针尖化学方法研究单壁碳纳米管末端羧基的解离性质

针尖化学利用化学手段对扫描探针显微镜 ( SPM)的针尖进行功能化修饰 ,将其作为化学反应的“探针”用于研究表面的局域化学反应性质、跟踪表面发生的化学反应过程等 [1] .用针尖化学技术来研究自组装膜 ( SAMs)表面酸碱基团的局域解离性质 ,称之为化学力滴定 [2～ 8] .利用表面缩合方法将单壁碳纳米管短管组装到 AFM针尖上 ,通过测定针尖上碳纳米管的末端基团与羟基自组装膜表面之间的粘滞力 ,研究碳纳米管末端羧基的解离性质 ,可得到碳纳米管结构与化学性质的信息 .1　实验部分1 .1　碳纳米管针尖和羟基末端自组装膜的制备　基底 [Si( …     

Patterning functional materials using channel diffused plasma-etched self-assembled monolayer templates

A simple and cost-effective methodology for large-area micrometer-scale patterning of a wide range of metallic and oxidic functional materials is presented. Self-assembled monolayers (SAM) of alkyl thiols on Au were micropatterned by channel-diffused oxygen plasma etching, a method in which selected areas of SAM were protected from plasma oxidation via a soft lithographic stamp. The patterned SAMs were used as templates for site-selective electrodeposition, electroless deposition and solution-phase deposition of functional materials such as ZnO, Ni, Ag thin films, and ZnO nanowires. The patterned SAMs and functional materials were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), atomic force microscopy (AFM), and tunneling AFM (TUNA).     

Alternative method for fabricating chemically functionalized AFM tips: silane modification of HF-treated Si3N4 probes

An alternative method for fabricating functionalized, atomic force microscopy (AFM) tips is presented. This technique is simple and requires only minimal preparation and tip modification to generate chemically sensitive probes that have a robust organic monolayer of flexible terminal chemistry attached to the surface. Specifically, commercially microfabricated Si3N4 AFM tips were modified with self-assembled monolayers (SAMs) of octadecyltrichlorosilane and (11-bromoundecyl)trichlorosilane after removing the native silicon oxide surface layer with concentrated hydrofluoric acid. The structure of these SAM films on solid silicon nitride surfaces was studied using contact angle goniometry and Fourier transform infrared spectroscopy. Pull-off force measurements on various bare (mica, graphite, and silicon) and SAM-functionalized substrates confirm that mechanically robust, long-chain organic silane SAMs can be formed on HF-treated Si3N4 tips without the presence of an intervening oxide layer. Adhesion experiments show that the integrity of the organic film on the chemically modified tips is maintained over repeated measurements and that the functionalized tips can be used for chemical sensing experiments since strong discrimination between different surface chemistries is possible.     

Quantitative analysis of protein adsorption via atomic force microscopy and surface plasmon resonance

Surface properties have a significant influence on the performance of biomedical devices. The influence of surface chemistry on the amount and distribution of adsorbed proteins has been evaluated by a combination of atomic force microscopy (AFM) and surface plasmon resonance (SPR). Adsorption of albumin, fibrinogen, and fibronectin was analyzed under static and dynamic conditions, employing self-assembled monolayers (SAMs) as model surfaces. AFM was performed in tapping mode with antibody-modified tips. Phase-contrast images showed protein distribution on SAMs and phase-shift entity provided information on protein conformation. SPR analysis revealed substrate-specific dynamics in each system investigated. When multi-protein solutions and diluted human plasma interacted with SAMs, SPR data suggested that surface chemistry governs the equilibrium composition of the protein layer.     

Selective depositions on polyelectrolyte multilayers: self-assembled monolayers of m-dPEG acid as molecular template

This paper describes the fabrication of self-assembled monolayer (SAM) patterns of m-d-poly(ethylene glycol) (m-dPEG) acid molecules onto polyelectrolyte multilayers (PEMs). The patterned SAMs on PEMs were created by ionic interactions using microcontact printing (microCP) technique. The created m-dPEG acid monolayer patterns on PEMs act as resistive templates, and thus further depositions of consecutive poly(anion)/poly(cation) pairs of charged particles result in the formation of three-dimensional (3-D) patterned PEM films or selective particle depositions atop the original multilayer thin films. In this study, we illustrate nonlithographic methods of patterning and controlling 3-D PEM architectures and selective particle depositions. We investigated the effect of variables--the choice of solvent, concentration, pH, substrate pretreatment, and stamp contact times--on microcontact printing of m-dPEG acid molecules onto PEM films to determine the optimal conditions for these parameters to achieve efficient transfer of m-dPEG acid patterns onto PEMs. Among the variables, the pH of the m-dPEG acid ink solution played the most important role in the transfer efficiency of the patterns onto the multilayer films. The patterned films were characterized by optical microscopy and atomic force microscopy (AFM).     

