Enlightening the Path to Protein Engineering: Chemoselective Turn-On Probes for High-Throughput Screening of Enzymatic Activity

Many successful stories in enzyme engineering are based on the creation of randomized diversity in large mutant libraries, containing millions to billions of enzyme variants. Methods that enabled their evaluation with high throughput are dominated by spectroscopic techniques due to their high speed and sensitivity. A large proportion of studies relies on fluorogenic substrates that mimic the chemical properties of the target or coupled enzymatic assays with an optical read-out that assesses the desired catalytic efficiency indirectly. The most reliable hits, however, are achieved by screening for conversions of the starting material to the desired product. For this purpose, functional group assays offer a general approach to achieve a fast, optical read-out. They use the chemoselectivity, differences in electronic and steric properties of various functional groups, to reduce the number of false-positive results and the analytical noise stemming from enzymatic background activities. This review summarizes the developments and use of functional group probes for chemoselective derivatizations, with a clear focus on screening for enzymatic activity in protein engineering.

Near-Infrared Probes for Biothiols (Cysteine, Homocysteine, and Glutathione): A Comprehensive Review

Biothiols (cysteine, homocysteine, and glutathione) are an important class of compounds with a free thiol group. These biothiols plays an important role in several metabolic processes in living bodies when present in optimum concentration. Researchers have developed several probes for the detection and quantification of biothiols that can absorb in UV, visible, and near-infrared (NIR) regions of the electromagnetic spectrum. Among them, NIR organic probes have attracted significant attention due to their application in in vivo and in vitro imaging. In this review, we have summarized probes for these biothiols, which could work in the NIR region, and discussed their sensing mechanism and potential applications. Along with focusing on the pros and cons of the reported probes we have classified them according to the fluorophore used and summarized their photophysical and sensing properties (emission, response time, limit of detection).

A novel hydrogen peroxide fluorescent probe for bioimaging detection and enables multiple redox cycles

In this subject, a novel hydrogen peroxide (H₂O₂) fluorescent probe (MNG) was designed and developed using naphthalimide derivatives and selenomorpholine. In PBS buffer (10 mM, pH = 7.4, 1 %DMSO), the selenomorpholine on the probe is capable of qualitatively and quantitatively detecting (H₂O₂) at a small amount under a detection limit of 61 nM. The probe follows a mechanism that Se (Ⅱ) in selenomorpholine is transformed to Se (Ⅳ), thus changing the spectra of the probe MNG. It is noteworthy that MNG can continuously make a cyclic response to H₂O₂ and glutathione (GSH), so it can potentially achieve redox process imaging in vivo. Moreover, this subject verified the redox process of the probe's continuous redox response in the Gaussian 09 programme through simulation calculation and mass spectrometry. The probe exhibits high biocompatibility. Moreover, it can detect H₂O₂ in MCF-7 cells and Argentine Bloodfin.

Synthesis of Metal–Organic Frameworks Quantum Dots Composites as Sensors for Endocrine-Disrupting Chemicals

Hazardous chemical compounds such as endocrine-disrupting chemicals (EDCs) are widespread and part of the materials we use daily. Among these compounds, bisphenol A (BPA) is the most common endocrine-disrupting chemical and is prevalent due to the chemical raw materials used to manufacture thermoplastic polymers, rigid foams, and industrial coatings. General exposure to endocrine-disrupting chemicals constitutes a serious health hazard, especially to reproductive systems, and can lead to transgenerational diseases in adults due to exposure to these chemicals over several years. Thus, it is necessary to develop sensors for early detection of endocrine-disrupting chemicals. In recent years, the use of metal–organic frameworks (MOFs) as sensors for EDCs has been explored due to their distinctive characteristics, such as wide surface area, outstanding chemical fastness, structural tuneability, gas storage, molecular separation, proton conductivity, and catalyst activity, among others which can be modified to sense hazardous environmental pollutants such as EDCs. In order to improve the versatility of MOFs as sensors, semiconductor quantum dots have been introduced into the MOF pores to form metal–organic frameworks/quantum dots composites. These composites possess a large optical absorption coefficient, low toxicity, direct bandgap, formidable sensing capacity, high resistance to change under light and tunable visual qualities by varying the size and compositions, which make them useful for applications as sensors for probing of dangerous and risky environmental contaminants such as EDCs and more. In this review, we explore various synthetic strategies of (MOFs), quantum dots (QDs), and metal–organic framework quantum dots composites (MOFs@QDs) as efficient compounds for the sensing of ecological pollutants, contaminants, and toxicants such as EDCs. We also summarize various compounds or materials used in the detection of BPA as well as the sensing ability and capability of MOFs, QDs, and MOFs@QDs composites that can be used as sensors for EDCs and BPA.

