Mitochondria-targeted ratiometric fluorescent probes for micropolarity and microviscosity and their applications

Mitochondria-Targeting Multifunctional Fluorescent Probe toward Polarity, Viscosity, and ONOO- and Cell Imaging

Abnormal changes occurring in the mitochondrial microenvironment are important markers indicating mitochondrial and cell dysfunction. Herein, we designed and synthesized a multifunctional fluorescent probe DPB that responds to polarity, viscosity, and peroxynitrite (ONOO^-). DPB is composed of an electron donor (diethylamine group) and electron acceptor (coumarin, pyridine cations, and phenylboronic acid esters), in which the pyridine group with a positive charge is responsible for targeting to mitochondria. D-π-A structure with strong intramolecular charge transfer (ICT) and twisted intramolecular charge transfer (TICT) properties give rise to respond to polarity and viscosity. The introduction of cyanogroup and phenylboronic acid esters increases the electrophilicity of the probe, which is prone to oxidation triggered by ONOO^-. The integrated architecture satisfies the multiple response requirements. As the polarity increases, the fluorescence intensity of probe DPB at 470 nm is quenched by 97%. At 658 nm, the fluorescence intensity of DPB increases with viscosity and decreases with the concentration of ONOO^-. Furthermore, the probe is not only successfully used to monitor mitochondrial polarity, viscosity, and endogenous/exogenous ONOO^- level fluctuations but also to distinguish cancer cells from normal cells by multiple parameters. Therefore, as-prepared probe provides a reliable tool for better understanding of the mitochondrial microenvironment and also a potential approach for the diagnosis of disease.

Polarity-Sensitive and Membrane-Specific Probe Quantitatively Monitoring Ferroptosis through Fluorescence Lifetime Imaging

As a new form of regulated cell death, ferroptosis is closely related to various diseases. To interpret this biological behavior and monitor related pathological processes, it is necessary to develop appropriate detection strategies and tools. Considering that ferroptosis is featured with remarkable lipid peroxidation of various cell membranes, it is logical to detect membranes' structural and environmental changes for the direct assessment of ferroptosis. For this sake, we designed novel polarity-sensitive fluorescent probes Mem-C₁C₁₈ and Mem-C₁₈C₁₈, which have superior plasma membrane anchorage, high brightness, and sensitive responses to environmental polarity by changing their fluorescence lifetimes. Mem-C₁C₁₈ with much less tendency to aggregate than Mem-C₁₈C₁₈ outperformed the latter in high resolution fluorescence labeling of artificial vesicle membranes and plasma membranes of live cells. Thus, Mem-C₁C₁₈ was selected to monitor plasma membranes damaged along ferroptosis process for the first time, in combination with the technique of fluorescence lifetime imaging (FLIM). After treating HeLa cells with Erastin, a typical ferroptosis inducer, the mean fluorescence lifetime of Mem-C₁C₁₈ displayed a considerable increase from 3.00 to 4.93 ns, with a 64% increase (corresponding to the polarity parameter Δf increased from 0.213 to 0.232). Therefore, our idea to utilize a probe to quantitate the changes in polarity of plasma membranes proves to be an effective method in the evaluation of the ferroptosis process.

Approaching high performance Ni(Co) molybdate electrode materials for flexible hybrid devices

Transition metal molybdates are widely used as electrode materials for supercapacitors due to their high theoretical specific capacitance and excellent electrochemical activity. It is an effective strategy to improve the electrochemical performance by reasonably designing electrode materials. Herein, we prepared several Ni_1-xCo_xMoO₄·nH₂O nanostructures through facile hydrothermal strategies. Benefiting from the synergistic effects between Ni and Co elements, the obtained electrode materials delivered a specific capacitance of 1390 F g⁻¹ at 1 A g⁻¹. The as-assembled device shows an energy density of 72.45 W h kg⁻¹ at a power density of 2688.8 W kg⁻¹. It also presents excellent cycling and mechanical stability after many times of charging and discharging processes and bending.

We prepare several Ni_1−xCo_xMoO₄·nH₂O nanostructures through facile hydrothermal strategies. The obtained electrode materials delivered a specific capacitance of 1390 F g⁻¹ at 1 A g⁻¹. The as-assembled device shows an energy density of 72.45 W h kg⁻¹ at a power density of 2688.8 W kg⁻¹.

