Chemical force microscopy of self-assembled monolayers on sputtered gold films patterned by phase separation

Spontaneous Intercluster Electron Transfer X2- + X0 → 2 X- (X = PtAu24(SCnH2n+1)18) in Solution: Promotion by Long Alkyl Chains

In this work, we systematically investigated the ligand effects on spontaneous electron transfer (ET) between alkanethiolate-protected metal clusters in solution. The donor and acceptor clusters used were [PtAu₂₄(SC_nH_2n+1)₁₈]^2- (8e(Cn)) and [PtAu₂₄(SC_mH_2m+1)₁₈]⁰ (6e(Cm)) (n, m = 2-16), which have icosahedral Pt@Au₁₂ cores with eight and six valence electrons, respectively. The ET rate constant (k_ET) from 8e(Cn) to 6e(Cm) in benzene exhibited a novel turnover behavior as a function of the total chain length n + m: the k_ET decreased with n + m in the range of 4-12, whereas it monotonically increased with n + m in the range of 12-32. Electrospray ionization mass spectrometry of the mixture of 8e(Cn) and 6e(Cm) detected the dimer complex 8e(Cn)·6e(Cm), the relative population of which increased with n + m. The activation energy (E_a), determined based on the Arrhenius plots for n = m, monotonically decreased with n (≥ 6). Based on these results, we proposed that the promotion of ET by longer alkanethiolates was ascribed to two effects on the key intermediate 8e(Cn)·6e(Cm): (1) elongation of the lifetime and (2) the contraction of the distance between 8e(Cn) and 6e(Cm) due to the stronger van der Waals interaction between the longer alkyl chains. Such alkyl-chain-promoted ET is specific to ultrasmall clusters in solution because a nonuniform ligand layer could be formed due to the large curvature of the cluster core.

Simulations of Friction Anisotropy on Self-Assembled Monolayers in Water

Molecular dynamics simulations were performed to study nanoscale friction on hydrophilic and hydrophobic self-assembled monolayers (SAMs) immersed in water. Sliding was simulated in two different directions to capture anisotropy due to the direction of motion relative to the inherent tilted orientation of the molecules. It was shown that friction depends on both hydrophobicity and sliding direction, with the highest friction observed for sliding on hydrophobic SAM in the direction against the initial orientation of the molecules. The origins of the friction trends were analyzed by differentiating the tip-SAM and tip-water force contributions to friction. The tip-water force was higher on the hydrophilic SAM, and this was shown to be due to the presence of a dense layer of water adjacent to the surface and hydrogen bonding. In contrast, the tip-SAM force was higher on the hydrophobic SAM due to a water depletion layer, which enabled the tip to be closer to the SAM terminal group. The higher-friction cases all exhibited greater penetration of the tip below the surface of the SAM, accommodated by further tilting and reorientation of the SAM molecules.

A Comprehensive Study of the Bridge Site and Substrate Relaxation Asymmetry for Methanethiol Adsorption on Au(111) at Low Coverage

We use dispersion-corrected density functional theory to explore the bridge-site asymmetry for methanethiol adsorbed on Au(111) with two different S-C bond orientations. We attribute the asymmetry to the intrinsic character of the Au(111) surface rather than the adsorbate. The preference for bridge-fcc versus bridge-hcp SCH3 adsorption sites is controlled by the S-C bond orientation. The system energy difference favors the bridge-fcc site by 8.1 meV on the unrelaxed Au(111) surface. Relaxing the Au substrate increased this energy difference to 26.1 meV. This asymmetry is also reflected in the atomic displacement of the relaxed Au surface. Although in both cases, the bridge-site Au atoms shift away from the fcc 3-fold hollow site, the motion is greater for the bridge-fcc allowing a more favorable geometry for the sulfur atom to bond to the bridging atoms. We confirm that the adsorption energy is strongly dependent on the S-C bond orientation and position, which can be understood in terms of a simple coordination geometry model. This work has important implications for alkanethiol surface diffusion and the structure of their self-assembled monolayers.

Interaction of the (2√3 × 3)rect. Adsorption-Site Basis and Alkyl-Chain Close Packing in Alkanethiol Self-Assembled Monolayers on Au(111): A Molecular Dynamics Study of Alkyl-Chain Conformation

We show that the adsorption site basis of the (2√3 × 3)rect. phase of n-alkanethiol self-assembled monolayers plays a key role in determining the molecular conformation of the close-packed alkyl chains. Ten proposed reconstructed Au-S interfaces are used to explore the minimized energy alkyl-chain packing of n-decanethiol molecules using molecular dynamics with the all-atom description. In this comparative study, all models have the same alkyl-chain surface density of four molecules per unit cell; thus, differences are due to the headgroup spacing within the 4-molecule basis as opposed to the average surface density. We demonstrate for the first time the 4-molecule-basis twist structure driven by the packing of alkanethiol molecules in a large simulation box (100 molecules, 25 unit cells) using molecular dynamics. Our results validate the prediction put forward by Mar and Klein that to achieve the 4-molecule-basis twist symmetry observed by the experiment, the headgroups must deviate from the high-symmetry (√3 × √3)R30° sites. The key structural parameters: tilt, twist, and end-group height, as well as their spatial order, are compared with experimental results, which we show is a highly sensitive approach that can be used to vet proposed Au-S interfacial models.

