Optimization of Three-Layered AuâAg Bimetallic Nanoshells for Triple

Article pubs.acs.org/JPCC

Optimization of Three-Layered Au−Ag Bimetallic Nanoshells for Triple-Bands Surface Plasmon Resonance Jian Zhu,† Jian-Jun Li,† Lin Yuan,‡ and Jun-Wu Zhao*,† †

The Key Laboratory of Biomedical Information Engineering of Ministry of Education, School of Life Science and Technology, Xi’an Jiaotong University, Xi’an 710049, China ‡ Department of Optical Information Science and Technology, School of Science, Xi’an Jiaotong University, Xi’an 710049, China ABSTRACT: Plasmonic light absorption properties of three-layered Au−Ag bimetallic nanoshells are investigated using the quasi-static electricity and plasmon hybridization theory. Comparing with Audielectric-Au and Ag-dielectric-Au multilayer nanoshells, Au-dielectric-Ag multilayer nanoshells are demonstrated more suitable for triple-bands surface plasmon resonance (SPR). Calculation results indicated that the triple plasmon resonance in Au-dielectric-Ag multilayer nanoshells could be optimized by choosing appropriate geometrical dimensions of the inner gold sphere and outer silver shell. The corresponding physical mechanism has also been illustrated by analyzing the surface charge distribution and intercoupling of the polarization fields from different metal−dielectric interfaces. This triple-bands SPR in the bimetallic multilayer nanostructure provides potential for multiplex biosensing based on SPR absorption, surface enhanced fluorescence, and surface enhanced Raman scattering (SERS). of inner and outer interfaces.8 However, the shorter wavelength SPR bands in nanorods and nanoshells are too weak to be used. Although a multiplex biosensor using Au nanorods has already been reported, Au nanorods with different aspect ratios must be fabricated.9 Because the frequency dependent longitudinal SPR is affected by the aspect ratio of Au nanorods, multiplex sensing was demonstrated by the plasmonic response to binding events of three targets.9 In order to achieve double-channel plasmonic sensing, two equal intensity plasmon resonances are necessary. Recent research demonstrated that the Au−Ag bimetallic nanostructures can provide two clearly separated SPR peaks. Chakravadhanula et al. reported the equal intensity double SPR of Au−Ag bimetallic quasi-nanocomposites based on sandwich geometry.10 By constructing Au−Ag dual-metal array three-electrode on-chip, Du et al. also developed a microfluidic electrochemical aptamer-based sensor for multiplex detection of small molecules.11 In recent years, metallic nanostructures with triple-bands SPR have also been studied. Because of the in-plane and out-ofplane dipole and quadrupole resonance, metallic triangular nanoplates have at most three SPR bands.12 However, the shorter wavelength peak is also very weak. Three-layered metaldielectric nanoshells are a new type of nanostructure. Because of the plasmon coupling between the inner metal sphere and outer metal shell, the SPR properties are more abundant and tunable.13−15 In Au-dielectric-Au multilayer nanoshells, there are at most three SPR bands corresponding to the |ω−+ ⟩, |ω+−⟩,

1. INTRODUCTION Surface plasmon resonance (SPR) in noble nanostructures has found great potential applications in optical sensing, biologic detection, and medical imaging due to the tunable intensity and peak wavelength of SPR by changing the shape, structure, and local environment of nanoparticles. Because of their high sensitivity to very small variations of the local environmental dielectric function, SPR has been widely used in chemical and biologic sensing.1,2 SPR is collective oscillation of the conduction band electrons, which also induces the electric field enhancement close to the nanoparticles. This SPR induced local field enhancement (LFE) is generally agreed that an important contribution to the surface-enhanced Raman spectroscopy (SERS) and surface enhanced fluorescence (SEF). Because the SPR and LFE of metal nanostructures are frequency dependent, the overlap of the excitation or emission bands of the luminescent molecular with the SPR band of the metal nanoparticles is an important factor affecting the intensity of SERS and SEF.3−5 Thus, metallic nanostructures with multiple SPR bands at different wavelengths could be used to design multiplex nanosensors for detecting several different kinds of binding targets at the same time by altering the light wavelength. It is well-known that there is only one SPR band in spherical metallic nanoparticles (520 nm for Au nanosphere and 400 nm for Ag nanosphere). Therefore, most of the study about plasmonic nanoparticle enhanced chemical and biologic sensing and detecting are single channeled.6 Further investigations show the rod-like metallic nanoparticles exhibit two SPR bands because of the shape anisotropy induced transverse and longitudinal resonances.7 Furthermore, metallic nanoshells also have two SPR bands because of the plasmon hybridization © 2012 American Chemical Society

