ADVANCES
16 Scientific American, December 2020 Illustration by Thomas Fuchsmagnetite. Furthermore, he found that pili
he gathered from the cells could indeed
conduct electricity.
Researchers have already developed
applications that use living conductive
microbes, but Lovley wants to harvest the
nanowires themselves to build environmen-
tally friendly electronics. He recently co-au-
thored two papers on sensors made from
Geobacter nanowires: One, described in
Nano Research, detects ammonia; the other,
detailed in Advanced Electronic Materials,
picks up changes in humidity. Another
device, which his group described in Nature,
uses nanowires to pull electrons from water
molecules in the air—thus producing elec-
tricity from humidity. “It has some advantag-
es over the other sustainable forms of elec-
tricity production, such as solar or wind,
because it’s a 24/7 continuous process,” Lov-
ley explains. “And it will work in just about
any environment on earth.”
He suggests nanowires, instead of bat-
teries, could power some devices. “Al ready
we can use the protein nanowires [to pro-
duce power] for small-scale electronics, like
a wearable patch for medical monitoring,”
he says, adding that nanowires can function
in living tissue without triggering a bad reac-
tion and are more biodegradable than metals.
Lovley says companies have expressed
interest in such applications. But some sci-
entists are skeptical about separating
nanowires from the bacteria that generate
them. “Taking proteins that have electrical
properties out of their natural context—
they [then] have to compete with synthetic
materials” for efficiency, explains Sarah
Glaven, a U.S. Naval Research Laboratory
biologist. Nanowires would “be hard-
pressed to compete with something like a
conductive metal.” She has previously
worked with Lovley but is not involved in his
current research, instead focusing on
genetically modifying conductive bacteria
for applications such as marine sensors.
Glaven notes that nanowires would have
an advantage in environments such as the
ocean or human body, which corrode tradi-
tional electronics. But even in that setting,
she says, nanowires would still vie with ma -
terials such as biocompatible polymers. She
prefers working with living microbes be -
cause “you don’t just have an electron-car-
rying material—you have the whole infor-
mation-processing suite within the cell itself.”
Although researchers are already find-
ing applications for both living cells and har-
vested nanowires—and have even explored
modifying the prolific bacterium Escherichia
coli to produce pili—questions remain about
which proteins make up the most produc-tive nanowires. Understanding whether pili
or another type of nanowire carries most of
Geobacter’ s electricity could guide scientists
choosing the best material for electronics.
“Everybody, including us, thought [the
key nanowires] were pili,” says Nikhil Mal-
vankar, a biophysicist who previously worked
with Lovley but currently has his own labora-
tory at Yale University. Last year, however,
Malvankar and his colleagues imaged Geo-
bacter with an electron microscope; they
concluded that rather than stringlike pilin
proteins, stacks of proteins called cyto-
chromes form the microbes’ main electrici-
ty-transmission method. The researchers
went on to examine a biofilm of the bacteria
via genetic-modification experiments, as
well as several imaging methods—Glaven
says they “really threw the kitchen sink” at
getting an accurate picture of the nanowires
Geobacter was using. The Yale team pin-
pointed a specific hyperefficient conductor
cytochrome called OmcZ, which Geobacter
produces in response to an electrical field, as
the biofilm’s primary method of shedding
electrons. “Seeing is believing, so I think
microscope imaging is very important,” says
co-author and Yale physicist Sibel Yalcin.
But researchers still do not agree on
which nanowire is most significant. Some
come down on the side of pili, others forPHYSICSChilling
Mystery
Lasers slow molecules for a
glimpse of the quantum worldBecause humans are large and warm,
we can rarely see quantum mechanics
in action. To do so, physicists use lasers
to cool atoms to just a trillionth of a degree
above absolute zero. This slows the atoms’
movement enough to watch them follow
quantum physics rules. But cooling mole-
cules made of more than one atom has
proved more difficult: somehow these
ultracold molecules tend to sneakily heat
up again, so researchers can no longer
keep track of them—a phenomenon
physicists call “ultracold molecule loss.”
A study published in Nature Physics reveals
how it happens.Being able to better see and control
ultracold molecules would help scientists
assemble a quantum machine piece by
piece, says Jun Ye, a physicist at the
University of Colorado Boulder, who
was not involved in the study. But mol-
ecule heating throws a wrench in this
process. A pioneer of ultracold mole-
cule experiments, Ye observed early on
that reactions—a matter of quantum
chemistry instead of quantum physics—
were somehow heating molecules up.
Yu Liu, a researcher at Harvard Univer-
sity and co-leader of the study, says the
researchers had planned to investigate the
reactions themselves. But, Liu says, “what
we saw during the process turns out to give
the answer to this question” of ultracold
molecule loss. The scientists slowed down
the chemical reactions between molecules
enough to observe their behavior while in
a state called “the complex,” which occurs in
the middle of the reaction—before the mol-ecules fully transform into the reaction’s
products. Because molecules interact with
light through electrical forces, the team
used lasers to keep them from flying away.
At room temperature the complex
exists too briefly to observe. At low tem-
peratures it sticks around longer, but the
researchers found that this longevity has a
cost: it gives the ultracold complex time to