Characterization of self-assembled monolayers of thiols on Au(111)

Self-assembled monolayers (SAMs) of n-butanethiol, n-dodecanethiol and their equimolar mixture on Au(111) were prepared and characterized by ellipsometry, contact angle measurement, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Results revealed that these SAMs are oriented ultrathin films with the thickness of nanometer scale, and the SAMs were influenced by the molecular chain length, the lattice orientation and cleanliness of the substrates. The surface of the longer chain SAM is hydrophobic. The thicknesses of three SAMs of n-butanethiol, n-dodecanethiol and their mixture revealed by ellipsometry and XPS are about 0.59 - 0.67nm, 1.60- 1.69 nm and 1.23 - 1.32nm, respectively. AFM images further demonstrated that the SAM formed by the mixture has some microdomains with two different thicknesses.     

Supramolecular microcontact printing and dip-pen nanolithography on molecular printboards

The transfer of functional molecules onto self-assembled monolayers (SAMs) by means of soft and scanning-probe lithographic techniques-microcontact printing (muCP) and dip-pen nanolithography (DPN), respectively-and the stability of the molecular patterns during competitive rinsing conditions were examined. A series of guests with different valencies were transferred onto beta-cyclodextrin- (beta-CD-) terminated SAMs and onto reference hydroxy-terminated SAMs. Although physical contact was sufficient to generate patterns on both types of SAMs, only molecular patterns of multivalent guests transferred onto the beta-CD SAMs were stable under the rinsing conditions that caused the removal of the same guests from the reference SAMs. The formation of kinetically stable molecular patterns by supramolecular DPN with a lateral resolution of 60 nm exemplifies the use of beta-CD-terminated SAMs as molecular printboards for the selective immobilization of printboard-compatible guests on the nanometer scale through the use of specific, multivalent supramolecular interactions. Electroless deposition of copper on the printboard was shown to occur selectively on the areas patterned with dendrimer-stabilized gold nanoparticles.     

Nanotribological properties of alkanephosphonic acid self-assembled monolayers on aluminum oxide: effects of fluorination and substrate crystallinity

Two phosphonic acid (PA) self-assembled monolayers (SAMs) are studied on three aluminum oxide surfaces: the C and R crystallographic planes of single crystal alpha-alumina (sapphire) and an amorphous vapor-deposited alumina thin film. SAMs are either fully hydrogenated CH3(CH2)17PO3H2 or semifluorinated CF3(CF2)7(CH2)11PO3H2. Atomic force microscope (AFM) topographic imaging reveals that the deposited films are homogeneous, atomically smooth, and stable for months in the laboratory environment. Static and advancing contact angle measurements agree with previous work on identical or similar films, but receding measurements suggest reduced coverage here. To enable reproducible nanotribology measurements with the AFM, a scanning protocol is developed that leads to a stable configuration of the silicon tip. Adhesion for the semifluorinated films is either comparable to or lower than that for the hydrogenated films, with a dependence on contact history observed. Friction between each film and the tips depends strongly upon the type of molecule, with the fluorinated species exhibiting substantially higher friction. Subtle but reproducible differences in friction are observed for a given SAM depending on the substrate, revealing differences in packing density for the SAMs on the different substrates. Friction is seen to increase linearly with load, a consequence of the tip's penetration into the monolayer.     

Tunable resistive m-dPEG acid patterns on polyelectrolyte multilayers at physiological conditions: template for directed deposition of biomacromolecules

This paper describes a new class of salt-responsive poly(ethylene glycol) (PEG) self-assembled monolayers (SAMs) on top of polyelectrolyte multilayer (PEMs) films. PEM surfaces with poly(diallyldimethylammonium chloride) as the topmost layer are chemically patterned by microcontact printing (muCP) oligomeric PEG molecules with an activated carboxylic acid terminal group (m-dPEG acid). The resistive m-d-poly(ethylene glycol) (m-dPEG) acid molecules on the PEMs films were subsequently removed from the PEM surface with salt treatment, thus converting the nonadhesive surfaces into adhesive surfaces. The resistive PEG patterns facilitate the directed deposition of various macromolecules such as polymers, dyes, colloidal particles, proteins, liposomes, and nucleic acids. Further, these PEG patterns act as a universal resist for different types of cells (e.g., primary cells, cell lines), thus permitting more flexibility in attaching a wide variety of cells to material surfaces. The patterned films were characterized by optical microscopy and atomic force microscopy (AFM). The PEG patterns were removed from the PEM surface at certain salt conditions without affecting the PEM films underneath the SAMs. Removal of the PEG SAMs and the stability of the PEM films underneath it were characterized with ellipsometry and optical microscopy. Such salt- and pH-responsive surfaces could lead to significant advances in the fields of tissue engineering, targeted drug delivery, materials science, and biology.     

