Conjugate Addition/Cyclization Sequence Enables Selective and Simultaneous Fluorescence Detection of Cysteine and Homocysteine

Multiple fluorescence and hydrogen peroxide-responsive properties of novel triphenylamine-benzothiazole derivatives

A novel fluorescent dye molecule - triphenylamine (TPA)-benzothiazole (BZT) - based on excited state intramolecular proton transfer (ESIPT) was prepared by the Suzuki coupling reaction. The photophysical property assay indicates that BZT-TPA appeared in distinguishable colors in mixed solvents with different water contents. Moreover, BZT-TPA exhibited observable AIE behavior. On this basis, a fluorescent probe BZT-TPA-BO was synthesized for detecting H2O2. This probe molecule was found to have excellent selectivity, rapid response, and good linear relationship (R2 = 0.989) for detecting H2O2 in aqueous medium. Through DFT calculation, fluorescence spectrum, nuclear magnetic titration and HR-MS, the mechanism of recognition of H2O2 by the probe BZT-TPA-BO is proposed. In addition, the probe BZT-TPA-BO to some extent exhibited better performance for detecting exogenous H2O2 in HeLa cells.

Triazine based fluorescent sensor for sequential detection of Hg2+ and L-Cysteine in real samples and application in logic Gate: A combination of Extensive experimental and theoretical analysis

Triazine based fluorescent sensor TBT was rationally designed and synthesized to achieve sequential detection of Hg²⁺ and L-cysteine based on the presence of sulfur moiety and suitable cavity in the molecule. Sensor TBT exhibited excellent sensing potential for the selective detection of Hg²⁺ ions and L-cysteine (Cys) in real samples. Upon addition of Hg²⁺ to sensor TBT, enhancement in emission intensity of sensor TBT was observed which was accredited to the presence of sulfur moiety and size of cavity in the sensor. Upon interaction with Hg²⁺ blockage of intramolecular charge transfer (ICT) along with chelation-enhanced fluorescence (CHEF) resulted in the increase in fluorescence emission intensity of sensor TBT. Further, TBT-Hg²⁺ complex was employed for the selective detection of Cys through fluorescence quenching mechanism. This was attributed to the significantly stronger interaction of Cys with Hg²⁺, which resulted in the formation of Cys-Hg²⁺ complex and subsequently sensor TBT was released from TBT-Hg²⁺ complex. The nature of interaction between TBT-Hg²⁺ and Cys-Hg²⁺ complex was evaluated through ¹H NMR titration experimentations. Extensive DFT studies were also carried out which include thermodynamic stability, frontier molecular orbitals (FMO), density of states (DOS), non-covalent interaction (NCI), quantum theory of atom in molecule (QTAIM), electron density differences (EDD) and natural bond orbital (NBO) analyses. All the studies supported the non-covalent type of interaction between analytes and sensor TBT. The limit of detection for Hg²⁺ ions was found to be as low as 61.9 nM. Sensor TBT was also employed for the quantitative detection of Hg²⁺ and Cys in real samples. Additionally, logic gate was fabricated by using sequential detection strategy.

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.

Coumarin-Based Fluorescence Probe for Differentiated Detection of Biothiols and Its Bioimaging in Cells

In this work, a coumarin derivative, SWJT-14, was synthesized as a fluorescence probe to distinguish cysteine (Cys), homocysteine (Hcy) and glutathione (GSH) in aqueous solutions. The detection limit of Cys, Hcy and GSH for the probe was 0.02 μM, 0.42 μM and 0.92 μM, respectively, which was lower than biothiols in cells. The probe reacted with biothiols to generate different products with different conjugated structures. Additionally, it could distinguish Cys, Hcy and GSH using fluorescence and UV-Vis spectra. The detection mechanism was confirmed by MS. SWJT-14 was successfully used in cellular experiments and detected both endogenous and exogenous biothiols.

