Spectrochimica Acta Part A 68 (2007) 778–782
Ultraviolet Raman study of thymine on the Au electrode Yanling Hao, Yan Fang ∗ Beijing Key Laboratory for Nano-Photonics and Nano-Structure, Capital Normal University, Beijing 100037, PR China Received 4 September 2006; received in revised form 30 December 2006; accepted 31 December 2006
Abstract We record the potential-dependent Raman spectra of thymine adsorbed on the roughened Au electrode by ultraviolet (UV) excitation at 325 nm, and we find that the surface-enhanced Raman spectra of thymine changed intensely with the negative shift of the applied potential. When the vibrational mode changes, the resonance potential (potential of maximum intensity) varies accordingly, indicating that the thymine molecules were chemisorbed on the roughened Au surface. The charge transfer (CT) mechanism could probably explain the experiment results in the present work. © 2007 Elsevier B.V. All rights reserved. Keywords: Ultraviolet (UV) Raman; Thymine; Au electrode; Charge transfer (CT) process
1. Introduction Since the first observation made by Fleischmann et al. , surface-enhanced Raman scattering (SERS), with its very high surface sensitivity and selectivity, has become a potentially ideal technique for studying chemical and physical interface phenomena [2–4]. It has been applied as a powerful technique for innovative and extensive analytical applications in surface science, electrochemistry, biology and materials research [5–7]. But we know that the conventional SERS spectroscopy usually works well with the excitation lines from the visible (450 nm) region to the near-infrared (NIR, 1064 nm) region [2–10]; however, there are inherent difficulties in the SERS experiment with ultraviolet (UV) excitation. Because of the experimental limitations, the reports of UV-SERS are very few. Due to its high energy, UV line could lead to resonant Raman transition or near resonant Raman transition among the electron energy levels, which increases the great sensitivity and acquires more abundant information about the molecule’s vibration and structure . Therefore, resonance enhancement can be employed to enhance the Raman signals of some biological molecules selectively in order to reach the low detection limit. Consequently, resonance Raman spectroscopy, especially ultraviolet resonance Raman spectroscopy (UVRRS), has been applied in the biological fields increasingly
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[12,13]. Moreover, ultraviolet resonance and surface-enhanced Raman effects can be combined together to produce the ultraviolet surface-enhanced resonance Raman spectroscopy (UV-SERRS). Therefore, it is significant to obtain more fruitful data in the UV region and reveal the experimental rule. As for the proteins and nucleic acids, the surface-enhanced Raman spectra excited by visible light are overlapped, which makes them complicated to analyze. Recently, it has been shown that more information about proteins and DNA (or RNA) can be obtained from the UV resonance Raman spectroscopy [14–20]. Thymine is not only one of the important pyrimidine bases of DNA, but also a frequent model of theoretical studies due to its biochemical importance [21,22]. Though the SERS spectra of thymine have been reported many times [23–28], the excitation lines are all located in the visible region. We obtained the UVSERS of thymine on the roughen Au electrode for the first time. In the present work, we studied the UV-SERS of thymine molecules adsorbed onto roughened Au electrode under different electrode potential. The potential-dependent surface Raman spectra of thymine are very different to the UV Raman spectra of thymine aqueous solution both in intensity and in the location of the Raman peaks. Especially with the change of the potential from −0.1 to −1.3 V, the surface Raman spectra changed obviously and when the vibrational mode changes, the resonance potential varies accordingly. Correspondingly, it can be assumed that the variation of SERS intensity as the applied voltage changes is a reflection of the electronic behavior, which was related to the charge transfer mechanism. So, we conclude that thymine molecules are chemisorbed on the Au electrode and
Y. Hao, Y. Fang / Spectrochimica Acta Part A 68 (2007) 778–782
charge transfer (CT) effect could probably explain the experimental results in the present work. 2. Experimental Thymine (99% pure) was purchased from Sigma–Aldrich and used without further purification. The UV Raman spectra were recorded with a Renishaw RM2000 micro-spectrophotometer, with a holographic notch filter and a CCD detector. A 15× objective was used to achieve a 180◦ backward scattering configuration. The excitation source was the 325 nm line of a K3301 R-GHelium–Cadmium laser. The power of laser on the sample was about 3.5 mW. The slit-width was 50 m and the resolution was 2 cm−1 . The optical absorption spectra were measured with a SHIMADZU Model UV-2401 PC UV-visible spectrophotometer. A three-electrode spectro-electrochemical was used to perform the oxidation–reduction cycles (ORCs). The counter electrode was a platinum ring (99.9%). An Ag/AgCl electrode was used as reference. The working electrode was made from 99.9% polycrystalline gold rod (Φ = 3.5 mm), which was mounted in a Teflon holder. A CHI660A electrochemical instrument was used to control the applied potential of the working electrode. The gold electrode was first mechanically polished with 1.0, 0.3 and 0.05 m alumina powder in turn to a mirror finish followed by ultrasonic cleaning with Milli-Q water, then it was mounted in the cell filled with 0.1 M KCl solution. The Sweepstep Functions was used. At the roughened surface of Au electrode, 24–25 oxidation–reduction circles were performed with a double potential step: −0. 25 to 1.25 V and kept for 8 s at 1.25 V, then back to −0.25 V and kept for 30 s. Sweep speed was 500 mV/s. When the process was finished, we can see the color of Au electrode surface was dark brown. This ex situ pretreatment was performed to form a relatively stable gold surface, which could sustain a rather constant Raman intensity for measurement. After being subjected to the roughing pretreatment, the electrode was immersed in the spectroelectrochemical cell that was filled with 0.01 M thymine and 0.1 M KCl for measurement. A Raman spectrum was normally measured after keeping the Au electrode at a fixed potential for 2 min, and the potentialdependent Raman spectra were acquired by moving the potential stepwise in the negative direction. All the potentials were quoted versus Ag/AgCl electrode.
