Hierarchically Driven IrO2 Nanowire Electrocatalysts for Direct

Mar 21, 2012 - Electrochemical Methods for the Analysis of Clinically Relevant Biomolecules. Mahmoud Labib , Edward H. Sargent , and Shana O. Kelley...
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Hierarchically Driven IrO2 Nanowire Electrocatalysts for Direct Sensing of Biomolecules Jun Ho Shim,†,‡ Yumin Lee,† Minkyung Kang,† Jaeyeon Lee,† Jeong Min Baik,§ Youngmi Lee,† Chongmok Lee,† and Myung Hwa Kim*,† †

Department of Chemistry & Nano Science, Ewha Womans University, Seoul 120-750, Korea Department of Chemistry, Daegu University, Gyeongsan 712-714, Korea § School of Mechanical and Advanced Materials Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan, Korea ‡

S Supporting Information *

ABSTRACT: Applying nanoscale device fabrications toward biomolecules, ultra sensitive, selective, robust, and reliable chemical or biological microsensors have been one of the most fascinating research directions in our life science. Here we introduce hierarchically driven iridium dioxide (IrO2) nanowires directly on a platinum (Pt) microwire, which allows a simple fabrication of the amperometric sensor and shows a favorable electronic property desired for sensing of hydrogen peroxide (H2O2) and dihydronicotinamide adenine dinucleotide (NADH) without the aid of enzymes. This rational engineering of a nanoscale architecture based on the direct formation of the hierarchical 1-dimensional (1D) nanostructures on an electrode can offer a useful platform for high-performance electrochemical biosensors, enabling the efficient, ultrasensitive detection of biologically important molecules.

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electrocatalysts for highly effective light driven water oxidation.18,19 Furthermore, it is exceptionally a biocompatible metal oxide approved by the Food and Drug Administration (FDA) for a stimulating and recording electrode with the benefits of superior charge storing capacity as well as structural stability over a commercial Pt electrode.20 However, the relating practical applications with IrO2 nanostructures are rarely found mainly due to experimental difficulties in fixing and integrating the synthesized nanomaterials into an electrical transducer to address the signal changes induced at the nanostructure.21 In this study, we report a powerful strategy to grow hierarchically driven single crystalline IrO2 1-D nanostructures on a Pt microwire, which is directly used as an electric signal transducer for highly effective sensing of biologically relevant molecules such as H2O2 and NADH (Figure 1). In fact, the direct electrochemical oxidation of H2O2 and NADH, both of which are important species involved in various biological enzymatic reactions, has been the subject of numerous studies related to the development of an amperometric biosensor. Problems inherent to such anodic detection are that these oxidation reactions, in general, are very irreversible at conventional electrodes and require high overpotentials. Such high potentials applied for the oxidation of H2O2 and NADH

n modern electrochemical science, electrodes of micrometer dimensions attract considerable attention in sensing since they minimize the required sample amount, provide high spatiotemporal resolution, and allow more straightforward analysis of the acquired experimental data.1−4 Their simple mode of operation is greatly promising for microelectronic applications including a high-performance amperometric sensor where the diffusion-controlled limiting current is directly proportional to the concentration of the electroactive analyte and miniaturized energy devices such as fuel cells.5 Unfortunately, it is still challenging to make miniaturized sensors having the sensitive delectability to biologically relevant species, despite various attempts, due to limitations in terms of catalytic efficiency and sustainability.6−9 The most critical drawback in using microelectrodes is that they generate weak electronic signals caused by a limited small surface area, which require a very sensitive instrumental detection system. One efficient way to address this limitation is to deposit nanomaterials, such as carbon-based nanostructures,10−12 nanoparticles,12,13 etc. on a supporting matrix to increase its specific surface area. Very recently, 1-D nanostructure materials14,15 such as nanowires and nanotubes have been considered as the most promising architectures to enhance the electrocatalytic activity due to their much higher surface area and amendable control of more complex hybrid nanostructures. IrO2 is of considerable interest as promising candidates as electrodes in a variety of applications owing to high catalytic activity, low resistivity, and superior chemical and thermal stability.16,17 It has been also known as being among the best © 2012 American Chemical Society

