Magnetic Sensing Potential of Fe3O4 Nanocubes Exceeds That of Fe3O4 Nanospheres.

This paper highlights the relation between the shape of iron oxide (Fe3O4) particles and their magnetic sensing ability. We synthesized Fe3O4 nanocubes and nanospheres having tunable sizes via solvothermal and thermal decomposition synthesis reactions, respectively, to obtain samples in which the volumes and body diagonals/diameters were equivalent. Vibrating sample magnetometry (VSM) data showed that the saturation magnetization (Ms) and coercivity of 100-225 nm cubic magnetic nanoparticles (MNPs) were, respectively, 1.4-3.0 and 1.1-8.4 times those of spherical MNPs on a same-volume and same-body diagonal/diameter basis. The Curie temperature for the cubic Fe3O4 MNPs for each size was also higher than that of the corresponding spherical MNPs; furthermore, the cubic Fe3O4 MNPs were more crystalline than the corresponding spherical MNPs. For applications relying on both higher contact area and enhanced magnetic properties, higher-Ms Fe3O4 nanocubes offer distinct advantages over Fe3O4 nanospheres of the same-volume or same-body diagonal/diameter. We evaluated the sensing potential of our synthesized MNPs using giant magnetoresistive (GMR) sensing and force-induced remnant magnetization spectroscopy (FIRMS). Preliminary data obtained by GMR sensing confirmed that the nanocubes exhibited a distinct sensitivity advantage over the nanospheres. Similarly, FIRMS data showed that when subjected to the same force at the same initial concentration, a greater number of nanocubes remained bound to the sensor surface because of higher surface contact area. Because greater binding and higher Ms translate to stronger signal and better analytical sensitivity, nanocubes are an attractive alternative to nanospheres in sensing applications.

Near-infrared-responsive, superparamagnetic Au@Co nanochains.

This manuscript describes a new type of nanomaterial, namely superparamagnetic Au@Co nanochains with optical extinctions in the near infrared (NIR). The Au@Co nanochains were synthesized via a one-pot galvanic replacement route involving a redox-transmetalation process in aqueous medium, where Au salt was reduced to form Au shells on Co seed templates, affording hollow Au@Co nanochains. The Au shells serve not only as a protective coating for the Co nanochain cores, but also to give rise to the optical properties of these unique nanostructures. Importantly, these bifunctional, magneto-optical Au@Co nanochains combine the advantages of nanophotonics (extinction at ca. 900 nm) and nanomagnetism (superparamagnetism) and provide a potentially useful new nanoarchitecture for biomedical or catalytic applications that can benefit from both activation by light and manipulation using an external magnetic field.

Synthesis of Hexagonal FeMnP Thin Films from a Single-Source Molecular Precursor.

The first heterobimetallic phosphide thin film containing iron, manganese, and phosphorus, derived from the single-source precursor FeMn(CO)8 (µ-PH2 ), has been prepared using a home-built metal-organic chemical vapor deposition apparatus. The thin film contains the same ratio of iron, manganese, and phosphorus as the initial precursor. The film becomes oxidized when deposited on a quartz substrate, whereas the film deposited on an alumina substrate provides a more homogeneous product. Powder X-ray diffraction confirms the formation of a metastable, hexagonal FeMnP phase that was previously only observed at temperatures above 1200 °C. Selected area electron diffraction on single crystals isolated from the films was indexed to the hexagonal phase. The effective moment of the films (µeff =3.68 µB ) matches the previously reported theoretical value for the metastable hexagonal phase, whereas the more stable orthorhombic phase is known to be antiferromagnetic. These results not only demonstrate the successful synthesis of a bimetallic, ternary thin film from a single-source precursor, but also the first low temperature approach to the hexagonal phase of FeMnP.

Rapid Growth of Zinc Oxide Nanotube-Nanowire Hybrid Architectures and Their Use in Breast Cancer-Related Volatile Organics Detection.

