Chemically modified halloysite nanotubes as a solid–phase microextraction coating

Highlights

• Halloysite nanotubes were modified and used as a SPME coating.

• Modification process was included etching, hydroxylation and amino grafting.

• Modified halloysite nanotubes were chemically bonded to fused-silica support by sol–gel process.

• The modified halloysite nanotubes fiber had high extraction efficiency with excellent thermal stability and high durability.

Abstract

Halloysite nanotubes were modified in three simple steps including etching, hydroxylation and amino grafting. The sol–gel technique was used for the chemical bonding of the modified halloysite nanotubes (MHNTs) to fused–silica support. The MHNTs, as a novel adsorbent was applied as a SPME coating. Diazinon, parathion and fenthion were selected as the model compounds to study the extraction efficiency of the coating. Gas chromatography–corona discharge ion mobility spectrometry was applied for the analysis of the extracted analytes. The parameters influencing the extraction efficiency of the method, such as stirring rate, salt effect, extraction temperature and time were optimized. The results showed that the MHNTs fiber had better extraction efficiency than the commercial SPME (PA, PDMS, and PDMS–DVB), bare silica, silica–based HNTs and HNTs–titanium dioxide fibers. The limits of detection were found to be in the range of 0.01–0.03 μg L⁻¹. The limits of quantification were in the range of 0.03–0.07 μg L⁻¹. Also, a good linearity in the range of 0.03–3.0, 0.07–2.0 and 0.03–3.0 μg L⁻¹, was found for diazinon, fenthion and parathion, respectively. The method precision was lower than 7.0 and 8.7% as the intra- and inter-day relative standard deviations, respectively. Agricultural wastewater, cucumber and apple were chosen as the real samples. The spiking recovery values were between 84 (±9) and 97% (±6). The results showed that the method was applicable and suitable for real samples analysis.

Introduction

Solid–phase microextraction (SPME) is one of the most interesting sample preparation methods, introduced by Pawliszyn in 1989 [1]. SPME is performed based on partitioning compounds between the sample matrix (gaseous, liquid and solid) and an adsorbent coated on a support. Since 1989, many efforts have been made to enhance the applicability and professional objectivity of SPME. The main studies in SPME method have been focused on the preparation of fibers with higher thermal and chemical stability [2], the use of metal wires as support [3], strategies for the coupling of SPME with other separation methods [4], [5], green analytical chemistry [6], automation [7], in vivo analysis [8], and the use of nanosized materials as SPME coatings [9]. One disadvantage of the method is the limited number of commercially available fibers.

During the last decade, researchers have prepared a variety of nanomaterials, as new coatings for SPME. Generally, to enhance the extraction capability of SPME coatings and promote their chemical, physical and thermal properties, the coating materials have been modified. Modification can be performed chemically using a suitable reagent and/or through the hybridization of nanomaterials with other substances. Chemical modification has been performed using different chemical reagents such as 3-mercaptopropyltrimethoxysilane, N-octadecyltrichlorosilane, ethylenediamine and carboxymethyl dextran hydroxamate [10], [11], [12], [13]. To prepare hybrid nanomaterials, different substances such as organic and inorganic polymers, carbon nanotubes (CNTs), graphene, ionic liquids, metals and metal oxides have been used [9], [14]. Besides the high extraction performance of modified nanomaterials, some disadvantages can be pointed out for these coating materials. The synthesis of the coatings usually involves a number of steps with difficult processes; the reagents and precursors used for modifications are expensive and sometimes not commercially available. Also, reagents and materials are not very environmentally friendly.

For the majority of modified SPME coatings, the improvement of extraction efficiency has just been obtained by the hybridization of nanomaterials. Compared to the hybridized SPME nanomaterials, there are limited reports using chemically modified nanomaterials as a suitable SPME coating [15], [16], [17]. In fact, a chemically modified nanomaterial can rarely be found with excellent chemical, mechanical and thermal properties, such that it can be applied as a high performance SPME coating.

Halloysite nanotubes (HNTs) are low cost materials with a morphology very similar to that of CNTs. Compared to CNTs, they are not toxic, and are environmentally friendly, abundant, and naturally available. HNTs have an aluminosilicate structure with the molecular formula of Al₂Si₂O₅(OH)₄·2H₂O. Nanotubes of halloysite consist of 10–15 nanostructure layers as a roll shape. Like CNTs, HNTs have a high aspect ratio (the inner diameter of 10–15 nm and the length of 1–2 μm). The interior and exterior surface of HNTs consists of Al₂O₃ and SiO₂, respectively [18]. The advantages that make the original HNTs convenient nanohybrid sorbents for SPME, are high thermal and mechanical resistance, cheapness and high adsorption capacity. Due to the unique chemical structure of HNT, its inner and outer surface can be simply chemically modified [19], [20].

In order to enhance the surface area and the adsorption capacity of original HNTs, some approaches such as intercalation, chemical activation and acid treatment have been reported [19], [21], [22]. Moreover, the diameter of lumen could be increased up to 2–3 times by the selective etching interior surface of HNTs in acidic media [19]. The etching process initiates with diffusion of hydronium ions into the lumen of HNTs. They then interact with alumina groups, and finally, the reaction products diffuse out of the nanotubes. Accordingly, the porosity of interior surface is enhanced. By controlling the etching time and temperature, single or more layers of alumina can be etched and the inner diameter may be expanded. Meanwhile, acid etching has no effect on the outer surface of nanotubes.

The exterior surface of HNTs naturally consists of SiOSi groups. However, the density of silanol groups on the surface of HNTs is very low. SiOSi groups on the external surface of HNTs can be activated using low concentrations of sodium hydroxide at room temperature. Due to the lower reactivity of alumina, as compared to silica groups, the hydroxylation modification is merely performed at the interior surface of HNTs [20], [22]. The hydroxylated surface of alumina can be simply chemically modified to increase the extraction efficiency of the sorbent. Among different reagents, organosilane compounds are good candidates to modify the metal oxide surface. In this regard, the interactions between the specific functional groups of grafted organosilanes and targeted analytes can enhance the adsorption efficiency and extraction capability of the sorbent [23].

Very recently, in our research group, the original HNTs were hybridized with titanium dioxide and used as a SPME coating [24]. In this study, both outer and inner surfaces of original HNTs were modified to increase the specific surface area of the sorbent. The sorbent was also chemically modified with an organosilane compound to enhance the extraction capability of the material. The extraction efficiency of the modified HNTs was evaluated as an adsorbent in SPME method. The inner diameter of the nanotubes was enhanced by sulfuric acid etching. The outer surface of HNTs was hydroxylated with sodium hydroxide; it was then grafted by (3-aminopropyl)triethoxysilane (APTES). The modified HNTs (MHNTs) were chemically coated on a fused-silica support by the sol–gel process, using tetraethoxysilane (TEOS) and methyltriethoxysilane (MTMOS). The fabricated fiber was used for the extraction of three organophosphorus pesticides (OPPs) including diazinon, parathion and fenthion (as model compounds) from wastewater and fruit samples. Effective parameters on the extraction efficiency, such as stirring rate, ionic strength, extraction time and temperature were also optimized. Gas chromatography–corona discharge ion mobility spectrometry (GC–CD–IMS) was used as separation–detection system.