Red pepper oil, also known as chili oil, is made by hot maceration of dried chilies in cooking oil (Rui et al. 2020; Xza et al. 2020). As a flavoring oil, red pepper oil is often used to process various snack foods to give them spicy taste (Ornelas-Paz et al. 2010; Zhang et al. 2021). Depending on the intensity of spiciness, red pepper oil is usually classified as slightly spicy, medium spicy, and extra spicy. However, due to regional and individual differences, consumers have different cognition and acceptance of spicy flavor, leading to this classification method is too subjective. This makes its application in food more based on experience, which is not conducive to the standardization of food production. Therefore, it is necessary to establish a more scientific classification method for grading the spiciness of red pepper oil. In fact, the main source of spiciness in red pepper oil is capsaicin, the content of which is directly related to spiciness of food and is commonly used to evaluate spiciness of food. Therefore, the establishment of determination method for capsaicin content is of great significance to the classification of red pepper oil.
A variety of methods for the analysis of capsaicin have been reported and generally recognized, such as spectrophotometry (Perucka and Oleszek 2000), ELISA (Yang et al. 2016), high-performance liquid chromatography (Yang et al. 2016), liquid chromatography-mass spectrometry (Kozukue et al. 2005), gas chromatography (Cisneros-Pineda et al. 2006; Peña-Alvarez et al. 2008), and TLC-ESI-MS (Santos et al. 2021). Spectrophotometry is simple and inexpensive, but it lacks specificity and sensitivity, and cannot achieve the purpose of trace detection. ELISA has higher specificity, but due to the difficulty in screening specific antibodies, so it cannot be widely used in the detection of capsaicin (Yang et al. 2016). Chromatography and mass spectrometry methods have the characteristics of high precision and good accuracy, but they need long detection period, frequent operation, expensive equipment and labor costs (Kaale et al. 2002; Peña-Alvarez et al. 2008; Wu et al. 2018).
Compared to usual tests, electrochemical analysis has significant advantages in terms of economy, timeliness, accuracy, miniaturization, and sensitivity (Sheela et al. 2011; Da Silva Antonio et al. 2019; Supchocksoonthorn et al. 2020) Up to now, there have been reports on the detection of capsaicin by electrochemical sensors based on metals and their oxides (Wang et al. 2016), carbon nanomaterials (Salimi et al. 2008; Yardm and entürk 2013), and other semiconductor substances (Yu et al. 2012; Søpstad et al. 2018). Among them, rGO is considered as a good modification material for detecting capsaicin because it not only has excellent electrical conductivity, but also can produce a large number of topological defects and continuous changes of carbon vacancies, thus enhancing the adsorption, embedding and activation properties of target molecules (Zhou et al. 2010; Zhong et al. 2019). However, rGO sheets tend to accumulate and lose the advantage of nanomaterials because of their van der Waals' forces (Wang et al. 2013; Mutyala and Mathiyarasu 2016). β-CD is an oligosaccharides consisting of seven glucose units, which can effectively disperse various nanomaterials to enhance their electrochemical detection performance. At the same time, β-CD has a toroidal structure with a hydrophobic inner cavity and a hydrophilic outer side, enabling them to selectively combine with some organic molecules (Przybyla et al. 2020). This interesting property will undoubtedly enrich the organic molecules on the electrode surface, thus improving the sensitivity of electrochemical detection (Díaz et al. 2018; Gu et al. 2021; Xu et al. 2021). Therefore, we speculate that β-CD can further enhance the detection performance of capsaicin by electrochemical sensors based on rGO.
In our work, a sensitive electrochemical sensor for detecting capsaicin was constructed by introducing β-CD into the polymerization solution of rGO. Firstly, the mixed solution of β-CD and graphene oxide (GO) was scanned by cyclic voltammetry (CV) to deposited β-CD/rGO composite material on the surface of GCE. Secondly, the electrochemical reaction mechanism of capsaicin on the sensor we prepared (β-CD/rGO/GCE) and the optimal deposition parameters of β-CD/rGO composite material were further investigated. Finally, the methodological evaluation and the application potential of β-CD/rGO/GCE for the detection of capsaicin were studied.