Luminescent and absorptive metal-coated emulsions for micro-velocimetry
Fluorescent latex beads have been widely used as tracers in microfluidics over for the last decades. They have the advantages to be density matched with water and to be easily localizable using fluorescence microscopy. We have recently synthesized silver-coated oil droplets that are both luminescent and absorptive, by first coating the oil interface with a polydopamine layer and then depositing a silver layer by a redox process. They have a mean diameter of 6 µm and a their density has been matched to the density of water by adjusting the thickness of the metallic layer. In this work we used these particles as tracers to measure the velocity profile of a water flow in a PDMS microchannel with a rectangular cross-section, which allowed us to confirm the predictions of the Stokes equation with results comparable to those of common submicronic polystyrene particles.
Reference : O. Mesdjian, Y. Chen, J. Fattaccioli. Micro Nano Engineering (MNE) Conference (2015).
Spatially-Controlled Protein Crystallization in Microfluidic Chambers
We developped a simple microfluidic device able to trigger the nucleation of the crystals at specific locations on the microchip for the statistical study of protein crystallization. The microsystem is an array of independent PDMS microchambers connected to a fluid-dispensing channel. The chambers are filled with a crystallizing aqueous protein solution and then sealed with a fluorinated oil phase. Each chamber presents a small oil/water interface at the connection with the main channel. The crystals most likely grow near the interface, allowing a microscopic observation of the nucleation events at specific positions on the chip. For the sake of demonstration, the method is applied the crystallization of HEW lysozyme.
Transport of Nanodroplets by Molecular Motors
In collaboration with Dr. Celine Bottier, during my postdoctoral stay at the University of Tokyo, we have studied the transport properties of emulsion droplets by linear biomolecular motors, kinesins, on oriented microtubules in microchannels. Although being small, with a diameter of around one micron, the droplets were far bigger than the solid particles used commonly for this kind of studies, thus able to bear much more kinesins on their surface. We explored how far and how quick the transport occur as a function of the droplet’s size and we showed that if the speed of the droplets is independent of their size, counterintuitively their run length increases. This results unravel the possibility to use molecular motors as a transportation mean for the miniaturization of BioMEMS.
Localized Anodization of Aluminum and Patterning
We investigated the localized anodization of aluminum evaporated on silicon substrate using micro scale patterns. Positive (S1818) and negative (SU-8 2035, KMPR 1010, KMPR 1035) photoresist patterns as well as SiO2 micro patterns were fabricated on aluminum substrates respectively. Then, anodization of each substrate was performed in oxalic acid electrolyte. We observed the formation of aluminum/alumina patterns on a flat substrate by SEM images of its crosssection.This work provides an easy and simple technique for the fabrication of aluminum/alumina micro patterns by localized anodization.
MicroContact Priting and RICM
Microcontact printing is a versatile technology used to create molecular micropatterns through the conformal contact of a microstructured elastomeric stamp with a wide range of substrates. Low-cost and straightforward, this technique is used in several engineering domains, from chips microfabrication to biophysical studies. An elastomeric, PDMS, is molded in a hard microfabricated template, usually in silicon, leading to a high precision soft replica of the master. After fabrication, the stamp is inked with the material that is to be printed, pushed toward the substrate with a controlled pressure and then removed. The limitation of the resolution of the microcontact printing technique is a consequence of two phenomena occurring during the stamping : the deformation of the stamp under pressure and the diffusion of the ink molecules on the substrate during the contact. I introduced the Reflection Interference Contrast Microscopy (RICM) to visualize in-situ the deformation of the PDMS stamp during the inking process on a glass surface. RICM allows imaging the Newton rings created by a object in close vicinity of a transparent surface and then reconstructing its three-dimensional profile . We showed that the collapse of the stamp tips in air lead to a strong increase of the surface area of the patterns independently of the diffusion process and we showed that the behavior of the patterns with the stamping pressure is similar to the behavior of the small-scale deformation of the stamp tip.
Temperature Measurement of Silicon Nanowires
Silicon nanowires, used as field-effect transistors, are now commonly integrated in CMOS devices for the detection of biomolecules. Their small width, a few tenth of nanometers, is suitable for the study of single molecules, and since their small volume shows very few thermal inertia, nanowires could constitute good candidates for the study of protein folding and unfolding dynamics. For such application, a robust, highly spatially resolved temperature mapping technique is necessary for a proper calibration of the device. Using proteins conjugated with a fluorescence probe, Rhodamine B, whose quantum yield decreases with temperature, we functionnalized the nanowires and we calibrated them in a liquid physiological buffer by fluorescence thermometry, with the help of an epifluorescence microscope solely.