The Nanoscopic Principles of Capacitive Ion Sensing Interfaces

The present communication discusses the operational principles of molecular interfaces that specifically recruit ions from an electrolyte solution and report this in a reagentless capacitive manner. We show that this capability arises through the natively mesoscopic properties that are presented when molecular scale receptors are tethered to an underlying electron reservoir. Building on the fundamentals of electrochemical capacitance we are able to directly associate the free-energy of an individual ion binding process with the electric field subsequently developed across the receptor such that the interfacial response is capacitive and analogous to that generated in a field-effect device. The magnitude of this signal scales with site occupancy of the receptive film and is related to the electric field screening length at the interface. At low ionic occupancy the response of the interface obeys a Debye-type phenomenon akin to classic “image charge” effects. At higher levels of occupancy, the response follows Thomas-Fermi screening and, significantly, is dependent on the electronic structure of the mesoscopic ion-receptor host-guest ensemble. This reagentless field-effect ionic detection is sensitive, specific in a chemically tunable manner and operates with just a single electrical contact.