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Water-soluble single-walled carbon nanotubes films: Preparation, characterization and applications as electrochemical sensing films
Journal of
Electroanalytical Chemistry Journal of Electroanalytical Chemistry 586 (2006) 77–85 www.elsevier.com/locate/jelechem
Water-soluble single-walled carbon nanotubes films: Preparation, characterization and applications as electrochemical sensing films Chengguo Hu b
a,b
, Xiaoxia Chen
a,b
, Shengshui Hu
a,b,*
a Department of Chemistry, Wuhan University, Wuhan, Hubei 430072, China State Key Laboratory of Transducer Technology, Chinese Academy of Sciences, Beijing 100080, China
Received 10 June 2005; received in revised form 9 August 2005; accepted 16 September 2005 Available online 26 October 2005
Abstract Water-soluble single-walled carbon nanotubes (SWNTs) were prepared via noncovalent functionalization by Congo red through a physical grinding treatment. Based on the unique property of strong rebundling when dried, water-soluble SWNTs were firmly immobilized on the surface of a glassy carbon electrode by a simple casting method. The prepared films were characterized by scanning electron microscopy (SEM), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results showed that watersoluble SWNTs formed uniform films with porous network structures of nanosizes on the electrode surface, which were stable in neutral and acidic solutions but were unstable in basic media. In addition, the structural properties and the negative charge density of these films can be conveniently controlled by choosing proper solvents during the washing procedure, providing a simple approach for adjusting their properties to specific applications. For instance, the films prepared from water-soluble SWNTs by N,N 0 -dimethyl formamide (DMF) washing (SWNTs–CRDMF) had looser structures and lower negative charge density than those by water washing (SWNTs– CRwater), which allowed the entry and the succedent electrochemical reactions of both positively and negatively charged species inside the films. The potential applications of these films in electroanalytical chemistry were examined. The enhanced response of dopamine (DA) and the separation of DA oxidation potential from those of uric acid (UA) and ascorbic acid (AA) at these films demonstrated that the water-soluble SWNTs were the ideal materials for constructing SWNTs-based electrochemical sensing films. 2005 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes; Sensing films; Glassy carbon electrode; Dopamine
1. Introduction Since the discovery in 1991 by Iijima [1], carbon nanotubes (CNTs) have become the subject of intense researches, due to their unique structural, electronic and mechanical properties and the potential applications in almost any aspect of nanotechnologies [2]. For example, the excellent biocompatibility, the good conductivity, the large specific area and the modifiable sidewall made CNTs ideal materials for constructing electrochemical sensors that can significantly improve the responses of biomolecules like *
Corresponding author. Tel.: +86 27 8721 8904; fax: +86 27 6875 4067. E-mail address: [email protected] (S. Hu).
0022-0728/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2005.09.008
dopamine [3–6], 5-hydroxytryptamine [5,6] and NADH [7]. The promoted electron transfer and direct electrochemistry of proteins at CNTs-based electrochemical sensing films were also reported previously [8–10]. However, as a result of the strong intertube interactions, CNTs generally existed as highly tangled ropes and were insoluble in all solvents, which greatly hindered their promising practical applications. To overcome this limitation, CNTs were either treated with acids [11–13] or heated in air [14,15] to produce hydroxyl, carboxyl and ketone groups at both the sidewall and the terminus, and to exfoliate them into small bundles or individual nanotubes. The resulting CNTs can be dispersed in organic solvents [13,15], concentrated sulfuric acid [11], polymer [16] or the suspension of
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poorly soluble surfactants [17], on the basis of the interactions between solvents and CNTs [11,13,15] or the wrapping of CNTs by polymers [16] or surfactant aggregates [17]. The CNTs suspensions were then cast on the electrode surfaces to form various electrochemical sensors. However, previous dispersing methods of CNTs usually suffered from some inevitable disadvantages due to the weak interactions between CNTs and the dispersing agents, such as low loadings and poor stability (e.g. surfactants or solvents dispersed CNTs), low exfoliation (e.g. polymers wrapped CNTs) and the presence of free additives, which greatly limited their applications in practice, especially for SWNTs. Due to the smaller size, the larger specific area, the stronger intertube attractions and the composition of only one layer of carbon atoms for SWNTs, the dispersing methods of multi-walled carbon nanotubes (MWNTs) that were widely used in electroanalytical chemistry were usually unsuitable for SWNTs. For instance, the treatments of MWNTs with mixed acids or concentrated nitric acid can effectively produce functional groups on the outer sidewalls of MWNTs and exfoliate them into individual tubes while preserving their inherent structures because of the retaining of the inner sidewalls. Whereas, the treatments of SWNTs by similar procedures cannot be precisely controlled as a long period of treatment might seriously destruct their structures and a short period of treatment would produce low solubility and exfoliation efficiency. This contradiction might account for the rare applications of SWNTs in electroanalytical chemistry. Thus, the development of new dispersing methods with high solubility and exfoliation efficiency that can be directly employed in electroanalytical chemistry has become great challenges. Recently, we reported a new noncovalent approach for preparing highly water-soluble single-walled carbon nanotubes (SWNTs) by a small planar and conjugated diazo dye, Congo red (CR) (Scheme 1) [18]. Through just a simple physical grinding treatment, the mixture of HNO3 purified SWNTs and CR was readily dissolved in water with high solubility (3.5 mg/ml). The complete elimination of free CR from the mixture hardly changed this excellent solubility. The strong p-stacking interaction between adsorbed CR and the sidewall of SWNTs was ascertained, which was regarded to be responsible for the high solubility of SWNTs in water. In this work, the possibility of constructing electrochemical sensing films by these water-soluble SWNTs was explored. On the basis of their specific property of strong rebundling when dried, water-soluble SWNTs were firmly immobilized on the surface of a glassy
Scheme 1. Molecular structure of Congo red.
