Self-assembled amphiphilic fluorescent probe: detecting pH-fluctuations within cancer cells and tumour tissues

Ketocyanine-Based Fluorescent Probe Revealing the Polarity Heterogeneity of Lipid Droplets and Enabling Accurate Diagnosis of Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) has gradually become a pronoun for terrifying death owing to its high mortality rate. With the progression of HCC, lipid droplets (LDs) in HCC cells exhibit specific variations such as increased LDs number and decreased polarity, which can serve as the diagnostic target. However, developing an effective method to achieve HCC diagnosis and reveal LDs polarity heterogeneity is still a crucial challenge. Herein, the first high-performance LDs-targeting probe (1) is reported based on ketocyanine strategy with ultrasensitive polarity-responding ability and near-infrared emission. Probe 1 shows excellent sensitivity to polarity parameter Δf (0.027-0.290) with 808-fold fluorescence enhancement and the emission wavelength red-shifts 91 nm. In HCC cells, probe 1 shows a 2.5- to 5.9-fold fluorescence enhancement compared with normal and other cancer cells which exceeds clinical threshold of 2.0, indicating probe 1 can distinguish HCC cells. The LDs polarity heterogeneity is revealed and it displays a sequence, HCC cells < other cancer cells < normal cells, which may provide useful insight to engineer LDs-targeting probes for HCC cell discrimination. Finally, probe 1 realizes accurate HCC diagnosis on the cellular, organ, and in vivo levels, providing a satisfying tool for clinical HCC diagnosis and surgical navigation.

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.

Polarity-Driven Two-Photon Fluorescent Probe for Monitoring the Perturbation in Lipid Droplet Levels during Mitochondrial Dysfunction and Acute Pancreatitis

Lipid droplets (LDs) act as an energy reservoir in cancer cells; on the other hand, mitochondria are hyperactive to fulfill the energy demand to accelerate cell proliferation. We are interested in unfolding the relationship between the cellular energy reservoir and energy producer through fluorescence labeling. Thus, a dual organelle-targeted fluorescent probe MLD-1 has been rationally developed. It visualized the crosstalk between mitochondrial dysfunction and the fluctuation of LDs in live cells. Its two-photon ability allowed us to acquire deep tissue images. For the first time, we have shown that the probe has the ability to track the accumulation of LDs in different mouse organs during pancreatic inflammation. MLD-1, being a selectively polarity-driven, chemo- and photostable LD probe, may offer great possibilities for studying LD-associated biology in due course.

A pH-Sensitive Fluorescent Chemosensor Turn-On Based in a Salen Iron (III) Complex: Synthesis, Photophysical Properties, and Live-Cell Imaging Application

pH regulation is essential to allow normal cell function, and their imbalance is associated with different pathologic situations, including cancer. In this study, we present the synthesis of 2-(((2-aminoethyl)imino)methyl)phenol (HL1) and the iron (III) complex (Fe(L1)2Br, (C1)), confirmed by X-ray diffraction analysis. The absorption and emission properties of complex C1 were assessed in the presence and absence of different physiologically relevant analytes, finding a fluorescent turn-on when OH- was added. So, we determined the limit of detection (LOD = 3.97 × 10-9 M), stoichiometry (1:1), and association constant (Kas = 5.86 × 103 M-1). Using DFT calculations, we proposed a spontaneous decomposition mechanism for C1. After characterization, complex C1 was evaluated as an intracellular pH chemosensor on the human primary gastric adenocarcinoma (AGS) and non-tumoral gastric epithelia (GES-1) cell lines, finding fluorescent signal activation in the latter when compared to AGS cells due to the lower intracellular pH of AGS cells caused by the increased metabolic rate. However, when complex C1 was used on metastatic cancer cell lines (MKN-45 and MKN-74), a fluorescent turn-on was observed in both cell lines because the intracellular lactate amount increased. Our results could provide insights about the application of complex C1 as a metabolic probe to be used in cancer cell imaging.

