Charge-Dependent Strategy Enables a Single Fluorescent Probe to Study the Interaction Relationship between Mitochondria and Lipid Droplets

A robust H2O2-responsive AIEgen with multiple-task performance: Achieving food analysis, visualization of dual organelles and diagnosis of liver injury

Drug- and alcohol-induced liver injury poses a serious threat to human health with long-lasting clinical, social and economic consequences. As a reactive oxygen species (ROS), hydrogen peroxide (H2O2) is widely-used as a molecular biomarker of liver injury. H2O2 is also utilized as a processing aid in the food industry. Therefore, monitoring the levels of H2O2 in liver and food samples is important for disease diagnosis and food safety demanding the development of efficient sensing and visualization tools. To address this challenge, we report a novel aggregation-induced emission (AIE) fluorescent probe for detecting H2O2 named TVQ-B, featuring near-infrared (NIR) emission (740 nm), large Stokes shift (195 nm), fast response time and high selectivity. Interestingly, H2O2 can activate the dual organelles targeting ability of TVQ-B for mitochondria and lipid droplets, enabling dual-channel imaging of H2O2. Importantly, the TVQ-B probe has been successfully used to monitor H2O2 in food samples, living cells, zebrafish and mice. This fluorescence imaging approach using endogenous H2O2 as a marker provides a robust and convenient diagnostic protocol for drug- and alcohol-induced liver injury. Based on this protocol, TVQ-B-based visualization provided potent evidence for the therapeutic efficacy of natural flavonolignan silybin for the reduction of negative effects of alcohol consumption.

An intelligent NIR fluorescent dye with dual response to pH and viscosity for investigating the interactions between LDs and mitochondria

The interaction between lipid droplets and mitochondria plays a pivotal role in biological processes including cellular stress, metabolic homeostasis, cellular autophagy and apoptosis. Deciphering the complex interplay between lipid droplets and mitochondria is essential for gaining insights into the fundamental workings of the cell and can have broad implications for the development of therapeutic strategies for various diseases, including metabolic disorders, neurodegenerative diseases, and cancer. In this study, we develop a pH and viscosity-responsive near-infrared (NIR) fluorescent probe PTOH to investigate the interaction between lipid droplets and mitochondria. This probe demonstrates a significant enhancement in fluorescence intensity at 470 nm when the pH increases, while under acidic conditions, its fluorescence intensity at 730 nm intensifies by a factor of 35 with rising system viscosity. Cell imaging experiments revealed that PTOH can effectively discriminate between normal and cancerous cells, as well as detect intracellular pH and viscosity alterations induced by drugs. Additionally, PTOH is utilized to visualize the interaction between lipid droplets and mitochondria and to differentiate between cellular autophagy and apoptosis phenomena, providing a valuable tool for elucidating the mechanisms underlying lipid droplet-mitochondria interactions and their associated diseases.

Rational design of an AIEgen for imaging lipid droplets polarity change during ferroptosis

Ferroptosis can regulate cell death by accumulating lipid peroxides, affecting the structure and polarity of lipid droplets (LDs), but clear evidence is still lacking. Fluorescence imaging is the most powerful technique for studying LDs' function. However, developing AIE fluorescent probes with high selectivity and sensitivity for targeting LDs remains challenging. In this study, we rationally designed an AIEgen, as a novel fluorescent probe TPE-BD, by constructing a push-pull electron structure. The probe has benzo[b]thiophene-3(2H)-one 1,1-dioxide as the electron acceptor, tetraphenylethylene (AIE skeleton) as the electron donor, and thiophene as the bridging group. The optical performance of probe TPE-BD indicated that the UV-visible absorption spectrum of the probe was minimally affected by solvent polarity (except for glycerol and PBS solvents), but the fluorescence of probe is very sensitive to changes in polarity, achieving the goal of polarity detection in LDs. CCK-8 assay and cell imaging experiments demonstrated that probe TPE-BD exhibited good cell compatibility and effectively targeted LDs, enabling the monitoring of LDs' polarity and quantity during ferroptosis.

A membrane-anchored fluorescent probe for the detection of pH in living cells and NAFLD

The abnormal pH in cell membranes can lead to disorder in membrane structure and permeability, and is also an important signal of cell cancer. The acidification of the cell membrane can lead to the disorder of cell lipid metabolism and lead to non-alcoholic fatty liver disease (NAFLD). However, fluorescent probes to detect the cell membrane pH have rarely been reported, let alone used to study NAFLD. For this, we developed a fluorescent probe (Mem-pH) that can firmly anchor the cell membrane based on lipophilic action and electrostatic action forces, and successfully detect membrane pH by fluorescence intensity. More importantly, the probe Mem-pH can quantify the pH of different kinds of cell membranes, further demonstrating that the pH of cancer cell membranes is lower than that of normal cell membranes. Furthermore, Mem-pH successfully differentiates and detects different degrees of NAFLD tissues, offering hope for timely diagnosis of NAFLD.

