Qualitative and quantitative determination of biologically active low-molecular-mass thiols in human blood by reversed-phase high-performance liquid chromatography with photometry and fluorescence detection

Analytical developments in the synergism of copper particles and cysteine: a review

Cysteine, a sulfur-containing amino acid, is a vital candidate for physiology. Coinage metal particles (both clusters and nanoparticles) are highly interesting for their spectacular plasmonic properties. In this case, copper is the most important candidate for its cost-effectiveness and abundance. However, rapid oxidation destroys the stability of copper particles, warranting the necessity of suitable capping agents and experimental conditions. Cysteine can efficiently carry out such a role. On the contrary, cysteine sensing is a vital step for biomedical science. This review article is based on a comparative account of copper particles with cysteine passivation and copper particles for cysteine sensing. For the deep understanding of readers, we discuss nanoparticles and nanoclusters, properties of cysteine, and importance of capping agents, along with various synthetic protocols and applications (sensing and bioimaging) of cysteine-capped copper particles (cysteine-capped copper nanoparticles and cysteine-capped copper nanoclusters). We also include copper nanoparticles and copper nanoclusters for cysteine sensing. As copper is a plasmonic material, fluorometric and colorimetric methods are mostly used for sensing. Real sample analysis for both copper particles with cysteine and copper particles for cysteine sensing are also incorporated in this review to demonstrate their practical applications. Both cysteine-capped copper particles and copper particles for cysteine sensing are the main essence of this review. The aspect of the synergism of copper and cysteine (unlike other amino acids) is quite promising for future researchers.

A ratiometric fluorescent probe based on a novel fluorophore with high selectivity for imaging cysteine in living cells

As a biothiol, cysteine (Cys) is essential to both physiological and pathological processes and has been associated with many diseases, including neurological disorders, rheumatoid arthritis, and renal dysfunction. Therefore, the development of a high-performance probe for detecting Cys levels can help prevent and diagnose disease. In this study, a ratiometric fluorescent probe based on a novel fluorophore was developed for detecting Cys, and it showed high specificity and a rapid response time toward Cys. This probe demonstrates excellent biocompatibility and has been utilized effectively for the imaging of Cys in living cells.

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

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

Design, synthesis and cell imaging of a new 3-thiolflavone fluorescent probe for biothiols

Abnormal levels of intracellular biothiol species, including glutathione (GSH), homocysteine (Hcy), and cysteine (Cys), are closely relevant to a variety of diseases. In this study, we first report the design and synthesis of a 3-thiolflavone-based probe 3-STF with an S-S linkage both as the fluorescence quenching agent and the recognition site. Upon treatment with biothiols, the S-S linkage was cleaved by nucleophilic attack of RS-, and 3-STF exhibited a specific "turn-on" fluorescence response at 543.8 nm upon 390 nm excitation. Meanwhile, 3-STF was proved to be highly sensitive and selective for the detection of biothiols over other nucleophiles such as amino acids and H2S. The sensing mechanism was further verified by 1H NMR and ESI-MS analysis. In addition, 3-STF with low cytotoxicity can be successfully applied to detect endogenous and exogenous biothiols in HepG-2 cells.

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

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

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.

Simultaneous Determination of Human Serum Albumin and Low-Molecular-Weight Thiols after Derivatization with Monobromobimane

Biothiols are extremely powerful antioxidants that protect cells against the effects of oxidative stress. They are also considered relevant disease biomarkers, specifically risk factors for cardiovascular disease. In this paper, a new procedure for the simultaneous determination of human serum albumin and low-molecular-weight thiols in plasma is described. The method is based on the pre-column derivatization of analytes with a thiol-specific fluorescence labeling reagent, monobromobimane, followed by separation and quantification through reversed-phase high-performance liquid chromatography with fluorescence detection (excitation, 378 nm; emission, 492 nm). Prior to the derivatization step, the oxidized thiols are converted to their reduced forms by reductive cleavage with sodium borohydride. Linearity in the detector response for total thiols was observed in the following ranges: 1.76–30.0 mg mL⁻¹ for human serum albumin, 0.29–5.0 nmol mL⁻¹ for α-lipoic acid, 1.16–35 nmol mL⁻¹ for glutathione, 9.83–450.0 nmol mL⁻¹ for cysteine, 0.55–40.0 nmol mL⁻¹ for homocysteine, 0.34–50.0 nmol mL⁻¹ for N-acetyl-L-cysteine, and 1.45–45.0 nmol mL⁻¹ for cysteinylglycine. Recovery values of 85.16–119.48% were recorded for all the analytes. The developed method is sensitive, repeatable, and linear within the expected ranges of total thiols. The devised procedure can be applied to plasma samples to monitor biochemical processes in various pathophysiological states.

