Advancing super-resolution microscopy with metal complexes: functional imaging agents for nanoscale visualization

Super-resolution microscopy (SRM) has transformed biological imaging by overcoming the diffraction limit, offering nanoscale visualization of cellular structures and processes. However, the widespread use of organic fluorescent probes is often hindered by limitations such as photobleaching, short photostability, and inadequate performance in deep-tissue imaging. Metal complexes, with their superior photophysical properties, including exceptional photostability, tuneable luminescence, and extended excited-state lifetimes, address these challenges, enabling precise subcellular targeting and long-term imaging. Beyond imaging, their theranostic potential unlocks real-time diagnostics and treatments for diseases such as cancer and bacterial infections. This review explores recent advancements in applying metal complexes for SRM, focusing on their utility in visualizing intricate subcellular structures, capturing temporal dynamics in live cells and elucidating in vivo spatial organization. We emphasize how rational design strategies optimize biocompatibility, organelle specificity, and deep-tissue penetration, expanding their applicability in multiplexed imaging. Furthermore, we discuss the design of various metal nanoparticles (NPs) for SRM, along with emerging hybrid nanoscale probes that integrate metal complexes with gold (Au) scaffolds, offering promising avenues for overcoming current limitations. By highlighting both established successes and potential frontiers, this review provides a roadmap for leveraging metal complexes as versatile tools in advancing SRM applications.

Release of damaged mitochondrial DNA: A novel factor in stimulating inflammatory response

Mitochondrial DNA (mtDNA) is a circular double-stranded genome that exists independently of the nucleus. In recent years, research on mtDNA has significantly increased, leading to a gradual increase in understanding of its physiological and pathological characteristics. Reactive oxygen species (ROS) and other factors can damage mtDNA. This damaged mtDNA can escape from the mitochondria to the cytoplasm or extracellular space, subsequently activating immune signaling pathways, such as NLR family pyrin domain protein 3 (NLRP3), and triggering inflammatory responses. Numerous studies have demonstrated the involvement of mtDNA damage and leakage in the pathological mechanisms underlying various diseases including infectious diseases, metabolic inflammation, and immune disorders. Consequently, comprehensive investigation of mtDNA can elucidate the pathological mechanisms underlying numerous diseases. The prevention of mtDNA damage and leakage has emerged as a novel approach to disease treatment, and mtDNA has emerged as a promising target for drug development. This article provides a comprehensive review of the mechanisms underlying mtDNA-induced inflammation, its association with various diseases, and the methods used for its detection.

Role of the Environment Polarity on the Photophysical Properties of Mesogenic Hetero-Polymetallic Complexes

New hetero-polynuclear coordination complexes based on a pentacoordinated Zn(II) metal center with tridentate terpyridine-based ligands and monoanionic gallates functionalized with long alkyl chains containing ferrocene units were designed, synthesized and characterized using spectroscopic and analytical methods. The complexes are mesomorphic, exhibiting columnar hexagonal mesophases. The photophysical properties in a solution and in an ordered condensed state were accurately investigated and the influence of the polarity of the solvent was evidenced.

In vitro and in vivo antitumor activities of Ru and Cu complexes with terpyridine derivatives as ligands

Six terpyridine ligands(L1-L6) with chlorophenol or bromophenol moiety were obtained to prepare metal terpyridine derivatives complexes: [Ru(L1)(DMSO)Cl2] (1), [Ru(L2)(DMSO)Cl2] (2), [Ru(L3)(DMSO)Cl2] (3), [Cu(L4)Br2]·DMSO (4), Cu(L5)Br2 (5), and [Cu(L6)Br2]⋅CH3OH (6). The complexes were fully characterized. Ru complexes 1-3 showed low cytotoxicity against the tested cell lines. Cu complexes 4-6 exhibited higher cytotoxicity against several tested cancer cell lines compared to their ligands and cisplatin, and lower toxicity towards normal human cells. Copper(II) complexes 4-6 arrested T-24 cell cycle in G1 phase. The mechanism studies indicated that complexes 4-6 accumulated in mitochondria of T-24 cells and caused significant reduction of the mitochondrial membrane potential, increase of the intracellular ROS levels and the release of Ca2+, and the activation of the Caspase cascade, finally inducing apoptosis. Animal studies showed that complex 6 obviously inhibited the tumor growth in a mouse xenograft model bearing T-24 tumor cells without significant toxicity.

