A sensor for detecting a target substance, in particular carbon dioxide, in a gas stream comprises a sensing element (8) disposed to be exposed to the gas stream, the sensing element comprising a working electrode (12); a counter electrode (14); and a solid electrolyte precursor (16) extending between and in contact with the working electrode and the counter electrode; whereby the gas stream may be caused to impinge upon the solid electrolyte precursor such that water vapour in the gas stream at least partially hydrates the precursor to form an electrolyte in electrical contact with the working electrode and the counter electrode. A method of sensing a target substance in a gas stream comprises causing the gas stream comprising water vapour to impinge upon a solid electrolyte precursor; allowing the solid electrolyte precursor to at least partially hydrate, so as to form an electrolyte bridge beA sensor for detecting a target substance, in particular carbon dioxide, in a gas stream comprises a sensing element disposed to be exposed to the gas stream, the sensing element comprising a working electrode; a counter electrode; and a solid electrolyte precursor extending between and in contact with the working electrode and the counter electrode; whereby the gas stream may be caused to impinge upon the solid electrolyte precursor such that water vapour in the gas stream at least partially hydrates the precursor to form an electrolyte in electrical contact with the working electrode and the counter electrode. A method of sensing a target substance in a gas stream comprises causing the gas stream comprising water vapour to impinge upon a solid electrolyte precursor; allowing the solid electrolyte precursor to at least partially hydrate, so as to form an electrolyte bridge between a working electrode and a counter electrode; applying a electric potential across the working electrode and counter electrode; measuring the current flowing between the working electrode and counter electrode as a result of the applied potential; and determining from the measured current flow an indication of the concentration of the target substance. The sensor and method are particularly suitable for analyzing tidal carbon dioxide concentrations in the exhaled breath of a person.