Coupling effects of NaYF4Yb,Er upconversion nanoshells and au ag metallic nanoshells 2

Chapter Experimental Methods   2.1 Material synthesis: UC nanoshells UC nanoparticles with hcp phase and diameter size ~10 nm were synthesized using thermal decomposition.71,72 A high UC emission was achieved for the UC nanoparticles produced using this method.73 In this method, precursors of trifluoroacetates of metal ions were decomposed at high temperature above 300 oC in high boiling point solvents (e.g 1-octadecene) and surfactants (e.g. oleylamine and oleic acid), leading to the formation of UC nanoparticles stabilized by the surfactants. Recent reports showed the hollow nanoparticles were synthesized via Kirkendall effect.74,75 The first Kirkendall effect was reported in 1942 and the result was confirmed in 1947.76,77 The Kirkendall effect was first studied for the synthesis of hollow structures of cobalt sulfide nanoparticles from room temperature to 182oC.74,78 The formation mechanism of voids inside the particles was dominated by outward diffusion of cobalt cations and balanced by inward diffusion of vacancies. The small voids in each particle were observed between the cobalt core and sulfide shell due to condensation of vacancies at the boundary. These small voids coalesced into bigger ones, followed by disappearance of the cobalt cores. Finally, a single void in the center of the cobalt sulfide nanoparticles was formed. More recently, the Kirkendall effect has been applied to synthesize hollow UC nanoshells. For example, hollow NaYF4:Yb,Er UC nanoshells were synthesized using a controlled ion exchange process from cubic-phase Y2O3 nanospheres79 or thermal decomposition of a mixture of trifluoroacetate 24    precursors.80 These hollow nanoshells were formed due to the Kirkendall effect. In this thesis, NaYF4:Yb,Er nanoshells with hcp crystal structure were synthesized using thermal decomposition of trifluoroacetate precursors in oleylamine at 340 °C.81    2.1.1 Preparation of trifluoroacetate precursors Trifluoroacetate precursors of Y, Yb, and Er [(CF3COO)3M, M = Y, Yb, and Er ions] were prepared by dissolving their respective oxides or hydroxides in trifluoroacetic acid (CF3COOH), followed by drying in oven at 80 oC.73 Sodium trifluoroacetate (CF3COONa) was prepared by dissolving sodium carbonate (Na2CO3) in trifluoroacetic acid, followed by drying in oven at 80 oC. 2.1.2 Synthesis of NaYF4:20%Yb,2%Er UC nanoshells In the synthesis of NaYF4:20%Yb,2%Er nanoshells, 20 mL of oleylamine was first reacted with mL of trifluoroacetic acid in a 50-mL three-neck flask under a continuous flow of Ar gas. A mixture of (CF3COO)3Y (0.488 mmol), (CF3COO)3Yb (0.125 mmol), (CF3COO)3Er (0.013 mmol), and CF3COONa (1.252 mmol) was then added and followed by 0.6 mL of deionized water under vigorous stirring at 60 °C for min. This mixture was heated to 340 °C using a heating mantle under refluxing condition and in the presence of Ar gas for protection from oxidation. After 30 min, the mixture was allowed to cool to 80 °C. The oleylamine-capped UC nanoshells were isolated by centrifugation at 10000 rpm for min, followed by washing and redispersing in hexane for characterizations. To investigate the formation 25    mechanism of the UC nanoshells, the samples were collected at 300 °C, min, 10 min, and 30 at 340 °C for structure and microstructure analyses. 2.1.3 Synthesis of solid NaYF4 core/NaYF4:20%Yb,2%Er shell nanoparticles In the synthesis of solid NaYF4 core (~10 nm)/NaYF4:20%Yb,2%Er UC shell (~3 nm), the solid NaYF4 core was first prepared and then coated by UC shell, following the reported methods.32,35 In the preparation of solid NaYF4 core, CF3COONa (1.252 mmol) and (CF3COO)3Y (0.626 mmol) was dissolved in oleylamine (20 ml) in a 50-mL three-neck flask at 90 ºC under Ar until a clear solution was formed. The mixture was then heated to 340 °C using a heating mantle and kept at such temperature for 30 min. The solution was allowed to cool to 100 ºC. A mixture of (CF3COO)3Y (0.976 mmol), (CF3COO)3Yb (0.250 mmol), (CF3COO)3Er (0.026 mmol), and CF3COONa (2.504 mmol) were added to the solution. This mixture was heated to 340 °C under refluxing condition and Ar. After 30 min, the mixture was allowed to cool to 80 °C. The particles were isolated by centrifugation at 10000 rpm for min, followed by washing and redispersing in hexane. 