Seok-Hwan Chung

Biological sensors based on Brownian relaxation of magnetic nanoparticles

We experimentally demonstrate a biomagnetic sensor scheme based on Brownian relaxation of magnetic nanoparticles suspended in liquids. The characteristic time scale of the Brownian relaxation can be determined directly by ac susceptibility measurements as a function of frequency. The peak in the imaginary part of the ac susceptibility shifts to lower frequencies upon binding the target molecules to the magnetic nanoparticles. The frequency shift is consistent with an increase of the hydrodynamic radius corresponding to the size of the target molecule.

Biological sensing with magnetic nanoparticles using Brownian relaxation (invited)

Magnetic nanoparticles coated with biochemical ligands are enabling many biological and medical applications. In particular biomagnetic sensors have potential advantages of simplicity and rapidity. We demonstrate a substrate-free biomagnetic sensing approach using the magnetic ac susceptibility of ferromagnetic particles suspended in a liquid. The magnetic relaxation of these particles is mainly due to Brownian rotational diffusion, which can be modified by binding the particles to the intended target. This scheme has several advantages: (i) it requires only one binding event; (ii) there is an inherent check of integrity; and (iii) the signal contains additional information about the target size.

Substrate-free Biosensing using Brownian Rotation of Bio-conjugated Magnetic Nanoparticles

The recent development of bio-conjugated magnetic nanoparticles offers many opportunities for applications in the field of biomedicine. In particular, the use of magnetic nanoparticles for biosensing has generated widespread research efforts following the progress of various magnetic field sensors. Here we demonstrate substrate-free biosensing approaches based on the Brownian rotation of ferromagnetic nanoparticles suspended in liquids. The signal transduction is through the measurement of the magnetic ac susceptibility as a function of frequency, whose peak position changes due to the modification of the hydrodynamic radius of bio-conjugated magnetic nanoparticles upon binding to target bio-molecules. The advantage of this approach includes its relative simplicity and integrity compared to methods that use substrate-based stray-field detectors.

Shaken Not Stirred: Magnetic Viruses for Biomagnetic Sensing

Magnetic nanoparticles coated with biochemical surfactants have emerged as an important component for enabling many biological and medical applications. A general challenge in the fabrication of these nanoparticles is to generate a high degree of uniformity both in the magnetic and biochemical properties of the nanoparticles. We address this issue through biological templating of inorganic nanoparticles, which offers promising opportunities for realizing the full potential of self-assembled materials. We implemented such biotemplating to create magnetic nanoparticles by utilizing native protein capsid shells derived in high yield from T7 bacteriophage viruses.The magnetic nanoparticles are grown inside of hollowed-out capsids that retain their original chemical recognition properties. The resultant magnetic viruses are uniform in geometry, physical properties, and biochemical functionality. We first coax the DNA out of the T7 virus by means of an alkaline treatment, and then grow cobalt or ironoxide particles inside the remaining hollow capsid shell. The advantage of our approach is twofold: (i) the use of native virus capsids allows for a straight-forward and economical production of the magnetic nanoparticles in large quantities, which enables practical applications, and (ii) given the richness of protein types that form the native T7 capsid, our magnetic virus can be tailored via phage display libraries to tune the bio-functionality and/or bio-tagging of a sample. Thus in our approach the biological functionality of the particles is established first before the magnetism is subsequently added. This is in contrast to the traditional fabrication of bio-functionalized magnetic nanoparticles, where the particles are typically functionalized after the preparation of the magnetic particles.