ARTICLE pubs.acs.org/Langmuir
Reductive-Oxidation Electrogenerated Chemiluminescence (ECL) Generation at a Transparent Silver Nanowire Electrode Yan Zhu,† Caleb M. Hill, and Shanlin Pan* The University of Alabama, Department of Chemistry, Tuscaloosa, Alabama 35487-0336, United States
bS Supporting Information ABSTRACT: We present the fabrication of a conductive, transparent electrode composed of Ag nanowires (NW) for spectroelectrochemical studies. Reductive-oxidation electrogenerated chemiluminescence (ECL) of Ru(bpy)32þ is generated at the Ag NW electrode in the presence of hydrogen peroxide and collected through the new transparent electrode. The ECL performance at the new nanostructured electrode is compared with several other electrodes, including bulk silver wire, glassy carbon disk, and thermally reduced transparent graphene oxide (tr-GO) electrodes. The Ag NW electrode is found to be the best electrode for the reductiveoxidation ECL generation because of its catalytic properties with respect to the reduction of hydrogen peroxide and its high surface area.
1. INTRODUCTION The application of nanoelectrodes (NE) has been exploited in many areas because the physical and chemical properties of nanostructured electrodes often differ dramatically from those of the corresponding bulk materials. For example, nanostructured electrodes have a high surface area, which can allow for fast mass transfer of redox species.1 Such enhanced mass transfer makes the study of faster electrochemical processes possible because mass transport is less of a limiting factor. A nanostructured electrode can also promote surface-catalyzed processes, which result in larger current densities with a lower overpotential compared to a bulk electrode.2-5 This allows one to achieve lower detection limits for quantitative analysis.6 Besides the enhanced catalytic activities and rapid mass transfer characteristics, optical characteristics of nanostructured electrodes such as light transparency and localization of incident electromagnetic field7 can be also be exploited for device fabrication or spectroscopic studies. For example, single-walled carbon nanotubes (CNT),8 graphene nanosheets,9,10 metal nanowires (NW),11-13 and hybrids of these nanomaterials have recently been used to fabricate low-cost transparent electrode applications.14-16 These electrodes can potentially replace the expensive indium-tin-oxide (ITO) electrodes that are widely used in optoelectronics, such as organic solar cells17 and lightemitting diodes.18 Random networks formed by metallic NWs are a promising replacement of ITO due to their good conductivity and optical transmittance.19 Yi Cui and co-workers have demonstrated the initial performance of transparent Ag NW electrodes on glass substrates and their use in organic in solar cells.7 They synthesized high aspect ratio Ag NWs with controlled size and conductivity, which led to large performance improvements for Ag NW based transparent electrodes.20 Electrochemical properties of this type of transparent working electrode have not yet been studied to r 2011 American Chemical Society
demonstrate its stability and the effect on an electrochemical reaction in an electrochemical cell. Our group recently used combined methods of electrochemistry and optical spectroscopy to understand the electrochemistry of single Ag NW electrode at the nanometer scale.21 We have demonstrated previously that photoluminescence of Ag NWs and electrochemistry can be used to understand their structural heterogeneities and photochemical reaction on their surface at nanometer scale. In this report, we present electrochemical performances of transparent Ag NW electrode, and demonstrate that Ag NWs can be used as a stable transparent electrode for the generation of electrogenerated chemiluminescence (ECL) in a reductive-oxidation scheme. ECL refers to the generation of chemiluminescence from an electrochemical reaction.22 These reactions frequently involve simultaneous electrogeneration of oxidized and reduced reactants, where the electron-transfer reactions between them generate excited states which relax radiatively. ECL has now become a powerful analytical technique.23-26 The applications of ECL include light emitting diodes and ultrasensitive biosensors.27 A widely used system for ECL is the Ru(bpy)32þ/tripropylamine (TPrA) system, in which Ru(bpy)32þ is the emissive species and TPrA is the coreactant.28 Recently, it was reported that hydrogen peroxide (H2O2) can be used as a coreactant to generate ECL of Ru(bpy)32þ in an aqueous solution via reductive-oxidation.29 Here, we study the ECL activity of Ru(bpy)32þ in the presence of H2O2 in order to evaluate the stability of a novel Ag NW transparent electrode for electrochemical studies. In addition, the catalytic activities of Ag NW for ECL generation and electrochemical characteristics are demonstrated. Received: September 4, 2010 Revised: January 14, 2011 Published: February 21, 2011 3121
