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

Eu/Tb-MOF as fluorescence sensors for the detection homocysteine in human serum performance and mechanistic investigation

Abnormal homocysteine (Hcy) levels in human serum have been associated with serious or vital diseases, making the reliable and easy detection of Hcy important to clinical analysis and biological study. In this work, five phosphorescent Ir(C^N)2(N^N) complexes (Irn) having aldehyde group were synthesized as probes (C^N and N^N denoted ligands). A discussion was conducted on their molecular structure, electronic structure, photophysical parameters, and Hcy sensing ability, revealing the correlations between their molecular structures and performances. Irn emission was enhanced (by ∼ two folds) and blue-shifted (by 100 nm) after meeting Hcy (free state), via a cyclization reaction between the -CHO group (from Irn) and Hcy. In addition, using RE(BTC) as a supporting material (RE = Tb and Eu), the Ir(III) probe was loaded onto a supporting material of RE(BTC) (H3BTC = 1, 3, 5-benzenetricarboxylic acid). The emission color was changed by increasing Hcy concentration. Straight working curves were obtained with LOD (limit of detection) of 1.9 μM and a response time of ∼200 s. The novelty of this work was the combination of Irn with RE(BTC), which offered enhanced and blue-shifted emission upon Hcy via a cyclization reaction. This demonstrated a high level of sensitivity towards homocysteine detection.

Synthesis and performance of a nanosensing platform for homocysteine detection: A series of iridium(III) complexes containing aldehyde group as probe and MOF as supporting substrate

The low cost and simple detection method for Hcy (homocysteine) is highly desired in analytical and biological fields since Hcy has been regarded as a bio-marker for multiple diseases. In this work, five Ir(C^N)2(N^N)+ compounds having -CHO group in their C^N or N^N ligand were synthesized and tried for Hcy sensing. Electron-donating groups such as -NH2 and -CH3 were incorporated into the C^N or N^N ligand. Their geometric structure, electronic structure, and optical parameters (with or without Hcy) were analyzed and compared carefully to explore their Hcy sensing potential. The sensing mechanism was revealed by NMR titration and theoretical simulation as a cyclization reaction between the -CHO group and Hcy. The optimal compounds, which showed increased emission quantum yield (2.5-fold) and emission blue-shift (by ∼ 100 nm) upon Hcy, were then covalently grafted into a porous host bio-MOF-1. Linear working plots were fitted, with good selectivity, LOD of 0.15 μM, and response time of 33 s. The novelty of this work was the eye-sensitive emission color change of this nanosensing platform from red (without Hcy) to green (with Hcy).

A novel sensitive fluorescent probe with double channels for highly effective recognition of biothiols

Biothiols play a crucial role in maintaining redox balance in organisms, and anomalous levels of biothiols in human organs can lead to various sicknesses and biological disorders. This work developed a novel sensitive fluorescent probe TZ-NBD with double channels for highly efficient recognition of biothiols. TZ-NBD adopts 4-Chloro-7-nitrobenzofurazan (NBD-Cl) as the recognition moiety with simultaneous fluorescence output. By incorporating NBD-Cl with the other fluorophore, benzothiazole dihydrocyclopentachromene derivative (TZ-OH), the dual-channel sensitive fluorescence probe TZ-NBD was built. The existence of Cys/ Hcy could significantly trigger both the green and red fluorescent emissions, which were derived from fluorophores amine-substituted NBD and TZ-OH, respectively. While exposing to GSH, only the red-channel fluorescence signal could be detected, indicating the release of TZ-OH. The phenomena was mainly attributed to the fact that sulfur-substituted NBD has nearly no fluorescence, while amine-substituted NBD shows obvious green fluorescence. In our study, TZ-NBD exhibited dual-channel sensitivity, fast response, and excellent selectivity to biothiols in vitro. Moreover, TZ-NBD was favorably utilized for recognition of biothiols in vivo. We believe that the sensitive fluorescence probe with double channels can afford an alternate approach for monitoring biothiols in organisms and would be useful for studying diseases associated with biothiols.

