NIST Tech Beat - May 10, 2007

Magnetic switches like those in computers also might be used to manipulate individual strands of DNA for high-speed applications such as gene sequencing, experiments at the National Institute of Standards and Technology (NIST) suggest.

As described in a recent paper,* NIST researchers found that arrays of switches called “spin valves”—commonly used as magnetic sensors in the read heads of high-density disk drives—also show promise as tools for controlled trapping of single biomolecules. The arrays might be used in chip-scale, low-power microfluidic devices for stretching and uncoiling, or capturing and sorting, large numbers of individual biomolecules simultaneously for massively parallel medical and forensic studies—a sort of magnetic random access memory (MRAM) for biosciences.

Spin valves are made by stacking thin layers of materials with different magnetic properties. Their net magnetization can be switched on and off by applying an external magnetic field of sufficient strength to align the electron “spins” in the magnetic layers in the same (on) or opposite (off) directions. NIST researchers made an array of spin valves, each about one by four micrometers in size, patterned on a 200-nanometer-thick silicon nitride membrane in fluid. When the spin valves are turned on, a local magnetic field is created that is strongest near the ends of the magnetic stack below the membrane—a field strong enough to trap nanoscale magnetic particles.

The NIST experiments demonstrated that the spin valves not only can trap magnetic particles, but also can be used as the pivot point for rotating strands of particles when a rotating magnetic field is applied. According to the researchers, these experimental results, combined with computer modeling, suggest that if biomolecules such as proteins or DNA strands were attached to the magnetic particles, the spin-valve array could apply torsional forces strong enough to alter the structure or shape of the biomolecules. The NIST group is now working on a microfluidic chip that will accomplish this electronically, which would be a significant milestone for applications.

Parallel processing of single biomolecules would be a significant advance over existing techniques limited to studying one molecule at a time. Optical tweezers, which use lasers to trap and manipulate biomolecules, tend to be slow and limited in force, and the particles need to be micrometer sized or larger. Existing magnetic tweezers can trap smaller particles and apply torque, but typically require permanent immobilization of biomolecules, which is time consuming and prevents subsequent analysis.

* E. Mirowski, J. Moreland, S. Russek, M. Donahue and K. Hsieh. Manipulation of magnetic particles by patterned arrays of magnetic spin-valve traps. Journal of Magnetism and Magnetic Materials, Vol. 311, pp. 401-404, (2007).

Imagine being able to rapidly identify tiny biological molecules such as DNA and toxins using less than a drop of salt water in a system that can fit on a microchip. It’s closer than you might believe, say a team of researchers at the National Institute of Standards and Technology (NIST), Brazil’s Universidade Federal de Pernambuco, and Wright State University in Dayton, Ohio.

In a paper appearing next week in the Proceedings of the National Academy of Sciences,* the team proves for the first time that a single nanometer-scale pore in a thin membrane can be used to accurately detect and sort different-sized polymer chains (a model for biomolecules) that pass through or block the channel.

Traditionally, unknown molecules are measured and identified using mass spectrometry, a process that involves ionizing and disintegrating large numbers of a target molecule, then analyzing the masses of the resulting molecules to produce a “molecular fingerprint” for the original sample. This equipment can cover a good-sized desk. By contrast, the “single-molecule mass spectrometry” system described in the PNAS paper is a non-destructive technique that in principle can measure one molecule at a time in a space small enough to fit on a single microchip device.

The technique involves creating a lipid bilayer membrane similar to those in living cells, and “drilling” a pore in it with a protein (alpha-hemolysin) produced by the Staphyloccoccus aureus bacteria specifically to penetrate cell membranes. Charged molecules (such as single-stranded DNA) are forced one-at-a-time into the nanopore, which is only 1.5 nanometers (the diameter of a human hair is about 10,000 nanometers) at its smallest point, by an applied electric current. As the molecules pass through the channel, the current flow is reduced in proportion to the size of each individual chain, allowing its mass to be easily derived.

In this experiment, various-sized chains in solution of the uncharged polymer polyethylene glycol (PEG) were substituted for biomolecules. Each type of PEG molecule reduced the nanopore’s electrical conductance differently as it moved through, allowing the researchers to distinguish one size of PEG chain from another.

As a control, a solution of a highly purified PEG of a specific size was characterized with the nanopore. The resulting “fingerprint” closely matched the one identifying samples of the same size polymer in the mixed chain solution.

Further enhancement of the data from both the experimental and control tests yielded mass measurements and identifications of the different PEG chains that correlate with those made by traditional mass spectrometry.

