NIST Tech Beat - February 1, 2007

Nanoscale magnets in the form of iron-containing molecules might be used to improve the contrast between healthy and diseased tissue in magnetic resonance imaging (MRI)—as long as the concentration of nanomagnets is carefully managed—according to a new report* by researchers at the National Institute of Standards and Technology (NIST) and collaborators. Molecular nanomagnets are a new class of MRI contrast agents that may offer significant advantages, such as versatility in design, over the compounds used today.

Contrast agents are used to highlight different tissues in the body or to help distinguish between healthy and diseased tissue. NIST is working with two universities and a hospital to design, produce and test nanomolecules that might make MRI imaging more powerful and easier to perform. The new paper resolves a debate in the literature by showing that iron-containing magnets just two nanometers wide, dissolved in water, do provide reasonable contrast in non-clinical MRI images—as long as the nanomagnet concentration is below a certain threshold. (A nanometer is one billionth of a meter.) Previous studies by other research groups had reached conflicting conclusions on the utility of molecular nanomagnets for MRI, but without accounting for concentration. NIST scientists, making novel magnetic measurements, were able to monitor the molecules’ decomposition and magnetic properties as the composition was varied.

The injectable dyes currently used as MRI contrast agents are of two types. Magnetic ions, which alter the nuclear properties of hydrogen in water, offer the advantage of consistent identical design but provide low contrast. The second category encompasses particles of thousands of atoms or crystals, which alter local magnetic fields; they provide contrast variation in a larger region but have irregular designs and magnetic properties that are difficult to control. By comparison, molecular nanomagnets can be designed to have consistent properties and high contrast. In addition, they might be modified to act as “smart” materials whose contrast could be turned on only when bonded to a target molecule or cell. Toxicity is not believed to be an issue, because iron is naturally found in the body and other studies have found that these materials are non-toxic at the concentrations used in MRI.

NIST works with Florida State University to make single-molecule magnets less than five nanometers (nm) in diameter, and works with the University of Colorado at Boulder to make nanocrystals in the 10-50 nm range. The agency is pioneering methods for manipulating and measuring the magnetic properties of these compounds and is developing instrumentation for understanding how contrast agents work and how to control contrast properties. Researchers correlate the measured properties to the observed MRI response under non-clinical conditions using imagers at The Children’s Hospital in Denver. The information gained is fed back into recipes for making even better nanomagnets. The work described in the new paper was supported in part by the National Science Foundation.

* B. Cage, S. Russek, R. Shoemaker, A. Barker, C. Stoldt, V. Ramachandarin and N. Dalal. Efficacy of the single-molecule magnet Fe₈ for magnetic resonance imaging contrast agent over a broad range of concentration. Polyhedron. In press, corrected proof available online.

Researchers at the National Institute of Standards and Technology (NIST) have developed a sensitive new method for rapidly assessing the quality of carbon nanotubes. Initial feasibility tests show that the method not only is faster than the standard analytic technique but also effectively screens much smaller samples for purity and consistency and better detects sample variability.

Carbon nanotubes have unique properties, and thermal and electrical conductance, that could be useful in fields such as aerospace, microelectronics and biotechnology. However, these properties may vary widely depending on nanotube dimensions, uniformity and chemical purity. Nanotube samples typically contain a significant percentage of more ordinary forms of carbon as well as metal particles left over from catalysts used in manufacturing. The new NIST method, described at a conference last week,* involves spraying nanotube coatings onto a quartz crystal, gradually heating the coated crystal, and measuring the change in its resonant frequency as different forms of carbon vaporize. The frequency changes in proportion to the mass of the coating, and scientists use this as a measure of stability at different temperatures to gauge consistency among samples. The quartz crystal technique, which can reveal mass changes of just a few nanograms, already is used in other contexts to detect toxic gases and measure molecular interactions.

NIST researchers tested dozens of samples from a batch of commercial single-walled carbon nanotubes, comparing results of the new method with those from a standard technique, thermogravimetric analysis, and confirming results with scanning electron microscopy. Both methods revealed that the samples contained large amounts of amorphous carbon as well as residual metal particles. But the quartz crystal method could obtain results from just micrograms of material, compared to milligrams for thermogravimetric analysis, and also revealed several orders of magnitude more variability in the samples tested. The new technique also uses simpler equipment.

