Sensing at the Nano-Level
At least once in every episode of Fox’s popular medical drama “House”, we are treated to a brief but beautiful journey through the human body, traveling with a virus or cell awaiting discovery by the renowned doctor and his team later in the show. Reduced to nano-size, we see—and sense—at a level impossible to discern in the macro, or even micro, world.
Fantasy, perhaps, but what may be mere good special effects is just one step away from reality as progress in incorporating and manipulating nano-particles into useful devices moves to the brink of practical application. One day soon, most experts agree, a nano-based sensor may course through the bloodstream to sense, monitor and diagnose a variety of health issues. Indeed, sensing developments at the nano-level already are impacting a variety of fields, from security and energy to chemical and pharmaceutical processing, as well as health care.
Nano-based elements have the power and potential for changing the way materials, components, devices and systems behave. Compare, for example, a piece of chalk with a seashell. Both are predominately calcium carbonate. However, if you drop a piece of chalk, it will break apart, while the seashell most likely will not. They are made of the same materials, but the seashell is nano-engineered by nature for strength; the chalk is not. That difference is in the structure.
The principles behind such structure manipulation on atomic and molecular levels form the foundation for nanotechnology. Applying those concepts has led scientists and technologists to new materials with properties as diverse as exceptional strength, durability and temperature resistance. As scientists learn more about this exciting and often surprising field, progress continues toward the development of products that go beyond materials to devices and systems, among them sensors with unusual and exceptional properties. In some respects, says at least one expert, nanotechnology and nano-sensing are a step away from providing a new perspective on the laws of physics.
Discerning the benefits
Today, nano-materials are already available for application in packaging components, says Suresh Nair, director of engineering, safety, sensing and connectivity business for Rockwell Automation. “Nano-coatings are helping to maintain cleanliness in the food and beverage industries,” he explains. “Materials are available that help keep bacteria off surfaces. In the future, photo sensors might incorporate nano-materials to prevent dust from settling on the lens, a feature that would reduce maintenance. No wiping or cleaning would be required. The sensor would operate better and for longer periods of time. The tradeoff, of course, is cost. It is a good idea, but is it cost-effective?”
Embryonic applications also extend into security, health care and energy. According to Brian Wirth, global products manager of MEMS (microelectromechanical systems), microstructures and nanotechnologies for GE Sensing, these include the detection of molecular level substances in security situations. These could be anything “from detection of gases to sensing of nano-particle-sized contaminants,” Wirth says. “Anything that outgases a measurable molecule might be detected—plastic explosives, for example, or perhaps airborne contaminants used for biological warfare. A lot of security programs are moving to the nano-sensing level.”
He admits, however, that there are not a lot of products “out there” yet, but insists many are in the development stage. “A lot depends on your definition of the term,” he says. “We have products used in security equipment today that detect nano-size substances. Some are in use today in airport security areas. Nano-materials/coatings science, applied within other types of structures to make them more efficient or more capable of measuring other substances, might reasonably be classified as nano-sensing by some.”
Wirth also sees significant efforts under way to miniaturize health care detection and diagnostics. “These techniques enable continuous monitoring instead of sending tests to a lab,” he explains. “With the right types of nano-sensing applications, it may be possible to measure continuously. Applications range from diabetic monitoring to pain management to blood science.”
In addition, Wirth sees nano-sensing as potentially having a significant impact on traditional instrumentation. Validation equipment for the pharmaceutical arena is one example. “In a pharma process, chemicals move through a production line to create a particular drug,” he says. “The line must be cleaned out regularly to the part per billion particle count level to ensure cleanliness. In the past, the process would involve taking samples, sending them to a lab and waiting for the results. Now the process is something akin to the airport security approach. A technician takes a swab, which acts like a sniffer to detect particles or contamination. The check is done in real time, using appropriate nano-based coatings and technology that are dedicated to sniffing for what must be detected. It works faster and more accurately, and it is portable.”
