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Raman spectroscopy of p-hydroxybenzoic acid aqueous solution and surface-unenhanced Raman scattering on silver colloid with ultraviolet excitation
Journal of Colloid and Interface Science 274 (2004) 122–125 www.elsevier.com/locate/jcis
Raman spectroscopy of p-hydroxybenzoic acid aqueous solution and surface-unenhanced Raman scattering on silver colloid with ultraviolet excitation Lili Xu and Yan Fang ∗ Beijing Key Lab of Nanophotonics and Nanostructure, Capital Normal University, Beijing 100037, People’s Republic of China Received 3 August 2003; accepted 13 December 2003
Abstract A high-quality Raman spectrum of p-hydroxybenzoic acid (PHBA) aqueous solution (10−2 M) under ultraviolet (UV) excitation at 325 nm was obtained, which could not be observed with visible and near infrared (NIR) excitations due to the low concentration in aqueous solution. However, the surface-unenhanced Raman scattering of PHBA in silver colloid excited by ultraviolet was unexpectedly observed, which was quite different from the cases excited with NIR and visible light, by which the SERS effect was very remarkable. This indicated that the SERS of the PHBA–silver colloid system showed selectivity to excitation wavelength. The enhancement mechanisms at different excitation wavelength regions are discussed. 2004 Elsevier Inc. All rights reserved. Keywords: Ultraviolet (UV) Raman spectrum; p-Hydroxybenzoic acid (PHBA); Surface-unenhanced Raman scattering (SUERS); Absorption
1. Introduction Employing photons as probes, Raman spectroscopy is potentially an ideal technique for studying chemical structures and physical interface phenomena [1]. Owing to the limitations of experimental instrument technology, conventional Raman spectroscopy usually works well at visible and NIR excitations [2,3]. However, UV Raman spectroscopy has recently been developed. It provides a great opportunity for Raman studies in the UV region, in which more significant phenomena would be revealed. There are advantages in using UV as excitation source. Photons with ultraviolet frequencies have high energy, which could induce resonance or near resonance transitions between electron energy levels of molecules [4]. Moreover, UV Raman could avoid fluorescence [5–7]. Therefore, UV Raman has usually been used in the investigation of biological macromolecules, especially for amide vibrations, which depend on the secondary structure [8,9]. SERS has the possibility of greatly increasing sensitivity of Raman spectroscopy, even to 105 –1010 fold. The SERS * Corresponding author.
E-mail address: [email protected] (L. Xu). 0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2003.12.041
in the UV region could not only assist in understanding the SERS enhancement mechanisms, but also develop a new application area of Raman spectroscopy. However, up to now, there are few reports about SERS in the UV region. The Raman spectra of PHBA in aqueous solution under NIR or visible excitation are difficult to observe due to the low concentration of PHBA even in saturated aqueous solutions. To obtain a Raman spectrum of a PHBA solution, PHBA is usually dissolved in organic solvents to increase solubility. However, PHBA is a type of acid and its properties are likely to change in organic solutions. On the other hand, the strong signals ascribable to the organic solvents would influence, or dominate over, the Raman spectrum of PHBA. But it is evident that there is no chemical interaction between PHBA and aqueous solution and the Raman spectrum is actual and credible. In this work, we have reported a high-quality Raman spectrum of PHBA aqueous solution excited by ultraviolet, as expected. It is worthwhile to note that no SERS effect is observed for PHBA adsorbed on silver colloid excited by ultraviolet, whereas the SERS is notable at NIR or visible excitation. The results mentioned above show that the degree of SERS enhancement of PHBA on silver surface is strongly dependent on excitation wavelength. The possible enhance-
L. Xu, Y. Fang / Journal of Colloid and Interface Science 274 (2004) 122–125
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ment mechanisms at different excitations have been discussed.
