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Surface-Enhanced Vibrational Study (SEIR and SERS) of Dithiocarbamate Pesticides on Gold Films S. Sa´nchez-Corte´s,*,† C. Domingo,† J. V. Garcı´a-Ramos,† and J. A. Azna´rez‡ Instituto de Estructura de la Materia, CSIC, Serrano 121, 28006-Madrid, Spain, and Instituto de Fı´sica Aplicada, CSIC, Serrano 144, 28006-Madrid, Spain Received September 5, 2000. In Final Form: November 8, 2000 Surface-enhanced infrared (SEIR) and surface-enhanced Raman scattering (SERS) spectroscopies are applied to the study of the adsorption on Au films of the dimethyldithiocarbamate derivative fungicides thiram and ziram. The specificity and sensitivity of both techniques in relation to the detection and surface stability of the above compounds is analyzed comparatively. We have found that both fungicides undergo a breakdown when adsorbed on Au films, although this breakdown takes place to a different extent; thiram seems to be less stable than ziram. The sensitivity of infrared techniques is higher, although a lower enhancement in relation to SERS is observed. Whereas the SEIR technique allows the detection of all the adsorbed fungicide forms, SERS displays a high sensitivity toward only certain adsorbed molecules, those that undergo a strong adsorption induced by the fungicide breakdown.
Introduction Alkyl dithiocarbamate compounds have been widely used as protective fungicides on field crops, vegetables, and fruits. The biological activity of these compounds is based on the chemical properties of the dithiocarbamate (DTC) group, which can react with HS-containing enzymes and coenzymes of fungal cells, thus blocking their catalytic activity. Enzyme inhibition may also occur by complex formation of the active DTC group with metal atoms of metal-containing enzymes.1 Once the pesticides are introduced into the environment, physical, chemical, and biological processes cause them to be transported or transformed to reaction products.2 The poor solubility of DTC fungicides in water makes their elimination from the natural environment difficult. There is much interest in the potential environmental impact of alkyl DTC usage in agriculture and its effect on human health. Although acute toxicity of these compounds is low, they are irritants to skin and mucous membranes3 and upon chronic exposure they are suspected carcinogens and teratogens.4,5 It should be of great interest to find a sensitive method to analyze and detect small concentrations of these fungicides in soils, water, and foods as well as the chemical state at which they are retained in these systems. So far, these compounds have been analyzed by methods involving degradation of the active ingredient to CS2, which is detected by spectrophotometric and gas chromatography.6 Surface-enhanced vibrational spectroscopies (SEVS) are nowadays an active area with a broad range of analytical * Corresponding author. Fax: +34 91 5 64 55 57. Phone: +34 91 5 61 68 00. E-mail:
[email protected]. † Instituto de Estructura de la Materia, CSIC. ‡ Instituto de Fı´sica Aplicada, CSIC. (1) Matolcsy, G.; Ndasy, M.; Andriska, V. Pesticide Chemistry; Elsevier: Amsterdam, 1988. (2) SSSA Special Publication Number 22; Sawhney, B. L., Brown, K., Eds.; Soil Science Society of America: Madison, WI, 1989; p 305. (3) The Pesticide Manual; Worthing, C. R., Ed.; British Crop Protection Council: 1991. (4) Agraval, R. C.; Shukla, Y.; Mehotra, N. K. Food Chem. Toxicol. 1997, 35, 523. (5) Hemavathi, E.; Rahiman, M.; Abdul, M. J. Environ. Biol. 1996, 17, 171. (6) Woodrow, J. E.; Seiber, J. N. J. Agric. Food Chem. 1995, 43, 1524.
