Discerning the Chemistry in Individual Organelles with Small-Molecule Fluorescent Probes

A cell-permeant small molecule for the super-resolution imaging of the endoplasmic reticulum in live cells

A simple BODIPY-based small molecule has been identified to selectively label the endoplasmic reticulum for high-resolution imaging with negligible cytotoxicity.

Sydnone-based turn-on fluorogenic probes for no-wash protein labeling and in-cell imaging

Fluorogenic sydnone-based turn-on probes allow efficient labeling of proteins and cell imaging through a bioorthogonal strained promoted sydnone–alkyne cycloaddition reaction.

A ratiometric fluorescent probe for detection of exogenous mitochondrial SO2 based on a FRET mechanism

A novel imidazo[1,5-a]pyridine-hemicyanine based ratiometric fluorescent probe for detection of mitochondrial SO₂ was designed and synthesized. The probe is based on a fluorescence resonance energy transfer (FRET) mechanism. It exhibits high selectivity and sensitivity towards SO₃²⁻ with a fast response time (3 min) and detection limit of 0.13 μM. Further, it showed low cytotoxicity and was successfully applied to image exogenous mitochondrial SO₂ in cells.

A novel imidazo[1,5-a]pyridine-hemicyanine based ratiometric fluorescent probe for detection of mitochondrial SO₂ was designed and synthesized.

Multifunctional pyrazoline based AIEgens: real-time tracking and specific protein “fishing” of lipid droplets

A series of multifunctional pyrazoline based AIEgens were developed for real-time tracking of lipid metabolism, reversibly monitoring intracellular pH in dual-color mode and specific labeling of lipid droplet related protein.

Rationally designed organelle-specific thermally activated delayed fluorescence small molecule organic probes for time-resolved biological applications

A new strategy for TADF-based probes to maintain long fluorescence emission lifetime in TRFI studies in cells was developed.

A semisynthetic fluorescent protein assembly-based FRET probe for real-time profiling of cell membrane protease functions in situ

A semisynthetic fluorescent protein assembly-based FRET probe (sFPAP) was proposed for cell membrane protease function assay.

A fluorogenic C4N4 probe for azide-based labelling

A new fluorogenic probe based on the recently identified 2,5-diaminopyrimidine (C4N4) fluorophore is introduced for azide-specific labelling.

Fluorescent Tracking of the Endoplasmic Reticulum in Live Pathogenic Fungal Cells

In fungal cells, the endoplasmic reticulum (ER) harbors several of the enzymes involved in the biosynthesis of ergosterol, an essential membrane component, making this organelle the site of action of antifungal azole drugs, used as a first-line treatment for fungal infections. This highlights the need for specific fluorescent labeling of this organelle in cells of pathogenic fungi. Here we report on the development and evaluation of a collection of fluorescent ER trackers in a panel of Candida, considered the most frequently encountered pathogen in fungal infections. These trackers enabled imaging of the ER in live fungal cells. Organelle specificity was associated with the expression of the target enzyme of antifungal azoles that resides in the ER; specific ER labeling was not observed in mutant cells lacking this enzyme. Labeling of live Candida cells with a combination of a mitotracker and one of the novel fungal ER trackers revealed sites of contact between the ER and mitochondria. These fungal ER trackers therefore offer unique molecular tools for the study of the ER and its interactions with other organelles in live cells of pathogenic fungi.

A reaction-based near-infrared fluorescent probe that can visualize endogenous selenocysteine in vivo in tumor-bearing mice

Monitoring the fluctuations of endogenous selenocysteine (Sec) in vivo is of significant interest to understand the physiological roles of Sec and the mechanisms of Sec-relevant diseases. Herein, a new near-infrared fluorescent probe, Fsec-1, has been developed for the determination of endogenous Sec in living cells and in vivo. Fsec-1 exhibits large fluorescence enhancement (136-fold) and a remarkably large Stokes shift (195 nm) when reacted with Sec. With the advantages of high sensitivity (a detection limit of 10 nM), good selectivity and low cytotoxicity, Fsec-1 was able to recognize both exogenous and endogenous Sec in living cells. The probe was also successfully applied in visualizing both exogenous and endogenous Sec in living mice. Notably, endogenously generated Sec in living tumors xenografted in nude mice was selectively detected by our reaction-based NIR probe for the first time. These results indicated that our new probe could serve as an efficient tool in monitoring endogenous Sec in vivo and exploring the anticancer mechanism of selenium.

