Electrochemical sensor based on molecularly imprinted polymer film via sol–gel technology and multi-walled carbon nanotubes-chitosan functional layer for sensitive determination of quinoxaline-2-carboxylic acid

Highlights

• A novel MIP sensor for quinoxaline-2-carboxylic acid detection was established.

• MWNTs-CS functional layer was introduced to improve performance of the sensor.

• Sol–gel technology was applied to form recognition element of the sensor.

• In detail, performance of the sensor was discussed through various methods.

• The established MIP sensor could be promising in food safety analysis.

Abstract

Quinoxaline-2-carboxylic acid (QCA) is difficult to measure since only trace levels are present in commercial meat products. In this study, a rapid, sensitive and selective molecularly imprinted electrochemical sensor for QCA determination was successfully constructed by combination of a novel modified glassy carbon electrode (GCE) and differential pulse voltammetry (DPV). The GCE was fabricated via stepwise modification of multi-walled carbon nanotubes (MWNTs)-chitosan (CS) functional composite and a sol–gel molecularly imprinted polymer (MIP) film on the surface. MWNTs-CS composite was used to enhance the electron transfer rate and expand electrode surface area, and consequently amplify QCA reduction electrochemical response. The imprinted mechanism and experimental parameters affecting the performance of MIP film were discussed in detail. The resulting MIP/sol–gel/MWNTs-CS/GCE was characterized using various electrochemical methods involving cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and DPV. The sensor using MIP/sol–gel/MWNTs-CS/GCE as working electrode showed a linear current response to the target QCA concentration in the wide range from 2.0×10⁻⁶ to 1.0×10⁻³ mol L⁻¹ with a low detection limit of 4.4×10⁻⁷ mol L⁻¹ (S/N=3). The established sensor with excellent reproductivity and stability was applied to evaluate commercial pork products. At five concentration levels, the recoveries and standard deviations were calculated as 93.5–98.6% and 1.7–3.3%, respectively, suggesting the proposed sensor is promising for the accurate quantification of QCA at trace levels in meat samples.

Introduction

Recently, due to characteristics of high selectivity, chemical stability and easy preparation (Hu et al., 2008), molecular imprinted polymer (MIP) have been extensively utilized in various field including sensors, drug delivery, catalysis and waste management (Liang et al., 2009, Parmpi and Kofinas, 2004, Ramanaviciene and Ramanavicius, 2004, Wang et al., 2001). The high selectivity and affinity of MIP for the template molecules make them become ideal candidates as recognition element for sensors. Up to now, many sensors with MIP recognition element based on various mechanism have been reported. Although MIP used as sensing materials expand the field of sensor application, the shortcomings such as low rate of mass and electronic transfer and low sensitivity still exist (Hu et al., 2011). Sol–gel imprinting process is a promising way to improve the performance of MIP film on sensor surface. Briefly, a sol–gel inorganic framework is formed around a suitable template via non-covalently/covalently interaction between functional monomer and the template in sol–gel process, possessing excellent permeability and uniformly porous structure (Olwill et al., 2004, Zhang et al., 2010c). Surface electrodeposition (Shacham et al., 1999) is a simple and effective method to deposit a uniform sol–gel MIP film with good adherence to the transducer surface (Hu et al., 2011) and thickness of the film could be easily controlled by varying the amount of circulated charge (Huang et al., 2011). Therefore, the combination of molecularly imprinting technology (MIT) and sol–gel technique is ideally appropriate to construct electrochemical sensing devices.

Since the discovery of MWNTs by Iijima (1991), its application prospect has attracted world-wide interest of researchers. MWNTs was employed as medium for electron transfer and the electrocatalyst to enhance the sensitivity of the electrochemical detection in electrochemical sensor (Kan et al., 2008, Kumar et al., 2010, Li et al., 2010). Several solvents such as concentrated sulfuric acid (Musameh et al., 2002), acetone (Wu et al., 2002), dihexadecyl hydrogen phosphate (DHP) (Wu et al., 2003) and N,N-dimethylformamide (DMF) (Wang et al., 2001, Wang et al., 2004) have been used to disperse MWNTs in modification of electrodes in previous reports. Nevertheless, the poor stability, weak adhesion and biological incompatibility of modified layer using these dispersants hindered the applications of MWNTs. Chitosan is an abundant natural cationic biopolymer with excellent film-forming ability, high mechanical strength and adhesion and biocompatibility, which had been used in the chemical modification of electrodes (Tkac et al., 2007). Many studies have demonstrated that MWNTs-based CS, which could overcome the darwbacks of MWNTs dispersion in some traditional dispersants, is a promising material (Zeng et al., 2005, Chen et al., 2009).

Carbadox (CBX) is widely used as feed additives in livestock and aquaculture. However, due to strong mutagenic and carcinogenic effects of CBX (Čihák and Vontorková, 1983, Yoshimura et al., 1981), many countries have banned the use of the drug. Quinoxaline-2-carboxylic acid (QCA) is the marker residues of CBX in tissues (Hutchinson et al., 2005), and it has been reported that QCA was found at trace levels in various samples including pork, chicken and fish. Various analytical methods like high performance liquid chromatography (HPLC) have been developed to assay QCA in complex samples. Nevertheless, a number of shortcomings still exists, including time-consuming procedures, expensive instrumentation and complex pretreatment steps. Consequently, the development of a convenient, rapid and accurate method for the determination of QCA is highly desirable. Electrochemical methods are considered as one of the most potential approaches because of their high sensitivity and simplicity.

In this study, we have developed an electrochemical sensor for QCA determination using a sol–gel MIP film as recognition element on GCE surface decorated by MWNTs-CS composite. The effect of MWNTs-CS composite on the modified electrode and performance of the developed MIP sensor were systematically evaluated by various experiments. This research was aimed at overcoming some shortcomings of weak signal response, long incubation time with MIP sensor, and also offering a rapid, sensitive and accurate electrochemical method for QCA determination.