applications. In particular, SEIR (surface-enhanced infrared) and SERS (surface-enhanced Raman scattering) spectroscopies have attracted much attention from the first observation of both phenomena.7,8 Although these techniques are based on the enormous enhancement of the electromagnetic field occurring in the vicinity of metallic nanoparticles, mainly of Ag and Au,7,9 it has also been suggested that chemical contributions to the total enhancement affect both SEIR and SERS.10,11 The short-range enhancement occurring in SEIR and SERS as well as the surface selection rules can be used to investigate the electrodynamical behavior of organic molecules adsorbed on charged metal surfaces, thus providing very valuable information regarding the structure of the adsorbed molecules.12 Pesticides in general are susceptible to undergoing chemical changes which may modify their stability and biological activity in the environment.13,14 Thus, SEIR and SERS spectroscopies may be applied to investigate the chemical processes undergone by these substances in the environment. The aim of this work is a double comparative study: (i) the different stability of two related DTC fungicides, thiram and ziram (Figure 1), when adsorbed on a Au film and (ii) a comparison between two related interfacial surface techniques (SEIR and SERS) in the analytical and structural study of these important molecules. Experimental Section Film Preparation. Ten nanometers (mass thickness) of gold was evaporated onto polished CaF2 windows employed as substrates. The evaporation was made in a vacuum chamber held at a pressure of 10-6 Torr. The film thickness was monitored (7) Harstein, A.; Kirtley, J. R.; Tsang, J. C. Phys. Rev. Lett. 1980, 45, 201. (8) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Chem. Phys. Lett. 1974, 26, 163. (9) Jeanmaire, D. L.; Van Duyne, R. J. Electroanal. Chem. 1977, 84, 1. (10) Osawa, M.; Ikeda, M. J. Phys. Chem. 1991, 95, 9914. (11) Albrecht, M. G.; Creighton, J. A. J. Am. Chem. Soc. 1977, 99, 5215. (12) Furtak, T. E. J. Electroanal. Chem. 1983, 150, 375. (13) Pusino, A.; Micera, G.; Gessa, C.; Petretto, S. Clays Clay Miner. 1989, 37, 558. (14) Oyamada, M.; Kuwatsuka, S. J. Pestic. Sci. 1989, 14, 321.
10.1021/la001269z CCC: $20.00 © 2001 American Chemical Society Published on Web 01/25/2001
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Figure 1. Structure of (a) thiram and (b) ziram.
Figure 2. UV-visible absorption spectrum of Au film deposited on CaF2 (a) compared to that of CaF2 without metal (b). using a quartz crystal oscillator. The typical deposition rate was 0.65 nm/min. Sample Preparation. Thick organic films were generated by depositing an aliquot (20 µL) of solutions, with the belowmentioned conditions, onto the respective surfaces and allowing the solvent to evaporate under cover. The solutions employed and the sample preparations were as follows: (i) 0.001% methanol (in the case of thiram) or ethanol (in the case of ziram) adsorbate solutions on Au-coated surfaces for enhanced spectra and (ii) 0.01% methanol or ethanol adsorbate solutions on uncoated CaF2 windows for the unenhanced spectra. Intrumentation. Fourier transform infrared (FTIR) transmission spectra were recorded with a Bruker IFS 66 instrument. The spectral resolution was 8 cm-1, and 100 scans were obtained from each sample. FT-Raman spectra were obtained by using a RFS 100/S Bruker spectrophotometer. The 1064 nm line, provided by a Nd:YAG laser, was used. The resolution was set at 4 cm-1, and a 180° geometry was employed. UV-vis absorption spectra were recorded with a Cintra 5 UVvis double beam spectrophotometer. The micrograph was obtained with a ELMISKOP 1A Siemens transmission electron microscope operating at 80 kv.
