Oxidation-Responsive, EuII/III-Based, Multimodal Contrast Agent for Magnetic Resonance and Photoacoustic Imaging

Europium(II/III) coordination chemistry toward applications

Europium is an f-block metal with two easily accessible oxidation states (+2 and +3) that have vastly different magnetic and optical properties from each other. These properties are tunable using coordination chemistry and are useful in a variety of applications, including magnetic resonance imaging, luminescence, and catalysis. This review describes important aspects of coordination chemistry of Eu from the Allen Research Group and others, how ligand design has tuned the properties of Eu ions, and how those properties are relevant to specific applications. The review begins with an introduction to the coordination chemistry of divalent and trivalent Eu followed by examples of how the coordination chemistry of Eu has made contributions to magnetic resonance imaging, luminescence, catalysis, and separations. The article concludes with a brief outlook on future opportunities in the field.

Outersphere Approach to Increasing the Persistance of Oxygen-Sensitive Europium(II)-Containing Contrast Agents for Magnetic Resonance Imaging with Perfluorocarbon Nanoemulsions toward Imaging of Hypoxia

Radiographic mapping of hypoxia is needed to study a wide range of diseases. Complexes of Eu(II) are a promising class of molecules to fit this need, but they are generally limited by their rapid oxidation rates in vivo. Here, a perfluorocarbon-nanoemulsion perfused with N2 , forms an interface with aqueous layers to hinder oxidation of a new perfluorocarbon-soluble complex of Eu(II). Conversion of the perfluorocarbon solution of Eu(II) into nanoemulsions results in observable differences between reduced and oxidized forms by magnetic resonance imaging both in vitro and in vivo. Oxidation in vivo occurrs over a period of ≈30 min compared to <5 min for a comparable Eu(II)-containing complex without nanoparticle interfaces. These results represent a critical step toward delivery of Eu(II)-containing complexes in vivo for the study of hypoxia.

Dual-Mode Tumor Imaging Using Probes That Are Responsive to Hypoxia-Induced Pathological Conditions

Hypoxia in solid tumors is associated with poor prognosis, increased aggressiveness, and strong resistance to therapeutics, making accurate monitoring of hypoxia important. Several imaging modalities have been used to study hypoxia, but each modality has inherent limitations. The use of a second modality can compensate for the limitations and validate the results of any single imaging modality. In this review, we describe dual-mode imaging systems for the detection of hypoxia that have been reported since the start of the 21st century. First, we provide a brief overview of the hallmarks of hypoxia used for imaging and the imaging modalities used to detect hypoxia, including optical imaging, ultrasound imaging, photoacoustic imaging, single-photon emission tomography, X-ray computed tomography, positron emission tomography, Cerenkov radiation energy transfer imaging, magnetic resonance imaging, electron paramagnetic resonance imaging, magnetic particle imaging, and surface-enhanced Raman spectroscopy, and mass spectrometric imaging. These overviews are followed by examples of hypoxia-relevant imaging using a mixture of probes for complementary single-mode imaging techniques. Then, we describe dual-mode molecular switches that are responsive in multiple imaging modalities to at least one hypoxia-induced pathological change. Finally, we offer future perspectives toward dual-mode imaging of hypoxia and hypoxia-induced pathophysiological changes in tumor microenvironments.

Methylated tetra-amide derivatives of paramagnetic complexes for magnetic resonance biosensing with both BIRDS and CEST