Mitochondria-Specific Fluorescent Probe for Revealing the Interaction between Mitochondria and Lysosomes during Apoptosis

The mitochondria, as one of the essential organelles in cells, are closely associated with numerous biological processes. Therefore, the realization of clear and real-time imaging for tracking mitochondria is of profound significance. Here, we present a mitochondria-targeting fluorescent probe, N(CH2)3-PD-NEt, for the real-time fluorescence imaging of mitochondria in living cells. Using the probe, the fluorescence changes of mitochondria stimulated by different drugs were successfully observed by fluorescence imaging. In addition, the dynamic processes of mitochondria and lysosomes during apoptosis were also explored. Importantly, we observed several novel dynamic interaction patterns between mitochondria and lysosomes. Among them, the most prominent pattern involved the noncontact movements of two lysosomes, that is, one lysosome gradually approached the other lysosome over time, eventually coming into contact and merging with it while gradually combining with mitochondria to form new mitochondria. Notably, the protrusions of the mitochondria became increasingly evident during this process. Meanwhile, we successfully observed the dynamic changes of mitochondria with SIM super-resolution imaging. The study provides promising help for the in-depth study of the dynamic processes of mitochondrial physiology and pathology and the study of the interactions between organelles.

An acridone based fluorescent dye for lipid droplet tracking and cancer diagnosis

Lipid droplets (LDs) are spherical organelles that localize in the cytosol of eukaryotic cells. Different proteins are embedded on the surface of LDs, so LDs play a vital role in the physiological activities of cells. The dysregulation of LDs is associated with various human diseases, such as diabetes and obesity. Therefore, it is essential to develop a fluorescent dye that labels LDs to detect and monitor illnesses. In this study, we developed the compound BDAA12C for staining LDs in cells. BDAA12C exhibits excellent LD specificity and low toxicity, enabling us to successfully stain and observe the fusion of LDs in A549 cancer cells. Furthermore, we also successfully distinguished A549 cancer cells and MRC-5 normal cells in a co-culture experiment and in normal and tumour tissues. Interestingly, we found different localizations of BDAA12C in well-fed and starved A549 cancer cells and consequently illustrated the transfer of fatty acids (FAs) from LDs to mitochondria to supply energy for β-oxidation upon starvation. Therefore, BDAA12C is a promising LD-targeted probe for cancer diagnosis and tracking lipid trafficking within cells.

pH-triggered hydrophility-adjustable fluorescent probes for simultaneously imaging lipid droplets and lysosomes and the application in fatty liver detection

To study the collaboration between lipid droplets (LDs) and lysosomes, and the lipid change in nonalcoholic fatty liver disease (NAFLD), herein two pH-triggered hydrophility-adjustable fluorescent probes (LD-Lyso and LD-Lyso 1) are designed. The mechanism is based on cyclization and ring-opening with thorough consideration of pH and hydrophilic differences between LDs and lysosomes. Both of the two probes exist in ring-opening form and emit red fluorescence in acidic environment, while they exist in cyclized form and the emission is blueshifted in alkaline environment due to reduced conjugate planes. Moreover, LD-Lyso exhibits near infrared fluorescence at 740 nm under ring-opening form, which facilitates further cell, tissue, and in vivo imaging. The cell imaging results show that LD-Lyso can simultaneously target LDs and lysosomes by two different colors. Impressively, LD-Lyso cannot only detect NAFLD tissues from the normal tissue, but also distinguish different degrees of NAFLD tissues and mice, which provides a very promising tool for timely diagnosis of early NAFLD.

