Dynamic Monitoring of Phase-Separated Biomolecular Condensates by 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.

Coacervate or precipitate? Formation of non-equilibrium microstructures in coacervate emulsions

Non-equilibrium processing of aqueous polyelectrolyte complex (PEC) coacervates is critical to many applications. In particular, many coacervate-forming systems are known to become trapped in out-of-equilibrium states (e.g., precipitation). The mechanism and conditions under which these states form, and whether they age, is not clearly understood. Here, we elucidate the influence of processing on the PEC coarsening mechanism as it varies with flow during mixing for a model system of poly(allylamine hydrochloride) and poly(acrylic acid sodium salt) in water. We demonstrate that flow conditions can be used to toggle the formation of rough, precipitate-like aggregates of micron-scale PEC structures. These structures form at compositions with viscous-dominant equilibrium rheology, and observations of their formation via optical microscopy suggest that they comprise colloidal aggregates of PEC coacervate droplets. We further show that these aggregates exhibit micron-scale coarsening, with a mixing time-dependent characteristic aging time scale. The results show that the formation of precipitate-like structures is not solely determined by composition, but is instead highly sensitive to mass transport and colloidal instability effects. Our observations suggest that the details of mixing flow can provide non-equilibrium structural control of a broad range of PEC coacervate materials orthogonally to structure-property inspired polymeric design. We anticipate that these findings will open the door for future studies on the control of non-equilibrium PEC formation and structure.

Small molecule fluorescent probes for the study of protein phase separation

Liquid-liquid phase separation (LLPS) and liquid-solid phase transitions (LSPT) play crucial roles in biological systems, including sorting biomolecules, facilitate the transport of substrates for assembly, and accelerate the formation of metabolic and signaling complexes. Efforts towards improved characterization and quantification of phase separated species remain of outstanding interest and priority. In this review, we cover recent advances and the strategies used with small molecule fluorescent probes for the study of phase separation.

A study on theoretical models for investigating time-resolved photoluminescence in halide perovskites

Time-resolved photoluminescence (TRPL) is an effective experimental technique to study charge carrier dynamic processes in halide perovskites on different time scales. In the past decade, several models have been proposed and employed to study the TRPL curves in halide perovskites, but there is still a lack of systematic summarization and comparative discussion. Here, we reviewed the widely employed exponential models to fit the TRPL curves, and focused on the physical meaning of the extracted carrier lifetimes, as well as the existing debates on the definition of the average lifetime. Emphasis was placed on the importance of the diffusion process in the carrier dynamics, especially for the halide perovskite thin films having transport layers. The solving of the diffusion equation, using both analytical and numerical methods, was then introduced to fit the TRPL curves. Furthermore, the newly proposed global fit and direct measurement of radiative decay rates were discussed.

CCT2 is an aggrephagy receptor for clearance of solid protein aggregates

Protein aggregation is a hallmark of multiple human pathologies. Autophagy selectively degrades protein aggregates via aggrephagy. How selectivity is achieved has been elusive. Here, we identify the chaperonin subunit CCT2 as an autophagy receptor regulating the clearance of aggregation-prone proteins in the cell and the mouse brain. CCT2 associates with aggregation-prone proteins independent of cargo ubiquitination and interacts with autophagosome marker ATG8s through a non-classical VLIR motif. In addition, CCT2 regulates aggrephagy independently of the ubiquitin-binding receptors (P62, NBR1, and TAX1BP1) or chaperone-mediated autophagy. Unlike P62, NBR1, and TAX1BP1, which facilitate the clearance of protein condensates with liquidity, CCT2 specifically promotes the autophagic degradation of protein aggregates with little liquidity (solid aggregates). Furthermore, aggregation-prone protein accumulation induces the functional switch of CCT2 from a chaperone subunit to an autophagy receptor by promoting CCT2 monomer formation, which exposes the VLIR to ATG8s interaction and, therefore, enables the autophagic function.

Phase separation in immune signalling

Immune signalling pathways convert pathogenic stimuli into cytosolic events that lead to the resolution of infection. Upon ligand engagement, immune receptors together with their downstream adaptors and effectors undergo substantial conformational changes and spatial reorganization. During this process, nanometre-to-micrometre-sized signalling clusters have been commonly observed that are believed to be hotspots for signal transduction. Because of their large size and heterogeneous composition, it remains a challenge to fully understand the mechanisms by which these signalling clusters form and their functional consequences. Recently, phase separation has emerged as a new biophysical principle for organizing biomolecules into large clusters with fluidic properties. Although the field is still in its infancy, studies of phase separation in immunology are expected to provide new perspectives for understanding immune responses. Here, we present an up-to-date view of how liquid-liquid phase separation drives the formation of signalling condensates and regulates immune signalling pathways, including those downstream of T cell receptor, B cell receptor and the innate immune receptors cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) and retinoic acid-inducible gene I protein (RIG-I). We conclude with a summary of the current challenges the field is facing and outstanding questions for future studies.

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.

The pH responsive upconversion fluorescence and photothermal conversion properties of NaYF4:Yb3+/Er3+@NaYF4@MnO2@Au

While photothermal therapy is widely applied in phototherapy, there are still challenges in developing new generation phototherapy materials with precise diagnostic functions. Here we report the construction of a pH responsive upconversion fluorescence imaging precisely guided photothermal therapy system, namely NaYF4:Yb3+/Er3+@NaYF4@MnO2@Au nanocomposites, which can effectively avoid light damage to non-target tissues. Owing to the fluorescence resonance energy transfer between the upconversion nanocrystal donor and MnO2 and Au acceptor, the upconversion fluorescence is completely quenched. However, in pH 5.3 PBS buffer, MnO2 is gradually broken down, and the upconversion fluorescence is partially recovered, which could be used for upconversion fluorescence imaging to precisely guide photothermal therapy under 980 nm excitation. Simultaneously, due to the absorption of 980 nm excitation light and the emission bands of Er3+ (2H11/2→4I15/2 and 4S3/2→4I15/2 transition), temperature increment of core@shell@MnO2@Au could reach 35.5 °C under 980 nm excitation at 0.8 W cm-2. The core@shell@MnO2@Au nanocomposites are supposed to contribute significantly in the biological applications of photoluminescence imaging and photothermal therapy.

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.