Reproducible Fabrication of Robust, Renewable Vertically Aligned

Sep 25, 2011 - School of Biological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand. bS Supporting Information...
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Reproducible Fabrication of Robust, Renewable Vertically Aligned Multiwalled Carbon Nanotube/Epoxy Composite Electrodes David J. Garrett,†,|| Paula A. Brooksby,‡ Frankie J. Rawson,‡,^ Keith H. R. Baronian,§ and Alison J. Downard*,† †

MacDiarmid Institute for Advanced Materials and Nanotechnology, Department of Chemistry, University of Canterbury, Private Bag 4800, Christchurch, New Zealand ‡ Department of Chemistry, University of Canterbury, Private Bag 4800, Christchurch, New Zealand § School of Biological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand

bS Supporting Information ABSTRACT: We describe the reproducible fabrication of robust, vertically aligned multiwalled carbon nanotube (VACNT)/epoxy composite electrodes. The electrodes are characterized by cyclic voltammetry, impedance spectroscopy, and scanning electron and atomic force microscopies. Low background currents are obtained at the electrodes, and common redox probe molecules and NADH show excellent voltammetric behavior. When electrode performance deteriorates due to fouling, the electrode surfaces can be reproducibly renewed by mechanical polishing followed by O2 plasma treatment. The electrochemical performance of the electrodes is maintained after more than 100 cycles of use and renewal.

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ertical alignment of carbon nanotubes (CNTs) can increase the electron transfer rate compared with rates through randomly dispersed CNT mats, an effect attributed to electron transfer being more “direct” through aligned CNTs.1 Further, CNT tips have been shown to give fast electron transfer rates with many redox molecules and analytes.2 4 Although recent studies have reported that fast electron transfer rates can also be achieved at the side-walls of single-walled CNTs,5,6 practical CNT electrodes for voltammetric analysis should be robust and renewable with reproducible surface area. These properties can be achieved by vertical alignment of CNTs and encapsulation in a tough, inert, and insulating matrix, so that only the CNT tips are exposed. In addition to the mechanical strength conferred by such a matrix, we7,8 and others9 11 have shown that encapsulation of vertically aligned multiwalled CNTs (VACNTs) in insulating epoxy lowers the capacitance of the electrodes, improving the signal to background ratio. Clearly, low background currents are desirable for voltammetric sensors as this increases the signal-to-noise ratio thereby improving sensitivity. Several strategies have been reported for preparing encapsulated CNT electrodes. Yun et al. describe 1 mm diameter towers of multiwalled carbon nanotubes (MWCNTs) that were peeled off the growth substrate, cast into epoxy, glued to a wire with conducting epoxy, and finally polished with diamond paste to reveal the CNT tips.12 Li et al. prepared low-density VACNT nanotube arrays that were encapsulated in a layer of SiO2, deposited using chemical vapor deposition (CVD). The constructs were mechanically polished to reveal the CNT tips.13,14 A simple approach to nonaligned MWCNT epoxy electrodes was reported by Esplandiu et al.15 and Pumera et al.16 These authors r 2011 American Chemical Society

mixed commercial MWCNTs with epoxy and cured the slurry in a plastic tube to make an electrode. A similar method was used by Chen et al. who cured an epoxy-MWCNT slurry in a capillary for use as a microelectrode.17 The above methods generate efficient microscale CNT based electrodes but are all complicated to prepare and require a high degree of process control. This paper describes an optimized, broadly accessible fabrication strategy for producing VACNT/epoxy composite macroscale electrodes with only the CNT tips exposed. A forest of multiwalled VACNTs is grown by CVD on a catalyst layer deposited by e-beam evaporation on a planar carbon substrate. The substrate is a thin layer of conducting graphitic carbon (pyrolyzed photoresist film, PPF) on a silicon wafer. With the use of a silicone mold, the VACNTs are sealed into a solid epoxy resin block which is subsequently polished using emery paper, diamond paste and finally treated with O2 plasma yielding macroscale electrodes with excellent electrochemical characteristics. Compared with our earlier VACNT electrodes,7,8 the fabrication method is significantly more reliable and reproducible and the electrodes show greater durability and reusability. During a period of more than 3 months, several of the described electrodes were renewed hundreds of times by polishing with diamond paste followed by O2 plasma treatment without any change in electrochemical performance. Received: July 8, 2011 Accepted: September 24, 2011 Published: September 25, 2011 8347

