Langmuir-Blodgett Patterning of Phospholipid Microstripes: Effect of the Second Component Xiaodong Chen, ² Nan Lu, ‡ Hui Zhang, ‡ Michael Hirtz, ² Lixin Wu, ‡ Harald Fuchs, ² and Lifeng Chi* ,² Physikalisches Institut and Center for Nanotechnology (CeNTech), Westfa ¨lische Wilhelms-UniVersita ¨t, D-48149 Mu ¨nster, Germany, and Key Laboratory for Supramolecular Structure and Materials of Ministry of Education, Jilin UniVersity, 130023, Changchun, People’s Republic of China ReceiVed: January 13, 2006; In Final Form: March 6, 2006 We systematically describe the striped pattern formation of the mixed monolayers of 1,2-di(2,4-octadeca- dienoyl)-sn-glycero-3-phosphocholine (DOEPC) and L-R-dipalmitoylphosphatidylcholine (DPPC) from the liquid expanded (LE) phase onto a mica surface by Langmuir-Blodgett (LB) transfer. The addition of the second component, DOEPC, strongly affects the formation of DPPC stripe patterns. When the molar ratio of DPPC and DOEPC is 1:0.1, the horizontal stripes dominate, while in the case of pure DPPC monolayer, there are three kinds of patterns: horizontal stripes, grids, and vertical stripes. The width and periodicity of stripes formed from the mixed monolayers are ca. 4-5 times smaller than those formed from pure DPPC patterns at the same transfer conditions, while the widths of channels are similar. A phase shift of substrate- mediated microphase separation in the two-component system during LB transfer is considered to be the mechanism for the influence of the second component on the formation of DPPC stripe patterns. Introduction Self-assembly and self-organization provide interesting routes toward the fabrication of patterned structures (from a few nanometers to micrometers in feature size) via a bottom-up approach. 1 The self-assembly and self-organization processes and the properties of the surface patterns (shape, size, function, etc.) can be controlled by tailoring the properties of building blocks. Two typical examples are block copolymer lithography, 2 where structures depend on the preparation conditions as well as the chemical properties of participating polymer fragments, and nanosphere lithography, 3 where the lateral dimensions can be controlled by varying the diameter of spherical colloids. Recently, it has been realized that the well-known Langmuir- Blodgett (LB) technique is another efficient way toward the fabrication of laterally patterned structures on solid supports that are chemically or physically differentiated on the micron or submicron scale, termed LB patterning here. Normally, laterally structured LB monolayers are generated by the deposition of regular two-dimensional (2D) domains formed at the air-water interface onto solid substrates. The shape of domains at the air-water interface can be extensively controlled by subphase conditions (e.g., composition and temperature), external electrical fields, molecular chirality, and the molecular composition of the monolayer. 4-6 The domains of shapes depend on two competing effects: interfacial line tension between the two lipid phases that favors circular domains, and repulsive dipole forces between the lipid molecules that favor elongated shapes. 4,5 Alternatively, the LB transfer process itself can be used to induce phase transitions and pattern formation near the three phase contact line from a homogeneous Langmuir monolayer, which was directly observed by fluores- cence microscopy. 7,8 Schwartz et al. observed dendritic patterns when fatty acids deposited from the liquid expanded (LE) phase undergo a substrate-induced phase transition on substrates to the liquid condensed (LC) phase. 9,10 Previously, we described the formation of L-R-dipalmitoylphosphatidylcholine (DPPC) stripe patterns by the LB transfer of the DPPC monolayer at the surface pressures below the LE-LC phase transition pressure. 11,12 The pattern formation was attributed to substrate- mediated first-order phase transition of DPPC, being a typical example of “self-organization”, and meniscus oscillation during LB transfer. Just as its name implies, the molecule-substrate interaction plays a very important role in the condensation process. Recently, Badia et al. 13,14 generated the pattern with parallel stripes by the LB transfer of a mixed phospholipid (DPPC and DLPC (L-R-dilauroylphosphatidylcholine)) mono- layer from the air-water interface onto a mica substrate; however, they suggested that the stripe formation in the mixed films is due to a cyclical condensed-phase nucleation and depletion process that is coupled to dynamic wetting instabili- ties, 13 not due to substrate-mediated condensation. 9-12 The difference is that Badia et al. transferred the mixed monolayer at high surface pressure, where DPPC is already in the LC state, while we transfer the monolayer below the phase transition point where DPPC is in the LE state. 11,12 Similar to the case of Badia et al., Purrucker et al. 15 reported a stripe micropattern on a solid substrate from a lipid/lipopolymer mixed monolayer by the LB transfer; however, the stripes are aligned perpendicular to the meniscus, which has also been observed in the case of pure DPPC. 16,17 Analogous stripe patterns (parallel to the meniscus) of fatty acid monolayers on mica surface, depending on the pH, were found by Vollhardt et al. 18,19 In this case, there is no phase transition (as the floating monolayer is already in a condensed phase) taking place during the LB transfer but rather a transition * Corresponding author. Phone: +49-251-83-33651. Fax: +49-251-83- 33602. E-mail: chi@uni-muenster.de. ² Westfa ¨lische Wilhelms-Universita ¨t. ‡ Jilin University. 8039 J. Phys. Chem. B 2006, 110, 8039-8046 10.1021/jp0602530 CCC: $30.25 © 2006 American Chemical Society Published on Web 03/31/2006