Applied Surface Science 258 (2012) 5166–5174 Contents lists available at SciVerse ScienceDirect Applied Surface Science jou rn al h om epa g e: www.elsevier.com/locate/apsusc Effect of surface structure and wettability of DLC and N-DLC thin ﬁlms on adsorption of glycine Mukhtar H. Ahmed ∗ , John A. Byrne Nanotechnology Integrated Bio-Engineering Centre, University of Ulster, Jordanstown, BT37 0QB, Belfast, UK a r t i c l e i n f o Article history: Received 29 November 2011 Received in revised form 4 January 2012 Accepted 29 January 2012 Available online 9 February 2012 Keywords: Diamond-like carbon (DLC) Wettability Surface free energy Glycine adsorption Raman spectroscopy Fourier transform infrared spectroscopy (FTIR) a b s t r a c t Diamond-like carbon (DLC) is known to have excellent biocompatibility. Various samples of DLC and nitrogen-doped DLC thin ﬁlms (N-DLC) were deposited onto silicon substrates using plasma-enhanced chemical vapour deposition (PECVD). Subsequently, the adsorption of amino acid glycine onto the sur- faces of the thin ﬁlms was investigated to elucidate the mechanisms involved in protein adhesion. The physicochemical characteristics of the surfaces, before and after adsorption of glycine, were investigated using Fourier transfer infrared (FTIR), Raman spectroscopy, spectroscopic ellipsometry (SE) and contact angle (). The Raman study highlighted decrease slightly in the ID/IG ratio at low levels of N (5.4 at.%), whilst increasing the nitrogen dopant level (>5.4 at.%) resulted in a increase of the ID/IG ratio, and the FTIR band at related to C N. Following exposure to glycine solutions, the presence of Raman bands at 1727 cm -1 and 1200 cm -1 , and FTIR bands at 1735 cm -1 indicates that the adsorption of glycine onto the surfaces has taken place. These results which obtained from SE and surface free energy, show that low levels of nitrogen doping in DLC enhances the adsorption of the amino acid, while, increased doping led to a reduced adsorption, as compared to undoped DLC. Glycine is bound to the surface of the DLC ﬁlms via both de-protonated carboxyl and protonated amino groups while, in the case of N-DLC gylcine was bound to the surface via anionic carboxyl groups and the amino group did not interact strongly with the surface. Doping of DLC may allow control of protein adsorption to the surface. © 2012 Elsevier B.V. All rights reserved. 1. Introduction The implantation of biomaterials into the human body allows it restructure function and hence to enhance the quality of life. The highly corrosive surroundings and the low tolerance of the body to some dissolution products restrict the materials to be used for implants [1]. Diamond-like carbon (DLC) is an excellent candidate for use as biocompatible coatings on biomedical implants [2] such as rotary blood pumps [3], artiﬁcial hearts, mechanical heart valves [4–6], coronary artery stents [7,8], hip and knee replacements [9,10], due to its remarkable properties such as high mechanical properties, high wear resistance, and chemical inertness [11,12]. Comparative studies showed that DLC has better biocompatibility and wear resistance than stainless steel [13], titanium and tita- nium alloys [14], poly-methyl mehtacrylate (PMMA) [15], cobalt chrome alloys, and alumina ceramics [16]. Several studies were per- formed to observe the dependence of hemocompatibility on the Raman D-band to G-band intensity ratio (ID/IG) of the DLC ﬁlms [17]. Therefore, the structure of DLC ﬁlms plays a vital role on the platelet adhesion on DLC surfaces [18]. ∗ Corresponding author. E-mail address: ahmed-m@ulster.ac.uk (M.H. Ahmed). In order to improve of it’s properties, DLC ﬁlms elementally modiﬁed by addition of third elements, such as nitrogen, silicon, ﬂu- orine, oxygen, and titanium [19,20]. Furthermore, nitrogen doped DLC ﬁlms (N-DLC) are considered for widespread clinical use as biocompatible coatings due to their excellent mechanical proper- ties including; surface roughness, elasticity, high hardness, infrared transparency and low friction coefﬁcient [21,22]. It was found that hydrogen content of the DLC ﬁlm as well as the ratio of sp2 to sp3 bonds can have signiﬁcant effects on friction and wear [23]. The replacement of CH with NH bonds in N-DLC reduces the aver- age coordination number and enhances the sp 2 hybridise bonding, leading to decrease in both internal stresses and the sp 3 hybridiza- tion fraction, due to presence of C N bonds [24]. Investigations have found that the factors including nitrogen concentration, C N ﬁlm roughness and types of bond between C and N, play a signiﬁ- cant role in clotting time and amount of adhered platelets [25]. 2. Experimental details 2.1. Film deposition Prior to ﬁlm deposition, Silicon wafers 1.5 cm × 1.5 cm were washed ultrasonically in pure acetone to remove residual organic 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.01.162