Functional nanostructures from surface chemistry patterning

Biological responses to physicochemical properties of biomaterial surface

Biomedical scientists use chemistry-driven processes found in nature as an inspiration to design biomaterials as promising diagnostic tools, therapeutic solutions, or tissue substitutes. While substantial consideration is devoted to the design and validation of biomaterials, the nature of their interactions with the surrounding biological microenvironment is commonly neglected. This gap of knowledge could be owing to our poor understanding of biochemical signaling pathways, lack of reliable techniques for designing biomaterials with optimal physicochemical properties, and/or poor stability of biomaterial properties after implantation. The success of host responses to biomaterials, known as biocompatibility, depends on chemical principles as the root of both cell signaling pathways in the body and how the biomaterial surface is designed. Most of the current review papers have discussed chemical engineering and biological principles of designing biomaterials as separate topics, which has resulted in neglecting the main role of chemistry in this field. In this review, we discuss biocompatibility in the context of chemistry, what it is and how to assess it, while describing contributions from both biochemical cues and biomaterials as well as the means of harmonizing them. We address both biochemical signal-transduction pathways and engineering principles of designing a biomaterial with an emphasis on its surface physicochemistry. As we aim to show the role of chemistry in the crosstalk between the surface physicochemical properties and body responses, we concisely highlight the main biochemical signal-transduction pathways involved in the biocompatibility complex. Finally, we discuss the progress and challenges associated with the current strategies used for improving the chemical and physical interactions between cells and biomaterial surface.

Photothermal laser fabrication of micro- and nanostructured chemical templates for directed protein immobilization

Photothermal patterning of poly(ethylene glycol) terminated organic monolayers on surface-oxidized silicon substrates is carried out using a microfocused beam of a CW laser operated at a wavelength of 532 nm. Trichlorosilane and trimethoxysilane precursors are used for coating. Monolayers from trimethoxysilane precursors show negligible unspecific protein adsorption in the background, i.e., provide platforms of superior protein repellency. Laser patterning results in decomposition of the monolayers and yields chemical templates for directed immobilization of proteins at predefined positions. Characterization is carried out via complementary analytical methods including fluorescence microscopy, atomic force microscopy, and scanning electron microscopy. Appropriate labeling techniques (fluorescent markers and gold clusters) and substrates (native and thermally oxidized silicon substrates) are chosen in order to facilitate identification of protein adsorption and ensure high sensitivity and selectivity. Variation of the laser parameters at a 1/e(2) spot diameter of 2.8 μm allows for fabrication of protein binding domains with diameters on the micrometer and nanometer length scale. Minimum domain sizes are about 300 nm. In addition to unspecific protein adsorption on as-patterned monolayers, biotin-streptavidin coupling chemistry is exploited for specific protein binding. This approach represents a novel facile laser-based means for fabrication of protein micro- and nanopatterns. The routine is readily applicable to femtosecond laser processing of glass substrates for the fabrication of transparent templates.

Oxadiazole-2-thiol adsorption on gold nanorods: a joint theoretical and experimental study by using SERS, XPS, and DFT

The chemisorption of 1,3,4-oxadiazole-2-thiol (ODT) on gold nanorods has been investigated by using surface-enhanced Raman spectroscopy (SERS) and density functional theory (DFT). Although most of the SERS spectra have remarkable similarity to the normal Raman spectra of the pure analyte, the adsorption of ODT on a gold surface leads to a drastic change in its Raman spectrum and distinct vibrational features are obtained with gold nanorods and spherical nanoparticles. Simulated Raman spectra for hybrid systems that consist of an oxadiazole moiety coordinated to a Au20 gold cluster provided valuable information about the coordination mode and enabled us to assign vibration modes.

