Organelle-specific detection of phosphatase activities with two-photon fluorogenic probes in cells and tissues

Fluorescent Probes for Disease Diagnosis

The identification and detection of disease-related biomarkers is essential for early clinical diagnosis, evaluating disease progression, and for the development of therapeutics. Possessing the advantages of high sensitivity and selectivity, fluorescent probes have become effective tools for monitoring disease-related active molecules at the cellular level and in vivo. In this review, we describe current fluorescent probes designed for the detection and quantification of key bioactive molecules associated with common diseases, such as organ damage, inflammation, cancers, cardiovascular diseases, and brain disorders. We emphasize the strategies behind the design of fluorescent probes capable of disease biomarker detection and diagnosis and cover some aspects of combined diagnostic/therapeutic strategies based on regulating disease-related molecules. This review concludes with a discussion of the challenges and outlook for fluorescent probes, highlighting future avenues of research that should enable these probes to achieve accurate detection and identification of disease-related biomarkers for biomedical research and clinical applications.

High-Fidelity Spatiotemporal Recognition of Golgi ALP through an Initial-Accumulation and Postactivation Strategy

Various signal molecules mediate complex physiological processes collectively in the Golgi. However, most currently accessible probes are questionable in illuminating the functions of these reactive species in Golgi because of the inability to irradiate these probes only at the desired Golgi location, which compromises specificity and accuracy. In this study, we rationally designed the first photocontrollable and Golgi-targeted fluorescent probe to in situ visualize the Golgi alkaline phosphatase (ALP). The designed probe with natural yellow fluorescence can provide access into Golgi and monitor the exact timing of accumulation in Golgi. On-demand photoactivation at only the desired Golgi location affords a significant emission response to ALP with illuminating red fluorescence at 710 nm. Through the photocontrollable fluorescence responsiveness to ALP, precise spatiotemporal recognition of Golgi ALP fluctuations is successfully performed. With this probe, for the first time, we revealed the Golgi ALP levels during cisplatin-induced acute kidney injury (AKI), which will further facilitate and complement the comprehensive exploration of ALP kinetics during physiological and pathological processes.

Fluorogenic Chemical Probes for Wash-free Imaging of Cell Membrane Damage in Ferroptosis, Necrosis, and Axon Injury

Selectively labeling cells with damaged membranes is needed not only for identifying dead cells in culture, but also for imaging membrane barrier dysfunction in pathologies in vivo. Most membrane permeability stains are permanently colored or fluorescent dyes that need washing to remove their non-uptaken extracellular background and reach good image contrast. Others are DNA-binding environment-dependent fluorophores, which lack design modularity, have potential toxicity, and can only detect permeabilization of cell volumes containing a nucleus (i.e., cannot delineate damaged volumes in vivo nor image non-nucleated cell types or compartments). Here, we develop modular fluorogenic probes that reveal the whole cytosolic volume of damaged cells, with near-zero background fluorescence so that no washing is needed. We identify a specific disulfonated fluorogenic probe type that only enters cells with damaged membranes, then is enzymatically activated and marks them. The esterase probe MDG1 is a reliable tool to reveal live cells that have been permeabilized by biological, biochemical, or physical membrane damage, and it can be used in multicolor microscopy. We confirm the modularity of this approach by also adapting it for improved hydrolytic stability, as the redox probe MDG2. We conclude by showing the unique performance of MDG probes in revealing axonal membrane damage (which DNA fluorogens cannot achieve) and in discriminating damage on a cell-by-cell basis in embryos in vivo. The MDG design thus provides powerful modular tools for wash-free in vivo imaging of membrane damage, and indicates how designs may be adapted for selective delivery of drug cargoes to these damaged cells: offering an outlook from selective diagnosis toward therapy of membrane-compromised cells in disease.

