An apparatus and method wherein the apparatus comprises: a graphene field effect transistor comprising quantum dots (7) coupled to a graphene channel (5); wherein the graphene field effect transistor is configured to be illuminated by a pulse of electromagnetic radiation (9) and configured to be exposed to a sample (11) such that an output provided by the graphene field effect transistor, in response to the pulse of electromagnetic radiation, is dependent upon at least one analyte within the sample.