Synthesis, characterization and anticancer properties: A series of highly selective palladium(II) substituted-terpyridine complexes

Ten new palladium(II) complexes [PdCl(L^1-10)]Cl have been synthesized by the reaction of palladium(II) chloride and ten 4'-(substituted-phenyl)-2,2':6',2''-terpyridine ligands bearing hydrogen(L¹), p-hydroxyl(L²), m-hydroxyl (L³), o-hydroxyl (L⁴), methyl (L⁵), phenyl (L⁶), fluoro (L⁷), chloro (L⁸), bromo (L⁹), or iodo (L¹⁰). Their structures were confirmed by FT-IR, ¹H NMR, elemental analysis and/or single crystal X-ray diffraction analysis. Their in vitro anticancer activities were investigated based on five cell lines, including four cancer cell lines (A549, Eca-109, Bel-7402, MCF-7) and one normal cell line (HL-7702). The results show that these complexes possess a strong killing effect on the cancer cells but a weak proliferative inhibition on the normal cells, implying their high inhibitory selectivity for the proliferation of the cancer cell lines. Flow cytometry characterization reveals that these complexes affect cell proliferation mainly in the G0/G1 phase and induce the late apoptotic of the cells. The quantity of palladium(II) ion in extracted DNA was determined by ICP-MS, which proved that these complexes target genomic DNA. And the strong affinity of the complexes with CT-DNA were confirmed by UV-Vis spectrum and circular dichroism (CD). The possible binding modes of the complexes with DNA were further explored by molecular docking. As the concentration of complexes 1-10 gradually increases, the fluorescence intensity of bovine serum albumin (BSA) decreases by a static quenching mechanism.

Small-molecule fluorogenic probes for mitochondrial nanoscale imaging

Mitochondria are inextricably linked to the development of diseases and cell metabolism disorders. Super-resolution imaging (SRI) is crucial in enhancing our understanding of mitochondrial ultrafine structures and functions. In addition to high-precision instruments, super-resolution microscopy relies heavily on fluorescent materials with unique photophysical properties. Small-molecule fluorogenic probes (SMFPs) have excellent properties that make them ideal for mitochondrial SRI. This paper summarizes recent advances in the field of SMFPs, with a focus on the chemical and spectroscopic properties required for mitochondrial SRI. Finally, we discuss future challenges in this field, including the design principles of SMFPs and nanoscopic techniques.

Bright and Light-Up Sensing of Benzo[c,d]indole-oxazolopyridine Cyanine Dye for RNA and Its Application to Highly Sensitive Imaging of Nucleolar RNA in Living Cells

Small molecular weight probes that can show a fluorescence signaling response upon binding to RNAs are promising for RNA imaging in living cells. Live-cell RNA imaging probes that can achieve a large light-up ability (>100-fold) and high Φ_bound value for RNA (>0.50) have been rarely reported to date. Here, benzo[c,d]indole-oxazolopyridine (BIOP), an unsymmetrical monomethine cyanine analogue, was newly developed as a bright and large light-up probe for imaging of nucleolar RNA in living cells. BIOP served as a yellow-emissive probe (λ_em = 570 nm) and exhibited a significant light-up response upon RNA binding (770-fold) with a high Φ_bound value (0.52). We demonstrated the advantages of BIOP over a commercially available RNA-staining probe, SYTO RNA select, for robust and sensitive RNA sensing by a systematic comparison of fluorescent properties for RNA. In addition, BIOP was found to possess high membrane permeability and low cytotoxicity in living cells. The examination of live-cell imaging revealed that BIOP exhibited emission in the nucleolus upon binding to nucleolar RNA much stronger than that of SYTO RNA select. Furthermore, BIOP facilitated the highly sensitive imaging of nucleolar RNA, in which 50 nM BIOP can stain nucleolar RNA in living cells with a 20 min incubation.

Organelle Interaction and Drug Discovery: Towards Correlative Nanoscopy and Molecular Dynamics Simulation

The inter-organelle interactions, including the cytomembrane, endoplasmic reticulum, mitochondrion, lysosome, dictyosome, and nucleus, play the important roles in maintaining the normal function and homeostasis of cells. Organelle dysfunction can lead to a range of diseases (e.g., Alzheimer's disease (AD), Parkinson's disease (PD), and cancer), and provide a new perspective for drug discovery. With the development of imaging techniques and functional fluorescent probes, a variety of algorithms and strategies have been developed for the ever-improving estimation of subcellular structures, organelle interaction, and organelle-related drug discovery with accounting for the dynamic structures of organelles, such as the nanoscopy technology and molecular dynamics (MD) simulations. Accordingly, this work summarizes a series of state-of-the-art examples of the recent progress in this rapidly changing field and uncovering the drug screening based on the structures and interactions of organelles. Finally, we propose the future outlook for exciting applications of organelle-related drug discovery, with the cooperation of nanoscopy and MD simulations.

