A new approach for bioassays based on frequency- and time-domain measurements of magnetic nanoparticles.

We demonstrate a one-step wash-free bioassay measurement system capable of tracking biochemical binding events. Our approach combines the high resolution of frequency- and high speed of time-domain measurements in a single device in combination with a fast one-step bioassay. The one-step nature of our magnetic nanoparticle (MNP) based assay reduces the time between sample extraction and quantitative results while mitigating the risks of contamination related to washing steps. Our method also enables tracking of binding events, providing the possibility of, for example, investigation of how chemical/biological environments affect the rate of a binding process or study of the action of certain drugs. We detect specific biological binding events occurring on the surfaces of fluid-suspended MNPs that modify their magnetic relaxation behavior. Herein, we extrapolate a modest sensitivity to analyte of 100 ng/ml with the present setup using our rapid one-step bioassay. More importantly, we determine the size-distributions of the MNP systems with theoretical fits to our data obtained from the two complementary measurement modalities and demonstrate quantitative agreement between them.

Towards an electrowetting-based digital microfluidic platform for magnetic immunoassays.

We demonstrate ElectroWetting-On-Dielectric (EWOD) transport and SQUID gradiometer detection of magnetic nanoparticles (MNPs) suspended in a 2 microl de-ionized water droplet. This proof-of-concept methodology constitutes the first development step towards a highly sensitive magnetic immunoassay platform with SQUID readout and droplet-based sample handling. Magnetic AC-susceptibility measurements were performed on MNPs with a hydrodynamic diameter of 100 nm using a high-Tc dc Superconducting Quantum Interference Device (SQUID) gradiometer as detector. We observed that the signal amplitude per unit volume is 2.5 times higher for a 2 microl sample droplet compared to a 30 microl sample volume.

Biomolecular reactions studied using changes in Brownian rotation dynamics of magnetic particles.

We have used magnetic particles to study specific binding of prostate specific antigen (PSA) to the surfaces of the bioparticles. The used particles have a mean diameter of about 130 nm and are placed in phosphate buffer saline (PBS). Each particle is composed of clusters of magnetic single domains of magnetite, which are covered with dextran. Changes in surface chemistry of the particles give rise to a change in the hydrodynamic volume of the particles. The later is mirrored by the changed frequency response of the complex magnetic susceptibility of a fluid containing these particles. Using ordinary induction coils and the lock-in amplifier technique it is possible to measure the complex magnetic susceptibility of the particle solution in a frequency range from about 10 Hz up to 10 kHz. From the measurement of the complex susceptibility versus the excitation frequency (both at the excitation frequency as well as at higher harmonics) we have shown that it is possible to quantitatively study the binding of PSA to the surfaces of the magnetic particles and thus to determine the PSA concentration in solution containing known concentration of nanoparticles functionalised with a monoclonal PSA antibody. Our method allows to perform an immunoassay in a single step and is much faster and cheaper compared to conventional ELISA procedures.