Fig. 1. (a) The UV Raman spectrum of 0.01 M aqueous thymine and 0.1 M KCl. (b) Surface Raman spectrum of Thymine adsorbed on a gold electrode from abulk solution of 0.01 M thymine and 0.1 KCl. Electrode potential = −1.2 V. Excitation line = 325 nm.
two modes can be observed from the Au electrode at the given potentials. From the solution, the ring stretch mode appeared at 1238 cm−1 is much greater than the CH3 -rocking mode at 1174 cm−1 ; from the gold electrode surface, however, the similar mode at 1238 cm−1 is weaker than the 1174 cm−1 peak at the given potential. The same phenomenon occurs at 1597 and 1670 cm−1 , which are both assigned to the in-plane C O strech mode. Very interestingly, the bands at 622, 749 and 809 cm−1 in the thymine solution Raman spectrum exhibited a blueshift in the surface Raman spectrum of thymine on the Au electrode. The blueshift of these bands may indicate a weak interaction of the aromatic rings on the gold surfaces. Fig. 2 shows the potential-dependent surface spectra of thymine adsorbed on the roughened Au electrode with ultraviolet 325 nm excitation. The solution was 0.01 M thymine and 0.1 M KCl. It can be observed that with a negative potential shift
3. Results and discussion By the UV line excitation, the high quality Raman spectrum of thymine aqueous solution (0.005 M) was obtained, as is shown in Fig. 1a. Since the quality of the UV Raman spectrum of thymine aqueous solution is much better than those obtained with NIR and visible excitations, the notable surface-enhanced Raman signal with UV excitation would be desired. In Fig. 1, the UV surface-enhanced Raman spectrum of 0.01 M thymine aqueous solution in 0.1 M KCl on the roughed gold electrode at −1.2 V (Fig. 1b) is compared with the UV Raman spectrum of 0.01 M aqueous thymine and 0.1 M KCl (Fig. 1a). The bands at 1022 and 1486 cm−1 are hardly observed in Fig. 1a; however, the
Fig. 2. The potential-dependent surface spectra of thymine adsorbed on the Au electrode with ultraviolet 325 nm excitation. The solution was 0.01 M thymine and 0.1 M KCl.
Y. Hao, Y. Fang / Spectrochimica Acta Part A 68 (2007) 778–782
Table 1 Vibrational assignment of thymine UV Raman Solid
On Au electrode −0.1 Va
618 741 804 985
614 750 810 988
616 755 809 990
616 755 809 987
616 755 809 987
616 755 812 987
1157 1217 1250 1370 1428 1494
1241 1364 1421
1238 1362 1419
1238 1362 1416
1238 1362 1416
1238 1362 1416
619 755 816 987 1019 1174 1235 1362
622 758 816 987 1022 1174 1219 1238 1362
625 758 816 990 1022 1174 1219 1238 1362
1486 1597 1667
1486 1597 1667
C O bending (i.p.) Ring breathing (i.p.) Ring def bend (i.p.) Wagging (NH)(o.p.) Wagging (CH3 ) (o.p.) Rocking (CH3 ) (o.p.) Str (C CH3 ) (i.p.) Ring str (i.p.) CH3 def bend (i.p.) N3 H def bend (i.p.) Ring str (i.p.) C O str (i.p.) C O str (i.p.)