Contact angles of oils on solid substrates in aqueous media: correlation with AFM data on protein adhesion

This study presents a method to measure the contact angles of oils on a substrate in water. Diiodomethane and perfluorodecalin were used as model oils. Self-assembled monolayers (SAMs) were prepared by adjusting the mole ratio of CH 3- and OH-terminated alkanethiols. The contact angles of the two oils in water increased with increasing hydrophilicity of the SAMs, and the results are contrasted with the contact angles of oils on these surfaces in air. In addition, perfluorodecalin showed higher contact angles than diiodomethane on the same surface. On the poly(N-isopropylacrylamide) (PNiPAAM) monolayer surface, the contact angles of the two oils in water decreased sharply at the transition temperature of PNiPAAM (approximately 30 degrees C), but the surface retained fairly high hydrophilicity even after the transition. The above results are correlated with atomic force microscopy (AFM) measurements of the adhesion force between protein-immobilized AFM tips (human fibrinogen and bovine serum albumin) and these monolayers.     

Direct detection by atomic force microscopy of single bond forces associated with the rupture of discrete charge-transfer complexes

Atomic force microscopy (AFM) was used to measure the chemical binding force of discrete electron donor-acceptor complexes formed at the interface between proximal self-assembled monolayers (SAMs). Derivatives of the well-known electron donor N,N,N',N'-tetramethylphenylenediamine (TMPD) and the electron acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ) were immobilized on Au-coated AFM tips and substrates by formation of SAMs of N,N,N'-trimethyl-N'-(10-thiodecyl)-1,4-phenylenediamine (I) and bis(10-(2-((2,5-cyclohexadiene-1,4-diylidene)dimalonitrile))decyl) disulfide (II), respectively. Pull-off forces between modified tips and substrates were measured under CHCl(3) solvent. The mean pull-off forces associated with TMPD/TCNQ microcontacts were more than an order of magnitude larger than the pull-off forces for TMPD/TMPD and TCNQ/TCNQ microcontacts, consistent with the presence of specific charge-transfer interactions between proximal TMPD donors and TCNQ acceptors. Furthermore, histograms of pull-off forces for TMPD/TCNQ contacts displayed 70 +/- 15 pN periodicity, assigned to the rupture of individual TMPD-TCNQ donor-acceptor (charge-transfer) complexes. Both the mean pull-off force and the 70 pN force quantum compare favorably with a contact mechanics model that incorporates the effects of discrete chemical bonds, solvent surface tensions, and random contact area variations in consecutive pull-offs. From the 70 pN force quantum, we estimate the single bond energy to be approximately 4-5 kJ/mol, in reasonable agreement with thermodynamic data. These experiments establish that binding forces due to discrete chemical bonds can be detected directly in AFM pull-off measurements employing SAM modified probes and substrates. Because SAMs can be prepared with a wide range of exposed functional groups, pull-off measurements between SAM-coated tips and substrates may provide a general strategy for directly measuring binding forces associated with a variety of simple, discrete chemical bonds, e.g., single hydrogen bonds.     

Molecular recognition forces between immunoglobulin G and a surface protein adhesin on living Staphylococcus aureus

We report AFM measurements of binding events between immunoglobulin G (IgG) and protein A (PA) on the surface of live Staphylococcus aureus bacteria. The experiments were carried out with IgG molecules tethered via CM-amylose linkers to thiol SAMs on gold-coated AFM tips. For comparison, a model system consisting of protein A molecules tethered to thiol SAMs on gold-coated silicon substrates was also investigated. Histograms of binding forces for the PA-IgG bond showed comparable rupture forces of 59 and 64 pN for the model system and live bacteria, respectively. We suggest that linker molecules with a length comparable to the AFM tip radius should make it possible to detect specific binding events on the surface of live bacteria with a lateral resolution of a few tens of nanometers. Furthermore, because S. aureus is an important human pathogen, especially methicillin-resistant strains (MRSA), it is possible that additional virulence factors beyond PA can be probed using this technique.     

