ARTICLE pubs.acs.org/Langmuir
Incorporation of Single-Walled Carbon Nanotubes into Ferrocene-Modified Linear Polyethylenimine Redox Polymer Films Tu O. Tran,†,‡,^ Emily G. Lammert,^ Jie Chen,†,‡ Stephen A. Merchant,^ Daniel B. Brunski,|| Joel C. Keay,|| Matthew B. Johnson,|| Daniel T. Glatzhofer,§ and David W. Schmidtke*,†,‡,^ Carbon Nanotube Technology Center, ‡University of Oklahoma Bioengineering Center, §Department of Chemistry and Biochemistry, Homer L. Dodge Department of Physics and Astronomy, and ^School of Chemical, Biological and Materials Engineering, University of Oklahoma, 100 East Boyd, Norman, Oklahoma 73019, United States
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ABSTRACT: In this study, we describe the effects of incorporating single-walled carbon nanotubes (SWNTs) into redox polymerenzyme hydrogels. The hydrogels were constructed by combining the enzyme glucose oxidase with a redox polymer (Fc-C6-LPEI) in which ferrocene was attached to linear poly(ethylenimine) by a six-carbon spacer. Incorporation of SWNTs into these films changed their morphology and resulted in a significant increase in the enzymatic response at saturating glucose concentrations (3 mA/cm2) as compared to films without SWNTs (0.6 mA/cm2). Likewise, the sensitivity at 5 mM glucose was significantly increased in the presence of SWNTs (74 μA/cm2 3 mM) as compared to control films (26 μA/cm2 3 mM). We demonstrate that the increase in the electrochemical and enzymatic response of these films depends on the amount of SWNTs incorporated and the method of SWNT incorporation. Furthermore, we report that the presence of SWNTs in thick films allows for more of the ferrocene redox centers to become accessible. The high current densities of the hydrogels should allow for the construction of miniature biosensors and enzymatic biofuel cells.
’ INTRODUCTION The incorporation of single-walled carbon nanotubes (SWNTs) into polymer matrices has attracted a growing interest because they allow for enhanced electrical, mechanical, and optical properties. SWNTpolymer composites are being developed for a wide-range of applications such as neuronal1,2 and bone3 tissue engineering constructs, supercapacitors,4 solar cells,5,6 and actuators.7 The enhanced properties of SWNTpolymer composites are dependent upon a number of factors including the SWNT type,8,9 SWNT length,10 SWNT loadings,1113 SWNT dispersion,1416 and SWNTpolymer interactions.9,1719 Given the importance of SWNTpolymer interactions in regulating the unique properties of SWNTpolymer composites, both theoretical and experimental studies have been performed. Recent molecular dynamics simulations have suggested that a polymer’s affinity for SWNTs is strongly influenced by the monomer structure,9,20 such as aromatic rings. Similarly, experimental studies have demonstrated that amines and polyamines have a high affinity for the sidewalls of single-walled carbon nanotubes.2123 Consequently, both branched (BPEI) and linear (LPEI) poly(ethylenimine) have been frequently combined with SWNTs because they have the highest density of amines among polyamine polymers. For example, Munoz et al. demonstrated that PEI-SWNT fibers have increased mechanical and electrical properties,23 while other groups have combined PEI and SWNTs for the development of field-effect transistors,24 substrates for neuronal cell adhesion,2 or CO2 adsorption.25 r 2011 American Chemical Society
