Elsevier

Colloids and Surfaces B: Biointerfaces

Volume 114, 1 February 2014, Pages 218-224
Colloids and Surfaces B: Biointerfaces

Anomalous antibacterial activity and dye degradation by selenium doped ZnO nanoparticles

https://doi.org/10.1016/j.colsurfb.2013.10.007Get rights and content

Highlights

  • Band gap of ZnO NPs increased by Se doping, and exhibited defects due to oxygen vacancies.

  • Enhanced photoinduced ROS generation in Se doped ZnO NPs than the pristine ZnO NPs.

  • Enhanced photocatalytic dye degradation of Se doped ZnO NPs than pristine ZnO NPs.

  • Reduced antibacterial activity of Se doped ZnO NPs was due to Se leaching from NPs in culture media.

Abstract

Selenium doped ZnO nanoparticles synthesized by mechanochemical method were spherically shaped of size distribution of 10.2 ± 3.4 nm measured by transmission electron microscopy. Diffused reflectance spectroscopy revealed increase in the band gap, ranging between 3.47 eV and 3.63 eV due to Se doping in ZnO nanoparticles. The antibacterial activity of pristine and Se doped ZnO nanoparticles was attributed to ROS (reactive oxygen species) generation in culture media confirmed by TBARS assay. Compared to complete inhibition of growth by 0.45 mg/mL of pristine ZnO nanoparticles, the batches of 0.45 mg/mL of selenium doped ZnO nanoparticles exhibited only 51% inhibition of growth of Escherichia coli. The reduced antibacterial activity of selenium doped ZnO nanoparticles was attributed to two opposing factors, e.g., ROS generation for inhibition of growth, countered by sustaining growth of E. coli due to availability of Se micronutrients in culture media, confirmed by inductively coupled plasma mass spectrometer measurement. Higher ROS generation by selenium doped ZnO nanoparticles was attributed to creation of oxygen vacancies, confirmed from green emission peak observed at 565 nm. The impact of higher ROS generation by selenium doped ZnO nanoparticles was evident from enhanced photocatalytic degradation of trypan blue dye, than pristine ZnO nanoparticles.

Introduction

Antibacterial activity of zinc oxide nanoparticles (ZnO NPs) has been widely studied due to their advantages over conventional antimicrobial agents, complimented with better stability at wider ranges of temperature and pressure, higher shelf-life, re-usability, ease of storage and transportation [1], [2], [3]. Various parameters, e.g., size, shape, stability, capping agents, structural defects and exposure conditions of ZnO NPs were correlated with antibacterial activity of ZnO nanoparticles [4], [5], [6], [7], [8], [9], [10]. The antimicrobial action of ZnO nanoparticles is a complex process comprising generation of reactive oxygen species (ROS) and its impact on cellular membrane dysfunction, cellular internalization of nanoparticles, mechanical abrasive action of nanoparticles and leaching of zinc ions from ZnO nanoparticles to toxic level [4], [11], [12], [13], [14], [15]. The ROS generation is attributed to creation of photoinduced charge carriers in ZnO NPs and their interactions with oxygen and water molecules at the surface of the NPs. The energy of the photons required to trigger the generation of charge carriers depends on the band gap of ZnO nanoparticles. It is noted that the band gap of ZnO nanoparticles depend on size, shape and structural aspects of ZnO nanoparticles and these factors depend on the method of preparation of nanoparticles [16], [17], [18].

Antibacterial activity of ZnO NPs could be enhanced by increasing ROS generation, which might be accomplished by inhibiting recombination of charge carriers. It is likely to be achieved by transporting charge carriers to the surface of the nanoparticles via defect levels in the forbidden gap where they can interact with oxygen and water molecules to generate more ROS. Doping ZnO with transition metals (e.g., Fe, Mn, Co, Ag) and rare earth elements (e.g., Ce) led enhanced antibacterial effect, attributed to formation of defects e.g., oxygen and zinc vacancies in doped ZnO NPs [19], [20], [21], [22]. Notably the band gaps of these metal doped ZnO nanoparticles were less than the un-doped batches of ZnO nanoparticles. On the contrary, studies on antibacterial effect of ZnO nanoparticles doped with non-metals are meager. A study on sulphur (S) doped ZnO NPs revealed enhanced band gap and were characterized with oxygen vacancies [23]. However, effect of enhanced band gap on antibacterial activity of S-doped ZnO is not known. Looking at the scientific prospect of non-metal doped ZnO NPs, we present here preparation, characterization and antibacterial activity of selenium doped ZnO nanoparticles. Selenium is chosen in this study as it is in the same group element, similar to sulphur and expected to exhibit structural defects due to doping in ZnO. In addition, selenium is a micronutrient for bacterial growth at a certain concentration range [24], and hence it would be worth investigating its antibacterial effect. The batches of selenium (5 wt% and 10 wt%) doped ZnO nanoparticles were prepared by facile mechanochemical method. The morphological, structural and optical features of the as-synthesized selenium doped ZnO NPs were studied by an array of techniques, and correlated with their antibacterial activity. The mechanism of antibacterial effect was quantitatively studied by estimating ROS generated in the culture medium by TBARS assay, which indicated oxidative stress induced lipid peroxidation [25], [26]. Further, the ROS generation effect of selenium doped ZnO nanoparticles was extended to study the photocatalytic dye degradation.

Section snippets

Materials

The precursors of synthesizing selenium doped ZnO nanoparticles, e.g., zinc acetate dihydrate (Zn(COOCH3)2·2(H2O)2) and oxalic acid dihydrate (H2C2O4·2H2O) were procured from Himedia Pvt. Ltd., India. The selenium (Se) metal powder (99.9%) was procured from SD Fine Chemicals Ltd., India. The particle size and purity of the Se powder used in the synthesis was studied by transmission electron microscopy (TEM) coupled with EDAX (energy dispersive X-ray analyzer). The TEM image revealed aggregation

Structural, morphological and optical characterization of Se doped ZnO NPs

The X-ray diffractogram of the as-synthesized Se doped ZnO NPs and pristine ZnO NPs revealed characteristic peaks at (1 0 0), (1 0 1), (1 0 2), (1 1 0), (1 0 3) and (1 1 2) planes (supporting information, Fig. S3) corresponding to the hexagonal wurtzite phase of ZnO, JCPDS card No. 5-0664. The XRD peaks of the pristine ZnO NPs and the Se doped ZnO NPs were broad, attributable to structural defects in the nanomaterials [28]. The peak intensities decreased with increase in the concentration of selenium

Conclusions

The Se doped ZnO NPs were synthesized by simple mechanochemical method which exhibited enhanced band gap, increase in oxygen vacancies and generated higher reactive oxygen species (ROS). The impact of enhanced ROS generation was consistent with higher photocatalytic dye degradation but the antibacterial activity of Se doped ZnO NPs was less than the pristine ZnO NPs. Such anomalous behavior was attributed to release of selenium from the doped ZnO NPs in the culture medium which acted as

Acknowledgments

This work has been carried out under the BRNS project 2007/37/47/BRNS and authors thank BRNS for the support. Mr. N. Bhavani Prasad is thankful to MHRD, Govt. of India for awarding senior research fellowship. Authors thank Dr. A.V.R. Reddy, Bhabha Atomic Research Centre, Mumbai, India for his immense scientific interest in this research work. Authors also thank Institute Instrumentation Centre of IIT Roorkee for allowing us to use various instrumental facilities.

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