Light-Up Probes Based on Fluorogens with Aggregation-Induced Emission Characteristics for Monoamine Oxidase-A Activity Study in Solution and in Living Cells

Self-Assembled Tetraphenylethene-Based Nanoaggregates with Tunable Electrochemiluminescence for the Ultrasensitive Detection of E. coli

As an effective ECL emitter, tetraphenylethene (TPE)-based molecules have recently been reported with aggregation-induced electrochemiluminescence (AIECL) property, while it is still a big challenge to control its aggregation states and obtain uniform aggregates with intense ECL emission. In this study, we develop three TPE derivatives carrying a pyridinium group, an alkyl chain, and a quaternary ammonium group via the Menschutkin reaction. The resulting molecules exhibit significantly red-shifted FL and enhanced ECL emissions due to the tunable reduction of the energy gap between the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs). More importantly, the amphiphilicity of the as-developed molecules enables their spontaneous self-assembly into well-controlled spherical nanoaggregates, and the ECL intensity of nanoaggregates with 3 -CH2- (named as C3) is 17.0-fold higher compared to that of the original 4-(4-(1,2,2-triphenylvinyl)phenyl)pyridine (TPP) molecule. These cationic nanoaggregates demonstrate a high affinity toward bacteria, and an ECL sensor for the profiling of Escherichia coli (E. coli) was developed with a broad linear range and good selectivity in the presence of an E. coli-specific aptamer. This study provides an effective way to enhance the ECL emission of TPE molecules through their derivatization and a simple way to prepare well-controlled AIECL nanoaggregates for ECL application.

Ultrafast Detection of Monoamine Oxidase A in Live Cells and Clinical Glioma Tissues Using an Affinity Binding-Based Two-Photon Fluorogenic Probe

Abnormal expression of monoamine oxidase A (MAO-A) has been implicated in the development of human glioma, making MAO-A a promising target for therapy. Therefore, a rapid determination of MAO-A is critical for diagnosis. Through in silico screening of two-photon fluorophores, we discovered that a derivative of N,N-dimethyl-naphthalenamine (pre-mito) can effectively fit into the entrance of the MAO-A cavity. Substitutions on the N-pyridine not only further explore the MAO-A cavity, but also enable mitochondrial targeting ability. The aminopropyl substituted molecule, CD1, showed the fastest MAO-A detection (within 20 s), high MAO-A affinity and selectivity. It was also used for in situ imaging of MAO-A in living cells, enabling a comparison of the MAO-A content in human glioma and paracancerous tissues. Our results demonstrate that optimizing the affinity binding-based fluorogenic probes significantly improves their detection rate, providing a general approach for rapid detection probe design and optimization.

Molecular Design Strategy of Protein Isoform-Specific Fluorescent Probes by Considering Molecule in Its Entirety

Generally, different isoforms of proteins exert separate biological functions. However, due to similar structures and identical catalysis functions, distinguishing isoforms is challenging. Summarizing a molecular design strategy has great significance in developing a protein-specific fluorescent probe. Usually, recognition of a group was deemed to be the key to a protein isoform-specific response. However, some novel literature reported that fluorophore could play a vital role in the protein isoform-specific response. It means that any part of the fluorescent probe could affect the detected properties. In this work, we report the generation of the first probe to specifically recognize HexA(β-N-acetylhexosaminidase A), Hex-C4, by adjusting the length of the linker. Hex-C4 exhibits specific recognition of HexA both in vitro and in living cells. The integration of the fluorescent spectrum and the MD (molecular dynamics) results provide two factors for the molecular design of isoform-specific fluorescent probes. One is the interaction between tetraphenyl ethylene (AIE fluorogen) and amino acid residues, and the other is the interaction between amino acid residues and the binding group. In this work, a powerful tool to detect HexA in living cells is reported for the first time. Further, a workable molecular design strategy for protein isoform-specific fluorescent probes is summarized.

Tailored Biosensors for Drug Screening, Efficacy Assessment, and Toxicity Evaluation

Biosensors have been flourishing in the field of drug discovery with pronounced developments in the past few years. They facilitate the screening and discovery of innovative drugs. However, there is still a lack of critical reviews that compare the merits and shortcomings of these biosensors from a pharmaceutical point of view. This contribution presents a critical and up-to-date overview on the recent progress of tailored biosensors, including surface plasmon resonance, fluorescent, photoelectrochemical, and electrochemical systems with emphasis on their mechanisms and applications in drug screening, efficacy assessment, and toxicity evaluation. Multiple functional nanomaterials have also been incorporated into the biosensors. Representative examples of each type of biosensors are discussed in terms of design strategy, response mechanism, and potential applications. In the end, we also compare the results and summarize the major insights gained from the works, demonstrating the challenges and prospects of biosensors-assisted drug discovery.

