Environ. Sci. Technol. 2003, 37, 761-765
Electrophoretic Assembly of Naturally Occurring Humic Substances as Thin Films K . V I N O D G O P A L , * ,† VAIDYANATHAN SUBRAMANIAN,‡ SHEILA CARRASQUILLO,† AND P R A S H A N T V . K A M A T * ,‡ Notre Dame Radiation Laboratory, Notre Dame, Indiana 46556, and Department of Chemistry, Indiana University Northwest, Gary, Indiana 46408
Polydisperse humic acid thin films on optically transparent electrodes (OTEs) have been prepared by electrophoretic deposition from a solution of Suwanee River humic acid (SHA) in ethanol/acetonitrile. The thickness of the film and the rate of deposition of SHA are dependent on the applied voltage and the concentration of the solution. Tappingmode atomic force microscopy (TM-AFM) confirms the assembly of SHA aggregates on the electrode surface. The ability of these thin films to incorporate redox-active species such as ferrocene from solution is demonstrated by cyclic voltammetry experiments. A linear dependence of the peak current for the oxidation of ferrocene as a function of scan rate indicates that the ferrocene is incorporated into the humic membrane.
Introduction Humic substances are heterogeneous and polydisperse macromolecules with properties that depend on their point of origin and specific conditions such as pH and ionic strength. They have been variously described as complex polyelectrolytes or as random coils that could form spheres (1). The abiding interest in the chemical and physical properties of these humic substances arises in part from their important role in the transport and fate of metals and hydrophobic organic pollutants in the environment (2). Complexation with metal ions such as cadmium alters the metal’s bioavailability, while organic pollutants could be sequestered in hydrophobic domains within these large biopolymers. In recent years, advances have been made in the fabrication of multicomposite polyelectrolyte membranes using oppositely charged polymers such as poly(styrene sulfonate)s and poly(diallyldimethylammonium chloride) on a variety of substrates (3). The methods of preparation of these membranes are quite simple and usually involve simple sequential spraying or dip immersion (3-5). Numerous applications for such thin polyelectrolyte films have been demonstrated ranging from separations to sensors to electronic devices. These layered polyelectrolytes are particularly attractive for use as membranes given their permeability and robustness (6). A number of groups have examined the transport of redox-active ions through these multilayers (3* To whom correspondence should be addressed. Phone: 219980-6688; Fax: 219-980-6673; E-mail:
[email protected]. † Indiana University Northwest. ‡ Notre Dame Radiation Laboratory. 10.1021/es0260667 CCC: $25.00 Published on Web 12/24/2002
2003 American Chemical Society
5). The polyelectrolyte nature of humic acids makes them suitable candidates for such applications and thereby provides a means of understanding the transport of metal ions and other redox species in natural environments. Tapping-mode atomic force microscopy (TM-AFM) is particularly convenient for obtaining topographic information on “soft” biopolymers such as humic substances (7). The reported AFM results are unique to the particular conditions used, with one group reporting 1-3-nm-sized rigid globules of humic acid (8) and others reporting somewhat larger ring-shaped aggregates (9). These reported variations in size are not unexpected given the polydisperse nature of the humic substance and the particular experimental conditions used in each case. A relatively large body of work can be found in the literature concerning the deposition of insoluble electroactive thin films on electrodes (10-12). In general, most of these methods involve either reductive or oxidative electrochemical polymerization of redox-active metal complexes. Our approach here is different. We employ an electrophoretic method for obtaining a porous membrane consisting of threedimensional arrays of humic acid aggregates on a conducting glass surface. This unique approach of electrically assembling naturally occurring humic substances as a film has potential applications in the design of natural humic-based membranes, which can then be used to incorporate electroactive species and control the diffusion of metal ions across them. AFM studies evaluating the morphology of such electrophoretically prepared polydisperse humic acid films are also described.
