A dual lock-and-key two photon fluorescence probe in response to hydrogen peroxide and viscosity: Application in cellular imaging and inflammation therapy

Near-infrared fluorescent probe for the imaging of viscosity in fatty liver mice and valuation of drug efficacy

Fatty liver disease affects at least 25 percent of the population worldwide and is a severe metabolic syndrome. Viscosity is closely related to fatty liver disease, so it is urgent to develop an effective tool for monitoring viscosity. Herein, a NIR fluorescent probe called MBC-V is developed for imaging viscosity, consisting of dimethylaniline and malonitrile-benzopyran. MBC-V is non-fluorescent in low viscosity solutions due to intramolecular rotation. In high viscosity solution, the intramolecular rotation of MBC-V is suppressed and the fluorescence is triggered. MBC-V has long emission wavelength at 720 nm and large Stokes shift about 160 nm. Moreover, MBC-V can detect changes in cell viscosity in fatty liver cells, and can image the therapeutic effects of drug in fatty liver cells. By taking advantage of NIR emission, MBC-V can be used as an imaging tool for fatty liver disease and a way to evaluate the therapeutic effect of drug for fatty liver disease.

Visualizing the crucial roles of plasma membrane and peroxynitrite during abdominal aortic aneurysm using two-photon fluorescence imaging

Peroxynitrite (ONOO-) and cell plasma membrane (CPM) are two key factors in cell pyroptosis during the progression of abdominal aortic aneurysm (AAA). However, their combined temporal and spatial roles in initiating AAA pathogenesis remain unclear. Herein, we developed a two-photon fluorescence probe, BH-Vis, enabling real-time dynamic detection of CPM and ONOO- changes, and revealing their interplay in AAA. BH-Vis precisely targets CPM with reduced red fluorescence intensity correlating with diminished CPM tension. Concurrently, a blue shift of the fluorescence signal of BH-Vis occurs in response to ONOO- offering a reliable ratiometric detection mode with enhanced accuracy by minimizing external testing variables. More importantly, two photon confocal imaging with palmitic acid (PA) and ganglioside (GM1) manipulation, which modulating cell pyroptosis, showcases reliable fluorescence fluctuations. This groundbreaking application of BH-Vis in a mouse AAA model demonstrates its significant potential for accurately identifying cell pyroptosis levels during AAA development.

Targeted platelet with hydrogen peroxide responsive behavior for non-alcoholic steatohepatitis detection

The most common chronic liver illness, non-alcoholic fatty liver disease (NAFLD), refers to a range of abnormalities of the liver with varying degrees of steatosis. When the clinical symptoms including liver damage, inflammation, and fibrosis, are added to the initial steatosis, NAFLD becomes non-alcoholic steatohepatitis (NASH), the problematic and severe stage. The diagnosis of NASH at the right time could therefore effectively prevent deterioration of the disease. Considering that platelets (PLTs) could migrate to the sites of inflamed liver sinusoids with oxidative stress during the development of NASH, we purified the PLTs from fresh blood and engineered their surface with hydrogen peroxide (H2O2) responsive fluorescent probe (5-DP) through lipid fusion. The engineered PLT-DPs were recruited and trapped in the inflammation foci of the liver with NASH through interaction with the extracellular matrix, including hyaluronan and Kupffer cells. Additionally, the fluorescence of 5-DP on the surface of PLT-DP was significantly enhanced upon reacting with the elevated level of H2O2 in the NASH liver. Thus, PLT-DP has great promise for NASH fluorescence imaging with high selectivity and sensitivity.

