Peptide-Bridged Dinuclear Ru(II) Complex for Mitochondrial Targeted Monitoring of Dynamic Changes to Oxygen Concentration and ROS Generation in Live Mammalian Cells

The development of ruthenium(ii) polypyridyl complexes and conjugates for in vitro cellular and in vivo applications

Ruthenium(ii) [Ru(ii)] polypyridyl complexes have been the focus of intense investigations since work began exploring their supramolecular interactions with DNA. In recent years, there have been considerable efforts to translate this solution-based research into a biological environment with the intention of developing new classes of probes, luminescent imaging agents, therapeutics and theranostics. In only 10 years the field has expanded with diverse applications for these complexes as imaging agents and promising candidates for therapeutics. In light of these efforts this review exclusively focuses on the developments of these complexes in biological systems, both in cells and in vivo, and hopes to communicate to readers the diversity of applications within which these complexes have found use, as well as new insights gained along the way and challenges that researchers in this field still face.

To cage or to be caged? The cytotoxic species in ruthenium-based photoactivated chemotherapy is not always the metal

In metal-based photoactivated chemotherapy (PACT), two photoproducts are generated by light-triggered photosubstitution of a metal-bound ligand: the free ligand itself and an aquated metal complex. By analogy with cisplatin, the aquated metal complex is usually presented as the biologically active species, as it can typically bind to DNA. In this work, we show that this qualitative assumption is not necessarily valid by comparing the biological activity, log P, and cellular uptake of three ruthenium-based PACT complexes: [Ru(bpy)₂(dmbpy)]²⁺, [Ru(bpy)₂(mtmp)]²⁺, and [Ru(Ph₂phen)₂(mtmp)]²⁺. For the first complex, the photoreleased dmbpy ligand is responsible for the observed phototoxicity, whereas the second complex is not phototoxic, and for the third complex it is the ruthenium bis-aqua photoproduct that is the sole cytotoxic species.

Pyridazine-bridged cationic diiridium complexes as potential dual-mode bioimaging probes

A novel diiridium complex [(N^C^N)2Ir(bis-N^C)Ir(N^C^N)2Cl]PF6 (N^C^N = 2-[3-tert-butyl-5-(pyridin-2-yl)phenyl]pyridine; bis-N^C = 3,6-bis(4-tert-butylphenyl)pyridazine) was designed, synthesised and characterised. The key feature of the complex is the bridging pyridazine ligand which brings two cyclometallated Ir(iii) metal centres close together so that Cl also acts as a bridging ligand leading to a cationic complex. The ionic nature of the complex offers a possibility of improving solubility in water. The complex displays broad emission in the red region (λ em = 520-720 nm, τ = 1.89 μs, Φ em = 62% in degassed acetonitrile). Cellular assays by multiphoton (λ ex = 800 nm) and confocal (λ ex = 405 nm) microscopy demonstrate that the complex enters cells and localises to the mitochondria, demonstrating cell permeability. Further, an appreciable yield of singlet oxygen generation (Φ Δ = 0.45, direct method, by 1O2 NIR emission in air equilibrated acetonitrile) suggests a possible future use in photodynamic therapy. However, the complex has relatively high dark toxicity (LD50 = 4.46 μM), which will likely hinder its clinical application. Despite this toxicity, the broad emission spectrum of the complex and high emission yield observed suggest a possible future use of this class of compound in emission bioimaging. The presence of two heavy atoms also increases the scattering of electrons, supporting potential future applications as a dual fluorescence and electron microscopy probe.

Rational design of a luminescent nanoprobe for hypoxia imaging in vivo via ratiometric and photoluminescence lifetime imaging microscopy

A luminescent nanoprobe has been designed for detection of oxygen. The nanoprobe exhibits high sensitivity, selectivity and excellent reversibility, and has been employed for hypoxia imaging in vitro and in vivo by ratiometric and photoluminescence lifetime imaging techniques.

