o-Carborane-based fluorophores as efficient luminescent systems both as solids and as water-dispersible nanoparticles

Molecular engineering of AIE-active boron clustoluminogens for enhanced boron neutron capture therapy

The development of boron delivery agents bearing an imaging capability is crucial for boron neutron capture therapy (BNCT), yet it has been rarely explored. Here we present a new type of boron delivery agent that integrates aggregation-induced emission (AIE)-active imaging and a carborane cluster for the first time. In doing so, the new boron delivery agents have been rationally designed by incorporating a high boron content unit of a carborane cluster, an erlotinib targeting unit towards lung cancer cells, and a donor-acceptor type AIE unit bearing naphthalimide. The new boron delivery agents demonstrate both excellent AIE properties for imaging purposes and highly selective accumulation in tumors. For example, at a boron delivery agent dose of 15 mg kg-1, the boron amount reaches over 20 μg g-1, and both tumor/blood (T/B) and tumor/normal cell (T/N) ratios reach 20-30 times higher than those required by BNCT. The neutron irradiation experiments demonstrate highly efficient tumor growth suppression without any observable physical tissue damage and abnormal behavior in vivo. This study not only expands the application scopes of both AIE-active molecules and boron clusters, but also provides a new molecular engineering strategy for a deep-penetrating cancer therapeutic protocol based on BNCT.

Precious metal clusters as fundamental agents in bioimaging usability

Fluorescent nanomaterials (NMs) are widely used in imaging techniques in biomedical research. Especially in bioimaging systems, with the rapid development of imaging nanotechnology, precious metal clusters such as Au, Ag, and Cu NMs have emerged with different functional agents for biomedical applications. Compared with traditional fluorescent molecules, precious metal clusters have the advantages of high optical stability, easy regulation of shape and size, and multifunctionalization. In addition, NMs possess strong photoluminescent properties with good photostability, high release rate, and sub-nanometer size. They could be treated as fundamental agents in bioimaging usability. This review summarizes the recent advances in bioimaging utilization, it conveys that metal clusters refer to Au, Ag, and Cu fluorescent clusters and could provide a generalized overview of their full applications. It includes optical property measurement, precious metal clusters in bioimaging systems, and a rare earth element-doped heterogeneous structure illustrated in biomedical imaging with specific examples, that provide new and innovative ideas for fluorescent NMs in the field of bioimaging usability.

Ratiometric Nanothermometer Based on a Radical Excimer for In Vivo Sensing

Ratiometric fluorescent nanothermometers with near-infrared emission play an important role in in vivo sensing since they can be used as intracellular thermal sensing probes with high spatial resolution and high sensitivity, to investigate cellular functions of interest in diagnosis and therapy, where current approaches are not effective. Herein, the temperature-dependent fluorescence of organic nanoparticles is designed, synthesized, and studied based on the dual emission, generated by monomer and excimer species, of the tris(2,4,6-trichlorophenyl)methyl radical (TTM) doping organic nanoparticles (TTMd-ONPs), made of optically neutral tris(2,4,6-trichlorophenyl)methane (TTM-αH), acting as a matrix. The excimer emission intensity of TTMd-ONPs decreases with increasing temperatures whereas the monomer emission is almost independent and can be used as an internal reference. TTMd-ONPs show a great temperature sensitivity (3.4% K-1 at 328 K) and a wide temperature response at ambient conditions with excellent reversibility and high colloidal stability. In addition, TTMd-ONPs are not cytotoxic and their ratiometric outputs are unaffected by changes in the environment. Individual TTMd-ONPs are able to sense temperature changes at the nano-microscale. In vivo thermometry experiments in Caenorhabditis elegans (C. elegans) worms show that TTMd-ONPs can locally monitor internal body temperature changes with spatio-temporal resolution and high sensitivity, offering multiple applications in the biological nanothermometry field.

