Enhanced Sensitivity for Biosensors: Multiple Functions of DNA

Enhanced Sensitivity for Biosensors: Multiple Functions of DNA-Wrapped Single-Walled ... It has been applied widely in chemical sensors2-4 but not as ...
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J. Phys. Chem. B 2006, 110, 16359-16365

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Enhanced Sensitivity for Biosensors: Multiple Functions of DNA-Wrapped Single-Walled Carbon Nanotubes in Self-Doped Polyaniline Nanocomposites Yufeng Ma, Shah R. Ali, Afua S. Dodoo, and Huixin He* Chemistry Department, Rutgers UniVersity, 73 Warren Street, Newark, New Jersey 07102 ReceiVed: March 10, 2006; In Final Form: June 18, 2006

A nanocomposite of poly(anilineboronic acid), a self-doped polyaniline, with ss-DNA-wrapped single-walled carbon nanotubes (ss-DNA/SWNTs) was fabricated on a gold electrode by in situ electrochemical polymerization of 3-aminophenylboronic acid monomers in the presence of ssDNA/SWNTs. We used this nanocomposite to detect nanomolar concentrations of dopamine and found that the sensitivity increased 4 orders of magnitude compared to the detection at an electrode modified with only poly(anilineboronic acid). For the first time, this work reports the multiple functions of the ss-DNA/SWNTs in the fabrication and biosensor application of a self-doped polyaniline/ss-DNA/SWNT nanocomposite. First, the ss-DNA/SWNTs acted as effective molecular templates during polymerization of self-doped polyaniline so that not only was the polymerization speed increased but also the quality of the polymer was greatly improved. Second, they functioned as novel actiVe stabilizers after the polymerization, significantly enhancing the stability of the film. Furthermore, the ss-DNA/SWNTs also acted as conductiVe polyanionic doping agents in the resulting polyaniline film, which showed enhanced conductivity and redox activity. Finally, the large surface area of carbon nanotubes greatly increased the density of the functional groups available for sensitive detection of the target analyte. We envision that polyaniline with other functional groups as well as other conducting polymers may be produced for different targeted applications by this approach.

Introduction Conducting polymers are attractive for sensor applications because their electronic and electrochemical properties are highly sensitive to molecular interactions, which provide excellent signal transduction for molecular detection.1 Among conducting polymers, polyaniline is unique since it is environmentally stable and easy to fabricate. It has been applied widely in chemical sensors2-4 but not as much in biosensors.5-7 The reason is that native polyaniline is neither electrochemically active nor conductive in neutral solutions, which is a prerequisite for biosensor applications. It is also limited both in the variety of molecules that can be detected and in the selectivity of the detection. Major breakthroughs in this field were the discoveries of self-doped polyaniline8-11 and polyelectrolyte-anion-doped polyaniline,6,7,12 which brought polyaniline into the biosensor field due to the improved redox activity and conductivity in neutral pH solutions. However, compared to the parent polyaniline, the electrochemical activity, conductivity, and the chemical and mechanical stabilities of both self-doped polyaniline and bulky polyelectrolyte-doped polyaniline are greatly reduced due to steric effects. Here we report that the stability of a self-doped polyaniline, poly(anilineboronic acid) in this work, is greatly improved when it is polymerized in situ with ss-DNA-wrapped single-walled carbon nanotubes (ss-DNA/SWNTs) (see Scheme 1). The redox properties of the polyaniline backbone are conserved in neutral solutions (pH 7.4), and the sensitivity for biomolecular detection is significantly enhanced. We found that the ss-DNA/ SWNTs performed multiple roles in the greatly improved properties of the self-doped polyaniline both during and after * Corresponding author. Tel: (973) 353-1254. Fax: (973) 353-1264. E-mail: [email protected].