Received: December 9, 2011 Accepted: March 21, 2012 Published: March 21, 2012 3827

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achieve the homogeneous growth of IrO2 nanowires on the surface of an entire Pt microwire, the end part of a Pt microwire was raised to the surface normal of a SiO2 substrate. The quartz boat and its IrO2 charge were cleansed of impurities by first placing it at the center of the quartz tube furnace under He (99.999%) carrier gas flowing for ∼10 min at a gas flow rate of 400 sccm before heating. After that, the furnace temperature was rapidly increased to a temperature in the range of 1000− 1050 °C with flowing He (99.999%) carrier gas of 400 sccm at the rate of 100 °C min−1. The nanowire growth proceeded with flowing high purity He (99.999%, 300 sccm) and O2 (99.9%, 10 sccm) for 1 h. The temperature of the region which obtained the nanowires on the substrate was measured by 700 °C. The furnace was then allowed to cool to room temperature under flowing He. Nanostructure Characterizations. The Pt microwires that were collected on the SiO2 substrate were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). The crystal structures of IrO2 nanowires on a Pt microwire were also imaged by high-resolution transmission electron microscopy (HRTEM, FEI Titan TEM/STEM at 300 kV) at room temperature. Samples for TEM imaging were prepared by touching the nanowire-covered Pt microwire to a TEM grid, thereby transferring some of nanowires to the grid. Raman scattering measurements were directly carried out on a Pt microwire with IrO2 nanowires transferred to a Pyrex glass slide. Raman spectra were recorded in the backscattering configuration using a confocal microscope (Renishaw InVia System) using a 100× (0.9 NA) microscope objective, which both focused the laser beam (∼1 μm) and collect the backscattered light. Raman spectra were acquired by the excitation with 632.8 nm He−Ne laser light. Low powers were used to ensure that the nanowires did not decompose by localized laser heating. Optimal results were obtained with 50 mW laser power and 300 s integration times. Microelectrode Fabrication and Electrochemical Measurements. The prepared IrO2 nanowire-Pt or bare Pt mirowire was electrically contacted to a W rod using Ag conductive epoxy. The IrO2 nanowire-Pt/W or Pt/W was heated in an oven at 50 °C for ∼20 min to cure the Ag epoxy. Cyclic voltammetry (CV) and amperometry were performed using a CHI 920C scanning electrochemical microscope (SECM, CH Instruments, Austin, TX). A three-electrode system was employed for the electrochemical measurements, which includes a saturated calomel electrode (SCE) as the reference electrode and a platinum wire as the counter electrode. The Pt microwires (with or without IrO2 nanowires) were immersed in the 0.05 M phosphate buffered solution (PBS, pH = 7.4) followed by regulation of the vertical position relative to the solution surface. Indeed, the microwire electrode was vertically approached to the solution surface using a piezoelectric step motor of the SECM instrument until the electrode gently touched the solution surface, which could be recognized easily from the sharp noise peaks in the monitored electrode currents. Assuming the contact position as distance zero, the microwire electrode was immersed with the desired depths into the solution. The amperometric response of the prepared electrode to varying target concentration was measured at EH2O2 = 0.2 V or ENADH = 0.3 V (vs SCE), respectively, with successive additions of an aliquot of target standard solution into PBS under constant stirring. The solutions were magnetically stirred throughout and the currents

Figure 1. Schematic diagram of the experiments for amperometric sensing of biomolecules using hierarchically driven single crystalline IrO2 1-D nanostructures on a Pt microwire electrode.

usually lead to fouling of the majority of electrode surfaces, which diminishes the stability of the amperometric response signal.22−24 Considerable effort has been devoted toward the goal of finding the new materials for electrode modification that will reduce the overpotential as well as minimize surface passivation effects at the metal electrode surface leading to extremely slow electron-transfer kinetics;11,13,24 there have been so far no reported attempts in growing highly single crystalline IrO2 nanowires directly on the surface of bare Pt in the manner of well-defined nanoscale architectures for a miniaturized microsensor. Attractive features of the present method are thus simplicity, low cost, and high throughput as well as high crystalline properties of the hierarchical 1-D nanostructures with unique multiscale architectures toward high-performance nano or microsensor fabrications.

