pubs.acs.org/Langmuir © 2009 American Chemical Society
Gaseous Adsorption in Melanins: Hydrophilic Biomacromolecules with High Electrical Conductivities A. Bernardus Mostert,† Karl J. P. Davy,† Jeremy L. Ruggles,‡ Ben J. Powell,† Ian R. Gentle,*,‡ and Paul Meredith† †
Centre for Organic Photonics and Electronics, School of Mathematics and Physics and ‡School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, Queensland 4072, Australia Received April 12, 2009. Revised Manuscript Received September 11, 2009
The melanins are an important class of multifunctional biomacromolecules that possess a number of intriguing physical and chemical properties including electrical and photoconductivity. Unusually for a conducting organic material, eumelanin is hydrophilic and its electrical properties are strongly dependent on its hydration state. We have therefore measured adsorption isotherms for two polar adsorbates, water and ethanol, in the pressed powder pellets of synthetic eumelanin typically used in electrical studies. We show that a simple kinetic monolayer Langmuir model describes the adsorption and find that there are strong adsorbate-eumelanin interactions in both cases. These isotherms allow the proper scaling of electrical conductivity data and in doing so make progress toward a better understanding of eumelanin electrical properties, which is a critical prerequisite to the design of new eumelanin-like bioelectronic materials.
Introduction The melanins are an important class of multifunctional biomacromolecules found throughout the biosphere.1 The main forms of melanin in humans are brown-black eumelanin and yellowreddish pheomelanin. Both eumelanin and pheomelanin are present to varying degrees in the skin where they are photoprotectants, acting as our primary defense against ultraviolet radiation, one of the most ubiquitous and potent environmental carcinogens.2 Eumelanin is composed of two basic monomeric unit types (as shown in Figure 1), 5,6-dihydroxyindole (DHI) and 5,6dihydroxyindole-2-carboxylic acid (DHICA).3 The heterogeneous macromolecular system derived from these monomers contains variously cross-linked oligomers and quinine redox states. Eumelanin possesses a unique collection of physical and chemical properties3 that include electrical and photoconductivity; broad, monotonic optical absorption across the UV, visible, and near IR; almost unity nonradiative conversion of absorbed photon energy; and free-radical and antioxidant behavior. Several of these properties are consistent with eumelanin’s biological roles (particularly as a photoprotectant). The optical properties have been recently explained using the so-called “chemical disorder model”,4,5 which posits that the eumelanin system consists of a distribution of oligomers each chemically distinct and with a range of optical gaps. The macroscopic properties of such a system are an ensemble average of the individual components. Electrical and photophysical studies on eumelanin have a long history in the literature. Indeed, eumelanin was one of the earliest bio-organic materials to have been studied in this way both *To whom correspondence should be addressed. E-mail: i.gentle@ uq.edu.au. (1) Prota, G. Melanins and melanogenesis; Academic Press: San Diego, CA, 1992. (2) Lin, J.; Fisher, D. Nature 2007, 445(7130), 843–850. (3) Meredith, P.; Sarna, T. Pigm. Cell Res. 2006, 19, 572–594. (4) Tran, M. L.; Powell, B. J.; Meredith, P. Biophys. J. 2006, 90, 743–752. (5) Meredith, P.; Powell, B. J.; Riesz, J.; Nighswander-Rempel, S. P.; Pederson, M. R.; Moore, E. G. Soft Matter 2006, 2, 37–44. (6) Longuet-Higgins, H. Arch. Biochem. Biophys. 1960, 86, 231–232. (7) Pullman, A.; Pullman, B. Biochim. Biophys. Acta 1961, 54, 384–385. (8) McGinness, J. Science 1972, 177, 896–897. (9) Potts, A. M.; Pinchit, A. Agressologie 1967, 9, 225–226.
