Changes in Surface Free Energy and Surface Conductivity of Carbon Nanotube/Polyimide Nanocomposite Films Induced by UV Irradiation.

Changes in surface energy and electrical conductivity of polyimide (PI)-based nanocomposite films filled with carbon nanotubes (CNTs) induced by UV exposure are gaining considerable interest in microelectronic, aeronautical, and aerospace applications. However, the underlying mechanism of PI photochemistry and oxidation reactions induced by UV irradiation upon the surface in the presence of CNTs is still not clear. Here, we probed the interplay between CNTs and PIs under UV exposure in the surface properties of CNT/PI nanocomposite films. Changes in contact angles and surface electrical conductivity at the surface of CNT/PI nanocomposite films after UV exposure were measured. The unpaired electron intensity of free radicals generated by UV exposure was monitored by electron paramagnetic resonance. Our study indicates that the covalent interactions between CNTs and radicals generated by UV irradiation on the PI surfaces tailor the surface energy and surface conductivity through anchoring radicals on CNTs. Surprisingly, adding CNTs into PI films exposed to UV leads to antagonistic contributions of dispersion and polar components to the surface energy. The surface electrical conductivity of the CNT/PI nanocomposite films has been improved due to an enhanced hopping behavior with dense π-conjugated CNT sites. To explain the observed changes in surface energy and surface conductivity of CNT/PI nanocomposite films induced by UV exposure, a qualitative model was put forward describing the covalent interactions between UV-induced PI free radicals and CNTs, which govern the chemical nature of surface components. This study is helpful for characterizing and optimizing nanocomposite surface properties by tuning the covalent interactions between components at the nanoscale.

Surface-enhanced Raman spectroscopy characterisation of functionalised multi-walled carbon nanotubes.

Multi-walled nanotube (MWNT) functionalisation was investigated by surface-enhanced Raman spectroscopy (SERS). The MWNTs were deposited as dilute dispersions on SERS-active substrates. We used nano-structured gold surfaces with various morphologies for our measurements. The surface enhancement effect was used to amplify the Raman signal from functional molecules bound to the nanotube walls. The recorded spectral features allowed for discrimination between the differently functionalised MWNTs. Although the present study is limited to a few examples, our measurements indicate the higher specificity obtained by the SERS approach and its possible use for a systematic study of functionalisation effects on MWNT structures.

Nanomaterial-based biosensors for a real-time detection of biological damage by UV light.

In this work, the design and fabrication of a miniaturized and light-weight biosensor that can be used to monitor the biological effects of hostile ultraviolet radiation in earth and space are presented. The biosensor is generated by embedding a sensitive element to UV radiation, DNA, in a hybrid carbon-based nanomaterial. In particular, we present results on the fabrication and characterization of hybrid nanostructured films containing graphene nanoplatelets (GNPs) and double-stranded DNA for the in situ and real-time detection of UV radiation damaging effects from the changes of the film electrical properties induced by exposure to UV-C radiation. The biosensor is realized by the deposition of the sensitive unit GNP/DNA on a supporting substrate made of flexible polymers or glass.

A supramolecular two-photon-active hydrogel platform for direct bioconjugation under near-infrared radiation.

A supramolecular hydrogel assembled from partially methacrylated polyethyleneimine (PEI) and with direct photopatterning capabilities at near-infrared (NIR) wavelengths is presented. The chemically modified branched PEI macromolecules were characterized by FTIR and NMR spectroscopy to quantify the degree of methacrylation. A highly hydrophilic polymer network with a water content up to 95% was prepared. The hydrogel microstructure in an aqueous solution was characterized using confocal laser scanning microscopy (CLSM), which revealed a porous network with large interconnected cavities. The photo-sensitive PEIMA hydrogel was activated by two-photon laser irradiation and micropatterns formed at its interface when probes with free carboxylic acid or hydroxyl groups were present in solution. Direct patterning of the hydrogel matrix with different biomolecules, and without additional photoinitiators, is demonstrated in two-photon microscopy experiments.

Smart polymer brush nanostructures guide the self-assembly of pore-spanning lipid bilayers with integrated membrane proteins.

