Fluorescent sensors for selective detection of thiols: expanding the intramolecular displacement based mechanism to new chromophores

Nanoassemblies of heptamethine cyanine dye-initiated poly(amino acid) enhance ROS generation for effective antitumour phototherapy

Phototherapy shows great potential for pinpoint tumour treatment. Heptamethine cyanine dyes like IR783 have high potential as agents for antitumour phototherapy due to their inherent tumour targeting ability, though their effectiveness in vivo is unsatisfactory for clinical translation. To overcome this limitation, we present an innovative strategy involving IR783-based polymeric nanoassemblies that improve the dye's performance as an antitumoural photosensitizer. In the formulation, IR783 is modified with cysteamine and used to initiate the ring-opening polymerization (ROP) of the N-carboxyanhydride of benzyl-L-aspartate (BLA), resulting in IR783-installed poly(BLA). Compared to free IR783, the IR783 dye in the polymer adopts a twisted molecular conformation and tuned electron orbital distribution, remarkably enhancing its optical properties. In aqueous environments, the polymers spontaneously assemble into nanostructures with 60 nm diameter, showcasing surface-exposed IR783 dyes that function as ligands for cancer cell and mitochondria targeting. Moreover, the nanoassemblies stabilized the dyes and enhanced the generation of reactive oxygen species (ROS) upon laser irradiation. Thus, in murine tumor models, a single injection of the nanoassemblies with laser irradiation significantly inhibits tumour growth with no detectable off-target toxicity. These findings highlight the potential for improving the performance of heptamethine cyanine dyes in antitumor phototherapy through nano-enabled strategies.

All-Nontoxic Fluorescent Probe for Biothiols and Its Clinical Applications for Real-Time Glioblastoma Visualization

Fluorescence-guided surgery (FSG) is a surgical method to selectively visualize the tumor site using fluorescent materials with instrumental setups in the operation rooms. It has been widely used in the surgery of brain tumors, such as glioblastoma (GBM), which is difficult to distinguish from normal tissue. Although FSG is crucial for GBM surgery, the commercially available fluorescent materials for FSG have shown serious adverse effects. To satisfy the clinical demand, we recently reported reaction-based fluorescent probes based on a 4-chloro-7-nitrobenzofurazan (NBD) fluorophore that can detect cysteine (Cys) and homocysteine (Hcy), a biomarker of GBM, and their applications for the GBM diagnosis and FSG. However, our probes have cellular toxicity issues arising from the leaving group (LG) that is generated after the reaction of the fluorescent probe and the analytes. In this study, we disclosed a nontoxic fluorescent probe for sensing biothiols and their clinical applications for real-time human glioblastoma visualization. Systematic toxicity analysis of several LGs was conducted on several cell lines. Among the LGs, 2-hydroxy-pyridine showed negligible toxicity, and its fluorescent probe derivative (named NPO-o-Pyr) showed high specificity and sensitivity (LOD: 0.071 ppm for Cys; 0.189 ppm for Hcy), a fast response time (<5 min) to Cys and Hcy, and high biocompatibility. In addition, NPO-o-Pyr can significantly detect the GBM site both in actual clinical samples as well as in the GBM-xenografted mouse model. We are confident that NPO-o-Pyr will become a new substitute in FSG due to its capability to overcome the limitations of the current fluorescent probes.

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.

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.

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.

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.

Simultaneous sensing of cysteine/homocysteine and glutathione with a fluorescent probe based on a single atom replacement strategy

In recent years, there have been many reports of fluorescent probes for multi-channel detection of Cys, Hcy and GSH. Particularly, reports of fluorescent probes using NBD (7-nitro-1,2,3-benzoxadiazole) or SNBD (7-nitro-1,2,3-benzothiadiazole) moieties as fluorophores are particularly common. Unfortunately, their 4-sulfhydryl derivatives exhibited negligible fluorescence, which makes them incapable of detecting GSH directly. Herein, by performing single selenium-for-oxygen atom replacement within 4-chloro-substituted NBD (NBD-Cl), we developed a small molecule fluorescent probe based on a single atom replacement strategy, which enables the probe to be used for simultaneously distinguishing Cys/Hcy and GSH, along with fluorescence imaging of Cys/Hcy and GSH in live cells from red and green emission channels, respectively.

