We report on the development of a simple and cost-effective potentiometric sensor array that is based on manual “drawing” on the polymeric support with the pencils composed of graphite and different types of zeolites. The sensor array demonstrates distinct sensitivity towards a variety of inorganic ions in aqueous media. This multisensor system has been successfully applied to quantitative analysis of 100 real-life surface waters sampled in Mahananda and Hooghly rivers in the West Bengal state (India). Partial least squares regression has been utilized to relate responses of the sensors to the values of different water quality parameters. It has been found that the developed sensor array, or electronic tongue, is capable of quantifying total hardness, total alkalinity, and calcium content in the samples, with the mean relative errors below 18%.

Symmetrical disubstituted carbohydrazides (H₄L^1–6) when reacted with [MoO₂(acac)₂] (in 1 : 2 molar ratio) afford the corresponding discrete complexes, [(MoO₂)₂L^1,5,6(MeOH)₂], [(MoO₂)₂L^1,5,6(Im)₂] (Im = imidazole), and/or polymeric ensembles, [(MoO₂)₂L^1–6]_n, depending on the reaction solvent, the ligand framework and the presence of ancillary coordinating species. In these assemblies, asymmetrical dimolybdenum(VI) entities, [(MoO₂)₂L^1–6], wherein the corresponding carbohydrazide acts as an adaptable two-compartment ligand offering ONO and ONN coordination environments, serve as the main building blocks. The solid-state structures of the obtained materials were elucidated via X-ray diffraction and infrared spectroscopy, whereas the behaviour of polymeric ensembles in DMSO-d₆ solution was investigated by nuclear magnetic resonance. Thermal studies revealed that the discrete species transform to the corresponding polymeric ensembles upon heating, whereas upon grinding they either remain intact or afford their reactive and coordinatively unsaturated (pentacoordinated) counterparts. While the discrete methanol-based complexes have a comparable spatial arrangement of ancillary ligands, imidazole-based ones offer substantially different scenario which was addressed in detail via density functional theory calculations. Finally, the polymeric ensembles were shown to be efficient catalysts for cyclooctene epoxidation under eco-friendly conditions while employing aqueous tert-butyl hydroperoxide as an oxidant.

Environmental monitoring of pollutants, such as NO_x and CO_x, which can be facilitated by a range of gas sensors, is of considerable fundamental and practical importance. This work has been focused on the synthesis and evaluation of zeolite β with tin (Sn-BEA) and dealuminated β (DeAl-BEA) zeolites. The zeolite samples have been extensively investigated by IR, UV-VIS and NMR spectroscopy, XRD, TGA, and N₂ adsorption-desorption. The prepared Sn-BEA sample is characterised by the submicron particle size, an almost defect-free structure, and high hydrophobicity. Sensors containing selective microporous layers based on Sn-BEA and DeAl-BEA zeolites have been prepared and extensively tested. Both the Sn-BEA and DeAl-BEA zeolites have been deposited in thin films and evaluated as gas sensors for CO, CO₂, NO, and NO₂ in the presence of water vapour at room temperature. The Sn-BEA zeolite-based sensor showed high selectivity towards NO₂, while the DeAl-BEA is selective towards CO₂ and NO₂.

A range of modified MAZ and LTL zeolites have been prepared and ion-exchanged with a series of alkali metal cations. It has been shown that K, Rb and Cs can be readily introduced into the MAZ structure with ~2 cations ion-exchanged per unit cell. In contrast, less than one cation of Li or Na per unit cell has been introduced under similar conditions. Ion-exchanged zeolites have been characterised using Fourier transform infrared spectroscopy, X-ray powder diffraction, scanning electron microscopy, X-ray fluorescence and N₂ physisorption in order to gain a better understanding of their structural and acidic properties. The FTIR data indicate considerable heterogeneity of the bridging OH groups in mazzite. The concentration of both Brønsted and Lewis acid sites detected in MAZ using pyridine as a probe molecule is lower than expected from its chemical composition, with the relative accessibility of the bridging OH-groups varying from 16% for H-MAZ to 28% for K-exchanged samples. This is in agreement with the N₂ adsorption-desorption data showing a rather low micropore volume for the ion-exchanged materials and with the NH₃-TPD results implying considerable transport limitations. This work demonstrates that the channel structure of mazzite is partially blocked resulting in a decreased micropore volume and limited access to the acid sites.

This work is focused on the application of Cu-containing zeolites as potential environmental sensors for monitoring carbon monoxide. A number of commercial zeolites with different structural properties (NaX, NaY, MOR, FER, BEA and ZSM-5) were modified using CuSO₄, Cu(NO₃)₂ and Cu(OAc)₂ solutions as copper sources to prepare Cu⁺-containing zeolites, since Cu⁺ forms stable complexes with CO at room temperature that can be monitored by infrared spectroscopy. Zeolite impregnation with Cu(NO₃)₂ resulted in the highest total Cu-loadings, while the Cu(OAc)₂-treated samples had the highest Cu⁺/Cu_total ratio. Cu(NO₃)₂-impregnated MOR, which displayed the highest concentration of Cu⁺, was subjected to a number of tests to evaluate its performance as a potential CO sensor. The working temperature and concentration ranges of the sensor were determined to be from 20 to 300 °C and from 10 to 10,000 ppm, respectively. The stepwise CO desorption experiments indicated that the sensor can be regenerated at 400 °C if required. Additional analyses under realistic flow conditions demonstrated that for hydrophilic zeolites, the co-adsorption of water can compromise the sensor’s performance. Therefore, a hydrophobic Sn-BEA was utilised as a parent material for the preparation of an impregnated Cu-Sn-BEA zeolite, which exhibited superior resistance to interfering water while maintaining its sensing properties. Overall, the prepared Cu-modified zeolites showed promising potential as environmental CO sensors, displaying high sensitivity and selectivity under representative testing conditions.