Exploring a Mitochondria Targeting, Dinuclear Cyclometalated Iridium (III) Complex for Image-Guided Photodynamic Therapy in Triple-Negative Breast Cancer Cells.

Photodynamic therapy (PDT) has emerged as an efficient and noninvasive treatment approach utilizing laser-triggered photosensitizers for combating cancer. Within this rapidly advancing field, iridium-based photosensitizers with their dual functionality as both imaging probes and PDT agents exhibit a potential for precise and targeted therapeutic interventions. However, most reported classes of Ir(III)-based photosensitizers comprise mononuclear iridium(III), with very few examples of dinuclear systems. Exploring the full potential of iridium-based dinuclear systems for PDT applications remains a challenge. Herein, we report a dinuclear Ir(III) complex (IRDI) along with a structurally similar monomer complex (IRMO) having 2-(2,4-difluorophenyl)pyridine and 4'-methyl-2,2'-bipyridine ligands. The comparative investigation of the mononuclear and dinuclear Ir(III) complexes showed similar absorption profiles, but the dinuclear derivative IRDI exhibited a higher photoluminescence quantum yield (Φp) of 0.70 compared to that of IRMO (Φp = 0.47). Further, IRDI showed a higher singlet oxygen generation quantum yield (Φs) of 0.49 compared to IRMO (Φs = 0.28), signifying the enhanced potential of the dinuclear derivative for image-guided photodynamic therapy. In vitro assessments indicate that IRDI shows efficient cellular uptake and significant photocytotoxicity in the triple-negative breast cancer cell line MDA-MB-231. In addition, the presence of a dual positive charge on the dinuclear system facilitates the inherent mitochondria-targeting ability without the need for a specific targeting group. Subcellular singlet oxygen generation by IRDI was confirmed using Si-DMA, and light-activated cellular apoptosis via ROS-mediated PDT was verified through various live-dead assays performed in the presence and absence of the singlet oxygen scavenger NaN3. Further, the mechanism of cell death was elucidated by an annexin V-FITC/PI flow cytometric assay and by investigating the cytochrome c release from mitochondria using Western blot analysis. Thus, the dinuclear complex designed to enhance spin-orbit coupling with minimal excitonic coupling represents a promising strategy for efficient image-guided PDT using iridium complexes.

Mitochondria-targeted cyclometalated iridium (III) complexes: Dual induction of A549 cells apoptosis and autophagy.

In this study, we synthesized 4 cyclometalated iridium complexes using N-(1,10-phenanthrolin-5-yl)picolinamide (PPA) as the main ligand, denoted as [Ir(ppy)2PPA]PF6 (ppy = 2-phenylpyridine, Ir1), [Ir(bzq)2PPA]PF6 (bzq = benzo[h]quinoline, Ir2), [Ir(dfppy)2PPA]PF6 (dfppy = 2-(3,5-difluorophenyl)pyridine, Ir3), and [Ir(thpy)2PPA]PF6 (thpy = 2-(thiophene-2-yl)pyridine, Ir4). Compared to cisplatin and oxaliplatin, all four complexes exhibited significant anti-tumor activity. Among them, Ir2 demonstrated higher cytotoxicity against A549 cells, with an IC50 value of 1.6 ± 0.2 µM. The experimental results indicated that Ir2 primarily localized in the mitochondria, inducing a large amount of reactive oxygen species (ROS) generation, that decreased in mitochondrial membrane potential (MMP), reduced ATP production, and further impaired mitochondrial function, leading to cytochrome c release. Additionally, Ir2 caused cell cycle arrest at the S phase and induced apoptosis through the AKT-mediated signaling pathway. Further investigations revealed that Ir2 could simultaneously induce both apoptosis and autophagy in A549 cells, with the latter acting as a non-protective mechanism that promoted cell death. More importantly, Ir2 exhibited low toxicity to both normal LO2 cells in vitro and zebrafish embryos in vivo. Consequently, these newly developed Ir(III) complexes show great potential in the development of novel and low-toxicity anticancer agents.

An N-heterocyclic carbene iridium(III) complex as a potent anti-cancer stem cell therapeutic.

