ARTICLE pubs.acs.org/ac
Single Nanoskived Nanowires for Electrochemical Applications Karen Dawson,§ J€org Strutwolf,† Ken P. Rodgers,§ Gregoire Herzog,§ Damien W. M. Arrigan,‡ Aidan J. Quinn,§ and Alan O’Riordan*,§ §
Tyndall National Institute, University College Cork, Lee Maltings, Cork, Ireland Institut f€ur Organische Chemie, Universit€at T€ubingen, Auf der Morgenstelle 18, D-72076 T€ubingen, Germany ‡ Nanochemistry Research Institute, Department of Chemistry, Curtin University, Perth, WA 6845, Australia †
bS Supporting Information ABSTRACT: In this work, we fabricate gold nanowires with well controlled critical dimensions using a recently demonstrated facile approach termed nanoskiving. Nanowires are fabricated with lengths of several hundreds of micrometers and are easily electrically contacted using overlay electrodes. Following fabrication, nanowire device performance is assessed using both electrical and electrochemical characterization techniques. We observe low electrical resistances with typical linear Ohmic responses from fully packaged nanowire devices. Steady-state cyclic voltammograms in ferrocenemonocarboxylic acid demonstrate scan rate independence up to 1000 mV s1. Electrochemical responses are excellently described by classical ButlerVolmer kinetics, displaying a fast, heterogeneous electron transfer kinetics, k0 = 2.27 ( 0.02 cm s1, R = 0.4 ( 0.01. Direct reduction of hydrogen peroxide is observed at nanowires across the 110 pM to 1 mM concentration range, without the need for chemical modification, demonstrating the potential of these devices for electrochemical applications.
A
dvances in nanotechnology have opened up numerous possibilities and opportunities in analytical science. For example, emerging devices that employ nanowires for direct, sensitive, and rapid detection of biological and chemical species represent a new and powerful class of sensors.15 Noble metal nanowires such as platinum, gold and silver are especially important because of their excellent electrical and optical properties and are finding increased use as sensors based on electrochemical, surface enhanced plasmon resonance and surface enhanced Raman scattering detection mechanisms.68 In particular, gold nanowires offer a number of advantages in electrochemical applications. As the critical dimensions of an electrode enter the nano regime, radial analyte diffusion to the electrode dominates, resulting in increased rates of mass transport, higher current densities, reduced double layer capacitance, higher signal-to-noise ratios, and steady-state voltammograms at high measurement rates.9 In addition, the small size of nanoelectrodes enables direct low-volume analysis,10 high-resolution scanning electrochemical microscopy,11 enhanced sensitivity in a wide range of sensing applications, and high-speed electron transfer kinetic studies.9 However, despite the large research interest, there is no widely accepted general technique for fabrication of gold nanowires. Various approaches for fabrication and synthesis of nanowires have been reported, including electron beam lithography,12 lithography patterned nanowire electrodeposition,13 template electrochemical deposition of nanowires using laser interference lithography,14 angled metal deposition onto V-grooved substrates,15 electrochemical step r 2011 American Chemical Society
edge decoration,16 electrodeposition in block copolymer,17 commercial templates,18 and in microfluidic channels.19 Recently, “nanoskiving”, which is a new paradigm for fabrication of complex nanostructures with well-defined geometry, has been demonstrated.20 In this approach, a thin layer of material of choice, such as gold, is evaporated/deposited onto an epoxy substrate, which is then further encapsulated in epoxy to form a block. Following curing, sections are sliced (skived) from the block, using ultramicrotomy, to yield nanomembranes, each containing gold nanostructures with their width and height controlled by the thickness of the deposited layer and the thickness of the skived