pH-Sensitive Near-IR Emitting Dinuclear Ruthenium Complex for Recognition, Two-Photon Luminescent Imaging, and Subcellular Localization of Cancer 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.

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.

Imidazoles as Serotonin Receptor Modulators for Treatment of Depression: Structural Insights and Structure-Activity Relationship Studies

Serotoninergic signaling is identified as a crucial player in psychiatric disorders (notably depression), presenting it as a significant therapeutic target for treating such conditions. Inhibitors of serotoninergic signaling (especially selective serotonin reuptake inhibitors (SSRI) or serotonin and norepinephrine reuptake inhibitors (SNRI)) are prominently selected as first-line therapy for the treatment of depression, which benefits via increasing low serotonin levels and norepinephrine by blocking serotonin/norepinephrine reuptake and thereby increasing activity. While developing newer heterocyclic scaffolds to target/modulate the serotonergic systems, imidazole-bearing pharmacophores have emerged. The imidazole-derived pharmacophore already demonstrated unique structural characteristics and an electron-rich environment, ultimately resulting in a diverse range of bioactivities. Therefore, the current manuscript discloses such a specific modification and structural activity relationship (SAR) of attempted derivatization in terms of the serotonergic efficacy of the resultant inhibitor. We also featured a landscape of imidazole-based development, focusing on SAR studies against the serotoninergic system to target depression. This study covers the recent advancements in synthetic methodologies for imidazole derivatives and the development of new molecules having antidepressant activity via modulating serotonergic systems, along with their SAR studies. The focus of the study is to provide structural insights into imidazole-based derivatives as serotonergic system modulators for the treatment of depression.

Comprehensive Insights into Medicinal Research on Imidazole-Based Supramolecular Complexes

The electron-rich five-membered aromatic aza-heterocyclic imidazole, which contains two nitrogen atoms, is an important functional fragment widely present in a large number of biomolecules and medicinal drugs; its unique structure is beneficial to easily bind with various inorganic or organic ions and molecules through noncovalent interactions to form a variety of supramolecular complexes with broad medicinal potential, which is being paid an increasing amount of attention regarding more and more contributions to imidazole-based supramolecular complexes for possible medicinal application. This work gives systematical and comprehensive insights into medicinal research on imidazole-based supramolecular complexes, including anticancer, antibacterial, antifungal, antiparasitic, antidiabetic, antihypertensive, and anti-inflammatory aspects as well as ion receptors, imaging agents, and pathologic probes. The new trend of the foreseeable research in the near future toward imidazole-based supramolecular medicinal chemistry is also prospected. It is hoped that this work provides beneficial help for the rational design of imidazole-based drug molecules and supramolecular medicinal agents and more effective diagnostic agents and pathological probes.

Optical properties, bioimaging and theoretical calculation of a Zn(II) complex based on triphenylamine derivative

A luminescent material with various optical properties based on a triphenylamine Zn(II) complex is described. The ultraviolet-visible absorption, one-photon excited fluorescence (OPEF) and two-photon excited fluorescence (TPEF) of the complex indicate that the material has good OPEF and TPEF properties. And the results of one- and two-photon HepG2 cells imaging experiments show the potential of the complex in fluorescence microscopy bioimaging. The experimental Stokes shift and the FWHM (full-width at half-maximum) in different solvents were correlated with the rMPI polarity of the solvent, and the perfect Boltzmann curves were obtained, where the Boltzmann correlation between Stokes shift and solvent polarity is reported for the second time. But the Boltzmann correlation between FWHM and solvent polarity is reported for the first time. In addition, the computational results indicate that, the covalent bond within the salt ZnBr₂ is strengthened by the coordination, and the newly formed coordination bond Zn-N is stronger than the original covalent bond Zn-Br.

Determination and Imaging of Small Biomolecules and Ions Using Ruthenium(II) Complex-Based Chemosensors

Luminescence chemosensors are one of the most useful tools for the determination and imaging of small biomolecules and ions in situ in real time. Based on the unique photo-physical/-chemical properties of ruthenium(II) (Ru(II)) complexes, the development of Ru(II) complex-based chemosensors has attracted increasing attention in recent years, and thus many Ru(II) complexes have been designed and synthesized for the detection of ions and small biomolecules in biological and environmental samples. In this work, we summarize the research advances in the development of Ru(II) complex-based chemosensors for the determination of ions and small biomolecules, including anions, metal ions, reactive biomolecules and amino acids, with a particular focus on binding/reaction-based chemosensors for the investigation of intracellular analytes' evolution through luminescence analysis and imaging. The advances, challenges and future research directions in the development of Ru(II) complex-based chemosensors are also discussed.

Dinuclear Organoruthenium Complex for Mitochondria-Targeted Near-Infrared Imaging and Anticancer Therapy to Overcome Platinum Resistance

New mononuclear and dinuclear Ru(II) coordination compounds with the 2,7-bisbenzoimidazolyl-naphthyridine ligand have been synthesized and characterized by UV-vis, NMR, and MALDI-TOF. The molecular structures for Ru(II) compounds were determined by single-crystal X-ray diffraction. With the expansion of ligand π-conjugation and the increase in the complexed Ru number, the maximum emission wavelength red-shifted from 696 to 786 nm. The binding mode between complexes and DNA was predicted by molecular docking, which is intercalations and π-π stacking interactions with the surrounding bases. The intercalation mode of DNA binding was then determined by DNA titration and ethidium bromide (EB) displacement experiments. The antigrowth effects of complexes RuY, RuY1, and RuY2 were tested in HaCat (normal cells), HeLa (cervical cancer), A549 (lung cancer), and A549/DDP (cisplatin-resistant lung cancer) through the MTT assay. The dinuclear complex RuY2 was superior to mononuclear complexes and cisplatin in the cisplatin-resistant cell line. Confocal imaging proved that the subcellular localization of Ru(II) complexes was mitochondria; moreover, apoptosis was detected by flow cytometry. All three complexes showed a dose-dependent manner in all four cell lines. All Ru(II) complexes were found to have reactive oxygen species (ROS). The finding indicated that these Ru(II) complexes caused cell death by both DNA disruption and ROS. This study helps to explore the potential of the polynuclear Ru(II) complexes for the combination of NIR imaging and Pt-resistant cancer therapy.