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Surface-enhanced Raman scattering study of alizarin red S
Vibrational Spectroscopy 54 (2010) 137–141
Contents lists available at ScienceDirect
Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec
Surface-enhanced Raman scattering study of alizarin red S Michele L. de Souza, Paola Corio ∗ Departamento de Química Fundamental - Instituto de Química, Universidade de São Paulo, CP 26.077, 05513-970 São Paulo, SP, Brazil
a r t i c l e
i n f o
Article history: Received 16 January 2010 Received in revised form 19 May 2010 Accepted 31 July 2010 Available online 11 August 2010 Keywords: 1-Propanethiol Anthraquinone dyes Alizarin red S SERS substrates Silver nanoparticles
a b s t r a c t This paper presents the application of surface-enhanced resonance Raman spectroscopy (SERRS) for the structural study of alizarin red S (ARS) and the nature of its interaction with silver nanoparticles. SERRS data for ARS over nanostructured silver electrodes suggest a surface-induced reaction of the adsorbed dye and the formation of an ion stabilized by the dye and alkali ions adsorbed at the metal surface. We found that precoating the SERS active substrate with 1-propanethiol inhibits the surface-induced modification of ARS. In addition to preventing structural modifications of ARS, the coating also concentrates the hydrophobic dye close enough to the SERS active interface enabling the observation of excellent Raman spectra of ARS in aqueous environment at ppm levels. The influence of resonance Raman effect and of the pH on the SERS spectra of ARS was also investigated. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Surface-enhanced Raman scattering (SERS) represented a great advance in the field of Raman spectroscopy during the 1970s since the inherent weakness of the Raman signal can be increased by many orders of magnitude [1]. The SERS effect is based on the enhancement of the Raman scattering of molecules located within the proximities of certain nanostructured metallic surfaces, having an enormous potential for applications where high sensitivity needs to be combined with good discrimination between molecular targets [2]. It is now well understood that the main contribution to the enhancement of the Raman signal comes from strongly enhanced optical fields in the vicinity of the metallic surface derived from localized plasmon resonance in metal nanoparticles (electromagnetic SERS mechanism). In addition, the SERS enhancement can also have a contribution from the socalled chemical mechanism, which is based on specific chemical interaction between the adsorbed molecule and the metal [3]. The SERS effect is recognized nowadays as an important analytical surface technique for the characterization of adsorbates and of the chemical transformations that occur on metallic surfaces [4]. An important issue to be considered for the development of practical applications of surface-enhanced spectroscopy is the possibility of enhanced photochemical reactions for surface-adsorbed molecules [5,6]. In fact, direct observation of enhanced photochemistry for various molecules adsorbed on metallic nanostructures has been reported, such as the photoreduction of methyl violo-
∗ Corresponding author. Tel.: +55 11 3091 3865; fax: +55 11 3091 3890. E-mail address: [email protected] (P. Corio). 0924-2031/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2010.07.010
gen [7], the photodecomposition of azo compounds [8], and the photoinduced degradation of aromatic amino acids [9]. In this paper, we investigate the SERS effect of the hydroxyanthraquinone derivative alizarin red S (ARS, 1,2-dihydroxy-3-sulfonate-9,10anthraquinone, Fig. 1) over silver nanoparticles. Quinone derivatives are known to play very important roles in biological systems, and their redox properties have been studied using electrochemical and spectroscopic techniques [10]. The most popular naturally occurring quinoid compounds are anthraquinone and hydroxyanthraquinone derivatives, such as alizarin (1,2dihydroxyanthraquinone). Anthraquinone dyes are known to be biologically active compounds, with remarkable antigenotoxic activity as an inhibitor of the human recombinant cytochrome P450 isozymes [11,12]. Hydroxyanthraquinones have attracted the attention owing to their possible applications related to the photoactivity based on their chromatic properties. In particular, dihydroxyanthraquinones are known as a prominent family of pharmaceutically active and biologically relevant chromophores [12]. Anthraquinone dyes are also of importance as photosensitizers for dye-sensitized solar cells [13]. Alizarin in particular is one of the dyes extensively investigated as an example of a molecule capable of serving as a light absorber and an electron donor in model systems designed for solar cells [14]. In this context, the knowledge of the physical and chemical properties of such class of dyes adsorbed on a nanoparticle can be of interest for the understanding of their behavior in a dye-sensitized photovoltaic device [15]. Although recent literature reports SERS studies of alizarin adsorbed over a variety of metallic nanoparticles [16], SERS investigations of the ARS dye are scarce. The difficulties in obtaining Raman and SERS data from this compound can be related to its
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M.L. de Souza, P. Corio / Vibrational Spectroscopy 54 (2010) 137–141
and 785 nm laser lines were acquired with a Renishaw In Via System spectrometer coupled to a Leica microscope. Raman spectra excited with 514.5 and 488.0 nm lines of an Ar+ laser were acquired with a Renishaw System 3000 spectrometer coupled to a Olympus microscope (BTH2), both equipped with a CCD detector that allows a rapid accumulation of Raman spectra with a spatial resolution of about 1 m (micro-Raman technique). In this work four scans spectra were acquired with 4 cm−1 resolution. The laser beam was focused on the sample by a 50× lens. The experiments were performed at ambient conditions using a back-scattering geometry. FT-Raman spectra were recorded at room temperature in a RSF 100 FT-Raman Bruker spectrometer using the 1064 nm radiation from Nd:YAG laser, and 16 scans with 4 cm−1 resolution. 2.2. SERS substrates 2.2.1. Silver colloidal particles preparation The silver colloid was obtained according to the method described by Lee and Meisel [17]. A total of 1 mL of a 1% (w/v) trisodium citrate aqueous solution was added to 50 mL of a boiling 1.0 × 10−3 mol L−1 silver nitrate aqueous solution, and boiling was continued for 1 h. The obtained colloidal suspension showed a turbid greenish-yellow aspect with maximum absorbance centered at 405 nm and had a final pH of 6.5.