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theoretically6-8 and experimentally.9-12 In 1974, McGinnes et al.13 reported in Science bistable switching behavior in solidstate eumelanin, which enshrined the amorphous semiconductor model as the dominant paradigm for interpreting eumelanin’s electrical and photophysical properties. This proposition had been widely accepted, but it has recently been argued that the conductivity may be protonic rather than electronic.3 The confusion over eumelanin’s electrical properties is only added to by the observation that the solid-state electrical conductivity changes as a function of hydration state14-25 (e.g., 103 increase in conductivity with a 50% relative humidity change). However, to date, measurements of eumelanin’s electrical properties have not been made under controlled environmental conditions, such as constant humidity. Strong humidity dependence has also been reported for other hydrophilic biomolecular systems by Rosenberg and co-workers.10,12,26 They suggested, in the context of a semiconductor model, that the hydration dependent (10) Rosenberg, B.; Postow, E. Ann. N.Y. Acad. Sci. 1969, 158, 161–190. (11) Trukhan, E. M.; Perevozchikon, N. F.; Ostrovskii, M. A. Biophysika 1970, 15, 1090–1094. (12) Powell, M. R.; Rosenberg, B. Bioenergetics 1970, 1, 493–509. (13) McGinness, J.; Corry, P.; Proctor, P. Science 1974, 183, 853–855. (14) Baraldi, P.; Capelletti, R.; Crippa, P.; Romeo, N. J. Electrochem. Soc. 1979, 126, 1207–1212. (15) Bridelli, M.; Capelletti, R.; Crippa, P. Bioelectrochem. Bioenerg. 1981, 8, 555–567. (16) Capozzi, V.; Perna, G.; Carmone, P.; Gallone, A.; Lastella, M.; Mezzenga, E.; Quartucci, G.; Ambrico, M.; Augelli, V.; Biagi, P.; Ligonzo, T.; Minafra, A.; Schiavulli, L.; Pallara, M.; Cicero, R. Thin Solid Films 2006, 511-512, 362–366. (17) Gonc-alves, P. J.; Baffa Filho, O.; Graeff, C. F. O. J. Appl. Phys. 2006, 99, 104701. (18) Jastrzebska, M.; Jussila, S.; Isotalo, H. J. Mater. Sci. 1998, 33, 4023–4028. (19) Jastrzebska, M.; Isotalo, H.; Paloheimo, J.; Stubb, H. J. Biomater. Sci., Polym. Ed. 1995, 7, 577–86. (20) Jastrzebska, M.; Kocot, A.; Tajber, L. J. Photochem. Photobiol., B 2002, 66, 201–206. (21) Jastrzebska, M.; Kocot, A.; Vij, J.; Zalewska-Rejdak, J.; Witecki, T. J. Mol. Struct. 2002, 606, 205–210. (22) Osak, W.; Tkacz, K.; Czternastek, H.; Slawinski, J. Biopolymers 1989, 28, 1885–1890. (23) Osak, W.; Tkacz-Smiech, K.; Elbanowski, M.; Slawinski, J. J. Biol. Phys. 1995, 21, 51–65. (24) Strzelecka, T. Physiol. Chem. Phys. 1982, 14, 223–231. (25) Strzelecka, T.; Band, A Physiol. Chem. Phys. 1982, 14, 219–222. (26) Rosenberg, B. J. Chem. Phys. 1962, 36, 816–823.
Published on Web 11/11/2009
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Figure 1. Building blocks of eumelanin. The two major units are DHI (5,6-dihydroxyindole) (HQ-hydroquinone) and DHICA (5,6-dihydroxyindole, 2-carboxylic acid). The other units SQ (semiquinone), QI (quinone-imine), and IQ (tautomer of QI) are different oxidation states and are thought to be incorporated in the macromolecular structure.
