Photostable pH-Sensitive Near-Infrared Aggregation-Induced Emission Luminogen for Long-Term Mitochondrial Tracking

Aggregation-induced emission nanoparticles facilitating multicolor lateral flow immunoassay for rapid and simultaneous detection of aflatoxin B1 and zearalenone

Herein we developed a multicolor lateral flow immunoassay (LFIA) test strip for rapid and simultaneous quantitative detection of aflatoxin B1 (AFB1) and zearalenone (ZEN). Three differently colored aggregation-induced emission nanoparticles (AIENPs) were designed as LFIA signal tags, with red and green AIENPs for targeting AFB1 and ZEN at the test line, and yellow AIENPs for indicating the validity of the test strip at the control (C) line. After surface functionalization with antibodies, the developed AIENP-based multicolor LFIA allows simultaneous and accurate quantification of AFB1 and ZEN using an independent C-line assisted ratiometric signal output strategy. The detection limits of AFB1 and ZEN were 6.12 and 26 pg/mL, respectively. The potential of this method for real-world applications was well demonstrated in corn and wheat. Overall, this multicolor LFIA shows great potential for field screening of multiple mycotoxins and can be extended to rapid and simultaneous monitoring of other small molecule targets.

A mitochondria-targeted fluorescent sensor for imaging endogenous peroxynitrite changes in acute lung injury

Acute lung injury (ALI) is a serious pulmonary inflammatory disease resulting from excessive reactive oxygen species (ROS) which could cause the damage of the alveolar epithelial cells and capillary endothelial cells. Peroxynitrite, as one of short-lived reactive oxygen species, is closely related to the process of ALI. Thus, it is important to monitor the fluctuation of peroxynitrite in living system for understanding the process of ALI. Herein, the novel mitochondria-targeted fluorescent probe BHMT was designed to respond to peroxynitrite and pH with distinct fluorescence properties respectively. The absorption spectrum of the probe BHMT exhibited a notable red shift as the pH value declined from 8.8 to 2.6. Upon reaction with peroxynitrite, BHMT had a significant increase of fluorescence intensity (63-fold) with maintaining a detection limit of only 43.7 nM. Furthermore, BHMT could detect the levels of endogenous peroxynitrite and image the intracellular pH in ratiometric channels utilizing cell imaging. In addition, BHMT was successfully applied to revealing the relationship between the peroxynitrite and the extent of ALI. Thus, these results indicated the probe BHMT could be a potential tool for diagnosing the early stage of ALI and revealed the peroxynitrite was likely to be a crucial therapeutic target in ALI treatment.

Ultrabright Xanthene Fluorescence Probe for Mitochondrial Super-Resolution Imaging

Mitochondria are important organelles that provide energy for cellular physiological activities. Changes in their structures may indicate the occurrence of diseases, and the super-resolution imaging of mitochondria is of great significance. However, developing fluorescent probes for mitochondrial super-resolution visualization still remains challenging due to insufficient fluorescence brightness and poor stability. Herein, we rationally synthesized an ultrabright xanthene fluorescence probe Me-hNR for mitochondria-specific super-resolution imaging using structured illumination microscopy (SIM). The rigid structure of Me-hNR provided its ultrahigh fluorescence quantum yield of up to 0.92 and ultrahigh brightness of up to 16,000. Occupying the para-position of the O atom in the xanthene skeleton by utilizing the smallest methyl group ensured its excellent stability. The study of the photophysical process indicated that Me-hNR mainly emitted fluorescence via radiative decay, and nonradiative decay and inter-system crossing were rare due to the slow nonradiative decay rate and large energy gap (ΔEst = 0.55 eV). Owing to these excellent merits, Me-hNR can specifically light up mitochondria at ultralow concentrations down to 5 nM. The unprecedented spatial resolution for mitochondria with an fwhm of 174 nm was also achieved. Therefore, this ultrabright xanthene fluorescence probe has great potential in visualizing the structural changes of mitochondria and revealing the pathogenesis of related diseases using SIM.

