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

Synthesis, characterization, and supramolecular architectures of two distinct classes of probes for the visualization of endogenously generated hypochlorite ions in response to cellular activity

Two distinct classes of compounds, (E)-2-(((3-amino-4-nitrophenyl) imino) methyl)-5-(diethylamino) phenol (SB) and 5-(diethylamino)-2-(5-nitro-1H-benzo[d]imidazol-2-yl) phenol (IM) were synthesized. SB, a bright red colored compound was crystallized in acetonitrile as a triclinic crystal system while IM, yellow colored compound crystallized as a monoclinic crystal system in dimethylformamide by vapor diffusion of diethylether. These compounds were characterized using spectroscopic techniques (IR, UV-visible, ¹H, and ¹³C NMR), and X-ray crystallography. SB and IM displayed classical and non-classical H-bonding involving C-H…O and π…π interactions. These compounds detected hypochlorite ions in aqueous DMSO (1: 9, v/v, HEPES buffer, pH 7.4), and detection was visible via color changes by naked eye. We also performed UV-visible and fluorescence titrations, showing detection limits of 8.82 × 10^-7 M for SB and 2.44 × 10^-7 M for IM. The fluorometric responses from SB and IM were also studied against different ROS and anions. DFT calculations were performed to strengthen the proposed sensing mechanisms of both SB and IM. Hypochlorite, which is endogenously generated by myeloperoxidase in endosomes, was specifically visualized using SB and IM in lipopolysaccharide-treated RAW264.7 cells. These probes were also used to image the generation of hypochlorite by RAW264.7 cells during phagocytosis of non-fluorescent polystyrene beads.

Guiding Drugs to Target-Harboring Organelles: Stretching Drug-Delivery to a Higher Level of Resolution

The ratio between the dose of drug required for optimal efficacy and the dose that causes toxicity is referred to as the therapeutic window. This ratio can be increased by directing the drug to the diseased tissue or pathogenic cell. For drugs targeting fungi and malignant cells, the therapeutic window can be further improved by increasing the resolution of drug delivery to the specific organelle that harbors the drug's target. Organelle targeting is challenging and is, therefore, an under-exploited strategy. Here we provide an overview of recent advances in control of the subcellular distribution of small molecules with the focus on chemical modifications. Highlighted are recent examples of active and passive organelle-specific targeting by incorporation of organelle-directing molecular determinants or by chemical modifications of the pharmacophore. The outstanding potential that lies in the development of organelle-specific drugs is becoming increasingly apparent.

New β-carboline fluorophores with superior sensitivity and endoplasmic reticulum specificity for tracking ER changes

Autofluorescing carboline-fluors were efficiently and rationally developed via a Pictet-Spengler involved one-pot multi-component reaction. The carboline-fluors demonstrate specific targeting towards the endoplasmic reticulum in living cells, and superior sensitivity to commercial ER-Trackers. Importantly, they were also successfully used to visualize changes in the ER during cell apoptosis and ER stress.

Boosting Cancer Therapy with Organelle-Targeted Nanomaterials

The ultimate goal of cancer therapy is to eliminate malignant tumors while causing no damage to normal tissues. In the past decades, numerous nanoagents have been employed for cancer treatment because of their unique properties over traditional molecular drugs. However, lack of selectivity and unwanted therapeutic outcomes have severely limited the therapeutic index of traditional nanodrugs. Recently, a series of nanomaterials that can accumulate in specific organelles (nucleus, mitochondrion, endoplasmic reticulum, lysosome, Golgi apparatus) within cancer cells have received increasing interest. These rationally designed nanoagents can either directly destroy the subcellular structures or effectively deliver drugs into the proper targets, which can further activate certain cell death pathways, enabling them to boost the therapeutic efficiency, lower drug dosage, reduce side effects, avoid multidrug resistance, and prevent recurrence. In this Review, the design principles, targeting strategies, therapeutic mechanisms, current challenges, and potential future directions of organelle-targeted nanomaterials will be introduced.