carbon electrode by a simple casting method. The resultant films were observed to have uniform appearances with porous network structures of nanosizes. Similar to other CNTs-based electrochemical sensors, these SWNTs films showed excellent activity towards the electrochemical reactions of small biomolecules (e.g. dopamine). Compared with the earlier CNTs dispersing methods, the CR noncovalent functionalization approach had some overwhelming advantages, such as high water solubility, high exfoliation, high stability and the absence of free additives, which permitted the fabrication of nanostructures from the watersoluble SWNTs using various techniques. For example, the presence of amino and sulfonic groups on CR molecules that were strongly attached to the sidewall of SWNTs provided a chance for the further modifications of watersoluble SWNTs by covalent bonding and self-assembling, which would greatly extend their applications in electrochemistry. At the same time, it was impossible for surfactants or Nafion dispersed SWNTs to achieve this goal since the presence of free surfactants or Nafion would disturb the assembling processes. As for the dispersing methods of CNTs by organic solvents, the denaturation of macrobiomolecules (e.g. proteins) in these solvents hindered their practical applications in constructing CNTsbased electrochemical biosensors. 2. Experimental 2.1. Chemicals and apparatus SWNTs (purity P90 wt%, Timesnano Co., Chengdu, China) were used as received. Dopamine (DA) was the product of Fluka Chemical Corporation. Congo red (CR), uric acid (UA), ascorbic acid (AA), potassium ferricyanide (K3Fe(CN)6) and potassium ferrocyanide (K4Fe(CN)6) were purchased from Shanghai Reagent Co., Shanghai, China. All chemicals were of analytical grade quality and used as received. All solutions were prepared from double-distilled water. Cyclic voltammetry (CV), differential pulse voltammetry (DPV) and amperometry were performed on an EG&G Model 283 electrochemical workstation (Princeton Applied Research, PAR, USA) controlled by a M270 software. Electrochemical impedance spectroscopy (EIS) was carried out with the EG&G Model 283 electrochemical workstation and EG&G Model 5210 lock in amplifier (Princeton Applied Research, PAR, USA) powered by the Powersuit software. In EIS, the frequency ranged from 100 mHz to 100 kHz, the DC potential was the average of the oxidation and reduction peak potentials, and the amplitude was set as 10 mV. The parameters in DPV were as follows: pulse width, 50 ms; pulse height, 50 mV; scan rate 20 mV/s. The electrode system contained a water-soluble SWNTs film coated glassy carbon electrode, a platinum wire counter electrode and a potassium chloride (KCl) saturated calomel reference electrode (SCE). All the potentials in this work were reported vs. SCE. The high-resolution transmission electron
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microscopy (HRTEM) was obtained on JEM-2010 at a voltage of 200 kV and the scanning electron microscopy (SEM) was recorded on Hitachi X 650 at a voltage of 25 kV. 2.2. Fabrication of water-soluble SWNTs-based electrochemical sensing films The preparation of water-soluble SWNTs has been described previously [18]. Briefly, SWNTs were mixed with CR on certain weight ratio (e.g. SWNTs/CR, 5/1) in an agate mortar and ground for 4 h with the addition of little water to avoid agglomeration of the dry CR powder, producing a greenish black mixture (SWNTs + CR), which was readily dissolved in water with solubility as high as 2.6 mg/ml by just vigorously shaking the solution. SWNTs + CR was dissolved in water and washed on a polytetrafluoroethylene (PTFE) filter disc of 0.22 lm pore size by water to remove excessive free CR until the filtrate became colorless. The obtained black product was denoted as SWNTs–CRwater, which had almost the same solubility as SWNTs + CR. SWNTs–CRwater was directly dissolved and stored in water immediately after the washing procedure to avoid the rebundling and the solubility loss of the exfoliated nanotubes when dried. The exact content of SWNTs–CRwater in solution can be easily estimated from the UV absorption at 245 nm (due to the p-plasmon absorption of SWNTs) using precisely weighed SWNTs + CR as the standard [18]. In this work, most adsorbed CR in SWNTs–CRwater was removed by thorough DMF washing. The resulting product was denoted as SWNTs–CRDMF, whose exact content can be estimated by the method similar to SWNTs–CRwater. Glassy carbon electrode (GCE) (3.0 mm in diameter, CHI, USA) was polished on a slush of 0.05 lm alumina (Al2O3), and washed in water, 1:1 HNO3 and ethanol with sonication, each for 3 min. Then, 5 ll of 0.2 mg/ml water-soluble SWNTs was cast onto the clean surface of GCE and air-dried. The resulting SWNTs films were rather firmly attached to the surface of GCE and cannot be easily wiped off in the dry state. In the process of glutaric dialdehyde (GDI) treatment, the water-soluble SWNTs films were immersed in 5% GDI aqueous solution for 30 min and then thoroughly washed with water to remove any physically adsorbed GDI. 3. Results and discussion 3.1. Construction of electrochemical sensing films based on water-soluble SWNTs Fig. 1 shows the microscopic images of water-soluble SWNTs. As expected, the starting SWNTs are assembled into bundles or ropes that contain numerous well-aligned SWNTs and the long ropes further entangle into networks (Fig. 1(a)), resulting in their insolubility in all solvents. Obviously, some irregular impurities are strongly attached to the sidewall of SWNTs ropes and cannot be removed
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even for a long period of sonication in water. With the noncovalent functionalization by CR, SWNTs are readily dissolved in water with high solubility and efficiently exfoliated into individual nanotubes (Fig. 1(b)). In addition, most of the irregular impurities on the sidewall of SWNTs are removed, indicating the cleaning role of adsorbed CR. Another interesting feature of CR decorated SWNTs is that one of the individual nanotube is bended to nearly a right angle at the middle part and tightly bound with another individual tube at the terminus via sidewall coupling, foreseeing the ability of water-soluble SWNTs to form stable and complicated network nanostructures. In fact, when cast on the surface of a glassy carbon electrode and air-dried, water-soluble SWNTs form uniform films on the electrode surface, which possess three dimensional network structures with plenty of interstices inside the films (Fig. 1(c)). In contrast to the additive-containing CNTs-based sensors, e.g. the Nafion-containing CNTs modified electrode [16], these porous network structures allow the entry of both macromolecules and micromolecules inside the films and make full use of the active sites of the SWNTs films. The stability of the water-soluble SWNTs films was examined. Due to the strong attachment of CR onto the sidewall of SWNTs and the chemical inertia of CR under ambient conditions, these films had excellent stability in air or water. When used in electrochemical measurements, the water-soluble SWNTs films remained stable in neutral and acidic media but would peel off from the electrode surface if used in basic solutions. For comparison, the dispersing capacity of SWNTs by 5% sodium dodecyl sulphate (SDS) aqueous solution and DMF has been evaluated [18]. The results showed that the solubility of SWNTs in these media was rather low (less than 0.1 mg/ml) and no useful sensing films can be prepared. Thus, the conventional approach for dispersing MWNTs in DMF that was extensively used in electroanalytical chemistry was unsuitable for SWNTs used here. 3.2. Voltammetric responses of water-soluble SWNTs films modified glassy carbon electrode Fig. 2 shows the successive voltammograms of the SWNTs–CRwater films modified glassy carbon electrode (SWNTs–CRwater/GCE) in 0.1 M phosphate buffer (pH 7.0). In the potential range of 1.0 to 1.0 V, several pairs of redox peaks are observed in the first cycle, which decrease in the following cycles except for the redox couples O1/R1 and O3/R3 and the reduction peak R6. The process O1/R1 is due to the intrinsic responses of SWNTs according to previous reports [4,17], and might be produced in the purification process by the manufacturer. The welldefined shapes of peaks O1 and R1 (DEp = 50 mV) indicate that the water-soluble SWNTs films have a uniform distribution on the electrode surface. The close of the redox potentials of O3/R3 to those of dopamine indicates that this process might originate from the decomposition aniline moieties of CR. As for the other peaks, they are attributed
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Fig. 1. Microscopic images of SWNTs: (a) HRTEM of the starting SWNTs; (b) HRTEM of SWNTs–CRwater; (c) SEM of SWNTs–CRDMF. The HRTEM samples were prepared by dipping a 200 mesh lacey carbon grid on a copper support in the aqueous solution of SWNTs or SWNTs–CRwater and being air-dried. The SEM sample was prepared by casting a drop of SWNTs–CRDMF aqueous solution on the surface of a glassy carbon electrode and being air-dried.
Fig. 2. Successive voltammograms of SWNTs–CRwater/GCE in 0.1 M phosphate buffer (pH 7.0). Inset shows the voltammograms of 0.5 mg/ml CR at GCE in 0.1 M phosphate buffer (pH 7.0). Scan rate, 100 mV/s.
to the redox reactions of CR or its decomposition products. This conclusion is supported by the electrochemical behaviors of CR at GCE (inset in Fig. 2). CR exhibits two oxidation peaks (P1 and P2) and two reduction peaks (P3 and P4) in the first cycle. Among these peaks, P4 was proved to be the reduction peak of dissolved oxygen as this peak can be removed by nitrogen purge. In the second cycle, the oxidation currents of P1 and P2 apparently decrease and P2 is split into two small peaks. In addition, a new oxidation peak (P5) appears, which forms a redox couple with P3. The similar responses of CR at GCE and at SWNTs– CRwater/GCE suggest that P1 might correspond to the oxidation process O2 while P2 should be the overlapped responses of O3 and O4. The absence of the reduction processes of CR at GCE except for P3 indicates that strong interactions might exist between the decomposition products of CR and the sidewall of SWNTs.
