Two-Photon Cellular and Intravital Imaging of Hypochlorous Acid by Fluorescent Probes That Exhibit a Synergistic Excited-State Intramolecular Proton Transfer-Intramolecular Charge Transfer Mechanism Enabling Near-Infrared Emission with a Large Stokes Shift

To develop highly effective molecular tools for intravital imaging of hypochlorous acid (HOCl), in this study, we initially designed two-photon hybrid fluorophores, SDP and P-SDP, by conjugating the classical dye 2-(2'-hydroxyphenyl)benzothiazole with the two-photon hydroxylphenyl-butadienylpyridinium fluorophore. The designed fluorophores exhibit a synergistic interaction between excited-state intramolecular proton transfer and intramolecular charge transfer mechanisms, enabling near-infrared (NIR) emission and significant Stokes shifts. Subsequently, using these fluorophores, we developed two HOCl fluorescent probes, SDP-SN and P-SDP-SN, by further incorporating N,N-dimethylthiocarbamate as a specific recognition group for HOCl. Toward HOCl, both SDP-SN and P-SDP-SN demonstrate an ultrafast response (less than 3 s), NIR emission at wavelengths of 714 and 682 nm, and remarkable Stokes shifts of 303 and 271 nm, respectively. Leveraging these advantages in conjunction with their ability to cross the blood-brain barrier, the probes find successful application in two-photon cellular and intravital imaging of HOCl. This includes visualizing endogenous generation of HOCl in cellular models related to inflammation, hyperglycemia, and ferroptosis, as well as mapping in vivo generation of HOCl within the brain and abdominal cavity using a murine model of systemic inflammation.

Near-Infrared Fluorescent Turn-On Probe for Selective Detection of Hypochlorite in Aqueous Medium and Live Cell Imaging

Hypochlorite, as an important reactive oxygen species (ROS), plays a vital role in many physiological and pathological processes, but an excess concentration of hypochlorite (ClO-) may become toxic to humans and cause disease. Hence, the selective and rapid detection of hypochlorite (ClO-) is necessary for human safety. Here, we report a novel near-infrared (NIR) fluorescence "turn-on" and highly selective benzophenoxazinium chloride-based fluorescent probe, BPH (benzophenoxazinium dihydroxy benzaldehyde), for hypochlorite detection. Due to hypochlorite-induced vicinal diol oxidation to the corresponding ortho benzoquinone derivative, the photoinduced electron transfer (PET) process, which was operating from vicinal diol to the benzophenoxazinium chloride receptor moiety, was suddenly inhibited, as a result of which strong NIR fluorescence "turn-on" emission was observed. The detection limit of BPH was found to be 2.39 × 10-10 M, or 0.23 nM. BPH was successfully applied for exogenous and endogenous hypochlorite detection in live MDA-MB 231 cells.

Sensing Hypochlorite or pH variations in live cells and zebrafish with a novel dual-functional ratiometric and colorimetric chemosensor

Both HClO and pH are essential players in multiple biological processes, which thus need to be controlled properly. Dysregulated HClO or pH correlates with many diseases. To meet these challenges, we need to develop highly competent probes for monitoring them. Over the years, despite a rich history of the development of HClO or pH probes, those that can do both jobs are still deficient. Herein, we present a novel dual-functional chemosensor, CMHN, which exhibits a blue and red shift of its fluorescence emission upon reacting with HClO or OH^-, respectively. CMHN was successfully harnessed in the imaging detection of HClO or OH^- in aqueous solutions, live cells, and zebrafish. Results indicated CMHN can detect HClO with high sensitivity (LOD -132 nM), a quick response time (<70 s), and high selectivity over dozens of interfering species through a colorimetric and ratiometric response. Besides, CMHN can probe pH changes sensitively and reversibly. Its working mechanism was verified by DFT calculations. These superior features make CMHN excel among the HClO or pH probes reported so far. Taken together, CMHN replenishes the deficiency in currently developed HClO or pH probes and paves the way for developing multifunctional HClO or pH probes in the future.

Direct Amidation to Access 3-Amido-1,8-Naphthalimides Including Fluorescent Scriptaid Analogues as HDAC Inhibitors

Methodology to access fluorescent 3-amido-1,8-naphthalimides using direct Buchwald-Hartwig amidation is described. The protocol was successfully used to couple a number of substrates (including an alkylamide, an arylamide, a lactam and a carbamate) to 3-bromo-1,8-naphthalimide in good yield. To further exemplify the approach, a set of scriptaid analogues with amide substituents at the 3-position were prepared. The new compounds were more potent than scriptaid at a number of histone deacetylase (HDAC) isoforms including HDAC6. Activity was further confirmed in a whole cell tubulin deacetylation assay where the inhibitors were more active than the established HDAC6 selective inhibitor Tubastatin. The optical properties of these new, highly active, compounds make them amenable to cellular imaging studies and theranostic applications.