SCHEME 1. A ss-DNA-Wrapped Single-Walled Carbon Nanotube (ss-DNA/SWNT)a

a The schematic is only a graphical presentation and does not represent the precise way ss-DNA binds on SWNTs.

the polymerization, which makes this work unique compared to previously reported conducting polymer/carbon nanotube composites.13-19 First, they acted as effective molecular templates during polymerization of the self-doped polyaniline to increase the polymerization speed and improve the quality of the polymer. Second, they functioned as novel actiVe stabilizers after the polymerization. One difficulty in working with selfdoped polyaniline is that the fully oxidized pernigraniline form is not stable in aqueous solution, and it is difficult to fully reduce back to the emeraldine and leucoemeraldine states.7,20-23 In our case, pernigraniline was readily reduced in the electrochemical

10.1021/jp0614897 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/02/2006

16360 J. Phys. Chem. B, Vol. 110, No. 33, 2006 experiments due to the electrocatalytic reductive ability of ssDNA/SWNTs.24,25 This ease in reduction significantly enhanced the stability of the film. Furthermore, the ss-DNA/SWNTs also acted as conductiVe polyanionic doping agents in the resulting polyaniline film that showed enhanced conductivity and redox activity. Finally, the very large surface area of the carbon nanotubes greatly increased the density of the functional groups available for sensitive detection of the target analytes. We demonstrate that dopamine concentrations as low as 1 nM can be detected without optimization of the sensing technique. The sensitivity is 104 times greater than at electrodes modified only with poly(anilineboronic acid).44 This approach thus holds great potential for molecular diagnosis of Parkinson’s disease. We envision that polyaniline with different functional groups, as well as other conducting polymers, can be produced for many different targeted applications using this approach.

Ma et al. The immobilization of the ss-DNA/SWNTs and ds-DNA, and the polymerization of 3-aminophenylboronic acid were performed following the exact experimental procedure in our previous work2 as summarized below. (1) A piece of mica surface was first modified with (aminopropyl)triethoxysilane28 using a vapor phase method to provide good adhesion between the ssDNA-wrapped carbon nanotube (or the ds-DNA) and the substrate surface. (2) ss-DNA/SWNTs (or ds-DNA) were immobilized on the modified mica surface through electrostatic attraction. (3) Then the mica surface with the ss-DNA/SWNTs (or ds-DNA) was incubated in 3-aminophenylboronic acid monomer solution at pH 4.0. (4) Free monomers were carefully washed away with an electrolyte solution. (5) The 3-aminophenylboronic acid monomers were enzymatically polymerized into poly(anilineboronic acid) along the ss-DNA/SWNTs (or ds-DNA) in the same solution by adding horseradish peroxidase and H2O2 successively.29

Experimental Section Materials. Purified HiPco single-walled carbon nanotubes were purchased from Carbon Nanotechnologies, Inc. Houston, TX. Single-stranded DNA with sequence d(T)30 was purchased from Integrated DNA Technologies, Inc. Coralville, IA. λ-DNA, 48,500 bp were purchased from New England Biolabs Inc. 3-Aminophenylboronic acid hemisulfate salt, 3-hydroxytyramine hydrochloride (dopamine), potassium fluoride, and all other reagents used were of analytical grade purity and were used as received from Aldrich Chemicals Inc., Milwaukee, WI. All solutions were prepared using Nanopure water (18.2 MΩ) (Nanopure water, Barnstead), which was also used to rinse and clean the sample after polymerization and before any characterization. Dispersion of Single-Walled Carbon Nanotubes in Water Solution. The bundled single-walled carbon nanotubes were first dispersed into water using the method described by Zheng et al.26,27 Briefly, 1 mg of purified HiPco SWNT was suspended in 1 mL aqueous ss-DNA solution (1 mg mL-1 in 0.1 M NaCl). The mixture was kept in an ice-water bath and sonicated (Branson, 2510) for 90 min. After sonication, the samples were centrifuged (Eppendorf 5415 C) for 90 min at 16 000g to remove insoluble material, leaving DNA-dispersed nanotube solutions at a mass concentration in the range of 0.2-0.5 mg/mL. Electrochemistry. The gold substrates were cleaned with freshly made piranha solution (caution: piranha solution should be handled with extreme care), rinsed with water, and finally rinsed with ethanol. Then the substrates were immersed into an ethanol solution of 10 mM 2-aminoethanethiol for 24 h, followed by thoroughly washing with ethanol and Nanopure water (Barnstead). The 2-aminoethanethiol-modified Au substrates were exposed to a drop of ss-DNA-wrapped carbon nanotube solution until dry. Electropolymerization of 3-aminophenylboronic acid with and without ss-DNA/SWNTs and electrochemical characterization of the resulting films were carried out at a CH Instrument 750 series electrochemical workstation. Cyclic voltammograms were recorded using a homemade Teflon cell with the modified gold film as the working electrode, a platinum wire as counter electrode, and an Ag wire as quasi reference electrode. The quasi reference electrode was calibrated against the more widely used Ag/AgCl/ saturated KCl reference, and all the potentials quoted in this work are in terms of the Ag/AgCl scale. Atomic Force Microscopy. Atomic force images were obtained using a Nanoscope IIIA (Digital Instruments) in tapping mode operating in ambient air. The height differences on the surface are color-coded. Lighter regions indicate larger heights.

