Adsorption of phenylalanine on single crystal rutile TiO 2 (1 1 0) surface A.G. Thomas a, * , W.R. Flavell a , C.P. Chatwin a , A.R. Kumarasinghe a , S.M. Rayner a , P.F. Kirkham a , D. Tsoutsou a , T.K. Johal b , S. Patel b a School of Physics and Astronomy, University of Manchester, Sackville Street Building, Manchester, M60 1QD, UK b CLRC Daresbury Laboratory, Warrington, Cheshire WA4 4AD, UK Available online 19 April 2007 Abstract The adsorption of the aromatic amino acid, phenylalanine on a TiO 2 rutile (1 1 0) single crystal surface has been investigated with photoemission and NEXAFS (near edge X-ray absorption fine structure) spectroscopy. The results indicate initial adsorption via the carboxylate group in a bidentate configuration with the phenyl ring oriented at approximately 25° to the surface normal. The amino group remains as NH 2 . Subsequent layers of phenylalanine appear to adsorb as neutral molecules with H-bonding between NH 2 and C@O groups. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Titanium dioxide; Single crystal; Rutile; Photoemission; NEXAFS; Amino acid; Adsorption geometry 1. Introduction Titanium dioxide has a multitude of technological appli- cations, including its use in dye sensitised solar cells [1], photocatalysts [2], biosensor supports [3,4], bactericides [5,6] and for water purification [5]. In a number of the applications mentioned above TiO 2 is coated with an organic species of some sort, for example in solar cells, a photoactive dye, N3 (cis-bis[4,4 0 -dicarboxy- 2,2 0 -bipyridine]-bis(isothiocynato)ruthenium(II)) is ad- sorbed onto the TiO 2 substrate. In biosensors, enzymes are adsorbed at the surface. In water purification and bac- tericidal applications, complex organic or biological mole- cules interact with TiO 2 , and in the presence of UV radiation are broken down. The fundamental interaction of these types of molecules with TiO 2 is of great interest in trying to understand how the photocatalytic process (in the case of bactericides and water purification) or elec- tron transfer mechanisms (in the case of biosensors and solar cells) function, and importantly, how they may be made more efficient. There have been many studies of amino acids and DNA/ RNA bases adsorbed on metal surfaces, particularly Cu surfaces [7–11]. Although there are differences in bonding between particular acids and surface combinations, strongly bound, ordered arrays of amino acids and DNA base molecules have been observed on Cu surfaces using STM (scanning tunnelling microscopy) and RAIRS (reflec- tion absorption infrared spectroscopy) [7–9]. Experimental data on adsorption of amino acids on single crystal oxide surfaces, on the other hand, are limited to a handful of studies of glycine adsorption. Glycine adsorption on TiO 2 has been the subject of a several studies [12–14]. TPD (tem- perature programmed desorption) measurements of co-ad- sorbed glycine and water on rutile TiO 2 showed that the glycine adsorbs intact with a desorption energy of around 1.2 eV per molecule [12]. It was also shown that around 60% of the glycine molecules adsorbed directly onto the surface underwent decomposition as they were removed from the surface. Techniques which use photons as the probe, such as photoemission and photoelectron diffraction show a gradual desorption and decomposition of the 0039-6028/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2007.04.085 * Corresponding author. Tel.: +44 161 3068764; fax: +44 161 3063941. E-mail address: a.g.thomas@manchester.ac.uk (A.G. Thomas). www.elsevier.com/locate/susc Surface Science 601 (2007) 3828–3832