Metal Peptide Conjugates in Cell and Tissue Imaging and Biosensing

Metal complex luminophores have seen dramatic expansion in application as imaging probes over the past decade. This has been enabled by growing understanding of methods to promote their cell permeation and intracellular targeting. Amongst the successful approaches that have been applied in this regard is peptide-facilitated delivery. Cell-permeating or signal peptides can be readily conjugated to metal complex luminophores and have shown excellent response in carrying such cargo through the cell membrane. In this article, we describe the rationale behind applying metal complexes as probes and sensors in cell imaging and outline the advantages to be gained by applying peptides as the carrier for complex luminophores. We describe some of the progress that has been made in applying peptides in metal complex peptide-driven conjugates as a strategy for cell permeation and targeting of transition metal luminophores. Finally, we provide key examples of their application and outline areas for future progress.

Terpyridine Zn(II) Complexes with Azide Units for Visualization of Histone Deacetylation in Living Cells under STED Nanoscopy

Histones are the alkali proteins in eukaryotic somatic chromatin cells which constitute the nucleosome structure together with DNA. Their abnormality is often associated with multiple tumorigenesis and other human diseases. Nevertheless, a simple and efficient super-resolution method to visualize histone distribution at the subcellular level is still unavailable. Herein, a Zn(II) terpyridine complex with rich-electronic azide units, namely, TpZnA-His, was designed and synthesized. The initial in vitro and in silico studies suggested that this complex is able to detect histones rapidly and selectively via charge-charge interactions with the histone H3 subunit. Its live cell nuclear localization, red-emission tail, and large Stokes shift allowed super-resolution evaluation of histone distributions with a clear distinction against nuclear DNA. We were able to quantitatively conclude three histone morphology alternations in live cells including condensation, aggregation, and cavity during activating histone acetylation. This work offers a better understanding as well as a versatile tool to study histone-involved gene transcription, signal transduction, and differentiation in cells.

Progress on the Physiological Function of Mitochondrial DNA and Its Specific Detection and Therapy

Mitochondrial DNA (mtDNA) is the genetic information of mitochondrion, and its structure is circular double-stranded. Despite the diminutive size of the mitochondrial genome, mtDNA mutations are an important cause of mitochondrial diseases which are characterized by defects in oxidative phosphorylation (OXPHOS). Mitochondrial diseases are involved in multiple systems, particularly in the organs that are highly dependent on aerobic metabolism. The diagnosis of mitochondrial disease is more complicated since mtDNA mutations can cause various clinical symptoms. To realize more accurate diagnosis and treatment of mitochondrial diseases, the detection of mtDNA and the design of drugs acting on it are extremely important. Over the past few years, many probes and therapeutic drugs targeting mtDNA have been developed, making significant contributions to fundamental research including elucidation of the mechanisms of mitochondrial diseases at the genetic level. In this review, we summarize the structure, function, and detection approaches for mtDNA. The most current topics in this field, such as mechanistic exploration and treatment of mtDNA mutation-related disorders, are also reviewed. Specific attention is given to discussing the design and development of these probes and drugs for mtDNA. We hope that this review will provide readers with a comprehensive understanding of the importance of mtDNA, and promote the development of effective molecules for theragnosis of mtDNA mutation-related diseases.

Strategies to promote permeation and vectorization, and reduce cytotoxicity of metal complex luminophores for bioimaging and intracellular sensing

Transition metal luminophores are emerging as important tools for intracellular imaging and sensing. Their putative suitability for such applications has long been recognised but poor membrane permeability and cytotoxicity were significant barriers that impeded early progress. In recent years, numerous effective routes to overcoming these issues have been reported, inspired in part, by advances and insights from the pharmaceutical and drug delivery domains. In particular, the conjugation of biomolecules but also other less natural synthetic species, from a repertoire of functional motifs have granted membrane permeability and cellular targeting. Such motifs can also reduce cytotoxicity of transition metal complexes and offer a valuable avenue to circumvent such problems leading to promising metal complex candidates for application in bioimaging, sensing and diagnostics. The advances in metal complex probes permeability/targeting are timely, as, in parallel, over the past two decades significant technological advances in luminescence imaging have occurred. In particular, super-resolution imaging is enormously powerful but makes substantial demands of its imaging contrast agents and metal complex luminophores frequently possess the photophysical characteristics to meet these demands. Here, we review some of the key vectors that have been conjugated to transition metal complex luminophores to promote their use in intra-cellular imaging applications. We evaluate some of the most effective strategies in terms of membrane permeability, intracellular targeting and what impact these approaches have on toxicity and phototoxicity which are important considerations in a luminescent contrast or sensing agent.

Stimulated emission depletion microscopy for biological imaging in four dimensions: A review

Stimulated emission depletion (STED) microscopy allows high lateral and axial resolution, long term imaging in living cells. Here we review recent technical advances in STED microscopy, with emphasis on resolution and measurement range of XYZt four dimensions. Different STED technical advances and novel STED probes are discussed with their respective application in biological subcellular imaging. This review may serve as a practical guide for choosing a suitable approach to the advanced STED super-resolution imaging.