Cationic octahedral molybdenum cluster complexes functionalized with mitochondria-targeting ligands: photodynamic anticancer and antibacterial activities

Octahedral molybdenum cluster complexes have recently come forth as pertinent singlet oxygen photosensitizers towards biological applications. Still, their phototoxic efficiency in the absence of nanocarriers remains limited due to their poor cellular uptake. Here, two cationic octahedral molybdenum cluster complexes, bearing carboxylate ligands with triphenylphosphonium (1) or N-methyl pyridinium (2) mitochondria-targeting terminal functions, have been designed and synthesized. Their photophysical properties in water and in vitro biological activity were investigated in the context of blue-light photodynamic therapy of cancer and photoinactivation of bacteria. Upon blue light irradiation, complex 1 displays red luminescence with a quantum yield of 0.24 in water, whereas complex 2 is much less emissive (ΦL < 0.01). Nevertheless, both complexes efficiently produce singlet oxygen, O2(1Δg). Complex 1 is rapidly internalized into HeLa cells and accumulated in mitochondria, followed by relocation to lysosomes and clearance at longer times. In contrast, the more hydrophilic 2 is not internalized into HeLa cells, highlighting the effect of the apical ligands on the uptake properties. The treatment with 1 results in an intensive phototoxic effect under 460 nm irradiation (IC50 = 0.10 ± 0.02 μM), which exceeds by far those previously reported for octahedral cluster-based molecular photosensitizers. The ratio between phototoxicity and dark toxicity is approximately 50 and evidences a therapeutic window for the application of 1 in blue-light photodynamic therapy. Complex 1 also enters and efficiently photoinactivates Gram-positive bacteria Enterococcus faecalis and Staphylococcus aureus, documenting its suitability as a blue-light photosensitizer for antimicrobial applications.

A mitochondria-targetable two-photon fluorescent probe with a far-red to near-infrared emission for sensing hypochlorite in biosystems

Hypochlorite (ClO^-), one of reactive oxygen species (ROS), is closely related with many physiological and pathological processes. Especially as one of cellular reactive oxygen species in mitochondria, ClO^- can induce mitochondrial permeability, which leads to apoptosis. Thus, developing an effective method which is able to sense ClO^- in mitochondria is important. Although fluorescent probe has become a powerful tool for imaging ClO^- in mitochondria, most of them suffered from phototoxicity to biosamples, autofluorescence, and photobleaching phenomenon due to their short-wavelength excitations and emissions. Based on advantages of two-photon fluorescent probe and far-red to NIR fluorescent probe, a mitochondria-targetable two-photon fluorescent probe with a turn-on signal in far-red to NIR region, Mito-TP-ClO, was developed for ClO^- in this paper. Mito-TP-ClO is consisted of a triphenylphosphonium cations as a mitochondria-targetable unit and a structure of dibenzoylhydrazine as a response unit to ClO^-. Mito-TP-ClO exhibited a high sensitivity and a high selectivity to ClO^-, with a linear range from 6.0 × 10^-8 to 1.0 × 10^-5 M and a detection limit of 2.5 × 10^-8 M. Due to its large two-photon cross section (267 GM) and far-red to NIR emission, Mito-TP-ClO exhibits excellent performances including low autofluorescence, photostable fluorescence signal, and deep tissue penetration (230 μM). Moreover, Mito-TP-ClO was successfully used to detect endogenous ClO^- in bacteria-infected cells and inflammatory mouse model, which confirmed that Mito-TP-ClO is a powerful tool to monitor ClO^- in mitochondria and study on effects of hypochlorite on mitochondria.

A Golgi-targeted off-on fluorescent probe for real-time monitoring of pH changes in vivo

We present the first Golgi-targeted small-molecular pH-sensitive fluorescent probe RSG, which allows an off-on fluorescence response to Golgi acidification with high sensitivity and specificity. RSG has been successfully used for real-time monitoring of Golgi pH changes induced by drug treatment at the cellular level, as well as by the LPS-mediated inflammation in vivo.

