Shepherding DNA with light and entropy

To analyze single molecules, including DNA, scientists need to manipulate and transport them through fluidic pathways. Using entropic nanoconfinement, they can pin DNA and other nano-sized species near constrictions in fluidic channels. To drive transport, researchers also exploit plasmons, electron oscillations in metals that resonate when excited by light. Localized heat from light-induced plasmon excitations produces thermal gradients and fluid motion that propel molecules forward. However, combining nanoconfinement with plasmonic transport has been a challenge. Nanoconfinement demands fluid channels that are less than the radius of gyration of the desired molecule—for many DNA molecules, that’s about 0.5 μm—whereas plasmonic flow-based transport requires channel heights greater than 1 μm.

Now Anders Kristensen at the Technical University of Denmark and colleagues have succeeded in joining the two techniques. Their demonstration of directed DNA transport opens the door for light-controlled manipulation of biopolymers in readily available microscope systems. The trick was embedding metallic V-shaped grooves in the nanofluidic channel. The tapered design of the grooves brings multiple length scales, and thus both mechanisms, into play: Large trapping potentials confine molecules, while plasmon-induced thermal gradients shuttle them along. The improved approach, with its impressive control and flexibility, prevents clumping and photodamage of biomolecules and enables propulsion of DNA. The researchers are looking to use their apparatus to perform site-specific chemical analysis of DNA and other biomolecules. (C. L. C. Smith et al., ACS Nano 11, 4553, 2017, doi:10.1021/acsnano.6b08563 .)