Near-Infrared Fluorescent Turn-on Probe with a Remarkable Large Stokes Shift for Imaging Selenocysteine in Living Cells and Animals

A near-infrared fluorescent probe with a large Stokes shift for the detection and imaging of biothiols in vitro and in vivo

In this study, a new near-infrared (NIR) fluorescent turn-on probe featuring a large Stokes shift (198 nm) was developed for the detection of biothiols. The probe was based on a dicyanoisophorone derivative serving as the fluorophore and a 2,4-dinitrobenzenesulfonyl (DNBS) group functioning as both a recognition site and a fluorescence quencher. In the absence of biothiols, the fluorescence of the probe was low due to the photoinduced electron transfer (PET) effect between the fluorophore and DNBS. Upon the presence of biothiols, the DNBS group underwent a nucleophilic aromatic substitution reaction with the sulfhydryl group of biothiols, leading to the release of the fluorophore and a notable emission peak at 668 nm. This developed probe exhibited exceptional selectivity and sensitivity to biothiols in solution, with an impressive detection limit of 28 nM for cysteine (Cys), 22 nM for homocysteine (Hcy), and 24 nM for glutathione (GSH). Furthermore, the probe demonstrated its applicability by successfully visualizing both endogenous and exogenous biothiols in living systems.

A terbium(III) complex-based time-resolved luminescent probe for selenocysteine as an inhibitor of selenoproteins

A terbium(III) complex-based time-resolved luminescence probe for selenocysteine can inhibit selenoprotein activity via a selenolate-triggered cleavage reaction of sulfonamide bonds in living cells.

Protein-Nanocaged Selenium Induces t(8;21) Leukemia Cell Differentiation via Epigenetic Regulation

The success of arsenic in degrading PML-RARα oncoprotein illustrates the great anti-leukemia value of inorganics. Inspired by this, the therapeutic effect of inorganic selenium on t(8; 21) leukemia is studied, which has shown promising anti-cancer effects on solid tumors. A leukemia-targeting selenium nanomedicine is rationally built with bioengineered protein nanocage and is demonstrated to be an effective epigenetic drug for inducing the differentiation of t(8;21) leukemia. The selenium drug significantly induces the differentiation of t(8;21) leukemia cells into more mature myeloid cells. Mechanistic analysis shows that the selenium is metabolized into bioactive forms in cells, which drives the degradation of the AML1-ETO oncoprotein by inhibiting histone deacetylases activity, resulting in the regulation of AML1-ETO target genes. The regulation results in a significant increase in the expression levels of myeloid differentiation transcription factors PU.1 and C/EBPα, and a significant decrease in the expression level of C-KIT protein, a member of the type III receptor tyrosine kinase family. This study demonstrates that this protein-nanocaged selenium is a potential therapeutic drug against t(8;21) leukemia through epigenetic regulation.

Engineering of a NIR fluorescent probe for high-fidelity tracking of lipid droplets in living cells and nonalcoholic fatty liver tissues

LDs (Lipid droplets) are key organelles for lipid metabolism and storage, which are closely related to ferroptosis and fatty liver. Due to its small size and highly dynamic nature, developing high-fidelity fluorescent probes for imaging of LDs is crucial for observing the dynamic physiological processes of LDs and investigating LDs-associated diseases. Herein, we synthesized three dicyanoisophorone-based fluorescent probes (DCIMe, DCIJ, and DCIQ) with different electron-donating groups and studied their imaging performance for LDs. The results show that DCIQ is highly polarity sensitive and can perform high-fidelity imaging for LDs, with significantly better performance than DCIMe, DCIJ, and commercial LD probe BODIPY 493/503. Based on this, DCIQ was successfully applied to real-time observe the interplays between LDs and other organelles (mitochondria, lysosomes, and endoplasmic reticulum), and to image the dynamics of LDs with fast scanning mode (0.44 s/frame) and the generation of oleic acid-induced LDs with high-fidelity. Finally, DCIQ was used to study the changes of LDs in the ferroptosis process and nonalcoholic fatty liver disease tissues. Overall, this study provided a powerful tool for high-fidelity imaging of LDs in cells and tissues.

