Quantum-dot-functionalized scanning probes for fluorescence-energy-transfer-based microscopy

Semiconductor nanocrystals are used to functionalize atomic force microscope (AFM) tips. Such tips may be useful for fluorescence resonance energy transfer (FRET) microscopy. CdSe/ZnS nanocrystals are chemically bound to the surface of an AFM tip by three coating methods utilizing organosilane linker molecules. Binding of nanocrystals by these methods was characterized on silicon and glass surfaces by AFM, scanning electron microscopy (SEM), and optical measurements. The use of mercaptopropyltrimethoxysilane as the linker molecule is found to provide the optimal linking scheme. Via the linking of the nanocrystals to the tip, the unique photophysical properties of the quantum dots and their tunability via chemical synthesis are exploited to create light-emitting scanning probes with controlled emission color, using a single excitation source. The functionalized probes retain their sharpness for high-resolution AFM topography acquisition. These properties are desirable for FRET schemes where the nanocrystals on the tip serve as either FRET donors or acceptors interacting with chromophores on the scanned sample. This interaction provides a contrast mechanism for high-resolution optical imaging in the near field. The potential of these coated tips for FRET-based imaging is demonstrated by localized binding of acceptor dye molecules to the functionalized tips resulting in distinctive FRET signals.