Selenium-Based Fluorescence Probes for the Detection of Bioactive Molecules

Chemistry of organoselenium reagents have now become an important tool of synthetic organic and medicinal chemistry. These reagents activate the olefinic double bonds and used to archive the number of organic transformations under mild reaction conditions. A number of organoselenium compounds have been identified as potent oxidants. Recently, various organoselenium species have been employed as chemical sensors for detecting toxic metals. Moreover, a number of selenium-based fluorescent probes have been developed for detecting harmful peroxides and ROS. In this review article, the synthesis of selenium-based fluorescent probes will be covered including their application in the detection of toxic metals and harmful peroxides including ROS.

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

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

A Raman/fluorescence dual-modal imaging guided synergistic photothermal and photodynamic therapy nanoplatform for precision cancer theranostics

A nanoplatform that integrates hypoxia-responsive fluorescent probe function as well as imaging and therapeutic functions is developed.

Unexpected ring closures leading to 2-N,N-dialkylaminoareno[1,3]tellurazoles

One step, up to 78% isolated yield, six examples. Facile access to 2-N,N-dialkylbenzo[1,3]tellurazoles.

Fluorescent chameleon labels for bioconjugation and imaging of proteins, nucleic acids, biogenic amines and surface amino groups. a review

Chameleon labels (ChLs) possess the unique property of changing (visible) color and fluorescence on binding to amino groups of biomolecules. MostChLs react with primary aliphatic amino groups such as those in lysine or with amino groups artificially introduced into polynucleic acids or saccharides, but someothers also react with secondary amino groups. Under controlled circumstances, the reactions are fairly specific. The review is subdivided into the following sections: (1) An introduction and classification of fluorescent labels; (2) pyrylium labels that undergo shortwave color changes upon labelling, typically from blue to red; (3) polymethine type of labels (that also undergo shortwave color changes, typically from green to blue; (4) various other (less common) chromogenic and fluorogenic systems; (5) hemicyanine labels that undergolongwavecolor changes, typically from yellow to purple; (6) the application of ChLs to labeling of proteins and oligonucleotides; (7) applications to fluorometric assays and sensing; (8) applications to fluorescence imaging of biomolecules; (9) applications in studies on affinity interactions (receptor-ligand binding); (10) applications in surface and interface chemistry; and (11) applications in chromatography, electrophoresis and isotachophoresis of biomolecules.

Using a 3-hydroxyflavone derivative as a fluorescent probe for the indirect determination of aminothiols separated by ion-pair HPLC

Homocysteine, cysteine, cysteinyl-glycine, and glutathione are significant biological aminothiols (ATs) that are marker-molecules in Down syndrome, Alzheimer's disease, or have been implicated as risk factors in atherosclerosis and other vascular diseases, and therefore rapid determination of these molecules is desirable. After reduction of the disulfides, a widely used method utilizes derivatization with ammonium 7-fluorobenzo-2-oxa-1,3-diazole-4-sulfonate (SBD-F) as a fluorogenic probe prior to reversed-phase HPLC separation followed by fluorescence detection. The traditional HPLC determination of ATs is time consuming and economically expensive. We have developed an ion-pair HPLC method coupled with indirect fluorescence detection after post-column reaction with a 2,4-dinitrobenzenesulfonate derivative of a 3-hydroxyflavone. The accuracy, precision, post-column temperature and residence time, and limit-of-detection were evaluated. Sample throughput and reduced sample preparation time of over an hour for the existing methods to less than 20 minutes for the new method is also demonstrated. No statistical differences in HCy, Cys, or Cys-Gly determinations in plasma samples were observed between our method and the traditional HPLC method.

