Goran Mihajlović

Detection of single magnetic bead for biological applications using an InAs quantum-well micro-Hall sensor

Room-temperature detection of a single commercial superparamagnetic bead (1.2μm in diameter) suitable for biological applications has been realized using an InAs quantum-well micro-Hall sensor. The detection was demonstrated using phase-sensitive detection on a single Hall cross as well as in a Hall gradiometry setup. The high signal to noise ratio, obtained in both configurations, promises detection of single nanometer-size particles by further miniaturization of the device to submicron dimensions.

Magnetic characterization of a single superparamagnetic bead by phase-sensitive micro-Hall magnetometry

Employing phase sensitive micro-Hall magnetometry at room temperature, we map the susceptibility of a single superparamagnetic bead, 1.2μm in diameter, as a function of magnetic field. We find that the dependence can be explained by modeling the bead as an ensemble of noninteracting superparamagnetic nanoparticles with log-normal distribution of magnetic moments. We also discuss the effect of possible dipolar interactions between the nanoparticles on the obtained results.

All-electrical switching and control mechanism for actomyosin-powered nanoactuators

A fast all-electrical activation and control mechanism for biomolecular motor-powered nanoactuators has been developed. Rapid and reversible on–off control of actomyosin biomolecular motors was experimentally demonstrated using in vitro motility assays. The results show that the motility of the actin filaments can be cycled repeatedly by electrically controlled thermal activation in the temperature range from 10°C to 50°C without functional loss. The fast response of the filaments upon rapid temperature switching suggests that thermal activation provides an effective method for turning actomyosin-powered nanoactuators on and off.

The detection of specific biomolecular interactions with micro-Hall magnetic sensors

The detection of reagent-free specific biomolecular interactions through sensing of nanoscopic magnetic labels provides one of the most promising routes to biosensing with solid-state devices. In particular, Hall sensors based on semiconductor heterostructures have shown exceptional magnetic moment sensitivity over a large dynamic field range suitable for magnetic biosensing using superparamagnetic labels. Here we demonstrate the capability of such micro-Hall sensors to detect specific molecular binding using biotin-streptavidin as a model system. We apply dip-pen nanolithography to selectively biotinylate the active areas of InAs micro-Hall devices with nanoscale precision. Specific binding of complementarily functionalized streptavidin-coated superparamagnetic beads to the Hall crosses occurs via molecular recognition, and magnetic detection of the assembled beads is achieved at room temperature using phase sensitive micro-Hall magnetometry. The experiment constitutes the first unambiguous demonstration of magnetic detection of specific biomolecular interactions with semiconductor micro-Hall sensors, and the selective molecular functionalization and resulting localized bead assembly demonstrate the possibility of multiplexed sensing of multiple target molecules using a single device with an array of micro-Hall sensors.

Enhanced spin signals due to native oxide formation in Ni80Fe20/Ag lateral spin valves

Large nonlocal spin valve signals are reported in mesoscopic Ni80Fe20/Ag lateral spin valves upon exposing them to air. Magnetotransport measurements combined with transmission electron microscopy show that the formation of a native oxide layer at the Ni80Fe20/Ag interface is responsible for the large signals. The results indicate that lateral spin valves with superior performance to those based on high-resistance tunnel barriers can be achieved via controllable growth of native permalloy oxides.

Facilitated Cross-Bridge Interactions with Thin Filaments by Familial Hypertrophic Cardiomyopathy Mutations in α-Tropomyosin

Familial hypertrophic cardiomyopathy (FHC) is a disease of cardiac sarcomeres. To identify molecular mechanisms underlying FHC pathology, functional and structural differences in three FHC-related mutations in recombinant α-Tm (V95A, D175N, and E180G) were characterized using both conventional and modified in vitro motility assays and circular dichroism spectroscopy. Mutant Tms exhibited reduced α-helical structure and increased unordered structure. When thin filaments were fully occupied by regulatory proteins, little or no motion was detected at pCa 9, and maximum speed (pCa 5) was similar for all tropomyosins. Ca2+-responsiveness of filament sliding speed was increased either by increased pCa50 (V95A), reduced cooperativity n (D175N), or both (E180G). When temperature was increased, thin filaments with E180G exhibited dysregulation at temperatures ~10°C lower, and much closer to body temperature, than WT. When HMM density was reduced, thin filaments with D175N required fewer motors to initiate sliding or achieve maximum sliding speed.

InAs quantum well Hall devices for room-temperature detection of single magnetic biomolecular labels

Hall sensors with cross width of ∼1μm were fabricated from InAs∕AlSb quantum well semiconductor heterostructures containing two-dimensional electron gas. The room-temperature device characteristics were examined by Hall effect and electronic noise measurements along with analytical calculations. In the low-frequency range, from 20Hzto1.6kHz, the noise-equivalent magnetic field resolution was found to be limited by 1∕f and generation-recombination noise from 22to3.5μT∕Hz. The corresponding noise-equivalent magnetic moment resolution reached 106μB∕Hz at ∼700Hz and was even lower at higher frequencies. Using a phase-sensitive measurement technique, detection of a single 1.2μm diameter bead, suitable for biological applications, was achieved with a signal to noise ratio of ∼33.3dB, as well as detection of six 250nm beads with a signal to noise of ∼2.3dB per bead. The work demonstrates the efficacy of InAs quantum well Hall devices for application in high sensitivity detection of single magnetic biomolecular l...

