A highly sensitive and selective analysis of alkali metal and ammonium ions was investigated on the basis of a Förster-type excitation energy transfer between two quantum dots (QDs) with different sizes. QDs are semiconductor nanoparticles and their emission wavelengths depend on their sizes. For the molecular recognition of potassium ion, mercaptoalkanoic 15-crown-5 ether was introduced onto the surface of QDs with their diameter of 2.1 (15C5-QDs2.1) and 2.6 nm (15C5-QDs2.6). When K
+ is added to a mixed suspension of 15C5-QDs2.1 and 15C5-QDs2.6, the efficiency of the excitation energy transfer will be enhanced since the recognition of K
+ brings a 1:2 sandwich-type complexation between the different size of QDs. As the concentration of K
+ was increased, actually, the fluorescence intensity at longer wavelength derived from 15C5-QDs2.6 was increased. Thus, the signal transduction can be accomplished through the energy transfer resulting in a ratiometric change in the emission intensity. The limit of detection (LOD) of K
+ was evaluated to be 0.50 μM, so that the sensitive detection of K
+ was successful. When Na
+ was added to the mixed suspension, the change in the fluorescence spectrum was not observed. Therefore, it was found that 15C5-QDs was K
+ selective fluorescent probe. We also prepared Na
+ or NH
4+ selective QDs probes by inducing mercaptoalkanoic 12-crown-4 ether or 18-crown-6 ether on QDs, respectively. The obtained LODs for Na
+ and NH
4+ were 1.3 μM and 0.31 μM, respectively.
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