A Mitochondrial-Targeting Near-Infrared Fluorescent Probe for Visualizing and Monitoring Viscosity in Live Cells and Tissues

Human serum albumin-encapsulated near-infrared hemicyanine photosensitizers for viscosity imaging and enhanced photodynamic therapy

The research successfully developed a novel dual-function near-infrared photosensitizer, CyI, which is encapsulated with human serum albumin (HSA) to form CyI-HSA nanocomplex that can simultaneously monitor the viscosity of cancer cell and enhance the efficacy of photodynamic therapy (PDT). In vitro experiments demonstrated that compared with CyI alone, CyI-HSA has superior water solubility and high photostability, and can effectively prevent aggregation-caused quenching (ACQ) caused by π-π stacking. Additionally, singlet oxygen quantum yield of CyI-HSA as high as 27.4 % under 658 nm laser irradiation, indicating significant PDT potential. Cellular experiments further revealed that CyI-HSA can monitor intracellular viscosity with high sensitivity, while generating a substantial amount of singlet oxygen, promoting PDT and cell apoptosis. Treatment of HepG-2 cells with 1 µmol/L CyI-HSA reduced cell viability by only 13 % (Laser = 100 mW cm-2). Therefore, as a kind of nano-photosensitizer integrating diagnostic and therapeutic functions, CyI-HSA has a broad prospect in the application of near-infrared hemicyanine photosensitizers in photodynamic therapy.

A dual-channel fluorescent probe with mitochondria-immobilization: Detecting polarity and viscosity during mitophagy

Mitophagy is a key pathway for regulating mitochondrial quality and quantity which is essential for the preservation of cellular homeostasis. Mitophagy process may be accompanied by changes of the mitochondrial microenvironments. The multifunctional fluorescent probe is crucial for the precise detection of multiple microenvironments, which is vital for the visualization of mitophagy. Herein, a mitochondria-immobilized fluorescent probe DPP was designed and fabricated to visualize mitophagy by monitoring polarity and viscosity in dual-channel. The DPP is characterized by "D-π-A″ structure, which provides the basis for the intramolecular charge transfer (ICT) and twisted intramolecular charge transfer (TICT) platform, enabling dual-channel responses to polarity and viscosity at emission wavelengths of 487 nm and 656 nm, respectively. The significant wavelength gap (169 nm) between the above channels prevents signal crosstalk. Additionally, the incorporation of 1, 4-dibenzyl chloride grants the probe mitochondrial immobilization capabilities, avoiding the leak of probe due to mitochondrial depolarization during autophagy. The DPP accumulates in mitochondria and monitors polarity and viscosity changes in green and red channels, respectively. Notably, the investigation of the relationship between polarity and viscosity revealed that an increase in viscosity is accompanied by a decrease in polarity. The mitophagy was effectively observed through the induction of DPP by rapamycin, with a particular emphasis on the increase in viscosity and decrease in polarity. Thus, DPP offers a powerful tool for a deeper understanding of the physiological and pathological processes associated with mitophagy and are regulated by various microenvironmental parameters.

A novel AIEgen fluorescent probe based on quinoline-malononitrile for monitoring and imaging ClO- and viscosity in biosystem

Hypochlorite (ClO-) and viscosity play a pivotal role in biological immune defense systems, it has been demonstrated that alterations in ClO- may link to the modifications in intracellular viscosity. Consequently, it is crucial to develop a tool that can accurately assess these changes of ClO- and viscosity concurrently. In this work, a dual-responsive fluorescent probe QM-WV based on the ICT effect for simultaneous detection of ClO- and viscosity was constructed. QM-WV exhibited a signally turn-on fluorescence signal at 552 nm for ClO- with favorable selectivity. Meanwhile, QM-WV displayed an exceptional 'off-on' fluorescence response at 650 nm to viscosity changes in glycerol-ethanol system. More importantly, QM-WV is capable of visually detecting ClO- by using smartphone as a portable signal processing device. Based on its low cytotoxicity and stability under physiological conditions, QM-WV further enabled the detection and imaging of ClO- and viscosity in HeLa cells and zebrafish.

A mitochondria-targeted fluorescent probe based on an anti-diffusion strategy for in situ imaging of fatty liver, inflammation and cancer

Abnormal mitochondrial viscosity is closely associated with a wide range of diseases and cellular dysfunction. It is crucial to develop fluorescent probes for precisely monitoring changes of mitochondrial viscosity in the detection and treatment of associated diseases. However, mitochondria-targeted fluorescent probes currently faced off-target problems because their high water-solubility could hinder the accurate detection of mitochondrial viscosity. Herein, a viscosity-sensitive fluorescent probe, HPQ-MV was designed and synthesized in this work. The indole cation and HPQ (2-(2'-hydroxyphenyl)-4(3H)-quinazolinone) moiety were introduced could make the probe HPQ-MV have excellent mitochondrial targeting properties and reduce the aqueous solubility of HPQ-MV made the probe less susceptible to diffusion, respectively. When the mitochondrial membrane potential was decreased, HPQ-MV could remain stable in the mitochondria and not cause false-negative signals. HPQ-MV had a signal-to-noise ratio of up to 2900-fold with respect to viscosity which was unaffected by pH and polarity. Additionally, HPQ-MV possessed a tissue permeability of up to 62.6 μM and had effectively facilitated in vivo imaging of fatty liver, inflammation, and in situ tumors.

Detecting Changes in Singlet Oxygen and Viscosity during Apoptosis-Ferroptosis Mediated Photodynamic Therapy and Establishing Visual Imaging of Fatty Liver

Cancer is a serious global health threat, and photodynamic therapy (PDT) is an effective treatment method for cancer. This therapy works by generating a large amount of singlet oxygen (1O2) under the influence of oxygen and light, which induces apoptosis in tumor cells, leading to their destruction. However, the resistance of cells to apoptosis limits the development of PDT, and thus the combination of ferroptosis and apoptosis provides a new perspective for PDT. During PDT and ferroptosis, the levels of 1O2 and the microenvironment (viscosity) within cells often change. To address this, this study developed a novel fluorescent probe, NI-QM-OH, based on the TICT-ICT effect, capable of monitoring changes in 1O2 and viscosity during PDT. The probe exhibits excellent selectivity, high sensitivity, and a low LOD (0.38 μM), and has been successfully applied for bioimaging in HepG2, HeLa, and MCF-7 cells, as well as for monitoring viscosity and 1O2 levels in zebrafish. Most importantly, NI-QM-OH also enables the visualization of the diagnosis of fatty liver disease (both alcoholic and nonalcoholic).

Dynamic visualization of endoplasmic reticulum viscosity in living cells with a multirotor-based fluorescent probe

Changes in viscosity can significantly influence the functionality of the endoplasmic reticulum (ER), and viscosity-responsive fluorescent probes can provide valuable feedback on its physiological state. In this study, three multirotor-based fluorescent probes (1-3) were designed and synthesized. Probes 1 and 2 exhibited remarkable viscosity sensitivity and specific targeting ability towards the ER. Probe 1 was utilized for in situ dynamic visualization of ER viscosity changes during inflammatory responses, drug treatments, and reticulophagy.