dx.doi.org/10.1021/ac201769t | Anal. Chem. 2011, 83, 8347–8351

Analytical Chemistry

’ EXPERIMENTAL SECTION The preparation of 2 3 μm thick layers of PPF on a Si wafer by pyrolysis of photoresist resin at 1100 C under a forming gas atmosphere (5% H2 in N2) has been previously described.18 The general procedures and equipment used for e-beam evaporation of metal catalyst layers on PPF and the water assisted growth of VACNTs by CVD have also been previously described.7 In this work, one-third of the PPF surface was masked with metal foil tape during e-beam deposition of the catalyst metals; this area (after removal of the metal tape) was used for electrical contact to the completed electrodes. The catalyst layers were either 10 nm Al2O3 followed by 2 nm of Fe or 10 nm Al followed by 2 nm of Fe (high purity Al2O3 and Fe pellets, Kurt J. Lesker Company). The gas flow rates and temperatures for preparation of VACNTs by CVD were 1.2 L min 1 Ar (Southern Gas Ltd.) and 0.8 L min 1 H2 (BOC) at 750 C for 30 min, followed by 1.0 L min 1 Ar, 0.4 L min 1 C2H4 (BOC) and ,0.1 L min 1 H2 at 750 C for 10 min, and finally cooling to 3.5  1014 Ω cm) and was used for encapsulation of all electrodes. Vacuum was applied to the uncured epoxy in the mold to remove air bubbles and ensure that the epoxy fully penetrated the VACNT forest. Three cycles over 10 min of gradual application of vacuum from atmospheric pressure to 60 mTorr was sufficient to remove all air bubbles from the forests. The vacuum was applied gradually to avoid formation of large air bubbles which could lift the VACNTs off the substrate. When vacuum treatment was omitted, electrodes showed irreproducible electrochemical behavior. This is attributed to incomplete insulation of VACNTs and exposure of trapped air bubbles during surface renewal. Further, without vacuum treatment, VACNT forests had poor adhesion to the PPF surface during polishing and when exposed to solvents, indicating that epoxy did not fully penetrate to the base of the forest. Imaging of the surface by SEM revealed large areas of VACNT/epoxy composite interspersed with micrometer sized “seams” of epoxy (Figure 2). Seam formation is most likely due to some VACNTs clumping together creating spaces which are filled by epoxy. Densification, caused by capillary forces, is commonly observed when aligned CNT forests are exposed to liquids.19 The electrochemical performance of the VACNT/epoxy electrodes was examined after polishing with diamond paste and again after O2 plasma treatment. Cyclic voltammograms (CVs) of Fe(CN)63 and Ru(NH3)63+ recorded at polished and O2 plasma treated VACNT/epoxy electrodes at a scan rate = 100 mV s 1 are shown in Figure 3A,B. The CV obtained for the Fe(CN)63 /4 couple at the polished VACNT/epoxy electrode (Figure 3A, gray dashed line) shows a large anodic and cathodic peak potential separation (ΔEp > 800 mV) indicating slow electron transfer kinetics.20 In contrast, the CV of the Ru(NH3)62+/3+ couple obtained at the polished electrode (Figure 3B, gray dashed line) is better defined. These results are consistent with the much greater sensitivity of the kinetics of the Fe(CN)63 /4 couple than the Ru(NH3)63+/2+ couple to the nature of the electrode surface.21 However the shape of the CV for Ru(NH3)63+ indicates that there is a mixture of linear and radial diffusion of the redox probe,2 suggesting that some area(s) of the surface are functioning as a

Figure 3. CVs (scan rate = 100 mV s 1) of (A) 1 mM Fe(CN)63 , (B) 1 mM Ru(NH3)63+, and (C) 1 mM NADH in pH 7 PBS, recorded at VACNT/epoxy electrodes. Gray dashed lines, freshly polished electrodes; black solid lines, three different electrodes after polishing and O2 plasma treatment. Black dashed line (A) CV of 1 mM Fe(CN)63 obtained at polished GC plate electrode with the same geometric area as VACNT/epoxy electrodes.

widely spaced microelectrode array.2 Evidently not all VACNT tips are fully exposed. After O2 plasma treatment of the polished VACNT/epoxy electrodes, the CVs for Fe(CN)63 and Ru(NH3)63+ are welldefined and consistent with semi-infinite linear diffusion of the redox probe2 (Figure 3A,B, black lines). Figure 3A also shows (black dashed line) the CV obtained for 1 mM Fe(CN)63 at a polished glassy carbon (GC) plate electrode with the same geometric area as the VACNT/epoxy electrodes. The very similar current magnitudes at the VACNT and GC electrodes confirm that the VACNT/epoxy surfaces give macroelectrode behavior at this scan rate (100 mV s 1). The electrochemical response of the Fe(CN)63 /4 couple in PBS at the VACNT/epoxy electrodes was examined in more detail and compared with that obtained at polished GC electrodes. Impedance data collected in a Fe(CN)63 solution were used to calculate the heterogeneous electron transfer rate constant, giving k = 0.012 and 0.0094 cm s 1 at O2-plasma treated VACNT/epoxy and polished GC electrodes, respectively. CVs obtained at both electrodes gave responses consistent with semiinfinite linear diffusion at scan rates between 1 and 400 mV s 1. On the basis of cyclic voltammetric peak currents, the surface areas of the electrodes were the same within experimental 8349