Selective protein-peptide interactions at surfaces

Protein-surface interactions are of critical significance in both biological and man-made systems. While the term "specific binding" is normally reserved for the description of well-structured interactions, it is often the case in biology that there are unstructured interactions that greatly favor some protein interactions over others, a necessity in the highly crowded environment of the cell. In this study, surface-bound peptide arrays were used as a model to explore the range of protein-surface interactions and to better understand the kinds of "nonspecific" or unstructured interactions that take place at chemically complex surfaces. Three samples, β-galactosidase, α1-antitrypsin and a mixture of nine different proteins, were bound to arrays of nearly 5000 different peptides with a wide range of hydrophobicity, charge and peptide length. All three protein samples show higher binding affinity to positively charged peptides. While β-galactosidase binds poorly to very hydrophobic peptides, in terms of either absolute binding or relative to the mixture of proteins, α1-antitrypsin binds with higher affinity to more hydrophobic peptides. More surprising is the observation that β-galactosidase affinity for the surface does not simply increase with the length of the peptide, as one might expect, even when only the best binders are considered. Instead, its affinity (both absolute and relative to the protein mixture) peaks in the four-to-nine amino acid residue range and then decreases substantially by 12 amino acids. In contrast, α1-antitrypsin increases nearly monotonically with peptide length, in terms of both apparent affinity and binding relative to other proteins. Of particular significance in a practical sense, it was possible to obtain quite specific binding; the identity of the 100 peptides that showed the best apparent affinity for each of the three protein samples overlapped very little. Thus, using this approach it would be straightforward to develop surfaces covered with specific short peptide sequences with relatively specific protein interaction profiles.

Electrochemical engineering of hollow nanoarchitectures: pulse/step anodization (Si, Al, Ti) and their applications

Hollow nanoarchitectured materials with straight channels play a crucial role in the fields of renewable energy, environment and biotechnology due to their one-dimensional morphology and extraordinary properties. The current challenge is the difficulty on tailoring hollow nanoarchitectures with well-controlled morphology at a relatively low cost. As a conventional technique, electrochemistry exhibits its unique advantage on machining nanostructures. In this review, we present the progress of electrochemistry as a valuable tool in construction of novel hollow nanoarchitectures through pulse/step anodization, such as surface pre-texturing, modulated, branched and multilayered pore architectures, and free-standing membranes. Basic principles for electrochemical engineering of mono- or multi-ordered nanostructures as well as free-standing membranes are extracted from specific examples (i.e. porous silicon, aluminum and titanium oxide). The potential of such nanoarchitectures are further demonstrated for the applications of photovoltaics, water splitting, organic degradation, nanostructure templates, biosensors and drug release. The electrochemical techniques provide a powerful approach to produce nanostructures with morphological complexity, which could have far-reaching implications in the design of future nanoscale systems.

Fabrication of molecular nanopatterns at aluminium oxide surfaces by nanoshaving of self-assembled monolayers of alkylphosphonates

Nanoshaving, by tracing an atomic force microscope probe across a surface at elevated load, has been used to fabricate nanostructures in self-assembled monolayers of alkylphosphonates adsorbed at aluminium oxide surfaces. The simple process is implemented under ambient conditions. Because of the strong bond between the alkylphosphonates and the oxide surface, loads in excess of 400 nN are required to pattern the monolayer. Following patterning of octadecylphosphonate SAMs, adsorption of aminobutyl phosphonate yielded features as small as 39 nm. Shaving of monolayers of aryl azide-terminated alkylphosphonates, followed by attachment of polyethylene glycol to unmodified regions in a photochemical coupling reaction, yielded 102 nm trenches into which NeutrAvidin coated, dye-labelled, polymer nanospheres could be deposited, yielding bright fluorescence with little evidence of non-specific adsorption to other regions of the surface. Structures formed in alkylphosphonate films by nanoshaving were used to etch structures into the underlying metal. Because of the isotropic nature of the etch process, and the large grain size, some broadening was observed, but features 25-35 nm deep and 180 nm wide were fabricated.

Writing on superhydrophobic nanopost arrays: topographic design for bottom-up assembly