Activatable fluorescent probes for imaging and diagnosis of rheumatoid arthritis

Rheumatoid arthritis (RA) is a systemic autoimmune disease that is primarily manifested as synovitis and polyarticular opacity and typically leads to serious joint damage and irreversible disability, thus adversely affecting locomotion ability and life quality. Consequently, good prognosis heavily relies on the early diagnosis and effective therapeutic monitoring of RA. Activatable fluorescent probes play vital roles in the detection and imaging of biomarkers for disease diagnosis and in vivo imaging. Herein, we review the fluorescent probes developed for the detection and imaging of RA biomarkers, namely reactive oxygen/nitrogen species (hypochlorous acid, peroxynitrite, hydroxyl radical, nitroxyl), pH, and cysteine, and address the related challenges and prospects to inspire the design of novel fluorescent probes and the improvement of their performance in RA studies.

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

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

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 simple lysosome-targeted fluorescent probe based on flavonoid for detection of cysteine in living cells

Cysteine (Cys) is one of the most important biothiols that plays a crucial role in many physiological and pathological processes, and therefore it is of great importance to detect and analyze Cys in subcellular environments, such as in lysosomes. However, only a few fluorescent probes were reported to be capable of detecting Cys in lysosomes selectively. In this wok, we designed and developed a simple, accessible flavone-based fluorescent probe LFA for detecting Cys in lysosomes. Morpholine was employed as the targeting unit for lysosome, and acrylate group was chosen as the Cys-response unit. The probe was easily prepared by a two-step procedure and displayed large Stokes shift, high sensitivity, turn-on response toward Cys over homocysteine (Hcy), glutathione (GSH), and other amino acids. With low cytotoxicity and good cell permeability, the probe could be successfully applied for fluorescence imaging of Cys in living cells. Furthermore, colocalization experiment revealed that lysosomal-targetable ability of LFA was significant. These results indicated that such simple fluorescent probe could provide a promising tool for detection of lysosomal Cys in living biological systems.

Schiff Base Aggregation-Induced Emission Luminogens for Sensing Applications: A Review

Fluorescence sensing can not only identify a target substrate qualitatively but also achieve the purpose of quantitative detection through the change of the fluorescence signal. It has the advantages of immense sensitivity, rapid response, and excellent selectivity. The proposed aggregation-induced emission (AIE) concept solves the problem of the fluorescence of traditional fluorescent molecules becoming weak or quenched in high concentration or aggregated state conditions. Schiff base fluorescent probes have the advantages of simple synthesis, low toxicity, and easy design. They are often used for the detection of various substances. In this review we cover late developments in Schiff base compounds with AIE characteristics working as fluorescence sensors.

A Novel Reversible Fluorescent Probe for Cu2+and S2-Ions and Imaging in Living Cells

A novel fluorescent probe TSOC (thiazole salicylaldehyde oxazole chlorinated) was synthesized based on benzothiazole conjugated olefinic bond with salicylicaldehyde unit as fluorophore and a phenyl oxazole unit as bonding unit. The probe could reversibly detect of Cu2+ and S2- over other common ions with longer emission and large stokes shift in an aqueous solution at pH 7.3 (DMSO-Hepes, v/v, 5:1, 10 mM). The bonding mechanism was supported through the titration experiment of fluorescence and absorption spectroscopy, 1H-NMR titration, HR-MS and DFT calculations. Moreover, the probe further exhibited good cell permeability and were successfully used to visualize Cu2+ and S2- in living cells.

Research progress in pharmacological activities and structure-activity relationships of tetralone scaffolds as pharmacophore and fluorescent skeleton

The tetralone and tetralone derivatives, as crucial structural scaffolds of potential novel drugs targeted at multiple biological end-points, are normally found in several natural compounds and also, it can be used as parental scaffold and/or intermediate for the synthesis of a series of pharmacologically active compounds with a broad-spectrum of bioactivities including antibacterial, antitumor, CNS effect and so on. Meanwhile, SAR information of its analogues has drawn attentions among medicinal chemists, which could contribute to the further research related to tetralone derivatives aimed at multiple targets. This review encompasses pharmacological activities, SAR analysis and docking study of tetralone and its derivatives, expecting to provide a general retrospect and prospect on tetralone derivatives.