dx.doi.org/10.1021/ac201769t |Anal. Chem. 2011, 83, 8347–8351

Analytical Chemistry uncertainty (there is ∼10% experiment-to-experiment variation in electrode area even for GC, which we attribute to variable “flattening” of the O-ring when assembling the electrochemical cell); the only apparent difference between the electrodes was that ΔEp values were larger at GC than at the VACNT/epoxy electrode at high scan rates (Figure S-2 in the Supporting Information). However, CVs obtained in blank PBS at O2plasma treated VACNT/epoxy and polished GC electrodes gave significantly lower currents for the VACNT/epoxy electrode (Figure S-3 in the Supporting Information). From EIS in a solution containing Fe(CN)63 , double-layer capacitances of 15.61 and 29.66 μF cm 2 were calculated for O2-plasma treated VACNT/epoxy and polished GC electrodes, respectively. The experimental results described above demonstrate that the O2-plasma treated VACNT/epoxy electrodes have desirable characteristics as voltammetric electrodes: well-behaved voltammetry is obtained over a range of scan rates, the electron transfer rate for the Fe(CN)63 /4 couple is very similar to that at the polished GC, and the capacitance is lower than at polished GC. However, the origin for the lower capacitance at the O2-plasma treated VACNT/epoxy electrodes is not entirely clear. Changes in the CVs of the redox probes at polished VACNT/epoxy electrodes after O2 plasma treatment are attributed to removal of organic residues from the diamond polishing compound22 and exposure of a greater number of VACNT tips than by polishing alone. O2 plasma efficiently etches epoxy8 and has been shown to form a variety of oxygen functionalities on the surface and ends of CNTs, with the relative amounts of different functionalities depending on the oxygen percentage and pressure used in the plasma generator.23 The changes induced by O2 plasma are expected to increase the capacitance of the VACNT/epoxy surface over that of polished GC; however, lower capacitance is observed for the former electrode material. The observations that O2 plasma treatment leads to semi-infinite linear diffusion of the redox probes at VACNT/epoxy electrodes and to equal apparent electrode areas for O2-plasma treated VACNT/epoxy and GC electrodes of the same geometric area, over the scan rate range 1 400 mV s 1, suggests that the O2-plasma treated VACNT/epoxy surface is comprised of closely spaced, but epoxy-separated, CNT tips, or bundles of CNT tips. The close spacing of active CNT tips leads to overlapping diffusion fields, semi-infinite linear diffusion of redox species, and equal Faradaic current magnitudes at the VACNT/epoxy and GC electrodes under all conditions examined in this work.2 On the other hand, the epoxy content of the surface lowers the overall capacitance. Together, these factors produce electrodes with excellent electrochemical properties. Figure 3A,B demonstrates a further important feature of our electrode fabrication procedure: excellent electrode electrode reproducibility and reproducibility of surface renewal. The CVs of the Fe(CN)63 /4 couple in Figure 3A (black lines) were obtained using three, separately prepared, plasma-treated VACNT/epoxy electrodes. (Deposition of the catalyst layers and VACNT growth were each undertaken at different times for these electrodes.) The close similarity of the CVs demonstrates that electrodes can be prepared with excellent reproducibility. After the CVs for Fe(CN)63 were obtained, the surfaces were renewed by polishing with diamond paste and treatment with O2 plasma and then CVs of the Ru(NH3)63+/4+ were obtained (Figure 3B, black lines). Evidently the electrode renewal process is also reproducible, and we found that electrodes could be reproducibly renewed at least 100 times

TECHNICAL NOTE

Figure 4. (A, B) SEM images of as grown VACNTs forests, before epoxy encapsulation at (A) low and (B) high magnification; (C) AFM images of a VACNT/epoxy surface after polishing (upper panel) and after polishing followed by O2 plasma treatment (lower panel). (D) SEM image of VACNTs exposed by O2 plasma treatment.

