Biosynthesis of cadmium sulfide nanoparticles by photosynthetic bacteria Rhodopseudomonas palustris

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Abstract

A simple route for the synthesis of cadmium sulfide nanoparticles by photosynthetic bacteria Rhodopseudomonas palustris has been demonstrated in this work. The cadmium sulfate solution incubated with R. palustris biomass changed to a yellow color from 48 h onward, indicating the formation of cadmium sulfide nanoparticles. The purified solution yielded the maximum absorbance peak at 425 nm due to CdS particles in the quantum size regime. Also, X-ray analysis of the purified nanoparticles confirmed the formation of cadmium sulfide. Transmission electron microscopic analysis of the samples showed a uniform distribution of nanoparticles, having an average size of 8.01 ± 0.25 nm, and its corresponding electron diffraction pattern confirmed the face-centered cubic (fcc) crystalline structure of cadmium sulfide. Furthermore, it was observed that the cysteine desulfhydrase producing S2− in the R. palustris was located in cytoplasm, and the content of cysteine desulfhydrase depending on the growth phase of cells was responsible for the formation of CdS nanocrystal, while protein secreted by the R. palustris stabilized the cadmium sulfide nanoparticles. In addition, R. palustris was able to efficiently transport CdS nanoparticles out of the cell.

Introduction

The optoelectronic and physicochemical properties of nonmaterials are size and shape dependent. So the synthesis of cadmium sulfide nanoparticles of different sizes and shapes is of great importance for their applications in optical devices, electronics and biotechnologies [1], [2]. Living organisms have the endogenous ability to exquisitely regulate synthesis of inorganic materials such as amorphous silica (diatoms), magnetite (magnetotactic bacteria), gypsum, and calcium carbonate layers (S-layer bacteria) and minerals such as calcite into functional superstructures [3]. Because of this ability to precisely direct the shape and crystallinity of a developing inorganic material, there is great interest in exploiting living organisms such as bacteria and fungi for inorganic materials synthesis.

An earlier study found that Clostridium thermoaceticum could precipitate CdS at the cell surface as well as in the medium from CdCl2 in the presence of cysteine hydrochloride in the growth medium. Most probably, cysteine acts as the source of sulfide [4]. Klebsiella pneumoniae exposed to Cd2+ ions in the growth medium were found to form 20–200 nm CdS on the cell surface [5]. Intracellular CdS nanocrystals, composed of a wurtzite crystal phase, are formed when Escherichia coli is incubated with cadmium chloride and sodium sulfide. Nanocrystal formation varies dramatically depending on the growth phase of the cells and increases about 20-fold in E. coli grown in the stationary phase as compared with that grown in the late logarithmic phase [6].

It has long been recognized that among the eukaryotes, yeasts are explored mostly in the biosynthesis of the semiconductor nanoparticles. Exposure of Candida glabrata to Cd2+ ions leads to the intracellular formation of CdS quantum dots [7]. Kowshik et al. have shown that CdS quantum dots synthesized intracellularly in Schizosaccharomyces pombe yeast cells exhibit ideal diode characteristics. Biogenic CdS nanoparticles in the size range 1–1.5 nm have been used in the fabrication of a heterojunction with poly (p-phenylenevinylene) [8]. Even more exciting is the finding that the exposure of F. oxysporum to the aqueous CdSO4 solution yields CdS quantum dots extracellularly [9]. The particles are reasonably monodisperse and range in size from 5 to 20 nm. Reaction of the fungal biomass with the aqueous CdNO3 solution for an extended period of time does not yield CdS nanoparticles, indicating the possibility of the release of a sulfate reductase enzyme into the solution. However, the intracellular biosynthesis of CdS nanoparticles and then effluence from cells is still scarce.

Phototrophic bacteria are ubiquitous in fresh and marine water, soil, wastewater, and activated sludge. They are metabolically the most versatile among all procaryotes: anaerobically photoautotrophic and photoheterotrophic in the light and aerobically chemoheterotrophic in the dark, so they can use a broad range of organic compounds as carbon and energy sources [10]. In this study, phototrophic bacteria Rhodopseudomonas palustris, a typical purple non-sulfur bacterium, have been chosen to synthesize CdS nanocrystals at room temperature through a single step process.

Section snippets

Materials

Photosynthetic bacteria R. palustris were cultured in the medium containing purvate, yeast extract, NaCl, NH4Cl and K2HPO4 at pH 7 and 30 °C. Cells were harvested at a various time points by centrifugation (4000 × g) at 4 °C for 10 min. After a minimum of 42 h of growth, the cells were no longer dividing, and the optical density (OD) at 650 nm was approximately 2.0. The culture was considered to be in stationary phase. After about 36 h of growth, the culture was close to but did not yet reach

Results and discussion

UV–vis spectra (Fig. 1) recorded from the R. palustris-cadmium sulfate phosphate-buffered saline reaction medium at different growth stages, namely, stationary, late logarithmic and mid-logarithmic phase, exhibit the appearance of an absorption peak, which progressively increases in intensity as the growth progresses. An increase of about 3-fold in nanoparticles formation by R. palustris in the stationary phase compared to the late logarithmic phase was observed. The presence of the absorption

Conclusion

In this study, a photosynthetic bacteria R. palustris was used for the synthesis of stable and uniform CdS nanoparticles. TEM analysis confirmed the uniform distribution of nanoparticles, having an average size of 8.01 ± 0.25 nm. The electron diffraction pattern confirmed the “fcc” crystalline structure of metallic CdS. Characterization by other techniques confirmed the presence of a protein matrix as a stabilizing agent. Furthermore, C-S-lyase, an intracellular enzyme located in the cytoplasm,

Acknowledgments

We thank Dr. Wen-Li Ma and Dr. Yue-jun Fu from College of Life Science and Technology, Shanxi University, PR China for technical support in carrying out the experiments. We acknowledge the service rendered by the Sophisticated Analytical Instrumentation Facility, Institute of Coal Chemistry, CAS, Taiyuan, China, in analyzing the samples by TEM.

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