**1. Introduction**

Pesticides are effective ingredients that have been extensively used to control agricultural diseases. However, their excessive use poses a challenge to the sustainable development of agriculture, and the administration of pesticide residues on the surface of food is harmful to human health. For this reason, considerable progress has been made recently in the determination of pesticides. Current approaches are mainly carried out by laboratory-scale analytical methods, such as HPLC, MC, GC, etc. [1,2]. Nevertheless, their inherent disadvantages, such as complicated pre-treatment, high costs, and timeconsuming procedures, generally limit their applications [3]. Therefore, research into other effective, convenient, and reliable detection strategies has gained increasing attention in recent years [4–6]. Specifically, rapid detection based on enzyme acetylcholinesterase (AChE) inhibition has become widely accepted for pesticide residue analysis, ascribing to its simple principle and visual evaluation [7]. In this regard, the rapid detection card, generally made of glass fiber, qualitative filter paper, or absorbent paper, has been identified as the preferred and direct method for determining pesticides. Nevertheless, the challenge associated with these cards is the discrepancy between the sensitivity of the analysis and the increased awareness of food safety, making it imperative to seek new types of carriers for the production of detection cards.

Recently, extensive evidence has demonstrated the grea<sup>t</sup> potential of nano/micromaterials in the field of catalysis, owing to their unique structure and surface properties [8,9]. Herein,

**Citation:** Feng, K.; Zhai, M.-Y.; Wei, Y.-S.; Zong, M.-H.; Wu, H.; Han, S.-Y. Fabrication of Nano/Micro-Structured Electrospun Detection Card for the Detection of Pesticide Residues. *Foods* **2021**, *10*, 889. https://doi.org/ 10.3390/foods10040889

Academic Editor: Roberto Romero-González

Received: 17 March 2021 Accepted: 13 April 2021 Published: 19 April 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

we try to further improve the conventional detection card's performance by supplying the nano/micromaterials as the immobilization carrier for the enzyme and substrate. Electrospinning—a mild, convenient, versatile, and cost-effective technique for developing micro-/nano-structured vehicles—is associated with a broad range of applications in pharmacy, tissue engineering, food packaging, etc. [10]. Specifically, the superiority of electrospun fibers as the carrier for chemical biosensors and immobilization has been highlighted in recent years due to their desirable properties, such as a high surface area to volume ratio, porous structure, and tunable porosity [11,12]. However, electrospun fibers used as the detection card matrix for the pesticide detection card have been less explored, except for in our previous research study [13]. It is known that the electrospun fiber mat could not only be used as the encapsulation vehicle but also the absorption carrier [14]. Our published work has already designed a novel detection card by encapsulating the enzyme (AChE) and substrate (indolyl acetate, IA) into the electrospun nanofibers and we also demonstrated their proper role for determining pesticides more sensitively. Unfortunately, the deformation problem of this card, stemming from its hydrophilic feature, calls for the exploration of hydrophobic materials. Additionally, it is also interesting to determine the pesticide detection performance of the card by using the electrospun fiber mat as the absorption matrix for the enzyme and substrate.

Herein, a biocompatible and biodegradable polymer, Poly(ε-caprolactone) (PCL), was applied to prepare the detection card. To improve enzyme absorption on the fiber mat, hydrophilic modification of the hydrophobic PCL fiber mat was performed, and the related properties were investigated systematically. Subsequently, the plain and modified PCL fiber mat were employed to absorb IA and AChE, respectively. Furthermore, the key factors including the concentration of the enzyme and substrate, inhibition time, and color development time were optimized to improve the performance of the detection card. The detection ability of this card for different pesticides was examined and compared with the corresponding values specified by the Standardization Administration of China (SAC). Lastly, the feasibility of this card for detecting pesticide residues in real foods was studied by comparison with the commercial card. This study offers new access for the sensitive and convenient detection of pesticide residues, which is expected to inspire the further application of electrospinning in the field of food and agriculture.
