Lignin from Sugar Cane Bagasse: Extraction, Fabrication of

In addition, FTIR analysis showed that this lignin LB film is structured in a way ...... Barros, A. M.; Dhanabalan, A.; Constantino, C. J. L.; Balogh,...
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Langmuir 2007, 23, 6652-6659

Lignin from Sugar Cane Bagasse: Extraction, Fabrication of Nanostructured Films, and Application A. A. Pereira,† G. F. Martins,† P. A. Antunes,‡ R. Conrrado,† D. Pasquini,§ A. E. Job,† A. A. S. Curvelo,‡ M. Ferreira,† A. Riul, Jr.,| and C. J. L. Constantino*,† DFQB, Faculdade de Cieˆ ncias e Tecnologia, UNESP, Presidente Prudente, 19060-900, SP, Brazil, Faculdade de Cieˆ ncias, Letras e Educac¸ a˜ o, UNOESTE, Presidente Prudente, 19050-900, SP, Brazil, Instituto de Quı´mica de Sa˜ o Carlos, USP, Sa˜ o Carlos, SP, 13560-970, Brazil, and UniVersidade Federal de Sa˜ o Carlos, campus Sorocaba, 18043-970, SP, Brazil ReceiVed December 11, 2006. In Final Form: March 27, 2007 Four lignin samples were extracted from sugar cane bagasse using four different alcohols (methanol, ethanol, n-propanol, and 1-butanol) via the organosolv-CO2 supercritical pulping process. Langmuir films were characterized by surface pressure vs mean molecular area (Π-A) isotherms to exploit information at the molecular level carrying out stability tests, cycles of compression/expansion (hysteresis), subphase temperature variations, and metallic ions dissolved into the water subphase at different concentrations. Briefly, it was observed that these lignins are relatively stable on the water surface when compared to those obtained via different extraction processes. Besides, the Π-A isotherms are shifted to smaller molecular areas at higher subphase temperatures and to larger molecular areas when the metallic ions are dissolved into the subphase. The results are related to the formation of stable aggregates (domains) onto the water subphase by these lignins, as shown in the Π-A isotherms. It was found as well that the most stable lignin monolayer onto the water subphase is that extracted with 1-butanol. Homogeneous Langmuir-Blodgett (LB) films of this lignin could be produced as confirmed by UV-vis absorption spectroscopy and the cumulative transfer parameter. In addition, FTIR analysis showed that this lignin LB film is structured in a way that the phenyl groups are organized preferentially parallel to the substrate surface. Further, these LB films were deposited onto gold interdigitated electrodes and ITO and applied in studies involving the detection of Cd+2 ions in aqueous solutions at low concentration levels throughimpedance spectroscopy and electrochemical measurements. FTIR spectroscopy was carried out before and after soaking the thin films into Cd+2 aqueous solutions, revealing a possible physical interaction between the lignin phenyl groups and the heavy metal ions. The importance of using nanostructured systems is demonstrated as well by comparing both LB and cast films.

Introduction Lignins are among the most abundant macromolecules in plant tissues, formed by phenylpropane units (C9 units) presenting different chemical groups linked in their structure according to the plant species, whose morphology might be bi- or threedimensional, also depending on the plant tissue.1 The extraction process is another parameter that interferes strongly in the structure of lignins.2,3 However, all this renewable biomass resource is wasted from papermaking and sugar production processes, being simply burnt and used as an energy source. For the industrial processes (Kraft and Soda), the burning step is of fundamental importance to recovery the inorganic chemicals employed in the pulping. In the organosolv process, the exclusive utilization of organic solvent/water mixtures eliminates the need to burn the liquor and allows the isolation of the lignins (by distillation of the organic solvent). In organosolv-CO2 pulping, as employed in this work, an additional advantage is the utilization of pressurized (liquid) carbon dioxide as an important part of the pulping liquor (50% alcohol/water mixture and 50% carbon dioxide). This process combines the utilization of a lower amount of organic solvent and facilitates the lignin recovery, by the * Corresponding author: [email protected]. † UNESP. ‡ UNOESTE. § USP. | Universidade Federal de Sa ˜ o Carlos. (1) Goring, D. A. I. ACS Symp. Ser. 1987, 397, Chapter I. (2) Fengel, D.; Wegener, G. Wood-Chemistry, Ultrastructure, Reactions; Walter de Gruyter: Berlin, 1984. (3) Sarkanen, K. V.; Ludwig, C. H. Lignins, Occurrence, Formation, Structure and Reactions; Wiley: New York, 1971.

