A unique 19F MRI agent for the tracking of non phagocytic cells in vivo

Protein-Engineered Fibers For Drug Encapsulation Traceable via 19F Magnetic Resonance

Theranostic materials research is experiencing rapid growth driven by the interest in integrating both therapeutic and diagnostic modalities. These materials offer the unique capability to not only provide treatment but also track the progression of a disease. However, to create an ideal theranostic biomaterial without compromising drug encapsulation, diagnostic imaging must be optimized for improved sensitivity and spatial localization. Herein, we create a protein-engineered fluorinated coiled-coil fiber, Q2TFL, capable of improved sensitivity to 19F magnetic resonance spectroscopy (MRS) detection. Leveraging residue-specific noncanonical amino acid incorporation of trifluoroleucine (TFL) into the coiled-coil, Q2, which self-assembles into nanofibers, we generate Q2TFL. We demonstrate that fluorination results in a greater increase in thermostability and 19F magnetic resonance detection compared to the nonfluorinated parent, Q2. Q2TFL also exhibits linear ratiometric 19F MRS thermoresponsiveness, allowing it to act as a temperature probe. Furthermore, we explore the ability of Q2TFL to encapsulate the anti-inflammatory small molecule, curcumin (CCM), and its impact on the coiled-coil structure. Q2TFL also provides hyposignal contrast in 1H MRI, echogenic signal with high-frequency ultrasound and sensitive detection by 19F MRS in vivo illustrating fluorination of coiled-coils for supramolecular assembly and their use with 1H MRI, 19F MRS and high frequency ultrasound as multimodal theranostic agents.

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.

Pivotal role of the protein corona in the cell uptake of fluorinated nanoparticles with increased sensitivity for 19F-MR imaging

In vivo cell tracking by non-invasive imaging technologies is needed to accelerate the clinical translation of innovative cell-based therapies. In this regard, ¹⁹F-MRI has recently gained increased attention for unbiased localization of labeled cells over time. To push forward the use of ¹⁹F-MRI for cell tracking, the development of highly performant ¹⁹F-probes is required. PLGA-based NPs containing PERFECTA, a multibranched superfluorinated molecule with an optimal MRI profile thanks to its 36 magnetically equivalent fluorine atoms, are promising ¹⁹F-MRI probes. In this work we demonstrate the importance of the surface functionalization of these NPs in relation to their interaction with the biological environment, stressing the pivotal role of the formation of the protein corona (PC) in their cellular labelling efficacy. In particular, our studies showed that the formation of PC NPs strongly promotes the cellular internalization of these NPs in microglia cells. We advocate that the formation of PC NPs in the culture medium can be a key element to be used for the optimization of cell labelling with a considerable increase of the detection sensitivity by ¹⁹F-MRI.

Preparation of PFPE-Based Polymeric Nanoparticles via Polymerization-Induced Self-Assembly as Contrast Agents for 19F MRI

Fluorine-19 magnetic resonance imaging (¹⁹F MRI) probes have received considerable research interest as imaging contrast agents (CAs), but they remain neglected and underutilized due to the limited fluorine content or poor performance of fluorinated tracers. Here, we present polymeric nanoparticles (NPs) as ¹⁹F MRI CAs with a simple synthesis method and promising imaging performance. First, hydrophilic random copolymers were synthesized from oligo(ethylene glycol) methyl ether acrylate and perfluoropolyether methacrylate by reversible addition-fragmentation chain transfer (RAFT) polymerization. The optimal fluorine content, polymer concentration, and cytotoxicity as ¹⁹F MRI CAs were investigated in detail. Then, the optimal copolymer was selected as the macromolecular chain transfer agent, and the chain extension was performed with 2-(perfluorooctyl ethyl methacrylate). Subsequently, the NPs with different morphologies, such as ellipsoidal, spherical nanoparticles and vesicles, were prepared in situ by the RAFT-mediated polymerization-induced self-assembly method. In addition, the ¹⁹F MRI signal and cytotoxicity studies further confirmed that these polymeric NPs are nontoxic and have great potential as promising ¹⁹F MRI CAs for biological applications.

