Discriminatory Detection of Cysteine and Homocysteine Based on Dialdehyde-Functionalized Aggregation-Induced Emission Fluorophores

Real-time monitoring of endogenous cysteine in LPS-induced oxidative stress process with a novel lysosome-targeted fluorescent probe

Cysteine (Cys), one of essential small-molecule-based biothiols in the human body, contributes to the regulation of redox reactions and is closely associated with many physiological and pathological metabolic processes. Herein, a novel fluorescent probe, hydroxyphenyl-conjugated benzothiazole (HBT-Cys) capable of detecting Cys was constructed, where acrylate served as the recognition group and hydroxyphenyl-linked benzothiazole acted as the fluorophore. The fluorescence of the probe was negligible in the absence of Cys, and an intense blue fluorescence was observed upon addition of Cys. The Cys-sensing mechanism could be ascribed to the Cys-involved hydrolysis reaction with acrylate, leading to light up the emission at 430 nm with about 80-fold enhancement. In addition, HBT-Cys exhibited a fast response time, remarkable selectivity and low detection limit. HBT-Cys also worked well in real-time monitoring of Cys in three different food samples (wolfberry, hawthorn, and red dates). Importantly, our probe had an excellent lysosomes-targeted ability, which was successfully employed to real-time visualize the fluctuation of both exogenous and endogenous Cys in living cells and zebrafish under lipopolysaccharide (LPS)-induced oxidative stress. Hopefully, the work shown here provides a potent candidate for the real-time tracking of Cys fluctuations in various biological samples.

Tuning the selectivity of amino acid recognition with dynamic covalent bond constrained fluorophores in aqueous media

The recognition and discrimination of amino acids are generating continuous interest due to their importance. Herein we developed a series of dynamic covalent reaction constrained aldehyde-derived fluorescent probes for the binding of amino acids with tunable selectivity. Diverse emission behaviors were obtained via pH triggered movement of ring-chain tautomerization equilibrium of aldehyde probes. By taking advantage of the distinct pK_a and reactivity of aldehyde probes and amino acids, unique fluorescence signaling patterns were generated, and the selectivity for amino acid recognition was further modulated. The selective recognition of Cys/Hcy was attained at pH 7.4 as a result of thiazolidine formation. The manipulation of the reactivity at pH 10 enabled the realization of high selectivity for His and Cys, respectively. Moreover, pH and redox stimuli-responsive dynamic covalent networks were constructed for the regulation of amino acid recognition. The strategies and results described should be appealing in many aspects, including dynamic assemblies, molecular sensing, biological labeling, and smart materials.

Chemistry of Substituted Thiazinanes and Their Derivatives

Thiazinanes and its isomeric forms represent one of the most important heterocyclic compounds, and their derivatives represented a highly potent drug in disease treatment such as, 1,1-dioxido-1,2-thiazinan-1,6-naphthyridine, which has been shown to have anti-HIV activity by a mechanism that should work as anti-AIDS treatment, while (Z)-methyl 3-(naphthalen-1-ylimino)- 2-thia-4-azaspiro[5 5]undecane-4-carbodithioate showed analgesic activity, cephradine was used as antibiotic and chlormezanone was utilized as anticoagulants. All publications were interested in the chemistry of thiazine (partially or fully unsaturated heterocyclic six-membered ring containing nitrogen and sulfur), but no one was dealing with thiazinane itself which encouraged us to shed new light on these interesting heterocycles. This review was focused on the synthetic approaches of thiazinane derivatives and their chemical reactivity.