The adscription of the redox peaks in Fig. 2 is confirmed by the variations of these responses with different treatments (Fig. 3). As mentioned above, SWNTs–CRwater/ GCE exhibits several pairs of redox peaks in the whole potential range (Fig. 3(a)). With the thorough washing by DMF, the voltammograms of SWNTs–CRwater/GCE are greatly simplified and most of the redox peaks disappear except for peaks O1 and R1 (Fig. 3(b)), demonstrating that these peaks should arise from the intrinsic responses of acidtreated SWNTs. The still existence of the weak CR characteristic features in the first cycle indicates the presence of small amount of adsorbed CR on the surface of SWNTs. Fig. 3(c) shows the response of SWNTs–CRwater/GCE with GDI treatment. Different from the voltammograms of SWNTs–CRwater/GCE, the responses of CR are greatly repressed, especially for peaks O2 and R2. Since GDI would selectively react with the amino groups on CR, the repression of peaks O2 and R2 is a direct evidence for attributing them to the redox reactions of the aniline moieties on CR. The reaction between GDI and the CR molecules that are strongly attached to the sidewall of SWNTs also foretells the possibility of immobilizing macrobiomolecules (e.g. proteins) on the sidewall of SWNTs via covalent bonding. The reaction of GDI with adsorbed CR can also been observed at SWNTs–CRDMF films modified glassy carbon electrode (SWNTs–CRDMF/GCE) (Fig. 3(d)), leading to the further decrease of the CR response and the further simplification of the background. 3.3. Electrochemical characterization of water-soluble SWNTs films modified glassy carbon electrode The electrochemical and structural properties of watersoluble SWNTs films are characterized by CV and EIS using [Fe(CN)6]3/4 as the probe (Fig. 4). Fig. 4(a) shows
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Fig. 3. Cyclic voltammograms in 0.1 M phosphate buffer (pH 7.0) of SWNTs–CRwater/GCE (a), SWNTs–CRDMF/GCE (b), SWNTs–CRwater/GCE with GDI treatment (c) and SWNTs–CRDMF/GCE with GDI treatment (d) for the first (solid lines) and the 10th cycles (short dot lines). Scan rate, 100 mV/s.
the CV of [Fe(CN)6]3/4 at different electrodes. As expected, [Fe(CN)6]3/4 exhibits a well-defined redox peaks at GCE. However, with the modification of SWNTs– CRwater films, the redox currents apparently decrease and the peak shapes become poor, due to the electrostatic repulsion of [Fe(CN)6]3/4 from the electrode surface by negatively charged sulfonic groups on CR attached to the sidewall of SWNTs. Interestingly, the electrochemical response of [Fe(CN)6]3/4 is obviously improved at SWNTs–CRDMF/GCE, close to that at GCE. The removing of most adsorbed CR from the sidewall of SWNTs can effectively reduce the negative charges on SWNTs, make the SWNTs sidewall hydrophobic and provide more reaction sites for the substrates. This result also suggests the low density of negative charges on sidewall of SWNTs, consistent with the disappearance of most CR signals at SWNTs–CRDMF films. The electrochemical impedance spectra of Fe(CN)6]3/4 at these electrodes are also studied. The Nyquist plot of [Fe(CN)6]3/4 at GCE is characteristic of a semicircle at high frequencies and a straight line with a slope of about 1 at low frequencies, corresponding to the kinetical and the diffusional processes, respectively. At SWNTs–CRwater/GCE, an obviously larger semicircle at high frequencies and a depressed straight line at low frequencies appear. Moreover, no apparent transition is observed between the semicircle and the straight line, suggesting the complicated structures of the films. The Nyquist plot behaves midway at SWNTs–CRDMF/GCE, reflected by the approach of its shape to that at SWNTs–CRwater/ GCE and the parallelism of the straight line at low frequencies with that at GCE. The former indicates the structural
similarity between the SWNTs–CRwater and the SWNTs– CRDMF films while the later suggests that the diffusional behavior of [Fe(CN)6]3/4 at SWNTs–CRDMF/GCE is similar to that at GCE, i.e. the amount of adsorbed CR at the SWNTs–CRDMF films is small and the diffusion process of [Fe(CN)6]3/4 is hardly influenced by the negative charges at these films. These conclusions are supported by the Bode plots (h (phase angle) vs. log f (frequency)) (Fig. 4(c)). At GCE, a symmetrical peak with a maximum value of 39 at about 100 Hz is observed, corresponding to the relaxation process of the GCE|solution interface. With the modification of either SWNTs–CRwater or SWNTs– CRDMF films, this relaxation completely disappears and a new one situated at 5 Hz appears, reflecting the replacement of the GCE|solution interface by the SWNTs films|solution interface. The complete disappearance of the relaxation associated with the GCE|solution interface also suggests that the electrochemical reactions of [Fe(CN)6]3/4 should take place at the surface of SWNTs instead of GCE. The appearance of the relaxations for SWNTs–CRwater/GCE and SWNTs–CRDMF/GCE at the same position confirms that these water-soluble SWNTs films should possess similar structural properties. Nevertheless, apparent differences still exist between the structures of these two films, i.e. the SWNTs–CRwater films might have denser and more regular structures than the SWNTs–CRDMF films. This is supported by the following facts: (a) the much larger maximum of the relaxation at the SWNTs–CRwater films (67) than that at the SWNTs–CRDMF films (52) suggests that the SWNTs– CRwater films|solution interface is closer to a pure capacitor than the SWNTs–CRDMF films|solution interface, i.e. the
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regulated films on the electrode surface with a metallic luster whereas the films of SWNTs–CRDMF are loose and porous (not shown). This conclusion is reasonable since the rebundling of water-soluble SWNTs is mainly achieved through the coupling of adsorbed CR on the sidewall of SWNTs via p–p interactions [18]. 3.4. Electrochemical characterization of the intrinsic redox of acid-treated SWNTs The electrochemical properties of acid-treated CNTs have been explored previously [4]. The conclusion is that the oxygen-containing groups (e.g. carboxyl groups) on the defects or opened caps are responsible for the unique redox signals of CNTs. Here, the influence of solution acidity on the redox behaviors of water-soluble SWNTs is examined (Fig. 5). Generally, the cyclic voltammograms of SWNTs regularly vary with solution pH. In the range of pH 3.0–9.0, the oxidation and reduction peak potentials exhibit excellent linear relationships with pH, which have almost the same slope (0.051 V/pH). This result is consistent with the conclusion that equal protons and electrons take part in the redox of SWNTs [4]. The variations of redox currents with solution pH are also obvious. Interestingly, both the reduction and oxidation currents reach the maximum values at around pH 7.0 and decrease with either the increase or the decrease of solution pH. In addition, a new redox couple situated at about 0.2 V (DEp = 40 mV) appears in acidic media, which is attributed to the decomposition products of CR based on the variations of the voltammograms at SWNTs–CRwater/GCE with solution pH. Similar to those at SWNTs–CRDMF/GCE, the positions of the processes O1/R1 and O3/R3 at SWNTs– CR CRwater/GCE regularly shifted to more positive potentials with the decrease of solution pH (not shown). Moreover, no new redox processes appeared and the redox
Fig. 4. Electrochemical responses of 0.5 mM [Fe(CN)6]3/4 in 0.1 M phosphate buffer (pH 7.0) at GCE, SWNTs–CRwater/GCE and SWNTs– CRDMF/GCE: (a) CV; (b) Nyquist plots; (c) Bode plots.
structures of the SWNTs–CRwater films should be denser and more regular than those of the SWNTs–CRDMF films; (b) the narrower distribution of the relaxation at the SWNTs–CRwater films than that at the SWNTs–CRDMF films indicates that the former should have denser and more regular structures. In fact, it can be clearly observed from naked eyes that SWNTs–CRwater forms smooth and
Fig. 5. Variation of cyclic voltammograms at SWNTs–CRDMF/GCE with solution pH in 0.1 M phosphate buffer. The acidity of the buffer solutions beyond conventional phosphate buffers was adjusted to the desired values with 1.0 M HCl. Scan rate, 100 mV/s.
