A group of chemists at Northwestern University, Chicago, claim they developed a diagnostic method that is much faster, easier to use, more accurate, and less expensive than polymerase chain reaction (PCR). The method, called bio-barcode amplification (BCA), is based on gold nanoparticles and DNA and can be used to detect multiple disease targets.

The group led by Chad Mirkin, PhD, director of Northwestern's Institute for Nanotechnology and professor of biochemistry, has been working in the area of nanoparticle detection for more than seven years. They discovered a way of using oligonucleotides as "barcodes" for different kinds of recognition events and as tools to amplify the signal associated with those events. The method works for the recognition of DNA, RNA, proteins, and small molecules.

The approach uses a combination of magnetic particle probes and nanoparticles used as bio-barcodes to create what Mirkin says is "the highest sensitivity protein detection system ever used, with about one million times more sensitivity than the next best thing, which are commercial ELISA-based assays," he says.

In a previous study [Nam et al., Science, vol. 301, pp. 1884-1886 (2003)], they demonstrated that they could use the bio-barcode approach to detect protein markers for disease. "We essentially had a PCR for proteins, but without the PCR," says Mirkin.

That success led them to test the barcode approach for other targets. "We wanted to test the approach to detect DNA, where synthetic DNA that is designed ahead of time is used as a barcode and an amplification scheme for recognizing target DNA in an unknown sample," says Mirkin.

In their latest study [Nam et al., Journal of the American Chemical Society, DOI: 10.1021/ja049384+], the team tested the approach using a strain of the anthrax bacterium. Development of detection systems for bioterrorism and biowarfare is an area of interest for the group.

The challenge, Mirkin says, has always been to develop a method with very high sensitivity comparable to PCR, without sacrificing the accuracy and selectivity. "PCR is an amazing technology. It allows you to take a sample and duplicate a portion of it so that you have enough of it to detect. That said, we still don't have detection systems that can be in a doctor's office, that allow you take a sample of blood, saliva, or urine and screen it for diseases or genetic predispositions."

The barcode system is more suitable for use at the point of care, says Mirkin, because it does not use easily degraded enzymes and the process is not as complex as PCR, where you have to limit contamination and be concerned about false positives. "It's built-in separation, amplification, and detection all in one," says Mirkin.

"The gold nanoparticle and the magnetic particle can recognize the target of interest," says Mirkin. "When the target is present, it zips those particles together. Applying a magnetic field separates the target with the barcodes from the extra solution. Water or heat is then added to release the barcode DNA. The barcode DNA can then be identified in a gene chip."

The approach, which has been licensed to Nanosphere Inc., Northbrook, Ill., could be applied to the detection of multiple diseases or genetic predispositions, says Mirkin. "It offers a single platform for detection of all small-molecule, DNA, RNA, and protein targets, where only probe is changed to detect the barcode, which are signatures for the different targets of interest."

- Elizabeth Tolchin