Dual-channel fluorescent signal readout strategy for cysteine sensing

Cysteine (Cys) is a biological thiol. Aberrant changes in thiol levels are associated with the development and pathogenesis of various diseases, including liver damage, Alzheimer's disease, weakness, and cardiovascular diseases. Therefore, thiol detection in biological samples has great importance in health monitoring and disease prediction. In this study, we developed a ratiometric fluorescence nanosensor combined with carbon dots (CDs)-doped mesoporous silica and fluorescein-based fluorescent probes loaded in pores for Cys detection. The nanosensor emitted fluorescence at 450 nm upon excitation at 370 nm. In the presence of Cys, the fluorescence emission from the probe could be selectively enhanced, whereas that from CDs could be changed. Thus, a ratiometric fluorescent sensor was developed. This sensor can eliminate the potential influence of background fluorescence and other analyte-independent external environmental factors. The nanosensor was utilized to monitor Cys levels in human serum, and satisfactory results were obtained. Results indicated that the nanosensor can be utilized as an excellent fluorescent nanocomposite material in practical biological applications.

A novel benzo[a]phenazin-based fluorescence probe for selective detection of cysteine with anti-cancer potency

Among the physiological and pathological sulfur-containing species, cysteine (Cys) is the most typical one which is an important component of the REDOX system in vivo. Monitoring the level of Cys from other competing species seems quite important in pre-clinical diagnosis and therapeutic evaluation. Herein, we developed a selective fluorescent probe, BPCys, for Cys from the benzo[a]phenazin backbone which had the potential of anti-cancer potency. BPCys suggested advantages including high specificity (40 fold over other species), high sensitivity (detection limit: 18 nM), wide pH adaptability (6.0-11.0) and in particular, the anti-cancer effect. Biological assays and in silico simulation hinted the potency of the detecting product on Topoisomerase I/II. In brief, this study raised a practical strategy for monitoring the Cys level in living cells, especially in cancer models with its anti-cancer potential, thus opened the mind of exploring more specific tool for specific applications.

Photoluminescence and Electrochemiluminescence Dual-Signaling Sensors for Selective Detection of Cysteine Based on Iridium(III) Complexes

Cysteine (Cys) is important in biosynthesis, detoxification, and metabolism. The selective detection of Cys over structurally similar homocysteine (Hcy) or glutathione (GSH) remains an immense challenge. Although there are many methods for detecting Cys, photoluminescence (PL) and electrochemiluminescence (ECL) techniques are well-suited for clinical diagnostics and analytical technology because of their high sensitivities. Herein, we report PL and ECL dual-channel sensors using cyclometalated iridium(III) complexes for the discrimination of Cys from Hcy and GSH. The sensors react with cysteine preferentially because of kinetic differences in intramolecular conjugate addition/cyclization, enabling phosphorescence enhancement and ECL decrease in the blue-shifted region. Sensor 1 shows ratiometric PL turn-on and ECL turn-off for Cys. In addition, unique ECL-enhancing behavior of sensor 1 toward GSH enables discrimination between Cys and GSH. Sensor 1 was successfully applied to the detection of Cys in human serum by the ECL method. We demonstrate the first case of a Cys-selective PL and ECL dual-channel chemodosimetric sensor based on cyclometalated iridium(III) complexes and expect that the rational design of efficient PL and ECL dual-channel sensors will be useful in diagnostic technology.