Visual monitoring of biocatalytic processes using small molecular fluorescent probes: strategies-mechanisms-applications

Real-time monitoring of biocatalytic-based processes is significantly improved and simplified when they can be visualized. Visual monitoring can be achieved by integrating a fluorescent unit with the biocatalyst. Herein, we outline the design strategies of fluorescent probes for monitoring biocatalysis: (1) probes for monitoring biocatalytic transfer: γ-glutamine is linked to the fluorophore as both a recognition group and for intramolecular charge transfer (ICT) inhibition; the probe is initially in an off state and is activated via the transfer of the γ-glutamine group and the release of the free amino group, which results in restoration of the "Donor-π-Acceptor" (D-π-A) system and fluorescence recovery. (2) Probes for monitoring biocatalytic oxidation: a propylamine is connected to the fluorophore as a recognition group, which cages the hydroxyl group, leading to the inhibition of ICT; propylamine is oxidized and subsequently β-elimination occurs, resulting in exposure of the hydroxyl group and fluorescence recovery. (3) Probes for monitoring biocatalytic reduction: a nitro group attached to a fluorophore as a fluorescence quenching group, this is converted to an amino group by catalytic reduction, resulting in fluorescence recovery. (4) Probes for monitoring biocatalytic hydrolysis: β-D-galactopyranoside or phosphate acts as a recognition group attached to hydroxyl groups of the fluorophore; the subsequent biocatalytic hydrolysis reaction releases the hydroxyl group resulting in fluorescence recovery. Following these 4 mechanisms, fluorophores including cyanine, coumarin, rhodamine, and Nile-red, have been used to develop systems for monitoring biocatalytic reactions. We anticipate that these strategies will result in systems able to rapidly diagnose and facilitate the treatment of serious diseases.

Styrylpyrimidine chromophores with bulky electron-donating substituents: experimental and theoretical investigation

Styrylpyrimidines with bulky 9,9-dimethylacridan, phenoxazine and phenothiazine electron-donating fragments were designed. Thermally activated delayed fluorescence (TADF) properties were expected for these structures. These chromophores exhibit peculiar emission properties. For 9,9-dimethylacridan and phenoxazine derivatives, a single emission highly sensitive to the polarity is observed in solution whereas for phenothiazine derivative a dual emission is observed in solution and is attributed to the coexistence of quasi-axial (Qax) and quasi-equatorial (Qeq) conformers. This study intends to understand through theoretical and experimental works, why the studied chromophores do not exhibit TADF properties, contrary to what was expected. The absence of phosphorescence both at room temperature and 77 K tends to indicate the impossibility to harvest triplet states in these systems. Wave-function based calculations show that for both conformers of the three chromophores the S1-T1 splitting is significantly larger than 0.2 eV. The second triplet state T2 of Qeq conformers is found very close in energy to the singlet S1 state, but S1 and T2 states possess similar charge transfer characters. This prevents efficient spin-orbit coupling between the states, which is consistent with the absence of TADF.

Ionic Solvent-Assisted MAPbBr3 Perovskite Film for Two-Photon Pumped Single-Mode Laser

Miniaturized coherent light sources on the nanoscale are highly desired for on-chip photonics integration. However, when approaching the diffraction limit, the sub-wavelength-scale all-dielectric lasers are difficult to realize due to the trade-off between lasing performance and physical size. Especially for a thin-film laser, usually an externally complex cavity is required to provide the necessary optical feedback. Herein, we successfully shrink the MAPbBr3 perovskite thin-film laser to sub-wavelength scale (300 nm) with simplified cavity design using only an ultraviolet glue layer and a quartz glass. The morphology quality and the gain properties of the film are enhanced by introducing ionic liquid. Consequently, the stable and low-threshold single-mode laser with a highly linear polarization degree of 78.6% and a narrow line width of 0.35 nm is achieved under two-photon excitation. The excellent single-mode laser with sub-wavelength scale and ultrasimplified structure could provide a facile and versatile platform for future integrated optoelectronic devices.

An activatable near-infrared molecular reporter for fluoro-photoacoustic imaging of liver fibrosis

Noninvasive and accurate detection of liver fibrosis is extremely significant for well-timed intervention and treatment to prevent or reverse its progression. Fluorescence imaging probes hold great potential for imaging of liver fibrosis, but they always encounter the inherent limitation of shallow penetration depth, which compromises their ability of in vivo detection. To overcome this issue, an activatable fluoro-photoacoustic bimodal imaging probe (IP) is herein developed for specific visualization of liver fibrosis. The probe IP is constructed on a near-infrared thioxanthene-hemicyanine dye that is caged with gamma-glutamyl transpeptidase (GGT) responsive substrate and linked with integrin-targeted peptide (cRGD). Such molecular design permits IP to effectively accumulate in the liver fibrosis region through specific recognition of cRGD towards integrin and activate its fluoro-photoacoustic signal after interaction with overexpressed GGT to precisely monitor the liver fibrosis. Thus, our study presents a potential strategy to design dual-target fluoro-photoacoustic imaging probes for noninvasive detection of early-stage liver fibrosis.

