Symbiotic assembly of peptide nano-mosaics at solid interfaces.

The spontaneous co-organization of distinct biomolecules at interfaces enables many of Nature's hierarchical organizations involving both hard and soft materials. Engineering efforts to mimic such hybrid complexes rely on our ability to rationally structure biomolecules at inorganic interfaces. Control over the nanoscale structure of patterned biomolecules remains challenging due to difficulties in controlling the multifarious interactions involved. This work discusses binary peptide assembly as a means to fabricate biomolecular nano-mosaics at graphite surfaces with predictable structures. Distinct peptide-substrate interactions lead to divergent crystallographic growth directions, molecular scale immiscibility, and a symbiotic assembly phenomenon. We present a symbiotic assembly model that accurately predicts the binary assembly structure relying solely on the constituent peptide nucleation kinetics and molar fractions. The ability to tune such biomolecular nano-mosaic structures facilitates the bottom up fabrication of high-density, multifunctional interfaces for nanotechnology.

Thermal Selection of Aqueous Molecular Conformations for Tailored Energetics of Peptide Assemblies at Solid Interfaces.

Key to the development of functional bioinorganic soft interfaces is the predictive control over the micron-scale assembly structure and energetics of biomolecules at solid interfaces. While assembly of labile biomolecules, such as short peptides, at interfaces is a great deal affected by the shape of the molecule, biomolecular conformations are prompted by external solution conditions, involving temperature, pH, and salt concentration. In this light, one can expect that the environmental conformational selection of aqueous biomolecules could potentially allow for fine-tuning of the equilibrium assembly structure at interfaces, as well as, the binding strength and molecular mobility within these assemblies. Here, we demonstrate the energetic and structural tailoring of two-dimensional surface assemblies of graphite-binding dodecapeptides, through the thermal selection of aqueous peptide conformations. Our findings based on a scanning probe energetic analysis, supplemented by molecular dynamics modeling, show that peptide-graphite and peptide-peptide intermolecular interactions strongly depend on the thermally selected molecular conformation and that the extent of the conformational change is directly related to the observed assembled structure. Enabled by these results was the design of a peptide with predictable binding and assembled structure, thus, suggesting environmental preconditioning of peptides as a means for controlling self-assembling active bioinorganic interfaces for bioelectronic implementations such as biomolecular fuel cells and biosensors.

Conformationally directed assembly of peptides on 2D surfaces mediated by thermal stimuli.

Dynamic and environmentally directed assembly of molecules in biological systems is essential for the fabrication of micronscale, hierarchical, functional structures. Here, we demonstrate the directed assembly of genetically selected graphite binding peptides on 2D solid surfaces upon thermal stimuli. Structural and kinetic analyses as well as molecular dynamics simulations yield the self-assembly process as thermally controllable upon tuning the solvated peptide conformational states. The ability to tailor the structure of two-dimensional soft bio/nano interfaces via external stimuli shows the potential for the bottom-up fabrication of complex materials with nanotechnological importance, such as biosensors, bioelectronics, and biomolecular fuel cells.

Communication: Local energetic analysis of the interfacial and surface energies of graphene from the single layer to graphite.

Recent advances in scanning probe methods that provide direct access to the surface free energy of inorganic layered materials in terms of the Hamaker constant yield energetic values for monolayer graphene that differ substantially, by a factor of around 0.4, from bulk graphite. The onset of bulk deviating energy values was observed at a multilayer slab thickness of ∼3 nm, corresponding to a layer number of 10. The findings, complemented with extractions from water contact angle measurements and calculated interlayer binding energies, find short-range ordinary van der Waals interactions to be most prominently affected by dimensional constraints and many-body interactions.

Direct determination of the local Hamaker constant of inorganic surfaces based on scanning force microscopy.

The energetics involved in the bonding fluctuations between nanometer-sized silicon dioxide (SiO2) probes and highly oriented pyrolytic graphite (HOPG) and molybdenum disulfide (MoS2) could be quantified directly and locally on the submicron scale via a time-temperature superposition analysis of the lateral forces between scanning force microscopy silicon dioxide probes and inorganic sample surfaces. The so-called "intrinsic friction analysis" (IFA) provided direct access to the Hamaker constants for HOPG and MoS2, as well as the control sample, calcium fluoride (CaF2). The use of scanning probe enables nanoscopic analysis of bonding fluctuations, thereby overcoming challenges associated with larger scale inhomogeneity and surface roughness common to conventional techniques used to determine surface free energies and dielectric properties. A complementary numerical analysis based on optical and electron energy loss spectroscopy and the Lifshitz quantum electrodynamic theory of van der Waals interactions is provided and confirms quantitatively the IFA results.

