Single-Walled Carbon Nanotube as 1D Array of Reacting Sites

ARTICLE pubs.acs.org/JPCC

Single-Walled Carbon Nanotube as 1D Array of Reacting Sites: Reaction Kinetics of Reduction of Cytochrome c in Tris Buffer Takafumi Nakashima and Masahito Sano* Department of Polymer Science and Engineering, Yamagata University, 3-4-16 Jyonan, Yonezawa, Yamagata 992-8510, Japan ABSTRACT: Dispersed single-walled carbon nanotubes (SWCNTs) reduce cytochrome c (Cyt-c) when simply mixed in Tris buffer. Atomic force microscopy, Raman spectroscopy, and UV
vis absorption spectroscopy show that SWCNTs are individually dispersed and are highly n-doped in Tris buffer, which accounts for the reducing ability. Because the size of Cyt-c is significantly larger than the diameter of SWCNT, each SWCNT can be modeled as a 1D array of reacting segments. The kinetic study reveals that the early stage follows the Langmuir adsorption model, implying that each reacting segment is equivalent and reacts independently. Several hours later, a number of adsorbed Cyt-c reaches significant to change the kinetics to logarithmic. The velocity starts to decrease exponentially, indicating a high sensitivity of the 1D system to adsorption and suggesting autocatalytic adsorption. The reaction ceases after 40 h due to saturated adsorption, corresponding to a poisoning effect in catalysis.

1. INTRODUCTION The structure of a single-walled carbon nanotube (SWCNT) is characterized by a graphene sheet of carbon hexagons rolled into a cylindrical shape with a diameter of 1
2 nm and a length as long as a few micrometers.1 These sizes mean different things depending on the reacting molecule, as they span both molecular and colloidal scales. For small molecules, only a small part of the SWCNT surface, corresponding to an area of a few hexagonal units, is sufficient for reaction. Because the equivalent hexagonal units exist on the same SWCNT almost everywhere, the reaction may be considered as being between small molecules and the surface of a three-dimensional (3D) uniform solid. In contrast, for a molecule larger than the tube diameter, the contacting area extends for a significant length along the tube axis. Other molecules are effectively excluded from this area while it is reacting. In this respect, a SWCNT looks like a very thin wire which allows only one molecule to react at a given length segment but has many reacting segments along its length. In other words, a SWCNT may be considered as a 1D array of reacting sites (Figure 1). Reactions between small molecules in solution are usually described by homogeneous reaction kinetics,2 which presumes isotropic 3D solution. When reactions take place at solid surfaces or liquid interfaces, the kinetics becomes heterogeneous in 2D. For the first time, SWCNTs offer an opportunity to study heterogeneous reactions involving 1D solid in solution. In particular, we are interested in questions like the following: Does each site react equivalently? Does a reaction of one site affect those of the neighboring sites? What is the effect of deactivating a reactive site on the kinetics? In this study, a protein is employed as a large reacting molecule and electron r 2011 American Chemical Society

transfer reaction is studied because it does not change the structures of both protein and SWCNT. Cytochrome c (Cyt-c) is a heme protein with a diameter of approximately 3.5 nm, which is significantly larger than the diameter of SWCNTs used in this study (0.7
1.2 nm). The ferric (Cyt-c3+) or ferrous (Cyt-c2+) states can be probed easily either electrochemically or spectroscopically. Although Cyt-c denatures easily on metal surfaces, it remains intact on carbon nanotubes.3,4 Cyt-c was chosen for this study, as we have found that SWCNTs acquire a negative ζ-potential at neutral pH after acid treatments5 and reduce Cyt-c by simply mixing in Tris buffer.6 Elucidating why simply mixing SWCNTs in buffer reduces Cyt-c is also a subject of the present study. Reactivity of native SWCNTs in electron transfer reactions is known to depend on electronic structure7 and chirality.8
11 The SWCNT sample used in this study contains various chiral tubes. As a result, the electrochemical reduction potential of Cyt-c lies in the middle of those of native SWCNTs.8
12 One of the major problems associated with SWCNTs is the difficulty in dispersing SWCNTs in solution. The most common method to suspend SWCNTs in solution is to add surfactants or polymers, which form surface layers around the SWCNT body. Because small molecules are able to migrate through the additive layer to reach the SWCNT surface, the experiment is still feasible. On the other hand, large molecules like proteins are not free to penetrate through the additive layer. Also, most surfactants affect the protein structure severely. Thus, all previous studies of Received: June 24, 2011 Revised: September 21, 2011 Published: September 27, 2011 20931

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Figure 1. A SWCNT is modeled as a 1D array of reacting segments. A red molecule reacts with the SWCNT to produce a blue molecule. An incoming molecule cannot reach the SWCNT surface if the segment is occupied by other molecules. A SWCNT represents a thin wire consisting of linearly connected reacting segments, each of which can react with one molecule at a time.

