pubs.acs.org/Langmuir © 2010 American Chemical Society
Surface Functionalization of Zinc Oxide by Carboxyalkylphosphonic Acid Self-Assembled Monolayers Beibei Zhang,† Tao Kong,† Wenzhi Xu,† Ruigong Su,† Yunhua Gao,*,‡ and Guosheng Cheng*,† †
Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, Jiangsu 215125, PR China, and ‡Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China Received August 10, 2009. Revised Manuscript Received January 20, 2010 Two carboxyalkylphosphonic acids (HOOC(CH2)nP(O)(OH)2, n = 2 for 3-PPA and n = 9 for 10-PDA) have been deposited onto 1D zinc oxide (ZnO) nanowires and bare ZnO wafers to form stable self-assembled monolayers (SAMs). The samples were systematically characterized using wettability, atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). 3-PPA was bound to the ZnO surfaces mainly through the CO2H headgroup, and 10-PDA formed self-assembled monolayers on the nanoscaled ZnO surface through the PO3H2 headgroups. To verify the potential utilization of the functionalized surfaces in the construction of biosensors or bioelectronics, IgG (immunoglobulin G) protein immobilization through SAM bridging was demonstrated. This work expands the application of phosphonic acid-based surface functionalization on sensing and optoelectronic devices.
Introduction Field-effect transistor (FET) sensors with the conductance depending upon binding detecting molecules (the chemical gating effect) demonstrated a real-time, robust, complementary metal oxide semiconductor (CMOS)-compatible detection system for electrically based sensing.1 Much effort has been expended in the international nanotechnology community to address the challenges of the sensing system in terms of sensitivity, selectivity, stability, and compatibility. Because of a very large surface-tovolume ratio, nanoscaled structures are believed to improve the sensitivity and/or selectivity of 1D FET sensors dramatically. For example, nanowire-based gas sensors with high sensitivity have been fabricated to detect extremely low concentrations of target gases, such as environmental polluting species CO, NO2, and ethanol gas.2,3 However, the application of gas sensors is inevitably limited by their poor selectivity. To improve and tailor the selectivity of nanoscaled FET sensors, surface functionalization is critical. SAMs have been successfully used as a powerful method of surface functionalization to form a stable, flexible, convertible surface for sensing, corrosion resistance, nanopatterning, and *To whom correspondence should be addressed. E-mail:
[email protected]. ac.cn,
[email protected]. (1) (a) Patolsky, F.; Lieber, C. M. Mater. Today 2005, 8, 20. (b) Cui, Y.; Wei, Q. Q.; Park, H. K.; Lieber, C. M. Science 2001, 293, 1289. (c) Stern, E.; Klemic, J. F.; Routenberg, D. A.; Wyrembak, P. N.; Turner-Evans, D. B.; Hamilton, A. D.; LaVan, D. A.; Fahmy, T. M.; Reed, M. A. Nature 2007, 445, 519. (d) Li, C.; Curreli, M.; Lin, H.; Lei, B.; Ishikawa, F. N.; Datar, R.; Cote, R. J.; Thompson, M. E.; Zhou, C. W. J. Am. Chem. Soc. 2005, 127, 12484. (2) Wan, Q.; Li, Q. H.; Chen, Y. J.; Wang, T. H.; He, X. L.; Li, J. P.; Lin, C. L. Appl. Phys. Lett. 2004, 84, 3654. (3) Comini, E.; Faglia, G.; Sberveglieri, G.; Pan, Z. W.; Wang, Z. L. Appl. Phys. Lett. 2002, 81, 1869. (4) Curreli, M.; Li, C.; Sun, Y. H.; Lei, B.; Gundersen, M. A.; Thompson, M. E.; Zhou, C. W. J. Am. Chem. Soc. 2005, 127, 6922. (5) Bram, C.; Jung, C.; Stratmann, M. Fresenius’ J. Anal .Chem. 1997, 358, 108. (6) Ulman, A. Chem. Rev. 1996, 96, 1533. (7) An, X. Q.; Cao, C. B.; Zhu, H. S. J. Cryst. Growth 2007, 308, 340. (8) (a) Mrksich, M.; Whitesides, G. M. Trends Biotechnol. 1995, 13, 228. (b) Sun, S. Q.; Leggett, G. J. Nano Lett. 2007, 7, 3753. (9) (a) Chidsey, C. E. D. Science 1991, 251, 919. (b) Cotton, C.; Glidle, A.; Beamson, G.; Cooper, J. M. Langmuir 1998, 14, 5139.
