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1.  Surface Confined Metallosupramolecular Architectures: Formation and STM Characterization 
Accounts of chemical research  2009;42(2):249-259.
CONSPECTUS
Metallosupramolecular compounds have attracted a great deal of attention over the past two decades largely because of their unique, highly-complex structural characteristics, the fact that they can be prepared with relative ease using coordination-driven self-assembly techniques, and their potential electronic, magnetic, optical, and catalytic properties. In particular, the use of electron-poor square planar Pt(II) transition metals in conjunction with rigid, electron-rich pyridyl donors had enabled the spontaneous self-assembly of a rich library of 2D metallacyclic and 3D metallacage assemblies via the directional-bonding approach. With the tremendous progress that has been made in the preparation and characterization of metallosupramolecules, much attention is now being turned toward fully exploring and developing their materials properties.
Assembling metallosupramolecular compounds on solid supports represents a vitally important step toward developing their materials properties. Surfaces provide a means of uniformly aligning and orienting these highly symmetric metallacycles and metallacages, which increases the level of coherence between molecules above that which can be achieved in the solution phase and affords a means of integrating adlayers into a solid-state materials setting. The dynamic nature of kinetically labile Pt(II)-N coordination bonds, however, requires that deposition and imaging conditions be appropriate to retain the assemblies' stability. Toward these aims it is imperative to understand the factors that govern surface self-assembly and the interactions that influence the structure and stability of the resulting adlayer. Such insight can be obtained from Scanning Tunneling Microscopy (STM), which has emerged as a powerful technique for the imaging and characterization of self-assembled adlayers.
This account describes the means by which 2D rectangular and square metallacycles and 3D trigonal bipyrimidal and chiral trigonal prism metallacages can be deposited on Highly Oriented Pyrolytic Graphite (HOPG) and Au(111) substrates such that the assemblies remain intact and result in ordered adlayers. The effects of varying the size, symmetry, and dimensionality of supramolecular adsorbates, the choice of substrate, the use of a molecular template, and the effects of chirality have been investigated. These systematic investigations provide much insight into the various adsorbate-adsorbate and substrate-adsorbate interactions that largely determine the architecture of each assembly and affect their performance in a materials setting. Exhibiting the ability to rationally control adlayer formation and structure will greatly enhance the potential of these supramolecules to be used in a variety of applications such as in host-guest sensing/diagnostic systems, molecular electronic devices, and in heterogeneous stereoselective synthesis and catalysis.
doi:10.1021/ar800117j
PMCID: PMC2654282  PMID: 19072706
2.  CeO2-based catalysts with engineered morphologies for soot oxidation to enhance soot-catalyst contact 
Nanoscale Research Letters  2014;9(1):254.
As morphology plays a relevant role in solid/solid catalysis, where the number of contact points is a critical feature in this kind of reaction, three different ceria morphologies have been investigated in this work as soot oxidation catalysts: ceria nanofibers, which can become organized as a catalytic network inside diesel particulate filter channels and thus trap soot particles at several contact points but have a very low specific surface area (4 m2/g); solution combustion synthesis ceria, which has an uncontrolled morphology but a specific surface area of 31 m2/g; and three-dimensional self-assembled (SA) ceria stars, which have both high specific surface area (105 m2/g) and a high availability of contact points. A high microporous volume of 0.03 cm3/g and a finer crystallite size compared to the other morphologies suggested that self-assembled stars could improve their redox cycling capability and their soot oxidation properties. In this comparison, self-assembled stars have shown the best tendency towards soot oxidation, and the temperature of non-catalytic soot oxidation has dropped from 614°C to 403°C in tight and to 552°C in loose contact conditions, respectively. As far as the loose contact results are concerned, this condition being the most realistic and hence the most significant, self-assembled stars have exhibited the lowest T10% onset temperature of this trio (even after ageing), thus proving their higher intrinsic activity. Furthermore, the three-dimensional shape of self-assembled stars may involve more of the soot cake layer than the solution combustion synthesis or nanofibers of ceria and thus enhance the total number of contact points. The results obtained through this work have encouraged our efforts to understand soot oxidation and to transpose these results to real diesel particulate filters.
doi:10.1186/1556-276X-9-254
PMCID: PMC4037285  PMID: 24940178
Soot oxidation; Diesel particulate filter; Ceria; Catalyst morphology
3.  The Self-Assembled Behavior of DNA Bases on the Interface 
A successful example of self-assembly in a biological system is that DNA can be an excellent agent to self-assemble into desirable two and three-dimensional nanostructures in a well-ordered manner by specific hydrogen bonding interactions between the DNA bases. The self-assembly of DNA bases have played a significant role in constructing the hierarchical nanostructures. In this review article we will introduce the study of nucleic acid base self-assembly by scanning tunneling microscopy (STM) at vacuum and ambient condition (the liquid/solid interface), respectively. From the ideal condition to a more realistic environment, the self-assembled behaviors of DNA bases are introduced. In a vacuum system, the energetic advantages will dominate the assembly formation of DNA bases, while at ambient condition, more factors such as conformational freedom and the biochemical environment will be considered. Therefore, the assemblies of DNA bases at ambient condition are different from the ones obtained under vacuum. We present the ordered nanostructures formed by DNA bases at both vacuum and ambient condition. To construct and tailor the nanostructure through the interaction between DNA bases, it is important to understand the assembly behavior and features of DNA bases and their derivatives at ambient condition. The utilization of STM offers the advantage of investigating DNA base self-assembly with sub-molecular level resolution at the surface.