2. Experimental 2.1. Preparation of silver colloid Silver colloid is prepared according to Lee and Meisel’s method [10]. A weight of 90 mg of AgNO3 is dissolved in 500 ml of deionized water and the solution is heated to boiling. Then 10 ml of 1% aqueous solution of trisodium citrate is added into the boiling AgNO3 solution drop by drop, accompanied by vigorous stirring. The mixed solution is kept boiling for a further 10 min. Finally, a gray-green silver colloid is obtained, which shows a single visible absorbance band at about 430 nm. 2.2. Instrumentation The UV Raman spectra are recorded by a RENISHAW H13325 spectrophotometer, and the UV laser line is at 325 nm. The output laser power, which could not induce changes of the sample, is about 12 mW. The near infrared spectra are obtained through a Bruker RFS100/S NIR-FT spectrophotometer. The exciting laser wavelength is 1064 nm. Resolution is 4 cm−1 and a 180◦ geometry is employed. The UV–vis absorption spectra from the 800- to the 320nm region are directly obtained on a SHIMADZU UV2401pc spectrophotometer.
3. Results and discussion 3.1. UV Raman spectrum of PHBA aqueous solution The NIR or visible Raman spectrum of PHBA in aqueous solution is hardly obtained because of the quite low solubility of PHBA. Organic solvents are usually used to increase the molecule concentration. However, PHBA is a type of acid and interaction with organic solvents might change its character. At the same time, the Raman signals attributed to organic solvents could disturb or even submerge the Raman signals of PHBA. It is obvious that the Raman spectrum of PHBA in aqueous solution could provide more creditable information about structure and properties. UV Raman spectroscopy makes it possible to obtain the Raman signals of PHBA in aqueous solution. As exhibited in Fig. 1a, the UV Raman spectrum of PHBA aqueous solution is much better than the NIR FT Raman of PHBA in organic solvents such as ethanol, where the concentration of PHBA is about 0.29 M (40 mg/ml) [11]. In general, there is possible interaction between alcohol and acid, which results in a Raman spectrum of PHBA in ethanol that cannot always provide credible information for PHBA. However,
Fig. 1. The UV Raman spectra of PHBA aqueous solution at 1 × 10−2 M (a) and of PHBA solid powder (b).
the Raman spectrum obtained in aqueous solution can supply credible information by reason of having no interference from the solvent. In comparison with the UV Raman spectrum of PHBA solid powder, an additional band appears at 1687 cm−1 , which is due to the C=O stretching mode in Fig. 1a. And Fig. 1b does not show the weak band at 1388 cm−1 that appears in the solution. The 1288 cm−1 band in Fig. 1b undergoes a downshift to 1279 cm−1 in Fig. 1a, and a shoulder peak appears at its left side. Another new shoulder appears at 822 cm−1 in the spectrum of the aqueous solution. In the UV Raman spectrum of the PHBA aqueous solution, a band at 1687 cm−1 attributed to (C=O) stretching can be observed, which also can be observed in the NIR FT Raman spectrum of PHBA in ethanol but not detected in that of PHBA solid powder [11]. In crystals, pairs of PHBA molecules are linked through hydrogen bonds (2.635 Å in pure acid) between carboxyl groups to form cyclic dimers [12]. However, in aqueous solution, the hydrogen bonds between carboxyl groups break down. The weak band at 1388 cm−1 in Fig. 1a, due to the carboxyl symmetric stretching mode, also confirms the dissociation of dimers [11]. In addition, the band at 1388 cm−1 could not be observed in ethanol at low intensity, which exhibits the high sensitivity of UV Raman spectroscopy. The 1524 cm−1 band appears in the spectrum of PHBA in aqueous solution, while the 1444 cm−1 band emerges from that of solid powder, which are, respectively, due to 19a and 19b of C–C stretching modes [13]. The downshift of the 1279 cm−1 band indicates the breakdown of hydrogen bonds between dimers. The dimers in PHBA are held together by hydrogen bonds (2.897 Å) between phenolic groups. The hydrogen-bonded molecules spiral around the twofold screw axes to make up layers of dimmers parallel to (401) [12]. For the PHBA in aqueous solution, hydrogen bonds established between phenolic groups are destroyed and the downshift undergone by the band is due to C–OH stretching.
