Near-Infrared Probe Based on Rhodamine Derivative for Highly Sensitive and Selective Lysosomal pH Tracking

Aggregation-enhanced photothermal therapy of organic dyes

Photothermal therapy (PTT) represents a groundbreaking approach to targeted disease treatment by harnessing the conversion of light into heat. The efficacy of PTT heavily relies on the capabilities of photothermal agents (PTAs). Among PTAs, those based on organic dyes exhibit notable characteristics such as adjustable light absorption wavelengths, high extinction coefficients, and high compatibility in biological systems. However, a challenge associated with organic dye-based PTAs lies in their efficiency in converting light into heat while maintaining stability. Manipulating dye aggregation is a key aspect in modulating non-radiative decay pathways, aiming to augment heat generation. This review delves into various strategies aimed at improving photothermal performance through constructing aggregation. These strategies including protecting dyes from photodegradation, inhibiting non-photothermal pathways, maintaining space within molecular aggregates, and introducing intermolecular photophysical processes. Overall, this review highlights the precision-driven assembly of organic dyes as a promising frontier in enhancing PTT-related applications. This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Therapeutic Approaches and Drug Discovery > Nanomedicine for Oncologic Disease Diagnostic Tools > In Vivo Nanodiagnostics and Imaging.

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.

Rapid and visual evaluation the internal corruption of meat tissue by a designed near-infrared fluorescence probe with a broad pH response range

Rapid and visual evaluation the internal corruption of meat tissue is closely related to public health. The pH change caused by glycolysis and amino acid decomposition is an important indicator of meat freshness. Herein, we designed a pH-responsive NIR fluorescent probe (Probe-OH) based on protonation/deprotonation for monitoring the internal corruption of meat tissue. Probe-OH was synthesized by a stable hemicyanine skeleton with phenolic hydroxyl group, which exhibited excellent performances such as high selectivity, high sensitivity, fast response time (60 s), a broad pH-responsive range of 4.0-10.0, and superior spatio-temporal sampling ability. In addition, we conducted a paper chip platform to measure pH value in different meat samples (pork and chicken), which is convenient to evaluate pH value of meat by observing the color changes of paper strips. Furthermore, in combination with the NIR advantages of fluorescence imaging, Probe-OH was successfully applied to assess the freshness of pork and chicken breasts, and the structural changes of muscle tissue can be clearly observed under confocal microscope. The results of Z-axis scanning showed that Probe-OH could penetrate into the interior to monitor the internal corruption of meat tissue, the fluorescence intensity changes with scanning height in the meat tissue section, and reaches its maximum at 50 μm. To the best of our knowledge, there have been no reports of fluorescence probe being used to image the inside of meat tissue section so far. It is expected that we can provide a new rapid, sensitive, near-infrared fluorescence method for assessment of the freshness in the internal organization of meat.

TPE-based fluorescent probe for dual channel imaging of pH/viscosity and selective visualization of cancer cells and tissues

The development of efficient fluorescence-based detection tools with high contrast and accuracy in cancer diagnosis has recently attracted extensive attention. Changes in the microenvironments between cancer and normal cells provide new biomarkers for precise and comprehensive cancer diagnosis. Herein, a dual-organelle-targeted probe with multiple-parameter response is developed to realize cancer detection. We designed a tetraphenylethylene (TPE)-based fluorescent probe TPE-PH-KD connected with quinolinium group for simultaneous detection of viscosity and pH. Due to the restriction on the double bond's rotation, the probe respond to viscosity changes in the green channel with extreme sensitivity. Interestingly, the probe exhibited strong emission of red channel in acidic environment, and the rearrangement of ortho-OH group occurred in the basic form with weak fluorescence when pH increased. Additionally, cell colocalization studies revealed that the probe was located in the mitochondria and lysosome of cancer cells. Following treatment with carbonyl cyanide m-chloro phenylhydrazone (CCCP), chloroquine, and nystatin, the pH or viscosity changes in the dual channels are also monitored in real-time. Furthermore, the probe TPE-PH-KD could effectively discriminate cancer from normal cells and organs with high-contrast fluorescence imaging, which sparked more research on an efficient tool for highly selectively visualizing tumors at the organ level.

