Core-shell like Cu<Subscript>2</Subscript>O nanocubes enfolded with Co(OH)<Subscript>2</Subscript> on reduced graphene oxide for the amperometric detection of caffeine

The authors report on the fabrication of Co(OH)₂-enfolded Cu₂O nanocubes on reduced graphene oxide (rGO), and the use of this material in an electrochemical caffeine sensor. The rGO/Cu₂O/Co(OH)₂ composite was characterized by X-ray powder diffraction pattern analysis, field emission scanning electron microscopy, energy dispersive X-ray spectroscopy and Raman spectroscopy. A rotating disc glassy carbon electrode covered with the nanocomposite displays enhanced electrocatalytic activity towards the electro-oxidation of caffeine. The peak oxidation potential is at 1.4  V (vs. Ag/AgCl) and hence is strongly shifted to the negative side when compared to other modified electrodes. The calibration plot is linear in the 0.83 to 1200 μM concentration range, with a 0.4 μM detection limit (at a signal-to-noise ratio of 3). The modified electrode is sensitive, selective and stable. It was successfully applied to the determination of caffeine in (spiked) caffeine-containing beverages and coffee powder and gave recoveries that ranged from 95.7 to 98.3 %.

Open image in new window

Graphical abstract Co(OH)₂ enfolded Cu₂O nanocubes on reduced graphene oxide (rGO) for the caffeine sensor

     

A Lagrangean based branch-and-cut algorithm for global optimization of nonconvex mixed-integer nonlinear programs with decomposable structures

In this work we present a global optimization algorithm for solving a class of large-scale nonconvex optimization models that have a decomposable structure. Such models, which are very expensive to solve to global optimality, are frequently encountered in two-stage stochastic programming problems, engineering design, and also in planning and scheduling. A generic formulation and reformulation of the decomposable models is given. We propose a specialized deterministic branch-and-cut algorithm to solve these models to global optimality, wherein bounds on the global optimum are obtained by solving convex relaxations of these models with certain cuts added to them in order to tighten the relaxations. These cuts are based on the solutions of the sub-problems obtained by applying Lagrangean decomposition to the original nonconvex model. Numerical examples are presented to illustrate the effectiveness of the proposed method compared to available commercial global optimization solvers that are based on branch and bound methods.     

Solar-driven electrochemically assisted semiconductor-catalyzed iodide ion oxidation. Enhanced efficiency by oxide mixtures

Oxidation of iodide ion from an air-saturated solution under natural sunlight (900±50 W m⁻²) on the surfaces of TiO₂, ZnO, Fe₂O₃, MoO₃ and CeO₂ enhances by 6 to 12-fold on application of a cathodic bias of −0.2 to −0.3 V (vs NHE) to the semiconductors; light, the semiconductor and dissolved oxygen are essential for iodine generation. The semiconductors under an anodic bias of +0.2 to +0.3 V (vs NHE) fail to oxidize iodide ion from air-saturated solution under sunlight. Under cathodic bias, semiconductor mixtures like TiO₂-ZnO, TiO₂-Fe₂O₃ and ZnO-Fe₂O₃ show enhanced photocatalytic activity, indicating improved charge separation in oxide mixtures. The mechanism of photocatalysis under cathodic bias is discussed.      

Selectivity in photocatalysis by particulate semiconductors

TiO₂, Fe₂O₃, CuO, ZnO, ZnS, Nb₂O₅, MoO₃, CdO, CdS, Sb₂O₃, CeO₂, HgO, Pb₂O₃, PbO₂ and Bi₂O₃ microparticles exhibit band gap excitation with UV-A light but they are selective to photodegrade phenols. While TiO₂ anatase and ZnO photocatalyze the degradation of phenol, o-aminophenol, m-aminophenol, p-aminophenol, o-chlorophenol, m-chlorophenol, p-chlorophenol, o-nitrophenol, p-nitrophenol, o-cresol, m-cresol, p-cresol, catechol, resorcinol and quinol, MoO₃ does not photodegrade any of the fifteen phenols. Fe₂O₃, CuO, ZnS, Nb₂O₅, CdO, CdS, Sb₂O₃, CeO₂, HgO, Pb₂O₃, PbO₂ and Bi₂O₃ are selective in photodegrading the fifteen phenols; however, the phenols get adsorbed over all sixteen particulate semiconductors.      

