A Hydrogen Bonded Non-Porous Organic-Inorganic Framework for Measuring Cysteine in Blood Plasma and Endogenous Cancer Cell

An imbalance in cysteine (Cys) levels in the cells and plasma has been identified as the risk indicator for various human diseases. The structural similarity of cysteine with its congener homocysteine and glutathione offers challenges in its measurement. Herein, we report a hydrogen-bonded organic-inorganic framework of Cu(II) (HOIF) for the selective detection of cysteine over other biothiols. The non-fluorescent HOIF showed 12-fold green emission in the presence of cysteine. The monomeric unit of HOIF is stabilized via intermolecular hydrogen bonds, resulting in a non-porous network structure. Non-interference from homocysteine, glutathione, and other competitive bio-analytes revealed explicit affinity of HOIF for cysteine. Fluorimetric titration showed a wide working concentration window (650 nM-800 μM) for measuring cysteine in an aqueous medium. The mechanistic investigation involving HRMS, EPR, and UV-vis spectroscopic studies revealed the decomplexation of HOIF with Cys, resulting in a fluorescence turn-on response from the luminescent ligand. Validation using a commercial dye, "Cysteine Green", confirmed the prospect of HOIF for early diagnostic purposes. Utilizing the fluorescence turn-on property of HOIF in the presence of cysteine, we measured cysteine quantitatively in the blood plasma samples. Bio-imaging of endogenous cysteine in cancer cells indicated the ability of HOIF to monitor the intracellular cysteine.

Flourescence sensors for heavy metal detection: major contaminants in soil and water bodies

Due to the increasing consumption of heavy metals, there is a rising need for specific and useful methods that are employed for the detection of heavy metals. Fluorescence sensing is a highly selective, rapid and biosensing technique that is employed in the determination of some heavy metals in any sample of soil or water, any other living person, the food being consumed or any other substance which are being used daily. These fluorescent methods are a type of analytical technique and they are mainly based on detection. Many types of metal conjugated molecules have been used of the detection of these heavy metals with various mechanisms. We have taken into account some specific sensor molecules as they were more suitable and easily accessible. These techniques that were employed in the detection of various heavy metals such as copper, lead and mercury have been discussed in the following review article.

Coumarin-Ensembled Vanadium(V) Compounds and Their Affinity Studies Toward Biological Thiols Probed by Fluorescence Spectroscopy

Fluorescence spectroscopic studies of a pair of new oxido-vanadium(V) compounds with biological thiols, such as homocysteine (Hcy), cysteine (Cys), and glutathione (GSH), have been investigated in this article. Despite notable progress in vanadium-thiol chemistry, no attention has been paid to exploring vanadium-based optical probes to study their interaction with biothiols. For this purpose, two oxido-vanadium(V) compounds, 1 and 2, have been prepared involving a tridentate ONO donor-based luminescent coumarin-derived ligand. Single crystal X-ray diffraction analysis, NMR (¹ H, ¹³ C, and ⁵¹ V) spectroscopy, XPS, and DFT calculations have been used to establish their identities. The vanadium center in these compounds has a distorted octahedral environment. In compound 2, a methanol molecule is coordinated to the vanadium(V) center in the trans position of the terminal oxido moiety. The latter exerts a strong trans-labilizing influence on the coordinating methanol. Both 1 and 2 are weakly fluorescent. Photophysical investigations of the vanadium complexes in aqueous media at physiological pH (7.4) in the presence of various biothiols and amino acids showed significant fluorescence enhancement (83-fold) of the vanadium complexes, specifically with Hcy. The specific affinity of the complexes for Hcy remained unchanged even in the presence of other biothiols and amino acids. Kinetic investigation reveals pseudo-first order behavior of the compound with Hcy. Mechanistic studies have manifested that Hcy-induced reduction triggers the decomplexation of the vanadium compound, followed by hydrolysis and subsequent cyclization. Time-correlated single photon counting suggested that the radiative rate constant (k_r ) of 1 and 2 in the presence of Hcy serves as the prime factor for the fluorescence enhancement of the medium. Compound 1 has been tested efficiently for Hcy measurement in blood plasma rendering it suitable for practical applications.

