The Ultimate MRI: Magnetic Resonance Force Microscopy

Nanometer-scale magnetic resonance imaging (Nano-MRI) has the potential to be a powerful tool in the investigation and characterization of a large class of materials. In particular, the extension of the three-dimensional imaging capability of MRI to the nanometer scale, combined with the ability to image materials non-destructively and with chemical specificity would enable structure determination of single biomolecules and virus particles—an advance that would lead to fundamentally new approaches in biomedicine. Nano-MRI would also provide the basis for the design and characterization of molecular assemblies and nanostructures which could be used to synthesize novel composite materials or devices. In the past decade, magnetic resonance force microscopy (MRFM) has emerged as a powerful technique for Nano-MRI; in 2004, MRFM was used to detect single electron spins in silica [1], and more recently, to image protons in single tobacco mosaic virus particles with 5-nm spatial resolution [2]. While these advances clearly demonstrate that MRFM is a viable technique for Nano-MRI, new breakthroughs are necessary for MRFM to become a useful tool for structural biology. The fundamental challenge to MRFM detection is improving the signal-to-noise ratio. Here, we present a fundamentally new approach to force-detected magnetic resonance imaging that utilizes ultrasensitive self-assembled rf-frequency silicon nanowire mechanical resonators and greatly improves force detection sensitivity. To generate the coupling between spins and the cantilever, the sample is attached near the tip of the nanowire resonator and brought into close proximity to a nanometer scale metallic constriction. Intense time-dependent fields generated by pulsing current through the constriction are used to generate the coupling between the spins and the mechanical resonator, as well as to spatially encode spin information in reciprocal space. Much like conventional MRI, our nano-MRI device uses time-dependent field gradients for Fourier transform imaging. I will present some of our recent work imaging proton spins in polystyrene with 10-nm spatial resolution, and discuss prospects for the future.

1. Rugar, D., Budakian, R., Mamin, H.J., and Chui, B.W. Single spin detection by magnetic resonance force microscopy, Nature 430, 239 (2004).

2. Degen, C.L., Poggio, M., Mamin, H.J., Rettner, C.T., and Rugar, D. Nanoscale magnetic resonance imaging, PNAS 106, 1313 (2009).