Cancer-Targeting Functionalization of Selenium-Containing Ruthenium Conjugate with Tumor Microenvironment-Responsive Property to Enhance Theranostic Effects

A mutifunctional ruthenium-based conjugate Ru-BSe was designed and synthesized. The Ru complex with favorable bioimaging function was covalently linked with a cancer-targeted molecule that could be effectively internalized by the tumor to realize enhanced theranostic effects. The pH-response of the Ru conjugate in tumor acidic microenvironment causes ligand substitution and release of therapeutic complex. This activated complex remains inert to the reducing biomolecule-glutathione and terminally locates in mitochondria, in which it triggers oxidative stress, and activates intrinsic apoptosis. Real-time monitoring reveals that this Ru conjugate could selectively accumulate in tumor tissue in vivo, which significantly suppresses tumor progression and alleviate the damage to normal organs, realizing the precise cancer theranosis.

Quantitatively monitoring oxygen variation in endoplasmic reticulum with a fluorophore–phosphor energy transfer cassette

EET cassette Ir-Np-OH has been constructed by connecting an Ir(iii) complex as acceptor to a naphthalimide dye as donor.

Multimodal Probes: Superresolution and Transmission Electron Microscopy Imaging of Mitochondria, and Oxygen Mapping of Cells, Using Small-Molecule Ir(III) Luminescent Complexes

We describe an Ir(III)-based small-molecule, multimodal probe for use in both light and electron microscopy. The direct correlation of data between light- and electron-microscopy-based imaging to investigate cellular processes at the ultrastructure level is a current challenge, requiring both dyes that must be brightly emissive for luminescence imaging and scatter electrons to give contrast for electron microscopy, at a single working concentration suitable for both methods. Here we describe the use of Ir(III) complexes as probes that provide excellent image contrast and quality for both luminescence and electron microscopy imaging, at the same working concentration. Significant contrast enhancement of cellular mitochondria was observed in transmission electron microscopy imaging, with and without the use of typical contrast agents. The specificity for cellular mitochondria was also confirmed with MitoTracker using confocal and 3D-structured illumination microscopy. These phosphorescent dyes are part of a very exclusive group of transition-metal complexes that enable imaging beyond the diffraction limit. Triplet excited-state phosphorescence was also utilized to probe the O₂ concentration at the mitochondria in vitro, using lifetime mapping techniques.

Drug-Porous Silicon Dual Luminescent System for Monitoring and Inhibition of Wound Infection

Wound monitoring and curing is of great importance in biomedical research. This work created a smart bandage that can simultaneously monitor and inhibit wound infection. The main components of the smart bandage are luminescent porous silicon (LuPSi) particles loaded with ciprofloxacin (CIP). This dual luminescent system can undergo accelerated fluorescent color change from red to blue upon the stimulation of reactive oxygen species (ROS) and elevated pH, which are main biomarkers in the infected wound. The mechanism behind the chemical-triggered fluorescent color change was studied in detail. In vitro experiment showed that the ratiometric fluorescent intensity (I_Red/I_Blue) of CIP-LuPSi particles decreased from 10 to 0.03 at pH 7.5 after 24 h, while the value deceased from 10 to 2.15 at pH 7.0. Strong correlation can be also found between the I_Red/I_Blue value and ROS concentration ranging from 0.1 to 10 mM. In addition, the oxidation of LuPSi also simultaneously triggered the release of CIP molecules, which exhibited bacterial inhibition activity. Therefore, the ratiometric fluorescent intensity change at red and blue channels can indicate not only the wound infection status but also the release of antibiotics. In vivo test proved that the smart bandage could distinguish infected wounds from acute wounds, just relying on the naked eyes or a cell phone camera. On the basis of the Si nanotechnology established in this work, theranostic wound care will be realized in future.

