Application of Silver-Coated Magnetic Microspheres to a SERS-Based

7 Mar 2011 - To immobilize the silver-coated microspheres, mini-solenoid bars have been ..... Wilson , R.; Bowden , S. A.; Parnell , J.; Cooper , J. M...
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Application of Silver-Coated Magnetic Microspheres to a SERS-Based Optofluidic Sensor Byunghee Han, Namhyun Choi, Ki Hyung Kim, Dong Woo Lim, and Jaebum Choo* Department of Bionano Engineering, Hanyang University, Ansan 426-791, South Korea ABSTRACT: Silver-coated magnetic microspheres (Fe3O4/Ag core/shell) were fabricated for use as a surface-enhanced Raman scattering (SERS) substrate. The surface morphology of the microspheres was characterized via field emission scanning electron microscopy and field emission transmission electron microscopy. Energy-dispersive spectroscopy was also employed to analyze the elemental composition of the structures. Malachite green was used as a model analyte to evaluate the performance of the microspheres as a SERS-active substrate and validate the SERS effect. The prepared microspheres possessed both magnetic and SERS properties. In addition, a microfluidic device with solenoids was designed and fabricated for a facile trace analysis. The solenoid chip allowed us to control the alignment of the magnetic microspheres on the wall of the microfluidic channel. Target samples were introduced into the channel and adsorbed on the surface of trapped silver-coated magnetic microspheres. SERS signals were then measured using a confocal Raman microscope. It is believed that a SERS-based optofluidic sensor with silver-coated magnetic microspheres can be successfully applied to microenvironmental analysis and other highly sensitive bioanalyses.

’ INTRODUCTION Surface-enhanced Raman scattering (SERS) has recently garnered a great deal of attention due to its powerful microanalytical capabilities.1-3 SERS can be used to obtain the molecular fingerprint information of an analyte with a high degree of sensitivity and low detection limits. When compared to conventional fluorescence detection techniques, SERS also possesses other intrinsic benefits, such as reduced photobleaching, narrow spectroscopic bands for detecting multiple species, and the ability to operate over a wide excitation wavelength range.4-6 SERS effects are generally observed in metal substrates that are very rough on a microscale.7,8 Thus, much effort has been devoted to the design and fabrication of highly sensitive and reproducible SERS-active metal substrates. Structures employed in previous studies include a rough electrode surface,9,10 aggregated nanoparticles,11,12 nanostructured metallic films,13,14 nanoparticle arrays,15-18 and metal nanoshells.19-21 Among the aforementioned material forms, metal nanoshells with a magnetic core and a metallic shell, such as Fe3O4/Au or Fe3O4/Ag, have received much attention due to their wide range of potential applications.22-25 Such structures can be easily magnetized by an external magnetic field and can simultaneously serve as an efficient SERS substrate. In the present work, silver-coated magnetic microspheres have been prepared by a two-step synthetic route. Gold nanoparticles (Au NPs) were first deposited on the surface of microsized magnetic spheres. The Au seeds on the magnetic spheres were then reduced by Ag ions and grown into a complete coating. Here, silver ions serve as a substitute for gold nanoparticles because the Raman enhancement factor of silver nanoparticles is 100-1000 times higher than that of Au NPs.26 Malachite green (MG) has been selected as a model compound to validate the SERS effect of the fabricated substrate. MG is a dye that has been employed as an r 2011 American Chemical Society

