Quantitative study of the gold-enhanced fluorescence of CdSe/ZnS nanocrystals as a function of distance using an AFM probe.

We report total internal reflection fluorescence microscopy (TIRFM) measurements using an atomic force microscope (AFM) probe to investigate the quantitative fluorescence profiles of CdSe/ZnS nanocrystals (NCs), which interact with gold surfaces, as a function of the gold-NC distance. First, a silica bead was glued to an AFM cantilever. CdSe/ZnS NCs were then immobilized on the silica-bead-functionalized AFM probe for control of the gold-NC distance, while a gold thin film served as the metal surface on a glass substrate. As a result of coexistent fluorescence resonance energy transfer and localized surface plasmon resonance (LSPR) coupling, a strong enhancement was observed at 15 nm, while fluorescence suppression was seen at a proximal distance. For further quantitative discussions of the effects of the metal surface on the fluorescence behavior of single NCs, we introduced a gold-deposited silica bead onto the AFM cantilever to regulate the gold-NC distance. Subsequently, the CdSe/ZnS NCs were immobilized on a glass surface. As a result of compensation for the contribution of photobrightening, the NC fluorescence profile for the glass surface also clearly displayed the LSPR-coupled enhancement at 10 nm. In contrast, it was discovered that positioning of the gold surface at a proximal single NC gave rise to quenching of the NC fluorescence. Our findings suggest that a method using an NC-functionalized AFM probe allows us to quantitatively investigate the NC fluorescence under the effects not only of a metal surface but also of an evanescence field.