Cancer stem cells (CSCs) represent a difficult to treat cellular niche within tumours due to their unique characteristics, which give them a high propensity for resistance to classical anti-cancer treatments and the ability to repopulate the tumour mass. An attribute that may be implicated in the high rates of recurrence of certain tumours. However, other characteristics specific to these cells, such as their high dependence on mitochondria, may be exploited for the development of new therapeutic agents that are effective against the niche. As such, a previously described phosphorescent N-heterocyclic carbene iridium(III) compound which showed a high level of cytotoxicity against classical tumour cell lines with mitochondria-specific effects was studied for its potential against CSCs. The results showed a significantly higher level of activity against several CSC lines compared to non-CSCs. Mitochondrial localisation and superoxide production were confirmed. Although the cell death involved caspase activation, their role in cell death was not definitive, with a potential implication of other, non-apoptotic pathways shown. A cytostatic effect of the compound was also displayed at low mortality doses. This study thus provides important insights into the mechanisms and the potential for this class of molecule in the domain of anti-CSC therapeutics.

Vagus nerve stimulation-induced cognitive enhancement: Hippocampal neuroplasticity in healthy male rats.

BACKGROUND: Vagus nerve stimulation (VNS) improves cognition in humans and rodents, but the effects of a single session of VNS on performance and plasticity are not well understood. OBJECTIVE: Behavioral performance and hippocampal (HC) electrophysiology/neurotrophin expression were measured in healthy adult rats after VNS paired training to investigate changes in cognition and synaptic plasticity. METHODS: Platinum/iridium electrodes were surgically implanted around the left cervical branch of the VN of anesthetized male Sprague-Dawley rats (N = 47). VNS (100 µs biphasic pulses, 30 Hz, 0.8 mA) paired Novel Object Recognition (NOR)/Passive Avoidance Task (PAT) were assessed 24 h after training and post-mortem tissue was collected 48 h after VNS (N = 28). Electrophysiology recordings were collected using a microelectrode array system to assess functional effects on HC slices 90 min after VNS (N = 19). Sham received the same treatment without VNS and experimenters were blinded. RESULTS: Stimulated rats exhibited improved performance in NOR (p < 0.05, n = 12) and PAT (p < 0.05, n = 14). VNS enhanced long-term potentiation (p < 0.05, n = 7-12), and spontaneous spike amplitude (p < 0.05, n = 7-12) and frequency (p < 0.05, n = 7-12) in the CA1. Immunohistochemical analysis found increased brain-derived neurotrophic factor expression in the CA1 (p < 0.05, n = 8-9) and CA2 (p < 0.01, n = 7-8). CONCLUSION: These findings suggest that our VNS parameters promote synaptic plasticity and target the CA1, which may mediate the positive cognitive effects of VNS. This study significantly contributes to a better understanding of VNS mediated HC synaptic plasticity, which may improve clinical utilization of VNS for cognitive enhancement.

Cyclometalated iridium(III) dithioformic acid complexes as mitochondria-targeted imaging and anticancer agents.

Four neutral cyclometalated iridium(III) (IrIII) dithioformic acid complexes ([(ppy)2Ir(S^S)], Ir1-Ir4) were designed and synthesized. Toxicity assay revealed that these complexes showed favorable anticancer activity, especially for human non-small cell lung cancer cells (A549). Ir1 exhibited the best anticancer activity (11.0 ± 0.4 µM) was about twice that of cisplatin, meanwhile, which could availably restrain A549 cells migration. Complexes could target mitochondria, induce a decrease in mitochondrial membrane potential (MMP), result in an increase of intracellular reactive oxygen species (ROS) and disruption of the cell cycle, and ultimately generate apoptosis. Western blotting experiment indicated that complexes could inhibit the expression of B cell CLL/lymphoma-2 protein (Bcl-2), induce the expression of BCL2-associated X protein (Bax) and lead to a massive release of Cytochrome C (Cyt-c), which amplified apoptosis signals by activating downstream pathway to promote apoptosis. All these confirmed the existence of mitochondrial anticancer channels for these complexes. Above all, cyclometalated iridium(III) dithioformic acid complexes possess the prospect of becoming a multifunctional cancer therapeutic platform, including mitochondria-targeted imaging, anti-migration, and anticancer agents.

Enhanced in vitro cytotoxicity and antitumor activity in vivo of iridium(III) complexes liposomes targeting endoplasmic reticulum and mitochondria.