Role of hydrophobicity in tuning the intracellular uptake of dendron-based fluorophores for in vitro metal ion sensing

Fluorophores are used for sensing biologically relevant ions, toxic metals or pathogenic markers. However, the mode of entry of such fluorophores into the cell greatly depends on their size, shape, surface charge, functional groups, and hydrophobicity. In particular, the influence of hydrophobicity on the intracellular uptake of fluorophores is poorly investigated. Self-assembly is a recent strategy to tune the intracellular uptake of fluorophores, facilitating increased intracellular sensing and fluorescence. Herein, self-assembly of three novel poly(aryl ether) dendron derivatives that contain rhodamine units was used to investigate the effect of hydrophobicity on the intracellular uptake of self-assembled fluorophores. The results suggest that monomer hydrophobicity plays an important role in the uptake. The dendron-based fluorophores, which upon self-assembly, formed stable spherical aggregates ranging from 300 to 500 nm. The rhodamine-based dendrons could selectively sense Hg²⁺ ions in the presence of other competing metal cations. Intracellular imaging of the dendron-based fluorophores displayed bright red fluorescence in human embryonic kidney cells. The rate of intracellular uptake of the three dendron-based fluorophores was analyzed by flow cytometry. The results establish the importance of the hydrophilic-lipophilic balance of the self-assembled amphiphiles for tuning the intracellular uptake.

A NIR fluorescent probe for detection of viscosity and lysosome imaging in live cells

Lysosomes, as the cellular recycling center, are filled with numerous hydrolases that can degrade most cellular macromolecules. Studies have shown that the abnormality of viscosity in lysosomes will disrupt the normal function of lysosomes. Herein, a D-π-A structure near-infrared fluorescent probe containing N,N-dimethylamino benzene as an electron donor, benzothiozole as an electron acceptor, and a vinyl group as a π unit, Lyso-BTC, is explored for fluorescence imaging of lysosomes and detection of lysosomal viscosity changes. Lyso-BTC exhibits a large Stokes shift (∼180 nm), NIR emission (685 nm), good biocompatibility, excellent photostability, and fluorescence response to viscosity. Moreover, the results of in vitro studies reveal that Lyso-BTC is lysosome-targeted and could be applied for the detection of viscosity changes in lysosomes caused by chloroquine treatment. These results confirm that Lys-BTC could be employed to monitor lysosomal viscosity changes in living cells.

A novel 2,5-bis(benzo[d]thiazol-2-yl)phenol scaffold-based ratiometric fluorescent probe for sensing cysteine in aqueous solution and serum

An efficient and novel 2,5-bis(benzo[d]thiazol-2-yl)phenol scaffold-based ratiometric fluorescent probe BTP-Cys for the sensing of cysteine has been developed. The probe BTP-Cys with acrylates moiety, as recognition site, has been successfully constructed on account of the excited state intramolecular proton transfer (ESIPT) mechanism. Upon the treatment with Cys (0-250 μM), this probe BTP-Cys exhibits a dramatic fluorescent intensity ratios enhancement (from 0.03 to 18.3) and a large emission shift (113 nm). The detection limit of this probe is as low as 3.8 × 10^-7 M. Importantly, the concentration and time dependent of Cys in bovine serum albumin (BSA) has also been measured, indicating that BTP-Cys could be a biocompatible and rapid probe for Cys in vitro.

Utilising an anilido-imino ligand to stabilise zinc-phosphanide complexes: reactivity and fluorescent properties

A series of zinc complexes bearing the anilido-imino ligand [(o-C6H4{N(C6H3iPr2)}{C(CH3) = NC6H3iPr2})] [(LDipp)ZnX] has been generated. This includes two amide derivatives, [(LDipp)Zn(N{SiMe3}2)] and [(LDipp)Zn(NH{Dipp})] and two phosphanide derivatives, [(LDipp)ZnPCy2] and [(LDipp)ZnPPh2]. The chemistry of the phosphanide complexes towards chalcogens was examined, with sulfur, selenium and tellurium oxidising the phosphorus centre of the dicylohexylphosphanide complex [(LDipp)ZnPCy2] to form [(LDipp)Zn(E)2PCy2] (E = S, Se, Te). Addition of tellurium to the diphenylphosphanide complex [(LDipp)ZnPPh2] results in formation of Ph2PPPh2 and [(LDipp)ZnTeZn(LDipp)]. The absorption and emission properties of these complexes was examined and the quantum yields are highly dependent upon the non-ancillary ligand X.

