Magnetic nanoparticles (NPs) have been used to obtain NMR-based sensors for analytes ranging from small molecules to viruses by the conjugation of biomolecules (antibodies, proteins, oligonucleotides) to the surface of NPs. In the presence of an analyte, the NPs form clusters that alter the relaxation time of the surrounding water protons. Here, we show that an organic molecule that binds calcium ions of nombiological origin, rather than a biomolecule, can be employed to modify the surface of a magnetic NP. When calcium ions are added, they induce NP clustering, providing an NMR-based sensor for these ions. Our work suggests that the many chemistries of nonbiological origin, such as those employed for ion-selective electrodes, can be adapted to obtain NMR-based sensors for ions.