nanomembrane, respectively. Nanostructures may then be released from the nanomembrane using oxygen plasma ashing to dry etch the epoxy resin. The approach is very fast and reproducible and is an inexpensive process for the fabrication of nanostructures that does not require access to high-end fabrication equipment for nanostructure fabrication, such as e-beam lithography, and has to date been employed to fabricate both individual nanowires and arrays of nanowires separated by well-defined gaps.20 In this work, we employ the nanoskiving technique to fabricate discrete gold nanowires for potential use in electrochemical applications. A combination of scanning electron microscopy (SEM) atomic force microscopy (AFM), and optical microscopy Received: February 16, 2011 Accepted: May 30, 2011 Published: May 30, 2011 5535
dx.doi.org/10.1021/ac2004086 | Anal. Chem. 2011, 83, 5535–5540
Analytical Chemistry is used to characterize the nanowires. Nanowires hundreds of micrometers in length and with critical dimensions of 207 ( 12 nm in width (SEM analysis) and 254 ( 11 nm in height (AFM analysis) are routinely fabricated. Using this approach, several thousand nanowires may be skived from each epoxy block. Due to their long lengths, as-fabricated nanowires may be easily electrically contacted by depositing overlaying electrodes using low-cost techniques, such as inkjet printing,21 screen printing,22 soft lithography,23 and shadow mask evaporation.24 However, to demonstrate the potential of nanoskived wires for use in electrochemical applications, we employ micrometer-scale photolithography, which is widely available, for overlay of contact electrodes. Following fabrication and integration, nanowire devices are characterized using a combination of electrical and electrochemical techniques.
’ EXPERIMENTAL SECTION Nanoskiving. Nanowires were fabricated using a technique known as nanoskiving20 (see Supporting Information Figure S1). Gold was selected as the electrode material of choice because of its high chemical stability,25 high electrical conductivity, and biocompatibility.26 Briefly, a thin film of gold was evaporated onto an epoxy (Araldite 506, Agar Scientific) substrate, which was further encapsulated in epoxy to form a block. Following curing, sections were sliced from the block using ultramicrotomy to yield nanomembranes each containing one gold nanowire. Nanowires, with lengths up to several hundred micrometers, were then liberated from the epoxy membrane by O2 plasma dry etching. SEM and AFM Measurements. Scanning electron microscope images of liberated single Au nanowires were acquired using a field emission SEM (JSM-6700F, JEOL UK Ltd.) operating at beam voltages between 3 and 5 kV. SEM analysis was undertaken for visual characterization of nanowires and determination of nanowire width. The topography of individual nanowires assembled on Si/SiO2 chips was characterized using a calibrated atomic force microscope (AFM; Dimension 3100, Veeco Instruments Inc.) in tapping mode with commercial tapping mode probes (MP-11100, Veeco Instruments Inc.; typical radius of curvature ∼10 nm and front/side cone angles of 15°/17.5°, respectively). Optical Microscopy. Optical micrographs were acquired using a calibrated microscope (Axioskop II, Carl Zeiss Ltd.) equipped with a charge-coupled detector camera (CCD; DEI750, Optronics). Nanowire Integration and Device Fabrication. For electrochemical device fabrication, nanomembranes were first deposited on 8.1 mm 8.1 mm silicon chips (with a 90 nm layer of thermally grown oxide) patterned with a 4 4 array of binary alignment marks (see Supporting Information Figure S2). Using the binary mark identifiers for alignment and registration, contact electrodes were then overlaid using optical lithography, metal evaporation (Ti 10 nm, Au 200 nm), and liftoff. A layer of photoresist (1 μm) was spin-coated onto the chip, and a trench opened in this passivation layer directly over the nanowire by photolithography to allow contact between the nanowire and the electrolyte solution. To complete device packaging, chips were assembled onto printed circuit boards, electrically contacted using wedge wire bonding (25 μm aluminum wires), and the bond wires protected by epoxy. Control devices (electrodes