Development of photolabile protecting groups and their application to the optochemical control of cell signaling

Many biological processes are naturally regulated with spatiotemporal control. In order to perturb and investigate them, optochemical tools have been developed that convey similar spatiotemporal precision. Pivotal to optochemical probes are photolabile protecting groups, so called caging groups, and recent developments have enabled new applications to cellular processes, including cell signaling. This review focuses on the advances made in the field of caging groups and their application in cell signaling through caged molecules such as neurotransmitters, lipids, secondary messengers, and proteins.

Fluorescent probes for organelle-targeted bioactive species imaging

The dynamic fluctuations of bioactive species in living cells are associated with numerous physiological and pathological phenomena. The emergence of organelle-targeted fluorescent probes has significantly facilitated our understanding on the biological functions of these species. This review describes the design, applications, challenges and potential directions of organelle-targeted bioactive species probes.

Bioactive species, including reactive oxygen species (ROS, including O₂˙^–, H₂O₂, HOCl, ¹O₂, ˙OH, HOBr, etc.), reactive nitrogen species (RNS, including ONOO^–, NO, NO₂, HNO, etc.), reactive sulfur species (RSS, including GSH, Hcy, Cys, H₂S, H₂S_n, SO₂ derivatives, etc.), ATP, HCHO, CO and so on, are a highly important category of molecules in living cells. The dynamic fluctuations of these molecules in subcellular microenvironments determine cellular homeostasis, signal conduction, immunity and metabolism. However, their abnormal expressions can cause disorders which are associated with diverse major diseases. Monitoring bioactive molecules in subcellular structures is therefore critical for bioanalysis and related drug discovery. With the emergence of organelle-targeted fluorescent probes, significant progress has been made in subcellular imaging. Among the developed subcellular localization fluorescent tools, ROS, RNS and RSS (RONSS) probes are highly attractive, owing to their potential for revealing the physiological and pathological functions of these highly reactive, interactive and interconvertible molecules during diverse biological events, which are rather significant for advancing our understanding of different life phenomena and exploring new technologies for life regulation. This review mainly illustrates the design principles, detection mechanisms, current challenges, and potential future directions of organelle-targeted fluorescent probes toward RONSS.

Uptake and Fate of Fluorescently Labeled DNA Nanostructures in Cellular Environments: A Cautionary Tale

Fluorescent dye labeling of DNA oligonucleotides and nanostructures is one of the most used techniques to track their fate and cellular localization inside cells. Here, we report that intracellular fluorescence, and even FRET signals, cannot be correlated with the cellular uptake of intact DNA structures. Live cell imaging revealed high colocalization of cyanine-labeled DNA oligos and nanostructures with phosphorylated small-molecule cyanine dyes, one of the degradation products from these DNA compounds. Nuclease degradation of the strands outside and inside the cell results in a misleading intracellular fluorescent signal. The signal is saturated by the fluorescence of the degradation product (phosphorylated dye). To test our hypothesis, we synthesized a range of DNA structures, including Cy3- and Cy5-labeled DNA cubes and DNA tetrahedra, and oligonucleotides with different stabilities toward nucleases. All give fluorescence signals within the mitochondria after cellular uptake and strongly colocalize with a free phosphorylated dye control. Kinetics experiments revealed that uptake of stable DNA structures is delayed. We also studied several parameters influencing fluorescent data: stability of the DNA strand, fixation methods that can wash away the signal, position of the dye on the DNA strand, and design of FRET experiments. DNA nanostructures hold tremendous potential for biomedical applications and biotechnology because of their biocompatibility, programmability, and easy synthesis. However, few examples of successful DNA machines in vivo have been reported. We believe this contribution can be used as a guide to design better cellular uptake experiments when using fluorescent dyes, in order to further propel the biological development, and application of DNA nanostructures.

