Joseph A. Christodoulides

Large induced magnetic anisotropy in manganese spinel ferrite films

The oxygen pressure dependence of magnetic anisotropy in pulse laser deposited manganese ferrite (MnFe2O4) films was investigated. Magnetic anisotropy fields (Ha) are shown to exceed 5kOe when films were processed at oxygen pressures below 5mTorr. Further, it is shown that the magnetically preferred direction of Ha can be aligned either along the film plane (pO2 8mTorr). The ability to induce large perpendicular magnetic anisotropy in spinel ferrites allows for new applications (i.e., phase shifters, filters, isolators, and circulators) near or above X-band frequencies to be considered.

A high surface area ordered mesoporous BiFeO3 semiconductor with efficient water oxidation activity

Bismuth ferrite (BiFeO3) is an important multiferroic oxide material because of its unique magnetic and ferroelectric properties. Here, we synthesize for the first time a highly ordered mesoporous BiFeO3 semiconductor using tartaric acid-assisted growth of the BiFeO3 compound inside the pores of a carbon template. Powder X-ray diffraction (XRD), transmission electron microscopy (TEM) and N2 physisorption measurements reveal that the template-free material possesses a three-dimensional hexagonal mesostructure with a large internal BET surface area (141 m2 g−1) and narrow sized pores (ca. 4 nm). Also, the pore walls comprise single-phase BiFeO3 nanocrystals according to the high-resolution TEM, electron diffraction and magnetic experiments. The mesoporous BiFeO3 shows high activity for the photocatalytic oxygen evolution reaction (OER) under UV-visible light (λ > 380 nm), affording an average oxygen evolution rate of 66 μmol h−1 g−1. We also show that the propensity of photogenerated holes for the OER can be significantly enhanced when 1 wt% Au nanoparticles are deposited on the BiFeO3 surface. The Au/BiFeO3 heterostructure exerts excellent OER activity (586 μmol h−1 g−1) and long-term cycling stability, raising the possibility for the design of effective and robust OER photocatalysts.

A New Methodology for Quantitative LSPR Biosensing and Imaging

A new quantitative analysis methodology for localized surface plasmon resonance (LSPR) biosensing which determines surface-receptor fractional occupancy, as well as an LSPR imaging technique for the spatiotemporal mapping of binding events, is presented. Electron beam nanolithography was used to fabricate 20 × 20 arrays of gold nanostructures atop glass coverslips. A single biotinylated array was used to measure the association kinetics of neutravidin to the surface by spectroscopically determining the fractional occupancy as a function of time. By regenerating the same array, a reliable comparison of the kinetics could be made between control samples and neutravidin concentrations ranging from 1 μM to 50 nM. CCD-based imagery of the array, taken simultaneously with the spectroscopic measurements, reveals the binding of neutravidin to the surface as manifested by enhanced scattering over the majority of the resonance peak. The temporal resolution of the LSPR imaging technique was 200 ms and the spatial resolution was 8 μm(2).

Quantitative imaging of protein secretions from single cells in real time.

Protein secretions from individual cells create spatially and temporally varying concentration profiles in the extracellular environment, which guide a wide range of biological processes such as wound healing and angiogenesis. Fluorescent and colorimetric probes for the detection of single cell secretions have time resolutions that range from hours to days, and as a result, little is known about how individual cells may alter their protein secretion rates on the timescale of minutes or seconds. Here, we present a label-free technique based upon nanoplasmonic imaging, which enabled the measurement of individual cell secretions in real time. When applied to the detection of antibody secretions from single hybridoma cells, the enhanced time resolution revealed two modes of secretion: one in which the cell secreted continuously and another in which antibodies were released in concentrated bursts that coincided with minute-long morphological contractions of the cell. From the continuous secretion measurements we determined the local concentration of antibodies at the sensing array closest to the cell and from the bursts we estimated the diffusion constant of the secreted antibodies through the extracellular media. The design also incorporates transmitted light and fluorescence microscopy capabilities for monitoring cellular morphological changes and intracellular fluorescent labels. We anticipate that this technique can be adapted as a general tool for the quantitative study of paracrine signaling in both adherent and nonadherent cell lines.

Magnetic and transport properties of Co2MnSnxSb1−x Heusler alloys

We present the magnetic, structural, and transport properties of the quaternary Heusler alloys Co2MnSnxSb1−x (x=0, 0.25, 0.50, 0.75, and 1.0), which have been theoretically predicted to be half-metallic. Magnetization measurements as a function of applied field show that the saturation moment for x=1 (Co2MnSn) is near the Slater–Pauling value of 5μB; however, the moment for x=0 (Co2MnSb) falls far short of its predicted value of 6μB. Resistivity as a function of temperature was measured from 5 to 400 K, and a phase transition from a half-metallic ferromagnetic phase to a normal ferromagnetic phase was observed between 50 and 80 K for all of the alloys. At low temperature (10 K<T<40 K), the resistivity ratio was found to vary as R(T)/R(T=5 K)=A+BT2+CT9/2, where the T2 term results from electron-electron scattering, whereas the T9/2 term is a consequence of double magnon scattering.