Synchronous Photoactivation-Imaging Fluorophores Break Limitations of Photobleaching and Phototoxicity in Live-cell Microscopy

Fluorescence microscopy is one of the most important tools in the studies of cell biology and many other fields, but two fundamental issues, photobleaching and phototoxicity, associated with the fluorophores have still limited its use for long-term and strong-illumination imaging of live cells. Here, we report a new concept of fluorophore engineering chemistry, synchronous photoactivation-imaging (SPI) fluorophores, activating and exciting fluorophores by a single light source to thus avoid the repeated switches between activation and excitation lights. The chemically reconstructed, nonemissive fluorophores can be photolyzed to allow continuous replenishing of "bright-state" probes detectable by standard fluorescent microscopes in the imaging process so as to bypass the photobleaching barrier to greatly extend the imaging period. Equally importantly, SPI fluorophores substantially reduce photocytotoxicity due to the scavenging of reactive oxygen species (ROS) by a photoactivable group and the slow release of "bright-state" probes to minimize ROS generation. Using SPI fluorophores, the time-lapsed confocal (>16 h) and super-resolution (>3 h) imaging of subcellular organelles under intensive illumination (50 MW/cm2) were achieved in live cells.

Lysosome-targeted fluorescent probes: Design mechanism and biological applications

As an integral organelle in the eukaryote, the lysosome is the degradation center and metabolic signal center in living cells, and partakes in significant physiological processes such as autophagy, cell death and cellular senescence. Fluorescent probe has become a favorite tool for studying organelles and their chemical microenvironments because of its high specificity and non-destructive merits. Over recent years, it has been reported that increasingly new lysosome-targeted probes play a major role in the diagnosis and monitor of diseases, in particular cancer and neurodegenerative diseases. In order to deepen the relevant research on lysosome, it is challenging and inevitability to design novel lysosomal targeting probes. This review first introduces the concepts of lysosome and its closely related biological activities, and then introduces the fluorescent probes for lysosome in detail according to different detection targets, including targeting mechanism, biological imaging, and application in diseases. Finally, we summarize the specific challenges and discuss the future development direction facing the current lysosome-targeted fluorescent probes. We hope that this review can help biologists grasp the application of fluorescent probes and broaden the research ideas of researchers targeting fluorescent probes so as to design more accurate and functional probes for application in diseases.

A water-soluble fluorescent pH probe and its application for monitoring lysosomal pH changes in living cells

Intracellular pH plays a crucial role in many cellular processes, and abnormal intracellular pH has been linked to common diseases such as cancer and Alzheimer's. To address this issue, a water-soluble fluorescent pH probe was designed based on the protonation/deprotonation of the 4-methylpiperazin-1-yl group, using dicyanoisophorone as the fluorophore. In the neutral form of the probe, fluorescence is quenched due to charge transfer from the 4-methylpiperazin-1-yl group to the fluorophore upon excitation. Under acidic conditions, protonation of the 4-methylpiperazin-1-yl group inhibits the photoinduced electron transfer process, leading to an increase in fluorescence intensity. Density-functional theory calculations also verified the fluorescence OFF-ON mechanism. The probe exhibits high selectivity, photostability, fast response to pH changes, and low cytotoxicity to cells. Additionally, the probe selectively accumulates in lysosomes, with a high Pearson coefficient (0.95) using LysoTracker Green DND-26 as a reference. Notably, the probe can monitor lysosomal pH changes in living cells and track pH changes stimulated by chloroquine. We anticipate that the probe has potential for diagnosing pH-related diseases.

THQ-Xanthene: An Emerging Strategy to Create Next-Generation NIR-I/II Fluorophores

Near-infrared fluorescence imaging is vital for exploring the biological world. The short emissions (<650 nm) and small Stokes shifts (<30 nm) of current xanthene dyes obstruct their biological applications since a long time. Recently, a potent and universal THQ structural modification technique that shifts emission to the NIR-I/II range and enables a substantial Stokes shift (>100 nm) for THQ-modified xanthene dyes is established. Thus, a timely discussion of THQ-xanthene and its applications is extensive. Hence, the advent, working principles, development trajectory, and biological applications of THQ-xanthene dyes, especially in the fields of fluorescence probe-based sensing and imaging, cancer theranostics, and super-resolution imaging, are introduced. It is envisioned that the THQ modification tactic is a simple yet exceptional approach to upgrade the performance of conventional xanthene dyes. THQ-xanthene will advance the strides of xanthene-based potentials in early fluorescent diagnosis of diseases, cancer theranostics, and imaging-guided surgery.

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.

