A colorimetric, ratiometric and water-soluble fluorescent probe for simultaneously sensing glutathione and cysteine/homocysteine

Water-Soluble Small Organic Fluorophores for Oncological Theragnostic Applications: Progress and Development

Cancer is one of the major noncommunicable diseases, responsible for millions of deaths every year worldwide. Though various cancer detection and treatment modalities are available today, many deaths occur owing to its late-stage detection and metastatic nature. Noninvasive detection using luminescence-based imaging tools is considered one of the promising techniques owing to its low cost, high sensitivity, and brightness. Moreover, these tools are unique and valuable as they can detect even the slightest changes in the cellular microenvironment. To achieve this, a fluorescent probe with strong tumor uptake and high spatial and temporal resolution, especially with high water solubility, is highly demanded. Recently, several water-soluble molecules with emission windows in the visible (400-700 nm), first near-infrared (NIR-I, 700-1000 nm), and second near-infrared (NIR-II, 1000-1700 nm) windows have been reported in literature. This review highlights recently reported water-soluble small organic fluorophores/dyes with applications in cancer diagnosis and therapeutics. We systematically highlight and describe the key concepts, structural classes of fluorophores, strategies for imparting water solubility, and applications in cancer therapy and diagnosis, i.e., theragnostics. We discuss examples of water-soluble fluorescent probes based on coumarin, xanthene, boron-dipyrromethene (BODIPY), and cyanine cores. Some other emerging classes of dyes based on carbocyclic and heterocyclic cores are also discussed. Besides, emerging molecular engineering methods to obtain such fluorophores are discussed. Finally, the opportunities and challenges in this research area are also delineated.

A novel fluorescent probe with a large Stokes shift for colorimetric and selective detection of cysteine in water, milk, cucumber, pear and tomato

Cysteine is an important amino acid that is related to human health and food safety. How to effectively detect Cys in food has received widespread attention. Compared with other methods, fluorescent probes have the advantages of simple operation, high sensitivity, and good selectivity. Therefore, a selective fluorescence probe 2 for Cys in food was designed and synthesized. Probe 2 employed the acrylate group as a thiol-recognition site for Cys, which endowed probe 2 with better selectivity for Cys over Hcy and GSH. The recognition pathway underwent Michael addition, intramolecular cyclization, and concomitant release of the piperideine-based fluorophore, along with a chromogenic change from yellow to orange. This pathway was supported by 1H NMR analysis and DFT calculations. In addition, probe 2 displays a linear response to Cys concentrations (0-30 μM), low detection limit (0.89 μM), and large Stokes shift (125 nm). Overall, probe 2 showed great application potential for the quantitative determination of Cys in water, milk, cucumber, pear and tomato.

An aqueous mediated ultrasensitive facile probe incorporated with acrylate moiety to monitor cysteine in food samples and live cells

A colorimetric probe TQA ((E)-4-(((8-(sec-butoxy)-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-9-yl)methylene)amino)benzylacrylate) possessing greater potent towards the sensing of cysteine was successfully synthesized and characterized. The aqueous soluble probe TQA detects Cys based on "ON-OFF" effect with excellent absorbance and emission properties. The probe TQA detects Cys up to its ultra-low level concentration of 1.5 nM and also quantifies the Cys up to 5.05 nM with the quicker response time of 140 s (2.3 min). In addition, the color change produced by the probe TQA on integrated with Cys was also identified easily via paper strip, cotton wool buds and RGB color picker app in smart mobiles. Further, the admirable selectivity and sensitivity of the probe TQA towards Cys extends its utility towards food samples and imaging of live HeLa cells.

