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Microscope Setup Rapidly Images Plasmon Sensor Array

Photonics Spectra
Dec 2005
Hank Hogan

When it comes to genomic and proteomic arrays, researchers want fast measurements and techniques free of fluorescent labels. These characteristics, noted University of California, Berkeley, bioengineering professor Luke P. Lee, can reduce the time and expense of using such arrays for systems biology and for molecular medicine in research and clinical settings.

The good news is that a candidate technology is available: metallic thin-film-based surface plasmon resonance sensors, which offer real-time, label-free chemical and biomolecular sensing. The bad news is that imaging the surface plasmon resonance sensor arrays has not been fast. Now Lee and students Gang L. Liu and Joseph C. Doll have demonstrated a solution to the problem based on a standard dark-field microscope setup.

In typical surface plasmon resonance imaging, researchers detect changes in the reflected spectrum of nanoparticles caused by nearby molecules. Current sensor arrays have, at most, 100 elements, and spectroscopic-based measurement techniques have been confined to only a few particles at a time. A large-scale microarray, however, has thousands of elements. The only solution has been to perform time-consuming multiple partial scans.

The Berkeley investigators used multispectral imaging to circumvent this problem. They sent white light into an Acton Research monochromator, automatically adjusting the instrument under software control to create a beam about 2 nm in width that swept over the spectral range of interest.

They used the resulting light as a source for a dark-field microscope and focused this onto a nanoplasmonic array of surface plasmon resonance sensors. They captured the spectra generated by the sensors with a 512 × 512-pixel Roper Scientific CCD camera, measuring the spectral response of all the sensors in the field of view, which was hundreds of microns across.

Using this setup, the researchers imaged gold nanoparticles and nanowires of various sizes. They easily differentiated between the diverse nanostructures based on the scattering resonances, which track particle size. The spatial resolution was about the diffraction limit. Based upon the time to sweep the approximately 100-nm range that would be needed to cover the spectrum of a typical nanoplasmonic array, they estimate that the approach would have an imaging speed of two frames per minute — fast enough to follow many biomolecular events.

Lee noted that the speed of the imaging technique justifies making a 10,000-element nanoplasmonic array. With the prototype instrument done, his team is working on just such an array.

Optics Express, Oct. 17, 2005, pp. 8520-8525.


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