Dual Fluorescence and NMR Study for the Interaction between Xanthene Dyes and Nanoparticles

Fluorescent dyes and nanoparticles (NPs) have been widely used together to make novel biosensors, taking advantage of their unique characteristics. It is crucial to have techniques that enable us to gain detailed and high-resolution information regarding the interaction between NPs and fluorescent dyes. In this work, we chose rhodamine B (RhB) and amidine- and carboxylate-modified polystyrene (CML) NPs as models and employed both NMR (¹H and STD-NMR) and optical (UV-vis and fluorescence) techniques to investigate the interaction between NPs and fluorescent dyes. From UV-vis and fluorescence spectroscopy, we see that there are larger red shifts when rhodamine B binds to carboxylate-modified polystyrene NPs than amidine-modified NPs. Correspondingly, RhB has broader NMR peaks and a larger STD effect when binding to CML NPs than amidine NPs. Results from these two techniques validate each other. It is notable that the NMR techniques provide more reliable data than UV-vis and fluorescence methods. Moreover, we show that NMR techniques, especially STD-NMR, can provide more atomic-level binding geometry information. The higher STD effect of the smaller aromatic ring of RhB implies that this aromatic ring is closer to the surface of NPs when binding to polystyrene NPs.