3.2 Experimenal
3.3.5 Glucose Sensing
In order to compare electro-catalysis of glucose oxidation by the nanostructured Cu electrodes, CV studies of both NB and NP electrodes were performed. As shown in Figure 3.8A, in a blank alkaline solution (50 mM NaOH(aq)), clear current increases corresponding to a Cu(II)/Cu(III) redox couple are observed above 0.7 and 0.75 V for the NB and the NP electrodes, respectively.14 For both electrodes in 0.1 M glucose mixed with 50 mM NaOH(aq), additional waves, corresponding to irreversible glucose oxidation, appear. For the Cu NB electrode, dramatic enhancement of the oxidation current can be observed between 0.2 - 0.7 V. For the NP electrode, the oxidation current increase starts only at 0.4 V.
Amperometric sensing is a technique commonly applied for glucose detection.2-16 It provides the response of oxidation current to analyte concentration variations. Figure 3.8B illustrates amperometric measurements of glucose by the nanostructured Cu electrodes (0.6 V in aerated 50 mM NaOH(aq)). The NB electrode provides a much higher current response than the NP electrode does, as demonstrated in Figure 3.8C. A sensitivity of 79.8 µA/mM with a linear dependence (R2 value, 0.998) of oxidation current to glucose concentration
Figure 3.8. Electrochemical data of Cu electrodes in 50 mM NaOH at 50 mV/s. (A) CV diagram of (a) NP and (b) NB electrodes without glucose, and (c) NP and (d) NB electrodes in 0.1 M glucose; (B) amperometric responses (at 0.6 V) of (a) NP and (b) NB electrodes (0.018 cm2) to successive additions of glucose; (C) current responses of (a) NP and (b) NB electrodes to glucose concentrations from the data in (B); (D) amperometric responses (at 0.6 V) of a NB electrode to interferences from ascorbic acid (AA, 10 µM) and uric acid (UA, 10 µM) prior to successive additions of glucose.
(10.0 µM - 1.13 mM) is observed for the NB electrode. The limit of detection (LOD) is 0.1 µM with a signal to noise ratio of 3. Performance of the NP electrode agrees with the reported data (sensitivity: 6.2 µA/mM; LOD: 0.96 µM).14-15 Compared to other nanostructured electrodes reported previously, our Cu NB electrodes showed superior performance. Some literature examples, as listed in Table 3.1, are Cu NPs/CNT (sensitivity:
17.76 µA/mM; LOD: 0.2 µM), Pt NPs/CNT (sensitivity: 2.11 µA/mM; LOD: 0.5 µM) composite electrodes and Pt NTs electrodes (sensitivity: 7.58 µA/mM; LOD: 1.0 µM).4,9,14
Table 3.1 The comparison of the performance of nanostructured electrodes for glucose detection.
Electrode Geometric area (cm2) Sensitivity (µA/mM) LOD (µM) Reference
Cu NBs 0.018 79.8 0.14 This study
a the data was calculated from conversion of specific sensitivity (µA/cm2mM) producing real surface area of electrodes
In addition, the design and fabrication steps of our Cu NB electrodes are simple and straightforward while low-cost materials are employed. These are the other advantages relative to the reported enzymatic sensors and template-assisted nanoelectrodes.2-7,9-11 In real physiological samples, interfering species such as L-ascorbic acid (AA) and uric acid (UA) normally co-exist with glucose. Their concentrations are about one-tenth of that of glucose levels. Amperometric responses of the NB electrode towards the addition of these two species (10 µM) followed by glucose (100 µM successively) were examined. As shown in Figure 3.8D, presence of AA and UA produces insignificant responses only, compared to the addition of glucose. The glucose response is still consistent with the calibration curve in Figure 3.8C, with a deviation of only 5%.
3.4 Conclusion
In conclusion, our results indicate that the Cu NB electrode can enhance electro-catalytic ability of glucose oxidation significantly. The high performance may be attributed to the large electrochemical surface area of the NBs and the exposed (111) belt surface planes.34 Presence of better contacts between the NBs and the substrate may also show positive effects. Thus, the kinetically-controlled electro-oxidation of glucose is amplified and the response current is increased. We anticipate that these low-cost and easy to fabricate Cu NB electrodes will perform exceptionally in sensing glucose concentrations in real biological samples. Research is in progress.
