Gaelle OFFRANC-PIRET

The purpose of this post-doc is to investigate the effects of surface topography and chemistry on cell growth in order to develop highly biocompatible neural implants. Various surface topographies will be achieved using either bottom-up or top-down approaches. In particular III-V nanowire surfaces and silicon nanowire surfaces will be investigated.

* 1st project
Guided excitation is an attractive strategy for in situ real time use of fluorescence biochips. This post-doctoral work dealt with the deposition of an amorphous silicon carbon nanofilm on a fluorescent biochip based on this guided excitation. This nanofilm allows for a well controlled surface functionalization with limited increase of waveguide losses.
The biochip works with a secondary excitation using Eu(TTA)3(H20)3 to absorb UV and emit at the excitation wavelength. The last one is guided thanks to the TiO2 planar waveguide which promotes the excitation and the detection of interactions at the surface of the biochip. During a real time acquisition, the fluorescence noise of the targets solution is then limited as compared to classical biochip.
The deposition of an amorphous silicon carbon nanofilm by Plasma Enhanced Chemical Vapor Deposition is realized onto the TiO2 biochip surface. The nanofilm has to be optimized in order to keep good waveguide properties. This nanofilm allows the binding of biomolecules (DNA, proteins) through well controlled surface functionalization processes. The quality of the surface biochemistry and the quality of the waveguide will give access to additional valuable quantities on biological interaction mechanisms during fluorescent real-time measurements, and allow new diagnostic applications.

*2nd project
Nanowires made of crystalline silicon, demonstrate a high potential for applications (biosensors, batteries, solar cells, labs on chip,...). We reported, for the first time, the elaboration of similar nanostructures from amorphous silicon and silicon-carbon alloys.
We studied the formation of hydrogenated amorphous silicon-carbon alloy (a-Si1-xCx:H) nanowires for different carbon concentrations (0 – 7 %) by using Ag-assisted electroless etching of the thin a-Si1-xCx:H films deposited by plasma-enhanced chemical vapour deposition from silane/methane gas mixtures. The nanowires morphologies (length, density, ...), studied by scanning electron microscopy, strongly depend on the concentration of the etchant (aqueous solution of hydrofluoric acide and silver nitrate), the etching time, and the carbon concentration of the deposited layer.
Since these alloys are inexpensive and can be deposited in large area on a variety of substrates (glass, plastics, metals or semiconductors), this opens the way to the integration of such nanostructures in different devices for a variety of applications.

PhD "Matrix-free laser desorption/ionization mass spectrometry on silicon nanowires and applications " - IRI (Institut de Recherche Interdisciplinaire), collaboration with IEMN (Institut d'Electronique, de Microelectronique et de Nanotechnologie), IBL (Institut de Biologie de Lille), INRA (Institut National de Recherche Agronomique) and UGSF (Unité de Glycobiologie Structurale et Fonctionnelle).

My PhD work dealt with the fabrication of an inorganic silicon nanowires substrate dedicated to the sensitive detection of biomolecules by laser desorption/ionisation mass spectrometry (LDI-MS).

I varied the morphology, composition and surface chemistry of silicon nanowires. For this, two different synthesis of silicon nanowires have been used: the Vapor Liquid Solid one and the silicon electroless chemical etching one. The parameters of synthesis have been changed in order to lead to various types of homogeneous silicon nanowires arrays, that have been observed by Scanning Electron Microscopy. The surface chemistry of silicon nanowires has been modified by photochemical grafting of alcene or acid on silicon and has been investigated by X-ray photoelectron spectroscopy.

I looked at the importance of optical and thermal properties, wetting properties and accessibility of analytes to the laser beam on the performance of silicon nanowires for LDI-MS. The optimized nanowire substrate exhibits a high sensitivity for the detection of low mass molecules (50 times higher than classical MALDI) and of peptides mixtures resulting from digestion of proteins. The silicon nanowires substrate was successfully used in a biochip format to immobilize peptides and to follow the course of the methylation reaction of those peptides. Furthermore, the silicon nanowires substrate integration in a lab on chip was investigated: a 1 µL droplet of a peptide mixture (50.10-15M) was displaced by digital microfluidics electrowetting on dielectric (EWOD) and successfully analyzed by LDI-MS.

I finally developed an original method combining the chemistry and topography of silicon nanowire surface, using optical lithography and chemical plasma etching technique. This method leads to areas with different liquid/solid surface tensions and enable localized adhesion of proteins, cells and bacteria. In complement to this last work and in accordance with a future development of a cell on chip, I studied the behavior of fibroplast cells in contact with silicon nanowires: viability, shape and behavior of cells have been investigated by fluorescence microscopy, Scanning Electron Microscopy and Transmission Electron Microscopy.