Multimodal Imaging Iridium(III) Complex for Hypochlorous Acid in Living Systems

Cationic fluorescent carbon dots with solution ultra-stability and its rapid/on-site sensing application for HClO

Carbon dots (CDs) as a remarkable fluorescent nanomaterial have the advantages of easy preparation, good photostability and high sensitivity. However, the poor aqueous solution stability of carbon dots largely limited their practical application due to the characteristic of easily forming precipitation for long time storage. Here, a kind of cationic fluorescent carbon dots CDs-P(Ph)3 was designed by introducing a cationic compound, (4-carboxybutyl) triphenyl phosphonium bromide, to construct an electrostatic shell outside the dots. Such electrostatic shell could highly improve carbon dots stability in an aqueous solution to make CDs-P(Ph)3 stable for long-term storage with negligible aggregation. Meanwhile, the sensitivity of CDs-P(Ph)3 for hypochlorous acid (HClO) was also enhanced on the basis of the electron-withdrawing effect of cationic substituents on the surface of carbon dots. The limit of detection of CDs-P(Ph)3 for HClO was as low as ∼0.32 μM. Additionally, the fluorescence of CDs-P(Ph)3 could be rapid quenched by HClO with a quenching efficiency of more than 80% within 30 s. The excellent stability of CDs-P(Ph)3 in an aqueous solution made it suitable for on-site detecting HClO in real samples, such as tap, well and lake water. Such designed fluorescent nanomaterial would provide a practical application pathway for optical sensing detection in environmental samples.

Recent Advances in Organometallic NIR Iridium(III) Complexes for Detection and Therapy

Iridium(III) complexes are emerging as a promising tool in the area of detection and therapy due to their prominent photophysical properties, including higher photostability, tunable phosphorescence emission, long-lasting phosphorescence, and high quantum yields. In recent years, much effort has been devoted to develop novel near-infrared (NIR) iridium(III) complexes to improve signal-to-noise ratio and enhance tissue penetration. In this review, we summarize different classes of organometallic NIR iridium(III) complexes for detection and therapy, including cyclometalated ligand-enabled NIR iridium(III) complexes and NIR-dye-conjugated iridium(III) complexes. Moreover, the prospects and challenges for organometallic NIR iridium(III) complexes for targeted detection and therapy are discussed.

"Iridium Signature" Mass Spectrometric Probes: New Tools Integrated in a Liquid Chromatography-Mass Spectrometry Workflow for Routine Profiling of Nitric Oxide and Metabolic Fingerprints in Cells

Nitric oxide (NO) is a highly reactive signaling molecule involved in diverse biological processes. Simultaneous profiling of NO and associated metabolic fingerprints in a single assay allows more accurate assessments of cell states and offers the possibility to better understand its exact biological roles. Herein, a multiplexing LC-MS workflow was established for simultaneous detection of intracellular NO and various metabolites based on a novel "iridium signature" mass spectrometric probe (Ir-MSP841). This Ir-MSP841 can convert highly liable NO to a stable permanently charged triazole product (Ir-TP852), enabling direct MS detection of NO. This ^191/193Ir-signature mass spectrometric probe-based approach is endowed with overwhelming advantages of interference-free, high quantitative accuracy, and great sensitivity (limit of detection down to 0.14 nM). It also reveals good linearity over a wide concentration range 12.5-500 nM and has been successfully employed for exploring the release behaviors of three representative NO donors in cells. Meanwhile, metabolic profiling results reveal that varying the concentrations of NO has distinct effects on various cellular metabolites. This study provides a robust, sensitive, and versatile method for simultaneous detection of NO and numerous metabolites in a single LC-MS run and expands its applications in biomedical research.

Novel Near-Infrared Iridium(III) Complex for Chemiluminescence Imaging of Hypochlorous Acid

Chemiluminescence (CL) probes that possess near-infrared (NIR) emission are highly desirable for in vivo imaging due to their deeper tissue penetration ability and intrinsically high sensitivity. Herein, a novel iridium-based CL probe (NIRIr-CL-1) with direct NIR emission was reported as the result of hypochlorous acid (HClO)-initiated oxidative deoximation. To improve its biocompatibility and extend the CL time for in vivo imaging applications, this NIRIr-CL-1 was prepared as a CL nanoparticle probe (NIRIr-CL-1 dots) through encapsulation by an amphiphilic polymer Pluronic F127 (F127). All results demonstrate that the NIRIr-CL-1 dots have good selectivity and sensitivity for visualization of HClO even at the depth of 1.2 cm. Owing to these advantages, the CL imaging of exogenous and endogenous HClO in mice was achieved. This study could provide new insights into the construction of new NIR emission CL probes and expand their applications in biomedical imaging.