A simple direct method for the rapid fabrication of zinc oxide nanotube-nanowire hybrid structure in an environmentally friendly way is described here. Zinc foils were anodized in an aqueous solution of washing soda and baking soda at room temperature in order to obtain the hybrid architecture. At the beginning of the process nanowires were formed on the substrate. The wider nanowires transformed into nanotubes in about a minute and grew in length with time. The morphological integrity was maintained upon heat treatment at temperatures up to the melting point of the substrate (∼400 °C) except that the nanotube wall became porous. The chemiresistor devices fabricated using the heat-treated structure exhibited high response to low-concentration volatile organic compounds that are considered markers for breast cancer. The response was not significantly affected by high humidity or presence of hydrogen, methane, or carbon dioxide. The devices are expected to find use as breath sensors for noninvasive early detection of breast cancer.

Silver-Free Gold Nanocages with Near-Infrared Extinctions.

This article reports the preparation of silver-free Au nanocages from cubic palladium templates. Pd nanocubes were subjected to galvanic replacement with Au3+ to produce Pd@Au nanocages having tunable dimensions (i.e., edge length, gold layer thickness, and hollow pore size), which allowed selectable positioning of the optical extinction maxima from the visible to the near infrared. These new nanocages circumvent the problems associated with previous Ag-derived gold alloy nanocages, which suffer from the toxicity of residual silver and the possible fragmentation of such alloyed nanostructures, thereby limiting their potential applications. In contrast, the present materials represent stable, nontoxic, tunable, and hollow plasmonic nanostructures.

Enzymatic synthesis of magnetic nanoparticles.

We report the first in vitro enzymatic synthesis of paramagnetic and antiferromagnetic nanoparticles toward magnetic ELISA reporting. With our procedure, alkaline phosphatase catalyzes the dephosphorylation of l-ascorbic-2-phosphate, which then serves as a reducing agent for salts of iron, gadolinium, and holmium, forming magnetic precipitates of Fe45±14Gd5±2O50±15 and Fe42±4Ho6±4O52±5. The nanoparticles were found to be paramagnetic at 300 K and antiferromagnetic under 25 K. Although weakly magnetic at 300 K, the room-temperature magnetization of the nanoparticles found here is considerably greater than that of analogous chemically-synthesized LnxFeyOz (Ln = Gd, Ho) samples reported previously. At 5 K, the nanoparticles showed a significantly higher saturation magnetization of 45 and 30 emu/g for Fe45±14Gd5±2O50±15 and Fe42±4Ho6±4O52±5, respectively. Our approach of enzymatically synthesizing magnetic labels reduces the cost and avoids diffusional mass-transfer limitations associated with pre-synthesized magnetic reporter particles, while retaining the advantages of magnetic sensing.

Bismuth@US-tubes as a Potential Contrast Agent for X-ray Imaging Applications.

The encapsulation of bismuth as BiOCl/Bi2O3 within ultra-short (ca. 50 nm) single-walled carbon nanocapsules (US-tubes) has been achieved. The Bi@US-tubes have been characterized by high-resolution transmission electron microscopy (HR-TEM), energy-dispersive X-ray spectroscopy (EDS), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Bi@US-tubes have been used for intracellular labeling of pig bone marrow-derived mesenchymal stem cells (MSCs) to show high X-ray contrast in computed tomography (CT) cellular imaging for the first time. The relatively high contrast is achieved with low bismuth loading (2.66% by weight) within the US-tubes and without compromising cell viability. X-ray CT imaging of Bi@US-tubes-labeled MSCs showed a nearly two-fold increase in contrast enhancement when compared to unlabeled MSCs in a 100 kV CT clinical scanner. The CT signal enhancement from the Bi@US-tubes is 500 times greater than polymer-coated Bi2S3 nanoparticles and several-fold that of any clinical iodinated contrast agent (CA) at the same concentration. Our findings suggest that the Bi@US-tubes can be used as a potential new class of X-ray CT agent for stem cell labeling and possibly in vivo tracking.

