Friday, March 10, 2006

A Fast and Simple Cocaine Detector

New biosensors could provide an inexpensive and portable way to detect everything from drugs to signs of cancer.

By Emily Singer

At-home pregnancy tests are the model of diagnostic simplicity: a tester just pees on a stick and within minutes knows if she has to buy a crib. Imagine if one could just as easily detect HIV infection or a drug overdose. Scientists at the University of California, Santa Barbara (UCSB) say they are close to developing devices that can do just that.

Biochemist Kevin Plaxco and colleagues at UCSB create sensors using specific DNA sequences combined with off-the-shelf components. They previously made devices that could detect proteins and bits of DNA linked to viral infections or other pathogens. In their latest feat, published last month in the Journal of the American Chemical Society, they created a sensor that can detect cocaine in the blood.

Scientists have been searching for ways to make cheap and easy detection kits -- with limited success. DNA sensors to detect other segments of DNA are fairly easy to design. "But DNA sensors that can indicate the presence of proteins are more challenging," says Paula Hammond, a chemical engineer at MIT. Simple assays for small molecules are also difficult, since they usually require reactions with other reagents, says Hammond.

"[This work] opens the doors for a new generation of engineered biosensors for the rapid and selective detection of a plethora of proteins, viruses, nucleic acid, and even traditionally difficult targets, such as...small molecules like cocaine," says Ciara O'Sullivan, a research professor at the Universitat Rovira i Virgili in Tarragona, Spain.

The UCSB researchers say their design has some major benefits over devices already on the market. Their device does not require any additional reagents, making it very easy to use. Other sensors use similar detection molecules, but require extra chemicals or bulky optical readers to detect the target. "We can really make a palmtop device," says Plaxco. The device can also be easily modified to sense a wide variety of substances, by swapping in different pieces of detecting DNA.

The sensor consists of a gold electrode covered in specific strands of DNA. When the target molecule, in this case cocaine, binds to the DNA, it changes conformation. That change increases current flow through the electrode, creating a measurable electronic signal that can be read by the device. The magnitude of the change in current indicates the concentration of the substance -- the fraction of DNA molecules that change conformation is proportional to the number of cocaine molecules in the sample.

The sensors also have the ability to work in contaminated samples, a goal that has been difficult to achieve in the biosensor field. Most detection systems in use today require a pure sample. But if such tests are to move from sterile lab settings to the doctor's office or patient's home, they must be able to work in the real world. In soon-to-be-published research, Plaxco's team showed that a sensor for DNA could detect the DNA target sequence in straight blood serum -- and even in mud. "Our sensor is immune to contaminants," says Plaxco. "We can scrape some dirt off the ground and add DNA and the sensor still works."