Lysosome-Targeting Fluorescence Sensor for Sequential Detection and Imaging of Cu2+ and Homocysteine in Living Cells

A conjugated polymer-based fluorescence sensor, namely, PTNPy, was constructed on the basis of a polythiophene scaffold coupled with dimethylpyridylamine (DPA) groups in side chains for the consecutive detection and quantification of Cu²⁺ and Hcy in a perfect aqueous medium. A dramatic fluorescence quenching of PTNPy by the addition of Cu²⁺ was observed in Tris-HCl buffer solution (2 mM, pH 7.4), demonstrating a quick (<1 min) and highly selective response to Cu²⁺ with a low limit of detection of 6.79 nM. Subsequently, the Cu²⁺-quenched fluorescence of PTNPy can be completely recovered by homocysteine (Hcy), showing excellent selectivity to Hcy over other competitive species such as cysteine and glutathione. Thanks to the low cytotoxicity and lysosomal targeting ability of PTNPy, it was further applied as an optical sensor for the sequential imaging of Cu²⁺ and Hcy in HeLa cells. More importantly, Hcy concentration was linearly related to the fluorescence intensity of PTNPy in living cells, demonstrating huge potential for real-time monitoring the fluctuation of Hcy levels in living cells.

A rapid “on–off–on” mitochondria-targeted phosphorescent probe for selective and consecutive detection of Cu2+ and cysteine in live cells and zebrafish

The detection of mitochondrial Cu²⁺ and cysteine is very important for investigating cellular functions or dysfunctions. In this study, we designed a novel cyclometalated iridium(iii) luminescence chemosensor Ir bearing a bidentate chelating pyrazolyl-pyridine ligand as a copper-specific receptor. The biocompatible and photostable Ir complex exhibited not only mitochondria-targeting properties but also an “on–off–on” type phosphorescence change for the reversible dual detection of Cu²⁺ and cysteine. Ir had a highly sensitive (detection limit = 20 nM) and selective sensor performance for Cu²⁺ in aqueous solution due to the formation of a non-phosphorescent Ir–Cu(ii) ensemble through 1 : 1 binding. According to the displacement approach, Ir was released from the Ir–Cu(ii) ensemble accompanied with “turn-on” phosphorescence in the presence of 0–10 μM cysteine, with a low detection limit of 54 nM. This “on–off–on” process could be accomplished within 30 s and repeated at least five times without significant loss of signal strength. Moreover, benefiting from its good permeability, low cytotoxicity, high efficiency, and anti-interference properties, Ir was found to be suitable for imaging and detecting mitochondrial Cu²⁺ and cysteine in living cells and zebrafish.

An iridium(iii) complex-based mitochondria targeting phosphorescent probe for selectively detecting Cu²⁺ and Cys in aqueous solution, living cells and zebrafish has been developed.

The colorimetric and microfluidic paper-based detection of cysteine and homocysteine using 1,5-diphenylcarbazide-capped silver nanoparticles

We have prepared a microfluidic paper-based analytical device (μPAD) for the determination of cysteine and homocysteine based on 1,5-diphenylcarbazide-capped silver nanoparticles. The μPAD was developed to identify and quantify the levels of cysteine and homocysteine. The proposed μPAD enabled the detection of cysteine and homocysteine using a colorimetric reaction based on modified silver nanoparticles. The color of the modified AgNPs in the test zone immediately changed after the addition of cysteine and homocysteine. Based on this change, the quantification of these two amino acids was achieved using an RGB color model and ImageJ software. Under optimized conditions, the proposed device enabled the determination of cysteine in the 0.20–20.0 μM concentration range with a limit of detection (LOD) of 0.16 μM. In addition, the LOD of homocysteine was calculated to be 0.25 μM with a linear range of 0.50–20.0 μM. In this work, we focused on the use of the μPAD for the analysis of a series of human urine samples.

A simple and novel portable method for the quantitative measurement of cysteine and homocysteine in human urine samples is presented.

