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Self-assembled monolayers of thiolates on metals as a form of nanotechnology.

The field of self-assembled monolayers (SAMs) has witnessed tremendous growth in synthetic sophistication and depth of characterization over the past 15 years.1 However, it is interesting to comment on the modest beginning and on important milestones. The field really began much earlier than is now recognized. In 1946 Zisman published the preparation of a monomolecular layer by adsorption (self-assembly) of a surfactant onto a clean metal surface.2 At that time, the potential of self-assembly was not recognized, and this publication initiated only a limited level of interest. Early work initiated in Kuhn’s laboratory at Gottingen, applying many years of experience in using chlorosilane derivative to hydrophobize glass, was followed by the more recent discovery, when Nuzzo and Allara showed that SAMs of alkanethiolates on gold can be prepared by adsorption of di-n-alkyl disulfides from dilute solutions.3 Getting away from the moisture-sensitive alkyl trichlorosilanes, as well as working with crystalline gold surfaces, were two important reasons for the success of these SAMs. Many self-assembly systems have since been investigated, but monolayers of alkanethiolates on gold are probably the most studied SAMs to date. The formation of monolayers by self-assembly of surfactant molecules at surfaces is one example of the general phenomena of self-assembly. In nature, self-assembly results in supermolecular hierarchical organizations of interlocking components that provides very complex systems.4 SAMs offer unique opportunities to increase fundamental understanding of self-organization, structure-property relationships, and interfacial phenomena. The ability to tailor both head and tail groups of the constituent molecules makes SAMs excellent systems for a more fundamental understanding of phenomena affected by competing intermolecular, molecular-substrates and molecule-solvent interactions like ordering and growth, wetting, adhesion, lubrication, and corrosion. That SAMs are well-defined and accessible makes them good model systems for studies of physical chemistry and statistical physics in two dimensions, and the crossover to three dimensions. SAMs provide the needed design flexibility, both at the individual molecular and at the material levels, and offer a vehicle for investigation of specific interactions at interfaces, and of the effect of increasing molecular complexity on the structure and stability of two-dimensional assemblies. These studies may eventually produce the design capabilities needed for assemblies of three-dimensional structures.5 However, this will require studies of more complex systems and the combination of what has been learned from SAMs with macromolecular science. The exponential growth in SAM research is a demonstration of the changes chemistry as a disciAbraham Ulman was born in Haifa, Israel, in 1946. He studied chemistry in the Bar-Ilan University in Ramat-Gan, Israel, and received his B.Sc. in 1969. He received his M.Sc. in phosphorus chemistry from Bar-Ilan University in 1971. After a brief period in industry, he moved to the Weizmann Institute in Rehovot, Israel, and received his Ph.D. in 1978 for work on heterosubstituted porphyrins. He then spent two years at Northwestern University in Evanston, IL, where his main interest was onedimensional organic conductors. In 1985 he joined the Corporate Research Laboratories of Eastman Kodak Company, in Rochester, NY, where his research interests were molecular design of materials for nonlinear optics and self-assembled monolayers. In 1994 he moved to Polytechnic University where he is the Alstadt-Lord-Mark Professor of Chemistry. His interests encompass self-assembled monolayers, surface engineering, polymers at interface, and surfaces phenomena. 1533 Chem. Rev. 1996, 96, 1533−1554

/pdf/formation-and-structure-of-self-assembled-monolayers-5eorfomxfu.pdf

Formation and Structure of Self-Assembled Monolayers.

One of the most pervasive problems afflicting people throughout the world is inadequate access to clean water and sanitation. Problems with water are expected to grow worse in the coming decades, with water scarcity occurring globally, even in regions currently considered water-rich. Addressing these problems calls out for a tremendous amount of research to be conducted to identify robust new methods of purifying water at lower cost and with less energy, while at the same time minimizing the use of chemicals and impact on the environment. Here we highlight some of the science and technology being developed to improve the disinfection and decontamination of water, as well as efforts to increase water supplies through the safe re-use of wastewater and efficient desalination of sea and brackish water.

Science and technology for water purification in the coming decades

http://experimentationlab.berkeley.edu/sites/default/files/AFMImages/butt_AFM.pdf

Force measurements with the atomic force microscope: Technique, interpretation and applications

The ability to control the size, shape, and material of a surface has reinvigorated the field of surface-enhanced Raman spectroscopy (SERS). Because excitation of the localized surface plasmon resonance of a nanostructured surface or nanoparticle lies at the heart of SERS, the ability to reliably control the surface characteristics has taken SERS from an interesting surface phenomenon to a rapidly developing analytical tool. This article first explains many fundamental features of SERS and then describes the use of nanosphere lithography for the fabrication of highly reproducible and robust SERS substrates. In particular, we review metal film over nanosphere surfaces as excellent candidates for several experiments that were once impossible with more primitive SERS substrates (e.g., metal island films). The article also describes progress in applying SERS to the detection of chemical warfare agents and several biological molecules.