Application of Butylamine as a Conjugative Reagent to On-Column Derivatization for the Determination of Antioxidant Amino Acids in Brain Tissue, Plasma, and Urine Samples

(1) Antioxidants are involved in body protection mechanisms against reactive oxygen species. Amino acids such as glutathione (GSH) and N-acetylcysteine (NAC) are known to be involved in providing protection against oxidative lethality. A quick and simple method for the determination of NAC and GSH in various biological matrices such as urine, plasma, and homogenates of brain tissues has been developed and described in this work. (2) The assay is based on reversed phase high performance liquid chromatography with spectrofluorimetric detection and on-column derivatization. Butylamine and o-phthaldialdehyde have been used as derivatization reagents. Since o-phthaldialdehyde constitutes a part of the mobile phase, the derivatization reaction and chromatographic separation occur simultaneously. (3) Linearity in the detector response for NAC in human urine was observed in the range of 5–200 nmol mL⁻¹, and NAC and GSH in the brain tissue homogenates were observed in the range of 0.5–5 nmol mL⁻¹ and 0.5–15 nmol mL⁻¹, respectively. Human plasma linearity ranges covered 0.25–5.00 nmol mL⁻¹ and 0.5–15 nmol mL⁻¹ for NAC and GSH, respectively. The LODs for NAC and GSH were 0.01 and 0.02 nmol mL⁻¹ while the LOQs were 0.02 and 0.05 nmol mL⁻¹, respectively. The usefulness of the proposed method was proven through its application to real samples.

Fluorescence turn-on sensing of L-cysteine based on FRET between Au-Ag nanoclusters and Au nanorods

The applications of metallic nanoclusters and nanoparticles in biological sensing have attracted special attention owing to their optical interaction based on fluorescence and surface plasmon resonance (SPR). In this work, we designed a fluorescent nanoprobe for the determination of L-cysteine (L-Cys) based on fluorescence resonance energy transfer (FRET) from gold‑silver bimetallic nanoclusters (Au-Ag NCs) to gold nanorods (AuNRs). Firstly, the negatively charged Au-Ag NCs protected by bovine serum albumin (BSA) are directly adsorbed on the surface of the positively charged AuNRs through electrostatic interaction, and the FRET effect leads to distinct fluorescence quenching of Au-Ag NCs at 615nm. The SPR wavelength of AuNRs is dependent on the aspect ratio, so the SPR of AuNRs could be tuned to have a better spectral overlap with fluorescence of Au-Ag NCs, which enhances the fluorescence quenching effect. Because the SH group of L-Cys has an affinity with gold, the addition of L-Cys can result in the release of Au-Ag NCs from the surface of AuNRs via forming AuS bonds. Thus, the introduction of L-Cys could effectively restore the fluorescence emission of the AuNRs/Au-Ag NCs system because of the restraint of FRET effect. Under the optimized conditions, the fluorescence recovery of AuNRs/Au-Ag NCs probe exhibits a linear response to L-Cys concentration ranging from 5 to 100μM, and the corresponding theoretical detection limit (LOD) is 1.73μM. Meanwhile, this method displays excellent sensitivity and selectivity for L-Cys over other amino acids, and it has been successfully applied to detect L-Cys in real samples.

Fluorescence red-shift of gold-silver nanoclusters upon interaction with cysteine and its application

In this work, gold-silver alloy nanoclusters (AuAg NCs) were demonstrated as a novel probe for fluorescent detection of cysteine (Cys). The alloy nanoclusters were fabricated by bovine serum albumin as a template and NaBH₄ as a reducer. They showed a red emission at 650 nm. The interaction between AuAg NCs and Cys was investigated. The thiol group in Cys molecules has strong affinity on the surface of metals, which results in variation of fluorescence peak wavelength. It was further demonstrated that this red-shift of fluorescence had a good linear relationship with the concentration of Cys in the range of 2-100 μM. The method was successfully applied for human plasma analysis with satisfactory results. This novel strategy was expected to provide a potential opportunity for extending the application of novel metal nanoclusters in fluorescence.

A dinitro-functionalized metal-organic framework featuring visual and fluorogenic sensing of H2S in living cells, human blood plasma and environmental samples