ARTICLE
overlaid onto substrates without a nanowire present) were fabricated in a similar manner. Electrical Characterization. Two-point electrical measurements were performed using a probe station (PML-8000, Wentworth Laboratories, U.K.) in combination with a parameter analyzer (HP4156A, Agilent Technologies, U.K.) programmed using Agilent VEE Pro 6.0 software. For these currentvoltage (IV) measurements, the source electrode was grounded, a bias sweep up to (10 mV was applied to the drain electrode, and the current through the nanowire was measured. Electrochemical Characterization. A CHI660a electrochemical analyzer and Faraday cage CHI200b (CH Instruments) was employed for all electrochemical studies. Experiments were carried out in a three-electrode cell employing the single nanowire electrodes as working electrodes, with a platinum wire counter electrode and saturated Ag/AgCl/KCl (saturated) reference electrode (CH Instruments). Cyclic voltammograms in sulphuric acid (H2SO4, 0.1 M) were recorded at nanowire electrodes. Studies on ferrocenemonocarboxylic acid, FcCOOH (1 mM, Alfa Aesar) were carried out in phosphate buffered saline (PBS, 10 mM, Sigma Aldrich) solutions at pH 7.4. Hydrogen peroxide (H2O2) stock solutions were prepared daily using 30% hydrogen peroxide (Sigma Aldrich). Standards were prepared by dilution from a 10 mM in 100 mM PBS stock solution. Predetermined aliquots were added sequentially to 100 mM PBS and agitated until homogeneous. All solutions were prepared with deionized water of resistivity 18.2 MΩ cm (Elga Pure Lab Ultra) and deaerated with N2 gas prior to electrochemical analysis
’ RESULTS AND DISCUSSION Following fabrication, nanowires were characterized using a combination of SEM, AFM, and optical microscopy. Figure 1a shows a SEM image (top panel) and AFM (bottom panel) micrographs of a portion of a fabricated nanowire with the statistical analysis of the critical dimensions measured at multiple locations at 14 different nanowires. Nanowires with widths of 207 ( 12 nm (SEM analysis), heights of 254 ( 11 nm (AFM analysis), and lengths of several hundred micrometers were routinely fabricated. Following fabrication and liberation, nanowires were integrated at silicon chip substrates and electrically contacted to form functional devices (see Figure 1b). Control devices (electrodes overlaid onto substrates without a nanowire present) were fabricated in a similar manner. For quality control purposes to assess the functionality of assembled and packaged nanowire devices, electrical characterization using standard two-point IV measurements was performed in the voltage range of 10 to þ10 mV. This low-voltage range was employed to prevent potential damage occurring to a nanowire from electromigration effects and to confirm both Ohmic behavior and low contact resistances. Nanowires characterized in this manner displayed linear responses, confirming electrical contact to the nanowires by the interconnection tracks. Packaged nanowire devices exhibited very low resistance (6893 Ω) for ∼80 μm long electrically contacted sections of nanowires (see Figure 1c). The measured resistance of the lithographically patterned interconnection tracks was 16 ( 2 Ω. Ignoring contact resistances, an average resistivity of ∼8 108 Ω m (based on 20 nanowires) was estimated for wires using a cross-sectional area of 5.3 1014 m2. This value is in good agreement with the bulk resistivity of gold, 2.21 108 Ω m.27 5536
dx.doi.org/10.1021/ac2004086 |Anal. Chem. 2011, 83, 5535–5540
Analytical Chemistry
ARTICLE
Figure 2. Cyclic voltammogram (CV) of a typical gold nanowire in 0.1 M H2SO4, from 0.4 to þ1.6 V at 100 mV s1. Inset: gold oxide reduction peaks obtained from 10 sequential CV scans. The arrow shows the direction in which the peak current increases upon repeated cycling.
