Conjugate addition/cyclization sequence enables selective and simultaneous fluorescence detection of cysteine and homocysteine

Cysteine triggered cascade reaction forming coumarin: Visualization of cysteine fluctuation in alcoholic liver disease by a NIR fluorescent probe

Alcoholic liver disease (ALD) is a chronic toxic liver injury caused by long-term heavy drinking. Due to the increasing incidence, ALD is becoming one of important medical tasks. Many studies have shown that the main mechanism of liver damage caused by large amounts of alcohol may be related to antioxidant stress. As an important antioxidant, cysteine (Cys) is involved in maintaining the normal redox balance and detoxifying metabolic function of the liver, which may be closely related to the pathogenesis of ALD. Therefore, it is necessary to develop a simple non-invasive method for rapid monitoring of Cys in liver. Thus, a near-infrared (NIR) fluorescent probe DCI-Ac-Cys which undergoes Cys triggered cascade reaction to form coumarin fluorophore is developed. Using the DCI-Ac-Cys, decreased Cys was observed in the liver of ALD mice. Importantly, different levels of Cys were monitored in the livers of ALD mice taking silybin and curcumin with the antioxidant effects, indicating the excellent therapeutic effect on ALD. This study provides the important references for the accurate diagnosis of ALD and the pharmacodynamic evaluation of silybin and curcumin in the treatment of ALD, and support new ideas for the pathogenesis of ALD.

Coumarin-aurone based fluorescence probes for cysteine sensitive in-situ identification in living cells

The quantification of cysteine (Cys) levels in the organisms holds paramount significance in biological research and disease diagnosis, which can give the correlation between abnormal Cys levels and diseases. In this study, two fluorescent probes, designated as DEA-OH and DEA-AC, featuring a coumarin-aurone backbone specifically engineered for Cys detection, were meticulously designed and synthesized. The diethylamino coumarin-aurone probe DEA-OH and the acrylate-substituted probe DEA-AC demonstrated remarkable sensitivity in detecting cysteine by means of copper displacement (DEA-OH) and acrylate hydrolysis mechanisms (DEA-AC) with fluorescence detection limits of 7.25 μM and 1.65 μM, respectively. Moreover, the fluorescence peak wavelength of the two probes displayed a linear relationship with solvent polarity in the ET (30) range of 30-65 kcal•mol-1, indicating the potential for monitoring changes in environmental polarity within this ET (30) range. The outstanding attributes exhibited by DEA-AC including superior photostability, remarkable selectivity, and swift response (kinetic rate constant: 0.00747 s-1), coupled with the exceptional anti-interference ability, have significantly broadened its scope of applications, for example detecting alterations in Cys within biological systems.

Highly selective fluorescent probe for cysteine via a tandem reaction and its bioimaging application in HeLa cells

Cysteine, as a vital endogenous small molecule mercaptan, plays a crucial role in various physiological processes. The high sensitivity and selectivity of fluorescent probes provide a method to monitor cysteine, which is helpful to understand the mechanism of cysteine in physiological processes more comprehensively. However, the detection of cysteine can be interfered by other small molecule biothiols. Therefore, the design of fluorescent probe based on the structural characteristics and reactivity of cysteine has become research focus currently. Given the biological compatibilities, biological targets, the metabolic pathway of 3-hydoxythalidomide, and its unique fluorescent properties, herein, we have designed a chemodosimeter, 2-(2,6-dioxopiperidin-3-yl)-1,3-dioxoisoindolin-4-yl acrylate, for the detection of cysteine based on a tandem reaction of thiol-ene click chemistry and aminolysis involving 3-hydroxythalidomide as a parent compound. Experimental data exhibited that the probe showed unique selectivity and sensitivity for cysteine over other amino acid and biothiols. In addition, the fluorescent intensity at 511 nm increased linearly as a function of cysteine concentration in the range of 0-6 × 10-7 M (regression factor, R2 = 0.999), with a limit of detection of 6.1 nM. The sensing mechanism was confirmed through 1H NMR titration and density functional theory calculations. Additionally, the probe was also successfully utilized for the detection of cysteine in sewage and for bioimaging in HeLa cells.

Simultaneous detection of cysteine and glutathione in food with a two-channel near-infrared fluorescent probe

L-Cysteine (Cys) and glutathione (GSH) are closely related biological species that widely exist in food and living cells. To simultaneously detect Cys and GSH from different emission channels, we developed a fluorescent probe (BDP-NBD) based on near-infrared BODIPY and 7-nitrobenzofurazan (NBD). Upon nucleophilic substitution reaction with GSH, BDP-NBD generated an emission band at 713 nm, which can be used to determine GSH (0-100 μM) with a low detection limit (34 nM). Different from GSH, BDP-NBD underwent a nucleophilic substitution-rearrangement reaction with Cys, affording two emission bands at 550 nm and 713 nm, respectively. BDP-NBD was successfully employed to quantify Cys and GSH in various food samples with good recoveries (86.6%-104.6%). Besides, BDP-NBD can image Cys and GSH in living cells from two emission channels. Therefore, this work developed a tool for the simultaneous determination of Cys and GSH in both food and living cells so as to ensure food safety and human health.

