Magnetic Viruses: Utilizing Self-Assembly for Biomedical Applications*

Magnetic nanoparticles coated with biochemical surfactants have emerged recently as an important component for enabling many biological and medical applications. We implemented a biotemplating approach to create such magnetic nanoparticles by utilizing native protein capsid shells derived in high yield from the T7 bacteriophage virus.\footnote{C.~Liu, S.-H. Chung, Q.~Jin, A.~Sutton, F.~Yan, B.~K.~Kay, S.~D.~Bader, L.~Makowski, and L.~Chen, J. Magn. Magn. Mater, in press.} The magnetic nanoparticles are grown via bio-mineralization reactions 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. This makes these viruses ideally suited for many biomedical applications among which we investigated specifically a novel sensing scheme for target recognition based on Brownian relaxation.\footnote{S.H.~Chung, A.~Hoffmann, S.~D.~Bader, C.~Liu, B.~Kay, L.~Makowski, and L.~Chen, Appl.\ Phys.\ Lett.\ {\bf 85}, 2971 (2004); S.~H.~Chung, A.~Hoffmann, K.~Guslienko, S.~D.~Bader, C.~Liu, B.~Kay, L.~Makowski, and L.~Chen, J.\ Appl.\ Phys.\ {\bf 97}, 10R101 (2005).} For this scheme we use the {\it ac}-susceptibility of the functionalized magnetic nanoparticles suspended in liquid. Upon binding the target of interest to the particles, their Brownian relaxation time is modified, which is readily detected by a change of the frequency dependence of the magnetic susceptibility. This scheme has several advantages; (i) it requires only one binding event for sensing; (ii) there is a useful signal both in the absence and presence of the target; (iii) the signal contains information about the size of the target besides the biochemical affinity; and (iv) since the binding modifies the magnetic susceptibility of the magnetic particles there is no need for removing unbound labels.