Activatable19F MRI Nanoparticle Probes for the Detection of Reducing Environments

A Set of Three GdIII Spin Labels with Methanethiosulfonyl Groups for Bioconjugation Covering a Wide Range of EPR Line Widths

Spin labels based on GdIII complexes are important tools for the elucidation of the structure, dynamics and interaction of biomolecules by electron paramagnetic resonance (EPR) spectroscopy. Their EPR spectroscopic properties line width and relaxation times influence their performance in a particular application. To be able to apply a complex well-suited for a specific application, a set of GdIII complexes with different EPR spectroscopic properties ready-made for spin labeling will be highly useful. We prepared three GdIII complexes with DO3APic, NO3Pic, and PyMTA as the basic ligand units. They cover a wide range of EPR line widths but have in common a cysteine-targeting methanethiosulfonyl (MTS) group connected to a pyridine ring, which is an intrinsic part of the ligand. The reaction with a cysteine-containing pentapeptide (0.45 mM in the peptide, pH ∼ 7) was complete within 90 s and chemoselective. The MTS group hydrolyzed with half-lives of >24, 8, 2, and 1 h at pH 5, 6, 7, and 8, respectively. The structurally related nicotinic acid-substituted disulfide (NDS) group was found to be hydrolytically much more stable. However, the MTS spin label clearly won the competition for the pentapeptide over the NDS spin label. If high reactivity is essential, MTS is clearly the better choice.

Systems Engineering Approach Towards Sensitive Cellular Fluorine-19 MRI

In vivo fluorine-19 MRI using F-based tracer media has shown utility and versatility for a wide range of biomedical uses, particularly immune and stem cell detection, as well as biosensing. As with many advanced MRI acquisition techniques, the sensitivity and limit of detection (LOD) in vivo is a key consideration for a successful study outcome. In this review, we analyze the primary factors that limit cell LOD. The achievable sensitivity is strongly dependent on the specific composition of tracer, cell type of interest, cell activity, data acquisition and reconstruction methods, and MRI hardware design. Recent innovations in molecular 19F tracer design and image acquisition-reconstruction methods have achieved significant leaps in 19F MRI sensitivity, and integration of these new materials and methods into studies can result in > 10-fold improvement in LOD. These developments will help unlock the full potential of clinical 19F MRI for biomedical applications.

The role of responsive MRI probes in the past and the future of molecular imaging

Magnetic resonance imaging (MRI) has become an indispensable tool in biomedical research and clinical radiology today. It enables the tracking of physiological changes noninvasively and allows imaging of specific biological processes at the molecular or cellular level. To this end, bioresponsive MRI probes can greatly contribute to improving the specificity of MRI, as well as significantly expanding the scope of its application. A large number of these sensor probes has been reported in the past two decades. Importantly, their development was done hand in hand with the ongoing advances in MRI, including emerging methodologies such as chemical exchange saturation transfer (CEST) or hyperpolarised MRI. Consequently, several approaches on successfully using these probes in functional imaging studies have been reported recently, giving new momentum to the field of molecular imaging, also the chemistry of MRI probes. This Perspective summarizes the major strategies in the development of bioresponsive MRI probes, highlights the major research directions within an individual group of probes (T 1- and T 2-weighted, CEST, fluorinated, hyperpolarised) and discusses the practical aspects that should be considered in designing the MRI sensors, up to their intended application in vivo.

A Reactive and Specific Sensor for Activity-Based 19F-MRI Sensing of Zn2

The rapid fluctuations of metal ion levels in biological systems are faster than the time needed to map fluorinated sensors designed for the 19F-MRI of cations. An attractive modular solution might come from the activity-based sensing approach. Here, we propose a highly reactive but still ultimately specific synthetic fluorinated sensor for 19F-MRI mapping of labile Zn2+. The sensor comprises a dipicolylamine scaffold for Zn2+ recognition conjugated to a fluorophenyl acetate entity. Upon binding to Zn2+, the synthetic sensor is readily hydrolyzed, and the frequency of its 19F-functional group in 19F-NMR is shifted by 12 ppm, allowing the display of the Zn2+ distribution as an artificial MRI-colored map highlighting its specificity compared to other metal ions. The irreversible Zn2+-induced hydrolysis results in a "turn-on" 19F-MRI, potentially detecting the cation even upon a transient elevation of its levels. We envision that additional metal-ion sensors can be developed based on the principles demonstrated in this work, expanding the molecular toolbox currently used for 19F-MRI.

Dual Nanoparticle Conjugates for Highly Sensitive and Versatile Sensing Using 19 F Magnetic Resonance Imaging

Fluorine magnetic resonance imaging (19 F MRI) has emerged as an attractive alternative to conventional 1 H MRI due to enhanced specificity deriving from negligible background signal in this modality. We report a dual nanoparticle conjugate (DNC) platform as an aptamer-based sensor for use in 19 F MRI. DNC consists of core-shell nanoparticles with a liquid perfluorocarbon core and a mesoporous silica shell (19 F-MSNs), which give a robust 19 F MR signal, and superparamagnetic iron oxide nanoparticles (SPIONs) as magnetic quenchers. Due to the strong magnetic quenching effects of SPIONs, this platform is uniquely sensitive and functions with a low concentration of SPIONs (4 equivalents) relative to 19 F-MSNs. The probe functions as a "turn-on" sensor using target-induced dissociation of DNA aptamers. The thrombin binding aptamer was incorporated as a proof-of-concept (DNCThr ), and we demonstrate a significant increase in 19 F MR signal intensity when DNCThr is incubated with human α-thrombin. This proof-of-concept probe is highly versatile and can be adapted to sense ATP and kanamycin as well. Importantly, DNCThr generates a robust 19 F MRI "hot-spot" signal in response to thrombin in live mice, establishing this platform as a practical, versatile, and biologically relevant molecular imaging probe.