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

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

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

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

A Facile Probe for Fluorescence Turn-on and Simultaneous Naked-Eyes Discrimination of H2S and biothiols (Cys and GSH) and Its Application

Hydrogen sulfide and biothiol molecules such as Cys and GSH acted important roles in many physiological processes. To simultaneously detect and distinguish them was quite necessary by a suitable fluorescent probe. A novel chemosensor 4-(4-(benzo[d]thiazol-2-yl)-2-methoxyphenoxy)-7-nitrobenzo[c][1,2,5]oxadiazole (BMNO) was designed to detect H₂S/Cys/GSH using the combination of nitrobenzofurazan (NBD) and benzothiazole fluorophores linked by a facile ether bond. The probe BMNO was developed for simultaneous identification of H₂S, Cys and GSH. Noticeably, the color changes (from colorless to light purple, light orange and light yellow) of probe BMNO solutions for sensing H₂S, Cys and GSH could be observed by naked eyes, respectively. The probe BMNO exhibited high selectivity and sensitivity for H₂S, Cys and GSH showing distinct optical signal with detection limit as low as 0.15 μM, 0.03 μM and 0.14 μM, respectively. The sensing mechanism was clarified by spectrum analysis and some controlled experiments. In addition, these outstanding properties of probe BMNO enabled its practical applications in detection H₂S in beer, and in cell imaging for Cys and GSH as well.

Visualizing and evaluating mitochondrial cysteine via near-infrared fluorescence imaging in cells, tissues and in vivo under hypoxia/reperfusion stress

Increasingly grim environmental pollutions are closely related with the occurrence and development of diseases. However, it's obscure how environmental stress disturbs the normal physiological process, and then how endogenous reactive species mend the cases. Hypoxia/reperfusion (H/R), a common and intractable injury in aquaculture and clinic, can induce oxidative stress and ultimately cause irreversible injury to organism. Cysteine (Cys) plays essential roles in maintaining transduction of numerous reactive species and redox homeostasis in subcellular structures, cells and organisms. A great deal of fluorescence research about Cys are focusing on development of selective probes but with poor exploration of the biofunction under environmental stress. Therefore, it is of great significance to examine the bio-effects of Cys against H/R stress. In the present work, we design a fluorescent probe BCy-AC for in situ detecting Cys, the unique Enol-Keto tautomerization of fluorophore BCy-Keto propels the reaction process which will improve the sensitivity and potential application performance of the probe. BCy-AC is conveniently applied to visualize Cys in HT-22 cells, zebrafish and mice tissues. Moreover, imaging results obtained from H/R models reveal that endogenous Cys changes with hypoxia and reperfusion time and Cys pretreatment effectively defend H/R injury in cells and in vivo.

Fluorescein Based Fluorescence Sensors for the Selective Sensing of Various Analytes

Fluorescein molecules are extensively used to develop fluorescent probes for various analytes due to their excellent photophysical properties and the spirocyclic structure. The main structural modification of fluorescein occurs at the carboxyl group where different groups can be easily introduced to produce the spirolactam structure which is non-fluorescent. The spirolactam ring opening accounts for the fluorescence and the dual sensing of analytes using fluorescent sensors is still a topic of high interest. There is an increase in the number of dual sensors developed in the past five years and quite a good number of fluorescein derivatives were also reported based on reversible mechanisms. This review analyses environmentally and biologically important cations such as Cu2+, Hg2+, Fe3+, Pd2+, Zn2+, Cd2+, and Mg2+; anions (F-, OCl-) and small molecules (thiols, CO and H2S). Structural modifications, binding mechanisms, different strategies and a comparative study for selected cations, anions and molecules are outlined in the article.

Native Chemical Ligation-Based Fluorescent Probes for Cysteine and Aminopeptidase N Using meso-thioester-BODIPY

meso-Carboxyl-BODIPY responds to small electronic changes resulting from acyl substitution reactions with a marked change in fluorescence. Herein, the minute changes that accompany the thioester to amide conversion encountered in native chemical ligation (NCL) are exploited in the construction of fluorescent "turn-on" probes. Two fluorogenic probes, 1 a and 4, derived from a meso-thioester-BODIPY scaffold, were designed for the selective detection of cysteine (1 a) and aminopeptidase N (4), respectively. The aromatic (1 a) and aliphatic (4) thioesters of meso-carboxyl-BODIPY are nonfluorescent. However, specific analyte-induced conversion to the meso-amide derivative caused significant spectral changes and a dramatic fluorescence enhancement. Probe 1 a exhibited a large fluorescence "turn-on" response with high selectivity toward cysteine via a tandem NCL reaction. Probe 4 was successfully applied to the monitoring and imaging of endogenous aminopeptidase N in live cancer cells.

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.