Thio-conjugation of substituted benzofurazans to peptides: molecular sieves catalyze nucleophilic attack on unsaturated fused rings

An efficient procedure to selectively introduce benzofurazan moieties into peptides was developed. It employs zeolite to catalyze the S-conjugation reaction.

The chronological evolution of small organic molecular fluorescent probes for thiols

Abnormal concentrations of biothiols such as cysteine, homocysteine and glutathione are associated with various major diseases. In biological systems, the structural similarity and functional distinction of these three small molecular thiols has not only required rigorous molecular design of the fluorescent probes used to detect each thiol specifically, but it has also inspired scientists to uncover the ambiguous biological relationships between these bio-thiols. In this minireview, we will discuss the evolution of small organic molecular fluorescent probes for the detection of thiols over the past 60 years, highlighting the potent methodologies used in the design of thiol probes and their particular applications in the semi-quantification of cellular thiols and real-time labelling. At the same time, the present challenges that limit their further application will be discussed. We hope that this minireview will promote future research to enable deeper insight into the crucial role of thiols in biological systems.

The chronological evolution of small organic molecular fluorescent probes for thiols: from separation dependency analysis to cellular specific analysis, what's next?

Nucleophilicity of cysteine and related biothiols and the development of fluorogenic probes and other applications

Biothiols such as l-cysteine, l-homocysteine, and glutathione play essential roles in many biological processes, and are directly associated with several health conditions. Therefore, the development of fast, selective, sensitive, and inexpensive methods for quantitatively analyzing biothiols in aqueous solution, but especially in biological samples, is a very attractive research field. In this feature review, we have approached the relevance of biothiols' nucleophilicity to develop selective fluorogenic probes. Since biothiols have considerable structural similarity, relevant strategies are in full development, including several fluorescent molecular platforms, specific receptor sites, reaction conditions, and optical responses. All of these features are properly presented and discussed. Biothiol sensing protocols are based on traditional organic chemistry reactions such as (hetero)aromatic nucleophilic substitution, addition, and substitution at carbonyl carbon, conjugate addition, and nucleophilic substitution at saturated carbon, amongst others including combined processes; furthermore, mechanistic aspects are detailed herein, including some interesting historical contexts. The feasibility of related fluorogenic probes is illustrated by analysis in complex matrices such as serum, cells, tissues, and animal models. Applications of these reactions in more complex systems such as sulfhydryl-based peptides and proteins are also presented, aiming at functionalizing and detecting these nucleophiles. Most literature cited in this review is recent; however, some other prominent works are also detailed. It is believed that this review may be accessible for many academic levels and may efficiently contribute not only to popularizing science but also to the rational development of fluorogenic probes for biothiol sensing.

A two-component charge transfer hydrogel with excellent sensitivity towards the microenvironment: a responsive platform for biogenic thiols

A two-component charge transfer (CT) hydrogel has been derived from a supramolecular heteroassembly of a pyrene amino acid conjugate (PyHisOH, donor) with a 4-chloro-7-nitrobenzofurazan (NBD-Ox, acceptor) derivative in aqueous medium. The mechanical stiffness, as well as the thermal stability of the CT hydrogels largely depend on the relative ratios of donor and acceptor units as well as on their overall concentration. Moreover, the gel-to-sol transition is found to be susceptible to various external stimuli such as heat, pH, metal ions, etc. Circular dichroism and morphological investigation reveal the formation of left-handed helical fibers in the CT gel network. XRD studies show the lamellar packing of the interactive units in the 3D network of the CT hydrogel. The determination of different rheological parameters confirms the viscoelastic as well as the thixotropic nature of the CT gel. Furthermore, the CT gel is employed for turn-on sensing of biogenic thiols, cyan fluorescence was observed with cysteine/homocysteine, while blue fluorescence with glutathione. Nucleophilic attack at the NBD moiety leads to the formation of thermodynamically stable amino-linked derivatives for cysteine or homocysteine and kinetically controlled thiol-linked adduct for glutathione. Thus, the current system presents a unique opportunity, where a CT hydrogel sample is involved for discriminating biogenic thiols via specific chemodosimetric interactions.

Nanomolar detection of biothiols via turn-ON fluorescent indicator displacement

A simple, visual colour and turn-ON fluorescent method for the detection of biothiols under physiological conditions is reported. The chemosensing is achieved on the basis of the displacement of BODIPY dyes from the surface of gold nanoparticles.

