Cross-linked copolymers based on poly(acrylic acid) and functionalised with crown-ethers have been designed and synthesised. The materials selectively absorb K+ over other Group I ions such as Li+ and Na+. The copolymers have been used as the basis of a quartz crystal microbalance, QCM, to form a sensor for use in aqueous solutions. Acrylic acid polymers cross-linked with ethylene glycol dimethacrylate, (EGDMA) and substituted with 18-crown-6 rings were coated onto the crystal and the sensing of K-(aq)(+) investigated. The detection limit of the developed sensor is < 1 ppm for K-(aq)(+) with a linear range extending to over 1000 ppm although further optimisation should significantly improve this performance. (c) 2005 Elsevier B.V. All rights reserved.