Molecular self-assembly: smart design of surface and interface via secondary molecular interactions

On-Surface Molecular Recognition Driven by Chalcogen Bonding

Chalcogen bonding interactions (ChBIs) have been widely employed to create ordered noncovalent assemblies in solids and liquids. Yet, their ability to engineer molecular self-assembly on surfaces has not been demonstrated. Here, we report the first demonstration of on-surface molecular recognition solely governed by ChBIs. Scanning tunneling microscopy and ab initio calculations reveal that a pyrenyl derivative can undergo noncovalent chiral dimerization on the Au(111) surface through double Ch···N interactions involving Te- or Se-containing chalcogenazolo pyridine motifs. In contrast, reference chalcogenazole counterparts lacking the pyridyl moiety fail to form regular self-assemblies on Au, resulting in disordered assemblies.

Optimization of green and environmentally-benign synthesis of isoamyl acetate in the presence of ball-milled seashells by response surface methodology

Ball-milled seashells, as a nano-biocomposite catalyst and natural source of CaCO₃ in its aragonite microcrystalline form with fixed CO₂, was optimized for the synthesis of isoamyl acetate (3-methylbutyl ethanoate) by response surface methodology with a five-level three-factor rotatable circumscribed central composite design. The seashells nano-biocomposite has proved to be an excellent heterogeneous multifunctional catalyst for the green and environmentally-benign synthesis of isoamyl acetate from acetic acid and isoamyl alcohol under solvent-free conditions. A high yield of 91% was obtained under the following optimal conditions: molar ratio of alcohol: acetic acid (1:3.7), catalyst loading (15.7 mg), the reaction temperature (98 °C), and the reaction time (219 min). The outstanding advantages of this protocol are the use of an inexpensive, naturally occurring and easily prepared nano-biocomposite material having appropriate thermal stability and without any modifications using hazardous reagents, lower catalyst loading and reaction temperature, no use of corrosive Bronsted acids as well as toxic azeotropic solvents or water adsorbents, and simplicity of the procedure.

Biofunctionalization of Porous Titanium Oxide through Amino Acid Coupling for Biomaterial Design

Porous transition metal oxides are widely studied as biocompatible materials for the development of prosthetic implants. Resurfacing the oxide to improve the antibacterial properties of the material is still an open issue, as infections remain a major cause of implant failure. We investigated the functionalization of porous titanium oxide obtained by anodic oxidation with amino acids (Leucine) as a first step to couple antimicrobial peptides to the oxide surface. We adopted a two-step molecular deposition process as follows: self-assembly of aminophosphonates to titanium oxide followed by covalent coupling of Fmoc-Leucine to aminophosphonates. Molecular deposition was investigated step-by-step by Atomic Force Microscopy (AFM) and X-ray Photoemission Spectroscopy (XPS). Since the inherent high roughness of porous titanium hampers the analysis of molecular orientation on the surface, we resorted to parallel experiments on flat titanium oxide thin films. AFM nanoshaving experiments on aminophosphonates deposited on flat TiO₂ indicate the formation of an aminophosphonate monolayer while angle-resolved XPS analysis gives evidence of the formation of an oriented monolayer exposing the amine groups. The availability of the amine groups at the outer interface of the monolayer was confirmed on both flat and porous substrates by the following successful coupling with Fmoc-Leucine, as indicated by high-resolution XPS analysis.

Optomechanical controlling of intermolecular interaction and the application in molecular self-assembly

In this paper, we combined cavity optomechanics and quantum mechanical mechanism of van der Waals force to study the dynamic behavior of interacting bimolecules in the plasmonic localized field, and extend it to the interacting multi-molecular system. We explored how plasmonic optomechanical coupling affects the strength of intermolecular interactions. Based on our results, we propose to use optical field to modulate the intermolecular interaction potential in plasmonic cavity, which can be utilized in the enhancement of the efficiency of the molecular self-assembly process and controlling the yield of the reaction in an optical environment. This research extends molecular optomechanics from intramolecular interactions to intermolecular interactions and may has high application potential in some nanostructure synthesis.

