Imagine a genomics or proteomics microarray that can be analyzed with surface-enhanced Raman scattering, a technique that can detect single molecules. What if the array could be designed to produce a maximally enhanced signal? Christopher J. Addison and Alexandre G. Brolo at the University of Victoria in British Columbia, Canada, have determined that surface enhancement is affected by varying the composition of nanoparticles on a glass slide, an early step toward establishing such a microarray.

In their experiment, the scientists alternately deposited gold nanoparticles and a dithiol molecule onto glass surfaces and characterized them with UV-visible spectroscopy, atomic force microscopy and surface-enhanced Raman scattering. Using a spectrometer from Varian Inc. of Lexington, Mass., they examined the gold nanoparticle-coated glass slides in air and in water at a resolution of 2 nm. At a rate of 333 nm/min, they scanned a range of wavelengths from 400 to 900 nm. Then they studied the slides with an atomic force microscope and noncontact silicon microscope tips, both from Veeco Instruments Inc. Finally, they added oxazine 720 dye to probe the array with a Raman spectrometer from Renishaw.

As reported in the Oct. 10 issue of Langmuir, the spectra became more redshifted with increasing amounts of nanoparticles, and atomic force microscopy showed structures that progressively grew with nanoparticle deposition. Those results suggest that the nanoparticles became more aggregated with increasing nanoparticle deposition.

For surface-enhanced Raman scattering measurements at 632 and 785 nm, nine and 13 nanoparticle depositions, respectively, exhibited a maximally enhanced signal. The signal increased to 100 times that of a single deposition, but the surface area increased only 20 percent, as measured by atomic force microscopy.

The researchers concluded that varying the number of depositions could enable maximum enhancement at various excitation wavelengths. It also could permit the creation of an array that produces various wavelengths that represent different cellular information.