Plasmonic nanopores combined with Raman spectroscopy
are emerging
as platforms for single-molecule detection and sequencing in label-free
mode. Recently, the ability of identifying single DNA bases or amino
acids has been demonstrated for molecules adsorbed on plasmonic particles
and then delivered into the plasmonic pores. Here, we report on bowl-shaped
plasmonic gold nanopores capable of direct Raman detection of single
λ-DNA molecules in a flow-through scheme. The bowl shape enables
the incident laser to be focused into the nanopore to generate a single
intense hot spot with no cut off in pore size. Therefore, we achieved
ultrasmall focusing of NIR light in a spot of 3 nm. This enabled us
to detect 7 consecutive bases along the DNA chain in flow-through
conditions. Furthermore, we found a novel electrofluidic mechanism
to manipulate the molecular trajectory within the pore volume so that
the molecule is pushed toward the hot spot, thus improving the detection
efficiency.