In this paper, two new iridium(III) complexes [Ir(ppy)2(CBIP)](PF6) (ppy = 2-phenylpyridine, CBIP = 2-(4'-chloro-(1,1'-biphenyl))-1H-imidazo[4,5-f][1,10]phenanthroline) (Ir1) and [Ir(piq)2(CBIP)](PF6) (piq = 1-phenylisoquinoline) (Ir2) were synthesized and characterized. The anticancer activity of the complexes against cancer A549, HepG2, SGC-7901, BEL-7402, HeLa and LO2 cells was evaluated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) method. Unexpectedly, the complexes exhibit no or low cytotoxic activity toward the selected cancer cells. To increase the anticancer activity, complexes Ir1 and Ir2 were encapsulated into the liposome to form Ir1lipo and Ir2lipo, while Ir1lipo and Ir2lipo show high cytotoxic efficacy against BEL-7402, SGC-7901 and HeLa cells and Ir2lipo displays moderate cytotoxic activity against A549 and HepG2. The anticancer mechanism was explored through wound healing, cell cycle arrest, apoptosis, the change of mitochondrial membrane potential and antitumor activity in vivo. The antitumor in vivo showed that Ir1Lipo (3.9 mg/kg) exhibited significant antitumor activity with an inhibitory rate of 62.16%. Additionally, the expression of B-cell lymphoma-2 family proteins was studies by western blotting analysis. The results demonstrate that Ir1lipo and Ir2lipo induce apoptosis in BEL-7402 cells via endoplasmic reticulum stress-mitochondrial pathway.

Induction of Paraptosis by Cyclometalated Iridium Complex-Peptide Hybrids and CGP37157 via a Mitochondrial Ca2+ Overload Triggered by Membrane Fusion between Mitochondria and the Endoplasmic Reticulum.

We previously reported that a cyclometalated iridium (Ir) complex-peptide hybrid (IPH) 4 functionalized with a cationic KKKGG peptide unit on the 2-phenylpyridine ligand induces paraptosis, a relatively newly found programmed cell death, in cancer cells (Jurkat cells) via the direct transport of calcium (Ca2+) from the endoplasmic reticulum (ER) to mitochondria. Here, we describe that CGP37157, an inhibitor of a mitochondrial sodium (Na+)/Ca2+ exchanger, induces paraptosis in Jurkat cells via intracellular pathways similar to those induced by 4. The findings allow us to suggest that the induction of paraptosis by 4 and CGP37157 is associated with membrane fusion between mitochondria and the ER, subsequent Ca2+ influx from the ER to mitochondria, and a decrease in the mitochondrial membrane potential (ΔΨm). On the contrary, celastrol, a naturally occurring triterpenoid that had been reported as a paraptosis inducer in cancer cells, negligibly induces mitochondria-ER membrane fusion. Consequently, we conclude that the paraptosis induced by 4 and CGP37157 (termed paraptosis II herein) proceeds via a signaling pathway different from that of the previously known paraptosis induced by celastrol, a process that negligibly involves membrane fusion between mitochondria and the ER (termed paraptosis I herein).

Cyclometalated Iridium(III) Complexes as Mitochondria-targeting Photosensitizers against Cisplatin-resistant Cells.

Four iridium (III) complexes Ir1-Ir4 were synthesized and characterized. Possessing high singlet oxygen production ability and specific mitochondria-localization, Ir1 was developed as a mitochondria-targeting photosensitizer. Ir1 exhibited strong phototoxicity against cancer cell line A549 and its corresponding cisplatin-resistant one A549R. In contrast, Ir1 showed low cytotoxicity toward normal cell HLF. This selectivity resulted from the different uptake amount. With 405 nm irradiation, Ir1 induced mitochondria-mediated cell death in A549R cells, achieving the overcome of drug-resistant.

Parahydrogen hyperpolarization of minimally altered urine samples for sensitivity enhanced NMR metabolomics.

Parahydrogen hyperpolarization has been shown to enhance NMR sensitivity in urine analysis by several orders of magnitude if urine samples are prepared by solid phase extraction (SPE). We present a different approach, developed for minimal sample alteration before analysis. Removing SPE from the workflow allows to retain a wider range of metabolites and paves the way towards more universal hyperpolarized NMR metabolomics of low abundance metabolites.

A bioactive ligand-conjugated iridium(III) metal-based complex as a Keap1-Nrf2 protein-protein interaction inhibitor against acetaminophen-induced acute liver injury.