Nanotechnology-Assisted Cell Tracking

The usefulness of nanoparticles (NPs) in the diagnostic and/or therapeutic sector is derived from their aptitude for navigating intra- and extracellular barriers successfully and to be spatiotemporally targeted. In this context, the optimization of NP delivery platforms is technologically related to the exploitation of the mechanisms involved in the NP–cell interaction. This review provides a detailed overview of the available technologies focusing on cell–NP interaction/detection by describing their applications in the fields of cancer and regenerative medicine. Specifically, a literature survey has been performed to analyze the key nanocarrier-impacting elements, such as NP typology and functionalization, the ability to tune cell interaction mechanisms under in vitro and in vivo conditions by framing, and at the same time, the imaging devices supporting NP delivery assessment, and consideration of their specificity and sensitivity. Although the large amount of literature information on the designs and applications of cell membrane-coated NPs has reached the extent at which it could be considered a mature branch of nanomedicine ready to be translated to the clinic, the technology applied to the biomimetic functionalization strategy of the design of NPs for directing cell labelling and intracellular retention appears less advanced. These approaches, if properly scaled up, will present diverse biomedical applications and make a positive impact on human health.

Monitoring the Remodeling of Biohybrid Tissue-Engineered Vascular Grafts by Multimodal Molecular Imaging

Tissue-engineered vascular grafts (TEVGs) with the ability to grow and remodel open new perspectives for cardiovascular surgery. Equipping TEVGs with synthetic polymers and biological components provides a good compromise between high structural stability and biological adaptability. However, imaging approaches to control grafts' structural integrity, physiological function, and remodeling during the entire transition between late in vitro maturation and early in vivo engraftment are mandatory for clinical implementation. Thus, a comprehensive molecular imaging concept using magnetic resonance imaging (MRI) and ultrasound (US) to monitor textile scaffold resorption, extracellular matrix (ECM) remodeling, and endothelial integrity in TEVGs is presented here. Superparamagnetic iron-oxide nanoparticles (SPION) incorporated in biodegradable poly(lactic-co-glycolic acid) (PLGA) fibers of the TEVGs allow to quantitatively monitor scaffold resorption via MRI both in vitro and in vivo. Additionally, ECM formation can be depicted by molecular MRI using elastin- and collagen-targeted probes. Finally, molecular US of α_v β₃ integrins confirms the absence of endothelial dysfunction; the latter is provocable by TNF-α. In conclusion, the successful employment of noninvasive molecular imaging to longitudinally evaluate TEVGs remodeling is demonstrated. This approach may foster its translation from in vitro quality control assessment to in vivo applications to ensure proper prostheses engraftment.

19 F MRI Nanotheranostics for Cancer Management: Progress and Prospects

Fluorine magnetic resonance imaging (¹⁹ F MRI) is a promising imaging technique for cancer diagnosis because of its excellent soft tissue resolution and deep tissue penetration, as well as the inherent high natural abundance, almost no endogenous interference, quantitative analysis, and wide chemical shift range of the ¹⁹ F nucleus. In recent years, scientists have synthesized various ¹⁹ F MRI contrast agents. By further integrating a wide variety of nanomaterials and cutting-edge construction strategies, magnetically equivalent ¹⁹ F atoms are super-loaded and maintain satisfactory relaxation efficiency to obtain high-intensity ¹⁹ F MRI signals. In this review, the nuclear magnetic resonance principle underlying ¹⁹ F MRI is first described. Then, the construction and performance of various fluorinated contrast agents are summarized. Finally, challenges and future prospects regarding the clinical translation of ¹⁹ F MRI nanoprobes are considered. This review will provide strategic guidance and panoramic expectations for designing new cancer theranostic regimens and realizing their clinical translation.

Nanotechnology as a Versatile Tool for 19F-MRI Agent’s Formulation: A Glimpse into the Use of Perfluorinated and Fluorinated Compounds in Nanoparticles