doi:10.3390/ijms15021901
PMCID: PMC3958828  PMID: 24473140
DNA base; self-assembly; interface chemistry; scanning tunneling microscopy
4.  An Investigation of the Effects of Self-Assembled Monolayers on Protein Crystallisation 
Most protein crystallisation begins from heterogeneous nucleation; in practice, crystallisation typically occurs in the presence of a solid surface in the solution. The solid surface provides a nucleation site such that the energy barrier for nucleation is lower on the surface than in the bulk solution. Different types of solid surfaces exhibit different surface energies, and the nucleation barriers depend on the characteristics of the solid surfaces. Therefore, treatment of the solid surface may alter the surface properties to increase the chance to obtain protein crystals. In this paper, we propose a method to modify the glass cover slip using a self-assembled monolayer (SAM) of functional groups (methyl, sulfydryl and amino), and we investigated the effect of each SAM on protein crystallisation. The results indicated that both crystallisation success rate in a reproducibility study, and crystallisation hits in a crystallisation screening study, were increased using the SAMs, among which, the methyl-modified SAM demonstrated the most significant improvement. These results illustrated that directly modifying the crystallisation plates or glass cover slips to create surfaces that favour heterogeneous nucleation can be potentially useful in practical protein crystallisation, and the utilisation of a SAM containing a functional group can be considered a promising technique for the treatment of the surfaces that will directly contact the crystallisation solution.
doi:10.3390/ijms140612329
PMCID: PMC3709788  PMID: 23749116
protein crystallisation; self-assembled monolayer; methyl; sulfydryl; amino
5.  Controlling Self-Assembly of Engineered Peptides on Graphite by Rational Mutation 
ACS Nano  2012;6(2):1648-1656.
Self-assembly of proteins on surfaces is utilized in many fields to integrate intricate biological structures and diverse functions with engineered materials. Controlling proteins at bio-solid interfaces relies on establishing key correlations between their primary sequences and resulting spatial organizations on substrates. Protein self-assembly, however, remains an engineering challenge. As a novel approach, we demonstrate here that short dodecapeptides selected by phage display are capable of self-assembly on graphite and form long-range ordered biomolecular nanostructures. Using atomic force microscopy and contact angle studies, we identify three amino-acid domains along the primary sequence that steer peptide ordering and lead to nanostructures with uniformly displayed residues. The peptides are further engineered via simple mutations to control fundamental interfacial processes, including initial binding, surface aggregation and growth kinetics, and intermolecular interactions. Tailoring short peptides via their primary sequence offers versatile control over molecular self-assembly, resulting in well-defined surface properties essential in building engineered, chemically rich, bio-solid interfaces.
doi:10.1021/nn204631x
PMCID: PMC3304023  PMID: 22233341
molecular self-assembly; inorganic binding peptides; nanotechnology; sequence mutation; atomic force microscopy; molecular recognition; liquid-solid interface
6.  Solid supported lipid membranes: New concepts for the biomimetic functionalization of solid surfaces 
Biointerphases  2008;3(2):FA125.
Surface-layer (S-layer) supported lipid membranes on solid substrates are interfacial architectures mimicking the supramolecular principle of cell envelopes which have been optimized for billions of years of evolution in most extreme habitats. The authors implement this biological construction principle in a variety of layered supramolecular architectures consisting of a stabilizing protein monolayer and a functional phospholipid bilayer for the design and development of new types of solid-supported biomimetic membranes with a considerably extended stability and lifetime—compared to existing platforms—as required for novel types of bioanalytical sensors. First, Langmuir monolayers of lipids at the water/air interface are used as test beds for the characterization of different types of molecules which all interact with the lipid layers in various ways and, hence, are relevant for the control of the structure, stability, and function of supported membranes. As an example, the interaction of S-layer proteins from the bulk phase with a monolayer of a phospholipid synthetically conjugated with a secondary cell wall polymer (SCWP) was studied as a function of the packing density of the lipids in the monolayer. Furthermore, SCWPs were used as a new molecular construction element. The exploitation of a specific lectin-type bond between the N-terminal part of selected S-layer proteins and a variety of glycans allowed for the buildup of supramolecular assemblies and thus functional membranes with a further increased stability. Next, S-layer proteins were self-assembled and characterized by the surface-sensitive techniques, surface plasmon resonance spectroscopy and quartz crystal microbalance with dissipation monitoring. The substrates were either planar gold or silicon dioxide sensor surfaces. The assembly of S-layer proteins from solution to solid substrates could nicely be followed in-situ and in real time. As a next step toward S-layer supported bilayer membranes, the authors characterized various architectures based on lipid molecules that were modified by a flexible spacer separating the amphiphiles from the anchor group that allows for a covalent coupling of the lipid to a solid support, e.g., using thiols for Au substrates. Impedance spectroscopy confirmed the excellent charge barrier properties of these constructs with a high electrical resistance. Structural details of various types of these tethered bimolecular lipid membranes were studied by using neutron reflectometry. Finally, first attempts are reported to develop a code based on a SPICE network analysis program which is suitable for the quantitative analysis of the transient and steady-state currents passing through these membranes upon the application of a potential gradient.