Theoretical Study on the Effect of Cyano- and Dimethylamine-Group on ESIPT Behavior and Luminescent Properties of Novel Flavone-Based Fluorophore

Recently, a new fluorescent senor based on 3-hydroxy-2-(naphthalen-2-yl)-4 H-chromen-4-one (HFN) for selective detection of H2Sn was obtained in the experiment (Spectrochim. Acta Part A 271(2022)120962). Based on HFN, three new compounds (HFN1, HFN2 and HFN3) are designed to explore the influences of dimethylamine (-N(CH3)2) and cyano (-CN) groups on the excited-state intramolecular proton transfer (ESIPT) process and luminescent features of HFN. After analyzing the mainly geometrical parameters, electron densities and infrared spectra, we discovered that the intramolecular hydrogen bonds (IHBs) in the target molecules become stronger upon photo-excitation. Introducing -CN or/and -N(CH3)2 groups into HFN indeed influences its ESIPT behavior and luminescent properties. The -N(CH3)2 group enhances IHB, reduces ESIPT barrier and caused absorption/ fluorescence (at T form) peak blue-shift, while the -CN group shows a counterproductive effect. The coincidence of -N(CH3)2 and -CN made the absorption/fluorescent wavelength of HFN red-shift more than single -N(CH3)2 or -CN group does.

Construction of a mitochondria-targeted probe to monitor cysteine levels in cancer cells and zebrafish

Cysteine (Cys) plays an indispensable role as an antioxidant in the maintenance of bioredox homeostasis. We have constructed an efficient fluorescent probe Mito-Cys based on the binding of indole and naphthol. The acrylic ester group serves as a recognition switch for specific detection of Cys, which undergoes Michael addition and intramolecular cyclization reactions, thereby ensuring the chemical kinetics priority of Cys compared to other biothiols. The probe has good water solubility, large Stokes shift (137 nm), with a detection limit of 21.81 nM. In addition, cell imaging experiments have shown that the probe has excellent mitochondrial targeting ability (R = 0.902). The probe can distinguish between Cys, homocysteine (Hcy) and glutathione (GSH), and can detect Cys specifically and quickly (100 s) to ensure accurate quantitative analysis of Cys changes in cells. More importantly, the probe confirms that ferroptosis inducing factors trigger thiol starvation in mitochondria, which helps to gain a deeper understanding of the physiological and pathological functions related to Cys and ferroptosis.

A Near-Infrared Fluorescent Dye with Tunable Emission Wavelength and Stokes Shift as a High-Sensitivity Cysteine Nanoprobe for Monitoring Ischemic Stroke

Sulfur-substituted dicyanomethylene-4H-chromene (DCM) derivatives based on the intramolecular charge transfer (ICT) mechanism were designed as near-infrared (NIR) fluorescent dyes. Using the Knoevenagel condensation method, the S-DCM-OH(835) fluorescence dye was synthesized, which had an emission wavelength exceeding 800 nm and 220 nm of a Stokes shift. Compared to commercial ICG, S-DCM-OH(835) was not only synchronized in emission wavelength but also far superior in Stokes shifts. These advantages made the design of S-DCM-NIR(835) based on this dye potentially valuable for biological applications. Based on this chemical structure, a fluorescent S-DCM-NIR(835) nanoprobe with a mean diameter of 17.69 nm was fabricated as the NIR imaging nanoprobe. Results showed that the nanoprobe maintained the high-specificity identification of cysteine (Cys) via the Michael addition reaction, with the detection limitation of 0.11 μM endogenous Cys. More importantly, in an ischemic stroke mouse model, the S-DCM-NIR(835) nanoprobe could monitor the Cys concentration change at stroke lesion due to the disruption of Cys metabolism under the ischemic stroke condition. Such a S-DCM-NIR(835) nanoprobe could not only differentiate the severity of the ischemic stroke using response time but also quantify the concentration of Cys in real-time in vivo.

Efficient turn-on fluorescent probe cooperated by cascade response for disclosing the fluctuation of cysteine in cells

BACKGROUND

The research on cysteine (Cys) determination is deemed as a hot topic, since it has been reported to be connected with various physiological processes and disease prediction. However, existing Cys-responding probes may expose some defects such as long reaction time, disappointing photostability, and suboptimal sensitivity. Under such a circumstance, our team has proposed an efficient fluorescent probe with novel sensing mechanism to perfectly cope with the above-mentioned drawbacks.

RESULTS

A novel cascade reaction-based probe 9-(2,2-dicyanovinyl)-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-8-yl acrylate (DPQA) has been synthesized for the first time. Undergoing addition-cleavage and cyclization-rearrangement processes, DPQA reacts with Cys to generate an iminocoumarin product with relucent green fluorescence, namely 11-imino-2,3,6,7-tetrahydro-1H,5H,11H-pyrano[2,3-f]pyrido[3,2,1-ij]quinoline-10-carbonitrile (IMC-J), and the relative fluorescence quantum yield (Φf) soars from 0.007 to 0.793. Utilizing such a mechanism, DPQA shows a superb turn-on signal (172-fold), low detection limit (4.1 nM), and wide detection range (5-6000 nM) toward Cys detection. Encouraged by the admirable sensing performance of DPQA, bioimaging of endogenous Cys has been attempted in HeLa cells with satisfactory results. Moreover, cell model of H2O2-induced oxidative stress has been established and the Cys fluctuation during this process has been inspected, elucidating how living cells confront with the eruption of reactive oxygen species (ROS) storm.

SIGNIFICANCE

The probe DPQA with such an intriguing cascade responding process for Cys detection has been endowed with many merits, such as fast reaction and superior sensitivity, conducive to improving responsiveness and rendering it more suitable for further applications. Thereby, we expect that the DPQA would be an efficient tool for detecting Cys fluctuation in living cells of different physiological processes.