Results and Discussion Increased Electrochemical Polymerization Speed. A gold (Au) electrode was first modified with a self-assembled monolayer (SAM) of 2-aminoethanethiol to promote adsorption of the ss-DNA/SWNTs onto the electrode. Poly(anilineboronic acid) (PABA) was then deposited onto the modified Au substrate by sweeping the electrochemical potential from -0.16 to 0.94 V (versus Ag/AgCl) in 0.05 M 3-aminophenylboronic acid monomer, 0.04 M KF, 0.5 M H2SO4. It was reported that the presence of F- in the monomer solution can decrease the potential required for the polymerization of 3-aminophenylboronic acid,11,30 thus its addition minimized overoxidation of the polyaniline backbone. After the second cycle, the polymerization potential was decreased to 0.89 V to further reduce the possibility of overoxidizing the polyaniline backbone. Continued cycling of the potential resulted in repeated deposition of PABA onto the electrode surface with a slightly lower deposition rate after the first seven cycles (Figure 1a,f). (We noticed that if we kept the same potential range (-0.16 to 0.94 V) during polymerization, the growing speed of the polymer decreased more rapidly with cycles and the obtained film also required a longer time to be stabilized.) Figure 1a shows the CV curves during the polymerization. For comparison, the CV curves that were obtained without immobilization of ss-DNA/SWNTs on gold surfaces are shown in Figure 1b. To clearly read the initial potentials for the polymerization and compare the polymerization current, we illustrate the first cycles for the electrodes with different modifications in Figure 1d. The large irreversible anodic peak observed in the first cycle belongs to the polymerization of 3-aminophenylboronic acid. We found that the monomer polymerized more readily on the ss-DNA/SWNTmodified electrode, indicated by a negative shift (about 150 mV) in the initial polymerization potential (indicated by the arrows in Figure 1d). The maximum current for the polymerization was six times higher in the presence of ss-DNA/SWNTs, providing further support on this point (Figure 1d). In subsequent cycles, two pairs of peaks appeared (Figure 1a,b), which correspond to the redox reactions of the polyaniline backbone: its transformation from insulating leucoemeraldine to conducting protonated emeraldine first and then from conducting protonated emeraldine to insulating pernigraniline.22 Figure 1e displays the CV curves for the last (21st) cycles, which clearly demonstrates that the amount of PABA deposited on the ss-DNA/SWNT-modified electrode was much larger than at the electrode without ss-DNA/SWNT. Figure 1f shows the current (the faradaic current of the second peak) as a function

DNA-Wrapped Single-Walled Carbon Nanotubes

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Figure 1. Cyclic voltammograms recorded during the electropolymerization of 3-aminophenylboronic acid on electrodes from the second cycle to the 21st cycles at (a) ss-DNA/SWNT-modified, (b) 2-aminoethanethiol-SAM-modified, and (c) ds-DNA-modified gold electrodes. Potential scan rate: 100 mV s-1, (d) The first cycles of the polymerization in (a)-(c), (e) The last (21st) cycles of the polymerization in (a)-(c), (f) The faradaic current of the second oxidation peaks of PABA on the modified electrodes as a function of cycles. m is the slope of the fitted lines, in µA/cycle units. The color coding in (a)-(c) corresponds to the different electrode modifications, and is used in (d)-(f).

