Ex Vivo Tracking of Endogenous CO with a Ruthenium(II) Complex

A series of anthracene-derived dyes for Cu2+-assisted CO sensing and bio-imaging: synthesis, performance, and mechanism

Endogenous CO acts as an important messenger for signal transduction and therapeutic effect in the human body. Fluorescent imaging appears to be a promising method for endogenous CO recognition, but traditional luminescent probes based on Pd-complexes suffered from defects of high cost. In this work, four anthracene-derived dyes having an = N-N = group were synthesized for Cu2+-assisted CO sensing. Their molecular structure, photophysical performance and spectral response to Cu2+ and CO were analyzed in detail. The optimal probe showed good selectivity and quenching effect to Cu2+, with PLQY (photoluminescence quantum yield) decreased from 0.33 to 0.04. The quenching mechanism was found as a static quenching mechanism by forming a non-fluorescent complex with Cu2+ (stoichiometric ratio = 1:1), as revealed by single crystal, EPR (electron paramagnetic resonance), and XPS (X-ray photoelectron spectroscopy) analysis. Such quenching effect could be reversed by CO, showing recovered fluorescence, with PLQY recovered to 0.32 within 328 s. Discussion on cellular endogenous CO imaging was included as well.

Fe(III)-Based Fluorescent Probe for High-Performance Recognition, Test Strip Analysis, and Cell Imaging of Carbon Monoxide

Fluorescence sensing and imaging techniques are being widely studied for detecting carbon monoxide (CO) in living organisms due to their speed, sensitivity, and ease of use to biological systems. Most fluorescent probes used for this purpose are based on heavy metal ions like Pd, with a few using elements like Ru, Rh, Ir, Os, Tb, and Eu. However, these metals can be expensive and toxic to cells. There is a need for more affordable and biologically safe fluorescent probes for CO detection. Drawing inspiration from the robust affinity exhibited by heme iron toward CO, in this work, a rhodamine derivative called RBF was developed for imaging CO in living cells by binding to Fe(III) and could be used for CO sensing. A Fe(III)-based fluorescent probe for CO imaging in living cells offers advantages of cost effectiveness, low toxicity, and ease of use. The fluorescence detection using the RBF-Fe system showed a direct correlation with increasing levels of CORM-3 (LOD = 146 nM) or the exposure time of CO gas, displaying reduced fluorescence. A CO test paper based on RBF-Fe was created for simple on-site CO detection, where fluorescence would diminish in response to CO exposure, allowing rapid (2 min) visual identification. Imaging of CO in living cells was successfully conducted using the probe system, showing a decrease in fluorescence intensity as CORM-3 concentrations increased, indicating its effectiveness in monitoring CO levels accurately within living cells.

A series of triphenylamine-derived fluorophores attached to a Cu-based MOF for gaseous CO optical sensing: synthesis, performance, and mechanism

A series of triphenylamine-derived fluorescent dyes were attached to a Cu2+-containing MOF (metal-organic framework), denoted as Pm@CuMOF. The molecular structures of these dyes were discussed by the single crystal structures. Their major absorption bands peaked at 410-450 nm, showing emission bands ranging from 556 to 586 nm with emission quantum yields ranging from 8.0 to 15.1%. It was found that the [-N(C2H5)2] group generally improved sensing performance, and the -OH group in the dyes helped the Cu2+ quenching effect. Pm@CuMOF was observed by SEM as nanorods with a width of ~100 nm and a length of 300 nm. Their XRD patterns and N2 adsorption/desorption isotherms were recorded to confirm their porous structure. A low probe loading level of ~4% was determined by TGA result. The CO sensing mechanism was revealed as a Cu2+/Cu+-involved sensing mechanism based on the result of NMR titration, IR, XPS, and EPR. The fluorescence of these triphenylamine-derived dyes was firstly quenched by CuMOF. In contact with CO, Cu2+ was reduced to Cu+, accompanied by the release and fluorescence recovery of the fluorescent dyes, showing emission turn-on effect towards CO gas. Pm@CuMOF showed increased emission intensity at CO level of 0.005% (versus N2), with response times ranging from 123 s to 280 s (depending on various temperatures). Good selectivity was observed over competing alkane gases, with stable emission for at least 5 days, but no linear calibration plots were observed.

