Hierarchical Collagen-Hydroxyapatite Nanostructures Designed through Layer-by-Layer Assembly of Crystal-Decorated Fibrils.

A comprehensive understanding of the mechanism by which type I collagen (Col) interacts with hydroxyapatite nanoparticles (Hap NPs) in aqueous solutions is a pivotal step for guiding the design of biologically relevant nanocomposites with controlled hierarchical structure. In this paper we use a variety of Hap NPs differing by their shape (rod vs platelet) and their size (∼30 vs ∼130 nm) and investigate their mechanism(s) of interaction with collagen. The addition of collagen to the Hap suspensions induces different effects that strongly depend on the nanoparticle type. Interestingly, the use of small rods, typically with ∼30 nm of length (R30), leads to the formation of assembled collagen fibrils decorated with Hap nanocrystals which, in turn, self-assemble progressively to form larger fibrillar Hap-Col composite. The crystals decorating collagen provide "intrinsic" negative charges to the fibrillar objects that allow their incorporation in three-dimensional structure using layer-by-layer (LbL) assembly. This offers a straightforward way to construct a collagen-based hybrid material with well-defined hierarchy under near-physiological conditions. In situ, QCM-D monitoring revealed the buildup of soft and highly hydrated hybrid (PAH/R30-Col)n multilayers for which the mechanism of growth was very different from that observed for polyelectrolytes and nanoparticles without collagen (PAH/R30). The LbL assembly of crystal-decorated collagen yields a hierarchical nanostructured film whose thickness and roughness can be modulated by the addition of salt and incorporate fibrillar objects of about 400 nm in width and few micrometers in length, as probed by AFM. The approach described in this work provides a relevant way to better control the (supra)molecular assembly of Col and Hap NPs with the perspective of developing hierarchical Hap-Col nanocomposites with tuned properties for various biomedical applications.

Preparation of an electrochemical biosensor based on lipid membranes in nanoporous alumina.

Model lipid bilayers are versatile tools to investigate the molecular processes occurring at the membrane level. Among the model membranes, substrate supported bilayers have attracted much interest because they are robust and they can be investigated by powerful surface sensitive techniques such as electrochemical measurements. In a biosensor, lipid films can be used not only as a support for the biological sensing elements but also as sensing elements themselves to detect molecules that are able to alter the structure and the properties of biomembranes. In this work, we have prepared a tethered lipid membrane-based biosensor able to detect the alterations of membrane structure and fluidity. This tethered lipid membrane was prepared in a nanoporous aluminium oxide that provides a high surface area and a protective environment against dewetting. The membrane contained PEG-PE lipids as hydrating, protective and tethering agents and ubiquinone which is a redox lipophilic mediator embedded within the acyl chains of the lipid bilayer. The lipid membrane was prepared inside the pores of the nanoporous support by a PEG-triggered fusion of liposomes. This sensing system was efficient to detect the alterations of lipid membranes that are induced by the addition of a commonly used non-ionic detergent: Triton X-100.