Measurement of the kinetics of photo-oxidation of self-assembled monolayers using friction force microscopy

The advancement of molecular nanotechnology requires new tools for the characterization of surface chemical reactivity with nanometer spatial resolution. While spectroscopy on sub-100 nm length scales remains challenging, friction force microscopy (FFM) is a promising tool for the characterization of molecular materials, although to date it has been little used in studies of surface reactivity. Here we report the use of FFM to measure the kinetics of photo-oxidation of self-assembled monolayers (SAMs) of alkanethiols adsorbed on gold surfaces. Two alternative approaches (analysis of friction-load plots and the use of line sections through images of patterned materials) are compared and found to yield data in very good agreement, with rate constants being found to be in good agreement despite being carried out on different microscopes. The use of line-section analysis provides a convenient method for the quantification of the extent of reaction in nanometer-scale patterns created in SAMs by the novel approach of scanning near-field photolithography.     

Mixed self-assembled monolayers of semirigid tetrahydro-4H-thiopyran end-capped oligo(cyclohexylidenes)

Single-component and mixed self-assembled monolayers (SAMs) of one- and three-ring semirigid tetrahydro-4H-thiopyran end-capped oligo(cyclohexylidenes)-that is, thiopyran (1), 4-(4-cyclohexylidene-cyclohexylidene)tetrahydro-4H-thiopyran (2), and 4-(tetrahydro-4H-thiopyran-4-cyclohexylidene-4'-ylidene)tetrahydro-4H-thiopyran (3)--on Au(111) substrates have been prepared and studied by cyclic voltammetry (CV), atomic force microscopy (AFM), and scanning tunneling microscopy (STM). It was found that the shortest adsorbate 1 more readily forms a SAM than 2 or 3. Notwithstanding, the SAMs of 2 or 3 are thermodynamically more stable due to favorable intermolecular attractions. Holes were made with the AFM tip establishing tilt angles of 30-50 degrees with respect to the surface normal for all SAMs. STM imaging showed well-ordered, line-shaped packing patterns with molecular resolution for the SAM of 2. Similar patterned structures were not observed for 1 and 3. Mixed SAMs were prepared by exposing a SAM of 1 to ethanol solutions of either 2 or 3. STM imaging revealed that domains of molecules of 2 or 3 amidst a monolayer of 1 are formed in both cases. Whereas in the mixed SAM of 1 and 2 the domains are irregularly shaped, circular islands of uniform size are found in the mixed SAM of 1 and 3.     

Self-assembled monolayers of 2-(thienyl)hexylphosphonic acid on native oxide surface of silicon fabricated by air-liquid interface-assisted method

A simple, fast, and low-compound-consuming procedure based on the air-liquid interface-assisted method for preparing self-assembled monolayers (SAMs) of organic molecules with phosphonic acid head groups on the native oxide surface of silicon was demonstrated. The SAMs thus prepared were characterized by contact angle measurement, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and atomic force microscopy (AFM). This approach enabled the fabrication of ordered SAMs in a large-area substrate.     

自组装制备二氧化铪图案化薄膜

采用短波紫外光(UV)在十八烷基三氯硅烷(OTS)自组装单分子层刻蚀不同的微型图案,利用液相自组装技术在OTS模板表面沉积HfO₂图案化薄膜。通过XRD、AFM、SEM、EDS等测试手段对OTS膜和HfO₂薄膜进行表征,结果表明：以OTS为模板利用液相自组装技术成功制备出边缘轮廓清晰、粗糙度较小、条纹宽度为10μm的立方晶型的HfO₂图案化薄膜。     

Fabrication and characterization of metal-molecule-metal junctions by conducting probe atomic force microscopy

Metal-molecule-metal junctions were fabricated by contacting Au-supported alkyl or benzyl thiol self-assembled monolayers (SAMs) with an Au-coated atomic force microscope (AFM) tip. The tip-SAM microcontact is approximately 15 nm(2), meaning the junction contains approximately 75 molecules. Current-voltage (I-V) characteristics of these junctions were probed as a function of SAM thickness and load applied to the microcontact. The measurements showed: (1) the I-V traces were linear over +/-0.3 V, (2) the junction resistance increased exponentially with alkyl chain length, (3) the junction resistance decreased with increasing load and showed two distinct power law scaling regimes, (4) resistances were a factor of 10 lower for junctions based on benzyl thiol SAMs compared to hexyl thiol SAMs having the same thickness, and (5) the junctions sustained fields up to 2 x 10(7) V/cm before breakdown. I-V characteristics determined for bilayer junctions involving alkane thiol-coated tips in contact with alkane thiol SAMs on Au also showed linear I-Vs over +/-0.3 V and the same exponential dependence on thickness. The I-V behavior and the exponential dependence of resistance on alkyl chain length are consistent with coherent, nonresonant electron tunneling across the SAM. The calculated conductance decay constant (beta) is 1.2 per methylene unit ( approximately 1.1 A(-)(1)) for both monolayer and bilayer junctions, in keeping with previous scanning tunneling microscope and electrochemical measurements of electron transfer through SAMs. These measurements show that conducting probe-AFM is a reliable method for fundamental studies of electron transfer through small numbers of molecules. The ability to vary the load on the microcontact is a unique characteristic of these junctions and opens opportunities for exploring electron transfer as a function of molecular deformation.     