Mechanism of periodic field driven self-assembly process

Dissipative self-assembly, a ubiquitous type of self-assembly in biological systems, has attracted a lot of attention in recent years. Inspired by nature, dissipative self-assembly driven by periodic external fields is often adopted to obtain controlled out-of-equilibrium steady structures and materials in experiments. Although the phenomena in dissipative self-assembly have been discovered in the past few decades, fundamental methods to describe dynamical self-assembly processes and responsiveness are still lacking. Here, we develop a theoretical framework based on the equations of motion and Floquet theory to reveal the dynamic behavior changing with frequency in the periodic external field driven self-assembly. Using the dissipative particle dynamics simulation method, we then construct a block copolymer model that can self-assemble in dilute solution to confirm the conclusions from the theory. Our theoretical framework facilitates the understanding of dynamic behavior in a periodically driven process and provides the theoretical guidance for designing the dissipative conditions.

Investigating Polymer Transformation during the Encapsulation of Metal Nanoparticles by Polystyrene-b-poly(acrylic acid) in Colloids

The colloidal self-assembly method holds great potential for large-scale synthesis at low expense of energy as compared to methods that assemble molecules by manipulating building blocks one after another. The development of the colloidal method, however, requires careful and intelligent design of the single building blocks as numerous degrees of freedom like isotropic nanoparticles (NPs) generally form highly repetitive, lattice-like structures or random aggregates upon self-assembly because of their identical surfaces throughout. Specifically, it is an interesting direction that if one can precisely control the localization of surface functionalities (i.e., ligands or polymer shells) on the NPs, a plethora of self-assembled structures (e.g., chains, sheets, rings, twisted, and even staircase structures) would be possible. Despite numerous simulations and modeling for this type of NPs, just a handful literature studies reported the controlling synthesis of metal-polymer patchy NPs through polymer shell shrinking/transformation in colloids. However, there are no detailed control experiments showing the mechanism of this polymer shell shrinking or transformation phenomenon. With the absence of a fundamental understanding of the driving forces and interactions between metal NP surface ligands and the hydrophobic polymer shell domain, simple and efficient design and synthesis of unique metal-polymer hybrid nanostructures are still obscure. Here, we report a detailed mechanistic study on the polymer shell transformation by using different types of surface ligands in encapsulation of metal NPs by polymer shells. The polymer shell transformation dynamic is studied after postheating treatment. The polymer shell transformation/shrinking on the metal NP surface depends on its surface ligand size being applied in the encapsulation step (polymer-ligand hydrophobic interaction effect). Longer-chain ligands provide stronger interactions between NPs and the hydrophobic domain of the polymer shell, which inhibits the polymer shell transformation. In contrast, short-chain ligands lead to weaker interactions, which assist in the polymer shell transformation. By understanding the underlying mechanisms, many new types of NPs, such as metal-polymer core-shell NPs, metal-polymer Janus NPs, silica-metal-polymer hybrid NPs, and silica-metal-polymer flower-like NPs have been synthesized for the first time. A new bottom-up platform for the synthesis of anisotropic NPs with the ability to control the patches in a precise manner has been created, which will benefit both nanotechnology (such as self-assembly in the nanoscale) and applications such as selective detection of the underlying ligands on the metal surface by using a surface-enhanced Raman spectrum study.