SWCNT and Cyt-c have been done with the SWCNTs fixed in the form of solid films, for instance, as an electrode for electrochemical measurements.3,12
16 We have carried out a series of experiments to find out a condition to disperse SWCNTs stably over a month in buffer solution without using additives. The paper consists of two parts. The SWCNTs dispersed in Tris buffer are characterized by atomic force microscopy (AFM), UV
vis absorption spectroscopy, and resonant Raman spectroscopy. We show that SWCNTs in buffer are individually dispersed and highly n-doped, which accounts for the reducing ability. The second part is the heterogeneous reaction kinetics of reduction of Cyt-c by doped SWCNTs in buffer. The kinetics is shown to proceed in two stages. Initially, the kinetics follows the Langmuir adsorption model, implying equivalency and independence of each reacting site. The 1D system, however, is highly sensitive to adsorption, resulting in deactivation of the reacting sites. The kinetics, then, shifts to a logarithmic regime.

2. EXPERIMENTAL SECTION 2.1. Dispersion in Tris Buffer. Purified SWCNTs (HiPco) were sonicated in a H2SO4/HNO3 (3:1 v/v) mixture for 3 h and etched in H2O2/H2SO4 for 10 min. The solid was collected on a Teflon filter and washed until the filtrate became a neutral pH. Water was deoxygenated prior to use. The acid-treated SWCNTs were redispersed in buffer solutions, and the supernatant after centrifugation at 45000g applied for 60 min was retained. Having examined various buffers, temperatures, and SWCNT concentrations, a stable dispersion was obtained in 2-amino-2-hydroxymethyl-1,3-propanediol (Tris) buffer (50 mmol/mL, pH 7.4) at low temperatures with concentrations less than 5 ng/mL. Resonant Raman spectroscopy (Nicolet Almega XR) was measured using 532 and 780 nm laser excitations. Because the measurement on the SWCNT solution gave very low S/N ratio due to water scattering, the measurement was made on the dried SWCNT samples on glass. For AFM imaging, a drop of the reaction solution was cast on mica. After the mica surface was dried completely, it was rinsed with water to wash out excess buffer materials before being imaged. 2.2. Reactions with Cyt-c. The SWCNT dispersion (1.0 ng/mL) was mixed with Cyt-c3+ solution in a quartz cell equipped with a Thunberg tube.6 After the cell was vacuum-pumped and refilled with N2 gas three times, it was completely sealed and kept

Figure 2. AFM image (10 μm  10 μm) of SWCNTs on mica cast from the buffer solution.

at 4 °C. The absorbances at 530 and 550 nm (Q-band) were used to calculate the concentration of each Cyt-c species ([Cyt-c]) using a standard procedure.12,17 The concentration of SWCNTs was low enough so that the absorption due to SWCNTs was negligible at these wavelengths.

3. RESULTS AND DISCUSSION 3.1. SWCNTs in Tris Buffer. Figure 2 shows a typical AFM image of SWCNTs cast from the buffer solution. The width of tubular features is broadened due to tip convolution. The length and height measured from many images are found to be in the range 0.3
1.0 μm and 1
3 nm, respectively. Since the diameter distribution of as-grown SWCNT is 0.7
1.2 nm, they are either individually dispersed or bundled only slightly. Regardless of whether they are individually dispersed or bundled, the reduction reaction with Cyt-c involves only the outermost surface. Thus, as long as a bundle is narrower than Cyt-c, the bundle can be considered as a thick SWCNT for the present study. Raman spectra of dried SWCNTs from the buffer (solid curve) and the pristine sample (dotted curve), excited by 532 and 780 nm lasers, are presented in Figure 3. At 532 nm excitation, both semiconducting and metallic tubes resonate, whereas 780 nm excitation probes mostly semiconducting tubes. The spectra at the radial breathing mode (RBM) region (Figure 3a and c) are normalized at the 270 cm
1 peak, which is known to be affected strongly by bundling.18 In both cases, there is no systematic change of RBM peaks. The spectra around the G band are normalized around the 1590 cm
1 peak. At 532 nm excitation, the buffered sample shows a G band downshift of 6 cm
1 compared with the pristine sample, indicating the increased electron density. The Breit
Wigner
Fano shoulder around 1530 cm
1, which is produced by phonon
electron coupling mainly due to metallic SWCNTs, broadens and downshifts as the electron density increases. At 780 nm excitation, the same G band downshift of 6 cm
1 is observed. These spectral characteristics have been established as the evidence of heavy n-doping by numerous studies using alkali metals19
22 as well as nitrogencontaining compounds.23,24 20932

