Nanostructured octadecylsilica chemically coated stainless-steel fiber for vacuum-assisted HS-SPME sampling of PAHs in soil
Graphical abstract
Introduction
Development of novel sorbents with higher sorption capacity and better physicochemical stability for microextraction, is nowadays an emerging area of research in separation science. A valuable option that has further flourished these studies is the use of nanoparticles for preparation of new sorbents with improved functionalities [1]. The conventional sorbents like silica are ubiquitously available at low-cost and their physiochemical characters have been thoroughly studied [2]. Consequently, alongside the efforts devoted to investigating new nanomaterials, a great attention has also been paid to improve the performance of the conventional sorbents by developing their reconstituted and nanostructured forms. Regarding the nontoxicity, low-cost, easy preparation, and possibility of simple surface modification, silica nanoparticles have been widely investigated and utilized in separation techniques. A widely used form of silica sorbent is octadecylsilica, that plays an important role in the chromatographic separation technologies [3]. Nanostructured octadecylsilica (NODS) is a promising sorbent for different fiber-based and needle-based configurations of SPME [4]. For instance, it was successfully utilized as the sorbent for in-tube SPME of polycyclic aromatic hydrocarbons (PAHs) in water samples [5].
Fiber SPME is the most used microextraction strategy, due to its technical and application features [6]. Extensive efforts have been taken to develop new fiber coatings to address the limitations of the existing commercial SPME fibers. Although most of the SPME fibers are silica-based, but recently attention has mainly focused on metal-based fibers due to their higher mechanical strength [7]. Nevertheless, preparation of fibers with tight adhesion of the coating to the metallic substrate isn’t an easy task. There are limited number of materials with acceptable sorption features that can firmly attach to the metallic fiber substrates. Among the examined metals, the best choice for fiber core is stainless-steel. It is mechanically strong and very resistance in harsh aqueous and organic environments [8]. The presence of remarkable amounts of chromium (18–20%), nickel (8–12%), manganese (2%), silicon (0.75%), and carbon (0.08%) in stainless-steel has made its prone to be superficially modified using chemical procedures [9]. Accordingly, chemical bonding of a silica to steel’s surface is possible by creating an oxide layer on its surface. The surface oxidation can be done by oxidizing in air at high temperature. It creates a thin oxide film, mainly consists of Fe2O3 and Cr2O3. This oxide superficial layer contains a substantial amount of hydroxyl species, that can be used as the active sites for binding with silanes [10]. Sol-gel procedure offers a simple and low-cost strategy to prepare uniform SPME coatings, with strong adhesion between the silica-based materials and stainless-steel substrate [11].
During the recent years, several modified forms of SPME including electroenhanced [12], cold-fiber [13], magnetic field assisted [14], microwave-assisted [15], ultrasonic-assisted [16], total vaporization [17], and vacuum-assisted headspace SPME (VA-HS-SPME) [18] have been developed to improve its performance and functionality. The improving effect of vacuum on the analytical performance of HS-SPME has been convincingly ascertained in a few reports [19], [20]. Increasing extraction efficiency, shortening sampling time, and lowering extraction temperature are the main inherent features of VA-HS-SPME.
This study aimed to develop a robust and efficient NODS coated fiber for the ultrasensitive sampling of organic volatile and semi-volatile compounds using VA-HS-SPME technique. Superficial hydroxyl groups of the stainless-steel fiber were activated by surface oxidation and sodium hydroxide treatment. The modified fiber was coated with NODS using a modified sol–gel Stöber route. Seven representative PAHs, with a wide range of volatility, were considered as the model analytes. Separation and quantification of PAHs was done using GC-FID. The fiber was employed for the analysis of PAHs in polluted soil samples through a VA-HS-SPME strategy. A response surface methodology (RSM) based Box-Behnken design (BBD) was utilized to optimize the experimental variables.
Section snippets
Chemicals
The analytes including naphthalene (Nap), acenaphthene (Ace), phenanthrene (Phn), fluoranthene (Flt), fluorene (Flr), anthracene (Ant), and pyrene (Pyr) were purchased from Sigma-Aldrich (Steinheim, Germany). A mixture standard solution (1000 µg mL−1) of the PAHs was prepared in methanol. Fresh working solutions were prepared weekly by appropriate diluting of the stock standard solution with water or methanol. Tetraethylorthosilicate (TEOS, ≥99.0%) and octadecyltrichlorosilane (OTS, ≥90%) were
Characterization of the NODS sorbent
The FT-IR spectrum of the developed NODS sorbent was recorded for characterization of functional groups and demonstrate bonding between NODS and modified stainless-less substrate (Fig. 1). The absorption bands at 3385, 2938, and 1485 cm−1 are attributed to the stretching vibrations of O–H, C–H, and Si-C bonds, respectively. The shoulder appeared at 954 cm−1 has resulted from the overlap of Si-O–H and Si-O-Fe stretching vibrations [24]. The Si-O-Fe vibration can be also concluded from the weak
Concluding remarks
An unbreakable, robust, and efficient SPME fiber was developed by coating a thin film of NODS on the surface of a stainless-steel wire using sol–gel-strategy. To activate superficial hydroxyl groups, the surface of the fiber was modified by oxidizing at high temperature and treatment with sodium hydroxide solution. The hydrolysis and condensation of TEOS in the presence of OTS, resulted in chemical binding between the fiber and the silica polymeric structure. The NODS coated fiber was employed
CRediT authorship contribution statement
Ebrahim Akbari: Methodology, Validation, Investigation, Writing - original draft. Alireza Ghiasvand: Supervision, Project administration, Conceptualization, Methodology, Validation, Data curation, Resources, Writing - review & editing, Visualization. Kolsoum Dalvand: Formal analysis, Software, Writing - original draft.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The authors are grateful for the support of Lorestan University.
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