Table 1. C9 Formula and Functional Group Contents Per C9 Unit lignin

C9 formula

CdOa

ML EL PL BL

C9H9.00O1.90(OCH3)0.66(OHphen.)0.69 C9H7.99O1.93(OCH3)0.72(OHphen.)0.59 C9H8.15O2.47(OCH3)0.72(OHphen.)0.58 C9H8.11O2.44(OCH3)0.73(OHphen.)0.64

2.16 1.75 2.02 3.64

a

Relative values obtained from FTIR spectra.

Table 2. Molecular Weight (Mn and Mw), Polidispersivity (Mw/Mn), Extrapolated Area (Aext), and Collapse Pressure (Πc)

ML EL PL BL

Mn g/mol

Mw g/mol

Mw/Mn

Aext Å2

Πc mN.m-1

1150 1180 1080 1060

1820 1970 1650 1590

1.58 1.67 1.53 1.50

63 93 50 42

30 32 30 33

release of pressure after pulping. This process produces pulp with lower strength properties but in similar yields and in shorter times when compared with the industrial processes. Various efforts have been made to better understand the structure and properties of these macromolecules to find nobler applications.4 In this sense, the fabrication of monomolecular films through the Langmuir technique5 has been used to exploit (4) Glasser, W.; Kelley, S. S. In Encyclopedia of Polymer Science and Engineering; Mark, H. F., Bikales, N. M., Overberger, C. G., Menges, G., Eds.; Wiley: New York, 1987; Vol. 8. (5) Petty, M. C. Langmuir-Blodgett Films - an Introduction; Cambridge University Press: Cambridge, 1996.

10.1021/la063582s CCC: $37.00 © 2007 American Chemical Society Published on Web 05/10/2007

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Figure 1. Π-A isotherms recorded for ML, EL, PL, and BL. The inset shows the Mw vs Aext for each lignin.

both the molecular structure and the spatial arrangement of the lignins.6-13 On the other hand, the fabrication of LangmuirBlodgett (LB) films of lignins is not a straightforward procedure, and few papers are reported in the literature.14-17 Mixed LB films of lignins and fatty acids are an alternative for that.15,18 The fabrication of ultrathin film using other methods such as layerby-layer19 or spin-coating techniques20 has been performed as well. In this work, the main goal in producing LB films is not only fundamental studies concerning the lignin molecular structure but also their use involving new technological applications such as transducer materials in a sensing device. To illustrate, LB films provide a lower dispersion of the data and better distinction of tastes in an electronic tongue system 3 orders of magnitude (6) Luner, P.; Kempf, U. Tappi 1970, 53, 2069&. (7) Chakrabarty, R. R.; Neuman, R. D.; Sarkanen, S. Proceedings of the International Symposium On Wood and Pulping Chemistry; Vancouver, BC, 1985; p 35. (8) Luner, P.; Roseman, G. Holzforschung 1986, 40, 61-66. (9) Gilardi, G.; Cass, A. E. G. Langmuir 1993, 9, 1721-1726. (10) Baumberger, S.; Aguie-Beghin, V.; Douillard, R.; Lapierre, C.; Monties, B. Ind. Crops Prod. 1997, 6, 259-263. (11) Oliveira, O. N., Jr.; Constantino, C. J. L.; Balogh, D. T.; Curvelo, A. A. S. Cellul. Chem. Technol. 1994, 28, 541-549. (12) Barros, A. M.; Dhanabalan, A.; Constantino, C. J. L.; Balogh, D. T.; Neto, C. P.; Oliveira, O. N., Jr. Thin Solid Films 1999, 354, 215-221. (13) Gundersen, S. A.; Ese, M. H.; Sjo¨blom, J. Colloids Surf., A 2001, 182, 199-218. (14) Constantino, C. J. L.; Juliani, L. P.; Botaro, V. R.; Balogh, D. T.; Pereira, M. R.; Ticianelli, E. A.; Curvelo, A. A. S.; Oliveira, O. N., Jr. Thin Solid Films 1996, 284, 191-194. (15) Constantino, C. J. L.; Dhanabalan, A.; Cotta, M. A.; Pereira-Da-Silva, M. A.; Curvelo, A. A. S.; Oliveira, O. N., Jr. Holzforschung 2000, 54, 55-60. (16) Pasquini, D.; Balogh, D. T;. Antunes, P. A.; Constantino, C. J. L.; Curvelo, A. A. S.; Aroca, R. F.; Oliveira, O. N., Jr. Langmuir 2002, 18, 6593-6596. (17) Pasquini, D.; Balogh, D. T.; Oliveira, O. N., Jr.; Curvelo, A. A. S. Colloids Surf., A 2005, 252, 193-200. (18) Constantino, C. J. L.; Dhanabalan, A.; Curvelo, A. A. S.; Oliveira, O. N., Jr. Thin Solid Films 1998, 327-328, 47-51. (19) Paterno, L.; Constantino, C. J. L.; Oliveira, O. N., Jr.; Mattoso, L. H. C. Colloids Surf., B 2002, 23, 257-262. (20) Norgren, M.; Notley, S. M.; Majtnerova, A.; Gellerstedt, G. Langmuir 2006, 22, 1209-1214.

