Microscopy Tip Enhances Quantum

Richard Gaughan

Quantum dots are valuable as fluorophores because they have a broad excitation spectrum and a relatively narrow emission spectrum that depends on the size of the particles. Because of these properties and their stability, they are finding application as markers in fluorescence microscopy.

Using quantum dots with an apertureless scanning near-field optical microscopy probe, researchers at King’s College London have demonstrated an imaging technique that offers resolution well beyond the diffraction limit. It relies on the electromagnetic field enhancement created by the needlelike structure of the gold probe. In the vicinity of the 30-nm-diameter probe tip, the electromagnetic field can be up to 100 times larger than the unmodified incident field. The probe also acts as a fluorescence quencher, modifying the quantum dots’ fluorescence lifetime and reducing the intensity by up to 100 times.

By balancing the two effects, David Richards and his colleagues predicted that an optimum probe/dot separation of approximately 17 nm would provide a net fluorescence enhancement of a factor of five. He said that the enhancement is not critically sensitive to the tip-sample separation for separations between 10 and 20 nm, so measurement artifacts induced by sample topography are unlikely to be as important as in other scanning near-field optical microscopy techniques.

To demonstrate how this effect can enhance image resolution, Richards deposited onto a coverslip a suspension containing several 2- to 3-nm-diameter quantum dots with CdSe cores and ZnS shells. A confocal microscope imaged a cluster of a few dots with a resolution of 200 nm.

When the gold apertureless probe tip was brought within a couple nanometers of the sample surface and centered in the laser excitation beam as the sample was scanned again, the peak fluorescence increased by a factor of four. Moreover, the resolution improved to 60 nm. The theoretical limit of the technique is better than this, but typical probe tips have radii of 20 to 30 nm, which limits the spatial resolution of the technique.

Although the resolution enhancement effect is interesting in itself, Richards also is intrigued by the possible applications of the technique involving single molecules on surfaces, protein events in cell membranes and nanostructured fluorescent materials such as light-emitting polymers.

Applied Physics Letters, Oct. 31, 2005, 183101.