Received: February 14, 2012 Revised: May 2, 2012 Published: May 8, 2012 11734

dx.doi.org/10.1021/jp301470p | J. Phys. Chem. C 2012, 116, 11734−11740

The Journal of Physical Chemistry C

Article

and |ω−−⟩ modes, respectively.16 However, the |ω−+ ⟩ mode that resulted from symmetric coupling between the antibonding shell plasmon mode and the inner sphere plasmon is usually too weak to be observed. Thus, only two plasmon bands could be used for pure metallic three-layered nanoshells. If the |ω−+ ⟩ plasmon mode could be enhanced, there will be three intense and separate SPR bands for multiplex sensing and detecting. In this paper, the advantage of both multishell structure and bimetallic combination are combined to achieve triple-bands SPR. By using different metals, gold or silver, as the inner sphere and outer shell, triple-bands SPR in Au-dielectric-Ag multilayer nanoshells are obtained. Furthermore, the triple plasmon resonance in Au-dielectric-Ag multilayer nanoshells has also been optimized by tuning the geometrical dimensions.

where εb(ω) is the dielectric function of bulk metal (Au or Ag) which is dependent on the light frequency.18 τ is the size limit relaxation time of Au−Ag multishells. ωp denotes the plasmon frequency of the bulk metal (Au or Ag), and ω is the frequency of the incident electromagnetic wave. These numerical parameters of gold and silver can be found from refs 18−20.

3. RESULTS AND DISCUSSION 3.1. Absorption Spectra of Three-Layered Au−Ag Bimetallic Nanoshells. The absorption spectra of Au-

2. THE MODEL AND PROCEDURE In this study, we examine the SPR properties of metaldielectric-metal multilayer nanoshells. Figure 1 illustrates the

Figure 1. Schematic representation of three-layered Au−Ag bimetallic nanoshells.

geometrical structure of bimetallic multilayer nanoshells. This multilayer nanostructure consists of a metallic core with a radius r1 and the dielectric constant ε1, a concentric metallic shell with a inner radius r2, outer radius r3, and the dielectric constant ε3. The metal core and shell are separated by a dielectric layer with a thickness of r2−r1 and dielectric constant ε2 = 4.2. In our previous study,16 it was found that the |ω−+ ⟩ mode is more easily obtained in the absorption spectrum when ε2 is large. Therefore, ε2 = 4.2 has been chosen in the calculation. The total multilayer nanostructure is embedded in a dielectric media with the dielectric constant ε4. For noble metal sphere and shell, the dielectric constants ε1 and ε3 are complex numbers and functions of light wavelength.17 In the study, the overall diameter of the multilayer nanoshells was kept less than 50 nm, which is much smaller than the light wavelength, and consequently, the absorption spectral properties can be understood with the quasi-static theory.16 In this quasi-static calculation, the dielectric functions of gold and silver are obtained by using the Drude model17 ωp2

εmetal(ω) = εb(ω) −

ω2

1+

1 ω2τ 2

Figure 2. Absorption spectra of Au-dielectric-Ag, Ag-dielectric-Au, and Au-dielectric-Au multilayer nanoshells, calculation results by using (A) quasi-static theory and (B) FDTD simulations. [r1, r2, r3] = [8, 17, 20] nm; ε2 = 4.2 and ε4 = 1.3.

dielectric-Ag, Ag-dielectric-Au, and Au-dielectric-Au multilayer nanoshells are compared in Figure 2A. In the case of Audielectric-Au multilayer nanoshells, one can find three absorption peaks at 515, 620, and 885 nm, which are corresponding to |ω+−⟩, |ω−+ ⟩, and |ω−−⟩ plasmon modes, respectively. The physical origin of these three plasmon modes has already been illustrated by the plasmonic hybridization theory.14,16 The |ω−+ ⟩ plasmon mode corresponds to the symmetric coupling between the outer antibonding shell plasmon and the inner sphere plasmon; the |ω+−⟩ plasmon mode corresponds to the symmetric coupling between the outer bonding shell plasmon mode and the inner sphere plasmon; and the |ω−−⟩ plasmon mode corresponds to the antisymmetric coupling between the outer bonding shell plasmon mode and the inner sphere plasmon.14,16 In the case

ωp2

+i

ω2

(ωτ + ωτ1 )

