Impact of metal-modified AFM tips on the fluorescence of single nanocrystals

Abstract

The influence of metal surfaces and nanoparticles on the fluorescence emission of fluorophores in close proximity is of particular interest for biophysical applications, near field optics and biosensing. For instance, the quenching of fluorophores by gold nanoparticles can be used for the investigation of biomolecular conformational changes or interactions and silver coated metal tips are potent scanning near field optical microscopy tips. Apart from the quenching effects, nanoparticles are used for fluorescence enhancement in biosensor applications. Here we use a setup combining total internal reflection fluorescence microscopy (TIRFM) with the piezo-controlled nanometer-sensitive movement of an atomic force microscope (AFM) in order to measure and quantify the fluorescence emission as a function of distance between single fluorophores and metal nanoparticles or tiny metal tips. By using CdSe/ZnS nanocrystals as fluorophores and gold as metal we observed significant fluorescence quenching as well as enhancement due to exciton-plasmon coupling. In the future, these experiments will be extended to metal nanoparticles of different elements, alloys, sizes and shapes, giving insight into the related energy transfer processes and quenching mechanisms.