3.2. Characterizations and Spectroscopic Measurements
3.3.2 SERS Analysis
Correlations between SERS and surface plasmon resonance (SPR) have been studied and connected both theoretically and experimentally.42-44 It has been proved that optimizing the correlation between the SPR of the substrate and the excitation
Figure 3.3 SERS (Excitation: 532 nm, power: 5 mW, data collection: 5 s) of R6G (10 µL, in ethanol) on the urchin-like Ag NWs. The R6G concentrations are 10-13 M (red), 10-14 M(cyan), 10-15 M (blue), 10-16 M (green), and 10-17 M (black).
wavelength provides an efficient way to increase SERS performance.45 Since R6G has large Raman scattering cross section at 532 nm,46 which is near the longitudinal modes of the NWs. The excitation wavelength of 532 nm was applied for the SERS experiments discussed below. In Figure 3.3, the vibration signals from R6G (10-13 - 10-17 M) on a urchin-like Ag NW on SPC substrate (with a growth time 6 h) can be observed clearly by Raman. Assignments to specific vibrational modes were listed in Table 3.1.47,48 As the concentration decreases, the SERS signal intensities decrease accordingly. Yet, the signals are still visible even at the concentration as low as 10-16
Table 3.1 Assignments of Raman frequencies of R6G in the spectra.
Frequency (cm-1) Assignments
614 ip XRD, op XRD
776 op C-H bend, ip XRD
1127 ip C-H bend
1184 ip XRD, C-H bend, N-H bend 1309 ip XRB, N-H bend, CH2 wag 1366 XRS, ip C-H bend
1509 XRS, C-N str, C-H bend, N-H bend 1650 XRS, ip C-H bend
ip: in plane, op: out of plane, XRD: xanthene ring deformations, XRB: xanthene ring breath, XRS: xanthene ring stretch, str: stretch.
Figure 3.4 Raman spectra of R6G (1 µM, 10 µL, in ethanol) on urchin-like silver NWs with different growth time periods. (Excitation: 532 nm, power: 0.05 mW, data collection: 1 s)
1000 counts
1 hr 3 hr 6 hr
1 hr
3 hr
6 hr
Table 3.2 Reported detection limits and Analytical Enhancement Factors (AEF) of R6G on different substrates.
a NP: nanoparticle, NR: nanorod, NW: nanowire, SPC: screen-printed carbon.
b Not reported.
M. To our knowledge, this is one of the lowest detectable R6G concentration reported so far (See Table 3.2 for other examples).49-57 When the growth time was shortened,
Substrate
Detection Limit of R6G (M)
Analytical Enhancement
Factor
Ref
Ag nanocrystals on Si 10-8 105 49
Ag NPs on glassy carbon 10-9 -b 50
Au-coated ZnO NRs 10-9 106 51
Au flowerlike nanoarchitectures 10-12 -b 52
Ag NR arrays on Si 10-14 -b 53
Ag NPs-coated Si NWs 10-14 2.3 x 108 54
Roughened Ag substrate 2 x 10-15 -b 55
Ag nanodesert roses on Si 10-15 2 x 1010 56
Ag and Au NPs containing substrates 10-16 -b 57
Ag urchin-like NWs on SPC 10 -16 1013 This study
Figure 3.5 Images of randomly selected areas (60 µm x 60 µm) of urchin-like Ag NWs on a SPC electrode. Series (A) - (F), optical images and corresponding Raman mappings of the R6G signals at 614 cm-1, 776 cm-1, 1366 cm-1, 1509 cm-1, and 1650 cm-1.
short and thin Ag NWs were obtained on the substrates (Figure 3.1CDE). The enhancement capability diminished drastically too (Figure 3.4). This can be rationalized by the following reasons. As we know, the SERS effect correlates extensively to the electric magnetic field enhancement by the so-called “hot-spots”. In the urchin-like Ag NWs fabricated in this study, SERS hot-spots may originate from metal gaps, slits, vacancies and crossovers.20-24,40 Short and sparse Ag NWs would
614 cm-1 776 cm-1 1366 cm-1 1509 cm-1 1650 cm-1 Optical
images
(A)
(B)
(C)
(D)
(F)
(E)
create few gaps, slits, and vacancies. Consequently, these substrates showed much inferior enhancement performance.
