Intrinsic Enzyme-like Activities of Cerium Oxide Nanocomposite and Its Application for Extracellular H2O2 Detection Using an Electrochemical Microfluidic Device.

Artificial enzyme mimics have gained considerable attention for use in sensing applications due to their high stability and outstanding catalytic activity. We show that cerium oxide nanosheets (NSs) exhibit triple-enzyme mimetic activity. The oxidase-, peroxidase-, and catalase-like activities of the proposed nanoparticles are demonstrated using both colorimetric and electron paramagnetic resonance (EPR) spectroscopy. On the basis of the excellent catalytic activity of cerium oxide NSs toward hydrogen peroxide, an electrochemical approach for the high-throughput detection of H2O2 in living cells was established. This report presents an analytical microfluidic chip integrated with a cerium oxide NS mimic enzyme for the fabrication of a simple, sensitive, and low-cost electrochemical sensor. Three Au microelectrodes were fabricated on a glass substrate using photolithography, and the working electrode was functionalized using cerium oxide NSs. The operation of this biosensor is based on cerium oxide NSs and presents a high sensitivity over a wide detection range, between 100 nM and 20 mM, with a low detection limit of 20 nM and a high sensitivity threshold of 226.4 µA·cm-2·µM-1. This microfluidic sensor shows a strong response to H2O2, suggesting potential applications in monitoring H2O2 directly secreted from living cells. This sensor chip provides a promising platform for applications in the field of diagnostics and sensing.

Solid-State Chemiresistors from Two-Dimensional MoS2 Nanosheets Functionalized with l-Cysteine for In-Line Sensing of Part-Per-Billion Cd2+ Ions in Drinking Water.

Sensing of metal contaminants at ultralow concentrations in aqueous environments is vital in today's overpopulated world, with an extremely stringent limit (<5 ppb) for Cd2+ ions in drinking water. Here, we utilize sonochemically exfoliated molybdenum disulfide (MoS2) nanosheets functionalized with l-cysteine (Cys) as highly sensitive and selective two-dimensional (2D) materials for solid-state chemiresistors. We specifically targeted Cd2+ ions due to their high toxicity at low concentrations. MoS2-Cys nanosheets are fabricated using an ad hoc, low-complexity, one-pot synthesis method. Porous MoS2-Cys thin films with a high surface area are assembled from these nanosheets. Two-terminal chemiresistors incorporating MoS2-Cys films are demonstrated to be preferentially sensitive to Cd2+ ions at neutral pH, irrespective of other metal ions present in water flowing through the device. A 5 ppb concentration of the Cd2+ ions in the water stream increases the device resistivity by 20 times. Our devices operate at broad (1-500 ppb) range and fast (∼1 s) response times. Cd2+ is selectively detected because of preferential, size-driven adsorption at the interstitials between l-cysteine functional groups, combined with pH-controlled charge transfer that removes electronic gap states from MoS2. MoS2-Cys-based chemiresistors can be deployed in-line to detect metal ions without any need for additional offline measurements.

Thermal conductivity of thin film-substrate systems from two-side scanning photothermal deflection measurements: Theoretical model and validation.

Photothermal deflection (PTD) has been frequently utilized to measure the thermal properties of thin solid films on a substrate. In the models commonly used to interpret PTD data, the substrate is assumed to be an ideal thermal insulator. This assumption poses important restrictions on the reliability of these thermal measurements and limits the possibility to use PTD for also measuring the specific heat of the samples. Simultaneous knowledge of specific heat and thermal diffusivity is necessary to determine the thermal conductivity of thin solid films. In this work, we calculated the phase and amplitude of the PTD signal at the two opposites sides (film-side and substrate-side) of a thin-film substrate system. We find that, on both sides, the phases of the PTD signal primarily depend on the thermal diffusivity of the thin film, while the amplitudes primarily depend on the specific heat. By using the phases and amplitudes at the two sides, we show that the accuracy of thermal conductivity measurements by PTD can be dramatically improved. We validate our theoretical model by measuring, in a scanning PTD apparatus, the thermal properties of gold thin films, which are in excellent agreement with, and improve on, existing data from the literature.

Porous graphene-based membranes for water purification from metal ions at low differential pressures.

