Outside Washington, D.C., just a stone’s throw up the road in Gaithersburg, Md., lies the sprawling 580-acre headquarters of the National Institute of Standards and Technology (NIST), an unheralded government agency that has played a part in some of the biggest advances in science, manufacturing and communications in the last century.
To be precise, the front gate of the institute’s wooded, rural campus is exactly 20.664 miles from the center of the U.S. Capitol Building. It’s worth mentioning, because NIST is in the business of precision: everything the institute does revolves around standards, measurements and comparisons, all reliant on a meticulous attention to detail. Since it was established in 1901, the agency has worked to promote U.S. innovation and industry by developing tools that help speed up research, ensure the reliability of manufactured goods, and improve our quality of life.
Everything from the recipe for steel used in automobile production to earthquake safety standards for buildings to the nation’s official atomic clock stemmed from the efforts of NIST researchers.
The institute’s bucolic setting — complete with wildlife like deer and foxes — belies a modern core of state-of-the-art facilities like the Center for Nanoscale Science and Technology. This massive concrete, steel and glass building houses laboratories isolated as much as 40 feet underground, tightly temperature- and humitity-controlled so as not to perturb the exquisitely sensitive experiments conducted there.
It’s in this pantheon of precision that researchers Gordon Shaw and Jon Pratt are working to develop new standards for measuring extremely tiny forces, all the way down at the molecular level. The centerpiece of their Small Force Metrology Lab is the atomic force microscope, a phenomenally sensitive instrument that can create an ultra-high resolution image by dragging a silicon strip tipped with a diamond point (much like a phonograph needle) back and forth over a surface, and measuring the force exerted on the silicon strip as it bends and flexes with the contours of the surface.
What Shaw and Pratt are looking for is a new way for scientists using atomic force microscopes around the world to cheaply and efficiently calibrate their machines. Their current research is exploring the hypothesis that tiny snippets of DNA could be used as a “standard reference material” for just such a purpose.
Shaw, a stocky, enthusiastic man with a ready smile and long brown hair pulled back into a ponytail, explained the concept in the cramped atomic force microscope control room. DNA, he said, has some basic characteristics that make it appealing as a reference material: it’s cheap to produce, it can be replicated identically every time, and it appears to break when stretched with a predictable amount of force.
“Anyone can really make this DNA molecule,” Shaw said. “Our goal is to come up with a recipe that people can repeat anywhere in the world and get the same force.”
To test this idea, Shaw places under the microscope DNA that has been specially treated so one end of each strand sticks to the surface being scanned, while the other is attracted to the tip of the silicon probe. Then, he goes fishing. When the probe has snagged the loose end of a DNA molecule, the researchers gradually pull on it until it comes apart, then record the force at which it broke.
With any luck, their research will be a success, and before long yet another NIST recipe for precision will be circulating the globe, helping spur scientists and entrepreneurs to break new ground and push the boundaries of technology ever further forward.
Showing posts with label atomic force microscope. Show all posts
Showing posts with label atomic force microscope. Show all posts
Friday, August 20, 2010
Saturday, August 14, 2010
Setting the standard for subatomic forces
Everyday language makes plenty of references to “force” – a person with a strong personality is a ‘force of nature,’ or we worry about unseen ‘forces of evil.’ But the physics version came before all those figures of speech – force as a push or pull that changes the motion of an object. Accurate evaluation of physical force matters constantly in every day life – a car moves forward when its bulk is propelled by gas being burned in a combustion engine, or an arm thrusts a ball forward to the first baseman or wide receiver. The car only works if all its component parts, made in many different places, fit together properly, and if its chosen engine generates enough – you guessed it – force to move the assembled body.
So, how do people compare forces? Way back in high school science, you might have heard of Newton’s Second Law. The simple mathematical description of physical force boils down to a famous formula, in which Force = Mass multiplied by Acceleration. By convention, the unit of measurement of force is now the appropriately-named "Newton," which is defined as the force required to accelerate a 1 kilogram mass at a particular rate (1 square meter, to be exact). This standard definition, coupled with known reference materials like that kilogram, allows engineers in the United States to describe amounts of force in ways in the same as engineers in, say, Korea.
Interestingly, the kilogram remains the only international measurement unit that is defined by an arbitrary “artifactual” standard – a hunk of metal made in 1889 that resides outside of Paris. All the others, such as length, time, and temperature, are linked to some natural phenomenon that’s the same the world over. For example, the chief unit of time, the second, is linked to properties of cesium atoms, an “intrinsic” standard. The intrinsic approach has obvious advantages, unless you really find it convenient to travel to France every time you need to check the whether your kilogram of car parts is the same as someone else’s. That’s why researchers at the National Institute of Standards and Technology (NIST), a subdivision of the U.S. federal government’s Department of Commerce in Gaithersburg, MD, are working hard on a new definition for the kilogram (http://www.nist.gov/mel/mmd/mf/rekilo.cfm). And the problems are even harder when the objects involved are on the nanoscale, thousands of times smaller than an engine or a baseball.
Enter NIST researchers Jon Pratt and Gordon Shaw. Dr. Pratt and Dr. Gordon are developing a new and novel means of standardizing the definition of very small forces between atoms (http://www.nist.gov/mel/mmd/mf/sfmet.cfm). Their work, soon to be published in detail, was initially described during Dr. Pratt’s keynote address to the 2010 Annual Meeting of the Society for Experimental Mechanics (see http://www.nist.gov/manuscript-publication-search.cfm?pub_id=905416).