Subcellular visualization: Organelle-specific targeted drug delivery and discovery

Organelles perform critical biological functions due to their distinct molecular composition and internal environment. Disorders in organelles or their interacting networks have been linked to the incidence of numerous diseases, and the research of pharmacological actions at the organelle level has sparked pharmacists' interest. Currently, cell imaging has evolved into a critical tool for drug delivery, drug discovery, and pharmacological research. The introduction of advanced imaging techniques in recent years has provided researchers with richer biological information for viewing and studying the ultrastructure of organelles, protein interactions, and gene transcription activities, leading to the design and delivery of precision-targeted drugs. Therefore, this reviews the research on organelles-targeted drugs based upon imaging technologies and development of fluorescent molecules for medicinal purposes. We also give a thorough analysis of a number of subcellular-level elements of drug development, including subcellular research instruments and methods, organelle biological event investigation, subcellular target and drug identification, and design of subcellular delivery systems. This review will make it possible to promote drug research from the individual/cellular level to the subcellular level, as well as give a new focus based on newly found organelle activities.

Ratiometric, pH-Sensitive Probe for Monitoring siRNA Delivery

Many RNA delivery strategies require efficient endosomal uptake and release. To monitor this process, we developed a 2'-OMe RNA-based ratiometric pH probe with a pH-invariant 3'-Cy5 and 5'-FAM whose pH sensitivity is enhanced by proximal guanines. The probe, in duplex with a DNA complement, exhibits a 48.9-fold FAM fluorescence enhancement going from pH 4.5 to pH 8.0 and reports on both endosomal entrapment and release when delivered to HeLa cells. In complex with an antisense RNA complement, the probe constitutes an siRNA mimic capable of protein knockdown in HEK293T cells. This illustrates a general approach for measuring the localization and pH microenvironment of any oligonucleotide.

Photoinduced electron transfer (PeT) based fluorescent probes for cellular imaging and disease therapy

Typical PeT-based fluorescent probes are multi-component systems where a fluorophore is connected to a recognition/activating group by an unconjugated linker. PeT-based fluorescent probes are powerful tools for cell imaging and disease diagnosis due to their low fluorescence background and significant fluorescence enhancement towards the target. This review provides research progress towards PeT-based fluorescent probes that target cell polarity, pH and biological species (reactive oxygen species, biothiols, biomacromolecules, etc.) over the last five years. In particular, we emphasise the molecular design strategies, mechanisms, and application of these probes. As such, this review aims to provide guidance and to enable researchers to develop new and improved PeT-based fluorescent probes, as well as promoting the use of PeT-based systems for sensing, imaging, and disease therapy.

Highly efficient carbon dots for quantitatively visualizing pH fluctuations in cells, zebrafish, mice and tumors

In vivo pH is closely related to complicated physiological and pathological processes. Quantitatively probing pH and visualizing pH variation via fluorescence (FL) imaging technique in living cells and organisms is crucial but difficult to accomplish. Herein, green fluorescent carbon dots (CDs) were synthesized, and their distinct advantages of extraordinarily high fluorescence quantum yield (FQY, 71.4%), unique photostability, high selectivity, sterling biocompatibility, appropriate pKa for biosensing, enable CDs to serve as pH-activatable probes for real time quantitative detection of continuous pH fluctuation in living cells, zebrafish, mice and tumors. We believe CDs are presently among the best-of-breed pH probes for comprehensive biomedical applications.

Rational Design of a Dual-Channel Fluorescent Probe for the Simultaneous Imaging of Hypochlorous Acid and Peroxynitrite in Living Organisms

Hypochlorous acid (HOCl) and peroxynitrite (ONOO^-) are two important highly reactive oxygen/nitrogen species, which commonly coexist in biosystems and play pivotal roles in many physiological and pathological processes. To investigate their function and correlations, it is urgently needed to construct chemical tools that can track the production of HOCl and ONOO^- in biological systems with distinct fluorescence signals. Here, we found that the coumarin fluorescence of coumarin-benzopyrylium (CB) hydrazides (spirocyclic form) is dim, and their fluorescence properties are controlled by their benzopyran moiety via an intramolecular photo-induced electron transfer (PET) process. Based on this mechanism, we report the development of a fluorescent probe CB2-H for the simultaneous detection of HOCl and ONOO^-. ONOO^- can selectively oxidize the hydrazide group of CB2-H to afford the parent dye CB2 (Abs_max/Em_max = 631/669 nm). In the case of HOCl, it undergoes an electrophilic attack on the benzopyran moiety of CB2-H to give a chlorinated product CB2-H-Cl, which inhibits the PET process within the probe and thus affords a turn-on fluorescence response at the coumarin channel (Abs_max/Em_max = 407/468 nm). Due to the marked differences in absorption/emission wavelengths between the HOCl and ONOO^- products, CB2-H enables the concurrent detection of HOCl and ONOO^- at two independent channels without spectral cross-interference. CB2-H has been applied for dual-channel fluorescence imaging of endogenously produced HOCl and ONOO^- in living cells and zebrafish under different stimulants. The present probe provides a useful tool for further exploring the distribution and correlation of HOCl and ONOO^- in more biosystems.

