Ion-detecting sensors for detecting a target ion in a sample are provided. The sensor comprises a plasticizer-free copolymer comprised of polymerized units of methacrylate monomers having pendent alkyl groups of different length and a functionalized ionophore of said ion, wherein at least a portion of the functionalized ionophore is grafted into the copolymer through covalent linkages. Sensors may comprise ionophores such as hydrophilic crown ethers or functionalized derivative of 3-oxapentandiaminde-type ionophores. This invention further provides sensors for detecting target ions in a sample, comprising plasticizer-free molecularly imprinted polymers, wherein the polymers comprise polymerized units of methacrylate monomers having pendent alkyl groups of different length and a functionalized ionophore of said ion. In particular, a magnesium ion sensor comprising a functionalized derivative of a 3-oxapentandiaminde-type calcium ion-selective ionophore is provided. Sensors of this invention include carrier-based ion-selective electrodes or optodes such as thin film ion-specific optodes, particle-based optodes, or bulk optodes.