Recent progress in the development of small-molecule fluorescent probes for detection and imaging of selenocysteine and application in thyroid disease diagnosis

Selenocysteine (SeCys) is the 21st genetically encoded amino acid present in proteins and is involved in various biological functions. Inappropriate levels of SeCys can be considered as a sign of various diseases. Therefore, small molecular fluorescent probes for the detection and imaging of SeCys in vivo in biological systems are considered to be of significant interest for understanding the physiological role of SeCys. Thus, this article mainly provides a critical evaluation of recent advances made in SeCys detection along with the biomedical applications based on small molecular fluorescent probes published in the literature during the past half a dozen years. Therefore, the article primarily deals with the rational design of fluorescent probes, wherein these were selective towards SeCys over other biologically abundant molecules, in particular the thiol-based ones. The detection has been monitored by different spectral techniques, such as fluorescence and absorption spectroscopy and in some cases even visual color changes. Further, the detection mechanism and the utility of fluorescent probes for in vitro and in vivo cell imaging applications are addressed. For clarity, the main features have been conveniently divided into four categories based on the chemical reactions of the probe, viz., in terms of the cleavage of the responsive group by the SeCys nucleophile: (i) 2,4-dinitrobene sulphonamide group, (ii) 2,4-dinitrobenesulfonate ester group, (iii) 2,4-dinitrobenzeneoxy group and (iv) miscellaneous types. Overall this article deals with the analysis of more than two dozen fluorescent probes demonstrated for selective detection of SeCys along with their applications towards disease diagnosis.

An imidazo[1,5-α]pyridines-based ratiometric fluorescent probe for sensing sulfur dioxide derivatives in real samples based on a FRET mechanism

A novel fluorescence resonance energy transfer (FRET)-based ratiometric emission fluorescent probe AT was designed and developed in which the imidazo[1,5-α]pyridine was served as a FRET donor and tricyanofuran (TCF) as the FRET acceptor to detect SO₃^2-/HSO₃^- based on the Michael addition reaction. Probe AT had a high energy transfer efficiency (95%) and a large pseudo-Stokes shift (259 nm) in EtOH/PBS buffer (5/5, v/v). It also possessed good selectivity and quick response to SO₃^2-/HSO₃^-. There was good linearity between the ratio of fluorescence intensity (F₄₉₉/F₆₄₅) and the concentrations of SO₃^2-/HSO₃^- in the ranges of 1.5-7.5 μM and 9-20 μM, with calculated detection limits (LOD) of 55 nM. In addition, the probe could also detect the concentrations of SO₃^2-/HSO₃^- in real samples such as environmental water and sugar, allowing the probe to be used in a variety of applications.

Rational construction of a new water soluble turn-on colorimetric and NIR fluorescent sensor for high selective Sec detection in Se-enriched foods and biosystems

As a naturally occurring amino acid, selenocysteine (Sec) plays a key role in a variety of cellular functions and Se-enriched foods. In this work, a robust water soluble fluorescence turn-on near-infrared (NIR) sensor NIR-Sec was constructed for Sec detection over biothiols in Se-enriched foods. Specifically, NIR-Sec contains a readily prepared water soluble NIR dicyanoisophorone fluorophore and a well-known response-site 2,4-dinitrobenzenesulfonyl moiety with strong intramolecular charge transfer (ICT) effect to quench the fluorescence intensity of NIR fluorophore. Upon addition of Sec, the NIR dicyanoisophorone fluorophore was released and a bright red emission at 663 nm was observed. Moreover, NIR-Sec toward Sec exhibited rapid response time (∼1 min), a large stoke shift (183 nm), and high selectivity and sensitivity (LOD: 52 nM). Impressively, NIR-Sec was successfully employed to detect and image Sec in Se-enriched foods and shrimp, indicating NIR-Sec could provide a robust tool for investigating the role of Sec in complex real-food samples.

Super-resolution analyzing spatial organization of lysosomes with an organic fluorescent probe

Lysosomes are multifunctional organelles involved in macromolecule degradation, nutrient sensing and autophagy. Live imaging has revealed lysosome subpopulations with dynamics and characteristic cellular localization. An as-yet unanswered question is whether lysosomes are spatially organized to coordinate and integrate their functions. Combined with super-resolution microscopy, we designed a small organic fluorescent probe, TPAE, that targeted lysosomes with a large Stokes shift. When we analyzed the spatial organization of lysosomes against mitochondria in different cell lines with this probe, we discovered different distance distribution patterns between lysosomes and mitochondria during increased autophagy flux. By using SLC25A46 mutation fibroblasts derived from patients containing highly fused mitochondria with low oxidative phosphorylation, we concluded that unhealthy mitochondria redistributed the subcellular localization of lysosomes, which implies a strong connection between mitochondria and lysosomes.

A naphthimide fluorescent probe for the detection of selenols in selenium-enriched Tan sheep

An "off-on" fluorescent probe, Nap-DNB, which is based on naphthimide, was designed and developed for the detection of biological selenols in vitro. We have adopted a combination of a low-pH detection environment and reaction sites that are more difficult to destroy to avoid the interference of a large number of biological thiols in biological samples. Nap-DNB can completely respond to selenocysteine within 15 mins, with a detection limit of 92 nM. Nap-DNB was successfully used for the detection of selenols in the serum, liver, and longissimus dorsi of selenium-enriched Tan sheep. Through comparison, we found that the detection of selenols by the Nap-DNB is similar to that by thioredoxin reductase and glutathione peroxidase in a commercial kit method. Nap-DNB can be used for the detection of selenols in selenium-enriched Tan sheep.