Nanoscale phase analysis of molecular cooperativity and thermal transitions in dendritic nonlinear optical glasses.

A broad nanoscopic study of a wide-range of dendritic organic nonlinear optical (NLO) self-assembly molecular glasses reveals an intermediate thermal phase regime responsible for both enhanced electric field poling properties and strong phase stabilization after poling. In this paper, the focus is on dendritic NLO molecular glasses involving quadrupolar, liquid crystal, and hydrogen bonding self-assembly mechanisms that, along with chromophore dipole-dipole interactions, dictate phase stability. Specifically, dendritic face-to-face interactions involving arene-perfluoroarene are contrasted to coumarin-containing liquid crystal mesogen and cinnamic ester hydrogen interactions. Both the strength of dendritic interactions and the impact of dipole fields on the relaxation behavior have been analyzed by nanoscale energetic probing and local thermal transition analysis. The presence of dendritic groups was found to fundamentally alter transition temperatures and the molecular relaxation behavior. Thermal transition analysis revealed that molecules with dendritic groups possess an incipient transition (T(1)) preceding the glass transition temperature (T(2)) that provides increased stability and a well-defined electric field poling regime (T(1) < T < T(2)), in contrast to molecular groups lacking dendrons that exhibit only single transitions. On the basis of enthalpic and entropic energetic analyses, thermally active modes below T(1) were found to be intimately connected to the dendron structure. Their corresponding activation energies, which are related to thermal stability, increased moving from cinnamic ester groups to coumarin moieties to arene-perfluoroarene interacting groups. While dendritic NLO materials were found to possess only enthalpic stabilization energies at temperatures relevant for device operation (T < T(1)), the apparent molecular binding energies above T(1) contain a substantial amount (up to ~80%) of cooperative entropic energy. The multiple interactions (from dipole-dipole interactions to local noncovalent dendritic interactions) are discussed and summarized in a model that describes the thermal transitions and phases.

Nano-engineering lattice dimensionality for a soft matter organic functional material.

A high performing electro-optic (EO) chromophore with covalently attached coumarin-based pendant groups exhibits intermolecular correlation of coumarin units through molecular dynamics (MD) simulations. Unique, orthogonal molecular orientations of the chromophore and coumarin units are also evident when investigated optically. Such molecular orientation translates to reduced lattice dimensionality of the bulk C1 soft matter material system, leading to increased acentric order and EO activity. Results are corroborated by nanorheological experimental methods.

Molecular friction dissipation and mode coupling in organic monolayers and polymer films.

The impact of thermally active molecular rotational and translational relaxation modes on the friction dissipation process involving smooth nano-asperity contacts has been studied by atomic force microscopy, using the widely known Eyring analysis and a recently introduced method, dubbed intrinsic friction analysis. Two distinctly different model systems, i.e., monolayers of octadecyl-phosphonic acid (ODPA) and thin films of poly(tert-butyl acrylate) (PtBA) were investigated regarding shear-rate critical dissipation phenomena originating from diverging mode coupling behaviors between the external shear perturbation and the internal molecular modes of relaxation. Rapidly (ODPA) versus slowly (PtBA) relaxing systems, in comparison to the sliding rate, revealed monotonous logarithmic and nonmonotonous spectral shear rate dependences, respectively. Shear coupled, enthalpic activation energies of 46 kJ∕mol for ODPA and of 35 and ∼65 kJ∕mol for PtBA (below and above the glass transition) were found that could be attributed to intrinsic modes of relaxations. Also, entropic energies involved in the cooperative backbone mobility of PtBA could be quantified, dwarfing the activation energy by more than a factor of five. This study provides (i) a material specific understanding of the molecular scale dissipation process in shear compliant substances, (ii) analyses of material intrinsic shear-rate mode coupling, shear coordination and energetics, (iii) a verification of Eyring's model applied to tribological systems toward material intrinsic specificity, and (iv) a valuable extension of the Eyring analysis for complex macromolecular systems that are slowly relaxing, and thus, exhibit shear-rate mode coupling.

Molecular mobility in self-assembled dendritic chromophore glasses.

