A non-linear harmonic analysis of potential induced fluorescence modulation of a DNA self assembled monolayer

Redox-Controlled Energy Transfer Quenching of Fluorophore-Labeled DNA SAMs Enables In Situ Study of These Complex Electrochemical Interfaces

Interfaces modified by a molecular monolayer can be challenging to study, particularly in situ, requiring novel approaches. Coupling electrochemical and optical approaches can be useful when signals are correlated. Here we detail a methodology that uses redox electrochemistry to control surface-based fluorescence intensity for detecting DNA hybridization and studying the uniformity of the surface response. A mixed composition single-strand DNA SAM was prepared using potential-assisted thiol exchange with two alkylthiol-modified ssDNAs that were either labeled with a fluorophore (AlexaFluor488) or a methylene blue (MB) redox tag. A significant change in fluorescence was observed when reducing MB to colorless leuco-MB. In situ fluorescence microscopy on a single-crystal gold bead electrode showed that fluorescence intensity depended on (1) the potential controlling the oxidation state of MB, (2) the surface density of DNA, (3) the MB:AlexFluor488 ratio in the DNA SAM, and (4) the local environment around the DNA SAM. MB efficiently quenched AlexaFluor488 fluorescence. Reduction of MB showed a significant increase in fluorescence resulting from a decrease in quenching or energy transfer efficiency. Hybridization of DNA SAMs with its unlabeled complement showed a large increase in fluorescence due to MB reduction for surfaces with sufficient DNA coverage. Comparing electrochemical-fluorescence measurements to electrochemical (SWV) measurements showed an improvement in detection of a small fraction of hybridized DNA SAM for surfaces with optimal DNA SAM composition and coverage. Additionally, this coupled electrochemical redox-fluorescence microscopy method can measure the spatial heterogeneity of electron-transfer kinetics and the influence of the local interfacial environment.

Electron ratcheting in self-assembled soft matter

Ratcheted multi-step hopping electron transfer systems can plausibly produce directional charge transport over very large distances without requiring a source-drain voltage bias. We examine molecular strategies to realize ratcheted charge transport based on multi-step charge hopping, and we illustrate two ratcheting mechanisms with examples based on DNA structures. The charge transport times and currents that may be generated in these assemblies are also estimated using kinetic simulations. The first ratcheting mechanism described for nanoscale systems requires local electric fields on the 10⁹ V/m scale to realize nearly 100% population transport. The second ratcheting mechanism for even larger systems, based on electrochemical gating, is estimated to generate currents as large as 0.1 pA for DNA structures that are a few μm in length with a gate voltage of about 5 V, a magnitude comparable to currents measured in DNA wires at the nanoscale when a source-drain voltage bias of similar magnitude is applied, suggesting an approach to considerably extend the distance range over which DNA charge transport devices may operate.

Application of FRET Microscopy to the Study of the Local Environment and Dynamics of DNA SAMs on Au Electrodes

Immobilized DNA probe strands self-assembled on an electrode surface are the bases of many electrochemically based biosensors. Control or measurement of the local environment around each DNA molecule tethered to the electrode surface is needed because the local environment can influence the binding or hybridization efficiency of the target in solution. Measurement of this local environment in buffer or under electrochemical control can be challenging. Here we demonstrate the use of fluorescence microscopy and a Förster resonance energy transfer (FRET) methodology to characterize multicomponent DNA SAMs. The DNA SAMs that were studied were composed of a series of mole fraction ratios of alkylthiol-modified DNA which was labeled with either AlexaFluor488 or AlexaFluor647, a FRET donor and acceptor, respectively. The DNA SAMs were hybridized before assembly onto the electrode surface. Wide-field filter-based FRET microscopy was used to study the assembly of DNA SAMs onto gold bead electrodes. These single-crystal gold bead electrodes contain many surface crystallographic regions which enable the comparison of the adsorbed DNA local environment. These surfaces show that most surface modifications are uniformly prepared, and the FRET efficiency can be explained through simple surface density considerations. The FRET efficiency for different compositions of the donor and acceptor for these regions is also explained through 2D FRET modeling. Not all surfaces were similar to the (111) and (110) regions showing deviations from the expected FRET behavior. Also demonstrated is FRET imaging using a confocal microscope. This approach proves useful in the analysis of a more dynamic system, such as the analysis of reductive desorption of the mixed-component DNA SAM. FRET microscopy is useful for surface analysis of the DNA local environment, enabling a measure of the surface modification, local density, and clustering and eventually a new detection modality.