surface-enhanced vibrational microspectroscopy of fulvic acid micelles

Surface-Enhanced Vibrational Microspectroscopyof Fulvic Acid Micelles

Ramon 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 Arrosadıa, E-31006 Pamplona, Spain

Micro-Raman spectroscopy, infrared absorption microspec-troscopy, and AFM images of nano- or microsized micellesformed by fulvic acid (FA) solutions, prepared at differentpHs, and cast on glass slides or gold island films, arereported. FA films cast on gold islands are characterizedby surface-enhanced infrared absorption (SEIRA), surface-enhanced infrared reflection absorption, and surface-enhanced Raman scattering (SERS). Based on spectralevidence, it is expected that the chemisorption of FA ongold island films takes place through thiol groups, whichbecome more active as pH increases. The SEIRA spectraof these films show increased peak intensity, as well asimproved band resolution. Microspectroscopy SERS stud-ies show that, at pH 5, FA form small aggregates on goldsurfaces. At pH 8, FA tends to expand due to electrostaticrepulsion, giving rise to a fractal surface composed ofdifferent domains. SERS studies of these domains revealthat the most polar molecules are located on the externalfaces. At pH 11, fractal conformations are even morepronounced and give rise to radial patterned structures.At this pH, the position of fulvic acid molecules in thefractal micelles is the same as observed at pH 8. In thisway, SERS can be viewed as a powerful tool for theanalysis of the composition, apparent contribution of thesurface functional groups of FA films, and the FA buildingblocks (i.e., catechol, gallic, salicylic, or ftalic acids) inthe structures of these materials.

Fulvic acids (FA) are natural macromolecules that can be foundubiquitously in nature, although, their origins are not well defined.They are macromolecular products derived from the humificationof organic molecules from plants, animals, microorganisms, andtheir metabolic products,1,2 FA can affect soil fertility, mineralweathering, and water acidity. They are also involved in thetransport, sequestration, and mitigation of contaminants, impactingatmospheric chemistry through the carbon cycle, in which carbonis constantly recycled among plants, animals, soil, air, and water.1

FA have a great tendency for adsorption onto mineral surfaces(i.e., clays, metals, metal oxides, etc.), modifying their chemicalbehavior.3,4 Notably, for a large range of environmental particlesand matrixes, fractal geometries have been shown, includinghumic acids, soot, mineral surfaces, biological aggregates, andaerosols.5 Systematic studies of the structures of these substancescontribute to the understanding of the role of their interactionswith other elements and compounds. Such knowledge is necessaryto predict and control the impact of chemical and biologicalchanges in the environment. To gain more insight into the preciserole, function, and structure of FA, there has been much effort inrecent years directed toward the determination of the chemicalstructures and geometric conformations of fulvic macromoleculesand aggregates.6-8 These previous studies have been carried outon the bulk materials or in solution. In the present work, weinvestigate thin solid films of FA to study the structure andorganization of the aggregation. Thin solid film studies permitthe collection of information on the structure that these materialsmay form when coating mineral surfaces.9 Given the nature ofthese materials, the use of thin films decreases the fluorescencecontribution that can interfere with signal collection in Ramanspectroscopy. The published work on humic substances as thinfilms has been primarily morphological, employing atomic forcemicroscopy (AFM).10,11 It is demonstrated here that FA associateto form nano- and microsized micelles with fractal geometries,dependent upon the pH value of the FA solution cast onto glassslides or gold island films. Spectroscopic characterization isachieved using micro-Raman and infrared absorption microspec-troscopy, and the enhanced results obtained in surface-enhancedvibrational spectroscopy experiments allow for the extraction ofinformation on both the structure and organization of thin solid

* To whom correspondence should be addressed. E-mail: [email protected].

† University of Windsor.‡ Universidad Publica de Navarra.

(1) Stevenson, F. J. Humus chemistry: Genesis, composition and reactions; JohnWiley & 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; pp1011-1069.

