A sensor for the determination of metal ions in aqueous solution based on a quartz crystal microbalance (QCM) was developed. The lack of selectivity of the QCM was alleviated by the chemical attachment to the crystal of water-soluble glycidyl methacrylate–vinylpyrrolidinone copolymers modified with 4-(3-aminopropyl)morpholine that selectively absorb metal ions from solution over a wide range of concentrations.