Morphology and optical responses of SERS active π-conjugated poly(N-ethyl-2-ethynylpyridinium iodide)/Ag nanocomposite systems

The influence of the poly(N-ethyl-2-ethynylpyridinium iodide) (PEEP-I) concentration on the morphology and optical properties of nanocomposite systems prepared by mixing the polymer solution with a hydrosol of ca. 9 nm Ag nanoparticles (NPs) was investigated by a combination of surface plasmon extinction (SPE) measurements, transmission electron microscope (TEM) imaging and surface-enhanced Raman spectroscopy (SERS). The PEEP-I concentration was found to have a strong impact on the assembly of Ag NPs and, consequently, on the optical responses of the composite systems. At low polymer concentrations in the composite (corresponding to ca. 50–1800 monomer units/NP), the formation of fractal aggregates was observed. In particular, the average fractal dimension D = 1.9 ± 0.1 was determined for aggregates in the system with 5 × 10−6 M polymer concentration. By contrast, in systems with polymer concentrations higher than about 1 × 10−5 M, relatively small aggregates of Ag NPs with large interparticle distances were formed. The differences in the morphology of the composite systems with various polymer concentrations manifested themselves clearly in their SPE spectra. Furthermore, upon optical excitation with appropriate wavelengths (488.0 and 514.5 nm), the fractal aggregates acted as carriers of “hot spots”, i.e. strong, localized, nanoscale optical fields, from which intense and well resolved SERS spectra of the polymer were obtained.