(3) Alvarez-Puebla, R. A.; Valenzuela-Calahorro, C.; Garrido, J. J. Langmuir2004, 20, 3657-3664.

(4) Alvarez-Puebla, R. A.; Valenzuela-Calahorro, C.; Garrido, J. J. J. ColloidInterface 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 Societyof 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.

Anal. Chem. 2004, 76, 7118-7125

7118 Analytical Chemistry, Vol. 76, No. 23, December 1, 2004 10.1021/ac049076u CCC: $27.50 © 2004 American Chemical SocietyPublished on Web 11/04/2004

films.12,13 Previously published work on the surface-enhancedRaman scattering (SERS) of humic substances has been carriedout on colloids,14-17 electrodes,18-21 and silver island films.21-23 Thesurface-enhanced infrared absorption (SEIRA)24,25 and surface-enhanced infrared reflection absorption (SEIRRA)26 of FA, re-ported here for the first time, give rise to better resolved IR spectrafor characterization of surface functional groups under conditionsdifferent from normal infrared absorption measurements. TheSERS spectra of humic substances previously reported14,17,21,27 wereobtained using 514.5- or 647-nm excitation on silver surfaces. Inthe present work, we have used several laser lines (442, 514.5,633, and 785 nm), and it was found that the best SERS resultswere obtained with 785-nm excitation on gold island films. Inaddition, the high spatial resolution of Raman microscopy (1 µm2)makes it possible to probe directly different sections of fractal-like structures, such as those of FA films.

EXPERIMENTAL SECTIONFA from a commercial humic substance by Acros Organics

(Geel, Belgium) was fractionated by adjusting the pH of a 40 gL-1 HS solution to 1.0. The FA was then purified using a XAD-8resin column, converted to the protonated form by passing itthrough a proton-saturated resin, and then freeze-dried, in ac-cordance with the procedure proposed by the International HumicSubstance Society (IHSS).2 C, H, N, and S contents weredetermined by elemental analysis (CHNS EA1108-by Carlo Erba,Milan, Italy). Carboxylic acidic groups and total acidity of FA weredetermined by calcium acetate and barium hydroxide methods,1

respectively. Phenolic acidic groups were calculated from thedifference between the total acidity and that from the carboxylicacidic groups. The results of the characterization of the FA arelisted below in Table 1 and are in agreement with what is expectedfor these kind of substances.1,2

Gold island films of 9-nm mass thickness were prepared in aBalzers BSV 080 glow discharge evaporation unit. The metal filmswere deposited on preheated (200 °C) glass (7059 Corning) andKRS-5 (Aldrich) slides. During film deposition, the backgroundpressure was maintained at ∼10-6 Torr, and the deposition rate(0.5 Å s-1) was monitored using an XTC Inficon quartz crystaloscillator. FA films were prepared by casting 10 µL of the FA

solutions (0.2500 g L-1) at pH values of 5, 8, and 11, on glass orgold island films. UV-visible extinction spectra (300-1100 nm)of the gold island films and FA films were recorded with a VarianCary 50 UV-visible spectrophotometer. Transmission SEIRAspectra were obtained on KRS-5 windows. SEIRRA on glass slideswas obtained using a Bruker Equinox 55 equipped with amicroscope (Bruker Hyperion 3000). The enhancement factor forSEIRA was calculated by comparing the measured intensities ofthe transmission spectrum of FA on gold island film on KRS-5windows with that recorded from FA film on KRS-5. Raman andSERS spectra were collected with either a Renishaw ResearchRamanscope 2000 or a Renishaw Invia system, both equipped withPeltier CCD detectors and Leica microscopes. The spectrographsuse 1800 g/mm gratings with additional band-pass filter optics.Excitation lines of 442, 514.5, 633, and 785 nm were used. Spectrawere collected in Renishaw’s continuous collection mode withaccumulation times of 10 s and five spectra being coadded in eachexperiment.