Fig. 1. (A) SERS spectra of ARS adsorbed on a silver electrode after (a) 20 min of sample preparation (inset: ARS structure) and (b) 3 days of sample preparation (0 = 632.8 nm). (B) UV–vis spectra of ARS aqueous solution (6.0 × 10−5 mol L−1 ) before and after exposure to the silver electrode.
strong fluorescence background under the laser excitation (even at the near-infrared wavelength region) and to its decomposition over traditionally assembled SERS active substrates, where direct interaction between the dye and a metallic surface occurs. In this way, the development of a well-suited SERS active surface for the study of such dye with becomes necessary. This work reports on the application of SERS to the study of ARS adsorbed over coated and uncoated silver nanostructures.
2.2.2. Colloidal silver supported on thin solid films Initially, glass slides were dipped in a solution containing nitric acid, sulfuric acid and deionized water (1:1:1, v/v) for 15 min in an ultrasonic bath. After that, these slides were washed abundantly with deionized water and rinsed with methanol, then oven-dried (110 ◦ C) for 30 min. They were placed in solution 0.5% APTMS ((3aminopropyl)trimetoxysilane) in n-hexane for 2 h with stirring, rinsed with methanol and deionized water and cured for 30 min at 110 ◦ C. After this procedure, one drop of silver colloid has been placed over the slides and this system was kept at desiccators in vacuum for 12 h. When dried, solution 1-propanethiol in methanol (0.5%, m/m) was placed dropwise over the silver nanoparticles. One drop of ARS 6.0 × 10−5 mol L−1 aqueous solution was placed over the substrate and the spectra were recorded after 30 min. Some spectra were recorded without the last stage of surface modification (addition of 1-propanethiol) to show the alizarin red S–silver (ARS–Ag) interaction. 2.2.3. Silver electrodes The metallic electrodes used as SERS substrates consisted of a silver electrode with 0.2 cm2 of geometrical area prepared by electrochemical roughing using three oxidation reduction cycles in the −0.3 to +0.25 V potential range in a 0.1 mol L−1 KCl solution, at 50 mV s−1 . The electrode is then removed from the spectroeletrochemical cell, rinsed abundantly with deionized water and immersed on an aqueous solution of the target analyte for 24 h. After this treatment, the silver electrode is rinsed with water and dried with nitrogen.
2. Experimental 3. Results and discussion 2.1. Materials and instrumentation Chemical and solvents were supplied by Aldrich and Merck. All chemicals were analytical grade and were used as received without any further purification. The organic solvents were spectroscopic grade. The working solutions were prepared using analytical grade chemicals and deionized water. UV–vis spectra were obtained with a Shimadzu UVPC-3101 scanning spectrophotometer. Raman spectra excited with 632.8
3.1. Raman spectra and SERS of ARS on silver electrodes—the effect of adsorption on a metallic surface The Raman and FT-Raman spectra of ARS show high fluorescence background and this behavior limits the application of Raman spectroscopy as an investigation method for such dye. SERS technique makes fluorescent quenching possible, and was applied for the vibrational study of ARS molecule.
M.L. de Souza, P. Corio / Vibrational Spectroscopy 54 (2010) 137–141
Fig. 2. UV–vis spectra of ARS aqueous solution (6.0 × 10−5 mol L−1 ) in the indicated pH values and after exposure to the SERS active silver electrode.
Fig. 1A shows the SERS spectra of ARS adsorbed on an electrochemically roughened silver electrode recorded with excitation at 632.8 nm. While the observed SERS signal is quite strong, the chemical interaction between the adsorbed molecule and the nanostructured SERS active surface causes significant changes in the observed SERS spectrum as a function of adsorption time. The observed SERS signal is due to a variety of chromophores generated by the surface-induced reaction at the silver surface. These changes are also reflected on the absorption spectra of dye solution which is in contact with the electrode shown in Fig. 1B. The maximum in the electronic spectra shifts from 422 to 517 nm. These changes in the absorption spectra are due to the equilibrium established between the adsorbed species on the electrode and in solution. To understand these results, it is important to consider the acid–base properties of the investigated molecule. Anthraquinonic species exists in three different forms, depending on the pH value: the neutral form, the monoanionic and the dianionic, for ARS the respective values are pK1 = 5.98 and pK2 = 9.88 [18]. The electronic spectra of each of these species present characteristic bands, as shown in Fig. 2. For pH = 5.6, two bands are observed, at 307 and 433 nm. At more acidic media (pH 1.6), no significant changes are
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observed, suggesting that the protonation occurs at a very low pH value. For pH values above 8.4, the band in 433 disappears, and a band at ca. 500 nm is observed. These changes are associated with the ionization of the phenolic group. ˜ In a work investigating the behavior of alizarin, Canamares et al. [19] report a similarity between the Raman spectra of the basic form of the dye (pH above 6) and the surface-adsorbed species, interacting through the ketonic and hydroxyl moieties with the surface. So, the interaction with the metallic surfaces induces the deprotonation of the OH group, and the formed species is similar to the dianionic form of the dye. Tentative assignment for alizarine red S, the most prominent Raman feature is proposed in Table 1, considering the comparison with the vibrational assignment proposed in the literature for the alizarine molecule in alkaline medium. The main changes observed in the SERS spectrum of adsorbed ARS according to adsorption time are related to the relative intensities of the bands in 1188, 1323 and 1245 cm−1 , associated with CC and COH stretching modes and to CO bending mode, respectively, and between the bands in 1424, 1446 and 1461 cm−1 . Also, after 3 days of adsorption time, the bands in 1627, 1188 and 566 cm−1 emerge and can be considered characteristic vibrational features of the adsorption product. In addition, the band at 630 cm−1 shifts to 648 cm−1 with the surface-induced reaction. Importantly, no significant modifications are observed in the vibration features associated with the sulfonate group. Thus, considering the decrease in the intensity of the vibrational mode assigned to CO stretching and bending modes, and the enhancement of the Raman bands associated with COH, CC, CCC and CH stretching modes caused by adsorption of the dye (Fig. 1A), it is reasonable to propose the interaction of the molecule and the metallic surface occurs through the ketonic and hydroxylic groups. So, the interaction between the dye and the metallic surface induces deprotonation of OH group, causing an electronic delocalization involving the aromatic region of the molecule (in a structure similar to the dianionic structure). The observed results in Fig. 1 thus suggest that the interaction between the dye and the enhancing metallic substrate causes the formation of an ARS ion stabilized in solution involving the adsorbed species and K+ ions adsorbed at the electrode surface during the oxidation–reduction cycles performed to activate the silver electrode. The surface-induced reaction is reflected in structural modifications of the dye which are observed both on the electrode surface (through changes in the vibrational Raman spectrum) and on the aqueous solution after contact with the electrode (through changes in its absorption spectrum).
