A new approach for developing a fluorimetric aptasensor has been described and applied for determi-
nation of a highly toxic cation, As(III). In this method an aptamer was used to aggregate cationic cy-
steamine-stabilized CdTe/ZnS core/shell quantum dots, as a result fluorescence quenching was accrued.
In the presence of As(III), the aptamer and As(III) make a complex, which prevents aggregation of the
quantum dots. Thus, the fluorescence intensity of the quantum dots was enhanced upon the de-ag-
gregation, which depends on the concentration of As(III). The fluorimetric assay has a very low detection
limit of 1.3 pmol L
1 As(III) with a dynamic range of 1.0  10 –11 to 1.0  10 –6 mol L
1 . The interference
effect of a wide variety of cations and anions was investigated, and the obtained results confirm high
selectivity of the aptasensor for As(III) detection. The present assay was successfully applied for the
determination of As(III) in several water samples.