A Fluorescent Probe for Early Detection of Melanoma and Its Metastasis by Specifically Imaging Tyrosinase Activity in a Mouse Model

Recent progress towards the development of fluorescent probes for the detection of disease-related enzymes

Normal physiological functions as well as regulatory mechanisms for various pathological conditions depend on the activity of enzymes. Thus, determining the in vivo activity of enzymes is crucial for monitoring the physiological metabolism and diagnosis of diseases. Traditional enzyme detection methods are inefficient for in vivo detection, which have different limitations, such as high cost, laborious, and inevitable invasive procedures, low spatio-temporal resolution, weak anti-interference ability, and restricted scope of application. Because of its non-destructive nature, ultra-environmental sensitivity, and high spatiotemporal resolution, fluorescence imaging technology has emerged as a potent tool for the real-time visualization of live cells, thereby imaging the motility of proteins and intracellular signalling networks in tissues and cells and evaluating the binding and attraction of molecules. In the last few years, significant advancements have been achieved in detecting and imaging enzymes in biological systems. In this regard, the high sensitivity and unparalleled spatiotemporal resolution of fluorescent probes in association with confocal microscopy have garnered significant interest. In this review, we focus on providing a concise summary of the latest developments in the design of fluorogenic probes used for monitoring disease-associated enzymes and their application in biological imaging. We anticipate that this study will attract considerable attention among researchers in the relevant field, encouraging them to pursue advances in the development and application of fluorescent probes for the real-time monitoring of enzyme activity in live cells and in vivo models while ensuring excellent biocompatibility.

Human NQO1 as a Selective Target for Anticancer Therapeutics and Tumor Imaging

Human NAD(P)H-quinone oxidoreductase1 (HNQO1) is a two-electron reductase antioxidant enzyme whose expression is driven by the NRF2 transcription factor highly active in the prooxidant milieu found in human malignancies. The resulting abundance of NQO1 expression (up to 200-fold) in cancers and a barely detectable expression in body tissues makes it a selective marker of neoplasms. NQO1 can catalyze the repeated futile redox cycling of certain natural and synthetic quinones to their hydroxyquinones, consuming NADPH and generating rapid bursts of cytotoxic reactive oxygen species (ROS) and H2O2. A greater level of this quinone bioactivation due to elevated NQO1 content has been recognized as a tumor-specific therapeutic strategy, which, however, has not been clinically exploited. We review here the natural and new quinones activated by NQO1, the catalytic inhibitors, and the ensuing cell death mechanisms. Further, the cancer-selective expression of NQO1 has opened excellent opportunities for distinguishing cancer cells/tissues from their normal counterparts. Given this diagnostic, prognostic, and therapeutic importance, we and others have engineered a large number of specific NQO1 turn-on small molecule probes that remain latent but release intense fluorescence groups at near-infrared and other wavelengths, following enzymatic cleavage in cancer cells and tumor masses. This sensitive visualization/quantitation and powerful imaging technology based on NQO1 expression offers promise for guided cancer surgery, and the reagents suggest a theranostic potential for NQO1-targeted chemotherapy.

Biosensors for melanoma skin cancer diagnostics

Skin cancer is a critical global public health concern, with melanoma being the deadliest variant, correlated to 80% of skin cancer-related deaths and a remarkable propensity to metastasize. Despite notable progress in skin cancer prevention and diagnosis, the limitations of existing methods accentuate the demand for precise diagnostic tools. Biosensors have emerged as valuable clinical tools, enabling rapid and reliable point-of-care (POC) testing of skin cancer. This review offers insights into skin cancer development, highlights essential cutaneous melanoma biomarkers, and assesses the current landscape of biosensing technologies for diagnosis. The comprehensive analysis in this review underscores the transformative potential of biosensors in revolutionizing melanoma skin cancer diagnosis, emphasizing their critical role in advancing patient outcomes and healthcare efficiency. The increasing availability of these approaches supports direct diagnosis and aims to reduce the reliance on biopsies, enhancing POC diagnosis. Recent advancements in biosensors for skin cancer diagnosis hold great promise, with their integration into healthcare expected to enhance early detection accuracy and reliability, thereby mitigating socioeconomic disparities.

