Mitochondria-targeted ratiometric fluorescent probe for real time monitoring of pH in living cells

A ratiometric fluorescent probe with dual-targeting capability for heat shock imaging

HSO3- is an important reactive sulfur species that maintains the normal physiological activities of living organisms and participates in a variety of redox homeostatic processes. It has been found that changes in HSO3- levels is closely related to the heat stroke phenomenon of the organism. Heat stroke causes damage to normal cells, which in turn causes damage to the body and even death. It is crucial to accurately monitor and track the physiological behavior of HSO3- during heat stroke. Herein, a ratiometric multifunctional fluorescent probe DRM-SO2 with dual-targeting ability to rapidly and precisely recognize HSO3- being constructed based on the FRET mechanism. DRM-SO2 has extra Large Stokes shift (216 nm), very high sensitivity (DL = 12.2 nM), fast response time and good specificity. When DRM-SO2 undergoes Michael addition with HSO3-, the fluorescence emission peak was blue-shifted from 616 nm to 472 nm, and a clear ratiometric signal appeared. The interaction between lysosomes and mitochondria in maintaining cellular homeostasis was investigated by the dual-targeting ability of the probe using HSO3- as a mediator. DRM-SO2 achieved successful targeting and real-time monitoring of exogenous and endogenous HSO3- in the cells. More importantly, imaging experiments in heat stroke mice revealed high HSO3- expression in intestinal tissues. This provides new ideas and research tools for early prevention of heat stroke-induced diseases such as intestinal injuries. In addition, the semi-quantitative monitoring experiments for paper-based visualization of HSO3- make the probe promising for the design of portable detectors.

Organic fluorophores-based molecular probes with dual-fluorescence ratiometric responses to in-vitro/in-vivo pH for biosensing, bioimaging and biotherapeutics applications

In recent years, organic fluorophores-based molecular probes with dual-fluorescence ratiometric responses to in-vitro/in-vivo pH (DFR-MPs-pH) have been attracting much interest in fundamental application research fields. More and more scientific publications have reported the exploration of various DFR-MPs-pH systems that have unique dual-fluorescence ratiometry as the signal output, in-built and signal self-calibration functions to improve precise detection of targets. DFR-MPs-pH systems possess high-performance applications in biosensing, bioimaging and biomedicine fields. This review has comprehensively summarized recent advances of DFR-MPs-pH for the first time. First of all, the compositions and types of DFR-MPs-pH are introduced by summarizing different organic fluorophores-based molecule systems. Then, construction strategies are analyzed based on specific components, structures, properties and functions of DFR-MPs-pH. Afterward, biosensing and bioimaging applications are discussed in detail, primarily referring to pH sensing and imaging detection at the levels of living cells and small animals. Finally, biomedicine applications are fully summarized, majorly involving bio-toxicity evaluation, bio-distribution, biomedical diagnosis and therapeutics. Meanwhile, the current status, challenges and perspectives are rationally commented after detailed discussions of representative and state-of-the-art studies. Overall, this present review is comprehensive, in-time and in-depth, and can facilitate the following further exploration of new and versatile DFR-MPs-pH systems toward rational design, facile preparation, superior properties, adjustable functions and highly efficient applications in promising fields.

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.

Acid-base responsive multifunctional poly(formyl sulfide)s through a facile catalyst-free click polymerization of aldehyde-activated internal diynes and dithiols

Acid-base equilibria play a critical role in biological processes and environmental systems. The development of innovative fluorescent polymeric materials to monitor acid-base equilibria is highly desirable. Herein, a novel catalyst-free click polymerization of aldehyde-activated internal diynes and dithiols was established, and exclusively Markovnikov poly(formyl sulfide)s (PFSs) with high molecular weights and moderate stereoregularity were produced in high yields. Because of the aromatic units and sulfur atoms in their main chains, these polymers possessed high refractive index values. By introducing the fluorene and aldehyde moieties, the resulting PFSs could act as a fluorescent sensor for sensitive hydrazine detection. Taking advantage of the reaction of the aldehyde group and hydrazine, imino-PFSs with remarkable and reversible fluorescence change through alternating fumigation with HCl and NH3 were easily acquired and further applied in multicolor patterning, a rewritable material and quadruple-mode information encryption. Additionally, a test strip of protonated imino-polymer for the tracking of bioamines in situ generated from marine product spoilage was also demonstrated. Collectively, this work not only provides a powerful click polymerization to enrich the multiplicity of sulfur-containing materials, but also opens up enormous opportunities for these functional polysulfides in diverse applications.

A pH-responsive sensor based on intramolecular internal standard for reproducible detection of strong acids and bases via 19F NMR spectroscopy

Numerous methods, including pH meters and optical sensors, have been developed for the detection of pH, which is an important indicator in various fields. However, those methods are susceptible to errors in strongly acidic and basic ranges and inaccurate pH measurement due to sample turbidity, hindering their application such as photographic industries and wastewater treatment facilities. Eco-friendly and non-invasive ¹⁹F NMR spectroscopy is a promising technique for measurement of strong acids and bases owing to its high sensitivity and little interference; nevertheless, inconsistencies in reproducibility impede its widespread adoption. Herein, we developed a¹⁹F NMR-based pH sensor by introducing an intramolecular internal standard strategy into a pH-responsive fluorinated material. Based on the acceptable deviation (Δδ_F = 17-19 ppb) in the evaluation of the internal standard signal, this pH-sensing platform enabled reproducible pH measurements in strongly acidic and basic environments. Moreover, its ¹⁹F NMR response showed reversibility and high stability to potential interfering factors, and the low absolute difference (0.026-0.086 in pH) for real samples such as diet Coke suggests its potential suitability for various acidic beverages.

