Modifying the Electron-Trapping Process at the BiVO4 Surface States via the TiO2 Overlayer for Enhanced Water Oxidation.

BiVO4 is one of the most promising photoanode candidates to achieve high-efficiency water splitting. However, overwhelming charge recombination at the interface limits its water oxidation activity. In this study, we show that the water oxidation activity of the BiVO4 photoanode is significantly boosted by the TiO2 overlayer prepared by atomic layer deposition. With a TiO2 overlayer of an optimized thickness, the photocurrent at 1.23 VRHE increased from 0.64 to 1.1 mA·cm-2 under front illumination corresponding to 72% enhancement. We attribute this substantial improvement to enhanced charge separation and suppression of surface recombination due to surface-state passivation. We provide direct evidence via transient photocurrent measurements that the TiO2 overlayer significantly decreases the photogenerated electron-trapping process at the BiVO4 surface. Electron-trapping passivation leads to enhanced electron photoconductivity, which results in higher photocurrent enhancement under front illumination rather than back illumination. This feature can be particularly useful for wireless tandem devices for water splitting as the higher band gap photoanodes are typically utilized with front illumination in such configurations. Even though the electron-trapping process is eliminated completely at higher TiO2 overlayer thicknesses, the charge-transfer resistance at the surface also increases significantly, resulting in a diminished photocurrent. We demonstrate that the ultrathin TiO2 overlayer can be used to fine tune the surface properties of BiVO4 and may be used for similar purposes for other photoelectrode systems and other photoelectrocatalytic reactions.

Carbon nanotubes part I: preparation of a novel and versatile drug-delivery vehicle.

INTRODUCTION: It is 23 years since carbon allotrope known as carbon nanotubes (CNT) was discovered by Iijima, who described them as "rolled graphite sheets inserted into each other". Since then, CNTs have been studied in nanoelectronic devices. However, CNTs also possess the versatility to act as drug- and gene-delivery vehicles. AREAS COVERED: This review covers the synthesis, purification and functionalization of CNTs. Arc discharge, laser ablation and chemical vapor deposition are the principle synthesis methods. Non-covalent functionalization relies on attachment of biomolecules by coating the CNT with surfactants, synthetic polymers and biopolymers. Covalent functionalization often involves the initial introduction of carboxylic acids or amine groups, diazonium addition, 1,3-dipolar cycloaddition or reductive alkylation. The aim is to produce functional groups to attach the active cargo. EXPERT OPINION: In this review, the feasibility of CNT being used as a drug-delivery vehicle is explored. The molecular composition of CNT is extremely hydrophobic and highly aggregation-prone. Therefore, most of the efforts towards drug delivery has centered on chemical functionalization, which is usually divided in two categories; non-covalent and covalent. The biomedical applications of CNT are growing apace, and new drug-delivery technologies play a major role in these efforts.

Carbon nanotubes part II: a remarkable carrier for drug and gene delivery.

INTRODUCTION: Carbon nanotubes (CNT) have recently been studied as novel and versatile drug and gene delivery vehicles. When CNT are suitably functionalized, they can interact with various cell types and are taken up by endocytosis. AREAS COVERED: Anti-cancer drugs cisplatin and doxorubicin have been delivered by CNT, as well as methotrexate, taxol and gemcitabine. The delivery of the antifungal compound amphotericin B and the oral administration of erythropoietin have both been assisted using CNT. Frequently, targeting moieties such as folic acid, epidermal growth factor or various antibodies are attached to the CNT-drug nanovehicle. Different kinds of functionalization (e.g., polycations) have been used to allow CNT to act as gene delivery vectors. Plasmid DNA, small interfering RNA and micro-RNA have all been delivered by CNT vehicles. Significant concerns are raised about the nanotoxicology of the CNT and their potentially damaging effects on the environment. EXPERT OPINION: CNT-mediated drug delivery has been studied for over a decade, and both in vitro and in vivo studies have been reported. The future success of CNTs as vectors in vivo and in clinical application will depend on achievement of efficacious therapy with minimal adverse effects and avoidance of possible toxic and environmentally damaging effects.