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currents of O3/R3 apparently increased whereas those of O1/R1 hardly changed. These results suggested that the new redox couple at 0.2 V might be the enhanced process of O3/R3 at SWNTs–CRDMF/GCE in acidic media. The influence of scan rate is also investigated (Fig. 6). As expected, the redox currents of SWNTs linearly increase with scan rate at low scan rate (6150 mV/s), representing a typical surface process [19]. When scan rate is higher 150 mV/s, both the reduction and the oxidation potentials exhibit good linear relationships with the logarithm of scan rate, which can be expressed as Ep,a = 0.014 + 0.068 log m (R = 0.9970) and Ep,c = 0.146 + 0.061 log m (R = 0.9952) for the oxidation and the reduction processes, respectively. The solving of these two equations produces a value of 0.52 for an, where a is the electron transfer coefficient and n is the electron transferred in the rate-determining step. It is obvious that one electron might involve in the rate-determining step, which is in accordance with the previous report [4]. 3.5. Promising applications of water-soluble SWNTs film modified glassy carbon electrode in electroanalytical chemistry The potential application of water-soluble SWNTs films as electrochemical sensing films was examined. Fig. 7 shows the responses of 1 · 105M DA, UA and AA at different electrodes. At GCE, all these biomolecules exhibit low and wide oxidation peaks, which overlap with each other and none can be selectively determined (inset in Fig. 7). The responses of these biomolecules are much different at SWNTs–CRDMF/GCE. No discernable response of AA is observed in this potential range and the background apparently increases. In fact, AA exhibited an insensitive wide oxidation peak at around 0.04 V (vs. SCE), which partly overlapped with the intrinsic response of SWNTs (not shown). At the same time, UA still shows
Fig. 6. Influence of scan rate on cyclic voltammograms of SWNTs– CRDMF/GCE in 0.1 M phosphate buffer (pH 7.0). Insets showed the variations of redox currents and potentials of SWNTs–CRDMF/GCE with scan rate.
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Fig. 7. Cyclic voltammograms of 1 · 105 M DA, UA and AA at SWNTs–CRDMF/GCE in 0.1 M phosphate buffer (pH 7.0). Inset showed cyclic voltammograms of DA, UA and AA at GCE in 0.1 M phosphate buffer (pH 7.0). Scan rate, 100 mV/s; accumulation conditions, 300 s at an open circuit.
an apparently enhanced oxidation response at SWNTs– CRDMF/GCE in 0.1 M phosphate buffer (pH 7.0). This confirms that the density of negatively charged CR on the sidewall of SWNTs is low because UA (pKa1 5.75) is negatively charged in the electrolyte used here (PBS, pH 7.0) and the presence of high density of negative charges on the electrode surface would depress the responses of UA. This result also indicates that the large portion of SWNTs sidewall is uncovered by CR, which might be the reactive sites of water-soluble SWNTs towards the substrates. Similar to UA, the width of the oxidation peak of DA is apparently narrowed and the oxidation current is greatly enlarged, leading to the separation of the DA oxidation potential from those of UA and AA for 126 and 125 mV, respectively. This provides a chance for the selective determination of DA in the presence of UA and AA. The dependence of the oxidation current on DA concentration at SWNTs–CRDMF/GCE is examined using DPV (Fig. 8(a)). In the range of 2.0 · 107–2.0 · 106 M, the oxidation current of DA shows a linear dependence on its concentration (Fig. 8(b)), which can be expressed as Ip(lA) = 3.86CDA(lM) + 0.28 (n = 6, R = 0.9981). At higher concentrations, the oxidation current gradually deviates from this linear relationship, due to the saturation of DA adsorption at SWNTs–CRDMF/GCE. The adsorption of DA was supported by the gradual decrease of its oxidation current with the increase of scan cycle in successive voltammograms after 300 s accumulation at an open circuit and the restoring of the oxidation current with a similar accumulation process. The reproducibility of SWNTs–CRwater/GCE was examined and a low RSD of 4.5% was obtained for six measurements of 5.0 · 107 M DA. The detection limit, defined as a signal-to-noise ratio of 3:1, was found to be 8.5 · 108 M for 300 s accumulation at an open circuit.. For nine successive additions of 5.0 · 107 M DA, rapid and stable amperometric responses
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Fig. 8. Differential pulse voltammograms (DPV) for 0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.5, 2.0, 3.0 and 4.0 lM DA (from inner to outer) at SWNTs–CRDMF/ GCE in 0.1 M phosphate buffer (pH 7.0) (a), dependence of DA oxidation current on its concentration (b) and amperometric responses of successive addition of 0.5 lM DA at an interval of 60 s (c). The accumulation conditions for DPV measurements were 300 s at an open circuit and the oxidation potential in amperometric measurement was set at 0.18 V (vs. SCE).
are obtained at SWNTs–CRDMF/GCE (Fig. 8(c)). This result suggests that SWNTs–CRDMF/GCE has the merit of being rapid, sensitive and stable as electrochemical sensing films. The interferences of 200-fold of UA and AA on the amperometric response of DA were evaluated. The addition of 1.0 · 104 M UA did not cause any changes on the signal of 5.0 · 107 M DA whereas the addition of 1.0 · 104 M AA led to an increase of the oxidation current for more than eight times. The different variations of the background with the addition of high concentrations of UA and AA might account for their different interferences on DA. The interferences of UA and AA on the DPV response of 2.0 · 107 M DA are also examined (Fig. 9). The addition of 200-fold of UA causes only a neglectable
decrease of DA response (3.5%) but the interference of 400-fold of UA is serious and the oxidation current of DA decreases for 39.7%. In contrast to UA, the addition of 200-fold AA leads to an increase of the DA response for 25.3%. The interferences of AA and UA on the DPV response of DA are majorly controlled by two opposite factors: the competitive adsorption causing the reduction of the reactive sites of DA and the overlapping of the oxidation peaks leading to the increase of DA oxidation current. The adsorption of UA at the hydrophobic sidewall of SWNTs should be stronger than that of AA due to the much lower solubility of UA in aqueous solutions. At the same time, as indicated in Fig. 7, the oxidation peak width of UA is much narrower than that of AA. Thus, the addition of UA mainly results in the reduction of the reactive sites at SWNTs–CRDMF/GCE by competitive adsorption whereas the addition of AA mainly contributes to the increase of the oxidation current through the overlapping of the oxidation peaks. The interference from AA could be eliminated by coating SWNTs–CRDMF/GCE with a film of negatively charged polymers (e.g. Nafion). 4. Conclusion In conclusion, water-soluble SWNTs were prepared via noncovalent functionalization by CR through a physical grinding process. These soluble SWNTs can be firmly immobilized on the surface of GCE by a simple casting method. The obtained SWNTs films were characterized by several techniques, including CV, EIS and SEM. The results showed that water-soluble SWNTs formed uniform films with network structures of nanosizes on the electrode surface. In addition, the structural properties and the negative charge density of these films can be conveniently controlled by choosing proper solvents in the washing procedure, which provided a simple approach for adjusting their properties to specific applications. As an example, the potential applications of the water-soluble SWNTs with DMF washing (SWNTs–CRDMF) in electroanalytical chemistry were explored. The enhanced response of DA as well as the separation of the DA oxidation peak from those of UA and AA at SWNTs–CRDMF/GCE demonstrated that these water-soluble SWNTs were promising materials for constructing CNTs-based electrochemical sensing films. Acknowledgement The authors are grateful for the financial support of National Natural Science Foundation of China (Nos. 30370397 and 60571042). References
Fig. 9. Interferences of UA and AA on the DPV response of 0.2 lM DA at SWNTs–CRDMF/GCE in 0.1 M phosphate buffer (pH 7.0) for 300 s accumulation at an open circuit.
[1] S. Iijima, Nature 354 (1991) 56. [2] M.S. Dresselhaus, G. Dresselhaus, P. Avouris, Carbon Nanotubes: Synthesis, Structure, Properties, and Applications, Springer Verlag, New York, 2001.