A red-emitting Europium(III) complex as a luminescent probe with large Stokes shift for the sequential determination of Cu2+ and biothiols in real samples

In this work, a novel Eu³⁺-DTPA-bis(AMC) complex with red luminescence was designed and synthesized for sequential detection of Cu²⁺ and biothiols (Cys/Hcy/GSH) based on the displacement strategy with the good selectivity, high sensitivity, and large Stokes shift (288 nm). The possible detection mechanism was verified by UV-vis, the high-resolution mass spectrometry, and the fluorescence decay curve. The experimental parameters, including the solution pH, the incubation time, the concentration ratio of Eu³⁺-DTPA-bis(AMC) to Cu²⁺ and biothiols concentration, were optimized. Under the optimal conditions, it shows a good linear relationship between the concentration (0-10 μM) of Cu²⁺ and the fluorescence intensity of Eu³⁺-DTPA-bis(AMC), with a low detection limit of 0.065 μM. The linear range and the limit of detection of the Eu³⁺-DTPA-bis(AMC)/Cu²⁺ system for Cys/Hcy/GSH were 2.5-22.5/5-45/5-50 μM and 0.11/0.07/0.05 μM, respectively. Surprisingly, the high or low concentration of Eu³⁺-DTPA-bis(AMC)/Cu²⁺ can significantly affect the selectivity of the sensing system to biothiols (Cys/GSH/Hcy). When the concentration of the Eu³⁺-DTPA-bis(AMC)/Cu²⁺ system is 10.0 μΜ, it could recognize biothiols (Cys/GSH/Hcy) from other substances, but when the concentration is as low as 3.3 μM, it could further specifically distinguished Cys from Hcy/GSH. Owing to the high anti-interference characteristics, accuracy and specificity, the sensing system was well applied to the cascade detection of Cu²⁺ in actual environmental samples and Cys in biological and food samples, including FBS, urine, milk, beverage, fresh juice with the satisfactory recoveries from 96.20 to 106.80 %.

Real-time, sensitive and simultaneous detection of GSH and Cys/Hcy by 8-substituted phenylselenium BODIPYs: a structure-activity relationship

Real-time and sensitive detection of biothiols is the key to biomedical research and clinical diagnosis. It is necessary to develop a highly sensitive and selective fluorescent probe for the detection of biothiols. In this paper, we have developed a series of meso-arylselenium BODIPY probes for the rapid and sensitive detection of biothiols and the dual-channel discrimination of GSH and Cys/Hcy. A structure-activity relationship was established from five p-substituted phenylselenium (R = NO₂, F, H, OCH₃ or N(CH₂CH₂)₂O) BODIPYs. Compared with most reported fluorescent probes, such as meso-BODIPY sulfur ethers, these probes display much lower LODs (∼nM levels) and more rapid responses, which are ascribed to the higher fluorescence efficiencies of the sensing products (Φ_f = 0.48 for GSH, 0.18 for Cys and 0.14 for Hcy) and the introduction of arylselenium, which is more active than arylthiol. Among them, the best sensing performance is that of probe 2a (R = NO₂); therefore, a structure-activity relationship of these fluorescent probes was also obtained. The excellent sensing performance was further revealed in the detection of GSH and Cys/Hcy in live cells.

Synthesis of BINOL-xylose-conjugates as "Turn-off" fluorescent receptors for Fe3+ and secondary recognition of cysteine by their complexes

A novel chiral fluorescence "turn-off" sensor was synthesised using the click reaction. The sensor was a BINOL-xylose derivative, modified at the 2-position and linked by 1,2,3-triazole. It was structurally characterized by ¹HNMR, ¹³CNMR, ESI-MS and IR analysis. The selectivity of R-β-d-2 in methanol solution has been studied. Among the 19 transition metal ions, alkaline metal ions and alkaline earth metal ions studied, R-β-d-2 had a selective fluorescence quenching reaction for Fe³⁺. The detection limit of R-β-d-2 for Fe³⁺ was 0.91 μmol L^-1. Complexation between R-β-d-2 and Fe³⁺ was investigated by ESI-MS and ¹HNMR. The stoichiometric ratio of R-β-d-2 was 1 : 1. In addition, the R-β-d-2-Fe³⁺ complex was titrated with 20 naturally occurring amino acids and Hcy with GSH. It was found that the complex R-β-d-2-Fe³⁺ had a secondary recognition effect on Cys by switching to fluorescence.

