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Controllable biosynthesis and characterization of α-ZnS and β-ZnS quantum dots: Comparing their optical properties

https://doi.org/10.1016/j.mssp.2016.07.015Get rights and content

Abstract

The controllable biosynthesis of α-ZnS and β-ZnS quantum dots by Clostridiaceae sp. using different concentration of hydroxypropyl starch as dispersant are demonstrated. The structure and properties are characterized by XRD, TEM, HRTEM, UV–Vis and fluorescence spectra, and so on. The results indicate that the α-ZnS quantum dots with wurtzite structure and diameter of 8±2 nm were attained under low dose of hydroxypropyl starch, while high dose obtained the β-ZnS quantum dots with sphalerite structure and diameter of 4±1 nm. The UV–Vis absorption of β-ZnS quantum dots have an evident red shift compared with α-ZnS quantum dots, and the as-biosynthesized β-ZnS quantum dots have a stronger fluorescence activity than α-ZnS quantum dots. This work displays a simple biosynthetic method to control the crystal structure and particle size of metal sulfide nanoparticles under mild conditions.

Introduction

Semiconductor quantum particles have attracted increasing attentions in many fields such as solar cells, optoelectronic transistor components and fluorescent biological labels because of their unique size-dependent electronic and optical properties [1]. Among various quantum dots (QDs), zinc sulfide (ZnS) QDs have been widely used in fluorescence, sensors, photo-catalysis and solar cells because of their unique optical and photo-chemical properties [2]. Moreover, ZnS is a significant layered semiconductor with a large direct band gap of about 3.6 eV as well as large absorption coefficient, making it an ideal candidate for tunable photosensitization and considerable photoactivity in the visible region [3]. Until recently, a variety of chemical methods have been developed to fabricate ZnS QDs, but these methods usually exist high cost and serious safety risk [4], [5], [6]. Therefore, the development of a cheap, effective, and reproducible method to prepare ZnS QDs with controlled structures and properties on a large scale remains a great challenge.

More recently, the biosynthesis of QDs has attracted widespread attention because of its simple preparation process, mild reaction conditions, low cost, the controllability of the growth of microorganisms and so on, representing an environmentally friendly method of producing industrially and technologically relevant QDs [7], [8], [9]. For example, several metal sulfide QDs including CdS, ZnS, CuS and PbS have been successfully synthesized by various microorganisms, such as fungus, yeast and bacteria [10], [11], [12], [13]. However, because biosynthesis process could not supply extreme conditions, it is generally hard to regulate the structure and size of QDs. Hence, it is of great importance to develop a simple biosynthesis process, which can obtain QDs with different structure and size to extend their range of applications.

In this work, a simply bio-reduction process was used to synthesize ZnS QDs under mild conditions by Clostridiaceae sp., and α-ZnS and β-ZnS QDs were controllably prepared via changing the concentration of hydroxypropyl starch in the above microbial system.

Section snippets

Materials and methods

All chemical reagents were analytical grade, and purchased from the Beijing Chemical Industry. Deionized water was used in all the experiments. Biosynthesis of metal sulfide nanoparticles was initiated in EDTA-M and sulfate by Clostridiaceae sp. under mild conditions by a low cost green methodologies described by our team [14], [15]. 0.08 g/L (low dose) and 0.8 g/L (high dose) of hydroxypropyl starch were added to the biosynthesis medium as a dispersant, respectively. The biosynthesized ZnS QDs

Results and discussion

The ZnS QDs were biosynthesized by incubating the Na2SO4, EDTA-Zn and different doses of hydroxypropyl starch mixed solution with Clostridiaceae sp. cells in a neutral medium at mild temperature (25–32 °C). The EDS analysis that both products synthesized at different hydroxypropyl starch concentrations were composed of the element Zn and S, and the atomic ratios of Zn:S were 0.96:1 and 0.95:1, respectively (Fig. S1 and S2), which were very close to the theoretic expectation. The results

Conclusions

In this study, both α-ZnS and β-ZnS QDs were successfully biosynthesized by the reaction of Zn-EDTA with Na2SO4 solution containing Clostridiaceae sp. using different doses of hydroxypropyl starch as dispersant at mild conditions. The as-biosynthesized α-ZnS QDs possessed the structure of hexagonal phase and the particle diameter were 8±2 nm, while β-ZnS QDs with diameter of 4±1 nm belonged to cubic phase. Compared with α-ZnS QDs, the β-ZnS QDs had a better absorption in visible range and a

Acknowledgments

We really appreciate the National Natural Science Foundation of China (21277012) and Shandong Fund of Sciences and Technology for environment Protection and Basic Research Fund of BIT for providing us the financial support.

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