Devices and methods for measuring a concentration of a target analyte in a biological fluid in vivo are provided herein. In some examples, a device includes an indwelling sensor and sensor electronics. The sensor may include a substrate; a first electrode disposed on the substrate; an ionophore disposed on the substrate to selectively transport the target ion to or within the first electrode; and a second electrode disposed on the substrate. The sensor electronics is configured to generate a signal corresponding to an electromotive force which is at least partially based on a potential difference that is generated between the first electrode and the second electrode responsive to the ionophore transporting the target ion to the first electrode.