BODIPY-Based Fluorescent Probes for Biothiols

Fluorescent probes for biothiols have aroused increasing interest owing to their potential to enable better understanding of the diverse physiological and pathological processes related to the biothiol species. BODIPY fluorophores exhibit excellent optical properties, which can be readily tailored by introducing diverse functional units at various positions of the BODIPY core. In the present review, the development of fluorescent probes based on BODIPYs for the detection of biothiols are systematically summarized, with emphasis on the preferable detection of individual biothiols, as well as simultaneous discrimination among cysteine (Cys), homocysteine (Hcy), reduced glutathione (GSH). In addition, organelle-targeting probes for biothiols are also highlighted. The general design principles, various recognition mechanisms, and biological applications are elaboratively discussed, which could provide a useful reference to researchers worldwide interested in this area.

Designing an "Off-On" Fluorescence Sensor Based on Cluster-Based CaII-Metal-Organic Frameworks for Detection of l-Cysteine in Biological Fluids

Recently, luminescent metal-organic framework (MOF) materials have attracted considerable attention in fluorescence sensing. In this essay, we prepared a new cluster-based Ca^II-MOFs {[Ca_1.5(μ₈-HL₁)(DMF)₂]·DMF}_n (1) with good water dispersibility, excellent photoluminescence properties (FL quantum yield of 20.37%) and great fluorescence stability. Further, it was employed to design as an "off-on" fluorescence sensor for sensitive detection of l-cysteine. This proposed strategy was that fluorescence of Ca^II-MOFs 1 was quenched for providing a low fluorescence background by the introduction of Pb²⁺ forming the Ca^II-MOFs 1/Pb²⁺ hybrid system. The quenching effect could be ascribed to the static quenching mechanism because of the formation of ground-state complexes and coordination interactions between the free carboxyl of H₄L₁ ligands of Ca^II-MOFs 1 and Pb²⁺. Then, with the addition of l-cysteine into the Ca^II-MOFs 1/Pb²⁺ hybrid system, the fluorescence signal was immediately restored. This result was because the Pb²⁺ was gradually released from the hybrid system by chelation interactions between the -SH groups of l-cysteine and Pb²⁺. This method received a relative wide linear range varying from 0.05 to 40 μM and a low detection limit of 15 nM for detection of l-cysteine. This proposed strategy was also successfully applied to detect l-cysteine in human serum samples with satisfactory recoveries from 95.9 to 101.5%.

GdPO4-Based Nanoprobe for Bioimaging and Selective Recognition of Dipicolinic Acid and Cysteine by a Sensing Ensemble Approach

Multiple functions incorporated in one single-component nanoplatform pave the way for important biomedicine applications. Herein, a multifunctional terbium-doped gadolinium orthophosphate (GdPO₄:Tb-EDTA) nanoplatform was prepared through a simple, ecofriendly, one-step hydrothermal method. Results showed that dipicolinic acid (DPA), the biomarker of bacterial spores, significantly increased the fluorescence intensity of this nanoplatform and conferred it with rapid response and excellent selectivity. Subsequently, the fluorescence of the ensemble GdPO₄:Tb-EDTA-DPA can be remarkably quenched by Cu²⁺, which led to a rewritable nanosensor used in the detection of cysteine (Cys) with excellent sensitivity. In addition, GdPO₄:Tb-EDTA can also be a potential T₁-weighted magnetic resonance imaging (MRI) contrast agent, which indicated a satisfactory in vitro MRI with r₁ relaxivity values of 13.9 mM^-1 s^-1 and in vivo MRI through intravenous administration on a rat model. Overall, the proposed assay may have great theoretical and practical significance for designing multifunctional biomaterials.

Acid-Induced Shift of Intramolecular Hydrogen Bonding Responsible for Excited-State Intramolecular Proton Transfer

The significant progress recently achieved in designing smart acid-responsive materials based on intramolecular charge transfer inspired us to utilize excited-state intramolecular proton transfer (ESIPT) for developing a turn-on acid-responsive fluorescent system with an exceedingly large Stokes shift. Two ESIPT-active fluorophores, 2-(2-hydroxyphenyl)pyridine (HPP) and 2-(2-hydroxyphenyl)benzothiazole (HBT), were fused into a novel dye (HBT-HPP) fluorescent only in the protonated state. Moreover, we also synthesized three structurally relevant control compounds to compare their steady-state fluorescence spectra and optimized geometric structures in neutral and acidic media. The results suggest that the fluorescence turn-on was caused by the acid-induced shift of the ESIPT-responsible intramolecular hydrogen bond from the HPP to HBT moiety. This work presents a systematic comparison of the emission efficiencies and basicity of HBT and HPP for the first time, thereby utilizing their differences to construct an acid-responsive smart organic fluorescent material. As a practical application, red fluorescent letters can be written using the acid as an ink on polymer film.

