262
H. Schulz
droxyflavone) [56]. Whereas the SERS spectrum of flavones was weak both on the
silver colloid and on the applied silver electrode, the correspondent spectra of the
hydroxyl derivatives showed intense bands, indicating that the oh-groups assist
the silver colloids to attach on the surface of the analyt molecules. SERS Raman
spectroscopy on silver colloids has been also successfully applied to identify yellow
dyes occurring in fruits of sumac ( Rhus sp.) and pomegranate ( Punica granatum)
[59]. the SERS spectrum of sumac extract shows a very good correspondence with
the hydroxyflavylium structure of anthocyanins. According to the literature bands
registered at 1,637, 1,585, 1,534, 1,485, 1,430 and 1,357 cm
−1
were assigned to
ring stretching vibrational modes, whereas the band at 1,247 cm
−1
was attributed
to the C-o stretching mode [44, 45, 52]. Contrary to that, the aqueous extract of
pomegranate rind shows predominantly the spectral feature of ellagic acid, indicating that there is a strong interaction between this phenolic plant substance and the
silver colloids. Consequently, a general shift of stretching vibrations related to the
aromatic ring system to lower wavenumbers is observed.
Raman spectra of hydroxycinnamic and hydroxybenzoic esters (caffeates, ferulates and gallates), possessing antioxidant and anticarcinogenic properties, with
special focus on the analysis of the ring substituents and the nature of the ester alkyl
chain was performed [60]. the individual vibrational pattern was assigned in the
light of dFt calculations resulting in a good agreement between calculated and
experimental data sets.
two SERS procedures were developed for the evaluation of alizarin in madder
root ( Rubia tinctorium), which usually shows high fluorescence during normal Raman measurements [58]. the first procedure involves thermal evaporation of silver
nanoparticles on filter paper stained with the individual natural dye to induce the
SERS effect. For the other procedure a special silver-alumina-based substrate (AgAl 2 o 3 ) was prepared by spin-coating an alumina nanoparticle layer onto a glass
slide to provide the nanostructure of the substrate, followed by thermally evaporating a layer of silver on top of the alumina layer. the dye samples to be analysed
were directly delivered onto this substrate. the authors found that the Ag-Al 2 o 3
substrate proved to present numerous advantages over other SERS materials and
guaranteed a certain surface roughness necessary to induce a strong SERS effect.
A fast and reliable quantification method for the determination of naturally occurring indigo dye in dyer’s knotweed ( Polygonum tinctorium) based on Ft-Raman
spectroscopy and reference hPLC data was developed [61, 62]. the samples used
for the spectroscopic calibration model were prepared by “diluting” pure indigo
with cellulose in the range from 0.1 to 2.0 mass %. For this, the indigo was carefully mixed with pure cellulose and 0.2 g of this mixture was used to prepare sample
pellets of distinct thickness and diameter by a commercial manual hydraulic press.
Alternatively, a conventional sample preparation was performed by pressing a certain amount of the indigo-cellulose mixture in small aluminium pans. Both sample
preparation techniques show only small differences in prediction quality. generally,
an excellent correlation within the regarded concentration range could be achieved
resulting in correlation coefficient values of 0.997 (metal pan) and 0.988 (pressed
pellets).
H. Schulz
droxyflavone) [56]. Whereas the SERS spectrum of flavones was weak both on the
silver colloid and on the applied silver electrode, the correspondent spectra of the
hydroxyl derivatives showed intense bands, indicating that the oh-groups assist
the silver colloids to attach on the surface of the analyt molecules. SERS Raman
spectroscopy on silver colloids has been also successfully applied to identify yellow
dyes occurring in fruits of sumac ( Rhus sp.) and pomegranate ( Punica granatum)
[59]. the SERS spectrum of sumac extract shows a very good correspondence with
the hydroxyflavylium structure of anthocyanins. According to the literature bands
registered at 1,637, 1,585, 1,534, 1,485, 1,430 and 1,357 cm
−1
were assigned to
ring stretching vibrational modes, whereas the band at 1,247 cm
−1
was attributed
to the C-o stretching mode [44, 45, 52]. Contrary to that, the aqueous extract of
pomegranate rind shows predominantly the spectral feature of ellagic acid, indicating that there is a strong interaction between this phenolic plant substance and the
silver colloids. Consequently, a general shift of stretching vibrations related to the
aromatic ring system to lower wavenumbers is observed.
Raman spectra of hydroxycinnamic and hydroxybenzoic esters (caffeates, ferulates and gallates), possessing antioxidant and anticarcinogenic properties, with
special focus on the analysis of the ring substituents and the nature of the ester alkyl
chain was performed [60]. the individual vibrational pattern was assigned in the
light of dFt calculations resulting in a good agreement between calculated and
experimental data sets.
two SERS procedures were developed for the evaluation of alizarin in madder
root ( Rubia tinctorium), which usually shows high fluorescence during normal Raman measurements [58]. the first procedure involves thermal evaporation of silver
nanoparticles on filter paper stained with the individual natural dye to induce the
SERS effect. For the other procedure a special silver-alumina-based substrate (AgAl 2 o 3 ) was prepared by spin-coating an alumina nanoparticle layer onto a glass
slide to provide the nanostructure of the substrate, followed by thermally evaporating a layer of silver on top of the alumina layer. the dye samples to be analysed
were directly delivered onto this substrate. the authors found that the Ag-Al 2 o 3
substrate proved to present numerous advantages over other SERS materials and
guaranteed a certain surface roughness necessary to induce a strong SERS effect.
A fast and reliable quantification method for the determination of naturally occurring indigo dye in dyer’s knotweed ( Polygonum tinctorium) based on Ft-Raman
spectroscopy and reference hPLC data was developed [61, 62]. the samples used
for the spectroscopic calibration model were prepared by “diluting” pure indigo
with cellulose in the range from 0.1 to 2.0 mass %. For this, the indigo was carefully mixed with pure cellulose and 0.2 g of this mixture was used to prepare sample
pellets of distinct thickness and diameter by a commercial manual hydraulic press.
Alternatively, a conventional sample preparation was performed by pressing a certain amount of the indigo-cellulose mixture in small aluminium pans. Both sample
preparation techniques show only small differences in prediction quality. generally,
an excellent correlation within the regarded concentration range could be achieved
resulting in correlation coefficient values of 0.997 (metal pan) and 0.988 (pressed
pellets).