Aggregation-Induced Emission Luminogens for Activity-Based Sensing

Fluorescent sensing has emerged as a powerful tool for detecting various analytes and visualizing numerous biological processes by virtue of its superb sensitivity, rapidness, excellent temporal resolution, easy operation, and low cost. Of particular interest is activity-based sensing (ABS), a burgeoning sensing approach that is actualized on the basis of dynamic molecular reactivity rather than conventional lock-and-key molecular recognition. ABS has been recognized to possess some distinct advantages, such as high specificity, extraordinary sensitivity, and accurate signal outputs. A majority of ABS sensors are constructed by modifying conventional fluorogens, which are strongly emissive when molecularly dissolved in solvents but experience emission quenching upon aggregate formation or concentration increase. The aggregation-caused quenching (ACQ) phenomenon leads to a limited amount of labeling of the analyte with the sensor and low photobleaching resistance, which could impede practical applications of the ABS protocol. As an anti-ACQ phenomenon, aggregation-induced emission (AIE) provides a straightforward solution to the ACQ problem. Thanks to their intrinsic advantages, including high photobleaching threshold, high signal-to-noise ratio, fluorescence turn-on nature, and large Stokes shift, AIE-active luminogens (AIEgens) represent a class of extraordinary fluorogen alternatives for the ABS protocol. The use of AIEgen-involved ABS can integrate the advantages of AIEgens and ABS, and additionally, the AIE process offers some unique properties to the ABS approach. For instance, in some cases of water-soluble AIEgen-involved ABS, chemical reaction not only leads to a chang in the emission color of the AIEgens but also causes solubility variations, which could result in specific "light-up" signaling. In this Account, the basic concepts and mechanistic insights of the ABS approach involving the AIE principle are briefly summarized, and then we highlight the new breakthroughs, seminal studies, and trends in the area that have been most recently reported by our group. This emerging sensing protocol has been successfully utilized for detecting an array of targets including ions, small molecules, biomacromolecules, and microenvironments, all of which closely relate to human health, medical, and public concerns. These detections are smoothly achieved on the basis of various reactions (e.g., hydrolysis, boronate cleavage, dephosphorylation, addition, cyclization, and rearrangement reactions) through different sensing principles. In these studies, the AIEgen-involved ABS strategy generally shows good biocompatibility, high selectivity, excellent reliability and high signal contrast, strongly indicating its great potential for high-tech innovations in the sensing field, among which bioprobing is of particular interest. With this Account, we hope to spark new ideas and inspire new endeavors in this emerging research area, further promoting state-of-the-art developments in the field of sensing.

The Biological Applications of Two Aggregation-Induced Emission Luminogens

Fluorescence imaging, as a commonly used scientific tool, is widely applied in various biomedical and material structures through visualization technology. Highly selective and sensitive luminescent biological probes, as well as those with good water solubility, are urgently needed for biomedical research. In contrast to the traditional aggregation-caused quenching of fluorescence, in the unique phenomenon of aggregation-induced emission (AIE), the individual luminogens have extremely weak or no emissivity because they each have free intramolecular motion; however, when they form aggregates, these components immediately "light up". Since the discovery of "turn-on" mechanism, researchers have been studying and applying AIE in a variety of fields to develop more sensitive, selective, and efficient strategies for the AIE dyes. There are numerous advantages to the use of AIE-based methods, including low background interference, strong contrast, high performance in intracellular imaging, and the ability for long-term monitoring in vivo. In this review, two typical examples of AIEgens, TPE-Cy and TPE-Ph-In, are described, including their structure properties and applications. Recent progress in the biological applications is mainly focused on. Undoubtedly, in the near future, an increasing number of encouraging and practical ideas will promote the development of more AIEgens for broad use in biomedical applications.

Rational design of a ratiometric two-photon fluorescent probe for real-time visualization of apoptosis

A dialdehyde-functionalized ratiometric two-photon fluorescent probe (Mito-DCHO) based on a symmetric carbazole-containing two-dimensional ICT system was rationally designed for cysteine-specific detection in mitochondria, which was utilized for real-time assessing and dual-channel visualization of the early stage of apoptosis by monitoring mitochondrial oxidative stress levels.

