Mimicking the silicon surface: reactivity of silyl radical cations toward nucleophiles

Radical cations of selected low molecular-weight silicon model compounds were obtained by photoinduced electron transfer. These radical cations react readily with a variety of nucleophiles, regularly used in monolayer fabrication onto hydrogen-terminated silicon. From time-resolved kinetics, it was concluded that the reactions proceed via a bimolecular nucleophilic attack to the radical cation. A secondary kinetic isotope effect indicated that the central Si-H bond is not cleaved in the rate-determining step. Apart from substitution products, also hydrosilylation products were identified in the product mixtures. Observation of the substitution products, combined with the kinetic data, point to an bimolecular reaction mechanism involving Si-Si bond cleavage. The products of this nucleophilic substitution can initiate radical chain reactions leading to hydrosilylation products, which can independently also be initiated by dissociation of the radical cations. Application of these data to the attachment of organic monolayers onto hydrogen-terminated Si surfaces via hydrosilylation leads to the conclusion that the delocalized Si radical cation (a surface-localized hole) can initiate the hydrosilylation chain reaction at the Si surface. Comparison to monolayer experiments shows that this reaction only plays a significant role in the initiation, and not in the propagation steps of Si-C bond making monolayer formation.     

Nanomechanics of the formation of DNA self-assembled monolayers and hybridization on microcantilevers

Biomolecular interactions over the surface of a microcantilever can produce its bending motion via changes of the surface stress, which is referred to nanomechanical response. Here, we have studied the interaction forces responsible for the bending motion during the formation of a self-assembled monolayer of thiolated 27-mer single-stranded DNA on the gold-coated side of a microcantilever and during the subsequent hybridization with the complementary nucleic acid. The immobilization of the single-stranded DNA probe gives a mean surface stress of 25 mN/m and a mean bending of 23 nm for microcantilevers with a length and thickness of about 200 microm and 0.8 microm, respectively. The hybridization with the complementary sequence could not be inferred from the nanomechanical response. The nanomechanical response was compared with data from well-established techniques such as surface plasmon resonance and radiolabeling, to determine the surface coverage and study the intermolecular forces between neighboring DNA molecules anchored to the microcantilever surface. From both techniques, an immobilization surface density of 3 x 10(12) molecules/cm(2) and a hybridization efficiency of 40% were determined. More importantly, label-free hybridization was clearly detected in the same conditions with a conventional sensor based on surface plasmon resonance. The results imply that the nanomechanical signal during the immobilization process arises mainly from the covalent attachment to the gold surface, and the interchain interactions between neighboring DNA molecules are weak, producing an undetectable surface stress. We conclude that detection of nucleic acid hybridization with nanomechanical sensors requires reference cantilevers to remove nonspecific signals, more sensitive microcantilever geometries, and immobilization chemistries specially addressed to enhance the surface stress variations.     

Structural characterization of organic multilayers on silicon(111) formed by immobilization of molecular films on functionalized Si-C linked monolayers

Silicon(111)-H surfaces were derivatized with omega-functionalized alkenes in UV-mediated and thermal hydrosilylation reactions to give Si-C linked monolayers. Additional molecular layers of organic compounds were coupled either directly or via linker molecules to the functionalized alkyl monolayers. In the first instance, amino-terminated monolayers were prepared from a tert-butoxycarbonyl-protected omega-aminoalkene followed by removal of the protecting group. Various thiols were coupled to the monolayer using a heterobifunctional linker, which introduced maleimide groups onto the surface. In the second system, N-hydroxysuccinimide (NHS) ester-terminated monolayers were formed by reaction of Si-H with N-succinimidyl undecenoate. The reactivity of the NHS ester groups was confirmed by further modification of the monolayer. The stepwise assembly of these multilayer structures was characterized by X-ray reflectometry and X-ray photoelectron spectroscopy.     

