Ag@Au core-shell dendrites: a stable, reusable and sensitive surface enhanced Raman scattering substrate

Abstract

Surface enhanced Raman scattering (SERS) substrate based on fabricated Ag@Au core-shell dendrite was achieved. Ag dendrites were grown on Si wafer by the hydrothermal corrosion method and Au nanofilm on the surface of Ag dendritic nanostructure was then fabricated by chemical reduction. With the help of sodium borohydride in water, Au surface absorbates such as thiophene, adenine, rhodamine, small anions (Br(-) and I(-)), and a polymer (PVP, poly(N-vinylpyrrolidone)) can be completely and rapidly removed. After four repeatable experiments, the substrate SERS function did not decrease at all, indicating that the Ag@Au dendrite should be of great significance to SERS application because it can save much resource. Six-month-duration stability tests showed that the Ag@Au core-shell dendrite substrate is much more stable than the Ag dendrite substrates. We have also experimented on fast detection of Cd(2+) at 10(-8) M concentration by decorating single-stranded DNA containing adenine and guanine bases on the surface of this Ag@Au dendrite. Finite-difference time-domain simulations were carried out to investigate the influence of Au nanolayer on Ag dendrites, which showed that the local electric fields and enhancement factor are hardly affected when a 4 nm Au nanolayer is coated on Ag dendrite surface.

Figure 1

(a) Large-scale and (b) magnified FE-SEM images of Ag dendrites. After Ag dendrite coated with Au, the (c) lager-scale and (d) magnified FE-SEM images. (e) HRTEM image of the surface region of Au nanofilm covered Ag dendrite. (f) XPS spectra of Ag@Au core-shell dendrites.

Figure 2

(a) SERS spectra of R6G obtained at different concentrations (from 10⁻³ to 10⁻⁸ M) with Ag@Au core-shell dendrites. (b) The linear relationship between log I of the band peaking at 1510 cm⁻¹ and log C. (c,d) SERS spectra of 10⁻⁵ M R6G detected at Ag dendrites and Ag@Au core-shell dendrites respectively, here new-prepared substrates and substrates after six months were tested.

Figure 3. Recycle SERS behaviors of Ag@Au core-shell dendrites:

(a) 10⁻⁵ M R6G with four cycles and (b) the alternating analysis of R6G, MO, PATP and CV with all 10⁻⁵ M concentration. Same labels of the odd and even curves in (a,b) respectively.

Figure 4

(a) Raman signals of Ag@Au core-shell dendrites decorated with ssDNA and different concentration of Cd²⁺ added on the surface of the substrate (from 10⁻⁸ to 10⁻³ M) compared with previous substrate without Cd²⁺. The inset shows the relationship between concentration of Cd²⁺ and the intensity ratio of the bands peaking at 710 and 629 cm⁻¹. (b) Recycle SERS behaviors of Ag@Au core-shell dendrites after ssDNA decorated on the surface to detect Cd²⁺. Same labels of the odd and even curves in (b).

Figure 5

(a) Images of distribution of surface local electric field for Ag@Au dendritic nanostructure and image of distribution of surface local electric fields on second-level branch, (b) with and (c) without Au nanofilm.