Modification of micro-channel filling flow by poly(dimethylsiloxane) surface functionalization with fluorine—Substituted aminonaphthols G. Cortese a , F. Martina a , G. Vasapollo a , R. Cingolani b , G. Gigli a,b , G. Ciccarella a,b, * a Universita` del Salento, Dipartimento di Ingegneria dell’Innovazione, via Arnesano, I-73100, Lecce, Italy b National Nanotechnology Laboratory (NNL) of CNR, INFM, Distretto Tecnologico ISUFI, via Arnesano, I-73100, Lecce, Italy 1. Introduction Microfabrication is essential to modern science and technology. Miniaturization and integration of a range of devices result in portability, reduction in time, costs, reagents, sample size and power consumption [1]. Furthermore, improvements in detection limits and a series of other new functions are also observed. In particular the possibility to implement mixers, diluters, and valves as components of a PDMS-based microfluidic circuit opens new perspectives for lab-on-chip applications such as bioassays and chemical sensors. This is possible mainly having the capability of modulating the filling flow of liquids within micro-channels by adopting the consolidated microfabrication technology. The flow of a liquid in micro-channels is a thermodynamically driven capillary process, explained in terms of interfacial free energy between solid/vapour and solid/liquid interfaces. It occurs because of a pressure difference between two hydraulically connected regions of the liquid mass, the flow direction resulting from the decreasing of the pressure [2]. A system based on capillary action as driving force is suitable for portable diagnostic devices designed without additional mechanical or electrical micropumps [3]. Microfluidics based on the capillarity-induced filling of elasto- meric channels by a suitable liquid or solution represents a very efficient method for patterning biomolecules and for the selective delivery of proteins from aqueous solutions, such as blood and saliva [4–6]. However, since the filling process depends on the interfacial free energies of micro-channel walls, there are several requirements for a successful patterning of such bio-derivatives [7,8]: the micro-channels have to be sufficiently hydrophilic to promote filling by capillary action and at the same time, they should possess protein repellent surfaces, in order to prevent any loss of proteins to micro-channels walls. Therefore, since water motion strongly depends on the channel walls conditions, the wetting of solid surfaces by the liquid can be controlled by varying the interfacial free energies of one wall and/or all walls of the micro-channels. In general, the filling rate of the fluidic channels is proportional to the reciprocal length of the capillary, z: dz/dt = Rgcos u/4hz, where h, R, g, and u indicate the liquid viscosity, the radius of the capillary, the liquid–vapour interfacial free energy of the fluid and Journal of Fluorine Chemistry 131 (2010) 357–363 ARTICLE INFO Article history: Received 30 April 2009 Received in revised form 19 November 2009 Accepted 20 November 2009 Available online 1 December 2009 Keywords: Microfluidics Poly(dimethylsiloxane) Surface functionalization Aminonaphthols Fluorine groups ABSTRACT Microfluidics based on the capillarity-induced filling of elastomeric channels by a suitable liquid or solution represents a useful route for realizing portable diagnostic devices designed without additional mechanical or electrical micropumps. In this study, an elastomeric mold made of poly(dimethylsiloxane) (PDMS), containing relief patterns placed in intimate contact with a silicon substrate, is utilized to create a continuous network of rectangular micro-channels for the motion of water fluid. The immobilization on activated PDMS surface of suitable functional molecules such as hydrophilic and hydrophobic fluorine-containing aminonaphthols, obtained through a straightforward and versatile synthetic procedure, allowed us to modulate PDMS surface properties depending on the structural characteristics of the employed derivative. In this context, the incorporation of fluorine groups is important for improving biocompatibility of the resulting device, providing surfaces that could be chemically and biologically inert as well as resistant to surface adhesion phenomena. The functionalization from liquid phase of PDMS replicas, involving a covalent derivatization via silanization reaction of the above mentioned compounds to an oxidized PDMS surface, resulted in a successful modification of microfluidic motion of water in rectangular capillaries, moreover contact angle values evidence also how wettability of PDMS films could be modulated, with the fluorinated aminonaphthols fuctionalized PDMS exhibiting higher contact angles. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author at: Universita` del Salento, Dipartimento di Ingegneria dell’Innovazione, via Arnesano, I-73100, Lecce. Italy.Tel.: +39 0832 298233. E-mail address: giuseppe.ciccarella@unisalento.it (G. Ciccarella). Contents lists available at ScienceDirect Journal of Fluorine Chemistry journal homepage: www.elsevier.com/locate/fluor 0022-1139/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jfluchem.2009.11.017