Amino Acid-Functionalized Polyelectrolyte Films as Bioactive Surfaces for Cell Adhesion

Surfaces were prepared with polyelectrolyte derivatives of poly(styrene- alt-maleic anhydride) (PSMA) functionalized with amino acids of different hydropathy indices, with the aim of evaluating the effect of the chemical functionality of polyelectrolytes on SH-SY5Y neuroblastoma cell adhesion. Functionalizing PSMA derivatives with l-glutamine, l-methionine, and l-tyrosine yielded PSMA-Gln, PSMA-Met, and PSMA-Tyr polyelectrolytes, respectively. We first studied the adsorption behavior of PSMA functionalized with amino acids on silicon wafer surfaces modified with 3-aminopropyltriethoxysilane at pH 4.0 and 7.0 and at low and high ionic strengths. The highest rate of polyelectrolyte adsorption was at pH 4.0 and high ionic strength and was higher with the glutamine and tyrosine films. The advance contact angles (θ_A) of the polyelectrolyte surfaces showed a moderate effect of ionic strength and pH on polyelectrolyte film wettability, with PSMA-Tyr being slightly more hydrophobic. Atomic force microscopy images of the polyelectrolyte surfaces showed two types of morphology: the well-defined globular nanostructure of PSMA-Met and PSMA-Tyr and densely packed nanofibrous-like structure of PSMA-Gln. The highest level of ionic strength caused a slight decrease in the size of the nanostructure that formed the surface domains, which was reflected in the degree of surface roughness. Cell adhesion assays with the polyelectrolyte film showed that SH-SY5Y neuroblastoma cells cultured on PSMA-Met present a well-extended morphology characterized by a stellate shape, with five or more actin-rich thin processes, whereas SH-SY5Y cells that were seeded on PSMA-Gln and PSMA-Tyr have a round morphology, with fewer and shorter processes. These results indicate that it is possible to modulate the surface characteristics of polyelectrolyte films based on their chemical functionality and environmental parameters such as pH and ionic strength in order to evaluate their effect on cell adhesion. Thus, surfaces prepared from polyelectrolytes functionalized with amino acids are an attractive and simple platform for cell adhesion, which can be used in developing biomaterials with modulated surface properties.

A review on versatile applications of blends and composites of CNC with natural and synthetic polymers with mathematical modeling

Cellulose is world's most abundant, renewable and recyclable polysaccharide on earth. Cellulose is composed of both amorphous and crystalline regions. Cellulose nanocrystals (CNCs) are extracted from crystalline region of cellulose. The most attractive feature of CNC is that it can be used as nanofiller to reinforce several synthetic and natural polymers. In this article, a comprehensive overview of modification of several natural and synthetic polymers using CNCs as reinforcer in respective polymer matrix is given. The immense activities of CNCs are successfully utilized to enhance the mechanical properties and to broaden the field of application of respective polymer. All the technical scientific issues have been discussed highlighting the recent advancement in biomedical and packaging field.

Surface-Mediated Supramolecular Self-Assembly of Protein, Peptide, and Nucleoside Derivatives: From Surface Design to the Underlying Mechanism and Tailored Functions

Among the many parameters that have been explored to exercise control over self-assembly processes, the influence of surface properties on self-assembly has been recognized as important but has received considerably less attention than other factors. This is particularly true for biomolecule-derived self-assembling molecules such as protein, peptide, and nucleobase derivatives. Because of their relevance to biomaterial and drug delivery applications, interest in these materials is increasing. As the formation of supramolecular structures from these biomolecule derivatives inevitably brings them into contact with the surfaces of surrounding materials, understanding and controlling the impact of the properties of these surfaces on the self-assembly process are important. In this feature article, we present an overview of the different surface parameters that have been used and studied for the direction of the self-assembly of protein, peptide, and nucleoside-based molecules. The current mechanistic understanding of these processes will be discussed, and potential applications of surface-mediated self-assembly will be outlined.