below the human threshold, when compared to cast films.21 Since lignins have a strong affinity to heavy metals,13,22-24 they can be favorably explored as a good transducer in the detection of heavy metal ions in aqueous solutions via e-tongue analysis.25-28 Such a combination might provide a faster assessment of pollutants in the environment, which is of vital importance nowadays in the monitoring of water. Four lignins obtained from sugar cane bagasse with the organosolv-CO2 supercritical pulping process using as extraction solvents methanol, ethanol, n-propanol, or 1-butanol were studied, since they may present different chemical characteristics from those extracted via the traditional pulping process. Langmuir films were produced and characterized by surface pressuremean molecular area (Π-A) isotherms considering stability tests, consecutive cycles of compression/expansion of the monolayers, subphase temperature variation, and the presence of Cd+2 metallic ions at different concentrations in the aqueous subphase, prior to the LB film fabrication. LB films were fully characterized by UV-vis absorption spectroscopy in order to check the growth and adhesion of the film onto the solid substrates, prior to sensor testing. Impedance spectroscopy and electrochemistry techniques were applied to the LB films transferred onto interdigitated electrodes and ITO, respectively, forming sensing units of an e-tongue device, which were dipped in liquid systems containing different concentrations of Cd+2 at different concentrations. FTIR (21) Ferreira, M.; Riul, A., Jr.; Wohnrath, K.; Fonseca, F. J.; Oliveira, O. N., Jr.; Mattoso, L. H. C. Anal. Chem. 2003, 75, 953-955. (22) Martin-Dupont, F.; Gloaguen, V.; Guilloton, M.; Granet, R.; Krausz, P. J. EnViron. Sci. Health, Part A 2006, 41, 149-160. (23) Celik, A.; Demirbas, A. Energy Sources 2005, 27, 1167-1177. (24) Dupont, L.; Bouanda, J.; Dumonceau, J.; Aplincourt, M. J. Colloid Interface Sci. 2003, 263, 35-41. (25) Riul, A., Jr.; Santos, D. S., Jr.; Wohnrath, K.; Di Tommazo, R.; Carvalho, A. C. P. L. F.; Fonseca, F. J.; Oliveira, O. N., Jr.; Taylor, D. M.; Mattoso, L. H. C. Langmuir 2002, 18, 239-245. (26) Santos, D. S., Jr.; Riul, A., Jr.; Malmegrim, R. R.; Josepetti, J. F.; Oliveira, O. N., Jr.; Mattoso, L. H. C. Macromol. Biosci. 2003, 3, 591-595. (27) Crespilho, F. N.; Zucolotto, V.; Siqueira, J. R.; Constantino, C. J. L.; Nart, F. C.; Oliveira, O. N., Jr. EnViron. Sci. Technol. 2005, 39, 5385-5389. (28) Antunes, P. A.; Santana, C. M.; Aroca, R. F.; Oliveira, O. N., Jr.; Constantino, C. J. L.; Riul, A., Jr. Synth. Met. 2005, 148, 21-24.