Because the dimensions of the lipid bilayer and the alpha-hemolysin pore, as well as the required amount of electrical current, are at the nanoscale level, the “single-molecule mass spectrometry” technology may one day be incorporated into “lab-on-a-chip” molecular analyzers and single-strand DNA sequencers.

* J.W.F. Robertson, C.G. Rodriguez, V.M. Stanford, K.A. Rubinson, O.V. Krasilnikov and J.J. Kasianowicz. Single-molecule mass spectrometry in solution using a solitary nanopore. Proceedings of the National Academy of Sciences 104 (20): 8207, May 15, 2007.

A tiny device for calibrating or stabilizing precision lasers has been designed and demonstrated at the National Institute of Standards and Technology (NIST). The prototype device could replace table-top-sized instruments used for laser calibration in atomic physics research, could better stabilize optical telecommunications channels, and perhaps could replace and improve on the precision of instrumentation used to measure length, chemicals or atmospheric gases.

The new spectrometer, described in the May 7 issue of Optics Express,* is the latest in a NIST series of miniaturized optical instruments such as chip-scale atomic clocks and magnetometers. The spectrometer is about the size of a green pea and consists of miniature optics, a microfabricated container for atoms in a gas, heaters and a photodetector, all within a cube about 10 millimeters on a side. The package could be used to calibrate laser instruments, or, if a miniature laser were included in the device, could serve as a wavelength or frequency reference.

Most of the optical components are commercially available. The key to the device is a tiny glass-and-silicon container—designed and fabricated at NIST—that holds a small sample of atoms. The sample chambers were micromachined in a clean room and filled and sealed inside a vacuum to ensure the purity of the atomic gas, but they can be mass-produced from silicon wafers into much smaller sizes, requiring less power and potentially cheaper than the traditional blown-glass containers used in laboratories. Although shrinking container size creates some limitations, NIST scientists have accommodated these difficulties by adding special features, such as heaters to keep more atoms in the gas state. NIST tests predict that the stability and signal performance of the tiny, portable device can be comparable to standard table-top setups.

The instrument works by measuring the intensity of a laser beam after it interacts with the atoms. The amount of light absorbed at a particular wavelength produces a characteristic signature. NIST has demonstrated the spectrometer with rubidium and cesium atoms, which absorb light at infrared, near-visible wavelengths, commonly used in atomic physics research. Different atoms or molecules, such as potassium or iodine, could be used for different applications. Or, a waveguide could be added to the device to double the frequency to stabilize lasers used in fiber-optic telecommunications. The mini-spectrometer would offer greater precision than the physical references now used to separate fiber-optic channels, with the advantage that more channels might be packed into the same spectrum.

* S.A. Knappe, H.G. Robinson and L. Hollberg. Microfabricated saturated absorption laser spectrometer. Optics Express. May 7, 2007.

With a nod to one of nature’s best surface chemists—an obscure desert beetle—polymer scientists at the National Institute of Standards and Technology (NIST) have devised a convenient way to construct test surfaces with a variable affinity for water, so that the same surface can range from superhydrophilic to superhydrophobic, and everything in between. Their technique, reported in a recent issue of the journal Langmuir,* may be used for rapid evaluation of paints and other materials that need to stick to surfaces.

The NIST team developed a flexible technique, based on ultraviolet light and photosensitive materials, to mimic one of nature’s cleverest feats of surface chemistry. The Stenocara beetle of Africa’s Namib Desert is able to thrive in a habitat so parched that not even the morning fog will condense. All the beetle has to do is raise its warty-looking wing covers into the breeze. Because the bumps are hydrophilic, or water-attracting, while the rest of the surface is hydrophobic, or water-repelling, the few water molecules that do strike the wing covers tend to get pushed uphill and collect on the bumps—where they eventually condense into artificial dewdrops that roll into the insect’s mouth. The insect’s trick is to use both surface structure and chemistry to create a surface that shifts rapidly from hydrophobic to hydrophilic.

The NIST researchers begin by coating the surface with a matrix of silica granules about 11 nanometers across. As with the beetle, whose wing covers are coated with organic particles about a thousand times larger, the spacing of the matrix provides a first, purely physical level of control over wettability: a water droplet placed atop the granules can sag only just so far into the gaps before it is stopped by surface tension.

The researchers then add a second level of control by coating the granules with a compound that changes their water affinity, in much the same way that a waxy substance makes some of the beetle’s microparticles hydrophobic. This step in itself is not unique; other research groups have added such compounds to granular surfaces using electrochemical techniques. The NIST group’s innovation is to use an optical technique that is much easier to modulate, and that can be carried out in air. They simply coat the granules with a photosensitive material, and expose it to ultraviolet light: the longer and more intense the exposure in a given area, the more hydrophilic that area becomes.

