A water-soluble fluorescence resonance energy transfer probe for hypochlorous acid and its application to cell imaging

Ex Tenebris Lux: Illuminating Reactive Oxygen and Nitrogen Species with Small Molecule Probes

Reactive oxygen and nitrogen species are small reactive molecules derived from elements in the air─oxygen and nitrogen. They are produced in biological systems to mediate fundamental aspects of cellular signaling but must be very tightly balanced to prevent indiscriminate damage to biological molecules. Small molecule probes can transmute the specific nature of each reactive oxygen and nitrogen species into an observable luminescent signal (or even an acoustic wave) to offer sensitive and selective imaging in living cells and whole animals. This review focuses specifically on small molecule probes for superoxide, hydrogen peroxide, hypochlorite, nitric oxide, and peroxynitrite that provide a luminescent or photoacoustic signal. Important background information on general photophysical phenomena, common probe designs, mechanisms, and imaging modalities will be provided, and then, probes for each analyte will be thoroughly evaluated. A discussion of the successes of the field will be presented, followed by recommendations for improvement and a future outlook of emerging trends. Our objectives are to provide an informative, useful, and thorough field guide to small molecule probes for reactive oxygen and nitrogen species as well as important context to compare the ecosystem of chemistries and molecular scaffolds that has manifested within the field.

Pyrazolone-Protein Interaction Enables Long-Term Retention Staining and Facile Artificial Biorecognition on Cell Membranes

Cell membrane genetic engineering has been utilized to confer cell membranes with functionalities for diagnostic and therapeutic purposes but concerns over cost and variable modification results. Although nongenetic chemical modification and phospholipid insertion strategies are more convenient, they still face bottlenecks in either biosafety or stability of the modifications. Herein, we show that pyrazolone-bearing molecules can bind to proteins with high stability, which is mainly contributed to by the multiple interactions between pyrazolone and basic amino acids. This new binding model offers a simple and versatile noncovalent approach for cell membrane functionalization. By binding to cell membrane proteins, pyrazolone-bearing dyes enabled precise cell tracking in vitro (>96 h) and in vivo (>21 days) without interfering with the protein function or causing cell death. Furthermore, the convenient anchor of pyrazolone-bearing biotin on cell membranes rendered the biorecognition to avidin, showing the potential for artificially creating cell targetability.

Rational Design of Activatable Lanthanide NIR-IIb Emissive Nanoprobe for In Situ Specific Imaging of HOCl In Vivo

Hypochlorous acid (HOCl), as an indispensable signaling molecule in organisms, is one of the key members of reactive oxygen species (ROS). However, in vivo, real-time dynamic near-infrared fluorescence imaging of HOCl levels in the 1400-1700 nm sub-window (NIR-IIb) remains a major challenge due to the lack of suitable detection methods. Herein, a general design of HOCl-responsive NIR-IIb fluorescence nanoprobe is proposed by integrating NaLuF4Yb/Er@NaLuF4 downshift nanoparticles (DSNPs) and HOCl recognition/NIR-IIb emissive modulation unit of M2-xS (M = Cu, Co, Pb) nanodots for real-time monitoring of HOCl levels. The fluorescence modulation unit of M2-xS nanodots presents remarkably enhanced absorption than Yb sensitizer at 980 nm and greatly inhibits the NIR-IIb fluorescence emission via competitive absorption mechanism. While, the M2-xS nanodots are easily degraded after triggering by HOCl, resulting in HOCl responsive turn-on (≈ten folds) NIR-IIb emission at 1532 nm. More importantly, in vivo highly precise and specific monitoring of inflammatory with abnormal HOCl expression is successfully achieved. Thus, the explored competitive absorption mediated quenching-activation mechanism provides a new general strategy of designing HOCl-responsive NIR-IIb fluorescence nanoprobe for highly specific and sensitive HOCl detection.

Fluorescent dyes based on rhodamine derivatives for bioimaging and therapeutics: recent progress, challenges, and prospects

Since their inception, rhodamine dyes have been extensively applied in biotechnology as fluorescent markers or for the detection of biomolecules owing to their good optical physical properties. Accordingly, they have emerged as a powerful tool for the visualization of living systems. In addition to fluorescence bioimaging, the molecular design of rhodamine derivatives with disease therapeutic functions (e.g., cancer and bacterial infection) has recently attracted increased research attention, which is significantly important for the construction of molecular libraries for diagnostic and therapeutic integration. However, reviews focusing on integrated design strategies for rhodamine dye-based diagnosis and treatment and their wide application in disease treatment are extremely rare. In this review, first, a brief history of the development of rhodamine fluorescent dyes, the transformation of rhodamine fluorescent dyes from bioimaging to disease therapy, and the concept of optics-based diagnosis and treatment integration and its significance to human development are presented. Next, a systematic review of several excellent rhodamine-based derivatives for bioimaging, as well as for disease diagnosis and treatment, is presented. Finally, the challenges in practical integration of rhodamine-based diagnostic and treatment dyes and the future outlook of clinical translation are also discussed.