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EXPERIMENTAL SECTION

Materials. K3[Fe(CN)6], Ru(NH3)6Cl3, H2O2, NADH, NaH2PO4·H2O, and Na2HPO4 were supplied by Sigma-Aldrich (St. Louis, MO). Pt microwire (99.99%) and W rod (0.05 in. × 3 in.) were acquired from Good Fellow (Oakdale, PA) and FHC (Bowdoin, ME), respectively. A silver conductive epoxy was purchased from Alfa Aesar (Haverhill, MA). All other analytical grade reagents and solvents were used as received. All aqueous solutions were prepared with deionized H2O (resistivity ≥ 18 MΩ cm). Growth of Hierarchically Driven IrO2 Nanowires on a Single Pt Microwire. The growth of IrO2 nanowires was directly carried out on a Pt microwire (diameter = 25 μm). Specifically, IrO2 nanowires were synthesized by the vapor transport without catalyst in a single zone quartz tube furnace, 2.5 cm in diameter and 60 cm long under atmospheric pressure. A total of 10 mg of fine meshed IrO2 (99.9%, Aldrich) powder was first loaded at the center of a 6 cm long quartz boat without further purification. Several Pt microwires on the SiO2 substrate were then introduced into the furnace at a point approximately ∼15 cm downstream of the IrO2 powder source. In order to 3828

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electrochemical electrode at our growth temperature (Figure 2a and Supplementary Figure S1 in the Supporting Information). Interestingly, a recent study of the growth of IrO2 nanowires using MOCVD (metal organic chemical vapor deposition) reported that they were not grown on Pt, Au, and Ir novel metals. However, it is necessary to explore the mechanistic behaviors in great detail in order to fully demonstrate the preferential and unidirectional crystal growth of IrO2 on a Pt wire. It is clearly seen that the Raman bands measured at 561, 729, and 751 cm−1 can be readily assigned to the first order Eg, B2 g, and A1 g phonon bands of rutile IrO2 (Figure 2e), based on previous measurements.26 Our measured band frequencies correspond closely to those observed with bulk samples; moreover, the full width at half-maximum of the 561 cm−1 Eg phonon mode at 12 cm−1 is very similar to the value reported27 for the bulk single crystal, implying high crystalline quality for hierarchically grown IrO2 nanowires on a Pt microwire. Additionally, the typical XRD pattern and EDX measurements of the IrO2 nanowires grown on a Pt microwire were consistent with rutile IrO2 with the tetragonal phase, which is in agreement with the standard data (JCPDS 88-0288) as shown in Figure S2 in the Supporting Information. It is clearly seen from a TEM image that an IrO2 nanowire on a Pt microwire is terminated by a prismatic form with the rectangular cross sections and each well-defined facet (Figure 3,

induced by the target molecule oxidation were recorded. Stock solutions of H2O2 or NADH were prepared daily in buffer solution and stored in the dark at 4 °C before use.

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RESULTS AND DISCUSSION The growth of IrO2 nanowires was directly carried out on a commercial Pt microwire (diameter = 25 μm) by the vapor transport without catalyst at 700 °C under atmospheric pressure. SEM images (Figure 2) clearly show a high density

Figure 2. As grown IrO2 nanowires on a Pt microwire (diameter = 25 μm). (a−d) SEM images with different magnifications showing hierarchically grown IrO2 nanowires on a 25 μm Pt microwire. (e) Micro Raman spectrum at 632.8 nm taken from the same sample shown in (a−d).

of the hierarchically grown IrO2 nanowires directly on a Pt microwire (diameter = 25 μm). High-magnification SEM images indicate IrO2 nanowires were well grown from out of plane of a Pt microwire with the length of more than 5 μm, the lateral dimensions of 20−100 nm near a nanowire tip, and the random orientations. The shapes of nanowires are quite straight but their lateral dimensions are not uniform along the growth direction, representing the needle-like shape so that the lateral dimension of the end of a nanowire is generally much smaller than that of the root of a nanowire strongly bounded on a Pt microwire (Figure 2a−d). In addition, the nanowires do not show the catalyst particles but the sharp facets at the end of tip, suggesting that the growth mechanism may resemble sublimation followed by recrystallization, a process referred to as vapor-solid (VS) growth.25 Thus, the formation of the hierarchical nanostructures of IrO2 can be rationalized in terms of the preferential nucleation of gas phase IrOx precursors on the relatively rough surface of a Pt microwire used as the