Two new cobalt(II) rhodamine 6G hydrazone complexes: structure, fluorescence and magnetism

Two new Co^II complexes, namely bis{N-[(6-bromopyridin-2-yl)methylidene]-2-[6-ethylamino-3-(ethyliminiumyl)-2,7-dimethyl-3H-xanthen-9-yl]benzene-1-carbohydrazonate}cobalt(II) bis(perchlorate)-dichloromethane-methanol (1/1/2), [Co(C₃₂H₃₀BrN₅O₂)₂](ClO₄)₂·CH₂Cl₂·2CH₃OH or [Co^II(L)₂](ClO₄)₂·CH₂Cl₂·2CH₃OH, (1), and the bis(tetrafluoridoborate) salt, [Co(C₃₂H₃₀BrN₅O₂)₂](BF₄)₂·CH₂Cl₂·2CH₃OH or [Co^II(L)₂](BF₄)₂·CH₂Cl₂·2CH₃OH, (2) (L is commonly 6-bromopyridine-2-carbaldehyde rhodamine 6G hydrazone), have been successfully constructed and characterized. The crystal structure analysis revealed that complexes (1) and (2) are mononuclear and have a Co^IIN₄O₂ distorted octahedral structure. The large π-conjugated xanthene moiety of the L ligand causes strong intermolecular π-π stacking interactions, yielding a supramolecular one-dimensional chain. Complexes (1) and (2) display an obvious fluorescence emission near 560 nm in the solid state. Magnetic investigations show that both (1) and (2) are paramagnetic, dominated by the structural distortion and spin-orbit coupling of Co^II.

Visualization of oxidative injury in the mouse kidney using selective superoxide anion fluorescent probes

Drug-induced acute kidney injury (AKI), caused by renal drug metabolism, has been regarded as a main problem in clinical pharmacology and practice. However, due to the lack of effective biomarkers and noninvasive real-time tools, the early diagnosis of drug-induced AKI is still a crucial challenge. The superoxide anion (O2˙-), the preliminary reactive oxidative species, is closely related to drug-induced AKI. In this paper, we reported two new mitochondria-targeted fluorescent probes for investigating AKI via mapping the fluctuation of O2˙- with high sensitivity and selectivity by the combination of rational design and a probe-screening approach. Small-molecule fluorescent probes (Naph-O2˙- and NIR-O2˙- ) with high accuracy and excellent selectivity were successfully applied to detect endogenously produced O2˙- in living cells and tissues by dual-model confocal imaging, and to trap the fluctuation of the O2˙- level during the drug-induced nephrotoxicity. Moreover, probe NIR-O2˙- was also used to elucidate the protective effects of l-carnitine (LC) against drug-induced nephrotoxicity for the first time. Therefore, these probes may be potential chemical tools for exploring the roles of O2˙- in complex nephrotoxicity disease systems.

Two-photon semiconducting polymer nanoparticles as a new platform for imaging of intracellular pH variation

Intracellular pH (pHi) plays a crucial role in cell physiological and pathological processes. We herein report an efficient pH-sensitive sensor based on two-photon excitable semiconducting polymer nanoparticles (PFV/PSMA-DA NPs) for pHi sensing. PFV/PSMA NPs were functionalized with redox-active dopamine (DA) and the obtained PFV/PSMA-DA NPs showed sensitive and reversible pH response over the pH range of 5.0-9.0. Owning to the high biocompatibility and pH-responsive DA, PFV/PSMA-DA NPs show low cytotoxicity and the quantification and imaging of intracellular pH changes of HeLa cells were successfully realized. Moreover, the detection of intracellular pH fluctuation induced by redox species such as NAC (N-acetylcysteine) and H₂O₂ was also achieved by both one- and two-photon excitation of the PFV/PSMA-DA NPs probe. This work clearly shows that nanoprobe based on two-photon PFV/PSMA-DA NPs could serve as a promising platform for quantitatively monitoring the intracellular pH fluctuations.

