Atomic force microscopy (AFM)-based nanografting for the study of self-assembled monolayer formation of organophosphonic acids on Al2O3 single-crystal surfaces

Adsorption, stability, and organization kinetics of organophosphonic acids on single-crystalline alumina surfaces were investigated by means of atomic force microscopy (AFM)-based imaging, nanoshaving, and nanografting. AFM friction and phase imaging have shown that chemical etching and subsequent annealing led to heterogeneities on single-crystalline surfaces with (0001) orientation. Self-assembly and stability of octadecylphosphonic acid (ODPA) were shown to be strictly dependent upon the observed heterogeneities of the surface termination, where it was locally shown that ODPA can loosely or strongly bind on different terminations of the crystal surface. Organization kinetics of ODPA was monitored with nanografting on (0001) surfaces. Supported by measurements of surface wettability and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), it was demonstrated that the lack of organization within the protective adsorbed hexylphosphonic acid (HPA) monolayer on alumina surfaces facilitated the reduced confinement effect during nanografting, such that kinetics information on the organization process of ODPA could be obtained.     

Surface‐initiated atom transfer radical polymerization from porous poly(tetrafluoroethylene) membranes using the C?F groups as initiators

This work reports the surface‐initiated atom transfer radical polymerization (ATRP) from hydrogen plasma‐treated porous poly(tetrafluoroethylene) (PTFE) membranes using the C? F groups as initiators. Hydrogen plasma treatment on PTFE membrane surfaces changes their chemical environment through defluorination and hydrogenation reactions. With the hydrogen plasma treatment, the C? F groups of the modified PTFE membrane surface become effective initiators of ATRP. Surface‐initiated ATRP of poly(ethylene glycol) methacrylate (PEGMA) is carried out to graft PPEGMA chains to PTFE membrane surfaces. The chain lengths of poly(PEGMA) (PPEGMA) grafted on PTFE surfaces increase with increasing the reaction time of ATRP. Furthermore, the chain ends of PPEGMA grown on PTFE membrane surfaces then serve as macroinitiators for the ATRP of N‐isopropylacrylamide (NIPAAm) to build up the PPEGMA‐b‐PNIPAAm block copolymer chains on the PTFE membrane surfaces. The chemical structures of the modified PTFE membranes are characterized using X‐ray photoelectron spectroscopy. The modification increases the surface hydrophilicity of the PTFE membranes with reductions in their water‐contact angles from 120° to 60°. The modified PTFE membranes also show temperature‐responsive properties and protein repulsion features owing to the presence of PNIPAAM and PPEGMA chains. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2076–2083, 2010     

Grafting of regenerated cellulose membrane by surface‐initiated atom transfer radical polymerization and its pH‐resposive behavior

Regenerated cellulose (RC) membranes which have pH modulated permeability have been prepared by anchoring the hydroxyl groups on the membrane surface with 2‐bromoisobutyryl bromide, followed by grafting with acrylic acid (AA) using atom transfer radical polymerization (ATRP). The obtained membranes were analyzed by X‐ray photoelectron spectroscopy (XPS), Fourier transform infrared attenuated total reflection spectrometer (ATR‐FTIR), scanning electron microscopy (SEM), TGA and the results showed that AA had been grafted onto the membrane surfaces successfully. Then the pH modulated permeability properties were tested by water flux measurement. All results show that the pH modulated permeability properties of a RC membrane can be obtained by surface‐initiated ATRP. Copyright © 2010 John Wiley & Sons, Ltd.     

Quantitative determination of surface energy using atomic force microscopy: the case of hydrophobic/hydrophobic contact and hydrophilic/hydrophilic contact

Atomic force microscopy (AFM) has been used to determine the surface energy of chemically modified surfaces at a local scale. In order to achieve this aim, it was necessary to graft both the AFM tip and the substrate with the same chemical functional groups. Two different organothiols terminated either by hydrophilic or hydrophobic chemical functionalities were used. Grafting process classically reported shows that after UV/ozone treatment for 30 min, the tip is coated by thermal deposition with 4‐5‐nm‐thick titanium layer followed by a 30‐nm‐thick gold layer. Finally, the tip is grafted by organothiols. The thickness of the layer deposited on the tip is of the same order of magnitude as the tip radius. To avoid the use of Ti and to decrease the thickness of the gold layer, we have developed a new way of grafting by using organic molecules like (3‐mercaptopropyl)triethoxysilane (MPS) as a linkage agent. Then this way of grafting was checked. Finally, AFM force‐distance curves, between grafted tips and chemically modified surface, were carried out in contact mode. Calibration of the various parts of the apparatus and especially of the cantilever (spring constant and tip radius) is of major importance to reach quantitative data. Finally, by applying a suitable theory of contact, we were able to determine the surface energy of our system. Copyright © 2005 John Wiley & Sons, Ltd.     

