Rational Design of Cyclometalated Iridium(III) Complexes for Three‐Photon Phosphorescence Bioimaging

Aqueous emissive cyclometalated iridium photoreductants: synthesis, computational analysis and the photocatalytic reduction of 4-nitrophenol

Herein, we present luminescent mononuclear iridium complexes [1]3+-[4]3+ using NEt3-appended C^N chelating benzimidazole (L1-L4) and semi-flexible phenanthroline-pyrazine-based (phpy) ligands exhibiting photocatalytic reduction of 4-nitrophenol (4-NP) in the presence of NEt3 in an aqueous medium. The formation of [1]3+-[4]3+ was confirmed by HRMS, 1H-1H COSY, and 13C and 19F NMR spectroscopy. The complex [4]3+ is water soluble, whereas the others ([1]3+-[3]3+) are partially soluble. The complexes are luminescent in both CH3CN and H2O media. The DFT study reveals that the HOMO of [1]3+ resides on the C^N chelating benzimidazole and iridium center. However, it moves to the pyrazine-pyridine of the phpy unit in the case of [2]3+-[4]3+. The LUMOs are localized on the phenanthroline unit of phpy for all the complexes. This suggests an important role of the fluorine atom on electron density distribution. Spin density analysis demonstrates that the emission bands of the complexes arise from 3MLLCT states. The complex [4]3+ displays promising photocatalytic activity towards 4-NP photoreduction, whereas complexes [1]3+-[3]3+ exhibit lower reactivity. The mechanistic study suggests that the reaction proceeds through an oxidative quenching pathway, where 4-NP is reduced by accepting an electron from excited [Ir(III)] and gets oxidized to Ir(IV), which comes back to its original Ir(III) state by accepting an electron from the sacrificial electron donor NEt3.

Ligand Regulation Strategy to Modulate ROS Nature in a Rhodamine-Iridium(III) Hybrid System for Phototherapy

The efficacy of photodynamic therapy (PDT) is highly dependent on the photosensitizer features. The reactive oxygen species (ROS) generated by photosensitizers is proven to be associated with immunotherapy by triggering immunogenic cell death (ICD) as well. In this work, we establish a rhodamine-iridium(III) hybrid model functioning as a photosensitizer to comprehensively understand its performance and potential applications in photodynamic immunotherapy. Especially, the correlation between the ROS generation efficiency and the energy level of the Ir(III)-based excited state (T1'), modulated by the cyclometalating (C∧N) ligand, is systematically investigated and correlated. We prove that in addition to the direct population of the rhodamine triplet state (T1) formed through the intersystem crossing process with the assistance of a heavy Ir(III) metal center, the fine-tuned T1' state could act as a relay to provide an additional pathway for promoting the cascade energy transfer process that leads to enhanced ROS generation ability. Moreover, type I ROS can be effectively produced by introducing sulfur-containing thiophene units in C∧N ligands, providing a stronger M1 macrophage-activation efficiency under hypoxia to evoke in vivo antitumor immunity. Overall, our work provides a fundamental guideline for the molecular design and exploration of advanced transition-metal-based photosensitizers for biomedical applications.

Combining Multiple Photosensitizer Modules into One Supramolecular System for Synergetic Enhanced Photodynamic Therapy

Photodynamic therapy (PDT), as an emerging cancer treatment, requires the development of highly desirable photosensitizers (PSs) with integrated functional groups to achieve enhanced therapeutic efficacy. Coordination-driven self-assembly (CDSA) would provide an alternative approach for combining multiple PSs synergistically. Here, we demonstrate a simple yet powerful strategy of combining conventional chromophores (tetraphenylethylene, porphyrin, or Zn-porphyrin) with pyridinium salt PSs together through condensation reactions, followed by CDSA to construct a series of novel metallo-supramolecular PSs (S1-S3). The generation of reactive oxygen species (ROS) is dramatically enhanced by the direct combination of two different PSs, and further reinforced in the subsequent ensembles. Among all the ensembles, S2 with two porphyrin cores shows the highest ROS generation efficiency, specific interactions with lysosome, and strong emission for probing cells. Moreover, the cellular and living experiments confirm that S2 has excellent PDT efficacy, biocompatibility, and biosafety. As such, this study will enable the development of more efficient PSs with potential clinical applications.