Kinetics of Ag/TiO2-photocatalyzed iodide ion oxidation

Abstract  

Ag-doped TiO₂ (anatase) samples (mass fraction w _Ag = 0.01 and w _Ag = 0.02) of 15.9 and 14.5 nm mean particle size and 11.46 and 10.14 m² g⁻¹ BET surface area were prepared by photodeposition. Doping results in surface plasmon resonance of the metallic silver nanoclusters at around 500 nm, but the absorption edge remains unaltered at 365 nm. Ag-doping remarkably enhances the photooxidation of iodide ion under UV light; iodine formation with Ag/TiO₂ with w _Ag = 0.01 is 16 times greater than with bare TiO₂. The reaction conforms to Langmuir–Hinshelwood kinetics with regard to both I⁻ and O₂. Increase of pH slows down iodine formation and sacrificial electron donors arrest the reaction. Pre-sonication of the catalyst slurry hinders the photocatalysis. Generation of iodine is much greater in acetonitrile than in water. Under the experimental conditions, Ag/TiO₂ with w _Ag = 0.01 is more efficient than Ag/TiO₂ with w _Ag = 0.02, and the enhanced photocatalysis is likely to be because of suppression of electron–hole pair recombination. Kinetic analysis reveals that increasing the Ag mass fraction from 0.01 to 0.02 enhances the surface pseudo-first-order rate constant but inhibits the adsorption of iodide ion and the oxygen molecule on the illuminated oxide surface.     

A Comparative Study of Modifying Gold and Carbon Electrode with 4‐Sulfophenyl Diazonium Salt

A comparison of the reductive adsorption behavior of 4‐sulfophenyl diazonium salt and subsequent electrochemical reactivity on gold relative to carbon was studied with some significant differences observed. The ability of the 4‐sulfophenyl layer adsorbed onto gold to block access of the redox probe ferricyanide to the underlying electrodes, as determined via cyclic voltammetry was inferior to the same layers formed on glassy carbon electrodes thus indicating a more open, porous layer formed on gold. More significantly, the 4‐sulfophenyl layers are shown to be far less electrochemically stable on gold than on glassy carbon. Electrochemical and X‐ray photoelectron spectroscopy (XPS) evidence suggests the instability is due to cleavage of the bond between sulfonate functional group and phenyl ring. These results provide further evidence that although aryl diazonium salt layers are relatively stable on gold surfaces compared with alkanethiol based self‐assembled monolayer (SAMs), the stability is not as high as is observed on carbon.     

Multi-vortex versus interstitial vortices scenario in superconducting antidot arrays

In superconducting thin films, engineered lattice of antidots (holes) act as an array of columnar pinning sites for the vortices and thus lead to vortex matching phenomena at commensurate fields guided by the lattice spacing. The strength and nature of vortex pinning is determined by the geometrical characteristics of the antidot lattice (such as the lattice spacing a₀, antidot diameter d, lattice symmetry, and orientation) along with the characteristic length scales of the superconducting thin films, viz., the coherence length (ξ) and the penetration depth (λ). There are at least two competing scenarios: (i) multiple vortices sit on each of the antidots at a higher matching period and (ii) there is nucleation of vortices at the interstitial sites at higher matching periods. Furthermore, it is also possible for the nucleated interstitial vortices to reorder under suitable conditions. We present our experimental results on NbN antidot arrays in the light of the above scenarios.     

Photocatalytic bacteria inactivation by polyethylene glycol-assisted sol–gel synthesized Cd-doped TiO2 under visible light

Cd-doped TiO₂ nanoparticles have been obtained by polyethylene glycol-assisted sol–gel synthesis and characterized by powder X-ray diffraction, energy dispersive X-ray analysis, high-resolution scanning electron microscopy and UV–visible diffuse reflectance, photoluminescence and impedance spectroscopies. Use of polyethylene glycol as templating agent provides club-shaped particles. Doping TiO₂ with Cd decreases the average crystallite size and charge transfer resistance, increases the capacitance, and leads to blue emission. Cd-doping enhances the visible light photocatalytic disinfection of bacteria but not dye degradation.