Mitochondrial-Targeting Near-Infrared Fluorescent Probe for Visualizing Viscosity in Drug-Induced Cells and a Fatty Liver Mouse Model

Mitochondria, as "cell energy stations", are involved in the regulation of various cell functions. Recent investigations revealed that mitochondrial dysfunction that can cause an intracellular viscosity mutation, a process that is associated with an increasing number of diseases that are not curable or manageable. However, conventional viscometers cannot be used to monitor the viscosity changes in living cells and in vivo. In order to cater to the complex biological environment, we present a chemical toolbox, MI-BP-CC, that employs N,N-diethyl and double bonds as sensitive sites for viscosity based on the TICT mechanism (twisted intramolecular charge transfer) to monitor the viscosity of living cells and fatter liver mice. MI-BP-CC features good mitochondrial targeting and a near-infrared emission. Surprisingly, in the presence of viscosity, the MI-BP-CC probe exhibited an ultrasensitive model for viscosity detection showing a red fluorescence signal from a silent "off" state to "on". More importantly, utilizing the satisfactory detection performance of MI-BP-CC, we have successfully visualized increased viscosity under the pathological models of Parkinson's (PD) and fatty liver mice. We anticipate that these findings will provide a convenient and efficient tool to understand physiological functions of viscosity in more biosystems.

A Critical Review on Organic Small Fluorescent Probes for Monitoring Carbon Monoxide in Biology

Endogenous carbon monoxide (CO) is an important intracellular gas messenger that is intimately involved in many physiological and pathological processes. The abnormal concentration of CO in living organisms can cause many diseases. Therefore, it is of great significance to monitor CO in biological samples. Fluorescent probe technology provides an effective and convenient method for CO monitoring, with the advantages of high selectivity and sensitivity, fast response time and in situ fluorescence imaging in biological tissues, which is favored by the majority of researchers. In this paper, the research progress of CO fluorescent probes since 2018 is reviewed, and the design, detection mechanism and biological application of the related fluorescent probes are summarized. And the relationship between the structure and performance of the probes is discussed. Furthermore, the development trend and application prospect of CO fluorescent probes are prospected.

Two phenanthroimidazole turn-on probes for the rapid detection of selenocysteine and its application in living cells imaging

Detection of selenocysteine (Sec) content in cells by fluorescence probe is of great significance for the identification of human related diseases. To achieve fast and sensitive detection of Sec, two isomers A4 and B4 as turn-on fluorescent probes to detect Sec were designed and synthesized. Both A4 and B4 display fast turn-on response, high selectivity and sensitivity toward Sec, which can be applied for fluorescence imaging of Sec in living cells. Compared with B4, A4 has a larger Stokes shift (125 nm), wider pH range (5-10) and lower detection limit (65.4 nM) due to its ESIPT (excited state intramolecular proton transfer) effect. In view of the detection performance of these two probes, they can be used as effective tools for detecting Sec in biological systems.

Integration of a Diselenide Unit Generates Fluorogenic Camptothecin Prodrugs with Improved Cytotoxicity to Cancer Cells

A diselenide/disulfide unit was introduced into camptothecin (CPT), and two selenoprodrugs (e.g., CPT-Se3 and CPT-Se4) were identified to show improved potency in killing cancer cells and inhibiting tumor growth in vivo. Interestingly, the intrinsic fluorescence of CPT was severely quenched by the diselenide bond. Both the selenoprodrugs were activated by glutathione with a nearly complete recovery of CPT's fluorescence. The activation of prodrugs was accompanied by the production of selenol intermediates, which catalyzed the constant conversion of glutathione and oxygen to oxidized glutathione and superoxides. The diselenide unit is widely employed in constructing thiol-responsive materials. However, the selenol intermediates were largely ignored in the activation process prior to this study. Our work verified that integration of the diselenide unit may further enhance the parent drug's efficacy. Also, the discovery of the fluorescence quenching property of the diselenide/disulfide bond further shed light on constructing novel theranostic agents.