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

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

A lysosome-targetable fluorescent probe for real-time imaging cysteine under oxidative stress in living cells

As an effective lysosomal biomarker for oxidative stress status, cysteine (Cys) plays an important role in lysosomal proteolysis. Herein, we report the first lysosome-targetable fluorescence probe (MCAB) for Cys-selective detection based on nucleophilic addition reaction of sulfhydryl toward a α, β-unsaturated ketone and demonstrate its application to lysosomal-targetable imaging. MCAB is designed based on a α, β-unsaturated ethanoylcarbazole as the fluorophore and the thiols reaction site, and a methylcarbitol unit as a lysosome-targetable group. Upon reacting with Cys, this probe turns on highly specific fluorescence signals linearly proportional to Cys concentrations over the range of 0-300 μM. MCAB detects Cys with a rapid response time (within 12 min) and low limit of detection (0.38 μM). MCAB is highly selective to Cys over other similar biothiols including homocysteine (Hcy) and glutathione (GSH). Moreover, it also exhibits significant lysosomal-targetable ability, which is ideal for lysosomal Cys-selective imaging. Using MCAB, we have successfully visualized the fluctuation endogenous Cys in lysosomes under oxidative stress status in real-time.

Development of a new fluorescent probe for cysteine detection in processed food samples

Cysteine is a crucial amino acid, found in a huge amount in protein-rich foods. We focused our research to determine the amount of free cysteine consumed highly in foods such as pork, beef, poultry, eggs, dairy, red peppers, soybeans, broccoli, brussels sprouts, oats, and wheat germs. A newly designed carbazole-pyridine-based fluorescent probe (CPI) has been introduced for quantitative estimation of cysteine (Cys) with a "turn on" fluorescence in some popular processed food samples chosen from our daily diet. CPI shows both naked eye and UV-visible color changes upon interaction with cysteine. The binding approach between CPI and Cys at biological pH has been thoroughly explored by UV-visible and fluorescence spectroscopy. From Job's plot analysis, 1:1 stoichiometric reaction between CPI and Cys is observed with a detection limit of 3.8 μM. NMR, ESI mass spectrometry, and time-dependent density functional theory (TD-DFT) study enlightens the formation of more stable product CPI-Cys. The "turn on" response of the probe CPI occurs due to the interruption of intra-molecular charge transfer (ICT) process upon reacting with cysteine. Moreover, CPI is a very stable, cost-effective compound and exhibits excellent real-time selectivity towards Cys over all other comparative biorelevant analytes. Interestingly, our proposed method is much advantageous as it is able to estimate cysteine predominantly by screening out other comparative biocomponents found in different protein-rich foods.

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.

Fluorescent probe for Cu2+ and the secondary application of the resultant complex to detect cysteine

A special fluorescent probe has been developed, one that demonstrated excellent "off-on-type" change in fluorescence with high selectivity toward Cu2+. Interestingly, the probe-Cu2+ complex could detect cysteine due to the ability of this amino acid to strongly coordinate Cu2+, and no obvious interference was observed from other amino acids and anions. According to the proposed mechanism, addition of cysteine induced decomplexation of the probe-Cu2+ form. Furthermore, the results of confocal microscopy experiments demonstrated the potential of using the probe to image Cu2+ in living cells and mice.

A human vision inspired adaptive platform for one-on-multiple recognition

The fluorescence of a coordinative molecule DCM displaying an intramolecular charge transfer (ICT) effect is regulated by several metal ions. These DCM-metal complexes were adopted to recognize different chemicals, including the recognition of triethylenetetramine, thiol-containing amino acids, and H2S upon binding DCM with Zn2+, Ag+, and Pb2+, respectively. This is in analogy to the general mode of human trichromatic color vision.