Exploring cysteine regulation in cancer cell survival with a highly specific "Lock and Key" fluorescent probe for cysteine

Using a highly specific “lock and key” fluorescent Cys probe, we confirmed that targeting Cys metabolism to deplete intracellular Cys is a more potent strategy to sensitize cancer cells to chemotherapies.

To probe the regulatory roles of cysteine (Cys) in cancer cell survival, a highly selective and sensitive fluorescent Cys probe SiR was developed by employing a novel “lock and key” strategy, which allows Cys to be detected without any interference or probe consumption caused by the intracellular high concentration of glutathione (GSH). Using SiR, we confirmed that inhibiting cystine (Cys₂) transporter system x_c^– to deplete intracellular Cys is more efficient than inhibiting glutamate–cysteine ligase GCL to deplete intracellular GSH for sensitizing cancer cells to chemotherapy. Moreover, with the probe, a possible self-protection mechanism of cancer cells was indicated: when extracellular Cys sources are blocked, cancer cells could still survive by multidrug resistance protein transporter (Mrp1)-mediated export of intracellular GSH/GSSG as sources to supply intracellular Cys for resisting detrimental oxidative stress. Based on this finding, we further confirmed that abrogating the self-protection mechanism is an even more efficient strategy for sensitizing cancer cells to chemotherapy.

Thiol-specific fluorogenic agent for live cell non-protein thiol imaging in lysosomes

Thiol molecules play a significant role in cellular structures and functions. These molecules are distributed in cells unevenly at the subcellular level. Disturbance of cellular thiols has been associated with various diseases and disorders. Probes that are able to detect subcellular thiol density in live cells are valuable tools in determining thiols' roles at the subcellular level. Lysosomes are a subcellular organelle involved in the degradation of macromolecules through the action of proteolytic enzymes. The degradation not only serves as a way to dispose of unwanted macromolecules but also a way to regulate a variety of cellular functions such as autophagy, endocytosis, and phagocytosis to maintain cell homeostasis. A probe that can detect lysosomal thiols in live cells will be useful in unveiling the roles of thiols in lysosomes. Currently, limited probes are available to detect lysosomal thiols in live cells. We would like to report 4,4'-{[7,7'-thiobis(benzo[c][1,2,5]oxadiazole-4,4'-sulfonyl)]bis(oxy))bis(naphthalene-2,7-disulfonicacid) (TBONES) as a thiol-specific fluorogenic agent for lysosomal thiol imaging in live cells through fluorescence microscopy. TBONES exhibits no fluorescence and readily reacts with non-protein thiols to form fluorescent thiol adducts with λ_ex = 400 nm and λ_em = 540 nm. No reaction was observed when TBONES was mixed with compounds containing nucleophilic functional groups other than thiols such as -OH, -NH₂, and -COOH. No reaction was observed either when TBONES was mixed with protein thiols. When incubated with cells, TBONES selectively and effectively imaged lysosomal thiols in live cells. Imaging of lysosomal thiols was confirmed by a co-localization experiment with LysoTracker™ Blue DND-22.

A New Quinone Based Fluorescent Probe for High Sensitive and Selective Detection of Biothiols and Its Application in Living Cell Imaging

In view of the vital role of biothiols in many physiological processes, the development of simple and efficient probe for the detection of biothiols is of great medical significance. In this work, we demonstrate the use of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), which respond rapidly to biothiols especially to glutathione, as a new fluorescent probe for the selective detection and bioimaging of biothiols. This new fluorescent probe can distinguish glutathione from cysteine and homocysteine easily under physiological concentration and detect glutathione quickly within three minutes. This probe exhibits high selectivity to biothiols and the detection limit was determined to be 3.08 × 10⁻⁹ M for glutathione, 8.55 × 10⁻⁸ M for cysteine, and 2.17 × 10⁻⁹ M for homocysteine, respectively. The sensing mechanism was further explored by density functional theory (DFT) and nuclear magnetic resonance (NMR) experiment; results showed that the interaction forces between the probe and biothiols were electrostatic interaction. In addition, the probe has been successfully applied to the detection of biothiols in Eca9706 cells by fluorescence confocal imaging technology.