Hepatotoxicity caused by an overdose of acetaminophen (APAP) is the leading reason for acute drug-related liver failure. Nuclear factor erythroid-2-related factor 2 (Nrf2) is a protein that helps to regulate redox homeostasis and coordinate stress responses via binding to the Kelch-like ECH-associated protein 1 (Keap1). Targeting the Keap1-Nrf2 interaction has recently emerged as a potential strategy to alleviate liver injury caused by APAP. Here, we designed and synthesized a number of iridium (III) and rhodium (III) complexes bearing ligands with reported activity against oxidative stress, which is associated with Nrf2 transcriptional activation. The iridium (III) complex 1 bearing a bioactive ligand 2,9-dimethyl-1,10-phenanthroline and 4-chloro-2-phenylquinoline, a derivative of the bioactive ligand 2-phenylquinoline, was identified as a direct small-molecule inhibitor of the Keap1-Nrf2 protein-protein interaction. 1 could stabilize Keap1 protein, upregulate HO-1 and NQO1, and promote Nrf2 nuclear translocation in normal liver cells. Moreover, 1 reversed APAP-induced liver damage by disrupting Keap1-Nrf2 interaction and without inducing organ damage and immunotoxicity in mice. Our study demonstrates the identification of a selective and efficacious antagonist of Keap1-Nrf2 interaction possessed good cellular permeability in cellulo and ideal pharmacokinetic parameters in vivo, and, more importantly, validates the feasibility of conjugating metal complexes with bioactive ligands to generate metal-based drug leads as non-toxic Keap1-Nrf2 interaction inhibitors for treating APAP-induced acute liver injury.

Spectroscopic characterization of the interactions of bovine serum albumin with medicinally important metal ions: platinum (IV), iridium (III) and iron (II).

Serum albumin protein plays a key role in the transportation and distribution of bioactive species including metal ions and metal-based drugs and, therefore, the nature of their binding could provide important insight for the development of new drugs. In the present investigation, binding interactions of bovine serum albumin (BSA) with three biologically important metal ions: Pt4+, Ir3+ and Fe2+ were screened using easy-to-use and cost-effective Fourier-Transform Infrared (FT-IR) and Ultraviolet-Visible (UV-Vis) spectroscopic techniques. Prior to the screening, the protein and metal ions were allowed to interact at physiological pH (7.4) and the spectral changes were monitored upon interaction. In FT-IR spectrum, the position of amide I band (C=O stretching) was shifted from 1652 cm-1 in case of free BSA to 1659, 1657 and 1656 cm-1 in BSA-Pt4+, BSA-Ir3+ and BSA-Fe2+ complexes, respectively. This spectral shifting was due to the binding of metal ions to N and O atoms of BSA peptide bonds. The interaction was further demonstrated by a remarkable reduction in spectral intensities of amide I and II bands. Secondary protein structure analysis revealed conformational changes characterized by a substantial decrease in α-helix (11.29-27.41%) accompanied by an increase in ß-sheet and ß-antiparallel contents. The absorption of BSA at a constant concentration at 280 nm was successively reduced as the concentration of Pt4+ and Ir3+ ions increased. On the other hand, the absorption of BSA-Fe2+ complex successively increased with the increase in the concentration of Fe2+ in the test solution. The binding constants for BSA-Pt4+, BSA-Ir3+ and BSA-Fe2+ complexes were calculated to be 1.55×104, 5.67×104 and 3.78×104 M-1, respectively. The results revealed that the three metal ions showed binding affinities with the BSA protein in the order: Ir3+>Fe2+>Pt4+.

Intrinsic Peroxidase-Mimicking Ir Nanoplates for Nanozymatic Anticancer and Antibacterial Treatment.

The study of inorganic nanozymes to overcome the disadvantages of bio-enzymes, such as the requirement of optimized reaction conditions and lack of durability against environmental factors, is one of the most significant research topics at present. In this work, we comprehensively analyzed the intrinsic peroxidase-like activity of Ir-based nanoparticles, the biological and nanozymatic potentials of which have not yet been explored. These particles were synthesized by the galvanic replacement of Ag nanoplates with Ir. Through the confirmed peroxidase-like activity and hydrogen peroxide decomposition with free radical generation facilitated by these particles, the antibacterial and anticancer effects were successfully verified in vitro. The nanozyme-based therapeutic effect observed at concentrations at which these nanoparticles do not show cytotoxicity suggests that it is possible to achieve more precise and selective local treatment with these particles. The observed highly efficient peroxidase-like activity of these nanoparticles is attributed to the partially mixed composition of Ir-Ag-IrO2 formed through the galvanic replacement reaction in the synthetic process.

