A Ratiometric, Fast-Responsive and Single-Wavelength Excited Fluorescent Probe for the Discrimination of Cys and Hcy

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

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

Lysosomal-targeted fluorescent probe based pH regulating reactivity for tracking cysteine dynamics under oxidative stress

The ability to detect and visualize cellular events and associated biological analytes is essential for the understanding of their physiological and pathological functions. Cysteine (Cys) plays a crucial role in biological systems and lysosomal homeostasis. This puts forward higher requirements on the performance of the probe. Herein, we rationally designed a coumarin-based probe for the reversible, specific, sensitive, and rapid detection of Cys based on pH regulating reactivity. The obtained probe (ECMA) introduces a morpholine moiety to target lysosomes, and α,β-unsaturated-ketone with an electron-withdrawing CN group served as a reversible reaction site for Cys. Importantly, ECMA was successfully applied to the real-time monitoring of Cys dynamics in living cells. Furthermore, cell imaging clearly revealed that exogenous Cys could induce the up-regulation of lysosomal ROS, which provided a powerful tool for investigating the relationship between oxidative stress and lysosomal Cys.

Calcium-enriched carbon nanoparticles loaded with indocyanine green for near-infrared fluorescence imaging-guided synergistic calcium overload, photothermal therapy, and glutathione-depletion-enhanced photodynamic therapy

Combining phototherapy with other treatments has significantly advanced cancer therapy. Here, we designed and fabricated calcium-enriched carbon nanoparticles (Ca-CNPs) that could effectively deplete glutathione (GSH) and release calcium ions in tumors, thereby enhancing the efficacy of photodynamic therapy (PDT) and the calcium overload effect that leads to mitochondrial dysfunction. Due to the electrostatic interaction, π-π stacking interaction, multiple hydrogen bonds, and microporous structures, indocyanine green (ICG) was loaded onto the surface of Ca-CNPs with a high loading efficiency of 44.7 wt%. The obtained Ca-CNPs@ICG can effectively improve the photostability of ICG while retaining its ability to generate singlet oxygen (1O2) and undergo photothermal conversion (Ca-CNPs@ICG vs. ICG, 45.1% vs. 39.5%). In vitro and in vivo experiments demonstrated that Ca-CNPs@ICG could be used for near-infrared fluorescence imaging-guided synergistic calcium overload, photothermal therapy, and GSH depletion-enhanced PDT. This study sheds light on the improvement of 1O2 utilization efficiency and calcium overload-induced mitochondrial membrane potential imbalance in tumor cells.

A lysosome-targeted fluorescent probe based on a BODIPY structure for Cys/Hcy detection

Cysteine (Cys) and homocysteine (Hcy) are important biothiols in living organisms. They play important roles in a variety of physiological and pathological processes. Therefore, it is very important to design an optical probe for the selective detection of Cys/Hcy. Herein, we report the design and synthesis of a fluorescent probe NBD-B-T based on a boron-dipyrromethene (BODIPY) structure, which showed an excellent lysosome targeting ability and an outstanding Cys/Hcy detection capacity. For NBD-B-T, the sensing group 7-nitro-2,1,3-benzoxadiazole (NBD) and the lysosomal targeting group morpholine were introduced. The results show that the NBD-B-T probe can detect Cys/Hcy with fluorescence emission turn-on performance. The low detection limits of this probe are about 76.0 nM for Hcy and 97.6 nM for Cys, respectively. The NBD-B-T probe has a low detection limit, high stability, and excellent selectivity and sensitivity. More importantly, the NBD-B-T can target lysosome, and simultaneously detect the Cys/Hcy in living cells.

