Peter Svedlindh

Sensitive molecular diagnostics using volume-amplified magnetic nanobeads

In this letter, we demonstrate a new principle for diagnostics based on DNA sequence detection using single-stranded oligonucleotide tagged magnetic nanobeads. The target DNA is recognized and volume-amplified to large coils by circularization of linear padlock probes through probe hybridization and ligation, followed by rolling circle amplification (RCA). Upon hybridization of the nanobeads in the RCA coils, the complex magnetization spectrum of the beads changes dramatically, induced by the attached volume-amplified target molecules. We show that the magnetization spectrum of the nanobeads can be used for concentration determination of RCA coils down to the pM range, thus creating the opportunity for nonfluorescence-based cost-efficient high-sensitivity diagnostics tool. We also show that the bead incorporation in the coils is diffusion-controlled and consequently may be accelerated by incubating the sample at higher temperatures.

Energy barrier distribution of a noninteracting nano-sized magnetic particle system

Abstract AC-susceptibility and magnetic relaxation data are analysed to determine the distribution of energy barriers ( E b ) of an assembly of small magnetic particles. The assembly, a dilute frozen ferrofluid, consists of spherical nano-sized maghemite ( γ -Fe 2 O 3 ) particles with a median diameter of 70 A. Assuming a direct proportionality between E b and particle volume, the volume distribution is found to be described by a normalised gamma distribution. The system is not adequately described by the commonly adopted log-normal distribution function, which comparably should yield an excess amount of large particles sizes. From the analyses, accurate values of the Arrhenius law prefactor ( τ 0 = 4 × 10 −10 s) and the magnetic anisotropy constant are derived.

Synthesis, structural and magnetic characterisation of the double perovskite A2MnMoO6 (A=Ba, Sr)

Abstract A study of the crystallographic structure and magnetic properties of the double perovskites Ba2MnMoO6 and Sr2MnMoO6 in polycrystalline form has been carried out by means of neutron powder diffraction (NPD) and magnetization measurements. The Rietveld analysis of room temperature data shows that the Mn2+ and Mo6+ ions are B-site ordered, i.e. the structure is a NaCl-type ordered double perovskite. Ba2MnMoO6 crystallizes in the cubic space group Fm 3 m (a=8.1680(1)) and Sr2MnMoO6 crystallizes in the space group P42/n (a=7.9575(5), c=7.9583(9)). Bond valence sum (BVS) calculation revealed that these compounds have the valency pair of {Mn2+(3d5;t32ge2g), Mo6+(4d0)}. The magnetic measurements suggest that these compounds transform to an antiferromagnetic state below 10 K.

Gold Nanowire Based Electrical DNA Detection Using Rolling Circle Amplification

We present an electrical sensor that uses rolling circle amplification (RCA) of DNA to stretch across the gap between two electrodes, interact with metal nanoparticle seeds to generate an electrically conductive nanowire, and produce electrical signals upon detection of specific target DNA sequences. RCA is a highly specific molecular detection mechanism based on DNA probe circularization. With this technique, long single-stranded DNA with simple repetitive sequences are produced. Here we show that stretched RCA products can be metalized using silver or gold solutions to form metal wires. Upon metallization, the resistance drops from TΩ to kΩ for silver and to Ω for gold. Metallization is seeded by gold nanoparticles aligned along the single-stranded DNA product through hybridization of functionalized oligonucleotides. We show that combining RCA with electrical DNA detection produces results in readout with very high signal-to-noise ratio, an essential feature for sensitive and specific detection assays. Finally, we demonstrate detection of 10 ng of Escherichia coli genomic DNA using the sensor concept.

