A targeted activatable NIR-II nanoprobe for positive visualization of anastomotic thrombosis and sensitive identification of fresh fibrinolytic thrombus

Anastomotic thrombosis prevalently causes anastomosis failure, accompanied with ischemia and necrosis, the early diagnosis of which is restricted by inherent shortcomings of traditional imaging techniques in clinic and lack of appropriate prodromal biomarkers for thrombosis initiation. Herein, a fresh thrombus-specific molecular event, protein disulfide isomerase (PDI) is innovatively chosen as the activating factor, and a thrombosis targeting and PDI-responsive turn-on near infrared II (NIR-II) fluorescence nanoprobe is firstly developed. The supramolecular complex-based nanoprobe IR806-PDA@BSA-CREKA is fabricated by assembling NIR-II emitting cyanine derivative IR806-PDA with bovine serum albumin (BSA), which could ameliorate the stability and pharmacokinetics of the nanoprobe, addressing the contradiction in the balance of brightness and biocompatibility. The NIR-II-off nanoprobe exhibits robust turn-on NIR-II fluorescence upon PDI-specific activation, in vitro and in vivo. Of note, the constructed nanoprobe demonstrates superior photophysical stability, efficient fibrin targeting peptide-derived thrombosis binding and a maximum signal-to-background ratio (SBR) of 9.30 for anastomotic thrombosis in NIR-II fluorescent imaging. In conclusion, the exploited strategy enables positive visualized diagnosis for anastomotic thrombosis and dynamic monitoring for thrombolysis of fresh fibrinolytic thrombus, potentially contributes a novel strategy for guiding the therapeutic selection between thrombolysis and thrombectomy for thrombosis treatment in clinic.

Enlightening the Path to Protein Engineering: Chemoselective Turn-On Probes for High-Throughput Screening of Enzymatic Activity

Many successful stories in enzyme engineering are based on the creation of randomized diversity in large mutant libraries, containing millions to billions of enzyme variants. Methods that enabled their evaluation with high throughput are dominated by spectroscopic techniques due to their high speed and sensitivity. A large proportion of studies relies on fluorogenic substrates that mimic the chemical properties of the target or coupled enzymatic assays with an optical read-out that assesses the desired catalytic efficiency indirectly. The most reliable hits, however, are achieved by screening for conversions of the starting material to the desired product. For this purpose, functional group assays offer a general approach to achieve a fast, optical read-out. They use the chemoselectivity, differences in electronic and steric properties of various functional groups, to reduce the number of false-positive results and the analytical noise stemming from enzymatic background activities. This review summarizes the developments and use of functional group probes for chemoselective derivatizations, with a clear focus on screening for enzymatic activity in protein engineering.

Photoinduced electron transfer (PeT) based fluorescent probes for cellular imaging and disease therapy

Typical PeT-based fluorescent probes are multi-component systems where a fluorophore is connected to a recognition/activating group by an unconjugated linker. PeT-based fluorescent probes are powerful tools for cell imaging and disease diagnosis due to their low fluorescence background and significant fluorescence enhancement towards the target. This review provides research progress towards PeT-based fluorescent probes that target cell polarity, pH and biological species (reactive oxygen species, biothiols, biomacromolecules, etc.) over the last five years. In particular, we emphasise the molecular design strategies, mechanisms, and application of these probes. As such, this review aims to provide guidance and to enable researchers to develop new and improved PeT-based fluorescent probes, as well as promoting the use of PeT-based systems for sensing, imaging, and disease therapy.

Methylated Chromenoquinoline as a Novel Nucleus Fluorescent Probe for Nucleic Acid Imaging

Two chromenoquinoline-based fluorescent probes 1a-b have been synthesized and investigated. Photofading behaviors of compounds 1a-b showed that at least 89% absorption remained after 6 h irradiating, meanwhile, many of ions and amino acids had negligible impacts on their fluorescence intensity, which meant they had excellent photostability and selectivity. Probes 1a-b exhibited strong absorption and emission in organic solvents with large fluorescence quantum yields, even in water probe 1a still had a relatively large fluorescence quantum yield (20%). Combined with DFT calculation, the influence of alkylation on optical properties of 1b was elucidated. In addition, the fluorescence intensity of probe 1b with red emission enhanced by 5.4-fold and 5.3-fold after DNA and RNA added, and the fluorescence quantum yield increased from 3% to 17% and 14%, respectively, but the neutral molecule 1a had no response to nucleic acid. Furthermore, confocal microscopy imaging of probes 1a-b showed that 1a targeted lipid droplets while the methylated probe 1b to nucleus in living HeLa cells. The results indicated that the subcellular targeting zone could be changed by alkylation of nitrogen atom on chromenoquinoline-based conveniently, which provided a new idea for designing and synthesizing new subcellular labeled probes.

