Spectroscopic investigations on the binding of Methylene Blue and Nile Blue to negatively charged gold nanorods

The effect of longitudinal surface plasmon (l-SP) electric field of gold nanorods (AuNRs) on the optical (absorption and fluorescence) properties of two dyes Methylene Blue (MB) and Nile Blue (NB) has been studied by tuning and detuning the l-SP band with the absorption maxima of the dyes. Binding between dyes and nanorods were established by electrostatic interaction by making the nanorods negatively charged after coating them with polystyrene sulfonate (PSS). The absorption spectra of the dye-nanorod complex showed two prominent absorption bands in the 550–700 nm regions. For the detuned condition these changes are attributed to the nanorod induced aggregation of the dyes. However for the tuned condition, with increasing dye concentration the energy gap between the bands were observed to increase and then saturate. This is attributed to resonance coupling between the l-SP of the nanorod with the dye absorption. Although the fluorescence intensity of the dyes in the presence of increasing amount of AuNRs were observed to be quenched their lifetimes were observed to increase. Both the radiative (kr) and nonradiative (knr) rates of the dyes decreases in the presence of AuNRs. The magnitude of decrease for kr is much higher than knr, which is attributed to the formation of the non-fluorescent dimeric species. The increase in the fluorescence lifetime is attributed to the suppression of the excited state nonradiative pathways of these dyes adsorbed on the surface of the AuNRs. In addition, the changes of kr and knr were observed to be greater for the tuned condition.