Control over wettability via surface modification of porous gradients Y. L. Khung a , M. A. Cole b , S. J. P. McInnes a and N. H. Voelcker a a School of Chemistry, Physics and Earth Sciences, Flinders University, Adelaide, SA, Australia b Ian Wark Research Institute, University of South Australia, Adelaide SA, Australia ABSTRACT The control over surface wettability is of concern for a number of important applications including chromatography, microfluidics, biomaterials, low-fouling coatings and sensing devices. Here, we report the ability to tailor wettability across a surface using lateral porous silicon (pSi) gradients. Lateral gradients made by anodisation of silicon using an asymmetric electrode configuration showed a lateral distribution of pore sizes, which decreased with increasing distance from the electrode. Pore sizes were characterised using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Pore diameters ranged from micrometres down to less than 10 nanometres. Chemical surface modification of the pSi gradients was employed in order to produce gradients with different wetting or non-wetting properties. Surface modifications were achieved via silanisation of oxidised pSi surfaces introducing functionalities including polyethylene glycol, terminal amine and fluorinated hydrocarbon chains. Surface modifications were characterised using infrared spectroscopy. Sessile drop water contact angle measurements were used to probe the wettability in regions of different pore size across the gradient. For the fluorinated gradients, a comparison of equilibrium and dynamic contact angle measurement was undertaken. The fluorinated surface chemistry produced gradients with wettabilities ranging from hydrophobic to near super-hydrophobic whereas pSi gradients functionalised with polyethylene glycol showed graded hydrophilicity. In all cases investigated here, changes in pore size across the gradient had a significant effect on wettability. KEYWORDS: Wettability, porous silicon, lateral gradients, silanisation 1. INTRODUCTION The solid-liquid interface is important in defining the degree of wettability (hydrophobicity/hydrophilicity) of a surface and subsequent interactions between the surface and components in the liquid contacting the surface. For example, the increasing demand for sophistication on biochip surfaces has motivated extensive studies of liquid-surface interaction [1, 2]. It is important to note that biochip and material design usually involves tuning of wettability using surface chemistry [3-6] and in particular surface roughness [7, 8] or a combination of texture and chemistry [9, 10]. Surface wettability is a key factor in mediating fluid transport in confined spaces and across surfaces and it also influences adsorption of biomolecules and subsequent biological response. Therefore, control over wettability has significant applications in areas such as biomedical diagnostics and biomaterials. In recent years, considerable attention had been directed towards self assembled monolayers (SAMs) and polymers with different wettability profiles to influence and modulate the levels of protein adsorption[11]. However it should be noted that wettability is not the only factor mediating protein adsorption. The latter is also susceptible to electrostatic effects and thus is sensitive to the pH of solution and protein isoelectric point [12]. Commonly used methods for tailoring wettability include introduction of chemical moieties on the surface and micro/nano fabrication of patterned and textured surfaces [13]. Generally for smooth flat surfaces a maximal water contact angle of around 110º - 120º is attainable [9, 14, 15]. Beyond this, roughening or surface structuring is required for an increase in contact angle [8, 9]. A well-known example is the so called Lotus effect where surface topography of the hydrophobic lotus leaves and other plant surfaces enable contact angles of >150º and low sliding angles [16]. Whilst many studies have focused on the engineering of surfaces to mimic nature and on the effects of topography and chemistry, the mechanisms involving the simultaneous effects of both chemical and topographical modification on BioMEMS and Nanotechnology III, edited by Dan V. Nicolau, Derek Abbott, Kourosh Kalantar-Zadeh, Tiziana Di Matteo, Sergey M. Bezrukov, Proc. of SPIE Vol. 6799, 679909, (2007) · 0277-786X/07/$18 · doi: 10.1117/12.759377 Proc. of SPIE Vol. 6799 679909-1