Potassium bromate (KBrO3) modulates oxidative stress and inflammatory biomarkers in sodium hydroxide (NaOH) - induced Crohn's colitis in Wistar rats

Potassium bromate (KBrO₃) present in consumed ozonised water was recently documented to exacerbate experimental gastric ulcer. Information, however, is vague as regards its effects in the colon where water reabsorption occurs. In this study, we observed the possible effects of KBrO₃ on oxidative stress and inflammatory biomarkers in sodium hydroxide (NaOH) - induced Crohn's colitis (CC). Wistar rats (180-200 g) were divided into six groups (n = 10): (i) control; (ii) untreated CC (induced by 1.4% NaOH; intra-rectal administration); and (iii-vi) CC treated with vitamin E, KBrO₃, vitamin E+KBrO₃, and sulphazalazine, respectively, for 7 days. Body weight and stool score were monitored daily. By day 3 and 7, excised colon was evaluated for ulcer scores and biochemical and histological analysis. Blood samples collected on days 3 and 7 were assayed for haematological indices using standard methods. Data were subjected to analysis of variance (ANOVA) and p ≤ 0.05 considered significant. Platelet/lymphocyte ratio, colonic ulcer score, malondialdehyde, and mast cells were significantly decreased while colonic sulfhydryl, and Ca²⁺- and Na⁺/K⁺-ATPase activities were increased following KBrO₃ treatment compared with untreated CC. These findings suggest that KBrO₃ may mitigate against NaOH-induced CC via inhibiting mast cell population and oxidative and inflammatory content but stimulating colonic sulfhydryl and Ca²⁺- and Na⁺/K⁺-ATPase activities.

Ebselen-Agents for Sensing, Imaging and Labeling: Facile and Full-Featured Application in Biochemical Analysis

Phenyl-1,2-benzoselenazol-3(2H)-one (ebselen) is a classical mimic of glutathione peroxidase (GPx). Thioredoxin interaction endows ebselen attractive biological functions, such as antioxidation and anti-infection, as well as versatile therapeutic usage. Accordingly, application of ebselen analogues in biosensing, chemical labeling, imaging analysis, disease pathology, drug development, clinical treatment, etc. have been widely developed, in which mercaptans, reactive oxygen species, reactive sulfur species, peptides, and proteins were involved. Herein, focusing on the application of ebselen-agents in biochemistry, we have made a systematic summary and comprehensive review. First, we summarized both the classical and the innovative methods for preparing ebselen-agents to present the synthetic strategies. Then we discussed the full functional applicability of ebselen analogues in three fields of biochemical analysis including the fluorescence sensing and bioimaging, derivatization for high throughput fluorescence analysis, and the labeling gents for proteomics. Finally, we discussed the current challenges and perspectives for ebselen-agents as analytical tools in biological research. By presenting the multifunctional applicability of ebselen, we hope this review could appeal researchers to design the ebselen-related biomaterials for biochemical analysis.

Fluorescent probes based on nucleophilic aromatic substitution reactions for reactive sulfur and selenium species: Recent progress, applications, and design strategies

Reactive sulfur species (RSS) and reactive selenium species (RSeS) are important substances for the maintenance of physiological balance. Imbalance of RSS and RSeS is closely related to a series of human diseases, so it is considered to be an important biomarker in early diagnosis, treatment, and stage monitoring. Fast and accurate quantitative analysis of different RSS and RSeS in complex biological systems may promote the development of personalized diagnosis and treatment in the future. One way to explore the physiological function of various types of RSS and RSeS in vivo is to detect them at the molecular level, and one of the most effective methods for this is to use fluorescent probes. Nucleophilic aromatic substitution (SNAr) reactions are commonly exploited as a detection mechanism for RSS and RSeS in fluorescent probes. In this review, we cover recent progress in fluorescent probes for RSS and RSeS based on SNAr reactions, and discuss their response mechanisms, properties, and applications. Benzenesulfonate, phenyl-O ether, phenyl-S ether, phenyl-Se ether, 7-nitro-2,1,3-benzoxadiazole (NBD), benzoate, and selenium-nitrogen bonds are all good detection groups. Moreover, based on an integration of different reports, we propose the design and synthesis of RSS- and RSeS-selective probes based on SNAr reactions, current challenges, and future research directions, considering the selection of active sites, the effect of substituents on the benzene ring, and the introduction of other functional groups.

