Fluorescence detection of intracellular pH changes in the mitochondria-associated process of mitophagy using a hemicyanine-based fluorescent probe

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.

An overview on recent advances of reversible fluorescent probes and their biological applications

Due to the simplicity and low detection limit, fluorescent probes are widely used in both analytical sensing and optical imaging. Compared to conventional fluorescent probes, reversibility endows the reversible fluorescent probe outstanding advantages and special properties, making reversible fluorescent probes with capable of quantitative, repetitive or circulatory. Reversible fluorescent probes can also monitor the concentration dynamics of target analytes in real time, such as metal ions, proteins and enzymes, as well as intracellular redox processes, which have been widely applied in various fields. This review summarized the types and excellent properties of reversible fluorescent probes designed and developed in recent years. It also summarized the applications of reversible fluorescent probe in fluorescence imaging, biological testing, monitoring redox cycles, and proposed the remaining challenges and future development directions of the reversible fluorescent probe. This review provided comprehensive overview of reversible fluorescent probe, which may provide valuable references for the design and fabrication of the reversible fluorescent probe.

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.

Fluorescence-On Imaging of Reticulophagy Enabled by an Acidity-Reporting Solvatochromic Probe

Aberrant autophagy of the endoplasmic reticulum (reticulophagy) is engaged in diverse pathological disorders. Herein, we reported sensitive imaging of reticulophagy with ER-Green-proRed, a diad combining a solvatochromic entity of trifluoromethylated naphthalimide for long-term ER tracking by green fluorescence and an entity of rhodamine-lactam fluorogenic to lysosomal acidity. Stringently accumulated in the ER to give green fluorescence, ER-Green-proRed exhibits robust red fluorescence upon codelivery with the ER subdomain into lysosomes. The relevance of turn-on red fluorescence to reticulophagy was validated by reticulophagy modulated by starvation, reticulophagic receptors, and autophagy inhibition. This imaging method was successfully employed to discern reticulophagy induced by various pharmacological agents. These results show the potential of ER-targeted pH probes, as exemplified by ER-Green-proRed, to image reticulophagy and to identify reticulophagy inducers.

Mitochondria-Targeted Fluorescent Nanoparticles with Large Stokes Shift for Long-Term BioImaging

Mitochondria (MITO) play a significant role in various physiological processes and are a key organelle associated with different human diseases including cancer, diabetes mellitus, atherosclerosis, Alzheimer's disease, etc. Thus, detecting the activity of MITO in real time is becoming more and more important. Herein, a novel class of amphiphilic aggregation-induced emission (AIE) active probe fluorescence (AC-QC nanoparticles) based on a quinoxalinone scaffold was developed for imaging MITO. AC-QC nanoparticles possess an excellent ability to monitor MITO in real-time. This probe demonstrated the following advantages: (1) lower cytotoxicity; (2) superior photostability; and (3) good performance in long-term imaging in vitro. Each result of these indicates that self-assembled AC-QC nanoparticles can be used as effective and promising MITO-targeted fluorescent probes.

Organic Fluorescent Probes for Monitoring Micro-Environments in Living Cells and Tissues

As a vital parameter in living cells and tissues, the micro-environment is crucial for the living organisms. Significantly, organelles require proper micro-environment to achieve normal physiological processes, and the micro-environment in organelles can reflect the state of organelles in living cells. Moreover, some abnormal micro-environments in organelles are closely related to organelle dysfunction and disease development. So, visualizing and monitoring the variation of micro-environments in organelles is helpful for physiologists and pathologists to study the mechanisms of the relative diseases. Recently, a large variety of fluorescent probes was developed to study the micro-environments in living cells and tissues. However, the systematic and comprehensive reviews on the organelle micro-environment in living cells and tissues have rarely been published, which may hinder the research progress in the field of organic fluorescent probes. In this review, we will summarize the organic fluorescent probes for monitoring the microenvironment, such as viscosity, pH values, polarity, and temperature. Further, diverse organelles (mitochondria, lysosome, endoplasmic reticulum, cell membrane) about microenvironments will be displayed. In this process, the fluorescent probes about the "off-on" and ratiometric category (the diverse fluorescence emission) will be discussed. Moreover, the molecular designing, chemical synthesis, fluorescent mechanism, and the bio-applications of these organic fluorescent probes in cells and tissues will also be discussed. Significantly, the merits and defects of current microenvironment-sensitive probes are outlined and discussed, and the development tendency and challenges for this kind of probe are presented. In brief, this review mainly summarizes some typical examples and highlights the progress of organic fluorescent probes for monitoring micro-environments in living cells and tissues in recent research. We anticipate that this review will deepen the understanding of microenvironment in cells and tissues and facilitate the studies and development of physiology and pathology.

