Luminescent Neutral Cyclometalated Iridium(III) Complexes Featuring a Cubic Polyhedral Oligomeric Silsesquioxane for Lipid Droplet Imaging and Photocytotoxic Applications

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.

Tracking Plasma Membrane Damage Using a Ruthenium(II) Complex Phosphorescent Indicator Paired with Cholesterol

Long-term in situ plasma membrane-targeted imaging is highly significant for investigating specific biological processes and functions, especially for the imaging and tracking of apoptosis processes of cells. However, currently developed membrane probes are rarely utilized to monitor the in situ damage of the plasma membrane. Herein, a transition-metal complex phosphorescent indicator, Ru-Chol, effectively paired with cholesterol, exhibits excellent properties on staining the plasma membrane, with excellent antipermeability, good photostability, large Stokes shift, and long luminescence lifetime. In addition, Ru-Chol not only has the potential to differentiate cancerous cells from normal cells but also tracks in real time the entire progression of cisplatin-induced plasma membrane damage and cell apoptosis. Therefore, Ru-Chol can serve as an efficient tool for the monitoring of morphological and physiological changes in the plasma membrane, providing assistance for drug screening and early diagnosis and treatment of diseases, such as immunodeficiency, diabetes, cirrhosis, and tumors.

An AIE Metal Iridium Complex: Photophysical Properties and Singlet Oxygen Generation Capacity

Photodynamic therapy (PDT) has garnered significant attention in the fields of cancer treatment and drug-resistant bacteria eradication due to its non-invasive nature and spatiotemporal controllability. Iridium complexes have captivated researchers owing to their tunable structure, exceptional optical properties, and substantial Stokes displacement. However, most of these complexes suffer from aggregation-induced quenching, leading to diminished luminous efficiency. In contrast to conventional photosensitizers, photosensitizers exhibiting aggregation-induced luminescence (AIE) properties retain the ability to generate a large number of reactive oxygen species when aggregated. To overcome these limitations, we designed and synthesized a novel iridium complex named Ir-TPA in this study. It incorporates quinoline triphenylamine cyclomethylated ligands that confer AIE characteristics for Ir-TPA. We systematically investigated the photophysical properties, AIE behavior, spectral features, and reactive oxygen generation capacity of Ir-TPA. The results demonstrate that Ir-TPA exhibits excellent optical properties with pronounced AIE phenomenon and robust capability for producing singlet oxygen species. This work not only introduces a new class of metal iridium complex photosensitizer with AIE attributes but also holds promise for achieving remarkable photodynamic therapeutic effects in future cellular experiments and biological studies.

Polyhedral Oligomeric Silsesquioxane-Based Nanoparticles for Efficient Chemotherapy of Glioblastoma

Glioblastoma (GBM) is the most common lethal brain tumor with dismal treatment outcomes and poor response to chemotherapy. As the regulatory center of cytogenetics and metabolism, most tumor chemotherapeutic molecules exert therapeutic effects in the nucleus. Nanodrugs showing the nuclear aggregation effect are expected to eliminate and fundamentally suppress tumor cells. In this study, a nanodrug delivery system based on polyhedral oligomeric silsesquioxane (POSS) is introduced to deliver drugs into the nuclei of GBM cells, effectively enhancing the therapeutic efficacy of chemotherapy. The nanoparticles are modified with folic acid and iRGD peptides molecules to improve their tumor cell targeting and uptake via receptor-mediated endocytosis. Nuclear aggregation allows for the direct delivery of chemotherapeutic drug temozolomide (TMZ) to the tumor cell nuclei, resulting in more significant DNA damage and inhibition of tumor cell proliferation. Herein, TMZ-loaded POSS nanoparticles can significantly improve the survival of GBM-bearing mice. Therefore, the modified POSS nanoparticles may serve as a promising drug-loaded delivery platform to improve chemotherapy outcomes in GBM patients.

In Vivo Imaging of Lipid Droplets and Oxygen Status in Hepatic Tissues of Nonalcoholic Fatty Liver Model Mice Using a Lipophilic Ir(III) Complex

Nonalcoholic fatty liver disease (NAFLD) is becoming common worldwide. In pathophysiological studies of NAFLD, an in vivo optical probe that enables visualization of lipid droplets (LDs) and imaging of oxygen status in hepatic tissues simultaneously would be very useful. Here, we present the phosphorescent Ir(III) complex BTP ((btp)₂Ir(acac) (btp = benzothienylpyridine, acac = acetylacetone)) as the first probe that meets this requirement. BTP was efficiently taken up into cultured 3T3-L1 adipocytes and selectively accumulated into LDs. Quantifying oxygen levels in LDs based on the phosphorescence lifetime of BTP allowed us to track changes in cellular oxygen tension after treatment with metabolic stimulants. Phosphorescence lifetime imaging microscopy combined with intravenously administered BTP in mice enabled specific visualization of LDs in hepatic lobules and simultaneous imaging of the oxygen gradient that decreased from the portal vein (PV) to the central vein (CV). NAFL model mice were created by feeding a high-fat diet (HFD) to mice for 3 or 7 days. The mice fed an HFD showed a marked increase in the amount and size of LDs in hepatocytes compared with those fed a normal diet, leading to abnormal microvascular structures. In addition, HFD-fed mice also exhibited reduced oxygen tension in areas other than the CV. Multicolor imaging with the LD-accumulated oxygen probe BTP and vasculature-staining FITC-lectin suggested that structural distortions of the sinusoidal microvasculature caused by enlarged LDs were associated with partial hypoxia in NAFL.

