Amplification of surface potential by long range induced molecular dipole moment reorientation – AMPLI

AMPLI addresses the general questions How does a weak signal be translated into a measurable one? How does an isolated event can induce a cascade of events that lead to a major effect? In cell biology, molecular conformational signaling is often the primary process by which cells respond to changes in their physical and chemical environments. It is reported that very small conformational changes (~1 Å) induced by external stress at the level of a molecular receptor outside a living cell can be transmitted across the cell membrane and amplified. A similar process, involving target entities (molecules, ions…) interacting with molecular probes on the surface of a sensor, can be envisaged as a mean of amplifying the transduction signal of host-guest recognition events, the molecular probe layer on the sensor surface playing the role of amplifier. Such a strategy opens new perspectives for trace detection through pushing away the detection limit.
In a previous project (ANR-16-JTIC-0003-01) we developed an organic field effect sensor (O-FET) for the detection of alkali ions in solution with a sensitive layer composed of molecular probes grafted atop a lipid monolayer. Exceptional performances were demonstrated, including a limit of detection (LOD) in the attomolar range. However, the variations of the surface potential measured for such low ion concentrations cannot be explained purely by local variations of charge density associated to the ion – probes complexation events; we hypothesis that an amplification phenomenon must be involved.
AMPLI’s general objective is to combine experimental work and molecular dynamics simulations to understand the effects leading to the unexpected macroscopic surface potential changes resulting from a few ion-probe complexation events at the surface of an organic monolayer and correlate this changes to the FET sensor response.
Our main hypothesis is that surface potential amplification could result from a scenario involving three consecutive mechanisms : 1) a host-guest interaction inducing a molecular conformation change of the host molecule, leading to a reorientation of its molecular dipole moment, 2) a propagation of the conformation change within the sensitive layer through molecular interactions in a domino-like cascade process, 3) a reorganization of the charges distribution in the electrical double layer at the active layer-solution interface at large scale, responsible for an amplification of the surface potential changes measured with the FET sensor.
Because there is no simple way to show that the amplified FET response is due to the propagation of the probes conformational change, we have defined a plan to reach this objective. Our strategy is to build well controlled surfaces of selected ionic probe and determine conditions for which a molecular conformational change, induced by the complexation of the probes with specific ions, would propagate in the layer through molecular interactions. These surfaces will be investigated by ATR-FTIR to study the change in molecular conformation and by KPFM to measure the change in surface potential. They will allow us to establish the relationship between the propagation of conformation change and surface potential variations. These surfaces will then be implemented in the FET to establish the relationship between the propagation of conformational change and FET response, i.e. surface potential variations.