Thiol-terminated monolayers on oxide-free Si: assembly of semiconductor-alkyl-S-metal junctions

Self-assembled monolayers formed by thermal hydrosilylation of a trifluoroacetyl-protected alkenylthiol on Si-H surfaces, followed by removal of the protecting groups, yield essentially oxide-free monolayers suitable for the formation of Si-C11H22-S-Hg and Si-C11H22-S-Au junctions in which the alkyl chains are chemically bound to the silicon surface (via Si-C bonds) and the metal electrode (via Hg-S or Au-S bonds). Two barriers to charge transport are present in the system: at low bias the current is temperature activated and hence limited by thermionic emission over the Schottky barrier in the silicon, whereas as at high bias transport is limited by tunneling through the organic monolayer. The thiol-terminated monolayer on oxide-free silicon provides a well-characterized system allowing a careful study of the importance of the interfacial bond to the metal electrode for current transport through saturated molecules.     

SORPTION OF OXYANIONS BY SURFACTANT-MODIFIED ZEOLITE

Zeolite has high internal and external surface areas and high internal and external cation exchange capacities suitable for surface modification by cationic surfactants. When the initial surfactant concentration is less than the critical micelle concentration, the sorted surfactant molecules primarily form a monolayer. Limited chromate sorption indicates that patchy bilayer may also form. When the initial surfactant concentration is greater than the critical micelle concentration and enough surfactant exists in the system, the sorbed surfactant molecules form bilayers, producing maximum chromate sorption. On a meq/kg basis, planar nitrate sorbs more on surfactant-modified zeolite surfaces than tetrahedral chromate. In the presence of sulfate or nitrate, chromate sorption is hindered due to competition for sorption sites. Quantitative sorption of nitrate and chromate and desorption of bromide indicate that the sorption of oxyanions is primarily due to surface anion exchange.     

Semicarbazide-functionalized Si(111) surfaces for the site-specific immobilization of peptides

The covalent attachment of semicarbazide-functionalized layers to hydrogen-terminated Si(111) surfaces is reported. The surface modification, based on the photoinduced hydrosilylation of a Si(111) surface with protected semicarbazide-functionalized alkenes, was investigated by means of X-ray photoelectron spectroscopy (XPS), contact angle measurements, and atomic force microscopy (AFM). The removal of the protecting group yielded a semicarbazide-terminated monolayer which was reacted with peptides bearing a glyoxylyl group for site-specific alpha-oxo semicarbazone ligation.     

1,6-Hexanedithiol monolayer as a receptor for specific recognition of alkylmercury

1,6-Hexanedithiol monolayer acts as an unusually specific recognition agent for CH3Hg+ when the microcantilever is used as the transducer; the mechanism of the sensor is discussed.     

A general microcantilever surface modification method using a multilayer for biospecific recognition

The key to microcantilever surface modification technology is to selectively modify one side of a microcantilever surface with molecular recognition layers. In this paper, we report a general microcantilever modification method using a multilayer film.     

Optimization of DNA hybridization efficiency by pH-driven nanomechanical bending

The accessibility and binding affinity of DNA are two key parameters affecting the hybridization efficiency in surface-based biosensor technologies. Better accessibility will result in a higher hybridization efficiency. Often, mixed ssDNA and mercaptohexanol monolayers are used to increase the hybridization efficiency and accessibility of surface-bound oligonucleotides to complementary target DNA. Here, no mercaptohexanol monolayer was used. We demonstrate by differential microcantilever deflection measurements at different pH that the hybridization efficiency peaks between pH 7.5 and 8.5. At low pH 4.5, hydration and electrostatic forces led to tensile surface stress, implying the reduced accessibility of the bound ssDNA probe for hybridization. In contrast, at high pH 8.5, the steric interaction between neighboring ssDNA strands was decreased by higher electrostatic repulsive forces, bending the microcantilever away from the gold surface to provide more space for the target DNA. Cantilever deflection scales with pH-dependent surface hybridization efficiency because of high target DNA accessibility. Hence, by changing the pH, the hybridization efficiency is adjusted.     

