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New amphiphilic near-infrared fluorescent probes (λem = 710 nm) are reported for monitoring viscosity of cell membranes. The probes were found to be selective and sensitive to viscosity, and to quickly (within 2 minutes) and stably anchor onto the cell membrane. Moreover, the applicability of the probe MYN-BS (a superior representative) was demonstrated, revealing a reversible viscosity change with temperature change and an increase in viscosity during cell foaming. The proposed probes may serve as a reliable tool for studying viscosity-related processes of cell membranes. [ABSTRACT FROM AUTHOR]

Lipid droplets (LDs) have been known as a non-negligible cellular organelle for lipid storage and metabolism. Fluorescent probes for imaging LDs would be paramount for depicting their functions in cells. Although commercially available Nile Red and BODIPYtm 493/503 have been widely used for labelling LDs, they exhibit unsatisfactory specificity and spectroscopic properties. This feature article reviews the recent advances in organic fluorescent probes for imaging LDs. We first introduce the key points for probe design, including regulating hydrophobicity and enhancing fluorescence quantum yield in LDs. Then, we summarize the structural features and biological applications of some representative LD probes classified by their frameworks. In addition, the current challenges and future research trends for the fluorescent probes of LDs are discussed as well. [ABSTRACT FROM AUTHOR]

Nitroxyl (HNO) is a promising regulator for cancer therapy. Here, we develop a tumor-targeted near-infrared fluorescent probe for HNO and utilize it in the real-time imaging of HNO release in vivo. [ABSTRACT FROM AUTHOR]

Tyrosinase plays a pivotal role in the hyperpigmentation of wounds. Here, we develop a new fluorescent probe and with it, we reveal an increase of tyrosinase activity at the wound site in zebrafish. [ABSTRACT FROM AUTHOR]

The hydroxyl radical (˙OH) in the endoplasmic reticulum has not been studied thoroughly due to the lack of proper methods. Here, an endoplasmic reticulum-targeting fluorescent probe for detecting ˙OH is reported. With this probe, the action of ˙OH in the endoplasmic reticulum has been imaged in living cells. [ABSTRACT FROM AUTHOR]

γ-Glutamyl transpeptidase (GGT), overexpressed in various cancer cells, has been perceived as a latent tumor biomarker. Thus, developing near-infrared (NIR) fluorescent GGT probes is highly desired for in vivo tumor imaging and studies. To our knowledge, however, such a GGT probe is still rare. Herein, we construct a new GGT-activatable NIR fluorescent probe HCAGlu by incorporating γ-glutamyl group as a recognition unit directly into a NIR hemicyanine fluorophore. HCAGlu exhibits a highly sensitive and selective NIR fluorescence off-on response to GGT. The probe has been applied to cell and histological section imaging, which demonstrates the ability of HCAGlu in distinguishing different GGT-expression levels in situ in biological samples. Notably, in vivo fluorescence imaging in tumor-bearing mice has been performed, verifying that probe HCAGlu can rapidly produce a distinct fluorescence signal in the tumor site via both intravenous and intratumoral injections. The simplicity and excellent performance of HCAGlu make it of high potential in studying the physiological function of GGT in vivo.A near-infrared fluorescent probe for imaging γ-glutamyl transpeptidase in biological samples [ABSTRACT FROM AUTHOR]

Abstract: The first sensitive fluorescence .OH probe (1) that is capable of monitoring the generation of trace .OH in iron autoxidation is presented. The probe 1 was designed by utilizing both the unique aromatic hydroxylation and the electrophilicity of .OH, and prepared by incorporating a strong electron‐donating methoxy group into a cyanine fluorochrome to enhance the trapping ability for .OH. Reaction of 1 with .OH leads to a larger π‐conjugation formation and near‐infrared fluorescence off–on response. The capability of probe 1 has been demonstrated by imaging .OH generated in living cells under iron autoxidation as well as various stimuli, which reveals that the basal level of .OH in RAW 264.7 cells is lower than that in HeLa cells. The superior analytical performance of probe 1 makes it useful for detecting trace .OH in some critical physiological and pathological processes associated with iron autoxidation. [ABSTRACT FROM AUTHOR]

Abstract: The first sensitive fluorescence .OH probe (1) that is capable of monitoring the generation of trace .OH in iron autoxidation is presented. The probe 1 was designed by utilizing both the unique aromatic hydroxylation and the electrophilicity of .OH, and prepared by incorporating a strong electron‐donating methoxy group into a cyanine fluorochrome to enhance the trapping ability for .OH. Reaction of 1 with .OH leads to a larger π‐conjugation formation and near‐infrared fluorescence off–on response. The capability of probe 1 has been demonstrated by imaging .OH generated in living cells under iron autoxidation as well as various stimuli, which reveals that the basal level of .OH in RAW 264.7 cells is lower than that in HeLa cells. The superior analytical performance of probe 1 makes it useful for detecting trace .OH in some critical physiological and pathological processes associated with iron autoxidation. [ABSTRACT FROM AUTHOR]