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Fig. 2. Raman spectra for PHBA aqueous solution at different concentrations, 1 × 10−2 (a), 5 × 10−3 (b), 2.5 × 10−3 (c), 1 × 10−3 (d), and 5 × 10−4 M (e) and for deionized water (f).
A new shoulder band appears at 822 cm−1 in the solution spectrum, which is possibly associated with ionization [14]. The dependence of UV Raman spectra on the concentration of the PHBA solution is exhibited in Fig. 2. It is clear that the lower the concentration of PHBA in aqueous solution, the less information in the UV Raman spectra. When the solution is diluted to 10−3 M, only several strong bands at 1610, 1279, 1171, and 846 cm−1 could be observed. When the solution is diluted to 5 × 10−4 M, the spectrum is similar to that of deionized water, just shown in Figs. 2e and 2f. In a word, analysis of the difference between the Raman spectrum of PHBA aqueous solution and that of the solid demonstrates more about the structure of the PHBA solid. The dimers, formed by the carboxyl groups between pairs of PHBA, are held to each other by the hydrogen bonds between phenolic groups.
Fig. 3. SUERS for PHBA at different concentration in silver colloid; concentration of PHBA is 1 × 10−2 (a), 5 × 10−3 (b), 2.5 × 10−3 (c), and 1 × 10−3 M (d).
Fig. 4. Absorbance spectra of colloid (a) and after addition of PHBA (b).
3.2. Surface-unenhanced Raman scattering (SUERS) of PHBA in silver colloid excited by ultraviolet If no SERS effect occurs when molecules adsorb on the metal surface, it is usually called surface-unenhanced Raman scattering (SUERS). The SUERS spectra for PHBA solution at different concentrations are shown in Fig. 3. Since the UV Raman spectrum of PHBA aqueous solution is strongly predominant over those at NIR and visible excitations, notable SERS excited by ultraviolet would be expected. However, only a SUERS effect of PHBA at excitation 325 nm on silver colloid was observed. When PHBA is added into silver colloid, the interaction between them takes place immediately. We observe not only color change of the silver colloid, but also changes in the absorption spectra in the region 800 to 320 nm, which ascertain the interaction. The absorbance peak of silver colloid at 430 nm weakens, whereas the long-wavelength region exhibits an increased absorbance after PHBA is added into silver colloid, as shown in Fig. 4. The interaction is also corroborated by the NIR SERS of PHBA in silver colloid, as shown in Fig. 5. There is no Raman signal in aqueous solution, but
Fig. 5. NIR Raman spectra of PHBA aqueous solution (a) and SERS in silver colloid (b).
abundant lines appear after the silver colloid is added. The case under visible excitation is similar to that under NIR. An aliquot of PHBA solution at a different concentration is added into the silver colloid to form mixtures, in which concentrations of PHBA are 1 × 10−2 , 5 × 10−3 , 2.5 × 10−3, and 1 × 10−3 M, respectively. The UV spectra of them are
L. Xu, Y. Fang / Journal of Colloid and Interface Science 274 (2004) 122–125
shown in Fig. 3. Comparing with Fig. 2, we find there is no SERS effect in Fig. 3 after PHBA is added into silver colloid and the Raman intensity is the same as that of the PHBA aqueous solution of the same concentration. Thus, we regard it as SUERS of PHBA on silver colloid excited by ultraviolet. The SERS of PHBA on silver colloid under visible excitation is possibly due to electromagnetic enhancement that occurs in the vicinity of small, interacting metal particles that are illuminated with light resonant or near resonant with the localized surface-plasmon frequency of the metal structure [15]. The absorbance peak of silver colloid is in the visible region. So the SERS effect of PHBA in silver colloid is significant at visible excitation. Furthermore, some literature