Synthesis, Biological Evaluation of Fluorescent 23-Hydroxybetulinic Acid Probes, and Their Cellular Localization Studies

23-Hydroxybetulinic acid (23-HBA) is a complex lupane triterpenoid, which has attracted increasing attention as an anticancer agent. However, its detailed mechanism of anticancer action remains elusive so far. To reveal its anticancer mode of action, a series of fluorescent 23-HBA derivatives conjugated with coumarin dyes were designed, synthesized, and evaluated for their antiproliferative activities. Subcellular localization and uptake profile studies of representative fluorescent 23-HBA probe 26c were performed in B16F10 cells, and the results suggested that probe 26c was rapidly taken up into B10F10 cells in a dose-dependent manner and mitochondrion was the main site of its accumulation. Further mode of action studies implied that the mitochondrial pathway was involved in 23-HBA-mediated apoptosis. Together, our results provided new clues for revealing the molecular mechanism of natural product 23-HBA for its further development into an antitumor agent.

A near-infrared fluorescent probe based on photostable Si-rhodamine for imaging hypochlorous acid during lysosome-involved inflammatory response

Hypochloric acid (HClO) is mainly distributed in acidic lysosomes of phagocytes and closely associated with numerous physiological and pathological processes, especially inflammatory response. Fluorescent probe has become an important tool for imaging HClO in lysosomes, but suffered from interference from autofluorescence in vivo, phototoxicity to biosamples and photobleaching phenomenon due to their short-wavelength excitation and emission. Unfortunately, up to now, no near-infrared (NIR) lysosome-targetable fluorescent probe has been reported for imaging HClO. In this paper, a near-infrared fluorescent probe Lyso-NIR-HClO for imaging lysosomal HClO was reported for the first time. Lyso-NIR-HClO based on Si-rhodamine is consisted of a morpholine unit as a lysosome-targetable group and a HClO-mediated cyclization reaction site as a response group, which was applied for highly selective and sensitive detection and imaging for endogenous and exogenous HClO in lysosomes, with a linear range from 5.0 × 10^-8 to 1.0 × 10^-5 M and a detection limit of 2.0 × 10^-8 M in vitro. Attributed to NIR emission and excellent photostability of Si-rhodamine, Lyso-NIR-HClO exhibits excellent performances in vivo, such as low interference from intracellular autofluorescence, stable and persistent fluorescence signal and good tissue penetration, which are in favor of accurate, time-lapse and long-term imaging for HClO. Finally, we applied the probe Lyso-NIR-HClO to visualize endogenous HClO during lysosome-involved inflammatory response including bacteria-infected cells and inflamed mouse model with satisfactory results. The above results proved that Lyso-NIR-HClO would be a potentially useful tool for the study of biological functions and pathological roles of HClO in lysosomes, especially role of lysosome in the inflammatory response.

A ratiometric Raman probe for live-cell imaging of hydrogen sulfide in mitochondria by stimulated Raman scattering

Stimulated Raman Scattering (SRS) coupled with alkyne tags has been an emerging imaging technique to visualize small-molecule species with high sensitivity and specificity. Here we describe the development of a ratiometric Raman probe for visualizing hydrogen sulfide (H2S) species in living cells as the first alkyne-based sensor for SRS microscopy. This probe uses an azide unit as a selective reactive site, and it targets mitochondria with high specificity. The SRS ratiometric images show a strong response to H2S level changes in living cells.