A review of bioselenol-specific fluorescent probes: Synthesis, properties, and imaging applications

Bioselenols are important substances for the maintenance of physiological balance and offer anticancer properties; however, their causal mechanisms and effectiveness have not been assessed. One way to explore their physiological functions is the in vivo detection of bioselenols at the molecular level, and one of the most efficient ways to do so is to use fluorescent probes. Various types of bioselenol-specific fluorescent probes have been synthesized and optimized using chemical simulations and by improving biothiol fluorescent probes. Here, we review recent advances in bioselenol-specific fluorescent probes for selenocysteine (Sec), thioredoxin reductase (TrxR), and hydrogen selenide (H₂Se). In particular, the molecular design principles of different types of bioselenols, their corresponding sensing mechanisms, and imaging applications are summarized.

Development of a water-soluble near-infrared fluorescent probe for endogenous cysteine imaging

We designed and synthesized a water-soluble near-infrared (NIR) fluorescent probe with the recognition unit of the cyanine-like structure and acrylate group. Through an aromatic ring nucleophilic substitution reaction based on sulfhydryl moiety, an off-on fluorescence response toward cysteine (Cys) was realized. The probe exhibited excellent spectral performance with an emission wavelength of 720nm and a detection limit of 0.20μM. The spectral properties, selectivity and anti-interference performance of the probe were systematically investigated. Density functional theory (DFT) calculations were conducted to clarify the luminescence mechanism of the probe. Furthermore, the probe was successfully applied to the detection of free Cys in human serum and the NIR imaging of endogenous Cys in living cells. Thus, the probe has a promising application prospect in clinical diagnosis and fluorescence imaging.

NBD-based fluorescent probes for separate detection of cysteine and biothiols via different reactivities

A NBD-based fluorescent probe is developed to seperately detect Cys and all biothiols via different reactivity.

A dual-responsive fluorescent probe for detection of fluoride ion and hydrazine based on test strips

Hydrazine (N₂H₄) and fluoride ion (F^-) are regarded as environmental pollutants and potential carcinogens. A dual-functional fluorescent probe (probe 1) was developed for both F^- and N₂H₄ with high selectivity and sensitivity. 1 was based on nucleophilic aromatic substitution reaction for N₂H₄ detection and selective cleavage of 4-nitrobenzenesulphonyl group for the determination of F^-. The limits of detection of probe for F^- and N₂H₄ were 77.82 nM and 29.34 nM, respectively, which are far below the threshold limit value (TLV) of United States Environmental Protection Agency (EPA). The home-made test strips of 1 provided the positive tool for F^- and gaseous N₂H₄ in different system. And the confocal fluorescence images indicated that 1 can quantitatively detect N₂H₄ in living PC12 cells. Promisingly, 1 has great prospects for N₂H₄ imaging and determining in living system.

A novel fluorescent turn-on probe for imaging biothiols based on SNAr substitution-skeletal rearrangement strategy

2is a novel fluorescent turn-on probe for imaging biothiols based on S_NAr substitution-skeletal rearrangement strategy with dramatic fluorescence enhancement and high sensitivity.

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.

Antioxidant activity of selenenamide-based mimic as a function of the aromatic thiols nucleophilicity, a DFT-SAPE model

The mechanism of action of the selenenamide 1 as a mimic of the glutathione peroxidase (GPx) was investigated by the density functional theory. The solvent-assisted proton exchange procedure was applied to model the catalytic behavior and antioxidant activity of this mimic. To have an insight into the charge transfer effect, different aromatic thiols, including electron donating substituents on the phenyl ring were considered. The catalytic behavior of the selenenamide was modeled in a four-step mechanism, described by the oxidation of the mimic, the reduction of the obtained product, selenoxide, the reduction of the selenenylsulfide and dehydration of selenenic acid. On the basis of the activation parameters, the final step of the proposed mechanism is the rate determining states of the catalytic cycle. Turnover frequency (TOF) analysis showed that the electron donating groups at the para-position of the phenyl ring of the PhSH do not affect the catalytic activity of the selenenamide in contrast to p-methyl thiophenol which indicates the highest nucleophilicity. The evaluation of the electronic contribution of the various donating groups on the phenyl ring of the aromatic thiols shows that the antioxidant activity of the selenenamide sufficiently increases in the presence of the electron-donating substitutions. Finally, the charge transfer process at the rate-determining state was investigated based on the natural bond orbital analysis.

