Functionalization of gold-coated carbon nanotubes with self-assembled monolayers of thiolates

Multi-walled carbon nanotubes (CNT) were synthesized by chemical vapor deposition using Co–Fe as a catalyst and ethylene as a carbon source. Afterward, a simple method combining wet-chemistry and chemical reduction was used to prepare carbon nanotube/gold material (CNT/Au). Pristine nanotubes and CNT/Au were characterized by transmission electron microscopy micrographs. It appeared that gold formed nanoparticles on CNTs endings and their sidewalls. Further functionalization was carried out by using thiols of different chemical properties and molecule sizes. Thiols formed self-assembled monolayer on gold surface that led to formation of CNT/gold/thiol-functionalized material. The amounts of chemisorbed thiols were measured by elemental analysis and thermogravimetry.     

Redox-active monolayers on nano-scale silicon electrodes

Uniform arrays of nano-scale electrolyte-molecule-silicon capacitors have been successfully fabricated. This was done by a combination of reactive ion etch and a selective wet etch through an anodic aluminium oxide mask to form nano-holes in silicon oxide/silicon nitride insulator layers on silicon. Self-assembled monolayers of 4-ferrocenylbenzyl alcohol were then attached to the exposed silicon surfaces at the bottom of the nano-holes. Characterization by conventional capacitance and conductance techniques showed very high capacitance and conductance peaks near -0.6?V, that were attributed to the charging and discharging of electrons into and from discrete levels in the monolayer owing to the presence of the redox-active ferrocenes.     

Dynamic microcantilever sensors for discerning biomolecular interactions

Response of a conductive micromechanical cantilever placed in close proximity to a surface undergoing electrical excitation near the resonance frequency of the cantilever is influenced by the presence of microscopic dielectrics in the gap between the cantilever and the sample surface. The variations of the resonance response of unmodified cantilevers at gap distances below a few hundred nanometers are used to discern biomolecular differences of oligomeric nucleic acids in an array format without the use of extrinsic labels. The resonance response variation paves the way for the development of high throughput detection of biomolecular reactions, such as DNA hybridization reactions or antibody-antigen interactions without the use of external labels, in which the need is only to see the presence or absence of interaction. This dynamic method is simple, does not require immobilizing individual elements on a cantilever array, and is compatible with current generation DNA chips in which DNA spots are deposited in micro- and nanoarray format.     

Intermolecular interactions in monolayers of porphyrins

Monolayers containing solely porphyrins with or without aliphatic chains or porphyrins mixed with phospholipids are studied on water on solid surfaces. The systems are manipulated on the water surface and characterized by film balance techniques, fluorescence microscopy, fluorescence and absorption spectroscopy and, after transfer onto solid supports, by optical and electron microscopy techniques. The data presented contain information on the interrelation between molecular and monolayer structure and on porphyrin-porphyrin and porphyrin-lipid interactions.Homogeneous monolayers can be formed only for porphyrins containing aliphatic chains or in dilution with lipids in the fluid phase. An ordered structure is obtained for zinc 3,8-bis(1'-heptadecenyl)deuteroporphyrin dimethylester (ZnHDPDME) that exhibits two aliphatic chains attached to one edge and two (hydrophilic) ester groups to the opposite edge of the porphyrin moiety. If these groups are bound to the same edge, a disordered structure with little tendency to aggregate formation is obtained.None of the porphyrins are soluble in the solid phase of phospholipids. As this is presumably due to the high packing density given by the area per hydrocarbon chain this is suggested to hold also for other aliphatic matrices including fatty acids.A strong decrease in fluorescence yield with increasing molecular density is observed for ZnHDPDME diluted in phospholipids. This suggests a change in the coordination of the central zinc atom of the porphyrin. As the environment can be changed in a defined and continuous way these systems offer the possibility of studying the relation between the microscopic surroundings and the optical properties.     

Liquid phase deposition of BiFeO3 thin film on self-assembly monolayers

BiFeO₃ (BFO) thin films were successfully deposited on self-assembled monolayers (SAMs) by the liquid phase deposition method. The measurement of contact angle and atomic force microscopy (AFM) showed that after immersion in an octadecyl trichlorosilane (OTS) solution for 30 min, the surface of the substrate was covered with a smooth, hydrophobic layer. After UV irradiation for 30 min, the smooth hydrophobic layer changed into a serrated hydrophilic layer. This indicated that the OTS-SAMs played an active role as chemical templates in controlling nucleation and growth of the BFO thin film. The phase and the surface topography of the BFO film were investigated respectively by X-ray diffraction, Field emission scanning electron microscopy (FE-SEM) and AFM. The results showed that the optimum annealing temperature and deposition temperature for preparing the BFO thin film were 600 and 70 °C respectively. The films were annealed at 600 °C for 2 h. As-prepared thin films were smooth, uniform, and dense with the height varying between 20 and 100 nm. Moreover, patterned BFO nanoarrays were prepared.     

