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.”
Jon Faiz Kayyem, cofounder of Clinical Micro Sensors, a biotech company that was bought by Motorola, says Plaxco’s design has strong potential for broad use because it’s cheap to make and easy to use. “For years, scientists have been reporting fantastic results, such as detecting a single particle of anthrax in a lung full of air. But you don’t find those out there in real use. Most devices don’t give attention to the person running the test, paying for the test, or making the test,” he says.
The versatility of the design is also important. “There are lots of one-off individual tests for proteins and small molecules that rely on molecular weight or binding tests,” says Kayyem. “But there is no good way to do small molecule and protein testing that allows you to take a single sensor and test for lots of different things. It would be a leap forward in efficiency to have a single machine to test for almost any small molecule using the folding DNA approach,” he says.
One of the primary uses that Plaxco envisions for the device is to detect the levels of different drugs in a patient’s blood. This kind of information could help doctors if they suspect a drug overdose. It could also be used to monitor the levels of prescription drugs that must be kept at a specific concentration in the body.
Scientists still have a few problems to work out before the tests can be used in the clinic. “This work is a clear step forward…However, the reported sensing technique must be further refined to enable clinically useful detection limits,” says Aimee Rose, a specialist in organic-based chemical sensors at Nomadics, a security technology company with offices in Cambridge, MA. The device can currently detect cocaine in blood or saliva to a few micromolars. For routine drug testing, though, it would need to be able to detect picomolar concentrations, or a million times more dilute.
Rose speculates it will be challenging to make the device three orders of magnitude more sensitive. However, Kayyem says “there are lots of tricks in the yard for improving sensitivity, so I don’t see that being a permanent barrier.”
The sensitivity issue will play a role in other applications as well. The team has created DNA sensors to detect viral DNA present in the blood stream. The device has a lower detection limit of approximately 60 million DNA molecules per milliliter, which is about four orders of magnitude poorer than that needed for clinically relevant HIV detection, says Plaxco.
The team is now working on a sensor to detect a protein that appears to be diagnostic for several forms of cancer. Plaxco says proteins may be a better target for diagnostic purposes, because they tend to be detectable at clinically relevant concentrations in the blood.