AFM (Digital Instruments NanoScope IV) topographical mea-surements were performed in tapping mode with a siliconcantilever (NSC 14 model, Ultrasharp) operating at a resonantfrequency of 256 kHz. Images were collected with high resolution(512 lines/sample) at a scan rate of 0.5 Hz. The data were collectedunder ambient conditions, and each scan was replicated to ensurethat any features observed were reproducible.

RESULTS AND DISCUSSIONFA 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. Thesolutions prepared with controlled pH values of 8 and 11 formedfilms with fractal-like structures, as can be seen in Figure 1. Filmscast from the pH 5 solution show aggregation but not theorganization found for the solutions at the other two pH values.The patterns of these microheterogeneous systems are reminis-cent of the ones formed by gold and silver colloids cast onto glasssubstrates.28 Figure 2 shows the transmission infrared spectra ofFA in KBr and that of a FA film cast from a FA solution at pH 11on KRS-5 window recorded with a 15× microscope objective. Thespectrum of the bulk, which has been previously reported,29,30 isslightly different from that of the film, which is better resolved.Broad and high-intensity bands that characterized the spectrum

(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.

(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, 135-

140.

Table 1. Selected Physical and Chemical Properties ofthe FA Employed

C (%) 40.1 E4/E6 5.76H (%) 3.57 <Mw> (g mol-1)a 1058N (%) 0.67 Cstrong acidic groups (mol kg-1) 5.71S (%) 0.65 Cweak acidic groups (mol kg-1) 2.73ash b pK 3.41O (%) 55.0 pK 9.39

a Average molecular weight estimated from Mw ) 3.99ε280 + 490according to Chin et al.39 b Not detected.

Analytical Chemistry, Vol. 76, No. 23, December 1, 2004 7119

of bulk humic substances are explained by the overlapping ofclosely related functional groups.31 In addition, bulk humicsubstances are hygroscopic and the trapped water moleculescontribute 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 surfacestudied using IR microscopy ranges from 500 × 500 to 50 × 50

µm2 with an average height in the 0.2-0.5-µm range. The infraredspectra of the thin solid films may be seen as a small organizedcross section of the bulk with a different distribution of functionalgroups when compared with the spectrum of the bulk. In addition,the apparent decrease of trapped water substantially reduces therelative intensity of the OH band.

Plasmon Absorption and Topography of Gold and Gold-Coated films. FA solutions were cast onto gold island films toexplore SEIRA as a sensitive technique for the structural char-acterization of these films. The surface plasmon absorption of Auisland film and those of coated gold film with FA are shown inFigure 3. The plasmon absorption32,33 centered at 702 nm in theneat gold film is affected by the dielectric constant of the FAcoating film and is displaced to lower wavelength. Increases inthe pH of the FA solution correlates with a decrease in thewavelength of the absorption maximums and the relative intensityof these maximums. A dielectric coating changes the plasmonabsorption of metal particles,34,35 and the differences reflect thequalitatively different nature of the films formed from solutionswith varying pH values. It is also possible that the nature of themolecule-metal interactions may be different, for instance, in theextent of chemical adsorption, or the coordination of gold withthe FA. This coordination likely involves thiol groups, whichbecome more active as pH increases. Whatever the interaction,the trend in the formation of fractal-like structure observed in theneat films is also seen in the films cast onto gold island films ascan be seen in Figure 4. This figure shows optical images obtainedwith a 20× and 50× microscope objectives for gold and gold-fulvic films formed from stock solutions at a pH 5, 8, and 11. Whilegold film appears to have a quite homogeneous surface (Figure4a), the gold-fulvic film at pH 5 shows aggregates randomlydistributed throughout the whole surface (Figure 4b). Gold-fulvicfilm at pH 8 and 11 (Figure 4c and d) clearly show the innerstructure of the fractal-like pattern. The AFM image (Figure 5a)shows a gold film composed by gold islands with a size that rangesfrom 20 to 50 nm and a height between 15 and 40 nm. The root-mean-square roughness of the film is 2.71 nm. The averageinterisland space is 20 nm. Fulvic film coated at pH 5 (Figure 5b)

(31) Bruccoleri, A. G.; Sorenson, B. T.; Langford, C. H. In Humic substances:Structures, models and functions; Gabbour, E. A., Davies, G., Eds.; The RoyalSociety of Chemistry: Cambridge, U.K., 2001; pp 193-208.