4514 DOI: 10.1021/la9042827
controlled wetting in the last few decades.4-8 Alkanethiolates SAMs on metal surfaces, especially Au surfaces,9 have been extensively studied because of their promising structure stability. Moreover, SAMs were employed to form favorable functionalized surfaces on metal oxide surfaces, for example, carboxylic acid, phosphonic acids (X(CH2)nPO(OH)2, X = CH3, OH; n = 12, 18...),10 and phosphoric acids (X(CH2)nOPO(OH)2, X = CH3, OH; n = 12, 18...) on TiO2,11 AgO,12 Al2O3,13 ZrO2,14 and ITO.15 Among them, alkyl phosphonates and phosphonic acid were proved to bind much more strongly than carboxylic acids to a wide range of metal oxides by forming well-packed SAMs with excellent thermal and hydrolytic stability.16 Nanoscaled ZnO is widely used in sensors, optoelectronic devices, and solar cells because of its excellent physical and chemical properties.2,17-22 For such purposes, controlling the surface functionalization is important to each individual utilization. Recently, Taratula and co-workers performed a series of studies on the functionalization on ZnO surfaces by different (10) Textor, M.; Ruiz, L.; Hofer, R.; Rossi, A.; Feldman, K.; H€ahner, G.; Spencer, N. D. Langmuir 2000, 16, 3257. (11) Gawalt, E. S.; Lu, G.; Bernasek, S. L.; Schwartz, J. Langmuir 1999, 15, 8929. (12) Tao, Y. T.; Huang, C. Y.; Chiou, D. R.; Chens, L. J. Langmuir 2002, 18, 8400. (13) Laibinis, P. E.; Hickman, J. J.; Wrighton, M. S.; Whitesides, G. M. Science 1989, 245, 845. (14) (a) Pawsey, S.; Yach, K.; Halla, J.; Reven, L. Langmuir 2000, 16, 3294. (b) Gao, W.; Dickinson, L.; Grozinger, C.; Morin, F. G.; Reven, L. Langmuir 1997, 13, 115. (15) (a) Moses, S.; Brewer, S. H.; Kraemer, S.; Fuierer, R. R.; Lowe, L. B.; Agbasi, C.; Sauthier, M.; Franzen, S. Sens. Actuators, B 2007, 125, 574. (b) Paniagua, S. A.; Hotchkiss, P. J.; Jones, S. C.; Marder, S. R.; Mudalige, A.; Marrika, F. S.; Pemberton, J. E.; Armstrong, N. R. J. Phys. Chem. C 2008, 112, 7809. (16) (a) Shafi, K. V. P. M.; Ulman, A.; Yan, X. Z.; Yang, N. L.; Estournes, C.; White, H.; Rafailovich, M. Langmuir 2001, 17, 5093. (b) Pawsey, S.; Yach, K.; Reven, L. Langmuir 2002, 18, 5205. (c) McElwee, J.; Helmy, R.; Fadeev, A. Y. J. Colloid Interface Sci. 2005, 285, 551. (d) Silverman, B. M.; Wieghaus, K. A.; Schwartz, J. Langmuir 2005, 21, 225. (17) Pang, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (18) Qiu, Y. F.; Yang, S. H. Adv. Funct. Mater. 2007, 17, 1345. (19) Johnson, J. C.; Yan, H. Q.; Schaller, R. D.; Haber, L. H.; Saykally, R. J.; Yang, P. D. J. Phys. Chem. B 2001, 105, 11387. (20) Wang, J. X.; Sun, X. W.; Yang, Y.; Huang, H.; Lee, Y. C.; Tan, O. K.; Vayssieres, L. Nanotechnology 2006, 17, 4995. (21) Wang, X. D.; Summers, C. J.; Wang, Z. L. Nano Lett. 2004, 4, 423. (22) Lao, C. S.; Li, Y.; Wong, C .P.; Wang, Z. L. Nano Lett. 2008, 7, 1323.
Published on Web 02/10/2010
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Zhang et al.
molecular linkers.23 Norton and co-workers also reported the functionalization of Zn- and O-terminated ZnO surfaces with thiols.24a Many other SAMs, including chlorosilanes, methoxysilanes, ethoxysilanes, and carboxylic acid, have been studied to modify ZnO surfaces.24b,25,26 However, most of this work is focused on ZnO nanoparticles and films,27,28 with quite a few studies directed at the surface functionalization of ZnO nanowires or nanorods.29 In this article, we demonstrate a promising technique for forming carboxyalkylphosphonic acid (HOOC(CH2)nP(O)(OH)2 (n = 2, 9)) SAMs on ZnO nanowires and bare wafers with the goal of improving the selectivity of 1D FETs.
