An encoded carrier comprises a code region having a code and a separate reaction region. The reaction region has a substantially circular diffraction grating on its surface and is configured to support surface plasmons. Incident radiation couples to surface plasmons at a peak angle of incidence. Present on the surface of the reaction region are known probe molecules such as strands of DNA which are exposed to unknown target molecules in a sample. Where hybridisation between probe and target molecules has occurred, the angle of incidence at which surface plasmon resonance occurs shifts, thus indicating the presence of the known molecule in the sample. Use of a circular grating allows multiple carriers to be used easily in a system by simplifying the alignment of the grating with the light source.