Mechanism, Regulation of Chalcogen and Chalcogenide Nanomaterials Biosynthesis and Their Application

生物合成硫族納米材料的機理、調控及應用

Student thesis: Doctoral Thesis

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Award date12 Dec 2018

Abstract

Environmental pollution by cadmium (Cd) and selenium (Se) has aroused widespread concerns today. Recently, there is a growing interest in the transformation of these contaminants into valuable nanomaterials through biosynthesis. One nanomaterial of particular interest is semiconductor quantum dots (QDs), which have the potential to be applied in bio-medicine, cancer treatment, solar cells, detectors, biosensors, imaging and light emitting diodes (QLED) due to their excellent fluorescence and semiconductor properties. Nano-selenium also has great potential for environmental remediation applications. Compared with the conventional chemical processes including organometallic and aqueous phase syntheses that generally suffer from high cost, intensive energy consumption, complicated operation, and harsh reaction conditions, biosynthesis enables a more sustainable production process. Biosynthesized QDs (Bio-QDs) have already been found in various kinds of organisms including bacteria, fungi, phage, plant extracts and earthworm. However, the sluggish synthetic process and low productivity of such biogenic nanomaterials limit their practical application. Thus, new synthetic processes and regulation methods to improve the yield and application performance of biogenic nanomaterials are highly desired.

The main contents and results of this dissertation are listed below:
1. Different organisms synthesize nano-selenium with different properties. Biosynthesis of nano-selenium in protozoa has not been investigated before. Tetrahymena, as an important biological model, are widely found in freshwater systems all over the world. They can also be cultivated at an industrial scale. Therefore, we investigated the biosynthesis of nano-selenium in Tetrahymena thermophila SB210 (T. thermophila SB210), and elucidated the biosynthesis mechanism. TEM (transmission electron microscope) and SEM (scanning electron microscope) show the formation of irregular and particle-shaped nano-selenium, with diameters ranging from 50 to 500 nm. HRTEM (high-resolution electron microscope) and Raman spectra showed that the nano-selenium produced by T. thermophile SB210 was amorphous. The results of real-time PCR demonstrated that thiol-rich peptides-related genes were overexpressed during the synthesis of nano-selenium. GSH results demonstrated that elevated amounts of –SH were consumed during the nano-selenium formation process. In vitro studies showed that GSH and cysteine were capable of reducing selenite into nano-selenium, pointing to the capacity of thiol-rich protein production. All of these results strongly suggested that thiol-rich peptides and proteins played key roles in the nano-selenium production.

2. Bio-QDs synthesized in different organisms typically have different characteristics. We investigated the biosynthesis of CdS1-XSeX Bio-QDs in protozoa Tetrahymena for the first time. EXAFS (extended X-Ray absorption fine structure) and HRTEM were applied for characterization. A green, simple, and cost-effective method was developed to synthesize CdS1-xSex Bio-QDs in Tetrahymena pyriformis (T. pyriformis), yielding CdS1-xSex Bio-QDs with an average diameter of 8.27 ± 0.77 nm. The fluorescence of Bio-QDs demonstrated excellent selectivity toward Cd2+ detection and the linear detection range was from 20 to 80 µM. This method has a great potential to be applied in Cd2+ detection. Furthermore, Tetrahymena have the potential to remediate heavy metal Cd2+ by synthesizing CdS1-xSex Bio-QDs.

3. The widespread applications of Bio-QDs are limited by the low production rate and poor fluorescent properties. Previous studies mainly relied on elevating the chemical energy utilization in the culture medium (optimization of the Bio-QDs synthesis process, genetic engineering). Regulating properties of Bio-QDs were seldom reported before. In this study, we improved the bio-assembly process and the properties of the resulting CdS1-XSeX Bio-QDs in Escherichia coli (E. coli) by using solar energy as an extra driving force. Raman spectra and HRTEM were performed to characterize the formation of CdS1-XSeX Bio-QDs. Fluorescence spectra were used to determine the amount of biosynthesized CdS1-XSeX QDs. The fluorescent lifetime of CdS1-XSeX Bio-QDs synthesized under sunlight irradiation in E. coli was 24.8 ns while the fluorescent lifetime of Bio-QDs synthesized under dark condition was 18.45 ns. CdS1-XSeX Bio-QDs with prolonged fluorescent lifetime were assembled under sunlight irradiation at a significantly higher reaction rate than that under dark condition. The enhancement was associated with the photocatalytic reduction of precursors of CdS1-XSeX Bio-QDs by self-assembled CdS1-XSeX Bio-QDs. This facile and sustainable regulation method could also be applied in a eukaryotic model T. pyriformis. It is the first time to report the utilization of solar energy in the synthesis of Bio-QDs. This study also suggests that light irradiation has the potential to facilitate Cd2+ remediation by promoting CdS1-XSeX Bio-QDs synthesis.

4. Incorporating artificial photosensitizers into microorganisms is considered a promising way to efficiently harvest solar energy. HPLC (high performance liquid chromatography) was applied for quantifying the nitrobenzene and aniline while GC-MS (gas chromatography-mass spectrometer) was used to confirm the formation of aniline. We found that the intracellularly self-precipitated CdS semiconductors in T. pyriformis significantly improved the nitrobenzene reduction by this organism under visible light irradiation. The intracellular CdS even triggered nitrobenzene reduction in TpyCSE mutant strains. The intracellular hybrid system could broaden the function of microorganisms and diversify solar energy conversion pathways. As well, the CdS-Tetrahymena hybrid system has the potential to promote nitrobenzene remediation.

In this dissertation, we demonstrated the biosynthesis of nano-Se and QDs in protozoa Tetrahymena, as well as their environmental application potentials. Nevertheless, there are still considerable challenges for their large-scale production and practical application. Although the contributions of Bio-QDs in solar-to-chemical conversion and biological hydrogen production have already received a lot of attention, there are several problems needed to be resolved before large-scale industrial production can be achieved. Purification of Bio-QDs is a complicated, time consuming and expensive procedure. Therefore, it is important to develop extracellular Bio-QDs synthesis or to develop simple purification procedure for Bio-QDs. Furthermore, it is necessary to continue to improve the yield of QDs biosynthesis. Regulation of properties including fluorescent lifetime, quantum yield needs to be developed. Finally, complicated modification of Bio-QDs using other elements may also broaden the application of Bio-QDs.