Application of a Specific and Sensitive NQO1 Turn-On Near-Infrared Fluorescence Probe for Live Cancer Cell and Xenografted Tumor Imaging in Nude Mice

Sensitive activity stains for enzymes selectively expressed in human cancers offer valuable tools for imaging with wide applications in experimental, diagnostic, and therapeutic settings. The scant expression of the antioxidant enzyme NQO1 in normal tissues and its great abundance in malignant counterparts due to the increased redox stress and hypoxia is one such example. Previously, we described a potent nontoxic probe that remains nonfluorescent but releases an intense fluorogenic compound after intracellular cleavage by NQO1 catalysis. This infrared probe with a 644 nm emission has excellent tissue penetrating ability and low background absorption. Described here are methods (fluorescence microscopy, flow cytometry, and in vivo animal imaging) to rapidly image NQO1 activity in hypoxic and non-hypoxic cancer cells and tumors developed in live mouse xenograft models. The specificity of the dye for NQO1 in all three procedures was verified, and the methods should be useful for both in vitro and in vivo studies.

Nile Blue: A Red-Emissive Fluorescent Dye That Displays Differential Self-Assembly and Binding to G-Quadruplexes

Nile Blue (NB) is a red-emissive dye that is well-known for imaging and staining applications. In this work, we describe the interaction of NB with various types of G-quadruplexes belonging to different topologies, molecularities, and conformations. Using spectroscopic techniques, we have determined the preferential binding of NB to c-Myc G-quadruplex and the other aspects of its binding. Concentration- and temperature-dependent studies showed that NB exists in a dynamic equilibrium between monomeric and H-aggregated states, which could be modulated by the addition of external agents such as anionic surfactants. NB displayed differential self-assembly with different types of G-quadruplex and duplex DNAs modulating its dynamic equilibrium between the monomeric and H-aggregated states. Fluorescence-based displacement studies revealed a 1:1 binding stoichiometry upon interaction with c-Myc G-quadruplex and an association constant of Kapp = 6.7 × 106 M-1. Circular dichroism studies indicated that NB does not cause changes in the overall conformation of either G-quadruplexes or duplexes; however, it does indicate nucleic acid-dependent self-assembly at higher concentrations. Heat capacity measurement showed a more negative change when compared to that in DNA duplex, indicating more burial of the polar surface area by NB to the G-quadruplex host.

Activatable probes with potential for intraoperative tumor-specific fluorescence-imaging guided surgery

Owing to societal development and aging population, the impact of cancer on human health and quality of life has increased. Early detection and surgical treatment are the most effective approaches for most cancer patients. As the scope of conventional tumor resection is determined by auxiliary examination and surgeon experience, there is often insufficient recognition of tiny tumors. The ability to detect such tumors can be improved by using fluorescent tumor-specific probes for surgical navigation. This review mainly describes the design principles and mechanisms of activatable probes for the fluorescence imaging of tumors. This type of probe is nonfluorescent in normal tissue but exhibits obvious fluorescence emission upon encountering tumor-specific substrates, such as enzymes or bioactive molecules, or changes in the microenvironment, such as a low pH. In some cases, a single-factor response does not guarantee the effective fluorescence labeling of tumors. Therefore, two-factor-activatable fluorescence imaging probes that react with two specific factors in tumor cells have also been developed. Compared with single biomarker testing, the simultaneous monitoring of multiple biomarkers may provide additional insight into the role of these substances in cancer development and aid in improving the accuracy of early cancer diagnosis. Research and progress in this field can provide new methods for precision medicine and targeted therapy. The development of new approaches for early diagnosis and treatment can effectively improve the prognosis of cancer patients and help enhance their quality of life.

Novel Green Fluorescent Probe Stem From Carbon Quantum Dots for Specific Recognition of Tyrosinase in Serum and Living Cells

Tyrosinase (TYR), an important biomarker for melanoma, offered significant information early detection of melanoma and may decrease the likelihood of mortality. Therefore, this article constructed a highly sensitive and selective green fluorescent functionalized carbon quantum dots (TYR-CQDs) for tyrosinase (TYR) activity detection by one-step hydrothermal protocol utilizing catechol, citric acid and urea as precursors. The prepared TYR-CQDs illustrated excellent linear relationship and broad linear range with a low detection limit, which exhibited high accuracy and recovery in quantitative determination of TYR in human serum samples. Furthermore, the TYR-CQDs had successfully realized intracellular TYR detection owing to excellent biocompatibility, high anti-interference ability and good cellular imaging capability, suggesting the potential biomedical applications in early diagnosis of melanoma and other tyrosinase-related diseases.