Robust carboxylic acid-terminated organic thin films and nanoparticle protectants generated from bidentate alkanethiols.

A new carboxylic acid-terminated alkanethiol having bidentate character, 16-(3,5-bis(mercaptomethyl)phenoxy)hexadecanoic acid (BMPHA), was designed as an absorbate and protectant to form thermally stable carboxylic acid-terminated organic thin films on flat gold and nanoparticles, respectively. The structural features of the organic thin films derived from BMPHA were characterized by ellipsometry, X-ray photoelectron spectroscopy (XPS), and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), and compared to those derived from mercaptohexadecanoic acid (MHA) and 16-(4-(mercaptomethyl)phenoxy)hexadecanoic acid (MMPHA). This study demonstrates that films derived from BMPHA are less densely packed than films derived from MHA and MMPHA. However, the results of solution-phase thermal desorption tests revealed that the carboxylic acid-terminated films generated from BMPHA exhibit an enhanced thermal stability compared to those generated from MHA and MMPHA. Furthermore, as a nanoparticle protectant, BMPHA can be used to stabilize large gold nanoparticles (~45 nm diameter) in solution, and BMPHA-protected gold nanoparticles exhibited a high thermal stability in solution thermolysis studies.

Monodisperse SnO2-coated gold nanoparticles are markedly more stable than analogous SiO2-coated gold nanoparticles.

This manuscript describes the synthesis of uniform monodisperse SnO2-coated gold nanoparticles and examines their colloidal stability as function of pH, with direct comparison to better known and widely used SiO2-coated gold nanoparticles. Aqueous acidic and basic colloidal SnO2-coated and SiO2-coated Au nanoparticle solutions were prepared, and their stability was monitored visually and by UV-vis spectroscopy. Notably, the SnO2-coated Au nanoparticle solutions were stable up to pH 12.5. However, at pH 13 and 14, the SnO2-coated Au nanoparticles underwent aggregation, which could be fully reversed upon neutralization of the solutions. In contrast, the SiO2-coated Au nanoparticle solutions were unstable at pH>10.5, irreversibly producing a precipitate composed of bare Au nanoparticle aggregates having little or no silica coating. Under acidic conditions, sedimentation was observed from both the colloidal SnO2-coated and SiO2-coated Au nanoparticle solutions, but the colloidal solutions could be reconstituted upon neutralization of the acidic solutions. The sedimentation at low pH coincided with the reported isoelectric pH values of SiO2 and SnO2, respectively. From an applications perspective, we are seeking to develop SnO2-coated metal nanoparticles as stable alternatives to the more widely employed SiO2-coated nanoparticles, with a particular emphasis on their use in sensor devices and solar cells.

Cisplatin@US-tube carbon nanocapsules for enhanced chemotherapeutic delivery.

The use of chemotherapeutic drugs in cancer therapy is often limited by problems with administration such as insolubility, inefficient biodistribution, lack of selectivity, and inability of the drug to cross cellular barriers. To overcome these limitations, various types of drug delivery systems have been explored, and recently, carbon nanotube (CNT) materials have also garnered attention in the area of drug delivery. In this study, we describe the preparation, characterization, and in vitro testing of a new ultra-short single-walled carbon nanotube (US-tube)-based drug delivery system for the treatment of cancer. In particular, the encapsulation of cisplatin (CDDP), a widely-used anticancer drug, within US-tubes has been achieved, and the resulting CDDP@US-tube material characterized by high-resolution transmission electron microscopy (HR-TEM), energy-dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and inductively-coupled optical emission spectrometry (ICP-OES). Dialysis studies performed in phosphate-buffered saline (PBS) at 37 °C have demonstrated that CDDP release from CDDP@US-tubes can be controlled (retarded) by wrapping the CDDP@US-tubes with Pluronic-F108 surfactant. Finally, the anticancer activity of pluronic-wrapped CDDP@US-tubes has been evaluated against two different breast cancer cell lines, MCF-7 and MDA-MB-231, and found to exhibit enhanced cytotoxicity over free CDDP after 24 h. These studies have laid the foundation for developing US-tube-based delivery of chemotherapeutics, with drug release mainly limited to within cancer cells only.

