**1. Introduction**

In recent years, the need to coordinate the growing environmental pollution problems with technological progress has become very urgen<sup>t</sup> [1]. Hence, the use of biodegradable polymer materials to develop functional food packaging materials has been widely concerned and studied, in order to reduce environmental pollution and meet people's needs for active food packaging [2,3]. Among many biopolymers, alginate, as a carbohydrate, is often used as a raw material for biodegradable food packaging, which has the characteristics of low cost, good biocompatibility, and excellent film-forming properties [4,5]. Sodium alginate (SA) is a naturally linear water-soluble polysaccharide extracted from brown algae, which is composed of *β*-D-mannuronic acid and *α*-l-guluronic acid (1–4) linking units [6,7]. SA has been widely used in the field of food packaging and biomedical fields because of its low price, easy access, and easy processing. As a food packaging material, SA has advantages, such as good mechanical properties and high transparency [8,9].

Pathogenic microorganisms can cause food spoilage and food-borne diseases, as well as consumers' concerns about chemical residues in food, making the food industry more and more concerned about the research of natural antibacterial agents in food packaging [10,11]. In order to maximize the function of packaging, extend the use period of food and protect consumers from the threat of food-borne disease outbreaks, antibacterial active substances (such as essential oils) are usually added to increase its function [12]. Cinnamon

**Citation:** Cui, R.; Zhu, B.; Yan, J.; Qin, Y.; Yuan, M.; Cheng, G.; Yuan, M. Development of a Sodium Alginate-Based Active Package with Controlled Release of Cinnamaldehyde Loaded on Halloysite Nanotubes. *Foods* **2021**, *10*, 1150. https://doi.org/10.3390/ foods10061150

Academic Editors: Hong Wu and Hui Zhang

Received: 23 March 2021 Accepted: 11 May 2021 Published: 21 May 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**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/).

essential oil is widely used in the field of food, which can be used to protect food without causing harm to human health. There have been many studies that add cinnamon essential oil to food packaging as an antimicrobial agen<sup>t</sup> for food preservation [13,14]. Cinnamaldehyde (CIN) has been approved by the Food and Drug Administration (FDA) and can be used in food products. It is the main ingredient extracted from cinnamon essential oil, which is not only harmless, but also has a broad spectrum of antibacterial and antifungal activities [15]. However, it must be taken into account that CIN is liquid and volatile at room temperature, which is a severe challenge we are currently facing. In this regard, there have been many solutions, such as using porous and other special structure carriers to load active compounds or encapsulating the active substance in chitosan or cyclodextrin [16,17]. In this context, it is one of the most convenient strategies to load CIN with nanoparticles of special structure, which can not only reduce CIN volatilization losses, but also impart the characteristics of nanoparticles to the food packaging system.

Among many nanoparticle carriers, halloysite nanotubes (HNTs) have aroused grea<sup>t</sup> interest of researchers due to their ability to trap, protect, and control the release of active substances [18]. HNTs, whose chemical formula is Al2Si2O5(OH)4.nH2O, which is a subgroup of kaolin. In addition, HNTs have been listed by the FDA as a safe food packaging material [19]. HNTs can be used as a carrier for drug delivery in the medical field because of their unique hollow tubular structure, low cost, and good biocompatibility [12,20]. In particular, HNTs have been shown to have higher adsorption capacity than montmorillonite [21]. Moreover, it has been shown that the addition of HNTs as fillers can improve the barrier and mechanical properties of polymers [10].

The purpose of this study is to develop SA antibacterial composite film with controlled release. The lumen of the original HNTs is limited in volume, and the enlargement of the pores will allow more active chemicals to be loaded. Therefore, in this study, HNT were etched with sulfuric acid to further increase its drug loading, and then CIN was loaded on acidified nanoparticles (T-HNTs) to prepare composite film by method of solution casting. The effects of T-HNTs loaded with CIN on the microstructure, barrier performance, transparency, and antibacterial properties of the composite film were studied, and the release behavior of CIN in the composite film was also investigated.
