Bioorthogonal “Labeling after Recognition” Affording an FRET-Based Luminescent Probe for Detecting and Imaging Caspase-3 via Photoluminescence Lifetime Imaging

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.

Utilising a 1,8-naphthalimide probe for the ratiometric fluorescent visualisation of caspase-3

The development of selective and sensitive probes for monitoring caspase-3 activity-a critical enzyme involved in apoptosis-remains an area of significant interest in biomedical research. Herein, we report the synthesis and characterisation of a novel ratiometric fluorescent probe, Ac-DEVD-PABC-Naph, designed to detect caspase-3 activity. The probe utilises a 1,8-naphthalimide fluorophore covalently linked to a peptide sequence via a self-immolative p-aminobenzyl alcohol (PABA) linker. Upon enzymatic cleavage by caspase-3, the probe undergoes spontaneous degradation, releasing the free naphthalimide fluorophore, resulting in a ratiometric change in fluorescence emission. Spectroscopic studies revealed a time-dependent ratiometric fluorescent response, demonstrating the probe's ability to visualise caspase-3 activity with high sensitivity. Enzyme kinetics such as K m (Michaelis constant), k cat (turnover number), and LOD (Limit of Detection) were obtained, suggesting that the probe possesses comparable kinetic data to other probes in literature, but with the added benefits of ratiometric detection. Selectivity studies also demonstrated the probe's specificity for caspase-3 over other endogenous species and enzymes. Ac-DEVD-PABC-Naph may be a promising tool for the quantitative detection and fluorescent visualisation of caspase-3 activity in biological systems, with potential applications in apoptosis research and drug development.

Covalently hybridized carbon dots@mesoporous silica nanobeads as a robust and versatile phosphorescent probe for time-resolved biosensing and bioimaging

Phosphorescence analyses have attracted broad attention due to their remarkable merits of the elimination of auto-fluorescence and scattering light. However, it remains a great challenge to develop novel materials with uniform size and morphology, stability, long lifetime, and aqueous-phase room temperature phosphorescence (RTP) characteristics. Herein, monodisperse and uniform RTP nanobeads were fabricated by an in situ covalent hybridization of carbon dots (CDs) and dendritic mesoporous silicon nanoparticles (DMSNs) via silane hydrolysis. The formation of Si-O-C and Si-C/N covalent bonds is beneficial for the fixation of vibrations and rotations of the luminescent centers. Specially, the nanopores of DMSNs provide a confined area that can isolate the triplet state of CDs from water and oxygen and thus ensure the occurrence of aqueous-phase RTP with an ultra-long lifetime of 1.195 s (seen by the naked eye up to 9 seconds). Through surface modifying folic acid (FA), CDs@DMSNs can serve as a probe to distinguish different cell lines that feature varying FA receptor expression levels. In addition, taking MCF-7 as the model, highly sensitive and quantitative detection (linear range: 103-106 cells per mL) has been achieved via an RTP probe. Furthermore, their potential applications in cellular and in vivo time-gated phosphorescence imaging have been proposed and demonstrated, respectively. This work would provide a new route to design CD-based RTP composites and promote their further applications in the medical and biological fields.

Exchangeable Self-Assembled Lanthanide Antennas for PLIM Microscopy

Lanthanides have unique photoluminescence (PL) emission properties, including very long PL lifetimes. This makes them ideal for biological imaging applications, especially using PL lifetime imaging microscopy (PLIM). PLIM is an inherently multidimensional technique with exceptional advantages for quantitative biological imaging. Unfortunately, due to the required prolonged acquisitions times, photobleaching of lanthanide PL emission currently constitutes one of the main drawbacks of PLIM. In this study, we report a small aqueous-soluble, lanthanide antenna, 8-methoxy-2-oxo-1,2,4,5-tetrahydrocyclopenta[de]quinoline-3-phosphonic acid, PAnt, specifically designed to dynamically interact with lanthanide ions, serving as exchangeable dye aimed at mitigating photobleaching in PLIM microscopy in cellulo. Thus, self-assembled lanthanide complexes that may be photobleached during image acquisition are continuously replenished by intact lanthanide antennas from a large reservoir. Remarkably, our self-assembled lanthanide complex clearly demonstrated a significant reduction of PL photobleaching when compared to well-established lanthanide cryptates, used for bioimaging. This concept of exchangeable lanthanide antennas opens new possibilities for quantitative PLIM bioimaging.

