STUDIA UNIVERSITATIS BABEŞ-BOLYAI, PHYSICA, SPECIAL ISSUE, 2003
FOOD ADDITIVES STUDY: VIBRATIONAL APPROACH
S. Cîntă Pinzaru1, N. Peica2, I. Pavel2, W. Kiefer2
1
Babes-Bolyai
University,
Physics
Faculty,
Kogalniceanu 1,RO 3400 Cluj-Napoca, Romania
2
Institut für Physikalische-Chemie, Am Hubland, DE
97074 Würzburg, Germany
Abstract
E 102 food additive is characterised by means of FT-IR, FTRaman and SERS spectroscopy in order check the detection limit, process
control and monitoring the dye content in food. The dye presence was
detectable under nanomolar level using SERS on silver colloid.
Introduction
Recent studies [1,2] showed that food additives are responsible for a large
number of health affections like allergic reactions, migraine, blurred vision, etc.
Asthmatics and aspirin intolerant sufferers appear to be particularly at risk. Tests
have shown that in large doses, such compounds will raise the plasma histamine
levels of healthy adults. Determination of tolerance level (maximum level of
additive allowed in food) requires sensitivity, selectivity and fast experimental
protocol involved in the process control. Such requirements are provided by the
resonance Raman (RR) or surface enhanced Raman scattering (SERS) techniques.
In order to detect, control and monitor the food additives, FT-IR, FT-Raman, RR,
and SERS spectroscopy were employed here. For example, E 102 additive (Fig. 1)
is characterised in order to get insight into its vibrational behaviours in different
environments. E 102 often give rise to a phenomenon named “metachromasia”, in
the presence of anions and other macromolecule.
O3 S
N
N
OH
3Na+
N
O
C
N
O
S O3
Fig. 1. The molecular structure of the E 102.
This is a characteristic colour change when certain tissue elements are
stained by cationic dye. Therefore, the first step in the characterisation of this
molecule would require its free state, than its aqueous solutions at different pH
values or adsorbed onto a surface and finally, in conjunction with other cationic
dyes up to the tissue level. The preliminary study is presented in this paper. E-102
S. CINTA PINZARU, N. PEICA, I. PAVEL, W. KIEFER
in Europe or FD&C Yellow Number 5, the molecule consists essentially of
trisodium 5-hydroxy-1-(4-sulfonatophenyl) (4-sulfonatophenylazo)-Hpyrazole-3-carboxylate and subsidary colour matters together with sodium
chloride and/or sodium sulphate as the principal uncoloured components. E
102 is described as the sodium salt. The calcium and the potassium are also
permitted.
Experimental
FT-IR and FT-Raman spectra of E 102 solid powder were recorded using
an Equinox 55 FT-IR Bruker spectrometer with an integrated FT-Raman
FRA 106 S module. SERS spectra of E 102 on silver colloid [3] were
obtained on a Labram integrated Raman system. An Ar+ laser (514.5 nm) coupled
by optic fiber to the 50x NA 0.7 objective of the microscope was used. Peltier
cooled CCD detection was performed. For the SERS samples, E 102 solutions in
distilled water and sodium citrate-reduced Ag colloid were freshly prepared. On the
fresly prepared Ag colloid 0.3 ml 10-1 mol·l-1 NaCl solution was added in order to
adjust the aggregation status and hence the optimal enhancement of the Raman
signal. All reagents used were of analytical purity.
Results and Discussions
An interesting behaviour in the electronic absorption was observed since
the bands of E 102 and the silver colloid presents the same position (Fig. 2).
Moreover, adding the E 102 solution to the colloid, the plasmon resonance remains
unchanged. This fact reflects that the modification in the aggregation status of
silver particles with the adsorbed molecules is unchanged.
200
300
Wavelength/nm
400
500
600
700
(c)
(a)
724
852
524
852
724
ecnabrosbA
(b)
Fig. 2. Comparative UV-VIS spectra of E102 10-1 mol l-1 aqueous solution (a), E102 with
Lee Meisel (b), and Lee- Meisel Ag sol (c).
The vibrational spectra of E 102 are presented in Fig. 3. The most prominent FT-IR
bands were observed at 692, 1007, 1035 (fingerprint), 1124, 1154, 1179, 1475,
FOOD ADDITIVES STUDY: VIBRATIONAL APPROACH
3422 cm-1, besides the medium intensity bands at 714, 836, 1346, 1413,
1555,1562, 1598, 1642, 1725 cm-1. The FT-Raman spectrum reveals a sharp
marker band at 1601 cm-1 (para-substituted benzene stretching), several intense
bands at 1136, 1365 and 1503 cm-1assigned as SO3 asym. stretch and N=N stretch
for trans- and cis- form respectively. Bands with medium intensity at 622, 633,
1051, 1095, 1181, 1219,1419, 1691 cm-1 were also observed.
FT-IR
FT-Raman
3500
3000
2500
2000
1500
Wavenumber / cm
1000
500
0
-1
Fig. 3. FT-IR (micro-ATR) and FT-Raman spectra of E 102, solid powder.
As a monoazo compound, E 102 reveals the characteristic azochromophore
group –N=N- subject to azo hydrazone form in appropriate solutions [4, 5]. The
vibrational behaviour of the aqueous solution will be discussed into a next paper.
SERS spectrum of the title compound obtained at usual concentration of
about 10-5-10-6 mol l-1presents roughly the same band positions and intensities as
the corresponding Raman spectrum of aqueous solution (not given here), reflecting
a most probably physisorption on the nanoparticles.
Fig. 4 presents the SERS spectrum of E 102 on Ag colloid at picomolar
concentration. As we can see in comparison with the FT-Raman spectrum, large
differences are present either in band positions or relative intensities. The adsorbed
species presents intense SERS bands at 1366, 1511 and 1653 cm-1, bands involved
into the adsorption process through the azo and C=O functional groups.
1129
1366
1316
E 102
(-9)
1600
1216
1177
0
1800
1501
1415
1357
500
1129
238
617
1000
SERS 10
1598
Raman intensity / arb. units
1500
1653
1578
1511
S. CINTA PINZARU, N. PEICA, I. PAVEL, W. KIEFER
1400
1200
FT-Raman
1000
800
600
400
200
-1
Wavenumber / cm
Fig. 4. SERS spectrum of E 102 on Ag colloid at picomolar concentration in comparison
with its corresponding FT-Raman spectrum.
Conclusions
FT-IR and FT-Raman spectra of E 102 were recorded and used as data bank for
further applications in trace analysis of food.
With a conventional SERS setup E 102 was found to be detectable under
nanomolar level (10-10 mol l-1). A strong chemical interaction of E 102 with the
colloidal particles was concluded mainly at very low concentrations.
The surface selection rules along with our theoretical results are subject to explain
the adsorbate structures on the metal surface at different concentrations and pH
values. SERS and SERRS may be promising techniques for detecting and
monitoring the presence of such additives in food.
References
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