- Email: [email protected]
Photoinduced coupling and adsorption of caffeic acid on silver surface studied by surface-enhanced Raman spectroscopy
Spectrochimica Acta Part A 55 (1999) 2935 – 2941 www.elsevier.nl/locate/saa
Letter
Photoinduced coupling and adsorption of caffeic acid on silver surface studied by surface-enhanced Raman spectroscopy S. Sa´nchez-Corte´s *, J.V. Garcı´a-Ramos Instituto de Estructura de la Materia, CSIC, Serrano, 121, 28006 Madrid, Spain Received 17 June 1999; received in revised form 16 August 1999; accepted 20 August 1999
Abstract The effect of light on the caffeic acid (CA) oxidative coupling is studied in aqueous solution and on silver by surface-enhanced Raman spectroscopy (SERS). CA can polymerize in aqueous solution or on a metal surface through an oxidative mechanism involving the formation of the corresponding quinone giving rise to characteristic Raman features in each case. We show here that the effect of light in relation to this oxidative coupling is crucial taking place mainly in the solution bulk. The products derived from such polymerization can then adsorb on the silver surface employed for SERS measurements, thus allowing its detection by Raman spectroscopy. The influence of irradiation time and the wavelength of the light employed for the photoinduced coupling was investigated. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Caffeic acid; SERS; FT-Raman; Photopolymerization; Browning
1. Introduction Phenolic compounds largely contribute to browning in plant-derived food products [1]. Oxidation reactions can be catalyzed by polyphenol oxidases or peroxidases, or can occur in an spontaneous way by autoxidation, if the enzymes are inactivated or removed during the food process. The fundamental step in browning is the transfor* Corresponding author. Tel.: +34-91-561-6800; fax: +3491-564-5557. E-mail address: [email protected] (S. Sa´nchez-Corte´s)
mation of o-diphenols to the corresponding oquinones by reaction with the O2 existent in the medium [2]. In particular, caffeic acid (CA) (Fig. 1a) and their derivatives are o-diphenolic cinnamic acid compounds which are common metabolites in the plant kingdom and, thus, are present in most of the foods derived from plants. In spite of its considerable interest with regard to browning, few studies on polymerization reactions of CA and its related compounds are available. This is probably due to the low stability of the intermediate condensation products [3].
1386-1425/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 9 9 ) 0 0 1 9 5 - X
2936
S. Sa´nchez-Corte´s, J.V. Garcı´a-Ramos / Spectrochimica Acta Part A 55 (1999) 2935–2941
The chemical reactions involved in the production of dark color have already been studied specially in wine. It is well established that CA starts it oxidation with the formation of the corresponding o-quinone [4], a very reactive specie which may polymerize, oxidize other substances or be reduced to the initial o-diphenol [5]. The study of CA polymerization by Raman spectroscopy is limited due to the high fluorescence and the poor stability of the resulting products. Nevertheless, the surface-enhanced Raman spectroscopy (SERS) can be successfully applied to such study since this technique is able to characterize very low concentrated and fluorescent compounds in a mixture [6]. Briefly, this technique is based on the employ of metal surfaces to enhance the Raman signal of a scatter molecule. In some cases it has been demonstrated that the presence of such surface, which usually is made of silver, gold or copper, can catalyze chemical reactions of molecules adsorbed on it. The chemical modifications that can occur on metal surfaces employed in SERS spectroscopy may involve the oxidation [7,8] and the oxidative polymerization of the adsorbates linked to the surface [9,10]. Likewise, CA can undergo catalytic modifications on the silver colloid which stabilize the unstable products derived from the chemical change. As the SERS
technique is more sensitive to the species which are in the vicinity of the metal surface, it is a powerful method to analyze small amounts of the products formed on the surface. In a previous work we have found that CA tends to polymerize in aqueous solution or in the presence of silver surface [11]. This chemical reaction involves the formation of furanoid (through the coupling of two double bonds of the CA side chain) or benzoquinone moieties (through the coupling of the side chain double bond with the catechol moiety) occurring in the first stages of the oxidative process. If CA is dissolved in water, the coupling reactions of CA take place before its adsorption on the metal, while if CA is dissolved in ethanol the CA coupling mainly occurs on the metal surface, thus giving rise to very different Raman spectra. In this work we demonstrate that the light has an important role in the oxidative coupling of CA. The influence of light on processes involving photopolymerization has been investigated for several phenols and polyphenols [12,13]. Nevertheless, the influence of light in the reactions undergone by CA has not been investigated in detail so far, despite the great importance of this molecule under the agricultural and industrial point of views.
2. Experimental CA was purchased from Sigma and employed without further purification. All the reagents were of analytical degree. All solutions were prepared by using triply distilled water.
