Ferroptosis Accompanied by •OH Generation and Cytoplasmic Viscosity Increase Revealed via Dual-Functional Fluorescence Probe

A viscosity-sensitivity probe for cross-platform multimodal imaging from mitochondria to animal

Viscosity in biological systems is a critical factor for various physiological process, including signal transduction and metabolisms of substance and energy. Abnormal viscosity has been proven as a key feature of many diseases, thereby real-time monitoring of viscosities in cells and in vivo is of great significance for the diagnosis and therapy of related diseases. Up to date, it is still challenging to monitor viscosity cross-platform from organelles to cells to animals with a single probe. Here, we report a benzothiazolium-xanthene probe with rotatable bonds that switch on the optical signals in high viscosity environment. The enhancements of absorption, fluorescence intensity and lifetime signals allow to dynamically monitoring the viscosity change in mitochondria and cells, while near infrared absorption and emission facilitate imaging the viscosity with both fluorescence and photoacoustic imaging in animals. The cross-platform strategy is capable of monitoring the microenvironment with multifunctional imaging across various levels.

Mitochondria-targeted fluorescent probe for imaging viscosity in hepatic ischemia-reperfusion injury cell model

The viscosity of the cell microenvironment is a parameter that affects cell physiological processes. A fluorescent probe X-V was designed to detect the viscosity changes of a hepatic ischemia-reperfusion injury (HIRI) cell model with high selectivity and sensitivity. The fluorescence emission wavelength is 615 nm and the Stokes shift can be up to 125 nm, which can be used not only for intracellular viscosity changes stimulated by different drugs but also for the detection of cell viscosity changes in the HIRI cell model. Probe X-V provides a useful tool to study the relationship between mitochondrial viscosity and related diseases.

Polarity-Ultrasensitive and Lipophilicity-Enhanced Structurally Modified Hemicyanine for Two-Color Staining to Reveal Cell Apoptosis during Chemotherapy

Programmed cell death (PCD) is a precisely controlled physiological process to sustain tissue homeostasis. Even though the PCD pathways have been explicitly subdivided, the individual cell death process seems to synergistically operate to eliminate cells rather than separately execute signal transduction. Apoptosis is the dominant intracellular PCD subtype, which is intimately regulated and controlled by mitochondria, thus tracing mitochondrial actions could reveal the dynamic changes of apoptosis, which may provide important tools for screening preclinical therapeutic agents. Herein, we exploited an innovative fluorophore Cy496 based on the light-initiated cleavage reaction. Cy496 bears the typical D-π-A structure and serves as a versatile building block for chemosensor construction through flexible side chains. By regulating lipophilicity and basicity through bis-site substitution, we synthesized a series of fluorescence probes and screened a novel mitochondria-targeted ratiometric probe Cy1321, which can real-time evaluate the dynamic changes of mitochondrial micropolarity mediated by bis-cholesterol anchoring. Cy1321 has realized two-color quantification and real-time visualization of polarity fluctuations on chemotherapy agent (cisplatin)-induced apoptosis through flow cytometry and confocal imaging and also achieved the purpose of detecting mitochondria-related apoptosis at the level of tissues. It is envisioned that Cy1321 has sufficient capability as a promising and facile tool for the evaluation of apoptosis and contributing to therapeutic drug screening.

Design strategies and applications of smart optical probes in the second near-infrared window

Over the last decade, a series of synergistic advances in the synthesis chemistries and imaging instruments have largely boosted a significant revolution, in which large-scale biomedical applications are now benefiting from optical bioimaging in the second near-infrared window (NIR-II, 1000-1700 nm). The large tissue penetration and limited autofluorescence associated with long-wavelength imaging improve translational potential of NIR-II imaging over common visible-light (400-650 nm) and NIR-I (750-900 nm) imaging, with ongoing profound effects on the studies of precision medicine. Unfortunately, the majority of NIR-II probes are designed as "always-on" luminescent imaging contrasts, continuously generating unspecific signals regardless of whether they reach pathological locations. Thus, in vivo imaging by traditional NIR-II probes usually suffers from weak detect precision due to high background noise. In this context, the advances of optical imaging now enter into an era of precise control of NIR-II photophysical kinetics. Developing NIR-II optical probes that can efficiently activate their luminescent signal in response to biological targets of interest and substantially suppress the background interferences have become a highly prospective research frontier. In this review, the merits and demerits of optical imaging probes from visible-light, NIR-I to NIR-II windows are carefully discussed along with the lens of stimuli-responsive photophysical kinetics. We then highlight the latest development in engineering methods for designing smart NIR-II optical probes. Finally, to appreciate such advances, challenges and prospect in rapidly growing study of smart NIR-II probes are addressed in this review.

