Correlated topographic and spectroscopic imaging beyond diffraction limit by atomic force microscopy metallic tip-enhanced near-field fluorescence lifetime microscopy

Abstract

A near-field optical imaging approach is demonstrated for simultaneous topographic and spectroscopic imaging with spatial resolution beyond the optical diffraction limit. The method combines metallic-tip-based tapping-mode atomic force microscopy (AFM) with fluorescence lifetime imaging microscopy (FLIM). The AFM metallic tip was formed by sputter coating a Si tapping mode tip with Au, in a way that forms a globular tip apex. Such tip apex generates high local electric field enhancement under laser illumination, which provides a strong electric-field interaction between the AFM tip and the fluorescent molecules under the tip. The tip perturbation of fluorescence gives the fluorescence lifetime changes that provide the AFM–FLIM imaging contrast. A finite element method simulation was used to further evaluate the electric near-field enhancement and electric field distribution originating from the metallic Au-coated AFM tapping-mode tip. We have demonstrated that spatially mapping the change in fluorescence lifetime and intensity is a promising approach to spectroscopic imaging at an AFM spatial resolution typically defined by the apex diameter of the AFM tips. The globular Au-coated AFM tip not only gives adequate spatial AFM tapping-mode imaging spatial resolution but also is “environmentally friendly” to soft samples, such as polymeric dye-labeled nanospheres and even biological specimens such as POPO-3 labeled DNA.