Although the differences among nanotube samples may appear subtle, they may still affect product viability, because even small variations in material composition can affect electrical and thermal behavior, and lack of uniformity may demand higher loads of nanotubes, which are expensive. NIST scientists carried out the tests with the help of students from the University of Colorado-Boulder and Rensselaer Polytechnic Institute, Troy, N.Y.

* S.A. Hooker, R. Geiss, R. Schilt and A. Kar. Rapid inspection of carbon nanotube quality. Paper presented Jan. 25 at the 31st International Cocoa Beach Conference and Exposition on Advanced Ceramics and Composites, Daytona Beach, Fla.

Call it a break in the case of “hidden order and the unconventional superconductor.” Writing in the journal Nature Physics,* U.S. and Canadian researchers report a major step toward solving a two-decades-old materials science mystery and progress toward the ultimate goal of engineering materials optimized for magnetic and electric properties.

The advance is the result of investigative work done at the National Institute of Standards and Technology’s Center for Neutron Research (NCNR), and at the National High Magnetic Field Laboratory (NHMFL) at Florida State University (FSU).

Stray magnetic fields suppress superconductivity, the resistance-free passage of electric current. But the object of the team’s scrutiny—a uranium-ruthenium-silicon compound (URu₂Si₂)—somehow accommodates the normal adversity between magnetism and superconductivity. At 17.5 degrees above absolute zero, once-nomadic electrons that had roamed freely about the compound’s lattice-like atomic structure—and generated their own magnetic fields—behave in a more orderly and cooperative fashion. This coherence sets the stage for superconductivity.

URu₂Si₂ belongs to a class of materials called heavy fermions, known to be reluctant superconductors. This is because current-carrying electrons in the intermetallic material interact with surrounding particles and truly gain from the experience. The association adds mass—making the electrons behave as though they were a few hundred times more massive than “normal.” The heavy electrons once were thought to make superconductivity impossible.

However, numerous heavy fermion superconductors now are known, and URu₂Si₂ ranks among the most curious of the lot.

Unexplained was how a “hidden order” suddenly arose in the wake of the magnetic instabilities caused by the roving electrons, each one spinning and producing its own miniature magnetic field. With neutron probes, researchers managed to track electron movements and determined that the wandering particles work out an unexpected accommodation in the spacing of their energy levels.

This research is supported by the National Science Foundation, NIST, the Natural Sciences and Engineering Research Council of Canada, the State of Florida and the Canadian Institute for Advanced Research.

*C.R. Wiebe, J.A. Janik, G.J. MacDougall, G.M.Luke, J.D. Garrett, H.D. Zhou, Y.J. Jo, L. Balicas, Y. Qiu, J.R.D. Copley, Z. Yamani and W.J.L. Buyers, Gapped itinerant spin excitations account for missing entropy in the hidden order state of URu₂Si₂, Nature Physics, Feb. 2007.

The National Institute of Standards and Technology (NIST) yesterday issued a draft profile that will assist federal agencies in developing plans to acquire and deploy products that implement Internet Protocol version 6 (IPv6). The profile recommends IPv6 capabilities for common network devices, including hosts, routers, intrusion detection systems and firewalls, and includes a selection of IPv6 standards and specifications needed to meet the minimum operational requirements of most federal agencies.

The Internet Protocol (IP)—actually a suite of protocols—is an international communications standard that defines how and where information such as text, voice and video move across the Internet. IPv6 is the next generation IP developed in the 1990’s to replace the current version which has been in use for more than 20 years. The IPv6 protocols offer several significant improvements over the current IPv4 protocols, including a vastly greater number of “addresses” and faster routing.

In 2005, the Office of Management and Budget advised federal agencies that they must upgrade or modify systems to handle IPv6 by June 2008. (See OMB memo at www.whitehouse.gov/omb/memoranda/fy2005/m05-22.pdf.) The NIST profile was developed to help ensure that IPv6-enabled federal information systems are interoperable and secure, and also addresses how such systems can interoperate and co-exist with the current IPv4 systems. Agencies with unique information technology requirements are expected to use the NIST profile as a basis for further refined specifications and policies.

The draft report, A Profile for IPv6 in the U.S. Government—Version 1.0 (Special Publication 500-267), also includes findings from NIST’s analysis of the current state of IPv6 standards, emerging commercial implementations and testing programs. NIST found that a core set of IPv6 standards have stabilized and viable commercial implementations are emerging. However, products currently are at varying levels of maturity and completeness. NIST also found that improved IPv6 security technologies and testing services are needed to ensure the safety of federal information systems using IPv6. Over the next year, NIST plans to develop guidelines for the secure adoption of IPv6 and develop a testing strategy for IPv6 for federal agencies.

