Simultaneous Silicon Oxide Growth and Electrophoretic Deposition of Graphene Oxide.

During electrophoretic deposition of graphene oxide (GO) sheets on silicon substrates, not only deposition but also simultaneous anodic oxidation of the silicon substrate takes place, leading to a three-layered material. Scanning electron microscopy images reveal the presence of GO sheets on the silicon substrate, and this is also confirmed by X-ray photoelectron spectroscopy (XPS), albeit that the carbon portion increases with increasing emission angle, hinting at a thin carbon layer. With increasing applied potential and increasing conductivity of the GO solution, the carbon signal decreases, whereas the overall thickness of the added layer formed on top of the silicon substrate increases. Through XPS spectra in which the Si 2p peaks shifted under those conditions to 103-104 eV, we were able to conclude that significant amounts of oxygen are present, indicative of the formation of an oxide layer. This leads us to conclude that GO can be deposited using electrophoretic deposition, but that at the same time, silicon is oxidized, which may overshadow effects previously assigned to GO deposition.

Systematic Comparison of Zwitterionic and Non-Zwitterionic Antifouling Polymer Brushes on a Bead-Based Platform.

Nonspecific adsorption of biomolecules to solid surfaces, a process called biofouling, is a major concern in many biomedical applications. Great effort has been made in the development of antifouling polymer coatings that are capable of repelling the nonspecific adsorption of proteins, cells, and micro-organisms. In this respect, we herein contribute to understanding the factors that determine which polymer brush results in the best antifouling coating. To this end, we compared five different monomers: two sulfobetaines, a carboxybetaine, a phosphocholine, and a hydroxyl acrylamide. The antifouling coatings were analyzed using our previously described bead-based method with flow cytometry as the read-out system. This method allows for the quick and automated analysis of thousands of beads per second, enabling fast analysis and good statistics. We report the first direct comparison made between a sulfobetaine with opposite charges separated by two and three methylene groups and a carboxybetaine bearing two separating methylene groups. It was concluded that both the distance between opposite charges and the nature of the anionic groups have a distinct effect on the antifouling performance. Phosphocholines and simple hydroxyl acrylamides are not often compared with the betaines. However, here we found that they perform equally well or even better, yielding the following overall antifouling ranking: HPMAA ≥ PCMA-2 ≈ CBMAA-2 > SBMAA-2 > SBMAA-3 ≫ nonmodified beads (HPMAA being the best).

Efficient and Tunable Three-Dimensional Functionalization of Fully Zwitterionic Antifouling Surface Coatings.

To enhance the sensitivity and selectivity of surface-based (bio)sensors, it is of crucial importance to diminish background signals that arise from the nonspecific binding of biomolecules, so-called biofouling. Zwitterionic polymer brushes have been shown to be excellent antifouling materials. However, for sensing purposes, antifouling does not suffice but needs to be combined with the possibility to efficiently modify the brush with recognition units. So far this has been achieved only at the expense of either antifouling properties or binding capacity. Herein we present a conceptually new approach by integrating both characteristics into a single tailor-made monomer: a novel sulfobetaine-based zwitterionic monomer equipped with a clickable azide moiety. Copolymerization of this monomer with a well-established standard sulfobetaine monomer results in highly antifouling surface coatings with a large yet tunable number of clickable groups present throughout the entire brush. Subsequent functionalization of the azido brushes via widely used strain-promoted alkyne azide click reactions yields fully zwitterionic 3D-functionalized coatings with a recognition unit of choice that can be tailored for any specific application. Here we show a proof of principle with biotin-functionalized brushes on Si3N4 that combine excellent antifouling properties with specific avidin binding from a protein mixture. The signal-to-noise ratio is significantly improved over that of traditional chain-end modification of sulfobetaine polymer brushes, even if the azide content is lowered to 1%. This therefore offers a viable approach to the development of biosensors with greatly enhanced performance on any surface.

Local Light-Induced Modification of the Inside of Microfluidic Glass Chips.

The ability to locally functionalize the surface of glass allows for myriad biomedical and chemical applications. This would be the case if the surface functionalization can be induced using light with wavelengths for which standard glass is almost transparent. To this aim, we present the first example of a photochemical modification of hydrogen-terminated glass (H-glass) with terminal alkenes. Both flat glass surfaces and the inside of glass microchannels were modified with a well-defined, covalently attached organic monolayer using a range of wavelengths, including sub-band-gap 302 nm ultraviolet light. A detailed characterization thereof was conducted by measurements of the static water contact angle, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and scanning Auger microscopy (SAM). Germanium attenuated total reflection Fourier transform infrared (GATR-FTIR) indicates that the mechanism of the surface modification proceeds via an anti-Markovnikov substitution. Reacting H-glass with 10-trifluoro-acetamide-1-decene (TFAAD) followed by basic hydrolysis affords the corresponding primary amine-terminated monolayer, enabling additional functionalization of the substrate. Furthermore, we show the successful formation of a photopatterned amine layer by the specific attachment of fluorescent nanoparticles in very discrete regions. Finally, a microchannel was photochemically patterned with a functional linker allowing for surface-directed liquid flow. These results demonstrate that H-glass can be modified with a functional tailor-made organic monolayer, has highly tunable wetting properties, and displays significant potential for further applications.

pH response and molecular recognition in a low molecular weight peptide hydrogel.

In this article we report the preparation and characterization of a peptide-based hydrogel, which possesses characteristic rheological properties, is pH responsive and can be functionalized at its thiol function. The tripeptide N-(fluorenyl-9-methoxycarbonyl)-L-Cys(acetamidomethyl)-L-His-L-Cys-OH 1 forms stable supramolecular aggregates in water leading to hydrogels above 1.5 wt%. Rheological analysis of the hydrogel revealed visco-elastic and shear thinning properties of samples containing 1.5 wt% of peptide 1. The hydrogel reversibly responds to pH changes. Below and above pH 6, electrostatic repulsion of the peptide results in a weakening of the three-dimensional gel network. Based on atomic force microscopy, small angle X-ray scattering and molecular dynamics simulations, it is proposed that the peptide assembles into nanostructures that tend to entangle at higher concentrations in water. The development of functional materials based on the peptide assemblies was possible via thiol-ene-click chemistry of the free thiol function at the C-terminal cysteine unit. As a proof of concept, the functionalization with adamantyl units to give 1-Ad was shown by molecular recognition of ß-cyclodextrin vesicles. These vesicles were used as supramolecular cross-linkers of the assemblies of peptide 1 mixed with peptide 1-Ad leading to gel networks at a reduced peptide concentration.

Highly Specific Protein Identification by Immunoprecipitation-Mass Spectrometry Using Antifouling Microbeads.

A common method to study protein complexes is immunoprecipitation (IP), followed by mass spectrometry (thus labeled: IP-MS). IP-MS has been shown to be a powerful tool to identify protein-protein interactions. It is, however, often challenging to discriminate true protein interactors from contaminating ones. Here, we describe the preparation of antifouling azide-functionalized polymer-coated beads that can be equipped with an antibody of choice via click chemistry. We show the preparation of generic immunoprecipitation beads that target the green fluorescent protein (GFP) and show how they can be used in IP-MS experiments targeting two different GFP-fusion proteins. Our antifouling beads were able to efficiently identify relevant protein-protein interactions but with a strong reduction in unwanted nonspecific protein binding compared to commercial anti-GFP beads.