Cyclometalated iridium(III) complexes as anti-breast cancer and anti-metastasis agents via STAT3 inhibition

Breast cancer is the most commonly diagnosed cancer and second‑leading cause of cancer deaths in women. Signal transducer and activator of transcription 3 (STAT3) plays a critical role in promoting breast cancer cell proliferation, invasion, angiogenesis, and metastasis, and the high expression of STAT3 is related to the occurrence and poor chemotherapy sensitivity of breast cancer. Iridium(III) complexes Ir-PTS-1- 4 containing a pterostilbene-derived ligand were synthesized to inhibit the STAT3 pathway in breast cancer. Ir-PTS-4 inhibited the proliferation of breast cancer cells by suppressing the expression of phosphorylated STAT3 and STAT3-related cyclin D1, arresting cell cycle in the S-phase, inducing DNA damage and reactive oxygen species (ROS) generation, eventually leading to autophagic cell death. The cell metastasis and invasion were also inhibited after Ir-PTS-4 treatment. Besides, Ir-PTS-4 exhibited excellent anti-proliferation activity in 3D multicellular tumor spheroids, showing potential for the treatment of solid tumors. This work presents the rational design of metal-based anticancer agents to block the STAT3 pathway for simultaneously inhibiting breast cancer proliferation and metastasis.

Neural Networks Push the Limits of Luminescence Lifetime Nanosensing

Luminescence lifetime-based sensing is ideally suited to monitor biological systems due to its minimal invasiveness and remote working principle. Yet, its applicability is limited in conditions of low signal-to-noise ratio (SNR) induced by, e.g., short exposure times and presence of opaque tissues. Herein this limitation is overcome by applying a U-shaped convolutional neural network (U-NET) to improve luminescence lifetime estimation under conditions of extremely low SNR. Specifically, the prowess of the U-NET is showcased in the context of luminescence lifetime thermometry, achieving more precise thermal readouts using Ag2 S nanothermometers. Compared to traditional analysis methods of decay curve fitting and integration, the U-NET can extract average lifetimes more precisely and consistently regardless of the SNR value. The improvement achieved in the sensing performance using the U-NET is demonstrated with two experiments characterized by extreme measurement conditions: thermal monitoring of free-falling droplets, and monitoring of thermal transients in suspended droplets through an opaque medium. These results broaden the applicability of luminescence lifetime-based sensing in fields including in vivo experimentation and microfluidics, while, hopefully, spurring further research on the implementation of machine learning (ML) in luminescence sensing.

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.

The Application of Bio-orthogonality for In Vivo Animal Imaging

The application of bio-orthogonality has greatly facilitated numerous aspects of biological studies in recent years. In particular, bio-orthogonal chemistry has transformed biological research, including in vitro conjugate chemistry, target identification, and biomedical imaging. In this review, we highlighted examples of bio-orthogonal in vivo imaging published in recent years. We grouped the references into two major categories: bio-orthogonal chemistry-related imaging and in vivo imaging with bio-orthogonal nonconjugated pairing. Lastly, we discussed the challenges and opportunities of bio-orthogonality for in vivo imaging.

Site-specific dual encoding and labeling of proteins via genetic code expansion

The ability to selectively modify proteins at two or more defined locations opens new avenues for manipulating, engineering, and studying living systems. As a chemical biology tool for the site-specific encoding of non-canonical amino acids into proteins in vivo, genetic code expansion (GCE) represents a powerful tool to achieve such modifications with minimal disruption to structure and function through a two-step "dual encoding and labeling" (DEAL) process. In this review, we summarize the state of the field of DEAL using GCE. In doing so, we describe the basic principles of GCE-based DEAL, catalog compatible encoding systems and reactions, explore demonstrated and potential applications, highlight emerging paradigms in DEAL methodologies, and propose novel solutions to current limitations.

Fluorescent Probes Based on AIEgen-Mediated Polyelectrolyte Assemblies for Manipulating Intramolecular Motion and Magnetic Relaxivity

Uniting photothermal therapy (PTT) with magnetic resonance imaging (MRI) holds great potential in nanotheranostics. However, the extensively utilized hydrophobicity-driven assembling strategy not only restricts the intramolecular motion-induced PTT, but also blocks the interactions between MR agents and water. Herein, we report an aggregation-induced emission luminogen (AIEgen)-mediated polyelectrolyte nanoassemblies (APN) strategy, which bestows a unique "soft" inner microenvironment with good water permeability. Femtosecond transient spectra verify that APN well activates intramolecular motion from the twisted intramolecular charge transfer process. This de novo APN strategy uniting synergistically three factors (rotational motion, local motion, and hydration number) brings out high MR relaxivity. For the first time, APN strategy has successfully modulated both intramolecular motion and magnetic relaxivity, achieving fluorescence lifetime imaging of tumor spheroids and spatio-temporal MRI-guided high-efficient PTT.

