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Synthesis and characterization of novel nanomaterials :
b gold nanoshells with organic-inorganic hybrid cores
h [electronic resource] /
by Alisha Peterson.
[Tampa, Fla] :
University of South Florida,
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Thesis (MSES)--University of South Florida, 2010.
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ABSTRACT: Gold nanoshells, a material generally composed of a core of silica surrounded by a thin shell of gold, are of great interest due to their unique and tunable optical properties. By varying the shell thickness and core size, the absorption and scattering properties are greatly enhanced. The nanoshells can be made to absorb or scatter light at various regions across the electromagnetic spectrum, from visible to the near infrared. The ability to tune the optical properties of nanoshells allows for their potential use in many different areas of research such as optical imaging, tumor ablation, drug delivery, and solar energy conversion. The research in this thesis focused on the synthesis and characterization of two novel gold nanoshell materials containing thermally-responsive, organic-inorganic hybrid layers. One type of material was based on a two-layer particle with a thermally responsive hybrid core of N-isopropylacrylamide (NIPAM)copolymerized with 3-(trimethoxysilyl)propyl methacrylate (MPS) that was then coated with a thin layer of gold. The second material was a three-layer particle with a silica core, a thermally responsive copolymer of NIPAM and MPS middle layer and an outer shell of gold. Various techniques were used to characterize both materials. Transmission electron microscopy (TEM) was used to image the particles and dynamic light scattering(DLS) was used to determine particle size and the temperature response. Additionally,UV-Vis spectroscopy was used to characterize the optical properties as a function of temperature.
Advisor: Vinay K. Gupta, Ph.D.
x Chemical & Biomedical Engineering
t USF Electronic Theses and Dissertations.
Synthesis and Characterization of Novel Nanom aterials: Gold Nanoshells with OrganicInorganic Hybrid Cores by Alisha D. Peterson A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Engineering Science Department of Chemical a nd Biomedical Engineering College of Engineering University of South Florida Major Professor: Vinay K. Gupta, Ph.D. John T. Wolan, Ph.D. Mark Jaroszeski, Ph.D. Date of Approval: June 23, 2010 Keywords: thermally-responsive, poly N-is opropylacrylamide, optical absorption, metallic, multilayered nanomaterial Copyright 2010, Alisha D. Peterson
DEDICATION This thesis is dedicated to my entire loving and supportive family. To the influential women in my life: My grandmother whose streng th encourages me daily My mother whose guidance keeps me moving forward in a positive direction & My sister whose constant encouragement helps me to reach all of my goals
ACKNOWLEDGEMENTS I would first like to thank my advisor, Dr. Vinay Gupta, for his guidance, support and patients over the past two years. I woul d like to express grat itude to my committee members, Dr. John Wolan and Dr. Mark Jaroszes ki, who have played an important role in my success as a graduate student. I would also like to thank my lab partners who have all helped me succeed in one way or another: Mr. Bijith Mankidy, Ms. Kristina Tran, and Ms. Fedena Fanord. Special thanks to Mrs. Yvonne Williams, Dr. Shekhar Bhansali and Mr. Bernard Batson for their constant support and guidance. Lastly, I would like to acknowledge the NSF Florida-Ge orgia Louis Stokes Alliance for Minority Participants (HRD#0929435) for financial support throughout my graduate career.
i TABLE OF CONTENTS LIST OF FIGURES ........................................................................................................... iii ABSTRACT .................................................................................................................... v i CHAPTER 1: INTRODUC TION AND BACKGROUND .................................................1 1.1 Traditional Meta llic Nanoshells .........................................................................1 1.1.1 Synthesis of Traditional Metallic Nanoshells .....................................3 1.1.2 Optical Properties of Tr aditional Metallic Nanoshells .......................7 1.1.3 Applications of Gold Nanoshells ........................................................8 1.2 Project Summary ..............................................................................................10 CHAPTER 2: SYNTHESIS AND CHAR ACTERIZATION OF TRADITIONAL GOLD NANOSHELLS ...................................................................................14 2.1 Synthesis and Functiona lization of Stber Silica.............................................14 2.1.1 Experimental .....................................................................................14 2.1.2 Results and Discussion .....................................................................16 2.2 Gold Nanoshells Using De position Precipitation Method ...............................17 2.2.1 Experimental .....................................................................................17 2.2.2 Results and Discussion .....................................................................19 CHAPTER 3: SYNTHESIS AND CHAR ACTERIZTION OF TWO LAYERED HYBRID CORE GOLD NANOSHELLS .......................................................27 3.1 Synthesis of PNIPAM-Siloxa ne Hybrid Core Particles ...................................27 3.1.1 Experimental .....................................................................................27 3.1.2 Results and Discussion .....................................................................29 3.2 Gold Seeding Using DP and LBL and Nanoshell Growth ..............................29 3.2.1 Experimental .....................................................................................30 3.2.2 Results and Discussion .....................................................................33 CHAPTER 4: SYNTHESIS AND C HARACTERIZATION OF THREE LAYERED SILICA CORE GOLD NANOSHELLS ......................................46 4.1 Synthesis and Functiona lization of Stber Silica.............................................46 4.1.1 Experimental .....................................................................................46 4.1.2 Results and Discussion .....................................................................48 4.2 Polymerization of NIPAM in the Presence of Stber Silica ............................48 4.2.1 Experimental .....................................................................................49 4.2.2 Results and Discussion .....................................................................50 4.3 Gold Seeding and Shell Growth on PNIPAM Coated Silica ...........................51
ii 4.3.1 Experimental .....................................................................................52 4.3.2 Results and Discussion .....................................................................53 CHAPTER 5: SUMMARY OF RESEARCH....................................................................72 REFERENCES ..................................................................................................................75
iii LIST OF FIGURES Figure 1.1: Schematic of the LBL synthesi s of traditional gold nanoshells. .....................11 Figure 1.2: Synthesis scheme fo r Stber silica particles. ...................................................12 Figure 1.3: Electronic charge displacemen t of gold nanoparticles (GNP) (top) compared to gold nanoshells (bottom) due to the electric field of incident radiation. .........................................................................................13 Figure 2.1: DLS measurements of large ba re silica particles (~350nm) compared to small bare silica particles (~190nm). .........................................................21 Figure 2.2: TEM images of bare silica particles with a m ean size ~190nm. .....................22 Figure 2.3: FTIR spectrum of bare silica (t op) compared to IR spectrum of amine functionalized (bottom) silica particles. .........................................................23 Figure 2.4: TEM images of gold seeded silica particles. ...................................................24 Figure 2.5: TEM images of gold nanoshells w ith different K-Gold to seed ratios. ...........25 Figure 2.6: UV-vis spectra of gold nanoshe lls corresponding to the ratios 250:1, 300:1, and 400:1 .............................................................................................26 Figure 3.1: Variation of the hydrodynamic diameter of PNIPAM-siloxane hybrid particles as a function of temperature. ............................................................36 Figure 3.2: TEM images of PNIP AM-siloxane hybrid cores. ...........................................37 Figure 3.3: DLS size distri bution plots for gold nanopa rticles shown as volume scattering (top) and intens ity scattering (bottom). ..........................................38 Figure 3.4: UV-vis spectrum of fres hly prepared gold nanoparticles ................................39 Figure 3.5: TEM images of PNIPAM-siloxa ne hybrid particle gold seeded with the DP method. ...............................................................................................40 Figure 3.6: UV-vis spectra of fresh gold nanoparticles vs. aged gold nanoparticles .........41
iv Figure 3.7: TEM images of PNIPAM-siloxane hybrid particles seeded using aged gold nanoparticles (left) and fres hly prepared gold nanoparticles (right). .............................................................................................................42 Figure 3.8: TEM of PNIPAM-siloxane hybrid gold nanoshells (50: 1) at different intervals of the shell growth: 7 inj ections (top left), 9 injections (top right), 12 injections (middle left), 15 injections (middle right), 20 injections (bottom). ........................................................................................43 Figure 3.9: UV-vis spectra of PNIPAM-s iloxane gold nanoshells at various intervals of shell growth as indicated in the legend. .....................................44 Figure 3.10: UV-vis spectra of PNIPAM-siloxa ne particles in solution as it cools from an initial T=55oC in 5 minute increments. ...........................................45 Figure 4.1: Schematic synthesis of three layered gold nanoshells.....................................57 Figure 4.2: FTIR spectrum of bare silica pa rticles (top) compared to MPS grafted silica particles (bottom). .................................................................................58 Figure 4.3: TEM images of bare silica pa rticles (left column) compared to MPS grafted silica particles (right column). ...........................................................59 Figure 4.4: DLS measurements showing in tensity scattering (top) and volume distribution (bottom) of bare silica particles. .................................................60 Figure 4.5: FTIR spectrum of bare silica particles (top) compared to PNIPAM coated silica particles (bottom). ......................................................................61 Figure 4.6: DLS distribution graph of blank silica compared to PNIPAM coated silica particles: in tensity (top) and volume (bottom). .....................................62 Figure 4.7: Variation of hydrodynamic diameter from DLS of PNIPAM coated silica particles as a function of temperature. .................................................63 Figure 4.8: TEM images of bare silica pa rticles (left) compared to PNIPAM coated silica particles (right). .......................................................................64 Figure 4.9: TEM images of gold se eded PNIPAM-silica particles ...................................65 Figure 4.10: TEM images of nanoshells with K-Gold to seed ratio of 500:1 (left) and 250:1 (right) prepared usi ng sodium borohydride route. .......................66 Figure 4.11: TEM images of nanoshells with K-Gold to seed ratio of 50:1 (left) and 20:1 (right) prepared using formaldehyde. .............................................67
v Figure 4.12: UV-vis spectra of 500:1 rati o nanoshells prepared using sodium borohydride. ................................................................................................68 Figure 4.13: UV-vis spectra of 500:1 rati o nanoshells prepared using sodium borohydride. ................................................................................................69 Figure 4.14: UV-vis spectra of 50:1 ratio nanoshells prepared using formaldehyde and taken every 5 minutes for 20 mi nutes total as sample cooled from 55oC. .....................................................................................................70 Figure 4.15: UV-vis spectra of 20:1 ratio nanoshells prepared using formaldehyde and taken every 5 minutes for 20 minut es total as sample cooled from 55oC. ..............................................................................................................71
vi Synthesis and Characterization of Novel Nanom aterials: Gold Nanoshells with OrganicInorganic Hybrid Cores Alisha Peterson ABSTRACT Gold nanoshells, a material generally co mposed of a core of silica surrounded by a thin shell of gold, are of great interest due to their unique and tuna ble optical properties. By varying the shell thickness and core size, the absorption and scattering properties are greatly enhanced. The nanoshells can be made to absorb or scatter light at various regions across the electromagnetic spectrum, from visible to the near infrared. The ability to tune the optical properties of nanoshells allows for their potential use in many different areas of research such as optic al imaging, tumor ablation, drug delivery, and solar energy conversion. The research in this thesis focused on the synthesis and characterization of two novel gold nanoshell materials cont aining thermally-responsive, organic-inorganic hybrid layers. One type of material was based on a two-layer particle with a thermally responsive hybrid co re of N-isopropylacrylamide (NIPAM) copolymerized with 3-(trimethoxysilyl)propyl methacrylate (MPS) th at was then coated with a thin layer of gold. The second material was a three-layer particle with a silica core, a thermally responsive copolymer of NIPAM and MPS middle layer and an outer shell of gold. Various techniques were used to characterize both materials. Transmission electron microscopy (TEM) was used to imag e the particles and dynamic light scattering
vii (DLS) was used to determine particle size and the temper ature response. Additionally, UV-Vis spectroscopy was used to characterize the optical propertie s as a function of temperature.