A tyrosinase fluorescent probe with large Stokes shift and high fluorescence enhancement for effective identification of liver cancer cells

Tyrosinase is widely regarded as an important biomarker for melanocytic and liver cancer. However, most currently reported tyrosinase probes have been focused on malignant melanoma study, and few tyrosinase probe have been applied for liver cancer investigation. Herein, we developed a novel probe HFC-TYR for sensitive and selective tracking of tyrosinase activity at enzyme and cellular level, and investigated its application for liver cancer diagnosis. As expected, HFC-TYR has excellent response ability for tyrosinase sensing at enzyme level, such as large Stokes shift (170 nm), high fluorescence enhancement (178-fold), low detection limit (0.12 U/mL), which indicates its potential for efficient identification of endogenous tyrosinase activity at cellular levels. Unsurprisingly, HFC-TYR is proved to be able detect endogenous tyrosinase levels in various living cells. More importantly, HFC-TYR is successfully used to distinguish HepG2 cells from other cells (SKOV3, HeLa and 293T), indicating that tyrosinase is overexpressed in HepG2 cells and HFC-TYR can specifically identify HepG2 cells at cellular level. Meanwhile, HFC-TYR is able to further monitor the endogenous tyrosinase activity in zebrafish models. Therefore, all the findings confirm that HFC-TYR has the application potential of liver cancer diagnosis.

Persistent and Efficient Multimodal Imaging for Tyrosinase Based on Two-Photon Excited Fluorescent and Room-Temperature Phosphorescent Probes

Tyrosinase is crucial to regulate the metabolism of phenol derivatives, playing an important role in the biosynthesis of melanin pigments, whereas an abnormal level of tyrosinase would lead to severe diseases. It is rather necessary to develop a sensitive and selective imaging tool to assess the level of tyrosinase in vivo. We thoroughly researched the luminous mechanism of the existing TPTYR probe and provided design strategies to improve its two-photon excited fluorescence properties. The designed probes benza2-TPTYR and product benza2-TPTYR-coumarin have large two-photon absorption cross sections at the NIR spectral region (41 GM/706 nm, 71 GM/852 nm), while benza2-TPTYR-coumarin possesses easily distinguishable spectrum in the visible region and a high fluorescence efficiency (Φ_F = 0.27). What is more, novel two-photon excited multimodal imaging based on the pure organic small molecule benza1-TPTYR-coumarin (61 GM/936 nm) is proposed first, simultaneously possessing strong instantaneous fluorescent (563.79 nm) and persistent room-temperature phosphorescent emissions (767.68 nm, 0.54 ms).

Recent Advances in the Enzyme-Activatable Organic Fluorescent Probes for Tumor Imaging and Therapy

The exploration of advanced probes for cancer diagnosis and treatment is of high importance in fundamental research and clinical practice. In comparison with the traditional "always-on" probes, the emerging activatable probes enjoy advantages in promoted accuracy for tumor theranostics by specifically releasing or activating fluorophores at the targeting sites. The main designing principle for these probes is to incorporate responsive groups that can specifically react with the biomarkers (e. g., enzymes) involved in tumorigenesis and progression, realizing the controlled activation in tumors. In this review, we summarize the latest advances in the molecular design and biomedical application of enzyme-responsive organic fluorescent probes. Particularly, the fluorophores can be endowed with ability of generating reactive oxygen species (ROS) to afford the photosensitizers, highlighting the potential of these probes in simultaneous tumor imaging and therapy with rational design. We hope that this review could inspire more research interests in the development of tumor-targeting theranostic probes for advanced biological studies.

Ultrasensitive Fibroblast Activation Protein-α-Activated Fluorogenic Probe Enables Selective Imaging and Killing of Melanoma In Vivo

