Researchers develop ’fingerprinting’ for biological agents

Scientists at Northwestern University have developed a powerful new method for detecting infectious diseases, including those associated with many bioterrorism and warfare threats such as anthrax, tularemia, smallpox and HIV.

A research team led by Chad A. Mirkin, director of Northwestern’s Institute for Nanotechnology, has invented a technique for creating thousands of DNA detection probes made of gold nanoparticles with individual molecules attached. Much like human fingerprints, these molecules act as unique signals for the presence of different biological agents. The new detection method, for instance, can easily distinguish smallpox’s distinct “fingerprint” from that of HIV.

“By providing a near infinite number of signals, this advance allows researchers to quickly and accurately screen a sample for an extraordinarily large number of diseases simultaneously,” said Mirkin, also George B. Rathmann Professor of Chemistry.

Results, which include testing for genetic markers for six biological agents including hepatitis A, smallpox and HIV, will be published in the Aug. 30 issue of the journal Science. The new technology, which takes advantage of a technique called Raman spectroscopy, improves upon optical detection methods reported previously by Northwestern in Science.

Mirkin’s group has been pioneering the use of nanoparticles as a potential replacement for the more expensive polymerase chain reaction (PCR) and conventional fluorescence probes, the most widely used detection technology. It currently take days and sometimes weeks for results of genetic screening and disease diagnosis to come back from the laboratory.

“PCR was an extraordinary advance in diagnostics, but its complexity prohibits the development of easy-to-use diagnostic systems that can produce quick results in the field or in the doctor’s office,” said Mirkin. “Once a disruptive technology like PCR is invented, it creates a challenge for scientists to develop something even better.”

The new detection method involves designing probes for each disease agent. Each probe consists of a tiny gold particle approximately 13 nanometers in diameter. (In comparison, a human hair is 10,000 nanometers wide.) Attached to the particles are two key items: molecules that provide a unique signal (the “fingerprint”) when a light is shined on them and a single strand of DNA designed to recognize and bind a target of interest, such as smallpox or hepatitis A.

These designer probes are used in conjunction with a chip spotted with strands of DNA designed to recognize different disease targets. If a disease target is present in the sample being tested, it binds to the appropriate spot on the chip. Corresponding nanoparticle probes latch onto any matches. The chip is then washed and treated with ordinary photographic developing solution. Silver coats the gold nanoparticles where a match has taken place. A laser is scanned across the chip, and the signals for the probes are recorded. A unique “fingerprint” can be designed for each biological agent.

“The silver enhances the signal by many orders of magnitude, creating a highly sensitive method for detecting DNA,” Mirkin said. “Our technique seems to surpass conventional fluorescence-based methods in almost every category — sensitivity, selectivity, ease of use and speed — and has the potential to be very inexpensive.” The “fingerprinting” method also offers a greater number of distinct signals than conventional methods, meaning more diseases can be tested for at one time.