This content is excerpted from Sensor Technology Alert and Newsletter, a sensor intelligence service published by the Technical Insights unit of Frost & Sullivan.
With demonstrated performance capabilities and reductions in cost and complexity, electronic nose sensor arrays have realistic opportunities in such areas as homeland security and detection of infectious diseases.
Nano-sized carbon tubes coated with strands of DNA have the potential to create miniature sensors with the ability to detect odors and tastes, according to researchers at the University of Pennsylvania and Monell Chemical Sciences Center.
Arrays of such nanosensors could potentially detect molecules on the order of one part per million. The researchers have tested the nanosensors on five different chemical odorants, including methanol and dinitrotoluene, a common chemical that can be a component of military-grade explosives. The nanosensors could sniff molecules out of the air or taste them in a liquid, suggesting applications ranging from domestic security to medical detectors.
"What we have here is a hybrid of two molecules that are extremely sensitive to outside signals: single-stranded DNA, which serves as the 'detector,' and a carbon nanotube, which functions as 'transmitter,'" stated Charlie Johnson, associate professor in Penn's department of physics and astronomy. "Put the two together and they become an extremely versatile type of sensor, capable of finding tiny amounts of a specific molecule."
Owing to the size of such sensors--each carbon nanotube is about a billionth of a meter wide, Johnson and his colleagues believe arrays of the sensors could serve as passive detection systems in virtually any location. The sensor surface is, moreover, self-regenerating, with each sensor lasting for more than 50 exposures to the targeted substances. Therefore, the sensors would not need to be replaced frequently.
The specificity of single-stranded DNA boosts the capability of these sensors. The biomolecules can be engineered, in a process called directed evolution, to recognize a variety of targets, including small molecules and specific proteins.
The nanotubes are suitable for signaling when the DNA has captured a target molecule. Single-walled nanotubes are formed from a single sheet of carbon molecules connected together and then rolled. It is described as a unique material in which every atom is on both the surface and the interior. Although nanotubes could find diverse applications, they are highly sensitive to electrostatic variations in their environment, whether the nanotube is in a liquid or in air.
"When the DNA portion of the nanosensor binds to a target molecule, there will be a slight change in the electric charge near the nanotube," Johnson stated. "The nanotube will then pick up on that change, turning it into an electric signal that can then be reported."
According to Johnson, an array of 100 sensors with different response characteristics and an appropriate pattern recognition program would be able to identify a weak known odor in the context of a strong and variable background.
"There are few limits as to what we could build these sensors to detect, whether it is a molecule wafting off an explosive device or the protein byproduct of a cancerous growth," he stated.
Researchers involved in the project include Cristian Staii, a graduate student in the department of physics and astronomy in Penn's School of Arts and Sciences; Michelle Chen, a graduate student in the department of material science and engineering in Penn's School of Engineering and Applied Science; and Alan Gelperin of the Monell Chemical Senses Center.
Funding for the research was furnished by the US Department of Energy, grants to Penn's laboratory on the Research of the Structure of Matter through the National Science Foundation, and Monell.
Johnson told Sensor Technology that the transistor geometry used in his team's nano e-nose sensors intrinsically contains more information that can be used to classify the measured signals, compared to a two-wire conductive polymer chemiresistor geometry. He noted that, "Polymer-based systems often detect odors using a swelling mechanism--the polymer binds the analyte, whose major effect is to swell the polymer. We know that the nanotube-based sensor can function by detecting ionization of the analyte, which lets us easily distinguish between gases that become positively charged--for example, trimethylamine--from those that become negatively charged--for example, propionic acid."
Furthermore, Johnson noted that polymer sensors often do not refresh completely when exposure to the analyte is concluded. Sometimes, a voltage pulse is used to drive off a bound analyte and refresh the sensor. In contrast, it has been shown that that the DNA nanotube sensor's response was highly reproducible through many cycles over about two hours of use.
Johnson explained that considerably more experimentation is required to definitely determine the types of analytes that could be readily sensed by carbon nanotubes coated with DNA strands. He mentioned that researchers have engineered carbon nanotubes with single-stranded DNA to have an affinity to a variety of small molecule analytes, proteins, and single-cell organisms.
Johnson believes that there is no significant technical issue that prevents the researchers from making an array of 100 devices that can be functionalized with 100 different DNA strands. They are working toward demonstrating such an array in the next few months.
The researchers are interested in a variety of applications for their carbon nanotube DNA sensors, including detection of trace amounts of explosive gases and chemical warfare agents, as well as analysis of breath for diagnosis of infections and cancer in the lung.
Key challenges with respect to commercialization of the carbon nanotube DNA sensors include manufacturability, and the pattern recognition algorithms for e-nose applications. Johnson envisions that a five-year time for commercialization of a carbon nanotube DNA e-nose sensor array is essentially reasonable. He believes that trace gas detection will most likely be the first application for such an e-nose array.