A Click Cage: Organelle-Specific Uncaging of Lipid Messengers

Lipid messengers exert their function on short time scales at distinct subcellular locations, yet most experimental approaches for perturbing their levels trigger cell-wide concentration changes. Herein, we report on a coumarin-based photocaging group that can be modified with organelle-targeting moieties by click chemistry and thus enables photorelease of lipid messengers in distinct organelles. We show that caged arachidonic acid and sphingosine derivatives can be selectively delivered to mitochondria, the ER, lysosomes, and the plasma membrane. By comparing the cellular calcium transients induced by localized uncaging of arachidonic acid and sphingosine, we show that the precise intracellular localization of the released second messenger is crucial for the signaling outcome. Ultimately, we anticipate that this new class of caged compounds will greatly facilitate the study of cellular processes on the organelle level.

Mitochondrial Specific H2S n Fluorogenic Probe for Live Cell Imaging by Rational Utilization of a Dual-Functional-Photocage Group

Reactive sulfur species play a very important role in modulating neural signal transmission. Hydrogen polysulfides (H₂S _n, n > 1) are recently suggested to be the actual signaling molecules. There are still few spatiotemporal controllable-based probes to detect H₂S _n. In this work, for the first time, we proposed the photocleavage product of the common photoremovable protecting group (2-nitrophenyl moiety) capable of trapping H₂S _n. Taking advantage of this, we constructed the probe H1 containing a photocontrollable group, a mitochondrial directing unit and a signal reporter fluorescein dye. H1 exhibited excellent fluorescence enhancement (50-fold) in response to H₂S _n under the aqueous buffer only after UV irradiation. H1 also showed high selectivity and sensitivity for H₂S _n over other reactive sulfur species, reactive oxygen species, and other analytes, especially biothoils including hydrogen sulfide, cysteine, homocysteine, and glutathione. We showed the utility of H1 to image H₂S _n in living cells with high spatiotemporal resolution.

Cellular Uptake of Nanoparticles versus Small Molecules: A Matter of Size

The primary function of the cell membrane is to protect cells from their surroundings. This entails a strict regulation on controlling the exchange of matter between the cell and its environment. A key factor when considering potential biological applications of a particular chemical structure has to do with its ability to internalize into cells. Molecules that can readily cross cell membranes are frequently needed in biological research and medicine, since most therapeutic entities are designed to modulate intracellular components. However, the design of molecules that do not penetrate cells is also relevant toward, for example, extracellular contrast agents, which are most widely used in clinical diagnosis. Small molecules have occupied the forefront of biomedical research until recently, but the past few decades have seen an increasing use of larger chemical structures, such as proteins or nanoparticles, leading to unprecedented and often unexpectedly novel research. Great achievements have been made toward understanding the rules that govern cellular uptake, which show that cell internalization of molecules is largely affected by their size. For example, macromolecules such as proteins and nucleic acids are usually unable to internalize cells. Intriguingly, in the case of nanoparticles, larger sizes seem to facilitate internalization via endocytic pathways, through which the particles remain trapped in lysosomes and endosomes. In this Account, we aimed at presenting our personal view of how different chemical structures behave in terms of cell internalization due to their size, ranging from small drugs to large nanoparticles. We first introduce the properties of cell membranes and the main mechanisms involved in cellular uptake. We then discuss the cellular internalization of molecules, distinguishing between those with molecular weights below 1 kDa and biological macromolecules such as proteins and nucleic acids. In the last section, we review the biological behavior of nanoparticles, with a special emphasis on plasmonic nanoparticles, which feature a high potential in the biomedical field. For each group of chemical structures, we discuss the parameters affecting their cellular internalization but also strategies that can be applied to achieve the desired intracellular delivery. Particular attention is paid to approaches that allow conditional regulation of the cell internalization process using external triggers, such as activable cell penetrating peptides, due to the impact that these systems may have in drug delivery and sensing applications. The Account ends with a "Conclusions and Outlook" section, where general lessons and future directions toward further advancements are briefly presented.