Biomass-based indole derived fluorescence sensor composited with cellulose paper: Detection of picric acid in food and environment samples

Picric acid (PA) is highly water-soluble, the fact makes it stand out as the most hazardous environment pollutant. Therefore, accurate determination of PA is of great significance for human health and environmental protection. Herein, a novel indole-based fluorescent sensor (H1) with good water solubility and fluorescence stability was reported. H1 exhibited 'turn-off' fluorescence response for PA with fast reaction rate (<30 s), unique specificity and excellent selectivity and high sensitivity (limit of detection = 34 nM). Further, H1 was successfully applied to detect PA in real samples (tap water, Yangtze River, Xuanwu Lake, soil, food, fish and shrimp) with satisfactory recoveries at three spiking levels ranging from 98.0 to 112.0 %. In addition, H1 displayed high biocompatibility in mung beans and fresh blood. Moreover, aiming to attain portable analysis, H1 was composited with biomass cellulose paper (H1-FP) and integrated with smartphone for construction as a solid-state fluorescence platform to achieve fast and visual detection of PA in suit with significant stability, high sensitively and selectivity. The establishment of this sensing approach is expected to offer new insight into rapid, selective, and sensitive detection of major pollutants for food and environmental safety.

Meso pyridinium BODIPY-based long wavelength infrared mitochondria-targeting fluorescent probe with high photostability

Mito-tracker deep red (MTDR) as a commercially available mitochondria-targeting probe was easily bleached upon imaging. We designed and synthesized a family of meso-pyridinium BODIPY and introduced lipophilic methyl or benzyl as the head moiety to develop a mitochondria-targeting deep red probe. Moreover, we changed the substitution of the 3,5-phenyl moieties with the methoxy or methoxyethoxyethyl group to balance hydrophilicity. The designed BODIPY dyes possessed long absorption and good fluorescence emission. Among them, meso ortho-pyridinium BODIPYs with benzyl head and glycol substitution on phenyl moiety (3h) with favorable Stokes shift were found to have the best mitochondrial targeting performance. 3h was easily uptaken by cells and was less toxic and more photostable than MTDR. An immobilizable probe (3i) was further developed, and nice mitochondria targeting properties under the damaging condition of mitochondria membrane potential were maintained. BODIPY 3h or 3i may become alternative long-wavelength mitochondria targeting probes apart from MTDR and be suitable for long-term mitochondrial tracking studies.

Design and synthesis of photostable triphenylamine based neutral AIE nano luminogens: specific and long-term tracking of mitochondria in cells

With the increasing dependence on fluorescence bioimaging, luminogens with aggregation-induced emission (AIE) properties have gained significant attention due to their excellent photostabilization, minimal photobleaching, high reliability, and superior biocompatibility. Since mitochondria are crucial subcellular organelles in eukaryotic cells with important biological functions, organelle-specific AIE emitters with distinct functions have been highly sought after, but with limited success using simple synthetic methods. Here, we describe a strategy for synthesizing two triphenylamine (TPA) based acrylonitriles, tethered to different donor groups, TPA and phenothiazine (PTZ), respectively, with superior AIE properties using Suzuki coupling. We conducted a systematic and detailed experimental analysis of the structural characteristics of both AIE luminogens, which exhibited excellent photostability, a large Stokes shift, and bright solid-state emission. A cell viability study carried out with F1 and F2 dyes revealed that both luminogens exhibited excellent biocompatibility. Based on fluorescence experiments, F2 displayed excellent AIE characteristics, permeability, biocompatibility, and photostability compared to rhodamine 123, allowing it to selectively stain and track mitochondria in cancer cells over an extended period of time. The Pearson correlation coefficient of F2 and rhodamine 123 was estimated to have an r-value of 0.99. Our findings are expected to provide insight into the synthesis of an extensive archive of AIE-based acrylonitriles with fascinating properties for mitochondrial staining.

Highly Sensitive Immunochromatographic Detection of Zearalenone Based on Ultrabright Red-Emitted Aggregation-Induced Luminescence Nanoprobes

Highly luminescent nanospheres have been demonstrated in enhancing the sensitivity of lateral flow immunoassay (LFIA) due to their loading numerous luminescent dyes. However, the photoluminescence intensities of existing luminescent nanospheres are limited due to the aggregation-caused quenching effect. Herein, highly luminescent aggregation-induced emission luminogens embedded nanospheres (AIENPs) with red emission were introduced as signal amplification probes of LFIA for quantitative detection of zearalenone (ZEN). Optical properties of red-emitted AIENPs were compared with time-resolved dye-embedded nanoparticles (TRNPs). Results showed that red-emitted AIENPs have stronger photoluminescence intensity on the nitrocellulose membrane and superior environmental tolerance. Additionally, we benchmarked the performance of AIENP-LFIA against TRNP-LFIA using the same set of antibodies, materials, and strip readers. Results showed that AIENP-LFIA exhibits good dynamic linearity with the ZEN concentration from 0.195 to 6.25 ng/mL, with half competitive inhibitory concentration (IC₅₀) and detection of limit (LOD) at 0.78 and 0.11 ng/mL, respectively. The IC₅₀ and LOD are 2.07- and 2.36-fold lower than those of TRNP-LFIA. Encouragingly, the precision, accuracy, specificity, practicality, and reliability of this AIENP-LFIA for ZEN quantitation were further characterized. The results verified that the AIENP-LFIA has good practicability for the rapid, sensitive, specific, and accurate quantitative detection of ZEN in corn samples.