Specific two-photon fluorescent probe for cysteine detection in vivo

Cysteine (Cys), an essential amino acid, plays several crucial functions in numerous biological processes. Notably, the detection of Cys is critical to disease diagnosis. Fluorescent probes that can quickly detect Cys will help to study the mechanism of certain diseases. Herein, a new fluorescent probe, ANP, which is based on 6-acetyl-N-methyl-2-naphthyl amine, has been developed for Cys detection over Hcy and GSH in vivo. The addition of thiol on α,β-unsaturated ketone promotes 87-fold fluorescence turn-on response with a 65 nM limit of detection. The high two-photon efficiency of the probe ANP (cross-section = 22.3) makes it a suitable probe for evaluating Cys in living cells without background fluorescence interference. Its application was extended to monitor the Cys distribution in live cells and tissues.

Coumarin-based fluorescent probe with 4-phenylselenium as the active site for multi-channel discrimination of biothiols

Biological mercaptans, also known as biothiols, play their own roles in a number of important physiological processes, and the abnormal levels of biothiols are closely associated with a variety of...

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.

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%).

An 8-arylselenium BODIPY fluorescent probe for rapid and sensitive discrimination of biothiols in living cells

By introducing 8-arylselenium as the active group, a BODIPY fluorescent probe ASeBD was constructed for rapid and sensitive detection and dual-channel discrimination of GSH and Cys/Hcy in solution and in living cells, and its mechanism was demonstrated.

Fluorescent Probes for Live Cell Thiol Detection

Thiols play vital and irreplaceable roles in the biological system. Abnormality of thiol levels has been linked with various diseases and biological disorders. Thiols are known to distribute unevenly and change dynamically in the biological system. Methods that can determine thiols' concentration and distribution in live cells are in high demand. In the last two decades, fluorescent probes have emerged as a powerful tool for achieving that goal for the simplicity, high sensitivity, and capability of visualizing the analytes in live cells in a non-invasive way. They also enable the determination of intracellular distribution and dynamitic movement of thiols in the intact native environments. This review focuses on some of the major strategies/mechanisms being used for detecting GSH, Cys/Hcy, and other thiols in live cells via fluorescent probes, and how they are applied at the cellular and subcellular levels. The sensing mechanisms (for GSH and Cys/Hcy) and bio-applications of the probes are illustrated followed by a summary of probes for selectively detecting cellular and subcellular thiols.

Structure modulation on fluorescent probes for biothiols and the reversible imaging of glutathione in living cells

The detection of small molecular biothiols (cysteine, homocysteine and glutathione) is of great importance, as they involve in a series of physiological and pathological processes and are associated with many diseases. To realize the real-time monitoring of a specific biothiol, a rapid and reversible probe is required. Therefore, three probes, namely, o-MNPy, m-MNPy and p-MNPy, with pyridine substituted α, β-unsaturated ketone as the recognition site, were reported here, and the reactivity of the recognition site was finely tuned by the connection mode of the pyridine unit. To single out the optimal one, the response performances of three probes toward each biothiol were systemically studied, taking the differences of the intracellular contents of three biothiols into account during the evaluation. Biothiols reacted with the probes through Michael addition, and results showed that the slight structural variations could affect the performances of the probes obviously. p-MNPy with the pyridine unit connected to the recognition site through the para-position of the nitrogen atom, revealed the best sensing ability among the three probes. It demonstrated rapid response, good selectivity and sensitivity, excellent pH adaptability to Cys and GSH, and displayed reversible detection toward GSH. Finally, p-MNPy was successfully applied to track the GSH fluctuations under the oxidative stress stimulated by H₂O₂ in living cells.

A reversible fluorescent probe for GSH was obtained through structure modulation, by which the intracellular GSH fluctuation was imaged.

