Application Of Metal Nanostructure Materials In Surface Enhanced Spectroscopy

Metal nanostructures can produce strong surface plasmon resonance in visible and near infrared bands.The light field energy can be localized in the surface of the metal nanostructures,leading to enhanced electric field in the surface of the metal nanostructure.Then,the spectra intensity of the optical materials laid near to the metal surface can be dramatically enhanced.Metal nanostructure materials can enhance the intensity of fluorescence and Raman spectra substantially.The investigation of metal nanostructure materials becomes a research focus because it is meaningful and useful in the field of biochemical detection,food safety,environmental protection and photoelectric device.However,the problems in the metal nanostructures'fabrication process such as complex production process,high cost,poor stability limit the practical applications for the substrates.To solve the above problems,this dissertation develops studies on metallic nanostructure substrates with good universality,high sensitivity,good stability and low cost.Firstly,we investigated the related theory of surface enhanced spectra technology with metal.We fond that the coupling of multiple plasmon resonance models can lead to high spectra enhancement in wide light band and nanostructures with many“hot spots”can highly enhance spectra intensity.Based on this,we designed the flower-like silver nanostructure array substrate.In order to solve the problems of complex production process and high cost in the metal nanostructure's synthesis process,we proposed the method of electrodeposition process with liquid crystalline phase as the soft template combining self-assembly of silver irons to fabricate the flower-like silver nanostructures.Aerosol OT?AOT?is a amphiphilic molecule contains a hydrophilic group and two oleophilic groups.The hydrophilic group can connect with water and oleophilic groups can connect with oil.Due to the amphiphilic property of the AOT molecule,the lyotropic liquid crystal molecule template with column arrays can be formed in the system of AOT/p-xylene/water.The water phase was replaced with the AgNO₃ solution.The aqueous solution of silver nitrate was isolated in the channels due to self-assembly effect.When the lyotropic liquid crystal system was laid in the electrochemical device,the channels were aligned parallel to the electric field.Under the action of electric field,the silver irons in AgNO₃ solution were electrophoresis to the cathode and reduced to Ag cores in quantity.As the deposition time went by,the size of the Ag cores became larger and larger.When the Ag cores were large enough,they broke through the limitation of the lyotropic liquid crystal template and forming flower-like silver nanostructure array.The flower-like silver nanostructure arrays with 750 nm-1.9?m diameters were fabricated successfully by controlling the deposition time.Scanning electromn microscopy?SEM?photos show that the flower-like nanostructures were composed of many petals whose thickness is about 20-50 nm.The density of petals is larger than the structures reported before.Between the petals there are many horns and thin gaps.The specific surface area of the whole flower-like nanostructure is large,which is beneficial to the surface plasmon oscillation and improvement of the spectra enhancement effect.To investigate the surface enhanced fluorescence?SEF?effect of the flower-like silver nanostructure substrates,the experimental setups for SEF experiment was set up.The SEF of Rhodamine 6G?R6G?which is a biomarker was measured.The R6G was excited by a solid state laser with 532 nm wavelength.The R6G solution was filled into the gap between the glass and the silver flower-like nanostructure film coated subatrate which froms a sandwich structure.The measured spectra of R6G show that the best enhancement factor is 181 at 551 nm wavelength,which is improved 4 times than reported before.This substrate was used in organic distributed feedback lasers.The enhancement effect of poly?2-methoxy-5-?2'-ethyl-hexyloxy?-p-phenylenevinylene??MEH-PPV?solution was measured and the enhancement factor is 41 at 569 nm wavelength.The amplified spontaneous emission?ASE?of MEH-PPV is enhanced dramatically which means the reduced threshold and improved slope efficiency in organic lasers.We studied the distance–dependent property of SEF with flower-like silver nanostructure substrates we fabricated based on the reaction theory between fluorophore and metal nanostructure substrate.The distance between silver nanostructures and fluorophore was controlled strictly by the layer-by-layer?LbL?technology constructing alternate layer of poly?allylamine hydrochloride??PAH?and poly?sodium 4-styrenesulfonate??PSS?.The fluorescence intensities were measured.The best enhanced fluorescence intensity is obtained at a distance of 5.2 nm.To investigate the surface enhanced Raman scattering?SERS?effect of the flower-like silver nanostructure substrates,we measured the SERS spectra of 4-mercaptobenzoic acid?4-MBA?which is a biomarker and melamine.The excitation wavelength is 785 nm.The best SERS enhancement factor of 1.17×10⁷ was achieved with the flower-like nanostructure substrate.The limit of detection is in 10^-1ppm magnitude which is lower than the minimum standard stated.Meanwhile,we also verified the applicability for different molecules'detection of the flower-like silver nanostructures with wide localized surface plasmon resonance property.Besides,the chemical stability of the flower-like silver nanostructures were studied by taking the nanostructures react with H₂O₂ solution for different time.The localized surface plasmon resonance spectrum of the silver flower nanostructures did nearly not change after reacting with H₂O₂ solution for 30 min,while the localized surface plasmon resonance spectrum of the silver nanocubes have also disappeared with the same reacting with H₂O₂ solution for 3 min,which shows that the chemical stability of the flower-like nanostructures is better than the nanocubes.But,when the flower-like nanostructure substrates were laid in air for several days,the flower-like nanostructures were oxidized to yellow.In order to improve the chemical stability and spectra enhancement effect of the flower-like nanostructures,we studied the plasmon resonance properties of the metal with good chemical stability and designed the flower-like 3D Ag-Au hetero-nanostructures.Based on the galvanic reaction between elemental Ag and HAuCl₄,we chose the flower-like silver nanostructures for 5 h growth time to react with HAuCl₄.The flower-like Ag-Au hetero-nanostructures with different morphologies and Au:Ag ratios were obtained by taking Ag flower to react with different concentration of HAuCl₄ for different time.We measured the SERS enhancement effects of different flower-like Ag-Au nanostructure substrates,the best SERS enhancement effect is 8.6×10⁷,which is 8 times larger than the flower-like Ag nanostructure substrate.We studied the chemical stability of the samples.The SERS enhancement effect with flower-like silver nanostructures reacting with H₂O₂ for 24 h became zero,while the SERS enhancement factor with flower-like Ag-Au nanostructures reacting with H₂O₂for 24 h was also 10⁴.Results show that the chemical stability of flower-like Ag-Au nanostructures is markedly improved compared with the Ag ones.In this dissertation,the fabrication,localized surface plasmon resonance properties and application in surface enhanced spectroscopy technology of flower-like silver nanostructures and flower-like Ag-Au nanostructures were studied.The nanostructure fabrication process is simple and cost-effective.The wide localized surface plasmon resonance properties of the flower-like nanostructures improved the universality for different molecules'detection.The sensitivity and chemical stability are improved,which lay a solid foundation for the practical process of surface enhanced spectra technology.