Azoanthracene-core structure as Cu2+-assisted CO sensing probe: Characterization, performance, and bioimaging

Detection of endogenous CO (carbon monoxide) is an interesting topic in biology because it has been discovered as a messenger for signal transduction and therapeutic effects in vital biological activities. Fluorescence imaging has proven a powerful tool for detecting endogenous CO, which drives the development of low-cost and easy-to-use fluorescent probes. In this study, four azobenzene derivatives (A1, A2, A3, and A4) with various substituents were reported, including their geometric structures, photophysical parameters, and spectral responses to Cu2+ and CO. The relationship between substituent structure and performance was discussed along with Cu2+ quenching and CO sensing mechanisms. The optimal probe (A1), which had no substituent, efficiently quenched fluorescence in the presence of Cu2+, with its PLQY decreased from 0.33 to 0.02, PLQY = photoluminescence quantum yield. Upon CO deoxidization, A1's fluorescence could be recovered (PLQY recovered to 0.32) within 180 s. Its sensing mechanism was static by forming a non-fluorescent complex with Cu2+ (with a stoichiometric ratio of 1:1). The bioimaging performance of A1 for endogenous CO in HeLa cells was reported.

Derivatization of 2,1,3-Benzothiadiazole via Regioselective C-H Functionalization and Aryne Reactivity

Despite growing interest in 2,1,3-benzothiadiazole (BTD) as an integral component of many functional molecules, methods for the functionalization of its benzenoid ring have remained limited, and many even simply decorated BTDs have required de novo synthesis. We show that regioselective Ir-catalyzed C-H borylation allows access to versatile 5-boryl or 4,6-diboryl BTD building blocks, which undergo functionalization at the C4, C5, C6, and C7 positions. The optimization and regioselectivity of C-H borylation are discussed. A broad reaction scope is presented, encompassing ipso substitution at the C-B bond, the first examples of ortho-directed C-H functionalization of BTD, ring closing reactions to generate fused ring systems, as well as the generation and capture reactions of novel BTD-based heteroarynes. The regioselectivity of the latter is discussed with reference to the Aryne Distortion Model.

A "turn-on" red cyclometalated iridium (III) complex for long-term tracking the diffusion of CORM-2 in cells and zebrafish

Transition metal carbonyl compound of CO releasing molecules (CORMs) are widely used to treat arthritis, tumor and immune. They play a physiological role by directly acting on target tissues to release CO for disease treatment without matrix metabolism after dissolution. It is important to track the level and diffusion process of CORMs in vivo to control CO dose and distribution, facilitating to understand the roles of CORMs in disease treatment. Herein, we designed two red ring Ir1/2 complexes with a large stokes shift. Both Ir1 and Ir2 complexes probes can sensitively and selectively respond to CORM-2. The probe Ir1 exhibits rapid reaction with CORM-2 in Phosphate Buffered Saline within 1 min, showing a detection limitation of 0.13 μM and manifesting a linear relationship with the CORM-2 concentration from 0 to 70 μM at λem = 618 nm. Due to low toxicity even after 12 h exposure and fluorescence stability, this probe has been successfully used for continuous tracking the diffusion process of CORM-2 in living cells for up to 60 min and visualizing CORM-2 distribution in zebrafish. Additionally, this probe showed a good capacity for deep penetration (126 μm), suggesting the potential in detecting CORM-2 in living tissues.

Near-infrared fluorogenic imaging of carbon monoxide in live cells using palladium-mediated carbonylation

Here we develop a near infrared (NIR) fluorogenic probe for carbon monoxide (CO) detection and imaging based on palladium-mediated carbonylation using a NIR boron-dipyrromethene difluoride as a fluorophore and tetraethylene glycols as aqueous moieties. The probe is utilized to image exogenous and endogenous CO under different stimulated conditions in live cells.

Endogenous CO imaging in bacterial pneumonia with a NIR fluorescent probe

Bacterial pneumonia is a serious respiratory illness that poses a great threat to human life. Rapid and precise diagnosis of bacterial pneumonia is crucial for symptomatic clinical treatment. Endogenous carbon monoxide (CO) is regarded as a significant indicator of bacterial pneumonia; herein, we developed a near-infrared (NIR) probe for fluorescence and photoacoustic (PA) dual-mode imaging of endogenous CO in bacterial pneumonia. NO2-BODIPY could rapidly and specifically react with CO to produce strong NIR fluorescence as well as ratiometric PA signals. NO2-BODIPY has outstanding features including fast response, fluorescence/PA dual mode signals, good specificity, and a low limit of detection (LOD = 20.3 nM), which enables it to image endogenous CO in cells and bacterial pneumonia mice with high sensitivity and high contrast ratio. In particular, NO2-BODIPY has two-photon excited (1340 nm, σ1 = 1671 GM) NIR fluorescence and has been utilized to image endogenous CO in bacterial pneumonia mice with deep tissue penetration. NO2-BODIPY has been demonstrated a good capability of fluorescence/PA dual-mode imaging of CO in bacterial pneumonia mice, providing a precise manner to diagnose bacterial pneumonia.