For electrochemical experiments, nanoelectrodes were first electrochemically subjected to repeated potentiodynamic cycling in 0.1 M sulphuric acid electrolyte. This served as both an electrochemical cleaning step and characterization of the nanowires. The data presented in Figure 2 shows a typical voltammogram obtained from the first cycle, scan rate 100 mV s1. In the positive sweep, the voltammograms exhibited two clearly defined peaks assigned to monolayer oxide formation at the electrode surface according to the following reaction: Au þ 3H2 O f Au2 O3 þ 6Hþ 6e
Figure 1. (a) SEM (top panel) and tapping-mode AFM (bottom panel) images of a portion of a gold nanowire on an oxidized silicon chip substrate following release from the nanomembrane. Histograms showing the distribution of widths and heights of nanowires obtained from SEM and AFM analysis, respectively. Solid red lines are Gaussian fits to the data. (b) Optical micrograph of a fully integrated and packaged nanowire device. The contact electrodes are passivated using an insulating layer of photoresist. A trench in the photoresist is opened over the nanowire electrode to allow direct contact between the nanowire and the external electrolyte. Scale bar, 25 μm. (c) Two-point IV characteristic measured for a typical fully integrated and packaged nanowire exhibiting Ohmic behavior and low resistance (81 Ω).
The slightly higher resistivity value may be attributed to grain boundary scattering within the polycrystalline nanowires.28 Standard open- and short-circuit control experiments were also performed on the packaged devices to confirm that the observed electrical characteristics were exclusively due to the nanowires.
The anodic peaks visible at ∼1.13 and 1.4 V may be attributed to oxide formation at gold (110) and gold (100) crystal planes, respectively.25,29 In the negative sweep, CVs exhibited a single characteristic gold reduction peak at ∼0.78 V. On repeated cycling, an increase in reduction current was initially observed (inset); however, after 1214 cycles, the reduction current reached constant values, suggesting that the nanowires had been fully cleaned. At this point, the cleaning process was terminated and the wires were used for further electrochemical studies. The charge for gold oxide monolayer reduction may be converted to real surface area using the recommended conversion factor 386 μC cm 2 . 27 Using this approach, it is estimated that nanowires had an average roughness factor of ∼6; that is, the electrochemical active area was ∼6 times higher than the calculated surface area. This high value is consistent with values reported in the literature for gold ultramicroelectrodes at low scan rates (∼100 mV s 1 )30 and may arise from oxygen adsorption (by a place-exchange mechanism), leading to the formation of thin oxide multilayers of undefined stoichiometry rather than single monolayers.31 The growth of oxide layers on gold electrodes depends strongly on the polarization conditions and time applied. In general, the longer the time, the thicker the oxide film, which is consistent with lower scan rates.32 Although parasitic faradic currents from adventitious trace impurities in sulphuric acid solutions may also exist and overlap with the electroreduction peak, these are unlikely to dominate the observed signal. Figure 3a shows a typical cyclic voltammogram (CV) acquired using a gold nanowire device in 1 mM ferrocene monocarboxylic 5537
dx.doi.org/10.1021/ac2004086 |Anal. Chem. 2011, 83, 5535–5540
Analytical Chemistry
ARTICLE
Figure 4. Fit of ButlerVolmer kinetics (eq 1) to an experimental cyclic voltammogram for different k0 values in 1 mM FcCOOH in 10 mM PBS, pH 7.4, at 5 mV s1. Two simulated voltammograms calculated for reversible waves with exaggerated k0 values of 1.10 and 3.30 cm s1 are included. Inset: close-up view of the dashed region in the main figure.
Figure 3. (a) CVs obtained for a nanowire electrode device in the presence (navy line) and absence (black triangles) of 1 mM FcCOOH in PBS from 0.1 to þ0.6 V, at 5 mV s1. Data for a control device (contact electrodes only, no nanowires) are also shown (red squares). Inset: the measured current for a control device and a nanowire in the absence of FcCOOH were less than 1 and 12 pA, respectively. (b) Steady-state voltammetric response acquired in 1 mM FcCOOH for two scan rates: 5 and 1000 mV s1, respectively.