of the CV cycle numbers for each of the electrodes with different modifications. Without ss-DNA/SWNTs, the faradaic currents grew in intensity during the first 17 cycles and leveled off afterward (Figure 1f, blue), indicating that PABA was deposited onto the electrodes only in the initial stages of the experiment. At the ss-DNA/SWNT modified electrodes, the PABA current increased during all cycles, with a slower increase rate after the seventh cycle. From the slopes of the current vs cycle numbers in Figure 1f, we can estimate the current increase speed, which was 10 times faster than that at the electrode without ss-DNA/SWNTs. After seven cycles, the current increase rate lowered to 10 µA per cycle, but it was still five times higher than that at the electrode without ss-DNA/SWNTs, where the rate remained 2 µA per cycle throughout. Although the potential for polymerization was shifted negatively and the redox current for the polyaniline backbone increased on the electrode with ss-DNA/SWNTs, the oxidation potential of the polymer backbone shifted positively by about 150 mV (Figure 1a,b). It was reported previously that the growth speed of polyaniline can be enhanced by adding polymeric anions, e.g., DNA, into the monomer solution. In those cases a negative shift of the polymerization potential was always accompanied with a positive shift of the oxidation potential of the polyaniline backbone,7,31,32 which are similar to the observa-

tions described here. The effects were attributed to the negative charges along the polyanionic chains that acted as molecular anchors to align the monomers and facilitate the polymerization. The resulting polyaniline required a more positive potential for oxidation than the polyaniline obtained with small doping agents because the bulky structure of the polyanions made the conformational change accompanying the redox reactions more difficult.32 Considering the negative phosphate groups on the surface of the ss-DNA-wrapped carbon nanotubes, one may assume that the observed increased polymerization speed and the positive potential shift of the polyaniline backbone is due to the ss-DNA/SWNTs serving the same function as a polyelectrolyte, such as DNA. To understand whether the ss-DNA or the carbon nanotubes in the ss-DNA/SWNTs played the major role on the increased polymerization speed observed in this system, we deposited a layer of λ-phage DNA on the 2-aminoethanethiol modified Au substrate and then performed the polymerization of PABA under the same conditions as described above. We chose doublestranded λ-phage DNA instead of single-stranded DNA so that the DNA could adsorb onto the positively charged electrode surface and expose the negative phosphate groups to the monomer solution as does the ss-DNA in the ss-DNA/SWNTs template. Surprisingly, we found polymerization of PABA to

16362 J. Phys. Chem. B, Vol. 110, No. 33, 2006 be more difficult on the ds-DNA-modified electrode (Figure 1c,d), as the initial polymerization potential shifted positively by ∼200 mV compared to the electrode with ss-DNA/SWNTs and 90 mV compared to the Au substrate modified only with the 2-aminoethanethiol SAM. The polymerization peak current (at E)0.72V) was slightly increased relative to the latter case, but the increase speed of the redox current (at E)0.38V) from the polyaniline backbone in subsequent cycles was still slower (1.3 µA per cycle) compared to polymerization on the electrode with only 2-aminoethanethiol SAM (2 µA per cycle), as shown in Figure 1f. This relatively lower redox current increase speed may be due to diminished redox activity and conductivity of PABA caused by the bulky polyanionic-doping agent role of DNA, even though a larger amount of PABA polymerized and deposited in the first cycle. Earlier, we reported that ds-DNA molecules could be used as templates to fabricate polyaniline molecular wires on insulating substrates,2 a procedure that relies on the electrostatic attraction of protonated aniline monomers to the negative charges along a DNA template. However, very surprisingly, a nanowire of poly(anilineboronic acid) did not form along the DNA chain when the same experiments were performed for 3-aminophenylboronic acid. PABA deposited everywhere on the surface, and, as a result, individual DNA chains were almost invisible after the polymerization, as shown in Figure 2a,b. All these results indicate that DNA itself did not have enough affinity for the 3-aminophenylboronic acid monomers to preconcentrate them onto the electrode surface, and thus it could not serve as a template for fabrication of poly(anilineboronic acid) nanowires either. The different degrees of pre-emulsification of aniline and 3-aminophenylboronic acid along the DNA templates are possibly due to electrostatic repulsive forces or steric effects caused by the presence of the boronic acid groups on the monomers in the latter case. Herein, we show that nanowires of poly(anilineboronic acid) can be fabricated on mica surfaces by using ss-DNA/SWNTs as growing templates. Figure 2c,d shows the tapping-mode AFM images of the ss-DNA/SWNTs on mica surfaces before and after polymerization of 3-aminophenylboronic acid. The diameters of the ss-DNA/SWNTs are 1.3 nm ( 0.4 nm, measured from their cross sections in the AFM image, averaged from measurements of 30 nanotubes. The apparent heights of the nanotubes after polymerization are around 3.2 nm, 2 nm higher than the ss-DNA/SWNTs alone. The thickness of the PABA coating was greater than that of the parent polyaniline formed on ds-DNA templates in our earlier work, which was only about 0.7-0.8 nm,2 indicating that the surface of the ss-DNA/SWNTs26,27,33 was more efficient than the ds-DNA surface in emulsifying monomers before polymerization. Since the electrostatic attraction provided by the phosphate groups was not strong enough to emulsify and align the monomers of 3-aminophenylboronic acid, we believe that the strong π-π interaction between the monomeric benzene ring and the graphite regions on the carbon nanotube played an especially important role in preconcentrating the 3-aminophenylboronic acid monomers before the polymerization. It was evident from the data that the monomers of PABA emulsify along the nanotube surface graphite regions, but the question arose as to whether they also pre-emulsified along the interior of the nanotubes. Downs et al.34 studied electrochemical polymerization of aniline on a Pt electrode modified with multiwalled-carbon nanotubes. Similar phenomena, such as increased polymerization speed, were observed in their experiment. They were not certain if the carbon nanotubes electro-