I2/DMSO-Promoted Synthesis of Chromeno[4,3-b]quinolines through an Imine Formation/Aza-Diels–Alder/Aromatization Tandem Reaction under Metal-Catalyst- and Photosensitizer-Free Conditions

A tandem approach was developed for the efficient synthesis of substituted chromeno[4,3-b]quinolines from arylamines and O-cinnamyl salicylaldehydes under metal-catalyst- and photosensitizer-free reaction conditions. Our protocol is based on an inexpensive I2/DMSO system in which molecular iodine first acts as a Lewis acid to promote the formation of the corresponding imine bearing the alkene moiety; then, this species fulfills a second role by catalyzing an intramolecular aza-Diels–Alder cycloaddition to generate the respective tetrahydrochromenoquinoline as an intermediate. Finally, the dual behavior of DMSO as an oxidant and as a solvent proved crucial at this stage, allowing the regeneration of I2 and promoting the aromatization of the tetrahydrochromenoquinoline intermediates to yield the desired 7-aryl-6H-chromeno[4,3-b]quinolines. This protocol is mild and easy to perform, features high step-economy (tandem process) and provides a new access to biologically important nitrogen- and oxygen-containing heterocyclic molecules.

Molecular Probes, Chemosensors, and Nanosensors for Optical Detection of Biorelevant Molecules and Ions in Aqueous Media and Biofluids

Synthetic molecular probes, chemosensors, and nanosensors used in combination with innovative assay protocols hold great potential for the development of robust, low-cost, and fast-responding sensors that are applicable in biofluids (urine, blood, and saliva). Particularly, the development of sensors for metabolites, neurotransmitters, drugs, and inorganic ions is highly desirable due to a lack of suitable biosensors. In addition, the monitoring and analysis of metabolic and signaling networks in cells and organisms by optical probes and chemosensors is becoming increasingly important in molecular biology and medicine. Thus, new perspectives for personalized diagnostics, theranostics, and biochemical/medical research will be unlocked when standing limitations of artificial binders and receptors are overcome. In this review, we survey synthetic sensing systems that have promising (future) application potential for the detection of small molecules, cations, and anions in aqueous media and biofluids. Special attention was given to sensing systems that provide a readily measurable optical signal through dynamic covalent chemistry, supramolecular host-guest interactions, or nanoparticles featuring plasmonic effects. This review shall also enable the reader to evaluate the current performance of molecular probes, chemosensors, and nanosensors in terms of sensitivity and selectivity with respect to practical requirement, and thereby inspiring new ideas for the development of further advanced systems.

Long wavelength emission fluorescent probe for highly selective detection of cysteine in living cells

We developed a fluorescent probe, named 2-(4-(acryloyloxy) phenyl)-4-(2-carboxyphenyl)-7-(diethylamino) chromenylium (PA-A), for detecting Cys using the -OH protection/deprotection strategy, which can react with Cys to form a red-emitting anthocyanidin derivative fluorophore. The probe has high selectivity to Cys over Hcy and GSH in phosphate buffer solution (PBS, 10 mM, pH = 7.4), high sensitivity, a low detection limit of 4.48 × 10^-8 mol/L, and it can be recognized with the naked eye. Fluorescence imaging experiment of Cys with PA-A at the cellular successfully showed excellent tissue penetration.

A Flexible Synthetic Approach to Fluorescent Chromeno[4,3-b]pyridines and Pyrano[3,2-c]chromenes from Electron-Deficient 3-Vinylchromones

We report a flexible approach to the synthesis of phenanthrene-like heterocycles through organocatalytic ANRORC (Addition of the Nucleophile, Ring Opening, and Ring Closure) reaction of electron-deficient 3-vinylchromones with cyanoacetamide. Addition of highly basic DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) or tetramethylguanidine (TMG) at 80 °C leads to chromeno[4,3-b]pyridines in good yields, whereas Et3 N at 20 °C made it possible to obtain the less accessible pyrano[3,2-c]chromenes and their 2-imines. The synthesis proceeds in mild conditions (EtOH, 20-80 °C), is versatile and applicable for a wide scope of reactants. The obtained compounds show bright fluorescence in the range 460-595 nm with high quantum yields (up to 0.84) in various solvents (MeCN, DMSO, EtOH, H2 O).