the foot of the wave may have arisen from incomplete deaeration of the electrolyte solution.35 Steady-state CVs allow kinetic information for an electronic transfer process occurring at an electrode surface to be explored. The measured voltammetric response from a nanoelectrode includes contributions from both mass transport and electrode kinetics of the electrochemical reaction.25 The steady state voltammogram of an uncomplicated quasireversible, one-electron oxidation reaction may be expressed by ButlerVolmer type kinetics, enabling the estimation of the lower limit for the standard heterogeneous rate constant, 25 ibv ¼
acid (FcCOOH) in 10 mM PBS, pH 7.4, and 25 °C. CVs display the typical steady-state behavior observed at nanoelectrodes for the single-electron oxidation of FcCOOH. Due to the small critical dimensions of the nanowires, mass transport to a nanowire is highly effective and becomes comparable to or larger than the rate of electron transfer from the FcCOOH to the electrode surface.33 The magnitude of the steady-state current was typically on the order of 37 nA. Control CV experiments were undertaken using both nanowire electrodes in 10 mM PBS buffer (in the absence of FcCOOH) and also using control devices (without nanowires) in the presence of FcCOOH in PBS electrolyte solution. The measured currents for a control device and a nanowire in the absence of FcCOOH were less than 1 and 12 pA, respectively (see Figure 3a inset). This indicates that the measured faradic current may be attributed to the electrochemical oxidation of FcCOOH at nanowire electrodes and that the interconnection tracks were sufficiently insulated, by the resist layer, from the electrolytic solution. The slight observed hysteresis (120 pA) between the forward and reverse sweeps may be attributed to nonfaradic currents arising from dielectric relaxations occurring in trace residual epoxy remaining on a nanowire following the fabrication process.34 Hysteresis was observed only in data obtained from one nanowire and was absent in data obtained from seven other nanowire devices. CVs in FcCOOH were observed to be independent of applied scan rates up to 1000 mV s1, permitting rapid signal acquisition with well resolved steady-state currents (see Figure 3b). The small rise in current at
"
F E E 1 þ exp RT
00
#
imt
" # Fð1 RÞ E E00 m þ 0 exp k RT ð2Þ
where imt is the mass transfer diffusion-limited current, F is the Faraday constant, R is the molar gas constant, T is temperature (K), E is the applied potential, E00 is the formal potential of the redox couple, R is the transfer coefficient, k0 is the standard heterogeneous rate constant (cm s1), and m is the mass transfer coefficient (cm s1). Equation 1 involves the assumption of identical diffusion coefficients of the oxidized and reduced species and a uniformly accessible electrode surface. The mass transfer coefficient may then be expressed as m = imt(AFC)1, where A is the electrode surface area and C is the bulk concentration of the electroactive species.33 Nonlinear leastsquares fitting of experimental voltammograms by eq 2 was performed by the LevenbergMarquardt algorithm using k0, R, and imt as fitting parameters (see Figure 4).36 The fitting range was restricted to potentials below 0.4 V so that only the features of the voltammogram containing information about the electrode kinetics (i.e., the rising part) were included. The other parameters in eq 1 were fixed at E00 = 0.306 mV, C = 1 mM, and A = 2.92 107 cm2 (geometric area of a nanowire). Equation 1 provides an excellent description of the experimental curve shape for FcCOOH. This fit yielded apparent values of k0 = 2.27 ( 0.02 cm s1, R = 0.4 ( 0.01, and imt = 6.802 ( 0.003 nA. This fitted k0 value is 2 orders of magnitude higher than that 5538
dx.doi.org/10.1021/ac2004086 |Anal. Chem. 2011, 83, 5535–5540
Analytical Chemistry
ARTICLE
overpotential and an increased response current to hydrogen peroxide.18 The peak currents were proportional to the H2O2 concentration, and a quasi-linear response across a dynamic range spanning eight decades of H 2 O 2 concentrations (10 10 10 3 M) was observed.