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Lignin Isolations. The lignins employed in this work were obtained in the organosolv-CO2 supercritical pulping process as described in Pasquini et al.29 The experiments were carried out in a 100 mL stainless steel reactor employing 10 g of sugar cane bagasse fibers and 50 mL of alcohol/water (3:1/v:v) as cosolvent, 150 bar of pressure, 170 °C, and 75 min of reaction time. The alcohols employed were methanol, ethanol, n-propanol, and 1-butanol. The

liquors obtained in the pulping processes were acidified with H2SO4 concentrated to pH ) 2, with the lignins precipitated, filtered, and dried. The four lignins obtained by employing different alcohols were named methanol lignin (ML), ethanol lignin (EL), n-propanol lignin (PL), and 1-butanol lignin (BL). Lignin Analysis. FTIR spectra were recorded from KBr pellets in a BOMEM MB102 spectrometer. C9 formula (phenylpropane units) was calculated on the basis of C, H, and O elemental analysis, methoxyl contents were determined by 13C NMR spectroscopy, and phenolic hydroxyl content determined by conductometric titration (Table 1). Relative carbonyl contents were estimated by dividing the area under the absorption peak at 1700 cm-1 (CdO) by that of the peak at 1500 cm-1 (aromatic ring), which is taken as reference (Table 1). Molecular weight distributions were determined by size exclusion chromatography (SEC) using three Plgel columns (500, 103 e 104 Å), THF as solvent at 1.0 mL‚min-1, and polystyrene standards (Table 2). These molecular features are reproducible considering different lignin isolations using the same process as well. Langmuir Films. The four lignins extracted from sugar cane were characterized through Langmuir films fabricated using a KSV 2000 trough. The Π-A isotherms were recorded using the Wilhelmy method, in which the plate was placed perpendicularly to the barriers to avoid displacements due to the rigidity of the lignin monolayer.30 The films were symmetrically compressed with a barrier speed of 10 mm‚min-1. The ultrapure water (18.2 MΩ‚cm) subphase from a Millipore Simplicity system was kept at 20 °C. All lignins were diluted in tetrahydrofuran (THF) whose concentrations and spread volumes onto the subphase were 0.80 mg‚mL-1 and 200 µL for ML; 0.20 mg‚mL-1 and 400 µL for EL; 0.86 mg‚mL-1 and 200 µL for PL; and 0.86 mg‚mL-1 and 200 µL for BL. The solutions were kept at 23 °C (room temperature) a couple of hours before starting the experiments, being slowly dropped onto the water subphase. A time interval of ca. 20 min was required for the solvent evaporation before starting compression of the films. LB Films of BL. The BL was transferred onto different substrates at constant surface pressure of 25 mN‚m-1, forming Y-type LB films. The dipping speed control along the deposition process is a key parameter to reach a transfer ratio close to 1. In our case, the dipping speed was varied from 1.0 to 4.0 mm‚min-1. UV-vis absorption spectra were recorded using a Varian spectrophotometer, model Cary 50, for 5, 9, 13, and 17 layers deposited onto quartz substrates. FTIR measurements were conducted in a Brucker spectrometer, model Vector 22, for both 30-layer LB film and cast film deposited onto ZnSe substrate in nitrogen atmosphere, with 64 scans, 4 cm-1 spectral resolution, and DTGS detector. The FTIR spectra were also recorded for the cast film before and after soaking in aqueous solution containing Cd+2 at 200.0 mg‚L-1. Impedance spectroscopy was carried out with a Solartron 1260A impedance/ gain-phase analyzer in the frequency range from 1 Hz to 1 MHz for 5-layer LB films deposited onto gold interdigitated electrodes (50 pair of digits, each having 10 µm width and 0.1 µm height, 10 µm apart from each other), which was left soaking in the aqueous solutions at 0.2, 2.0, and 20.0 mg‚L-1 of Cd+2 ions for 20 min prior impedance measurements. Five independent measurements were taken in all solutions to check reproducibility. Comparatively, electrochemical measurements were performed with an Autolab PGSTAT 30. The reference electrode was Hg/Hg2Cl2/KCl(sat.) (SCE); a 1.5 cm2 platinum foil was used as auxiliary electrode, while the working electrode was the 21-layer LB film of BL deposited onto ITO (1.0 cm2). The experiments were conducted in phosphate buffer solution (pH 7.0) at room temperature (23 °C). Cyclic voltammograms were carried out in the presence of Cd+2 at concentrations varying from 1.0 to 20.0 mg‚L-1 and sweeping ratio of 50.0 mV‚s-1. The bare ITO was tested in solution at 20.0 mg‚L-1 Cd+2. After the measurements, the films tested were exhaustively washed with the electrolytic solution to check the reproducibility. Tests of the sensor response to Cd+2 were carried out using cyclic voltammetry experiments.

