Electrochemical determination of formaldehyde via reduced AuNPs@PPy composites modified electrode
Introduction
Formaldehyde has the functions of anticorrosion, prolonging shelf life, increasing water holding capacity, toughness and so on. However, formaldehyde is a protoplast poison which can react with proteins. Formaldehyde could cause the irritation of the eyes, nose and throat, as well as the irritation and allergic contact dermatitis of the skin [1]. Milk is a common drink of consumption and dominates global dairy production [2,3]. It is popular with consumers because of its unique nutrition and health care function [4]. With the prevalence of dairy industry and dairy processing industry, the quality and safety problems of milk and dairy products have gradually emerged [5,6]. Milk would quickly deteriorate especially in the higher temperature season or region during storage or transport process if lack of refrigerating facilities [7]. In order to prolong the shelf life of fresh milk and add harmful preservatives to milk, formaldehyde is one of the common preservatives [8,9].
At present, the more common test methods for detecting formaldehyde are high performance liquid chromatography (HPLC) [10], gas chromatography (GC) [11] and spectrophotometry [12]. These methods were used to make formaldehyde react with derivative under certain conditions to form a stable one. The reaction conditions of different derivatives and formaldehyde were different, and the properties of the generated derivatives were diverse [13]. As a result, different derivatives are used in different detection methods. Application of acetylacetone as derivatives in spectrophotometry and 2,4-dinitrophenylhydrazine as a derivatizing agent for chromatography. In addition, many other methods are available in other fields for the detection of formaldehyde. For example, Formaldehyde dehydrogenase, reduced glutathione and nicotinamide adenine dinucleotide were placed on piezoelectric quartz crystal. A reversible reaction occurs with formaldehyde in the gas phase. The purpose of formaldehyde detection has been achieved [14].
The standard of the determination of exceeding formaldehyde in milk has attracted worldwide attention in recent years. With the development of industry, the traditional methods of formaldehyde detection have been faced with challenges, and new detection methods have emerged. The most common method for the determination of formaldehyde in milk is by HPLC with 2,4-dinitrophenylhydrazine as the derivative [15]. A standard technique to measure the concentration of formaldehyde in food is HPLC. DNPH(2,4-dinitrophenylhydrazine) cartridges were used to liquid chromatography analysis of sample. The sampling time is a few hours and the analysis takes typically 30 h [16]. These procedures are a long and hard road. Formaldehyde can also be endogenous, background levels of the compound are found in foods, ranging from 0.3 to 10 M in milk. More sensitive detection methods are needed to detect formaldehyde in milk. So, an analytical method based on electrochemical sensor with high sensitivity, convenience, and low-cost is a good alternative to achieve more rapid and effective real-time monitoring of formaldehyde in food [17,18].
PPy is a good tool for preparing nanocomposites. It possesses biocompatibility and capability to convert the energy generated by the interaction between the analyte and the identified part of the analyte into an electrical signal that can be easily monitored. PPy might be electrochemically generated and deposited on the conducting surfaces [19,20]. PPy can be mainly used in catalytic biosensors, versatile immobilization matrix in design of biosensors, immunosensors, DNA sensors and molecular imprinting technology. PPy is also successfully exploited for development of various types of electrochemical sensors. Usually, Pt and Pt-based electrocatalysts have been used for electrochemical oxidation of formaldehyde [21]. Although AuNPs is a noble metal, its resource is more abundant compared with Pt, and thus its price is much lower than Pt [22]. AuNPs and PPy has important utility via electrodeposition technique [23]. The traditional preparation procedure of Au is complicated and strict. The preparation of AuNPs by the reduction of polypyrrole can make the process easier.
In the present work, we describe a AuNPs /PPy/GCE, which shows high catalytic activity for formaldehyde. The PPy and AuNPs were modified on GCE by electrochemical method to obtain Au/PPy/GCE. The material was characterized by SEM, EDX and XPS. The electrochemical oxidation behavior of formaldehyde on AuNPs/PPy/GCE was studied. Electrochemical characterization for the AuNPs/PPy/GCE in NaOH with formaldehyde was performed using cyclic voltammetry(CV) and differential pulse voltammetry(DPV). The formaldehyde in milk was quantified.
Section snippets
Materials
Potassium ferrocyanide, formaldehyde solution (37%−40%), sodium hydrate (NaOH), formic acid and methanol anhydrous were purchased from Xilong Chemical Co. LTD (Shantou, China). Gold chloride solution (HAuCl4), pyrrole, 5-hydroxymethylfurfural, teans-2,4-decadienal and n-nonaldehyde were bought from Aladdin Industrial Corporation (U.S.A.). Potassium chloride were purchased from Zhiyuan Chemical Reagent Co. LTD (Tianjin, China). Sodium sulfate anhydrous were purchased from Fengchuan Chemical
Surface characterization of PPy/GCE and AuNPs/PPy/GCE
The SEM morphology of PPy/GCE on the electrode surface was displayed in Fig. 1. And it presents typical images of a bare smooth GCE(A) film, a rough PPy/GCE (B) film and AuNPs /PPy/GCE (C) film. The PPy grows in three dimensions and increased contact area with reactants, thus increasing the conductivity of the electrode [25]. In AuNPs /PPy/GCE (C), gold nanoparticles have a particle size of about 40 nm.
The elemental analysis of the material by EDS (Fig. 2) shows that AuNPs is successfully
Conclusions
In conclusion, we proposed an electrochemical sensing platform based on AuNPs modified PPy/GCE for the detection of formaldehyde. The large specific surface area of polypyrrole increased the adsorption capacity of formaldehyde, and the capacity of reduction Au3+ was improved by modifying polypyrrole with large overpotential. The modification conditions of AuNPs and PPy film were optimized. The results showed that the AuNPs/PPy/GCE sensor has a favorable catalytic effect on formaldehyde.
Funding information
This work was supported by the National Natural Science Foundation of China (31,360,387).
Ethical approval
This study does not contain any studies with human participants performed by any of the authors.
Informed consent
Not applicable.
CRediT authorship contribution statement
Huiting Xi: Formal analysis, Writing - original draft, Writing - review & editing. Xingguang Chen: Methodology. Yao Cao: Data curation. Junjun Xu: Data curation. Cuizhu Ye: Data curation. Danwen Deng: Validation. Jinsheng Zhang: Validation. Ganhui Huang: Supervision, 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.
Acknowledgments
This study was supported by the National Natural Science Foundation of China (project number 31360387). We would also like to thank Dongdong Wang, Siyu Lin and Tiantian Wu for their help with the experiment.
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