Functionalization of Si(111) surfaces with alkyl chains terminated by electrochemically polymerizable thienyl units.

A Si(111) surface has been derivatized with a thiophene-terminated alkyl monolayer which was subsequently photoanodically oxidized in the presence of thiophene to yield a strongly adherent and smooth conducting film.

Self-Assembly of Lamellar and Expanded Lamellar Coordination Networks.

Structural similarity with clay-type minerals is apparent in the self-assembled layered coordination network [{Ag(1)(MeCN)2 }∞ ][BF4 ]∞ , which is stabilized by a lamellar chelate effect (section of structure shown on the right). The interlayer region is expandable through the use of larger ligands such as PhCN. The void space is occupied by one PhCN molecule per repeat unit.

Evidence for initiation of thermal reactions of alkenes with hydrogen-terminated silicon by surface-catalyzed thermal decomposition of the reactant.

New insights into the mechanism of thermal reactions of alkenes with hydrogen terminated silicon are presented. Scanning tunneling microscopy (STM) imaging at the early stages of the reaction of 1-decene with H/Si(111) at 150 degrees C confirm this reaction occurs via a propagating radical chain mechanism. In addition, evidence is presented for an initiation mechanism involving degradation of hydrocarbon molecules catalyzed by the silanol surface of Schlenk tubes commonly used in carrying out these reactions. Hydrogen-terminated silicon surfaces are found to be unstable in the "inert" solvent dodecane when heated at 150 degrees C in a Pyrex Schlenk tube. By contrast, the surfaces were significantly more stable at the same temperature when reactions were carried out in Teflon (polytetrafluoroethylene or PTFE). The thermal reaction of decene with H/Si(111) was found to proceed more rapidly in Pyrex than in PTFE, consistent with an impurity-based initiation mechanism.

Reaction of alkenes with hydrogen-terminated and photooxidized silicon surfaces. A comparison of thermal and photochemical processes.

Reagentless micropatterning of hydrogen-terminated Si(111) via UV irradiation through a photomask has proven to be a convenient strategy for the preparation of ordered bicomponent monolayers. The success of this technique relies upon the differential rate of reaction of an alkene with the hydrogen-terminated and photooxidized regions of the surface. Monolayer formation can be accomplished under either thermal or photochemical conditions. It was observed that, after 3 h, reaction in neat alkene solution irradiation (Rayonet, 300 nm) afforded the expected patterned surface, while thermal conditions (150 degrees C) resulted in a partial loss of pattern fidelity. Monolayer properties and formation were studied on oxidized and hydrogen-terminated silicon under thermal and photochemical initiation, by contact angle, ellipsometry, Fourier transform infrared spectroscopy, high-resolution electron energy loss spectroscopy, and X-ray photoelectron spectroscopy. Results show that alkenes add to silanol groups on the silica surface in a manner consistent with acid catalysis: once attached to the surface, the silica oxidized the hydrocarbon.

A simple approach to micropatterning and surface modification of poly(dimethylsiloxane).

Ozone treatment is an efficient economical, alternative method for surface activation of poly(dimethylsiloxane) (PDMS). This is illustrated by the derivatization of a PDMS surface with (3-aminopropyl)triethoxysilane (APTES). The apparent surface concentration of amino groups was found to be ca. 10(-8) mol/cm2 using UV/visible spectroscopy of the product from the reaction of the amino groups and fluorescamine. Potential application for micropatterning of biologically active interfaces was demonstrated by the covalent immobilization of oligonucleotides. A simple process for photolithographic patterning on PDMS surfaces has been developed.

Formation, characterization, and chemistry of undecanoic acid-terminated silicon surfaces: patterning and immobilization of DNA.

