Editorial
Analytical Chemistry of Soft Material Surfaces
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oft materials are.. .welll.. soft. They are deformable and may contain volatile constituents. Examples include polymerfilms,contact lenses,fluiddroplets, hydrogels, cell membranes, people, and plants. The emergence of the collective term "soft materials" in the materials science community has coincided with the recognition that there are some common measurement needs, particularly with regard to characterizing soft material surfaces. The needs include surface adhesion, viscoelasticity, and lateral and depth imaging of chemical constituency; these are tough but fascinating problems. In this frontier editorial I point to some of the analytical tools that have or are being adapted to address these needs. Force microscopy offers potent methodologies for soft materials. In the tapping mode varrant of atomic force microscopy (AFM), the phase relations between cantilever excitation and response provide surface images of viscoelasticity and adhesion forces that complement the height image measurements. In chemical force microscopy (CFS), the AFMtipis modified wiih a thin molecular film with which to probe the adhesion (force curves) and the frictional forces between chemical groups on the tip and the soft material surface. An exciting prospect is the combination of these kinds of CFS responses with AFM phase contrast images in order to interpret the latter images in chemically defined terms. Thermal imaging is relatively unknown to chemists but is widely used in materials science and process monitoring for nondestructive, rapid measurements. An optical beam of modulated intensity excites an oscillating thermal wave at a soft (or hard) surface. The thermal wave can then be detected by photothermal reflectance, the mirage and photothermal elastic effects, and IR photothermal radiometry. Images from these responses reveal lateral and subsurface patterns of thermal conductivity that, for example, are markers for chemical heterogeneities and adhesion loss that signal delamination of thinfilmsfrom a substrate. An analytical challenge in thisfieldis to correlate the thermal effects with chemical content For example the
variant photoacoustic spectroscopy does bring some chemical sensitivity to the "analytical table". There are many other approaches. Small angle neutron and X-ray scattering can discern compositional patterns on surfaces at a resolution of 1-100 nm. Scanning electrochemical microscopy is a powerful approach to liquid/liquid interfaces—the ultimate soft surface. An unusual way to image magnetic domains makes use oo the swimming directions of magnetotropic bacteria. Secondary ion mass spectra (SIMS) provide a rich bounty of chemical information from surfaces bombarded with various atomic and ionic projectiles. X-ray photoelectron spectroscopy (XPS) covers a wide range of elements at a very shallow sampling depth. However SIMS and XPS require ultrahigh vacuum conditions placing a premium on research compatible with soft volatile materials Light microscopy, on the other hand, is a longstanding workhorse for soft materials, especially the "bio-kind"; the advent of near-field optical microscopy (NFOM) has seriously upped the ante for image resolution. Equally important, NFOM offers the potential for high-resolution images of both electronic spectroscopy and dynamic observations of soft surfaces. A significant challenge in this methodology seems to be the routine preparation of near-field tips with high optical throughput. I did not realize when initially planning this editorial that the list of soft materials measurement tools would be as long or as potent as it is, and their continuing evolution will bring even more exciting capacities. .I is sbviously important to attempt correlations of images of physical properties with those of chemical content. Such correlations lie at the pioneer edge of understanding soft materials and will require the wedding of tools and measurement parameters developed for materials science with those of analytical chemistry. These are great interdisciplinary opportunities for analytical chemists.
Analytical Chemistry News & Features, November 1, 1998 6 8 9 A
