Selective surface chemistry using alumina nanoparticles generated from block copolymers

Developing orthogonal surface chemistry techniques that perform at the nanoscale is key to achieving precise control over molecular patterning on surfaces. We report the formation and selective functionalization of alumina nanoparticle arrays generated from block copolymer templates. This new material provides an alternative to gold for orthogonal surface chemistry at the nanometer scale. Atomic force microscopy and X-ray photoelectron spectroscopy confirm these particles show excellent selectivity over silica for phosphonic and carboxylic acid adsorption. As this is the first reported synthesis of alumina nanoparticles from block copolymer templates, characterizations via Fourier transform infrared spectroscopy, Auger electron spectroscopy, and transmission electron microscopy are presented. Reproducible formation of alumina nanoparticles was dependent on a counterintuitive synthetic step wherein a small amount of water is added to an anhydrous toluene solution of block copolymer and aluminum chloride. The oxidation environment of the aluminum in these particles, as measured by Auger electron spectroscopy, is similar to that of native aluminum oxide and alumina grown by atomic layer deposition. This discovery expands the library of available surface chemistries for nanoscale molecular patterning.     

Surfaces Decorated with Polymeric Nanocompartments for pH Reporting

Here we present a novel active surface that demonstrates pH responsiveness and can be used as a platform for designing ‘smart labels’. To generate our active surfaces, we immobilized polymer nanocompartments onto glass surfaces using thiol–ene chemistry. Prior to surface attachment, a pH responsive model dye was encapsulated within nanocompartments at two different pH values. We confirmed the attachment and distribution of dye‐loaded polymersomes and established the pH responsiveness of the active surface construct. The strategy presented here was carefully chosen to obtain small sized functional surfaces from commercially available materials that can be easily integrated into intelligent packaging systems. The ability to miniaturize such smart labels, while still being able to detect their response to the environment, is a crucial step towards developing active surfaces suitable for food packaging applications.     

Interface chemistry of nanostructured materials: ion adsorption on mesoporous alumina

This paper presents a part of our work on understanding the effect of nanoscale pore space confinement on ion sorption by mesoporous materials. Acid-base titration experiments were performed on both mesoporous alumina and alumina particles under various ionic strengths. The point of zero charge (PZC) for mesoporous alumina was measured to be approximately 9.1, similar to that for nonmesoporous alumina materials, indicating that nanoscale pore space confinement does not have a significant effect on the PZC of pore surfaces. However, for a given pH deviation from the PZC, (pH-PZC), the surface charge per mass on mesoporous alumina was as much as 45 times higher than that on alumina particles. This difference cannot be fully explained by the surface area difference between the two materials. Our titration data have demonstrated that nanoscale confinement has a significant effect, most likely via the overlap of the electric double layer (EDL), on ion sorption onto mesopore surfaces. This effect cannot be adequately modeled by existing surface complexation models, which were developed mostly for an unconfined solid-water interface. Our titration data have also indicated that the rate of ion uptake by mesoporous alumina is relatively slow, probably due to diffusion into mesopores, and complete equilibration for sorption could take 4-5 min. A molecular simulation using a density functional theory was performed to calculate ion adsorption coefficients as a function of pore size. The calculation has shown that as pore size is reduced to nanoscales (<10 nm), the adsorption coefficients of ions can vary by more than two orders of magnitude relative to those for unconfined interfaces. The prediction is supported by our experimental data on Zn sorption onto mesoporous alumina. Owing to their unique surface chemistry, mesoporous materials can potentially be used as effective ion adsorbents for separation processes and environmental cleanup.     

