Anal. Chem. 2004, 76, 7118-7125
Surface-Enhanced Vibrational Microspectroscopy of Fulvic Acid Micelles Ramo´n A. Alvarez-Puebla,† Julian J. Garrido,‡ and Ricardo F. Aroca*,†
Materials and Surface Science Group, School of Physical Science, University of Windsor, Windsor, ON, Canada, N9B 3P4, and Department of Applied Chemistry, Universidad Publica de Navarra, Campus Arrosadı´a, E-31006 Pamplona, Spain
Micro-Raman spectroscopy, infrared absorption microspectroscopy, and AFM images of nano- or microsized micelles formed by fulvic acid (FA) solutions, prepared at different pHs, and cast on glass slides or gold island films, are reported. FA films cast on gold islands are characterized by surface-enhanced infrared absorption (SEIRA), surfaceenhanced infrared reflection absorption, and surfaceenhanced Raman scattering (SERS). Based on spectral evidence, it is expected that the chemisorption of FA on gold island films takes place through thiol groups, which become more active as pH increases. The SEIRA spectra of these films show increased peak intensity, as well as improved band resolution. Microspectroscopy SERS studies show that, at pH 5, FA form small aggregates on gold surfaces. At pH 8, FA tends to expand due to electrostatic repulsion, giving rise to a fractal surface composed of different domains. SERS studies of these domains reveal that the most polar molecules are located on the external faces. At pH 11, fractal conformations are even more pronounced and give rise to radial patterned structures. At this pH, the position of fulvic acid molecules in the fractal micelles is the same as observed at pH 8. In this way, SERS can be viewed as a powerful tool for the analysis of the composition, apparent contribution of the surface functional groups of FA films, and the FA building blocks (i.e., catechol, gallic, salicylic, or ftalic acids) in the structures of these materials. Fulvic acids (FA) are natural macromolecules that can be found ubiquitously in nature, although, their origins are not well defined. They are macromolecular products derived from the humification of organic molecules from plants, animals, microorganisms, and their metabolic products,1,2 FA can affect soil fertility, mineral weathering, and water acidity. They are also involved in the transport, sequestration, and mitigation of contaminants, impacting atmospheric chemistry through the carbon cycle, in which carbon is constantly recycled among plants, animals, soil, air, and water.1 * To whom correspondence should be addressed. E-mail: G57@ uwindsor.ca. † University of Windsor. ‡ Universidad Publica de Navarra. (1) Stevenson, F. J. Humus chemistry: Genesis, composition and reactions; John Wiley & Sons: New York, 1994. (2) Swift, R. S. In Method of soil science analysis: Chemical methods. Part 3; Sparks, D. L., Ed.; Soil Science Society of America: Madison, WI, 1996; pp 1011-1069.
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FA have a great tendency for adsorption onto mineral surfaces (i.e., clays, metals, metal oxides, etc.), modifying their chemical behavior.3,4 Notably, for a large range of environmental particles and matrixes, fractal geometries have been shown, including humic acids, soot, mineral surfaces, biological aggregates, and aerosols.5 Systematic studies of the structures of these substances contribute to the understanding of the role of their interactions with other elements and compounds. Such knowledge is necessary to predict and control the impact of chemical and biological changes in the environment. To gain more insight into the precise role, function, and structure of FA, there has been much effort in recent years directed toward the determination of the chemical structures and geometric conformations of fulvic macromolecules and aggregates.6-8 These previous studies have been carried out on the bulk materials or in solution. In the present work, we investigate thin solid films of FA to study the structure and organization of the aggregation. Thin solid film studies permit the collection of information on the structure that these materials may form when coating mineral surfaces.9 Given the nature of these materials, the use of thin films decreases the fluorescence contribution that can interfere with signal collection in Raman spectroscopy. The