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Figure 5. The Q-band region of Cyt-c before (solid curve) and 36 h after (dotted curve) the addition of SWCNTs.

Figure 3. Raman spectra of the dried SWCNTs from Tris buffer (solid curve) and the as-grown sample (dotted curve), excited by 532 and 780 nm lasers. Tris causes the G band to downshift and the BWF shoulder to broaden, as indicated by the arrows.

Figure 6. Temporal development of reduction of Cyt-c. The number on each curve is the initial ferric Cyt-c concentration. The solid curves are the best fit to the Langmuir kinetic equation.

Figure 4. UV
vis absorption spectrum of the acid-treated SWCNTs in Tris buffer (solid curve) and in water (dotted curve). The peaks due to van Hove singularity seen in water are missing in Tris buffer.

The UV
vis absorption spectrum of the acid-treated SWCNT in Tris is given in Figure 4. The concentration of SWCNT was made higher than the one used for the reaction to gain sufficient S/N ratio. The low concentration sample had the same spectral shape as the concentrated one. Also shown is that of the acid treated SWCNT in water. Individual peaks due to different chirality seen in water are not observable in Tris buffer. This kind of featureless spectra has been obtained from highly doped SWCNTs25 as well as heavily bundled samples. All three measurements consistently indicate that SWCNTs in Tris buffer are nearly individually dispersed and highly doped. This is consistent with the previous finding that SWCNTs are doped easily by amines.26,27 The excess charges also strengthen the electrostatic repulsion between SWCNTs to stabilize dispersion.5

The acid-treated SWCNTs can be dispersed in water for a few days without adding surfactants.28 When the acid-treated SWCNTs and Cyt-c were mixed in water, no reduction reaction was observed. Apparently, the defects produced by the acid treatment alone cannot reduce Cyt-c. Thus, reduction of Cyt-c by SWCNTs is driven by the electrons supplied by Tris, which exist in far excess quantity than other solutes. 3.2. Reaction Kinetics. Figure 5 depicts the spectral change at the Q-band region of Cyt-c. An oxidized form having a peak around 530 nm (solid curve) is replaced by a reduced form having peaks at 520 and 550 nm (dotted curve). These changes are identical with other nanotube studies17 as well as when Cyt-c was reduced by sodium dithionite. A temporal development of the reduction reaction for various initial [Cyt-c3+] is presented in Figure 6. A control sample of Cyt-c3+ in Tris buffer without SWCNTs shows no change in absorption spectrum over the same time period. [Cyt-c2+] increases fast initially and then reaches a limited value [Cyt-c2+]0 after nearly 24 h. A higher [Cyt-c3+] gives similar behavior but with the larger limited concentration. It has been reported that electron transfer between small molecules and surfactant-covered SWCNTs completes within a few minutes.10 We expect that the present case of the surfactantfree SWCNTs should react even faster with small molecules. The rate constants between Cyt-c and solid SWCNT films are in the range 1
10 s
1,14,15 which is explained by the fact that the heme group is exposed in Cyt-c and the solid SWCNT films have 20933

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Figure 7. Langmuir isotherm of the limited concentrations obtained from Figure 6.

large surface areas. In solution, each SWCNT behaves as a colloidal particle,5 meaning that the reaction is heterogeneous. Although the length is on the colloidal scale, the diameter is smaller than the reacting molecule, which is considered to be a reason for the slow kinetics. In other words, the rate determining step is adsorption of Cyt-c on SWCNTs. 3.3. Langmuir Model. We first analyze the data by assuming that the adsorbed Cyt-c does not affect the reaction. Among various adsorption models (Langmuir, Freundlich, Slygin
Frumkin, Brunauer
Emmett
Teller, and Harkins
Jura),2 the Langmuir kinetic equation of the form ½Cyt-c2þ  ¼ ½Cyt-c2þ 0 f1
expð
ktÞg