1 CHAPTER 1: INTRODUC TION AND BACKGROUND 1.1 Traditional Metallic Nanoshells Gold nanoshells are of great interest due to their nanosized dimensions and uniquely tunable optical properties. Their ability to absorb and scatter light in the visible and near-infrared regions stems from coll ectively oscillating conduction electrons stimulated by the electric field in the incident beam [1-3]. For isotropic colloidal gold nanoparticles, localized surface plasmon resonance is seen only in the visible region and is t ypically around 520 nm for isolated particles between 10-20 nm. Changing the shape or a ttaching spherical particles to a larger colloidal substrate, forming nanoshells, allows the optical properties to be easily tuned to various ranges across the elect romagnetic spectrum [1, 4]. For example, nanorods are elongated nanoparticles that display two plas mon bands, as opposed to the single band seen in spherical nanoparticles. There is a transverse band in the visible and a longitudinal band located in the near infrared region (> 700 nm). The exact location of the longitudinal band depends on the aspect ratio of the elongated nanorods [3, 5]. In the case of gold nanoshells, where the gold nanopart icles are attached to a large colloidal core of a dielectric material, the coupli ng of localized surfaced plasmons for the nanoparticles that make up the sh ell leads to absorptio n in the near infrared region. Silica and polystyrene are commonly used as colloidal substrates for trad itional gold nanoshells. Polystyrene has a low decomposition temperat ure and a high solubil ity rate in most
2 commonly used solvents. These characteri stics make polystyrene ideal for creating hollow gold nanoshells. Silica is often used because it is chemically inert, optically transparent, and structurally stable against degradation and coagulation. Therefore, it is used most often to build two layered gold na noshells. Many techniques have been studied to deposit gold nanoparticles onto the surf ace of silica and polystyrene. Synthesis methods including, sol-gel condensation, layer by layer (LBL), and deposition precipitation (DP) have been developed to construct both hollow and two-layered metallic nanostructures [2, 6-10]. Gold nanoshells are optically versatile, capable of bot h absorbing and scattering electromagnetic radiation at various wavelengt hs. Their scattering abilities are great for use in optical imaging applications and ab sorption properties give gold nanoshells the ability to convert light energy into heat, namely photothermal conversion. Photothermal conversion has been studied for a variety of biomedical applic ations. Exploration of both absorption and scattering prope rties of gold nanoshell will greatly benefit many different applications including, drug delivery syst ems, cancer treatment, and solar energy conversion [2, 5, 7, 11]. The overall goal of this research is to develop gold nanoshells constructed with organic-inorganic hybrid cores. The use of hybr id cores will enhance the tunability of the metallic nanoshells optical properties since it pr ovides a versatile route to controlling the dielectric core. In addition, the use of or ganic polymers such as PNIPAM can lead to novel effects since the cross-li nked PNIPAM matrix is capable of swelling and collapsing in response to thermal stimulus. Thus, by us ing temperature responsive polymeric layers
3 to create metallic nanoshells one can tune the optical properties of metallic nanoshells during and after the synthesis process. 1.1.1 Synthesis of Traditional Gold Nanoshells Although several methods [2, 7, 9, 12] ha ve been used to synthesize gold nanoshells, two of the most popular methods will be described in this section. One method uses multiple steps to separately sy nthesize and functionaliz e each component, then combine them into one par ticle in a final step. Here, this method is referred to as layer-by-layer technique (LBL) [6, 9, 13, 14] Deposition-precipitation (DP) is an alternative, and sometimes favored, route to produce traditional gold nanoshells [7, 15, 16]. The DP process is sometimes preferred because it minimizes the number of steps used, which ultimately makes the synthesis process shorter. Despite the fact that the deposition-precipitation method is shorter, controlling ever y parameter, such as pH, temperature, and reaction time, needed to make the synthesis a success can be difficult. Therefore, it is sometimes necessary to ta ke the longer LBL route to achieve desired results. Figure 1.1 shows step by step sche matic of the synthesis process for silica core gold nanoshells. The first two steps of either method, LB L or DP, are the same. First, the core particle is synthesized and then in a second step, its surface is modified. Synthesizing silica is most commonly done using the we ll-known Stber method developed by Werner Stber and Arthur Fink in the 1960s . This method uses ammonium hydroxide as a catalyst in a hydrolysis-condensation reac tion of tetraethylort hosilicate (TEOS). Hydrolysis is the first step of the Stber method wherein the ethoxy groups are replaced
4 with hydroxyl groups. The s econd step is polycondensati on, where water is removed from the silicon hydroxide groups to form silicon dioxides (SiO2). Figure 1.2 shows the mechanism of this reaction. By varying one or more of the reaction parameters the size, porosity and morphology can be altered as desi red. Altering the con centrations of TEOS, ammonium hydroxide, and even the total re action volume produces different sized particles. Varying the ammonium hydroxide concentration also alters the porosity and morphology of the silica [2, 17]. Experiment s discussed in chapters 1 show that manipulating the reaction temperature as well as the total reaction time also played a major role in the size of the particles produced. The second step in the LBL or DP pro cesses is the modification of the silica surface. Surface modifiers are often used to a lter the surface charge of silica. Varying the surface charge promotes the attachment of additional materials to the silica surface. Many functional groups have been studied fo r the purpose of surf ace modifying silica particles. Amine (-NH2), thiol (-SH), a nd vinyl (-C=C) groups are among some of the most commonly used functional groups [15, 18-22]. Determining the appropriate group to use depends on the species that is to be attached to the silica surface. For instance, vinyl groups are often used as surface modifiers to induce the growth of polymeric chains on silica surfaces. In this case, an initiator is used for free radi cal polymerization of a monomer and grafting of the polymer to the silica surface [23, 24]. For gold nanoshells, it is necessary to a ttach gold nanoparticles to the silica surface. To achieve this, reagents bearin g positively charged amine groups, such as 3aminopropyltrimethoxysilane (APTMS) or 3aminopropyltriethoxysilane (APTES) are commonly used as coupling agents on th e silica surfaces [2, 6, 7, 11, 16, 25]. Although
5 literature methods exist [16, 26] where surface modification is not necessary to attach metallic nanoparticles, the use of surface m odified cores ensures the attachment of metallic particles in a dense manner [2, 16]. The reason for this difference lies in the fact that the isoelectric point (IEP) or point of no electric charge of bare silica is ~2. Above the IEP the surface charge of silica is negative, which makes it difficult for negatively charged gold nanoparticles to at tach to its surface [7, 27]. There have been reports where the IEP of silica was manipulated, to al ter the surface char ge, through prolonged exposure to various solutions. For exampl e, ammonium hydroxide used during silica synthesis, can shift the surface charge from negative to positive, if used in excess amounts. Positively charged ammonia cations attach to the silicon hydroxide species which eliminates the need for an additiona l APTMS or APTES functionalization step [16, 26, 27]. The layer-by layer (LBL) method involve s, synthesizing and surface modifying the core silica particles, as described previously, followed by the synthesis and attachment of gold nanoparticles, and lastly the growth of the metallic shell around the core. Gold nanoparticles, ~2nm in size, are synthesized by reducing a solution of tetrachloroauric acid (HAuCl4) in an aqueous medium. Go ld ions precipitate into metallic gold nanoparticles by using reducing agents such as, formaldehyde, sodium borohydride, sodium hydroxide, or ascorbic acid [2, 6, 7]. This process often incorporates a capping agent, sodium c itrate or tetrakis (hydroxymethyl) phosphonium chloride (THPC), to stabili ze the nanoparticles and preven t aggregation  Gold nanoparticles are then attached to the surface modified sili ca; this is known as core
6 seeding. The metallic nanopartic les serve as nucleat ion sites to promote further reduction of gold salt in the form of a shell. There are three key ingredients used in the gold shell growth process. The first is a gold solution known as K-Gold, it consists of tetrachloauric acid and potassium carbonate mixed in an aqueous medium. Sili ca core particles are dispersed in this solution prior to shell growth. Next a re ducing agent, commonly sodium borohydride or formaldehyde, is used to reduce the gold in the K-Gold solution from Au3+ to Au0. The gold seeds on the core surface will attract the additional reduced gold, from the K-Gold solution, to form a layer around the silica surf ace. Prior to adding the reducing agent, a capping agent is added to slow the reacti on and stabilize the pa rticles [6, 7, 13, 25]. In order to seed the silica core part icles without separately synthesizing gold nanoparticles the deposition precipitation (D P) method is used. The DP method is commonly used for synthesis of catalytic oxide materials, but Phonthammachai and coworkers have demonstrated the use of this method to seed silica particles with gold nanoparticles [7, 16, 26]. Here, gold salt soluti on is hydrolyzed in the presence of amine functionalized silica particle s under slightly basi c conditions, pH 7-9. Basic conditions are necessary because at this pH range th e more prevalent gold anions are (Au (OH)3Cl)-, and precipitation is most efficien t using this specious of chloroauric anions. Also, a basic pH is above the IEP of the support material, as a result negatively charged chloride ions are not attracted to its surface. The inability for chloride i ons to attach to the silica surface encourages the growth of favorably small gold nanoparticles [7, 29, 30]. After seeding, the shell is grown using the same me thod described previously in the section for the LBL method, using K-Gold, reducing agents, and capping agents .
