Peña-Rodríguez and Pal Nanoscale Research Letters 2011, 6:279 http://www.nanoscalereslett.com/content/6/1/279 N A N O E X P R E S S Open Access Enhanced plasmonic behavior of bimetallic (Ag-Au) multilayered spheres Ovidio Peña-Rodríguez1*† and Umapada Pal2† Abstract In this article we study the plasmonic behavior of some stable, highly biocompatible bimetallic metal-dielectric- metal (MDM) and double concentric nanoshell (DCN) structures. By simply switching the material of the inner structure from Au to Ag, the intensity of their surface plasmon resonance could be increased in the optical transparency region of the human tissues up to 20 and 60 percent for the MDM and DCN, respectively, while the biocompatibility is retained. The obtained results indicate that these novel structures could be highly suitable for surface enhanced Raman scattering and photothermal cancer therapy. Background Surface plasmon resonance (SPR), which comes from the collective oscillation of conduction electrons, domi- nates the optical spectra of metallic nanoparticles (NPs), making them attractive for many potential applications. For example, the dependence of their SPR frequency on the dielectric constant of embedding medium [1] can be used for cancer treatments [2,3], and biological or che- mical sensing [4-6]. Moreover, the intense electromag- netic fields produced by the SPR in the surroundings of the nanoparticle are essential in surface-enhanced Raman scattering (SERS) [7], which in turn has impor- tant applications in areas such as medical diagnostics [8] and immunoassay [9,10]. Traditionally, gold and silver have been the preferred materials for the synthesis of nanoparticles [11-14], and both of them have some advantages and disadvantages. Gold NPs are easier to synthesize, have better biocompat- ibility and long-term stability but silver NPs have a more intense SPR, which is of great advantage for SERS and sensing applications. On the other hand, it can be attrac- tive [15] to use bimetallic NPs, where the advantages of both materials can be combined to obtain structures with improved optical response. Nevertheless, while it has been demonstrated [15] that three-layered nanoshells of SiO2-Au-Ag can theoretically improve their optical * Correspondence: ovidio@bytesfall.com † Contributed equally 1Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, Bellaterra, Barcelona, 08193, Spain Full list of author information is available at the end of the article response, those structures present some practical pro- blems related to the difficulty of maintaining separate gold and silver layers, as they tend to form alloys [16-18]. Multilayered nanoshells or nanomatryushkas are a new kind of particles which have been synthesized recently [19-21], and studied theoretically [22-25]. In addition to their better SPR tunability, there appears no problem of alloying in such nanostructures as the metal- lic layers are separated by dielectric ones. In spite of this potential advantage, there exists no systematic study in literature on the advantages of replacing monometallic multilayered structures with bimetallic ones. In this work, we have used classical Mie calculations [26-28] to study two different kinds of bimetallic multi- layered nanoshells: metal-dielectric-metal (MDM) struc- tures and double concentric nanoshells (DCNs). The bulk values of Ag and Au dielectric functions reported by Johnson and Christy [29] were used to calculate their opti- cal responses after applying the usual size correction [30]. It has been observed that the configurations containing sil- ver at the inner layer and gold at the outer are particularly advantageous as the SPR intensity can be increased without compromising biocompatibility and stability of the nano- particles. Obtained results have been explained in terms of the theory of plasmon hybridization. These bimetallic struc- tures could be used as excellent replacements for monome- tallic ones in most sensing and SERS-based applications. Procedure In the present article, we have studied bimetallic MDM structures and DCNs with geometries as shown in © 2011 Peña-Rodríguez and Pal; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.