Fluorinated Polymer Nanoparticles as a Novel 19F MRI Contrast Agent Prepared by Dendrimer-Initiated Living Radical Polymerization

Fluoropolymer Nanoparticles Synthesized via Reversible-Deactivation Radical Polymerizations and Their Applications

Fluorinated polymeric nanoparticles (FPNPs) combine unique properties of fluorocarbon and polymeric nanoparticles, which has stimulated massive interest for decades. However, fluoropolymers are not readily available from nature, resulting in synthetic developments to obtain FPNPs via free radical polymerizations. Recently, while increasing cutting-edge directions demand tailored FPNPs, such materials have been difficult to access via conventional approaches. Reversible-deactivation radical polymerizations (RDRPs) are powerful methods to afford well-defined polymers. Researchers have applied RDRPs to the fabrication of FPNPs, enabling the construction of particles with improved complexity in terms of structure, composition, morphology, and functionality. Related examples can be classified into three categories. First, well-defined fluoropolymers synthesized via RDRPs have been utilized as precursors to form FPNPs through self-folding and solution self-assembly. Second, thermally and photoinitiated RDRPs have been explored to realize in situ preparations of FPNPs with varied morphologies via polymerization-induced self-assembly and cross-linking copolymerization. Third, grafting from inorganic nanoparticles has been investigated based on RDRPs. Importantly, those advancements have promoted studies toward promising applications, including magnetic resonance imaging, biomedical delivery, energy storage, adsorption of perfluorinated alkyl substances, photosensitizers, and so on. This Review should present useful knowledge to researchers in polymer science and nanomaterials and inspire innovative ideas for the synthesis and applications of FPNPs.

Exploiting the Fluxionality of Lanthanide Complexes in the Design of Paramagnetic Fluorine Probes

Fluorine-19 MRI is increasingly being considered as a tool for biomolecular imaging, but the very poor sensitivity of this technique has limited most applications. Previous studies have long established that increasing the sensitivity of ¹⁹F molecular probes requires increasing the number of fluorine nuclei per probe as well as decreasing their longitudinal relaxation time. The latter is easily achieved by positioning the fluorine atoms in close proximity to a paramagnetic metal ion such as a lanthanide(III). Increasing the number of fluorine atoms per molecule, however, is only useful inasmuch as all of the fluorine nuclei are chemically equivalent. Previous attempts to achieve this equivalency have focused on designing highly symmetric and rigid fluorinated macrocyclic ligands. A much simpler approach consists of exploiting highly fluxional lanthanide complexes with open coordination sites that have a high affinity for phosphated and phosphonated species. Computational studies indicate that Ln^III-TREN-MAM is highly fluxional, rapidly interconverting between at least six distinct isomers. In neutral water at room temperature, Ln^III-TREN-MAM binds two or three equivalents of fluorinated phosphonates. The close proximity of the ¹⁹F nuclei to the Ln^III center in the ternary complex decreases the relaxation times of the fluorine nuclei up to 40-fold. Advantageously, the fluorophosphonate-bound lanthanide complex is also highly fluxional such that all ¹⁹F nuclei are chemically equivalent and display a single ¹⁹F signal with a small LIS. Dynamic averaging of fluxional fluorinated supramolecular assemblies thus produces effective ¹⁹F MR systems.

19F-nanoparticles: Platform for in vivo delivery of fluorinated biomaterials for 19F-MRI

Fluorine-19 (¹⁹F) magnetic resonance imaging (MRI) features one of the most investigated and innovative techniques for quantitative and unambiguous cell tracking, providing information for both localization and number of cells. Because of the relative insensitivity of the MRI technique, a high number of magnetically equivalent fluorine atoms are required to gain detectable signals. However, an increased amount of ¹⁹F nuclei induces low solubility in aqueous solutions, making fluorine-based probes not suitable for in vivo imaging applications. In this context, nanoparticle-based platforms play a crucial role, since nanoparticles may carry a high payload of ¹⁹F-based contrast agents into the relevant cells or tissues, increase the imaging agents biocompatibility, and provide a highly versatile platform. In this review, we present an overview of the ¹⁹F-based nanoprobes for sensitive ¹⁹F-MRI, focusing on the main nanotechnologies employed to date, such as fluorine and theranostic nanovectors, including their design and applications.

Fluorinated MRI contrast agents and their versatile applications in the biomedical field

MRI has been recognized as one of the most applied medical imaging techniques in clinical practice. However, the presence of background signal coming from water protons in surrounding tissues makes sometimes the visualization of local contrast agents difficult. To remedy this, fluorine has been introduced as a reliable perspective, thanks to its magnetic properties being relatively close to those of protons. In this review, we aim to give an overall description of fluorine incorporation in contrast agents for MRI. The different kinds of fluorinated probes such as perfluorocarbons, fluorinated dendrimers, polymers and paramagnetic probes will be described, as will their imaging applications such as chemical exchange saturation transfer (CEST) imaging, physico-chemical changes detection, drug delivery, cell tracking and inflammation or tumors detection.