Recent Advances in Organometallic NIR Iridium(III) Complexes for Detection and Therapy

Iridium(III) complexes are emerging as a promising tool in the area of detection and therapy due to their prominent photophysical properties, including higher photostability, tunable phosphorescence emission, long-lasting phosphorescence, and high quantum yields. In recent years, much effort has been devoted to develop novel near-infrared (NIR) iridium(III) complexes to improve signal-to-noise ratio and enhance tissue penetration. In this review, we summarize different classes of organometallic NIR iridium(III) complexes for detection and therapy, including cyclometalated ligand-enabled NIR iridium(III) complexes and NIR-dye-conjugated iridium(III) complexes. Moreover, the prospects and challenges for organometallic NIR iridium(III) complexes for targeted detection and therapy are discussed.

Metallopolymer strategy to explore hypoxic active narrow-bandgap photosensitizers for effective cancer photodynamic therapy

Practical photodynamic therapy calls for high-performance, less O2-dependent, long-wavelength-light-activated photosensitizers to suit the hypoxic tumor microenvironment. Iridium-based photosensitizers exhibit excellent photocatalytic performance, but the in vivo applications are hindered by conventional O2-dependent Type-II photochemistry and poor absorption. Here we show a general metallopolymerization strategy for engineering iridium complexes exhibiting Type-I photochemistry and enhancing absorption intensity in the blue to near-infrared region. Reactive oxygen species generation of metallopolymer Ir-P1, where the iridium atom is covalently coupled to the polymer backbone, is over 80 times higher than that of its mother polymer without iridium under 680 nm irradiation. This strategy also works effectively when the iridium atom is directly included (Ir-P2) in the polymer backbones, exhibiting wide generality. The metallopolymer nanoparticles exhibiting efficient O2•- generation are conjugated with integrin αvβ3 binding cRGD to achieve targeted photodynamic therapy.

Molecularly Engineered Room-Temperature Phosphorescence for Biomedical Application: From the Visible toward Second Near-Infrared Window

Phosphorescence, characterized by luminescent lifetimes significantly longer than that of biological autofluorescence under ambient environment, is of great value for biomedical applications. Academic evidence of fluorescence imaging indicates that virtually all imaging metrics (sensitivity, resolution, and penetration depths) are improved when progressing into longer wavelength regions, especially the recently reported second near-infrared (NIR-II, 1000-1700 nm) window. Although the emission wavelength of probes does matter, it is not clear whether the guideline of "the longer the wavelength, the better the imaging effect" is still suitable for developing phosphorescent probes. For tissue-specific bioimaging, long-lived probes, even if they emit visible phosphorescence, enable accurate visualization of large deep tissues. For studies dealing with bioimaging of tiny biological architectures or dynamic physiopathological activities, the prerequisite is rigorous planning of long-wavelength phosphorescence, being aware of the cooperative contribution of long wavelengths and long lifetimes for improving the spatiotemporal resolution, penetration depth, and sensitivity of bioimaging. In this Review, emerging molecular engineering methods of room-temperature phosphorescence are discussed through the lens of photophysical mechanisms. We highlight the roles of phosphorescence with emission from visible to NIR-II windows toward bioapplications. To appreciate such advances, challenges and prospects in rapidly growing studies of room-temperature phosphorescence are described.

Three-Photon Infrared Stimulation of Endogenous Neuroreceptors in Vivo

To interrogate neural circuits and crack their codes, in vivo brain activity imaging must be combined with spatiotemporally precise stimulation in three dimensions using genetic or pharmacological specificity. This challenge requires deep penetration and focusing as provided by infrared light and multiphoton excitation, and has promoted two-photon photopharmacology and optogenetics. However, three-photon brain stimulation in vivo remains to be demonstrated. We report the regulation of neuronal activity in zebrafish larvae by three-photon excitation of a photoswitchable muscarinic agonist at 50 pM, a billion-fold lower concentration than used for uncaging, and with mid-infrared light of 1560 nm, the longest reported photoswitch wavelength. Robust, physiologically relevant photoresponses allow modulating brain activity in wild-type animals with spatiotemporal and pharmacological precision. Computational calculations predict that azobenzene-based ligands have high three-photon absorption cross-section and can be used directly with pulsed infrared light. The expansion of three-photon pharmacology will deeply impact basic neurobiology and neuromodulation phototherapies.