The Effect of Molecular Conformations and Simulated "Self-Doping" in Phenothiazine Derivatives on Room-Temperature Phosphorescence

The research of purely organic room-temperature phosphorescence (RTP) materials has drawn great attention for their wide potential applications. Besides single-component and host-guest doping systems, the self-doping with same molecule but different conformations in one state is also a possible way to construct RTP materials, regardless of its rare investigation. In this work, twenty-four phenothiazine derivatives with two distinct molecular conformations were designed and their RTP behaviors in different states were systematically studied, with the aim to deeply understand the self-doping effect on the corresponding RTP property. While the phenothiazine derivatives with quasi-axial (ax) conformation presented better RTP performance in aggregated state, the quasi-equatorial (eq) ones were better in isolated state. Accordingly, the much promoted RTP performance was achieved in the stimulated self-doping state with ax-conformer as host and eq-one as guest, demonstrating the significant influence of self-doping on RTP effect.

Designs and Applications of Multi-stimuli Responsive FRET Processes in AIEgen-Functionalized and Bi-fluorophoric Supramolecular Materials

Materials capable of displaying strong ratiometric fluorescence with Förster resonance energy transfer (FRET) processes have attracted much research interest because of various chemosensor and biomedical applications. This review highlights several popular strategies in designing FRET-OFF/ON mechanisms of ratiometric fluorescence systems. In particular, the developments of organic and polymeric FRET materials featuring aggregation-induced emission-based luminogens (AIEgens), supramolecular assemblies, photochromic molecular switches and surfactant-induced AIE/FRET mechanisms are presented. AIEgens have been frequently employed as FRET donor and/or acceptor fluorophores to obtain enhanced ratiometric fluorescences in solution and solid states. Since AIE effects and FRET processes rely on controllable distances between fluorophores, many interesting fluorescent properties can be designed by regulating aggregation states in polymers and supramolecular systems. Photo-switchable fluorophores, such as spiropyran and diarylethene, provide drastic changes in fluorescence spectra upon photo-induced isomerizations, leading to photo-switching mechanisms to activate/deactivate FRET processes. Supramolecular assemblies offer versatile platforms to regulate responsive FRET processes effectively. In rotaxane structures, the donor-acceptor distance and FRET efficiency can be tuned by acid/base-controlled shuttling of the macrocycle component. The tunable supramolecular interactions are strongly influenced by external factors (such as pH values, temperatures, analytes, surfactants, UV-visible lights, etc.), which induce the assembly and disassembly of host-guest systems and thus their FRET-ON/FRET-OFF behavior. In addition, the changes in donor or acceptor fluorescence profiles upon detections of analytes can also sufficiently alter the FRET behavior and result in different ratiometric fluorescence outputs. The strategies and examples provided in this review offer the insights and toolkits for future FRET-based material developments.

Phosphorescent Ir(III) Complexes for Biolabeling and Biosensing

Cyclometalated Ir(III) complexes exhibit strong phosphorescence emission with lifetime of submicroseconds to several microseconds at room temperature. Their synthetic versatility enables broad control of physical properties, such as charge and lipophilicity, as well as emission colors. These favorable properties have motivated the use of Ir(III) complexes in luminescent bioimaging applications. This review examines the recent progress in the development of phosphorescent biolabels and sensors based on Ir(III) complexes. It begins with a brief introduction about the basic principles of the syntheses and photophysical processes of cyclometalated Ir(III) complexes. Focus is placed on illustrating the broad imaging utility of Ir(III) complexes. Phosphorescent labels illuminating intracellular organelles, including mitochondria, lysosomes, and cell membranes, are summarized. Ir(III) complexes capable of visualization of tumor spheroids and parasites are also introduced. Facile chemical modification of the cyclometalating ligands endows the Ir(III) complexes with strong sensing ability. Sensors of temperature, pH, CO₂, metal ions, anions, biosulfur species, reactive oxygen species, peptides, and viscosity have recently been added to the molecular imaging tools. This diverse utility demonstrates the potential of phosphorescent Ir(III) complexes toward bioimaging applications.

Luminescent and Photofunctional Transition Metal Complexes: From Molecular Design to Diagnostic and Therapeutic Applications

There has been emerging interest in the exploitation of the photophysical and photochemical properties of transition metal complexes for diagnostic and therapeutic applications. In this Perspective, we highlight the major recent advances in the development of luminescent and photofunctional transition metal complexes, in particular, those of rhenium(I), ruthenium(II), osmium(II), iridium(III), and platinum(II), as bioimaging reagents and phototherapeutic agents, with a focus on the molecular design strategies that harness and modulate the interesting photophysical and photochemical behavior of the complexes. We also discuss the current challenges and future outlook of transition metal complexes for both fundamental research and clinical 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.