A Surface Masking Technique for the Determination of Plasma Polymer Film Thickness by AFM

A method for the determination of coating film thicknesses at nanometer resolution based on surface masking and atomic force microscopy (AFM) is described. A polymeric mask is used to cover part of a substrate during the deposition of thin polymeric coatings by plasma polymerization, allowing the production of well defined polymer steps of heights of a few tens of nanometers. Tapping mode AFM has been employed to analyze the topography of these steps at high resolution. This method has also allowed accurate measurement of the kinetics of the deposition of plasma polymer films over a range of exposure times. XPS analysis of different substrate surfaces following mask removal found barely detectable residues, suggesting that the underlying surface chemistry remains unchanged, and accessible for further modification. In combination with quartz crystal microgravimetry, the method has been applied to the measurement of the density of plasma polymer coatings in the thickness range 4–50 nm.     

Towards dynamic coating of glass microchip chambers for amplifying DNA via the polymerase chain reaction

As microchip technology evolves to allow for the integration of more complex processes, particularly the polymerase chain reaction (PCR), it will become necessary to define simple approaches for minimizing the effects of surfaces on the chemistry/processes to be performed. We have explored alternatives to silanization of the glass surface with the use of additives that either dynamically coat or adsorb to the glass surface. Polyethylene glycol, polyvinylpyrrolidone (PVP), and hydroxyethylcellulose (HEC) have been explored as potential dynamic coatings and epoxy (poly)dimethylacrylamide (EPDMA) evaluated as an adsorbed coating. By carrying out analysis of the PCR products generated under different conditions via microchip electrophoresis, we demonstrate that these coating agents adequately passivate the glass surface in a manner that prevents interference with the subsequent PCR process. While several of the agents tested allowed for PCR amplification of DNA in glass, the EPDMA was clearly superior with respect to ease of preparation. However, more efficient PCR (larger mass of amplified product) could be obtained by silanizing the glass surface.     

Facile preparation of superhydrophobic and oleophobic surfaces via the combination of Cu(0)‐mediated reversible‐deactivation radical polymerization and click chemistry

The fabrication of novel hydrophobic, superhydrophobic, and oleophobic surfaces on glass using nanosilica particles modified with polymer brushes prepared via surface initiated Cu(0)‐mediated reversible‐deactivation radical polymerization was demonstrated. Monomers including n‐butyl acrylate, 2,2,2‐trifluoroethyl methacrylate, and 1,1,1,3,3,3‐hexafluoroisopropyl acrylate were used to synthesize a series of nanosilica–polymer organic/inorganic hybrid materials. Products were analyzed using infrared spectroscopy, thermogravimetric analysis, scanning and transmission electron microscopy. The coated nanosilica showed core–shell structures that contains polymer brushes up to 67 wt %. The application of these particles for modifying surface wettability was examined by covalently attaching them to glass via a recently developed one‐pot “grafting to” methodology using “thio‐bromo click” chemistry. Atomic force microscopy topographic images show up to 25 times increase in roughness of the coated glass compared to blank glass sample. Contact angle measurements showed that nanosilica coated with PBA and PTFEM produced hydrophobic glass surfaces, while a superhydrophobic and oleophobic surface was generated using nanosilica functionalized with PHFIPA. This novel methodology can produce superhydrophobic and oleophobic surfaces in an easy and fast way without the need for tedious and time‐consuming processes, such as layer‐by‐layer deposition, high temperature calcination, and fluorinated oil infusion. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018     

Adsorption and desorption of stearic acid self-assembled monolayers on aluminum oxide

