Modification of hydrogen-free diamond-like carbon (DLC) is presented, with acrylic acid (AA) vapor carried into a vacuum chamber by argon and with the in situ assistance of low-power radio frequency (RF) plasma at a temperature below 100°C. Measured by atomic force microscopy (AFM) technique, the roughness ( ) of the DLC was ？nm. XPS and FT-IR spectra analysis showed that carboxyl groups were immobilized on the surface of the DLC films, with about 40% of carboxyl group area coverage. It was found that the RF plasma and reaction time are important in enhancing the modification rate and efficiency. 1. Introduction Diamond-like carbon (DLC) films, which are hard, stable, inexpensive, and biocompatible, have been widely used in the fields of biosensors and electrochemistry for many years [1, 2]. With the development of modification technology, DNA and protein have been immobilized on DLC films [3, 4]. In the future, DLC films are promising in the applications of electrodes and bioactivity protected films in biosensors. To do so, one challenge is to improve carboxyl or amino modification techniques for the DLC surface to improve protein or DNA immobilization covalently. Recently, a photochemical functionalization technique was carefully studied, which makes possible a functionalized organic monolayer by UV light irradiation immobilization on the hydrogen-terminated surfaces of the nanocrystalline and single-crystal diamond (111), in a thin layer of liquid reactants [5, 6]. However, this technique requires hydrogen-terminated DLC surface at a temperature higher than 800°C (to remove the surface oxygen [7]), which is too high for some applications, such as GMR biosensors, and would damage the GMR elements [8]. Furthermore, the functionalization time was always over 10？hrs to obtain a functional group coverage of 10% [6, 9] (for the trifluoroethyl ester of -undecenoic acid (TFU), and the coverage is defined by the ratio of carboxyl group content over that of the carbon monolayer on the surface of the DLC). Ababou-Girard et al. [10] reported a thermal functionalization method to graft ethyl undecylenate molecules onto the hydrogen-terminated amorphous carbon surface. However, this method showed a poor efficiency, with obtained DLC？:？H rate of merely 4%. Hovis et al. [11] found that vinyl group (C=C) would react with hydrogen-free diamond surface in ultravacuum ( ？Pa) due to the active sites (dangling bands [12], surface π bonds [11], etc.) on the surface of hydrogen-free diamond. However, the reaction rate for vinyl groups is about the order of 10？3 on diamond,
References
[1]
A. H？rtl, E. Schmich, J. A. Garrido et al., “Protein-modified nanocrystalline diamond thin films for biosensor applications,” Nature Materials, vol. 3, no. 10, pp. 736–742, 2004.
[2]
M. Wang, N. Simon, G. Charrier et al., “Distinction between surface hydroxyl and ether groups on boron-doped diamond electrodes using a chemical approach,” Electrochemistry Communications, vol. 12, no. 3, pp. 351–354, 2010.
[3]
W.-S. Yang, O. Auciello, J. E. Butler et al., “DNA-modified nanocrystalline diamond thin-films as stable, biologically active substrates,” Nature Materials, vol. 1, pp. 253–258, 2002.
[4]
P. Christiaens, V. Vermeeren, S. Wenmackers et al., “EDC-mediated DNA attachment to nanocrystalline CVD diamond films,” Biosensors and Bioelectronics, vol. 22, no. 2, pp. 170–177, 2006.
[5]
B. M. Nichols, J. E. Butler Jr., J. N. Russell, and R. J. Hamers, “Photochemical functionalization of hydrogen-terminated diamond surfaces: a structural and mechanistic study,” Journal of Physical Chemistry B, vol. 109, no. 44, pp. 20938–20947, 2005.
[6]
T. Strother, T. Knickerbocker, J. N. Russell, J. E. Butler, L. M. Smith, and R. J. Hamers, “Photochemical functionalization of diamond films,” Langmuir, vol. 18, no. 4, pp. 968–971, 2002.