2.1. Preparation of the metal colloid and samples for SERS
Fig. 1. Caffeic acid (CA) (a), benzodioxane derivative produced by CA coupling (b) (different isomers are possible), isoferulic acid (c) and catechol (d).
The silver colloid was prepared by following the method described by Lee and Meisel [14]: 200 ml of a 10 − 3 M AgNO3 aqueous solution was heated to boiling, then 4 ml of a 1% trisodium citrate solution were added, keeping the mixture boiling for 1 h. Stock solutions of CA in ethanol were prepared at a final concentration of 5× 10 − 2 M. The addition of aliquots of the ethanol CA solution to the colloid up to the concentrations em-
S. Sa´nchez-Corte´s, J.V. Garcı´a-Ramos / Spectrochimica Acta Part A 55 (1999) 2935–2941
ployed here induces its aggregation and reduces its pH from 6.5 to 4.0. A volume of 10 ml of the last mixture were then placed in a 2 mm width capilar for SERS measurements. The samples employed for investigation of the light effect were prepared in the same way: addition of CA on the silver colloid and, then, irradiated with laser visible light at 514.5 nm before SERS measurements at 1064 nm. This irradiation was accomplished by placing 10 ml of the corresponding sample in a 2 mm capilar. The radiation power at the sample was about 40 mW. At these conditions the maximum light effect is attained, as a power increase did not alter the SERS intensity of the photoproducts. The influence of natural light on the CA aqueous solution employed for recording the SERS of CA without laser irradiation is negligible, as demonstrated by the UV-vis spectrum (result not shown) which does not change during the time comprised between the solution preparation and the spectrum acquisition. CA is poorly soluble in water, thus, the aqueous solution of CA was prepared at a 5×10 − 2 M final concentration by adding NaOH up to a pH of 7.0.
2.2. Instrumentation FT-Raman and FT-SERS spectra were obtained by using a RFS 100/S Brucker spectrophotometer. The 1064 nm line, provided by an Nd:Yag laser, was used as excitation line. The resolution was set to 4 cm − 1 and a 180° geometry was employed. The output laser power was 150 mW in the case of SERS measurements and 300 mW in the case of the solution. The variation of the power did not induce any significant change on the SERS spectral profiles, thus indicating that the irradiation with near infrared light does not influence significantly the chemical changes undergone by CA. For the liquid samples 5 mm optical path quartz cells were employed. We recorded ten 100 scans FT-SERS spectra during the first 20 min after the sample preparation to control possible variations occurring with the time in SERS measurements, while the FT-Raman of the solutions were the result of 1000 scans accumulation recorded during 20 min.
2937
Raman spectra with excitation in the visible were recorded in a U-1000 Jobin-Yvon Spectrophotometer by using the 457.9 and 514.5 nm radiation lines of a Spectra Physics Model 165 argon ion laser and the 647.1 nm radiation line of a Spectra Physics Model 165 krypton ion laser. Resolution was set at 4 cm − 1 and a 90° geometry was used to record the data. The laser power at the sample was 20 mW in case of using the 457.9 nm line and 40 mW for the 514.5 nm lines. All the spectra were recorded at 1 cm − 1 step intervals with an integration time of 1 s.
3. Results and discussion The intensity of the CA SERS spectrum grows slowly with the time when the adsorbate is at a relatively low concentration (10 − 3 M), thus indicating that the diffusion of the molecule to the metal surface takes a long period of time and controls the appearance of SERS signal just after the addition of CA to the colloid. This effect will be of a great importance in relation to the lightdependent CA condensation. At this concentration we did not observe any SERS signal at the beginning in a fresh prepared SERS sample (Fig. 2a). However, if this sample is previously irradiated with light at 514.5 nm for 30 min we obtain the spectrum of Fig. 2b, which shows many intense features appearing at 436, 492, 1108, 1151 and a broad feature centered at 1421 cm − 1. Other weaker bands are observed at 1212 and 1339 cm − 1. This spectrum differs completely from the CA FT-Raman spectrum obtained in water (Fig. 2c). We have attributed the new bands appearing in the SERS to the CA condensation products induced by the light. In fact, they can be assigned to benzodioxane moiety [15,16] created during the coupling process as it has been proposed by several authors studying the autoxidation of CA in solution [17,18]. The formation of such product involves both the side chain and the o-diphenol moieties of two CA molecules with the formation of a dioxane cycle (Fig. 1b). In accordance to this assignment, we have seen a notable intensity decrease of the side chain n(CC) vibration appearing at 1638 cm − 1 and a simultaneous decrease
2938
S. Sa´nchez-Corte´s, J.V. Garcı´a-Ramos / Spectrochimica Acta Part A 55 (1999) 2935–2941
ring avoids the formation of a dioxane cycle (results not shown). The comparison with these parent molecules suggests that the photoinduced CA coupling needs both the oxidation of the o-diphenol to the corresponding quinone and the photoactivation of the side chain double bond of CA. After 30 min, the aged non-irradiated CA 10 − 3 M sample begun to show a SERS spectrum (Fig. 3a). This spectrum shows significant changes in relation to the SERS of the irradiated sample (Fig. 2b) and the FT-Raman of CA in solution (Fig. 2c), observing shifts in almost all the bands and appearance of new bands in the 1300–1550 cm − 1 region. The differences observed between the different polymers that are produced both in
Fig. 2. FT-SERS of CA (10 − 3 M) exciting at 1064 nm before (a) and after irradiating the sample for 30 min with the 514.5 nm line (b). (c) FT-Raman spectrum of the CA aqueous solution (0.05 M).