Ratiometric fluorescent probes for pH mapping in cellular organelles

The intracellular pH (pHi) in organelles, including mitochondria, endoplasmic reticulum, lysosomes, and nuclei, differs from the cytoplasmic pH, and thus maintaining the pH of these organelles is crucial for cellular homeostasis. Alterations in the intracellular pH (ΔpHi) in organelles lead to the disruption of cell proliferation, ion transportation, cellular homeostasis, and even cell death. Hence, accurately mapping the pH of organelles is crucial. Accordingly, the development of fluorescence imaging probes for targeting specific organelles and monitoring their dynamics at the molecular level has become the forefront of research in the last three decades. Among them, ratiometric fluorescent probes minimize the interference from the excitation wavelength of light, auto-fluorescence from probe concentration, environmental fluctuations, and instrument sensitivity through self-correction compared to monochromatic fluorescent probes, which are known as turn-on/off fluorescent probes. Small-molecular ratiometric fluorescent probes for detecting ΔpHi are challenging yet demanding. To date, sixty-two ratiometric pH probes have been reported for monitoring internal pH alterations in cellular organelles. However, a critical review on organelle-specific ratiometric probes for pH mapping is still lacking. Thus, in the present review, we report the most recent advances in ratiometric pH probes and the previous data on the role of mapping the ΔpHi of cellular organelles. The development strategy, including ratiometric fluorescence with one reference signal (RFRS) and ratiometric fluorescence with two reversible signals (RFRvS), is systematically illustrated. Finally, we emphasize the major challenges in developing ratiometric probes that merit further research in the future.

Two quinoline-based two-photon fluorescent probes for imaging of viscosity in subcellular organelles of living HeLa cells

Two viscosity-sensitive two-photon fluorescent probes (QL and QLS) were designed and synthesized, which can be localized in lysosome and mitochondria in living HeLa cells, respectively. As the increases of viscosity from 2.55 to 1150 cP, the fluorescence quantum yield (Φ) of QL and QLS was increased by 28-fold and 37-fold, respectively. At the same time, its effective two-photon absorption cross section (Φδ_TPA) was enhanced by 15-fold and 16-fold, respectively. Fluorescence lifetime imaging (FLIM) of living HeLa cells stained with QL and QLS, revealed that lysosomal viscosity ranged from 100.76 to 254.74 cP and mitochondrial viscosity ranged from 92.21 to 286.79 cP. This type of fluorescent probe is helpful in the design and application of materials for monitoring diseases associated with abnormal viscosity.

PET- and ICT-Based Ratiometric Probe: An Unusual Phenomenon of Morpholine-Conjugated Fluorophore for Mitochondrial pH Mapping during Mitophagy

Mitochondrial functions are heavily influenced by acid-base homeostasis. Hence, elucidation of the mitochondrial pH is essential in living cells, and its alterations during pathologies is an interesting question to be addressed. Small molecular fluorescent probes are progressively applied to quantify the mitochondrial pH by fluorescence imaging. Herein, we designed a unique small molecular fluorescent probe, PM-Mor-OH, based on the lipophilic morpholine ligand-conjugated pyridinium derivative of "IndiFluors". The morpholine-conjugated fluorescent probe usually localized the lysosome. However, herein, we observed unusual phenomena of morpholine-tagged PM-Mor-OH that localized mitochondria explicitly. The morpholine ligand also plays a pivotal role in tuning optical properties via photoinduced electron transfer (PET) during internal pH alteration (ΔpHi). In the mitophagy process, lysosomes engulf damaged mitochondria, leading to ΔpHi, which can be monitored using our probe. It exhibited "ratiometric" emission at single wavelength excitation (ex. 488) and is suitable for monitoring and quantifying the ΔpHi using confocal microscope high-resolution image analysis during mitophagy. The bathochromic emission shifts due to intramolecular charge transfer (ICT) in basic pH were well explained by the time-dependent density functional theory (TD-DFT/PCM). Similarly, the change in the emission ratio (green/red) with pH variations was also validated by the PET process. In addition, PM-Mor-OH can quantify the pH change during oxidative stress induced by rapamycin, mutant A53T α-synuclein-mediated protein misfolding stress in mitochondria, and during starvation. Rapamycin-induced mitophagy was further elucidated by the translocation of mCherry Parkin to damaged mitochondria, which well correlates with our probe. Thus, PM-Mito-OH is a valuable probe for visualizing mitophagy and can act as a suitable tool for the diagnosis of mitochondrial diseases.

A novel mitochondria-targetable NIR fluorescent probe for monitoring intracellular hypobromous acid levels

The development of ultrasensitive in situ detection techniques for monitoring hypobromous acid (HBrO) levels in the biological systems is of great significance to reveal its complex pathological and physiological effects. A simple mitochondria-targetable hydrazine-based near-infrared (NIR) fluorescent probe (Mito-NIR) for detecting HBrO in the mitochondria of live cells is presented in this paper. Probe Mito-NIR displays the ultrafast (< 5 s) response for HBrO. It can detect HBrO with high sensitivity. Additionally, it shows high selectivity towards HBrO over other biologically important substances. Finally, it can monitor the changes of endogenous/exogenous HBrO levels in the mitochondria of live cells. A simple mitochondria-targetable NIR fluorescent probe with picomolar sensitivity for HBrO was developed to specifically track mitochondrial HBrO.

A water-soluble 1, 8-naphthalimide-based fluorescent pH probe for distinguishing tumorous tissues and inflammation mice

A water-soluble fluorescent probe BPN, by introducing a piperazine as the pH-sensitive fluorescence signaling motif to the hydrophilic propionic acid-substituted 1, 8-naphthalimide fluorophore, is highly sensitive to pH changes within cytoplasm matrix in living cells, as well as pH-related diseases models. Owing to the protonation-induced inhibition of the photoinduced electron transfer (PET) from piperazine to naphthalimide fluorophore, BPN displayed a significant fluorescence enhancement (more than 131-fold) upon the pH decreasing from 11.0 to 3.0. The linear rang was between pH 6.4 to 8.0 with a pK_a value of 6.69 near the physiological pH, which was suitable for cytosolic pH research. Furthermore, BPN exhibited a large Stokes shift (142 nm), good water solubility, excellent photostability, high selectivity and low cytotoxicity. All these advantages were particularly beneficial for intracellular pH imaging. Using BPN, we demonstrated the real-time monitoring of cytosolic pH changes in living cells. Most importantly, BPN has not only been successfully applied for distinguishing inflammation mice, but also the surgical specimens of cancer tissue, making it of great potential application in the cancer diagnosis.