Development and Application of Ruthenium(II) and Iridium(III) Based Complexes for Anion Sensing

Improvements in the design of receptors for the detection and quantification of anions are desirable and ongoing in the field of anion chemistry, and remarkable progress has been made in this direction. In this regard, the development of luminescent chemosensors for sensing anions is an imperative and demanding sub-area in supramolecular chemistry. This decade, in particular, witnessed advancements in chemosensors based on ruthenium and iridium complexes for anion sensing by virtue of their modular synthesis and rich chemical and photophysical properties, such as visible excitation wavelength, high quantum efficiency, high luminescence intensity, long lifetimes of phosphorescence, and large Stokes shifts, etc. Thus, this review aims to summarize the recent advances in the development of ruthenium(II) and iridium(III)-based complexes for their application as luminescent chemosensors for anion sensing. In addition, the focus was devoted to designing aspects of polypyridyl complexes of these two transition metals with different recognition motifs, which upon interacting with different inorganic anions, produces desirable quantifiable outputs.

Tracking HOCl by an incredibly simple fluorescent probe with AIE plus ESIPT in vitro and in vivo

Hypochlorous acid is an important active substance involved in a variety of physiological processes in living organisms, while abnormal concentrations of HOCl are strongly associated with a variety of diseases such as cancer, inflammation, atherosclerosis, and Alzheimer's disease. As a result, it's crucial to establish a reliable method for tracking HOCl in vivo in order to investigate its physiological consequences. In this work, we developed a fluorescent probe DFSN with both AIE and ESIPT for imaging HOCl in vivo. DFSN not only has a basic structure and is easy to synthesize, but also has superior performance. The probe responds to HOCl in less than 10 s and has good selectivity and sensitivity to HOCl (DL = 6.3 nM), with a 110-fold increase in fluorescence intensity following response. In addition, DFSN can realize the rapid detection of hypochlorous acid with naked eyes. Moreover, DFSN can be used for the detection of exogenous and endogenous HOCl in RAW264.7 cells, and additionally enables the tracking of HOCl in cancer cells (Hela cells and HepG2 cells). More notably, it has been utilized to image hypochlorous acid in zebrafish with great success. The probe DFSN will be useful in determining the physiological significance of HOCl.

Persistent and Efficient Multimodal Imaging for Tyrosinase Based on Two-Photon Excited Fluorescent and Room-Temperature Phosphorescent Probes

Tyrosinase is crucial to regulate the metabolism of phenol derivatives, playing an important role in the biosynthesis of melanin pigments, whereas an abnormal level of tyrosinase would lead to severe diseases. It is rather necessary to develop a sensitive and selective imaging tool to assess the level of tyrosinase in vivo. We thoroughly researched the luminous mechanism of the existing TPTYR probe and provided design strategies to improve its two-photon excited fluorescence properties. The designed probes benza2-TPTYR and product benza2-TPTYR-coumarin have large two-photon absorption cross sections at the NIR spectral region (41 GM/706 nm, 71 GM/852 nm), while benza2-TPTYR-coumarin possesses easily distinguishable spectrum in the visible region and a high fluorescence efficiency (Φ_F = 0.27). What is more, novel two-photon excited multimodal imaging based on the pure organic small molecule benza1-TPTYR-coumarin (61 GM/936 nm) is proposed first, simultaneously possessing strong instantaneous fluorescent (563.79 nm) and persistent room-temperature phosphorescent emissions (767.68 nm, 0.54 ms).

Designing of a high-performance fluorescent small molecule enables dual-mode and ultra-sensitive fluorescence visualizing of HSO3- and HClO in dried fruit, beverage, and water samples

Herein, a novel hemicyanine derivative (E)-3-(1,1-dimethyl-2-(4-(methylthio)styryl)-1H-benzo[e]indol-3-ium-3-yl)propane-1-sulfonate (BIS) has been reasonably designed. Compound BIS is long-wavelength emissive and water-soluble with a large Stokes shift. Intriguingly, probe BIS provides a dual-mode fluorescence response pattern for the sensing of bisulfite (HSO₃^-) and hypochlorous acid (HClO) with great limit of detections (3.6 and 57.4 nM). First, the 1,4-Michael addition of HSO₃^- on the conjugated double bond triggers a ratiometric response (I₄₆₅/I₅₇₅). Second, the rapid oxidation of HClO on the thioether moiety provides a turn-on response (I₅₇₅). Evaluation of HSO₃^- and HClO levels in dried fruit, beverage, and water samples has been carried out with satisfactory results. Moreover, motivated by an impressive chromatic variation (red to blue), smartphone-assisted signal readout system and thin-film sensing platform are facilely constructed for real-time and on-site measurement of HSO₃^- levels. Furthermore, probe BIS is used for the in vivo imaging of HSO₃^- in edible fish models.