Photothermal visual sensing of alkaline phosphatase based on the etching of Au@MnO2 core-shell nanoparticles

Alkaline phosphatase (ALP), as a crucial enzyme involved in many physiological activities, is always used as one of the significant biomarkers in clinical diagnosis. Herein, a novel, simple, and effective photothermal quantitative method based on the etching of MnO₂-coated gold nanoparticles (Au@MnO₂ NPs) was established for ALP activity assay with a household thermometer-based visual readout. The photothermal effect of Au@MnO₂ NPs is much higher than that of MnO₂ NPs or Au NPs. The MnO₂ shell of Au@MnO₂ NPs can be etched by ascorbic acid, a product of ALP-catalyzed hydrolysis of 2-phospho-l-ascorbic acid. With the etching of Au@MnO₂ NPs, the photothermal conversion efficiency decreased gradually, causing the decrease of the temperature increment of the solutions by degrees. A household thermometer, instead of large-scale and professional instruments, was used as a signal reader to realize the visual quantitative detection. The photothermal platform was used successfully for the determination of ALP with a wide linear range from 2.0 to 50 U/L and a detection limit as low as 0.75 U/L. Moreover, the inhibition efficiency of sodium vanadate for ALP activity was investigated, proving the photothermal quantitative method will be a potential platform for screening enzyme inhibitors. Such a sensitive, facile, and low-cost sensing assay provides a new prospect to develop platforms for point-of-care testing.

A fluorescent probe to simultaneously detect both O-GlcNAcase and phosphatase

O-GlcNAc modification of proteins often has crosstalk with protein phosphorylation. These posttranslational modifications are highly dynamic events that modulate a wide range of cellular processes. Owing to the physiological and pathological significance of protein O-GlcNAcylation and phosphorylation, we designed the fluorescent probe, βGlcNAc-CM-Rhod-P, to differentially detect activities of O-GlcNAcase (OGA) and phosphatase, enzymes that are responsible for these modifications. βGlcNAc-CM-Rhod-P was comprised of a βGlcNAc-conjugated coumarin (βGlcNAc-CM) acting as an OGA substrate, a phosphorylated rhodol (Rhod-P) as a phosphatase substrate and a piperazine bridge. Because the emission wavelength maxima of CM and Rhod liberated from the probe are greatly different (100 nm), spectral interference is avoided. The results of this study revealed that treatment of βGlcNAc-CM-Rhod-P with OGA promotes formation of the GlcNAc-cleaved probe, CM-Rhod-P, and a consequent increase in the intensity of fluorescence associated with free CM. Also, it was found that exposure of the probe to phosphatase produces a dephosphorylated probe, βGlcNAc-CM-Rhod, which displays strong fluorescence arising from free Rhod. On the other hand, when incubated with both OGA and phosphatase, βGlcNAc-CM-Rhod-P was converted to CM-Rhod which lacked both βGlcNAc and phosphoryl groups, in conjunction with increases in the intensities of fluorescence arising from both free CM and Rhod. This probe was employed to detect activities of OGA and phosphatase in cell lysates and to fluorescently image both enzymes in cells. Collectively, the findings indicate that βGlcNAc-CM-Rhod-P can be utilized as a chemical tool to simultaneously determine activities of OGA and phosphatase.

The arylvinylpyrimidine scaffold: a tunable platform for luminescent and optical materials

The incorporation of electron-withdrawing pyrimidine rings within π-extended systems allows access to a wide variety of fluorescent push-pull molecules that display emission properties highly sensitive to external stimuli. A suitable design of these compounds leads to interesting materials for a variety of optoelectronic applications. In this context, a vast number of arylvinylpyrimidine-based chromophores have been extensively studied during the last two decades. Along with the main synthetic pathways, this review summarizes the photophysical features of these active compounds having great potential and their most important applications as sensors and luminescence materials.

