Fluorescent labeling of biological molecules is used widely for analytical purposes in biotechnology and bioengineering. It typically involves the use of an organic dye linked to a moiety that selectively bonds a particular biological molecule, allowing the detection of the latter by the fluorescence of the dye. Typical organic fluorescent dyes have narrow excitation spectra and broad emission spectra that exhibit red tailing.

Semiconductor nanocrystals or quantum dots have emerged as a new class of fluorescent markers with distinct advantages over the traditional organic dyes [1-4]. Their attractive properties include a narrow, symmetric, and strong emission that is size-tunable, continuous excitation by any wavelength smaller than the emission wavelength, resistance to photobleaching, as well as excellent optical and chemical stability that allows their use in lengthy experiments, both in vitro and in vivo. The ability to synthesize different populations of quantum dots with narrow emission spectra permits multiplexing, a property that is very important for simultaneous detection of several analytes, that would be very tedious and expensive if done sequentially.

In addition to their use as multi-color "passive" tags for biomolecules, quantum dots have significant potential as "active" sensors, because their optical spectra change when the detection molecules attached to their surface hydridize with complementary ones in solution. The focus of this work is the development of new biological detection and DNA analysis schemes by using the changes in the optical spectra of ZnSe quantum dots.

We have synthesized highly-fluorescent ZnSe quantum dots, modified their surface with functional molecules to render them water-dispersible, and conjugated them with oligonucleotides. Hybridization of complementary oligonucleotides conjugated with ZnSe quantum dots results in a significant increase in the fluorescence intensity of the quantum dots and a measurable red shift in the emission wavelength.

A series of experiments was performed in which DNA hybridization was studied using ZnSe quantum dots conjugated with oligonucleotides as active sensors and free oligonucleotides in solution as targets. An increase in the emission intensity of the quantum dots was observed upon hybridization of the tagged oligonucleotides with free oligonucleotides. A dependence of the detected fluorescence emission intensity on the length and location of the hybridized part was detected, indicating that spectral changes can be used to identify the DNA stuctures attached to the quantum dots. Ongoing experiments are focusing on determining the sensitivity of this techique for detecting DNA mutations. Applications of these phenomena in the development of novel DNA analysis strategies will be discussed.

References

1. M. Bruchez, M. Moronne, P. Gin, S. Weiss and A.P. Alivisatos, Science 281, 2013 (1998).

2. W.C.W. Chan and S. Nie, Science 281, 2016 (1998).

3. M.-Y. Han, X. Gao, J. Z. Su, and S. Nie, Nature Biotechnology 19, 631 (2001).

4. W. C. W. Chan, D. J. Maxwell, X. Gao, R. E. Bailey, M. Han and S. Nie, Current Opinion in Biotechnology, 13, 40 (2002).