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.

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 ratiometric pH probe for acidification tracking in dysfunctional mitochondria and tumour tissue in vivo

With an ideal pKa (7.4) for mitochondrial pH monitoring, CouDa could immobilize in mitochondria independent of MMP. Acidification tracking was realized in dysfunctional mitochondria and tumour tissue.

A heptamethine cyanine with meso-N-induced rearrangement for acid-activated tumour imaging and photothermal therapy

Photothermal therapy has been developed as one of the most attractive strategies for tumour therapy. However, most of the reported photothermal probes still suffer from poor selectivity or specificity for...

Charge Neutralization Strategy to Construct Salt-Tolerant and Cell-Permeable Nanoprobes: Application in Ratiometric Sensing and Imaging of Intracellular pH

Intracellular pH homeostasis is essential for the survival and function of biological cells. Negatively charged molecular probes, such as pyranine (HPTS), tend to exhibit poor salt tolerance and unsatisfactory cell permeability, limiting their widespread use in intracellular assays. Herein, we explored a charge neutralization strategy using multicharged cationic nanocarriers for an efficient and stable assembly with the pH-sensitive HPTS. Through immobilization and neutralization with poly(allylamine hydrochloride)-stabilized red-emitting gold nanoclusters (PAH-AuNCs), the resulting nanoprobes (HPTS-PAH-AuNCs) offered improved salt tolerance, satisfactory cell permeability, and dual-emission properties. The fluorescence ratio exhibited a linear response over the pH range of 3.0-9.0. Moreover, the proposed HPTS-PAH-AuNCs were successfully applied to determine and visualize lysosomal pH variations in living cells, which indicated great potential for biosensing and bioimaging applications in living systems. Benefiting from the charge neutralization strategy, various types of probes can be expected to achieve broader analytical applications.

Evolution of Supramolecular Systems Towards Next-Generation Biosensors

Supramolecular materials, which rely on dynamic non-covalent interactions, present a promising approach to advance the capabilities of currently available biosensors. The weak interactions between supramolecular monomers allow for adaptivity and responsiveness of supramolecular or self-assembling systems to external stimuli. In many cases, these characteristics improve the performance of recognition units, reporters, or signal transducers of biosensors. The facile methods for preparing supramolecular materials also allow for straightforward ways to combine them with other functional materials and create multicomponent sensors. To date, biosensors with supramolecular components are capable of not only detecting target analytes based on known ligand affinity or specific host-guest interactions, but can also be used for more complex structural detection such as chiral sensing. In this Review, we discuss the advancements in the area of biosensors, with a particular highlight on the designs of supramolecular materials employed in analytical applications over the years. We will first describe how different types of supramolecular components are currently used as recognition or reporter units for biosensors. The working mechanisms of detection and signal transduction by supramolecular systems will be presented, as well as the important hierarchical characteristics from the monomers to assemblies that contribute to selectivity and sensitivity. We will then examine how supramolecular materials are currently integrated in different types of biosensing platforms. Emerging trends and perspectives will be outlined, specifically for exploring new design and platforms that may bring supramolecular sensors a step closer towards practical use for multiplexed or differential sensing, higher throughput operations, real-time monitoring, reporting of biological function, as well as for environmental studies.

Immunomodulatory activity of brown algae Turbinaria ornata derived sulfated polysaccharide on LPS induced systemic inflammation

BACKGROUND

Inflammation and oxidative stress are common pathologies in a wide range of chronic diseases. Polysaccharides are known to exhibit antioxidant and anti-inflammatory potential and are suggested to possess immunomodulatory potential.

PURPOSE

Herein, the immunomodulatory activity of a sulfated polysaccharide (PS) separated from a brown marine algae Turbinaria ornata is studied in LPS instigated systemic inflammation in experimental rats.

