Molecular Insight Into Redox Properties Of Extracellular Polymer Substances Of Electroactive Bacteria By Surface-enhanced Raman Spectroscopy

Microbial bioelectrochemical systems?BES?are unique devices capable of interconverting electrical and chemical energy while employing metabolism of electroactive microorganisms.This BES are viewed as a burgeoning biotechnology for electricity production,environmental remediation or the synthesis of chemicals such as alcohols,organic acids and hydrogen.Electroactive bacteria,as the most crucial composition of the BES,which can participate in ambient activities by exchange electron with extracellular solid carriers?electrodes,iron oxides,etc?by oxide organics.The efficiency of electron transfer of elecactive bacteria has determined the function of BES.Electroactive microorganisms mainly exist in the forms of planktonic bacteria and biofilm in external environment.Extracellular polymeric substances?EPS?,which coated with surface of bacteria and play an important role in microbial extracellular electron transport?EET?.Extracellular polymeric substances?EPS?,a complex high-molecular-weight mixture of polymers,are secreted by microorganisms;they determine the physicochemical properties and morphological structure of bacterial cell as well as protect the bacterial cell from unfavorable environments.Several researches have proved that the EPS of electroactive microorganisms have excellent redox properties and electrochemical activity,and could take part in meidiate extracellular electron transport.However,lillte is known about the electron transfer molecular mechanism of EPS of planktonic electroactive bacteria and electroactive biofilm.Lacking of efficient and simple techniques to in-situ investigate molecular information of EPS of electroactive microorganisms during electron transport process.Surface-enhanced raman spectroscopy?SERS?,a nondestructive,high-sensitive and preparation-convinient analytical technique is widely used in the fields of material,biology,chemistry,medicine and food.Ag,Au and Ni nanoparticles were firstly synthesised and worked as double functionally SERS active substrate in this paper to in situ acquire redox molecular information of EPS of electroactive bacteria during extracellular electron transport process.The main research results are as follows:1.For the planktonic bacteria,two typical planktonic electroactive bacteria:Shewanella oneidensis and Geobacter sulfurreducens and two nonelectroactive bactria:Escherichia coli and Bacillus subtilis were cultured.EPS were extracted severally,then physicochemical analysis,electrochemical analysis and SERS were conducted respectively.Expriment results are as follows:?1?EPS of electroactive bacteria have more content of redox protein and humic subctances and have higher EET capabilities and electrochemical activity than nonelectroactive strains.?2?Ni and Ag NPs were introduced to act as SERS substrates and serve as reductant/oxidant.The SERS spectra show that the EPS of electroactive bacteria are consistent with purified horse cytochrome c in terms of the changes in the position and relative intensity of those bands,indicating that cytochromes might be the main redox component in these EPS.2.For the mixed electroactive biofilm,different nanoparticles with positive and negative charges?,-Au,+Ag and–Ag?were systhisised and worked as SERS active substrate to assemble with electroactive biofilm by electrostatic interaction.Results are as follows:?1?biofilm coated positively charged NPs present better SERS signals and reproducibility as a result of the formation of abundant and homogeneous hot spots and?2?most of these signals originate from the EPS surrounded with biofilm,avoiding negative effect during complicated ex-extraction of EPS.?3?Positive relation was also found between the values of relative intensity(I₁₁₂₅/I₁₁₆₅)and electron transfer capacity?ETC?among EPS of biofilm,which could used to evaluate the ETC of EPS of biofilm.?4?SERS measurement demonstrated that Ag NPs could not only oxidize EPS of mixed electroactive biofilm,but magnify EPS Raman signals when SERS focused on 532 nm excitation wavelength.A new method which based on SERS combined with various nanoparticles have been proposed to in situ probe redox properities of EPS of electroactive bacteria.Investigating the molecular mecainsm of electron transfer of EPS of planktonic bacteria and biofilm during electron transfer process.A new idea was provided for the applicaition of SERS in microbial electrochemistry.In the meanwhile,it affords a theoretical basis for the practical application of BES.