High-stability spherical lanthanide nanoclusters for magnetic resonance imaging

High-nuclear lanthanide clusters have shown great potential for the administration of high-dose mononuclear gadolinium chelates in magnetic resonance imaging (MRI). The development of high-nuclear lanthanide clusters with excellent solubility and high stability in water or solution has been challenging and is very important for expanding the performance of MRI. We used N-methylbenzimidazole-2-methanol (HL) and LnCl3·6H2O to synthesize two spherical lanthanide clusters, Ln32 (Ln = Ho, Ho32; and Ln = Gd, Gd32), which are highly stable in solution. The 24 ligands L- are all distributed on the periphery of Ln32 and tightly wrap the cluster core, ensuring that the cluster is stable. Notably, Ho32 can remain highly stable when bombarded with different ion source energies in HRESI-MS or immersed in an aqueous solution of different pH values for 24 h. The possible formation mechanism of Ho32 was proposed to be Ho(III), (L)- and H2O → Ho3(L)3/Ho3(L)4 → Ho4(L)4/Ho4(L)5 → Ho6(L)6/Ho6(L)7 → Ho16(L)19 → Ho28(L)15 → Ho32(L)24/Ho32(L)21/Ho32(L)23. To the best of our knowledge, this is the first study of the assembly mechanism of spherical high-nuclear lanthanide clusters. Spherical cluster Gd32, a form of highly aggregated Gd(III), exhibits a high longitudinal relaxation rate (1 T, r1 = 265.87 mM-1·s-1). More notably, compared with the clinically used commercial material Gd-DTPA, Gd32 has a clearer and higher-contrast T1-weighted MRI effect in mice bearing 4T1 tumors. This is the first time that high-nuclear lanthanide clusters with high water stability have been utilized for MRI. High-nuclear Gd clusters containing highly aggregated Gd(III) at the molecular level have higher imaging contrast than traditional Gd chelates; thus, using large doses of traditional gadolinium contrast agents can be avoided.

The Potential of Gadolinium Ascorbate Nanoparticles as a Safer Contrast Agent

There have been health concerns raised against the use of gadolinium (Gd)-based magnetic resonance imaging contrast agents. The primary observation is that Gd ions are prone to leaking into the bloodstream, causing nephrogenic systemic fibrosis as one of the side effects. In addition, such leakage of the ions inhibits easy clearance from the body. Herein we propose that Gd-ascorbate nanoparticles could be one of the safer choices as they are rather stable in aqueous dispersion and they do not get affected by Zn or Fe ions in the medium. The magnetic properties of the ions are preserved in the nanoparticles, and particles when sufficiently small may be amenable to renal clearance from the human body. Thus, when an aqueous solution of Gd-acetate and ascorbic acid was left to evolve with time, a Gd-ascorbate complex was formed that led to the formation of nanoparticles with time. The sizes of the nanoparticles increased with time, and when the particles were sufficiently large, they precipitated out of the medium. In addition, smaller nanoparticles were consistently present at all times of observations. UV-vis, photoluminescence and FTIR spectroscopy, mass spectrometry, and transmission electron microscopy analyses confirmed the formation of nanoparticles of Gd-ascorbate complex. In addition, magnetic measurements confirmed the high relaxivity of the nanoparticles as compared to the parent salt, indicating the effectiveness of the nanoparticles as contrast agents. Density functional theory-based calculations of the molecular complex-based nanoparticles accounted for the experimental observations.

Hyper-CEST NMR of metal organic polyhedral cages reveals hidden diastereomers with diverse guest exchange kinetics

Guest capture and release are important properties of self-assembling nanostructures. Over time, a significant fraction of guests might engage in short-lived states with different symmetry and stereoselectivity and transit frequently between multiple environments, thereby escaping common spectroscopy techniques. Here, we investigate the cavity of an iron-based metal organic polyhedron (Fe-MOP) using spin-hyperpolarized ¹²⁹Xe Chemical Exchange Saturation Transfer (hyper-CEST) NMR. We report strong signals unknown from previous studies that persist under different perturbations. On-the-fly delivery of hyperpolarized gas yields CEST signatures that reflect different Xe exchange kinetics from multiple environments. Dilute pools with ~ 10⁴-fold lower spin numbers than reported for directly detected hyperpolarized nuclei are readily detected due to efficient guest turnover. The system is further probed by instantaneous and medium timescale perturbations. Computational modeling indicates that these signals originate likely from Xe bound to three Fe-MOP diastereomers (T, C₃, S₄). The symmetry thus induces steric effects with aperture size changes that tunes selective spin manipulation as it is employed in CEST MRI agents and, potentially, impacts other processes occurring on the millisecond time scale.

