Surface Enhanced Raman Spectroscopy

The Raman microscopy is a well-established technique that provides information on the vibrational frequencies of molecules from regions as small as 1 cubic micrometer. Such frequencies depend on the masses of atoms in the molecules and on the strength of interatomic bonds, and thus the Raman signal yields a chemically sensitive signature of the specimen. Coupled with the non-destructive nature of the measurement, and the ability to perform the analysis on the micrometer scale, Raman microscopy has become a standard analytical tool. Indeed there are many Raman microscopes in Australia which are heavily used, including a UV Raman system located at Macquarie University and Visible- and FT-Raman systems at the University of Sydney and UNSW.

Conventional Raman spectroscopy with all its versatility suffers from low signal strength, compared with fluorescence. As such it is not well suited to identify minor components of mixtures, which is a frequent requirement in life sciences. The Surface Enhanced Raman Scattering (SERS) technique is designed to overcome two of the present limitations of the Raman microspectroscopy. The first is the spatial resolution of Raman microscopy of 1 mm, which is increased to a nanometre range through the use of near-field approach. The second is a relatively weak signal making it difficult to examine detailed chemical content of many complex specimens. This will be overcome by applying the concept of Surface Enhanced Raman scattering. Results dating back over 20 years show that the Raman signal can be enhanced when small metal particles are located in close proximity to molecules being examined. The enhancement has been shown to be as much as 8 orders of magnitude on Cu, Ag and Au surfaces, and many authors have since demonstrated the feasibility of single molecule detection.

The SERS technique is of utility in many areas of science. In forensic sciences it can be used, for example, to examine the microstructure of forged handwriting that can be directly correlated with the chemistry of ink that was used. Such measurements can also be extended to dyes on paper with topography and micro-fluorescence of the paper correlated with the Raman data. In the chemistry of polymers, the degree of cross-linking in the polymer can be investigated by monitoring the micro-domains in the film with linearly polarised light. In life sciences the utility of the SERS system is related to its extreme sensitivity and from the fact that it can operate in an intermittent contact mode with liquid specimens and thus biological materials in their physiological media can now be examined.

The SERS measurements can be carried out in a conventional Raman system such as our Renishaw Raman microscope, but importantly also using the Nanonics NSOM system based on an atomic force microscope (AFM) controller with a cantilevered fibre optics probe. The SERS operation can, in principle, be implemented by the use of a special gold nanoparticle which is attached to the fibre probe and scanned over the specimen surface. In addition to SERS the Nanonics system can be used for near field spectroscopy and imaging simultaneously with the AFM imaging. Thus chemical variations in a specimen across an area in the order of 1 micrometre by 1 micrometre can be detected with greatly enhanced sensitivity (10⁴ enhancement over and above that of the corresponding Raman microscope) and spatial resolution in the order of tens of nanometres.

A special feature of the Nanonics instrument is that it can be fitted under a conventional optical microscope. It can therefore cooperate with a Raman microscope system and with the fluorescence excitation system. This is achieved by a special construction of the optic fibre aperture that is suspended from one side enabling simultaneous viewing in near –field and in far-field using a conventional objective. This makes it possible to aim at and examine selected microscopic objects within the field of view of a standard microscope. For example, researchers in biology can now use near field Raman signals from a membrane when addressing critical questions of near membrane molecular changes, while using the micro-Raman to monitor deep alterations in the cell. This can be correlated with the detection of mechanical movements (within less than 0.05 nm) of the membrane.