A COMPARISON OF ELECTROCHEMICAL AND COLLOIDAL SURFACE
ENHANCED RESONANCE RAMAN SCATTERING (SERRS) FROM TWO ANALYTES
DESIGNED TO CHEMISORB TO SILVER SURFACES
   
R. E. Littleford1*, G. Dent2, D. Graham1, C. McHugh1 and W. E. Smith1

1 University of Strathclyde, Department of Pure and Applied Chemistry
295 Cathedral Street, Glasgow G1 1XL, Scotland
2Avecia Ltd., Hexagon House, Blackley, Manchester, M9 8ZS
    
In this study two dyes, designed to chemisorb to silver metal surfaces, are
compared using electrochemical and colloidal SERRS. One adsorbate is an
azine. Electrochemical SERRS from this dye gives signals which are stable
over two hours. The maximum signal intensity is observed after 9
oxidation-reduction cycles and a potential of -900 mV applied to the working
electrode, with the lowest observable concentration being 10-5 M. The other
adsorbate, a derivative of benzotriazole, gives maximum signal intensity at
-300 mV, with the intensity of the spectrum falling rapidly with time. The
lowest observable concentration was about 10-4 M. However, both dyes give
time stable and intense signals with colloidal SERRS, and the lowest
observable concentration was about 10-8 M. At higher concentrations (10-4
M), the electrochemical method was semi-quantitative (2.69 %) whereas the
colloidal method gave very high RSDs of 20.95 %. At lower concentrations
with electrochemical SERS the RSDs increased (6.62 % for 10-5 M) whereas
with colloidal SERS they decreased (10.05 % for 10-8 M). The large RSDs in
the colloidal method at high concentration are attributed to multilayer
analyte coverage, causing over aggregation. Hence, electrochemical SERS
would be the preferred technique for semi-quantitative analysis at higher
concentrations, whereas the greater sensitivity of colloidal SERS would make
it the preferred method if low concentrations of analyte are to be
determined.

The difference in the time stability of the signal with the two methods
using the benzotriazole dye is attributed to the different silver metal
surfaces used. Electrochemical SERRS uses a freshly roughened surface for
each analysis, which may be chemically active to specific ligands. Colloidal
however has a surface which has been pre-prepared in a manner that makes it
time stable and for which the enhancement is mainly caused by aggregation of
the particles.

A large difference in the voltage at which maximum intensity is achieved for
each ligand suggest a significant charge transfer enhancement.