published work on humic substances as thin films has been primarily morphological, employing atomic force microscopy (AFM).10,11 It is demonstrated here that FA associate to form nano- and microsized micelles with fractal geometries, dependent upon the pH value of the FA solution cast onto glass slides or gold island films. Spectroscopic characterization is achieved using micro-Raman and infrared absorption microspectroscopy, and the enhanced results obtained in surface-enhanced vibrational spectroscopy experiments allow for the extraction of information on both the structure and organization of thin solid (3) Alvarez-Puebla, R. A.; Valenzuela-Calahorro, C.; Garrido, J. J. Langmuir 2004, 20, 3657-3664. (4) Alvarez-Puebla, R. A.; Valenzuela-Calahorro, C.; Garrido, J. J. J. Colloid Interface Sci. 2004, 277, 55-61. (5) Dachs, J.; Eisenreich, S. J. Langmuir 2001, 17, 2533-2537. (6) Gunasekara, L.; Dickinson, C.; Xing, B. In Humic substances: Structures, models and functions; Ghabbour, E. A., Davies, G., Eds.; The Royal Society of Chemistry: Cambridge, U.K., 2001. (7) Chin, W. C.; Orellana, M. V.; Verdugo, P. Nature 1998, 391, 568-571. (8) Myneni, S. C. B.; Brown, J. T.; Martinez, G. A.; Meyer-Ilse, W. Science 1999, 286, 1335-1337. (9) Sanchez-Cortes, S.; Francioso, O.; Ciavatta, C.; Garcia-Ramos, J. V.; Gessa, C. J Colloid Interface Sci. 1998, 198, 308-318. (10) Mertig, M.; Klemm, D.; Zanker, H.; Pompe, W. Surf. Interface Anal. 2002, 33, 113-117. (11) Vinodgopal, K.; Subramaniam, V.; Carrasquillo, S.; Kamat, P. Environ. Sci. Technol. 2003, 37, 761-765. 10.1021/ac049076u CCC: $27.50
© 2004 American Chemical Society Published on Web 11/04/2004
films.12,13 Previously published work on the surface-enhanced Raman scattering (SERS) of humic substances has been carried out on colloids,14-17 electrodes,18-21 and silver island films.21-23 The surface-enhanced infrared absorption (SEIRA)24,25 and surfaceenhanced infrared reflection absorption (SEIRRA)26 of FA, reported here for the first time, give rise to better resolved IR spectra for characterization of surface functional groups under conditions different from normal infrared absorption measurements. The SERS spectra of humic substances previously reported14,17,21,27 were obtained using 514.5- or 647-nm excitation on silver surfaces. In the present work, we have used several laser lines (442, 514.5, 633, and 785 nm), and it was found that the best SERS results were obtained with 785-nm excitation on gold island films. In addition, the high spatial resolution of Raman microscopy (1 µm2) makes it possible to probe directly different sections of fractallike structures, such as those of FA films. EXPERIMENTAL SECTION FA from a commercial humic substance by Acros Organics (Geel, Belgium) was fractionated by adjusting the pH of a 40 g L-1 HS solution to 1.0. The FA was then purified using a XAD-8 resin column, converted to the protonated form by passing it through a proton-saturated resin, and then freeze-dried, in accordance with the procedure proposed by the International Humic Substance Society (IHSS).2 C, H, N, and S contents were determined by elemental analysis (CHNS EA1108-by Carlo Erba, Milan, Italy). Carboxylic acidic groups and total acidity of FA were determined by calcium acetate and barium hydroxide methods,1 respectively. Phenolic acidic groups were calculated from the difference between the total acidity and that from the carboxylic acidic groups. The results of the characterization of the FA are listed below in Table 1 and are in agreement with what is expected for these kind of substances.1,2 Gold island films of 9-nm mass thickness were prepared in a Balzers BSV 080 glow discharge evaporation unit. The metal films were deposited on preheated (200 °C) glass (7059 Corning) and KRS-5 (Aldrich) slides. During film deposition, the background pressure was maintained at ∼10-6 Torr, and the deposition rate (0.5 Å s-1) was monitored using an XTC Inficon quartz crystal oscillator. FA films were prepared by casting 10 µL of the FA (12) Tolaieb, B.; Constantino, C. J. L.; Aroca, R. F. Analyst 2004, 129, 337-341. (13) Goulet, P. J. G.; Pieczonka, N. P. W.; Aroca, R. F. Anal. Chem. 2003, 75, 1918-1923. (14) Francioso, O.; Sanchez-Cortes, S.; Tugnolic, V.; Marzadoria, C.; Ciavatta, C. J. Mol. Struct. 