Published: August 29, 2011 r2011 American Chemical Society 12506 dx.doi.org/10.1021/la202087u | Langmuir 2011, 27, 12506–12514 ARTICLE pubs.acs.org/Langmuir Structure of DNAÀCationic Surfactant Complexes at Hydrophobically Modified and Hydrophilic Silica Surfaces as Revealed by Neutron Reflectometry Marit  eC  ardenas,* ,† Hanna Wacklin, ‡,§ Richard A. Campbell, ‡,|| and Tommy Nylander || † Nanoscience Center and Institute of Chemistry, Copenhagen University, Universitetsparken 5, DK-2100 Copenhagen E, Denmark ‡ Institut Laue-Langevin, 6 rue Jules Horowitz, BP 156, 38042 Grenoble, France § European Spallation Source ESS AB, P.O. Box 176, 221 00 Lund, Sweden ) Physical Chemistry, Department of Chemistry, Lund University, P.O. Box 124, SE-22100 Lund, Sweden ’ INTRODUCTION Complexes of DNA with cationic species are important in living systems as well as in a range of applications, which has promoted a large variety of studies in the last decades. DNA separation and purification methods were developed as early as 1967 on the basis of the characteristic phase separation that occurs in the presence of cationic surfactants. 1,2 The potential of using DNAÀcationic surfactant (or lipid) systems (lipoplexes) as nonviral vectors for gene delivery has been recognized, and the number of studies of such systems has therefore increased during recent years. 3,4 The stability of lipoplexes at biological interfaces is crucial given that gene delivery particles must circulate in the bloodstream for several hours, 4 where they will encounter different types of interfaces that they are likely to interact with, which can influence the uptake of DNA at the target. Hence, determining the interfacial behavior of lipoplexes is a key parameter in designing efficient gene delivery systems. In mixed solutions of oppositely charged polymers and surfactants, asso- ciative phase separation as a consequence of the attractive interaction between the surfactant and the polymer is a well- known phenomenon, 5,6 and many of the features from such system are also observed for the DNA/cationic surfactant systems. In the bulk, surfactant addition to a DNA solution will therefore lead to an initial complexation on the single-molecule scale, at a so-called critical association concentration (cac) that is below the cmc of the surfactant. The cooperativity has been demonstrated for a range of DNA condensing agents such as multivalent ions, cationic lipids, and surfactants, and the con- sequence is the collapse of the DNA coil on the single-chain level as a discrete all-or-none type of transition to a globule. 7À10 If the surfactant concentration is further increased above this point, then the complexes formed by individual DNA molecules aggregate and precipitate out of the solution. Such phase separation is believed to occur via a compensation of the elec- trical charges of the DNA molecule, which decreases the aqueous solubility of the complexes. The structure of the precipitated phase resembles the liquid-crystalline structures formed by the surfactant (complexing agent) and includes lamellar, hexagonal, cubic, and other types of mesophases. 11À15 The formation of DNA and cationic lipid/surfactant complexes at interfaces differs from the corresponding process in the bulk solution in that complex formation occurs over a different Received: November 28, 2010 Revised: August 13, 2011 ABSTRACT: In this article, we discuss the structure and composition of mixed DNAÀcatio- nic surfactant adsorption layers on both hydrophobic and hydrophilic solid surfaces. We have focused on the effects of the bulk concentrations, the surfactant chain length, and the type of solid surface on the interfacial layer structure (the location, coverage, and conformation of the DNA and surfactant molecules). Neutron reflectometry is the technique of choice for revealing the surface layer structure by means of selective deuteration. We start by studying the interfacial complexation of DNA with dodecyltrimethylammonium bromide (DTAB) and hexadecyltrimethylammonium bromide (CTAB) on hydrophobic surfaces, where we show that DNA molecules are located on top of a self-assembled surfactant monolayer, with the thickness of the DNA layer and the surfactantÀDNA ratio determined by the surface coverage of the underlying cationic layer. The surface coverages of surfactant and DNA are determined by the bulk concentration of the surfactant relative to its critical micelle concentration (cmc). The structure of the interfacial layer is not affected by the choice of cationic surfactant studied. However, to obtain similar interfacial structures, a higher concentration in relation to its cmc is required for the more soluble DTAB surfactant with a shorter alkyl chain than for CTAB. Our results suggest that the DNA molecules will spontaneously form a relatively dense, thin layer on top of a surfactant monolayer (hydrophobic surface) or a layer of admicelles (hydrophilic surface) as long as the surface concentration of surfactant is great enough to ensure a high interfacial charge density. These findings have implications for bioanalytical and nanotechnology applications, which require the deposition of DNA layers with well-controlled structure and composition.