De novo design of a novel AIE fluorescent probe tailored to autophagy visualization via pH manipulation

BACKGROUND

Macroautophagy is an essential cellular self-protection mechanism, and defective autophagy has been considered to contribute to a variety of diseases. During the process, cytoplasmic components are transported via autophagosomes to acidic lysosomes for metabolism and recycling, which represents application niches for lysosome-targeted fluorescent probes. Additionally, in view of the complexity of the autophagy pathway, it entails more stringent requirements for probes suitable for monitoring autophagy. Meanwhile, aggregation-induced emission (AIE) fluorescent probes have been impressively demonstrated in the biomedical field, which bring fascinating possibilities to the autophagy visualization.

METHODS

We reported a generalizable de novo design of a novel pH-sensitive AIE probe ASMP-AP tailored to lysosome targeting for the interpretation of autophagy. Firstly, the theoretical calculation was carried out followed by the investigation of optical properties. Then, the performance of ASMP-AP in visualizing autophagy was corroborated by starvation or drugs treatments. Furthermore, the capability of ASMP-AP to monitor autophagy was demonstrated in ex vivo liver tissue and zebrafish in vivo.

RESULTS

ASMP-AP displays a large stokes shift, great cell permeability and good biocompatibility. More importantly, ASMP-AP enables a good linear response to pH, which derives from the fact that its aggregation state can be manipulated by the acidity. It was successfully applied for imaging autophagy in living cells and was proved capable of monitoring mitophagy. Moreover, this novel molecular tool was validated by ex vivo visualization of activated autophagy in drug-induced liver injury model. Interestingly, it provided a meaningful pharmacological insight that the melanin inhibitor 1-phenyl-2-thiourea (PTU)-induced autophagy was clearly presented in wild-type zebrafish.

CONCLUSIONS

ASMP-AP offers a simple yet effective tool for studying lysosome and autophagy. This is the first instance to visualize autophagy in zebrafish using a small-molecule probe with AIE characters, accurate lysosome targeting and simultaneous pH sensitivity. Ultimately, this novel fluorescent system has great potential for in vivo translation to fuel autophagy research.

Mitochondrial Targeting and Imaging with Small Organic Conjugated Fluorophores: A Review

The last decade has seen an increasingly large number of studies reporting on the development of novel small organic conjugated systems for mitochondrial imaging exploiting optical signal transduction pathways. Mitochondria are known to play a critical role in a number of key biological processes, including cellular metabolism. Importantly, irregularities on their working function are nowadays understood to be intimately linked to a range of clinical conditions, highlighting the importance of targeting mitochondria for therapeutic benefits. In this work we carry out an in-depth evaluation on the progress to date in the field to pave the way for the realization of superior alternatives to those currently existing. The manuscript is structured by commonly used chemical scaffolds and comprehensively covers key aspects factored in design strategies such as synthetic approaches as well as photophysical and biological characterization, to foster collaborative work among organic and physical chemists as well as cell biologists.

Real-Time Fluorescence Imaging of the Abscisic Acid Receptor Allows Nondestructive Visualization of Plant Stress

Environmental stress greatly decreases crop yield. The application of noninvasive techniques is one of the most practical and feasible ways of monitoring the health condition of plants under stress. However, it remains largely unsolved. A chemical fluorescent probe can be applied as a typical nondestructive method, but it has not been applied in living plants for stress detection to date. The abscisic acid (ABA) receptor plays a central role in conferring tolerance to environmental stresses and is an excellent target for developing fluorescent probes. Herein, we developed a fluorescence molecular imaging technology to monitor live plant stress by visualizing the protein expression level of the ABA receptor PYR1. A computer-aided designed indicator dye, flubactin, exhibited an 8-fold enhancement in fluorescence intensity upon interaction with PYR1. In vitro and in vivo experiments showed that flubactin is suitable to be used to detect salt stress in plants in real time. Moreover, the low toxicity of flubactin promotes its application in the future. Our work opens a new era for the nondestructive visualization of plant stress in vivo.