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Scheme 1. Schematic of the Procedures Used for Ag NW Electrode Fabrication
2. EXPERIMENTAL SECTION 2.1. Ag NW Preparation. A 5 mL aliquot of ethylene glycol (EG) was heated in a silicon oil bath at 160 °C for 15 min under magnetic stirring (300 rpm). Then, 40 μL of a 4 mM CuCl2 3 2H2O/EG solution was then added, and the solution was heated for another 15 min. Then, 1.5 mL of 114 mM PVP/EG was then added to the vial, followed by the addition of 1.5 mL of 94 mM AgNO3/ EG solution. The solution became gray and wispy in an hour, and then the reaction was cooled to room temperature and kept for another 2 h. The product was washed twice with acetone and three times with deionized (DI) water and subsequently stored in 2-propanol. 2.2. Ag NW Electrode Fabrication. As shown in Scheme 1, a transparent Ag NW electrode was prepared by spraying the 2propanol containing Ag NWs onto a 1.5 cm 1.5 cm bare glassy substrate surface with a gap formed by two strips of ITO on top. The strips of ITO serve as physical contacts to the Ag NWs to ensure good connection to Ag NWs. Meanwhile, this design allowed us to check the conductivity of the Ag NW film during the deposition process. After deposition of the conductive Ag NW film, Ag NWs on one of the ITO strips were removed by simply using Scotch-brand tape. This allows for electrical connections to Ag NWs indirectly through one of the ITO electrodes for electrochemical studies without exposing the ITO directly in electrolytes. The two ITO strips were prepared by selectively etching an ITO glass substrate in concentrated hydrochloric acid. Ag NWs in 2-propanol were sprayed on the treated substrate by using an airbrush gunpiece (FUSO SEIKI CO., Ltd., Japan). 2.3. Thermally Reduced Graphene Oxide (tr-GO) Transparent Electrode Fabrication. tr-GO electrodes were prepared by using an aqueous solution of GO nanosheets. GO was prepared by a modified Hummer’s method.30 Briefly, graphite of 45 μm particle size is preoxidized with several strong oxidizing agents. The final product was washed, vacuum filtered, and then subjected to dialysis in order to remove ions and prevent aggregation of the final graphene oxide product after the final stages of synthesis. The GO solution was then subjected to ultrasonication with the 400 V Brandson digital sonifer for 30 min at 30% amplitude. After sonication, the graphene oxide was centrifuged for 1 h at 3200 rpm. This process of sonication followed by centrifugation was repeated up to five times to ensure the smallest and thinnest GO sheets.31 To prepare the transparent tr-GO electrode, a square quartz substrate was used as the substrate base for the electrode. The 5% GO solution was spray-gunned using an airbrush gunpiece (Fuso Seiko Co. Ltd.) onto the surface while heating the quartz on a hot plate. The coated quartz substrate was placed into a tube furnace (Barnstead International type F21100) and heated under nitrogen to 1000 °C with a ramp rate of 5 °C/min. Tr-GO prepared using this method exhibits good stability and transparency as well as good conductivity.
Figure 1. Typical AFM image of Ag NW electrode deposited on a glass substrate.
2.4. Cyclic Voltammetry and ECL Measurements. A conventional three-electrode electrochemical cell was used for the electrochemistry and ECL experiments. Ag NW, bulk Ag wire, glassy C, and tr-GO electrodes were employed as working electrodes. Pt wire and Ag/ AgCl were used as counter and reference electrodes, respectively. Cyclic voltammograms (CVs) were obtained with a CHI760 electrochemical workstation (CH Instruments, Austin, TX). All solutions used were purged with nitrogen gas for 20 min to remove oxygen. To obtain a simultaneous CV and integrated ECL signal, a photomultiplier tube (PMT, Hamamatsu C4220) was used, with -750 V supplied to the PMT with a high-voltage power supply (series 225 Bertan High Voltage Co., Hicksville, NY). ECL signals were produced by scanning potentials from 0 V to a potential sufficiently negative to reduce both H2O2 and Ru(bpy)32þ. The ECL was also produced by stepping the potential from 0 to -1.5 V. A home-built electrochemical cell was used in these experiments as shown in Supporting Information (Figure 1S). The counter electrode was shielded in a separate compartment to avoid detection of ECL that might be generated at the counter electrode during the ECL generation at the working electrode. SEM images of the Ag nanowires were obtained with a JEOL-7000 scanning electron microscope (SEM), and a Digital Instruments Nanoscope IV atomic force microscope (AFM) was used to obtain AFM images of the Ag NW electrode. A FEI 200 kV transmission electron microscope (TEM) equipped with a charge-coupled device (CCD) camera for energy dispersive X-ray (EDX) detection. EDX was used for high resolution imaging of Ag NWs.
