Harnessing the optical properties of gold nanorods: fluorescence enhancement, biosensing and photothermal therapy
In recent years, there has been great interest in the synthesis, characterization, and application of gold nanomaterials, especially gold nanorods. Due to their small size, ease of functionalization, and uniquely tunable optical properties, gold nanorods have potential application in solar cells, sensing, bioimaging, drug delivery, and cancer therapy. Excitation of a gold nanorod localized surface plasmon resonance (LSPR) can result in the enhancement of many photophysical processes such as light absorption, Rayleigh (Mie) scattering, and fluorescence. The presence of a strong electromagnetic field has been observed to enhance spectroscopic molecular signals from fluorophores, Raman-active molecules, and two-photon chromophores bound to or near the metal surface. Additionally, strong light absorption and nonradiative dissipation of absorbed energy allows for the use of gold nanorods in photothermal heating applications. Optical applications such as these may be realized by carefully engineering the size, shape, surface chemistry of gold nanorods. The focus of this dissertation is in exploring how gold nanorod surface properties can be modified for various optical applications. In each chapter, gold nanorods are prepared, the surface coating is modified, and the materials are characterized by a variety of methods. This thesis seeks to demonstrate how these materials may be useful, with a focus on three particular applications: fluorescence enhancement, biosensing and photothermal therapy. Chapter 1 is an introduction to gold nanorods and contains a thesis overview. The unique optical properties, which are the basis for most applications, are discussed. This is followed by a general overview of the possible optical applications of gold nanorods. The history of advancements in gold nanorod synthesis and techniques for surface modification are described. There is a particular focus on the surface modification techniques most often used in this thesis, including thiol functionalization, silica coating and layer-by-layer wrapping of polyelectrolytes. Methods for characterization of gold nanorods, and their surface coatings are described, and the chapter ends with a dissertation overview. Plasmonic nanoparticles can strongly interact with adjacent fluorophores, resulting in plasmon-enhanced fluorescence or fluorescence quenching. Chapter 2 explores how fluorescence behavior is altered near a gold nanorod surface. Fluorescence coupling is dependent upon nanoparticle composition, the distance between the fluorophore and the plasmonic surface, the transition dipole orientation, and the degree of spectral overlap between the fluorophore’s absorbance/emission and the surface plasmon band of the nanoparticles. We examine the distance and plasmon wavelength dependent fluorescence of an infrared dye (“IRDye”) bound to silica-coated gold nanorods. Nanorods with plasmon band maxima ranging from 530 to 850 nm are synthesized and then coated with mesoporous silica shells 11–26 nm thick. IRDye is covalently attached to the nanoparticle surface via a click reaction. Steady-state fluorescence measurements demonstrate plasmon wavelength and silica shell thickness dependent fluorescence emission. Maximum fluorescence intensity, with approximately 10-fold enhancement, is observed with 17 nm shells when the nanorod plasmon maximum is resonant with IRDye absorption. Time-resolved photoluminescence reveals multi-exponential decay and a sharp reduction in fluorescence lifetime with decreasing silica shell thickness, and when the plasmon maximum is closer to IRDye absorption/emission. Control experiments are carried out to confirm that the observed changes in fluorescence are due to plasmonic interactions, not simply surface attachment. There is no change in fluorescence intensity or lifetime when IRDye is bound to mesoporous silica nanoparticles. In addition, IRDye loading is limited to maintain a distance between dye molecules on the surface to more than 9 nm, well above the Förster radius. This assures minimal dye–dye interactions on the surface of the nanoparticles. The interface between nanoparticles and bacterial surfaces is of great interest for applications in nanomedicine and food safety. In Chapter 3, we investigate how nanoparticles might interact with bacteria by monitoring the binding of bacterial lipopolysaccharides to gold nanorods. We demonstrate that interactions between gold nanorods and lipopolysaccharides are governed by the nanoparticle surface coating. Polymer-coated gold nanorod substrates are exposed to lipopolysaccharides extracted from Pseudomonas aeruginosa, Salmonella enterica and Escherichia coli, and attachment is monitored using localized surface plasmon resonance refractometric sensing. The number of lipopolysaccharide molecules attached per nanorod is calculated from the shift in the plasmon maximum, which results from the change in refractive index after analyte binding. Colloidal gold nanorods in water are also incubated with lipopolysaccharides to demonstrate the effect of lipopolysaccharide