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School of Biological Sciences

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School of Earth and Atmospheric Sciences

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School of Mathematics

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School of Physics

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Some optical and catalytic properties of metal nanoparticles

(Georgia Institute of Technology, 2009-08-20) Tabor, Christopher Eugene

The strong electromagnetic field that is induced at the surface of a plasmonic nanoparticle can be utilized for many important applications, including spectroscopic enhancement and electromagnetic waveguides. The focus of this thesis is to study some of the properties of induced plasmonic fields around metal nanoparticles. Current methodologies for fabricating nanoparticles are discussed, including lithography and colloidal synthesis. This dissertation includes studies on plasmonic driven nanoparticle motion of surface supported gold nanoprisms from a substrate into solution via a femtosecond pulse. The mechanism of particle motion is discussed and the stability of the unprotected nanoprisms in solution is studied. Fundamental plasmonic near-field coupling between two plasmonic nanoparticles is also examined. Experimental results using electron beam lithography fabricated samples are used to explicitly describe the plasmonic coupling between dimers as a function of the nanoparticle size, shape, and orientation. These variables are systematically studied and the dependence is compared to mathematically derived functional dependencies in order to model and predict the effects of plasmonic coupling. As an extension, the coupling between plasmonic nanoparticles is shown in a common application, surface enhanced Raman scattering. The final chapter is devoted to an investigation of the nature of nanocatalysis, homogeneous and heterogeneous, for several reactions using metal nanoparticles.
Plasmonic field effects on the spectroscopic and photobiological function of the photosynthetic system of bacteriorhodopsin

(Georgia Institute of Technology, 2009-03-06) Biesso, Arianna

The first section of this thesis concerns the study of interactions between the intense local plasmonic field generated by nanostructure and a well known photosynthetic protein system, bacteriorhodopsin (bR). bR is a membrane protein responsible for proton transport. Among the many intermediates formed upon photoexcitations, two of the most relevant have been studied. The intermediates under studies were I460 and M412, and their decay dynamics were measured in presence of the plasmonic field generated by the excitation of their surface electrons using visible photons. Both intermediates decay lifetime were affected when the plasmonic field was turned on, and it was verify that thermal effect were not the source of the change in dynamic. The second part concerns the investigation of third-order nonlinearity of a series of extended conjugated squaraine dyes in the telecommunication spectral region. Their nonlinearity is measured via Degenerate Four Wave Mixing and Z-scan as function of the dyes increasing conjugation length and number of squarylium groups. The dyes produced large real and imaginary values for the third order nonlinearity in the 1300-1500 nm range which makes them attractive for optical limiting type of applications.
Plasmons in assembled metal nanostructures: radiative and nonradiative properties, near-field coupling and its universal scaling behavior

(Georgia Institute of Technology, 2008-01-10) Jain, Prashant K.

Noble metal nanostructures possess unique properties including large near-field enhancement and strong light scattering and absorption due to their plasmon resonance - the collective coherent oscillation of the metal free electrons in resonance with the electromagnetic field of light. The effect of nanostructure size, shape, composition, and environment on the plasmon resonance frequency and plasmonic enhancement is well known. In this thesis, we describe the effect of inter-particle coupling in assembled plasmonic nanostructures on their radiative and non-radiative properties. When metal nanoparticles assemble, plasmon oscillations of neighboring particles couple, resulting in a shift in the plasmon resonance frequency. Our investigation of plasmon coupling in gold nanorods shows that the coupling between the plasmons is "bonding" in nature when the plasmon oscillations are polarized along the inter-particle axis, whereas an "anti-bonding" interaction results when the polarization is perpendicular. We studied the distance-dependence of plasmon coupling using electrodynamic simulations and experimental plasmon resonances of lithographically fabricated gold nanoparticle pairs with systematically varying inter-particle separations. The strength of plasmon bonding, reflected by the fractional plasmon shift, decays near-exponentially with the inter-particle separation (in units of particle size) according to a universal trend independent of the nanoparticle size, shape, metal type, or medium. From the universal scaling model, we obtain a "plasmon ruler equation" which calculates (in good agreement with the experiments of Alivisatos and Liphardt) the inter-particle separation in a gold nanosphere pair from its plasmon resonance shift, making it applicable to the determination of inter-site distances in biological systems. Universal size-scaling is valid also in the metal nanoshell structure, a nanosphere trimer, and pairs of elongated nanoparticles, thus making it a generalized fundamental model, which is useful in optimizing plasmon coupling for achieving tunable plasmon resonances, enhanced plasmonic sensitivities, and large SERS cross-sections. Ultrafast laser pump-probe studies of non-radiative electronic relaxation in coupled metal nanospheres in aggregates and in gold nanospheres conjugated to thiol SAMs are also reported. We also show that the relative contribution of scattering (radiative) to absorption (non-radiative) part of the plasmon relaxation, respectively useful in optical and photothermal applications, can be increased by increasing the nanostructure size.
Fundamental studies of the interaction between femtosecond laser and patterned monolayer plasmonic nanostructures