(1) 11735



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Au-dielectric-Au multilayer nanoshells. Thus, there are three intense and clearly separated plasmonic absorption bands in this bimetallic multilayer nanostructure, which provide potential for multiplex biosensing based on plasmonic absorption and surface enhanced Raman scattering (SERS). This multiplex plasmonic nanostructure can be used to detect three kinds of binding targets simultaneously by choosing the incident wavelength. As is known, the excitation of a higher order mode due to retardation will not take place by using the quasi-static calculation. However, in order to prove the absorption peak at shorter wavelength is not the excitation of a bonding quadruple mode, we also calculate the absorption spectra by using the finite difference time domain (FDTD) simulation.21,22 In this study, the FDTD simulations are calculated by using FDTD solutions (version 7.5) software provided by Lumerical Solutions, Inc. As shown in Figure 2B, the absorption spectral properties from FDTD calculation are similar to the calculation results based on quasi-static theory. For Au-dielectric-Ag multilayer nanoshells, the absorption peak corresponding to the |ω−+ ⟩ plasmonic mode still arises at about 400 nm and is intense and distinct. Therefore, we believe that the absorption peak at shorter wavelength is not the higher order SPR mode. 3.2. The Physical Origin of SPR Modes in Au−Ag Bimetallic Multilayer Nanoshells. The physical picture of the plasmon modes in pure metallic three-layer nanoshells has already been illustrated by the plasmonic hybridization

Figure 3. Schematic representation of the plasmonic hybridization of the |ω+−⟩ mode and polarization fields in the Au−Ag bimetallic multilayer nanoshells.

of Ag-dielectric-Au multilayer nanoshells, only two plasmonic peaks corresponding to the |ω+−⟩ and |ω−−⟩ modes have been observed in the absorption spectrum. The |ω−+ ⟩ plasmonic mode is absent as the inserted sphere is changed from gold to silver. However, in the case of Au-dielectric-Ag multilayer nanoshells, the |ω +− ⟩ plasmonic mode arises and the corresponding absorption peak is much greater than that of

Figure 4. Surface charge distribution and local field enhancement of three-layered Au−Ag bimetallic nanoshells: (A) surface charge distribution and (B) local field enhancement of Au-dielectric-Au multilayer nanoshells at 515 nm; (C) surface charge distribution and (D) local field enhancement of Au-dielectric-Ag multilayer nanoshells at 400 nm. 11736



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Figure 5. The effect of inner gold sphere radius on the environmental refractive index dependent absorption spectra of Au-dielectric-Ag multilayer nanoshells: (A) r1 = 3 nm; (B) r1 = 6 nm; (C) r1 = 9 nm; (D) r1 = 12 nm.

theory.14,16 However, there are some new observations when the material (gold or silver) of inner sphere or outer shell has been changed to other metal (silver or gold). In the case of the |ω−+ ⟩ mode, the antibonding plasmon in the outer gold shell has a very small dipole moment because the cavity plasmon (the polarization field P2 corresponding to the inner surface of the gold shell) is oppositely aligned with the sphere plasmon (the polarization field P1 corresponding to the outer surface of the gold shell),13 as shown in Figure 3. Although the inner sphere plasmon (corresponding to P3) is conformably aligned with the sphere plasmon of the outer gold shell and enhances the dipole moment of the |ω−+ ⟩ mode, the effect from the cavity plasmon of the outer gold shell is greater. Therefore, the absorption peak corresponding to the |ω−+ ⟩ mode is very weak, which can be demonstrated by the absorption spectrum of Au-dielectric-Au multilayer nanoshells. When the inner gold sphere has been changed to a silver sphere, one can obtain a bimetallic Ag-dielectric-Au multilayer nanoshell. Because the plasmonic coupling between two different metals is relatively weak, the positive effect from the inner silver sphere plasmon to the sphere plasmon of the outer gold shell is very weak. Therefore, the dipole moment of the |ω−+ ⟩ mode will not be enhanced and is too weak to be observed in the absorption spectrum. On the other hand, one can obtain a bimetallic Au-dielectric-Ag multilayer nanoshell, as the outer gold shell has been changed to a silver shell. Because silver nanoparticles have a stronger and sharper SPR than that of gold23 and the plasmon of the outer silver shell has a greater