Other benefits could impact the transportation and energy industries. GE’s Global Research Center (GRC) has a program exploring nano-ceramic materials, which could increase efficiency in airplanes by decreasing the weight of aircraft engines, says Todd Alhart, spokesperson for the center. “That would be good for the environment while also increasing performance. We also have a research program in nano-metal alloys, which could enable engines and turbines to run at higher temperatures, again increasing efficiency. And we’re looking at nano-materials to improve alternative energy technologies such as wind and solar energy. In wind, a lighter blade would allow for more efficient wind capture. In solar, potentially new materials would capture more sunlight to make solar energy more viable.”
A wide range of possibilities
Despite the promise of nano-sensing, commercialization of products has a ways to go. “The home runs that nanotechnology has hit so far are in materials,” says Roger Grace, president of Roger Grace Associates. “Nanotechnology sensors must either have nanotechnology geometries (components &100 nm or less in size) or be capable of accurately performing measurements at the nano-level. I don’t believe there are many products like that out there that meet those criteria.”
Most MEMS devices, Grace adds, “have taken 17 to 20 years to become reality. Nano-devices will take just as long.” But the possibilities are there, and, according to some, maybe not quite that far from finding application. At GE’s GRC, researchers have turned to nature for clues for developing and applying nano-sensing concepts.
“We are in the process of identifying the capabilities of some amazing features and seeing how they may be applied,” says Dr. Radislav Potyrailo, principal scientist at GE GRC, who outlines a recent discovery based on the nanostructure of tropical butterfly wings. “These nanostructures exhibit acute optical properties and acute chemical-sensing properties. … We are trying to determine if we can take advantage of new physical and chemical phenomena on the nanoscale and apply them to sensing applications so that we can make better sensors.”
One possible application, Potyrailo says, involves the use of dyes that change color or fluorescence as a function of analyzed concentrations. Right now, Potyrailo says, “using them for detection has advantages and limitations. The most prominent limitation is that they tend to eventually photo-bleach from sun or laser light. So we are turning to nanostructures, such as these iridescent butterfly wings. Now the colors come not from organic molecules, but from physical structural phenomena. What we have learned thus far is that this intricate nanostructure allows us to selectively detect different types of gases in air. And it outperforms conventional approaches.”
It is not surprising that everyone is trying to develop biological- and chemical-detection systems, adds Jonathan Tucker, lead marketing engineer of nanotechnology for Keithley Instruments. “With nano, the materials are on a molecular or nano-scale, 10 to 9 m. Materials at that scale behave much differently than they do at the macro and micro scale, making devices like biosensors a very viable application for chemical- and biological-sensing systems. If you functionalize a nano material, like a carbon nanotube, to be more sensitive to a specific biological or chemical material so that it would react upon contact or in proximity to something, and using a basic transistor as the device to detect it, developing a commercializable product quickly for security applications becomes highly possible.”
Tucker is a bit more positive about the proliferation of commercial products. “We’re seeing activity in the chemical and biological areas right now, and we will see applications far beyond that in the next few years,” he says. “In the controls area, I see the development of nano-sensors for detecting toxic chemicals that might be used in a chemical plant or other process because they’d be so much more sensitive. There is interest in electrically characterizing [current versus voltage measurements] these sensors.
“Because many sensors are being designed around a transistor structure, designers have to characterize the response of the device with nano-materials on them. They will not behave the way a traditional sensor would because the materials are reacting on a quantum scale. The rules are different. It requires a lot of testing under real-life conditions to understand how nano-equipped sensors will respond electrically and how to turn those electrical signals into useful information.”
Keithley plays a major role in providing the electrical characterization instrumentation critical for those conducting nano-research and development. (See “Nano-measurement in action,” which describes a nano-measurement system.) The function is widely done in the semiconductor industry. In the sensor arena, it is necessary to determine how a device will perform electrically in real situations.
Tucker explains: “Our tools help researchers understand the electrical operation of a device. We call them ‘source measure units (SMU).’ They provide a stimulus or source (current or voltage) to bias the devices. Then, after the application of the chemical or biological element that is of interest to detect, the SMU measures the resulting voltage or current so you can evaluate the response of the sensor. It helps answer a variety of questions. What kinds of currents need to be measured when my sensor comes in contact with the element that it is trying to detect? How do you differentiate between air and what you’re trying to detect? What are the electrical signals and how do they differ? It also goes a long way toward understanding how to prevent false positives.”