Synthesis of a new environment-sensitive fluorescent probe based on TICT and application for detection of human serum albumin and specific lipid droplets imaging

Environment-sensitive fluorescent probes have always been as forceful tools to understand the pathophysiological processes of relevant diseases. In this work, a new fluorescent probe with typical D-π-A structure was designed and showed high sensitivity to polarity and viscosity changes. DPAR could selectively detect human serum albumin (HSA) with turn-on orange emission in aqueous PBS buffer (pH 7.4), which showed advantages such as rapid response (4 min), high sensitivity (LOD 0.98 μg/mL). Therefore, it was successfully used for achieving HSA levels in urine samples and HSA imaging in HeLa cells. DPAR also exhibited the capability to recognize the cancer cells over the normal cells by lower polarity guided lipid droplets (LDs) imaging (in green emission channel). The detection mechanism for HSA and cancer diagnosis was convinced that DPAR encountered the lower-polarity and higher-viscosity microenvironment, resulting in the confinement of the TICT process and intramolecular rotation. These facts showed that DPAR had good application prospects in environment-related biomedical research and clinical diagnosis.

TICT-Based Microenvironment-Sensitive Probe with Turn-on Red Emission for Human Serum Albumin Detection and for Targeting Lipid Droplet Imaging

Fluorescent probes sensitive to microenvironment have always been fascinating due to their tremendous advantages in tracking changes in the pathophysiological microenvironment and potential application in the early diagnosis of related diseases. In this study, a fluorescent luminogen, triphenylamine-thiophene-rhodanine (TPA-TRDN), with high sensitivity to changes in polarity and viscosity was designed and could be applied to detecting human serum albumin (HSA) in actual urine, as well as lipid droplets (LDs) in cells and in vivo with turn-on red emission. TPA-TRDN could selectively detect HSA with fast response (10 min), superior sensitivity (LOD 0.34 μg/mL, about 60-fold fluorescence enhancement), and wide detection range (0.00-0.30 mg/mL). The detection mechanism was demonstrated: TPA-TRDN encountered the hydrophobic IB domain of HSA, leading to the inhibition of the twisted intramolecular charge transfer (TICT) phenomenon and intramolecular rotation. Moreover, TPA-TRDN demonstrated satisfactory ability to identify cancer cells and noncancer cells by microenvironment-guided specific LD bioimaging. This evidence indicated that TPA-TRDN has promising application in the microenvironment-related biomedical field and clinical diagnosis.

Small molecule based fluorescent chemosensors for imaging the microenvironment within specific cellular regions

The microenvironment (local environment), including viscosity, temperature, polarity, hypoxia, and acidic-basic status (pH), plays indispensable roles in cellular processes. Significantly, organelles require an appropriate microenvironment to perform their specific physiological functions, and disruption of the microenvironmental homeostasis could lead to malfunctions of organelles, resulting in disorder and disease development. Consequently, monitoring the microenvironment within specific organelles is vital to understand organelle-related physiopathology. Over the past few years, many fluorescent probes have been developed to help reveal variations in the microenvironment within specific cellular regions. Given that a comprehensive understanding of the microenvironment in a particular cellular region is of great significance for further exploration of life events, a thorough summary of this topic is urgently required. However, there has not been a comprehensive and critical review published recently on small-molecule fluorescent chemosensors for the cellular microenvironment. With this review, we summarize the recent progress since 2015 towards small-molecule based fluorescent probes for imaging the microenvironment within specific cellular regions, including the mitochondria, lysosomes, lipid drops, endoplasmic reticulum, golgi, nucleus, cytoplasmic matrix and cell membrane. Further classifications at the suborganelle level, according to detection of microenvironmental factors by probes, including polarity, viscosity, temperature, pH and hypoxia, are presented. Notably, in each category, design principles, chemical synthesis, recognition mechanism, fluorescent signals, and bio-imaging applications are summarized and compared. In addition, the limitations of the current microenvironment-sensitive probes are analyzed and the prospects for future developments are outlined. In a nutshell, this review comprehensively summarizes and highlights recent progress towards small molecule based fluorescent probes for sensing and imaging the microenvironment within specific cellular regions since 2015. We anticipate that this summary will facilitate a deeper understanding of the topic and encourage research directed towards the development of probes for the detection of cellular microenvironments.