7 1.1.2 Optical Properties of Gold Nanoshells The properties of metallic nanoshells include, but are not limited to, optical, magnetic, photothermal, and catalytic [2, 31-33 ]. The coating material used for shell formation determines which of these properties will be exhibited. For nanoshells coated with gold, photothermal and optical propertie s, consisting of light absorption and scattering are most commonly displayed. The expression of optical properties or ability for gold nanoshells to scatter or absorb light is based on wh at is called surface plasmon resonance [2, 3, 34, 35]. In metal films, plasmons are described as positively charged clouds of ions and negatively charged clouds of electrons overlapping one another. Disturbance, from external ra diation, causes the electron clo ud to become polarized. To reinstate equilibrium, the electrons will generate kinetic energy, which causes the conduction electrons to exceed their steady stat e value. The excitation of the electrons causes them to collectively oscillate at a particular frequency. For spherical gold nanoparticles the electrons are localized onl y on the outer surface causing the electron oscillations to be seen only in the vi sible region, ~520nm. In gold nanoshells the coupling of LSPR can be shifted to the n ear infrared region, between 800-1200 nm as a result of interactions between the electrons of the two, inner and outer, surfaces of the shell. These oscillation patterns are show n in figure 1.3. Varying the shell thickness during synthesis determines how much of ho w little the electrons of the two surfaces interact. [2, 3, 12, 33, 35-38]. Li terature studies have shown that there is a red shift, towards the NIR region, in the L SPR band if the shell thickness is less than the radius of the core particle. If the shell thickness is gr eater than the core radius a blue shift towards the visible region is observed. The absorbance sh ift is illustrated later in this thesis in
8 chapter two, figure 2.7. This trend is only considered feasib le if the gold nanoshell is coated in a uniform manner around the entire core particle  In short, as the number of gold nanoparticles on the sili ca surface increase there is a red shift, towards the NIR region, but once the shell thic kens particle separation is increased and the coupling between particles is suppressed and a blue shift is detected [2, 39]. Determining particle size and optical pr operties are very important when deciding which applications will benefit most from the use of gold nanoshells. The detection of optical absorption, scattering and photothermal properties, of gold nanoshells has been well studied [1, 12, 39-41]. Th ese properties can be easily characterized using UV-vis spectroscopy. Typically, nanoshells are disper sed in an aqueous or organic medium and UV-vis spectroscopy is used to characterize the optical extincti on of gold nanoshells. Photothermal properties of nanoshells have b een tested by NIR laser irradiation of an aliquot of nanoshell solution in a small amount of water and measurement of the temperature change by using a thermocouple . 1.1.3 Applications of Gold Nanoshells Due to the unique optical pr operties of gold nanoshells, they have b een explored for a variety of potential applications [3, 4, 42-45]. Among these potential applications, most attention has focused on cell imaging, photothermal ablation, and drug delivery. The popularity of metallic nanoshells for use in biomedical applications is due to the fact that biological tissue penetration is possible, without damage, at NIR wavelengths. In this region gold nanoshells are able to absorb or scatter light efficiently. This material also offers a, cheap, minimally invasive me thod to investigate biol ogical tissues .
9 The optical scattering properties of metallic nanoshells have been studied to enhance current tissue imaging techniques. Fo r example, using a dark field microscope white light can be used to excite the conducti on electrons causing scattering to occur at a distinct frequency. Through the use of biomarkers, the partic les can be attached to one desired cell type, and when the nanoshells are illuminated the attached cells can be easily imaged [11, 46]. Photothermal tumor ablation is another biomedical application of great interest. Ablation of cancerous tissue is achieved by first at taching the nanoshells to the cells through the use of biomarkers and then applying a radiation source. The localized heating causes destruction to cancerous tissue in the proximity of the nanoshells. This technique is beneficial because it allows for healthy cells to remain un-affected [11, 42, 46-48]. Although, biomedical applications seem to dominate the research surrounding gold nanoshells, a few theoretical studies have been completed to determine the ability of gold nanoshells to be used in harvesting and conversion of solar energy [4, 49]. Incorporating gold nanoshells into building st ructures, through the use of coatings or paints, can greatly reduce the amount of electri city used for heating, both industrially and domestically. Using gold nanos hells to convert solar energy for heating purposes will greatly advance the search to find alternative energy methods Gold nanoshells can be optically tuned to absorb or s catter light at a desired wavele ngth or across various region of the solar spectrum. The sun emits half of its light in the visible region 400-700 nm, where gold nanoparticles are great absorber s, and the other half at longer NIR wavelengths, that cannot be abso rbed or scattered by existing ma terials. This void can be met through the use of tunable gold nanoshells . Using the Mie theory, an optimal
10 combination of gold nanoshells and gold nano particles can be dete rmined that would allow for the absorption or scattering of most of the solar spectrum [4, 36]. 1.2 Project Summary This thesis focuses on the developmen t and characterization of novel gold nanoshells capable of absorbing light across th e electromagnetic spectrum. As described earlier, traditional gold nanoshe lls offer tunable optical pr operties via changes in the geometry of the shell relative to the core which is controlled during the synthesis process. In this research pr oject, we have focused on a mate rial that provid es a different level of versatility by combining organic ma terials with inorgani c components for the core. Because the core polymeric material consists of a thermally responsive polymer, the optical properties can also be manipulat ed using an external stimulus such as temperature. The thesis is organized as follows. Chapter 2 focuses on the traditional gold nanoshells that were synthesized in our lab for comparison with the novel hybrid materials. Chapter 3 discusses the synthe sis and characterizati on of a two-layer gold nanoshell and Chapter 4 presents the work on a three-layer gold nanoshell material. Chapter 5 of the thesis provides a summary of the research results.
11 Figure 1.1: Schematic of the LBL synthe sis of traditional gold nanoshells.
12 Figure 1.2: Synthesis scheme for Stber silica particles.
13 Figure 1.3: Electronic charge displacemen t of gold nanoparticles (GNP) (top) compared to gold nanoshells (bottom) due to the electric field of incident radiation.
14 CHAPTER 2: SYNTHESIS AND CHARA CTERIZATION OF TRADITIONAL GOLD NANOSHELLS 2.1 Synthesis and Functionalization of Stber Silica Traditional gold nanoshells were synthesized and ch aracterized for comparison against two and three layered hybrid gold nanoshells. Thes e particles were prepared using the deposition precipitation method [7, 16 ]. This method involved the synthesis of a silica core particles, using the Stber method, that were amine (-NH2) terminated through surface modification with 3-ami nopropyltrimethoxysilane (APTMS). Amine groups bear a positive charge that helps in the attachment of negatively charged gold nanoparticles. The amount of APTMS added must be sufficient enough to coat the entire surface of each particle; therefore, it was a dded in excess. Functi onalized core particles were then seeded in one step using a basic gold solution and the sh ell was grown using a strong reducing agent, namely NaBH4. 2.1.1 Experimental Materials: Ammonia hydroxide (28-30%) was purchased from Sigma Aldrich. Tetra ethyl orthosilicate (TEOS 98%) and 3-aminopropyltrimethoxysilane were both purchased from Acros Organics and ethanol from Pharmaco-AAPER & Commercial Alcohols. Deionized wa ter was purified using an Easypure UV water system.
