An iridium complex-based probe for photoluminescence lifetime imaging of human carboxylesterase 2 in living cells

Shining New Light on Biological Systems: Luminescent Transition Metal Complexes for Bioimaging and Biosensing Applications

Luminescence imaging is a powerful and versatile technique for investigating cell physiology and pathology in living systems, making significant contributions to life science research and clinical diagnosis. In recent years, luminescent transition metal complexes have gained significant attention for diagnostic and therapeutic applications due to their unique photophysical and photochemical properties. In this Review, we provide a comprehensive overview of the recent development of luminescent transition metal complexes for bioimaging and biosensing applications, with a focus on transition metal centers with a d6, d8, and d10 electronic configuration. We elucidate the structure-property relationships of luminescent transition metal complexes, exploring how their structural characteristics can be manipulated to control their biological behavior such as cellular uptake, localization, biocompatibility, pharmacokinetics, and biodistribution. Furthermore, we introduce the various design strategies that leverage the interesting photophysical properties of luminescent transition metal complexes for a wide variety of biological applications, including autofluorescence-free imaging, multimodal imaging, organelle imaging, biological sensing, microenvironment monitoring, bioorthogonal labeling, bacterial imaging, and cell viability assessment. Finally, we provide insights into the challenges and perspectives of luminescent transition metal complexes for bioimaging and biosensing applications, as well as their use in disease diagnosis and treatment evaluation.

Combination of two-photon fluorescent probes for carboxylesterase and ONOO- to visualize the transformation of nonalcoholic fatty liver to nonalcoholic steatohepatitis in liver orthotopic imaging

As the most common cause of liver diseases, nonalcoholic fatty liver disease (NAFLD) can be classified into nonalcoholic fatty liver (NAFL) and nonalcoholic steatohepatitis (NASH). While NAFL is generally benign, the transition from NAFL to NASH is a cardinal feature of the non-benign liver disease that leads to cirrhosis and cancer, which indicates that tracking the transformation of NAFL to NASH timely is significant for precision management of liver diseases. Therefore, two fluorescent probes (CNFCl and DRNO) have been developed to visualize this pathological event. α-Fluorochloroacetamide and α-ketoamide was employed as the recognition site for carboxylesterase (CE) in CNFCl and peroxynitrite (ONOO-) in DRNO, respectively. CNFCl (λem = 445 nm) and DRNO (λem = 560 nm) showed high specificity and sensitivity towards CE and ONOO- respectively. By incubating with CE/ONOO- for 0.5 h respectively, both the emission intensity of CNFCl (linear range: 0-0.2 U/mL) and DRNO (linear range: 0-17.5 μM) displayed significant enhancement. As a result, the detection limit of CNFCl and DRNO for CE and ONOO- was calculated as 4.2 mU/L and 0.05 μM respectively. More importantly, the emission spectra of CNFCl and DRNO in the presence of CE and ONOO- respectively were cross-talk free under the two-photon excitation of 720 nm. This greatly facilitated the simultaneous detection of CE and ONOO- at distinctive channel, thus ensuring the high fidelity of the detection. These two probes were combined to image the fluctuation of CE and ONOO- during the conversion of NAFL to NASH in vitro and in vivo. It was found that while CE displayed a tendency to rise and then reduce during the transition from NAFL to NASH, ONOO- increased continuously, confirming that the combined imaging by CNFCl and DRNO might visualize the transformation of NAFL to NASH. The results provide robust visual tool to decipher the relationship between the stage of NAFLD and the level of CE/ONOO-. We anticipate this study may open new avenues to distinguish NASH from NAFL, which may further promote the study of intracellular biological activities of CE and the development of NAFLD diagnostic methods.

Luminescent iridium(III) porphyrin complexes as near-infrared-emissive biological probes

We report herein the design, synthesis and characterisation of a series of luminescent iridium(III) porphyrin complexes [Ir(ttp)(CH2CH2OH)] (H2ttp = 5,10,15,20-tetra-4-tolylporphyrin) (1), [Ir(tpp-Ph-NO2)(CO)Cl] (H2tpp-Ph-NO2 = 5-(4-((4-nitrophenoxy)carbonyloxymethyl)phenyl)-10,15,20-triphenylporphyrin) (2), [Ir(tpp-COOMe)(Py)2](Cl) (H2tpp-COOMe = 5-(4-methoxycarbonylphenyl)-10,15,20-triphenylporphyrin; Py = pyridine) (3) and [Ir(tpp-COOH)(Py)2](Cl) (H2tpp-COOH = 5-(4-carboxylphenyl)-10,15,20-triphenylporphyrin) (4). All the complexes displayed long-lived near-infrared (NIR) emission attributed to an excited state of mixed triplet intraligand (3IL) (π → π*) (porphyrin) and triplet metal-to-ligand charge transfer (3MLCT) (dπ(Ir) → π*(porphyrin)) character. The cytotoxicity of the complexes toward HeLa cells was examined by the 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyltetrazolium bromide (MTT) assay. The cationic complexes 3 and 4 exhibited higher cytotoxic activity toward HeLa cells than their neutral counterparts 1 and 2. Cellular uptake studies by inductively coupled plasma-mass spectrometry (ICP-MS) and laser-scanning confocal microscopy (LSCM) indicated that complexes 3 and 4 showed higher cellular uptake efficiencies than complexes 1 and 2 due to their cationic charge, and they were enriched in the perinuclear region of the cells with negligible nuclear uptake. Additionally, the carboxyl complex 4 was used to label a model protein bovine serum albumin (BSA) via an amidation reaction. The resultant luminescent protein conjugate 4-BSA displayed similar photophysical properties and intracellular localisation behaviour to its parent complex. The results of this work will contribute to the development of luminescent iridium(III) porphyrin complexes and related bioconjugates as NIR-emissive probes for bioimaging applications.