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[11] M. Musameh, J. Wang, A. Merkoci, Y. Lin, Electrochem. Commun. 4 (2002) 743. [12] G. Wang, J. Xu, H. Chen, Electrochem. Commun. 4 (2002) 506. [13] J. Wang, S.B. Hocevar, B. Ogorevc, Electrochem. Commun. 6 (2004) 176. [14] Z. Shi, Y. Lian, X. Zhou, Z. Gu, Y. Zhang, S. Iijima, Solid State Commun. 112 (1999) 35. [15] Q. Zhao, Z. Gu, Q. Zhuang, Electrochem. Commun. 6 (2004) 83. [16] J. Wang, M. Musameh, Y. Lin, J. Am. Chem. Soc. 125 (2003) 2409. [17] Y. Sun, J. Fei, K. Wu, S. Hu, Anal. Bioanal. Chem. 375 (2003) 544. [18] C. Hu, Z. Chen, A. Shen, X. Shen, J. Li, S. Hu, Carbon, in press. [19] E. Laviron, J. Electroanal. Chem. 101 (1979) 19.
Electroanalytical Chemistry Journal of Electroanalytical Chemistry 586 (2006) 77–85 www.elsevier.com/locate/jelechem
Water-soluble single-walled carbon nanotubes films: Preparation, characterization and applications as electrochemical sensing films Chengguo Hu b
a,b
, Xiaoxia Chen
a,b
, Shengshui Hu
a,b,*
a Department of Chemistry, Wuhan University, Wuhan, Hubei 430072, China State Key Laboratory of Transducer Technology, Chinese Academy of Sciences, Beijing 100080, China
Received 10 June 2005; received in revised form 9 August 2005; accepted 16 September 2005 Available online 26 October 2005
Abstract Water-soluble single-walled carbon nanotubes (SWNTs) were prepared via noncovalent functionalization by Congo red through a physical grinding treatment. Based on the unique property of strong rebundling when dried, water-soluble SWNTs were firmly immobilized on the surface of a glassy carbon electrode by a simple casting method. The prepared films were characterized by scanning electron microscopy (SEM), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results showed that watersoluble SWNTs formed uniform films with porous network structures of nanosizes on the electrode surface, which were stable in neutral and acidic solutions but were unstable in basic media. In addition, the structural properties and the negative charge density of these films can be conveniently controlled by choosing proper solvents during the washing procedure, providing a simple approach for adjusting their properties to specific applications. For instance, the films prepared from water-soluble SWNTs by N,N 0 -dimethyl formamide (DMF) washing (SWNTs–CRDMF) had looser structures and lower negative charge density than those by water washing (SWNTs– CRwater), which allowed the entry and the succedent electrochemical reactions of both positively and negatively charged species inside the films. The potential applications of these films in electroanalytical chemistry were examined. The enhanced response of dopamine (DA) and the separation of DA oxidation potential from those of uric acid (UA) and ascorbic acid (AA) at these films demonstrated that the water-soluble SWNTs were the ideal materials for constructing SWNTs-based electrochemical sensing films. 2005 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes; Sensing films; Glassy carbon electrode; Dopamine
1. Introduction Since the discovery in 1991 by Iijima [1], carbon nanotubes (CNTs) have become the subject of intense researches, due to their unique structural, electronic and mechanical properties and the potential applications in almost any aspect of nanotechnologies [2]. For example, the excellent biocompatibility, the good conductivity, the large specific area and the modifiable sidewall made CNTs ideal materials for constructing electrochemical sensors that can significantly improve the responses of biomolecules like *
Corresponding author. Tel.: +86 27 8721 8904; fax: +86 27 6875 4067. E-mail address: [email protected] (S. Hu).
0022-0728/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2005.09.008
dopamine [3–6], 5-hydroxytryptamine [5,6] and NADH [7]. The promoted electron transfer and direct electrochemistry of proteins at CNTs-based electrochemical sensing films were also reported previously [8–10]. However, as a result of the strong intertube interactions, CNTs generally existed as highly tangled ropes and were insoluble in all solvents, which greatly hindered their promising practical applications. To overcome this limitation, CNTs were either treated with acids [11–13] or heated in air [14,15] to produce hydroxyl, carboxyl and ketone groups at both the sidewall and the terminus, and to exfoliate them into small bundles or individual nanotubes. The resulting CNTs can be dispersed in organic solvents [13,15], concentrated sulfuric acid [11], polymer [16] or the suspension of
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poorly soluble surfactants [17], on the basis of the interactions between solvents and CNTs [11,13,15] or the wrapping of CNTs by polymers [16] or surfactant aggregates [17]. The CNTs suspensions were then cast on the electrode surfaces to form various electrochemical sensors. However, previous dispersing methods of CNTs usually suffered from some inevitable disadvantages due to the weak interactions between CNTs and the dispersing agents, such as low loadings and poor stability (e.g. surfactants or solvents dispersed CNTs), low exfoliation (e.g. polymers wrapped CNTs) and the presence of free additives, which greatly limited their applications in practice, especially for SWNTs. Due to the smaller size, the larger specific area, the stronger intertube attractions and the composition of only one layer of carbon atoms for SWNTs, the dispersing methods of multi-walled carbon nanotubes (MWNTs) that were widely used in electroanalytical chemistry were usually unsuitable for SWNTs. For instance, the treatments of MWNTs with mixed acids or concentrated nitric acid can effectively produce functional groups on the outer sidewalls of MWNTs and exfoliate them into individual tubes while preserving their inherent structures because of the retaining of the inner sidewalls. Whereas, the treatments of SWNTs by similar procedures cannot be precisely controlled as a long period of treatment might seriously destruct their structures and a short period of treatment would produce low solubility and exfoliation efficiency. This contradiction might account for the rare applications of SWNTs in electroanalytical chemistry. Thus, the development of new dispersing methods with high solubility and exfoliation efficiency that can be directly employed in electroanalytical chemistry has become great challenges. Recently, we reported a new noncovalent approach for preparing highly water-soluble single-walled carbon nanotubes (SWNTs) by a small planar and conjugated diazo dye, Congo red (CR) (Scheme 1) [18]. Through just a simple physical grinding treatment, the mixture of HNO3 purified SWNTs and CR was readily dissolved in water with high solubility (3.5 mg/ml). The complete elimination of free CR from the mixture hardly changed this excellent solubility. The strong p-stacking interaction between adsorbed CR and the sidewall of SWNTs was ascertained, which was regarded to be responsible for the high solubility of SWNTs in water. In this work, the possibility of constructing electrochemical sensing films by these water-soluble SWNTs was explored. On the basis of their specific property of strong rebundling when dried, water-soluble SWNTs were firmly immobilized on the surface of a glassy
Scheme 1. Molecular structure of Congo red.