antiseptic in the aquaculture industry, but its use has been banned in some countries because it is suspected of being genotoxic and carcinogenic. Thus, SERS is a very effective technique to monitor trace amounts of MG in water.27 A microfluidic platform that uses the prepared silver-coated microspheres for the SERS-based microanalysis has been adopted in this study. Recently, SERS-based optofluidic sensors have been widely used for the highly sensitive analysis of many different types of chemical/biological targets.28-33 If a continuous flow and homogeneous mixing conditions between the nanoparticles and the analytes are maintained in a microfluidic channel, a highly accurate and reproducible analysis is possible.34,35 In the present work, silver-coated microspheres were used as SERS templates in a microfluidic channel. To immobilize the silver-coated microspheres, mini-solenoid bars have been designed and fabricated.36,37 By applying an electric field through the solenoid bars, magnetic fields for immobilizing the SERS templates were successfully generated. Consequently, the microspheres can be used as SERS templates for sequential biological/environmental assays. Using this solenoid chip, tedious manual handling of analyte samples can be avoided and all analytic procedures can be performed under flowing conditions. To the best of our knowledge, this is the first report about a core-shell microsphere-embedded microfluidic channel for a highly sensitive and reproducible SERS detection. Here, silvercoated magnetic microspheres, prepared by a simple two-step synthetic route, are expected to be an excellent SERS platform for the quantitative analysis of various analytes. Received: December 25, 2010 Revised: February 1, 2011 Published: March 07, 2011 6290

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Figure 1. Schematic illustration of the fabrication process for Fe3O4/Ag core/shell microspheres: (a) amine-modified Fe3O4, (b) Au NP-seeded Fe3O4, (c) Ag-coated Fe3O4, and (d) Fe3O4 microspheres attracted to the wall of vials by a bar magnet. FE-SEM images of three different Fe3O4 microspheres: (e) Fe3O4, (f) Fe3O4/Au, and (g) Fe3O4/Ag.

’ EXPERIMENTAL SECTION Materials. Trisodium citrate dehydrate (Na3 citrate), silver nitrate (AgNO3), ethanolamine (C2H7NO), gold(III) chloride trihydrate (HAuCl4), and malachite green (MG) were purchased from SigmaAldrich. Dynabeads M270 (Fe3O4 beads) with a diameter of 2.8 μm were used as magnetic microsphere templates. The surface of the magnetic microspheres was modified with amino functional groups using an Invitrogen kit. All materials were used as received. Fabrication of Fe3O4/Ag Core/Shell Structures. Au NPs (16 nm diameter) were prepared according to a method reported by Frens.38 A 50 mL portion of a 10-2% HAuCl4 solution in a volumetric flask was first heated. When the HAuCl4 solution started to boil, 1 mL of a 1% Na3 citrate solution was added, and heating was maintained until the solution changed to a reddish colloid. After the color change, heating was stopped. The diameter of the Au NPs was estimated via UV/vis spectroscopy and transmission electron microscopy (TEM). For the gold seeding, 200 μL of commercial Fe3O4 microspheres (1.7  10-13 M) were dispersed by ultrasonication and placed into a 1.7 mL microtube. The total volume was maintained at 1 mL by adding 400 μL of distilled water and 400 μL of the 16 nm gold colloidal solution. After 30 min of vigorous stirring, Fe3O4/Au microspheres were formed. To prepare the Fe3O4/Ag microspheres, 200 μL of Fe3O4/Au microspheres was placed in a microtube. A 30 μL portion of a 0.1 M AgNO3 solution and 725 μL of distilled water were then added into the microtube. After 45 μL of 0.1 M ethanolamine was introduced into the original

