Hybrid Noble Metal Core/Shell Nanocrystals: Controlled Synthesis, Characterization, Catalytic And Surface Enhancement Raman Scattering (SERS) Applications

Bi/tri or multi-metallic nanocrystals(NCs)with Core/Shell heterostructure,or inter-metallic and alloyed structures are emerging as more important materials than monometallic NCs.Core/shell nanostructured materials,are considered as an effective way to develop highly active,selective,robust,low-cost and environmental friendly nanoelectrocatalytic systems.Moreover,Core/Shell nanostructures based on Noble Metal Nanoparticles(NMNPs)have played an important role in the development of new biosensors and/or in the enhancement of existing biosensing techniques to fulfill the demand for more specific and highly sensitive biomolecular diagnostics.This thesis is focused on the controllable synthesis of Hybrid Core/Shell Noble Metal Nanocrystals with different morphologies and functionalities.Bimetallic Ag@Pt and Tri-metallic Au@Ag@Pt nanocrystals consisting of a controllable thin Pt shell,via interface-mediated galvanic displacement have been successfully synthesized.Through Water-Ethanol-Oil(W-E-O)interface mediation,the controllable “out to in” displacement of Ag atoms to Pt enables the formation of a thin Pt shell on monodisperse sub-ten-nanometer Au@Ag and Ag nanocrystals.The synthesized nanoparticles with a thin Pt shell exhibited an important catalytic activity towards the oxygen reduction reaction(ORR)due to the high exposure of Pt atoms.Moreover,Sharp-featured aqueous Au@Ag core/shell nanocuboids(NCs)have been synthesized successfully.FEM simulation and experimental results demonstrated that the controllable sharp features and the self-assembly of Au@Ag NCs enabled better E-field enhancement and SERS activities.The single-point mutations of TDP-43 and IAPP8-37 proteins at a very low concentration(10 p M)were detected clearly for the first time.Related detailed research works are summarized as follows:1.Synthesis of Bimetallic Subten-Nanometer Ag@Pt with Ultrathin Pt Shell and Catalytic Activity towards Oxygen Reduction Reaction.The idea of core/shell structure is to improve the utilization of Pt atoms by depositing a thin Pt-based shell around a less expensive cores.To decrease Pt loading and keep Pt atoms to catalysis in maximum,coating ultrathin Pt layer on Ag nanocrysal is highly needed,especially size around 5 nm with good monodispersity.In such way,the Pt catalysts could be used efficiently in atomic level.In this work,highly monodispersed Ag@Pt core/shell nanoparticles with size of 5 nm have been successfully synthesized.Ag NPs capped with oleic acid have been synthesized on a large scale which is important for Pt shell growth.The introduction of certain quantity of ethanol and ascorbic acid solution promotes the phase transfer of 2-6Pt Cl ions to the hydrophobic Ag NPs surface leading to the formation of Ag@Pt core/shell structure.The addition of drops of oleylamine is critical in order to ensure the formation and stabilization of the ultrathin Pt shell.The formation and distribution of Pt atoms on the Ag NP surface was confirmed by HAADF-STEM characterization of arbitrarily chosen particle and its EDS elemental mapping.The theoretical calculations carried on one layer Pt shell proved that the experimental amount of Pt in 5 nm sized Ag@Pt corresponds closely to one atomic layer.The electrochemical test of the as-prepared Ag@Pt,without Carbon support shows that ORR likely occurs on the ultrathin Pt shell,in comparison to Pt/C support.Using 50 μg loading amount of Ag@Pt NPs without carbon support,48.4 μg/cm2 electrochemical active surface area,the potentials of ORR and limiting current are 0.54 V and – 1.6 m A/cm2 respectively.These values are less positive than that of commercial Pt/C 0.76 V,– 3.9 m A/cm2.However,these half wave potentials are more positive than that