The new technique’s most immediate application is for testing paints, adhesives and other coatings: instead of daubing the compounds on dozens of surfaces one by one, researchers can now spread them over a single surface that tests the entire range of wettability within the space of a few centimeters. Other applications also are possible, ranging from water collection in dry regions to open-air microchannel devices. Indeed, the same technique can be used to create surfaces that vary in their affinity for alcohol and many other small molecule liquids.

* J.T. Han, S. Kim and A. Karim. UVO-tunable superhydrophobic to superhydrophilic wetting transition on biomimetic nanostructured surfaces. Langmuir 2007, 23, 2608-2614.

A super stable fiber-optic network that can be tuned across a range of visible and near-infrared frequencies while synchronizing the oscillations of light waves from different sources has been demonstrated at the National Institute of Standards and Technology (NIST). The flexible network design can simplify accurate comparisons of the latest atomic clocks operating at different frequencies and in different locations. The research also may have applications in remote sensing and secure communications.

Described in the May issue of Nature Photonics,* the prototype NIST network demonstrates the first remote synchronization of light waves from two “frequency combs”—advanced laboratory tools for precisely measuring frequencies of light. The two combs have fine “teeth” marking precise frequencies in different but overlapping bands. If light waves at identical frequencies are merged, they can either overlap exactly or be “out of phase” (that is, their oscillations are at the same frequency but start at different times). Light waves at different frequencies never overlap exactly but, with great effort, can be made to overlap out of phase in the same patterns in repeated experiments. The NIST network is designed to do exactly that, thus reducing channel “noise” that would result from mismatches. The stability of the lasers and low “jitter” of the synchronized waves means the original signal character is always preserved.

The network also showcases record performance in a frequency comb produced from an erbium fiber laser, an alternative to the original frequency comb generated from a titanium-sapphire crystal, also developed at NIST. Scientists recently reduced the noise in the fiber-based comb enough to improve its stability 30-fold, achieving performance comparable to the state-of-the-art Ti:Sapphire frequency comb used as the second comb in the new NIST network. Fiber-based frequency combs have the potential to be more compact and less expensive; they also measure the lower, near-infrared frequencies of light that are used in telecommunications.

The prototype network spans three-quarters of a kilometer and connects three different laboratories on the NIST Boulder, Colo., campus. The designers say it could be extended to 50 km or more without any loss in performance. To showcase the capability of the two frequency combs (which operate on different principles) to precisely compare vastly disparate optical frequencies across great distances, both combs are stabilized by the same source of 1126 nm laser light, so that each tooth of each comb is locked to a single frequency. In addition, laser light at 1535 nm laser, stabilized by one comb, is compared to 1535 nm light generated from the second comb, and the stability of the beat frequency (representing the difference between them) is analyzed to evaluate network performance.

The optical fiber was provided by OFS Laboratories, Somerset, N.J. The project was supported in part by the Defense Advanced Projects Agency.

* I. Coddington, W.C. Swann, L. Lorini, J.C. Bergquist, Y. Le Coq, C.W. Oates, Q. Quraishi, K.S. Feder, J.W. Nicholson, P.S. Westbrook, S.A. Diddams and N.R. Newbury. Coherent optical link over hundreds of meters and hundreds of terahertz with subfemtosecond timing jitter. Nature Photonics 1, 283 - 287 (2007), published online: May 1, 2007.

Builders interested both in conservation and thrift can benefit from the latest updates to an innovative software package released this week by the National Institute of Standards and Technology (NIST). BEES 4.0, the new version of NIST’s software tool for selecting environmentally preferred, cost-effective building products, updates data on more than 200 products and adds 30 new products for review. It also offers users the option of a new set of consensus weights for scoring the environmental impact of individual building products.

BEES 4.0 (Building for Environmental and Economic Sustainability version 4) measures both the environmental and economic performance of building products with life-cycle assessment techniques developed respectively by the International Organization of Standardization (ISO) and ASTM International.

With BEES a user can ascertain, for instance, the environmental impact of a product at any stage of its existence—raw material acquisition, manufacture, transportation, installation, use, and recycling and waste management. The environmental ramifications of the product at each of these stages is provided for each of 12 categories: global warming, acidification, eutrophication, fossil fuel depletion, indoor air quality, habitat alteration, human health, ecological toxicity, ozone depletion, smog, criteria air pollutants and water intake. The new consensus weight option, developed by a panel of building product manufacturers, green building designers and environmental assessment experts, allows users to evaluate environmental impacts considering short-, medium- and long-term effects.