Green-Emitting Rhodamine Dyes for Vital Labeling of Cell Organelles Using STED Super-Resolution Microscopy

Fluorescence microscopy reveals the localization, spatial distribution, and temporal dynamics of the specifically labeled organelles in living cells. Labeling with exogenous conjugates prepared from fluorescent dyes and small molecules (ligands) is an attractive alternative to the use of fluorescent proteins, but proved to be challenging due to insufficient cell-permeability of the probes, unspecific staining, or low dye brightness. We evaluated four green-emitting rhodamine dyes and their conjugates intended for the specific labeling of lysosomes, mitochondria, tubulin, and actin in living cells. The imaging performance of the probes in living human fibroblasts has been studied by using confocal and stimulated emission depletion (STED) super-resolution microscopy with a commercial 595 nm STED laser. Two bright and photostable dyes (LIVE 510 and LIVE 515) provide specific and versatile staining.

A lysosome-targeting and polarity-specific fluorescent probe for cancer diagnosis

A lysosome-targeting and polarity-specific fluorescent probe CPM has been rationally designed for cancer diagnosis and imaging. We have successfully shown that lysosome polarity may serve as an ubiquitious marker for cancer detection. The potential of CPM for cancer diagnosis has also been demonstrated at the levels of live cells, organs, whole animal, and clinical patient tissue samples.

Differently Tagged Probes for Protein Profiling of Mitochondria

The mitochondrion is one of the most important organelles in the eukaryotic cell. Characterization of the mitochondrial proteome is a prerequisite for understanding its cellular functions at the molecular level. Here we report a proteomics method based on mitochondrion-targeting groups and click chemistry. In our strategy, three different mitochondrion-targeting moieties were each augmented with a clickable handle and a cysteine-reactive group. Fluorescence-based bioimaging and fractionation experiments clearly showed that most signals arising from the labels were localized in the mitochondria of cells, as a result of covalent attachment between probe and target proteins. The three probes had distinct profiling characteristics. Furthermore, we successfully identified more than two hundred mitochondrial proteins. The results showed that different mitochondrion-targeting groups targeted distinct proteins with partial overlap. Most of the labeled proteins were localized in the mitochondrial matrix and inner mitochondrial membrane. Our results provide a tool for chemoproteomic analysis of mitochondrion-related proteins.

Conformationally Induced Off-On Two-Photon Fluorescent Bioprobes for Dynamically Tracking the Interactions among Multiple Organelles

Unveiling the synergism among multiple organelles for fully exploring the mysteries of the cell has drawn more and more attention. Herein, we developed two two-photon fluorescent bioprobes (Lyso-TA and Mito-QA), of which the conformational change triggered an "off-on" fluorescent response. Lyso-TA can real-time monitor the fusion and movement of lysosomes as well as unveil the mitophagy process with the engagement of lysosomes. Mito-QA was transformed from Lyso-TA by one-step ambient temperature reaction, visualizing the dysfunctional mitochondria through a shift from mitochondria to nucleoli. With superior two-photon absorption cross section, good biocompatibility, and greater penetration depth, two small bioprobes were both applied in in vivo bioimaging of brain tissues and zebrafish.

Lysosome-targeted carbon dots for ratiometric imaging of formaldehyde in living cells

Formaldehyde (FA) is involved in many biological processes and is closely connected with many diseases including Alzheimer's disease and cancer. Therefore, methods for sensitive and selective detection of FA in living cells are highly demanded. As a new class of carbon nanomaterials, carbon dots (CDs) have attracted great attention owing to their robust photostability, good biocompatibility and environmental friendliness. In this manuscript, the first lysosome-targeted CDs for ratiometric fluorescence detection of FA were efficiently prepared from dexamethasone and 1,2,4,5-tetraaminobenzene through the microwave-assisted hydrothermal method. These CDs show highly selective and sensitive sensing ability towards FA with fast response and great changes of ratio values. The CDs exhibit robust photostability and good biocompatibility and were successfully employed in ratiometric fluorescence bioimaging of FA fluctuations in lysosomes of living cells, which demonstrates their great practicability in FA-related bioanalysis and biological studies.

