Biophysicists long for an ideal material-something more structured and less
sticky than a standard glass surface-to anchor and position individual biomolecules.
Gold is an alluring possibility, with its simple chemistry and the ease with
which it can be patterned. Unfortunately, gold also tends to be sticky and can
be melted by lasers. Now, biophysicists at JILA
have made gold more precious than ever-at least as a research tool-by creating
nonstick gold surfaces and laser-safe gold nanoposts, a potential boon to laser
trapping of biomolecules.
The gold posts in this colorized micrograph, averaging 450 nanometers in diameter, are used to anchor individual biomolecules such as DNA for studies of their mechanical properties. The background surface is glass coated with a protein to prevent unwanted sticking. Source: National Institute of Standards and Technology.
JILA is a joint institute of the National Institute of Standards and Technology
(NIST) and the University of Colorado at Boulder.
JILA’s successful use of gold in optical-trapping experiments, reported
in Nano Letters,* could lead to a 10-fold increase in numbers of single molecules
studied in certain assays, from roughly five to 50 per day, according to group
leader Tom Perkins of NIST. The ability to carry out more experiments with greater
precision will lead to new insights, such as uncovering diversity in seemingly
identical molecules, and enhance NIST’s ability to carry out mission work,
such as reproducing and verifying piconewton-scale force measurements using
DNA, Perkins says. (A one-kilogram mass on the Earth’s surface exerts
a force 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,
Sept. 13, 2007.)
Perkins and other biophysicists use laser beams to precisely manipulate, track
and measure molecules like DNA, which typically have one end bonded to a surface
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 experiments has generally
involved nonspecifically absorbing fragile molecules onto a sticky glass surface,
producing random spacing and sometimes destroying biological activity. “It’s
like dropping a car onto a road from 100 feet up and hoping 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 an
otherwise nonstick surface. Gold posts are easy to lay down in desired patterns
at the nanometer scale. Perkins’ group attached the DNA to the gold with
sulfur-based chemical units called thiols (widely used in nanotechnology), an
approach that is mechanically stronger than the protein-based bonding techniques
typically used in biology. The JILA scientists used six thiol bonds instead
of just one between the DNA and the gold posts. These bonds were mechanically
strong enough to withstand high-force laser trapping and chemically robust enough
to allow the JILA team to coat the unreacted gold on each nanopost with a polymer
cushion, which eliminated undesired sticking. “Now you 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
to 500 nanometers and a height of 20 nanometers-that the scientists could
avoid hitting the posts directly with lasers. “Like oil and water, traditionally
laser tweezers and gold don’t mix. By making very small islands of gold,
we positioned individual molecules where we wanted them, and with a mechanical
strength that enables more precise and additional types of studies,” Perkins
says.
The research was supported by a W.M. Keck Grant in the RNA Sciences, the National
Science Foundation, and NIST.
* D.H. Paik, Y. Seol, W. Halsey and T.T. Perkins. Integrating a high-force
optical trap with gold nanoposts and a robust gold-DNA bond. Nano Letters. Articles
ASAP (As Soon As Publishable) Publication Date (Web): June 3, 2009 DOI: 10.1021/nl901404s.