Advances in Organic Small Molecule-Based Fluorescent Probes for Precision Detection of Liver Diseases: A Perspective on Emerging Trends and Challenges

Liver disease poses a significant challenge to global health, and its early diagnosis is crucial for improving treatment outcomes and patient prognosis. Since fluctuation of key biomarkers during the onset and progression of liver diseases can directly reflect liver health and normal/abnormal function, biomarker-based assays are vital tools for the early detection of liver disease. In this context, small molecule fluorescent probes have undeniably emerged as indispensable tools for diagnosis and analysis, with an ever-growing number of small molecule-based fluorescent probes being developed over recent years, with the sole aim of monitoring relevant biomarkers of liver disease. This perspective will focus on the development and application of probes developed primarily over the last 10 years for diagnosing a range liver disease-related processes. It will outline the foundational design strategies for developing promising probes, their optical response to key biomarkers, and how they have been demonstrated in proof-of-concept imaging applications. Current challenges and new developments in the field will be discussed, with the aim of providing insights and highlighting opportunities in the field.

In situ observation of mitochondrial viscosity in liver of mice with drug-induced liver injury by near-infrared fluorescence imaging

Drug-induced liver injury (DILI) has emerged as among of the undesirable drug effects, posing significant threats to human health. However, in clinical practice, there remains a shortage of dependable and pre-diagnosis tools for DILI. Numerous studies indicated that the elevated intrahepatic viscosity levels were closely linked to the onset and progression of DILI. Therefore, establishing reliable tools to monitor mitochondrial viscosity are crucial for prompt diagnosis of DILI in situ. Herein, we proposed a new near-infrared (NIR) fluorescence probe (Wyry-M-V) for detecting mitochondrial viscosity, in which consisted of xanthene, multiple viscosity-responsive rotors (diphenyl, vinyl cyanide, and benzyl chloride) and mitochondrial targeting site (pyridinium cation). Furthermore, the Wyry-M-V was triumphantly utilized in mitochondrial viscosity imaging upon treatment with lipopolysaccharide, nystatin and acetaminophen (APAP). Notably, based on the advantages of NIR emission wavelength, the Wyry-M-V was resoundingly used for the detection of mitochondrial viscosity in APAP-induced DILI mice model.

Precise photorelease in living cells by high-viscosity activatable coumarin-based photocages

Intracellular viscosity is a critical microenvironmental factor in various biological systems, and its abnormal increase is closely linked to the progression of many diseases. Therefore, precisely controlling the release of bioactive molecules in high-viscosity regions is vital for understanding disease mechanisms and advancing their diagnosis and treatment. However, viscosity alone cannot directly trigger chemical reactions. Inspired by molecular rotor fluorophores, we have developed a series of high-viscosity activated photocages by modifying the C3 position of the coumarin scaffold with electron-withdrawing groups. In low-viscosity environments, both fluorescence and photocleavage of the photocages are inhibited by nonradiative decay caused by intramolecular free rotation. In contrast, in high-viscosity environments, the restriction of this intramolecular rotation restores fluorescence and photocleavage. These unique photolysis properties enable the selective photorelease of these photocages in high-viscosity conditions. As a proof of concept, we have developed a drug delivery system that targets abnormal mitochondria with high viscosity. This system demonstrates enhanced photolysis efficiency in abnormal mitochondria compared to normal ones, allowing for precise drug release in diseased mitochondria while ensuring excellent biological safety in healthy mitochondria. We anticipate that these photocages will serve as convenient and efficient tools for the precise release of active molecules in high-viscosity environments.

Dual-State Emissive Mitochondrial Viscosity Probe for Long-Term Imaging of Rheumatoid Arthritis

Rheumatoid arthritis (RA) is a destructive autoimmune disease that seriously affects human health. Due to the lack of a cure for RA, a good prognosis largely depends on early diagnosis and effective treatment monitoring of RA. Therefore, the development of fluorescent probes capable of real-time detection of RA is of great significance. Dual-state emission (DSE) molecules can emit light in both dilute solution and solid state, making them ideal fluorophores for constructing fluorescent probes. However, there are currently no reports of DSE molecule-based fluorescent probes for RA imaging. Herein, we report a fluorescent probe MQP-Boc based on a novel DSE molecule for effective RA imaging. MQP-Boc selectively responds to viscosity with sensitive fluorescence changes at 667 nm and is mitochondria targetable. Cell imaging studies show that MQP-Boc can detect changes in mitochondrial viscosity and perform long-term imaging of mitochondria, which is significantly superior to that of the control probe MQP-Ac. Imaging studies on a mouse model of RA induced by λ-carrageenan show that MQP-Boc has excellent real-time and long-term imaging capabilities for RA. Besides, with MQP-Boc, significant increases in joint tissue viscosity were found during the RA process. All results indicate that MQP-Boc is an effective new tool for studying and diagnosing RA.

Biotinylated Viscosity Sensitive Cell Membrane Probe for Targeted Imaging and Precise Visualization of Tumor Cells and Tumors

Cancer is a global health challenge that urgently requires more sensitive and effective cancer detection methods. Fluorescence imaging with small molecule fluorescent probes has shown great promise for cancer detection but most of the developed probes lack active tumor cell targeting, which makes them unable to selectively target tumors, thereby reducing the accuracy of in vivo tumor detection. Herein, we report a novel probe Bio-S that combines a viscosity-sensitive and cell membrane targetable fluorescent group with biotin for targeted imaging and precise visualization of tumor cells and tumors. Bio-S exhibits sensitive fluorescence changes for viscosity at ∼660 nm and excellent cell membrane localization and imaging ability (red fluorescence, wash-free, and long-term imaging). Moreover, compared with the nonbiotinylated control probe C6-S, the biotinylated Bio-S can specifically target tumor cell membranes, thereby achieving much higher selectivity and sensitivity in distinguishing tumor cells from normal cells. Mice imaging experiments show that tail vein injection of Bio-S can target tumors and monitor lung cancer metastasis at the in vivo level. Therefore, this work provides an effective new strategy and tool for tumor-targeted detection and precise diagnosis.

Lipid droplet specific BODIPY based rotors with viscosity sensitivity to distinguish normal and cancer cells: impact of molecular conformation

Lipid droplets (LDs) are dynamic, multifunctional organelles critical for regulating energy balance, cell signaling, membrane formation, and trafficking. Recent studies have highlighted LDs as emerging cancer biomarkers, with cancer cells typically exhibiting a higher number and viscosity of LDs compared to normal cells. This discovery paves the way for developing molecular probes that can monitor intracellular viscosity changes within LDs, offering a powerful tool for early cancer diagnosis, recurrence monitoring, and therapeutic interventions. In this study, we designed and synthesized two series of donor-acceptor (D-A) conjugated BODIPY-cyanostilbene based fluorophores (5a-c and 6a-c) by fine-tuning the cyanostilbene unit with three distinct substituents (OMe, H, Cl) and modulating the molecular conformation via rigidifying the indacene core. While the terminal substituents had a minimal effect on the optical properties, changes in molecular conformation significantly impacted the photophysical behavior of the fluorophores. Compounds 5a-c function as molecular rotors, with the free rotation of the meso-biphenyl rings leading to non-radiative deactivation of the excited state, resulting in weak emission. Additionally, this structural feature makes them highly responsive to changes in viscosity. As the glycerol concentration increased from 0% to 99%, the fluorescence intensity of compounds 5a, 5b, and 5c increased dramatically by 17-fold, 78-fold, and 43-fold, respectively. In contrast, compounds 6a-c, with restricted phenyl ring rotation due to tetra-methyls on the indacene unit, showed only a modest 2-3-fold increment in fluorescence intensity under similar conditions. These fluorophores possess several key advantages, including high selectivity for LDs, good photostability, sensitivity to viscosity, and responsiveness to polarity and pH. Moreover, they effectively differentiate between normal and cancer cells, making them valuable tools for cancer diagnosis and potential therapeutic applications.