15 Stber Silica: In a 200mL round bottom flask, 10mL of water and 5mL of ammonium hydroxide were added to 100mL of ethanol and vigorously mixed at 30oC for 30minutes. 6mL of TEOS wa s then added and the reaction was continued, undisturbed overnight, ~24 hour s. To remove the particles from the ethanol medium they were centrifuged a nd washed using water in a series of centrifugations. After purifi cation the particles were dried for a minimum of 6 hours in a vacuum oven and weighed. Surface Modification: To f unctionalize the surface of th e silica particles APTMS was used. 200mg of the dried bare partic les were redispersed, using sonication, in 10mL of toluene. The turbid soluti on was then added to a 25mL round bottom flask and placed in a 75oC oil bath. An excess of APTMS, 65uL, was added to the solution and the reaction conti nued with mixing for 12 hours. Amine functionalized silica particles were purif ied to remove any un-reacted APTMS. To accomplish this centrifugation was us ed. They were first centrifuged to remove the original medium, redispersed an d centrifuged twice in toluene. This was followed by four washes in ethanol to remove any exce ss toluene and four washes in water to remove any remaining ethanol. Lastly, surface modified silica particles were dried in a vacuum oven overnight. Characterization Techniques: Dynamic li ght scattering (DLS) was carried out using a Malvern Zetasizer Nano-S inst rument. DLS investigated the size distribution of the silica particles. Sa mple preparation included, dispersing 20uL of particle solution in 1mL of water inside of a 2mL polystyrene cuvette. FEI Morgagni 268D transmission electron micr oscope was used to image the particles
16 to determine the shape and polydispersity in size. For imaging, sample preparation included depositing and dryi ng ~20uL of sample on a formvar copper grid. A Nicolet Magna IR Spectrometer 860 was used characterize the silica particles. The samples were prepared by mixing dried silica particles with potassium bromide (KBr) and crushi ng the powder to form a pellet. 2.1.2 Results and Discussion Silica core particles were successfu lly prepared by mixing TEOS at a warm temperature of 30oC for a 24 hour period. Heating the solution and prolonging the reaction time resulted in the production of small nanoparticles below 200nm in diameter. For size comparison, additional silica particle s were made using the same Stber method but the reaction was done at room temperatur e and the time was reduc ed to 6 hours. DLS size distribution graphs shown in figure 2.1 co mpares the difference in silica size based on reaction time and temperature. Fo r those particles synthesized at 30oC the diameter is measured to be ~190nm. By slightly alte ring the reaction time and temperature as discussed previously, the particle diameter was doubled to ~390nm. TEM images of bare silica, shown in figure 2.2, confirm th at the particles prepared at 30oC are less than 200nm in diameter and have lo w polydispersity in size. Images of amine functionaliz ed silica particles are almo st identical to those of bare silica particles indicating that the APTMS functionalization does not lead to aggregation. FTIR spectrum was used to characterize the presence of APTMS on silica particles. Figure 2.3 shows the IR spectra, which are dominated by the Si-O-Si peak around 1000cm-1. The spectrum of the amine functiona lized silica part icles revealed no
17 significant peaks from the organic molecule s; presumably the small amount of APTMS on the surface was not sufficient for detection by IR absorption. The indication of amine presence on silica surfaces wa s later evident thro ugh the production of evenly co ated gold nanoshells that are describe d in the next section. 2.2 Gold Nanoshells Using Deposition Precipitation Method As discussed earlier, the DP process is a synthesis technique favored because it uses fewer steps than most other methods. This method avoids the excess step of synthesizing an additional gold nanopartic le solution. Instead, the nanoparticles are precipitated in the presence of amine functionalized silica particles. Several different Kgold solution to core particle solution ratios were used when synthe sizing traditional gold nanoshells to achieve a di fferent shell thicknesses 2.2.1 Experimental Materials: Chloroauric acid (HAuCl43H2O 99%) was purchased from Sigma Aldrich, sodium hydroxide and potassium carbonate were both purchased from Fischer Scientific, and lastly sodium c itrate and sodium borohydride (98%) were purchased from Acros Organics. DP Gold seeding of surface modified silica: Using sonication, 100mg of amine grafted silica particles were dispersed in 1mL of water. Separately, an aliquot of 0.1M NaOH was added drop by dr op to 4mL of 6.35mM HAuCl4 to adjust its pH to 8. The silica solution was added to the basic gold chloride solution and mixed vigorously in a 10mL round bottom flask at 75oC. The reaction continued for
18 45minutes. The gold seeded silica particle s were washed multiple times in water to remove any un-reacted gold, and finally redispersed in water. Gold shell growth: In a150mL amber gla ss bottle, 100mL of K-Gold solution was made by dispersing 1.5mL of 25mM HAuCl 4 in 100mLof water and adjusting the pH through the addition of 60mg of K2CO3. For the 250:1 ratio of K-Gold to silica particles, 100mL of gold seeded silica solution was added to 25mL of KGold solution and mixed for 3 minutes to di sperse the particles. Then, 1.25mL of capping agent, sodium citrate were mixed into the reaction for 2-3 minutes prior to the addition of 2.5mL of reducing agent, NaBH4. For 300:1 and 400:1 ratios, the amount of seeded particle solution remained the same as in the case for 250:1. The amount of K-Gold was increased to 30mL for 300:1 and 40mL for 400:1. Capping and reducing agents were increased proportionately to the increase in Kgold solution. The ratio of K-gold to cappi ng agent is 20:1 so, for 300:1 shell ratios, 1.5mL was used and for 400:1 ratios 2mL was used. As for the reducing agent, its ratio to K-gold is 10:1, therefore 3 and 4mL of NaBH4 was used for 300:1 and 400:1 ratio s, respectively. Characterization Techniques: FEI Mo rgagni 268D transmission electron microscopy was used for imaging the nanopa rticles after seeding and final shell growth. UV-vis spectra were r ecorded using a Jasco V-530 UV/Vis Spectrophotometer. The spect ra determined the light absorption pattern of the particles based on their shell thickness. Gold nanoshell deposition onto a formvar copper grid surface was the method of preparation for TEM grids and UV-vis
19 samples were prepared by simply adding 1mL of particle solution to a 2mL polystyrene cuvette. 2.2.2 Results and Discussion Successful deposition of gold nanoparticle s on silica cores was achieved by using the DP method. High temperature of 75oC was used to promote bonding between the terminated amine groups on the silica surf ace and the gold nanoparticles. Basic conditions were used to ensure that the gold ions precipitated to form gold nanoparticles efficiently, and the total reaction time was limited to 45 minutes to prevent the formation of large gold clusters in solution and on the core surface [7, 16]. During shell growth, a capping agent, sodium citrate, was used to decrease the chances of aggregate formation and because NaBH4 is a strong reducing agent, the sodi um citrate also played a role in slowing the reduction reaction. Another way the reaction was slowed was to add the reducing agent (NaBH4) in increments of 500uL every 20-30 seconds. TEM was used to image this material at both the seeding and shel l growth steps. Images of gold seed silica particles are shown in figure 2.4. These imag es display silica particles with small gold nanoparticles, ~2-6nm, absorbed to their su rfaces. Gold seeds 2-6nm in diameter are good for producing evenly coated gold nanoshells The different gold nanoshells, with ratios of 250:1, 300:1, and 400:1, are shown in figure 2.5. Each of these nanoshell materials exhibit fairly uniform but incomple te shells. These particles were not highly aggregated and the shell thickne ss appeared to be the same fo r most of the particles found on the grid. The images of each sample, 250:1, 300:1, and 400:1, look very similar in appearance but a distinction be tween each can be seen in the UV-vis spectra presented in
20 figure 2.6. Each peak expands ac ross the visible and near infr ared regions from 600nm to 1000nm. As the K-gold to silica particle solution ratio increases the shell thickness increases. An increase in the shell thickne ss causes a red shift in the absorption band. Results discussed in this chapter demons trate that the traditional gold nanoshells were successfully prepared using the DP method and their characterization by UV-Vis and TEM provide the basis for comparison to the prope rties of the novel organicinorganic hybrid nanoshells, discus sed in the next two chapters.
21 30 25 20 15 10 5 0Intensity(%) 100 101 102 103 104Size(nm) Â— Small Silica Particles Â— Large Silica Particles 25 20 15 10 5 0Volume(%) 100 101 102 103 104Size(nm) Â— Small Silica Particles Â— Large Silica Particles Figure 2.1: DLS measurements of large ba re silica particles (~350nm) compared to small bare silica particles (~190nm). Bo th the volume distribution (top) and the intensity distribution (bottom) are shown.
22 Figure 2.2: TEM images of bare silica pa rticles with a mean size ~190nm.
23 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6Absorbance 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber(cm-1) Â— Bare Silica 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6Absorbance 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber(cm-1) Â— Amine grafted Silica Figure 2.3: FTIR spectrum of bare silica (top) compared to IR spectrum of amine functionalized (bottom) silica particles.
24 Figure 2.4: TEM images of gold seeded silica pa rticles. In both images, the scale bar shown corresponds to 100nm.
25 Figure 2.5: TEM images of gold nanoshells with different K-Gold to seed ratios. 250:1 (top), 300:1 (middle), and 400:1 (bottom).