Results and Discussion Absorption Spectrum and Transmission Electron Microscopic of Au Film. Figure 2 shows the UV-vis spectrum of the employed Au film. This spectrum shows a broad absorption band centered at 750 nm that may correspond to the plasmon excitation of the Au particles observed in the micrograph of Figure 3. These are rather long particles with a width of 10-40 nm and a length which oscillates in a broad interval between 40 and even
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1000 nm. The plasmon resonance absorption of these particles is extended toward the near-infrared and infrared regions; hence, they can be applied to obtain the SEIR and SERS spectra with the above instrumentation. Thiram. Figure 4c displays the SEIR spectrum of thiram deposited on Au film, and Figure 4d shows the FTIR spectrum of the Au film. The SEIR of thiram is compared with its FTIR spectrum deposited onto CaF2 (Figure 4b) and dispersed in KBr (Figure 4a). The two last spectra show a very similar profile. They are characterized by the following bands: at 849 cm-1, attributed to the methyl groups; at 970 and 1040 cm-1, attributed to ν(C-S) and ν(CdS) motions, respectively; at 1236 and 1149 cm-1 attributed to δ(CNC), δ(HCN), ν(C-N), and t(CH3) vibrations; at 1375 cm-1, attributed to the ν(C-N) motion coupled to δs(CH3); and at 1501 cm-1, resulting from δas(CH3). In contrast, the SEIR spectrum (Figure 4c) shows many changes induced by the interaction of thiram with the metal. Among these changes, we highlight the following: (i) the disappearance of the bands at 849 and 1040 cm-1, (ii) the intensity inversion of the bands at 1149 and 1236 cm-1, which are shifted toward lower and higher wavenumbers, respectively; (iii) the appearance of a new intense band at 1567 cm-1. The last band is related to the formation of a CdN bond in the thioureide tautomeric form of dithiocarbamate groups.15 The existence of this bond is connected to the interaction with a metal, which induces the attraction of the free electron pair in the N atom and the creation of the CdN bond. The double-bond character of the C-N bond is highly dependent on the metal nature,16,17 because of the different strength of the interaction with the metal. In fact, the high wavenumber observed for this vibration in the SEIR spectrum indicates a strong interaction with the Au surface. Such an interaction may imply a subsequent S-S bond cleavage of thiram when the fungicide is adsorbed on the metal, giving rise to two dimethyl DTC residues (Figure 5), as has also been found on Ag colloids.18 On the other hand, the disappearance of the band attributed to the CdS bond seen at 1040 cm-1 19 in free thiram points out that the interaction of thiram with the Au surface takes place via the SCS group (Figure 5a) through a bidentate complex with Au that involves both S atoms instead of a monodentate interaction that would involve only one S atom (Figure 5b). The large shift to higher wavenumbers of the ν(CdN) vibration in the Au surface in comparison with that observed on Ag colloids18 indicates a strong interaction of thiram with Au in films of this metal, and this further justifies a bidentate configuration of the fungicide adsorption. An analysis of the SEIR enhancement factors of thiram (Table 1) is not easy because of the chemical change undergone by the fungicide upon its adsorption. The different enhancement factor deduced for each band can be attributed either to the different orientation of each vibrational mode with respect to the surface, as deduced from the SEIR selection rules,20 or to the contribution to the enhancement phenomenon of the chemical effect,10,21 (15) Coleman, M. M.; Koenig, J. L.; Shelton, J. R. J. Polym. Sci. 1974, 12, 1001. (16) Brown, D. A.; Glass, W. K.; Burke, M. A. Spectrochim. Acta 1976, 32A, 137. (17) Frigerio, A.; Halac, B.; Perec, M. Inorg. Chim. Acta 1989, 164, 149. (18) Sa´nchez-Corte´s, S.; Vasina, M.; Francioso, O.; Garcı´a-Ramos, J. V. Vib. Spectrosc. 1998, 17, 133. (19) Bonati, F.; Ugo, R. J. Organomet. Chem. 1967, 10, 257. (20) Osawa, M.; Ataka, K.-I.; Yoshi, K.; Nishikawa, Y. Appl. Spectrosc. 1993, 47, 1497. (21) Badilescu, S.; Ashrit, P. V.; Truong, V.-V.; Badilescu, I. I. Appl. Spectrosc. 1989, 43, 549.
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Figure 3. Transmission electron micrograph of Au film (×150 000).
