A device for identifying the presence of a specific target molecule or biomarker by the detection of a change in an electrical property includes a measurement sensor 8 comprising a semiconducting sensor structure 12 capable of conjugating with the biomarker, thus giving rise to the said change in electrical property, and an electrode system 3, 4 for conducting a signal from the device. According to the invention there is a further such sensor 9, of substantially identical form but having its sensor structure 14 already conjugated with the biomarker, or otherwise capped, e.g. using a further oligonucleotide strand, so as to act as an internal reference. When a biological sample, e.g. saliva, is applied to the electrodes, the reference enables the discounting of all environmental effects other than the biomarker. The invention provides a simple, cheap and accurate text for one or more biomarkers that can be used in the field without complex equipment.