Response of Non-Polar Oil Component on Low Salinity Effect in Carbonate Reservoirs: Adhesion Force Measurement Using Atomic Force Microscopy

While the effect of polar-oil component on oil-brine-carbonate system wettability has been extensively investigated, there has been little quantitative analysis of the effect of non-polar components on system wettability, in particular as a function of pH. In this context, we measured the contact angle of non-polar oil on calcite surface in the presence of 10,000 ppm NaCl at pH values of 6.5, 9.5 and 11. We also measured the adhesion of non-polar oil group (–CH3) and calcite using atomic force microscopy (AFM) under the same conditions of contact angle measurements. Furthermore, to gain a deeper understanding, we performed zeta potential measurements of the non-polar oil-brine and brine-calcite interfaces, and calculated the total disjoining pressure. Our results show that the contact angle decreases from 125° to 78° with an increase in pH from 6.5 to 11. AFM measurements show that the adhesion force decreases with increasing pH. Zeta potential results indicate that an increase in pH would change the zeta potential of the non-polar oil-brine and calcite-brine interfaces towards more negative values, resulting in an increase of electrical double layer forces. The total disjoining pressure and results of AFM adhesion tests predict the same trend, showing that adhesion forces decrease with increasing pH. Our results show that the pH increase during low-salinity waterflooding in carbonate reservoirs would lift off non-polar components, thereby lowering residual oil saturation. This physiochemical process can even occur in reservoirs with low concentration of polar components in crude oils.

Tuning Oleophobicity of Silicon Oxide Surfaces with Mixed Monolayers of Aliphatic and Fluorinated Alcohols

We demonstrate the formation of mixed monolayers derived from a microwave-assisted reaction of alcohols with silicon oxide surfaces in order to tune their surface oleophobicity. This simple, rapid method provides an opportunity to precisely tune the constituents of the monolayers. As a demonstration, we sought fluorinated alcohols and aliphatic alcohols as reagents to form monolayers from two distinct constituents for tuning the surface oleophobicity. The first aspect of this study sought to identify a fluorinated alcohol that formed monolayers with a relatively high surface coverage. It was determined that 1H,1H,2H,2H-perfluoro-1-octanol yielded high quality monolayers with a water contact angle (WCA) value of ∼110° and contact angle values of ∼80° with toluene and hexadecane exhibiting both an excellent hydrophobicity and oleophobicity. Tuning of the oleophobicity of the modified silicon oxide surfaces was achieved by controlling the molar ratio of 1H,1H,2H,2H-perfluoro-1-octanol within the reaction mixtures. Surface oleophobicity progressively decreased with a decrease in the fluorinated alcohol content while the monolayers maintained their hydrophobicity with WCA values of ∼110°. The simple and reliable approach to preparing monolayers of a tuned composition that is described in this article can be utilized to control the fluorocarbon content of the hydrophobic monolayers on silicon oxide surfaces.

In situ probing the interior of single bacterial cells at nanometer scale

We report a novel approach to probe the interior of single bacterial cells at nanometre resolution by combining focused ion beam (FIB) and atomic force microscopy (AFM). After removing layers of pre-defined thickness in the order of 100 nm on the target bacterial cells with FIB milling, AFM of different modes can be employed to probe the cellular interior under both ambient and aqueous environments. Our initial investigations focused on the surface topology induced by FIB milling and the hydration effects on AFM measurements, followed by assessment of the sample protocols. With fine-tuning of the process parameters, in situ AFM probing beneath the bacterial cell wall was achieved for the first time. We further demonstrate the proposed method by performing a spatial mapping of intracellular elasticity and chemistry of the multi-drug resistant strain Klebsiella pneumoniae cells prior to and after it was exposed to the 'last-line' antibiotic polymyxin B. Our results revealed increased stiffness occurring in both surface and interior regions of the treated cells, suggesting loss of integrity of the outer membrane from polymyxin treatments. In addition, the hydrophobicity measurement using a functionalized AFM tip was able to highlight the evident hydrophobic portion of the cell such as the regions containing cell membrane. We expect that the proposed FIB-AFM platform will help in gaining deeper insights of bacteria-drug interactions to develop potential strategies for combating multi-drug resistance.