(Georgia Institute of Technology, 2007-07-09) Huang, Wenyu

This dissertation is focused on the interaction between femtosecond laser and patterned two-dimensional gold nanostructures. The sample was prepared by two different lithographic techniques, the nanosphere lithography and the electron beam lithography. Characterization was carried out with scanning electron microscopy, transmission electron microscopy, atomic force microscopy, and UV-vis absorption spectroscopy. Femtosecond transient absorption spectroscopy was used to answer a number of fundamental questions regarding the laser-nanostructure interaction. Under a low density irradiation of a femtosecond laser, we examined the effect of the lattice crystallinity on the electron-phonon relaxation in monolayer periodic array nanoparticles prepared with nanosphere lithography. We found that the electron-phonon relaxation rate was faster in polycrystalline nanoparticles and decreases greatly in single crystalline nanospheres, which is explained by the presence of high density grain boundaries. The ultrafast laser-induced coherent phonon oscillations in patterned gold nanoparticles are also fully characterized. We studied the effect of size, shape, thickness, monitoring wavelength, and materials of the prismatic array nanoparticles on the period of their coherent phonon oscillations. In a gold nanodisk pair system, we found that the fractional change in the vibration frequency increases exponentially with decreasing the ratio of the interparticle separation to the particle diameter, which is explained by the coupling of the induced electric field in one nanodisk by the strong surface plasmon field of its pair partner. Based on the coherent phonon oscillation of gold caps on a polystyrene sphere monolayer array, a new all-optical gigahertz modulation technique is developed. Under a high density irradiation of a femtosecond laser, the melting and ablation processes can be induced in gold nanoparticles. We studied femtosecond laser induced shape and localized surface plasmon resonance band changes of gold prismatic array nanoparticles. We also observed that the femtosecond laser irradiation of the nanoprisms at the surface plasmon resonance absorption maximum can cause them to detach from the substrate and 'fly away'. Atomic force microscopy and scanning electron microscopy measurements revealed that the displaced nanoparticles are thinner and smaller than the undisplaced ones, which supports an atomic ablation mechanism.
Surface effects on the ultrafast electronic relaxation of some semiconductor and metallic nanoparticles

(Georgia Institute of Technology, 2006-06-28) Darugar, Qusai A.

The research presented has been focused on understanding the surface effects on the optical and electronic properties of some metallic and semiconductor nanomaterials. When the particle sizes are on the nanometer length scale, a large fraction of atoms in the particles are on the surface. The bonding of the surface atoms being unsaturated could cause trapping and introduce defects that interact with the excited electrons. The effect of the surface on the optical and electronic properties of some semiconductor and metallic nanoparticles is investigated. When the size and shape of nanomaterials change, both the electron density of the excited electrons on the surface and the electronic structure change. Therefore, it becomes important to understand how these changes affect the electronic motion in the particles in order to exploit their full potential in a variety of applications. Semiconductor nanoparticles studied include cadmium selenide (CdSe) and cadmium sulfide (CdS). Effect of changing CdSe shape and size on optical and electronic properties has been investigated and the ability for the CdS nanoparticles to show optical gain (stimulated emission) in solution at room temperature is reported. Effect of surface phonon contribution on the exited electron relaxation in copper nanoparticles is investigated. For the particles size smaller than the mean free path of the electrons in the metal, electron-surface phonon coupling becomes an important factor (contribution) for hot electron relaxation. In the thesis presented, it is shown for the first time the size depended electronic relaxation in copper nanoparticles. Fluorescence due to surface plasmon field enhancement is observed for copper nanoparticles to be million times stronger than the fluorescence observed from bulk copper.
Interesting Electronic and Dynamic Properties of Quantum Dot Quantum Wells and other Semiconductor Nanocrystal Heterostructures