Figure 3. Detailed crystal structures of as grown IrO2 nanowires. The low and high magnification TEM images of a representative IrO2 nanowire and selected area diffraction (SAED) pattern taken from the same sample shown in Figure 2. A SAED pattern taken along the [100] zone axis indicates that an IrO2 nanowire is a single crystalline tetragonal structure with a [001] growth direction.

inset). High-resolution TEM images of a tip of a IrO2 nanowire represent that the lattice spacing of adjacent planes is about 0.250 nm corresponding to that between the (101) planes of tetragonal IrO2. The nanowires grow along the [001] direction parallel to a {110} family plane, in exact agreement with the fast Fourier transform (FFT) of the lattice-resolved image (Figure 2, inset). Interestingly, the sharp facets seen at the tip of the nanowire imply that these low-index facets arise from the low surface energy during the growth process.27 It is well-known 3829

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Figure 4. CVs of 25 μm Pt wire (a) and IrO2-Pt (c) in an aqueous solutions containing 10 mM [Fe(CN)6]3− and 1.0 M KCl by varying the potential scan rate (5, 10, 20, and 50 mV s−1). CVs of 25 μm Pt wire (b) and IrO2-Pt (d) in an aqueous solutions containing [Fe(CN)6]3− at various concentrations (0.1, 0.5, 1, 5, 10 mM) and 1.0 M KCl with a scan rate of 10 mV s−1. CVs of IrO2-Pt (e) in an aqueous solutions containing Ru(NH3)63+ at various concentrations (1, 5, 10 mM) and 1.0 M KCl with a scan rate of 10 mV s−1. For all, immersion depth, 100 μm. (f) The corresponding calibration plots of the quasi-steady-state currents vs concentration of electroactive species: (○) 25 μm Pt wire and (●) IrO2-Pt with [Fe(CN)6]3− couple (at 0.0 V); (Δ) 25 μm Pt wire and (▲) IrO2-Pt with Ru(NH3)63+/2+ couple (at −0.3 V due to H2 evolution).

Figure 5. (a) CVs of IrO2-Pt in 1.0 M KCl by varying the potential scan rate (10, 20, 50, 100, 150, and 200 mV s−1). (b) Plots for the difference of capacitive currents (Δi) vs scan rate: (○) 25 μm Pt wire, (Δ) 76 μm Pt wire, and (□) IrO2-Pt microelectrodes.

electrolyte were reported using microelectrodes.30−32 The quasi-steady state currents showed a linear proportionality to the concentration of [Fe(CN)6]3−, yielding a slope of 23.7 nA mM−1 (Figure 4f). The CV shape of the IrO2-Pt microelectrode with the [Fe(CN)6]3− couple with v = 5 mV s−1 was similar to that of Pt wire v = 20 mV s−1, showing quasi-steady state currents at the longer experimental time scale than the bare Pt wire (Figure 4a,c). Indeed, this observation indicates that the diffusion layer of [Fe(CN)6]3−/4− grows along with the real surface plane of jagged local IrO2 nanowires on a Pt microwire for shorter experimental time. On the other hand, the diffusion layer grows along with the smooth cylindrical geometric surface plane for longer experimental time, similar to the one at a Pt microwire substrate with a diameter extended from the original one by the length of IrO2 nanowires.29 A plot of current values at 0.0 V (shown in Figure 4d) against the concentration of

that the grow rate is strongly dependent on the surface energies of crystal planes. Thus, we successfully grew high-quality single crystalline hierarchical IrO2 nanowires directly on a Pt microwire in a single step for a microelectrode fabrication. Before evaluating the electrochemical behavior of the IrO2 nanowires-Pt (IrO2-Pt) microelectrode, cyclic voltammograms (CVs) of Pt wire (diameter = 25 μm) were recorded with the immersion depth of 100 μm in an aqueous solution containing 10 mM [Fe(CN)6]3− and 1.0 M KCl by varying the potential scan rate (v) (Figure 4a).28 The shape of the CVs changes from the typical steady-state sigmoidal one with slower v to the peakshaped one with faster v, indicating the changes in the diffusion layer growth pattern, i.e., so-called “quasi-steady state”29 was obtained with slower v. Note that 1.0 M KCl was the supporting electrolyte because the “anomalous” CV responses of [Fe(CN)6]3−/4− (75 times upon modification of IrO2 on Pt from the TEM image, indicating the electrical double layer is not thin enough to reflect the RSA of this electrode at this ionic strength.33 Electrocatalytic activity of the single crystalline IrO2-Pt was further investigated for the kinetic-controlled oxidations of H2O2 and NADH by amperometry, where two variables are important: intrinsic catalytic activity and surface area. Figure 6

displays the amperometric responses of the bare Pt microwire (red) and IrO2-Pt microwire (blue) electrodes to successive additions of H2O2 (parts a and b) and NADH (part c) aliquots into a stirred PBS solution (pH 7.4) at EH2O2 = 0.3 V (a), 0.2 V (b), and at ENADH = 0.3 V vs SCE (c), respectively. The current response of IrO2-Pt at EH2O2 = 0.3 V is 6 times larger than that of bare Pt; however, the increment would not surpass the increase of RSA (Figure 6a), which implies the enhancement of surface area played a key role. At EH2O2 = 0.2 V, the electrode based on the pristine Pt microwire displayed no detectable anodic current flows (Figure 6b). It can be ascribed to the smaller oxidation overpotential of IrO2-Pt than that of Pt. The Pt microwire with the IrO2 decoration, in contrast, exhibited remarkably enhanced current signals, induced by the oxidation of NADH, in proportion to the target molecule concentration: sensitivity of 2.9 nA mM−1 (r = 0.999) for NADH, detection limit of 5.0 μM for NADH (Figure 6c). In this case, we believe the intrinsic catalytic activity or small overpotential of NADH oxidation played a key role rather than RSA enhancement, which was evidenced by the linear sweep voltammograms (LSVs) for the oxidation of NADH (Figure 6d). Note, the small oxidation overpotential for electrochemical detection is of importance to enhance the selectivity against the oxidation of biological interferences, i.e., ascorbic acid, dopamine, uric acid, and acetamidophenol. Furthermore, the hierarchical IrO2 nanowires show a very fast response, as the response reaches 95% of the steady-state value within 10 s. Overall, increase of RSA of a hierarchically grown 1-D nanostructure electrode equipped with an appropriate catalytic material decoration, we believe, enhances accessibility relative to smooth structures for favorable and efficient binding of the analyte.35 It is also notable that the distinct geometric design of the nanostructured catalyst affects the catalytic activity, i.e., the 3831

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(12) Singhal, R.; Orynbayeva, Z.; Sundaram, R. V. K.; Niu, J. J.; Bhattacharyya, S.; Vitol, E. A.; Schrlau, M. G.; Papazoglou, E. S.; Friedman, G.; Gogotsi, Y. Nat. Nanotechnol. 2011, 6, 57. (13) Orozco, J.; Jiménez-Jorquera, C.; Fernández-Sánchez, C. Bioelectrochemistry 2009, 75, 176. (14) Bierman, M. J.; Lau, Y. K. A.; Kvit, A. V.; Schmitt, A. L.; Jin, S. Science 2008, 320, 1060. (15) Kawasaki, J. K.; Arnold, C. B. Nano Lett. 2011, 11, 781. (16) Kwon, S. J.; Fan, F. R. F.; Bard, A. J. J. Am. Chem. Soc. 2010, 132, 13165. (17) Youngblood, A. J.; Lee, S.-H. A.; Kobayashi, Y.; HernandezPagan, E. A.; Hoertz, P. G.; Moore, T. A.; Moore, A. L.; Gust, D.; Mallouk, T. E. J. Am. Chem. Soc. 2009, 131, 926. (18) Nam, Y. S.; Magyar, A. P.; Lee, D.; Kim, J.-W.; Yun, D. S.; Park, H. Jr; T., S. P.; Weitz, D. A.; Belcher, A. M. Nat. Nanotechnol. 2010, 5, 340. (19) Frame, F. A.; Townsend, T. K.; Chamousis, R. L.; Sabio, E. M.; Dittrich, T.; Browning, N. D.; Osterloh, F. E. J. Am. Chem. Soc. 2011, 133, 7264. (20) Zhang, F.; Reddy, B. U. R. K.; Venkatraman, V. L.; Prasad, S.; Vu, T. Q.; Hsu, S.-T. Jpn. J. Appl. Phys. 2008, 47, 1147. (21) Chen, Y.-L.; Hsu, C.-C.; Song, Y.-H.; Chi, Y.; Carty, A. J.; Peng, S.-M.; Lee, G.-H. Chem. Vap. Deposition 2006, 12, 442. (22) Moiroux, J.; Elving, P. J. Anal. Chem. 1978, 50, 1056. (23) Katakis, I.; Domínguez, E. Mikrochim. Acta 1997, 126, 11. (24) Evans, S. A. G.; Elliott, J. M.; Andrews, L. M.; Bartlett, P. N.; Doyle, P. J.; Denuault, G. Anal. Chem. 2002, 74, 1322. (25) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (26) Korotcov, A. V.; Huang, Y.-S.; Tiong, K.-K.; Tsai, D.-S. J. Raman Spectrosc. 2007, 38, 737. (27) Lee, Y.; Ye, B.-U.; Yu, H. K.; Lee, J.-L.; Kim, M. H.; Baik, J. M. J. Phys. Chem. C 2011, 115, 4611. (28) Brett, C. M. A.; Brett, A. M. O. Electrochemistry: Principles, Methods, and Applications; Oxford University Press: Oxford, U.K., 1993. (29) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001; pp 168, 232, 174. (30) Lee, C.; Anson, F. C. J. Electroanal. Chem. 1992, 323, 381. (31) Rooney, M. B.; Coomber, D. C.; Bond, A. M. Anal. Chem. 2000, 72, 3486. (32) Beriet, C.; Pletcher, D. J. Electroanal. Chem. 1993, 361, 93. (33) Boo, H.; Park, S.; Ku, B.; Kim, Y.; Park, J. H.; Kim, H. C.; Chung, T. D. J. Am. Chem. Soc. 2004, 126, 4524. (34) Thanawala, S. S.; Baird, R. J.; Georgiev, D. G.; Auner, G. W. Appl. Surf. Sci. 2008, 254, 5164. (35) Squires, A. M. O.; Messinger, R. J.; Manalis, S. R. Nat. Biotechnol. 2008, 26, 417. (36) Shim, J. H.; Kim, Y. S.; Kang, M.; Lee, C.; Lee, Y. Phys. Chem. Chem. Phys. 2012, 14, 3974.

smaller RSA catalyst shows better activity than the larger RSA one in some cases.36 Indeed, IrO2 nanowires grown vertically to a Pt microwire surface plane, resulting in cactus-like structures (Figure 2a), provide one of the optimized geometries to detect biologically important species with greatly improved sensitivity and stability.

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CONCLUSIONS Here we report that the hierarchically driven IrO2 nanowires on Pt microwires have been fabricated directly in a single step by the simple surface growing method. We demonstrated that the extremely dense IrO2 nanowires on the surface of Pt microwire have an excellent electrocatalytic activity and could be used in the electrochemical detection of H2O2 and NADH. This rational engineering of a nanoscale architecture based on the direct formation of the hierarchical 1-D nanostructures on an electrode can offer a useful platform for high-performance electrochemical biosensors, enabling the efficient, sensitive detection of biological important molecules.

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ASSOCIATED CONTENT

S Supporting Information *

Additional SEM images and XRD pattern of as grown IrO2 nanowires on a Pt microwire. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.H.S. and Y.L. contributed equally to this work. This research was supported by MEST & DGIST (Grant 10-BD-0101, Convergence Technology with New Renewable Energy and Intelligent Robot) and by Basic Science Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (Grant No. 2010-0022028). This work was also supported by the Ewha Global Top 5 Grant 2011 of Ewha Womans University.

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