Ratiometric Imaging of Cysteine Level Changes in Endoplasmic Reticulum during H2O2-Induced Redox Imbalance

Selective delivery of imaging probes and therapeutics to the endoplasmic reticulum or Golgi apparatus: Current strategies and beyond

To maximize therapeutic effects and minimize unwanted effects, the interest in drug targeting to the endoplasmic reticulum (ER) or Golgi apparatus (GA) has been recently growing because two organelles are distributing hubs of cellular building/signaling components (e.g., proteins, lipids, Ca2+) to other organelles and the plasma membrane. Their structural or functional damages induce organelle stress (i.e., ER or GA stress), and their aggravation is strongly related to diseases (e.g., cancers, liver diseases, brain diseases). Many efforts have been developed to image (patho)physiological functions (e.g., oxidative stress, protein/lipid-related processing) and characteristics (e.g., pH, temperature, biothiols, reactive oxygen species) in the target organelles and to deliver drugs for organelle disruption using organelle-targeting moieties. Therefore, this review will overview the structure, (patho)physiological functions/characteristics, and related diseases of the organelles of interest. Future direction on ER or GA targeting will be discussed by understanding current strategies and investigations on targeting, imaging/sensing, and therapeutic systems.

Efficient turn-on fluorescent probe cooperated by cascade response for disclosing the fluctuation of cysteine in cells

BACKGROUND

The research on cysteine (Cys) determination is deemed as a hot topic, since it has been reported to be connected with various physiological processes and disease prediction. However, existing Cys-responding probes may expose some defects such as long reaction time, disappointing photostability, and suboptimal sensitivity. Under such a circumstance, our team has proposed an efficient fluorescent probe with novel sensing mechanism to perfectly cope with the above-mentioned drawbacks.

RESULTS

A novel cascade reaction-based probe 9-(2,2-dicyanovinyl)-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-8-yl acrylate (DPQA) has been synthesized for the first time. Undergoing addition-cleavage and cyclization-rearrangement processes, DPQA reacts with Cys to generate an iminocoumarin product with relucent green fluorescence, namely 11-imino-2,3,6,7-tetrahydro-1H,5H,11H-pyrano[2,3-f]pyrido[3,2,1-ij]quinoline-10-carbonitrile (IMC-J), and the relative fluorescence quantum yield (Φf) soars from 0.007 to 0.793. Utilizing such a mechanism, DPQA shows a superb turn-on signal (172-fold), low detection limit (4.1 nM), and wide detection range (5-6000 nM) toward Cys detection. Encouraged by the admirable sensing performance of DPQA, bioimaging of endogenous Cys has been attempted in HeLa cells with satisfactory results. Moreover, cell model of H2O2-induced oxidative stress has been established and the Cys fluctuation during this process has been inspected, elucidating how living cells confront with the eruption of reactive oxygen species (ROS) storm.

SIGNIFICANCE

The probe DPQA with such an intriguing cascade responding process for Cys detection has been endowed with many merits, such as fast reaction and superior sensitivity, conducive to improving responsiveness and rendering it more suitable for further applications. Thereby, we expect that the DPQA would be an efficient tool for detecting Cys fluctuation in living cells of different physiological processes.

Treatment evaluation of Rheumatoid arthritis by in situ fluorescence imaging of the Golgi cysteine

Rheumatoid arthritis (RA) is a long-term systemic inflammatory disease that causes severe joint pain. Golgi stress caused by redox imbalance significantly involves in acute and chronic inflammatory diseases, in which cysteine (Cys), as a representative reducing agent, may be an effective biomarker for RA. Hence, in order to achieve RA early detection and drugs evaluation, based on our previous work about innovative Golgi-targeting group, we established a phenylsulfonamide-modified fluorescence probe, Golgi-Cys, for the selective fluorescence imaging of Cys in Golgi apparatus in vivo. By application of Golgi-Cys, the Cys changes under Golgi stress in cells were elucidated. More importantly, we found that the probe can be effectively utilized for the RA detection and treatment evaluation in situ.

Oxidation Analysis of l-Cysteine with a Chiral Sensor Based on Quantum Weak Measurement

l-Cysteine, distinguished by its possession of reactive sulfhydryl groups within its molecular structure, plays a significant role in both biological systems and the pharmaceutical industry. It stands not only as a natural component integral to the constitution of glutathione but also as the principal precursor for the synthesis of l-cystine through an oxidation reaction. This study endeavors to introduce a novel approach to l-cysteine analysis, capitalizing on its optical activity, whereby an optical rotation detection system grounded in the principles of quantum weak measurement is proffered. The optical rotation angle corresponding to the concentration of chiral solutions can be accurately ascertained through spectral analysis. In practical implementation, a chiral sensing system, boasting a sensitivity of 372 nm/rad, was meticulously constructed, leveraging the concept of weak value amplification. Then, the real-time monitoring of chemical reactions involving l-cysteine and dimethyl sulfoxide was performed. Under the specific experimental conditions outlined in this investigation, it was observed that the oxidation process culminated within approximately 12 h. The application of weak measurement-based chiral sensors holds immense potential, providing robust technical support for real-time monitoring in fields such as chiral analysis and the synthesis of chiral pharmaceutical compounds.