Polarity-Sensitive Probe for Two-Photon Fluorescence Lifetime Imaging of Lipid Droplets In Vitro and In Vivo

Lipid droplets (LDs) are crucial organelles used to store lipids and participate in lipid metabolism in cells. The abnormal aggregation and polarity change of LDs are associated with the occurrence of diseases, such as steatosis. Herein, the polarity-sensitive probe TBPCPP with a donor-acceptor-π-acceptor (D-A-π-A) structure was designed and synthesized. The TBPCPP has a large Stokes shift (∼220 nm), excellent photostability, high LD targeting, and considerable two-photon absorption (TPA) cross-section (∼226 GM), enabling deep two-photon imaging (∼360 μm). In addition, the fluorescence lifetime of TBPCPP decreases linearly with increasing solvent polarity. Therefore, with the assistance of two-photon fluorescence lifetime imaging microscopy (TP-FLIM), TBPCPP has successfully achieved not only the visualization of polarity changes caused by LD accumulation in HepG-2 cells but also lipid-specific imaging and visualization of different polarities in lipid-rich regions in zebrafish for the first time. Furthermore, TP-FLIM revealed that the polarity gradually decreases during steatosis in HepG-2 cells, which provided new insights into the diagnosis of steatosis.

Unprecedented hetero-coordinated Ir(C^N)2tmd complexes containing both five- and six-membered Ir-(C^N) rings based on phenanthrylpyridine ligands: syntheses, crystal structures and photophysical properties

By using 2-(9-phenanthryl)pyridine (phpyr) and its derivatives as cyclometalated ligands, we synthesized a set of isomeric red-emitting complexes Ir(phpyr-R)2tmd (R = -H, -CF3, -F and -CH3, tmd = 2,2,6,6-tetramethylheptane-3,5-dione) with different coordinated modes, including bis-five-membered and five- + six-membered Ir-(C^N) ring chelating modes. The latter are the first examples of hetero-coordinated Ir(C^N)2(L^X)-type complexes containing both five- and six-membered Ir-(C^N) metallocycles. Their coordination geometries were distinctly determined using X-ray crystallographic analysis. Compared to typical bis-five-membered ring-chelated complexes, these novel hetero-coordinated isomers show bathochromic emission and lower quantum yields. On careful analysis of their electrochemical behavior and DFT calculations, it has been found that the regulatory effects of the solitary six-membered metallocycles in Ir(phpyr-R)2tmd could not only stabilize the LUMO but also destabilize the HOMO, leading to a narrower energy gap. More importantly, DFT calculations of the relative energies of these isomeric complexes demonstrated that bis-five-membered and five- + six-membered chelating modes are more stable compared to bis-six-membered rings, consistent with experiments. This work provides guidance for the structural design of Ir(C^N)2(L^X)-type complexes.

Near-infrared metal agents assisting precision medicine: from strategic design to bioimaging and therapeutic applications

Metal agents have made incredible strides in preclinical research and clinical applications in recent years, but their short emission/absorption wavelengths continue to be a barrier to their distribution, therapeutic action, visual tracking, and efficacy evaluation. Nowadays, the near-infrared window (NIR, 650-1700 nm) provides a more accurate imaging and treatment option. Thus, there has been ongoing research focusing on developing multifunctional NIR metal agents for imaging and therapy that have deeper tissue penetration. The design, characteristics, bioimaging, and therapy of NIR metal agents are covered in this overview of papers and reports published to date. To start with, we focus on describing the structure, design strategies, and photophysical properties of metal agents from the NIR-I (650-1000 nm) to NIR-II (1000-1700 nm) region, in order of molecular metal complexes (MMCs), metal-organic complexes (MOCs), and metal-organic frameworks (MOFs). Next, the biomedical applications brought by these superior photophysical and chemical properties for more accurate imaging and therapy are discussed. Finally, we explore the challenges and prospects of each type of NIR metal agent for future biomedical research and clinical translation.

Tailoring the π-system of benzimidazole ligands towards stable light-harvesting cyclometalated iridium(III) complexes

The synthesis, structure, optical and redox properties as well as photovoltaic studies of iridium(III) complexes with cyclometalated 2-arylbenzimidazoles decorated with various polyaromatic fragments and an ancillary aromatic β-diketone are reported. Despite the strong preference of the iridium(III) ion to form bis- or tris-cyclometalated complexes in which the metal participates in five-membered metallacycles, the cyclometalation of the benzimidazole ligands containing rigid π-extended systems yields dimeric complexes containing strained five- or six-membered metallacycles and allows for generating an extremely rare monocyclometalated complex. X-ray crystallography shows that the steric strain observed in the dimers is retained in heteroleptic diketonate complexes which is also corroborated by gas-phase DFT calculations. While emission maxima and redox potentials of the heteroleptic complexes exhibit just a moderate variation upon the change of the cyclometalated ligands, the extension of the π-system of the benzimidazole ligands give the complexes remarkable light absorption in the visible spectral range, which meets the requirements for application in dye-sensitized solar cells. At the titania photoanodes, these iridium dyes retain their optical properties and exhibit power conversion efficiencies under standard AM 1.5 G conditions comparable to those of other iridium-based sensitizers. These results demonstrate that the size and position of the π-extended fragment in cyclometalated ligands can modulate not only the electronic structure of the corresponding iridium(III) complexes, but also affect their composition, structure and reactivity that may find implications in future design of emerging iridium dyes, emitters and catalysts.