Fluorescent probe for sensitive discrimination of Hcy and Cys/GSH in living cells via dual-emission

Abnormal levels of Cys, Hcy and GSH are associated with various diseases, thus monitoring biothiols is of great significance. In this work, a dual-emission responsive near-infrared fluorescent probe NIR-NBD for detecting Hcy and Cys/GSH was developed based on the conjugation of a dicyanoisophorone based fluorophore (NIR-OH) and 7-nitrobenzofurazan (NBD). To our surprise, the addition of Hcy induced significant fluorescence enhancement at both 549 and 697 nm; while Cys/GSH resulted in major fluorescence emission at 697 nm. The detection limit was determined to be 33.2 nM for Cys, 33.5 nM for Hcy, and 34.4 nM for GSH. Therefore, the probe can be used for discriminative detection of Hcy and Cys/GSH. Moreover, fluorescence imaging of HeLa cells indicated that the probe was cell membrane permeable and could be used for visualizing Hcy and Cys/GSH in living cells.

Site-specific near-infrared fluorescent labelling of proteins on cysteine residues with meso-chloro-substituted heptamethine cyanine dyes

Near-infrared (NIR) fluorescence imaging is a promising new medical imaging modality. Associated with a targeting molecule, NIR fluorophores can accumulate selectively in tissues of interest and become valuable tools for the diagnosis and therapy of various pathologies. To facilitate the design of targeted NIR imaging agents, it is important to identify simple and affordable fluorescent probes, allowing rapid labelling of biovectors such as proteins, ideally in a site-specific manner. Here, we demonstrate that heptamethine cyanine based fluorophores, such as IR-783, that contain a chloro-cyclohexyl moiety within their polymethine chain can react selectively, at neutral pH, with cysteine residues in proteins to give stable, site-specifically labelled conjugates, that emit in the NIR spectral window. This reaction is exemplified with the labelling of peptides and two protein models: albumin and a Fab' antibody fragment. The resulting fluorescent proteins are stable and suitable for in vivo NIR imaging applications, as shown on a mice model. This straightforward one-step procedure, that does not require the prior derivatisation of the fluorophore with a bioconjugatable handle, should facilitate the production and use of near-infrared labelled proteins in life sciences.

Coordination geometry-induced optical imaging of l-cysteine in cancer cells using imidazopyridine-based copper(ii) complexes

Overexpression of cysteine cathepsins proteases has been documented in a wide variety of cancers, and enhances the l-cysteine concentration in tumor cells. We report the synthesis and characterization of copper(ii) complexes [Cu(L1)₂(H₂O)](SO₃CF₃)₂, 1, L1 = 3-phenyl-1-(pyridin-2-yl)imidazo[1,5-a]pyridine, [Cu(L2)₂(SO₃CF₃)]SO₃CF₃, 2, L2 = 3-(4-methoxyphenyl)-1-pyridin-2-yl-imidazo[1,5-a]pyridine, [Cu(L3)₂(H₂O)](SO₃CF₃)₂, 3, L3 = 3-(3,4-dimethoxy-phenyl)-1-pyridin-2-yl-imidazo[1,5-a]pyridine and [Cu(L4)₂(H₂O)](SO₃CF₃)₂, 4, L4 = dimethyl-[4-(1-pyridin-2-yl-imidazo[1,5-a]pyridin-3-yl)phenyl]amine as 'turn-on' optical imaging probes for l-cysteine in cancer cells. The molecular structure of complexes adopted distorted trigonal pyramidal geometry (τ, 0.68-0.87). Cu-N_py bonds (1.964-1.989 Å) were shorter than Cu-N_imi bonds (2.024-2.074 Å) for all complexes. Geometrical distortion was strongly revealed in EPR spectra, showing g_‖ (2.26-2.28) and A_‖ values (139-163 × 10^-4 cm^-1) at 70 K. The d-d transitions appeared around 680-741 and 882-932 nm in HEPES, which supported the existence of five-coordinate geometry in solution. The Cu(ii)/Cu(i) redox potential of 1 (0.221 V vs. NHE) was almost identical to that of 2 and 3 but lower than that of 4 (0.525 V vs. NHE) in HEPES buffer. The complexes were almost non-emissive in nature, but became emissive by the interaction of l-cysteine in 100% HEPES at pH 7.34 via reduction of Cu(ii) to Cu(i). Among the probes, probe 2 showed selective and efficient turn-on fluorescence behavior towards l-cysteine over natural amino acids with a limit of detection of 9.9 × 10^-8 M and binding constant of 2.3 × 10⁵ M^-1. The selectivity of 2 may have originated from a nearly perfect trigonal plane adopted around a copper(ii) center (∼120.70°), which required minimum structural change during the reduction of Cu(ii) to Cu(i) while imaging Cys. The other complexes, with their distorted trigonal planes, required more reorganizational energy, which resulted in poor selectivity. Probe 2 was employed for optical imaging of l-cysteine in HeLa cells and macrophages. It exhibited brighter fluorescent images by visualizing Cys at pH 7.34 and 37 °C. It showed relatively less toxicity for these cell lines as ascertained by the MTT assay.

