Plasmonic enhancement of fluorescence and Raman scattering by metal nanotips

TL;DR

This paper demonstrates how noble metal nanotips can significantly enhance fluorescence and Raman scattering signals, improving optical resolution and sensitivity in nanoscale imaging and spectroscopy.

Contribution

It introduces new experimental techniques and nanofabrication methods to achieve substantial plasmonic enhancement of optical signals near metal nanotips.

Findings

Fluorescence enhancement depends strongly on tip-sample distance.

Raman scattering signals are amplified by over 5 orders of magnitude.

High-resolution spatial mapping of fluorescence lifetime shows increased decay rates near nanotips.

Abstract

We report modifications to the optical properties of fluorophores in the vicinity of noble metal nanotips. The fluorescence from small clusters of quantum dots has been imaged using an apertureless scanning near-field optical microscope. When a sharp gold tip is brought close to the sample surface, a strong distance-dependent enhancement of the quantum dot fluorescence is observed, leading to a simultaneous increase in optical resolution. These results are consistent with simulations of the electric field and fluorescence enhancement near plasmonic nanostructures. Highly ordered periodic arrays of silver nanotips have been fabricated by nanosphere lithography. Using fluorescence lifetime imaging microscopy, we have created high resolution spatial maps of the lifetime components of vicinal fluorophores; these show an order of magnitude increase in decay rate from a localized volume around the nanotips, resulting in a commensurate enhancement in the fluorescence emission intensity. Spatial maps of the Raman scattering signal from molecules on the nanotips shows an enhancement of more than 5 orders of magnitude.