Near-Infrared Probes Designed with Hemicyanine Fluorophores Featuring Rhodamine and 1,8-Naphthalic Derivatives for Viscosity and HSA Detection in Live Cells

This paper presents the development of near-infrared (NIR) fluorescent probes, A and B, engineered from hemicyanine dyes with 1,8-naphthalic and rhodamine derivatives for optimized photophysical properties and precise mitochondrial targeting. Probes A and B exhibit absorption peaks at 737 nm and low fluorescence in phosphate-buffered saline (PBS) buffer. Notably, their fluorescence intensities, peaking at 684 (A) and 702 nm (B), increase significantly with viscosity, as demonstrated through glycerol-to-PBS ratio experiments. This increase is attributed to restricted rotational freedom in the fluorophore and its linkages to rhodamine or 1,8-naphthalic groups. Theoretical modeling suggests nonplanar configurations for both probes, with primary absorptions in the rhodamine and hemicyanine cores (A: 543; B: 536 nm), and additional transitions to 1,8-naphthalic (A: 478 nm) and rhodamine (B: 626 nm) groups. Probe A is also responsive to human serum albumin (HSA), a key biomarker, with fluorescence increasing in HeLa cells as HSA concentrations rise. In contrast, probe B shows no response to HSA, likely due to steric hindrance from its bulky rhodamine group, illustrating a selectivity difference between the probes. Probe B, however, excels in mitochondrial imaging, confirmed through cellular and in vivo studies. In HeLa cells, it tracked viscosity changes following treatment with monensin, nystatin, and lipopolysaccharide (LPS), with fluorescence increasing in a dose-dependent manner. In fruit flies, probe B effectively detected monensin-induced viscosity changes, demonstrating its stability and in vivo applicability. These findings highlight the versatility and sensitivity of probes A and B as tools in biological research, with potential applications in monitoring mitochondrial health, detecting biomarkers like HSA, and investigating mitochondrial dynamics in disease.

Mitochondria-targeted fluorescent probes based on coumarin-hemicyanine for viscosity changes and their applications in cells and mice

As an important parameter of the cellular microenvironment, the changes in mitochondrial viscosity are closely related to various life activities. Therefore, the development of fluorescent probes for test the changes of mitochondrial viscosity has great significance. In this study, we developed two fluorescent probes for the detection of the mitochondrial viscosity changes. The probes exhibited different fluorescence intensities at different viscosity based on the twisted intramolecular charge transfer process. The characteristics of high anti-interference performance, wide pH applicability, low cytotoxicity and excellent mitochondrial targeting performance made the probes successfully used to distinguish normal cells from cancer cells, achieving visualization of viscosity changes. Furthermore, probes P1 and P2 can also be used as early diagnosis of tumors in mice and reveal the pathology of tumor development. The probes could be serve as a promising viscosity detection tool for discriminating normal cells and cancer cells in biology-related research.

Roadmap for Designing Donor-π-Acceptor Fluorophores in UV-Vis and NIR Regions: Synthesis, Optical Properties and Applications

Donor acceptor (D-π-A) fluorophores containing a donor unit and an acceptor moiety at each end connected by a conjugated linker gained attention in the last decade due to their conjugated system and ease of tunability. These features make them good candidates for various applications such as bioimaging, photovoltaic devices and nonlinear optical materials. Upon excitation of the D-π-A fluorophore, intramolecular charge transfer (ICT) occurs, and it polarizes the molecule resulting in the 'push-pull' system. The emission wavelengths of fluorophores can be altered from UV-vis to NIR region by modifying the donor unit, acceptor moiety and the π linker between them. The NIR emitting fluorophores with restricted molecular rotations are used in aggregation-induced emission (AIE). D-π-A fluorophores with carboxylic acid and cyano groups are preferred in photovoltaic applications, and fluorophores with large surface area are used for two photon absorbing applications. Herein, we report the synthesis, optical properties, and applications of various D-π-A fluorophores in UV-vis and NIR region.

A novel viscosity sensitive hemicyanine fluorescent dye for real-time imaging of amyloid-β aggregation

Alzheimer's Disease (AD) is a neurodegenerative disease, of which β-amyloid (Aβ) deposition is one of the most important pathological features. It has been reported that during Aβ aggregation, the microenvironment around the Aβ protein is altered in terms of viscosity and polarity. In this work, we developed five novel hemicyanine fluorescent probes (MZs). After screening the photochemical properties, MZ-2 and 3 were found to enable the rapid detection of Aβ42 aggregates, which were also sensitive to ambient viscosity. After comparison the structure of probes, we also observed that extensions of conjugated π-systems effectively cause redshifts of excitation wavelength. In the meanwhile, hydroxyl groups with weaker ionization strengths are more responsive to Aβ42 aggregates than sulfonate groups, probably due to the small size of the hydroxyl group and the acidity. Overall, MZ-2 showed the best response to Aβ42 aggregates (15.35-fold) and viscosity (17.6-fold). MZ-2 can quickly cross the blood-brain barrier (BBB), enabling high-fidelity imaging of Aβ42 aggregates in the mice brain.

NIR emissive probe for fluorescence turn-on based dead cell sorting and in vivo viscosity mapping in C. elegans

Dead cell sorting is pivotal and plays a very significant role in homeostasis. Apoptosis and ferroptosis are the two major regulatory cell death processes. Apoptosis is a programmed cell death process, while ferroptosis is a regulatory cell death process. Monitoring the dead cells coming out from these processes is extremely important to stop various cellular dysfunctions. Here, we present a single NIR emissive probe that can observe both apoptotic and ferroptosis regulatory cell deaths. We were able to directly visualize the dead cells in both animal and plant cells upon a significant increase in the fluorescence intensity of the probe. During cell death, the increased cytoplasm viscosity restricted the rotor motion and helped in the fluorescence turn-on of the probe. Lysosomal viscosity was found to play a crucial role in the ferroptosis pathway. On the other hand, the probe was not only efficient in mapping the viscosity in various parts of live Caenorhabditis elegans (C. elegans) bodies but also able to differentiate between live and dead animals.

Mitochondrial membrane potential-independent near-infrared fluorescent probes for viscosity-exclusive imaging

Elucidating the intrinsic relationship between disease and mitochondrial viscosity is crucial for early diagnosis. However, current mitochondrial viscosity fluorescent probes are highly dependent on mitochondrial membrane potential (MMP) and are sensitive to other mitochondrial microenvironment parameters. To address these issues, a mitochondria-targeting MMP-independent and viscosity exclusive near-infrared (NIR) fluorescent probe, ACR-DMA, was developed. ACR-DMA consists of thiophene acetonitrile as the skeleton and viscosity-sensitive unit, a pyridinium cation for the mitochondria-targeting group, and a benzyl bromide subunit for mitochondrial immobilization. It is very sensitive to viscosity and shows significant "turn-on" fluorescence behavior at 710 nm with a more than 150-fold fluorescence intensity increase. Furthermore, ACR-DMA can be firmly immobilized in mitochondria and can monitor viscosity changes induced by nystain, monensin, and lipopolysaccharide. Additionally, it was successfully used to visualize mitochondrial viscosity changes resulting from tumors, inflammation, and drug-induced acute kidney injury, revealing the relationship between viscosity and disease both in vitro and in vivo. ACR-DMA is expected to be a promising candidate for diagnosing mitochondrial viscosity-related diseases.