NIST is asking for comments on the draft report by March 2, 2007. The report is available at www.antd.nist.gov/usgv6-v1-comments.html. Comments should be sent to sp500-267-comments@antd.nist.gov or Chief, Advanced Network Technologies Division, Attention: SP500-267, National Institute of Standards and Technology, Mail Stop 8920, Gaithersburg, MD 20899-8920.

Chemists at the National Institute of Standards and Technology (NIST) have created a standardized form of common house dust to support environmental scientists studying our everyday exposure to a catalog of potentially hazardous chemicals.

Although a “standard house dust” may sound funny, environmental scientists are quite serious about the potential for household grime to harbor harmful chemicals. A 2004 study by NIST and the Environmental Protection Agency (EPA), for example, found high concentrations of polybrominated diphenyl ethers (PBDEs) in household dust*. PBDEs were widely used as flame retardants in consumer products but have been phased out due to concerns over their toxicity. Polychlorinated biphenyls (PCBs), once commonly used in electrical equipment as an insulator, have not been produced since 1977 because of their toxicity, but still are found in the environment.

Accurate assessments of everyday exposure to many of these contaminants are difficult because of both the complexity of the analysis and the small quantities involved. “PCBs,” for example, is shorthand for dozens of chemically similar compounds that may be found in hundreds of combinations of only a few micrograms each in a kilogram of dust. To provide environmental scientists with an accurate baseline for calibrating their tests, NIST prepared a reference sample of typical house dust that has been certified for the concentrations of over 80 potentially hazardous chemicals.

The dust was collected, with assistance from the EPA, from vacuum cleaner bags collected from homes, cleaning services, motels and hotels in the states of North Carolina, Maryland, Ohio, New Jersey, Montana and Wisconsin during 1993 and 1994, and sterilized, filtered, homogenized and analyzed. Each 10-gram sample of Standard Reference Material 2585, “Organic Contaminants in House Dust,” is certified by NIST for the concentrations of 33 selected polycyclic aromatic hydrocarbons (PAHs), 30 PCBs, four chlorinated pesticides and 15 PBDEs. Reference values—measurements believed to be accurate but not meeting NIST criteria for a “certified” value—are provided for 33 additional PAHs, 12 additional PCBs, 10 additional chlorinated pesticides and three additional PBDEs.

Standard Reference Materials (SRMs) are among the most widely distributed and used products from NIST. The agency prepares, analyzes and distributes well over a thousand different materials that are used throughout the world to check the accuracy of instruments and test procedures used in manufacturing, clinical chemistry, environmental monitoring, electronics, criminal forensics and dozens of other fields.

The National Institute of Standards and Technology (NIST) is planning a competition to develop one or more cryptographic “hash” algorithms to augment and revise the current Secure Hash Standard (Federal Information Processing Standard 180-2). As a first step in this process, NIST is looking for comments on its recently published draft minimum acceptability requirements, submission requirements and evaluation criteria for candidate algorithms.

Hashing algorithms are mathematical procedures that take data, usually a message, and chop and combine it down into a much shorter number that is a “fingerprint” of the original data. Good hash algorithms have two features—two different inputs are overwhelmingly likely to generate two different fingerprints, and given a specific fingerprint, there is no practical way of calculating a set of input data that will have the same fingerprint. Hash algorithms are used widely by the federal government and others in various applications, such as digital signatures and message authentication. FIPS 180-2 specifies five cryptographic hash algorithms—SHA-1, SHA-224, SHA-256, SHA-384 and SHA-512. Because serious attacks have been reported in recent years against cryptographic hash algorithms, including SHA-1, NIST is preparing the groundwork for a more secure hash standard.

For more information on the proposed competition, including a copy of the Federal Register announcement issued Jan. 23, 2007, and to submit comments on the draft hash algorithm requirements and evaluation criteria, see www.nist.gov/hash-function. Since the submission requirements and the evaluation criteria may change as a result of the comments that NIST receives, candidate algorithms should not be submitted until the competition is announced later this year.

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Editor: Michael Baum

Date created: February 1, 2007
Date updated: February 1, 2007
Contact: inquiries@nist.gov