866 DOI: 10.1021/la9023103 Langmuir 2010, 26(2), 866–872 Published on Web 09/03/2009 pubs.acs.org/Langmuir © 2009 American Chemical Society Controlled Oxidation, Biofunctionalization, and Patterning of Alkyl Monolayers on Silicon and Silicon Nitride Surfaces using Plasma Treatment Michel Rosso, †,‡ Marcel Giesbers, † Karin Schroen, ‡ and Han Zuilhof * ,† † Laboratory of Organic Chemistry, Wageningen University, Dreijenplein 8, 6703 HB Wageningen, The Netherlands, and ‡ Laboratory of Food and Bioprocess Engineering, Wageningen University, Bomenweg 2, 6703 HD Wageningen, The Netherlands Received June 27, 2009 A new method is presented for the fast and reproducible functionalization of silicon and silicon nitride surfaces coated with covalently attached alkyl monolayers. After formation of a methyl-terminated 1-hexadecyl monolayer on H- terminated Si(100) and Si(111) surfaces, short plasma treatments (1-3 s) are sufficient to create oxidized functionalities without damaging the underlying oxide-free silicon. The new functional groups can, e.g., be derivatized using the reaction of surface aldehyde groups with primary amines to form imine bonds. In this way, plasma-treated monolayers on silicon or silicon nitride surfaces were successfully coated with nanoparticles, or proteins such as avidin. In addition, we demonstrate the possibility of micropatterning, using a soft contact mask during the plasma treatment. Using water contact angle measurements, ellipsometry, XPS, IRRAS, AFM, and reflectometry, proof of principle is demonstrated of a yet unexplored way to form patterned alkyl monolayers on oxide-free silicon surfaces. Introduction Plasma treatments of organic materials have been widely studied to prepare modified surfaces for organic membranes 1 or materials of biotechnological and biomedical interests. 2-4 Indeed, a short expo- sure of organic surfaces, usually polymers, to plasma can create directly new surface functionalities. The gas present in the plasma chamber determines the obtained functionalization: 5,6 oxygen or water plasma leads to the oxidation of surfaces and to the formation of polar surface groups (-OH, CdO, O-CdO), whereas exposure to ammonia, for instance, will mainly yield surface amine groups (-NH 2 ). In most cases, these treatments produce surfaces with a higher biocompatibilty, 7-9 but another interesting application in- volves the subsequent functionalization of these surfaces with bioac- tive molecules for specific recognition and sensing. 10 While plasma treatments yield lower densities of surface functional groups than classical chemical reactions, this can be compensated by the size of the subsequently grafted moieties, whether they consist of biomolecules 2 (enzymes, antibodies, DNA, etc.) or polymer brushes. 11,12 Beside the work carried out on polymer surfaces, several groups have also studied the effects of oxygen plasma, 13-15 as well as atomic oxygen 16,17 or ion beams 18,19 on organic thiol monolayers on gold. In comparison, very little has been done concerning the further functionalization of plasma-treated monolayers. This is somewhat surprising, as plasma treatment provides an easy and fast activation of chemically inert organic monolayers (e.g., methyl-terminated), with potential applications in biosensing. Recently, carbon dioxide 20 and oxygen 21,22 plasma treatments were used to functionalize alkylsilane monolayers on glass and silica surfaces, and one of the latter works demonstrated the attachment of antibodies. 22 In this work, we extended the plasma functionalization to alkyl monolayers on oxide-free silicon, produced from the reaction of alkenes with hydrogen-terminated silicon surfaces and with HF- etched silicon-enriched silicon nitride (Si 3.9 N 4 ). These high-quality alkyl monolayers, prepared by thermal or photochemical reaction of alkenes with hydrogen-terminated silicon surfaces, 23-34 have been studied for their high potential in sensing and nanotechnology *E-mail: Han.Zuilhof@wur.nl. (1) Durand, J.; Rouessac, V.; Roualdes, S. Ann. Chim.-Sci. Mater. 2007, 32, 141–158. 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