A Recent Update on Rhodamine Dye Based Sensor Molecules: A Review

Herein we have discussed such important modified rhodamine compounds which have been used as chemosensors for the last 7-8 years. This review covered some chemosensors for the detection of metal ions like Al(III), Cu(II), Hg(II), Co(II), Fe(III), Au(III), Cr(III), and some anion like CN-. The selectivity, sensitivity, photophysical properties (i.e., UV-Vis spectral studies, fluorescence studies giving special emphasis to absorption wavelength in UV-Vis spectra and excitation and emission wavelength in fluorescence spectra), binding affinity, the limit of detection, and the application of those chemosensors are described clearly. Here we have also discussed some functionalized rhodamine-based chemosensors that emit in the near-infrared region (NIR) and can target lysosomes and detect lysosomal pH. Their versatile applicability in the medicinal ground is also delineated. We have focused on the photophysical properties of spirolactam rhodamine photoswitches and applications in single-molecule localization microscopy and volumetric 3D light photoactivable dye displays. The real-time detection of radical intermediates has also been exemplified.

pH-Switched Near-Infrared Fluorescent Strategy for Ratiometric Detection of ONOO- in Lysosomes and Precise Imaging of Oxidative Stress in Rheumatoid Arthritis

Rheumatoid arthritis (RA) is well-known as a kind of autoimmune disease, which brings unbearable pain to the patients by multiple organ complications besides arthritis. To date, RA can be hardly cured, but early diagnosis and standard treatment can relieve symptoms and pain. Therefore, an effective tool to assist the early diagnosis of RA deserves considerable attention. On account of the overexpressed ONOO^- during the early stage of RA, a near-infrared (NIR) receptor, Lyso-Cy, is proposed in this work by linker chemistry to expand the conjugated rhodamine framework by cyanine groups. Contributed by the pH-sensitive spiral ring in rhodamine, receptor Lyso-Cy has been found to be workable in lysosomes specifically, which was confirmed by the pH-dependent spectra with a narrow responding region and a well-calculated pK_a value of 5.81. We presented an excellent ratiometric sensing protocol for ONOO^- in an acidic environment, which was also available for targeting ONOO^- in lysosomes selectively. This innovative dual-targeting responsive design is expected to be promising for assisting RA diagnosis at an early stage with respect to the joint inflammatory model established in this work at the organism level.

Near-Infrared Fluorescent Probe for H2S Detection: Will pH Affect the Intracellular Sensing?

Near-infrared (NIR) fluorescent probe has exhibited unique advantages for in vitro and in vivo detection of hydrogen sulfide (H₂S), an important endogenous gasotransmitter in redox homeostasis and multiple life processes. However, both the pH-dependent emission of NIR probes and H₂S conversions would normally affect the accurate detection in cellular environments in different acidic conditions. Herein, both experiments and theoretical calculations were carried out to examine the effect of pH on intracellular sensing of H₂S by the NIR probe. Selecting a NIR probe of R1 with dual-excited NIR responses to H₂S as the model, the pH-dependent R1 emission was confirmed by optical measurements, whose structural changes were further examined by mass spectrometry (MS). Significantly, the dynamic changes versus pH increase were employed for the online monitoring of ambient MS (AMS), observing important intermediate species without sample pretreatments. Thereby, intermediates and transition states were confirmed by theoretical calculations, which proposed the mechanism of nucleophilic substitution, followed by the hydrolysis process with increasing pH. As examined, R1 exhibited a relatively stable NIR emission at pH 4-8, while a dramatic change in signals occurred at higher-pH conditions. Therefore, R1 was demonstrated to be reliable for intracellular sensing of H₂S and had been confirmed by cell imaging. This work has initiated a comprehensive strategy for evaluating fluorescence (FL) probes, showing potential for the development of fluorescent probes.

Natural flavylium-inspired far-red to NIR-II dyes and their applications as fluorescent probes for biomedical sensing

Fluorescent probes that emit in the far-red (600-700 nm), first near-infrared (NIR-I, 700-900 nm), and second NIR (NIR-II, 900-1700 nm) regions possess unique advantages, including low photodamage and deep penetration into biological samples. Notably, NIR-II optical imaging can achieve tissue penetration as deep as 5-20 mm, which is critical for biomedical sensing and clinical applications. Much research has focused on developing far-red to NIR-II dyes to meet the needs of modern biomedicine. Flavylium compounds are natural colorants found in many flowers and fruits. Flavylium-inspired dyes are ideal platforms for constructing fluorescent probes because of their far-red to NIR emissions, high quantum yields, high molar extinction coefficients, and good water solubilities. The synthetic and structural diversities of flavylium dyes also enable NIR-II probe development, which markedly advance the field of NIR-II in vivo imaging. In the last decade, there have been huge developments in flavylium-inspired dyes and their applications as far-red to NIR fluorescent probes for biomedical applications. In this review, we highlight the optical properties of representative flavylium dyes, design strategies, sensing mechanisms, and applications as fluorescent probes for detecting and visualizing important biomedical species and events. This review will prompt further research not only on flavylium dyes, but also into all far-red to NIR fluorophores and fluorescent probes. Moreover, this interest will hopefully spillover into applications related to complex biological systems and clinical treatments, ranging in focus from the sub-organelle to whole-animal levels.