Multiplex Detection of DNA Sequences Using the Volume-Amplified Magnetic Nanobead Detection Assay

The possibility for conducting multiplex detection of DNA-sequences using the volume-amplified magnetic nanobead detection assay [Stromberg, M.; Goransson, J.; Gunnarsson, K.; Nilsson, M.; Svedlindh, P. and Strømme, M. Nano Lett. 2008 , 8, 816-821] was investigated. In this methodology, a batch consisting of a mixture of several sizes of probe-tagged magnetic beads was used for detection of several types of targets in the same compartment. Furthermore, a nonlinear least-squares deconvolution procedure of the composite imaginary part of complex magnetization vs frequency spectra based on the Cole-Cole model was applied to analyze the data. The results of a quantitative biplex analysis experiment were compared with the corresponding separate single-target assays. Finally, triplex analysis was briefly demonstrated qualitatively. Biplex and triplex detection were found to perform well qualitatively. Biplex detection was found to enable a rough target quantification. Multiplex detection may become a complement to performing multiple separate single-target assays for, e.g., parallel detection of multiple infectious pathogens. Multiplex detection also permits robust relative quantification and inclusion of an internal control to improve quantification accuracy.

Critical dynamics of an interacting magnetic nanoparticle system

Effects of dipole-dipole interactions on the magnetic relaxation have been investigated for three Fe-C nanoparticle samples with volume concentrations of 0.06, 5 and 17 vol%. While both the 5 and 17 vol% samples exhibit collective behaviour due to dipolar interactions, only the 17 vol% sample displays critical behaviour close to its transition temperature. The behaviour of the 5 vol% sample can be attributed to a mixture of collective and single-particle dynamics.

Fabrication and characterization of highly pure and homogeneous YBa2Cu3O7 superconductors from sol‐gel derived powders

The high Tc superconducting oxide YBa2Cu3O7−d has been fabricated by a simple colloidal sol‐gel precursor technique based on citrate complexes uniformly dispersed into ethyleneglycol and water. The present method permits easier fabrication of highly pure and homogeneous superconducting materials when compared with other more elaborate preparation procedures based on solution techniques as it eliminates many steps such as centrifugation, filtration, and pH control. The purity of the final product was estimated to be >99.5% based on the combined results of x‐ray diffraction and Raman scattering analyses. The composition of the sample was determined to be Y1.00Ba1.98Cu3.05O6.85 by inductively coupled plasma spectroscopy and idiometry. The sol‐gel process produced homogeneous materials with particle sizes smaller than ∼3 μm. The superconducting transition determined with resistivity measurements was shown to be sharp with a transition width narrower than 1 K. The magnetic susceptibility measurements exhibited...

Towards equilibrium in spin glasses (invited)

New nonequilibrium features of the magnetic susceptibility of spin glasses are reviewed. Experimental observations on the time variation of the zero‐field‐cooled (ZFC) and field‐cooled (FC) susceptibilities indicate that the same equilibrium value is obtained after the same characteristic time teq(T). ZFC and ac susceptibility measurements on metallic spin glasses unambiguously reveal the existence of aging of the zero‐field spin‐glass state. The possibility to probe the aging process from these measurements is outlined. The aging of the zero‐field state ends at the characteristic time teq(T), where a spin‐glass state at dynamic equilibrium is retained.

Postgrowth annealing of (Ga,Mn) As under As capping: An alternative way to increase TC

In situ postgrowth annealing of (Ga,Mn)As layers under As capping is adequate for achieving high Curie temperatures (TC) in a similar way as ex situ annealing in air or in N2 atmosphere practiced earlier. Thus, the first efforts give an increase of TC from 68 to 145 K after 2 h annealing at 180 °C. These data, in combination with lattice parameter determinations and photoemission results, show that the As capping acts as an efficient sink for diffusing Mn interstitials.

Structural and magnetic properties of GaMnAs layers with high Mn-content grown by migration-enhanced epitaxy on GaAs(100) substrates

Ferromagnetic GaMnAs containing up to 10% Mn has been grown by migration-enhanced epitaxy at a substrate temperature of 150 °C. The lattice constant of hypothetical zinc-blende structure MnAs is determined to be 5.90 A, which deviates somewhat from previously reported values. This deviation is ascribed to growth-condition-dependent density of point defects. Magnetization measurements showed an onset of ferromagnetic ordering around 75 K for the GaMnAs layer with 10% Mn. This means that the trend of falling Curie temperatures with increasing Mn concentrations above 5.3% is broken.