A fluorescent probe with dual acrylate sites for discrimination of different concentration ranges of cysteine in living cells

Monitoring of cysteine (Cys) is of significant importance for studying Cys-involved biological functions and clinically diagnosing Cys-related diseases. Recently, few fluorescent probes with two different reacting sites were reported to be capable of sensing different concentration ranges of Cys with distinct fluorescence signals, particularly suiting for bioimaging. However, due to relative sophisticated synthesis and moderate selectivity, the applications of these probes were still severely restricted. In this work, we proposed a novel probe design strategy by utilizing two same reacting groups, instead of two different reacting groups, to simplify the synthesis route and minimize the interference from competing species. Same reacting groups in a probe with different steric hindrances could exhibit different reactivities to Cys. This probe showed distinguishable fluorescence peak wavelengths towards low and high concentration ranges of Cys, giving green and blue emissions, respectively. Moreover, this probe was successfully applied for monitoring of Cys concentration in living cells. We believe this work provided a simpler strategy for dual-site fluorescent probes to sense difference concentration ranges of Cys, which may inspire more probe design in future.

An insight into the in vivo imaging potential of curcumin analogues as fluorescence probes

Curcumin and its derivatives have good electrical and optical properties due to the highly symmetric structure of delocalized π electrons. Apart from that, curcumin and its derivatives can interact with numerous molecular targets, thereby exerting less side effects on human body. The fluorescence emission wavelength and fluorescence intensity of curcumin can be enhanced by modifying its π-conjugated system and ß-diketone structure. Some curcumin-based fluorescent probes have been utilized to detect soluble/insoluble amyloid-ß protein, intracranial reactive oxygen species, cysteine, cancer cells, etc. Based on the binding characteristics of curcumin-based fluorescent probes with various target molecules, the factors affecting the fluorescence intensity and emission wavelength of the probes are analyzed, in order to obtain a curcumin probe with higher sensitivity and selectivity. Such an approach will be greatly applicable to in vivo fluorescence imaging.

A lysosome-targetable two-photon excited near-infrared fluorescent probe for visualizing hypochlorous acid-involved arthritis and its treatment

Lysosome of phagocyte is the main site of hypochlorous acid (HClO) production, and HClO can be employed as the biomarker for the diagnosis and treatment evaluation of arthritis. In recent years, developing fluorescent probes for lysosomal HClO has attracted considerable attention, but most of them still have some defects, such as autofluorescence, phototoxicity and photobleaching because of their excitation and emission located in short-wavelength region. Due to the advantages of two-photon fluorescent probes with near-infrared emissions, a lysosome-targetable two-photon fluorescent probe (Lyso-TP-HClO) with a near-infrared emission was reported in this paper. Lyso-TP-HClO has a high selectivity and a high sensitivity to HClO in the linear range (10.0 × 10^-8 to 5.0 × 10^-6 M), with a detection limit of 3.0 × 10^-8 M. Due to the two-photon excited near-infrared emission, Lyso-TP-HClO has excellent imaging performances, such as small autofluorescence, excellent photostability, and large imaging depth. Furthermore, Lyso-TP-HClO was successfully employed for visualizing lysosomal HClO in bacteria-infected cells. At last, we have successfully used Lyso-TP-HClO to image the arthritis and evaluate the treatment of arthritis in mice. All the results confirm that Lyso-TP-HClO is a useful chemical tool for imaging of lysosomal HClO, the diagnosis of arthritis, and treatment evaluation of arthritis.

Fluorescent small organic probes for biosensing

Small-molecule based fluorescent probes are increasingly important for the detection and imaging of biological signaling molecules due to their simplicity, high selectivity and sensitivity, whilst being non-invasive, and suitable for real-time analysis of living systems. With this perspective we highlight sensing mechanisms including Förster resonance energy transfer (FRET), intramolecular charge transfer (ICT), photoinduced electron transfer (PeT), excited state intramolecular proton transfer (ESIPT), aggregation induced emission (AIE) and multiple modality fluorescence approaches including dual/triple sensing mechanisms (DSM or TSM). Throughout the perspective we highlight the remaining challenges and suggest potential directions for development towards improved small-molecule fluorescent probes suitable for biosensing.