A NBD-S-rhodamine dyad for dual-color discriminative imaging of biothiols and Cys/Hcy

A fluorescent probe based on fast thiolysis of NBD thioether is developed for dual-color discriminative imaging of Cys and GSH.

Reaction-based ratiometric fluorescent probe for selective recognition of sulfide anions with a large Stokes shift through switching on ESIPT

Ratiometric fluorescent probe BNPT has been synthesized and characterized for S²⁻ sensing via ESIPT mechanism.

A Michael addition–cyclization-based switch-on fluorescent chemodosimeter for cysteine and its application in live cell imaging

We designed and synthesized a fast response fluorescent probe, BTAC (benzothiazol-azacoumarin), for detection of cysteine (Cys).

A novel mitochondria-targeted two-photon fluorescent probe for dynamic and reversible detection of the redox cycles between peroxynitrite and glutathione

Redox homeostasis is important for maintenance of normal physiological functions within cells. Redox state of cells is primarily a consequence of precise balance between levels of reducing equivalents and reactive oxygen species. Redox homeostasis between peroxynitrite (ONOO^-) and glutathione (GSH) is closely associated with physiological and pathological processes, such as prolonged relaxation in vascular tissues and smooth muscle preparations, attenuation of hepatic necrosis, and activation of matrix metalloproteinase-2. We report a two-photon fluorescent probe (TP-Se) based on water-soluble carbazole-based compound, which integrates with organic selenium, to monitor changes in ONOO^-/GSH levels in cells. This probe can reversibly respond to ONOO^- and GSH and exhibits high selectivity, sensitivity, and mitochondrial targeting. The probe was successfully applied to visualize changes in redox cycles during ONOO^- outbreak and antioxidant GSH repair in cells. The probe will lead to significant development on redox events involved in cellular redox regulation.

A ratiometric fluorescent probe for imaging and quantifying anti-apoptotic effects of GSH under temperature stress

A ratiometric fluorescent probe for imaging and quantifying concentration fluctuations and anti-apoptotic effects of GSH under hypothermia and hyperthermia in HepG2 and HepG2/DDP xenografts.

Hypothermia and hyperthermia are cell stressed states resulting from environmental temperature changes, which can abnormally decrease intracellular glutathione (GSH) concentrations and induce apoptosis. As the most abundant intracellular non-protein biothiol, GSH can protect cells from apoptosis. Considering the important roles of GSH in the anti-apoptotic process in cells and in vivo, we strive to develop a powerful chemical tool for the direct detection of GSH concentration changes under temperature stress. Herein, we report a ratiometric fluorescent probe (CyO-Dise) based on a selenium–sulfur exchange reaction for the qualitative and quantitative detection of GSH concentration fluctuations in cells and in vivo. The probe has been successfully used to assess the changes of GSH levels in HepG2 and HL-7702 cells using the stimulations of hypothermia and hyperthermia. In terms of the anti-apoptotic effect of GSH under hypothermic and hyperthermic conditions, human normal liver HL-7702 cells have stronger abilities to fight against temperature stress than human liver carcinoma HepG2 cells. Hypothermia and hyperthermia can also improve the drug resistance of cis-dichlorodiamineplatinum(ii) (DDP)-resistant HepG2/DDP cells. The CyO-Dise probe has been employed to image GSH concentration changes in HepG2 and HepG2/DDP xenografts on nude mice. With the adjuvant therapy effects of hypothermia and hyperthermia, the chemotherapy drug DDP exhibits good ability for the treatment of HepG2 and HepG2/DDP xenografts. The above applications make our probe a potential new candidate for the accurate diagnosis of cancer and efficacy evaluation of treatment.