Various approaches to modify biomaterial surfaces for improving hemocompatibility

In this paper, the mechanism of thrombus formation on the surface of polymeric materials and the various approaches of modifying biomaterial surfaces to improve their hemocompatibility are reviewed. Moreover, the blood compatibility of the cellulose membrane grafted with O-butyrylchitosan (OBCS) by using a radiation grafting technique was studied. Surface analysis of grafted cellulose membrane was verified by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and electron spectroscopy for chemical analysis (ESCA), which confirmed that OBCS was successfully grafted onto the cellulose membrane surfaces. Blood compatibility of the grafted cellulose membranes was evaluated by platelet rich plasma (PRP) contacting experiments and protein adsorption experiments using blank cellulose membranes as the control. The blood compatibility of OBCS grafted cellulose membranes is better than that of blank cellulose membranes. These results suggest that the photocrosslinkable chitosan developed here has the potential of serving in blood-contacting applications in medical use.     

Surface-grafting of phosphates onto a polymer for potential biomimetic functionalization of biomaterials

In the human body, phosphate groups play important roles in signaling and the biological functions of proteins and peptides. Despite the importance of phosphate groups, polymer surfaces have not been directly grafted with phosphate groups by chemical reactions because the usual organic solvents used to graft phosphate groups can dissolve or swell polymers. We focused this study on grafting phosphate groups onto a poly(ethylene-co-acrylic acid) (PEAA) surface in an aqueous solution. O-phospho L-serine and O-phosphoethanolamine were grafted on PEAA surfaces to introduce phosphate groups by activating carboxylic acid groups of PEAA using N-hydroxysuccinimide (NHS) and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) in an aqueous environment. X-ray photoelectron spectroscopy (XPS) was used to elucidate the process by which surface grafting occurs and the process that the phosphate group is cleaved into a phosphate ion and a hydrolyzed molecule at high pH. It was found that under appropriate reaction conditions the phosphate groups could be successfully grafted on the polymer surfaces. The phosphate-grafted polymer surfaces showed lower water contact angles than the initial polymer surfaces likely due to their highly mobile and hydrophilic phosphate side groups. This work demonstrates a technique to successfully graft phosphate groups onto organic polymer surfaces in a biocompatible aqueous environment, which may open new avenues to functionalizing synthetic polymeric and natural macromolecule derived biomaterials.     

Interactions of benzoic acid and phosphates with iron oxide colloids using chemical force titration

Colloidal iron oxides are an important component in soil systems and in water treatment processes. Humic-based organic compounds, containing both phenol and benzoate functional groups, are often present in these systems and compete strongly with phosphate species for binding sites on the iron oxide surfaces. Here, we examine the interaction of benzoate and phenolic groups with various iron oxide colloids using atomic force microscopy (AFM) chemical force titration measurements. Self-assembled monolayers (SAMs) of 4-(12-mercaptododecyloxy)benzoic acid and 4-(12-mercaptododecyloxy)phenol were used to prepare chemically modified Au-coated AFM tips, and these were used to probe the surface chemistry of a series of iron oxide colloids. The SAMs formed were also characterized using scanning tunneling microscopy, reflection-absorption infrared spectroscopy, and X-ray photoelectron spectroscopy. The surface pK(a) of 4-(12- mercaptododecyloxy)benzoic acid has been determined to be 4.0 +/- 0.5, and the interaction between the tip and the sample coated with a SAM of this species is dominated by hydrogen bonding. The chemical force titraton profile for an AFM probe coated with 4-(12- mercaptododecyloxy)benzoic acid and a bare iron oxide colloid demonstrates that the benzoic acid function group interacts with all three types of iron oxide sites present on the colloid surface over a wide pH range. Similar experiments were carried out on colloids precipitated in the presence of phosphoric, gallic, and tannic acids. The results are discussed in the context of the competitive binding interactions of solution species present in soils or in water treatment processes.     

