggests that the AIE luminogens dock in the hydrophobic cleft between subdomains IIA and IIIA of HSA with the aid of hydrophobic effect, charge neutralization, and hydrogen bonding interactions, offering mechanistic insight into the microenvironment inside the hydrophobic cavity.