solution, the mixture was allowed to react in an ultrasonicator for 50 min at 50 °C. After this reaction, Fe3O4/Ag microspheres were formed. Fabrication of the Solenoid-Embedded Microfluidic Channel. A solenoid-embedded PDMS microfluidic channel was fabricated via soft lithography. A Sylgrad 184 PDMS prepolymer was thoroughly mixed with its curing agent at a ratio of 10:1 (w/w) and then degassed using a vacuum pump. A solenoid was then placed along the channel. The resulting mold was baked in a vacuum oven, and the structured PDMS layer was peeled off from the master. Inlet and outlet holes were punched in the structured replica so as to provide fluidic access. The replica was then bonded on a plasmatreated glass slide. The solenoid on the chip was composed of an iron wire core with a copper wire coiled around it. To fabricate the solenoid, an iron core with a diameter of 1 mm was shaped into a type of yoke. Copper wire with a diameter of 0.45 mm was then wrapped around the core about 30 times. The maximum magnetic force between the N pole and S pole of the solenoid was estimated to be approximately 32 mT. Here, the field intensity could be controlled by the power supply. The magnetic field generated by the solenoid is sufficient to trap the magnetic microspheres on the channel wall. SERS Detection. Two different types of Raman microscopes were used for the Raman measurements. Confocal Raman measurements for optimizing the Fe3O4/Ag process and a quantitative analysis of the MG were performed with a Renishaw 2000 Raman microscopy system. A Melles Griot He-Ne laser operating at λ = 632.8 nm with a power of 5 mW was employed as the excitation 6291

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Figure 2. (a) FE-TEM image and (b) EDS profile for Fe3O4/Ag core/shell microspheres. Mapping images for (c) Fe3O4, (d) Fe3O4/Au, and (e) Fe3O4/Ag.

source. The Rayleigh line was removed from the collected Raman signal using a holographic notch filter located in the collection path. Raman scattering was measured with a charge-coupled device (CCD) camera with a spectral resolution of 1 cm-1. An additional CCD camera was fitted to a microscope so that optical images could be obtained. SERS signals of the Fe3O4/Ag microspheres were collected after drying the aqueous samples on a glass slide. A 20 objective lens was used to focus a laser spot on the surface of a Fe3O4/Ag bead; the exposure time was 10 s. Here, the laser beam size was estimated to be 1 μm, and the SERS signals were collected on a different microsphere for each measurement. The SERS signals of the magnetic microspheres in a solenoid-embedded PDMS channel were measured with a home-built Raman microscopy system. This system consists of an inverted Olympus IX 70 microscope, a SpectraPro-2500i monochromator, a Princeton Pixis 400 CCD camera, and a Coherent Innova 70C ion laser. An Innova 70C Ar-Kr ion laser operating at λ = 647.1 nm with a power of 5 mW was used as the excitation source. A 600 g/mm grating was also employed; the spectral resolution was 1.2 cm-1.

’ RESULTS AND DISCUSSION A schematic of the fabrication process for the Fe3O4/Ag core/ shell microspheres is shown in Figure 1. Au NPs with a size of 16 nm were first deposited onto the surface of positive amine-

Figure 3. SERS spectra for 250 ppb MG adsorbed on (a) Fe3O4, (b) Fe3O4/Au, and (c) Fe3O4/Ag microspheres.

modified Fe3O4 microspheres by electrostatic interactions. For the amine-modified Fe3O4, the ζ potential was initially measured to be þ18.21 mV. This value changed to -6.28 mV after the gold nanoparticle seeds were deposited. When the surface was substituted with silver nanoparticles, the potential value 6292

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Figure 4. Concentration-dependent SERS spectra of MG adsorbed on Fe3O4/Ag microspheres. The MG concentration ranges from 10 to 350 ppb. The corresponding calibration curve is displayed in the inset. Error bars are the standard deviations from a total of three measurements.