A dual-emission fluorescent probe with independent polarity and viscosity responses: The synthesis, spectroscopy and bio-imaging applications

Viscosity and polarity are essential parameters that play critical roles in various physiological processes. Thus, dual-emission fluorescent probes that respond to both polarity and viscosity are highly sought-after tools for studying these processes. In addressing this need, a novel fluorescent probe (L), with dual emissions centered at 460 nm and 780 nm, which can sensitively respond to polarity and viscosity respectively, has been developed. Probe (L) is constructed through rational molecular design, utilizing two conjugated synthons connected by a π-bond to form a D-π-A system. The twisted intramolecular charge transfer (TICT) state is dominant in low-viscosity environments, resulting in weak near-infrared (NIR) fluorescence. Conversely, the intramolecular charge transfer (ICT) state is expected to prevail in high-viscosity environments, leading to strong NIR fluorescence. The polarity-sensitive fluorescence centered at 460 nm can be attributed to the emission of the coumarin unit. Moreover, probe (L) exhibits low cytotoxicity and primarily targets mitochondria. By leveraging the dual-emission properties of probe (L), real-time imaging of polarity and viscosity fluctuations within cells has been achieved. Additionally, probe (L) can be used for in situ and in vivo imaging of rheumatoid arthritis (RA) with good imaging resolution.

Xylan-based near-infrared fluorescent probes for monitoring viscosity abnormalities in living cells and zebrafish

Viscosity is a crucial indicator of the flow state of proteins, lipids, and polysaccharides in the cell microenvironment and plays a vital role in maintaining normal cellular activities. Abnormal viscosity in any part of the cell constituents can lead to various diseases in the organism. For instance, abnormal mitochondrial viscosity can lead to diseases, such as diabetes and Parkinson's disease. Therefore, real-time monitoring of changes in mitochondrial viscosity in both pathological and physiological environments is relevant. This study describes a water-soluble xylan-based near-infrared fluorescence probe that can detect changes in cellular viscosity. The designed mitochondria-targeting near-infrared fluorophores were introduced into modified xylan to form a viscosity-sensing fluorescent probe (NI-XylV). The fluorescence intensity of NI-XylV at 590 and 670 nm gradually increases with an increase in viscosity caused by environmental changes, enabling the sensitive detection of viscosity changes in mitochondria within living cells. NI-XylV exhibits good photostability, biocompatibility, excellent mitochondrial targeting, and broad application prospects as a bio-based fluorescence probe.

Viscosity-activated carbon dots for noninvasive disease diagnosis, therapeutic efficacy assessment and anticancer drug screening

Abnormal levels of intracellular microviscosity have a close relationship with some diseases and pathologies. Therefore, quantitative imaging of viscosity at the cellular and organ levels is beneficial for disease diagnosis, curative effect evaluation, and anticancer drug screening. Herein, we synthesized viscosity-activated orange-red emitting carbon dots (named as LP-CDs) from lignin and p-phenylenediamine using a one-step method, whose fluorescence intensity, lifetime, and quantum yield significantly increase with rising viscosity. Meanwhile, the fluorescence intensity of LP-CDs also has a good linear relationship with the solution viscosity. Taking the advantages of high sensitivity, noninvasiveness, and rapid result output of fluorescence imaging, LP-CDs can visualize the cellular viscosity changes induced by inflammation and hyperglycemia, and image tumor evolution. More importantly, LP-CDs can be used as powerful sensors to screen anticancer drugs in vivo by evaluating therapeutic effects. This work provides a new scheme for constructing robust nanoprobes to achieve the diagnosis and imaging-guided surgery of viscosity related diseases.

A near-infrared fluorescent probe for simultaneous detection of pH and viscosity

In this work, we developed a ratiometric fluorescent probe (NT) based on ICT framework in near-infrared (NIR) which could detect pH and viscosity simultaneously. Long emission wavelength in NIR could protect the probe from interference of background fluorescence and improve the accuracy of the test. Due to the presence of thiazole-salt, the probe possessed good water solubility and could respond immediately to pH in water system. The pH values measured by NT in the actual samples were not much different from that measured by the pH meter, therefore, NT could give excellent accuracy. NT realized the reversible detection of pH by protonation and deprotonation. NT was used successfully to detect the pH of actual water samples, human serum and meat, as well as the viscosity variation caused by thickeners. Additionally, NT could monitor the changes of pH and viscosity in living cells. Therefore, the novel probe exhibited potential application in the fields of the environment, human health and food safety evaluation.

Near-Infrared Fluorescent Probe Reveals Elevated Mitochondrial Viscosity during Acute Alcoholic Liver Injury

Acute alcoholic liver injury (AALI) has become an important cause of liver disease worldwide, and there is an urgent need to develop noninvasive and sensitive methods to detect and evaluate AALI. We report herein three novel but readily available mitochondrial targeting fluorescence probes (ICR, ICJ, and ICQ) for AALI detection. These probes contain different electron-donating groups, among which ICQ exhibits NIR fluorescence (740 nm), a large Stokes shift (110 nm), and a sensitive response to viscosity (73-fold enhancement in fluorescence from water to glycerol), making it suitable for in vivo imaging. ICQ also exhibits an excellent ability to image mitochondrial viscosity changes in cells. More importantly, ICQ can target the liver selectively and image the viscosity changes in the liver noninvasively. Through establishing an AALI mouse model, ICQ was successfully applied to the in situ imaging changes in liver viscosity during the AALI process. The results showed a significant increase in liver viscosity in AALI mice, indicating that viscosity can serve as a marker for AALI, and ICQ is a promising noninvasive and sensitive tool for detecting and evaluating AALI.

Design and synthesis of a TICT-based red-emissive fluorescent probe for the rapid and selective detection of HSA in human biofluids and live cell imaging

Here, we report the design and synthesis of a D⋯π⋯A-based fluorescent probe, (E)-4-(4-(dibutylamine)-2-hydroxystyryl)-1-methylquinolin-1-ium (DHMQ), which is nonfluorescent in ∼100% PBS buffer medium due to a twisted intra molecular charge transfer (TICT) phenomenon and it becomes highly fluorescent (∼149 fold) in the presence of human serum albumin (HSA), owing to the restriction of its intramolecular free rotation inside the hydrophobic binding cavity of HSA. The site-selective fluorescence displacement assay and molecular docking studies clearly reveal that DHMQ selectively binds at subdomain IB of HSA. The 3σ/slope method was adopted to determine the limit of detection (LOD) value, which was as low as 2.39 nM in ∼100% PBS medium, indicating its high sensitivity towards HSA. The low dissociation constant value [Kd = (1.066 ± 0.017) μM] suggests a strong complexation between the DHMQ and HSA. Importantly, it has been demonstrated that DHMQ is capable of detecting HSA in real human serum and urine samples and was found to be suitable for live cell imaging of HSA.