Real-time tracking of ER turnover during ERLAD by a rhenium complex via lifetime imaging

Endoplasmic reticulum (ER) degradation by autophagy (ER-phagy) is a recently revealed selective autophagy pathway that plays important roles in organelle turnover and protein degradation, but the biological functions of ER-phagy are largely unknown. Here, we present an ER-targeting Re(I) tricarbonyl complex (Re-ERLAD) that can accumulate in the ER, induce ER-to-lysosome-associated degradation (ERLAD) upon visible light irradiation, and label ER buds and track their morphological alterations during ER-phagy. The emission of Re-ERLAD is sensitive to viscosity, which is a key parameter reflecting the amount of unfolded protein in the ER. Quantitative detection using two-photon fluorescence lifetime imaging microscopy shows that ER viscosity initially increases and then decreases during ERLAD, which reveals that ERLAD is a pathway for alleviating ER stress caused by unfolded proteins. In conclusion, our work presents the first specific photoinducer and tracker of ERLAD, which can be used in studying the regulatory mechanism and function of this process.

Deep-Red/Near-Infrared to Blue-Green Phosphorescent Iridium(III) Complexes Featuring Three Differently Charged (0, -1, and -2) Ligands: Structures, Photophysics, and Organic Light-Emitting Diode Application

We have designed and synthesized a new family of neutral phosphorescent iridium(III) complexes (Ir1-Ir6) featuring three differently charged (0, -1, and -2) ligands, in which biphenyl (bp) is used as a dianionic (-2) ligand, 4,6-difluorophenylpyridine (dfppy) or 1-phenylisoquinoline (piq) is used as a monoanionic (-1) ligand, and 2,2'-bipyridyl (bpy), 1,10-phenanthroline (phen), 1,2-bis(diphenylphosphanyl)benzene (dppb), or 1,2-bis(diphenylphosphanyl)ethane (dppe) is used as a neutral (0) ligand. The X-ray structures confirm that three coordination carbon atoms of all complexes assume a facial geometry, which can be beneficial to the stability of the structure. More importantly, the emitting color of the complexes can be tuned from deep red/near-infrared (NIR) (680-710 nm) to blue-green (466-496 nm) with different monoanionic (-1) ligands and neutral (0) ligands. Interestingly, the complex Ir5 shows a significant aggregation-induced phosphorescent emission effect, while Ir6 with a similar structure shows an opposite aggregation-caused quenching effect, mainly due to slight differences in the neutral (0) ligand structure. Notably, all deep red/NIR-emitting complexes (Ir1-Ir4) exhibit a distinct charge transfer (CT) excited state from the dianionic (-2) ligand to the neutral (0) ligand according to density functional theory calculations, whereas the excited state of blue-green-emitting complexes (Ir5-Ir6) displays the CT from the dianionic (-2) ligand to the monoanionic (-1) ligand. Considering better stability and optical performance, the deep red-emitting complexes (Ir2 and Ir4) with a simple structure are used as emitting layers of organic light-emitting diode devices and achieved good maximum external quantum efficiency (4.9 and 5.8%) peaking at 676 and 655 nm, respectively, with a very low turn-on voltage (2.5 V). This research provides a good strategy for the design of phosphorescent iridium complexes based on three differently charged (0, -1, and -2) ligands and their optoelectric applications.

Millisecond-Range Time-Resolved Bioimaging Enabled through Ultralong Aqueous Phosphorescence Probes

Probes featuring room-temperature phosphorescence (RTP) are promising tools for time-resolved imaging. It is worth noting that the time scale of time-resolved bioimaging generally ranges around the microsecond level, because of the short-lived emission. Herein, the first example of millisecond-range time-resolved bioimaging is illustrated, which is enabled through a kind of ultralong aqueous phosphorescence probes (i.e., cyclo-(Arg-Gly-AspD-Tyr-Cys)-conjugated zinc-doped silica nanospheres), with a RTP emission lasting for ≈5 s and a lifetime as long as 743.7 ms. We demonstrate that live cells and deep tumor tissue in mice can be specifically targeted through immune-phosphorescence imaging, with a high signal-to-background ratio (SBR) value of ≈69 for in vitro imaging, and ≈627 for in vivo imaging, respectively. We further show that, compared to that of fluorescence imaging, the SBR enhancement of millisecond-range time-resolved in vivo bioimaging is up to 105 times.

Stimulus-responsive room temperature phosphorescence materials with full-color tunability from pure organic amorphous polymers

Achieving stimulus-responsive ultralong room temperature phosphorescence (RTP) in organic materials especially with full-color tunable emissions is attractive and important but rarely reported. Here, a strategy was reported to realize stimulus-responsive RTP effect with color-tunable emissions by using water as solvent in the preparation process without any organic solvent through covalent linkage of arylboronic acids with different π conjugations and polymer matrix of polyvinyl alcohol. The yielded polymer films exhibit outstanding RTP performance (2.43 s). Furthermore, an excitation-dependent RTP film was obtained, and the afterglow color changes from blue to green, then to red as the excitation wavelength increases. The RTP property of all the above materials is sensitive to water and heat stimuli, because the rigidity of the system could be broken by water. Last, they were successfully applied in a multilevel information encryption and multicolor paper and ink.