Lysosome-Targeted Gold Nanotheranostics for In Situ SERS Monitoring pH and Multimodal Imaging-Guided Phototherapy

The integration of surface-enhanced Raman spectrum (SERS) and fluorescence-photoacoustic multimodal imaging in near-infrared photothermal therapy is highly desirable for cancer theranostic. However, typically, gold nanotheranostics usually require an additional modification of fluorophores and complex design refinements. In this work, by integrating surface-modified cysteine-hydroxyl merocyanine (CyHMC) molecules onto AuNRs, a novel lysosome-targeted gold-based nanotheranostics AuNRs-CyHMC that combines the specificity of Raman spectrum, the speed of fluorescence imaging, and deep penetration of photoacoustic imaging was successfully fabricated. Interestingly, fluorescence and Raman signals in this AuNRs-CyHMC system do not interfere, but it has pH-sensitive Raman signals and self-fluorescence localization ability under different excitation wavelengths. Fluorescence co-localization experiments further confirmed the lysosome-targeting ability of AuNRs-CyHMC. Typically, the proposed nanotheranostics were capable of SERS monitoring pH changes in both phosphate-buffered saline and living cells. Meanwhile, in vitro and in vivo experiments revealed that AuNRs-CyHMC possessed excellent fluorescence-photoacoustic performance and could be used for multimodal imaging-guided photothermal therapy. Furthermore, our work implied that gold nanotheranostics can provide great potential for cancer diagnosis and treatment.

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.

A novel fluorescent probe with dual-sites for simultaneously monitoring metabolisms of cysteine in living cells and zebrafishes

Understanding cellular metabolism holds immense potential for developing new drugs that regulate metabolic pathways. Two gas signal molecules, SO₂ and H₂S, are the main metabolites from cysteine (Cys) via oxidation and desulfurization pathways, respectively. However, a few fluorescent probes for real-time monitor of the metabolic pathways of cysteine have been reported. To understand metabolic alterations of cysteine, we have rationally designed and prepared a dual-signal fluorescent probe HN, which could differentiate SO₂ and H₂S through two different fluorescence channels simultaneously, along with similar reaction kinetics and both "off-on" fluorescence responses. Probe HN exhibits the potential to monitor the metabolism pathways of cysteine, and the distinguishment of cancer cells from normal cells could be realized. This methodology will promote further understanding of the physiological and pathological roles of cysteine.

A two-photon excited near-infrared fluorescent probe for imaging peroxynitrite during drug-induced hepatotoxicity and its remediation

Drug-induced liver injury (DILI) has been a hot issue of public health, owing to its unpredictability and serious harm to public health. Peroxynitrite (ONOO^-) is an important biomarker for the assessment and diagnosis of DILI. In this article, based on a kind of rhodamine analogue with a near-infrared (NIR) emission (610 nm-800 nm) and a two-photon absorption cross section (54 GM), a two-photon excited NIR fluorescence probe (NIR-ONOO) for ONOO^- was developed. With a high selectivity and a high sensitivity to ONOO^-, NIR-ONOO has a linear range for detection of ONOO^- from 5.0 × 10^-8 to 1.0 × 10^-5 M, a good detection limit (15 nM) and a large fluorescence enhancement (340-fold). In addition, NIR-ONOO has been used to monitor ONOO^- in cells with satisfactory results. Because of its two-photon excied NIR emission, NIR-ONOO also showed excellent performances for imaging ONOO^- including low autofluorescence, stable and persistent fluorescence, and a deep penetration (204 μm). Finally, NIR-ONOO was successfully employed to image ONOO^- in inflammatory mouse, drug-induced hepatotoxicity in cells and its remediation. All the results indicated that NIR-ONOO is a powerful chemical tool to image ONOO^- and assay drug-induced hepatotoxicity.

In Vivo Imaging of Senescent Vascular Cells in Atherosclerotic Mice Using a β-Galactosidase-Activatable Nanoprobe

Senescence-associated diseases have severely diminished the quality of life and health of patients. However, a sensitive assay of these diseases remains limited due to a lack of straightforward methods. Considering that senescence-associated β-galactosidase (SA-β-Gal) is overexpressed in senescent cells, the detection of SA-β-Gal in senescent cells and tissues might be a feasible strategy for the early diagnosis of SA diseases. In this study, a β-galactosidase-activatable nanoprobe BOD-L-βGal-NPs was developed for the imaging of senescent cells and vasculature in atherosclerotic mice via real-time monitoring of β-Gal. BOD-L-βGal-NPs was fabricated by encapsulating a newly designed NIR ratiometric probe BOD-L-βGal within a poly(lactic-co-glycolic) acid (PLGA) core. Nanoprobe BOD-L-βGal-NPs showed good accumulation in arteries, thus successfully visualizing senescent cells and vasculature in atherosclerotic mice by tail vein injection. Our findings indicated that nanoprobe BOD-L-βGal-NPs holds great potential for the early diagnosis and therapy of atherosclerosis and other aging-associated diseases.