A dual-selective fluorescent probe for discriminating glutathione and homocysteine simultaneously

Homocysteine (Hcy) and glutathione (GSH) play important roles in a variety of physiological and pathological processes. Abnormal levels of Hcy and GSH are related to various diseases. Fluorescent probes for detecting them with sensitive and selective are highly desirable. However, efficient discrimination of Hcy and GSH is still a challenge for their similar molecular structures and chemical properties. Herein, we report a naphthalimide and sulfonyl benzoxadiazole (SBD) based dual-selective fluorescent probe for Hcy and GSH over other amino acids. The probe exhibited weak fluorescence (Φ = 0.075, in DMSO) at 490 nm and fluorescence enhancement upon addition of GSH (1-20 μM) with a detection limit of 0.8 μM. The probe also exhibited ratiometric fluorescence responses for Hcy (fluorescence at 490 nm decreased and fluorescence at 552 nm increased). The fluorescence intensity ratio (I₅₅₂/I₄₉₀) showed a good linear correlation with the Hcy concentrations in range of 3-20 μM and the detection limit was 0.1 μM. Moreover, this probe was successfully utilized for monitoring Hcy and GSH in living cells.

A novel NBD-based fluorescent turn-on probe for the detection of cysteine and homocysteine in living cells

Biothiols, such as cysteine (Cys), homocysteine (Hcy) and glutathione (GSH), are involved in a number of biological processes and play crucial roles in biological systems. Thus, the detection of biothiols is highly important for early diagnosis of diseases and evaluation of disease progression. Herein, we developed a new turn-on fluorescent probe 1 based on 7-nitro-2,1,3-benzoxadiazole (NBD) with high selectivity and sensitivity for Cys/Hcy on account of nucleophilic substitution and Smiles rearrangement reaction. The probe could sense Cys/Hcy rapidly, the intensity of fluorescence increased immediately within 1min. Furthermore, the probe is low toxic and has been successfully applied to detect intracellular Cys/Hcy by cell fluorescence imaging in living normal and cancer cells.

Enhanced Doubly Activated Dual Emission Fluorescent Probes for Selective Imaging of Glutathione or Cysteine in Living Systems

The development of novel fluorescent probes for monitoring the concentration of various biomolecules in living systems has great potential for eventual early diagnosis and disease intervention. Selective detection of competitive species in biological systems is a great challenge for the design and development of fluorescent probes. To improve on the design of fluorescent coumarin-based biothiol sensing technologies, we have developed herein an enhanced dual emission doubly activated system (DACP-1 and the closely related DACP-2) for the selective detection of glutathione (GSH) through the use of one optical channel and the detection of cysteine (Cys) by another channel. A phenylselenium group present at the 4-position completely quenches the fluorescence of the probe via photoinduced electron transfer to give a nonfluorescent species. Probes are selective for glutathione (GSH) in the red region and for cysteine/homocysteine (Cys/Hcy) in the green region. When they were treated with GSH, DACP-1 and DACP-2 showed strong fluorescence enhancement in comparison to that for closely related species such as amino acids, including Cys/Hcy. Fluorescence quantum yields (Φ_F) increased for the red channel (<0.001 to 0.52 (DACP-1) and 0.48 (DACP-2)) and green channel (Cys) (<0.001 to 0.030 (DACP-1) and 0.026 (DACP-2)), respectively. Competing fluorescent enhancements upon addition of closely related species were negligible. Fast responses, improved water solubility, and good cell membrane permeability were all properly established with the use of DACP-1 and DACP-2. Live human lung cancer cells and fibroblasts imaged by confocal microscopy, as well as live mice tumor model imaging, confirmed selective detection.