A Fast Dual-responsive OFF-ON Fluorescent Probe for Cysteine and Glutathione without Interference from Homocysteine

Abnormal levels of biothiols, such as cysteine (Cys), homocystine (Hcy), and glutathione (GSH), are generally known to result in various diseases. A fast dual-responsive OFF-ON fluorescent probe HBO-AC was synthesized and developed. Non-fluorescent HBO-AC can sense Cys by regaining fluorescence at 444 nm within 10 min and a response to GSH by restoring fluorescence at 349 nm within 20 min. There is no mutual interference with Δλ ca. 100 nm. A novel method was developed by utilizing a low reaction rate between HBO-AC and Hcy to eliminate common interference from Hcy. A successful determination of Cys and GSH in fetal bovine serum (FBS) indicated that the probe had potential application for clinical diagnosis. Moreover, it was confirmed that HBO-AC can resist interference from protein to some extent, since FBS was not pretreated before use.

A single benzene fluorescent probe for efficient formaldehyde sensing in living cells using glutathione as an amplifier

Formaldehyde (FA), a simple reactive carbonyl molecule, is endogenously produced in the cell at various physiological condition. At elevated level, FA causes severe cell toxicity as well as damage in macromolecules such proteins and DNA. For detecting FA in living cell, we identify a small but effective fluorescent turn on probe comprising single benzene-based orothophenylenediamine compound. Further study reveals that carboxylic group in orothophenylenediamine plays the important role in enhancing fluorescent signal than another electron withdrawing group. It is even interesting to observe the occurrence of fluorescent enhancement in glutathione (GSH) environment which is generally abundant in every cell. Our probe enables to detect FA over other bio-analytes efficiently with limit of detection of 123 nM and 355-fold of enhancement in cellular mimicking conditions. Moreover, this probe could be useful in discriminating cell that has high concentration of FA as well as GSH.

Hairpin DNA assisted dual-ratiometric electrochemical aptasensor with high reliability and anti-interference ability for simultaneous detection of aflatoxin B1 and ochratoxin A

The simultaneous detection of multiple mycotoxins in grains is significant due to the enhanced toxicity induced by their synergistic effects. In this work, a dual-ratiometric electrochemical aptasensing strategy for the simultaneous detection of aflatoxin B1 (AFB1) and ochratoxin A (OTA) was developed. Here, an anthraquinone-2-carboxylic acid (AQ)-labelled complementary DNA (cDNA) was used to provide separate and specific binding sites to assemble the ferrocene-labelled AFB1 aptamer (Fc-Apt1) and methylene blue-labelled OTA aptamer (MB-Apt2). The target-induced current ratios of I_Fc/I_AQ and I_MB/I_AQ were then used to quantitatively relate to AFB1 and OTA, respectively. Following this principle, two types of aptasensors involving the hairpin DNA (hDNA) and linear single-stranded DNA (ssDNA) as the cDNA were fabricated for performance comparisons. The results revealed that hairpin DNA with a rigid 2D structure can greatly improve the assembly and recognition efficiency of the sensing interface, which makes the hDNA-based aptasensor possess high sensitivity, reliability and anti-interference ability. The hDNA-based aptasensor exhibited a detection range of 10-3000 pg mL^-1 for AFB1 and 30-10000 pg mL^-1 for OTA, respectively, with no observable cross-reactivity. Furthermore, the aptasensor was applied to analyze corn and wheat samples, and the reliability was validated by HPLC-MS/MS. Our work has presented a novel way for fabricating a high-performance aptasensor for simultaneous detection of multiple mycotoxins.

Penta-fluorophenol: a Smiles rearrangement-inspired cysteine-selective fluorescent probe for imaging of human glioblastoma

Two of the most critical factors for the survival of glioblastoma (GBM) patients are precision diagnosis and the tracking of treatment progress. At the moment, various sophisticated and specific diagnostic procedures are being used, but there are relatively few simple diagnosis methods. This work introduces a sensing probe based on a turn-on type fluorescence response that can measure the cysteine (Cys) level, which is recognized as a new biomarker of GBM, in human-derived cells and within on-site human clinical biopsy samples. The Cys-initiated chemical reactions of the probe cause a significant fluorescence response with high selectivity, high sensitivity, a fast response time, and a two-photon excitable excitation pathway, which allows the imaging of GBM in both mouse models and human tissue samples. The probe can distinguish the GBM cells and disease sites in clinical samples from individual patients. Besides, the probe has no short or long-term toxicity and immune response. The present findings hold promise for application of the probe to a relatively simple and straightforward following of GBM at clinical sites.