In the field of biosensors, BPEI has primarily been used with both SWNTs and multiwalled carbon nanotubes (MWNTs) as a way to disperse carbon nanotubes2630 and/or link biological molecules such as antibodies,31,32 enzymes,26,28 or DNA27 to the nanotubes. Consequently, sensors for glucose,26,28 hydrogen peroxide,26,28,29 neurotransmitters,29 DNA damage,27 and immunosensors for carcinoembryonic antigen31 and human immunoglobulin G32 have been developed. In an alternative scheme, Yan et al. dispersed SWNTs in BPEI, using the imine groups as a platform to tether ferrocene redox centers to the SWNT-BPEI matrix before adsorbing the enzyme glucose oxidase.30 Recently, our group has synthesized redox polymers based on attaching ferrocene to linear poly(ethylenimine) (LPEI).3336 These polymers have been shown to efficiently communicate with the redox centers of enzymes (e.g., glucose oxidase,34,35 horseradish peroxidase34), producing current densities as high as 2 mA/cm2. Our group has also previously demonstrated that the addition of SWNTs into cross-linked films of the redox polymer poly[(vinylpyridine)Os(bipyridyl)2Cl2þ/3þ] (PVP-Os) and glucose oxidase increased the oxidation and reduction peak currents during cyclic voltammetry 210 fold, while the glucose electrooxidation current was increased 3-fold.13 In this work, we characterize biosensors constructed by incorporating SWNTs into solution cast films of the ferrocene redox polymer Fc-C6-LPEI Received: December 16, 2010 Revised: March 19, 2011 Published: April 11, 2011 6201
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Figure 1. Structure of the Fc-C6-LPEI redox polymer.
(Figure 1) and the enzyme glucose oxidase (GOX). Films of Fc-C6-LPEI and GOX, with and without SWNTs, were formed by cross-linking the secondary amine groups on the LPEI backbone with the free amino groups of the lysine residues of GOX through epoxide chemistry. In addition to reporting the electrochemical and enzymatic properties of these composite films, we also investigate two different ways of SWNT incorporation.
’ EXPERIMENTAL SECTION Chemicals and Solutions. Glucose oxidase (GOX) from Aspergillus niger (EC 1.1.3.4, Type X-S, 136.1 units/mg of solid, 75% protein) and salts were purchased from Sigma-Aldrich. Ethylene glycol diglycidyl ether (EGDGE) was purchased from Polysciences Inc., Warrington, PA. All chemicals were used as received. The redox polymer, designated as Fc-C6-LPEI, was synthesized by using the previously described procedure35 and had a ferrocene substitution of ∼15%. Solutions of 2 M glucose were allowed to mutarotate for 24 h before use and subsequently kept refrigerated at 4 C. A phosphate buffer solution (50 mM) was prepared by dissolving 6 g of NaH2PO4 in 1 L of Nanopure deionized water. SWNTs produced by the CoMoCAT synthesis method and purified by air oxidation and hydrofluoric acid treatment37 were kindly provided to us by SouthWest Nanotechnologies. The SWNTs once received were sonicated in deionized water with a homogenizer (22% amplitude, Cole-Parmer model CPX) to debundle the tube aggregates. Electrochemical Instrument. Cyclic voltammetry and constant potential experiments were performed with a CH Instruments model 832 bipotentiostat (Austin, TX). Electrochemical impedance measurements were performed with a Solartron SI 1260 impedance/gain-phase analyzer (Solartron Analytical, Oak Ridge, TN) in conjunction with a SI 1287 potentiostat (Solartron Analytical, Oak Ridge, TN). Unless otherwise stated, all electrochemical impedance spectroscopy experiments were performed at a DC potential of 0.35 V and an AC perturbation of 10 mV. All experiments were conducted in a three-electrode cell configuration with a saturated calomel reference electrode (SCE) and a platinum wire counter electrode in 50 mM phosphate buffer pH 7 at room temperature (25 C). Preparation of Enzyme Sensors. Incubation Method. A GOX/ SWNT solution was prepared by dissolving sufficient enzyme in the premade SWNTs solution (13.8 mg/mL in H2O) to make a 10 mg/mL GOX solution. The GOX/SWNT solution was then kept in the refrigerator at 4 C for 18 h. The redox hydrogel was made by mixing 14 μL of a Fc-C6-LPEI polymer solution (10 mg/mL in H2O), 6 μL of the vortexed GOX/SWNT solution, and 0.75 μL of an vortexed aqueous EGDGE (10% v/v) solution. A 3 μL aliquot of the Fc-C6-LPEI/SWNTGOX/EGDGE mixture was then deposited onto the glassy carbon
