Colloidal processes such as nucleation, growth, ripening, and dissolution are fundamental to the synthesis and application of engineered nanoparticles, as well as numerous natural systems. In ...nanocolloids consisting of a dispersion of nanoparticles in solution, colloidal stability is influenced by factors including the particle surface facet and capping layer, and local temperature, chemistry, and acidity. In this paper, we investigate colloidal stability through the real-time manipulation of nanoparticles using in situ liquid cell Scanning Transmission Electron Microscopy (STEM). In a distribution of uniform iron oxide nanoparticles, we use the electron beam to precisely control the local chemistry of the solution and observe the critical role that surface chemistry plays in nanoparticle stability. By functionalizing the nanoparticle surfaces with charged amino acids and peptides, stability can be tuned to promote dissolution, growth, or agglomeration, either permanently or reversibly. STEM imaging is used to quantify kinetics of individual nanoparticles subject to local variations in chemistry. These measurements of dissolution and growth rates of iron oxide nanoparticles provide insights into nanoparticle stability relevant to synthesis and functionalization for biomedical applications.
We demonstrate ground state tunability for a hybrid artificial spin ice composed of Fe nanomagnets which are subject to site-specific exchange-bias fields, applied in integer multiples of the lattice ...along one sublattice of the classic square artificial spin ice. By varying this period, three distinct magnetic textures are identified: a striped ferromagnetic phase; an antiferromagnetic phase attainable through an external field protocol alone; and an unconventional ground state with magnetically charged pairs embedded in an antiferromagnetic matrix. Monte Carlo simulations support the results of field protocols and demonstrate that the pinning tunes relaxation timescales and their critical behavior.
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•Developed a nanoparticle growth model from fundamental equations.•This model bridges the two limiting cases of currently accepted model.•This model comprehensively describes ...nanoparticle growth and provides new insights.•Established the required conditions for fabricating monodisperse nanoparticles.
A nanoparticle growth model is developed to predict and guide the syntheses of monodisperse colloidal nanoparticles in the liquid phase. The model, without any a priori assumptions, is based on the Fick’s law of diffusion, conservation of mass and the Gibbs–Thomson equation for crystal growth. In the limiting case, this model reduces to the same expression as the currently accepted model that requires the assumption of a diffusion layer around each nanoparticle. The present growth model bridges the two limiting cases of the previous model i.e. complete diffusion controlled and adsorption controlled growth of nanoparticles. Specifically, the results show that a monodispersion of nanoparticles can be obtained both with fast monomer diffusion and with surface reaction under conditions of small diffusivity to surface reaction constant ratio that results is growth ‘focusing’. This comprehensive description of nanoparticle growth provides new insights and establishes the required conditions for fabricating monodisperse nanoparticles critical for a wide range of applications.
Sensitivity and spatial resolution in Magnetic Particle Imaging are affected by magnetic properties of the nanoparticle tracers used during imaging. Here, we have carried out a comprehensive magnetic ...characterization of single-core iron oxide nanoparticles that were designed for MPI. We used ac susceptometry, fluxgate magnetorelaxometry, and magnetic particle spectroscopy to evaluate the tracer's magnetic core size, hydrodynamic size, and magnetic anisotropy. Our results present a self-consistent set of magnetic and structural parameters for the tracers that is consistent with direct measurements of size using transmission electron microscopy and dynamic light scattering and that can be used to better understand their MPI performance.
Emergency room visits due to traumatic brain injury (TBI) is common, but classifying the severity of the injury remains an open challenge. Some subjective methods such as the Glasgow Coma Scale ...attempt to classify traumatic brain injuries, as well as some imaging based modalities such as computed tomography and magnetic resonance imaging. However, to date it is still difficult to detect and monitor mild to moderate injuries. In this report, we demonstrate that the magnetic particle imaging (MPI) modality can be applied to imaging TBI events with excellent contrast. MPI can monitor injected iron nanoparticles over long time scales without signal loss, allowing researchers and clinicians to monitor the change in blood pools as the wound heals.
Abstract Magnetic Particle Imaging (MPI) is a novel non-invasive biomedical imaging modality that uses safe magnetite nanoparticles as tracers. Controlled synthesis of iron oxide nanoparticles (NPs) ...with tuned size-dependent magnetic relaxation properties is critical for the development of MPI. Additional functionalization of these NPs for other imaging modalities ( e.g . MRI and fluorescent imaging) would accelerate screening of the MPI tracers based on their in vitro and in vivo performance in pre-clinical trials. Here, we conjugated two different types of poly-ethylene-glycols (NH2 -PEG-NH2 and NH2 -PEG-FMOC) to monodisperse carboxylated 19.7 nm NPs by amide bonding. Further, we labeled these NPs with Cy5.5 near infra-red fluorescent (NIRF) molecules. Bi-functional PEG (NH2 -PEG-NH2 ) resulted in larger hydrodynamic size (∼98 nm vs. ∼43 nm) of the tracers, due to inter-particle crosslinking. Formation of such clusters impacted the multimodal imaging performance and pharmacokinetics of these tracers. We found that MPI signal intensity of the tracers in blood depends on their plasmatic clearance pharmacokinetics. Whole body mice MPI/MRI/NIRF, used to study the biodistribution of the injected NPs, showed primary distribution in liver and spleen. Biodistribution of tracers and their clearance pathway was further confirmed by MPI and NIRF signals from the excised organs where the Cy5.5 labeling enabled detailed anatomical mapping of the tracers.in tissue sections. These multimodal MPI tracers, combining the strengths of each imaging modality ( e.g . resolution, tracer sensitivity and clinical use feasibility) pave the way for various in vitro and in vivo MPI applications.
In magnetic particle imaging (MPI), the changing magnetization of magnetic nanoparticle (MNP) tracers subjected to an alternating magnetic field is detected. The physical properties of the MNP ...tracers have a direct effect on the quality of the resulting signal. In order to improve MPI image resolution and sensitivity, optimizing these properties, in particular the MNP core size, is essential. In this work, we investigate the existence of an optimal MNP core size for MPI using stochastic simulations of Langevin equations, supported by magnetic particle spectroscopy (MPS) measurements of highly monodisperse single-core nanoparticles with carefully tailored core sizes. We demonstrate that once the MNP core diameter exceeds around 28 nm (with the exact value depending on applied field properties and non-magnetic nanoparticle coating), relaxation effects will begin to dominate. Furthermore, as nanoparticle size is increased, interparticle interactions make it difficult to stabilize the particles in water and maintain their monodispersity. Taken together, we conclude that 28 nm in core diameter is an optimal size for single-core, monodisperse, magnetite particles used in MPI.
We report a combined experimental and computational study on the critical role of surfactants in the nucleation and growth of Co nanoparticles synthesized by chemical routes. By varying the ...surfactant species, Co nanoparticles of different morphologies under similar reaction conditions (e.g., temperature and Co−precursor concentration) were produced. Depending on the surfactant species, the growth of Co nanoparticles followed three different growth pathways. For example, with surfactants oleic acid (OA) and trioctylphosphine oxide (TOPO) used in combination, Co nanoparticles followed a diffusional growth pathway, leading to single crystalline nanoparticles. Multiple-grained nanoparticles, through an aggregation process, were formed with the combination of surfactants OA and dioctylamine (DOA). Further, an Ostwald ripening process was observed in the case of TOPO alone. Complementary electronic structure calculations were used to predict the optimized Co−surfactant complex structures and to quantify the binding energy between the surfactants (ligands) and the Co atoms. These calculations were further applied to predict the Co nanoparticle nucleation and growth processes based on the stability of Co−surfactant complexes.
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