Dual-Functional AIE Fluorescent Probe for Visualization of Lipid Droplets and Photodynamic Therapy of Cancer

Abnormal lipid droplets (LDs) are known to be intimately bound with the occurrence and development of cancer, allowing LDs to be critical biomarkers for cancers. Aggregation-induced emission luminogens (AIEgens), with efficient reactive oxygen species (ROS) production performance, are prime photosensitizers (PSs) for photodynamic therapy (PDT) with imaging. Therefore, the development of dual-functional fluorescent probes with aggregation-induced emission (AIE) characteristics that enable both simultaneous LD monitoring and imaging-guided PDT is essential for concurrent cancer diagnosis and treatment. Herein, we reported the development of a novel LD-targeting fluorescent probe (TDTI) with AIE performance, which was expected to realize the integration of cancer diagnosis through LD visualization and cancer treatment via PDT. We demonstrated that TDTI, with typical AIE characteristics and excellent photostability, could target LDs with high specificity, which enables the dynamic tracking of LDs in living cells, specific imaging of LDs in zebrafish, and the differentiation of cancer cells from normal cells for cancer diagnosis. Meanwhile, TDTI exhibited fast ROS generation ability (achieving equilibrium within 60 s) under white light irradiation (10 mW/cm2). The cell apoptosis assay revealed that TDTI effectively induced growth inhibition and apoptosis of HeLa cells. Further, the results of PDT in vivo indicated that TDTI had a good antitumor effect on the tumor-bearing mice model. Collectively, these results highlight the potential utility of the dual-functional fluorescent probe TDTI in the integrated diagnosis and treatment of cancer.

An efficient lipid droplet-targeted fluorescent probe for detection of intracellular viscosity

Lipid droplet, an intracellular lipid reservoir, is vital for energy metabolism and signal transmission in cells. The viscosity directly affects the metabolism of lipid droplets, and the abnormal viscosity is associated with the occurrence and development of various diseases. Therefore, it is indispensable to develop techniques that can detect viscosity changes in intracellular lipid droplets. Based on twisted intramolecular charge transfer (TICT) mechanism, a novel small-molecule lipid droplet-targeted viscosity fluorescence probe PPF-1 was designed. The probe was easy to synthesize, it had a large Stokes shift, stable optical properties, and low bio-toxicity. Compared to being in methanol solution, the fluorescence intensity of PPF-1 in glycerol solution was increased 26.7-fold, and PPF-1 showed excellent ability to target lipid droplets. Thus, the probe PPF-1 could provide an effective means of detecting viscosity changes of lipid droplets and was of great value for physiological diagnosis of related diseases, pathological analysis, and medical research.

Visualization of the Ferroptosis in Atherosclerotic Plaques with Nanoprobe Engineered by Macrophage Cell Membranes

Atherosclerosis (AS) is the root cause of cardiovascular diseases. Ferroptosis is characterized by highly iron-dependent lipid peroxidation and has been reported to play an important role in the pathogenesis of AS. Visualization of the ferroptosis process in atherosclerotic plaques is of great importance for diagnosing and treating AS. In this work, the rationally designed fluorescent probe FAS1 exhibited excellent advantages including large Stokes shift, sensitivity to environmental viscosity, good photostability, and improved water solubility. It also could co-locate with commercial lipid droplets (LDs) probes (BODIPY 493/503) well in RAW264.7 cells treated by the ferroptosis inducer. After self-assembly into nanoparticles and then encapsulation with macrophage membranes, the engineered FAS1@MM NPs could successfully target the atherosclerotic plaques in Western diet-induced apolipoprotein E knockout (ApoE-/-) mice and reveal the association of ferroptosis with AS through fluorescence imaging in vivo. This study may provide additional insights into the roles of ferroptosis in the diagnosis and treatment of AS.

Visualizing intracellular membrane interactions and cell type-specific differentiation in ferroptosis and apoptosis with Boranil-Carbazole derivative

Intracellular lipid systems play essential roles in various physiological functions and cell growth processes. However, our understanding of the intricate interactions within this system, especially between mitochondria and lipid droplets, is limited, particularly in the context of cancer cells' altered lipid metabolism. To address this, our study introduces an N-B-O BODIPY-hexylcarbazole derivative, named Cz-Boranil, that sets a new benchmark in visualizing these critical interactions. Cz-Boranil's unique capability lies in its ability to display distinct intracellular distribution patterns in both normal and cancer cells, offering nuanced cell type-specific differentiation. More impressively, this probe tracks the coordinated interactions of lipid droplets and mitochondria during the critical processes of ferroptosis and apoptosis. We believe that the innovative capabilities of Cz-Boranil will revolutionize our understanding of intracellular lipid interactions and prove pivotal in identifying and studying cancerous cells.

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.