A specific fluorescent probe for fast detection and cellular imaging of cysteine based on a water-soluble conjugated polymer combined with copper(II)

In pure water system, the specific and rapid detection of cysteine (Cys) is very important and challenging. Herein, a new optical probe was developed for the purpose based on the complex of cupric ion (Cu²⁺) with a water-soluble conjugated polymer, poly[3-(3-N,N-diacetateaminopropoxy)-4-methyl thiophene disodium salts] (PTCO₂). The fluorescence of PTCO₂ in 100% aqueous solution can almost completely extinguished by Cu²⁺ ions due to its intrinsic paramagnetic properties. Among various amino acids, only Cys causes immediately the efficient recovery of the Cu²⁺-quenched fluorescence of PTCO₂ with ~31-folds fluorescence enhancement because of the stronger affinity of Cys to Cu²⁺ leading to the formation of Cu²⁺-Cys complex through Cu-S bond and separation of Cu²⁺ from weak-fluorescent PTCO₂-Cu(II) ensemble and thereby restoring the free PTCO₂ fluorescence. In tris-HCl buffer solution (2 mM, pH 7.4), the intensity of the restored fluorescence is linear with the concentration of Cys, ranging from 0 to 120 μM and the estimated detection limit of Cys is 3.3 × 10^-7 M with the correlation coefficient R = 0.9981. In addition, the PTCO₂-Cu(II) ensemble probe exhibits low cytotoxicity and good membrane penetration, and its application in living cell imaging of Cys has also been explored.

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 two-photon ratiometric fluorescent probe for highly selective sensing of mitochondrial cysteine in live cells

We report herein a two-photon ratiometric fluorescent probe (DNEPI) for mitochondrial cysteine (Cys) detection on the basis of a merocyanine (compound 1) as the two-photon fluorophore and a 2,4-dinitrobenzensulfonyl (DNBS) unit as the biothiol reaction site. Upon reaction with Cys in DMSO/PBS (1/1, v/v), DNEPI showed a distinct ratiometric fluorescence emission characteristic (F_{583 nm}/F_{485 nm}) linearly proportional to Cys concentrations over the range of 2-10 μM, which was attribute to the enhanced intramolecular charge transfer (ICT) effect by cleavage of the sulfonic acid ester bond of DNEPI to release compound 1. More importantly, the probe could detect Cys with a fast response time (within 2 min) and the detection limit was quantitatively calculated as 0.29 μM. Furthermore, DNEPI not only exhibited high selectivity toward Cys over other similar biothiols, including homocysteine (Hcy) and glutathione (GSH), but also displayed significant mitochondrial-targeting ability, which were favorable for mitochondrial Cys-selective imaging. Subsequently, application of DNEPI to Cys imaging in live cells was successfully achieved by two-photon fluorescence microscopy, suggesting that the probe proposed here could be used to monitor mitochondrial Cys concentration changes in live cells with negligible interference from other biological thiols.

A highly sensitive luminescent probe based on Ru(II)-bipyridine complex for Cu2+, l-Histidine detection and cellular imaging

A ruthenium(II) bipyridyl complex conjugated with functionalized Schiff base (RuA) has been synthesized and functioned as a luminescent probe. The luminescence of RuA was greatly quenched by Cu²⁺ due to its molecular coordination with paramagnetic Cu²⁺. Subsequently, the addition of l-Histidine can turn on the luminescence of the RuA-Cu(II) ensemble, which can be attributed to the replacement of RuA in RuA-Cu(II) ensemble by l-Histidine. On the basis of the quenching and recovery of the luminescence of RuA, we proposed a rapid and highly sensitive on-off-on luminescent assay for sensing Cu²⁺ and l-Histidine in aqueous solution. Under the optimal conditions, Cu²⁺ and l-Histidine can be detected in the concentration range of 5 nM-9.0 μM and 50 nM-30 μM, respectively, and the corresponding detection limits were calculated to be 0.35 and 0.44 nM (S/N=3), separately. The proposed luminescent probe has been successfully utilized for the analysis of Cu²⁺ and l-Histidine in real samples (drinking water and biological fluids). Furthermore, the probe revealed good photostability, low cytotoxicity and excellent permeability, making it a suitable candidate for cell imaging and labeling in vitro.

A novel glucosamine-linked fluorescent chemosensor for the detection of pyrophosphate in an aqueous medium and live cells

A glucosamine-linked Cu²⁺ ensemble has been successfully developed for detection of pyrophosphate (PPi) in aqueous medium and in live MD-AMB-231 cells.

