Structure Rigidification Promoted Ultrabright Solvatochromic Fluorescent Probes for Super-Resolution Imaging of Cytosolic and Nuclear Lipid Droplets

A Membrane-Targeting Aggregation-Induced Emission Probe for Monitoring Lipid Droplet Dynamics in Ischemia/Reperfusion-Induced Cardiomyocyte Ferroptosis

Myocardial ischemia/reperfusion injury (MIRI) is the leading cause of irreversible myocardial damage. A pivotal pathogenic factor is ischemia/reperfusion (I/R)-induced cardiomyocyte ferroptosis, marked by iron overload and lipid peroxidation. However, the impact of lipid droplet (LD) changes on I/R-induced cardiomyocyte ferroptosis is unclear. In this study, an aggregation-induced emission probe, TPABTBP is developed that is used for imaging dynamic changes in LD during myocardial I/R-induced ferroptosis. TPABTBP exhibits excellent LD-specificity, superior capability for monitoring lipophagy, and remarkable photostability. Molecular dynamics (MD) simulation and super-resolution fluorescence imaging demonstrate that the TPABTBP is specifically localized to the phospholipid monolayer membrane of LDs. Imaging LDs in cardiomyocytes and myocardial tissue in model mice with MIRI reveals that the LD accumulation level increase in the early reperfusion stage (0-9 h) but decrease in the late reperfusion stage (>24 h) via lipophagy. The inhibition of LD breakdown significantly reduces the lipid peroxidation level in cardiomyocytes. Furthermore, it is demonstrated that chloroquine (CQ), an FDA-approved autophagy modulator, can inhibit ferroptosis, thereby attenuating MIRI in mice. This study describes the dynamic changes in LD during myocardial ischemia injury and suggests a potential therapeutic target for early MIRI intervention.

Fluorescent Probe as Dual-Organelle Localizer Through Differential Proton Gradients Between Lipid Droplets and Mitochondria

Dual-organelle molecular localizers represent powerful new tools allowing the exploration of interorganelle physical contacts and subcellular chemical communication. Here, we describe new dynamic molecular probes to localize mitochondria and lipid droplets taking advantage of the differential proton gradients present in these organelles as well as the activity of mitochondrial esterase. We unveil their potential utility when organelle retention mechanisms and proton gradients are synchronized, an insight that has not been documented previously. Our discoveries indicate that dual-organelle probes serve as a valuable multiplexing assay during starvation-induced autophagy. The pioneering molecular mechanism they employ opens doors to avoid using labile esters such as acetoxymethyl derivatives which are not optimal in imaging microscopy assays.

Polymeric microenvironment enhancing polarity response sensitivity for discriminating lipid droplets in cancer cells

Cellular micro-environment analysis via fluorescence probe has become a powerful method to explore the early-stage cancer diagnosis and pathophysiological process of relevant diseases. The polarity change of intracellular lipid droplets (LDs) is closely linked with disorders or diseases, which result in various physiological and pathological processes. However, the efficient design strategy for lipid droplet polarity probes with high sensitivity is lacking. To overcome this difficulty, two kinds of LDs-targeting and polarity-sensitive fluorescent probes containing carbazole and siloxane groups were rationally designed and synthesized. With the carbazole-based rotor and bridge-like siloxanes, two probes (P1 and P2) behave high sensitivity to polarity changes and show different fluorescent intensity in normal and cancer cells. Notably, polysiloxanes groups promoted the response sensitivity of the probes dramatically for the polymeric microenvironment. In addition, due to the polarity changes of LDs in cancer cells, the distinct fluorescent intensities in different channels of laser scanning confocal microscope were observed between NHA cell and U87 cells. This work could offer an opportunity to monitor the dynamic behaviors of LDs and further provide a powerful tool to be potentially applied in the early-stage diagnosis of cancer.

Synthesis, structure, photophysical property, stability of tetraphenylethylene-based boranil, and applications in cell imaging

A new family of tetraphenylethylene-based N,O-chelated boranil complexes (TPE-BAs) with aggregation-induced emission (AIE) characteristics were developed. X-ray crystallographic analysis indicated that the terminal substituents on the aniline moiety significantly affected the intermolecular stacking mode, thereby influencing the photophysical properties. The stabilities of these compounds are closely related to the substituents on the aniline moiety. Electron-donor-substituted TPE-BA-OMe exhibited the best stability, whereas the electron-acceptor-substituted compounds exhibited poor stability. Benefitting from its AIE properties and suitable lipophilicity, TPE-BA-OMe served as an excellent fluorescent probe for the specific bioimaging of lipid droplets in living cells.