Theranostics based on AIEgens

The utilization of luminogens with aggregation-induced emission (AIE) characteristics has recently been developed at a tremendous pace in the area of theranostics, mainly because AIE luminogens (AIEgens) hold various distinct advantages, such as good biocompatibility, excellent fluorescence properties, simple preparation and modification, perfect size of nano-aggregation for enhanced permeability and retention effect, promoted efficiencies of photodynamic and photothermal therapies, efficient photoacoustic imaging, and ready constructions of multimodal imaging and therapy. Significant breakthroughs and developments of theranostics based on AIEgens have been achieved in the past few years, and great progress has been witnessed in many theranostic modalities, indicating that AIEgens remarkably complement conventional theranostic materials and promote the development of theranostics. This review provides theoretical insights into the advantages of AIEgens in theranostics, and systematically summarizes the basic concepts, seminal studies, recent trends and perspectives in theranostics based on AIEgens. We believe that AIEgens would be promising multifunctional theranostic platforms in clinical fields and facilitate significant advancements in this research-active area.

Manipulating and visualizing the dynamic aggregation-induced emission within a confined quartz nanopore

Aggregation-induced emission (AIE) as a unique photophysical process has been intensively explored for their features in fields from optical sensing, bioimaging to optoelectronic devices. However, all AIE luminogens (AIEgens) hardly recover into the initial dispersed state after illuminating at the ultimate aggregated state, which limits AIEgens to achieve reversible sensing and reproducible devices. To real-time manipulate the emission of AIEgen, here we take the advantage of confined space in the quartz nanopore to achieve a nanopore-size-dependent restriction of AIEgens for reversible conversions of “on-to-off” and “off-to-on” emission. By electrochemically manipulating 26 fL AIEgen solution inside nanopore confinement, AIE illuminates while moves along nanopore from the constricted tip to inside cavity at a velocity of 1.4–2.2 μm s⁻¹, and vice versa. We further apply this dynamic manipulation for a target delivery of AIEgen into single cells, which opens up new possibility to design powerful and practical AIE applications.

The difficulty in recovering the aggregation-induced emission fluorogens (AIEgens) to the initial dispersed state upon illuminating has limited their applications. Here, the authors employ the confined space in the quartz nanopore to achieve a nanopore-size dependent restriction of AIEgens.

A highly sensitive fluorescent probe based on the Michael addition mechanism with a large Stokes shift for cellular thiols imaging

A novel fluorescent probe IPY-MAL for thiols was developed based on imidazo[1,5-α]pyridine derivative, which was decorated with a maleimide group. The probe IPY-MAL showed a rapid response (30 s), high sensitivity and selectivity for thiols with a large Stokes shift (140 nm), which was triggered by the Michael addition reaction of thiols toward the C=C double bond of the maleimide group. Moreover, this probe IPY-MAL could quantitatively detect the concentrations of thiols ranging from 0 to 50 μM, and the detection limit was found to be as low as 28 nM. Cell imaging results indicated that the probe IPY-MAL could detect and visualize thiols in the living cells. Graphical abstract A novel imidazo[1,5-α]pyridine-based fluorescent probe was developed for sensitively monitoring and imaging thiols in living A549 cells with a large Stokes shift.

A Fluorescent Coumarin-Based Probe for the Fast Detection of Cysteine with Live Cell Application

A new coumarin-based fluorescent probe, containing an allylic esters group, has been designed and synthesized for sensing cysteine in physiological pH. In this fluorescent probe, the coumarin was applied as the fluorophore and an allylic esters group was combined as both a fluorescence quencher and a recognition unit. The probe can selectively and sensitively detect cysteine (Cys) over homocysteine, glutathione, and other amino acids, and has a rapid response time of 30 min and a low detection limit of 47.7 nM. In addition, the probe could be applied for cell imaging with low cytotoxicity.