Figure 5. Adsorption mechanisms of thiram (a and b) and ziram (c and d) deduced from the SEIR and SERS spectra. Table 1. SEIR Enhancement Factors (EF) of the Thiram and Ziram Main Bands
Figure 4. (a) FTIR of thiram in KBr, (b) FTIR of thiram on CaF2 (×100), (c) SEIR of thiram on Au film (×100), and (d) FTIR of Au film (×0.1).
which differently affects each SEIR band through the |∂µ/ ∂Q| term (where µ is the dipole moment and Q is the normal coordinate). We observed a greater enhancement of the 1367 cm-1 band, attributed to the methyl group bending coupled to ν(C-N) motion, and of the band at 1491 cm-1, also
band (cm-1)
thiram (EF)
band (cm-1)
ziram (EF)
1491 1367 1237 1137 971
60 100 10 60 6
1495 1369 1256 1100 1015
4 6 10 10 70
attributed to the methyl groups. The strong enhancement observed for the bands at 1491, 1367, and 1137 cm-1 can be attributed to the high perpendicular orientation of the methyl vibrations that contribute to such modes, sup-
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Figure 7. (a) FTIR of ziram on CaF2 and (b) SEIR of ziram on Au film.
Figure 6. (a) FT-SERS of thiram on Au film (150 mW), (b) FT-Raman of solid thiram dispersed in KBr (50 mW), (c) FTRaman of Au film (50 mW), and (d) FT-Raman of thiram on CaF2 (150 mW).
posing a bidentate interaction of the DTC moiety with the metal. Under such orientation, the ν(CdN) vibration is also perpendicular, thus contributing to the enhancement of the 1367 cm-1 band and most probably to that at 1137 cm-1. In contrast, the band at 971 cm-1 is not so enhanced, and this could be related to the predominant parallel orientation of the δ(CNC), δ(NCS), and ν(CS) modes derived from the direct interaction of the DTC group with the Au film. The FT-SERS spectrum (exciting at 1064 nm) of the same sample employed in the SEIR experiment is shown in Figure 6a, together with the Raman of the solid thiram (Figure 6b), the Raman of the Au film (Figure 6c), and the unenhanced Raman of thiram on CaF2 (Figure 6d). In the SERS of thiram, one can observe bands that are not attributed to the fungicide and are due to the Au film (see Figure 6c). The comparison between the SERS and thiram in the solid state (Figure 6b) also indicates the molecular breakdown upon adsorption on the metal surface. This fact is demonstrated by the important changes observed in the region below 600 cm-1, where one can observe an intensity decrease of the bands at 558 and 394 cm-1, which are assigned to the ν(S-S) vibration. Moreover, the strong bands observed in the SEIR spectrum of thiram at 1491, 1237, and 1137 cm-1 are very weak in the SERS of thiram. The high contribution of the methyl group vibrations to such bands suggests an important role of the chemical mechanism in the SERS enhancement. This is also supported by the relative weakening of the ν(C-H) modes (result not shown).
The importance of the chemical mechanism in the SERS of thiram also justifies the high intensity of the band observed at 1378 cm-1, owing to the considerable contribution of the ν(C-N) mode to this band. The last assumption is supported by the observation of a strong band appearing at 270 cm-1 that can be attributed to the ν(S-Au) vibration. This mechanism is not operative in SEIR under the same terms, because of the resonant character of the Raman effect when a charge transfer between the metal and the adsorbate is taking place. Moreover, the FT-Raman spectrum of thiram on CaF2 displays only bands attributed to CaF2: at 321 cm-1 (strong) and a triplet in the 1050-1150 cm-1 region. Ziram. The SEIR spectrum of ziram is shown in Figure 7a. The most important feature observed in the SEIR of ziram in comparison to that of thiram is the much lower enhancement factor (Table 1). The unenhanced FTIR spectrum of ziram deposited on CaF2 (Figure 7b) shows the same profile as the FTIR of the solid ziram dispersed in KBr (spectrum not shown). Compared to the latter spectrum, the SEIR one also shows deep differences that can be attributed to a chemical modification of the fungicide, as in the case of thiram. We suggest that in the case of ziram a partial breakdown of the metallic complex could occur because of the interaction with the Au film. The main differences observed in the SEIR of ziram compared to the unenhanced spectrum are (i) a shift to lower wavenumbers of the bands appearing at 1520 and 1385 cm-1 in the solid up to 1495 and 1369 cm-1 in the SEIR, respectively, (ii) a shift of the 1241 cm-1 band to 1256 cm-1, (iii) an intensity decrease of the 1139 cm-1 band, which probably is shifted to 1100 cm-1, and, finally, (iv) the great intensity decrease of the strong band appearing at 976 cm-1 accompanied by an enhancement of the band appearing at 1015 cm-1.
