Using a refined technique for trapping and manipulating
nanoparticles, researchers at NIST have extended the trapped particles’ useful
life more than tenfold. This new approach, which one researcher likens to
“attracting moths,” promises to give experimenters the trapping time
they need to build nanoscale structures and may open the way to working with
nanoparticles inside biological cells without damaging the cells with intense
laser light.
Scientists routinely trap and move nanoparticles in a
solution with “optical tweezers”—a laser focused to a very small
point. The tiny dot of laser light creates a strong electric field, or
potential well, that attracts particles to the center of the beam. Although the
particles are attracted into the field, the molecules of the fluid they are
suspended in tend to push them out of the well. This effect only gets worse as
particle size decreases because the laser’s influence over a particle’s
movement gets weaker as the particle gets smaller. One can always turn up the
power of the laser to generate a stronger electric field, but doing that can
fry the nanoparticles too quickly to do anything meaningful with them—if it can
hold them at all.
NIST researchers’ new approach uses a control and feedback
system that nudges the nanoparticle only when needed, lowering the average
intensity of the beam and increasing the lifetime of the nanoparticle while
reducing its tendency to wander. According to Thomas LeBrun, they do this by
turning off the laser when the nanoparticle reaches the center and by
constantly tracking the particle and moving the tweezers as the particle moves.
“You can think of it like attracting moths in the dark
with a flashlight,” says LeBrun. “A moth is naturally attracted to
the flashlight beam and will follow it even as the moth flutters around
apparently at random. We follow the fluttering particle with our flashlight
beam as the particle is pushed around by the neighboring molecules in the fluid.
We make the light brighter when it gets too far off course, and we turn the
light off when it is where we want it to be. This lets us maximize the time
that the nanoparticle is under our control while minimizing the time that the
beam is on, increasing the particle’s lifetime in the trap.”
Using this method at constant average beam power, 100-nm
gold particles remained trapped 26 times longer than had been seen in previous
experiments. Silica particles 350 nm in diameter lasted 22 times longer, but
with the average beam power reduced by 33%. LeBrun says that their approach
should be able to be combined with other techniques to trap and hold even
smaller nanoparticles for extended periods without damaging them.
“We’re more than an order of magnitude ahead of where
we were before,” says LeBrun. “We now hope to begin building complex
nanoscale devices and testing nanoparticles as sensors and drugs in living
cells.”