Paramagnetic agents that utilize two mechanisms to provide physiological information by magnetic resonance imaging (MRI) and magnetic resonance spectroscopic imaging (MRSI) are described. MRI with chemical exchange saturation transfer (CEST) takes advantage of the agent's exchangeable protons (e.g., -OH or -NH_x , where 2 ≥ x ≥ 1) to create pH contrast. The agent's incorporation of non-exchangeable protons (e.g., -CH_y , where 3 ≥ y ≥ 1) makes it possible to map tissue temperature and/or pH using an MRSI method called biosensor imaging of redundant deviation in shifts (BIRDS). Hybrid probes based upon 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate chelate (DOTA^4- ) and its methylated analog (1,4,7,10-tetraazacyclododecane-α, α', α″, α‴-tetramethyl-1,4,7,10-tetraacetate, DOTMA^4- ) were synthesized, and modified to create new tetra-amide chelates. Addition of several methyl groups per pendent arm of the symmetrical chelates, positioned proximally and distally to thulium ions (Tm³⁺ ), gave rise to favorable BIRDS properties (i.e., high signal-to-noise ratio (SNR) from non-exchangeable methyl proton peaks) and CEST responsiveness (i.e., from amide exchangeable protons). Structures of the Tm³⁺ probes elucidate the influence of methyl group placement on sensor performance. An eight-coordinate geometry with high symmetry was observed for the complexes: Tm-L1 was based on DOTA^4- , whereas Tm-L2 and Tm-L3 were based on DOTMA^4- , where the latter contained an additional carboxylate at the distal end of each arm. The distance of Tm³⁺ from terminal methyl carbons is a key determinant for sustaining BIRDS temperature sensitivity without compromising CEST pH contrast; however, water solubility was influenced by introduction of hydrophobic methyl groups and hydrophilic carboxylate. Combined BIRDS and CEST detection of Tm-L2, which features two high-SNR methyl peaks and a strong amide CEST peak, should enable simultaneous temperature and pH measurements for high-resolution molecular imaging in vivo.

Human-Body-Temperature Triggerable Phase Transition of W-VO2@PEG Nanoprobes with Strong and Switchable NIR-II Absorption for Deep and Contrast-Enhanced Photoacoustic Imaging

The immense potential of temperature-responsive nanomaterials for use as contrast agents has propelled much recent research and development in the field of photoacoustic (PA) imaging, while the exorbitant transition temperature exceeding the human-tolerable range and the low reversibility of the reported temperature-sensitive nanosystems are still two severe issues that hinder effective imaging and long-term monitoring in practical applications. Herein, we propose a high-performing thermoresponsive polyethylene glycol-coated tungsten-doped vanadium dioxide (W-VO₂@PEG) nanoprobe (NP) with strong and switchable optical absorption in the near-infrared-II (NIR-II) biowindow (1000-1700 nm) near human-body temperature, to achieve deep and contrast-enhanced PA imaging. Our study shows that the PA signal amplitude of W-VO₂@PEG NPs at 1064 nm increases up to 260% when the temperature increases from 35 °C to 45 °C, with a signal fluctuation of less than 10% after 10 temperature cycles, therefore enabling great potential of "off-to-on" dynamic contrast-enhanced imaging capability in deep-seated tissues. Experiments on tissue-mimicking phantoms and in vitro chicken breast showed that, by levering the prepared W-VO₂@PEG NPs and dynamically modulating the temperature field with an external NIR optical stimulus, contrast-enhanced PA images of the target can be obtained with an imaging depth up to 1.5 cm. Furthermore, in vivo potential of the prepared thermoresponsive NPs for the detection and identification of deep-seated tumors by directly comparing to conventional "always on" NPs has been demonstrated. Our work will offer feasible guidance for the development of smart temperature-activatable PA NPs with improved imaging depth and imaging contrast.

Systematic tuning of the emission colors and redox potential of Eu(ii)-containing cryptates by changing the N/O ratio of cryptands

The λmax, excited-state lifetimes, and the anodic peak potential of Eu2+/Eu3+ for Eu(ii)-containing cryptates depend linearly on the number of N atoms.

Systematic Tuning of the Optical Properties of Discrete Complexes of EuII in Solution Using Counterions and Solvents

We describe a systematic study of the influence of halides and solvents on the optical properties of Eu^II-containing complexes in solution starting from well-defined crystalline precursors. Anionic halides, chloride and bromide, blue-shift the spectroscopic properties of Eu^II, whereas neutral ligands, methanol and acetonitrile, cause a red shift. This system provides evidence that Eu^II has a stronger affinity for chloride, and to some extent bromide, relative to acetonitrile but not methanol. We also describe a simple procedure using an ion-exchange resin for the exchange of iodide counterions to hexafluorophosphate. These findings are a step toward designing ligands that can tune the optical properties of Eu^II-containing complexes for solution-based applications.