Melanoma is a malignant cancer with a high risk of metastasis and continued increase in death rates over the past decades, and its prognosis is highly related to the disease's stage, while early detection and treatment of melanoma are significant to the improvement of its therapy outcome. Different from the traditional methods for disease diagnosis, enzyme-activated fluorescent probes were developed rapidly due to their high sensitivity and temporal-spatial ratio and have been widely applied in tumor detection, surgical navigation, and cancer-related research. Fibroblast activation protein-α (FAPα), a serine-type cell surface protease that plays important roles in cell invasion and extracellular matrix degradation, is widely involved in tumor progression such as malignant melanoma, so developing a FAPα activity-based molecular tool would be of great potential for the early diagnosis and therapy of melanoma. However, few fluorescent probes targeting FAPα have been applied in melanoma-related studies, and thus, the construction of FAPα activity-based fluorescent probes for melanoma detection is in urgent need. By incorporating the selective recognition unit with a red-emission fluorophore, cresyl violet, we herein report an ultrasensitive (limit of detection = 5.3 ng/mL) fluorogenic probe for FAPα activity sensing, named CV-FAP; the acquired probe showed a significantly higher binding affinity (15.7-fold) and overall catalytic efficiency (2.6-fold) when compared with those of the best reported FAPα probes. The good performance of CV-FAP made it possible to discriminate malignant melanoma cells and tumor-bearing mice from normal cells and mice with high contrast. More importantly, CV-FAP showed significant antitumor activity toward melanoma in cultured cells and tumor-bearing nude mice (over 95% inhibited tumor growth) with good safety, which made it an ideal theranostic agent for melanoma.

Self-assembled peptides-modified flexible field-effect transistors for tyrosinase detection

Flexible biosensors have received intensive attention for real-time, non-invasive monitoring of cancer biomarkers. Highly sensitive tyrosinase biosensors, which are important for melanoma screening, remained a hurdle. Herein, high-performance tyrosinase-sensing field-effect transistor-based biosensors (bio-FETs) have been successfully achieved by self-assembling nanostructured tetrapeptide tryptophan-valine-phenylalanine-tyrosine (WVFY) on n-type metal oxide transistors. In the presence of target tyrosinase, the phenolic hydroxyl groups in WVFY are rapidly converted to benzoquinone with the consumption of protons, which could be detected potentiometrically by bio-FETs. As a result, the WVFY-modified bio-FETs exhibited an ultra-low detection limit of 1.9 fM and an optimal detection range of 10 fM to 1 nM toward tyrosinase sensing. Furthermore, flexible devices fabricated on ∼2.9-μm-thick polyimide (PI) substrates illustrated robust mechanical flexibility, which could be attached to human skin conformally. These achievements hold promise for wearable melanoma screening and provide designing guidelines for detecting other important cancer biomarkers with bio-FETs.

Turn on chemiluminescence-based probes for monitoring tyrosinase activity in conjunction with biological thiols

We report a chemiluminescent probe (CLPT1) that permits the paired detection of tyrosinase (Tyr) and biological thiols. Tyr only leads to a poor chemiluminescence response, a finding ascribed to the formation of a stable o-benzoquinone intermediate. The addition of glutathione (GSH), or ascorbate to the o-benzoquinone intermediate results in thiol conjugation or reduction to this intermediate, respectively. This produces a strong chemiluminescence response. Thiol co-dependence was demonstrated in live cells using the cell permeable analogue, CLPT3. The present chemiluminescence-based strategy allows the concurrent detection of tyrosinase activity and biological thiols.

An HDAC8-selective fluorescent probe for imaging in living tumor cell lines and tissue slices

Histone deacetylase 8 (HDAC8) has been used as a therapeutic target for many cancers as it is highly expressed in neuroblastoma cells and breast cancer cells. HDAC8-selective fluorescent probes need to be urgently developed. Herein, two novel fluorescent probes, namely NP-C6-PCI and AM-C6-PCI, based on the conjugation of 1,8-naphthalimide with a highly selective inhibitor of HDAC8 (PCI-34051) were reported. Compared with PCI-34051 (KD = 6.25 × 10-5 M), NP-C6-PCI (KD = 8.05 × 10-6 M) and AM-C6-PCI (KD = 7.42 × 10-6 M) showed great selectivity toward HDAC8. Two fluorescent probes exhibited high fluorescence intensity under λex = 450 nm and a large Stokes shift (100 nm). NP-C6-PCI was selected for cell and tissue imaging due to the similarity in the bioactivity of NP-C6-PCI with PCI-34051. The ability of NP-C6-PCI to target imaging HDAC8 in SH-SY5Y and MDA-MB-231 tumor cells was demonstrated. Furthermore, NP-C6-PCI was applied to imaging SH-SY5Y tumor tissue slices to indicate the relative expression level of HDAC8. Therefore, this HDAC8-selective fluorescent probe can be expected for applications in HDAC8-targeted drug screening as well as in pathologic diagnoses.