19F MRI of Polymer Nanogels Aided by Improved Segmental Mobility of Embedded Fluorine Moieties

Using fluorinated probes for 19F MRI imaging is an emerging field with potential utility in cellular imaging and cell tracking in vivo, which complements conventional 1H MRI. An attractive feature of 19F-based imaging is that this is a bio-orthogonal nucleus and the naturally abundant isotope is NMR active. A significant hurdle however in the 19F MRI arises from the tendency of organic macromolecules, with multiple fluorocarbon substitutions, to aggregate in the aqueous phase. This aggregation results in significant loss of sensitivity, because the T2 relaxation times of these aggregated 19F species tend to be significantly lower. In this report, we have developed a strategy to covalently trap nanoscopic states with an optimal degree of 19F substitutions, followed by significant enhancement in T2 relaxation times through increased segmental mobility of the side chain substituents facilitated by the stimulus-responsive elements in the polymeric nanogel. In addition to NMR relaxation time based evaluations, the ability to obtain such signals are also evaluated in mouse models. The propensity of these nanoscale assemblies to encapsulate hydrophobic drug molecules and the availability of surfaces for convenient introduction of fluorescent labels suggest the potential of these nanoscale architectures for use in multimodal imaging and therapeutic applications.

Fluorinated Paramagnetic Complexes: Sensitive and Responsive Probes for Magnetic Resonance Spectroscopy and Imaging

Fluorine magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI) of chemical and physiological processes is becoming more widespread. The strength of this technique comes from the negligible background signal in in vivo¹⁹F MRI and the large chemical shift window of ¹⁹F that enables it to image concomitantly more than one marker. These same advantages have also been successfully exploited in the design of responsive ¹⁹F probes. Part of the recent growth of this technique can be attributed to novel designs of ¹⁹F probes with improved imaging parameters due to the incorporation of paramagnetic metal ions. In this review, we provide a description of the theories and strategies that have been employed successfully to improve the sensitivity of ¹⁹F probes with paramagnetic metal ions. The Bloch-Wangsness-Redfield theory accurately predicts how molecular parameters such as internuclear distance, geometry, rotational correlation times, as well as the nature, oxidation state, and spin state of the metal ion affect the sensitivity of the fluorine-based probes. The principles governing the design of responsive ¹⁹F probes are subsequently described in a "how to" guide format. Examples of such probes and their advantages and disadvantages are highlighted through a synopsis of the literature.

Fe- and Ln-DOTAm-F12 Are Effective Paramagnetic Fluorine Contrast Agents for MRI in Water and Blood

A series of fluorinated macrocyclic complexes, M-DOTAm-F12, where M is La^III, Eu^III, Gd^III, Tb^III, Dy^III, Ho^III, Er^III, Tm^III, Yb^III, and Fe^II, was synthesized, and their potential as fluorine magnetic resonance imaging (MRI) contrast agents was evaluated. The high water solubility of these complexes and the presence of a single fluorine NMR signal, two necessary parameters for in vivo MRI, are substantial advantages over currently used organic polyfluorocarbons and other reported paramagnetic ¹⁹F probes. Importantly, the sensitivity of the paramagnetic probes on a per fluorine basis is at least 1 order of magnitude higher than that of diamagnetic organic probes. This increased sensitivity is due to a substantial-up to 100-fold-decrease in the longitudinal relaxation time (T₁) of the fluorine nuclei. The shorter T₁ allows for a greater number of scans to be obtained in an equivalent time frame. The sensitivity of the fluorine probes is proportional to the T₂/T₁ ratio. In water, the optimal metal complexes for imaging applications are those containing Ho^III and Fe^II, and to a lesser extent Tm^III and Yb^III. Whereas T₁ of the lanthanide complexes are little affected by blood, the T₂ are notably shorter in blood than in water. The sensitivity of Ln-DOTAm-F12 complexes is lower in blood than in water, such that the most sensitive complex in water, Ho^III-DOTAm-F12, could not be detected in blood. Tm^III yielded the most sensitive lanthanide fluorine probe in blood. Notably, the relaxation times of the fluorine nuclei of Fe^II-DOTAm-F12 are similar in water and in blood. That complex has the highest T₂/T₁ ratio (0.57) and the lowest limit of detection (300 μM) in blood. The combination of high water solubility, single fluorine signal, and high T₂/T₁ of M-DOTAm-F12 facilitates the acquisition of three-dimensional magnetic resonance images.

Multifunctional hyperbranched polymers for CT/19F MRI bimodal molecular imaging

Multifunctional hyperbranched polymers containing iodine and fluorine were synthesised by reversible addition–fragmentation chain transfer (RAFT) polymerisation, and evaluated as novel contrast agents for CT/¹⁹F MRI bimodal molecular imaging.

Ligation driven (19)F relaxation enhancement in self-assembled Ln(III) complexes

Strong bidentate ligation between a fluorinated isophthalate and binuclear lanthanide-DO3A species yields a new class of (19)F NMR agent with very high nuclear relaxation rates at physiologically-relevant pH.

19F-lanthanide complexes with increased sensitivity for 19F-MRI: optimization of the MR acquisition

Fluorine-19 magnetic resonance methods offer advantages for molecular or cellular imaging in vivo due to the absence of radioactivity, lack of naturally occurring background signal, and the ability to easily combine measurements with anatomical MRI. Previous studies have shown that (19) F-MRI sensitivity is limited to millimolar concentrations by slow longitudinal relaxation. In this study, a new class of macrocyclic fluorinated lanthanide complexes is investigated where relaxation rates are significantly shortened by proximity of the fluorine group to a paramagnetic lanthanide ion located within the same molecule. Longitudinal and transverse relaxation rates are field dependent and in the range 50-150 s(-1) and 70-200 s(-1), respectively, at 7 T. Relaxation rates in these complexes are a function of the molecular structure and are independent of concentration at biologically relevant levels, so can be used as criteria to optimize imaging acquisition. Phantom experiments at 7 T indicate a lower limit for detection by imaging of 20 μM.