Methods Reference Suwannee River humic acid (SHA) was obtained from the International Humic Substances Society (IHSS). Solutions of SHA in a mixture of 4% ethanol and 96% acetonitrile at a concentration of 100 mg/L were used for the electrophoretic deposition. This choice of solvent was a function of the desire for easy dissolution of the SHA and the need for a solvent of low dielectric constant, thereby ensuring that the applied voltage can be kept to a minimum during the deposition process. The optically transparent electrodes (OTEs) were cut from a transparent electrically conducting glass sheet (TEC Glass) obtained from Pilkington, Toledo, OH. Three milliliters of the humic acid solution in the ethanol-acetonitrile mixture was transferred to a small cell in which two OTEs were kept at a distance of ∼6 mm using a Teflon spacer. A dc voltage of between 25 and 75 V was applied between the two electrodes using a Fluka 415 dc power supply. The schematic of the cell used for casting the humic acid films is shown as an inset in Figure 1. Formation of a light yellow film on the electrode surface connected to the positive terminal confirms the deposition of SHA on the OTE. The film was then dried in air. The thickness of the film can be controlled by varying the time of deposition. These electrophoretically deposited films were quite robust and did not deteriorate during AFM measurements. AFM images were obtained using a Digital Nanoscope IIIa in tapping mode. An etched silicon tip was used as the AFM probe for imaging the SHA films on the OTE in air. Particle size measurements (depth profile) were performed using section analysis. An AFM image of the blank OTE shows typical and fairly uniform 30-nm-sized particles of indium tin oxide, which is the conductive coating on the OTE. SHA was also deposited on cover glass slips. The glass slips were pretreated by immersion in a 3% aqueous solution VOL. 37, NO. 4, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Absorbance of SHA films cast on OTE at different time intervals (30, 90, 120, 180, and 240 s) following the application of a 50-V dc electric field. Note the increasing absorption with increasing time. The solution contained 100 mg/L SHA in 4% ethanol/ 96% acetonitrile. The inset shows a schematic of the electrophoretic cell employed for casting humic acid films on the OTE. of 3-aminopropyltrimethoxysilane for 10 min. These pretreated slips were then dipped into an aqueous solution of SHA (concentration 100 mg/L) at pH 7.0 and dried in air under ambient conditions. The thickness of the SHA film can be controlled by varying the dipping time. Because of the presence of amino groups, the aqueous APS solution serves to make the glass surface positively charged, thereby ensuring that the negatively charged humic acid particles will adhere durably to the glass surface (13). The AFM image of a blank APS-coated cover glass slip shows a uniform film with no isolated particles. AFM images of the blank OTE and APS film on cover glass are part of the Supporting Information provided with this paper. Absorption spectra were recorded using a Shimadzu 3101 PC spectrophotometer. All electrochemical measurements were carried out in a standard three-electrode cell containing a saturated calomel reference electrode (SCE) and a platinum wire counter electrode. The working electrode was the OTE coated with SHA (OTE/SHA) or the OTE/SHA electrode with ferrocene (Fc) incorporated into the SHA membrane (OTE/ SHA/Fc). SHA was deposited electrophoretically as described earlier. To ensure maximum coverage, the deposition voltage was raised to 200 V, and deposition was carried out for 20 min. The Fc was incorporated into the membrane by dipping the OTE/SHA electrode thus prepared into a 5mM solution of Fc in acetonitrile for 30 min, followed by careful washing with acetonitrile. The electrolyte solution was a 0.1 M solution of tetrabutylammonium perchlorate (TBAP) in acetonitrile. A BAS 100 electrochemical analyzer was used to perform the cyclic voltammetry measurements.
