Surface Grafting Density Research Articles

Surface plasmon optical study of the interfacial phase transition of elastinlike polypeptide grafted on gold

The conformational changes in elastinlike polypeptides (ELPs) grafted to a solid/solution interface via different architectures were studied using surface plasmon resonance spectroscopy and surface plasmon field-enhanced fluorescence spectroscopy (SPFS). SPFS provides a simple and convenient optical method to study the influence of the grafting method and the graft density on the conformational changes in ELPs at the solid-solution interface as a function of environmental variables. A typical response of the ELP, consistent with its stimuli responsiveness, was a gradual collapse upon increasing the ionic strength; this effect was inversely correlated with the surface graft density of the ELP.

Effect of Fluctuations in the Brush Conformation on the Interaction Between Polymer Brushes in a Good Solvent

This study presents a novel approach for analyzing the interaction between two parallel surfaces grafted with polymer brushes in a good solvent. In the proposed approach, molecular dynamics simulations are performed to establish the mean brush height and the standard deviation of the brush height distribution for a given value of the surface separation. The corresponding probability density function (PDF) of the brush height is then determined and a statistical technique is applied to compute the corresponding interaction free energy per unit area of the grafted substrates. Finally, the Derjaguin approximation is employed to determine the corresponding value of the interaction force between the two surfaces. At relatively high surface grafting density as well as under low to moderate compressions of these two parallel plates, the interdigitation effect of the brushes is quite weak and is not considered in the present study. The results obtained for the interaction free energy and interaction force are compared with those derived using the Alexander and de Gennes (AdG) model [1977, “Adsorption of Chain Molecules With a Polar Head. A Scaling Approach,” J. Phys. (Paris), 38, pp. 983–989, 1985, “Films of Polymer-Solutions,” C. R. Acad. Sci., 300, pp. 839–843] and the Milner, Witten, and Cates (MWC) model [1988, “Theory of the Grafted Polymer Brush,” Macromolecules, 21, pp. 2610–2619], respectively. The value of the normalized interaction free energy computed using the present method is higher than that obtained from the AdG and MWC models at larger surface separations. However, the three sets of results are in good agreement particularly at smaller values of the surface separation. In addition, the results obtained by the current method for the interaction force are found to be in better agreement with the experimental data than those obtained using the AdG or MWC models. The enhanced performance of the proposed method is attributed primarily to the use of an adaptive non-Gaussian PDF of the brush height to model the effects of fluctuations in the brush conformation at different distances from the grafting plane.

Layer-by-Layer Deposition of Polyelectrolyte Macroinitiators for Enhanced Initiator Density in Surface-Initiated ATRP

The layer-by-layer (L-b-L) deposition of oppositely charged polyelectrolytic macroinitiators has been demonstrated on planar silica substrates. The build-up of the macroinitiator multilayers was monitored by ellipsometry (up to 21 layers) and dual polarization interferometry (up to 17 layers) and good agreement was found between these techniques. The increase in L-b-L thickness was approximately linear, with an average thickness of 2.3 A per layer of deposited macroinitiator. Surface-initiated ATRP of a model nonionic methacrylic monomer, 2-hydroxyethyl methacrylate (HEMA) in a 1:1 methanol/water mixture was conducted at ambient temperature. Increasing the number of macroinitiator layers led to a significant increase in PHEMA brush thickness up to 110 nm, which is attributed to the greater surface grafting density. PHEMA brush thicknesses obtained after 22 h showed a linear dependence on the number of layers of deposited macro-initiator, with all layers exhibiting near-identical growth kinetics. X-ray photoelectron spectroscopy was used to monitor L-b-L assembly and also to confirm PHEMA growth. This technique indicated the loss of small counterions from the multilayers during L-b-L deposition and confirmed an increase in the surface density of bromoester initiator groups as the number of deposited macroinitiator layers was increased. For 17 macroinitiator layers, the bromoester initiator density is estimated to be approximately 4.9 +/- 0.2 nm (-2) from the DPI data. This is comparable to that calculated for ATRP initiator monolayers obtained by either thiol or silane chemistry. Ellipsometry suggested that the macroinitiator multilayers were weakly hydrated prior to the in situ HEMA polymerization. AFM studies indicated that the PHEMA brushes had appreciable surface roughness, but this roughness became negligible compared to the brush thickness with increasing macroinitiator layers.