without any degradation in performance or change in apparent surface area. The performance of the VACNT/epoxy electrodes for NADH oxidation was also examined. Oxidation of NADH at low potentials is important for development of biosensors that rely on the NADH/NAD+ couple.24 Figure 3C, gray dashed line, shows that after polishing only, there is a small response to NADH at ∼0.9 V. CVs obtained at the three electrodes, after renewal by polishing with diamond paste and O2 plasma treatment (black solid trace), show a well-defined peak for NADH oxidation with Epa ≈ 0.54. The catalysis of NADH oxidation at CNT electrodes has been attributed to quinone functionalities on the CNTs,25 and it is assumed that these are introduced to the surfaces of VACNT/epoxy electrodes by O2 plasma treatment. With the use of net-like SWCNT electrodes based on SWCNTs that had been oxidized by a variety of methods, peak potentials as low as 0.04 V vs Ag/AgCl (pH 7.4) have been recorded for oxidation of NADH.24 While the overpotential required using our electrodes is higher (0.54 V), our electrodes show an estimated 7-fold greater current density (based on geometric area) and 7-fold greater signal/background current ratio than the net-like SWCNT electrodes, which is advantageous for NADH sensing. SEM and AFM images of VACNT forests are shown in Figure 4A,B. The VACNTs have an estimated density (from SEM images) of 2.7 ( 0.5  109 CNT cm 2. MWCNTs grown using the same equipment and similar conditions were previously found to have less than 15 walls and to have highly defective outer walls, as expected for growth by water-assisted CVD.8 Comparison of AFM and SEM investigations of the epoxy-encapsulated VACNTs after polishing with diamond paste (Figure 4C, upper) and after polishing followed by O2 plasma treatment (Figure 4C, lower and D) show exposure of the VACNT tips by O2 plasma etching. After polishing alone, the epoxy seams were recessed compared to the VACNT epoxy composite and the depth of the recess increased after O2 plasma treatment (Supporting Information, Figure S-4), consistent with a faster etch rate for epoxy than the CNT/epoxy composite.

’ CONCLUSIONS VACNT/epoxy composite electrodes with excellent electrochemical characteristics can be reproducibly fabricated by 8350

dx.doi.org/10.1021/ac201769t |Anal. Chem. 2011, 83, 8347–8351

Analytical Chemistry optimizing the procedures for encapsulating the VACNTs with epoxy and activating the surface by polishing with diamond paste and O2 plasma treatment. The multiwalled VACNTs are grown directly on a conducting carbon substrate and are then encapsulated with an insulating epoxy using a purpose-made mold. After exposure of the VACNT tips by O2 plasma treatment, the electrodes give very similar electron transfer rates and Faradaic currents for the Fe(CN)63 /4 couple to those obtained at polished GC electrodes of the same geometric area. Importantly, the capacitance and hence the background current densities of the O2 plasma-treated VACNT/epoxy electrodes are lower than those of polished GC electrodes. The electrodes are robust and can be reproducibly renewed many times. Reliable, reproducible fabrication of VACNT/epoxy composite electrodes that can be used for numerous electrochemical measurements will facilitate further investigations of their use for voltammetric analysis.

’ ASSOCIATED CONTENT

bS

Supporting Information. Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: +64-3-3642501. Fax: +64-3-3642110. E-mail: alison. [email protected].

TECHNICAL NOTE

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Present Addresses

Department of Physics, The University of Melbourne, The David Caro Building, Corner Swanston Street and Tin Alley, VIC 3010 Australia. ^ School of Chemical Engineering, University of Birmingham, Edgbaston, Birmingham, B15 2TT, U.K.

’ ACKNOWLEDGMENT The research was funded by the RSNZ Marsden Fund (Contract UOC 0605) and the MacDiarmid Institute for Advanced Materials and Nanotechnology. D.J.G. would like to thank the New Zealand Tertiary Education Commission for a Doctoral Scholarship and the Department of Electrical and Computer Engineering, University of Canterbury, for use of facilities. ’ REFERENCES (1) Gooding, J. J.; Chou, A.; Liu, J. Q.; Losic, D.; Shapter, J. G.; Hibbert, D. B. Electrochem. Commun. 2007, 9, 1677–1683. (2) Banks, C. E.; Davies, T. J.; Wildgoose, G. G.; Compton, R. G. Chem. Commun. 2005, 829–841. (3) Nugent, J. M.; Santhanam, K. S. V.; Rubio, A.; Ajayan, P. M. Nano Lett. 2001, 1, 87–91. (4) Moore, R. R.; Banks, C. E.; Compton, R. G. Anal. Chem. 2004, 76, 2677–2682. (5) Dumitrescu, I.; Dudin, P. V.; Edgeworth, J. P.; Macpherson, J. V.; Unwin, P. R. J. Phys. Chem. C 2010, 114, 2633–2639. (6) Dumitrescu, I.; Unwin, P. R.; Wilson, N. R.; Macpherson, J. V. Anal. Chem. 2008, 80, 3598–3605. (7) Liu, X.; Baronian, K. H. R.; Downard, A. J. Anal. Chem. 2008, 80, 8835–8839. (8) Liu, X. M.; Baronian, K. H. R.; Downard, A. J. Carbon 2009, 47, 500–506. 8351

dx.doi.org/10.1021/ac201769t |Anal. Chem. 2011, 83, 8347–8351