STUDY DESIGN AND METHODS

Male SD rats are pretreated with different doses of PS (2.5, 5, 10 mg/kg bw) for a week followed by inducing systemic inflammation using LPS (10 mg/kg i.p.). Blood withdrawn after 8 h of LPS injection is subjected to hematological analysis (WBC, HCT, and PLT). After 24 h of LPS induction, cardiac tissue was isolated and subjected to biochemical, molecular, and histopathological analysis. Effect of PS pre-treatment (2.5, 5, 10 mg/kg bw) was checked by assessing serum parameters (AST, CK-MB, and γGT), antioxidant markers (LPO, GSH, SOD, Grx) and inflammatory markers (IL1β, IL6, IL10, NFκB), followed by analyzing the iNOS, PI3k and Akt to identify the probable mode of action.

RESULTS

Elevated levels of AST, CK-MB, and γGT in serum were significantly reduced on PS pretreatment. LPS significantly raised the LPO and Grx levels in heart tissue whereas, PS pre-treatment significantly reduced LPO and Grx levels. GSH and SOD levels were reduced upon LPS induction and were brought to near normal by HD of PS. PS also reduced the mRNA levels of IL6, Trx, and increased IL10 levels in the heart tissue substantiating its anti-inflammatory and antioxidant potency. Further, IL1β, NFκB, iNOS, and pPI3k/pAkt expressions were significantly modulated by PS in the cardiac tissue substantiating the immunomodulatory effect. A trend of improvement in the inflammatory pathology was also observed in the heart tissue compared to LPS control, as confirmed by histopathology analysis.

CONCLUSION

Altogether, this study concludes the immunomodulatory potential of PS from the marine macroalgae Turbinaria ornata significantly and prevents LPS induced systemic inflammation in the cardiac tissue presumably influenced by the glucopyranose and fucopyranose subunits in the polysaccharide.

A pH-Sensitive Spirocyclization Strategy for Constructing a Single Fluorescent Probe Simultaneous Two-Color Visualizing of Lipid Droplets and Lysosomes and Monitoring of Lipophagy

Lipid droplets (LDs) and lysosomes are crucial for maintaining intracellular homeostasis. But single fluorescent probes (SFPs) capable of simultaneous and discriminative visualizing of two organelles above and their interaction in living cells are still challenging due to the lack of rational design strategies. To break this bottleneck, herein, we develop a reliable strategy based on a pH-sensitive intramolecular spirocyclization. As a proof of concept, an SFP CMHCH, which possesses a switchable hemicyanine/spiro-oxazine moiety induced by pH, has been designed and synthesized. In acidic environments, the ring-open form CMHCH exhibits red-shift emission and low logP value, whereas the ring-closed form CMHC displays blue-shift emission and high logP value in neutral or basic environments. Thus, the distinct different hydrophilicity/hydrophobicity and absorption/emission properties of these two forms enable targeting LDs and lysosomes simultaneously and discriminatingly. Very importantly, the dynamic process of lipophagy can be directly monitored with CMHCH. The success of CMHCH indicated that the spirocyclization strategy is efficient for constructing SFPs to LDs and lysosomes.

A New Spiropyran-Based Fluorescent Probe for Dual Sensing of Ferrous Ion and pH

A new spiropyran-based fluorescent probe was developed for dual detection of Fe2+ ion and pH. Addition of Fe2+ and Ag+ to the probe solution enhanced the fluorescence intensity by 6 and 5 fold, respectively. Addition of Fe3+, Hg2+ and Ni2+ caused slight increase in the fluorescence intensity of the probe. While addition of other common metal ions did not bring about substantial change of the fluorescence. Thus the probe can be used for fluorescence turn-on detection of Fe2+ ion in ethanol/water (9:1) medium. The detection limit of the probe for Fe2+ is 0.77 µM. The suitable pH range for the probe to detect Fe2+ was pH 3 - 9. Other metal ions including Li+, Na+, K+, Ag+, Cu2+, Zn2+, Co2+, Ni2+, Mn2+, Sr2+, Hg2+, Ca2+, Mg2+, Al3+, Cr3+, and Fe3+ did not cause marked interference with Fe2+ recognition. The color of the probe solution was yellow at pH 1 - 2 and colorless at other pH values. The fluorescence intensity of the probe was low at pH 1 - 12 and increased significantly when the pH was 13 and 14, indicating that the probe can be used as a colorimetric and fluorescent probe for sensing extremely acidic or extremely alkaline conditions through different channels.