A dual-lock-controlled mitochondria-targeted ratiometric fluorescence probe for simultaneous detection of atherosclerosis-related HClO and viscosity

Precise detection of inflammatory microenvironment-related viscosity and hypochlorous acid (HClO) contributes to illuminating the pathogenesis and further diagnosing of atherosclerosis (AS). Herein, a dual-lock-controlled mitochondria-targeted fluorescence probe (NS) for simultaneous imaging of HClO and viscosity in AS-related foam cells is presented. NS performs linear increase in green-fluorescence along with increased viscosity (excited at 425 nm), permitting "off-on" fluorescence imaging of viscosity. Meanwhile, upon HClO activation, NS exhibits red-shifted and enhanced fluorescence in orange, thus leading to ratiometric fluorescence quantification of HClO (excited at 465 nm). Such dual-lock-controlled effect makes NS realize simultaneous imaging of viscosity and HClO with high sensitivity and selectivity via "off-on" and ratiometric fluorescence readouts, respectively. Besides, endowed with mitochondria-targeting capacity, NS achieves in situ imaging of mitochondria viscosity and HClO in living RAW264.7 cells. Importantly, for the first time, NS realizes simultaneous imaging of mitochondria viscosity and HClO in macrophage-derived foam cells, revealing the close association between HClO level and viscosity change in mitochondria during foaming translation of macrophages in atherogenesis. This work not only provides a novel strategy and tool to image organelle-located viscosity and HClO in living systems, but also holds great potential in early diagnosis of AS.

A novel AIE material for sensing of Erythromycin in pure water by hydrogen bond in portable test strips and cellular imaging applications

Erythromycin could be used to treat various bacterial infection, but it was harmful to the colonic microflora. Therefore, it is highly desirable to develop a fluorescence probe that could selectively and sensitively detect Erythromycin in pure water. In this work, a fluorescent probe named EHMC, which exhibited aggregation-induced emission (AIE) characteristic in solid state and water/EtOH binary solvent was developed for "turn on" sensing Erythromycin in pure water with high selectivity and sensitivity (detection limit: 1.78 × 10-8 M). Also, there are fewer interference from other antibiotics in the detection process of probe EHMC for Erythromycin. Moreover, probe EHMC could as a portable test strips for highly selective detection of Erythromycin and identification of different concentrations of Erythromycin. In addition, living cells imaging experiments displayed that probe EHMC could detect Erythromycin in A549 cells and BEAS-2B cells successfully. Combined with the theoretical calculation results The sensing mechanisms that the CO in Erythromycin and OH in EHMC formed intermolecular hydrogen bond and further formed new aggregates were confirmed by job' plot, 1H NMR, FT-IR, ESI-MS, DLS and TEM and DFT calculation.

A general design of pyridinium-based fluorescent probes for enhancing two-photon microscopy

Two-photon absorbing fluorescent probes have emerged as powerful imaging tools for subcellular-level monitoring of biological substances and processes, offering advantages such as deep light penetration, minimal photodamage, low autofluorescence, and high spatial resolution. However, existing two-photon absorbing probes still face several limitations, such as small two-photon absorption cross-section, poor water solubility, low membrane permeability, and potentially high toxicity. Herein, we report three small-molecule probes, namely MSP-1arm, Lyso-2arm, and Mito-3arm, composed of a pyridinium center (electron-acceptor) and various methoxystyrene "arms" (electron-donor). These probes exhibit excellent fluorescence quantum yield and decent aqueous solubility. Leveraging the inherent intramolecular charge transfer and excitonic coupling effect, these complexes demonstrate excellent two-photon absorption in the near-infrared region. Notably, Lyso-2arm and Mito-3arm exhibit distinct targeting abilities for lysosomes and mitochondria, respectively. In two-photon microscopy experiments, Mito-3arm outperforms a commercial two-photon absorbing dye in 2D monolayer HeLa cells, delivering enhanced resolution, broader NIR light excitation window, and higher signal-to-noise ratio. Moreover, the two-photon bioimaging of 3D human forebrain organoids confirms the successful deep tissue imaging capabilities of both Lyso-2arm and Mito-3arm. Overall, this work presents a rational design strategy in developing competent two-photon-absorbing probes by varying the number of conjugated "arms" for bioimaging applications.