Peptide probes for proteases - innovations and applications for monitoring proteolytic activity

Proteases are excellent biomarkers for a variety of diseases, offer multiple opportunities for diagnostic applications and are valuable targets for therapy. From a chemistry-based perspective this review discusses and critiques the most recent advances in the field of substrate-based probes for the detection and analysis of proteolytic activity both in vitro and in vivo.

Manipulating Electroluminochromism Behavior of Viologen-Substituted Iridium(III) Complexes through Ligand Engineering for Information Display and Encryption

Electrically controlling photoluminescence has attracted great research interest and offers many opportunities for technological developments. Electroluminochromic materials undergo redox reactions under low-voltage stimuli to achieve reversible luminescence switching. Till now, photoluminescence switching of a single molecule caused by electrical stimuli is restricted to intensity response because the redox-active moieties are good electron donors or acceptors and electrical stimuli can regulate the photoinduced electron-transfer and affect the luminescence intensity. In this work, the manipulation of the electroluminochromism behavior of a series of viologen-substituted iridium(III) complexes through the regulation of ligand orbital energy levels and electronic communication between the viologen pendants and the iridium(III) complex core is reported. Electrochemical redox reactions reversibly modulate either the luminescence quenching effect or the push-pull electronic effect of the viologen substituents, achieving multicolor "on-off" luminescence response toward electrical stimuli and luminescence manipulation between two emissive states with different wavelengths and lifetimes. To illustrate the promising applications of these electroluminochromic materials, recording and displaying luminescence information under electrical stimuli are demonstrated. Information encryption is realized by letting the electroluminochromism occur in the near-infrared region or in the time domain. Near-infrared camera or time-resolved luminescence analysis can be used to help read the invisible information.

Completely aqueous processable stimulus responsive organic room temperature phosphorescence materials with tunable afterglow color

Many luminescent stimuli responsive materials are based on fluorescence emission, while stimuli-responsive room temperature phosphorescent materials are less explored. Here, we show a kind of stimulus-responsive room temperature phosphorescence materials by the covalent linkage of phosphorescent chromophore of arylboronic acid and polymer matrix of poly(vinylalcohol). Attributed to the rigid environment offered from hydrogen bond and B-O covalent bond between arylboronic acid and poly(vinylalcohol), the yielded polymer film exhibits ultralong room temperature phosphorescence with lifetime of 2.43 s and phosphorescence quantum yield of 7.51%. Interestingly, the RTP property of this film is sensitive to the water and heat stimuli, because water could destroy the hydrogen bonds between adjacent poly(vinylalcohol) polymers, then changing the rigidity of this system. Furthermore, by introducing another two fluorescent dyes to this system, the color of afterglow with stimulus response effect could be adjusted from blue to green to orange through triplet-to-singlet Förster-resonance energy-transfer. Finally, due to the water/heat-sensitive, multicolor and completely aqueous processable feature for these three afterglow hybrids, they are successfully applied in multifunctional ink for anti-counterfeit, screen printing and fingerprint record.

Ferroptosis-Enhanced Cancer Immunity by a Ferrocene-Appended Iridium(III) Diphosphine Complex

Ferroptosis is a programmed cell death pathway discovered in recent years, and ferroptosis-inducing agents have great potential as new antitumor candidates. Here, we report a Ir(III) complex (Ir1) containing a ferrocene-modified diphosphine ligand that localizes in lysosomes. Under the acidic environments of lysosomes, Ir1 can effectively catalyze Fenton reaction, produce hydroxyl radicals, induce lipid peroxidation, down-regulate glutathione peroxidase 4, and induce ferroptosis. RNA sequencing analysis shows that Ir1 can significantly affect pathways related to ferroptosis and cancer immunity. Accordingly, Ir1 can induce immunogenic cells death and suppress tumor growth in vitro, regulate T cell activity and immune microenvironments in vivo. In conclusion, we show the potential of small molecules with ferroptosis-inducing capabilities for effective cancer immunotherapy.

Three Types of Charged Ligands Based Carboxyl-Containing Iridium(III) Complexes: Structures, Photophysics, and Solution Processed OLED Application

A novel family of three types of charged (0, -1, -2) ligands based phosphorescent iridium(III) complexes with different carboxyl-containing dianionic (-2) ligands have been synthesized. Their single-crystal structures show that all neutral complexes (Ir1, Ir2, and Ir3) show a trans-N^N configuration between dianionic (-2) and monoanionic (-1) ligands, which is in contrast with the trans-N^C configuration in cationic complex Ir4, which has an interesting hydrogen bond in the solid state. Notably, Ir4 shows higher luminescence efficiency and an obvious blue shift emission relative to those in Ir1, Ir2, and Ir3. DFT calculations demonstrate that all neutral complexes (Ir1, Ir2, and Ir3) exhibit ligand-to-ligand charge transfer (LLCT) excited state character from the dianionic (-2) ligand to the neutral (0) ligand, which are completely different from the cationic complex Ir4 that exhibits an LLCT excited state from the monoanionic (-1) ligand to the neutral (0) ligand. Considering better solubility, Ir1 was eventually used in solution-processed OLED and achieved moderate efficiency (6.6%, 14.3 cd A^-1, 2.8 lm W^-1) with an orange light displaying CIE_x,y coordinates of (0.53, 0.46). This work provides a new strategy to construct three types of charged (0, -1, -2) ligands based phosphorescent iridium(III) complexes and extends the range of iridium complex luminescent materials.