Table 1 Tentative assignment for the observed SERS bands for alizarine red after 20 min and after 3 days of adsorption to the silver electrode. Wavenumber/cm−1
Tentative assignment
Alizarine [19]
Alizarine red [20]
ARS (20 min adsorption time)
ARS (3 days adsorption time)
– 1624 1600 – 1461 1458 1423 – 1320 1296 1272 – 1186 – –
1665 1634 1590 – 1459 1441 – 1350 1330 1289 1265 1235 1203 1160 1068
– – 1603 1480 1458 – 1423 1340 1323 1295 1245 – 1188 1162 1057
– 1627 1605 – 1461 1446 1424 – 1325 1297 1246 – 1188 1160 1058
: stretching; ı: in-plane bending; : out-of-plane bending.
(C O) (C O) (CC) (C O)/(CC)/ı(CH) (CC)/ı(COH)/ı(CH) – (CC)/ı(COH) ı(CO) (CC) (C O) (C O) SO3 H (CC)/ı(CH)/ı(CCC) (SO3 ) assimétrico (SO3 ) simétrico
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Fig. 3. SERS spectra of ARS adsorbed over silver colloidal particles at the indicated pH values, and SERS spectrum of ARS over silver electrode after 3 days of adsorption time (0 = 632.8 nm).
In the present study, it was observed that the ARS solution turns pink colored by contact with a metallic electrode (either gold or silver) activated through oxidation–reduction cycles. It is important to mention that no changes are induced in the bulk solution by the immersion of a polished metallic electrode, even after a period of 3 months. A reasonable explanation for such behavior is based on the presence of alkali metal ions adsorbed at the surface during the ORC which, upon interaction of the dye with the surface stabilizes the alizarine red S ion that diffuses to the solution bulk. In fact, it has been reported that one of the features of hidroxyanthraquinones compounds is the fact that this compounds are able to strongly interact with the surface with the formation of a chelate complex with metal [21] and quinone anions can be significantly stabilized for alkali ions [22]. To better understand the dependence of characteristic Raman features of ARS on its protonation state, SERS of ARS in aqueous solution as a function of pH was investigated (Fig. 3). Fig. 3 shows a similarity between the characteristic spectra of deprotonated ARS (observed in basic media) and the dye after 3 days of interaction with the metallic surface. This result corroborates the adsorption model proposed, where complexation of ARS is compared to its deprotonation reaction. It is thus important to mention that such results suggest that changes in the SERS spectra results from a surface-induced reaction, rather than from changes in the structure of the adsorbed molecule. This conclusion is supported by the change in the absorption spectra of the solution induced by the presence of the activated electrode, which is assigned to the formation of an anion stabilized due the presence of the anionic form of ARS and K+ ions from the substrate. The use an analyte-specific affinity coating to avoid such surface-reaction was investigated, as shown in the next section. 3.2. SERS on colloidal silver nanostructures supported on thin solid films Given that the interaction between ARS and electrochemically roughened silver electrode promoted structural changes on the investigated dye, SERS of ARS was also investigated employing a different substrate, constructed by the immobilization of colloidal silver particles by silanization of a glass slide. The use of these systems as SERS active substrates is of considerable interest. The transference of the colloids to solid supports stabilizes them, lead-
Fig. 4. SERS spectra of the CH3 (CH2 )2 SH coated silver substrate and of ARS (6.0 × 10−5 mol L−1 ) adsorbed over alkyl modified and unmodified silver substrate (0 = 632.8 nm).