Here, we describe a new dinitro-functionalized Zr(iv) MOF (MOF = metal-organic framework) having a UiO-66 (UiO = University of Oslo) framework topology called UiO-66-(NO₂)₂ (1). It shows fluorescence turn-on behavior towards H₂S in simulated biological medium (HEPES buffer, pH = 7.4). By employing solvothermal conditions, 1 was successfully synthesized by reacting ZrCl₄, H₂BDC-(NO₂)₂ [H₂BDC-(NO₂)₂ = 2,5-dinitro-1,4-benzenedicarboxylic acid] ligand and benzoic acid with a molar ratio of 1 : 1 : 10 in DMF (DMF = N,N-dimethylformamide) at 130 °C for 24 h. The material was characterized by infrared spectroscopy, X-ray powder diffraction (XRPD) and thermogravimetric (TG) analyses. The compound not only displays highly sensitive fluorometric sensing of H₂S but also exhibits a visually detectable colorimetric change towards H₂S in daylight. Moreover, the high selectivity of 1' towards H₂S is retained even when several other biologically intrusive species co-exist in the sensing medium. The limit of detection (LOD) of the compound is 14.14 μM which lies in the range of the H₂S concentration found in biological systems. Fluorescence microscopy studies on J774A.1 cells revealed the efficacy of the probe for imaging H₂S in living cells. Moreover, this material can detect H₂S in human blood plasma (HBP) and monitor the sulfide concentration in real water samples. All these features clearly demonstrate that the material has huge potential for highly selective sensing of both extracellular and intracellular H₂S.

Near-infrared turn-on fluorescent probe for discriminative detection of Cys and application in in vivo imaging

Near-infrared (NIR) fluorescent probes are widely employed in biological detection because of their lower damage to biological samples, low background interference, and high signal-to-noise ratio. Herein, a highly water-soluble NIR probe (NIRHA) based on a hemicyanine skeleton and bearing an acrylate moiety was synthesized. The probe showed high selectivity toward cysteine (Cys) over homocysteine (Hcy) and glutathione (GSH). The probe also had low cytotoxicity and was successfully applied in HeLa cells and mouse experiments. Results of bioimaging experiments indicated that the probe was effective for visualizing endogenous Cys in vitro and in vivo.

Near-infrared (NIR) fluorescent probes are widely employed in biological detection because of their lower damage to biological samples, low background interference, and high signal-to-noise ratio.

A label-free fluorimetric detection of biothiols based on the oxidase-like activity of Ag+ ions

In this work, a label-free and sensitive fluorimetric method has been developed for the detections of biothiols including cysteine (Cys), homocysteine (Hcy), and glutathione (GSH), based on the specific biothiol-induced inhibition of the oxidase-like activity of silver ions (Ag⁺). It is well established that o-phenylenediamine (OPD) can be oxidized by Ag⁺ ions to generate fluorescent 2,3-diaminophenazine (OPDox). The introduction of biothiols would inhibit the oxidation of OPD by Ag⁺ due to the strong coordination between biothiols and Ag⁺. The changes of fluorescence intensities obtained in the Ag⁺-OPD system exhibited good linear correlations in the ranges of 0.50-30.0μM for Cys, 1.0-45.0μM for Hcy and 0.50-40.0μM for GSH. The detection limits (S/N=3) of Cys, Hcy and GSH were 110nM, 200nM and 150nM, respectively. Subsequently, the developed fluorimetric method was successfully applied for the detection of biothiols in human serum.

Development of a two-photon turn-on fluorescent probe for cysteine and its bio-imaging applications in living cells, tissues, and zebrafish

A two-photon fluorescent probe Co-Cys for detecting cysteine has been designed to monitor cysteine in cells, tissues and zebrafish.

A selective colorimetric sensing strategy for cysteine based on an indicator-displacement mechanism

Rapid determination of cysteine in aqueous solution is important for the diagnosis and treatment of some diseases.

High-bright fluorescent carbon dot as versatile sensing platform

The surface functionalization will introduce additional functional groups on carbon dots (CDs) surface and then enrich the properties of CDs. Here, we show the various surface functionalized CDs (-COOH, -OH, -SH, -NH₂, etc, named as NS-CDs) were synthesized with fascinating features, including high quantum efficiency (38.9%), long-term stability and good biocompatibility. Notably, it can serve as multifunction fluorescent probe in sensing system, including label-free detections in hydrogen peroxide (H₂O₂) with a wide linear range (1.20 × 10^-3 - 8.80 × 10^-12M) and a low limit of detection (LOD, 1.00 × 10^-12M); and glutathione, covering a concentration range of 2.00 × 10^-3 - 1.00 × 10^-7M and LOD of 1.00 × 10^-7M. In addition, the NS-CDs as fluorescent probe could selectively detect metal ions (such as, Hg²⁺, 1.00 × 10^-8 - 1.50 × 10^-3M, 1.00 × 10^-7M), antibiotics (tetracycline, 1.00 ×10^-10 - 2.50 × 10^-5M, 1.00 ×10^-10M) and toxic pollutant (nitrobenzene, 5.00 × 10^-7 to 1.00 × 10^-3gL^-1, 5.00 × 10^-7gL^-1) with wide linear range and satisfactory detection limits.