Molecular self-assembly at solid surfaces

Self-assembly, the process by which objects initially distributed at random arrange into well-defined patterns exclusively due to their local mutual interactions without external intervention, is generally accepted to be the most promising method for large-scale fabrication of functional nanostructures. In particular, the ordering of molecular building-blocks deposited at solid surfaces is relevant for the performance of many organic electronic and optoelectronic devices, such as organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs) or photovoltaic solar cells. However, the fundamental knowledge on the nature and strength of the intermolecular and molecule-substrate interactions that govern the ordering of molecular adsorbates is, in many cases, rather scarce. In most cases, the structure and morphology of the organic-metal interface is not known and it is just assumed to be the same as in the bulk, thereby implicitly neglecting the role of the surface on the assembly. However, this approximation is usually not correct, and the evidence gathered over the last decades points towards an active role of the surface in the assembly, leading to self-assembled structures that only in a few occasions can be understood by considering just intermolecular interactions in solid or gas phases. In this work we review several examples from our recent research demonstrating the apparently endless variety of ways in which the surface might affect the assembly of organic adsorbates.     

基于硅基自组装膜的分子电子器件

文章综述了以硅为基底的自组装有机单层膜在分子电子器件中的应用,重点介绍了自组装膜的电子传导性,包括各种理论模型,如隧穿效应、热电子激发、Poole-Frankel激发以及跨越传导。此外,以烷基链(σ-分子),共轭链(π-分子)体系组成的自组装膜为基础的各种分子电子器件,如二极管、共振隧穿二极管,分子记忆和分子晶体管的概念、结构及工作原理也一并被讨论。     

Glucose biosensor based on the microcantilever

Diagnosis and management of diabetes require quantitative and selective detection of blood glucose levels. We report a technique for micromechanical detection of biologically relevant glucose concentrations by immobilization of glucose oxidase (GOx) onto a microcantilever surface. Microfabricated cantilevers have traditionally found utility in atomic force microscope imaging. During the past decade, however, microcantilevers have been increasingly used as transducers in chemical-sensing systems. This paper describes the combination of this technology with enzyme specificity to construct a highly selective glucose biosensor. The enzyme-functionalized microcantilever undergoes bending due to a change in surface stress induced by the reaction between glucose in solution and the GOx immobilized on the cantilever surface. Experiments were carried out under flow conditions. The common interferences for glucose detection in other detection schemes have been tested and have shown to have no effect on the measurement of blood glucose level by this technique.     

Molecular dynamics studies of microscopic wetting phenomena on self-assembled monolayers

Molecular dynamics calculations have been used to investigate the behavior of overlayers of water or n-alkane fluids on solid surfaces formed from “self-assembled” monolayers of long-chain hydrocarbons. A microscopic analog of the wetting contact angle is used to measure the surface wetting characteristics. On a nonpolar surface, formed by close packed chains having -CH₃ tailgroups, the water molecules aggregate to form a compact droplet. The calculated contact angle of the droplet is similar to experimental values for macroscopic water droplets. Contrary to intuition, the overlayers of hexadecane or decane form droplets with smaller contact angles on the same surface. However, the calculated contact angles are again in reasonable accord with experimental values.     

Controlled dielectrophoretic nanowire self-assembly using atomic layer deposition and suspended microfabricated electrodes

Effects of design and materials on the dielectrophoretic self-assembly of individual gallium nitride nanowires (GaN NWs) onto microfabricated electrodes have been experimentally investigated. The use of TiO(2) surface coating generated by atomic layer deposition (ALD) improves dielectrophoretic assembly yield of individual GaN nanowires on microfabricated structures by as much as 67%. With a titanium dioxide coating, individual nanowires were placed across suspended electrode pairs in 46% of tests (147 out of 320 total), versus 28% of tests (88 out of 320 total tests) that used uncoated GaN NWs. An additional result from these tests was that suspending the electrodes 2.75 μm above the substrate corresponded with up to 15.8% improvement in overall assembly yield over that of electrodes fabricated directly on the substrate.     