Ma et al.

Figure 2. TappingMode AFM images of double standed (ds)-λ phage DNA (a) before and (b) after 3-aminophenylboronic acid polymerization. TappingMode AFM images of ss-DNA/SWNTs (c) before and (d) after 3-aminophenylboronic acid polymerization and their corresponding section analysis showing the thickness of the nanowires. The substrates for all these images were APTES modified mica surface.

chemically catalyzed the polymerization of aniline, and they attributed the phenomena to the confinement of pre-emulsified aniline monomers inside the carbon nanotubes and to the relatively large surface area of the nanotube electrodes. However, in their study they also found that the redox potentials of the polyaniline backbone were negatively shifted compared to polyaniline on a conventional Pt electrode, which is in contrast to this study. The potentials for the polyaniline backbone oxidation shifted positively (about 150 mV) in this study (in the presence of ss-DNA/SWNTs), which is a typical electrochemical behavior of the conducting polymer when it was doped with polyelectrolyte. In addition, considering the pore of a single-walled carbon nanotube is only several angstroms in diameter, and 3-aminophenylboronic acid is more hydrophilic than aniline, we believe that the monomers of poly(anilineboronic acid) are unlikely to pre-emulsify inside of the carbon nanotubes in this study. They possibly just adsorbed and preconcentrated on the surface of the ss-DNA/SWNTs, which acted as conductive polyanionic doping agents after the polymerization. Therefore, we attribute the faster deposition in the first seven cycles, especially in the first cycle, to the preconcentration of the monomers by the carbon nanotubes and to the larger surface area of the electrode. At this stage, we are not certain if the ss-DNA/SWNTs electrochemically catalyzed the polymerization. A prerequisite for continuous electrochemical deposition of film is that the produced film be conductive in each cycle of deposition; otherwise, the electrochemical deposition would selfterminate. The redox current of the polyaniline backbone