(Georgia Institute of Technology, 2006-06-01) Schill, Alexander Wilhem

Some interesting electronic and dynamic properties of semiconductor nanocrystal heterostructures have been investigated using various spectroscopic methods. Semiconductor nanocrystal heterostructures were prepared using colloidal synthesis techniques. Ultrafast transient absorption spectroscopy was used to monitor the relaxation of hot electrons in CdS/HgS/CdS quantum dot quantum wells. Careful analysis of the hot electron relaxation in CdS/HgS/CdS quantum dot quantum wells reveals an energy dependent relaxation mechanism involving electronic states of varying CdS and HgS composition. The composition of the electronic states, combined with the layered structure of the nanocrystal permits the assignment of CdS localized and HgS localized excited states. The dynamic effect of surface passivation is then shown to have the strongest influence on excited states that are localized in the HgS layer. New quantum dot quantum well heterostructures of different sizes and compositions were also prepared and studied. The dynamic properties of CdS/CdSe/CdS colloidal quantum wells suggest simultaneous relaxation of excited electrons within the CdS core and CdSe shell on the sub-picosecond time scale. Despite the very different electronic structure of CdS/CdSe/CdS compared to CdS/HgS/CdS, the time scales of the relaxation and electron localization were very similar. Enhancement of trap luminescence was observed when CdS quantum dots were coated with silver. The mechanism of the enhancement was investigated using time-resolved spectroscopic techniques.
Gold and Silver Nanoparticles: Characterization of their Interesting Optical Properties and the Mechanism of their Photochemical Formation

(Georgia Institute of Technology, 2006-05-30) Eustis, Susie

A new method is developed referred to as Gold Nanorod Optical Modeling Equations (GNOME) for determining the average aspect ratio of gold nanorods in solution. In this method, the observed inhomogeneously broadened optical spectrum is fitted to a number of calculated homogeneously broadened spectra with different aspect ratios having different contributions. From this method, the average aspect ratio is determined. This is a more accurate than the presently used method of TEM. The surface plasmon enhanced fluorescence spectra of gold nanorods are calculated as a function of the aspect ratio and compared to experimental spectra. In this calculation, the inclusion of both the aspect ratio distribution calculated from the GNOME method as well as the incorporation of the intrinsic fluorescence of bulk gold are found necessary to model the enhanced fluorescence spectrum of gold nanorods using previously published equations. The enhanced spectrum decreases rapidly as the aspect ratio increases and the surface plasmon band shift away from the gold interband absorption. Photochemical methods are used to synthesize silver nanoparticles on silica surfaces and gold nanoparticles in solution. The formation silver nanoparticles utilizes benzophenone as a photosensitizing agent to initiate the reaction. The effects of the light source and irradiation time are investigated. The presence of different forms of silica are investigated in the formation of metal nanoparticles. This method produced silver nanoparticles on silica that can be in the form of film or powder that are useful in heterogeneous catalysis. Direct photochemical methods are applied to generate gold nanoparticles from chloroauoroic acid in ethylene glycol in the presence of polyvinylpyrrolidone as a capping material. A detailed mechanism of the formation of the gold nanoparticle is determined. This is done by following the kinetics of formation of the gold nanoparticles after irradiation under different conditions. The disproportionation of the gold ions as well as their reduction by ethylene glycol is found to be important in the formation of the nanoparticles. Photochemical synthesis provides room temperature techniques to generate metal nanoparticles in a variety of environments.
Gold Nanoparticles Used in Cancer Cell Diagnostics, Selective Photothermal Therapy and Catalysis of NADH Oxidation Reaction

(Georgia Institute of Technology, 2006-04-12) Huang, Xiaohua

Gold nanoparticles strongly absorb and scatter visible and near infrared light because of the strongly enhanced electric fields at the surface. This provides the potential of designing novel optically active reagents for simultaneous molecular imaging and photothermal cancer therapy. In this thesis, gold nanospheres and nanorods conjugated with anti-epidermal growth factor receptor (anti-EGFR) antibodies that specifically target EGFR on the cell surface are shown to be used for dual diagnostics and therapy. Using micro-absorption spectroscopy and light scattering imaging, cancerous (HOC 313 and HSC 3) and noncancerous cells (HaCat) can be differentiated due to the overexpression of EGFR on the surface of cancer cells. By irradiating the cells with a CW laser, selective photothermal cancer therapy is realized in visible region by using gold nanospheres and in near infrared region by using gold nanorods. The use of nanorods allow for in vivo therapy due to the fact that their absorption is in the near infrared region at which the laser light meets less interference from the tissue absorption. In addition, the catalytic effect of gold nanoparticles on the oxidization of NADH to NAD+ is investigated. The addition of gold nanoparticles is found to quench the NADH fluorescence intensities but has no effect on the fluorescence lifetime. This suggests that the fluorescence quenching is not due to coupling with the excited state, but due to changing the ground state of NADH. The intensity of the 340 nm absorption band of NADH is found to decrease while that of the 260 nm band of NAD+ is found to increase as the concentration of gold nanoparticles increase. This conversion reaction is further supported by nuclear magnetic resonance and mass spectroscopy. The linear relationship between the initial reaction rate of NADH and the concentration of gold nanoparticles strongly supports that NADH is surface catalyzed by the gold nanoparticles. The catalytic property of this important reaction might have important future applications in biological and medical fields.
Application of Spectroscopy to Protein Characterization