2001, 565-566, 481-485. (15) Francioso, O.; Sanchez-Cortes, S.; Ciavatta, V.; Tugnoli, C.; Gessa, C. Appl. Spectrosc. 1998, 52, 270-277. (16) Francioso, O.; Sanchez-Cortes, S.; Tugnoli, V.; Ciavatta, C.; Sitti, L.; Gessa, C. Appl. Spectrosc. 1996, 50, 1165-1174. (17) Francioso, O.; Sanchez-Cortes, S.; Casarini, D.; Garcia-Ramos, J. V.; Ciavatta, C.; Gessa, C. J. Mol. Struct. 2002, 609, 137-147. (18) Yang, Y.-H.; Zhou, Q.; Yu, J.-Y. J. Environ. Sci. Health, A 1996, A31. (19) Wang, T.; Zhong, F.-P.; Yang, Y.-H.; Zhang, D.-H. Spectrosc. Lett. 1996, 29. (20) Wang, T.; Xiao, Y.-J.; Yang, Y.; Chase, H. A. J. Environ. Sci. Health, A 1999, A43, 749-765. (21) Vogel, E.; Ge[ss]ner, R.; Hayes, M. H. B.; Kiefer, W. J. Mol. Struct. 1999, 482-483, 195-199. (22) Wang, T.; Yang, Y. H. Toxicol. Environ. Chem. 1996, 57, 137-144. (23) Yang, Y. H.; Zhang, D. H. Toxicol. Environ. Chem. 1996, 56, 273-282. (24) Osawa, M. In Handbook of Vibrational Spectroscopy; Chalmers, J. M., Griffiths, P. R., Eds.; John Wiley: New York, 2002; pp 785-799. (25) Ross, D.; Aroca, R. J. Chem. Phys. 2002, 117, 8095-8103. (26) Roy, D.; Fendler, J. Adv. Mater. 2004, 16, 479-508. (27) Liang, E. J.; Yang, Y.; Kiefer, W. Spectrosc. Lett. 1999, 32, 689-701.
Table 1. Selected Physical and Chemical Properties of the FA Employed C (%) H (%) N (%) S (%) ash O (%)
40.1 3.57 0.67 0.65 b 55.0
E4/E6 (g mol-1)a Cstrong acidic groups (mol kg-1) Cweak acidic groups (mol kg-1) pK pK
5.76 1058 5.71 2.73 3.41 9.39
a Average molecular weight estimated from M ) 3.99 w 280 + 490 according to Chin et al.39 b Not detected.
solutions (0.2500 g L-1) at pH values of 5, 8, and 11, on glass or gold island films. UV-visible extinction spectra (300-1100 nm) of the gold island films and FA films were recorded with a Varian Cary 50 UV-visible spectrophotometer. Transmission SEIRA spectra were obtained on KRS-5 windows. SEIRRA on glass slides was obtained using a Bruker Equinox 55 equipped with a microscope (Bruker Hyperion 3000). The enhancement factor for SEIRA was calculated by comparing the measured intensities of the transmission spectrum of FA on gold island film on KRS-5 windows with that recorded from FA film on KRS-5. Raman and SERS spectra were collected with either a Renishaw Research Ramanscope 2000 or a Renishaw Invia system, both equipped with Peltier CCD detectors and Leica microscopes. The spectrographs use 1800 g/mm gratings with additional band-pass filter optics. Excitation lines of 442, 514.5, 633, and 785 nm were used. Spectra were collected in Renishaw’s continuous collection mode with accumulation times of 10 s and five spectra being coadded in each experiment. AFM (Digital Instruments NanoScope IV) topographical measurements were performed in tapping mode with a silicon cantilever (NSC 14 model, Ultrasharp) operating at a resonant frequency of 256 kHz. Images were collected with high resolution (512 lines/sample) at a scan rate of 0.5 Hz. The data were collected under ambient conditions, and each scan was replicated to ensure that any features observed were reproducible. RESULTS AND DISCUSSION FA solutions (0.2500 g L-1) prepared at pH values of 5, 8, and 11 were used to cast FA films on glass slides or gold island films. A fixed amount of solution, 10 µL, was employed in all cases. The solutions prepared with controlled pH values of 8 and 11 formed films with fractal-like structures, as can be seen in Figure 1. Films cast from the pH 5 solution show aggregation but not the organization found for the solutions at the other two pH values. The patterns of these microheterogeneous systems are reminiscent of the ones formed by gold and silver colloids cast onto glass substrates.28 Figure 2 shows the transmission infrared spectra of FA in KBr and that of a FA film cast from a FA solution at pH 11 on KRS-5 window recorded with a 15× microscope objective. The spectrum of the bulk, which has been previously reported,29,30 is slightly different from that of the film, which is better resolved. Broad and high-intensity bands that characterized the spectrum (28) Lemma, T.; Aroca, R. F. J. Raman Spectrosc 2002, 33, 197-201. (29) Agnelli, A.; Celi, L.; Degl’Innocenti, G.; Corti, G.; Ugolini, F. C. Soil Sci. 2000, 165, 314-327. (30) Niemeyer, J.; Chen, Y.; Bollag, J. M. Soil Sci. Soc. Am. J. 1992, 56, 135140.