Fluorescent probes for visualizing ROS-associated proteins in disease

Abnormal expression of proteins, including catalytic and expression dysfunction, is directly related to the development of various diseases in living organisms. Reactive oxygen species (ROS) could regulate protein expression by redox modification or cellular signal pathway and thus influence the development of disease. Determining the expression level and activity of these ROS-associated proteins is of considerable importance in early-stage disease diagnosis and the identification of new drug targets. Fluorescence imaging technology has emerged as a powerful tool for specific in situ imaging of target proteins by virtue of its non-invasiveness, high sensitivity and good spatiotemporal resolution. In this review, we summarize advances made in the past decade for the design of fluorescent probes that have contributed to tracking ROS-associated proteins in disease. We envision that this review will attract significant attention from a wide range of researchers in their utilization of fluorescent probes for in situ investigation of pathological processes synergistically regulated by both ROS and proteins.

Spectrophotometric Assays for Sensing Tyrosinase Activity and Their Applications

Tyrosinase (TYR, E.C. 1.14.18.1), a critical enzyme participating in melanogenesis, catalyzes the first two steps in melanin biosynthesis including the ortho-hydroxylation of L-tyrosine and the oxidation of L-DOPA. Previous pharmacological investigations have revealed that an abnormal level of TYR is tightly associated with various dermatoses, including albinism, age spots, and malignant melanoma. TYR inhibitors can partially block the formation of pigment, which are always used for improving skin tone and treating dermatoses. The practical and reliable assays for monitoring TYR activity levels are very useful for both disease diagnosis and drug discovery. This review comprehensively summarizes structural and enzymatic characteristics, catalytic mechanism and substrate preference of TYR, as well as the recent advances in biochemical assays for sensing TYR activity and their biomedical applications. The design strategies of various TYR substrates, alongside with several lists of all reported biochemical assays for sensing TYR including analytical conditions and kinetic parameters, are presented for the first time. Additionally, the biomedical applications and future perspectives of these optical assays are also highlighted. The information and knowledge presented in this review offer a group of practical and reliable assays and imaging tools for sensing TYR activities in complex biological systems, which strongly facilitates high-throughput screening TYR inhibitors and further investigations on the relevance of TYR to human diseases.

Construction of a near-infrared fluorescent probe for ratiometric imaging of peroxynitrite during tumor progression

Malignant tumors are one of the main causes for human death and are tightly associated with overexpression of reactive oxygen species (ROS) in pathological processes. Therefore, in vivo monitoring of ROS, especially ONOO-, remains of great significance for diagnosis and therapy of tumors to improve the survival rate. Herein, we designed and constructed a reliable near-infrared (NIR) ratiometric fluorescent biosensor CDMS for monitoring the fluctuations of ONOO- in the process of tumor progression. CDMS featured outstanding stability to photoirradiation, substantial quantum yields, rapid response (<5 s), high selectivity and excellent biocompatibility. Moreover, CDMS exhibited distinct ratiometric fluorescence signal changes after reacting with ONOO-. Fluorescence imaging in immune stimulated cells indicated that CDMS was competent to determine the levels of ONOO- in the cellular level. Remarkably, CDMS was further applied in monitoring the expression of ONOO- in a peritonitis mouse model and tumor-bearing mouse model. Based on the excellent properties of CDMS, the probe exhibited the potential for noninvasive in vivo visualization of ONOO- in the occurrence and process of tumor development. It is envisioned that CDMS can be employed as a promising tool for monitoring the ONOO- fluxes in tumor pathological progression, especially for tumor diagnosis and therapy.