Nanopores in arrays on silicon chips are functionalized with pH-responsive poly(methacrylic acid) (PMAA) brushes and used as supports for pore-spanning lipid bilayers with integrated membrane proteins. Robust platforms are created by the covalent grafting of polymer brushes using surface-initiated atom transfer radical polymerization (ATRP), resulting in sensor chips that can be successfully reused over several assays. His-tagged proteins are selectively and reversibly bound to the nitrilotriacetic acid (NTA) functionalization of the PMAA brush, and consequently lipid bilayer membranes are formed. The enhanced membrane resistance as determined by electrochemical impedance spectroscopy and free diffusion of dyed lipids observed as fluorescence recovery after photobleaching confirmed the presence of lipid bilayers. Immobilization of the His-tagged membrane proteins on the NTA-modified PMAA brush near the pore edges is characterized by fluorescence microscopy. This system allows us to adjust the protein density in free-standing bilayers, which are stabilized by the polymer brush underneath. The potential application of the integrated platform for ion channel protein assays is demonstrated.

Switching transport through nanopores with pH-responsive polymer brushes for controlled ion permeability.

Several nanoporous platforms were functionalized with pH-responsive poly(methacrylic acid) (PMAA) brushes using surface-initiated atom transfer radical polymerization (SI-ATRP). The growth of the PMAA brush and its pH-responsive behavior from the nanoporous platforms were confirmed by scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, and atomic force microscopy (AFM). The swelling behavior of the pH-responsive PMAA brushes grafted only from the nanopore walls was investigated by AFM in aqueous liquid environment with pH values of 4 and 8. AFM images displayed open nanopores at pH 4 and closed ones at pH 8, which rationalizes their use as gating platforms. Ion conductivity across the nanopores was investigated with current-voltage measurements at various pH values. Enhanced higher resistance across the nanopores was observed in a neutral polymer brush state (lower pH values) and lower resistance when the brush was charged (higher pH values). By adding a fluorescent dye in an environment of pH 4 or pH 8 at one side of the PMAA-brush functionalized nanopore array chips, diffusion across the nanopores was followed. These experiments displayed faster diffusion rates of the fluorescent molecules at pH 4 (PMAA neutral state, open pores) and slower diffusion at pH 8 (PMAA charged state, closed pores) showing the potential of this technology toward nanoscale valve applications.

Probing biofouling resistant polymer brush surfaces by atomic force microscopy based force spectroscopy.

The protein repellency and biofouling resistance of zwitterionic poly(sulfobetaine methacrylate)(pSBMA) brushes grafted via surface initiated polymerization (SIP) from silicon and glass substrata was assessed using atomic force microscopy (AFM) adherence experiments. Laboratory settlement assays were conducted with cypris larvae of the barnacle Balanus amphitrite. AFM adherence includes the determination of contact rupture forces when AFM probe tips are withdrawn from the substratum. When the surface of the AFM tip is modified, adherence can be assessed with chemical specifity using a method known as chemical force microscopy (CFM). In this study, AFM tips were chemically functionalized with (a) fibronectin- here used as model for a nonspecifically adhering protein - and (b) arginine-glycine-aspartic acid (RGD) peptide motifs covalently attached to poly(methacrylic acid) (PMAA) brushes as biomimics of cellular adhesion receptors. Fibronectin functionalized tips showed significantly reduced nonspecific adhesion to pSBMA-modified substrata compared to bare gold (2.3±0.75 nN) and octadecanethiol (ODT) self-assembled monolayers (1.3±0.75 nN). PMAA and PMAA-RGD modified probes showed no significant adhesion to pSBMA modified silicon substrata. The results gathered through AFM protein adherence studies were complemented by laboratory fouling studies, which showed no adhesion of cypris larvae of Balanus amphitrite on pSBMA. With regard to its unusually high non-specific adsorption to a wide variety of materials the behavior of fibronectin is analogous to the barnacle cyprid temporary adhesive that also binds well to surfaces differing in polarity, charge and free energy. The antifouling efficacy of pSBMA may, therefore, be directly related to the ability of this surface to resist nonspecific protein adsorption.

Reversible pH-controlled switching of poly(methacrylic acid) grafts for functional biointerfaces.

