Barcoded metal nanowires: Optical reflectivity and patterned fluorescence

Metallic "barcodes" have been reported recently in which the size and location of distinguishable metal segments (e.g., Au and Ag) are used to encode information [Nicewarner-Peña et al., Science 2001, 294, 137-141]. Barcode readout is accomplished by conventional brightfield reflectance optical microscopy. Herein we report the wavelength-dependent optical reflectivity of individual stripes in metallic barcodes, and how this wave-length-dependence impacts the intensity of fluorescence from sandwich immuno- and hybridization assays performed on the particle surface. The encoded particles used in this study were striped nanowires on the order of 4-8 μm in overall length, with individual stripes typically on the order of 1-2 μm, and diameters ∼320 nm. Reflectivity measurements were made for several metals (Ag, Cu, Co, Ni, Pd, and Pt) relative to Au, which was used as an internal standard. Despite the subwavelength diameters of these nanowires, good agreement was found between experimentally determined reflectivities and bulk metal values. Under some conditions, fluorescence intensity patterns corresponding to the underlying metal segments could be observed. We find that the ratio of fluorescence intensities on different metal segments correlate with the metal reflectivity ratios at the excitation and emission wavelengths for the dye. Surface roughness and chemical effects may also play a role for some metals. We have shown that by choice of the underlying metal, particle striping patterns can be accentuated or hidden in the fluorescence image. This is demonstrated in a triplexed DNA hybridization assay.