In situ detection of alkaline phosphatase in a cisplatin-induced acute kidney injury model with a fluorescent/photoacoustic bimodal molecular probe

Kidneys play an important part in drug metabolism and excretion. High local concentration of drugs or drug allergies often cause acute kidney injury (AKI). Identification of effective biomarkers of initial stage AKI and constructing activable molecular probes with excellent detection properties for early evaluation of AKI are necessary, yet remain significant challenges. Alkaline phosphatase (ALP), a key hydrolyzing protease, exists in the epithelial cells of the kidney and is discharged into the urine following kidney injury. However, no studies have revealed its level in drug-induced AKI. Existing ALP fluorescent molecular probes are not suitable for testing and imaging of ALP in the AKI model. Drug-induced AKI is accompanied by oxidative stress, and many studies have indicated that a large increase in reactive oxygen species (ROS) occur in the AKI model. Thus, the probe used for imaging of AKI must be chemically stable in the presence of ROS. However, most existing near-infrared fluorescent (NIRF) ALP probes are not stable in the presence of ROS in the AKI model. Hence, we built a chemically stable molecular sensor (CS-ALP) to map ALP level in cisplatin-induced AKI. This novel probe is not destroyed by ROS generated in the AKI model, thus allowing high-fidelity imaging. In the presence of ALP, the CS-ALP probe generates a new absorbance peak at 685 nm and a fluorescent emission peak at 716 nm that could be used to "turn on" photoacoustic (PA) and NIRF imaging of ALP in AKI. Levels of CS-ALP build up rapidly in the kidney, and CS-ALP has been successfully applied in NIRF/PA bimodal in vivo imaging. Through the NIRF/PA bimodal imaging results, we demonstrate that upregulated expression of ALP occurs in the early stages of AKI and continues with injury progression.

In Vivo Simultaneous Imaging of Plasma Membrane and Lipid Droplets in Hepatic Steatosis using Red-Emissive Two-Photon Probes

The plasma membrane, which is a phosphoglyceride bilayer at the outer edge of the cell, plays diverse and important roles in biological systems. Visualization of the plasma membrane in live samples is important for various applications in biological functions. We developed an amphiphilic two-photon (TP) fluorescent probe (THQ-Mem) to selectively monitor the plasma membrane in live samples. This probe exhibited red emission (620-700 nm), large TP absorption cross sections (δ_max > 790 GM), and high selectivity to the plasma membrane. In cultured cells and in vivo hepatic tissue imaging, THQ-Mem showed bright TP-excited fluorescence (TPEF) and remarkable selectivity for the plasma membrane. Furthermore, simultaneous in vivo imaging with THQ-Mem and a TP lipid droplet probe could serve as an efficient tool to monitor morphological and physiological changes in the plasma membrane and lipid droplets.

Development of Nucleoside Diphosphate-Bearing Fragile Histidine Triad-Imaging Fluorescence Probes with Well-Tuned Hydrophobicity for Intracellular Delivery

Fluorescence-guided cancer surgery can dramatically improve recurrence rates and postoperative quality of life of patients by accurately distinguishing the boundary between normal and cancer tissues during surgery, thereby minimizing excision of normal tissue. One promising target in early stage cancer is fragile histidine triad (FHIT), a cancer suppressor protein with dinucleoside triphosphate hydrolase activity. In this study, we have developed fluorescence probes containing a nucleoside diphosphate moiety, which dramatically improves the reactivity and specificity for FHIT, and a moderately lipophilic ester moiety to increase the membrane permeability. The ester moiety is cleaved by ubiquitous intracellular esterases, and then, FHIT in the cells specifically cleaves nucleoside monophosphate. The remaining phosphate moiety is rapidly cleaved by ubiquitous intracellular phosphatases to release the fluorescent dye. We confirmed that this probe can detect FHIT activity in living cells. A comprehensive evaluation of the effects of various ester moieties revealed that probes with CLogP = 5-7 showed good membrane permeability and were good substrates of the target enzyme; these findings may be helpful in the rational design of other multiple phosphate-containing probes targeting intracellular enzymes.

Enzyme-triggered click chemistry combined with surface-enhanced Raman spectroscopy for the simple and sensitive detection of alkaline phosphatase activity from complex biological samples

Schematic of the enzyme-triggered click chemistry combined with the SERS technique for ALP detection.

Two-photon fluorogenic probe for visualizing PGP-1 activity in inflammatory tissues and serum from patients

A PGP-1-specific one/two-photon fluorogenic probe (BH1), capable of high sensitivity, super selectivity, and visual imaging of endogenous PGP-1 activity from live mammalian cells and serum/skin tissues from patients by using one/two-photon fluorescence microscopy (O/TPFM).