Imaging and Measuring Vesicular Acidification with a Plasma Membrane-Targeted Ratiometric pH Probe

Tracking the pH variation of intracellular vesicles throughout the endocytosis pathway is of prior importance to better assess the cell trafficking and metabolism of cells. Small molecular fluorescent pH probes are valuable tools in bioimaging but are generally not targeted to intracellular vesicles or are directly targeted to acidic lysosomes, thus not allowing the dynamic observation of the vesicular acidification. Herein, we designed Mem-pH, a fluorogenic ratiometric pH probe based on chromenoquinoline with appealing photophysical properties, which targets the plasma membrane (PM) of cells and further accumulates in the intracellular vesicles by endocytosis. The exposition of Mem-pH toward the vesicle's lumen allowed to monitor the acidification of the vesicles throughout the endocytic pathway and enabled the measurement of their pH via ratiometric imaging.

A multimodal fluorescent probe for portable colorimetric detection of pH and it's application in mitochondrial bioimaging

Mitochondria, as vital energy supplying organelles, play important roles in cellular metabolism, which are closely related with mitochondrial pH (∼8.0). In this work, a novel multimodal fluorescent probe was employed for ratiometric and colorimetric detection of pH. The probe is designed to work by controlling benzothiazole phenol-hemicyanine system as the interaction site and hemicyanine connected by conjugate bonds as the mitochondrial targeting, which also could make the fluorescence of probe red-shifted. This system results in a perfect ratiometric fluorescent response, whose emission changed from red to blue under pH 2.0-10.0, having a broad linear range (pH = 3.0-10.0). And the marked colour change (light yellow to deep purple via naked eye under pH 2.0-11.0) could be used to construct the test strip colorimetry and smartphone APP detection method, realizing the fast, portable, and accurate detection of pH in vitro and environment. Besides, the probe owns the characteristics of easy loading, high selectivity and staining ability of mitochondria, and low cytotoxicity, thereby allowing imaging of pH values and real-time monitor the subcellular mitochondria pH changes caused by drugs in living cells. It thus could be used to monitor the organ-specific dynamics related to transitions between pathological and physiological states.

A highly sensitive ratiometric fluorescent sensor for copper ions and cadmium ions in scallops based on nitrogen doped graphene quantum dots cooperating with gold nanoclusters

Based on the electrostatic interaction, we constructed a ratiometric fluorescence nanomixture of graphene quantum dots-gold nanoclusters (GQDs-AuNCs) for the quantitative detection of Cu²⁺ and Cd²⁺. When Cu²⁺ or Cd²⁺ was added into the reaction system, the fluorescence of GSH-AuNCs at 565 nm can be quenched by Cu²⁺ and enhanced by Cd²⁺ while the intensity of N-GQDs at 403 nm stayed constant. Under the optimized conditions, the fluorescence intensity ratio (I₅₆₅/I₄₀₃) of the GQDs-AuNCs system was proportional to the concentration of Cu²⁺ and Cd²⁺ in the range of 8×10^-8 mol/L-6×10^-6 mol/L and 1×10^-6 mol/L-4×10^-5 mol/L, respectively, with detection limits of 4.12×10^-9 mol/L and 9.43×10^-7 mol/L, respectively. In the presence of Cu²⁺ and Cd²⁺, the paper-based vision sensor would produce visible fluorescent color changes, which can be used for rapid detection on site. The method has been successfully applied to the determination of Cu²⁺ and Cd²⁺ in scallops with satisfactory results.

A new fluorescent probe for neutral to alkaline pH and imaging application in live cells

A new pH-sensitive fluorescent probe NAP-MDA was designed and synthesized. NAP-MDA consists of 1,8-naphthalimide as fluorophore, morpholine and N,N-dimethylethylenediamine as pH-responsive groups. Due to the photoinduced electron transfer (PET) mechanism, the fluorescence of 1, 8-naphthalimide was thoroughly quenched under alkaline condition (pH > 10.0), however, NAP-MDA displayed increasing fluorescence as the rise of acidity. Notably, NAP-MDA possessed an excellent linear dependence with neutral to alkaline pH (7.2-9.4), with a pKa of 8.38. NAP-MDA had good photostability and reversibility. Meanwhile, the probe was selective to pH without interference from common reactive species, temperature and viscosity. Fluorescent testing strips were fabricated with NAP-MDA and were successfully utilized to visualize the different pH with a handhold UV lamp. Confocal fluorescence imaging in live cells demonstrated that NAP-MDA mainly fluoresced in lysosomes, and could be applied for quantification of the pH within live cells.

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.

Mitokyne: A Ratiometric Raman Probe for Mitochondrial pH

Mitochondrial pH (pH_mito) is intimately related to mitochondrial function, and aberrant values for pH_mito are linked to several disease states. We report the design, synthesis, and application of mitokyne 1-the first small molecule pH_mito sensor for stimulated Raman scattering (SRS) microscopy. This ratiometric probe can determine subtle changes in pH_mito in response to external stimuli and the inhibition of both the electron transport chain and ATP synthase with small molecule inhibitors. In addition, 1 was also used to monitor mitochondrial dynamics in a time-resolved manner with subcellular spatial resolution during mitophagy providing a powerful tool for dissecting the molecular and cell biology of this critical organelle.