A fluorescent probe for imaging nitroreductase with signal amplification in high-viscosity environments

Herein, we developed a fluorescent probe ENBT for in vitro detection of nitroreductase (NTR) as well as imaging intracellular NTR. ENBT itself is non-fluorescent and it could be catalyzed by NTR to generate a viscosity-sensitive fluorophore EBT. The fluorescence intensity of EBT could be further enhanced in cancer cells with relatively high viscosity due to the inhibition of the twisted intramolecular charge transfer effect. The probe ENBT has a good response to NTR with a detection limit of 36.8 ng mL-1, and EBT has a good response to viscosity. Furthermore, different concentrations of NTR (0-1.4 μg mL-1) were used to react with the probe and the reaction systems were subjected to different viscosity solutions, and the fluorescence signals of the products in the viscosity range of 45.86-163.60 cP were increased up to 1.69-fold. ENBT was successfully used to image NTR in cells under different hypoxic conditions as well as in Staphylococcus aureus. Finally, lipopolysaccharide was added to stimulate an increase in cellular viscosity after ENBT was catalyzed by intracellular NTR into EBT, and the fluorescence signals were observed to increase by 1.72-fold. The signal amplification capability gives ENBT higher sensitivity and immunity to interference. Moreover, it has the advantages of mitochondrial targeting, large Stokes shift (190 nm), high selectivity, and can be easily synthesized.

A large Stokes shift NIR fluorescent probe for visual monitoring of mitochondrial peroxynitrite during inflammation and ferroptosis and in an Alzheimer's disease model

The excessive formation of peroxynitrite (ONOO-) in mitochondria has been implicated in various pathophysiological processes and diseases. However, owing to short emission wavelengths and small Stokes shifts, previously reported fluorescent probes pose significant challenges for mitochondrial ONOO- imaging in biological systems. In this study, a near-infrared (NIR) fluorescent probe, denoted as DCO-POT, is designed for the visual monitoring of mitochondrial ONOO-, displaying a remarkable Stokes shift of 170 nm. The NIR fluorophore of DCO-CHO is released by DCO-POT upon the addition of ONOO-, resulting in off-on NIR fluorescence at 670 nm. This phenomenon facilitates the high-resolution confocal laser scanning imaging of ONOO- generated in biological systems. The practical applications of DCO-POT as an efficient fluorescence imaging tool are verified in this study. DCO-POT enables the fluorometric visualization of ONOO- in organelles, cells, and organisms. In particular, ONOO- generation is analyzed during cellular and organism-level (zebrafish) inflammation during ferroptosis and in an Alzheimer's disease mouse model. The excellent visual monitoring performance of DCO-POT in vivo makes it a promising tool for exploring the pathophysiological effects of ONOO-.

Activity-Based Sensing for Chemistry-Enabled Biology: Illuminating Principles, Probes, and Prospects for Boronate Reagents for Studying Hydrogen Peroxide

Activity-based sensing (ABS) offers a general approach that exploits chemical reactivity as a method for selective detection and manipulation of biological analytes. Here, we illustrate the value of this chemical platform to enable new biological discovery through a case study in the design and application of ABS reagents for studying hydrogen peroxide (H2O2), a major type of reactive oxygen species (ROS) that regulates a diverse array of vital cellular signaling processes to sustain life. Specifically, we summarize advances in the use of activity-based boronate probes for the detection of H2O2 featuring high molecular selectivity over other ROS, with an emphasis on tailoring designs in chemical structure to promote new biological principles of redox signaling.

Regulating the activity of boronate moiety to construct fluorescent probes for the detection of ONOO-in vitro and in vivo

Abnormal intracellular peroxynitrite (ONOO^-) concentration is related to oxidative damage, which is correlated with many pathological consequences, such as local inflammation and other diseases. In this work, a series of resorufin benzyl ether-based fluorescent probes were designed using boronate as a recognizing moiety installed on a phenyl moiety for ONOO^- detection via a self-immolation mechanism. The location of the boronate as well as the substitution patterns on the phenyl moiety were investigated and the responding behaviors of the designed probes to ONOO^-, other reactive oxygen species, and biothiols were examined. It was found that all the immolative probes were inevitably dominated by ONOO^-. Compared with other probes, p-Borate possessed favorable selectivity and high sensitivity to ONOO^-. Moreover, p-Borate was successfully used to detect ONOO^- in cells and inflamed mice.