Real-time detection and imaging of exogenous and endogenous Zn2+ in the PC12 cell model of depression with a NIR fluorescent probe

Depression is closely related to overactivation of N-methyl-d-aspartic acid (NMDA) receptors, and Zn2+ is a vital NMDA receptor modulator involved in the pathophysiological and physiological processes of depression. Therefore, quantitative and real-time detection of Zn2+ is very important for understanding the pathogenesis of depression. In this work, a near-infrared (NIR) fluorescent probe ISO-DPA was designed and synthesized for Zn2+ detection with a large Stokes shift (185 nm), high quantum yield (up to 44%), high sensitivity (LOD = 0.106 μM) and good pH stability. The probe showed rapid response within 10 s, accompanied by a distinct fluorescence change from faint to bright pink with the fluorescence intensity increasing 4.5-fold. Moreover, the sensing mechanism of ISO-DPA towards Zn2+ was supported by MALDI-TOF-MS and Job's plot. The probe ISO-DPA could detect instantaneous variation of exogenous and endogenous Zn2+ in PC12 cells. The bioimaging results reveal the increase of the endogenous Zn2+ concentration in PC12 cells under the oxidative stress induced by glutamate and confirm that overactivation of NMDA receptors results in an increase of the Zn2+ level. All the results proved that ISO-DPA is an excellent probe for detecting Zn2+ in solution and living cells and could help us better understand Zn2+ associated pathogenesis of depression.

Highly Photostable Fluorescent Tracker with pH-Insensitivity for Long-Term Imaging of Lysosomal Dynamics in Live Cells

Visualizing and tracking lysosomal dynamic changes is crucially important in the fields of physiology and pathology. Most currently used pH-dependent small-molecule lysotrackers and sensors usually fail to visualize and track the changes due to (1) their leakage from lysosomes when the lysosomal pH increases and (2) their low photostability. Therefore, it is of significant interest to develop lysosomal probes for visualizing and tracking lysosomal dynamics independent of pH fluctuations and with high photostability. Herein, we found that the popular dicyanomethylene-4H-pyran (DCM) derivative DCM-NH₂ can selectively target and label lysosomes with bright red fluorescence regardless of pH changes. The fluorescence enhancement in lysosomes has probably resulted from their microenvironment of polarity and viscosity. Compared with the commonly used commercial lysosomal molecular probes (LysoTracker Deep Red (LTDR) and LysoTracker Red DND-99), DCM-NH₂ was demonstrated to exhibit a much stronger tolerance in lysosomes against various treatments and microenvironmental changes, and lysosomal membrane permeability could not cause DCM-NH₂ to lose imaging of their targets as well. Moreover, DCM-NH₂ exhibited a superior anti-photobleaching ability and low (photo-) cytotoxicity, which, along with pH-insensitivity, ensured its capability of long-term visualizing and tracking lysosomal dynamics. Lysosomal dynamic events such as the kiss-and-run process, fusion-fission, and mitophagy were successfully recorded with DCM-NH₂. Our study thus confirms that DCM-NH₂ is highly competitive for lysosomal imaging by overcoming the limitations of the commercial LysoTrackers and highlights the unexplored application of DCM-NH₂ in bioimaging.

In Vivo Imaging and Tracking Carbon Monoxide-Releasing Molecule-3 with an NIR Fluorescent Probe

As a water-soluble carbon monoxide-releasing molecule, CORM-3 is widely used as a CO donor to study CO in the life system. CORM-3 can also replace gaseous CO as a therapeutic drug molecule to reveal the physiological and pathological effects of CO in life. Therefore, it is of great importance to visualize and track CORM-3 in the life system. We develop herein a near-infrared (NIR) fluorescent probe CORM3-NIR that can detect CORM-3 both in living cells and in vivo effectively. The probe is based on the unique fluorescent QCy7 and uses a 4-nitrobenzyl group to trap CORM-3, and importantly, it shows good water solubility and responds rapidly, selectively, and sensitively to CORM-3, releasing QCy-7 and producing distinct colorimetric and significant NIR fluorescence change signals at 743 nm. The Stokes shift is up to 81 nm. The probe is also able to detect CORM-3 ratiometrically with fluorescence at 743 and 600 nm. Besides, with low cytotoxicity, the probe also shows good NIR fluorescence bioimaging ability for CORM-3 in live cells and mice, which indicates that CORM3-NIR is an effective probe for tracking and studying CORM-3 in the life system.

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.

Highly specific nuclear labeling via in situ formation of fluorescent copper nanoparticles

When imaging cells, nuclear counterstaining is imperative; however, many commercial nuclear-staining dyes based on nucleic acid intercalation result in nonspecific signals in the cytoplasm. Here, we propose a new strategy that stains the nucleus with high specificity by in situ formation of DNA-templated copper nanoparticles (CuNPs). We demonstrated that genomic DNA in the nucleus enabled rapid formation of highly fluorescent CuNPs immediately following addition of a copper ion source and ascorbate as a reducing agent. Moreover, we found that RNA and mitochondrial DNA, largely responsible for nonspecific cytoplasmic signals from commercial nuclear-staining dyes, did not mediate the formation of the highly fluorescent CuNPs, resulting in highly specific nuclear staining at a reduced cost relative to commercially available methods. Furthermore, we verified the compatibility of the proposed method with other fluorescence-labeling techniques. These results demonstrated the efficacy of this method and its promise as a powerful tool for cell imaging.

