Highly sensitive detection to gallic acid by polypyrrole-based MIES supported by MOFs-Co2+@Fe3O4
Introduction
Gallic acid (3,4,5-trihydroxybenzoic acid) (GA) is one of the main phenolic components occurring naturally in plants (gallnut, blueberries, tea leaves, black tea and other plants) and possesses antioxidant, anti-inflammatory, anti-microbial, radical scavenging properties, DNA polymerase activity and ribonucleotide reductase activity [1]. GA is widely used in food and cosmetics because it can prevent lipid peroxide and rancidity [1]. However, GA has a slight toxicity at low concentrations, therefore, its accumulation in the environment should be avoided as far as possible, especially in water samples that degrade slowly [2]. To this end, determination of GA in different systems with enhanced selectivity, sensitivity, and limit of detection (LOD) has become a major research topic in analytical chemistry over the last decade [3].
Many analytical methods for the determination of GA have been developed, such as reverse flow injection analysis [4], resonance light scattering [5], fluorescence [6], chemiluminescence [7], spectrophotometric [8], thin-layer chromatography [9], gas chromatography with mass spectrometry (GC–MS) [10] and high performance liquid chromatography (HPLC) [11]. However, these methods generally have some such problems as high price, long operation time, poor sensitivity and insufficient specificity that they cannot be popularized.
Electrochemical sensors are recognized as more reliable, selective, and sensitive devices in the determination of GA [3]. Gopal et al. used MWCNT/graphene/GCE to monitor GA in drinks [12]. Feminus et al. successfully achieved the determination of GA with graphene modified electrode [1]. Ghaani et al. and Sivakumar et al. respectively used the catalytic oxidation reaction of the AgNPs/Delph/GCE sensor and NiAl2O4-modified electrodes to detect GA in food [13,14]. However, their selectivity and sensitivity were not ideal and need further promotion. Fortunately, molecular imprinting electrochemical sensor (MIES) is a new type of sensor which uses polymer with specific adsorption capacity as recognition element [15]. It has the common advantages of molecular imprinting technology and electrochemical analysis methods, such as short analysis time, simple operation, trace detection, strong specificity and so on [16].
Molecular imprinting is a biomimetic identification system that simulates a biological recognition system, e.g. antigen-antibody recognition [17]. The target molecules were used as template molecule and combined with functional monomer to prepare solid copolymer, and then the template molecule is eluted to obtain the three-dimensional specific imprinting sites matching the shape, size and functionality of target molecules. Finally, the specific recognition of the target molecules is realized by the imprinting sites on the polymer [11]. Thus, molecular imprinted polymers (MIPs) have advanced specificity. However, traditional MIPs have many limitations including incomplete removal of the template, low binding efficiency and poor imprinting site accessibility.
On one hand, polypyrrole (Ppy), an important conducting polymer, was adulterated as films by electropolymerization in the presence of template to achieve a complete removal of the template [18]. Researchers began to use ppy as a functional monomer to synthesize special thin films due to its good biocompatibility, easy synthesis and unique “doping” characteristics [[19], [20], [21]]. Polypyrrole-based MIPs is widely used in the detection of small molecules including theophylline [18], caffeine [22], ciprofloxacin hydrochloride [23] and levofloxacin [24], weight molecules including DNA [17] and glycoproteins [25]. Therefore, we select electropolymerized ppy to construct the MIPs film.
On the other hand, mesoporous materials, an imprinting support material with high surface area, simplest template binding and rebinding capabilities, were used as a supporting material for MIPs film to overcome the shortcomings included low binding efficiency and poor imprinting site accessibility [26]. Mesoporous silica [27], multi-walled carbon nanotubes [16,26] and graphene@carbon nanotube [28] have been reported to modify electrode. In addition, metal-organic frameworks (MOFs), a mesoporous material, established as a relatively new class of crystalline porous materials with high surface area and tailorability, attract extensive interest and exhibit a variety of applications [29]. However, there are few reports on using MOFs as a supporting material for MIPs film.