Effects of chain architectures on the surface structures of conjugated rod-coil block copolymer brushes

Rod-coil block copolymers are of unique and interesting characteristics since their physical properties can be reversibly tuned in response to the external stimuli, such as change in solvent quality. In this study, dissipative particle dynamics is used to investigate the surface structures of rod-coil polymer brushes tethered onto a surface. When immersed in the selective solvent for the coil blocks, rod blocks tend to form aggregates. Our results show that linear and Y-shaped polymer brushes exhibit similar aggregative behavior. However, some of the surface structures can be acquired within experimentally attainable surface grafting density only for Y-shaped polymer brushes. On the other hand, comblike polymer brushes are found to possess more diverse aggregative manners than linear brushes. Surface structures with aggregates taking the forms of cones, cylinders, or layers of spheres are found. By controlling the aggregative structures, it is possible for us to adjust the physical properties, such as optical function, of the material.

Chemical Functionalization of Diamond Surfaces by Reaction with Diaryl Carbenes

A rapid route to the chemical functionalization of hydrogen-terminated diamond surfaces deposited by chemical vapor deposition involving their reaction with substituted diaryl carbenes has been investigated. To avoid difficulties in the handling of highly reactive compounds, the carbene is generated in situ from the thermal decomposition at 400 K of a thin film of the corresponding diaryl diazomethane precursor deposited at the diamond interface. X-ray photoelectron spectroscopy (XPS) has been used to verify that surface functionalization using two starting compounds, bis(4-iodophenyl) diazomethane and bis(4-nitrophenyl) diazomethane, can be achieved using this approach in agreement with recent theoretical studies. The surface grafting density is measured to be around 10(14) cm(-2) in each case. The chemistry observed is found to be insensitive to the detailed properties of the diamond film and to the presence of oxygen contamination at the hydrogen-terminated diamond surface. We further demonstrate the utility of the approach, in the case of the bound nitrophenyl compound, by its reduction to the corresponding primary amine followed by reaction with fluorescein isothiocyanate to achieve fluorescent tagging of the diamond interface.

A Langevin dynamics simulation study of the tribology of polymer loop brushes

The tribology of surfaces modified with doubly bound polymer chains (loops) has been investigated in good solvent conditions using Langevin dynamics simulations. The density profiles, brush interpenetration, chain inclination, normal forces, and shear forces for two flat substrates modified by doubly bound bead-necklace polymers and equivalent singly bound polymers (twice as many polymer chains of 12 the molecular weight of the loop chains) were determined and compared as a function of surface separation, grafting density, and shear velocity. The doubly bound polymer layers showed less interpenetration with decreasing separation than the equivalent singly bound layers. Surprisingly, this difference in interpenetration between doubly bound polymer and singly bound polymer did not result in decreased friction at high shear velocity possibly due to the decreased ability of the doubly bound chains to deform in response to the applied shear. However, at lower shear velocity, where deformation of the chains in the flow direction is less pronounced and the difference in interpenetration is greater between the doubly bound and singly bound chains, some reduction in friction was observed.