A simple fluorescent probe for sensing cysteine over homocysteine and glutathione based on PET

A big challenge is the discrimination of sulfhydryl-containing amino acids due to their structural similarity. We designed and synthesized a simple fluorescent probe 3 for specific detection of cysteine based on photo-induced electron transfer (PET). The acrylate and BODIPY moieties in probe 3 act as a reaction site and reporter group, respectively. So the synergistic effect of the substituent groups endows probe 3 very strong green fluorescence at 525nm (λ_ex=500nm). The cleavage reaction induced by cysteine leads to acrylate hydrolysis, and thereby triggers PET on, which effectively quench the fluorescence of 3. Probe 3 exhibited a rapid response towards cysteine over homocysteine and glutathione. Probe 3 is successfully applied for sensing and imaging cysteine in vitro or in vivo cells with low cytotoxicity.

A simple colorimetric and fluorescent probe with high selectivity towards cysteine over homocysteine and glutathione

A novel turn-on fluorescent sensor AQDA with high selective towards cysteine.

A gold nanocluster-based ratiometric fluorescent probe for cysteine and homocysteine detection in living cells

A novel organic–inorganic ratiometric fluorescent probe AuNCs–NBD was developed for the detection of cysteine (Cys) and homocysteine (Hcy).

Ratiometric Detection of β-Amyloid and Discrimination from Lectins by a Supramolecular AIE Glyconanoparticle

While the development of AIE (aggregation-induced-emission) based fluorimetric probes for biological applications has been an active research area, probes with a ratiometric signal for biomolecular recognition have been rare. Here, a ratiometric AIE glyconanoparticle formed by the supramolecular assembly between a silole-based AIEgen and fluorescent glycoprobes for the detection of amyloid β (Aβ) peptides and fibrils, which are a signature of neurological disorders such as the Alzheimer's disease, is shown. Complexation of glycoprobes with the AIEgen produces an intensive fluorescence emission of the former because of a Förster resonance energy transfer between the two molecules. Subsequently, the presence of Aβ dissembles the particle, producing a fluorescence emission of the AIEgen. Interestingly, the addition of lectins that selectively recognize the glycoprobes results in a different ratiometric response of the particle, thereby enabling a discrimination from Aβ detection. This research offers insight into the simple construction of multifunctional ratiometric probes based on the supramolecular hybridization of a wide variety of AIEgens with fluorescent molecular probes.

A Simple and Effective Ratiometric Fluorescent Probe for the Selective Detection of Cysteine and Homocysteine in Aqueous Media

Biothiols such as cysteine (Cys) and homocysteine (Hcy) are essential biomolecules participating in molecular and physiological processes in an organism. However, their selective detection remains challenging. In this study, ethyl 2-(3-formyl-4-hydroxyphenyl)-4-methylthiazole-5-carboxylate (NL) was synthesized as a ratiometric fluorescent probe for the rapid and selective detection of Cys and Hcy over glutathione (GSH) and other amino acids. The fluorescence intensity of the probe in the presence of Cys/Hcy increased about 3-fold at a concentration of 20 equiv. of the probe, compared with that in the absence of these chemicals in aqueous media. The limits of detection of the fluorescent assay were 0.911 μM and 0.828 μM of Cys and Hcy, respectively. ¹H-NMR and MS analyses indicated that an excited-state intramolecular proton transfer is the mechanism of fluorescence sensing. This ratiometric probe is structurally simple and highly selective. The results suggest that it has useful applications in analytical chemistry and diagnostics.

A mitochondria-targeted turn-on fluorescent probe for the detection of glutathione in living cells

A novel turn-on red fluorescent BODIPY-based probe (Probe 1) for the detection of glutathione was developed. Such a probe carries a para-dinitrophenoxy benzyl pyridinium moiety at the meso position of a BODIPY dye as self-immolative linker. Probe 1 responds selectively to glutathione with the detection limit of 109nM over other amino acids, common metal ions, reactive oxygen species, reactive nitrogen species, and reactive sulfur species. A novel electrostatic interaction to modulate the SNAr attack of glutathione was believed to play significant role for the observed selective response to glutathione. The cleavage of dinitrophenyl ether by glutathione leads to the production of para-hydroxybenzyl moiety which is able to self-immolate through an intramolecular 1,4-elimination reaction to release the fluorescent BODIPY dye. The low toxic probe has been successfully used to detect mitochondrial glutathione in living cells.