Basic Fluorescent Protonation-Type pH Probe Sensitive to Small ΔpKa of Methanol and Ethanol

An optical pH probe is a simple and effective tool for determining an accurate pH value in its localized area. However, basic pH probes with pK_BH+ values above 8 have rarely been reported, although many components with high pK_a such as arginine play important roles in vivo. Herein, we introduce novel colorimetric and fluorescent basic probes 1-5, which are designed using push-pull-type aminoquinoline and aminobenzoquinoline fluorophores, with pK_BH+ values ranging from 8.4 to 9.9. After the basicity of the remarkably sensitive basic probe 4 was tuned, it was able to successfully distinguish between the pK_a values of MeOH (15.5) and EtOH (15.9), thus displaying selective protonation and fluorescence enhancement in MeOH over EtOH. Our pH probes can be used to detect MeOH poisoning in commercial EtOH products such as hand sanitizers, providing an effective solution to this problem observed during the COVID-19 pandemic.

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.

Comparison of rhodamine 6G, rhodamine B and rhodamine 101 spirolactam based fluorescent probes: A case of pH detection

Ring-opening reaction of rhodamine spirolactam has been widely applied to construct fluorescent probes. The fluorescence properties of the probe were finely tuned for specific purpose through changing the rhodamine fluorophore. However, the influence on response range and kinetic parameters of the probe during the change has been seldom discussed. Herein, we took pH detection as an example and constructed spirolactam based probes (RLH A-C) with Rhodamine 6G, Rhodamine B and Rhodamine 101. The pK_a values and observed rate constant k_obs of RLH A-C were determined and found to negatively correlated with the calculated Gibbs free energy differences ΔG_C-O and ΔG_TS respectively. The potential applications of RLH A-C in imaging acidic microenvironment were also investigated in cells. We expect the comparison of rhodamine fluorophores will facilitate the quantitative optimization of rhodamine spirolactam based fluorescent probes.

A novel Near-Infrared fluorescent probe for Zn2+ and CN- double detection based on dicyanoisfluorone derivatives with highly sensitive and selective, and its application in Bioimaging

We have successfully synthesized NIRF as a near-infrared fluorescence probe for relay recognition of zinc and cyanide ions. The probe possesses well selectivity and anti-interference ability over common ions towards Zn²⁺ and CN^-. The results showed that Zn²⁺ and the probe formed [NIRF-Zn²⁺] complex after added Zn²⁺ into the probe NIRF solution, which emited red fluorescence. The probe can be used for quantitative detection of Zn²⁺ with a detection limit of 4.61 × 10^-8 M. It was determined that the binding stoichiometry between the NIRF and Zn²⁺ was 1:1 according to the job^,s curve. Subsequently, CN^- was added to the NIRF-Zn²⁺ solution, CN^- combined with Zn²⁺ to generate [Zn(CN^-)_x]^1-x due to the stronger binding ability between zinc ion and cyanogen, which lead to the red fluorescence disappeared. The quantitative detection of CN^- was realized with a detection limit of 7.9 × 10^*7 M. In addition, the probe has excellent specificity and selectivity for Zn²⁺ and CN^-. And the probe can be stable in a wide range of pH. Through biological experiments, we found that it can complete cell imaging in macrophages and imaging of living mice, which has application prospects in Bioimaging. In addition, the probe NIRF has good applicability for Zn²⁺ and CN^- detection in actual samples.