(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 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 inKBr and from a film cast on KRS-5 at pH 11 (b). The absorbancescale is only applicable for the bottom spectrum.

Figure 3. Plasmon absorption of a neat Au island film and goldfilms coated with FA solutions prepared at pH 5, 8, and 11.

7120 Analytical Chemistry, Vol. 76, No. 23, December 1, 2004

shows globular domains ranging between 0.2 and 1.3 µm. Filmscoated at pH 8 (Figure 5c) and pH 11 (Figure 5d) show thebranches of the fractal structure observed on the sample by opticalmicroscopy. The value for the roughness degrease from 41.4 nmat pH 5 to 30.2 nm at pH 8 and 21.4 nm at 11, which indicatesthat the fulvic supramolecular structure spreads on the surfacewith the increase of the pH may be due to the increase in theelectrostatic repulsion.

SEIRA and SEIRRA. SEIRA25 on gold island films is observedand is illustrated in Figure 6, where the transmission infraredspectrum of a FA film on KRS-5 windows is overlaid with that ofan FA at pH 11, cast on gold islands that had been evaporated onthe KRS-5 substrate. The neat film spectrum, multiplied by a factorof 6, is also shown using a dash line. It can be seen that the SEIRAspectrum shows enhancement of the main band in the originalspectrum, along with some additional features, such as themaximum seen at 1297 cm-1. Thus, an average enhancementfactor of 6 can be calculated for this particular observation. TheSEIRA spectrum not only shows an increased intensity of thebands (which experimentally can be translated into a decrease inthe number of scans necessary to obtain a quality IR spectrumand, consequently, to a decrease in experimental time) but alsoan increase in the number of characteristic vibrational bands thatcan be observed in the IR spectrum. The SEIRRA spectra collectedfor all three FA samples are shown in Figure 7, where the mostcharacteristic infrared bands are detected.36 They are due to O-H

stretching (3378 cm-1), aliphatic C-H stretching (2939 and 2850cm-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 (1427cm-1), symmetric COO- stretching (1297 cm-1), C-O stretching(1266 cm-1), and out-of-plane deformations of aromatic C-Hgroups (879 and 842 cm-1).3,37 Experimentally, SEIRRA spectraof various FA films on gold were recorded with a microscopeattachment with a fixed incident angle in reflection geometry. FAfilms at different pHs were fabricated on Au islands evaporatedonto glass slides and spectra, and images were obtained using a15× objective. The results are shown in Figure 7, together withthe images obtained from the section probed by micro-FT-IR. TheSEIRRA spectra show the same behavior with variation of pH thathave been shown in previous FT-IR work9,15 but with an improvedspectral resolution. The SEIRRA of FA films at pH 5 show strongbands due to v(O-H), ∼3500 cm-1, v(CdO), 1767 cm-1, aliphaticC-H bending and polymeric aromatic rings, 1427 cm-1, thesymmetric vs(COO-) at ∼1300 cm-1, and v(C-O) of aliphaticalcohols and ethers at 1151 cm-1. The aromatic waggingω(C-H) is also seen at 802 cm-1. As pH increases, the intensityof the bands due to carboxylic acids v(O-H) at ∼3500 cm-1, andv(CdO) at ∼1767 cm-1, decreases, while the intensity of theantisymmetric vas and symmetric vs stretching modes of COO-,at ∼1620- and ∼1350-cm-1 bands, respectively, increases. Theincrease in the ionic forms likely lead to the formation of “ionic

(36) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; GRaselli, J. G. The Handbook ofInfrared and Raman Characteristic Frequencies of Organic Molecules;Academic Press: San Diego, 1991.

