Preferential Biofunctionalization of Carbon Nanotubes Grown by

May 10, 2010 - Multiwalled carbon nanotubes (CNTs) were grown by microwave plasma .... of negative bias (0, -100, -200, and -300 V) and four .... by T...
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Preferential Biofunctionalization of Carbon Nanotubes Grown by Microwave Plasma-Enhanced CVD Sungwon S. Kim,†,‡ Placidus B. Amama,‡ and Timothy S. Fisher*,†,‡ School of Mechanical Engineering, Purdue UniVersity, 585 Purdue Mall, West Lafayette, Indiana 47905, and Birck Nanotechnology Center, Purdue UniVersity, 1205 West State Street, West Lafayette, Indiana 47905 ReceiVed: December 22, 2009; ReVised Manuscript ReceiVed: April 20, 2010

Multiwalled carbon nanotubes (CNTs) were grown by microwave plasma chemical vapor deposition using dendrimer-templated Fe nanoparticles as a catalyst. Variation of the dc bias voltage and the calcination temperature of the dendrimer-templated Fe2O3 catalyst yielded a matrix of CNT arrays. Samples were immobilized with glucose oxidase, which were used for amperometric cyclic voltammetry experiments. Spectroscopic and electron microscopic characterizations indicate that enzyme adsorption per unit CNT area is higher for samples with lower quality, suggesting that CNTs with higher levels of defect densities are desirable for biosensing applications. I. Introduction 1

Since the discovery of carbon nanotubes (CNTs), scientists and engineers have sought to exploit their exceptional mechanical, electrical, thermal, and chemical properties. Emphasis has largely concentrated on synthesis methods and procedures2,3 and developing engineering applications, such as transistors,4 sensors,5 and thermal enhancement materials.6 The need for enhanced functional performance has been a motivator for research in improvement of material characteristics.7-9 This need has also motivated attempts to investigate the efficacy of CNTs in biosensor and biomaterials applications. Although many published reports exist on the use of CNTs in biosensors,10-27 only the end effect (i.e., the sensing mechanism itself and observed changes thereof) is commonly reported, with little attention given to the material aspects or synthesis procedures that often have a strong effect on the observed results. In this respect, it is not surprising that a disconnect exists between the synthesis process of CNTs2,3 and interpretation of biofunctionalization experimental results in that many of the biosensor experiments are conducted with CNTs procured from vendors or collaborators. The development of new approaches for the controlled growth of CNTs is expected to positively impact the field of biosensing as several studies have shown that the physical properties of CNTs affect the performance of these CNT-based electronic sensors. For instance, Lin et al. showed that a biosensor developed using CNT nanoelectrode ensembles performs efficiently during selective electrochemical analysis of glucose in the presence of interferers.25 Also, Heng et al.26 have shown that electrodes modified with bamboo structured CNTs have clear superiority over those modified with single-walled carbon nanotubes (SWCNTs) because of the presence of edge planes of graphene at regular intervals along the walls of bamboo structured CNTs. A sound understanding and control of the synthesis process and related effects on functional performance characteristics is * To whom correspondence should be addressed. E-mail: tsfisher@ purdue.edu. Phone: (765) 494-5627. Fax: (765) 496-8299. † School of Mechanical Engineering. ‡ Birck Nanotechnology Center.