A highly sensitive, fast responsive and reversible naphthalimide-based fluorescent probe for hypochlorous acid and ascorbic acid in aqueous solution and living cells

It is very important to exploit real-time, ultrasensitive and specific visualization detection methods for hypochlorous acid/hypochlorite (HOCl/ClO^-) in biological systems as they are the guardians of the human immune system against pathogens invasion. In our work, we designed a novel reversible naphthalimide-based fluorescent probe NAP-OH to recognize HClO/ClO^- with a unique selective colorimetric and fluorescent response, a short response time (<8 s) and a high sensitivity (10.3 nM). In addition, NAP-OH exhibits a novel on-off-on fluorescence response to ClO^-/ascorbic acid (AA) with good cycle stability. The fluorescence signal is quenched because HClO/ClO^- oxidizes the subunit of NAP-OH to the segment 2,2,6,6-tetramethyl-1-oxo-piperidinium in NAP-O, which can be reduced by AA with the recovery of fluorescence. Finally, the confocal fluorescence imaging has been performed, which proves that NAP-OH can satisfactorily monitor intracellular endogenous and exogenous HClO/AA redox cycles.

CeO2 Nanomaterials from Diesel Engine Exhaust Induce DNA Damage and Oxidative Stress in Human and Rat Sperm In Vitro

Cerium dioxide nanomaterials (CeO₂ NMs) are widely used in nano-based diesel additives to decrease the emission of toxic compounds, but they have been shown to increase the emission of ultrafine particles as well as the amount of released Ce. The Organization for Economic Cooperation and Development included CeO₂ NMs in the priority list of nanomaterials that require urgent evaluation, and the potential hazard of aged CeO₂ NM exposure remains unexplored. Herein, human and rat sperm cells were exposed in vitro to a CeO₂ NM-based diesel additive (called Envirox^TM), burned at 850 °C to mimic its release after combustion in a diesel engine. We demonstrated significant DNA damage after in vitro exposure to the lowest tested concentration (1 µg·L⁻¹) using the alkaline comet assay (ACA). We also showed a significant increase in oxidative stress in human sperm after in vitro exposure to 1 µg·L⁻¹ aged CeO₂ NMs evaluated by the H₂DCF-DA probe. Electron microscopy showed no internalization of aged CeO₂ NMs in human sperm but an affinity for the head plasma membrane. The results obtained in this study provide some insight on the complex cellular mechanisms by which aged CeO₂ NMs could exert in vitro biological effects on human spermatozoa and generate ROS.

The reduction performance of double bonds regulated by the competition of push-pull electron groups to realize the colorimetric and fluorescence recognition of hypochlorous acid

Based on its reducibility, the double bond can act as a reaction site for hypochlorous acid (HOCl), which had been demonstrated by a great deal of work. Nevertheless, the reactivity is influenced by the adjacent chemical environment. Therefore, in this work, we constructed a probe (QI) by methoxy-substituted quinoline conjugating dicyanoisoflurone, in which dicyano and pyridine N act as electron-withdrawing groups and the methoxy acts as an electron-donating group, to regulate their adjacent C[double bond, length as m-dash]C reactivity. The "push-pull" electron effect between the methoxy group and the pyridine N led to the C[double bond, length as m-dash]C bond being passivated. On the other hand, another C[double bond, length as m-dash]C bond was activated by the strong electron-pulling effect of the dicyano group. Thus, the previously weak intramolecular charge transfer became stronger after the dicyano adjacent to the C[double bond, length as m-dash]C was oxidized by HOCl, and showed a strong emission shifted from 570 to 520 nm along with a color change. The reaction mechanism was verified by mass spectrometry, NMR and theoretical calculation, and further bioimaging demonstrated the practical application of the probe.