Metal Complexes for Cancer Sonodynamic Therapy

Sonodynamic therapy (SDT) for cancer treatment is gaining attention owing to its non-invasive property and ultrasound's (US) deep tissue penetration ability. In SDT, US activates the sonosensitizer at the target deep-seated tumors to generate reactive oxygen species (ROS), which ultimately damage tumors. However, drawbacks such as insufficient ROS production, aggregation of sonosensitizer, off-target side effects, etc., of the current organic/nanomaterial-based sonosensitizers limit the effectiveness of cancer SDT. Very recently, metal complexes with tunable physiochemical properties (such as sonostability, HOMO to LUMO energy gap, ROS generation ability, aqueous solubility, emission, etc.) have been devised as effective sonosensitizers, which could overcome the limitations of organic/nanomaterial-based sonosensitizers. This concept introduces all the reported metal-based sonosensitizers and delineates the prospects of metal complexes in cancer sonodynamic therapy. This new concept of metal-based sonosensitizer can deliver next-generation cancer drugs.

Synthesis, characterization and biological evaluation of two cyclometalated iridium(III) complexes containing a glutathione S-transferase inhibitor

The cyclometalated iridium(III) compounds have been intensively studied for health-related applications due to their outstanding luminescent properties and multiple anticancer modes of action. Herein, two iridium(III) compounds Ir-1 and Ir-3 containing glutathione S-transferase inhibitor (GSTi) were developed and studied together with two unfunctionalized compounds Ir-2 and Ir-4 as a comparison. Biological study indicated that GSTi-bearing complexes Ir-1 and Ir-3 exert a synergistic effect on the inhibition of cancer cells. The photophysical properties of Ir-1 ∼ Ir-4 were investigated by UV/vis absorption and fluorescence spectroscopy and rationalized with TD-DFT calculations. As expected, GSTi-bearing complexes Ir-1 and Ir-3 exhibited considerable cytotoxicity against both A549 and cisplatin-resistant A549/cis cancer cells, much higher than the unfunctionalized iridium compounds Ir-2 and Ir-4. Further study indicated that Ir-1 and Ir-3 mainly localize in the mitochondria of tumor cells, and exert their cytotoxicity via generating ROS and inhibiting GST activity. The flow cytometry investigations demonstrated that Ir-1 and Ir-3 can arrest the cell cycle in S phase and induce the cell death through apoptosis process. Overall, the complexation of GST inhibitors with cyclometalated iridium(III) agents provides an effective way for potentiating the cytotoxicity of iridium(III) anticancer agents and resensitizing the efficacy against cisplatin resistant cancer cells.

Mitochondria-targeted cyclometalated iridium (III) complex for H2S-responsive intracellular redox regulation as potent photo-oxidation anticancer agent

Owing to the safety and low toxicity, photodynamic therapy (PDT) for cancer treatment has received extensive attention. However, the excess H₂S in cancer cells reduces the PDT efficiency, because H₂S indirectly depletes the reactive oxygen species (ROS). To improve anticancer efficiency, a mitochondria-targeted iridium(III) complex Ir-MMB has been developed as H₂S consumer and photo-oxidation anticancer agent. On the one hand, complex Ir-MMB can consume H₂S with sensitive phosphorescence turn-on, which has been successfully applied to exogenous and endogenous H₂S response imaging in living cells. On the other hand, Ir-MMB can enhance its anticancer activity and cause photo-oxidation damage via catalyzing the oxidation of reduced form of nicotinamide-adenine dinucleotide (NADH) to NAD⁺ and producing H₂O₂ under light, and ultimately results in cell apoptosis through mitochondrial depolarization and ROS production.