A Multi-signal Fluorescent Probe with Multiple Binding Sites for Simultaneous Sensing of Cysteine, Homocysteine, and Glutathione

A novel fluorescent probe was developed by integrating chlorinated coumarin and benzothiazolylacetonitrile and exploited for simultaneous detection of cysteine (Cys), homocysteine (Hcy), and glutathione (GSH). Featuring four binding sites and different reaction mechanisms for different biothiols, this probe exhibited rapid fluorescence turn-on for distinguishing Cys, Hcy, and GSH with 108-, 128-, 30-fold fluorescence increases at 457, 559, 529 nm, respectively, across different excitation wavelengths. Furthermore, the probe was successfully applied to the fluorescence imaging of endogenous Cys and GSH and exogenous Cys, Hcy, and GSH in living cells.

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

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

A Single Fluorescent Probe to Visualize Hydrogen Sulfide and Hydrogen Polysulfides with Different Fluorescence Signals

Hydrogen sulfide (H2 S) and hydrogen polysulfides (H2 Sn , n>1) are endogenous regulators of many physiological processes. In order to better understand the symbiotic relationship and cellular cross-talk between H2 S and H2 Sn , it is highly desirable to develop single fluorescent probes which enable dual-channel discrimination between H2 S and H2 Sn . Herein, we report the rational design, synthesis, and evaluation of the first dual-detection fluorescent probe DDP-1 that can visualize H2 S and H2 Sn with different fluorescence signals. The probe showed high selectivity and sensitivity to H2 S and H2 Sn in aqueous media and in cells.

A new quinoline-based fluorescent probe for Cd2+ and Hg2+ with an opposite response in a 100% aqueous environment and live cell imaging

A new quinoline-based dual fluorescent probe L was synthesized for the detection of Cd²⁺ (turn-on) and Hg²⁺ (turn-off) in a 100% aquatic system. Its in vivo sensitivities to Cd²⁺ and Hg²⁺ were demonstrated in the EC9706 cell line.

The Development of Fluorescent Probes for Visualizing Intracellular Hydrogen Polysulfides

Endogenous hydrogen polysulfides (H2Sn; n>1) have been recognized as important regulators in sulfur-related redox biology. H2Sn can activate tumor suppressors, ion channels, and transcription factors with higher potency than H2S. Although H2Sn are drawing increasing attention, their exact mechanisms of action are still poorly understood. A major hurdle in this field is the lack of reliable and convenient methods for H2Sn detection. Herein we report a H2Sn-mediated benzodithiolone formation under mild conditions. This method takes advantage of the unique dual reactivity of H2Sn as both a nucleophile and an electrophile. Based on this reaction, three fluorescent probes (PSP-1, PSP-2, and PSP-3) were synthesized and evaluated. Among the probes prepared, PSP-3 showed a desirable off/on fluorescence response to H2Sn and high specificity. The probe was successfully applied in visualizing intracellular H2Sn.

AMP/GMP Analogs as Affinity ESIPT Probes for Highly Selective Sensing of Alkaline Phosphatase Activity in Living Systems

Current probes for alkaline phosphatase (ALP) detection had been developed mainly by adding a phosphate group to a dye, which would lead to indistinct performance when implemented in a living system as several phosphatases exist together. In this study, the nucleotides adenosine monophosphate (AMP) and guanosine monophosphate (GMP) were introduced into 2'-(2'-hydroxyphenyl)-benzothiazole-based probes, and highly fluorescent turn-on probes with good selectivity towards ALP over several phosphatases, as well as high affinity and low toxicity were obtained. In the presence of L-phenylalanine, an ALP inhibitor, a strong decrease in fluorescence recovery was observed. These probes allowed for real-time imaging of endogenous ALP activity in living cells as well as in a zebrafish model.