Simultaneously being a non-radiative and non-invasive technique makes magnetic resonance imaging (MRI) one of the highly sought imaging techniques for the early diagnosis and treatment of diseases. Despite more than four decades of research on finding a suitable imaging agent from fluorine for clinical applications, it still lingers as a challenge to get the regulatory approval compared to its hydrogen counterpart. The pertinent hurdle is the simultaneous intrinsic hydrophobicity and lipophobicity of fluorine and its derivatives that make them insoluble in any liquids, strongly limiting their application in areas such as targeted delivery. A blossoming technique to circumvent the unfavorable physicochemical characteristics of perfluorocarbon compounds (PFCs) and guarantee a high local concentration of fluorine in the desired body part is to encapsulate them in nanosystems. In this review, we will be emphasizing different types of nanocarrier systems studied to encapsulate various PFCs and fluorinated compounds, headway to be applied as a contrast agent (CA) in fluorine-19 MRI (¹⁹F MRI). We would also scrutinize, especially from studies over the last decade, the different types of PFCs and their specific applications and limitations concerning the nanoparticle (NP) system used to encapsulate them. A critical evaluation for future opportunities would be speculated.

Biological Utility of Fluorinated Compounds: from Materials Design to Molecular Imaging, Therapeutics and Environmental Remediation

The applications of fluorinated molecules in bioengineering and nanotechnology are expanding rapidly with the controlled introduction of fluorine being broadly studied due to the unique properties of C-F bonds. This review will focus on the design and utility of C-F containing materials in imaging, therapeutics, and environmental applications with a central theme being the importance of controlling fluorine-fluorine interactions and understanding how such interactions impact biological behavior. Low natural abundance of fluorine is shown to provide sensitivity and background advantages for imaging and detection of a variety of diseases with ¹⁹F magnetic resonance imaging, ¹⁸F positron emission tomography and ultrasound discussed as illustrative examples. The presence of C-F bonds can also be used to tailor membrane permeability and pharmacokinetic properties of drugs and delivery agents for enhanced cell uptake and therapeutics. A key message of this review is that while the promise of C-F containing materials is significant, a subset of highly fluorinated compounds such as per- and polyfluoroalkyl substances (PFAS), have been identified as posing a potential risk to human health. The unique properties of the C-F bond and the significant potential for fluorine-fluorine interactions in PFAS structures necessitate the development of new strategies for facile and efficient environmental removal and remediation. Recent progress in the development of fluorine-containing compounds as molecular imaging and therapeutic agents will be reviewed and their design features contrasted with environmental and health risks for PFAS systems. Finally, present challenges and future directions in the exploitation of the biological aspects of fluorinated systems will be described.

The sensitivity of magnetic particle imaging and fluorine-19 magnetic resonance imaging for cell tracking

Magnetic particle imaging (MPI) and fluorine-19 (19F) MRI produce images which allow for quantification of labeled cells. MPI is an emerging instrument for cell tracking, which is expected to have superior sensitivity compared to 19F MRI. Our objective is to assess the cellular sensitivity of MPI and 19F MRI for detection of mesenchymal stem cells (MSC) and breast cancer cells. Cells were labeled with ferucarbotran or perfluoropolyether, for imaging on a preclinical MPI system or 3 Tesla clinical MRI, respectively. Using the same imaging time, as few as 4000 MSC (76 ng iron) and 8000 breast cancer cells (74 ng iron) were reliably detected with MPI, and 256,000 MSC (9.01 × 1016 19F atoms) were detected with 19F MRI, with SNR > 5. MPI has the potential to be more sensitive than 19F MRI for cell tracking. In vivo sensitivity with MPI and 19F MRI was evaluated by imaging MSC that were administered by different routes. In vivo imaging revealed reduced sensitivity compared to ex vivo cell pellets of the same cell number. We attribute reduced MPI and 19F MRI cell detection in vivo to the effect of cell dispersion among other factors, which are described.

19 F MRI Probes for Multimodal Imaging*

Magnetic resonance imaging (MRI) is one of the most powerful imaging tools today, capable of displaying superior soft-tissue contrast. This review discusses developments in the field of ¹⁹ F MRI multimodal probes in combination with optical fluorescence imaging (OFI), ¹ H MRI, chemical exchange saturation transfer (CEST) MRI, ultrasonography (USG), X-ray computed tomography (CT), single photon emission tomography (SPECT), positron emission tomography (PET), and photoacoustic imaging (PAI). In each case, multimodal ¹⁹ F MRI probes compensate for the deficiency of individual techniques and offer improved sensitivity or accuracy of detection over unimodal counterparts. Strategies for designing ¹⁹ F MRI multimodal probes are described with respect to their structure, physicochemical properties, biocompatibility, and the quality of images.