2.1.4 Synthesis of solid NaYF4:20%Yb,2%Er nanoparticles In the typical synthesis of solid NaYF4:20%Yb,2%Er nanoparticles (~15 nm), (CF3COO)3Y (0.976 mmol), (CF3COO)3Yb (0.250 mmol), (CF3COO)3Er (0.026 mmol), and CF3COONa (2.504 mmol) were dissolved in oleylamine (20 ml) in a 50-mL three-neck flask at 90 ºC under the presence of Ar gas until a clear solution was formed. The mixture was then heated to 26    340 °C using a heating mantle and held at this temperature for 40 min. The solution was allowed to cool to 80 ºC. The particles were isolated by centrifugation at 10000 rpm for min, followed by washing and redispersing in hexane. 2.1.5 Surface coatings of NaYF4:20%Yb,2%Er UC nanoshells Undoped NaYF4 nanoshells were first synthesized using the synthesis method of NaYF4:20%Yb,2%Er nanoshells (Sec. 2.1.2). For the preparation of undoped NaYF4 nanoshells, only (CF3COO)3Y (0.626 mmol) and CF3COONa (1.252 mmol) were used in the first step without (CF3COO)3Yb and (CF3COO)3Er. The surface coatings were done by coating the undoped NaYF4 nanoshells with Yb,Er doped NaYF4 shell, followed by another undoped NaYF4 shell on top. The detailed procedure is given as follows. A mixture of (CF3COO)3Y (0.976 mmol), (CF3COO)3Yb (0.250 mmol), (CF3COO)3Er (0.026 mmol), and CF3COONa (2.504 mmol) were added to the solution of as-synthesized undoped NaYF4 nanoshells in a 50-mL three-neck flask and then heated to 340 °C using a heating mantle under refluxing condition and Ar. After 30 min, the mixture was allowed to cool to 100 °C. Then (CF3COO)3Y (1.952 mmol) and CF3COONa (5.008 mmol) were added and followed by heating to 340 oC. After 30 min, the mixture was allowed to cool to 80 °C. The particles were isolated by centrifugation at 10000 rpm for min, followed by washing and redispersing in hexane. 27    2.2 Material synthesis: Au-Ag metallic nanoshells Galvanic replacement reaction is a powerful method to produce hollow metallic nanostructures.52,82 In galvanic replacement reaction, Ag nanoparticles are commonly used as the sacrificial metal template in synthesis of hollow Au-Ag nanoshells. Au would be formed via the reduction of HAuCl4 and deposited on Ag templates being oxidized.83 The deposition of Au and oxidation of Ag solids lead to the formation of Au-Ag nanoshells with interior cavity. 2.2.1 Synthesis of Ag nanoparticles In a typical synthesis of Ag nanoparticles consisted of decahedral (~43 nm in size) and triangular prism (~53 nm in edge length) shapes, mL of 1octadecene (ODE) and mL of oleylamine were mixed using a magnetic stirrer at a spin rate of 700 rpm in a 25-mL three-neck flask. ODE was selected because of its high boiling point (~315 oC) and good compatibility with oleylamine, allowing the reaction at 160 oC. The ODE-oleylamine mixture was then heated to 160 oC using a heating mantle under a continuous flow of N2 gas. A solution of 10 mg of AgNO3 (58.9 mol) was immediately injected to the mixture. The temperature decreased to ~155 oC after injection of the AgNO3 solution, then increased to 160 oC again within a few minutes. After 30 at 160 oC, the solution was cooled to 60 oC, followed by dilution with mL of toluene. The solution of Ag nanoparticles was kept in a vial wrapped with aluminum foil and stored in the dark until further use. Assynthesized Ag nanoparticles were used as a sacrificial template in synthesis of Au-Ag nanoshell via the galvanic replacement reaction with HAuCl4. 