Magnetic moment degradation of nanowires in biological media: real-time monitoring with SQUID magnetometry

Magnetic nanoparticles are used throughout biology for applications from targeted drug and gene delivery to the labeling of cells. These nanoparticles typically react with the biological medium to which they are introduced, resulting in a diminished magnetic moment. The rate at which their magnetic moment is diminished limits their utility for targeting and can signal the unintended release of surface-functionalized biomolecules. A foreknowledge of the time-dependent degradation of the magnetic moment in a given medium can aid in the selection of the optimal buffering solution and in the prediction of a reasonable experimental time frame. With this goal in mind, we have developed a SQUID magnetometer based methodology for measuring the saturation magnetic moment of nanoparticles in real time while immersed in a biological medium. Measurements on Co and Ni nanowires in a variety of commonly used buffered salines demonstrated that the technique has the dynamic range and sensitivity to detect the rapid reduction in moment due to active corrosion as well as much more subtle changes from the formation of a passivating surface oxide layer. In order to correlate the magnetic moment reductions to these specific chemical processes, samples were additionally characterized using x-ray photoelectron spectroscopy, inductively coupled plasma spectroscopy and scanning electron microscopy. The most reactive buffers studied were found to be phosphate and carbonate based, which caused active corrosion of the Co nanowires but only a comparatively slow passivation of the Ni nanowires by oxidation.

The use of DNA molecular beacons as nanoscale temperature probes for microchip-based biosensors

Todays biosensors and drug delivery devices are increasingly incorporating lithographically patterned circuitry that is placed within microns of the biological molecules to be detected or released. Elevated temperatures due to Joule heating from the underlying circuitry cannot only reduce device performance, but also alter the biological activity of such molecules (i.e. binding, enzymatic, folding). As a consequence, biochip design and characterization will increasingly require local measurements of the temperature and temperature gradients on the biofunctionalized surface. We have developed a technique to address this challenge based on the use of DNA molecular beacons as a nanoscale temperature probe. The surface of fused-silica chips with lithographically patterned, current-carrying gold rings have been functionalized with a layer of molecular beacons. We utilize the temperature dependence of the molecular beacons to calibrate the temperature at the center of the rings as a function of applied current from 25 to 50 degrees C. The fluorescent images of the rings reveal the extent of heating to the surrounding chip due to the applied current while resolving temperature gradients over length scales of less than 500nm. Finite element analysis and analytic calculations of the distribution of heat in the vicinity of the current-carrying rings agree well with the experimental results. Thus, molecular beacons are shown to be a viable tool for temperature calibration of micron-sized circuitry and the visualization of submicron temperature gradients.

Optimizing Nanoplasmonic Biosensor Sensitivity with Orientated Single Domain Antibodies

Localized surface plasmon resonance (LSPR) spectroscopy and imaging are emerging biosensor technologies which tout label-free biomolecule detection at the nanoscale and ease of integration with standard microscopy setups. The applicability of these techniques can be limited by the restrictions that surface-conjugated ligands must be both sufficiently small and orientated to meet analyte sensitivity requirements. We demonstrate that orientated single domain antibodies (sdAb) can optimize nanoplasmonic sensitivity by comparing three anti-ricin sdAb constructs to biotin-neutravidin, a model system for small and highly orientated ligand studies. LSPR imaging of electrostatically orientated sdAb exhibited a ricin sensitivity equivalent to that of the biotinylated LSPR biosensors for neutravidin. These results, combined with the facts that sdAb are highly stable and readily produced in bacteria and yeast, build a compelling case for the increased utilization of sdAbs in nanoplasmonic applications.

Address line-assisted switching of vertical magnetoresistive random access memory cells

Vertical magnetoresistive random access memory (VMRAM) is a high-density, nonvolatile memory that employs current perpendicular to the plane to switch soft (read) and hard (write) magnetic layers of a giant-magnetoresistive memory element. VMRAM cells consist of closed-flux toroid-shaped elements and intersecting address lines situated above and beneath the elements [J.-G. Zhu, Y. Zheng, and G. A. Prinz, J. Appl. Phys. 87, 6668 (2000)]. Experiments performed on 64-element strings show that the intersecting address lines effectively assist in VMRAM cell switching. With projected density scaling to 400Gbits∕in.2 [J.-G. Zhu, Y. Zheng, and G. A. Prinz, J. Appl. Phys. 87, 6668 (2000)], VMRAM has the potential to compete with both semiconductor memories and mechanical hard disks.

A Label-free Technique for the Spatio-temporal Imaging of Single Cell Secretions.

Inter-cellular communication is an integral part of a complex system that helps in maintaining basic cellular activities. As a result, the malfunctioning of such signaling can lead to many disorders. To understand cell-to-cell signaling, it is essential to study the spatial and temporal nature of the secreted molecules from the cell without disturbing the local environment. Various assays have been developed to study protein secretion, however, these methods are typically based on fluorescent probes which disrupt the relevant signaling pathways. To overcome this limitation, a label-free technique is required. In this paper, we describe the fabrication and application of a label-free localized surface plasmon resonance imaging (LSPRi) technology capable of detecting protein secretions from a single cell. The plasmonic nanostructures are lithographically patterned onto a standard glass coverslip and can be excited using visible light on commercially available light microscopes. Only a small fraction of the coverslip is covered by the nanostructures and hence this technique is well suited for combining common techniques such as fluorescence and bright-field imaging. A multidisciplinary approach is used in this protocol which incorporates sensor nanofabrication and subsequent biofunctionalization, binding kinetics characterization of ligand and analyte, the integration of the chip and live cells, and the analysis of the measured signal. As a whole, this technology enables a general label-free approach towards mapping cellular secretions and correlating them with the responses of nearby cells.