Trends in small organic fluorescent scaffolds for detection of oxidoreductase

Oxidoreductases are diverse class of enzymes engaged in modulating the redox homeostasis and cellular signaling cascades. Abnormal expression of oxidoreductases including thioredoxin reductase, azoreductase, cytochrome oxidoreductase, tyrosinase and monoamine oxidase leads to the initiation of numerous disorders. Thus, enzymes are the promising biomarkers of the diseased cells and their accurate detection has utmost significance for clinical diagnosis. The detection method must be extremely selective, sensitive easy to use, long self-life, mass manufacturable and disposable. Fluorescence assay approach has been developed potential substitute to conventional techniques used in enzyme's quantification. The fluorescent probes possess excellent stability, high spatiotemporal ratio and reproducibility represent applications in real sample analysis. Therefore, the enzymatic transformations have been monitored by small activatable organic fluorescent probes. These probes are generally integrated with enzyme's substrate/inhibitors to improve their binding affinity toward the enzyme's catalytic site. As the recognition unit bio catalyzed, the signaling unit produces the readout signals and provides novel insights to understand the biochemical reactions for diagnosis and development of point of care devices. Several structural modifications are required in fluorogenic scaffolds to tune the selectivity for a particular enzyme. Hence, the fluorescent probes with their structural features and enzymatic reaction mechanism of oxidoreductase are the key points discussed in this review. The basic strategies to detect each enzyme are discussed. The selectivity, sensitivity and real-time applications are critically compared. The kinetic parameters and futuristic opportunities are present, which would be enormous benefits for chemists and biologists to understand the facts to design and develop unique fluorophore molecules for clinical applications.

Review on the recent progress in the development of fluorescent probes targeting enzymes

Enzymes are very important for biological processes in a living being, performing similar or multiple tasks in and out of cells, tissues and other organisms at a particular location. The abnormal activity of particular enzyme usually caused serious diseases such as Alzheimer's disease, Parkinson's disease, cancers, diabetes, cardiovascular diseases, arthritis etc. Hence, nondestructive and real-time visualization for certain enzyme is very important for understanding the biological issues, as well as the drug administration and drug metabolism. Fluorescent cellular probe-based enzyme detectionin vitroandin vivohas become broad interest for human disease diagnostics and therapeutics. This review highlights the recent findings and designs of highly sensitive and selective fluorescent cellular probes targeting enzymes for quantitative analysis and bioimaging.

Nitrogen and fluorine co-doped green fluorescence carbon dots as a label-free probe for determination of cytochrome c in serum and temperature sensing

As an important biomarker, the analysis of cytochrome c (Cyt c) plays a crucial role in cell-apoptosis or even cancer diagnosis. This work develops a label-free probe for Cyt c using the nitrogen and fluorine co-doped carbon dots (N, F-CDs) which were facile prepared through solvothermal method with 3, 4-difluorophenylhydrazine as precursor. The N, F-CDs have an average diameter of 3.4 nm, and can form a quite stable colloidal solution. The N, F-CDs show bright yellow-green fluorescence, excitation/emission wavelengths 475/530 nm, and a relatively high fluorescence quantum yield of 16.9%. Interestingly, the N, F-CDs indicate a linear and reversible variation of emission intensity with a sensitivity of -1.11% per ℃ in the temperature range from 25 to 60 ℃. Inner filter effect (IFE) between N, F-CDs and Cyt c turns the fluorescence of N, F-CDs from "on" to "off". The sensor possesses the excellent anti-interference ability towards the main components of plasma. Under optimum conditions, there is a linear relationship between fluorescence intensity function (F₀-F) and the concentration of Cyt c in the range of 0.5-25 μΜ with a limit of detection (LOD) (S/N = 3) of 0.25 μM. Finally, the developed method has been successfully used to detect Cyt c in human serum sample with satisfactory recoveries in a range of 93.14-110.40%.

Detection of Tyrosinase in Real Food Samples and Living Cells by a Novel Near-Infrared Fluorescence Probe

A new near-infrared fluorescence probe was developed and applied to the fluorescence detection of tyrosinase in real food samples and living cells. The probe (E)-2-(2-(6-((3-hydroxybenzyloxy)carbonylamino)-2,3-dihydro-1H-xanthen-4-yl)vinyl)-3,3-dimethyl-1-propyl-3H-indolium (1) was designed and synthesized by coupling 3-hydroxybenzyl alcohol via carbamate bond with an amino hemicyanine skeleton, based on the high anti-interference ability of 3-hydroxybenzyl alcohol to reactive oxygen species and its binding affinity to tyrosinase. Compared with the existing tyrosinase probes, the proposed probe exhibits superior analytical performance, such as high selectivity, high sensitivity, superior spatiotemporal sampling ability, fluorescence signal switching at 706 nm, and low detection limit of 0.36 U mL^-1. More importantly, the probe has been successfully used to monitor tyrosinase in the browning of apple slices for the first time, and the results indicated that the strongest fluorescence intensity could be achieved at 2.5 h to realize precise visual recognition of tyrosinase. Notably, the probe determined tyrosinase in real food samples (apple, banana, cheese, and red wine) with a stable average recovery range of 95.7-108.3% and has been successfully used to monitor tyrosinase in the living B16 cells. The superior properties of the probe make it of great potential use in food nutritional value evaluation and clinical diagnosis of melanin-related diseases.