[7]
J. A. Menéndez, J. Phillips, B. Xia, and L. R. Radovic, “On the modification and characterization of chemical surface properties of activated carbon: in the search of carbons with stable basic properties,” Langmuir, vol. 12, no. 18, pp. 4404–4410, 1996.
[8]
M. Hecker, D. Tietjen, H. Wendrock et al., “Annealing effects and degradation mechanism of NiFe/Cu GMR multilayers,” Journal of Magnetism and Magnetic Materials, vol. 247, no. 1, pp. 62–69, 2002.
[9]
Y. L. Zhong, K. F. Chong, P. W. May, Z. K. Chen, and K. P. Loh, “Optimizing biosensing properties on undecylenic acid-functionalized diamond,” Langmuir, vol. 23, no. 10, pp. 5824–5830, 2007.
[10]
S. Ababou-Girard, F. Solal, B. Fabre, F. Alibart, and C. Godet, “Covalent grafting of organic molecular chains on amorphous carbon surfaces,” Journal of Non-Crystalline Solids, vol. 352, no. 9–20, pp. 2011–2014, 2006.
[11]
J. S. Hovis, S. K. Coulter, R. J. Hamers, M. P. D'Evelyn, J. N. Russell, and J. E. Butler, “Cycloaddition chemistry at surfaces: reaction of alkenes with the diamond(001)-2 × 1 surface,” Journal of the American Chemical Society, vol. 122, no. 4, pp. 732–733, 2000.
[12]
Y. Yamauchi, Y. Sasai, S. I. Kondo, and M. Kuzuya, “Chemical diagnosis of DLC by ESR spectral analysis,” Thin Solid Films, vol. 518, no. 13, pp. 3492–3496, 2010.
[13]
S. Takabayashi, K. Okamoto, H. Motoyama, T. Nakatani, H. Sakaue, and T. Takahagi, “X-ray photoelectron analysis of surface functional groups on diamond-like carbon films by gas-phase chemical derivatization method,” Surface and Interface Analysis, vol. 42, no. 2, pp. 77–87, 2010.
[14]
J. C. Lascovich, R. Giorgi, and S. Scaglione, “Evaluation of the sp2/sp3 ratio in amorphous carbon structure by XPS and XAES,” Applied Surface Science, vol. 47, no. 1, pp. 17–21, 1991.
[15]
N. Paik, “Raman and XPS studies of DLC films prepared by a magnetron sputter-type negative ion source,” Surface and Coatings Technology, vol. 200, no. 7, pp. 2170–2174, 2005.
[16]
A. Zebda, H. Sabbah, S. Ababou-Girard, F. Solal, and C. Godet, “Surface energy and hybridization studies of amorphous carbon surfaces,” Applied Surface Science, vol. 254, no. 16, pp. 4980–4991, 2008.
[17]
H. P. Boehm, “Surface oxides on carbon and their analysis: a critical assessment,” Carbon, vol. 40, no. 2, pp. 145–149, 2002.
[18]
T. Kondo, Y. Niwano, A. Tamura et al., “Enhanced electrochemical response in oxidative differential pulse voltammetry of dopamine in the presence of ascorbic acid at carboxyl-terminated boron-doped diamond electrodes,” Electrochimica Acta, vol. 54, no. 8, pp. 2312–2319, 2009.
[19]
C. Coyle, R. P. Gandiraman, V. Gubala et al., “Tetraethyl orthosilicate and arylic acid forming robust carboxylic functionalities on plastic surfaces for biodiagnostics,” Plasma Processes and Polymers, vol. 9, no. 1, pp. 28–36, 2012.
[20]
J.-H. Hu and X.-F. Zheng, “Organic silicon compound,” in Practical Infrared Spectroscopy, Z. Yang, S.-X. Zhang, and R. Liu, Eds., pp. 364–367, Science Press, Beijing, China, 2011.
[21]
L. J. Bellamy, Advances in Infrared Group Frequencies, Methuen, London, UK, 1968.
[22]
C. J. Powell and A. Jablonski, “NIST Electron Inelastic-Mean-Free-Path Database version 1.2,” 2010.