A new near-infrared fluorescent probe for sensing extreme acidity and bioimaging in lysosome

Since the intracellular pH plays an important role in the physiological and pathological processes, however, the probes that can be used for monitoring pH fluctuation under extreme acidic conditions are currently rare, so it is necessary to construct fluorescent probes for sensing pH less than 4. In this work, we developed a new near-infrared (NIR) fluorescent probe Cy-SNN for sensing pH fluctuation under extremely acidic conditions. For the preparation of this probe, benzothiozolium moiety was chosen as lysosomal targeting unit and NIR fluorophore, and barbituric acid moiety was fused in the polymethine chain of probe to introduce protonation center. Surprisingly, on the basis of the balance of quaternary ammonium salts and free amines, the pKa value of Cy-SNN was calculated as low as 2.96, implying that Cy-SNN can be used in acidic conditions with pH < 4. Moreover, Cy-SNN exhibited highly selective response to H+ over diverse analytes in real-time with dependable reversibility. Importantly, Cy-SNN can be used to specifically target lysosome, providing potential tools for monitoring the function of lysosome in autophagy process.

A bifunctional sensitive fluorescence probe based on pyrene for the detection of pH and viscosity in lysosome

Lysosome is one of the important organelles in intracellular transport. It plays a significant role in the physiological process. The lysosomal microenvironment affects the functions of lysosome. When the original acidic environment of lysozyme is destroyed or the fluid viscosity increases gradually, various diseases are easily induced. However, most fluorescent probes can only locate in cells. The fewer probes of subcellular organelles were found and their functions are often single. So, it is of great importance to design multifunctional fluorescent probes with the capable of localizing in lysosome. In this study, a novel lysosome probe, 4-(4-Pyren-1-yl-but-3-enyl)-morpholine (PIM), was synthesized using pyrene as a fluorescent group and morpholine as a target group. The introduction of morpholine group made PIM localize in lysosome with high selectivity. The fluorescence will be enhanced with the increased viscosity because of restricting the rotation of CC bond and CN in PIM, and the detecting linear range is from 4.05 cP to 393.48 cP, which qualified the requirement of the viscosity monitoring in body. Meanwhile, the fluorescence intensity of PIM declines with the decrease of pH because the Schiff base of PIM is hydrolyzed, which was affirmed by ¹H NMR, LC-MS and fluorescence spectra. Moreover, cell imaging and MTT experiments confirmed that PIM as a novel bifunctional probe can be used to detect pH and endogenous viscosity in lysosome.

Immune responses of oyster hemocyte subpopulations to in vitro and in vivo zinc exposure

Oysters are an excellent biomonitor of coastal pollution and the hyper-accumulator of toxic metals such as copper and zinc (Zn). One unique feature of molluscs is their hemocytes which are mainly involved in immune defenses. Different subpopulations of hemocytes have been identified, but their functions in metal transport and detoxification are not clear. In this study, we examined the immune responses of different subpopulations of oyster Crassostrea hongkongensis hemocytes under different periods of Zn exposure by using flow cytometer and confocal microscopy. In vitro exposure to Zn resulted in acute immune responses by increasing the reactive oxygen species (ROS) production and phagocytosis and decreased number of granulocytes and mitochondrial membrane potential (MMP) within 3 h. Granulocyte mortality and lysosomal pH increased whereas glutathione (GSH) decreased within 1 h of in vitro exposure, indicating the immune stimulation of granulocytes. Within the first 7 days of in vivo exposure, immunocompetence of granulocytes was inhibited with increasing granulocyte mortality but decreasing ROS production and phagocytosis. However, with a further extension of Zn exposure to 14 days, both phagocytosis and lysosomal content increased with an increasing number of granulocytes, indicating the increase of hemocyte-mediated immunity. Our study demonstrated that granulocytes played important roles in oyster immune defenses while other subpopulations may also participate in immune functions. The degranulation and granulation due to transition between semigranulocytes and granulocytes after Zn exposure were important in metal detoxification. The study contributed to our understanding of the immune phenomena and the adaptive capability of oysters in metal contaminated environments.

A pH-activated fluorescent probe via transformation of azo and hydrazone forms for lysosomal pH imaging

A pyridone-based hydrazone probe Sth-NH is established for monitoring the lower lysosomal pH of cancer cells via a turn-on fluorescence mode.