A long-lived Donor-Acceptor fluorescent probe for sequential detection of Cu2+ and biothiols

A new long-lived Donor-Acceptor (D-A) fluorophore based on carbazolyl dicyanobenzene was developed as an ON-OFF-ON multifunctional fluorescent probe 1 for sequential detection of Cu²⁺ and biothiols (Cys, Hcy and GSH). The fluorescence of probe 1 can be significantly and selectively quenched by Cu²⁺. Meanwhile, the fluorescence lifetime decreased from 2.1 μs to 18.5 ns. The limit of detection was determined to be 33.6 nM. Upon addition of biothiols (Cys, Hcy and GSH), the generated ensemble 1-Cu²⁺ displayed a "turn-on" fluorescent response at 555 nm and an obvious recovery in fluorescence lifetime and UV-vis absorption within 1 min. The limit of detection for Cys, Hcy and GSH were calculated by fluorescence titration experiments to be 0.19, 0.21 and 0.29 μM, respectively. The ensemble 1-Cu²⁺ was further successfully applied in bioimaging.

A turn-on Schiff base fluorescent probe for the exogenous and endogenous Fe3+ ion sensing and bioimaging of living cells

A Schiff base fluorescent probe, namely naphthalic anhydride – (2-pyridine) hydrazone (NAH), has been synthesized and developed for the highly selective and sensitive monitoring of Fe³⁺ ions in an aqueous solution and living cells.

Domino Transformations of Ene/Yne Tethered Salicylaldehyde Derivatives: Pluripotent Platforms for the Construction of High sp3 Content and Privileged Architectures

Diversity-oriented synthesis (DOS) has become a powerful synthetic tool that facilitates the construction of nature-inspired and privileged chemical space, particularly for sp³ -rich non-flat scaffolds, which are needed for phenotypic screening campaigns. These diverse compound collections led to the discovery of novel chemotypes that can modulate the protein function in underrepresented biological space. In this context, starting material-driven DOS is one of the most important tools used to build diverse compound libraries with rich stereochemical and scaffold diversity. To this end, ene/yne tethered salicylaldehyde derivatives have emerged as a pluripotent chemical platform, the products of which led to the construction of a privileged chemical space with significant biological activities. In this review, various domino transformations employing o-alkene/alkyne tethered aryl aldehyde/ketone platforms are described and discussed, with emphasis on the period from 2011 to date.

Heterogeneous Amberlyst-15-catalyzed synthesis of complex hybrid heterocycles containing [1,6]-naphthyridine under metal-free green conditions

An efficient green protocol for the synthesis of complex hybrid heterocycles containing [1,6]-naphthyridine and coumarin/pyrazole moieties was established, involving an intramolecular [4 + 2] hetero Diels-Alder reaction as the key step. The biologically significant 12,13-dihydro-6H-benzo[h]chromeno[3,4-b][1,6]naphthyridin-6-ones and 6,10-dihydro-5H-benzo[h]pyrazolo[3,4-b][1,6]naphthyridines were synthesized starting from 2-(N-propargylamino)-arylaldehydes and 3-aminocoumarins or 3-methyl-1-aryl-1H-pyrazol-5-amines in the presence of an Amberlyst-15 catalyst in PEG-200 in good yields. The easy access to diverse complex molecules in a single operation from readily available starting materials, a commercially available, transition metal-free and recyclable catalyst, the use of a green solvent, a very high atom economy and the release of water as the only side product are the highlights of this protocol.

A novel and simple imidazo[1,2-a]pyridin fluorescent probe for the sensitive and selective imaging of cysteine in living cells and zebrafish

Cysteine (Cys), homocysteine (Hcy) and glutathione (GSH) play many crucial physiological roles in organisms. Their abnormal levels can cause and indicate various diseases. In the present study, a small-molecule fluorescent probe 2-(imidazo[1,2-a]pyridin-2-yl)phenyl acrylate (IPPA) was designed, synthesized and characterized by NMR, FT-IR and HRMS. IPPA can selectively detect Cys over other analytes because of an approximately 76 times enhancement in fluorescence intensity. The limit of detection of IPPA for Cys was 0.33 μM. The pseudo-first-order rate constant of the reaction between IPPA and Cys was approximately 10 times that of the reaction between IPPA and Hcy (KCys 3.18 × 10-3 S-1vs KHcy 4.92 × 10-4 S-1), indicating that Cys can be distinguished from Hcy. In addition, IPPA exhibits strong anti-interference ability, small molecular weight, high efficiency, low toxicity and good cell permeability. It was successfully used in imaging HepG2 cells and zebrafish. The fluorescence response of IPPA for calf serum are powerful proofs for practical application. Therefore, IPPA has high potential for bioassay applications.