dropped to -37.29 mV. This change means that a reduction reaction occurs when Au seeds are substituted by Ag ions. In Figure 1a-c, three different colors are used to denote aminemodified Fe3O4 (yellow), Fe3O4/Au (red), and Fe3O4/Ag (gray) core/shell structures. As shown in Figure 1d, the paramagnetic properties of all three microspheres are consistently maintained. The surface morphologies of the three different magnetic beads were analyzed by FE-TEM, FE-SEM, and EDS mapping. Shown in Figure 1e-g are FE-SEM images for Fe3O4, Fe3O4/Au, and Fe3O4/Ag core/shell structures, respectively. The magnetic microspheres in Figure 1e are composed of many small particles, and their surfaces are not smooth. The deposition of 16 nm Au NPs onto the surfaces of the magnetic microspheres can be confirmed in Figure 1f. The Fe3O4 microspheres in Figure 1g were coated with silver clusters; their average thickness was estimated to be 40 nm. The surface composition of each type of Fe3O4 core/shell was analyzed via FE-TEM imaging and EDS mapping; the results are shown in Figure 2. An FE-TEM image of a Fe3O4/Ag core/shell structure is displayed in Figure 2a. Unfortunately, quantitative data for the core/shell structures cannot be obtained with their FE-TEM images alone. Therefore, surface images of each type of microsphere were analyzed using EDS mapping data. Shown in Figure 2c-e are the EDS mapping images of Fe3O4, Fe3O4/Au, and Fe3O4/Ag core/shell structures, respectively; Fe, Au, and Ag are shown in blue, red, and green, respectively. The quantitative profile for a Fe3O4/Ag core/shell structure is displayed in Figure 2b. According to the EDS mapping analysis, the concentrations of Fe, Au, and Ag were found to be 29 020, 3 064, and 98 080, respectively. Such a result means that most of the magnetic microsphere surfaces have been covered with Ag through a successful reduction reaction. The thickness of the Ag was estimated to be about 40-50 nm.

To compare enhancement effects, the SERS spectrum of malachite green (MG) for each type of magnetic microsphere was measured and compared. Shown in Figure 3, spectra a-c, are the SERS spectra for 250 ppb MG adsorbed on the surface of Fe3O4, Fe3O4/Au, and Fe3O4/Ag, respectively. As evident in this figure, the SERS signals for Fe3O4/Ag are much stronger than those measured for Fe3O4/Au. As such, silver-coated magnetic microspheres are considered to be a very suitable SERS template. To validate the feasibility of this SERS template, a quantitative analysis of MG using Fe3O4/Ag microspheres was performed. Although the use of MG has been banned in several countries, it is still being used in many parts of the world due to its low cost, availability, and efficacy. Thus, it is very important to develop a highly sensitive detection system for monitoring trace amounts of MG in water. The SERS-based analysis of MG using Fe3O4/ Ag microspheres is a good candidate for such a detection system. For a quantitative analysis of the MG concentration in this study, individual stock solutions of MG from 10 to 350 ppb were prepared. For the SERS measurement, the individual stock solutions were mixed with Fe3O4/Ag microspheres in a microtube. A sample solution (5 μL) was then dropped on a glass slide and allowed to be dried for the Raman measurements. The SERS spectra were measured with a confocal Raman microscope equipped with a 632.8 nm wavelength laser; the laser power was approximately 5 mW. A 20 objective lens was used to focus the laser light on the sample, and the exposure time was 10 s. Shown in Figure 4 are the SERS spectra for different concentrations of MG; the corresponding calibration curve is displayed in the inset. The relative Raman intensity at 1615 cm-1 was monitored and used as a quantitative evaluation of the MG content. Here, the error bars are the standard deviations from a 6293

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Figure 5. (a) Layout and (b) image of a SERS-based optofluidic sensor integrated with a solenoid. (c) A microscopic image of trapped Fe3O4/Ag microspheres on the walls of the microfluidic channel.

Figure 6. Time-dependent SERS spectrum of MG adsorbed on trapped Fe3O4/Ag microspheres in a microfluidic channel. The detection time for the SERS measurement corresponds to (a) 1, (b) 5, (c) 7, (d) 10, (e) 13, (f) 15, (g) 16, and (h) 19 min. The calibration curve is displayed in the inset.