Synthesis, patterning and applications of star-shaped poly(ethylene glycol) biofunctionalized surfaces

Poly(ethylene) glycol (PEG) is an excellent material to modify surfaces to resist non-specific protein adsorption. Linear PEG has been extensively studied both theoretically and experimentally and it has been found that resistance of PEG-coated surfaces to protein adsorption depends mainly on the molecular weight of the polymer and the surface grafting density. End-functionalized star-shaped PEGs allow for interpolymer crosslinking to form a dense layer. An excellent example of such a system consists of a 6-arm PEG/PPG (4 : 1) star polymer functionalized with isocyanate using IPDI. The end functionalization may be further biofunctionalized to recognize specific biomolecules such as streptavidin, His-tagged proteins, amino-terminated oligonucleotides and cell receptors. This functionalization may be patterned into specific geometries using stamping techniques or randomly distributed by statistical reaction of the end group with the biofunctional molecule in solution. The surface preparation uses simple spin-, dip- or spray-coating and produces smooth layers with low background fluorescence. These properties, together with the advantageous chemical properties of PEG, render the surfaces ideal for immobilizing proteins on surfaces with detection limits down to the single molecule level. Proteins immobilized on such surfaces are able to maintain their folded, functional form and are able to completely refold if temporarily exposed to denaturing conditions. Immobilized enzyme molecules were able to perform their function with the same activity as the enzyme in solution. Future directions of using surfaces coated with such crosslinked star polymers in highly sensitive and robust biotechnology applications will be discussed.

Scaling Behavior of Nanopatterned Polymer Brushes

Brushing up: Experimental polymer-brush heights obtained for different footprint sizes, molecular weights, and surface-grafting densities are analyzed using recent scaling relations obtained from a coarse-grained model. The good agreement between the experimental and theoretical results indicates that the scaling predictions are useful in the design and fabrication of polymeric and bio-macromolecular nanostructures. The image shows a molecular-dynamics simulation of nanopatterned polymer brushes.

Diffusion-Limited Patterning of Molecules in Nanofluidic Channels

Diffusion-limited patterning (DLP) is a new technique that enables patterning of labile molecular species in solution phase onto surfaces that are not easily accessible. This technique is self-aligning and is simple to implement for patterning multiple species. We demonstrated DLP by patterning alternating bands of fluorescently labeled and unlabeled streptavidin in biotin-functionalized nanofluidic channels with spatial resolution better than 1 microm. The methodology of DLP also enables experimental measurement of a unique parameter that relates molecular surface grafting density, concentration, diffusivity, and channel geometry.

Role of Surface-Anchored Polymer Chains on the Adhesion of an Elastomer

We study the adhesion between an elastomer of cross-link index, P, and a flat solid surface where polymer chains of polymerisation index, N, have been end-grafted. To understand the adhesive feature of such a system, one has to study both the origin of the grafted layer interdigitation with the network and the end-grafted chain extraction out of the elastomer when it separates from the solid surface. Both aspects are addressed here. We develop an analytical model that allows one to predict a critical surface grafting density, σ*≃P 1/10/N 3/5, beyond which only the thermal fluctuations allow the layer to interdigitate with the elastomer. A review of the possible mechanisms of chain extraction is proposed.

Switchable surface traps for injectable bead-based chromatography in PDMS microfluidic channels

We report here a reversible microchannel surface capture system for stimuli-responsive grafted bioanalytical beads. Poly(N-isopropylacrylamide) (PNIPAAm) was grafted onto polydimethylsiloxane (PDMS) surfaces by a UV-mediated graft polymerization from a photoinitiator that was preadsorbed in the channel wall. The surface grafting density and resulting switchable hydrophilic/hydrophobic properties were controlled by varying the photo-illumination times and/or the initiator concentration. At limiting PNIPAAm-graft densities, the surfaces demonstrated minimal contact angles of 35 degrees below the lower critical solution temperature (LCST) and maximal contact angles of 82 degrees above it. These contact angles could be varied depending on the graft density. The surface grafts are spatially limited to the photo-illuminated region to define where the trap is constructed. The surface traps capture PNIPAAm-grafted nanobeads uniformly above the LCST and facilitate their rapid release as the temperature is reversed to below the LCST. This dual surface trap and injectable chromatography system could be useful in many applications, such as affinity separations, immunoassays, and enzyme bioprocesses, by providing for the controlled capture and release of chromatography beads.

