Yuping Bao

Water-Soluble Iron Oxide Nanoparticles with High Stability and Selective Surface Functionality

The water dispensability and stability of high quality iron oxide nanoparticles synthesized in organic solvents are major issues for biomedical and biological applications. In this paper, a versatile approach for preparing water-soluble iron oxide nanoparticles with great stability and selective surface functionality (-COOH, -NH(2), or -SH) was demonstrated. The hydrophobic nanoparticles were first synthesized by the thermal decomposition of an iron oleate complex in organic solvent. Subsequently, the hydrophobic coatings of nanoparticles were replaced with poly(acrylic acid) , polyethylenimine, or glutathione, yielding charged nanoparticles in aqueous solution. Two parameters were found to be critical for obtaining highly stable nanoparticle dispersions: the original coating and the surfactant-to-nanoparticle ratio. These charged nanoparticles exhibited different stabilities in biological buffers, which were directly influenced by the surface coatings. This report will provide significant practical value in exploring the biological or biomedical applications of iron oxide nanoparticles.

Magnetochromatic Polydiacetylene by Incorporation of Fe3O4 Nanoparticles

Conjugated polymers have been explored for a broad spectrum of applications, particularly in optoelectronic and sensing materials, because the p-electron delocalization in the backbones provides them with intriguing electronic and optical properties. For example, polydiacetylene (PDA) exhibits an intense chromatic transition, typically from blue to red, in response to various external stimuli, such as temperature, pH, ion, solvent, and ligand interaction. This colorimetric change can be easily perceived by the naked eye, making PDA an ideal candidate for sensing applications. The color changes of PDAs are caused by the shortening of the effective PDA conjugation length as a result of changes in the conformation of the polymer backbone under stimuli. To improve their practical applications, increasing interest has been extensively attracted to explore PDAs with new sensing functionalities. For example, electronand light-induced PDA/carbon nanotube fibers and PDA/azobenzene nanomaterials were recently discovered by the groups of Peng and Zou, 16] respectively. The common and critical strategy is to form nanocomposites by introducing the second phases with specifically designed structures and functionalities. However, to the best of our knowledge, no magnetisminduced chromatism has ever been realized in PDA. However, magnetochromatic PDA materials could be advantageous for many applications ranging from small devices to aircraft as they are low-cost in fabrication, simple in structure, high-efficiency in sensitivity, and convenient and safe in operation. A possible approach to fabricate magnetochromatic PDA systems is to introduce other nanomaterials that may induce PDA conformational changes upon exposure to a magnetic field. Iron oxide nanoparticles have been widely studied for their superparamagnetic properties in recent years and represent one of the ideal candidates to meet this requirement. Herein, we first incorporated Fe3O4 nanoparticles into PDA to produce high-quality composites that change colors in AC magnetic field through a ready and efficient selfassembly process. The synthetic procedures are shown in Figure 1. Diacetylenic monomers are first connected to the

The critical role of surfactants in the growth of cobalt nanoparticles.

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.

Synthesis and Growth Mechanism of Iron Oxide Nanowhiskers

Iron oxide nanowhiskers with dimensions of approximately 2 × 20 nm were successfully synthesized by selectively heating an iron oleate complex. Such nanostructures resulted from the difference in the ligand coordination microenvironments of the Fe(III) oleate complex, according to our electronic structure calculations and thermogravimetric analysis. A ligand-directed growth mechanism was subsequently proposed to rationalize the growth process. The formation of the nanowhiskers provides a unique example of shape-controlled nanostructures, offering additional insights into nanoparticle synthesis.

Superstructures of self-assembled cobalt nanocrystals

Uniform three-dimensional superstructures of spherical cobalt nanocrystals are produced by the interplay between dipolar interaction and applied magnetic field. An anomalous low-temperature magnetic behavior is observed, indicating that uncompensated surface spins become ordered below 10 K, as evidenced by the presence of two magnetic phases that superimpose in hysteresis loops as compared to measurements at 20 K. The approach discussed here provides a framework for applications such as high-performance mesomagnets, microelectronic and magnetic devices fabrication, and can be extended to other nanocomposite materials fabrication if cobalt particles can act as carriers for other nanoparticles.