ing to the formation of self-sustained films that can be considered portable SERS sensors for specific analytical applications [23]. To avoid direct interaction between the dye and the silver nanostructure, the SERS active substrate was coated with a self-assembled monolayer (SAM) of CH3 (CH2 )2 SH. In aqueous systems, these systems provide surfaces for the detection of hydrophobic analytes [24,25], and it has been found that coatings with thiol groups are strongly anchored to the noble metal substrate and do not desorb under acidic or basic conditions [25]. SERS spectra (excited by 632.8 nm laser line) of ARS adsorbed over coated and uncoated silver substrate are shown in Fig. 4. Substrates A and C shown in Fig. 4 consist of silver colloid immobilized on glass silanized coated with 1-propanethiol and ARS solution. The observed spectra of ARS over the coated substrates A and C show a series of surface-enhanced Raman lines assignable to the monolayers of CH3 (CH2 )2 SH superimposed on the intense surface-enhanced resonance Raman vibrational spectrum of ARS dye. The adsorption CH3 (CH2 )2 SH to the nanostructured surface allows the observation of SERS spectra of unperturbed ARS, without the changes induced by the complexation reaction between the dye and the metal. The Raman bands of the CH3 (CH2 )2 SH in the coated substrate are stronger in the region below 1025 cm−1 , thus interfering little with the observation of the more characteristic Raman features of ARS, which occur in higher wavenumber region. The coated film therefore is suitable for sensitive spectroscopic analysis of ARS dye, allowing for the detection of the dye at very low concentration range (6.0 × 10−5 mol L−1 ). Thus, in addition to preventing structural modifications of ARS, the coating also concentrates the hydrophobic dye close enough to the SERS active interface. So, in addition to strong intensification of the Raman signal, the coated substrate shows very high reproducibility and stability over time (up to 20 days), enabling the observation of excellent Raman spectra of ARS in aqueous environment at ppm levels. 3.3. Dependence on the wavelength excitation energy: resonance Raman and SERS It has been observed that the performance of the substrate towards the analysis of ARS is strongly dependent of the frequency of the exciting radiation. Fig. 5 shows the SERS spectra of ARS
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electrode surface (through changes in the vibrational Raman spectrum) and on the aqueous solution in contact with the electrode (through changes in its absorption spectrum). SERS of ARS was also investigated employing a substrate constructed by the immobilization of colloidal silver particles by silanization of a glass slide. It was found that the use of CH3 (CH2 )2 SH as an analyte-specific affinity coating avoids direct contact between the dye and the metallic particles thus allowing the observation of SERS spectra of unperturbed ARS. The coated film therefore is suitable for sensitive spectroscopic analysis of ARS dye, allowing for the detection of the dye at very low concentration range (6.0 × 10−5 mol L−1 ). Acknowledgements This work was supported by FAPESP, CNPq and CAPES. Fellowships from FAPESP and CNPq are gratefully acknowledged. References
Fig. 5. SERS spectra of ARS (6,0 × 10−5 mol L−1 ) over 1-propanethiol coated silver surface obtained at the indicated laser wavelengths.
adsorbed over the coated silver substrate obtained in excitation wavelength between 488 and 1064 nm. Considering the Raman signal of the CH3 (CH2 )2 SH as an internal intensity standard, Fig. 5 shows that higher energy excitation wavelengths enhance the dye signal. It is possible to observe a weak signal from ARS on spectra excited by 488 and 514 nm, the most intense substrate band in 1025 cm−1 is not present in these spectra. This behavior can be assigned to the contribution of resonance Raman effect to the observed signal. For lower energy exciting wavelengths, in the red and infrared regions, the obtained spectra shows principally the Raman features of the thiol SAM, indicating that the association of surface enhancement and resonance Raman effect is necessary for the observation of the ARS vibrational features (surface-enhanced resonance Raman effect, SERRS). 4. Conclusions SERRS of ARS was investigated over silver electrodes and over colloidal silver nanoparticles supported on thin solid films coated and uncoated with a self-assembled monolayer of 1-propanethiol. Direct chemical interaction between the dye and a metallic nanostructured surface causes significant changes in the observed SERS spectrum as a function of time. Results suggest that the interaction between the dye and the metallic surface induces deprotonation of OH group and the formation of an ion stabilized by the adsorbed species and K+ ions. The surface-induced reaction is reflected in structural modifications of the dye which are observed both on the