and down shift of the benzene nb mode from 1606 to 1593 cm − 1. On the other hand, the very strong bands appearing at 1151 and 1108 cm − 1 can be attributed to n(C – O), d(C – H) and n(C – C) vibrations of the dioxane moiety formed during the photoinduced condensation of CA. The formation of the benzodioxane moiety from CA increases on rising the pH, as indicated by the intensity increase of the above features [11]. However, the light irradiation seems to induce the formation of such groups even at the low pH of the samples prepared here. Furthermore, no light effect was found neither for catechol (Fig. 1c), where no side chain exists in the molecule, nor for isoferulic acid (Fig. 1d), where the methoxi group existent in the benzene
Fig. 3. FT-SERS of CA (10 − 3 M) of the same sample than Fig. 2a aged for 30 min (exciting with 1064 nm) (a) and after irradiating the sample for (b) 5 and (c) 30 min with the 514.5 nm line.
S. Sa´nchez-Corte´s, J.V. Garcı´a-Ramos / Spectrochimica Acta Part A 55 (1999) 2935–2941
Fig. 4. FT-SERS of CA (2× 10 − 3 M) before (a) and after (b) irradiating the sample for 30 min with the 514.5 nm line.
presence or absence of light are not drastic since they are polymers originated from the same block. However, those differences are important enough as to indicate that CA undergoes a coupling through a different mechanism in the dark and, most probably, on the metal [11]. Nevertheless, when this non-irradiated sample is illuminated for increasing periods of time (Fig. 3b and c) we observed a progressive enhancement of those bands seen in Fig. 2b, pointing out again that they can be attributed to the photoinduced CA adducts. In addition, we observed a general intensity enhancement of the spectrum after the irradiation. All this facts suggest that the light not only induces the CA coupling in the bulk, but also the adsorption of the resulting products on the silver surface, which display a higher affinity to be adsorbed on the metal.
2939
If the same experiment is carried out by using a higher CA concentration (2×10 − 3 M) we observe from the very beginning, and without light irradiation, an intense SERS spectrum (Fig. 4a) similar to that of the non-irradiated sample of Fig. 3a. At these conditions, the light irradiation has a negligible influence as demonstrated by the SERS spectrum of Fig. 4b. This demonstrated once again that the effect of light is manifested mainly before the adsorption of CA on the metal surface. The poor effect of light at this concentration is then attributed to the much higher diffusion rate of CA from the bulk to the surface. Moreover, the SERS obtained at this higher concentration corresponds to the adsorbed CA, which also undergoes oxidative coupling leading to a mixture of products comprising furanoid, dioxane and ring-coupled derivatives [11,18,19]. The spectrum of Fig. 4a cannot be attributed to polymers induced by natural light irradiation existing as impurities in the solid, since the absorbance above 400 nm of a fresh solution is very low, and the SERS of Fig. 4a undergoes an evolution in time [11] towards a different spectral profile in comparison to the light induced SERS spectrum, thus indicating again that the polymerization undergone by CA is different in absence or presence of laser irradiation. CA shows a very different SERS spectrum depending on the excitation line (Fig. 5). This effect is typical for photoactive molecules susceptible of giving a resonance Raman effect in the visible region, due to the resonance selectivity of the cromophoric groups. The selection of vibrational modes depends on the absorption properties of the molecule at different excitation wavelengths. The changes observed for CA when decreasing the excitation wavelength can be summarize as follows: (a) increase of the bands appearing at 1564, 1606, 1384 and 1208 cm − 1; (b) decrease of the bands at 1504, 1468, 1231, 1161 and 1118 cm − 1, and (c) disappearance of the bands at low wavenumber observed at 508 and 431 cm − 1 when exciting at higher excitation wavelengths. Nevertheless, in the case of CA, the changes observed at varying the excitation line can be ascribed to both the photochemical modification of CA upon irradiation with visible light and a