(29) Pasquini, D.; Pimenta, M. T. B.; Ferreira, L. H.; Curvelo, A. A. S. J. Supercrit. Fluids 2005, 36, 31-39.

(30) Constantino, C. J. L.; Dhanabalan, A.; Oliveira, O. N., Jr. ReV. Sci. Instrum. 1999, 70, 3674-3680.

Figure 2. Π-A isotherms recorded for ML, PL, and BL with three consecutive cycles of compression and expansion.

spectroscopy was applied to determine both the molecular organization of the lignins in LB films and the metallic ion/ lignin interaction that made possible the detection of ions by the sensing units. Experimental Section

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Figure 3. Π-A isotherms recorded for ML, PL, and BL with the water subphase kept at 10, 20, and 30 °C.

Figure 4. Π-A isotherms recorded for BL with and without Cd+2 at 0.2, 2.0, and 20.0 mg‚L-1 into the water subphase.

Results and Discussion The Π-A isotherms recorded for all lignins are presented in Figure 1. The extrapolated area (Aext) determined by extending the condensed phase of the isotherm to the x-axis and the collapse pressure (Πc), which corresponds to the point where the ratio ∂Π/∂A decreases, are also shown in Figure 1, and their values presented in Table 2. It should be clear here that the collapse for these complex molecules is understood as a stacking of the molecules rather than the breakdown of the film after reaching its maximum packing.11 It can be seen in Figure 1 that the BL presents a well-defined condensed phase followed by PL, ML, and EL (i.e., the liquid-expanded phase goes in the opposite way), which might be directly related to the carbonyl content in the lignin molecular structure. The characterization of the four lignins through FTIR to estimate the relative contents of carbonyl

groups revealed that the carbonyl content increases in the following order: EL < PL ≈ ML < BL (Table 1). However, according to previous results related to other lignins,12 it was suggested that the higher content of polar groups would lead to a more expanded isotherm. In that case, the carbonyl groups, for instance, would have been lifted off from the water surface during the compression, leading to larger areas whose result is a more expanded isotherm. On the other hand, in a recent publication17 was found only a slight displacement of the Π-A isotherms for lignins also extracted from sugar cane bagasse but using different extraction methods. In that case, the content of carbonyl polar groups varied from 1.5 to 4.6, which is an interval greater than that in this work. Therefore, stability tests were carried out having in mind both the better comprehension of the role of the carbonyl groups in the lignin Langmuir films and the possibility of

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Figure 5. UV-vis absorption spectra for LB films of BL with 5, 9, 13, and 17. Inset: absorbance at 280 nm vs the number of deposited layers.

fabricating LB films using some of these lignins. The test consists basically of compressing the Langmuir film until a chosen surface pressure within the condensed phase is reached and then checking the displacement of the barriers over time to keep the surface pressure constant. Higher stability is attributed to monolayers presenting a lower decrease in area for a certain period of time. The results for the Langmuir films of the lignins and the percentage of decreased area versus time (figure not shown) revealed that for a period of ca. 2 h BL presents the lower percentage of decreased area, followed by PL, ML, and EL. This trend shows that the carbonyl groups might promote a better interaction between lignin and the water surface, leading to a more stable film. Therefore, it seems that the content of carbonyl groups might play a more important role in the stability of the Langmuir film on the water surface than in the shape of the Π-A isotherms (more expanded or condensed). In general, it can be concluded that these lignins present a good packing on the water surface, which is also supported by the high compressibility presented by them. In the same way, the reproducibility for Πc (figure not shown) related to other lignins12 indicates good stability of the materials tested on the water surface. In addition, by considering the similar Πc values, the increase of the Aext values with the molecular weight (inset in Figure 1), and the small polydispersity found for the lignins (Table 2), it could be speculated that they reach a similar arrangement in the condensed phase with the growth of the molecule occurring in two dimensions. A similar trend for Mw vs Aext has been found for other lignins,14 however, when submitted to fractionation process, which is not the case here. It must be mentioned that EL requires a more careful analysis, since its Π-A isotherms shift in terms of Aext and Πc, as shown by the arrows in Figure 1, considering several measurements recorded. The lignin-lignin intermolecular interaction was investigated by cycles of consecutive compression and expansion of the Langmuir films (results are illustrated in Figure 2) for three consecutive cycles. Strong hysteresis can be observed for all lignins, which shows that the molecules interact strongly during the compression of the film, leading to the formation of aggregates (domains). Moreover, the fact that for successive compression/ expansion cycles the Π-A isotherms start increasing at smaller areas during the compressions, but start decreasing in the same areas during the expansions, indicates that the aggregates formed during the first compression are stable, with the consecutive