Fluorescent probe for Cu2+ and the secondary application of the resultant complex to detect cysteine

A special fluorescent probe has been developed, one that demonstrated excellent "off-on-type" change in fluorescence with high selectivity toward Cu2+. Interestingly, the probe-Cu2+ complex could detect cysteine due to the ability of this amino acid to strongly coordinate Cu2+, and no obvious interference was observed from other amino acids and anions. According to the proposed mechanism, addition of cysteine induced decomplexation of the probe-Cu2+ form. Furthermore, the results of confocal microscopy experiments demonstrated the potential of using the probe to image Cu2+ in living cells and mice.

A chromenoquinoline-based two-photon fluorescent probe for the highly specific and fast visualization of sulfur dioxide derivatives in living cells and zebrafish

Sulfur dioxide (SO₂) derivatives play critical roles in various biological processes. Therefore, effective methods for monitoring SO₂ are of vital importance in bisulfite/sulfite biology. In this study, a two-photon (TP) imaging probe (CQ-SO₂) for detecting SO₂ derivatives was designed and constructed, based on the chromenoquinoline (CQ) fluorophore and a β-chlorovinyl aldehyde sensing moiety. The TP properties of the CQ derivatives were revealed for the first time in this study. This study enriched the biological application range of CQ derivatives and also provided a new choice for the development of TP dyes. In particular, the CQ-SO₂ probe exhibited a fast response time (about 5 s), low detection limit (16 nM) and ultrahigh specificity towards SO₂ derivatives. Furthermore, the probe was successfully applied to the highly specific TP bioimaging of SO₂ derivatives in living cells and zebrafish.

Covalent Modification of Biomolecules through Maleimide-Based Labeling Strategies

Since their first use in bioconjugation more than 50 years ago, maleimides have become privileged chemical partners for the site-selective modification of proteins via thio-Michael addition of biothiols and, to a lesser extent, via Diels-Alder (DA) reactions with biocompatible dienes. Prominent examples include immunotoxins and marketed maleimide-based antibody-drug conjugates (ADCs) such as Adcetris, which are used in cancer therapies. Among the key factors in the success of these groups is the availability of several maleimides that can be N-functionalized by fluorophores, affinity tags, spin labels, and pharmacophores, as well as their unique reactivities in terms of selectivity and kinetics. However, maleimide conjugate reactions have long been considered irreversible, and only recently have systematic studies regarding their reversibility and stability toward hydrolysis been reported. This review provides an overview of the diverse applications for maleimides in bioconjugation, highlighting their strengths and weaknesses, which are being overcome by recent strategies. Finally, the fluorescence quenching ability of maleimides was leveraged for the preparation of fluorogenic probes, which are mainly used for the specific detection of thiol analytes. A summary of the reported structures, their photophysical features, and their relative efficiencies is discussed in the last part of the review.

Tuning the Photoinduced Electron Transfer in a Zr-MOF: Toward Solid-State Fluorescent Molecular Switch and Turn-On Sensor

The immobilization of fluorescent photoinduced electron transfer (PET) switches/sensors into solid state, which usually cannot maintain their identical properties in solution, has remained a big challenge. Herein, a water-stable anthracene and maleimide appended zirconium-based-metal-organic framework (Zr-MOF; UiO-68-An/Ma) is reported. Unlike the regular intramolecular "fluorophore-spacer-receptor" format, the separated immobilization of fluorescent (anthracene) and acceptor (maleimide) groups into the framework of a multivariate MOF can also favor a pseudo-intramolecular fluorescent PET process, resulting in UiO-68-An/Ma with very weak fluorescence. Interestingly, after Diels-Alder reaction or thiol-ene reaction of maleimide groups, the pseudo-intramolecular fluorescent PET process in UiO-68-An/Ma fails and the solid-state fluorescence of the crystals is recovered. In addition, UiO-68-An/Ma shows an interesting application as solid-state fluorescent turn-on sensor for biothiols, with the naked eye response at a low concentration of 50 µmol L-1 within 5 min. This study represents a general strategy to enable the efficient tuning of fluorescent PET switches/sensors in solid state, and considering the fluorescence of the PET-based MOFs can be restored after addition of analyte/target species, this research will definitely inspire to construct stimuli-responsive fluorescent MOFs for interesting applications (e.g., logic gate) in future.