Ultrabright Green-Emissive Nanodots for Precise Biological Visualization

Developing general methods to fabricate water-dispersible and biocompatible fluorescent probes will promote different biological visualization applications. Herein, we report a metal-facilitated method to fabricate ultrabright green-emissive nanodots via the one-step solvothermal treatment of rose bengal, ethanol, and various metal ions. These metal-doped nanodots show good water dispersity, ultrahigh photoluminescence quantum yields (PLQYs) (e.g., the PLQY of Fe-doped nanodots (FeNDs) was ∼97%), and low phototoxicity. Owing to the coordination effect of metal ions, the FeNDs realize glutathione detection with outstanding properties. Benefiting from the high endoplasmic reticulum (ER) affinity of the chloride group, the FeNDs can act as an ER tracker with long ER imaging capacity (FeNDs: >24 h; commercial ER tracker: ∼1 h) and superb photostability and can achieve tissue visualization in living Caenorhabditis elegans. The metal-doped nanodots represent a general nanodot preparation method and may shed new light on diverse biological visualization uses.

Hypochlorite activatable ratiometric fluorescent probe based on endoplasmic reticulum stress for imaging of atherosclerosis

Endoplasmic reticulum (ER) stress can induce reactive oxygen (ROS) generation which is directly associated with the emergence of atherosclerosis. Foam cells could promote atherogenesis by inducing ER stress. To understand hypochlorite (ClO-) levels in foam cells under ER stress, novel ER-targeted ClO- activatable ratiometric fluorescence probes Rx-NE and Rx-NCE were designed using a classical rhodamine dye and coumarin dye bridge moiety as the fluorescent skeleton. Both Rx-NE and Rx-NCE demonstrated ratiometric detection capabilities for ClO-, with Rx-NCE showing better sensitivity compared to Rx-NE. The probe Rx-NCE could detect the upregulation of ClO- in foam cells under ER stress and clearly outline delineation of the boundary of atherosclerotic plaques by dual-color imaging. Importantly, the hypochlorite-activated ratiometric probe Rx-NCE had been innovatively applied to the distinction of atherosclerotic blood vessels in atherosclerosis-bearing transgenic (tg) (flk1: eGFP) zebrafish. The probe Rx-NCE is of significant value for investigating the pathological role of ER stress and atherosclerotic diseases, as well as offering new insights into the identification of atherosclerosis.

DBDMH-Promoted Methylthiolation in DMSO: A Metal-Free Protocol to Methyl Sulfur Compounds with Multifunctional Groups

Organic thioethers play an important role in the discovery of drugs and natural products. However, the green synthesis of organic sulfide compounds remains a challenging task. The convenient and efficient synthesis of 5-alkoxy-3-halo-4-methylthio-2(5H)-furanones from DMSO is performed via the mediation of 1,3-dibromo-5,5-dimethylhydantoin (DBDMH), affording a facile route for the sulfur-functionalization of 3,4-dihalo-2(5H)-furanones under transition metal-free conditions. This new approach has demonstrated the functionalization of non-aromatic Csp2-X-type halides with unique structures containing C-X, C-O, C=O and C=C bonds. Compared with traditional synthesis methods using transition metal catalysts with ligands, this reaction has many advantages, such as the lower temperature, the shorter reaction time, the wide substrate range and good functional group tolerance. Notably, DMSO plays multiple roles, and is simultaneously used as an odorless methylthiolating reagent and safe solvent.

Recent Progress in the Rational Design of Biothiol-Responsive Fluorescent Probes

Biothiols such as cysteine, homocysteine, and glutathione play significant roles in important biological activities, and their abnormal concentrations have been found to be closely associated with certain diseases, making their detection a critical task. To this end, fluorescent probes have become increasingly popular due to their numerous advantages, including easy handling, desirable spatiotemporal resolution, high sensitivity, fast response, and favorable biocompatibility. As a result, intensive research has been conducted to create fluorescent probes for the detection and imaging of biothiols. This brief review summarizes recent advances in the field of biothiol-responsive fluorescent probes, with an emphasis on rational probe design, including the reaction mechanism, discriminating detection, reversible detection, and specific detection. Furthermore, the challenges and prospects of fluorescence probes for biothiols are also outlined.