important to improve biomaterial performance. An attempt to bridge this gap through a comprehensive methodology starting from nanomaterial synthesis, continuing to enzyme immobilization, and culminating with functional characterization is presented in this work. Successful biofunctionalization of glucose oxidase on CNTs is demonstrated, and a relationship between peak amperometric current and CNT structure, resulting from varying CNT synthesis conditions, is established. The quality of CNTs in terms of the relative number of defect sites, resulting from the nanostructure created by controlled synthesis conditions, is shown to strongly influence the adsorption of glucose oxidase on CNTs. II. Experimental Methods Carbon nanotubes were grown on Ti/SiO2/Si substrates using dendrimer-templated Fe2O3 nanoparticles as a catalyst in a MPCVD (microwave plasma-enhanced chemical vapor deposition) reactor. CNTs were functionalized with glucose oxidase, after which they were exposed to glucose in a fluidic cell where cyclic voltammetry tests were conducted. Enzyme-functionalized CNTs were used as the working electrode to record electric current when voltage was applied. Also, CNTs were observed with scanning electron microscopy (SEM) for external characteristics as well as transmission electron microscopy (TEM) for internal characteristics. Finally, Raman spectroscopy was employed to characterize the quality of the CNTs. This general methodology was repeated for an experimental matrix consisting of 16 samples, and results were compared. CNT growth, glucose oxidase immobilization, and cyclic voltammetry test procedures are explained in more detail below. A. Catalyst Preparation. The catalyst solution was prepared by stabilizing excess Fe3+ using a fourth-generation, amineterminated poly(amidoamine) (PAMAM) dendrimer herein after referred to as G4-NH2, through a interdendritic templating mechanism.34,35 The Fe3+/G4-NH2 composite was synthesized by separately dissolving FeCl3 · 6H2O (5.7 mmol) and the G4NH2 dendrimer (0.003 mmol) into deionized water and subsequently mixing the solutions. Evaporation is a popular method to deposit catalyst material onto substrate surfaces, but this method can be prone to inconsistencies, such as catalyst contamination due to the different metals used in general-purpose

10.1021/jp912092n  2010 American Chemical Society Published on Web 05/10/2010

Biofunctionalization of CNTs Grown by MPCVD evaporation systems. A wet chemistry approach can offer the benefit of consistency, convenience, and relative immunity to catalyst poisoning. A highly reliable, reproducible, and flexible synthesis method33 involving the use of a dendrimer as a nanotemplate that yields a nearly monodispersed transition metal has been used in previous work35 and was also employed here. Silicon wafers (100 mm diameter) with a silicon dioxide top layer were coated with a 30 nm layer of Ti to prevent the possibility of silicide poisoning of the Fe2O3 nanoparticles and to promote their adhesion to the substrate surface. The Ti layer was evaporated using a Leybold electron-beam evaporator. After this metal deposition step, the wafer was cut into squares, approximately 10 mm ×10 mm in size. These Si/SiO2/Ti squares served as the substrates for catalyst immobilization and subsequent CNT growth and biofunctionalization experiments. The individual substrates were first cleaned with methanol and deionized water to remove any impurities and then dried with a stream of N2. The precleaned substrates were dipped into the catalyst solution for 10 s and dried in a stream of N2, resulting in the coating of the Fe3+/G4-NH2 composite on the substrate surface. Finally, the supported Fe3+/G4-NH2 composites were calcined at different temperatures for 20 min, ultimately resulting in the formation of a monolayer of exposed Fe2O3 nanoparticles. B. CNT Growth Procedure. CNT growth occurred in a microwave plasma chemical vapor deposition (MPCVD) chamber under a specific set of controlled conditions, including substrate temperature, chamber pressure, gas composition, bias voltage, and plasma power.28-32 A detailed procedure for CNT growth by the methods employed in the present work has been previously reported.39 In brief, both substrates for each of the 16 runs were placed on the center of a Mo puck, and the chamber was evacuated to 1.5-1.6 Torr and purged with N2 for 5 min. The substrate was annealed in ambient N2 in order to stabilize the Fe2O3 nanoparticles.35 This process was followed by heating of the susceptor to 900 °C, after which the chamber was pressurized to 10 Torr with a 50 sccm stream of H2. At this point, the plasma power was turned on to 200 W to ignite the hydrogen plasma, and a 10 sccm stream of CH4 was introduced within 30 s. Directly after the introduction of the CH4 into the plasma, stability of the plasma was checked, and the bias voltage was turned on in the appropriate cases (excluding the 0 V cases). These growth conditions were sustained for 10 min. During the initial minute of each CNT growth process, the surface color of the substrates was observed to turn dark, indicating that CNT growth had initiated. After the growth had persisted for 10 min, the bias voltage and the plasma were turned off, the H2 and the CH4 flows were cut off, the susceptor heater was turned off, in that order, and the chamber was allowed to cool to room temperature. Thereafter, the substrates were removed, and one sample was labeled for biofunctional testing, while the other was set aside for SEM/TEM/Raman spectroscopy characterization. Figure 1 shows an SEM image of a growth product yielding CNTs. The Fe2O3 nanoparticles can be seen to have deposited only on the Ti underlayer. Also, CNTs can be seen to have initiated growth from these nanoparticles with good adhesion to the substrate surface. C. Variation of Growth Conditions - Sample Matrix. The bias voltage of the MPCVD and the catalyst calcination temperature were the two independent growth parameters employed in controlling the general quality of CNTs in terms of defect density for enzyme immobilization. The bias voltage in the MPCVD controls the position of the plasma over the substrate. Macroscopically, higher negative bias voltage has the