Influence of chain ordering on frictional properties of self-assembled monolayers (SAMs) in nano-lubrication

Adhesion of organic films to substrates is important in applications that involve solid surfaces in sliding contact. Although the thickness of self-assembled monolayers (SAMs) is only a few nanometers, they can drastically modify the frictional properties of the underlying substrate, and thus have great potential for serving as boundary lubricants on micro- and nano-scales. This review focuses on the relationship between the structural and compositional properties of SAMs and their frictional response. Adhesion of SAMs to the substrate surface usually occurs through chemisorption of the head groups on the constituent molecules, with molecular interactions such as van der Waals interactions playing important roles in organizing the molecules into surface films, and in controlling their tribological behavior. The durability and wear resistance of SAMs depend on the nature and strength of the binding forces between the head groups and the substrate surfaces, while the adhesion and friction forces are strongly influenced by the interactions of the terminal groups with the counterfaces. Results from both experimental measurements and molecular dynamics simulations consistently indicate that structural ordering of alkyl chains in SAMs reduces their frictional response, and that SAMs formed by molecules with alkyl chains longer than 8 to 10 methylene units are well organized, exhibiting low levels of friction. Less densely packed or more disordered monolayers inherently possess greater numbers of conformational defects at room temperature and present lower barriers to defect creation under the action of a contacting surface, and thus exhibit higher friction. Cross-linking of the spacer chains can reduce the frictional response of disordered films by increasing the chain ordering, but has little impact on SAMs that are already well ordered. On the other hand, introduction of sterically demanding terminal groups and dissimilar molecules reduces molecular ordering in SAMs and increases their frictional response. Significant growth in the application of SAMs to control the adhesion, friction, and wear of materials is expected with better understanding of the frictional properties of SAMs controlled by their structures and compositions and with technological progresses.

Effects of humidity and sample surface free energy on AFM probe-sample interactions and lateral force microscopy image contrast

Contrast between hydrophilic and hydrophobic domains and probe-sample adhesion forces as a function of relative humidity (RH) and sample surface free energy have been investigated using hydrophilic and hydrophobic atomic force microscopy (AFM) probes. For hydrophobic probes, the adhesion force is low, and the AFM image contrast between hydrophilic and hydrophobic domains is poor over the 0-93% RH. For hydrophilic probes, the image contrast between the hydrophilic and hydrophobic domains is poor at low RH but improved at high RH. This image contrast change is related to adhesion force differences between the two domains. In turn, the enhanced adhesion and image contrasts at elevated RH are attributed to capillary forces, which are large over the hydrophilic domains but greatly diminished over the hydrophobic domains. The adhesion force increases slightly with sample surface free energy at low RH, but increases rapidly with increasing sample surface free energy at high RH. The results indicate that for AFM in air, tailoring the RH of the probe-sample environment and utilizing a hydrophilic probe can enhance imaging of materials chemical heterogeneity with nanoscale spatial resolution.

Photoswitching Behavior of a Novel Single Molecular Tip for Noncontact Atomic Force Microscopy Designed for Chemical Identification

A tripod molecule with an azobenzene arm was designed as a single molecular tip for noncontact atomic force microscopy (NC-AFM). The azobenzene moiety showed photoisomerization that enabled measurements of the same position of the sample by different tip apexes with different interactions. Photoswitching behavior of the molecule synthesized and adsorbed on Au surfaces was examined and reversible switching between the trans- and cis forms was successfully confirmed by NC-AFM measurements.

Monitoring of glass derivatization with pulsed force mode atomic force microscopy

Non-specific adsorption of proteins at solid/liquid interfaces is a major problem in the use of synthetic biomaterials and in ultrasensitive detection methods. Grafting surfaces with a dense layer of poly(ethylene glycol) (PEG) or other polymers is a most widely used strategy to solve this task. While such modified surfaces have been characterized by their ability to resist protein adsorption, the polymer layers themselves have rarely been studied in fine detail. Atomic force microscopy (AFM) using the pulsed force mode (PFM), is an ideal technique to investigate structural features and physiochemical properties of surfaces because topology and adhesion are simultaneously detected with high lateral resolution. In the present study, PFM-AFM was applied to thoroughly characterize different stages of glass derivatization, up to the formation of a dense PEG layer. Lateral inhomogeneities in topology and/or adhesion were observed at all stages before PEG attachment. The covalently bound PEG, however, was seen to form a densely packed monolayer with maximal thickness, smooth surface, and weak adhesion. Thus, PFM-AFM appears to be a valuable tool for the characterization of protein-repelling surfaces in solution.