Mechanistic Study on a BINOL-Coumarin-Based Probe for Enantioselective Fluorescent Recognition of Amino Acids

A detailed investigation was conducted on the reaction of a 1,1'-bi-2-naphthol-coumarin-based fluorescent probe with amino acids. On the basis of the studies, including fluorescence spectroscopy, ¹H NMR, UV-vis, mass spectroscopy, single-crystal X-ray analysis, and molecular modeling, it was found that the distinctively different fluorescent responses of the probe toward the amino acid at the two excitation wavelengths are due to two different reaction pathways that generate different intermediates and products.

A review of bioselenol-specific fluorescent probes: Synthesis, properties, and imaging applications

Bioselenols are important substances for the maintenance of physiological balance and offer anticancer properties; however, their causal mechanisms and effectiveness have not been assessed. One way to explore their physiological functions is the in vivo detection of bioselenols at the molecular level, and one of the most efficient ways to do so is to use fluorescent probes. Various types of bioselenol-specific fluorescent probes have been synthesized and optimized using chemical simulations and by improving biothiol fluorescent probes. Here, we review recent advances in bioselenol-specific fluorescent probes for selenocysteine (Sec), thioredoxin reductase (TrxR), and hydrogen selenide (H₂Se). In particular, the molecular design principles of different types of bioselenols, their corresponding sensing mechanisms, and imaging applications are summarized.

Quinolone-based fluorescent probes for distinguished detection of Cys and GSH through different fluorescence channels

Dual-channel discrimination of Cys and GSH using a red fluorescent probe.

A new fluorescent probe for sensing of biothiols and screening of acetylcholinesterase inhibitors

An axial N₂O-type BODIPY probe has been proposed for sensitive and selective sensing of biothiols and screening of AChE inhibitors using a fluorescence turn-on assay.

A novel strategy for rhodamine B-based fluorescent probes with a selective glutathione response for bioimaging in living cells

The aim of this study was to overcome the reported shortcomings of the glutathione (GSH) detection of rhodamine-based fluorescent probes, such as poor selectivity to thiol groups and reversible unstable covalent binding with the thiol groups.

A GSH Fluorescent Probe with a Large Stokes Shift and Its Application in Living Cells

Intracellular GSH is the most abundant non-protein biothiol and acts as a central antioxidant to defend against aging toxins and radicals. Meanwhile abnormal level of intracellular GSH concentration is directly related to some diseases. In this case, detecting intracellular GSH rapidly and sensitively is of great significance. We synthesize a simple fluorescent probe (named GP) which can discriminate GSH from Cys (cysteine) or Hcy (homocysteine) and presents a 50-fold fluorescence increasing. The response time of GP to GSH was only 5 min and the product GO (the product of GP after reacting with GSH) after reacting with GSH possesses a larger Stokes shift for 135 nm than that in reported work. Probe GP can detect intracellular effectively and shows obvious yellow fluorescence. Briefly, probe GP can detect intracellular GSH rapidly and effectively both in vitro and in living cells.

Structurally regular arrangement induced fluorescence enhancement and specific recognition for glutathione of a pyrene chalcone derivative

Glutathione (GSH) is an important antioxygen and free radical scavenger in the organism. Level of GSH in vivo is associated with many diseases and specific recognition for GSH is very important. Here, a pyrene chalcone derivative 1 1-(2-hydroxyphenyl)-3-(1-pyrenyl)-2-propen-1-one as specific probe for GSH was developed. The probe can give rise to rapid blue fluorescence enhancement for GSH based on Michael addition reaction in pure PBS solution with high sensitivity, fast response rate and high specificity. The compound also can be applied for GSH detection in HeLa cell. Simultaneously, the compound exhibits blue fluorescence emission enhancement in methanol-water (1:1, v/v) solution with fluorescence quantum yield being 0.45 due to the competition of water molecules for hydrogen bonds between hydroxyl and carbonyl and the formation of structurally regular rodlike crystals, which allows regulating fluorescence emission by different solvent condition.

