**1. Introduction**

Cherry tomatoes (*Lycopersicon esculentum var. cerasiforme cv.*) consist of beneficial nutrients, such as minerals, vitamins, phenolic compounds, and lycopene [1]. However, cherry tomatoes deteriorate rapidly without proper preservation, and thus their shelf life ranges from only 5 to 7 days after harvest [2]. Therefore, it is very important to reduce decay incidence and extend the postharvest life of cherry tomatoes. Currently, different treatments, such as drying, edible coatings, modified atmosphere packaging, etc., have been applied to maintain the postharvest quality of cherry tomatoes [2–4]. However, modern consumers' convenient lifestyle and their demand for minimally processed fruits and vegetables put forward higher requirements for freshness preservation [4]. Modified atmosphere packaging, as one of the most widely used techniques for fresh produce preservation, has attracted significant research interest for its time-saving convenience and ease of use. It reduces gas exchange, moisture evaporation, and respiration reaction rates and suppresses physiological disorders of fruits and vegetables [5]. Existing modified atmosphere packaging usually involves the use of polymer materials, such as polyethylene (PE), polyvinyl chloride (PVC), and polypropylene (PP) [4]. However, these films have limited gas selectivity or gas permeability, which can no longer meet the requirements

Xiao, K. Preparation of an Amidated Graphene Oxide/Sulfonated Poly Ether Ether Ketone (AGO/SPEEK) Modified Atmosphere Packaging for the Storage of Cherry Tomatoes. *Foods* **2021**, *10*, 552. https://doi.org/ 10.3390/foods10030552

**Citation:** Cheng, Y.; Dong, H.; Wu, Y.;

Academic Editor: Eleni Tsantili

Received: 7 December 2020 Accepted: 28 February 2021 Published: 7 March 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/).

for the preservation of cherry tomatoes [6]. From this perspective, it is of great practical significance to develop an atmosphere-modified packaging that matches the respiration rate of cherry tomatoes [7].

Among diverse polymers, sulfonated poly ether ether ketone (SPEEK), as an aromatic thermoplastic engineering plastic, has attracted increasing attention for the development of food packaging materials due to its excellent gas permeability [8]. For instance, He et al. [9] prepared a SPEEK/PVDF self-balancing packaging film, which significantly shortened the equilibrium time of CO2 or O2 concentration in the package, making the shelf life of broccoli more than double. However, there is still much room to improve the selective permeability of SPEEK. Noteworthily, these polymers can be further improved as functional packaging materials by incorporating some new compounds, such as nanoparticles [10]. Graphene oxide (GO), as a newly emerging carbon nanomaterial with a two-dimensional layered structure, has a large specific surface area and strong CO2 adsorption capacity, and thus acts as one of the ideal materials [11,12]. At present, the methods for the surface modification of GO can be categorized as covalent and non-covalent functionalization [13]. Among them, the amidated form of GO (AGO) better improves the adsorption of CO2 and effectively prevents the deterioration of mechanical properties of the film. However, the effects of AGO/SPEEK films on postharvest attributes and flavor characteristics in cherry tomatoes have not been reported yet.

In the present study, GO was modified by amidation and then added to SPEEK in the form of inorganic fillers to prepare a film with a high CO2/O2 separation ratio. The produced films were then characterized to analyze the efficacy of modification of SPEEK with AGO. Subsequently, the mechanical properties, water uptake ratio (WUR), swelling ability (SA), water vapor permeability (WVP), and gas permeability of the pure SPEEK film and AGO/SPEEK films were also examined and compared. Finally, the efficacy of the resulting package was assessed by performing storage trials of cherry tomatoes.