Cysteine-Activatable Near-Infrared Fluorescent Probe for Dual-Channel Tracking Lipid Droplets and Mitochondria in Epilepsy

Dual-channel fluorescent probes could respond to a specific target and emit different wavelengths of fluorescence before and after the response. Such probes could alleviate the influence caused by the variation of the probe concentration, excitation intensity, and so on. However, for most dual-channel fluorescent probes, the probe and fluorophore faced spectral overlap, which reduced sensitivity and accuracy. Herein, we introduced a cysteine (Cys)-responsive and near-infrared (NIR) emissive AIEgen (named TSQC) with good biocompatibility to dual-channel monitor Cys in mitochondria and lipid droplets (LDs) during cell apoptosis through wash-free fluorescence bio-imaging. TSQC can label mitochondria with bright fluorescence around 750 nm, and after reacting with Cys, the reaction product TSQ could spontaneously target LDs with emissions around 650 nm. Such spatially separated dual-channel fluorescence responses could significantly improve detection sensitivity and accuracy. Furthermore, the Cys-triggered dual-channel fluorescence imaging in LDs and mitochondria during apoptosis induced by UV light exposure, H₂O₂, or LPS treatment is clearly observed for the first time. Besides, we also report here that TSQC can be used to image subcellular Cys in different cell lines by measuring the fluorescence intensities of different emission channels. In particular, TSQC shows superior utility for the in vivo imaging of apoptosis in acute and chronic epilepsy mice. In brief, the newly designed NIR AIEgen TSQC can respond to Cys and separate two fluorescence signals to mitochondria and LDs, respectively, to study Cys-related apoptosis.

Monitoring cell viability in N-nitrosodiethylamine induced acute hepatitis and detection of hydrazine in solution and gas phase with Dual-function fluorescent probes

The highly toxic N-nitrosodiethylamine (NDEA) and hydrazine (N₂H₄) caused severe environmental contamination and serious health risks. Herein, we designed the two-photon ratiometric fluorescent probe (Nap-2), emission maximum shifted from 466 nm to 571 nm, to monitor cell viability of NDEA induced acute hepatitis via esterase activity detection. Furthermore, the probe Nap-2 evaluate the hydrazine (N₂H₄) content in the solution and gas phase. It is worth mentioning that we used NDEA induced acute hepatitis in the mice and evaluated the negative correlation of esterase activity in the tissue cells and serum with Nap-2. The probe Nap-2 exhibited that acute hepatitis induced by NDEA decreased cell viability. Furthermore, we made convenient test papers using Nap-2 to detect N₂H₄ in solution and gas phase. After adding N₂H₄, the fluorescence color changed from blue to yellow and was visible to the naked eye. This work provides a convenient tool and method for evaluating the toxicity of NDEA induced acute hepatitis and detecting N₂H₄ in the environment.

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.

Hypochlorite Detection by Fluorescent Sensors Bearing Long Alkyl Chains: The Role of Chain Length in Sensing Properties

Three pyridinium derivatives bearing alkyl chains of different lengths (C1, C8, and C18) that show aggregation-enhanced emission were synthesized. These compounds can be used to detect ClO- ion as the reaction releases the fluorescent core with an increase in emission intensity and change in absorption wavelength. The lowest detection limit of TPA-Pyr-18C was 6.04 μM. The length of the alkyl chain and resulting lipophilicity allowed the targeting of different subcellular structures. TPA-Pyr-18C could be used for staining yolk lipids in zebrafish.

An indanone-based fluorescent probe for detection and imaging of Cys/Hcy in living cells

Effective detection of Cys and Hcy plays an important role in the diagnosis of diseases. In this work, a novel indanone-based fluorescent probe INIAc-CN for sensitively and effectively detecting Cys and Hcy was developed. The probe exhibited weak fluorescence, but obvious fluorescent enhancement after reacted with Cys/Hcy. Moreover, the good anti-interference and low cytotoxicity of the probe made it successfully applied for monitoring Cys and Hcy of in living cells.

Gold Nanocluster-Based Fluorometric Banoxantrone Assay Enabled by Photoinduced Electron Transfer

Monitoring the blood concentration of banoxantrone (AQ4N) is important to evaluate the therapeutic efficacy and side effects of this new anticancer prodrug during its clinical applications. Herein, we report a fluorescence method for AQ4N detection through the modulation of the molecule-like photoinduced electron transfer (PET) behavior of gold nanoclusters (AuNCs). AQ4N can electrostatically bind to the surface of carboxylated chitosan (CC) and dithiothreitol (DTT) co-stabilized AuNCs and quench their fluorescence via a Coulomb interaction-accelerated PET process. Under optimized experimental conditions, the linear range of AQ4N is from 25 to 200 nM and the limit of detection is as low as 5 nM. In addition, this assay is confirmed to be reliable based on its successful use in AQ4N determination in mouse plasma samples. This work offers an effective strategy for AQ4N sensing based on fluorescent AuNCs and widens the application of AuNCs in clinical diagnosis and pharmaceutical analysis.