Responsive polymeric brushes of poly(methacrylic acid) (PMAA) were grafted from silicon surfaces using controlled surface-initiated atom-transfer radical polymerization (SI-ATRP). The growth kinetics of PMAA was investigated with respect to the composition of the ATRP medium by grafting the polymer in mixtures of water and methanol with different ratios. The dissociation behavior of the polymer layers was characterized by FTIR titration after incubating the polymer-grafted substrates in PBS buffer solutions with different pH values. PMAA layers show a strong pH-dependent behavior with an effective pK(a) of the bulk polymer brush of 6.5 ± 0.2, which is independent of the polymer brush thickness and methanol content of the ATRP grafting medium. The pH-induced swelling and collapse of the grafted polymer layers were quantified in real time by in situ ellipsometry in liquid environment. Switching between polymer conformations at pH values of 4 and 8 is rapid and reversible, and it is characterized by swelling factors (maximum thickness/minimum thickness) that increase with decreasing the methanol content of the SI-ATRP medium.

In situ monitoring of the catalytic activity of cytochrome C oxidase in a biomimetic architecture.

Cytochrome c oxidase (CcO) from Paracoccus denitrificans was immobilized in a strict orientation via a his-tag attached to subunit I on a gold film and reconstituted in situ into a protein-tethered bilayer lipid membrane. In this orientation, the cytochrome c (cyt c) binding site is directed away from the electrode pointing to the outer side of the protein-tethered bilayer lipid membrane architecture. The CcO can thus be activated by cyt c under aerobic conditions. Catalytic activity was monitored by impedance spectroscopy, as well as cyclic voltammetry. Cathodic and anodic currents of the CcO with cyt c added to the bulk solution were shown to increase under aerobic compared to anaerobic conditions. Catalytic activity was considered in terms of repeated electrochemical oxidation/reduction of the CcO/cyt c complex in the presence of oxygen. The communication of cyt c bound to the CcO with the electrode is discussed in terms of a hopping mechanism through the redox sites of the enzyme. Simulations supporting this hypothesis are included.

Binding of alkyl polyglucoside surfactants to bacteriorhodopsin and its relation to protein stability.

The binding of alkyl polyglucoside surfactants to the integral membrane protein bacteriorhodopsin (BR) and the formation of protein-surfactant complexes are investigated by sedimentation equilibrium via analytical ultracentrifugation and by small-angle neutron scattering (SANS). Contrast variation techniques in SANS enable measurement of the composition of the protein-surfactant complexes and determination of the thickness of the surfactant shell bound to the protein. The results indicate that alkyl polyglucosides can bind to BR as single surfactant layers or as a thicker shell. The thickness of the surfactant shell increases with increasing surfactant tail length, and it is generally unrelated to the aggregation number of the micelles even for a small and predominantly hydrophobic membrane protein such as BR. The aggregation numbers determined by sedimentation equilibrium methods match those measured by SANS, which also allows reconstruction of the shape of the protein-detergent complex. When the surfactant is present as a single layer, the BR loses activity, as measured by absorption spectroscopy, more quickly than it does when the surfactant forms a thicker shell.

Self-assembly of medium-chain alkyl monoglucosides in ammonium sulfate solutions with poly(ethylene glycol).

We study the phase behavior and microstructure of alkyl-beta-monoglucosides with intermediate chain lengths (octyl- and nonyl-beta-glucoside) in aqueous solutions containing ammonium sulfate and poly(ethylene glycol) (PEG). When the glucoside surfactants are mixed with PEG of molecular weight 3350 or larger, two different phase transitions are observed in the temperature range 0-100 degrees C, with lower and upper miscibility gaps separated by a one-phase isotropic region. Isothermal titration calorimetry is used to quantify the effect of PEG on the micellization properties of the alkyl monoglucosides, whereas small-angle neutron scattering gives insight into the microstructure of the surfactant/polymer mixtures near the liquid-liquid phase boundary. Results show that the range and the strength of the interactions in these solutions are highly affected by the presence of PEG. Solutions with nonyl-beta-glucoside contain larger micelles than those with octyl-beta-glucoside, and the intermicellar interactions are much stronger and longer ranged. The relevance of these findings for membrane protein crystallization is discussed.