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The team uses a gold-coated glass slide as an electrode, generating a surface plasmon on the electrode with either a LED or a laser. The surface plasmon resonance is strongly affected by molecular binding to the electrode surface or local concentrations in proximity to the electrode. Changes in SPR in turn affect the light intensity, which imaged with a charge-coupled device (CCD) camera. Because the detector converts photons to a pixel-by-pixel current, the camera produces a map of current density for the entire sample at video speed. The intensity of the optical signal, rather than the area measured, drives the amplitude of the local-current signal, allowing the system to interrogate sub-micron regions while maintaining high sensitivities.
The group has imaged areas as small as 0.2 μm x 3 μm to obtain a noise ceiling of approximately 0.3 pA. In tests, they were able to detect TNT to levels as low as 0.5 ng; they estimate that with their current system they could achieve detection levels as low as 0.3 fg. With the widespread availability of low-cost LEDs, diode lasers, and CMOS image sensors, the technology could be both portable and economical enough for handheld airport security or defense applications.
At the Massachusetts Institute of Technology (MIT; Cambridge, Mass.), researchers are combining magnetic resonance imaging (MRI) and atomic force microscopy (AFM) to achieve high-resolution, subsurface 3D imaging for applications like structural biology. MRI can provide nondestructive glimpses inside the human body, for example, but because signals are very weak resolution is limited to a few microns, at best. The MIT group enhances the performance by placing a tiny magnet in close proximity to the sample so that it interacts with the nuclear spins of the atoms. An AFM cantilever monitors the force exerted between the two with attonewton sensitivity, generating images with a resolution up to 1,000 times better than the best conventional MRI. Perhaps more important, the hybrid system can gather data on the elemental composition of the sample, enhancing traditional force microscopy with chemical contrast.
“AFM contributes the sensitivity and MRI gives you capability to look below the surface,” says Christian Degen, assistant professor at MIT. “Using this technology, we can measure on the order of about 100 atoms, or 100 nuclei, as compared to about 1012, which is the best clinical MRI can do.”
To build the system, dubbed the magnetic resonance force microscope (or MRFM), the team starts with a sub-millimeter silicon AFM cantilever. They mount the sample to the end of the cantilever and place it in close proximity to a magnetic tip (see Figure 2). An alternating RF field repeatedly flips the spins of the atoms at well-defined locations within the sample. That variation drives the cantilever into sub-angstrom oscillations, which are tracked by a laser interferometer. As the system scans the sample, it builds up a 3D image. The group has used the system to image a single virus (see Figure 3).