Elimination Reaction-Based Benzimidazole Probe for Cysteine Detection and Its Application in Serum Sample Analysis

Benzimidazole-based compound 2-(p-tolyl)-1H-benzo[d]imidazole (3) and its derivative probe A-B have been synthesized for the highly selective detection and quantification of Cys in human serum. The photophysical properties of A-B and compound 3 were evaluated by UV-vis absorption and fluorescence spectroscopy. A-B showed high selectivity and sensitivity for Cys among tested analytes, including amino acids, anions, and cations. A-B selectively reacts with Cys and results in compound 3 with fluorescence turn-on effect. A-B did not show any interference from the components in the serum matrix for Cys detection in the human serum sample. A-B detects Cys in serum samples with 2.3-5.4-fold better LOD than reported methods. The detection limit of 86 nM and 43 nM in HEPES buffer using UV-visible and fluorescence spectroscopy, respectively, makes A-B an excellent chemosensor for Cys detection.

A dual-response fluorescent probe for discriminative sensing of hydrazine and bisulfite as well as intracellular imaging with different emission

Bisulfite and hydrazine are harmful to the environment safety and human health. Therefore, it is of great value to develop a smart fluorescent probe with high selectivity for detection of bisulfite and hydrazine. In our report, a dual-response fluorescent probe EDBI with high selectivity, rapid response, and low detection limit for discriminative determination HSO₃^- and N₂H₄ was exploited. The probe EDBI is capable of distinctive sensing HSO₃^- and N₂H₄ based on nucleophilic addition reactions by taking advantage of ratiometric fluorescence and fluorescence "on-off" mode, respectively. The dual-responses behaviors of probe EDBI toward HSO₃^- and N₂H₄ were attribute to different reaction sites, which it has been confirmed by HRMS. More importantly, cytotoxicity experiment authenticated that probe possesses low toxicity and good penetration. The probe EDBI with excellent performance, it was successfully employed to distinguishable sense HSO₃^- and N₂H₄ in living cells by diverse channel patterns. Therefore, this simple dual-response fluorescence probe is expected to be used for real-time monitoring bisulfite and hydrazine in biological samples.

Recent progress in small-molecule fluorescent probes for endoplasmic reticulum imaging in biological systems

Endoplasmic reticulum (ER) is an indispensable organelle in eukaryotic cells involved in protein synthesis and processing, as well as calcium storage and release. Therefore, maintaining the quality of ER is of great importance for cellular homeostasis. Aberrant fluctuations of bioactive species in the ER will result in homeostasis disequilibrium and further cause ER stress, which has evolved to contribute to the pathogenesis of various diseases. Therefore, the real-time monitoring of various bioactive species in the ER is of high priority to ascertain the mysterious roles of ER, which will contribute to unveiling the corresponding mechanism of organism disturbances. Recently, fluorescence imaging has emerged as a robust technique for the direct visualization of molecular events due to its outstanding sensitivity, high temporal-spatial resolution and noninvasive nature. In this review, we comprehensively summarize the recent progress in design strategies, bioimaging applications, potential directions and challenges of ER-targetable small-molecular fluorescent probes.

Fluorescent probes for targeting endoplasmic reticulum: design strategies and their applications

Advances in developing organic fluorescent probes and fluorescence imaging techniques have enhanced our understanding of cell biology. The endoplasmic reticulum (ER) is a dynamic structure that plays a crucial role in protein synthesis, post-translational modifications, and lipid metabolism. The malfunction of ER contributes to several physiological and pathological conditions. Therefore, the investigations on the imaging and role of ER have attracted a lot of attention. Due to their simplicity, synthetic tunability, photostability, high quantum yields, easier cellular uptake, and lower cytotoxicity, organic fluorophores offer invaluable tools for the precision targeting of various cellular organelles and probe ER dynamics. The precision staining is made possible by incorporating specific functional groups having preferential and local organelle biomolecular interactions. For instance, functional moieties such as methyl sulfonamide, sulfonylurea, and pentafluorophenyl assist in ER targeting and thus have become essential tools to probe a deeper understanding of their dynamics. Furthermore, dual-function fluorescent probes that simultaneously image ER and detect specific physiological parameters or biological analytes were achieved by introducing special recognition or chemically reactive sites. This article attempts to comprehensively capture various design strategies currently employed by researchers utilizing small organic molecules to target the ER and detect specific analytes.