Construction of a Rational-Designed Multifunctional Platform Based on a Fluorescence Resonance Energy Transfer Process for Simultaneous Detection of pH and Endogenous Peroxynitrite

Peroxynitrite (ONOO^-), a kind of reactive oxygen species, plays an indispensable role in many physiological processes. The stability and reactivity of ONOO^- are significantly affected by the pH of the environment. A novel fluorescent probe RN-NA that can simultaneously respond to ONOO^- and pH was proposed and constructed based on a rational-designed multifunctional fluorescence resonance energy transfer (FRET) platform. The RN-NA probe exhibited a remarkably different fluorescence change in response to ONOO^- and pH. The fluorescence signals at 525 and 710 nm increased about 4-fold with a pH change from 8.0 to 3.0. The changes in fluorescence at 525 nm are mainly attributed to photo-induced electron transfer, and the fluorescence enhancement at 710 nm was mainly due to acid-induced open-closed circulation. In the presence of ONOO^-, the fluorescence at 525 nm increased 5-fold, while the fluorescence at 710 nm was almost completely diminished. Up to 70-fold fluorescence enhancement was observed in the ratiometric channel F₅₂₅/F₇₁₀. In the cell imaging experiment, the intracellular pH was adjusted using H⁺/K⁺ ionophore and nigericin, and the endogenous ONOO^- was generated by lipopolysaccharide (LPS) and γ-interferon (IFN-γ). The RN-NA probe can respond to cellular pH and endogenous ONOO^- with remarkable fluorescence changes in both red/green and ratiometric channels.

A Single Fluorescent pH Probe for Simultaneous Two-Color Visualization of Nuclei and Mitochondria and Monitoring Cell Apoptosis

Subcellular organelles play indispensable roles in diverse biological processes by their precise mutual cooperation. Thus, the development of a single fluorescent probe (SF-probe) for simultaneous and discriminable visualization of different organelles and their dynamics during certain bioprocess is significant, yet remains greatly challenging. Herein, for the first time, we rationally prepared a pH-sensitive SF-probe (named HMBI) for the simultaneous two-color visualization of nuclei and mitochondria and monitoring cell apoptosis. HMBI shows remarkable ratiometric fluorescence changes toward pH changes. Due to different pH environments in subcellular organelles, HMBI can image nuclei and mitochondria with green and red emission, respectively. HMBI can monitor drug-induced cell apoptosis with dramatically decreased red emission in mitochondria but almost unchanged green emission in nuclei, and the shrinking and pyknotic nuclei are also observed during cell apoptosis. HMBI possesses tremendous potential in two-color biomedical imaging of the dynamic changes of nuclei and mitochondria in many physiological processes.

Ratiometric Near-Infrared Fluorescent Probes Based on Hemicyanine Dyes Bearing Dithioacetal and Formal Residues for pH Detection in Mitochondria

Ratiometric near-infrared fluorescent probes (AH⁺ and BH⁺) have been prepared for pH determination in mitochondria by attaching dithioacetal and formal residues onto a hemicyanine dye. The reactive formyl group on probe BH⁺ allows for retention inside mitochondria as it can react with a protein primary amine residue to form an imine under slightly basic pH 8.0. Probes AH⁺ and BH⁺ display ratiometric fluorescent responses to pH changes through the protonation and deprotonaton of a hydroxy group in hemicyanine dyes with experimentally determined pKa values of 6.85 and 6.49, respectively. Calculated pK_a values from a variety of theoretical methods indicated that the SMD_BONDI method of accounting for solvent and van der Waals radii plus including a water molecule located near the site of protonation produced the closest overall agreement with the experimental values at 7.33 and 6.14 for AH⁺ and BH⁺ respectively.

A near-infrared fluorescent probe based on a hemicyanine dye with an oxazolidine switch for mitochondrial pH detection

A near-infrared fluorescent probe (AH+) has been prepared by incorporating an oxazolidine switch into a near-infrared hemicyanine dye. The probe shows fast and sensitive responses to pH from an oxazolidine switch to the hemicyanine dye upon pH decreases from 10.0 to 5.0. The probe shows good photostability, low cytotoxicity, and reversible fluorescence responses to pH changes with a pKa value of 7.6. It has been successfully used to determine pH changes in mitochondria.

Revealing Minor pH Changes of Mitochondria by a Highly Sensitive Molecular Fluorescent Probe

Mitochondrial pH is an important factor associated with cellular metabolism and pathological states. Thus, sensitively monitoring its minor change was essential. However, it was challengeable due to the lack of suitable probes. Here, a mitochondria-targeted probe (NIR-OH-1) was synthesized. Based on the protonation/deprotonation of the hydroxy group and the assistance of carboxyl group on NIR-OH-1 molecular structure, a dramatic NIR activated signal was generated for sensing pH. Probe NIR-OH-1 displayed a good photo-stability and reversibility and could detect pH change without interference by other biologically active species. Importantly, NIR-OH-1 had an appropriate pK_a value (7.77) and tiny acid-base transition range, which was allowed to map the small pH changes of cellular mitochondrial. Moreover, NIR-OH-1 was also successfully applied in real-time monitoring mitochondrial pH-related pathological events in living cells under different stimulation, demonstrating the prospect of its clinical application in accurate mitochondrial pH detection under related physiological and pathological conditions.

A novel ratiometric fluorescent probe for the detection of mitochondrial pH dynamics during cell damage

A sensitive fluorescent probe (E)-4-(3-(benzo[d]thiazol-2-yl)-4-hydroxy-5-methylstyryl)-1-methylpyridin-1-ium iodide (HBTMP) for the monitoring of pH in mitochondria was rationally exploited.

An ESIPT based versatile fluorescent probe for bioimaging live-cells and E. coli under strongly acidic conditions

A BTNN probe undergoes a 146 times increase in fluorescence intensity at 530 nm on lowering the pH from 7.0 to 2.0 and has been deployed for the bioimaging of MG-63 live cells and E. coli bacteria at different pH levels.