Figure 5. Cyclic voltammograms for different H2O2 concentrations: 110 pM, 550 pM, 50 nM, 1 μM, and 1 mM obtained using an unmodified nanowire electrode.
previously reported for FcCOOH using a 2 mm diameter platinum disk electrode (0.014 cm s1)37 and is in agreement with other ferrocene molecular derivatives measured using carbon nanotubes33 and Pt nanoelectrodes.38 We also fit the ButlerVolmer equation to CVs obtained using higher scan rates. A slight increase in k0, from 2.27 ( 0.02 cm s1 to 2.39 ( 0.12 cm s1 was observed when the scan rate was increased from 5 to100 mV s1, demonstrating the relative independence of the apparent k0 value on scan rate. To explore the quality of the fit, we simulated two voltammograms with exaggerated k0 values of 1.10 and 3.30 cm s1, respectively. These voltammograms had similar CV characteristics but were clearly offset from experimental results (see Figure 4). However, from the inset in Figure 4, it is clear that the estimated fit of k0 = 2.27 ( 0.02 cm s1 was in excellent agreement with the experimental data. In this manner, these nanowire devices permit the analysis of fast electrode reactions, demonstrating their applicability for use in electrochemical kinetic studies. To explore the potential of these nanowire devices as a general platform for future electrochemical sensing applications, pristine nanowire devices were employed for detection of hydrogen peroxide (H2O2), a key target analyte in environmental analysis, food spoilage, biomedical applications, and homeland security.3943 The direct reduction of hydrogen peroxide at gold electrodes is known to be difficult due to a low catalytic activity and the high overpotentials required for the redox reaction to proceed.44 For this reason, gold electrodes are traditionally modified with redox electron transfer mediators; for example, Prussian blue, to surmount these obstacles.45,46 However, it has recently been shown that rough electrodes typically have a higher catalytic activity than annealed gold due to edges playing an important role as active sites for reduction reactions.47 This implies that nanoskived nanowires could be suitable for direct electrochemical reduction of H2O2 without the need for chemical modification. In this regard, Figure 5 shows typical CVs acquired for a range of different concentrations of H2O2 (110 pM1 mM) in 100 mM PBS. On addition of successive aliquots of H2O2, increasing cathodic and anodic currents, attributed to the electrocatalytic reduction and oxidation, were observed. Reduction currents were achieved with applied voltages as low as 0.05 V, much lower than those values obtained using conventional gold macroelectrodes.18,48 This suggests that the higher catalytic activity of the surface of a nanoskived nanowire offers a decreased
’ CONCLUSION In conclusion, we demonstrate a robust low-cost and a straightforward approach for fabrication, integration, and packaging of gold nanowires at silicon chip substrates for use as nanoelectrodes in electrochemical applications. Nanowire devices exhibited steady-state sigmoidal cyclic voltammograms for FcCOOH that were observed to be independent of scan rate up to 1000 mV s1. The heterogeneous electron transfer-rate constant, k0, for the oxidation of FcCOOH in PBS was found to be 2.27 ( 0.02 cm s1, R = 0.4 ( 0.005, 2 orders of magnitude greater than previously determined at a 2 mm diameter platinum disk electrode. The high catalytic activity of the nanowires enabled direct detection of hydrogen peroxide, without the need for surface chemical modification, across a wide concentration range (110 pM1 mM), showing the potential of these devices for future analytical applications. It is expected that further optimization in materials processing and device design, such as including a flow injection cell, will permit future realization of nanowire-based electrochemical devices. Finally, combining nanoskiving-based nanowire fabrication with low-cost electrical contacting techniques, such as inkjet printing, could enable low-cost fabrication and open the door for disposable, throwaway electrochemical nanosensor devices. ’ 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
*E-mail:
[email protected]; www.tyndall.ie/nanotech.