Coordination-Directed Self-Assembly of Functional Polynuclear Lanthanide Supramolecular Architectures

Lanthanide supramolecular chemistry is a fast growing and intriguing research field due to the unique photophysical, magnetic, and coordination properties of lanthanide ions (Ln^III). Compared with the intensively investigated mononuclear Ln-complexes, polymetallic lanthanide supramolecular assemblies offer more structural superiority and functional advantages. In recent decades, significant progress has been made in polynuclear lanthanide supramolecules, varying from structural evolution to luminescent and magnetic functional materials. This review summarizes the design principles in ligand-induced coordination-driven self-assembly of polynuclear Ln-structures and intends to offer guidance for the construction of more elegant Ln-based architectures and optimization of their functional performances. Design principles concerning the water solubility and chirality of the lanthanide-organic assemblies that are vital in extending their applications are emphasized. The strategies for improving the luminescent properties and the applications in up-conversion, host-guest chemistry, luminescent sensing, and catalysis have been summarized. Magnetic materials based on supramolecular assembled lanthanide architectures are given in an individual section and are classified based on their structural features. Challenges remaining and perspective directions in this field are also briefly discussed.

Mesoporous Silica Nanoparticles-Embedded Lanthanide Organic Polyhedra for Enhanced Stability, Luminescence and Cell Imaging

We report here a simple but efficient “ship-in-a-bottle” synthetic strategy for increasing the stability and luminescence performance of LOPs by embedding them into mesoporous silica nanoparticles (MSNs). Three types of...

Base- and Metal-Dependent Self-Assembly of Lathanide-Organic Coordination Polymers or Macrocycles with Tetradentate Acylhydrazone-based Ditopic Ligands

Herein, we report a comprehensive study on the lanthanide-directed coordination self-assembly with two bis-tetradentate acylhydrazone ligands H₄ L¹ and H₄ L² . Multifarious outcomes, which are base- and metal-dependent, were revealed by NMR, ESI-TOF-MS and X-ray crystallography. In the absence of base, bent H₄ L¹ was assembled into dinuclear double-strand helicate Ln₂ (H₂ L¹ )₂ by partially-deprotonated assembly with La, Sm or Eu, while trinuclear Ln₃ (H₂ L¹ )₃ with Yb or Lu. For linear H₄ L² , infinite 1D zig-zag metal-organic polymeric chain (Ln₂ H₂ L² )_n was obtained. However, complete deprotonated L¹ and L² assembled into discrete trinuclear Ln₃ (L^{1 /2} )₃ and tetranuclear Ln₄ (L^{1 /2} )₄ macrocyclic structures under the basic condition. For these, there are multiple possible isomers coexisting in the solution which were enumerated and simulated with molecular mechanic modeling. Visible-light sensitized NIR emissions on the Yb complexes have been observed, endowing them potential application in photofunctional materials.

Coordination-Assembled Water-Soluble Anionic Lanthanide Organic Polyhedra for Luminescent Labeling and Magnetic Resonance Imaging

Lanthanide-containing functional complexes have found a variety of applications in materials science and biomedicine because of their unique electroptical and magnetic properties. However, the poor stability and solubility in water of multicomponent lanthanide organic assemblies significantly limit their practical applications. We report here a series of water-stable anionic Ln_2nL_3n-type (n = 2, 3, 4, and 5) lanthanide organic polyhedra (LOPs) constructed by deprotonation self-assembly of three fully conjugated ligands (H₄L¹ and H₄L^2a/b) featuring a 2,6-pyridine bitetrazolate chelating moiety. The outcomes of the LOPs formation reactions were found to be very sensitive toward the reaction conditions including base, metal source, solvents, and concentrations as characterized by a combination of NMR, high-resolution ESI-MS and X-ray crystallography. Ligands H₄L^2a/b manifested an excellent sensitization toward lanthanide ions (Ln = Eu^III and Tb^III), with high luminescent quantum yields for Tb₈L^2a₁₂ (Φ = 11.2% in water) and Eu₈L^2b₁₂ (Φ = 76.8% in DMSO) measured in polar solvents. Furthermore, due to the giant molecular weight and rigidity of the polyhedral skeleton, Gd₈L^2b₁₂ showed a very high longitudinal relaxivity (r₁) of 400.53 mM^-1S^-1. The performance of Gd₈L^2b₁₂ as potential magnetic resonance imaging contrast agents (CAs) in vivo was evaluated with much longer retention time in the tumor sites compared with the commercial Gd^III-based CAs. Dual-modal imaging potential has also been demonstrated with the mixed Eu/Gd LOPs. Our results not only provide a new design route toward water-stable multinuclear lanthanide organic assemblies but also offer potential candidates of supramolecular-edifices for bioimaging and drug delivery.

One-/Two-Photon Excited Cell Membrane Imaging and Tracking by a Photoactive Nanocage

Cell membrane imaging by predesigned molecular and supramolecular photoluminescence probes is of great importance in understanding the nano-biointeractions for potential applications in cellular tracking, drug delivery, cancer diagnosis, and treatment. Herein, we report an effective strategy for cell membrane imaging in both living cell and tissue levels on the basis of a multifunctional nanocage (MOC-16) integrating one-/two-photon active phosphorescence, high charges, balanced hydrophobicity/lipophilicity, and proton sensitivity attributes. The intrinsic optical characters, including strong one-/two-photon excitation and pH-dependent red emission, make MOC-16 powerful optical probes for membrane imaging in living cell and tissue levels under both visible and near-infrared irradiations. Meanwhile, the highly positive charges of +28 endow MOC-16 with adequate water solubility and deprotonation ability while still maintaining its hydrophobicity, thus enabling balanced hydrophobic-lipophilic interactions at the nano-biointerface to facilitate a pH-dependent membrane absorption within the biological pH range of 5.3-7.4. However, the low-charged RuL₃ metalloligand or polyethylene glycol (PEG)-modified MOC-16^PEG with less hydrophobicity cannot offer enough nano-biointeractions for cell membrane tracking. These findings advance the fundamental understanding of nano-biointerface interactions of MOCs with cell membranes and provide further guidance in their biological applications.