Note: def, deformation; str, stretching; i.p., in-plane; o.p., out-of-plane. a Electrode potential
from −0.1 to −1.3 V, the bands at 618 cm−1 (C O bending), 755 cm−1 (ring breathing) are blue shifted about 3–19 cm−1 , indicating that the surface Raman spectra are very sensitive to the electrode potential. With a negative potential shift, the intensity of the band at 1174 cm−1 , which is assigned to the CH3 rocking mode is augmented gradually, but the intensity of the band at 1238 cm−1 (ring stretching) is minimized. When the applied potential is at −0.1 V, the former band is somewhat weaker than the latter. However, the band at 1174 cm−1 is already a little stronger than the one at 1238 cm−1 when the potential is moved negatively to −0.8 V. And it can be seen that the 1174 cm−1 band has already submerged the 1238 cm−1 band when the potential reaches −0.9 V. The same phenomenon occurs at 1597 and 1670 cm−1 , which are both assigned to the C O stretch mode. From Fig. 2, it is indicated that, when with the applied potential at −0.8 V, the band at 1361 cm−1 present a shoulder at 1346 cm−1 , and it turns to one of the highest peak when the applied potential reached to −1.2 and −1.3 V. And when the applied potential is shifted to −0.8 V, the bands at 1022 cm−1 (CH3 Wagging), 1219 cm−1 (C–CH3 stretch) and 1485 cm−1 (ring stretch) appeared, although they are all very weak. All the variations of the spectra on the electrode potential are further evidence that thymine molecules can interact with the Au surface. Table 1 lists the frequencies of Raman bands assignments for thymine. The assignments are made according to [27,28]. It has been reported that absorption of ultraviolet light by adjacent thymine molecules yields a cyclobutane photodimer in DNA [29–31]. Can the thymine molecules in the solution yield cyclobutane photodimer in our experiment? The following experiment had been done to test whether the cyclobutane photodimer was formed in our experiment. Firstly, we got the optical absorption spectra of 5 × 10−3 M thymine aqueous solution with different lengths of UV irradiate time, and found that they are all the same. Secondly, Fig. 3 shows the Raman spectra of 0.01 M thymine aqueous solutions in 0.1 M KCl aqueous solutions with
different lengths of UV irradiate time. No changes were observed in the surface Raman spectra, so we concluded that the cyclobutane photodimer was not formed in the experiment. That is to say, the change of the spectra at the different potential is due to the interaction of thymine and the Au electrode. According to the theories about SERS, there are two main mechanisms to explain SERS : one is the electromagnetic (EM) mechanism, which implies the increase of the electric field near the metal surface through the resonant surface plasmon excitation. Under the assumption, the EM effect is independent of the electrode potential and the Raman shift of these bands that enhanced by this mechanism will not change obviously. Moreover, the surface plasmons could not be excited in the ultraviolet region on a noble metal such as Au , because the higher energy of UV line is beyond the plasmon resonance, this will reduce the relative importance of the electromagnetic
Fig. 3. The UV Raman spectra of thymine aqueous solution with different irradiate time: (a) 0 min, (b) 10 min, (c) 20 min, (d) 30 min, (e) 40 min, (f) 50 min, (g) 60 min and (h) 90 min.
Y. Hao, Y. Fang / Spectrochimica Acta Part A 68 (2007) 778–782
between the adsorbate and the metal surface, the Raman shift of these bands enhanced by this mechanism will change more or less. The partial vibration frequencies (e.g. the bands at 618 and 755 cm−1 ) of thymine on the Au electrode are shifted about 3–19 cm−1 with the negative shift of potential. 4. Conclusions In this work, we first obtained the UV-SERS on Au/thymine system. And the high quality Raman spectra of thymine in aqueous solution on the Au electrode surface at different potential is reported. It is showed that several intensity reversals occur, and the change of intensity maxima versus potential are observed not to be coincident. With the potential shift from −0.1 to −1.3 V, the relative intensities of most bands changed obviously, and partial vibration frequencies are shifted about 3–19 cm−1 . The electrode potential strongly dependence of the spectra is the evidence that thymine are chemisorbed on Au surface. Charge transfer effect could probably contribute to the experimental results. Fig. 4. Scheme of the charge transfer mechanism in the SERS.
Acknowledgements contribution to the SERS effect [33,34]. The other is chemical mechanism, which is due to a photo-driven charge transfer process. The CT mechanism has been proposed by Lombardi et al. , which is suggested that the transfer of an electron from the Fermi level of the metal to an unoccupied molecular orbital of the adsorbate or vice versa is possible depending on the energy of the photon and the electric potential of the inter-phase. The CT mechanism in this process is shown schematically in four steps in Fig. 4: (1) the incident UV laser photon is annihilated exciting the electron in the Fermi level (FL) of Au surface, and the electron is excited to the higher energy level. At the meantime, the electron–hole (e–h) pair in the metal generated. (2) The excited electron is transferred to vacant orbital (LUMO) in the adsorbate, and the corresponding radical anion is formed, on whose potential energy surface (PES) the system transitorily remains until the electron returns to the metal by another nonradiative process. (3) The combined effect of the energy of the UV photon together with that of the Fermi level in the gold electrode, which can be regulated by controlling the electric potential of the interphase, acts obviously in the strong dependence of the relative intensity and vibrational frequencies on the electrode potential in this case. (4) If the neutral molecule appears to be vibrationally excited the annihilation of the electron–hole pair in the metal gives rise to a Raman photon. A SERS–CT mechanism similar to a resonance Raman (RR) process between the electronic ground state of the adsorbate and charge transfer levels has been proposed [36,37]. Therefore, the potential energy surface of the involved states will be the key factor to influence the relative intensities of the bands. Moreover, it is conjectured that the variation of SERS intensity as the applied voltage changes reflects the electronic behavior, which, in turn, is related to the CT mechanism. While from Fig. 3, it can be observed that the intensity of the bands near 1022, 1174, 1238, 1346, 1597 and 1670 cm−1 are all dramatically changed with the potential from −0.1 to −1.3 V. Considering the chemical effect
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