Facile preparation of surface-exchangeable core@shell iron oxide@gold nanoparticles for magnetic solid-phase extraction: Use of gold shell as the intermediate platform for versatile adsorbents with varying self-assembled monolayers

Highlights

• The core@shell Fe₃O₄@Au nanoparticles functionalized with SAMs were successfully constructed.

• The SAMs could be transformed from one kind to another via thiol exchange process.

• The developed nanomaterials could be applied in mode switching MSPE.

Abstract

The core@shell Fe₃O₄@Au nanoparticles (NPs) functionalized with exchangeable self-assembled monolayers have been developed for mode switching magnetic solid-phase extraction (MSPE) using high performance liquid chromatography with ultraviolet detection. The adsorbents were synthesized by chemical coprecipitation to prepare magnetic cores followed by sonolysis to produce gold shells. Functionalization of Fe₃O₄@Au NPs surface was realized through self-assembly of commercially available low molecular weight thiol-containing ligands using gold shells as intermediate platform and the dynamic nature of Au–S chemistry allowed substituent of one thiol-containing ligand with another simply by thiol exchange process. The resultant adsorbents were characterized by transmission electronic microscopy, Fourier transform infrared spectroscopy, elemental analysis, contact angle measurement, and vibrating sample magnetometry. To evaluate the versatile performance of the developed MSPE adsorbents, they were applied for normal-phase SPE followed by reversed-phase SPE. A few kinds of diphenols and polycyclic aromatic hydrocarbons (PAHs) were employed as model analytes, respectively. The predominant parameters affecting extraction efficiency were investigated and optimized. Under the optimum experimental conditions, wide dynamic linear range (6.25–1600 μg L⁻¹ for diphenols and 1.56–100 μg L⁻¹ for PAHs) with good linearity (r² ≥ 0.989) and low detection limits (0.34–16.67 μg L⁻¹ for diphenols and 0.26–0.52 μg L⁻¹ for PAHs) were achieved. The advantage of the developed method is that the Fe₃O₄@Au NPs could be reutilized for preconcentrating diverse target analytes in different SPE modes sequentially simply through treatment with desired thiol-containing ligands.

Introduction

Trace analysis of target species in biological or environmental matrices does pose great challenges to researchers as a result of their low concentration levels and concomitant existence of matrix interferences. Therefore, sample pretreatment is often required for preconcentrating analytes from various samples prior to analysis [1], [2]. Solid phase extraction (SPE) is a powerful tool to preconcentrate analytes of interest from sample matrices. It has obvious advantages of high extraction efficiency and low consumption of organic solvents, but is still time-consuming and labor intensive due to its limited diffusion and mass transfer rate [3], [4]. To overcome the limitations of traditional SPE, many new approaches have been designed and dispersive SPE (dSPE) is one of the promising sample pretreatment approaches. In dSPE, the adsorbents are dispersed in a sample solution containing the target analytes rather than packed in the cartridge, which avoids channeling or blocking cartridges occurred in traditional SPE [5], [6]. Among numerous dSPE adsorbents, magnetic nanoparticles (MNPs) have obtained popularity resulted from their large specific surface area, short diffusion route, and ease of magnetic separation. The technology based on dSPE using MNPs is known as magnetic solid-phase extraction (MSPE) [7], [8]. The distinct virtue of this technology is that magnetic adsorbents can be easily recovered by a magnet without additional centrifugation or filtration of the sample, which makes sample treatment more convenient and timesaving [9], [10]. Among the most widely used magnetic adsorbents, bare Fe₃O₄ NPs has played a major role. However, the bare Fe₃O₄ NPs suffer from several inherent limitations as the aggregation caused by their high surface-to-volume ratio and attractive forces reduces their superparamagnetic properties. With some exceptions, they usually lack target selectivity and therefore are unsuitable for sample pretreatment in complicated matrices [11]. Thus, functionalization of bare Fe₃O₄ NPs has been required to protect them from aggregation and introduce desired surface properties [12]. For instance, carbonaceous materials [13], [14], [15], [16], ionic liquids [7], [17], [18], metal [12], surfactants [19], [20], polymers [21], [22], and proteins [23] were used for modification of Fe₃O₄ NPs as the MSPE adsorbents to extract the target compounds in environmental and biological samples. Nevertheless, most of these MSPE adsorbents could only be used for extracting a certain class of hydrophobic analytes based on hydrophobic interactions in reversed-phase SPE (RP-SPE), which caused the bottleneck in sample pretreatment in non-polar matrices containing hydrophilic compounds, precluding greatly the widespread applications of MSPE technology. Hence, there is a high demand for development of a versatile MSPE adsorbent with exchangeable surface functionalities according to different classes of target analytes. That is, the MSPE adsorbent could be reutilized for selectively extracting one series of analytes followed by extracting another distinct series through a simple treatment for the adsorbent.

During the last decade, self-assembled monolayers (SAMs) have generated a new research avenue for solid surface functionalization due to comparatively simple preparation, high reproducibility, and availability of various SAM molecules [24], [25]. Metallic and semiconductor surfaces are thought to be the most versatile self-assembly building blocks by manipulating the interfacial interactions [26], [27]. One of the most attractive contemporary endeavors which has stimulated intensive researches into potential high-impact applications is SAMs of alkanethiolates on gold surfaces. Au has been considered as the research hotspot due to the advantages of chemical stability, biocompatibility, versatility in surface modification, especially that fine-tune gold surfaces can be designed not only by selecting SAM alkanethiolates featuring different head groups, but also by switching from one kind of SAM to another via a simple thiol exchange process [28], [29]. Thus, functionalization of Fe₃O₄ NPs with gold coating casts a positive light for exploiting an adsorbent with exchangeable surfaces based on the dynamic nature of Au–S bindings for mode switching MSPE. Although several recent literatures reported preparation of core@shell Fe₃O₄@Au NPs, most of them focused on the synthesis methods [30], [31], [32] rather than practical applications [33], [34].

In this work, core@shell Fe₃O₄@Au NPs functionalized with exchangeable SAMs were developed for mode switching MSPE based on the dynamic nature of Au–S bindings. The proposed method further exerted the merit of MNPs and extended the scope of MSPE applications, since the adsorbents could be reutilize in different SPE modes via a simple thiol exchange process. To demonstrate the feasibility of the method, the analytical performance of the adsorbents was successfully switched from normal-phase SPE (NP-SPE) for diphenols to RP-SPE for polycyclic aromatic hydrocarbons (PAHs). Coupling this versatile MSPE technique with high performance liquid chromatography separation (HPLC) and UV detection, a multipurpose MSPE–HPLC–UV analytical method was established.