BODIPY Based OFF-ON Fluorescent Probe for Endogenous Carbon Monoxide Imaging in Living Cells

Carbon monoxide (CO) is one of the signaling molecules that are ubiquitous in humans, which involves in the regulation of human physiology and pathology. In this work, the probe PEC was designed and synthesized based on BODIPY fluorophore that can selectively detect CO through reducing the nitro group to amino group, resulting in a "turn-on" fluorescence response with a simultaneous increase in the concentration of CO. The response is selective over a variety of relevant reactive free radicals, ions, and amino acid species. PEC has the advantages of good stability, good water solubility, and obvious changes in fluorescence signals. In addition, PEC can be used to detect and track endogenous CO in living cells.

The importance of and difficulties involved in creating molecular probes for a carbon monoxide gasotransmitter

As one of the triumvirate of recognized gasotransmitter molecules, namely NO, H2S, and CO, the physiological effects of CO and its potential as a biomarker have been widely investigated, garnering particular attention due to its reported hypotensive, anti-inflammatory, and cytoprotective properties, making it a promising therapeutic agent. However, the development of CO molecular probes has remained relatively stagnant in comparison with the fluorescent probes for NO and H2S, owing to its inert molecular state under physiological conditions. In this review, starting from elucidating the definition and significance of CO as a gasotransmitter, the imperative for the advancement of CO probes, especially fluorescent probes, is expounded. Subsequently, the current state of development of CO probe methodologies is comprehensively reviewed, with an overview of the challenges and prospects in this burgeoning field of research.

Light activated simultaneous release and recognition of biological signaling molecule carbon monoxide (CO)

The therapeutic action of carbon monoxide (CO) is very well known and has been studied on various types of tissues and animals. However, real-time spatial and temporal tracking and release of CO is still a challenging task. This paper reported an amphiphilic CO sensing probe NP and phospholipid 1,2-Dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) based nanoscale vesicular sensing system Ves-NP consisting of NP. The liposomal sensing system (Ves-NP) showed good selectivity and sensitivity for CO without any interference from other relevant biological analytes. Detection of CO is monitored by fluorescence OFF-ON signal. Ves-NP displayed LOD of 5.94 µM for CO detection with a response time of 5 min. Further, in a novel attempt, Ves-NP is co-embedded with the amphiphilic CO-releasing molecule 1-Mn(CO)₃ to make an analyte replacement probe Ves-NP-CO. Having a both CO releasing and sensing moiety at the surface of the same liposomal system Ves-NP-CO play a dual role. Ves-NP-CO is used for the simultaneous release and recognition of CO that can be controlled by light. Thus, in this novel approach, for the first time we have attached both the release and recognition units of CO in the vesicular surface, both release and recognition simultaneously monitored by the change in fluorescent OFF-ON signal.

A Review for In Vitro and In Vivo Detection and Imaging of Gaseous Signal Molecule Carbon Monoxide by Fluorescent Probes

Carbon monoxide (CO) is a vital endogenous gaseous transmitter molecule involved in the regulation of various physiological and pathological processes in living biosystems. In order to investigate the biological function of CO, many technologies have been developed to monitor the level of endogenous CO in biosystems. Among them, the fluorescence detection technology based on the fluorescent probe has the advantages of high sensitivity, excellent selectivity, simple operation, especially non-invasive damage to biological samples, and the possibility of real-time in situ detection, etc., which is considered to be one of the most effective and applicable detection techniques. Therefore, in the last few years, a lot of work has been carried out on the design, synthesis and in vivo fluorescence imaging studies of CO fluorescent probes. Furthermore, using fluorescent probes to detect the changes in CO concentrations in living cells and tissues as well as in organisms has been one of the hot research topics in recent years. However, it is still a challenge to rationally design CO fluorescent probe with excellent optical performance, structural stability, low background interference, good biocompatibility, and excellent water solubility. Therefore, this review focuses on the research progress of CO fluorescent probes in the detection mechanism and biological applications in recent years. However, this popular and leading topic has rarely been summarized comprehensively to date. Thus, the research progress of CO fluorescent probes in recent years is reviewed in terms of their design concept, detection mechanism, and their biological applications. In addition, the relationship between the structure and performance of the probes was also discussed. More significantly, we hope that more excellent optical properties fluorescent probes for gaseous transmitter molecule CO detection and imaging will overcome the current problems of high biotoxicity and limited water solubility in future.

Strategy for Detecting Carbon Monoxide: Cu2+-Assisted Fluorescent Probe and Its Applications in Biological Imaging

Herein, a novel strategy was proposed for identifying carbon monoxide (CO), which plays a crucial part in living systems. For the first time, we have managed to design, synthesize, and characterize successfully this new Cu²⁺-assisted fluorescent probe (DPHP) in detecting CO. Compared with the commonly adopted Pd⁰-mediated Tsuji-Trost reaction recognition method, such a new strategy did not engage costly palladium (II) salt and generated no leaving group, indicating a satisfactory anti-interference ability. The recognition mechanism was confirmed by IR, ¹H NMR titration, HR-MS, cyclic voltammetry, X-ray photoelectron spectroscopy, electron paramagnetic resonance, and optical properties. Surprisingly, it was found that the new method achieved high selectivity and rapid identification of CO with a lower limit of detection (1.7 × 10^-8 M). More intriguingly, it could recognize endogenous and exogenous CO in HeLa cells. The cytotoxicity of this new method was so low that it allowed the detection of CO in mice and zebrafish. Basically, our results trigger a novel viewpoint of rationally designing and synthesizing advanced materials for CO detection with unique features, impelling new research in detection chemistry.