Synthesis and comparison study of electrochemiluminescence from mononuclear and corresponding heterodinuclear Ir-Ru complexes via an amide bond as a bridge

A mononuclear iridium-based complex with a primary amine group (named Ir-NH₂), a mononuclear ruthenium-based complex with a butanoic acid group (named Ru-COOH) and the corresponding heterodinuclear complex containing an iridium and ruthenium center via an amide bond bridge (named Ir-Ru) were designed and successfully synthesized in this study. The photophysical and electrochemical properties and ECL performances of these three metal complexes under various experimental conditions were well characterized. For the first time, the insights from this comprehensive comparison study indicate that the two metal-based subunits with comparable luminescent properties are significant in the design of bimetallic-based multicolor luminophores at the molecular level, which helps us to further understand the emission performances of bimetallic complexes and to rationally design more efficient corresponding organometallic luminophores with multicolor emission for wide applications in the future.

Phosphorescent Ir(III) Complexes for Biolabeling and Biosensing

Cyclometalated Ir(III) complexes exhibit strong phosphorescence emission with lifetime of submicroseconds to several microseconds at room temperature. Their synthetic versatility enables broad control of physical properties, such as charge and lipophilicity, as well as emission colors. These favorable properties have motivated the use of Ir(III) complexes in luminescent bioimaging applications. This review examines the recent progress in the development of phosphorescent biolabels and sensors based on Ir(III) complexes. It begins with a brief introduction about the basic principles of the syntheses and photophysical processes of cyclometalated Ir(III) complexes. Focus is placed on illustrating the broad imaging utility of Ir(III) complexes. Phosphorescent labels illuminating intracellular organelles, including mitochondria, lysosomes, and cell membranes, are summarized. Ir(III) complexes capable of visualization of tumor spheroids and parasites are also introduced. Facile chemical modification of the cyclometalating ligands endows the Ir(III) complexes with strong sensing ability. Sensors of temperature, pH, CO₂, metal ions, anions, biosulfur species, reactive oxygen species, peptides, and viscosity have recently been added to the molecular imaging tools. This diverse utility demonstrates the potential of phosphorescent Ir(III) complexes toward bioimaging applications.

Luminescent Metal Complexes for Bioassays in the Near-Infrared (NIR) Region

Near-infrared (NIR, 700-1700 nm) luminescent imaging is an emerging bioimaging technology with low photon scattering, minimal autofluorescence, deep tissue penetration, and high spatiotemporal resolution that has shown fascinating promise for NIR imaging-guided theranostics. In recent progress, NIR luminescent metal complexes have attracted substantially increased research attention owing to their intrinsic merits, including small size, anti-photobleaching, long lifetime, and metal-centered NIR emission. In the past decade, scientists have contributed to the advancement of NIR metal complexes involving efforts to improve photophysical properties, biocompatibility, specificity, pharmacokinetics, in vivo visualization, and attempts to exploit new ligand platforms. Herein, we summarize recent progress and provide future perspectives for NIR metal complexes, including d-block transition metals and f-block lanthanides (Ln) as NIR optical molecular probes for bioassays.

Phosphorescent Ir(III) complexes derived from purine nucleobases

We report the preparation and the study of new types of neutral and cationic phosphorescent heteroleptic Ir(III) complexes derived from 6-phenylpurine nucleosides and nucleotides. Neutral complexes of general formula Ir(C^N)₂(acac) 7, and 8a-c (HC^N = 9-substituted-6-phenyl purine) are orange-red emissive upon photoexcitation, with short lifetimes and good quantum yields (0.42-0.65) in both PMMA films and 2-MeTHF at room temperature. In turn, cationic complexes [Ir(C^N)₂(dtb-bpy)][PF₆] 9, 12a and 12c (dtb-bpy = 4,4'-di-tert-butyl-2,2'-dipyridine) are yellow-green emitters with moderate quantum yields (0.24-0.32).

Millisecond-Range Time-Resolved Bioimaging Enabled through Ultralong Aqueous Phosphorescence Probes

Probes featuring room-temperature phosphorescence (RTP) are promising tools for time-resolved imaging. It is worth noting that the time scale of time-resolved bioimaging generally ranges around the microsecond level, because of the short-lived emission. Herein, the first example of millisecond-range time-resolved bioimaging is illustrated, which is enabled through a kind of ultralong aqueous phosphorescence probes (i.e., cyclo-(Arg-Gly-AspD-Tyr-Cys)-conjugated zinc-doped silica nanospheres), with a RTP emission lasting for ≈5 s and a lifetime as long as 743.7 ms. We demonstrate that live cells and deep tumor tissue in mice can be specifically targeted through immune-phosphorescence imaging, with a high signal-to-background ratio (SBR) value of ≈69 for in vitro imaging, and ≈627 for in vivo imaging, respectively. We further show that, compared to that of fluorescence imaging, the SBR enhancement of millisecond-range time-resolved in vivo bioimaging is up to 105 times.