Conjugated polymer with carboxylate groups-Hg2+ system as a turn-on fluorescence probe for label-free detection of cysteine-containing compounds

In this work, a turn on fluorescent sensor, based on Hg²⁺ coordination conjugated polymer, was developed to detect cysteine-containing compounds. The fluorescence of conjugated polymer (poly(2,5-bis (sodium 4-oxybutyrate) -1,4 - phenylethynylene-alt-1,4-phenyleneethynylene; PPE-OBS) would be quenched by Hg²⁺ because of the coordination-induced aggregation and electron transfers of PPE-OBS toward Hg²⁺. When there were some cysteine-containing compounds in PPE-OBS-Hg²⁺ system, the fluorescence of PPE-OBS would be recovered. It indicated that the PPE-OBS-Hg²⁺ system could be used to detect cysteine-containing compounds. Under the optimized conditions, the experiment results showed that there were particularly linear range, high sensitivity and selectivity over other amino acids. The limit of detection (LOD) of cysteine (Cys), homocysteine (Hcy) and glutathione (GSH) were 0.725μmolL^-1, 0.982μmolL^-1 and 1.21μmolL^-1 by using this sensor. In addition, Cys standard recovery in several green tea drink and honey samples was also demonstrated. The recovery of Cys was range from 96.3 to 105.0% and RSD was less than 3.25%. The satisfactory results demonstrated that the proposed method could be as a potential fluorescent method for determining cysteine-containing compounds in real samples.

A simple colorimetric and fluorescent probe with high selectivity towards cysteine over homocysteine and glutathione

A novel turn-on fluorescent sensor AQDA with high selective towards cysteine.

A highly water-soluble, sensitive, coumarin-based fluorescent probe for detecting thiols, and its application in bioimaging

In this study, a coumarin-based probe (probe 1) bearing a maleimide group was used to rapidly and selectively detect thiols.

A new fluorescence turn-on probe for biothiols based on photoinduced electron transfer and its application in living cells

A new biothiol-selective fluorescent probe 1 based on photoinduced electron transfer (PET) mechanism was designed and synthesized. The UV-Vis absorption and fluorescent emission properties of probe 1 towards various analytes were studied in detail. The probe exhibited a large stokes shift (~200nm) after reacted with biothiols and could selectively detect cysteine (Cys) in dimethyl sulfoxide (DMSO)/H2O solution (9:1, v/v, 10mM phosphate buffer saline, pH3.5) over glutathione (GSH), homocysteine (Hcy) and other analytes with a detection limit of 0.117μM. In addition, probe 1 responded well to GSH, Hcy and Cys in the same above solution with pH5.5 and got the detection limits of 0.151μM, 0.128μM and 0.037μM, respectively. Probe 1 was of very low cytotoxicity and successfully applied for imaging of thiols in living cells.

Fluorescence ‘on–off–on’ chemosensor for the sequential recognition of Hg2+and cysteine in water

Fluorescent chemosensor showed the sequential detection of Hg²⁺and cysteine, and could be applied for quantification of Hg²⁺in water samples.

Recent advances in thiol and sulfide reactive probes

Because of the biological relevance of thiols and sulfides such as cysteine, homocysteine, glutathione and hydrogen sulfide, their detection has attracted a great deal of research interest. Fluorescent probes are emerging as a new strategy for thiol and hydrogen sulfide analysis due to their high sensitivity, low cost, and ability to detect and image thiols in biological samples. In this short review, we have summarized recent advances in the development of thiol and hydrogen sulfide reactive fluorescent probes. These probes are compared and contrasted with regard to their designing strategies, mechanisms, photophysical properties, and/or reaction kinetics. Biological applications of these probes are also discussed.

Retracted Article: Upconversion luminescent logic gates and turn-on sensing of glutathione based on two-photon excited quantum dots conjugated with dopamine

This article reported the two-photon excited quantum dots-based novel upconversion luminescent logic gates for turn-on sensing of glutathione.