Selective detection of peroxynitrite in living cells by a near-infrared diphenyl phosphinate-based dicyanoisophorone probe

A new NIR fluorescent probe for detection of ONOO^- has been developed, which possesses a large Stokes shift, good selectivity and low cytotoxicity. This NR-ONOO probe exhibits a strong turn-on near-infrared fluorescence response toward ONOO^- ion under excitation at 560 nm and has been successfully applied in detecting ONOO^- in living HeLa cells.

Design and application of near-infrared fluorophore based on a novel thiazolidinedione-functionalized dicyanoisophorone

A novel dicyanoisophorone (DCI)-based NIR fluorophore employing 2, 4-thiazolidinediones as the modification site was designed for fluorescence imaging. The fluorophore was assessed as a switchable reporter for H₂O₂ and the probe exhibited lysosomes-targeted, a large turn-on fluorescence signal at 720 nm with a large stokes shift (150 nm) and can be used in biological systems. The ability of the novel fluorophore to emit NIR fluorescence through a "turn-on" activation mechanism makes it a promising fluorophore for in vivo imaging applications. The strategy of introducing the thiazolidinediones with the easy modification site into the fluorophore has a good application prospect to expand the application of the NIR fluorophore.

A FRET-ICT Dual-Modulated Ratiometric Fluorescence Sensor for Monitoring and Bio-Imaging of Cellular Selenocysteine

Since the fluctuation of cellular selenocysteine (Sec) concentration plays an all-important role in the development of numerous human disorders, the real-time fluorescence detection of Sec in living systems has attracted plenty of interest during the past decade. In order to obtain a faster and more sensitive small organic molecule fluorescence sensor for the Sec detection, a new ratiometric fluorescence sensor Q7 was designed based on the fluorescence resonance energy transfer (FRET) strategy with coumarin fluorophore as energy donor and 4-hydroxy naphthalimide fluorophore (with 2,4-dinitrobenzene sulfonate as fluorescence signal quencher and Sec-selective recognition site) as an energy acceptor. The sensor Q7 exhibited only a blue fluorescence signal, and displayed two well distinguished emission bands (blue and green) in the presence of Sec with ∆λ of 68 nm. Moreover, concentrations ranging of quantitative detection of Sec of Q7 was from 0 to 45 μM (limit of detection = 6.9 nM), with rapid ratiometric response, high sensitivity and selectivity capability. Impressively, the results of the living cell imaging test demonstrated Q7 has the potentiality of being an ideal sensor for real-time Sec detection in biosystems.

Cyanine-based fluorescent indicator for mercury ion and bioimaging application in living cells

A commercial cyanine dye IR-780 and a thioether-containing dicarboxylic acid ligand were used to construct the near-infrared fluorescent probe, which was used as a near-infrared fluorescent indicator for the determination of mercury ions in water and in living cells. This indicator displayed high specificity towards Hg²⁺ without any interference from other detecting species. Especially, the emission at 790 nm dramatically increased more than 25 times after interacting with Hg²⁺. The binding experiment showed that the indicator formed 1:1 complex with Hg²⁺. More, this indicator could be applied in the visualization of Hg²⁺ in living cells and measuring the Hg²⁺ concentration of tap-water sample.

pH-responsive Ag2S nanodots loaded with heat shock protein 70 inhibitor for photoacoustic imaging-guided photothermal cancer therapy

Heat-treated cancer cells have thermo-resistance due to the up-regulated levels of heat shock proteins (HSP) resulting in low therapeutic efficiency and ineffective ablation of tumors. In this work, we report pH-responsive Ag₂S nanodots (Ag₂S NDs) loaded with HSP70 inhibitor (QE-PEG-Ag₂S) for enhanced photothermal cancer therapy. QE-PEG-Ag₂S was easily prepared via self-assembly of hydrophobic Ag₂S NDs, amphiphilic pH-responsive PEG_5k-PAE_10k polymer, and an HSP70 inhibitor quercetin (QE). QE-PEG-Ag₂S has ideal water-solubility and biocompatibility, can rapidly enter cells, and preferentially accumulate in cell lysosomes. The slightly acidic environment of tumor cells and the acidity of lysosomes as well as the high temperature generated by photothermal therapy under irradiation of NIR light (808 nm) promote the release of the inhibitor molecules to reduce the heat resistance of cancer cells and improve the in vivo photothermal therapy efficiency. Moreover, QE-PEG-Ag₂S has good photoacoustic imaging (PAI) ability; this QE-PEG-Ag₂S concentration dependent signal can precisely follow the accumulation of the nanomaterials in tumors and dictate the correct time for light therapy. As a result, QE-PEG-Ag₂S achieved complete tumor ablation effect with no recurrence when only irradiated with NIR light for 10 min. This approach offers a new approach for the theranostic applications of Ag₂S NDs. STATEMENT OF SIGNIFICANCE: In this work, pH-responsive Ag₂S nanodots loaded with the heat shock protein inhibitor for enhanced photothermal cancer therapy have been simply prepared via self-assembly process. This nanoagent possesses ideal water-solubility and biocompatibility, can rapidly enter cells, and preferentially accumulate in cell lysosomes. The acidic environment of tumor cells and the acidity of lysosomes, as well as the high temperature generated by photothermal therapy under irradiation of NIR light promote the release of the inhibitor molecules from the nanoagent to improve the in vivo photothermal therapy efficiency. Moreover, the photoacoustic imaging (PAI) of the nanoagent can precisely follow the accumulation of the nanomaterials in tumors and dictate the light therapy time to guarantee the complete tumor ablation effect with no recurrence.