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Figure 3. Plasmon absorption of a neat Au island film and gold films coated with FA solutions prepared at pH 5, 8, and 11.
Figure 1. Micelle formation of fulvic acid solutions, cast on glass, prepared at (a) pH 8 and (b) pH 11.
Figure 2. (a) Transmission FT-IR spectra of bulk FA dispersed in KBr and from a film cast on KRS-5 at pH 11 (b). The absorbance scale is only applicable for the bottom spectrum.
of bulk humic substances are explained by the overlapping of closely related functional groups.31 In addition, bulk humic substances are hygroscopic and the trapped water molecules contribute to the broadening of the structural vibrational bands. The fulvic films were produced by casting 10 µL of 250 mg L-1 solution of FA on a ∼10-mm2 surface (107 µm2) and the surface studied using IR microscopy ranges from 500 × 500 to 50 × 50 (31) Bruccoleri, A. G.; Sorenson, B. T.; Langford, C. H. In Humic substances: Structures, models and functions; Gabbour, E. A., Davies, G., Eds.; The Royal Society of Chemistry: Cambridge, U.K., 2001; pp 193-208.
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µm2 with an average height in the 0.2-0.5-µm range. The infrared spectra of the thin solid films may be seen as a small organized cross section of the bulk with a different distribution of functional groups when compared with the spectrum of the bulk. In addition, the apparent decrease of trapped water substantially reduces the relative intensity of the OH band. Plasmon Absorption and Topography of Gold and GoldCoated films. FA solutions were cast onto gold island films to explore SEIRA as a sensitive technique for the structural characterization of these films. The surface plasmon absorption of Au island film and those of coated gold film with FA are shown in Figure 3. The plasmon absorption32,33 centered at 702 nm in the neat gold film is affected by the dielectric constant of the FA coating film and is displaced to lower wavelength. Increases in the pH of the FA solution correlates with a decrease in the wavelength of the absorption maximums and the relative intensity of these maximums. A dielectric coating changes the plasmon absorption of metal particles,34,35 and the differences reflect the qualitatively different nature of the films formed from solutions with varying pH values. It is also possible that the nature of the molecule-metal interactions may be different, for instance, in the extent of chemical adsorption, or the coordination of gold with the FA. This coordination likely involves thiol groups, which become more active as pH increases. Whatever the interaction, the trend in the formation of fractal-like structure observed in the neat films is also seen in the films cast onto gold island films as can be seen in Figure 4. This figure shows optical images obtained with a 20× and 50× microscope objectives for gold and goldfulvic films formed from stock solutions at a pH 5, 8, and 11. While gold film appears to have a quite homogeneous surface (Figure 4a), the gold-fulvic film at pH 5 shows aggregates randomly distributed throughout the whole surface (Figure 4b). Gold-fulvic film at pH 8 and 11 (Figure 4c and d) clearly show the inner structure of the fractal-like pattern. The AFM image (Figure 5a) shows a gold film composed by gold islands with a size that ranges from 20 to 50 nm and a height between 15 and 40 nm. The rootmean-square roughness of the film is 2.71 nm. The average interisland space is 20 nm. Fulvic film coated at pH 5 (Figure 5b) (32) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Berlin, 1995. (33) Jensen, T. R.; Malinsky, M. D.; Haynes, C. L.; van Duyne, R. P. J. Phys. Chem. B 2000, 104, 10549-10556. (34) Mulvaney, P. Langmuir 1996, 12, 788-800. (35) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668-677.
