Single-Molecule Measurements of Gold-Quenched Quantum Dots

We report the study of the quenching of quantum dots (CdSe) by gold nanoparticles at the single-molecule level. Double-stranded DNA is used as a rigid spacer to tune the distance between the two nanoparticles. The width of the fluorescent intensity distribution, monitored at different interparticle distances, reflects both the nanoparticle heterogeneity and the fluorescence intermittency of the quantum dot. The fluorescence distribution emitted by single CdSe nanocrystals can easily be distinguished from the fluorescence of partially quenched CdSe. Our results show that the distance-dependence quenching is compatible with a Förster-type process.

Figure 1

Inset: Fluorescent observation of four Qdots adsorbed on a surface (acquisition time 500 ms). Histogram of the integrated intensity of individual Qdots and Qdot-dsDNA-Au complexes. The two measurements were performed successively and reported on the same plot. The gap between the Qdot-Au pair is 5.9 nm, which lead to a quenching efficiency of $82.3\pm 15.9\%$.

Figure 2

Intensity distribution of single Qdot (top) and Qdot-dsDNA-Au complexes for, respectively, 7-11-15-21 bp DNA linker. The solid line corresponds to a Gaussian fit.

Figure 3

Quenching efficiency as a function of the distance (circle: single-molecule studies; square: bulk measurements). The solid line corresponds to the resonant energy transfer function with $R_0=7.5\mathrm{n}\mathrm{m}$. In the schematic DPPE represents the phospholipids and PEG, the polyethylene glycol, constituting the surrounding Qdot micelle.

Figure 4

Emission spectrum of a solution of ssDNA-Qdots (circle), a mixture ssDNA-Qdots and ssDNA-Au with no complementarity between the ssDNA (square), and Qdot-dsDNA-Au complexes (diamond).