28    2.2.2 Synthesis of Au–Ag nanoshells As-synthesized Ag nanoparticles (16 mL in the solution) were added to a 50-mL three-neck flask (with magnetic stirrer at a spin rate of ~700 rpm) and then heated to 60 oC in a water bath. A mM HAuCl4 solution was prepared by dissolving 29.5 mg of HAuCl4•3H2O (75 mol) in 13.5 mL of toluene and 1.5 mL of oleylamine. Fresh HAuCl4 solution was prepared and kept in the dark before use. The mM HAuCl4 solution was added to the 50-mL reaction flask at an injection rate of 0.25 mL/min. The samples were collected after injection of various amounts of mM HAuCl4 solution as shown in Table 2.1. The samples were isolated by centrifugation at 10,000 rpm for min. The obtained particles were washed three times with mL of toluene, followed by centrifugation, and re-dispersed in mL of toluene for characterizations. Table 2.1 Samples were collected after the injection of a variable amount of mM HAuCl4 solution for the measurement of microstructure, morphology, chemical composition, and extinction spectra. Sample Volume of mM HAuCl4 solution (mL) Amount of HAuCl4 (mol) 2.0 3.0 4.0 10 15 20 5.0 6.0 10.0 15.0 25 30 50 75 29    2.3 Assembly of Au-Ag nanoshells and UC nanoshells To study the plasmonic effects of Au-Ag nanoshells on fluorescence properties of UC nanoshells, single layer of Au-Ag nanoshells on glass substrates were prepared using spin-coating, followed by the coating of silica film using sputtering. UC nanoshells were further deposited on the silica filmcoated Au-Ag nanoshell layer to form an assembly of Au-Ag nanoshell layer/silica film/UC nanoshell layer. In this assembly, the distance between Au-Ag nanoshell layer and UC nanoshell layer was controlled by the thickness of silica film. The assemblies with different surface coverage % of Au-Ag nanoshell layer were also prepared. The surface coverage- and distance- dependent fluorescence of the UC nanoshells was investigated. 2.3.1 Preparation of Au-Ag nanoshell layer Glass substrate was prior soaked in aqua regia (3 parts concentrated HCl (37%)/1 part concentrated HNO3 (65%)) for removal of contamination at the glass surface, followed by washing three times with ethanol. The washed glass was then dried in oven at 80 oC for 24 h. The glass was kept in vacuum chamber before use. Au-Ag nanoshell layer on glass substrate was prepared using spincoating. The Au-Ag nanoshell solution (25 L) of was dropped on a glass substrate (1.5 cm x 1.5 cm), followed by spin-coating at a speed of 1500 rpm for 30 seconds for each cycle. Samples were prepared by – 40 cycles of spin coating to obtain Au-Ag nanoshell layer with different surface coverage % on the substrates. The samples were kept in the vacuum chamber before use. Scanning electron microscopy (SEM) was performed for each sample. The 30    surface coverage % of the Au-Ag nanoshells on the substrate was calculated from the SEM images using Java image processing and analysis program (ImageJ). The surface coverage % is defined by the ratio of area occupied by the nanoshells to the total analyzed area. 2.3.2 Assembly of Au-Ag nanoshell layer/silica film/UC nanoshell layer Silica deposition was performed using a magnetron sputtering system at a deposition rate of 15 nm per h at room temperature. The operating conditions were 150 W (RF power), 0.5 Pa (chamber pressure), and 50 standard cubic centimeter per minute (sccm) (Ar flow rate). The silica deposition rate was calibrated using surface profiler (KLA Tencor Alpha-Step Q). The glass and silica have similar appearance and properties. For the thickness measurement using the surface profiler, the glass substrate was first coated with Ti film and followed by silica film. The silica thickness was obtained from the total (Ti + silica) thickness normalized by the thickness of Ti film. The average silica deposition rate was measured to be ~15 nm per h. A similar silica deposition rate (~15 nm per h) was obtained from the silica film sputtered on silicon substrates using the same operating conditions of the magnetron sputtering system (Fig. 2.1). In this study, silica film was selected due to its good chemical and thermal stability, and low thermal conductivity (1.4 Wm−1K−1).84 Its inertness to solvents (e.g. hexane and toluene) would allow the deposition of hexane solution of UC nanoshells on the silica filmcoated Au-Ag nanoshells by solvent evaporation method, whereas the low thermal conductivity would reduce the photothermal effects of Au-Ag nanoshells to UC nanoshells. 