Equivalent Response Strategy for Sensing Total Biothiols in Human Serums and Living Cells Using a Hemicyanine-Based Self-Immolative Probe

Biothiols including cysteine (Cys), homocysteine (Hcy), and glutathione (GSH) are crucial in maintaining the redox balance in the body, and the metabolism and transportation of biothiols rely on the coreaction of diverse proteins and enzymes. The abnormal concentrations and metabolism of biothiols are closely associated with many diseases. However, due to the same active reaction site of the sulfydryl group in biothiols, it is inevitable to bear a confused signal of mutual influence on both nonselective detection and discriminate detection, which presents a serious challenge of accurately sensing or imaging the three biothiols. By assigning an α,β-unsaturated ketone moiety as a Michael acceptor to trigger thiols to complete the irreversible equivalent domino response processes of nucleophilic addition, olefinic bond migration, and self-immolation, a targeted strategy was rationally pointed out, and herein, a hemicyanine-based probe CyOCy was prepared as a proof of strategy demonstration. The new probe could be equivalently lit up by Cys, Hcy, GSH, and even biothiol combinations (Cys/Hcy, Cys/GSH, Hcy/GSH, or Cys/Hcy/GSH) with unified linear ranges, detection limits, and response times. The probe CyOCy has been successfully used for the accurate quantification of total biothiols in the serum samples of healthy persons and coronary heart disease patients. In addition, the probe has been applied for cell screening, exogenous biothiol imaging, and monitoring drug-induced biothiol fluctuations. The purposive thinking of this work may provide an effective avenue for the accurate sensing of multicomponent samples.

An extra-large Stokes shift near-infrared fluorescent probe for specific detection and imaging of cysteine

Cysteine (Cys) plays a crucial role in numerous physiological and pathological processes. Therefore, it is imperative to design a highly selective and sensitive near-infrared (NIR) fluorescent probe to monitor Cys. In this study, we have developed a novel NIR fluorescent probe XA based on Xanthene hybrid tetrahydro-acridine salt dye for specifically tracking of Cys, where a chlorine-substituted tetrahydro-acridine acts as a high Cys-reactive site and water-soluble group. Probe XA exhibits a remarkable "turn-on" NIR emission (830 nm) with an extra-large Stokes shift (305 nm) for monitoring Cys. It also has a high selectivity, rapid response time (6 min) and high sensitivity (LOD as 0.5 μM). We fully characterized and discussed the sensing mechanism of XA toward Cys using HPLC and MS spectrums, as well as quantum theory calculations. Furthermore, the excellent properties of NIR fluorescent detection allow this novel probe to successfully monitor fluctuations of exogenous and endogenous Cys concentration levels in living cells and in vivo.

A coumarin based ratiometric fluorescent probe for the detection of Cu2+ and mechanochromism as well as application in living cells and vegetables

In this paper, a novel coumarin-derived fluorescent probe NY was designed and synthesized. NY displayed a significant ratiometric fluorescence response towards Cu2+ in PBS buffer (10 mM, pH = 7.4), with the emission wavelength blue-shifted from 580 to 495 nm, and a fluorescence change from orange to green was evident under a 365 nm UV light. Meanwhile, NY had the advantages of high selectivity, short response time (5 min), low detection limit (1.3 × 10-8 M) and large binding constant (1.45 × 105 M-1) towards Cu2+. The binding mechanism between NY and Cu2+ was elucidated by FT-IR, 1H NMR titration, TOF-MS and Job's plot analysis. In addition, NY was successfully employed in the detection of Cu2+ within environmental water and vegetable samples with satisfactory results. Laser confocal microscopy imaging results showed that NY could easily penetrate HeLa cells membrane to target mitochondria and image Cu2+ in living cells. Furthermore, NY demonstrated mechanochromic properties by exhibiting orange-red fluorescence when subjected to mechanical grinding.

A novel turn-on fluorescent probe for detection of pH in extremely acidic environment and its application

A water-soluble turn-on fluorescent probe PNAP for pH has been designed and synthesized. PNAP was consist of pyrene as fluorophore and morpholine as receptor. Owing to the photoinduced electron transfer (PET) effect, the fluorescence of PNAP was quenched, while PNAP exhibited a remarkable "turn-on" fluorescence with the increase of acidity. Notably for its pKa of 2.15, PNAP was one of the pH fluorescent probes used in extremely acidic environments. Furthermore, PNAP also displayed good repeatability, strong anti-ion interference ability, high sensitivity and selectivity toward pH. In addition, PNAP has been successfully applied to the test strips and monitor the pH of environment water samples and realistic samples, showing its good promising prospect.

