Principal Investigator Moungi Bawendi
Semiconductor NC quantum dots are typically isolated both chemically and electronically from their surroundings by inorganic and organic passivating layers, and these features are responsible in part for the photostability that has made them an increasingly popular choice for biological imaging and tracking studies. However, these coatings tend to make NCs unresponsive to their environment. A sensing function may be derived by overlaying the NC with a coating containing a lumophore whose emission is environmentally responsive. This type of construct takes advantage of the NC’s robustness for NC-based sensors in which the unchanging fluorescence behavior of the NC core is compared against the environmentally-responsive coating. However, additional advantages may be achieved if the NC becomes actively involved in the response of the coating via a resonant energy transfer relationship established between the QD donor and the appended molecules. The continuous excitation spectrum of the NC is conferred upon the construct, obviating the need for independent excitation of the sensing fluorophore. NCs are also well-suited as FRET donors for sensor applications because of the very high valency presented by a suitably functionalized NC surface: that is, a large and tunable number of acceptors can be associated with each NC so as to achieve the optimal overall FRET efficiency to maximize sensitivity.
The sensor depicted at right is constructed from CdSe/ZnS core/shell NCs linked to squaraine-derived acceptor molecules. Sensing action results from the engineered overlap of the pH-sensitive dye absorption spectrum with the (pH-insensitive) quantum dot emission spectrum. Whereas strong overlap leads to efficient FRET at low pH, deprotonation of the phenolic proton(s) at basic pH (pKa=8.8) reduces the overlap and thus lowers the FRET efficiency. The solution pH can be quantified by taking the ratio of each emission peak intensity (dot and dye) to the intensity at the isosbestic point, which functions as an internal reference. Self-referencing is preserved under a varying excitation power and the imposition of a strongly scattering medium, making the method a robust sensing approach for a variety of biological applications.