˜ [1] M.V. Canamares, J.V. Garcia-Ramos, S. Sanchez-Cortes, M. Castillejo, M. Oujja, J. Colloid Interface Sci. 326 (1) (2008) 103–109. [2] S.E.J. Bell, N.M.S. Sirimuthu, Chem. Soc. Rev. 37 (2008) 1012–1024. [3] M. Moskovits, L.L. Tay, J. Yang, T. Haslett, Top. Appl. Phys. 82 (2002) 215–226. [4] R. Aroca, Surface Enhanced Vibational Espectroscopy, John Wiley & Sons, Ltd., West Sussex, 2006. [5] K. Kwan, S.J. Lee, K.L. Kim, J. Phys. Chem. B 108 (2004) 16208–16212. [6] R.A. Wolkow, M. Moskovits, J. Chem. Phys. 87 (1987) 5858–5869. [7] H. Feilchenfeld, G. Chumanov, T.M. Cotton, J. Phys. Chem. 100 (1996) 4937–4943. [8] D. Franzke, A. Wokaun, J. Phys. Chem. 96 (1992) 6377–6381. [9] E.J. Bjerneld, F. Svedberg, P. Johansson, M. Käll, J. Phys. Chem. A 108 (2004) 4187–4193. [10] S.H. Han, S.W. Joo, T.H. Ha, Y. Kim, K. Kim, J. Phys. Chem. B 104 (2000) 11987–11995. [11] E. Takahashi, K. Fujita, T. Kamataki, S. Arimoto-Kobayashi, K. Okamoto, T. Negishi, Mutat. Res. 508 (2002) 147–156. [12] A. Baran, B. Wrzosek, J. Bukowska, L.M. Proniewicz, M. Baranska, J. Raman Spectrosc. 40 (4) (2009) 436–441. [13] C. Li, X.Y.R. Chen, J. Pan, H. Tian, H. Zhu, X. Wang, A. Hagfeldt, L. Sun, Sol. Energy Mater. Sol. Cells 91 (2007) 1863–1871. [14] A. Nawrocka, S. Krawczyk, J. Phys. Chem. C 112 (2008) 10233–10241. [15] P.M. Jayaweera, R.M.S.P. Rajapaksha, K. Tennakone, Chem. Phys. Lett. 412 (2005) 29–34. ˜ [16] M.V. Canamares, J.V. Garcia-Ramos, S. Sanchez-Cortes, M. Castillejo, M. Oujja, J. Colloid Interface Sci. 326 (2008) 103–109. [17] P.C. Lee, D. Meisel, J. Phys. Chem. 86 (1982) 3391–3395. [18] A. Holmgren, L. Wu, W. Forsling, Spectrochim. Acta A 55 (1999) 1721–1730. ˜ [19] M.V. Canamares, J.V. Garcia-Ramos, C. Domingo, S. Sanchez-Cortes, J. Raman Spectrosc. 35 (2004) 921–927. [20] P.M. Jayaweera, T.A.U. Jayarathne, Surf. Sci. 600 (2006) L297–L300. ˜ [21] M.V. Canamares, J.V. Garcia-Ramos, J.D. Gómez-Varga, D. Domingo, S. SanchezCortes, Langmuir 23 (2007) 5210–5215. [22] J.S. Jaworski, M.K. Kalinowski, J. Electroanal. Chem. 76 (1977) 301–314. [23] R.F. Aroca, R.A. Alvarez-Puebla, N. Pieczonka, S. Sanchez-Cortez, J.V. GarciaRamos, Adv. Colloid Interface Sci. 116 (1–3) (2005) 45–61. [24] J.C.S. Costa, A.C. Sant’Ana, P. Corio, M.L.A. Temperini, Talanta 70 (2006) 1011–1016. [25] D.N. Strekal’, A.E. German, G.A. Gachko, S.A. Maskevich, J. Mol. Spectrosc. 89 (6) (2000) 911–917.
Contents lists available at ScienceDirect
Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec
Surface-enhanced Raman scattering study of alizarin red S Michele L. de Souza, Paola Corio ∗ Departamento de Química Fundamental - Instituto de Química, Universidade de São Paulo, CP 26.077, 05513-970 São Paulo, SP, Brazil
a r t i c l e
i n f o
Article history: Received 16 January 2010 Received in revised form 19 May 2010 Accepted 31 July 2010 Available online 11 August 2010 Keywords: 1-Propanethiol Anthraquinone dyes Alizarin red S SERS substrates Silver nanoparticles
a b s t r a c t This paper presents the application of surface-enhanced resonance Raman spectroscopy (SERRS) for the structural study of alizarin red S (ARS) and the nature of its interaction with silver nanoparticles. SERRS data for ARS over nanostructured silver electrodes suggest a surface-induced reaction of the adsorbed dye and the formation of an ion stabilized by the dye and alkali ions adsorbed at the metal surface. We found that precoating the SERS active substrate with 1-propanethiol inhibits the surface-induced modification of ARS. In addition to preventing structural modifications of ARS, the coating also concentrates the hydrophobic dye close enough to the SERS active interface enabling the observation of excellent Raman spectra of ARS in aqueous environment at ppm levels. The influence of resonance Raman effect and of the pH on the SERS spectra of ARS was also investigated. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Surface-enhanced Raman scattering (SERS) represented a great advance in the field of Raman spectroscopy during the 1970s since the inherent weakness of the Raman signal can be increased by many orders of magnitude [1]. The SERS effect is based on the enhancement of the Raman scattering of molecules located within the proximities of certain nanostructured metallic surfaces, having an enormous potential for applications where high sensitivity needs to be combined with good discrimination between molecular targets [2]. It is now well understood that the main contribution to the enhancement of the Raman signal comes from strongly enhanced optical fields in the vicinity of the metallic surface derived from localized plasmon resonance in metal nanoparticles (electromagnetic SERS mechanism). In addition, the SERS enhancement can also have a contribution from the socalled chemical mechanism, which is based on specific chemical interaction between the adsorbed molecule and the metal [3]. The SERS effect is recognized nowadays as an important analytical surface technique for the characterization of adsorbates and of the chemical transformations that occur on metallic surfaces [4]. An important issue to be considered for the development of practical applications of surface-enhanced spectroscopy is the possibility of enhanced photochemical reactions for surface-adsorbed molecules [5,6]. In fact, direct observation of enhanced photochemistry for various molecules adsorbed on metallic nanostructures has been reported, such as the photoreduction of methyl violo-
∗ Corresponding author. Tel.: +55 11 3091 3865; fax: +55 11 3091 3890. E-mail address: [email protected] (P. Corio). 0924-2031/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2010.07.010
gen [7], the photodecomposition of azo compounds [8], and the photoinduced degradation of aromatic amino acids [9]. In this paper, we investigate the SERS effect of the hydroxyanthraquinone derivative alizarin red S (ARS, 1,2-dihydroxy-3-sulfonate-9,10anthraquinone, Fig. 1) over silver nanoparticles. Quinone derivatives are known to play very important roles in biological systems, and their redox properties have been studied using electrochemical and spectroscopic techniques [10]. The most popular naturally occurring quinoid compounds are anthraquinone and hydroxyanthraquinone derivatives, such as alizarin (1,2dihydroxyanthraquinone). Anthraquinone dyes are known to be biologically active compounds, with remarkable antigenotoxic activity as an inhibitor of the human recombinant cytochrome P450 isozymes [11,12]. Hydroxyanthraquinones have attracted the attention owing to their possible applications related to the photoactivity based on