Advances in the Development of Water-Soluble Fluorogenic Probes for Bioimaging of Hypochlorite/Hypochlorous Acid in Cells and Organisms

This mini-review summarizes the development of intracellular fluorogenic probes for biological investigations of hypochlorous acid/hypochlorite (HOCl/OCl^-) in living cells and tissues. Monitoring the formation or effects of reactive oxygen species (ROS) inside living systems is critical in determining their roles in human physiology. HOCl/OCl^- is considered as an important member of the nonradical ROS family for its decisive microbicidal action in the innate immune system. Even though HOCl/OCl^- plays a defensive role in human health, abnormal or overexpression may have detrimental effects on the host physiology leading to many diseases, including neurodegeneration and cancer. In recent years, progress in the development of fluorescent imaging probes for observing HOCl/OCl^- levels in live cells and tissues has been made. Despite considerable advancement, challenges still exist in areas like working solvent/media, pH, response time, buffer selection, emission region, and others. In addition, this account aims to discuss the design strategies and sensing mechanisms of the representative fluorogenic probes for bioimaging of HOCl/OCl^-, endogenously and exogenously. Herein, we also have tried to provide the future direction to develop HOCl/OCl^- specific probes for disease diagnosis with particular attention to the requirement of the recognition group, solvent, and buffer media, which will be beneficial for those working in the domain of biomedical research.

A ratiometric water-soluble fluorescent probe for the detection of sulfur dioxide derivative in sinusitis mice model

Sulfur dioxide (SO₂) plays an extremely important role in the basic processes of physiology and pathology. As an antioxidant, SO₂ can maintain the redox homeostasis in the cell. Excessive inhalation of SO₂ would lead to irreparable respiratory damage, resulting in respiratory diseases, neurological disorders, and even cardiovascular disease. Thus, it is urgent to exploit an excellent way to monitor SO₂ derivatives in biological system. Herein, we design a water-soluble ratiometric fluorescent probe to fast detect the level of SO₂ derivatives in living cells in vivo. The probe displays obvious fluorescence signal at long wavelength, which is helpful for imaging of biological system. After respond to SO₂ derivatives, the fluorescence signal at 465 nm increases rapidly due to the Michael addition reaction is triggered, further causing the disruption of large conjugated system. The probe exhibits high selectivity and fast respond to SO₂ derivatives, which can be able to sensitive and real-time monitoring of SO₂ derivatives level in living cells. Moreover, the probe reveals a low detection limit and a great linear relationship to SO₂ derivatives. Based on the negligible cytotoxicity and good biocompatibility of the probe, which is employed to detect exogenous and endogenous SO₂ derivatives in living cells. In addition, it is also served as a potential chemical tool to detect SO₂ derivatives in mice model of sinusitis.

A phenothiazine coumarin based ratiometric fluorescent probe for real-time detection of lysosomal hypochlorite in living cell and zebra fish

Hypochlorite (ClO^-), a type of reactive oxygen species (ROS), plays an essential role in complex biological systems. Real-time detection of the content and distribution of ClO^- in cells or subcellular organelle is critically essential. In this paper, a lysosomal-targeted fluorescent probe, Cou-Lyso, was constructed for real-time detection of ClO^- in a ratiometric manner, achieving high sensitivity with a low detection limit (0.58 μM). Upon reaction with ClO^-, this probe was subjected to a significant fluorescence change from red emission (λ_maxem = 610 nm) to green emission (λ_maxem = 535 nm) with the ratio of I_{535 nm}/I_{610 nm} displaying a 76-fold enhancement from 0.04 to 3.03. The confocal imaging experiments for Cou-Lyso showed that this probe could detect ClO^- in living cell and zebra fish. This probe has been successfully applied to stain lysosome and image lysosomal ClO^- based on co-localization imaging experiments.

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.

Fast-Response Fluorogenic Probe for Visualizing Hypochlorite in Living Cells and in Zebrafish

A fast-response fluorogenic probe-compound D1-for monitoring hypochlorite (ClO^- ), based on specific ClO^- cleavage of a C=N bond and producing results observable to the naked eye, has been developed. The response of the probe to ClO^- increases linearly, and the fluorescence intensity was heightened by a factor of about 25. D1 responses to ClO^- , with high selectivity and sensitivity, were observable by naked eye within 10 s. D1 can not only detect levels of hypochlorite in vitro, such as in urine, but is also capable of monitoring hypochlorite content under extremely cold conditions, as low as -78 °C. Meanwhile, its good biocompatibility permitted the use of D1 to detect intracellular ClO^- by confocal microscopy. Moreover, D1 was successfully applied to monitor exogenous and endogenous ClO^- in zebrafish through fluorescence imaging.