Specific and sensitive imaging of basal cysteine over homocysteine in living cells

Biological thiols play important roles in maintaining appropriate redox status of organisms. Accepting the challenge to differentiate structurally similar cysteine (Cys) and homocysteine (Hcy), we have successfully developed a miniature synthetic turn-on fluorescent probe based on 6-(2-benzothiazolyl)-2-naphthalenol for Cys. This probe is able to specifically react with Cys to yield its naphthalenol derivative, accompanied by remarkable green fluorescence enhancement with a detection limit of 14.8 nM. Besides, this probe displays much greater selectivity for Cys over other biological thiols, including homocysteine (Hcy) and glutathione (GSH). Practically, good cell permeability and low cytotoxicity make it suitable for monitoring basal Cys in living cells.

A turn-on fluorescent probe conjugating with a reactive acrylate for visualization of basal Cys specifically in living cells was developed.

Real-Time Monitoring of Endogenous Cysteine Levels In Vivo by near-Infrared Turn-on Fluorescent Probe with Large Stokes Shift

Cysteine (Cys), as an important biothiol, plays a major role in many physiological processes like protein synthesis, detoxification and metabolism, and also is closely associated with a variety of diseases; thus the design of novel highly selective and sensitive near-infrared (NIR) fluorescent probes for Cys detection in vivo is of great significance. Herein, we report a selective and sensitive NIR turn-on fluorescent probe (CP-NIR) with large Stokes shift for detecting Cys in vivo. Upon addition of Cys to the solution of the probe, it is absorption wavelength shifts from 550 to 600 nm, accompanying with an obvious enhancement of NIR fluorescence emission centering around 760 nm. This Michael-addition reaction-based probe shows a large Stokes shift (160 nm), low detection limit (48 nM), fast response time, and low toxicity. Moreover, this novel NIR probe with good cell permeability was successfully applied to monitoring endogenous Cys in living cells and in a mouse model.

Fluorescein-Based Chromogenic and Ratiometric Fluorescence Probe for Highly Selective Detection of Cysteine and Its Application in Bioimaging

A dual mode fluorescent probe, which is based on an integration of fluorescein and coumarin fluorophores, was developed for the discrimination of Cys from Hcy and GSH. This probe (2) shows the advantage of quick reaction (5 min) with Cys, resulting in a strong fluorescence turn-on response when excited at 450 nm. Notably, it also demonstrates the ratiometric fluorescence property while excited by a shorter wavelength (332 nm). All of results suggest probe 2 has a high selectivity toward Cys even in the presence of other amino acids, cations and anions. The detection limit of Cys was calculated as 0.084 μM, which was much lower than the intracellular concentration. ¹H NMR, MS and DFT calculation were used to reveal the detection mechanism further. Finally, this low cytotoxic probe was successfully applied in bioimaging within HepG2 cells.

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.

Synthesis and Application of an Aldazine-Based Fluorescence Chemosensor for the Sequential Detection of Cu²⁺ and Biological Thiols in Aqueous Solution and Living Cells

A fluorescence chemosensor, 2-hydroxy-1-naphthaldehyde azine (HNA) was designed and synthesized for sequential detection of Cu(2+) and biothiols. It was found that HNA can specifically bind to Cu(2+) with 1:1 stoichiometry, accompanied with a dramatic fluorescence quenching and a remarkable bathochromic-shift of the absorbance peak in HEPES buffer. The generated HNA-Cu(2+) ensemble displayed a "turn-on" fluorescent response specific for biothiols (Hcy, Cys and GSH) based on the displacement approach, giving a remarkable recovery of fluorescence and UV-Vis spectra. The detection limits of HNA-Cu(2+) to Hcy, Cys and GSH were estimated to be 1.5 μM, 1.0 μM and 0.8 μM, respectively, suggesting that HNA-Cu(2+) is sensitive enough for the determination of thiols in biological systems. The biocompatibility of HNA towards A549 human lung carcinoma cell, was evaluated by an MTT assay. The capability of HNA-Cu(2+) to detect biothiols in live A549 cells was then demonstrated by a microscopy fluorescence imaging assay.

