Sensors and Organic Electronic Devices

Our research activities in the field of chemical sensors go from molecules to materials and from materials to devices and systems. Sensing materials are mainly phthalocyanines, porphyrins and polymers. The analytical targets are chosen with respect to their environmental impact, or importance in the health or agro-food sectors, e. g. ammonia, ozone and volatile organic compounds in air and redox-active species in solution. We focus on the electrosynthesis of new materials for the conception of new sensors and biosensors. In fine the multi-functional material can assume several roles essential to the device: a) recognition (condition of a specific response towards a given species); b) conduction (ability to transport the charges inside the material); c) electrocatalysis (ability to transfer the charges from the electrode to redox-active species in solution).

- In the field of conductometric sensors, we oriented our research work towards three directions, the preparation of polymer-macrocycle hybrid materials, the development of new transducers and the work in real conditions, in particular in wet environment.

The combination of phthalocyanines or related derivatives with polymers or carbonaceous materials led to efficient chemical sensors. We showed how the incorporation of macrocyclic molecules in hybrid materials highly modifies the structural and morphological characteristics of the materials. Rugosity, specific surface and porosity being key parameters in the analyte-sensing material interactions, these modifications highly improve the performance of chemical sensors. Thus, hybrid materials combining polypyrrole with sulfonated phthalocyanines as counterions were electrosynthesized at the surface of platinum interdigitated electrodes. The films exhibit a higher sensitivity to ammonia than pure phthalocyanines, with a very good reversibility, showing the synergetic effects in the structure and properties of polypyrrole-phthalocyanine materials. (J. Mater. Chem., 2012).

We highly study Molecular Semiconductor-Doped Insulator (MSDI) heterojunctions that we patented and that combine two materials with very different conductivities (Analyst 2009). By means of impedance spectroscopy we showed that the Schottky contacts between the sublayer and the electrodes play a key role, additionally to the interface between the two organic layers (Org. Electron. 2015). MSDIs made from lutetium bisphthalocyanine (LuPc₂), a unique stable radical species with semiconducting properties, combined with the copper perfluorophthalocyanine, a n-type low conductive material, revealed to be the best sensor in terms of insensitivity to relative humidity variations and of lifespan. We built MSDIs not only with p-type and n-type behaviors but also with an ambipolar behavior (Sens. & Actuators B, 2018).

- We take advantage of electrochemical methods to develop new sensors and biosensors.

Thus, pursuing the innovative work related to the direct electrografting of porphyrins on electrode materials, we develop well-controlled surface processing techniques. We prepare well-defined and stable molecular materials authorizing structural diversity on porphyrin (metal center, peripheral substituents, …), due to a suitable design of porphyrin precursors. New transformations on the porphyrin immobilized on the electrode are studied considering that, nowadays, post-functionalization strategies of electro-deposited materials are extremely challenging.

The electrosynthesis of polyaniline derivatives, in particular the poly(tetrafluoroaniline), obtained in a low conductive state, allowed building double lateral heterojunctions, by combination with the lutetium bisphthalocyanine (ACS Appl. Mater. & Interfaces 2018). They exhibit a particularly interesting sensitivity to ammonia, even under humid atmospheres, with sub ppm limits of detection.

- Modification of the electrode work function: We have shown that potential-assisted deposition of thioalkanes on gold is more selective and also 100-fold faster than passive adsorption. This technique enables the formation of layers of any shape by selectively coating suitably polarized electrodes. Electrodeposition also gives access to modified electrodes by depositing only one molecular layer on their surface instead of a film, to investigate Schottky barriers and electrode work function. Electrografting allows the modification of the work function of electrodes, which is of interest in many electronic devices, among which photovoltaïc cells, organic light emitting diodes and organic field-effect transistors. This parameter is important because it determines the energy barrier at the metal / semiconductor interfaces (Schottky contacts). These surface modifications can also modify the arrangement of molecules that are then deposited. In all cases, they allow tuning the charge transport in molecular materials and devices, and the performances of chemical sensors.

- The final step is the prototyping of gas sensor systems for air quality and processes monitoring, based on our devices and on our know-how in electronics and computer science.