Influence of the Steric Bulk and Solvent on the Photoreactivity of Ruthenium Polypyridyl Complexes Coordinated to l-Proline

Ruthenium polypyridyl complexes are good candidates for photoactivated chemotherapy (PACT) provided that they are stable in the dark but efficiently photosubstitute one of their ligands. Here the use of the natural amino acid l-proline as a protecting ligand for ruthenium-based PACT compounds is investigated in the series of complexes Λ-[Ru(bpy)₂(l-prol)]PF₆ ([1a]PF₆; bpy = 2,2′-bipyridine and l-prol = l-proline), Λ-[Ru(bpy)(dmbpy)(l-prol)]PF₆ ([2a]PF₆ and [2b]PF₆; dmbpy = 6,6′-dimethyl-2,2′-bipyridine), and Λ-[Ru(dmbpy)₂(l-prol)]PF₆ ([3a]PF₆). The synthesis of the tris-heteroleptic complex bearing the dissymmetric proline ligand yielded only two of the four possible regioisomers, called [2a]PF₆ and [2b]PF₆. Both isomers were isolated and characterized by a combination of spectroscopy and density functional theory calculations. The photoreactivity of all four complexes [1a]PF₆, [2a]PF₆, [2b]PF₆, and [3a]PF₆ was studied in water (H₂O) and acetonitrile (MeCN) using UV–vis spectroscopy, circular dichroism spectroscopy, mass spectrometry, and ¹H NMR spectroscopy. In H₂O, upon visible-light irradiation in the presence of oxygen, no photosubstitution took place, but the amine of complex [1a]PF₆ was photooxidized to an imine. Contrary to expectations, enhancing the steric strain by the addition of two ([2b]PF₆) or four ([3a]PF₆) methyl substituents did not lead, in phosphate-buffered saline (PBS), to ligand photosubstitution. However, it prevented photoxidation, probably as a consequence of the electron-donating effect of the methyl substituents. In addition, whereas [2b]PF₆ was photostable in PBS, [2a]PF₆ quantitatively isomerized to [2b]PF₆ upon light irradiation. In pure MeCN, [2a]PF₆ and [3a]PF₆ showed non-selective photosubstitution of both the l-proline and dmbpy ligands, whereas the non-strained complex [1a]PF₆ was photostable. Finally, in H₂O–MeCN mixtures, [3a]PF₆ showed selective photosubstitution of l-proline, thus demonstrating the active role played by the solvent on the photoreactivity of this series of complexes. The role of the solvent polarity and coordination properties on the photochemical properties of polypyridyl complexes is discussed.

Three ruthenium polypyridyl l-proline complexes with increasing strain (R, R′ = H or Me) were synthesized and their photoreactivities studied in phosphate-buffered saline, pure acetonitrile (MeCN), and water−MeCN mixtures. Depending on the number of methyl groups, on the presence of air, and on the nature of the solvent, either photoisomerization, photooxidation of l-proline, selective photosubstitution, or nonselective photosubstitution was observed.

Novel organotin(iv) complexes derived from 4-fluorophenyl-selenoacetic acid: synthesis, characterization and in vitro cytostatic activity evaluation

A cluster of novel organotin(iv) complexes were designed, synthesized, and characterized by elemental analysis, FT-IR, and NMR (¹H, ¹³C, and ¹¹⁹Sn) spectroscopy as well as single-crystal X-ray diffraction.

Rational design of polymeric core shell ratiometric oxygen-sensing nanostructures

A new approach for the fabrication of luminescent ratiometric sensing nanosensors is described using core–shell nanoparticles in which the probe and reference are spatially separated into the shell and core of the nanostructure respectively.

Tumor-targeted supramolecular nanoparticles self-assembled from a ruthenium-β-cyclodextrin complex and an adamantane-functionalized peptide

A supramolecular strategy was presented to form water-soluable Ru(ii) nanoparticles. The particles can induce cell death in integrin α_vβ₃-rich tumor cells with high selectivity.