Phosphorescent iridium (III) complex with covalent organic frameworks as scaffolds for highly selective and sensitive detection of homocysteine

Introduction: Developing a convenient and cost-effective platform for detecting homocysteine (Hcy) is of great interest as Hcy has been found to be a biomarker for Alzheimer's disease, gastric cancer, and other diseases. Methods: In this study, we synthesized five phosphorescent Ir(C∧N)2(N∧N)+ compounds (Irn, n = 1-5) with various substituents (-CHO or -CHO/-NH2), which were then doped into a covalent organic framework (COF) host via covalent bonding. Results and Discussion: The resulting optimal composites (denoted as Ir4/5@EBCOF) with -CHO/-NH2 substituents not only overcame the self-quenching issue of the bare Ir4/5 complexes but also showed rapid, highly selective, and sensitive detection of Hcy, with a limit of detection (LOD) of 0.23 μM and reaction time of 88 s. The sensing mechanism was revealed as the unique cyclization reaction between Ir(III) and Hcy that forms a six-membered ring. During the process, the color changes in the composites can be observed visually. It is expected that these phosphorescent Iridium (III) complexes with COFs will have the potential to serve as promising platforms for detecting thiols.

A novel dual-function fluorescent probe for the detection of cysteine and its applications in vitro

A fluorescent probe of both colorimetric and ratiometric type for highly selective and sensitive detection of Cys (cysteine) is very important in biological analysis. In this work, a new colorimetric and ratiometric fluorescent probe ((E)-2-(2-(5-(4-(acryloyloxy)phenyl)furan-2-yl)vinyl)-3-methylbenzo[d]thiazol-3-ium iodide, LP-1) was designed and synthesized for the detection of Cys. The reaction mechanism of LP-1 toward Cys involves a conjugate addition reaction between Cys and the α,β-unsaturated carbonyl group, leading to the formation of an intermediate thioether, followed by intramolecular cyclization to produce the desired compounds LP-1-OH. At this point, the ICT process is activated, significantly increasing the fluorescence intensity of the molecules. Meanwhile, LP-1 is highly selective and sensitive to Cys identification under optimized experimental conditions. LP-1 shows a good linear relationship in the range of Cys concentration from 0.40 μM to 40 μM (R2 = 0.9942) and the limit of detection (LOD) of Cys is 0.19 μM. In addition, we have developed a simple, portable and low-cost smartphone-based high-sensitivity Cys detection method based on naked eye obvious color detection. LP-1 also has low cell toxicity and can be successfully used for biological imaging of Cys, suggesting that it is a promising biological application tool for Cys detection.

Simultaneous SERS Sensing of Cysteine and Homocysteine in Blood Based on the CBT-Cys Click Reaction: Toward Precisive Diagnosis of Schizophrenia

At present, there is a lack of sufficiently specific laboratory diagnostic indicators for schizophrenia. Serum homocysteine (Hcy) levels have been found to be related to schizophrenia. Cysteine (Cys) is a demethylation product in the metabolism of Hcy, and they always coexist with highly similar structures in vivo. There are few reports on the use of Cys as a diagnostic biomarker for schizophrenia in collaboration with Hcy, mainly because the rapid, economical, accurate, and high-throughput simultaneous detection of Cys and Hcy in serum is highly challenging. Herein, a click reaction-based surface-enhanced Raman spectroscopy (SERS) sensor was developed for simultaneous and selective detection of Cys and Hcy. Through the efficient and specific CBT-Cys click reaction between the probe containing cyan benzothiazole and Cys/Hcy, the tiny methylene difference between the molecular structures of Cys and Hcy was converted into the difference between the ring skeletons of the corresponding products that could be identified by plasmonic silver nanoparticle enhanced molecular fingerprint spectroscopy to realize discriminative detection. Furthermore, the SERS sensor was successfully applied to the detection in related patient serum samples, and it was found that the combined analysis of Cys and Hcy can improve the diagnostic accuracy of schizophrenia compared to a single indicator.

Treatment evaluation of Rheumatoid arthritis by in situ fluorescence imaging of the Golgi cysteine

Rheumatoid arthritis (RA) is a long-term systemic inflammatory disease that causes severe joint pain. Golgi stress caused by redox imbalance significantly involves in acute and chronic inflammatory diseases, in which cysteine (Cys), as a representative reducing agent, may be an effective biomarker for RA. Hence, in order to achieve RA early detection and drugs evaluation, based on our previous work about innovative Golgi-targeting group, we established a phenylsulfonamide-modified fluorescence probe, Golgi-Cys, for the selective fluorescence imaging of Cys in Golgi apparatus in vivo. By application of Golgi-Cys, the Cys changes under Golgi stress in cells were elucidated. More importantly, we found that the probe can be effectively utilized for the RA detection and treatment evaluation in situ.

High-Contrast Photoacoustic Imaging of Localized Cysteine in Orthotopic Breast Cancer Enabled by A Totally-Caged Methylene Blue Probe

High-contrast photoacoustic sensing imaging (PASI) was greatly determined by optical absorption changes of the absorbers usually enabled by activatable probes via controllably converting the absorbed electromagnetic energy to ultrasound waves. However, most of current photoacoustic probes still suffer from limited imaging contrast towards specific species because of their small absorption spectral changes in the near infrared (NIR) region. Herein, we developed a methylene blue-based photoacoustic probe with its NIR optical absorption totally caged, which could afford dramatical "OFF-to-ON" absorption transition for high-contrast photoacoustic imaging towards the localized cysteine. The rationally designed methylene blue-based probe for cysteine (MB-Cys) would keep in off state with almost no absorption in NIR region, while upon activated by cysteine through cyclization reaction with acrylates, it would reconstruct the π-conjugation system to release the free methylene blue with strong absorption centered at 665 nm (>130-fold enhancement). The unique responsive behavior could enable the PASI for photoacoustic mapping the cysteine in orthotopic breast cancer in a high-contrast manner. Therefore, this work established an up-to-date strategy to originally eliminate the background photoacoustic signal for PASI to accurately monitor cysteine in vivo.