Metal complexes for mitochondrial bioimaging.

Mitochondria are essential organelles in eukaryotic cells, containing various signaling molecules and important enzymes associated with cell growth, death, and proliferation. The visualization of mitochondria and their biochemistry with confocal microscopy, fluorescence (phosphorescence) lifetime microscopy (FLIM, PLIM), and super-resolution microscopy has therefore been of great interest in recent years. In particular, transition metal complexes have emerged as excellent mitochondria-targeting probes, due to their high photostabilities, large Stokes shifts, tunable chemical structures and long luminescence lifetimes. In this review, we focus on platinum, ruthenium and iridium complexes, and their application as detectors of micro-environmental alterations as well as for the imaging of signaling molecules inside mitochondria.

Iridium-based probe for luminescent nitric oxide monitoring in live cells.

Nitric oxide (NO) is an intra- and extracellular messenger with important functions during human physiology process. A long-lived luminescent iridium(III) complex probe 1 has been designed and synthesized for the monitoring of NO controllably released from sodium nitroprusside (SNP). Probe 1 displayed a 15-fold switch-on luminescence in the presence of SNP at 580 nm. The probe exhibited a linear response towards SNP between 5 to 25 µM with detection limit at 0.18 µM. Importantly, the luminescent switch-on detection of NO in HeLa cells was demonstrated. Overall, complex 1 has the potential to be applied for NO tracing in complicated cellular environment.

Imaging of a clickable anticancer iridium catalyst.

Iridium-based anticancer reagents are receiving increasing attention for their high cytotoxicity. Herein, by activating CH bonds in the well-known antioxidant α-phenyl-N-tert-butylnitrone (PBN), we synthesized and characterized a series of new iridium complexes. Complex 1-AMP exhibited the best antiproliferation activity towards human ovarian cancer A2780 cells. The azide group in complex 1-AMP underwent the Cu(I)-catalysed azide-alkyne cycloaddition (CuAAC) reaction and the resulting fluorescent imaging in cells suggested it mainly accumulated in mitochondria. In comparison, to eliminate cytotoxicity of Cu(I) catalyst, we conducted a reaction between complex 1-AMP and a commercialized dye via strain-promoted alkyne-azide cycloaddition (SPAAC) reaction in live cells, confirming its targeting mainly in the mitochondria. Iridium-based anticancer complexes containing a nitrone ligand and azide group may offer a useful tool to probe the mechanism of metallodrugs.

Turn-on Luminescent Probe for Hydrogen Peroxide Sensing and Imaging in Living Cells based on an Iridium(III) Complex-Silver Nanoparticle Platform.

A sensitive turn-on luminescent sensor for H2O2 based on the silver nanoparticle (AgNP)-mediated quenching of an luminescent Ir(III) complex (Ir-1) has been designed. In the absence of H2O2, the luminescence intensity of Ir-1 can be quenched by AgNPs via non-radiative energy transfer. However, H2O2 can oxidize AgNPs to soluble Ag+ cations, which restores the luminescence of Ir-1. The sensing platform displayed a sensitive response to H2O2 in the range of 0-17 µM, with a detection limit of 0.3 µM. Importantly, the probe was successfully applied to monitor intracellular H2O2 in living cells, and it also showed high selectivity for H2O2 over other interfering substances.

A G-quadruplex-selective luminescent iridium(III) complex and its application by long lifetime.