Results and Discussion The humic substances have a large negative surface potential, thereby enabling their movement under the influence of an electric field (14). The negative surface charge of these humic substances arises from the carboxylic, phenolic hydroxyl, and alcoholic hydroxyl functional groups in humic substances. We have exploited the electrical property of humic substances to electrophoretically deposit polydisperse films of humic substances on OTEs. The surface charge of these humic substances is expected to increase with molecular size and remain very sensitive to ionic strength. Hence, electrophoretic deposition provides a convenient method for the creation of a porous film. A similar approach was adopted earlier to prepare films of fullerene clusters (15, 16) and gold nanoclusters (17) on electrode surfaces. It has been shown that the porosity of the films and the packing of the 762
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FIGURE 2. Absorbance of the SHA solution in the electrophoretic cell (applied voltage ) 50 V dc) monitored during deposition from the solution onto the electrode. The spectra were recorded at time intervals of 30, 90, 120, 180, and 240 s. Note the decreasing absorption with increasing time. gold clusters on the surface can be controlled by varying either the applied electric dc voltage or the time of deposition (17). Electrophoretic Deposition of SHA as Thin Films. Under the influence of an electric field (>25 V dc) the aggregates of SHA from acetonitrile/ethanol solution are driven toward the positive OTE. The deposition process is attributed to an electrophoretic phenomenon in which the negatively charged humic particles migrate and assemble on the OTE surface, which is maintained under positive bias. Within 2 min of electrophoretic deposition, the positive OTE terminal begins to show a yellow coloration with a corresponding loss of color in the solution. This electrode, referred to as OTE/SHA, was thoroughly washed before further analysis. The negative OTE, on the other hand, did not show any deposition and remained transparent throughout the electrophoretic process. Absorption spectra of the humic films deposited on the OTE at different deposition times are shown in Figure 1. The spectra are identical to that obtained from literature (18). The dc voltage was maintained constant at 50 V for these deposition experiments. The electrophoretically deposited films of SHA exhibit absorption spectra that are similar to that observed for the SHA solution. With increasing time of deposition, the absorbance of the film increases as greater amounts of humic acid are deposited onto the electrode surface. This observation is also complemented by the fact that the absorbance of the SHA solution in the cell decreases in an analogous fashion (Figure 2). The absorption spectrum of the SHA solution (100 mg/L) in the 4% ethanol/96% acetonitrile solution from the electrophoresis cell was monitored as the deposition of the film from the solution continued. The decrease in the net absorption of the solution and increase in the OTE absorption confirm the deposition of SHA as a thin layer. The negatively charged SHA aggregates are driven under the electric field to provide a robust coverage on the electrode surface. The film thus formed was stable, as no pealing or cracking occurred, even after washing with acetonitrile. The rate of SHA deposition on the OTE surface is dependent on both the applied voltage and the initial SHA concentration. Figure 3 shows the rate of deposition on the OTE (indicated by the increase in the absorption at 400 nm) as a function of applied voltage and SHA concentration. The deposition rate increases with increasing SHA concentration and deposition voltage. A simple exponential fit of the absorption growth shows an increase in the rate of deposition from 0.262 min-1 at 100 mg/L SHA at 25 V to 0.732 min-1 at 75 V. Although the SHA absorption spectra are featureless,
FIGURE 3. Growth of SHA absorption at 400 nm during the electrophoretic deposition process. The experimental conditions were as follows: (a, f) 40 mg/L SHA and 50 V, (b, 9) 100 mg/L SHA and 25 V, (c, b) 100 mg/L SHA and 50 V, (d, 2) 100 mg/L SHA and 75 V using 4% ethanol/96% acetonitrile as the solvent. they serve as an excellent probe for obtaining a quantitative estimate of the deposition process. Atomic Force Microscopy of SHA Films. The morphology of the electrophoretically deposited humic acid films was probed using TM-AFM. The AFM image (Figure 4) of the films obtained after 2 min of deposition at 50 V shows predominantly 40-60-nm-sized particles. In some cases, we have also observed larger-sized (100-200 nm) globular aggregates (see, for example, the aggregate in the top left corner of the image in Figure 4). The sectional analysis of the AFM images of these low-coverage films shows a porous, three-dimensional assembly of the SHA on the electrode surface. Increasing the deposition time or the applied voltage has a pronounced effect on the morphology of the electrophoretically deposited humic acid films. Figure 5 is a threedimensional image of an SHA film on the OTE obtained after 12 min of electrophoretic deposition at 50 V. This threedimensional image of the surface topography indicates a much higher degree of aggregation on the electrode surface. As we go to longer deposition times, larger particles predominate, and the particles are more closely packed, indicating more uniform deposition. (The AFM image of the blank OTE is presented in Figure 1 of the Supporting Information.) The humic acid aggregates obtained by electrophoretic deposition have much larger lateral diameters and occupy larger surface areas as deposition times and voltages are increased. It is well-known that lateral dimensions are often overestimated as a result of tip geometries. As in the case of humic particles adsorbed on mica (19), the SHA particles on the OTE tend to have a pancake rather than spheroidal shape. The electrophoretic deposition does not generate the ultrasmall spheroidal particles that other groups have obtained by straight deposition on mica or by dipping (8, 9, 20-22). Because the particle sizes shown in Figure 4 are significantly larger, we postulate that, under these conditions, several smaller SHA aggregates must be coalescing to form a larger particle during the electrodeposition process. Such coalescence is probably enhanced under the given nonaqueous solvent conditions. We have compared this electrophoretic deposition method with a dipping method in which a pretreated cover glass surface is dipped into an aqueous SHA solution. The pretreatment with APS makes the glass surface is positively charged, thereby ensuring that the negatively charged humic particles will adhere durably to the surface (13). (The AFM
FIGURE 4. AFM image and sectional analysis of SHA particles assembled as thin films on OTE using an SHA concentration of 100 mg/L, an applied voltage of 50 V dc, and a 2-min deposition time. The AFM image was recorded in the tapping mode using an etched silicon probe.
