Abstract: A method of monitoring a fluid is described comprising introducing a luminescent nanoparticle into the fluid; removing a fluid sample from the fluid; adding a reagent to the fluid sample to vary the luminescence behaviour of the luminescent nanoparticles and/or of other luminescent species present in the fluid; analysing the luminescence of the modified fluid sample to determine an amount of the nanoparticle present therein. A use of a tracer based on these principles is also described.

Abstract: A method of producing a security device (60) is disclosed. The method comprises inkjet printing a liquid crystal material onto a first region (61) of a substrate and ink jet printing the same liquid crystal material onto a second region (62) of the substrate. The volume of the liquid crystal material printed per unit area of the substrate in the first region of the substrate is different to the volume of the liquid crystal material printed per unit area of the substrate in the second region of the substrate such that the wavelength of the peak reflectance of the liquid crystal material on the first region is different to the wavelength of the peak reflectance of the liquid crystal material on the second region.