Polarity-Driven Two-Photon Fluorescent Probe for Monitoring the Perturbation in Lipid Droplet Levels during Mitochondrial Dysfunction and Acute Pancreatitis

Lipid droplets (LDs) act as an energy reservoir in cancer cells; on the other hand, mitochondria are hyperactive to fulfill the energy demand to accelerate cell proliferation. We are interested in unfolding the relationship between the cellular energy reservoir and energy producer through fluorescence labeling. Thus, a dual organelle-targeted fluorescent probe MLD-1 has been rationally developed. It visualized the crosstalk between mitochondrial dysfunction and the fluctuation of LDs in live cells. Its two-photon ability allowed us to acquire deep tissue images. For the first time, we have shown that the probe has the ability to track the accumulation of LDs in different mouse organs during pancreatic inflammation. MLD-1, being a selectively polarity-driven, chemo- and photostable LD probe, may offer great possibilities for studying LD-associated biology in due course.

Synthesis of Pyridin-1(2H)-ylacrylates and the Effects of Different Functional Groups on Their Fluorescence

While fluorescent organic materials have many potential as well as proven applications and so have attracted significant attention, pyridine-olefin conjugates remain a less studied subset of such systems. Herein, therefore, we report on the development of the straightforward syntheses of pyridin-1(2H)-ylacrylates and the outcomes of a study of the effects of substituents on their fluorescent properties. Such compounds were prepared using a simple, metal-free and three-component coupling reaction involving 2-aminopyridines, sulfonyl azides and propiolates. The fluorescent properties of the ensuing products are significantly affected by the positions of substituents on the cyclic framework, with those located in central positions having the greatest impact. Electron-withdrawing groups tend to induce blue shifts while electron-donating ones cause red shifts. This work highlights the capacity that the micro-modification of fluorescent materials provides for fine-tuning their properties such that they may be usefully applied to, for example, the study of luminescent materials.

The rational utilization of organelle microenvironment for imaging of lysosomal SO2 with high fidelity

Nowadays, more and more studies have linked the abnormal expression of active molecules in organelles with the occurrence of diseases, so there is an urgent need to develop tools for detecting active molecules in specific organelles. However, the recognition receptors of most organelle-targeting probes currently developed always remain active, which easily causes them to react with the analyte in the cytoplasm, thus misjudging the role of the analyte in the physiological and pathological processes. Therefore, it is of great significance to develop a new strategy for the design of probes capable of high-fidelity imaging of the analyte in specific organelles. Herein, we propose a new strategy that the activation of recognition receptors that can be triggered by the microenvironment of targeting organelles. Based on this strategy, we develop a novel lysosome-targeting fluorescent probe (Lyso-SO₂) for imaging of sulfur dioxide (SO₂) with high-fidelity in lysosomes. The inert probe is activated by the acidic environment in the lysosome and then responds quickly (<2 s) and sensitively (LOD = 0.34 μM) to SO₂. This paradigm by taking full advantage of the features of the organelle microenvironment provides a promising methodology for developing organelle-targeting probes for high-fidelity imaging.

Exploring the microscopic changes of lipid droplets and mitochondria in alcoholic liver disease via fluorescent probes with high polarity specificity

Alcoholic liver disease (ALD) has received extensive attention because of the increasing alcohol consumption globally as well as its high morbidity. It is reported that absorbed alcohol can cause lipid metabolism disorder and mitochondria dysfunction, so here in this work, we planned to study the microscopic changes of the two organelles, lipid droplets (LDs) and mitochondria in hepatocyte, under the stimulation of alcohol, hoping to present some meaningful information for the theranostics of ALD by the technique of fluorescence imaging. Guided by theoretical calculation, two fluorescent probes, named CBu and CBuT, were rationally designed. Although constructed by the same chromophore scaffold, they stained different organelles efficiently and emitted distinctively. CBu with high lipophilicity, ascribed to the two butyl groups, can selectively localize in LDs with green fluorescence, while CBuT bearing a triphenylphosphine unit can specifically target mitochondria due to electrostatic interactions with near-infrared (NIR) fluorescence. Both probes displayed remarkable selectivity and sensitivity to polarity, free from the environmental interferences including viscosity, pH and other bio-species. With these two probes, the accumulation of LDs and polarity decrease in mitochondria were clearly monitored at the green and red channels, respectively, in the ALD cell model. CBuT was further applied to image the mice with ALD in vivo. In short, we have confirmed the valuable organelles, LDs and mitochondria, for ALD study and provided two potent molecular tools to visualize their changes through fluorescence imaging, which would be favorable for the further development of theranostics for ALD.