26 0.8 0.7 0.6 0.5Absorbance 1100 1000 900 800 700 600 500 400 Wavelength(nm) Â—250:1 Â—300:1 Â—400:1 250:1 300:1 400:1 Figure 2.6: UV-vis spectra of gold nanoshe lls corresponding to the ratios 250:1, 300:1, and 400:1
27 CHAPTER 3: SYNTHESIS AND CHARA CTERIZATION OF TWO LAYERED HYBRID CORE GOLD NANOSHELLS 3.1 Synthesis of PNIPAM-Siloxane Hybrid Core Particles In the presented work, two types of nove l gold nanoshells were synthesized. The first is a two component material composed of a PNIPAM-Siloxane microgel coated with a layer of gold. The core hybrid particle s were synthesized us ing a precipitation polymerization technique [24, 50, 51]. In this method N-isopropylacrylamide was copolymerized with 3-trimethoxysilylpropy l methacrylate (MPS) using potassium persulfate as the reaction ini tiator. The polymerization was done in the presence of N, methylenebisacrylamide (MBA) to create a cross-linked polymeric network containing siloxane groups . This method is common in literature and has been used frequently to produce spherical, thermally responsive polymeric particles. 3.1.1 Experimental Materials: N-isopropylacrylamide (NIPAM ) was purchased from TCI America, purified and re-crystallized in our lab. N, N, methylenebisacrylamide (MBA 98%) and potassium persulfate (KPS 99%) we re purchased from Sigma Aldrich. 3-trimethoxysiylpropylmethacrylate (MPS 98%) was purchased from Acros Organics. Synthesis of Microgels: Using a 300mL round-bottom flask, 200mL of deionized water was degassed, by bubbling with N2, for 1hour. Then, 1g of re-crystallized
28 NIPAM and 50mg of MBA, which is 5% of the monomer concentration, was added and bubbling continued for an additional 1hour at 75oC. Separately, 20mg of KPS, which is 2% of the monomer concentration, was bubbled in 10mL of water at room temperature for 1hour. Once the monomer mixture was degassed and heated, the KPS solution was added to initiate the reacti on. Two hours after adding the initiator, 240uL of the co -monomer, MPS, was added and the polymerization continued for an additiona l 2hours. The total reaction time was 6 hours. The microgel solution is cleaned in water using centrifugation. The particles were centrifuged at 6500rpm three to five times and between each centrifugation they were redispersed in deionized water. After the final water wash the microgels were redispersed in 50mL of water and stored at room temperature until use. Characterization Techniques: Dynamic light scattering (DLS) r eadings were taken using a Malvern Zetasizer Nano series inst rument. The measurements were used to determine the size of the PNIPAM-siloxane core particles. Also, the microgels were imaged using transmission electr on microscopy or TEM on a FEI Morgagni 268D model. DLS samples were prepared by dispersing 20uL of sample solution into 1mL of deionized water. This was done in a 2.5mL polysty rene cuvette. The cuvette was slightly shaken to ensure the dispersion of the nanoshells. TEM sample preparation involved dispersi ng 20uL of microgel solution in 1mL of deionized water and drying a few microliters of this solution on a formvar copper grid.
29 3.1.2 Results and Discussion Thermally responsive PNIPAM-siloxane mi crogels were successfully synthesized using precipitation polymerization of NIPAM and MPS. An increase in co-monomer MPS can cause a decrease in thermal response and in some cases it has been noted in literature that the addition of large am ounts of co-monomer (~40% of the monomer concentration) will produce microgels coated w ith a thin outer layer that show very little thermal response, if any . Figure 3.1 shows the DLS trend graph for PNIPAMsiloxane microgels synthesized as per the pro cedure described in the experimental section above. The trend graph confirms the temper ature dependent size response of the core microgel particles at temperatures between 40oC and 20oC. It displays that at high temperatures, above the volume phase transitio n temperature of cross-linked particles, 32oC, the particle diameter is small around 225n m. This is due to the collapse of the polymer matrix and expulsion of wate r. For lower temperatures, below 32oC, the matrix swells with water creating an increase in pa rticleÂ’s diameter to ~300nm. A gradual diameter increase can be seen as the temperatur e of the particle solution decreases. TEM images in figure 3.2, show spherically shap ed particles that are uniform in size. 3.2 Gold Seeding Using DP and LBL and Shell Growth Nanoshells were grown on hybrid core partic les that were seeded using the layer by layer technique (LBL) and the depositi on precipitation method (DP). In the LBL process gold nanoparticles were synt hesized through the reduction of hydrogen tetrachloroaurate (III) with a basic solu tion of NaOH [6, 13, 25, 52, 53]. For the DP method, synthesizing an additiona l gold nanoparticle solution was not necessary. Instead
30 the gold was directly precipitated onto the surface of the core pa rticle [7, 16, 26, 30]. Gold seeded hybrid cores were first atte mpted using deposition precipitation. This technique was chosen because it diminishes the need to synthesize a separate gold nanoparticle solution and also because it was proven, in chapter 2, to be successful in synthesizing traditional sili ca core gold nanoshells. Because the attachment of small nanoparticles, ~2nm, to the microgel core was difficult to ac hieve using the DP technique, the LBL method was adopted. Using the LBL method under the right parameters produced uniformed gold decorate d PNIPAM-siloxane hybrid particles. The shells were grown around the hybrid cores at different thicknesses to determine which exhibited the best optical properties, while maintaining the thermal responsiveness of the core. 3.2.1 Experimental Materials: Ammonium hydroxide ( 28-30%), chloroauric acid (HAuCl43H2O 99.9%), and tetrakis (hydroxymethyl) phosphonium chloride (THPC 80%) were purchased from Sigma Aldrich. Potassium carbonate and sodium hydroxide were purchased from Fischer Scientific. Sodi um borohydride and sodium citrate were purchased from Acros Organics. Synthesis of Gold Nanoparticles : 2.54mM solution of HAuCl4 was made by diluting 715uL of a 25mM gol d solution in 6.5mL of water. The stock solution was kept in the dark at room temperatur e until used. A 1.2mM stock solution of THPC was made by dispersing 20uL of 80% THPC in 95mL of water. This solution was refrigerated when not in us e. 1M NaOH stock solution was made by
31 dissolving NaOH salt in an aliquot of wate r. After preparing each stock solution gold nanoparticles were synthesized by di spersing 1.2mL of 1M NaOH and 4mL of THPC in 180mL of water. The solution was mixed continuously for 57minutes. 6.75mL of 2.54mM HAuCl4 was added in a single injection, quickly, using an electronic pipette. Particles we re refrigerated in an amber glass bottle until used. One-step Gold Seeding (DP Process): In the DP process the gold nanoparticles are precipitated in the presence of the core particles under slightly basic conditions. The pH of 10mL of 6.35mM HAuCl4 was adjusted to approximately 8 through the slow addition of 0.1M NaOH. 1mL of hybr id core particles was mixed overnight in 2mL of NH4OH. Through centrifugation mo st of the excess ammonium hydroxide solution is removed and the par ticles were dispersed in water. The particle solution was then added to th e basic gold chloride solution and the mixture was vigorously stirred at 65oC for 15 minutes. The solution was cooled to room temperature for 1hr. To remove any un-attached gold nanoparticles the solution was centrifuged at 1000rpm 3 times for 30 minutes each. The cleaned particles were dispersed in water and stored at room temperature. LBL Gold Seeding: The LBL method requi red the separate synthesis of gold nanoparticles described previously in this section. For this procedure, 3mL of hybrid microgels were mixed overnight in 6mL of NH4OH. The ammonium hydroxide was then removed using centrifuga tion and the particles are redispersed in 4mL of water. 1.5mL of the ammonium soaked aqueous polymer solution was then incubated, at room temperature, with 10mL of freshly prepared gold
32 nanoparticles in a 20mL scintillation vial overnight. Centrifugation was used to separate any unattached gold nanoparticles and the seeded hybrid particles were redispersed in water. Gold Shell Growth: Gold seeded core particles, prepared using the LBL procedure, were used in the shell formation step. A gold precursor solution commonly called K-Gold is needed to grow the gold shell. It consists of, 1.5mL of 25mM HAuCl4 mixed in 100mL of water for 1-2 minutes. 60mg of potassium carbonate was added to adjust the pH to approximately 10.1. The solution was mixed overnight in an amber glass bottle and the pH was taken after 24 hours of mixing. 10mM of sodium citrate was used as a capping agent. It was synthesized by dissolving an aliquot of s odium citrate salt in water. A strong reducing agent, 6.6mM sodium borohydride, was used to add additional gold onto the seeded hybrid core particle. Th is reagent was made by a dding an aliquot of NaBH4 to ice cold water. The reducing solution was kept in the freezer when not in use. Several core to shell ratios were synthesi zed for each, the amount of core particle solution was kept constant and the K-Go ld amount was varied. The experiment described here is a 500:1, K-gold to core seed, ratio. 100uL of gold seeded hybrid core particle solution was dispersed in 50mL of K-Gold for 2-4 minutes. 2.5mL of 10mM sodium citrate was added to th e core particle-K-gold solution and mixed for 2 minutes. 5mL of cold 6.6mM sodium borohydride was added in 250uL increments with 20 seconds between each addition. The solution was centrifuged and washed repeatedly in water to rem ove any un-reacted gold nanoparticles and finally dispersed in water.
