Self-Assembly strategies for controlled spatial organization
A pure self-assembly approach towards spatial organization, to provide a complementary paradigm to the currently available top-down methodologies, including lithographic techniques, would in itself be a very desirable goal. Other parameters like cost and ease of fabrication only seem to be speaking more in favor of such an approach. Self-assembly, defined as the spontaneous, thermodynamically controlled, organization of individual molecules into a (meta) stable and (spatially) well-defined aggregate usually is a complicated and not very well understood phenomenon. It critically relies on a delicate interplay of many interactions, often of different nature. It is exactly for this reason that self-assembly approaches have been put forward as an alternative to top-down methodology in nanotechnology. Self-assembly promises to be a cheap and potentially extremely easy methodology to decorate surfaces with all kinds of interesting templates. In general it would be highly desirable to be able to control interfacial properties, as interfaces play such a prominent role in many processes, e.g. in catalysis, molecular recognition, and as sites of nucleation, both in natural and synthetic systems. We developed new techniques to pattern chemically and topographically polymeric and silicon substrate on nanometer scale for biological investigations. In particular three different methods have been used to nanostructure surface by using the self-assembly of molecules.
Nanocontainers at Surfaces
Two-dimensional (2D) porous substrates have attracted great attention for a wide range of applications including chemical microcontainers, surface-plasmon resonance biosensors, catalytic supports and photonic crystals. We present a versatile and simple approach for rapidly fabricating nanopatterned surfaces on micrometer scale. The nanopatterned surfaces consisted of 2D nanopore arrays, having internal area of gold surrounded by polymeric matrix. The nanopores depth can be modulated by using UV-ozone treatment, in particular by increase time treatment, decrease pore depth. The preferential adsorption of proteins and other biological molecules can be investigated onto the nanostructured surfaces prepared from different spin-coated polymers. The adsorption is studied as a function of the pore geometrical features, including volume, aspect ratio and diameter, as well as the chemical contrast. The driving chemical factors were identified in terms of surface free energy gradients and chemical termination of the pore bottom and walls.
Self-structuring Langmuir-Blodgett films
Langmuir-Blodgett technique was used to obtain a regular patterned large-area with mesostructured features. This strategy uses a simple fabrication technique to control the alignment, size, shape, and periodicity of self-organized phospholipid monolayer patterns with feature sizes down to 100 nm over surface areas of square centimeters. Because of the anisotropic wetting behavior of the patterns, they can be used as templates to direct the self-assembly of functional molecules and nanocrystals. The described mesoscopic structured surfaces may serve as a platform in engineering the biological/material interface and constructing biofunctionalized structures. The striped pattern formation of the mixed monolayers of L--dipalmitoylphosphatidylcholine (DPPC) and poly--caprolactone (PCL) onto a mica surface by Langmuir-Blodgett (LB) transfer have been investigated. The addition of the second component, PCL, strongly affects the formation of DPPC stripe patterns in term of pressure and transfer rate values. Self-assembled monolayers of DPPC and DPPC/PCL adsorbed on mica are examined by dynamic force spectroscopy under ambient conditions. By a systematic recording of the frequency shift caused by the tip–sample interaction we determine the corresponding tip–sample potential and force curves. Due to the systematic mapping of the tip–sample interaction it is possible to compute contour maps of the tip-surface potential of the DPPC films and to extract local properties like contact stiffness and adhesion force.
Self-organization of amphiphilic peptides
TMolecular self-assembly has become a widely used method for fabrication of biological and biocompatible structures at the nano- and micrometer range. In particular, amphiphilic peptides have been shown to self-assemble into a variety of cylindrical nanostructures, with diameter of a few nanometer and micrometer scale lengths. The self-assembling behavior on surfaces of two classes such peptides, both composed of alanine (A), Aspartic acid (D) and Lysine (D), with different structure has been studied by using Atomic Force Microscopy. These compounds were a 5-amino acid amphiphilic peptide (AcA4DOH), characterized by single tail, and a 10-amino acid one ((AcA4)2KDOH) having a double tail. The peptides showed different organization patterns ranging from unstructured round shape aggregates to 1D-wires. The following surfaces of different wettability and charge state have been investigated: mica as reference surface, two positively charged polyelectrolyte surfaces, and a negatively charged one. The effect of pH values on the aggregation state of the single and double-tail peptides has been also investigated. The self-organization process is shown to be severely affected by the type of peptide and surface properties, including specifically the charge state at a given pH. The results strongly support a self-assembly mechanism based onto a very specific organization processes at the liquid-solid interface, based onto the active role played by the surfaces to promote the orientation and organization of single molecules as the prime processes at nanometer scale, followed by the aggregation process at the mesoscopic scale.