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compressions leading to Langmuir films more and more packed. By comparing the three lignins, it was found that the decrease in the relative area given by (∆A/Aext × 100) is in the range 16-19% for all of them, where ∆A is the difference between the area at 10 mN‚m-1 for the third expansion and the area at 10 mN‚m-1 for the first compression. This is consistent with the conclusion above that these lignins might reach similar arrangements in the condensed phase on the water surface. Figure 3 presents the Π-A isotherms recorded for different temperatures of the water subphase: 10, 20, and 30 °C. The temperature might play two roles in these systems containing nonconventional amphiphilic molecules: increasing the mobility of the molecules, allowing a better packing on the water surface being helped by the π-π interactions, and consequently leading to smaller molecular areas, or improving the disorder of the system, leading to larger molecular areas.31 In our case, it seems that the first effect is predominant, producing a shift in the Π-A isotherms toward smaller areas for higher temperatures. By considering both the possibility of lignin molecules to complex with metallic ions13,22-24 and their application in devices, Π-A isotherms were recorded with the presence of Cd+2 in the water subphase at different concentrations (0.2, 2.0, and 20.0 mg‚L-1), as shown in Figure 4. BL was shown due to its higher stability on the water subphase. A trend of the Π-A isotherms being shifted to higher molecular areas for higher concentrations of Cd+2 in the subphase can be seen. This might be related to the electrostatic repulsion among the metallic ions when they are trapped within the lignin structure, leading to molecular rearrangements in the Langmuir films at the air/water interface. The later results revealed a good perspective of applying this lignin to detect heavy metal ions in aqueous solutions. Accordingly, LB films were produced in a controllable and reproducible way. Figure 5 shows the UV-vis absorption spectra for LB films containing 5, 9, 13, and 17 deposited layers of BL on quartz substrates. The inset presents the absorbance at 280 nm assigned to the phenyl groups.32 The linear growth of the absorbance indicates that the same amount of lignin is transferred per deposited layer. On top of that, the cumulative transfer monitored during the deposition process through the dipper speed indicates that the transfer of the film is homogeneous along the substrate surface. The cumulative transfer, defined as the ratio between the area displaced by the barriers to keep the surface pressure constant during the whole deposition process and the covered area of the substrate, was kept close to 1 during the LB film fabrication. The structure of the LB films of BL was then investigated at the molecular level using FTIR spectroscopy. It is known that the absorption of light by a molecule fixed in space depends on the coupling of the transition dipole to an external electromagnetic field. The probability for absorption is thus proportional to the square of the dipole moment matrix element along the direction Ej of light polarization.33,34 Crystals, adsorbates, or thin solid films may present spatial anisotropy introduced by molecular alignment. Therefore, the observed intensity of the allowed infrared modes can be modulated by a well-defined spatial orientation of the incident electric field (Ej). It means that allowed infrared modes of given symmetry species will be seen (31) Antunes, P. A.; Constantino, C. J. L.; Aroca, R. F. Langmuir 2001, 17, 2958-2964. (32) Lin, S. Y.; Dence, C. W. Methods in Lignin Chemistry; Springer-Verlag: Berlin, 1992. (33) Antunes, P. A.; Constantino, C. J. L.; Duff, J.; Aroca, R. Appl. Spectrosc. 2001, 55, 1341-1346 and references therein. (34) Born, M.; Wolf, E. Principles of Optics, 5th ed.; Pergamon Press: Oxford, 1975.

