By Frances Galvin

Chinese scientists can detect genetically modified food with a new DNA
detector.
DNA sensors have many applications, including in disease diagnosis and
environmental monitoring. The aim of the team led by Kui Jiao, at the
Qingdao University of Science and Technology, was to develop a device for
detecting genetically modified organisms. Their device combines an
electrochemical sensor with the polymerase chain reaction (PCR), a method
which amplifies target DNA sequences by repeated copying.

While PCR is a very sensitive technique, methods of analysing the result are
not perfect, often requiring toxic chemicals or extra steps, which increase
the sample contamination risk and the time and cost of analysis. Jiao's new
system detects successful amplification simply by dipping an electrode into
the post-PCR sample.

Using an electrode coated with carbon nanotubes, the system determines the
consumption of dGTP - one of DNA's building blocks - by the PCR. This is
possible because the electrode can oxidise free dGTP, but is less able to
oxidise dGTP when it is embedded in the DNA double helix. This results in a
reduction in the system current proportional to the extent of the
amplification reaction.

Jiao says the team had previously looked to the electrochemical properties
of the finished DNA for their detector. 'But the need for a sensor with
super-sensitivity, made us consider the other side of the coin,' explains
Jiao, monitoring the loss of starting material rather than the build up of
product.

The DNA detection arises because a PCR is started by adding DNA fragments
complementary to parts of a target sequence. Only if the target sequence is
present will the added fragments initiate the copying process, and
amplification and the current decrease be observed. Using this method, the
team has successfully identified transgenic maize samples, obtaining results
agreeing with a traditional PCR analysis method.

Edmond Magner, who researches bioelectrochemistry at the University of
Limerick, Ireland, sees the technique's potential. He warns there are a
number of hurdles which will need to be overcome, such as producing carbon
nanotubes reliably and cheaply. But once these challenges are met 'it could
provide a relatively fast and possibly cost effective means of monitoring
PCRs if it could be applied on a wider scale,' says Magner.

www.checkbiotech.org