Novel Dual-Organelle-Targeting Probe (RCPP) for Simultaneous Measurement of Organellar Acidity and Alkalinity in Living Cells

Many organelles, such as lysosomes and mitochondria, maintain a pH that is different from the cytoplasmic pH. These pH differences have important functional ramifications for those organelles. Many cellular events depend upon a well-compartmentalized distribution of H⁺ ions spanning the membrane for the optimal function. Cells have developed a variety of mechanisms that enable the regulation of organelle pH. However, the measurement of organellar acidity/alkalinity in living cells has remained a challenge. Currently, most existing probes for the estimation of intracellular pH show a single -organelle targeting capacity. Such probes provide data that fails to comprehensively reveal the pathological and physiological roles and connections between mitochondria and lysosomes in different species. Mitochondrial and lysosomal functions are closely related and important for regulating cellular homeostasis. Accordingly, the design of a single fluorescent probe that can simultaneously target mitochondria and lysosomes is highly desirable, enabling a better understanding of the crosstalk between these organelles. We report the development of a novel fluorescent sensor, rhodamine-coumarin pH probe (RCPP), for detection of organellar acidity/alkalinity. RCPP simultaneously moves between mitochondrion and lysosome subcellular locations, facilitating the simultaneous monitoring of pH alterations in mitochondria and lysosomes.

Fluorescent probes for imaging bioactive species in subcellular organelles

Luminescent molecular probes and nanoscale materials have become important tools in biosensing and bioimaging applications because of their high sensitivity, fast response, specificity, and methodological simplicity. In recent years, there has been a notable advancement in fluorescent probes that respond to the subtle changes in subcellular microenvironments (e.g., polarity, pH, and viscosity) or distribution of certain crucial biomarkers (e.g., reactive oxygen species, ions, amino acids, and enzymes). The dynamic fluctuations of these bio-molecules in subcellular microenvironments control cellular homeostasis, immunity, signal conduction, and metabolism. Their abnormal expressions are linked to various biological disorders and disease states. Thus, the real-time monitoring of such bioactive species is intimately linked to clinical diagnostics. Appropriately designed luminescent probes are ideally suited for desired organelle specificity, as well as for reporting intracellular changes in biochemicals/microenvironmental factors with the luminescence ON response. In this perspective, we review our recent work on the development of fluorescent probes for sensing and imaging within sub-cellular organelles. We have also discussed the design aspects for developing a prodrug with a fluorescent probe as an integral part of possible theranostic applications. An overview of the design principles, photophysical properties, detection mechanisms, current challenges, and potential future directions of fluorescent probes is presented in this feature article. We have also discussed the limitations and challenges of developing the solution platform for sensing technologies in clinical diagnostics.

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.

Recent Advances in Small Molecule-Based Intracellular pH Probes

Intracellular pH plays an important role in many biological and pathological processes. Small-molecule based pH probes are found to be the most effective for pH sensing because of ease of preparation, high sensitivity, and quick response. They have many advantages such as small perturbation to the functions of the target, functional adaptability, cellular component-specific localization, etc. The present review highlights the flurry of recent activity in the development of such probes. The probes are categorized based on the type of fluorophore used like quinoline, coumarin, BODIPY, rhodamine, indolium, naphthalimide, etc., and their analytical performance is discussed.

Real-time in vitro monitoring of the subcellular toxicity of inorganic Hg and methylmercury in zebrafish cells

Mercury (Hg) is a prominent environmental contaminant and can cause various subcellular effects. Elucidating the different subcellular toxicities of inorganic Hg (Hg²⁺) and methylmercury (MeHg) is critical for understanding their overall cytotoxicity. In this study, we employed aggregation-induced emission (AIE) probes to investigate the toxicity of Hg at the subcellular level using an aquatic embryonic zebrafish fibroblast cell line ZF4 as a model. The dynamic monitoring of lysosomal pH and the mapping of pH distribution during Hg²⁺ or MeHg exposure were successfully realized for the first time. We found that both Hg²⁺ and MeHg decreased the mean lysosomal pH, but with contrasting effects and mechanisms. Hg²⁺ had a greater impact on lysosomal pH than MeHg at a similar intracellular concentration. In addition, Hg²⁺ in comparison to MeHg exposure led to an increased number of lysosomes, probably because of their different effects on autophagy. We further showed that MeHg (200 nM) exposure had an inverse effect on mitochondrial respiratory function. A high dose (1000 nM) of Hg²⁺ increased the amount of intracellular lipid droplets by 13%, indicating that lipid droplets may potentially play a role in Hg²⁺detoxification. Our study suggested that, compared with other parameters, lysosome pH was most sensitive to Hg²⁺ and MeHg. Therefore, lysosomal pH can be used as a potential biomarker to assess the cellular toxicity of Hg in vitro.