Development of BODIPY-based fluorescent probes for imaging Aβ aggregates and lipid droplet viscosity

Alzheimer's disease (AD), gradually recognized as an untreatable neurodegenerative disorder, has been considered to be closely associated with Aβ plaques, which consist of β-amyloid protein (Aβ) and is one of the crucial pathological features of AD. There are no obvious symptoms in the initial phase of AD, and thus the therapeutic interventions are important for early diagnosis of AD. Moreover, recent researches have indicated that lipid droplets might serve as a potential ancillary biomarker, and its viscosity changes are closely associated to the pathological process of AD. Herein, two newly fluorescent probes 5QSZ and BQSZ have been developed and synthesized for identifying Aβ aggregates and detecting the viscosity of lipid droplet. After selectively binding to Aβ aggregates, 5QSZ and BQSZ exhibited linear and obvious fluorescence enhancements (32.58 and 36.70 folds), moderate affinity (Kd = 268.0 and 148.6 nM) and low detection limits (30.11 and 65.37 nM) in aqueous solutions. Further fluorescence staining of 5QSZ on brain tissue sections of APP/PS1 transgenic mouse exhibited the higher selectivity of 5QSZ towards Aβ aggregates locating at the core of the plaques. Furthermore, 5QSZ and BQSZ displayed strong linear fluorescence emission enhancements towards viscosity changes and would be utilized to monitor variation in cellular viscosity induced by LPS or monensin. These two probes were non-cytotoxic and showed good localization in lipid droplets. Therefore, 5QSZ and BQSZ could serve as potential bi-functional fluorescent probes to image Aβ aggregates and monitor the viscosity of lipid droplets, which have significant implications for the early diagnosis and progression of AD.

Dynamic insights into mitochondrial function: Monitoring viscosity and SO2 levels in living cells

Mitochondria, central organelles pivotal for eukaryotic cell function, extend their influence beyond ATP production, encompassing roles in apoptosis, calcium signaling, and biosynthesis. Recent studies spotlight two emerging determinants of mitochondrial functionality: intramitochondrial viscosity and sulfur dioxide (SO2) levels. While optimal mitochondrial viscosity governs molecular diffusion and vital processes like oxidative phosphorylation, aberrations are linked with neurodegenerative conditions, diabetes, and cancer. Similarly, SO2, a gaseous signaling molecule, modulates energy pathways and oxidative stress responses; however, imbalances lead to cytotoxic sulfite and bisulfite accumulation, triggering disorders such as cancer and cardiovascular anomalies. Our research focused on development of a dual-channel fluorescent probe, applying electron-withdrawing acceptors within a coumarin dye matrix, facilitating monitoring of mitochondrial viscosity and SO2 in live cells. This probe distinguishes fluorescence peaks at 650 nm and 558 nm, allowing ratiometric quantification of SO2 without interference from other sulfur species. Moreover, it enables near-infrared viscosity determination, particularly within mitochondria. The investigation employed theoretical calculations utilizing Density Functional Theory (DFT) methods to ascertain molecular geometries and calculate rotational energies. Notably, the indolium segment of the probe exhibited the lowest rotational energy, quantified at 7.38 kcals/mol. The probe featured heightened mitochondrial viscosity dynamics when contained within HeLa cells subjected to agents like nystatin, monensin, and bacterial lipopolysaccharide (LPS). Overall, our innovative methodology elucidates intricate mitochondrial factors, presenting transformative insights into cellular energetics, redox homeostasis, and therapeutic avenues for mitochondrial-related disorders.

Cancer-Targeting and Viscosity-Activatable Near-Infrared Fluorescent Probe for Precise Cancer Cell Imaging

Small-molecule fluorescent probes have emerged as potential tools for cancer cell imaging-based diagnostic and therapeutic applications, but their limited selectivity and poor imaging contrast hinder their broad applications. To address these problems, we present the design and construction of a novel near-infrared (NIR) biotin-conjugated and viscosity-activatable fluorescent probe, named as QL-VB, for selective recognition and imaging of cancer cells. The designed probe exhibited a NIR emission at 680 nm, with a substantial Stokes shift of 100 nm and remarkably sensitive responses toward viscosity changes in solution. Importantly, QL-VB provided an evidently enhanced signal-to-noise ratio (SNR: 6.2) for the discrimination of cancer cells/normal cells, as compared with the control probe without biotin conjugation (SNR: 1.8). Moreover, we validated the capability of QL-VB for dynamic monitoring of stimulated viscosity changes within cancer cells and employed QL-VB for distinguishing breast cancer tissues from normal tissues in live mice with improved accuracy (SNR: 2.5) in comparison with the control probe (SNR: 1.8). All these findings indicated that the cancer-targeting and viscosity-activatable NIR fluorescent probe not only enables the mechanistic investigations of mitochondrial viscosity alterations within cancer cells but also holds the potential as a robust tool for cancer cell imaging-based applications.

Near-infrared fluorescent indolizine-dicyanomethylene-4H-pyran hybrids for viscosity imaging in living cells

The development of near-infrared organic fluorescent dyes with tunable emission profiles is highly required in the field of biological sensing and imaging. In this paper, we designed and synthesized two organic fluorescent dyes, DCM-1 and DCM-2, through the hybridization of indolizine and dicyanomethylene-4H-pyran skeleton. These two compounds show near-infrared fluorescence with emission maximum approximately at 640 and 680 nm, respectively. Notably, both DCM-1 and DCM-2 have specific responses to viscosity without being interfered by biological relevant species. Cell experiments demonstrate that DCM-1 and DCM-2 can detect dynamic changes in viscosity within living cells, suggesting their potential applications in chemical biology research.

A TICT-AIE activated dual-channel fluorescence-on probe to reveal the dynamics mechanosensing of lipid droplets during ferroptosis

Mechanical forces play a crucial role in cellular processes, including ferroptosis, a form of regulated cell death associated with various diseases. However, the mechanical aspects of organelle lipid droplets (LDs) during ferroptosis are poorly understood. In this study, we designed and synthesized a fluorescent probe, TPE-V1, to enable real-time monitoring of LDs' viscosity using a dual-channel fluorescence-on model (red channel at 617 nm and NIR channel at 710 nm). The fluorescent imaging of using TPE-V1 was achieved due to the integrated mechanisms of the twisted intramolecular charge transfer (TICT) and aggregation-induced emission (AIE). Through dual-emission channel fluorescence imaging, we observed the enhanced mechanical energy of LDs triggering cellular mechanosensing, including ferroptosis and cell deformation. Theoretical calculations confirmed the probe's behavior, showing that high-viscosity media prevented the rotation processes and restored fluorescence quenching in low viscosity. These findings suggest that our TICT-TPE design strategy provides a practical approach to study LDs' mechanical properties during ferroptosis. This development enhances our understanding of the interplay between mechanical forces and LDs, contributing to the knowledge of ferroptotic cell death and potential therapeutic interventions targeting dysregulated cell death processes.

Construction of a Near-Infrared Fluorescent Probe for Dynamic Monitoring and Early Diagnosis of Heart Failure

Cardiac hypertrophy characterized by abnormal cardiomyocyte viscosity is a typical sign of heart failure (HF) with vital importance for early diagnosis. However, current biochemical and imaging diagnostic methods are unable to detect this subclinical manifestation. In this work, we developed a series of NIR-I fluorescence probes for detecting myocardial viscosity based on the pyridazinone scaffold. The probes showed weak fluorescence due to free intramolecular rotation under low-viscosity conditions, while they displayed strong fluorescence with limited intramolecular rotation in response to a high-viscosity environment. Among them, CarVis2 exhibited higher stability and photobleaching resistance than commercial dyes. Its specific response to viscosity was not influenced by the pH and biological species. Furthermore, CarVis2 showed rapid and accurate responses to the viscosity of isoproterenol (ISO)-treated H9C2 cardiomyocytes with good biocompatibility. More importantly, CarVis2 demonstrated excellent sensitivity in monitoring myocardial viscosity variation in HF mice in vivo, potentially enabling earlier noninvasive identification of myocardial abnormalities compared to traditional clinical imaging and biomarkers. These findings revealed that CarVis2 can serve as a powerful tool to monitor myocardial viscosity, providing the potential to advance insights into a pathophysiological mechanism and offering a new reference strategy for early visual diagnosis of HF.