Recent Advances in Small Molecule-Based Intracellular pH Probes

Intracellular pH plays an important role in many biological and pathological processes. Small-molecule based pH probes are found to be the most effective for pH sensing because of ease of preparation, high sensitivity, and quick response. They have many advantages such as small perturbation to the functions of the target, functional adaptability, cellular component-specific localization, etc. The present review highlights the flurry of recent activity in the development of such probes. The probes are categorized based on the type of fluorophore used like quinoline, coumarin, BODIPY, rhodamine, indolium, naphthalimide, etc., and their analytical performance is discussed.

Molecular Imaging-Guided Sonodynamic Therapy

Low-intensity ultrasound-triggered sonodynamic therapy (SDT) is a promising noninvasive therapeutic modality due to its strong tissue penetration ability. In recent years, with the development of nanotechnology, nanoparticle-based sonosensitizer-mediated SDT has been widely investigated. With the increasing demand for precise and personalized treatment, abundant novel sonosensitizers with imaging capabilities have been developed for determining the optimal therapeutic window, thus significantly enhancing treatment efficacy. In this review, we focus on the molecular imaging-guided SDT. The prevalent mechanisms of SDT are discussed, including ultrasonic cavitation, sonoluminescence, reactive oxygen species, and mechanical damage. In addition, we introduce the major molecular imaging techniques and the design principles based on nanoparticles to achieve efficient imaging. Furthermore, the imaging-guided SDT for the treatment of cancer, bacterial infections, and vascular diseases is summarized. The ultimate goal is to design more effective imaging-guided SDT modalities and provide novel ideas for clinical translation of SDT.

Glycopeptide-Conjugated Aggregation-Induced Emission Luminogen: A pH-Responsive Fluorescence Probe with Tunable Self-Assembly Morphologies for Cell Imaging

pH values play an important role in various cell biological processes. Abnormal pH values in living systems are frequently associated with the development of diseases such as cancers, infection, and other diseases. Real-time monitoring of the changes of pH values in vivo will give us the significant indication for these diseases' progression. Within those pH-sensitive imaging probes, aggregation-induced emission (AIE) molecules exhibit great potential in aqueous imaging environment due to their high fluorescence quantum yield and stability. However, the modulation of the AIE probe with pH sensitivity and light-up property face challenges. Here, we introduced a new glycopeptide-modified AIE probe (TGO) based on the optimized solid-phase peptide synthesis approach. The response to pH of the peptide: DDDD progression changed hydrophobicity and hydrophilicity, resulting in the change of the amphipathicity balance. When modulating the pH from 5.5 to 8.0, the adverse protonation of the peptide induced assembled nanostructure transformation from nanolamellae to nanomicelles. Meanwhile, the pH-induced charge change in peptides can greatly influence the microenvironment of the AIEgen, resulting in the increase of fluorescence intensity.

Stimuli-Responsive AIEgens

The unique advantages and the exciting application prospects of AIEgens have triggered booming developments in this area in recent years. Among them, stimuli-responsive AIEgens have received particular attention and impressive progress, and they have been demonstrated to show tremendous potential in many fields from physical chemistry to materials science and to biology and medicine. Here, the recent achievements of stimuli-responsive AIEgens in terms of seven most representative types of stimuli including force, light, polarity, temperature, electricity, ion, and pH, are summarized. Based on typical examples, it is illustrated how each type of systems realize the desired stimuli-responsive performance for various applications. The key work principles behind them are ultimately deciphered and figured out to offer new insights and guidelines for the design and engineering of the next-generation stimuli-responsive luminescent materials for more broad applications.

Real-Time Visualization and Monitoring of Physiological Dynamics by Aggregation-Induced Emission Luminogens (AIEgens)

Physiological dynamics in living cells and tissues are crucial for maintenance and regulation of their normal activities and functionalities. Tiny fluctuations in physiological microenvironments can leverage significant influences on cell growth, metabolism, differentiation, and apoptosis as well as disease evolution. Fluorescence imaging based on aggregation-induced emission luminogens (AIEgens) exhibits superior advantages in real-time sensing and monitoring of the physiological dynamics in living systems, including its unique properties such as high sensitivity and rapid response, flexible molecular design, and versatile nano- to mesostructural fabrication. The introduction of canonic AIEgens with long-wavelength, near-infrared, or microwave emission, persistent luminescence, and diversified excitation source (e.g., chemo- or bioluminescence) offers researchers a tool to evaluate the resulting molecules with excellent performance in response to subtle fluctuations in bioactivities with broader dimensionalities and deeper hierarchies.