Experimental Section Synthesis of 10-Phosphonodecanoic Acid (10-PDA). The reagents used in synthesis experiments were of analytical grade or higher. Solvents were dried and distilled under nitrogen by standard procedures. 3-Phosphonopropionic acid (HOOC(CH2)2P(O)(OH)2, 3-PPA) was purchased from Sigma-Aldrich and used without any further purification. 10-Phosphonodecanoic acid (HOOC(CH2)9P(O)(OH)2, 10-PDA) was synthesized according to the literature16b,30 (Supporting Information). ZnO Nanowires and Wafers. Surface functionalization was performed on ZnO surfaces (nanowires and wafers) and Si wafers. ZnO nanowires were synthesized using hydrothermal methods (Supporting Information).31 The wafers used in this work were commercial single-crystalline ZnO wafers (Æ0001æ; 10 mm 5 mm 0.5 mm) polished with a surface roughness of less than 0.5 nm and 2-in.-diameter Si wafers coated with a 300 nm oxide layer. Prior to functionalization, SiO2 wafers were sonicated in trichloroethylene, acetone, and ethanol for 15 min, respectively, followed by additional sonication in DI water for 10 min and blown dry with high-purity Ar. Finally, both ZnO and SiO2 wafers were O2 plasma cleaned for 60 s and rinsed with DI water. Formation of Self-Assembled Monolayers. The cleaned wafers and nanowires were immersed into either a 3-PPA aqueous solution (5 mM) or a 10-PDA methanol solution (5 mM) for 72 h to form SAMs on their surfaces. After that, these samples were rinsed with DI water and methanol and blown dry with N2. Water contact angle values of the wafers were measured right after the self-assembly procedure. The surfaces of ZnO and SiO2 wafers were further characterized by AFM, FT-IR, and XPS. Immobilization of Antibody. The ZnO wafers with SAMs were loaded into a solution of N-hydroxysuccinimide (NHS) (50 mg/mL) and 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) (50 mg/mL) in MES (2-(N-morpholine)-ethane sulfonic acid, pH 6) buffer for 30 min, followed by rinsing with (23) (a) Taratula, O.; Galoppini, E.; Wang, D.; Chu, D.; Zhang, Z.; Chen, H. H.; Saraf, G.; Lu, Y. C. J. Phys. Chem. B 2006, 110, 6506. (b) Taratula, O.; Galoppini, E.; Mendelsohn, R.; Reyes, P. I.; Zhang, Z.; Duan, Z. Q.; Zhong, J.; Lu, Y. C. Langmuir 2009, 25, 2107. (24) (a) Sadik, P. W.; Pearton, S. J.; Norton, D. P.; Lambers, E.; Ren, F. J. Appl. Phys. 2007, 101, 104514. (b) Hou, X.; Zhou, F.; Yu, B.; Liu, W. Mater. Sci. 2007, 452-453, 732. (25) Yakimova, R.; Steinhoff, G.; Petoral, R. M. J.; Vahlberg, C.; Khranovskyy, V.; Yazdi, G. R.; Uvdal, K.; Spetz, A. L. Biosens. Bioelectron. 2007, 22, 2780. (26) (a) Jeon, K. A.; Son, H. H.; Kim, C. E.; Shon, M. S.; Yoo, K. H.; Choi, A. M.; Jung, H. I.; Lee, S. Y. 5th IEEE Sensors Conference, Daegu, South Korea, IEEE 2006.(b) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. D. Nat. Mater. 2005, 4, 455. (27) (a) Galoppini, E. Coord. Chem. Rev. 2004, 248, 1283. (b) Grasset, F.; Saito, N.; Li, D.; Park, D.; sakaguchi, I.; Ohashi, N.; Haneda, H.; Roisnel, T.; Mornet, S.; Duguet, E. J. Alloys Compd. 2003, 360, 298. (28) Allen, C. G.; Baker, D. J.; Albin, J. M.; Oertli, H. E.; Gillaspie, D. T.; Olson, D. C.; Furtak, T. E.; Collins, R. T. Langmuir 2008, 24, 13393. (29) Pauporte, T.; Lincot, D. Electrochim. Acta 2000, 45, 3345. (30) Adden, N.; Gamble, L. J.; Castner, D. G.; Hoffmann, A.; Gross, G.; Menzel, H. Langmuir 2006, 22, 8197. (31) Wei, A.; Sun, X. W.; Xu, C. X.; Dong, Z. L.; Yang, Y.; Tan, S. T.; Huang, W. Nanotechnology 2006, 17, 1740.