2940
S. Sa´nchez-Corte´s, J.V. Garcı´a-Ramos / Spectrochimica Acta Part A 55 (1999) 2935–2941
resonant effect of the coupling products, which show an increasing adsorbance towards the visible region [18,19]. Thus, the spectra obtained at 457.9 and 514.5 nm may correspond to the surface-enhanced resonance Raman spectra (SERRS) of the very fluorescent CA photoproducts originated from the light irradiation. These products bring about the FT-SERS spectrum of Fig. 2a, which is the SERS spectrum obtained after recording those of Fig. 5c and d. The medium intense band observed at 1631 cm − 1, attributed to the side chain n(CC) vibration, reveals the still presence of side chains in the CA products, as it would occur supposing the formation of a benzodioxane moiety. Furthermore, the intense bands observed at 1564, 1384 and 1208 cm − 1 may indicate the formation of phenolic polycondensated dyes, whose Raman spectrum is resonantly enhanced when
decreasing the excitation wavelength, since similar bands have been observed also in the SERS of structurally related molecules [20]. The similarity found between the SERS spectrum obtained at 647.1 nm (Fig. 5b) and that obtained at 1064 nm (Fig. 5a) suggests that the red light has a low influence on the photoinduced coupling of CA The above results points out that the effect of light on CA takes place mainly in the solution, inducing a subsequent adsorption of the coupled CA products on the metal surface one formed in the bulk solution. This effect has a significant importance in relation to the extraction, storage, handle and consume of food products containing CA, and can have a notable relevance also in processes involving the formation of photoinduced self-assembled monolayers from photoactived natural products.
Acknowledgements This work has been supported by the Spanish Ministerio de Educacio´n y Cultura (Direccio´n General de Ensen˜anza Superior e Investigacio´n Cientı´fica) project number PB97-1221. We also acknowledge the Consejo Superior de Investigaciones Cientı´ficas for a contract to S. S.-C.
References
Fig. 5. SERS of CA exciting at (a) 1064 nm; (b) 647.1 nm; (c) 514.5 nm, and (e) 457.9 nm.
[1] V.L. Singleton, Adv. Food Res. Suppl. 3 (1972) 143. [2] A.G. Mathew, H.A.B. Parpia, Adv. Food Res. 19 (1971) 137. [3] V.L. Singleton, Am. J. Enol. Vitic. 38 (1987) 69. [4] P. Hapiot, A. Neudeck, J. Pinson, H. Fulcrand, P. Neta, C. Rolando, J. Electroanal. Chem. 405 (1997) 169. [5] W.S. Pierpoint, Biochem. J. 98 (1966) 567. [6] T.M. Cotton. The application of surface-enhanced Raman scattering to biochemical systems, in: R.J.H. Clark, R.E. Hester (Eds.), Spectroscopy of Surfaces, John Wiley and Sons, Chichester, 1988 (Chapter 3). [7] M.L. Patterson, M.J. Weaver, J. Phys. Chem. 89 (1985) 1331. [8] X.J. Gu, K.L. Akers, M. Moskovits, J. Phys. Chem. 95 (1991) 3696. [9] J.S. Suh, K.H. Michaelian, J. Raman Spectrosc. 18 (1987) 409. [10] R.S. Venkatachalam, F.J. Boerio, P.G. Roh, J. Raman Spectrosc. 19 (1988) 281.
S. Sa´nchez-Corte´s, J.V. Garcı´a-Ramos / Spectrochimica Acta Part A 55 (1999) 2935–2941 [11] S. Sa´nchez-Corte´s, J.V. Garcı´a-Ramos, Appl. Spectrosc. (1999) in press. [12] K. Ichimura, Y. Akita, H. Akiyama, K. Kudo, Y. Hayashi, Macromolecules 30 (1997) 903. [13] J. Zhao, K. Abe, H. Akiyama, Z. Liu, F. Nakanishi, Langmuir 15 (1999) 2543. [14] P.C. Lee, D. Meisel, J. Phys. Chem. 84 (1982) 339. [15] Bruker Analytische Messtechnik GMBH Library, 1995. [16] C.J. Pouchert, The Aldrich Library of Infrared Spectra,
.
2941
second ed., Aldrich Chemical Company, 1975. [17] K. Matsumoto, H. Takahashi, Y. Miyake, Y. Fukuyama, Tetrahedron Lett. 40 (1999) 3185. [18] J.L. Cilliers, V.L. Singleton, J. Agric. Food Chem. 39 (1991) 1298. [19] V. Cheynier, M. Moutounet, J. Agric. Food Chem. 40 (1992) 2038. [20] S. Schneider, H. Grau, P. Halbig, P. Freunscht, U. Nickel, J. Raman Spectrosc. 27 (1996) 57.