Light-induced covalent immobilization of monolayers of magnetic nanoparticles on hydrogen-terminated silicon.

Specifically tailored ω-alkenyl-1-carboxylic acids were synthesized for use as surfactants in the single-step preparation of manganese ferrite (MnFe2O4) nanoparticles (NPs). Monodisperse manganese ferrite NPs terminated with ω-alkenyl moieties were prepared via a one-pot reaction at high temperature without the need of ligand exchange. Using this approach, simple adjustment of the rate of heating allowed precise tuning of the size of the nanoparticles, which were characterized in bulk form by transmission electron microscopy (TEM), Fourier-transform infrared (FT-IR) spectroscopy, and X-ray diffraction (XRD). These surfactant-coated magnetic nanoparticles were then deposited onto hydrogen-terminated silicon(111) wafers and covalently anchored to the surface by UV-initiated covalent bonding. Analysis by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) confirmed that the UV treatment led to covalent immobilization of the NPs on the silicon surface with a consistent packing density across the surface. The magnetic properties of the stable, surface-bound nanoparticle arrays were characterized using a superconducting quantum interference device (SQUID) magnetometer. The materials and methods described here are being developed for use in bit-patterned ultrahigh density magnetic recording media and nanoscale biomagnetic sensing.

Iron phosphide nanostructures produced from a single-source organometallic precursor: nanorods, bundles, crosses, and spherulites.

Single-source molecular precursors were found to produce iron phosphide materials. In a surfactant system of trioctylamine and oleic acid, H2Fe3(CO)9PtBu reacted to form Fe4(CO)12(PtBu)2, which decomposed to give Fe2P nanorods and "bundles." Control of the morphology obtained was possible by varying the surfactant system; addition of increasing amounts of oleic acid resulted in crystal splitting, while the addition of microliter amounts of an alkane enhanced the crystal splitting to give sheaflike structures. The different morphologies seen were attributed to imperfect crystal growth mechanisms.

Injectable in situ cross-linkable nanocomposites of biodegradable polymers and carbon nanostructures for bone tissue engineering.

This study investigates the effects of nanostructure size and surface area on the rheological properties of un-cross-linked poly(propylene fumarate) (PPF) nanocomposites and the mechanical properties of cross-linked nanocomposites as a function of the nanostructure loading. Three model carbon nanostructures were examined, C(60) fullerenes, ultra-short single-walled carbon nanotubes (US-tubes) and single-walled carbon nanotubes (SWNTs). Rheological measurements showed that C60 and US-tube un-cross-linked nanocomposites exhibited viscous-like characteristics with the complex viscosity independent of frequency for nanostructure concentrations up to 1 wt%. Compressive and flexural mechanical testing demonstrated significant mechanical reinforcement of US-tube and SWNT nanocomposites as compared to cross-linked polymer alone, with an up to twofold increase in the mechanical properties. Scanning electron microscopy examination of the fracture surface of cross-linked US-tube nanocomposite revealed lack of aggregation of US-tubes. Although sol fraction studies did not provide any evidence of additional cross-linking, due to the presence of US-tubes in the nanocomposites, transmission electron microscopy studies suggested the crystallization of PPF on the surface of US-tubes which can contribute to the mechanical reinforcement of the US-tube nanocomposites. These results demonstrate that the rheological properties of un-cross-linked nanocomposites depend mainly on the carbon nanostructure size, whereas the mechanical properties of the cross-linked nanocomposites are dependent on the carbon nanostructure surface area. The data also suggest that US-tube nanocomposites are suitable for further consideration as injectable scaffolds for bone tissue engineering applications.