J. Bukowska and P. Piotrowski
38
SERS spectra on metal colloids suffer from some irreproducibility [71–73] caused
by aggregation of metallic nanoparticles. on the other hand, aggregating agents
are indispensable to achieve better surface enhancement, because huge electromagnetic fields responsible for surface enhancement are created at particle junctions
(nanogaps). A common problem in these experiments is the inhomogeneity of the
colloids, which in turn provides local differences in the SERS enhancement factors.
most SERS studies of amino acids aimed at establishing the mode of interaction
with the metal and the geometry of the molecule at the surface. SERS spectra allow determining the molecular form of amino acid present at the metal support
(cationic, zwitterionic, or anionic), owing to high sensitivity of the spectrum to
the molecular structure of the adsorbed species [74–79]. Some experimental SERS
results are supported by theoretical calculations using dFt approach, making interpretation of the spectra more credible [80].
the most stable and reproducible SERS spectra of aminoacids have been recorded for sulphur containing molecules such as cysteine or methionine [77, 81–83].
SERS spectra of cysteine were recorded on various substrates such as colloidal
nanoparticles [84] and rough silver electrodes, on which cysteine spontaneously
self-assembles [81, 83]. Cysteine molecules are chiral—they can exist in one of two
enantiomeric forms (L- or d-), that only differ in that their structures are mirror images of each other. Because only one enantiomer tends to be physiologically active
while the other one is inactive or even toxic, drug compounds are produced in an
enantiomerically pure form. Chiral surfaces may be applied as sensors for chiral recognition. In principle, both: the normal Raman and the SERS spectra are not sensitive to molecule chirality. however, in carefully designed experiments SERS spectra
of cysteine monolayers on the Ag electrode surfaces have been shown to be sensitive
to chirality of adjacent cysteine molecules at the metal surface [83]. It was demonstrated that there is a range of electrode potentials in which cysteine molecules are
adsorbed mainly in zwitterionic form with the Coo– groups close to the surface,
while at more negative potentials Nh 3
+
groups deprotonate at the surface with simultaneous weakening of the interaction of the carboxylic groups with the surface.
As expected, the potential-induced effects for d-cysteine were similar to these for
L-cysteine. however, for racemic mixture at acidic ph, the changes in the spectral pattern corresponding to potential-induced transition from adsorbed zwitterions
to neutral molecule were considerably smaller. the origin of the observed effects
were explained in terms of the most stable adsorption configuration for cysteine that
involves metal-sulphur, metal-nitrogen and two hydrogen bonds between carboxylic groups (cyclic dimer) as indicated by dFt calculations [85]. In the case of Ld
cysteine dimers, there is a mismatch in the carboxylic bonds of the neighbouring
molecules, which results in considerable weakening of the hydrogen bonds between
them. the L- and d- cysteine enantiomers adsorb at the metal surface with equal
probability, when adlayer is self-assembled from the solution. thus, in spite of higher stability of the homochiral dimers, molecules of opposite chirality may exist at the
surface in the immediate vicinity. In such a case intermolecular interactions between
adjacent molecules are considerably weaker than for monolayer formed by the molecules of the same chirality, what is reflected in the band positions in the respective
38
SERS spectra on metal colloids suffer from some irreproducibility [71–73] caused
by aggregation of metallic nanoparticles. on the other hand, aggregating agents
are indispensable to achieve better surface enhancement, because huge electromagnetic fields responsible for surface enhancement are created at particle junctions
(nanogaps). A common problem in these experiments is the inhomogeneity of the
colloids, which in turn provides local differences in the SERS enhancement factors.
most SERS studies of amino acids aimed at establishing the mode of interaction
with the metal and the geometry of the molecule at the surface. SERS spectra allow determining the molecular form of amino acid present at the metal support
(cationic, zwitterionic, or anionic), owing to high sensitivity of the spectrum to
the molecular structure of the adsorbed species [74–79]. Some experimental SERS
results are supported by theoretical calculations using dFt approach, making interpretation of the spectra more credible [80].
the most stable and reproducible SERS spectra of aminoacids have been recorded for sulphur containing molecules such as cysteine or methionine [77, 81–83].
SERS spectra of cysteine were recorded on various substrates such as colloidal
nanoparticles [84] and rough silver electrodes, on which cysteine spontaneously
self-assembles [81, 83]. Cysteine molecules are chiral—they can exist in one of two
enantiomeric forms (L- or d-), that only differ in that their structures are mirror images of each other. Because only one enantiomer tends to be physiologically active
while the other one is inactive or even toxic, drug compounds are produced in an
enantiomerically pure form. Chiral surfaces may be applied as sensors for chiral recognition. In principle, both: the normal Raman and the SERS spectra are not sensitive to molecule chirality. however, in carefully designed experiments SERS spectra
of cysteine monolayers on the Ag electrode surfaces have been shown to be sensitive
to chirality of adjacent cysteine molecules at the metal surface [83]. It was demonstrated that there is a range of electrode potentials in which cysteine molecules are
adsorbed mainly in zwitterionic form with the Coo– groups close to the surface,
while at more negative potentials Nh 3
+
groups deprotonate at the surface with simultaneous weakening of the interaction of the carboxylic groups with the surface.
As expected, the potential-induced effects for d-cysteine were similar to these for
L-cysteine. however, for racemic mixture at acidic ph, the changes in the spectral pattern corresponding to potential-induced transition from adsorbed zwitterions
to neutral molecule were considerably smaller. the origin of the observed effects
were explained in terms of the most stable adsorption configuration for cysteine that
involves metal-sulphur, metal-nitrogen and two hydrogen bonds between carboxylic groups (cyclic dimer) as indicated by dFt calculations [85]. In the case of Ld
cysteine dimers, there is a mismatch in the carboxylic bonds of the neighbouring
molecules, which results in considerable weakening of the hydrogen bonds between
them. the L- and d- cysteine enantiomers adsorb at the metal surface with equal
probability, when adlayer is self-assembled from the solution. thus, in spite of higher stability of the homochiral dimers, molecules of opposite chirality may exist at the
surface in the immediate vicinity. In such a case intermolecular interactions between
adjacent molecules are considerably weaker than for monolayer formed by the molecules of the same chirality, what is reflected in the band positions in the respective