Detection and imaging of Carbon monoxide releasing Molecule-2 in HeLa cells and zebrafish using a Metal-Free Near-Infrared fluorescent off-on probe

By incorporating 4-nitrobenzene as the recognition unit into a stable hemicyanine skeleton based on IR 780, a designed near-infrared fluorescent off-on probe DNXI shows excellent analytical performance in real-time monitoring of carbon monoxide releasing molecule-2 (CORM-2) in living HeLa cells and zebrafish without transition metals. The response mechanism of CORM-2 with DNXI may reduce the nitro group to an amino group, followed by 1,6-rearrangement elimination reaction, resulting in the recovery of both color and fluorescence signal. As a result, DNXI shows very low background signal, which is rather desired for achieving sensitive detection of CORM-2. Compared with the existing CORM-2 probes, DNXI shows excellent optical performance in vitro and in vivo, high selectivity and sensitivity to CORM-2, as well as near-infrared fluorescence emission 712 nm, with a low detection limit of 103 nM. More importantly, DNXI is low cytotoxic, cell membrane permeable, and its applicability has been demonstrated for monitoring CORM-2 in living HeLa cells and zebrafish. These superior properties of the probe enable it have great potential to be used in biological systems or in vivo related studies.

A Critical Review on Organic Small Fluorescent Probes for Monitoring Carbon Monoxide in Biology

Endogenous carbon monoxide (CO) is an important intracellular gas messenger that is intimately involved in many physiological and pathological processes. The abnormal concentration of CO in living organisms can cause many diseases. Therefore, it is of great significance to monitor CO in biological samples. Fluorescent probe technology provides an effective and convenient method for CO monitoring, with the advantages of high selectivity and sensitivity, fast response time and in situ fluorescence imaging in biological tissues, which is favored by the majority of researchers. In this paper, the research progress of CO fluorescent probes since 2018 is reviewed, and the design, detection mechanism and biological application of the related fluorescent probes are summarized. And the relationship between the structure and performance of the probes is discussed. Furthermore, the development trend and application prospect of CO fluorescent probes are prospected.

Functional Polysiloxane Enables Visualization of the Presence of Carbon Monoxide in Biological Systems and Films

As an essential gasotransmitter, carbon monoxide (CO) had gradually become a research hotspot in that it possessed important physiological functions and unique pharmacological properties. However, to date, no report has focused on the topic of detecting CO both in vivo and using films. To open up a new field of CO probes, for the first time, we designed a probe (PMAH-CO) that showed a distinctive ratio emission characteristic and displayed the quantitative distribution of CO in HeLa cells and zebrafish with a higher signal-to-noise ratio. Meanwhile, the fluorescent polysiloxane-based film (PMF) containing PMAH-CO exhibited an excellent response to CO. Due to the addition of the Si-O bond, the probe exhibited a broad transparency in the visible light range and had excellent photostability. Moreover, the probe was economically viable, easy to handle, and suitable for biological research. Hence, PMAH-CO and PMF would open up the road to broaden the application of silicone materials in the field of fluorescence imaging.

CO/light dual-activatable Ru(ii)-conjugated oligomer agent for lysosome-targeted multimodal cancer therapeutics

Stimuli-activatable and subcellular organelle-targeted agents with multimodal therapeutics are urgently desired for highly precise and effective cancer treatment. Herein, a CO/light dual-activatable Ru(ii)-oligo-(thiophene ethynylene) (Ru-OTE) for lysosome-targeted cancer therapy is reported. Ru-OTE is prepared via the coordination-driven self-assembly of a cationic conjugated oligomer (OTE-BN) ligand and a Ru(ii) center. Upon the dual-triggering of internal gaseous signaling molecular CO and external light, Ru-OTE undergoes ligand substitution and releases OTE-BN followed by dramatic fluorescence recovery, which could be used for monitoring drug delivery and imaging guided anticancer treatments. The released OTE-BN selectively accumulates in lysosomes, physically breaking their integrity. Then, the generated cytotoxic singlet oxygen (¹O₂) causes severe lysosome damage, thus leading to cancer cell death via photodynamic therapy (PDT). Meanwhile, the release of the Ru(ii) core also suppresses cancer cell growth as an anticancer metal drug. Its significant anticancer effect is realized via the multimodal therapeutics of physical disruption/PDT/chemotherapy. Importantly, Ru-OTE can be directly photo-activated using a two-photon laser (800 nm) for efficient drug release and near-infrared PDT. Furthermore, Ru-OTE with light irradiation inhibits tumor growth in an MDA-MB-231 breast tumor model with negligible side effects. This study demonstrates that the development of an activatable Ru(ii)-conjugated oligomer potential drug provides a new strategy for effective subcellular organelle-targeted multimodal cancer therapeutics.