As a subclass of MOFs, the zeolitic imidazolate frameworks (ZIFs) are prepared on the basis of the nets of aluminosilicate zeolites [30]. ZIF-67 is composed of cobalt ion and imidazole ligand [31,32]. Sun et al. reported that the ZIF-67 synthesized in methanol has a high specific surface area of 1258.6m2/g and micropores volume of 0.576cm3/g [33]. Therefore, to increase specific imprinting sites and enhance sensitivity of molecular imprinting electrochemical sensor, we employed ZIF-67 taken cobalt ion chelating magnetic beads (Co2+@Fe3O4) as the support point of receptor. This can effectively extend and fix polypyrrole film, and endow the sensor with the common advantages of both nanomaterials and porous materials, thus significantly improving the properties of the material.
In this study, we constructed a new sensing platform by combining electrochemical technology, molecular imprinting technology, polypyrrole materials, nanomaterials and mesoporous materials for highly sensitive detection of GA. Thus, the MIP/Fe3O4@ZIF-67/Au electrochemical sensor realized the common advantages of short time, simple operation, strong specificity, high sensitivity and low LOD for the detection of GA. Through the validation experiments, we confirmed these advantages of the prepared sensor. Furthermore, the modified electrodes were characterized as using scanning electron microscope (SEM), energy dispersive spectrometer (EDS), fourier transform infrared (FTIR) spectrometer and x ray diffraction (XRD) techniques. The electrochemical behaviors of the modified electrodes were investigated by differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS). The sensor has also successfully realized the trace GA detection in real sample and has good application prospect.
Section snippets
Chemicals and instrumentation
GA, P-hydroxybenzoic acid and tannic acid were purchased from Shanghai Aladdin biochemical Technology Co., Ltd. (Shanghai, China). l-tyrosine were purchased from Beijing Suolaibao Technology Co., Ltd. (Beijing, China). Salicylic acid and p-toluene sulfonic acid were purchased from Shanghai McLean Biochemistry Co., Ltd. (Shanghai, China). 2-methylglyoxaline was purchased from Sigma Aldrich Trading Co., Ltd. (Shanghai, China). Potassium ferricyanide and potassium ferrocyanide were purchased from
SEM and EDS characterization
The surface morphology of the Co2+@Fe3O4/Au, Fe3O4@ZIF-67/Au and MIP/Fe3O4@ZIF-67/Au sensors were characterized by the JSM-6701F field emission SEM (Fig. 1). The Co2+@Fe3O4 (Φ ≈ 50 nm, 30 nm) were distributed evenly on the surface of the gold electrode (5.0 kV, ×50,000, WD4.6 mm, 100 nm) (Fig. 1A). After being soaked in 2-methylimidazole methanol solution, the outer layer of Co2+@Fe3O4 was coated with a shell and changed into core-shell structure, which resulted in the increase of particle size
Conclusion
In this paper, a new molecular imprinted electrochemical sensing platform based on MOFs (ZIF-67) supported by Co2+@Fe3O4 was developed, characterized and applied to the detection of GA. Through optimization, the specific imprinting sites of the MIP/Fe3O4@ZIF-67/Au sensor were increased, which is reflected in the Δi response of Fe2+/3+ redox reaction strengthened from 25.38 μA to 30.84 μA. The MIP/Fe3O4@ZIF-67/Au sensor showed a superior sensitivity with a lower LOD of 0.297pM (S/N = 3) and a
CRediT authorship contribution statement
Cuizhu Ye: Formal analysis, Writing-original draft, Writing review & editing. Xingguang Chen: Methodology. Junjun Xu: Data curation. Huiting Xi: Data curation. Tiantian Wu: Software. Danwen Deng: Validation. Jinsheng Zhang: Validation. Ganhui Huang: Supervision and Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (31360387). We express our thanks to Yanyun Li, Yao Cao, Siyu Lin, Dongdong Wang and Jun Yang for their assistance in the experiment.
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