Molecularly imprinted polymers coated on multi-walled carbon nanotubes through a simple indirect method for the determination of 2,4-dichlorophenoxyacetic acid in environmental water

Highlights

• 2,4-D MIPs were grafted on CNTs by a facile indirect method.

• The prepared CNTs@SiO₂@MIPs were characterized by FT-IR, SEM and TG.

• The prepared CNTs@SiO₂@MIPs possess good selectivity and quick mass transfer.

• CNTs@SiO₂@MIPs can be used for 2,4-D determination in environmental water.

Abstract

A new and facile method was presented to graft molecularly imprinted polymers (MIPs) on carbon nanotubes (CNTs) for 2,4-dichlorophenoxyacetic acid (2,4-D) analysis. In brief, CNTs were firstly coated with a layer of vinyl group modified silica, followed by a common precipitation polymerization with 2,4-D as the template, ethylene glycol dimethacrylate (EGDMA) as the crosslinker and 2,2-azobisisobutyronitrile (AIBN) as the initiator. The imprinted effects obtained by using different monomers were investigated, and the results showed that acrylamide (AM) and styrene as mixed monomers was the best choice. This functionalized material was characterized by Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and thermogravimetry (TG), which demonstrated a successful polymerization reaction on CNTs with MIPs grafting ratio of about 80%. The results of static adsorption experiments indicated the imprinted material possessed fast kinetics and good selectivity for 2,4-D molecules. A corresponding analytical method was developed and demonstrated to be applicable for the determination of 2,4-D in environmental water. The recoveries were in the range from 74.6% to 81.2% with relative standard deviation below 7.0%. To be emphasized, the method for MIPs coating proposed herein also provides a significant reference for other radical polymerization reactions based on CNTs.

Introduction

Along with the benefit for modern agriculture, however, the large-scale application of herbicides also results in the contamination of surface and ground waters because of their potential toxic, carcinogenic, mutagenic effects and mobility, which has become a great environmental concern. 2,4-dichlorophenoxyacetic acid (2,4-D) is a selective herbicide of the phenoxyacetic acid group, with weak aromatic acid properties [1]. The chemical structure of 2,4-D resembles indoleacetic acid, a naturally occurring hormone produced by plants to regulate their own growth [2]. Due to its low cost and good selectivity, 2,4-D is most extensively used for combating a wide range of broad-leaved weeds in agriculture and forestry [3]. 2,4-D is relatively high water-soluble and has a low tendency to accumulate in organic matter [4]. As a result, it can be often detected in many natural bodies of water in different areas of the world, posing a direct health threat to living organisms due to its established genotoxicity and endocrine disruption properties [5]. Therefore, its determination from environmental water is likely to become increasingly important.

Some methods reported for 2,4-D determination are based on gas chromatography (GC), high performance liquid chromatography (HPLC), enzyme-linked immunosorbent assay (ELISA), surface-enhanced Raman scattering (SERS) etc. [2], [6], [7], [8]. Among which HPLC is a widely used determination method with low cost compared to ELISA and SERS, and with no derivatization step that needed in GC. However, an enrichment step by solid-phase extraction (SPE) is usually needed when especially for environmental sample analysis by HPLC method. Amongst the best candidates as SPE sorbents are molecularly imprinted polymers [9]. MIPs are synthetic polymers with a predetermined selectivity for the target molecule, involving a retention mechanism based on molecular recognition regardless of the complexity of the matrix it is in [10]. Due to their favorable molecular recognition capabilities and stability, MIPs have been applied in chromatography [11], catalysis [12], sensors [13], adsorption [14], solid-phase extraction [15], to name but a few. In the previous reports, 2,4-D MIPs were often prepared by the conventional bulk polymerization [16], [17], [18], which suffers some intrinsic limitations, such as the heterogeneous distribution of the binding sites, the deep buried binding sites in bulk, and poor site accessibility for the template recognition and elution [19]. To solve these problems, the surface molecular imprinting technique [20], [21], [22] has been developed to improve the mass transfer between recognition sites and target molecules by designing the molecular recognition sites on the surfaces of supported materials.

Recently, significant attention has been paid to the surface molecular imprinting technique based on the modification of carbon nanotubes [23], [24], [25], [26]. CNTs, with unique mechanical properties and extremely large surface area, should be an ideal candidate as the supported material, which would endow MIPs with large surface area. Generally, CNTs need to be functionalized with vinyl groups before when MIPs are grafted on by radical polymerization approach. However, In order to graft vinyl groups, CNTs in most reported paper were initially treated with strong acid to introduce carboxyl groups [27], [28], [29], [30], [31], which would in turn cause some damages to the structure of CNTs. To resolve this problem, Zhang et al. also reported a mild synthesis route to introduce carboxyl groups, and used aniline as a patch for the preparation of vinyl group functionalized CNTs [32]. However, this synthesis route seems long and time consuming, while SOCl₂ is still necessary and not feasible for practical operation. Therefore, developing new synthesis routes to introduce vinyl groups on CNTs is of great importance, not only for the MIPs coating, but also for other radical polymerization reactions based on CNTs.

In this paper, a simple indirect method was developed to introduce vinyl groups on CNTs. Then 2,4-D MIPs based on CNTs was successfully prepared using 2,4-D as template, AM and styrene as mixed monomers, EGDMA as a cross-linker and AIBN as an initiator. The adsorption dynamics, adsorption capacity and selective recognition of CNTs-MIPs were investigated. Then the prepared CNTs-MIPs were used as SPE absorbents, coupling with HPLC to determine 2,4-D in environmental water.