Ultrabright organic fluorescent probe for quantifying the dynamics of cytosolic/nuclear lipid droplets

Lipid droplets (LDs) are extremely active organelles that play a crucial role in energy metabolism, membrane formation, and the production of lipid-derived signaling molecules by regulating lipid storage and release. Nevertheless, directly limited by the lack of superior fluorescent probes, studies of LDs dynamic motion velocity have been rarely reported, especially for nuclear LDs. Herein, a novel organic fluorescent probe Lipi-Bright has been rationally developed based on bridged cyclization of distyrylbenzene. The fully ring-fused molecule structure endows the probe with high photostability. Moreover, this new fluorescent probe displays the features of excellent LDs staining specificity as well as ultrahigh fluorescence brightness. Lipi-Bright labeled LDs was dozens of times brighter than representative probes BODIPY 493/503 or Nile Red. Consequently, by in-situ time-lapse fluorescence imaging, the dynamics of LDs have been quantitatively studied. For instance, the velocities of cytosolic LDs (37 ± 15 nm/s) are found to be obviously faster than those of nuclear LDs (24 ± 4 nm/s), and both the cytosolic LDs and the nuclear LDs would be moved faster or slower depend on the various stimulations. Overall, this work providing plentiful information on LDs dynamics will greatly facilitate the in-depth investigation of lipid metabolism.

Engineering of a NIR fluorescent probe for high-fidelity tracking of lipid droplets in living cells and nonalcoholic fatty liver tissues

LDs (Lipid droplets) are key organelles for lipid metabolism and storage, which are closely related to ferroptosis and fatty liver. Due to its small size and highly dynamic nature, developing high-fidelity fluorescent probes for imaging of LDs is crucial for observing the dynamic physiological processes of LDs and investigating LDs-associated diseases. Herein, we synthesized three dicyanoisophorone-based fluorescent probes (DCIMe, DCIJ, and DCIQ) with different electron-donating groups and studied their imaging performance for LDs. The results show that DCIQ is highly polarity sensitive and can perform high-fidelity imaging for LDs, with significantly better performance than DCIMe, DCIJ, and commercial LD probe BODIPY 493/503. Based on this, DCIQ was successfully applied to real-time observe the interplays between LDs and other organelles (mitochondria, lysosomes, and endoplasmic reticulum), and to image the dynamics of LDs with fast scanning mode (0.44 s/frame) and the generation of oleic acid-induced LDs with high-fidelity. Finally, DCIQ was used to study the changes of LDs in the ferroptosis process and nonalcoholic fatty liver disease tissues. Overall, this study provided a powerful tool for high-fidelity imaging of LDs in cells and tissues.

Lipid droplet-specific near-infrared fluorescent probe for discriminating cancer and normal cells and diagnosing fatty liver

Lipid droplets (LDs) is a newly essential organelle, which participates in carious physiological and pathological processes. LDs are considered as potential markers for disease diagnosis. Specific imaging of LDs is useful to understand their basic biological function and to diagnose diseases. Here we designed and synthesized two fluorescent probes based on the low polarity and high viscosity in LDs. The terminal probe ZH-2 exhibits lipophilicity, NIR emission, viscosity sensitivity, and LDs specificity. The probe has been successfully used for visualizing LDs metabolism, discriminating between normal and cancerous cells, and diagnosing fatty liver disease.

Xanthene, cyanine, oxazine and BODIPY: the four pillars of the fluorophore empire for super-resolution bioimaging

In the realm of biological research, the invention of super-resolution microscopy (SRM) has enabled the visualization of ultrafine sub-cellular structures and their functions in live cells at the nano-scale level, beyond the diffraction limit, which has opened up a new window for advanced biomedical studies to unravel the complex unknown details of physiological disorders at the sub-cellular level with unprecedented resolution and clarity. However, most of the SRM techniques are highly reliant on the personalized special photophysical features of the fluorophores. In recent times, there has been an unprecedented surge in the development of robust new fluorophore systems with personalized features for various super-resolution imaging techniques. To date, xanthene, cyanine, oxazine and BODIPY cores have been authoritatively utilized as the basic fluorophore units in most of the small-molecule-based organic fluorescent probe designing strategies for SRM owing to their excellent photophysical characteristics and easy synthetic acquiescence. Since the future of next-generation SRM studies will be decided by the availability of advanced fluorescent probes and these four fluorescent building blocks will play an important role in progressive new fluorophore design, there is an urgent need to review the recent advancements in designing fluorophores for different SRM methods based on these fluorescent dye cores. This review article not only includes a comprehensive discussion about the recent developments in designing fluorescent probes for various SRM techniques based on these four important fluorophore building blocks with special emphasis on their effective integration into live cell super-resolution bio-imaging applications but also critically evaluates the background of each of the fluorescent dye cores to highlight their merits and demerits towards developing newer fluorescent probes for SRM.