DNA-Wrapped Single-Walled Carbon Nanotubes increased much faster on the electrode with ss-DNA/SWNTs than at the electrodes with ds-DNA and with the 2-aminoethanethiol monolayer only (Figure 1a,b,c,f). Figure 1e clearly shows that more PABA was deposited on the electrode with ss-DNA/SWNTs. We attribute the increase in deposition in the subsequent cycles to the increased surface area of the electrodes imparted by the carbon nanotubes and to the better conductivity of the resulting nanocomposite film. After polymerization, the ss-DNA/SWNTs acted as conductive doping agents, which caused the film to be more conductive and electrochemically active compared to the PABA film doped with a nonconductive polyanionic doping agent (i.e. on the electrode with ds-DNA) and small doping agents (on the electrode with the 2-aminoethanethiol SAM only).35 The better conductivity of the nanocomposite film may also be due to better conductivity of the PABA component. Although full characterization of the electronic and the molecular structures of the PABA are still under study, this assumption is supported by our preliminary UVvis analysis of the chemical polymerization of 3-aminophenylboronic acid in the absence and presence of ss-DNA/SWNTs. PABA obtained with ss-DNA/SWNTs absorbs light at a longer wavelength, indicating that longer conjugated structures were obtained. (The details will be published elsewhere soon.) In fact, this assumption is also supported by the reports on the fabrication of conducting polymer nanowires or nanotubes using polycarbonate nanopores as templates.36-38 According to these reports, conducting polymers preferentially nucleated and grew on the walls of the nanopores due to the coexistence of anionic sites and hydrophobic components on the pore walls. The resulting conducting polymer nanotubes normally had higher conductivity than conducting polymers produced by traditional methods due to the longer conjugated structures and better orientation of the polymer chains.36-38 Therefore we conclude that the template effects of the ss-DNA/SWNTs not only include increased polymer growth rate, as described above, but also facilitation of more head-to-tail coupling during polymer growth,32 resulting in PABA with longer conjugated molecular structures. Very possibly, the ss-DNA/SWNT templates could be used in the synthesis of other conducting polymers, which could provide great flexibility in tuning their properties to suit the needs of many applications, including sensors and nanoelectronic devices. Enhanced Stability. Toward the goal of biosensor applications, which require operation in physiological conditions, the deep green PABA film obtained at the ss-DNA/SWNTs electrode was first stabilized in 0.5 M H2SO4 and then in 0.01 M phosphate buffered saline (PBS, pH 7.4) by sweeping the potential between 0.04 and 0.79 V until the cyclic voltammetry (CV) curves were stabilized. Interestingly we found that the stability of the resulting films was also increased by the ssDNA/SWNTs, as is indicated by the fact that the CV curves were easily stabilized in both H2SO4 and PBS for the PABA on the ss-DNA/SWNTs modified electrodes. Figure 3a,b shows the CV curves in H2SO4 solution obtained for PABA at electrodes with and without the ss-DNA/SWNTs. In the absence of the ss-DNA/SWNTs, the current decreased continuously upon cycling, and the oxidation and reduction peaks of the polyaniline backbone also shifted significantly to negative potentials. Figure 3b shows that after 180 cycles, the redox peaks of polyaniline are still decreasing and the peak positions are still shifting. Holding the film at -0.3 V for 5 min prior to the scan caused the CV responses to be recovered to a certain extent. A similar unstable behavior was observed when the polymerization was performed on the ds-DNA

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Figure 3. Successive cyclic voltammograms of poly(anilineboronic acid) in H2SO4 solutions on electrodes (a) with and (b) without ssDNA/SWNTs; potential scan rate: 100 mV s-1.

modified electrodes (data not shown). This behavior has often been observed when the parent polyaniline film was cycled in higher pH solutions,7,20 and it was attributed to the inability to reduce the fully oxidized pernigraniline completely over the time scale of the cyclic voltammetry experiment. On the contrary, with the ss-DNA/SWNTs on the electrode, the CV curve of the ninth cycle is almost identical to the curves obtained from previous cycles, indicating that the film was stabilized by the ninth cycle. It was reported that incorporation of Nafion during polymerization improved the stability of PABA films, presumably due to electrostatic interaction between PABA and Nafion, which enhanced the adhesion between PABA film and the electrodes.7,39 Although the negative phosphate groups alone on the surface of the ss-DNA-wrapped SWNTs could not hold PABA by electrostatic interaction, the stability of the nanocomposite film is likely due to the ss-DNA/SWNTs serving the same function as Nafion, in this case by the strong π-π interaction between PABA and the carbon nanotubes. The enhanced stability of the PABA in the presence of carbon nanotubes is also possibly due to the longer conjugated molecular structures in the resulting PABA film, as discussed above, which may be more stable. Another possible reason for the enhanced stability may be the synergetic interaction of ss-DNA and SWNTs, which results in the electrocatalytic reductive ability of the ss-DNA/ SWNTs.24,25 Different from carbon nanotubes alone, ss-DNAwrapped carbon nanotubes are surprisingly effective electron donors, instead of electron acceceptors,13,40 and can be readily oxidized by strong oxidants such as KMnO4 and K2IrCl6.24 Recently the electrocatalytic reduction ability of the ss-DNA/ SWNT was demonstrated in the study of electrochemical oxidation of Ru(bpy)32+. The ss-DNA/SWNTs reduced electrogenerated Ru(III) during electrooxidation of chemical Ru(bpy)32+.25 Polyaniline has three states, and the fully oxidized pernigraniline state is not stable in aqueous solutions and it is difficult to be fully reduced. Both these characteristics could

