Arka Saha

An amperometric cholesterol biosensor with excellent sensitivity and limit of detection based on an enzyme-immobilized microtubular ZnO@ZnS heterostructure

The controlled synthesis of porous nanostructure materials and their use as active materials for cholesterol sensing are of key importance in current research due to their exceptionally high surface area, their optical and electrical properties, and their good electron transport characteristics. In the present study we have synthesized a microtubular ZnO@ZnS heterostructure from the corresponding ZnO microtubes by a simple aqueous chemical sulphidation process. ZnS microtubes have also been synthesized by removal of ZnO from the ZnO@ZnS heterostructure using acetic acid. Both ZnO@ZnS and ZnS microtubes have a high surface area (56 and 68 m2 g−1, respectively) and a modified electronic structure. A ZnO@ZnS heterostructure-modified electrode has excellent amperometric cholesterol-sensing performance, with sensitivity 52.67 mA M−1 cm−2, signal to noise (S/N) ratio = 15, and limit of detection (LOD) 0.02 mM with S/N = 3. The sensing performance of the ZnO@ZnS heterostructure, including both sensitivity and LOD, is superior to that reported either for ZnO-based or Au- or Pt-modified sensors. Its superior performance originates both from its high microstructure-based surface area and its modified electronic structure, which facilitate electron transport to the electrode.

Rectangular ZnO porous nano-plate assembly with excellent acetone sensing performance and catalytic activity

The controlled synthesis of a hierarchically assembled porous rectangular ZnO plate (2.5–3.5 μm length, 1.5–2.5 μm width and 100–150 nm thickness) from bulk ZnO without using any organic substrates, such as solvents/surfactants/structure-directing agents, is presented. The synthesized ZnO plates are single crystalline with exposed (100) facets on the flat surface, porous and formed through the calcination of a hydrozincite [Zn5(CO3)2(OH)6] intermediate. A gas sensor based on the synthesized porous ZnO architecture exhibited high sensitivity towards acetone even in low concentration (S = 3.4 in 1 ppm acetone) with good selectivity. The ZnO nanostructured material as a heterogeneous catalyst also showed excellent catalytic activity for the synthesis of 5-substituted-1H-tetrazoles (yield = 94%). Both the activities are superior than those of other reported ZnO based acetone sensors and heterogeneous catalysts. We believe that the improved properties of the synthesized ZnO nanostructure is due to the exposed (100) facets, and its porous and assembled structure, which provides a reasonably large accessible surface area, and facilitates diffusion and mass transport of gas or substrate molecules.

Generalized synthesis and evaluation of formation mechanism of metal oxide/sulphide@C hollow spheres

Herein, we report a generalized novel soft-template approach for the synthesis of a variety of uniform metal oxide (MoO2, Fe3O4, V2O3) and sulphide (MoS2, ZnS)@C hollow spheres using sucrose and cetyl trimethylammonium bromide (CTAB) as a soft template. The synthesized hollow spheres are uniform in size, with a size range of 800 nm to 1.3 μm. The developed methodology allows for altering the carbon content by just varying the amount of sucrose in the precursor solution. The formation mechanism of the soft template was also studied by using isothermal titration calorimetry (ITC), infrared (IR) spectroscopic analysis and scanning electron microscopy (SEM) of the intermediate. The strategy was developed based on the in situ formation of the spherical soft template by the interaction of sucrose and CTAB under the experimental conditions, formation of an inorganic shell by the interaction of a metal salt and template surface, carbonization of sucrose under hydrothermal conditions, and finally formation of the desired metal oxide/sulphide@C through calcination under 5% H2 in a flow of N2. Using the synthesized MoS2@C hollow sphere as a typical representative of such hollow spheres, its performance was examined as an anode for a lithium-ion battery to determine the applicability of the developed procedure; it exhibited a high specific capacity (∼1100 mA h g−1 at 150 mA g−1) as a lithium-ion battery (LiB) anode.

A facile method for the synthesis of a C@MoO2 hollow yolk–shell structure and its electrochemical properties as a faradaic electrode

Transition-metal oxide hollow yolk–shell micro/nanostructures combined with a conducting substance have gained significant attention as efficient electrode materials for electrochemical energy storage applications due to their large surface area, internal void space, and structural stability. Herein, we report a facile aqueous solution-based soft template method using sucrose–CTAB for the synthesis of a hollow yolk–shell structure of carbon-incorporated MoO2 (C@MoO2) with a diameter of 0.9–1.1 μm, wall thickness of 100 nm, inner yolk size of 400–450 nm, and BET surface area of 40 m2 g−1. During the synthesis process, sucrose plays a dual role, both as a template and a carbon source. The electrochemical charge storage mechanism follows a battery-type behaviour when tested as a faradaic electrode in 3.0 M KOH electrolyte. C@MoO2 exhibits a high specific capacity of 188 C g−1 at the current density of 0.5 A g−1, good rate performance (50.6 C g−1 at 10 A g−1), and 78% retention of capacity after 5000 cycles at 5 A g−1. The obtained performance is superior to those obtained for pure MoO2 hollow spheres (137.1 C g−1 at 0.5 A g−1) as well as previously reported MoO2 and MoO3, indicating the potential applicability of the as-synthesised yolk–shell C@MoO2.

Efficient oxidation of hydrocarbons over nanocrystalline Ce1−xSmxO2 (x = 0–0.1) synthesized using supercritical water

Selective oxidation of hydrocarbons to more functional oxygenated compounds is a challenging task for industrial research. Here we report the synthesis of highly crystalline Ce1−xSmxO2 (x = 0–0.1) using supercritical water and their excellent catalytic activity for selective oxidation of hydrocarbons (ethyl benzene, n-butylbenzene, biphenyl methane, 1,2,3,4-tetrahydro naphthalene, cyclohexene and cyclopentene) to corresponding ketone through the oxidation of activated proton. Materials characterization results revealed the formation of highly crystalline small cube shaped nanoparticles (∼8–10 nm) with highly exposed (100) facet and exhibiting a surface area of 83–96 m2 g−1. The catalytic study revealed that Ce0.95Sm0.05O2 is highly active towards selective oxidation of stable sp3 hybridized C–H bond of different hydrocarbons. The superior activity is most probably due to its high surface area, high degree of crystallinity with exposed high energy active (100) facet and presence of large amount Ce3+. In optimized condition as high as 90% conversion of ethyl benzene with 87% selectivity of acetophenone was observed. Among other different substrates n-butylbenzene and cyclopentene showed 100% selectivity towards corresponding ketone with the conversion of 60% and 73% respectively. The catalyst is re-usable for minimum 5 times without any deactivation of its activity.

An alternative hydrolytic synthesis route for uniform metal selenide nanoparticles

A generalized size selective synthetic protocol for uniform metal (Cd, Zn, Pb, Cu, In) selenide nanoparticles in hydrothermal conditions, using decanoic acid (C10) as a ligand, aqueous metal ammonium/ammonium carbonate complex and sodium hydrogen selenide (NaHSe), is developed. The materials show improvement in size dependent properties and hold promise for various practical applications.