Dynamic variations in adhesion of self-assembled monolayers on nanoasperities probed by atomic force microscopy

Utilizing atomistic simulations to map pressure distributions and contact areas in molecular adlayers within nanoscale surface-asperity junctions: a demonstration with octadecylsilane-functionalized silica interfaces

To achieve a better understanding of the mechanical effects of adsorbed films at surface contacts, methods were developed to map and examine the pressure distribution of nanoasperity contacts, modeled by molecular dynamics simulation. The methods employ smoothing functions to project the atomic forces obtained in contact simulation onto the contact plane for fitting to standard continuum contact models and subsequent analysis. Importantly, these methods allow for contact evolution between nanoscopic asperity-asperity contacts to be examined because these are the central load-bearing junctions at interfaces. To demonstrate the application and features of this approach, it was employed to examine the evolution of contact between silica nanoasperities, with an increasing density of octadecyltrichlorosilane (OTS) films employed as a model adsorbate film. Linearly increasing contact radius and linearly decreasing maximal pressure were observed as a function of the film packing density. Because contact between the underlying, high-energy silica surfaces is undesirable, the evolution of silica contact was also examined using these same methods. As more molecules were introduced into the contact, a sharp transition was observed from the narrow, high-pressure interaction between the underlying substrates, to a broad, substantially lower pressure interaction, indicating a sharp transition from the dry to lubricated condition. To study the dependence of these behaviors on contact morphology, silica nanoasperities in contact with a flat silica surface were also examined. Similar behavior, including the broadening of the contact area and the minimization of direct surface contact, were observed. The method developed herein is applicable to a variety of systems and can be employed to optimize surface protection and pressure redistribution by boundary lubricants. This method can also be extended to AFM adhesion measurements where a detailed understanding of the true contact area is critical for the quantitative determinations of molecular forces and local surface mechanics.

Molecular tailoring of interfacial failure

Self-assembled monolayers (SAMs) provide an enabling platform for molecular tailoring of the chemical and physical properties of an interface. In this work, we systematically vary SAM end-group functionality and quantify the corresponding effect on interfacial failure between a transfer printed gold (Au) film and a fused silica substrate. SAMs with four different end groups are investigated: 11-amino-undecyltriethoxysilane (ATES), dodecyltriethoxysilane (DTES), 11-bromo-undecyltrimethoxysilane (BrUTMS), and 11-mercapto-undecyltrimethoxysilane (MUTMS). In addition to these four end groups, mixed monolayers of increasing molar ratio of MUTMS to DTES in solution are investigated. The failure of each SAM-mediated interface is initiated by a noncontact laser-induced spallation method at strain rates in excess of 10(6) s(-1). By making multiple measurements at increasing stress amplitudes (controlled by the laser fluence), we measure interface strengths of 19 ± 1.7, 20 ± 1.3, 52 ± 5.4, and 80 ± 6.5 MPa for interfaces functionalized with ATES, DTES, BrUTMS, and MUTMS, respectively. The interface strength is effectively tuned between the low strength of DTES and the high strength of MUTMS by controlling the concentration of MUTMS in solution. X-ray photoelectron spectroscopy of the failed interfaces reveals the influence of end group functionality on molecular dissociation, which significantly alters the failure process.

Bioinspired catechol-terminated self-assembled monolayers with enhanced adhesion properties

The role of the catechol moiety in the adhesive properties of mussel proteins and related synthetic materials has been extensively studied in the last years but still remains elusive. Here, a simplified model approach is presented based on a self-assembled monolayer (SAM) of upward-facing catechols thiol-bound to epitaxial gold substrates. The orientation of the catechol moieties is confirmed by spectroscopy, which also showed lack of significant amounts of interfering o-quinones. Local force-distance curves on the SAM measured by atomic force microscopy (AFM) shows an average adhesion force of 45 nN, stronger than that of a reference polydopamine coating, along with higher reproducibility and less statistical dispersion. This is attributed to the superior chemical and topographical homogeneity of the SAM coating. Catechol-terminated SAMs are also obtained on high-roughness gold substrates that show the ability to assemble magnetic nanoparticles, despite their lack of enhanced adhesion at the molecular level. Finally, the influence of the catechol group on the formation and quality of the SAM is explored both theoretically (molecular dynamics simulations) and experimentally using direct-write AFM lithography.