Recent Advances of Bioresponsive Nano-Sized Contrast Agents for Ultra-High-Field Magnetic Resonance Imaging

The ultra-high-field magnetic resonance imaging (MRI) nowadays has been receiving enormous attention in both biomaterial research and clinical diagnosis. MRI contrast agents are generally comprising of T1-weighted and T2-weighted contrast agent types, where T1-weighted contrast agents show positive contrast enhancement with brighter images by decreasing the proton's longitudinal relaxation times and T2-weighted contrast agents show negative contrast enhancement with darker images by decreasing the proton's transverse relaxation times. To meet the incredible demand of MRI, ultra-high-field T2 MRI is gradually attracting the attention of research and medical needs owing to its high resolution and high accuracy for detection. It is anticipated that high field MRI contrast agents can achieve high performance in MRI imaging, where parameters of chemical composition, molecular structure and size of varied contrast agents show contrasted influence in each specific diagnostic test. This review firstly presents the recent advances of nanoparticle contrast agents for MRI. Moreover, multimodal molecular imaging with MRI for better monitoring is discussed during biological process. To fasten the process of developing better contrast agents, deep learning of artificial intelligent (AI) can be well-integrated into optimizing the crucial parameters of nanoparticle contrast agents and achieving high resolution MRI prior to the clinical applications. Finally, prospects and challenges are summarized.

Spectroscopic and Electrochemical Trends in Divalent Lanthanides through Modulation of Coordination Environment

Due to the importance of both visible-light luminescence and lanthanides in modern society, the influence of the ligand environment on complexes of Yb^II were studied and compared with analogous complexes of Eu^II. Four ligands with systematically varied electronic and steric characteristics were used to probe the coordination environment and electronic and redox properties of the corresponding Yb^II-containing complexes. Strong-field nitrogenous donors gave rise to bathochromic shifts, leading to visible-light absorption by Yb^II. Trends in properties across the series of Yb^II-containing complexes were compared to trends reported for the analogous Eu^II-containing complexes, revealing the translatability of coordination environment effects across the divalent lanthanide series. These studies provide valuable information regarding the behavior of small and medium-sized divalent lanthanides outside of the solid state.

Measurement of the Dissociation of EuII-Containing Cryptates Using Murexide

The dissociation rates of five Eu^II-containing cryptates in water were measured using UV-visible spectroscopy and murexide at pH 6.5, 7, 7.5, 8, and 9. Murexide was used as a coordinating dye for Eu^II. The results for a known cryptate were within experimental error of the value obtained using other methods and enabled the measurement of other cryptates. This validation of the use of murexide to study the dissociation of Eu^II-containing cryptates enables its use with other complexes of Eu^II.

Chemistry of MRI Contrast Agents: Current Challenges and New Frontiers

Tens of millions of contrast-enhanced magnetic resonance imaging (MRI) exams are performed annually around the world. The contrast agents, which improve diagnostic accuracy, are almost exclusively small, hydrophilic gadolinium(III) based chelates. In recent years concerns have arisen surrounding the long-term safety of these compounds, and this has spurred research into alternatives. There has also been a push to develop new molecularly targeted contrast agents or agents that can sense pathological changes in the local environment. This comprehensive review describes the state of the art of clinically approved contrast agents, their mechanism of action, and factors influencing their safety. From there we describe different mechanisms of generating MR image contrast such as relaxation, chemical exchange saturation transfer, and direct detection and the types of molecules that are effective for these purposes. Next we describe efforts to make safer contrast agents either by increasing relaxivity, increasing resistance to metal ion release, or by moving to gadolinium(III)-free alternatives. Finally we survey approaches to make contrast agents more specific for pathology either by direct biochemical targeting or by the design of responsive or activatable contrast agents.

The first example of fluorescence of the solid individual compounds of Eu2+ ion: EuCl2 , EuI2 , EuBr2

Fluorescence (FL) from Eu²⁺ ion solid individual compounds was observed for the first time using the examples EuCl₂ , EuI₂ and EuBr₂ . Fluorescence was detected at excitation by ultraviolet (UV) light from powdered samples of Eu (II) halides at room temperature (RT) in an argon atmosphere. In air, FL of all Eu²⁺ compounds studied was stable, and intensity persisted for weeks. Depending on the nature of the halide anion and due to a nephelauxetic effect, the position of the maxima in the FL spectra underwent a red shift in the series: EuCl₂ (409 nm) < EuBr₂ (428 mm) < EuI₂ (432 nm). The lifetimes of the excited states of the Eu²⁺ * ions for EuI₂ , EuCl₂ and EuBr₂ were 355, 76 and 54 ns, respectively.