Glycan Metabolic Fluorine Labeling for In Vivo Visualization of Tumor Cells and In Situ Assessment of Glycosylation Variations

The abnormality in the glycosylation of surface proteins is critical for the growth and metastasis of tumors and their capacity for immunosuppression and drug resistance. This anomaly offers an entry point for real-time analysis on glycosylation fluctuations. In this study, we report a strategy, glycan metabolic fluorine labeling (MEFLA), for selectively tagging glycans of tumor cells. As a proof of concept, we synthesized two fluorinated unnatural monosaccharides with distinctive 19 F chemical shifts (Ac4 ManNTfe and Ac4 GalNTfa). These two probes could undergo selective uptake by tumor cells and subsequent incorporation into surface glycans. This approach enables efficient and specific 19 F labeling of tumor cells, which permits in vivo tracking of tumor cells and in situ assessment of glycosylation changes by 19 F MRI. The efficiency and specificity of our probes for labeling tumor cells were verified in vitro with A549 cells. The feasibility of our method was further validated with in vivo experiments on A549 tumor-bearing mice. Moreover, the capacity of our approach for assessing glycosylation changes of tumor cells was illustrated both in vitro and in vivo. Our studies provide a promising means for visualizing tumor cells in vivo and assessing their glycosylation variations in situ through targeted multiplexed 19 F MRI.

The internal structure of gadolinium and perfluorocarbon-loaded polymer nanoparticles affects 19F MRI relaxation times

19F magnetic resonance imaging (19F MRI) is an emerging technique for quantitative imaging in novel therapies, such as cellular therapies and theranostic nanocarriers. Nanocarriers loaded with liquid perfluorocarbon (PFC) typically have a (single) core-shell structure with PFC in the core due to the poor miscibility of PFC with organic and inorganic solvents. Paramagnetic relaxation enhancement acts only at a distance of a few angstroms. Thus, efficient modulation of the 19F signal is possible only with fluorophilic PFC-soluble chelates. However, these chelates cannot interact with the surrounding environment and they might result in image artifacts. Conversely, chelates bound to the nanoparticle shell typically have a minimal effect on the 19F signal and a strong impact on the aqueous environment. We show that the confinement of PFC in biodegradable polymeric nanoparticles (NPs) with a multicore structure enables the modulation of longitudinal (T1) and transverse (T2) 19F relaxation, as well as proton (1H) signals, using non-fluorophilic paramagnetic chelates. We compared multicore NPs versus a conventional single core structure, where the PFC is encapsulated in the core(s) and the chelate in the surrounding polymeric matrix. This modulated relaxation also makes multicore NPs sensitive to various acidic pH environments, while preserving their stability. This effect was not observed with single core nanocapsules (NCs). Importantly, paramagnetic chelates affected both T1 and T219F relaxation in multicore NPs, but not in single core NCs. Both relaxation times of the 19F nucleus were enhanced with an increasing concentration of the paramagnetic chelate. Moreover, as the polymeric matrix remained water permeable, proton enhancement additionally was observed in MRI.

How to 19F MRI: applications, technique, and getting started

Magnetic resonance imaging (MRI) plays a significant role in the routine imaging workflow, providing both anatomical and functional information. 19F MRI is an evolving imaging modality where instead of 1H, 19F nuclei are excited. As the signal from endogenous 19F in the body is negligible, exogenous 19F signals obtained by 19F radiofrequency coils are exceptionally specific. Highly fluorinated agents targeting particular biological processes (i.e., the presence of immune cells) have been visualised using 19F MRI, highlighting its potential for non-invasive and longitudinal molecular imaging. This article aims to provide both a broad overview of the various applications of 19F MRI, with cancer imaging as a focus, as well as a practical guide to 19F imaging. We will discuss the essential elements of a 19F system and address common pitfalls during acquisition. Last but not least, we will highlight future perspectives that will enhance the role of this modality. While not an exhaustive exploration of all 19F literature, we endeavour to encapsulate the broad themes of the field and introduce the world of 19F molecular imaging to newcomers. 19F MRI bridges several domains, imaging, physics, chemistry, and biology, necessitating multidisciplinary teams to be able to harness this technology effectively. As further technical developments allow for greater sensitivity, we envision that 19F MRI can help unlock insight into biological processes non-invasively and longitudinally.

Nanoprobe-based molecular imaging for tumor stratification

The responses of patients to tumor therapies vary due to tumor heterogeneity. Tumor stratification has been attracting increasing attention for accurately distinguishing between responders to treatment and non-responders. Nanoprobes with unique physical and chemical properties have great potential for patient stratification. This review begins by describing the features and design principles of nanoprobes that can visualize specific cell types and biomarkers and release inflammatory factors during or before tumor treatment. Then, we focus on the recent advancements in using nanoprobes to stratify various therapeutic modalities, including chemotherapy, radiotherapy (RT), photothermal therapy (PTT), photodynamic therapy (PDT), chemodynamic therapy (CDT), ferroptosis, and immunotherapy. The main challenges and perspectives of nanoprobes in cancer stratification are also discussed to facilitate probe development and clinical applications.

19 F VT NMR: Novel Tm3+ and Ce3+ Complexes Provide New Insight into Temperature Measurement Using Molecular Sensors

In the past few decades, magnetic resonance spectroscopy (MRS) and MR imaging (MRI) have developed into a powerful non-invasive tool for medical diagnostic and therapy. Especially 19 F MR shows promising potential because of the properties of the fluorine atom and the negligible background signals in the MR spectra. The detection of temperature in a living organism is quite difficult, and usually external thermometers or fibers are used. Temperature determination via MRS needs temperature-sensitive contrast agents. This article reports first results of solvent and structural influences on the temperature sensitivity of 19 F NMR signals of chosen molecules. By using this chemical shift sensitivity, a local temperature can be determined with a high precision. Based on this preliminary study, we synthesized five metal complexes and compared the results of all variable temperature measurements. It is shown that the highest 19 F MR signal temperature dependence is detectable for a fluorine nucleus in a Tm3+ -complex.

Recent Research Progress of 19 F Magnetic Resonance Imaging Probes: Principle, Design, and Their Application

Visualization of biomolecules, cells, and tissues, as well as metabolic processes in vivo is significant for studying the associated biological activities. Fluorine magnetic resonance imaging (19 F MRI) holds potential among various imaging technologies thanks to its negligible background signal and deep tissue penetration in vivo. To achieve detection on the targets with high resolution and accuracy, requirements of high-performance 19 F MRI probes are demanding. An ideal 19 F MRI probe is thought to have, first, fluorine tags with magnetically equivalent 19 F nuclei, second, high fluorine content, third, adequate fluorine nuclei mobility, as well as excellent water solubility or dispersity, but not limited to. This review summarizes the research progresses of 19 F MRI probes and mainly discusses the impacts of structures on in vitro and in vivo imaging performances. Additionally, the applications of 19 F MRI probes in ions sensing, molecular structures analysis, cells tracking, and in vivo diagnosis of disease lesions are also covered in this article. From authors' perspectives, this review is able to provide inspirations for relevant researchers on designing and synthesizing advanced 19 F MRI probes.

An Fe complex for 19F magnetic resonance-based reversible redox sensing and multicolor imaging

We report a first-in-class responsive, pentafluorosulfanyl (-SF₅)-tagged ¹⁹F MRI agent capable of reversibly detecting reducing environments via an Fe^II/III redox couple. In the Fe^III form, the agent displays no ¹⁹F MR signal due to paramagnetic relaxation enhancement-induced signal broadening; however, upon rapid reduction to Fe^II with one equivalent of cysteine, the agent displays a robust ¹⁹F signal. Successive oxidation and reduction studies validate the reversibility of the agent. The -SF₅ tag in this agent enables 'multicolor imaging' in conjunction with sensors containing alternative fluorinated tags and this was demonstrated via simultaneous monitoring of the ¹⁹F MR signal of this -SF₅ agent and a hypoxia-responsive agent containing a -CF₃ group.

Multi-Channel Lanthanide Nanocomposites for Customized Synergistic Treatment of Orthotopic Multi-Tumor Cases

Simultaneous photothermal ablation of multiple tumors is limited by unpredictable photo-induced apoptosis, caused by individual intratumoral differences. Here, a multi-channel lanthanide nanocomposite was used to achieve tailored synergistic treatment of multiple subcutaneous orthotopic tumors under non-uniform whole-body infrared irradiation prescription. The nanocomposite reduces intratumoral glutathione by simultaneously activating the fluorescence and photothermal channels. The fluorescence provides individual information on different tumors, allowing customized prescriptions to be made. This enables optimal induction of hyperthermia and dosage of chemo drugs, to ensure treatment efficacy, while avoiding overtherapy. With an accessional therapeutic laser system, customized synergistic treatment of subcutaneous orthotopic cancer cases with multiple tumors is possible with both high efficacy and minimized side effects.

Development and application of animal models to study thyroid-associated ophthalmopathy

Thyroid-associated ophthalmopathy (TAO), also known as Graves' ophthalmopathy, is an autoimmune disease that is usually accompanied by hyperthyroidism. Its pathogenesis involves the activation of autoimmune T lymphocytes by a cross-antigen reaction of thyroid and orbital tissues. The thyroid-stimulating hormone receptor (TSHR) is known to play an important role in the development of TAO. Because of the difficulty of orbital tissue biopsy, the establishment of an ideal animal model is important for developing novel clinical therapies of TAO. To date, TAO animal modeling methods are mainly based on inducing experimental animals to produce anti-thyroid-stimulating hormone receptor antibodies (TRAbs) and then recruit autoimmune T lymphocytes. Currently, the most common methods are hTSHR-A subunit plasmid electroporation and hTSHR-A subunit adenovirus transfection. These animal models provide a powerful tool for exploring the internal relationship between local and systemic immune microenvironment disorders of the TAO orbit, facilitating the development of new drugs. However, existing TAO modeling methods still have some defects, such as low modeling rate, long modeling cycles, low repetition rate, and considerable differences from human histology. Hence, the modeling methods require further innovation, improvement, and in-depth exploration.

Metabolic fluorine labeling and hotspot imaging of dynamic gut microbiota in mice

Real-time localization and microbial activity information of indigenous gut microbiota over an extended period of time remains a challenge with existing visualizing methods. Here, we report a metabolic fluorine labeling (MEFLA)-based strategy for monitoring the dynamic gut microbiota via ¹⁹F magnetic resonance imaging (¹⁹F MRI). In situ labeling of different microbiota subgroups is achieved by using a panel of peptidoglycan-targeting MEFLA probes containing ¹⁹F atoms of different chemical shifts, and subsequent real-time in vivo imaging is accomplished by multiplexed hotspot ¹⁹F MRI with high sensitivity and unlimited penetration. Using this method, we realize extended visualization (>24 hours) of native gut microbes located at different intestinal sections and semiquantitative analysis of their metabolic dynamics modulated by various conditions, such as the host death and different β-lactam antibiotics. Our strategy holds great potential for noninvasive and real-time assessing of the metabolic activities and locations of the highly dynamic gut microbiota.

Fabrication of magnetic nanoprobes for ultrahigh-field magnetic resonance imaging

Ultrahigh-field magnetic resonance imaging (UHF-MRI) has been attracting tremendous attention in biomedical imaging owing to its high signal-to-noise ratio, superior spatial resolution, and fast imaging speed. However, at UHF-MRI, there is a lack of proper imaging probes that can impart superior imaging sensitivity of disease lesions because conventional contrast agents generally produce pronounced susceptibility artifacts and induce very strong T₂ decay effects, thus hindering satisfactory imaging performance. This review focused on the recent development of high-performance nanoprobes that can improve the sensitivity and specificity of UHF-MRI. Firstly, the contrast enhancement mechanism of nanoprobes at UHF-MRI has been elucidated. In particular, the strategies for modulating nanoprobe performance, including size effects, metal alloying and magnetic-dopant effects, surface effects, and stimuli-response regulation, have been comprehensively discussed. Furthermore, we illustrate the remarkable advances in the design of UHF-MRI nanoprobes for medical diagnosis, such as early-stage primary tumor and metastasis imaging, angiography, and dynamic monitoring of biosignaling factors in vivo. Finally, we provide a summary and outlook on the development of cutting-edge UHF-MRI nanoprobes for advanced biomedical imaging.

Fluorinated Ionic Liquid Based Multicolor 19 F MRI Nanoprobes for In Vivo Sensing of Multiple Biological Targets

Multicolor imaging, which maps the distribution of different targets, is important for in vivo molecular imaging and clinical diagnosis. Fluorine 19 magnetic resonance imaging (¹⁹ F MRI) is a promising technique because of unique insights without endogenous background or tissue penetration limit. Thus multicolor ¹⁹ F MRI probes, which can sense a wide variety of molecular species, are expected to help elucidate the biomolecular networks in complex biological systems. Here, a versatile model of activatable probes based on fluorinated ionic liquids (ILs) for multicolor ¹⁹ F MRI is reported. Three types of ILs at different chemical shifts are loaded in nanocarriers and sealed by three stimuli-sensitive copolymers, leading to "off" ¹⁹ F signals. The coating polymers specifically respond to their environmental stimuli, then degrade to release the loaded ILs, causing ¹⁹ F signals recovery. The nanoprobes are utilized for non-invasive detection of tumor hallmarks, which are distinguished by their individual colors in one living mouse, without interference between each other. This multicolor imaging strategy, which adopts modular construction of various ILs and stimuli-responsive polymers, will allow more comprehensive sensing of multiple biological targets, thus, opening a new realm in mechanistic understanding of complex pathophysiologic processes in vivo.

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.

Activatable 19 F MRI Nanoprobes for Visualization of Biological Targets in Living Subjects

Visualization of biological targets such as crucial cells and biomolecules in living subjects is critical for the studies of important biological processes. Though ¹ H magnetic resonance imaging (MRI) has demonstrated its power in offering detailed anatomical and pathological information, its capacity for in vivo tracking of biological targets is limited by the high biological background of ¹ H. ¹⁹ F distinguishes itself from its competitors as an exceptional complement to ¹ H in MRI through its high sensitivity, low biological background, and broad chemical shift range. The specificity and sensitivity of ¹⁹ F MRI can be further boosted with activatable nanoprobes. The advantages of ¹⁹ F MRI with activatable nanoprobes enable in vivo detection and imaging at the cellular or even molecular level in deep tissues, rendering this technique appealing as a potential solution for visualization of biological targets in living subjects. Here, recent progress over the past decades on activatable ¹⁹ F MRI nanoprobes made from three major ¹⁹ F-containing compounds, as well as present challenges and potential opportunities, are summarized to provide a panoramic prospective for the people who are interested in this emerging and exciting field.

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.

Accessing lanthanide-based, in situ illuminated optical turn-on probes by modulation of the antenna triplet state energy

Luminescent lanthanides possess ideal properties for biological imaging, including long luminescent lifetimes and emission within the optical window. Here, we report a novel approach to responsive luminescent Tb(iii) probes that involves direct modulation of the antenna excited triplet state energy. If the triplet energy lies too close to the 5D4 Tb(iii) excited state (20 500 cm-1), energy transfer to 5D4 competes with back energy transfer processes and limits lanthanide-based emission. To validate this approach, a series of pyridyl-functionalized, macrocyclic lanthanide complexes were designed, and the corresponding lowest energy triplet states were calculated using density functional theory (DFT). Subsequently, three novel constructs L3 (nitro-pyridyl), L4 (amino-pyridyl) and L5 (fluoro-pyridyl) were synthesized. Photophysical characterization of the corresponding Gd(iii) complexes revealed antenna triplet energies between 25 800 and 30 400 cm-1 and a 500-fold increase in quantum yield upon conversion of Tb(L3) to Tb(L4) using the biologically relevant analyte H2S. The corresponding turn-on reaction can be monitored using conventional, small-animal optical imaging equipment in presence of a Cherenkov radiation emitting isotope as an in situ excitation source, demonstrating that antenna triplet state energy modulation represents a viable approach to biocompatible, Tb-based optical turn-on probes.

Customized Photothermal Therapy of Subcutaneous Orthotopic Cancer by Multichannel Luminescent Nanocomposites

Photothermal therapy (PTT) is a potentially advanced strategy for highly precise cancer treatment. Tumor-microenvironment-activatable agents provide useful tools for PTT, but their photothermal conversion capacities vary and cannot be evaluated in vivo; thus, a general PTT prescription does not work with individual activatable agents. Here, glutathione (GSH)-activatable nanocomposites, silicomolybdate-functionalized NaLuF₄ :Yb,Er@NaLuF₄ @NaLuF₄ :Nd are prepared, for customized PTT of subcutaneous orthotopic cancer. By simultaneously determining intratumoral GSH concentration and the amount of accumulated agent using multiple orthogonal luminescent emissions of nanocomposites, near-infrared absorbance of photothermal conversion agents is evaluated in vivo, based on the optimized irradiating prescriptions (irradiating power density and time) established. This allows customized PTT of each individual case with high efficacy and viability. This work also includes a method for investigating individual intratumoral variation, and the development of the next generation of customized nanomedicine for efficacious PTT of subcutaneous orthotopic cancer.

Multiplex and In Vivo Optical Imaging of Discrete Luminescent Lanthanide Complexes Enabled by In Situ Cherenkov Radiation Mediated Energy Transfer

Recently, we pioneered the application of Cherenkov radiation (CR) of radionuclides for the in situ excitation of discrete Eu(III) and Tb(III) complexes. CR is produced by isotopes decaying under emission of charged particles in dielectric media and exhibits a maximum intensity below 400 nm. We have demonstrated that luminescent lanthanide antenna complexes are ideal acceptors for Cherenkov radiation-mediated energy transfer (CRET). Here, we develop and assess peptide-functionalized Tb(III) and Eu(III) complexes in conjunction with CRET excitation by the positron emissive radioisotope ¹⁸F for simultaneous, multiplexed imaging and in vivo optical imaging. This work demonstrates, for the first time, that the detection of the luminescence emission of a discrete Eu(III) complex in vivo is feasible. Our results open possibilities for discrete luminescent lanthanide complexes to be used as diagnostic, optical tools for the intrasurgical guidance of tumor resection.

The Design of Abnormal Microenvironment Responsive MRI Nanoprobe and Its Application

Magnetic resonance imaging (MRI) is often used to diagnose diseases due to its high spatial, temporal and soft tissue resolution. Frequently, probes or contrast agents are used to enhance the contrast in MRI to improve diagnostic accuracy. With the development of molecular imaging techniques, molecular MRI can be used to obtain 3D anatomical structure, physiology, pathology, and other relevant information regarding the lesion, which can provide an important reference for the accurate diagnosis and treatment of the disease in the early stages. Among existing contrast agents, smart or activatable nanoprobes can respond to selective stimuli, such as proving the presence of acidic pH, active enzymes, or reducing environments. The recently developed environment-responsive or smart MRI nanoprobes can specifically target cells based on differences in the cellular environment and improve the contrast between diseased tissues and normal tissues. Here, we review the design and application of these environment-responsive MRI nanoprobes.

Investigation of the Post-Synthetic Confinement of Fluorous Liquids Inside Mesoporous Silica Nanoparticles

Perfluorocarbon (PFC) filled nanoparticles are increasingly being investigated for various biomedical applications. Common approaches for PFC liquid entrapment involve surfactant-based emulsification and Pickering emulsions. Alternatively, PFC liquids are capable of being entrapped inside hollow nanoparticles via a postsynthetic loading method (PSLM). While the methodology for the PSLM is straightforward, the effect each loading parameter has on the PFC entrapment has yet to be investigated. Previous work revealed incomplete filling of the hollow nanoparticles. Changing the loading parameters was expected to influence the ability of the PFC to fill the core of the nanoparticles. Hence, it would be possible to model the loading mechanism and determine the influence each factor has on PFC entrapment by tracking the change in loading yield and efficiency of PFC-filled nanoparticles. Herein, neat PFC liquid was loaded into silica nanoparticles and extracted into aqueous phases while varying the sonication time, concentration of nanoparticles, volume ratio between aqueous and fluorous phases, and pH of the extraction water. Loading yields and efficiency were determined via ¹⁹F nuclear magnetic resonance and N₂ physisorption isotherms. Sonication time was indicated to have the strongest correlation to loading yield and efficiency; however, method validation revealed that the current model does not fully explain the loading capabilities of the PSLM. Confounding variables and more finely controlled parameters need to be considered to better predict the behavior and loading capacity by the PSLM and warrants further study.

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.

Synthesis of triethoxysilylated cyclen derivatives, grafting on magnetic mesoporous silica nanoparticles and application to metal ion adsorption

The synthesis through click chemistry of triethoxysilylated cyclen derivative-based ligands is described. Different methods were used such as the copper catalyzed Huisgen's reaction, or thiol–ene reaction for the functionalization of the cyclen scaffold with azidopropyltriethoxysilane or mercaptopropyltriethoxysilane, respectively. These ligands were then grafted on magnetic mesoporous silica nanoparticles (MMSN) for extraction and separation of Ni(ii) and Co(ii) metal ions from model solutions. The bare and ligand-modified MMSN materials revealed high adsorption capacity (1.0–2.13 mmol g⁻¹) and quick adsorption kinetics, achieving over 80% of the total capacity in 1–2 hours.

The adsorption of metal ions through ligand-functionalized magnetic mesoporous silica nanoparticles is described.

Responsive fluorinated nanoemulsions for 19F magnetic resonance detection of cellular hypoxia

We report two highly fluorinated Cu-based imaging agents, CuL1 and CuL2, for detecting cellular hypoxia as nanoemulsion formulations. Both complexes retained their initial quenched 19F MR signals due to paramagnetic Cu2+; however, both complexes displayed a large signal increase when the complex was reduced. DLS studies showed that the CuL1 nanoemulsion (NECuL1) had a hydrodiameter of approximately 100 nm and that it was stable for four weeks post-preparation. Hypoxic cells incubated with NECuL1 showed that 40% of the Cu2+ taken up was reduced in low oxygen environments.

Perfluorocarbons-Based 19F Magnetic Resonance Imaging in Biomedicine

Fluorine-19 (¹⁹F) magnetic resonance (MR) molecular imaging is a promising noninvasive and quantitative molecular imaging approach with intensive research due to the high sensitivity and low endogenous background signal of the ¹⁹F atom in vivo. Perfluorocarbons (PFCs) have been used as blood substitutes since 1970s. More recently, a variety of PFC nanoparticles have been designed for the detection and imaging of physiological and pathological changes. These molecular imaging probes have been developed to label cells, target specific epitopes in tumors, monitor the prognosis and therapy efficacy and quantitate characterization of tumors and changes in tumor microenvironment noninvasively, therefore, significantly improving the prognosis and therapy efficacy. Herein, we discuss the recent development and applications of ¹⁹F MR techniques with PFC nanoparticles in biomedicine, with particular emphasis on ligand-targeted and quantitative ¹⁹F MR imaging approaches for tumor detection, oxygenation measurement, smart stimulus response and therapy efficacy monitoring, et al.

Silica nanoparticle coated perfluorooctyl bromide for ultrasensitive MRI

MRI with hyperpolarized ¹²⁹Xe can achieve low-concentration detection. Herein, nanoparticle-coated perfluorooctyl bromide (PFOB) was developed as a ¹²⁹Xe MRI contrast agent with a moderate exchange rate, sufficient stability and feasible surface modification. The α_vβ₃ integrin overexpressed by non-small-cell lung cancer A549 cells was successfully detected by ¹²⁹Xe MRI with high specificity through adequate surface modifications.

A Fluorinated Ionic Liquid-Based Activatable 19F MRI Platform Detects Biological Targets

¹⁹F magnetic resonance imaging (¹⁹F MRI) is a promising technique for in vivo molecular imaging and clinical diagnosis, benefiting from its negligible background and unlimited tissue penetration depth. However, the development of ¹⁹F probes with good water solubility and versatile functions for bioresponsive and practical applications remains a challenge. Here, we report fluorinated ion liquids (ILs) as a new type of fluorine agents and build a fluorinated ionic liquid-based activatable ¹⁹F MRI platform (FILAMP), which relies on the phase transition of ILs. Upon exposure to environmental stimulation, coating polymer dissolves or degrades to release the fluorinated ILs payload, which rapidly enhances ¹⁹F signal. This "turn-on" response is verified by the successful detection of biological targets (for example, dysregulated pH and MMP overexpression) at the cellular level and in mice, demonstrating the potential of FILAMP as a robust activatable ¹⁹F probe for diagnosis and monitoring of biological and pathological processes.

19F Magnetic Resonance Activity-Based Sensing Using Paramagnetic Metals

Fluorine magnetic resonance imaging (¹⁹F MRI) is a promising bioimaging technique due to the favorable magnetic resonance properties of the ¹⁹F nucleus and the lack of detectable biological background signal. A range of imaging agents have been developed for this imaging modality including small molecule perfluorocarbons, fluorine-rich macromolecules and nanoparticles, and paramagnetic metal-containing agents. Incorporation of paramagnetic metals into fluorinated agents provides a unique opportunity to manipulate relaxation and chemical shift properties of ¹⁹F nuclei. Paramagnetic centers will enhance relaxation rates of nearby ¹⁹F nuclei through paramagnetic relaxation enhancement (PRE). Further, metals with anisotropic unpaired electrons can induce changes in ¹⁹F chemical shift through pseudocontact shift (PCS) effects. PRE and PCS are dependent on the nature of the metal center itself, the molecular scaffold surrounding it, and the position of the ¹⁹F nucleus relative to the metal center. One intriguing prospect in ¹⁹F magnetic resonance molecular imaging is to design responsive agents that can serve to provide a read out biological activity, including the activity of enzymes, redox activity, the activity of ions, etc. Paramagnetic agents are well suited for this activity-based sensing as metal complexes can be designed to respond to specific biological activities and give a corresponding ¹⁹F response that results from changes in the metal complex structure and subsequently PRE/PCS. Broadly speaking, when designing paramagnetic ¹⁹F MR biosensors, one can envision that in response to changes in analyte activity, the number of unpaired electrons of the metal changes or the ligand conformation/chemical composition changes. This Account highlights activity-based probes from the Que lab that harness paramagnetic metals to modulate ¹⁹F signal. We discuss probes that use conversion from Cu²⁺ to Cu⁺ in response to reducing environments to dequench the ¹⁹F MR signal. Probes in which oxidants convert Co²⁺ to Co³⁺, resulting in chemical shift responses, are also described. Finally, we explore our foray into using Ni²⁺ coordination switching to furnish probes with different ¹⁹F signals when they are converted between 4-coordinate square planar and higher coordination numbers. A major barrier for ¹⁹F MR molecular imaging is in vivo application, as signal sensitivity is relatively low, requiring long imaging times to detect imaging agents. Nanoparticle and macromolecular agents show promise due to their higher fluorine density and longer circulation times; however, their analyte scope is limited to analytes that induce cleavage events. A grand challenge for researchers in this area is adapting lessons learned from small molecule paramagnetic probes with promising in vitro activities for the development of probes with enhanced in vivo utility for basic biological and clinical applications.

Cascaded Multiresponsive Self-Assembled 19F MRI Nanoprobes with Redox-Triggered Activation and NIR-Induced Amplification

Molecular probes featuring promising capabilities including specific targeting, high signal-to-noise ratio, and in situ visualization of deep tissues are in great demand for tumor diagnosis and therapy. ¹⁹F magnetic resonance imaging (MRI) techniques incorporating stimuli-responsive probes are anticipated to be highly beneficial for specific detection and imaging of tumors because of negligible background and deep tissue penetration. Herein, we report a cascaded multiresponsive self-assembled nanoprobe, which enables sequential redox-triggered and near-infrared (NIR) irradiation-induced ¹⁹F MR signal activation/amplification for sensing and imaging. Specifically, we designed and synthesized a cascaded multiresponsive ¹⁹F-bearing nanoprobe based on the self-assembly of amphiphilic redox-responsive ¹⁹F-containing polymers and NIR-absorbing indocyanine green (ICG) molecules. It could realize the activation of ¹⁹F signals in the reducing tumor microenvironment and subsequent signal amplification via the photothermal process. This stepwise two-stage activation/amplification of ¹⁹F signals was validated by ¹⁹F NMR and MRI both in vitro and in vivo. The multiresponsive ¹⁹F nanoprobes capable of cascaded ¹⁹F signal activation/amplification and photothermal effect exertion can provide accurate sensing and imaging of tumors.

Reversible redox-responsive 1H/19F MRI molecular probes

The redox-responsive manganese(ii)/(iii) complexes serve as a pair of reversible probes for ¹H MRI and ¹⁹F MRI of biological redox species.

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 Glycopolymers as Reduction-responsive 19F MRI Agents for Targeted Imaging of Cancer

Imaging agents that can be targeted to specific diseases and respond to the microenvironment of the diseased tissue are of considerable interest due to their potential in diagnosing and managing diseases. Here we report a new class of branched fluorinated glycopolymers as ¹⁹F MRI contrast agents that respond to a reductive environment, for targeted imaging of cancer. The fluorinated glycopolymers can be readily prepared by a one-pot RAFT polymerization of glucose- and fluorine-containing monomers in the presence of a disulfide-containing cross-linking monomer. The incorporation of glucose units along the polymer chain enables these fluorinated glycopolymers to effectively target cancer cells due to interactions with the overexpressed sugar transporters present on the cell surface. In addition, the polymers exhibit an enhanced ¹⁹F MRI signal in response to a reductive environment, one of the unique hallmarks of many cancer cells, demonstrating their potential as promising candidates for targeted imaging of cancer.

A fluorinated bihydrazide conjugate for activatable sensing and imaging of hypochlorous acid by 19F NMR/MRI

A fluorinated bihydrazide conjugate as a ¹⁹F NMR/MRI probe with a “turn-on” character for activatable sensing and imaging of HClO was reported.