noted peak Raman enhancement at 540 nm for silver due to an electromagnetic mechanism [16]. However, according to the literature, the electromagnetic contribution to the enhancement is insignificant [16]. And the interparticle interaction sometimes leads to larger enhancement. This could possibly explain the preponderance of the NIR SERS over the visible SERS. As the NIR region is concerned, the enhancement is due to both electromagnetic and chemical mechanism called charge transfer. “A part of excitation is therefore a metal–molecule charge transfer process. The trapping of electron in the empty virtual molecular orbital may be accompanied by a relaxation along the normal coordinates of molecule. So that when the electron leaves the virtual orbital the molecule is left in a vibrational excited state” [15]. The charge transfer can be demonstrated by the band ascribable to the metal–molecule vibration at about 200 cm−1 , which is not shown in Fig. 5 [17]. The UV SUERS of PHBA in silver colloid is possible because there is neither electromagnetic nor chemical enhancement in the UV region where the absorbance is weak, especially at about 320 nm, as shown in Fig. 4. And it is well known that the energy of ultraviolet is too large to induce charge transfer between the vibrational energy levels of the molecule and the metal. The photon energy of ultraviolet is about 3.8 eV, while the energy to induce transition between the vibrational energy levels is no more than 0.5 eV in our experiment. The result of wavelength dependence confirms the electromagnetic mechanism and charge transfer process in the NIR and visible regions.
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4. Conclusion Due to the low solubility of p-hydroxybenzoic acid in aqueous solution, it is difficult to detect the Raman signals in the visible and NIR regions. However, in this report, we present the high-quality UV Raman spectrum of PHBA aqueous solution (0.01 M). The appearance of the band at 1687 cm−1 and the weak band at 1388 cm−1 demonstrates the dissociation of the dimers. Another interesting result is the SUERS excited by ultraviolet in the PHBA–silver colloid system, whereas the SERS effect is notable in the visible and NIR regions. As for the enhancement mechanism at different excitation, the possible reason for the SUERS is that there is neither electromagnetic nor chemical enhancement in the UV region, which could be corroborated by the absorption spectrum.
References [1] L. Can, C.S. Peter, Catal. Today 33 (1997) 353. [2] E. Duval, G. Mariotto, M. Montagna, O. Pilla, G. Viliani, M. Barland, Europhys. Lett. 3 (1987) 333. [3] K. Katrin, K. Harald, I. Irving, R.D. Ramachandra, S.F. Michael, Chem. Phys. 247 (1999) 155. [4] A.W. Joseph, Alaa J. 37 (1999) 1015. [5] Y. Yi, X. Guang, L. Can, X. Fengshou, J. Catal. 194 (2000) 487. [6] X. Guang, Y. Yi, F. Zhaochi, X. Qin, X. Fengshou, L. Can, Micropor. Mesopor. Mater. 42 (2001) 317. [7] Y. Yi, Y. Jihong, X. Guang, L. Can, X. Fengshou, Phys. Chem. Chem. Phys. 3 (2001) 2692. [8] C. John, D.S. Alastair, FEBS Lett. 503 (2001) 30. [9] C. Zhenhuan, X.G. Chen, S.W.H. Janet, A.A. Sanford, Biochemistry 37 (1998) 2854. [10] P.C. Lee, D. Meisel, J. Phys. Chem. 86 (1982) 3391. [11] S. Sanchez-Cortes, J.V. Garcia-Ramos, J. Colloid Interface Sci. 231 (2000) 98. [12] E.A. Heath, P. Singh, Y. Ebisuzaki, Acta Crystallogr. Sect. C 48 (1992) 1960. [13] G. Varsanyi, Assignments for Vibrational Spectra of Seven Hundred Benzene Derivations, Wiley, New York, 1974. [14] B. Humbert, M. Alnot, F. Quiles, Spectrochim. Acta Part A 54 (1998) 465. [15] M. Moskovits, Rev. Mod. Phys. 57 (1985) 783. [16] E.J. Zeman, G.C. Schatz, J. Phys. Chem. 91 (1987) 634. [17] W. Chaozhi, H. Tianjing, G. Xiaoping, L. Jungqing, X. Houwen, L. Fanchen, J. Mol. Struct. 140 (1996) 227.