’ RESULTS AND DISCUSSION 3.1. SEM Characterization of Ag NWs. Figure 1 shows a typical AFM image of the Ag NWs used for Ag electrode fabrication. . SEM measurement (Supporting Information, Figure 2S) that the obtained Ag NWs have a length in the range of 1 to 10 μm, with uniform diameters of over 50 nm produced from batch to batch. AFM image of the Ag NW electrode indicate the Ag NW electrode contains abundant Ag NWs overlapped in a random orientation. The electrode surface has a Z range of about 1 μm as shown by the AFM images. The obtained Ag NWs were not purified. Therefore, there is certain amount of Ag nanoparticles present as shown in the AFM and SEM images. Further purification 3122
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Figure 2. Transmittance spectra of a typical Ag NW thin film electrode. Figure 4. CVs of Ag NW coated glassy carbon electrode in 0.2 M Na2SO4. Ag NWs were dispersed in 0.3% Nafion solution prior to being spin-coated and dried on a 3 mm (in diam.) glassy carbon electrode. Scan rate: 100 mV/s. Counter electrode: Pt. Initial potential for all CVs is -0.08 V (vs Ag/AgCl). The first segments of all CVs are scanned in negative direction to -0.35 V. The high potential was extended from 0.12 V to 0.15, 0.2, 0.3, 0.4, and 0.5 V for each CV in order to resolve the effect of forced oxidation of silver NWs on their reduction peak.
Figure 3. Cyclic voltammetry of (a) bulk Ag electrode, (b) Ag NW electrode, and ITO thin film electrode coated with Ag NWs in 0.5 M NaOH.
of these nanowires will help form more reliable contact between individual wires to form excellent network for charge transport. 3.2. Transmittance Spectrum of the Ag NW Electrode. Figure 2 presents the visible transmittance spectrum of Ag NW electrode. Absorption spectrum of Ag NW solution in 2-propanol is shown in Supporting Information (Figure S3). The Ag NWs exhibited absorption peaks around 450 nm and another extending into the near-infrared range; these are attributed to the major surface Plasmon resonance of silver in visible range32 and strong Plasmon coupling and overlapping of NWs.33 Such strong interaction between NWs with visible light is extremely important for ultrasensitive detection of trace amount of molecule by using Raman spectroscopy.34 This is another application of Ag NW transparent electrode we made in addition to the spectroelectrochemical applicationsin this report. 3.3. Voltammetric Response of the Ag NW Electrode in NaOH. Figure 3 shows typical CVs of Ag NW electrodes and a bulk Ag electrode in 0.5 M NaOH. The anodic current response was characterized by two major anodic current peaks (A1, A2). The reverse potential scan exhibited two cathodic peaks (C1, C2). The first small anodic peak (A1) can be assigned to the oxidation of Ag to Ag2O and the second anodic current peak (A2) is attributed to the oxidation of Ag2O and the formation of AgO.35 The cathodic peak C1 is attributed to the electro-reduction of AgO to Ag2O, the more negative cathodic peaks C2 is related to processes involved in electroreduction of Ag2O species to Ag. In comparison to bulk the Ag electrode, the Ag NW electrode has much broader anodization and reduction peaks. The peak potentials are shifted in more positive direction for anodization and more negative direction for reduction. The randomly distributed NWs are interconnected to form network for electrical current flow but the nature of the contact between the
individual NWs is not clear. The increased overpotential could be attributed to (1) a silver oxide layer formed in air on the surface of the Ag NWs that may limit charge transfer in the NW network and (2) the loose contacts of NWs to each other in the presence of solvent since there is no electrode binders present in the NW electrode to form rigid network of Ag NWs (this will inhibit the charge transfer through the NW film in some areas of the electrode when solvent diffuses into the NW network), and (3) the presence of silver nanoparticles that can increase the film roughness resulting poor conductivity of the Ag NW network. To show the effect on surface oxide layer of Ag NWs on the film conductivity, we first used electrochemical method to detect the presence of oxide layer but this attempt was not successful as shown in Figure 4. Surface oxide silver layer was expected to be reduced to generate cathodic current as we scan the electrode potential from its rest potential to negative direction while keeping the inside silver layer intact. However, we found no cathodic current presence in Figure 4. This indicates that electrochemical approach is not