Fine-tuning benzazole-based probe for the ultrasensitive detection of Hg2+ in water samples and seaweed samples

Developing potentially toxic metal ion probes is significant for environment and food safety. Although Hg²⁺ probes have been extensively studied, small molecule fluorophores that can integrate two applications of visual detection and separation into one unit remain challenging to access. Herein, by incorporating triphenylamine (TPA) into tridentate skeleton with an acetylene bridge, 2,6-bisbenzimidazolpyridine-TPA (4a), 2,6-bisbenzothiazolylpyridine-TPA (4b) and 2,6-bisbenzothiazolylpyridine-TPA (4c) were first constructed, expectably showing distinct solvatochromism and dual-state emission properties. Since the diverse emission properties, the fluorescence detection of 4a-4b can be achieved with an ultrasensitive response (LOD = 10^-11 M) and efficient removal of Hg²⁺. More interestingly, 4a-4b can not only be developed into paper/film sensing platform, but also reliably detect Hg²⁺ in real water and seaweed samples, with recoveries ranging from 97.3% to 107.8% and a relative standard deviation of less than 5%, indicating that they have excellent application potential in the field of environmental and food chemistry.

Super-resolution dynamic tracking of cellular lipid droplets employing with a photostable deep red fluorogenic probe

Lipid droplets (LDs) are critical organelles involved in many physiological processes in eukaryotic cells. To visualize and study LDs, particular the small/nascent LDs, the emerging super-resolution fluorescence imaging techniques with nanoscale resolution would be much more powerful in comparison to the conventional confocal/wide-field imaging techniques. However, directly limited by the availability of advanced LDs probes, super-resolution fluorescence imaging of LDs is a practically challenging task. In this context, a superior LDs fluorescent probe named Lipi-Deep Red is newly developed for structured illumination microscopy (SIM) super-resolution imaging. This fluorescent probe features with the advantages of strong deep red/NIR emission, fluorogenic character, high LDs specificity, and outstanding photostability. These advantages enable the fluorescent probe to be finely applied in SIM super-resolution imaging, e.g. time-lapse imaging (up to 1000 frames) to monitor the LDs dynamics at nanoscale (159 nm), two-color time-lapse imaging to discover the nearby contact/interaction between LDs and mitochondria. Consequently, the fusion processes of LDs are impressively visualized at a high spatial and temporal resolution. Two kinds of contact models between LDs and mitochondria (dynamic contact and stable contact) newly proposed in the recent literatures are successfully revealed.

An endoplasmic reticulum-specific ratiometric fluorescent probe for imaging esterase in living cells

Esterase is primarily distributed in the endoplasmic reticulum (ER) and often overexpressed in cancer cells. Therefore, the detection of esterase in ER is significant for monitoring the metabolic process of various esters and evaluating the efficacy of chemotherapeutic prodrugs. However, only few fluorescent probes can detect esterase in the ER due to the lack of ER-specificity. More seriously, these probes are often limited by low pearson's colocalization coefficient and one single wavelength emission. To solve those problems, an ER-specific ratiometric fluorescent probe (ER-EST) is designed for detecting esterase in living cells. The ER-EST shows a ratiometric and red-shifted emission (125 nm) from 435 to 560 nm after hydrolysis by esterase. The fluorescence intensity ratio of ER-EST displays quantitative response to the esterase activity (0-0.5 U/mL) with low detection limit of 1.8 × 10^-4 U/mL. Importantly, the ER-EST with good biocompatibility and excellent ER-targeted ability was successfully employed to ratiometric image the endogenous endoplasmic reticulum esterase in living cells.

A Turn-On Lipid Droplet-Targeted Near-Infrared Fluorescent Probe with a Large Stokes Shift for Detection of Intracellular Carboxylesterases and Cell Viability Imaging

Carboxylesterases (CEs) play important physiological roles in the human body and are involved in numerous cellular processes. Monitoring CEs activity has great potential for the rapid diagnosis of malignant tumors and multiple diseases. Herein, we developed a new phenazine-based "turn-on" fluorescent probe DBPpys by introducing 4-bromomethyl-phenyl acetate to DBPpy, which can selectively detect CEs with a low detection limit (9.38 × 10^-5 U/mL) and a large Stokes shift (more than 250 nm) in vitro. In addition, DBPpys can also be converted into DBPpy by carboxylesterase in HeLa cells and localized in lipid droplets (LDs), emitting bright near-infrared fluorescence under the irradiation of white light. Moreover, we achieved the detection of cell health status by measuring the intensity of NIR fluorescence after co-incubation of DBPpys with H₂O₂-pretreated HeLa cells, indicating that DBPpys has great potential applications for assessing CEs activity and cellular health.