AIPE-Active Ir(III) complexes with tuneable photophysical properties and application in mitochondria-targeted dual-mode photodynamic therapy

Aggregation-induced phosphorescence emission (AIPE) materials based on transition metal Ir(III) complexes have significant advantages in bioimaging and photodynamic therapy (PDT) due to the long lifetime, the reduced photobleaching and the good reactive oxygen species (ROS) generation. Herein, four cationic Ir(III) complexes (Ir1-Ir4) have been synthesized and studied. Tunable phosphorescence from green to red with the excellent properties of AIPE and long lifetimes can be achieved by varying the substituents. Moreover, these phosphorescence Ir(III) complexes exhibited dual-mode PDT potential (type I and type II). Complex Ir4 showed great prospect in bioimaging and PDT with the large Stokes shift (259 nm), the long lifetime (9.85 μs) and the high ROS yield (0.73). Confocal microscopy demonstrated that Ir4 accumulated in the mitochondria selectively and possessed remarkable photostability (reduced photobleaching up to 600 s). The results indicate that Ir4 may be used in dual-mode PDT guided by mitochondria-targeted imaging. This work provides an in-depth understanding of the relationship between structure and photophysical properties and facilitates the study in PDT applications.

pH-Responsive Iridium(III) Two-Photon Photosensitizers Loaded CaCO3 Nanoplatform for Combined Ca2+ Overload and Photodynamic Therapy

Intracellular calcium levels are closely related to cell survival. The disruption of the calcium buffering capacity or an overload of the calcium levels enhances the susceptibility of cells towards external...

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.

Recent progress in metal-based molecular probes for optical bioimaging and biosensing

Biological imaging and biosensing from subcellular/cellular level to whole body have enabled non-invasive visualisation of molecular events during various biological and pathological processes, giving great contributions to the rapid and impressive advances in chemical biology, drug discovery, disease diagnosis and prognosis. Optical imaging features a series of merits, including convenience, high resolution, good sensitivity, low cost and the absence of ionizing radiation. Among different luminescent probes, metal-based molecules offer unique promise in optical bioimaging and biosensing in vitro and in vivo, arising from their small sizes, strong luminescence, large Stokes shifts, long lifetimes, high photostability and tunable toxicity. In this review, we aim to highlight the design of metal-based molecular probes from the standpoint of synthetic chemistry in the last 2 years for optical imaging, covering d-block transition metal and lanthanide complexes and multimodal imaging agents.

Multi-photon absorption organotin complex for bioimaging and promoting ROS generation

Compared to general fluorescent dyes, multi-photon fluorescent dyes exhibit deeper tissue penetration and lower auto-fluorescence in the bio-imaging field. Therefore, it is necessary to develop an efficient multiphoton imaging agent for deep tissue imaging. In this work, an organotin derivative (HSnBu₃) has been designed and synthesized, which shows multiphoton absorption activity. In constrast to the ignorable three-photon activity of the ligand, the complex (HSnBu₃) exhibits three-photon activity under NIR excitation (1500 nm). Results of chemical and biological tests confirmed that HSnBu₃ was more easily activated by oxygen resulting in a higher level of ¹O₂, which could induce a decrease in mitochondrial membrane potential in HepG2 cells. It suggests that HSnBu₃ has potential in photodynamic therapy.

Nano-photosensitizers for enhanced photodynamic therapy

Photodynamic therapy (PDT) utilizes photosensitizers (PSs) together with irradiation light of specific wavelength interacting with oxygen to generate cytotoxic reactive oxygen species (ROS), which could trigger apoptosis and/or necrosis-induced cell death in target tissues. During the past two decades, multifunctional nano-PSs employing nanotechnology and nanomedicine developed, which present not only photosensitizing properties but additionally accurate drug release abilities, efficient response to optical stimuli and hypoxia resistance. Further, nano-PSs have been developed to enhance PDT efficacy by improving the ROS yield. In addition, nano-PSs with additive or synergistic therapies are significant for both currently preclinical study and future clinical practice, given their capability of considerable higher therapeutic efficacy under safer systemic drug dosage. In this review, nano-PSs that allow precise drug delivery for efficient absorption by target cells are introduced. Nano-PSs boosting sensitivity and conversion efficiency to PDT-activating stimuli are highlighted. Nano-PSs developed to address the challenging hypoxia conditions during PDT of deep-sited tumors are summarized. Specifically, PSs capable of synergistic therapy and the emerging novel types with higher ROS yield that further enhance PDT efficacy are presented. Finally, future demands for ideal nano-PSs, emphasizing clinical translation and application are discussed.