A new dual-function fluorescent probe is developed for detecting nitroreductase (NTR) and adenosine triphosphate (ATP) with different responses. Imaging application of the probe reveals that intracellular NTR and ATP display an adverse changing trend during a hypoxic process and ATP can serve as a new sign for cell hypoxia. [ABSTRACT FROM AUTHOR]

Monoamine oxidase (MAO) has two isoforms, MAO-A and MAO-B, which show different functions, and thus selective fluorescence imaging is important for biological studies. Currently, however, specific detection of MAO-A remains a great challenge. Herein, we report a new strategy for specific imaging of MAO-A through the design of fluorogenic probes combining the characteristic structure of an inhibitor of the target enzyme along with propylamine as a recognition moiety. The high specificity of our representative probe is demonstrated by imaging MAO-A in different live cells such as SH-SY5Y (high levels of MAO-A) and HepG2 (high levels of MAO-B), and further validated by western blot analyses. The superior specificity of the probe may enable the accurate detection of MAO-A in complex biosystems. Importantly, the use of the characteristic structure of an inhibitor, as demonstrated in this work, may serve as a general strategy to design specific recognition moieties for fluorogenic probes for enzymes. [ABSTRACT FROM AUTHOR]

We have, for the first time, developed a near-infrared fluorescent probe for aminopeptidase N by combining a hemicyanine and an alanyl residue. The probe exhibits high selectivity and sensitivity with a detection limit of 0.8 ng mL−1. With the probe, aminopeptidase N has been imaged in both cells and mice in vivo, indicating a promising tool for studying the function of the enzyme. [ABSTRACT FROM AUTHOR]

Peroxynitrite (ONOO - ), an important reactive oxygen species (ROS), may be generated in the mitochondria under various physiological and pathological conditions. In this work, we designed and synthesized a mitochondria-targetable fluorescence probe RPTPP by using rhodamine as the fluorophore, phenylhydrazine as the recognition moiety, and triphenyl-phosphonium cation as the mitochondria-targeting moiety. Upon reaction of the probe with ONOO - , the oxidation of phenylhydrazine by ONOO - and the subsequent hydrolysis opens the nonfluorescent spirocyclic structure and, thus, triggers a fluorescence turn-on response, which provides a sensitive and selective method for the detection of ONOO - . The mitochondria-targeting property of RPTPP was confirmed by the colocalization experiments as well as the mitochondria uncoupling treatments. Moreover, the applications of the probe for imaging intracellular ONOO - were performed in living cells, which reveals that the ONOO - level in RAW264.7 cells undergoes an about 9-fold increase with the stimulation of LPS and IFN-γ for 15 h. [ABSTRACT FROM AUTHOR]

A resorufin-based highly sensitive and selective fluorescence off–on probe with a new recognition moiety for tyrosinase is developed, and applied to detect and image endogenous tyrosinase activity in different living cells. [ABSTRACT FROM AUTHOR]

Correction for 'Recent advances in fluorescent probes for lipid droplets' by Yanyan Zhao et al., Chem. Commun., 2022, 58, 1495–1509, DOI: 10.1039/D1CC05717K. [ABSTRACT FROM AUTHOR]

Abstract: Enzyme-coupling reactions play an important role in the assay of analytes. In this manuscript, we developed a new fluorescent probe for the detection of glucose through the enzyme-coupling reaction of DT-diaphorase (DTD). The probe was synthesized through a mild and simple synthetic procedure, and showed good fluorescence response to DTD. The reactions for the detection of glucose proceed as follows: glucose dehydrogenase oxidizes glucose to gluconolactone with NAD+ as the electron acceptor to yield NADH, and NADH can be utilized by DTD to further react with the probe releasing resorufin. As a result of these tandem reactions, fluorescence off–on response will occur. The method showed high selectivity for glucose with a detection limit of 0.2µM, which may provide a potential way for fluorescence detection of glucose through enzyme-coupling reactions. Furthermore, the applicability of the method has been demonstrated by detecting glucose in human urine samples. [Copyright &y& Elsevier]