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electrode surface and allowed to cure for at least 18 h at room temperature. Mixing Method. The redox hydrogel was made by mixing the following solutions in order. First, 14 μL of a Fc-C6-LPEI polymer solution (10 mg/mL in H2O) and 3 μL of SWNTs solution (27.6 mg/mL in H2O) were mixed by vortexing, next 3 μL of GOX solution (20 mg/mL in H2O) was added and vortexed, and finally 0.75 μL of an aqueous EGDGE (10% v/v) solution was added and vortexed. A 3 μL aliquot of the Fc-C6-LPEI/SWNT/GOX/EGDGE mixture was then deposited onto the glassy carbon electrode surface and allowed to cure for at least 18 h at room temperature. It should be noted that, although the stock concentrations of SWNTs and GOX were different in the incubation and mixing methods described above, the final concentrations of both SWNTs and GOX in the composite films were the same. Control Sensor. Sensors made without incorporating SWNTs into the redox hydrogel film were made by mixing the following solutions: 14 μL of a Fc-C6-LPEI polymer solution (10 mg/mL in H2O), 6 μL of GOX solution (10 mg/mL in H2O), and 0.75 μL of an aqueous EGDGE (10% v/v) solution. A 3 μL aliquot of the Fc-C6-LPEI/GOX/EDGE mixture was then deposited onto the glassy carbon electrode surface and allowed to cure for at least 18 h at room temperature. SWNT and Film Loading Experiments. To determine the effect of SWNT loading, solutions of SWNTs solutions were premade at different concentrations (3.45, 6.9, 10.35, 13.8, 20.7, and 27.6 mg/mL) and incorporated into the redox polymer/enzyme films by the mixing method. The corresponding SWNTs weight percents in films were computed on the basis of the total water-free film weight to be 3.5%, 6.7%, 9.8%, 12.6%, 17.8%, and 22.4%. For the film loading experiments, solutions with 22.4 wt % SWNT were prepared by the mixing method, and the solution drop size (0.25, 0.5, 1.0, 1.5, 3.0, or 5.0 μL) deposited on the electrode was varied to have different film thicknesses. Calculations and Statistics. Values are presented as mean ( standard error of the mean unless otherwise specified.
’ RESULTS Effect of SWNT Incorporation Method on Electrochemistry, Electron Transport, and Communication with Enzymes of Fc-C6-LPEI Hydrogels. Previously, we demonstrated that the
incorporation of conjugates of GOX and SWNTs (GOX-SWNTs) into cross-linked films of the redox polymer PVP-Os increased both the electrochemical and the enzymatic response of these films.13 Because PEI has been shown to strongly interact with carbon nanotubes and the properties of SWNTpolymer composites are dependent upon the polymer type, we investigated whether incorporation of GOX-SWNT conjugates into Fc-C6LPEI hydrogels would have similar effects. In our original “incubation method” of SWNT incorporation, conjugates of GOX and SWNTs were produced by first incubating the SWNTs in a GOX solution for 18 h prior to mixing with the redox polymer and cross-linker. In this study, we also tested an alternative procedure to incorporate SWNTs into the redox hydrogels. In the new “mixing method”, we replaced the time-consuming incubation procedure of SWNTs in enzyme solution with a simple mixing procedure in which SWNTs were added to the polymer solution followed by addition of the enzyme and cross-linker. The mixing procedure reduced the preparation time from 18 h to less than an hour. Figure 2A shows the cyclic voltammograms of glassy carbon electrodes coated with: (i) Fc-C6-LPEI/GOX hydrogels without SWNTs (control), (ii) Fc-C6-LPEI/GOX/SWNT hydrogels made by the incubation method (22 wt % SWNT), and (ii) Fc-C6-LPEI/ GOX/SWNT hydrogels made by the mixing method (22 wt % 6202
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Figure 2. Effect of SWNT incorporation method on electrochemical and enzymatic Response. (A) Representative CVs of glassy carbon electrodes (GCE) coated with redox polymer (Fc-C6-LPEI) and enzyme (GOX) alone (control), a GCE coated with a composite of Fc-C6-LPEI-GOX-SWNT made by the mixing method (22 wt % SWNT), and a GCE coated with a composite of Fc-C6-LPEI-GOX-SWNT made by the incubation method (22 wt % SWNT) (scan rate = 50 mV/s, T = 25 C). (B) Nyquist plots corresponding to the impedance spectra for the three types of sensors described in (A). Impedance measurements were performed at a potential of 0.35 V vs SCE, using an AC perturbation of 10 mV in the frequency range from 100 mHz to 100 kHz. (C) Effect of film type on the value of cDe1/2 as measured from the EIS experiments. (D) Glucose calibration curves for the three types of sensors described in (A) (T = 25 C, E = 0.5 V vs SCE).
