Regular articleThe biosynthesis of cadmium selenide quantum dots by Rhodotorula mucilaginosa PA-1 for photocatalysis
Graphical abstract
Introduction
QDs are semiconductor crystals with Quantum confinement effect when their size is on the same order as the size of the exciton Bohr radius. The QDs are mainly formed by elements from group IIVI (such as cadmium selenide, cadmium sulfide, cadmium telluride, zinc sulfide, etc.) and group III-V (such as indium phosphide, indium arsenide, etc.) [1]. Owning to their unique optical and electronic properties, QDs attract wide interest in the field of bioimaging [2], photocatalysis [3] and sensors [[4], [5], [6]]. CdSe QDs belong to the IIVI group of the semiconductor. They can absorb a considerable part of the UV–vis region in sunlight, and subsequently generate election-hole pairs [7]. Based on their properties of narrow bandgap (∼ 1.74 eV) and rapid electron-hole pair generation [8], CdSe QDs have a good potential for photocatalytic degradation of organic pollutants.
The synthesis methods of QDs are mainly chemical approach in the aqueous phase or the oil phase. Although the oil-phase synthesis method can synthesize QDs with uniform particle size and high quantum yield, the reaction requires harsh conditions. The organic reagents used in the oil-phase synthesis are flammable and explosive, and obtained QDs show poor water-solubility and requires further surface modification before application [9]. The water-solubility problem can be solved by the method of aqueous phase synthesis, but this method still has many problems such as high energy consumption and poor uniformity of the particle size [10]. In addition, the bare Cd-based QDs demonstrates cytotoxicity effects due to the release of cadmium ion [[11], [12], [13]]. These deficiencies in the chemical synthesis seriously distort applications of the commercial synthesis of CdSe QDs. Microbial factories have great potential in the environment-friendly synthesis and size control of QDs.
In the past few years, several bacteria, fungi, and yeast have been found to be able to synthesize CdSe QDs under mild conditions such as normal temperature and pressure [14]. Microorganisms present several specific and non-specific pathways to chelate, methylate, reduce or oxidize ionic compounds [1]. Glutathione plays an important role in the synthesis of CdSe QDs by Pseudomonas stutzeri TS44 [15]. Several reduced thiol-containing proteins play a key role in the synthesis of CdSxSe1-x QDs by Escherichia coli [16]. Compared with hydrothermally synthesized CdSe and CdTe, biosynthetic CdSe QDs had lower cytotoxicity [17]. Compared with chemical synthesis, biosynthesis of CdSe does not require complex organic reagents, reducing energy and material consumption.
Up to now, biosynthesized CdSe QDs has not been successfully used in the photocatalytic reactions although they demonstrate the application in bio-imaging and some other areas. In this study, we reported CdSe QDs biosynthesis by a yeast R. mucilaginosa PA-1. The effect of precursors and the medium pH on the QDs synthesis was explored. Moreover, the photocatalytic properties of synthesized CdSe QDs were examined using a dye malachite green (MG) as a model pollutant. This work demonstrates a green, safe, environment-friendly method to synthesize CdSe QDs that demonstrate a great photocatalytic activity toward the degradation of organic pollutants.
Section snippets
Microbial strains and culture conditions
The strain R. mucilaginosa PA-1 in this study was isolated from Zhoushan, Zhejiang province, China. The R. mucilaginosa PA-1 was cultured into the R2A medium (0.5 g/L tryptone, 0.5 g/L yeast extract, 0.5 g/L acid hydrolysate of casein, 0.5 g/L glucose, 0.5 g/L soluble starch, 0.3 g/L sodium pyruvate, 13.42 g/L K2HPO4, 0.05 g/L MgSO4·7H2O, 11.80 g/L citric acid and pH is 4.0) for 36 h at 30 ℃ under shaking at 200 rpm.
Biosynthesis of CdSe QDs
Culture of R. mucilaginosa PA-1 grown to the mid-logarithmic phase was
Synthesis and characterization of CdSe QDs
After simultaneously exposed to Na2SeO3 and CdCl2, R. mucilaginosa PA-1 cells accumulated a large number of granules (Fig. 1a) and emitted green fluorescence (Fig. 1b). The merged image indicated that the green fluorescence was emitted from the granules (Fig. 1c). Small granules were also observed in cells exposed to Na2SeO3 only (Fig. 1d), which might be aggregates of Se(0) nanoparticles. A large granule was observed in most cells exposed to CdCl2 only (Fig. 1g), which might be the
Conclusion
CdSe QDs were synthesized by R. mucilaginosa PA-1 through self-assembly and in the state of clusters associating with cells. The concentrations of cadmium ions rather than selenium ions regulate the biosynthesis of CdSe QDs, implying a detoxification mechanism underlying the biosynthesis. Purified QDs showed a narrow size distribution and strong green fluorescence. Moreover, they demonstrated a good photocatalytic activity toward organic chemicals, using MG as a model. Given the wide
Declaration of Competing Interest
The authors declare that there are no conflicts of interest.
Acknowledgments
This work was supported by National Natural Science of Foundation of China (31670126, 31770139), the Natural Science Foundation of Anhui Province of China (1508085ME92, 1608085QC47), the Scientific Research Foundation of Talented Scholars from Anhui University (J01006042), the Open Fund for Discipline Construction, Institute of Physical Science and Information Technology, Anhui University (S01003121), Supporting Plan for Excellent Young Talents in Colleges of Anhui Province (gxyqZD2017002,
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They contribute equally to this manuscript.