Green and Red Fluorescent Dyes for Translational Applications in Imaging and Sensing Analytes: A Dual-Color Flag

Red and green are two of the most-preferred colors from the entire chromatic spectrum, and red and green dyes are widely used in biochemistry, immunohistochemistry, immune-staining, and nanochemistry applications. Selective dyes with green and red excitable chromophores can be used in biological environments, such as tissues and cells, and can be irradiated with visible light without cell damage. This critical review, covering a period of five years, provides an overview of the most-relevant results on the use of red and green fluorescent dyes in the fields of bio-, chemo- and nanoscience. The review focuses on fluorescent dyes containing chromophores such as fluorescein, rhodamine, cyanine, boron-dipyrromethene (BODIPY), 7-nitobenz-2-oxa-1,3-diazole-4-yl, naphthalimide, acridine orange, perylene diimides, coumarins, rosamine, Nile red, naphthalene diimide, distyrylpyridinium, benzophosphole P-oxide, benzoresorufins, and tetrapyrrolic macrocycles. Metal complexes and nanomaterials with these dyes are also discussed.

Fast and Selective Two-Stage Ratiometric Fluorescent Probes for Imaging of Glutathione in Living Cells

Two fluorescent, m-nitrophenol-substituted difluoroboron dipyrromethene dyes have been designed by nucleophilic substitution reaction of 3,5-dichloro-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY). Nonsymmetric and symmetric probes, that is. BODIPY 1 (with one nitrophenol group at the position 3) and BODIPY 2 (with two nitrophenol groups at the positions 3 and 5) were applied to ratiometric fluorescent glutathione detection. The detection is based on the two-step nucleophilic aromatic substitution of the nitrophenol groups of the probes by glutathione in buffer solution containing CTAB. In the first stage, probe 1 showed ratiometric fluorescent color change from green (λ_em = 530 nm) to yellow (λ_em = 561 nm) because of monosubstitution with glutathione (I_561nm/I_530nm). Addition of excess glutathione caused the second stage of ratiometric fluorescent color change from yellow to reddish orange (λ_em = 596 nm, I_596nm/I_561nm) due to disubstitution with glutathione. Therefore, different concentration ranges of glutathione (from less to excess) could be rapidly detected by the two-stage ratiometric fluorescent probe 1 in 5 min. While, probe 2 shows single-stage ratiometric fluorescent detection to GSH (from green to reddish orange, I_596nm/I_535nm). Probes 1 and 2 exhibit excellent properties with sensitive, specific colorimetric response and ratiometric fluorescent response to glutathione over other sulfur nucleophiles. Application to cellular ratiometric fluorescence imaging indicated that the probes were highly responsive to intracellular glutathione.

A multi-signal fluorescent probe for simultaneously distinguishing and sequentially sensing cysteine/homocysteine, glutathione, and hydrogen sulfide in living cells

A multi-signal fluorescent probe was engineered for simultaneously distinguishing and sequentially sensing cysteine/homocysteine, glutathione, and hydrogen sulfide in living cells.

Biothiols, which have a close network of generation and metabolic pathways among them, are essential reactive sulfur species (RSS) in the cells and play vital roles in human physiology. However, biothiols possess highly similar chemical structures and properties, resulting in it being an enormous challenge to simultaneously discriminate them from each other. Herein, we develop a unique fluorescent probe (HMN) for not only simultaneously distinguishing Cys/Hcy, GSH, and H₂S from each other, but also sequentially sensing Cys/Hcy/GSH and H₂S using a multi-channel fluorescence mode for the first time. When responding to the respective biothiols, the robust probe exhibits multiple sets of fluorescence signals at three distinct emission bands (blue-green-red). The new probe can also sense H₂S at different concentration levels with changes of fluorescence at the blue and red emission bands. In addition, the novel probe HMN is able to discriminate and sequentially sense biothiols in biological environments via three-color fluorescence imaging. We expect that the development of the robust probe HMN will provide a powerful strategy to design fluorescent probes for the discrimination and sequential detection of biothiols, and offer a promising tool for exploring the interrelated roles of biothiols in various physiological and pathological conditions.

Copper(ii)-benzimidazole complexes as efficient fluorescent probes forl-cysteine in water

Copper(ii)-benzimidazole complexes could detectl-cysteine over other natural amino acids at pH 7.34 by a ‘turn-on’ fluorescence mechanismviathe reduction of Cu(ii) to Cu(i) followed by displacement with excellent selectivity.

A simple fluorescent off-on probe for the discrimination of cysteine from glutathione

A simple and stable fluorescent off-on probe for discrimination of cysteine (Cys) from glutathione (GSH) has been developed by combining resorufin with 7-nitrobenzofurazan. The probe, displaying distinct emission patterns for Cys and GSH at just one excitation wavelength, can be used for simultaneous determination of Cys and GSH in human plasma.