HClO-Activated Fluorescence and Photosensitization from an AIE Nanoprobe for Image-Guided Bacterial Ablation in Phagocytes

Bacteria hiding in host phagocytes are difficult to kill, which can cause phagocyte disorders resulting in local and systemic tissue damage. Effective accumulation of activatable photosensitizers (PSs) in phagocytes to realize selective imaging and on-demand photodynamic ablation of bacteria is of great scientific and practical interests for precise bacteria diagnosis and treatment. Herein, HClO-activatable theranostic nanoprobes, DTF-FFP NPs, for image-guided bacterial ablation in phagocytes are introduced. DTF-FFP NPs are prepared by nanoprecipitation of an HClO-responsive near-infrared molecule FFP and an efficient PS DTF with aggregation-induced emission characteristic using an amphiphilic polymer Pluronic F127 as the encapsulation matrix. As an energy acceptor, FFP can quench both fluorescence and production of reactive oxygen species (ROS) of DTF, thus eliminating the phototoxicity of DTF-FFP NPs in normal cells and tissues. Once delivered to the infection sites, DTF-FFP NPs light up with red fluorescence and efficiently generate ROS owing to the degradation of FFP by the stimulated release of HClO in phagocytes. The selective activation of fluorescence and photosensitization is successfully confirmed by both in vitro and in vivo results, demonstrating the effectiveness and theranostic potential of DTF-FFP NPs in precise bacterial therapy.

Luminescent probes for hypochlorous acid in vitro and in vivo

HClO/ClO^- is the most effective antibacterial active oxygen in neutrophils. However, its excessive existence often leads to the destruction of human physiological mechanisms. In recent years, the developed luminescent probes for the detection of HClO/ClO^- are not only conducive to improve the sensitivity and selectivity of HClO/ClO^- detection, but also play a crucial role in understanding the biological functions of HClO/ClO^-. In addition, luminescent probe-based biological imaging for HClO/ClO^- at sub-cellular resolution has become a powerful tool for biopathology and medical diagnostic research. This article reviews a variety of luminescent probes for the detection of HClO/ClO^-in vitro and in vivo with different design principles and mechanisms, including fluorescence, phosphorescence, and chemiluminescence. The photophysical/chemical properties and biological applications of these luminescent probes were outlined. Finally, we summarized the merits and demerits of the developed luminescent probes and discussed their challenges and future development trends. It is hoped that this review can provide some inspiration for the development of luminescent probe-based strategies and to promote the further research of biomedical luminescent probes for HClO/ClO^-.

Imaging of Hypochlorous Acid by Fluorescence and Applications in Biological Systems

In recent decades, HOCl research has attracted a lot of scientists from around the world. This chemical species is well known as an important player in the biological systems of eukaryotic organisms including humans. In the human body, HOCl is produced by the myeloperoxidase enzyme from superoxide in very low concentrations (20 to 400 μm); this species is secreted by neutrophils and monocytes to help fight pathogens. However, in the condition called "oxidative stress", HOCl has the capability to attack many important biomolecules such as amino acids, proteins, nucleotides, nucleic acids, carbohydrates, and lipids; these reactions could ultimately contribute to a number of diseases such as neurodegenerative diseases (AD, PD, and ALS), cardiovascular diseases, and diabetes. In this review, we discuss recent efforts by scientists to synthesize various fluorophores which are attached to receptors to detect HOCl such as: chalcogen-based oxidation, oxidation of 4-methoxyphenol, oxime/imine, lactone ring opening, and hydrazine. These synthetic molecules, involving rational synthetic pathways, allow us to chemoselectively target HOCl and to study the level of HOCl selectivity through emission responses. Virtually all the reports here deal with well-defined and small synthetic molecular systems. A large number of published compounds have been reported over the past years; this growing field has given scientists new insights regarding the design of the chemosensors. Reversibility, for example is considered important from the stand point of chemosensor reuse within the biological system; facile regenerability using secondary analytes to obtain the initial probe is a very promising avenue. Another aspect which is also important is the energy of the emission wavelength of the sensor; near-infrared (NIR) emission is favorable to prevent autofluorescence and harmful irradiation of tissue; thus, extended applicability of such sensors can be made to the mouse model or animal model to help image internal organs. In this review, we describe several well-known types of receptors that are covalently attached to the fluorophore to detect HOCl. We also discuss the common fluorophores which are used by chemist to detect HOCl, Apart from the chemical aspects, we also discuss the capabilities of the compounds to detect HOCl in living cells as measured through confocal imaging. The growing insight from HOCl probing suggests that there is still much room for improvement regarding the available molecular designs, knowledge of interplay between analytes, biological applicability, biological targeting, and chemical switching, which can also serve to further sensor and theurapeutic agent development alike.