A well-known property of superhydrophobic surfaces, such as an array of hydrophobic nanoposts, is to allow only limited surface contact of a liquid to the tips of the nanoposts. Herein we demonstrate that material deposition from solution, whether solid precipitation, surface adsorption or colloidal adhesion in static system, or dynamic "writing", can be limited to these specific areas of the surface when in this nonwetting state. As an example of solid precipitation, we show that nucleation of CaCO(3) results in the growth of small, uniform, amorphous deposits (which can merge and recrystallize) instead of disordered, large crystals due to the abundance of identical, small heterogeneous nucleation sites. The growth of amorphous CaCO(3) can be used to trap molecules from solution, as a potential application for controlled drug release. To demonstrate the localized surface adsorption, we show that chemical functionalization of the post tips can make them "sticky" for specific attachment of species (such as colloidal particles) from solution. The electrostatic charge and relative size ratio of the particle/post diameters control the attachment of particles to the post tips with great specificity. Dynamic conditions have also been shown for writing using droplets translated across the nonwetting surface at controlled speeds during deposition. These methods offer unprecedented control over the heterogeneous nucleation and localized growth of crystals from solution and avoid nonspecific adsorption. There is selective control of colloidal or molecular attachment to the nanopost tips, whereby the contact area, time of contact, and tip surface chemistry for reaction are all independently tunable parameters.

Photopatterning of multilayer n-alkylsilane films

Surface photopatterning of organosilane self-assembled monolayers (SAM) has received increasing attention since its introduction 20 years ago. Herein we report for the first time a cost-efficient soft photopatterning technique affording amplified 3D multilayer structures. The essential chemistry relies on a spatially controlled photoacid-catalyzed hydrolysis and polycondensation of n-alkyltrimethoxysilane precursors (n-C(12)H(25)Si(OCH(3))(3),). Amphiphilic siloxane species are photogenerated locally and are able to self-assemble spontaneously into a long-range-ordered lamellar mesostructure.

Nanostructures of functionalized gold nanoparticles prepared by particle lithography with organosilanes

Periodic arrays of organosilane nanostructures were prepared with particle lithography to define sites for selective adsorption of functionalized gold nanoparticles. Essentially, the approach for nanoparticle lithography consists of procedures with two masks. First, latex mesospheres were used as a surface mask for deposition of an organosilane vapor, to produce an array of holes within a covalently bonded, organic thin film. The latex particles were readily removed with solvent rinses to expose discrete patterns of nanosized holes of uncovered substrate. The nanostructured film of organosilanes was then used as a surface mask for a second patterning step, with immersion in a solution of functionalized nanoparticles. Patterned substrates were fully submerged in a solution of surface-active gold nanoparticles coated with 3-mercaptopropyltrimethoxysilane. Regularly shaped, nanoscopic areas of bare substrate produced by removal of the latex mask provided sites to bind silanol-terminated gold nanoparticles, and the methyl-terminated areas of the organosilane film served as an effective resist, preventing nonspecific adsorption on masked areas. Characterizations with atomic force microscopy demonstrate the steps for lithography with organosilanes and functionalized nanoparticles. Patterning was accomplished for both silicon and glass substrates, to generate nanostructures with periodicities of 200-300 nm that match the diameters of the latex mesospheres of the surface masks. Nanoparticles were shown to bind selectively to uncovered, exposed areas of the substrate and did not attach to the methyl-terminal groups of the organosilane mask. Billions of well-defined nanostructures of nanoparticles can be generated using this high-throughput approach of particle lithography, with exquisite control of surface density and periodicity at the nanoscale.

Ions redistribution and meniscus relaxation during Langmuir wetting process

Nonstationary kinetics of the ion redistribution within the meniscus region during deposition of a charged Langmuir monolayer after beginning or stopping of the substrate motion is analyzed on the basis of the results of numerical simulations. The time evolution of the ions concentration profiles forming at the contact line and propagating toward the bulk solution is considered. It is shown that the diffusion front propagates much slower within the region of overlapping diffuse layers than outside of this region. At the beginning of the deposition process a region characterized by quasi-stationary behavior of the ion concentration and electric potential distributions is formed in close vicinity to the contact line. A stationary deposition regime is established when the region of quasi-stationary distributions reaches the external boundary of the Nernst layer provided that the substrate motion is not very fast. For the substrate velocities higher than the critical one the concentration near the contact line can decrease to such small values which do not allow a stable deposition process. The developed mathematical model allows addressing to transient regimes of the monolayer deposition which are very important for understanding the mechanisms leading to meniscus instability.