Dextran Functionalized Surfaces via Reductive Amination: Morphology, Wetting, and Adhesion

Dextran surface grafting density was systematically varied via a two-step process involving SiO(2) amination by aminopropyltriethoxy silane (APTES) followed by oxidized dextran (M(w) = 110 kDa) chemisorption. Dextran oxidation kinetics with sodium metaperiodate (NaIO(4)) were quantified by (1)H NMR and pH measurements. Aldehyde group formation increased with increasing oxidation time. For 0.5 h oxidation time, dried film ellipsometric thickness was constant for solution concentrations ranging from 1 to 4 mg/mL. Dextran layers with the lowest grafting density wetted fastest and displayed the lowest contact angle (theta(APTES) > theta(1 h) > theta(2,4 h) > theta(0.5 h)). Under aqueous conditions, AFM force versus displacement measurements on 0.5 and 4.0 h surfaces exhibited a single displacement jump upon retraction. The 1.0 and 2.0 h surfaces showed two jumps consistent with two populations of chains, namely, loosely and strongly bound dextran. Overall, film morphology and wetting behavior were relatively invariant with grafting density, confirming the method's robustness for preparing biomimetic coatings with consistent properties.

Chemomechanics of Surface Stresses Induced by DNA Hybridization

When biomolecular reactions occur on one surface of a microcantilever beam, changes in intermolecular forces create surface stresses that bend the cantilever. While this phenomenon has been exploited to create label-free biosensors and biomolecular actuators, the mechanisms through which chemical free energy is transduced to mechanical work in such hybrid systems are not fully understood. To gain insight into these mechanisms, we use DNA hybridization as a model reaction system. We first show that the surface grafting density of single-stranded probe DNA (sspDNA) on a surface is strongly correlated to its radius of gyration in solution, which in turn depends on its persistence length and the DNA chain length. Since the persistence length depends on ionic strength, the grafting density of sspDNA can be controlled by changing a solution's ionic strength. The surface stresses produced by the reaction of complementary single-stranded target DNA (sstDNA) to sspDNA depend on the length of DNA, the grafting density, and the hybridization efficiency. We, however, observe a remarkable trend: regardless of the length and grafting density of sspDNA, the surface stress follows an exponential scaling relation with the density of hybridized sspDNA.

Effects of Ionic Strength and Surface Charge on Protein Adsorption at PEGylated Surfaces

PEGylated Nb2O5 surfaces were obtained by the adsorption of poly(L-lysine)-g-poly(ethylene glycol) (PLL-g-PEG) copolymers, allowing control of the PEG surface density, as well as the surface charge. PEG (MW 2 kDa) surface densities between 0 and 0.5 nm(-2) were obtained by changing the PEG to lysine-mer ratio in the PLL-g-PEG polymer, resulting in net positive, negative and neutral surfaces. Colloid probe atomic force microscopy (AFM) was used to characterize the interfacial forces associated with the different surfaces. The AFM force analysis revealed interplay between electrical double layer and steric interactions, thus providing information on the surface charge and on the PEG layer thickness as a function of copolymer architecture. Adsorption of the model proteins lysozyme, alpha-lactalbumin, and myoglobin onto the various PEGylated surfaces was performed to investigate the effect of protein charge. In addition, adsorption experiments were performed over a range of ionic strengths, to study the role of electrostatic forces between surface charges and proteins acting through the PEG layer. The adsorbed mass of protein, measured by optical waveguide lightmode spectroscopy (OWLS), was shown to depend on a combination of surface charge, protein charge, PEG thickness, and grafting density. At high grafting density and high ionic strength, the steric barrier properties of PEG determine the net interfacial force. At low ionic strength, however, the electrical double layer thickness exceeds the thickness of the PEG layer, and surface charges "shining through" the PEG layer contribute to protein interactions with PLL-g-PEG coated surfaces. The combination of AFM surface force measurements and protein adsorption experiments provides insights into the interfacial forces associated with various PEGylated surfaces and the mechanisms of protein resistance.

