Nanoparticle-templated hierarchically porous polymer/zeolitic imidazolate framework as a solid-phase microextraction coatings

doi:10.1016/j.chroma.2018.06.059

Journal of Chromatography A

Volume 1567, 14 September 2018, Pages 55-63

https://doi.org/10.1016/j.chroma.2018.06.059 Get rights and content

Highlights

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Polymer/zeolitic imidazolate framework monolithic as a SPME coatings.
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On fiber metal oxide nanoparticle conversion to porous crystals.
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Homogenous porous crystal SPME coatings with hierarchical pore system.
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Enhanced extraction capacity for BTEX.

Abstract

A two-step ZnO nanoparticle-directed method has been implemented to prepare polymer monolith/zeolitic imidazolate framework (ZIF) solid-phase microextraction (SPME) fiber coatings with hierarchical micro-meso-macroporosity. The polymer/ZIF monolith was prepared on the surface of a stainless steel wire from a polymerization mixture containing dispersed ZnO nanoparticles. The embedded ZnO nanoparticles in the precursor polymer monolith coating were converted on-fiber to submicrometric porous crystals of the prototypical ZIF-8, based on the coordination of Zn(II) with 2-methylimidazole. The polymer/ZIF monolith coating was applied to the headspace SPME of benzene, toluene, ethylbenzene, and xylenes (BTEX) from water samples, followed by gas chromatography-flame ionization detection (GC-FID).

Hierarchically porous polymer/ZIF monolithic coatings showed a superior performance for BTEX extraction in comparison to coatings based on pure macroporous organic polymer monoliths, silicone glue/ZIF-8 coatings or commercial PDMS coatings. Experimental parameters such as desorption temperature, desorption time, salt concentration, temperature effect, equilibrium time and extraction time were investigated. Under the selected experimental conditions, limits of detection of 0.02–0.11 μg L⁻¹, linear ranges of 0.2–200 μg L⁻¹, relative standard deviations of 4.3–8.2%, and a fiber-to-fiber reproducibility of 8.9–9.8% (n = 3) were obtained. Recoveries higher than 88% were obtained for BTEX analysis in tap water, wastewater and landfill leachates.

Introduction

The selection of an appropriate sample pretreatment method is an essential step in the field of separation science [[1], [2], [3]]. Sample preparation techniques are usually divided in solid-phase extraction and liquid-phase extraction methods [4]. Different miniaturized solid-phase extraction (SPE) modes for improved sample preparation have been developed including stir bar sorptive extraction (SBSE) [5], thin film microextraction (TFME) [6], magnetic solid-phase extraction (MSPE) [7,8], dispersive solid-phase extraction (dSPE) [9,10], or solid-phase microextraction (SPME) [11,12], among others. SPME became a popular solvent-free microextraction technique, which is a sensitive, portable, environmentally friendly, and easy to operate tool for efficient and versatile sample preparation [13,14]. SPME relies on analyte partitioning between a stationary phase coated on a fiber and the sample matrix, and operates in two major modes including direct-immersion (DI) [15], and headspace (HD) [16]. HS-SPME is an especially attractive sample preparation technique for the analysis of volatile organic compounds due to the effective removal of non-volatile potential interferences, especially relevant when complex and dirty matrices are analyzed [17,18]. The need to maximize the performance of the SPME technique led to the development of robust coating materials, minimizing memory effects, and providing highly reproducible results and a long lifetime of the SPME support, as well as the exploration of different micro/nanomaterials as SPME coatings [13,14,19].

Metal-organic frameworks (MOFs) [20,21], including the highly robust and stable zeolitic imidazolate frameworks (ZIFs) [22,23], emerged as a class of porous crystalline materials obtained by linking metal atoms or clusters with organic ligands. MOFs exhibit prominent properties such as high specific surface area, high thermal stability, ordered crystalline structure, possibility of functionalization, and excellent adsorption capacity [21,24]. In recent studies the potential of MOFs was evaluated for the development of analytical applications [[25], [26], [27], [28]], including coatings for SPME [29]. SPME coatings have been developed incorporating prototypical MOFs for the extraction of volatile benzene homologues [30], polychlorinated biphenyls [31], polycyclic aromatic hydrocarbons [32], BTEX [33,34], or phenols [35], among others.

