Substrate effect on the plasmonic sensing ability of hollow nanoparticles of different shapes.

Gold hollow nanospheres (AuHSs) and gold hollow nanocubes (AuHCs) were synthesized by the galvanic replacement technique using silver nano templates. Colloidal AuHSs are found to have a higher sensitivity factor than that of AuHCs. This value decreases for both shapes when the nanoparticles are assembled on a quartz substrate by using the Langmuir-Blodgett technique. AuHSs are observed to have the larger effect. It is observed that as the separation gap between AuHCs nanoparticles decreases, their localized surface plasmon resonance band red shifts more than AuHSs. This is accounted for by the discrete dipole approximation (DDA) calculations. The coupling between the plasmon fields of the AuHCs pair is stronger than that between AuHSs pair. Using the DDA calculation, this is found to be due to geometric factors, as well as to the difference in the plasmonic field intensity. The calculation also showed that the plasmon field distributions of both AuHCs and AuHSs were distorted by the quartz substrate in a different manner. It is also observed that the surface-enhanced Raman spectrum of thiophenol is stronger when measured on AuHCs than on AuHSs. This is due to the difference in the plasmon field distribution as well as the fact that the AuHCs have a higher scattering/absorption yield ratio.