A highly potent ruthenium(II)-sonosensitizer and sonocatalyst for in vivo sonotherapy

As a basic structure of most polypyridinal metal complexes, [Ru(bpy)₃]²⁺, has the advantages of simple structure, facile synthesis and high yield, which has great potential for scientific research and application. However, sonodynamic therapy (SDT) performance of [Ru(bpy)₃]²⁺ has not been investigated so far. SDT can overcome the tissue-penetration and phototoxicity problems compared to photodynamic therapy. Here, we report that [Ru(bpy)₃]²⁺ is a highly potent sonosensitizer and sonocatalyst for sonotherapy in vitro and in vivo. [Ru(bpy)₃]²⁺ can produce singlet oxygen (¹O₂) and sono-oxidize endogenous 1,4-dihydronicotinamide adenine dinucleotide (NADH) under ultrasound (US) stimulation in cancer cells. Furthermore, [Ru(bpy)₃]²⁺ enables effective destruction of mice tumors, and the therapeutic effect can reach deep tissues over 10 cm under US irradiation. This work paves a way for polypyridinal metal complexes to be applied to the noninvasive precise sonotherapy of cancer.

Sonodynamic therapy has therapeutic promise due to its safety and good tissue penetration, but is currently bottlenecked due to a lack of efficient and safe sonosensitizers. Here the authors show that [Ru(bpy)₃]²⁺ can produce singlet oxygen and sonooxidize NADH in deep tissue, and destroy mouse tumors effectively.

Water-Soluble Iridic-Porphyrin Complex for Non-invasive Sonodynamic and Sono-oxidation Therapy of Deep Tumors

Due to conventional photodynamic therapy encountering serious problems of phototoxicity and low tissue-penetrating depth of light, other dynamic therapy-based therapeutic methods such as sonodynamic therapy (SDT) are expected to be developed. To improve the therapeutic response to SDT, more effective sonosensitizers are imperative. In this study, a novel water-soluble iridium(III)-porphyrin sonosensitizer (IrTMPPS) was synthesized and used for SDT. IrTMPPS generated ample singlet oxygen (¹O₂) under US irradiation and especially showed distinguished US-activatable abilities at more than 10 cm deep-tissue depths. Interestingly, under US irradiation, IrTMPPS sonocatalytically oxidized intracellular NADH, which would enhance SDT efficiency by breaking the redox balance in the tumor. Moreover, IrTMPPS displayed great sonocytotoxicity toward various cancer cells, and in vivo experiments demonstrated efficient tumor inhibition and anti-metastasis to the lungs in the presence of IrTMPPS and US irradiation. This report gives a novel idea of metal-based sonosensitizers for sonotherapy by fully taking advantage of non-invasiveness, water solubility, and deep tumor therapy.

Cancer cell membrane camouflaged iridium complexes functionalized black-titanium nanoparticles for hierarchical-targeted synergistic NIR-II photothermal and sonodynamic therapy

The diagnosis and treatment of cancer is one of the biggest medical challenges of the century. Despite significant improvements, there remains an urgent need for novel anticancer procedures. Among the most promising approaches, increasing attention has been devoted towards photothermal and sonodynamic therapy in which sensitizers are activated upon light/ultrasound radiation to generate a cytotoxic effect. While these methods have undoubtedly shown a high therapeutic success, these techniques are intrinsically limited. Herein, the functionalization of black-titanium nanoparticle with iridium complexes and cancer cell membranes in a nanoplatform for hierarchical targeted synergistic photothermal and sonodynamic cancer imaging and therapy is proposed. The particles showed to generate efficiently heat upon irradiation and catalytically form reactive oxygen species upon ultrasound radiation. The nanoparticle formulation demonstrated to selectively localize in the mitochondria as well as to preferentially accumulate in cancerous over non-cancerous cells as well as in the tumor inside a mouse model, presenting a hierarchical targeting strategy. Upon synergistic irradiation at 1064 nm and ultrasound radiation, the nanoparticles were able to act as an imaging agent and identify the tumor site with high spatial resolution as well as act as a therapeutic agent and completely eradicate a tumor inside a mouse model.