Optical/electrochemical methods for detecting mitochondrial energy metabolism

This review highlights the biological importance of mitochondrial energy metabolism and the applications of multiple optical/electrochemical approaches to determine energy metabolites. Mitochondria, the main sites of oxidative phosphorylation and adenosine triphosphate (ATP) biosynthesis, provide the majority of energy required by aerobic cells for maintaining their physiological activity. They also participate in cell growth, differentiation, information transmission, and apoptosis. Multiple mitochondrial diseases, caused by internal or external factors, including oxidative stress, intense fluctuations of the ionic concentration, abnormal oxidative phosphorylation, changes in electron transport chain complex enzymes and mutations in mitochondrial DNA, can occur during mitochondrial energy metabolism. Therefore, developing accurate, sensitive, and specific methods for the in vivo and in vitro detection of mitochondrial energy metabolites is of great importance. In this review, we summarise the mitochondrial structure, functions, and crucial energy metabolic signalling pathways. The mechanism and applications of different optical/electrochemical methods are thoroughly reviewed. Finally, future research directions and challenges are proposed.

Acylation of antimicrobial peptide-plasmid DNA vectors formulation for efficient gene delivery in cancer therapy

Antimicrobial peptides/DNA complexes were designed based on AMPs chensinin-1b and its corresponding lipo-chensinin-1b conjugated with an aliphatic acid with different chain lengths and therapeutic genes. The main goal of such a complex includes two aspects: first, antimicrobial peptides deliver therapeutic genes to cancer cells and genes expressed in targeted tissue for cancer gene therapy, and, second, the antimicrobial peptide kills cancer cells when used alone as an anticancer agent. This study presents a model composed of chensinin-1b and its lipo-chensinin-1b and eGFP plasmids, which were used as reporter genes, and the final peptide/eGFP plasmid complexes were analyzed by TEM and DLS. The gene transfection efficiency of the complex was evaluated by a microplate reader, FACS and CLSM. Compared with Lipo2000, the antimicrobial peptide showed specific selectivity for transfection against cancer cells and mammalian cells. The peptides chensinin-1b and lipo-chensinin-1b binding with the eGFP plasmid displayed optimal transfection efficiencies at a mass ratio of 8. In addition, PA-C1b can deliver p53-eGFP plasmids into MCF-7 cancer cells, and the proliferation of cells was inhibited and even caused cell death. Overall, PA-C1b was screened and found to have the highest transfection efficiency for gene delivery and good cellular uptake capability. The in vivo transfection ability of PA-C1b was investigated using a tumor-bearing mouse model, and the transfection efficiency reflected by the fluorescence of expressed GFP was determined by in vivo imaging. Conclusively, the antimicrobial peptide PA-C1b could be used as the nonviral vector with high efficiency for cancer gene therapy.

Innovative fluorescent probes for in vivo visualization of biomolecules in living Caenorhabditis elegans

Caenorhabditis elegans (C. elegans) as a well-established multicellular model organism has been widely used in the biological field for half a century. Its numerous advantages including small body size, rapid life cycle, high-reproductive rate, well-defined anatomy, and conserved genome, has made C. elegans one of the most successful multicellular model organisms. Discoveries obtained from the C. elegans model have made great contributions to research fields such as development, aging, biophysics, immunology, and neuroscience. Because of its transparent body and giant cell size, C. elegans is also an ideal subject for high resolution and high-throughput optical imaging and analysis. During the past decade, great advances have been made to develop biomolecule-targeting techniques for noninvasive optical imaging. These novel technologies expanded the toolbox for qualitative and quantitative analysis of biomolecules in C. elegans. In this review, we summarize recently developed fluorescent probes or labeling techniques for visualizing biomolecules at the cellular, subcellular or molecular scale by using C. elegans as the major model organism or designed specifically for the applications in C. elegans. Combining the technological advantages of the C. elegans model with the novel fluorescent labeling techniques will provide new horizons for high-efficiency quantitative optical analysis in live organisms.