A novel "dual-locked" fluorescent probe for ONOO- and viscosity enables serum-based rapid disease screening

Peroxynitrite (ONOO^-) plays important roles in the progression of important disease such as inflammation, cancer, and diabetes, which made it an attractable target for biosensor development. However, to detect ONOO^- solely is highly dependent on the sensitivity of the detection method and may be disturbed by unwillingly false-positive signal. Cellular viscosity is an important microenvironmental parameter and its abnormal changes are closely related to diseases such as diabetes and cancer. In this case, to construct a "dual-locked" molecular tool for both ONOO^- and viscosity sensing and to evaluate the performance of such strategy in disease diagnosis is of great importance. We herein firstly reported the construction of a novel "dual-locked" probe DCI-OV which showed capability for simultaneous measuring ONOO^- concentration and system viscosity with high sensitivity (LOD = 4.7 nM) and high specificity. Moreover, both exogenous and low level of endogenous ONOO^- in living cells could be detected using DCI-OV due to viscosity amplified signal. Furthermore, cancer cells and insulin-resistant cells could be easily distinguished using DCI-OV. By taking advantage of the "dual-locked" sensing strategy, a total of 85 samples of human serum were screened using DCI-OV based rapid disease screening method and it was capable of differentiated and subdivided patients into specific type of disease, indicating the great potential of application of DCI-OV into clinical related disease diagnosis.

Recent advances in dual-target-activated fluorescent probes for biosensing and bioimaging

Fluorescent probes have been powerful tools for visualizing and quantifying multiple dynamic processes in living cells. However, the currently developed probes are often constructed by conjugation a fluorophore with a recognition moiety and given signal-output after triggering with one singly target interest. Compared with the single-target-activated fluorescent probes mentioned above, the dual-target-activated ones, triggering with one target under stimulus (such as photoirradiation, microenvironment) or another targets, have the advantages of advoiding nonspecific activation and "false positive" results in complicated environments. In recent years, many dual-target-activated fluorescent probes have been developed to detect various biologically relevant species. In view of the importance of a comprehensive understanding of dual-target- activated fluorescent probes, a thorough summary of this topic is urgently needed. However, no comprehensive and critical review on dual target activated fluorescent probes has been published recently. In this review, we focus on the dual-target-activated fluorescent probes and briefly outline their types and current state of development. In each type, the chemical structure, proposed responsive mechanism and application of probes are highlighted. At last, the challenges and prospective opportunities of every type were proposed.

Simultaneous tracking of autophagy and oxidative stress during stroke with an ICT-TBET integrated ratiometric two-photon platform

Over recent years, fluorescent probes exhibiting simultaneous responses to multiple targets have been developed for in situ, real-time monitoring of cellular metabolism using two photon fluorescence sensing techniques due to numerous advantages including ease of operation, rapid reporting, high resolution, long visualization time and being non-invasive. However, due to interference from different fluorescence channels during simultaneous monitoring of multiple targets and the lack of ratiometric capability amongst the available probes, the accuracy in tracing metabolic processes has been restricted. With this research, using a through-bond energy transfer (TBET) mechanism, we designed a viscosity and peroxynitrite (ONOO^-) mitochondria-targeting two-photon ratiometric fluorescent probe Mito-ONOO. Our results indicated that with decreasing levels of mitochondrial viscosity and increasing levels of ONOO^-, the maximum of the emission wavelength of the probe shifted from 621 nm to 495 nm under 810 nm two-photon excitation. The baselines for the two emission peaks were significantly separated (Δλ = 126 nm), improving the resolution and reliability of bioimaging. Moreover, by ratiometric analysis during oxygen-glucose deprivation/reoxygenation (OGD/R, commonly used to simulate cell ischemia/reperfusion injury), the real-time visualization of the metabolic processes of autophagy and oxidative stress was possible. Our research indicated that during cellular oxygen-glucose deprivation/reoxygenation, cells produce ONOO^-, causing cellular oxidative stress and cellular autophagy after 15 min, as such Mito-ONOO exhibits the potential for the monitoring and diagnosis of stroke, as well as providing insight into potential treatments, and drug design.