Computational Modeling of the Catalytic Cycle of Glutathione Peroxidase Nanomimic

To elucidate the role of a derivative of ebselen as a mimic of the antioxidant selenoenzyme glutathione peroxidase, density functional theory and solvent-assisted proton exchange (SAPE) were applied to model the reaction mechanism in a catalytic cycle. This mimic plays the role of glutathione peroxidase through a four-step catalytic cycle. The first step is described as the oxidation of 1 in the presence of hydrogen peroxide, while selenoxide is reduced by methanthiol at the second step. In the third step of the reaction, the reduction of selenenylsulfide occurs by methanthiol, and the selenenic acid is dehydrated at the final step. Based on the kinetic parameters, step 4 is the rate-determining step (RDS) of the reaction. The bond strength of the atoms involved in the RDS is discussed with the quantum theory of atoms in molecules (QTAIM). Low value of electron density, ρ(r), and positive Laplacian values are the evidence for the covalent nature of the hydrogen bonds rupture (O₃₀-H₃₁, O₃₃-H₃₄). A change in the sign of the Laplacian, L(r), from the positive value in the reactant to a negative character at the transition state indicates the depletion of the charge density, confirming the N₅-H₁₀ and O₁₁-Se₁ bond breaking. The analysis of electron location function (ELF) and localized orbital locator (LOL) of the Se₁-N₅ and Se₁-O₁₁ bonds have been done by multi-WFN program. High values of ELF and LOL at the transition state regions between the Se, N, and O atoms display the bond formation. Finally, the main donor-acceptor interaction energies were analyzed using the natural bond orbital analysis for investigation of their stabilization effects on the critical bonds at the RDS.

Keto-polymethines: a versatile class of dyes with outstanding spectroscopic properties for in cellulo and in vivo two-photon microscopy imaging

The synthesis of keto-heptamethine derivatives has been expanded to various new symmetrical and asymmetrical structures, including an unprecedented di-anionic keto-polymethine. The spectroscopic behavior of these new dyes has been systematically and thoroughly investigated, revealing that the formation of hydrogen bond interactions with protic solvents is responsible for a dramatic enhancement of the fluorescence quantum yield in the far-red spectral region. The existence of these strong hydrogen-bond interactions was further confirmed by molecular dynamics simulations. These bis-dipolar polymethines exhibit large two-photon absorption (TPA) cross-sections (σ² in GM) in the near-infrared, making them ideal candidates for NIR-to-NIR two-photon microscopy imaging applications. We demonstrate that the molecular engineering of the hydrophilic/hydrophobic balance enables targeting of different cellular components, such as cytoplasm or cell membranes. Addition of appropriate substituents provides the molecule with high-water-solubility, affording efficient two-photon probes for angiography.

Quantification of cysteine hydropersulfide with a ratiometric near-infrared fluorescent probe based on selenium-sulfur exchange reaction

Cysteine hydropersulfide (Cys-SSH) plays primary roles in the synthesis of sulfur-containing cofactors, regulation of cellular signaling, activation or inactivation of enzyme activities, and modulation of cellular redox milieu. However, its biofunctions need to be further addressed due to the fact that many issues remain to be clarified. Herein, we conceive a novel ratiometric near-infrared fluorescent probe Cy-Dise for the sensitive and selective detection of Cys-SSH in living cells and in vivo for the first time. Cy-Dise is composed of three moieties: bis(2-hydroxyethyl) diselenide, heptamethine cyanine, and d-galactose. Cy-Dise exhibits a satisfactory linear ratio response to Cys-SSH via a selenium-sulfur exchange reaction in the range of 0-12 μM Cys-SSH. The experimental detection limit is determined to be 0.12 μM. The results of ratio imaging analyses confirm the qualitative and quantitative detection capabilities of Cy-Dise in HepG2 cells, HL-7702 cells, and primary hepatocytes. The level changes of Cys-SSH in cells stimulated by some related reagents are also observed. The probe is also suitable for deep tissue ratio imaging. Organ targeting tests with Cy-Dise in normal Spraque-Dawley (SD) rats and Walker-256 tumor SD rats verify its predominant localization in the liver. The probe is promising for revealing the roles of Cys-SSH in physiological and pathological processes.

