Generation of a stable surface concentration of amino groups on silica coated onto titanium substrates by the plasma enhanced chemical vapour deposition method Endre J. Szili a,b , Sunil Kumar c, *, Roger St. C. Smart d , Nicolas H. Voelcker a, ** a School of Chemistry, Physics and Earth Sciences, Flinders University, Bedford Park, SA 5042, Australia b Mawson Institute, University of South Australia, Mawson Lakes, SA 5095, Australia c Ian Wark Research Institute, University of South Australia, Mawson Lakes, SA 5095, Australia d Applied Centre for Structural and Synchrotron Studies, University of South Australia, Mawson Lakes, SA 5095, Australia 1. Introduction Biomaterials are materials intended to interface with biological systems to evaluate, treat, augment, or replace any tissue, organ, or function of the body [1a,2]. Among the metals used in biomaterials research, titanium possesses the best combination of mechanical and biocompatible properties. Titanium owes its good biocompat- ibility to a dense surface oxide layer, which is approximately 4 nm thick and forms spontaneously in the presence of oxygen [1b]. However, the performance of titanium biomaterials, such as those used as orthopaedic implants, can still be improved because some titanium implants integrate incorrectly into the surrounding bone tissue, loosen over time or cause severe inflammatory responses, which often requires corrective surgery. To address this problem, titanium surfaces can be modified to enhance cellular behaviour on the biomaterial. Some common surface modifica- tions include surface oxidation and hydroxylation, ion implanta- tion and biomolecule immobilisation [1c,3–8]. A commercial product, Bioglass 1 , is one of the more widely used surface coatings for the modification of titanium biomater- ials. The composition of 45S5-type Bioglass 1 by wt.% is 45% SiO 2 , 24.5% Na 2 O, 24.5% CaO, 5% P 2 O 5 . Bioglass 1 and other bioactive glasses, having variations in the elemental composition to that of Bioglass 1 , are not only biocompatible with the body but also possess bioactive properties leading to enhanced cellular activity on the material’s surface [9,10]. The common techniques for generating bioactive glass ceramic coatings on titanium include enamelling, electrophoresis, sol–gel and plasma spraying. How- ever, bioactive glass ceramic coatings possess some disadvantages such as poor adhesion to the substrate surface [11]. We have developed another form of bioactive glass coatings by utilising plasma enhanced chemical vapour deposition (PECVD) technology to generate strongly adherent silica films (PECVD-SiO 2 ) on titanium substrates [12–15]. We have recently reported the characteristic surface features of PECVD-SiO 2 , which was depos- ited using a deposition procedure that we optimised for coating titanium biomaterials on a semi-commercial basis. The films were shown to be strongly adherent and conformal to the underlying titanium substrate, contained stoichiometric SiO 2 , low in carbon (below 10 at.%) and were very hydrophilic (water contact angle below 108) [16]. However, if PECVD-SiO 2 biomaterial coating technology is to realise its full potential, the surface also needs to be easily chemically modified so that biological interactions (e.g. cell attachment, proliferation and differentiation) can be controlled on the implant surface. Therefore, in this report we investigated if PECVD-SiO 2 films coated on titanium substrates can be modified with a stable surface concentration of reactive functional groups Applied Surface Science xxx (2009) xxx–xxx ARTICLE INFO Article history: Received 14 October 2008 Accepted 25 February 2009 Available online xxx Keywords: 3-Aminopropyl triethoxysilane Plasma enhanced chemical vapour deposition Silica Titanium ABSTRACT This report describes the amino functionalisation of the surface of plasma enhanced chemically vapour deposited silica films (PECVD-SiO 2 ), which were coated onto titanium substrates. Amino groups were linked to PECVD-SiO 2 via 3-aminpropyl triethoxysilane (APTES). We showed that the APTES functionalised PECVD-SiO 2 surfaces contained a high packing density of amino groups (67–92 NH 2 groups per nm 2 ), indicative of a multilayered and highly cross-linked APTES film. 65–66% of the original surface concentration of APTES was retained on the PECVD-SiO 2 surface after incubation under physiological conditions, indicating that APTES films are relatively stable on PECVD-SiO 2 in these environments. The stability of the amino groups obtained on PECVD-SiO 2 in this study is much higher compared to other hydroxyl-bearing materials, such as titanium. Therefore, PECVD-SiO 2 films may find use as functional biomaterial coatings and as intermediate adhesion layers in silanisation processes. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +61 8 8302 3169; fax: +61 8 8302 3683. ** Corresponding author. Tel.: +61 8 8201 5338; fax: +61 8 8201 2905. E-mail addresses: sunil.kumar@unisa.edu.au (S. Kumar), nico.voelcker@flinders.edu.au (N.H. Voelcker). G Model APSUSC-18380; No of Pages 5 Please cite this article in press as: E.J. Szili, et al., Generation of a stable surface concentration of amino groups on silica coated onto titanium substrates by the plasma enhanced chemical vapour deposition method, Appl. Surf. Sci. (2009), doi:10.1016/ j.apsusc.2009.02.092 Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.02.092