No abstract

Microelectromechanical system (MEMS)-based mass sensors are proposed as potential candidates for highly sensitive chemical and gas detection applications owing to their miniaturized structure, low power consumption, and ease of integration with readout circuits. This paper presents a new approach in developing micromachined mass sensors based on capacitive and piezoelectric transducer configurations for use in low concentration level gas detection in a complex environment. These micromachined sensors operate based on a shift in their center resonant frequencies. This shift is caused by a change in the sensor’s effective mass when exposed to the target gas molecules, which is then correlated to the gas concentration level. In this work, capacitive and piezoelectric-based micromachined sensors are investigated and their principle of operation, device structures and configurations, critical design parameters and their candidate fabrication techniques are discussed in detail.

The chromium­(III) amino-bis­(phenolate) “ate” complex, {Na­[CrCl2 L1]}, 1, where L1 = 6,6′-((1,4-diazepane-1,4-diyl)­bis­(methylene))­bis­(2,4-di-tert-butylphenolato) and its derivatives 2–4 (where the phenolate substituents and the diamino linker were varied) were prepared. Single crystals obtained by recrystallization of 1 were shown by X-ray diffraction to be a related octahedral complex, 1′, where one of the O-donors in 1′ is protonated, resulting in a monoanionic ligand and affording a neutral Cr­(III) dichloride complex. The ligand [L1] chelates in a meridional fashion and anionic chloride donors reside in a trans axial orientation. Cr­(III) amino-bis­(phenolate) complexes 1–4 showed good activity toward ring-opening polymerization (ROP) of cyclohexene oxide (CHO) without the use of an added cocatalyst, producing high-molecular weight poly­(cyclohexene oxide) where 1 was the most active catalyst. Arrhenius data obtained for the ROP of CHO using 1 revealed an activation barrier of 64.0 ± 9.8 kJ mol–1 and the nonlinear polymer molecular weight growth versus conversion relationship suggests a step-growth polymerization mechanism.

This work is a comprehensive review of sensing materials, which interact with several target gases pertinent to agricultural monitoring applications. Sensing materials which interact with carbon dioxide, water vapor (relative humidity), hydrogen sulfide, ethylene and ethanol are the focus of this work. Performance characteristics such as dynamic range, recovery time, operating temperature, long-term stability and method of deposition are discussed to determine the commercial viability of the sensing materials considered in this work. In addition to the sensing materials, deposition methods are considered to obtain the desired sensing material thickness based on the sensor’s mechanism of operation. Various material classes including metal oxides, conductive polymers and carbon allotropes are included in this review. By implementing multiple sensing materials to detect a single target analyte, the issue of selectivity due to cross sensitivity can be mitigated. For this reason, where possible, it is desirable to utilize more than one sensing material to monitor a single target gas. Among those considered in this work, it is observed that PEDOT PSS/graphene and TiO2-coated g-C3N4 NS are best suited for CO2 detection, given their wide dynamic range and modest operating temperature. To monitor the presence of ethylene, BMIM-NTf2, SWCNTs and PtTiO2 offer a dynamic range most suitable for the application and require no active heating. Due to the wide dynamic range offered by SiO2/Si nanowires, this material is best suited for the detection of ethanol; a gas artificially introduced to prolong the shelf life of the harvested crop. Finally, among all other sensing materials investigated, it observed that both SWCNTs and CNTs/SnO2/CuO are most suitable for H2S detection in the given application.

Chromium diamino-bis(phenolate) complexes, CrXL [where L = 6,6′-((1,4-diazepane-1,4-diyl)bis(methylene))bis(2,4-dimethylphenolato) and X = Cl − (1), OH − (2), and N 3 − (3)], were prepared and characterized by MALDI-TOF MS and single-crystal Xray diffraction. Complex 1 crystallized as two linkage isomers, specifically a green chloride-bridged dimer (1) and a pink asymmetrically bridged isomer exhibiting one chloride bridging atom and one bridging phenolate oxygen (1′). Adventitious moisture during sample handling causes the formation of hydroxide-containing complex 2. The reaction of 1 with PPNN 3 (where PPN = bis(triphenylphosphine)iminium) permits the isolation of a crystalline chromium azide complex, 3, which was structurally authenticated. Complex 1 showed good activity toward the ring-opening copolymerization of cyclohexene oxide and carbon dioxide with an added chloride, azide, or 4-(dimethylamino)pyridine (DMAP) cocatalyst to give a completely alternating polycarbonate with a narrow molecular weight dispersity.