To demonstrate the capability of the urchin-like Ag NWs in sensing R6G further, the following SERS mapping studies was carried out. We estimate that about 6000 R6G molecules exist on the urchin-like Ag NW on an SPC substrate when R6G (1 fM,
10 µL) is applied. Since the substrate has a surface area 0.196 cm2, we can determine that there are only thirty R6G molecules on an area 106 µm2. Thus, we anticipate locating only a single R6G molecule in a mapped-area 3600 µm2 (60 x 60 µm2). By using the Ag NWs, the Raman mappings of R6G peaks at 614 cm-1, 776 cm-1, 1309 cm-1, 1366 cm-1, 1509 cm-1, 1650 cm-1 are displayed in Figure 3.5. From six randomly selected areas (60 x 60 µm2 each) on the substrate, six sets of R6G signals are observed in four areas. Among them, one area displayed in the series B in Figure 3.5, a cluster of three sets of R6G signals is shown. In series C, E, F, each image displays only a single signal at the same point within the mapped-area 3600 µm2. On the other hand, two areas (A and D) are totally silent from any R6G response. This means that no R6G molecules exist in these two areas. Since the R6G molecules were randomly adsorbed on the substrate, the observation agrees with the estimated R6G density on the surface. That is on average, one molecule was adsorbed on each of the mapped areas. We conclude that each set of SERS mapping data were generated by a single R6G molecule. The Raman spectra of the single R6G molecule signals in Figure 3.5 are displayed in Figure 3.6. Further investigations of time-resolved surface-enhanced Raman spectra are shown in Figure 3.7. As the table in Figure 3.7 displayed, Raman peaks at 1658 cm-1, 1516 cm-1, 1314 cm-1, 1368 cm-1, and 1098 cm-1 are changed in both wavelength and intensity. In literature, it is found that the orientation of molecules on the surface affects the Raman intensities of specific vibrational modes strongly.58 This is because certain modes, such as in-plane and out-of-plane vibrations,
Figure 3.6 Raman spectra of the R6G signals in Figure 3.5. (A) (B) (C) spectra are the signals in Figure 3.5(B) from left to right. (D), (E), (F) spectra are the signals come from Figure3.5 (C), (E), (F), respectively.
are highly influenced by the local electrical field parallel or perpendicular to the molucles.58,59 Another possible reason is that when molecules are trapped in the hot
(A) (B)
(E) (F)
(C) (D)
Figure 3.7 Time-resolved surface-enhanced Raman spectra of R6G (10 µL, 1 fM) molecule recorded at 3-s intervals. Over 100 spectra were recorded before the signals disappeared. Ten spectra were selected to highlight sudden spectral changes. The table displayed five main Raman signals in these spectra. The Raman signals abruptly changed in both frequency and intensity. The laser excitation wavelength was 532 nm and the power was about 5 mW.
spots, some physical stress may be generated so that the adsorbed molecules undergo certain structural transformations60 or photochemical decomposition.25 In previous reports, most single molecule detections were carried out in special environment (i.e.
on nanocrystal aggregates) or required multiple step modifications of sensing surfaces.30-35 Our Ag NW substrate clearly shows the advantage of detecting a single molecule easily over the entire treated surface.
Intrinsic SERS enhancement factor (EF) is difficult to estimate because many variables, such as adsorbed molecules and laser scattering volume, are difficult to obtain.61 Thus, we use the analytical enhancement factor (AEF), defined by the following equation: AEF = (ISERS/CSERS)/(IRS/CRS),61 to estimate the SERS performance of the urchin-like Ag NWs. Here, IRS represents the Raman intensity of an analyte with
1
Figure 3.8 SERS of R6G on urchin-like Ag NWs (10 fM, red) and on a glass slide (0.1 M, black). (Excitation: 532 nm, power: 5 mW, data collection: 5 s).
a concentration CRS on a non-SERS substrate. ISERS is obtained from a SERS-active substrate with an analyte concentration CSERS. In the studies, all the other parameters, including laser wavelength, laser power, microscope magnification, and spectrometer, are identical. In our experiment, a glass plate was used as the non-SERS substrate while the urchin-like Ag NWs was employed as the SERS-active one. The Raman responses of R6G on these substrates are compared in Figure 3.8. Using IRS and ISERS of the peaks at 614 cm-1, 776 cm-1, 1366 cm-1, 1509 cm-1, and 1650 cm-1 of R6G, the averaged AEF of the urchin-like Ag NWs substrate is estimated to be about 1013. The value is superiior to most of the literature data listed in Table 3.2. In addition, on a commercial SERS-active substrate KlariteTM, the signals from R6G (1 nM) almost vanished.40 At higher R6G concentration such as 1 pM, fifteen randomly selected spots on urchin-like Ag NWs substrate ususally displayed similar SERS performance.