doi:10.1116/1.2913612
PMCID: PMC2876326  PMID: 20408662
7.  Recent Advances in Colloidal and Interfacial Phenomena Involving Liquid Crystals 
This article describes recent advances in several areas of research involving the interfacial ordering of liquid crystals (LCs). The first advance revolves around the ordering of LCs at bio/chemically functionalized surfaces. Whereas the majority of past studies of surface-induced ordering of LCs have involved surfaces of solids that present a limited diversity of chemical functional groups (surfaces at which van der Waals forces dominate surface-induced ordering), recent studies have moved to investigate the ordering of LCs on chemically complex surfaces. For example, surfaces decorated with biomolecules (e.g. oligopeptides and proteins) and transition metal ions have been investigated, leading to an understanding of the roles that metal-ligand coordination interactions, electrical double-layers, acid-base interactions, and hydrogen bonding can have on the interfacial ordering of LCs. The opportunity to create chemically-responsive LCs capable of undergoing ordering transitions in the presence of targeted molecular events (e.g., ligand exchange around a metal center) has emerged from these fundamental studies. A second advance has focused on investigations of the ordering of LCs at interfaces with immiscible isotropic fluids, particularly water. In contrast to prior studies of surface-induced ordering of LCs on solid surfaces, LC- aqueous interfaces are deformable and molecules at these interfaces exhibit high levels of mobility and thus can reorganize in response to changes in interfacial environment. A range of fundamental investigations involving these LC-aqueous interfaces have revealed that (i) the spatial and temporal characteristics of assemblies formed from biomolecular interactions can be reported by surface-driven ordering transitions in the LCs, (ii) the interfacial phase behaviour of molecules and colloids can be coupled to (and manipulated via) the ordering (and nematic elasticity) of LCs, and (iii) confinement of LCs leads to unanticipated size-dependent ordering (particularly in the context of LC emulsion droplets). The third and final advance addressed in this article involves interactions between colloids mediated by LCs. Recent experiments involving microparticles deposited at the LC-aqueous interface have revealed that LC-mediated interactions can drive interfacial assemblies of particles through reversible ordering transitions (e.g., from one-dimensional chains to two-dimensional arrays with local hexagonal symmetry). In addition, recent single nanoparticle measurements suggest that the ordering of LCs about nanoparticles differs substantially from micrometer-sized particles and that the interactions between nanoparticles mediated by the LCs are far weaker than predicted by theory (sufficiently weak that the interactions are reversible and thus enable self-assembly). Finally, LC-mediated interactions between colloidal particles have also been shown to lead to the formation of colloid-in-LC gels that possess mechanical properties relevant to the design of materials to interface with living biological systems. Overall, these three topics serve to illustrate the broad opportunities that exist to do fundamental interfacial science and discovery-oriented research involving LCs.
doi:10.1021/la103301d
PMCID: PMC3089817  PMID: 21090596
8.  STM visualisation of counterions and the effect of charges on self-assembled monolayers of macrocycles 
Summary
Despite their importance in self-assembly processes, the influence of charged counterions on the geometry of self-assembled organic monolayers and their direct localisation within the monolayers has been given little attention. Recently, various examples of self-assembled monolayers composed of charged molecules on surfaces have been reported, but no effort has been made to prove the presence of counterions within the monolayer. Here we show that visualisation and exact localisation of counterions within self-assembled monolayers can be achieved with scanning tunnelling microscopy (STM). The presence of charges on the studied shape-persistent macrocycles is shown to have a profound effect on the self-assembly process at the liquid–solid interface. Furthermore, preferential adsorption was observed for the uncharged analogue of the macrocycle on a surface.
doi:10.3762/bjnano.2.72
PMCID: PMC3201620  PMID: 22043456
counterions; liquid–solid interface; macrocycles; scanning tunnelling microscopy; self-assembly
9.  Profile structures of the voltage-sensor domain and the voltage-gated K+-channel vectorially oriented in a single phospholipid bilayer membrane at the solid-vapor and solid-liquid interfaces determined by x-ray interferometry 
One subunit of the prokaryotic voltage-gated potassium ion channel from Aeropyrum pernix (KvAP) is comprised of six transmembrane α helices, of which S1–S4 form the voltage-sensor domain (VSD) and S5 and S6 contribute to the pore domain (PD) of the functional homotetramer. However, the mechanism of electromechanical coupling interconverting the closed-to-open (i.e., nonconducting-to-K+-conducting) states remains undetermined. Here, we have vectorially oriented the detergent (OG)-solubilized VSD in single monolayers by two independent approaches, namely “directed-assembly” and “self-assembly,” to achieve a high in-plane density. Both utilize Ni coordination chemistry to tether the protein to an alkylated inorganic surface via its C-terminal His6 tag. Subsequently, the detergent is replaced by phospholipid (POPC) via exchange, intended to reconstitute a phospholipid bilayer environment for the protein. X-ray interferometry, in which interference with a multilayer reference structure is used to both enhance and phase the specular x-ray reflectivity from the tethered single membrane, was used to determine directly the electron density profile structures of the VSD protein solvated by detergent versus phospholipid, and with either a moist He (moderate hydration) or bulk aqueous buffer (high hydration) environment to preserve a native structure conformation. Difference electron density profiles, with respect to the multilayer substrate itself, for the VSD-OG monolayer and VSD-POPC membranes at both the solid-vapor and solid-liquid interfaces, reveal the profile structures of the VSD protein dominating these profiles and further indicate a successful reconstitution of a lipid bilayer environment. The self-assembly approach was similarly extended to the intact full-length KvAP channel for comparison. The spatial extent and asymmetry in the profile structures of both proteins confirm their unidirectional vectorial orientation within the reconstituted membrane and indicate retention of the protein’s folded three-dimensional tertiary structure upon completion of membrane bilayer reconstitution. Moreover, the resulting high in-plane density of vectorially oriented protein within a fully hydrated single phospholipid bilayer membrane at the solid-liquid interface will enable investigation of their conformational states as a function of the transmembrane electric potential.