Carbohydrate-protein interactions by "clicked" carbohydrate self-assembled monolayers

A Huisgen 1,3-dipolar cycloaddition "click chemistry" was employed to immobilize azido sugars (mannose, lactose, alpha-Gal) to fabricate carbohydrate self-assembled monolayers (SAMs) on gold. This fabrication was based on preformed SAM templates incorporated with alkyne terminal groups, which could further anchor the azido sugars to form well-packed, stable, and rigid sugar SAMs. The clicked mannose, lactose, and alpha-Gal trisaccharide SAMs were used in the analysis of specific carbohydrate-protein interactions (i.e., mannose-Con A; ECL-lactose, alpha-Gal-anti-Gal). The apparent affinity constant of Con A binding to mannose was (8.7 +/- 2.8) x 10(5) and (3.9 +/- 0.2) x 10(6) M(-1) measured by QCM and SPR, respectively. The apparent affinity constants of lactose binding with ECL and alpha-Gal binding with polyclonal anti-Gal antibody were determined to be (4.6 +/- 2.4) x 10(6) and (6.7 +/- 3.3) x 10(6) M(-1), respectively by QCM. SPR, QCM, AFM, and electrochemistry studies confirmed that the carbohydrate SAM sensors maintained the specificity to their corresponding lectins and nonspecific adsorption on the clicked carbohydrate surface was negligible. This study showed that the clicked carbohydrate SAMs in concert with nonlabel QCM or SPR offered a potent platform for high-throughput characterization of carbohydrate-protein interactions. Such a combination should complement other methods such as ITC and ELISA in a favorable manner and provide insightful knowledge for the corresponding complex glycobiological processes.     

A Monte Carlo calculation of lateral interactions in silver monolayers on W(110)

The lateral interactions in silver submonolayers on W(110) are estimated by comparing phase diagrams calculated by means of Monte Carlo techniques with an experimental phase diagram inferred from work function and thermal desorption spectroscopy measurements. Pairwise interactions alone, at least within the ranges considered in this paper, do not seem to be able to reproduce part of the experimental phase diagram. The additional use of three-body interactions, however, introduces a desired asymmetry into the calculated phase diagrams, resulting in a reasonably close fit with the experimental phase diagram. Best-fit estimates for the lateral interactions were obtained by assuming attractive first-nearest-neighbour and repulsive second- and third-nearest-neighbour interactions, together with two different types of attractive three-body interaction. Ground state phase diagrams for adsorbates with pairwise interactions extending up to the fifth nearest neighbour and two different types of three-body interaction are discussed in detail for attractive first-nearest-neighbour interactions and b.c.c. (110) substrates.     

Nanomechanical detection of DNA melting on microcantilever surfaces

We observe surface stress changes in response to thermal dehybridization, or melting, of double-stranded DNA (dsDNA) oligonucleotides that are grafted on one side of a microcantilever beam. Changes in surface stress occur when one complementary DNA strand melts and diffuses away from the other, resulting in alterations of the electrostatic, counterionic, and hydration interaction forces between the remaining neighboring surface-grafted DNA molecules. We have been able to distinguish changes in the melting temperature of dsDNA as a function of salt concentration and oligomer length. This technique also highlights differences between surface immobilized and solution DNA melting dynamics, which allows us to better understand the stability of DNA on surfaces. The transduction of phase transitions into a mechanical signal is ubiquitous for DNA, making cantilever-based detection a widely useful and complementary alternative to calorimetric and fluorescence measurements.     

Scanning electrochemical microscopy monitoring in microcantilever platforms

The deflection of cantilever systems may be performed by an indirect electrochemical method that consists of measuring the local cantilever activity and deflection in a feedback generation-collection configuration of the SECM. This is illustrated during the electrochemically assisted adsorption of Br onto a gold-coated cantilever, either in its pristine state or previously coated with a thin organic barrier. It is further extended to the adsorption of an antibody in a heterogeneous immunoassay at an allergen-coated microcantilever platform. In both reactions, the cantilever deflection is qualitatively detected from the SECM tip current measurement and a quantitative estimate is obtained through modeling. This electroanalytical strategy provides an alternative approach to standard optical detection. It can overcome some limitations of the optical method by allowing electrochemical characterization of nonconductive cantilevers and appropriate use for closed systems.