A review: Small organic molecule dual/multi-organelle-targeted fluorescent probes

In recent years, the dual/multi-organelle-targeted fluorescent probe based on small organic molecules has good biocompatibility and can visualize the interaction between different organelles, which has attracted much attention. In addition, these probes can also be used to detect small molecules in the organelle environment, such as active sulfur species (RSS), reactive oxygen species (ROS), pH, viscosity and so on. However, the review of dual/multi-organelle-targeted fluorescent probe for small organic molecules lacks a systematic summary, which may hinder the development of this field. In this review, we will focus on the design strategies and bioimaging applications of dual/multi-organelle-targeted fluorescent probe, and classify them into six classes according to different organelles targeted. The first class probe targeted mitochondria and lysosome. The second class probe targeted endoplasmic reticulum and lysosome. The third class probe targeted mitochondria and lipid droplets. The fourth class probe targeted endoplasmic reticulum and lipid droplets. The fifth class probe targeted lysosome and lipid droplets. The sixth class multi-targeted probe. The mechanism of these probes targeting organelles and the visualization of the interaction between different organelles are emphasized, and the prospect and future development direction of this research field are prospected. This will provide a systematic idea for the development and functional research of dual/multi-organelle-targeted fluorescent probe, and promote its research in related physiological and pathological medicine field in the future.

Orange-Emissive Carbon Dots with High Photostability for Mitochondrial Dynamics Tracking in Living Cells

Mitochondria play significant roles in maintaining a stable internal environment for cell metabolism. Hence, real-time monitoring of the dynamics of mitochondria is essential for further understanding mitochondria-related diseases. Fluorescent probes provide powerful tools for visualizing dynamic processes. However, most mitochondria-targeted probes are derived from organic molecules with poor photostability, making long-term dynamic monitoring challenging. Herein, we design a novel mitochondria-targeted probe based on carbon dots with high performance for long-term tracking. Considering that the targeting ability of CDs is related to surface functional groups, which are generally determined by the reaction precursors, we successfully constructed mitochondria-targeted O-CDs with emission at 565 nm through solvothermal treatment of m-diethylaminophenol. The O-CDs are bright with a high quantum yield of 12.61%, high mitochondria-targeting ability, and good stability. The O-CDs possess a high quantum yield (12.61%), specific mitochondria-targeting ability, and outstanding optical stability. Owing to the abundant hydroxyl and ammonium cations on the surface, O-CDs showed obvious accumulation in mitochondria with a high colocalization coefficient of up to 0.90 and remained steady even after fixation. Besides, O-CDs showed outstanding compatibility and photostability under various interruptions or long-time irradiation. Therefore, O-CDs are preferable for the long-term tracking of dynamic mitochondrial behavior in live cells. We first observed the mitochondrial fission and fusion behaviors in HeLa cells, and then, the size, morphology, and distribution of mitochondria in physiological or pathological conditions were clearly recorded. More importantly, we observed different dynamics interactions between mitochondria and lipid droplets during the apoptosis and mitophagy processes. This study provides a potential tool for exploring interactions between mitochondria and other organelles, further promoting the research on mitochondria-related diseases.

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.

An efficient tetrahydroquinazolin-2-amine derivative-grafted cellulose fluorescent probe for detection of Cu2+ and Zn2

Cu²⁺ and Zn²⁺ play crucial roles in many physiological processes, and their disorder will cause harm to human health. An efficient difunctional fluorescent probe CMC-GE-PQA for simultaneous detection of Cu²⁺ and Zn²⁺ was synthesized based on carboxymethyl cellulose. The probe CMC-GE-PQA exhibited a moderate blue fluorescence color. Interestingly, this probe showed a distinct fluorescence enhancement response toward Zn²⁺, while it displayed a significant fluorescence quenching response toward Cu²⁺. The detection limits of CMC-GE-PQA for Cu²⁺ and Zn²⁺ were calculated as low as 5.0 × 10^-8 M and 1.0 × 10^-7 M, respectively. The detection mechanisms of CMC-GE-PQA for Cu²⁺ and Zn²⁺ were fully verified by Job's plot, X-ray photoelectron spectroscopy analysis. The probe CMC-GE-PQA was applied to determine the trace amounts of Cu²⁺ and Zn²⁺ in environmental water samples. In addition, the probe CMC-GE-PQA-based fluorescent film and hydrogel were manufactured to achieve the portable detection of Cu²⁺ and Zn²⁺.

pH-Triggered Charge Reversible Fluorescent Probe for Simultaneous Imaging of Lipid Droplets and Nucleoli in Living Cells