Aggregation-caused dual-signal response of gold nanoclusters for ratiometric optical detection of cysteine

Designing ratiometric sensors for cysteine (Cys) monitoring with high accuracy is of great significance for disease diagnosis and biomedical studies. The current ratiometric methods mainly rely on multiplex probes, which not only complicates the operation but also increases the cost, making it difficult for quantitative Cys detection in resource-limited areas. Herein, one-pot prepared gold nanoclusters (Au NCs) that glow red fluorescent were synthesized by employing glutathione as the stabilizer and reducing agent. When Fe3+ is present with Au NCs, the fluorescence is quenched and the scattering is strong because of the aggregation of Au NCs. With introduction of Cys, Cys can efficiently compete with glutathione-modified Au NCs for Fe3+, which leads to increase of fluorescence and decrease of scattering. The ratiometric determination of Cys can be thereby realized by collecting the fluorescence and SRS spectrum simultaneously. The linear range for Cys was 5-30 µM with a detection limit of 1.5 µM. In addition, the sensing system exhibits good selectivity for Cys and shows potential application in biological samples.

Tumor-Targeting Probe for Dual-Modal Imaging of Cysteine In Vivo

Cysteine (Cys) is a crucial biological thiol that has a vital function in preserving redox homeostasis in organisms. Studies have shown that Cys is closely related to the development of cancer. Thus, it is necessary to design an efficient method to detect Cys for an effective cancer diagnosis. In this work, a novel tumor-targeting probe (Bio-Cy-S) for dual-modal (NIR fluorescence and photoacoustic) Cys detection is designed. The probe exhibits high selectivity and sensitivity toward Cys. After reaction with Cys, both NIR fluorescence and photoacoustic signals are activated. Bio-Cy-S has been applied for the dual-modal detection of Cys levels in living cells, and it can be used to distinguish normal cells from cancer cells by different Cys levels. In addition, the probe is capable of facilitating dual-modal imaging for monitoring changes in Cys levels in tumor-bearing mice. More importantly, the excellent tumor-targeting ability of the probe greatly improves the signal-to-noise ratio of imaging. To the best of our knowledge, this is the first Cys probe to combine targeting and dual-modal imaging performance for cancer diagnosis.

Recent Progress in the Rational Design of Biothiol-Responsive Fluorescent Probes

Biothiols such as cysteine, homocysteine, and glutathione play significant roles in important biological activities, and their abnormal concentrations have been found to be closely associated with certain diseases, making their detection a critical task. To this end, fluorescent probes have become increasingly popular due to their numerous advantages, including easy handling, desirable spatiotemporal resolution, high sensitivity, fast response, and favorable biocompatibility. As a result, intensive research has been conducted to create fluorescent probes for the detection and imaging of biothiols. This brief review summarizes recent advances in the field of biothiol-responsive fluorescent probes, with an emphasis on rational probe design, including the reaction mechanism, discriminating detection, reversible detection, and specific detection. Furthermore, the challenges and prospects of fluorescence probes for biothiols are also outlined.

Recent progress in the development of fluorescent probes for imaging pathological oxidative stress

Oxidative stress is closely related to the physiopathology of numerous diseases. Reactive oxygen species (ROS), reactive nitrogen species (RNS), and reactive sulfur species (RSS) are direct participants and important biomarkers of oxidative stress. A comprehensive understanding of their changes can help us evaluate disease pathogenesis and progression and facilitate early diagnosis and drug development. In recent years, fluorescent probes have been developed for real-time monitoring of ROS, RNS and RSS levels in vitro and in vivo. In this review, conventional design strategies of fluorescent probes for ROS, RNS, and RSS detection are discussed from three aspects: fluorophores, linkers, and recognition groups. We introduce representative fluorescent probes for ROS, RNS, and RSS detection in cells, physiological/pathological processes (e.g., Inflammation, Drug Induced Organ Injury and Ischemia/Reperfusion Injury etc.), and specific diseases (e.g., neurodegenerative diseases, epilepsy, depression, diabetes and cancer, etc.). We then highlight the achievements, current challenges, and prospects for fluorescent probes in the pathophysiology of oxidative stress-related diseases.