A two-photon fluorescence probe with endoplasmic reticulum targeting ability for turn-on sensing photosensitized singlet oxygen in living cells and brain tissues

Endoplasmic reticulum (ER) is an indispensable organelle responsible for protein synthesis, transportation, and maintenance of Ca²⁺ homeostasis in eukaryotic cells. Recent studies highlighted that ER-targeted photosensitizers with high yield of singlet oxygen (¹O₂) are effective in selectively disrupting ER function and are promising candidates for anticancer therapy. Unfortunately, no ER targetable fluorescent probes for determining ¹O₂ photosensitized in this photodynamic therapy process is available. In this work, we synthesized an ER-targetable, two-photon fluorescence probe, ER-¹O₂, for fluorescence turn-on sensing of ¹O₂. ER-¹O₂ demonstrated high sensitivity to ¹O₂ sensing with a wide detection range (0-2.75 μM) and a low detection limit (0.11 μM). ER-¹O₂ also displayed excellent selectivity toward ¹O₂ out of other ROS and metal ions. Notably, ER-¹O₂ exhibited low cytotoxicity but with specific ER targetable capability. On account of these advantageous features, fluctuations of ¹O₂ in living cells and brain tissues were effectively visualized by ER-¹O₂.

Michael addition–elimination–cyclization based turn-on fluorescence (MADELCY TOF) probes for cellular cysteine imaging and estimation of blood serum cysteine and aminoacylase-1

Michael addition–elimination–cyclization based turn-on fluorescence (MADELCY TOF) probes for the highly sensitive estimation of Cys and aminoacylase-1 (ACY-1).

A novel NIR fluorescent probe with ratiometric imaging of cysteine in endoplasmic reticulum

We develop a large Stoke's shift ratiometric probe for imaging endoplasmic reticulum oxidative stress induced by Cytoxan.

A reversible coumarin-based sensor for intracellular monitoring cysteine level changes during Cu2+-induced redox imbalance

Biological thiols are crucial small molecule amino acids widely existing in cells, which play indispensable roles in maintaining redox homeostasis of living systems. Owing to their abnormal levels have close relation with many diseases, thus, developing more convenient, rapid and practical in-vivo detection tools is imminent. Herein, a reversible coumarin-based probe (HNA) was successfully constructed through a simple two-step synthesis. HNA can detect Cys/Hcy with high response speed and desirable selectivity based on Michael addition recognition mechanism. Free HNA has an orange emission at 580 nm, but after addition of Cys/Hcy, the conjugated structure of probe HNA was destroyed by the attack of sulfhydryl, resulting in a new green emission at 507 nm. Further, HNA has been applied to monitor Cys/Hcy in HeLa cells and zebrafish. Notably, HNA has also been successfully applied for real-time tracing Cys levels changes in living cells and zebrafish during the imbalance in redox status caused by copper (II). This provides a new strategy for studying the process of oxidative stress in cells.

A redox reversible endoplasmic reticulum-targeted fluorescent probe for revealing the redox status of living cells

The endoplasmic reticulum (ER) is one of the most important organelles in cells and is involved in protein synthesis, folding and orderly transport. Redox balance is the key to its normal function. In this work, we designed and synthesized an endoplasmic reticulum-targeted fluorescent probe N-Se with selenomorpholine as the redox reversible detection moiety. N-Se could selectively respond to ClO^- within only 8 s with a LOD of 28.8 nM. Furthermore, such a response is reversible in the regulation of GSH. Confocal fluorescence imaging confirmed the excellent endoplasmic reticulum targeting ability of N-Se. Thus, it could real-time monitor the dynamic changes of the redox status in the endoplasmic reticulum through the variation of the fluorescence intensity.

Ultrasensitive fluorescent probe for visual biosensing of esterase activity in living cells and its imaging application

Esterase activity is often used as an index to evaluate the health status of cells and plays an important role in cell metabolism and apoptosis. Herein, we develop two fluorescent probes for visual biosensing of esterase activity and imaging in living cells. In vitro, after the introduction of esterase, enzymolysis destroys the ester bond of the probe, causing the fluorescent color of probe changes from yellow to red, thus realizing the visual strategy for determination of esterase activity, with high sensitivity and selectivity. Especially, probe VA, 2-(4-acetoxystyryl)-3-ethyl-1,1-dimethyl- 1H-benzo[e]indol-3-ium, exhibits higher sensitivity with a lower detection limit (up to 7.15 × 10^-6 U/mL). In the cell experiment, the fluorescent probe VA also shows good biocompatibility and high spatial resolution, and is successfully applied to the intracellular fluorescent imaging and biosensing of esterase in living cells. More importantly, the probe VA can judge the unhealthy state of H₂O₂-induced HeLa cells using dual-fluorescence signals. The results confirm that the fluorescence method is a reliable tool for detecting endogenous esterase in living biological system.