Fluorescence copolymer-based dual-signal monitoring tyrosinase activity and its inhibitor screening via blue-green emission transformation

Tyrosinase (TYR) is a crucial enzyme in melanin metabolism and catecholamine production, its abnormal overexpression is closely associated with many human diseases involving melanoma cancer, vitiligo, Parkinson's disease and so on. Herein, a dual-signal fluorescence sensing system for monitoring TYR activity is constructed depending on the transformation of blue-green fluorescence emission of copolymer. The developed sensing system is based on TYR catalyzing the hydroxylation of mono-phenol to o-diphenol and the conversion of fluorescence copolymer (FCP) blue emission (430 nm) and green emission (535 nm) in the presence of PEI. In the system, both blue and green emission exhibit a high selectivity and sensitivity (S/B up to 300 and 30 for blue and green emission, respectively) toward TYR in the range from 0.5 to 2.5 U/mL with the detection limit of 0.002 U/mL and 0.06 U/mL, respectively. Additionally, this assay is used to detect TYR in human serum with excellent recovery even at 30% human serum concentrations. Furthermore, it still has been successfully applied to TYR inhibitor screening by taking kojic acid as a model. We believe that our developed sensor has great potential application in TYR-associated disease diagnosis and treatment and drug discovery.

A Simple Aggregation-Induced Emission Nanoprobe with Deep Tumor Penetration for Hypoxia Detection and Imaging-Guided Surgery in Vivo

The pan-cancer detection and precise visualization of tiny tumors in surgery still face great challenges. As tumors grow aggressively, hypoxia is a common feature of solid tumors and has supplied a general way for detecting tumors. Herein, we report a simple aggregation-induced emission nanoprobe-TPE-4NE-O that can specifically switch on their fluorescence in the presence of cytochrome P450 reductase, a reductase which is overexpressed under hypoxia conditions. The probe can selectively light up the hypoxia cells and has shown enhanced deep tumor penetration via charge conversion both in vitro and in vivo. After being modified with FA-DSPE-PEG, higher tumor uptake can be seen and FA-DSPE/TPE-4NE-O showed specific visualization to the hypoxia cancer cells. Excitingly, much brighter fluorescence was accumulated at the tumors in the FA-DSPE/TPE-4NE-O group, even though the tumor was as small as 2.66 mm. The excellent performance of FA-DSPE/TPE-4NE-O in detecting tiny tumors has made it possible for imaging-guided tumor resection. More importantly, the probe exhibited good biocompatibility with negligible organ damage and eliminated a hemolysis risk. The simple but promising probe has supplied a new strategy for pan-cancer detection and tiny tumor visualization, which have shown great potential in clinical translation.

Catalysis-based specific detection and inhibition of tyrosinase and their application

Tyrosinase is an important enzyme in controlling the formation of melanin in melanosome, and plays a key role in the pigmentation of hair and skin. The abnormal expression or activation of tyrosinase is associated with several diseases such as albinism, vitiligo, melanoma and Parkinson disease. Excessive deposition of melanin could cause diseases such as freckles and brown spots in the human body, and it is also closely related to browning of fruits and vegetables and insect molting. Detecting and inhibiting the activity of tyrosinase is of extraordinary value in the progress of diagnosis and treatment of these diseases. Therefore, many selective optical detection probes and small molecular inhibitors have been developed, and have made significant contributions to the basic and clinical research on these diseases. In this paper, the detection and inhibition of tyrosinase and their application in whitening products are reviewed, with special emphasis on development of fluorescent probes and inhibitors. Hopefully, this review will help design more efficient and sensitive tyrosinase probes and inhibitors, as well as shed light on novel treatment of diseases such as melanoma.

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The abnormal expression or activation of tyrosinase is the pathogenesis of several diseases such as albinism, vitiligo, and melanoma.

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Detecting and inhibiting tyrosinase activity is of great value in the diagnosis and treatment of these diseases.

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The detection/inhibition of tyrosinase and its application in whitening products are reviewed, with special emphasis on probes/inhibitors.