Surface and bulk dissolution properties, and selectivity of DNA-linked nanoparticle assemblies

Using a simple mean-field model, we analyze the surface and bulk dissolution properties of DNA-linked nanoparticle assemblies. We find that the dissolution temperature and the sharpness of the dissolution profiles increase with the grafting density of the single-stranded DNA "probes" on the surface of colloids. The surface grafting density is controlled by the linker occupation number, in analogy with quantum particles obeying fractional statistics. The dissolution temperature increases logarithmically with the salt concentration. This is in agreement with the experimental findings [R. Jin, G. Wu, Z. Li, C. A. Mirkin, and G. C. Schatz, J. Am. Chem. Soc. 125, 1643 (2003)]. By exploiting the unique phase behavior of DNA-coated colloids, it should be possible to detect multiple "targets" in a single experiment by essentially mapping the DNA base-pair sequence onto the phase behavior of DNA-linked nanoparticle solution.

Characterization of Grafting Density and Binding Efficiency of DNA and Proteins on Gold Surfaces

The surface grafting density of biomolecules is an important factor for quantitative assays using a wide range of biological sensors. We use a fluorescent measurement technique to characterize the immobilization density of thiolated probe DNA on gold and hybridization efficiency of target DNA as a function of oligonucleotide length and salt concentration. The results indicate the dominance of osmotic and hydration forces in different regimes of salt concentration, which was used to validate previous simulations and to optimize the performance of surface-stress based microcantilever biosensors. The difference in hybridization density between complementary and mismatched target sequences was also measured to understand the response of these sensors in base-pair mismatch detection experiments. Finally, two different techniques for immobilizing proteins on gold were considered and the surface densities obtained in both cases were compared.

Synthesis and Characterization of Novel Silicon‐Functional Polyisobutylenes and Their Applications: Polyisobutylene Brushes on Silicate Substrates via Living Cationic Polymerization

A series of novel chlorosilyl functional initiators have been prepared and applied for the first time in the living cationic polymerization of isobutylene (IB). Well‐defined polyisobutylenes (PIBs) carrying mono‐, di‐, and tri‐chlorosilyl head‐group, and a tert‐chloro end‐group were synthesized using newly designed silyl‐functional initiators in conjunction with TiCl4 in Hex:MeCl (60:40, v:v) at −80°C. End‐group analysis by 1H NMR spectroscopy verified the product structure and the survival of the Si‒Cl head‐groups during the polymerization. The chlorosilyl functional initiators and chlorosilyl functional PIBs have been employed for the synthesis of PIB brushes on planar silicate substrates by the “grafting from” and “grafting to” techniques. The products were characterized by x‐ray photoelectron spectra (XPS), atomic force microscopic (AFM), and ellipsometry. The “grafting from” approach yielded surface‐bound polymer brushes with more uniform surface and higher grafting density compared to the “grafting to” technique. The film thickness of PIB brushes formed by “grafting from” increased linearly with increasing molecular weight of PIB. In contrast, PIB brushes produced by “grafting to” gave constant thickness independent of the molecular weight of PIB.

Entanglement Network of the Polypropylene/Polyamide Interface. 3. Deformation to Fracture