A new generation of membranes for water purification based on weakly oxidized and nanoporous few-layer graphene is here introduced. These membranes dramatically decrease the high energy requirements of water purification by reverse osmosis. They combine the advantages of porous and non-oxidized single-layer graphene, offering energy-efficient water filtration at relatively low differential pressures, and highly oxidized graphene oxide, exhibiting high performance in terms of impurity adsorption. In the reported fabrication process, leaks between juxtaposed few-layer graphene flakes are sealed by thermally annealed colloidal silica, in a treatment that precedes the opening of (sub)nanometre-size pores in graphene. This process, explored for the first time in this work, results in nanoporous graphene flakes that are water-tight at the edges without occluding the (sub)nanopores. With this method, removal of impurities from water occurs through a combination of size-based pore rejection and pore-edge adsorption. Thinness of graphene flakes allows these membranes to achieve water purification from metal ions in concentrations of few parts-per-million at differential pressures as low as 30 kPa, outperforming existing graphene or graphene oxide purification systems with comparable flow rates.

Photon management properties of rare-earth (Nd,Yb,Sm)-doped CeO2 films prepared by pulsed laser deposition.

CeO2 is a promising material for applications in optoelectronics and photovoltaics due to its large band gap and values of the refractive index and lattice parameters, which are suitable for silicon-based devices. In this study, we show that trivalent Sm, Nd and Yb ions can be successfully inserted and optically activated in CeO2 films grown at a relatively low deposition temperature (400 °C), which is compatible with inorganic photovoltaics. CeO2 thin films can therefore be efficiently functionalized with photon-management properties by doping with trivalent rare earth (RE) ions. Structural and optical analyses provide details of the electronic level structure of the films and of their energy transfer mechanisms. In particular, we give evidence of the existence of an absorption band centered at 350 nm from which energy transfer to rare earth ions occurs. The transfer mechanisms can be completely explained only by considering the spontaneous migration of Ce(3+) ions in CeO2 at a short distance from the RE(3+) ions. The strong absorption cross section of the f-d transitions in Ce(3+) ions efficiently intercepts the UV photons of the solar spectrum and therefore strongly increases the potential of these layers as downshifters and downconverters.

Selective Area Band Engineering of Graphene using Cobalt-Mediated Oxidation.

This study reports a scalable and economical method to open a band gap in single layer graphene by deposition of cobalt metal on its surface using physical vapor deposition in high vacuum. At low cobalt thickness, clusters form at impurity sites on the graphene without etching or damaging the graphene. When exposed to oxygen at room temperature, oxygen functional groups form in proportion to the cobalt thickness that modify the graphene band structure. Cobalt/Graphene resulting from this treatment can support a band gap of 0.30 eV, while remaining largely undamaged to preserve its structural and electrical properties. A mechanism of cobalt-mediated band opening is proposed as a two-step process starting with charge transfer from metal to graphene, followed by formation of oxides where cobalt has been deposited. Contributions from the formation of both CoO and oxygen functional groups on graphene affect the electronic structure to open a band gap. This study demonstrates that cobalt-mediated oxidation is a viable method to introduce a band gap into graphene at room temperature that could be applicable in electronics applications.

Self-ordering properties of functionalized acenes for annealing-free organic thin film transistors.

Presented here is a study of the molecular self-ordering properties of four bis(phenylethynyl) anthracene based organic semiconductors related to their electronic structure employing X-ray spectroscopy techniques and density functional theory (DFT) calculations. The local molecular order through polarization dependence of C 1s → π* transitions revealed ordered π-stacking nearly perpendicular to the substrate due to van der Waals interactions between alkyl groups. DFT calculations were used to deconvolute the measured electronic structure and examine effects of small changes in molecular geometry in relation to measured charge carrier mobility in top contact field effect transistors. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are found to be conjugated from the anthracene core across the bridging ethynyl groups to the thiophene and phenyl end groups. The inclusion of ethynyl bridges connecting the thiophenes has a twofold effect of both reducing the rotational freedom of this functional group and increasing HOMO/LUMO conjugation across the molecules. These features help create a more rigid upright structure for HB-ant-THT with better molecular orbital conjugation and subsequent higher mobility. With this understanding of how different functional groups interact with an acene core, future synthesis of new materials may be directed toward annealing-free organic semiconducting materials.