The monitoring of single molecule reactions promises unrivalled insight into chemical reaction mechanisms, but represents one of the most challenging tasks in chemistry. Surface-enhanced Raman scattering (SERS) is a particularly attractive single molecule (SM) technique due to its high chemical specificity, which allows to directly detect relevant intermediates and molecular subpopulations. However, SM-SERS is still at a premature state due to the highly challenging task to place single molecules precisely in nanoscale gaps of plasmonic nanostructures. These are required to provide sufficiently high electromagnetic field enhancement to reach SM sensitivity. The aim of SMART-DNA was to exploit artificial DNA nanostructures to provide sufficient structural control to assemble both, nanoparticles and target molecules, with nanometer precision. By means of novel DNA origami nanostructures the distance between two nanoparticles is controlled, and at the same time target molecules are placed at the positions of highest Raman enhancement through DNA aptamers.
Apart from Raman enhancement the excitation of the localized surface plasmon resonance of the metallic nanostructures results in other plasmonic effects such as heating and possibly the transfer of hot electrons. This can lead to diffusion, conformational changes or even dissociation of the target molecules. These issues do not only concern SM-SERS, but also make quantitative SERS and the SERS analysis of complex (bio)molecules very challenging. By the improved structural control achieved by SMART-DNA, nanoscale heating and hot electron transfer and their effect on SERS spectra was studied on an ensemble and a SM level. Finally, reactions induced by plasmonically generated electrons in DNA and DNA modified with electrophilic molecules was studied by SERS with the aim to develop novel strategies to improve cancer radiation therapies such as photothermal therapy.
Objectives of SMART-DNA:
I. Provide control over multiple structural parameters using bionanotechnology. SMART-DNA will create and apply advanced DNA origami-based SERS substrates with aptamer binding sites.
II. Establish reliable, continuous SM-SERS detection for a broad range of molecules.
III. Explore nanoscale interactions between NPs, molecules and excitation light.
IV. Explore electron-transfer induced reactions in DNA and modified DNA.
Conclusions: The main goals of the project have been reached, i.e. a DNA origami nanofork has been designed, characterised and optimized for SM-SERS measurements. SM-SERS measurements have been performed for a range of dye molecules, different proteins and small molecules such as hemin under both, dry and liquid conditions. Chemical modifications induced by ligands present in solution have been characterized on the SM level and SMs can now be monitored over several minutes to monitor their behaviour in the SERS hot spot and possible chemical transformations. Furthermore, hot electron reactions have been studied in great detail using small aromatic molecules as well as modified DNA. Furthermore, strategies have been explored to improve cancer radiation therapy.