A new pentacyclic pyrylium fluorescent probe that responds to pH imbalance during apoptosis

Efficient fluorophores with easy synthetic routes and fast responses are of great importance in clinical diagnostics. Herein, we report a new, rigid pentacyclic pyrylium fluorophore, PS-OMe, synthesised in a single step by a modified Vilsmeier-Haack reaction. Insights into the reaction mechanism facilitated a new reaction protocol for the efficient synthesis of PS-OMe which upon demethylation resulted in a "turn-on" pH sensor, PS-OH. This new fluorescent probe has been successfully used to monitor intracellular acidification at physiological pH. From the fluorescence image analysis, we were able to quantify the intracellular dynamic pH change during apoptosis. This new pH probe is a potential chemical tool for screening, drug discovery and dose determination in cancer therapy.

Sensing mechanism of a fluorescent probe for thiophenols: Invalidity of excited-state intramolecular proton transfer mechanism

Simple and effective detection of thiophenols has attracted great attention. A fluorescent probe 1 with high selectivity and sensitivity is designed and synthesized based on the excited-state intramolecular proton transfer (ESIPT) in experiment. However, we conclude that the ESIPT process fails to happen actually based on the calculation results. In the present work, the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods are employed to investigate the real sensing mechanism. The calculated absorption and emission spectra agree well with the experimental results. By comparing the energy of enol and keto configurations and the constructed potential energy surfaces (PESs) in the ground (S₀) and excited (S₁) states of 3-(benzo[d]thiazol-2-yl)-10-butyl-10H-phenothiazin-2-ol (dye 2), the ESIPT process is confirmed impossible because of the relatively high keto form energy and potential energy barrier. Besides, the transition state of dye 2 is optimized to offer the accurate potential energy barrier. The results of calculated frontier molecular orbitals (FMOs) and spectra indicate that it is the photoinduced electron transfer (PET) process that results in the fluorescence quenching of probe 1. After adding thiophenols, the thiolysis of 2,4-dinitrophenyl ether bond is triggered and dye 2, which emits strong fluorescence because of the absence of PET process, is obtained. Consequently, our study has demonstrated that probe 1 can act as a fluorescent probe to detect thiophenols through the off-on fluorescence variation based on the PET mechanism but not the ESIPT process.

A bond energy transfer based difunctional fluorescent sensor for Cys and bisulfite

In living cells, cysteine (Cys) and bisulfite are involved in many important physiological processes. Their unbalance in vivo would lead to multiple diseases. So, it is vital to develop difuntional sensor for Cys and bisulfite. As we known, cysteine could metabolized into bisulfite by the metabolic processes of cysteine in the animal level. Therefore, we designed and synthesized a mitochondria-targeted long-wavelength ratio fluorescence sensor Z2 for Cys and bisulfite simultaneous detection. Z2 exhibitted excellent selectivity, good anti-interference, fast response and low detection limit. The sensor exhibited obviously two channels fluorescence response for Cys and bisulfite orderly. Z2 is widely used for imaging Cys and bisulfite in MCF-7 cells, zebrafish, and mice, and successfully imaging Cys metabolism in these livings. We hope this bifunctional ratio fluorescence sensor Z2 will be a useful tool to monitor Cys and SO₂ levels in living systems.

A long-lived Donor-Acceptor fluorescent probe for sequential detection of Cu2+ and biothiols

A new long-lived Donor-Acceptor (D-A) fluorophore based on carbazolyl dicyanobenzene was developed as an ON-OFF-ON multifunctional fluorescent probe 1 for sequential detection of Cu²⁺ and biothiols (Cys, Hcy and GSH). The fluorescence of probe 1 can be significantly and selectively quenched by Cu²⁺. Meanwhile, the fluorescence lifetime decreased from 2.1 μs to 18.5 ns. The limit of detection was determined to be 33.6 nM. Upon addition of biothiols (Cys, Hcy and GSH), the generated ensemble 1-Cu²⁺ displayed a "turn-on" fluorescent response at 555 nm and an obvious recovery in fluorescence lifetime and UV-vis absorption within 1 min. The limit of detection for Cys, Hcy and GSH were calculated by fluorescence titration experiments to be 0.19, 0.21 and 0.29 μM, respectively. The ensemble 1-Cu²⁺ was further successfully applied in bioimaging.