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Figure 1. High-magnification SEM image of a typical sample grown from Fe2O3 nanoparticles, showing Fe2O3 nanoparticles on the Ti layer.

effect of pulling the plasma closer to the substrate surface, as observed through a side port window in the MPCVD chamber. The increase of positive ions bombarding the substrate surface in the presence of a negative bias is thought to have an effect on the quality and the nanostructure of the CNTs.36,37 Experimentally, it was shown in prior work38 that CNTs grown under negative bias were observed to better adsorb fluorescently marked BSA (bovine serum albumin). The catalyst calcination temperatures used herein are similar to those used in our previous work5 because these specific temperatures allowed us to vary the mean diameters and quality of the CNTs. The calcination temperatures used were 250, 550, 700, and 900 °C. The calcination step plays another important role, as it determines the morphology of the Fe2O3 nanoparticles formed on the substrate and the temperature at which this step is conducted can have a large impact on subsequent CNT growth. The present hypothesis is that the quality of CNTs, dictated by local lattice defect structures, affects the degree of adsorption of enzymes on CNTs, which can be produced with relatively continuous wall structures with few defects or with a relatively high defect density. The defects create preferred sites for chemical bonding to occur. We suggest that a larger defect site density will improve the adsorption of enzymes, leading to superior biosensor material performance. To test this hypothesis, an experimental matrix consisting of four values for the degree of negative bias (0, -100, -200, and -300 V) and four calcination temperatures (250, 550, 700, and 900 °C) was constructed to form a total of 16 different cases. All other growth conditions and processes were held constant. The process involved characterization of all samples via field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and Raman spectroscopy, as well as biofunctionalization of all samples with glucose oxidase for cyclic voltammetry experiments. To facilitate this approach, two substrates were prepared for each growth run, resulting in two samples prepared under identical conditions. One was used for the biofunctionalization experiment, whereas the other was set aside for further microscopy and spectroscopy. D. Biofunctionalization Procedure. Each sample was placed into 24-well polystyrene culture plates. The well holding the CNT sample was initially filled with 1 mL of pH 7.2 PBS (phosphate buffer saline) buffer so that the samples were completely immersed. A 0.4 mL portion of diluted glucose oxidase with a concentration of 100 mg in 10 mL of PBS buffer

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Figure 2. Cyclic voltammograms obtained to verify the enzymatic reaction between glucose and glucose oxidase.