Letter
Photoinduced coupling and adsorption of caffeic acid on silver surface studied by surface-enhanced Raman spectroscopy S. Sa´nchez-Corte´s *, J.V. Garcı´a-Ramos Instituto de Estructura de la Materia, CSIC, Serrano, 121, 28006 Madrid, Spain Received 17 June 1999; received in revised form 16 August 1999; accepted 20 August 1999
Abstract The effect of light on the caffeic acid (CA) oxidative coupling is studied in aqueous solution and on silver by surface-enhanced Raman spectroscopy (SERS). CA can polymerize in aqueous solution or on a metal surface through an oxidative mechanism involving the formation of the corresponding quinone giving rise to characteristic Raman features in each case. We show here that the effect of light in relation to this oxidative coupling is crucial taking place mainly in the solution bulk. The products derived from such polymerization can then adsorb on the silver surface employed for SERS measurements, thus allowing its detection by Raman spectroscopy. The influence of irradiation time and the wavelength of the light employed for the photoinduced coupling was investigated. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Caffeic acid; SERS; FT-Raman; Photopolymerization; Browning
1. Introduction Phenolic compounds largely contribute to browning in plant-derived food products [1]. Oxidation reactions can be catalyzed by polyphenol oxidases or peroxidases, or can occur in an spontaneous way by autoxidation, if the enzymes are inactivated or removed during the food process. The fundamental step in browning is the transfor* Corresponding author. Tel.: +34-91-561-6800; fax: +3491-564-5557. E-mail address: [email protected] (S. Sa´nchez-Corte´s)
mation of o-diphenols to the corresponding oquinones by reaction with the O2 existent in the medium [2]. In particular, caffeic acid (CA) (Fig. 1a) and their derivatives are o-diphenolic cinnamic acid compounds which are common metabolites in the plant kingdom and, thus, are present in most of the foods derived from plants. In spite of its considerable interest with regard to browning, few studies on polymerization reactions of CA and its related compounds are available. This is probably due to the low stability of the intermediate condensation products [3].
1386-1425/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 9 9 ) 0 0 1 9 5 - X
2936
S. Sa´nchez-Corte´s, J.V. Garcı´a-Ramos / Spectrochimica Acta Part A 55 (1999) 2935–2941
The chemical reactions involved in the production of dark color have already been studied specially in wine. It is well established that CA starts it oxidation with the formation of the corresponding o-quinone [4], a very reactive specie which may polymerize, oxidize other substances or be reduced to the initial o-diphenol [5]. The study of CA polymerization by Raman spectroscopy is limited due to the high fluorescence and the poor stability of the resulting products. Nevertheless, the surface-enhanced Raman spectroscopy (SERS) can be successfully applied to such study since this technique is able to characterize very low concentrated and fluorescent compounds in a mixture [6]. Briefly, this technique is based on the employ of metal surfaces to enhance the Raman signal of a scatter molecule. In some cases it has been demonstrated that the presence of such surface, which usually is made of silver, gold or copper, can catalyze chemical reactions of molecules adsorbed on it. The chemical modifications that can occur on metal surfaces employed in SERS spectroscopy may involve the oxidation [7,8] and the oxidative polymerization of the adsorbates linked to the surface [9,10]. Likewise, CA can undergo catalytic modifications on the silver colloid which stabilize the unstable products derived from the chemical change. As the SERS
technique is more sensitive to the species which are in the vicinity of the metal surface, it is a powerful method to analyze small amounts of the products formed on the surface. In a previous work we have found that CA tends to polymerize in aqueous solution or in the presence of silver surface [11]. This chemical reaction involves the formation of furanoid (through the coupling of two double bonds of the CA side chain) or benzoquinone moieties (through the coupling of the side chain double bond with the catechol moiety) occurring in the first stages of the oxidative process. If CA is dissolved in water, the coupling reactions of CA take place before its adsorption on the metal, while if CA is dissolved in ethanol the CA coupling mainly occurs on the metal surface, thus giving rise to very different Raman spectra. In this work we demonstrate that the light has an important role in the oxidative coupling of CA. The influence of light on processes involving photopolymerization has been investigated for several phenols and polyphenols [12,13]. Nevertheless, the influence of light in the reactions undergone by CA has not been investigated in detail so far, despite the great importance of this molecule under the agricultural and industrial point of views.
2. Experimental CA was purchased from Sigma and employed without further purification. All the reagents were of analytical degree. All solutions were prepared by using triply distilled water.