total of three measurements. A very good linear response was found in the concentration range of 10-350 ppb. The obtained data demonstrate that Fe3O4/Ag microspheres can be used for a fast and sensitive trace analysis of MG in water. In this case, the limit of detection (LOD) was determined to be 10 ppb and the correlation coefficient was 0.9912. Recently, a SERS-based microfluidic platform was extensively used for biological and environmental analysis. Such a platform was

employed because microfluidic systems allow for a SERS analysis to be performed in a continuous flow regime and homogeneous mixing conditions to be generated in a microfluidic channel. Consequently, a more reproducible quantitative analysis can be conducted under microfluidic conditions when performing SERS. The quantitative analysis of MG using the SERS-based microfluidic platform has been reported in previous studies.26,35 In the previous works, alligatorteeth-shaped PDMS channels and silver nanoparticles with a 6294

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The Journal of Physical Chemistry C diameter of 40 nm were used for the highly sensitive analysis of MG. The silver nanoparticles and the MG analytes were mixed together in a microfluidic channel, and their SERS signals were detected under flowing conditions. However, in many cases, a fixed substrate platform is needed to adsorb analyte molecules. For example, the immune analysis of specific antigens includes sequential assay steps, such as the immobilization of primary antibodies, washing, binding of target antigens, immobilization of secondary antibodies, and enzyme reactions for colorimetric detection. Thus, the trapping of microspheres on the channel wall is important so that they can be used as an effective detection substrate. To trap the microspheres, a mini-solenoid was embedded in the microfluidic channel. This solenoid generates a magnetic field that is sufficient to trap the Fe3O4/Ag microspheres. A schematic illustration of the integrated solenoid channel is shown in Figure 5a. The dimensions of the microfluidic channel are 2 cm  4 cm  0.7 cm (width  length  height). The flow rate was 0.5 μL/min, and the magnetic force applied to the solenoid bars was approximately 15 mT. The direction and intensity of the induced magnetic fields generated by the solenoid can be used to control the trapping arrangement of the magnetic microspheres. Fe3O4/Ag microspheres are first trapped on the wall of the channel and function as SERS templates. Target samples then flow through the channel and are adsorbed on the surface of the microspheres. Here, “hot spots” generated by the Fe3O4/Ag microspheres are detected by SERS. A selective valve system was used for the sequential introduction of the microspheres and target samples. A power supply was also employed to control the magnetic field generated by the yoke-type solenoid. The experimental setup for the Raman measurements on the Fe3O4/ Ag microspheres in the channel is shown in Figure 5b, whereas an actual image of the integrated solenoid channel and the Fe3O4/Ag microspheres trapped inside the channel by the magnetic fields is displayed in Figure 5c. The time-dependent Raman spectrum for 1 ppm MG adsorbed on the surface of Fe3O4/Ag is shown in Figure 6. The entire spectrum was measured for 20 min with 1 min intervals. The SERS intensity at 1615 cm-1 did not appear to change after 16 min; this is marked in the inset of Figure 6 with dotted lines. This observation confirms that the Fe3O4/Ag microspheres could be successfully used as an effective SERS template in a solenoid microfluidic channel. The complete adsorption time is estimated to be less than 20 min.

’ CONCLUSIONS Core/shell microspheres with a magnetic core and a silver shell are very interesting composite systems because they are easily magnetized by an external magnetic field and simultaneously serve as a SERS substrate. In this work, silver-coated magnetic microspheres were prepared for use as an effective SERS substrate. MG dye, a fungicide and antiseptic widely used in the aquaculture industry, was employed as a model compound to validate the SERS effect of the fabricated substrate. According to the obtained experimental data, Fe3O4/Ag microspheres are well suited for use as a SERS substrate and can be utilized for a fast and reproducible trace analysis. For MG, the LOD was estimated to be 10 ppb and the correlation coefficient was 0.9912. A novel SERS-based optofluidic sensor using Fe3O4/Ag microspheres was also developed. A mini-valve system to control the sequential flows and a mini-solenoid system to trap magnetic microspheres were integrated into the channel. To validate its potential applicability, a trace analysis of MG was performed.

