Self-assembly of highly oriented lamellar nanoparticle-phospholipid nanocomposites on solid surfaces.

When nanoparticles (NPs) are embedded in organic matrices, the materials can show not only combined properties of the original components but also improved performances not seen in the original components.1 To this end, it is important to control the size, shape, and dispersion of the NPs on a chemically diverse range of supports. However, controlling materials at the nanometer scale is of great challenge, particularly when long-range ordering is pursued. In most of the studies, the NPs are synthesized in situ, and it is difficult to control their size distribution and dispersion on the support.1,2 As monodispersive NPs can nowadays be routinely synthesized, more researchers are trying to use self-assembly,3 field-directed deposition,4 template-directed self-assembly,5 and layer-by-layer assembly6 to fabricate NP-containing composites. In most of the methods, complicated processing procedures and/or charge matching are required, but a well-dispersed structure with long-range ordering has seldom been obtained. In this work, we exploit the self-assembly property of amphiphilic molecules to disperse water-soluble NPs. The self-assembly allows the production of ordered lamellar structure without the requirement of high-precision patterning and/or opposite-charge matching. In our method, a drop of a mixed solution of NPs and phospholipids was cast onto the surface of a silicon or glass wafer. Multilayered nanocomposite was formed after the rapid evaporation of water in the evacuated chamber (Figure 1a). The phospholipid used is dioleoyl-sn-glycero-phophocholine (DOPC), and the original NP solution is CdTe suspension in water (∼3 nm in diameter, stabilized with 3-mercaptopropionic acid, and negatively charged because of the carboxy groups on the surface). The process begins with a DOPC solution of an initial concentration (co ) 2.0 mg/mL) higher than the critical micelle concentration, and sonication results in formation of micelles and liposomes. The added NPs adsorb onto the membranes to form stable nanoparticle-liposome hybrids in the solution.7 When cast onto a hydrophilic surface, the solution outspreads over the solid surface. Evaporation-induced enrichment of the phospholipids leads to conglutination and fusion of the liposomes (with the adsorbed NPs), and promotes cooperative assembly of the molecules into a liquid-crystalline mesophase with the NPs being confined in the hydrophilic interfaces. The rapid solvent evaporation is believed to be crucial for the assembly process (Supporting Information), making them different from the assembly of DNA/lipid complexes in solutions.8 The evaporation is faster at the edge than in the interior of the spreading fluid. An outward flow of the solvent develops because the solvent removed via evaporation must be replenished by a flow from the interior.3c The flow endows a shear on the lamellar mesophase, aligning all the interfaces parallel to the solid surface. The cross-section TEM images in Figure 1 show clearly welldefined lamellar distribution of the NPs. The phospholipids are invisible in the TEM images, but the observed fluctuation of the layers and variation of the interlayer distances are intrinsic to them. The X-ray reflectivity (XRR) analysis confirms that the structure is homogeneous and highly ordered. The grazing incident X-ray diffraction (GIXRD) measurements reveal a peak at 1.63° (inset in Figure 2a), indicating that the NPs are two dimensionally ordered at the interfaces between the lipid bilayers. The period of the film increases gradually from 42.7 Å (for pure lipid film) to 46.9 Å Figure 1. (a) Cartoons showing the fabrication process and the structural model of the nanocomposite. (b, c) Cross-section TEM images of two typical samples.