’ ACKNOWLEDGMENT This work was supported by the European Commission under the FP6 NMP project “Hydromel” (NMP2-CT-2006-026622), by Science Foundation Ireland under the Research Frontiers Programme (SFI/09/RFP/CAP2455) and the Irish HEA PRTLI program (Cycle 3 “Nanoscience” and Cycle 4 “Inspire”). ’ REFERENCES (1) Hahm, J.; Lieber, C. M. Nano Lett. 2004, 4, 51. (2) Wang, X.; Ozkan, C. S. Nano Lett. 2008, 8, 398. (3) Ramanathan, K.; Bangar, M. A.; Yun, M.; Chen, W.; Myung, N. V.; Mulchandani, A. J. Am. Chem. Soc. 2005, 127, 496. (4) Bangar, M. A.; Hangarter, C. M.; Yoo, B.; Rheem, Y.; Chen, W.; Mulchandani, A.; Myung, N. V. Electroanalysis 2009, 21, 61. (5) Wang, R. H.; Ruan, C. M.; Kanayeva, D.; Lassiter, K.; Li, Y. B. Nano Lett. 2008, 8, 2625. (6) Neumann, T.; Johansson, M. L.; Kambhampati, D.; Knoll, W. Adv. Funct. Mater. 2002, 12, 575. (7) Moskovits, M. Rev. Mod. Phys. 1985, 57, 783. 5539
dx.doi.org/10.1021/ac2004086 |Anal. Chem. 2011, 83, 5535–5540
Analytical Chemistry (8) Zhang, X. Y.; Li, D.; Bourgeois, L.; Wang, H. T.; Webley, P. A. ChemPhysChem 2009, 10, 436. (9) Arrigan, D. W. M. Analyst 2004, 129, 1157. (10) Conyers, J. L.; White, H. S. Anal. Chem. 2000, 72, 4441. (11) Shao, Y. H.; Mirkin, M. V.; Fish, G.; Kokotov, S.; Palanker, D.; Lewis, A. Anal. Chem. 1997, 69, 1627. (12) Keebaugh, S.; Kalkan, A. K.; Nam, W. J.; Fonash, S. J. Electrochem. Solid State Lett. 2006, 9, H88. (13) Menke, E. J.; Thompson, M. A.; Xiang, C.; Yang, L. C.; Penner, R. M. Nat. Mater. 2006, 5, 914. (14) Ji, R.; Lee, W.; Scholz, R.; Gosele, U.; Nielsch, K. Adv. Mater. 2006, 18, 2593. (15) Jorritsma, J.; Gijs, M. A. M.; Kerkhof, J. M.; Stienen, J. G. H. Nanotechnology 1996, 7, 263. (16) Walter, E. C.; Murray, B. J.; Favier, F.; Kaltenpoth, G.; Grunze, M.; Penner, R. M. J. Phys. Chem. B 2002, 106, 11407. (17) Chai, J.; Wang, D.; Fan, X. N.; Buriak, J. M. Nat. Nanotechnol. 2007, 2, 500. (18) Zhang, X.; Wang, H.; Bourgeois, L.; Pan, R.; Zhao, D.; Webley, P. A. J. Mater. Chem. 2008, 18, 463. (19) Yun, M.; Myung, N. V.; Vasquez, R. P.; Lee, C.; Menke, E.; Penner, R. M. Nano Lett. 2004, 4, 419. (20) Xu, Q.; Rioux, R. M.; Whitesides, G. M. ACS Nano 2007, 1, 215. (21) Hart, A. L.; Turner, A. P. F.; Hopcroft, D. Biosens. Bioelectron. 1996, 11, 263. (22) Laschi, S.; Palchetti, I.; Mascini, M. Sens. Actuators, B 2006, 114, 460. (23) Delamarche, E.; Vichiconti, J.; Hall, S. A.; Geissler, M.; Graham, W.; Michel, B.; Nunes, R. Langmuir 2003, 19, 6567. (24) O’Brien, G. A.; Quinn, A. J.; Tanner, D. A.; Redmond, G. Adv. Mater. 