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Analysis Method for Measuring Submicroscopic Distances with Blinking Quantum Dots

B. Christoffer Lagerholm ^*, Laurel Averett , Gabriel E. Weinreb ^*, Ken Jacobson ^*  and Nancy L. Thompson 

^* Department of Cell and Developmental Biology, Department of Physics, Lineberger Comprehensive Cancer Center, and Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina

Correspondence: Address reprint requests to Nancy L. Thompson, Dept. of Chemistry, Campus Box 3290, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3290. Tel.: 919-962-0328; Fax: 919-966-3675; E-mail: nlt{at}unc.edu.

A method is described that takes advantage of the intermittency ("blinking") in the fluorescence of quantum dots (QDs) to measure absolute positions of closely spaced QDs. The concept is that even if two QDs are separated by only tens of nanometers, the position of each QD is resolvable if the point spread function of each can be imaged independently of the other. In the case of QDs, this is possible if each QD separately blinks completely on and off during a time-lapse sequence. To demonstrate the principle of this method, time-lapse sequences of single blinking QDs were acquired and the centroids of the point spread functions determined. Images of the blinking QDs were then overlapped in software, pixel by pixel, generating a range of submicroscopic distances between QD pairs. Methods were developed for analyzing the overlapped time sequences of the QD pairs so that the positions of the QDs and the distances between them could be determined without prior knowledge of the single QD positions. We subsequently used this method to measure the end-to-end length of a 122-basepair double-stranded DNA fragment.