Synthesis and application of coumarin fluorescence probes

In recent years, the research on fluorescent probes has developed rapidly. Coumarin fluorescent probes have also been one of the hot topics in recent years. For the synthesis and application of coumarin fluorescent probes, great progress has been made. Coumarin fluorescent probes have become more and more widely used in biochemistry, environmental protection, and disease prevention, and have broad prospects. This review introduces the three main light emitting mechanisms (PET, ICT, FRET) of fluorescent probes, and enumerates some probes based on this light emitting mechanism. In terms of the synthesis of coumarin fluorescent probes, the existing substituents on the core of coumarin compounds were modified. Based on the positions of the modified substituents, some of the fluorescent probes reported in the past ten years are listed. Most of the fluorescent probes are formed by modifying the 3 and 7 position substituents on the mother nucleus, and the 4 and 8 position substituents are relatively less modified. In terms of probe applications, the detection and application of coumarin fluorescent probes for Cu²⁺, Hg²⁺, Mg²⁺, Zn²⁺, pH, environmental polarity, and active oxygen and sulfide in the past ten years are mainly introduced.

In recent years, the research on fluorescent probes has developed rapidly.

Internal standard fluorogenic probe based on vibration-induced emission for visualizing PTP1B in living cells

Herein, as a proof of concept, we developed the first enzymatic VIE fluorogenic probe for protein tyrosine phosphatase 1B (PTP1B).

Branching effect on the linear and nonlinear optical properties of styrylpyrimidines

The branching effect on the photophysical properties of styrylpyrimidines is studied experimentally and theoretically.

Photostable Fluorescent Tracker for Imaging Mitochondria with Super Resolution

The power factories in cells, mitochondria, play important roles in all physiological processes. It is reported that progressive mitochondrial swelling and outer mitochondrial membrane rupture could be induced by a wide variety of apoptotic and necrotic stimuli. Regrettably, although a variety of mitochondrial probes have been developed, most of them are based on the detection of active species in mitochondria. Probes that can monitor the status and distribution of mitochondria for a long time are still urgently needed. In this study, a fluorescent sensor with excellent properties, EtNBEn, is described. Outstanding performance allows it to be observed not only in cells but also in living Daphnia and zebrafish under confocal microscopy for a long time. Moreover, the swelling process of mitochondria under light stimulation is also visualized under super-resolution (SR) microscopy. All these results suggest that EtNBEn could be employed for tagging mitochondria in various physiological processes, which makes a great contribution to the cure of diseases.

Phosphorylated lipid-conjugated oligonucleotide selectively anchors on cell membranes with high alkaline phosphatase expression

Attachment of lipid tails to oligonucleotides has emerged as a powerful technology in constructing cell membrane-anchorable nucleic acid-based probes. In practice, however, conventional lipid-conjugated oligonucleotides fail to distinguish among different cell membranes. Herein, a phosphorylated lipid-conjugated oligonucleotide (DNA-lipid-P) is reported for alkaline phosphatase (ALP)-dependent cell membrane adhesion. In the absence of ALP, DNA-lipid-P with its poor hydrophobicity shows only weak interaction with cell membrane. However, in the presence of the highly expressed plasma membrane-associated ALP, DNA-lipid-P is converted to lipid-conjugated oligonucleotide (DNA-lipid) by enzymatic dephosphorylation. As a result of such conversion, the generated DNA-lipid has greater hydrophobicity than DNA-lipid-P and is thus able to insert into cell membranes in situ. Accordingly, DNA-lipid-P enables selective anchoring on cell membranes with elevated ALP level. Since elevated ALP level is a critical index of some diseases and even cancers, DNA-lipid-P holds promise for cell membrane engineering and disease diagnostics at the molecular level.

Membrane-anchored DNA probes have been used to study molecular interactions and control cell assembly, but are not selective for different cell membranes. Here the authors develop a lipid-conjugated oligonucleotide for alkaline phosphatase-dependent cell membrane anchorage and use it to distinguish different cancer cells.

Bio-inspired iron-catalyzed oxidation of alkylarenes enables late-stage oxidation of complex methylarenes to arylaldehydes

It is a long-standing challenge to achieve efficient and highly selective aerobic oxidation of methylarenes to benzaldehydes, owing to overoxidation problem stemming from the oxidizability of benzaldehyde far higher than the toluene under usual aerobic conditions. Herein we report a bio-inspired iron-catalyzed polymethylhydrosiloxane-promoted aerobic oxidation of methylarenes to benzaldehydes with high yields and selectivities. Notably, this method can tolerate oxidation-labile and reactive boronic acid group, which is normally required to be transformed immediately after its introduction, and represents a significant advance in the area of the chemistry of organoboronic acids, including the ability to incorporate both aldehyde and ketone functionalities into unprotected arylboronic acids, a class that can be difficult to access by current means. The robustness of this protocol is demonstrated on the late-stage oxidation of complex bioactive molecules, including dehydroabietic acid, Gemfibrozil, Tocopherol nicotinate, a complex polyol structure, and structurally complex arylboronic acids.

