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Resistive-based gas sensors have been considered as the most favorable gas sensors for detection of toxic gases and volatile organic compounds (VOCs) because of their simple structure, low cost, high sensitivity, ease of use, and high stability. Unfortunately, wide application of resistive-based gas sensors is limited by their low selectivity. In this article, we present the fabrication of ultrahigh selective NH3 gas sensor based on tin–titanium dioxide/reduced graphene/carbon nanotube (Sn–TiO2@rGO/CNT) nanocomposites. The Sn–TiO2@rGO/CNT nanocomposites with different molar ratios of Sn/Ti (1:10, 3:10, and 5:10) were synthesized via the solvothermal method. Characterizations by scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy confirmed the decoration of Sn–TiO2 nanoparticles on rGO/CNT nanocomposite surfaces. The Sn–TiO2@rGO/CNT nanocomposite gas sensor exhibited high response and ultrahigh selectivity to NH3 against toluene, dimethylformamide, acetone, ethanol, methanol, isopropanol, formaldehyde, hydrogen, carbon dioxide, acetylene, and VOCs in paint thinners at room temperature. The Sn–TiO2@rGO/CNT nanocomposite gas sensor with molar ratio of Sn/Ti = 1:10 showed the highest response to NH3 over other molar ratios of Sn/Ti as well as pure rGO/CNT and Sn–TiO2 gas sensors. The ammonia-sensing mechanisms of the Sn–TiO2@rGO/CNT gas sensor were proposed based on the formation of p–n heterojunctions of p-type rGO/CNT and n-type Sn–TiO2 nanoparticles via a low-temperature oxidizing reaction process.

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This research focuses on understanding the nature of the Fe ions substituted in hydroxyapatite ((HAP) Ca 10-x Fe x( PO 4)6( OH )2-xÿx: x = 0, 0.2 and 0.4) powder synthesized at temperatures between 500°C and 1000°C and their crystallization. The DSC scan indicates a solid state phase transition at about 757°C for the as-prepared powder HAP. The transformation of HAP to β-tricalcium phosphate (β- TCP , β- Ca 3( PO 4)2) is seen when the powder were heated to 750°C. The sizes of the crystallites were determined to be about 46–51 nm, 33–40 nm and 33–59 nm for Fe content of 0, 0.2 and 0.4 mol%, respectively. The ESR parameters for the Fe 3+ ions, g = 4.23 and 8.93 indicated that the ions were subjected to a rhombic ion crystal field within the HAP structures. The g values of ~2.01 indicated that the particles were super-paramagnetic and ferromagnetic iron nanoparticles, having an average size about 0.2–0.5 μm in length.

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Magnetic nanoparticle composite with a cobalt ferrite (CoFe2O4, (CF)) core and an apatite (Ap) coating was synthesized using a biomineralization process in which a modified simulated body fluid (1.5SBF) solution is the source of the calcium phosphate for the apatite formation. The core–shell structure formed after the citric acid–stabilized cobalt ferrite (CFCA) particles were incubated in the 1.5 SBF solution for 1 week. The mean particle size of CFCA-Ap is about 750 nm. A saturation magnetization of 15.56 emug-1 and a coercivity of 1808.5 Oe were observed for the CFCA-Ap obtained. Bovine serum albumin (BSA) was used as the model protein to study the adsorption and release of the proteins by the CFCA-Ap particles. The protein adsorption by the CFCA-Ap particles followed a more typical Freundlich than Langmuir adsorption isotherm. The BSA release as a function of time became less rapid as the CFCA-Ap particles were immersed in higher pH solution, thus indicating that the BSA release is dependent on the local pH.

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