Experimental - Spectroscopy

42 Spectroscopy 26(6) June 2011
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(d)
Intensity (a.u)
60
40
20
0
(c)
60
Intensity (a.u)
(b)
Intensity (a.u)
(a)
Intensity (a.u)
40
20
0
12
8
4
0
12
8
4
0
1140
1140
1140
1140
1195
1195
1193
1174
1191
1222
1310
1311
1310
1310
1368
1368
1368
1370
1407
1406
1408
1441
1463
1407
1441
nation (6). The UV–vis absorption spectra
shown in Figure 2 indicate that the contribution
of molecular resonance to the SERS
is small. The surface plasmon resonance is
excited by laser radiation and may result
from the aggregations of silver nanoscale
particles. Some new lines appear in the
SERS spectra, indicating strong chargetransfer
effects between the molecules and
the silver surface. Therefore, the enhancement
of SERS signals is determined mainly
by surface plasmon resonance, chargetransfer
resonance, and their combination.
The change in SERS intensities with
variations in the pH may result from the
change in the contributions of the various
resonances. At high pH levels (pH 4–7), the
azo form of methyl yellow molecules predominates,
because these molecules adsorb
onto the silver surface through Ag-N bonding.
At low pH levels (pH 2–3), the hydrazo
form predominates, because these molecules
are adsorbed on silver surface through
the bridge of N + -Cl - -Ag + . The adsorption
1463
1441
1462
1442
1463
1492
1598
1619
1598
1619
1597
1619
1596
1619
1200 1400 1600 1800
Raman shift (cm -1 )
Figure 5: SERS spectra of methyl yellow (5 ×
10 -5 M) in 1100–1800 cm -1 range at pH (a) 2,
(b) 3, (c) 4, and (d) 7.
Intensity (a.u)
change of the molecules may make the molecular
orientation change, thus causing the
intensities to change. This is consistent with
the surface selection rules based on surface
plasmon resonance (6). The interaction of
molecules with a silver surface may change
primarily as a result of the charge of the
methyl yellow molecule (MYH + ).
Conclusion
The SERS spectra of methyl yellow in
varied pH solutions were studied. The enhancement
of SERS signals is determined
mainly by surface plasmon resonance,
charge-transfer resonance, and their
combination. A relative intensity analysis
of the Raman bands indicated that the
intensities change primarily when the pH
changes from 4 to 3. This may be a result
of a change in the contributions of their
combined system, such as the charge of
methyl yellow molecules and adsorption
of molecules on the silver surface.
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pH
Figure 6: Relative intensity variation of methyl yellow SERS bands at (a) 1140, (b) 1408, and (c)
824 cm -1 with pH variation from 2 to 7.
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Zhen Long Zhang and Yu Jun Mo
are with the School of Physics and Electronics
at Henan University, in Kaifeng, China.
Da Hu Chang is with the Department
of Mathematics and Science at the Luoyang
Institute of Science and Technology, in
Luoyang, China. ◾
pH
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