CdS Nanoparticles For Photo-catalyzed Cell Redox Reaction

Nano-semiconductor materials have potential application value in many fields because of their unique optical,acoustic,electrical,and thermal properties.The band gap width of nanocrystalline CdS can be adjusted between 4.5 eV and 2.5 eV,and energy level transitions can occur under the excitation of ultraviolet and visible light,so it can be used as photocatalyst to catalyze biological reactions.In the enzyme-catalyzed reactions,the coenzyme nicotinamide adenine dinucleotide(NADH)and its phosphorylated form(NADPH)cycle is essential for enzyme-catalyzed reactions,but it is expensive and difficult to apply for large-scale production and application.Therefore,it is essential to develop an efficient regeneration system that can convert solar energy into NADH.In this project,nano-cadmium sulfide(CdS),a semiconductor material,was selected to transfer electrons to coenzyme NAD+to regenerate NADH,and form a coenzyme self-circulation reaction with the enzyme to promote the progress of the biocatalytic reaction.The research contents include the following aspects:1.CdS nanocrystals were synthesized by different methods.Three different types of CdS nanoparticles were synthesized in the experiment:CdS quantum dots were synthesized at high temperature,CdS network framework structure(CdS-NFs)nanomaterials and CdS-NFs shuttle-type nanopolymers(CdS-NFs-A)were synthesized at room temperature with organic reagents.Moreover,multi-angle laser light scattering,SEM and TEM were used for characterization.Those found that the particle size distribution of CdS QDs was less than 5 nm,CdS-NFs formed a nano-network structure,in which the particle size of CdS is about 5nm,and CdS-NFs-A formed a 20nm shuttle nanometer material.2.Research on CdS-NADH-cell photocatalytic reaction.Firstly,existing Leucine Dehydrogenase strains were resuscitated,expressed and purified.Secondly,the optimal reaction conditions of LDH were optimized:pH,buffer solution,temperature and substrate concentration.CdS-NADH-cell photocatalytic reaction system was established under the optimal reaction conditions,and three kinds of electron donors were screened:triethanolamine(TEOA),ascorbic acid(VC),2-[4-(2-hydroxyethyl)piperazin-1-yl]ethanesulfonic acid(HEPES)buffer solution,and the light,light intensity,concentration of photocatalyst were optimized.In the best light catalytic conditions,Trimethylpyruvate(TMP)is converted into L-tert-Leucine.Finally,resting cells react without exogenous coenzyme,TEOA as electron donor,under the excitation of 460 nm illumination.Added 5mM CdS-NFs in reaction system,then the conversion efficiency of TMP as high as 95%;achieved substrate conversion efficiency up to 55%when used CdS-NFs-A as photocatalyst,and catalyzed substrate conversion 75%for CdS QDs.In addition,the cofactor cycle constant TOF of these three materials was increased by 19 times,11 times and 15 times which compared with the control group without materials.3.Construction of CdS-NADH-LDH photocatalytic reaction system.The rest cell combined with three kinds of CdS to explore the influence of different CdS materials on the immobilization rate and enzyme activity of LDH.It was found that the immobilization efficiency of CdS-NFs reached 80%and the recovery rate of immobilized enzyme activity was 77%.Finally,under the optimal reaction condition,0.5mM NADH was added exogenously,5mM CdS-NFs photocatalyzed 10 mM trimethylpyruvate conversion efficiency reached 95%after 72 h,the TOF increased by 8 times;the conversion efficiency of CdS-NFs-A photocatalyzed substrate was 65%,the TOF increased by 5.4 times.Due to the self-polymerization of CdS QDs,the substrate conversion efficiency was only 75%,TOF increased by 6.25 times.The results of photocatalytic LDH and resting cells were similar.4.The CdS-NADH photocatalytic system was constructed to verify the reaction efficiency of the photocatalyst.It was found in the experiment that NAD+could be regenerated into NADH under the light excitation of 460 nm LED,but the regeneration efficiency was low,with only 0.02 TTn h-1 of TOF.This mainly because the unstable presence of coenzyme in the photocatalytic system,which could be easily degraded into nicotinamide and adenine ribonucleotide(AMP).The experiments verified the existence of coenzyme in CdS-NADH-LDH system,and found that coenzyme still degraded,so it was speculated that coenzyme regeneration became the rate-limiting step in the photocatalytic stage.In summary,all the three CdS can promote the self-cycle of coenzyme through photocatalysis,and then promote the redox reaction of cells.The special porous network structure of CdS-NFs is conducive to the combination of substrate and enzyme in the redox reaction,thus improving the efficiency of photocatalysis.The synthesis of CdS-NFs-A and CdS-NFs not only improves the solubility of CdS,but also significantly improves the energy-driven electron transfer efficiency.The results of this study provide a new idea for the preparation of highly efficient semiconductor materials and photocatalytic redox reactions based on the organic metal frames.