Rhelogical and antibacterial performance of sodium alginate/zinc oxide composite coating for cellulosic paper
Graphical abstract
Introduction
Antibacterial cellulosic paper has drawn growing interest from both engineers and researchers to counteract the surface contamination and disease transmission caused by bacterial interaction. It holds great promise applications for many places, such as food packaging factories, hospitals, restaurants and public places. However, natural cellulosic paper does not have any antibacterial activity. The surface coating is a simple and effective way to incorporate organic and/or inorganic antibacterial agents, rendering cellulosic papers with antibacterial properties.
Zinc oxide (ZnO) is an exceptionally important material, possessing unique optical, electrical, chemical and catalytic properties. In fact, as an effective antibacterial agent, ZnO exhibits good biocompatibility, easy fabrication, less toxicity and heat stability [1], [2], [3]. Extensive work has been reported on the antibacterial activity of ZnO against various bacteria, including E. coli [4], C. jejuni [5], S. aureus [6], and B. subtilis [7]. It is well known that the dispersion and stabilization of ZnO nanoparticles play very crucial roles in their antibacterial activities, and hence the use of suitable bio-based binders have been investigated. For instance, Cheng et al. [8] prepared carboxymethyl-starch-stabilized ZnO nanoparticles in situ in water, and deposited the ZnO coating on cellulose paper by using aqueous solution. Martinsna et al. [9] used the electrostatic assembly method to fabricate nanofibrillated cellulose/ZnO nanocomposite coating. Pang et al. [7] synthesized the sodium lignosulfonate stabilized ZnO nanoparticles through a two step dip-coating approach. These results showed that the sodium lignosulfonate modified ZnO could be homogeneously dispersed in water.
Sodium alginate (SA) is one of the most widely used natural polysaccharides, which is extracted from brown seaweeds [10]. Owing to its nontoxicity, biodegradability and biocompatibility, SA has been found widespread applications in tissue scaffolds, drug delivery and food industry. Recently, several studies have reported the preparation of SA/ZnO composites by various methods. Baskoutas et al. [11] used zinc nitrate or zinc acetate as the starting material to prepare zinc alginate gels, and subsequently heated the gels at high temperatures (800 °C and 450 °C) to get the ZnO nanocrystals. Trandafilovic et al. [1] showed that the spherical ZnO nanoparticles could be synthesized within the alginate solution by using microwave treatment. Shalumon et al. [12] obtained ZnO nanoparticles by using a wet chemical method, and then developed SA/ZnO/poly (vinyl alcohol) (PVA) nanofibrous mats with the electrospinning of SA/PVA from aqueous solutions. Varaprasad et al. [13] prepared rod-shape ZnO particles with precipitation technique, and deposited on the surface of cellulose fibers through the SA matrix with impregnation. These works proved that the ZnO particles could homogeneously disperse in the SA hydrogel. Therefore, the development of SA/ZnO composite coating as an antibacterial layer on the cellulosic paper has wide application potentials. Moreover, it has been reported that the treatment of ZnO at a lower temperature leads to a higher antibacterial activity [14], [15].
In this present work, ZnO nanoparticles were prepared within the SA matrix at 120 °C. In this SA/ZnO composite coating, ZnO nanoparticles were used as antibacterial agents, and SA served as a binder to disperse ZnO nanoparticles. The effect of ZnO concentrations on the rheological behavior of the SA/ZnO composite coating was investigated. Subsequently, the change in thermal stability and tensile property of the modified cellulosic papers was identified. The antibacterial activity of the coated cellulosic paper was further tested by using E. coli (Gram-positive bacteria) and S. aureus (Gram-negative bacteria).
Section snippets
Materials
Sodium alginate (SA), ammonium hydroxide (NH4OH) and zinc oxide (ZnO) were obtained from Sigma-Aldrich. The viscosity of 1 wt% SA in water solution was 15–25 cP. Commercially available filter-papers (Double-circle™, Grade 101) were supplied by Whatman-Xinhua Filter Paper Co. Ltd. (Hangzhou, China). All the materials were used as received without further purification.
Preparation of SA/ZnO composite coatings modified paper sheets
The preparation of ZnO nanoparticles was based on the report [16] with modification. Briefly, different concentration of ZnO
Steady shear rheological behavior of SA/ZnO composite coatings
The measurement of rheological properties of composite coatings is crucial to infer the microstructure of the fluid matrix and understand the surface coating processing. Fig. 1a shows the steady shear viscosity of the composite coatings with various ZnO concentrations versus the shear rate. The ZnO-0 exhibited a typical Newtonian behavior at low shear rates, i.e., the viscosity increased slightly in the shear rate range of 0.01 ∼ 10 s−1 [19]. At the high shear rate regime, the decrease in the
Conclusions
The ZnO particles were successfully prepared and dispersed homogeneously on the surface of cellulosic papers at 120 °C using sodium alginate (SA) served as a binder. The rheology test revealed that there were cross-linkings between Zn2+ and SA molecular chains in the aqueous solution, resulting in the viscosity of ZnO/SA composite coating increased in the low shear frequency region and decreased in the high shear frequency region, hence, benefit to industrial applications. The thermal stability
Acknowledgement
This work was supported by a grant from the City University of Hong Kong (SRG-7004377).
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