(37) Alvarez-Puebla, R. A.; Valenzuela-Calahorro, C.; Garrido, J. J. J. ColloidInterface Sci. 2004, 270, 47-55.

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).


micelles”, where the repulsion between the different fulvicmacromolecules gives rise to expanded fractal-like structures (seeFigures 4 and 5). This fractal behavior is not induced by theAu-FA interaction, since it is also present when FA is cast on aglass surface, as shown in Figure 1.

Raman Microscopy and SERS. The spontaneous Ramanscattering of FA, in both bulk and film form, is hampered by strongfluorescence. The Raman spectra obtained with 633- and 785-nmexcitation is typically weak scattering on a strong fluorescencebackground, consistent with the results obtained by Vogel et al.21

with 647-nm laser excitation. The Raman spectra are poorlyresolved, with bands similar to those reported by Sanchez-Corteset al.9 Raman spectra were recorded with several laser lines (442,488, 514, 633, and 785 nm) in the search for optimal experimentalvariables. For example, the Raman spectra recorded using 442-nm excitation show two broad bands (∼1600 and ∼1400 cm-1)characteristic of low-structure graphite-like carbons.21,38 SERS ongold islands was attained using excitation lines at 633 and 785nm. However, the best results were obtained using the 785-nmlaser line, and only these results will be discussed.

To facilitate the discussion, the most characteristic groupfrequencies 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 6. (a) FT-IR spectrum of FA film on glass slide (dashed lineshows spectrum multiplied by a factor of 6) and (b) SEIRA spectrumof FA on an Au island film.

Figure 5. AFM height images of (a) gold and gold-fulvic films at pH (b) 5, (c) 8, and (d) 11.


been collected in Table 2. The table is constructed to include thecharacteristic vibrational modes that allow for the identificationof 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,

1000, and 809 are characteristic of m-hydroxybenzoic acid; and1619, 1381, 1310, 1251, 1035, and 809 cm-1 are characteristic ofsalicylic acid groups.9

The micro-SERS spectra of the cast FA film on gold wererecorded with the 50× objective permitting a spatial resolution of∼1 µm2. The results are shown in Figure 8, where SERS spectra

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 filmwhere spectra were collected.

Table 2. Characteristic SERS Bands for Humic Substances

wavenumber (cm-1) assignment ref

3240 ν(N-H) of bonded amines 212956, 2885 ν(C-H) 212135 ν(CtC) or accumulated v(CdC) or a vibrational mode of -NH3

+ 211700-1650 ν (CdO) 15, 361618, 1590 νA (COO-) and benzene ring stretching 9, 17∼1580 aromatic ring stretching vibrations in plane stretching

vibrations of highly substituted phenols17

1574, 1475, 498 polymeric benzene rings 171540 ring stretching vibrations of aromatic moieties 151450 δ (C-H2) aliphatic 151379 νS (COO-) 9, 171315 νS (COO-) and benzene substituted ring 9, 171300-1000 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 molecule21

1211 ν (C-O) ether 171171 δ (C-H) 91165 ν (C-O) alcohol and aliphatic ethers 151060 ν (C-C) 15700-400 in-plane deformation of the -COO- group and torsional

motion of a -NH3+ moiety

21

365 skeletal deformation mode 21


recorded on distinct spots within one microstructure are shown.The SERS spectra of the FA cast at pH 5 (Figure 8a), and collectedusing the 785-nm laser line, show different relative intensities incomparison with the FA cast at pH 8 (Figure 8b). Changes inrelative intensities may be due to the ionization of the carboxylicacids with pH, as is shown by the increase in the relative intensityof the bands from 1390 to 1300 cm-1, which can be assigned tovs(COO-), and of the 640-cm-1 band assigned to an in planedeformation of the COO- group. Another important differencebetween the spectra collected at pH 5 and those collected at pH8 is the strong increase in the intensity of the band at 890 cm-1,which may be due to benzene derivatives, likely substituted withcarboxylic acid groups, which are ionized at this pH and interact-ing with the gold surface.