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.

Development of a fast-responsive and turn on fluorescent probe with large Stokes shift for specific detection of cysteine in vivo

Cysteine has a great effect on the physiological and pathological processes, which could bring out various diseases such as skin lesions, edema, hair depigmentation, Alzheimer's, Parkinson's, and liver damage due to the abnormal concentrations of cysteine. Therefore, it is of great impoatance to develop a method for imaging Cys. Herein, a novel fluorescent probe was developed for imaging Cys in vivo specially. This turn-on probe exhibited favorable advantages including large Stokes shift (90 nm), fast response (10 min), good selectivity, low cytotoxicity and so on. Furthermore, the probe could be applied to monitoring cysteine in living HeLa cells, which indicates that this turn-on probe could penetrate viable cell membranes and image Cys over other analystes especially HCy and GSH.

A novel berberine-based colorimetric and fluorimetric probe for hydrazine detection

Hydrazine in water and soil has caused serious diseases for human health. In this work, a simple fluorescent probe (BP) for hydrazine detection was synthesized from berberine. The probe has excellent fluorescence properties and naked-eye detection.

A near-infrared biothiol-specific fluorescent probe for cancer cell recognition

Cancer is a global health issue and a leading cause of death. The discrimination of cancer cells from normal cells is of significant importance for the early diagnosis of cancers. As one of the useful biomarkers for developing cancer diagnosis and chemotherapy resistance systems, biothiols not only play an essential role in physiological and pathological processes but also exhibit cytoprotective effects in the susceptibility to carcinogenesis. It would be highly desirable to explore near-infrared biothiol-specific fluorescent probes for cancer diagnosis with outstanding specificity. In this study, a novel near-infrared fluorescent probe BPO-THAZ decorated with thiazole as a recognition site was presented for sensitive and selective detection of endogenous biothiols. BPO-THAZ can be used to not only evaluate the biothiol level in living HeLa cells upon treatment with H2O2 or anti-cancer drugs but also assess endogenous biothiols in stem cells. Furthermore, BPO-THAZ was successfully utilized to discriminate cancer cells from normal cells showing great promise for cancer diagnosis.

A mitochondria-targeted fluorescent probe based on coumarin-pyridine derivatives for hypochlorite imaging in living cells and zebrafish

Hypochlorite plays a critical role in various physiological processes and is involved in many diseases. Thus, real-time, rapid, and accurate monitoring of hypochlorite has important medical and physiological significance. Herein, a novel coumarin-pyridine derivative (CPD) probe was designed and synthesized, which exhibited fantastic advantages, such as a rapid response (within 10 s), naked eye recognition, large Stokes shift (185 nm), dual-channel detection, and high selectivity and sensitivity toward OCl^- (detection limit 0.012 μM, S/N = 3). Furthermore, the current CPD probe was successfully used to image OCl^- in the mitochondria of both A549 cells and zebrafish, which further demonstrated its suitability for practical applications in biological systems.

A mitochondria-targetable two-photon fluorescent probe with a far-red to near-infrared emission for sensing hypochlorite in biosystems

Hypochlorite (ClO^-), one of reactive oxygen species (ROS), is closely related with many physiological and pathological processes. Especially as one of cellular reactive oxygen species in mitochondria, ClO^- can induce mitochondrial permeability, which leads to apoptosis. Thus, developing an effective method which is able to sense ClO^- in mitochondria is important. Although fluorescent probe has become a powerful tool for imaging ClO^- in mitochondria, most of them suffered from phototoxicity to biosamples, autofluorescence, and photobleaching phenomenon due to their short-wavelength excitations and emissions. Based on advantages of two-photon fluorescent probe and far-red to NIR fluorescent probe, a mitochondria-targetable two-photon fluorescent probe with a turn-on signal in far-red to NIR region, Mito-TP-ClO, was developed for ClO^- in this paper. Mito-TP-ClO is consisted of a triphenylphosphonium cations as a mitochondria-targetable unit and a structure of dibenzoylhydrazine as a response unit to ClO^-. Mito-TP-ClO exhibited a high sensitivity and a high selectivity to ClO^-, with a linear range from 6.0 × 10^-8 to 1.0 × 10^-5 M and a detection limit of 2.5 × 10^-8 M. Due to its large two-photon cross section (267 GM) and far-red to NIR emission, Mito-TP-ClO exhibits excellent performances including low autofluorescence, photostable fluorescence signal, and deep tissue penetration (230 μM). Moreover, Mito-TP-ClO was successfully used to detect endogenous ClO^- in bacteria-infected cells and inflammatory mouse model, which confirmed that Mito-TP-ClO is a powerful tool to monitor ClO^- in mitochondria and study on effects of hypochlorite on mitochondria.