2.1. Preparation of the metal colloid and samples for SERS
Fig. 1. Caffeic acid (CA) (a), benzodioxane derivative produced by CA coupling (b) (different isomers are possible), isoferulic acid (c) and catechol (d).
The silver colloid was prepared by following the method described by Lee and Meisel [14]: 200 ml of a 10 − 3 M AgNO3 aqueous solution was heated to boiling, then 4 ml of a 1% trisodium citrate solution were added, keeping the mixture boiling for 1 h. Stock solutions of CA in ethanol were prepared at a final concentration of 5× 10 − 2 M. The addition of aliquots of the ethanol CA solution to the colloid up to the concentrations em-
S. Sa´nchez-Corte´s, J.V. Garcı´a-Ramos / Spectrochimica Acta Part A 55 (1999) 2935–2941
ployed here induces its aggregation and reduces its pH from 6.5 to 4.0. A volume of 10 ml of the last mixture were then placed in a 2 mm width capilar for SERS measurements. The samples employed for investigation of the light effect were prepared in the same way: addition of CA on the silver colloid and, then, irradiated with laser visible light at 514.5 nm before SERS measurements at 1064 nm. This irradiation was accomplished by placing 10 ml of the corresponding sample in a 2 mm capilar. The radiation power at the sample was about 40 mW. At these conditions the maximum light effect is attained, as a power increase did not alter the SERS intensity of the photoproducts. The influence of natural light on the CA aqueous solution employed for recording the SERS of CA without laser irradiation is negligible, as demonstrated by the UV-vis spectrum (result not shown) which does not change during the time comprised between the solution preparation and the spectrum acquisition. CA is poorly soluble in water, thus, the aqueous solution of CA was prepared at a 5×10 − 2 M final concentration by adding NaOH up to a pH of 7.0.
2.2. Instrumentation FT-Raman and FT-SERS spectra were obtained by using a RFS 100/S Brucker spectrophotometer. The 1064 nm line, provided by an Nd:Yag laser, was used as excitation line. The resolution was set to 4 cm − 1 and a 180° geometry was employed. The output laser power was 150 mW in the case of SERS measurements and 300 mW in the case of the solution. The variation of the power did not induce any significant change on the SERS spectral profiles, thus indicating that the irradiation with near infrared light does not influence significantly the chemical changes undergone by CA. For the liquid samples 5 mm optical path quartz cells were employed. We recorded ten 100 scans FT-SERS spectra during the first 20 min after the sample preparation to control possible variations occurring with the time in SERS measurements, while the FT-Raman of the solutions were the result of 1000 scans accumulation recorded during 20 min.
2937
Raman spectra with excitation in the visible were recorded in a U-1000 Jobin-Yvon Spectrophotometer by using the 457.9 and 514.5 nm radiation lines of a Spectra Physics Model 165 argon ion laser and the 647.1 nm radiation line of a Spectra Physics Model 165 krypton ion laser. Resolution was set at 4 cm − 1 and a 90° geometry was used to record the data. The laser power at the sample was 20 mW in case of using the 457.9 nm line and 40 mW for the 514.5 nm lines. All the spectra were recorded at 1 cm − 1 step intervals with an integration time of 1 s.
3. Results and discussion The intensity of the CA SERS spectrum grows slowly with the time when the adsorbate is at a relatively low concentration (10 − 3 M), thus indicating that the diffusion of the molecule to the metal surface takes a long period of time and controls the appearance of SERS signal just after the addition of CA to the colloid. This effect will be of a great importance in relation to the lightdependent CA condensation. At this concentration we did not observe any SERS signal at the beginning in a fresh prepared SERS sample (Fig. 2a). However, if this sample is previously irradiated with light at 514.5 nm for 30 min we obtain the spectrum of Fig. 2b, which shows many intense features appearing at 436, 492, 1108, 1151 and a broad feature centered at 1421 cm − 1. Other weaker bands are observed at 1212 and 1339 cm − 1. This spectrum differs completely from the CA FT-Raman spectrum obtained in water (Fig. 2c). We have attributed the new bands appearing in the SERS to the CA condensation products induced by the light. In fact, they can be assigned to benzodioxane moiety [15,16] created during the coupling process as it has been proposed by several authors studying the autoxidation of CA in solution [17,18]. The formation of such product involves both the side chain and the o-diphenol moieties of two CA molecules with the formation of a dioxane cycle (Fig. 1b). In accordance to this assignment, we have seen a notable intensity decrease of the side chain n(CC) vibration appearing at 1638 cm − 1 and a simultaneous decrease
2938
S. Sa´nchez-Corte´s, J.V. Garcı´a-Ramos / Spectrochimica Acta Part A 55 (1999) 2935–2941
ring avoids the formation of a dioxane cycle (results not shown). The comparison with these parent molecules suggests that the photoinduced CA coupling needs both the oxidation of the o-diphenol to the corresponding quinone and the photoactivation of the side chain double bond of CA. After 30 min, the aged non-irradiated CA 10 − 3 M sample begun to show a SERS spectrum (Fig. 3a). This spectrum shows significant changes in relation to the SERS of the irradiated sample (Fig. 2b) and the FT-Raman of CA in solution (Fig. 2c), observing shifts in almost all the bands and appearance of new bands in the 1300–1550 cm − 1 region. The differences observed between the different polymers that are produced both in
Fig. 2. FT-SERS of CA (10 − 3 M) exciting at 1064 nm before (a) and after irradiating the sample for 30 min with the 514.5 nm line (b). (c) FT-Raman spectrum of the CA aqueous solution (0.05 M).