of Ag;hence this strategy provides high performance Pt catalysts with minimal Pt content and cost.2.Synthesis of Trimetallic Subten-Nanometer Au@Ag@Pt with Ultrathin Pt Shell and Catalytic Activity towards Oxygen Reduction Reaction.Despite great advances in the field of electrocatalysis,the controlled synthesis of NPs consisting of multiple noble metal components has been relatively unexplored in comparison to bimetallic NPs.Although they can provide new insights into the structure composition property relationships in noble metal NPs.They can also provide a deeper understanding of layer-by-layer engineering of fundamental electron and phonon coupling interactions in metal core/shell nanostructures.In this work,we demonstrated that trimetallic Au@Ag@Pt core/shell/shell nanostructures with a well-controlled thin Pt shell can be synthesized in a high yield in Water-Ethanol-Oleylamine(W-E-O)System.The epitaxial growth of silver shell onto Au NPs,layer by layer leads to the synthesis of Au@Ag Core/shell in large scale.By displacement reaction,an ultrathin Pt shell is deposited onto Au@Ag to produce Au@Ag@Pt trimetallic nanostructure.HAADF-STEM characterization of arbitrarily chosen particle and its EDS elemental mapping confirmed the formation of Au@Ag@Pt core/shell structure.In addition,the Pt composition in as-prepared 7 nm sized Au@Ag@Pt NPs is close to one atomic layer,varying in the range between less than one layer,and two layers.Using 50 μg loading amount of the as-prepared Au@Ag@Pt NPs without carbon support,43.36 μg/cm2 electrochemical active surface area,the potentials of ORR and limiting current are 0.56 V and – 2.0 m A/cm2 respectively.These half wave potentials values are more positive than that of silver.Hence,the as prepared Au@Ag@Pt core shell nanostructures exhibits also an important performance towards Oxygen Reduction Reaction.3.Controllable Synthesis of sharp featured Au@Ag Core/Shell Nanocuboids and the Label-free Ultrasensitive Protein Single-point Mutation SERS Detection.Highly monodispersed Au@Ag core/shell nanocuboids(NCs)with sharp features and different aspect ratios(AR)have been synthesized in aqueous phase.We exploited the possibility of using aromatic additives such as sodium salicylate,salicylic acid,sodium oleate to synthesize highly monodispersed gold nanorods with different aspect ratios,through seed mediated growth mechanism.The as-prepared gold nanorods,with a reduced amount of Cetyltrimethylammonium bromide (CTAB),benzyldimethylamonium chloride(BDAC)surfactants and ascorbic acid as reducing agent,gave a fine control over the growth kinetics of silver NPs;resulting in the formation of Au@Ag nanocuboids with sharp edges.Au@Ag core/shell NCs also can self-assembly on the substrate with a ca.2 nm nanogap between adjacent NCs.The sharp features and in-cuboid,out-of-cuboid Localized Surface Plasmon Resonance(LSPR)coupling of Au@Ag core/shell NCs enabled better E-field enhancement and better SERS activities.In addition,the Finite Element Method(FEM)simulation of E-field enhancement simulation have been carried out to reveal the Au@Ag core/shell NCs assemblies induced improved the E-field enhancement and LSPR properties.We demonstrated the application for label-free SERS detection for the Amyotrophic Lateral Sclerosis(ALS)related protein TAR DNA-binding Protein-43(TDP-43)and the core region of type II diabetes related protein islet amyloid polypeptide 8-37(IAPP8-37)under a low concentration(10 p M)for the first time.More significantly,we can distinct the wild-type TDP-43(namely Wt),and its single-point mutations: phosphorylated A315 T mutant(p A315T),A315 E mutant TDP-43(A315E)proteins,and human-IAPP8-37(h IAPP8-37),rat-IAPP8-37(r IAPP8-37)via label-free SERS with concentration of 10 p M,respectively.This is a big step forward for the low-concentration protein ultrasensitive and rapid detection.