Abstract: This invention relates to an assay label comprising an amorphous carbon particle, a functionalised dextran polymer attached to the surface of the carbon particle and a first member of a complementary binding pair covalently bonded to the functionalised dextran polymer. The invention also provides a device incorporating the assay label, which further comprises a radiation source adapted to generate a series of pulses of electromagnetic radiation at a wavelength such that the absorption of the radiation by the label generates energy by non-radiative decay; a sample chamber containing a transducer having a pyroelectric or piezoelectric element and electrodes which is capable of transducing energy generated by non-radiative decay into an electrical signal; and a detector which is capable of detecting the electrical signal generated by the transducer.