Surface-Guided Chemical Processes on Self-Assembled DNA Nanostructures

Solid-liquid interfaces have been of great significance in the activation of chemical reactions via restricting the conformation or orientation of the reactants. Self-assembled DNA nanostructures encoded with tremendous chemical and physical information provide an efficient platform to unravel and regulate mechanisms of surface chemical processes. In this review, we discuss the surface addressability, morphological features, and charged properties of DNA nanostructures as well as the recognition, catalytic, and dynamic properties of DNA molecules. We highlight the synergies between the surface properties of DNA nanostructures and the molecular features of DNA strands, which is a key to the synthesis of conductive polymer nanomaterials with well-defined shapes or electronic/optical properties. We also focus on the control over the substrate channeling pathways of enzyme networks or metal nucleation on DNA nanostructures toward the production of specifically emissive metal nanoclusters. In the end, we provide an outlook of future possible directions based on the rational design of DNA-based self-assembly, including dynamic energy transfer, stimuli-responsive synthesis, and programmable activation of the mechanophores on the surfaces of DNA nanostructures.

Amphiphilic peptides as novel nanomaterials: design, self-assembly and application

Designer self-assembling peptides are a category of emerging nanobiomaterials which have been widely investigated in the past decades. In this field, amphiphilic peptides have received special attention for their simplicity in design and versatility in application. This review focuses on recent progress in designer amphiphilic peptides, trying to give a comprehensive overview about this special type of self-assembling peptides. By exploring published studies on several typical types of amphiphilic peptides in recent years, herein we discuss in detail the basic design, self-assembling behaviors and the mechanism of amphiphilic peptides, as well as how their nanostructures are affected by the peptide characteristics or environmental parameters. The applications of these peptides as potential nanomaterials for nanomedicine and nanotechnology are also summarized.

On-Surface Self-Assembly of a C3 -Symmetric π-Conjugated Molecule Family Studied by STM: Two-Dimensional Nanoporous Frameworks

The on-surface self-assembled behavior of four C₃ -symmetric π-conjugated planar molecules (Tp, T12, T18, and Ex) has been investigated. These molecules are excellent building blocks for the construction of noncovalent organic frameworks in the bulk phase. Their hydrogen-bonded 2D on-surface self-assemblies are observed under STM at the solid/liquid interface; these structures are very different to those in the bulk crystal. Upon combining the results of STM measurements and DFT calculations, the formation mechanism of different assemblies is revealed; in particular, the critical role of hydrogen bonding in the assemblies. This research provides us with not only a deep insight into the self-assembled behavior of these novel functional molecules, but also a convenient approach toward the construction of 2D multiporous networks.

Kinetics of Surface-Driven Self-Assembly and Fatigue-Induced Disassembly of a Virus-Based Nanocoating

Self-assembling protein layers provide a "bottom-up" approach for precisely organizing functional elements at the nanoscale over a large solid surface area. The design of protein sheets with architecture and physical properties suitable for nanotechnological applications may be greatly facilitated by a thorough understanding of the principles that underlie their self-assembly and disassembly. In a previous study, the hexagonal lattice formed by the capsid protein (CA) of human immunodeficiency virus (HIV) was self-assembled as a monomolecular layer directly onto a solid substrate, and its mechanical properties and dynamics at equilibrium were analyzed by atomic force microscopy. Here, we use atomic force microscopy to analyze the kinetics of self-assembly of the planar CA lattice on a substrate and of its disassembly, either spontaneous or induced by materials fatigue. Both self-assembly and disassembly of the CA layer are cooperative reactions that proceed until a phase equilibrium is reached. Self-assembly requires a critical protein concentration and is initiated by formation of nucleation points on the substrate, followed by lattice growth and eventual merging of CA patches into a continuous monolayer. Disassembly of the CA layer showed hysteresis and appears to proceed only after large enough defects (nucleation points) are formed in the lattice, whose number is largely increased by inducing materials fatigue that depends on mechanical load and its frequency. Implications of the kinetic results obtained for a better understanding of self-assembly and disassembly of the HIV capsid and protein-based two-dimensional nanomaterials and the design of anti-HIV drugs targeting (dis)assembly and biocompatible nanocoatings are discussed.

