A sensor is provided for measuring the presence or amount of an ionic analyte in an electrolyte solution. The sensor comprises (i) two electrically conducting electrodes on an electrically insulating substrate, and separated by a distance L, where L is between 10 nm and 100 μm; (ii) a chemiresistor film, wherein the impedance of the chemiresistor film changes in the presence of an ionic analyte; and wherein the chemiresistor film is in contact with the electrodes; (iii) a gate electrode formed from an electrically conductive material; (iv) a potential generator means configured to apply an electrical potential difference between the gate electrode and the chemiresistor film; (v) a controller to selectively control the potential generator means to apply either a positive electric potential or a negative electric potential to the gate electrode relative to the potential of the chemiresistor film; and (vi) a voltage means adapted to apply a voltage signal between said pair of electrically conducting electrodes to enable measurement of the resistance of the chemiresistor film.