ð1Þ

with k = 0.1
0.2 h
1 gave the best fit, as indicated by the solid curves in Figure 6. Furthermore, [Cyt-c2+]0 is found to follow the Langmuir isotherm given by ½Cyt-c2þ 0 ¼

ab½Cyt-c3þ  1 þ b½Cyt-c3þ 

ð2Þ

with a = 3.90 μmol/L and b = 0.063 L/μmol (Figure 7). The Langmuir isotherm is based on the assumptions that (A) every site is equivalent and adsorbs independent of its neighbors, (B) only one molecule adsorbs at one site, and (C) equilibrium exists between adsorption and desorption. When these adsorption characteristics are translated into the reaction, (A) indicates equivalence and independence of the reacting sites. The featureless UV
vis spectrum of Figure 4 indicates that the 1D nature of the electronic state as represented by van Hove singularity (not to be confused with the 1D array of the reacting sites that signifies geometrical arrangement) is lost in Tris buffer, meaning that the electrons no longer have strong long-range coupling. In addition, since the kinetics is very slow, the electron transferred to Cyt-c at a particular reacting site can be compensated well before the next reaction by the additional electron density provided by Tris anywhere along the whole length. Furthermore, the present SWCNTs were so heavily doped in buffer that localized electrons at defects did not play a major role. (B) is reasonable for the present reaction, since electron transfer takes place only with molecules on the first layer, even if multilayer adsorption occurs. As long as the molecule is desorbed immediately after the reaction, (C) is satisfied. It is also possible to relate (C) to redox equilibrium theoretically, since the electrochemical reduction potential of Cyt-c lies in the middle of those of SWCNTs.8
12

Figure 8. Addition experiments where the fresh solution of (a) SWCNT and (b) Cyt-c3+ was added at 40 h after initial mixing, respectively. The slight increase after 24 h in part b is within experimental error.

There are many examples of adsorption on both SWCNTs and multiwalled CNTs that follow the Langmuir isotherm.29
33 Typically, the adsorbing molecules are gases, inorganic ions, and low molecular weight organic compounds. CNTs are in the form of solid films either by themselves or on supports. In other words, these systems represent adsorption of small molecules on the 2D assembly of CNTs. The van der Waals interactions, which are extremely strong among CNTs,34 remain insignificant between CNTs and small molecules. Also, other than the defect sites, the reactivity of CNT surfaces is very low. Thus, unless small molecules have the tendency to self-aggregate or CNTs contain significant amounts of defects, Langmuir adsorption is expected. In the present case of 1D solid, there is no a priori reason that the reaction follows Langmuir type. Dimensionality does not enter into Langmuir adsorption, so the result indicates that there is no kinetic difference between 1D and 2D arrays. One of the processes that depend critically on dimensionality is surface diffusion. Since there is only one path along the 1D array, surface diffusion is severely impeded by an adsorbed molecule. In the present case, each reacting segment on SWCNT is long enough to contain many reactive units (hexagonal units that perform electron transfer with heme) so that Cyt-c need not diffuse along SWCNT before reaction. Apparently, the kinetics is so slow that other processes become irrelevant. At very low reactant concentrations, SWCNTs do not interact with each other and the simultaneously reacting Cyt-c molecules are far apart on the same SWCNT. This is equivalent to Cyt-c reacting with an isolated SWCNT segment. Then, the kinetics becomes homogeneous, and in fact, the initial velocity obtained from Figure 6 is fitted well by the pseudo-first-order equation. 3.4. Logarithmic Model. A previous electrochemical study13 has indicated that adsorption of Cyt-c on CNT saturates after 48 h. To examine the effect of saturated adsorption, we added fresh solutions of SWCNT and Cyt-c3+ to the reaction mixtures 40 h after mixing, respectively. An addition of fresh SWCNT restarted the reaction, whereas an addition of Cyt-c3+ hardly increased the product (Figure 8). This implies that the reducing 20934

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Figure 9. (a) The same data as in Figure 6, fitted by the logarithmic law. (b) A comparison of logarithmic (dotted lines) and Langmuir (solid curves) fits for the first 10 h at 6.3 μmol/L. Other initial concentrations also show a “dip” at the first several hours and are fitted better by the Langmuir model.

