By Salvatore Salamone
June 15, 2003 | Small enough to be placed in crucial testing areas like building airshafts, a new nanotechnology-based system from Integrated Nano-Technologies (INT) is designed to speed discovery of pathogens and dangerous biological agents. The company showed a prototype of its BioDetect system last month.
Demand for fast and small detection systems is on the rise thanks to the SARS scare and the bioterrorism threat. For instance, a recent article in The New York Times (“Sniffing New York’s Air Ducts for Signs of Terror,” April 22, 2003) noted that, after the start of the war in Iraq, a specially trained team of National Guard soldiers tested New York City hotels, tourist sites, and government buildings for biological and chemical agents. Members of this team Òclimbed through a small access panel in [a building’s] sub-basement,Ó took a cotton swab, drew it across an air filter, sealed the specimen in a bottle, and sent it to a lab for testing, the article explained.
With INT’s BioDetect system, which the company says will be commercially available by the end of this year, samples -- taken from air, solids, or liquids -- would probably be collected in much the same manner as done by the National Guard in New York. But, according to INT, results would be obtained faster with BioDetect.
“The core technology in BioDetect is the ‘metallization’ of DNA,” says Stephen Nazarian, director of communications at INT. “We coat [captured] DNA strands of the pathogen of interest with metal and essentially make the DNA into a wire.” The metallization takes place once a sample’s DNA strands have attached to nanometer-sized DNA probes on a silicon chip, thus creating a complete circuit that indicates the pathogen of interest is indeed present.
The complete system consists of a briefcase-size analyzer and disposable test cards. A single card can be used to simultaneously check for hundreds of pathogens by placing probes for each one of interest on a test card. Different cards can be developed for different biological agents, including anthrax, smallpox, and SARS. To develop a test card for a specific pathogen requires that the DNA sequence of that pathogen or biological agent be known. Both the BioDetect system and current DNA-based detection methods would start by extracting DNA fragments to compare to the known DNA structure of the pathogen of interest.
Current detection systems that try to identify biological agents by their DNA composition typically involve a multistep process. First, the sample’s DNA fragments must be increased by about a millionfold using techniques like polymerase chain reaction (PCR). The fragments are then run through a microarray analysis system. Final identification is confirmed by scanning the microarray. The entire process can take from several hours to a couple of days.
With the BioDetect system, “there’s no PCR involved, and no scanning or looking at an array,” Nazarian says. The sample’s original fragments are simply passed through the BioDetect system, taking 20 to 30 minutes for the entire detection process versus up to two days using conventional methods.
Probing for Pathogens
Each disposable test card has a chip with wires called probes. They contain the complementary nucleic acid of the targeted biological agent. The probes are arched, with a gap between two halves of each probe. A sample’s collected DNA fragments are passed back and forth over the probes on the sensor chip. If a sample’s DNA strand bonds to two adjacent elements containing the complementary nucleotides of a pathogen of interest, it means that fragment has the same DNA composition as the pathogen. Those fragments are then coated with metal, making them highly conductive.
The metallic coating of the sample’s strand completes an electrical circuit, allowing for detection of the suspected pathogen. “there’s a 10,000-fold drop in resistance [when a sample fragment bonds to the two probe halves, indicating the fragment has the same DNA as the pathogen of interest],” Nazarian says. This makes it fairly easy to determine if a pathogen is present. INT has developed systems with 14 structures on each chip; each structure can have thousands of nano-size probes.
BioDetect can, in theory, be used to detect any biological entity that has been sequenced. “We would need to develop a chip set for that [agent], something that takes about two to three weeks,” Nazarian says. When the system was being designed, no one knew of SARS. INT could take the sequence and quickly build a detection card, Nazarian says.
The field of nanotechnology biodetectors is very small, but INT is not the only company using nanotech to improve DNA-based detection of biological agents. Genicon Sciences uses nanometer-size particles on microarrays to enhance the light emitted by an array. This makes it easier to detect the presence of specific genes (see “Making the Most of Microarrays,” Sept. 2002 Bio-IT World, page 1). Genicon’s approach still requires that a sample’s DNA fragments be run through a PCR and microarray system, which adds to the time to make a positive identification of a pathogen.