Si/SiO2-templated formation of ultraflat metal surfaces on glass, polymer, and solder supports: their use as substrates for self-assembled monolayers

This paper describes the use of several methods of template stripping (TS) to produce ultraflat films of silver, gold, palladium, and platinum on both rigid and polymeric mechanical supports: a composite of glass and ultraviolet (UV)-curable adhesive (optical adhesive, OA), solder, a composite of poly(dimethyl siloxane) (PDMS) and OA, and bare OA. Silicon supporting its native oxide layer (Si/SiO2) serves as a template for both mechanical template stripping (mTS), in which the metal film is mechanically cleaved from the template, and chemical template stripping (cTS), in which the film-template composite is immersed in a solution of thiols, and the formation of the SAM on the metal film causes the film to separate from the template. Films formed on all supports have lower root-mean-square (rms) roughness (as measured by atomic force microscopy, AFM) than films used as-deposited (AS-DEP) by electron-beam evaporation. Monolayers of n-dodecanethiolate formed by the mTS and cTS methods are effectively indistinguishable by scanning tunneling microscopy (STM); molecularly resolved images could be obtained using both types of surfaces. The metal surfaces, before being cleaved, are completely protected from contact with the atmosphere. This protection allows metal surfaces intended to support SAMs to be prepared in large batch lots, stored, and then used as needed. Template stripping thus eliminates the requirement for evaporation of the film immediately before use and is a significant extension and simplification of the technology of SAMs and other areas of materials science requiring clean metal surfaces.     

Chemical force microscopy investigation of phosphate adsorption on the surfaces of iron(III) oxyhydroxide particles

Phosphate-modified AFM tips were prepared by the deposition of self-assembled monolayers (SAMs) of bis(11-thioundecyl) phosphate on Au-coated silicon nitride cantilevers. The properties of the PO(2)H-terminated SAMs were investigated by studying the pH-dependent force interactions of the tips with phosphate- and carboxylic acid-terminated SAM control surfaces. The PO(2)H functional groups had a pK(a) of approximately 5.0. A chemical force microscopy (CFM) study was conducted on the interactions between the probes and the surfaces of hydrous ferric oxide particles prepared in our laboratory by hydrolytic precipitation from FeCl(3). The forces between PO(2)H probes and the hydrous ferric oxide surfaces were seen to exhibit a strong pH dependence, with maximum attractive forces occurring for pH values between 5 and 8. The effects of postprecipitation of the hydrous ferric oxide colloids with orthophosphate, H(2)PO(4)(-), dimethylphosphate, (CH(3)O)(2)PO(2)H (DMP), and tannic acid (TA) on the adhesive interactions between the PO(2)H tips and the solids were also investigated. Attenuated total reflectance infrared spectroscopy (ATR-IR) was used to verify the presence of surface-adsorbed species and zeta potentiometric measurements to determine surface charge on the colloids. We show that the method of chemical force titration using phosphate-terminated tips can differentiate between these various colloids and that it shows promise as a general method for studying this environmentally important class of compounds.     

Structural characterization of aldehyde-terminated self-assembled monolayers

The structure of aldehyde-terminated alkanethiol self-assembled monolayers (SAMs) on Au(111) is investigated using scanning tunneling microscopy (STM), atomic force microscopy (AFM), and density functional theory (DFT) calculations. For the first time, the structures of aldehyde-terminated SAMs are revealed with molecular resolution. SAMs of 11-mercapto-1-undecanal exhibit the basic (radical3xradical3)R30 degrees periodicity and form various c(4x2) superstructures upon annealing. In conjunction with DFT studies, the models of the (radical3xradical3)R30 degrees and the c(4x2) superstructures are constructed. In comparison with alkanethiol SAMs, the introduction of aldehyde-termini results in smaller domain size, lower degree of long-range order, large coverage of disordered areas, and higher density of missing molecules and other point defects within domains of closely packed molecules. The origin of these structural differences is mainly attributed to the strong dipole-dipole interactions among the aldehyde termini.