Mitochondria-Anchored Molecular Thermometer Quantitatively Monitoring Cellular Inflammations

Temperature in mitochondria can be a critical indicator of cell metabolism. Given the highly dynamic and inhomogeneous nature of mitochondria, it remains a big challenge to quantitatively monitor the local temperature changes during different cellular processes. To implement this task, we extend our strategy on mitochondria-anchored thermometers from "on-off" probe Mito-TEM to a ratiometric probe Mito-TEM 2.0 based on the Förster resonance energy transfer mechanism. Mito-TEM 2.0 exhibits not only a sensitive response to temperature through the ratiometric changes of dual emissions but also the specific immobilization in mitochondria via covalent bonds. Both characters support accurate and reliable detection of local temperature for a long time, even in malfunctioning mitochondria. By applying Mito-TEM 2.0 in fluorescence ratiometric imaging of cells and zebrafishes, we make a breakthrough in the quantitative visualization of mitochondrial temperature rises in different inflammation states.

A novel dual-site ICT/AIE fluorescent probe for detecting hypochlorite and polarity in living cells

A novel dual-site fluorescent probe (CTPA) was rationally designed and synthesized for the detection of hypochlorite (ClO−) and polarity.

Microenvironment-sensitive iridium(iii) complexes for disease theranostics

Microenvironmental parameters, including hypoxia, pH, polarity, viscosity and temperature, play pivotal roles in controlling the biological, physical or chemical behaviors of local molecules. Abnormal changes in these parameters would cause cellular malfunction or become a hallmark of the occurrence of severe diseases. Recently, a number of phosphorescent Ir(iii) complexes have been designed to respond to such parameters due to their attractive properties such as high photostability, long emission lifetimes, and environment-sensitive emission profiles. This review aims to provide a summary of the progress achieved in developing iridium-based probes responding to microenvironmental parameters in biological systems in recent years for diagnosis and treatment of diseases such as cancer and diabetes.

A Threshold-Limited Fluorescence Probe for Viscosity

Viscosity of body fluid is an established biomarker of pathological conditions. Abnormality of cellular viscosity occurs when cells are challenged with external stresses. Small molecule probes to assess the viscosity are sought after for both disease diagnostics and basic studies. Fluorescence based probes are particular attractive due to their potentials for convenient and high spatiotemporal resolution microscopic monitoring of biological samples. The dyes with a floppy push-pull backbone or dyes with a rotatable substituent exhibits a viscosity responsive fluorescence enhancement and therefore viable viscosity probes. The scaffold of the existing viscosity probes contains typically one such floppy site. Therefore, they typically linearly respond to log(viscosity). We argue that minor viscosity fluctuation could potentially be physiological as the biological system is dynamic. We wish to develop a type of conceptually-new, threshold-limited viscosity probes, to complement the existing probes. Such probes do not exhibit a fluorescence enhancement when challenged with minor and presumably physiological enhancement of viscosity. When the viscosity is higher than a certain threshold, their fluorescence turns on. We hypothesize that a dye with two far-apart floppy sites could potentially yield such a threshold-limited signal and designed VPZ2 and VPZ3. Through spectral titration, VPZ3 was found to yield the desired threshold-limited signal. VPZ3 was suitable for in vitro bioimaging of viscosity under one-photon or two-photon excitation. VPZ3 is potentially useful in many downstream applications. Future work includes fine-tune of the threshold to allow tailored limit for fluorescence turn-on to better meet the need of different applications. Besides the implications in the real-world applications, the design concept could also be translated to design of alternative substrates.

Transition metal oxide@hydroxide assemblies as electrode materials for asymmetric hybrid capacitors with excellent cycling stabilities

In this work, three-dimensional cactus-like Co₃O₄@Ni(OH)₂ electrode materials are grown directly on Ni foam via a two-step hydrothermal method. The as-prepared products possess a specific capacitance of 464.5 C g⁻¹ at 0.5 A g⁻¹ and 91.67% capacitance retention after 20 000 cycles. The as-assembled device using the as-synthesized samples as positive electrodes delivers an energy density of 112.5 W h kg⁻¹ at a power density of 1350 W h kg⁻¹. The superior electrochemical performance of the electrode materials can be attributed to their unique structure, the synergistic effect between Co₃O₄ and Ni(OH)₂ materials and reversible reaction kinetics. It suggests that the products are potential alternatives in future energy storage devices.

In this work, three-dimensional cactus-like Co₃O₄@Ni(OH)₂ electrode materials are grown directly on Ni foam via a two-step hydrothermal method.