Dependence of SERS enhancement on the chemical composition and structure of Ag/Au hybrid nanoparticles

Abstract

Noble metal nanoparticles (NPs) such as silver (Ag) and gold (Au) have unique plasmonic properties that give rise to surface enhanced Raman scattering (SERS). Generally, Ag NPs have much stronger plasmonic properties and, hence, provide stronger SERS signals than Au NPs. However, Ag NPs lack the chemical stability and biocompatibility of comparable Au NPs and typically exhibit the most intense plasmonic resonance at wavelengths much shorter than the optimal spectral region for many biomedical applications. To overcome these issues, various experimental efforts have been devoted to the synthesis of Ag/Au hybrid NPs for the purpose of SERS detections. However, a complete understanding on how the SERS enhancement depends on the chemical composition and structure of these nanoparticles has not been achieved. In this study, Mie theory and the discrete dipole approximation have been used to calculate the plasmonic spectra and near-field electromagnetic enhancements of Ag/Au hybrid NPs. In particular, we discuss how the electromagnetic enhancement depends on the mole fraction of Au in Ag/Au alloy NPs and how one may use extinction spectra to distinguish between Ag/Au alloyed NPs and Ag-Au core-shell NPs. We also show that for incident laser wavelengths between ∼410 nm and 520 nm, Ag/Au alloyed NPs provide better electromagnetic enhancement than pure Ag, pure Au, or Ag-Au core-shell structured NPs. Finally, we show that silica-core Ag/Au alloy shelled NPs provide even better performance than pure Ag/Au alloy or pure solid Ag and pure solid Au NPs. The theoretical results presented will be beneficial to the experimental efforts in optimizing the design of Ag/Au hybrid NPs for SERS-based detection methods.

Hollow Au–Ag Alloy Nanorices and Their Optical Properties

“…The scattering spectra of a hollow Au–Ag alloy nanorice (with a 570 nm length and a 90 nm width) exhibits three higher-order plasmon resonances at approximately 583 nm, 716 nm, and 916 nm, respectively, as shown in Figure 2 c. The simulated scattering spectrum and the corresponding hollow nanorice model are shown in Figure 2 d. The results show that the main peak of the hollow Au–Ag alloy nanorice (the red line) is satisfactory, in agreement with the experimental result ( Figure 2 c). In addition, with the increase of the Au component, the plasmon resonance red-shifts and the resonant intensity decreases, which is in agreement with the simulation results of previous studies [ 34 ]. This indicates that the multipolar plasmon resonances modes of the hollow alloy nanorices can be excited and regulated by changing their Au component.…”

Cross‐Wavelength Hierarchical Metamaterials Enabled for Trans‐Scale Molecules Detection Simultaneously

“…The result shows that our sample is stable for up to 4 days and the signal begins to attenuate on the fifth day (Figure S7 , Supporting Information). This limitation could be replaced by using other relatively stable nanoparticles [ 36 , 37 ] or adding an external shield on the surface of nanoparticles [ 38 , 39 ] to design more stable metamaterials in future studies. In this work, alternative materials including gold nanoparticles and gold film coated on ordered polystyrene beads were used to enhance the stability of the metamaterial (Figures S8 and S9 , Supporting Information).…”

Modulation of interparticle gap for enhanced SERS sensitivity in chemically stable Ag@Au hetero-architectures

“…In contrast, Au nanoparticles have better chemical stability, but lower SERS enhancement capabilities than Ag nanoparticles of a similar size. 3 As a result, many studies have tried to improve the stability of Ag nanoparticles by forming Ag-Au alloys [4][5][6] or Ag@Au core-shell structures. [7][8][9][10][11] The surface plasmon resonance (SPR) of Ag-Au alloy nanoparticles can be tuned to fall between that of Ag nanoparticles and that of Au nanoparticles based on the Ag to Au atomic ratio.…”