Real-time monitoring of abnormal mitochondrial viscosity in glioblastoma with a novel mitochondria-targeting fluorescent probe

Glioblastoma is the most fatal and insidious malignancy, due to the existence of the blood-brain barrier (BBB) and the high invasiveness of tumor cells. Abnormal mitochondrial viscosity has been identified as a key feature of malignancies. Therefore, this study reports on a novel fluorescent probe for mitochondrial viscosity, called ZVGQ, which is based on the twisted intramolecular charge transfer (TICT) effect. The probe uses 3-dicyanomethyl-1,5,5-trimethylcyclohexene as an electron donor moiety and molecular rotor, and triphenylphosphine (TPP) cation as an electron acceptor and mitochondrial targeting group. ZVGQ is highly selective, pH and time stable, and exhibits rapid viscosity responsiveness. In vitro experiments showed that ZVGQ could rapidly recognize to detect the changes in mitochondrial viscosity induced by nystatin and rotenone in U87MG cells and enable long-term imaging for up to 12 h in live U87MG cells. Additionally, in vitro 3D tumor spheres and in vivo orthotopic tumor-bearing models demonstrated that the probe ZVGQ exhibited exceptional tissue penetration depth and the ability to penetrate the BBB. The probe ZVGQ not only successfully visualizes abnormal mitochondrial viscosity changes, but also provides a practical and feasible tool for real-time imaging and clinical diagnosis of glioblastoma.

A Red-Emission Fluorescent Probe with Large Stokes Shift for Detection of Viscosity in Living Cells and Tumor-Bearing Mice

Abnormal viscosity is closely related to the occurrence of many diseases, such as cancer. Therefore, real-time detection of changes in viscosity in living cells is of great importance. Fluorescent molecular rotors play a critical role in detecting changes in cellular viscosity. Developing red emission viscosity probes with large Stokes shifts and high sensitivity and specificity remains an urgent and important topic. Herein, a novel viscosity-sensitive fluorescent probe (TCF-VIS1) with a large stokes shift and red emission was prepared based on the 2-dicyanomethylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (TCF) skeleton. Due to intramolecular rotation, the probe itself does not fluorescence at low viscosity. With the increase in viscosity, the rotation of TCF-VIS1 is limited, and its fluorescence is obviously enhanced. The probe has the advantages of simple preparation, large Stokes shift, good sensitivity and selectivity, and low cytotoxicity, which make it successfully used for viscosity detection in living cells. Moreover, TCF-VIS1 showed its potential for cancer diagnosis at the cell level and in tumor-bearing mice by detecting viscosity. Therefore, the probe is expected to enrich strategies for the detection of viscosity in biological systems and offer a potential tool for cancer diagnosis.

A novel fluorescent probe for visualizing viscosity changes in lipid droplets during chemotherapy-induced ferroptosis

BACKGROUND

Ferroptosis, as a novel form of cell death, is becoming one of the hot topics in cancer treatment research. It differs from necrosis and autophagy in that it involves the accumulation of lipid peroxides and is triggered by iron dependency. Recent studies have suggested that this mechanism may alter the viscosity or structure of lipid droplets (LDs). The relationship between LDs viscosity and ferroptosis remains an active area of research with limited reports at present. Additionally, there is a lack of effective anticancer drugs targeting the ferroptosis pathway to promote ferroptosis in tumour cells. Therefore, the development of tools to detect viscosity changes during ferroptosis and targeted therapeutic strategies is of great significance.

RESULTS

By coupling 1,3-indandione with naphthalimide, including decamethylamine as a LDs recognition group, we designed and synthesized an environmental fluorescent probe that induces intramolecular charge transfer (TICT) effects. Notably, the diffusion and transport of intracellular substances may be affected in highly viscous environments. Under such conditions, intracellular iron ions may accumulate, leading to peroxide production and cellular damage, which can trigger ferroptosis. Therefore, WD-1 achieved excellent in situ bioimaging of LDs targeting and its viscosity during ferroptosis in HeLa cells and zebrafish. Furthermore, it was observed that WD-1 effectively differentiated between malignant and normal cells during this process, highlighting its potential significance in distinguishing cellular states. In addition, we used the antitumour drug paclitaxel to study ferroptosis in cancer cells. These findings not only provide an excellent tool for the development of the ferroptosis response, but also are crucial for understanding the biological properties of LDs during the ferroptosis response.

SIGNIFICANCE AND NOVELTY

Based on a powerful tool of fluorescent probe with in vivo bioimaging, we developed WD-1 to track the impact of paclitaxel on the process of ferroptosis in living cells. Therefore, we preliminarily believe that paclitaxel may affect the occurrence of ferroptosis and control apoptosis in cancer cells. These findings not only serve as an exceptional tool for advancing our understanding of the ferroptosis response, but furthermore play a vital role in comprehending the biological characteristics of LDs in relation to ferroptosis.

Monitoring the Mitochondrial Viscosity Changes During Cuproptosis with Iridium(III) Complex Probe via In Situ Phosphorescence Lifetime Imaging

Cuproptosis is a novel copper-dependent form of programmed cell death, displaying important regulatory functions in many human diseases, including cancer. However, the relationship between the changes in mitochondrial viscosity, a key factor associated with cellular malfunction, and cuproptosis is still unclear. Herein, we prepared a phosphorescent iridium (Ir) complex probe for precisely monitoring the changes of mitochondrial viscosity during cuprotosis via phosphorescence lifetime imaging. The Ir complex probe possessed microsecond lifetimes (up to 1 μs), which could be easily distinguished from cellular autofluorescence to improve the imaging contrast and sensitivity. Benefiting from the long phosphorescence lifetime, excellent viscosity selectivity, and mitochondrial targeting abilities, the Ir complex probe could monitor the increase in the mitochondrial viscosity during cuproptosis (from 46.8 to 68.9 cP) in a quantitative manner. Moreover, through in situ fluorescence imaging, the Ir complex probe successfully monitored the increase in viscosity in zebrafish treated with lipopolysaccharides or elescolomol-Cu2+, which were well-known cuproptosis inducers. We anticipate that this new Ir complex probe will be a useful tool for in-depth understanding of the biological effects of mitochondrial viscosity during cuproptosis.

A flavonoid salt probe for distinguishing between tumor and normal cells

YH-2 represents an innovative, non-invasive fluorescent probe featuring a structure based on flavonoid onium salts. It is characterized by a well-suited Stokes shift and emits in the near-infrared (NIR) wavelength range. Its capacity to distinguish between HeLa cells, HepG2 cells, and LO2 cells is attributed to differential intracellular viscosity. Experimental results validate the heightened viscosity of organelles, such as the endoplasmic reticulum (ER), mitochondria and lysosomes in tumor cells compared to LO2 cells. Of paramount importance, YH-2 demonstrates the capability to swiftly image tumors within a mere 20 min following tail vein injection and this imaging ability can be sustained for an extended period of up to 5 h. This method offers a potential tumor diagnostic strategy in vivo.

