Schiff base receptor 1a has been synthesised and attached to the surface of preformed CdSe/ZnS Quantum Dots (QDs) to form QD-conjugate 2a. While 1a was determined to be selective for Mg2+, 2a demonstrated selectivity for both K+ and Ca2+ when tested against a range of physiologically and environmentally relevant cations by changes in the fluorescence spectra. Thus, the nanoparticle surface functions as a scaffold for the organisation of receptors enabling semi-selective binding. The fluorescence response was shown to be linear between 15-50 mu M for K+ and 2-35 mu M for Ca2+. It was also demonstrated that 2a could measure both K+ and / or Ca2+ in solutions containing both ions.