Longitudinal monitoring of cancer cell subpopulations in monolayers, 3D spheroids, and xenografts using the photoconvertible dye DiR

A central challenge in cancer biology is the identification, longitudinal tracking, and -omics analysis of specific cells in vivo. To this aim, photoconvertible fluorescent dyes are reporters that are characterized by a set of excitation and emission spectra that can be predictably altered, resulting in a distinct optical signature following irradiation with a specific light source. One such dye, DiR, is an infrared fluorescent membrane probe that can irreversibly undergo such a switch. Here, we demonstrate a method using DiR for the spatiotemporal labeling of specific cells in the context of cancer cell monolayer cultures, 3D tumor spheroids, and in vivo melanoma xenograft models to monitor the proliferation of cellular subpopulations of interest over time. Importantly, the photoconversion process is performed in situ, supporting the pursuit of novel avenues of research in molecular pathology.

A dual-modal nanoprobe based on Eu(iii) complex-MnO2 nanosheet nanocomposites for time-gated luminescence-magnetic resonance imaging of glutathione in vitro and in vivo

Dual-modal fluorescence-magnetic resonance imaging (MRI) techniques have gained great interest in biomedical research and clinical practice, since they integrate the advantages of both imaging techniques and provide a useful approach to simultaneously investigate both molecular and anatomical information at the same biological structures. Herein, we report the construction of a dual-modal time-gated luminescence (TGL)/MRI nanoprobe, BHHBB-Eu3+@MnO2, for glutathione (GSH) by anchoring luminescent β-diketone-Eu3+ complexes on layered MnO2 nanosheets. The fabricated nanoprobe exhibited very week luminescence and MR signals since the luminescence of the Eu3+ complex was quenched by MnO2 nanosheets and Mn atoms were isolated from water. Upon exposure to GSH, the MnO2 nanosheets were rapidly and selectively reduced to Mn2+ ions, resulting in remarkable enhancements of TGL and MR signals simultaneously. The combination of TGL and MR detection modes enables the nanoprobe to be used for detecting GSH in a wide concentration range (1-1000 μM) and imaging GSH at different resolutions and depths ranging from the subcellular level to the whole body. Furthermore, the as-prepared nanoprobe exhibited a low cytotoxicity and good biocompatibility, rapid response rate, long-lived luminescence, and high sensitivity and selectivity for responding to GSH. These features allowed it to be successfully used for the TGL detection of GSH in human sera, TGL imaging of GSH in living cells and zebrafish, as well as dual-modal TGL/MR imaging of GSH in tumor-bearing mice. All of these results highlighted the applicability and advantages of the nanoprobe for detecting GSH in vitro and in vivo.

Self-Assembled Biocompatible Fluorescent Nanoparticles for Bioimaging

Fluorescence is a powerful tool for mapping biological events in real-time with high spatial resolution. Ultra-bright probes are needed in order to achieve high sensitivity: these probes are typically obtained by gathering a huge number of fluorophores in a single nanoparticle (NP). Unfortunately this assembly produces quenching of the fluorescence because of short-range intermolecular interactions. Here we demonstrate that rational structural modification of a well-known molecular fluorophore N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl) (NBD) produces fluorophores that self-assemble in nanoparticles in the biocompatible environment without any dramatic decrease of the fluorescence quantum yield. Most importantly, the resulting NP show, in an aqueous environment, a brightness which is more than six orders of magnitude higher than the molecular component in the organic solvent. Moreover, the NP are prepared by nanoprecipitation and they are stabilized only via non-covalent interaction, they are surprisingly stable and can be observed as individual bright spots freely diffusing in solution at a concentration as low as 1 nM. The suitability of the NP as biocompatible fluorescent probes was demonstrated in the case of HeLa cells by fluorescence confocal microscopy and MTS assays.