Synthesis and application of the fluorescent furan and imidazole probes for selective in vivo and in vitro cancer cell imaging

Development of imaging probes for identification of tumors in the early stages of growth can significantly reduce the tumor-related health hazards and improve our capacity for treatment of cancer. In this work, three different furan and imidazole fluorescent derivatives abbreviated as Cyclo X, SAC and SNO are introduced for in vivo and in vitro imaging of cancer cells. The fluorescence quantum yield values were 0.226, 0.400 and 0.479 for Cyclo X, SAC and SNO, respectively. The excitation and emission wavelengths of maximum intensity were (360, 452), (350, 428) and (350, 432) nm for Cyclo X, SAC and SNO, respectively. The MTT reduction assay was used to estimate the cytotoxic activity of the proposed derivatives against HT-29 (cancer) and Vero (normal) cell lines. Cyclo X showed no cytotoxic effect, while SAC and SNO showed significantly higher cytotoxicity against the tested cell lines than cisplatin as a well-known anticancer drug. In vitro fluorescence microscopic images obtained using HT-29 cells showed that Cyclo X produced very bright images. The in vivo cancer cell imaging using 4T1 tumor-bearing mice revealed that Cyclo X is selectively accumulated in the tumor without distribution in the mice body organs. The spectral and structural stability, large Stokes shift, non-cytotoxicity and high level of selectivity for in vivo imaging are properties that make Cyclo X a suitable candidate to be used for long-term monitoring of cancer cells.

Novel Meso-Benzothiazole-Substituted BODIPY-Based AIE Fluorescent Rotor for Imaging Lysosomal Viscosity and Monitoring Autophagy

Meso-substituted boron dipyrromethenes (BODIPYs) provide a potential and innovative strategy for the synergistic construction of aggregation-induced emission (AIE) probes and fluorescent rotors for monitoring cellular viscosity changes, which play critical roles in understanding the function of viscosity in its closely associated diseases. Therefore, for the first time, a BODIPY-based fluorescent probe (1) with a rotatable meso-benzothiazole group was rationally designed and synthesized, showing both good viscosity-responsive and AIE properties. Probe 1 through direct linkage with the thiazole group, showed nearly no emission in low viscous solvents; however, a strong emission at 534 nm appeared and increased gradually with the increase in viscosity, attributing to the efficient restriction of the rotatable meso-benzothiazole group. The intensity (log I₅₃₄) displayed a good linear relationship with viscosity (log η) in the viscous range of 0.59-945 cP in methanol/glycerol mixtures. Interestingly, 1 showed enhanced emission at 534 nm in 70% water compared to pure acetonitrile due to the aggregation-induced inhibited rotations. Cellular imaging suggested that 1 could successfully sense lysosomal viscosity changes induced by lipopolysaccharide, nystatin, low temperature, and dexamethasone in living cells, which could be further applied in autophagy monitoring by tracing viscosity changes. As a comparison, its analogue 2 directly linking with the phenyl group showed no viscosity-responsive or AIE properties. Therefore, for the first time, we reported a meso-benzothiazole-BODIPY-based fluorescent rotor with AIE and lysosomal viscosity-responsive properties in nervous cells, which was further applied in monitoring autophagy, and this work thus could provide an innovative strategy for the design of potential AIE and viscosity-responsive probes.

Bichromatic Imaging with Hemicyanine Fluorophores Enables Simultaneous Visualization of Non-alcoholic Fatty Liver Disease and Metastatic Intestinal Cancer

Simultaneous detection of different diseases via a single fluorophore is challenging. We herein report a bichromatic fluorophore named Cy-914 for the simultaneous diagnosis of non-alcoholic fatty liver disease (NAFLD) and metastatic intestinal cancer by leveraging its NIR-I/NIR-II dual-color imaging capability. Cy-914 with a pK_a of 6.98 exhibits high sensitivity to pH and viscosity, showing turn-on NIR-I fluorescence at 795 nm in an acidic tumor microenvironment, meanwhile displaying intense NIR-II fluorescence at 914/1030 nm under neutral to slightly basic viscous conditions. Notably, Cy-914 could sensitively and noninvasively monitor viscosity variations in the progression of NAFLD. More importantly, it was able to simultaneously visualize NAFLD (ex/em = 808/1000-1700 nm) and intestinal metastases (ex/em = 570/810-875 nm) in two independent channels without spectral cross interference after topical spraying, further improving fluorescence-guided surgery of tiny metastases less than 3 mm. This strategy may provide an understanding for developing multi-color fluorophores for multi-disease diagnosis.