To create a new standard, they’ve studied the behavior of piece of double stranded DNA dissolved in a fluid under defined conditions of temperature and pH. Ordinarily DNA has a loose structure, rather like a woven rope that coils and flops randomly in solution. But the DNA can be anchored to a surface at one end, then picked up and pulled from the other end by a tiny lever in a large instrument designed to measure forces called an atomic force microscope (AFM). This arrangement simultaneously stretches out the DNA rope and measures the force being exerted, when the AFM lever senses resistance. When the pulling action first begins to stretch the DNA, the molecule resists the pull, and increasing force is measured via the AFM’s lever. But then at a certain point, the rope begins to unravel or fray instead of the resistance increasing – and for a fairly long period of more pulling, the force being measured doesn’t change. Eventually, when pulled hard enough and long enough, the rope puts up a last gasp of resistance and very large forces are measured, until the rope breaks apart altogether.
That intermediate zone, in which the force remains constant despite increasing pull, can be taken advantage of to use as a standard means of defining a particular amount of molecular force. “Our goal,” says Dr. Pratt, “is a recipe for people to be able to repeat anywhere in the world and get a known force.” The standard DNA can be made and used by any group anywhere in the world who has access to what are now relatively cheap and common materials and equipment. In fact, Dr. Shaw points out, “In an afternoon’s work you can make enough [DNA] to give everybody on the planet 5000 force references.” And the intrinsic property of that piece of DNA is the same whether stretched by an atomic force microscope in Montreal or in Moscow.
The technical catch is that the properties of the levers used in the atomic force microscope have to also be comparable and known, or “calibrated,” in order to get the same results everywhere. To solve that problem, Dr. Pratt built the world’s first electrostatic force balance (“Five years of my life,” jokes Dr. Pratt), a large and delicate piece of equipment that’s used to calibrate the levers used in atomic force microscopes. A calibrated set of levers can be shipped throughout the world to other reference agencies. Thus, NIST is on the forefront of furthering even more new applications for materials made on the nanoscale, which are now ubiquitous in products as varied as medicines, suntan lotion, and semi-conductors.
So, how do people compare forces? Way back in high school science, you might have heard of Newton’s Second Law. The simple mathematical description of physical force boils down to a famous formula, in which Force = Mass multiplied by Acceleration. By convention, the unit of measurement of force is now the appropriately-named "Newton," which is defined as the force required to accelerate a 1 kilogram mass at a particular rate (1 square meter, to be exact). This standard definition, coupled with known reference materials like that kilogram, allows engineers in the United States to describe amounts of force in ways in the same as engineers in, say, Korea.
Interestingly, the kilogram remains the only international measurement unit that is defined by an arbitrary “artifactual” standard – a hunk of metal made in 1889 that resides outside of Paris. All the others, such as length, time, and temperature, are linked to some natural phenomenon that’s the same the world over. For example, the chief unit of time, the second, is linked to properties of cesium atoms, an “intrinsic” standard. The intrinsic approach has obvious advantages, unless you really find it convenient to travel to France every time you need to check the whether your kilogram of car parts is the same as someone else’s. That’s why researchers at the National Institute of Standards and Technology (NIST), a subdivision of the U.S. federal government’s Department of Commerce in Gaithersburg, MD, are working hard on a new definition for the kilogram (http://www.nist.gov/mel/mmd/mf/rekilo.cfm). And the problems are even harder when the objects involved are on the nanoscale, thousands of times smaller than an engine or a baseball.
Enter NIST researchers Jon Pratt and Gordon Shaw. Dr. Pratt and Dr. Gordon are developing a new and novel means of standardizing the definition of very small forces between atoms (http://www.nist.gov/mel/mmd/mf/sfmet.cfm). Their work, soon to be published in detail, was initially described during Dr. Pratt’s keynote address to the 2010 Annual Meeting of the Society for Experimental Mechanics (see http://www.nist.gov/manuscript-publication-search.cfm?pub_id=905416).
To create a new standard, they’ve studied the behavior of piece of double stranded DNA dissolved in a fluid under defined conditions of temperature and pH. Ordinarily DNA has a loose structure, rather like a woven rope that coils and flops randomly in solution. But the DNA can be anchored to a surface at one end, then picked up and pulled from the other end by a tiny lever in a large instrument designed to measure forces called an atomic force microscope (AFM). This arrangement simultaneously stretches out the DNA rope and measures the force being exerted, when the AFM lever senses resistance. When the pulling action first begins to stretch the DNA, the molecule resists the pull, and increasing force is measured via the AFM’s lever. But then at a certain point, the rope begins to unravel or fray instead of the resistance increasing – and for a fairly long period of more pulling, the force being measured doesn’t change. Eventually, when pulled hard enough and long enough, the rope puts up a last gasp of resistance and very large forces are measured, until the rope breaks apart altogether.
That intermediate zone, in which the force remains constant despite increasing pull, can be taken advantage of to use as a standard means of defining a particular amount of molecular force. “Our goal,” says Dr. Pratt, “is a recipe for people to be able to repeat anywhere in the world and get a known force.” The standard DNA can be made and used by any group anywhere in the world who has access to what are now relatively cheap and common materials and equipment. In fact, Dr. Shaw points out, “In an afternoon’s work you can make enough [DNA] to give everybody on the planet 5000 force references.” And the intrinsic property of that piece of DNA is the same whether stretched by an atomic force microscope in Montreal or in Moscow.
The technical catch is that the properties of the levers used in the atomic force microscope have to also be comparable and known, or “calibrated,” in order to get the same results everywhere. To solve that problem, Dr. Pratt built the world’s first electrostatic force balance (“Five years of my life,” jokes Dr. Pratt), a large and delicate piece of equipment that’s used to calibrate the levers used in atomic force microscopes. A calibrated set of levers can be shipped throughout the world to other reference agencies. Thus, NIST is on the forefront of furthering even more new applications for materials made on the nanoscale, which are now ubiquitous in products as varied as medicines, suntan lotion, and semi-conductors.
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