Rational design of a new near-infrared fluorophore and apply to the detection and imaging study of cysteine and thiophenol

The development of a near-infrared fluorophore with excellent fluorescence performance, a large Stokes shift, and good biocompatibility has become a focus in the field of fluorescence imaging in recent years. Based on quantum chemistry calculations and reasonable molecular design strategies, a new NIR fluorophore was developed and characterized by simple synthesis, easy structural modification, and a large Stokes shift (105 nm). Furthermore, two new "activatable" fluorescent probes QN-Cys and QN-DNP were synthesized using a simple structural modification. The probe QN-Cys can recognize Cys with high sensitivity (LOD = 128 nM) and high selectivity, and its fluorescence intensity has a good linear relationship with the Cys concentration in the range of 5-35 μM. Furthermore, probe QN-Cys can effectively distinguish Cys from Hcy and GSH, and was successfully applied to the detection and imaging of Cys in human serum, cells, and zebrafish. The probe QN-DNP showed a good specific and sensitive (LOD = 78 nM) fluorescence response to thiophenol, and its fluorescence intensity has a good linear relationship with the thiophenol concentration in the range of 5-30 μM. Furthermore, it was successfully applied to detect thiophenol in real water samples with good recoveries (97-102%), and image thiophenol in living cells, zebrafish and mice. Notebly, the QN-DNP probe could be applied to visualize the distribution of thiophenol in the mice.

A near-infrared fluorescence turn-on probe based on Michael addition-intramolecular cyclization for specific detection of cysteine and its applications in environmental water and milk samples and living cells

Owing to its important biological functions in many physiological and pathological processes, it is necessary to develop efficient and appropriate detection methods for monitoring the levels of Cys in biological systems. Based on this, a novel rhodol-isophorone derivative (RHI) was designed and synthesized as a reaction-based fluorescence probe for specific detection of Cys with high sensitivity and large Stokes shift (155 nm). This probe was composed of an acrylate moiety (recognition group) and a rhodol-isophorone derivative (fluorophore). Probe RHI could react with Cys rapidly (15 min) with a 100-fold fluorescence enhancement. The limit of detection value was calculated to be 0.168 μM. When Cys was added, the color of the probe RHI solution turned from yellow to blue, indicating that Cys could be monitored by the naked eye. In addition, probe RHI was successfully utilized for detecting Cys in environmental water and milk samples. More importantly, the probe could be applied to imaging Cys in living cells with low cytotoxicity and good biocompatibility.

Substitution-rearrangement-cyclization strategy to construct fluorescent probe for multicolor discriminative analysis biothiols in cells and zebrafish

Discriminative detection of biothiols (Cysteine, homocysteine and glutathione) is of great significance to clarificate their complex physiological processes, the occurrence and development of related diseases. However, similar structure and reactivity among such species pose huge challenges in developing fluorescent probes to distinguish among of them. In this work, a dual-site probe CTT reacted with the analytes to regulate molecular conjugation through substitution-rearrangement-cyclization strategy, utilizing a multi-channel signal combination mode to realize the distinguishing detection of the three biothiols. Cell and zebrafish imaging experiments sufficiently demonstrated that CTT could semiquantify biothiols, which will provide valuable chemical tool for elucidating the complex biological functions of biothiols.

Development of a Novel Benzimidazole-Based Probe and Portable Fluorimeter for the Detection of Cysteine in Human Urine

The measurement of cysteine in human urine and live cells is crucial for evaluating biological metabolism, monitoring and maintaining the immune system, preventing tissue/DNA damage caused by free radicals, preventing autoimmune diseases, and diagnosing disorders such as cystinuria and cancer. A method that uses a fluorescence turn-on probe and a portable fluorescence spectrometer device are crucial for highly sensitive, simple, rapid, and inexpensive cysteine detection. Herein, we present the synthesis and application of a benzimidazole-based fluorescent probe (ABIA) along with the design and development of a portable fluorescence spectrometer device (CysDDev) for detecting cysteine in simulated human urine. ABIA showed excellent selectivity and sensitivity in detecting cysteine over homocysteine, glutathione, and other amino acids with the response time of 1 min and demonstrated a detection limit of 16.3 nM using the developed CysDDev. Further, ABIA also demonstrated its utility in detecting intracellular cysteine, making it an excellent probe for bio-imaging assay.