Photoresponsive metallopolymer nanoparticles for cancer theranostics

Over the past decades, transition metal complexes have been successfully used in anticancer phototherapies. They have shown promising properties in many different areas including photo-induced ligand exchange or release, rich excited state behavior, and versatile biochemical properties. When encorporated into polymeric frameworks and become part of nanostructures, photoresponsive metallopolymer nanoparticles (MPNs) show enhanced water solubility, extended blood circulation and increased tumor-specific accumulation, which greatly improves the tumor therapeutic effects compared to low-molecule-weight metal complexes. In this review, we aim to present the recent development of photoresponsive MPNs as therapeutic nanomedicines. This review will summarize four major areas separately, namely platinum-containing polymers, zinc-containing polymers, iridium-containing polymers and ruthenium-containing polymers. Representative MPNs of each type are discussed in terms of their design strategies, fabrication methods, and working mechanisms. Current challenges and future perspectives in this field are also highlighted.

Robust Packing of a Self-Assembling Iridium Complex via Endocytic Trafficking for Long-Term Lysosome Tracking

Live cell imaging of lysosome positioning and motility is critical to studying lysosome status and function for pharmacological interventions. To create a super stable lysosomal probe for long-term live cell imaging, we have designed and synthesized an aromatic-peptide-conjugated cyclometalated iridium(III) complex that emits light via π-π stacking oriented self-assembly in water at extremely low concentration. Through endocytic trafficking, self-assemblies are transformed from nanoparticles into sturdily packed networks that are stabilized in lysosomal acidic environment. Upon short time/low dose treatment of the iridium complex at passage 0, live cell lysosomal tracking is applicable beyond the 14th passage of cells with high labelling rate and a mild decline in luminescence intensity. The illuminated lysosomes are trackable using super-resolution imaging to study their response to cellular processes.

Fluorescent Probes for Imaging Protein Disulfides in Live Organisms

Cellular redox homeostasis is predominantly controlled by the ratio of thiols and disulfides, and reversible thiol-disulfide exchange reactions are fundamental of the biological redox regulation. However, due to the dynamic exchanges of thiols and disulfides, the detection, especially the in situ detection, of protein disulfides (PDS) is challenging. We employ the strategy, i.e., the increase of emission upon an environment-sensitive dye binding to proteins, to design PDS probes and discover a two-photon probe PDSTP590 (S6) that selectively recognizes PDS in live organisms. With the aid of the probe, we further disclose the elevation of PDS in brains of the mouse stroke model.

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.

Recent progress in photosensitizers for overcoming the challenges of photodynamic therapy: from molecular design to application

Photodynamic therapy (PDT), a therapeutic mode involving light triggering, has been recognized as an attractive oncotherapy treatment. However, nonnegligible challenges remain for its further clinical use, including finite tumor suppression, poor tumor targeting, and limited therapeutic depth. The photosensitizer (PS), being the most important element of PDT, plays a decisive role in PDT treatment. This review summarizes recent progress made in the development of PSs for overcoming the above challenges. This progress has included PSs developed to display enhanced tolerance of the tumor microenvironment, improved tumor-specific selectivity, and feasibility of use in deep tissue. Based on their molecular photophysical properties and design directions, the PSs are classified by parent structures, which are discussed in detail from the molecular design to application. Finally, a brief summary of current strategies for designing PSs and future perspectives are also presented. We expect the information provided in this review to spur the further design of PSs and the clinical development of PDT-mediated cancer treatments.

Necroptosis-inducing iridium(iii) complexes as regulators of cyclin-dependent kinases

Mitochondria-targeted Ir(iii) complexes induce necroptosis and downregulate certain cell cycle proteins to achieve cell cycle arrest and an anti-proliferation effect in drug-resistant lung cancer.

A mitochondrial-targeting iridium(iii) complex for H2O2-responsive and oxidative stress amplified two-photon photodynamic therapy

The mitochondrial-targeting iridium(iii) complex Ir–B(OH)2 is activated in the presence of endogenous H2O2 to release the two-photon photosensitizer Ir–OH and the GSH scavenger methylquinone for amplified two-photon photodynamic therapy.

Rational design of iridium–porphyrin conjugates for novel synergistic photodynamic and photothermal therapy anticancer agents

Iridium–porphyrin conjugates assembled in nanoparticles are photosensitizers that exhibit excellent photothermal imaging and synergistic PDT and PTT in vivo.