Lifetime-Based Responsive Probes: Design and Applications in Biological Analysis

With the development of modern biomedicine, biological analysis and detection are very important in disease diagnosis, detection of curative effect, prognosis and prediction of tumor recurrence. Compared with the currently widely used optical probes based on intensity signals, the lifetime signal does not depend on the influence of conditions such as the concentration of luminophore, tissue penetration depth and measurement method. Therefore, biological detection methods based on lifetime-based responsive probes have attracted great attention from the scientific community. Here, we briefly review the key advances in lifetime-based responsive probes in recent years (2017-2022). The review focuses on the design strategies of lifetime-based responsive probes and the research progress of their applications in the field of bioanalysis, and discusses the challenges they face. We hope it will further promote the development of lifetime-based responsive probes in the field of bioanalysis. With the development of modern biomedicine, biological analysis and detection are very important in disease diagnosis, detection of curative effect, prognosis and prediction of tumor recurrence. Compared with the currently widely used optical probes based on intensity signals, the lifetime signal does not depend on the influence of conditions such as the concentration of luminophore, tissue penetration depth and measurement method. Therefore, biological detection methods based on lifetime-based responsive probes have attracted great attention from the scientific community. Here, we briefly review the key advances in lifetime-based responsive probes in recent years (2017-2022). The review focuses on the design strategies of lifetime-based responsive probes and the research progress of their applications in the field of bioanalysis, and discusses the challenges they face. We hope it will further promote the development of lifetime-based responsive probes in the field of bioanalysis.

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.

Multifunction-Harnessed Afterglow Nanosensor for Molecular Imaging of Acute Kidney Injury In Vivo

Afterglow is superior to other optical modalities for biomedical applications in that it can exclude the autofluorescence background. Nevertheless, afterglow has rarely been applied to the high-contrast "off-to-on" activatable sensing scheme because the complicated afterglow systems hamper the additional inclusion of sensory functions while preserving the afterglow luminescence. Herein, a simple formulation of a multifunctional components-incorporated afterglow nanosensor (MANS) is developed for the superoxide-responsive activatable afterglow imaging of cisplatin-induced kidney injury. A multifunctional iridium complex (Ir-OTf) is designed to recover its photoactivities (phosphorescence and the ability of singlet oxygen-generating afterglow initiator) upon exposure to superoxide. To construct the nanoscopic afterglow detection system (MANS), Ir-OTf is incorporated with another multifunctional molecule (rubrene) in the polymeric micellar nanoparticle, where rubrene also plays dual roles as an afterglow substrate and a luminophore. The multiple functions covered by Ir-OTf and rubrene renders the composition of MANS quite simple, which exhibits superoxide-responsive "off-to-on" activatable afterglow luminescence for periods longer than 11 min after the termination of pre-excitation. Finally, MANS is successfully applied to the molecular imaging of cisplatin-induced kidney injury with activatable afterglow signals responsive to pathologically overproduced superoxide in a mouse model without autofluorescence background.

Near-infrared emitting iridium complexes: Molecular design, photophysical properties, and related applications

Organic light-emitting diodes (OLEDs) have become popular displays from small screens of wearables to large screens of televisions. In those active-matrix OLED displays, phosphorescent iridium(III) complexes serve as the indispensable green and red emitters because of their high luminous efficiency, excellent color tunability, and high durability. However, in contrast to their brilliant success in the visible region, iridium complexes are still underperforming in the near-infrared (NIR) region, particular in poor luminous efficiency according to the energy gap law. In this review, we first recall the basic theory of phosphorescent iridium complexes and explore their full potential for NIR emission. Next, the recent advances in NIR-emitting iridium complexes are summarized by highlighting design strategies and the structure-properties relationship. Some important implications for controlling photophysical properties are revealed. Moreover, as promising applications, NIR-OLEDs and bio-imaging based on NIR Ir(III) complexes are also presented. Finally, challenges and opportunities for NIR-emitting iridium complexes are envisioned.