Near-Infrared in and out: Observation of Autophagy during Stroke via a Lysosome-Targeting Two-Photon Viscosity-Dependent Probe

Fluorescence imaging using probes with two-photon excitation and near-infrared emission is currently the most popular in situ method for monitoring biological species or events, with a large imaging depth, low background fluorescence, low optical damage, and high spatial and temporal resolution. Nevertheless, current fluorescent dyes with near-infrared emission still have some disadvantages such as poor water solubility, low fluorescence quantum yield, and small two-photon absorption cross sections. These drawbacks are mainly caused by the structural characteristics of dyes with large conjugation surfaces but lacking strong and rigid structures. Herein, a lysosome-targeted and viscosity-sensitive probe (NCIC-VIS) is designed and synthesized. The protonation of morpholine not only helps anchor NCIC-VIS to the lysosome but also significantly enhances its water solubility. More importantly, its viscosity can increase the rigid structure of NCIC-VIS, which will improve the fluorescence quantum yield and the two-photon absorption cross section due to the imposed restrictions on molecular torsion. Based on the abovementioned characteristics, the real-time imaging of cellular autophagy (could increase the viscosity of lysosomes) was realized using NCIC-VIS. The results demonstrated that the level of autophagy was significantly enhanced in mice during stroke, while the inhibition of oxidative stress significantly reduced the degree of autophagy. The study corroborates that oxidative stress induced by stroke can lead to the development of autophagy.

Development of a ratiometric nitric oxide probe with baseline resolved emissions by an ESIPT and rhodol ring opened-closed integrated two-photon platform

In recent years, reflecting the degree of cellular inflammation through in situ monitoring of nitric oxide using fluorescence sensing has received much attention due to many merits such as non-invasiveness and easy operation. In particular, two-photon excitation microscopy can significantly improve the imaging resolution and visualization time. In the meantime, a ratiometric-based nitric oxide fluorescent sensor can avoid the interference of many factors, including light source intensity, solvent scattering degree, solvent color, solvent viscosity, probe distribution, and instrument performance, and improve the accuracy of the result. However, the mutual interference of two emission peaks is still an issue restricting the development of this field. In this work, the Rh-NO-F dye obtained by modifying the rhodol dye with benzothiazole exhibited excited state intramolecular proton transfer (ESIPT) in the closed ring state. In the open ring state, however, the emission wavelength can be significantly red-shifted by increasing the degree of dye conjugation. By introducing o-phenylenediamine, the recognition domain of NO, we successfully designed and synthesized a ratiometric two-photon NO fluorescent probe, Rh-NO-P, which showed a 154 nm increase in the maximum emission wavelength before and after the response and almost no interference between the two emission peaks. Confocal imaging showed that the probe could achieve in situ detection of exogenous NO fluctuations in cells. The probe was also successfully applied to detect the changes in NO content during wound healing in mice.

A NO-specific two-photon ratiometric probe for evaluation of inflammation in the process of diabetic mouse skin ulcers was constructed.

Near-infrared fluorescent probe for hydrogen sulfide: high-fidelity ferroptosis evaluation in vivo during stroke

Ferroptosis is closely associated with cancer, neurodegenerative diseases and ischemia-reperfusion injury and the detection of its pathological process is very important for early disease diagnosis. Fluorescence based sensing technologies have become excellent tools due to the real-time detection of cellular physiological or pathological processes. However, to date the detection of ferroptosis using reducing substances as markers has not been achieved since the reducing substances are not only present at extremely low concentrations during ferroptosis but also play a key role in the further development of ferroptosis. Significantly, sensors for reducing substances usually consume reducing substances, instigating a redox imbalance, which further aggravates the progression of ferroptosis. In this work, a H₂S triggered and H₂S releasing near-infrared fluorescent probe (HL-H₂S) was developed for the high-fidelity in situ imaging of ferroptosis. In the imaging process, HL-H₂S consumes H₂S and releases carbonyl sulfide, which is then catalyzed by carbonic anhydrase to produce H₂S. Importantly, this strategy does not intensify ferroptosis since it avoids disruption of the redox homeostasis. Furthermore, using erastin as an inducer for ferroptosis, the observed trends for Fe²⁺, MDA, and GSH, indicate that the introduction of the HL-H₂S probe does not exacerbate ferroptosis. In contrast, ferroptosis progression was significantly promoted when the release of H₂S from HL-H₂S was inhibited using AZ. These results indicate that the H₂S triggered and H₂S releasing fluorescent probe did not interfere with the progression of ferroptosis, thus enabling high-fidelity in situ imaging of ferroptosis.