(Georgia Institute of Technology, 2005-11-11) Sanii, Laurie Shireen

There are two contributions of this thesis. The first contribution, described in chapters one through six, involves studing the relationship between the protein packing structure of bacteriorhodopsin (bR) and its function as a proton pump. In 2002, a novel crystallization method published by Bowie and Farham resulted in an unusual antiparallel monomeric packing structure of bicelle bacteriorhodopsin (bcbR) crystals, the spectroscopic properties of which had not been studied. In this thesis, these bicelle bR crystals are investigated to better understand how the changes in the protein tertiary structure affect the function. Specifically: Does the retinal Schiff base retain its ability to isomerize in this unusual protein packing structure of bR? How is the hydration of its binding pocket affected? Does the protein retain the ability to undergo the photocycle and pump protons? If so, how are the rates of the deprotonation/reprotonation of the Schiff base affected by the antiparallel monomer packing structure of the protein? Is Asp85 still the proton acceptor during the deprotonation process of the photocycle? The second contribution of the thesis, described in chapter seven, describes the surface attachment and growth of the biofilm formed by the pathogenic bacterium Streptococcus pneumoniae using attenuated total reflection/Fourier transform infrared spectroscopy (ATR/FTIR). This organism was chosen for its clinical significance; it is one of the organisms suspected in forming biofilms in individuals who develop otitis media, one of the most common causes of ear infections of childhood. In contrast to previous ATR/FTIR experiments examining the formation of biofilms on surfaces, this method is unique in that it combines two techniques - ATR/FTIR and Epifluorescence microscopy which when used together allow for the simultaneous monitoring of the IR spectrum of the S. pneumoniae biofilm as it develops and as provides a method for quantifying total and viable cell counts at various stages during the development.
Shape-Dependent Nanocatalysis and the Effect of Catalysis on the Shape and Size of Colloidal Metal Nanoparticles

(Georgia Institute of Technology, 2005-03-30) Narayanan, Radha

From catalytic studies in surface science, it has been shown that the catalytic activity is dependent on the type of metal facet used. Nanocrystals of different shapes have different facets. This raises the possibility that the use of metal nanoparticles of different shapes could catalyze different reactions with different efficiencies. The catalytic activity is found to correlate with the fraction of surface atoms located on the corners and edges of the tetrahedral, cubic, and spherical platinum nanoparticles. It is observed that for nanoparticles of comparable size, the tetrahedral nanoparticles have the highest fraction of surface atoms located on the corners and edges and also have the lowest activation energy, making them the most catalytically active. Nanoparticles have a high surface-to-volume ratio, which makes them attractive to use compared to bulk catalytic materials. However, their surface atoms are also very active due to their high surface energy. As a result, it is possible that the surface atoms are so active that their size and shape could change during the course of their catalytic function. It is found that dissolution of corner and edge atoms occurs for both the tetrahedral and cubic platinum nanoparticles during the full course of the mild electron transfer reaction and that there is a corresponding change in the activation energy in which both kinds of nanoparticles strive to behave like spherical nanoparticles. When spherical palladium nanoparticles are used as catalysts for the Suzuki reaction, it is found that the nanoparticles grow larger after the first cycle of the reaction due to the Ostwald ripening process since it is a relatively harsh reaction due to the need to reflux the reaction mixture for 12 hours at 100 oC. When the tetrahedral Pt nanoparticles are used to catalyze this reaction, the tetrahedral nanoparticles transform to spherical ones, which grow larger during the second cycle. In addition, studies on the effect of the individual reactant have also provided clues to the surface catalytic process that is taking place. In the case of the electron transfer reaction, the surface catalytic process involves the thiosulfate ions binding to the nanoparticle surface and reacting with the hexacyanoferrate (III) ions in solution. In the case of the Suzuki reaction, the surface catalytic mechanism of the Suzuki reaction involves the phenylboronic acid binding to the nanoparticle surface and reacting with iodobenzene via collisional processes.