Figure 4. Optical images of (a) gold and gold-fulvic films at pH (b) 5, (c) 8, and (d) 11. Images captured with a 20× or 50× objectives).
shows globular domains ranging between 0.2 and 1.3 µm. Films coated at pH 8 (Figure 5c) and pH 11 (Figure 5d) show the branches of the fractal structure observed on the sample by optical microscopy. The value for the roughness degrease from 41.4 nm at pH 5 to 30.2 nm at pH 8 and 21.4 nm at 11, which indicates that the fulvic supramolecular structure spreads on the surface with the increase of the pH may be due to the increase in the electrostatic repulsion. SEIRA and SEIRRA. SEIRA25 on gold island films is observed and is illustrated in Figure 6, where the transmission infrared spectrum of a FA film on KRS-5 windows is overlaid with that of an FA at pH 11, cast on gold islands that had been evaporated on the KRS-5 substrate. The neat film spectrum, multiplied by a factor of 6, is also shown using a dash line. It can be seen that the SEIRA spectrum shows enhancement of the main band in the original spectrum, along with some additional features, such as the maximum seen at 1297 cm-1. Thus, an average enhancement factor of 6 can be calculated for this particular observation. The SEIRA spectrum not only shows an increased intensity of the bands (which experimentally can be translated into a decrease in the number of scans necessary to obtain a quality IR spectrum and, consequently, to a decrease in experimental time) but also an increase in the number of characteristic vibrational bands that can be observed in the IR spectrum. The SEIRRA spectra collected for all three FA samples are shown in Figure 7, where the most characteristic infrared bands are detected.36 They are due to O-H (36) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; GRaselli, J. G. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press: San Diego, 1991.
stretching (3378 cm-1), aliphatic C-H stretching (2939 and 2850 cm-1), S-H stretching (2492 cm-1), CdO stretching (1764 cm-1), aromatic ring and antisymmetric COO- stretching (1677 cm-1), aliphatic C-H bendings and polymeric aromatic rings (1427 cm-1), symmetric COO- stretching (1297 cm-1), C-O stretching (1266 cm-1), and out-of-plane deformations of aromatic C-H groups (879 and 842 cm-1).3,37 Experimentally, SEIRRA spectra of various FA films on gold were recorded with a microscope attachment with a fixed incident angle in reflection geometry. FA films at different pHs were fabricated on Au islands evaporated onto glass slides and spectra, and images were obtained using a 15× objective. The results are shown in Figure 7, together with the images obtained from the section probed by micro-FT-IR. The SEIRRA spectra show the same behavior with variation of pH that have been shown in previous FT-IR work9,15 but with an improved spectral resolution. The SEIRRA of FA films at pH 5 show strong bands due to v(O-H), ∼3500 cm-1, v(CdO), 1767 cm-1, aliphatic C-H bending and polymeric aromatic rings, 1427 cm-1, the symmetric vs(COO-) at ∼1300 cm-1, and v(C-O) of aliphatic alcohols and ethers at 1151 cm-1. The aromatic wagging ω(C-H) is also seen at 802 cm-1. As pH increases, the intensity of the bands due to carboxylic acids v(O-H) at ∼3500 cm-1, and v(CdO) at ∼1767 cm-1, decreases, while the intensity of the antisymmetric vas and symmetric vs stretching modes of COO-, at ∼1620- and ∼1350-cm-1 bands, respectively, increases. The increase in the ionic forms likely lead to the formation of “ionic (37) Alvarez-Puebla, R. A.; Valenzuela-Calahorro, C.; Garrido, J. J. J. Colloid Interface Sci. 2004, 270, 47-55.
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Figure 5. AFM height images of (a) gold and gold-fulvic films at pH (b) 5, (c) 8, and (d) 11.