Quinoline-derived fluorescent probes for the discrimination of Cys from Hcys/GSH and bioimaging in living cells

Development of thiol-specific fluorescent probes with selectivity in different thiol compounds is more practical and significant than those without that capacity. In this work, a new quinoline-derived fluorophore, hydroxyl-substituted quinoline-benzo[d]oxazole 6 with high fluorescence quantum yield is synthesized and esterified with acrylic acid to afford two fluorescent probes, BQA-1 and BQA-2 for selectively discriminating Cys from Hcys/GSH based on conjugate addition-cyclization mechanism. BQA-1 exhibits a large ratiometric fluorescence response toward Cys in aqueous pH 7.4 solution with big emission peak-shifting from 383 nm to 518 nm, over 130 nm. The detection limit is determined to be as low as 0.59 μM. In contrast to BQA-1, BQA-2 whose acrylic ester moiety is further modified with pyridine group, displays a turn-on fluorescence response to Cys with detection limit of 0.98 μM. Both BQA-1 and BQA-2 have relatively weak response to another two biothiols, Hcys and GSH and nearly no response to other nucleophiles. Furthermore, the potential application for the detection of biothiols in living cells has been demonstrated by cell imaging experiment.

A Single Composition Architecture-Based Nanoprobe for Ratiometric Photoacoustic Imaging of Glutathione (GSH) in Living Mice

As one of the reduction species, glutathione (GSH) plays a tremendous role in regulating the homeostasis of redox state in living body. Accurate imaging of GSH in vivo is highly desired to provide a real-time visualization of physiological and pathological conditions while it is still a big challenge. Recently developed photoacoustic imaging (PAI) with high resolution and deep penetration characteristics is more promising for in vivo GSH detection. However, its application is dramatically limited by the difficult designation of photoacoustic probes with changeable near-infrared (NIR)-absorption under reductive activation. A cyanine derivative-based activatable probe is developed for in vivo ratiometric PAI of GSH for the first time. The probe is structurally designed to output ratiometric signals toward GSH in NIR-absorption region based on the cleavage of disulfide bond followed by a subsequent exchange between the secondary amine and sulfydryl group formed. Such a ratiometric manner provides high signal-to-noise imaging of blood vessels and their surrounding areas in tumor. Concomitantly, it also exhibits good specificity toward GSH over other thiols. Furthermore, the single composition architecture of the probe effectively overcomes the leakage issue compared with traditional multicomposition architecture-based nanoprobe, thus enhancing the imaging accuracy and fidelity in living body.

A simple fluorescent probe for the fast sequential detection of copper and biothiols based on a benzothiazole derivative

A simple benzothiazole fluorescent chemosensor was developed for the fast sequential detection of Cu²⁺ and biothiols through modulating the excited-state intramolecular proton transfer (ESIPT) process. The compound 1 exhibits highly selective and sensitive fluorescence "on-off" recognition to Cu²⁺ with a 1:1 binding stoichiometry by ESIPT hinder. The in situ generated 1-Cu²⁺ complex can serve as an "on-off" fluorescent probe for high selectivity toward biothiols via Cu²⁺ displacement approach, which exerts ESIPT recovery. It is worth pointing out that the 1-Cu²⁺ complex shows faster for cysteins (within 1min) than other biothiols such as homocysteine (25min) and glutathione (25min). Moreover, the compound 1 displays 160nm Stoke-shift for reversibly monitoring Cu²⁺ and biothiols. In addition, the probe is successfully used for fluorescent cellular imaging. This strategy via modulation the ESIPT state has been used for determination of Cu²⁺ and Cys with satisfactory results, which further demonstrates its value of practical applications.

Methylated chromenoquinoline dyes: synthesis, optical properties, and application for mitochondrial labeling

Methylated chromenoquinoline dyes were designed and synthesized, and their photophysical properties were investigated.

Specific and sensitive imaging of basal cysteine over homocysteine in living cells

Biological thiols play important roles in maintaining appropriate redox status of organisms. Accepting the challenge to differentiate structurally similar cysteine (Cys) and homocysteine (Hcy), we have successfully developed a miniature synthetic turn-on fluorescent probe based on 6-(2-benzothiazolyl)-2-naphthalenol for Cys. This probe is able to specifically react with Cys to yield its naphthalenol derivative, accompanied by remarkable green fluorescence enhancement with a detection limit of 14.8 nM. Besides, this probe displays much greater selectivity for Cys over other biological thiols, including homocysteine (Hcy) and glutathione (GSH). Practically, good cell permeability and low cytotoxicity make it suitable for monitoring basal Cys in living cells.