BACKGROUND: The G-quadruplex motif has been widely used for the construction of analytical detection platforms due to its rich structural polymorphism and flexibility. Luminescent assays are often limited due to the interference from endogenous fluorophores in biological samples. METHODS: To address this challenge, a novel long lifetime iridium(III) complex 1 was synthesized and used to construct a G-quadruplex-based assay for detecting prostate specific antigen (PSA) in aqueous solution. PSA is a common biomarker in serum and used as a model for demonstration in this work. RESULTS: The PSA assay has achieved a detection limit of 40.8pg·mL-1, and shows high selectivity towards PSA over other proteins. Additionally, the assay could function in diluted human serum by using time-resolved luminescent spectroscopy, with good linearity from 1 to 10ng·mL-1 of PSA, which is adequate to detect the PSA levels for physiological (<4ng·mL-1) and clinical (4-10ng·mL-1) applications. CONCLUSIONS: The assay was successfully constructed. As revealed from time-resolved method, the long lifetime property of iridium(III) complex 1 plays an important role in distinguishing phosphorescence signals from short-life auto-fluorescence of human serum. GENERAL SIGNIFICANCE: Luminescent transition metal complexes offer several advantages over other widely used organic fluorophores, such as long phosphorescence lifetime, large Stokes shift and modular syntheses. In addition, the assay could work effectively in diluted human serum using time-resolved luminescent spectroscopy, it therefore could be potentially developed to monitor PSA in biological samples. This article is part of a Special Issue entitled "G-quadruplex" Guest Editor: Dr. Concetta Giancola and Dr. Daniela Montesarchio.

An Aldol Reaction-Based Iridium(III) Chemosensor for the Visualization of Proline in Living Cells.

A long-lived aldol reaction-based iridium(III) chemosensor [Ir(ppy)2(5-CHOphen)]PF6 (1, where ppy = 2-phenylpyridine and 5-CHOphen = 1,10-phenanthroline-5-carbaldehyde) for proline detection has been synthesized. The iridium(III) complex 1, incorporating an aldehyde group in N^N donor ligand, can take part in aldol reaction with acetone mediated by proline. The transformation of the sp2-hybridized carbonyl group into a sp3-hybridized alcohol group influences the metal-to-ligand charge-transfer (MLCT) state of the iridium(III) complex, resulting in a change in luminescence in response to proline. The interaction of the iridium(III) complex 1 with proline was investigated by 1H NMR, HRMS and emission titration experiments. Upon the addition of proline to a solution of iridium(III) complex 1, a maximum 8-fold luminescence enhancement was observed. The luminescence signal of iridium(III) complex 1 could be recognized in strongly fluorescent media using time-resolved emission spectroscopy (TRES). The detection of proline in living cells was also demonstrated.

Library design and screening protocol for artificial metalloenzymes based on the biotin-streptavidin technology.

Artificial metalloenzymes (ArMs) based on the incorporation of a biotinylated metal cofactor within streptavidin (Sav) combine attractive features of both homogeneous and enzymatic catalysts. To speed up their optimization, we present a streamlined protocol for the design, expression, partial purification and screening of Sav libraries. Twenty-eight positions have been subjected to mutagenesis to yield 335 Sav isoforms, which can be expressed in 24-deep-well plates using autoinduction medium. The resulting cell-free extracts (CFEs) typically contain >1 mg of soluble Sav. Two straightforward alternatives are presented, which allow the screening of ArMs using CFEs containing Sav. To produce an artificial transfer hydrogenase, Sav is coupled to a biotinylated three-legged iridium pianostool complex Cp*Ir(Biot-p-L)Cl (the cofactor). To screen Sav variants for this application, you would determine the number of free binding sites, treat them with diamide, incubate them with the cofactor and then perform the reaction with your test compound (the example used in this protocol is 1-phenyl-3,4-dihydroisoquinoline). This process takes 20 d. If you want to perform metathesis reactions, Sav is coupled to a biotinylated second-generation Grubbs-Hoveyda catalyst. In this application, it is best to first immobilize Sav on Sepharose-iminobiotin beads and then perform washing steps. Elution from the beads is achieved in an acidic reaction buffer before incubation with the cofactor. Catalysis using your test compound (in this protocol, 2-(4-(N,N-diallylsulfamoyl)phenyl)-N,N,N-trimethylethan-1-aminium iodide) is performed using the formed metalloenzyme. Screening using this approach takes 19 d.

Cellular uptake of PLA nanoparticles studied by light and electron microscopy: synthesis, characterization and biocompatibility studies using an iridium(III) complex as correlative label.

We present the synthesis of polylactide by ring-opening polymerization using a luminescent iridium(III) complex acting as initiator. The polymer was formulated into nanoparticles, which were taken up by HEK-293 cells. We could show that the particles provided an appropriate contrast in both superresolution fluorescence and electron microscopy, and, moreover, are non-toxic, in contrast to the free iridium complex.