Ratiometric Detection of Glutathione Based on Disulfide Linkage Rupture between a FRET Coumarin Donor and a Rhodamine Acceptor

Abnormal levels of glutathione, a cellular antioxidant, can lead to a variety of diseases. We have constructed a near-infrared ratiometric fluorescent probe to detect glutathione concentrations in biological samples. The probe consists of a coumarin donor, which is connected through a disulfide-tethered linker to a rhodamine acceptor. Under the excitation of the coumarin donor at 405 nm, the probe shows weak visible fluorescence of the coumarin donor at 470 nm and strong near-infrared fluorescence of the rhodamine acceptor at 652 nm due to efficient Forster resonance energy transfer (FRET) from the donor to the acceptor. Glutathione breaks the disulfide bond through reduction, which results in a dramatic increase in coumarin fluorescence and a corresponding decrease in rhodamine fluorescence. The probe possesses excellent cell permeability, biocompatibility, and good ratiometric fluorescence responses to glutathione and cysteine with a self-calibration capability. The probe was utilized to ratiometrically visualize glutathione concentration alterations in HeLa cells and Drosophila melanogaster larvae.

Fluorescent Probes for Live Cell Thiol Detection

Thiols play vital and irreplaceable roles in the biological system. Abnormality of thiol levels has been linked with various diseases and biological disorders. Thiols are known to distribute unevenly and change dynamically in the biological system. Methods that can determine thiols' concentration and distribution in live cells are in high demand. In the last two decades, fluorescent probes have emerged as a powerful tool for achieving that goal for the simplicity, high sensitivity, and capability of visualizing the analytes in live cells in a non-invasive way. They also enable the determination of intracellular distribution and dynamitic movement of thiols in the intact native environments. This review focuses on some of the major strategies/mechanisms being used for detecting GSH, Cys/Hcy, and other thiols in live cells via fluorescent probes, and how they are applied at the cellular and subcellular levels. The sensing mechanisms (for GSH and Cys/Hcy) and bio-applications of the probes are illustrated followed by a summary of probes for selectively detecting cellular and subcellular thiols.

Real-time tracking lysosomal pH changes under heatstroke and redox stress with a novel near-infrared emissive probe

Lysosomes are important subcellular organelles with acidic pH. The change of lysosomal pH can affect the normal function and activity of cells. To conveniently detect and visualize lysosomal pH changes, we designed herein a novel fluorescent probe NIR-Rh-LysopH. The probe is based on a Rhodamine 101 derivative, which was modified to include a fused tetrahydroquinoxaline ring to obtain near-infrared fluorescence and a methylcarbitol moiety to locate the lysosome. Based on the proton-induced spirolactam ring-opening mechanism, NIR-Rh-LysopH showed rapid, selective, sensitive, and reversible near-infrared fluorescence responses around 686 nm (Stokes shift 88 nm) with a pKa value of 5.70. From pH 7.4 to 4.0, about 285 folds of fluorescence enhancement was observed. Cell experiments showed that NIR-Rh-LysopH has low cytotoxicity and excellent lysosome-targeting ability. Moreover, NIR-Rh-LysopH was applied successfully to track lysosomal pH changes induced by drugs (such as chloroquine and dexamethasone), heatstroke, and redox stress. Thus, NIR-Rh-LysopH is very promising for conveniently tracking lysosomal pH changes and studying the related life processes.

Poly(amino acid)s-based star AIEgens for cell uptake with pH-response and chiral difference

Chiral aggregation-induced emission luminogens (AIEgens) are the new-generation chiral sensors that regulate chiral signals from the molecular level to the macroscopic assembly. Expanding applications of chiral AIEgens and in-depth understanding of their chiral recognition in biological systems are meaningful. Herein, two star chiral AIEgens, consisting of tetraphenylethene (TPE) as core and poly(N-acryloyl-L(D) valine) (PLV or PDV) as arms, were precisely synthesized via atom transfer radical polymerization (ATRP) technique and named TPE-PLV and TPE-PDV. They possessed typical AIE characteristics and exhibited an increase in concentration-dependent fluorescence intensity. The two AIEgens were pH-responsive and had strong AIE-related emission in acidic solution. Importantly, AIEgens can enter the living cells by ATP dependent endocytosis, then light them up. The interactions between the AIEgens and living human hepatocarcinoma (HepG2) cells revealed that the internalization process of TPE-PLV and TPE-PDV was both chiral-dependent and pH-responsive. This novel strategy for synthesizing poly(amino acid)s functionalized AIEgens could inspire the development of promising fluorescent materials with chirality.