Cooperation between organelles is essential to maintain the normal functions of cells. Lipid droplets (LDs) and nucleoli, as important organelles, play an important role in the normal activities of cells. However, due to the lack of appropriate tools, in situ observation of the interaction between them has been rarely reported. In this work, taking into full consideration the pH and charge differences between LDs and nucleoli, a pH-triggered charge reversible fluorescent probe (LD-Nu) was constructed based on a cyclization-ring-opening mechanism. The in vitro pH titration experiment and ¹H NMR showed that LD-Nu gradually transferred from the charged form to the electroneutral form with the increase of pH, and thus, the conjugate plane was reduced and its fluorescence blue-shifted. Most importantly, the physical contact between LDs and nucleoli was visualized for the first time. Meanwhile, the relationship between LDs and nucleoli was also further investigated, and the results showed that their interaction was more liable to be affected by the abnormality of LDs than those of nucleoli. Moreover, the cell imaging results displayed that the LDs both in the cytoplasm and nucleus were observed using the probe LD-Nu, and interestingly, the LDs in the cytoplasm were more susceptible to external stimuli than those in the nucleus. In a word, the probe LD-Nu can serve as a powerful tool for further exploration of the interaction mechanism between LDs and nucleoli in living cells.

One Stone, Three Birds: A Smart Single Fluorescent Probe for Simultaneous and Discriminative Imaging of Lysosomes, Lipid Droplets, and Mitochondria

Complex intracellular life processes are usually completed through the cooperation of multiple organelles. Real-time tracking of the interplays between multiple organelles with a single fluorescent probe (SFP) is very helpful to deepen our understanding of complex biological processes. So far, SFP for simultaneously differentiating and visualizing of more than two different organelles has not been reported. Herein, we report an SFP (named ICM) that can be used for simultaneously differentiating and visualizing three important organelles: mitochondria, lysosomes, and lipid droplets (LDs). The probe can simultaneously light up mitochondria/lysosomes (∼700 nm) and LDs (∼480 nm) at significantly different emission wavelengths with high fidelity, and mitochondria and lysosomes can be effectively distinguished by their different shapes and fluorescence intensities. With this smart probe, real-time and simultaneous tracking of the interplays of these three organelles was successfully achieved for the first time.

Construction of a TICT-AIE-Integrated Unimolecular Platform for Imaging Lipid Droplet-Mitochondrion Interactions in Live Cells and In Vivo

Inter-organelle interactions play a vital role in diverse biological processes. Thus, chemical tools are highly desirable for understanding the spatiotemporal dynamic interplay among organelles in live cells and in vivo. However, designing such tools is still a great challenge due to the lack of universal design strategies. To break this bottleneck, herein, a novel unimolecular platform integrating the twisted intramolecular charge transfer (TICT) and aggregation-induced emission (AIE) dual mechanisms was proposed. As a proof of concept, two organelles, lipid droplets (LDs) and mitochondria, were selected as models. Also, the first TICT-AIE integration molecule, BETA-1, was designed for simultaneous and dual-color imaging of LDs and mitochondria. BETA-1 can simultaneously target LDs and mitochondria due to its lipophilicity and cationic structure and emit cyan fluorescence in LDs and red fluorescence in mitochondria. Using BETA-1, for the first time, we obtained long-term tracking of dynamic LD-mitochondrion interactions and identified several impressive types of dynamic interactions between these two organelles. More importantly, the increase in LD-mitochondrion interactions during ferroptosis was revealed with BETA-1, suggesting that intervening in the LD-mitochondrion interactions may modulate this cell death. BETA-1 was also successfully applied for in vivo imaging of LD-mitochondrion interactions in C. elegans. This study not only provides an effective tool for uncovering LD-mitochondrion interactions and deciphering related biological processes but also sheds light on the design of new probes with an integrated TICT-AIE mechanism for imaging of inter-organelle interactions.

A novel fluorescent probe with ACQ-AIE conversion by alkyl chain engineering for simultaneous visualization of lipid droplets and lysosomes

The AIE bio-probes have attracted extensive attention because of their good brightness, long-term in situ retention ability, photostability and low cytotoxicity. Recently, the transformation of ACQ to AIE has become very popular, which is very important for the further development of AIE probes. Herein, a series of novel dyes (NR-Lyso-Ⅰ, NR-Lyso-Ⅱ, NR-Lyso-III, NR-Lyso-IV) were designed and synthesized. It was found that alkylation of 4-aminonaphthalimide could achieve the transformation of the dye from ACQ to AIE effect due to the growth of carbon chain. Moreover, the AIE probe NR-Lyso-IV exhibited dual-state emission (DSE) and large Stokes shift (＞100 nm), excellent selectivity, photostability, and low cytotoxicity, which was able to simultaneous visualize the lipid droplets (LDs) and lysosomes of HeLa cells and zebrafish.

