Enhancing the Extinction Efficiency and Plasmonic Response of Bimetallic Nanoparticles of Au-Ag in Robust Thin Film Sensing Platforms

Abstract

The extinction efficiency of noble metal nanoparticles (NPs), namely gold (Au) and silver (Ag), are dependent on their size and surrounding dielectric. Exploiting the Localized Surface Plasmon Resonance (LSPR) phenomenon, the composition and structure of the NPs might be tailored to achieve a configuration that optimizes their response (sensitivity) to environmental changes. This can be done by preparing a bimetallic system, benefiting from the chemical stability of Au NPs and the higher scattering efficiency of Ag NPs. To enhance the LSPR sensing robustness, incorporating solid supports in the form of nanocomposite thin films is a suitable alternative. In this context, the NPs composed of gold (Au), silver (Ag), and their mixture in bimetallic Au-Ag NPs, were grown in a titanium dioxide (TiO₂) matrix using reactive DC magnetron sputtering. Thermal treatment at different temperatures (up to 700 °C) tuned the LSPR response of the films and, consequently, their sensitivity. Notably, the bimetallic film with Au/Ag atomic ratio 1 exhibited the highest refractive index sensitivity (RIS), with a value of 181 nm/RIU, almost one order of magnitude higher than monometallic Au-TiO₂. The nanostructural analysis revealed a wide NP size distribution of bimetallic NPs with an average size of 31 nm, covering about 20% of the overall surface area. These findings underscore the significant potential of bimetallic film systems, namely AuAg-TiO₂, in LSPR sensing enhancement.

Keywords: Au-Ag nanoparticle; AuAg-TiO2 thin film; Localized Surface Plasmon Resonance (LSPR); plasmonics; refractive index sensitivity (RIS).

Figure 1

Calculated extinction, scattering, and absorption efficiencies of a spherical nanoparticle with variable size D (D = 10, 20, 30 nm), composed of (a) Ag and (b) Au, surrounded by a dielectric medium with a constant refractive index of 2.3 (simulating a TiO₂ matrix). Calculations were performed using a software from Nanohub.org [22].

Figure 2

Simulated refractive index sensitivity (RIS) of a spherical nanoparticle (D = 10, 20, 30 nm) surrounded by a TiO₂ finite layer (d = 2, 4, 8 nm), immersed into two environments with different refractive index units, η₁ (RIU) = 1.333 and η₂ (RIU) = 1.381 (Δη = 0.048) [22]. RIS was calculated by the difference in the wavelength of extinction maxima, divided by the difference in refractive index units, according to RIS = (λ₂ − λ₁)/Δη.

Figure 3

Optical transmittance spectra of the Au-TiO₂, AuAg-TiO₂, and Ag-TiO₂ films with different Au and/or Ag contents and subjected to different annealing temperatures.

Figure 4

LSPR band minimum wavelength shift of AuAg-TiO₂ thin films (Au/Ag atomic ratio of 1 and 2), annealed at 400 °C, during the RIS assessment, concerning each cycle. The surrounding media of the plasmonic nanocomposite thin film changed between DI water and sucrose solution of 30% (w/w) of sucrose.

Figure 5

LSPR band minimum wavelength shift of Au-TiO₂, and AuAg-TiO₂ thin films (Au/Ag atomic ratio of 1 and 2), annealed at 700 °C, during the RIS assessment, concerning each cycle. The surrounding media of the plasmonic nanocomposite thin film changed between DI water and sucrose solution of 30% (w/w) of sucrose.

Figure 6

The SEM micrographs of the AuAg-TiO₂ thin film annealed at 700 °C, at increasing amplifications in (a), and the bimetallic NPs size distributions analysis in (b), considering the Feret diameter, nearest neighbor distance, and aspect ratio parameters.

Figure 7

The Morphological analysis of the AuAg-TiO₂ thin films annealed at 700 °C: (a) two-dimensional (2D) AFM height images of 10 × 10 µm² and 5 × 5 µm², with roughness and height parameters. Considering the 2D AFM height image 5 × 5 µm², the topographic profile (delimited in the black dashed line in (a)), average height estimation (delimited by orange dotted line), and height distribution profile are represented in (b), and (c), respectively.