X‐ray action on polymeric membrane surfaces: a chemical and morphological characterization

The surfaces of six polymeric membranes—two polysulphone membranes, two composite reverse‐osmosis polyamide/polysulphone membranes having polyamide as the active layer and two activated membranes containing di‐2‐ethylhexylphosphoric acid and di‐2‐ethylhexyldithiophosphoric acid as carriers, respectively—have been characterized before and after irradiation with an x‐ray source, both chemically and topographically by XPS and atomic force microscopy (AFM), respectively. Changes in atomic concentrations of the characteristic elements of the membranes and in the shape of XPS spectra as a function of irradiation time can be related to chemical modifications on the membrane surface. The most significant changes have been observed for polysulphone, which is reduced by x‐ray action; this fact also shows the inhomogeneity of the surface of the di‐2‐ethylhexyldithiophosphoric‐activated membrane. In contrast, polyamide top layers of composite membranes have been shown to be the most stable. Chemical modifications are not related directly to changes in membrane roughness because for all membranes only small changes have been observed for AFM images recorded before and after membrane irradiation. Moreover, the roughness of both polysulphone membranes decreases slightly due to x‐ray radiation but increases slightly for all polyamide‐containing membranes (composite and activated membranes). Copyright © 2003 John Wiley & Sons, Ltd.     

Surface study of sintered alumina substrates using solution‐processed ZnO nanorods as a microscopic wettability indicator

Wettability and its distribution are crucial factors that indicate the surface conditions of substrates. We report a surface study of sintered alumina substrates using solution‐processed ZnO nanorods as a microscopic wettability indicator. The alumina substrates comprising of micrometer‐sized sintered grains were treated separately with ultraviolet/ozone (dry process) or ozone water (wet process), and their surface conditions were characterized by conventional surface analysis methods, such as water contact angle, X‐ray photoelectron spectroscopy and grazing angle attenuated total reflection Fourier transform infrared spectroscopy. The results showed that the alumina substrates treated with ultraviolet/ozone and ozone water had distinct clean surfaces compared to those without treatments, but no significant differences were noted between these two ozone‐based treatments. Then, as a wettability‐sensitive deposition technique, Pd‐catalyzed chemical deposition of ZnO nanorods was performed on the alumina substrates, which involved dip coating of Pd nanoparticles on the substrates in aqueous solutions, followed by the chemical solution growth of ZnO. Vertically aligned ZnO nanorods of ~85 nm in diameter were densely formed along a rough surface of the substrates. Morphological uniformity of the nanorods varied depending on the treatment condition; local surfaces with sufficient wettability provided uniform nanorods but those with insufficient wettability gave irregular nanorods, making the visualization of the microscopic surface wettability possible. Copyright © 2016 John Wiley & Sons, Ltd.     

Surface initiated ring-opening polymerization of l-proline N-carboxy anhydride from single and multi walled carbon nanotubes

Ring-opening surface initiated polymerization of l-proline N-carboxyanhydride was performed from amine functionalized single (SWNTs) and multi walled carbon nanotubes (MWNTs). The primary amines were grafted on the surfaces via a well-studied Diels–Alder cycloaddition. The initiator attachment helped the debundling of carbon nanotubes as shown by atomic force microscopy (AFM) studies where only small aggregates were observed. Thermogravimetric analysis revealed high wt% of grafted polyproline on the carbon nanotubes surface after the ring-opening polymerization. AFM studies showed a rather uniform layer of grafted polyproline from both MWNTs and SWNTs. The grafting of PLP on the surface was also verified by FTIR and Raman spectroscopy as well as ¹H NMR in CDCl₃/d-TFA. The polyproline grafted carbon nanotubes (CNTs) were readily dissolved in organic solvents in contrast to the insoluble pristine and amine-functionalized CNTs.     

Nanoscale patterning of flat carbon surfaces by scanning probe lithography and electrochemistry

We report the formation of carbon surfaces patterned at the nanoscale with organic functionalities. Thin (<10 nm) films are covalently grafted to the surface via the electrochemical reduction of aryl diazonium salts. Areas of the film are removed with an AFM tip, and a second modifier is electrochemically grafted to the exposed surface. The pattern can incorporate different chemical functionalities, or alternatively topographical patterns can be assembled, where the same functionality is present throughout the pattern.     

Glass bead grafting with poly(carboxylic acid) polymers and maleic anhydride copolymers