FIGURE 5. Three-dimensional AFM image of SHA film obtained after 12 min of electrophoretic deposition at 50 V dc from a solution of 100 mg of SHA /L. image of the blank cover glass treated with APS is presented in Figure 2 of the Supporting Information.) Just as in the electrophoretic case, we can control the deposition by varying the dipping times. The AFM image of the SHA on a cover glass surface obtained after dipping for 5 min is shown in Figure 6. The image obtained after 5 min shows predomiVOL. 37, NO. 4, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. AFM image of SHA particles adsorbed on positively charged cover slips from an aqueous SHA solution (100 mg/L) at pH 7.0 (5-min adsorption time). The AFM image was recorded in the tapping mode using an etched silicon probe.
FIGURE 7. Cyclic voltammograms of (a) OTE/SHA and (b) OTE/ SHA/Fc0 in a 0.1 M solution of TBAP in acetonitrile. Scan rate ) 50 mV/s nantly 15-30-nm-sized particles. However, at longer dipping times, we see the presence of large aggregates on the surface, similar to those observed during electrophoresis. This is an interesting observation in that the deposition here is from aqueous solution in contrast to the nonaqueous conditions used in the electrophoresis. Electrochemical Activity of Ferrocene-Incorporated SHA Films. Significant effort has been made in the past to probe the interaction between humic substances and metal ions and organic molecules that coexist (naturally or as a pollutant) (23, 24). Assembling the humic aggregates as thin films on an electrode surface provides a convenient way to probe the redox activity of the humic-bound species. In this initial attempt, we have succeeded in incorporating ferrocene (Fc0) into the SHA film by immersing the OTE/ SHA electrode into an acetonitrile solution containing 5 mM Fc. After immersing the electrode for 30 min, we carefully washed it with acetonitrile to remove unbound Fc. This electrode, referred to as OTE/SHA/Fc0, was inserted into a three-compartment electrochemical cell containing 0.1 M tetrabutylammonium perchlorate (TBAP) in acetonitrile as electrolyte, a Pt counter electrode, and a saturated calomel electrode (SCE) as the reference. The cyclic voltammograms of OTE/SHA and OTE/SHA/Fc are shown in Figure 7. The SHA film alone shows limited electrochemical activity with no noticeable peak currents in the range of 0-800 mV versus SCE. A plain OTE dipped into Fc solution and then washed 764
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FIGURE 8. Cyclic voltammograms of OTE/SHA/Fc0 in a 0.1 M solution of TBAP in acetonitrile as a function of the scan rate. Note the increase in the redox peak with increasing scan rate. Inset shows the linear dependence of the peak current as a function of the scan rate. with acetonitrile showed no electrochemical activity over the same range. The broad nature of the forward and reverse scans is indicative of the capacitive current. In contrast, the Fc0-bound SHA films show cyclic voltammograms corresponding to Fc0 oxidation. The oxidation potential of the humic-bound Fc is 0.30 V versus SCE. This potential is similar to the oxidation potential of 0.35 V vs SCE in acetonitrile. The cyclic voltammograms can be reproduced during repetitive scans as well as over a period of several hours. The robustness of this SHA film was also evident from the stability of the current observed in rotating disk electrode (up to 8000 rpm) experiments. Figure 8 shows the cyclic voltammograms of an OTE/ SHA/Fc0 electrode recorded at different scan rates. The peak current increases proportionately with the increasing scan rate. The linear dependence of the peak current on the scan rate (inset in Figure 8) confirms that the redox reaction of Fc0/Fc+ occurs within the SHA membrane. As we go to higher scan rates, we begin to see deviations from linearity. Such deviations are often observed in membrane- or polymercoated electrodes and has been attributed to film resistance or to diffusion-like behavior of the substrate within the film, particularly in thick layers (25). Although the cyclic voltammograms for the Fc0/Fc+ couple are reversible, it is interesting to note that the voltammetric peaks during the forward and reverse scans are nonsymmetrical. This discrepancy is likely to arise from the differing degrees of interaction of Fc0 and Fc+ species with SHA causing their individual mobilities to differ within the film. Although we cannot conclusively comment on the mobility of the electroactive species, the oxidation wave ((Fc0 f Fc+ + e-) resembles that observed for diffusing species. The diffusion of Fc0 within the film or an electron-hopping process can contribute to this effect. On the other hand, the sharper reduction peak (Fc+ + e- f Fc0) resembles more of a surface wave as commonly observed for species bound to the electrode surface. Rotating disk experiments are underway to probe the mobility of other electroactive species within the SHA film. In any event, our preliminary electrochemical experiments confirm the ability of SHA membranes to act as a host to redox species such as Fc0. This simple approach of modifying SHA films with electrochemically active membranes further opens up new avenues to probe the mobility and interactions of a variety of naturally occurring metal ions and pollutant molecules.
