Antibacterial nanostructures derived from oxidized sodium alginate-ZnO

Abstract

The present study describes synthesis, characterization and antibacterial application of oxidized sodium alginate (OSA)-zinc oxide (ZnO) hybrid nanostructures (OSA-ZnO). In continuation to our previous study on oxidized guar gum (OGG)-ZnO (OGG-ZnO) nanocomposite, in the present study we have chosen OSA to understand the role of polysaccharide charge type in designing the antibacterial material. The nanomaterial has been characterized using UV–visible, FTIR, XRD, SEM and TEM analyses. The nanostructure has shown crystalline nature having hexagonal phase with preferred (101) orientation, while TEM image indicated that the material has ~6 nm particle size. It exhibited very good antibacterial performance against Bacillus subtilis (B. subtilis), Cellulomonas cellulans (C. cellulans), Staphylococcus typhi (S. typhi), and Escherichia coli (E. coli) bacterial strains, ZOI for B. subtilis, C. cellulans, S. typhi, and E. coli being 22, 18, 19.5 and 18.5 mm respectively. Under identical conditions, pure ZnO showed significantly lower ZOI for the corresponding bacterial strains (14, 12.5, 12 and 13.5 mm respectively), while native SA and OSA did not exhibit any biological activity.

Introduction

These days, nanotechnology has made its special place in the fields of biomedicine and pharmacy. Nanotechnology has been utilized as an alternative antimicrobial strategy to cope with the re-emergence of infectious diseases and to tackle with the antibiotic-resistant bacterial strains [1]. As opposed to micro- or macro-sized metal oxide particles, corresponding nanoparticles show enhanced antimicrobial activity due to their large surface to volume ratio [2]. Now a day's biopolymer modified nanoparticles have attracted attention in the field of nanotechnology [3] as these natural polymers not only reduce the toxicity of the nanoparticles but also they enhance their antimicrobial potential [4].

Zinc oxide is a promising antimicrobial and antitumor material [5,6]. It has been reported to inhibit the adhesion and internalization of enterotoxigenic E. coli bacteria [7]. In addition, ZnO nanoparticles (ZnO-NPs) are known for reducing the attachment and viability of microbes on biomedical surfaces [8]. The particle size has an important role in determining the antibacterial performance of ZnO-NPs [9]. Size control of ZnO-NPs has been accomplished via thermal decomposition of zinc acetate coated on organic additives [10]. Several methods are known for obtaining ZnO-NPs e.g. microwave assisted synthesis [11], sol–gel technique [12], hydrothermal synthesis [13], solution combustion method [14], laser technique [15], mechanochemical synthesis [16], solvothermal process [17], and sonochemical synthesis [18]. The co-precipitation is an inexpensive and environmentally friendly technique which allows room temperature growth and fabrication of the nanoparticles.

SA, sourced from brown marine algae, is an anionic and hydrophilic polysaccharide [19]. It consists of β-d-mannuronic acid (M-unit), and α-l-guluronic acid residues (G-unit) which are arranged as the interspersed blocks of M and G units [20]. The high molecular weight alginate polymers are difficult to degrade through hydrolytic and enzymatic chain breakage; however their oxidation degradation is reported [21]. Hydrogen peroxide is a popular and an environmentally friendly oxidant for oxidizing polysaccharides [[22], [23], [24], [25], [26], [27]]. Polysaccharides may be depolymerized through oxidation while their structures get modified e.g. during cellulose oxidation, high content of ketone groups are introduced [26]. Similarly oxidation of starch by hydrogen peroxide introduces carboxyl and carbonyl groups at the C-6 positions [24]. Hydrogen peroxide oxidizes chitosan to result carboxyl groups and deamination with simultaneous ring-opening [22]. There are only rare reports on the oxidation of SA through H₂O₂ [28,29].

Owing to high relevance of SA in biomedical field [30,31], it has been chosen for stabilization and binding nano ZnO [32]. In our previous study we have revealed that OGG-ZnO hybrid nanostructures have excellent antibacterial activity against B. subtilis and Salmonella typhi (S. typhi) [33]. In continuation to this study, in present study we have taken OSA instead OGG to understand the effect of polysaccharide charge type in deriving antibacterial nano ZnO.