The dramatic local electric-field enhancement property of Ag nanoparticles was used as the basis to significantly increase the signal output of a novel label-free (or "self-labeled") fluorescence-based DNA detection system. In response to identical amounts of analyte, nanostructured Ag substrates provided a posthybridization fluorescent sensor response over 10-fold larger than the response from planar Au substrates. Detection performance strongly depended upon the Ag substrate roughness. Consistent with work by others on metal-enhanced fluorescence, fluorescence intensity also depended strongly on the distance between the fluorophore and the Ag substrate surface. Adjusting the surface roughness, amount of the Ag deposited on the surface, and the DNA probe length allowed for production of an optimized response. In addition to constituting a novel label-free DNA sensor, we anticipate that these structures will provide a unique platform for testing concepts in plasmonics.

Correlated measurements of fluorescence and topography were performed for individual single-walled carbon nanotubes (SWNTs) on quartz using epifluorescence confocal microscopy and atomic force microscopy (AFM). Surprisingly, only ~11% of all SWNTs in DNA-wrapped samples were found to be highly emissive on quartz, suggesting that the ensemble fluorescence quantum yield is low because only a small population of SWNTs fluoresces strongly. Qualitatively similar conclusions were obtained from control studies using a sodium cholate surfactant system. To accommodate AFM measurements, excess surfactant was removed from the substrate. Though individual SWNTs on nonrinsed and rinsed surfaces displayed differences in fluorescence intensities and line widths, arising from the influence of the local environment on individual SWNT optical measurements, photoluminescence data from both samples displayed consistent trends.

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As illuminated in 1991, 1,2 carbon nanotubes possess unique electronic and mechanical properties that have received much attention. 3 The breakthrough finding that semiconducting single-walled carbon nanotubes (SWNTs) fluoresce 4 has generated intense interest in their optical properties as well. For example, the fact that semiconducting SWNTs have a size-tunable energy gap (E g ) spanning a wide wavelength range from the visible to the infrared spectral regions 5 and are highly robust emitters 6 allows for potential applications in nanometer-scale optoelectronics, 7,8 biotechnology, 9-11 and quantum optics. 12,13 Typically, as-synthesized nanotubes form tightly bundled ropes with a mixture of metallic and semiconducting SWNTs. 14,15 The electronic properties of these ropes are different from that of individual SWNTs: bundling broadens SWNT energy levels and red-shifts their overall band gap energy. 16 Bundling of nanotubes also results in intertube energy transfer that completely quenches their fluorescence. 4,17 However, dispersing and isolating nanotubes in surfactant micellar structures allows them to fluoresce. 4 These suspensions have some limitations because they are highly sensitive to environmental conditions, such as surfactant concentration, and the presence of salts in the solution. 18 For example, drying or cooling the SWNT suspension causes aggregation and forces the majority of suspended nanotubes to rebundle into ropes. In addition to general problems with stability, for aqueous suspensions it is very difficult to observe fluorescence from SWNTs with diameters larger than 1.2 nm due to strong absorption by water in the near-and midinfrared. 4

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