New Near-infrared Fluorescent Probes with Single-photon Anti-Stokes-shift Fluorescence for Sensitive Determination of pH Variances in Lysosomes with a Double-Checked Capability

Two near-infrared luminescent probes with Stokes-shift and single-photon anti-Stokes-shift fluorescence properties for sensitive determination of pH variance in lysosomes have been synthesized. A morpholine residue in probe A which serves as a targeting group for lysosomes in viable cells was attached to the fluorophores via a spirolactam moiety while a mannose residue was ligated to probe B resulting in increased biocompatibility and solubility in water. Probes A and B contain closed spirolactam moieties, and show no Stokes-shift or anti-Stokes-shift fluorescence under neutral or alkali conditions. However, the probes incrementally react to pH variance from 7.22 to 2.76 with measurable increases in both Stokes-shift and anti-Stokes-shift fluorescence at 699 nm and 693 nm under 645 nm and 800 nm excitation, respectively. This acid-activated fluorescence is produced by the breaking of the probe spirolactam moiety, which greatly increased overall π-conjugation in the probes. These probes possess upconversion near-infrared fluorescence imaging advantages including minimum cellular photo-damage, tissue penetration, and minimum biological fluorescence background. They display excellent photostability with low dye photobleaching and show good biocompatibility. They are selective and capable of detecting pH variances in lysosomes at excitation with two different wavelengths, i.e., 645 and 800 nm.

Visualization of methylglyoxal in living cells and diabetic mice model with a 1,8-naphthalimide-based two-photon fluorescent probe

Methylglyoxal (MGO), a dicarbonyl metabolite, is the most studied precursor of advanced glycation end-products (AGEs) and its elevated levels have also been associated with various pathologies. Hence, the development of effective methods for monitoring MGO in live cells and in vivo is of great importance for ascertaining the onset and progress of related diseases. Herein, we designed and synthesized an endoplasmic reticulum-targeting two-photon fluorescent probe called NI-OPD for the detection of MGO with high selectivity, sensitivity, and hypotoxicity. The probe was successfully applied for monitoring MGO in living cells and a diabetic mice model. The two-photon fluorescence images confirmed that the endogenous MGO in the liver and kidney tissues of diabetic mice is higher than that of normal mice. Furthermore, it revealed that after treatment with metformin, a widely used hypoglycemia drug, the diabetic mice showed a decreased concentration of MGO in liver and kidney tissues. Thus, NI-OPD may serve as a useful tool for the detection of MGO and for studying the relationships between MGO and pathological and biological processes in biosystems.

A Multifunctional Chemical Probe for the Measurement of Local Micropolarity and Microviscosity in Mitochondria

The measurement of physicochemical parameters in living cells can provide information on individual cellular organelles, helping us to understand subcellular function in health and disease. While organelle-specific chemical probes have allowed qualitative evaluation of microenvironmental variations, the simultaneous quantification of mitochondrial local microviscosity (η_m ) and micropolarity (ϵ_m ), along with concurrent structural variations, has remained an unmet need. Herein, we describe a new multifunctional mitochondrial probe (MMP) for simultaneous monitoring of η_m and ϵ_m by fluorescence lifetime and emission intensity recordings, respectively. The MMP enables highly precise measurements of η_m and ϵ_m in the presence of a variety of agents perturbing cellular function, and the observed changes can also be correlated with alterations in mitochondrial network morphology and motility. This strategy represents a promising tool for the analysis of subtle changes in organellar structure.

Positively Charged Hyperbranched Polymers with Tunable Fluorescence and Cell Imaging Application

Fluorescence-tunable materials are becoming increasingly attractive because of their potential applications in optics, electronics, and biomedical technology. Herein, a multicolor molecular pixel system is realized using a simple copolymerization method. Bleeding of two complementary colors from blue and yellow fluorescence segments reproduced serious multicolor fluorescence materials. Interestingly, the emission colors of the polymers can be fine-tuned in the solid state, solution phase, and in hydrogel state. More importantly, the positive fluorescent polymers exhibited cell-membrane permeable ability and were found to accumulate on the cell nucleus, exhibiting remarkable selectivity to give bright fluorescence. The DNA/RNA selectivity experiments in vitro and in vivo verified that [tris(4-(pyridin-4-yl)phenyl)amine]-[1,8-dibromooctane] has prominent selectivity to DNA over RNA inside cells.

