A Generalized Model on the Effects of Nanoparticles on Fluorophore Fluorescence in Solution

Abstract

Nanoparticles (NP) can modify fluorophore fluorescence in solution through multiple pathways that include fluorescence inner filter effect (IFE), dynamic and static quenching, surface enhancement, and fluorophore quantum yield variation associated with structural and conformational modifications induced by NP binding. The combined contribution of the latter three effects is termed the collective near-field effect because (1) they affect only fluorophore fluorescence in molecules close to the NPs, and (2) it is impossible to differentiate these effects with steady-state fluorescence measurements. A generalized model (F0corr/FNPcorr = (1 + K[NP])/(1 + K[NP]S) was developed for the determination of the NP collective near-field effect S on the fluorophore fluorescence in the surface-adsorbed molecules. The popular Stern–Volmer equation (F0corr/FNPcorr = (1 + K[NP]) used in current fluorescence studies of NP interfacial interactions is a special case of this generalized model, valid only under situations in which the surface-bound molecules are completely fluorescence inactive (S = 0). In addition, we excluded the possibility of NPs inducing significant dynamic fluorescence quenching under realistic experimental conditions on the basis of a simple back-of-the-envelope calculation. Furthermore, using an external reference fluorescence IFE correction method developed in this work, we demonstrated that gold nanoparticles (AuNPs) only slightly attenuate, but do not completely quench the fluorescence signal of the protein, bovine serum albumin (BSA), on AuNP. This result undermines the reliability of the BSA/AuNP binding constants calculated using the Stern–Volmer equation in earlier studies of BSA/AuNP interfacial interactions. The methodology and insights provided in this work should be of general importance for fluorescence study of nanoparticle interfacial interactions.