electrical properties of many biomaterials, including eumelanin, may result from the modification of the dielectric constant by polar adsorbates, for example, water, that leads to a reduction in the local activation energy for conduction. Alternatively, the charge transport may be ionic (e.g., protonic).3 Neither model has yet been ruled out, and identifying the main charge carrier in eumelanin remains an important open problem in the field.3 The observations of hydration dependent properties are quite natural given that eumelanin, like many biomaterials, contains many hydrophilic units (see Figure 1). However, a hydrophilic organic based conductor remains an oddity, since the most prominent organic/polymer conductors and their derivatives (e.g., poly(acetylene)s, poly(pyrrole)s, poly(thiophene)s, poly(aniline)s, etc.) are hydrophobic materials. Therefore, an inherently hydrophilic conductor is of significant scientific interest: for example, one may envisage eumelanin to be a useful biocompatible electrical material in applications such as biosensing.27 If the charge transport does involve an ionic mechanism, then eumelanin could represent an interesting and useful bridge between electronic semiconductors and biological and biomimetic systems, where ion transport dominates electrical behavior. Determining the origin of eumelanin’s electrical conductivity is now a priority if the field is to make progress in achieving this bioelectronic potential. In this regard, the first question one must ask is how do relevant adsorbates interact with eumelanin? To date, there has been only one systematic study12 of the physical adsorption of water vapor into solid-state eumelanin, and no other small polar molecules have been studied (but note that Crippa et al. have reported a systematic measurement of a nitrogen isotherm28). In this paper, we report the kinetic and equilibrium adsorption behavior of eumelanin pellets in the presence of pure water and ethanol vapors as a function of relative pressure. We study pellets, rather than powders, as measurements of eumelanin’s electrical properties are typically made on pressed powder samples. We observe very strong adsorbate-melanin interactions in both cases and explain the behavior using a Langmuir single-layer adsorption model. These eumelanin-water and eumelanin-alcohol adsorption isotherms represent a critical step toward explaining the electrical properties of the eumelanin system and the potential realization of their use in bioelectronic devices and chemi-sensors.
Experimental Section Eumelanin Synthesis. Auto-oxidized eumelanin was produced according to previously published methodologies.29 (27) Bothma, J.; de Boor, J.; Divakar, U.; Schwenn, P.; Meredith, P. Adv. Mater. 2008, 20, 3539–3542. (28) Crippa, P. R.; Giorcelli, C.; Zeise, L. Langmuir 2003, 19, 348–353. (29) Felix, C.; Hyde, J.; Sarna, T.; Sealy, R. J. Am. Chem. Soc. 1978, 100, 3922– 3926.
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dl-Dopa (Sigma-Aldrich, 5 g) was dissolved in 2 L of deionized water. The solution was adjusted to pH 8 using ammonia solution (AnalaR, 25% NH3 Merck). Air was bubbled through the solution for 3 days, periodically adding ammonia to maintain the solution at pH 8. Following this, the solution was adjusted to pH 2 with concentrated hydrochloric acid (Univar Analytical reagent HCl 32%), resulting in a black precipitate which was then filtered and washed with deionized water. The resulting powder, which resembled crushed obsidian, was dried overnight at 80 C. Although it is not possible to identify specific chemical markers to unequivocally confirm this material to be a melanin polyindolequinone, this preparation produces a powder with all the characteristic properties of eumelanin including strong, monotonic broad band absorbance, characteristic fluorescence, and the typical C, H, N, and O ratios as measured by X-ray photoelectron spectroscopy. Pellet Preparation. For the adsorption studies, approximately 200 mg of powder was pressed into a 13 mm diameter pellet by applying a small amount of pressure, applying a vacuum to remove residual moisture and air for 1 min, and then increasing the pressure in increments of 1 t every 15 s for 1 min. Finally, the pressure was increased to 10 t (770 MPa) and held for 3 min. The resulting pellet appeared smooth and black. Adsorption Experimental Procedure. The determination of water adsorption behavior was performed using a vacuum microbalance (CI Electronics Ltd.) with an MK2 vacuum head and an external CI STABAL control unit and computer logging system. The vacuum line uses a simple vapor delivery system (see the Supporting Information) in which water vapor is introduced from a flask containing Milli-Q water that had been degassed with three freeze-pump-thaw cycles. The volume of the gas handling system was deliberately increased through the addition of two large flasks in order to maintain pressure stability and provide a sufficient vapor volume. Pressure measurements were performed with a MKS Baratron pressure transducer with a range of 1000 mbar. The pumping was done by a diffusion and rotary pump combination which achieved pressures of 10-4 Torr and monitored using a Penning gauge. Possible contamination from pump oil on the sample was prevented by having the cold traps (approx. 