Aggregation induced emission (AIE) materials for mitochondria imaging

Mitochondria are energy producing organelle of the eukaryotic cells. The main activities of mitochondria monitored by various marker molecules are autophagy detection, estimation of Reactive Oxygen Species (ROS), mitochondrial death and Photodynamic therapy in cancer cells. Due to the advantages of specificity and sensitivity, aggregation induced emission (AIE) is now popular for the mitochondria labeling. In this chapter, we would like to discuss three major types of AIEgens probe used in mitochondrial staining. There are three different types of AIEgens available for mitochondrial detection and sensing based on their different structural motifs. The first type of AIEgens is tetraphenylethene (TPE) based molecules. Due to simple engineering architecture, TPE based AIEgens are widely employed in bioimaging applications. AIEgen such as triphenylphosphine (TPP), and triphenylamine (TPA) are also employed as a novel building block. These are successfully used as exceptional lipid droplet (LD)-specific bio probes in cell imaging, assurance of cell combination, and photodynamic cancer cell removal. The third group is the miscellaneous AIEgens probe involved in mitochondria imaging.

Target-triggered and controlled release plasmon-enhanced fluorescent AIE probe for conformational monitoring of insulin fibrillation

In this work, we constructed a target-triggered and controlled-release plasmon-enhanced fluorescent AIE probe to realize the purpose of conformational monitoring of insulin fibrillation. We synthesized a novel water-soluble anthracene derivative, 4,4',4'',4'''-(anthracene-9,10-diylbis(ethene-2,1,1-triyl))tetrakis(N,N,N-trimethylbenzenaminium) iodide (BDVAI), with AIE properties, high biocompatibility and good self-assembly effect. Gold nanocages (AuNCs) were selected as the substrate for PEF, and the inner space of hollow AuNCs was filled with BDVAI. Thiol-modified DNA chains were bonded to the surface of AuNCs by Au-S bonds, and an insulin aptamer was combined with the sulfhydryl chain to seal the AuNCs. This PEF-AIE sensor produces different fluorescence signals when interacting with native insulin and fibrillar insulin; thus, monitoring conformational changes in insulin can be realized by detecting fluorescence intensity changes during insulin fibrillation. Based on this design, this system realized sensitive detection of fibrillar insulin with a detection limit of 23.6 pM. This AIE molecular-based PEF fluorescence enhancement system improves the optical properties of fluorescent substances, which is of great significance in improving the detection sensitivity of amyloid fibrils conformational changes and providing a reliable basis for further understanding the pathogenesis of amyloidosis.

Molecular engineering of a near-infrared fluorescent ligand for tracking mitochondrial DNA G-quadruplexes

G-quadruplexes are supposed to exist in mitochondrial DNA, which may participate in the regulation of mitochondrial metabolism. However, their formation dynamics and cellular roles are still unknown, calling for the development of promising small-molecule fluorescent probes for detecting mitochondrial DNA (mtDNA) G4s in live cells. In this study, we engineered a NIR fluorescent probe termed IZIN-1 by conjugating a typical G4 ligand and a mitochondrion-targeting AIE luminogen, which was then found to be able to emit NIR fluorescence by binding to mtDNA G4s via a distinctive mode. Cell-based experiments demonstrated that IZIN-1 may track mtDNA G4 formation in live cells. Collectively, this work provides a promising example about how to design a fluorescent probe targeting mtDNA G4s, and also offers a feasible chemical tool for investigating mtDNA G4s in live cells.

β-Naphthothiazolium-based ratiometric fluorescent probe with ideal pKa for pH imaging in mitochondria of living cells

The weakly alkaline microenvironment (pH ~8.0) in mitochondria plays a vital role in maintaining its morphology and function. Thus monitoring mitochondrial pH (pH_mito) is of great significance. Herein, a ratiometric fluorescent probe (ENBT) for pHmito imaging in mitochondria of living cells is reported. pH variation closely correlates to intramolecular charge transfer (ICT) from naphthol to β-naphthothiazolium. ENBT exhibits a remarkable decrease on ratiometric fluorescence at λ_em1/λ_em2 = F₅₉₅/F₇₀₀ in response to pH variation within 6.30-9.29. In addition, ENBT has an ideal pKa value of 7.94 ± 0.08, which is advantageous in accurate sensing of pH_mito. Moreover, ENBT has a Stokes shift of >150 nm, which effectively eliminates the potential interference from the excitation irradiation. ENBT shows excellent capability for specific staining of mitochondria with low cytotoxicity, which is most suitable for pH_mito imaging in live cells. The probe was applied for monitoring pH_mito variation in mitochondria of live cells caused by H₂O₂, NAC (N-Acetyl-l-cysteine), NH₄Cl, carbonyl cyanide m-chlorophenyl hydrazone (CCCP) and lactate/pyruvate. The morphological alterations of mitochondria in living cells after treatment by CCCP were further evaluated.