A novel water-soluble fluorescent probe, 1-(2,3-epoxypropoxy)-2-phenylazo-8-acetylamino-naphthalene-3,6-disulfonic acid sodium salt (EPANS), was designed and synthesized by reacting epichlorohydrin with azophloxine, and its properties for labeling of histidine were studied. In alkaline media, EPANS reacted selectively with histidine rather than with other amino acids, causing a large decrease in fluorescence intensity and thereby allowing a selective detection of histidine without a tedious separation. The linear range was 4–500 μM with a detection limit of 1 μM (S/N=3). The proposed method has been successfully used to determine histidine in human serum. The obtained results were in good agreement with those given by using histidine–nickel complex adsorptive voltammetry. [Copyright &y& Elsevier]

Fluorescent probes combined with confocal microscopy are recognized as a powerful tool for imaging living cells and even organelles due to their high sensitivity and resolution. However, many of analyte-activatable and organelle-targetable fluorescent probes are developed via tedious attempts, and a relatively predictable method to design such probes is still lacking. Herein, we put forward an effective synthetic strategy to construct both targetable and responsive probes for organelles based on the cresyl violet scaffold. The approach allows access to a variety of organelle-targeting fluorescent probes for an analyte of interest via introducing the corresponding targeting and recognition groups to the 5- and 9-positions of cresyl violet, respectively. The potency of the approach is exemplified by its application to develop four cresyl violet-based fluorophores with different organelle-targeting groups, and a mitochondrion-targeting ratiometric probe capable of imaging Pd0 in living cells. •A general approach is put forward to develop both targetable and responsive fluorescent probes based on the cresyl violet scaffold. •The approach simplifies the structure identification of cresyl violet-based probes. •The potency of the method is exemplified by its application to develop four fluorophores with different organelle-targeting units, and a mitochondrion-targeting ratiometric probe capable of imaging Pd0 in living cells. [ABSTRACT FROM AUTHOR]

In this article, we presented a new rapid, sensitive and selective method for the determination of hyperin (Hyp) based on the fluorescence quenching of fluorescent carbon dots (CDs). The CDs were prepared by simply mixing an aqueous solution of citric acid with diphosphorus pentoxide. This one-step synthetic route is fast and simple with neither high temperature nor complicated synthesis steps is involved. When Hyp was added to CDs solution, the fluorescence intensity of the CDs significantly decreased. The CDs display high selectivity for Hyp over many potentially interfering substances. Under the optimized conditions, a good linear relationship between the fluorescence intensity ratio F o / F and the concentration of Hyp is obtained in a range of 0.22–55 µM with a detection limit ( S / N  = 3) of 78.3 nM. The method was successfully applied for the determination of Hyp in fufangmuji granules and human serum samples with recoveries in a range of 93.3–107.0%. This paper highlights the usefulness of CDs as an effective fluorescence probe for the Hyp detection due to its easy preparation, low-cost, excellent photostability, favorable biocompatibility and low cytotoxicity. [ABSTRACT FROM AUTHOR]

Phosphorylation mediated by protein kinases plays a pivotal role in metabolic and cell-signaling processes, and the dysfunction of protein kinases such as protein kinase A (PKA) may induce several human diseases. Therefore, it is of great significance to develop a facile and effective method for PKA activity assay and high-throughput screening of inhibitors. Herein, we develop a new fluorescent off-on method for PKA assay based on the assembly of anionic polyuridylic acid (polyU) and cationic fluorescent peptide. The phosphorylation of the peptide disrupts its electrostatic binding with polyU, suppresses the concentration quenching effect of polyU, and thus causes fluorescence recovery. The recovered fluorescence intensity at 585 nm is directly proportional to the PKA activity in the range of 0.1-3.2 U/mL with a detection limit of 0.05 U/mL. Using our method, the PKA activity in HeLa cell lysate is determined to be 58.2 ± 5.1 U/mg protein. The method has also been employed to evaluate the inhibitory effect of PKA inhibitors with satisfactory results and may be expected to be a promising candidate for facile and cost-effective assay of kinase activity and high-throughput inhibitor screening. [ABSTRACT FROM AUTHOR]

Heat shock is a heat-related pathology characterized by a high body temperature and an obvious change of many enzymatic activities. Carboxylesterase (CE), as the major hydrolase in liver, is responsible for the hydrolysis of many drugs or the detoxification of various toxins from all organs. However, the correlation between heat shock and the CE activity in cells remains unknown, mainly due to the lack of a suitable research approach. Herein, a new water-soluble fluorescence probe, MYO-CE, with a specific bipiperidinyl recognition moiety has been developed for detecting the CE activity. MYO-CE reacted selectively with CE instead of other esterase, causing a large fluorescence off-on response at 560 nm with a detection limit of 0.39 U/mL. The applicability of MYO-CE for cell imaging was demonstrated by monitoring the alteration of the hepatocellular CE activity under inflammation. More importantly, we investigated the change of the CE activity during heat shock, uncovering a significant increase for the first time. This finding was further validated by a commercial colorimetric kit assay. The proposed probe shows a promising prospect for the CE study in cells under different pathological conditions. [ABSTRACT FROM AUTHOR]