1 m from the sample) immersed in liquid nitrogen, which also served to improve the vacuum generated by the pumps. For adsorption measurements, the desired pressure of water was admitted into the system, and the reservoir closed off. The sample was allowed to equilibrate for at least 4 h while the sample mass was monitored with the microbalance. For the next data point, the eumelanin sample was dried by pumping for 1 h, after which the same procedure was repeated but at a higher pressure. The experiment was conducted on two pellets to ensure consistency of the eumelanin response. The water experiments were conducted at a constant temperature of 22.5 ( 0.2 C. Ethanol adsorption/desorption measurements were made following the same procedure as for water, replacing water with absolute ethanol (Ajax FineChem, Ethanol Absolute Univar). One significant difference was that the response of the eumelanin to ethanol was much slower, such that equilibration times of 50-120 h were required. The length of the experiment resulted in experimental challenges that were not present in the water studies, chiefly that it was impossible to avoid minor air leaks that became significant over such long times. In order to compensate for this effect, a control experiment was performed without ethanol, and the change in baseline noted over the same period of the adsorption experiment (see the Supporting Information). This was subtracted from the data to obtain the corrected adsorption curve. All ethanol experiments were conducted at a constant temperature of 20.3 ( 0.2 C. Data Treatment. Because of the slow response of the material, we have attempted to estimate how far the system was from thermodynamic equilibrium at the conclusion of each adsorption measurement at a given relative pressure. In order to do this, the temporal data were simulated using Langmuir’s monolayer DOI: 10.1021/la901290f
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adsorption kinetics model.30 Note that one can use this model to derive the Brunauer, Emmet, and Teller (BET) isotherms (see below), ensuring consistency between temporal and equilibrium data. In the Langmuir model, the adsorbent is modeled as a flat surface of total area Atot in the presence of adsorbate at pressure P. We assume that only monolayer adsorption is permitted. If the area covered by the adsorbate is designated Aad and the uncovered area Aun, then Atot = Aun þ Aad. The rate of adsorption is -
dAun ¼ a1 Aun P -b1 Aad e -Ed =RT dt
ð1Þ
where a1 and b1 are constants and Ed is the enthalpy of desorption. The sample mass is mðtÞ ¼ RAad ðtÞ þ mðt ¼ 0Þ
ð2Þ
where R is the mass density per unit area of the adsorbate, which is determined by the molecular mass and packing of adsorbate molecules on the surface and m(t = 0) is the dry mass of the pellet. It is now possible to integrate the rate equation from an initial condition where the entire area Atot is uncovered to a point where the covered area at time t is Aad(t) and the mass is 0
mðtÞ - mðt ¼ 0Þ
1 -ða1 P þ b1 e -Ed =RT Þt -Ed =RT a P e þ b e 1 1 A ¼ RAtot @1 a1 P þ b1 e -Ed =RT
ð3Þ
The data sets at each individual pressure value were fitted to eq 3, which yielded values for a1, RAtot, and b1 eEd/RT. Upon investigation, we found the term b1 eEd/RT to be of order 10-20 s-1 across all water data sets, whereas a1P was of order 10-4 s-1. The equivalent values for ethanol were for a1P ≈ 10-5 s-1 and for b1 eEd/RT ≈ 10-19 s-1. Therefore, the desorption term is negligible compared to adsorption, indicating that the interaction between water and eumelanin is very strong (as the BET curves also demonstrate; see below). Clearly, our experiment is not accurate to one part in 1016. Therefore, we work in the limit b1 eEd/RT f 0, whence mðtÞ - mðt ¼ 0Þ ≈ RAtot 1 - e -a1 Pt
ð4Þ
