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Published online: 18 March 2009 | doi:10.1038/nchina.2009.52

Analytical chemistry: Finding singles

Felix Cheung

Introduction

© (2009) RSC

Surface-enhanced Raman scattering (SERS) is a spectroscopic technique used to study the behaviour and properties of molecules. Although the technology has proved sensitive enough to probe single-molecule systems, its applicability is still limited because SERS spectra often have strong fluctuations. Now, Jiannian Yao at the Chinese Academy of Sciences in Beijing and co-workers¹ have successfully performed SERS on a nonpolar organic molecule by reducing unwanted spectral fluctuations.

The researchers used a low-power laser to illuminate a perylene molecule adsorbed on a substrate that had its surface treated with silver nanoparticles. Scattered light from the sample was collected and measured at room temperature, and the result (see top image) was compared with the calculated spectrum (see bottom image) of a single perylene molecule.

In theory, the highly symmetrical perylene molecule should have three vibrational frequencies, or modes, in its spectrum that are greatly enhanced by the silver surface. The experimental spectrum exhibits not only these three modes, but also several sharp spectral features and 'blinkings' thought to be caused by thermal effects. The researchers found that these fluctuations could be effectively eliminated by cooling the sample down to -150 °C for a period of time.

The study represents the first example of SERS of a nonpolar molecule and, more importantly, the first blinking-free SERS of a single molecule. The technique not only widens the applicability of SERS, but also provides a general protocol for stabilizing SERS spectra.

The authors of this work are from:
Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Science, Beijing, China; Department of Theoretical Chemistry, Royal Institute of Technology, Stockholm, Sweden; Hefei National Laboratory of Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, China; Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, China.

Reference

1. Luo, Z. et al. Stabilizing single-molecular Raman spectrum of a nonbonding molecule on Ag nanoparticles. Chem. Comm. 11, 1342–1344 (2009).
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