Covalent assembly of silver nanoparticles on hydrogen-terminated silicon surface

Synthesis of ω-alkenyl-terminated silver nanoparticles (AgNPs) and then their immobilization on a hydrogen-terminated silicon surface in two-dimensional arrangement through covalent interaction are demonstrated. The thermal-induced hydrosilylation at mild conditions facilitate nanoparticles assembly through interaction between terminal alkenyl (CH(2)=CH-) groups of AgNPs and hydrogen-terminated silicon surface. The assembly of AgNPs on a silicon surface is characterized by FESEM and XPS. Adequate coating of 10-undecene-1-thiol (UDT) on AgNPs and mild temperature hydrosilylation impede the fusion or aggregation of nanoparticles, while they immobilized on a silicon surface, which is very crucial to preserve the discrete entities of nanoparticles. This elegant and facile approach provides stable monolayer of AgNPs with very good coverage area and promises potential to fabricate electronic devices and solar cells, where nanoparticles needs to be directly attached to the silicon surface without an interfacial oxide thin film.     

The initiation mechanisms for surface hydrosilylation with 1-alkenes

Hydrosilylation provides a route to form substituted silanes in solution. A similar reaction has been observed in the formation of covalent organic monolayers on a hydrogen-terminated silicon surface and is called thermal hydrosilylation. In solution, the mechanism requires a catalyst to add the basal silicon and saturating hydrogen to the C=C double bond. On the silicon surface, however, the reaction proceeds efficiently at 200 °C, initiated by visible light, and more slowly at room temperature in the dark. Such low activation energy barriers for the reactions on a surface relative to that required for solution hydrosilylation are remarkable, and although many explanations have been suggested, controversy still exists. In this work using a constrained molecular dynamics approach within the density functional theory framework, we show that the free energy activation barrier for abstraction of a hydrogen from silicon by an alkene molecule can be overcome by visible light or thermal excitation. Furthermore, we show that by concerted transfer of a hydrogen from the α-carbon to the β-carbon, a 1-alkene can insert its α-carbon into a surface Si-H bond to accomplish hydrosilylation.     

Effect of surface viscosity on the vibration of microcantilevers

Using the theory of beam bending and the theory of surface rheology, the equation describing the effect of a surface viscous film on the vibration of an elastic microcantilever is derived. A generalized solution for the vibration of the microcantilever is obtained, which depends on the boundary conditions and the surface viscosity. There is a phase lag between the externally harmonic excitation and the microcantilever response due to the energy loss from surface flow. For small energy dissipation with negligible surface energy, the maximum energy loss occurs approximately at the natural frequencies of the microcantilever.     

Direct photochemical functionalization of Si(111) with undecenol

Direct UV photochemical functionalization of H-terminated Si(111) with bifunctional 10-undecen-1-ol was achieved with selective attachment via its vinyl end, resulting in the formation of a compact monolayer with free terminal alcohol groups. This is due to the faster radical propagation mechanism in hydrosilylation with alkene compared to the nucleophilic attack mechanism of alcohol, which is impeded by intermolecular hydrogen bonding present at room temperature. Evidence from X-ray photoelectron spectroscopy, infrared spectroscopy, and resistance to fluoride etching shows that Si-C is the interfacial bond, and atomic force microscopy shows the presence of a smooth, uniform monolayer conforming to the atomic terraces of the Si(111) surface. The application of such a hydroxyl-terminated monolayer was demonstrated by tethering a bromoinitiator through surface esterification and thereafter subjecting the surface to the surface-initiated atom-transfer radical polymerization of butyl methacrylate. The poly(butyl methacrylate) brushes formed were found to be smooth (R(a) < 0.3 nm) and uniform even for a thin film of 4.0 nm.     

An anti E. coli O157:H7 antibody-immobilized microcantilever for the detection of Escherichia coli (E. coli).

A silicon microcantilever sensor was developed for the detection of Escherichia coli O157:H7. The microcantilever was modified by anti-E. coli O157:H7 antibodies on the silicon surface of the cantilever. When the aquaria E. coli O157:H7 positive sample is injected into the fluid cell where the microcantilever is held, the microcantilever bends upon the recognition of the E. coli O157:H7 antigen by the antibodies on the surface of the microcantilever. A negative control sample that does not contain E. coli O157:H7 antigen did not cause any bending of the microcantilever. The detection limit of the sensor was 1 x 10(6) cfu/mL when the assay time was < 2 h.     