Recently (Macromolecules 2000, 33, 1385, 1396), we have proposed a novel algorithm for generating entanglement network specimens of interfacial polymeric systems. The specimens are created by sampling the configurational distribution functions derived from a self-consistent mean field lattice model. Although overstretched strands can be relaxed by a Monte Carlo (MC) method, the specimens generated are not in detailed mechanical equilibrium. In this paper, we develop a method for relaxing the network with respect to its density distribution, and thereby imposing the condition of mechanical equilibrium, without changing the network topology. Our method rests on minimizing a free energy function characterizing the network, with respect to the coordinates of all entanglement points and chain ends. Contributions to the free energy include (a) the elastic energy due to stretching of the chain strands and (b) the free energy due to the repulsive and attractive (cohesive) interactions between segments. To calculate the interaction energy, a simple cubic grid is superimposed on the network. The repulsive interactions are calculated within each grid cell. The attractive interactions are calculated from contributions between cells and within each cell. We first apply the free energy minimization procedure to a simple system (bulk polypropylene) and obtain results which agree satisfactorily with experimental data and serve as a basis for parametrizing the interaction model. The free energy minimization is then applied to mechanically relax computer specimens of a polypropylene/polyamide6 (PP/PA6) interfacial system, strengthened by PP chains grafted onto the PA6 surface. The fully relaxed networks serve as a starting point for the mesoscopic simulation of fracture phenomena, caused by the application of tensile stress perpendicular to the interface. The network is deformed at a constant strain rate and the network topology evolves according to elementary mechanical processes of chain scission, chain slippage, disentanglement, and reentanglement. Chain slippage across an entanglement point occurs according to a Zhurkov activated rate equation with parameters derived from viscosity data. Each cycle of the kinetic MC algorithm used to track the deformation process consists of the imposition of a small incremental strain on the network, relaxation to mechanical equilibrium, introduction of the micromechanical processes mentioned above, and again relaxation to mechanical equilibrium. The MC cycles are repeated until fracture occurs. Results of the fracture experiments confirm the assertion, previously founded on structural grounds, that optimal adhesion in the PP/PA6 system examined is achieved for a relatively low surface grafting density of 0.1 chains/nm2.

Thermodynamic Properties and Swelling Behavior of Glycolipid Monolayers at Interfaces

Synthetic glycolipids with lactose headgroups (N = 1, 2, and 3) were synthesized, and their thermodynamic properties were systematically studied by Langmuir isotherms using a film balance. The molar transition entropy and the molar latent heat were calculated by applying the Clausius−Clapeyron equation. It has been demonstrated that the phase behavior of the glycolipid monolayers is comparable to that of ordinary phospholipids, despite the lower degree of cooperativity between the larger headgroups. The glycolipid monolayer was transferred onto a solid surface by Langmuir−Blodgett deposition, and the swelling behavior was investigated by ellipsometry. The surface grafting density was precisely controlled, and the water disjoining pressure inside the lactose layer was quantitatively measured. The measured swelling curves were analyzed in terms of the theoretical descriptions for the grafted polymer “brushes”. For the lipids with lactose units of N = 2 and 3, the disjoining pressure−thickness relation could fit very well to these theoretical approaches, even though the statistical limit N ≫ 1 is hardly fulfilled. The results suggest entropic effects of the headgroups on the interaction between the neighboring molecules. On the other hand, the theoretical description of the swelling behavior of the lipids with one lactose unit failed due to the “rodlike” structure of lactose. The unique properties of these glycolipids at interfaces, such as (i) the phase behavior comparable to that of ordinary phospholipids and (ii) the “polymer-like” swelling behavior, play very important roles in biological systems. Mimicking the complex interactions between oligosaccharide headgroups in the plasma membranes, the synthetic glycolipids designed in this study are quite realistic models for the glycocalix.

A hydroxyapatite coating covalently linked onto a silicone implant material.

A novel composite consisting of hydroxyapatite (HAp) microparticles covalently coupled onto a silicone sheet was developed. Initially, an acrylic acid (AAc) -grafted silicone sheet with a 16.7 microg/cm(2) surface graft density was prepared by corona-discharge treatment. The surface of sintered, spherical, carbonated HAp particles with an average diameter of 2.0 microm was subsequently modified with amino groups. The amino group surface density of the HAp particles was calculated to be approximately one amino molecule per 1.0 nm(2) of particle surface area. These samples were characterized with Fourier transform infrared spectrometry and X-ray photoelectron spectroscopy. After the formation of ammonium ionic bonds between both samples under aqueous conditions, they were reacted at 180 degrees C for 6 h in vacuo to form covalent bonds through a solid-phase condensation. The HAp particles were coupled to the AAc-grafted silicone surface by a covalent linkage. Further improvements in the adhesive and bioactive properties of the HAp-coated silicone material are expected.