Raman spectroscopy of p-hydroxybenzoic acid aqueous solution and surface-unenhanced Raman scattering on silver colloid with ultraviolet excitation Lili Xu and Yan Fang ∗ Beijing Key Lab of Nanophotonics and Nanostructure, Capital Normal University, Beijing 100037, People’s Republic of China Received 3 August 2003; accepted 13 December 2003
Abstract A high-quality Raman spectrum of p-hydroxybenzoic acid (PHBA) aqueous solution (10−2 M) under ultraviolet (UV) excitation at 325 nm was obtained, which could not be observed with visible and near infrared (NIR) excitations due to the low concentration in aqueous solution. However, the surface-unenhanced Raman scattering of PHBA in silver colloid excited by ultraviolet was unexpectedly observed, which was quite different from the cases excited with NIR and visible light, by which the SERS effect was very remarkable. This indicated that the SERS of the PHBA–silver colloid system showed selectivity to excitation wavelength. The enhancement mechanisms at different excitation wavelength regions are discussed. 2004 Elsevier Inc. All rights reserved. Keywords: Ultraviolet (UV) Raman spectrum; p-Hydroxybenzoic acid (PHBA); Surface-unenhanced Raman scattering (SUERS); Absorption
1. Introduction Employing photons as probes, Raman spectroscopy is potentially an ideal technique for studying chemical structures and physical interface phenomena [1]. Owing to the limitations of experimental instrument technology, conventional Raman spectroscopy usually works well at visible and NIR excitations [2,3]. However, UV Raman spectroscopy has recently been developed. It provides a great opportunity for Raman studies in the UV region, in which more significant phenomena would be revealed. There are advantages in using UV as excitation source. Photons with ultraviolet frequencies have high energy, which could induce resonance or near resonance transitions between electron energy levels of molecules [4]. Moreover, UV Raman could avoid fluorescence [5–7]. Therefore, UV Raman has usually been used in the investigation of biological macromolecules, especially for amide vibrations, which depend on the secondary structure [8,9]. SERS has the possibility of greatly increasing sensitivity of Raman spectroscopy, even to 105 –1010 fold. The SERS * Corresponding author.
E-mail address: [email protected] (L. Xu). 0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2003.12.041
in the UV region could not only assist in understanding the SERS enhancement mechanisms, but also develop a new application area of Raman spectroscopy. However, up to now, there are few reports about SERS in the UV region. The Raman spectra of PHBA in aqueous solution under NIR or visible excitation are difficult to observe due to the low concentration of PHBA even in saturated aqueous solutions. To obtain a Raman spectrum of a PHBA solution, PHBA is usually dissolved in organic solvents to increase solubility. However, PHBA is a type of acid and its properties are likely to change in organic solutions. On the other hand, the strong signals ascribable to the organic solvents would influence, or dominate over, the Raman spectrum of PHBA. But it is evident that there is no chemical interaction between PHBA and aqueous solution and the Raman spectrum is actual and credible. In this work, we have reported a high-quality Raman spectrum of PHBA aqueous solution excited by ultraviolet, as expected. It is worthwhile to note that no SERS effect is observed for PHBA adsorbed on silver colloid excited by ultraviolet, whereas the SERS is notable at NIR or visible excitation. The results mentioned above show that the degree of SERS enhancement of PHBA on silver surface is strongly dependent on excitation wavelength. The possible enhance-
L. Xu, Y. Fang / Journal of Colloid and Interface Science 274 (2004) 122–125
123
ment mechanisms at different excitations have been discussed.
