Silica Nanoparticles Doped with Multiple Fluorescent Dyes Demonstrated as Potential New Barcoding Tags
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cover layer on a silicon substrate. While normal atomic force microscopy (AFM) showed a smooth, featureless surface of the top polymeric layer, the SNFUH phase image clearly showed the dispersed, buried gold nanoparticles. SNFUH is particularly well suited for detecting defects and voids in microelectronics structures. For example, the scale of fabrication continues to shrink and interconnect metal lines are approaching 60-nm widths. Model shallow trench structures were fabricated in a dielectric material with a 50-nm-thick silicon nitride layer deposited as a capping layer and etched into the trenches. A 500-nm-thick lowdielectric polymer layer was deposited by spin-coating and annealing. A conventional topography scan showed a uniform and continuous polymer coating at the bottoms of the trenches as well as on the tops of the lines, as seen in Figure 1 (p. 169). However, the corresponding SNFUH phase image revealed embedded voids within the polymer and at the SiN–polymer interfaces. The SNFUH technique can also be used to image embedded or buried sub-
structures in biological samples. A nearcontact mode was used for imaging soft structures, wherein the probe tip is lifted by 2–5 nm after it touches the surface. This was used to obtain high-contrast, high-resolution images of malaria parasites inside infected red blood cells. The direct real-space in vitro imaging was performed without labeling or sectioning the cells under physiologically viable conditions. SNFUH was thus demonstrated to be a versatile technique for nondestructive, high-resolution, real-space imaging of diverse materials systems. In particular, it fills the spatial resolution gap at the 10–100-nm scale for nondestructive subsurface imaging. GOPAL RAO
Silica Nanoparticles Doped with Multiple Fluorescent Dyes Demonstrated as Potential New Barcoding Tags Often in biological imaging experiments, it is desirable to view several components of a structure simultaneously in real time. This can be done with small-molecule fluo-
rescent tags, but this technique is limited by the number of available dyes that are excited at the same wavelength but have distinguishable emission spectra. Quantum dots, polymer microspheres, and other materials have been used to overcome this limitation and create “bio-barcoding” systems, but many of these have problems with performance. In the January 11 issue of Nano Letters (p. 84; DOI: 10.1021/ nl052105b), researchers L. Wang and W. Tan from the University of Florida at Gainesville have presented a novel solution to this problem. The researchers incorporated combinations of three common fluorophores into silica nanoparticles (NPs) to create NPs that respond to monochromatic illumination with a range of distinguishable emission signatures. They also demonstrated the potential for using these materials as tags for biological molecules by functionalizing the NPs with biotin and binding them to avidinfunctionalized microspheres. The researchers incorporated the organic dyes FITC, R6G, and ROX into silica NPs in a variety of r
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