33 Characterization Techniques: FEI Mo rgagni 268D transmission electron microscopy was used to image each materi al described in this section, the DP seeded cores, LBL seeded cores, and the complete shell particles. Samples were prepared by drying ~20 microliters of d iluted nanoshell solution onto a copper formvar grid. Jasco Spectrophotometer model V-530 was used to determine the position of the absorption band of the gol d nanoparticles and the gold nanoshells at various shell thicknesses. 3.2.2 Results and Discussion Reduction of gold chloride solution ha s been proven to be a good method for synthesizing nanosize gold particles capable of absorbing light in the visible region of the electromagnetic spectrum. Here, gold nanopa rticles were synthesized using a simple reduction method. The formation of gold nanopa rticles could be identified visually by a change is solution color from clear to brown. To determine the size of these particles, DLS was utilized. The size di stribution graph, figure 3.3, show s particles with diameters of ~10nm or less, small partic les are desired when seeding pa rticles for nanoshell growth. Surface plasmon resonance for small gold na noparticles is commonly seen around 520nm . The optical absorption of the nanopartic les synthesized here was analyzed using UVvis spectroscopy, shown in figure 3.4. These part icles exhibit an absorption peak in the visible region at a wavelength of 518nm, t ypical for gold nanopartic les less than 20nm in size. Gold nanoparticle seeding of polymeric microgels using deposition precipitation produced large clustered gold particles. TEM images, figure 3.5, display core particles
34 with a minimal amount of small uniformed gold nanoparticles attached. Many of the smaller gold nanoparticles found in the sample we re unattached from the core particles. Much of the gold actually attached to a core consisted of large clus ters of various sizes and shapes. Successfully creating nanoshells requires the gold seeds to be very small, ~2nm, and uniformed in size and shape. To accomplish this goal the seeding technique was switched from DP to LBL. The main sti pulation when using the LBL technique is to seed hybrid core particles with freshly pr epared gold nanoparticles. Over time gold nanoparticles will begin to aggregate in solution, this is indicated by a color change of the solution from brown to pale purple or darker depending on the size of the aggregates. Figure 3.6 shows the UV-vis spec tra of freshly prepared gol d nanoparticles compared to those that have been aged. A red shift is seen in the aged particles indicating an increase in particle size. For this reason, gold nanopa rticles are stored at temperatures below 4oC, but despite proper storage aggregation may s till occur. If gold nanoparticles that are extremely aged (~weeks) are used for seeding, large gold clusters similar to those observed when using DP process, will attach to the core surface. Demonstrated in figure 3.7 are hybrid core particles seeded with ag ed gold nanoparticles compared with those seeded with freshly prepared nanoparticles. For the latter it is clear that the gold nanoparticle density is high an d the particles are uniformed in shape and size, between 24nm. Nanoshell growth is greatly dependent on the rate in which the reducing agent works. Sodium borohydride is a strong reducing agent, adding it in small increments is a way of controlling how fast the gold reducti on happens. Using a capping agent is also helpful in reducing the speed of this proces s. Adding the reduci ng agent too quickly it
35 will result in the production of very thick nanoshells with optical properties similar to those of gold nanoparticles. TEM images, figur e 3.8, were taken at different intervals of shell growth with a final ratio of 500:1. A sample was taken after the addition of 7 (1.75Ml), 9 (2.25mL), 12 (3mL), 15 (3.75m L), and 20 (5mL) injections of NaBH4. The corresponding UV-vis spectra are shown in figure 3.9. From TEM it is obvious that increasing K-gold amount increas es the amount of gold coati ng the core particle. UV spectra show that as the shell grows a red shif t is seen but once a complete shell is formed a blue shift is observed. Figure 3.10 displays UV-vis spec tra taken of a 500:1 core to shell ratio at various temperatures, ranging from room temperature to 55oC. These graphs were used to determine if the collaps e and swelling of the co re particle played a role in the optical properties. It can be seen in these spectra that heating causes a slight red shift to occur and as the samp le cools a blue shift is seen.
36 300 280 260 240Size Peak(nm) 40 35 30 25 20 Temperature(oC) Figure 3.1: Variation of the hydrodynamic diameter of PNIPAM-siloxane hybrid particles as a function of temperature. Th e dashed line is drawn to guide the eye.
37 Figure 3.2: TEM images of PNIP AM-siloxane hybrid cores.
38 25 20 15 10 5 0Volume(%) 100 101 102 103 104Size(nm) 25 20 15 10 5 0Intensity(%) 100 101 102 103 104Size(nm) Figure 3.3: DLS size distribution plots for gold nanoparticles shown as volume scattering (top) and intensity scattering (bottom).
39 0.20 0.18 0.16 0.14 0.12 0.10 0.08 0.06Absorbance 1100 1000 900 800 700 600 500 400 Wavelength (nm) Figure 3.4: UV-vis spectrum of fres hly prepared gold nanoparticles.
40 Figure 3.5: TEM images of PNIPAM-siloxane hy brid particle gold seeded with the DP method. All scale bars are 200nm.
41 0.25 0.20 0.15 0.10 0.05Absorbance 1100 1000 900 800 700 600 500 400 Wavelength(nm) Â— Gold Nanoparticles: aged 3 weeks Â— Gold Nanoparticles: freshly prepared Figure 3.6: UV-vis spectra of fresh gold nanoparticles vs aged gold nanoparticles.
42 Figure 3.7: TEM images of PNIPAM-siloxane hybrid particles seeded using aged gold nanoparticles (left) and freshly prepared gold nanopa rticles (right).
43 Figure 3.8: TEM of PNIPAM-siloxane hybrid gold nanoshells (50:1) at different intervals of the shell growth: 7 injections (top left), 9 injections (top right), 12 injections (middle left), 15 injections (middl e right), 20 injections (bottom). All scale bars are 200nm.
44 0.40 0.35 0.30 0.25Absorbance 1000 900 800 700 600 500 Wavelength(nm) 500:1 Gold nanoshells 7 injections of NaBH4 9 injections of NaBH4 12 injections of NaBH4 15 injections of NaBH4 20 injections of NaBH4 Figure 3.9: UV-vis spectra of PNIPAM-siloxan e gold nanoshells at various intervals of shell growth as indicated in the legend.
45 0.30 0.29 0.28 0.27 0.26 0.25 0.24Absorbance 1100 1000 900 800 700 600 500 400 Wavelength(nm) 500:1 Gold nanoshell T=55oC T<55oC (5minutes) T<55oC (10minutes) T<55oC (15minutes) T<55oC (20minutes) Figure 3.10: UV-vis spectra of PNIPAM-siloxane particles in solution as it cools from an initial T=55oC in 5 minute increments.
46 CHAPTER 4: SYNTHESIS AND CHARA CTERIZATION OF THREE LAYERED SILICA CORE GOLD NANOSHELLS 4.1 Synthesis and Functionalization of Stber Silica Similar to core particles described in ch apter 2, silica part icles discussed here were synthesized using the we ll known Stber method. As e xplained earlier, the Stber method consists of two steps, first hydr olysis of ethoxy groups followed by the condensation of silicon hydroxide groups of tetraethylorthosili cate. Parameters of this reaction are fairly simple to control, and ar e easily manipulated to achieve particles of a desired size and morphology. The silica particles prepared here were later coated with a layer of poly N-isopropylacrylamide (PNIPA M). To encourage polymerization around its surface, silica was functionalized directly after synthesis using 3methacryloxypropyltrimethoxysilane (MPS). MPS provides the silica surface with terminal vinyl groups, or carbon-carbon double bonds, that break to copolymerize with oligomers of NIPAM. A schematic of this is displayed in figure 4.1. 4.1.1 Experimental Materials : Ammonia hydroxide (28-30%) was purchased from Sigma Aldrich, ethanol was purchased from Pharm aco-AAPER & Commercial Alcohols, Deionized water was purified using Easypure UV water system, and both tetraethyl orthsilicate (TEOS 98%) and 3-trimethoxysilypropylmethacrylate (MPS 98%) were purchased from Acros Organics.
47 Synthesis of Stber Silica: In a 350mL round bottom flask, 30mL of water and 11mL of ammonia hydroxide were disper sed in 200mL of etha nol. The solution was allowed to mix vigorously for 30 minutes in a 70oC oil bath. At that point, 16mL of TEOS was added and the r eaction continued for 1 hour. For functionalization purposes, 50mL of the soluti on was kept in its original medium and the remainder was centrifuged and wash ed repeatedly in water to remove the ethanol residue. MPS Functionalization of Stber Silica: In the same 350mL round bottom flask used for silica synthesis, 50mL of fr eshly prepared silic a particles were functionalized in its original medium us ing MPS. 6mL of MPS was added to the 50mL solution and the solution was mi xed vigorously overnight at room temperature. The mixture was then heated to 80oC for 1 hour to promote covalent bonding. The product was then centrifuged and washed in water to separate out any un-reacted MPS and finally disp ersed in fresh deionized water. Characterization techniques: Nicolet Ma gna IR Spectrophotometer (FTIR) was used to detect the presence of MPS on the surface of the silica particles surface. Samples were prepared by pelletizing a powder mixture containing sample particles and potassium bromide (KBr). Dynamic light scattering measurements were done using a Malvern Zetasizer Nano Series. The measurements were used to determine the size distribution of the silica particles. These samples were prepared by simply disper sing 20uL of the aqueous pa rticles soluti on in 1mL of deionized water, within a 2.5mL polysty rene cuvette. TEM samples taken using a FEI Morgagni 268D were used to im age the particles before and after
48 functionalization. Samples were prepared by drying ~20uL of sample solution on a copper formvar grid. 4.1.2 Results and Discussion Bare silica particles and MPS functionaliz ed silica particles we re compared using FTIR spectrum, shown in figure 4.2. Similar to amine functionalized silica discussed in chapter 2, the IR spectra for MPS functiona lized silica particles did not show any significant peak difference when compared to bare silica particles. Bare and MPS functionalized silica particles were imaged using TEM shown in figure 4.3. When compared to the bare silica, MPS silica particles were very similar in appearance. Although, the presence of MPS cannot be de termined through IR absorption or TEM imaging characterization techniques, the succe ssful formation of polymer coated silica, discussed later in this chapter, led us to conclude that te rminal double bonds were present on the surface of the silica particles. Si ze distribution of bare Stber silica was determined by DLS. The distribution grap hs illustrated in figure 4.4 display the hydrodynamic diameter of silica in water to be ~170nm. 4.2 Polymerization of NIPAM in the Presence of Stber Silica As discussed in chapter one silica particles are capable of promoting polymerization of a monomer ar ound its surface thr ough the use of modifiers, preferably vinyl groups (-C=C). The use of MPS helps to overcome the natura l hydrophilic nature of silica particles which encourages the at tachment of hydrophobic NIPAM oligomers to the silica surface. Small silica particles have more surface area than larger particles.
