A sensor for the detection of an analyte comprising a cis-diol moiety, which comprises a holographic element comprising a medium and a hologram disposed throughout the volume of the medium, wherein an optical characteristic of the element changes as a result of a variation of a physical property occurring throughout the volume of the medium, and wherein the medium is a polymer comprising a group of formula (i) wherein n is 0, 1, 2, 3 or 4; each X (if present) is independently is an atom or group which, via an electronic effect, promotes formation of a tetrahedral geometry about the boron atom; and Y is a spacer which, when n is 0 or otherwise optionally, is an atom or group which, via an electronic effect, promotes formation of a tetrahedral geometry about the boron atom. Such a sensor may be used for the detection of glucose.