The anticancer therapeutics of lysosome disruption/PDT/chemotherapy based on Ru-OTE complex was achieved, which provides a new strategy for developing multimodal and effective stimuli-activatable subcellular organelle-targeted cancer therapeutics.

Activatable Photoacoustic Probe for In Situ Imaging of Endogenous Carbon Monoxide in the Murine Inflammation Model

Photoacoustic (PA) imaging is an emerging biomedical imaging modality that combines the advantages of optical and ultrasound imaging. Carbon monoxide (CO), which is a vital endogenous cell-signaling molecule in the human body, exerts critical physiological functions such as anti-inflammatory, antiapoptotic, and antiproliferative. The imbalance of CO homeostasis is also associated with numerous diseases. Therefore, it is critically important to noninvasively monitor the steady-state changes of CO in vivo. However, the activatable photoacoustic (PA) probes for detecting CO-associated complicated diseases have not yet developed. In this work, we developed the first turn-on PA probe (MTR-CO) to visualize the CO level in the lipopolysaccharide (LPS)-induced acute inflammation murine model through PA imaging technology. MTR-CO is composed of a near-infrared absorption cyanine-like dye (MTR-OH) and allyl formate, showing a 10.2-fold PA signal enhancement at 690 nm upon activation by CO. Furthermore, the results revealed that MTR-CO has high sensitivity, excellent specificity, and good biocompatibility for CO in vivo. MTR-CO was then applied for PA imaging of CO in cells and for monitoring the development of acute inflammation in the murine model by tracking the changes of the CO level. These findings provide a promising strategy for accurately detecting the steady-state changes of CO in living organisms.

A Two-Photon Probe Based on Naphthalimide-Styrene Fluorophore for the In Vivo Tracking of Cellular Senescence

Cellular senescence is a state of stable cell cycle arrest that can negatively affect the regenerative capacities of tissues and can contribute to inflammation and the progression of various aging-related diseases. Advances in the in vivo detection of cellular senescence are still crucial to monitor the action of senolytic drugs and to assess the early onset or accumulation of senescent cells. Here, we describe a naphthalimide-styrene-based probe (HeckGal) for the detection of cellular senescence both in vitro and in vivo. HeckGal is hydrolyzed by the increased lysosomal β-galactosidase activity of senescent cells, resulting in fluorescence emission. The probe was validated in vitro using normal human fibroblasts and various cancer cell lines undergoing senescence induced by different stress stimuli. Remarkably, HeckGal was also validated in vivo in an orthotopic breast cancer mouse model treated with senescence-inducing chemotherapy and in a renal fibrosis mouse model. In all cases, HeckGal allowed the unambiguous detection of senescence in vitro as well as in tissues and tumors in vivo. This work is expected to provide a potential technology for senescence detection in aged or damaged tissues.

Two-photon small-molecule fluorescence-based agents for sensing, imaging, and therapy within biological systems

In this tutorial review, we will explore recent advances for the design, construction and application of two-photon excited fluorescence (TPEF)-based small-molecule probes.

Simultaneous Detection of Carbon Monoxide and Viscosity Changes in Cells

A new family of robust, non‐toxic, water‐compatible ruthenium(II) vinyl probes allows the rapid, selective and sensitive detection of endogenous carbon monoxide (CO) in live mammalian cells under normoxic and hypoxic conditions. Uniquely, these probes incorporate a viscosity‐sensitive BODIPY fluorophore that allows the measurement of microscopic viscosity in live cells via fluorescence lifetime imaging microscopy (FLIM) while also monitoring CO levels. This is the first example of a probe that can simultaneously detect CO alongside small viscosity changes in organelles of live cells.

Recent Advances in Fluorescence Light-Up Endogenous and Exogenous Carbon Monoxide Detection in Biology

Considerable attention has been paid by the scientific community to detect toxic carbon monoxide (CO) in sub-cellular organelles like mitochondria, lysosomes, nuclei, etc. due to their generation and accumulation through numerous biological processes and their role as signal transducer, therapeutics, etc. Various methods are also available for detection of CO, but fluorescence light-up detection is considered the best due to its easy and accurate sensing capability. As of now, no review is available in the literature dedicated to fluorescent detection of only CO both in vitro and in vivo, but considering the huge amount of work reporting every year, it is necessary to have an account of all the recent significant works devoted to it. This review will give special attention to the most noteworthy development of fluorescent light-up probes for the detection of cellular and sub-cellular targetable CO starting from 2012 and emphasizing also the mechanism of action and the applications.