19F-Grafted Fluorescent Carbonized Polymer Dots for Dual-Mode Imaging

Dual-modal imaging systems could provide complementary information by taking advantage of each imaging modality. Herein, a fluorescence and ¹⁹F magnetic resonance imaging nanoprobe was developed through preparation of ¹⁹F-grafted fluorescent carbonized polymer dots (FCPDs). Both fluorescence and ¹⁹F nuclear magnetic resonance intensities of these FCPDs can be modulated by controlling the carbonization processes. The strong yellow fluorescence renders these FCPDs capable of cell fluorescence imaging. The in vitro and in vivo assessments demonstrated that the as-prepared FCPDs were suitable for ¹⁹F magnetic resonance imaging (¹⁹F MRI), which would provide great potential for biological imaging and early diagnosis applications. Moreover, this fabrication strategy offers a new protocol for ¹⁹F MRI nanoprobe design.

Visualizing stem cells in vivo using magnetic resonance imaging

Stem cell (SC) therapies displayed encouraging efficacy and clinical outcome in various disorders. Despite this huge hype, clinical translation of SC therapy has been disheartening due to contradictory results from clinical trials. The ability to monitor migration and engraftment of cells in vivo represents an ideal strategy in cell therapy. Therefore, suitable imaging approach to track MSCs would allow understanding of migratory and homing efficiency, optimal route of delivery and engraftment of cells at targeted location. Hence, longitudinal tracking of SCs is crucial for the optimization of treatment parameters, leading to improved clinical outcome and translation. Magnetic resonance imaging (MRI) represents a suitable imaging modality to observe cells non-invasively and repeatedly. Tracking is achieved when cells are incubated prior to implantation with appropriate contrast agents (CA) or tracers which can then be detected in an MRI scan. This review explores and emphasizes the importance of monitoring the distribution and fate of SCs post-implantation using current contrast agents, such as positive CAs including paramagnetic metals (gadolinium), negative contrast agents such as superparamagnetic iron oxides and ¹⁹ F containing tracers, specifically for the in vivo tracking of MSCs using MRI. This article is categorized under: Diagnostic Tools > In Vivo Nanodiagnostics and Imaging Nanotechnology Approaches to Biology > Nanoscale Systems in Biology Therapeutic Approaches and Drug Discovery > Emerging Technologies.

Emergence of Hexagonally Close-Packed Spheres in Linear Block Copolymer Melts

The hexagonally close-packed (HCP) sphere phase is predicted to be stable across a narrow region of linear block copolymer phase space, but the small free energy difference separating it from face-centered cubic spheres usually results in phase coexistence. Here, we report the discovery of pure HCP spheres in linear block copolymer melts with A = poly(2,2,2-trifluoroethyl acrylate) ("F") and B = poly(2-dodecyl acrylate) ("2D") or poly(4-dodecyl acrylate) ("4D"). In 4DF diblocks and F4DF triblocks, the HCP phase emerges across a substantial range of A-block volume fractions (circa f_A = 0.25-0.30), and in F4DF, it forms reversibly when subjected to various processing conditions which suggests an equilibrium state. The time scale associated with forming pure HCP upon quenching from a disordered liquid is intermediate to the ordering kinetics of the Frank-Kasper σ and A15 phases. However, unlike σ and A15, HCP nucleates directly from a supercooled liquid or soft solid without proceeding through an intermediate quasicrystal. Self-consistent field theory calculations indicate the stability of HCP is intimately tied to small amounts of molar mass dispersity (Đ); for example, an HCP-forming F4DF sample with f_A = 0.27 has an experimentally measured Đ = 1.04. These insights challenge the conventional wisdom that pure HCP is difficult to access in linear block copolymer melts without the use of blending or other complex processing techniques.

Very low field 19F MRI of perfluoro-octylbromide: Minimizing chemical shift effects and signal loss due to scalar coupling

¹⁹F images have been obtained from perflurooctylbromide (PFOB) at very low magnetic field (50 mT). The small spectral dispersion (in Hz) means that all fluorine nuclei contribute to the signal without chemical shift artifacts or the need for specialized imaging sequences. Turbo spin echo trains with short interpulse intervals and full 180° refocussing pulses suppress scalar coupling, leading to long apparent T₂ values and highly efficient data collection. Overall, the detection efficiency of PFOB is very similar that of water in tissue.