effect on the total plasmon of the multishells, the dipole moment of the |ω−+ ⟩ mode becomes intense. In order to demonstrate the antibonding nature of the outer shell, we also plotted the surface charge distribution and local electric field patterns of the three-layered Au−Ag bimetallic nanoshells in Figure 4 by using the FDTD simulation. As shown in Figure 4A and B, in the antibonding shell plasmon, different kinds of charges signed on the inner and outer surfaces of the outer gold shell, and the intense electric field is focused in the gold shell. From Figure 4A, one can also find the surface charge distribution type of the inner gold sphere is the same as that of the outer surface of the gold shell. However, the effect from the cavity plasmon of the outer gold shell is greater; thus, both the charge density at the outer surface of the gold shell and the local field in the gold shell are relatively weak. The charge distribution and local field patterns of the Au-dielectricAg multilayer nanoshell are plotted in Figure 4C and D. One can find both the charge density at the outer surface of the silver shell and the local field in the silver shell are much greater than that of the Au-dielectric-Au multilayer nanoshell. Therefore, the absorption peak corresponding to the dipole moment of the |ω−+ ⟩ mode of the Au-dielectric-Ag multilayer nanoshell is more intense. However, the charge density at the inner gold core surface is too weak compared with that of the outer silver shell of the Au-dielectric-Ag multilayer nanoshell. Therefore, the charge distributions on the inner core surface are not visible in Figure 4C. In order to observe the charge distribution fashion, the charge distributions around the inner gold core 11737



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Figure 7. The effect of surface scattering on the absorption spectrum of Au-dielectric-Ag multilayer nanoshells. [r1, r2, r3] = [8, 17, 20] nm; ε2 = 4.2 and ε4 = 1.3.

Figure 6. The effect of outer silver shell radius on the environmental refractive index dependent absorption spectra of Au-dielectric-Ag multilayer nanoshells: (A) r3 = 18 nm; (B) r3 = 21 nm; (C) r3 = 24 nm.

Figure 8. The environmental refractive index dependent absorption spectra of Au-dielectric-Ag multilayer nanoshells with consideration of surface scattering: (A) [r1, r2, r3] = [9, 15, 20] nm; (B) [r1, r2, r3] = [8, 15, 18] nm. ε2 = 4.2 and ε4 = 1.3.

have been replotted as an inset of Figure 4C, and the color scale has been readjusted to make the contour plot visible. 3.3. Optimization of Triple Plasmon Resonance in AuDielectric-Ag Multilayer Nanoshells. The goal of this section is to explore the optimization of three-layered Au−Ag bimetallic nanoshells for triple-bands plasmonic sensing. Figure 5 shows the environmental refractive index dependent absorption spectra of Au-dielectric-Ag multilayer nanoshells with different inner gold sphere radii. It is obvious that the |ω+−⟩ and |ω−−⟩ modes are more sensitive to the environmental refractive index. Increasing the surrounding dielectric constant ε4 leads to the |ω−−⟩ mode red shifts and gets intense, whereas the |ω+−⟩ mode red shifts and fades down, which are similar to

that of Au-dielectric-Au multilayer nanoshells.16 For the |ω−+ ⟩ mode, the antibonding plasmon hybridization results in the strong coupling and intense local electric field in the metal shell. Thus, the influence from the environment polarization is relatively weak, and the |ω−+ ⟩ mode is not sensitive to the dielectric environment. Increasing ε4 only leads to the |ω−+ ⟩ mode red shifts and fades down slightly. One can find that the surrounding dielectric constant affects the plasmon intensity of the |ω−+ ⟩, |ω+−⟩, and |ω−−⟩ modes in different ways. Therefore, choosing an appropriate value of ε4 is important for making the 11738



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Au-dielectric-Ag multilayer nanoshell with consideration of surface scattering are plotted in Figure 7. In this case, the effective mean free path dependent relaxation time has been taken into account into the dielectric function of the silver shell.26−28 One can find the broadening of the surface plasmon absorption peaks due to the intrinsic size effect. However, there are still three intense and clearly separated SPR bands in the absorption spectrum. The effects of r1 and r3 on the frequency separation of those three plasmon absorption bands also have not changed with consideration of surface scattering. As shown in Figure 8, triple-bands plasmon resonance could also be obtained when the inner gold sphere radius is not too small or too large ([r1, r2, r3] = [9, 15, 20] nm), and the thickness of the outer silver shell is relatively small ([r1, r2, r3] = [8, 15, 18] nm). These calculation results with consideration of surface scattering indicate that, although the surface scattering leads to the widening of the absorption bands and defeats the purpose of multiplexing to some extent, there are still three intense and clearly separated plasmonic absorption bands in the Au-dielectric-Ag multilayer nanoshell, which provides potential for multiplex biosensing.

