FRET Imaging of Nonuniformly Distributed DNA SAMs on Gold Reveals the Role Played by the Donor/Acceptor Ratio and the Local Environment in Measuring the Rate of Hybridization.

Mixed DNA SAMs labeled with a fluorophore (either AlexaFluor488 or AlexaFluor647) were prepared on a single crystal gold bead electrode using potential-assisted thiol exchange and studied using Förster resonance energy transfer (FRET). A measure of the local environment of the DNA SAM (e.g., crowding) was possible using FRET imaging on these surfaces since electrodes prepared this way have a range of surface densities (Γ). The FRET signal was strongly dependent on Γ and on the ratio of AlexaFluor488 to AlexaFluor647 used to make the DNA SAM, which were consistent with a model of FRET in 2D systems. FRET was shown to provide a direct measure of the local DNA SAM arrangement on each crystallographic region of interest providing a direct assessment of the probe environment and its influence on the rate of hybridization. The kinetics of duplex formation for these DNA SAMs was also studied using FRET imaging over a range of coverages and DNA SAM compositions. Hybridization of the surface-bound DNA increased the average distance between the fluorophore label and the gold electrode surface and decreased the distance between the donor (D) and acceptor (A), both of which result in an increase in FRET intensity. This increase in FRET was modeled using a second order Langmuir adsorption rate equation, reflecting the fact that both D and A labeled DNA are required to become hybridized to observe a FRET signal. The self-consistent analysis of the hybridization rates on low and high coverage regions on the same electrode showed that the low coverage regions achieved full hybridization 5× faster than the higher coverage regions, approaching rates typically found in solution. The relative increase in FRET intensity from each region of interest was controlled by manipulating the donor to acceptor composition of the DNA SAM without changing the rate of hybridization. The FRET response can be optimized by controlling the coverage and the composition of the DNA SAM sensor surface and could be further improved with the use of a FRET pair with a larger (e.g., > 5 nm) Förster radius.