Development of a Concise Rhodamine-Formylhydrazine Type Fluorescent Probe for Highly Specific and Ultrasensitive Tracing of Basal HOCl in Live Cells and Zebrafish

Hypochlorous acid (HOCl) has received special attention by virtue of its pivotal antimicrobial nature, and the appropriate amount of HOCl is beneficial to innate immunity of host to cope with microbial invasion. However, the uncontrollable accumulation of HOCl is implicated in many human diseases and even cancers. Thus, to determine its deeper biological functions, it is significantly important to specifically monitor intracellular HOCl in biosystems. Herein, we rationally designed a simple fluorescent probe FH-HA on the basis of the formylhydrazine recognition receptor and rhodamine B fluorophore. It is worth noting that the formylhydrazine moiety for the first time is adopted as the recognition receptor for specifically recognizing HOCl. Additionally, probe FH-HA also exhibited excellent performance in many areas including satisfactory water-solubility, high specificity, and excellent sensitivity. Notably, probe FH-HA could quickly respond to HOCl (within 3 s), which facilitates the tracing of transient HOCl. More importantly, probe FH-HA was capable of specifically tracing the fluctuations of endogenous HOCl in living cells and zebrafish, and it could monitor basal HOCl in cancer cells to distinguish cancer cells from normal ones.

Small-Molecule-Based Fluorescent Sensors for Selective Detection of Reactive Oxygen Species in Biological Systems

Reactive oxygen species (ROS) encompass a collection of intricately linked chemical entities characterized by individually distinct physicochemical properties and biological reactivities. Although excessive ROS generation is well known to underpin disease development, it has become increasingly evident that ROS also play central roles in redox regulation and normal physiology. A major challenge in uncovering the relevant biological mechanisms and deconvoluting the apparently paradoxical roles of distinct ROS in human health and disease lies in the selective and sensitive detection of these transient species in the complex biological milieu. Small-molecule-based fluorescent sensors enable molecular imaging of ROS with great spatial and temporal resolution and have thus been appreciated as excellent tools for aiding discoveries in modern redox biology. We review a selection of state-of-the-art sensors with demonstrated utility in biological systems. By providing a systematic overview based on underlying chemical sensing mechanisms, we wish to highlight the strengths and weaknesses in prior sensor works and propose some guiding principles for the development of future probes.

Bioluminescent probe for detecting endogenous hypochlorite in living mice

As a kind of biologically important reactive oxygen species (ROS), hypochlorite (ClO^-) plays a crucial role in many physiological processes. As such, endogenous ClO^- is a powerful antibacterial agent during pathogen invasion. Nonetheless, excessive endogenous ClO^- could pose a health threat to mammalian animals including humans. However, the detection of endogenous ClO^- by bioluminescence probes in vivo remains a considerable challenge. Herein, based on a caged strategy, we developed a turn-on bioluminescent probe 1 for the highly selective detection of ClO^-in vitro and imaging endogenous ClO^- in a mouse inflammation model. We anticipate that such a probe could help us understand the role of endogenous ClO^- in a variety of physiological and pathological processes.

Rational Design of a Hepatoma-Specific Fluorescent Probe for HOCl and Its Bioimaging Applications in Living HepG2 Cells

Liver cancer is a kind of high mortality cancer due to the difficulty of early diagnosis. And according to the reports, the concentration of reactive oxygen species (ROS) was higher in cancer cells than normal cells. Therefore, developing an effective fluorescent probe for hepatoma-selective imaging of hypochlorous acid (HOCl) which is one of the vital ROS is of great importance for understanding the role of HOCl in liver cancer pathogenesis. However, the cell-selective fluorescent probe still remains a difficult task among current reports. Herein, a galactose-appended naphthalimide (Gal-NPA) with p-aminophenylether as a new receptor and galactose moiety as hepatoma targeting unit was synthesized and employed to detect endogenous HOCl in living HepG2 cells. This probe was proved to possess good water solubility and could respond specifically to HOCl. In addition, probe Gal-NPA could completely react to HOCl within 3 s meanwhile accompanied by tremendous fluorescence enhancement. The quantitative linear range between fluorescence intensities and the HOCl concentrations was 0 to 1 μM (detection limit = 0.46 nM). More importantly, fluorescence confocal imaging experiments showed that probe Gal-NPA could discriminate hepatoma cells over other cancer cells and simultaneously trace endogenous HOCl levels in living HepG2 cells. And we thus anticipate that probe Gal-NPA has the potential application for revealing the functions of HOCl in hepatoma cells.

A new ratiometric fluorescent probe for detecting endogenous HClO in living cells

A new benzimidazole-hemicyanine-based ratiometric fluorescent probe (ZBM-H) was developed, which showed high selectivity and sensitivity for detecting HClO.