A highly selective and recyclable NO-responsive nanochannel based on a spiroring opening-closing reaction strategy

Endogenous nitric oxide (NO) is an important messenger molecule, which can directly activate K⁺ transmission and cause relaxation of vascular smooth muscle. Here, inspired by the K⁺ channel of smooth muscle cells, we report, a novel NO-regulated artificial nanochannel based on a spiro ring opening-closing reaction strategy. This nanofluidic diode system shows an outstanding NO selective response owing to the specific reaction between o-phenylenediamine (OPD) and NO on the channel surface with high ion rectification ratio (~6.7) and ion gating ratio (~4). Moreover, this NO gating system exhibits excellent reversibility and stability as well as high selectivity response. This system not only helps us understand the process of NO directly regulating biological ion channels, but also has potential application value in the field of biosensors.

A chromenoquinoline-based two-photon fluorescent probe for the highly specific and fast visualization of sulfur dioxide derivatives in living cells and zebrafish

Sulfur dioxide (SO₂) derivatives play critical roles in various biological processes. Therefore, effective methods for monitoring SO₂ are of vital importance in bisulfite/sulfite biology. In this study, a two-photon (TP) imaging probe (CQ-SO₂) for detecting SO₂ derivatives was designed and constructed, based on the chromenoquinoline (CQ) fluorophore and a β-chlorovinyl aldehyde sensing moiety. The TP properties of the CQ derivatives were revealed for the first time in this study. This study enriched the biological application range of CQ derivatives and also provided a new choice for the development of TP dyes. In particular, the CQ-SO₂ probe exhibited a fast response time (about 5 s), low detection limit (16 nM) and ultrahigh specificity towards SO₂ derivatives. Furthermore, the probe was successfully applied to the highly specific TP bioimaging of SO₂ derivatives in living cells and zebrafish.

Simultaneous dual-colour tracking lipid droplets and lysosomes dynamics using a fluorescent probe

After entering a cell, most small molecule fluorescent probes are dispersed in the cytoplasm before they then accumulate in a specific organelle or subcellular zone. Molecules that can enter two or more organelles with high selectivity are all but unknown. In this work, we report a naphthalimide-based fluorescent probe, NIM-7, that allows lipid droplets and lysosomes to be labelled simultaneously and with high specificity. These subcellular entities can then be visualized readily through yellow and red fluorescence, using different excitation and detection channels. NIM-7 allows 3D imaging and quantitative visualizing of lipid droplets and lysosomes. It is also able to track simultaneously the movement of lipid droplets and lysosomes in real-time. We also report here that NIM-7 can be used to image both different cell lines and zebrafish embryos.

Reaction-free and MMP-independent fluorescent probes for long-term mitochondria visualization and tracking

Visualizing and tracking mitochondrial dynamic changes is crucially important in the fields of physiology, pathology and pharmacology. Traditional electrostatic-attraction based mitochondrial probes fail to visualize and track the changes due to their leakage from mitochondria when mitochondrial membrane potential (MMP) decreases. Reaction-based MitoTracker probes can realize visualization and tracking of mitochondria changes independent of MMP changes. However, such probes impair mitochondrial proteins and exhibit high cytotoxicity. Therefore, it still remains challenging to explore reaction-free and highly biocompatible probes for visualizing and tracking mitochondrial dynamics independent of MMP fluctuations. Herein we synthesized two reaction-free fluorescent mitochondrial probes ECPI-12 and IVPI-12 bearing a long C12-alkyl chain. These cationic probes can firmly immobilize in the mitochondrial inner membrane by strong hydrophobic interaction between the C12-alkyl chain and lipid bilayer, resulting in high specificity and long-term mitochondrial staining regardless of MMP changes. They also exhibit large two-photon absorption cross-sections and show deep penetration in live tissues in two-photon microscopy. Furthermore, they display excellent biocompatibility and realize in situ and real-time mitophagy tracking in live cells. These excellent properties could make ECPI-12 and IVPI-12 the first selective tools for long-term visualization and tracking of mitochondrial dynamics.