The values for the parameters for eq 4 are reported in the Supporting Information. The value of RAtot should be constant according to the model; however, eumelanin samples demonstrate history dependent behavior28 (also consistent with our observation of warping of the material beyond 0.8 relative pressure; see below). Therefore, we take RAtot and a1 as independent free parameters for each different pressure. The values obtained for the fitting parameters were then used to estimate the equilibrium mass [i.e., m(t f ¥)] at each relative pressure measured. We will see below that the experimental data fit remarkably well to eq 4. One should be careful not to overinterpret this success. First, we note that the fit has two free parameters. Second, we note that eq 1 has the same functional form as an arbitrary first order expansion of the adsorption rate in P. Therefore, any adsorption process that is only weakly dependent on P, such as diffusion, will have a rate governed by an equation with the same empirical form as eq 1. At the (low) pressures relevant to this experiment, it seems reasonable to assume that almost all adsorption processes would have an extremely small second order term. Third, and possibly most importantly, the model assumes that adsorption and desorption are rate determining, and in a system such as this diffusion would (30) Barnes, G.; Gentle, I. Interfacial Science: An Introduction; Oxford University Press: Oxford, UK, 2005.
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certainly be expected to be significant. Nevertheless, we proceed with this analysis as it is superior to fitting to an arbitrary function, but caution the reader not to interpret a1 or RAtot too literally. Uncertainties in the results were calculated by convoluting the random errors in the estimated dry weight of the pellet with that of the final weight measured (determined by the difference between the equilibrium mass and the mass at the end of an experimental run). However, it is likely that systematic errors in determining the actual equilibrium mass from the temporal data, and difficulties in accounting for any drifts in the measuring equipment over the long adsorption times are more important and also harder to quantify. The equilibrium data obtained were then analyzed using a finite layer BET model, since it is a standard method for analyzing adsorbent/adsorbate interactions, nσ ZX 1 -ðν þ 1ÞX ν þ νX νþ1 ¼ σ nm 1 -X 1 þ ðZ -1ÞX - ZX νþ1
ð5Þ
where nσ is the number of moles of adsorbate, nσm is the number of moles required to give monolayer coverage, Z = e(Ed-Ev)/RT (where Ev is the enthalpy of vaporization of the adsorbate), X is the relative pressure of the adsorbate, and ν is the number of layers. The analysis was conducted by first fitting the infinite layer BET model30 to the relative pressure ranges of 0.05-0.3 as is standard.30 The values of Ed - Ev obtained were then used in eq 5, which was used to fit the entire data set to obtain a value for ν.
Results and Discussion Water Adsorption. Figure 2 shows representative data for water adsorption obtained at a relative pressure of 0.45, with the fit using eq 4. Despite the very simple model used, surprisingly good fits were obtained for all our water data. In each case, it was observed that the sample had reached equilibrium after approximately 1 h. The one place our model fails to describe the data well is the initial rise in the data (seen in all our curves). This may indicate a lower energy site within the material, for example, dimples on a surface. These lower energy sites are saturated first, and then the more numerous but slightly higher energy sites are then populated, which is captured by the model. This indicates multilayer adsorption, though the number of layers are very close to one (as our BET analysis, below, suggests). The equilibrium adsorption isotherm for eumelanin pellets in the presence of water vapor can be seen in Figure 3. The BET parameters are reported in Table 1. The data suggest that there is a strong interaction between eumelanin and water vapor, consistent with our find that desorption is negligible, with a large weight gain at low relative pressure such that the mass had increased by 10% at a relative pressure of 0.3. The point of monolayer adsorption is not clearly defined in the isotherm, which lies between type 1 and type 2 behavior.30 Note that no data were obtained above a relative pressure of 0.8, as the pellets were observed to warp significantly. As described previously, eumelanin is a disordered material at the molecular and macromolecular levels.5 This leads to heterogeneity in the detailed chemical and physical properties,4 with the macroscopic behavior being an ensemble average over these microscopic entities. This heterogeneity makes definitive chemical analysis impossible, hence the truism “no two melanins are ever made the same”.1 Consequently, in order to determine the molar water adsorption, estimates were made by assuming that the sample is either pure poly-DHI or pure poly-DHICA. This enabled the determination of the number of moles of water adsorbed at a given relative pressure per mole of monomer, and Langmuir 2010, 26(1), 412–416
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Figure 2. Temporal adsorption data for water adsorption into a eumelanin pressed powder. The red line is the modeled fit using eq 4.