Recent advances in small-molecule fluorescent probes for studying ferroptosis

Ferroptosis is an iron-dependent, non-apoptotic form of programmed cell death driven by excessive lipid peroxidation (LPO). Mounting evidence suggests that the unique modality of cell death is involved in the development and progression of several diseases including cancer, cardiovascular diseases (CVDs), neurodegenerative disorders, etc. However, the pathogenesis and signalling pathways of ferroptosis are not fully understood, possibly due to the lack of robust tools for the highly selective and sensitive imaging of ferroptosis analytes in complex living systems. Up to now, various small-molecule fluorescent probes have been applied as promising chemosensors for studying ferroptosis through tracking the biomolecules or microenvironment-related parameters in vitro and in vivo. In this review, we comprehensively reviewed the recent development of small-molecule fluorescent probes for studying ferroptosis, with a focus on the analytes, design strategies and bioimaging applications. We also provided new insights to overcome the major challenges in this emerging field.

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.

Simultaneous Monitoring of HOCl and Viscosity with Drug-Induced Pyroptosis in Live Cells and Acute Lung Injury

Pyroptosis is a newly identified form of cell death that is closely correlated with many diseases. Recent studies have indicated that the inflammation in pyroptosis would accelerate the generation of reactive oxygen species (ROS). In addition, intracellular viscosity is another key microenvironmental parameter that reflects many physiological and pathological states in the early stage, hypochlorous acid (HOCl), as an important ROS, also plays significant roles in a variety of pathologies. However, the fluctuation of viscosity and HOCl in the process of pyroptosis is still unknown. Herein, we present a dual-responsive fluorescent probe (Lyso-VH) for simultaneously detecting viscosity and HOCl. Lyso-VH was successfully used to image the fluctuation of HOCl and viscosity in the lysosome of three kinds of cells with dependent and independent channels. Moreover, Lyso-VH can be employed to investigate the changes of HOCl and viscosity during the process of pyroptosis in living cells and acute lung injury (ALI). Thus, this work can not only serve as a powerful tool to simultaneously visualize the fluctuation of HOCl and viscosity in lysosomes, but also provide a new insight into drug-induced pyroptosis in living cells and acute lung injury.

The Mechanisms of Ferroptosis and the Applications in Tumor Treatment: Enemies or Friends?

Ferroptosis, as a newly discovered non-apoptotic cell death mode, is beginning to be explored in different cancer. The particularity of ferroptosis lies in the accumulation of iron dependence and lipid peroxides, and it is different from the classical cell death modes such as apoptosis and necrosis in terms of action mode, biochemical characteristics, and genetics. The mechanism of ferroptosis can be divided into many different pathways, so it is particularly important to identify the key sites of ferroptosis in the disease. Herein, based on ferroptosis, we analyze the main pathways in detail. More importantly, ferroptosis is linked to the development of different systems of the tumor, providing personalized plans for the examination, treatment, and prognosis of cancer patients. Although some mechanisms and side effects of ferroptosis still need to be studied, it is still a promising method for cancer treatment.

Precision Navigation of Hepatic Ischemia-Reperfusion Injury Guided by Lysosomal Viscosity-Activatable NIR-II Fluorescence

Hepatic ischemia-reperfusion injury (HIRI) is responsible for postoperative liver dysfunction and liver failure. Precise and rapid navigation of HIRI lesions is critical for early warning and timely development of pretreatment plans. Available methods for assaying liver injury fail to provide the exact location of lesions in real time intraoperatively. HIRI is intimately associated with oxidative stress which impairs lysosomal degradative function, leading to significant changes in lysosomal viscosity. Therefore, lysosomal viscosity is a potential biomarker for the precise targeting of HIRI. Hence, we developed a viscosity-activatable second near-infrared window fluorescent probe (NP-V) for the detection of lysosomal viscosity in hepatocytes and mice during HIRI. A reactive oxygen species-malondialdehyde-cathepsin B signaling pathway during HIRI was established. We further conducted high signal-to-background ratio NIR-II fluorescence imaging of HIRI mice. The contour and boundary of liver lesions were delineated, and as such the precise intraoperative resection of the lesion area was implemented. This research demonstrates the potential of NP-V as a dual-functional probe for the elucidation of HIRI pathogenesis and the direct navigation of HIRI lesions in clinical applications.

A High Spatiotemporal Iontronic Single-Cell Viscometer

Ideal single-cell viscometer has remained unachieved, leaving a gap in current palette of single-cell nanotools. Information of single-cell viscosity could contribute to our knowledge of fundamental biological processes, e.g., mass diffusion, biochemical interaction, and cellular responses to many diseases and pathologies. Although advances have been made to this end, existing methods generally suffer from limitations, e.g., low spatiotemporal resolution. Here, we describe a high spatiotemporal iontronic single-cell viscometer that operates upon a patch clamp integrated with double-barreled nanopores separated by a septum of ca. 32 nm. The system enables reversible electroosmotic manipulation of the adjacent small fluid bridging two nanopores, the viscous alternation of which could be sensitively monitored by the ionic responses. In practical cellular studies, significantly, our findings reveal not only the less deviated medium viscosities than those of lysosomes and mitochondria but also the highest viscosities in the near-nuclear region than those of mitochondrion-dense and lysosome-dense regions. This work has provided an accessible single-cell viscometer and enriched the armory of single-cell nanotools.

