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ANALYTICAL CURRENTS
R or S? Adhesion forces provide clues to handedness Techniques that measure intermolecular forces have been shown to discriminate be tween chiral forms of compounds. By attach ing chiral molecules to the scanning probe tip of an atomic force microscope, Rachel McKendry, Maria-Elena Theoclitou, and co-workers at the University of Cambridge (U.K.) demonstrate that two enantiomers of mandelic acid can be readily distinguished by chemical force microscopy (CFM). Adhesion force measurements provide information regarding the binding energy between molecules on the probe tip's sur face and those on the sample's surface. When combined with topographic and fric tion images, adhesion measurements can be used to create a functional map, show ing the spatial arrangement of ligands on the surface. The authors measure adhesion forces for various chiral tips and surfaces. Probe tips are coated with acylated phenylglycine (T), modified with an alkanethiol, and attached to
(A) S-enantiomer of the chiral ligand, T, attached to AFM tip. (B) Surface coated with enantiomers of mandelic acid. (C) R-enantiomer of M1, M2, and M3 (Adapted with permission. Copyright 1998 Macmillan Magazines.)
Carbon nanotubes get the picture
tube tips than with conventional tips, corresponding to a 12-30% improve ment in resolution. The nanotube tips also reveal greater height variations along the fibril than do conventional tips, presumably because of their high aspect ratio. The nanotube tips appear to approach the limit of tip-induced broadening of the structure. The improvement in resolution can be attributed to the smaller diameter of the nanotubes. Deconvolution methods reveal an average radius of 9 nm for the nanotubes compared with 13-20 nm for the silicon tips. Single-walled nanotubes have an even smaller effective radius of 3 nm. (J. Am. Chem. Soc. 1998, 120, 603-04)
Carbon nanotubes, with their long (rela tively speaking) thin cylindrical shape, could improve the lateral resolution of AFM imaging. Charles M. Lieber, Peter T. Lansbury, Jr., and colleagues at Har vard University and Brigham and Wom en's Hospital attach multiwall carbon nanotubes (MWNTs) and single wall carbon nanotubes to the ends of singlecrystal cantilever tips and use them to image amyloid S(l-40)-derived fibrils (the primary constituent of the amyloid plaques formed in Alzheimer's disease) with tapping mode AFM. To establish the resolution of the nanotube tips, the researchers com pared the diame ters and height variations along the length of the fibril as deter mined by the nanotubes and by single-crystal sili con tips. The ob served widths of AFM image of a type I A$40 fibril acquired with a MWNT tip. thefibrilsand proto The left arrow indicates a fibril V'-branch, the middle arrow fibrils sre 3-8 nm indicates a defect in the periodic structure, and the right arrow smaller with nano indicates a staggered fibril end.
a gold-coated probe. The tips are derivatized with either the S or the R enantiomer, or with a racemic mixture. Mandelic acid, hav ing only a single chiral center, ,s attached to a gold surface, either as an ether (Ml), through the aromatic ring (M2), or as an ester (M3). Altogether there are 18 possible combinations of chiral surfaces and tips. A significant difference (essentially 100%) in the adhesion force for different enantiomers was observed, demonstrating that CFM is sensitive enough to distin guish between simple chiral molecules. The authors are now attempting to gain a quantitative understanding of the results at a molecular level through the use of FT-IR (Nature 1998 391 566-68)
In the flow with μFNs When it comes to microfluidic networks (uFNs), photolithographically etched chan nels on silicon or plastic substrates imme diately leap to mind. A less familiar method of constructing uFNs is with an elastomeric polymer that can form a seal with the sub strate. Hans Biebuyck and co-workers at the IBM Research Division (Switzerland) describe the design, function, and applica tion of simple uFNs for delivering reactants onto a substrate. In this work, the researchers made the uFN of poly(dimethylsiloxane) (PDMS). The uFN was then applied by hand to a silicon substrate, where it was used to di rect the reaction of immunoglobulin G with
Analytical Chemistry News & Features, April 1, 1998 239 A
