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Microbe and Machine Merged to Create

First 'Cellborg'

By Ker Than

LiveScience Staff Writer

posted:

27 October 2005

02:09 pm ET

Fully

merging microbe and machine for the first time, scientists have created

gold-plated bacteria that can sense humidity.

The

breakthrough is the first "cellborg" in what might become an array of

devices that could sense dangerous gases or other hazardous substances.

The

bioelectronic device swells and contracts in response to how much water

vapor is in the air. It’s called a cellborg

humidity sensor,

and it is at least four times more sensitive than those that are solely

electronic. It even works even when its biological parts are long dead.

How

it was made

Scientists

first coated a silicon chip with a layer of live Bacillus cereus

bacteria. Some of the long, rod-shaped microbes lodged between two

etched electrodes on the chip’s surface, forming a bridge. The chip was

then washed in a solution containing tiny gold particles, each one

about 30 nanometers across.

A

nanometer is one billionth of a meter. A human hair is roughly 100,000

nanometers wide.

The

gold nanoparticles attached to long hair-like proteins on the surface

of the bacteria, transforming them into gold-plated bridges that

completed an electronic circuit.

The

hair-like proteins are called teichoic acid molecules. They are

negatively charged and provide a surface for the positively-charged

gold nanoparticles to attach to. Without them, the gold nanoparticles

would repel one another due to their like-charges and no bridge between

the two electrodes could ever form.

By

wrapping themselves around the gold nanoparticles, the teichoic acid

molecules therefore act as metal insulators, creating what engineers

call a “dielectric barrier.”

“To

any electronic person, that’s a field day,” said Ravi Saraf, a

University of Nebraska chemical engineer who led the discovery. “You

can go nuts with it.”

First

of its kind

The

bodies of the gold-plated bacteria swell as humidity increases and they

absorb moisture; they contract when humidity decreases. The swelling

causes the gold nanoparticles on the bacteria’s surface to grow farther

apart, like stickers on an inflating balloon.

Even

a

tiny separation of 0.2 nanometers between the gold nanoparticles was

enough to interfere with the flow of electric current between the

circuit’s two electrodes. That’s because the farther apart the gold

particles on the bacteria’s surface, the harder it becomes for

electrons to “hop” between particles and get from one electrode to the

other.

The

cellborg sensor is extremely sensitive: a drop from 20 percent to zero

humidity results in a 40-fold decrease in current flow. In humidity

sensors that are solely electronic, the decrease is only 10-fold.

According

to Saraf, their hybrid sensor is the first to incorporate

microorganisms into an electronic device.

In

the past, researchers have programmed bacteria to behave like biological

computers

or created electronic circuits that respond to glowing bacteria as a

way to detect chemicals, but in those cases, the line separating

microbe and machine was still distinct.

The

nearest other attempt to merge the two occurred in March, when

researchers at the University of Wisconsin-Madison reported using

electrodes to trap and examine bacteria. One researcher from that team

essentially predicted the experiment by Saraf and his graduate student,

Vikas Berry, saying that it might be possible to attach microscopic

gold particles to the shell of the bacteria to form “nanoscale gold

wire.”

Bacteria

zombies

Once

assimilated, the gilded bacteria can survive for only about two days,

but even when dead, their bodies still swell and contract in response

to changes in humidity. They can go on working this way for months,

Saraf said.

If

scientists could coat bacteria with gold nanoparticles without killing

them, it might be possible to make cellborg sensors that could power

an electronic circuit instead of just completing one, Saraf told LiveScience.

Another

possibility may be to tweak the bacteria so they respond to things

other than humidity. They could be made to swell or contract, for

example, when they feed on certain gases or hazardous chemicals.

The

study was detailed in the Oct. 21 issue of the journal Angewandte

Chemie.