Figure 3. Equilibrium adsorption isotherm for water adsorption by pressed powder eumelanin pellets with the modeled BET curve (eq 5). Table 1. Values Obtained from BET Modeling of the Water and Ethanol Adsorption Isotherms adsorbent
H2O
CH3CH2OH
Ed - Ev (kJ mol-1) nσm /g (mol g-1) ν (no.)
0.55 0.0047 3.65
0.5 1.87 10-7 ¥
the results obtained in this way are plotted in Figure 4. As the real material is a mixture of DHI and DHICA, the data points represent upper and lower bounds for our sample, and thus, we find that of order one water molecule is adsorbed per monomer at high relative pressure. This correspondence is high but not surprising, as it is expected that water will form hydrogen bonds with the polar groups in the eumelanin structure, specifically the catechol moieties.31 Ethanol Adsorption. The adsorption of ethanol vapor by eumelanin pellets is much slower than that of water, as can be seen in Figure 5. The time taken for the system to approach equilibrium is approximately 1/a1P. This is ∼24 min for most pressures of water and 10-30 h for ethanol. This is not unexpected given the very different sizes of water and ethanol molecules and the morphology of the eumelanin pellets, which means that diffusion (31) Powell, B. J.; Baruah, T.; Bernstein, N.; Brake, K.; McKenzie, R. H.; Meredith, P.; Pederson, M. R. J. Chem. Phys. 2004, 10, 8608–8615.
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Figure 4. Equilibrium adsorption isotherm of moles of water adsorbed per mole of eumelanin monomer assuming that sample to be either pure poly-DHI or pure poly-DHICA. As the real material is a mixture of DHI and DHICA, the true result should lie between the two data sets.
Figure 5. Representative temporal data for ethanol adsorption into pressed eumelanin pellets. The red line is the modeled fit to eq 4.
is likely to be a dominating factor in the adsorption rate. As stated earlier, we did not explicitly model diffusion, though all processes linear in P are accounted for regardless of their physical origin. However, given the larger mass of ethanol versus water, mass transport through the pore volume may well have a significant effect on the kinetics. As was the case for water, ethanol adsorption was modeled using the simple model described earlier, and the equilibrium adsorption determined for each value of relative pressure measured. The adsorption isotherm can be seen in Figure 6. Fits to the temporal data were generally not as good as those for water, and so the error bars in Figure 6 are correspondingly larger, which is merely indicative of the fact that it is more difficult to accurately determine equilibrium adsorption for a slowly adsorbing system. It can be observed that the time-dependent adsorption of ethanol shows no evidence of multiple phases as was observed for water; rather a smoothly increasing curve was seen. One sees from Figure 6 that a rapid increase in adsorption at relative pressure below 0.1, while between 0.2 and 0.4 little increase occurs. Above ∼0.4 relative pressure, the adsorption increases with pressure. The mass of ethanol adsorbed is comparable to the mass of water adsorbed (cf. Figure 3); although the isotherm shape is more clearly type 2, in contrast to water. The modeled BET curve in Figure 6 is indicated by the solid line, and its parameters are reported in Table 1. We also obtained a molar ratio adsorption curve as for the water data, which can be seen in Figure 7. The values for the DOI: 10.1021/la901290f
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of a powder. Another paper by Dolidovich et al.32 obtained water adsorption isotherms on melanin containing phytoadsorbents, though we are unable to make a meaningful comparison since there was no comparison between phytoadsorbents with and without melanin. Finally, water adsorption studies on carbon materials in general are a well-established field.33 Work done on such materials, as by Hou et al.,34 and the models used (extensively reviewed by Furmaniak et al.33) may contribute to the future development and understanding of the adsorption behavior in eumelanin, though it does not affect the central conclusions of our study.