Selective detection of nitrite ion by an AIE-active tetraphenylethene dye through a reduction step in aqueous media

We report selective and sensitive nitrite sensor in water based on AIE-active tetraphenylethene bearing amino functionality.

An aggregation-induced emission (AIE) ratiometric fluorescent cysteine probe with an exceptionally large blue shift

A novel colorimetric and ratiometric AIE probe for monitoring of cysteine based on a coumarin Schiff-base platform is developed.

Dual emission channels for sensitive discrimination of Cys/Hcy and GSH in plasma and cells

A new selective fluorescent and colorimetric chemosensor for the detection of GSH was developed. The discrimination of GSH from Cys and Hcy is achieved through two emission channel detection. The detection limit of probe 1 for GSH reached 10 nM (3 ppb). The excellent sensitivity and selectivity of probe 1 allow the selective detection of GSH over Cys and Hcy, which can be visualized colorimetrically and/or fluorescently. The sensitive detection of GSH allowed for convenient measurement of the GSH content in human plasma. The presence of GSH in cells was demonstrated through cell imaging.

Acid-Activatable Michael-Type Fluorescent Probes for Thiols and for Labeling Lysosomes in Live Cells

A Michael addition is usually taken as a base-catalyzed reaction. Most fluorescent probes have been designed to detect thiols in slightly alkaline solutions (pH 7-9). The sensing reactions of almost all Michael-type fluorescent probes for thiols are faster in a high pH solution than in a low pH solution. In this work, we synthesized a series of 7-substituted 2-(quinolin-2-ylmethylene)malonic acids (QMAs, substituents: NEt2, OH, H, Cl, or NO2) and their ethyl esters (QMEs) as Michael-type fluorescent probes for thiols. The sensing reactions of QMAs and QMEs occur in distinct pH ranges, pH < 7 for QMAs and pH > 7 for QMEs. On the basis of experimental and theoretic studies, we have clarified the distinct pH effects on the sensing reactivity between QMAs and QMEs and demonstrated that two QMAs (NEt2, OH) are highly sensitive and selective fluorescent probes for thiols in acidic solutions (pH < 7) and promising dyes that can label lysosomes in live cells.

A flavone-based turn-on fluorescent probe for intracellular cysteine/homocysteine sensing with high selectivity

A new type of flavone-based fluorescent probe (DMAF) capable of cysteine (Cys)/homocysteine (Hcy) sensing with high selectivity over other amino acids was developed. Such type of probe undergoes Cys/Hcy-mediated cyclization reaction with the involvement of its aldehyde group, which suppresses of the photoinduced electron transfer (PET) process of the probe molecule and consequently leads to the enhancement of fluorescence emission upon excitation using visible light. The formation of product of the Cys/Hcy-mediated cyclization reaction was confirmed and the preliminary fluorescence imaging experiments revealed the biocompatibility of the as-prepared probe and validated its practicability for intracellular Cys/Hcy sensing.

Design strategies of fluorescent probes for selective detection among biothiols

Simple thiol derivatives, such as cysteine (Cys), homocysteine (Hcy), and glutathione (GSH), play key roles in biological processes, and the fluorescent probes to detect such thiols in vivo selectively with high sensitivity and fast response times are critical for understanding their numerous functions. However, the similar structures and reactivities of these thiols pose considerable challenges to the development of such probes. This review focuses on various strategies for the design of fluorescent probes for the selective detection of biothiols. We classify the fluorescent probes for discrimination among biothiols according to reaction types between the probes and thiols such as cyclization with aldehydes, conjugate addition-cyclization with acrylates, native chemical ligation, and aromatic substitution-rearrangement.

Old is new again: a chemical probe for targeting mitochondria and monitoring mitochondrial membrane potential in cells

The tetraphenylethene-indolium molecule (TPE-indo) can both image the mitochondria and indicate mitochondrial activity by the fluorescence change of TPE-indo.