Dithiocarbamate Pesticides on Gold Film
Figure 8. (a) FT-SERS of ziram on Au film (150 mW), (b) baseline corrected spectrum of (a), (c) FT-Raman of ziram on CaF2 (150 mW), and (d) FT-Raman of solid ziram in KBr (50 mW).
The changes observed for ziram on Au film occur in the opposite direction as compared to those observed for thiram. In fact, the ν(CdN) band appearing at 1569 cm-1, that corresponds to the thiram band at 1567 cm-1, and the bands at 1142 cm-1, that may correspond to that at 1137 cm-1 of the SEIR of thiram, are very weak, and the band of solid ziram seen at 976 cm-1 almost disappears in the SEIR. The observation of bands at 1567 and 971 cm-1 in the SEIR of thiram was interpreted as due to a strong interaction of this molecule, or the products resulting from its breakdown, with Au in the film. The low intensity reached by the ν(CdN) band (at 1569 cm-1) in the SEIR of ziram suggests that the interaction of ziram with Au is weak and that this fungicide may be rather physisorbed on the metal film (Figure 5c). Moreover, this interaction seems not to take place through a bidentate complex, as indicated by the absence of the band at 976 cm-1 and the observation of a strong band at 1015 cm-1, assigned to ν(CdS). The opposite behavior of ziram in comparison to thiram indicates that the molecule changes from a strong metallic interaction configuration in the free molecule (Figure 1), because of the presence of a Zn2+ in the complex, to a situation of weak interaction in the Au film (Figure 5c). The latter interaction mechanism implies a completely different orientation of ziram that accounts for the different spectral profiles. The SERS spectra further confirm the striking difference observed for ziram in relation to thiram. As in the case of the SEIR spectrum, the SERS of ziram is weak and shows a high background (Figure 8a). The baseline corrected spectrum (Figure 8b) shows a spectral profile that is closer to that of thiram. This fact suggests that the
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SERS spectrum corresponds to ziram molecules that are probably broken, giving rise to two dimethyl DTC groups able to strongly interact with the Au film (Figure 5d). The similarity of the SERS spectra of both pesticides contrasts with the remarkable differences found between their SEIR spectra. In addition, as in the case of thiram, the Raman of ziram deposited on CaF2 (Figure 8c) displays only features corresponding to this crystal. Comparative Study of SEIR and SERS of Thiram and Ziram. The great differences observed in the surfaceenhanced vibrational spectra of thiram and ziram can be attributed to the different stability of these molecules in the presence of the Au film. The stability of thiram seems to be much lower than that of ziram, and this may explain its more intense SEVS spectra; from this result we inferred that the adsorption on the metal surface is a prerequisite to ensure a strong SEVS spectrum. From the point of view of the electromagnetic mechanism, this is related to the different distance from the surface at which the adsorbate is placed if it is chemisorbed or physisorbed, whereas from the point of view of the chemical effect this could be related to the variation of the |∂µ/∂Q|2 and |∂R/∂Q|2 terms (where R is the molecular polarizability) corresponding to the SEIR and SERS intensities, respectively, when the molecule is interacting with the metal.22 The vibrational modes for which the above terms become higher after the molecule is chemisorbed will be obviously enhanced in the SEVS spectra. In the case of the SERS spectra, the stability and adsorption of the above pesticides seems to be of great importance in relation to the enhancement. Because the molecular breakdown is required for the DTC chemisorption, the majority of ziram molecules are supposed to be physisorbed on the Au film, remaining farther from the surface than those of thiram. The SERS enhancement