Fluorescent nanorods based on 9,10-distyrylanthracene (DSA) derivatives for efficient and long-term bioimaging

Fluorescent nanoparticles based on 9,10-distyrylanthracene (DSA) derivatives (4,4'-((1E,1'E)-anthracene-9,10-diylbis(ethene-2,1-diyl))bis(N,N-dimethylaniline) (NDSA) and 4,4'-((1E,1'E)-anthracene-9,10-diylbis(ethene-2,1-diyl))dibenzonitrile (CNDSA)) were prepared using an ultrasound aided nanoprecipitation method. The morphologies of the fluorescent nanoparticles could be controlled by adjusting the external ultrasonication time. NDSA or CNDSA could form spherical nanodots (NDSA NDs, CNDSA NDs) in a THF-H2O mixture with an 80% or 70% water fraction when the ultrasonication time was 30 s. When the ultrasonication time was prolonged to 10 min, NDSA and CNDSA could assemble into nanorods (NDSA NRs, CNDSA NRs). Meanwhile, the sizes of NDSA NRs and CNDSA NRs could be controlled by adjusting the water content in the mixture. As the water fraction was increased from 60% to 80%, the sizes of NDSA and CNDSA nanorods or nanodots reduced from 238.4 nm to 140.3 nm, and 482 nm to 198.4 nm, respectively. When the water fraction was up to 90%, irregular morphologies of NDSA and CNDSA could be observed. The nanoparticles exhibited intense fluorescence emission, good anti-photobleaching properties, as well as excellent stability and biocompatibility. In vitro cell imaging experiments indicated that the nanorods prepared by this simple method had the potential to be used for efficient and noninvasive long-term bioimaging.

Revealing Minor pH Changes of Mitochondria by a Highly Sensitive Molecular Fluorescent Probe

Mitochondrial pH is an important factor associated with cellular metabolism and pathological states. Thus, sensitively monitoring its minor change was essential. However, it was challengeable due to the lack of suitable probes. Here, a mitochondria-targeted probe (NIR-OH-1) was synthesized. Based on the protonation/deprotonation of the hydroxy group and the assistance of carboxyl group on NIR-OH-1 molecular structure, a dramatic NIR activated signal was generated for sensing pH. Probe NIR-OH-1 displayed a good photo-stability and reversibility and could detect pH change without interference by other biologically active species. Importantly, NIR-OH-1 had an appropriate pK_a value (7.77) and tiny acid-base transition range, which was allowed to map the small pH changes of cellular mitochondrial. Moreover, NIR-OH-1 was also successfully applied in real-time monitoring mitochondrial pH-related pathological events in living cells under different stimulation, demonstrating the prospect of its clinical application in accurate mitochondrial pH detection under related physiological and pathological conditions.

AIE-Based Dynamic in Situ Nanoscale Visualization of Amyloid Fibrillation from Hen Egg White Lysozyme

Protein misfolding and denaturation, represented by amyloid fibrillation, are associated with many diseases. However, as a general chemical biological process, the dynamic structure information on amyloid fibrillation has not been demonstrated categorically. Herein, hen egg white lysozyme (HEWL) was used as the model protein of interest to realize in situ nanoscale imaging of protein fibrillation process using the fluorophores with aggregation-induced emission (AIE) activity. The AIE-active fluorophores exhibit the reversible capability of association and dissociation with β-sheet structure and thus dynamic binding-induced emission, which causes the spontaneous switching of fluorescence. The entire HEWL denaturation process induced by sodium dodecyl sulfate (SDS) at ambient conditions was demonstrated in detail by using two AIE-active fluorophores (TPE-NaSO₃ and PD-BZ-OH) through reversible electrostatic interaction and specific labeling between AIE probes and β-sheet structures of amyloid fibrils, respectively. The results indicate that PD-BZ-OH is more specific AIE probe for amyloid fibrils than TPE-NaSO₃. In comparison, the SEM and TEM results show the same denaturation process of protein fibrillation induced by SDS at different concentrations. The static super-resolution imaging of amyloid fibrils is performed with a resolution of 35 nm using PD-BZ-OH aqueous solution without additional auxiliary conditions. The dynamic evolution process of HEWL amyloid fibrillation is in situ visualized through super-resolution fluorescent microscopy with nanoscale resolution. Both static and dynamic super-resolution imaging of amyloid fibrillation provides detailed nanoscale structure information exceeding 50 nm resolution, which is of great significance in the exploration of amyloid fibrillation and related diseases.

