A sensor for the detection or measurement of a carbohydrate analyte in fluid comprises components of a competitive binding assay the readout of which is a detectable or measurable optical signal retained by a material that permits diffusion of the analyte but not the assay components, the assay components comprising: a carbohydrate binding molecule labelled with one of a proximity based signal generating/modulating moiety pair; and a carbohydrate analogue capable of competing with the analyte for binding to the carbohydrate binding molecule, the carbohydrate analogue being a flexible water-soluble polymer comprising: polymerized or co-polymerised residues of monomer units, the monomer unit residues bearing pendant carbohydrate or carbohydrate mimetic moieties and pendant moieties which are the other of the proximity based signal generating/modulating moiety pair.