PMCID: PMC3246680  PMID: 22060407
10.  Glycosylated Self-Assembled Monolayers for Arrays and Surface Analysis 
Methods in molecular biology (Clifton, N.J.)  2012;808:10.1007/978-1-61779-373-8_6.
Over the past few decades, carbohydrates (glycans) have received growing attention for their many roles in biological systems, including pathogenesis, receptor-ligand interactions, and cell signaling. To unravel the biology of this important category of biomolecules, a host of new tools have been developed for glycomics investigation. At the forefront is the carbohydrate microarray, developed to immobilize functional glycans on a solid substrate to rapidly screen a variety of potential binding partners (carbohydrates, proteins, nucleic acids, cells, and viruses).
The essential role played by surface modification on glycan microarray performance requires new methods to rigorously characterize glycan surface chemistries. Due to their highly reproducible nature and well-studied properties, self-assembled monolayers (SAMs) on gold are powerful models for presenting glycans on a solid substrate, engineering biomimetic microenvironments and exploring the bioactivity of immobilized carbohydrates via surface plasmon resonance (SPR). However, it can be challenging to prepare high quality glycosylated SAMs (glyco-SAMs) that retain their biological function following surface immobilization. Herein, a selection of versatile methods for the preparation of glyco-SAMs using natural and chemically modified glycans is described. This chapter will highlight the following three immobilization techniques: (1) direct self assembly using thiolated glycosides onto gold, (2) tethering aminated glycosides onto amine-reactive SAMs, and (3) conjugating natural glycan onto divinyl sulfone-activated SAMs.
doi:10.1007/978-1-61779-373-8_6
PMCID: PMC3832357  PMID: 22057519
Surface plasmon resonance; Self-assembled monolayer; Glyco-SAM; Thiolated glycoside; Divinyl sulfone
11.  Nanoscale Protein Arrays of Rich Morphologies via Self-assembly on Chemically Treated Diblock Copolymer Surfaces 
Nanotechnology  2013;24(9):095601.
Well-controlled assembly of proteins on supramolecular templates of block copolymers can be extremely useful for high-throughput biodetection. We report the adsorption and assembly characteristics of a model antibody protein to various polystyrene-block-poly(4-vinylpyridine) templates whose distinctive nanoscale structures are obtained through time-regulated exposure to chloroform vapor. Strong adsorption preference of the protein to the polystyrene segment in the diblock copolymer templates leads to an easily predictable, controllable, rich set of nanoscale protein morphologies through self-assembly. We also demonstrate that the chemical identities of various subareas within individual nanostructures can be readily elucidated by investigating the corresponding protein adsorption behavior to each chemically distinct area of the template. In our approach, a rich set of intricate nanoscale morphologies of protein arrays that cannot be easily attained through other means can be generated straightforwardly via self-assembly of proteins on chemically treated diblock copolymer surfaces, without the use of clean room-based fabrication tools. Our approach provides much-needed flexibility and versatility for the use of block copolymer-based protein arrays in biodetection. The ease of fabrication in producing well-defined and self-assembled templates can contribute to a high degree of versatility and simplicity in acquiring intricate nanoscale geometry and spatial distribution of proteins in arrays. These advantages can be extremely beneficial both for fundamental research and biomedical detection, especially in the areas of solid-state based, high-throughput protein sensing.
doi:10.1088/0957-4484/24/9/095601
PMCID: PMC3600641  PMID: 23395956
protein assembly; protein adsorption; protein array; diblock copolymer; polymeric nanotemplate
12.  The Anther Steps onto the Stigma for Self-Fertilization in a Slipper Orchid 
PLoS ONE  2012;7(5):e37478.
Background
Due to the spatial separation between male and female pollen grains from the anther of most flowering plants, including orchids, pollens are transported by wind or animals and deposited onto the receptive surface of the stigma of a different plant. However, self-pollination is common in pollinating animal-scarce habitats. In such habitats, self-pollinations require the assistance of a pollinating agent (e.g., wind, gravity, or floral assembly) to transport the pollen grains from the anther onto its own stigma.
