Optical assessment of chiral-achiral polymer blends based on surface plasmon resonance effects of gold nanoparticles

In this research, we investigated the chiroptical properties of chiral-achiral polymer blends consisting of poly[(9,9-bis((S)-3,7-dimethyloctyl)-9H-fluoren-2,7-diyl)-alt-(4-phenylene)] and poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)] (PFPh-F8BT). The chirality of the blend film decreased on increasing the composition of the achiral polymer. However, the measured circular dichroism values of the blends with high content of F8BT were lower than expected considering the composition of the chiral PFPh polymer, suggesting that the chirality of the blend film was governed by the steric hindrance of the achiral polymeric dopant. Next, we examined optical sensing of the chiral-achiral polymer blends using gold nanoparticles (AuNPs) prepared using the citrate reduction and Ostwald ripening methods. Transmission electron microscopy images showed that the as-made AuNPs had a monodispersed spherical shape with an average diameter of 89 nm. The optical absorption spectrum of the calcined AuNP film exhibited a surface plasmon resonance wavelength at 550 nm. Raman scattering intensities of the PFPh-F8BT blends on the AuNPs were higher than those on fused silica glass; the observed Raman enhancement could be attributed to the optical confinement effects of the AuNPs. Surface-enhanced Raman scattering spectra of the PFPh-F8BT blends revealed that the 1546 cm^-1 Raman peak could be used to estimate the composition of F8BT within the PFPh-F8BT blend.