Mitochondrial-Targeted Ratiometric Fluorescent Probe to Monitor ClO- Induced by Ferroptosis in Living Cells

Ferroptosis is a type of iron-dependent programmed cell death. Once such kind of death occurs, an individual cell would undergo a series of changes related to reactive oxygen species (ROS) in mitochondria. A mitochondrial-targeted ratiometric fluorescent probe (MBI-OMe) was developed to specifically detect ferroptosis-induced ClO⁻, whose recognition group is p-methoxyphenol, and the mitochondrial-targeted group is benzimidazole. The fluorescence of MBI-OMe was first quenched by 30 μM of Fe³⁺, and then MBI-OMe appeared as a ratiometric signal at 477 nm and 392 nm in response to ferroptosis-induced ClO⁻ in living cells. MBI-OMe was successfully used to evaluate changes in ClO⁻ induced by ferroptosis.

Recent Advances of Fluorescence Probes for Imaging of Ferroptosis Process

Ferroptosis is an iron−dependent form of regulated cell death. It has attracted more and more research interests since it was found because of its potential physiological and pathological roles. In recent years, many efforts have been made for the developments and applications of selective fluorescence probes for real−time and in situ tracking of bioactive species during ferroptosis process, which is necessary and significant to further study the modulation mechanisms and pathological functions of ferroptosis. In this review, we will focus on summarizing the newly developed fluorescence probes that have been applied for ferroptosis imaging in the recent years, and comprehensively discussing their design strategies, including the probes for iron, reactive oxygen species, biothiols and intracellular microenvironmental factors.

Activatable molecular rotor based on bithiophene quinolinium toward viscosity detection in liquids

The development of non-invasive and effective viscosity inspection methods during the liquid deterioration process is urgently needed since viscosity is one of the most important physical parameters of liquids. Methods featuring rapid detection, high sensitivity, cheap equipment, and fast result output are greatly desired. In this study, a viscosity-sensitive molecular rotor (BTPEQ) with a large Stokes shift (187 nm), and long emission wavelength (648 nm) has been developed. The rotor is comprised of a bithiophene donor and quinolinium acceptor, and displays a typical twisted intramolecular-charge transfer (TICT) feature, with good photostability, selectivity, and universality in various commercial liquids. With the aid of BTPEQ, the thickening effects of liquid thickeners can be determined. More importantly, BTPEQ was explored to visualize the viscosity variations in liquids at different metamorphic stages, and it was found that the viscosity level in microenvironments is highly dependent on the liquid food metamorphic period. It is worth noting that this approach can facilitate the continued perfection of fluorescent analytical methods for food quality and safety inspection.

Ratiometric imaging of butyrylcholinesterase activity in mice with nonalcoholic fatty liver using an AIE-based fluorescent probe

Butyrylcholinesterase (BChE) is an essential human biomarker which is related to liver and neurodegenerative diseases. It is of great significance to develop a fluorescent probe that can image BChE in vitro and in vivo. Unfortunately, most fluorescent probes that are based on a single change in fluorescence intensity are susceptible to environmental interference. Therefore, we reported an easily available ratiometric fluorescent probe, TB-BChE, with aggregation-induced emission (AIE) characteristics for ratiometric imaging of BChE. TB-BChE demonstrated excellent sensitivity (LOD = 39.24 ng mL^-1) and specificity for BChE. Moreover, we have successfully studied the ratiometric imaging of TB-BChE to BChE in a nonalcoholic fatty liver disease model. These results indicated that TB-BChE is expected to become a powerful analysis tool for butyrylcholinesterase research in basic medicine and clinical applications.

A mitochondria-targeted rhodol fluorescent probe for imaging of hydrogen peroxide in living cells

Hydrogen peroxide (H₂O₂) is a main member of reactive oxygen species (ROS) in cells that has a significant impact on various physiological and pathological processes of organisms. Here, we designed and synthesized a new type of fluorescent probe Rhodol-OAc for the specific detection of H₂O₂. The probe had good water solubility, high selectivity and sensitivity to H₂O₂, low cytotoxicity, excellent mitochondrial targeting ability, etc. It was successfully applied in the imaging of exogenous and endogenous H₂O₂ in living cells. In addition, theoretical calculations were carried out to clarify the luminescence mechanism of the probe. More importantly, we successfully applied the probe to indirectly detect xanthine and glucose, the metabolism of which generates H₂O₂, and achieved satisfactory results.