Dual Fluorimetric Sensor for Tandem Detection of Cadmium and Cysteine: An Approach for Designing a Molecular Keypad Lock System

A fluorimetric sensor for dual and sensitive detection of Cd2+ ion and Cysteine (based on 2-picolylamine platform) was developed.The sensor was designed and synthesized by simple condensation method and characterized by using common spectroscopic methods. The observations made from the kinetics of absorption and emission profile shows that probe Pdac behaves as ''ON-OFF'' fluorescent quenching sensor for cadmium ions. The probe exhibit selectivity in fluorescence quenching behaviour over other competitive metal ions, and also the Pdac-Cd2+ ensemble behave as an efficient ''OFF-ON'' type sensor for an essential amino acid Cysteine. Moreover, this dual sensing nature of the sensor makes it successfully applied for the designing of a molecular keypad lock system.

A D-π-A-based near-infrared fluorescent probe with large Stokes shift for the detection of cysteine in vivo

D-π-A dyes are an ideal strategy for building near-infrared fluorescent probes that have a large Stokes shift due to their excellent properties of adjustable emission wavelength and Stokes shift. Developing a near-infrared (NIR) fluorescent probe (JTPQ-Cys) capable of detecting cysteine (Cys) was the aim of this study. In JTPQ-Cys, julolidine served as the electron donor (D) and quinoline as the electron acceptor (A), with 3,4-ethylenedioxythiophene as the π-bridge. The π-conjugation and vibrational/rotational activity of the molecule were increased by the introduction of 3,4-ethylenedioxythiophene, causing the molecule to exhibit NIR emission and a large Stokes shift. When JTPQ-Cys was used to detect Cys, a clear fluorescence turn-on signal was observed at 741 nm, together with a Stokes shift of 268 nm. The limit of detection of JTPQ-Cys for Cys is 24 nM. Moreover, JTPQ-Cys has been utilized successfully for imaging studies of Cys in cells and zebrafish because it has good photostability, low cytotoxicity, and a high signal-to-noise ratio. Overall, our findings demonstrate the potential of JTPQ-Cys to be one of the best choices for detecting Cys in biological systems, and JTPQ is an ideal fluorophore to construct fluorescence dyes for bioimaging.

A novel sensitive fluorescent probe with double channels for highly effective recognition of biothiols

Biothiols play a crucial role in maintaining redox balance in organisms, and anomalous levels of biothiols in human organs can lead to various sicknesses and biological disorders. This work developed a novel sensitive fluorescent probe TZ-NBD with double channels for highly efficient recognition of biothiols. TZ-NBD adopts 4-Chloro-7-nitrobenzofurazan (NBD-Cl) as the recognition moiety with simultaneous fluorescence output. By incorporating NBD-Cl with the other fluorophore, benzothiazole dihydrocyclopentachromene derivative (TZ-OH), the dual-channel sensitive fluorescence probe TZ-NBD was built. The existence of Cys/ Hcy could significantly trigger both the green and red fluorescent emissions, which were derived from fluorophores amine-substituted NBD and TZ-OH, respectively. While exposing to GSH, only the red-channel fluorescence signal could be detected, indicating the release of TZ-OH. The phenomena was mainly attributed to the fact that sulfur-substituted NBD has nearly no fluorescence, while amine-substituted NBD shows obvious green fluorescence. In our study, TZ-NBD exhibited dual-channel sensitivity, fast response, and excellent selectivity to biothiols in vitro. Moreover, TZ-NBD was favorably utilized for recognition of biothiols in vivo. We believe that the sensitive fluorescence probe with double channels can afford an alternate approach for monitoring biothiols in organisms and would be useful for studying diseases associated with biothiols.

An ortho-activation strategy to develop NIR fluorescent probe for rapid imaging of biothiols in vivo

Biothiols are the main antioxidants in regulating the redox balance and resisting oxidative stress in various biological processes, but the long detection time of current fluorescent probes hinders their rapid imaging in vitro and in vivo. To reveal the influx of biothiols, we rationally develop an ortho-activation approach to accelerate the reaction between the probe and biothiols, by introducing electron-withdrawing fluorine atom into the ortho-site of the phenolic hydroxyl group in the NIR probe to generate an ortho-inductive effect. The ortho-fluorine helps to increase the chemical reactivity of the molecular structure, resulting in a significantly shorter detection time (within 5 min) as compared to previous reports (> 20 min for acrylates-based probes in aqueous solution). Based on this approach, our near-infrared probe 2F-RBX can sensitively and efficiently detect endogenous biothiols in living HepG2 cells and in vivo. These data suggest that ortho-activation is a simple and flexible approach to construct sensitive fluorescent probes for rapid imaging of biothiols, and perhaps other molecules in future, under biological circumstances.

Selection of regioselective DNA aptamer for detection of homocysteine in nondeproteinized human plasma

Small molecule-binding aptamers often suffer from high cross reactivity to structure analogues in biological samples, limiting their value for clinical diagnosis. Herein, we present a method to overcome this issue, by performing binding-inhibited organic reaction-based regioselective selection of aptamers against homocysteine (Hcy), which is a marker for diagnosing many disorders including stroke and Alzheimer's. This approach has led to isolation of a DNA aptamer that binds to the alkane thiol chain of Hcy with exceptional specificity against cysteine. It also binds with oxidized Hcy at weaker affinity. Using this new aptamer, we produced a reusable fluorescent optical fiber aptasensor for direct and validated detection of both free and total Hcy in nondeproteinized patient plasma in the diagnostic concentration range. The binding site-specific aptamer selection and optical-fiber-sensing strategy can expand the practical utility of aptamers in clinical diagnosis.