Highly Efficient Förster Resonance Energy Transfer Modulations of Dual-AIEgens between a Tetraphenylethylene Donor and a Merocyanine Acceptor in Photo-Switchable [2]Rotaxanes and Reversible Photo-Patterning Applications

A series of novel photo-switchable [2]rotaxanes (i.e., Rot-A-SP and Rot-B-SP before and after shuttling controlled by acid-base, respectively) containing one spiropyran (SP) unit (as a photochromic stopper) on the axle and two tetraphenylethylene (TPE) units on the macrocycle were synthesized via click reaction. Upon UV/visible light exposure, both mono-fluorophoric rotaxanes Rot-A-SP and Rot-B-SP with the closed form (i.e., non-emissive SP unit) could be transformed into the open form (i.e., red-emissive merocyanine (MC) unit) to acquire their respective bi-fluorophoric Rot-A-MC and Rot-B-MC reversibly. The aggregation-induced emission (AIE) properties of bi-fluorophoric TPE combined with MC AIEgens of these designed rotaxanes and mixtures in semi-aqueous solutions induced interesting ratiometric photoluminescence (PL) and Förster resonance energy transfer (FRET) behaviors, which were further investigated and verified by dynamic light scattering (DLS), X-ray diffraction (XRD), and time-resolved photoluminescence (TRPL) measurements along with theoretical studies. Accordingly, in contrast to the model axle (Axle-MC) and the analogous mixture (Mixture-MC, containing the axle and macrocycle components in a 1:1 molar ratio), more efficient FRET behaviors and stronger red PL emissions were obtained from dual-AIEgens between a blue-emissive TPE donor (PL emission at 468 nm) and a red-emissive MC acceptor (PL emission at 668 nm) in both novel photo-switchable [2]rotaxanes Rot-A-MC and Rot-B-MC under various external modulations, including water content, UV/Vis irradiation, pH value, and temperature. Furthermore, the reversible fluorescent photo-patterning applications of Rot-A-SP in a powder form and a solid film with excellent photochromic and fluorescent behaviors are first investigated in this report.

Dynamic Visible Monitoring of Heterogeneous Local Strain Response through an Organic Mechanoresponsive AIE Luminogen

Local strain concentration is critically important for damage formation of structural components. Therefore, it is of particular interest in developing the structural health monitoring (SHM) method for large-scale, full-field, and on-site monitoring of local strain response in complicated structural components in service. The present work investigated a SHM method based on a pure organic mechanoresponsive luminogen (MRL), 1,1,2,2-tetrakis(4-nitrophenyl)ethane, for heterogeneous local strain concentration. Invisible heterogeneous local strain response in complicated weld joints is transformed into visible fluorescence under monotonic tension and cyclic stress loading. The local strain (<15%) calculated by fluorescence intensity has a good agreement with the results obtained by the conventional digital image correlation method, indicating good measurement accuracy of the calibrated organic MRL method. The heterogeneity of local strain in complicated weld joints increases along with elongation and number of stress cycles. Moreover, the higher mean stress and stress amplitude can induce significantly higher accumulated local strain in the relatively soft fusion zone region. Compared with conventional strain measurement methods, the present organic MRL method opens up new possibilities for large-scale, full-field, and on-site monitoring of local strain concentration and damage in complicated structural components.

AIEgens Conjugation Improves the Photothermal Efficacy and Near-Infrared Imaging of Heptamethine Cyanine IR-780

Near-infrared (NIR) fluorescent probes can deeply penetrate through tissues with little damage. To facilitate image-guided theranostics, researchers usually apply a desired amount of photosensitizers to achieve effective photothermal responses. However, these probes could easily suffer from low photostability and aggregated-caused quenching effect in high concentrations. In this paper, the rational incorporation of an aggregated-induced emission (AIE) unit into the structure of heptamethine cyanine IR-780 is reported. Using tetraphenylethene (TPE) as an AIE core, we synthesize three TPE-modified IR-780 probes (IR-780 AIEgens) via different linkages. The IR-780 derivatives all show enhanced AIE features, in which the probe with an ether linkage (IR780-O-TPE) is superior in rapid cell uptake, high targeting capacity, and good photostability. Moreover, IR780-O-TPE exhibits the strongest cytotoxicity to HeLa cells (IC₅₀ = 3.3 μM). The three IR-780 derivatives displayed a photothermal response in a concentration-dependent manner, in which IR-780 AIEgens are more cytotoxic than IR-780, with IC₅₀ of 0.3 μM under 808 nm laser irradiation. In tumor-bearing mice, the optimal probe IR780-O-TPE also showed a more effective photothermal response than IR-780. By illustrating the relationship between aggregation state with photophysical properties, cell imaging, and cytotoxicity, this work is helpful in modulating NIR-based photosensitizers into AIE features for efficient image-guided theranostics.