was added to each well and incubated at room temperature. The polystyrene plate was shaken for 20 min. Next, three consecutive washing steps were conducted so that any unbound enzyme could be removed. In each of the washing steps, the wells were filled with 0.1 mL of PBS buffer, and the samples were placed inside the wells so that the surface was completely immersed. The plates were agitated for 5 min. This washing step was repeated three consecutive times so that excess unbound enzyme could be removed. After the washing steps were complete, the samples were functionalized with glucose oxidase immobilized on the CNT samples. The next step was to verify that enzyme had indeed immobilized onto the CNT samples and to quantify the sample-to-sample differences in adsorption through cyclic voltammetry (CV) experiments. E. Cyclic Voltammetry Procedure. Amperometric measurements were recorded using an Epsilon potentiostat system from BAS (Bioanalytical Systems Inc., West Lafayette, IN) in a three-electrode cell. A Ag/AgCl electrode was used as the reference electrode, and a platinum electrode was used as the counter electrode (both from BAS). A glassy carbon electrode is often used as the working electrode in CV experiments. In the current work, the biofunctionalized CNT samples were instead used as the working electrode. After the electrodes were set in place, 4 mL of diluted glucose solution with a concentration of 111 mM was injected into the cell. Using the BAS Epsilon EC software (from BAS), cyclic voltammetry experiments were conducted by setting the initial potential to 0 mV, the switching potential to 800 mV, and the final potential to 0 mV, where the voltage applied was with respect to the Ag/ AgCl reference electrode. A scan rate of 100 mV/s was selected for all cyclic voltammetry measurements. Preliminary cyclic voltammetry experiments were conducted to verify the enzymatic reaction of the glucose and the glucose oxidase. Results from these experiments are summarized in Figure 2, which shows cyclic voltammograms for a baseline sample exhibiting no current in the absence of glucose oxidase, compared to cases where current is picked up in the presence of glucose oxidase for a traditional glassy carbon electrode, and a CNT electrode similar to the ones used in this study. Also, preliminary cyclic voltammetry experiments verified that solutions containing higher concentrations of glucose yielded higher peak currents at the switching potential due to increased reaction between the glucose oxidase and higher concentrations of glucose, as shown in Figure 3. These preliminary cyclic voltammetry measurements gave verification to the fact that enzymatic reaction between the glucose and glucose oxidase was indeed occurring and that

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Figure 3. Cyclic voltammograms obtained to verify conformation to enzyme kinetics where reactions between glucose oxidase and higher concentrations of glucose lead to a higher current.

the reaction was conforming to conventional enzyme kinetics in which higher levels of current were being recorded for solutions containing higher concentrations of glucose. III. Results and Discussion CNTs were successfully grown on all substrates, and FESEM images acquired at various magnifications for CNTs grown at 0 V bias are shown in Figure 4. The low-magnification images reveal a vertical orientation of the CNTs and lengths of 10-50 µm, depending on the growth condition. Generally, CNTs grown from catalysts calcined at higher temperatures of over 700 °C were observed to yield longer CNTs with denser coverage. Also, higher-magnification SEM images revealed the presence of more carbonaceous particles in the samples with lower calcination temperatures, suggesting that the quality of the CNT growth was higher with higher calcination temperatures. Cyclic voltammograms (CVs) were generated for the 0, -100, -200, and -300 V bias cases, along with the four calcination temperatures (250, 550, 700, and 900 °C) for each. Figure 5 shows the results of the set of CVs under 0 V bias for the four calcination temperatures, 250, 550, 700, and 900 °C (abbreviated as Calc.). In the actual data acquisition process, the current was recorded, and this value was divided by the nominal substrate area of each square sample as a basis for comparison. Interestingly, the negative peak current density was observed to generally increase with increasing catalyst calcination temperature (Figure 6). Although higher levels of peak current density would result from enhanced enzyme adsorption on to the CNT surface, the peak current densities alone do not directly identify underlying contributing factors responsible for this trend. To examine differences in nanostructure and to explain possible structural differences leading to varying levels of enzyme adsorption by the CNTs, two samples were observed by TEM (Titan 80). In Figure 7a,b, low-magnification results show the CNTs lying on top of the TEM copper mesh. Figure 7c,d contains high-magnification images in which the concentric CNT walls are distinguishable. In Figure 7c, which corresponds to CNTs grown from catalysts calcined at 250 °C and a 0 V bias growth condition, the diameter of the CNT is approximately 20 nm, and the number of walls is approximately 20, verifying that the CNT is multiwalled. Further, discontinuities in the side walls of CNTs grown from these catalysts calcined at 250 °C are observed, whereas the walls of CNTs derived from catalysts calcined at 900 °C (Figure 7d) are observed to be more

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Figure 4. Consolidated low-magnification (top row) and high-magnification (bottom row) SEM images of 0 V bias cases for calcination temperatures of 250, 550, 700, and 900 °C (from left to right).