A bifunctional probe reveals increased viscosity and hydrogen sulfide in zebra fish model of Parkinson's disease

Altered H₂S levels and intracellular viscosity have both been seen in Parkinson's disease (PD). However, how H₂S and intracellular viscosity are involved in PD pathogenesis remains unknown. Herein, a dual-function fluorescent probe DF was designed and synthesized to analyze intracellular viscosity and hydrogen sulfide. It is a near-infrared fluorescence probe with improved photostability and large Stokes shift (110 nm). The probe reveals increased viscosity and hydrogen sulfide in zebrafish model of PD for the first time.

Fluorescent probes for bioimaging of potential biomarkers in Parkinson's disease

This review comprehensively summarizes various types of fluorescent probes for PD and their applications for detection of various PD biomarkers.

A novel ratiometric AIE-based fluorescent probe for specific detection of Hcy/Cys and imaging of living cells in vivo

The sensing mechanism toward Hcy/Cys is realized based on the condensation reaction, which breaks CN to form a thiazolidine adduct.

Recent advances of small molecule fluorescent probes for distinguishing monoamine oxidase-A and monoamine oxidase-B in vitro and in vivo

Monoamine oxidases (MAO-A and MAO-B) are the two flavin adenine dinucleotide (FAD) enzymes that play an important role in neurotransmitter homeostasis and in protection against biogenic amines. The two MAO enzymes are related to various diseases such as neurological disorders, cancer or other systemic diseases. It is crucial to distinguish these two subtypes in order to explore the pathogenesis and pathophysiology of different diseases. In this review, the relationship between MAOs and related diseases is briefly introduced. Additionally, we summarize the recent advances in small molecule fluorescent probes for specific detection of MAO-A and MAO-B.

Dual-functional AIE fluorescent probes for imaging β-amyloid plaques and lipid droplets

Alzheimer's disease (AD) is a chronic neurodegenerative disease. Better imaging and early diagnosis of biomarkers of AD is extremely important for therapeutic interventions. The amyloid cascade hypothesis and its revised version identify insoluble β-amyloid deposition as a good diagnostic biomarker for AD. Moreover, lipid droplets may also act as an auxiliary biomarker related to AD pathology based on recent studies. Herein, two quinoline-based AIE probes were designed and synthesized for the imaging of Aβ plaques and lipid droplets. The probes exhibited remarkable turn-on fluorescence enhancements with the Aβ aggregates. The lipid droplets-targeting probe FB exhibited high selectivity and binding affinity towards the Aβ aggregates with a detection limit as low as 26.9 nM. Furthermore, FB was capable of readily imaging Aβ plaques and lipid droplets at the cellular level and in brain sections of transgenic AD mice. The probe FB can serve as a promising tool for developing early diagnosis and innovative therapeutics of AD.

Mitochondrial-Targeted and Near-Infrared Fluorescence Probe for Bioimaging and Evaluating Monoamine Oxidase A Activity in Hepatic Fibrosis

Monoamine oxidase A (MAO-A) is a promising diagnostic marker for cancer, depression, Parkinson's disease, and liver disease. The fluorescence detection of MAO-A in living animals is of extreme importance for the early diagnosis of related diseases. However, the development of specific and mitochondrial-targeted and near-infrared (NIR) fluorescence MAO-A probes is still inadequate. Here, we designed and synthesized four NIR fluorescence probes containing a dihydroxanthene (DH) skeleton to detect MAO-A in complex biological systems. The specificity of our representative probe DHMP2 displays a 31-fold fluorescence turn-on in vitro, and it can effectively accumulate in the mitochondria and specifically detect the endogenous MAO-A concentrations in PC-3 and SH-SY5Y cell lines. Furthermore, the probe DHMP2 can be used to visualize the endogenous MAO-A activity in zebrafish and tumor-bearing mice. More importantly, it is the first time that the MAO-A activity of hepatic fibrosis tissues is detected through the probe DHMP2. The present study shows that the synthesized DHMP2 might serve as a potential tool for monitoring MAO-A activity in vivo and diagnosing related diseases.