Design and Application of Fluorescent Probes to Detect Cellular Physical Microenvironments

The microenvironment is indispensable for functionality of various biomacromolecules, subcellular compartments, living cells, and organisms. In particular, physical properties within the biological microenvironment could exert profound effects on both the cellular physiology and pathology, with parameters including the polarity, viscosity, pH, and other relevant factors. There is a significant demand to directly visualize and quantitatively measure the fluctuation in the cellular microenvironment with spatiotemporal resolution. To satisfy this need, analytical methods based on fluorescence probes offer great opportunities due to the facile, sensitive, and dynamic detection that these molecules could enable in varying biological settings from in vitro samples to live animal models. Herein, we focus on various types of small molecule fluorescent probes for the detection and measurement of physical parameters of the microenvironment, including pH, polarity, viscosity, mechanical force, temperature, and electron potential. For each parameter, we primarily describe the chemical mechanisms underlying how physical properties are correlated with changes of various fluorescent signals. This review provides both an overview and a perspective for the development of small molecule fluorescent probes to visualize the dynamic changes in the cellular environment, to expand the knowledge for biological process, and to enrich diagnostic tools for human diseases.

Rational construction and evaluation of a dual-functional near-infrared fluorescent probe for the imaging of Amyloid-β and mitochondrial viscosity

Alzheimer's disease is a fatal, incurable, chronic neurodegenerative disease. Diagnosis in its early and even preclinical stages will be beneficial for its prevention and treatment. In the accepted pathological theory, abnormal accumulation of Aβ protein and abnormal mitochondrial function, including changes in mitochondrial viscosity, is closely related to Alzheimer's disease. To date, rare fluorescent probes have been reported that can simultaneously image Aβ plaques and mitochondrial viscosity. Therefore, the development of a dual-functional fluorescent probe for real-time fluorescence imaging of Aβ plaques and mitochondrial viscosity is crucial to discover a novel approach and strategy for the treatment of Alzheimer's disease, and to understand the pathological process and crosstalk between different biomarkers of Alzheimer's disease. Herein, we rationally designed and synthesized a series of fluorescent probes QM-SF-1∼5 with dimethylamino-quinolinium as the skeleton and thiophene as the π bridge to connect the groups with different electron-push/pull capacities. Among them, QM-SF-2 exhibited excellent properties such as large Stokes shift (168 nm), near-infrared emission (689 nm), and high selectivity and sensitivity (limit of detection was 1.07 μM) to Aβ aggregate and mitochondrial viscosity changes, indicating its promising prospects as a dual-functional imaging tool in the pathological study of Alzheimer's disease.

Rational design and comparison of three curcumin-based fluorescent probes for viscosity detection in living cells and zebrafish

Viscosity is a crucial indicator of the cellular microenvironment, which can affect the normal level of cellular metabolism. Aberrant levels of viscosity can result in the emergence of a variety of physiological problems including diabetes, Parkinson's disease, inflammation, etc. Therefore, it is crucial to exploit effective assays that can detect viscosity levels in living cells and organisms. Three new nitrogen-containing heterocyclic fluorescent probes, CNO, CNN and CNNB, were designed and prepared by coupling curcumin with isoxazole, pyrazole, and phenylpyrazole rings, respectively. The fluorescence response properties of these probes to the viscosity level were analyzed in parallel. All the probes, CNO, CNN and CNNB, exhibited a significantly enhanced fluorescence response to viscosity in a broad pH range with excellent photostability, sensitivity and anti-interference ability. The sensing mechanisms of these probes for viscosity were verified by DFT calculations. In addition, these probes were successfully employed for detecting viscosity levels in living HeLa cells and zebrafish. This research compares the viscosity-responsive capabilities of curcumin-based fluorescent probes containing different nitrogen-containing heterocyclic structures, and provides a new design strategy and guidance for developing curcumin-based fluorescent probes for viscosity analysis.

Design a dual-response two-photon fluorescent probe for simultaneous imaging of mitochondrial viscosity and peroxynitrite in a thrombosis model

BACKGROUND

Venous thromboembolism is a sudden cardiovascular disease that can lead to death, and its pathologic development is closely related to vascular viscosity and inflammation. However, direct evidence from in vivo is really scarce. The key limitation is that the combined probes cannot detect multiple markers simultaneously, which may lead to unreliable results. Therefore, to develop a single probe that can simultaneously monitor the variations of viscosity in the vascular microenvironment as well as inflammation level during venous thrombosis.

RESULTS

A dual-responsive two-photon fluorescent probe, Cou-ONOO, was designed and synthesized. Cou-ONOO provides a visualization tool for monitoring the viscosity of the vascular as well as the inflammatory marker ONOO‾ during thromboembolism via dual-channel simultaneous imaging. As a single probe that can recognize dual targets, Cou-ONOO effectively avoids the problems from unreliable results caused by complex synthesis and differences in intracellular localization, diffusion, and metabolism of different dyes as using combinatorial probes. Using Cou-ONOO, simultaneous imaging the variations of viscosity and ONOO‾at the cellular and tissue levels was successfully performed. In addition, Cou-ONOO also successfully visualized and tracked the viscosity of the vascular microenvironment and ONOO‾ during venous embolism in mice.

SIGNIFICANCE

Experimental results show that both viscosity and inflammation are abnormally overexpressed in the microenvironment at the thrombus site during venous thrombosis. An intuitive visualization tool to elucidate the variations of viscosity as well as inflammation level in the vascular microenvironment during thrombosis was provided, which will facilitate a better clinical understanding of the pathological process of thrombosis.

A novel pathway for ratiometric hydrazine sensing in environmental samples and the detection of intracellular viscosity by a mitochondria-targeted fluorescent sensor

Mass and signal transfer, dispersion of reactive metabolites in living cells, and interactions between biomacromolecules are greatly affected by viscosity inside the cells. It is crucial to accurately determine viscosity for reliable results because of the complexities of live cells. Herein, we introduce a new fluorescence probe based on the cyanobiphenyl and benzothiazolium units. This probe not only responds to intracellular viscosity but also detects hydrazine, a widely used chemical that poses significant environmental and toxic risks to organisms. The proposed sensing mechanism provides a new pathway that includes intramolecular cyclization with hydrazine, which differs from other sensing mechanisms. A weak emission (at 590 nm) of the probe under excitation at 365 nm resulted in 25-fold higher emission at 488 nm after the addition of N2H4. The quantum yield of the probe (Φ = 0.089) increased to Φ = 0.199 with the addition of N2H4. In addition, the probe demonstrated 45-fold emission enhancement at 560 nm in viscous media, with a color change from non-fluorescence to yellow fluorescence. Good hydrazine sensing features with high adaptability, selectivity, sensitivity, ratiometric and fast response (90 s), low cytotoxicity (more than 90% of cell viability), low detection limit (86.0 nM), good linearity in the range of 0-35.0 μM, and high signal-to-noise ratio sensing capability were achieved. The hydrazine-sensing capability of the mitochondria-targetable probe in living cells makes it a strong candidate for various biological and environmental applications, including intracellular tracking and imaging. These results suggest that the present probe shows significant potential for the effective fluorescence detection of hydrazine.

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.

A novel near-infrared fluorescent probe for ultrasensitive and visual detection of mitochondrial viscosity

The development of fluorescent probes capable of detecting abnormal changes in cellular mitochondrial viscosity is of great significance, as these changes have been connected to many diseases. In this study, the conventional tetraphenylethylene (TPE) molecule was modified to fabricate a novel near-infrared fluorescent, TTPB, which was then used to measure the mitochondrial viscosity. Due to the introduction of TPE and pyridine groups, TTPB had an AIE effect and mitochondrial targeting function. Meanwhile, TTPB was extremely sensitive to variations in viscosity for the twisted intramolecular charge transfer (TICT) phenomenon. The logarithm of fluorescence intensity (logI620) of the probe demonstrated an excellent linear connection with the logarithm of viscosity (logη) in the viscosity range of 1.2 ∼ 956.0 cP, indicating the probe could quantitatively detect viscosity. Moreover, TTPB was able to visually track autophagy in addition to detecting the mitochondrial viscosity in the inflammatory cell model. These results showed that the probe was anticipated to be employed for the early diagnosis of related diseases.