Single near-infrared fluorescent probe with high- and low-sensitivity sites for sensing different concentration ranges of biological thiols with distinct modes of fluorescence signals

We describe a unique approach for the development of an interesting type of the fluorescent probes, which can show different modes of fluorescence signals to distinct concentration ranges of a target of interest. The key points for the design of the new type of the fluorescent probes include the judicious selection of the dye platforms and the corresponding high- and low-sensitivity sites. It is known that the normal concentrations of biological thiols have significant biological functions. However, up- or down-regulated concentrations of thiols may induce several diseases. Therefore, it is highly important to monitor the changes of thiol concentrations in living systems. Based on the proposed strategy, we engineer the novel NIR fluorescent probe, CHMC-thiol, which remarkably can display a turn-on signal to the low concentration range of thiols and a ratiometric response to the high concentration range of thiols for the first time. We anticipate that the intriguing strategy formulated herein will be widely useful for the development of concentration range-dependent fluorescent probes.

An aryl-thioether substituted nitrobenzothiadiazole probe for the selective detection of cysteine and homocysteine

An aryl-thioether substituted nitrobenzothiadiazole probe was synthesized and employed to detect cysteine and homocysteine selectively in living cells. Interestingly, both cysteine (Cys) and homocysteine (Hcy) promote an enhancement of the fluorescence intensity of the probe at pH 7.4 while only Cys gives rise to this enhancement under weakly acidic conditions (pH 6.0).

Selective Fluorescence Detection of Cysteine over Homocysteine and Glutathione Based on a Cysteine-Triggered Dual Michael Addition/Retro-aza-aldol Cascade Reaction

In this work, a cysteine (Cys)-triggered dual Michael addition/retro-aza-aldol cascade reaction has been exploited and utilized to construct a fluorescent probe for Cys for the first time. The resulting fluorescent probe 8-alkynylBodipy 1 contains an activated alkynyl unit as Michael receptor and a Bodipy dye as fluorescence reporter and can highly selectively detect Cys over homocysteine (Hcy)/glutathione (GSH) as well as other amino acids with a significant fluorescence off-on response (∼4500-fold) and an ultralow detection limit (0.38 nM). The high selectivity of 1 for Cys could be attributed to a kinetically favored five-membered cyclic intermediate produced by the dual Michael addition of Cys with the activated alkynyl unit of 1. The big fluorescence off-on response is due to the subsequent retro-aza-aldol reaction of the five-membered cyclic intermediate that results in the release of a highly fluorescent 8-methylBodipy dye 2. The probe has been successfully used to detect and image Cys in serum and cells, respectively.

Design strategies of fluorescent probes for selective detection among biothiols

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

A dual-response BODIPY-based fluorescent probe for the discrimination of glutathione from cystein and homocystein

In situ monitoring of intracellular thiol activity in cell growth and function is highly desirable. However, the discriminative detection of glutathione (GSH) from cysteine (Cys) and homocystein (Hcy) and from common amino acids still remains a challenge due to the similar reactivity of the thiol groups in these amino acids. Here we report a novel strategy for selectively sensing GSH by a dual-response mechanism. Integrating two independent reaction sites with a disulfide linker and a thioether function into a fluorescent BODIPY-based chemsensor can guarantee the synergetic dual-response in an elegant fashion to address the discrimination of GSH. In the first synergetic reaction process, the thiol group in GSH, Cys and Hcy induces disulfide cleavage and subsequent intramolecular cyclization to release the unmasked phenol-based BODIPY (discriminating thiol amino acids from other amino acids). In the second synergetic process, upon the substitution of the thioether with the nucleophilic thiolate to form a sulfenyl-BODIPY, only the amino groups of Cys and Hcy, but not that of GSH, undergo a further intramolecular displacement to yield an amino-substituted BODIPY. In this way, we make full use of the kinetically favorable cyclic transition state in the intramolecular rearrangement, and enable photophysical distinction between sulfenyl- and amino-substituted BODIPY for allowing the discriminative detection of GSH over Cys and Hcy and thiol-lacking amino acids under physiological conditions. Moreover, this probe exhibits a distinguishable ratiometric fluorescence pattern generated from the orange imaging channel to the red channel, which proves the differentiation of GSH from Cys and Hcy in living cells.