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Annals of Biomedical Engineering, Vol. 34, No. 1, January 2006 ( C 2006) pp. 15–22 DOI: 10.1007/s10439-005-9001-8

Metal Nanoshells

Rice University, Department of Bioengineering, Box 1892, MS 144, Houston, TX 77251-1892, USA and 2 Department of Electrical andComputer Engineering, Box 1892, MS 366, Houston, TX 77251-1892, USA
(Received 19 January 2005; accepted 20 May 2005; published online: 10 March 2006)

Abstract—Metal nanoshells are a new class of nanoparticles with highly tunable optical properties. Metal nanoshells consist of a dielectric core nanoparticle such as silica surrounded by an ultrathin metal shell, often composed of gold forbiomedical applications. Depending on the size and composition of each layer of the nanoshell, particles can be designed to either absorb or scatter light over much of the visible and infrared regions of the electromagnetic spectrum, including the near infrared region where penetration of light through tissue is maximal. These particles are also effective substrates for surface-enhanced Raman scattering(SERS) and are easily conjugated to antibodies and other biomolecules. One can envision a myriad of potential applications of such tunable particles. Several potential biomedical applications are under development, including immunoassays, modulated drug delivery, photothermal cancer therapy, and imaging contrast agents. Keywords—Nanotechnology, immunoassay, controlled release, optical imaging,cancer.

WHAT IS A METAL NANOSHELL? Medieval alchemists were some of the first to discover the plasmon resonance phenomenon when they successfully reduced gold from a salt solution into its reddish colloidal form. The reddish color arises from the metal colloid’s extinction at approximately 520 nm, which results from optical resonances of surface plasmons (or oscillating conducting electrons) in themetal induced by the incident light. Many bulk metals (i.e., Au, Ag, Ni, Pt) demonstrate a plasmon resonance with each metal having a characteristic peak within a defined region of the visible spectrum.5 Plasmon resonance phenomena of metal colloids have enabled several biomedical applications, but these have been limited by the fact that the plasmon resonances of conventionally available materialsfall within the visible range of the spectrum where penetration of light through blood and tissue is low. Nanoshells are a new class of nanoparticles with tunable plasmon resonance, allowing materials to be specifically
Address correspondence to Jennifer L. West , Rice University, Department of Bioengineering, Box 1892, MS 144, Houston, TX 77251-1892, USA. Electronic mail: jwest@rice.edudesigned to match the wavelength required for a particular application, for instance to fall within near infrared (NIR) regions where light penetration through tissue is optimal. Conceived of over 50 years ago1 but not realized until the 1990’s,3,26,50 a metal nanoshell consists of a spherical dielectric nanoparticle surrounded by an ultrathin, conductive, metallic layer. By varying the composition anddimensions of the layers of the nanoparticles, nanoshells can be designed and fabricated with plasmon resonances from the visible to infrared regions of the spectrum.27 For a given composition of core and metal shell, the plasmon resonances of the nanoparticle, which determines the particle’s optical absorption and scattering, may be tuned by changing the ratio of the nanoparticle’s core size toits shell thickness (Fig. 1). Experimentally, the first metal nanoshell, developed by Zhou et al.,50 consisted of an Au2 S dielectric core surrounded by a gold shell. Depending upon the size of the nanoparticles, it was possible to shift the plasmon resonance to longer wavelengths of light, ranging from the standard gold colloid peak of ∼520 out to ∼900 nm. Gold-gold sulfide (Au–Au2 S) nanoshells...
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