"Two Birds with One Stone" Ruthenium(II) Complex Probe for Biothiols Discrimination and Detection In Vitro and In Vivo

In this work, a "two birds with one stone" ruthenium(II) complex probe, Ru-NBD, is proposed as an effective tool for biothiols detection and discrimination in vitro and in vivo. Ru-NBD is nonluminescent due to the quenching of Ru(II) complex emission by photoinduced electron transfer (PET) from Ru(II) center to NBD and the quenching of NBD emission through 4-substitution with "O" ether bond. Ru-NBD is capable of reacting with Cys/Hcy to form long-lived red-emitting Ru-OH and short-lived green-emitting NBD-NR, while reacting with GSH to produce Ru-OH and nonemissive NBD-SR. The long lifetime emission of Ru(II) complex allows elimination of short lifetime background and NBD-NR fluorescence for total biothiols detection ("bird" one) by time-gated luminescence (TGL) analysis, and the remarkable difference in luminescence color response allows discrimination GSH and Cys/Hcy ("bird" two) through steady-state luminescence analysis. Ru-NBD features high sensitivity and selectivity, rapid luminescence response, and low cytotoxicity, which enables it to be used as the probe for luminescence and background-free TGL detection and visualization of biothiols in live cells, zebrafish, and mice. The successful development of this probe is anticipated to contribute to the future biological studies of biothiols roles in various diseases.

CO Sense and Release Flavonols: Progress toward the Development of an Analyte Replacement PhotoCORM for Use in Living Cells

Carbon monoxide (CO) is a signaling molecule in humans. Prior research suggests that therapeutic levels of CO can have beneficial effects in treating a variety of physiological and pathological conditions. To facilitate understanding of the role of CO in biology, molecules that enable fluorescence detection of CO in living systems have emerged as an important class of chemical tools. A key unmet challenge in this field is the development of fluorescent analyte replacement probes that replenish the CO that is consumed during detection. Herein, we report the first examples of CO sense and release molecules that involve combining a common CO-sensing motif with a light-triggered CO-releasing flavonol scaffold. A notable advantage of the flavonol-based CO sense and release motif is that it is trackable via fluorescence in both its pre- and postsensing (pre-CO release) forms. In vitro studies revealed that the PdCl₂ and Ru(II)-containing CORM-2 used in the CO sensing step can result in metal coordination to the flavonol, which minimizes the subsequent CO release reactivity. However, CO detection followed by CO release is demonstrated in living cells, indicating that a cellular environment mitigates the flavonol/metal interactions.

Small-molecule fluorescent probes for imaging gaseous signaling molecules: current progress and future implications

Endogenous gaseous signaling molecules including nitric oxide (NO), carbon monoxide (CO) and hydrogen sulfide (H2S) have been demonstrated to perform significant physiological and pharmacological functions and are associated with various diseases in biological systems. In order to obtain a deeper insight into their roles and mechanisms of action, it is desirable to develop novel techniques for effectively detecting gaseous signaling molecules. Small-molecule fluorescent probes have been proven to be a powerful approach for the detection and imaging of biological messengers by virtue of their non-invasiveness, high selectivity, and real-time in situ detection capability. Based on the intrinsic properties of gaseous signaling molecules, numerous fluorescent probes have been constructed to satisfy various demands. In this perspective, we summarize the recent advances in the field of fluorescent probes for the detection of NO, CO and H2S and illustrate the design strategies and application examples of these probes. Moreover, we also emphasize the challenges and development directions of gasotransmitter-responsive fluorescent probes, hoping to provide a general implication for future research.

Metal-free carbon monoxide-releasing micelles undergo tandem photochemical reactions for cutaneous wound healing

Carbon monoxide (CO) has shown broad biomedical applications. The site-specific delivery and controlled release of CO is of crucial importance to achieve maximum therapeutic benefits. The development of carbon monoxide (CO)-releasing polymers (CORPs) can increase the stability, optimize pharmacokinetic behavior, and reduce the side effects of small molecule precursors. However, almost all established CORPs were synthesized through a post functional approach, although the direct polymerization strategy is more powerful in controlling the chain compositions and architectures. Herein, a direct polymerization strategy is proposed toward metal-free CO-releasing polymers (CORPs) based on photoresponsive 3-hydroxyflavone (3-HF) derivatives. Such CO-releasing amphiphiles self-assemble into micelles, having excellent water-dispersity. Intriguingly, photo-triggered tandem photochemical reactions confer successive fluorescence transitions from blue-to-red-to-colorless, enabling self-reporting CO release in vitro and in vivo as a result of the incorporation of 3-HF derivatives. More importantly, the localized CO delivery of CORPs by taking advantage of the spatiotemporal control of light stimulus outperformed conventional metal carbonyls such as CORMs in terms of anti-inflammation and cutaneous wound healing. This work opens a novel avenue toward metal-free CORPs for potential biomedical applications.