2. Experimental 2.1. Preparation of silver colloid Silver colloid is prepared according to Lee and Meisel’s method [10]. A weight of 90 mg of AgNO3 is dissolved in 500 ml of deionized water and the solution is heated to boiling. Then 10 ml of 1% aqueous solution of trisodium citrate is added into the boiling AgNO3 solution drop by drop, accompanied by vigorous stirring. The mixed solution is kept boiling for a further 10 min. Finally, a gray-green silver colloid is obtained, which shows a single visible absorbance band at about 430 nm. 2.2. Instrumentation The UV Raman spectra are recorded by a RENISHAW H13325 spectrophotometer, and the UV laser line is at 325 nm. The output laser power, which could not induce changes of the sample, is about 12 mW. The near infrared spectra are obtained through a Bruker RFS100/S NIR-FT spectrophotometer. The exciting laser wavelength is 1064 nm. Resolution is 4 cm−1 and a 180◦ geometry is employed. The UV–vis absorption spectra from the 800- to the 320nm region are directly obtained on a SHIMADZU UV2401pc spectrophotometer.
3. Results and discussion 3.1. UV Raman spectrum of PHBA aqueous solution The NIR or visible Raman spectrum of PHBA in aqueous solution is hardly obtained because of the quite low solubility of PHBA. Organic solvents are usually used to increase the molecule concentration. However, PHBA is a type of acid and interaction with organic solvents might change its character. At the same time, the Raman signals attributed to organic solvents could disturb or even submerge the Raman signals of PHBA. It is obvious that the Raman spectrum of PHBA in aqueous solution could provide more creditable information about structure and properties. UV Raman spectroscopy makes it possible to obtain the Raman signals of PHBA in aqueous solution. As exhibited in Fig. 1a, the UV Raman spectrum of PHBA aqueous solution is much better than the NIR FT Raman of PHBA in organic solvents such as ethanol, where the concentration of PHBA is about 0.29 M (40 mg/ml) [11]. In general, there is possible interaction between alcohol and acid, which results in a Raman spectrum of PHBA in ethanol that cannot always provide credible information for PHBA. However,
Fig. 1. The UV Raman spectra of PHBA aqueous solution at 1 × 10−2 M (a) and of PHBA solid powder (b).
the Raman spectrum obtained in aqueous solution can supply credible information by reason of having no interference from the solvent. In comparison with the UV Raman spectrum of PHBA solid powder, an additional band appears at 1687 cm−1 , which is due to the C=O stretching mode in Fig. 1a. And Fig. 1b does not show the weak band at 1388 cm−1 that appears in the solution. The 1288 cm−1 band in Fig. 1b undergoes a downshift to 1279 cm−1 in Fig. 1a, and a shoulder peak appears at its left side. Another new shoulder appears at 822 cm−1 in the spectrum of the aqueous solution. In the UV Raman spectrum of the PHBA aqueous solution, a band at 1687 cm−1 attributed to (C=O) stretching can be observed, which also can be observed in the NIR FT Raman spectrum of PHBA in ethanol but not detected in that of PHBA solid powder [11]. In crystals, pairs of PHBA molecules are linked through hydrogen bonds (2.635 Å in pure acid) between carboxyl groups to form cyclic dimers [12]. However, in aqueous solution, the hydrogen bonds between carboxyl groups break down. The weak band at 1388 cm−1 in Fig. 1a, due to the carboxyl symmetric stretching mode, also confirms the dissociation of dimers [11]. In addition, the band at 1388 cm−1 could not be observed in ethanol at low intensity, which exhibits the high sensitivity of UV Raman spectroscopy. The 1524 cm−1 band appears in the spectrum of PHBA in aqueous solution, while the 1444 cm−1 band emerges from that of solid powder, which are, respectively, due to 19a and 19b of C–C stretching modes [13]. The downshift of the 1279 cm−1 band indicates the breakdown of hydrogen bonds between dimers. The dimers in PHBA are held together by hydrogen bonds (2.897 Å) between phenolic groups. The hydrogen-bonded molecules spiral around the twofold screw axes to make up layers of dimmers parallel to (401) [12]. For the PHBA in aqueous solution, hydrogen bonds established between phenolic groups are destroyed and the downshift undergone by the band is due to C–OH stretching.