Novel triphenylamine-based fluorescent chemo-sensors for fast detection of thiophenols in vitro and in vivo

A novel chemo-sensor based on triphenylamine derivative Probe-TPA for thiophenols (C₆H₅SH, p-NH₂-C₆H₄SH, p-OH-C₆H₄SH) detection was presented in this work. The target dye Probe-TPA displayed high selectivity and extremely fast response toward thiophenols in DMSO/PBS buffer (5/5, v/v) solution in the presence of other competitive species (such as K⁺, Na⁺, Ni²⁺, Fe³⁺, S^2-, HS^-, SO₄^2-, SO₃^2-, NaClO, H₂O₂, GSH, Cys, Hcy, etc.). The sensing property for thiophenols was studied by UV-Visible, fluorescence spectrophotometric analyses and DFT/TD-DFT calculations, those results indicated that the sensor Probe-TPA possessed high anti-interference ability, excellent sensitivity, higher specifity, dramatically "naked-eye" fluorescence enhancement (almost 200-folds) under 365 nm UV lamp, especially immediate response speed (within 15 s). In extended application aspect, the fluorescent chemo-sensor Probe-TPA could provide a new method of analysis to detect of thiophenol in real water samples and visualize monitoring in live cells with remarkable fluorescence variation.

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.

A water-soluble fluorescent probe for the detection of thiophenols in water samples and in cells imaging

Water pollution is the main cause of death of aquatic organisms such as fish et al. Content of thiophenols in water samples is an important indicator for assessing the degree of water pollution. The development of fluorescent probes with high selectivity and high sensitivity to detect thiophenols in water samples is extremely important in both environmental and life sciences. Although several fluorescent probes for thiophenols detection have been reported in recent years, most of them required the assistance of organic solvents to remedy the restriction caused by the poor water solubility of the probe, which did not fully reflect the actual situation of thiophenols in actual water samples. To fully overcome this shortage, we modified the 1,8-naphthylimide moiety with carboxyl to obtain a water-soluble fluorescent probe which could react with thiophenols specifically through nucleophilic aromatic substitution reaction (S_NAr) reaction with turn-on fluorescent responses. The corresponding detection limit was 71 nM. Supported by the spectroscopic changes, test strips based on the probe could detect thiophenols quantificationally and conveniently. At the same time, the probe could detect thiophenols in water sample with quantitative recovery. Besides, cell imaging experiments demonstrated the possibility of the probe to detect thiophenols in living cells.

A TP-FRET-based fluorescent sensor for ratiometric visualization of selenocysteine derivatives in living cells, tissues and zebrafish

Selenium is a biologically essential micronutrient element serving as an essential building block for selenoproteins (SePs), which is playing a key role in various cellular functions. Hence, it is of great significance to developing a reliable and rapid method for detection of Sec in biosystems. Compared with the previously reported probes that have been developed for selective detection of Sec, two-photon (TP) ratiometric Sec-specific probes would be advantageous for the NIR excitation and built-in correction of the dual emission bands. To quantitatively and selectively detect Sec over biothiols with rapid and sensitive response, we for the first time report a new fluorescence resonance energy transfer (FRET)-based TP ratiometric fluorescence probe CmNp-Sec, which was constructed by conjugating a TP fluorophore 6 (coumarin derivative with a D-π-A-structure) with a naphthalimide fluorophore 9 via a non-conjugated linker, and employed a 4-dinitrobenzene-ether (DNB) with a strong ICT effect as Sec responsive moiety. It exhibits quantitatively detect Sec in a wide range (0-50 μM) with a limit of detection of 7.88 nM within 10 min. More impressively, this probe can be conveniently used to detect Sec in living cells, tissues and zebrafish, demonstrating it has the latent capability in further biological applications.

Two isomeric and distinguishable H2S fluorescence probes for monitoring spoilage of eggs and visualizing exogenous and endogenous H2S in living cells

Accurate fabrication of fluorescence probes to efficiently monitor and detect H₂S levels in the fields of foodstuffs and physiology is crucial.