Precision targeted ruthenium(ii) luminophores; highly effective probes for cell imaging by stimulated emission depletion (STED) microscopy

Fluorescence microscopy has undergone a dramatic evolution over the past two decades with development of super-resolution far-field microscopy methods that break the light diffraction limited resolution of conventional microscopy, offering unprecedented opportunity to interrogate cellular processes at the nanoscale. However, these methods make special demands of the luminescent agents used for contrast and development of probes suited to super-resolution fluorescent methods is still relatively in its infancy. In spite of their many photophysical advantages, metal complex luminophores have not yet been considered as probes in this regard, where to date, only organic fluorophores have been applied. Here, we report the first examples of metal complex luminophores applied as probes for use in stimulated emission depletion (STED) microscopy. Exemplified with endoplasmic reticulum and nuclear targeting complexes we demonstrate that luminescent Ru(ii) polypyridyl complexes can, through signal peptide targeting, be precisely and selectively delivered to key cell organelles without the need for membrane permeabilization, to give high quality STED images of these organelles. Detailed features of the tubular ER structure are revealed and in the case of the nuclear targeting probe we exploit the molecular light switch properties of a dipyrido[3,2-a:2',3'-c]phenazine containing complex which emits only on DNA/RNA binding to give outstanding STED contrast and resolution of the chromosomes within the nucleus. Comparing performance with a member of the AlexaFluor family commonly recommended for STED, we find that the performance of the ruthenium complexes is superior across both CW and gated STED microscopy methods in terms of image resolution and photostability. The large Stokes shifts of the Ru probes permit excellent matching of the stimulating depletion laser with their emission whilst avoiding anti-Stokes excitation. Their long lifetimes make them particularly amenable to gated STED, giving a much wider window for gating than traditional probes. Our findings indicate that ruthenium polypyridyl peptide targeted probes are a powerful new partner to STED microscopy, opening up new approaches to probe design for STED microscopy.

Biological processing of dinuclear ruthenium complexes in eukaryotic cells

The biological processing - mechanism of cellular uptake, effects on the cytoplasmic and mitochondrial membranes, intracellular sites of localisation and induction of reactive oxygen species - of two dinuclear polypyridylruthenium(ii) complexes has been examined in three eukaryotic cells lines. Flow cytometry was used to determine the uptake of [{Ru(phen)2}2{μ-bb12}](4+) (Rubb12) and [Ru(phen)2(μ-bb7)Ru(tpy)Cl](3+) {Rubb7-Cl, where phen = 1,10-phenanthroline, tpy = 2,2':6',2''-terpyridine and bbn = bis[4(4'-methyl-2,2'-bipyridyl)]-1,n-alkane} in baby hamster kidney (BHK), human embryonic kidney (HEK-293) and liver carcinoma (HepG2) cell lines. The results demonstrated that the major uptake mechanism for Rubb12 and Rubb7-Cl was active transport, although with a significant contribution from carrier-assisted diffusion for Rubb12 and passive diffusion for Rubb7-Cl. Flow cytometry coupled with Annexin V/TO-PRO-3 double-staining was used to compare cell death by membrane damage or apoptosis. Rubb12 induced significant direct membrane damage, particularly with HepG2 cells, while Rubb7-Cl caused considerably less membrane damage but induced greater levels of apoptosis. Confocal microscopy, coupled with JC-1 assays, demonstrated that Rubb12 depolarises the mitochondrial membrane, whereas Rubb7-Cl had a much smaller affect. Cellular localisation experiments indicated that Rubb12 did not accumulate in the mitochondria, whereas significant mitochondrial accumulation was observed for Rubb7-Cl. The effect of Rubb12 and Rubb7-Cl on intracellular superoxide dismutase activity showed that the ruthenium complexes could induce cell death via a reactive oxygen species-mediated pathway. The results of this study demonstrate that Rubb12 predominantly kills eukaryotic cells by damaging the cytoplasmic membrane. As this dinuclear ruthenium complex has been previously shown to exhibit greater toxicity towards bacteria than eukaryotic cells, the results of the present study suggest that metal-based cationic oligomers can achieve selective toxicity against bacteria, despite exhibiting a non-specific membrane damage mechanism of action.

