Image of the interface of cell (blue) and nanopillar shows cell membranes wrapped around the pillar. Image: Stanford Univ. |
As words go,
evanescent doesn’t see enough use. It is an artful term whose beauty belies its
true meaning: fleeting or dying out quickly. James Dean was evanescent. The
last rays of a sunset are evanescent. All that evanesces, however, is not lost,
as a team of Stanford researchers demonstrated in a recent article
in Proceedings of the National Academy of
Sciences. In fact, in the right hands, evanescence can have a
lasting effect.
The Stanford
team—led by chemist Bianxiao Cui and engineer Yi Cui (no relation), with scholars
Chong Xie and Lindsey Hanson—have created a cellular research platform that
uses nanopillars that glow in such a way as to allow biologists, neurologists,
and other researchers a deeper, more precise look into living cells.
“This novel system of illumination is very precise,” said Bianxiao
Cui, the study’s senior author and assistant professor of chemistry at
Stanford. “The nanopillar structures themselves offer many advantages that
make this development particularly promising for the study of human
cells.”
Longstanding challenges
To comprehend the potential of this breakthrough, it is helpful to understand
the challenges to earlier forms of molecular imaging, which shine light
directly on the subject area rather than using backlighting, as in this approach.
Scientists
hoping for better, smaller molecular imaging have for years been handcuffed by
a physical limitation on how small an area they could focus on—an area known as
the observation volume. The minimum observation volume has long been limited to
the wavelength of visible light, about 400 nm. Individual molecules, even long
proteins common in biology and medicine, are much smaller than 400 nm.
This is where evanescence comes in. The Stanford team has successfully employed
quartz nanopillars that glow just enough to provide light to see by, but weak
enough to punch below the 400-nm barrier. The field of light surrounding the
glowing nanopillars—known as the “evanescence wave”—dies out within
about 150 nm of the pillar. Voilà—a light source smaller than the wavelength of
light. The Stanford researchers estimate that they have shrunk the observation
volume to one-tenth the size of previous methods.
Particular promise
The Stanford nanopillar imaging technique is particularly promising in cellular
studies for several reasons. First, it is non-invasive—it does not harm the
cell that is being observed, a downfall of some earlier technologies. For
instance, a living neuron can be cultured on the platform and observed over
long periods of time.
Second, the
nanopillars essentially pin the cells in place. This is promising for the study
of neurons in particular, which tend to move over time due to the repeated
firing and relaxation necessary for study.
Lastly, and
perhaps most importantly, the Stanford team found that by modifying the chemistry
on the surface of the nanopillars they could attract specific molecules they
want to observe. In essence they can handpick molecules to study even within
the crowded and complex environment of a human cell.
A scanning electron microscope image of a cell grown over and interacting with nanopillars. Arrows indicated three nanopillars. Photo: Stanford Univ. |
“We know that proteins and their antibodies attract each other,”
said Bianxiao Cui. “We coat the pillars with antibodies and the proteins
we want to look at are drawn right to the light source—like prima donnas to the
limelight.”
Setting the scene
To create their nanopillars, the Stanford team members begin with a sheet of
quartz, which they spray with fine dots of gold in a scattershot pattern—Jackson
Pollock-style. They then etch the quartz using a corrosive gas. The gold dots
shield the quartz directly below from the etching process, leaving behind tall,
thin pillars of quartz.
The
researchers can control the height of the nanopillars by adjusting the amount
of time the etching gas is in contact with the quartz and the diameter of the
nanopillars by varying the size of the gold dots. Once the etching process is
complete and the pillars are created, they add a layer of platinum to the flat
expanse of quartz at the base of the pillars.
The setting is
something out of a futuristic John Ford film—Monument Valley
rendered in quartz crystal. All that is missing is a stagecoach and John Wayne.
In this world, a wide desert of platinum stretches to the horizon, interrupted
on occasion by transparent spikes of crystalline quartz that rise several
hundred nanometers from the valley floor.
The Stanford
researchers then shine a light from below their creation. The opaque platinum
blocks most of the light, but a small amount travels up through the
nanopillars, which glow against the dark field of platinum.
“The
nanopillars look a bit like tiny light sabers,” said Yi Cui, associate
professor of materials science and engineering at Stanford, “but they
provide just the right amount of light to allow scientists to do some pretty
amazing stuff—like looking at individual molecules.”
The team has
created an exceptional platform for culturing and observing human cells. The
platinum is biologically inert and the cells grow over and closely adhere to
the nanopillars. The glowing spires then meet with fluorescent molecules within
the living cell, causing the molecules to glow—providing the researchers just
the light they need to peer inside the cells.
“So, not only have we found a way to illuminate volumes one-tenth as
small as previous methods—letting us look at smaller and smaller structures—but
we can also pick and choose which molecules we want to observe,” said Yi
Cui. “This could prove just the sort of transformative technology that
researchers in biology, neurology, medicine and other areas need to take the
next leap forward in their research.”
