Owing to the difficulty in controlling the dopant or defect types and their homogeneity in carbon materials, it is still a controversial issue to identify the active sites of carbon-based metal-free catalysts. Herein, we report a proof of concept study on the active-site evaluation for a highly oriented pyrolytic graphite catalyst with specific pentagon carbon defective patterns (D-HOPG).It is demonstrated that specific carbon defect types (edged pentagon in this work) could be selectively created via controllable N-doping. Work function analyses coupled with macro/microelectrochemical performance measurements suggest that the pentagon defects in D-HOPG served as major active sites for acidic oxygen reduction reaction (ORR), even much superior to the pyridinic nitrogen sites in N-doped highly oriented pyrolytic graphite (N-HOPG). This work enables us to elucidate the relative importance of the specific carbon defects vs N-dopant species and their respective contributions to the observed overall acidic ORR activity.

The effect of solvent on the formation of assemblies of partially fluorinated block copolymers in solution has been examined. Two classes of materials based respectively on 2,2,2-trifluoroethyl acrylate (TFEA) and 2,2,2-trifluoroethyl methacrylate (TFEMA) were dissolved in organic solvents, and the properties on successive addition of water were studied using NMR spectroscopy, NMR imaging, AFM, and TEM. The relatively high glass transition temperature of the methacrylate blocks resulted in the formation of kinetically trapped structures that could only be resolved following heating to temperatures well above the T g. The acrylate polymers formed loose assemblies in pure dimethylformamide, and on addition of water cylindrical micelles were observed. On the other hand, in pure acetone the partially fluorinated segments interacted more strongly with the solvent, with this structure inverting on addition of water. The NMR parameters were strongly dependent on the proposed structures in solution, and most markedly the mutual association of the fluorinated segments resulted in strong dipole–dipole interactions with the 19F spins. The importance of structure and solvent on the potential of these copolymers as MRI contrast agents is discussed, and broad design rules for effective 19F MRI agents are given.

Polycaprolactone (PCL) is frequently used as the base polymer in scaffolds targeted for tissue engineering applications. However, in the absence of further surface modification, the lack of functional moieties on the PCL chain results in non-ideal surface properties of such scaffolds, especially in terms of the inability to tailor the presentation of functional ligands for directed cell adhesion and growth. The current study investigates gamma irradiation-induced grafting as a means of improving the biofunctionality of the PCL surface. The surface presentation of carboxylic acid groups on 2D PCL films could be tailored by changing the acrylic acid (AAc) concentration and/or the solvent during grafting, as evaluated from X-ray photoelectron spectroscopy (XPS). From data obtained using Raman spectroscopy, it was concluded that the penetration depth of the grafted pAAc was affected by the solvent system with a mixed watermethanol system yielding high penetration. Grafted samples displayed a decreased elastic modulus of the surface correlating with pAAc penetration depth, as shown by nano-indentation using atomic force microscopy (AFM). The most promising grafting conditions found for the 2D PCL films were then applied to 3D thermally induced phase separation (TIPS) scaffolds and it was demonstrated using XPS that equivalent levels of grafting of pAAc could be achieved throughout the whole depth of the scaffold. The scaffolds maintained their overall integrity after grafting, even though we observed a decrease in the compressive modulus by 20% after surface modification. These combined studies confirm the utility of this surface modification methodology for scaffolds targeted at tissue engineering and cell culture applications.