Micro ElectroChimie et Optique couplées : vers des microLeviers pour l'Imagerie et l'Etude de Réactions de Surface – µecoliers

Context and Objectives Microcantilevers are promising mechanical objects for the development of physical chemical and biological sensors. The sensing of an analyte on a coated microcantilever sensor is based on the optical detection of a nanometric deformation of the cantilever. For its successful implementation into lab-on-a-chips, the specificity of the sensing detection must be improved. Different strategies are proposed. Among them, special efforts must be aimed at the chemistry of the sensing coating but also towards the implementation of multiple and complementary detection modes. Electrochemistry could help solve both issues. Indeed, on one hand, the electrochemistry of diazonium salts results in the formation at an electrode of strongly adherent and chemically sensitive coatings. On the other hand, as in many sensors, electrochemistry provides a convenient actuation/detection mode. Besides, full-field optical imaging of the microcantilever should generate richer information. In this context, our project wishes to couple innovative optical microscopic imaging techniques with micro-electrochemical techniques in order to image and investigate in-situ and in real-time micro-electrochemical surface processes. Description and methodology We have identified three different imagery modes that should be implemented with electrochemistry. The first one is the FT-IR microscopy that is particularly adapted to the fast and quantitative, in-situ and ex-situ, real time chemical characterization of micron scale surfaces modifications. The other two, electroreflectance and ellipsometry microscopies, are based upon the detection of changes in the reflectivity of a surface upon its chemical transformation. We have already built preliminary microscopic set-up and shown their ability to map surface modifications. We now propose to couple electrochemical techniques with FT-IR, electroreflectance and ellipsometry microscopies at the micron scale for the in-situ and real time inspection of microelectrode electrografting and sensing processes. For this purpose, we will first improve the sensitivity of our optical set-ups and validate their use in the in-situ and real-time monitoring of the electrografting of microelectrodes with coatings issuing from the reduction of diazonium ions. The choice of diazonium is particularly relevant as our second objective is to develop microsystems-based sensors made of strongly adherent coatings. The investigated coatings will be made from simple and known diazoniums that form monolayer to micrometer thick films. New mechanistic insights into the kinetic of film growth are expected with resolutions of respectively the sub-micrometer, (sub-)nanometer and few second in lateral dimension, thickness and time. This technique may provide an unprecedented alternative to Quartz-Crystal Microgravimetry. We will then use these electrografted layers and the coupled opto-electrochemical detections for microsensors. The sensing capacities will be obtained from -NO2 or -CO2H terminated diazonium. The coating will allow the detection of different analytes and more particularly metallic ions. The detection will be obtained from electrochemical measurement on a coating enriched with the analyte and from the combined in-situ optical mapping. Within a constant miniaturization effort, we will tend towards the transposition of this work, when technically possible, into thin electrochemical layer cells made from standard microfluidic chips. Here, the development of innovative, robust and versatile detection techniques coupling electrochemistry and optical imageries will compete efficiently with the more constraining Surface Plasmon Resonance detection regarding electrode and cell configurations. Finally we will transpose this work to microcantilever platforms, the in-situ optical detection of the lever bending associated with its electrografting or with the sensing process will demonstrate the high complemetarity of this new opto-electrochemico-mechanic tool. Expected Results This project will allow to couple different optical microscopies with electrochemical techniques using microelectrodes and to compare their sensibilities to existing analytical techniques. This new analytical tool will be particularly well adapted to the mechanistic investigation of thin film growth and sensing detection. Beyond its high potential in microcantilever platforms, it will provide an alternative to existing detection modes in various analytical microsystems or microfluidic chips. This project will also build a strong scientific gateway between the two laboratories at ESPCI by taking benefit of the complementary expertises and knowledge of each person involved. This is especially relevant as analytical physical chemistry requires such strong interaction between physics and chemistry.