Multi-Stimuli Responsive FRET Processes of Bifluorophoric AIEgens in an Amphiphilic Copolymer and Its Application to Cyanide Detection in Aqueous Media

A novel amphiphilic aggregation-induced emission (AIE) copolymer, that is, poly(NIPAM-co-TPE-SP), consisting of N-isopropylacrylamide (NIPAM) as a hydrophilic unit and a tetraphenylethylene-spiropyran monomer (TPE-SP) as a bifluorophoric unit is reported. Upon UV exposure, the close form of non-emissive spiropyran (SP) in poly(NIPAM-co-TPE-SP) can be photo-switched to the open form of emissive merocyanine (MC) in poly(NIPAM-co-TPE-MC) in an aqueous solution, leading to ratiometric fluorescence of AIEgens between green TPE and red MC emissions at 517 and 627 nm, respectively, via Förster resonance energy transfer (FRET). Distinct FRET processes of poly(NIPAM-co-TPE-MC) can be observed under various UV and visible light irradiations, acid-base conditions, thermal treatments, and cyanide ion interactions, which are also confirmed by theoretical studies. The subtle perturbations of environmental factors, such as UV exposure, pH value, temperature, and cyanide ion, can be detected in aqueous media by distinct ratiometric fluorescence changes of the FRET behavior in the amphiphilic poly(NIPAM-co-TPE-MC). Moreover, the first FRET sensor polymer poly(NIPAM-co-TPE-MC) based on dual AIEgens of TPE and MC units is developed to show a very high selectivity and sensitivity with a low detection limit (LOD = 0.26 μM) toward the cyanide ion in water, which only contain an approximately 1% molar ratio of the bifluorophoric content and can be utilized in cellular bioimaging applications for cyanide detections.

Single AIEgen for multiple tasks: Imaging of dual organelles and evaluation of cell viability

Fully understanding the complicated interplays among various chemical species and organelles is greatly important to unravel the mystery of life. However, fluorescent probes capable of visualizing multiple targets discriminatively are severely deficient, which extremely limit the investigation on intracellular interplays among various species. Towards this end and in consideration of the unique advantages of aggregation-induced emission luminogens (AIEgens), here we rationally designed and presented a single AIEgen, named TVQE, bearing lipophilic, cationic and hydrolyzable moieties, and this AIEgen was capable of illuminating mitochondria and lipid droplets with red and blue emission, respectively. In addition, TVQE was successfully used for evaluating cell viability due to its distinct two-color emission changes tuned by esterase-mediated hydrolysis. Of particular importance is that TVQE can selectively differentiate live, early apoptotic, late apoptotic, and dead cells by confocal microscopy and quantify cell viability statistically by flow cytometry.

Enhancement of the Aggregation-Induced Emission by Hydrogen Bond for Visualizing Hypochlorous Acid in an Inflammation Model and a Hepatocellular Carcinoma Model

As an important reactive oxygen species, hypochlorous acid (HClO) is produced in various physiological processes. The abnormal rise of the HClO level is associated with a large number of inflammatory diseases. In this work, we develop a simple, aqueous-soluble aggregration-induced emission (AIE) probe for sensing HClO with significant aggregation-induced fluorescence (>1000 times). Two probes, CH₃O-TPE-Py⁺-N⁺ (COTN) and OH-TPE-Py⁺-N⁺ (HOTN) (TPE, tetraphenylethylene), are synthesized for sensing HClO by the cleavage of the Py⁺-N⁺ group; the reaction products are CH₃O-TPE-CHO (COT) and OH-TPE-CHO (HOT), respectively. The hydrophobicity of the probes is changed with the increased aggregation-induced emission. During the process, HOTN shows significantly better response than COTN. The slightly different chemical structures of COTN and HOTN result in a significant response to HClO. The theoretical calculation data support the theory that the hydrogen bond contributes to the excellent sensitivity for HClO. On the basis of the good response to HClO in vitro, HOTN is used to image inflammation and hepatocellular carcinoma in vivo because these diseases always produce high HClO levels.

2-Styrylquinoline-based two-photon AIEgens for dual monitoring of pH and viscosity in living cells

A new class of aggregation-induced emission (AIE) fluorophores HAPHs with excellent two-photon properties is developed from styrylquinoline.