Oxidation of toluenes to benzaldehydes is usually accompanied by overoxidation products. Here, the authors report an iron-catalysed aerobic oxidation of methylarenes to benzaldehydes with high yields and selectivities, even in presence of boronic acid groups and in complex natural products and drugs.

Carbazole- and Triphenylamine-Substituted Pyrimidines: Synthesis and Photophysical Properties

A series of pyrimidine derivatives bearing one, two or three triphenylamine/9-ethylcarbazole substituents has been synthesized by Suzuki cross-coupling reaction. All compounds showed absorption bands in the UV region and the emission of violet-blue light upon irradiation. Protonation led to quenching of the fluorescence, although some derivatives remained luminescent with the appearance of a new red-shifted band in the spectra. Accurate control of the amount of acid enabled white photoluminescence to be obtained both in solution and in solid state.

Single-Particle Enzyme Activity Assay with Spectral-Resolved Dark-Field Optical Microscopy

In a clinical assay, enzymes are essential biomarkers for human disease diagnosis. In this work, a spectral-resolved single-particle detection (SPD) method is introduced to quantify alkaline phosphatase (ALP) activity in human serum with a supraparticle (SP) based on MnO₂-modified gold nanoparticle (denoted as GNP@MnO₂ SP) as the probe. In the presence of ALP, 2-phospho-l-ascorbic acid trisodium salt can be hydrolyzed into l-ascorbic acid, which serves as a good reduction agent to trigger the decomposition of the MnO₂ shell on the GNP surface. Given that a trace amount of ALP exists, noticeable scattering color change can be detected at the single-particle level due to the sensitive localized surface plasmon resonance (LSPR) effect from GNPs. With spectral-resolved dark-field optical microscopy, a linear dynamic range of 0.06 to 2.48 mU/mL ( R² = 0.99) and a very low limit of detection of 5.8 μU/mL for the ALP assay are readily achieved, which is more sensitive over the methods based on ensemble sample measurement. As a consequence, this strategy opens a new avenue for the design of an ultrasensitive detection method for disease-correlated biomarker diagnosis in the future.

Ligand-displacement-based two-photon fluorogenic probe for visualizing mercapto biomolecules in live cells, Drosophila brains and zebrafish

Investigating the change in expression level of mercapto biomolecules (GSH/Cys/Hcy) necessitates a rapid detection method for a series of physiological and pathological processes. Herein, we present a ligand-displacement-based two-photon fluorogenic probe based on an Fe(iii) complex, TPFeS, which is a GSH/Cys/Hcy rapid detection fluorogenic probe for in vitro analysis and live cell/tissue/in vivo imaging. The "in situ" probe is non-fluorescent and was prepared from a 1 : 2 ratio of Fe(iii) and TPS, a novel two-photon (TP) fluorophore with excellent one-photon (OP) and TP properties under physiological conditions, as a fluorescent ligand. This probe shows a rapid and remarkable fluorescence restoration (OFF-ON) property due to the ligand-displacement reaction of mercapto biomolecules in a recyclable manner in vitro. A significant two-photon action cross-section, good selectivity for biothiols, low cytotoxicity, and insensitivity to pH over the biologically relevant pH range allowed the direct visualization of mercapto biomolecules at different levels between normal/drug-treated live cells, as well as in Drosophila brain tissues/zebrafish based on the use of two-photon fluorescence microscopy.

ZnII Complexes for Bioimaging and Correlated Applications

Zinc is a biocompatible element that exists as the second most abundant transition metal ion and an indispensable trace element in the human body. Compared to traditional metal-organic complexes systems, d¹⁰ metal Zn^II complexes not only exhibit a large Stokes shift and good photon stability but also possess strong emission and low cytotoxicity with a relatively small molecular weight. The use of Zn^II complexes has emerged in the last decade as a versatile and convenient tool for numerous biological applications, including bioimaging, molecular and protein recognition, as well as photodynamic therapy. Herein, we review recent developments involving Zn^II metal complexes applied as specific subcellular compartment imaging probes and their correlated utilizations.

