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Analytical Chemistry

Meeting Briefs

by Stephen K. Ritter
January 8, 2007 | APPEARED IN VOLUME 85, ISSUE 2

Diamond-Like Films Are Hard And Slick

Thin carbon films that take on a diamond lattice structure are of interest as wear-resistant coatings and lubricants because of their exceptional tribological properties, they are hard and slick. Materials scientist Ali Erdemir of Argonne National Laboratory reported on his work to make these "near frictionless" films by chemical vapor deposition and to understand their surface chemistry. The films have ultralow wear rates and record coefficients of friction as low as 0.001, he noted. That's more slippery than graphite (0.15) and polytetrafluoroethylene (0.04), two of the slipperiest materials known.

Friction is the result of making and breaking chemical bonds between atoms in a sliding object and atoms on the surface it is sliding upon. Using photoelectron spectroscopy and mass spectrometry, Erdemir and his coworkers have discovered that films in which a moderate number of carbon atoms are hydrogenated have lower coefficients of friction than hydrogen-free films, because the hydrogen atoms prevent bonding at those carbon atoms. The researchers also have found that coefficients of friction are lowest in a vacuum or an inert atmosphere as opposed to air, because O2, H2O, and other molecules can interact with the carbon atoms. Argonne is looking to develop the coatings commercially for a variety of applications.

Credit: Accelr8
Credit: Accelr8

Microfluidic Device Pegs Drug-Resistant Bacteria

A "lab on a chip" device that identifies pathogenic bacteria and then determines the most effective antibiotic to counteract it in a patient was described by Michael J. Lochhead, a bioengineer at Denver-based biotechnology company Accelr8. The microfluidic system is expected to significantly speed up assessment and successful treatment of hospital-acquired pneumonia.

A patient's sample and reagents are loaded into a disposable cartridge via ports (holes at left in schematic), then the liquids flow into microchannels (center), Lochhead explained. Bacterial cells are captured on a functionalized poly(ethylene glycol) hydrogel in the channels with the aid of an electric field.

A mix of antibodies labeled with different fluorescent tags is introduced next, and the antibodies bind specifically to the different strains of bacteria present. An automated microscope peering into the channels identifies the bacteria, after which different antibiotics are pumped into the channels to determine which drug is most effective at killing the bacteria. The entire process takes less than eight hours, compared with up to three days for traditional lab culture methods, Lochhead noted. Accelr8's system is now ready to begin clinical testing.

Credit: Courtesy of Luke Hanley
Credit: Courtesy of Luke Hanley

Mass Spec Analysis Of Bacterial Biofilms

Mass spectrometry to characterize and image biological materials has become a hot topic for medical and homeland security research. In one example, chemistry graduate student Praneeth D. Edirisinghe of the University of Illinois, Chicago, described laser desorption post-ionization mass spectrometry (LDPI-MS) to study bacterial biofilms. These difficult-to-analyze microbial communities grow on human tissue or medical devices such as catheters and cause some two-thirds of all serious infections.

Edirisinghe and his adviser, Luke Hanley, and their coworkers studied surface-bound peptides in biofilms of the harmless bacterium Bacillus subtilis (Anal. Chem., DOI: 10.1021/ac0615605). The peptides form part of a chemical communication network that regulates growth of the biofilm, a process known as quorum sensing.

Mass spectral analysis of surface-bound biomolecules usually is hampered by the difficulty in forming ions, Edirisinghe said. The researchers reasoned that the two-laser LDPI-MS method offered a possible solution. A pulsed nitrogen laser was used to shear off peptides from the biofilm surface. Pretreatment of the peptides to attach a chemical tag such as anthracene allowed a molecular fluorine laser to selectively ionize the desorbed peptide chains for time-of-flight mass detection.

The data are allowing the team to map the quorum-sensing process in biofilms as shown, where the red peak corresponds to peptide ions localized at a microbial colony. This strategy could be used to monitor the effectiveness of antibiotics and thus limit or prevent biofilm-based infections and antibiotic resistance, Edirisinghe said.

Spinning-Wall Method Boosts Plasma Studies

A new approach for probing plasma interactions on surfaces has been developed by chemical engineer Vincent M. Donnelly and his coworkers at the University of Houston. Donnelly described a plasma reactor in which a wall incorporates a disklike substrate that can spin at up to 200,000 rpm. As the spinning substrate passes into the reaction chamber, it's exposed to the plasma. The exposed surface then passes out of the chamber and into differentially pumped vacuum chambers where plasma gas radicals and molecules that desorb from the substrate are detected by molecular-beam mass spectrometry, and plasma species that remain adsorbed are analyzed by Auger electron spectroscopy.

Plasmas are ionized gases used to deposit thin films and etch surfaces in the fabrication of silicon integrated circuits. Reactions involving plasma gas molecules concentrate on reactor wall surfaces, Donnelly noted, and understanding these complex interactions is important for controlling and optimizing plasmas, particularly as integrated circuit feature sizes continue to shrink.

Donnelly's group is using the spinning-wall reactor to more accurately study dynamic surface species such as oxygen and chlorine radicals and ClO and ClO2 formed in standard O2 and Cl2 plasmas or mixed Cl2-O2 plasmas. By varying the spin rate, the mechanisms and kinetics of radical loss and product formation can be determined, Donnelly said.



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