19F Magnetic Resonance Imaging and Spectroscopy in Neuroscience

¹H magnetic resonance imaging (MRI) has established itself as a key diagnostic technique, affording the visualization of brain anatomy, blood flow, activity and connectivity. The detection of other atoms (e.g. ¹⁹F, ²³Na, ³¹P), so called hetero-nuclear MRI and spectroscopy (MRS), provides investigative avenues that complement and extend the richness of information that can be gained from ¹H MRI. Especially ¹⁹F MRI is increasingly emerging as a multi-nuclear (¹H/¹⁹F) technique that can be exploited to visualize cell migration and trafficking. The lack of a ¹⁹F background signal in the brain affords an unequivocal detection suitable for quantification. Fluorine-based contrast material can be engineered as nanoemulsions, nanocapsules, or nanoparticles to label cells in vitro or in vivo. Fluorinated blood substitutes, typically nanoemulsions, can also carry oxygen and serve as a theranostic in poorly perfused brain regions. Brain tissue concentrations of fluorinated pharmaceuticals, including inhalation anesthetics (e.g. isoflurane) and anti-depressants (e.g. fluoxetine), can also be measured using MRS. However, the low signal from these compounds provides a challenge for imaging. Further methodological advances that accelerate signal acquisition (e.g. compressed sensing, cryogenic coils) are required to expand the applications of ¹⁹F MR imaging to, for instance, determine the regional pharmacokinetics of novel fluorine-based drugs. Improvements in ¹⁹F signal detection and localization, combined with the development of novel sensitive probes, will increase the utility of these multi-nuclear studies. These advances will provide new insights into cellular and molecular processes involved in neurodegenerative disease, as well as the mode of action of pharmaceutical compounds.

Tuning the thermoresponsive properties of PEG-based fluorinated polymers and stimuli responsive drug release for switchable 19F magnetic resonance imaging

Switching on of the 19F MRI signal via stimuli-responsive release of hydrophobic drug from PEG-based partly-fluorinated polymers due to change in thermoresponsive properties.

Fluoropolymers in biomedical applications: state-of-the-art and future perspectives

Biomedical applications of fluoropolymers in gene delivery, protein delivery, drug delivery, ¹⁹F MRI, PDT, anti-fouling, anti-bacterial, cell culture, and tissue engineering.

Tuning of the Aggregation Behavior of Fluorinated Polymeric Nanoparticles for Improved Therapeutic Efficacy

Incorporation of fluorinated moieties in polymeric nanoparticles has been shown in many instances to increase their uptake by living cells and, hence, has proven to be a useful approach to enhancing delivery to cells. However, it remains unclear how incorporation of fluorine affects critical transport processes, such as interactions with membranes, intracellular transport, and tumor penetration. In this study, we investigate the influence of fluorine on transport properties using a series of rationally designed poly(oligo(ethylene glycol) methyl ether acrylate)-block-perfluoropolyether (poly(OEGA)_m-PFPE) copolymers. Copolymers with different fluorine contents were prepared and exhibit aggregate in solution in a manner dependent on the fluorine content. Doxorubicin-conjugated poly(OEGA)₂₀-PFPE nanoparticles with lower fluorine content exist in solution as unimers, leading to greater exposure of hydrophobic PFPE segments to the cell surface. This, in turn, results in greater cellular uptake, deeper tumor penetration, as well as enhanced therapeutic efficacy compared to that with the micelle-state nanoaggregates (poly(OEGA)₁₀-PFPE and poly(OEGA)₅-PFPE) with higher fluorine content but with less PFPE exposed to the cell membranes. Our results demonstrate that the aggregation behavior of these fluorinated polymers plays a critical role in internalization and transport in living cells and 3D spheroids, providing important design criteria for the preparation of highly effective delivery agents.