sensitive enough to detect the trace amount of surface oxide layer should it be present or there is no surface oxide layer. We then applied a high potential than the rest potential to purposely coat the Ag NWs with oxide layer by anodizing them. As shown in Figure 4, at different extend of the anodization, the formed oxide layer can be reduced back to silver as indicated by the sharp cathodic current around 0.05 V (vs Ag/AgCl). Such forced oxidation of Ag NW can form much thick oxide layer on Ag NWs and could completely convert Ag NWs to silver oxide in an electrochemical cell. In addition to the electrochemical characterization, we used high resolution TEM to characterize the surface oxide layer of individual Ag NWs under vacuum condition. As shown in parts A and B of Figure 5, we are able to locate high quality Ag NWs and image single Ag NW with high resolution. EDX spectrum at the nanometer scale on a single NW was obtained by focusing the electron beam at a center spot and an edge spot as shown in Figure 5B. Both spots on the single Ag NW show the presence of only a trace amount of oxygen as shown in Figure 5, parts C and D. Therefore, we conclude from both TEM and electrochemical 3123
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Figure 5. TEM and EDX characterization of Ag NWs used for transparent electrode fabrication. A is a typical TEM image of several Ag NWs. B is a high resolution TEM of a single Ag NW used for surface chemical analysis. C and D are EDX spectra of two different spots 1 and 2, respectively, on single Ag NW as shown in Figure B.
experiment that such trace amount of surface oxide layer present on the surface of Ag NWs will not cause much change in the overpotential. The main source of the high overpotential should be from the loose contact between NWs in the presence of solvent. As shown by the TEM and AFM measurements, Ag NWs used to fabricate transparent electrode contains certain amount of Ag nanoparticles that will also increase the roughness of the electrode and might cause dead ends in the Ag NW network. Therefore, the presence of nanoparticles will also contribute to the overpotential as well. This can be further avoided by purifying the silver NWs and having a better control on the NW length distribution. 3.4. Double Layer Capacity of Ag NW Electrode. As shown in Figure 6A, we measured the double layer capacity of the new Ag NW electrode by CV performed in the presence of a PBS buffer (pH = 7.4). The current density has a platform in the potential range of 0.1 to 0.3 V and is found to be proportional to the scan rate. The current density was plotted as a function of scan rate (Figure 6B) and a slope of 2.89 10-4 F/cm2 was obtained, which is the double layer specific capacity of the Ag NW electrode in PBS buffer (pH = 7.4). This number is almost two orders magnitude higher than an ideal planar electrode; this discrepancy is found because the real surface area of our Ag NW
electrode was not used for this calculation. This Ag NW electrode has excellent electrochemical capacitive characteristics according to these measurements. The high specific capacitance results from the microstructure of the NW electrode, which exhibits facile electrolyte penetration, fast proton exchange, and excellent conductivity.36 3.5. Catalytic Reduction of H2O2 at the Ag NW Electrode. Prior to the ECL study, we investigated the electrochemical activities of H2O2 at different working electrodes, including Ag NW, bulk Ag, glassy carbon disk, and tr-GO thin film electrodes by CV. As shown in Figure 7, Both the Ag NW electrode and the bulk Ag electrode have much higher reduction current densities and lower overpotentials than the tr-GO and glassy carbon disk electrodes. In this process, decomposition of H2O2 occurs after accepting one electron from the electrode, resulting in the formation of a hydroxyl radical. The reaction can be represented as follows: H2 O2 þ e- f OH• þ OHHowever, the observed turn-on potential of this reaction changed from one electrode to another because the reaction rate is kinetically controlled. The reduction reaction of H2O2 appears to be kinetically favorable at Ag surface because both Ag 3124
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Figure 6. (A) CVs of Ag NW electrode in PBS buffer (pH = 7.4) with a scan rate of 25, 50, 100, 200, 500, and 1000 mV/s. (B) Linear relationship between scan rate and current density at 0.2 V (vs Ag/AgCl) for calculating double layer capacity C of the Ag NW electrode. C (2.89 10-4 F/cm-2) is obtained using Ag NW electrode’s geometric area instead of real surface area.