Methodology/Principal Findings
Based on observations on floral morphology and flowering phenology, tests of the breeding system, and a comparison of pollination mechanisms, a new self-pollination process was discovered in the hermaphroditic (i.e., possessing spatially separated male and female organs) flower of a slipper orchid, Paphiopedilum parishii. The anther changes from a solid to a liquid state and directly steps onto the stigma surface without the aid of any pollinating agent or floral assembly.
Conclusions
The mode of self-pollination discussed here is a new addition to the broad range of genetic and morphological mechanisms that have evolved in flowering plants to ensure their reproductive success. The present self-contained pollination mechanism is a possible adaptation to the insect-scarce habitat of the orchid.
doi:10.1371/journal.pone.0037478
PMCID: PMC3359306  PMID: 22649529
13.  Controlling the Surface Chemistry of Graphite by Engineered Self-Assembled Peptides 
Langmuir  2012;28(23):8589-8593.
The systematic control over surface chemistry is a long-standing challenge in biomedical and nanotechnological applications for graphitic materials. As a novel approach, we utilize graphite-binding dodecapeptides that self-assemble into dense domains to form monolayer thick long-range ordered films on graphite. Specifically, the peptides are rationally designed through their amino acid sequences to predictably display hydrophilic and hydrophobic characteristics while maintaining their self-assembly capabilities on the solid substrate. The peptides are observed to maintain a high tolerance for sequence modification, allowing the control over surface chemistry via their amino acid sequence. Furthermore, through a single step co-assembly of two different designed peptides, we predictably and precisely tune the wettability of the resulting functionalized graphite surfaces from 44 to 83 degrees. The modular molecular structures and predictable behavior of short peptides demonstrated here give rise to a novel platform for functionalizing graphitic materials that offers numerous advantages, including non-invasive modification of the substrate, bio-compatible processing in an aqueous environment, and simple fusion with other functional biological molecules.
doi:10.1021/la300268d
PMCID: PMC3374047  PMID: 22428620
14.  Surface induced nanofiber growth by self-assembly of a silk-elastinlike protein polymer 
Many synthetic and natural peptides are known to self-assemble to form various nanostructures such as nanofibers, hollow tubes, or ring-like structures. Some of the synthetic peptide molecules are specifically designed to produce well-defined nanostructures by controlling intermolecular interactions. Many environmental conditions such as salt concentration, pH, temperature, and surface characteristics influence intermolecular interactions, hence the process of the self-assembly. Here we studied self-assembly of a genetically engineered protein polymer composed of silk-like and elastin-like repeats on a mica surface. Silk-elastinlike protein polymers (SELPs) consist of tandem repeats of Gly-Ala-Gly-Ala-Gly-Ser from Bombyx mori (silkworm) and Gly-Val-Gly-Val-Pro from mammalian elastin. At a very low polymer concentration of 1 μg/ml, SELPs self-assembled into nanofibrous structures on a mica surface. Examination using atomic force microscopy (AFM) and dynamic light scattering techniques showed that SELPs self-assembled into nanofibers in the presence of the mica surface but not in the bulk state. Ionic strength had a significant influence on nanofiber growth, indicating the importance of electrostatic interactions between the polymer and the mica surface. At low ionic strength, the kinetics of nanofiber growth indicates that the mica surface effectively removed a lag phase by providing nucleating sites, facilitating nanofiber self-assembly of SELPs. Further examination of self-assembly on various surfaces such as silicon, positively charged surface, and hydrophobic surface revealed that negatively charged hydrophilic surface provides optimal surface to facilitate self-assembly of SELPs.
doi:10.1021/la9015993
PMCID: PMC2783466  PMID: 19803470
Silk-elastinlike protein polymer; Self-assembly; Fibrillization; Nanofiber; Amyloid fiber; Ionic strength; Growth kinetics; Atomic force microscopy; Dynamic light scattering
15.  Interfacial energetics of globular–blood protein adsorption to a hydrophobic interface from aqueous-buffer solution 
Adsorption isotherms of nine globular proteins with molecular weight (MW) spanning 10–1000 kDa confirm that interfacial energetics of protein adsorption to a hydrophobic solid/aqueous-buffer (solid–liquid, SL) interface are not fundamentally different than adsorption to the water–air (liquid–vapour, LV) interface. Adsorption dynamics dampen to a steady-state (equilibrium) within a 1 h observation time and protein adsorption appears to be reversible, following expectations of Gibbs' adsorption isotherm. Adsorption isotherms constructed from concentration-dependent advancing contact angles θa of buffered-protein solutions on methyl-terminated, self-assembled monolayer surfaces show that maximum advancing spreading pressure, Πamax, falls within a relatively narrow 10<Πamax<20mNm−1 band characteristic of all proteins studied, mirroring results obtained at the LV surface. Furthermore, Πa isotherms exhibited a ‘Traube-rule-like’ progression in MW similar to the ordering observed at the LV surface wherein molar concentrations required to reach a specified spreading pressure Πa decreased with increasing MW. Finally, neither Gibbs' surface excess quantities [Γsl−Γsv] nor Γlv varied significantly with protein MW. The ratio {[Γsl−Γsv]/Γlv}∼1, implying both that Γsv∼0 and chemical activity of protein at SL and LV surfaces was identical. These results are collectively interpreted to mean that water controls protein adsorption to hydrophobic surfaces and that the mechanism of protein adsorption can be understood from this perspective for a diverse set of proteins with very different composition.
doi:10.1098/rsif.2005.0087
PMCID: PMC1578746  PMID: 16849238
protein adsorption; surface; solid–water interface; blood proteins; air–water interface; interphase
16.  Two homologous genes coding for spore-specific proteins are expressed at different times during development of Myxococcus xanthus. 