Au-dielectric-Ag multilayer nanoshells exhibit triple plasmon resonance, especially equal three plasmon absorption bands. For example, in Figure 5B, three intense plasmon absorption bands with similar intensity could be obtained when ε4 = 2.0. Although getting three plasmon absorption bands with equal intensity needs an accurate environmental dielectric constant, triple-bands intense plasmons could be obtained within a wide surrounding dielectric range (from 1.5 to 5.0), as shown in Figure 5C. Figure 5 also indicates that the inner gold sphere radius r1 takes great effect on the frequency separation of those three plasmon absorption bands. As shown in Figure 5A, too small r1 (r1 = 3 nm) leads to the |ω+−⟩ and |ω−−⟩ modes blending together. With increasing r1, the |ω+−⟩ and |ω−−⟩ modes separate from each other (r1 = 6 nm), and they are optimized when r1 = 9 nm, as shown in Figure 5B and C. However, further increasing of r1 leads to the |ω−−⟩ mode fading down and disappearing (r1 = 12 nm), as shown in Figure 5D. Thus, too big or too small inner gold sphere radii are all not appropriate to obtain the triple plasmon resonance. The effects of the overall radius r3 of the outer silver shell on the environmental refractive index dependent absorption spectra of Au-dielectric-Ag multilayer nanoshells are shown in Figure 6. The changing of intensity and peak wavelength of the three plasmon bands as a function of the surrounding dielectric constant are similar to that of the effect from r1. However, the effect of r3 on the frequency separation of those three plasmon absorption bands is different from the effect from r1. Increasing r3 leads to the |ω−−⟩ mode blue shift, whereas the position of the |ω+−⟩ mode is not sensitive to r3. Therefore, the |ω+−⟩ and |ω−−⟩ modes blend together as r3 is increased. Thus, a too large outer silver shell radius is not suitable for obtaining the triple plasmon resonance. It is obvious that the shifting of the |ω−−⟩ mode takes great effect on the triple plasmon resonance. Therefore, our next goal is to illustrate the physical mechanism of the geometrical tenability of the shifting of the |ω−−⟩ mode. As is known, the |ω−−⟩ mode corresponds to the antisymmetric coupling between the outer bonding shell plasmon mode and the inner sphere plasmon.14 In the bonding interaction of the outer shell plasmon mode, the same kind of charges (negative or positive) signed on both the inner and outer surfaces of the outer silver shell and the electric field in the silver shell is very weak. Therefore, the bonding mode is a weak coupling. Because the coupling strength between the inner and outer surface of the silver shell is decreased as the shell thickness is increased, increasing the outer shell radius leads to the increasing of the shell thickness and the blue shift of the |ω−−⟩ mode. When the inserted gold sphere is antisymmetric coupled with the outer silver shell, the surface charge distribution of the inner gold sphere is opposite to that of the outer silver shell. Then, different kinds of charges sign on the inner and outer surfaces of the middle dielectric layer. The attractive Coulombic force between positive and negative charges will further weaken the bonding plasmon of the outer silver shell. Therefore, increasing the inner sphere radius enhances the attractive Coulombic force and weakens the bonding plasmon of the outer nanoshell, which leads to the red shift of the |ω−−⟩ mode. As is known, the damping of plasmonic resonance is due to the surface electron scattering. In small metallic particles, the scattering of the surface electrons results in a loss of electron phase coherence, which in turn results in a broadening of the plasmon absorption band.24,25 The absorption spectra of the

4. CONCLUSIONS In summary, we have studied the SPR properties of Au−Ag bimetallic multishell nanostructures. Theoretical calculations based on quasi-static electricity and plasmon hybridization show that there are three intense and clearly separated plasmonic absorption bands corresponding to the |ω−+ ⟩, |ω+−⟩, and |ω−−⟩ modes in Au-dielectric-Ag multilayer nanoshells. This multiplex plasmonic nanostructure can be used to detect three kinds of binding targets simultaneously by choosing the incident wavelength. The triple plasmon resonance in Audielectric-Ag multilayer nanoshells could be optimized by tuning the geometrical dimensions. It has been demonstrated that too large or too small inner gold sphere radii are not suitable to obtain the triple plasmon resonance. The relatively small thickness of the outer silver shell is suitable for obtaining the triple-bands plasmon resonance.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 86-29-82664224. Fax: 86-29-82664224. E-mail: [email protected] Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Program for New Century Excellent Talents in University under Grant No. NCET-100688 and the Fundamental Research Funds for the Central Universities, and the National Natural Science Foundation of China under Grant Nos. 11174232, 61178075, and 81101122.

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REFERENCES

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Optimization of Three-Layered AuâAg Bimetallic Nanoshells for Triple

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