Graphene is composed of a honeycomb arrangement of carbon
atoms. Scientists have enlisted the exotic properties of graphene to function like the film of an
incredibly sensitive camera system in visually mapping tiny electric
fields in a liquid.
Researchers hope the new method will allow more
extensive and precise imaging of the electrical signaling networks in
our hearts and brains.
‘Graphene could be used as a material to sense electrical fields in a liquid. The new method will allow more extensive and precise imaging of the electrical signaling networks in our hearts and brains.’
The ability to visually depict the strength and motion of very faint
electrical fields could also aid in the development of so-called
lab-on-a-chip devices that use very small quantities of fluids on a
microchip-like platform to diagnose disease or aid in drug development,
for example, or that automate a range of other biological and chemical
The setup could potentially be adapted for sensing or trapping
specific chemicals, too, and for studies of light-based electronics (a
field known as optoelectronics).
A new way to visualize electric fields
"This was a completely new, innovative idea that graphene could be
used as a material to sense electrical fields in a liquid," said Jason
Horng, a co-lead author of a study published in Nature Communications
that details the first demonstration of this graphene-based imaging
system. Horng is affiliated with the Kavli Energy NanoSciences
Institute, a joint institute at Lawrence Berkeley National Laboratory
(Berkeley Lab) and UC Berkeley, and is a postdoctoral researcher at UC
The idea sprang from a conversation between Feng Wang, a faculty
scientist in Berkeley Lab's Materials Sciences Division whose research
focuses on the control of light-matter interactions at the nanoscale,
and Bianxiao Cui, who leads a research team at Stanford University that
specializes in the study of nerve-cell signaling. Wang is also a UC
Berkeley associate professor of physics, and Cui is an associate
professor of chemistry at Stanford University.
"The basic concept was how graphene could be used as a very general
and scalable method for resolving very small changes in the magnitude,
position, and timing pattern of a local electric field, such as the
electrical impulses produced by a single nerve cell," said Halleh B.
Balch, a co-lead author in the work. Balch is also affiliated with the
Kavli Energy NanoSciences Institute and is a physics PhD student at UC
"One of the outstanding problems in studying a large network of
cells is understanding how information propagates between them," Balch
Other techniques have been developed to measure electrical signals
from small arrays of cells, though these methods can be difficult to
scale up to larger arrays and in some cases cannot trace individual
electrical impulses to a specific cell.
Also, Cui said, "This new method does not perturb cells in any way,
which is fundamentally different from existing methods that use either
genetic or chemical modifications of the cell membrane."
The new platform should more easily permit single-cell measurements
of electrical impulses traveling across networks containing 100 or more
living cells, researchers said.
Tapping graphene's light-absorbing properties
Graphene is the focus of intense R&D because of its incredible
strength, ability to very efficiently conduct electricity, high degree
of chemical stability, the speed at which electrons can move across its
surface, and other exotic properties. Some of this research is focused
on the use of graphene as a component in computer circuits and display
screens, in drug delivery systems, and in solar cells and batteries.
In the latest study, researchers first used infrared light produced
at Berkeley Lab's Advanced Light Source to understand the effects of an
electric field on graphene's absorption of infrared light.
In the experiment, they aimed an infrared laser through a prism to a
thin layer called a waveguide. The waveguide was designed to precisely
match graphene's light-absorbing properties so that all of the light was
absorbed along the graphene layer in the absence of an electric field.
Researchers then fired tiny electrical pulses in a liquid solution
above the graphene layer that very slightly disrupted the graphene
layer's light absorption, allowing some light to escape in a way that
carried a precise signature of the electrical field. Researchers
captured a sequence of images of this escaping light in
thousandths-of-a-second intervals, and these images provided a direct
visualization of the electrical field's strength and location along the
surface of the graphene.
The new imaging platform - dubbed CAGE for "Critically coupled
waveguide-Amplified Graphene Electric field imaging device" - proved
sensitive to voltages of a few microvolts (millionths of a volt). This
will make it ultrasensitive to the electric fields between cells in
networks of heart cells and nerve cells, which can range from tens of
microvolts to a few millivolts (thousandths of a volt).
Researchers found that they could pinpoint an electric field's
location along the graphene sheet's surface down to tens of microns
(millionths of a meter), and capture its fading strength in a sequence
of time steps separated by as few as five milliseconds, or thousandths
of a second.
In one sequence, researchers detailed the position and dissipation,
or fade, of a local electric field generated by a
10-thousandths-of-a-volt pulse over a period of about 240 milliseconds,
with sensitivity down to about 100 millionths-of-a-volt.
Next up: living heart cells
Balch said that there are already plans to test the platforms with
living cells. "We are working with collaborators to test this with real
heart cells," she said. "There are several potential applications for
this research in heart health and drug screening."
There is also potential to use other atomically thin materials besides graphene in the imaging setup, she said.
"The kind of elegance behind this system comes from its generality,"
Balch said. "It can be sensitive to anything that carries charge."