Evaluation of HOCl-generating anticancer agents by an ultrasensitive dual-mode fluorescent probe

Hypochlorous acid (HOCl), a reactive oxygen species (ROS), plays a crucial role in the process of pathogenic oxidative stress. Some powerful anticancer agents, such as elesclomol, specifically induce cancer cell apoptosis by increasing HOCl levels. However, sensitive tools to monitor subtle changes of biological HOCl in vivo are limited. To achieve this, we herein present rationally designed probes C1-C7 through introducing a bioorthogonal dimethylthiocarbamate receptor. All the probes were shown to sensitively and rapidly detect HOCl in the nanomolar/biologically relevant concentration range with fluorescence turn-on observed in their respective optical regions, resulting in a blue-to-red "fluorescence rainbow" and providing a broad selection of colors for imaging HOCl in vivo. Remarkably, probe C7 exhibited both a turn-on signal at biologically relevant concentrations (LOD1 = 18 nM) and a ratiometric response at the high risk pathogenic concentrations (LOD2 = 0.47 μM), which gives a higher reliability compared to a single signal and avoids cross-talk caused by the combined use of several probes. C7 was used to monitor the oxidative stress process induced by elesclomol in live cancer cells, and using this probe it was further discovered that an evodiamine derivative was capable of generating cancer-cell HOCl.

A chromenoquinoline-based two-photon fluorescent probe for the highly specific and fast visualization of sulfur dioxide derivatives in living cells and zebrafish

Sulfur dioxide (SO₂) derivatives play critical roles in various biological processes. Therefore, effective methods for monitoring SO₂ are of vital importance in bisulfite/sulfite biology. In this study, a two-photon (TP) imaging probe (CQ-SO₂) for detecting SO₂ derivatives was designed and constructed, based on the chromenoquinoline (CQ) fluorophore and a β-chlorovinyl aldehyde sensing moiety. The TP properties of the CQ derivatives were revealed for the first time in this study. This study enriched the biological application range of CQ derivatives and also provided a new choice for the development of TP dyes. In particular, the CQ-SO₂ probe exhibited a fast response time (about 5 s), low detection limit (16 nM) and ultrahigh specificity towards SO₂ derivatives. Furthermore, the probe was successfully applied to the highly specific TP bioimaging of SO₂ derivatives in living cells and zebrafish.

Broadband optical waveguide modulators based on strongly coupled hybrid graphene and metal nanoribbons for near-infrared applications

In this paper, we numerically demonstrate a variety of broadband optical waveguide modulators based on the hybrid surface plasmon polariton (HSPP) concept for near-infrared applications. The modulator is composed of strongly coupled double-layer graphene and double rectangle cross-sectional metal nanoribbons separated by three Al2O3 spacers, which are interpolated in a SiO2 waveguide. Owing to the unique strong coupling of HSPPs between metal nanoribbons, the subwavelength confinement, the in-plane electric field component, the light-graphene interaction, and the modulation effect of the modulator are significantly enhanced. The results show the proposed modulator achieves an outstanding performance with a modulation depth (MD) over 2.3 dB μm-1 and a small normalized mode area of ∼10-5 in a wide range of wavelength from 1.3 to 1.8 μm. By optimizing the separation of the double rectangle metal nanoribbons at the telecommunication wavelength of 1.55 μm, the modulator exhibits a high MD of 3.12 dB μm-1, a small footprint of 1.8 μm2, an ultra-wide 3 dB modulation bandwidth of 380.23 GHz, and an ultra-low energy consumption of 29.39 fJ per bit. Furthermore, we also demonstrate a modulator based on two properly apart semicircular (rhombus) metal nanoribbons with a drastically enhanced MD of 11.3 (6.32) dB μm-1 at 1.55 μm. Benefitting from the strong subwavelength confinement and excellent broadband modulation performance, the proposed optical waveguide modulators offer a significant potential to realize various long-wave near-infrared integrated modulators, interconnects and optoelectronic devices.