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Fig. 2. Raman spectra for PHBA aqueous solution at different concentrations, 1 × 10−2 (a), 5 × 10−3 (b), 2.5 × 10−3 (c), 1 × 10−3 (d), and 5 × 10−4 M (e) and for deionized water (f).
A new shoulder band appears at 822 cm−1 in the solution spectrum, which is possibly associated with ionization [14]. The dependence of UV Raman spectra on the concentration of the PHBA solution is exhibited in Fig. 2. It is clear that the lower the concentration of PHBA in aqueous solution, the less information in the UV Raman spectra. When the solution is diluted to 10−3 M, only several strong bands at 1610, 1279, 1171, and 846 cm−1 could be observed. When the solution is diluted to 5 × 10−4 M, the spectrum is similar to that of deionized water, just shown in Figs. 2e and 2f. In a word, analysis of the difference between the Raman spectrum of PHBA aqueous solution and that of the solid demonstrates more about the structure of the PHBA solid. The dimers, formed by the carboxyl groups between pairs of PHBA, are held to each other by the hydrogen bonds between phenolic groups.
Fig. 3. SUERS for PHBA at different concentration in silver colloid; concentration of PHBA is 1 × 10−2 (a), 5 × 10−3 (b), 2.5 × 10−3 (c), and 1 × 10−3 M (d).
Fig. 4. Absorbance spectra of colloid (a) and after addition of PHBA (b).
3.2. Surface-unenhanced Raman scattering (SUERS) of PHBA in silver colloid excited by ultraviolet If no SERS effect occurs when molecules adsorb on the metal surface, it is usually called surface-unenhanced Raman scattering (SUERS). The SUERS spectra for PHBA solution at different concentrations are shown in Fig. 3. Since the UV Raman spectrum of PHBA aqueous solution is strongly predominant over those at NIR and visible excitations, notable SERS excited by ultraviolet would be expected. However, only a SUERS effect of PHBA at excitation 325 nm on silver colloid was observed. When PHBA is added into silver colloid, the interaction between them takes place immediately. We observe not only color change of the silver colloid, but also changes in the absorption spectra in the region 800 to 320 nm, which ascertain the interaction. The absorbance peak of silver colloid at 430 nm weakens, whereas the long-wavelength region exhibits an increased absorbance after PHBA is added into silver colloid, as shown in Fig. 4. The interaction is also corroborated by the NIR SERS of PHBA in silver colloid, as shown in Fig. 5. There is no Raman signal in aqueous solution, but
Fig. 5. NIR Raman spectra of PHBA aqueous solution (a) and SERS in silver colloid (b).
abundant lines appear after the silver colloid is added. The case under visible excitation is similar to that under NIR. An aliquot of PHBA solution at a different concentration is added into the silver colloid to form mixtures, in which concentrations of PHBA are 1 × 10−2 , 5 × 10−3 , 2.5 × 10−3, and 1 × 10−3 M, respectively. The UV spectra of them are
L. Xu, Y. Fang / Journal of Colloid and Interface Science 274 (2004) 122–125
shown in Fig. 3. Comparing with Fig. 2, we find there is no SERS effect in Fig. 3 after PHBA is added into silver colloid and the Raman intensity is the same as that of the PHBA aqueous solution of the same concentration. Thus, we regard it as SUERS of PHBA on silver colloid excited by ultraviolet. The SERS of PHBA on silver colloid under visible excitation is possibly due to electromagnetic enhancement that occurs in the vicinity of small, interacting metal particles that are illuminated with light resonant or near resonant with the localized surface-plasmon frequency of the metal structure [15]. The absorbance peak of silver colloid is in the visible region. So the SERS effect of PHBA in silver colloid is significant at visible excitation. Furthermore, some literature noted peak Raman enhancement at 540 nm for silver due to an electromagnetic mechanism [16]. However, according to the literature, the electromagnetic contribution to the enhancement is insignificant [16]. And the interparticle interaction sometimes leads to larger enhancement. This could possibly explain the preponderance of the NIR SERS over the visible SERS. As the NIR region is concerned, the enhancement is due to both electromagnetic and chemical mechanism called charge transfer. “A part of excitation is therefore a metal–molecule charge transfer process. The trapping of electron in the empty virtual molecular orbital may be accompanied by a relaxation along the normal coordinates of molecule. So that when the electron leaves the virtual orbital the molecule is left in a vibrational excited state” [15]. The charge transfer can be demonstrated by the band ascribable to the metal–molecule vibration at about 200 cm−1 , which is not shown in Fig. 5 [17]. The UV SUERS of PHBA in silver colloid is possible because there is neither electromagnetic nor chemical enhancement in the UV region where the absorbance is weak, especially at about 320 nm, as shown in Fig. 4. And it is well known that the energy of ultraviolet is too large to induce charge transfer between the vibrational energy levels of the molecule and the metal. The photon energy of ultraviolet is about 3.8 eV, while the energy to induce transition between the vibrational energy levels is no more than 0.5 eV in our experiment. The result of wavelength dependence confirms the electromagnetic mechanism and charge transfer process in the NIR and visible regions.