Development of coatings to minimize unwanted surface adsorption is extremely important for their use in applications, such as sensors and medical implants. Self-assembled monolayers (SAMs) are an excellent choice for coatings that minimize nonspecific adsorption because they can be uniform and have a very high surface coverage. Another equally important characteristic of such coatings is their stability. In the present study, both the bonding mechanism and the stability of stearic acid SAMs on two aluminum oxides (single-crystal C-plane aluminum oxide (sapphire) and amorphous aluminum oxide (alumina)) are investigated. The adsorption mechanism is investigated by ex situ X-ray photoelectron spectroscopy and infrared (IR) spectroscopy. The results revealed that stearic acid binds to sapphire surfaces via a bidentate interaction of carboxylate with two oxygen atoms while it binds to alumina surfaces via both bidentate and monodentate interactions. Desorption kinetics of stearic acid self-organized on both aluminum oxide surfaces into water is explored by ex situ tapping mode atomic force microscopy, IR spectroscopy, and contact angle measurements. The results exhibit that the SAMs of stearic acid formed on sapphire are not stable in water and are continuously lost through desorption. Water contact angle measurements of SAMs that are immersed in water further indicate that the desorption rate of adsorbates from atomically smooth terrace sites is substantially faster than that of adsorbates from the sites of surface defects due to weaker molecular interaction with the smooth surface. A time-dependent desorption profile of SAMs grown on amorphous alumina reveals that contact angles decrease monotonically without any regional distinction, providing further evidence for the presence of adsorption sites with different types of affinity on the amorphous alumina surface.     

Parallel arrays of geometric nanowells for assembling curtains of DNA with controlled lateral dispersion

The analysis of individual molecules is evolving into an important tool for biological research, and presents conceptually new ways of approaching experimental design strategies. However, more robust methods are required if these technologies are to be made broadly available to the biological research community. To help achieve this goal we have combined nanofabrication techniques with single-molecule optical microscopy for assembling and visualizing curtains comprised of thousands of individual DNA molecules organized at engineered diffusion barriers on a lipid bilayer-coated surface. Here we present an important extension of this technology that implements geometric barrier patterns comprised of thousands of nanoscale wells that can be loaded with single molecules of DNA. We show that these geometric nanowells can be used to precisely control the lateral distribution of the individual DNA molecules within curtains assembled along the edges of the engineered barrier patterns. The individual molecules making up the DNA curtain can be separated from one another by a user-defined distance dictated by the dimensions of the nanowells. We demonstrate the broader utility of these patterned DNA curtains in a novel, real time restriction assay that we refer to as dynamic optical restriction mapping, which can be used to rapidly identify entire sets of cleavage sites within a large DNA molecule.     

Superhydrophobic surface from Cu-Zn alloy by one step O2 concentration dependent etching

The article reports on a very simple method to fabricate superhydrophobic surfaces with Cu-Zn alloy via changing the local oxygen concentration and formation of oxygen difference cell, which can be readily realized by covering or contacting the Cu-Zn alloy surface with a glass slide. This superhydrophobic film comes from the formation of a flower-like hierarchical structure due to the accelerated alloy etching. In contrast, the white film grown in the un-covered area showed a much lower hydrophobicity due to its different morphology. These superhydrophobic surfaces or superhydrophobic-hydrophobic surfaces are expected to find applications in making self-cleaning alloy surface, in metal anticorrosion, and in biomineralization, etc.     

Surface modification of silicate glass using 3-(mercaptopropyl)trimethoxysilane for thiol-ene polymerization

A thiol-ene polymerization was accomplished on silicate glass slides to graft a series of homopolymers and copolymers using 3-(mercaptopropyl)trimethoxysilane (MTS) as both a silane coupling agent and initiator. MTS was initially covalently bonded to an acid cleaned glass surface via a classical sol-gel reaction. Poly(acrylic acid) (PAA), poly(acrylamide) (PAAm), poly(methyl acrylate) (PMA), poly(acrylamido-2-methyl-propanesulfonic acid) (PAMPS), and the copolymer poly(AA-co-AAm-co-MA-co-AMPS) were grafted from the thiol group of MTS. The surface chemistry of the MTS modified slides and polymer grafts was characterized with attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). Surface texture was evaluated with tapping mode atomic force microscopy (TM-AFM). The Owens-Wendt-Kaelble (OWK) and Lifshitz-van der Waals acid-base (LW-AB) methods were used to evaluate surface energies by sessile drop contact angle method. The synthetic approach demonstrated a facile, rapid method for grafting to glass surfaces.     

