A novel morphological ion imprinted polymers for selective solid phase extraction of Cd(II): Preparation, adsorption properties and binding mechanism to Cd(II)

Highlights

• A unique morphological spheres-on-sphere silica particles for imprinting polymer carriers

• Isothermal titration calorimetry experiments explain the binding mechanism of MPTMS to cadmium ions.

• Ion-imprinted polymers exhibit good adsorption properties in real samples.

Abstract

A Cd(II) ion-imprinted polymers (SOS@Cd(II)-IIPs) with specific recognition property and high adsorption capacity was synthesized on a novel spheres-on-sphere (SOS) particle surface. Then, the obtained SOS@Cd(II)-IIPs were characterized and applied in solid phase extraction (SPE) column for the preconcentration of cadmium in real samples. The affinity constant (K_ITC), reaction enthalpy (ΔH), and entropy (ΔS) of Cd(II) binding to monomers (3-mercaptopropyl trimethoxysilane, MPTMS) were also determined by isothermal titration calorimetry (ITC) to analyse their adsorption mechanism. The coordination binding of Cd(II) to MPTMS was demonstrated as an exothermic process with a high K_ITC value of 1.60×10⁵ M⁻¹ and an enthalpy value of −9.54 kJ∙mol⁻¹. The binding reaction was mainly driven by both enthalpy and entropy, and can be completed spontaneously at any temperatures. The SOS@Cd(II)-IIPs synthesized on the SOS particle surface exhibited a maximum adsorption capacity of 41.212 mg∙g⁻¹ for Cd(II). Meanwhile, the SOS@Cd(II)-IIPs also showed a specificity recognition ability for Cd(II) using analogues Cu(II), Pb(II), Cr(II), and Ni(II) as interference. In real samples, SOS@Cd(II)-IIPs based SPE can extract approximately 95–105% of Cd(II) with a RSD of 3.41%–8.12%, indicating that it could have wide application prospects in Cd(II) adsorption and detection.

Introduction

As a toxic metal element, cadmium causes severe health hazards to humans and has been considered as one of the most serious food pollutants [1]. Therefore, it is vital to determine the trace amounts of cadmium quickly and effectively from food samples [2]. Flame atomic absorption spectrometry (FAAS) is one among conventional techniques for the determination of cadmium because of the operational facility, high sample throughput and low cost [3]. However, the direct determination of cadmium at trace levels by FAAS is limited, not only due to insufficient sensitivity, but also to matrix interference [4]. In recent years, various preconcentration and separation methods, including ion exchange [5], solid phase extraction (SPE), [6] and liquid-liquid extraction techniques [7], have been used to solve these problems. Among these methods, SPE has attracted substantial attention in eliminating interferences and enriching analytes [8]. However, some commonly used adsorbent materials for SPE, such as C₁₈, is lack of selectivity, which leads to poor separation and purification efficiency for specific targets [9]. As a new type of adsorbent material, ion-imprinted polymers (IIPs) have become a research hotspot because of their excellent selectivity [10].

Molecular imprinting, which was first proposed in 1949 by Dickey [11], is a technique to create template-shaped cavities in polymer matrices with the memory of the template molecules to be used in molecular recognition. Due to their specific selectivity, desirable recovery rates, and resistance to the environmental extremes, molecularly imprinted polymers (MIPs) have been wildly used in membrane separation, bio-sensors, solid phase extraction, and contaminant adsorption [12,13]. The ion imprinting technique is similar to molecular imprinting, but mainly uses ion as a template, and has been widely used in treating heavy metal pollution. IIPs are usually prepared by bulk polymerization, precipitation polymerization, and emulsion polymerization [14]. However, IIPs that are prepared by conventional methods have deficiencies, such as deeply embedded recognition sites and irregular shapes, which may result in difficult target ions recognition at binding sites because of the existence of a diffusion barrier. Moreover, the adsorption capacity of the imprinted polymers prepared by the traditional method for Cd(II) is usually less than 10 mg∙g⁻¹ or even lower [15,16]. Surface imprinting, in which imprinting cavities can be sufficiently exposed to the surface of the carrier, has become a research hotspot because of its high accessibility for binding sites. In surface imprinting, silica particles were usually used as a carrier for the preparation of IIPs [17]. However, it is difficult to form a large number of imprinting sites on the surface of these solid silica gel particles, which results in a low absorption capacity. Therefore, at present, there is an urgent need to find a carrier that can help increase the amount of imprinted cavities and then improve the adsorption capacity of IIPs [18].

Spheres-on-sphere (SOS) silica particles, in which many nanospheres are covalently anchored to a solid microsphere, were first synthesized using the one-pot method described by Ahmed [19]. Due to their excellent structural stability, proper size and high specific surface area, SOS silica particles have been applied to high-performance liquid chromatography (HPLC) with low back pressure for the rapid separation of peptides and proteins [20]. These characteristics of SOS particles also make it be a potential excellent adsorption material for the removal of the environmental and food pollutants. However, at present, SOS silica particles are mainly used to adsorb macromolecules in the mixture, and are not suitable for the adsorption and separation of small ions such as Cd(II). Furthermore, the surface of calcined SOS silica particles only contains a small number of hydroxyl groups, which limits their recognition ability to the target molecules in the mixture and leads to a sharp drop in the adsorption efficiency. The synthesis of imprinted polymers on the surface of SOS particles is an effective way to improve above-mentioned deficiencies, because the novel imprinted polymers have the advantages of both the specific recognition properties of traditional imprinted polymers and the large surface area of SOS particles. However, to date, there are no reports on the preparation of IIPs on the surface of SOS particles. The adsorption characteristics and mechanism of imprinted polymers synthesized on SOS particles surface toward Cd(II) are still unclear (Table 1).

Therefore, in this study, SOS particles were prepared using an improved one-pot method, and then their morphology and particle size distribution were characterized by scanning electron microscope (SEM) and dynamic light scattering (DLS). Subsequently, IIPs were synthesized on the surface of these optimized SOS particles using Cd(II) as the template, 3-mercaptopropyl trimethoxysilane (MPTMS) as the monomers, and epichlorohydrin (ECH) as the cross-linker. The morphological and chemical properties of the IIPs were characterized by SEM, fourier transform-infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA) and  nitrogen adsorption-desorption experiments. The adsorption capacity, adsorption model, and selective recognition mechanism were studied using adsorption experiments and isothermal titration calorimetry (ITC) experiments. The obtained SOS@Cd(II)-IIPs were further applied in the SPE column for the preconcentration and detection of cadmium in real samples.