Journal of Bacteriology  1985;163(1):121-125.
The ops and tps genes of Myxococcus xanthus have ca. 90% DNA and amino acid sequence homology and are in the same orientation separated by a spacer region of only 1.4 kilobases. The products of the two genes were found to cross-react immunologically, and both were capable of Ca2+-dependent self-assembly on the surface of myxospores. However, the ops and tps genes were expressed very differently during the developmental cycle of M. xanthus. The tps gene is induced early during fruiting body formation on a solid surface, and its product, protein S, is made in large quantities (up to 15% of total protein synthesis). When the cells turn into myxospores, protein S is assembled on the outer surface of the spore. We have now also found it in much smaller quantities inside the spores. The ops gene, on the other hand, appears to be induced later in development, after the cells have sporulated, since the ops gene product was found only inside the spores. When an ops gene under the control of a tps gene promoter was inserted into a wild-type strain, the ops gene product was synthesized at the same time as protein S and assembled onto the spore surface.
Images
PMCID: PMC219088  PMID: 3924890
17.  Asymmetric van der Waals Forces Drive Orientation of Compositionally Anisotropic Nanocylinders within Smectic Arrays: Experiment and Simulation 
ACS nano  2013;8(1):657-670.
Understanding how micro- and nanoparticles interact is important for achieving bottom-up assembly of desired structures. Here, we examine the self-assembly of two-component, compositionally asymmetric nanocylinders that sediment from solution onto a solid surface. These particles spontaneously formed smectic arrays. Within the rows of an array, nanocylinders tended to assemble such that neighboring particles had the same orientation of their segments. As a probe of interparticle interactions, we classified nanocylinder alignments by measuring the segment orientations of many sets of neighboring particles. Monte Carlo simulations incorporating an exact expression for the van der Waals (vdW) energy indicate that differences in the vdW interactions, even when small, are the key factor in producing observed segment alignment. These results point to asymmetrical vdW interactions as a potentially powerful means of controlling orientation in multicomponent cylinder arrays, and suggest that designing for these interactions could yield new ways to control self-assembly.
doi:10.1021/nn405312x
PMCID: PMC3926316  PMID: 24308771
Electrostatics; Self-Assembly; Monte Carlo; Nanowire; Nanotube; Computational; Hamaker
18.  Human Growth Hormone Adsorption Kinetics and Conformation on Self-Assembled Monolayers 
The adsorption process of the recombinant human growth hormone on organic films, created by self-assembly of octadecyltrichlorosilane, arachidic acid, and dipalmitoylphosphatidylcholine, is investigated and compared to adsorption on silica and methylated silica substrates. Information on the adsorption process of human growth hormone (hGH) is obtained by using total internal reflection fluorescence (TIRF). The intensity, spectra, and quenching of the intrinsic fluorescence emitted by the growth hormone’s single tryptophan are monitored and related to adsorption kinetics and protein conformation. For the various alkylated hydrophobic surfaces with differences in surface density and conformational freedom it is observed that the adsorbed amount of growth hormone is relatively large if the alkyl chains are in an ordered structure while the amounts adsorbed are considerably lower for adsorption onto less ordered alkyl chains of fatty acid and phospholipid layers. Adsorption on methylated surfaces results in a relatively large conformational change in the growth hormone’s structure, as displayed by a 7 nm blue shift in emission wavelength and a large increase in the effectiveness of fluorescence quenching. Conformational changes are less evident for hGH adsorption onto the fatty acid and phospholipid alkyl chains. Adsorption kinetics on the hydrophilic head groups of the self-assembled monolayers are similar to those on solid hydrophilic surfaces. The relatively small conformational changes in the hGH structure observed for adsorption on silica are even further reduced for adsorption on fatty acid head groups.
doi:10.1021/la970669s
PMCID: PMC4130235  PMID: 25125795
19.  In Situ Porous Structures: A Unique Polymer Erosion Mechanism in Biodegradable Dipeptide-based Polyphosphazene and Polyester Blends Producing Matrices for Regenerative Engineering 
Advanced functional materials  2010;20(17):2743-2957.