A coumarin-based fluorescent probe: single-wavelength excitation, discrimination of Cys/Hcy and GSH by naked eyes

Biothiols mainly include cysteine (Cys), homocysteine (Hcy) and glutathione (GSH), which play an important role in life activities and abnormal changes in their concentrations are closely related to certain diseases. Therefore, the quantitative tracking and analysis of biothiols in living organisms has become a hot research topic in recent years. In this work, a coumarin-based fluorescent probe COUN was designed and synthesized for the comparable color recognition of Cys/Hcy and GSH by introducing the phenylethynyl group as the recognition site of biothiols, which showed significant fluorescence enhancement and green fluorescence under the UV light at 365 nm. The probe specifically recognized Hcy, showing 40-fold fluorescence enhancement and strong green fluorescence at 492 nm. Moreover, there was a good linear relationship between the fluorescence intensity of the probe and certain concentrations of Cys/Hcy and GSH, with detection limits of 36.6 nM, 86.4 nM, and 174 nM, respectively. The recognition mechanism of COUN to distinguish Cys/Hcy and GSH was studied by TDDFT calculations. More importantly, COUN was successfully used for imaging biothiols in living cells. The results showed that this probe could provide an effective contribution to the understanding of the role of biothiols, especially Hcy.

Vision for Ratiometric Nanoprobes: In Vivo Noninvasive Visualization and Readout of Physiological Hallmarks

Lesion areas are distinguished from normal tissues surrounding them by distinct physiological characteristics. These features serve as biological hallmarks with which targeted biomedical imaging of the lesion sites can be achieved. Although tremendous efforts have been devoted to providing smart imaging probes with the capability of visualizing the physiological hallmarks at the molecular level, the majority of them are merely able to derive anatomical information from the tissues of interest, and thus are not suitable for taking part in in vivo quantification of the biomarkers. Recent advances in chemical construction of advanced ratiometric nanoprobes (RNPs) have enabled a horizon for quantitatively monitoring the biological abnormalities in vivo. In contrast to the conventional probes whose dependency of output on single-signal profiles restricts them from taking part in quantitative practices, RNPs are designed to provide information in two channels, affording a self-calibration opportunity to exclude the analyte-independent factors from the outputs and address the issue. Most of the conventional RNPs have encountered several challenges regarding the reliability and sufficiency of the obtained data for high-performance imaging. In this Review, we have summarized the recent progresses in developing highly advanced RNPs with the capabilities of deriving maximized information from the lesion areas of interest as well as adapting themselves to the complex biological systems in order to minimize microenvironmental-induced falsified signals. To provide a better outlook on the current advanced RNPs, nanoprobes based on optical, photoacoustic, and magnetic resonance imaging modalities for visualizing a wide range of analytes such as pH, reactive species, and different derivations of amino acids have been included. Furthermore, the physicochemical properties of the RNPs, the major constituents of the nanosystems and the analyte recognition mechanisms have been introduced. Moreover, the alterations in the values of the ratiometric signal in response to the analyte of interest as well as the time at which the highest value is achieved, have been included for most of RNPs discussed in this Review. Finally, the challenges as well as future perspectives in the field are discussed.

Carbon dot and silver nanoparticle-based fluorescent probe for the determination of sulfite and bisulfite via inner-filter effect and competitive redox reactions

A new sensitive fluorescent probe (CDs-AgNP/H₂O₂) for detecting sulfite and bisulfite (SO₃^2- and HSO₃^-) based on the inner-filter effect (IFE) between silver nanoparticles (AgNPs) and carbon dots (CDs) was developed. Because of the spectral overlap between the absorption of AgNPs and the excitation of CDs, the fluorescence of CDs can be quenched by AgNPs owing to the IFE. H₂O₂ weakens the IFE and restores the fluorescence due to the oxidation of AgNPs by H₂O₂. However, the existence of SO₃^2-/HSO₃^- can quench the fluorescence again as a result of redox reaction between SO₃^2-/HSO₃^- and H₂O₂. The results showed a broad linear range of 20-200 μM with a low limit of detection (3.02 μM) toward SO₃^2-/HSO₃^-. The combination of IFE and redox reaction led to improvement of the sensitivity and selectivity. The probe was implemented to measure SO₃^2-/HSO₃^- in various agricultural products and foods with acceptable results (80.6 to 118.9% recovery).