The detection for hypochlorite by UV-Vis and fluorescent spectra based on oxidized ring opening and successive hydrolysis reaction

In this work, two high selective and sensitive fluorescent probes for ClO(-), 7-Hydroxycoumarin and 4-Hydroxycoumarin were designed. The reaction mechanism that we speculated was the oxidized ring opening reaction and hydrolysis. The detection could be realized in quasi-aqueous phase and the detection limits of probe [7] and probe [4] for ClO(-) were found to be 56.8nM and 70.5nM. Furthermore, the probes can be used to cell imagings.

A high-resolution mitochondria-targeting ratiometric fluorescent probe for detection of the endogenous hypochlorous acid

Hypochlorite anion, one of the biologically important reactive oxygen species, plays an essential role in diverse normal biochemical functions and abnormal pathological processes. Herein, an efficient high-resolution mitochondria-targeting ratiometric fluorescent probe for hypochlorous acid detection has been designed, synthesized and characterized. It is easily synthesized by the condensation reaction (CC) of a 2-(2-hydroxyphenyl) quinazolin-4(3H)-one fluorophore and a cyanine group (mitochondria-targeting), which made the whole molecular a large Stokes shift (210nm) and the two well-resolved emission peaks separated by 140nm. As a result, it is considered as a good candidate for high resolution hypochlorous acid imaging in live cells. The ratiometric fluorescent probe exhibited outstanding features of high sensitivity, high selectivity, rapid response time (within 50s), and excellent mitochondria-targeting ability. Moreover, the probe can also be successfully applied to imaging endogenously hypochlorous acid in the mitochondria of living cells with low cytotoxicity, and high resolution.

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.

Sensitive detection of ozone by a practical resorufin-based spectroscopic probe with extremely low background signal

Ozone (O₃) has attracted much attention because of its key role in human health and disease, and its detection is of great importance for various biochemical studies as well as environmental evaluation. Here we develop a simple and practical spectroscopic off-on O₃ probe based on resorufin and the specific reaction of but-3-enyl with O₃. The probe shows an extremely low background spectroscopic signal, but reacts with O₃ producing a distinct color and fluorescence change. The detection limit of the probe for O₃ is 5.9 nM, which corresponds to an ozone concentration of 0.056 mg m⁻³ in air in this study and is lower than the international ambient air quality standard of 0.1 mg m⁻³. More importantly, the proposed probe is worth popularizing, and its applicability has been successfully demonstrated on both the determination of O₃ in real ambient air samples and the imaging of O₃ in biological cells.

Development of a ruthenium(II) complex-based luminescent probe for hypochlorous acid in living cells

A novel Ru(II) complex, [Ru(bpy)2(DNPS-bpy)](PF6)2 (bpy: 2,2'-bipyridine, DNPS-bpy: 4-(2,4-dinitrophenylthio)methylene-4'-methyl-2,2'-bipyridine), has been designed and synthesized as a highly sensitive and selective luminescence probe for the recognition and detection of hypochlorous acid (HOCl) in living cells by exploiting a "signaling moiety-recognition linker-quencher" sandwich approach. The complex possesses large stokes shift (170 nm), long emission wavelength (626 nm), and low cytotoxicity. Owing to the effective photoinduced electron transfer (PET) from Ru(II) center to the electron acceptor, 2,4-dinitrophenyl (DNP), the red-emission of bipyridine-Ru(II) complex was completely withheld. In aqueous media, HOCl can trigger an oxidation reaction to cleave the DNP moiety from the Ru(II) complex, which results in the formation of a highly luminescent bipyridine-Ru(II) complex derivative, [Ru(bpy)2(COOH-bpy)](PF6)2 (COOH-bpy: 4'-methyl-2,2'-bipyridyl-4-carboxylic acid), accompanied by a 190-fold luminescence enhancement. Cell imaging experimental results demonstrated that [Ru(bpy)2(DNPS-bpy)](PF6)2 is membrane permeable, and can be applied for capturing and visualizing the exogenous/endogenous HOCl molecules in living cell samples. The development of this Ru(II) complex probe not only provides a useful tool for monitoring HOCl in living systems, but also strengthens the application of transition metal complex-based luminescent probes for bioimaging.