49 Using small cores, less than 200nm decreased the chance for pure microgels, without a silica core, to form. Here, PNIPAM coated si lica particles were s ynthesized using 170nm sized MPS grafted silica particles, described in the previous section. Polymerization was done in the presence of functionalized silica particles using a precipitation polymerization technique. In this technique, MPS-silica particles interact with NIPAM oligomers through the use of an initiator. The initiator creates termin al radicals on the monomer ends by breaking its double bonds. These radicals then inter act with the double bonds on the functionalized si lica surface to break and copoly merize with the monomer . 4.2.1 Experimental Materials: Nisopropyl acrylamide (NIP AM) was purchased from TCI America, purified and re-crystallized in the la b. N, N`-methylenebisacrylamide (MBA 99%) and potassium persulfate were purchased from Sigma Aldrich. Silica Core NIPAM Polymerization: In a 100mL three-neck round bottom flask, 58mg of NIPAM and 4mg of MBA were mi xed in 10mL of water. This monomer solution was bubbled in a 45oC oil bath for one hour. Separately, 15mL of was warmed and degassed with nitrogen. Th is solution was added to the bubbled monomer solution to increase the final reac tion volume to ~25mL and in the same step an aqueous MPS-silica solution was a dded. At this point the temperature is also increased from 45oC to 70oC while N2 bubbling and purging continued for an additional hour. Independently, an init iator solution was prepared by dispersing 6mg of KPS in 2mL of water. The solution was degassed for 20 minutes, and added to the NIPAM solution to initia te the polymeriza tion reaction. The
50 reactants were mixed in a N2 atmosphere for 6 hours after the addition of the initiator solution. The particles were wa shed using water and centrifugation to remove any unattached m onomer and cross-linker. Characterization Techniques: Nicolet Magna IR Spectrometer 860 (FTIR) was used to detect the presence of PNIPAM on the surface of silic a particles. The thermal response and size distribution of PNIPAM-silica particles was determined by dynamic light scattering using a Malv ern Zetasizer Nano Series, and a FEI Morgagni 268F transmission electron micr oscope was used to image the silica particles before and after polymerization. 4.2.2 Results and Discussion Bare silica particles were compared to PNIPAM coated silica particles using FTIR, DLS, and TEM. In figur e 4.5, the FTIR spectrum of PNIPAM-silica displays an intense peak around 1000cm-1 which indicates the presence of Si-O-Si groups. In this same spectrum hydrocarbon peak s between 2500 and 3000cm-1 are present as well as peaks that are typically seen in amide bearing materials, namely around 1650, 1550, and 1460cm-1. When compared to the PNIPAM-sili ca, IR spectrum of bare silica only exhibits the Si-O-Si peak. DLS plots dem onstrate a particle diameter increase after attachment of the polymer shell. The si ze distribution of sili ca and PNIPAM coated silica at 20oC are presented in figure 4.6. The diameter of bare silica was measured to be around 170nm and this diameter increased to approximately 300nm for PNIPAM coated silica. Figure 4.7 displays the diameter in re sponse to decrease in temperature from 40oC to 20oC of the PNIPAM-silica particles. At room temperature (~20oC), PNIPAM-silica
51 particles possess a diameter of 345nm. Th is temperature is below the volume phase transition temperature of cr oss-linked NIPAM meaning that the polymer matrix is swollen with water. At 40oC, above the volume phase tran sition temperature the polymer matrix is collapsed and the water is expelled this results in a smaller hydrodynamic diameter, ~ 250nm. TEM images in figure 4.8 compare bare silica particles to PNIPAMsilica particles. In these images the polymer layer in PNIPAM-silica can be distinguished from the core due to the difference in densit y of each material. Silica is denser than PNIPAM therefore the core of the particle ap pears darker than the less dense outer shell. 4.3 Gold Seeding and Shell Growth on PNIPAM Coated Silica PNIPAM-SiO2 particles were gold seeded by re ducing gold salt solution in the presence of the polymeric core particles. Chloroauric acid was diffused into the polymer layers of the partic les and reduced, which resulted in gold nanoparticles within the polymeric layers and around the surface of th e PNIPAM-SiO2 cores. This process was done at room temperature or below to ensure the polymer matrix was expanded as much as possible, to increase the uptake of gold solution. Increasi ng the amount of gold solution in within or around to core partic les increases the number of gold nanoparticle nucleating sites formed. The nanoshells we re grown in using the same technique discussed in chapter two, with the excepti on that some samples were made using a different reducing agent. For this materi al sodium borohydride and formaldehyde were used as reducing agents to grow the metallic shell. Formaldehyde was chosen because it is a much weaker reducer compared to sodi um borohydride making it easier to control the shell growth rate.
52 4.3.1 Experimental Materials: Chlo roauric acid (HAuCl43H2O 99.9%), was purchased from Sigma Aldrich, potassium carbonate and sodium hydroxide were purchased from Fischer Scientific, and Sodium borohydride, sodi um citrate, and formaldehyde (37%) were all purchased from Acros Organics. Gold Seeding PNIPAM-silica: In an 8mL scintillati on vial, 1.88mL of 2.54mM HAuCl4 was mixed with 2mL of PNIPAM-SiO2 solution for 25 minutes. Separately, in a 50mL conical flask, 166uL of 1M NaOH and 550uL of THPC were vigorously mixed in 25mL of water at room temperature. Quickly, the goldPNIPAM-SiO2 solution was added to the basic aqueous solution and the reduction reaction continued for 1 hour. The particles were then centrifuged and washed in water to remove any unattached gold nanopar ticles. Seve ral core to shell ratios were synthesized but for explanation purposes, the 500:1 K-gold to core particle solution is described for the sodium borohydride shell growth and a 50:1 ratio is detailed for the formaldehyde grown nanoshells. Shell Growth on PNIPAM-silica using NaBH4: A K-Gold Solution consisting of 60mg of K2CO3 and 1.5mL of 25mM HAuCl4 dissolved in 100mL of water was aged for two days in an amber glass vial. After one day the pH was measured to be 10.06. A 10mM sodium citrate solution was prepared by dissolving 40mg of citrate salt in 13.6mL of water. A reduc ing agent stock solution of 75mM sodium borohydride was prepared by dissolving 26mg of NaBH4 powder in 9mL of water. The 6.6mM NaBH4 solution needed for the re action was diluted from the 75mM stock. In a 60mL glass bottl e, 50mL of K-gold was heated to 50oC for 5
53 minutes before adding 100uL of gold s eeded PNIPAM-silica particles. The solution continued to mix vigorously fo r 10 minutes before 2.5mL of 10mM of sodium citrate was added. One minute af ter the addition of sodium citrate, 5mL of 6.6mM NaBH4 was added in 20 increments of 250uL approximately 1 minute apart and the reaction co ntinued for 15 minutes. Shell Growth on PNIPAM-silica using Formaldehyde: In a 20mL scintillation vial, 10mL of K-gold and 200uL of gold s eeded PNIPAM-silica particle solution were mixed and heated to 55oC for 10minutes. 25uL of formaldehyde was then added and the reduction reaction continued for 15 minutes. Characterization Techniques: Transmissi on electron microscopy, FEI Morgagni 268D model, was used to image gold seeded PNIPAM-silica and gold shell coated PNIPAM-silica. Samples were prepared by placing ~20uL of particle solution on a formvar copper grid and allo wing the grid to dry under a visible lamp. A Jasco V-530 UV/Vis Spectropho tometer was used to take UV-vis spectrum of the complete nanoshell material The spectra were used to determine the optical properties of the nanoshell s at various shell thicknesses and temperatures. 4.3.2 Results and Discussion TEM images and UV-vis spectra were used to determine the presence of the gold nanoshells at various synthe sis stages. For gold seeded PNIPAM-silica, TEM images shown in figure 4.9, illustrate that gold na noparticles between 2-6nm densely populate the polymeric matrices of the core particles. This particle size was ideal for creating
54 uniformed gold shells. TEM images show n in figure 4.10 are of 500:1 gold nanoshells grown using sodium borohydride. It can be seen from these images that the shells were incomplete and that there were large gold aggr egates both attached to the core particles and free in the solution. TEM images of 250:1 gold nanoshells, also in figure 4.10, displayed core particles that were coated with more gold than that seen in nanoshells described above. The 250:1 particles were expected to have le ss gold than the 500:1 particles simply because the gold ratio was lower, therefore, it is assumed that the inconsistency in the shell formation is due to the excess aging (2 days) of the K-Gold solution used for the 500:1 gold nanoshells. Both the 500:1 and the 250:1 samples were prepared using the same procedure, excep t in the 250:1 the K-Gold solution was only aged for 1 day. These findings helped us to determine that the age of the K-Gold solution played a major role in the formation of th e gold shell. Images taken of formaldehyde grown nanoshell, 50:1 and 20:1, are shown in figure 4.11. Both type s of particles had rough and incomplete shells. Large gold aggr egates were also seen in both samples, especially 50:1, presumably due to the increas e in K-Gold. UV vis spectra were recorded at various temperatures for sodium bor ohydride (500:1), formaldehyde (50:1), and formaldehyde (20:1) grown gold nanoshells. For the 500:1 shells grown with NaBH4 two sets of spectra were taken, the first was ta ken on the day of synt hesis shown in figure 4.12, and the second was taken 4 days after synthesis, shown in figure 4.13. The spectra for 500:1 nanoshells grown using NaBH4 display two absorption peaks, a main peak in the visible region around 600nm and a sec ond in the near infrared region at approximately 960nm. The initial spectrum is taken at 45oC directly after synthesis and the following 3 are taken approximately every 5 minutes as the sample cooled. The fifth
55 and sixth spectra were taken after 55 minut es and 3 hours of cooling, respectively. For those spectra taken 4 days posy-synthesis the sample was initially reheated to 55oC and then allowed to cool. A spectrum was taken every 5 minutes for 15 minutes total. The overall collection of spectra, for figures 4.12 and 4.13, shows that as the temperature of the sample increase, the visible peak remains almost constant but th e near infrared peak intensifies to absorb more irradiated light. The shift in peak intensity was reversible and when the sample temperature cooled the NI R peak declined in intensity. The main difference between the spectra taken on the da y of synthesis and those taken after some time was that the intensity of the visible a nd NIR peaks in the latter were much less intense. The lack of intensity in the aged sample could be due to the aggregation of gold on the core particle surface or even the separation of these large clusters from the core particle. The nanoshells grown using form aldehyde offer slightly different optical properties, the properties for a 50:1 and a 20: 1 ratio are displayed in the UV-vis spectra shown in figure 4.14 and 4.15, respectively. Th e absorption spectra for both the 50:1 and 20:1 ratios show a broad peak that spans from the visible region, 600nm well into the near infrared region, 1000nm. The broadness of this peak increased as the sample cooled to room temperature. For each rati o, the first spectrum was taken at 55oC and at this temperature the peak is seen more in the vi sible region. Four additional spectra were taken in 5 minute intervals as the temperature of the sample decreased. Here, the peak broadened as the sample cooled. When a lower amount of formaldehyde, 20:1, was used the broadening of the peak as the sample c ooled was much greater when compared to those synthesized using a larger formaldehyde ratio, 50:1. The visible region peak is believed to be a result of the collapse in the polymeric matrix. When the matrix was
56 collapsed the gold aggregates were closer t ogether causing an optical absorption similar to that of gold nanoparticles. The broadening seen in the absorptio n peak as the sample cooled was presumably a result of the gold aggregates being forced apart due to the increase in the core particle diameter. Based on the results presented in this ch apter, PNIPAM coat ed silica particles were successfully synthesized and these hybrid core particles were able to be converted into three layered thermally responsive gold nanoshells with unique optical properties.