Visual Monitoring of Nucleic Acid Dynamic Structures during Cellular Ferroptosis Using Rationally Designed Carbon Dots with Robust Anti-Interference Ability to Reactive Oxygen Species

Ferroptosis triggered by an iron-dependent accumulation of lipid reactive oxygen species (ROS) has drawn widespread attention. Directly visualizing the dynamic structures of nucleic acids during the ferroptosis of cells is of great importance considering their vital roles in numerous biological functions. However, direct imaging remains challenging, largely due to the extremely high concentrations of ROS generated during ferroptosis, which can affect the imaging of nucleic acid targeted fluorescent probes. To overcome this challenge, nucleic acid-responsive carbon dots (CDs) providing favorable optical properties together with high chemical stability were synthesized. Furthermore, the CDs penetrated the cell membrane quickly and accumulated in the nuclei of cells. The robust anti-interference ability to ROS allows the CDs to visualize the dynamic structures of nucleic acids during ferroptosis. Moreover, the CDs were successfully employed in the imaging of nucleic acids during cell division. The nuclei-targeting CDs show great potential as a powerful tool for imaging nuclei in ferroptosis-related biological and clinical research.

Dual-channel detection of viscosity and pH with a near-infrared fluorescent probe for cancer visualization

Compared to ordinary cells, tumor cells have a unique microenvironment, characterized by high viscosity, low pH, high reactive oxygen species level and the overexpression of certain proteases. Therefore, viscosity and pH can be used as important parameters for visualizing cancer. We designed a spiro-oxazolidine compound (In-1) for the dual-channel detection of viscosity and pH, with the red channel for detecting viscosity and the blue channel for pH. Interestingly, In-1 can locate different organelles under different conditions. Under physiological conditions, In-1 efficiently targeted lysosomes and showed that the viscosity of lysosomes increases in cancer cells while the pH decreases, which can be used to distinguish and detect cancer cells and normal cells. When we treated HL-7702 cells with CCCP, the probe could effectively target the mitochondria, and the fluorescence intensity in the pH channel decreased. This indicates that In-1 can be used as a powerful tool to simultaneously monitor viscosity and pH in different organelles, and may have a guiding role in diseases caused by mitochondrial and lysosomal microenvironments.

An osmium-peroxo complex for photoactive therapy of hypoxic tumors

The limited therapeutic effect on hypoxic and refractory solid tumors has hindered the practical application of photodynamic therapy. Herein, we report our investigation of an osmium-peroxo complex (Os2), which is inactive in the dark, but can release a peroxo ligand O₂^•− upon light irradiation even in the absence of oxygen, and is transformed into a cytotoxic osmium complex (Os1). Os1 is cytotoxic in the presence or absence of irradiation in hypoxic tumors, behaving as a chemotherapeutic drug. At the same time, the light-activated Os2 induces photocatalytic oxidation of endogenous 1,4-dihydronicotinamide adenine dinucleotide in living cancer cells, leading to ferroptosis, which is mediated by glutathione degradation, lipid peroxide accumulation and down-regulation of glutathione peroxidase 4. In vivo studies have confirmed that the Os2 can effectively inhibit the growth of solid hypoxic tumors in mice. A promising strategy is proposed for the treatment of hypoxic tumors with metal-based drugs.

Photodynamic therapy has been a promising technique for the treatment of tumours. In this manuscript, the authors report on the photoactivation of the osmium peroxo complex and its potential use for chemotherapy and photodynamic therapy under blue light irradiation against tumours in their hypoxic environment.

A dual-response NIR probe reveals positive correlation between biothiols and viscosity under cellular stress change

A mitochondrial targeted NIR fluorescent probe NIR-NBD was designed and developed for the detection of biothiols and viscosity. Furthermore, a positive correlation between the biothiol level and viscosity under cellular stress change was found for the first time, which provides some important correlation analysis information in the pathophysiological state.

A "Self-Checking" pH/Viscosity-Activatable NIR-II Molecule for Real-Time Evaluation of Photothermal Therapy Efficacy

We present a second near-infrared (NIR-II) self-checking molecule, LET-1052, for acidic tumor microenvironment (TME) turn-on photothermal therapy (PTT), followed by viscosity based therapeutic efficacy evaluation by itself in two independent channels, denoted as "self-checking" strategy. In acidic TME, LET-1052 was protonated and turned on NIR-II absorption for PTT under 1064 nm laser irradiation. Subsequently, PTT-induced cellular death increases intracellular viscosity, which inhibited the intramolecular rotation of LET-1052, resulting in the enhancement of NIR-I fluorescence for real-time evaluation of PTT efficacy. After PTT of tumor-bearing mice for different periods of NIR-II laser irradiation, NIR-I fluorescence in the tumor region showed positive correlation with tumor growth inhibition rate, demonstrating reliable and prompt prediction of PTT efficacy. The strategy may be expanded for instant evaluation of other therapeutic modalities for personalized medicine.

