Silica aerogel coated on metallic wire by phase separation of polystyrene for in–tube solid phase microextraction

Highlights

• A sol–gel–based silica aerogel was coated on a copper wire by phase separation of polystyrene.

• The use of phase separation led to the formation of a porous and thick structure of silica aerogel.

• The coated wire was used as a superhydrophobic sorbent for in–tube solid phase microextraction.

Abstract

In this research, an attempt was made toward synthesizing a sol–gel–based silica aerogel and its subsequent coating on a copper wire by phase separation of polystyrene. Adaption of this new approach enabled us to coat the metallic wire with powder materials. The use of this method for coating, led to the formation of a porous and thick structure of silica aerogel. The coated wire was placed in a needle and used as the sorbent for in–tube solid phase microextraction of chlorobenzenes (CBs). The superhydrophobicity of sorbent on extraction efficiency was investigated by using different ratios of tetraethylorthosilicate/methyltrimethoxysilane. The surface coated with the prepared silica aerogel by the phase separation of polystyrene showed high contact angle, approving the desired superhydrophobic properties. Effects of major parameters influencing the extraction efficiency including the extraction temperature, extraction time, ionic strength, desorption time were investigated and optimized. The limits of detection and quantification of the method under the optimized condition were 0.1–1.2 and 0.4–4.1 ng L⁻¹, respectively. The relative standard deviations (RSD%) at a concentration level of 10 ng L⁻¹ were between 4 and 10% (n = 3). The calibration curves of CBs showed linearity from 1 to100 ng L⁻¹. Eventually, the method was successfully applied to the extraction of model compounds from real water samples and relative recoveries varied from 88 to 115%.

Introduction

Today, environmental pollution is a major global issue. Many pollutants, even in small amounts, are threatening human health and the whole environment. Therefore, identification and determination of these pollutants in rivers, ground– and sea–waters and even food is critical. Different analytical methods have been developed to address these issues but further improvements on the performance of these methods are needed. The major approach for the extraction and pre–concentration of contaminants is based upon on the use of a solid phase as a desired sorbent [1]. Solid phase microextraction (SPME) [2] is a typical approach for determination of various pollutants. In this method, the fiber coating, is fixed on a wire–like substrate and extraction is carried out by the least amount of sorbent. In tube solid phase microextraction (IT-SPME), regarded as another SPME approach, is employed as an efficient sampling technique in conjunction with gas chromatography (GC) and high performance liquid chromatography (HPLC). In this technique, the coating phase is deposited on a wire which is subsequently fixed in a needle while its position remains constant during extraction and sample is flown over the coating. Usually, the desired sorbents used in SPME must have high surface area, and preferred to be temperature resistant and capable of interacting with the target analytes, while could be easily prepared. Many materials such as conductive polymers [3], carbon allotropes [4], electrospun nanofibers [5] and metallic materials [6] have appropriate characteristics but their fixation on the wire and their use as the sorbent is quite challenging. Different approaches such as electrodepositing [7], electrospinning [8], chemical bonding with functional wire [9] and gluing [10] have been used for the wire substrate coating. Silica aerogel, as a powder material, is suitable to be used as a sorbent but difficult to be assembled on the metallic wires [11]. In this research the phase separation method was used for fixation of silica aerogel on the copper wire. So, the silica aerogels were dispersed in polystyrene solution and the wire dipped in this solution and subsequently in an anti-solvent. This approach led to crystallization of polystyrene and silica particles on the wire. Polystyrene, acts as glue to immobilize the silica particles on the wire. The silica aerogel was synthesized using various ratios of tetraethylorthosilicate (TEOS) and methyltrimethoxysilane (MTMS) to achieve the highest superhydrophobicity [12]. For creation of superhydrophobicity, it is necessary to have an appropriate surface with sufficient roughness and non–polar groups for functionalization [13]. These properties cause the surface to have less contact with water and achieve higher contact angels. Superhydrophobic structures have a strong tendency to absorb non–polar materials as they have been used to separate oil from water [14]. So superhydrophobic materials are perfect option for synthesizing suitable sorbents for extraction and determination of non–polar pollutants. In this research, polystyrene was used to assemble the superhydrophobic silica aerogel on the copper wire. This wire was eventually used for IT-SPME of chlorobenzenes, as the representatives of non–polar pollutants, from aquatic media.