Figure 5. Cyclic voltammograms for 0 V bias samples, with a sweep rate of 100 mV/s.

Figure 6. Peak current density plotted as a function of catalyst calcination temperature for 0, -100, -200, and -300 V bias cases.

continuous. These observations are important in that a defective wall structure and the presence of carbonaceous materials may provide a better environment for enzyme adsorption. Although TEM imaging reveals atomic structure locally, a more comprehensive approach using Raman spectroscopy was attempted to quantify the foregoing observations concerning nanostructure. A Raman spectroscopy system using a Renishaw Raman imaging microscope coupled to a 785 nm (1.58 eV) diode laser was used to perform the analysis. Three points were

randomly selected on each sample, and the data were acquired. The results for all locations and samples indicated strong peaks well above the noise floor near the 1312 cm-1 (D band) and 1600 cm-1 (G band), which are the indicative vibrational modes for CNTs. The G band represents the tangential stretching mode of highly ordered sp2 graphite, whereas the D band represents the degree of defects or amorphous carbon.6,40 The intensity of the G band relative to the D band (IG/ID) was used to quantify the quality of the CNTs,6,35 with higher IG/ID ratios interpreted as representing better quality CNTs. Because the CNT arrays were multiwalled, we expect the main contributors to the change in the IG/ID ratio to be the amorphous carbon content and the relative number of defect sites on the CNT walls. Results indicate that the values IG/ID ratios fall in the range of 0.39-0.70. These values are consistent with the IG/ID ratios reported for CNTs grown from the dendrimer catalyst in prior work,6 reflecting the consistency of the growth process. The variation of IG/ID ratio with calcination temperature is shown in Figure 8. For negatively biased samples, a general trend of increasing IG/ID ratios with increased calcination temperature is apparent, although an exception exists for the 700 °C calcination temperature. This increasing trend of IG/ID ratios suggest that, as the calcination temperature increases, the amorphous carbon content and the number of defect sites on the CNT walls decrease relative to the graphitic content. Initially, the peak current density was plotted as a function of IG/ID ratio in an attempt to correlate the quality of grown CNTs to the degree of glucose oxidase adsorption. This resulted in a broad data scatter with no discernible trend, leading to the posit that the projected substrate area was not the correct parameter in correlating cyclic voltammetry results to Raman spectroscopy results. However, when the same peak current was divided by the estimated total CNT area of each sample, as a function of IG/ID ratios, a clear trend emerged, as discussed in the following paragraphs. Although estimating the true surface area of a CNT array with high confidence is difficult, we have adopted a rational, although approximate, approach in which a characteristic CNT diameter, density, and length of an array are assigned based on SEM and TEM imaging. A simple combination of these terms gives an estimate of CNT area that is used in the subsequent normalization of current density. Using the imaging software ImageJ, the average CNT diameters were estimated by selecting and averaging 10 sampling areas using high-magnification SEM images for each of the 16 samples. A similar approach was used to estimate the average spacing

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Figure 7. TEM images of CNTs produced under 0 V bias using catalysts calcined at (a) 250 °C (low magnification), (b) 900 °C (low magnification), (c) 250 °C (high magnification), and (d) 900 °C (high magnification). The scale bars are 100 (a), 200 (b), and 10 nm (c, d).

Figure 8. IG/ID ratios plotted as a function of catalyst calcination temperature for bias voltages of 0, -100, -200, and -300 V.