Some assembly still required
Although the potential of nano-sensing products is apparent and undisputed, so is the admission that many hurdles remain before the benefits are inherent in manufacturing and process-sensing applications.
“There are still major hurdles to overcome in the evolution of this technology,” Rockwell’s Nair says. “Reliable commercial processes for cost-effective manufacturing, corresponding standards, and supporting metrology and tools are just beginning to evolve. Organizations such as IEEE [Institute of Electrical and Electronics Engineers], IEC [International Electrotechnical Commission], and NEMA [National Electrical Manufacturers Association] are beginning to address standardization, and tool vendors have some tools under development. But we are not there yet, and until we get past these hurdles, bringing products to fruition will be difficult.”
The pressure is on the manufacturing process, Grace says. Most researchers, he believes, are developing technology, not making it work successfully. “They need to start looking at the manufacturing issues. We need to learn to mass produce nano-products,” he insists. “Unless we can manufacture these products robustly, with high throughput, low cost and reproducibility, we will be unable to successfully commercialize them.” Nonetheless, Grace says he is confident that the products will come. “Too much research is going on for them not to.”
His views are in agreement with those of Harry E. Stephanou, Ph.D., director of the Automation & Robotics Research Institute and professor of electrical engineering at the University of Texas at Arlington. He has been involved in MEMS and nano-research for more than a decade and agrees that the manufacturing issue is paramount.
“We can develop many things in the lab, but unless we can make them, no one can buy them,” he says. “One bottleneck is our lack of ability to manufacture sensors and actuators cost-effectively. Unless products can be transitioned into industry, they are nothing more than a nice curiosity.”
Foundry services to help bring products to market more quickly are beginning to emerge, Stephanou says, “But it will take a paradigm shift in manufacturing to get nano-production going. You cannot self-assemble these things. I have not seen any major breakthroughs in this area, any totally different way of manufacturing at the nano level. We are still trying to scale down other paradigms. Methods involving printing, templates [and] lithography have been tried, but we haven’t devised a new nano-manufacturing paradigm. I think we will get there, but until and unless there is some disruptive innovation, I think we’re going to be where we are for a while.”
From materials to devices
Somewhat more optimistic, the GE GRC puts commercialization about five years down the road. Potyrailo gives the butterfly example as a reason for such optimism. “The nano-features of the butterfly wings are teaching us how to assemble nanomaterials based on larger building blocks to produce overall arrays that give us something we are more comfortable handling.”
Keithley’s Tucker believes the first stages of commercialization of nano-sensors is already here. “Right now, developers are using existing semiconductor architectures in an attempt to not re-invent the wheel. They are modifying those architectures, basically transistors—FETs (field effect transistors)—with nanoscale materials like carbon nanotubes or other nano-materials to create highly sensitive sensors. Sensors like these could be placed throughout a chemical process plant to provide faster warnings because they are so highly sensitive as to what they can capture. Sensors are some of the first real commercializable components based on nanotechnology.”
However, Tucker admits that cost remains an issue. “Part of the problem is that you’re dealing with elements invisible to the naked eye. Will new types of manufacturing devices be needed? Quite possibly. Efforts are under way to use existing semiconductor fabrication tools. At the same time, researchers are looking for something radically different or from a totally new perspective. The federal government and venture capitalists are investing a lot of money in this technology, and they expect results.”
Cost-effectiveness is the determining factor, says GE Sensing’s Wirth. “These technologies are expensive now, but as they develop, they will become less so. There are similarities between the processes being looked at and used for MEMS and for nano. Whenever you can incorporate known approaches to develop nano-level products, you are a step ahead and much closer to getting the product into the marketplace.”
Adds UT’s Stephanou, “In the commercial field, nanotechnology is still more associated with materials than with devices or systems. But at some point, it will reach the device level. When I was an undergraduate, I worked with individual transistors. Students today have never seen a transistor. They are all embedded in bigger things. We’re headed in the same direction with nano-sensing—from discrete sensors to embedded sensors that will take us to the system level.”