Scanning pH-metry for Observing Reversibility in Protein Folding

One of the main factors affecting protein structure in solution is pH. Traditionally, to study pH-dependent conformational changes in proteins, the concentration of the H+ ions is adjusted manually, complicating real-time analyses, hampering dynamic pH regulation, and consequently leading to a limited number of tested pH levels. Here, we present a programmable device, a scanning pH-meter, that can automatically generate different types of pH ramps and waveforms in a solution. A feedback loop algorithm calculates the required flow rates of the acid/base titrants, allowing one, for example, to generate periodic pH sine waveforms to study the reversibility of protein folding by fluorescence spectroscopy. Interestingly, for some proteins, the fluorescence intensity profiles recorded in such a periodically oscillating pH environment display hysteretic behavior indicating an asymmetry in the sequence of the protein unfolding/refolding events, which can most likely be attributed to their distinct kinetics. Another useful application of the scanning pH-meter concerns coupling it with an electrospray ionization mass spectrometer to observe pH-induced structural changes in proteins as revealed by their varying charge-state distributions. We anticipate a broad range of applications of the scanning pH-meter developed here, including protein folding studies, determination of the optimum pH for achieving maximum fluorescence intensity, and characterization of fluorescent dyes and other synthetic materials.

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.

Recent Advancements in Colorimetric and Fluorescent pH Chemosensors: From Design Principles to Applications

Due to the immense biological significance of pH in diverse living systems, the design, synthesis, and development of pH chemosensors for pH monitoring has been a very active research field in recent times. In this review, we summarize the designing strategies, sensing mechanisms, biological and environmental applications of fluorogenic and chromogenic pH chemosensors of the last three years (2018-2020). We categorized these pH probes into seven types based on their applications, including 1) Cancer cell discriminating pH probes; 2) Lysosome targetable pH probes; 3) Mitochondria targetable pH probes; 4) Golgi body targetable pH probes; 5) Endoplasmic reticulum targetable pH probes; 6) pH probes used in nonspecific cell imaging; and 7) pH probes without cell imaging. All these different categories exhibit diverse applications of pH probes in biological and environmental fields.

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.

Hydrothermal-Assisted Synthesis and Stability of Multifunctional MXene Nanobipyramids: Structural, Chemical, and Optical Evolution

Recent advancements in two-dimensional materials have brought MXene (Ti₃C₂) into attention due to its exciting properties as a very promising material for various applications. In this work, we report a novel Ti₃C₂ nanobipyramid (Ti₃C₂ NB) structure obtained through a three-step process involving exfoliation, delamination, and subsequent hydrothermal treatment. The morphological and textural properties at each step of synthesis were studied using an array of experimental techniques such as transmission electron microscopy, scanning electron microscopy, and atomic force microscopy and the chemical properties through X-ray diffraction, Raman, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy analysis. The Ti₃C₂ NBs exhibit fluorescence with an excitation-dependent emission. Further, the effect of temperature and pH on the fluorescence was also investigated, which opens up its scope in bioanalytical applications. Ti₃C₂ NBs showed a ∼43% increase in photoluminescence intensity from pH 3 to 11 while a ∼38% increase with the temperature from 20 to 80 °C. Usually, MXenes are highly susceptible to oxidation, but the Ti₃C₂ NBs were found to be chemically and optically stable even after 30 days. Bestowed with good hydrophilicity, the material exhibited high biocompatibility on the mouse fibroblast cell line L929. Further, L929 cells also showed good cellular adhesion on a Ti₃C₂ NB-modified glass substrate. These properties pave a way for its multifunctional ability as a sensor for pH and temperature as well as bioimaging.

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.

Organic fluorescent probes for monitoring autophagy in living cells

As a ubiquitous degradation process in cells, autophagy plays important roles in various biological activities. However, the abnormality of autophagy is closely related to many diseases, such as aging, neurological disorder, and cancer. Thus, monitoring the process of autophagy in living cells has high significance in biological studies and diagnosis of related diseases. In order to real-time and in situ monitor the process of autophagy, various organic fluorescent probes have been explored in recent years owing to the advantages such as handy staining processes, flexible molecular design strategies, and near-nondestructive detection. However, this interesting and frontier topic has not been reviewed so far. In this tutorial review, we will focus on the latest breakthrough results of organic fluorescent probes in monitoring autophagy of living cells, especially the probe design strategies based on the several microenvironment changes of the autophagy process, and the responding mechanisms and bio-imaging applications in the autophagy process. In addition, we will discuss the shortcomings and limitations of the probes developed, such as susceptible to interference, unable to monitor the whole process, and lack of clinical applications. Finally, we will highlight some challenges and further opportunities in this field. This tutorial review may promote the development of more robust fluorescent probes to further reveal the mechanisms of autophagy, which is the basis of degradation and recycling of cell components.

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.

A self-immobilizing near-infrared fluorogenic probe for sensitive imaging of extracellular enzyme activity in vivo

Reported herein is a self-immobilizing near-infrared fluorogenic probe that can be used to image extracellular enzyme activity in vivo. Using a fluorophore as a quinone methide precursor, this probe covalently anchors at sites of activation and greatly enhances the fluorescence intensity at 710 nm upon enzymatic stimulus, significantly boosting detection sensitivity in a highly dynamic in vivo system.

