A novel composite of SiO₂-coated graphene oxide and molecularly imprinted polymers for electrochemical sensing dopamine

Abstract

A novel imprinting route based on graphene oxide (GO) was proposed for preparing a composite of SiO₂-coated GO and molecularly imprinted polymers (GO/SiO₂–MIPs). In this route, SiO₂-coated GO sheets were synthesized in a water–alcohol mixture with sol–gel technique. Prior to polymerization, the vinyl groups were introduced onto the surface of GO/SiO₂ through chemical modification with γ-methacryloxypropyl trimethoxysilane (γ-MAPS), which can direct the selective polymerization on the GO/SiO₂ surface. Then a novel composite of GO/SiO₂–MIPs was successfully obtained by the copolymerization in presence of vinyl groups functionalized GO/SiO₂, dopamine (DA), methacrylic acid and ethylene glycol dimethacrylate. The GO/SiO₂–MIPs composite was characterized by FTIR, TGA, Raman spectroscopy, SEM and AFM. The properties such as special binding, adsorption dynamics and selective recognition ability using differential pulse voltammetry (DPV) were evaluated. The DPV current response of GO/SiO₂–MIPs sensor was nearly 3.2 times that of the non-imprinted polymers (NIPs). In addition, the GO/SiO₂–MIPs sensor could recognize DA from its relatively similar molecules of norepinephrine and epinephrine, while the sensors based on GO/SiO₂–NIPs and vinyl groups functionalized GO/SiO₂ did not have the ability. The GO/SiO₂–MIPs sensor had a wide linear range over DA concentration from 5.0×10⁻⁸ to 1.6×10⁻⁴ M with a detection limit of 3.0×10⁻⁸ M (S/N=3). The sensor based on this novel imprinted composite was applied to the determination of DA in injections and human urine samples with satisfactory results.

Introduction

Graphene, a two-dimensional monolayer of carbon atoms arranged in honeycomb lattice, has received tremendous attention from both the experimental and the theoretical scientific communities (Geim and Novoselov, 2007, Lu et al., 2011a). Because of their unique electrical, thermal and mechanical properties, graphene provides an ideal platform to prepare functional materials for electronics, electric devices and sensors (Li et al., 2008). Meanwhile, graphene oxide (GO), its derivative of graphene, not only owns the above mentioned properties like graphene, but also possesses some other properties different from graphene, such as hydrophilicity, multiple oxygen moieties, and controllable electronic properties (Andre Mkhoyan et al., 2009, Scheuermann et al., 2009). In view of these, GO has attracted great attention in preparing advanced materials (Gao et al., 2011, Zhu et al., 2012a). Among them, Scheuermann and co-workers reported palladium nanoparticles–GO composite for highly active catalysts for the Suzuki–Miyaura coupling reaction (Scheuermann et al., 2009). Kou et al. reported the SiO₂-coated GO as general building blocks for large-area superhydrophilic coatings (Kou and Gao, 2011). Ag nanoparticles-decorated SiO₂-coated GO was synthesized for H₂O₂ and glucose sensors (Lu et al., 2011b). GO could serve as the reinforcing element in a polymer matrix in fabricating composite materials (Gao et al., 2011). This combination between GO and polymer offers an attractive route to introduce some novel properties. Some researchers have already functionalized GO with various polymers to assemble the composites with desired properties (Liu et al., 2012). The efforts were mainly made on the effective dispersibility of the composites (Li et al., 2010a), and improvement of the electrical conductivity (Bai et al., 2011), thermal stability and mechanical strength of the composites (Cao et al., 2012). In addition, some groups have already synthesized composite materials based on the desirable merging of GO and molecularly imprinted polymers (MIPs), which make it possible to increase the selectivity, sensitivity and improve the binding kinetic properties (Li et al., 2010b, Qiu et al., 2012).

Molecular imprinting technique has become a promising way to prepare MIPs with tailored selectivity for analytes (Baggiani et al., 2012, Ge and Turner, 2009). MIPs exhibit an affinity for the template molecule over other structurally related compounds. MIPs have a wide application in chemical sensors (Mao et al., 2011, Riskin et al., 2008, Xing et al., 2012) and separation media (Ma et al., 2011). However, traditional MIPs have many limitations (Gao et al., 2010), such as incomplete template removal, low-affinity binding and slow mass transfer. Recently, a molecular imprinting technique on the surface of nanomaterials to prepare surface MIPs composites, which enables the template-imprinting sites to situate at the surface or in the proximity of the materials' surfaces, providing the advantages of favorable selectivity and fast association/dissociation kinetics. Some researchers have reported surface MIPs composites based on Fe₃O₄ nanoparticles (Li et al., 2011), carbon nanotubes (Gao et al., 2010, Kan et al., 2008, Zhang et al., 2011a), polyaniline nanofibers (Liang et al., 2011), and metal–organic frameworks (Qian et al., 2011) owing to their unique physicochemical properties and desired application. Many routes have been explored to develop these surface MIPs composites, such as free radical polymerization (FRP) (Kan et al., 2008, Zhang et al., 2011b), reversible addition fragmentation chain transfer (RAFT) polymerization (Li et al., 2011) and sol–gel method (Gao et al., 2010), etc. The widely used FRP has gained great attention for preparing these surface MIPs composites based on the vinyl groups functionalized supported materials (Ma et al., 2011). It has been demonstrated that the vinyl functional monomer layer of the supported materials can direct the selective occurrence of imprinting polymerization at the surface of supported materials through the copolymerization of vinyl end groups with functional monomers (Kan et al., 2008).

GO, with hydrophilicity, unique electrical properties and large area, should be an excellent candidate as a supported material for preparing surface MIPs composite based on nanomaterials (Li et al., 2010b). Many routes have been explored to develop surface MIPs composites based on GO via RAFT method (Li et al., 2010b, Qiu et al., 2012). To the best of our knowledge, surface MIPs composite based on the vinyl groups functionalized GO with FRP has not been reported. Herein, we proposed a novel imprinting route based on GO for preparing GO/SiO₂–MIPs composite. In this route, GO/SiO₂ sheets were synthesized in a water–alcohol mixture with sol–gel technique. Dopamine (DA), an important catecholamine neurotransmitter (Mao et al., 2012, Zhang et al., 2011b), was used as the template molecule in this work. Vinyl groups were the first time introduced onto the surface of GO/SiO₂ to direct the selective polymerization on the GO/SiO₂ surface. GO/SiO₂–MIPs, having specific response for DA, were successfully prepared in presence of vinyl groups functionalized GO/SiO₂, DA, monomers and cross-linkers. The properties such as special binding, adsorption dynamics and selective recognition ability of GO/SiO₂–MIPs using differential pulse voltammetry (DPV) were evaluated. The GO/SiO₂–MIPs sensor could recognize DA from its relatively similar molecules of norepinephrine and epinephrine. The GO/SiO₂–MIPs sensor had a wide linear range over DA concentration with a low detection limit. The sensor based on GO/SiO₂–MIPs was applied to the determination of DA in practical samples with satisfactory results.