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.

Dynamic Monitoring of Phase-Separated Biomolecular Condensates by Photoluminescence Lifetime Imaging

The formation of biomolecular condensates is driven by liquid-liquid phase separation, which is prevalent in cells to govern crucial cellular functions. However, understanding the properties of phase-separated condensates remains very challenging for the lack of suitable techniques. Here, we report a photoluminescence lifetime imaging method for real-time monitoring of phase-separated condensates, both in vitro and in living cells, using a microsecond-scale photoluminescence lifetime probe based on iridium complex. The probe has a large Stokes shift, excellent cell permeability, and minimal cell autofluorescence interference. With this method, the dynamic process of phase separation of fused in sarcoma protein has been well explored, showing high spatiotemporal resolution and high throughput. Beginning with initial formation, the protein droplets get bigger and more viscous, and then a final maturation to solidified aggregates has been characterized. This study paves the path for a deeper understanding of the properties of phase-separated biomolecular condensates.

Synthesis of Biocompatible Aliphatic Terpolymers via In Situ Fluorescent Monomers for Three-in-One Applications: Polymerization of Hydrophobic Monomers in Water

Biocompatible, nonconventional, multifunctional, purely aliphatic, light-emitting terpolymers, i.e., acrylonitrile-co-3-(N-isopropylacrylamido)propanenitrile-co-N-isopropylacrylamide (AN-co-NIPAMPN-co-NIPA, 1) and acrylonitrile-co-3-(N-hydroxymethylacrylamido)propanenitrile-co-N-hydroxymethylacrylamide (AN-co-NHMAMPN-co-NHMA, 2), were designed and synthesized via N-H-functionalized C-C + N-C-coupled in situ protrusions/grafting of fluorophore monomers, i.e., NIPAMPN and NHMAMPN, by solution polymerization of two highly hydrophobic nonemissive monomers in water. These scalable and reusable 1 and 2 were suitable for high-performance three-in-one applications, such as Fe(III) sensors, imaging of Madin-Darby canine kidney (MDCK) and human lung cancer (A549) cells, and security inks. The structures of 1 and 2, N-C-coupled in situ attachments/grafting of third fluorophore monomers, grafting events, and aggregation-enhanced emissions (AEEs), were analyzed by ¹H and ¹³C NMR spectroscopy, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy, ultraviolet-visible (UV-vis) spectroscopy, thermogravimetric (TG) analysis, high-resolution transmission electron microscopy (HRTEM), dynamic light scattering (DLS), fluorescence imaging, and fluorescence lifetime. The geometries, electronic structures, and absorption/emission properties of 1 and 2 at optimized compositions were examined by density functional theory (DFT), time-dependent DFT (TDDFT), and natural transition orbital (NTO) analyses. The limits of detection were 3.20 × 10^-7 and 1.37 × 10^-7 M for 1 and 2, respectively. The excellent biocompatibility of 1 and 2 was confirmed by >95% retention of MDCK and A549 cell morphologies.

Visualization of carboxylesterase 2 with a near-infrared two-photon fluorescent probe and potential evaluation of its anticancer drug effects in an orthotopic colon carcinoma mice model

We have established a near-infrared two-photon fluorescent probe for the detection of CE2 with high selectivity and sensitivity.

Endoplasmic Reticulum Targeting Ratiometric Fluorescent Probe for Carboxylesterase 2 Detection in Drug-Induced Acute Liver Injury

Carboxylesterase 2 (CES2), an endoplasmic reticulum (ER) located phase I enzyme, plays a vital role in the metabolism of various endogenous and exogenous substances, and is regarded as an important target for the design of prodrugs. Unfortunately, superior highly selective ER targeting fluorescent probes for monitoring of CES2 are not currently available. Herein, we report an ER targeting CES2 selective and sensitive ratiometric fluorescent probe ERNB based on the ER localizing group p-toluenesulfonamide. ERNB possessed high specificity, sensitivity, and exhibited excellent subcellular localization when compared to commercial ER tracker, and was used to image CES2 in the ER of living cells. Additionally, using ERNB we evaluated the CES2 regulation under d,l-dithiothreitol and tunicamycin-induced ER stress. Furthermore, we determined the down regulation of CES2 activity and expression in the acetaminophen-induced acute liver injury model. On the basis of these results, we conclude that ERNB is a promising tool for highlighting the role of CES2 in the ER and in exploring the role of CES2 in the development of diseases associated with ER stress.

Mitochondria-targeted Ir@AuNRs as bifunctional therapeutic agents for hypoxia imaging and photothermal therapy

Gold nanorods with surfaces modified by iridium(iii)-azo complexes (Ir@AuNRs) were developed as mitochondria-targeted bifunctional therapeutic agents for hypoxia-imaging and photothermal therapy.