Anion sensing with luminescent quantum dots - A modular approach based on the photoinduced electron transfer (PET) mechanism

A CdSe-ZnS quantum dot (QD) has been surface functionalised with 1-(2-mercapto-ethyl)-3-phenyl-thiourea in the fluorophore-spacer-receptor format typical of Photoinduced Electron Transfer (PET) based organic dye sensors. The resulting QD conjugate was tested for selectivity toward the tetrabutylammonium salts of fluoride, chloride, bromide, hydrogen sulfate and acetate. Addition of fluoride, chloride and acetate ions resulted in an approximate 90% quenching of the original fluorescence intensity, while bromide and hydrogen sulfate had almost no effect. The observed quench was attributed to an increase in the reduction potential of the receptor upon anion binding resulting in an increase in PET from the excited QD to the receptor and a concomitant reduction in fluorescence intensity. The selectivity and sensitivity were comparable to an analagous organic dye based sensor where a similar receptor was bound to an anthracene fluorophore. Thus a modular approach is evident where a receptor used in an organic dye based sensor can be adapted and successfully used with QD's.