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Figure 6. FTIR spectra for a BL 30-layer LB film and BL cast film under nitrogen atmosphere. Scheme 1. Molecular Organization of the BL Lignins in the LB Films

Table 3. Wavenumbers and Assignments for the Main FTIR Absorption Bands from the 30-Layer Lignin LB Film center cm-1

assignments

830 1034 1122 1220 1263 1458 1513 1598 1713

C-H aromatic bending (out-of-plane) C-H aromatic (bending in-plane) and C-O alcohol (bending) C-O-C ether (bending) C-O-C ether (bending) guaiacyl ring (bending) C-H (bending) aromatic ring (CdC stretching) aromatic ring (CdC stretching) CdO carbonyl (stretching)

with absorption intensity proportional to the square of the scalar product

Ej‚µ′ where Ej ) polarization of the vector E (electric field of the incident light) and µ′ or (∂µ/∂Q) ) dynamic dipole moment derivative. The corollary is that symmetry species of oriented molecules may be distinguished by the use of polarized radiation. Therefore, the use of FTIR spectroscopy allows one to identify anisotropy in molecular arrangements.33,34 For instance, in Figure 6 is presented the FTIR spectra for both cast and 30-layer LB films of BL using the transmission mode. The assignments of the main bands indicated in Figure 6 are given in Table 3.32 The concordance in terms of the wavenumbers for both shows that

the LB technique does not induce changes in the molecular structure of the lignin. However, differences in the relative intensities suggest that the BL presents distinct molecular organization in the films. The cast film is assumed to have a random molecular organization due to the way that it is produced. On the other hand, the LB technique can induce either a random or specific molecular organization leading to isotropic or anisotropic films, respectively. Therefore, taking into account both the fact that in the FTIR transmission mode the electric field lies at the surface plane of the substrate and the dominance of the aromatic ring in plane vibration (1598 cm-1) for the LB film, it is possible to conclude that this lignin is structured with the phenyl groups placed predominantly parallel to the substrate surface (Scheme 1). The weak relative intensity of the C-H aromatic bending out-of-plane (830 cm-1) in the FTIR transmission mode is additional evidence of this molecular organization once the dynamic dipole moment derivative related to this vibration mode is perpendicular to the plane of the benzene ring.33 This molecular organization is similar to that found in lignins extracted from sugar cane bagasse using the ethanol/water extraction process.16,17 In the latter, a perpendicular molecular orientation was found for lignins extracted from the same sugar cane bagasse, however, using a distinct method (acetone/water).16 The main difference found for these lignins that could affect their molecular organization when transferred from the water surface to the solid substrate was the content of carbonyl groups (2.1 for ethanol/water lignins and 1.6 for acetone/water lignins). Furthermore, in refs 16 and 17, it was also observed that the

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Figure 7. Impedance spectroscopy measurements for a BL 5-layer LB film immersed in Cd+2 aqueous solutions at different concentrations (0.2, 2.0, and 20.0 mg‚L-1).

higher the content of polar groups, the smoother the LB film surfaces, according to atomic force microscopy (AFM). In our case, the butanol/water lignins (BL) present the highest content of estimated carbonyl groups (3.64), consistent with the molecular organization found. Since nanostructured LB films of BL could be produced in a controllable way, five layers were deposited onto Au interdigitated electrodes forming a sensing unit, which was immersed in aqueous solutions containing Cd+2 at 0.2, 2.0, or 20.0 mg‚L-1. Figure 7 shows impedance spectroscopy data for Cd+2, with good reproducibility in five independent measurements. In a few words, at low frequencies the electrical response is dominated by double-layer effects; in an intermediate region (100 Hz to 10 kHz), it is ruled by a strong film/electrolyte interaction; and at higher frequencies, by the geometric capacitance following the equivalent circuit model proposed by Taylor et al.35 Ongoing studies including other water contaminants are being undertaken to detect the upper and lower limits of detection and to determine the dependence of the capacitance on the metal concentration used. Cyclic voltammetry experiments were performed for 21-layer LB films of BL deposited onto ITO soaked in aqueous solutions containing Cd+2 at 1.0, 2.5, 5.0, and 20.0 mg‚L-1 as shown in Figure 8. The voltammograms were obtained with a scan rate of 50.0 mV‚s-1 in phosphate buffer solution. It is readily seen that the LB films exhibit a different electrochemical response in comparison to the bare ITO in Cd+2 solution 20.0 mg‚L-1 (inset). The bare ITO substrate has no electrochemical response in the potential window used here using only phosphate buffer (results not show). The data suggest linear growth of the current with the concentration above 2.5 mg‚L-1. The lower limit is 1.0 mg‚L-1, and the upper limit was not tested, but the signal obtained by cyclic voltammetry is better with the increase of concentration of the Cd+2 ions. The oxidation/reduction peaks appear at ca. 0.20 and 0.07 V, respectively. In fact, the best signal is obtained at 20.0 mg‚L-1. All measurements were done with the same working electrode. On the basis of these results, it can be concluded that above 1.0 mg‚L-1 of Cd+2 the electrochemistry may be used as a complementary tool to impedance spectroscopy in sensing applications involving this lignin in LB films. (35) Taylor, D. M.; MacDonald, A. G. J. Phys. D: Appl. Phys. 1987, 20, 1277-1283.