Increasing complexity in bottom-up molecular designs of amorphous structures with multiple relaxation modes demands an integrated and cognitive design approach, where chemical synthesis is guided by both analytical tools and theoretical simulations. In particular, this is apparent for novel organic second-order nonlinear optical materials of self-assembling molecular glasses involving dendritic arene stabilization moieties (phenyl, naphthyl, and anthryl) with electro-optical activities above 300 pm/V. In this study, nanoscale thermo-mechanical analyses yield direct insight into the molecular enthalpic and entropic relaxation modes. Arene-perfluoroarene interactions for coarse self-assembly are found to impose three phase relaxation regimes, with intermediate regimes of 8-15 degrees C in width and apparent activation energies between 40 and 60 kcal/mol to be the most effective for poling. Energetic analyses based on intrinsic friction microscopy (IFA) identify increasing temporal stability with increasing arene size for the low-temperature regime. Electric field poling efficiency is found to be inversely proportional to entropic cooperative contributions that can make up 80% of the overall apparent relaxation energy for the high-temperature regime. The origin for the activation energies below the incipient glass transition temperature, based on complementary molecular dynamic simulations, is tied primarily to noncovalent interactions between chromophore (dipole), dendritic (quadrupole) moieties, and combinations thereof.

Intrinsic friction analysis--novel nanoscopic access to molecular mobility in constrained organic systems.

Condensed organic materials designed for nanotechnological applications are impacted significantly by internal and external constraints. Internal constraints are inherent to the molecular architecture, and generally result from direct bonding, electrostatic interactions, or steric effects. Internal constraints can be incorporated a priori into molecular designs, as a prescription for desired material properties. Today's challenge lies in obtaining information about the molecular mobility, in particular, in thin organic structured or unstructured films. A method that has been recently introduced, and is well suited for free surface analysis of complex organic thin film materials is intrinsic friction analysis (IFA), a spectroscopic analysis method based on scanning force microscopy (SFM). Here, we present a critical assessment of the method with practical illustrations towards the study of intramolecular and molecular mobilities in amorphous organic non-linear optical (NLO) materials, specifically self-assembling molecular organic glasses and complex organic polymer side-chain systems.

Cooperative and submolecular dissipation mechanisms of sliding friction in complex organic systems.

Energy dissipation in single asperity sliding friction was directly linked to submolecular modes of mobility by intrinsic friction analysis, involving time-temperature superposition along with thermodynamic stress and reaction rate models. Thereby, polystyrene served as a representative tribological sample for organic and amorphous complex systems. This study reveals the significance of surface and subsurface (alpha-, beta-, and gamma-) relaxational modes, which couple under appropriate external conditions (load, temperature, and rate) with shear induced disturbances, and thus gives rise to material specific frictional dissipation. At low pressures and temperatures below the glass transition point, the phenyl pendant side groups of polystyrene, known for their preferential orientation at the free surface, were noticed to be the primary channel for dissipation of kinetic sliding-energy. While this process was found to be truly enthalpic (activation energy of 8 kcalmol), energy dissipation was shown to possess both enthalpic and cooperative entropic contributions above the loading capacity of the surface phenyl groups (9.9 kcalmol) or above the glass transition. Apparent Arrhenius activation energies of frictional dissipation of 22 and 90 kcalmol, respectively, and cooperative contributions up to 80% were found. As such, this study highlights issues critical to organic lubricant design, i.e., the intrinsic enthalpic activation barriers of mobile linker groups, the evaluation of cooperative mobility phenomena, and critical tribological parameters to access or avoid coupling between shear disturbances and molecular actuators.

Interfacial mobility and bonding strength in nanocomposite thin film membranes.

The interfacial interaction strength and transition properties in a reverse selective thin film nanocomposite system, silica-poly[(trimethylsilyl)propyne] (SiO(x)-PTMSP), are investigated locally by heated tip atomic force microscopy. SiO(x)-PTMSP has recently been introduced as a new class of reverse selective membrane materials with extraordinarily high permeability and selectivity (reverse selectivity). Here, we examine the thermal transition properties of the polymer matrix and the debonding strength between PTMSP and silica. Transitions at 330 degrees C were identified as degradation processes. Criteria for debonding were found to include polymer viscoelastic responses, particle size, embedding depth, scan speed, and frequency of impact. Probe-particle impact forces revealed a debonding energy of 2.6 J/m(2) and an impact force transition that occurs 30 degrees C below the degradation temperature in the neat polymer, confirming the presence of enhanced polymer mobility at the SiO(x)-PTMSP interface.

Mesoscale dynamics and cooperativity of networking dendronized nonlinear optical molecular glasses.