carbon electrode by a simple casting method. The resultant films were observed to have uniform appearances with porous network structures of nanosizes. Similar to other CNTs-based electrochemical sensors, these SWNTs films showed excellent activity towards the electrochemical reactions of small biomolecules (e.g. dopamine). Compared with the earlier CNTs dispersing methods, the CR noncovalent functionalization approach had some overwhelming advantages, such as high water solubility, high exfoliation, high stability and the absence of free additives, which permitted the fabrication of nanostructures from the watersoluble SWNTs using various techniques. For example, the presence of amino and sulfonic groups on CR molecules that were strongly attached to the sidewall of SWNTs provided a chance for the further modifications of watersoluble SWNTs by covalent bonding and self-assembling, which would greatly extend their applications in electrochemistry. At the same time, it was impossible for surfactants or Nafion dispersed SWNTs to achieve this goal since the presence of free surfactants or Nafion would disturb the assembling processes. As for the dispersing methods of CNTs by organic solvents, the denaturation of macrobiomolecules (e.g. proteins) in these solvents hindered their practical applications in constructing CNTsbased electrochemical biosensors. 2. Experimental 2.1. Chemicals and apparatus SWNTs (purity P90 wt%, Timesnano Co., Chengdu, China) were used as received. Dopamine (DA) was the product of Fluka Chemical Corporation. Congo red (CR), uric acid (UA), ascorbic acid (AA), potassium ferricyanide (K3Fe(CN)6) and potassium ferrocyanide (K4Fe(CN)6) were purchased from Shanghai Reagent Co., Shanghai, China. All chemicals were of analytical grade quality and used as received. All solutions were prepared from double-distilled water. Cyclic voltammetry (CV), differential pulse voltammetry (DPV) and amperometry were performed on an EG&G Model 283 electrochemical workstation (Princeton Applied Research, PAR, USA) controlled by a M270 software. Electrochemical impedance spectroscopy (EIS) was carried out with the EG&G Model 283 electrochemical workstation and EG&G Model 5210 lock in amplifier (Princeton Applied Research, PAR, USA) powered by the Powersuit software. In EIS, the frequency ranged from 100 mHz to 100 kHz, the DC potential was the average of the oxidation and reduction peak potentials, and the amplitude was set as 10 mV. The parameters in DPV were as follows: pulse width, 50 ms; pulse height, 50 mV; scan rate 20 mV/s. The electrode system contained a water-soluble SWNTs film coated glassy carbon electrode, a platinum wire counter electrode and a potassium chloride (KCl) saturated calomel reference electrode (SCE). All the potentials in this work were reported vs. SCE. The high-resolution transmission electron
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microscopy (HRTEM) was obtained on JEM-2010 at a voltage of 200 kV and the scanning electron microscopy (SEM) was recorded on Hitachi X 650 at a voltage of 25 kV. 2.2. Fabrication of water-soluble SWNTs-based electrochemical sensing films The preparation of water-soluble SWNTs has been described previously [18]. Briefly, SWNTs were mixed with CR on certain weight ratio (e.g. SWNTs/CR, 5/1) in an agate mortar and ground for 4 h with the addition of little water to avoid agglomeration of the dry CR powder, producing a greenish black mixture (SWNTs + CR), which was readily dissolved in water with solubility as high as 2.6 mg/ml by just vigorously shaking the solution. SWNTs + CR was dissolved in water and washed on a polytetrafluoroethylene (PTFE) filter disc of 0.22 lm pore size by water to remove excessive free CR until the filtrate became colorless. The obtained black product was denoted as SWNTs–CRwater, which had almost the same solubility as SWNTs + CR. SWNTs–CRwater was directly dissolved and stored in water immediately after the washing procedure to avoid the rebundling and the solubility loss of the exfoliated nanotubes when dried. The exact content of SWNTs–CRwater in solution can be easily estimated from the UV absorption at 245 nm (due to the p-plasmon absorption of SWNTs) using precisely weighed SWNTs + CR as the standard [18]. In this work, most adsorbed CR in SWNTs–CRwater was removed by thorough DMF washing. The resulting product was denoted as SWNTs–CRDMF, whose exact content can be estimated by the method similar to SWNTs–CRwater. Glassy carbon electrode (GCE) (3.0 mm in diameter, CHI, USA) was polished on a slush of 0.05 lm alumina (Al2O3), and washed in water, 1:1 HNO3 and ethanol with sonication, each for 3 min. Then, 5 ll of 0.2 mg/ml water-soluble SWNTs was cast onto the clean surface of GCE and air-dried. The resulting SWNTs films were rather firmly attached to the surface of GCE and cannot be easily wiped off in the dry state. In the process of glutaric dialdehyde (GDI) treatment, the water-soluble SWNTs films were immersed in 5% GDI aqueous solution for 30 min and then thoroughly washed with water to remove any physically adsorbed GDI. 3. Results and discussion 3.1. Construction of electrochemical sensing films based on water-soluble SWNTs Fig. 1 shows the microscopic images of water-soluble SWNTs. As expected, the starting SWNTs are assembled into bundles or ropes that contain numerous well-aligned SWNTs and the long ropes further entangle into networks (Fig. 1(a)), resulting in their insolubility in all solvents. Obviously, some irregular impurities are strongly attached to the sidewall of SWNTs ropes and cannot be removed
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even for a long period of sonication in water. With the noncovalent functionalization by CR, SWNTs are readily dissolved in water with high solubility and efficiently exfoliated into individual nanotubes (Fig. 1(b)). In addition, most of the irregular impurities on the sidewall of SWNTs are removed, indicating the cleaning role of adsorbed CR. Another interesting feature of CR decorated SWNTs is that one of the individual nanotube is bended to nearly a right angle at the middle part and tightly bound with another individual tube at the terminus via sidewall coupling, foreseeing the ability of water-soluble SWNTs to form stable and complicated network nanostructures. In fact, when cast on the surface of a glassy carbon electrode and air-dried, water-soluble SWNTs form uniform films on the electrode surface, which possess three dimensional network structures with plenty of interstices inside the films (Fig. 1(c)). In contrast to the additive-containing CNTs-based sensors, e.g. the Nafion-containing CNTs modified electrode [16], these porous network structures allow the entry of both macromolecules and micromolecules inside the films and make full use of the active sites of the SWNTs films. The stability of the water-soluble SWNTs films was examined. Due to the strong attachment of CR onto the sidewall of SWNTs and the chemical inertia of CR under ambient conditions, these films had excellent stability in air or water. When used in electrochemical measurements, the water-soluble SWNTs films remained stable in neutral and acidic media but would peel off from the electrode surface if used in basic solutions. For comparison, the dispersing capacity of SWNTs by 5% sodium dodecyl sulphate (SDS) aqueous solution and DMF has been evaluated [18]. The results showed that the solubility of SWNTs in these media was rather low (less than 0.1 mg/ml) and no useful sensing films can be prepared. Thus, the conventional approach for dispersing MWNTs in DMF that was extensively used in electroanalytical chemistry was unsuitable for SWNTs used here. 3.2. Voltammetric responses of water-soluble SWNTs films modified glassy carbon electrode Fig. 2 shows the successive voltammograms of the SWNTs–CRwater films modified glassy carbon electrode (SWNTs–CRwater/GCE) in 0.1 M phosphate buffer (pH 7.0). In the potential range of 1.0 to 1.0 V, several pairs of redox peaks are observed in the first cycle, which decrease in the following cycles except for the redox couples O1/R1 and O3/R3 and the reduction peak R6. The process O1/R1 is due to the intrinsic responses of SWNTs according to previous reports [4,17], and might be produced in the purification process by the manufacturer. The welldefined shapes of peaks O1 and R1 (DEp = 50 mV) indicate that the water-soluble SWNTs films have a uniform distribution on the electrode surface. The close of the redox potentials of O3/R3 to those of dopamine indicates that this process might originate from the decomposition aniline moieties of CR. As for the other peaks, they are attributed
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Fig. 1. Microscopic images of SWNTs: (a) HRTEM of the starting SWNTs; (b) HRTEM of SWNTs–CRwater; (c) SEM of SWNTs–CRDMF. The HRTEM samples were prepared by dipping a 200 mesh lacey carbon grid on a copper support in the aqueous solution of SWNTs or SWNTs–CRwater and being air-dried. The SEM sample was prepared by casting a drop of SWNTs–CRDMF aqueous solution on the surface of a glassy carbon electrode and being air-dried.
Fig. 2. Successive voltammograms of SWNTs–CRwater/GCE in 0.1 M phosphate buffer (pH 7.0). Inset shows the voltammograms of 0.5 mg/ml CR at GCE in 0.1 M phosphate buffer (pH 7.0). Scan rate, 100 mV/s.
to the redox reactions of CR or its decomposition products. This conclusion is supported by the electrochemical behaviors of CR at GCE (inset in Fig. 2). CR exhibits two oxidation peaks (P1 and P2) and two reduction peaks (P3 and P4) in the first cycle. Among these peaks, P4 was proved to be the reduction peak of dissolved oxygen as this peak can be removed by nitrogen purge. In the second cycle, the oxidation currents of P1 and P2 apparently decrease and P2 is split into two small peaks. In addition, a new oxidation peak (P5) appears, which forms a redox couple with P3. The similar responses of CR at GCE and at SWNTs– CRwater/GCE suggest that P1 might correspond to the oxidation process O2 while P2 should be the overlapped responses of O3 and O4. The absence of the reduction processes of CR at GCE except for P3 indicates that strong interactions might exist between the decomposition products of CR and the sidewall of SWNTs.
