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Nanoshells are spherical shells (typically gold) on the order of ~5 nm to ~50 nm thick on solid (typically silica) or empty cores on the order of ~100 nm in diameter. These composite particles (coated spheres from the point of view of the light scattering theory, for example, Kerker 1969) have remarkable optical properties which can be modified over a wide spectral range from the the visible (>400 nm) to mid-infrared (~10 µm) by selecting the particle size and structure (for example, Loo et al 2004, Oldenburg et al 1998). As noted, for example, by Borghese et al (1987b), the shapes of attenuation spectra of nanoshells are controlled by the relative thickness of the metal shell. The shell thickness also controls relative contributions of light scattering or absorption to the attenuation of light by these particles. With their optical properties easily "tuned" and optical cross sections some 6 orders of magnitude greater than molecular markers and absorbers (for example, Loo et al 2004), the nanoshells are being vigorously researched with a view towards their applications, for example, as contrast agents in optical coherence tomography or as absorbers of light in cancer theraphy.
Nanoshells (a 100 nm polystyrene microsphere coated with a 15-20 nm thick gold crescent; a strong light absorption band at ~780 nm) have recently been used to propel a small water droplet on a hydrophobic substrate (Liu et al 2006). An edge of the droplet containing the nanoshells was illuminated with a 20 mW laser beam at a wavelength of 785 nm. Absorption of light by the nanoshells caused local heating and evaporation of the droplet. The vapor condensed at the substrate in the immediate vicinity of the droplet edge and quickly coalesced into microdroplets which eventually merged with the parent droplet, advancing its edge. This optically-controlled flow, albeit slow for an unconfined droplet, can be signifcantly accelerated when the droplet is confined to withing a microchannel. Liu et al achieved fluid velocities on the order of 10 µm s-1 in a 10 µm wide microchannel with a submilliwatt light beam.
| CITATION: Jonasz M. 2006. Spectroscopy of nanoshells (www.tpdsci.com/Tpc/OptNsh.php). In: Top. Part. Disp. Sci. (www.tpdsci.com). |
HISTORY: Published: 10-Apr-2006 Modified: 31-May-2006 Reviewed: PENDING |
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