A Novel Fluorescence Probe Based on Azamonardine for Detecting and Imaging Cysteine in Cells and Zebrafish with High Selectivity and Sensitivity

A novel fluorescent probe based on azamonardine (Aza) fluorophore was designed and synthesized for the highly selective detection of cysteine (Cys) in vivo and in vitro. After reacting with acryloyl chloride, the fluorescence of Aza is effectively quenched, resulting in the formation of the Aza-acryl probe. Upon the addition of Cys, the ester bond of Aza-acryl is cleaved, releasing a new compound (Compound 1) with strong fluorescence, thereby achieving fluorescence turn-on detection of Cys. The structure of Aza-acryl was characterized using X-ray crystallography and NMR spectroscopy. Additionally, density functional theory was employed to elucidate the quenching mechanism of the acyl group on the Aza. Aza-acryl exhibits high selectivity towards Cys and distinguishes it from other biothiols such as homocysteine (Hcy) and glutathione (GSH). The mechanism of Aza-acryl for detecting Cys was investigated through HPLC, NMR spectroscopy, high-resolution mass spectrometry, and reaction kinetics experiments. Aza-acryl demonstrates excellent imaging capabilities for Cys in cells and zebrafish, providing a reliable and selectable tool for the detection and imaging of Cys in biological systems.

Microperoxidase-11 functionalized nanozyme with enhanced peroxidase-mimicking activities for visual detection of cysteine

Various nanomaterials with peroxidase activity (nanozyme) have been designed for bio catalysis and biosensing, however, most of them need further design and modification of probe molecules for the specific binding reaction with targets. This results in a decrease in catalysis activity and hinders them to be perfect alternatives to natural enzyme in biosensing. In this work, an enhanced nanozyme was synthesized by functionalizing natural microperoxidase-11 (MP-11) on a hybrid graphene oxide-gold (GO-Au) material. The designed nanozyme showed an enhanced catalysis activity and realized a robust and efficient colorimetric detection of cysteine based on specific binding reaction between active iron center from MP-11 and thiol in cysteine. The enhanced properties show promising applications of complex nanozyme and provides a great opportunity for developing efficient sensing systems.

Rational design of a novel acryl-modified CQDs fluorescent probe for highly selective detection and imaging of cysteine in vitro and in vivo

A novel fluorescent nanoprobe CQDs-O-Acryl has been designed and synthesized to directly and accurately identify Cys over other biothiols in PBS (10 mM, pH 7.4) buffer. The carbon quantum dots (CQDs-OH) (λ_ex/em maxima = 495/525 nm) were fabricated by a solvothermal method using resorcinol as the carbon source. The CQDs-O-Acryl was achieved through covalently grafting the acryloyl group on the surface of carbon quantum dots by nuclear reaction based on static quenching. The structure and morphology of CQDs-OH and CQDs-O-Acryl have been characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and UV-vis absorption spectroscopy. Upon the addition of Cys, the ester bond of CQDs-O-Acryl has been broken, and the free CQDs were released by conjugated addition and cyclization reactions successively, emitting strong green fluorescence at 525 nm (λ_ex = 495 nm). Under the optimized conditions, CQDs-O-Acryl exhibited good sensing of Cys within the range 0.095-16 μM (the LOD of 0.095 μM). Due to the high sensitivity, reliability, fast fluorescence response (10 min), and low toxicity of CQDs-O-Acryl, it was successfully applied to fluorescence imaging of Cys in A549 cells and zebrafish.

Fluorescent probe for highly selective detection of cysteine in living cells

Cys play an important physiological role in the human body. Abnormal Cys concentration can cause many diseases. Therefore, it is of great significance to detect Cys with high selectivity and sensitivity in vivo. Because homocysteine (Hcy) and glutathione (GSH) have similar reactivity and structure to cysteine, few fluorescent probes have been reported to be specific and efficient for cysteine. In this study, we designed and synthesized an organic small molecule fluorescent probe ZHJ-X based on cyanobiphenyl, which can be used to specifically recognize cysteine. The probe ZHJ-X exhibits specific selectivity for cysteine, high sensitivity, short reaction response time, good anti-interference ability, and has a low detection limit of 3.8 × 10^-6 M. The probe ZHJ-X was successfully applied to the visualization of Cys in living cells and had great application prospects in cell imaging and detection.

Rational Design of Esterase-Insensitive Fluorogenic Probes for In Vivo Imaging

Small-molecule fluorogenic probes are indispensable tools for performing research in biomedical fields and chemical biology. Although numerous cleavable fluorogenic probes have been developed to investigate various bioanalytes, few of them meet the baseline requirements for in vivo biosensing for disease diagnosis due to their insufficient specificity resulted from the remarkable esterase interferences. To address this critical issue, we developed a general approach called fragment-based fluorogenic probe discovery (FBFPD) to design esterase-insensitive probes for in vitro and in vivo applications. With the designed esterase-insensitive fluorogenic probe, we successfully achieved light-up in vivo imaging and quantitative analysis of cysteine. This strategy was further extended to design highly specific fluorogenic probes for other representative targets, sulfites, and chymotrypsin. The present study expands the bioanalytical toolboxes available and offers a promising platform to develop esterase-insensitive cleavable fluorogenic probes for in vivo biosensing and bioimaging for the early diagnosis of diseases.

