Surface-enhanced fluorescence and Raman scattering study of antitumoral drug hypericin:: An effect of aggregation and self-spacing depending on pH

Surface-enhanced fluorescence (SEF) and surface-enhanced Raman scattering (SERS) techniques were applied in this work to study the ionization, aggregation, and adsorption onto the metal surface of the antitumoral drug hypericin (Hyp). Hyp is able to interact with Ag nanoparticles under different mechanisms depending on the ionization state of the drug. The monoanionic Hyp(-) can homoassociate giving rise to H-aggregates which render an intense SEF emission, in contrast to what usually occur in solution, where the fluorescence is quenched. At a pH below 6.0, the neutral Hyp species exist under the 1, 6-dixo tautomer on the metal surface. This structure is able to form J-aggregates by intermolecular H-bonds giving rise to molecular networks that can be adsorbed on the metal surface oriented parallel to this surface. In this case, the SEF enhancement is lower due to this parallel orientation. The SERS spectra were very important to calculate the different ionization pK of Hyp on the surface and to obtain information on key physicochemical processes that Hyp undergoes, such as the tautomerization, intermolecular interaction, and the orientation on the surface.