Measurements for molar extinction coefficients of aqueous quantum dots

Quantum dots (QDs) have been widely used in biological and medical fields as labeling probes due to their unique and fascinating optical properties. However, few reports have been found on how to characterize molar concentrations of aqueous QDs and their bioconjugates, mainly due to the lack of suitable analytical tools. In this paper, we describe a simple and efficient method for characterizing molar extinction coefficients of different sized QDs using fluorescence correlation spectroscopy (FCS). Different from previous methods based on FCS, the molar extinction coefficients of QDs were determined by comparing the count rate per molecule or brightness per particle (BPP) of QDs and chosen reference dyes. The feasibility of the method was verified in principle and by experiments.