Red-emitting fluorescent turn-on probe with specific isothiocyanate recognition site for cysteine imaging in living systems

Cysteine (Cys) is an effective biomarker in life systems and is closely related to a variety of diseases, so developing a specific and efficient detection method for Cys is of great significance. To date, extensive work has been undertaken toward this goal. However, the differentiation of Cys from other biothiols still represents a challenge from an experimental point of view. Toward this end, a selective and sensitive red-emitting probe (TMN-NCS) with an isothiocyanate (ITC)-based structure was proposed in this paper. A large Stokes shift (210 nm) was observed upon addition of Cys to a solution of TMN-NCS. In addition, TMN-NCS showed low toxicity, a low detection limit (120 nM), and excellent cell permeability. The results suggested that TMN-NCS holds great promise for biological applications.

Research on endoplasmic reticulum-targeting fluorescent probes and endoplasmic reticulum stress-mediated nanoanticancer strategies: A review

Subcellular localization of organelles can achieve accurate drug delivery and maximize drug efficacy. As the largest organelle in eukaryotic cells, the endoplasmic reticulum (ER) plays an important role in protein synthesis, folding, and posttranslational modification; lipid biosynthesis; and calcium homeostasis. Observing the changes in various metal ions, active substances, and the microenvironment in the ER is crucial for diagnosing and treating many diseases, including cancer. Excessive endoplasmic reticulum stress (ERS) can have a killing effect on malignant cells and can mediate cell apoptosis, proper modulation of ERS can provide new perspectives for the treatment of many diseases, including cancer. Therefore, the ER is used as a new anticancer target in cancer treatment. This review discusses ER-targeting fluorescent probes and ERS-mediated nanoanticancer strategies.

Three asymmetric BODIPY derivatives as fluorescent probes for highly selective and sensitive detection of cysteine in living cells

Biothiols are widely involved in various important physiological activities and play a significant role in maintaining redox homeostasis in living organisms. Herein, we designed and synthesized three new asymmetric fluorescent probes (BDP-S-Ph, BDP-S-ENE and BDP-S-R) to discriminate Cys from Hcy/GSH. These probes reacted with Cys to form meso-amino-BODIPYs via SNAr substitution-rearrangement, thereby inducing a fluorescence turn-on effect. Moreover, they could selectively and sensitively detect Cys in solution with low detection limits (50 nM, 28 nM and 87 nM, respectively). Through comparing the response rates of the three probes to Cys, we concluded that the increase of conformational restrictions led to a decrease in probe reactivity. Besides, the sensing mechanism was demonstrated by mass spectrometry. Furthermore, cell experiments indicated that the probes were able to image exogenous and endogenous Cys through green or red channels in living cells.

Photoactivated In Situ Generation of Near Infrared Cyanines for Spatiotemporally Controlled Fluorescence Imaging in Living Cells

Photoactivated trimerization of 2,3,3-trimethyl-3H-indole derivatives created near infrared fluorophore Cy5. The synthetic method is air-tolerant, photosensitizer free, metal free, and condensation agent free. Living cells make Cy5 on a time scale of minutes under white light irradiation at a low power intensity, with the monomer as the only exogenous agent. The new method is promising to find applications in cell studies for in situ spatiotemporally controlled fluorescence imaging in living cells.

Revealing the redox status in endoplasmic reticulum by a selenium fluorescence probe

As an important organelle, the endoplasmic reticulum (ER) participates in the synthesis and secretion of various proteins, glycogen, lipids and cholesterol in eukaryotic cells. In this work, an endoplasmic reticulum-targeted reversible fluorescent probe (ER-Se) was designed and synthesized. The probe, based on a selenide group, shows high sensitivity and good selectivity toward HClO (LOD = 0.85 μM). In addition, the probe has reversible capability towards HClO/GSH. Most importantly, co-location experiment results indicated that the probe exhibited a great ability to target the endoplasmic reticulum. Furthermore, the probe was successfully applied to detect exogenous and endogenous HClO in ER and monitored the redox status changes during ER stress.

Viscosity sensitive endoplasmic reticulum fluorescent probes based on oxazolopyridinium

A series of viscosity sensitive fluorescent probes 1a-e were synthesized by linking coumarin and oxazolopyridinium via dimethylene in this paper. The viscosity test of probes 1a-e indicated that the fluorescence intensity of the probes enhanced significantly with the increase of viscosity of the system (0.89-865 cP), and exhibited a nearly OFF-ON response to viscosity at 648 nm, 650 nm and 650 nm, respectively. In addition, cells still had a high survival rate after co-culturing with probes 1a-e for 12 h (94-98%). Meanwhile, the laser confocal experiment showed that the variation of the carbon chain length in the oxazolopyridinium could affect the subcellular region of the localization of the probes in cells. When the length of the carbon chain in oxazolopyridinium was between n-C7H15 and n-C12H23, probes 1b-d had the ability to target the endoplasmic reticulum in the cells. Moreover, probes 1b-d showed no significant change in fluorescence intensity after 35 min of continuous laser confocal irradiation, indicating that they had excellent anti-photobleaching properties.