Surface modification of PEN and PET substrates by plasma treatment and layer-by-layer assembly of polyelectrolyte multilayer thin films and their application in electroless deposition

Surface modification of PEN and PET substrates by plasma-treatment and LbL assembly of polyelectrolyte multilayers and subsequent electroless nickel deposition.

Synchronous Generation of Nano- and Microscaled Hierarchical Porous Polyelectrolyte Multilayers for Superwettable Surfaces

We created both a superhydrophilic polymer surface and a superhydrophobic surface by using the poly(acrylic acid) (PAA)/poly(allylamine hydrochloride) (PAH) multilayers with the synchronously generated hierarchical porous surface structures. The formation of surface and pore structures induced at acidic pH values is subject to the composition, distribution, and molecular weights of polyelectrolytes in the layer-by-layer (LbL) assembled film, leading to a variety of unique surface topographies and porous structures located on different scales. During the porous induction at pH 2.0, both nano- and microscaled features synchronously developed on the surface as a result of the unique combination of high-molecular-weight PAH (900K g/mol) and low molecular weight PAA (15K g/mol), along with a much reduced deposition time of 1 min. Although thermally cross-linked, the porous surface with hierarchical structure could achieve superhydrophilicity due to the remaining free amine and carboxylate groups on the porous structures. A complete switch from the superhydrophilic to the superhydrophobic surface was achieved via a simple chemical vapor deposition of trichloro(1H,1H,2H,2H-perfluoro-octyl)silane. In this work, the effects of molecular weight of polyelectrolytes (15K-900K g/mol), deposition time (10-900 s) during the LbL assembly, and pH (1.8 to 2.4) for the porous induction on the surface topography, pore structures, and wetting behavior were investigated in detail. A variety of unique porous surface structures on different length scales were systematically studied by controlling the above parameters.

Development of Layered Multiscale Porous Thin Films by Tuning Deposition Time and Molecular Weight of Polyelectrolytes

This work focuses on the design of porous polymeric films with nano- and micro-sized pores existing in distinct zones. The porous thin films are fabricated by the post-treatment of layer-by-layer assembled poly(allylamine hydrochloride) (PAH)/poly(acrylic acid) (PAA) multilayers. In order to improve the processing efficiency, the deposition time is shortened to ≈ 10 s. It is found that fine porous structures can be created even by significantly reducing the processing time. The effect of using polyelectrolytes with widely different molecular weights is also studied. The pore size is increased by using high molecular weight PAH, while high molecular weight PAA minimizes the pore size to nanometer scale. Having gained a precise control over the pore size, layered multiscale porous thin films are further built up with either a microsized porous zone on top of a nanosized porous zone or vice versa.

Hydrogen-Bonded Multilayer Films Based on Poly(N-vinylamide) Derivatives and Tannic Acid

Layer-by-layer (LbL) assembly based on hydrogen-bonding interactions is generating great interest for biomedical applications because it is composed of neutral polymers, while LbL assembly based on electrostatic interaction requires polycations which may induce toxicity issues. As a neutral polymer, poly(N-vinylamide), which has low toxicity compared to poly(acrylamide), has the potential to fabricate LbL thin films via hydrogen-bonding interactions. Herein we report interpolymer complexes of poly(N-vinylamide)s and natural polyphenol tannic acid to form the multilayered thin film. Poly(N-vinylformamide) and poly(N-vinylacetamide), which are water-soluble and insoluble in acetonitrile, could not form complexes with TA in water. On the other hand, N-alkylated poly(N-vinylamide) such as poly(N-ethyl-N-vinylformamide) and poly(N-methyl-N-vinylacetamide) was soluble in acetonitrile and allowed the LbL assembly to proceed with TA. Furthermore, the QCM frequency shift with films composed of poly(N-ethyl-N-vinylformamide) and TA were stable in water, while those of poly(N-methyl-N-vinylacetamide) and TA were instable in water, possibly because formamide has lower steric hindrance compared to acetamide to allow stronger hydrogen-bonding interactions to take place. Thus, LbL assembly reactions with alkylated poly(N-vinylamide)s and TA were investigated and revealed that poly(N-ethyl-N-formamide) and TA, which are water-soluble, effectively interacted with one another to generate water-stable hydrogen-bonded multilayered films.