Facile synthesis of a ratiometric oxygen nanosensor for cellular imaging

A new type of cell-penetrating ratiometric fluorescence oxygen sensing nanoparticle was prepared through a facile co-precipitation method. Amphiphilic polymer poly (styrene-co-maleic anhydride) (PSMA) was firstly cooperated with polystyrene (PS) to envelop the highly photostable phosphorescent oxygen indicator, platinum(II)-tetrakis(pentafluorophenyl)porphyrin (PtTFPP, emission at 648nm), and the reference fluorophore, poly(9, 9-dioctylfluorene) (PFO, emission at 440nm ), via hydrophobic interaction in aqueous solution. To improve the sensor biocompatibility, the biomacromolecule poly-l-lysine (PLL) was selected to act as a shell via electrostatic forces. The as-prepared PtTFPP doped core-shell nanoparticles (called PPMA/PLL NPs) exhibited an excellent ratiometric luminescence response to O2 content with high quenching efficiency and full reversibility in the oxygen sensing. More importantly, these oxygen nanosensors passed across the cell membrane after co-incubation without external force. Labeled cells exhibited high brightness in the matching blue and red channels of a digital camera. And most nanosensors were found locating in cytoplasm rather than being trapped in endosomes.

Osmium(II) polypyridyl polyarginine conjugate as a probe for live cell imaging; a comparison of uptake, localization and cytotoxicity with its ruthenium(II) analogue

A first investigation into the application of a luminescent osmium(ii) bipyridine complex to live cell imaging is presented. Osmium(ii) (bis-2,2-bipyridyl)-2(4-carboxylphenyl) imidazo[4,5f][1,10]phenanthroline was prepared and conjugated to octaarginine, a cell penetrating peptide. The photophysics, cell uptake and cytotoxicity of this osmium complex conjugate were performed and compared with its ruthenium analogue. Cell uptake and distribution of both ruthenium and osmium conjugates were very similar with rapid transmembrane transport of the osmium probe (complete within approx. 20 min) and dispersion throughout the cytoplasm and organelles. The near-infrared (NIR) emission of the osmium complex (λmax 726 nm) coincides well with the biological optical window and this facilitated luminescent and luminescence lifetime imaging of the cell which was well resolved from cell autofluorescence. The large Stokes shift of the emission also permitted resonance Raman mapping of the dye within CHO cells. Rather surprisingly, the osmium conjugate exhibited very low cytotoxicity when incubated both in the dark and under visible irradiation. This was attributed to the remarkable stability of this complex which was reflected by the complete absence of photo-bleaching of the complex even under extended continuous irradiation. In addition, when compared to its ruthenium analogue its luminescence was short-lived in water therefore rendering it insensitive to O2.

Electrochemiluminescent Array to Detect Oxidative Damage in ds-DNA Using [Os(bpy)2(phen-benz-COOH)]2+/Nafion/Graphene Films

Reactive oxygen species (ROS) oxidize guanosines in DNA to form 8-oxo-7,8-dihydro-2-deoxyguanosine (8-oxodG), a biomarker for oxidative stress. Herein we describe a novel 64-microwell electrochemiluminescent (ECL) array enabling sensitive multiplexed detection of 8-oxodG in ds-DNA without hydrolysis. Films of Nafion and reduced graphene oxide containing ECL dye [Os(bpy)₂(phen-benz-COOH)]²⁺ (OsNG, {bpy= 2,2'-bipyridine and phen-benz-COOH = (4-(1,10-phenanthrolin-6-yl) benzoic acid)}) were assembled into microwells on a pyrolytic graphite wafer to detect 8-oxodG in oligonucleotides by electrochemiluminescence (ECL). DNA oxidation by Fenton's reagent or by ROS formation during redox cycles involving NADPH, Cu^II, and model metabolites was monitored. UPLC-MS/MS of oxidized DNA samples were used for calibration. Detection limit for the fluidic arrays was one 8-oxodG per 670 intact nucleobases, or 0.15%. The method is sensitive enough to evaluate DNA oxidation from biologically relevant ROS-generating reactions of Cu^II, NADPH, and model metabolites.