SWNT). Incorporation of SWNTs into the cross-linked films by the incubation method or the mixing method had several similar effects. First, the presence of SWNTs shifted both the oxidation and the reduction potential peaks and resulted in a broader potential peak separation (ΔEp,incubation = 186 ( 6 mV, ΔEp,mixing = 180 ( 6 mV) as compared to the peak separation (ΔEp,no SWNTs = 113 ( 4 mV) in films without SWNTs. Second, while the oxidation (360 mV) and reduction peaks (250 mV) were well-defined in the control films without SWNTs, the corresponding peaks of the incubation and mixing SWNT-modified films were significantly broadened. Third, both the background and the anodic peak current of the incubation (ip,a = 108 ( 3 μA) and mixing (ip,a = 81 ( 4 μA) SWNT-modified films were significantly larger than those of films without SWNTs (ip,a = 21 ( 1 μA). Increases in background currents with SWNTs films have been observed by others38,39 and are attributed to the high surface area of SWNTs, which increases the electrode area as well as the double layer charging and background reactions. As suggested in our previous work on SWNT incorporation into redox polymer films of PVP-Os,13 we attribute the increased peak currents to the SWNTs making more of the redox centers electrochemically accessible. The potential cause(s) of the broadening of the peak separation potential will be discussed below in the section on SWNT loading.
To further investigate the nature of electron transfer in the incubation and mixing SWNT-modified film types, we measured the electron transport, cDe1/2, and interfacial electron transfer resistance using electrochemical impedance spectroscopy (EIS). Figure 2B shows Nyquist plots of control films without SWNTs and films containing SWNTs made by the incubation and mixing method. The control films exhibited characteristic linear curves in the entire frequency range tested, which was indicative of the diffusion-controlled process of electron transfer. In contrast, films made with SWNTs by both the incubation and the mixing methods exhibited a large diameter semicircle domain that is characteristic of an increased interfacial electron transfer resistance. Measurements of the rate of electron transport, cDe1/2 (c is the concentration of redox sites, De is electron diffusion coefficient), through the film were made by analyzing the impedance response in the low-frequency range using a Randles circuit and plotting the imaginary impedance, Im(Z), versus the inverse square root of frequency, ω1/2 (data not shown). From the slope of this graph (i.e., Warburg coefficient), the value of cDe1/2 for each film type was determined as previously described.33,40 As shown in Figure 2C, both the incubation and the mixing methods of SWNT incorporation significantly increased values of cDe1/2. 6203
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Table 1. Effects of Method of SWNT Incorporation on Biosensor Responsea film type (n = no. of electrodes) control (n = 10) incubation method
Jmax
Km
sensitivity
(μA/cm2)
(mM)
(μA/cm2 3 mM)
425 ( 12 3090 ( 150
15.6 ( 0.2 52.2 ( 4.2
26.0 ( 0.9 73.9 ( 3.9
2650 ( 200
41.9 ( 1.9
71.3 ( 3.9
(n = 10) mixing method (n = 4) a
Jmax is the maximum current obtained experimentally at saturating glucose concentrations. Km was determined by fitting the data with Excel Solver. Sensitivity was determined from the experimental current response at 5 mM glucose concentration. Values are expressed as mean ( standard error of the mean. The SWNT content in both the incubation and the mixing method films was 22 wt % (E = 0.5 V vs SCE, T = 25 C, pH 7.4).