and down shift of the benzene nb mode from 1606 to 1593 cm − 1. On the other hand, the very strong bands appearing at 1151 and 1108 cm − 1 can be attributed to n(C – O), d(C – H) and n(C – C) vibrations of the dioxane moiety formed during the photoinduced condensation of CA. The formation of the benzodioxane moiety from CA increases on rising the pH, as indicated by the intensity increase of the above features [11]. However, the light irradiation seems to induce the formation of such groups even at the low pH of the samples prepared here. Furthermore, no light effect was found neither for catechol (Fig. 1c), where no side chain exists in the molecule, nor for isoferulic acid (Fig. 1d), where the methoxi group existent in the benzene
Fig. 3. FT-SERS of CA (10 − 3 M) of the same sample than Fig. 2a aged for 30 min (exciting with 1064 nm) (a) and after irradiating the sample for (b) 5 and (c) 30 min with the 514.5 nm line.
S. Sa´nchez-Corte´s, J.V. Garcı´a-Ramos / Spectrochimica Acta Part A 55 (1999) 2935–2941
Fig. 4. FT-SERS of CA (2× 10 − 3 M) before (a) and after (b) irradiating the sample for 30 min with the 514.5 nm line.
presence or absence of light are not drastic since they are polymers originated from the same block. However, those differences are important enough as to indicate that CA undergoes a coupling through a different mechanism in the dark and, most probably, on the metal [11]. Nevertheless, when this non-irradiated sample is illuminated for increasing periods of time (Fig. 3b and c) we observed a progressive enhancement of those bands seen in Fig. 2b, pointing out again that they can be attributed to the photoinduced CA adducts. In addition, we observed a general intensity enhancement of the spectrum after the irradiation. All this facts suggest that the light not only induces the CA coupling in the bulk, but also the adsorption of the resulting products on the silver surface, which display a higher affinity to be adsorbed on the metal.
2939
If the same experiment is carried out by using a higher CA concentration (2×10 − 3 M) we observe from the very beginning, and without light irradiation, an intense SERS spectrum (Fig. 4a) similar to that of the non-irradiated sample of Fig. 3a. At these conditions, the light irradiation has a negligible influence as demonstrated by the SERS spectrum of Fig. 4b. This demonstrated once again that the effect of light is manifested mainly before the adsorption of CA on the metal surface. The poor effect of light at this concentration is then attributed to the much higher diffusion rate of CA from the bulk to the surface. Moreover, the SERS obtained at this higher concentration corresponds to the adsorbed CA, which also undergoes oxidative coupling leading to a mixture of products comprising furanoid, dioxane and ring-coupled derivatives [11,18,19]. The spectrum of Fig. 4a cannot be attributed to polymers induced by natural light irradiation existing as impurities in the solid, since the absorbance above 400 nm of a fresh solution is very low, and the SERS of Fig. 4a undergoes an evolution in time [11] towards a different spectral profile in comparison to the light induced SERS spectrum, thus indicating again that the polymerization undergone by CA is different in absence or presence of laser irradiation. CA shows a very different SERS spectrum depending on the excitation line (Fig. 5). This effect is typical for photoactive molecules susceptible of giving a resonance Raman effect in the visible region, due to the resonance selectivity of the cromophoric groups. The selection of vibrational modes depends on the absorption properties of the molecule at different excitation wavelengths. The changes observed for CA when decreasing the excitation wavelength can be summarize as follows: (a) increase of the bands appearing at 1564, 1606, 1384 and 1208 cm − 1; (b) decrease of the bands at 1504, 1468, 1231, 1161 and 1118 cm − 1, and (c) disappearance of the bands at low wavenumber observed at 508 and 431 cm − 1 when exciting at higher excitation wavelengths. Nevertheless, in the case of CA, the changes observed at varying the excitation line can be ascribed to both the photochemical modification of CA upon irradiation with visible light and a