Substrate-Photocaged Enzymatic Fluorogenic Probe Enabling Sequential Activation for Light-Controllable Monitoring of Intracellular Tyrosinase Activity

Tyrosinase (TYR) is a crucial enzyme involved in melanogenesis, and its overexpression is closely associated with melanoma. To precisely monitor intracellular TYR activity, remote control of a molecule imaging tool is highly meaningful but remains to be explored. In this work, we present the first photocaged tyrosinase fluorogenic probe by caging the substrate of the enzymatic probe with a photolabile group. Because of the sequential light and enzyme-activation feature, this probe exhibits photocontrollable "turn on" response toward TYR with good selectivity and high sensitivity (detection limit: 0.08 U/mL). Fluorescence imaging results validate that the caged probe possesses the capability of visualizing intracellular endogenous tyrosinase activity in a photocontrol fashion, thus offering a promising molecule imaging tool for investigating TYR-related physiological function and pathological role. Moreover, our sequential activation strategy has great potential for developing more photocontrollable enzymatic fluorogenic probes with spatiotemporal resolution.

Dual-Readout Tyrosinase Activity Assay Facilitated by a Chromo-Fluorogenic Reaction between Catechols and Naphthoresorcin

Analyte-responsive chromo-fluorogenic reactions under accessible conditions are important for designing small-molecule spectroscopic probes. We describe a series of newly constructed motifs based on the chromo-fluorogenic reaction between catechol derivatives (typically hydroxytyrosol, dopamine, and levodopa) and naphthoresorcin (NR) in aqueous solution under ambient conditions. The weakly absorptive and fluorogenic catechols/NR was converted to products having visible absorption and bright fluorescence within several minutes. The chromo-fluorophores produced from this reaction had a maximum absorbance at 458 nm and emission at 480 nm with high fluorescence quantum yields (30-84%). Inspired by the tyrosinase-catalyzed hydroxylation of monophenols to catechols, the tyrosinase-enabled chromo-fluorogenic reaction was verified by using monophenol (typically tyrosol) as the substrate. In this regard, a dual-readout tyrosinase activity assay was developed by virtue of the in situ "turn-on" optical signals. Furthermore, a test of tyrosinase inhibition, by using a common inhibitor kojic acid, demonstrated the potential of the chromo-fluorogenic reaction for developing other tyrosinase related assays and signal transduction.

Fast tyrosinase detection in early stage melanoma with nanomolar sensitivity using a naphthalimide-based fluorescent read-out probe

We report an expeditious approach for selective tyrosinase detection in early stage melanoma with nanomolar sensitivity using a napthalimide-based fluorescent probe.

Electrochemistry-Regulated Recyclable SERS Sensor for Sensitive and Selective Detection of Tyrosinase Activity

Tyrosinase (TYR) which can catalyze the oxidation of catechol is recognized as a significant biomarker of melanocytic lesions, thus developing powerful methods for the determination of TYR activity is highly desirable for the early diagnosis of melanin-related diseases, including melanoma. Herein, we develop a novel portable and recyclable surface-enhanced Raman scattering (SERS) sensor, prepared by assembling gold nanoparticles and p-thiol catechol ( p-TC) on an ITO electrode, for detecting TYR activity via the SERS spectral variation caused by the conversion of p-TC into its corresponding quinone under TYR catalysis. The developed SERS sensor has a rapid response to TYR within 1 min under the optimized conditions and shows high selectivity for TYR with the detection limit at 0.07 U/mL. Importantly, this SERS sensor can be easily regulated by applying negative voltage to achieve circular utilization, favoring the automation of SERS detection. Furthermore, the presented recyclable SERS sensor can perform well on both the determination of TYR activity in serum and the assessment of TYR inhibitor, demonstrating huge potential in the sensitive, selective, and facile detection of TYR activity for disease diagnosis and drug screening related with TYR.

A colorimetric and near -infrared ratiometric fluorescent probe for the determination of endogenous tyrosinase activity based on cyanine aggregation

A ratiometric fluorescent probe for TYR activity with high sensitivity was developed based on the H-aggregation of a cyanine dye.

A novel hydrosoluble near-infrared fluorescent probe for specifically monitoring tyrosinase and application in a mouse model

A novel hydrosoluble near-infrared fluorescent probe is applied to imaging and detection of endogenous tyrosinase in living cells, zebrafish and a mouse model.