Synthetic biodegradable polymers serve as temporary substrates that accommodate cell infiltration and tissue in-growth in regenerative medicine. To allow tissue in-growth and nutrient transport, traditional three-dimensional (3D) scaffolds must be prefabricated with an interconnected porous structure. Here we demonstrated for the first time a unique polymer erosion process through which polymer matrices evolve from a solid coherent film to an assemblage of microspheres with an interconnected 3D porous structure. This polymer system was developed on the highly versatile platform of polyphosphazene-polyester blends. Co-substituting a polyphosphazene backbone with both hydrophilic glycylglycine dipeptide and hydrophobic 4-phenylphenoxy group generated a polymer with strong hydrogen bonding capacity. Rapid hydrolysis of the polyester component permitted the formation of 3D void space filled with self-assembled polyphosphazene spheres. Characterization of such self-assembled porous structures revealed macropores (10-100 μm) between spheres as well as micro- and nanopores on the sphere surface. A similar degradation pattern was confirmed in vivo using a rat subcutaneous implantation model. 12 weeks of implantation resulted in an interconnected porous structure with 82-87% porosity. Cell infiltration and collagen tissue in-growth between microspheres observed by histology confirmed the formation of an in situ 3D interconnected porous structure. It was determined that the in situ porous structure resulted from unique hydrogen bonding in the blend promoting a three-stage degradation mechanism. The robust tissue in-growth of this dynamic pore forming scaffold attests to the utility of this system as a new strategy in regenerative medicine for developing solid matrices that balance degradation with tissue formation.
doi:10.1002/adfm.201090073
PMCID: PMC3141818  PMID: 21789036
Polymeric materials; Composite materials; Thin films; Tissue engineering; Biomedical applications
20.  S-Layer Protein Self-Assembly 
Crystalline S(urface)-layers are the most commonly observed cell surface structures in prokaryotic organisms (bacteria and archaea). S-layers are highly porous protein meshworks with unit cell sizes in the range of 3 to 30 nm, and thicknesses of ~10 nm. One of the key features of S-layer proteins is their intrinsic capability to form self-assembled mono- or double layers in solution, and at interfaces. Basic research on S-layer proteins laid foundation to make use of the unique self-assembly properties of native and, in particular, genetically functionalized S-layer protein lattices, in a broad range of applications in the life and non-life sciences. This contribution briefly summarizes the knowledge about structure, genetics, chemistry, morphogenesis, and function of S-layer proteins and pays particular attention to the self-assembly in solution, and at differently functionalized solid supports.
doi:10.3390/ijms14022484
PMCID: PMC3587997  PMID: 23354479
S-layer; self-assembly; fusion protein; surface functionalization; nanobiotechnology
21.  Preparation and characterization of core–shell battery materials for Li-ion batteries manufactured by substrate induced coagulation 
Journal of Power Sources  2011;196(6-4):3290-3295.
Graphical abstract
Research highlights
▶ Core-shell battery material preparation using a dip-coating method. ▶ Substrate induced coagulation for battery materials. ▶ Solid-state reaction of titania on lithium cobalt oxide. ▶ Formation of inorganic layer studied by XRD, Rietveld analysis, XPS and SEM. ▶ Surface reaction has little effect on the content of electroactive material.
In this work Substrate Induced Coagulation (SIC) was used to coat the cathode material LiCoO2, commonly used in Li-ion batteries, with fine nano-sized particulate titania. Substrate Induced Coagulation is a self-assembled dip-coating process capable of coating different surfaces with fine particulate materials from liquid media. A SIC coating consists of thin and rinse-prove layers of solid particles. An advantage of this dip-coating method is that the method is easy and cheap and that the materials can be handled by standard lab equipment. Here, the SIC coating of titania on LiCoO2 is followed by a solid-state reaction forming new inorganic layers and a core–shell material, while keeping the content of active battery material high. This titania based coating was designed to confine the reaction of extensively delithiated (charged) LiCoO2 and the electrolyte. The core–shell materials were characterized by SEM, XPS, XRD and Rietveld analysis.
doi:10.1016/j.jpowsour.2010.11.043
PMCID: PMC3029556  PMID: 21415909
Dip-coating; Core–shell; Lithium cobalt oxide; Li-ion battery; Titania
22.  In vitro characterization of polycaprolactone matrices generated in aqueous media 
Acta biomaterialia  2009;6(3):1061-1068.
In this study, a novel process of dissolving polycaprolactone (PCL) matrices in glacial acetic acid was explored in which matrices spontaneously formed upon contact with water. Scanning electron microscopy analysis showed rough architecture and holes on the self-assembled matrix relative to matrices formed after dissolving in chloroform. Immersion in the gelatin solution reduced its roughness and number of micropores. Atomic force microscopy (AFM) analysis confirmed the increased roughness of the self-assembled matrices. The roughness of the matrices decreased after incubation in 1 N NaOH for 10 min. AFM analysis also revealed that the self-assembled matrix had a net positive surface charge, whereas chloroform–cast matrix had a negative surface charge. The surface charge of self-assembled matrix after immersion in gelatin changed to negative. However, incubation in NaOH did not affect the surface charge. The tensile properties were tested in both the dry state (25 °C) and the wet state (37 °C) by immersion in phosphate-buffered saline. Self-assembled matrix had lower elastic modulus, break stress and break strain than chloroform–cast matrix in both states. The elastic modulus in the wet condition was reduced by half in self-assembled matrix but tensile strain increased. Samples were further analyzed by ramp-hold test for assessing stress relaxation behavior. Both self-assembled and chloroform–cast matrices had similar trends in stress relaxation behavior. However, stress accumulation in self-assembled matrix was half that of chloroform–cast matrix. In vitro cell cultures were conducted using human foreskin fibroblast (HFF-1) in serum-free medium. Cytoskeletal actin staining showed cell adhesion and spreading on all matrices. Cell retention was significantly increased in self-assembled matrix compared to chloroform–cast matrix. Addition of gelatin improved the retention of seeded cells on the surface. In summary, PCL matrices generated using this novel technique show significant promise in biomedical applications.
doi:10.1016/j.actbio.2009.08.002
PMCID: PMC2997440  PMID: 19664731
Matrix; PCL; Stress relaxation; Roughness; Surface property
23.  Folding Control and Unfolding Free Energy of Yeast Iso-1-cytochrome c Bound to Layered Zirconium Phosphate Materials Monitored by Surface Plasmon Resonance 
The journal of physical chemistry. B  2008;112(30):9201-9208.
