An analyte sensor apparatus and a corresponding fluid medium, the analyte sensor apparatus comprising a sensing element, the external surface of which comprises a membrane to inhibit exposure of the sensing element; the corresponding fluid medium comprising a receptor species and an activatable species, the receptor species for interacting with an analyte to activate the activatable species, activation of the activated species causing increased porosity of the membrane of an in-contact analyte sensor apparatus to correspondingly increase exposure of the sensing element to allow for production of a detectable electrical signal which can be used to sense the presence of the analyte.