Plant-Inspired Multifunctional Fluorescent Hydrogel: A Highly Stretchable and Recoverable Self-Healing Platform with Water-Controlled Adhesiveness for Highly Effective Antibacterial Application and Data Encryption-Decryption

In recent years, hydrogels as an attractive class of intelligent soft materials have been applied in various advanced fields, including electronic materials, wearable devices, and wound dressing materials. However, it still remains a critical challenge to integrate information encryption transmission capability, antibacterial activity, high mechanical performance, adhesiveness, and self-healable ability into one material and achieve the synergistic characteristics through a simple method. In our study, a facile strategy of a plant-inspired hydrogel was proposed, which provides a novel initiator-free photo-cross-linked hydrogel system by simply mixing the coumarin derivative Pho-CA and the monomer in water, and then obtaining the hydrogel Gel-C-Am under the irradiation of UV light without adding any other cross-linking agents and initiators, and this process is very similar to the growth process of plants in nature. This novel hydrogel presents desirable mechanical properties (including twist, stretchability, and recoverability), which exhibits elongation of approximately 1600%. More interestingly, Gel-C-Am hydrogel displays reversible adhesiveness to various substrates (such as glass, paper, leaves, and rubber), and its adhesion properties can be regulated by water: the viscosity disappears when its surface becomes wet, and the viscosity will recover after the water evaporates. In addition, the developed hydrogel has certain self-healable ability. Two pieces of the Gel-C-Am hydrogel can combine together and reshape into one piece in water, and the fused hydrogel has uniform and interconnected pores under SEM. Based on the characteristic of Pho-CA whose fluorescence get recovery after UV irradiation, the hydrogel can be used in the field of encryption and decryption. Also, the resulting Gel-C-Am hydrogel shows an effective antibacterial activity and can potentially be addressed as antibacterial coatings. Taken together, the formation of the novel fluorescent hydrogel system is just like the growth of a plant in the presence of water and light, Pho-CA and the monomer will form a highly stretchable and recoverable self-healing hydrogel with water-controlled adhesiveness. The developed Gel-C-Am hydrogel shows favorable attributes and is suitable for applications in antibacterial polymeric coatings and information encryption transmission.

The fluorescence toolbox for visualizing autophagy

Autophagy is an adaptive catabolic process functioning to promote cell survival in the event of inappropriate living conditions such as nutrient shortage and to cope with diverse cytotoxic insults. It is regarded as one of the key survival mechanisms of living organisms. Cells undergo autophagy to accomplish the lysosomal digestion of intracellular materials including damaged proteins, organelles, and foreign bodies, in a bulk, non-selective or a cargo-specific manner. Studies in the past decades have shed light on the association of autophagy pathways with various diseases and also highlighted the therapeutic value of autophagy modulation. Hence, it is crucial to develop effective approaches for monitoring intracellular autophagy dynamics, as a comprehensive account of methodology establishment is far from complete. In this review, we aim to provide an overview of the major current fluorescence-based techniques utilized for visualizing, sensing or measuring autophagic activities in cells or tissues, which are categorized firstly by targets detected and further by the types of fluorescence tools. We will mainly focus on the working mechanisms of these techniques, put emphasis on the insight into their roles in biomedical science and provide perspectives on the challenges and future opportunities in this field.

Cytochrome c light-up graphene oxide nanosensor for the targeted self-monitoring of mitochondria-mediated tumor cell death

Targeting mitochondria-mediated apoptosis has emerged as a promising strategy for tumor therapy. However, technologies used to treat tumors that enable the direct visualization of mitochondria-mediated apoptosis in living cells have not been developed to date. Cytochrome c (Cyt c) translocation from mitochondria is a central mediating event in cell apoptosis. In this study, we developed a multifunctional nanosensor that can monitor the real-time translocation of Cyt c from mitochondria in living cells to evaluate the antitumor effect of dihydroartemisinin (DHA). A fluorophore-tagged DNA aptamer is loaded on a graphene oxide (GO)-based nanovehicle, and the cytosolic release of Cyt c causes the dissociation of the aptamer from the GO nanovehicle and triggers the emission of a red fluorescence signal. Furthermore, DHA linked with a coumarin derivative is loaded on GO as a mitochondria-targeting ligand to improve its antitumor activity. This DHA prodrug also emits a green fluorescence signal when delivered to mitochondria. This nanosensor provides a convenient mechanism to monitor mitochondrial targeting by drugs and mitochondria-induced therapeutic efficacy, which may be possible to diagnose the drug efficacy to optimize the treatment for patients with cancer.

A benzothiazolium-based fluorescent probe with ideal pKa for mitochondrial pH imaging and cancer cell differentiation

A mitochondrial pH sensing fluorescent probe namely 2-(2-(6-hydroxynaphthalen-2-yl)vinyl)-3-(6-(triphenyl-phosphonio)hexyl)benzothiazol-3-ium bromide (HTBT2) was designed and facilely synthesized via the Knoevenagel condensation reaction. HTBT2 displayed a linear fluorescence enhancement at 612 nm in response to pH changes between 8.70 and 7.20. The pK_a value was determined to be 8.04 ± 0.02, which might be ideal for mitochondrial pH (pH_mito∼8.0) detection. HTBT2 also exhibited a remarkable large Stokes shift of 176 nm, which could diminish the interference of excitation light. The results of live cell imaging studies suggested that HTBT2 showed excellent targeting ability for mitochondria. Importantly, it was successfully applied to visualize mitochondrial pH changes in live cells and differentiate the pH_mito difference between cancer cell lines and normal cell lines. Our results consistently supported that HTBT2 held practical promise for the investigation of physiological processes related to pH_mito changes and clinical potential for cancer cell differentiation.