A novel Schiff base derivative: Synthesis, two-photon absorption properties and application for bioimaging

A novel donor-π-acceptor-π-donor type (D-π-A-π-D') Schiff base derivative (L) has been designed and synthesized. The structure of L is confirmed by single-crystal X-ray diffraction analysis as well. The photophysical properties of compound L were comprehensively investigated by using both experimental and theoretical methods. The results indicate that L exhibits large Stokes shift and moderate two-photon action (2PA) cross-section in the near infrared (NIR) region. Furthermore, the confocal microscopy imaging study demonstrates that compound L could penetrate into cells and target the cellular mitochondria compartment. Due to its low cytotoxicity, compound L provides a promising tool for directly lighting up the mitochondria compartment in living HepG2 cells.

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.

Localizing Antifungal Drugs to the Correct Organelle Can Markedly Enhance their Efficacy

A critical aspect of drug design is optimal target inhibition by specifically delivering the drug molecule not only to the target tissue or cell but also to its therapeutically active site within the cell. This study demonstrates, as a proof of principle, that drug efficacy can be increased considerably by a structural modification that targets it to the relevant organelle. Specifically, by varying the fluorescent dye segment an antifungal azole was directed from the fungal cell mitochondria to the endoplasmic reticulum (ER), the organelle that harbors the drug target. The ER-localized azole displayed up to two orders of magnitude improved antifungal activity and also dramatically reduced the growth of drug-tolerant fungal subpopulations in a panel of Candida species, which are the most prevalent causes of serious human fungal infections. The principle underlying the "target organelle localization" approach provides a new paradigm to improve drug potency and replenish the limited pipeline of antifungal drugs.

Enhancing the Anti-Solvatochromic Two-Photon Fluorescence for Cirrhosis Imaging by Forming a Hydrogen-Bond Network

Two-photon imaging is an emerging tool for biomedical research and clinical diagnostics. Electron donor-acceptor (D-A) type molecules are the most widely employed two-photon scaffolds. However, current D-A type fluorophores suffer from solvatochromic quenching in aqueous biological samples. To address this issue, we devised a novel class of D-A type green fluorescent protein (GFP) chromophore analogues that form a hydrogen-bond network in water to improve the two-photon efficiency. Our design results in two-photon chalcone (TPC) dyes with 0.80 quantum yield and large two-photon action cross section (210 GM) in water. This strategy to form hydrogen bonds can be generalized to design two-photon materials with anti-solvatochromic fluorescence. To demonstrate the improved in vivo imaging, we designed a sulfide probe based on TPC dyes and monitored endogenous H₂ S generation and scavenging in the cirrhotic rat liver for the first time.

Detection of analytes in mitochondria without interference from other sites based on an innovative ratiometric fluorophore

Mitochondria are vital organelles that not only produce cellular energy but also participate in many biological processes. Recently, various fluorescent probes have been developed for mitochondrial imaging. However, due to the lack of suitable dyes or strategies, it is difficult for most reported mitochondrial targeting probes to prove whether the analytes they detected are from mitochondria. In addition, positive charge on mitochondrial probes can seriously affect the mitochondrial environment. To address these issues, we herein put forward a novel strategy for probe design based on a smart NIR dye (HDFL) for mitochondrial targeting detection. Compared to general mitochondrial targeting probes that are modified with a target site and a reaction site, the new strategy is to combine the two sites together for a mitochondrial probe that would provide accurate detection of analytes in mitochondria without interference. As a proof of concept, we synthesized a mitochondrial-targetable probe HDFL-Cys for cysteine. Bioimaging studies have shown that the new type of probe HDFL-Cys can first accumulate in mitochondria and then react with the analyte (cysteine) accompanied by the departure of the targeting group (lipophilic cation moieties). Thus, it can specifically detect the analyte in mitochondria without interference from extra-mitochondrial analytes. We anticipate that the new strategy based on the novel NIR dye HDFL may be a potential platform for developing desirable ratiometric fluorescent probes for mitochondrial imaging.