The adscription of the redox peaks in Fig. 2 is confirmed by the variations of these responses with different treatments (Fig. 3). As mentioned above, SWNTs–CRwater/ GCE exhibits several pairs of redox peaks in the whole potential range (Fig. 3(a)). With the thorough washing by DMF, the voltammograms of SWNTs–CRwater/GCE are greatly simplified and most of the redox peaks disappear except for peaks O1 and R1 (Fig. 3(b)), demonstrating that these peaks should arise from the intrinsic responses of acidtreated SWNTs. The still existence of the weak CR characteristic features in the first cycle indicates the presence of small amount of adsorbed CR on the surface of SWNTs. Fig. 3(c) shows the response of SWNTs–CRwater/GCE with GDI treatment. Different from the voltammograms of SWNTs–CRwater/GCE, the responses of CR are greatly repressed, especially for peaks O2 and R2. Since GDI would selectively react with the amino groups on CR, the repression of peaks O2 and R2 is a direct evidence for attributing them to the redox reactions of the aniline moieties on CR. The reaction between GDI and the CR molecules that are strongly attached to the sidewall of SWNTs also foretells the possibility of immobilizing macrobiomolecules (e.g. proteins) on the sidewall of SWNTs via covalent bonding. The reaction of GDI with adsorbed CR can also been observed at SWNTs–CRDMF films modified glassy carbon electrode (SWNTs–CRDMF/GCE) (Fig. 3(d)), leading to the further decrease of the CR response and the further simplification of the background. 3.3. Electrochemical characterization of water-soluble SWNTs films modified glassy carbon electrode The electrochemical and structural properties of watersoluble SWNTs films are characterized by CV and EIS using [Fe(CN)6]3/4 as the probe (Fig. 4). Fig. 4(a) shows
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Fig. 3. Cyclic voltammograms in 0.1 M phosphate buffer (pH 7.0) of SWNTs–CRwater/GCE (a), SWNTs–CRDMF/GCE (b), SWNTs–CRwater/GCE with GDI treatment (c) and SWNTs–CRDMF/GCE with GDI treatment (d) for the first (solid lines) and the 10th cycles (short dot lines). Scan rate, 100 mV/s.
the CV of [Fe(CN)6]3/4 at different electrodes. As expected, [Fe(CN)6]3/4 exhibits a well-defined redox peaks at GCE. However, with the modification of SWNTs– CRwater films, the redox currents apparently decrease and the peak shapes become poor, due to the electrostatic repulsion of [Fe(CN)6]3/4 from the electrode surface by negatively charged sulfonic groups on CR attached to the sidewall of SWNTs. Interestingly, the electrochemical response of [Fe(CN)6]3/4 is obviously improved at SWNTs–CRDMF/GCE, close to that at GCE. The removing of most adsorbed CR from the sidewall of SWNTs can effectively reduce the negative charges on SWNTs, make the SWNTs sidewall hydrophobic and provide more reaction sites for the substrates. This result also suggests the low density of negative charges on sidewall of SWNTs, consistent with the disappearance of most CR signals at SWNTs–CRDMF films. The electrochemical impedance spectra of Fe(CN)6]3/4 at these electrodes are also studied. The Nyquist plot of [Fe(CN)6]3/4 at GCE is characteristic of a semicircle at high frequencies and a straight line with a slope of about 1 at low frequencies, corresponding to the kinetical and the diffusional processes, respectively. At SWNTs–CRwater/GCE, an obviously larger semicircle at high frequencies and a depressed straight line at low frequencies appear. Moreover, no apparent transition is observed between the semicircle and the straight line, suggesting the complicated structures of the films. The Nyquist plot behaves midway at SWNTs–CRDMF/GCE, reflected by the approach of its shape to that at SWNTs–CRwater/ GCE and the parallelism of the straight line at low frequencies with that at GCE. The former indicates the structural
similarity between the SWNTs–CRwater and the SWNTs– CRDMF films while the later suggests that the diffusional behavior of [Fe(CN)6]3/4 at SWNTs–CRDMF/GCE is similar to that at GCE, i.e. the amount of adsorbed CR at the SWNTs–CRDMF films is small and the diffusion process of [Fe(CN)6]3/4 is hardly influenced by the negative charges at these films. These conclusions are supported by the Bode plots (h (phase angle) vs. log f (frequency)) (Fig. 4(c)). At GCE, a symmetrical peak with a maximum value of 39 at about 100 Hz is observed, corresponding to the relaxation process of the GCE|solution interface. With the modification of either SWNTs–CRwater or SWNTs– CRDMF films, this relaxation completely disappears and a new one situated at 5 Hz appears, reflecting the replacement of the GCE|solution interface by the SWNTs films|solution interface. The complete disappearance of the relaxation associated with the GCE|solution interface also suggests that the electrochemical reactions of [Fe(CN)6]3/4 should take place at the surface of SWNTs instead of GCE. The appearance of the relaxations for SWNTs–CRwater/GCE and SWNTs–CRDMF/GCE at the same position confirms that these water-soluble SWNTs films should possess similar structural properties. Nevertheless, apparent differences still exist between the structures of these two films, i.e. the SWNTs–CRwater films might have denser and more regular structures than the SWNTs–CRDMF films. This is supported by the following facts: (a) the much larger maximum of the relaxation at the SWNTs–CRwater films (67) than that at the SWNTs–CRDMF films (52) suggests that the SWNTs– CRwater films|solution interface is closer to a pure capacitor than the SWNTs–CRDMF films|solution interface, i.e. the
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regulated films on the electrode surface with a metallic luster whereas the films of SWNTs–CRDMF are loose and porous (not shown). This conclusion is reasonable since the rebundling of water-soluble SWNTs is mainly achieved through the coupling of adsorbed CR on the sidewall of SWNTs via p–p interactions [18]. 3.4. Electrochemical characterization of the intrinsic redox of acid-treated SWNTs The electrochemical properties of acid-treated CNTs have been explored previously [4]. The conclusion is that the oxygen-containing groups (e.g. carboxyl groups) on the defects or opened caps are responsible for the unique redox signals of CNTs. Here, the influence of solution acidity on the redox behaviors of water-soluble SWNTs is examined (Fig. 5). Generally, the cyclic voltammograms of SWNTs regularly vary with solution pH. In the range of pH 3.0–9.0, the oxidation and reduction peak potentials exhibit excellent linear relationships with pH, which have almost the same slope (0.051 V/pH). This result is consistent with the conclusion that equal protons and electrons take part in the redox of SWNTs [4]. The variations of redox currents with solution pH are also obvious. Interestingly, both the reduction and oxidation currents reach the maximum values at around pH 7.0 and decrease with either the increase or the decrease of solution pH. In addition, a new redox couple situated at about 0.2 V (DEp = 40 mV) appears in acidic media, which is attributed to the decomposition products of CR based on the variations of the voltammograms at SWNTs–CRwater/GCE with solution pH. Similar to those at SWNTs–CRDMF/GCE, the positions of the processes O1/R1 and O3/R3 at SWNTs– CR CRwater/GCE regularly shifted to more positive potentials with the decrease of solution pH (not shown). Moreover, no new redox processes appeared and the redox
Fig. 4. Electrochemical responses of 0.5 mM [Fe(CN)6]3/4 in 0.1 M phosphate buffer (pH 7.0) at GCE, SWNTs–CRwater/GCE and SWNTs– CRDMF/GCE: (a) CV; (b) Nyquist plots; (c) Bode plots.
structures of the SWNTs–CRwater films should be denser and more regular than those of the SWNTs–CRDMF films; (b) the narrower distribution of the relaxation at the SWNTs–CRwater films than that at the SWNTs–CRDMF films indicates that the former should have denser and more regular structures. In fact, it can be clearly observed from naked eyes that SWNTs–CRwater forms smooth and
Fig. 5. Variation of cyclic voltammograms at SWNTs–CRDMF/GCE with solution pH in 0.1 M phosphate buffer. The acidity of the buffer solutions beyond conventional phosphate buffers was adjusted to the desired values with 1.0 M HCl. Scan rate, 100 mV/s.