Figure 6. The equilibrium adsorption isotherm for ethanol adsorption by pressed powder eumelanin pellets with modeled BET curve (eq 5).
Figure 7. Equilibrium adsorption isotherm of moles of ethanol adsorbed per mole of eumelanin monomer assuming that sample to be either pure ploy-DHI or pure poly-DHICA. As the real material contains both DHI and DHICA, the true result should lie between the two data sets.
molar ratios are similar to that of water, which leads us to similar conclusions as for water (see above). The molar ratios are about one-third smaller for ethanol than water, which is natural as the ethanol molecule is roughly three times the size of water. Comparison of our results with other published data is complicated by the fact that adsorption is dependent on the morphology and chemical makeup of the sample, both of which can vary from one study to another. Powell and Rosenberg12 obtained an adsorption isotherm of water on powdered eumelanin. They obtained a type 2 isotherm, and the amounts adsorbed were approximately double those observed in the current study. These differences are entirely reasonable given the different morphologies of the samples, in particular, the higher available surface area
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Conclusion We have performed a systematic study of adsorption of two common polar solvents, water and ethanol. To our knowledge, Figure 6 is the first reported isotherm for the adsorption of an alcohol into eumelanin. Furthermore, the water isotherm in Figure 3 is the first water-eumelanin adsorption isotherm for a pressed pellet, which is the morphology typically used for electrical studies. Our data indicate that both adsorbates are strongly adsorbed with a weight gain of up to 20% being possible. The main difference between the adsorbates is the time scale of the adsorption processes. A limited layers BET model showed that there is a strong binding and an equilibrium binding capacity of ∼1:1 (moles of water to moles of eumelanin starting monomer). Unlike other conducting organic/polymer materials, eumelanin is hydrophilic, and therefore, one is required to pay attention to the environment, which these adsorption isotherms codify. With these isotherms, one can begin to calibrate bulk electrical measurements, therefore allowing controlled studies of eumelanin’s behavior. This is a necessary prerequisite for the rational design of bioelectronic devices and chemi-sensors based on eumelanin. Acknowledgment. The work reported in this paper was funded through the Australian Research Council Discovery Program (DP0879944). P.M. is supported by the Queensland State Government Smart State Senior Fellowship program, and B.J.P. is supported by a Queen Elizabeth II fellowship from the Australian Research Council (DP0878523). P.M. also acknowledges support from the University of Queensland through the UQ Foundation Research Excellence scheme. The Centre for Organic Photonics and Electronics is a UQ Strategic Initiative. Supporting Information Available: Parameters and plots of adsorption data fits, and further details of time dependence. This material is available free of charge via the Internet at http://pubs.acs.org. (32) Dolidovich, A. F.; Akhremkova, G. S.; Lapina, V. A.; Rubanov, A. S. Russ. J. Phys. Chem. 2003, 77, 73–76. (33) Furmaniak, S.; Gauden, P. A.; Terzyk, A. P.; Rychlicki, G. Adv. Colloid Interface Sci. 2008, 137, 82–143. (34) Hou, P.-X.; Orikasa, H.; Yamazaki, T.; Matsuoka, K.; Tomita, A.; Setoyama, N.; Fukushima, Y.; Kyotani, T. Chem. Mater. 2005, 17, 5187–5193.
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