Touch sensitive liquid has future
as optical memory
Researchers have created a new material that
changes its colour from red to bright yellow when it's touched. It could be
used as a biosensor or even as optical memory in the future
A new material developed at the University of Michigan stays
liquid more than 200 degrees Fahrenheit below its expected freezing point, but
a light touch can cause it to form yellow crystals that glow under ultraviolet
light.
Even living cells sitting on a film of the supercooled liquid
produce crystal footprints, which means that it's about amillion times more
sensitive than other known molecules that change color in response to pressure.
The material could have applications as a new kind of sensor for living cells,
while the mechanism behind its unusual properties may guide the development of
electronics and medicines. “As you know, water freezes around zero degrees
Celsius. It changes to ice.It's as if the water remained a liquid down to -100
degrees Celsius,“ said Kyeongwoon Chung, first author of the paper published in
ACS Central Science.
Electronics manufacturers are interested in glass-like,
carbon-based materials known as amorphous, organic materials, these materials
are cheaper, easier to work with and more flexible than inorganic
semiconductors such as silicon. Because they don't have a crystal structure
that must be broken up, they also dissolve well in the body, which improves
their effectiveness as pharmaceuticals. “We would like to understand better
molecular design principles to control the crystallisation tendency of organic
molecules,“ said Jinsang Kim, whose group discovered the molecule.“Most organic
materials have a strong driving force to crystallise, but they don't always
form the same crystal, making quality control difficult.“
The team investigated a family of organic molecules widely used in
pigments and electronic devices like solar cells, LEDs and transistors looking
for ways to streamline the production of their amorphous forms. These molecules
can be described as a rigid core flanked by two flexible side chains. If the
chains are short, the core molecules drive crystallisation, but if they are
long, the chains interact to form a dif ferent kind of crystal.
The team found that by varying the lengths of the side chains,
they could cause a stalemate between two modes of crystallisation. “We found
that the core unit and side chains are working in opposite directions,“ said
Kim.
As a result, the molecule remained liquid even when cooled below
its melting temperature of 273 degrees Fahrenheit. Typically, this would also
be the material's freezing point. Instead, the molecules stay in a stable,
“supercooled“ liquid state down to 41 degrees Fahrenheit, at which point the
molecules solidify into a glass.
In addition to the unusually broad temperature range for the
supercooled liquid state, the group found that it crystallised when rubbed with
a stylus, changing from dark red to bright yellow. The rubbing broke the
stalemate between the two ways for the molecules to connect, allowing the side
chains to link up. At high temperatures around 212 degrees Fahrenheit, when the
molecules moved freely, just a touch could make the entire film or droplet
crystallise.
“It's like a domino effect,“ Kim said.
But at room temperature, the thicker supercooled liquid
crystallised only where the stylus made contact, allowing Chung to scrawl
messages such as “shear-triggered crystal“ or a secret note to his wife.
Kim's group is pursuing using this molecule in biosensors, which
might reveal characteristics of cells for medical diagnosis. The ability to
write and erase luminescent information also suggests the potential for use in
a memory that encodes information with light rather than magnetism. Such
“optical memory“ would require much more development.
Mm15may15
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