Glass beads were etched with acids and bases to increase the surface porosity and the number of silanol groups that could be used for grafting materials to the surfaces. The pretreated glass beads were functionalized using 3‐aminopropyltriethoxysilane (APS) coupling agent and then further chemically modified by reacting the carboxyl groups of carboxylic acid polymers with the amino groups of the pregrafted APS. Several carboxylic acid polymers and poly(maleic anhydride) copolymers, such as poly(acrylic acid) (PAA), poly(methacrylic acid) (PMA), poly(styrene‐alt‐maleic anhydride) (PSMA), and poly(ethylene‐alt‐maleic anhydride) (PEMA) were grafted onto the bead surface. The chemical modifications were investigated and characterized by FT‐IR spectroscopy, particle size analysis, and tensiometry for contact angle and porosity changes. The amount of APS and the different polymer grafted on the surface was determined from thermal gravimetric analysis and elemental analysis data. Spectroscopic studies and elemental analysis data showed that carboxylic acid polymers and maleic anhydride copolymers were chemically attached to the glass bead surface. The improved surface properties of surface modified glass beads were determined by measuring water and hexane penetration rates and contact angle. Contact angles increased and porosity decreased as the molecular weights of the polymer increased. The contact angles increased with the hydrophobicity of the attached polymer. The surface morphology was examined by scanning electron microscopy (SEM) and showed an increase in roughness for etched glass beads. Copyright © 2007 John Wiley & Sons, Ltd.     

Humidity-dependent surface tension measurements of individual inorganic and organic submicrometre liquid particles

Surface tension, an important property of liquids, is easily measured for bulk samples. However, for droplets smaller than one micron in size, there are currently no reported measurements. In this study, atomic force microscopy (AFM) and force spectroscopy have been utilized to measure surface tension of individual submicron sized droplets at ambient pressure and controlled relative humidity (RH). Since the surface tension of atmospheric aerosols is a key factor in understanding aerosol climate effects, three atmospherically relevant systems (NaCl, malonic and glutaric acids) were studied. Single particle AFM measurements were successfully implemented in measuring the surface tension of deliquesced particles on the order of 200 to 500 nm in diameter. Deliquesced particles continuously uptake water at high RH, which changes the concentration and surface tension of the droplets. Therefore, surface tension as a function of RH was measured. AFM based surface tension measurements are close to predicted values based on bulk measurements and activities of these three chemical systems. Non-ideal behaviour in concentrated organic acid droplets is thought to be important and the reason for differences observed between bulk solution predictions and AFM data. Consequently, these measurements are crucial in order to improve atmospheric climate models as direct measurements hitherto have been previously inaccessible due to instrument limitations.     

PSSA pore-filled PVDF membranes by simultaneous electron beam irradiation: Preparation and transport characteristics of protons and methanol

Polystyrene sulfonic acid (PSSA) pore-filled poly(vinylidene fluoride) (PVDF) membranes have been prepared using simultaneous electron irradiation method. Porous PVDF films were grafted by pre-swelling in styrene solution and subsequent irradiation with an electron beam (EB) under nitrogen atmosphere and at ambient temperature. The grafted films i.e. polystyrene (PS) pore-filled PVDF were subsequently sulfonated with a diluted mixture of chlorosufonic acid. The effects of the reaction parameters on the content of PS grafted in the pores of PVDF films were investigated. The chemical and morphological properties of the membranes in comparison with their un-grafted and grafted counterparts were studied by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The transport properties of these membranes such as ionic conductivity and methanol permeability were evaluated and correlated with the content of PS grafted in the pores of the PVDF films. The PSSA pore-filled PVDF membranes with PS content in the pores of 40% and above showed superior performance characteristics compared to Nafion 117 membrane and therefore can be potential alternatives to improve the performance of direct methanol fuel cell (DMFC).     

Photosensitive Azopolymer Brushes via Atom Transfer Radical Polymerization for Protein Sensing

Novel photosensitive azopolymer brushes were synthesized via surface initiated atom transfer radical polymerization using initiator self‐assembled on Au surface. The chemical structures of azobenzene derivatives were confirmed by Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance spectroscopy (NMR). The surface morphology of azopolymers via atom transfer radical polymerization (ATRP) for different time was investigated by atomic force microscopy (AFM). Additionally, the photoisomerization of azopolymer was measured by ultraviolet‐visible spectroscopy (UV‐Vis). The results indicate that such azopolymers can undergo trans‐cis‐trans photoisomerization efficiently by photo‐irradiation with UV light. Furthermore, this photoisomerization property could also induce the reversible adsorption of bovine serum albumin (BSA) adsorption on azopolymer brush surfaces. This adsorption kinetics of the reversible process can be measured by surface plasmon resonance (SPR) spectroscopy in situ. It suggests that the protein biochips could be regenerated safely by UV irradiation rather than by being rinsed with chemical reagents.     

