University of California, Berkeley, researchers have invented a variation on the standard electronic transistor, creating the first ‘nanofluidic’ transistor that allows them to control the movement of ions through sub-microscopic, water-filled channels.

University of California, Berkeley, researchers have
invented a variation on the standard electronic transistor, creating the
first ‘nanofluidic’ transistor that allows them to control the movement of
ions through sub-microscopic, water-filled channels.
A nanofluidic transistor, too small to see with the
naked eye, could allow the creation of microscopic chemical plants that operate
without moving parts. (Image courtesy Majumdar & Yang labs)

The researchers – a chemist and a mechanical engineer –
predict that, just as the electronic transistor became the main component
of microprocessors and integrated circuits, so will nanofluidic transistors
anchor molecular processors, allowing microscopic chemical plants on a chip
that operate without moving parts. No valves to get stuck, no pumps to blow,
no mixers to get clogged.

‘A transistor is like a valve, but you use electricity
to open or close it,’ explained Arun Majumdar, professor of mechanical engineering
at UC Berkeley. ‘Here, we use a voltage to open or close an ion channel.
Now that we’ve shown you can make this building block, we can hook it up to
an electronic chip to control the fluidics.’

One application Majumdar and colleague Peidong Yang, UC
Berkeley professor of chemistry, are exploring is cancer diagnosis. A nanoscale
chemical analysis chip could, theoretically, take the contents of as few
as 10 cancer cells and pull out protein markers that can tip doctors to the
best means of attacking the cancer.

‘This is an ideal way to open up cells and identify the
proteins or enzymes inside,’ he said. ‘An enzyme profile would tell doctors
a lot about the kind of cancer, especially in its early stages when there
are only a few cells around.’

Yang, who built a variation of the transistor using nanotubes,
is equally intrigued by the computational possibilities of the device.

‘It may sound a little bit far fetched, but we’re thinking
about whether we can do the same thing with nanofluidic transistors as we
can currently with MOSFETs,’ he said, referring to the Metal-Oxide Semiconductor
Field Effect Transistors used in most of today’s microprocessor chips. ‘Using
molecules to process information gives you a fundamentally different information
processing device.’

Majumdar, Yang and colleagues Rohit Karnik, a mechanical
engineering graduate student; Rong Fan, a chemistry graduate student; and
mechanical engineering students Min Yue and Deyu Li reported their success
– the product of three years of effort – in the May issue of the journal Nanoletters.
Yang and Majumdar are also faculty scientists at Lawrence Berkeley National

One big advantage of nanofluidic transistors, Majumdar
said, is that they could be made using the same manufacturing technology that
today produces integrated circuits. Nanofluidic channels could be integrated
with electronics on a single silicon chip, with the electronics controlling
the operation of the nanofluidics. The only microscale parts of the device
are the microchannels for injecting liquid.

Majumdar and Yang’s team constructed a 35-nanometer-high
channel between two silicon dioxide plates, then filled the channel with water
and potassium chloride salt. They showed that by applying a voltage across
the channel by means of electrodes attached to the plates, they could shut
off the flow of potassium ions through the water. This is analogous to the
control of electron flow through a transistor by means of a gate voltage.

Such ion manipulations are not possible through microscopic
channels because ions in the liquid quickly move to the plates and cancel
out the voltage, basically shielding the interior of the liquid from the
electric field. Channels less than 100 nanometers across, however, are so
small that this shielding doesn’t occur, so ions in the bulk liquid can be
pushed or pulled by electric voltages.

If the ions are proteins, they can be shuttled through
channels lined with fluorescent antibodies for detecting or sensing. If the
ions are pieces of DNA, they can be sorted and sequenced. In fact, the authors
say, any highly sensitive biomolecular sensing down to the level of a single
molecule could be performed with nanofluidic transistors. They demonstrated
that labeled, charged DNA fragments could be manipulated in their transistor.

Yang, who is adept at making nanoscale lasers, tubes, wires
and other devices, created a version of the transistor using nanotubes with
internal diameters of 20 nanometers, proving that the same sort of molecular
processing can be done with these innovative structures. While Majumdar foresees
putting electronic and nanofluidic transistors on the same chip to provide
computer control of chemical processing, Yang foresees the computing and chemical
processing being done by the same nanofluidic channels.

‘With nanotubes, you have access to much smaller dimensions
compared to conventional nanofabrication, but in terms of integration, it’s
more difficult,’ Yang said. ‘For the future, both processes are fundamentally
interesting, and eventually devices will combine both.’

Majumdar and Yang acknowledge that a lot more work needs
to be done, including understanding the surface effects inside nanochannels.
In addition, the voltage required to shut off ion flow is now 75 volts, far
too high for any of today’s integrated circuits. But their team has a few
other papers waiting to appear in Nanoletters and in the Physical Review Letters
that push the technology farther than this initial paper. They hope to beat
the time lag between invention of the transistor in 1947 and creation of
the first integrated circuit in 1960.

‘We want to be the first to build integrated circuits with
just three transistors able to do sorting and eluting, just as a two- or
three-bit processor can do multiplexing and addressing,’ Majumdar said.

The work was supported by the National Cancer Institute’s
Innovative Molecular Analysis Technologies program and by the Department of
Energy. Current work is being funded by the National Science Foundation.

Source :