Bacterial synthesis of nanocatalyst and biofilm-templated nanocatalytic system for water purification
Ng, Chun Kiat
Date of Issue2017-06-23
Interdisciplinary Graduate School (IGS)
Singapore Centre on Environmental Life Sciences Engineering (SCELSE)
Microorganisms have the potential to change the oxidation state of metals and these microbial processes have opened up a new window for novel applications including biosynthesis of metal nanomaterials. Planktonic cells based synthesis of metal nanomaterials has been studied previously; however, much of the mechanistic details are still relatively unknown. The application of planktonic cells-based metal nanomaterials synthesis is constrained to batch and fed-batch processes. Moreover, planktonic cells have low tolerance to toxic substrates or products which undermine the synthesis efficiency of metal nanomaterials. Microbial biofilms might offer a solution. Biofilms are structured, surface-associated, microbial communities that are prevalent in most natural environments, engineering systems and clinical settings. One of the most important features of a biofilm is the presence of extracellular polymeric substances (EPS) that form a matrix and encase the cells in the biofilm. Cells in biofilms often form structurally stable communities and show a remarkable resistance against various biocides. Hence, we hypothesize that biofilm may be a good candidate to use for the synthesis of metal nanomaterials. This project aims to develop a robust and efficient biofilm-based nano-catalytic system which is applicable to water purification and environmental remediation. To understand the mechanism of nanomaterial production by microorganism, we used model organism Shewanella oneidensis MR-1 to synthesize silver, silver sulfide and palladium nanoparticles. We showed that the synthesized metal and metalloid nanoparticles exhibit catalytic activity. Microbial synthesis of silver and silver sulfide is influenced by outer membrane cytochromes, while synthesis of palladium nanoparticles (NPs) is influenced by periplasmic [NiFe]-hydrogenase. In addition, we XXI also revealed that in extracellular biosynthesis of NPs, the usually neglected non-cell-associated NPs could have high catalytic activity, highlighting the need of novel methods that can efficiently retain extracellular NPs in the biosynthesis processes. The second part of the project focused on exploring the reductive formation of Pd(0) nanoparticles in biofilms under ambient conditions, understanding the metabolic responses of the biofilms during the process of palladium nanoparticles synthesis, and developing a robust nanocatalytic biofilm through genetic engineering. We showed that the S. oneidensis biofilms can reduce Pd(II) and accumulate Pd(0) nanocrystals with a size of 10-20 nm in the biofilm matrix and in the cell membrane even under bulk aerobic conditions, and that the S. oneidensis biofilms with Pd(0) nanocrystals exhibit nanocatalytic activity. However, we also observed that Pd(II) exposure inhibits cellular respiration and energy metabolism of S. oneidensis cells, posing a risk of biofilm detachment. To improve the robustness of biofilm-based nanocatalyst, we explored two approaches: (1) genetic manipulation via c-di-GMP pathway and (2) physical alteration via controlled pyrolysis. In previous studies, c-di-GMP has been reported to be able to influence the production of EPS and influence biofilm processes such as cell dispersal. In our study, we showed that we can increase the production of EPS and alleviate the biofilm detachment during the biofilm’s exposure to Pd(II). The insertion of plasmid pYedQ2 can increase the production of cytoplasmic c-di-GMP, an upstream signalling molecule which influences several key cellular processes, such as cellular motility, extracellular electron transport, iron uptake and biofilm formation in S. oneidensis MR-1. XXII Further, we also showed that controlled pyrolysis leads to carbonisation of biofilm and formation of stable, abiotic heteroatom (N and P)-doped carbon-palladium (C-Pd) nanocomposite catalyst. Moreover, the pyrolysed biofilm-templated C-Pd nanocomposite catalyst show high Cr(VI) reduction, and maintained high reduction rate even on the 5th catalytic cycle. Although the pyrolysed heteroatom-doped carbon on its own has no Cr(VI) reduction activity, it seems to enhance the catalytic activity of the Pd nanocrystals in the C-Pd nanocomposite.