Elucidation of photophysical changes and orientation of acridine orange dye on the surface of borate capped gold nanoparticles using multispectroscopic techniques

In the present work, we have carried out a detailed investigation on the binding interaction of acridine orange (AO) with borate capped gold nanoparticles (Au NPs) by absorption spectroscopy, steady state emission spectroscopy, time resolved fluorescence spectroscopy, high resolution transmission electron microscopy (HR-TEM), dynamic light scattering (DLS), zeta potential measurements, Fourier transform infrared (FT-IR) and Raman spectroscopy. The phenomena of hypochromism and the appearance of coupled localized surface plasmon resonance (LSPR) bands in the absorption spectral studies of the AO-Au NP system suggested the vital role of electrostatic interactions between AO dye and Au NPs. The results from HR-TEM and DLS measurements confirmed the formation of aggregated clusters of nanoparticles. The zeta potential studies of Au NPs and AO coated Au NPs suggested the occurrence of partial surface charge neutralization of negatively charged Au NPs by cationic AO dye. The interparticle edge to edge separation distance of adjacent Au NPs revealed the presence of AO molecules between the nano-gaps of Au NPs. Emission spectral studies of AO in the presence of increasing concentrations of Au NPs indicated the existence of a static quenching mechanism. Time resolved fluorescence spectroscopic study further validated the findings of steady state emission measurements. The comparison of experimental emission quenching data with the theoretically calculated value suggested the probable multi-layered assembly of AO on the surface of Au NPs. FT-IR and Raman spectral studies further revealed the existence of electrostatic interactions and possible horizontal orientation of AO dye molecules on Au NP surfaces.