4′-(5-N-Propylthiophen-2-yl)-2,2′:6′,2″-terpyridine

A new thiophene-substituted terpyridine derivative has been prepared through the reaction between 5-n-propylthiophene-2-carboxaldehyde and 2-acetylpyridine. This terpyridine derivative bears an alkyl chain linked via a thiophene heterocycle.

Recent advances in luminescent materials for super-resolution imaging via stimulated emission depletion nanoscopy

Recent progress on STED fluorophores for super-resolution imaging and also their characteristics are outlined here, thus providing some guidelines to select proper probes and even develop new materials for super-resolution imaging via STED nanoscopy.

4′-(N-(Propan-1,2-dienyl)pyrrol-2-yl)-2,2′:6′,2″-terpyridine

A new pyrrole-substituted terpyridine derivative that possesses an allene moiety was obtained as an “unexpected” sole product during an attempt to alkylate the N-atom of pyrrole with propargyl bromide in order to obtain an alkyne-functionalized terpyridine.

Mitochondrial DNA targeting and impairment by a dinuclear Ir–Pt complex that overcomes cisplatin resistance

A dinuclear complex [(ppy)Ir(tpy)PtCl]²⁺ (Ir–Pt) can exhibit strong antitumor activity towards cisplatin-resistant cancer cells and induce cell necrosis via mtDNA damage and mitochondrial dysfunction.

Organelle-Redirected Chameleon Sensor-Enabled Live Cell Imaging of Mitochondrial DNA

Mitochondrial DNA (mtDNA) plays important roles in diverse physiological processes and myriad diseases. We herein report mtDNA imaging with a chameleon sensor containing a cationic rhodamine B (RB) entity for mitochondria targeting and a fluorogenic SYBR Green-I (SG) entity for DNA sensing. SG-RB selectively binds to mtDNA and gives green SG fluorescence in mitochondria of living cells but gives red RB fluorescence upon delivery of mitochondria into lysosomes in mitophagy. With the dual-color imaging, mtDNA aggregation and elevated mitophagy were identified in HeLa cells stressed with anticancer doxorubicin. These results suggest the utility of organelle-redirected DNA sensors for live cell imaging of mtDNA involved in myriad pathological disorders.

Enhanced three-photon activity triggered by the AIE behaviour of a novel terpyridine-based Zn(ii) complex bearing a thiophene bridge

Multiphoton bioimaging benefits from good penetration of tissue, low phototoxicity and high resolution. Hence, development of efficient multiphoton imaging agents is highly desirable but remains challenging. Herein, a novel terpyridine-based Zn(ii) complex bearing a thiophene bridge was designed rationally and fabricated. Thanks to its aggregation-induced emission (AIE), DZ1 emitted bright yellow-green fluorescence (λ em = 575 nm) under physiological conditions. The three-photon spectral changes of DZ1 when binding with RNA unambiguously reflected its RNA-specific targeting behaviour, resulting in twofold enhancement in three-photon action cross-sections located at the second near-infrared window (1700 nm).

Organic nanoparticles with ultrahigh stimulated emission depletion efficiency for low-power STED nanoscopy

Stimulated emission depletion (STED) nanoscopy is a powerful sub-diffraction imaging tool to probe subcellular structures and organelles. Conventional organic dyes require high STED power (PSTED) to obtain sub-diffraction resolution, leading to serious photo-bleaching. Herein, this study demonstrates highly emissive silica-coated core-shell organic nanoparticles (CSONPs) as a new type of photostable probe with ultrahigh stimulated emission depletion efficiency for low-power super-resolution STED nanoscopy. The CSONPs offer (i) efficient red emission with high solid-state fluorescence quantum yields around 0.6, (ii) large Stokes shift of 150 nm and (iii) high photostability owing to silica shell protection. The stimulated emission depletion efficiency (η) of CSONPs was extremely high up to η = 99% (the highest value reported so far) with a saturation intensity as low as Isat = 0.18 MW cm-2. Moreover, this research demonstrates the super-resolution imaging of living HeLa cells stained using CSONPs with a lateral spatial resolution of 63 nm at an extremely low depletion power of ISTED = 0.89 MW cm-2 and a long-term stability >600 s at η = 80% without obvious fatigue. The excellent and comprehensive performances of the CSONPs are promising for super-resolution imaging in biological applications.

4,4″-Dichloro-4′-(2-thienyl)-2,2′:6′,2″-terpyridine

A new thiophene-substituted terpyridine derivative has been prepared and characterized. This ligand features a thiophene heterocycle (as an electrochemically polymerizable unit) as well as two chlorine atoms for further functionalization.

Identification of fatty liver disease at diverse stages using two-photon absorption of triphenylamine-based BODIPY analogues

03B can target 3T3-L1 model cells at different fatty liver disease (FLD)-induced stages and clearly distinguish FLD tissue and display a lesion in situ.