Figure 7. CVs of Ag NW electrode (1), bulk Ag (2), glassy carbon disk (3), and tr-GO thin film electrode (4) in 1 mM H2O2 of PBS buffer (pH = 7.4). Inset is the enlarging CV of glassy carbon electrode (3).
electrodes have most positive threshold potential in comparison to tr-GO and glassy carbon disk electrodes. These results show that the Ag NW electrode possessed the relatively remarkable catalytic ability to H2O2 reduction comparing with the other electrodes. The difference in the current density between silver NW electrode and Ag bulk electrode is due to the different surface area. 3.6. Reductive-Oxidation ECL of Ru(bpy)32þ with H2O2. Figure 8 shows the CVs and corresponding ECL response of 3 mM Ru(bpy)32þ in the presence of 1 mM H2O2 in pH 7.4 PBS for different electrodes. Similar to Figure 7, H2O2 starts to be reduced at -0.2 V for silver electrodes as shown in Figure 8A. ECL was not observed at this potential until the electrode potential approached or exceeded -1.25 V, where Ru(bpy)32þ is reduced. This demonstrates that Ru(bpy)3þ is required for ECL generation. The presence of H2O2 and its reduction at low potential range is important for efficient ECL generation. A suggested mechanism for such reductiveoxidation ECL process is shown in Supporting Information (Figure S4). First, Ru(bpy)32þ is reduced to form Ru(bpy)3þ while H2O2 is reduced to form OH•. The produced OH• was a sufficiently strong oxidizing agent to oxidize Ru(bpy)3þ to form excited state of Ru(bpy)32þ, Ru(bpy)32þ*, which emits light though radiative decay process to ground state. At the same time, the generated OH 3 could oxidize Ru(bpy)32þ to Ru(bpy)33þ. The annihilation of Ru(bpy)3þ and Ru(bpy)33þ produces an excited state of Ru(bpy)32þ, Ru(bpy)32þ*. It is not clear what the relative rates of
Figure 8. CV (A) and corresponding ECL (B) of 3 mM Ru(bpy)32þ in the presence of 1 mM H2O2 in pH 7.4 PBS at Ag NW electrode (1), bulk Ag (2), glassy carbon disk (3), and tr-GO (4) thin film electrode.
these two light emitting processes are. This could be understood by controlling the relative concentration of these redox species including the initial concentration of Ru(bpy)33þ in order to fully understand the kinetics of the light emitting process. Digital simulation can also be used to fit the experimental results to help understand the electrochemical mechanism. As shown in Figure 8 B, ECL starts to be collected at a more positive potential for both the Ag NW and bulk silver electrodes than for the tr-GO and glassy carbon disk electrodes, because they are kinetically favorable for H2O2 reduction (cf. Figure 7). The bulk Ag electrode shows only unstable ECL intensity than the Ag NW electrode when the potential scan was reversed. This is because mass transfer process for redox species near a bulk electrode is slow in the presence of the large concentration gradient of redox species and coreactant. The tr-GO electrode has much better ECL performance and higher H2O2 reduction rate, as shown by Figure 8 and 7, respectively, than the glassy carbon disk electrode; this can be explained by the strong interaction 3125
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’ ASSOCIATED CONTENT
bS
Supporting Information. Depiction of the additional electrochemical cell used for ECL measurements, SEM, absorption spectrum, and ECL data. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Present Addresses †
Current address: School of Chemical Engineering and Technology, Wuhan University of Technology, Wuhan, 430070, China.