Nanoparticles for "two color" 19F magnetic resonance imaging: Towards combined imaging of biodistribution and degradation

The use of polymeric nanoparticles (NPs) as therapeutics has been steadily increasing over past decades. In vivo imaging of NPs is necessary to advance the therapeutic performance. 19F Magnetic Resonance Imaging (19F MRI) offers multiple advantages for in vivo imaging. However, design of a probe for both biodistribution and degradation has not been realized yet. We developed polymeric NPs loaded with two fluorocarbons as promising imaging tools to monitor NP biodistribution and degradation by 19F MRI. These 200 nm NPs consist of poly(lactic-co-glycolic acid) (PLGA) loaded with perfluoro-15-crown-5 ether (PFCE) and PERFECTA. PERFECTA/PFCE-PLGA NPs have a fractal sphere structure, in which both fluorocarbons are distributed in the polymeric matrix of the fractal building blocks, which differs from PFCE-PLGA NPs and is unique for fluorocarbon-loaded colloids. This structure leads to changes of magnetic resonance properties of both fluorocarbons after hydrolysis of NPs. PERFECTA/PFCE-PLGA NPs are colloidally stable in serum and biocompatible. Both fluorocarbons show a single resonance in 19F MRI that can be imaged separately using different excitation pulses. In the future, these findings may be used for biodistribution and degradation studies of NPs by 19F MRI in vivo using "two color" labeling leading to improvement of drug delivery agents.

Supramolecular Fluorine Magnetic Resonance Spectroscopy Probe Polymer Based on Passerini Bifunctional Monomer

A water-soluble fluorine magnetic resonance spectroscopy host-guest probe, P(HPA-co-AdamCF₃A), was successfully constructed from the facile synthesis of a bifunctional monomer via a quantitative Passerini reaction. Supramolecular complexation with (2-hydroxypropyl)-β-cyclodextrin promoted a change in the chemical environment, leading to modulation of both the relaxation properties as well as chemical shift of the fluorine moieties. This change was used to probe the supramolecular interaction by ¹⁹F MRI spectroscopy and give insight into fluorine probe formulation. This work provides a fundamental basis for an ¹⁹F MR imaging tracer capable of assessing host-guest inclusion and a potential model to follow the fate of a drug delivery system in vivo.

Integrating Fluorinated Polymer and Manganese-Layered Double Hydroxide Nanoparticles as pH-activated 19 F MRI Agents for Specific and Sensitive Detection of Breast Cancer

¹⁹ F magnetic resonance imaging (¹⁹ F MRI) agents capable of being activated upon interactions with cancer triggers are attracting increasing attention, although challenges still remain for precise and specific detection of cancer tissues. In this study, a novel hybrid ¹⁹ F MRI agent for pH-sensitive detection of breast cancer tissues is reported, a composite system designed by conjugating a perfluoropolyether onto the surface of manganese-incorporated layered double hydroxide (Mn-LDH@PFPE) nanoparticles. The ¹⁹ F NMR/MRI signals from aqueous solutions of Mn-LDH@PFPE nanoparticles are quenched at pH 7.4, but "turned on" following a reduction in pH to below 6.5. This is due to partial dissolution of Mn²⁺ from the Mn-LDH nanoparticles and subsequent reduction in the effect of paramagnetic relaxation. Significantly, in vivo experiments reveal that an intense ¹⁹ F MR signal can be detected only in the breast tumor tissue after intravenous injection of Mn-LDH@PFPE nanoparticles due to such a specific activation. Thus pH-activated Mn-LDH@PFPE nanoparticles are a potential "smart" ¹⁹ F MRI agent for precise and specific detection of cancer diseases.

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.

Protein-Engineered Nanoscale Micelles for Dynamic 19F Magnetic Resonance and Therapeutic Drug Delivery

Engineered proteins provide an interesting template for designing fluorine-19 (19F) magnetic resonance imaging (MRI) contrast agents, yet progress has been hindered by the unpredictable relaxation properties of fluorine. Herein, we present the biosynthesis of a protein block copolymer, termed "fluorinated thermoresponsive assembled protein" (F-TRAP), which assembles into a monodisperse nanoscale micelle with interesting 19F NMR properties and the ability to encapsulate and release small therapeutic molecules, imparting potential as a diagnostic and therapeutic (theranostic) agent. The assembly of the F-TRAP micelle, composed of a coiled-coil pentamer corona and a hydrophobic, thermoresponsive elastin-like polypeptide core, results in a drastic depression in spin-spin relaxation ( T2) times and unaffected spin-lattice relaxation ( T1) times. The nearly unchanging T1 relaxation rates and linearly dependent T2 relaxation rates have allowed for detection via zero echo time 19F MRI, and the in vivo MR potential has been preliminarily explored using 19F magnetic resonance spectroscopy (MRS). This fluorinated micelle has also demonstrated the ability to encapsulate the small-molecule chemotherapeutic doxorubicin and release its cargo in a thermoresponsive manner owing to its inherent stimuli-responsive properties, presenting an interesting avenue for the development of thermoresponsive 19F MRI/MRS-traceable theranostic agents.