The ability to assemble humic substances as thin films on electrode surfaces demonstrates the usefulness of this technique in preparing ordered humic membranes. By anchoring them onto an electrode surface, it should be possible to probe the binding of metal ions as well, as the effect of ionic strength and pH, on the transport of ionic and other molecular species by electroanalytical techniques.
Acknowledgments The work described herein was supported by the Office of Basic Energy Sciences of the U.S. Department of Energy. This is Contribution No. NDRL 4399 from Notre Dame Radiation Laboratory.
Supporting Information Available AFM images of the blank OTE and APS film on cover glass. This material is available free of charge via the Internet at http://pubs.acs.org.
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(9) Maurice, P.; Namjesnik-Dejanovic, K. Environ. Sci. Technol. 1999, 33, 1538-1541. (10) Elliot, C. M.; Baldy, C. J.; Nuwaysir, L. M.; Wilkens, C. L. Inorg. Chem. 1990, 29, 389. (11) Calvert, J. M.; Schmehl, R. H.; Sullivan, B. P.; Facci, J. S.; Meyer, T. J.; Murray, R. W. Inorg. Chem. 1983, 22, 2151. (12) Cosnier, S.; Deronzier, A.; Moutet, J. C. J. Electroanal. Chem. 1985, 193, 193-204. (13) Jenney, C. R.; DeFife, K. M.; Colton, E.; Anderson, J. M. J. Biomed. Mater. Res. 1998, 41, 171-184. (14) Green, S.; Morel, F.; Blough, N. Environ. Sci. Technol. 1992, 26, 294-302. (15) Kamat, P. V.; Barazzouk, S.; George Thomas, K.; Hotchandani, S. J. Phys. Chem. B 2000, 104, 4014-4017. (16) Kamat, P. V.; Barazzouk, S.; Hotchandani, S. Adv. Mater. 2001, 13, 1614-1617. (17) Chandrasekharan, N.; Kamat, P. V. Nano Lett. 2001, 1, 67-70. (18) Choudhry, G. G. Toxicol. Environ. Chem. 1981, 4, 261. (19) Balnois, E.; Wilkinson, K. Colloids Surf. 2002, 207, 229-242. (20) Liu, C.; Huang, P. Atomic Force Microscopy of pH, ionic strength, and cadmium effects on surface features of humic substances; Royal Society of Chemistry: Cambridge, U.K., 1999, pp 87-89 (as given in ref 1). (21) Plaschke, M.; Romer, J.; Klenze, R.; Kim, J. Colloids Surf. A: Physicochem. Eng. Aspects 1999, 160, 269-279. (22) Plaschke, M.; Romer, J.; I., K. J. Environ. Sci. Technol. 2002, 36, 4483-4488. (23) Ghantous, L.; Lojou, E.; Bianco, P. Electroanalysis 1998, 10, 12491254. (24) Berbel, F.; Diaz-Cruz, J. M.; Arino, C.; Esteban, M.; Mas, F.; Garces, J. L.; Puy, J. Environ. Sci. Technol. 2001, 35, 1097-1102. (25) Henning, T. P.; White, H. S.; Bard, A. J. J. Am. Chem. Soc. 1982, 104, 5862-5868.
Received for review August 17, 2002. Revised manuscript received November 6, 2002. Accepted November 14, 2002. ES0260667
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