Synthesis of triphenylamine-containing conjugated polyelectrolyte and fabrication of fluorescence color-changeable, paper-based sensor strips for biothiol detection

A new emission color-changeable (green-to-blue) conjugated polyelectrolyte was synthesized to use in biothiol sensing with its paper-based strip.

Amine functionalized tetraphenylethylene: a novel aggregation-induced emission based fluorescent chemodosimeter for nitrite and nitrate ions

A novel AIE-based fluorescent probe for the detection of trace amounts of nitrite and nitrate ions in water has been developed, which spontaneously detects nitrites (or nitrates) by a fluorescence “turn-off” method.

Protein recognition using synthetic small-molecular binders toward optical protein sensing in vitro and in live cells

This review describes the recognition and sensing techniques of proteins and their building blocks by use of small synthetic binders.

A multi-functional probe to discriminate Lys, Arg, His, Cys, Hcy and GSH from common amino acids

A smart probe to classify and differentiate basic amino acids, thiol-containing amino acids and GSH in selected environments.

Fluorogen-peptide conjugates with tunable aggregation-induced emission characteristics for bioprobe design

Fluorogens with aggregation-induced emission (AIE) characteristics are attracting intense research interest, and an AIE-peptide conjugate strategy has been reported for developing turn-on probes based on hydrophilic peptide ligands. To build a model also suitable for hydrophobic ligands, we propose to fine-tune the AIE characteristics for probe design. In this work, an iconic AIE fluorogen tetraphenylethene (TPE) was designed to conjugate with peptide fragments containing different numbers of aspartic acid (D) units. Relationships between the numbers of D and the hydrophilicity, optical properties, and aggregate sizes and the AIE characteristics of TPE-peptide conjugates were investigated carefully. Five carboxyl groups were found to be the threshold to "turn off" the fluorescence of TPE. As a proof-of-concept, TPE-SS-D5 containing a cleavable disulfide bond was synthesized for thiol turn-on detection. The validated tunable AIE characteristic offers new opportunities to design fluorescence turn-on probes based on hydrophobic recognition elements and AIE fluorogens.

Aggregation-induced emission: the whole is more brilliant than the parts

"United we stand, divided we fall."--Aesop. Aggregation-induced emission (AIE) refers to a photophysical phenomenon shown by a group of luminogenic materials that are non-emissive when they are dissolved in good solvents as molecules but become highly luminescent when they are clustered in poor solvents or solid state as aggregates. In this Review we summarize the recent progresses made in the area of AIE research. We conduct mechanistic analyses of the AIE processes, unify the restriction of intramolecular motions (RIM) as the main cause for the AIE effects, and derive RIM-based molecular engineering strategies for the design of new AIE luminogens (AIEgens). Typical examples of the newly developed AIEgens and their high-tech applications as optoelectronic materials, chemical sensors and biomedical probes are presented and discussed.

Differences in heterocycle basicity distinguish homocysteine from cysteine using aldehyde-bearing fluorophores

We report the detection of homocysteine over cysteine based upon characteristic differences between 5- and 6-membered heterocyclic amines formed upon reaction with aldehyde-bearing compounds. Homocysteine-derived thiazinane-4-carboxylic acids are more basic than cysteine-derived thiazolidines-4-carboxylic acids. Fluorescence enhancement in response to homocysteine is achieved by tuning pH and excitation wavelength.

A simple and effective coumarin-based fluorescent probe for cysteine

Acrylic acid 3-acetyl-2-oxo-2 H-chromen-7-yl ester (ACA) was rationally designed and synthesized as a simple and effective fluorescent probe for sensing cysteine with high selectivity and naked-eye detection. The probe can detect cysteine by fluorescence spectrometry with a detection limit of 0.657 μM and can be used with calf serum and in live cell imaging. The conjugate addition/cyclization sequence mechanism of the reaction between ACA and cysteine was confirmed by ESI-MS and fluorescence spectra.