Phenotype-Based High-Content Screening Using Fluorescent Chemical Bioprobes: Lipid Droplets and Glucose Uptake Quantification in Live Cells

Phenotypic screening in live cells has emerged as a promising strategy for drug discovery in pharmaceutical communities. For relevant phenotype-based screening setups, it is critical to develop adequate reporters in order to selectively visualize subcellular compartments or phenotypic changes that represent disease-related characteristics during compound screening. In this chapter, we introduce two phenotype-based high-content/high-throughput assays using fluorescent bioprobes that have been designed and refined to selectively stain cellular lipid droplets (LDs) and to show cellular glucose uptake. In conjunction with target identification process for the hit compounds from phenotypic screening, these fluorescent chemical probe-based screening techniques are expected to drive a great advancement for the discovery of novel first-in-class therapeutics.

Fluorogen-activating proteins: beyond classical fluorescent proteins

Fluorescence imaging is a powerful technique for the real-time noninvasive monitoring of protein dynamics. Recently, fluorogen activating proteins (FAPs)/fluorogen probes for protein imaging were developed. Unlike the traditional fluorescent proteins (FPs), FAPs do not fluoresce unless bound to their specific small-molecule fluorogens. When using FAPs/fluorogen probes, a washing step is not required for the removal of free probes from the cells, thus allowing rapid and specific detection of proteins in living cells with high signal-to-noise ratio. Furthermore, with different fluorogens, living cell multi-color proteins labeling system was developed. In this review, we describe about the discovery of FAPs, the design strategy of FAP fluorogens, the application of the FAP technology and the advances of FAP technology in protein labeling systems.

Gold-carbon dots for the intracellular imaging of cancer-derived exosomes

As a novel fluorescent nanomaterial, gold-carbon quantum dots (GCDs) possess high biocompatibility and can be easily synthesized by a microwave-assisted method. Owing to their small sizes and unique optical properties, GCDs can be applied to imaging of biological targets, such as cells, exosomes and other organelles. In this study, GCDs were used for fluorescence imaging of exosomes. Tumor-specific antibodies are attached to the GCDs, forming exosome specific nanoprobes. The nanoprobes can label exosomes via immuno-reactions and thus facilitate fluorescent imaging of exosomes. When incubated with live cells, exosomes labeled with the nanoprobes can be taken up by the cells. The intracellular experiments confirmed that the majority of exosomes were endocytosed by cells and transported to lysosomes. The manner by which exosomes were taken up and the intracellular distribution of exosomes are unaffected by the GCDs. The experimental results successfully demonstrated that the presented nanoprobe can be used to study the intrinsic intracellular behavior of tumor derived exosomes. We believe that the GCDs based nanoprobe holds a great promise in the study of exosome related cellular events, such as cancer metastasis.

A highly sensitive fluorescent probe based on the Michael addition mechanism with a large Stokes shift for cellular thiols imaging

A novel fluorescent probe IPY-MAL for thiols was developed based on imidazo[1,5-α]pyridine derivative, which was decorated with a maleimide group. The probe IPY-MAL showed a rapid response (30 s), high sensitivity and selectivity for thiols with a large Stokes shift (140 nm), which was triggered by the Michael addition reaction of thiols toward the C=C double bond of the maleimide group. Moreover, this probe IPY-MAL could quantitatively detect the concentrations of thiols ranging from 0 to 50 μM, and the detection limit was found to be as low as 28 nM. Cell imaging results indicated that the probe IPY-MAL could detect and visualize thiols in the living cells. Graphical abstract A novel imidazo[1,5-α]pyridine-based fluorescent probe was developed for sensitively monitoring and imaging thiols in living A549 cells with a large Stokes shift.