Micro- and nanopatterning of functional organic monolayers on oxide-free silicon by laser-induced photothermal desorption

The photothermal laser patterning of functional organic monolayers, prepared on oxide-free hydrogen-terminated silicon, and subsequent backfilling of the laser-written lines with a second organic monolayer that differs in its terminal functionality, is described. Since the thermal monolayer decomposition process is highly nonlinear in the applied laser power density, subwavelength patterning of the organic monolayers is feasible. After photothermal laser patterning of hexadecenyl monolayers, the lines freed up by the laser are backfilled with functional acid fluoride monolayers. Coupling of cysteamine to the acid fluoride groups and subsequent attachment of Au nanoparticles allows easy characterization of the functional lines by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Depending on the laser power and writing speed, functional lines with widths between 1.1 μm and 250 nm can be created. In addition, trifluoroethyl-terminated (TFE) monolayers are also patterned. Subsequently, the decomposed lines are backfilled with a nonfunctional hexadecenyl monolayer, the TFE stripes are converted into thiol stripes, and then finally covered with Au nanoparticles. By reducing the lateral distance between the laser lines, Au-nanoparticle stripes with widths close to 100 nm are obtained. Finally, in view of the great potential of this type of monolayer in the field of biosensing, the ease of fabricating biofunctional patterns is demonstrated by covalent binding of fluorescently labeled oligo-DNA to acid-fluoride-backfilled laser lines, which--as shown by fluorescence microscopy--is accessible for hybridization.

Morphological wetting transitions at ring-shaped surface domains

The wetting behavior of ring-shaped (or annular) surface domains is studied both experimentally and theoretically. The ring-shaped domains are lyophilic and embedded in a lyophobic substrate. Liquid droplets deposited on these domains can attain a variety of morphologies depending on the liquid volume and on the dimensions of the ringlike surface domains. In the experiments, the liquid volume is changed in a controlled manner by varying the temperature of the sample. Such a volume change leads to a characteristic sequence of droplet shapes and to morphological wetting transitions between these shapes. The experimental observations are in good agreement with analytical and numerical calculations based on the minimization of the interfacial free energy. Small droplets form ringlike liquid channels (or filaments) that are confined to the ring-shaped domains and do not spread onto the lyophobic disks enclosed by these rings. As one increases the volume of the droplets, one finds two different morphologies depending on the width of the ring-shaped domains. For narrow rings, the droplets form nonaxisymmetric liquid channels with a pronounced bulge. For broad rings, the droplets form axisymmetric caps that cover both the lyophilic rings and the lyophobic disks.

Photothermal micro- and nanopatterning of organic/silicon interfaces

Photothermal laser processing of organic monolayers on oxide-free silicon substrates under ambient conditions is investigated. Organic monolayers on Si(100) and Si(111) substrates are prepared via hydrosilylation of H-terminated silicon samples in neat 1-hexadecene and 1-hexadecyne, respectively. Laser processing at lambda = 514 nm and a 1/e(2) spot diameter of 2.6 microm results in local decomposition of the monolayers and oxidation of the exposed substrate. In agreement with the high thermal and chemical stability of these monolayers, a thermokinetic analysis of the data from experiments at distinct laser powers and pulse lengths points to a highly activated process. As a result, processing is strongly nonlinear and allows for subwavelength patterning, with line widths between 0.4 and 1.4 microm. Most remarkably, upon fabrication of dense line patterns, narrow organic monolayer stripes with sharp edges and lateral dimensions of 80 nm are formed. This opens up new perspectives in photothermal engineering of organic/silicon interfaces, e.g., for hybrid microelectronic and sensor applications.

A versatile approach to fabricate ordered heterogeneous bull's-eye-like microstructure arrays

In this paper, ordered heterogeneous bull's-eye-like microstructure arrays were fabricated through a simple two-step method on gold substrate with patterned self-assemble monolayers (SAMs). First, we prepared ordered polymer dot arrays on the SAMs patterned gold substrate by SAMs-direct dewetting. Subsequently, by manipulating concentration-controlled dewetting process, ordered ring arrays were obtained on the dot arrays patterned surface under the protection of water droplets. Namely, ordered bull's-eye-like structure arrays were fabricated successfully. The mechanism of these two kinds of dewetting process has been investigated in detail. And due to these two steps were independent, different materials could be simply introduced to the current system. Therefore, ordered homogeneous and heterogeneous bull's-eye-like structure arrays such as poly(N-vinylcarbazole) (PVK) (dot)/PVK (ring), PVK/5,12-ditetradecylquinolino[2,3-b]acridine-7,14(5H,12H)-dione (DTQA), and PVK/Fe(3)O(4) nanoparticles were obtained. This straightforward method may open up new possibilities for practical use of microchips with binary heterogeneous structure arrays.