57 Figure 4.1: Schematic synthesis of three layered gold nanoshells.
58 1.4 1.2 1.0 0.8 0.6 0.4Absorbance 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber(cm-1) Â— MPS Silica 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6Absorbance 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber(cm-1) Â— Bare Silica Figure 4.2: FTIR spectrum of bare silica particles (top ) compared to MPS grafted silica particles (bottom).
59 Figure 4.3: TEM images of bare silica part icles (left column) compared to MPS grafted silica particles (right column).
60 20 15 10 5 0Intensity(%) 100 101 102 103 104Size(nm) 20 15 10 5 0Volume(%) 100 101 102 103 104Size(nm) Figure 4.4: DLS measurements showing in tensity scattering (top) and volume distribution (bottom) of bare silica particles.
61 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6Absorbance 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber(cm-1) Â— Bare Silica 0.27 0.26 0.25 0.24 0.23 0.22Absorbance 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber(cm-1) Â— PNIPAM Coated Silica Si-O-Si Hydrocarbons Amide Figure 4.5: FTIR spectrum of bare silica particles (t op) compared to PNIPAM coated silica particles (bottom).
62 20 15 10 5 0Volume(%) 100 101 102 103 104Size(nm) Â— Bare Silica Â— PNIPAM Coated Silica 25 20 15 10 5 0Intensity(%) 100 101 102 103 104Size(nm) Â— Bare Silica Â— PNIPAM Coated Silica Figure 4.6: DLS distribution gr aph of blank silica compared to PNIPAM coated silica particles: intensit y (top) and volume (bottom).
63 340 320 300 280 260Size Peak(nm) 40 35 30 25 20 Temperature (oC) Figure 4.7: Variation of hydr odynamic diameter from DLS of PNIPAM coated silica particles as a function of temperature. Th e dashed line is drawn to guide the eye.
64 Figure 4.8: TEM images of bare silica partic les (left) compared to PNIPAM coated silica particles (right)
65 Figure 4.9: TEM images of gold seed ed PNIPAM-silic a particles
66 Figure 4.10: TEM images of nanoshells with K-Gold to seed rati o of 500:1 (left) and 250:1 (right) prepared using sodium borohydride route.
67 Figure 4.11: TEM images of nanoshells with K-Gold to seed rati o of 50:1 (left) and 20:1 (right) prepared using formaldehyde.
68 0.105 0.100 0.095 0.090 0.085 0.080Absorbance 1100 1000 900 800 700 600 500 400 Wavelength(nm) T=45oC T<45oC(-5minutes) T<45oC(-13minutes) T<45oC(-19minutes) T<45oC(-55minutes) T<45oC(-3hrs) Figure 4.12: UV-vis spectra of 500:1 ratio nanoshells prepared using sodium borohydride. The spectra are measured as sample cooled from 45oC to room temperature.
69 75x10-3 70 65 60 55Absorbance 1100 1000 900 800 700 600 500 400 Wavelength(nm) 4 Days Post-synthesis T=55oC T<55oC (-5minutes) T<55oC (-10minutes) T<55oC (-15minutes) Figure 4.13: UV-vis spectra of 500:1 ratio nanoshells prepared using sodium borohydride. The spectra were taken 4 d ays after synthesis and are measured as sample cooled from 55oC to room temperature.
70 1.2 1.0 0.8 0.6 0.4Absorbance 1100 1000 900 800 700 600 500 400 Wavelength(nm) T=55oC T<55oC(-5minutes) T<55oC(-10minutes) T<55oC(-15minutes) T<55oC(-20minutes) Figure 4.14: UV-vis spectra of 50:1 ratio na noshells prepared using formaldehyde and taken every 5 minutes for 20 minutes total as sample cooled from 55oC.
71 1.0 0.9 0.8 0.7Absorbance 1100 1000 900 800 700 600 500 400 Wavelength T=55oC T<55oC (-5minutes) T<55oC (-10minutes) T<55oC (-15minutes) T<55oC (-20minutes) Figure 4.15: UV-vis spectra of 20:1 ratio na noshells prepared using formaldehyde and taken every 5 minutes for 20 minutes total as sample cooled from 55oC.
72 CHAPTER 5: SUMMARY OF RESEARCH In summary, the research in this thesis successfully developed two novel thermally responsive gold nanoshell materials. The novel nanoshells materials combined for the first time an inorganic and an orga nic component. Specifica lly, the first nanoshell is a two layered material constructed of a PNIPAM-siloxane hybrid core coated with an outer layer of gold. The second material consists of three layers, a silica core, PNIPAM middle layer, and an outer gold shell. Each of these materials was successfully synthesized using a layer by layer technique. By changing various parameters, such as temperature, concentration, pH, and reaction ti me, the core particle size, thermal response of the polymer layer, and the go ld shell thickness can be controlled. Controlling each of these parameters is very important because they each play a key role in the optical properties exhibited by the material. The two layered hybrid core nanoshells are described in chapter three. Results from this chapter confirm the synthesis of thermally responsive hybrid core particles. It was demonstrated using DLS that the hybrid microgels were free to collapse and swell with temperature change, prior to the addition of the gold shell. Thes e particles, although less dense than traditional nanoshell cores, ar e capable of supporti ng a gold nanoshells on their surfaces. Based on UV-vis spectroscopy results the incorporat ion of a shell around the hybrid core did not hinder the thermal re sponsiveness of the particles. UV-vis also revealed that a change in particle diameter induced a slight red shift in the optical
73 absorption spectrum. Although optically re sponsive particles we re synthesized TEM images displayed particles with incomplete a nd uneven shells. Further investigations are needed to determine the ex act amount of gold solution a nd reducing agent needed to create particles with uniform shells of a desi red thickness. It is assumed that creating uniform nanoshells will enhance the optical properties even further. Chapter four details the synthesis and prope rties of three layered gold nanoshells. These particles consist of silica cores, as s een in traditional gold nanoshells, coated with PNIPAM, and lastly a layer of gold. Silica pa rticles were incorporated because they are known in literature to produce stable gold nanoshells. The polymer coating was used to maintain the thermal responsiveness found in the hybrid core nanoshells detailed in chapter 3. Results from DLS revealed that the polymer middle layer is temperature responsive and there is no hindr ance from the silica core partic le. When compared to the hybrid core particles, described in chapte r three the polymeric layer of the three component nanoshell was more thermally respon sive presumably due to the lack of a comonomer. Images taken using TEM demonstrat e that many of the polymer coated silica particles contained two or mo re silica cores. TEM images taken of the gold nanoshells displayed incomplete shells and large gold aggr egates attached to the core particles as well as freely dispersed within the sample. Finally, by utilizing UV-vis spectroscopy a thermally responsive NIR peak was clearly de fined. Further research is needed to determine the optimum silica diameter to prevent the pr oduction of multi-core PNIPAMsilica particles. More studies are also need ed to perfect the shape and uniformity of the gold nanoshell.
74 Incorporating organic components into th is normally inorganic material was a novel contribution to the field of nanomaterials. Both ma terials presented here, the hybrid core and the PNIPAM-silica core gold nanoshells, exhibited unique thermal and optical properties, in solution. For both two and three layer gold nanoshells grown using a strong reducing agent, NaBH4, we saw a NIR peak whose absorption intensified proportional to temperature increase. The abso rption properties of these materials caused heat to dissipate from the particle surface and as a result the surrounding environment was heated. As the environmental temperature increased the particle absorbed more NIR light, this cyclic process crea ted a continuous heating system, in solution. Fo r three layer gold nanoshells grown using formaldehyde, a w eaker reducing agent, we saw a different set of thermal and optical properties. Here the absorbance band decreased and became broader as a result of a temperature decrease This property could contribute to the potential development of novel nanomateri als with the ability to automatically discontinue absorption as the surrou ndingl temperature increases. Solar conversion systems commonly exist as flat panels or as a coating dried on a linear surface therefore, in fu ture work the formation of these novel gold nanoshells into monolayers on flat substrates will need to be studied. Also, determining how the confinement of these particles to a flat substrate will influence their thermal and optical properties will also need to be thoroughly investigated. By exploiting these properties new and more efficient solar conversion systems can potentially be developed.
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