16364 J. Phys. Chem. B, Vol. 110, No. 33, 2006 cause instability of the polyaniline film.7,20-23 Since the electropolymerization potential of 3-aminophenylboronic acid is more positive than the oxidation potential of the emeraldine state to pernigraniline state transition, the product of the polymerization naturally is in the fully oxidized pernigraniline state. However, due to the ss-DNA/SWNTs, the pernigraniline produced during the potential cycling was readily reduced to the stable emeradine state, like the electrogenerated Ru(III) was reduced during the electrooxidation of Ru(bpy)32+. Therefore, their presence greatly decreased the possibility of the polyaniline backbone’s degradation. It also helped maintain the electrochemical activity of the film in the time scale of potential scanning. Without the ssDNA/SWNTs, the only way to reduce the pernigraniline is to shift the potential to more negative values or to increase the time for reduction. Furthermore, the redox potentials of the polyaniline backbone kept shifting to more negative values in the absence of carbon nanotubes, which would make formation of pernigraniline easier and thus increase its quantity during the potential cycles. Therefore, even more serious degradation is induced in the absence of ss-DNA/SWNTs. Enhanced Sensitivity for Biosensor Applications. The great stability of the nanocomposite film and its electrochemical activity in pH 7.4 PBS allow us to use the self-doped polyaniline for sensitive detection of biomolecules in solutions approximating physiological conditions. In this work, we take dopamine as an example, which is a biogenic amine that acts as a neurotransmitter in the brain.41 It is suspected that an abnormal loss of dopamine in the nanomolar range leads to various neurodegenerative diseases, such as Parkinson’s disease and schizophrenia.42,43 However, it remains challenging to detect dopamine in nanomolar concentrations, and methods are actively pursued.44-48 We found that dopamine concentrations as low as 1 nM can be detected by modifying the gold electrode surface with a thin layer of PABA/ss-DNA/SWNTs composite as described above. Detection of dopamine with different concentrations was performed after the composite film was stabilized in pH 7.4 PBS solutions and monitored by cyclic voltammetry (CV). Various concentrations of dopamine were added into the electrochemical cell, and the electrochemical current of the polyaniline backbone was measured. Dopamine chemically binds to the boronic acid group, as shown in the scheme, which hinders the electrochemical activity of the polyaniline backbone, perhaps due to steric effects.44

From Figure 4a we can see that dopamine can induce a reproducible current decrease in the CV curves, and the current decreases as the concentration of dopamine increases. Figure 4b shows that the relative current decreases are proportional to the increase of the concentration of dopamine from 1 nM to 10 nM. These results are reproducible in this concentration range as evidenced from repeating experiments, in which the data points from different experiments are coded with different colors and shapes in Figure 4b. However, we noticed that the relative current decrease leveled off at different concentrations of dopamine for different experiments. We do not know the exact reason yet at present. We think that the extent of the levelingoff depends on the degree of aggregation of the ss-DNAwrapped carbon nanotubes in the course of their adsorption and

Ma et al.

Figure 4. (a) Cyclic voltammograms of poly(anilineboronic acid) on an Au electrode with ss-DNA/SWNTs in PBS solution (pH)7.4). In the absence (black) and in the presence of nanomolar concentrations of dopamine (colored) as shown in the inset of this figure. Potential scan rate: 100 mV s-1. (b) Corresponding calibration curves for dopamine detection from four individual experiments, encoded with different colors and shapes (inset: the equation to which the data were fit). The relative current decreases are proportional to the concentration of dopamine from 1 nM to 10 nM in these four independent experiments. At higher concentrations, the relative current decrease saturates at different levels for different films.

drying on the electrodes. Currently we are working on better drying methods to prevent aggregation. Since the linear sensing range is in the appropriate regime for studying in vivo dopamine levels, our method holds great potential for molecular diagnosis of Parkinson’s disease. The sensitivity toward dopamine increased 104 times compared to when only PABA was used to modify the electrodes in a detection platform of microelectrochemical transistors.44 We believe that the ss-DNA/SWNTs in the composite increased the effective electrode surface area, causing a higher density of boronic acid groups available for dopamine binding and thus significantly enhancing the detection sensitivity. TappingMode AFM images (data not shown) clearly illustrate that the film fabricated with ss-DNA/SWNTs modification has a much greater surface roughness than the film obtained without modification. We should mention that higher detection sensitivity could be reached by optimizing the detection techniques. For example, a microelectrochemical transistor as a detection platform normally has higher sensitivity because the conductivity of polyaniline can change by many orders of magnitude when its redox states are switched.3,12,49-51 This large change in conductance of the polymer leads to amplification of the detection signal. Another technique, differential potential voltammetry,52 could greatly decrease the background charging currents and in turn also increase the detection sensitivity.