A novel lipid droplets/lysosomes-targeting colorimetric and ratiometric fluorescent probe for Cu2+ and its application

A novel multifunctional fluorescent probe LL2 was prepared via a one-step condensation reaction between 3-formyl-N-butylcarbazole and 2-Hydroxy-1-naphthylhydrazone. LL2 can work as a colorimetric probe for Cu²⁺, and can also selectively recognize Cu²⁺ via ratiometric fluorescence signal. After the addition of Cu²⁺, the probe LL2 responded rapidly within 5 s and reached stability within 30 s. In natural light, when Cu²⁺ were added to the solution, the color of probe LL2 changed from yellowish to colorless, while there was a discernible fluorescence changed from green to blue under a 365 nm UV lamp. The ratiometric fluorescence intensity (F₄₄₉/F₅₁₀) showed a good linear relationship (R² = 0.9902) with Cu²⁺ concentration in the concentration range of 0-5 μmol/L, and the minimum detection limit was 1.96 μM. Cell imaging experiments showed that LL2 could capture fluorescence signals in the green and blue channels of HepG2 cells through fluorescence confocal microscope, and successfully recognize exogenous Cu²⁺ in HepG2 cells. In addition, fluorescence co-localization experiments showed that LL2 could target both lipid droplets and lysosomes. Meanwhile, LL2 could be applied to filter paper strip assay and detection of Cu²⁺ in actual water samples. These results indicated that probe LL2 has a good capability for monitoring Cu²⁺ in environment and living cells.

TDDFT study on the simultaneous sensing mechanism for peroxynitrite and glutathione by a bifunctional fluorescent probe

Using time-dependent density functional theory (TDDFT) method, the response mechanism of a reported bifunctional fluorescent probe for simultaneous recognition of peroxynitrite and glutathione (Chem. Commun. 2018, 54, 11336) was theoretically studied. Calculated vertical excitation energies based on the ground-state and excited-state geometries were consistent with the corresponding experimental ultraviolet-visible and fluorescence spectra. In the ground state, electron delocalization in the probe was limited because its geometry was restrained by steric hindrance. Frontier molecular orbital analysis has shown that the probe should undergo photoinduced electron transfer (PET) from the benzothiazole moiety to the maleimide moiety after excitation. The nonplanar structure together with PET led to fluorescence quenching of the probe. The probe could be dealkylated by peroxynitrite anion. The resulting intramolecular hydrogen bond increasesd the planarity of the molecule, while also gave rise to an excited-state proton-transfer process. Moreover, the addition reaction between the probe and glutathione inhibited the PET process. These two analytes together contributed to the fluorescence enhancement of the final product. This theoretical sensing mechanism for peroxynitrite and glutathione may potentially be important for the design and enhancement of novel probes.

Fluorescent "AND" logic gates for simultaneous detection of thiols and protons: photophysical properties, mechanism and bioimaging of living cells

Five fluorescent probes TP1-5 were demonstrated as two-input "AND" molecular logic gates for the detection of thiols and protons. The molecules were designed based on "thiol receptor-spacer₁-fluorophore-spacer₂-proton receptor" mode. The logic gates were constructed by employing maleimide, naphthalimide and morpholine (TP1-3)/N-methyl piperazine (TP4-5) as the thiol receptor, fluorophore and proton receptor, respectively. All probes show significant fluorescence enhancements upon addition of both protons and thiols. However, much weaker spectral responses were observed with the addition of only one single analyte. The fluorescence outputs, based on photoinduced electron transfer (PET) and (twisted) intramolecular charge transfer (TICT/ICT), were modulated by the proton receptor and linker. The length of spacer₁ affects the responses toward thiols, whereas spacer₂ influences the sensing performance toward protons. The difference between the pK_a values of morpholine (∼5.80) and N-methyl piperazine (∼7.10) enables us to detect thiols in divergent pH circumstances. TP1-3 exhibit an excellent "AND" logic function for simultaneous detection of protons and thiols as well as bioimaging thiols in weakly acidic living cells. However, TP4 and TP5 are not good candidates for executing "AND" logic operation possibly due to the stronger electron donating properties and steric effect of N-methyl piperazine.