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Figure 8. Cyclic voltammograms for a BL 21-layer LB film in phosphate buffer solution in the presence of Cd+2 at concentrations ranging from 1.0 to 20.0 mg‚L-1. Inset shows the bare ITO in the solution Cd+2 at 20.0 mg‚L-1. Scan rate: 50.0 mV‚s-1.

Figure 9. FTIR spectra for a BL cast film before and after 20 min immersed into 200.0 mg‚L-1 Cd+2 aqueous solution.

Finally, the interaction between both lignin and metallic ions was carried out with FTIR spectroscopy in transmission mode, measured before and after soaking a cast film in aqueous solutions containing Cd+2 at 200.0 mg‚L-1 for 20 min (Figure 9), which was the time used for the electrical measurements. A comparison among the FTIR spectra allowed the identification of the effect, in the vibrational parameters (frequency, band shape, and relative intensity), of possible interactions with the metallic ions. The main change observed was the inversion of the relative intensities of the bands at 1598 and 1713 cm-1 assigned to CdC aromatic ring stretching and CdO carbonyl stretching, respectively (Table 3). The spectral changes (relative intensities, basically) observed due to the presence of the metallic ion could be related to a physical interaction between the Cd+2 and the lignin. The cleavage of the lignin molecules could be another possibility. However, the reversibility presented in the electrical measurements (Figure 7) after successive washings of the electrode corroborates the physical interaction hypothesis. The interaction between lignin molecules and metallic ions has been reported elsewhere;13,22-24 however, specific interactions have not been discussed. The LB film has presented the same kind of variation. Its spectrum was not shown once the cast film FTIR spectrum presented a higher signal/noise ratio. However, the importance of using ultrathin films could be observed by comparing FTIR spectra of 30-layer LB and cast films before and after soaking them in a Cd+2 solution.

Lignin from Sugar Cane Bagasse

The spectral differences mentioned above were apparent in the 30-layer LB film after 20 min of soaking in 20.0 mg‚L-1 solution, while in the cast film, they appear only after soaking in 200.0 mg‚L-1 solution for the same period of time. Therefore, the observed changes in impedance and cyclic voltammetry were mostly due to the increase of the Cd+2 ions in solution, as the absence of specific interactions between lignin and Cd+2 ions in the FTIR spectra emphasizes the nonspecificity of the sensor.

Conclusions Lignins extracted from sugar cane bagasse using the organosolv-CO2 supercritical pulping process have been characterized by size exclusion chromatography; C, H, and O elemental analysis; 13C NMR spectroscopy, conductometric titration, and FTIR to determine the C9 units and the content of different polar groups in their molecular structure. This information has been used to discuss the Π-A isotherms recorded for the Langmuir films formed on the ultrapure water surface. It was found that the presence of polar groups promotes better stability of the lignin molecules on the water subphase. The Π-A isotherm results also revealed that the lignins studied here form stable aggregates and can be well-packed on the water surface with a similar molecular arrangement in the Langmuir

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films where higher temperatures help the molecular packing. Moreover, the growth of the molecules might happen in two dimensions. Homogeneous LB films could be fabricated by controlling the dipping speed along the deposition course, in which the molecules were structured with the aromatic rings preferentially parallel to solid substrate, as determined by UV-vis and FTIR spectroscopies. As suggested already by Π-A isotherms, the LB films were successfully applied in the detection of Cd+2 at different concentrations in aqueous solutions using both impedance spectroscopy and electrochemical techniques. FTIR spectroscopy indicated that the origin of the good performance of the lignin as a transducer material in these sensor applications might be the physical interaction between the lignin and the metallic ion. The better performance of ultrathin LB films against cast films was also demonstrated. Acknowledgment. Financial assistance from CNPq/IMMP and FAPESP agencies. Authors are also grateful to LNLS (project LMF 4385) regarding the fabrication of the interdigitated electrodes. LA063582S