DNA-Wrapped Single-Walled Carbon Nanotubes In summary, we draw on the findings26,27 that single-stranded DNA could disperse bundled single-walled carbon nanotubes into aqueous solution, resulting in ss-DNA-helically wrapped carbon nanotubes with both negatively charged DNA phosphate groups and bare carbon nanotube graphite regions exposed to the solution. For the first time, it is demonstrated that ss-DNA/ SWNTs can act as a template for in situ electrochemical polymerization of self-doped polyaniline. The resulting nanocomposites show excellent properties through the combined synergistic effects of their component materials. For example, the electrocatalytic reductive ability of the ss-DNA/SWNTs and their strong interaction with the polyaniline backbone significantly improved the stability of PABA. The huge surface area of the carbon nanotubes greatly increased the density of the functional groups accessible for sensitive detection of the target analyte. We believe that the excellent properties of the ss-DNA/ SWNTs described in this report could also apply during their incorporation into substituted polyanilines or other polymers to devise new types of functional nanomaterials. Acknowledgment. Acknowledgment is made to the donors of the American Chemical Society Petroleum Research Fund for partial support of this research. Support from a Rutgers University Research Council Grant is gratefully acknowledged. Shah R. Ali is the recipient of a Rutgers Undergraduate Research Fellowship. References and Notes (1) Persaud, K. C.; Pelosi, P. Sensor Arrays Using Conducting Polymers for an Artificial Nose; Kluwer Academic Publishers: Boston, 1992; pp 237256. (2) Ma, Y. F.; Zhang, J. M.; Zhang, G. J.; He, H. X. J. Am. Chem. Soc. 2004, 126, 7097-7101. (3) Paul, E. W.; Ricco, A. J.; Wrighton, M. S. J. Phys. Chem. 1985, 89, 1441-1447. (4) Virji, S.; Huang, J. X.; Kaner, R. B.; Weiller, B. H. Nano Lett. 2004, 4, 491-496. (5) Sukeerthi, S.; Contractor, A. Q. Anal. Chem. 1999, 71, 2231-2236. (6) Bartlett, P. N.; Simon, E. Phys. Chem. Chem. Phys. 2000, 2, 25992606. (7) Bartlett, P. N.; Wang, J. H. J. Chem. Soc., Faraday Trans. 1996, 92, 4137-4143. (8) Yue, J.; Wang, Z. H.; Cromack, K. R.; Epstein, A. J.; MacDiarmid, A. G. J. Am. Chem. Soc. 1991, 113, 2665-2671. (9) Chan, H. S. O.; Ho, P. K. H.; Ng, S. C.; Tan, B. T. G.; Tan, K. L. J. Am. Chem. Soc. 1995, 117, 8517-8523. (10) Wei, X.-L.; Wang, Y. Z.; Long, S. M.; Bobeczko, C.; Epstein, A. J. J. Am. Chem. Soc. 1996, 118, 2545-2555. (11) Deore, B. A.; Hachey, S.; Freund, M. S. Chem. Mater. 2004, 16, 1426-1432. (12) Bartlett, P. N.; Astier, Y. Chem. Commun. 2000, 105-112. (13) Zengin, H.; Zhou, W.; Jin, J.; Czerw, R.; Smith, J. D. W.; Echegoyen, L.; Carroll, D. L.; Foulger, S. H.; Ballato, J. AdV. Mater. 2002, 14, 1480-1483. (14) Cochet, M.; Maser, W. K.; Benito, A. M.; Callejas, M. A.; Martı´nez, M. T.; Benoit, J.-M.; Schreiber, J.; Chauvet, O. Chem. Commun. 2001, 1450-1451. (15) Chen, G. Z.; Shaffer, M. S. P.; Coleby, D.; Dixon, G.; Zhou, W.; Fray, D. J.; Windle, A. H. AdV. Mater. 2000, 12, 522-526. (16) Li, X. H.; Wu, B.; Huang, J.-E.; Zhang, J.; Liu, Z. F.; Li, H. L. Carbon 2002, 411, 1670-1673.

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