Light controlled reversible Michael addition of cysteine: a new tool for dynamic site-specific labeling of proteins

Cysteine-based Michael addition is a widely employed strategy for covalent conjugation of proteins, peptides, and drugs. The covalent reaction is irreversible in most cases, leading to a lack of control over the process. Utilizing spectroscopic analyses along with X-ray crystallographic studies, we demonstrate Michael addition of an engineered cysteine residue in human Cellular Retinol Binding Protein II (hCRBPII) with a coumarin analog that creates a non-fluorescent complex. UV-illumination reverses the conjugation, yielding a fluorescent species, presumably through a retro-Michael process. This series of events can be repeated between a bound and non-bound form of the cysteine reversibly, resulting in the ON-OFF control of fluorescence. The details of the mechanism of photoswitching was illuminated by recapitulation of the process in light irradiated single crystals, confirming the mechanism at atomic resolution.

Research progress of auxiliary groups in improving the performance of fluorescent probes

In the design work of fluorescent probes, it is important to consider not only the factors of fluorescence properties but also the environment in which the fluorescent molecule works. This requires the design of auxiliary groups to refine the fluorescent molecule. Nowadays, more and more fluorescent molecules are not limited to the traditional fluorescent probe consisting of a fluorophore, linker arm and recognition group, but integrate the three into one, and introduce auxiliary groups where possible. Auxiliary groups are "catalytic groups" that do not interact with the substrate, or "catalyze" the interaction of the recognition group with the substrate. The introduced auxiliary groups can improve the sensitivity and selectivity of the detection to some extent, which has attracted great interest from researchers. Although previous work has focused on this aspect, no one has summarized it systematically and comprehensively. So this review summarizes the role of auxiliary groups that are classified into three categories according to the different mechanisms between the auxiliary groups and the substance, in improving the performance of fluorescent probes in recent years (2012-2022). In particular, we generalize the mechanisms of the auxiliary groups in improving the sensitivity and selectivity of fluorescent probes. Also, the fundamental principles of auxiliary groups to improve the sensitivity and selectivity of fluorescent probes are discussed and future research directions in this field are proposed.

Rational development of an ESIPT-based fluorescent probe with large Stokes shift for imaging of hydrogen sulfide in live cells

It is crucial to monitor hydrogen sulfide (H2S) because H2S plays a vital role in the regulation of many physiology and pathology processes. Many evidences indicate that endogenous H2S is closely associated with many diseases such as inflammation and cancers. Herein, we reported a novel fluorescent probe BTDI to monitor the fluctuation of H2S based on the excited-state intramolecular proton transfer (ESIPT) mechanism both ex vivo and in vivo. The selectivity of BTDI for H2S is significantly higher than that for biothiols and other potential anions. After the probe responded to H2S, the nucleophilic addition reaction of the H2S with probe BTDI resulted the shifting of maximum emission peak from 630 nm to 542 nm and the fluorescent signals change from red to green emission along with a large Stokes shift (240 nm). Moreover, BTDI can be successfully applied to detect extracellular and endogenous H2S in living cells through fluorescent cell-imaging, which provides a promising tool for the specific recognition of H2S in complex biological systems.

A unique water soluble probe for measuring the cardiac marker homocysteine and its clinical validation

A series of copper(II) compounds 1-4 were synthesized and developed as fluorogenic probes to measure the cardiac marker homocysteine (Hcy) without any interference from other bioanalytes prevalent in human blood plasma including, cysteine and glutathione. UV-vis and EPR studies have provided confirmatory evidence for reduction-induced-emission-enhancement of the probe, which is responsible for the observed "off-to-on" behaviour towards Hcy. Water solubility, remarkable fluorescence enhancement (55-111 fold), and low detection ability (nearly 2.5 μM) make the probe suitable for clinical testing of cardiac samples. Investigation of 1 against a few reductive interferents testifies its specificity for Hcy. Results from clinical examination of cardiac samples by 1 when combined with the outcome of the reliability testing involving a clinically approved commercial immunoassay kit, validates the prospect of the molecular probe for direct measurement of Hcy in human plasma, which is unprecedented.

Fluorescent probes for the detection of disease-associated biomarkers

Fluorescent probes have emerged as indispensable chemical tools to the field of chemical biology and medicine. The ability to detect intracellular species and monitor physiological processes has not only advanced our knowledge in biology but has provided new approaches towards disease diagnosis. In this review, we detail the design criteria and strategies for some recently reported fluorescent probes that can detect a wide range of biologically important species in cells and in vivo. In doing so, we highlight the importance of each biological species and their role in biological systems and for disease progression. We then discuss the current problems and challenges of existing technologies and provide our perspective on the future directions of the research area. Overall, we hope this review will provide inspiration for researchers and prove as useful guide for the development of the next generation of fluorescent probes.