To determine the effect of SWNT incorporation on the enzymatic response of the films to glucose, we performed constant-potential amperometry by poising the electrodes at 0.5 V vs SCE and measuring the output current as aliquots of a stock 2 M glucose were added to a well-stirred solution. As shown in Figure 2D and Table 1, all three films displayed MichaelisMenten-type behavior. Incorporation of SWNTs either by the mixing or by the incubation method led to a ∼3-fold increase in the sensitivity of the sensors at low glucose concentrations (i.e., 5 mM), a ∼7-fold increase in the maximum current output under saturating glucose, and a ∼3-fold increase in the Km. An increased value of Km in the films containing SWNTs may be the result of either (i) a change in the enzyme’s conformation that lowers the enzyme’s binding affinity for the substrate or (ii) an increased mass transfer resistance. As will be discussed below, both the film morphology and the Km of sensors, made by the mixing method, change as the wt % of SWNT increases, which suggests a correlation between them. Although we cannot differentiate between these two mechanisms at this time, mass transfer through the film will depend upon the film’s microstructure, and thus we hypothesize that the presence of the SWNTs results in an increased mass transfer resistance. In our previous study,13 we suggested that the increase in sensor response to glucose for sensors made by the incubation method could be the result of the enzyme, GOX, becoming partially unfolded as it absorbed onto the walls of the SWNTs during the 18 h incubation, which would allow for improved access to the FAD centers by the osmium redox centers. However, because the mixing and incubation method gave similar enzymatic results, this explanation seems less likely. In the mixing method, the SWNTs were first mixed with the Fc-C6-LPEI redox polymer before adding the enzyme. Because polyamines have a high affinity for the sidewalls of single-walled carbon nanotubes, we would expect that the amount of GOX that adsorbs directly to the SWNTs and/or partially denatures would be significantly reduced by the Fc-C6-LPEI redox polymer first adsorbing to the SWNTs in the mixing method. This would suggest that other steps in the bioelectrocatalytic oxidation of glucose are important. These steps include: diffusion of the glucose to the film, electron transfer from glucose to GOX, electron transfer from GOX to a nearby ferrocene redox center, electron transfer through the film (by neighboring ferrocene redox centers and/ or SWNTs), and electron transfer from a ferrocene redox center
Figure 3. Effect of the SWNT loading on film electrochemistry. (A) Representative CVs of GCEs modified with redox hydrogels containing different weight fractions of SWNTs by the mixing method (scan rate = 50 mV/s, T = 25 C). (B) Dependence of the peak oxidation currents during cyclic voltammetry and cDe1/2 from EIS experiments upon the SWNT weight fraction.
