Chun Mu

Tip-enhanced fluorescence microscopy of high-density samples

High-density samples of fluorescent quantum dots (QDs) were imaged using an apertureless near-field optical microscopy technique. QD fluorescence was modulated by oscillating a silicon atomic force microscope tip above an illuminated sample and a lock-in amplifier was used to suppress background from the excitation laser. Spatial resolution near 10nm and a peak signal-to-noise ratio (SNR) of ∼60 were achieved. Individual QDs within high-density ensembles were still easily resolved (SNR>5) at a density of 14QDs∕μm2. These results have favorable implications for the eventual nanoscale imaging of viable biological systems, such as cellular membranes.

Resolving single fluorophores within dense ensembles: contrast limits of tip-enhanced fluorescence microscopy.

We investigate the limits of one-photon fluorescence as a contrast mechanism in nanoscale-resolution tip-enhanced optical microscopy. Specifically, we examine the magnitude of tip-induced signal enhancement needed to resolve individual fluorophores within densely-packed ensembles. Modulation of fluorescence signals induced by an oscillating tip followed by demodulation with a lock-in amplifier increases image contrast by nearly two orders of magnitude. A theoretical model of this simple modulation/ demodulation scheme predicts an optimal value for the tip-oscillation amplitude that agrees with experimental measurements. Further, as an important step toward the eventual application of tip-enhanced fluorescence microscopy to the nanoscale structural analysis of biomolecular systems, we show that requisite signal enhancement factors are within the capabilities of commercially available silicon tips.

Three-Dimensional Mapping of Near-Field Interactions via Single-Photon Tomography

We demonstrate a near-field tomography method for investigating the coupling between a nanoscopic probe and a fluorescent sample. By correlating the arrival of single fluorescence photons with the lateral and vertical position of an oscillating tip, a complete three-dimensional analysis of the near-field coupling is achieved. The technique is used to reveal a number of interesting three-dimensional near-field features and to improve image contrast in tip-enhanced fluorescence microscopy.

Nanoscale Fluorescence Microscopy Using Carbon Nanotubes

We demonstrate the use of single-walled carbon nanotubes as nano-optical probes in apertureless near-field fluorescence microscopy. The carbon nanotubes strongly quench fluorescence leading to near-field contrast with spatial resolution of ~20 nm.

Nanoscale fluorescence microscopy using carbon nanotubes

We demonstrate the first reported use of single-walled carbon nanotubes as nano-optical probes in apertureless near-field fluorescence microscopy. We show that, in contrast to silicon probes, carbon nanotubes always cause strong fluorescence quenching when used to image dye-doped polystyrene spheres and Cd-Se quantum dots. For quantum dots, the carbon nanotubes induce very strong near-field contrast with a spatial resolution of 20 nm. Images of dye-doped spheres exhibit crescent-shaped artifacts caused by distortions in the surface water layer found in ambient conditions.

Nanoscale fluorescence imaging using a single-wall carbon nanotube

A single-wall carbon nanotube attached to an AFM probe is used for near-field optical imaging of 20 nm diameter fluorescent spheres and 5 nm diameter CdSe quantum dots.

Optimizing contrast of tip-enhanced fluorescence microscopy for imaging high-density samples

Quantum dots are imaged using tip-enhanced fluorescence microscopy. Optimization of the operation parameters leads to high-contrast images of high-density samples and a novel photon analysis improves contrast further.