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Tel. +49 (0) 731 140 700, Fax. +49 (0) 731 140 70200
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Confocal Raman Microscopy - Surface enhanced Raman
Spectroscopy (SERS) on Ag-Nanoparticles coated with Dipicolinic
Acidthe image in fig. 1. The total scan time was 35
minutes and the scan range was 50 µm x 50µm. As is typical for SERS, not only one
dedicated spectrum for the organic molecule,but many different spectra can be observed(Fig. 1 and Fig. 2).
This is due to the fact that the molecules
are attached at different positions to theAg-particle, which induces different
proportions in the chemical bonds of themolecule to contribute to the SERS effect. As
the field enhancement is confined to thenear-field, only the parts ofthe molecules that are tightly bonded to theAg-particle, show a strong Raman
enhancement. In additon, the resonanceenhancement depends also on the size andthe shape of the Ag-particle.

Raman Spectroscopy is a powerful techniqueused to investigate the chemical states of thebonds in carbon materials.
Surface enhanced Raman spectroscopy (SERS),
which enables the collection of Raman signals
from even a single molecule,makes thetechnique more useful for analysis ofmolecules deposited onto metal surfaces.
Enhancement in Raman intensities of a factor
up to 1014 may be observed with thistechnique. Two mechanisms are considered
responsible for the enhancement:
The primary contribution is an enlargementof the local electromagnetic field, due to theexcitation of a localised surface plasmon.
Another factor results from a charge transferstate between the surface complex of theadsorbed molecule and the metal surface.

Due to its extreme sensitivity, the Confocal
Raman Microscope alpha300 R is the ideal
instrument to perform high resolution
Raman imaging and SERS.

Fig. 1: Integral intensity of all Raman spectra acquired in
the Spectral Imaging Mode. Scan range: 50 x 50 µm

Fig. 2: Averaged Raman spectra of the marked regions in fig.1.

In this study, Ag-nanoparticles coated withorganic dipicolinic acid were investigated toshow the different arrangement of theparticles and its corresponding chemicalbehaviour. The particles should have adiameter of about 20 nm and wereagglomerated into groups of particles.
The alpha300 R was used in the Spectral
Imaging Mode where a complete Raman
spectrum was collected at every pixel of theimage. 40 000 spectra (200 lines consistingof 200 pixels per line) with an integration
time of 50 ms/spectrum were used to createapplication noteLise-Meitner-Straße 6, D-89081 Ulm, Germany
Tel. +49 (0) 731 140 700, Fax. +49 (0) 731 140 70200
www.witec.de, info@witec.de

Time Series
For time series the Laser beam waspositioned on a certain molecule and aseries of 900 spectra with 200 ms
integration time per spectrum was acquired(Fig.3).

The spectra can change completely within afraction of a second. The spectra can be very
stable for several seconds before
suddenly changing dramatically within a few
milliseconds. The reason for this behaviouuris that only a single molecule isinvolved in the Raman scattering process
and that the bonding between the Agparticle and the molecule changes due tothermal fluctuations and/or chemicalreactions.

Fig. 3: Five spectra of a series of 900 spectra shown with an integration time of 200 ms for a single molecule.

Samples courtesy of Dr. Joseph Miragliotta,
John Hopkins University, Baltimore, USA© 2009 by WITec Wissenschaftliche Instrumente
und Technologie GmbH, www.WITec.de