Development of a new ratiometric probe with near-infrared fluorescence and a large Stokes shift for detection of gasotransmitter CO in living cells

A near-infrared (NIR) ratiometric fluorescent probe, NIR-Ratio-CO, was developed for rapid detection of carbon monoxide (CO) in both solution and living cells through the strategy of Pd⁰-mediated Tsuji-Trost reaction. This probe shows a rapid, highly specific and sensitive detection process for CO, accompanied by colorimetric and distinct ratiometric fluorescence changes at 655 and 592 nm with a large Stokes shift up to 195 nm. The detection limit for CO was measured to be about 61 nM by the fluorescence method. In addition, this probe was successfully applied for ratiometric imaging of both exogenous and endogenous CO in living cells, indicating that it can be used as a novel tool for ratiometric fluorescent detection of CO in living systems.

A Nuclear-Localized Naphthalimide-Based Fluorescent Light-Up Probe for Selective Detection of Carbon Monoxide in Living Cells

A nuclear-localized fluorescent light-up probe, NucFP-NO₂, was designed and synthesized that can detect CO selectively in an aqueous buffer (pH 7.4, 37 °C) through the CO-mediated transformation of the nitro group into an amino-functionalized moiety. This probe triggered a more than 55-fold "turn-on" fluorescence response to CO without using any metal ions, e.g., Pd, Rh, Fe, etc. The enhanced response is highly selective over a variety of relevant reactive oxygen, nitrogen, and sulfur species and also various biologically important cationic, anionic, and neutral species. The detection limit of this probe for CO is as low as 0.18 μM with a linear range of 0-70 μM. Also, this fluorogenic probe is an efficient candidate for monitoring intracellular CO in living cells (RAW 264.7, A549 cells), and the fluorescence signals predominantly localize in the nuclear region.

The development of a hemicyanine-based ratiometric CO fluorescent probe with a long emission wavelength and its applications for imaging COin vitroandin vivo

A novel ratiometric fluorescent probe,Hcy-CO, with long-wavelength emission was developed for visualizing CO in living cells and zebrafish.

Recent Progress in the Development of Fluorescent Probes for Thiophenol

Thiophenol (PhSH) belongs to a class of highly reactive and toxic aromatic thiols with widespread applications in the chemical industry for preparing pesticides, polymers, and pharmaceuticals. In this review, we comprehensively summarize recent progress in the development of fluorescent probes for detecting and imaging PhSH. These probes are classified according to recognition moieties and are detailed on the basis of their structures and sensing performances. In addition, prospects for future research are also discussed.

A new mitochondrion targetable fluorescent probe for carbon monoxide-specific detection and live cell imaging

A new mitochondrion targetable molecular probe for carbon monoxide (CO)-specific detection based on palladium-free mediated opening of spirolactam was designed. The turn-on red fluorescence caused by CO enables a safe and powerful method for unravelling the function of CO in biological systems to be established.

Upconversion Nanoprobes for in Vitro and ex Vivo Measurement of Carbon Monoxide

Here, we have developed a new colorimetric and luminescence nanosensor, based on upconversion nanoparticles (UCNPs), for in vitro and ex vivo measurement of carbon monoxide (CO). The nanoprobe has two strong fluorescence emission peaks in the UCNP core to excite fluorophores at 540 and 800 nm. The CO-responsive palladium ion-bounded rhodamine B derivatives (Pd-RBDs) are encapsulated in the mesoporous silica (mSiO₂) shell and the particles outside the cyclodextrin (CD) layer. Reduction of palladium ions by CO results in the release of palladium from the Pd-RBDs, thereby inducing the closure of the spiro ring of the RBD and the accompanying reduction of rhodamine B (RB) absorption at 500-600 nm overlapping with the luminescence spectrum of UCNPs maximized at 540 nm. Therefore, the I₅₄₀/I₈₀₀ ratio of the nanoprobe will increase when CO is present, making it possible to quantitatively measure CO. Besides working in a clean buffer environment with known [CO], this method was evaluated using living cells and tissue sections. Additionally, these probes were also successfully used to investigate the CO-related protective activity of anti-hepatic ischemia-reperfusion injury (HIRI) oligopeptides.

A near-infrared fluorescent probe for imaging endogenous carbon monoxide in living systems with a large Stokes shift

Near-infrared (NIR) fluorescent probes with a large Stokes shift are very practical tools for bioimaging applications. Carbon monoxide (CO) is a key gaseous signal molecule and its imaging in living systems has attracted great attention in recent years. In this work, a very easy-to-get NIR fluorescent probe with a remarkable large pseudo-Stokes shift (238 nm) for detection of CO was reported. This probe was found to show a rapid NIR fluorescent turn-on response for CO with high selectivity, high sensitivity and a low detection limit (38 nM). Moreover, imaging CO in living cells and animals with this probe was successfully applied with a high signal-to-noise ratio. The results indicate that this probe can be used as a new practical tool for imaging of endogenous CO in living systems.

