Abstract: A hydrogel comprising a particle physically entrapped within a hydrogel matrix, a method for making the hydrogel, a particle for use in the hydrogel and the use of the hydrogel to sense a chemical species, especially anions in solution. The particle comprises an active material and a plurality of chains of a first polymeric material, each of the chains of the first polymeric material having a first end and a second end. The active material and the first ends of the chains form a core; and the second ends of the chains extend outwardly away from the core to form a shell. The hydrogel matrix comprises chains of a second polymeric material in the form of a three dimensional cross-linked network.