Apoptosis, autophagy, cell cycle arrest, cell invasion and BSA-binding studies in vitro of ruthenium(ii) polypyridyl complexes

Four new ruthenium(ii) polypyridyl complexes were synthesized and characterized. The anticancer activity was investigated by cytotoxicity in vitro, apoptosis, comet assay, ROS, autophagy, cell invasion and western blot analysis.

Peptide-mediated vectorization of metal complexes: conjugation strategies and biomedical applications

The rich chemical and structural versatility of transition metal complexes provides numerous novel paths to be pursued in the design of molecules that exert particular chemical or physicochemical effects that could operate over specific biological targets.

An efficient route to asymmetrically diconjugated tris(heteroleptic) complexes of Ru(ii)

A highly efficient and versatile route to the preparation of tris(heteroleptic) Ru(ii) polypyridyl complexes is described which permits access to two or more independently conjugatable termini in the final structure.

Development of a novel H2S and GSH detection cocktail for fluorescence imaging

A highly sensitive fluorescent detection cocktail has been developed for the simultaneous imaging of H₂S and GSH in live cells.

A Phosphorescent Iridium(III) Complex-Modified Nanoprobe for Hypoxia Bioimaging Via Time-Resolved Luminescence Microscopy

Oxygen plays a crucial role in many biological processes. Accurate monitoring of oxygen level is important for diagnosis and treatment of diseases. Autofluorescence is an unavoidable interference in luminescent bioimaging, so that an amount of research work has been devoted to reducing background autofluorescence. Herein, a phosphorescent iridium(III) complex-modified nanoprobe is developed, which can monitor oxygen concentration and also reduce autofluorescence under both downconversion and upconversion channels. The nanoprobe is designed based on the mesoporous silica coated lanthanide-doped upconversion nanoparticles, which contains oxygen-sensitive iridium(III) complex in the outer silica shell. To image intracellular hypoxia without the interferences of autofluorescence, time-resolved luminescent imaging technology and near-infrared light excitation, both of which can reduce autofluorescence effectively, are adopted in this work. Moreover, gradient O2 concentration can be detected clearly through confocal microscopy luminescence intensity imaging, phosphorescence lifetime imaging microscopy, and time-gated imaging, which is meaningful to oxygen sensing in tissues with nonuniform oxygen distribution.

Serum Albumin Binding Inhibits Nuclear Uptake of Luminescent Metal-Complex-Based DNA Imaging Probes

The DNA binding and cellular localization properties of a new luminescent heterobimetallic Ir(III) Ru(II) tetrapyridophenazine complex are reported. Surprisingly, in standard cell media, in which its tetracationic, isostructural Ru(II) Ru(II) analogue is localized in the nucleus, the new tricationic complex is poorly taken up by live cells and demonstrates no nuclear staining. Consequent cell-free studies reveal that the Ir(III) Ru(II) complex binds bovine serum albumin, BSA, in Sudlow's Site I with a similar increase in emission and binding affinity to that observed with DNA. Contrastingly, in serum-free conditions the complex is rapidly internalized by live cells, where it localizes in cell nuclei and functions as a DNA imaging agent. The absence of serum proteins also greatly alters the cytotoxicity of the complex, where high levels of oncosis/necrosis are observed due to this enhanced uptake. This suggests that simply increasing the lipophilicity of a DNA imaging probe to enhance cellular uptake can be counterproductive as, due to increased binding to serum albumin protein, this strategy can actually disrupt nuclear targeting.

Solvent switchable dual emission from a bichromophoric ruthenium–BODIPY complex

A ruthenium(ii) polypyridyl–BODIPY dyad is presented which exhibits a solvent switchable dual emission.