Figure 6. (a) FT-IR spectrum of FA film on glass slide (dashed line shows spectrum multiplied by a factor of 6) and (b) SEIRA spectrum of FA on an Au island film.
micelles”, where the repulsion between the different fulvic macromolecules gives rise to expanded fractal-like structures (see Figures 4 and 5). This fractal behavior is not induced by the Au-FA interaction, since it is also present when FA is cast on a glass surface, as shown in Figure 1. 7122
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Raman Microscopy and SERS. The spontaneous Raman scattering of FA, in both bulk and film form, is hampered by strong fluorescence. The Raman spectra obtained with 633- and 785-nm excitation is typically weak scattering on a strong fluorescence background, consistent with the results obtained by Vogel et al.21 with 647-nm laser excitation. The Raman spectra are poorly resolved, with bands similar to those reported by Sanchez-Cortes et al.9 Raman spectra were recorded with several laser lines (442, 488, 514, 633, and 785 nm) in the search for optimal experimental variables. For example, the Raman spectra recorded using 442nm excitation show two broad bands (∼1600 and ∼1400 cm-1) characteristic of low-structure graphite-like carbons.21,38 SERS on gold islands was attained using excitation lines at 633 and 785 nm. However, the best results were obtained using the 785-nm laser line, and only these results will be discussed. To facilitate the discussion, the most characteristic group frequencies reported in the literature of humic substances have (38) Liang, E. J.; Yang, Y.-H.; Kiefer, W. J. Environ. Sci. Health. A 1996, A31, 2477-2486.
Figure 7. SEIRRA spectra of a FA film on 9-nm gold island film at pH (a) 5, (b) 8, and (c) 11 and optical images for the sections of the film where spectra were collected. Table 2. Characteristic SERS Bands for Humic Substances wavenumber (cm-1) 3240 2956, 2885 2135 1700-1650 1618, 1590 ∼1580 1574, 1475, 498 1540 1450 1379 1315 1300-1000 1211 1171 1165 1060 700-400 365
assignment
ref
ν(N-H) of bonded amines ν(C-H) ν(CtC) or accumulated v(CdC) or a vibrational mode of -NH3+ ν (CdO) νA (COO-) and benzene ring stretching aromatic ring stretching vibrations in plane stretching vibrations of highly substituted phenols polymeric benzene rings ring stretching vibrations of aromatic moieties δ (C-H2) aliphatic νS (COO-) νS (COO-) and benzene substituted ring stretching modes of the C-C, C-O or C-N bonds and/or rocking and wagging modes of the C-H and N-H units of the molecule ν (C-O) ether δ (C-H) ν (C-O) alcohol and aliphatic ethers ν (C-C) in-plane deformation of the -COO- group and torsional motion of a -NH3+ moiety skeletal deformation mode
21 21 21 15, 36 9, 17 17
been collected in Table 2. The table is constructed to include the characteristic vibrational modes that allow for the identification of functionalities and building blocks. Fingerprint bands at 1569, 1479, 1329, 1224, and 667 cm-1 are characteristic of catechol17 groups; 1610, 1516, 1470, 1352, 1325, 1282, 1169, 962, 818, 747, and 547 cm-1 are characteristic of gallic acid groups;17 1610, 1372,
17 15 15 9, 17 9, 17 21 17 9 15 15 21 21
1000, and 809 are characteristic of m-hydroxybenzoic acid; and 1619, 1381, 1310, 1251, 1035, and 809 cm-1 are characteristic of salicylic acid groups.9 The micro-SERS spectra of the cast FA film on gold were recorded with the 50× objective permitting a spatial resolution of ∼1 µm2. The results are shown in Figure 8, where SERS spectra Analytical Chemistry, Vol. 76, No. 23, December 1, 2004
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Figure 8. Micro-SERS spectra of FA films cast from solutions at (a) pH 5, (b) 8, and (c) 11 on 9-nm Au gold island films. Laser excitation at 785 nm and spectra were collected from different spots as marked on the optical image.