ability of SWCNTs has been lost. The SWCNT surface is completely covered by Cyt-c, creating a situation similar to poisoning in catalysis. This poisoning effect leads us to reanalyze the same kinetic data by assuming that the adsorption affects the reaction. For 2D surfaces with reacting sites, each of which adsorbs a molecule independently and is deactivated as a result of adsorption, the velocity often decreases in proportion to the bulk concentration. Then the kinetics becomes parabolic. In the present case, however, the parabolic model or other common models like a reciprocal logarithmic model deviate significantly from the data points. The best fit is obtained by a logarithmic model given by ½Cyt-c2þ  ¼ A þ B lnðtÞ

ð3Þ

where A and B are some constants. It is shown in Figure 9a where the same data as presented in Figure 6 are used. This is the first finding of the logarithmic kinetics involving SWCNTs. Equation 3 implies that the velocity decreases exponentially with the concentration. Since the volume element scales with the spatial dimension, the number density in the 1D system is more sensitive to a change in the number of sites than in 2D. The additional “gain” on the concentration dependence by dimensionality may be described as a power law as long as the model is based on the density. In this case, the parabolic dependence in 2D is amplified to the higher power law dependence in 1D. The present exponential decrease, therefore, indicates that it must involve factors stronger than dimensionality. As discussed in the previous section, SWCNT is so heavily doped by Tris that the adsorbed Cyt-c2+ molecules are unlikely to affect the electronic structure of the entire SWCNT. Thus, the strong dependence is

not due to a change in redox potentials. Instead, we propose autocatalytic adsorption of Cyt-c on SWCNT; that is, the already adsorbed Cyt-c enhances further adsorption of Cyt-c. Although the interactions responsible for adsorption are not known, major interactions such as van der Waals, hydrophobic, and electrostatic require their close approach. A freely diffusing Cyt-c in solution is attracted to the adsorbed Cyt-c on SWCNT by the hydrophobic interaction between Cyt-c. Since the attracted Cyt-c molecule has a greater chance to be within the range of interactions with SWCNT than the freely diffusing one, it is adsorbed more easily. Usually, a sigmoid function is used to describe autocatalysis in 2D as well as 3D. Because the collision frequency of diffusing molecules with a 1D rod is much smaller than that with a 2D surface, 1D autocatalysis may have a stronger dependence than sigmoid. An additional enhancement is provided by the enlarged surface area of the SWCNT with the adsorbed Cyt-c compared with the adsorbent-free SWCNT. In 1D, this is related to the diameter of Cyt-c and the adsorbed amount. Since the dependence of each dimensionality, autocatalytic adsorption, and enlarged surface area is weaker than exponential, a cooperative contribution of all effects may be involved in the present case. 3.5. Total Kinetics. Both Langmuir and logarithmic models give similar curves, and they appear to fit the data. A close examination, however, reveals that the logarithmic model fits excellent during the middle period but poor during the first 4 h and after 40 h (Figure 9b). Since the reaction is essentially over by 40 h due to the poisoning effect, the logarithmic model, which continues to increases indefinitely, is no longer valid after 40 h. This observation suggests that the reaction proceeds with two kinetic stages. At the beginning, a SWCNT is free of strongly adsorbed Cyt-c. All reacting sites on the SWCNT are available to Cyt-c. Each site reacts equally and independently, following the Langmuir model. Some Cyt-c molecules, however, remain adsorbed and prevent other molecules from reacting. After several hours, the amount of strongly adsorbed Cyt-c molecules becomes large enough to start affecting the reaction and the kinetics is now described by the logarithmic model. The exponential decrease of velocity signifies the 1D kinetics. After approximately 40 h, all reacting sites are covered by the adsorbed Cyt-c molecules and the reaction is quenched.

4. CONCLUSIONS In conclusion, SWCNTs are highly doped in Tris buffer, which drives the reduction reaction of Cyt-c. Because compounds with amine functional groups are often employed in SWCNT dispersions, we must be aware that the reactivity of SWCNT may be altered significantly. For the reacting molecule whose size is considerably larger than the tube diameter, a SWCNT represents a 1D array of reacting sites. When the number of strongly adsorbed molecules is small, the kinetics is described by the Langmuir model. The 1D nature manifests itself as the high sensitivity to adsorption, shifting the kinetics to logarithmic. Because many kinds of biomolecules are known to adsorb on SWCNTs, the present result should be applicable to various reactions with biopolymers. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. 20935

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’ ACKNOWLEDGMENT This work is supported in part by Grants-in-Aid for Scientific Research 19054003.

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