Towards the Application of Purely Inorganic Icosahedral Boron Clusters in Emerging Nanomedicine

Traditionally, drugs were obtained by extraction from medicinal plants, but more recently also by organic synthesis. Today, medicinal chemistry continues to focus on organic compounds and the majority of commercially available drugs are organic molecules, which can incorporate nitrogen, oxygen, and halogens, as well as carbon and hydrogen. Aromatic organic compounds that play important roles in biochemistry find numerous applications ranging from drug delivery to nanotechnology or biomarkers. We achieved a major accomplishment by demonstrating experimentally/theoretically that boranes, carboranes, as well as metallabis(dicarbollides), exhibit global 3D aromaticity. Based on the stability-aromaticity relationship, as well as on the progress made in the synthesis of derivatized clusters, we have opened up new applications of boron icosahedral clusters as key components in the field of novel healthcare materials. In this brief review, we present the results obtained at the Laboratory of Inorganic Materials and Catalysis (LMI) of the Institut de Ciència de Materials de Barcelona (ICMAB-CSIC) with icosahedral boron clusters. These 3D geometric shape clusters, the semi-metallic nature of boron and the presence of exo-cluster hydrogen atoms that can interact with biomolecules through non-covalent hydrogen and dihydrogen bonds, play a key role in endowing these compounds with unique properties in largely unexplored (bio)materials.

Direct B-H Functionalization of Icosahedral Carboranes via Hydrogen Atom Transfer

The efficient and selective functionalization of icosahedral carboranes (C₂B₁₀H₁₂) at the boron vertexes is a long-standing challenge owing to the presence of 10 inert B-H bonds in a similar chemical environment. Herein, we report a new reaction paradigm for direct B-H functionalization of icosahedral carboranes via B-H homolysis enabled by a nitrogen-centered radical-mediated hydrogen atom transfer (HAT) strategy. Both the HAT process of the carborane B-H bond and the resulting boron-centered carboranyl radical intermediate have been confirmed experimentally. The reaction occurs at the most electron-rich boron vertex with the lowest B-H bond dissociation energy (BDE). Using this strategy, diverse carborane derivatization, including thiolation, selenation, alkynylation, alkenylation, cyanation, and halogenation, have been achieved in satisfactory yields under a photoinitiated condition in a metal-free and redox-neutral fashion. Moreover, the synthetic utility of the current protocol was also demonstrated by both the scale-up reaction and the construction of carborane-based functional molecules. Therefore, this methodology opens a radical pathway to carborane functionalization, which is distinct from the B-H heterolytic mechanism in the traditional strategies.

Fluorescent Multifunctional Organic Nanoparticles for Drug Delivery and Bioimaging: A Tutorial Review

Fluorescent organic nanoparticles (FONs) are a large family of nanostructures constituted by organic components that emit light in different spectral regions upon excitation, due to the presence of organic fluorophores. FONs are of great interest for numerous biological and medical applications, due to their high tunability in terms of composition, morphology, surface functionalization, and optical properties. Multifunctional FONs combine several functionalities in a single nanostructure (emission of light, carriers for drug-delivery, functionalization with targeting ligands, etc.), opening the possibility of using the same nanoparticle for diagnosis and therapy. The preparation, characterization, and application of these multifunctional FONs require a multidisciplinary approach. In this review, we present FONs following a tutorial approach, with the aim of providing a general overview of the different aspects of the design, preparation, and characterization of FONs. The review encompasses the most common FONs developed to date, the description of the most important features of fluorophores that determine the optical properties of FONs, an overview of the preparation methods and of the optical characterization techniques, and the description of the theoretical approaches that are currently adopted for modeling FONs. The last part of the review is devoted to a non-exhaustive selection of some recent biomedical applications of FONs.

