DNA and chips

The secret ingredient in a future biological computer is to add a little DNA. But, making hybrid devices from a silicon chip and a strand of genetic material means mixing hard-wired microelectronics technology with the softer world of molecular biology.

Now, chemists at the University of Newcastle upon Tyne have come up with a solution that could lead to new lab-on-a-chip devices and biological sensors for use in medicine and environmental analysis. It might even one day allow biology to compute or provide an interface between electronic devices and living things.

Ben Horrocks

Ben Horrocks

Newcastle chemists Benjamin Horrocks and Andrew Houlton and their colleagues have devised a way to automate the solid-phase synthesis of DNA on a semiconductor chip. They believe their method could readily be adapted to the conventional fabrication techniques of photolithography used in the microelectronics industry to pattern the microscopic transistors and circuitry on a computer chip.

The team recently reported how it has found a way to attach a DNA sequence of just seventeen nucleotides to a silicon surface modified with organic molecules. The key to unlocking hybrid DNA chips lies in the team’s use of bifunctional organic molecules. At one end the molecule has the right chemistry to allow it to be attached to an oxide-free silicon surface. The other end of the molecule has a functional chemical group on which a DNA strand can be grown using an automated DNA synthesizer of the kind found in biotech laboratories the world over.

Andrew Houlton

Andrew Houlton

The team is working with two aims in mind – first, the development of chemical sensors and secondly the synthesis of DNA on silicon surfaces for nanoscale molecular architecture. This addresses the projected reduction in the size of electronic components which by 2015 are predicted to be of the order of nanometres (i.e. built from molecules), explains Houlton.

Gel electrophoresis reveals DNA is attached to the silicon

Gel electrophoresis reveals DNA is attached to the silicon

Previous endeavours in this area have generally used glass in preference to silicon wafers and those that have focused on silicon have applied organic molecules to an oxidised surface rather than the naked silicon chip. The Newcastle team has now confirmed that it is possible to cover the surface of a silicon chip with DNA strands. Moreover, the 17-base DNA strands can be coupled with the complementary DNA strand making the familiar DNA double helix. From the nanodevice perspective the importance of the team’s work lies in their ability to pattern the surface of the silicon rather than simply randomly deposit DNA strands. Patterning using the printing and etching techniques of microelectronics fabrication means they can tightly control the arrangement of the DNA on the surface and so produce what might one day become molecular circuitry.

A DNA-patterned silicon surface

A DNA-patterned silicon surface

Sequential modification of a silicon surface with DNA

Sequential modification of a silicon surface with DNA

Further reading

Angew. Chem. Int. Ed., 41, 615 (2002)
http://www3.interscience.wiley.com/cgi-bin/abstract/90512278/ABSTRACT

DOI: 10.1002/1521-3773(20020215)41:4<615::AID-ANIE615>3.0.CO;2-Y

Benjamin Horrocks
http://www.ncl.ac.uk/chemistry/staff/profile/b.r.horrocks

Andrew Houlton
http://www.ncl.ac.uk/chemistry/staff/profile/andrew.houlton

Suggested searches

Molecular Electronics
Nanotechnology

Planetary shortlist

There is a bright disc of interplanetary dust particles that surrounds our solar system, starting beyond the orbit of Saturn and stretching beyond the outer reaches, according to European astronomers. The discovery could lead to a speedier way of determining whether distant stars have their own planets too.

Markus Landgraf of the European Space Agency and colleagues have obtained the first direct evidence of a bright dust disc surrounding our Solar System and they reckon it should help illuminate the search for planets orbiting other stars in our galaxy. The finding will help mission planners short list stellar candidates for observation with ESA’s future planet-search missions, Eddington and Darwin.

Markus Landgraf

Markus Landgraf

Planetary systems are thought to condense from a cloud of gas and dust. Planets form near the central star, where the material is densest. However, at great distances from the star, the gas and dust is sparse and can coalesce only into a vast band of small, icy bodies. A dust ring would therefore seem indicative of a mature star with a planetary system. In our Solar System, the dust particles that reach out beyond the orbit of Neptune form the Edgeworth-Kuiper belt. The dust is continually lost to deeper space though and according to Landgraf there has to be something to replenish it for the disk to be maintained. Indeed, 50 tonnes of dust have to be produced every second.

If you have a dust disc around a star that’s not particularly young, then it’s extremely interesting because the dust has to come from somewhere. The only explanation is that the star has planets, comets, asteroids or other bodies that collide and generate the dust, explains ESA’s Malcolm Fridlund.

