| Chirality is widespread in nature.Whether it is amino acids,sugars,micro DNA,or snails and conch in the macro world,those all are closely related to chirality.For natural chiral organisms,the circular dichroism（CD）signal of organic molecules is very weak and usually appears in the ultraviolet spectral range.Enhancing chiral response and extending it to the visible and infrared regions have become urgent scientific issues.Chiral inorganic nanomaterials are the most dynamic research directions in nanomaterials and chiral materials today.Noble metal nanoparticles can support local surface plasmon resonance,which can greatly enhance the radiative properties of the nanoparticles,and accompying strong optical,electrical,biological and catalytic effects,their optical activity can be easily modulated by controlling their size and morphology.The chiral inorganic nanomaterials prepared by chemically attaching a chiral ligand to the surface of achiral metal nanoparticles obtain optical chirality in the ultraviolet and visible spectral range,and show a strong chiral response,which is several orders of magnitude larger than that of chiral organic molecules,and has broader application prospects in real life.However,there are still challenges to further improve the chiral response in practical applications.In this thesis,noble metal gold-silver nanostructures are taken as the research object,and enhance the chiral response of chiral noble metal nanocomposites is the research goal.A variety of chiral molecules and noble metal gold-silver hybrid nanomaterials are designed and synthesized.The chiral response origin of hybrid nanomaterials and intrinsic relationship between material composition and properties are systematically studied.First,the coupling between noble metal nanostructures and chiral molecules will produce a strong chiral response,ranging from ultraviolet to visible spectral regions.In this work,Au/Ag ANTs were prepared by coupling cysteine with Au/Ag ANTs.The chiral response of this material is strongly dependent on the chirality of cysteine and shows a clear mirror image behavior.Unlike Ag nanorods,Au/Ag ANTs have a higher chiral response due to stronger local electromagnetic fields,which can be confirmed by FDTD simulation of local electric fields on these two plasmonic nanostructures.The chiral composites induced CD signal appears in the interband absorption region of Au/Ag ANTs,rather than in the local surface plasmon band,which can be attributed to both the extended cysteine helical network conformation on the surface of Au/Ag ANTs and the near-field enhancement effect of plasmonic nanotubes.Because the energy of the molecular transition matches the interband transition of the metal,and the coupling efficiency largely depends on the local electric field strength on the metal surface,which confirms that the Coulomb interaction induces the coupling between cysteine and Au/Ag ANTs,which leads that cysteine molecules form an extended helical network on the surface of Au/Ag ANTs.We systematically studied the effects of temperature,pH,and external ions on the chiral response of the composites,and confirmed the helical network conformation of cysteine molecules on the surface of the material.In addition,the-NH2 group of an amino acid molecule generates an oxidation current at 0.5-0.7 V in an electrochemical reaction.We studied the selective recognition ability of Au/Ag ANTs for amino acids.Due to the presence of chiral active sites and the steric effect of large groups,the material also exhibits excellent amino acid chiral recognition in catalytic electrochemical reactions.Secondly,Chiral Ag/Au-cysteine hybrid nanospheres（HNSs）were prepared by wet chemistry methods using cysteine as an inductive agent.Through TEM,SEM,CD,UV-vis and other characterization methods,we explored the formation mechanism of this hybrid nanospheres.Unlike the dipole approximation,the circular dichroism spectrum of this material can be divided into two parts:1)one region associated with the interband absorption enhanced optical activity of structural arrangement of Cysteine molecules at 200–320 nm;2)another region corresponding to a ligand-to-metal charge transfer mixed with ligand-to-metal-metal charge transfer excitation due to the synergetic interplay of the electrostatic interaction between Cysteine side chains and the Au（I）???Au（I）aurophilic attraction in the Au（I）-Cysteine complexes,located at 350-400 nm.The chiral signal of the material showed a dynamic change with the concentration of Cysteine and Au.The chiroptical response increases dramatically with increasing the concentration of Au from 0.1×10-3 M to 1.56×10-3 M,and subsequently decreases with further increasing the concentration of Au from 3.12×10-3 M to 12.5×10-3 M,which is similar to the sergeants and soldiers effect of chiral polymer materials.When the cysteine concentration gradually decreased,the chiral signal gradually decreased or even disappeared.Furthermore,the circinate-like morphology of the alloyed nanospheres is strongly dependent on the presence of Ag and the chiral bimetal HNSs present excellent enantioselective recognition for amino acids in catalytic electrochemical reactions due to the specifc activity and chiral active spots.Thirdly,a novel mesoporous SnO2 nanotube with uniform morphology can be synthesized by using Ag nanorod as a sacrificial template and Tin-alginate as Sn source via a self-assembly process.The void of mesoporous SnO2 nanotubes can be precisely controlled by tuning the coating thicknesses of Tin alginate（TA）in Ag@TA core-shell nanocables.In-situ variable temperature X-ray spectrum shows that the evolution process of mesoporous SnO2 nanotubes is driven by the minimization of free energy.We also calculated the free energy of the process of generating SnO2 nanotubes.The results show that the thickness of TA has a great influence on the free energy.At low reaction temperature,the change of free energy plays an important role in the formation of mesoporous SnO2 nanotubes.The formation of mesoporous SnO2 nanotubes is an exothermic reaction,which means that as the reaction proceeds,more reaction heat will be released.When the precursor contains more TA components,more heat will be released to further increase the local temperature,which causes the metal Ag core to dissolve when the reaction temperature of the precursor Ag@TA-IV nanocable reaches300°C,and the mesoporous SnO2 nanotubes can be completely formed until the reaction temperature reaches 450°C.In addition,Kirkendall diffusion plays an important role in the formation of mesoporous SnO2 nanotubes because the surface diffusion process helps to expand the internal pores.Unlike general SnO2nanostructures,the mesoporous SnO2 nanotubes exhibit efficacious methanol sensing behaviors.The dynamic transients revealing their significantly enhanced gas response for 1 ppm methanol is 1.88 and the rapid response and recovery time is 3 and 6 s,respectively.This simple strategy provides new insights to design and synthesize other inorganic metal oxide nanoarchitectures by using different templates and metal ions,which lays the foundation for the subsequent preparation of chiral mesoporous SnO2nanotubes and study their properties in chiral catalysis. |
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