Micromechanical Thermal Assays of Ca2+-Regulated Thin-Filament Function and Modulation by Hypertrophic Cardiomyopathy Mutants of Human Cardiac Troponin

Microfabricated thermoelectric controllers can be employed to investigate mechanisms underlying myosin-driven sliding of Ca2+-regulated actin and disease-associated mutations in myofilament proteins. Specifically, we examined actin filament sliding—with or without human cardiac troponin (Tn) and α-tropomyosin (Tm)—propelled by rabbit skeletal heavy meromyosin, when temperature was varied continuously over a wide range (∼20–63°C). At the upper end of this temperature range, reversible dysregulation of thin filaments occurred at pCa 9 and 5; actomyosin function was unaffected. Tn-Tm enhanced sliding speed at pCa 5 and increased a transition temperature (Tt) between a high activation energy (Ea) but low temperature regime and a low Ea but high temperature regime. This was modulated by factors that alter cross-bridge number and kinetics. Three familial hypertrophic cardiomyopathy (FHC) mutations, cTnI R145G, cTnI K206Q, and cTnT R278C, cause dysregulation at temperatures ∼5–8°C lower; the latter two increased speed at pCa 5 at all temperatures.

Submicrometer Hall Sensors for Superparamagnetic Nanoparticle Detection

Submicrometer Hall sensors, with Hall cross width of ~250 nm, were fabricated from InAs/AlSb quantum well semiconductor heterostructures. The room-temperature device characteristics were examined by experimental Hall effect and electronic noise measurements combined with analytical calculations. The noise-equivalent magnetic moment resolution of the order of 104muB/radicHz was obtained at frequencies above ~1 kHz. We show that the devices can achieve single superparamagnetic nanoparticle detection and thus be employed in experiments involving single magnetically labeled biomolecule detection.

Imaging of lateral spin valves with soft x-ray microscopy

Imaging of lateral spin valves with soft X-ray microscopy O. Mosendz, ∗ G. Mihajlovi´ , and J. E. Pearson c Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA P. Fischer and M.-Y. Im Center for X-ray Optics, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA S. D. Bader and A. Hoffmann † Materials Science Division and Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, USA (Dated: July 16, 2009) We investigated Co/Cu lateral spin valves by means of high-resolution transmission soft x-ray microscopy with magnetic contrast that utilizes x-ray magnetic circular dichroism (XMCD). No magnetic XMCD contrast was observed at the Cu L 3 absorption edge, which should directly image the spin accumulation in Cu. Although electrical transport measurements in a non-local geometry clearly detected the spin accumulation in Cu, which remained unchanged during illumination with circular polarized x-rays at the Co and Cu L 3 absorption edges. PACS numbers: 72.25.Hg, 73.40.Jn, 75.25.+z, 75.75.+a I. INTRODUCTION Recent developments in spintronics showed that the use of spin polarized currents can have a profound impact on applications such as magnetic information processing and storage. Furthermore, the study of pure spin currents instead of spin polarized charge currents is providing a promising new direction to advance the present technology. 1,2 Pure spin currents can be gener- ated via a spin polarized charge current injection from ferromagnets, 3 spin Hall effects, 4 or spin pumping. 5–7 In order to utilize pure spin currents it is necessary to un- derstand their propagation properties. To this end, the direct imaging of a non-equilibrium spin accumulation that accompanies pure spin currents can generate new insights into the dynamic behavior of spins. This be- came evident from prior research on semiconducting sys- tems, where such imaging is made possible due to strong magneto-optic effects . 8–12 From such magneto-optical measurements it was discovered in semiconductors that spin coherence times are relatively long and injected spins can be transported with charge currents over macroscopic distances; 8 spin Hall effects are detectible; 9,10 the imag- ing revealed the flow-pattern of electrically injected spins including a spin back-diffusion against the charge cur- rent at the draining contact; 11 and spin polarization can be generated via spin-dependent reflection from metal- semiconductor interfaces. 12 For metallic spin transport systems such direct imaging of the spin accumulation and spin currents remains elu- sive. One reason is that due to the smaller spin diffusion lengths, which are typically far in the submicron range, 13 standard optical techniques do not provide sufficient spa- tial resolution. Nevertheless, it can be expected that suc- cessful imaging of the spin accumulation in metals will greatly enhance our understanding of spin currents and dynamics. Some questions that could be immediately addressed are how spin currents couple to charge drift currents in metals 14 and what role inhomogeneous spin injection plays for contacts whose sizes are comparable to the spin diffusion length. 15 Soft x-ray microscopy with state-of-the-art Fresnel zone plates used as high resolution optics 16 is a promis- ing approach towards the goal of imaging spin accumula- tions in metals. The spatial resolution, down to 10 nm, is sufficiently high and it is possible to get magnetic con- trast via x-ray magnetic circular dichroism (XMCD). 17,18 Here we show investigations of Co/Cu lateral spin valves, which were imaged using magnetic transmission soft x- ray microscopy (MTXM) with circular polarized x-rays for detecting XMCD from the spin accumulation in Cu. Although the presence of a spin accumulation was ver- ified with non-local transport measurements, there was no detectable XMCD contrast at the Cu L 3 absorption edge. II. EXPERIMENTAL The Co/Cu lateral spin valves were fabricated by means of e-beam lithography utilizing a double-layer PMMA/PMGI resist on a SiN/Si substrate. Selectively removing the Si enabled the SiN layer to be used as a free-standing x-ray transparent membrane, which is re- quired to perform the MTXM imaging experiment. In order to minimize detrimental absorption we used a 100- nm thick SiN layer. Ohmic junctions for the lateral spin valves were formed by employing a lithographically con- trolled e-beam resist undercut technique with subsequent shadow mask evaporation of 25-nm Co and 80-nm Cu. 19 After their fabrication, the lateral spin valves were cov- ered by a 100 nm thick SiN protective layer deposited via rf -sputtering. Finally, the membrane windows were defined with photolithography on the substrate back-side