Chemically specific laser-induced patterning of alkanethiol SAMs: characterization by SEM and AFM

Self-assembled monolayers (SAMs) are widely used to modify the interfacial properties of solid surfaces in either a homogeneous or a patterned manner. One of the many techniques developed for patterning SAMs involves heating the surface with a focused laser beam. Localized heating can result in pattern formation through either the ablation of both the solid substrate and the SAM (chemically nonspecific patterning) or, at lower temperatures, the selective breaking of the chemical bonds between the SAM and the substrate (chemically specific patterning). The latter method is termed chemically specific laser-induced patterning and is demonstrated for alkanethiol monolayers on gold (Au). In this report, the interplay between alkanethiol desorption and nanoscale Au ablation is studied using atomic force microscopy to image both the topographical and the chemical features of laser patterned areas. Frequently the two processes occur simultaneously but with different spatial extents, as predicted theoretically, due to their different threshold temperatures. By tuning the exposure conditions (laser power and irradiation time), parameters are established where local heating causes alkanethiol desorption without any Au ablation, thus, allowing chemically specific desorption and patterning of alkanethiol SAMs. This allows chemical patterns to be created without changes in the surface topography. Using scanning electron microscopy, a linear dependence of pattern size on irradiation time is demonstrated for circular features 0.5-1 microm in diameter.

Site-selective optical coupling of PbSe nanocrystals to Si-based photonic crystal microcavities

A novel method for patterning optically active colloidal PbSe nanocrystals on Si surfaces is reported. Oleate-capped PbSe nanocrystals were found to adhere preferentially to H-terminated Si surfaces over oxide and alkyl-terminated Si surfaces. Scanning probe lithography was used to oxidize locally a dodecyl monolayer on the Si surface of a silicon-on-insulator wafer prepatterned with photonic crystal microcavities. Aqueous HF was then used to remove the oxide and expose H-terminated Si areas, yielding patterned PbSe nanocrystals on the Si surface after exposure to a nanocrystal solution. This patterning technique allows for the selective deposition of PbSe nanocrystals at the main antinode of the silicon-based microcavities. More than a 10-fold photoluminescence enhancement due to the cavity-nanocrystal coupling was observed.

Engineering the spatial selectivity of surfaces at the nanoscale using particle lithography combined with vapor deposition of organosilanes

Particle lithography is a practical approach to generate millions of organosilane nanostructures on various surfaces, without the need for vacuum environments or expensive instrumentation. This report describes a stepwise chemistry route to prepare organosilane nanostructures and then apply the patterns as a spatially selective foundation to attach gold nanoparticles. Sites with thiol terminal groups were sufficiently small to localize the attachment of clusters of 2-5 nanoparticles. Basic steps such as centrifuging, drying, heating, and rinsing were used to generate arrays of regular nanopatterns. Close-packed films of monodisperse latex spheres can be used as an evaporative mask to spatially direct the placement of nanoscopic amounts of water on surfaces. Vapor phase organosilanes deposit selectively at areas of the surface containing water residues to generate nanostructures with regular thickness, geometry, and periodicity as revealed in atomic force microscopy images. The area of contact underneath the mesospheres is effectively masked for later synthetic steps, providing exquisite control of surface coverage and local chemistry. By judicious selection in designing the terminal groups of organosilanes, surface sites can be engineered at the nanoscale for building more complex structures. The density of the nanopatterns and surface coverage scale predictably with the diameter of the mesoparticle masks. The examples presented definitively illustrate the capabilities of using the chemistry of molecularly thin films of organosilanes to spatially define the selectivity of surfaces at very small size scales.