to the electrode surface. The fact that both the mixing method and the incubation method had electron transfer rates (Figure 2C) that were significantly higher than the control films without SWNTs suggests that electron transfer through the film may be an important factor. However, because the incubation method had a 2-fold higher rate of electron transfer than the mixing method suggests that either (i) other steps are important as well or (ii) increasing the electron transfer rate above a critical value (i.e., mixing method level) has no additional effect. Effects of SWNT Loading on the Electrochemical and Enzymatic Properties of Fc-C6-LPEI/GOX Hydrogels. Because the enzymatic response of the mixing method was similar to that of the incubation method, but reduced the fabrication time from 18 to 1 h, the remaining studies were performed with sensors fabricated by the mixing method of SWNT incorporation. To explore the effects of SWNT loadings on the cross-linked Fc-C6LPEI/GOX hydrogels, different SWNT weight percents (022%) were incorporated into the cross-linked polymeric film by the mixing method while keeping the amounts of polymer and enzyme constant. Figure 3 shows the results of SWNT loading via the mixing method on the cyclic voltammograms. At low loadings of SWNTs (200 μg/cm2 the values remained relatively constant. These results mirror the CV peak current response (Figure 7D) and would suggest that above a critical thickness, additional ferrocene sites at the outer edge of the polymer film are no longer electrochemically accessible. In contrast, films made with SWNTs showed an almost linear increase in cDe1/2 with film loading, which suggests that the presence of SWNTs allows for more distant ferrocene redox centers to be accessible in thicker films. It is worth noting that there was only a small difference in cDe1/2 between films 6207
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Figure 10. Effect of SWNTs on the operational enzymatic stability of glucose biosensors. Cross-linked films of GOX and Fc-C6-LPEI films containing no SWNTs and 22.4 wt % SWNTs were operated continuously in PBS at pH 7.4 and 10 mM glucose. E = 0.4 V, T = 25 C. Films that contained SWNTs were made by the mixing method.
Figure 11. Electron transfer mechanisms. (A) In films containing only redox polymer (Fc-C6-LPEI) and enzyme (GOX), electrons are transferred from the enzyme’s FAD redox center through the film by selfexchange between neighboring reduced and oxidized redox centers. (B) In films containing SWNTs, electrons can be transferred not only by the self-exchange mechanism shown in (A), but also by electron transfer through the SWNTs to more distant redox centers (black line) or directly to the electrode surface through an interconnected SWNT network (blue line).
Figure 9. Effect of the film loading on enzymatic response to glucose. Glucose calibration curves for GCEs modified with redox hydrogels without SWNTs (A) and with SWNTs (B) at different film loadings. (C) Dependence of the maximum glucose current density on the film loading. Films that contained SWNTs were made by the mixing method.
without and with SWNTs films at low film loadings. However, as we increased the film’s thickness, the difference became more prevalent. These results are in agreement with our previous results of SWNTs enhancing the CV peak currents in thick PVP-Os films.13 The effects of film loading on the enzymatic response of films made with and without SWNTs are shown in Figure 9. For films without SWNTs (Figure 9A), the sensitivity to glucose over the entire glucose concentration range tested increased for low polymer loadings, and then was essentially constant for film loadings >200 μg/cm2. In contrast, increasing the loading of films containing SWNTs resulted in a significant increase in the response to glucose (Figure 9B). Similar to the measurements of cDe1/2,
the current response to glucose at saturating conditions initially increased with film loading and then plateaued when SWNTs were absent (Figure 9C), while the current steadily increased with film loadings containing SWNTs. Effects of SWNTs on Enzymatic Stability of Films. Stability tests were performed to determine whether the incorporation of SWNTs into the Fc-C6-LPEI/GOX films made by the mixing method would affect the stability of the films. Figure 10 shows continuous operation stability tests for sensors made without SWNTs and with 22 wt % SWNTs. After 24 h of operation, films without SWNTs retained 55% of their original current, while films with 22 wt % SWNTs retained 67%. The stabilities of the films without SWNTs are similar to those previously reported.35 These results suggest that incorporation of SWNTs into the films can increase the enzymatic stability of the films. The exact mechanism for this improved stability is unknown at this time, but recent studies have suggested that highly curved nanoscale supports as compared to flat surfaces allow for increase enzyme stability.44,45 Thus, one possibility is that during the film curing process GOX adsorbs onto the surface of the SWNTs, allowing for increased 6208
