Polymers as Templates for Au and Au@Ag Bimetallic Nanorods: UV-Vis and Surface Enhanced Raman Spectroscopy

In this study, we report on the fabrication of nanoassemblies composed by Au and bimetallic core shell Au@Ag nanorods immobilized onto pH-responsive poly(2-vinylpyridine) (P2VP) brushes. The general process includes the synthesis of gold nanorods (through the seed mediated method) and a subsequent reduction of AgNO3 on the gold surface. These colloidal particles were immobilized onto carboxy-terminated P2VP brushes (COOH-P2VP), previously grafted on underlying substrates (silicon wafers and glass slides). UV-vis spectroscopy and transmission electron microscopy (TEM) images confirmed the morphology of the gold nanorods and the core shell structure of the bimetallic Au@Ag nanorods. The presence of metal nanoparticles on the P2VP-functionalized substrates was demonstrated by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) images. The swelling-deswelling behavior of the pH responsive polymer brushes was confirmed using UV-vis spectroscopy over P2VP brushes functionalized glass slides with immobilized gold nanorods (P2VP-Au rod) at two different pH values. A significant displacement of 32 nm in the longitudinal plasmon band was observed between the swollen and the collapsed state of the pH-sensitive polymer brushes, proving the potential application of these nanostructures for the fabrication of pH nanosensors capable of detect changes in the pH of aqueous media. The surface enhanced Raman spectroscopy (SERS) activity of the nanoassemblies was demonstrated using a dye molecule, rhodamine 6G (R6G), as model analyte. The remarkable SERS intensity of silver compared to gold was confirmed. In addition, the SERS signal intensity depended on both the concentration of particles deposited on the functionalized substrate and the thickness of the silver shell surrounding the gold nanorod. Enhancement factors (EFs) on the order of 10(11) for immobilized Au@Ag nanoparticles demonstrated the ability of this novel nanostructured system for applications in the detection of conventional SERS analytes and offer the possibility to fabricate stable macroscopic devices for SEAS spectroscopy applications.