Topographic effects on adhesive force mapping of stretched DNA molecules by pulsed-force-mode atomic force microscopy

Adhesive interaction between a tip and a sample surface was examined on a microscopic scale by pulsed-force-mode atomic force microscopy (PFM-AFM). The signal measured by monitoring pull-off force is influenced by various factors such as topography, elasticity, electrostatic charges, and adsorbed water on surfaces. Here, we focus on the topographic effects on the adhesive interaction. To clarify the topographic influence, the adhesive force measurement of a stretched DNA molecule with a smaller radius of curvature than that of a tip was carried out at low relative humidity (RH) with an alkanethiol-modified tip. The experimental conditions such as low RH and the use of the alkanethiol-modified tip were required to minimise the influence of water capillary force on hydrated DNA strands. The hydrophobic modification of a substrate surface was also important to minimise the adsorbed water effect. The DNA molecules were stretched on the substrate surfaces by an immobilisation process called a dynamic molecular combing method. The two-component vapour-phase surface modification with an alkylsilane mixed with a silane derivative containing an amino end group enhanced the DNA adsorption due to the electrostatic interaction. The experimental results for the topographic effects on the adhesive force mapping were reproducible.

Imaging stretched single DNA molecules by pulsed-force-mode atomic force microscopy

The effect of a surface water layer on DNA strands deposited on a substrate was studied by atomic force microscopy (AFM). DNA molecules were deposited and stretched on chemically modified glass coverslips by a molecular combing method. Lambda bacteriophage DNA molecules were aligned on the organosilane-modified substrate surfaces by chemical and physical adsorption during the molecular combing. The combed DNA molecules were observed in humidity-controlled air and in aqueous solutions by pulsed-force-mode AFM (PFM-AFM). Chemical modification of cantilevers with an Au-coated tip by organothiol compounds was also applied to DNA observation. Mapping adhesive forces in aqueous media was useful to discriminate chemically the DNA strands from the substrate surface. The results suggest that PFM-AFM can be used widely to image the stretched DNA molecules on the silane-modified substrates.

A study of topographic effects on chemical force microscopy using adhesive force mapping

Origins of peak broadening in a histogram of measured adhesive forces were studied. The adhesive forces were measured in water by pulsed-force-mode atomic force microscopy. One sample was prepared by a microcontact printing method on a sputtered gold film with fine grains, on which CH(3)- and COOH-terminated regions were produced. Gold surfaces of other samples were chemically modified homogeneously by a self-assembling method in solution. Their surfaces were, however, topographically different, i.e. (i) an Au(111)-terrace-rich gold film prepared by vacuum vapor deposition at high temperature and (ii) sputtered gold films on cover glass with different grain sizes obtained by different deposition time. These sample surfaces and the probe tip surface were all CH(3)-terminated by self-assembled monolayers with CH(3)(CH(2))(19)SH. The main origin of peak broadening in the histogram was the topographic effect. Namely, the change in the grain sizes and the change in multiplicity of contacts between the tip and convexities of the grains resulted in the distribution of the observed adhesive forces.

Nanochemical surface analyzer in CMOS technology

We have developed an atomic force microscopy (AFM) cantilever system, fabricated using a standard CMOS process and a few post-processing steps, capable of detecting the difference between hydrophilic and hydrophobic samples for the purpose of nanochemical surface analysis. The fully integrated cantilever comprises a thermal actuator for cantilever deflection and a Wheatstone bridge to sense cantilever bending, thus obviating the need for cumbersome laser detection and external piezoelectric drives. Glass microspheres have been affixed to the cantilevers and, were either modified with a self-assembled monolayer to form hydrophobic tips, or left unmodified for hydrophilic tips. Force-distance curves have been used to measure the force between the functionalized/unfunctionalized tips and hydrophobic/hydrophilic sample surfaces. In an optimization step three different Wheatstone bridge sensors have been designed and characterized; best Wheatstone bridge sensitivity is 8.0 microV/nm with a 713 nm/mW actuator efficiency.

Investigation of nanotribological properties of self-assembled monolayers with alkyl and biphenyl spacer chains (invited)

Understanding the relationships between molecular structure and nanotribological performance of self-assembled monolayers (SAMs) are quite important for molecular tailoring for efficient lubrication. For this purpose, SAMs, having alkyl and biphenyl spacer chains with different surface terminal groups (-CH3, -COOH, and -OH), and head groups (-SH and -OH), were prepared. The influence of spacer chains, surface terminal groups, and head groups on adhesion, friction and wear properties were investigated by contact mode atomic force microscopy (AFM). The relative stiffness of SAMs was determined by force modulation mode AFM and indentation experiments using load-displacement curves. The friction properties of SAMs are explained using a molecular spring model in which local stiffness governs the friction properties. Micropatterned SAMs specimen were fabricated and studied to verify the molecular spring model. The influence of relative humidity, temperature and velocity on adhesion and friction was studied. The failure mechanisms of SAMs and substrates were investigated by wear and continuous microscratch AFM technique. Based on these studies, the adhesion, friction and wear mechanisms of SAMs at molecular scale are discussed.