AIEgen-Based Lifetime-Probes for Precise Furin Quantification and Identification of Cell Subtypes

Biochemical sensing probes based on aggregation-induced-emission luminogens (AIEgens) are widely used in biological imaging and therapy, chemical sensing, and material sciences. However, it is still a great challenge to quantify the targets through fluorescence intensity of AIEgen probes due to their undesirable aggregations. Here, a PyTPA-ZGO probe with three lifetime signals for precise quantification of furin is constructed: the lifetime signal 1 and signal 2 comes from AIEgen PyTPA-P (τ_Pn ) and inorganic nanoparticles Zn₂ GeO₄ :Mn²⁺ -NH₂ (τ_Zn ), respectively, while the lifetime signal 3 is marked as the composite dual-lifetime signal (CDLS_n , C D L S n = τ Z n τ P n ). In contrast, the fluorescence intensity signal of PyTPA-P shows defectively quantitative performance. Furthermore, it is found that the CDLS_n exhibits higher significant differences than the two other lifetime signals (τ_Pn and τ_Zn ) thanks to its wide range between the maximum and minimum signal values and small standard deviation. Therefore, CDLS_n is further used to accurately identify cell subtypes based on the specific concentration of furin in each subtype. The lifetime criterion can realize precise quantification, and it should be a promising direction of AIEgen-based quantitative analysis in the future.

Time-resolved analysis of photoluminescence at a single wavelength for ratiometric and multiplex biosensing and bioimaging

Simultaneous analysis of luminescence signals of multiple probes can improve the accuracy and efficiency of biosensing and bioimaging. Analysis of multiple signals at different wavelengths usually suffers from spectral overlap, possible energy transfer, and difference in detection efficiency. Herein, we reported a polymeric luminescent probe, which was composed of a phenothiazine-based fluorescent compound and a phosphorescent iridium(iii) complex. Both luminophores emitted at around 600 nm but their luminescence lifetimes are 160 times different, allowing time-resolved independent analysis. As the fluorescence was enhanced in response to oxidation by hypochlorite and the phosphorescence was sensitive toward oxygen quenching, a four-dimensional relationship between luminescence intensity, fluorescence/phosphorescence ratio, hypochlorite concentration, and oxygen content was established. In cellular imaging, time-resolved photoluminescence imaging microscopy clearly showed the independent fluorescence response toward hypochlorite and phosphorescence response toward oxygen in separated time intervals. This work opens up a new idea for the development of multiplex biosensing and bioimaging.

Hydrothermal Synthesis of Zinc-Doped Silica Nanospheres Simultaneously Featuring Stable Fluorescence and Long-Lived Room-Temperature Phosphorescence

Fluorescence and phosphorescence are known as two kinds of fundamental optical signals, which have been used for myriad applications. To date, simultaneous activation of stable fluorescence and long-lived room-temperature phosphorescence (RTP) emission in the aqueous phase remains a big challenge. We prepare zinc-doped silica nanospheres (Zn@SiNSs) with fluorescence and RTP properties using a facile hydrothermal synthetic strategy. For the as-prepared Zn@SiNSs, the recombination of electrons and holes in defects and defect-stabilized excitons derived from oxygen vacancy/C=N bonds lead to the production of stable fluorescence and long-lived RTP (emission lasting for ≈9 s, quantum yield (QY): ≈33.6 %, RTP lifetime: ≈236 ms). The internal Si-O bonded networks and hydrophilic surface in Zn@SiNSs can reduce nonradiative decay to form self-protective RTP, and also provide high water solubility, excellent pH- and photostability.