recorded on distinct spots within one microstructure are shown. The SERS spectra of the FA cast at pH 5 (Figure 8a), and collected using the 785-nm laser line, show different relative intensities in comparison with the FA cast at pH 8 (Figure 8b). Changes in relative intensities may be due to the ionization of the carboxylic acids with pH, as is shown by the increase in the relative intensity of the bands from 1390 to 1300 cm-1, which can be assigned to vs(COO-), and of the 640-cm-1 band assigned to an in plane deformation of the COO- group. Another important difference between the spectra collected at pH 5 and those collected at pH 8 is the strong increase in the intensity of the band at 890 cm-1, which may be due to benzene derivatives, likely substituted with carboxylic acid groups, which are ionized at this pH and interacting with the gold surface. 7124
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Assuming, from the standpoint of topological geometry,39 a micellar model for the FA aggregates (i.e., a micelle as a closed environment where the surface interactions are accessible and detectable by SERS), we can pursue a discussion of the observed spectra and the relative intensities of the characteristic vibrational modes. The comparison of spectra collected from different locations on the films shows that the observed building block mix forming the fulvic is slightly different from spot to spot. Spectrum 1 (Figure 8b) shows higher intensity than the other spectra in its carboxylate bands (1390-1300 and 640 cm-1), and a weaker absorption at 1471 cm-1, probably due to δ(C-H) and ring stretching of the aromatic rings. Since spectrum 1 was collected (39) Turro, N. J.; Garcia-Garibay, M. In Photochemistry in Organized & Constrained Media; Ramamurthy, V., Ed.; VCH: New York, 1991; pp 1-38.
on the border between two different domains (Figure 8b), it is likely that the more acidic molecules would be located at the surface, while more apolar or aromatic ones would remain inside the micelle. At pH 11, the fulvic acid films tend to expand on the gold island surface forming a radial fractal pattern (Figure 8c),40 which have been previously reported on surfaces other than gold islands.41,42 The basic driving force behind these phenomena is most likely an increase in the intermolecular electrostatic repulsion between different fulvic aggregates, and correspondingly in the SERS spectra, one can see in the intensity increase of the bands assigned to vS(COO-) ∼1608 cm-1, vA(COO-) ∼1369 cm-1, and δ(COO-) ∼641 cm-1. On the other hand, the displacement of both symmetric and antisymmetric stretches probably indicates that due to the great number of carboxylic groups it is likely that once the thiol groups are saturated some of these acidic groups will coordinate Au as Vogel et al. suggested.21 The films are composed of many FA units tightly bound at the molecular level, due to condensation or polymerization phenomena. SERS spectra collected from different spots of the fractal structures (Figure 8c) show slight differences with one another. The border FA shows a more prominent carboxylic functionality (spots 1 and 4), while the spectra “inside” the structure seems to show more aromatic content (spots 2 and 3). Both alkaline pH spectra (pH 8 and 11) show bands due to the same building blocks as those at pH 5, catechol, gallic acid, m-hydroxybenzoic acid, and salicylic acid groups, but with (40) Van Damme, H. In The fractal approach to heterogeneous chemistry: Surfaces, colloids and polymers; Avnir, D., Ed.; John Wiley & Sons: Chichester, U.K., 1989. (41) Senesi, N. Soil Sci. 1999, 164, 841-856. (42) Ren, S. Z. T. E.; Rice, J. A. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 1996, 53, 2980-2983.
different intensities because of the ionization of the carboxylic acid groups and because of the overlapping of some bands with the carboxylate stretching. CONCLUSIONS Films deposited onto glass and gold island films illustrate the formation of fractal geometries in FA films and their dependence on the pH of the stock solution. The SEIRA and SERS spectra of FA films on gold islands are reported here for the first time. Films cast from a stock solution at pH 8 give rise to a fractal surface that can be probed using micro-SERS, with spatial resolution of ∼1 µm2. The SERS spectra of these domains can provide information about the building blocks on the surface of the micellar structure. Using stock solutions at pH 11, the fractal geometry gives rise to radial pattern structures. The information extracted from the spatially resolved micro-SERS spectra of the fractal patterns formed from solutions at pH 11 and pH 8 are in full agreement and suggest that carboxylic groups predominate on the surface of FA micelles, at the edges of the fractal. Hence, the analytical application of surface-enhanced vibrational spectroscopy (SEIRA + SERS) to probe supramolecular structures such as those found in FA films has been demonstrated. As well, the door has been opened for future applications in structural analysis of FA, humic, or other supramolecular films using the powerful combination of microscopy and spatially resolved SERS spectroscopy.
Received for review June 24, 2004. Accepted September 20, 2004. AC049076U
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