The role of substrate interactions in the modification of surface forces by self-assembled monolayers

Detailed pressure and strain mapping of atomistic contact simulations elucidate the mechanical and tribochemical mechanisms of surface force modification with SAMs.

Effects of probe pollutants on morphological and mechanical measurements of muscle and collagen fibers using atomic force microscopy

Because of the interaction between probes and samples, pollutants in buffer solution or in the air would easily bind to probes and make the probe polluted, which might influence the morphological and mechanical measurements with atomic force microscopy. The polluted probes might transfer the pollutants onto the samples and thus change the surface ultrastructure of samples, or collect the deviated feedback signals to make the phantasm images. The former process is irreversible even if a new probe is employed, and the latter one is a reversible process as long as changing the used/polluted probe. Effects of polluted probes on morphological and mechanical characteristics of insect flight muscle and rat tail tendon collagen I fibers had been discussed in this study, in which, we constructed a series of methods to avoid/reduce the collecting of phantasm images and deviated mechanical information, such as changing the scanning direction and scanning force, replacing the new probes, or cleaning the polluted probes.

Self-organized porphyrinic materials

The self-assembly and self-organization of porphyrins and related macrocycles enables the bottom-up fabrication of photonic materials for fundamental studies of the photophysics of these materials and for diverse applications. This rapidly developing field encompasses a broad range of disciplines including molecular design and synthesis, materials formation and characterization, and the design and evaluation of devices. Since the self-assembly of porphyrins by electrostatic interactions in the late 1980s to the present, there has been an ever increasing degree of sophistication in the design of porphyrins that self-assemble into discrete arrays or self-organize into polymeric systems. These strategies exploit ionic interactions, hydrogen bonding, coordination chemistry, and dispersion forces to form supramolecular systems with varying degrees of hierarchical order. This review concentrates on the methods to form supramolecular porphyrinic systems by intermolecular interactions other than coordination chemistry, the characterization and properties of these photonic materials, and the prospects for using these in devices. The review is heuristically organized by the predominant intermolecular interactions used and emphasizes how the organization affects properties and potential performance in devices.

Gloss phenomena and image analysis of atomic force microscopy in molecular and cell biology

Proper sample preparation, scan setup, data collection and image analysis are key factors in successful atomic force microscopy (AFM), which can avoid gloss phenomena effectively from unreasonable manipulations or instrumental defaults. Fresh cleaved mica and newly treated glass cover were checked first as the substrates for all of the sample preparation for AFM. Then, crystals contamination from buffer was studied separately or combined with several biologic samples, and the influence of scanner, scan mode and cantilever to data collection was also discussed intensively using molecular and cellular samples. At last, images treatment and analysis with off-line software had been focused on standard and biologic samples, and artificial glosses were highly considered for their high probability. SCANNING 31: 49-58, 2009. (c) 2009 Wiley Periodicals, Inc.

Nano-indentation at the surface contact level: applying a harmonic frequency for measuring contact stiffness of self-assembled monolayers adsorbed on Au

In this study, the well-ordered alkanethiolate self-assembled monolayers (SAMs) of varied chain lengths and tail groups were employed as examples for nano-characterization on their mechanical properties. A novel nano-indentation technique with a constant harmonic frequency was applied on SAMs chemically adsorbed on Au to explore their contact mechanics, and furthermore to interpret how SAM molecules respond to an infinitesimal oscillation force without pressing them. Experimental results demonstrated that the harmonic contact stiffness along with the measured displacement of SAMs/Au was distinguishable using a dynamic contact modulus with the distinct feature of phase angles. Phase angles resulted from the relaxing continuation of an applied harmonic frequency and mostly influenced by the outermost tail group of SAM molecules. The harmonic contact stiffness of SAM molecules obviously increased with the densely packed alkyl chains and relatively intense agglomeration of the head group at the anchoring site. As a consequence, the result of this work is relevant to contact mechanics at the surface contact level for the distinction of molecular substances attached on a solid surface. Furthermore it is particularly anticipated to identify biological molecules of variable qualities under a fluid-like micro-environment.