Figure 9. Current (A) and ECL (B) responses of Ag NW electrode (1), bulk Ag (2), glassy carbon disk (3), and tr-GO thin film electrode (4) as the electrode potential was stepped from 0 V to -2.0 V (vs Ag/AgCl) in 3 mM Ru(bpy)32þ in the presence of 1 mM H2O2 in PBS (pH 7.4) . The multiple potential steps have a width of 1 s.
’ ACKNOWLEDGMENT This material is based upon work supported by the Department of Energy under Award Number (s) DE-SC0005392. This work is partially supported by the University of Alabama 2010 RGC award. We acknowledge D. Clayton for assistant with SEM imaging. We also thank R. Martens and J. Goodwin of the CAF of the University of Alabama for assisting with the TEM and EDX measurements. Z.Y. is grateful to the China Scholarship Council (CSC) for financial support of her work at the University of Alabama. C.H. acknowledges the GAANN fellowship support of the Department of Education (P200A100190) for his work. ’ REFERENCES
between redox species with the surface functional groups on the GO and the enhanced conductivity due to the thermal treatment. Detailed understanding of this enhancement is in progress. To test the ECL stability and dependence of different potential wave forms, we studied the step potential response of the reductive-oxidation ECL. As shown in Figure 9, we stepped the electrode potential from 0 to -2.0 V per 1 s and both current and ECL responses were recorded for the four different kinds of electrodes. Similar to the results obtained from CV as shown in Figure 8, the two Ag electrodes show the most current and ECL. The Ag NW current decays rapidly as shown in Figure 8A; this is because the mass transfer near a nanostructured electrode is rapid. We only see small current for trGO because H2O2 is less favorable at a carbon surface in comparison to Ag. The Ag NW, bulk Ag and tr-GO electrodes show intense ECL at -2.0 V, where the most intense ECL was observed in Figure 8.
4. CONCLUSIONS A transparent Ag NW electrode was fabricated for spectroelectrochemical studies such as ECL generation. It was found that ECL can be collected through the new transparent electrode. In comparison to thermally reduced graphene oxide (tr-GO), bulk silver wire, and glassy carbon electrodes, the Ag NW electrode is found to be the best electrode for ECL generation in the reductive-oxidation scheme because of its catalytic activity and high surface area. The new Ag NW electrode is transparent and able to catalyze certain electrochemical reactions, plus the Ag NW network could support strong local field enhancement for Raman spectroscopy, therefore it can potentially used for spectroelectrochemical studies and ultrasensitive optical and electrochemical sensing applications.
(1) Hirota, K.; Tajima, K.; Hashimoto, K. Langmuir 2005, 21, 11592–11595. (2) Streeter, I.; Baron, R.; Compton, R. G. J. Phys. Chem. C 2007, 111, 17008–17015. (3) Sljukic, B.; Baron, R.; Salter, C.; Crossley, A.; Compton, R. G. Anal. Chim. Acta 2007, 590, 67–74. (4) Genies, L.; Faure, R.; Durand, R. Electrochim. Acta 1998, 44, 1317–1326. (5) Hughes, M. D.; Xu, Y.; Jenkins, P.; McMorn, P.; Landon, P.; Enache, D. I.; Carley, A. F.; Attard, G.; Hutchings, G. J.; King, F.; Stitt, E. H.; Johnston, P.; Griffin, K.; Kiely, C. J. Nature 