The preparation of MOF-based SPME coatings is usually based on the direct immobilization of bulk MOF crystals to the SPME support using an adhesive like silicone [36], or epoxy glue [35]. MOF SPME coatings have been also prepared by MOF in situ growth [31], covalent bonding on modified SPME fibers [37], the incorporation of MOFs as metal-organic aerogels [33], or the immobilization of metal oxides followed by their conversion to MOFs [38].

In order to develop optimum MOF-based SPME devices, hierarchically porous coatings containing MOF crystals would be highly beneficial, creating large surface area and robust extraction supports with a more accessible pore network. A feasible approach to prepare such supports could be integrating MOFs in macroporous organic polymer monolithic coatings [39]. MOF crystals have incorporated into polymer monoliths in column format by direct embedding [40,41], or layer-by-layer approaches [42,43]. Alternatively, the use of hybrid polymer monolith/metal oxide nanoparticles showed a high simplicity, efficiency and introduction of a hierarchical pore system to the resulting support [44], overcoming the main limitations of classic MOF immobilization procedures (low efficiency, time consuming,…). However, no polymer/MOF coatings with a hierarchical pore system have been reported yet for SPME applications.

The aim of this work is to develop for the first time a fast and efficient procedure for the preparation of SPME coatings with hierarchical micro-meso-macropore system. The development of the hierarchical pore system is based on embedding metal oxide nanoparticles in precursor polymer monolith coating. On fiber conversion of the precursor polymer/ZnO monolith is carried out in the presence of an organic linker (2-methylimidazole) to obtain a polymer/zeolitic imidazolate framework-8 (ZIF-8) monolith [44]. Each component of the coating provided a different type of pore to the SPME support, including macropores from the polymer monolith, mesopores from the polymer templating effect of metal oxide nanoparticles, and micropores from the ZIF-8. As proof-of-concept of the hierarchically porous polymer/ZIF coatings have been applied to the HS-SPME of BTEX from water samples, followed by their quantification with GC-FID.

Section snippets

Chemicals and standard solutions

All reagents were from analytical grade provided by Sigma-Aldrich (Madrid, Spain). Zinc oxide nanoparticles (Sigma-Aldrich, 40 wt.% dispersion in ethanol, <130 nm particle size (DLS) were used as MOF precursor. Aluminum oxide (Sigma-Aldrich, activated, Brockmann I), was used for monomer purification to prepare polymer monolith coatings. Milli-Q water (Direct-8 purification system, resistivity >18 MΩ cm) was used. Stock standard solutions were prepared in methanol at a concentration of 2000 mg L

Characterization of polymer/ZIF monolith coating

The morphology of the prepared hierarchically porous polymer/ZIF SPME fibers was studied using SEM. A low-magnification image shows the overall appearance of the polymer/ZIF SPME device (Fig. 2a), observing a homogeneous and well distributed coating. A higher magnification image of the inset from Fig. 2a, shows the rough morphology of the SPME coating due to the globular shape of the monolithic polymer coating (Fig. 2b). A clear change in the morphology of the SPME coating is observed after ZIF

Conclusion

In the present study, the preparation of hierarchically porous polymer/ZIF monolithic SPME coatings is reported. The developed approach is based on the preparation of polymer/ZnO nanoparticle monolithic coatings followed by the on-fiber conversion of the non-porous ZnO nanoparticles into highly microporous ZIF-8 crystals. The developed coating combines a macroporous structure typical of organic polymer monoliths with additional mesopores templated by the incorporation of ZnO nanoparticles, and

Acknowledgements

The authors are grateful for the financial support of this work from the Iran National Science Foundation (INSF) (grant number 95849652) and the University of Kashan. The Spanish Agencia Estatal de Investigación (AEI – Spain) and the European Funds for Regional Development (FEDER – European Union) are gratefully acknowledged for financial support through Project CTQ2016-77155-R (AEI/FEDER, UE). Authors thank F. Hierro Riu (SCT) for scanning micrographs.