This paper describes a simple strategy for DNA immobilization on chemically modified and patterned silicon surfaces. The photochemical modification of hydrogen-terminated Si(111) with undecylenic acid leads to the formation of an organic monolayer covalently attached to the surface through Si-C bonds without detectable reaction of the carboxylic acid group, providing indirect support of a free radical mechanism. Chemical activation of the acid function was achieved by a simple chemical route using N-hydroxysuccinimide (NHS) in the presence of N-ethyl-N'-(3-dimethylaminopropyl) carbodiimide hydrochloride. Single strand DNA with a 5'-dodecylamine group was then coupled to the NHS-activated surface by amide bond formation. Using a previously reported chemical patterning approach, we have shown that DNA can be immobilized on silicon surfaces in spatially well-resolved domains. Methoxytetraethyleneglycolamine was used to inhibit nonspecific adsorption. The resulting DNA-modified surfaces have shown good specificity and chemical and thermal stability under hybridization conditions. The sequential reactions on the surface were monitored by ATR-FTIR, X-ray Photoelectron Spectroscopy, and fluorescence spectroscopy.

Use of atomic force microscopy for morphological and morphometric analyses of acrosome intact and acrosome-reacted human sperm.

The objective of this study was to use atomic force microscopy (AFM), with submicron resolution, for morphophologic and morphometric analyses of acrosome intact and acrosome-reacted human sperm heads. A mixed population of acrosome intact and reacted sperm was produced by treating capacitated sperm with A23187, which induced the acrosome reaction in approximately 50% of total sperm population. This A23187-treated sperm suspension was then plated onto a coverslip and acrosome reacted sperm were preidentified by their specific staining with rhodamine-conjugated Concanavalin A. The sperm coverslip was then air-dried and scanned by a Nanoscope IIIa atomic force microscope, using the contact mode. Top and side view images processed through the illuminate mode revealed three dimensional sperm head contour, with the highest point situated in the head posterior in both acrosome intact and acrosome reacted sperm. Maximum height, length, and width measured in 50 acrosome intact and 50 acrosome-reacted sperm were the same in both populations. However, head length at half maximum height was significantly decreased in acrosome reacted sperm (2.99 +/- 0.24 microm vs. 3.56 +/- 0.32 microm of acrosome intact sperm), due to the sudden change of the height contour from the maximum peak to the anterior tip of acrosome-reacted sperm. Our results described here can therefore be used to differentiate acrosome intact and reacted sperm from each other. This would allow future studies on subcellular changes, related to the acrosome reaction, at the submicron resolution level under more physiological conditions, since AFM does not require fixing or staining of the samples.

Spectroscopic and calorimetric studies on the mechanism of methylenecyclopropane rearrangements triggered by photoinduced electron transfer.

2-(dideuteriomethylene)-1,1-bis(4-methoxyphenyl)cyclopropane (d(2)-1) undergoes degenerate rearrangement in both singlet- and triplet-sensitized electron-transfer photoreactions. Nanosecond time-resolved absorption spectroscopy on laser flash photolysis of the unlabeled 1 with 9,10-dicyanoanthracene, 1,2,4,5-tetracyanobenzene, or N-methylquinolinium tetrafluoroborate as an electron-accepting photosensitizer gives rise to two transients with lambda(max) at 500 and 350 nm assigned to the dianisyl-substituted largely twisted trimethylenemethane cation radical (6.+) and the corresponding diradical (6..), respectively. These intermediates are also detected, respectively, by steady state and nanosecond time-resolved EPR with chloranil or anthraquinone as a sensitizer. The degenerate rearrangement of d(2)-1 thus proceeds via these two different types of intermediates in a cation radical cleavage-diradical cyclization mechanism. Energetics based on nanosecond time-resolved photoacoustic calorimetry support this mechanism. A comparison of the reactivities and the spectroscopic results of 1, 1,1-bis(4-methoxyphenyl)-2-methylenespiro[2.2]pentane (2), and 1-cyclopropylidene-2,2-bis(4-methoxyphenyl)cyclopropane (3) suggest that the reversible methylenecyclopropane rearrangement between 2 and 3 proceeds via a similar mechanism.