Rational Design of Novel Lipophilic Aggregation-Induced Emission Probes for Revealing the Dynamics of Lipid Droplets during Lipophagy and Ferroptosis

Lipid droplets (LDs), as crucial organelles, play a significant role in some physiological processes. Monitoring the concentration of LDs and dynamic behaviors between LDs and other organelles during some physiological processes is important for studying their biological function and medical diagnosis. Herein, we report a series of aggregation-induced emission (AIE) probes AIE-Cbz-LD-C_n (n = 1, 3, 5, 7, OMe) based on the conjugation of quinoline-malononitrile (QM) and carbazole for tracking the dynamic changes of LDs and studying the association between LDs and lysosome/endoplasmic reticulum (ER). To our great delight, AIE-Cbz-LD-C₃, AIE-Cbz-LD-C₅, and AIE-Cbz-LD-C₇ could aggregate in LDs accurately and light up the LDs with good photostability. Among them, AIE-Cbz-LD-C₇ was used to visualize the interplay between LDs and lysosomes during lipophagy due to the excellent LD-specificity. We also succeeded in tracking the number of newborn LDs generated near the endoplasmic reticulum regions revealing that the number increased considerably during ferroptosis by using AIE-Cbz-LD-C₇, which supplies useful evidence for the hypothesis that LDs generate from the ER. We expect the probe AIE-Cbz-LD-C₇ would be a practical tool for tracking the physiological and pathological processes contacted with LDs.

Highly Efficient Red/NIR-Emissive Fluorescent Probe with Polarity-Sensitive Character for Visualizing Cellular Lipid Droplets and Determining Their Polarity

Lipid droplets (LDs), which are ubiquitous organelles existing in almost all eukaryotic cells, have attracted a lot of attention in the field of cell biology over the last decade. For the biological study of LDs via fluorescence imaging, the superior LD fluorescent probes with environmental polarity-sensitive character are highly desired and powerful but are very scarce. Herein, we have newly developed such a kind of fluorescent probe named LDs-Red which enables us to visualize LDs and to further reveal their polarity information. This fluorescent probe displays the advantages of intense red/near-infrared emission, high LD staining specificity, and good photostability; thus, it would be very useful for LD fluorescence imaging application. As a result, the three-dimensional confocal imaging to visualize spatial distribution of LDs and the multicolor confocal imaging to simultaneously observe LDs and other cellular organelles have been realized using this new LD fluorescent probe. Furthermore, the polarity-sensitive emission character of this probe enables us to quantitatively determine the LD polarity via spectral scan imaging. Consequently, the cancer cells (HepG2, HeLa, and Panc02) displaying lower polarity of LDs than the normal cells (L929, U251, and HT22) have been systematically demonstrated. In addition, this polarity-sensitive probe displaying shorter fluorescence wavelengths in cancer cells than in normal cells has an important and potential ability to distinguish them.

Preparation of Chromeno[b]quinoline Derivatives and Their Application for Lipid Droplets Markers

Functional dyes with a chromeno[b]quinoline skeleton (3a-d) were synthesized by one-step cyclization between coumarin derivatives and aromatic amines under the promotion of anhydrous aluminum chloride in 41.2-45.8% yields. Their maximum absorption and emission wavelengths locate at 358-396 and 420-603 nm with large Stokes shifts (168-231 nm), and their intramolecular charge transfer has been corroborated by density functional theory calculations. Cell experiments have proved that the probes 3a-c possess the ability to target lipid droplets.

Polarity-Sensitive Probe: Dual-Channel Visualization of the "Chameleon" Migration with the Assistance of Reactive Oxygen Species

The real-time and differentiated visualization of the organelles is favorable for exploring the distribution and interaction. However, most visual probes emit monochromatic fluorescence and target a single organelle, which impedes the in-depth study of their interplay. To overcome this obstacle, we tactfully conceived a polarity-sensitive fluorescent DPDO-C that could accurately discriminate polarity changes in the cellular environment, exhibiting distinct fluorescence in lipid droplets (LDs) and mitochondria. Remarkably, the probe DPDO-C could migrate from mitochondria to LDs with the assistance of reactive oxygen species, which was conducive to further monitoring of the number and size of LDs as well as the interactions between LDs and other organelles. Moreover, the nuanced difference between normal and fatty liver tissues was also distinguished by two-color fluorescence imaging, which could act as a promising candidate for the early diagnosis of fatty liver.