Grafting of nanoporous alumina membranes and films with organic acids

In this study, direct surface grafting of nanoporous alumina membranes and glass‐supported alumina films was carried out with three different fluorinated organic acids: trifluoroacetic acid, perfluoropentanoic acid and 2,3,4,5,6‐pentafluorobenzoic acid. Elemental surface composition and chemical environment of alumina were investigated using X‐ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). Alumina surfaces grafted with fluoro‐organic acids exhibited increased hydrophobic properties compared to ungrafted surfaces when measured using goniometry and atomic force microscopy (AFM). This work describes the evidence for surface chemical modification of alumina using direct reaction with organic acids. An AFM study of the adsorption of the immunoglobulin G (IgG) molecules on the fluoro‐organic‐acid‐grafted surfaces is reported. The results show that an ordered arrangement of immunoglobulin G structures with in‐filling of pores could be achieved only on the more hydrophobic fluoro‐organic‐acid‐grafted alumina membranes. Copyright © 2007 John Wiley & Sons, Ltd.     

A Novel Method for Surface Free-Energy Determination of Powdered Solids

Interfacial solid/liquid interactions play a crucial role in wetting, spreading, and adhesion processes. In the case of a flat solid surface, contact angle measurements are commonly utilized for the determination of the solid surface free energy and its components. However, if such a surface cannot be obtained, then the contact angle can not be measured directly. Usually methods based on imbibition of probe liquids into a thin porous layer or column are applied. In this paper a novel method, also based on the capillary rise, is proposed for the solid surface free-energy components determination. Actually, it is a modification of the thin column wicking method; similar theoretical background can be applied together with that appropriate for the capillary rise method of liquid surface tension determination. The proposed theoretical approach and procedure are verified by using single glass capillaries, and then alumina and ground glass powders were used for the method testing. Thus obtained surface free-energy components for these solids, for both glass and alumina, agree well with the literature values.     

Fabrication of Au hybrid protein chips and its application to SERS-based bioassay

Hybrid micro/nanostructures composed with alternative Au nanoparticle (NP) arrays and protein dots were fabricated via layer-by-layer self-assembly and the microsphere lithography technique. These micro/nanostructures were novel protein chips which had applications in the surface-enhanced Raman spectroscopy (SERS) based immunoassay. The synthetic processes were to fabricate Au nanowell arrays initially by using the templates of ordered monolayers of polystyrene (PS) microsphere arrays. Then, the proteins of antibody (avidin) were imbedded in the Au nanowells. Lastly, the immune reaction was implemented by adding atto 610-biotin. SERS spectra were recorded as the immunoassay readout, which showed the lowest detective concentration of 100 pg/mL. These new kind of SERS-based protein chips were easy to fabricate, inexpensive and supersensitive, and exhibit the potential application in bioassays, forensics and biosensors.     

Probing the surface structure of hydroxyapatite using NMR spectroscopy and first principles calculations

The surface characteristics of hydroxyapatite (HA) are probed using a combination of NMR spectroscopy and first principles calculations. The NMR spectrum is taken from a bone sample and the first principles calculations are performed using a plane-wave density functional approach within the pseudopotential approximation. The computational work focuses on the (100) and (200) surfaces, which exhibit a representative range of phosphate, hydroxyl and cation bonding geometries. The shielding tensors for the 31P, 1H and 17O nuclei are calculated from the relaxed surface structures using an extension of the projector augmented-wave method. The calculated 31P chemical shifts for the surface slab are found to be significantly different from the bulk crystal and are consistent with the NMR data from bone and also synthetically prepared nanocrystalline samples of HA. Rotational relaxations of the surface phosphate ions and the sub-surface displacement of other nearby ions are identified as causing the main differences. The investigation points to further calculations of other crystallographic surfaces and highlights the potential of using NMR with ab initio modelling to fully describe the surface structure and chemistry of HA, which is essential for understanding its reactivity with the surrounding organic matrix.     