Traces of the disc surrounding our Solar System are visible in this image taken by COBE. The blue band curving across this image is created by the dust disc surrounding our Solar System (Copyright Michael Hauser (Space Telescope Science Institute)

Traces of the disc surrounding our Solar System are visible in this image taken by COBE. The blue band curving across this image is created by the dust disc surrounding our Solar System (Copyright Michael Hauser (Space Telescope Science Institute)

To prove the existence of the dust disk, Landgraf and colleagues sieved vast quantities of NASA’s Pioneer 10 and 11 data from the 1970s and early 1980s. These spacecraft found dust particles of unknown origin beyond Saturn’s orbit. It was initially suggested that the dust might come from comets but these objects only discard dust when they are close to the sun, they are frozen solid beyond Saturn. They compared the size of the dust particles from beyond Saturn with those measured by ESA’s Ulysses spacecraft, which has orbited the poles of the Sun for more than a decade.

Artist’s impression of Pioneer’s Jovian fly-by

Artist’s impression of Pioneer’s Jovian fly-by

The interstellar grains detected by Ulysses are typically ten to a hundred times smaller than the smallest grain that could be detected by Pioneer. Thus, the Pioneer grains have to be made somewhere within our Solar System.

The only plausible origin is collisions between the small, icy objects in the Edgeworth-Kuiper belt. Since these are the remnants of planet formation, the team believe that planetary systems around other stars will also produce constantly replenishing dust rings.

Brightly shining discs around the stars Vega and Epsilon Eridani have already been observed and the ESA results hint that these discs are evidence of their planets. If we see a similar dust ring around a main sequence star (a mature star, like the Sun), we’ll know it must have asteroids or comets. If we see gaps in the dust ring, it will probably have planets which are trapping the dust grains in resonant orbits, or ejecting them from the Solar System during close encounters, explains Landgraf.

The results appear in the May 2002 issue of The Astronomical Journal

Further reading

Suggested searches

Interplanetary Dust Particles
Extrasolar Planets

Flowing hot and cold

It is a relatively easy to control the direction of an electric current – use a rectifier – but heat usually flows from hot to cold. Now, Italian and French researchers have devised a rectifier for heat in a computer simulation. They reckon a heat rectifier might be made from DNA or another biological molecule.

Giulio Casati, Michael Peyrard and Marcello Terraneo of Insubria University in Como, Italy, have simulated the vibrations of atoms in a crystal lattice and reckon that given the right arrangement of atoms a device could be built that carried heat energy in one direction only. They looked at one-dimensional solids whose vibrational frequency depends on the size of the vibration. A material such as a long strand of DNA in the solid state might behave in this way for instance.

Thermal transfer Renaissance style (Georges de La Tour, particular of Saint Joseph, Paris, Musée du Louvre)

Thermal transfer Renaissance style (Georges de La Tour, particular of Saint Joseph, Paris, Musée du Louvre)

In such an anharmonic material, energy is transferred less efficiently from one end to the other than in a harmonic material where the vibrational frequency (temperature) is independent of the amplitude of the vibration.

At its simplest level, thermodynamics describes atoms in a solid as vibrating with heat energy flow as though they are connected to each other by springs. Casati believes that computational tools can provide a deeper insight and as such allow interesting and potentially surprising properties, such as allowing heat to travel in only one direction.

Giulio Casati

Giulio Casati

A thermal rectifier might consist of an anharmonic chain molecule tethered by a stiff molecular spring at one end and a loose spring at the other. When the spring is at a higher temperature, the chain molecule will vibrate more slowly because energy is not carried through the chain so effectively. Imagine the effect of plucking a guitar string on to which small pieces of children’s play clay have been stuck. The sound produced would be muffled because of the inefficiency of the string’s vibration. Suppose such a thermal rectifier material as we describe is constructed and you put some object inside this material, then if what is inside is cold then it remains cold; if instead it is warm it will cool down. The heat can flow only in one direction, Casati explained to Spotlight.

Computer simulated rectifier

Computer simulated rectifier

Such a thermal rectifier might have applications in electronics – as a cooling device for computer chips and in biotechnology as a heat flow controller for microscale chemical reactors. As to applications, I feel it is too early to say… much work needs to be done, confesses Casati, but what is important is that we now know that possibility exists.

Further reading

Phys. Rev. Lett. 88, 094302
http://link.aps.org/abstract/PRL/v88/e094302/

DOI: 10.1103/PhysRevLett.88.094302

Michael Peyrard
http://perso.ens-lyon.fr/michel.peyrard/

Suggested searches

Thermal Rectifiers
Thermodynamics