Highly Selective Two-Photon Fluorescent Probe for Ratiometric Sensing and Imaging Cysteine in Mitochondria

A novel ratiometric mitochondrial cysteine (Cys)-selective two-photon fluorescence probe has been developed on the basis of a merocyanine as the fluorophore and an acrylate moiety as the biothiol reaction site. The biocompatible and photostable acrylate-functionalized merocyanine probe shows not only a mitochondria-targeting property but also highly selective detection and monitoring of Cys over other biothiols such as homocysteine (Hcy) and glutathione (GSH) and hydrogen sulfide (H2S) in live cells. In addition, this probe exhibits ratiometric fluorescence emission characteristics (F518/F452), which are linearly proportional to Cys concentrations in the range of 0.5-40 μM. More importantly, the probe and its released fluorophore, merocyanine, exhibit strong two-photon excited fluorescence (TPEF) with two-photon action cross-section (Φσmax) of 65.2 GM at 740 nm and 72.6 GM at 760 nm in aqueous medium, respectively, which is highly desirable for high contrast and brightness ratiometric two-photon fluorescence imaging of the living samples. The probe has been successfully applied to ratiometrically image and detect mitochondrial Cys in live cells and intact tissues down to a depth of 150 μm by two-photon fluorescence microscopy. Thus, this ratiometric two-photon fluorescent probe is practically useful for an investigation of Cys in living biological systems.

A thiol fluorescent probe reveals the intricate modulation of cysteine's reactivity by Cu(II)

In this study, Cu(II)-mediated differential alteration of cysteine (Cys) reactivity is reported by using a Cys-specific fluorescent probe. The probe could react with Cys to give out strong fluorescence. When Cys was preincubated with Cu(II), the fluorescence of the probe was decreased due to the inhibition of Cys's reactivity by Cu(II). Remarkably, experimental results reveal that the probe could detect Cu(II) at subnanomolar concentrations. In contrast, Cu(II) could only partially inhibit the reaction between Cys and Ellman's reagent (DTNB). Furthermore, selectivity experiments show that Cu(II) is a much more potent inhibitor for Cys compared to other metal ions. Cell imaging experiments also confirm the inhibitory effects of Cu(II) on Cys's reactivity in living cells. We envision that the probe could add a useful tool for sensitive and selective detection of Cu(II) for biomedical research.

Spectroscopic studies on the interaction of terpyridine-CuCl2 with cysteine

Interaction of the classical coordination complex TpyCuCl₂ (Tpy = terpyridine) with cysteine was studied. Addition of cysteine to the solution of TpyCuCl₂ was found to give greatly enhanced fluorescence.

A new turn-on chemosensor for bio-thiols based on the nanoaggregates of a tetraphenylethene-coumarin fluorophore

In this work, a tetraphenylethene-coumarin hybrid fluorophore (TPE-Cou) that contains a Schiff base form is designed and synthesized. A combination of plentiful optical properties and chemical reactivity towards thiols allows TPE-Cou to work as an excellent turn-on probe of thiols with a wide linear range, revealing the great potential of this dye as a quantitative fluorescence indicator. By means of NMR and optical spectrum analyses, a mechanistic picture at the molecular level has been drawn to illustrate how this dye works as a bio-thiol-sensitive fluorescent probe.

A novel fluorescence probe for estimation of cysteine/histidine in human blood plasma and recognition of endogenous cysteine in live Hct116 cells

A new cell membrane permeable reagent for detection of endogenous Cys as well as quantitative estimation of Cys and His in a human blood plasma sample.

A dioxadithiaazacrown ether–BODIPY dyad Hg2+ complex for detection of l-cysteine: fluorescence switching and application to soft material

A Hg²⁺ coordinate complex of a 1,4-dioxa-7,13-dithia-10-azacyclopentadecane–BODIPY dyad recognises l-cysteine (cys) via reversible complexation/decomplexation and show switching of fluorescence upon sequential addition of Hg²⁺ and cys in solution as well as in hydrogel.