Perfluorocarbon-Based 19 F MRI Nanoprobes for In Vivo Multicolor Imaging

In vivo multicolor imaging is important for monitoring multiple biomolecular or cellular processes in biology. ¹⁹F magnetic resonance imaging (MRI) is an emerging in vivo imaging technique because it can non‐invasively visualize ¹⁹F nuclei without endogenous background signals. Therefore, ¹⁹F MRI probes capable of multicolor imaging are in high demand. Herein, we report five types of perfluorocarbon‐encapsulated silica nanoparticles that show ¹⁹F NMR peaks with different chemical shifts. Three of the nanoprobes, which show spectrally distinct ¹⁹F NMR peaks with sufficient sensitivity, were selected for in vivo multicolor ¹⁹F MRI. The nanoprobes exhibited ¹⁹F MRI signals with three colors in a living mouse. Our in vivo multicolor system could be utilized for evaluating the effect of surface functional groups on the hepatic uptake in a mouse. This novel multicolor imaging technology will be a practical tool for elucidating in vivo biomolecular networks by ¹⁹F MRI.

High F-Content Perfluoropolyether-Based Nanoparticles for Targeted Detection of Breast Cancer by 19F Magnetic Resonance and Optical Imaging

Two important challenges in the field of ¹⁹F magnetic resonance imaging (MRI) are the maintenance of high fluorine content without compromising imaging performance, and effective targeting of small particles to diseased tissue. To address these challenges, we have developed a series of perfluoropolyether (PFPE)-based hyperbranched (HBPFPE) nanoparticles with attached peptide aptamer as targeting ligands for specific in vivo detection of breast cancer with high ¹⁹F MRI sensitivity. A detailed comparison of the HBPFPE nanoparticles (NPs) with the previously reported trifluoroethyl acrylate (TFEA)-based polymers demonstrates that the mobility of fluorinated segments of the HBPFPE nanoparticles is significantly enhanced (¹⁹F T₂ > 80 ms vs 31 ms), resulting in superior MR imaging sensitivity. Selective targeting was confirmed by auto- and pair correlation analysis of fluorescence microscopy data, in vitro immunofluorescence, in vivo ¹⁹F MRI, ex vivo fluorescence and ¹⁹F NMR. The results highlight the high efficiency of aptamers for targeting and the excellent sensitivity of the PFPE moieties for ¹⁹F MRI. Of relevance to in vivo applications, the PFPE-based polymers exhibit much faster clearance from the body than the previously introduced perfluorocarbon emulsions ( t_1/2 ∼ 20 h vs up to months). Moreover, the aptamer-conjugated NPs show significantly higher tumor-penetration, demonstrating the potential of these imaging agents for therapeutic applications. This report of the synthesis of polymeric aptamer-conjugated PFPE-based ¹⁹F MRI CAs with high fluorine content (∼10 wt %) demonstrates that these NPs are exciting candidates for detecting diseases with high imaging sensitivity.

Elucidating the Impact of Molecular Structure on the 19F NMR Dynamics and MRI Performance of Fluorinated Oligomers

To understand molecular factors that impact the performance of polymeric ¹⁹F magnetic resonance imaging (MRI) agents, a series of discrete fluorinated oligoacrylates with precisely defined structure were prepared through the combination of controlled polymerization and chromatographic separation techniques. These discrete oligomers enabled thorough elucidation of the dependence of ¹⁹F NMR and MRI properties on molecular structure, for example, the chain length. Importantly, the oligomer size and dispersity strongly influence NMR dynamics (T₁ and T₂ relaxation times) and MR imaging properties with higher signal-to-noise ratio (SNR) observed for oligomers with longer chain length and shorter T₁. Our approach enables an effective pathway and thus opportunities to rationally design effective polymeric ¹⁹F MR imaging agents with optimized molecular structure and NMR relaxivity.