their chromatic properties. In particular, dihydroxyanthraquinones are known as a prominent family of pharmaceutically active and biologically relevant chromophores [12]. Anthraquinone dyes are also of importance as photosensitizers for dye-sensitized solar cells [13]. Alizarin in particular is one of the dyes extensively investigated as an example of a molecule capable of serving as a light absorber and an electron donor in model systems designed for solar cells [14]. In this context, the knowledge of the physical and chemical properties of such class of dyes adsorbed on a nanoparticle can be of interest for the understanding of their behavior in a dye-sensitized photovoltaic device [15]. Although recent literature reports SERS studies of alizarin adsorbed over a variety of metallic nanoparticles [16], SERS investigations of the ARS dye are scarce. The difficulties in obtaining Raman and SERS data from this compound can be related to its
138
M.L. de Souza, P. Corio / Vibrational Spectroscopy 54 (2010) 137–141
and 785 nm laser lines were acquired with a Renishaw In Via System spectrometer coupled to a Leica microscope. Raman spectra excited with 514.5 and 488.0 nm lines of an Ar+ laser were acquired with a Renishaw System 3000 spectrometer coupled to a Olympus microscope (BTH2), both equipped with a CCD detector that allows a rapid accumulation of Raman spectra with a spatial resolution of about 1 m (micro-Raman technique). In this work four scans spectra were acquired with 4 cm−1 resolution. The laser beam was focused on the sample by a 50× lens. The experiments were performed at ambient conditions using a back-scattering geometry. FT-Raman spectra were recorded at room temperature in a RSF 100 FT-Raman Bruker spectrometer using the 1064 nm radiation from Nd:YAG laser, and 16 scans with 4 cm−1 resolution. 2.2. SERS substrates 2.2.1. Silver colloidal particles preparation The silver colloid was obtained according to the method described by Lee and Meisel [17]. A total of 1 mL of a 1% (w/v) trisodium citrate aqueous solution was added to 50 mL of a boiling 1.0 × 10−3 mol L−1 silver nitrate aqueous solution, and boiling was continued for 1 h. The obtained colloidal suspension showed a turbid greenish-yellow aspect with maximum absorbance centered at 405 nm and had a final pH of 6.5.
Fig. 1. (A) SERS spectra of ARS adsorbed on a silver electrode after (a) 20 min of sample preparation (inset: ARS structure) and (b) 3 days of sample preparation (0 = 632.8 nm). (B) UV–vis spectra of ARS aqueous solution (6.0 × 10−5 mol L−1 ) before and after exposure to the silver electrode.
strong fluorescence background under the laser excitation (even at the near-infrared wavelength region) and to its decomposition over traditionally assembled SERS active substrates, where direct interaction between the dye and a metallic surface occurs. In this way, the development of a well-suited SERS active surface for the study of such dye with becomes necessary. This work reports on the application of SERS to the study of ARS adsorbed over coated and uncoated silver nanostructures.
2.2.2. Colloidal silver supported on thin solid films Initially, glass slides were dipped in a solution containing nitric acid, sulfuric acid and deionized water (1:1:1, v/v) for 15 min in an ultrasonic bath. After that, these slides were washed abundantly with deionized water and rinsed with methanol, then oven-dried (110 ◦ C) for 30 min. They were placed in solution 0.5% APTMS ((3aminopropyl)trimetoxysilane) in n-hexane for 2 h with stirring, rinsed with methanol and deionized water and cured for 30 min at 110 ◦ C. After this procedure, one drop of silver colloid has been placed over the slides and this system was kept at desiccators in vacuum for 12 h. When dried, solution 1-propanethiol in methanol (0.5%, m/m) was placed dropwise over the silver nanoparticles. One drop of ARS 6.0 × 10−5 mol L−1 aqueous solution was placed over the substrate and the spectra were recorded after 30 min. Some spectra were recorded without the last stage of surface modification (addition of 1-propanethiol) to show the alizarin red S–silver (ARS–Ag) interaction. 2.2.3. Silver electrodes The metallic electrodes used as SERS substrates consisted of a silver electrode with 0.2 cm2 of geometrical area prepared by electrochemical roughing using three oxidation reduction cycles in the −0.3 to +0.25 V potential range in a 0.1 mol L−1 KCl solution, at 50 mV s−1 . The electrode is then removed from the spectroeletrochemical cell, rinsed abundantly with deionized water and immersed on an aqueous solution of the target analyte for 24 h. After this treatment, the silver electrode is rinsed with water and dried with nitrogen.
2. Experimental 3. Results and discussion 2.1. Materials and instrumentation Chemical and solvents were supplied by Aldrich and Merck. All chemicals were analytical grade and were used as received without any further purification. The organic solvents were spectroscopic grade. The working solutions were prepared using analytical grade chemicals and deionized water. UV–vis spectra were obtained with a Shimadzu UVPC-3101 scanning spectrophotometer. Raman spectra excited with 632.8
3.1. Raman spectra and SERS of ARS on silver electrodes—the effect of adsorption on a metallic surface The Raman and FT-Raman spectra of ARS show high fluorescence background and this behavior limits the application of Raman spectroscopy as an investigation method for such dye. SERS technique makes fluorescent quenching possible, and was applied for the vibrational study of ARS molecule.
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Fig. 2. UV–vis spectra of ARS aqueous solution (6.0 × 10−5 mol L−1 ) in the indicated pH values and after exposure to the SERS active silver electrode.