An acrylate AIE-active dye with a two-photon fluorescent switch for fluorescent nanoparticles by RAFT polymerization: synthesis, molecular structure and application in cell imaging

This work reported the synthesis of a novel acrylate AIE-active dye with a reversible two-photon fluorescent switch and its amphiphilic PEG-TM polymers via RAFT polymerization, which are attractive for applications in cell imaging.

Inhibitor structure-guided design and synthesis of near-infrared fluorescent probes for monoamine oxidase A (MAO-A) and its application in living cells and in vivo

A series of near-infrared fluorescent probes based on inhibitor (clorgyline) structure-guided design were synthesized for the specific detection of MAO-A in cells and in vivo.

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.

Fine Tuning of Emission Behavior, Self-Assembly, Anion Sensing, and Mitochondria Targeting of Pyridinium-Functionalized Tetraphenylethene by Alkyl Chain Engineering

Compared to the many studies that focus on the development of novel molecular frameworks pertaining to functionalized fluorescent materials, there is lesser emphasis on side chains even though they have a significant impact on the properties and applications of fluorescent materials. In this study, a series of pyridinium-functionalized tetraphenylethene salts (TPEPy-1 to TPEPy-4) possessing different alkyl chains are synthesized, and the influence of chain length on their optical performance and applications is thoroughly investigated. By changing the alkyl chain, the fluorogens exhibit opposite emission behavior in aqueous media because of their distinct hydrophobic nature, and their solid-state emission can be fine-tuned from green to red owing to their distinct molecular configuration. In addition, by increasing the chain length, the microstructure of the self-assembled fluorogens converts from microplates to microrods with various emission colors. Moreover, TPEPy-1 exhibits dual-mode fluorescence "turn-on" response toward NO₃^- and ClO₄^- in aqueous media because the anions induce the self-assembly of fluorogens. Furthermore, the fluorogens display cellular uptake selectivity while the proper alkyl chain impels the fluorogens to penetrate the cell membrane and accumulate in the mitochondria with high specificity.

Enzyme-Responsive Bioprobes Based on the Mechanism of Aggregation-Induced Emission

Enzymes play an indispensable role in maintaining normal life activities. The abnormalities of content and activity in specific enzymes are usually associated with the occurrence and the development of major diseases. Correspondingly, fluorescent bioprobes with distinctive sensing mechanisms and different functionalities have attracted growing attention as convenient tools for optical probing and monitoring the activity of enzymes. Ideally and excitedly, the recently emerged luminogens with an aggregation-induced emission (AIE) feature could perfectly overcome the aggregation-caused quenching (ACQ) effect of conventional bioprobes. Based on the fantastic characteristics of AIE luminogens (AIEgens), specific enzyme bioprobes have been designed through integration with recognition units, demonstrating many advantages including low background interference, a high signal-to-noise ratio (SNR), and superior photostability. In this review, by presenting some typical examples, we summarize the working principle and structural design of specific AIEgen-based bioprobes that are triggered by enzymes and discuss their great potential in biomedical applications, with the aim to promote the future research of fluorescent bioprobes involving enzymes.

A sensitive fluorescent probe for alkaline phosphatase and an activity assay based on the aggregation-induced emission effect

A sensitive fluorescent probe (TPEQN-P) was designed and synthesized for detecting alkaline phosphatase and monitoring its enzymatic activity based on the specific aggregation-induced emission effect.

Recent progresses in small-molecule enzymatic fluorescent probes for cancer imaging

An overview of recent advances in small-molecule enzymatic fluorescent probes for cancer imaging, including design strategies and cancer imaging applications.

Fluorescent Sensors Based on Aggregation-Induced Emission: Recent Advances and Perspectives

Fluorescent sensors with advantages of excellent sensitivity, rapid response, and easy operation are emerging as powerful tools in environmental monitoring, biological research, and disease diagnosis. However, conventional fluorophores featured with π-planar structures usually suffer from serious self-quenching in the aggregated state, poor photostability, and small Stokes' shift. In contrast to conventional aggregation-caused quenching (ACQ) fluorophores, the newly emerged aggregation-induced emission fluorogens (AIEgens) are featured with high emission efficiency in the aggregated state, which provide unique opportunities for various sensing applications with advantages of high signal-to-noise ratio, strong photostability, and large Stokes' shift. In this review, we will first briefly give an introduction of the AIE concept and the turn-on sensing principles. Then, we will discuss the recent examples of AIE sensors according to types of analytes. Finally, we will give a perspective on the future developments of AIE sensors. We hope this review will inspire more endeavors to devote to this emerging world.