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currents of O3/R3 apparently increased whereas those of O1/R1 hardly changed. These results suggested that the new redox couple at 0.2 V might be the enhanced process of O3/R3 at SWNTs–CRDMF/GCE in acidic media. The influence of scan rate is also investigated (Fig. 6). As expected, the redox currents of SWNTs linearly increase with scan rate at low scan rate (6150 mV/s), representing a typical surface process [19]. When scan rate is higher 150 mV/s, both the reduction and the oxidation potentials exhibit good linear relationships with the logarithm of scan rate, which can be expressed as Ep,a = 0.014 + 0.068 log m (R = 0.9970) and Ep,c = 0.146 + 0.061 log m (R = 0.9952) for the oxidation and the reduction processes, respectively. The solving of these two equations produces a value of 0.52 for an, where a is the electron transfer coefficient and n is the electron transferred in the rate-determining step. It is obvious that one electron might involve in the rate-determining step, which is in accordance with the previous report [4]. 3.5. Promising applications of water-soluble SWNTs film modified glassy carbon electrode in electroanalytical chemistry The potential application of water-soluble SWNTs films as electrochemical sensing films was examined. Fig. 7 shows the responses of 1 · 105M DA, UA and AA at different electrodes. At GCE, all these biomolecules exhibit low and wide oxidation peaks, which overlap with each other and none can be selectively determined (inset in Fig. 7). The responses of these biomolecules are much different at SWNTs–CRDMF/GCE. No discernable response of AA is observed in this potential range and the background apparently increases. In fact, AA exhibited an insensitive wide oxidation peak at around 0.04 V (vs. SCE), which partly overlapped with the intrinsic response of SWNTs (not shown). At the same time, UA still shows
Fig. 6. Influence of scan rate on cyclic voltammograms of SWNTs– CRDMF/GCE in 0.1 M phosphate buffer (pH 7.0). Insets showed the variations of redox currents and potentials of SWNTs–CRDMF/GCE with scan rate.
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Fig. 7. Cyclic voltammograms of 1 · 105 M DA, UA and AA at SWNTs–CRDMF/GCE in 0.1 M phosphate buffer (pH 7.0). Inset showed cyclic voltammograms of DA, UA and AA at GCE in 0.1 M phosphate buffer (pH 7.0). Scan rate, 100 mV/s; accumulation conditions, 300 s at an open circuit.
an apparently enhanced oxidation response at SWNTs– CRDMF/GCE in 0.1 M phosphate buffer (pH 7.0). This confirms that the density of negatively charged CR on the sidewall of SWNTs is low because UA (pKa1 5.75) is negatively charged in the electrolyte used here (PBS, pH 7.0) and the presence of high density of negative charges on the electrode surface would depress the responses of UA. This result also indicates that the large portion of SWNTs sidewall is uncovered by CR, which might be the reactive sites of water-soluble SWNTs towards the substrates. Similar to UA, the width of the oxidation peak of DA is apparently narrowed and the oxidation current is greatly enlarged, leading to the separation of the DA oxidation potential from those of UA and AA for 126 and 125 mV, respectively. This provides a chance for the selective determination of DA in the presence of UA and AA. The dependence of the oxidation current on DA concentration at SWNTs–CRDMF/GCE is examined using DPV (Fig. 8(a)). In the range of 2.0 · 107–2.0 · 106 M, the oxidation current of DA shows a linear dependence on its concentration (Fig. 8(b)), which can be expressed as Ip(lA) = 3.86CDA(lM) + 0.28 (n = 6, R = 0.9981). At higher concentrations, the oxidation current gradually deviates from this linear relationship, due to the saturation of DA adsorption at SWNTs–CRDMF/GCE. The adsorption of DA was supported by the gradual decrease of its oxidation current with the increase of scan cycle in successive voltammograms after 300 s accumulation at an open circuit and the restoring of the oxidation current with a similar accumulation process. The reproducibility of SWNTs–CRwater/GCE was examined and a low RSD of 4.5% was obtained for six measurements of 5.0 · 107 M DA. The detection limit, defined as a signal-to-noise ratio of 3:1, was found to be 8.5 · 108 M for 300 s accumulation at an open circuit.. For nine successive additions of 5.0 · 107 M DA, rapid and stable amperometric responses
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Fig. 8. Differential pulse voltammograms (DPV) for 0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.5, 2.0, 3.0 and 4.0 lM DA (from inner to outer) at SWNTs–CRDMF/ GCE in 0.1 M phosphate buffer (pH 7.0) (a), dependence of DA oxidation current on its concentration (b) and amperometric responses of successive addition of 0.5 lM DA at an interval of 60 s (c). The accumulation conditions for DPV measurements were 300 s at an open circuit and the oxidation potential in amperometric measurement was set at 0.18 V (vs. SCE).
are obtained at SWNTs–CRDMF/GCE (Fig. 8(c)). This result suggests that SWNTs–CRDMF/GCE has the merit of being rapid, sensitive and stable as electrochemical sensing films. The interferences of 200-fold of UA and AA on the amperometric response of DA were evaluated. The addition of 1.0 · 104 M UA did not cause any changes on the signal of 5.0 · 107 M DA whereas the addition of 1.0 · 104 M AA led to an increase of the oxidation current for more than eight times. The different variations of the background with the addition of high concentrations of UA and AA might account for their different interferences on DA. The interferences of UA and AA on the DPV response of 2.0 · 107 M DA are also examined (Fig. 9). The addition of 200-fold of UA causes only a neglectable
decrease of DA response (3.5%) but the interference of 400-fold of UA is serious and the oxidation current of DA decreases for 39.7%. In contrast to UA, the addition of 200-fold AA leads to an increase of the DA response for 25.3%. The interferences of AA and UA on the DPV response of DA are majorly controlled by two opposite factors: the competitive adsorption causing the reduction of the reactive sites of DA and the overlapping of the oxidation peaks leading to the increase of DA oxidation current. The adsorption of UA at the hydrophobic sidewall of SWNTs should be stronger than that of AA due to the much lower solubility of UA in aqueous solutions. At the same time, as indicated in Fig. 7, the oxidation peak width of UA is much narrower than that of AA. Thus, the addition of UA mainly results in the reduction of the reactive sites at SWNTs–CRDMF/GCE by competitive adsorption whereas the addition of AA mainly contributes to the increase of the oxidation current through the overlapping of the oxidation peaks. The interference from AA could be eliminated by coating SWNTs–CRDMF/GCE with a film of negatively charged polymers (e.g. Nafion). 4. Conclusion In conclusion, water-soluble SWNTs were prepared via noncovalent functionalization by CR through a physical grinding process. These soluble SWNTs can be firmly immobilized on the surface of GCE by a simple casting method. The obtained SWNTs films were characterized by several techniques, including CV, EIS and SEM. The results showed that water-soluble SWNTs formed uniform films with network structures of nanosizes on the electrode surface. In addition, the structural properties and the negative charge density of these films can be conveniently controlled by choosing proper solvents in the washing procedure, which provided a simple approach for adjusting their properties to specific applications. As an example, the potential applications of the water-soluble SWNTs with DMF washing (SWNTs–CRDMF) in electroanalytical chemistry were explored. The enhanced response of DA as well as the separation of the DA oxidation peak from those of UA and AA at SWNTs–CRDMF/GCE demonstrated that these water-soluble SWNTs were promising materials for constructing CNTs-based electrochemical sensing films. Acknowledgement The authors are grateful for the financial support of National Natural Science Foundation of China (Nos. 30370397 and 60571042). References
Fig. 9. Interferences of UA and AA on the DPV response of 0.2 lM DA at SWNTs–CRDMF/GCE in 0.1 M phosphate buffer (pH 7.0) for 300 s accumulation at an open circuit.
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