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Final Report Summary - MAGNETOTUBE (1D magnetic nanostructures using mineralizing peptides)

Description of project context and objectives

Magnetite nanotubes on one hand are of interest for numerous applications including MRI, biological and molecular separation, removal of environmental pollution and catalysis (S. J. Son, et al.;JACS 2005). Peptides on the other hand are biologic macromolecules able to self-assemble into nanostructures in water (X.Zhao et al.; ChemSocRev2010). Their physico-chemical properties can easily be tuned and they have proven to be adequate templates for mineralizing materials like silica (E.Pouget et al.;NatureMat.2007). Recent technological advances allow to synthetize short peptides at lower cost and in larger amount. The possibility to combine peptide nanostructures with magnetite is expected to allow the fabrication of low cost, biocompatible, ecologically friendly, self-assembled magnetic nanotubes. The initial main objective of this project was thus to create low-cost magnetic peptidic nanotubes using a bottom-up approach. Additionally, increasing the knowledge on nanostructures formed by self-assembling peptides can be considered as a secondary objective.
The research fellow has a previous expertise in self-assembling peptidic nanotubes through his PhD while the scientist in charge is a specialist of magnetite nanoparticules formation through chemical and biological processes. The Max Planck host institution is specialised in biomaterials and offers access to a large range of material characterisation techniques.

Work performed since the beginning of the project and the main results achieved so far

We started the project by selecting short peptides for their ability to form nanotubes in water, potential ability to template silica and magnetite and low cost of production in order to be able to produce enough material. It quickly appeared that the production costs of Bis(N-alpha-amido-glycylglycine)-1,7-heptane dicarboxylate and cyclic alternating D- and L-alpha-peptides were too high to be used for nanomaterial production in appropriate quantities. Three peptides were chosen to make nanotubes at low cost: diphenylalanine (FF), AAAAAAK (A6K) and Lanreotide. To make 1D peptidic nanostructures covered with silica and/or magnetite, we tested for each of them the effect of the variation of numerous parameters on their assembly, including pH, concentration, solvent, counterions, reaction time, addition of surfactants, temperature, silica precursors and different types of magnetite nanoparticles.

1) Diphenylalanine
The diphenylalanine peptide (FF) formed easily crystalline nanotubes / nanowires of 500 nm to 1 μm diameter in milligram amounts when the solubilized powder is transferred from a hydroxyfluoropropanol or an ethyltrifluoroacetate solution to water. Synthetic magnetite particles as well as magnetosomes extracted from magnetotactic bacteria bind to the diphenylalanine nanotubes / nanowires through what appear to be a hydrophobic effect (Figure 1). For the same reason, the FF structures tend to entangle together but can be separated mechanically with a micromanipulator. This 1D magnetic nanostructures can be produced in milligrams quantities but they have a few disadvantages: 1) The need of toxic solvent to solubilize first, 2) The nanostructures slowly deteriorate if kept in water, and 3) Silica precursors do not bind specifically to the diphenylalanine and form a silica matrix around them.

2) A6K
A6K peptides with TFA salt form a gel of nanotubes at a concentration of 12% w/w (peptide mass/total mass) and above. The peptidic nanotubes have a diameter of 26 nm and a micrometric length (Figure 2). After numerous experiments, it appeared that this critical concentration limit seems too high to be compatible for proper silica or magnetite mineralization of the nanotubes. In order to have a proper silica mineralization using silica precursors, the dynamics of silica deposition and peptide growth must be finely tuned. However, nanotubes do not form below the critical concentration in the case of the A6K. Above the critical concentration, there are so many peptide nanotubes that silica precursors or magnetic particles do not mix properly with them and instead forms clusters. Also, the A6K peptidic gel at high concentration is very difficult to manipulate in thin glass capillaries. We have tried to solution this problem by mixing quickly the silica precursors with the A6K peptide at high concentration by vortexing in Eppendorf plastic tubes. In that case however, silica / peptide nanofibers of 2 nm diameter with a 100 nm length were observed but no nanotubes (Figure2).

3) Lanreotide
Lanreotide peptide, which was used during the fellow’s doctoral work to form centimeter long bundles of silica / peptide nanotubes, was also tested in numerous conditions. The Lanreotide forms micrometric long peptidic nanotubes with diameter of 24 nm. Lanreotide is mixed with TEOS or TMOS silica precursors to form silica nanotube bundles. Depending of the dynamics of the mineralization conditions, the hybrid nanotubes bundles can form centimeter long fibers or a short and thick block. Magnetic nanoparticles were incorporated in the peptide gel and trapped in by the forming silica matrix. It was not possible to form magnetite directly on peptidic nanotubes. Different magnetic particles have been tested but only commercial polyethyleneimine (PEI) coated iron oxide particles of 20 nm diameter mixed properly with the Lanreotide / silica system. The coated iron oxide nanoparticles incorporated in a homogenous way on the silica walls but the silica deposition layer became also thicker. The PEI cover helps the solubilisation and non-aggregation of iron oxide nanoparticles in water. The resulting self-assembled nanotubes are made of peptide / silica / PEI coated iron oxide, with an average diameter of 80 nm and an average length of 500 nm. Their formation is very simple: It only requires a simple two steps mix in water at room temperature. First, the mixture of the peptide with iron oxide nanoparticles is prepared and then this mixture is placed in a glass capillary with a solution of hydrolysed silica precursors on the top (Figure 3). The iron oxide particles observed in TEM images are incorporated in the silica wall. When 30 nm iron oxide nanoparticles were used, the resulting silica/peptide/iron oxide macroscopic fibers were magnetic.

One of the planned strategies of the project was also to incorporate the magnetic nanoparticles inside the silica nanotubes but the nanoparticles never seemed to enter inside preformed Lanreotide/silica nanotubes when mixed together. Empirical experimentation seems to indicate that nanoparticles can enter into a nanotube if the internal diameter is at least 5 times the diameter of the nanoparticle. In our case, the 24 nm diameter of Lanreotide nanotubes is too small even for 10 nm iron oxide nanoparticles to enter.

In conclusion, we were able to create two kinds of 1D magnetic nanostructures using two different self-assembling peptides. The resulting magnetic nanomaterials were not homogenous enough or in too small quantities for a macroscopic test of physical, magnetic and optical properties. We have shown that it is possible to form self-assembled peptidic nanotubes composed of silica and iron oxide nanoparticles in a simple two steps mix in ambient conditions. This may lead to more simple processes to form self-assembled magnetic nanotubes. A publication reporting these results is in preparation for a peer-reviewed journal.

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