Aggregation-Induced-Emission Materials with Different Electric Charges as an Artificial Tongue: Design, Construction, and Assembly with Various Pathogenic Bacteria for Effective Bacterial Imaging and Discrimination

Imaging-based total bacterial count and type identification of bacteria play crucial roles in clinical diagnostics, public health, biological and medical science, and environmental protection. Herein, we designed and synthesized a series of tetraphenylethenes (TPEs) functionalized with one or two aldehyde, carboxylic acid, and quaternary ammonium groups, which were successfully used as fluorescent materials for rapid and efficient staining of eight kinds of representative bacterial species, including pathogenic bacteria Vibrio cholera, Klebsiella pneumoniae, and Listeria monocytogenes and potential bioterrorism agent Yersinia pestis. By comparing the fluorescence intensity changes of the aggregation-induced-emission (AIE) materials before and after bacteria incubation, the sensing mechanisms (electrostatic versus hydrophobic interactions) were simply discussed. Moreover, the designed AIE materials were successfully used as an efficient artificial tongue for bacteria discrimination, and all of the bacteria tested were identified via linear discriminant analysis. Our current work provided a general method for simultaneous broad-spectrum bacterial imaging and species discrimination, which is helpful for bacteria surveillance in many fields.

Iron modulates the activity of monoamine oxidase B in SH-SY5Y cells

Both monoamine oxidase B (MAO-B) and iron accumulation are associated with neurologic diseases including Parkinson's disease. However, the association of iron with MAO-B activity was poorly understood. Here we took advantage of highly sensitive and specific fluorescence probes to examine the change in MAO-B activity in human dopaminergic neuroblastoma (SH-SY5Y) cells upon iron exposure. Both ferric and ferrous ions could significantly enhance the activity of MAO-B, instead of MAO-A, in SH-SY5Y cells. In addition, iron-induced increase in MAO-B probe fluorescence could be prevented by pargyline and other newly developed MAO-B inhibitors, suggesting that it was MAO-B activity-dependent. These findings may suggest MAO-B is an important sensor in iron-stressed neuronal cells.

Aggregation induced enhanced and exclusively highly Stokes shifted emission from an excited state intramolecular proton transfer exhibiting molecule

The inner filter effect due to self-quenching dominates the normal emission of dyes at higher concentrations, which would limit their applications. Since normal emission was also observed with aggregation induced emission enhancement (AIEE) active excited state intramolecular proton transfer (ESIPT) exhibiting molecules, two new molecules are synthesized and studied to obtain normal emission free AIEE. The molecules are 4-(3-(benzo[d]thiazol-2-yl)-5-tert-butyl-4-hydroxybenzyl)-2-(benzo[d]thiazol-2-yl)-6-tert-butyl phenol (bis-HPBT) and its oxazole analogue (bis-HPBO). Of these molecules, bis-HPBT, which is weakly fluorescent in tetrahydrofuran solution, shows a sudden high enhancement in fluorescence upon addition of 70% water due to the formation of aggregates. Though the normal emission is also observed in tetrahydrofuran, it is completely eliminated in the aggregates, and the aggregates display exclusive tautomer emission. However, bis-HPBO does not emit such an exclusive tautomer emission in the water/tetrahydrofuran mixture. The enhancement in the fluorescence quantum yield of bis-HPBT in 70% water is ∼300 times higher than that in tetrahydrofuran. The modulated molecular structure of bis-HPBT is the cause of this outstanding AIEE. The observation of almost exclusive tautomer emission is a new additional advantage of AIEE from bis-HPBT over other ESIPT molecules. Since the tautomer emission is highly Stokes shifted, no overlap with the absorption spectrum occurs and therefore, the inner filter effect is averted. The aggregated structure acts as a good fluorescence chemosensor for metal ions as well as anions. The aggregated structure is cell permeable and can be used for cell imaging.

A selective and light-up fluorescent probe for β-galactosidase activity detection and imaging in living cells based on an AIE tetraphenylethylene derivative

An AIE based tetraphenylethylene derivative (TPE-Gal) was designed for light-up fluorescence detection and imaging of β-galactosidase activity in living cells.

An AIE based tetraphenylethylene derivative for highly selective and light-up sensing of fluoride ions in aqueous solution and in living cells

MOTIPS-TPE displayed highly selectivity for F⁻ in PBS and was employed as an AIE light-up probe for F⁻ in living HeLa cells.