Green synthesis of silver nanoparticles using soluble soybean polysaccharide and their application in antibacterial coatings
Graphical abstract
Introduction
In the past decades, the preparation of noble metal nanoparticles by green synthesis approach has received considerable attention from researchers due to the global efforts to protect the environment. Unlike conventional methods, green synthetic methods allow one to avoid using environmental toxic chemicals during the synthesis process [1]. Among the various noble metal nanoparticles, silver nanoparticles (AgNPs) have attracted special interests because of their distinctive optical, electrical, catalytic and antibacterial properties compared to their macro scaled counterparts [2]. At present, AgNPs have been widely used in personal care products, textiles, food packaging, building materials, and medical instruments because of their outstanding antibacterial effect.
The green synthesis of AgNPs always involves several steps, each step should be in accordance with the basic principles of green chemistry [3]. In brief, the environmental safety of the selected solvent, reducing and stabilizing (capping) agents must be evaluated from green chemistry perspectives. So far many green routes have been developed for obtaining environmentally benign AgNPs, including the polysaccharide method, Tollens method, biological method [4], irradiation method and polyoxometalates method [1]. Compared with other methods, polysaccharide method has several advantages: (i) simplicity; (ii) polysaccharides are widely available and inexpensive; (iii) no additional reducing or capping agents are needed; (iv) no sophisticated experimental equipments are required; (v) the polysaccharide-stabilized AgNPs are suitable for biological and pharmaceutical applications.
To date, many studies have been conducted using polysaccharides and their derivatives for the synthesis of AgNPs that could be potentially useful as antimicrobial substances in packaging and biomedical fields. A list of the synthesis parameters and properties of silver nanoparticles synthesized using polysaccharides was given in Table S1. In those studies, water is normally used as an eco-friendly benign solvent and polysaccharides as reducing or stabilizing (or both) agents. For instance, stable AgNPs were synthesized by using starch as a capping/reducing agent under different conditions [[5], [6], [7]]. The reduction of Ag+ inside of the starch template avoids nanoparticle aggregation due to the binding interactions between starch and AgNPs. The synthesis of AgNPs using chitosan (CS) as a reducing and a capping agent has also been developed, and the CS-AgNPs composite was found to have better antimicrobial performance than its individual components [8,9]. In another study, Ulvan has been used as reducing and stabilizing agent for AgNPs synthesis, the Ulvan-based AgNPs showed antimicrobial activity toward both Gram-positive and Gram-negative bacteria [10]. However, from the parameters presented in Table S1, it can be concluded that it still remains a great challenge to develop a reliable route to synthesize small and uniform AgNPs under mild reaction conditions.
Soluble soybean polysaccharide (SSPS) is a water-soluble polysaccharide derived from soybean cotyledon. It can be extracted and refined from okara and soybean meal, which are residue materials resulting mainly from the manufacture of soybean oil and soy protein isolate. The structure of SSPS is characterized by a compact and globular form with long arabinan and galactan side chains [11]. SSPS has been reported to provide health benefits to humans through lowering blood cholesterol, improving laxation and reducing the risk of diabetes. Apart from its nutritional value, it has various functions such as film forming ability, antioxidative activity, dispersion, stabilization, emulsification and adhesion [12]. Therefore, it has already found broad applications in food and packaging industries [13,14]. In aqueous solution, SSPS has relatively low viscosity and high stability compared to other widely used polysaccharides such as guar gum, sodium alginate. Additionally, the viscosity of the SSPS solution does not significantly change upon heating and addition of salts or acidic substances [15]. These features, which were originated from the globular structure of SSPS in solution, would provide an ideal reducing/stabilizing system for synthesizing AgNPs in a green and sustainable manner.
In this study, we have demonstrated facile green synthesis of AgNPs by heating silver nitrate aqueous solution in the presence of SSPS. SSPS provides a new environmental-friendly reducing/stabilizing agent for rapid and low-cost synthesis of AgNPs with very small sizes. As compared to other polysaccharide-based silver nanoparticles, the obtained SSPS-stabilized AgNPs (SA) have smaller particle size and high stability by virtue of the unique globular structure of SSPS. The structure and antibacterial activity of the synthesized SA against Gram-positive and Gram-negative bacteria were investigated. Subsequently, the application of the SA colloidal dispersion (SACD) as coating agent on a commercial Kraft paper has been proposed to endow it with excellent antibacterial ability.
Section snippets
Materials
SSPS (Molecular weight: 470–520 kDa, measured by GPC; total carbohydrate ≥85.0%, crude protein ≤4.0%) was kindly provided by JinJing Bio. Co., Ltd. (Henan, China). Silver nitrate (AgNO3) was purchased from Civi Chemical Technology Co., Ltd. (Shanghai, China). Glycerol was obtained from Aladdin Co., Ltd. (Shanghai, China). A commercial unbleached Kraft paper, having a grammage of 200 g/m2 and an average thickness of 236 ± 21 μm, was purchased from Nine Dragons Paper Co., Ltd. (Taicang, China).
Synthesis of SSPS-stabilized AgNPs
Polysaccharides have been proposed as green reducing agents and stabilizers for the synthesis of silver nanoparticles in many studies (Table S1). In the present work, the formation of AgNPs was achieved by reduction of silver ions (Ag+) in the presence of SSPS. The schematic diagram of formation and stabilization of silver nanoparticles is illustrated in Fig. 1. According to the static and dynamic light scattering results, the radius of gyration of SSPS was about 23.5 ± 2.8 nm [18]. SSPS
Conclusion
In the present study, we have demonstrated a facile and green chemistry approach for the synthesis of silver nanoparticles by using SSPS as stabilizing and reducing agent. Concentrations of metallic source and SSPS, reaction time and temperature are main effective factors influencing size and yield of the AgNPs. The as-synthesized AgNPs were very small and the size could be controlled by varying the metal ion concentration. XRD, EDX and TEM analyses confirmed the presence of elemental silver
CRediT authorship contribution statement
Zhengxin Ma: Investigation, Writing - Original draft preparation. Jie Liu: Supervision, Conceptualization, Investigation, Writing - Reviewing & Editing. Yanchun Liu: Investigation. Xuejing Zheng: Visualization, Resources. Keyong Tang: Validation, Resources, Funding acquisition.
Declaration of competing interest
The authors declare no conflict of interest.
Acknowledgements
This work was supported by the Science and Technology Department of Henan Province [grant number 172102410022], and National Natural Science Foundation of China [grant numbers U1204504, 51473150].
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