Coumarin-based fluorescent probes toward viscosity in mitochondrion/lysosome

Viscosity is an important microenvironmental indicator that plays an important role in the process of information transmission in various regions. Herein, two coumarin-based viscosity-sensitive fluorescent probes (CHB, CHN) were synthesized and the photophysical properties of the two probes were studied. The fluorescence quantum yields of CHB and CHN in glycerol can be as high as 25.2% and 18.3% respectively. The two probes can linearly detect viscosity in the viscosity logarithm range of 0.83-2.07, which is not interfered with pH, metal ions, anions and biomolecules. Fluorescent confocal cell experiments show CHB and CHN have good targeting ability to mitochondrion, lysosome, Endoplasmic reticulum and Golgi apparatus, and can be used to detect viscosity in mitochondrion/lysosome.

A Near-Infrared Probe for Specific Imaging of Lipid Droplets in Living Cells

Lipid droplets (LDs) are involved in various physiological processes and associated with cancer development, and are regarded as a potential tumor marker for cancer diagnosis. Monitoring LDs is of prior importance to understand their involvement in biological mechanisms and the early detection of cancers. Highly sensitive and specific noninvasive fluorescent probes are particularly desirable for imaging LDs and cancer diagnosis. Herein, according to the high-viscosity and low-polarity microenvironment in LDs, we developed four easily prepared LDs-specific probes based on noncharged merocyanines. Among them, LD-1 absorbs and emits in the near-infrared (NIR) region with a large Stokes shift. Importantly, LD-1 displayed high sensitivity to high viscosity and low polarity, which allowed it to show high LDs-targeting ability. In cell imaging, LD-1 successfully probed the changes in LDs in the presence of oleic acid or during ferroptosis and was used to distinguish cancer cells from normal cells.

D-π-A structure fluorophore: NIR emission, response to viscosity, detection cyanide and bioimaging of lipid droplets

Intracellular viscosity, an important microenvironment factor, is closely involved in various cell processes as well as diseases. On the other hand, cyanide is one of the most hazardous chemicals for human health and environments. However, a NIR fluorescent probe for both response to viscosity and detection of cyanide remains vacant. Herein, we reported a D-π-A structure fluorophore (named CTR) which exhibited NIR emission and fluorescent enhancement response to viscosity via the molecular rotor strategy. Furthermore, CTR displayed fluorescent and colorimetric response to cyanide. Notably, test strips stained with CTR were fabricated, which could serve as an efficient and suitable cyanide test kit. Moreover, CTR could selectively accumulate in lipid droplets and visualize the metabolism of lipid droplets in live cells. These findings would provide new avenue to design fluorescent probe for effective response to viscosity, detection of cyanide, and bioimaging of lipid droplets in live cells.

Recent advances in fluorescent probes for lipid droplets

Lipid droplets (LDs) have been known as a non-negligible cellular organelle for lipid storage and metabolism. Fluorescent probes for imaging LDs would be paramount for depicting their functions in cells. Although commercially available Nile Red and BODIPY^tm 493/503 have been widely used for labelling LDs, they exhibit unsatisfactory specificity and spectroscopic properties. This feature article reviews the recent advances in organic fluorescent probes for imaging LDs. We first introduce the key points for probe design, including regulating hydrophobicity and enhancing fluorescence quantum yield in LDs. Then, we summarize the structural features and biological applications of some representative LD probes classified by their frameworks. In addition, the current challenges and future research trends for the fluorescent probes of LDs are discussed as well.

Construction of a ratiometric phosgene probe by chromophore formation from auxochrome

A new fluorescent probe for ratiometric detection of phosgene is reported. This probe was constructed with classic 1,8-naphthalimide and 2-(2-aminophenyl)benzimidazole by Ullmann coupling reaction. After exposure to phosgene, yellow fluorescence weakened while blue fluorescence enhanced significantly. There was a ratiometric response between 542 nm and 490 nm. The detection limit (LOD) was 6.7 nM and the response time was within 200 s in CHCl₃. Meaningfully, a new chromophore, benzo [4,5]imidazo[1,2-c]quinazolin-6(5H)-one, was formed after 2-(2-aminophenyl)benzimidazole unit reacted with phosgene, and the ratiometric response was achieved by two chromophores in which the mechanism was confirmed by ¹H NMR spectra, HRMS and theoretical calculation. Furthermore, test papers and nanofibers were fabricated with the probe, which could sensitive detection of phosgene within 10 min and 1 min respectively.

A polarity-sensitive fluorescent probe based on a difluoroboron derivative for monitoring the variation of lipid droplets

We design LD-L, a polarity-sensitive fluorescent probe, which can specifically detect lipid droplets and monitor variations in their number.