3.5 References
1. See World Health Organization (WHO) website: http://www.who.int/en/
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13. Wang, G.; Mantey, K.; Nayfeh, M. H.; Yau, S.-T. Appl. Phys. Lett. 2006, 89, 243901.
14. Kang, X.; Mai, Z.; Zou, X.-Y.; Cai, P.-X.; Mo, J.-Y. Anal. Biochem. 2007, 363, 143.
20. Zhao, N.; Wei, Y.; Sun, N.; Chen, Q.; Bai, J.; Zhou, L.; Qin, Y.; Li, M.; Qi, L.
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21. Zhang, J.; Liu, H.; Wang, Z.; Ming, N. Appl. Physics Lett. 2007, 91, 133112.
22. Chen, Y.; Milenkovic, S.; Hassel, A. W. Nano Lett. 2008, 8, 737.
23. Liu, Z.; Li, S.; Yang, Y.; Peng, S.; Hu, Z.; Qian, Y. Adv. Mater. 2003, 15, 1946.
24. Huang, T.-K., Cheng, T.-H.;Yen, M.-Y.;Hsiao, W.-H.; Wang, L.-S. Chen, F.-R.; Kai, J.-J. Lee, C.-Y.; H.-T. Chiu Langmuir 2007, 23, 5722.
25. Wang, Z. L. Adv. Mater. 2003, 15, 432.
26. Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947.
27. Shi, W. S.; Peng, H. Y.; Wang, N.; Xu, C. P.; Li, L.; Lee, C. S.; Lee, S. T. J. Am. Chem.
Soc. 2001, 123, 11095.
28. Ma, Y.; Qi, L.; Shen, W.; Ma, J. Langmuir 2005, 21, 6161.
29. Cao, X. B.; Xia, Y.; Zhang, S. Y.; Li, F. Q. Adv. Mater. 2004, 16, 649.
30. Mo, M.; Zeng, J.; Lin, X.; Yu, W.; Zhang, S.; Qian, Y. Adv. Mater. 2002, 14, 1658.
31. Zhang, M.; Wang, Z.; Xi, G.; Ma, D.; Zhang, R.; Qian, Y. J. Cryst. Growth 2004, 268, 215.
32. Law, M.; Sirbuly, D.; Johnson, J.; Goldberger, J.; Saykally, R.; Yang, P.-D. Science 2004, 305, 1269.
33. Zen, J.-M.; Hsu, C.-T.; Kumar, A. S.; Lyuu, H.-J.; Lin, K.-Y. Analyst 2004, 129, 841.
34. Takahashi, I.; Koga, O.; Hoshi, N.; Hori, Y. J. Electroanal. Chem. 2002, 533, 135.
Chapter 4
Growth of Pagoda-Topped Tetragonal Cu Nanopillar Arrays
4.1 Introduction
One-dimensional (1D) metal nanowires (NWs), nanorods (NRs), and nanobelts (NBs) have attracted considerable attentions because of their promising applications in future.[1-15]
However, facile and controllable fabrication of free-standing metal NW arrays is still a difficult challenge. Hard templates with 1D channels, such as anodic aluminum oxide (AAO) and iontracked membranes, were often employed for high density 1D metal growths.[8-13]
Hard template-free processes are rare. In some cases, penta-twinned Cu NWs grown upward on hard substrates by chemical vapor deposition (CVD) were reported.[14,15] These processes were rather complicated and relatively expensive equipments were needed. Recently, we have demonstrated that by using simple galvanic displacement reactions, novel nanostructures of Cu, Ag and Au can be grown directly on conducting substrates efficiently.[16-17] In these cases, morphology of the products were adjusted by suitable surfactants, acting as growth control agents. Here, we wish to report the growth and characterization of an unusual new type of 1D Cu in a form of pagoda-topped tetragonal nanopillar arrays, previously unknown in literature, by this route. In addition, our preliminary investigation demonstrates that the Cu nanopillars can emit electrons efficiently under applied electrical fields.
4.2 Experimental