2940
S. Sa´nchez-Corte´s, J.V. Garcı´a-Ramos / Spectrochimica Acta Part A 55 (1999) 2935–2941
resonant effect of the coupling products, which show an increasing adsorbance towards the visible region [18,19]. Thus, the spectra obtained at 457.9 and 514.5 nm may correspond to the surface-enhanced resonance Raman spectra (SERRS) of the very fluorescent CA photoproducts originated from the light irradiation. These products bring about the FT-SERS spectrum of Fig. 2a, which is the SERS spectrum obtained after recording those of Fig. 5c and d. The medium intense band observed at 1631 cm − 1, attributed to the side chain n(CC) vibration, reveals the still presence of side chains in the CA products, as it would occur supposing the formation of a benzodioxane moiety. Furthermore, the intense bands observed at 1564, 1384 and 1208 cm − 1 may indicate the formation of phenolic polycondensated dyes, whose Raman spectrum is resonantly enhanced when
decreasing the excitation wavelength, since similar bands have been observed also in the SERS of structurally related molecules [20]. The similarity found between the SERS spectrum obtained at 647.1 nm (Fig. 5b) and that obtained at 1064 nm (Fig. 5a) suggests that the red light has a low influence on the photoinduced coupling of CA The above results points out that the effect of light on CA takes place mainly in the solution, inducing a subsequent adsorption of the coupled CA products on the metal surface one formed in the bulk solution. This effect has a significant importance in relation to the extraction, storage, handle and consume of food products containing CA, and can have a notable relevance also in processes involving the formation of photoinduced self-assembled monolayers from photoactived natural products.
Acknowledgements This work has been supported by the Spanish Ministerio de Educacio´n y Cultura (Direccio´n General de Ensen˜anza Superior e Investigacio´n Cientı´fica) project number PB97-1221. We also acknowledge the Consejo Superior de Investigaciones Cientı´ficas for a contract to S. S.-C.
References
Fig. 5. SERS of CA exciting at (a) 1064 nm; (b) 647.1 nm; (c) 514.5 nm, and (e) 457.9 nm.
[1] V.L. Singleton, Adv. Food Res. Suppl. 3 (1972) 143. [2] A.G. Mathew, H.A.B. Parpia, Adv. Food Res. 19 (1971) 137. [3] V.L. Singleton, Am. J. Enol. Vitic. 38 (1987) 69. [4] P. Hapiot, A. Neudeck, J. Pinson, H. Fulcrand, P. Neta, C. Rolando, J. Electroanal. Chem. 405 (1997) 169. [5] W.S. Pierpoint, Biochem. J. 98 (1966) 567. [6] T.M. Cotton. The application of surface-enhanced Raman scattering to biochemical systems, in: R.J.H. Clark, R.E. Hester (Eds.), Spectroscopy of Surfaces, John Wiley and Sons, Chichester, 1988 (Chapter 3). [7] M.L. Patterson, M.J. Weaver, J. Phys. Chem. 89 (1985) 1331. [8] X.J. Gu, K.L. Akers, M. Moskovits, J. Phys. Chem. 95 (1991) 3696. [9] J.S. Suh, K.H. Michaelian, J. Raman Spectrosc. 18 (1987) 409. [10] R.S. Venkatachalam, F.J. Boerio, P.G. Roh, J. Raman Spectrosc. 19 (1988) 281.
S. Sa´nchez-Corte´s, J.V. Garcı´a-Ramos / Spectrochimica Acta Part A 55 (1999) 2935–2941 [11] S. Sa´nchez-Corte´s, J.V. Garcı´a-Ramos, Appl. Spectrosc. (1999) in press. [12] K. Ichimura, Y. Akita, H. Akiyama, K. Kudo, Y. Hayashi, Macromolecules 30 (1997) 903. [13] J. Zhao, K. Abe, H. Akiyama, Z. Liu, F. Nakanishi, Langmuir 15 (1999) 2543. [14] P.C. Lee, D. Meisel, J. Phys. Chem. 84 (1982) 339. [15] Bruker Analytische Messtechnik GMBH Library, 1995. [16] C.J. Pouchert, The Aldrich Library of Infrared Spectra,
.
2941
second ed., Aldrich Chemical Company, 1975. [17] K. Matsumoto, H. Takahashi, Y. Miyake, Y. Fukuyama, Tetrahedron Lett. 40 (1999) 3185. [18] J.L. Cilliers, V.L. Singleton, J. Agric. Food Chem. 39 (1991) 1298. [19] V. Cheynier, M. Moutounet, J. Agric. Food Chem. 40 (1992) 2038. [20] S. Schneider, H. Grau, P. Halbig, P. Freunscht, U. Nickel, J. Raman Spectrosc. 27 (1996) 57.