Surface-attached sensors for cation and anion recognition.

The development of surface-attached sensors for cationic and anionic guests is of intense current research interest. In addition to the environmental flexibility, robustness and reusability of such devices, surface-confined sensors typically exhibit an amplified response to target analytes owing to preorganization of the receptor. Whereas redox-active cations may be sensed by studying the cyclic voltammetry of host-guest systems containing ion-selective receptors attached to an appropriate electrode, redox-inactive ionic species require the use of electrochemical impedance spectroscopy, with appropriately functionalized electrodes and redox probes. Alternatively, receptors may be constructed that incorporate an electrochemical or optical reporter group within their structure to provide a macroscopic response to the presence of an ionic guest. This critical review seeks to present an up-to-date, although necessarily selective, account of the progress in the field, and provides insights into possible future developments, including the utilization of receptor-nanoparticle conjugates and mechanically interlocked receptors.