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.