Luminescent Metal Complexes as Emerging Tools for Lipid Imaging

Fluorescence microscopy is a key tool in the biological sciences, which finds use as a routine laboratory technique (e.g., epifluorescence microscope) or more advanced confocal, two-photon, and super-resolution applications. Through continued developments in microscopy, and other analytical methods, the importance of lipids as constituents of subcellular organelles, signalling or regulating molecules continues to emerge. The increasing recognition of the importance of lipids to fundamental cell biology (in health and disease) has prompted the development of protocols and techniques to image the distribution of lipids in cells and tissues. A diverse suite of spectroscopic and microscopy tools are continuously being developed and explored to add to the "toolbox" to study lipid biology. A relatively recent breakthrough in this field has been the development and subsequent application of metal-based luminescent complexes for imaging lipids in biological systems. These metal-based compounds appear to offer advantages with respect to their tunability of the photophysical properties, in addition to capabilities centred around selectively targeting specific lipid structures or classes of lipids. The presence of the metal centre also opens the path to alternative imaging modalities that might not be applicable to traditional organic fluorophores. This review examines the current progress and developments in metal-based luminescent complexes to study lipids, in addition to exploring potential new avenues and challenges for the field to take.

Half-Sandwich Cyclometalated RhIII Complexes Bearing Thiolate Ligands: Biomolecular Interactions and In Vitro and In Vivo Evaluations

A class of cyclometalated RhIII complexes [Cp*Rh(ppy)(SR)] bearing thiolate ligands, Cp* = pentamethylcyclopentadienyl, ppy = 2-phenylpyridinate, and R = pyridyl (Spy, 2), pyrimidyl (SpyN, 3), benzimidazolyl (Sbi, 4), and benzothiazolyl (Sbt, 5), were produced and identified by means of spectroscopic methods. The in vitro cytotoxicity of the RhIII compounds in three different human mortal cancerous cell lines (ovarian, SKOV3; breast, MCF-7; lung, A549) and a normal lung (MRC-5) cell line were evaluated, indicating the selectivity of these cyclometalated RhIII complexes to cancer cells. Complex 5, selected for in vivo experiment, has shown an effective inhibition of tumor growth in SKOV3 xenograft mouse model relative to control (p-values < 0.05 and < 0.01). Importantly, the outcomes of H&E (hematoxylin and eosin) staining and hematological analysis revealed negligible toxicity of 5 compared to cisplatin on a functioning of the main organs of mouse. Molecular docking, UV-vis, and emission spectroscopies (fluorescence, 3D fluorescence, synchronous) techniques were carried out on 1-5 to peruse the mechanism of the anticancer activities of these complexes. The obtained data help to manifest the binding affinity between the rhodium compounds and calf thymus DNA (CT-DNA) through the interaction by DNA minor groove and moderate binding affinity with bovine serum albumin (BSA), particularly with the cavity in the subdomain IIA. It can be concluded that the Rh-thiolate complexes are highly promising leads for the development of novel effective DNA-targeted anticancer drugs.

Development of a fluoride-anion sensor based on aggregation of a dye-modified polyhedral oligomeric silsesquioxane

We report a new concept for a turn-on fluoride sensor based on the aggregation and release of a dye-modified polyhedral oligomeric silsesquioxane.

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.

Novel Pyrazine-Bridged D-A-D Type Charge Neutral Probe for Membrane Permeable Long-Term Live Cell Imaging

A novel donor-acceptor-donor (D-A-D) type compound containing pyrazine as the acceptor and triphenylamine as the donor has been designed and synthesized. The photophysical properties and biocompatibility of this probe, namely (OMeTPA)2-Pyr for live cell imaging were systematically investigated, with observed large Stokes shifts, high photostability, and low cytotoxicity. Furthermore, we demonstrated that (OMeTPA)2-Pyr could permeate live cell membranes for labeling. The proposed mechanism of this probe was the binding and shafting through membrane integral transport proteins by electrostatic and hydrophobic interactions. These salient and novel findings can facilitate the strategic design of new pyrazine-fused charge-neutral molecular platforms as fluorescent probes, for long-term in situ dynamic monitoring in live cells.

Inherently dinuclear iridium(III) meso architectures accessed by cyclometalation of calix[4]arene-based bis(aryltriazoles)

Conventional cyclometalation of calix[4]arene bis(aryltriazoles) with iridium(III) chloride hydrate leads to unique meso architectures in which the Ir₂Cl₂ core is cross-bound by two (C^N)₂ ligands, which allows further replacement of the chloride bridges with ancillary ligands while maintaining the dinuclear structures of the complexes having independent or coupled iridium pairs.

Tuning the organelle specificity and cytotoxicity of iridium(III) photosensitisers for enhanced phototheranostic applications

Luminescent cyclometallated iridium(III) complexes with a polyhedral oligomeric silsesquioxane (POSS) unit were designed as efficient theranostic agents that displayed tuneable organelle-targeting properties, minimal dark cytotoxicity and substantial photocytotoxicity even under hypoxic conditions.