Time-Resolved Luminescent Sensing and Imaging for Enzyme Catalytic Activity Based on Responsive Probes

Enzymes, as a kind of biomacromolecules, play an important role in many physiological processes and relate directly to various diseases. Developing an efficient detection method for enzyme activity is important to achieve early diagnosis of enzyme-relevant diseases and high throughput screening of potential enzyme-relevant drugs. Time-resolved luminescence assay provide a high accuracy and signal-to-noise ratios detection methods for enzyme activity, which has been widely used in high throughput screening of enzyme-relevant drugs and diagnosis of enzyme-relevant diseases. Inspired by these advantages, various responsive probes based on metal complexes and metal-free organic compounds have been developed for time-resolved bioimaging and biosensing of enzyme activity owing to their long luminescence lifetimes, high quantum yields and photostability. In this review, we comprehensively reviewed metal complex- and metal-free organic compound-based responsive probes applied to detect enzyme activity through time-resolved imaging, including their design strategies and sensing principles. Current challenges and future prospects in this rapidly growing field are also discussed.

Novel heterobimetallic Ir(III)-Re(I) complexes: design, synthesis and antitumor mechanism investigation

The reasonable design of binuclear or multinuclear metal complexes has demonstrated their potential advantages in the anticancer field. Herein, three heterobimetallic Ir(III)-Re(I) complexes, [Ir(C^N)₂LRe(CO)₃DIP](PF₆)₂ (C^N = 2-phenylpyridine (ppy, in IrRe-1), 2-(2-thienyl)pyridine (thpy, in IrRe-2) and 2-(2,4-difluorophenyl)pyridine (dfppy, in IrRe-3); L = pyridylimidazo[4,5-f][1,10]phenanthroline; DIP = 4,7-diphenyl-1,10-phenanthroline), were designed and synthesized. The heterobimetallic IrRe-1-3 complexes show pH-sensitive emission properties, which can be used for specific imaging of lysosomes. Additionally, IrRe-1-3 display higher cytotoxicity against tested tumor cell lines than the clinical chemotherapeutic drug cisplatin. Further mechanisms indicate that IrRe-1-3 can induce apoptosis and autophagy, increase intracellular reactive oxygen species (ROS), depolarize the mitochondrial membrane (MMP), block the cell cycle at the G0/G1 phase and inhibit cell migration. To the best of our knowledge, this is the first example of the synthesis of heterobimetallic Ir(III)-Re(I) complexes with superior anticancer activities and evaluation of their anticancer mechanisms.

A smart probe for simultaneous imaging of the lipid/water microenvironment in atherosclerosis and fatty liver

A smart lipid droplet specific fluorescent probe (LDs-DM) with dual-emission in water (706 nm) and oil (535 nm) under single-excitation was prepared for revealing the ultra-structure of the aqueous and lipidic microenvironment in living cells, fatty liver tissues and atherosclerotic plaques without fluorescence emission crosstalk. This work is expected to provide inspiration for designing a novel probe with color switching ability.

Self-assembly-induced luminescence of Eu3+-complexes and application in bioimaging

Design and engineering of highly efficient emitting materials with assembly-induced luminescence, such as room-temperature phosphorescence (RTP) and aggregation-induced emission (AIE), have stimulated extensive efforts. Here, we propose a new strategy to obtain size-controlled Eu³⁺-complex nanoparticles (Eu-NPs) with self-assembly-induced luminescence (SAIL) characteristics without encapsulation or hybridization. Compared with previous RTP or AIE materials, the SAIL phenomena of increased luminescence intensity and lifetime in aqueous solution for the proposed Eu-NPs are due to the combined effect of self-assembly in confining the molecular motion and shielding the water quenching. As proof of concept, we also show that this system can be further applied in bioimaging, temperature measurement and HClO sensing. The SAIL activity of the rare-earth (RE) system proposed here offers a further step forward on the roadmap for the development of RE light conversion systems and their integration in bioimaging and therapy applications.