Real time monitoring of aminothiol level in blood using a near-infrared dye assisted deep tissue fluorescence and photoacoustic bimodal imaging

The development of molecular probes for the detection and imaging of biological thiols is a major step forward diagnosing various types of diseases. Previously reported thiol imaging strategies were mainly based on a single mode of imaging with a limited application in vivo. In this work, we introduced an unsymmetrical near-infrared (NIR) squaraine dye (USq) as an exogenous contrast agent for photoacoustic and fluorescence bimodal imaging of thiol variations in live animals. USq exhibits a narrow absorption band at 680 nm that generates a photoacoustic signal and a strong NIR emission at 700 nm (ΦF = 0.27), which is applicable for deep tissue optical imaging. Both photoacoustic and fluorescence signals could selectively disappear in the presence of different thiols. Through in vitro and in vivo imaging studies, unique imaging capability of USq was demonstrated, and the effect of food uptake on the increased level of aminothiols in blood was confirmed.

Phospholipid Encapsulated AuNR@Ag/Au Nanosphere SERS Tags with Environmental Stimulus Responsive Signal Property

Surface-enhanced Raman scattering (SERS) tags draw much attention due to the ultrasensitivity and multiplex labeling capability. Recently, a new kind of SERS tags was rationally designed by encapsulating metal nanoparticles with phospholipid bilayers, showing great potential in theranostics. The lipid bilayer coating confers biocompatibility and versatility to changing surface chemistry of the tag; however, its "soft" feature may influence SERS signal stability, which is rarely investigated. Herein, we prepared phospholipid-coated AuNR@Ag/Au nanosphere SERS tags by using three different kinds of Raman reporters, i.e., thio-containing 4-nitrothiophenol (NT), nitrogen-containing hydrophobic chromophore cyanine 7 monoacid (Cy7), and alkyl chain-chromophore conjugate 1,1'-dioctadecyl-3,3,3',3'-tetramethylindodicarbocyanine (DiD). It was found that signal responses were different upon additional stimulation which the tags may encounter in theranostic applications including the presence of detergent Triton X-100, lipid membrane, and photothermal treatment. Living-cell imaging also showed signal changing distinction. The different SERS signal performances were attributed to the different Raman reporter releasing behaviors from the tags. This work revealed that Raman reporter structure determined signal stability of lipid-coated SERS tags, providing guidance for the design of stimulus responsive tags. Moreover, it also implied the potential of SERS technique for real time drug release study of lipid based nanomedicine.

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

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

A Two-Photon Fluorescent Probe for Lysosomal Thiols in Live Cells and Tissues

Lysosome-specific fluorescent probes are exclusive to elucidate the functions of lysosomal thiols. Moreover, two-photon microscopy offers advantages of less phototoxicity, better three dimensional spatial localization, deeper penetration depth and lower self-absorption. However, such fluorescent probes for thiols are still rare. In this work, an efficient two-photon fluorophore 1,8-naphthalimide-based probe conjugating a 2,4-dinitrobenzenesulfonyl chloride and morpholine was designed and synthesized, which exhibited high selectivity and sensitivity towards lysosomal thiols by turn-on fluorescence method quantitatively and was successfully applied to the imaging of thiols in live cells and tissues by two-photon microscopy.

A two-photon off-on fluorescence probe for imaging thiols in live cells and tissues

An effective off-on two-photon fluorescence imaging probe for thiols was developed and successfully applied to image thiols both in live cells and tissues.

A near-infrared ratiometric fluorescent probe for cysteine detection over glutathione indicating mitochondrial oxidative stress in vivo

We establish a near-infrared (NIR) ratiometric fluorescent probe Cy-NB for the selective detection of cysteine (Cys) over glutathione (GSH) and homocysteine (Hcy) in mitochondria to indicate oxidative stress. Heptamethine cyanine dye is chosen as the fluorophore of Cy-NB whose emission locates in NIR region. And p-nitrobenzoyl is employed as the fluorescent modulator due to its capability of selective-Cys response. Once triggered by Cys, the uncaged p-nitrobenzoyl rearranges the polymethine π-electron system of the fluorophore, which leads to a remarkable spectrum shifts in absorption and emission profiles. Taking advantage of these spectroscopic properties, we construct a ratiometric fluorescent signal for the detection of Cys with a detection limit of 0.2 µM within 5 min. Our probe Cy-NB can sensitively detect the mitochondrial Cys pool changes under different oxidative stress status in HepG2 cells. We also successfully employ Cy-NB to imaging Cys level changes in living mice. It suggests that mitochondrial Cys can be used as an oxidative stress biomarker with simple potential clinical applications. And our probe Cy-NB is of great potential for further utilizing in exploring the physiological function of Cys in biological systems.