Multifunctional Fluorescent Probe for Simultaneously Detecting Microviscosity, Micropolarity, and Carboxylesterases and Its Application in Bioimaging

Based on OR logic gate, we proposed a smart near-infrared (NIR) fluorescent probe, named VPCPP, for simultaneously monitoring local microviscosity, micropolarity, and carboxylesterases (CEs) in living cells through blue and red channels. This proposed probe was capable of distinguishing cancer cells from normal cells and had good potential for identifying living liver cell lines. Furthermore, the fluctuations of the three analytes of interest in different cell status was successfully explored. Particularly, facilitated with high-content analysis (HCA) and VPCPP, a simple and efficient high-throughput screening (HTS) platform was first constructed for screening antitumor drugs and studying their effect on the analytes. For the first time, we found that sorafenib-induced ferroptosis led to an increase in the microviscosity and up-regulation of CEs at the same time. Additionally, the procedure that aristolochic acid (AA) induced the overexpression of CEs was verified. Besides, VPCPP was utilized for imaging the variations of the two microenvironment parameters and CEs in the inflammation model. Finally, VPCPP was able to image the tumor ex vivo and in vivo through two channels and one channel separately, as well as to visualize the kidneys and liver ex vivo with dual emissions, which indicated that the probe had great potential for imaging applications such as medical diagnosis, preclinical research, and imaging-guided surgery.

A PET-based fluorescent probe for monitoring labile Fe(II) pools in macrophage activations and ferroptosis

A fluorescent probe (COU-LIP-1) for monitoring labile Fe(II) pools (LIP) with high selectivity and sensitivity was developed utilizing coumarin 343 as the fluorophore and 3-nitrophenylazanyl ester as both the reactive group and the fluorescence quenching group. Fe(II)-induced reductive cleavage of the N-O bond results in a turn-on response via a photo-induced photon transfer (PET) mechanism. The probe was applied for monitoring labile iron(II) changes in M1 and M2a macrophage activations and also erastin-induced ferroptosis, providing a powerful tool for selectively sensing LIP under both physiological and stressed conditions.

Simultaneous visualization and quantification of copper (II) ions in Alzheimer's disease by a near-infrared fluorescence probe

The abnormal accumulation of copper ions (Cu²⁺) is considered to be one of the pathological factors of Alzheimer's disease (AD), but the internal relationship between Cu²⁺ and AD progression is still not fully clear. In this work, a sensitive and selective near-infrared fluorescent copper ion probe (DDP-Cu) was designed for quantification and visualization of Cu²⁺ level in lysates, living cells, living zebrafish and brain tissues of drosophila and mice with AD. By using this probe, we demonstrated that the content of Cu²⁺ in the brains of AD mice and drosophila enhanced nearly 3.5-fold and 4-fold than that of normal mice and drosophila, respectively. More importantly, pathogenesis analysis revealed that elevated Cu²⁺ led to changes in factors closely associated with AD, such as the increasing of reactive oxygen species(ROS), the aggregation of amyloid-β protein (Aβ) and nerve cell cytotoxicity. These findings could promote the understanding of the roles between Cu²⁺ and AD.

Resorufin-based fluorescent probe with elevated water solubility for visualizing fluctuant peroxynitrite in progression of inflammation

Inflammation is a significant protective response in biological systems and associated with various diseases. Peroxynitrite (ONOO^-) as a highly active oxidant participates in the inflammatory process of organisms. Thus, it is necessary to construct novel fluorescent probes for exploring inflammation-related diseases through detecting endogenous ONOO^-. Resorufin-based fluorescent probes for testing ONOO^- were rare and suffered from poor water solubility. In this work, we elaborately designed three resorufin-based incorporating isatin derivatives probes RF-ITs and successfully obtained two highly selective probes RF-IT-OC and RF-IT-EG for ONOO^-. Comparing the other two probes, RF-IT-EG containing triethylene glycol monomethyl ether on isatin moiety displayed better water solubility (3.2 mg/L), faster response rate (60 s), larger signal-to-noise ratio (103-fold) and lower detection limit (87 nM) for monitoring ONOO^-. The cells imaging results manifested that probe RF-IT-EG could be applied to trace endogenous ONOO^- with inappreciable cytotoxicity. Moreover, the RF-IT-EG was capable of tracking the fluctuation of endogenous ONOO^- in LPS-stimulated inflamed mouse leg models. This work will provide a faithful and promising probe for illustrating the roles of ONOO^- in various inflammation-related diseases.

