Gold-modified silver nanorod arrays: growth dynamics and improved SERS properties

Only a few remaining technical hurdles currently prevent the implementation of SERS as a mainstream detection technology. Although oblique-angle deposited silver nanorod arrays provide superior analytical figures of merit for SERS sensing, stability issues of silver surfaces can impede their use for real-world sensing applications within certain environments. To circumvent this issue, silver nanorod arrays are modified with a straight-forward, inexpensive Au-coating via a galvanic replacement reaction. The morphological, structural, compositional, and optical properties of the Au-modified Ag nanorod arrays are studied by multiple ex situ morphological characterization techniques and in situ optical absorbance spectroscopy. Depending on the reaction time, the Au coating experiences five different stages of the morphological and compositional changes. The porosity of the Au layer and the content of Ag decrease with reaction time. The optical measurements show that the representative localized plasmon resonance peak of the nanorod red-shifts as the reaction proceeds. The surface enhanced Raman scattering (SERS) intensity, tested using 4-mercaptophenol, decreases exponentially with reaction time, due to the compositional evolution of the nanostructure from pure Ag to a Au-Ag alloy with increasing Au content. We show that the Au-modified Ag nanorod is very stable in NaCl solution compared to the as-deposited Ag nanorod, and the 20 or 30 minute Au-modified Ag nanorod substrate shows an improved SERS sensitivity for air contamination detection. Such an improved SERS substrate can be used in more hostile environments where a pure Ag nanorod substrate cannot be used, and is good for practical sensing applications.