Controlled self-assembly of colloidal cobalt nanocrystals

Spherical cobalt particles are synthesized by injecting stock solution (0.54 g Co2(CO)8 dissolved in 3ml 1,2-dichlorobenzene) into refluxing 1,2-dichlorobenzene at 182 C in the presence of 0.2ml oleic acid and 0.1 g TOPO. Disks are prepared in a similar manner except 0.34 g linear amines are substituted for TOPO. We are developing a precipitation technique that is a variation of the solvent–non-solvent pair method, in which mixtures of solvents (toluene, hexane, dichlorobenzene) and non-solvents (methanol, butanol) with different boiling temperatures produce an evaporation rate gradient. This technique allows particles to remain in solution with sufficient thermal energy to slowly form highly ordered structures. The non-solvent mixture is added drop wise to a dilute solution of particles, which precipitate onto a SiO or amorphous carbon TEM film. This method produces 2D arrays most of the time. There is growing interest in organizing materials on the nanometer scale for fundamental investigations [1–3] and novel technological applications [5,6]. One of the most promising routes to create highly ordered structures on the nanometer scale is by the self-assembly of nanoscale building blocks comprised of colloidal nanocrystals [7,8]. However, the mechanisms influencing the self-assembly of such nanocrystals are not understood [9] and there is a critical need to develop general approaches for predicting and controlling this behavior. Here, using surfactant-stabilized Co nanocrystals [10] with narrow size distributions and controlled shapes as building blocks, we describe our ability to finely tune their selfassembled arrays. We can selectively achieve square packing, hexagonal close packing, linear chains, arrays spatially segregated as a function of particle size and lyotropic liquid-crystal-like arrays with orientation order. This richness in selfassembly is obtained in a single system as a function of size and shape, and in which one of a set of competing forces (steric, van der Waals, depletion, entropy or magnetostatic) dominates to determine the resulting organization. It is well known that colloidal nanoparticles suspended in a liquid will spontaneously self-assemble into highly ordered, close-packed hexagonal arrays when their packing densities approach a critical value [1,11]. This process is understood in terms of an entropy-driven, first order, liquid–solid phase transition in the limit of a hard sphere system [2,12]. Similar experiments, however, performed in microgravity [13] reveal significant differences from terrestrial experiments suggesting the possibility of using competing forces of similar magnitudes to influence the selfassembly process. In addition to such solvent

A general approach to synthesis of nanoparticles with controlled morphologies and magnetic properties

We present a systematic approach to fabricate a variety of magnetic nanoparticles with desirable structure and controlled magnetic properties based on our studies of the process kinetics. The morphology of binary alloy particles is dependent on their bulk thermodynamics—for immiscible heterogeneous systems (Co–Au) core-shell structures are obtained while miscible systems (Fe–Pt) lead to alloy nanoparticles. The annealing effects on FePt nanoparticles show that the coercivity and magnetic anisotropy increase dramatically after annealing at temperatures above 650°C. Studies of Co–Au core-shell structure show that the core is magnetic, but the Au shell does not significantly affect its magnetic properties.

Magnetic nanoparticles: material engineering and emerging applications in lithography and biomedicine

We present an interdisciplinary overview of material engineering and emerging applications of iron oxide nanoparticles. We discuss material engineering of nanoparticles in the broadest sense, emphasizing size and shape control, large-area self-assembly, composite/hybrid structures, and surface engineering. This is followed by a discussion of several nontraditional, emerging applications of iron oxide nanoparticles, including nanoparticle lithography, magnetic particle imaging, magnetic guided drug delivery, and positive contrast agents for magnetic resonance imaging. We conclude with a succinct discussion of the pharmacokinetics pathways of iron oxide nanoparticles in the human body—an important and required practical consideration for any in vivo biomedical application, followed by a brief outlook of the field.

Surface charge and dosage dependent potential developmental toxicity and biodistribution of iron oxide nanoparticles in pregnant CD-1 mice

Iron oxide nanoparticles have attracted much attention because of their potential applications, such as drug delivery, biomedical imaging, and photocatalysis. Due to their small size and the potential to cross the placental barrier, the risk to pregnant women and the developing fetus from exposure to nanoparticles is of great concern. The developmental toxicity and biodistribution of a single dose versus multiple doses of iron oxide nanoparticles with positive or negative surface charges were investigated in vivo. Multiple doses of positively-charged nanoparticles given over several days resulted in significantly increased fetal deaths and accumulation of iron in the fetal liver and placenta. These results indicate both positively and negatively charged iron oxide nanoparticles have the ability to cross the placenta and accumulate in the fetus, though greater bioaccumulation and toxicity was observed with a positively-charged surface coating.

Brownian magnetic relaxation of water-based cobalt nanoparticle ferrofluids

Following the synthesis of monodispersed 20nm cobalt nanoparticles via a thermal decomposition method we have successfully transferred these hydrophobic nanoparticles into the water phase using tetramethylamonium hydroxide pentahydrate as phase transfer agent and 12-aminododecanoic acid as a stabilizing agent. Frequency dependent ac susceptibility of water-based cobalt nanoparticle ferrofluids was measured at room temperature in the frequency range of 0.01–1000Hz. In addition to the “high” frequency magnetic relaxation peak at 200Hz, which is determined by the solvent viscosity and hydrodynamic volume of the nanoparticles, a lower frequency relaxation peak attributed to the interaction of surface coatings is observed. The position of the latter varies with the solution pH in the range of 0.02–0.05Hz. This variation is explained as due to the change of chemical and charge state of the surfactant molecules. The peak at low frequency may be potentially used to study protein configuration changes in solution ...