Surface‐initiated atom transfer radical polymerization grafting of poly(2,2,2‐trifluoroethyl methacrylate) from flat silicon wafer surfaces

Poly(2,2,2‐trifluoroethyl methacrylate) (PTFEMA), a partially fluorinated polymer, was directly grafted from silicon wafer surfaces by a surface‐initiated atom‐transfer radical polymerization (ATRP). The polymer layer thickness increased linearly with monomer conversion and molecular weight of free polymers in solution. The thickness was mainly determined by the experimental conditions such as activator/deactivator ratio, monomer/catalyst ratio, and monomer concentration. PTFEMA layers of more than 100‐nm thick were obtained. The grafted PTFEMA chains were “living” and allowed the extension of a second block of PMMA. X‐ray photoelectron spectroscopy study showed that the chemical compositions at the surfaces agreed well with their theoretical values. A novel surface‐attachable difunctional initiator was also synthesized and applied to the grafting of PTFEMA. The grafting density was doubled using this difunctional initiator, from 0.48 to 0.86 chains/nm². © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1252–1262, 2006     

Characterization of nano‐organized multilayers constituted by successive inorganic (gold) and organic (alkylthiols) layers

Nanostructured multilayers constituted by alternate metallic (gold) and organic (alkyldithiol) layers, and grafted onto glass or silicon substrates are prepared and analysed. Such complex layers could be of interest as a new type of surfaces but also as localized dissipative zones particularly in the field of adhesion science. The formation and the structure of these model systems are examined using a number of techniques such as atomic force microscopy (AFM), wetting analysis (contact angles), X‐ray photoelectron spectroscopy (XPS) and conductivity measurements. It is shown that, in terms of electrical conductivity, gold layers exhibit a percolation transition from an insulating granular structure to a conductive worm‐like structure at a threshold thickness of about 5 nm. XPS (and wettability) analyses clearly indicate that the fractional coverage of the gold surface is about 30% with alkyldithiol and that these molecules are either grafted in a stand‐up position or in the form of a loop. Moreover, a partial electrical connection between two successive gold layers is observed, confirming that the confined organic layer of alkyldithiol between them is too loosely organized to play the role of an insulating barrier. Copyright © 2007 John Wiley & Sons, Ltd.     

Post‐Plasma Grafting of AEMA as a Versatile Tool to Biofunctionalise Polyesters for Tissue Engineering

In the last decade, substantial research in the field of post‐plasma grafting surface modification has focussed on the introduction of carboxylic acids on surfaces by grafting acrylic acid (AAc). In the present work, we report on an alternative approach for biomaterial surface functionalisation. Thin poly‐ε‐caprolactone (PCL) films were subjected to a dielectric barrier discharge Ar‐plasma followed by the grafting of 2‐aminoethyl methacrylate (AEMA) under UV‐irradiation. X‐ray photoelectron spectroscopy (XPS) confirmed the presence of nitrogen. The ninhydrin assay demonstrated, both quantitatively and qualitatively, the presence of free amines on the surface. Confocal fluorescence microscopy (CFM), atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to visualise the grafted surfaces, indicating the presence of pAEMA. Static contact angle (SCA) measurements indicated a permanent increase in hydrophilicity. Furthermore, the AEMA grafted surfaces were applied for comparing the physisorption and covalent immobilisation of gelatin. CFM demonstrated that only the covalent immobilisation lead to a complete coverage of the surface. Those gelatin‐coated surfaces obtained were further coated using fibronectin. Osteosarcoma cells demonstrated better cell‐adhesion and cell‐viability on the modified surfaces, compared to the pure PCL films.

     

Surface grafted sulfobetaine polymers via atom transfer radical polymerization as superlow fouling coatings

One of the sulfobetaine methacrylate (SBMA) monomers, N-(3-sulfopropyl)-N-(methacryloxyethyl)-N,N-dimethylammonium betaine, was polymerized onto initiator-covered gold surfaces using atom transfer radical polymerization (ATRP) to form uniform polymer brushes. Self-assembled monolayers (SAMs) with ATRP initiators were characterized by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The thickness of grafted poly(SBMA) films was measured by ellipsometry. Fibrinogen adsorption on poly(SBMA) grafted surfaces was measured with a surface plasmon resonance (SPR) sensor. Two approaches were compared to graft ATRP initiators onto gold surfaces for surface polymerization and subsequent protein adsorption on these polymer grafted surfaces. The first was to prepare a SAM from omega-mercaptoundecyl bromoisobutyrate onto a gold surface. Superlow fouling surfaces with well-controlled poly(SBMA) brushes were achieved using this approach (e.g., fibrinogen adsorption <0.3 ng/cm2). The second approach was to react bromoisobutyryl bromide with a hydroxyl-terminated SAM on a gold surface. Although protein adsorption decreased as the density of surface initiators increased, the surface prepared using the second approach was not able to achieve as low protein adsorption as the first approach. Key parameters to achieve superlow fouling surfaces were studied and discussed.