The free energy change (ΔG°) for the unfolding of immobilized yeast iso-1-cytochrome c (Cyt c) at nanoassemblies was measured by surface plasmon resonance (SPR) spectroscopy. Data show that SPR is sensitive to protein conformational changes, and protein solid interface exerts a major influence on bound protein stability. First, Cyt c was self-assembled on the Au film via the single thiol of Cys-102. Then, crystalline sheets of layered α-Zr(O3POH)2 · H2O (α-ZrP) or Zr(O3PCH2CH2COOH)2 · xH2O (α-ZrCEP) were adsorbed to construct α-ZrP/Cyt c/Au or α-ZrCEP/Cyt c/Au nanoassemblies. The construction of each layer was monitored by SPR, in real time, and the assemblies were further characterized by atomic force microscopy and electrochemical studies. Thermodynamic stability of the protein nanoassembly was assessed by urea-induced unfolding. Surprisingly, unfolding is reversible in all cases studied here. Stability of Cyt c in α-ZrP/Cyt c/Au increased by ~4.3 kJ/mol when compared to the unfolding free energy of Cyt c/Au assembly. In contrast, the protein stability decreased by ~1.5 kJ/mol for α-ZrCEP/Cyt c/Au layer. Thus, OH-decorated surfaces stabilized the protein whereas COOH-decorated surfaces destabilized it. These data quantitate the role of specific functional groups of the inorganic layers in controlling bound protein stability.
doi:10.1021/jp7121642
PMCID: PMC3490220  PMID: 18598069
24.  The Ferritin Superfamily: Supramolecular Templates for Materials Synthesis 
Biochimica et biophysica acta  2009;1800(8):834-845.
Members of the ferritin superfamily are multi-subunit cage-like proteins with a hollow interior cavity. These proteins possess three distinct surfaces, i.e. interior and exterior surfaces of the cages and interface between subunits. The interior cavity provides a unique reaction environment in which the interior reaction is separated from the external environment. In biology the cavity is utilized for sequestration of irons and biomineralization as a mechanism to render Fe inert and sequester it from the external environment. Material scientists have been inspired by this system and exploited a range of ferritin superfamily proteins as supramolecular templates to encapsulate nanoparticles and/or as well-defined building blocks for fabrication of higher order assembly. Besides the interior cavity, the exterior surface of the protein cages can be modified without altering the interior characteristics. This allows us to deliver the protein cages to a targeted tissue in vivo or to achieve controlled assembly on a solid substrate to fabricate higher order structures. Furthermore, the interface between subunits is utilized for manipulating chimeric self-assembly of the protein cages and in the generation of symmetry-broken Janus particles. Utilizing these ideas, the ferritin superfamily has been exploited for development of a broad range of materials with applications from biomedicine to electronics.
doi:10.1016/j.bbagen.2009.12.005
PMCID: PMC3763752  PMID: 20026386
Ferritin; Dps (DNA binding protein from nutrient starved cells); Nanoparticle; Multi-functionalities; Biomimetic chemistry; Janus particle; Bio-template
25.  DNA Hairpin Stabilization on a Hydrophobic Surface 
DNA hybridization in the vicinity of surfaces is a fundamental process for self-assembled nanoarrays, nanocrystal superlattices, and biosensors. It is widely recognized that solid surfaces alter molecular forces governing hybridization relative to bulk solution, and these effects can either favor or disfavor the hybridized state depending on the specific sequence and surface. Results presented here provide new insights into the dynamics of DNA hairpin-coil conformational transitions in the vicinity of hydrophilic oligo(ethylene glycol) (OEG) and hydrophobic trimethylsilane (TMS) surfaces. Single-molecule methods are used to observe the forward and reverse hybridization hairpin-coil transition of adsorbed species while simultaneously measuring molecular surface diffusion in order to gain insight into surface interactions with individual DNA bases. At least 35,000 individual molecular trajectories are observed on each type of surface. We find that unfolding slows and the folding rate increases on TMS relative to OEG despite stronger attractions between TMS and unpaired nucleobases. These rate differences lead to nearly complete hairpin formation on hydrophobic TMS and significant unfolding on hydrophilic OEG, resulting in the surprising conclusion that hydrophobic surface coatings are preferable for nanotechnology applications that rely on DNA hybridization near surfaces.
doi:10.1002/smll.201202335
PMCID: PMC3741999  PMID: 23184340
conformational change; molecular diffusion; poly(ethylene glycol); self-assembly; single-molecule tracking

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