Red-emissive Dual-state Fluorogenic Probe for Wash-free Imaging of Lipid Droplets in Living Cells and Fatty Liver Tissues

Lipid droplet (LD) dysfunction can result in various diseases, such as nonalcoholic fatty liver disease. Imaging agents built on dual-state emission (DSE) molecules that fluoresce in both dilute solutions and the aggregated state are receiving attention as this type of probe could provide bright fluorescence signals at variable concentrations, avoiding false signal readout caused by the concentration fluctuation in living systems. Herein, we identified a red emissive molecule featuring DSE, from three newly synthesized molecules, for specific detection of LDs in live cells. The bioimaging abilities have been well confirmed by optical spectroscopies, theoretical calculations, cell experiments, as well as animal studies. The DSE probe is effective for LD detection at concentrations ranging from 1 μM to 100 μM while retaining high brightness and signal fidelity. This study provides a knowledge base for the future design of DSE-active fluorescent probes for understanding LD-related diseases.

Single-molecule characterization of a bright and photostable deep-red fluorescent squaraine-figure-eight (SF8) dye

Squaraine Figure Eight (SF8) dyes are a unique class of deep-red fluorescent dyes with self-threaded molecular architecture that provides structural rigidity while simultaneously encapsulating and protecting the emissive fluorochrome. Previous cell microscopy and bulk phase studies of SF8 dyes indicated order of magnitude enhancements in photostability over conventional pentamethine cyanine dyes such as Cy5. Studies conducted at the single molecule level now reveal that these ensemble level enhancements carry over to the single molecule level in terms of enhanced emission quantum yields, longer times to photobleaching, and enhanced total photon yields. When compared to Cy5, the SF8-based dye SF8(D4)2 possesses a three-fold larger single molecule emission quantum yield, exhibits order of magnitude longer average times before photobleaching, and exhibits twenty times larger photon yields. Additional features such as water solubility, fluorochrome encapsulation to protect it against nucleophilic attack, and selective biomarker targeting capability make SF8-based dyes promising candidates for biological labeling and microscopy applications and single molecule tracking.

Research Progress in Fluorescent Probes for Arsenic Species

Arsenic is a toxic non-metallic element that is widely found in nature. In addition, arsenic and arsenic compounds are included in the list of Group I carcinogens and toxic water pollutants. Therefore, rapid and efficient methods for detecting arsenic are necessary. In the past decade, a variety of small molecule fluorescent probes have been developed, which has been widely recognized for their rapidness, efficiency, convenience and sensitivity. With the development of new nanomaterials (AuNPs, CDs and QDs), organic molecules and biomolecules, the conventional detection of arsenic species based on fluorescence spectroscopy is gradually transforming from the laboratory to the portable kit. Therefore, in view of the current research status, this review introduces the research progress of both traditional and newly developed fluorescence spectrometry based on novel materials for arsenic detection, and discusses the potential of this technology in the rapid screening and field testing of water samples contaminated with arsenic. The review also discusses the problems that still exist in this field, as well as the expectations.

Reversible Near-Infrared Fluorescent Probe for Rapid Sensing Sulfur Dioxide and Formaldehyde: Recognition and Photoactivation Mechanism and Applications in Bioimaging and Encryption Ink

A novel near-infrared (NIR) fluorescent Probe 1 was successfully developed for the reversible detection of sulfur dioxide derivatives and formaldehyde. The purple solution of Probe 1 faded to colorless in 1.8 s with the addition of HSO₃^-. Meanwhile, its fluorescence signal disappeared instantaneously with a 39 nM detection limit. The probe exhibited excellent selectivity toward HSO₃^- over other potential interfering agents. Then, its absorption and fluorescence bands were able to effectively recover in response to formaldehyde. Remarkably, this reverse process was able to accelerate 84 times under UV light in 122 s and achieved a recovery rate of 98% by UV light, the photoactivation mechanism was fully determined by HRMS and theoretical calculation. Furthermore, we demonstrated that Probe 1 was successfully applied for the detection of sulfur dioxide derivatives and formaldehyde in living cells and data encryption.