editorial
Soft and Molecular Surfaces
E
very material has surfaces and interfaces with surrounding media, whether rail cars in air, dissolved nanoparticles, or biological cells living within an organism. The chemical behavior of surfaces is consequently an enormously important scientific and technological subject, which includes a host of analytical issuesOwhat elements are present, what molecules are present, are there gradients of composition or solvation below the immediate surface, etc.. A large portfolio of analytical surface measurements has emergedOthe oldest and best developed (both conceptually and commercially) deal with analysis of hard, solid surfaces. Methods like X-ray photoelectron spectroscopy, among others, populate surface analysis chapters in undergraduate textbooks. A new generation of analytical approaches is growing up that are applicable to “softer” surfaces, e.g., molecular surfaces ranging from monolayers of molecules to molecular films to biological cells and gels. In this Frontier Editorial, I highlight two methodologies that have experienced particular success in the past several yearsOnamely, DESI MS and scanning electrochemical microscopy (SECM). DESI, the newer (2004) of the two methodologies, has percolated through the MS community to spawn a number of variants with different acronyms. All have the common characteristic of applicability to surfaces in the open atmosphere, seriously changing longstanding paradigms of sample accessibility to MS inspection. SECM is an older (1989) experiment that over the last few years has surged in applications to soft, molecular interfaces. Analytical chemistsOand teachersOneed to take note of both developments. I will outline some of the essential characteristics; more complete reviews of both are available in the literature. Basic DESI consists of aiming an electrospray emitter at the sample surface, where ionization and desorption droplet-projectiles splash into the soft surface, throwing out charged droplets containing analyte, some of which enter the MS inlet capillary for droplet evaporation and mass analysis of produced analyte ions. This complex and as yet incompletely dissected process is being actively investigated as to mechanism(s), geometrical parameters, quantitation, sensitivity, and selectivity. One aspect, however, seems quickly clear: applicability to a wide variety of soft surfaces, including lifting off of surfaces molecular components of pharmaceutical products, proteins, peptides, polymers, evaporated chromatographic eluents, and living cells. Fascinating observations from the latter include
10.1021/AC901752X 2009 AMERICAN CHEMICAL SOCIETY
Published on Web 08/10/2009
surfactant cell membrane components of bacterial biofilms. Both fundamental and application studies seem destined to spread for a time, with the accompanying appearance of new equipment in the instrument marketplace. The SECM experiment involves a potential-controlled micro- or nanoelectrode tip (“probe”) near to or slowly approaching a sample surface on an ångstro¨m-resolved distance scale. The surface may itself be conducting and functioning as an electrode or instead simply be a molecular surface. The probe can variously detect, release, or recycle (feedback) chemicals that emanate from or react with the molecular surface. Because the probe⫺surface distances can be very small (nanometer-scale), mass transport times are short and timescales of detectable changes in the molecular surfaces accordingly short. The experiment thus has some molecular specificity in terms of redox entities detected, surface dynamics sensitivity as to timescale, and with rastering of the probe, a modestly good (ca. micrometer) lateral imaging resolution. The probe can be coupled with an atomic force microscope tip function to concurrently provide molecular surface topology. Applications of these ideas have included imaging of latent fingerprints, quantifying enzyme activity within single living cells, grafting polymers and fluorescent labels in surface patterns, titrating adsorbed molecules, and observing the dynamic thermal floppiness of tethered, terminally redox-labeled DNA strands. I found the latter application especially interesting since surface molecular floppiness was an enabling tenant of the earliest (mid-1970s) chemically modified electrodes, to which this Editor was a contributor. The SECM probe capability for detect/release/recycle functions will continue to enable soft surface investigations, with a solid theory foundation for quantitation. Like DESI, SECM profits from development of the instrument marketplace. DESI and SECM obviously are useful for different, important problems in soft surface analysis. These instrumental and conceptual developments deserve our plaudits; they were initiated, respectively, by R. Graham Cooks and Allen J. Bard.
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