10-14 M R6G on Ag NWs
10-1 M R6G on glass
1000 counts
10-14 M R6G on Ag NWs
10-1 M R6G on glass
1000 counts
1000 counts
Figure 3.9 Raman spectrum of R6G (1 µM, 10 µL, in ethanol) on urchin-like silver NWs (Detected by MiniRam™ II Raman Spectrometer System, excitation: 785 nm, power: 5 mW, data collection: 5 s).
Different urchin-like Ag NWs substrates showed similar results. We also employed a low-cost portable Raman instrument with an excitation wavelength 785 nm to study potential SERS application of our Ag NWs. As shown in Figure 3.9, the R6G (1 µM, 10 µL in ethanol) signals were observed clearly. The result suggests that by coupling our Ag NW substrate with a low-cost portable apparatus, it is possible to find economical and real-life sensing applications.62-65
1000 counts
3.4 Conclusion
In this study, we have developed a simple low-cost surfactant-assisted galvanic reduction process to grow urchin-like Ag NWs on carbon screen printed electrodes.
The urchin-like Ag NW substrate shows a high SERS performance. Using R6G as the probe molecule, the test needs only a minute quantity of sample solution (10 µL) with a short sensing time (5 s). The AEF is high (1013). The detection limit for R6G is below femto molar concentration. This means the sensing is at single molecular level for R6G. Consequently, we anticipate that by coupling the urchin-like Ag NWs with a low-cost portable instrument, the setup can be applied for rapid biological, medicine, and environmental pollutant sensing applications. The investigation is in progress.
3.5 References
1. Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Chem. Phys. Lett. 1974, 26, 163.
2. Albrecht, M. G.; Creighton, J. A. J. Am. Chem. Soc. 1977, 99, 5215.
3. Jeanmaire, D. L.; Van Duyne, R. P. J. Electroanal. Chem. 1977, 84, 1.
4. Wang, H. H.; Liu, C. Y.; Wu, S. B.; Liu, N. W.; Peng, C. Y.; Chan, T. H.; Hsu, C.
F.; Wang, J. K.; Wang, Y. L. Adv. Mater. 2006, 18, 491.
5. Lee, S. J.; Morrill, A. R.; Moskovits, M. J. Am. Chem. Soc. 2006, 128, 2200.
6. Tao, A.; Kim, F.; Hess, C.; Goldberger, J.; He, R.; Sun, Y.; Xia, Y.; Yang, P.
Nano Lett. 2003, 3, 1229.
7. Liu, Y. J.; Chu, H. Y.; Zhao, Y. P. J. Phys. Chem. C 2010, 114, 8176.
8. Sun, Y.; Wiederrecht, G. P. Small 2007, 3, 1964.
9. Lin, H.; Mock, J.; Smith, D.; Gao, T.; Sailor, M. J. J. Phys. Chem. B 2004, 108, 11654.
10. Wen, X.; Xie, Y.-T.; Mak, W. C.; Cheung, K. Y.; Li, X.-Y.; Renneberg, R.; Yang, S. Langmuir 2006, 22, 4836.
11. Song, W.; Cheng, Y.; Jia, H.; Xu, W.; Zhao, B. J. Colloid Interface Sci. 2006, 298, 765.
12. Gutés, A.; Carraro, C.; Maboudian, R. J. Am. Chem. Soc. 2010, 132, 1476.
13. Rashid, H.; Mandal, T. K. J. Phys. Chem. C 2007, 111, 16750.
14. Hong, B. H.; Bae, S. C.; Lee, C. W.; Jeong, S.; Kim, K. S. Science 2001, 294, 348.
15. Jin, R.; Cao, Y.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Science 2001, 294, 1901.
16. Jana, N. R.; Gearheart, L.; Murphy, C. J. Chem. Commun. 2001, 617.
17. Sun, Y.; Xia, Y. Adv. Mater. 2002, 14, 833.
18. Choi, J.; Sauer, G.; Nielsch, K.; Wehrspohn, R. B.; Gösele, U. Chem. Mater.
2003, 15, 776.
19. Sun, Y. Chem. Mater. 2007, 19, 5845.
20. Qin, L. D.; Zou, S. L.; Xue, C.; Atkinson, A.; Schatz, G. C.; Mirkin, C. A. Proc.
Natl. Acad. Sci. U. S. A. 2006, 103, 13300.
21. Wang, Z. B.; Luk'yanchuk, B. S.; Guo, W.; Edwardson, S. P.; Whitehead, D. J.;
Li, L.; Liu, Z.; Watkins, K. G. J. Chem. Phys. 2008, 128, 094705.
