Nonstick and laser safe gold aids laser trapping of biomolecules

Now, biophysicists at JILA have made gold more precious thanever - at least as a research tool - by creating nonstick gold surfaces andlaser-safe gold nanoposts, a potential boon to laser trapping of biomolecules.

JILA is a joint institute of the National Institute of Standards andTechnology (NIST) and the Universityof Colorado at Boulder.

JILA's successful use of gold in optical-trapping experiments, reported inNano Letters, could lead to a 10-fold increase in numbers of single moleculesstudied in certain assays, from roughly five to 50 per day, according to groupleader Tom Perkins of NIST. The ability to carry out more experiments withgreater precision will lead to new insights, such as uncovering diversity inseemingly identical molecules, and enhance NIST's ability to carry out missionwork, such as reproducing and verifying piconewton-scale force measurementsusing DNA, Perkins says. (A one-kilogram mass on the Earth's surface exerts aforce of roughly 10 newtons. A piconewton is 0.000 000 000 001 newtons. See'JILA Finds Flaw in Model Describing DNA Elasticity' NIST Tech Beat, 13September 2007.)

Perkins and other biophysicists use laser beams to precisely manipulate,track and measure molecules like DNA, which typically have one end bonded to asurface and the other end attached to a micron-sized bead that acts as a'handle' for the laser. Until now, creating the platform for such experimentshas generally involved nonspecifically absorbing fragile molecules onto asticky glass surface, producing random spacing and sometimes destroyingbiological activity. 'It's like dropping a car onto a road from 100 feet up andhoping it will land tires down. If the molecule lands in the wrong orientation,it won't be active or, worse, it will only partially work,' Perkins says.

Ideally, scientists want to attach biomolecules in an optimal pattern on anotherwise nonstick surface. Gold posts are easy to lay down in desired patternsat the nanometre scale. Perkins' group attached the DNA to the gold withsulphur-based chemical units called thiols (widely used in nanotechnology), anapproach that is mechanically stronger than the protein-based bondingtechniques typically used in biology. The JILA scientists used six thiol bondsinstead of just one between the DNA and the gold posts. These bonds weremechanically strong enough to withstand high-force laser trapping andchemically robust enough to allow the JILA team to coat the unreacted gold oneach nanopost with a polymer cushion, which eliminated undesired sticking. 'Nowyou can anchor DNA to gold and keep the rest of the gold very nonstick,'Perkins says.

Moreover, the gold nanoposts were small enough - with diameters of 100 to500 nanometres and a height of 20 nanometres - that the scientists could avoidhitting the posts directly with lasers. 'Like oil and water, traditionallylaser tweezers and gold don't mix. By making very small islands of gold, wepositioned individual molecules where we wanted them, and with a mechanicalstrength that enables more precise and additional types of studies,' Perkinssays.