The first near-infrared fluorescent probe with excellent water-solubility for γ -glutamyl transpeptidase (GGT) has been developed by combining glutathione (GSH) as a recognition unit with a near-infrared hemicyanine fluorophore through an acrylyl linker. The probe exhibits a highly selective and sensitive fluorescent off-on response to GGT with a detection limit of 0.50 U/L, and the response mechanism is based on the enzyme-catalyzed cleavage of the γ -glutamyl bond of GSH, followed by the spontaneous intramolecular cyclization and the release of the fluorophore. Notably, the probe has been used to image GGT in zebrafish and evaluate the inhibition ability of three common inhibitors of GGT both in vitro and in vivo , revealing that their inhibition efficiencies are acivicin >6-diazo-5-oxo-L-norleucine >L-serine-borate complex, and their corresponding IC 50 values are 0.11±0.01 mM, 0.34±0.04 mM and 2.06±0.24 mM, respectively. The proposed probe is simple, and may have great potential for screening GGT inhibitors. [ABSTRACT FROM AUTHOR]

Abstract: A highly selective Nile Red-based fluorescent probe has been designed, synthesized and applied for fluorescence imaging of reduced glutathione (GSH) in live cells. The probe is constructed by incorporating 2,4-dinitrobenzenesulfonyl (DBS) group as a quencher into the Nile Red fluorophore, and itself shows weak fluorescence due to the strong intramolecular charge transfer process. Nucleophilic aromatic substitution of GSH leads to the removal of the DBS unit in the probe, accompanying a large increase in fluorescence intensity. Under the optimized conditions, the fluorescence enhancement value is directly proportional to the concentration of GSH in the range of 0.5–6μM, with a detection limit of 43nM (k =3). Moreover, the probe features long-wavelength emission of about 650nm, which could avoid the interference of short-wavelength fluorescence from biological matrixes. These properties facilitate the probe''s applicability in bioimaging, and such an application has been demonstrated on fluorescence imaging of GSH in HeLa cells. [Copyright &y& Elsevier]

Abstract: A novel fluorescent probe is designed and synthesized for the determination of cysteine in biological samples by incorporating 2,4-dinitrobenzenesulfonyl (DBS) group as a quencher into the BODIPY skeleton. The BODIPY-based probe itself shows weak fluorescence due to the strong intramolecular charge transfer process. Upon reaction with cysteine, however, the probe produces a rapid and large fluorescence enhancement through the removal of the DBS unit by nucleophilic aromatic substitution. This valuable property leads to the development of a new and simple method for cysteine assay. Under the optimized conditions, the fluorescence enhancement value is directly proportional to the concentration of cysteine in the range 2–12μM, with a detection limit of 30nM (S/N=3). The applicability of the developed method has been successfully demonstrated on the determination of non-protein cysteine in human serum. Compared to most of the existing fluorescent probes proposed for cysteine, the BODIPY-based one exhibits an excellent overall performance in terms of selectivity, sensitivity and simplicity. [ABSTRACT FROM AUTHOR]

A fluorescence probe based on cyanine fluorophore was designed and synthesized in this work, which can be used to determine viscosity and reactive oxygen species (e.g., OCl−, ONOO−) at different wavelengths. Under a low viscosity medium, the fluorescence quantum yield of the probe is very low; however, with the increase of the medium viscosity, the probe's emission at 571 nm is enhanced by nearly 25-fold due to the inhibition of intramolecular rotations. On the other hand, the probe shows a rapid and linear fluorescence response at 710 nm to OCl− or ONOO− within 1 min. The different spectral response regions of the probe permit the selective detection of both viscosity and reactive oxygen species. Furthermore, the probe is demonstrated to be cell permeable and capable of detecting the viscosity and the total amount of OCl−/ONOO− in living cells with the help of confocal microscope fluorescence imaging. Unlabelled Image • A dual-functional fluorescence probe for monitoring the reactive oxygen species and viscosity has been developed. • The probe showed different spectral response regions to the reactive oxygen species and viscosity. • The probe can be used to the selective imaging of both the reactive oxygen species and viscosity in cells. [ABSTRACT FROM AUTHOR]