Fig. 1A shows the SERS spectra of ARS adsorbed on an electrochemically roughened silver electrode recorded with excitation at 632.8 nm. While the observed SERS signal is quite strong, the chemical interaction between the adsorbed molecule and the nanostructured SERS active surface causes significant changes in the observed SERS spectrum as a function of adsorption time. The observed SERS signal is due to a variety of chromophores generated by the surface-induced reaction at the silver surface. These changes are also reflected on the absorption spectra of dye solution which is in contact with the electrode shown in Fig. 1B. The maximum in the electronic spectra shifts from 422 to 517 nm. These changes in the absorption spectra are due to the equilibrium established between the adsorbed species on the electrode and in solution. To understand these results, it is important to consider the acid–base properties of the investigated molecule. Anthraquinonic species exists in three different forms, depending on the pH value: the neutral form, the monoanionic and the dianionic, for ARS the respective values are pK1 = 5.98 and pK2 = 9.88 [18]. The electronic spectra of each of these species present characteristic bands, as shown in Fig. 2. For pH = 5.6, two bands are observed, at 307 and 433 nm. At more acidic media (pH 1.6), no significant changes are
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observed, suggesting that the protonation occurs at a very low pH value. For pH values above 8.4, the band in 433 disappears, and a band at ca. 500 nm is observed. These changes are associated with the ionization of the phenolic group. ˜ In a work investigating the behavior of alizarin, Canamares et al. [19] report a similarity between the Raman spectra of the basic form of the dye (pH above 6) and the surface-adsorbed species, interacting through the ketonic and hydroxyl moieties with the surface. So, the interaction with the metallic surfaces induces the deprotonation of the OH group, and the formed species is similar to the dianionic form of the dye. Tentative assignment for alizarine red S, the most prominent Raman feature is proposed in Table 1, considering the comparison with the vibrational assignment proposed in the literature for the alizarine molecule in alkaline medium. The main changes observed in the SERS spectrum of adsorbed ARS according to adsorption time are related to the relative intensities of the bands in 1188, 1323 and 1245 cm−1 , associated with CC and COH stretching modes and to CO bending mode, respectively, and between the bands in 1424, 1446 and 1461 cm−1 . Also, after 3 days of adsorption time, the bands in 1627, 1188 and 566 cm−1 emerge and can be considered characteristic vibrational features of the adsorption product. In addition, the band at 630 cm−1 shifts to 648 cm−1 with the surface-induced reaction. Importantly, no significant modifications are observed in the vibration features associated with the sulfonate group. Thus, considering the decrease in the intensity of the vibrational mode assigned to CO stretching and bending modes, and the enhancement of the Raman bands associated with COH, CC, CCC and CH stretching modes caused by adsorption of the dye (Fig. 1A), it is reasonable to propose the interaction of the molecule and the metallic surface occurs through the ketonic and hydroxylic groups. So, the interaction between the dye and the metallic surface induces deprotonation of OH group, causing an electronic delocalization involving the aromatic region of the molecule (in a structure similar to the dianionic structure). The observed results in Fig. 1 thus suggest that the interaction between the dye and the enhancing metallic substrate causes the formation of an ARS ion stabilized in solution involving the adsorbed species and K+ ions adsorbed at the electrode surface during the oxidation–reduction cycles performed to activate the silver electrode. The surface-induced reaction is reflected in structural modifications of the dye which are observed both on the electrode surface (through changes in the vibrational Raman spectrum) and on the aqueous solution after contact with the electrode (through changes in its absorption spectrum).
Table 1 Tentative assignment for the observed SERS bands for alizarine red after 20 min and after 3 days of adsorption to the silver electrode. Wavenumber/cm−1
Tentative assignment
Alizarine [19]
Alizarine red [20]
ARS (20 min adsorption time)
ARS (3 days adsorption time)
– 1624 1600 – 1461 1458 1423 – 1320 1296 1272 – 1186 – –
1665 1634 1590 – 1459 1441 – 1350 1330 1289 1265 1235 1203 1160 1068
– – 1603 1480 1458 – 1423 1340 1323 1295 1245 – 1188 1162 1057
– 1627 1605 – 1461 1446 1424 – 1325 1297 1246 – 1188 1160 1058
: stretching; ı: in-plane bending; : out-of-plane bending.
(C O) (C O) (CC) (C O)/(CC)/ı(CH) (CC)/ı(COH)/ı(CH) – (CC)/ı(COH) ı(CO) (CC) (C O) (C O) SO3 H (CC)/ı(CH)/ı(CCC) (SO3 ) assimétrico (SO3 ) simétrico
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Fig. 3. SERS spectra of ARS adsorbed over silver colloidal particles at the indicated pH values, and SERS spectrum of ARS over silver electrode after 3 days of adsorption time (0 = 632.8 nm).
