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Quantum-dot fluorescence lifetime engineering with DNA origami constructs.

The ability to organize nanostructures of disparate types and materials—such as metal nanoparticles and semiconductor quantum dots—is challenging but essential for the creation of novel materials and devices. Metal nanoparticles (NPs) have interesting individual plasmonic properties and can be organized to exhibit useful collective responses. Quantum dots (Qdots) provide a powerful means to optically access the nanoscale. Bringing the two together in a well-controlled manner can create structures with interesting properties such as fluorescence enhancement/quenching and high efficiency Fçrster resonance energy transfer. It also has been an area of intense study, both theoretical and experimental, for a wide range of applications including photodetectors, optical modulators and nanoscale lasers. In particular, changing the fluorescence intensity and lifetime of Qdots, when proximate to metal NPs can be used in sensing applications because of the strong distance dependence of the interaction between the Qdots and the ability to engineer the properties (e.g. size, absorbance/emission spectrum) of the individual components over wide ranges. Numerous strategies have been used to connect and control the distance between Qdots and NPs with nanometer precision. 3,7–9] The structures used previously, however, exhibit a limited persistence length (about 50 nm), making the construction of complex geometries required for eventual use in real devices challenging. DNA origami offers a platform with significant advantages: a more rigid scaffold to organize various moieties, increased geometrical complexity, no need to control the stoichiometry of the spacer per NP, and more control over distances. We have therefore chosen to exploit DNA origami for this purpose. We have developed a novel, flexible approach to fluorescence lifetime engineering of CdSe/ZnS (core/shell) Qdots by controlling their coupling to adjacent gold nanoparticles (AuNPs) at geometrically different locations on the DNA origami. To examine these templates in their native state in solution, we use a three-dimensional (3D), real-time, single-particle tracking system. We determine the influence of AuNPs on the Qdot fluorescence lifetime by systematically varying the location, number, and size of AuNPs as well as the interparticle distance and spectral overlap between AuNPs and Qdots. The DNA origami template serves as a programmable nano-pegboard for heterogeneous integration of Qdots and AuNPs wherein complex geometries are created by using modified staple strands on the DNA origami to capture specific nanoparticles (Figure 1). Herein, we manipulate and control the average photon count rate and lifetime of Qdots by varying the geometrical configuration of Qdot–AuNP conjugates on DNA origami and observe good agreement between theory and measurement. Solution-based measurements are important, because they provide insight into how the templates will behave in a biological environment. The single-particle tracking system enables us to follow the 3D motion of individual diffusing