Lysosome-Targeted Single Fluorescence Probe for Two-Channel Imaging Intracellular SO₂ and Biothiols

As the members of reactive sulfur species, SO₂ and biothiols play a significant role in physiological and pathological processes and directly influence numerous diseases. Furthermore, SO₂ and biothiols can provide a reductive environment for lysosomes to carry out their optimal functionality. To this end, the development of single fluorescent probes for imaging SO₂ and biothiols from different emission channels is highly desirable for understanding their physiological nature. Here, a lysosome-targeted fluorescent probe (BPO-DNSP) with a dual reaction site for SO₂ and biothiols was presented. BPO-DNSP can sensitively and selectively respond to SO₂ in the green channel with a large Stokes shift over 105 nm, and to biothiols in the near-infrared emission channel with a large Stokes shift over 109 nm. The emission shift for the two channels was as high as 170 nm. Colocalization experiments verified that BPO-DNSP can selectively enrich lysosomes. Notably, BPO-DNSP can not only be used to image intracellular SO₂ and biothiols from two different channels, but also to monitor the conversion of biothiols to SO₂ without adding exogenous enzymes in living HeLa cells.

Two-photon imaging of the endoplasmic reticulum thiol flux in the brains of mice with depression phenotypes

Depression is a common mental illness with high morbidity and mortality. Mounting evidence suggests that an imbalance of the oxidant-antioxidant defence system is strongly correlated with depression and the dysfunction of the endoplasmic reticulum (ER) is strongly related to the oxidative stress. Therefore, as vital and abundant antioxidants in the ER, biothiols may contribute to the etiology of depression. However, ideal two-photon (TP) fluorescent probes for in vivo imaging of ER-associated thiols in the brains of mice with depression phenotypes are still lacking. Hence, we describe a fluorescent probe (ER-SH) to visualize thiols in living systems. ER-SH displays high sensitivity, excellent ER-targeting ability, outstanding TP properties and low cytotoxicity. Using this ER-SH probe, we succeeded in revealing an increase in the endogenous thiol levels under ER stress induced by DTT. Significantly, TP in vivo imaging showed for the first time that the thiol levels are reduced in brains of mice with depression phenotypes. Collectively, this work can assist in further understanding the molecular mechanism of depression and offers a crucial dimension for diagnosis and anti-depression treatments.

A near-infrared fluorescent probe for monitoring leucine aminopeptidase in living cells

A novel water-soluble near-infrared fluorescent probe (CHMC-M-Leu) for specific monitoring of LAP in vitro and in vivo.

A NIR fluorescent sensor for biothiols based on a dicyanoisophorone derivative with a large Stokes shift and high quantum yield

Probe N-Bio exhibited rapid response, high sensitivity and strong NIR fluorescence in the detection of biothiols in living cells.

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.

Functional synthetic probes for selective targeting and multi-analyte detection and imaging

In contrast to the classical design of a probe with one binding site to target one specific analyte, probes with multiple interaction sites or, alternatively, with single sites promoting tandem reactions to target one or multiple analytes, have been developed. They have been used in addressing the inherent challenges of selective targeting in the presence of structurally similar compounds and in complex matrices, as well as the visualization of the in vivo interaction or crosstalk between the analytes. Examples of analytes include reactive sulfur species, reactive oxygen species, nucleotides and enzymes. This review focuses on recent innovations in probe design, detection mechanisms and the investigation of biological processes. The vision is to promote the ongoing development of fluorescent probes to enable deeper insight into the physiology of bioactive analytes.

Constructing a far-red to near-infrared fluorescent probe for highly specific detection of cysteine and its bioimaging applications in living cells and zebrafish

A novel far-red to near-infrared fluorescent probe for highly specific detection of cysteine without interference.

A novel near-infrared fluorescent probe for detecting intracellular alkaline phosphatase and imaging of living cells

Design, synthesis and application of a fluorescent probe with a novel near-infrared fluorophore for in vivo imaging of alkaline phosphatase.