Highly Sensitive Detection of UV Radiation Using a Uranium Coordination Polymer

The accurate detection of UV radiation is required in a wide range of chemical industries and environmental or biological related applications. Conventional methods taking advantage of semiconductor photodetectors suffer from several drawbacks such as sophisticated synthesis and manufacturing procedure, not being able to measure the accumulated UV dosage as well as high defect density in the material. Searching for new strategies or materials serving as precise UV dosage sensor with extremely low detection limit is still highly desirable. In this work, a radiation resistant uranium coordination polymer [UO₂(L)(DMF)] (L = 5-nitroisophthalic acid, DMF = N,N-dimethylformamide, denoted as compound 1) was successfully synthesized through mild solvothermal method and investigated as a unique UV probe with the detection limit of 2.4 × 10^-7 J. On the basis of the UV dosage dependent luminescence spectra, EPR analysis, single crystal structure investigation, and the DFT calculation, the UV-induced radical quenching mechanism was confirmed. Importantly, the generated radicals are of significant stability which offers the opportunity for measuring the accumulated UV radiation dosage. Furthermore, the powder material of compound 1 was further upgraded into membrane material without loss in luminescence intensity to investigate the real application potentials. To the best of our knowledge, compound 1 represents the most sensitive coordination polymer based UV dosage probe reported to date.

A novel coelenterate luciferin-based luminescent probe for selective and sensitive detection of thiophenols

The first dual bioluminescent and chemiluminescent sensor for detecting highly toxic thiophenols has been developed. Such a probe was designed by using a coelenterazine analogue as the luminophore and dinitrophenyl ether as the recognition moiety. It should be noted that this probe displayed good sensitivity and selectivity toward thiophenols, and has been effectively applied for the quantitative detection of thiophenols in aqueous media and complex biological samples.

The Promise of Self-Assembled 3D Supramolecular Coordination Complexes for Biomedical Applications

In the supramolecular chemistry field, coordination-driven self-assembly has provided the basis for tremendous growth across many subdisciplines, spanning from fundamental investigations regarding the design and synthesis of new architectures to defining different practical applications. Within this framework, supramolecular coordination complexes (SCCs), defined as large chemical entities formed from smaller precursor building blocks of ionic metal nodes and organic multidentate ligands, resulting in intricate and well-defined supramolecular structures, hold great promise. Notably, interest in the construction of discrete 3D molecular architectures, such as those offered by SCCs, has experienced extraordinary progress because of their potential application as sensors, catalysts, probes, and containers and in basic host-guest chemistry. Despite numerous synthetic efforts and a number of inherent favorable properties, the field of 3D SCCs for biomedical applications is still in its infancy. This Viewpoint focuses on 3D SCCs, specifically metallacages and helicates, first briefly presenting the fundamentals in terms of the synthesis and characterization of their host-guest properties, followed by an overview of the possible biological applications with representative examples. Thus, emphasis will be given in particular to metallacages as drug delivery systems and to chiral helicates as DNA recognition domains. Overall, we will provide an update on the state-of-the-art literature and will define the challenges in this fascinating research area at the interface of different disciplines.

A gadolinium(iii) complex based dual-modal probe for MRI and fluorescence sensing of fluoride ions in aqueous medium and in vivo

A novel Gd(iii) complex based dual-modal probe, Gd(TTA)₃-DPPZ was designed and assembled for the simultaneous fluoride ion in aqueous media and in vivo.

Gadolinium nicotinate clusters as potential MRI contrast agents

Three multinuclear gadolinium(iii) clusters were synthesized and characterized, and their potential as MRI contrast agents was evaluated at 7 T.

Metal–organic molecular cages: applications of biochemical implications

New well-designed materials are highly demanded with the prospect of versatile properties, offering successful applications as alternates to conventional materials.

Environmentally responsive MRI contrast agents

Biomedical imaging techniques can provide a vast amount of anatomical information, enabling diagnosis and the monitoring of disease and treatment profile. MRI uniquely offers convenient, non-invasive, high resolution tomographic imaging. A considerable amount of effort has been invested, across several decades, in the design of non toxic paramagnetic contrast agents capable of enhancing positive MRI signal contrast. Recently, focus has shifted towards the development of agents capable of specifically reporting on their local biochemical environment, where a switch in image contrast is triggered by a specific stimulus/biochemical variable. Such an ability would not only strengthen diagnosis but also provide unique disease-specific biochemical insight. This feature article focuses on recent progress in the development of MRI contrast switching with molecular, macromolecular and nanoparticle-based agents.