Multi-Ligand-Binding Flavoprotein Dodecin as a Key Element for Reversible Surface Modification in Nano-biotechnology

In this paper the multiple (re)programming of protein-DNA nanostructures comprising generation, deletion, and reprogramming on the same flavin-DNA-modified surface is introduced. This work is based on a systematic study of the binding affinity of the multi-ligand-binding flavoprotein dodecin on flavin-terminated DNA monolayers by surface plasmon resonance and quartz crystal microbalance with dissipation (QCM-D) measurements, surface plasmon fluorescence spectroscopy (SPFS), and dynamic AFM force spectroscopy. Depending on the flavin surface coverage, a single apododecin is captured by one or more surface-immobilized flavins. The corresponding complex binding and unbinding rate constants kon(QCM) = 7.7 × 10(3) M(-1)·s(-1) and koff(QCM) = 4.5 × 10(-3) s(-1) (Kd(QCM) = 580 nM) were determined by QCM and were found to be in agreement with values for koff determined by SPFS and force spectroscopy. Even though a single apododecin-flavin bond is relatively weak, stable dodecin monolayers were formed on flavin-DNA-modified surfaces at high flavin surface coverage due to multivalent interactions between apododecin bearing six binding pockets and the surface-bound flavin-DNA ligands. If bi- or multivalent flavin ligands are adsorbed on dodecin monolayers, stable sandwich-type surface-DNA-flavin-apododecin-flavin ligand arrays are obtained. Nevertheless, the apododecin flavin complex is easily and quantitatively disassembled by flavin reduction. Binding and release of apododecin are reversible processes, which can be carried out alternatingly several times to release one type of ligand by an external redox trigger and subsequently replace it with a different ligand. Hence the versatile concept of reprogrammable functional biointerfaces with the multi-ligand-binding flavoprotein dodecin is demonstrated.

Hierarchical hydrodynamic flow confinement: efficient use and retrieval of chemicals for microscale chemistry on surfaces

We devised, implemented, and tested a new concept for efficient local surface chemistry that we call hierarchical hydrodynamic flow confinement (hierarchical HFC). This concept leverages the hydrodynamic shaping of multiple layers of liquid to address challenges inherent to microscale surface chemistry, such as minimal dilution, economical consumption of reagent, and fast liquid switching. We illustrate two modes of hierarchical HFC, nested and pinched, by locally denaturing and recovering a 26 bp DNA with as little as 2% dilution and by efficiently patterning an antibody on a surface, with a 5 μm resolution and a 100-fold decrease of reagent consumption compared to microcontact printing. In addition, valveless switching between nanoliter volumes of liquids was achieved within 20 ms. We believe hierarchical HFC will have broad utility for chemistry on surfaces at the microscale.

Robust and flexible free-standing films for unidirectional drug delivery

Robust and flexible free-standing polymer films for unidirectional drug delivery are fabricated by sandwiching drug-containing polyelectrolyte multilayer films between poly(lactic-co-glycolic acid) (PLGA) barrier and capping layers. The drug-containing films are fabricated by layer-by-layer (LbL) assembly of chemically cross-linked poly(allylamine hydrochloride)-dextran (PAH-D) microgel and hyaluronic acid (HA), which can load negatively charged cancer-inhibiting drug, methotrexate (MTX). Because the PLGA barrier layer effectively blocks MTX release, MTX can be predominantly released from the PLGA capping layer of the free-standing film. This increases the efficacy of released MTX to cancer cells while minimizing its side effects on the normal tissues. We believe that the unidirectional drug delivery free-standing films can open a new avenue to design of highly efficient drug delivery systems for biomedical application.