Glutathione Peroxidase-Activatable Two-Photon Ratiometric Fluorescent Probe for Redox Mechanism Research in Aging and Mercury Exposure Mice Models

Solid evidence confirms that glutathione peroxidase (GPx) is a kind of vital protease in the first-line antioxidant defense system and participates in regulation of redox homeostasis as well as the pentose phosphate pathway. However, the current methods cannot achieve real-time and in situ visualization studies of GPx. In addition, GPx is highly reactive and susceptible to external interference, and there is rare research for exploring the roles of GPx under environmental factor exposure. Herein, we report a novel two-photon ratiometric fluorescent probe (TP-SS) for GPx detection for the first time. Using TP-SS, we explore the reversible catalytic cycle and the antioxidant mechanisms of GPx/GSH redox pool in aging and mercury exposure models. We detect the concentration fluctuation of GPx in aging and mercury exposure mice models. Also, we perform GPx detection in deep brain tissue and the imaging depth up to 100 μm. We believe that the novel two-photon ratiometric fluorescent probe TP-SS can facilitate the development of GPx-targeting tools and offer great advances in exploring the physiological/pathological functions of GPx.

Bioluminescence Imaging of Selenocysteine in Vivo with a Highly Sensitive Probe

Selenocysteine (Sec), a vital member of reactive selenium species, is closely implicated in diverse pathophysiological states, including cancer, cardiovascular diseases, diabetes, neurodegenerative diseases, and male infertility. Monitoring Sec in vivo is of significant interest for understanding the physiological roles of Sec and the mechanisms of human diseases associated with abnormal levels of Sec. However, no bioluminescence probe for real-time monitoring of Sec in vivo has been reported. Herein, we present a novel bioluminescent probe BF-1 as an effective tool for the determination of Sec in living cells and in vivo for the first time. BF-1 has advantages of high sensitivity (a detection limit of 8 nM), remarkable bioluminescence enhancement (580-fold), reasonable selectivity, low cytotoxicity, and high signal-to-noise ratio imaging feasibility of Sec in living cells and mice. More importantly, BF-1 affords high sensitivity for monitoring Sec stimulated by Na₂SeO₃ in tumor-bearing mice. These results demonstrate that our new probe could serve as a powerful tool to selectively monitor Sec in vivo, thus providing a valuable approach for exploring the physiological and pathological functions and anticancer mechanisms of selenium.

A novel water-soluble naked-eye probe with a large Stokes shift for selective optical sensing of Hg2+ and its application in water samples and living cells

A water-soluble and colorimetric fluorescent probe with a large Stokes shift (139 nm) for rapidly detecting Hg2+, namely Hcy-mP, was synthesized by using an indole derivative and 2,4-dihydroxybenzaldehyde as starting materials. This probe demonstrates good selectivity for Hg2+ over other metal ions including Ag+, Pb2+, Cd2+, Cr3+, Zn2+, Fe3+, Co2+, Ni2+, Cu2+, K+, Na+, Mg2+, and Ca2+ in aqueous solution. With the increase in concentration of Hg2+, the color of the solution changed from pale yellow to pink and the fluorescence intensity decreased slightly. When 5-equivalents of EDTA were added to the solution with Hg2+, the fluorescence intensity of this probe was restored. The probe has been applied to the detection of Hg2+ in real water samples. Moreover, this probe was confirmed to have low cytotoxicity and excellent cell membrane permeability. The effect of Hcy-mP-Hg2+ towards living cells by confocal fluorescence was also investigated.

Engineering Self-Calibrating Nanoprobes with Two-Photon-Activated Fluorescence Resonance Energy Transfer for Ratiometric Imaging of Biological Selenocysteine

Selenocysteine (Sec) has proven to be the dominant active site of diverse selenoproteins that are directly linked with human health and disease. Thus, understanding the critical functions and dynamics of endogenous Sec at cellular and tissue levels is highly demanded. However, no method has been reported that is capable of providing reliable quantitative imaging analysis of Sec in living systems, especially in deep tissues, with low background signal and high sensitivity and imaging resolution simultaneously. To address this challenge, we herein report a novel class of engineered Sec-responsive fluorescent nanoprobes that combines two-photon excitation with Förster resonance energy transfer (FRET) mechanisms for direct, yet selective, sensing and imaging of biological Sec over abundant competing biothiols. Specifically, the two-photon excitation at the near-infrared window can minimize light scattering and background signals in tissues, thus offering improved spatial and temporal imaging of deep living tissues with reduced background interference. Moreover, a reasonable FRET donor-acceptor pair has further been designed and verified by theoretical calculation. The acceptor undergoes intramolecular rearrangement specifically in response to the nucleophilic attack of Sec, hence triggering remarkable FRET-mediated ratiometric fluorescence enhancement for sensitive and reliable quantification of Sec through self-calibration of two emission channels. These striking properties, along with good water solubility and biocompatibility, suggest that this strategy may serve as a valuable imaging tool for studying various Sec-related biological events in complex biological systems.

