Simple and accurate quantification of quantum dots via single-particle counting

Quantification of quantum dots (QDs) is essential to the quality control of QD synthesis, development of QD-based LEDs and lasers, functionalizing of QDs with biomolecules, and engineering of QDs for biological applications. However, simple and accurate quantification of QD concentration in a variety of buffer solutions and in complex mixtures still remains a critical technological challenge. Here, we introduce a new methodology for quantification of QDs via single-particle counting, which is conceptually different from established UV-vis absorption and fluorescence spectrum techniques where large amounts of purified QDs are needed and specific absorption coefficient or quantum yield values are necessary for measurements. We demonstrate that single-particle counting allows us to nondiscriminately quantify different kinds of QDs by their distinct fluorescence burst counts in a variety of buffer solutions regardless of their composition, structure, and surface modifications, and without the necessity of absorption coefficient and quantum yield values. This single-particle counting can also unambiguously quantify individual QDs in a complex mixture, which is practically impossible for both UV-vis absorption and fluorescence spectrum measurements. Importantly, the application of this single-particle counting is not just limited to QDs but also can be extended to fluorescent microspheres, quantum dot-based microbeads, and fluorescent nano rods, some of which currently lack efficient quantification methods.