factor exhibits a strong dependence upon the distance to the surface,23 that may account for the SERS intensity decrease in the case of ziram. However, the situation is different for the SEIR effect, because the physical process taking place in the latter case is the absorption of IR light. The higher cross section of the IR absorption compared to the Raman scattering explains why molecules that are not close to the surface may undergo a dramatic Raman intensity decrease while the IR absorption is still intense. We suggest that the weak SERS spectrum observed for ziram is actually due to the few ziram molecules that undergo a molecular breakdown and a chemisorption. In addition, the chemisorption of a larger amount of dissociated thiram on the film, as compared to the ziram, led to a further enhancement of the SERS signal through the chemical mechanism.The qualitatively different SEIR spectral profile of ziram in relation to thiram indicates that SEIR is more sensitive to all the possible forms of ziram adsorbed onto the Au films: dissociated and strongly attached (Figure 5d) or not dissociated and weakly attached to the Au film (Figure 5c). In contrast, the SERS technique seems to be sensitive only to the ziram molecules that are dissociated when adsorbed on the metal film. The different adsorbed species deduced for ziram on the Au film are probably related to the existence of adsorption sites with different properties on the metal surface. Similar conclusions were obtained by other authors, although on Ag films.24 This result indicates the higher importance of the electromagnetic enhancement mechanism in SEIR spec(22) Aroca, R.; Rodriguez-Llorente, S. J. Mol. Struct. 1997, 408/409, 17. (23) Moskovits, M. Rev. Mod. Phys. 1985, 57, 783 (24) Merklin, G. T.; Griffiths, P. R. J. Phys. Chem. B 1997, 101, 5810.
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tra, which is responsible for the different enhancement factors deduced for each vibrational mode (Table 1), depending on the different orientations adopted by both molecules on the Au film (Figure 5c,d). The importance of the monodentate configuration (Figure 5c) in the case of ziram is supported by the high enhancement factor reached by the δ(HCN) and δ(CNC) (1256 cm-1), F(CH3) (1100 cm-1), and ν(CdS) (1015 cm-1) modes, which may adopt a more perpendicular orientation under such a configuration. Conclusions Thiram is much less stable than ziram when adsorbed on a Au film. This fact determines a remarkable difference between these two molecules when they are analyzed with the two different interfacial vibrational techniques SEIR and SERS. The SERS spectrum of ziram is very weak and similar to that of thiram. This indicates that the chemisorption derived from the pesticide’s breakdown is needed to observe a strong SERS spectrum and that, in the case of ziram, only those molecules that are decomposed, giving
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rise to a dimethyl DTC moiety similar to that obtained in thiram, will induce an intense SERS signal. This is explained by a high contribution of the chemical effect to the SERS enhancement. The situation is different for the SEIR effect; the still intense infrared absorption of physisorbed ziram led to a SEIR spectrum with a considerable intensity, with a different spectral profile in comparison to that of thiram, because of a reorientation of the DTC moiety on the film. Whereas SERS is sensitive only to the ziram molecules that are dissociated when adsorbed on the metal film, SEIR seems to be sensitive to all the possible forms of ziram adsorbed onto the film. For this reason, SEIR can be considered as a more sensitive technique than SERS. Acknowledgment. This work has been supported by Direccio´n General de Ensen˜anza Superior e Investigacio´n Cientı´fica Project No. PB97-1221 and the Comunidad de Madrid Project No. 07M/0040/1999. We acknowledge a contract of CSIC to S.S.-C. LA001269Z