A ratiometric fluorescent probe for simultaneous detection of Cys/Hcy and GSH

GSH, Cys and Hcy are the main intracellular thiols to play crucial roles in human pathologies. It is a great challenge to differentiate these three biothiols using single molecular fluorescent probes due to their close similarities in chemical structure and reactivity. In this work, based on the fluorescence resonance energy transfer (FRET) mechanism, a fluorescent probe CPR was constructed to simultaneously distinguish GSH and Cys/Hcy by means of ratiometric fluorescence changes: from red (584 nm) to green (542 nm) for GSH and from red (584 nm) to blue (472 nm) for Cys/Hcy. This probe showed high sensitivity and selectivity with low limits of detection (LOD = 12 nm, 13 nm and 30 nm for Cys, Hcy and GSH, respectively) and was capable of imaging GSH and Cys/Hcy in cells and zebrafish in a ratiometric manner with low toxicity.

Coumarin-Based Small-Molecule Fluorescent Chemosensors

Coumarins are a very large family of compounds containing the unique 2H-chromen-2-one motif, as it is known according to IUPAC nomenclature. Coumarin derivatives are widely found in nature, especially in plants and are constituents of several essential oils. Up to now, thousands of coumarin derivatives have been isolated from nature or produced by chemists. More recently, the coumarin platform has been widely adopted in the design of small-molecule fluorescent chemosensors because of its excellent biocompatibility, strong and stable fluorescence emission, and good structural flexibility. This scaffold has found wide applications in the development of fluorescent chemosensors in the fields of molecular recognition, molecular imaging, bioorganic chemistry, analytical chemistry, materials chemistry, as well as in the biology and medical science communities. This review focuses on the important progress of coumarin-based small-molecule fluorescent chemosensors during the period of 2012-2018. This comprehensive and critical review may facilitate the development of more powerful fluorescent chemosensors for broad and exciting applications in the future.

A dual-responsive fluorescent probe for detection of fluoride ion and hydrazine based on test strips

Hydrazine (N₂H₄) and fluoride ion (F^-) are regarded as environmental pollutants and potential carcinogens. A dual-functional fluorescent probe (probe 1) was developed for both F^- and N₂H₄ with high selectivity and sensitivity. 1 was based on nucleophilic aromatic substitution reaction for N₂H₄ detection and selective cleavage of 4-nitrobenzenesulphonyl group for the determination of F^-. The limits of detection of probe for F^- and N₂H₄ were 77.82 nM and 29.34 nM, respectively, which are far below the threshold limit value (TLV) of United States Environmental Protection Agency (EPA). The home-made test strips of 1 provided the positive tool for F^- and gaseous N₂H₄ in different system. And the confocal fluorescence images indicated that 1 can quantitatively detect N₂H₄ in living PC12 cells. Promisingly, 1 has great prospects for N₂H₄ imaging and determining in living system.

“Turn-On” Fluorescent Assay of Biothiols Based on Nitrogen-Rich Polymer Carbon Nanostrips and Its Application in Cell Imaging

In this work, a sensitive and selective turn-on fluorimetric method has been developed for the determination of biothiols based on blocking Ag+-induced fluorescence quenching of nitrogen-rich polymer carbon nanostrips (NRPCNSs). Ag+ion can induce the fluorescence quenching of NRPCNSs due to the formation of nonfluorescent coordination complexes via robust Ag-N interaction. Once addition of biothiols, such as cysteine (Cys) and glutathione (GSH), Ag+ions prefer to interact with biothiols rather than NRPCNSs, which could be attribute to the formation of Ag-S bond, thus leading to effective fluorescent recovery of NRPCNSs. Under the optimized conditions, excellent linear relationships existed between the recovery degree of the NRPCNSs and the concentrations of Cys and GSH in the range of 0.05 μM to 10 μM and 0.2 μM to 30 μM, respectively. And, the limits of detection (LODs) for Cys and GSH are 16.5 nM and 65.1 nM, respectively. The detection system also shows high selectivity against other non-thiol amino acids. Moreover, the potential in practical applications of this proposed method has been demonstrated by detecting biothiols in human serum and fluorescence imaging of biothiols in living cells.

