Tip-Enhanced Raman Spectroscopy of Combed Double-Stranded DNA Bundles

Double-stranded (ds) DNA of a lambda-phage virus are combed on octadecyltrichlorosilane (OTS)-modified borosilicate glass substrates and investigated by means of tip-enhanced Raman spectroscopy (TERS) using tips coated with an Ag/Au bilayer. Owing to an enhancement factor higher than 6 x 102 and a lateral spatial resolution better than 9 nm (which is below the size of the tip apex radius), cross-sections of nanowire-shaped thin DNA bundles can be spatially resolved. TER spectra reveal vibrational modes typical of DNA nucleobases and backbone, as confirmed by confocal Raman measurements carried out on dense stacks of DNA strands. While the TER signature of nucleobases is congruent with observations in single-stranded (ss) DNA, additional modes tied to the DNA backbone can be discerned in ds DNA. TERS enables ss and ds DNA samples to be distinguished from each other and hence can be exploited for the detection of DNA hybridization. Moreover, no TER contribution of the OTS layer appears, suggesting that functionalized DNA strands could be studied without spectral perturbation from the substrate. This work paves the way toward the nanoscale spectral study of organized DNA-based nanostructures.