Monitoring the dynamics of hemeoxygenase-1 activation in head and neck cancer cells in real-time using plasmonically enhanced Raman spectroscopy

Real-time monitoring of the dynamics of pharmacologically generated HO-1 in mammalian cells by using plasmonically enhanced Raman spectroscopy (PERS).

We report for the first time the usage of plasmonically enhanced Raman spectroscopy (PERS) to directly monitor the dynamics of pharmacologically generated hemeoxygenase-1 (HO-1) by evaluating the kinetics of formation of carbon monoxide (CO), one of the metabolites of HO-1 activation, in live cells during cisplatin treatment. Being an endogenous signaling molecule, CO plays an important role in cancer regression. Many aspects of HO-1's and CO's functions in biology are still unclear largely due to the lack of technological tools for the real-time monitoring of their dynamics in live cells and tissues. In this study, we found that, together with nuclear region-targeted gold nanocubes (AuNCs), cisplatin treatment can dramatically trigger the activation of HO-1 and thereby the rate and production of CO in mammalian cells in a dose-dependent manner. Though quantitative molecular data revealed that a lower concentration of cisplatin up-regulates HO-1 expression in cancer cells, PERS data suggest that it poorly facilitates the activation of HO-1 and thereby the production of CO. However, at a higher dose, cisplatin along with AuNCs could significantly enhance the activation of HO-1 in cancer cells, which could be probed in real-time by monitoring the CO generation by using PERS. Under the same conditions, the rate of formation of CO in healthy cells was relatively higher in comparison to the cancer cells. Additionally, molecular data revealed that AuNCs have the potential to suppress the up-regulation of HO-1 in cancer cells during cisplatin treatment at a lower concentration. As up-regulation of HO-1 has a significant role in cell adaptation to oxidative stress in cancer cells, the ability of AuNCs in suppressing the HO-1 overexpression will have a remarkable impact in the development of nanoformulations for combination cancer therapy. This exploratory study demonstrates the unique possibilities of PERS in the real-time monitoring of endogenously generated CO and thereby the dynamics of HO-1 in live cells, which could expedite our understanding of the signaling action of CO and HO-1 in cancer progression.

Mitochondria-Targetable Ratiometric Time-Gated Luminescence Probe for Carbon Monoxide Based on Lanthanide Complexes

As a critical gasotransmitter, carbon monoxide (CO) has been demonstrated to be related with mitochondrial respiration, but the monitoring of CO in mitochondria remains a great challenge. In this work, a unique ratiometric time-gated luminescence (TGL) probe, Mito-NBTTA-Tb³⁺/Eu³⁺, that can specifically respond to mitochondrial CO has been developed. The probe was designed by incorporating a mitochondria-targeting moiety, triphenylphosphonium, into a CO-activatable terpyridine polyacid derivative, 4'-(4-nitrobenzyloxy-2,2':6',2''-terpyridine-6,6''-diyl) bis(methylenenitrilo) tetrakis(acetic acid), for coordinating to Eu³⁺ and Tb³⁺ ions to construct lanthanide complex-based probe for ratiometric TGL detection of CO. Upon reaction with CO, accompanied by the conversion of nitro group to amino group, a 1,6-rearrangement-elimination reaction occurs, which leads to the cleavage of 4-nitrobenzyl group from Mito-NBTTA-Tb³⁺/Eu³⁺, resulting in the significant increase of Tb³⁺ emission at 540 nm and moderate decrease of Eu³⁺ emission at 610 nm. After the reaction, the I₅₄₀/ I₆₁₀ ratio was found to be 48-fold enhanced. This feature allowed Mito-NBTTA-Tb³⁺/Eu³⁺ to be employed as a ratiometric TGL probe for CO detection with the I₅₄₀/ I₆₁₀ ratio as a signal. In addition, the probe showed outstanding mitochondria-localization characteristic, which enabled the probe to be successfully applied to imaging CO within mitochondria of living cells under TGL and ratiometric modes. The application of Mito-NBTTA-Tb³⁺/Eu³⁺ was demonstrated by the visualization and quantitative detection of exogenous and endogenous CO in living cells and mouse liver tissue slices, as well as in living Daphnia magna and mice. All of the results suggested the potential of Mito-NBTTA-Tb³⁺/Eu³⁺ for the quantitative monitoring of CO in vitro and in vivo.

A long-wavelength ultrasensitive colorimetric fluorescent probe for carbon monoxide detection in living cells

A long-wavelength ultrasensitive colorimetric fluorescent probe was developed to track carbon monoxide in living cells.