Colloids and Surfaces A: Physicochem. Eng. Aspects 290 (2006) 89–105 Synthesis and plasmonic properties of silver and gold nanoshells on polystyrene cores of different size and of gold–silver core–shell nanostructures Ken-Tye Yong a,c , Yudhisthira Sahoo a,b , Mark T. Swihart a,c,∗ , Paras N. Prasad a,b a Institute for Lasers, Photonics and Biophotonics, University at Buffalo, The State University of New York, Buffalo, NY 14260-4200, USA b Department of Chemistry, University at Buffalo, The State University of New York, Buffalo, NY 14260-4200, USA c Department of Chemical and Biological Engineering, University at Buffalo, The State University of New York, Buffalo, NY 14260-4200, USA Received 23 January 2006; accepted 2 May 2006 Available online 10 May 2006 Abstract Simple methods of preparing silver and gold nanoshells on the surfaces of monodispersed polystyrene microspheres of different sizes as well as of silver nanoshells on free-standing gold nanoparticles are presented. The plasmon resonance absorption spectra of these materials are presented and compared to predictions of extended Mie scattering theory. Both silver and gold nanoshells were grown on polystyrene microspheres with diameters ranging from 188 to 543 nm. The commercially available, initially carboxylate-terminated polystyrene spheres were reacted with 2- aminoethanethiol hydrochloride (AET) to yield thiol-terminated microspheres to which gold nanoparticles were then attached. Reduction of silver nitrate or gold hydroxide onto these gold-decorated microspheres resulted in increasing coverage of silver or gold on the polystyrene core. The nanoshells were characterized using transmission electron microscopy (TEM), scanning electron microscopy (SEM) and UV–vis spectroscopy. By varying the core size of the polystyrene particles and the amount of metal (silver or gold) reduced onto them, the surface plasmon resonance of the nanoshell could be tuned across the visible and the near-infrared regions of the electromagnetic spectrum. Necklace-like chain aggregate structures of gold core–silver shell nanoparticles were formed by reducing silver nitrate onto free citrate-gold nanoparticles. The plasmon resonance absorption of these nanoparticles could also be systematically tuned across the visible spectrum. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanoshell; Plasmon resonance; Silver; Gold 1. Introduction Metal colloids are well-known for their surface plasmon resonance (SPR) properties, which originate from collective oscillation of their conduction electrons in response to optical excitation [1–4]. The SPR frequency of a particular metal col- loid sample is different from that of the corresponding metal film and has been shown to depend on particle size [5–7], shape [8,9], and dielectric properties [10], aggregate morphology [11], sur- face modification [12], and refractive index of the surrounding medium [13]. For example, the SPR peak of 13 nm spherical gold colloids is around 520 nm and that of 5–6 nm silver nanoparticles around 400 nm [14]. The SPR peaks shift to the red depending ∗ Corresponding author. Tel.: +1 716 645 2911x2205; fax: +1 716 645 3822. E-mail address: swihart@eng.buffalo.edu (M.T. Swihart). on the particle shape, state of aggregation, and the surround- ing dielectric medium [15]. SPR has been explored for use in fabricating optical filters [16], photon energy transport devices [17], probes for scanning near-field optical microscopy [18], active surfaces for surface-enhanced Raman spectroscopy [19] and fluorescence scattering [20], and chemical or biological sen- sors [21]. Metal nanoshells have shown tremendous promise for sys- tematic engineering of SPR. These are composite nanoparticles that consist of a dielectric core coated with a few nanometers to a few tens of nanometers of a metal, usually gold or silver [22]. The SPR of these nanoparticles can be varied over hundreds of nanometers in wavelength, across the visible and into the infrared region of the spectrum, by varying the relative dimen- sions of the core and the shell. Using the Mie scattering theory and controllable colloidal growth chemistry, the optical reso- nance of a core–shell composite nanoparticle can be “designed” 0927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2006.05.004