A PET-based lysosome-targeted turn-on fluorescent probe for the detection of H2S and its bioimaging application in living cells and zebrafish

A turn-on fluorescent probe regulated by a PET mechanism has been engineered for the detection of H₂S in living cells and zebrafish.

Functional synthetic probes for selective targeting and multi-analyte detection and imaging

In contrast to the classical design of a probe with one binding site to target one specific analyte, probes with multiple interaction sites or, alternatively, with single sites promoting tandem reactions to target one or multiple analytes, have been developed. They have been used in addressing the inherent challenges of selective targeting in the presence of structurally similar compounds and in complex matrices, as well as the visualization of the in vivo interaction or crosstalk between the analytes. Examples of analytes include reactive sulfur species, reactive oxygen species, nucleotides and enzymes. This review focuses on recent innovations in probe design, detection mechanisms and the investigation of biological processes. The vision is to promote the ongoing development of fluorescent probes to enable deeper insight into the physiology of bioactive analytes.

A Triple-Emission Fluorescent Probe for Discriminatory Detection of Cysteine/Homocysteine, Glutathione/Hydrogen Sulfide, and Thiophenol in Living Cells

Thiols, such as cysteine (Cys), homocysteine (Hcy), glutathione (GSH), hydrogen sulfide (H₂S), and thiophenol are metabolically correlated with each other via redox reactions. As a result of the similarity of chemical properties between Cys, Hcy, GSH, H₂S, and thiophenol, it is very challenging to develop an effective methodology to differentiate them. In this work, a triple-emission fluorescent probe, NCQ, was reported for the simultaneous detection of Cys/Hcy, GSH/H₂S, and thiophenol with high sensitivity and selectivity. The solution of NCQ displayed distinct fluorescent signals toward Cys/Hcy, GSH/H₂S, and thiophenol: blue and green for Cys/Hcy, blue for GSH/H₂S, blue and red for thiophenol. Through the blue-green-red emission color combination, Cys/Hcy, GSH/H₂S, and thiophenol could be discriminatively detected in solution and in living cells.

Fluorescent probes for detecting cysteine

Cysteine plays a crucial role in physiological processes. Therefore, it is necessary to develop a method for detecting cysteine. Fluorimetry has the advantages of convenient detection, short response time, high sensitivity and good selectivity. In this review, fluorescent probes that detect cysteine over the past three years are summarized based on structural features of fluorophores such as coumarin, BODIPY, rhodamine, fluorescein, CDs, QDs, etc and reaction groups including acrylate, aldehyde, halogen, 7-nitrobenzofurazan, etc. Then, effects of different combinations between fluorophores and response groups on probe properties and detection performances are discussed.

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 multi-signal fluorescent probe for the discrimination of cysteine/homocysteine and glutathione and application in living cells and zebrafish

A multi-signal fluorescent probe for the discrimination of cysteine/homocysteine and glutathione was engineered in living cells by one-photon and two-photon modes and zebrafish by one-photon modes.

Fluorescent probes for the simultaneous detection of multiple analytes in biology

This review identifies and discusses fluorescent sensors that are capable of simultaneously reporting on the presence of two analytes for biological application.

Fluorescent coumarin-based probe for cysteine and homocysteine with live cell application

Cysteine (Cys) and homocysteine (Hcy) are two of important biological thiols and function as important roles in several biological processes. The development of Cys and Hcy probes will help to explore the functions of biothiols in biological systems. In this work, a new coumarin-based probe AC, containing an acryloyl moiety, was developed for Cys and Hcy detection in cells. Cys and Hcy undergo a nucleophilic addition and subsequent cyclization reaction to remove to the acryloyl group and yield a fluorescent product, 7-hydroxylcomuarin. The probe AC showed good selectivity for cysteine and homocysteine over glutathione and other amino acids and had low detection limits of 65nM for Cys and 79nM for Hcy, respectively. Additionally, confocal imaging experiments demonstrated that the probe AC can be applied to visualize Cys and Hcy in living cells.