Selective, Rapid, and Ratiometric Fluorescence Sensing of Homocysteine in Live Neurons via a Reaction-Kinetics/Sequence-Differentiation Strategy Based on a Small Molecular Probe

Small molecular biothiols, including cysteine (Cys), homocysteine (Hcy), and glutathione (GSH), play essential roles in maintaining the redox homeostasis of biological systems, the disorders of which are closely associated with neuropathology. To date, many probes have been developed to identify Cys and GSH; however, due to the relatively low content and the high structural homology with Cys, there is still a lack of effective strategies to design probes enabling Hcy detection in physiological environments with high selectivity, high sensitivity, and rapid response. Herein, we developed a reaction-kinetics/sequence-differentiation strategy based on a dual-binding-site boron-dipyrrin (BODIPY) fluorophore, which was able to selectively distinguish Hcy from Cys and GSH within 50 s though a ratiometric fluorescence response mode. Benefiting from these features, the probe is capable of real-time imaging and quantitative analysis of intracellular Hcy in living neurons. Moreover, results of the disease-model experiments at the cellular level indicated a gradual increase of the Hcy level in neurons during the processes of aggregated amyloid-β (Aβ) peptide or ischemia treatment, which would further promote the neuron apoptosis. These findings provide the first direct experimental evidence for the impact of Alzheimer's disease and ischemic stroke on the Hcy metabolism of brain neurons and the associated neuron injury.

Elimination Reaction-Based Benzimidazole Probe for Cysteine Detection and Its Application in Serum Sample Analysis

Benzimidazole-based compound 2-(p-tolyl)-1H-benzo[d]imidazole (3) and its derivative probe A-B have been synthesized for the highly selective detection and quantification of Cys in human serum. The photophysical properties of A-B and compound 3 were evaluated by UV-vis absorption and fluorescence spectroscopy. A-B showed high selectivity and sensitivity for Cys among tested analytes, including amino acids, anions, and cations. A-B selectively reacts with Cys and results in compound 3 with fluorescence turn-on effect. A-B did not show any interference from the components in the serum matrix for Cys detection in the human serum sample. A-B detects Cys in serum samples with 2.3-5.4-fold better LOD than reported methods. The detection limit of 86 nM and 43 nM in HEPES buffer using UV-visible and fluorescence spectroscopy, respectively, makes A-B an excellent chemosensor for Cys detection.

Rational design of a large Stokes shift xanthene-benzothiozolium dyad for probing cysteine in mitochondria

Xanthene-modified cyanine dyes are considered to be an effective means to extend the emission wavelength and improve the photo-stability of cyanine dyes. However, the fluorophores obtained by this strategy generally have narrow Stokes shifts, which severely limits their application in biological imaging. Herein, a reasonable design strategy is adopted to provide an effective strategy to commendably improve the Stokes shift of xanthene-benzothiozolium fluorophores via the simultaneous expansion of a molecular π-conjugated system and heteroatomic substitution. Combined with density functional theory calculation guidance, the O atom is replaced with the S atom in the xanthene moiety and a π-conjugated benzene ring is introduced in the benzothiozolium moiety. Surprisingly, the results of the spectroscopic experiment showed that the fluorescence emission wavelength of PhCy-OH was extended to 803 nm, and the Stokes shift was improved to 68 nm. PhCy-Cys can effectively distinguish Cys from homocysteine (Hcy) and glutathione (GSH) with an extremely low detection limit of 0.166 μM. Importantly, PhCy-Cys has the ability to image endogenous Cys in mitochondria, providing the potential for exploring the specific function and mechanism of Cys in regulating oxidative stress in mitochondria.

Peptide probes for proteases - innovations and applications for monitoring proteolytic activity

Proteases are excellent biomarkers for a variety of diseases, offer multiple opportunities for diagnostic applications and are valuable targets for therapy. From a chemistry-based perspective this review discusses and critiques the most recent advances in the field of substrate-based probes for the detection and analysis of proteolytic activity both in vitro and in vivo.

A turn-on fluorescent probe via substitution-rearrangement for highly sensitive and discriminative detection of cysteine and its imaging in living cells

Biothiols play an important role in many physiological and pathological processes, especially in the occurrence of oxidative stress caused by abnormal cysteine (Cys) concentration. Therefore, it is particularly critical to develop a method that can specifically identify Cys to avoid interference from other biological analytes. However, most Cys-specific fluorescent probes are difficult to distinguish between homocysteine (Hcy) and glutathione (GSH). In this work, to avoid the interference of Hcy and GSH, we developed a fluorescent probe triarylimidazole-naphthalimide-piperazine-sulfonyl benzoxadiazole (TNP-SBD-Cl) based on fluorescence resonance energy transfer (FRET) on platform of naphthalimide-sulfonyl benzoxadiazole (SBD), the main SBD 4-chlorine groups have mild reactivity to undergo substitution and rearrangement to distinguish Hcy and GSH. The TNP-SBD-Cl response to Cys would turn on FRET and generate a new yellow fluorescence with a large Stokes shift (157 nm), and with excellent selectivity and low detection limit (0.87 μM). Moreover, TNP-SBD-Cl can be used to monitor Cys in living HeLa cells with low cytotoxicity, suggesting that it has markedly diagnostic significance in physiological and pathological processes.

Michael addition–elimination–cyclization based turn-on fluorescence (MADELCY TOF) probes for cellular cysteine imaging and estimation of blood serum cysteine and aminoacylase-1

Michael addition–elimination–cyclization based turn-on fluorescence (MADELCY TOF) probes for the highly sensitive estimation of Cys and aminoacylase-1 (ACY-1).

Indole-substituted flavonol-based cysteine fluorescence sensing and subsequent precisely controlled linear CO liberation

This study describes the first indole-substituted flavonol-based fluorescent probe to effectively sense and image Cys in vivo, as a precursor of photoCORM, actuated by Cys, triggered by visible-light, release precisely controlled linear CO under O2.

Cysteine ratiometric fluorescence sensing reaction actuated B-ring naphthalene-substituted flavonol-based PhotoCORM: precisely controlled linear CO liberation

This study describes the first B-ring-naphthalene-substituted flavonol-based ratiometric fluorescent probe to efficiently detect and image endo/exo-genous Cys both in vivo, and subsequent Cys-driven, visible-light triggered linear CO delivery under O2.