AND logic gate based fluorescence probe for simultaneous detection of peroxynitrite and hypochlorous acid

Hypochlorous acid (HOCl) and peroxynitrite (ONOO^-) are two of the most important reactive species and associated with various diseases in various physiological and pathological processes. Nonetheless, many of their roles are still vague due to the shortage of methods for simultaneously detecting HOCl and ONOO^-. Herein, three simple yet useful fluorogenic probes, LG-1, LG-2 and LG-3, have been fabricated with facile synthesis route and used to monitor the coexistence of HOCl and ONOO^- as AND-based logic gate fluorescent probe firstly. LG-1 and LG-2, which consists of 1,3-oxathiolane group and boronate group respectively, were designed to verify the capacity of monitoring HOCl and ONOO^- without interference from each other. The result showed that these two groups are perfect reaction sites of detecting HOCl and ONOO^- respectively via specific analyte-induced reactions. Hence, LG-3, which is attached by these two groups to suppress the fluorophore core, can response to HOCl and ONOO^- simultaneously without mutual interference and generate the significant time-dependent fluorescence enhancement. By investigating the absorption and fluorescence properties of LG-3 towards HOCl and ONOO^- individually and collectively, the result confirmed clearly that LG-3 has the capacity of monitoring the coexistence of HOCl and ONOO^-, which could act as a two-input AND-based logic gate fluorescent probe.

A near-infrared rhodamine-based lysosomal pH probe and its application in lysosomal pH rise during heat shock

Heat shock is a potentially fatal condition characterized by high body temperature (>40 °C), which may lead to physical discomfort and dysfunctions of organ systems. Acidic pH environment in lysosomes can activate enzymes, thus facilitating the degradation of proteins in cellular metabolism. Owing to the lack of a practical research tool, it remains difficult to exploit relationship between heat shock and lysosome. Herein, a NIR lysosomal pH chemosensor (NRLH) was developed. One typical lysosome-locating group, morpholine, was incorporated into NRLH. The fluorescence intensity showed pH-dependent characteristics and responded sensitively to pH fluctuations in the pH range of 3.0-5.5. NRLH with a pKa of 4.24 displayed rapid response and high selectivity for H⁺ among common species. We also demonstrated NRLH was capable of targeting lysosomes. Importantly, NRLH was applied in cellular imaging and the data revealed that lysosomal pH increased but never decreased during the heat shock. Therefore, NRLH may act as an effective molecular tool for exploring the mechanisms of heat-related pathology in bio-systems.

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.

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.

Organelle-Redirected Chameleon Sensor-Enabled Live Cell Imaging of Mitochondrial DNA

Mitochondrial DNA (mtDNA) plays important roles in diverse physiological processes and myriad diseases. We herein report mtDNA imaging with a chameleon sensor containing a cationic rhodamine B (RB) entity for mitochondria targeting and a fluorogenic SYBR Green-I (SG) entity for DNA sensing. SG-RB selectively binds to mtDNA and gives green SG fluorescence in mitochondria of living cells but gives red RB fluorescence upon delivery of mitochondria into lysosomes in mitophagy. With the dual-color imaging, mtDNA aggregation and elevated mitophagy were identified in HeLa cells stressed with anticancer doxorubicin. These results suggest the utility of organelle-redirected DNA sensors for live cell imaging of mtDNA involved in myriad pathological disorders.

A Reaction-Based Fluorescent Probe for Imaging of Native Hypochlorous Acid

Hypochlorous acid (HOCl), one of the reactive oxygen species (ROS), is highly reactive and short-lived. It is a challenge to dynamic monitor HOCl activity in living systems. Hence, we synthesized a new fluoresce nt probe RF1 based on protection of the hydroxyl group by N,N-dimethylthiocarbamate recognition group, which reached a low fluorescence background signal and highly sensitive property. On account of the electrophilic addition of Cl⁺ to the sulfide of thiocarbamate moiety, probe RF1 was converted to resorufin and triggered emitting bright. RF1 showed not only the highly sensitive and selective response to HOCl in vitro, but also can be applied in environmental water samples and detected HOCl by test strips. Besides, the ability of RF1 monitoring HOCl in HeLa cells by exogenous simulation and tracing native HOCl in macrophages cells were also explored.

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.

Real-time visualization of autophagy by monitoring the fluctuation of lysosomal pH with a ratiometric two-photon fluorescent probe

A benzimidazole-decorated two-photon fluorescent probe (Lyso-MPCB) based on the p-methoxyphenylacetylene-substituted carbazole was developed for detecting lysosomal pH with a double-channel signal, which can be used to visualize autophagy by real-time imaging the fluctuation of the pH in the lysosomes.