Cavitand-based polymers for the detection and removal of organic analytes in water

Abstract

Water pollution and the reduced availability of clean water sources are global issues that are becoming relevant in concomitance with the incoming climate change. Among the several pollutants present in water, the attention is on organic aromatic pollutant, halogenated or not, and heavy metals. High chemical stability and low-biodegradability of persistent organic pollutants (POPs) like aromatic, chlorinated and polycyclic compounds makes their removal difficult with conventional treatments. In particular, advanced oxidation and ozonation, widely used for the POP pollutants, lead to the formation of extremely toxic partial oxidation by-products. Therefore, there is the need in the development of new technologies to remove them through selective membrane and adsorption processes. A challenging class of persistent organic pollutants are the so-called polycyclic aromatic hydrocarbons (PAH). PAHs are a class of hazardous organic compound made by two or more fused aromatic rings, bonded together in linear, angular or cluster arrangements. Currently, over 400 kinds of PAHs and their derivates have been identified and classified, but usually most regulations, analyses and data are focused on only 16 PAH compounds considered as priority-contaminants both by the United States Environmental Agency and by the European Environment Agency. The 16 most monitored PAHs are acenaphthene, benzo[ghi]perylene, chrysene, acenaphthylene, benz[a]anthracene, benzo[b]fluoranthene, anthracene, fluorene, pyrene benzo[k]fluoranthene, benzo[a]pyrene, fluoranthene, indeno[1,2,3-cd]pyrene, naphthalene, phenanthrene, dibenz[a,h]anthracene. In order to overcome the traditional drawbacks of advanced oxidation and ozonation processes towards complex aromatic compounds, we decided to design and synthetize a new molecular receptor able to selectively remove PAHs dispersed in water through the formation of supramolecular host-guest complexes. To achieve this purpose, a new deep cavitand was synthesized through the functionalization of a resorcinarene scaffold with four extended benzoquinoxaline walls at the upper rim. The extension of the classical quinoxaline walls using benzoquinoxaline groups led to a deeper and electron-rich hydrophobic cavity, suitable for the inclusion of large polyaromatic compounds such as PAHs. The molecular structure of the new tetrabenzoquinoxaline cavitand (BzQxCav) was determined via X-ray crystallographic and the size of the inner cavity estimated by computational studies. The molecular structure obtained via X-ray diffraction on single crystals highlighted the organization of the benzoquinoxaline walls yielding a close vase conformation. The introduction of an additional ring fused with the “standard” quinoxaline wall provided a cavity depth of ca. 8.3 Å and a large entrance of ca. 8.3 Å x 7 Å. The accessible free volume inside the cavity was calculated using Caver software obtaining as final result a volume of 247 Å3. Moreover, the vase-kite conformational equilibrium adopted by a BzQxCav after pH perturbation was monitored using 1H-NMR and UV/VIS-Fluorescence spectroscopy. The performed experiments highlighted the existence of a reversible vase-kite conformational equilibrium in the BzQxCav, an interesting feature for the decomplexation and subsequent regeneration of the hosting cavitand properties. The removal ability towards a 20 ppt solution of the 16 most monitored PAHs was evaluated by solid phase micro extractions (SPME) comparing the performances of the deep tetrabenzoquinoxaline cavitand with the standard quinoxaline cavitand and the commercial PDMS 30. The analysis showed the better performances of the deep BzQxCav than the standard Quinoxaline cavitand and the commercial polymer, in particular towards the lighter and linear pollutants. In addition, the new deep benzoquinoxaline cavitand was exploited in different concentrations (3% and 6% w/v) as molecular recognition component in polyacrylonitrile electrospun fibers. The new materials were tested as filters in the purification of PAHs contaminated waters, each in concentration of 100 ppt. GC-MS analyses conducted over the filtered waters by 6%-charged fibers exhibited a suppression up to 60% of the lowest molecular weight PAHs. In concomitance with the increasing in the molecular weight and steric hindrance of the aromatic pollutants we observed the growth of a parallel physisorption process, which, combined with extracting abilities of the deep BzQxCav, leads to an almost complete suppression of the heaviest PAHs pollutants. Inspired by the activated eletrospun fibers effective in the water purification, a new strategy to develop new filtrating materials was investigated. Using SLA 3D-printing technique, a series of new porous structures were printed and activated for the removal of aromatic and chlorinated aromatic pollutant, PAH and heavy metals in water. Trimethylolpropane triacrylate (TMPTA) was used as monomer for the creation of the printed scaffold. The choice of TMPTA as printing material was driven by a compromise between the necessity of an acceptable