Two-photon ratiometric fluorescent probe for imaging of hypochlorous acid in acute lung injury and its remediation effect

Acute lung injury (ALI) is a pulmonary inflammatory disease with high morbidity and mortality rates. However, owing to the unknown etiology and rapid progression of the disease, the diagnosis of ALI is full of challenges with no effective treatment. Since the inflammatory response and oxidative stress played vital roles in the development of ALI, we herein developed the largest emission cross-shift (△λ = 145 nm) two-photon ratiometric fluorescent probe of TPRS-HOCl with high selectivity and short response time toward hypochlorous acid (HOCl) for exploring the relevance between the degree of ALI and HOCl concentration in the development process of the disease. In addition, the inhibition effect of HOCl during different treatment periods was also evaluated. Moreover, the tendency of imaging results was basically in accordance with that of hematoxylin and eosin (H&E) staining and the treatment effect became better in the early stage when using N-acetylcysteine (NAC), demonstrating the sensitivity of TPRS-HOCl toward ALI response. Thus, TPRS-HOCl has great potential to diagnose ALI in the early stage and guide for effective treatment.

Recent progress in metal-based molecular probes for optical bioimaging and biosensing

Biological imaging and biosensing from subcellular/cellular level to whole body have enabled non-invasive visualisation of molecular events during various biological and pathological processes, giving great contributions to the rapid and impressive advances in chemical biology, drug discovery, disease diagnosis and prognosis. Optical imaging features a series of merits, including convenience, high resolution, good sensitivity, low cost and the absence of ionizing radiation. Among different luminescent probes, metal-based molecules offer unique promise in optical bioimaging and biosensing in vitro and in vivo, arising from their small sizes, strong luminescence, large Stokes shifts, long lifetimes, high photostability and tunable toxicity. In this review, we aim to highlight the design of metal-based molecular probes from the standpoint of synthetic chemistry in the last 2 years for optical imaging, covering d-block transition metal and lanthanide complexes and multimodal imaging agents.

Recent development and application of cyclometalated iridium(III) complexes as chemical and biological probes

Iridium complexes have been widely applied as molecular sensors because of their rich photophysical properties, including large Stokes shifts, long emission lifetimes, environment-sensitive emissions, and high luminescence quantum yields. In this paper, we review the recent development and application of iridium complexes as probes for ions, anions, gaseous species, organic molecules, small biomolecules, biomacromolecules, and subcellular organelles. Our outlook for iridium-based probes is also discussed.

A Two-Photon Excited Near-Infrared Iridium(III) Complex for Multi-signal Detection and Multimodal Imaging of Hypochlorite

Hypochlorite (ClO^-), as a type of reactive oxygen species (ROS), plays a crucial role in the process of oxidative stress and is closely related to many diseases. Thus, developing a method for detecting and imaging of ClO^- with high sensitivity and selectivity is of great significance. However, the applications of most luminescent probes are limited to the fact that the excitation and emission wavelengths of them are in the visible light region rather than in the near-infrared (NIR) region. Hence, an NIR iridium(III) complex (Mul-NIRIr) with two-photon excitation is designed for the detecting and imaging of ClO^-. In the presence of ClO^-, the luminescent intensity and lifetime of Mul-NIRIr are remarkably enhanced. Interestingly, Mul-NIRIr also exhibits excellent electrochemiluminescence (ECL) properties, and the ECL signal is significantly enhanced with the addition of ClO^-. What is more, Mul-NIRIr is also suitable for the detection and analysis ClO^- by flow cytometry. Therefore, Mul-NIRIr is developed to detect multiple signals and is successfully applied to detect exogenous and endogenous ClO^- in living cells with one-photon, two-photon, and phosphorescence lifetime image microscopy (PLIM). In addition, Mul-NIRIr was successfully used for imaging of ClO^- in tissues and inflammatory mouse models. All of the above results indicate that Mul-NIRIr is highly effective in detecting ClO^- in living systems.

Ultrasensitive Nucleic Acid Assay Based on Cyclometalated Iridium(III) Complex with High Electrochemiluminescence Efficiency

This work developed a sensitive electrochemiluminescence (ECL) biosensor based on a cyclometalated iridium(III) complex ((bt)₂Irbza), which was synthesized for the first time. Annihilation, reductive-oxidative, and oxidative-reductive ECL behaviors of (bt)₂Irbza were investigated, respectively. The oxidative-reductive ECL intensity was the strongest compared with the other two, which showed 16.7 times relative ECL efficiency compared with commercial [Ru(bpy)₃]²⁺ under the same experimental conditions. Therefore, an ECL biosensing system with (bt)₂Irbza as the anodic luminophore was established for miRNA detection based on a closed bipolar electrode (BPE). Combined with both steric hindrance and catalytic effects induced by hemin/G-quadruplex in the cathodic reservoir of BPE that changed the Faraday current of the cathode and thus mediated the ECL intensity of (bt)₂Irbza in the anode of BPE, the ECL sensor stated an ultrahigh sensitivity for microRNA (miRNA-122) analysis with a detection limit of 82 aM.