Unveiling Mechanism of Organic Photogenerator for Hydroxyl Radicals Generation by Molecular Modulation

Photodynamic therapy (PDT) with organic photosensitizers generally goes through the oxygen-dependent process, generating singlet oxygen and/or superoxide anion. However, the generation of reactive oxygen species is often suppressed as a result of hypoxia, one of the common features in tumors, therefore limiting the effectiveness of the tumor treatments. Consequently, it is urgent and significant to develop an oxygen-independent hydroxyl radical photogenerator and unveil the mechanism. In this work, a hydroxyl radical (·OH) photogenerator originating from the electron transfer process is engineered. Detailed mechanism studies reveal that the optimized photosensitizer, WS2D, which contains a bithiophene unit, could both promote charge carrier generation and accelerate reaction efficiency, resulting in the efficient production of ·OH. In addition, WS2D nanoparticles are constructed to improve the polydispersity and stability in aqueous solution, which exhibit excellent biocompatibility and mitochondrial targeting. Bearing the above advantages, WS2D is employed in phototheranostics, which could release ·OH effectively and damage mitochondria precisely, achieving high PDT efficiency in vitro and in vivo. Overall, this work successfully provides valuable insights into the structural design of a hydroxyl radicals (·OH) photogenerator with great practical perspectives.

A novel fluorescent probe with large Stokes shift for accurate detection of HOCl in mitochondria and its imaging application

Hypochlorous acid (HOCl), mainly generated in mitochondria, plays a critical role in various physiological processes. To better understand the role and function of HOCl in mitochondria, herein, we present the design and synthesis of a Mito-QL reporter for probing the HOCl within mitochondria without other interference generated in living cells. Through the combination of TICT/ICT mechanisms, probe Mito-QL, with large stokes shift (203 nm) and low background fluorescence, exhibited excellent sensitivity (900-fold fluorescence enhancement) and selectivity towards HOCl (LOD = 2.4 nM). The co-location experiments confirmed that probe Mito-QL can firstly localize in the mitochondria and then react with HOCl in mitochondria. Also, the probe is capable of imaging endogenous and exogenous HOCl even the generation of HOCl during the ferroptosis of cells, which is beneficial for more efficient application in biological imaging.

Generation of singlet oxygen via iron-dependent lipid peroxidation and its role in Ferroptosis

Ferroptosis is a cell death pathway mediated by iron-dependent accumulation of lipid peroxide. However, the specific downstream molecular events of iron-dependent lipid peroxidation are yet to be elucidated. In this study, based on various spectral analyses, we have found evidence that singlet oxygen is produced through the Russell mechanism during the self-reaction of lipid peroxyl radicals generated via iron-dependent lipid peroxidation regardless of the presence of cholesterol. Significantly reduced generation of singlet oxygen was observed in the absence of iron. The generated singlet oxygen accelerated the oxidative damage of lipid membranes by propagating lipid peroxidation and facilitated ferroptotic cancer cell death initiated by erastin. In this work, singlet oxygen has been revealed to be a new reactive species that participates in ferroptosis, thus improving the understanding on iron-dependent lipid peroxidation and the mechanism of ferroptosis.

Triphenylamine indanedione as an AIE-based molecular sensor with one-step facile synthesis toward viscosity detection of liquids

AIE-based triphenylamine indanedione molecular sensors were synthesized in a one-step facile manner and designed for viscosity detection in liquids.

New cell-membrane-anchored near-infrared fluorescent probes for viscosity monitoring

We developed new cell-membrane-anchored fluorescent probes for viscosity detection. Probe MYN-BS has been successfully applied in the monitoring of cell membrane viscosity during temperature change and cell foaming.

Near-infrared fluorescent probe for hydrogen sulfide: high-fidelity ferroptosis evaluation in vivo during stroke

Ferroptosis is closely associated with cancer, neurodegenerative diseases and ischemia-reperfusion injury and the detection of its pathological process is very important for early disease diagnosis. Fluorescence based sensing technologies have become excellent tools due to the real-time detection of cellular physiological or pathological processes. However, to date the detection of ferroptosis using reducing substances as markers has not been achieved since the reducing substances are not only present at extremely low concentrations during ferroptosis but also play a key role in the further development of ferroptosis. Significantly, sensors for reducing substances usually consume reducing substances, instigating a redox imbalance, which further aggravates the progression of ferroptosis. In this work, a H₂S triggered and H₂S releasing near-infrared fluorescent probe (HL-H₂S) was developed for the high-fidelity in situ imaging of ferroptosis. In the imaging process, HL-H₂S consumes H₂S and releases carbonyl sulfide, which is then catalyzed by carbonic anhydrase to produce H₂S. Importantly, this strategy does not intensify ferroptosis since it avoids disruption of the redox homeostasis. Furthermore, using erastin as an inducer for ferroptosis, the observed trends for Fe²⁺, MDA, and GSH, indicate that the introduction of the HL-H₂S probe does not exacerbate ferroptosis. In contrast, ferroptosis progression was significantly promoted when the release of H₂S from HL-H₂S was inhibited using AZ. These results indicate that the H₂S triggered and H₂S releasing fluorescent probe did not interfere with the progression of ferroptosis, thus enabling high-fidelity in situ imaging of ferroptosis.