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 Coumarin-Porphyrin FRET Break-Apart Probe for Heme Oxygenase-1

Heme oxygenase-1 (HO-1) is a vital enzyme in humans that primarily regulates free heme concentrations. The overexpression of HO-1 is commonly associated with cardiovascular and neurodegenerative diseases including atherosclerosis and ischemic stroke. Currently, there are no known chemical probes to detect HO-1 activity, limiting its potential as an early diagnostic/prognostic marker in these serious diseases. Reported here are the design, synthesis, and photophysical and biological characterization of a coumarin-porphyrin FRET break-apart probe to detect HO-1 activity, Fe-L1. We designed Fe-L1 to "break-apart" upon HO-1-catalyzed porphyrin degradation, perturbing the efficient FRET mechanism from a coumarin donor to a porphyrin acceptor fluorophore. Analysis of HO-1 activity using Escherichia coli lysates overexpressing hHO-1 found that a 6-fold increase in emission intensity at 383 nm was observed following incubation with NADPH. The identities of the degradation products following catabolism were confirmed by MALDI-MS and LC-MS, showing that porphyrin catabolism was regioselective at the α-position. Finally, through the analysis of Fe-L2, we have shown that close structural analogues of heme are required to maintain HO-1 activity. It is anticipated that this work will act as a foundation to design and develop new probes for HO-1 activity in the future, moving toward applications of live fluorescent imaging.

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.

Including fluorescent nanoparticle probes within injectable gels for remote strain measurements and discrimination between compression and tension

The ability to remotely and non-invasively monitor and measure the strain within injectable gels used to augment soft tissue is highly desirable. Such information could enable real-time monitoring of gel performance and bespoke gel design. We report progress towards this goal using two fluorescent particle probe systems included within two different injectable gels. The two injectable gels have been previously studied in the contexts of intervertebral disc repair and stretchable gels for cartilage repair. The two fluorophore particle probes are blue or near-infrared (NIR) emitting and are present at very low concentrations. The normalised photoluminescence (PL) intensity from the blue emitting probe is shown to equal the compressive deformation ratio of the gels. Furthermore, the normalised ratio of the PL intensities for the blue and NIR probes varies linearly with deformation ratio over a wide range (from 0.2 to 3.0) with a seamless transition from compression to tension. Hence, PL can discriminate between compression and tension. The new approach established here should apply to other gels and enable remote detection of whether a gel is being compressed or stretched as well as the extent. This study may provide an important step towards remotely and minimally invasively measuring the strain experienced by load-supporting gels in vivo.

B- and N-Embedded π-Conjugation Units Tuning Intermolecular Interactions and Optical Properties of Platinum(II) Complexes

A new series of neutral and cationic platinum(II) complexes containing a B- or N-embedded π-conjugation unit has been prepared. Notably, significantly different intermolecular interactions (Pt-Pt, π-π, head to tail, and head to head) and interesting optical properties exist in these complexes, which can be attributed to the difference in spatial structures and π-electron properties between B- and N-embedded π-conjugation units. Unexpectedly, under a hypoxic atmosphere, N-embedded neutral complex PtNacac can display a distinct dual-emission with both fluorescence and phosphorescence, whereas only a single fluorescence emission was observed in the air, which is different from the B-embedded neutral complex PtBacac with only a single phosphorescence emission at any atmosphere, as well confirmed by lifetime measurement and oxygen sensing experiments. DFT calculations reveal that unusual ligand-to-metal charge transfer (LMCT) excited state character and low spin orbit coupling (SOC) elements can be found in N-embedded complexes due to the strong electron-donating ability of the N-embedded unit. Based on this, as a novel ratiometric oxygen probe with a simple structure, PtNacac can be successfully used to examine intracellular oxygen levels by monitoring both fluorescence and phosphorescence signals via ratiometric photoluminescence imaging and time-resolved luminescence imaging (TRLI) technology. This work provides a completely new idea for designing fluorescence/phosphorescence dual-emissive complexes.

Photofunctional transition metal complexes as cellular probes, bioimaging reagents and phototherapeutics

This critical review summarises the recent biological applications of transition metal complexes as cellular probes, bioimaging reagents and phototherapeutics.

Conformation-Dependent Phosphorescence of Galactose-Decorated Phosphors and Assembling-Induced Phosphorescence Enhancement

Amorphous organic room-temperature phosphorescent (RTP) materials are promising for their facile preparation and processability, while the conformation effects of phosphors at amorphous state are lack of study in comparison with the rigid effects due to the commonly irregular assembling and dispersal of phosphors in rigid systems. Herein, we report a series of phosphorescent molecules modified by polyhydroxy galactose, whose RTP emission at the amorphous state can be regulated by controlling the conformational distortion of the phosphorescent segments. Further, a strong RTP emission is facilely obtained by the co-assembling between polyhydroxy phosphors and polyhydroxy matrices (α-CD, β-CD, and chitosan). Owing to the rigid effect of the enhanced hydrogen bonding cross-linking, the highest RTP quantum yield reaches 19.4%; whereas, the RTP emissions of assemblies become conformation insensitive. The conflicting relationship between the conformation effect and rigid effect is attributed to the differences between aggregated single-component systems and dispersed assembling systems. Besides, the unique and different moisture responsiveness of the co-assembling samples is discovered and further applied in data encryption. The research expands the scope for designing amorphous pure organic RTP materials with supramolecular strategies and shows a modularized approach for assembling-enhanced phosphorescence.