Figure 9. Current per CNT area plotted as a function of catalyst calcination temperature for bias voltages of 0, -100, -200, and -300 V.

between the CNTs and the average length of the CNTs. These estimates were used to approximate the surface area of the CNTs. CNTs with longer lengths, smaller spacings between CNTs, and smaller diameters yielded larger CNT surface areas. As a general trend, CNTs grown in the absence of negative bias and at higher catalyst calcination temperatures were observed to be denser and longer, leading to larger CNT surface areas. The results using CNT area as a basis for peak current density in CVs are shown with respect to calcination temperature and applied bias in Figures 9 and 10. Figure 9 reveals that the current per CNT area generally decreases as the catalyst calcination temperature increases. Higher current per CNT area implies more enzyme adsorption (activity). This means that samples

with lower catalyst calcination temperatures exhibit higher current densities at the CNT walls, which is a clear indicator of higher levels of enzyme adsorption per CNT area. This observation is consistent with the relative quality indicators of the CNT samples (based on TEM and Raman analysis). Similarly, Figure 10 shows a general increasing trend in current density as the negative bias voltage increases. This trend can be attributed to the plasma conditions, which, as the negative bias voltage increases, cause increased flux of energetic positive ions that likely create high defect densities on CNT walls.36,37 Again, higher levels of current per CNT area can be interpreted as more enzyme adsorption on a unit CNT area basis. When the peak current per CNT area is plotted as a function of IG/ID ratio, an inverse relation becomes apparent (see Figure

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Figure 10. Current per CNT area plotted as a function of negative bias voltage for 250, 550, 700, and 900 °C calcination temperature cases.

In the present work, CNTs were grown under a varying set of growth conditions using a dendrimer-templated Fe2O3 nanoparticle catalyst, after which glucose oxidase was immobilized onto the CNTs. The dc bias voltage in the MPCVD and catalyst calcination temperature were selected as the two key parameters thought to yield samples having differences in their nanostructure, to form a matrix of 16 samples. Analysis and correlation of cyclic voltammetry, SEM, TEM, and Raman spectroscopy results were used to deduce the effects of differences in nanostructure leading to varying degrees of glucose oxidase adsorption on CNT samples. The results indicate that CNTs with smaller IG/ID ratios, which have a larger amount of amorphous carbon and a relatively higher number of defect sites, yield a higher degree of enzyme adsorption on a per CNT area basis. The results suggest that longer CNTs with high defect densities are preferred and that these factors may not be independently controllable in a given synthesis process. Alternatives, such as postgrowth plasma treatment of long, high-purity CNT arrays, could produce more sensitive biosensors by increasing the defect density. Acknowledgment. The authors would like to sincerely thank Dr. Dmitri N. Zakharov for TEM assistance and Dr. Michael R. Ladisch for helpful discussions related to glucose oxidase adsorption. References and Notes

Figure 11. Current per CNT area plotted as a function of IG/ID ratios.

11). This implies that CNTs with a higher density of wall defects and/or more amorphous carbon (lower IG/ID ratios) exhibit improved adsorption of glucose oxidase, leading to a higher peak current per CNT area. Assuming that the immobilization of enzyme occurs and that the total number of defect sites dictates the magnitude of the peak current density, the total number of binding sites is expressible as the product of the binding sites per unit area of CNT and the total surface area of the CNT array. The former is determined by the quality of the growth and can be estimated from the IG/ID ratios, whereas the latter is dictated by the density and length of the CNT array. This proposition derives from the present data in that IG/ID ratios exhibit an increasing trend with increased calcination temperature. However, the length of the CNTs was observed via SEM to increase with calcination temperature, causing the total surface area of the CNTs to increase. The combined effect of these two competing factors is manifested in the CV results, and the present results should provide guidance to achieve increased immobilization of enzyme onto CNT surfaces. CNTs with smaller IG/ID ratios (implying more defect sites per unit area) and longer CNTs are preferred so that the total number of defect sites and the peak current density can be maximized. It is also plausible that high-quality CNTs could be grown at high rates and then subsequently treated to induce defects to improve biosensing performance, although this approach has not yet been pursued.

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