concentration on plasmon shift, ζ-potential, and association constant. Both gold nanorod surface charge and surface chemistry affect gold nanorod–lipopolysaccharide interactions. In general, anionic lipopolysaccharides are found to attach more effectively to cationic gold nanorods than to neutral or anionic gold nanorods. Some variation in lipopolysaccharide attachment is also observed between the three strains studied, demonstrating the potential complexity of bacteria–nanoparticle interactions. In recent years, there has been a great deal of interest in the preparation and application of nanoparticles for cancer therapy. Chapter 4 reviews the progress in thermal cancer treatments using gold nanoparticles. Gold nanoparticles are especially suited to thermal destruction of cancer due to their ease of surface functionalization and photothermal heating ability. We begin with an introduction to the properties of gold nanoparticles and heat-generating mechanisms which have been established. The pioneering work in photothermal therapy is discussed along with the effects of photothermal heating on cells in vitro. Additionally, radiofrequency-mediated thermal therapy is reviewed. We focus the discussion on the developments and progress in nanoparticle design for photothermal cancer therapy since 2010. This includes in vitro and in vivo studies, and the recent progression of gold nanoparticle photothermal therapy toward clinical cancer treatment. The chapter concludes with a perspective on the prospects of commercial application of photothermal-mediated cancer therapy with gold nanoparticles Chapter 5 expands the range of photothermal therapy applications to inactivation of vegetative cells and endospores of the bacterium Geobacillus stearothermophilus. Gold nanorods are prepared and characterized and are coated with four different neutral or charged polymers to investigate the impact of surface charge on cell attachment and inactivation. The effects of gold nanorod exposure and photothermal heating using a 785 nm laser on colony growth of spores and vegetative cells reveal greater reductions in colony formation with charged nanorods. Additionally, spore morphology is examined before and after treatment. There are small changes in morphology observed as increasing area per spore and decreased spore aspect ratio which might be correlated with inactivation. Although the inactivation of endospores is not as great as traditional sterilization techniques, these results demonstrate that there is potential in the application of photothermal heating with gold nanorods to inactivate heat-resistant bacterial endospores. In Chapter 6 we consider how gold nanorod surface chemistry may be modified by silica coating and silane functionalization to maintain optical stability and therefore increase the effectiveness of gold nanorods in optical applications. Heating in an oven to 150°C, and charging due to electron beam exposure cause shortening and widening of gold nanorods and result in decreased in aspect ratio. However, these changes in morphology and optical properties are greatly reduced by silica coating and silane functionalization. Pulsed laser irradiation also is found to alter gold nanorod optical properties, and interesting changes in gold nanorod morphology are observed. Together, these results suggest that silica coating and silane functionalization improve the shape stability of gold nanorods and therefore may help to preserve the optical properties, especially compared to CTAB gold nanorods. The increased prevalence of functionalized nanomaterials in a range of applications will inevitably lead to nanomaterial contamination of soil and groundwater. Chapter 7 moves past nanotechnology applications to consider the environmental fate of gold nanomaterials. We investigate how nanoparticle shape and surface chemistry influences their stability and transport within environmental systems. A library of spherical and rod-shaped gold nanoparticles is prepared with different surface chemistries. Nanoparticle stability against aggregation in simulated groundwater is investigated. The stability of gold nanoparticle probes in simulated groundwater depends on both the surface charge imparted by the capping agent, and the nature of the interaction between the nanoparticle surface and the capping agent. However, in the presence of natural organic matter, gold nanoparticles are found to form heteroaggregates, regardless of the initial surface coating. In addition, nanoparticle retention in columns of soil and alginate is quantified. The surface charge and capping agent interaction also influence retention of functionalized nanoparticles. Negatively-charged gold nanoparticles are only weakly retained in soil and alginate, and hence, are potentially much more mobile in environmental matrices than nanoparticles carrying positive surface charges. Together, these data suggest that the environmental fate of nanoparticles is strongly influenced by their surface chemistry, as well as core material and size.
Gold nanorods, Applications, Silica, Fluorescence, Biosensing, Photothermal
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign