Silica‐Encapsulated Gd3+‐Aggregated Gold Nanoclusters for In Vitro and In Vivo Multimodal Cancer Imaging

Gd³⁺‐aggregated gold nanoclusters (AuNCs) encapsulated by silica shell (Gd³⁺‐A‐AuNCs@SiO₂ NPs) were strategically designed and prepared. The as‐prepared nanoparticles exhibit aggregation‐enhanced fluorescence (AEF), with an intensity that is up to 3.8 times that of discrete AuNCs. The clusters served as novel nanoprobes for in vitro and in vivo multimodal (fluorescence, magnetic resonance, and computed X‐ray tomography) cancer imaging     

Multifunctional Core–Shell Silica Nanoparticles for Highly Sensitive 19F Magnetic Resonance Imaging

¹⁹F magnetic resonance imaging (¹⁹F MRI) is useful for monitoring particular signals from biological samples, cells, and target tissues, because background signals are missing in animal bodies. Therefore, highly sensitive ¹⁹F MRI contrast agents are in great demand for their practical applications. However, we have faced the following challenges: 1) increasing the number of fluorine atoms decreases the solubility of the molecular probes, and 2) the restriction of the molecular mobility attenuates the ¹⁹F MRI signals. Herein, we developed novel multifunctional core–shell nanoparticles to solve these issues. They are composed of a core micelle filled with liquid perfluorocarbon and a robust silica shell. These core–shell nanoparticles have superior properties such as high sensitivity, modifiability of the surface, biocompatibility, and sufficient in vivo stability. By the adequate surface modifications, gene expression in living cells and tumor tissue in living mice were successfully detected by ¹⁹F MRI.     

Mesoporous silica nanoparticles for 19F magnetic resonance imaging,fluorescence imaging,and drug delivery

Multifunctional mesoporous silica nanoparticles (MSNs) are good candidates for multimodal applications in drug delivery, bioimaging, and cell targeting. In particular, controlled release of drugs from MSN pores constitutes one of the superior features of MSNs. In this study, a novel drug delivery carrier based on MSNs, which encapsulated highly sensitive ¹⁹F magnetic resonance imaging (MRI) contrast agents inside MSNs, was developed. The nanoparticles were labeled with fluorescent dyes and functionalized with small molecule-based ligands for active targeting. This drug delivery system facilitated the monitoring of the biodistribution of the drug carrier by dual modal imaging (NIR/¹⁹F MRI). Furthermore, we demonstrated targeted drug delivery and cellular imaging by the conjugation of nanoparticles with folic acid. An anticancer drug (doxorubicin, DOX) was loaded in the pores of folate-functionalized MSNs for intracellular drug delivery. The release rates of DOX from the nanoparticles increased under acidic conditions, and were favorable for controlled drug release to cancer cells. Our results suggested that MSNs may serve as promising ¹⁹F MRI-traceable drug carriers for application in cancer therapy and bio-imaging.     

Dual‐Frequency Calcium‐Responsive MRI Agents

Responsive or smart magnetic resonance imaging (MRI) contrast agents are molecular sensors that alter the MRI signal upon changes in a particular parameter in their microenvironment. Consequently, they could be exploited for visualization of various biochemical events that take place at molecular and cellular levels. In this study, a set of dual‐frequency calcium‐responsive MRI agents are reported. These are paramagnetic, fluorine‐containing complexes that produce remarkably high MRI signal changes at the ¹H and ¹⁹F frequencies at varying Ca²⁺ concentrations. The nature of the processes triggered by Ca²⁺ was revealed, allowing a better understanding of these complex systems and their further improvement. The findings indicate that these double‐frequency tracers hold great promise for development of novel functional MRI methods.     

Specific Detection and Imaging of Enzyme Activity by Signal‐Amplifiable Self‐Assembling 19F MRI Probes

Specific turn‐on detection of enzyme activities is of fundamental importance in drug discovery research, as well as medical diagnostics. Although magnetic resonance imaging (MRI) is one of the most powerful techniques for noninvasive visualization of enzyme activity, both in vivo and ex vivo, promising strategies for imaging specific enzymes with high contrast have been very limited to date. We report herein a novel signal‐amplifiable self‐assembling ¹⁹F NMR/MRI probe for turn‐on detection and imaging of specific enzymatic activity. In NMR spectroscopy, these designed probes are “silent” when aggregated, but exhibit a disassembly driven turn‐on signal change upon cleavage of the substrate part by the catalytic enzyme. Using these ¹⁹F probes, nanomolar levels of two different target enzymes, nitroreductase (NTR) and matrix metalloproteinase (MMP), could be detected and visualized by ¹⁹F NMR spectroscopy and MRI. Furthermore, we have succeeded in imaging the activity of endogenously secreted MMP in cultured media of tumor cells by ¹⁹F MRI, depending on the cell lines and the cellular conditions. These results clearly demonstrate that our turn‐on ¹⁹F probes may serve as a screening platform for the activity of MMPs.     

Organic Nanoprobes for Fluorescence and 19F Magnetic Resonance Dual‐Modality Imaging

Multimodal imaging techniques have been demonstrated to be greatly advantageous in achieving accurate diagnosis and gained increasing attention in recent decades. Herein, we present a new strategy to integrate the complementary modalities of ¹⁹F magnetic resonance imaging (¹⁹F MRI) and fluorescence imaging (FI) into a polymer nanoprobe composed of hydrophobic fluorescent organic core and hydrophilic fluorinated polymer shell. The alkyne‐terminated fluorinated copolymer (Pn) of 2,2,2‐trifluoroethyl acrylate (TFEA) and poly(ethylene glycol) methyl ether acrylate (PEGA) was first prepared via atom transfer radical polymerization (ATRP). The PEGA plays an important role in both improving ¹⁹F signal and modulating the hydrophilicity of Pn. The alkynyl tail in Pn is readily conjugated with azide modified tetra‐phenylethylene (TPE) through click chemistry to form azo polymer (TPE‐azo‐Pn). The core‐shell nanoprobes (TPE‐P3N) with an average particle size of 57.2 ± 8.8 nm are obtained via self‐assembly with ultrasonication in aqueous solution. These nanoprobes demonstrate high water stability, good biocompatibility, strong fluorescence and good ¹⁹F MRI performance, which present great potentials for simultaneous fluorescence imaging and ¹⁹F–MR imaging.     

Biomineralization of Versatile CuS/Gd2O3 Hybrid Nanoparticles for MR Imaging and Antitumor Photothermal Chemotherapy

The versatile application of nanoparticles in integrating imaging and therapy has aroused extensive research interest in precision medicine. Of the various nanoparticles that have been studied, CuS has shown great potential in the construction of multifunctional agents, owing to its excellent photothermal heating properties. Herein, we report a facile one‐pot biomineralization approach for the preparation of versatile bovine‐serum‐albumin‐conjugated CuS/Gd₂O₃ hybrid nanoparticles (BSA?CuS/Gd₂O₃ HNPs), which simultaneously possessed strong longitudinal relaxivity, an outstanding photothermal effect, high drug‐loading capacity, and pH/temperature‐responsive drug release. The versatile nanoparticles were used for magnetic resonance imaging (MRI) and antitumor photothermal chemotherapy, both in vitro and in vivo. In vivo MRI showed that the BSA?CuS/Gd₂O₃ HNPs had a long circulation time and effective passive tumor‐uptake ability. More importantly, combined in vitro and in vivo therapy demonstrated that drug‐loaded BSA?CuS/Gd₂O₃ HNPs offered outstanding synergistic therapeutic efficacy for tumor inhibition.     

High-Field MRI Contrast Agents and their Synergy with Optical Imaging: the Evolution from Single Molecule Probes towards Nano-architectures

Contrast agents for magnetic resonance imaging have historically been based on paramagnetic metal complexes, particularly Gd³⁺ chelates, which tend to lose their contrast enhancement ability with increasing magnetic field strength. Emerging high-field MRI applications require the development of novel contrast agents that exhibit high relaxation enhancement as a function of magnetic field strength. Paramagnetic ions such as Dy³⁺, Tb³⁺ or Ho³⁺ incorporated into supramolecular or inorganic nano-architectures represent promising platforms for the development of high field MRI contrast agents. Furthermore, such platforms allow facile inclusion of multiple imaging modalities, therapeutic loading, and targeting vectors. This Minireview examines the application of contrast agents for high-field MRI, which range from single molecules to nanoparticles. Approaches to create multimodal agents by combining high-field MRI contrast properties with another imaging modality are also discussed.     

GdIII-19F Distance Measurements for Proteins in Cells by Electron-Nuclear Double Resonance

Studies of protein structure and dynamics are usually carried out in dilute buffer solutions, conditions that differ significantly from the crowded environment in the cell. The double electron-electron resonance (DEER) technique can track proteins’ conformations in the cell by providing distance distributions between two attached spin labels. This technique, however, cannot access distances below 1.8 nm. Here, we show that Gd^III-¹⁹F Mims electron-nuclear double resonance (ENDOR) measurements can cover part of this short range. Low temperature solution and in-cell ENDOR measurements, complemented with room temperature solution and in-cell Gd^III-¹⁹F PRE (paramagnetic relaxation enhancement) NMR measurements, were performed on fluorinated GB1 and ubiquitin (Ub), spin-labeled with rigid Gd^III tags. The proteins were delivered into human cells via electroporation. The solution and in-cell derived Gd^III-¹⁹F distances were essentially identical and lie in the 1–1.5 nm range revealing that both, GB1 and Ub, retained their overall structure in the Gd^III and ¹⁹F regions in the cell.     

Paramagnetic Relaxation Enhancements in Acetate and Its Fluorine Derivatives Interacting with Gd3+: Complex Formation,Structure, and Transmetallation

The relative spatial distribution and motion with respect to Gd³⁺ of the ¹H and ¹⁹F nuclei in the acetate ion and its fluorine derivatives are studied in D₂O solutions through the paramagnetic relaxation rate enhancements (PREs) of these nuclei. We derive general theoretical expressions of the longitudinal PRE in terms of the analytical concentrations of metal and ligands, formation constants of the complexes, metal–nucleus distances, and coordination lifetimes of the ligands. The observed formation constants of the 1 metal: 1 ligand complexes markedly decrease with increasing number of fluorine atoms, the electronegativity of which reduces the negative partial charge of the coordinating COO^? group. The coordination lifetimes are very short at the scale of the relaxation times of the protons of metal bound acetate, that is, shorter than about 10 μs. The average distance of the acetate protons from Gd³⁺ is in fair agreement with independent crystallographic determination. The release of free Gd³⁺ from the very stable Gddtpa (dtpa=diethylene‐triaminepentaacetate) complex caused by the competition of Zn²⁺ for dtpa, is evidenced by an increase of the PREs with Zn²⁺ concentration. The observed PRE increase is consistent with the known equilibrium constants governing the speciation involving Gd³⁺, Zn²⁺, and dtpa. The present case study illustrates a method which easily yields experimental tunable properties suitable to test the ongoing theories of lanthanide Ln³⁺ complexation in solution.     

Detecting Enzyme Activities with Exogenous MRI Contrast Agents

This review focuses on exogenous magnetic resonance imaging (MRI) contrast agents that are responsive to enzyme activity. Enzymes can catalyze a change in water access, rotational tumbling time, the proximity of a ¹⁹F‐labeled ligand, the aggregation state, the proton chemical‐exchange rate between the agent and water, or the chemical shift of ¹⁹F, ³¹P, ¹³C or a labile ¹H of an agent, all of which can be used to detect enzyme activity. The variety of agents attests to the creativity in developing enzyme‐responsive MRI contrast agents.     

Aryl‐Phosphonate Lanthanide Complexes and Their Fluorinated Derivatives: Investigation of Their Unusual Relaxometric Behavior and Potential Application as Dual Frequency 1H/19F MRI Probes

A series of low molecular weight lanthanide complexes were developed that have high ¹H longitudinal relaxivities (r₁) and the potential to be used as dual frequency ¹H and ¹⁹F MR probes. Their behavior was investigated in more detail through relaxometry, pH‐potentiometry, luminescence, and multinuclear NMR spectroscopy. Fitting of the ¹H NMRD and ¹⁷O NMR profiles demonstrated a very short water residence lifetime (<10 ns) and an appreciable second sphere effect. At lower field strengths (20 MHz), two of the complexes displayed a peak in r₁ (21.7 and 16.3 mM ^?1 s^?1) caused by an agglomeration, that can be disrupted through the addition of phosphate anions. NMR spectroscopy revealed that at least two species are present in solution interconverting through an intramolecular binding process. Two complexes provided a suitable signal in ¹⁹F NMR spectroscopy and through the selection of optimized imaging parameters, phantom images were obtained in a MRI scanner at concentrations as low as 1 mM . The developed probes could be visualized through both ¹H and ¹⁹F MRI, showing their capability to function as dual frequency MRI contrast agents.     

The Role of Equilibrium and Kinetic Properties in the Dissociation of Gd[DTPA‐bis(methylamide)] (Omniscan) at near to Physiological Conditions

[Gd(DTPA‐BMA)] is the principal constituent of Omniscan, a magnetic resonance imaging (MRI) contrast agent. In body fluids, endogenous ions (Zn²⁺, Cu²⁺, and Ca²⁺) may displace the Gd³⁺. To assess the extent of displacement at equilibrium, the stability constants of DTPA‐BMA^3? complexes of Gd³⁺, Ca²⁺, Zn²⁺, and Cu²⁺ have been determined at 37 °C in 0.15 M NaCl. The order of these stability constants is as follows: GdL≈CuL>ZnL?CaL. Applying a simplified blood plasma model, the extent of dissociation of Omniscan (0.35 mM [Gd(DTPA‐BMA)]) was found to be 17 % by the formation of Gd(PO₄), [Zn(DTPA‐BMA)]^? (2.4 %), [Cu(DTPA‐BMA)]^? (0.2 %), and [Ca(DTPA‐BMA)]^? (17.7 %). By capillary electrophoresis, the formation of [Ca(DTPA‐BMA)]^? has been detected in human serum spiked with [Gd(DTPA‐BMA)] (2.0 mM ) at pH 7.4. Transmetallation reactions between [Gd(DTPA‐BMA)] and Cu²⁺ at 37 °C in the presence of citrate, phosphate, and bicarbonate ions occur by dissociation of the complex assisted by the endogenous ligands. At physiological concentrations of citrate, phosphate, and bicarbonate ions, the half‐life of dissociation of [Gd(DTPA‐BMA)] was calculated to be 9.3 h at pH 7.4. Considering the rates of distribution and dissociation of [Gd(DTPA‐BMA)] in the extracellular space of the body, an open two‐compartment model has been developed, which allows prediction of the extent of dissociation of the Gd^III complex in body fluids depending on the rate of elimination of the contrast agent.     

Cyclodextrin‐Based Bimodal Fluorescence/MRI Contrast Agents: An Efficient Approach to Cellular Imaging

A novel bimodal fluorescence/MRI probe based on a cyclodextrin scaffold has been synthesized and characterized. The final agent employs the fluorescein (F) functionality as a fluorescence marker and the Gd^III complex of a macrocyclic DOTA‐based ligand (GdL) having one aminobenzyl‐phosphinic acid pendant arm as an MRI probe, and has a statistical composition of (GdL)_6.9‐F_0.1‐β‐CD. Slow rotational dynamics (governed by a very rigid cyclodextrin scaffold) combined with fast water exchange (ensured by the chosen macrocyclic ligand) resulted in a high relaxivity of ～22 s^?1 mM ^?1 per Gd^III or ～150 s^?1 mM ^?1 per molecule of the final conjugate (20 MHz, 25 °C). In vitro labelling of pancreatic islets (PIs) and rat mesenchymal stem cells has been successfully performed. The agent is not cytotoxic and is easily internalized into cells. The labelled cells can be visualized by MRI, as proved by the detection of individual labelled PIs. A fluorescence study performed on mesenchymal stem cells showed that the agent stays in the intracellular space for a long time.     

19F Paramagnetic Relaxation Enhancement: A Valuable Tool for Distance Measurements in Proteins

Fluorine NMR paramagnetic relaxation enhancement was evaluated as a versatile approach for extracting distance information in selectively F‐labeled proteins. Proof of concept and initial applications are presented for the HIV‐inactivating lectin cyanovirin‐N. Single F atoms were introduced at the 4‐, 5‐, 6‐ or 7 positions of Trp49 and the 4‐position of Phe4, Phe54, and Phe80. The paramagnetic nitroxide spin label was attached to Cys residues that were placed into the protein at positions 50 or 52. ¹⁹F‐T₂ NMR spectra with different relaxation delays were recorded and the transverse ¹⁹F‐PRE rate, ¹⁹F‐Γ₂, was used to determine the average distance between the F nucleus and the paramagnetic center. Our data show that experimental ¹⁹F PRE‐based distances correspond to 0.93 of the ¹H_N‐PRE distances, in perfect agreement with the gyromagnetic γ¹⁹F/γ¹H ratio, thereby demonstrating that ¹⁹F PREs are excellent alternative parameters for quantitative distance measurements in selectively F‐labeled proteins.     

19F 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 ¹⁹F MRI multimodal probes in combination with optical fluorescence imaging (OFI), ¹H 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 ¹⁹F MRI probes compensate for the deficiency of individual techniques and offer improved sensitivity or accuracy of detection over unimodal counterparts. Strategies for designing ¹⁹F MRI multimodal probes are described with respect to their structure, physicochemical properties, biocompatibility, and the quality of images.     

Selective Anchoring of GdIII Chelates on the External Surface of Organo‐Modified Mesoporous Silica Nanoparticles: A New Chemical Strategy To Enhance Relaxivity

The optimization of the physico‐chemical properties of both Gd^III chelates and nanocarriers is of great importance for the development of effective nanosystems for magnetic resonance imaging (MRI) applications. With this aim, macrocyclic Gd^III chelates were selectively attached to the pendant amino groups exposed to the external surface of spheroidal mesoporous silica nanoparticles (MSNs). This was achieved by treating the metal complexes with MSNs that contained the templating surfactant molecules confined within the silica channels (hexadecyltrimethylammonium (CTA)/MSN), followed by extraction of the surfactant. The nanoparticles showed greatly improved ¹H relaxometric efficiency relative to corresponding systems that also feature Gd^III chelates conjugated inside the pores. A further significant relaxivity enhancement was observed after chemical transformation of the free amino groups into amides. The ionic relaxivity of the final nanoparticles (r_1p=79.1 mM ^?1 s^?1; 0.5 T, 310 K) is one of the highest reported so far.     

Preparation of Highly Efficient MRI Contrast Agents through Complexation of Cationic GdIII‐Containing Metallosurfactant with Biocompatible Polyelectrolytes

Novel contrast agents were developed through assembling of Gd^III‐containing metallosurfactant (MS) with biocompatible polyelectrolytes sodium hyaluronate (HA), heparinsodium (HS) and dextran sulfate sodium (DSS). The formed polyelectrolyte–surfactant complexes showed different structural patterns as the charge ratio increased, including spherical aggregates, rod‐like aggregates and network patterns in monovalent HA system, while spherical structures emerged in multivalent HS and DSS systems. Energy dispersive spectroscopy analysis and scanning electron microscopy mapping showed the presence of Gd^III in these complexes. Inductively coupled plasma atomic emission spectrometry was further used to quantify the contents of Gd^III in the assemblies. T1 magnetic resonance imaging showed that these Gd^III‐loaded complexes exhibited relaxivity of up to 63.81 mM ^?1 s^?1, much higher than that of Ominiscan (4.64 mM ^?1 s^?1). The cytotoxicity test in vitro demonstrated the excellent biocompatibility of these complexes, which is essential for clinical application.     

A Chemical Strategy for the Relaxivity Enhancement of GdIII Chelates Anchored on Mesoporous Silica Nanoparticles

Functionalised MCM‐41 mesoporous silica nanoparticles were used as carriers of Gd^III complexes for the development of nanosized magnetic resonance imaging contrast agents. Three Gd^III complexes based on the 1,4,7,10‐tetraazacyclododecane scaffold (DOTA; monoamide‐, DOTA‐ and DO3A‐like complexes) with distinct structural and magnetic properties were anchored on the silica nanoparticles functionalised with NH₂ groups. The interaction between Gd^III chelates and surface functional groups markedly influenced the relaxometric properties of the hybrid materials, and were deeply modified passing from ionic ? NH₃⁺ to neutral amides. A complete study of the structural, textural and surface properties together with a full ¹H relaxometric characterisation of these hybrid materials before and after surface modification was carried out. Particularly for the anionic complex 2 attached to MCM‐41, an impressive increase in relaxivity (r_1p) was observed (from 20.3 to 37.8 mM ^?1 s^?1, 86.2 % enhancement at 20 MHz and 310 K), mainly due to a threefold faster water exchange rate after acetylation of the surface ? NH₃⁺ ions. This high r_1p value, coupled with the large molar amount of grafted 2 onto the silica nanoparticles gives rise to a value of relaxivity per particle of 29 500 mM ^?1 s^?1, which possibly allows it to be used in molecular imaging procedures. Smaller changes were observed for the hybrid materials based on neutral 1 and 3 complexes. In fact, whereas 1 shows a weak interaction with the surface and acetylation induced only some decrease of the local rotation, complex 3 appears to be involved in a strong interaction with surface silanols. This results in the displacement of a coordinated water molecule and in a decrease of the accessibility of the solvent to the metal centre, which is unaffected by the modification of ammonium ions to neutral amides.     

Gadolinium(III)-Based Dual 1H/19F Magnetic Resonance Imaging Probes

We present two novel octadentate cyclen-based ligands bearing one (L¹) or two (L²) phenylacetamide pendants with two CF₃ groups either at positions 3 and 5 (L¹) or 4 (L²). The corresponding Gd³⁺ complexes possess one coordinated water molecule, as confirmed by luminescence lifetime measurements on the Eu^III and Tb^III analogues. A detailed ¹H and ¹⁷O relaxometric characterization has revealed the parameters that govern the relaxivities of these complexes. The water-exchange rate of the mono-amide derivative GdL¹ (k_ex²⁹⁸=1.52×10⁶ s⁻¹) is faster than that determined for the bis-amide complex GdL² (k_ex²⁹⁸=0.73×10⁶ s⁻¹). ¹H and ¹⁹F NMR studies have indicated that the complexes are present in solution almost exclusively as the square-antiprismatic (SAP) isomers. ¹⁹F NMR relaxation studies indicated Gd ⋅⋅⋅ F distances of 7.4±0.1 and 9.1±0.1 Å for GdL¹ and GdL², respectively. Phantom MRI studies revealed the favorable properties of GdL² as a dual ¹H/¹⁹F magnetic resonance imaging (MRI) probe, whereas the shorter Gd ⋅⋅⋅ F distance of GdL¹ reduces the signal-to-noise ratio due to the very short transverse relaxation time of the ¹⁹F NMR signal.