A highly chromogenic selective Rhodamine-chloride-based fluorescence probe activated by cysteine and application in living cells and zebrafish

Cysteine (Cys), one of the biological thiols, which plays critical roles in biological system regulating the balance of redox homeostasis. In order to monitor the level of Cys in the living cells and organisms, a chromogenic fluorescence probe Rhocl-Cys based on Rhodamine chloride exhibiting the preferable performance of fluorescence turn-on response reacting with Cys was presented. Rhocl-Cys responded rapidly to Cys within 20 min, and had stable fluorescence intensity within pH 6.0-10.0, high selectivity towards Cys and the anti-inference capability with a low detection limit of 0.80 μM. In particular, Rhocl-Cys could qualitatively and quantitatively monitor the level of endogenous and exogenous Cys in living cells and successfully apply to zebrafish detecting Cys. Therefore, these results might further provide the basis exploring the role of Cys in biological system and facilitate as clinical diagnostic molecular tools.

Research Progress on Multifunctional Fluorescent Probes for Biological Imaging, Food and Environmental Detection

There has been rapid progress in the development of fast, sensitive, cheap and low-cytotoxicity micro-molecule fluorescent probes for application in various fields, including disease diagnosis, food safety and environmental safety. As an analytical tool, dual-function fluorescent probes with dual-emission responses have attracted considerable attention due to their cost-effectiveness and efficiency over single-function sensors. This review primarily describes research progress on multifunctional probes in terms of the reaction type and coordination type, as well as the general design principles of probes. The analytes include reactive oxygen species (ROS), reactive sulfur species (RSS), harmful cations and anions, etc. Multifunctional probes for food, medical and environmental applications are listed for future research. To improve the development of rapid detection methods, trends and strategies in the development of multifunctional fluorescent probes are also discussed.

Real-time imaging of intracellular cysteine level fluctuations during Cu2+ or H2O2 induced redox imbalance using a turn-on fluorescence sensor

Redox balance is a necessary guarantee to maintain the normal physiological activities of organisms. Cysteine (Cys), a critical biological thiol, has the effect of maintaining redox balance in the body. The concentration of intracellular Cys is abnormal under redox imbalance, thereby resulting in multiple diseases. Additionally, studies have revealed that Cu²⁺ can stimulate the body to produce excess reactive oxygen species (ROS, similar to H₂O₂), and the generated ROS will consume reducing substances (such as Cys) in the body, leading to redox imbalance. Thus, finding a simple and effective method to monitor Cys under redox imbalance is pressing. Here, a turn on probe (DDNO) was proposed by connecting SBD-Cl to a red dye (HDM). The probe can specifically recognize Cys with rapid response (180 s) and low detection limit (0.61 μM) through substitution-rearrangement reaction between sulfhydryl and chlorine atom. Bioimaging experiments indicated that the probe has good biocompatibility and cell membrane permeability, which can be applied to monitor the fluctuation of Cys levels in live cells and zebrafish under the redox imbalance induced by Cu²⁺ or H₂O₂.