wettability grade, essential to work in water, and fast curing rate during the printing process, necessary to guarantee a high pore printing definition. The decoration of the polymeric scaffold with molecular receptors was achieved through a surface post-printing modification. Three different molecular receptors, equipped with a methyl methacrylate moiety, were synthesized and covalently anchored over the surface of 3D printed scaffold through a radical reaction: tetraquinoxaline (QxCav) cavitand as selective receptor towards aromatic and chlorinated aromatic pollutant, tetrabenzoquinoxaline cavitand (BzQxCav) effective in the removal of PAHs, and dopamine as heavy metals complexing agent. Analytical measurements were performed to verify the uptake abilities of the QxCav decorated 3D-printed membranes in the removal in water of aromatic hydrocarbons such as benzene, toluene, ethylbenzene, xylenes and chlorobenzenes, each at 1 ppm concentration. The 3D-scaffold intrinsic physisorption behavior towards all the tested pollutants was evaluated conducting the same experiments using the not functionalized membranes. Surface functionalization led to a significant increase in the removal ability of all the aromatic chemicals, up to 65% towards the not-chlorinated pollutants. More in details, the activated surfaces compared with the untreated ones, showed the ability to remove an average additional 35% of aromatic compound. Using Fluorescence spectroscopy, additional tests were performed to confirm the existence of vase-kite conformational equilibrium on the surface of the QxCav functionalized membranes, a remarkable feature for the regeneration of the purifying materials. ICP optical analysis was performed to evaluate the purifying abilities of the dopamine-based porous scaffolds towards Tl+, Hg2+, Cr6+ and Pb2+ ions, each present in concentration of 1 ppm. Unfortunately, the analytical experiments did not provide evidence of effective removal abilities in the decorated dopamine-based membrane. An additional issue explored was the development of electrochemical devices for the detection of organic analytes in water, in particular ATS illicit drugs and aromatic pollutants. The conventional methods developed for the detection in water of ATS illicit drugs and the derived “designer drugs” rely on highly time-consuming methods and they are based on expensive equipment. As alternative, we explored the combination of molecular receptors with piezoelectric transducers. In particular, piezoelectric sensors are appealing, thanks to a series of advantages including simple operations, equipment miniaturization, fast response, low cost and high portability. So far, their use has been limited by the low selectivity. For this reason, we decided to deposit cavitands on QCM transducer through electropolymerization, in order to overcome the traditional limitations of the technique. Among the several molecular receptors available in the literature, tetraphosphonate cavitand (Tiiii) represents one of the most performant systems in the sensing of amphetamine drugs and their derivatives in water. In order to develop a new QCM sensor for the detection in water of ATS illicit drugs, the synthesis of a new thiophene-based conductive polymer functionalized with the tetraphosphonate cavitand (Tiiii) was attempted. The new tetraphosphonate cavitand monomer was synthetized with a 3-ethynyl-2,2’-bithiophene group at the lower rim. Unfortunately, electropolymerization tests performed over the QCM device highlighted the electrochemical inactivity of the synthesized Tiiii. The absence of activity is attributed to the steric hindrance exerted by the presence of four bulky phenyl groups at the lower rim of the cavitand, which inhibited the polymer formation onto the QCM electrodes. As last case, we explored the opportunity to exploit cavitands in the development of new electrochemical impedance sensors able to work in water. The electrochemical impedance spectroscopy is a not-destructive steady-state technique for the detection of several relaxation phenomena, which characterize the electroactive systems. All the relaxation phenomena are followed by the impedance changes in the electrochemical sample. Despite the advantages of the technique, one of the main challenges in the development of impedance sensors relies in the design of new water-resilient devices. One of the possible strategies followed to overcome the water interference in the impedance sensor is the introduction of selective molecular receptors towards the molecular targets. In this prospective, a tetraquinoxaline cavitand functionalized at the upper rim with a 2,2’-dithiophene was synthesized and afterwards polymerized through heterogeneous oxidative polymerization with FeCl3. 1H-NMR characterization performed on the polymer highlighted the kite conformation of the tetraquinoxaline cavitand. In addition, a tetraphosphonate cavitand functionalized at the lower rim with a 5,5’-dibromo-2,2’-bithiophene group was synthesized and furtherly polymerized through a Stille coupling reaction. Optical characterizations and GPC analyses are ongoing in order to start the following impedance experiments.