A nanoengineering approach for investigation and regulation of protein immobilization

It is known that protein attachment to surfaces depends sensitively upon the local structure and environment of the binding sites at the nanometer scale. Using nanografting and reversal nanografting, both atomic force microscopy (AFM)-based lithography techniques, protein binding sites with well-defined local environments are designed and engineered with nanometer precision. Three proteins, goat antibiotin immunoglobulin G (IgG), lysozyme, and rabbit immunoglobulin G, are immobilized onto these engineered surfaces. Strong dependence on the dimension and spatial distribution of protein binding sites are revealed in antibody recognition, covalent attachment via primary amine residues and surface-bound aldehyde groups. This investigation indicates that AFM-based nanolithography enables the production of protein nanostructures, and more importantly, protein-surface interactions at a molecular level can be regulated by changing the binding domains and their local environment at nanometer scale.

Nanostructured Biomaterials for Regeneration

Biomaterials play a pivotal role in regenerative medicine, which aims to regenerate and replace lost/dysfunctional tissues or organs. Biomaterials (scaffolds) serve as temporary 3D substrates to guide neo tissue formation and organization. It is often beneficial for a scaffolding material to mimic the characteristics of extracellular matrix (ECM) at the nanometer scale and to induce certain natural developmental or/and wound healing processes for tissue regeneration applications. This article reviews the fabrication and modification technologies for nanofibrous, nanocomposite, and nanostructured drug-delivering scaffolds. ECM-mimicking nanostructured biomaterials have been shown to actively regulate cellular responses including attachment, proliferation, differentiation and matrix deposition. Nano-scaled drug delivery systems can be successfully incorporated into a porous 3D scaffold to enhance the tissue regeneration capacity. In conclusion, nano-structured biomateials are a very exciting and rapidly expanding research area, and are providing new enabling technologies for regenerative medicine.

Elucidating the role of surface hydrolysis in preparing organosilane nanostructures via particle lithography

A new method of particle lithography is described for preparing rings or nanoporous films of organosilanes. Millions of exquisitely uniform and precisely spaced nanostructures with designed surface chemistry can be rapidly produced using vapor deposition through mesoparticle masks. Nanoscopic amounts of water are essential for initiating surface hydrosilation. Thus, the key step for preparing covalently bonded nanostructures of organosilanes is to control drying parameters to spatially direct the placement of water on surfaces.

Chemical surface reactions by click chemistry: coumarin dye modification of 11-bromoundecyltrichlorosilane monolayers

The functionalization of surfaces and the ability to tailor their properties with desired physico-chemical functions is an important field of research with a broad spectrum of applications. These applications range from the modification of wetting properties, over the alteration of optical properties, to the fabrication of molecular electronic devices. In each of these fields, it is of specific importance to be able to control the quality of the layers with high precision. The present study demonstrates an approach that utilizes the 1,3-dipolar cycloaddition of terminal acetylenes to prepare triazole-terminated monolayers on different substrates. The characterization of the precursor monolayers, the optimization of the chemical surface reactions as well as the clicking of a fluorescent dye molecule on such azide-terminated monolayers was carried out. A coumarin 343 derivative was utilized to discuss the aspects of the functionalization approach. Based on this approach, a number of potential surface reactions, facilitated via the acetylene-substituted functional molecules, for a broad range of applications is at hand, thus leading to numerous possibilities where surface modifications are concerned. These modifications can be applied on non-structured surfaces of silicon or glass or can be used on structured surfaces. Various possibilities are discussed.

Chemical template directed iodine patterns on the octadecyltrichlorosilane surface

A carboxylic-terminated nanometer-scale chemical pattern on an octadecyltrichlorosilane (OTS) surface can guide the deposition and crystallization of iodine, forming an iodine pattern on the chemical pattern. The iodine in the pattern is gel-like when fabricated by the solution-deposit method. In contrast, a dendritic, snowflake-shaped polycrystalline iodine sheet is formed by the vapor-phase condensation method. The data demonstrate that iodine is a good tracing and visualizing agent for studying liquid behavior at the nano scale. The topography of the iodine stain reveals that the "coffee ring" effect can be suppressed by reducing the pattern size and increasing the evaporation rate. The chemical template-bound iodine pattern has an unusually low vapor pressure and it can withstand prolonged baking at elevated temperature, which differs significantly from bulk iodine crystals.