Cavitand-based solid-phase microextraction coating for the selective detection of nitroaromatic explosives in air and soil

Structural characterization of the supra-molecular complex between a tetra-quinoxaline-based cavitand and benzo-nitrile

The structural characterization is reported of the supra-molecular complex between the tetra-quinoxaline-based cavitand 2,8,14,20-tetra-hexyl-6,10:12,16:18,22:24,4-O,O'-tetra-kis-(quinoxaline-2,3-di-yl)calix[4]resorcinarene (QxCav) with benzo-nitrile. The complex, of general formula C84H80N8O8·2C7H5N, crystallizes in the space group P with two independent mol-ecules in the asymmetric unit, displaying very similar geometrical parameters. For each complex, one of the benzo-nitrile mol-ecules is engulfed inside the cavity, while the other is located among the alkyl legs at the lower rim. The host and the guests mainly inter-act through weak C-H⋯π, C-H⋯N and dispersion inter-actions. These inter-actions help to consolidate the formation of supra-molecular chains running along the crystallographic b-axis direction.

Ultra-sensitive solid-phase Microextraction-Gas Chromatography-Mass spectrometry determination of polycyclic aromatic hydrocarbons in snow samples using a deep cavity BenzoQxCavitand

A very sensitive and selective solid-phase microextraction-gas chromatography-mass spectrometry method based on the use of a deep cavity BenzoQxCavitand as innovative coating was developed and validated for the simultaneous determination of the 16 US-EPA priority pollutants polycyclic aromatic hydrocarbons (PAHs) in snow samples at ultra-trace levels. The presence of a 8.3 Å deep hydrophobic cavity allowed the engulfment of all the 16 PAHs, providing enhanced selectivity also in presence of interfering aromatic pollutants at high concentration levels. Validation proved the reliability of the method for the determination of the investigated compounds achieving detection limits in the 0.03-0.30 ng/L range, good precision, with relative standard deviations <18% and recovery rates in the 90.8(±2.1)%-109.6(±1.0)%. The detection of low-molecular weight PAHs in snow samples from Antarctica and Alps confirms the widespread occurrence of these compounds, thus assessing the impact of anthropogenic activities onto the environment.

CTV Based Sensor for the Rapid Detection of Nitro Toluene With Computational Studies and Molecular Modelling

A new tri-naphthoylated Cyclotriveratrylene molecule has been synthesized for the rapid and sensitive detection of 4-nitrotoluene (4-NT) among various nitro aromatic compounds (NACs) by using a spectrofluorimetric method. The newly derrivatized cyclotriveratrylene compound is successfully confirmed by using the available techniques of ¹H-NMR, ¹³CNMR, and ESI-MS. This synthesised molecule is 1NC-CTV (1-Naptholy Chloride-Cyclotriveratrylene). Strong quenching in the fluorescence intensity of 1NC-CTV was observed upon the addition of 4-NT. Further quantum yield studies were carried out and by using the stern volmer it was concluded that the fluorescence quenching mechanism is dynamic or static. The molecule 1NC-CTV was further studied with the help of computational methods such as molecular docking to study the binding interactions and properties of the molecule.

Recent Applications and Newly Developed Strategies of Solid-Phase Microextraction in Contaminant Analysis: Through the Environment to Humans

The present review aims to describe the recent and most impactful applications in pollutant analysis using solid-phase microextraction (SPME) technology in environmental, food, and bio-clinical analysis. The covered papers were published in the last 5 years (2014–2019) thus providing the reader with information about the current state-of-the-art and the future potential directions of the research in pollutant monitoring using SPME. To this end, we revised the studies focused on the investigation of persistent organic pollutants (POPs), pesticides, and emerging pollutants (EPs) including personal care products (PPCPs), in different environmental, food, and bio-clinical matrices. We especially emphasized the role that SPME is having in contaminant surveys following the path that goes from the environment to humans passing through the food web. Besides, this review covers the last technological developments encompassing the use of novel extraction coatings (e.g., metal-organic frameworks, covalent organic frameworks, PDMS-overcoated fiber), geometries (e.g., Arrow-SPME, multiple monolithic fiber-SPME), approaches (e.g., vacuum and cold fiber SPME), and on-site devices. The applications of SPME hyphenated with ambient mass spectrometry have also been described.

Preparation of Methacrylate-based Polymers Modified with Chiral Resorcinarenes and Their Evaluation as Sorbents in Norepinephrine Microextraction

Aminomethylation reactions between chiral amino compounds (S)-(-)-1-phenylethylamine and l-proline with tetranonylresorcinarene and tetra-(4-hydroxyphenyl)resorcinarene in presence of formaldehyde were studied. The reaction between l-proline and resorcinarenes generated regioselectively chiral tetra-Mannich bases, due to the molecular incorporation of the fragment of the chiral amino acid. On the other hand, tetranonylresorcinarene and (S)-(-)-1-phenylethylamine formed regio- and diasteroselectively chiral tetrabenzoxazines, both by chiral auxiliary functionalization and by the transformation of the molecular structure that confers inherent chirality. The products obtained were characterized using IR, 1H-NMR, 13C-NMR, COSY, HMQC, and HMBC techniques. The reaction of (S)-(-)-1-phenylethylamine with tetra-(4-hydroxyphenyl)resorcinarene did not proceed under the experimental conditions. Once the chiral aminomethylated tetra-(4-hydroxyphenyl)resorcinarene was obtained, the chemical modification of poly(GMA-co-EDMA) was studied, and the results showed an efficient incorporation of the aminomethylated compound. For the physical modification, chiral aminomethylated tetranonylresorcinarenes were employed, finding that the incorporation of modified resorcinarenes occurs, but with less efficiency than that observed using chemical modification. The modified polymers were characterized via FT-IR, scanning electron microscopy imaging, and elemental analysis. Finally, polymers modified with chiral resorcinarenes were used as sorbents in norepinephrine microextraction; for practical purposes, artificial urine was prepared and used. To perform the microextraction, the decision was made to use the modern rotating-disk sorptive extraction technique (RDSE), because of its analytical attributes as a green, or eco-friendly, technique. According to the results, the method preliminarily validated for the determination of norepinephrine in artificial urine shows that the modified polymer with chiral derivative of tetra-(4-hydroxyphenyl)resorcinarene worked effectively as a new sorbent phase for the quantitative microextraction of norepinephrine, exhibiting high stability and homogeneity of composition and structure within the working range.