A H₂S triggered and H₂S releasing near-infrared fluorescent probe (HL-H₂S) was developed. HL-H₂S does not interfere with the progression of ferroptosis by consuming H₂S, thus enabling high-fidelity in situ imaging of ferroptosis.

Recent progress of organic small molecule-based fluorescent probes for intracellular pH sensing

Fluorescent probes along with fluorescence microscopy are essential tools for biomedical research. Various cellular ubiquitous chemical factors such as pH, H₂O₂, and Ca²⁺ are labeled and traced using specific fluorescent probes, therefore helping us to explore their physiological function and pathological change. Among them, intracellular pH value is an important factor that governs biological processes, generally ∼7.2. Furthermore, specific organelles within cells possess unique acid-base homeostasis, involving the acidic lysosomes, alkalescent mitochondria, and neutral endoplasmic reticulum and Golgi apparatus, which undergo various physiological processes such as intracellular digestion, ATP production, and protein folding and processing. In this review, recently reported fluorescent probes targeted toward the lysosomes, mitochondria, endoplasmic reticulum, Golgi apparatus, and cytoplasm for sensing pH change are discussed, which involves molecular structures, fluorescence behavior, and biological applications.

Ultrasensitive fluorescent probe for visual biosensing of esterase activity in living cells and its imaging application

Esterase activity is often used as an index to evaluate the health status of cells and plays an important role in cell metabolism and apoptosis. Herein, we develop two fluorescent probes for visual biosensing of esterase activity and imaging in living cells. In vitro, after the introduction of esterase, enzymolysis destroys the ester bond of the probe, causing the fluorescent color of probe changes from yellow to red, thus realizing the visual strategy for determination of esterase activity, with high sensitivity and selectivity. Especially, probe VA, 2-(4-acetoxystyryl)-3-ethyl-1,1-dimethyl- 1H-benzo[e]indol-3-ium, exhibits higher sensitivity with a lower detection limit (up to 7.15 × 10^-6 U/mL). In the cell experiment, the fluorescent probe VA also shows good biocompatibility and high spatial resolution, and is successfully applied to the intracellular fluorescent imaging and biosensing of esterase in living cells. More importantly, the probe VA can judge the unhealthy state of H₂O₂-induced HeLa cells using dual-fluorescence signals. The results confirm that the fluorescence method is a reliable tool for detecting endogenous esterase in living biological system.

SLC7A11/ xCT is a target of miR-5096 and its restoration partially rescues miR-5096-mediated ferroptosis and anti-tumor effects in human breast cancer cells

Breast cancer cells evade cell death by overexpressing SLC7A11, which functions by transporting cystine into cells in exchange for intracellular glutamate facilitating glutathione synthesis and reducing reactive oxygen species (ROS)-mediated stress. Using an in silico approach, we predicted an miRNA (miR-5096) that can target and downregulate SLC7A11. We demonstrated SLC7A11 as a target of miR-5096 by 3'UTR luciferase assay and further validated it by identifying reduced mRNA and protein levels of SLC7A11 upon miR-5096 overexpression. miR-5096-induced ferroptotic cell death in human breast cancer cells was confirmed by concurrently increased ROS, OH^-, lipid ROS, and iron accumulation levels and decreased GSH and mitochondrial membrane potential (MitoTracker™ Orange) with mitochondrial shrinkage and partial cristae loss (observed by TEM). miR-5096 inhibited colony formation, transwell migration, and breast cancer cell invasion, whereas antimiR-5096 promoted these tumorigenic properties. Ectopic expression of SLC7A11 partly reversed miR-5096-mediated effects on cell survival, ROS, lipid peroxides, iron accumulation, GSH, hydroxyl radicals, mitochondrial membrane potential, and colony formation. miR-5096 modulated the expression of epithelial-mesenchymal transition markers in vitro and inhibited the metastatic potential of MDA-MB-231 cells in a tumor xenograft model of zebrafish larvae. Our results demonstrate that miR-5096 is a tumor-suppressive miRNA in breast cancer cells, and this paper discusses its therapeutic implications.