Metal-Catalyzed and Metal-Free Nucleophilic Substitution of 7-I-B18H21

In this work, two pathways of reactivity are investigated to generate site-specific substitutions at the B7 vertex of the luminescent boron cluster, anti-B₁₈H₂₂. First, a palladium-catalyzed cross-coupling reaction utilizing the precursor 7-I-B₁₈H₂₁ and a series of model nucleophiles was developed, ultimately producing several B-N- and B-O-substituted species. Interestingly, the B-I bond in this cluster can also be substituted in an uncatalyzed fashion, leading to the formation of various B-N, B-O, and B-S products. This work highlights intricate differences corresponding to these two reaction pathways and analyzes the role of solvents and additives on product distributions. As a result of our synthetic studies, seven new B₁₈-based clusters were synthesized, isolated, and characterized by mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy. The photoluminescence properties of two structurally similar ether and thioether products were further investigated, with both exhibiting blue fluorescence in solution at 298 K and long-lived green or yellow phosphorescence at 77 K. Overall, this work shows, for the first time, the ability to perform substitution of a boron-halogen bond with nucleophiles in a B₁₈-based cluster, resulting in the formation of photoluminescent molecules.

Site-Selective Functionalization of Carboranes at the Electron-Rich Boron Vertex: Photocatalytic B-C Coupling via a Carboranyl Cage Radical

Functionalization of carboranes in a vertex-specific manner is a perennial challenge. Here, we report a photocatalytic B-C coupling for the selective functionalization of carboranes at the boron site which is most distal to carbon. This reaction was achieved by the photo-induced decarboxylation of carborane carboxylic acids to generate boron vertex-centered carboranyl radicals. Theoretical calculations also demonstrate that the reaction more easily occurs at the boron site bearing higher electron density owing to the lower energy barrier for a single-electron transfer to generate a carboranyl radical. By using this strategy, a number of functionalized carboranes could be accessed through alkylation, alkenylation, and heteroarylation under mild conditions. Moreover, both a highly efficient blue emitter with a solid-state luminous efficiency of 42 % and a drug candidate for boron neutron capture therapy (BNCT) containing targeting and fluorine units were obtained.

Multimodal Applications of Zinc Gallate-Based Persistent Luminescent Nanoparticles in Cancer Treatment: Tumor Margining, Diagnosis, and Boron Neutron Capture Therapy

On the basis of the boron neutron capture therapy (BNCT) modality, we have designed and synthesized a zinc gallate (ZnGa₂O₄)-based nanoformulation for developing an innovative theranostic approach for cancer treatment. Initially, the (ZnGa_1.995Cr_0.005O₄ or ZnGa₂O₄:(0.5%)Cr persistent luminescence nanoparticles (PLNPs) embedded on silica matrix were synthesized. Their surface functionalization was performed using organic synthesis strategies to attach the amine functional moieties which were further coupled with poly(vicinal diol). These diols were helpful for conjugation with ¹⁰B(OH)₃, which subsequently served to couple with an in-house-synthesized variant of pH-(low)-insertion peptide (pHLIP) finally giving a tumor-targeting nanoformulation. Most importantly, the polymeric diols helped in conjugation of a substantial number of ¹⁰B to provide the therapeutic dose required for effective BNCT. This nanoformulation internalized substantially (∼80%) to WEHI-164 cancer cells within 6 h. Tumor homing studies indicated that the accumulation of this formulation at the acidic tumor site was within 2 h. The in vitro evaluation of the formulation against WEHI-164 cancer cells followed by neutron irradiation revealed its potent cytotoxicity with IC₅₀ ∼ 25 μM. In the case of studies on animal models, the melanoma-induced C57BL/6 and fibrosarcoma-induced BALB/c mice were treated with formulations through intratumoral and intravenous injections, respectively, followed by neutron irradiation, leading to a significant killing of the cancer cells, which was evidenced by a reduction in tumor volume (75-80%) as compared with a control tumor. Furthermore, the histopathological studies confirmed a damaging effect only on tumor cells, while there was no sign of damage to the vital organs in treated mice as well as in controls.

Color-tuning and manipulation of aggregation-induced emission efficiency of o-carborane–tetraphenylethylene dyads through substituted o-carboranes

Two new carborane-containing tetraphenylethylene derivatives (1 and 2) were designed and synthesized. Together with our previously published compounds (3–5), we studied structure–activity relationships in detail. Results showed that compounds 1...