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.

pH-sensitive radiolabeled and superfluorinated ultra-small palladium nanosheet as a high-performance multimodal platform for tumor theranostics

Radiolabeled nanomaterials, especially those with ultra-small structures, have been the research focus in recent years, and thus may open up new prospects for clinical diseases theranostics. Herein, fluorinated Pd nanosheets labeled with Gd or radionuclides are developed as multimodal platforms for tumor theranostics. These nanomaterials decorated by functional polyethylene glycol demonstrate ultrahigh ¹⁹F MRI signal, ultrasmall size and good dispersibility. These ultrasmall materials exhibit good biocompatibility and easily to be modified for multimodal imaging (SPECT/MRI/PAI) by assembling the functional groups like building blocks. Furthermore, with high accumulation in tumor sites, under the guidance of multimodal imaging, combined photothermal therapy and radiotherapy are performed and synergistic effects are obtained. By comparing the in vivo behaviors of nanostructures labeled by different nuclides, the present study suggests the pH-sensitive radioiodinated Pd nanosheet which has unexpected T/NT ratio (>4-fold tumor-to-muscle ratio) in SPECT imaging and solves the critical high background issue of nanoprobes, could improve diagnostic accuracy and guide combination therapy. In summary, this functionalized nanoplatform with promising imaging and therapeutic efficacy has great potential for precision theranostic nanomedicines.

Fluorine Meets Amine: Reducing Microenvironment-Induced Amino-Activatable Nanoprobes for 19F-Magnetic Resonance Imaging of Biothiols

¹⁹F-magnetic resonance imaging (MRI) is of great significance for noninvasive imaging and detection of various diseases. However, the main obstacle in the application of ¹⁹F-MRI agents stems from the unmet signal sensitivity due to the poor water solubility and restricted mobility of segments with high number of fluorine atoms. Herein, we report a kind of intracellular reducing microenvironment-induced amino-activatable ¹⁹F-MRI nanoprobe, which can be used for specific imaging of biothiols. In principle, the nanoprobe has an initial architecture of hydrophobic core, where the trifluoromethyl-containing segments are compactly packed and ¹⁹F NMR/MRI signals are quenched ("OFF" state). Upon encountering sulfydryl, the strong electron-withdrawing 2,4-dinitrobenzenesulfonyl groups are excised to recover secondary amino groups, whose p K_a is proved to be 7.21. As a consequence, the molecular weight loss of the hydrophobic segment and the protonation of amino groups induce significant disturbance of hydrophilic/hydrophobic balance, leading to the disassembly of the nanoprobes and regain of spin-spin relaxation and ¹⁹F NMR/MRI signals ("ON" state, T₂ up to 296 ± 5.3 ms). This nanoprobe shows high sensitivity and selectivity to biothiols, enabling intracellular and intratumoral imaging of glutathione. Our study not only provides a new nanoprobe candidate for biothiols imaging in vivo but also a promising strategy for the molecular design of real water-soluble and highly sensitive ¹⁹F-MRI nanoprobes.

19F CEST imaging probes for metal ion detection

For detecting metal ions with ¹⁹F chemical exchange saturation transfer magnetic resonance imaging (¹⁹F CEST MRI), a class of novel fluorinated chelators with diverse fluorine contents and chelation properties were conveniently synthesized on gram scales. Among them, a DTPA-derived chelator with high sensitivity and selectivity was identified as a novel ¹⁹F CEST imaging probe for simultaneously detecting multiple metal ions.

Sensing caspase-1 activity using activatable 19F MRI nanoprobes with improved turn-on kinetics

Tandemly-repeated peptide design enhanced enzyme accessibility and cleavage efficiency of substrates on the nanoparticle surface, resulting in high MRI signal amplification.

Firefly Luciferin-Inspired Biocompatible Chemistry for Protein Labeling and In Vivo Imaging

Biocompatible reactions have emerged as versatile tools to build various molecular imaging probes that hold great promise for the detection of biological processes in vitro and/or in vivo. In this Minireview, we describe the recent advances in the development of a firefly luciferin-inspired biocompatible reaction between cyanobenzothiazole (CBT) and cysteine (Cys), and highlight its versatility to label proteins and build multimodality molecular imaging probes. The review starts from the general introduction of biocompatible reactions, which is followed by briefly describing the development of the firefly luciferin-inspired biocompatible chemistry. We then discuss its applications for the specific protein labeling and for the development of multimodality imaging probes (fluorescence, bioluminescence, MRI, PET, photoacoustic, etc.) that enable high sensitivity and spatial resolution imaging of redox environment, furin and caspase-3/7 activity in living cells and mice. Finally, we offer the conclusions and our perspective on the various and potential applications of this reaction. We hope that this review will contribute to the research of biocompatible reactions for their versatile applications in protein labeling and molecular imaging.

Novel Gd-Loaded Silicon Nanohybrid: A Potential Epidermal Growth Factor Receptor Expressing Cancer Cell Targeting Magnetic Resonance Imaging Contrast Agent

Continuing our research efforts in developing mesoporous silicon nanoparticle-based biomaterials for cancer therapy, we employed here porous silicon nanoparticles as a nanocarrier to deliver contrast agents to diseased cells. Nanoconfinement of small molecule Gd-chelates (L1-Gd) enhanced the T₁ contrast dramatically compared to distinct Gd-chelate (L1-Gd) by virtue of its slow tumbling rate, increased number of bound water molecules, and their occupancy time. The newly synthesized Gd-chelate (L1-Gd) was covalently grafted on silicon nanostructures and conjugated to an antibody specific for epidermal growth factor receptor (EGFR) via a hydrazone linkage. The salient feature of this nanosized contrast agent is the capability of EGFR targeted delivery to cancer cells. Mesoporous silicon nanoparticles were chosen as the nanocarrier because of their high porosity, high surface area, and excellent biodegradability. This type of nanosized contrast agent also performs well in high magnetic fields.