Cavitand-Decorated Silicon Columnar Nanostructures for the Surface Recognition of Volatile Nitroaromatic Compounds

Nanocolumnar Si substrates (porous silicon (PSi)) have been functionalized with a quinoxaline-bridged (EtQxBox) cavitand in which the quinoxaline moieties are bonded to each other through four ethylendioxy bridges at the upper rim of the cavity. The receptor, which is known to selectively complex aromatic volatile organic compounds (VOCs) even in the presence of aliphatic compounds, has been covalently anchored to PSi. The larger surface area of PSi, compared to that of flat substrates, allowed one to study the recognition process of the surface-grafted receptors through different techniques: Fourier-transform infrared spectroscopy, thermal desorption, and X-ray photoelectron spectroscopy. The experiments proved that surface-grafted cavitands retain the recognition capability toward aromatic VOCs. In addition, the affinities of EtQxBox for various aromatic compounds (i.e., benzene, toluene, nitrobenzene, and p-nitrotoluene) have been studied combining density functional theory computations and thermal desorption experiments. Computational data based on the crystal structures of the complexes indicate that this cavitand possesses a higher affinity toward aromatic nitro-compounds compared to benzene and toluene, making this receptor of particular interest for the detection of explosive taggants. The results of computational studies have been validated also for the surface-grafted receptor through competitive recognition experiments. These experiments showed that EtQxBox-functionalized PSi can recognize nitrobenzene in the presence of a significant excess of aromatic vapors such as benzene (1:300) or toluene (1:100).

Nanoporous Silica-Dye Microspheres for Enhanced Colorimetric Detection of Cyclohexanone

Forensic detection of non-volatile nitro explosives poses a difficult analytical challenge. A colorimetric sensor comprising of ultrasonically prepared silica-dye microspheres was developed for the sensitive gas detection of cyclohexanone, a volatile marker of explosives 1,3,5-trinitro-1,3,5-triazinane (RDX) and 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX). The silica-dye composites were synthesized from the hydrolysis of ultrasonically sprayed organosiloxanes under mild heating conditions (150 °C), which yielded microspherical, nanoporous structures with high surface area (~300 m2/g) for gas exposure. The sensor inks were deposited on cellulose paper and given sensitive colorimetric responses to trace the amount of cyclohexanone vapors even at sub-ppm levels, with a detection limit down to ~150 ppb. The sensor showed high chemical specificity towards cyclohexanone against humidity and other classes of common solvents, including ethanol, acetonitrile, ether, ethyl acetate, and ammonia. Paper-based colorimetric sensors with hierarchical nanostructures could represent an alternative sensing material for practical applications in the detection of explosives.

Environmental Gas Sensing with Cavitands

Environmental gas sensing needs stringent sensor requirements in terms of sensitivity, selectivity and ruggedness. One of the major issues to be addressed is combining in a single device the conflicting requirements of molecular-level selectivity and low-ppb sensitivity. The exploitation of synthetic molecular receptors as sensing materials is particularly attractive to address the selectivity issue, to single out the desired analytes in the presence of overwhelming amounts of interferents. This minireview summarizes the strategies in environmental gas and vapor sensing using molecular receptors as selective hosts for specific analytes, with the main focus on cavitands. In particular, we highlight the use of these macrocycles as selective preconcentrator units to be integrated into portable devices for environmental monitoring. Depending on the class of analytes to be detected, the molecular recognition properties of cavitands can be manipulated through the proper choice of the bridging groups at the upper rim, and their transducer integration can be implemented through the manifold functionalization options at the lower rim.

Probing Molecular Recognition at the Solid-Gas Interface by Sum-Frequency Vibrational Spectroscopy

Molecular recognition is among the most important chemical events in living systems and has been emulated in supramolecular chemistry, driven by chemical and biochemical sensing potential. Identifying host-guest association in situ at the interface, between the substrate-bound receptors and the analyte-containing media, is essential to predict complexation performances in term of the receptor conformation, orientation and organization. Herein, we report the first sum-frequency vibrational spectroscopy study of molecular recognition at the solid-gas interface. The binding capability of tetraquinoxaline cavitands toward volatile aromatic and aliphatic compounds, namely benzonitrile and acetonitrile, is investigated as test system. We prove the selective complexation of the receptors, organized in a solid-supported hybrid bilayer, toward aromatic compounds. Quantitative analysis allows to correlate the average orientations of the guest molecules and the host binding pockets, establishing "on-axis" complexation of benzonitrile within the cavitand cavity. The study is readily applicable to other receptors, molecular architectures, interfaces and analytes.