Dual role far red fluorescent molecular rotor for decoding the plasma membrane and mitochondrial viscosity

The dysfunctions in the mitochondria are associated with various pathological conditions like neurodegeneration, metabolic disorder, and cancer, leading to dysregulated cell death. Here, we have designed and synthesized a julolidine-based molecular rotor (JMT) to target mitochondria with far-red emission accounting for mitochondrial dysfunction. JMT showed viscosity sensitivity with 160-fold enhancement in fluorescence intensity. The origin of the dark state in a lower viscous environment was investigated through density functional calculations. We have employed JMT to monitor mitochondrial dysfunction induced by nystatin using confocal and fluorescence lifetime imaging microscopy. Further, we investigated mitochondrial abnormalities under inflammatory conditions triggered by lipopolysaccharide in live HeLa cells. The cellular uptake mechanisms of JMT were studied using various endocytosis inhibitors. Moreover, we reported tracking small fluorescent molecule switching from mitochondria to the plasma membrane upon introducing mitochondrial depolarizer in cells. On treating the mitochondria potential uncoupler, JMT relocates to the cell membrane and can be utilized for understanding the interplay between mitochondria and cell membranes. Moreover, JMT was applied to stain the RBC plasma membrane isolated from human blood.

A benzothiazole-salt-based fluorescent probe for precise monitoring the changes of pH and viscosity

This work presented a novel ratiometric fluorescent probe (NBO) based on benzothiazole dye, which could monitor the pH fluctuations with high sensitivity via the intramolecular charge transfer (ICT) process. NBO was developed with a good linear response in the pH range of 5.75-7.00 (pKa = 6.5) and a reversible structural change in acidic and alkaline environments. Besides, NBO also has the potential to detect the viscosity changes. Meanwhile, NBO has been successfully applied to the pH monitoring of a variety of water samples in natural environment and human serum. With the treatment of different solutions at pH 2.0 - pH 9.0, the test strips showed significant color changes under both 365 nm UV lamp and room light. When the test strips were applied to white wine, pH could be detected quickly and easily by the naked eyes. Therefore, a novel probe that can be used to detect pH in environment, human serum and food has been successfully developed.

Recent advances in fluorescent probes for dual-detecting ONOO- and analytes

Although peroxynitrite (ONOO-) plays an essential role in cellular redox homeostasis, its excess ONOO- will affect the normal physiological function of cells. Therefore, real-time monitoring of changes in local ONOO- will contribute to further revealing the biological functions. Reliable and accurate detection of biogenic ONOO- will definitely benefit for disentangling its complex functions in living systems. In the past few years, more fluorescent probes have been developed to help understand and reveal cellular ONOO- changes. However, there has been no comprehensive and critical review of multifunctional fluorescent probes for cellular ONOO- and other analytes. To highlight the recent advances, this review first summarized the recent progress of multifunctional fluorescent probes since 2018, focusing on molecular structures, response mechanisms, optical properties, and biological imaging in the detection and imaging of cellular ONOO- and analytes. We classified and discussed in detail the limitations of existing multifunctional probes, and proposed new ideas to overcome these limitations. Finally, the challenges and future development trends of ONOO- fluorescence probes were discussed. We hoped this review will provide new research directions for developing of multifunctional fluorescent probes and contribute to the early diagnosis and treatment of diseases.

Theoretical Insights into a Near-Infrared Fluorescent Probe NI-VIS Based on the Organic Molecule for Monitoring Intracellular Viscosity

So many biological functional disorders and diseases, such as atherosclerosis, hypertension, diabetes, Alzheimer's disease, as well as cell malignancy are closely related with the intracellular viscosity. A safe and effective intracellular viscosity detecting method is desired by the biomedical community. Recently, a novel near-infrared fluorescent probe NI-VIS with a twisting intramolecular charge transfer mechanism was developed. The capability of this probe to visualize the viscosity variation in cirrhotic liver tissues and map the micro viscosity in vivo were testified using an experiment. In this work, the twisting intramolecular charge transfer mechanism and fluorescent properties of the probe NI-VIS were studied in detail under quantum mechanical method. The low energy barrier among the different conformations of the probe indicated the occurrence of twisting intramolecular charge transfer due to the rotation of the aryl group in the probe molecule while within the low viscosity environment. The electronic structure analysis on different probe conformations revealed the electron transfer process of the probe under optical excitation. All these theoretical results could provide insights into understand in greater depth the principles and build highly effective fluorescent probe to monitor the viscosity in biological samples.

Detection of breast cancer cells by a near-infrared fluorescent probe targeting mitochondrial viscosity

Monitoring abnormal viscosity in biological systems is important for basic research and clinical applications. Fluorescence imaging technology is adaptable for the visualization of tumor tissues due to its comprehensive features. However, fluorescence detection of intracellular viscosity in clinical samples remains challenging. We developed a promising near-infrared fluorescent probe, M556, for viscosity measurement. M556, which targets mitochondria, was successfully applied to monitor the mitochondrial viscosity in living cells. Furthermore, M556 was demonstrated to effectively discriminate tumors from normal tissues in a mouse tumor model and in clinical specimens from breast cancer patients, thus indicating the potential perioperative use of this probe by clinicians to assist with biopsy procedures.

Specific Fluorescent Probes for Imaging DNA in Cell-Free Solution and in Mitochondria in Living Cells

New styryl dyes consisting of N-methylpyridine or N-methylquinoline scaffolds were synthesized, and their binding affinities for DNA in cell-free solution were studied. The replacement of heterocyclic residue from the pyridine to quinoline group as well as variation in the phenyl part strongly influenced their binding modes, binding affinities, and spectroscopic responses. Biological experiments showed the low toxicity of the obtained dyes and their applicability as selective dyes for mitochondria in living cells.

Comprehensive monitoring of mitochondrial viscosity variation during different cell death processes by a NIR mitochondria-targeting fluorescence probe

Cell death is a fundamental feature of multicellular organisms, in which mitochondria play crucial roles. Therefore, revealing and monitoring the microenvironment of mitochondria are significant to investigate cell death process. Herein, the mitochondrial viscosity variation behaviors of a series of different cell death processes were monitored with a NIR mitochondria-targeting fluorescence probe FLV. FLV was designed based on a rotatable flavylocyanine fluorophore that presented selective and sensitive NIR fluorescence enhancement response with the increase of environmental viscosity. Fluorescence imaging experiments of living cells incubated with nystatin or under different temperature indicated that FLV was capable of imaging the change of mitochondrial viscosity. Finally, FLV was applied for monitoring the mitochondrial viscosity variation during different cell death processes. It was found that there were obvious mitochondrial viscosity increases during apoptosis, necrosis and autophagy; however, no detectable mitochondrial viscosity variation was observed in ferroptosis process incubated with ferroptosis inducer erastin or RSL3 for 6 h. These results demonstrated that FLV is a viable tool for monitoring the mitochondrial viscosity variation and is likely to be used in the diagnosis of the mitochondrial viscosity-associated cell processes and diseases.

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.