125
4. Conclusion Due to the low solubility of p-hydroxybenzoic acid in aqueous solution, it is difficult to detect the Raman signals in the visible and NIR regions. However, in this report, we present the high-quality UV Raman spectrum of PHBA aqueous solution (0.01 M). The appearance of the band at 1687 cm−1 and the weak band at 1388 cm−1 demonstrates the dissociation of the dimers. Another interesting result is the SUERS excited by ultraviolet in the PHBA–silver colloid system, whereas the SERS effect is notable in the visible and NIR regions. As for the enhancement mechanism at different excitation, the possible reason for the SUERS is that there is neither electromagnetic nor chemical enhancement in the UV region, which could be corroborated by the absorption spectrum.
References [1] L. Can, C.S. Peter, Catal. Today 33 (1997) 353. [2] E. Duval, G. Mariotto, M. Montagna, O. Pilla, G. Viliani, M. Barland, Europhys. Lett. 3 (1987) 333. [3] K. Katrin, K. Harald, I. Irving, R.D. Ramachandra, S.F. Michael, Chem. Phys. 247 (1999) 155. [4] A.W. Joseph, Alaa J. 37 (1999) 1015. [5] Y. Yi, X. Guang, L. Can, X. Fengshou, J. Catal. 194 (2000) 487. [6] X. Guang, Y. Yi, F. Zhaochi, X. Qin, X. Fengshou, L. Can, Micropor. Mesopor. Mater. 42 (2001) 317. [7] Y. Yi, Y. Jihong, X. Guang, L. Can, X. Fengshou, Phys. Chem. Chem. Phys. 3 (2001) 2692. [8] C. John, D.S. Alastair, FEBS Lett. 503 (2001) 30. [9] C. Zhenhuan, X.G. Chen, S.W.H. Janet, A.A. Sanford, Biochemistry 37 (1998) 2854. [10] P.C. Lee, D. Meisel, J. Phys. Chem. 86 (1982) 3391. [11] S. Sanchez-Cortes, J.V. Garcia-Ramos, J. Colloid Interface Sci. 231 (2000) 98. [12] E.A. Heath, P. Singh, Y. Ebisuzaki, Acta Crystallogr. Sect. C 48 (1992) 1960. [13] G. Varsanyi, Assignments for Vibrational Spectra of Seven Hundred Benzene Derivations, Wiley, New York, 1974. [14] B. Humbert, M. Alnot, F. Quiles, Spectrochim. Acta Part A 54 (1998) 465. [15] M. Moskovits, Rev. Mod. Phys. 57 (1985) 783. [16] E.J. Zeman, G.C. Schatz, J. Phys. Chem. 91 (1987) 634. [17] W. Chaozhi, H. Tianjing, G. Xiaoping, L. Jungqing, X. Houwen, L. Fanchen, J. Mol. Struct. 140 (1996) 227.