31    Fig. 2.1 (a) SEM image of cross-section of silica film sputtered on silicon substrate for hours. (b) Average thickness of silica films obtained at different sputtering time. The average silica deposition rate was calculated to be ~15 nm per h. The Au-Ag nanoshell layer with different surface coverage % on the glass was coated by 30-nm silica films using a magnetron sputtering system. The silica film-coated Au-Ag nanoshell layer was then coated with UC nanoshells. The procedure is given as follows. The sample of silica filmcoated Au-Ag nanoshell layer was placed in a 10-mL beaker glass. Then mL of 0.07 wt% hexane solutions of UC nanoshells were added to the 10-mL beaker glass. The UC nanoshells were then deposited on the Au-Ag nanoshell layer/silica film by solvent evaporation in a vacuum chamber (Fig. 2.2). To obtain the similar concentration of UC nanoshells deposited on the Au-Ag nanoshell layer/silica film, same concentration and volume of hexane solutions of UC nanoshells was used to fabricate all the assemblies in this study. The effects of the Au-Ag surface coverage on the fluorescence of the UC nanoshells were studied. To investigate the distance-dependent fluorescence of UC nanoshells, the Au-Ag nanoshell layer (prepared by 20 cycles of spin-coating) was coated 32    by silica films with different thickness (5 - 180 nm) using the magnetron sputtering system. The UC nanoshells were then deposited on the Au-Ag nanoshell layer/silica film to form an assembly of Au-Ag nanoshell layer/silica film/UC nanoshell layer. The distance between the Au-Ag nanoshell layer and UC nanoshell layer was controlled by the thickness of the silica film.   Fig. 2.2 Schematic of deposition of UC nanoshells (0.07 wt% in hexane) by solvent evaporation in a vacuum chamber. 2.4 Material characterizations   2.4.1 Crystal structure X-ray diffraction (XRD) is a non-destructive characterization technique which can provide crystal structure information of materials. In this technique, monochromated X-ray striking a sample is scattered by the atoms in the sample. The scattered intensity of the X-ray is collected as a function of the incident and scattered angle. In this thesis, the crystal structures of the 33    samples were investigated using powder XRD diffractometer system (Cu K radiation) (Bruker AXS, Karlsruhe, Germany). XRD spectra of the samples were compared with their corresponding standard XRD spectra [Joint Committee on Powder Diffraction Standards (JCPDS), for example, file number PDF 16-334 for hcp sodium yttrium fluoride and PDF 4-783 for fcc silver]. 2.4.2 Microstructure and surface morphology Transmission electron microscopy (TEM) is a microscopy technique in which an electron beam is transmitted through an ultra-thin sample, interacting with the specimen as it passes through. An image is formed from the interaction of the electrons transmitted through the sample. In this thesis, the microstructure of the nanoparticle samples was studied using a JEOL JEM 2010F transmission electron microscope (JEOL, Tokyo, Japan) operated at 200 kV. Carbon-coated copper grids (400 meshes) were used to support the nanoparticles. The average particle size was estimated by random measurements of 100 – 200 particles from the TEM images. Elemental composition of the samples were performed using energy dispersive X-ray (EDX) in the transmission electron microscope. The average concentration of the elements in the samples was determined from the EDX data randomly collected at least different selected area. Different from the TEM which produces an image by detecting the electrons transmitted through the sample, scanning electron microscopy (SEM) produces an image by detecting the electrons such as secondary electrons which are emitted from the sample surface due to excitation by the 34    primary electron beam. The SEM can produce high-resolution images of the sample surface. In this thesis, the surface morphology of the samples was investigated using a Zeiss Supra 40 field emission scanning electron microscope (Carl Zeiss, Oberkochen, Germany). The samples were prepared by depositing the particle solution on silicon substrate, followed by drying in vacuum chamber. 