Ion-selective quantum dots (ISQDs) have been produced that are capable of measuring physiologically relevant ions (such as Na+, K+, Cl-, etc.). ISQDs consist of quantum dots, an ion-selective polymer matrix, and a biocompatible coating. ISQDs work by selective ion extraction by the polymer matrix which leads to a change in the absorbance intensity of an embedded dye. This attenuates the intensity of the quantum dot by directly absorbing its fluorescence emission. As an example of this technology, a sodium-selective ISQD measures sodium over the range of 1 mM to 1 M with 200-fold selectivity over potassium and a resolution of 80 mu M at 10 mM.