22. Chen, C.; Hutchison, J. A.; Clemente, F.; Kox, R.; Uji-I, H.; Hofkens, J.; Lagae, L.; Maes, G.; Borghs, G.; Van Dorpe, P. Angew. Chem. Int. Ed. 2009, 48, 9932.
23. Prokes, S. M.; Glembocki, O. J.; Rendell, R. W.; Ancona, M. G. Appl. Phys. Lett.
2007, 90, 093105.
24. Prokes, S. M.; Alexson, D.; Glembocki, O. J.; Park, H. D.; Rendell, R. W. J. Vac.
Sci. Technol. B 2009, 27, 2055.
25. Nie, S.; Emory, S. R., Science 1997, 275, 1102.
26. Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L. T.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Phys. Rev. Lett. 1997, 78, 1667.
27. Wilson, R.; Bowden, S. A.; Parnell, J.; Cooper, J. M. Anal. Chem. 2010, 82, 2119.
28. Rao, S.; Raj, S.; Balint, S.; Fons, C. B.; Campoy, S.; Llagostera, M.; Petrov, D.
Appl. Phys. Lett. 2010, 96, 213701.
29. Wang, Y.; Lee, K.; Irudayaraj, J. J. Phys. Chem. C 2010, 114, 16122.
30. Michaels, A. M.; Jiang; Brus, L. J. Phys. Chem. B 2000, 104, 11965.
31. Bosnick, K. A.; Jiang, J.; Brus, L. E. J. Phys. Chem. B 2002, 106, 8096.
32. Futamata, M.; Maruyama, Y.; Ishikawa, M. Vib. Spectrosc 2004, 35, 121.
33. Futamata, M.; Maruyama, Y.; Ishikawa, M. J. Phys. Chem. B 2004, 108, 13119.
34. Sawai, Y.; Takimoto, B.; Nabika, H.; Ajito, K.; Murakoshi, K. J. Am. Chem. Soc.
2007, 129, 1658.
35. Lim, D.-K.; Jeon, K.-S.; Kim, H. M.; Nam, J.-M.; Suh, Y. D. Nat. Mater. 2010, 9, 60.
36. Hsia, C.-H.; Yen, M.-Y.; Lin, C.-C.; Chiu, H.-T.; Lee, C.-Y. J. Am. Chem. Soc.
2003, 125, 9940.
37. Wang, L. S.; Buchholz, D. B.; Li, Y.; Li, J.; Lee, C. Y.; Chiu, H. T.; Chang, R. P.
H. Appl. Phys. A 2007, 87, 1.
38. Huang, T.-K.; Cheng, T.-H.; Yen, M.-Y.; Hsiao, W.-H.; Wang, L.-S.; Chen, F.-R.;
Kai, J.-J.; Lee, C.-Y.; Chiu, H.-T. Langmuir 2007, 23, 5722.
39. Huang, T.-K.; Chen, Y.-C.; Ko, H.-C.; Huang, H.-W.; Wang, C.-H.; Lin, H.-K.;
Chen, F.-R.; Kai, J.-J.; Lee, C.-Y.; Chiu, H.-T. Langmuir 2008, 24, 5647.
40. Yang, Y.-C.; Huang, T.-K.; Chen, Y.-L.; Mevellec, J.-Y.; Lefrant, S.; Lee, C.-Y.;
Chiu, H.-T. J. Phys. Chem. C 2011, 115, 1932.
41. Sun, Y.; Gates, B.; Mayers, B.; Xia, Y. Nano Lett. 2002, 2, 165.
42. Moskovits, M. Rev. Mod. Phys. 1985, 57, 783.
43. Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2003, 107, 7426.
44. Alvarez-Puebla, R.; Cui, B.; Bravo-Vasquez, J.-P.; Veres, T.; Fenniri, H. J. Phys.
Chem. C 2007, 111, 6720.
45. Jackson, J. B.; Halas, N. J. Proc. Natl. Acad. Sci. U. S. A. 2004, 101, 17930.
46. Shim, S.; Stuart, C. M.; Mathies, R. A. ChemPhysChem 2008, 9, 697.
47. Hildebrandt, P.; Stockburger, M. J. Phys. Chem. 1984, 88, 5935.
48. Jensen, L.; Schatz, G. C. J. Phys. Chem. A 2006, 110, 5973.
49. Qiu, T.; Wu, X. L.; Shen, J. C.; Chu, P. K. Appl. Phys. Lett. 2006, 89, 131914.
50. Chen, H.; Wang, Y.; Qu, J.; Dong, S. J. Raman Spectrosc. 2007, 38, 1444.
51. Sakano, T.; Tanaka, Y.; Nishimura, R.; Nedyalkov, N. N.; Atanasov, P. A.; Saiki, T.; Obara, M. J. Phys. D: Appl. Phys. 2008, 41, 235304.