A nanosensor made of sulfur–nitrogen co-doped carbon dots for “off–on” sensing of hypochlorous acid and Zn(ii) and its bioimaging properties

In this study, we present a facile one-step solvothermal strategy for the fabrication of sulfur–nitrogen co-doped carbon dots (SNCDs) using p-phenylenediamine and cysteamine hydrochloride as the precursors.

A naphthalimide based fast and selective fluorescent probe for hypochlorous acid/hypochlorite and its application for bioimaging

A reversible and mitochondria targetable fluorescent probe (Nap-Se) bearing 1,8-naphthalimide and a selenomorpholine fragment was designed and synthesized.

Lysosomal-Targeted Two-Photon Fluorescent Probe to Sense Hypochlorous Acid in Live Cells

A two-photon reversible fluorescent probe L1 was designed and synthesized. The fluorescence intensity of the probe solution was strong, while the fluorescence of the solution was obviously quenched and the color of the solution was changed after the addition of hypochlorous acid, indicating this is "naked-eye sensor" for the detection of HClO. The probe showed great selectivity for hypochlorous acid over other reactive oxygen species (ROS) and anions. Fluorescence titration experiments showed that the probe has a low detection limit of 0.674 μM. Because of a morpholine group introduced to the naphathalimide framework, probe L1 was successfully applied to detect intracellular HClO in lysosome.

An ultrafast responsive BODIPY-based fluorescent probe for the detection of endogenous hypochlorite in live cells

An ultrafast responsive BODIPY-based fluorescent probe was synthesized to selectively detect endogenous hypochlorite in live cells.

An efficient two-photon fluorescent probe for human NAD(P)H:quinone oxidoreductase (hNQO1) detection and imaging in tumor cells

The probe TPQ, which displays high selectivity and anti-interference ability, was successfully applied to endogenous hNQO1 imaging and for the identification of different cancer cells.

Development of a unique family of two-photon full-color-tunable fluorescent materials for imaging in live subcellular organelles, cells, and tissues

We outline the rational design, synthesis, optical property studies, and biological imaging studies of a unique family of two-photon full-color-tunable functional fluorescent materials.

Recent progress in chromogenic and fluorogenic chemosensors for hypochlorous acid

Due to the biological and industrial importance of hypochlorous acid, the development of optical probes for HOCl has been an active research area. Hypochlorous acid and hypochlorite can oxidize electron-rich analytes with accompanying changes in molecular sensor spectroscopic profiles. Probes for such processes may monitor HOCl levels in the environment or in an organism and via bio-labeling or bioimaging techniques. This review summarizes recent developments in the area of chromogenic and fluorogenic chemosensors for HOCl.

Development of a BODIPY-based ratiometric fluorescent probe for hypochlorous acid and its application in living cells

A BODIPY-based ratiometric fluorescent probe for HOCl has been designed based on the transduction of thioether to sulfoxide function. This probe features a marked absorption and emission blue-shift upon the HOCl-promoted rapid transduction, enabling the highly selective and ratiometric detection. In addition, the probe works excellently within a wide pH range of 4-10, addressing the existing pH dependency issue. Living cells studies demonstrate that the probe is cell membrane permeable and can be employed successfully to image endogenous HOCl generation in macrophage cells.

One small molecule as a theranostic agent: naphthalimide dye for subcellular fluorescence localization and photodynamic therapy in vivo

A small single-molecule theranostic agent based on naphthalimide was developed, which possessed both bright fluorescence imaging and effective photodynamic therapeutic treatment.

A mitochondria-targeted near-infrared probe for colorimetric and ratiometric fluorescence detection of hypochlorite in living cells

A mitochondria-targeted near-infrared probe for rapid, sensitive and specific detection of hypochlorite with colorimetric and ratiometric fluorescence dual responses.

A ratiometric fluorescence sensor for HOCl based on a FRET platform and application in living cells

A ratiometric fluorescent probe CRSH based on a FRET platform for detecting HOCl. CRSH showed high selectivity, excellent sensitivity and a fast response toward HOCl.

A highly selective and sensitive fluorescence probe for rapid detection of hypochlorite in tap water and cancer cells

A fluorescence probe for the rapid detection of hypochlorite in tap water and cancer cells.

A highly selective and sensitive photoinduced electron transfer (PET) based HOCl fluorescent probe in water and its endogenous imaging in living cells

A probe based on the phenothiazine–acridine orange conjugate (Ptz–AO) has been designed and synthesized for the sensitive and selective detection of HOCl.