Assuming, from the standpoint of topological geometry,39 amicellar model for the FA aggregates (i.e., a micelle as a closedenvironment where the surface interactions are accessible anddetectable by SERS), we can pursue a discussion of the observedspectra and the relative intensities of the characteristic vibrationalmodes. The comparison of spectra collected from differentlocations on the films shows that the observed building block mixforming the fulvic is slightly different from spot to spot. Spectrum1 (Figure 8b) shows higher intensity than the other spectra in itscarboxylate bands (1390-1300 and 640 cm-1), and a weakerabsorption at 1471 cm-1, probably due to δ(C-H) and ringstretching of the aromatic rings. Since spectrum 1 was collected

(39) Turro, N. J.; Garcia-Garibay, M. In Photochemistry in Organized & ConstrainedMedia; Ramamurthy, V., Ed.; VCH: New York, 1991; pp 1-38.

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 at785 nm and spectra were collected from different spots as marked on the optical image.


on the border between two different domains (Figure 8b), it islikely that the more acidic molecules would be located at thesurface, while more apolar or aromatic ones would remain insidethe micelle.

At pH 11, the fulvic acid films tend to expand on the gold islandsurface forming a radial fractal pattern (Figure 8c),40 which havebeen previously reported on surfaces other than gold islands.41,42

The basic driving force behind these phenomena is most likelyan increase in the intermolecular electrostatic repulsion betweendifferent fulvic aggregates, and correspondingly in the SERSspectra, one can see in the intensity increase of the bands assignedto vS(COO-) ∼1608 cm-1, vA(COO-) ∼1369 cm-1, and δ(COO-)∼641 cm-1. On the other hand, the displacement of bothsymmetric and antisymmetric stretches probably indicates thatdue to the great number of carboxylic groups it is likely that oncethe thiol groups are saturated some of these acidic groups willcoordinate Au as Vogel et al. suggested.21 The films are composedof many FA units tightly bound at the molecular level, due tocondensation or polymerization phenomena. SERS spectra col-lected from different spots of the fractal structures (Figure 8c)show slight differences with one another. The border FA showsa more prominent carboxylic functionality (spots 1 and 4), whilethe spectra “inside” the structure seems to show more aromaticcontent (spots 2 and 3).

Both alkaline pH spectra (pH 8 and 11) show bands due tothe same building blocks as those at pH 5, catechol, gallic acid,m-hydroxybenzoic acid, and salicylic acid groups, but with

different intensities because of the ionization of the carboxylicacid groups and because of the overlapping of some bands withthe carboxylate stretching.

CONCLUSIONS

Films deposited onto glass and gold island films illustrate theformation of fractal geometries in FA films and their dependenceon the pH of the stock solution. The SEIRA and SERS spectra ofFA films on gold islands are reported here for the first time. Filmscast from a stock solution at pH 8 give rise to a fractal surfacethat can be probed using micro-SERS, with spatial resolution of∼1 µm2. The SERS spectra of these domains can provideinformation about the building blocks on the surface of themicellar structure. Using stock solutions at pH 11, the fractalgeometry gives rise to radial pattern structures. The informationextracted from the spatially resolved micro-SERS spectra of thefractal patterns formed from solutions at pH 11 and pH 8 are infull agreement and suggest that carboxylic groups predominateon the surface of FA micelles, at the edges of the fractal. Hence,the analytical application of surface-enhanced vibrational spec-troscopy (SEIRA + SERS) to probe supramolecular structuressuch as those found in FA films has been demonstrated. As well,the door has been opened for future applications in structuralanalysis of FA, humic, or other supramolecular films using thepowerful combination of microscopy and spatially resolved SERSspectroscopy.

Received for review June 24, 2004. Accepted September20, 2004.

AC049076U

(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.


surface-enhanced vibrational microspectroscopy of fulvic acid micelles

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