Recent developments in bioorthogonal chemistry and the orthogonality within

The emergence of bioorthogonal reactions has greatly advanced research in the fields of biology and medicine. They are not only valuable for labeling, tracking, and understanding biomolecules within living organisms, but also important for constructing advanced bioengineering and drug delivery systems. As the systems studied are increasingly complex, the simultaneous use of multiple bioorthogonal reactions is equally desirable. In this review, we take a look at the different bioorthogonal reactions that have recently been developed, the methods of cellular incorporation and the strategies to create orthogonality within the bioorthogonal landscape.

Metal-Enhanced Fluorescence by Bifunctional Au Nanoparticles for Highly Sensitive and Simple Detection of Proteolytic Enzyme

Although fluorescence-based analytical methods have been used in intracellular analyses, their sensitivity is low for the precise analysis of intracellular proteolytic enzymes to observe cell apoptosis related to cancer and neurodegenerative diseases. In this study, a metal-enhanced-fluorescence (MEF)-based highly sensitive biosensor for the detection of proteolytic enzymes is proposed for the first time by using a bifunctional Au nanoparticle (AuNP), which is connected to the fluorophore by both single-stranded DNA (ssDNA) and a peptide. Once caspase-3, a proteolytic enzyme, cuts the peptide specifically, the fluorescence signal is drastically increased because the ssDNA maintains an optimal distance for the MEF. The proposed sensing method shows the highly sensitive detection of caspase-3 based on just a simple enzymatic cleavage reaction within 1 h, and caspase-3-related preapoptotic cell detection was successfully carried out with high sensitivity. The proposed sensing method is a rapid, simple, and one-step technique for the real-time monitoring of intracellular proteolytic enzymes and can be applied to the early diagnosis of cancer and neurodegenerative diseases.

A Tumor-Microenvironment-Responsive Lanthanide-Cyanine FRET Sensor for NIR-II Luminescence-Lifetime In Situ Imaging of Hepatocellular Carcinoma

Deep tissue imaging in the second near-infrared (NIR-II) window holds great promise for widespread fundamental research. However, inhomogeneous signal attenuation due to tissue absorption and scattering hampers its application for accurate in vivo biosensing. Here, lifetime-based in situ hepatocellular carcinoma (HCC) detection in NIR-II region is presented using a tumor-microenvironment (peroxynitrite, ONOO^- )-responsive lanthanide-cyanine Förster resonance energy transfer (FRET) nanosensor. A specially designed ONOO^- -responsive NIR-II dye, MY-1057, is synthesized as the FRET acceptor. Robust lifetime sensing is demonstrated to be independent of tissue penetration depth. Tumor lesions are accurately distinguished from normal tissue due to the recovery lifetime. Magnetic resonance imaging and liver dissection results illustrate the reliability of lifetime-based detection in single and multiple HCC models. Moreover, the ONOO^- amount can be calculated according to the standard curve.

MiR-379 relieves myocardial injury after acute myocardial infarction by regulating tumor necrosis factor-α-induced protein 8

BACKGROUND

Acute myocardial infarction (AMI) is the myocardial avascular necrosis syndrome caused by coronary atherosclerotic plaque rupture, thrombosis or coronary artery occlusion. Therefore, it is of great significance to find new targets for the treatment of myocardial infarction. The purpose of this study was to investigate the effect of microRNA-379 (miR-379) on AMI and its mechanism.

METHODS

MiR-379 mimic was used to transfect H9c2 cells and we determined the protective effect of miR-379 on H9c2 by detecting the level of apoptosis. TargetScan software was used to detect miR-379's downstream targets. We constructed siRNA to analyze the effect of miR-379's downstream targets on H9c2 cells. In addition, we used miR-379 agomir to inject the tail vein of AMI rats to verify the effect of miR-379 on rat cardiomyocytes.

RESULTS

TargetScan detected that miR-379 and Tumor necrosis factor-α-induced protein 8 (TNFAIP8) may have binding sites and the dual luciferase reporter assay found that miR-379 binds to TNFAIP8 and inhibits its activity. MiR-379 mimic was found to reduce the expression of caspase3 and caspase9 in H9c2 cells and thereby reduce H2O2-induced cell damage. Inhibition of TNFAIP8 also significantly reduced apoptosis level and inhibited the NF-κB signaling pathway in H9c2 cells. Finally, miR-379 agomir was used to inject the tail vein of AMI rats and verified the protective effect of miR-379 in the heart in vivo.

CONCLUSIONS

MiR-379 has a binding site with TNFAIP8 and can inhibit its activity by binding to TNFAIP8 mRNA. SiRNA-TNFAIP8 can inhibit the NF-κB signaling pathway and protect myocardial cells from AMI-induced myocardial damage by reducing the apoptosis level of myocardial cells.