In the present study, it was observed that the ARS solution turns pink colored by contact with a metallic electrode (either gold or silver) activated through oxidation–reduction cycles. It is important to mention that no changes are induced in the bulk solution by the immersion of a polished metallic electrode, even after a period of 3 months. A reasonable explanation for such behavior is based on the presence of alkali metal ions adsorbed at the surface during the ORC which, upon interaction of the dye with the surface stabilizes the alizarine red S ion that diffuses to the solution bulk. In fact, it has been reported that one of the features of hidroxyanthraquinones compounds is the fact that this compounds are able to strongly interact with the surface with the formation of a chelate complex with metal [21] and quinone anions can be significantly stabilized for alkali ions [22]. To better understand the dependence of characteristic Raman features of ARS on its protonation state, SERS of ARS in aqueous solution as a function of pH was investigated (Fig. 3). Fig. 3 shows a similarity between the characteristic spectra of deprotonated ARS (observed in basic media) and the dye after 3 days of interaction with the metallic surface. This result corroborates the adsorption model proposed, where complexation of ARS is compared to its deprotonation reaction. It is thus important to mention that such results suggest that changes in the SERS spectra results from a surface-induced reaction, rather than from changes in the structure of the adsorbed molecule. This conclusion is supported by the change in the absorption spectra of the solution induced by the presence of the activated electrode, which is assigned to the formation of an anion stabilized due the presence of the anionic form of ARS and K+ ions from the substrate. The use an analyte-specific affinity coating to avoid such surface-reaction was investigated, as shown in the next section. 3.2. SERS on colloidal silver nanostructures supported on thin solid films Given that the interaction between ARS and electrochemically roughened silver electrode promoted structural changes on the investigated dye, SERS of ARS was also investigated employing a different substrate, constructed by the immobilization of colloidal silver particles by silanization of a glass slide. The use of these systems as SERS active substrates is of considerable interest. The transference of the colloids to solid supports stabilizes them, lead-
Fig. 4. SERS spectra of the CH3 (CH2 )2 SH coated silver substrate and of ARS (6.0 × 10−5 mol L−1 ) adsorbed over alkyl modified and unmodified silver substrate (0 = 632.8 nm).
ing to the formation of self-sustained films that can be considered portable SERS sensors for specific analytical applications [23]. To avoid direct interaction between the dye and the silver nanostructure, the SERS active substrate was coated with a self-assembled monolayer (SAM) of CH3 (CH2 )2 SH. In aqueous systems, these systems provide surfaces for the detection of hydrophobic analytes [24,25], and it has been found that coatings with thiol groups are strongly anchored to the noble metal substrate and do not desorb under acidic or basic conditions [25]. SERS spectra (excited by 632.8 nm laser line) of ARS adsorbed over coated and uncoated silver substrate are shown in Fig. 4. Substrates A and C shown in Fig. 4 consist of silver colloid immobilized on glass silanized coated with 1-propanethiol and ARS solution. The observed spectra of ARS over the coated substrates A and C show a series of surface-enhanced Raman lines assignable to the monolayers of CH3 (CH2 )2 SH superimposed on the intense surface-enhanced resonance Raman vibrational spectrum of ARS dye. The adsorption CH3 (CH2 )2 SH to the nanostructured surface allows the observation of SERS spectra of unperturbed ARS, without the changes induced by the complexation reaction between the dye and the metal. The Raman bands of the CH3 (CH2 )2 SH in the coated substrate are stronger in the region below 1025 cm−1 , thus interfering little with the observation of the more characteristic Raman features of ARS, which occur in higher wavenumber region. The coated film therefore is suitable for sensitive spectroscopic analysis of ARS dye, allowing for the detection of the dye at very low concentration range (6.0 × 10−5 mol L−1 ). Thus, in addition to preventing structural modifications of ARS, the coating also concentrates the hydrophobic dye close enough to the SERS active interface. So, in addition to strong intensification of the Raman signal, the coated substrate shows very high reproducibility and stability over time (up to 20 days), enabling the observation of excellent Raman spectra of ARS in aqueous environment at ppm levels. 3.3. Dependence on the wavelength excitation energy: resonance Raman and SERS It has been observed that the performance of the substrate towards the analysis of ARS is strongly dependent of the frequency of the exciting radiation. Fig. 5 shows the SERS spectra of ARS
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electrode surface (through changes in the vibrational Raman spectrum) and on the aqueous solution in contact with the electrode (through changes in its absorption spectrum). SERS of ARS was also investigated employing a substrate constructed by the immobilization of colloidal silver particles by silanization of a glass slide. It was found that the use of CH3 (CH2 )2 SH as an analyte-specific affinity coating avoids direct contact between the dye and the metallic particles thus allowing the observation of SERS spectra of unperturbed ARS. The coated film therefore is suitable for sensitive spectroscopic analysis of ARS dye, allowing for the detection of the dye at very low concentration range (6.0 × 10−5 mol L−1 ). Acknowledgements This work was supported by FAPESP, CNPq and CAPES. Fellowships from FAPESP and CNPq are gratefully acknowledged. References
Fig. 5. SERS spectra of ARS (6,0 × 10−5 mol L−1 ) over 1-propanethiol coated silver surface obtained at the indicated laser wavelengths.
adsorbed over the coated silver substrate obtained in excitation wavelength between 488 and 1064 nm. Considering the Raman signal of the CH3 (CH2 )2 SH as an internal intensity standard, Fig. 5 shows that higher energy excitation wavelengths enhance the dye signal. It is possible to observe a weak signal from ARS on spectra excited by 488 and 514 nm, the most intense substrate band in 1025 cm−1 is not present in these spectra. This behavior can be assigned to the contribution of resonance Raman effect to the observed signal. For lower energy exciting wavelengths, in the red and infrared regions, the obtained spectra shows principally the Raman features of the thiol SAM, indicating that the association of surface enhancement and resonance Raman effect is necessary for the observation of the ARS vibrational features (surface-enhanced resonance Raman effect, SERRS). 4. Conclusions SERRS of ARS was investigated over silver electrodes and over colloidal silver nanoparticles supported on thin solid films coated and uncoated with a self-assembled monolayer of 1-propanethiol. Direct chemical interaction between the dye and a metallic nanostructured surface causes significant changes in the observed SERS spectrum as a function of time. Results suggest that the interaction between the dye and the metallic surface induces deprotonation of OH group and the formation of an ion stabilized by the adsorbed species and K+ ions. The surface-induced reaction is reflected in structural modifications of the dye which are observed both on the
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