Imidazolyl-benzocoumarins as ratiometric fluorescence probes for biologically extreme acidity

A rational approach to develop a fluorescent probe for sensing biologically "extreme" acidity (pH <3) is disclosed. The probe, a push-full type 3-(imidazolyl)benzocoumarin dye, has the lowest pK_a = 1.3 among ratiometric probes known so far, which is ascribed due to a unique sensing mechanism. The probe has high quantum yields, high chemical stability and good aqueous solubility. The probe was successfully applied to ratiometric fluorescence imaging of intrabacterial acidity from pH 4.0-1.0, offering a practical means for studying biological systems under the extreme pH conditions.

Morpholine-Substituted Rhodamine Analogue with Multi-Configurational Switches for Optical Sensing of pH Gradient under Extreme Acidic Environments

Superior pH-responsive molecules are required for the development of functional materials applicable to advanced molecular technologies. Despite having been widely developed, many rhodamine-based pH-responsive molecules exhibit a single configurational switch for "turn-on". Herein, we report a new type of rhodamine-based pH-responsive molecule with multi-configurational switches displaying stable two-step structural and color conversion in response to pH. This rhodamine analogue could be successfully applied to optical sensing of pH gradient under extreme acidic environments both in solution and on hydrogel through high-contrast color change. We demonstrated that this multi-responsive character enabled optical memory of different pH information.

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.

Lysosome-targeting pH indicator based on peri-fused naphthalene monoimide with superior stability for long term live cell imaging

Lysosomal pH is altered in many pathophysiological conditions. We describe synthesis and spectral properties of a new lysosomal fluorescent marker dye suitable for microscopic evaluation of lysosomal distribution and pH changes.

Resorufin-based responsive probes for fluorescence and colorimetric analysis

The fluorescence imaging technique has attracted increasing attention in the detection of various biological molecules in situ and in real-time owing to its inherent advantages including high selectivity and sensitivity, outstanding spatiotemporal resolution and fast feedback. In the past few decades, a number of fluorescent probes have been developed for bioassays and imaging by exploiting different fluorophores. Among various fluorophores, resorufin exhibits a high fluorescence quantum yield, long excitation/emission wavelength and pronounced ability in both fluorescence and colorimetric analysis. This fluorophore has been widely utilized in the design of responsive probes specific for various bioactive species. In this review, we summarize the advances in the development of resorufin-based fluorescent probes for detecting various analytes, such as cations, anions, reactive (redox-active) sulfur species, small molecules and biological macromolecules. The chemical structures of probes, response mechanisms, detection limits and practical applications are investigated, which is followed by the discussion of recent challenges and future research perspectives. This review article is expected to promote the further development of resorufin-based responsive fluorescent probes and their biological applications.

A FRET-based near-infrared ratiometric fluorescent probe for detection of mitochondria biothiol

A new fluorescent probe A with BODIPY as FRET donor and near-infrared rhodamine as FRET acceptor is constructed through disulfide bonding and use for ratiometric fluorescence detection of biothiol. Due to the efficient fluorescence resonance energy transfer (FRET) from BODIPY donor to near-infrared rhodamine acceptor, Probe A only displays near-infrared rhodamine fluorescence (λ_em = 656 nm) under BODIPY excitation at 480 nm. The presence of biothiol leads to BODIPY fluorescence increases (λ_em = 511 nm) and near-infrared rhodamine fluorescence decreases since the disulfide bond of the probe is broken by biothiols, effectively separating the donor from the acceptor, thus inhibiting the FRET process. Probe A exhibits remarkable high selectivity and excellent linear relationship from 10 μM to 100 μM of GSH, with low detection limit as 0.26 μM. Cellular imaging experiments shows that the probe is predominantly present in mitochondria and has been successfully applied to detect biothiol concentrations changes in mitochondria of living cells.

Visualizing semipermeability of the cell membrane using a pH-responsive ratiometric AIEgen

In clinical chemotherapy, some basic drugs cannot enter the hydrophobic cell membrane because of ionization in the acidic tumor microenvironment, a phenomenon known as ion trapping. In this study, we developed a method to visualize this ion trapping phenomenon by utilizing a pH-responsive ratiometric AIEgen, dihydro berberine (dhBBR). By observing the intracellular fluorescence of dhBBR, we found that non-ionized dhBBR can enter cells more easily than ionized forms, which is in accordance with the concept of ion trapping. In addition, dhBBR shows superior anti-photobleaching ability to Curcumin thanks to its AIE properties. These results suggest that dhBBR can serve as a bioprobe for ion trapping.