Homocysteine-specific fluorescence detection and quantification for evaluating S-adenosylhomocysteine hydrolase activity

The medium Ks value of copper complex contributed to the specific reduction of Cu2+ by homocysteine and the formation of a stable six-membered ring species.

The two-steps reaction fluorescent probe for the selective detection of cysteine and its applications

We reported the specific fluorescent probe (MC-BOD-XDS) with two-steps reaction based on monomercapto-coumarin-BODIPY for selective detection of cysteine，high activty mercapto-coumarin as the multiple reaction group instead of a group internal standard fluorophore. The reaction mechanism of MC-BOD-XDS for detecting cysteine was different from the reported probes about the nucleophilic  aromatic substitution reaction (SNAr) of chlorinated BODIPY. The fluorescent color of MC-BOD-XDS changed from yellow to red, and then to orange. The linear calibration diagram showed that it can potentially be used for quantitatively detection of Cys. Its potential applications were demonstrated by employing it for detection of Cys in artificial urine and in fluorescent imaging in HeLa cells.

A coumarin-based fluorescent probe for specific detection of cysteine in the lysosome of living cells

Cysteine (Cys), the only amino acid in the 20 natural amino acids that contains a reduced sulfhydryl group, plays important roles in the balance of redox homeostasis in biological systems. Lysosome is an important organelle containing a variety of hydrolases and has been proved to be the decomposition center of a variety of exogenous and endogenous macromolecular substances. In this research, a coumarin-based fluorescent probe MCA for the detection of Cys in lysosomes of living cells was developed. Due to the acrylate moiety, this probe exhibited high sensitivity (detection limit = 6.8 nM) and selectivity towards Cys superior to other analytes. Moreover, the probe was proved to be lysosome-targetable and showed good cell imaging ability and low cell toxicity.

Rational design of a new near-infrared fluorophore and apply to the detection and imaging study of cysteine and thiophenol

The development of a near-infrared fluorophore with excellent fluorescence performance, a large Stokes shift, and good biocompatibility has become a focus in the field of fluorescence imaging in recent years. Based on quantum chemistry calculations and reasonable molecular design strategies, a new NIR fluorophore was developed and characterized by simple synthesis, easy structural modification, and a large Stokes shift (105 nm). Furthermore, two new "activatable" fluorescent probes QN-Cys and QN-DNP were synthesized using a simple structural modification. The probe QN-Cys can recognize Cys with high sensitivity (LOD = 128 nM) and high selectivity, and its fluorescence intensity has a good linear relationship with the Cys concentration in the range of 5-35 μM. Furthermore, probe QN-Cys can effectively distinguish Cys from Hcy and GSH, and was successfully applied to the detection and imaging of Cys in human serum, cells, and zebrafish. The probe QN-DNP showed a good specific and sensitive (LOD = 78 nM) fluorescence response to thiophenol, and its fluorescence intensity has a good linear relationship with the thiophenol concentration in the range of 5-30 μM. Furthermore, it was successfully applied to detect thiophenol in real water samples with good recoveries (97-102%), and image thiophenol in living cells, zebrafish and mice. Notebly, the QN-DNP probe could be applied to visualize the distribution of thiophenol in the mice.

Substitution Effect on 2-(Oxazolinyl)-phenols and 1,2,5-Chalcogenadiazole-Annulated Derivatives: Emission-Color-Tunable, Minimalistic Excited-State Intramolecular Proton Transfer (ESIPT)-Based Luminophores

Minimalistic 2-(oxazolinyl)-phenols substituted with different electron-donating and -withdrawing groups as well as 1,2,5-chalcogenadiazole-annulated derivatives thereof were synthesized and investigated in regard to their emission behavior in solution as well as in the solid state. Depending on the nature of the incorporated substituent and its position, emission efficiencies were increased or diminished, resulting in AIE or ACQ characteristics. Single-crystal analysis revealed J- and H-type packing motifs and a so-far undescribed isolation of ESIPT-based fluorophores in the keto form.

Development of a Novel Benzimidazole-Based Probe and Portable Fluorimeter for the Detection of Cysteine in Human Urine

The measurement of cysteine in human urine and live cells is crucial for evaluating biological metabolism, monitoring and maintaining the immune system, preventing tissue/DNA damage caused by free radicals, preventing autoimmune diseases, and diagnosing disorders such as cystinuria and cancer. A method that uses a fluorescence turn-on probe and a portable fluorescence spectrometer device are crucial for highly sensitive, simple, rapid, and inexpensive cysteine detection. Herein, we present the synthesis and application of a benzimidazole-based fluorescent probe (ABIA) along with the design and development of a portable fluorescence spectrometer device (CysDDev) for detecting cysteine in simulated human urine. ABIA showed excellent selectivity and sensitivity in detecting cysteine over homocysteine, glutathione, and other amino acids with the response time of 1 min and demonstrated a detection limit of 16.3 nM using the developed CysDDev. Further, ABIA also demonstrated its utility in detecting intracellular cysteine, making it an excellent probe for bio-imaging assay.

A highly selective quinolizinium-based fluorescent probe for cysteine detection

A novel fluorescent quinolizinium-based turn-off probe has been developed for selective detection of cysteine. The probe showed high selectivity and sensitivity towards cysteine over other amino acids including the similarly structured homocysteine and glutathione with a detection limit of 0.18 μM (S/N = 3). It was successfully applied to cysteine detection in living cells with low cytotoxicity and quantitative analysis of spiked mouse serum samples with moderate to good recovery (96-109%).