Surface-Enhanced Raman Spectroscopy

The ability to understand and control molecular transport is critical to numerous chemical measurement strategies, especially as they apply to mass-limited samples in nanometer-scale structures. The characteristics of nanoscale structures and devices highlighted in the examples discussed in this article include enhanced mass transport, accessing novel physical behavior, large surface-to-volume ratio, diminished background signals, and the fact that molecular characteristics can dominate the behavior of the structure. The control of nanoscale transport is physically embodied in different structures and experiments. Those structures and experiments highlighted here are featured because of their centrality (nanochannels and nanopores), their connection to more familiar macroscale phenomena (nanoelectrodes), and/or their ability to introduce control (stimulus-responsive materials) or because they represent especially interesting possibilities (stochastic sensing structures).

Nanoscale Control and Manipulation of Molecular Transport in Chemical Analysis

The ultraviolet (UV)-photoinduced chemistry at low irradiances in O2 and the O3 thermal chemistry of alkanethiolate SAMs have been studied. At the low irradiances typical of low-pressure Hg lamps, UV photolytic generation of products soluble in polar solvents from hexadecanethiol (HDT) SAMs requires both O2 and irradiation below 200 nm. Significantly, exposure of HDT SAMs to ex situ generated O3 in the dark produced the same results as UV irradiation of HDT SAMs in the presence of O2. Both of these facts suggest a leading role for photogenerated O3 in the UV photochemistry of alkanethiol SAMs. Further, O3 was found to be at least an order of magnitude more effective than other active oxygen-containing species at generating labile products under the conditions employed. The products of reaction from either O3 exposure or UV irradiation in O2 appear to be oxidized sulfur headgroups, which are easily removed by subsequent rinsing in a polar solvent. The heterogeneous reaction of O3 with alkanethiols is remar...

Ultraviolet Photochemistry and ex Situ Ozonolysis of Alkanethiol Self-Assembled Monolayers on Gold

Active spatiotemporal control of electrochemical reactions through dynamic electrochemical potential gradients was explored by investigating three different types of reactions on Au:  alkanethiol SAM electrosorption, Cu deposition and stripping, and O2 evolution from H2O2 oxidation. Counterpropagating gradients composed of two different thiols differing either in terminal functionality or in chain length were prepared, and their kinetic and environmental stability was inferred from spatially resolved contact angle measurements for samples stored under varying environmental conditions for periods up to one month. Chain length was found to correlate strongly with stabilitya requirement for stability being that at least one of the chains be at least C8 or longer. Spatially directed Cu deposition on Au was demonstrated by forming Cu stripes on Au, establishing that a sequence of different potential gradients could be used to define an area of deposition in the center of a working electrode. Dynamic spatiotemp...

Active Spatiotemporal Control of Electrochemical Reactions by Coupling to In−Plane Potential Gradients†

A novel method of correlated imaging, combining confocal Raman microscopy (CRM) and matrix-assisted laser desorption ionization (MALDI) mass spectrometry imaging (MSI) was developed in order to investigate the structural and chemical diversity inherent in three-dimensional (3D) cell cultures. These 3D spheroidal cell cultures are high throughput in vitro model systems that recapitulate some of the chemical and physiological gradients characteristic of tissues. As a result, they are ideal for testing new imaging approaches due to the native diversity of cellular phenotypes found within a single culture. Individually, confocal Raman microscopy (CRM) and mass spectrometry imaging (MSI) produce different kinds of chemical information. CRM imaging reveals differences in cellular integrity and protein secretion across a typical near-equatorial transverse slice, while MSI shows localization of small molecules to discrete regions of the spheroid section. Correlating information obtained from these disparate imaging methods begins with an external fiducial mask, added to the spheroidal samples to orient image acquisition on the two orthogonal platforms. Rather than combine the images directly, principal component analysis is used to reveal the most chemically-informative elements, which are then combined using digital image correlation. Using this approach, relationships between the principal components of each method are visualized so that they may be compared on commensurate spatial length scales.

Correlated mass spectrometry imaging and confocal Raman microscopy for studies of three-dimensional cell culture sections.

A fluid circuit includes a membrane (22) having a first side, a second side opposite the first side, and a pore (42) extending from the first side to the second side. The circuit also includes a first channel (28) containing fluid extending along the first side of the membrane (22) and a second channel (30) containing fluid extending along the second side of the membrane (22) and crossing the first channel (28). The circuit also includes an electrical source (50) in electrical communication with at least one of the first fluid and second fluid for selectively developing an electrical potential between fluid in the first channel (28) and fluid in the second channel (30). This causes at least one component of fluid to pass through the pore (42) in the membrane (22) from one of the first channel (28) and the second channel (30) to the other of the first channel (28) and the second channel (30).

Paul W. Bohn

Papers

Nanoscale Control and Manipulation of Molecular Transport in Chemical Analysis

Ultraviolet Photochemistry and ex Situ Ozonolysis of Alkanethiol Self-Assembled Monolayers on Gold

Active Spatiotemporal Control of Electrochemical Reactions by Coupling to In−Plane Potential Gradients†

Correlated mass spectrometry imaging and confocal Raman microscopy for studies of three-dimensional cell culture sections.

Hybrid microfluidic and nanofluidic system