2.4.3 Dynamic light scattering (DLS) analysis DLS is a technique that can be used to determine the size distribution profile of small particles in suspension. In DLS analysis, the motion related to size of the particles is measured. The smaller particles move faster than the larger ones in the solution. The particle size obtained by this technique is the diameter of a sphere which is commonly referred to as the hydrodynamic diameter. In this study, the DLS measurement was performed using a Malvern Zetasizer Nano ZEN3600 (Malvern Instruments, Worcestershire, UK). 2.4.4 UC fluorescence The room UC fluorescence spectra were measured using a LS-55 luminescence spectrometer (Perkin Elmer Instruments, Cambridge, UK) with an external 980-nm laser diode (1 W, continuous wave with m fiber, Beijing Viasho Technology Co., Beijing, China) as the excitation source in place of the xenon lamp in the spectrometer. The spectrometer was operated in the bioluminescence mode, with a gate time of ms, delay time of 10 ms, cycle of 20 ms, and a flash count of 1. The UC fluorescence photographs were taken 35    using Canon Powershot A620 and Canon EOS 60D cameras (Canon, Tokyo, Japan). 2.4.5 UV–Vis–NIR extinction UV–Vis–NIR spectrometer measures the intensity of light passing through a sample and compares it to the intensity of incident light. For example, when light passes through a particle, the light can be absorbed, scattered, and/or transmitted by the particle (Fig. 2.3). The absorption and scattering of the particle can be expressed by absorption and scattering cross section, respectively. The sum of absorption and scattering cross section is referred to as the extinction cross section. The extinction spectra can be determined by recording the light passing through the sample using UV–Vis– NIR spectrometer. In this thesis, the UV–Vis–NIR extinction spectra were measured using a Cary-5000 UV–Vis–NIR spectrophotometer (Varian, Palo Alto, CA, USA). The extinction spectra of the samples were normalized to that of their surrounding mediums (e.g. solvents, substrates). Scattered Incident lights Absorbed Transmitted Particle Fig. 2.3 An illustration describes the transmission, absorption, and scattering processes of lights passing through a particle. 36    2.4.6 X-ray photoelectron spectroscopy (XPS) Photoelectron spectroscopy is based upon a single photon in/electron out process. In XPS, an X-ray photon of energy hv absorbed by an atom in a molecule or solid leads to ionization and the emission of a core (inner shell) electron. The kinetic energy (Ek) of the emitted photoelectron is measured by electron energy analyzer (electron spectrometer). Thus, the binding energy (Eb) of the electron can be calculated: Φ 2.1 where Φ is the spectrometer work function. From the XPS spectra, the composition of the elements can be calculated from the ratio of integrated peak areas normalized by respective sensitivity factors. The XPS analysis was performed using Kratos Axis Ultra DLD (Kratos analytical, Manchester, UK) with a monochromated Al Kα source (1486.6 eV). 2.4.7 Inductively coupled plasma optical emission spectrometry (ICPOES) ICP-OES is an analytical technique used for the detection of trace metals. It is a type of emission spectroscopy that uses the inductively coupled plasma to produce excited atoms or ions that emit electromagnetic radiation at wavelengths characteristic of a particular element. The intensity of this emission is indicative of the concentration of the element within the sample. The composition of metal ions was performed using a Dual-view Optima 5300 DV ICP-OES system (PerkinElmer, Shelton, USA). 37    [...]... bioluminescence mode, with a gate time of 1 ms, delay time of 10 ms, cycle of 20 ms, and a flash count of 1 The UC fluorescence photographs were taken 35    using Canon Powershot A 620 and Canon EOS 60D cameras (Canon, Tokyo, Japan) 2. 