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According to the time-dependent SERS data, the complete adsorption time is less than 20 min. Using this microfluidic sensor, tedious manual handling of analyte samples can be avoided and the total analysis time can be greatly reduced. We expect that this optofluidic sensor with Fe3O4/Ag microspheres as SERS substrates can be successfully used in immune analysis, including sequential binding steps. For example, we recently reported the immunoassay of a cancer marker, CEA, using the SERS-based detection and the sandwich immunocomplexes between hollow gold nanospheres and magnetic beads. If this assay is done using the currently designed SERS-based optofluidic sensor, it is expected to be a powerful clinical tool for the fast and sensitive medical diagnosis of a disease.39 In fact, research on a breast cancer marker detection system using this technology is currently underway.

’ AUTHOR INFORMATION Corresponding Author

*Phone: þ82-31-400-5201. Fax: þ82-31-436-8188. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (Grant Nos. R11-2009-044-1002-0 and K20904000004-09A0500-00410) and the National Cancer Center of Korea (Grant No. 0620400-1). This work was also partially supported by the Ministry of Knowledge Economy (MKE) and Korea Industrial Technology Foundation (KOTEF) through the Human Resource Training Project for Strategic Technology. ’ REFERENCES (1) Willets, K. A.; Van Duyne, R. P. Annu. Rev. Phys. Chem. 2007, 58, 267–297. (2) Yoon, I.; Kang, T.; Choi, W.; Kim, J.; Yoo, Y.; Joo, S. W.; Park, Q. H.; Ihee, H.; Kim, B. J. Am. Chem. Soc. 2009, 131, 758–762. (3) Zhang, P.; Guo, Y. J. Am. Chem. Soc. 2009, 131, 3808–3809. (4) Chan., S.; Kwon, T. W.; Koo, L. P.; Lee, L. P.; Berlin, A. A. Adv. Mater. 2003, 15, 1595–1598. (5) Doering, W. E.; Nie, S. Anal. Chem. 2003, 75, 6171–6176. (6) Mulvaney, S. P.; Musik, M. D.; Keating, C. D.; Natan, M. J. Langmuir 2003, 19, 4784–4790. (7) Yan, B.; Thubagere, A.; Premasiri, W. R.; Ziegler, L. D.; Negro, L. D.; Reinhard, B. M. ACS Nano 2009, 3, 1190–1202. (8) Wilson, R.; Bowden, S. A.; Parnell, J.; Cooper, J. M. Anal. Chem. 2010, 82, 2119–2123. (9) Abdelsalam, M. E.; Bartlett, P. N.; Baumberg, J. J.; Cintra, S.; Kelf, T. A.; Russel, A. E. Electrochem. Commun. 2005, 7, 741–744. (10) Gao, P.; Gosztola, D.; Leung, L. W. H.; Weaver, M. J. J. Electroanal. Chem. 1987, 233, 211–222. (11) Genov, D. A.; Shalaev, V. M.; Sarychev, A. K. Phys. Rev. B 2005, 72, 113102. (12) Lutz, B. R.; Dentinger, C. E.; Nguyen, L. N.; Sun, L.; Zhang, J.; Allen, A. N.; Chan, S.; Knudsen, B. S. ACS Nano 2008, 2, 2306–2314. (13) Dick, L. A.; McFarland, A. D.; Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2002, 106, 853–860. (14) Yu, Q. M.; Guan, P.; Qin, D.; Golden, G.; Wallace, P. M. Nano Lett. 2008, 8, 1923–1928. (15) Felidj, N.; Laurent, G.; Aubrad, J.; Levi, G.; Hohenau, A.; Krenn, J. R.; Aussenegg, F. R. J. Chem. Phys. 2005, 123, 221103. (16) Grand, J.; de la Chapelle, M. L.; Bijeon, J. L.; Adam, P. M.; Vial, A.; Royer, P. Phys. Rev. B 2005, 72, 033407. 6295

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