Fluorescence imaging of pathophysiological microenvironments

Abnormal microenvironments (viscosity, polarity, pH, etc.) have been verified to be closely associated with numerous pathophysiological processes such as inflammation, neurodegenerative diseases, and cancer. As a result, deep insights into these pathophysiological microenvironments are particularly beneficial for clinical diagnosis and treatment. However, the monitoring of pathophysiological microenvironments is unattainable by the traditional clinical diagnostic techniques such as magnetic resonance imaging, computed tomography, and positron emission tomography. Recently, fluorescence imaging has shown tremendous advantages and potential in the tracing of pathophysiological microenvironment variations. In this context, a general discussion is provided on the state-of-the-art progress of fluorescent probes for visualizing pathophysiological microenvironments (viscosity, pH, and polarity), since 2016, as well as the future perspectives in this challenging field.

Amphiphilic fluorescent probe self-encored in plasma to detect pH fluctuations in cancer cell membranes

We have developed an amphiphilic pH probe (P1CS) to detect pH levels in the plasma membrane in cancer cells. An elevated fluorescence signal at 550 nm at the cell surface of cancer cells (MDA-MB-231, HeLa cells) prompted the application of P1CS as a pH marker for the cancer cell surface, discriminating it from normal cells (WI-38). Moreover, the probe enables labeling of the surface of multilayered tumor spheroids, which promotes its use as a marker for the surface of tumor tissue.

Xanthene-based Fluorescence Turn-on Probe for Highly Acidic pH Range in Aqueous Solution

A xanthene-based probe, Xanth-NPr, is developed as a molecular system that exhibits sensitivity for the highly acidic environments with fluorescence turn-on behavior. Xanth-NPr is designed on the principle of photoinduced electron transfer (PET), which controls the fluorescence profile of the probe. The structure of Xanth-NPr contains the dipropylaniline group as a PET promoting unit. Xanth-NPr exhibited quenched fluorescence as long as it is present in neutral or moderately acidic conditions. However, in the highly acidic pH range, it displayed a strong red-colored fluorescence at 592 nm as the protonation of dipropylaniline moiety inhibits the PET process. A model probe Xanth-M without any PET promoting unit was also synthesized. The model probe along with theoretical calculations was employed to explain the role of the PET process in regulating the fluorescence behavior of Xanth-NPr. Xanth-NPr showed linear fluorescence response as a function of pH in the range of 1 to 4.1 with the pKa value of 2.72. Likewise, its fluorescence profile is not altered by the presence of biologically relevant cations.

Recent Progress in Small Spirocyclic, Xanthene-Based Fluorescent Probes

The use of fluorescent probes in a multitude of applications is still an expanding field. This review covers the recent progress made in small molecular, spirocyclic xanthene-based probes containing different heteroatoms (e.g., oxygen, silicon, carbon) in position 10'. After a short introduction, we will focus on applications like the interaction of probes with enzymes and targeted labeling of organelles and proteins, detection of small molecules, as well as their use in therapeutics or diagnostics and super-resolution microscopy. Furthermore, the last part will summarize recent advances in the synthesis and understanding of their structure-behavior relationship including novel computational approaches.

Fluorescent AIE-Active Materials for Two-Photon Bioimaging Applications

Fluorescence imaging has been widely used as a powerful tool for in situ and real-time visualization of important analytes and biological events in live samples with remarkably high selectivity, sensitivity, and spatial resolution. Compared with one-photon fluorescence imaging, two-photon fluorescence imaging exhibits predominant advantages of minimal photodamage to samples, deep tissue penetration, and outstanding resolution. Recently, the aggregation-induced emission (AIE) materials have become a preferred choice in two-photon fluorescence biological imaging because of its unique bright fluorescence in solid and aggregate states and strong resistance to photobleaching. In this review, we will exclusively summarize the applications of AIE-active materials in two-photon fluorescence imaging with some representative examples from four aspects: fluorescence detection, in vitro cell imaging, ex vivo tissue imaging, and in vivo vascular imaging. In addition, the current challenges and future development directions of AIE-active materials for two-photon bioimaging are briefly discussed.