Green synthesis of nanosilver coating on paper for ripening delay of fruits under visible light

Highlights

• Coated paper with AgNPs was green synthesized using spent green tea.

• AgNPs can degrade ethylene gas via photocatalytic reaction under visible light.

• Coated paper possessed an anti-bacterial activity and ripening delay of fruit.

Abstract

Active packaging containing photocatalyst is generally used to solve the issues of loss and quality deterioration of fresh fruits. However, only ultraviolet (UV) light can activate these photo-catalysts in ethylene photo-degradation. To overcome this limitation, green synthesized silver nanoparticles (AgNPs) that inhibits ethylene gas production under visible light has been developed. In this study, spent green tea was mixed with deionized water at various ratios ranging from liquid to solid to obtain spent green tea extract (ex-SGT) having different concentrations used for synthesis of AgNPs. The results indicated that a good distribution of particles with a bandgap energy of 2.17 eV which was in the visible region for ethylene photo-degradation was successfully obtained with the synthesized AgNPs-60 (ex-SGT at the ratio of 60:1). The coated paper possessed an antibacterial activity and decreased the ethylene concentration in mango’s packaging. Based on the finding in this work, it appears that the coated paper could have potential to be used for novel packaging of fruits to delay their ripening so that their freshness can be maintained.

Introduction

Ethylene (C₂H₄) is a natural aging hormone which accelerates the ripeness of fruits and vegetables [1]. A high concentration of ethylene results in the deterioration of overall quality and shortening of shelf-life of fruits and vegetables [2]. Hence, removing ethylene from the surrounding environment is one of the approaches that can prolong shelf-life and reduce postharvest losses [3]. Recently, photocatalytic oxidation of ethylene has been developed to preserve freshness of fruits and vegetables. Titanium dioxide (TiO₂) has been widely used in photocatalytic oxidation of ethylene. It is a well-known fact that the catalyst has the ability of producing reactive oxygen species (ROS) to oxidize ethylene into carbon dioxide and water under ultraviolet (UV) light [4], [5]. Photodegradation activity of ethylene was studied by a number of researchers including Kaewklin et al. [2]. The improvement of ethylene photodegradation efficiency was observed when the content of TiO₂ in chitosan film was increased from 0.25% to 1%wt under UV light. The presence of TiO₂ in nanofibers significantly decreased ethylene concentration inside packaging during 22 days [2]. Although TiO₂ was successfully applied in photocatalytic oxidation of ethylene, it still has a limitation; that is, TiO₂ only responds to UV light because of its wide bandgap energy (3.2 eV) [6]. The UV light is only 5% of solar energy whereas the visible light accounts for 43% of solar energy. Therefore, developing a new catalyst that can respond to both UV and visible light would be an approach to achieve novel packaging that can be utilized for numerous applications [7], [8].

Silver nanoparticles (AgNPs) are photocatalysts performing catalytic activity under the whole solar spectrum. AgNPs can absorb visible and UV light from solar irradiation via surface plasmon effect and internal band transition effect, respectively. Nowadays, AgNPs have gained great attention in both academic and industrial fields due to their excellent properties, that is, antimicrobial agent [9], [10], catalytic activity [11], [12], and electrical conductivity [13]. Recently, plant extract mediated green synthesis of AgNPs has been developed [14], [15]. The green synthesis of AgNPs is more ecofriendly than physical and chemical methods [16], [17]. Parthibavarman et al. [18] studied green synthesis of AgNPs using apple and grape extracts for dye degradation under visible light. The highest efficiency of dye degradation of 95–98% was found when 10 mg of AgNPs per liter of dye was used. Furthermore, the synthesized AgNPs also showed antimicrobial activities for Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli.) Panzella et al. [19] reported the new trend of green synthesis of AgNPs using waste from the food industry. They suggested that spent coffee ground could be used as a reducing agent for synthesis of AgNPs. The minimum inhibitory concentration against Pseudomonas aeruginosa (P.aeruginosa), E. coli, and S. aureus was observed at 0.3%wt of AgNPs. Among the waste of beverage industries, a large amount of tea waste at an approximate amount of 3 million tons per year is generated [20]. Ramdani et al. [21] suggested waste management through the extraction of active ingredient from the spent green tea. In their study, phenolic compounds and tannin were extracted using distilled water. These compounds were successfully applied as reducing and capping agents for the reduction of silver salt into AgNPs. Therefore, use of ex-SGT in the synthesis of AgNPs would contribute to the management of waste from the tea industry.

Green synthesized AgNPs have been widely incorporated into bio-based polymers to improve their antibacterial activities [22], [23], thermal, mechanical, barrier, and other properties [24]. Furthermore, the deposit of AgNPs on support has attracted much attention to achieve the free-standing AgNPs coated support for various applications such as sensor and heterogeneous catalyst. The environmentally friendly support such as cellulose has high specific surface area for deposit of nanoparticles. The cellulose in the form of filter paper has been developed as a support for green synthesized AgNPs [24]. Jung et al. [25] prepared coated filter paper with green synthesized AgNPs using starch as a reducing agent. The coated paper with 100 mM silver nitrate showed excellent antibacterial activity against E. coli and S. aureus. Kanikireddy et al. [26] studied green synthesis of carboxy methyl cellulose-guar gum (CG) film containing AgNPs. The CG film nanocomposite had antibacterial activities and could delay the decay of strawberries after being stored for 6 days.

Currently, there is little information on the effects of AgNPs on the photocatalytic and antimicrobial activities in fruit and vegetable packaging. Therefore, the present research aimed to develop multifunction packaging paper coated with green synthesized AgNPs. The ex-SGT at various concentrations and starch were used as reducing agents for green synthesis of AgNPs. Properties of coated paper with AgNPs, such as photocatalytic ethylene degradation, antibacterial activities, and physical properties were investigated. In addition, filter paper coated with AgNPs was placed in a plastic box containing mangoes to study the ripening delay of the fruit.