2006, 18, 2379. (25) Bard, A. J., Faulkner, L. R., Electrochemical Methods, Fundamentals & Applications; 2nd ed.; Wiley & Sons: New York, 2001. (26) Zayats, M.; Baron, R.; Popov, I.; Willner, I. Nano Lett. 2005, 5, 21. (27) Burke, L. D.; Nugent, P. F. Gold Bull. 1998, 31, 39. (28) Durkan, C.; Welland, M. E. Phys. Rev. B 2000, 61 (20), 14215–14218. (29) Electranlaytical Chemistry; Bard, A. J., Ed.; Marcel Dekker, Inc.: New York, 1976; Vol. 9. (30) Zuliani, C.; Walsh, D. A.; Keyes, T. E.; Forster, R. J. Anal. Chem. 2010, 82, 7135. (31) Trasatti, S.; Petrii, O. A. Pure Appl. Chem. 1991, 63 (5), 711–734. (32) Tremiliosi Filho, G.; Dall Antonia, L. H.; Jerkiewicz, G. J. Electroanal. Chem. 1997, 422 (12), 149–159. (33) Heller, I.; Kong, J.; Heering, H. A.; Williams, K. A.; Lemay, S. G.; Dekker, C. Nano Lett. 2005, 5, 137. (34) Ahmed, W.; Kooij, E. S.; van Silfhout, A.; Poelsema, B. Nano Lett. 2009, 9 (11), 3786–3794. (35) Zoski, C. G. Handbook of Electrochemistry; 1st ed.; Elsevier: Oxford, 2007. (36) Press, W. H.; Flannery, B. P.; Teukolsky, S. A.; Vetterling, W. T. Numerical Recipes in Fortran 77; 2nd ed.; Cambridge University Press: Cambridge, 1992. (37) Antiochia, R.; Lavagnini, I.; Magno, F.; Valentini, T.; Palleschi, G. Electroanalysis 2004, 16, 1451. (38) Watkins, J. J.; Chen, J. Y.; White, H. S.; Abruna, H. D.; Maisonhaute, E.; Amatore, C. Anal. Chem. 2003, 75, 3962. (39) Nowall, W. B.; Kuhr, W. G. Electroanalysis 1997, 9, 102. (40) Foulds, N. C.; Lowe, C. R. Anal. Chem. 1988, 60, 2473. (41) Brahim, S.; Narinesingh, D.; Guiseppi-Elie, A. Anal. Chim. Acta 2001, 448, 27. (42) Szatrowski, T. P.; Nathan, C. F. Cancer Res. 1991, 51, 794. (43) Huang, X. D.; Cuajungco, M. P.; Atwood, C. S.; Hartshorn, M. A.; Tyndall, J. D. A.; Hanson, G. R.; Stokes, K. C.; Leopold, M.; Multhaup, G.; Goldstein, L. E.; Scarpa, R. C.; Saunders, A. J.; Lim, J.; Moir, R. D.; Glabe, C.; Bowden, E. F.; Masters, C. L.; Fairlie, D. P.; Tanzi, R. E.; Bush, A. I. J. Biol. Chem. 1999, 274, 37111.
ARTICLE
(44) Salimi, A.; Hallaj, R.; Soltanian, S.; Mamkhezri, H. Anal. Chim. Acta 2007, 594, 24. (45) Karyakin, A. A.; Puganova, E. A.; Budashov, I. A.; Kurochkin, I. N.; Karyakina, E. E.; Levchenko, V. A.; Matveyenko, V. N.; Varfolomeyev, S. D. Anal. Chem. 2004, 76, 474. (46) Li, J.; Qiu, J. D.; Xu, J. J.; Chen, H. Y.; Xia, X. H. Adv. Funct. Mater. 2007, 17, 1574. (47) Schwank, J. Gold Bull. 1983, 16. (48) Guo, S. J.; Wen, D.; Dong, S. J.; Wang, E. K. Talanta 2009, 77, 1510.
5540
dx.doi.org/10.1021/ac2004086 |Anal. Chem. 2011, 83, 5535–5540