Simple synthesis of multifunctional photosensitizers for mitochondrial and bacterial imaging and photodynamic anticancer and antibacterial therapy

Photosensitizers (PSs), a critical drug administered for successful photodynamic therapy (PDT), have been well researched regarding their anticancer or bactericidal capability with high precision and low invasiveness. Although traditional PSs have been explored either in photodynamic anticancer or in antibiosis, they usually require synthesis with multiple steps, harsh synthetic conditions, and a complicated purification process for a single targeted product. Therefore, developing new multifunctional PSs with a simple synthesis and reactant flexibility which combine mitochondrial and bacterial imaging, efficient photodynamic anticancer and antibacterial effects is of the utmost urgency and of great importance for clinical applications. Herein, a large structural investigation of isoquinolinium-based PSs synthesized by a simple Rh-catalysed annulation reaction with high yields is presented. These lipophilic cationic PSs have a tunable photophysical property. LIQ-6 was found to perform not only as an ideal mitochondria targeting probe but also an effective cancer cell killing PS, and moreover, a tracker for bacterial imaging and ablation. LIQ-6 can be used to image a wide range of cancer cells and to monitor the photo-induced cell apoptosis, and simultaneously, it can also image and be a photodynamic germicide for both Gram-positive and Gram-negative bacteria. Furthermore, LIQ-6 shows great effectiveness in the wound healing process, showing its ability to be an ideal PS in vivo as well. This contribution is believed to offer a new platform for the construction of a theragnostic system for future practical applications in biology and biomedicine.

Bis-cyclometallated Ir(III) complexes containing 2-(1H-pyrazol-3-yl)pyridine ligands; influence of substituents and cyclometallating ligands on response to changes in pH

Bis-cyclometallated Ir(iii) complexes containing 2-(1H-pyrazol-3-yl)pyridine ligands have been synthesised. Their absorption is almost unchanged with changes in pH however the emission intensities vary by a factor of up to three and the complexes have emission pKas in the range 8.0 to 10.0. Substituents on the pyrazole have only a minor effect on the emission pKa. Surprisingly the complexes with phenylpyrazole cyclometallated ligands 3aL1-3 showed an intensity decrease with increasing pH (switch off) whilst the corresponding phenylpyridine ones 3cL1-3 showed an increase in emission intensity with increasing pH. Putting electron-withdrawing CF3 substituents on the cyclometallating phenyls reduced the pKa of the complexes to 6.8-7.8, thereby extending the useful pKa range; however, in general it tended to reduce the magnitude of the change in emission intensity. Surprisingly the CF3-substituted complexes also showed a complete reversal in the direction of the intensity change when compared to their respective unsubstituted congeners.

Ratiometric Fluorescent Microgels for Sensing Extracellular Microenvironment pH during Biomaterial Degradation

Bone regeneration has attracted extensive attention in the field of regenerative medicine. The influence of biomaterial on the extracellular environment is important for regulating the biological functions of cells for tissue regeneration. Among the various influencing factors, we had previously demonstrated that the extracellular pH value in the local microenvironment during biomaterial degradation affected the balance of bone formation and resorption. However, there is a lack of techniques for conveniently detecting the pH of the extracellular environment. In light of the development of fluorescent pH-sensing probes, herein, we fabricated a novel ratiometric fluorescent microgel (F-MG) for real-time and spatiotemporal monitoring of microenvironment pH. F-MGs were prepared from polyurethane with a size of around 75 μm by loading with pH-sensitive bovine serum albumin nanoparticles (BNPs) and pH-insensitive Nile red as a reference. The pH probes exhibited reversible fluorescence response to pH change and worked in a linear range of 6-10. F-MGs were biocompatible and could be used for long-term pH detection. It could be used to map interfacial pH on biomaterials during their degradation through pseudocolored images formed by the fluorescence intensity ratio between the green fluorescence of BNPs and the red fluorescence of Nile red. This study provided a useful tool for studying the influence of biomaterial microenvironment on biological functions of surrounding cells.

In Situ Monitoring of Mitochondria Regulating Cell Viability by the RNA-Specific Fluorescent Photosensitizer

Cell viability is greatly affected by external stimulus eliciting correlated dynamical physiological processes for cells to choose survival or death. A few fluorescent probes have been designed to detect whether the cell is in survival state or apoptotic state, but monitoring the regulation process of the cell undergoing survival to death remains a long-standing challenge. Herein, we highlight the in situ monitor of mitochondria regulating the cell viability by the RNA-specific fluorescent photosensitizer L. At normal conditions, L anchored mitochondria and interacted with mito-RNA to light up the mitochondria with red fluorescence. With external light stimulus, L generated reactive oxide species (ROS) and cause damage to mitochondria, which activated mitochondrial autophagy to prevent death, during which the red fluorescence of L witnessed dynamical distribution in accordance with the evolution of vacuole structures containing damaged mitochondria into autophagosomes. However, with ROS continuously increasing, the mitochondrial apoptosis was eventually commenced and L with red fluorescent was gradually accumulated in the nucleoli, indicating the programmed cell death. This work demonstrated how the delicate balance between survival and death are regulated by mitochondria.

Mitochondria-targeted drug delivery in cancers

Mitochondria are considered one of the most important subcellular organelles for targeting and delivering drugs because mitochondria are the main location for various cellular functions and energy (i.e., ATP) production, and mitochondrial dysfunctions and malfunctions cause diverse diseases such as neurodegenerative disorders, cardiovascular disorders, metabolic disorders, and cancers. In particular, unique mitochondrial characteristics (e.g., negatively polarized membrane potential, alkaline pH, high reactive oxygen species level, high glutathione level, high temperature, and paradoxical mitochondrial dynamics) in pathological cancers have been used as targets, signals, triggers, or driving forces for specific sensing/diagnosing/imaging of characteristic changes in mitochondria, targeted drug delivery on mitochondria, targeted drug delivery/accumulation into mitochondria, or stimuli-triggered drug release in mitochondria. In this review, we describe the distinctive structures, functions, and physiological properties of cancer mitochondria and discuss recent technologies of mitochondria-specific "key characteristic" sensing systems, mitochondria-targeted "drug delivery" systems, and mitochondrial stimuli-specific "drug release" systems as well as their strengths and weaknesses.

