Holographic sensors are a class of colorimetric optical probes that allow quantitative, real-time measurements of biomarkers. The sensing devices consist of metal or dielectric nanostructures periodically organised in a functionalised hydrogel film. Such hydrogels may be selected from poly(2-hydroxyethyl methacrylate), polyacrylamide or gelatin, and they can be functionalised to be sensitive to a wide range of analytes including pH, glucose, metal ions and antibodies. When the holographic sensor is illuminated with a broadband white-light source, the grating produces visible-light diffraction and displays a monochromatic colour. The sensor reports on the concentration of analytes by Bragg diffraction peak changes.