Red and Near-Infrared Fluorescent Probe for Distinguishing Cysteine and Homocysteine through Single-Wavelength Excitation with Distinctly Dual Emissions

Small-molecule biothiols, including cysteine (Cys), homocysteine (Hcy), and glutathione (GSH), participate in various pathological and physiological processes. It is still a challenge to simultaneously distinguish Cys and Hcy because of their similar structures and reactivities, as well as the interference from the high intramolecular concentration of GSH. Herein, a novel fluorescent probe, CySI, based on cyanine and thioester was developed to differentiate Cys and Hcy through a single-wavelength excitation and two distinctly separated emission channels. The probe exhibited a turn-on fluorescence response to Cys at both 625 nm (the red channel) and 740 nm (the near-infrared channel) but only showed fluorescence turn-on to Hcy at 740 nm (the near-infrared channel) and no fluorescent response to GSH. With the aid of built-in self-calibration of single excitation and dual emissions, simultaneous discriminative determinations of Cys and Hcy were realized through red and near-infrared channels. CySI exhibited excellent selectivity toward Cys and Hcy with a fast response. This probe was further exploited to visualize exogenous Cys and Hcy in cells through dual emission channels under one excitation. Moreover, it could efficiently target mitochondria and was applied to monitor the endogenous Cys fluctuations independently in mitochondria through the red emission channel.

Long-wavelength emission carbon dots as self-ratiometric fluorescent nanoprobe for sensitive determination of Zn2

A novel ratiometric fluorescence nanoprobe based on long-wavelength emission carbon dots (CDs) was designed for high sensitive and selective detection of Zn²⁺. The CDs were conveniently prepared by a one-step solvothermal treatment of formamide and glutathione (GSH). Under single excitation wavelength (420 nm), the obtained CDs exhibit three emission peaks at 470, 650, and 685 nm, respectively. For the long-wavelength emission region of the CDs, the fluorescence at 685 nm can be quenched with different levels upon the addition of most metal ions. However, the presence of Zn²⁺ not only results in the fluorescence quenching at 685 nm effectively but also enhances at 650 nm remarkably, which may be due to the formation of CD-Zn²⁺ chelate complex inducing the dispersion of CDs aggregates and changes in the group distribution on the surface of CDs. Taking the advantage of the unique fluorescence response induced by Zn²⁺, the prepared CDs were successfully employed as nanoprobe for self-ratiometric fluorescence determination of Zn²⁺ with F₆₅₀/F₆₈₅ as signal output. A good linear relationship in the concentration range 0.01 to 2 μM, and a detection limit as low as 5.1 nM has been obtained. The ratiometric nanoprobe was successfully applied to  Zn²⁺ determination  in human serum samples.

A novel quinoline-based fluorescent probe for real-time monitoring of Cys in glioma

The highly selective fluorescent probe ZS-C1 for imaging Cys in living cells and 3D tumor cell sphere.

Recent Research Progress of Red-Emitting/Near-Infrared Fluorescent Probes for Biothiols

Small-molecule biological thiols, including cysteine (Cys), homocysteine (Hcy) and glutathione (GSH), occupy a vital position in physiological and pathological activities. Abnormal fluctuations of their concentrations are often closely connected with...

A ratiometric fluorescent probe based on peptide modified MnFe2O4 nanoparticles for matrix metalloproteinase-7 activity detection in vitro and in vivo

A peptide modified MnFe2O4 ratiometric fluorescent nanoprobe is developed for noninvasively visualizing the distribution of matrix metalloproteinase-7 in vitro and in vivo.

A ratiometric fluorescent probe based on two-isophorone fluorophore for detecting cysteine

The key biological thiol, cysteine (Cys), which can participate in many physiological and pathological processes in the human body, has also been proofed to have considerable effects on redox homeostasis...

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

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

A dual responsive fluorescent probe for selective detection of cysteine and bisulfite and its application in bioimaging

A coumarin-based dual responsive fluorescent probe with a simple structure was developed for the detection of Cys and HSO₃ ^-. Under simulated physiological conditions, Cou-F displayed an on-off fluorescence response to Cys at 521 nm and an off-on fluorescence response to HSO₃ ^- at 500 nm. Furthermore, Cou-F had the advantages of high sensitivity, strong specificity and rapid response. The detection limits of Cou-F toward Cys and HSO₃ ^- were 0.54 μM and 0.65 μM, respectively. Cou-F enabled high selective responses to Cys and HSO₃ ^- over other biologically related species. The response times of Cou-F toward Cys and HSO₃ ^- were 80 s and 100 s. The fluorescence imaging of Cys and HSO₃ ^- was achieved in living RAW246.7 cells.