Enzyme-Activated Fluorogenic Probes for Live-Cell and in Vivo Imaging

Fluorogenic probes, small-molecule sensors that unmask brilliant fluorescence upon exposure to specific stimuli, are powerful tools for chemical biology. Those probes that respond to enzymatic catalysis illuminate the complex dynamics of biological processes at a level of spatiotemporal detail and sensitivity unmatched by other techniques. Here, we review recent advances in enzyme-activated fluorogenic probes for biological imaging. We organize our survey by enzyme classification, with emphasis on fluorophore masking strategies, modes of enzymatic activation, and the breadth of current and future applications. Key challenges such as probe selectivity and spectroscopic requirements are described alongside therapeutic, diagnostic, and theranostic opportunities.

Two-photon absorption spectroscopy of trans-stilbene, cis-stilbene, and phenanthrene: Theory and experiment

Two-photon absorption (2PA) spectroscopy provides complementary, and sometimes more detailed, information about the electronic structure of a molecule relative to one-photon absorption (1PA) spectroscopy. However, our understanding of the 2PA processes is rather limited due to technical difficulties in measuring experimental 2PA spectra and theoretical challenges in computing higher-order molecular properties. This paper examines the 2PA spectroscopy of trans-stilbene, cis-stilbene, and phenanthrene by a combined experimental and theoretical approach. The broadband 2PA spectra of all three compounds are measured under identical conditions in order to facilitate a direct comparison of the absolute 2PA cross sections in the range 3.5-6.0 eV. For comparison, the theoretical 2PA cross sections are computed using the equation-of-motion coupled-cluster method with single and double substitutions. Simulated 2PA spectra based on the calculations reproduce the main features of the experimental spectra in solution, although the quantitative comparison is complicated by a number of uncertainties, including limitations of the theoretical model, vibronic structure, broadening of the experimental spectra, and solvent effects. The systematic comparison of experimental and theoretical spectra for this series of structurally similar compounds provides valuable insight into the nature of 2PA transitions in conjugated molecules. Notably, the orbital character and symmetry-based selection rules provide a foundation for interpreting the features of the experimental 2PA spectra in unprecedented detail.

Interrogating Protein Phosphatases with Chemical Activity Probes

Protein phosphatases, while long overlooked, have recently become appreciated as drivers of both normal- and disease-associated signaling events. As a result, the spotlight is now turning torwards this enzyme family and efforts geared towards the development of modern chemical tools for studying these enzymes are well underway. This Minireview focuses on the evolution of chemical activity probes, both optical and covalent, for the study of protein phosphatases. Small-molecule probes, global monitoring of phosphatase activity through the use of covalent modifiers, and targeted fluorescence-based activity probes are discussed. We conclude with an overview of open questions in the field and highlight the potential impact of chemical tools for studying protein phosphatases.

In situ generation of photoactivatable aggregation-induced emission probes for organelle-specific imaging

Photoactivatable fluorescent probes are ideal tools for organelle study with a significant advantage of high spatiotemporal resolution. However, conventional photo-caged fluorophores for organelle-specific imaging suffer from several drawbacks, such as aggregation-caused quenching (ACQ), instability under ambient light, low photoactivation efficiency, and toxic photo-cleavage byproducts. Herein, we propose a strategy for in situ generation of photoactivatable aggregation-induced emission (AIE) probes of 2-(2-hydroxyphenyl)-benzothiazolines from easily available disulfide and thiol substrates through tandem S-S bond reduction and intramolecular cyclization reaction. Because the photoactivatable AIE probes can be in situ generated in a quantitative yield, they can be directly used for bio-imaging without complicated separation steps. Under both one- and NIR two-photon irradiation, excellent spatiotemporal resolution and high photoactivation efficiency were achieved for specific imaging of lipid droplets and lysosomes, respectively. Based on their in situ generation and adjustable organelle-targeting ability, the photoactivatable AIE probes could become an easy-to-use imaging tool in the study of the biological functions of organelles.

TPZ, a bright centrosymmetric two-photon scaffold for bioimaging

The development of biocompatible two-photon fluorophores with a large absorption cross-section is challenging, despite the presence of theoretical guidelines. By rendering asymmetric PRODAN dye centrosymmetric, we designed and synthesized a novel class of two-photon fluorophores (TPZ). Their photophysical properties were investigated and their imaging potentials in cells, tissues and zebrafish were showcased.