Photoacoustic and Magnetic Resonance Imaging Bimodal Contrast Agent Displaying Amplified Photoacoustic Signal

Progress in photoacoustic (PA) and magnetic resonance imaging (MRI) bimodal contrast agents has been achieved mainly by utilizing the imaging capability of single or multiple components and consequently realizing the desired application for both imaging modalities. However, the mechanism of the mutual influence between components within a single nanoformulation, which is the key to developing high-performance multimodal contrast agents, has yet to be fully understood. Herein, by integrating conjugated polymers (CPs) with iron oxide (IO) nanoparticles using an amphiphilic polymer, a bimodal contrast agent named CP-IO is developed, displaying 45% amplified PA signal intensity as compared to bare CP nanoparticle, while the performance of MRI is not affected. Further experimental and theoretical simulation results reveal that the addition of IO nanoparticles in CP-IO nanocomposites contributes to this PA signal amplification through a synergistic effect of additional heat generation and faster heat dissipation. Besides, the feasibility of CP-IO nanocomposites acting as PA-MRI bimodal contrast agents is validated through in vivo tumor imaging using mice models. From this study, it is demonstrated that a delicately designed structural arrangement of various components in a contrast agent could potentially lead to a superior performance in the imaging capability.

Synthesis, Characterization, and Handling of EuII-Containing Complexes for Molecular Imaging Applications

Considerable research effort has focused on the in vivo use of responsive imaging probes that change imaging properties upon reacting with oxygen because hypoxia is relevant to diagnosing, treating, and monitoring diseases. One promising class of compounds for oxygen-responsive imaging is Eu^II-containing complexes because the Eu^II/III redox couple enables imaging with multiple modalities including magnetic resonance and photoacoustic imaging. The use of Eu^II requires care in handling to avoid unintended oxidation during synthesis and characterization. This review describes recent advances in the field of imaging agents based on discrete Eu^II-containing complexes with specific focus on the synthesis, characterization, and handling of aqueous Eu^II-containing complexes.

Fluorinated EuII-based multimodal contrast agent for temperature- and redox-responsive magnetic resonance imaging

Magnetic resonance imaging (MRI) using redox-active, EuII-containing complexes is one of the most promising techniques for noninvasively imaging hypoxia in vivo. In this technique, positive (T1-weighted) contrast enhancement persists in areas of relatively low oxidizing ability, such as hypoxic tissue. Herein, we describe a fluorinated, EuII-containing complex in which the redox-active metal is caged by intramolecular interactions. The position of the fluorine atoms enables temperature-responsive contrast enhancement in the reduced form of the contrast agent and detection of the oxidized contrast agent via MRI in vivo. Positive contrast is observed in 1H-MRI with Eu in the +2 oxidation state, and chemical exchange saturation transfer and 19F-MRI signal are observed with Eu in the +3 oxidation state. Contrast enhancement is controlled by the redox state of Eu, and modulated by the fluorous interactions that cage a bound water molecule reduce relaxivity in a temperature-dependent fashion. Together, these advancements constitute the first report of in vivo, redox-responsive imaging using 19F-MRI.

Structural Features of Europium(II)-Containing Cryptates That Influence Relaxivity

Eu^II -containing complexes were studied with respect to properties relevant to their use as contrast agents for magnetic resonance imaging. The influences of molecular parameters and field strength on relaxivity were studied for a series of Eu^II -containing cryptates and their adducts with β-cyclodextrins, poly-β-cyclodextrins, and human serum albumin. Solid- and solution-phase characterization of Eu^II -containing complexes is presented that demonstrates the presence of inner-sphere molecules of water. Additionally, relaxivity, water-exchange rate, rotational correlation time, and electronic relaxation times were determined using variable-temperature ¹⁷ O NMR, nuclear magnetic relaxation dispersion, and electron paramagnetic resonance spectroscopic techniques. These results are expected to be instrumental in the design of future Eu^II -based contrast agents.