A H₂S triggered and H₂S releasing near-infrared fluorescent probe (HL-H₂S) was developed. HL-H₂S does not interfere with the progression of ferroptosis by consuming H₂S, thus enabling high-fidelity in situ imaging of ferroptosis.

Observation of macrophage autophagy in the healing of diabetic ulcers via a lysosome-targeting polarity-specific two-photon probe

As a disease with high incidence, mutilation, and fatality rates, diabetic ulcers (DUs) have become a difficult and complicated disease of widely concern in recent years due to the unclear healing mechanism. The main reason for the delayed healing in DU patients is the unduly long chronic inflammation window, and the polarization state of macrophages plays a key role in this process. Since autophagy is believed to be closely related to the polarization trend of macrophages, recent studies have shown that autophagy is closely related to the healing of DU. To this end, a lysosome-targeting polarity-sensitive probe, XZTU-VIS, was developed to monitor the changes in lysosomal polarity, thereby assessing the autophagy of macrophages in mice suffering from DU. The experimental results showed that under two-photon fluorescence microscopy, the green channel fluorescence signal of XZTU-VIS decreased significantly during autophagy. In the meantime, DU models established using BV-2 cells and mice showed a process that could cause inflammation and the release of ROS, thereby inducing autophagy.

A polarity-dependent two-photon fluorescent probe for evaluation of autophagy in the process of diabetic mouse skin ulcer-induced inflammation was constructed.

A near-infrared, colorimetric and ratiometric fluorescent sensor with high sensitivity to hydrogen peroxide and viscosity for solutions detection and imaging living cells

Considering to the importance of hydrogen peroxide (H₂O₂) in a number of pathological processes and fluorescence sensor as a powerful tool for imaging and therapy of H₂O₂-related diseases, herein, a near-infrared fluorescent sensor for colorimetric and ratiometric detecting H₂O₂ and viscosity, named NCR, was constructed and reported. Its long emission wavelength (670 nm) avoids the interference of biological autofluorescence. When NCR interacted with 10 equiv. of H₂O₂, colorimetric characteristic provides a clear naked eye recognition (colourless turned to green under UV light and light purple turned to colorless under visible light). In addition, NCR shows high sensitivity to viscosity with or without the addition of H₂O₂. It was also found alkaline environment contribute to enhance fluorescence response to H₂O₂. A suitable water-octanol partition coefficient (logP, 0.521 ± 0.003) and low cytotoxicity displayed that NCR is very favorable to image living cells. Furthermore, exogenous and endogenous H₂O₂ was detected successfully by NCR in living Hela cells. These studies indicate that NCR has great potential to develop as a practical tool for monitoring H₂O₂ level in biological process.

Recent Advances in Fluorescence Imaging of Pulmonary Fibrosis in Animal Models

Pulmonary fibrosis (PF) is a lung disease that may cause impaired gas exchange and respiratory failure while being difficult to treat. Rapid, sensitive, and accurate detection of lung tissue and cell changes is essential for the effective diagnosis and treatment of PF. Currently, the commonly-used high-resolution computed tomography (HRCT) imaging has been challenging to distinguish early PF from other pathological processes in the lung structure. Magnetic resonance imaging (MRI) using hyperpolarized gases is hampered by the higher cost to become a routine diagnostic tool. As a result, the development of new PF imaging technologies may be a promising solution. Here, we summarize and discuss recent advances in fluorescence imaging as a talented optical technique for the diagnosis and evaluation of PF, including collagen imaging, oxidative stress, inflammation, and PF-related biomarkers. The design strategies of the probes for fluorescence imaging (including multimodal imaging) of PF are briefly described, which can provide new ideas for the future PF-related imaging research. It is hoped that this review will promote the translation of fluorescence imaging into a clinically usable assay in PF.