The problems associated with applying nano-discoveries to products are perhaps summarized best by Potyrailo: “We need to find ways to connect the nanostructures we see under the microscope to the real world components in a sensor,” he says. “The big issue is how to make a sensor at reasonable cost and in sufficient quantity. We are trying to combine nanotechnology advances with traditional instrumentation so that we can drive costs down such that we bring the micro and macro worlds to the nano world.”
Building safety, standards
Beyond the manufacturing and fabrication, other nano-sensing issues remain, most particularly in terms of health and safety. Concerns surrounding working with and using nano-based products and about how they interact with the human body have not disappeared. Investigation and research at the academic and government levels are extensive and ongoing. Says Rockwell’s Nair, “We don’t know what the impact of breathing nano-particles is. We also don’t know much about any possible end-of-life problems with nano-particles. What are the disposal issues? Do they pose a hazard? These are among the uncertainties.” (Read more about the nanotechnology safety efforts of the U.S. National Institute for Occupational Safety and Health on the NIOSH Website at cdc.gov/niosh .)
Universally, those involved with nanotechnology efforts agree safety remains a critical issue. “One of the strategies of the federal government’s national nanotechnology initiative is to understand the societal impacts of nanotechnology,” Keithley’s Tucker says. “Safety issues are being investigated by government agencies and university endeavors, examining such issues as the dangers of materials like carbon nanotubes. How toxic are they? Are they toxic at all? What happens when you use them? As these efforts get under way, the research continues concurrently. It has to. You have to pursue progress.”
Another concern as product research and development progresses is standardization. “I’ve become a big advocate here,” Tucker says. “I work closely with the IEEE standards efforts. I am co-chair of the Nanoelectronics Standards Roadmap Initiative (NESR). One of the topics we keep talking about is sensors. Nanotechnology as a science has no standards. A big question is how do you develop standards—even for sensing and sensor technology? Who should write them? How are we all going to agree to them? If there is a hurdle that could get in the way of mass production, reproducibility, repeatability and verification, it is in the area of standards.”
GE’s Alhart agrees with Tucker. “Safety is obviously important. It is a priority in doing any research,” he says. “We are working to provide input to government agencies to help promote the safe use of nanotech and the appropriate and effective regulation of the field.”
Get involved, stresses Tucker. “Get involved in standards development and the activities related to it. Getting involved with standards can give you a competitive advantage. A lot of work remains to be done with nano-sensing and nano-products. Don’t let someone else do it.”
|Jeanine Katzel is a consulting editor for Control Engineering . Contact her at email@example.com .|
Nano-measurement in action
Development of tools critical to producing nano-devices, such as electrical characterization measurement systems, is progressing. One of those in use today is the Zephyr measurement system, developed at Nanomix (
The script language describes instrument configuration and measurement operations. The operations are arranged in sequences, which can be nested or run concurrently. The measurement results are sent to the application’s output components, which store the data in files and display them in the graphical user interface, and may be enabled from the menu when necessary. Most of the script properties (parameters) may be controlled from the user interface. For example, test duration, measurement resolution and general purpose instrument bus (GPIB) addresses can be defined as user-configurable properties. Each script may come with its own set of properties or share a subset with other scripts (for instance, the instruments’ GPIB addresses).
The devices are arranged in test groups; this approach is known as blocking in experiment design. Typically, all devices within a group are tested and stored in identical conditions. Information about individual devices is entered only once; after that, only a group ID must be entered to run an experiment.
Currently, this system supports about 20 instruments from a variety of vendors, including Agilent Technologies, Keithley Instruments, Nanomix, Omega Engineering, Sensirion, Teledyne Technologies and Vaisala. In addition, specialized virtual instruments are supported; one such instrument takes I-V (current-voltage) measurements of CNTFET devices, and special output components save and display the I-V data, while the script controls only the timing of the switch and measurement operations.
Data files can be uploaded to the database; viewed in Nanoscope, a Nanomix application; or parsed and analyzed using Matlab or other applications. This system has been in use for about two years and has proved highly flexible and scalable. An open-source version of the software will be released soon under a BSD (open source) license at
This material is based on work partially supported by the National Science Foundation under Grant No. 0450648. For more information, visit the Nanomix Website or e-mail Sergei Skarupo firstname.lastname@example.org.