Advanced surface modification of indium tin oxide for improved charge injection in organic devices

A new method is described for surface modification of ITO with an electroactive organic monolayer. This procedure was done to enhance hole injection in an electronic device and involves sequential formation of a monolayer of a pi-conjugated organic semiconductor on the indium tin oxide (ITO) surface followed by doping with a strong electron acceptor. The semiconductor monolayer is covalently bound to the ITO, which ensures strong adhesion and interface stability; reduction of the hole injection barrier in these devices is accomplished by formation of a charge-transfer complex by doping within the monolayer. This gives rise to very high current densities in simple single layer devices and double layer light emitting devices compared to those with untreated ITO anodes.     

Oligo(ethylene glycol)-terminated monolayers on silicon surfaces and their nanopatterning with a conductive atomic force microscope

Functionalization of silicon substrate surfaces with a stable monolayer for resisting non-specific adsorption of proteins has attracted great interest,since it is directly relevant to the development of miniature,silicon-based biosensors and implantable microdevices,such as silicon-neuron interfaces.This brief review summarizes our contribution to the development of robust monolayers grown by surface hydrosilylation on atomically flat,hydrogen-terminated silicon surfaces.The review also outlines our strateg...     

Improved surface modification approach for micromechanical biosensors

We have investigated the sensing performance of protein-based microcantilever biosensors prepared from multiple surface conjugation chemistries. The 11-mercaptoundecanoic acid monolayers were prepared according to both traditional and modified processes. In three protein-based biosensors, the modified process improved microcantilever sensing performance by increasing the bending amplitude, a critical step toward developing a cost-effective microcantilever-based sensor platform for medical diagnostics and environmental and drug screening applications. Scanning electron microscopy (SEM) images demonstrated that proteins immobilized on the microcantilever surface using the modified chemistry approach formed a compact layer.     

基于微悬臂梁生物传感器检测三磷酸腺苷

基于适配子构建了无标记检测三磷酸腺苷(ATP)的微悬臂梁生物传感器。 将ATP适配子修饰在微悬臂梁阵列中的传感悬臂镀金面上,用来识别ATP,而参比悬臂修饰巯基己醇(MCH)防止非特异性吸附。 ATP与其适配子发生特异性相互作用,使悬臂的上下两个表面产生应力差,导致传感悬臂产生偏转,扣除参比悬臂偏转后其偏转值与ATP的浓度在0.5~5 mmol/L范围内有良好的线性关系,相关系数为0.998,最低检出限为0.06 mmol/L。 该微悬臂梁生物传感器响应快速、操作简单,并且对ATP具有良好的特异性。     

Surface-mediated chromate-resistant mechanism of Enterobacter cloacae bacteria investigated by atomic force microscopy

The Enterobacter cloacae CYS-25 strain isolated from a chromate plant shows a strong capability for chromate resistance instead of chromate reduction in aerobic conditions. In this study, atomic force microscopy (AFM) was used for studying the morphology characteristics of bacterial properties during the chromate resistance process. The average length of E. cloacae bacteria in the stationary phase is about 2.3 +/- 0.6 microm, while under the stimulation of 400 mg/L CrO42-, the length of bacteria increases to 3.2 +/- 0.7 microm. Height and phase images showed that, with the addition of CrO42-, the smooth surface of bacteria changed into one with discontinuous features with characteristic dimension of 40-200 nm. Analysis reveals that these compact convex patches are organic components stimulated by CrO42-. A chromate resistance mechanism relating to the overexpression of extracellular biologic components for preventing the permeability of CrO42- into the cell is proposed as the survival strategy of E. cloacae in chromate situation.     

过渡金属膦配合物在硅氢化反应中的应用研究

过渡金属膦配合物在有机合成和催化反应中的应用非常广泛, 大量含膦杂原子配体被设计合成, 利用其特定的配位能力, 和过渡金属配位成过渡金属膦配合物, 并测试其对特定有机化学反应的催化性能. 硅氢加成反应是有机硅化学中的重要反应, 多种过渡金属包括铂、钯、铑、钌等的膦配合物对于硅氢加成反应均有催化活性. 综述了近几年来过渡金属膦配合物在硅氢加成反应中的应用进展.