First, molecular scale insight into the mobility of a novel class of organic materials for photonic applications with electro-optical activities larger than 300 pm/V is presented. A representative second order nonlinear optical (NLO) material of this class of self-assembling molecular glasses involving quadrupolar phenyl-perfluorophenyl (Ph-PhF) interactions is analyzed based on its molecular relaxation phenomena and phase behavior. Thereby, a new and straightforward nanoscale methodology, involving shear modulation force microscopy and intrinsic friction analysis is introduced. It provides both the submolecular enthalpic and entropic dynamics in nanoconstrained systems (e.g., ultrathin films), and thus, insight into local motion of single molecules due to dissociation of Ph-PhF pairs as well as the cooperative dynamics of the assembled network. This nanoscale model-independent thermomechanical methodology is shown to be very effective in fundamentally evaluating appropriate poling conditions of organic NLO materials. It promises to be a straightforward analysis tool to guide organic material synthesis from a molecular mobility perspective, particularly for applications that impose nanoscale constraints on the system.

Molecular dissipation phenomena of nanoscopic friction in the heterogeneous relaxation regime of a glass former.

Nanoscale sliding friction involving a polystyrene melt near its glass transition temperature Tg (373 K) exhibited dissipation phenomena that provide insight into the underlying molecular relaxation processes. A dissipative length scale that shows significant parallelism with the size of cooperatively rearranging regions (CRRs) could be experimentally deduced from friction-velocity isotherms, combined with dielectric loss analysis. Upon cooling to approximately 10 K above Tg, the dissipation length Xd grew from a segmental scale of approximately 3 A to 2.1 nm, following a power-law relationship with the reduced temperature Xd approximately TR-phi. The resulting phi=1.89+/-0.08 is consistent with growth predictions for the length scale of CRRs in the heterogeneous regime of fragile glass formers. Deviations from the power-law behavior closer to Tg suggest that long-range processes, e.g., the normal mode or ultraslow Fischer modes, may couple with the alpha relaxation, leading to energy dissipation in domains of tens of nanometers.

Interfacial glass transition profiles in ultrathin, spin cast polymer films.

Interfacial glass transition temperature (T(g)) profiles in spin cast, ultrathin films of polystyrene and derivatives were investigated using shear-modulated scanning force microscopy. The transitions were measured as a function of film thickness (delta), molecular weight, and crosslinking density. The T(g)(delta) profiles were nonmonotonic and exhibited two regimes: (a) a sublayer extending about 10 nm from the substrate, with T(g) values lowered up to approximately 10 degrees C below the bulk value, and (b) an intermediate regime extending over 200 nm beyond the sublayer, with T(g) values exceeding the bulk value by up to 10 degrees C. Increasing the molecular weight was found to shift the T(g)(delta) profiles further from the substrate interface, on the order of 10 nm/kDa. Crosslinking the precast films elevated the absolute T(g) values, but had no effect on the spatial length scale of the T(g)(delta) profiles. These results are explained in the context of film preparation history and its influence on molecular mobility. Specifically, the observed rheological anisotropy is interpreted based on the combined effects of shear-induced structuring and thermally activated interdiffusion.

Creeping friction dynamics and molecular dissipation mechanisms in glassy polymers.

The dissipation mechanism of nanoscale kinetic friction between an atomic force microscopy tip and a surface of amorphous glassy polystyrene has been studied as a function of two parameters: the scanning velocity and the temperature. Superposition of the friction results using the method of reduced variables revealed the dissipative behavior as an activated relaxation process with a potential barrier height of 7.0 kcal/mol, corresponding to the hindered rotation of phenyl groups around the C-C bond with the backbone. The velocity relationship with friction F(v) was found to satisfy simple fluctuation surface potential models with F proportional to const-ln(v) and F proportional to const-ln(v)2/3.

Effect of interfacial liquid structuring on the coherence length in nanolubrication.

The degree of interfacial structuring of n-hexadecane and octamethylcyclotetrasiloxane (OMCTS) was measured within a nanometer boundary regime to silicon surfaces. Boundary-layer effects on lubricating sliding (in terms of a thermodynamic stress activation parameter) and the layer thickness were determined by scanning force microscopy. A 2.0+/-0.3 nm thick, entropically cooled layer was found for n-hexadecane. Measurements on spherically shaped OMCTS molecules exhibited only an interfacial "monolayer," and identified the molecular shape of n-hexadecane responsible for augmented interfacial structuring. Interfacial liquid structuring was found to reduce friction.