Jakob Blomgren

Size-Dependent Relaxation Properties of Monodisperse Magnetite Nanoparticles Measured Over Seven Decades of Frequency by AC Susceptometry

Magnetic relaxation is exploited in innovative biomedical applications of magnetic particles such as magnetic particle imaging (MPI), magnetic fluid hyperthermia, and bio-sensing. Relaxation behavior should be optimized to achieve high performance imaging, efficient heating, and good SNR in bio-sensing. Using two AC susceptometers with overlapping frequency ranges, we have measured the relaxation behavior of a series of monodisperse magnetic particles and demonstrated that this approach is an effective way to probe particle relaxation characteristics from a few Hz to 10 MHz, the frequencies relevant for MPI, hyperthermia, and sensing.

Sensitive High Frequency AC Susceptometry in Magnetic Nanoparticle Applications

We report on the development of a sensitive high frequency susceptometer capable of measuring in the frequency range from 25 kHz up to 10 MHz with a volume susceptibility sensitivity of 3.5×l0−5 at 100 kHz corresponding to about 0.3% of the measured AC susceptibility. In combination with the previous reported DynoMag system capable of measuring dynamic magnetic properties in the range from 1 Hz to 200 kHz we are thus able to measure dynamic magnetic properties between 1 Hz to 10 MHz with high magnetic sensitivity. We will show AC susceptometry applications and results within the fields of magnetic hyperthermia and dynamic magnetic characterization of magnetic nanoparticle system with different particle sizes and magnetic properties.

A SQUID picovoltmeter working at 77 K

A SQUID picovoltmeter working at 77 K has been constructed with a bandwidth of 37 kHz. The white noise of the picovoltmeter was 32 pV//spl radic/Hz with the input shorted. For an optimal source resistance of 0.8 /spl Omega/ the noise temperature was 46 K. The picovoltmeter was constructed from a multilayer high-T/sub c/ SQUID-magnetometer coupled to the signal source via a 10-turn copper coil. The mutual inductance between the coil and the SQUID washer was 0.7 nH. To reduce the noise contributions of the first-stage room temperature amplifier, additional positive feedback (APF) was used together with direct read-out electronics. As a demonstration, the current-voltage characteristics of another SQUID was measured with the picovoltmeter.

An HTS SQUID picovoltmeter used as preamplifier for Rogowski coil sensors

Abstract In the design and development of high voltage insulation systems, partial discharge measurements are often used for system characterization and evaluation. One commonly used sensor for such measurements is the Rogowski sensor, which is a type of induction coil. Since the sensor itself has low resistance it produces very little voltage noise. Hence, the noise limit of the measurement is often set by the preamplifier. Recent development of SQUID picovoltmeters shows excellent noise performance, especially for sensors with low source impedances. We have evaluated the possibility to use the SQUID picovoltmeter as preamplifier for Rogowski type sensors. The designed voltmeter has a voltage noise of 110 pV/ Hz and a bandwidth of 250 kHz. Measurements of sensors with inductances between 0.5 and 40 μH have been performed.

An HTS SQUID picovoltmeter with a flip-chip flux transformer

An HTS SQUID picovoltmeter was constructed, which in two different configurations gave a voltage noise of e/sub n/=3.2pV//spl radic/Hz and a current noise of i/sub n/=310 pA//spl radic/Hz, and a voltage noise of e/sub n/=75 pV//spl radic/Hz and a current noise of i/sub n/=10 pA//spl radic/Hz, respectively. The low frequency noise was reduced to below 1 Hz using bias current reversal. The SQUID is coupled to the input coil via an intermediate flip-chip flux transformer. Measurements were performed to determine the input coil to SQUID mutual inductance for different input coil configurations.

Discharge measurements using a HTS-SQUID based amplifier system

In the design and development of high voltage insulation systems, partial discharge measurements are often used for system characterization and evaluation. The aim of this work was to investigate the SQUID sensor as an amplifier in a high voltage environment. An HTS SQUID amplifier has been developed and tested under laboratory conditions. Different SQUIDs and a number of different input configurations have been investigated. Preliminary data indicate that the system has a performance similar to a commercial system used as reference. However, calculations and simulations show that the new method has potential for further improvement.

Noise properties of an YBCO SQUID

Abstract We have measured the electrical and the white noise properties of a high-Tc SQUID at 65 K, using a room temperature low noise amplifier and a high-Tc SQUID-based voltage amplifier. Operating at 65 K, the voltage amplifier had a voltage noise of 50 pV/ Hz , a current noise of 15 pA/ Hz and a 3 dB bandwidth of 300 kHz. We have found that the measured white voltage noise exceeded the theoretically expected noise by a factor of 2.

Volume-amplified magnetic bioassay integrated with microfluidic sample handling and high-Tc SQUID magnetic readout

A bioassay based on a high-Tc superconducting quantum interference device (SQUID) reading out functionalized magnetic nanoparticles (fMNPs) in a prototype microfluidic platform is presented. The target molecule recognition is based on volume amplification using padlock-probe-ligation followed by rolling circle amplification (RCA). The MNPs are functionalized with single-stranded oligonucleotides, which give a specific binding of the MNPs to the large RCA coil product, resulting in a large change in the amplitude of the imaginary part of the ac magnetic susceptibility. The RCA products from amplification of synthetic Vibrio cholera target DNA were investigated using our SQUID ac susceptibility system in microfluidic channel with an equivalent sample volume of 3 μl. From extrapolation of the linear dependence of the SQUID signal versus concentration of the RCA coils, it is found that the projected limit of detection for our system is about 1.0 × 105 RCA coils (0.2 × 10−18 mol), which is equivalent to 66 fM in the 3 μl sample volume. This ultra-high magnetic sensitivity and integration with microfluidic sample handling are critical steps towards magnetic bioassays for rapid detection of DNA and RNA targets at the point of care.