Synthetic Bilayer Wetting on SiO 2 Surfaces Luiz Carlos Salay ‡ and Ana Maria Carmona-Ribeiro* ,² Departamento de Bioquı ´mica, Instituto de Quı ´mica, and Departamento de Engenharia Eletro ˆ nica, Escola Polite ´ cnica, UniVersidade de Sa ˜ o Paulo, CP 26077, 05599-970 Sa ˜ o Paulo SP, Brazil ReceiVed: October 10, 1997; In Final Form: January 22, 1998 Wetting on SiO 2 /Si wafers in the presence of dioctadecyldimethylammonium bromide (DODAB) bilayer vesicles is determined as a function of DODAB concentration and time of interaction between surface and vesicles. In 10 mM TRIS buffer at pH 7.4, the dynamic contact angle (θ) displays a maximum at an intermediate DODAB concentration (C) that corresponds to a minimum of contact angle hysteresis and is consistent with optimization of bilayer deposition. In pure water, θ is smaller than the contact angle for the bare surface consistently with adhesion of collapsed vesicles that carry water. In buffer, angles are larger than those for the bare surface, suggesting bilayer deposition under water and its conversion to a monolayer as the bilayer-bearing surface recedes into air. At a given C, θ against the interaction time (t) has an inverted- bell shape, indicating first vesicle adhesion, then bilayer deposition (at intermediate times), and finally further vesicle adhesion onto the deposited bilayer. Two major wettability parameters characterize bilayer deposition on the solid surface: minimization of contact angle hysteresis and maximization of the measured contact angle. Determination of dynamic contact angles is proposed as a quick and efficient technique to distinguish between vesicle adhesion and bilayer deposition on solid surfaces in general. Introduction The interaction of water-soluble surfactants with solid polymeric or mineral surfaces has been intensively and exten- sively studied. 1-3 However, the potential of bilayer-forming amphiphiles as interface agents able to modify solid surfaces remains hitherto poorly explored. Bilayer-forming amphiphiles assemble on solid particles or planar surfaces either as a bilayer, a monolayer, or adhered vesicles depending on the nature of the amphiphile and surface and on interactions driving am- phiphile deposition. 4-16 Bilayer deposition of dioctadecyldi- methylammonium bromide (DODAB) and sodium dihexadecyl phosphate (DHP) from vesicles onto oppositely charged poly- styrene microspheres was reported. 4 There is an electrostatically driven vesicle adhesion to the latex that is followed by bilayer deposition. 5 Thereafter, as amphiphile concentration increases, vesicles adhere to the bilayer-covered latex with or without disruption, depending on nature of the synthetic amphiphile. 5 In general, the interaction between bilayer vesicles and solid surfaces in the form of particulates 6-15 or planar surfaces 16,17 still lacks characterization from the point of view of exact physical parameters. Recently, the physical adsorption of bilayer-forming amphiphiles on hydrophilic silica, in particular, was shown to depend on previous centrifugation of the vesicle sample, pH, buffer, temperature, and physical state of the bilayer. 14 In fact, establishing suitable experimental conditions for the occurrence of bilayer deposition as well as an efficient and quick methodology to ascertain whether bilayer deposition indeed took place on a solid surface is still a problem for those interested in producing supported bilayers. Many applications of supported bilayers in biotechnology are presently hampered by the poor reproducibility of various bilayer deposition methods. The process of bilayer deposition on solid substrates is indeed thought as “magic”. Therefore, its description in terms of exact physical parameters such as the contact angle 10 or surface roughness 16 may significantly improve our understanding of the underlying physicochemical processes on a molecular scale. In this work, we propose the determination of dynamic contact angles as a major tool to distinguish between vesicle adhesion and bilayer deposition on a solid surface. Minimization of contact angle hysteresis and maximization of the contact angle value are established as precise criteria to identify bilayer deposition on solid surfaces in general. In addition, the influence of time and DODAB concentration on formation of bilayers or adhered vesicles on planar SiO 2 surfaces is described. Material and Methods Dioctadecyldimethylammonium bromide, DODAB (99.9% pure), was obtained from Sigma Chemical Co. (St. Louis, MO). Analytical determination of DODAB concentration was per- formed as previously described. 4,5 All other reagents were analytical grade and used without further purification. Water was Milli-Q quality. Small DODAB vesicles were prepared by sonication with the tip. 4,18 Thereafter, the dispersion was centrifuged (14 000 rpm/h) to eliminate multilamellar vesicles and titanium ejected from the tip. A typical vesicle mean diameter is 86 nm. 4 Silicon monocrystalline slides (〈100〉 for crystal orientation, P type, 10-20 Ω cm of resistivity) of 0.4 mm mean thickness were cut with a diamond tip in order to obtain rectangular planar surfaces (25 × 15 mm). Before thermic oxidation, cleaning of the silicon surface was performed in accordance with standard procedures for silicon uses in microelectronics. 19 Rapid thermic oxidation 20 was carried out under dried oxygen (less than 5 ppm water) at 1150 °C/500 s using a Heatpulse 410T (A G Associates). A thickness of about 30 nm for the SiO 2 layer * To whom correspondence should be addressed. FAX 055 11 815 5579; E-mail mcribeir@quim.iq.usp.br. ² Departamento de Bioquı ´mica, Instituto de Quı ´mica. ‡ Departamento de Engenharia Eletro ˆnica, Escola Polite ´cnica. 4011 J. Phys. Chem. B 1998, 102, 4011-4015 S1089-5647(97)03295-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/22/1998