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Article DI water and N2 drying. The modified wafers were then immersed into a solution of IgG (Sigma, with FITC, fluorescein isothiocyanate, 0.02 mg/mL) in MES buffer (pH 6) for 4 h at room temperature. The residual NHS esters were blocked by ethanolamine (50 mM) in PBS (phosphate-buffered saline) buffer (0.01 M, pH 8.0) for 60 min. After that, the wafers were cleaned in PBS buffer (0.01 M, pH 7.4) with 0.1% BSA (bovine serum albumin) and 0.1% Tween 20 and dried. Finally, the wafers were stored in a solution of BSA (1% w/v) in 10 mM PBS buffer (pH 7.4) and dried in a flow of N2 before analysis. The characterization of the immobilization of IgG was carried out by using a fluorescence microscope. Contact Angle Measurement. The surface wettability was investigated by measuring water contact angles (Dataphysics OCA20, Data Physics Instruments GmbH, Germany) with the sessile drop method (2.0 μL). Atomic Force Microscopy (AFM). Surface topography and roughness measurements of the ZnO and SiO2 wafers were performed by tapping mode AFM (Dimension 3100, Vecco) under ambient conditions. The AFM images presented are of the nature of the raw data, except for a flattening process to reduce the background slope. Fourier Transform Infrared Spectroscopy (FT-IR). FTIR spectroscopy (Nicolet 6700, Thermo) was applied to measure the surface properties of the functionalized ZnO surfaces. For ZnO nanowires, absorbance spectra were collected at a resolution of 4 cm-1 (32 scans) by dispersing the modified ZnO nanowires into a KBr pellet. For ZnO wafers, FT-IR ATR spectra were collected at a resolution of 2 cm-1 (128 scans). X-ray Photoelectron Spectroscopy (XPS). XPS analyses were performed using an Axis Ultra DLD system (Kratos, U.K.) equipped with a concentric hemispherical analyzer in the standard configuration. Spectra were acquired using a monochromatic Al KR source operating at 150 W at a base pressure of 10-9 mbar. Angle-resolved XPS measurements were conducted at different takeoff angles, namely, 15 and 75 with respect to the normal line of the surface plane, to obtain depth-dependent information. As a reference, 10-PDA powder and ZnO wafers were analyzed with a takeoff angle of 45 with respect to the surface. Spectra were referenced to the contamination of the C 1s hydrocarbon chain at 284.8 eV and were fit using Gaussian functions. Fluorescence Microscopy. A fluorescence microscope (NISElements Advanced Research, Nikon, Japan) was used for the fluorescence imaging of FITC-labeled IgG bound to ZnO using appropriate excitation and emission optical filters. Images were captured and analyzed with NIS-Elements software and Nikon ND2 Viewer software (Nikon, Japan).
Morphology and Structure Characterization of the ZnO Nanostructures. The morphology and structure of ZnO nanostructures were characterized by field-emission scanning electron microscopy (FE-SEM, Quanta 400 FEG, FEI) operating at 20 kV and an X-ray diffraction instrument (XRD, X’Pert PRO MPD, PANalytical, Holland) with a Cu KR source (λ =1.5406 A˚) using a step size of 0.02 deg/s over the range of 20 < 2θ < 80 at room temperature.
Nuclear Magnetic Resonance Spectroscopy and Mass Spectrometry. NMR spectra were recorded on a Varian Unity Inova 400 NMR spectrometer at room temperature. Mass spectra were recorded using a Waters Quattro Premier XE mass spectrometer with an electrospray (ESI) atmospheric pressure ionization (API) source.
Results and Discussion Morphology and Structure of As-Obtained ZnO Nanowires. Figure S2a in the Supporting Information illustrates that as-obtained ZnO nanowires have diameters from 200 to 500 nm, and an average length of 5 μm. Figure S2b in the Supporting Information shows the XRD patterns of the hydrothermal DOI: 10.1021/la9042827
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Zhang et al. Table 1. Roughness and Water Contact Angle of Wafers before and after Functionalization by 10-PDA roughness (nm)
wafers ZnO SiO2
water contact angle
preimmersion
postimmersion
Rq = 0.223 ( 0.009
Rq = 0.614 ( 0.02
Ra = 0.175 ( 0.006 Rq = 0.92 ( 0.15
Ra = 0.462 ( 0.005 Rq = 1.66 ( 0.23
Ra = 0.66 ( 0.12
Ra = 1.3 ( 0.15
preimmersion
postimmersion
13.5 ( 0.7
54.38 ( 1.53