Glass fracture surfaces seen with an atomic force microscope

Fracture surfaces of Suprasil 2, Herasil 2, AR glass and Duran glass rods have been studied by an atomic force microscope (AFM) in the contact mode. They could be characterized in the fracture mirror, the mist and the hackle zones. The RMS (root mean square) roughness in the fracture mirror of all glasses investigated increased with growing distance from the origin of the fracture. On several fracture surfaces of different glasses steps have been observed, due to fracture in shear mode. Furthermore changes in the fracture surfaces during scanning have also been observed. They are thought to stem from reactions of the freshly broken glass surface with the surrounding atmosphere and forces between the scanning tip and the soft surface.     

Substrate-independent dip-pen nanolithography based on reactive coatings

We report that nanostructuring via dip-pen nanolithography can be used for modification of a broad range of different substrates (polystyrene, Teflon, stainless steel, glass, silicon, rubber, etc.) without the need for reconfiguring the underlying printing technology. This is made possible through the use of vapor-based coatings that can be deposited on these substrates with excellent conformity, while providing functional groups for subsequent spatially directed click chemistry via dip-pen nanolithography. Pattern quality has been compared on six different substrates demonstrating that this approach indeed results in a surface modification protocol with potential use for a wide range of biotechnological applications.     

A Smart Superwetting Surface with Responsivity in Both Surface Chemistry and Microstructure

Recently, smart surfaces with switchable wettability have aroused much attention. However, only single surface chemistry or the microstructure can be changed on these surfaces, which significantly limits their wetting performances, controllability, and applications. A new surface with both tunable surface microstructure and chemistry was prepared by grafting poly(N‐isopropylacrylamide) onto the pillar‐structured shape memory polymer on which multiple wetting states from superhydrophilicity to superhydrophobicity can be reversibly and precisely controlled by synergistically regulating the surface microstructure and chemistry. Meanwhile, based on the excellent controllability, we also showed the application of the surface as a rewritable platform, and various gradient wettings can be obtained. This work presents for the first time a surface with controllability in both surface chemistry and microstructure, which starts some new ideas for the design of novel superwetting materials.     

Studies of electroosmotic flow and the effects of protein adsorption in plasma‐polymerized microchannel surfaces

This paper presents a study of EOF properties of plasma‐polymerized microchannel surfaces and the effects of protein (fibrinogen and lysozyme) adsorption on the EOF behavior of the surface‐modified microchannels. Three plasma polymer surfaces, i.e. tetraglyme, acrylic acid and allylamine, are tested. Results indicate EOF suppression in all plasma‐coated channels compared with the uncoated glass microchannel surfaces. The EOF behaviors of the modified microchannels after exposure to protein solutions are also investigated and show that even low levels of protein adsorption can significantly influence EOF behavior, and in some cases, result in the reversal of flow. The results also highlight that EOF measurement can be used as a method for detecting the presence of proteins within microchannels at low surface coverage (<1 ng/cm² on glass). Critically, the results illustrate that the non‐fouling tetraglyme plasma polymer is able to sustain EOF. Comparison of the plasma‐polymerized surfaces with conventionally grafted polyelectrolyte surfaces demonstrates the stabilities of the plasma polymer films, enabling multiple EOF runs over 3 days without deterioration in performance. The results of this study clearly demonstrate that plasma polymers enable the surface chemistry of microfluidic devices to be tailored for specific applications. Critically, the deposition of the non‐fouling tetraglyme coating enables stable EOF to be induced in the presence of protein.     

Surface properties and catalytic activity of pure and silica-coated aluminas

The surface properties of predominantly microporous, mesoporous, and nonporous alumina samples were studied and compared with samples modified by coating with a fixed amount of silica on their surfaces. The samples were characterized in terms of their specific surface areas, pore structure, and the chemistry of the surface, namely, surface acidity and surface OHs. An attempt was made to relate the activity toward cumene cracking with the chemistry as well as the predominant pore size. This study will hopefully reveal the role of the pore structure of alumina in determining its adsorption and catalytic activity and also the effect of doping with silica in modifying the surface properties of alumina.