Quantitative analysis of interactive behavior of mitochondria and lysosomes using structured illumination microscopy

Super-resolution optical microscopy has extended the spatial resolution of cell biology from the cellular level to the nanoscale, enabling the observation of the interactive behavior of single mitochondria and lysosomes. Quantitative parametrization of interactions between mitochondria and lysosomes under super-resolution optical microscopy, however, is currently unavailable, which has severely limited our understanding of the molecular machinery underlying mitochondrial functionality. Here, we introduce an M-value to quantitatively investigate mitochondria and lysosome contact (MLC) and mitophagy under structured illumination microscopy. We found that the M-value for an MLC is typically less than 0.4, whereas in mitophagy it ranges from 0.5 to 1.0. This system permits further investigation of the detailed molecular mechanism governing the interactive behavior of mitochondria and lysosomes.

Design and applications of a novel fluorescent probe for detecting glutathione in biological samples

This study aimed to develop a novel and practical fluorescent method for GSH detection in complex biological samples. To this end, a series of coumarin-based fluorescent probes was designed and synthesized using various aliphatic halogens as the sensing group. By using a new evaluation method of GSH/Cys/Hcy coexisting conditions, the probe with chloropropionate (CBF3) showed a high selectivity, excellent sensitivity, good stability for GSH detection. The reaction mechanism is proposed as nucleophilic substitution/cyclization and intramolecular charge transfer (ICT), which was confirmed by LC-MS and NMR analysis, as well as density functional theory calculations. In addition, CBF3 was demonstrated to be competent not only for the quantitative detection of GSH in real serum samples, but also for sensing GSH changes in different oxidative stress models in living cells and nematodes. This study showed a practical strategy for constructing GSH-specific fluorescent probes, and provided a sensitive tool for real-time sensing of GSH in real biological samples. The findings would greatly facilitate further investigations on GSH-associated clinical diagnosis and biomedical studies.

A multifunctional nanoprobe for targeting tumors and mitochondria with singlet oxygen generation and monitoring mitochondrion pH changes in cancer cells by ratiometric fluorescence imaging

Mitochondria are the main sites of cell metabolism. Even minor pH changes may lead to mitochondrial dysfunction and promote cell apoptosis. Mitochondrion-targeting photosensitizers can produce singlet oxygen in the mitochondria. In tumor photodynamic therapy (PDT), tumor cells are killed through singlet oxygen generation by photosensitizers, and optimally the process of cell apoptosis can be real-time monitored by monitoring the changes of mitochondrial pH value. To this end, a multifunctional nanoprobe that is not only able to produce singlet oxygen in mitochondria but also able to detect the changes in mitochondrial pH value has been developed in this work. The probe is a single-excited dual-emission biomass quantum dot (BQD-FA) prepared from Osmanthus leaves with folic acid (FA) and polyoxyethylene diamine as modifiers. The BQD-FAs can target tumor cells and mitochondria, and produce singlet oxygen in the mitochondria under near-infrared laser irradiation (λ _em = 660 nm). On the other hand, in the pH range of 3-8, the fluorescence intensity ratio of BQD-FAs at wavelengths 490 nm and 650 nm showed a good linear relationship with the pH value of mitochondria. The ratiometric fluorescence imaging of mitochondria using the prepared BQD-FAs showed that when the cells were chemically stimulated with chlorphenizone, the mitochondrial pH dropped from 7.9 to 7.2 within 15 min. Based on these characteristics, we envision that the prepared multifunctional nanoprobe will be of high significance in the biomedical research of mitochondria and PDT of tumors.

Acridinium Benzoates for Ratiometric Fluorescence Imaging

Acridinium benzoate was developed as a unique ICT-based fluorescent scaffold for both ratiometric and turn-on fluorescence imaging through decaging of the phenolic hydroxyl groups. Two fluorescent probes, Acr1-H₂ O₂ and Acr1-β-gal, were developed for the fluorescence imaging of H₂ O₂ and β-galactosidase in vivo.

Ratiometric fluorescent probes based on through-bond energy transfer of cyanine donors to near-infrared hemicyanine acceptors for mitochondrial pH detection and monitoring of mitophagy

Two ratiometric near-infrared fluorescent probes have been developed to selectively detect mitochondrial pH changes based on highly efficient through-bond energy transfer (TBET) from cyanine donors to near-infrared hemicyanine acceptors. The probes consist of identical cyanine donors connected to different hemicyanine acceptors with a spirolactam ring structure linked via a biphenyl linkage. At neutral or basic pH, the probes display only fluorescence of the cyanine donors when they are excited at 520 nm. However, acidic pH conditions trigger spirolactam ring opening, leading to increased π-conjugation of the hemicyanine acceptors, resulting in new near-infrared fluorescence peaks at 740 nm and 780 nm for probes A and B, respectively. This results in ratiometric fluorescence responses of the probes to pH changes indicated by decreases of the donor fluorescence and increases of the acceptor fluorescence under donor excitation at 520 nm due to a highly efficient TBET from the donors to the acceptors. The probes only show cyanine donor fluorescence in alkaline-pH mitochondria. However, the probes show moderate fluorescence decreases of the cyanine donor and considerable fluorescence increases of hemicyanine acceptors during the mitophagy process induced by nutrient starvation or under drug treatment. The probes display rapid, selective, and sensitive responses to pH changes over metal ions, good membrane penetration, good photostability, large pseudo-Stokes shifts, low cytotoxicity, mitochondria-targeting, and mitophagy-tracking capabilities.

In Situ Fluorescent and Photoacoustic Imaging of Golgi pH to Elucidate the Function of Transmembrane Protein 165

Golgi pH homeostasis affects many different biological processes, including glycosylation. Recent studies have demonstrated that transmembrane protein 165 (TMEM165) deficiency leads to Golgi glycosylation abnormalities by disturbing Golgi pH homeostasis. However, due to the lack of specific tools to measure Golgi pH in situ, evidence for TMEM165 involvement in H⁺ transport in the Golgi apparatus is still absent. Herein, the photoacoustic and fluorescent dual-mode probe CPH was developed for ratiometric detection of Golgi pH. CPH was proved to accumulate in the Golgi apparatus and reversibly image Golgi pH in real-time with high sensitivity in cells. Furthermore, we found that the absence of TMEM165 influenced H⁺ equilibrium and caused Golgi apparatus acidification. Our work provides strong evidence that TMEM165 regulates Golgi pH homeostasis. Moreover, we believe that CPH has the potential to be a practical tool to monitor Golgi pH in various biological processes.