The Role of Amino Acids in Neurotransmission and Fluorescent Tools for Their Detection

Neurotransmission between neurons, which can occur over the span of a few milliseconds, relies on the controlled release of small molecule neurotransmitters, many of which are amino acids. Fluorescence imaging provides the necessary speed to follow these events and has emerged as a powerful technique for investigating neurotransmission. In this review, we highlight some of the roles of the 20 canonical amino acids, GABA and β-alanine in neurotransmission. We also discuss available fluorescence-based probes for amino acids that have been shown to be compatible for live cell imaging, namely those based on synthetic dyes, nanostructures (quantum dots and nanotubes), and genetically encoded components. We aim to provide tool developers with information that may guide future engineering efforts and tool users with information regarding existing indicators to facilitate studies of amino acid dynamics.

A fluorescent probe based on reversible Michael addition-elimination reaction for the cycle between cysteine and H2O2

Cysteine oxidation by H₂O₂, generating either cysteine sulfenic acid (CSOH) or disulfide (CSSC), is involved in redox homeostasis and signaling. Compared with quantification of the cysteine content, monitoring the cysteine dynamics in real-time, in particular, takes on even greater importance. However, existing fluorescent probes suffer from low specificity or irreversible recognition mechanisms. In the present work, we have successfully developed a reversible fluorescent probe for the cycle between cysteine and H₂O₂ based on the Michael addition-elimination reaction. This probe features a specific and quantitative response to cysteine. The reversible detection was realized repeatedly with the addition of cysteine and H₂O₂ in order. We also demonstrated its usage for monitoring exogenous and endogenous cysteine in living cells. Eventually, this probe was capable of imaging cysteine dynamically in real-time.

Fluorescent Probe for Simultaneous Discrimination of GSH, Cys, and SO2 Derivatives

Biothiols and SO₂ derivatives, as essential reactive sulfur species (RSS), play vital roles in various physiological processes and have a close network of generation and metabolic pathways among them. To clarify their complex correlations, fluorescent probes to simultaneously detect GSH, Cys, and SO₂ derivatives are highly desirable. Herein, we develop the first fluorescent probe (BO-HEM) to simultaneously discriminate GSH, Cys, and SO₂ derivatives. The fluorescent probe is designed by integration of hemicyanine and BODIPY fluorophores through an ether bond. The ether bond of the probe is rapidly replaced by thiolates through nucleophilic aromatic substitution (S_NAr) to generate hemicyanine with NIR fluorescence and sulfur-BODIPY. The amino groups of Cys but not GSH then further replace the thiolate to form amino-BODIPY. As for SO₃^2-, nucleophilic addition to the double bond of BO-HEM generates adduct O-BODIPY with green fluorescence. To further improve the sensing performance, the nanoprobe with increased reactivity and biocompatibility is constructed by encapsulation of BO-HEM into the polymeric micelle. More importantly, by taking advantage of the hydrophilicity of the reaction products, the spectral discrimination was further enhanced to avoid signal interference. The nanoprobe is applied to discriminate biothiols and SO₂ derivatives in living cells through three-color fluorescence imaging.

Benzoxazine-based fluorescent probes with different auxochrome groups for cysteine detection

Three 5H-benzo[a]phenoxazin-5-one-based (benzoresorufin and nile-red) Cysteine (Cys) detection probes have been comparatively designed and synthesized in this paper. The optical experiments exhibit probe 1b with a crotonoyl group has no response toward Cys; while probes 1a and 1c have the same reaction site (acryloyl group), their optical responses to Cys are quite different. The benzoresorufin-based-probe 1a shows a turn-on fluorescence response (118-fold) to Cys at 631 nm and affords a very low detection limit (DL = 19.8 nM). Compared with probe 1a, the nile-red-based probe 1c displays gradually diminishing fluorescence intensity with increased Cys concentration at 665 nm. And the notable different fluorescence response mechanisms of probes 1a and 1c toward Cys can be interpreted by HRMS and time-dependent density functional theorety (TDDFT) calculations. Furthermore, both of the two probes indicate high sensitivity and selectivity toward Cys over other similar structured amino acids including homocysteine (Hcy) and glutathione (GSH). Further cellular applications of the two probes have been successfully performed in HeLa cells.