A turn-on fluorescent probe conjugating with a reactive acrylate for visualization of basal Cys specifically in living cells was developed.

A Michael addition–cyclization-based switch-on fluorescent chemodosimeter for cysteine and its application in live cell imaging

We designed and synthesized a fast response fluorescent probe, BTAC (benzothiazol-azacoumarin), for detection of cysteine (Cys).

A new simple phthalimide-based fluorescent probe for highly selective cysteine and bioimaging for living cells

A new turn-on phthalimide fluorescent probe has designed and synthesized for sensing cysteine (Cys) based on excited state intramolecular proton transfer (ESIPT) process. It is consisted of a 3-hydroxyphthalimide derivative moiety as the fluorophore and an acrylic ester group as a recognition receptor. The acrylic ester acts as an ESIPT blocking agent. Upon addition of cystein, intermolecular nucleophilic attack of cysteine on acrylic ester releases the fluorescent 3-hydroxyphthalimide derivative, thereby enabling the ESIPT process and leading to enhancement of fluorescence. The probe displays high sensitivity, excellent selectivity and with large Stokes shift toward cysteine. The linear interval range of the fluorescence titration ranged from 0 to 1.0×10^-5M and detection limit is low (6×10^-8M). In addition, the probe could be used for bio-imaging in living cells.

Fluorescent Probes with Multiple Binding Sites for the Discrimination of Cys, Hcy, and GSH

Biothiols such as cysteine (Cys), homocysteine (Hcy), and glutathione (GSH) play crucial roles in maintaining redox homeostasis in biological systems. This Minireview summarizes the most significant current challenges in the field of thiol-reactive probes for biomedical research and diagnostics, emphasizing the needs and opportunities that have been under-investigated by chemists in the selective probe and sensor field. Progress on multiple binding site probes to distinguish Cys, Hcy, and GSH is highlighted as a creative new direction in the field that can enable simultaneous, accurate ratiometric monitoring. New probe design strategies and researcher priorities can better help address current challenges, including the monitoring of disease states such as autism and chronic diseases involving oxidative stress that are characterized by divergent levels of GSH, Cys, and Hcy.

Ratiometric Fluorescent Probe for Lysosomal pH Measurement and Imaging in Living Cells Using Single-Wavelength Excitation

A novel lysosome-targeting ratiometric fluorescent probe (CQ-Lyso) based on the chromenoquinoline chromorphore has been developed for the selective and sensitive detection of intracellular pH in living cells. In acidic media, the protonation of the quinoline ring of CQ-Lyso induces an enhanced intramolecular charge transfer (ICT) process, which results in large red-shifts in both the absorption (104 nm) and emission (53 nm) spectra which forms the basis of a new ratiometric fluorescence pH sensor. This probe efficiently stains lysosomes with high Pearson's colocalization coefficients using LysoTrackerDeep Red (0.97) and LysoTrackerBlue DND-22 (0.95) as references. Importantly, we show that CQ-Lyso quantitatively measures and images lysosomal pH values in a ratiometric manner using single-wavelength excitation.

An excited state intramolecular proton transfer dye based fluorescence turn-on probe for fast detection of thiols and its applications in bioimaging

In this study, a new fluorescent probe 2-(2'-hydroxy-5'-N-maleimide phenyl)-benzothiazole (probe 1), was designed and synthesized by linking the excited state intramolecular proton transfer (ESIPT) fluorophore to the maleimide group for selective detection of thiols in aqueous solution. The fluorescence of probe 1 is strongly quenched by maleimide group through the photo-induced electron transfer (PET) mechanism, but after reaction with thiol, the fluorescence of ESIPT fluorophore is restored, affording a large Stokes shifts. Upon addition of cysteine (Cys), probe 1 exhibited a fast response time (complete within 30s) and a high signal-to-noise ratio (up to 23-fold). It showed a high selectivity and excellent sensitivity to thiols over other relevant biological species, with a detection limit of 3.78×10^-8M (S/N=3). Moreover, the probe was successfully applied to the imaging of thiols in living cells.

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.

Orchestration of dual cyclization processes and dual quenching mechanisms for enhanced selectivity and drastic fluorescence turn-on detection of cysteine

Synergistic dual cyclizations and dual quenching mechanisms of acrylate and maleimide groups for improved turn-on fluorescence detection of cysteine.