An enhanced fluorescence sensor for specific detection Cys over Hcy/GSH and its bioimaging in living cells

Cysteine (Cys) is not only the central matter of sulfur metabolism in cells but also the only amino acid with reduced thiol group in 20 kinds of natural amino acids. In animal cells, Cys is taking part in many important and essential biological functions including protein synthesis, detoxification and metabolism. The development and application of fluorescent probes for the detection of Cys have attracted more and more attention and interest. Herein, we report a new fluorescent probe NFA that utilized naphthyl carboxy fluorescein as fluorophore and acryloyl group as reaction site for Cys specific detection. The probe essentially has weak fluorescence. Cys addition to NFA containing system induced distinct enhanced fluorescence emission which was attributed to the nucleophilic reaction of cysteine and acryloyl to release the fluorophore. The signal fluorescent response detection system allows NFA to be a reliable tool for Cys detection with low detection limit (0.58 μM). And NFA has been successfully applied for Cys imaging specifically in live Hela cells, which promotes the probe as a potential tool to understand the pathology of Cys related diseases.

Mechanosensitive Fluorescent Probes to Image Membrane Tension in Mitochondria, Endoplasmic Reticulum, and Lysosomes

Measuring forces inside cells is particularly challenging. With the development of quantitative microscopy, fluorophores which allow the measurement of forces became highly desirable. We have previously introduced a mechanosensitive flipper probe, which responds to the change of plasma membrane tension by changing its fluorescence lifetime and thus allows tension imaging by FLIM. Herein, we describe the design, synthesis, and evaluation of flipper probes that selectively label intracellular organelles, i.e., lysosomes, mitochondria, and the endoplasmic reticulum. The probes respond uniformly to osmotic shocks applied extracellularly, thus confirming sensitivity toward changes in membrane tension. At rest, different lifetimes found for different organelles relate to known differences in membrane organization rather than membrane tension and allow colabeling in the same cells. At the organelle scale, lifetime heterogeneity provides unprecedented insights on ER tubules and sheets, and nuclear membranes. Examples on endosomal trafficking or increase of tension at mitochondrial constriction sites outline the potential of intracellularly targeted fluorescent tension probes to address essential questions that were previously beyond reach.

A novel mitochondrial-targeting near-infrared fluorescent probe for imaging γ-glutamyl transpeptidase activity in living cells

γ-Glutamyl transpeptidase (GGT) plays an essential role in regulating cellular glutathione and cysteine homeostasis, and its abnormal elevation is associated with different diseases including cancers. Here a novel mitochondrial-targeting near-infrared fluorescent probe was designed for GGT by conjugating glutamate acid to a newly synthesized amine hemicyanine fluorophore. The fluorescent probe was initially non-emissive due to the formation of an amide bond which destroyed the electronic-donating ability of the amine moiety and disrupted the push-pull structure. GGT-mediated cleavage of the γ-glutamyl bond regenerated the initial fluorophore with distinct intramolecular charge transfer (ICT) and activated the fluorescence signal. The fluorescent probe displayed a linear relationship to the concentration of GGT in the range of 1.0-90 U L-1, with an estimated limit of detection (LOD) of 0.4 U L-1. Its ability to target and image mitochondrial GGT activity was demonstrated in living cells with high specificity and fast response. We believe our near-infrared fluorescent probe could have great potential in imaging mitochondrial GGT activity and elucidating GGT-associated pathological consequences in living cells and even small animal models.

A NIR fluorescent sensor for biothiols based on a dicyanoisophorone derivative with a large Stokes shift and high quantum yield

Probe N-Bio exhibited rapid response, high sensitivity and strong NIR fluorescence in the detection of biothiols in living cells.

Ultrafast labeling and high-fidelity imaging of mitochondria in cancer cells using an aggregation-enhanced emission fluorescent probe

An aggregation-enhanced emission probe was developed for ultrafast labeling and high-fidelity imaging of mitochondria in cancer cells with a high signal-to-noise ratio.

Acidity-triggered zwitterionic prodrug nano-carriers with AIE properties and amplification of oxidative stress for mitochondria-targeted cancer theranostics

A TPE-based polyurethane prodrug has been established for mitochondria-targeting drug delivery and real-time monitoring.

Constructing a far-red to near-infrared fluorescent probe for highly specific detection of cysteine and its bioimaging applications in living cells and zebrafish

A novel far-red to near-infrared fluorescent probe for highly specific detection of cysteine without interference.