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Zeolites have been little investigated for food applications, despite their chemical composition is similar to clays and clay minerals, which have been extensively analyzed for various applications, including food. Zeolites can be distinguished from clay materials, since the former have a porous microstructure characterized by intracrystalline cavities and channels, while the latter have a laminar microstructure. The goal of this review paper was to give a comprehensive perspective in terms of the different food applications found so far for zeolites, namely: antimicrobial materials, ethylene scavengers, fillers for food packaging materials, food nanoreactors, food substance sensors, immobilizers and stabilizers of active compounds and enzymes, molecular sieves for the pretreatment of food samples, as well as intelligent food contact materials. The main food applications from zeolites are related to their good properties as adsorbent materials, and these properties can be altered and tuned by ion exchange, surface organo-modification, among others, for a specific designed application. Zeolites for food applications have been investigated primarily as antimicrobial materials, concentrators of target analytes and sensors for food substances. However, the other potential food applications indicated above from zeolites are booming, since they are harmless materials recognized by various organizations.
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However, many MOFs are very sensitive to water because their M−O (M = Cu, Zn) bonds are easy to degrade even subjected to a small amount of water. A branch of MOFs material, zeolite imidazolate frameworks (ZIFs), have higher thermal stability than MOFs due to their stronger M−O bonds, and excellent stability under water and alkaline solutions [14], its application in SPME can be found in several publications [15–20]. It is reported that there are a variety of strategies to prepare SPME fibers with MOFs as adsorbent coatings, including sol–gel [21], adsorption [15], adhesive [22], and in-situ growth [12].
At present, electrochemical deposition has become a novel approach to manufacture metal–organic framework (MOF) coatings. Here, zeolitic imidazolate framework (ZIF) coating solid-phase microextraction fiber was prepared by electrochemical deposition method with stainless steel wire as the support. The fabrication process was proceeded without adding any supporting electrolyte or regulator, and the ZIF-8 coating fiber could be obtained at room temperature within 20 min. The main factors affecting the preparation of ZIF-8 coated fiber were optimized, including the type of zinc salt in the deposition solution, deposition voltage, and time. Finally, eight polycyclic aromatic hydrocarbons were used as simulated pollutants, and the performance of the fiber had been evaluated using HS-SPME-FID. The optimum extraction conditions of the SPME-GC-FID methods were stirring for 40 min at 70 °C. The linear range of the method was 0.05–50 μg/L, limits of detection were 0.010–0.054 μg/L, inter-batch, intra-day, and inter-day relative standard deviation was lower than 4.2%, 8.5%, and 9.5%, respectively. The service life of a single fiber exceeds 80 extraction/desorption cycles.
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However, an adhesive can decrease the surface area of the MOF. Ghani et al., [98] discovered a decrease in the surface area of ZIF-8 from 903 to 2 m2/g after mixing glue with ZIF-8 particles. Also, the solubility of glue in water or organic solvents is a crucial parameter in developing stable MOF-based SPME coatings [113].
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Nanoparticle-templated hierarchically porous polymer/zeolitic imidazolate framework as a solid-phase microextraction coatings

Highlights

Abstract

Introduction

Section snippets

Chemicals and standard solutions

Characterization of polymer/ZIF monolith coating

Conclusion

Acknowledgements

Trends Analyt. Chem.

Trends Analyt. Chem.

J. Chromatogr. A

J. Magn. Magn. Mater.

Talanta

J. Chromatogr. B

J. Chromatogr A

Anal. Chim. Acta

J. Chromatogr. A

J. Chromatogr. A

Trends Analyt. Chem.

J. Chromatogr. A

Trends Analyt. Chem.

Trends Analyt. Chem.

Anal. Chim. Acta

J. Chromatogr. A

Anal. Chim. Acta

J. Chromatogr. A

Talanta

Anal. Chim. Acta

J. Chromatogr. A.

Anal. Chim. Acta

Emerging materials for sample preparation

J. Sep. Sci.

Stir bar sorptive extraction (SBSE), a novel extraction technique for aqueous samples: theory and principles

J. Microcolumn Sep.

Thin-film microextraction

Anal. Chem.