Large-area high-performance SERS substrates prepared with a combination of mesoporous silica film and AuAg Alloy film

Abstract

Uniform large-area and high-performance surface enhanced Raman scattering (SERS) substrates were prepared for the first time by using a simple three-step method. The method contains sputtering deposition of an AuAg alloy layer on the glass sheet and subsequent dip-coating of a sol-gel copolymer-templated silica film on the alloy layer and finally thermal treatment of the double-layer coated glass sheet in air at 450℃ for several hours. The thermal treatment leads to formation of the mesoporous silica (MS) film. The scanning electron microscope images show that the top layer of MS has the open-pore structure that facilitates rapid diffusion of small molecules into the film. The energy dispersive X-ray (EDX) spectroscopy analyses indicate that the thermal treatment of the substrate results in the loss of Ag atoms in the AuAg alloy film accompanied with the nanostructure formation in the film. The systematic measurements of SERS spectra for Nile blue (NB) and Crystal violet (CV) demonstrate that the insertion of a 20-nm-thick gold layer between the glass sheet and a 50-nm-thick AuAg alloy film can effectively increase the SERS enhancement factor of the substrate. Dependence of the SERS signal on the substrate immersion time was investigated with an aqueous solution of 50 nmol•L-1 CV, and the findings indicate that the intensity of SERS signal increases with increasing the immersion time up to 30 min after which the signal becomes stable. The SERS signal intensity detected 15 min after immersion is 85% of its stable value. The position of each peak in the SERS spectrum for the adsorbed CV molecules is perfectly identical to that in the conventional Raman spectrum obtained with the aqueous CV solution, giving a sign that the MS film enables to prevent the metal from interfering the positions of SERS peaks. The experimental results demonstrate that such novel SERS substrates have the detection limit of 1 nmol•L-1 NB.