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Langmuir enzyme stability. This explanation is tentative and currently under investigation in our lab. Proposed Mechanism. Taking all of these results together, we propose the following mechanism for the enhancement in both the electrochemical and the enzymatic responses of films made with SWNTs. In films without SWNTs, electrons are transferred from the FADH2 redox centers of GOX to ferricinium redox centers, which are within the electron transfer distance.46,47 Electrons are then transferred by self-exchange between neighboring reduced (electron-loaded) and oxidized (hole-loaded) redox centers until they reach the electrode surface.47,48 For example, in the schematic of Figure 11A, electrons must be relayed through 4 ferricinium redox centers (Figure 11A). This shuttling of electrons is a diffusion-like process, and when ferrocene redox centers or GOX molecules are beyond a critical distance from the electrode surface, the electrons from these distant centers cannot diffuse to the electrode surface within the time frame of the experiment, and thus are electrochemically inaccessible.49 However, we hypothesize that when SWNTs are present in the films, they provide an alternative and faster route of electron transfer from distant ferrocene and enzyme redox centers. For example, in Figure 11B, after an electron is transferred from the GOX to the neighboring ferricinium redox center, the electron is then transferred to a nearby SWNT. The electron in the initial SWNT can then be transferred to a distant ferricinium center that is close to the electrode surface (black arrow). Alternatively, the electron in the initial SWNT could be transferred to an interconnected SWNT that is also connected to the electrode surface (blue arrow). Either pathway should increase the rate of electron transfer by reducing the number of redox center self-exchange steps required from 4 to either 2 or 1, and thereby allow more of the distant ferrocene redox centers to be electrically accessible.
’ CONCLUSION In this study, we demonstrate that the incorporation of SWNTs into the Fc-C6-LPEI/GOX redox hydrogels increases both the electrochemical and the enzymatic response of the films. Incorporation of SWNTs by either the mixing or the incubation method led to 59-fold increases in the rate of electron transfer (cDe1/2) as well as a ∼5-fold increase in the enzymatic response to glucose. We also demonstrate that a critical SWNT concentration of 9 wt % was required for the electron transport and enzymatic response to increase as compared to control films. These data suggest that below this concentration the SWNTs may connect locally but form a noncontinuous network, whereas above this percolation threshold the SWNTs form a continuous network. Furthermore, we also demonstrate that the presence of SWNTs facilitated both electron transport through the films and collection of electrons from GOX in thick films. The maximum current densities to glucose in the Fc-C6LPEI/GOX/SWNT composites (2.53 mA/cm2) are approximately 3-fold higher than similar composite films made by our group with the redox polymer PVP-Os (1 mA/cm2).13 Likewise, the glucose response reported in this study is ∼10-fold higher than the response to glucose (20 μA or 280 μA/cm2) reported by Yan et al.30 who combined SWNTs with branched poly(ethylenimine) (BPEI) modified with ferrocene. We have previously shown that there are differences between the enzymatic response of films made with glucose oxidase and ferrocenemodified LPEI and ferrocene-modified BPEI.33 A second difference between the two studies is that we attached the ferrocene to
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the LPEI backbone before combining with SWNTs, while in the Yan study, ferrocene was attached to the BPEI backbone only after it had been adsorbed to the surface of the SWNTs. Taken together, these observations suggest that redox polymer structure plays an important role in the interactions between redox polymers and SWNTs. These high current densities are significant because they should allow for (i) the construction of biosensors with reduced dimensions, (ii) lower detection limits, and (iii) increased current and power outputs in redox polymer-based enzymatic biofuel cells.
’ AUTHOR INFORMATION Corresponding Author
*Phone: (405) 325-7944. Fax: (405) 325-5813. E-mail: dschmidtke@ ou.edu.