Visual monitoring of the lysosomal pH changes during autophagy with a red-emission fluorescent probe

Autophagy plays crucial roles in maintaining normal intracellular homeostasis. Molecular probes capable of monitoring lysosomal pH changes during autophagy are still highly required yet challenging to develop. Here, a lysosome-targeting fluorescent pH probe, RML, is presented by introducing a methylcarbitol unit as the lysosome-targeting group to rhodamine B, which is highly sensitive to pH changes. RML exhibits remarkable pH-dependent behavior at 583 nm with a fluorescent enhancement of more than 148-fold. The pK_a value is determined as 4.96, and the linear response with pH changes from 4.50-5.70, which is favorable for lysosomal pH imaging. We also confirm that RML diffuses selectively into lysosomes using confocal fluorescence microscopy. Using RML, we have successfully visualized autophagy by monitoring the lysosomal pH changes.

Multifunctional gold nanoparticles as smart nanovehicles with enhanced tumour-targeting abilities for intracellular pH mapping and in vivo MR/fluorescence imaging

A number of multimodal agents have been developed for tumour imaging and diagnosis, but most of them cannot be used to study the detailed physiological or pathological changes in living cells at the same time. Herein, a series of pH-responsive magnetic resonance and fluorescence imaging (MRI/FI) dual-modal "nanovehicles" are developed and tested. These new dual-modal materials allow for intercellular pH sensing, and those with units that are dually sensitive towards both acidic and basic environments have the ability for intracellular pH mapping and can be used to quantify pH at the cellular level. In addition, detailed pH changes in organelles (including lysosomes and mitochondria) can be investigated at the same time. On the other hand, with the tumour-targeting peptide (cRGD)-modified dual-modal nanovehicles, in vivo tumour MR and fluorescence imaging, which is suitable for cancer diagnosis, can be achieved. Moreover, it has been proved that these materials can pass through the blood brain barrier (BBB). By combining the above mentioned promising properties, these novel multifunctional "nanovehicles" may provide a new method for studying the role of pH during cancer diagnosis and treatment.

A mitochondria-targeted two-photon fluorescent probe for sensing and imaging pH changes in living cells

A novel two-photon pH probe, 3-benzimidazole-7-hydroxycoumarin (BHC), was designed and synthesized based on the structures of hydroxycoumarin and benzimidazole. BHC showed good linearity in the pH ranges of 3.30-5.40 (pKa = 4.20) and 6.50-8.30 (pKa = 7.20) at a maximum emission wavelength of 480 nm. BHC in acidic and alkaline media could be distinguished by an obvious spectral shift of the maximum absorption wavelength from 390 nm to 420 nm. In addition, BHC was well localized to mitochondria and successfully applied to one-photon and two-photon imaging of pH changes in the mitochondria of HeLa cells. The findings presented herein suggest that BHC can serve as an excellent fluorescent probe for selectively sensing mitochondrial pH changes with remarkable photostability and low cytotoxicity.

A red-emission fluorescent probe for visual monitoring of lysosomal pH changes during mitophagy and cell apoptosis

We report a red-emission pH fluorescent probe (MSO) for visual monitoring of lysosomal pH changes during mitophagy and cell apoptosis in living cells.

Photostable Fluorescent Tracker for Imaging Mitochondria with Super Resolution

The power factories in cells, mitochondria, play important roles in all physiological processes. It is reported that progressive mitochondrial swelling and outer mitochondrial membrane rupture could be induced by a wide variety of apoptotic and necrotic stimuli. Regrettably, although a variety of mitochondrial probes have been developed, most of them are based on the detection of active species in mitochondria. Probes that can monitor the status and distribution of mitochondria for a long time are still urgently needed. In this study, a fluorescent sensor with excellent properties, EtNBEn, is described. Outstanding performance allows it to be observed not only in cells but also in living Daphnia and zebrafish under confocal microscopy for a long time. Moreover, the swelling process of mitochondria under light stimulation is also visualized under super-resolution (SR) microscopy. All these results suggest that EtNBEn could be employed for tagging mitochondria in various physiological processes, which makes a great contribution to the cure of diseases.

A dual-site controlled ratiometric probe revealing the simultaneous down-regulation of pH in lysosomes and cytoplasm during autophagy

In this work, a unique dual-site controlled fluorescent probe was presented for the sensitive and concurrent detection of pH in the cytoplasm and lysosomes. With the probe, the simultaneous down-regulation of pH in the lysosomes and cytoplasm during autophagy has been successfully revealed for the first time.

The ratiometric fluorescent probe with high quantum yield for quantitative imaging of intracellular pH

Intracellular pH, especially cytoplasmic pH (~7.2) plays a crucial role in cell functions and metabolism. A ratiometric fluorescent probe namely, 6-(2-(benzothiazol-2-yl)vinyl)naphthalen-2-ol (BTNO) was facilely synthesized by the condensation of 6-hydroxy-2-naphthaldehyde and 2-methylbenzothiazole. BTNO exhibited a remarkable ratiometric emission (F₄₅₆/F₅₂₆) enhancement in response to a pH change with a linear range of pH = 9.50-7.00 and a pKa value of 7.91 ± 0.03, which is desirable for measuring and monitoring the cytoplasmic pH fluctuations. In addition, because of the high fluorescence quantum yield of BTNO (Φ = 0.88 in DMSO and 0.61 in water relative to quinine sulfate solution in 0.1 M H₂SO₄), the interferences of the probe on the physiological functions could be greatly reduced. This could also provide enhanced measurement sensitivity. The successful demonstration of BTNO in detecting and monitoring the intracellular pH changes in live HeLa cells via a ratiometric approach confirmed that BTNO held a practical potential in biomedical research.