Due to their unique physical, chemical and optical properties, quantum dots (QDs) have been widely applied in biological and biomedical research. However, the influence of cations on QDs has rarely been explored except for the quenching effect on QD fluorescence by some heavy metal ions (e.g. Ag+ ions and Cu2+ ions). Here we present a new approach to study the effect of physiological cations (e.g. Ca2+ ions, Mg2+ ions, Na+ ions and K+ ions) upon QDs by using single-particle detection. Our results demonstrate that divalent cations (e.g. Ca2+ ions and Mg2+ ions) can selectively induce the self-assembly of QDs in a concentration- and time-dependent manner, which is impossible to attain with ensemble fluorescence measurements. In comparison with conventional TEM, SEM and AFM measurements, this single-particle detection has significant advantages of simple/no sample preparation, rapid analysis, low cost, and easy measurement in the native environment. This single-particle detection technique might also provide a unique approach to study enzyme-responsive bimolecular assembly, and the characterization of early protein assembly states.
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