Comprehensive economic performance data are similarly available for the costs of initial investment, replacement, operation, maintenance and repair, and disposal. Environmental and economic performances are combined into an overall performance measure using the ASTM standard for Multi-Attribute Decision Analysis. For the entire BEES analysis, building products are defined and classified according to the ASTM standard classification for building elements known as UNIFORMAT II.

BEES 4.0 includes a number of new non-biobased products, including carpeting from several manufacturers who agree to purchase carbon credits to offset the product’s life-cycle greenhouse gas emissions. These and other products, such as biobased carpets, roof coatings, building maintenance products and fertilizers that qualify for a government “green” preferential purchase program, could increase builder participation in the nation’s green building drive.

The U.S. Department of Agriculture Chief Economist’s Office of Energy Policy and New Uses supported NIST’s BEES research on biobased products.

The National Institute of Standards and Technology (NIST) has established a new voluntary accreditation program for the laboratories that test radiation detection equipment used by first responders. The new program will help ensure that laboratories testing a wide variety of new radiation detection instruments produce comparable results, allowing homeland security personnel to better assess the best products for each application.

From personal radiation detectors the size of pagers to units large enough to scan trucks and trains, emergency responders can choose from a wide variety of radiation detection equipment for homeland security applications. To make informed decisions when buying equipment, they must have confidence that instrument test results from different laboratories are comparable. The new NIST program, developed with support from the Department of Homeland Security (DHS), offers laboratories the opportunity to be accredited for their ability to test radiation detection equipment in conformance with recognized industry standards. The new service is part of NIST’s National Voluntary Laboratory Accreditation Program (NVLAP).

Laboratories seeking accreditation under the new program will have to demonstrate their conformity with ISO/IEC 17025, General requirements for the competence of testing and calibration laboratories; NIST Handbook 150, NVLAP Procedures and General Requirements; and NIST Handbook 150-23, Homeland Security Applications—Radiation Detection Instruments.

Testing of radiation detectors at accredited laboratories will be based on a series of standards developed by the Institute of Electrical and Electronics Engineers (IEEE) facilitated by NIST radiation measurement experts with input from the user, manufacturer and regulatory communities. DHS provided financial support for the standards development. Currently, five standards have been published, covering a variety of radiation detectors, and several more are under development. This series of standards also includes standards for operator training programs and data format to enhance the confidence that this equipment will be effective, rugged, useable and interoperable. Coupled with the NVLAP laboratory accreditation program, these standards will enhance users’ ability to determine the radiation detector that best suits their purpose.

Testing laboratories interested in the new NVLAP program for radiation detection instruments should contact Betty Ann Sandoval at betty.sandoval@nist.gov or (301) 975-8446.

The National Institute of Standards and Technology (NIST) has announced that its new Center for Nanoscale Science and Technology (CNST) is now accepting proposals for work in its user facility for nanotechnology research. The CNST offers researchers from universities, industry and other government agencies access to state-of-the-art and beyond-state-of-the-art facilities to study a wide range of nanotechnology topics.

The new center focuses on overcoming major technical obstacles to cost-effective manufacturing of products made with components the size of atoms and molecules by developing measurement methods, standards and technology that help emerging nanotechnologies move from the laboratory to production.

The center is located within NIST’s Advanced Measurement Laboratory, one of the most advanced research facilities of its kind in the world. The national user facility component of the center includes a 1486 square-meter [16,000 square-foot] nanofabrication facility, about half of which is devoted to class 100 cleanroom space. The nanofabrication facility includes more than 30 state-of-the-art tools such as photolithography, ion beam and etching equipment capable of creating, measuring and inspecting nanoscale devices with dimensions as small as 10 nanometers (billionths of a meter).

In order to maintain high throughput and high product yields, the semiconductor industry requires online monitors of the widths of lines fabricated on wafers. Traditionally, scanning electron microscopy (SEM) has been the tool of choice. However, SEM requires that wafers be removed from the assembly line, which in turn creates a bottleneck in production. Recently, measurements of light reflected by test gratings have developed a lot of interest as an online alternative to SEM. The technique, referred to as scatterometry or optical critical dimension (OCD) metrology, can be incorporated into an assembly line, is non-destructive, and has proven very sensitive to small changes in line profile and dimension.

To solicit industry collaboration and participation in the development and evaluation of artifact and documentary standards for scatterometry, the National Institute of Standards and Technology (NIST) and SEMATECH will hold the Joint Workshop on Nanoscale OCD/Scatterometry Standards, July 18, 2007, at the Marriott Hotel in San Francisco. The workshop will be held during the same week as and nearby to the semiconductor industry trade show SemiconWest.

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Editor: Michael Baum

Date created: May 10, 2007
Date updated: May 10, 2007
Contact: inquiries@nist.gov