J. Phys. Chem. C 2008, 112, 11887–11892
11887
Photochemical Modification of a Boron-doped Diamond Electrode Surface with Vinylferrocene Takeshi Kondo,*,† Hikaru Hoshi,† Kensuke Honda,‡ Yasuaki Einaga,§ Akira Fujishima,| and Takeshi Kawai† Department of Industrial Chemistry, Faculty of Engineering, Tokyo UniVersity of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601 Japan, Department of Chemistry and Earth Sciences, Faculty of Science, Yamaguchi UniVersity, 1677-1 Yoshida, Yamaguchi-shi, Yamaguchi 753-8512 Japan, Department of Chemistry, Faculty of Science and Technology, Keio UniVersity, 3-14-1 Hiyoshi, Yokohama, 223-8522 Japan, and Kanagawa Academy of Science and Technology (KAST), 3-2-1 Sakado Takatsu-ku, Kawasaki, Kanagawa, 213-0012 Japan ReceiVed: April 3, 2008; ReVised Manuscript ReceiVed: May 29, 2008
Boron-doped diamond (BDD) surfaces were photochemically modified with vinylferrocene (VFC), and the modified surfaces were analyzed both qualitatively and quantitatively by electrochemical techniques. VFCmodified BDD (VFC-BDD) was prepared by ultraviolet (254 nm) irradiation of hydrogen-terminated BDD in mesitylene, acetonitrile, or n-alkane solutions of VFC. Cyclic voltammetry (CV) results indicated facile electron transfer between ferrocenyl groups and the BDD substrate. The surface coverage of VFC-modified polycrystalline and single-crystal (100) BDD was also estimated by CV and was found to reach saturation at a value possibly corresponding to monolayer coverage by ferrocenyl groups. The electrochemical stability of VFC-BDD to successive potential cycling in acetonitrile was found to be much higher than that of a selfassembled ferrocenylalkanethiol monolayer on an Au electrode surface. This study is the first to show that appropriate solutions of a terminal alkene can be used in the photochemical modification of diamond, yielding monolayer coverage of the surface. Introduction Covalent immobilization of biomolecules such as enzymes1–5 and DNA6–8 onto diamond surfaces has recently been investigated with a view to fabricating biosensors. These techniques may also be used to impart catalytic activity to and control the electronic properties of diamond surfaces. Covalent surface modification of diamond with organic molecules has been achieved by silane coupling, in which surface OH groups on diamond react with the coupling agent,2,9–11 and by grafting with electrochemical reduction of diazonium salts.12–17 Besides these methods, photochemical surface modification of diamond using vinyl derivatives is a promising method for stable immobilization of molecular monolayers linked to the diamond substrate surface by C-C bonds.5–7,13,18–27 Several groups have recently proposed a mechanism for such photochemical modification involving a photoemission-induced radical reaction,22–24,27 possibly enabled by the negative electron affinity (NEA) of hydrogen-terminated diamond surfaces. Quantitative estimation of coverage of the modified surface is essential for better understanding this mechanism and the properties of the resulting modified diamond surfaces. Surface coverage of modified diamond surfaces has often been estimated by quantitative X-ray photoelectron spectroscopic (XPS) analysis. However, this method can provide only relative atomic concentrations, often represented as the ratio to carbon atomic concentration (X/C ratio). Because carbon atoms on modified diamond surfaces may * Corresponding author. Phone: +81 3260 4271; fax: +81 5261 4631; e-mail:
[email protected]. † Tokyo University of Science. ‡ Yamaguchi University. § Keio University. | Kanagawa Academy of Science and Technology.
be from the diamond or the modifier molecules, saturation of the X/C atomic concentration ratio does not always mean termination of the modification reaction. If the modifier molecules polymerize on the diamond surface, the X/C ratio stays constant even though the polymerization reaction may still be in progress. Therefore, an absolute measurement of surface coverage is essential for understanding the surface modification of diamond. This goal may be achieved by the use of electroactive species as the modifier molecules, which are subsequently quantified by electrochemical measurements made on the modified surfaces of conductive diamond substrates. The surface modification of electrode materials by ferrocene derivatives in electrocatalysis has been widely studied,28,29 including examination of their roles as electron mediators for enzyme reactions,30,31 in molecular electronic devices,32,33 and so on. Becuase ferrocene derivatives are electroactive species exhibiting a simple reversible one-electron redox reaction, detailed characterization of ferrocene-modified surfaces is possible in electrochemical investigations. In the present study, conductive boron-doped diamond BDD thin-film surfaces were photochemically modified with vinylferrocene (FcCH)CH2) (Scheme 1), and the modified surfaces were analyzed both qualitatively and quantitatively by voltammetry. The results indicated that the modified diamond surface contained a monolayer of ferrocene derivatives, and facile electron transfer was found to take place between the ferrocenyl group and the BDD substrate. In addition, this is the first report of a terminal alkene in solution being immobilized on a diamond surface by photochemical modification. This method is likely to prove generally useful because many more types of alkenes can be used to modify surfaces than have previously been used in
10.1021/jp802875c CCC: $40.75 2008 American Chemical Society Published on Web 07/16/2008
11888 J. Phys. Chem. C, Vol. 112, No. 31, 2008
Kondo et al.
SCHEME 1: Photochemical Modification of VFC
conventional studies employing liquid molecules containing terminal alkenes. Experimental Section BDD surfaces were prepared on a conductive p-Si (111) wafer using a microwave plasma-assisted chemical vapor deposition (MPCVD) system. A mixture of acetone and a 70% trimethoxyborane/methanol solution (Kanto Chemical Co., Inc.) with a final B/C atomic concentration ratio of 20 000 ppm was used as the feed source.11 Typically, after a 15 h deposition at a microwave power of 1300 W, a polycrystalline diamond film of ca. 6 µm thickness (grain size