Rational design of NIR fluorescence probes for sensitive detection of viscosity in living cells

Developing near-infrared (NIR) fluorescence probes for detection of intracellular viscosity is still sufficiently challenging. In this work, three kinds of D-A-D type naphthyl and 2,1,3‑benzoxadiazol hybrid NIR dyes functionalized with amino (NY1), N‑methylamino (NY2) and N,N‑dimethylamino (NY3) groups for intracellular micro-viscosity detection were designed and synthesized. All the probes exhibited very weak NIR emission in low viscosity environment and obvious fluorescence enhancement with the increased viscosity. Different substituent groups had a high impact on the photophysical properties and response sensitive of the probes to viscosity. The structure-property relationships were systematic investigated. The results showed that stronger electron-donating ability and larger steric effect of N,N‑dimethylamino led to a narrower energy gap and more sensitive to viscosity environment. Therefore, NY3 exhibited higher signal noise ratio for viscosity detection and was successfully applied for imaging the changes of intracellular micro-viscosity. This work provides an efficient way to design powerful NIR fluorescence probes for viscosity detection.

A near-infrared fluorescent probe for imaging endogenous carbon monoxide in living systems with a large Stokes shift

Near-infrared (NIR) fluorescent probes with a large Stokes shift are very practical tools for bioimaging applications. Carbon monoxide (CO) is a key gaseous signal molecule and its imaging in living systems has attracted great attention in recent years. In this work, a very easy-to-get NIR fluorescent probe with a remarkable large pseudo-Stokes shift (238 nm) for detection of CO was reported. This probe was found to show a rapid NIR fluorescent turn-on response for CO with high selectivity, high sensitivity and a low detection limit (38 nM). Moreover, imaging CO in living cells and animals with this probe was successfully applied with a high signal-to-noise ratio. The results indicate that this probe can be used as a new practical tool for imaging of endogenous CO in living systems.

ESIPT-based fluorescent probe for cysteine sensing with large Stokes shift over homocysteine and glutathione and its application in living cells

An HBT-based fluorescent probe for Cys with a large Stokes shift and high selectivity was developed that operates by the ESIPT process.

Inhibitor structure-guided design and synthesis of near-infrared fluorescent probes for monoamine oxidase A (MAO-A) and its application in living cells and in vivo

A series of near-infrared fluorescent probes based on inhibitor (clorgyline) structure-guided design were synthesized for the specific detection of MAO-A in cells and in vivo.

A coumarin-based dual optical probe for homocysteine with rapid response time, high sensitivity and selectivity

In this study, a new coumarin-based fluorescent and chromogenic dual channel probe (DC) was used for the selective detection of homocysteine (Hcy) over other amino acids, especially for cysteine (Cys) and glutathione (GSH). When Hcy is present in the solution, the remarkable fluorescence enhancement and obvious blue shift in UV-vis spectra can be observed. In addition, the color change from purple to yellow can be observed clearly by unaided eyes. This probe DC has fast response time, excellent sensitivity and selectivity to Hcy. A linear relationship exists between the ratio of emissions at 486 and 625 nm, and Hcy can be detected in a wide concentration range (0-200 μM). The signal-to-background ratio of fluorescence at 486 nm can reach 8.4, and the detection limit is calculated to be 3.5 µM. The response mechanism is proved to be the Michael addition reaction by Hcy. Preliminary results on cell imaging enable the practical application of Hcy tracing in living cells.

A reaction-based near-infrared fluorescent probe that can visualize endogenous selenocysteine in vivo in tumor-bearing mice

Monitoring the fluctuations of endogenous selenocysteine (Sec) in vivo is of significant interest to understand the physiological roles of Sec and the mechanisms of Sec-relevant diseases. Herein, a new near-infrared fluorescent probe, Fsec-1, has been developed for the determination of endogenous Sec in living cells and in vivo. Fsec-1 exhibits large fluorescence enhancement (136-fold) and a remarkably large Stokes shift (195 nm) when reacted with Sec. With the advantages of high sensitivity (a detection limit of 10 nM), good selectivity and low cytotoxicity, Fsec-1 was able to recognize both exogenous and endogenous Sec in living cells. The probe was also successfully applied in visualizing both exogenous and endogenous Sec in living mice. Notably, endogenously generated Sec in living tumors xenografted in nude mice was selectively detected by our reaction-based NIR probe for the first time. These results indicated that our new probe could serve as an efficient tool in monitoring endogenous Sec in vivo and exploring the anticancer mechanism of selenium.