Using aggregates of gold nanorods in SER(R)S experiments: an empirical evaluation of some critical aspects

An empirical evaluation of some critical aspects resulting from aggregation of gold nanorods (AuNRs) used as surface enhanced resonant Raman scattering (SERRS)-active substrates was reported. Two types of AuNR substrates with longitudinal plasmon bands which either match (in-plasmon resonance) or not (off-plasmon resonance) the wavelength of the exciting laser source (lambda: 632.8 nm) were tested in resonant Raman detection of methylene blue (MB). The in-plasmon resonance condition proved to be significantly useful for detecting MB at very low concentration (less than 10(-10) M), whereas the off-plasmon resonance setup is more than enough for intermediate-low concentrations (down to 10(-8) M). Differently sized AuNR aggregates, obtained by sequential dilution of the AuNR solutions allowed us to investigate the dependence of for surface enhanced Raman scattering (SERS) intensity on the size of the aggregates, pointing out a simple strategy for preparing AuNR-based SERS substrates.