2005, 437, 1132–1141. (6) Arrigan, D. W. M. Analyst 2004, 129, 1157–1165. (7) Mohanty, P.; Yoon, I.; Kang, T.; Seo, K.; Varadwaj, K. S. K.; Choi, W.; Park, Q.; Ahn, J.; Suh, Y. D.; Ihee, H.; Kim, B. J. Am. Chem. Soc. 2007, 129, 9576–9577. (8) Hu, L.; Hecht, D. S.; Gruner, G. Nano Lett. 2004, 4, 2513–2517. (9) Becerril, H. A.; Mao, J.; Liu, Z.; Stoltenberg, R. M.; Bao, Z.; Chen, Y. ACS Nano 2008, 2, 463–470. (10) Eda, G.; Lin, Y. Y.; Miller, S.; Chen, C. W.; Su, W. F.; Chhowalla, M. App. Phys. Lett. 2008, 92, 233305–233310. (11) Ju, S.; Li, J. F.; Liu, J.; Chen, P, C.; Ha, Y. G.; Ishikawa, F.; Chang, H.; Zhou, C. W.; Faccetti, A.; Janes, D. B.; Marks, T. J. Nano Lett. 2008, 8, 997–1004. (12) Lee, J. Y.; Connor, S. T.; Cui, Y. Nano Lett. 2008, 689–692. (c) De, S.; Higgins, T. M.; Lyons, P. E.; Doherty, E. M.; Nirmalraj, P. N.; Blau, W. J.; Boland, J. J.; Coleman, J. N. ACS Nano 2009, 3, 1767–1774. (13) De, S.; Higgins, T. M.; Lyons, P. E.; Doherty, E. M.; Nirmalraj, P. N.; Blau, W. J.; Boland, J. J.; Coleman, J. N. ACS Nano 2009, 3, 1767–1774. (14) Hong, W. J.; Xu, Y. X.; Lu, G. W.; Li, C.; Shi, G. Q. Electrochem. Commun. 2008, 10, 1555–1558. (15) Tung, V. C.; Allen, M. J.; Yang, Y.; Kaner, R. B. Nat. Nanotechnol. 2009, 4, 25–29. (16) Tung, V. C.; Chen, L. M.; Allen, M. J.; Wassei, J. K.; Nelson, K.; Kaner, R. B.; Yang, Y. Nano Lett. 2009, 9, 1949–1955. (17) Hoppe, H.; Sariciftci, N. S. J. Mater. Chem. 2006, 16, 45–61. 3126
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(18) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913–915. (19) Azulai, D.; Belenkova, T.; Gilon, H.; Barkay, Z.; Markovich, G. Nano Lett. 2009, 9, 4246–4249. (20) Hu, L. B.; Kim, H. S.; Lee, J. Y.; Peumans, P.; Cui, Y. ACS Nano 2010, 4, 2955–2963. (21) Clayton, D. A.; Benoist, D. M.; Zhu, Y.; Pan, S. L. ACS Nano 2010, 4, 2363–2373. (22) Miao, W. J. Chem. Rev. 2008, 108, 2506–2553. (23) Knight, A. W. In Chemiluminescence in Analytical Chemistry; Grarcia-Campana, A. M., Baeyens, W. R. G., Eds.; Marcel Dekker: New York, 2001; pp 211. (24) Richter, M. M. In Optical Biosensors: Present and Future; Ligler, F. S., Taitt, C. A. R., Eds.; Elsevier: New York, 2002. (25) Knight, A. W. Trends Anal. Chem. 1999, 18, 47–53. (26) Andersson, A. M.; Schmehl, R. H. Mol. Supramol. Photochem. 2001, 7, 153–158. (27) Miao, W. J.; Bard, A. J. Anal. Chem. 2004, 76, 7109–7113. (28) Zu, Y.; Bard, A. J. Anal. Chem. 2000, 72, 3223–3231. (29) Choi, J.-P.; Bard, A. J. Anal. Chim. Acta 2005, 541, 143–150. (30) Hummers, W. S.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, 1339. (31) Kovtyukhova, N. I.; Patricia, J. O.; Benjamin, R. M.; Thomas, E. M.; Sergey, A. C.; Eugenia, V. B.; Alexandr, D. G. Chem. Mater. 1999, 11, 771–778. (32) An, C. H.; Peng, S.; Sun, Y. G. Adv. Mater. 2010, 22, 2570–2574. (33) Zhang, D. B.; Qi, L. M.; Yang, J. H.; Ma, J. M.; Cheng, H. M.; Huang, L. Chem. Mater. 2004, 16, 872–876. (34) Fang, Y.; Wei, H.; Hao, F.; Nordlander, P.; Xu, H. Nano Lett. 2009, 9, 2049–2053. (35) Hoar, T. P.; Dyer, C. K. Electrochim. Acta 1972, 17, 1563–1571. (36) Hu, C. C.; Chang, K. H.; Lin, M. C. Nano Lett. 2006, 6, 290–294.
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