Enhanced plasmonic behavior of bimetallic (Ag-Au) multilayered spheres
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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.
KeywordsSurface Plasmon Resonance SERS Extinction Efficiency Plasmon Hybridization Plasmon Resonance Mode
double concentric nanoshell
surface enhanced Raman scattering
surface plasmon resonance.
Surface plasmon resonance (SPR), which comes from the collective oscillation of conduction electrons, dominates 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  can be used for cancer treatments [2, 3], and biological or chemical sensing [4, 5, 6]. Moreover, the intense electromagnetic fields produced by the SPR in the surroundings of the nanoparticle are essential in surface-enhanced Raman scattering (SERS) , which in turn has important applications in areas such as medical diagnostics  and immunoassay [9, 10].
Traditionally, gold and silver have been the preferred materials for the synthesis of nanoparticles [11, 12, 13, 14], and both of them have some advantages and disadvantages. Gold NPs are easier to synthesize, have better biocompatibility 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 attractive  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  that three-layered nanoshells of SiO2-Au-Ag can theoretically improve their optical response, those structures present some practical problems related to the difficulty of maintaining separate gold and silver layers, as they tend to form alloys [16, 17, 18].
Multilayered nanoshells or nanomatryushkas are a new kind of particles which have been synthesized recently [19, 20, 21], and studied theoretically [22, 23, 24, 25]. In addition to their better SPR tunability, there appears no problem of alloying in such nanostructures as the metallic 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, 27, 28] to study two different kinds of bimetallic multilayered nanoshells: metal-dielectric-metal (MDM) structures and double concentric nanoshells (DCNs). The bulk values of Ag and Au dielectric functions reported by Johnson and Christy  were used to calculate their optical responses after applying the usual size correction . It has been observed that the configurations containing silver 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 nanoparticles. Obtained results have been explained in terms of the theory of plasmon hybridization. These bimetallic structures could be used as excellent replacements for monometallic ones in most sensing and SERS-based applications.
The energy level diagram for plasmon hybridization in the studied MDM (red) and DCN (blue) structures is depicted in Figure 1b. The plasmon resonance in a MDM [DCN] structure can be viewed as the interaction between the plasmon responses of the inner sphere [nanoshell] (|ωs〉 [ Open image in new window and Open image in new window ]) and the outer ( Open image in new window and Open image in new window ) nanoshell. Three hybridized modes are obtained for the MDM, the energy mode Open image in new window corresponds to the antisymmetric coupling between the symmetric plasmon resonance modes of the outer ( Open image in new window ) nanoshell and the sphere plasmon. The coupling between the higher-energy antibonding mode of the outer nanoshell and the nanosphere plasmon modes is very weak and only one hybridized mode is produced in this case ( Open image in new window ). In contrast, four hybridized modes are produced in the DCN structure because its inner nanoshell has two energy modes. The energy mode Open image in new window ( Open image in new window ) corresponds to the antisymmetric (symmetric) coupling between the symmetric plasmon resonance modes of the inner ( Open image in new window ) and outer ( Open image in new window ) nanoshells. On the other hand, the energy mode Open image in new window ( Open image in new window ) corresponds to the symmetric (antisymmetric) coupling between the antisymmetric plasmon resonance modes of the inner ( Open image in new window ) and outer ( Open image in new window ) nanoshells (Figure 1c). Although, in principle, there exists also a coupling between the antisymmetric and symmetric plasmons of the separate nanoshells, it has only a small influence on the hybridized modes, due to the large energy separation between those two modes .
The characteristics of the surface plasmon resonance obtained for the multilayer structures studied in this work primarily depend on the properties of the SPR of the two constituents metal layers (their composition and thickness) and the strength of the coupling between them (defined by the thickness of the dielectric layer that separates them). Since the dependence of the optical response in terms of geometrical parameters has been previously studied in detail for both structures [22, 23], we focus only on the influence of the composition. For this, we have selected a configuration where both the dielectric layer thickness and the total size of the particles are constant, while the thickness of the metal layers is simultaneously varied (inversely), in order to obtain an opposite behavior between the inner and outer energy modes (one red-shifts when the other does the contrary). For this configuration, the optical response of the bimetallic nanostructures can be tuned to find an optimum relation between t1 and t3 yielding the maximum gain in intensity for a given red-shift. Moreover, the optical response can be analyzed across the full range where it can vary in a single simulation. Calculated values were compared with those obtained for the equivalent Au-only structures in both cases.
Results and discussion
The intermediate region, which includes thicknesses of the outer layer ranging from 5 to 10 nm, is the most interesting because appreciable increases in the intensity of the Open image in new window mode are obtained for the bimetallic particles, at the expense of a small blue-shift. When both effects are weighted, a net gain of intensity is obtained, revealing the advantage of using bimetallic structures. The improvement in intensity can be explained by considering the characteristics of the geometry as well as the differences between the SPRs of silver and gold. Firstly, in this configuration the internal and external energy modes are closer to each other, resulting in a greater coupling between them, and consequently, both have some influence over the two hybridized modes. Moreover, the SPR of silver is considerably more intense than that of gold, and appears at higher energy (lower wavelength). Thus, the changes in the Open image in new window mode are driven by the increased influence of the inner metallic layer. Fortunately, the increase in intensity dominates over the red-shift, improving the optical response. On reducing the value of t3, the Open image in new window mode red-shifts, getting away from the Open image in new window mode and reducing their cross-influence over the hybridized modes, and then the differences in the Open image in new window mode between the Au-Au and Ag-Au configurations are erased (first region).
For the third region (t3 > 10 nm), again there are few differences between the two compositions; here the bonding hybridized mode keeps blue-shifting until it almost disappears, "absorbed" by the antibonding one. This effect is produced by the shielding caused by the thick outer layer, through which the light does not "see" the inner region and, therefore, the particle behaves almost like a single Au nanoshell.
By manipulating structural parameters of bimetallic MDM (DCN) structures, a gain in intensity of Open image in new window mode up to 20 (60) percent can be achieved over their Au-only counterparts in the region of transparency of human tissues. The condition for such gains is that the outer metal layer has a thickness in the range of 5 to 10 nm; in this configuration the internal and external energy modes are closer, so that the interaction between them is greater and, consequently, both have some cross-influence over the hybridized modes. Thus, the Open image in new window energy mode "inherits" the spectral position of the energy mode of the outer metallic layer and the intensity of the inner one. Our designed bimetallic nanostructures could be more suitable than the conventional mono metallic nanoparticles and nanoshell structures for SERS and cancer therapy applications.
O. Peña-Rodríguez thanks DGAPA-UNAM and ICMAB-CSIC for extending a postdoctoral fellowship through the UNAM-CSIC agreement.
This article is published under license to BioMed Central Ltd. 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.