52. Duan, G. T.; Cai, W. P.; Luo, Y. Y.; Li, Z. G.; Li, Y. Appl. Phys. Lett. 2006, 89, 211905.
53. Zhou, Q.; Li, Z.; Yang, Y.; Zhang, Z. J. Phys. D: Appl. Phys. 2008, 41, 152007.
54. Galopin, E.; Barbillat, J.; Coffinier, Y.; Szunerits, S.; Patriarche, G.;
Boukherroub, R. ACS Appl. Mater. Interfaces 2009, 1, 1396.
55. Liu, Y.-C.; Yu, C.-C.; Sheu, S.-F. J. Mater. Chem. 2006, 16, 3546.
56. Gutes, A.; Carraro, C.; Maboudian, R. ACS Appl. Mater. Interfaces 2009, 1, 2551.
57. Liu, Y.-C.; Yu, C.-C.; Hsu, T.-C. Electrochem. Commun. 2007, 9, 639.
58. Cai, W.-B.; Wan, L.-J.; Noda, H.; Hibino, Y.; Ataka, K.; Osawa, M. Langmuir 1998, 14, 6992.
59. Ru, E. C. L.; Meyer, M.; Blackie, E.; Etchegoin, P. G. J. Raman Spectrosc. 2008, 39, 1127.
60. Chen, T.; Wang, H.; Chen, G.; Wang, Y.; Feng, Y.; Teo, W. S.; Wu, T.; Chen, H.
ACS Nano 2010, 4, 3087.
61. Le Ru, E. C.; Blackie, E.; Meyer, M.; Etchegoin, P. G. J. Phys. Chem. C 2007, 111, 13794.
62. Gao, L.; Fan, L.; Zhang, J. Langmuir 2009, 25, 11844.
63. Sun, X.; Lin, L.; Li, Z.; Zhang, Z.; Feng, J. Mater. Lett. 2009, 63, 2306.
64. Chen, H.; Simon, F.; Eychmüller, A. J. Phys. Chem. C 2010, 114, 4495.
65. Lv, S.; Suo, H.; Zhao, X.; Wang, C.; Jing, S.; Zhou, T.; Xu, Y.; Zhao, C. Solid State Commun. 2009, 149, 1755.
Chapter 4
Urchin-like Ag Nanowires as a Non-enzymatic Hydrogen Peroxide Sensor
4.1 Introduction
H2O2 is not only widely used in paper, cleaning product, and food industries,1 but also generated as a by-product in several enzyme-catalyzed reactions.2-5 A lot of analytic techniques have been developed to detect minute quantities of H2O2 such as titrimetry,6 spectrophotometry,7,8 chemiluminescence,9-11 and electrochemistry.12-14 Among them, electrochemical analysis has been considered as a low-cost and effective method due to its simplicity and high sensitivity. Until now, a great deal of H2O2 sensors have been developed based on electocatalysis of H2O2 reduction by immobilized enzymes.15-17 However, these electrodes showed many disadvantages related to their stability and activity degradations of immobilized enzymes.18,19 Therefore, there are more and more attempts to develop non-enzymatic sensors constructed from nanostructured materials. For example, in recent years, electrodes modified with metal nanoparticles (NPs) such as Pt NPs,20,21 Au NPs,22 Pd NPs23 and Ag NPs,24 have been extensively used for non-enzymatic H2O2 sensors. They usually showed large specific surface areas, excellent conductivities, and outstanding electrocatalytic activities. Because Ag is a relatively inexpensive noble metal of all, several kinds of Ag nanostructures have been fabricated for non-enzymatic H2O2
sensing applications.25-27 Previously, we have reported the non-enzymatic glucose sensors such as Cu nanobelt28 and Au nanocoral29 on screen-printed carbon (SPC) electrodes via several heterogeneous reactions. We discovered that surfactant-assisted
galvanic reductions provide low cost, one step, and near room temperature growth routes. By using this strategy, recently, we demonstrated the growth of urchin-like Ag NWs on screen-printed carbon (SPC) electrodes.30 Using the new electrode, we wish to report the first case of using Ag NWs for non-enzymatic H2O2 sensing. Our discoveries are discussed below.
4.2 Experimantal