’ ACKNOWLEDGMENT This work was partially supported by NSF grants (CBET 0547619, CBET 0967988), the Center for Semiconductor Physics in Nanostructures (C-SPIN), an OU/UA NSF-funded MRSEC (DMR-0520550), a U.S. Department of Energy grant (DEFG0206ER64239), and an Oklahoma Bioenergy Center grant (Project No. 100020). ’ REFERENCES (1) Gheith, M. K.; Sinani, V. A.; Wicksted, J. P.; Matts, R. L.; Kotov, N. A. Adv. Mater. 2005, 17, 2663–2670. (2) Hu, H.; Ni, Y.; Mandal, S. K.; Montana, V.; Zhao, B.; Haddon, R. C.; Parpura, V. J. Phys. Chem. B 2005, 109, 4285–4289. (3) Shi, X.; Hudson, J. L.; Spicer, P. P.; Tour, J. M.; Krishnamoorti, R.; Mikos, A. G. Nanotechnology 2005, 16, S531–S538. (4) Wang, J.; Xu, Y.; Chen, X.; Sun, X. Compos. Sci. Technol. 2007, 67, 2981–2985. (5) Pasquier, A. D.; Unalan, H. E.; Kanwai, A.; Miller, S.; Chhowalla, M. Appl. Phys. Lett. 2005, 87, 203511. (6) Landi, B. J.; Castro, S. L.; Ruf, H. J.; Evans, C. M.; Bailey, S. G.; Raffaelle, R. P. Sol. Energy Mater. Sol. Cells 2005, 87, 733–746. (7) Landi, B. J.; Raffaelle, R. P.; Heben, M. J.; Alleman, J. L.; VanDerveer, W.; Gennett, T. Nano Lett. 2002, 2, 1329–1332. (8) Kanai, Y.; Grossman, J. C. Nano Lett. 2008, 8, 908–912. (9) Zheng, Q. B.; Xue, Q. Z.; Yan, K. O.; Hao, L. Z.; Li, Q.; Gao, X. L. J. Phys. Chem. C 2007, 111, 4628–4635. (10) Tasca, F.; Gorton, L.; Wagner, J. B.; Noll, G. Biosens. Bioelectron. 2008, 24, 272–278. (11) Das, N. C.; Liu, Y. Y.; Yang, K. K.; Peng, W. Q.; Maiti, S.; Wang, H. Polym. Eng. Sci. 2009, 49, 1627–1634. (12) MacDonald, R. A.; Laurenzi, B. F.; Viswanathan, G.; Ajayan, P. M.; Stegemann, J. P. J. Biomed. Mater. Res., Part A 2005, 74A, 489–496. (13) Joshi, P. P.; Merchant, S. A.; Wang, Y. D.; Schmidtke, D. W. Anal. Chem. 2005, 77, 3183–3188. (14) Boge, J.; Sweetman, L. J.; Panhuis, M. I. H.; Ralph, S. F. J. Mater. Chem. 2009, 19, 9131–9140. (15) Tsai, T. W.; Heckert, G.; Neves, L. F.; Tan, Y. Q.; Kao, D. Y.; Harrison, R. G.; Resasco, D. E.; Schmidtke, D. W. Anal. Chem. 2009, 81, 7917–7925. (16) Kashiwagi, T.; Fagan, J.; Douglas, J. F.; Yamamoto, K.; Heckert, A. N.; Leigh, S. D.; Obrzut, J.; Du, F. M.; Lin-Gibson, S.; Mu, M. F.; Winey, K. I.; Haggenmueller, R. Polymer 2007, 48, 4855–4866. (17) Guo, H. N.; Minus, M. L.; Jagannathan, S.; Kumar, S. ACS Appl. Mater. Interfaces 2010, 2, 1331–1342. (18) Sen, R.; Zhao, B.; Perea, D.; Itkis, M. E.; Hu, H.; Love, J.; Bekyarova, E.; Haddon, R. C. Nano Lett. 2004, 4, 459–464. 6209
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dx.doi.org/10.1021/la104999f |Langmuir 2011, 27, 6201–6210