4.5 UV–Vis–NIR extinction UV–Vis–NIR spectrometer measures the intensity of light passing through a sample and compares it to the intensity of incident light For example, when... image is formed from the interaction of the electrons transmitted through the sample In this thesis, the microstructure of the nanoparticle samples was studied using a JEOL JEM 20 10F transmission electron microscope (JEOL, Tokyo, Japan) operated at 20 0 kV Carbon-coated copper grids (400 meshes) were used to support the nanoparticles The average particle size was estimated by random measurements of. .. random measurements of 100 – 20 0 particles from the TEM images Elemental composition of the samples were performed using energy dispersive X-ray (EDX) in the transmission electron microscope The average concentration of the elements in the samples was determined from the EDX data randomly collected at least 5 different selected area Different from the TEM which produces an image by detecting the electrons... diffractometer system (Cu K radiation) (Bruker AXS, Karlsruhe, Germany) XRD spectra of the samples were compared with their corresponding standard XRD spectra [Joint Committee on Powder Diffraction Standards (JCPDS), for example, file number PDF 16-334 for hcp sodium yttrium fluoride and PDF 4-783 for fcc silver] 2. 4 .2 Microstructure and surface morphology Transmission electron microscopy (TEM) is a microscopy... of incident light For example, when light passes through a particle, the light can be absorbed, scattered, and/ or transmitted by the particle (Fig 2. 3) The absorption and scattering of the particle can be expressed by absorption and scattering cross section, respectively The sum of absorption and scattering cross section is referred to as the extinction cross section The extinction spectra can be determined... photon of energy hv absorbed by an atom in a molecule or solid leads to ionization and the emission of a core (inner shell) electron The kinetic energy (Ek) of the emitted photoelectron is measured by electron energy analyzer (electron spectrometer) Thus, the binding energy (Eb) of the electron can be calculated: Φ 2. 1 where Φ is the spectrometer work function From the XPS spectra, the composition of. .. spectrophotometer (Varian, Palo Alto, CA, USA) The extinction spectra of the samples were normalized to that of their surrounding mediums (e.g solvents, substrates) Scattered Incident lights Absorbed Transmitted Particle Fig 2. 3 An illustration describes the transmission, absorption, and scattering processes of lights passing through a particle 36    2. 4.6 X-ray photoelectron spectroscopy (XPS) Photoelectron... emission spectroscopy that uses the inductively coupled plasma to produce excited atoms or ions that emit electromagnetic radiation at wavelengths characteristic of a particular element The intensity of this emission is indicative of the concentration of the element within the sample The composition of metal ions was performed using a Dual-view Optima 5300 DV ICP-OES system (PerkinElmer, Shelton, USA) 37 ... drying in vacuum chamber 2. 4.3 Dynamic light scattering (DLS) analysis DLS is a technique that can be used to determine the size distribution profile of small particles in suspension In DLS analysis, the motion related to size of the particles is measured The smaller particles move faster than the larger ones in the solution The particle size obtained by this technique is the diameter of a sphere which is... from the ratio of integrated peak areas normalized by respective sensitivity factors The XPS analysis was performed using Kratos Axis Ultra DLD (Kratos analytical, Manchester, UK) with a monochromated Al Kα source (1486.6 eV) 2. 4.7 Inductively coupled plasma optical emission spectrometry (ICPOES) ICP-OES is an analytical technique used for the detection of trace metals It is a type of emission spectroscopy . 30  2. 3 Assembly of Au- Ag nanoshells and UC nanoshells To study the plasmonic effects of Au- Ag nanoshells on fluorescence properties of UC nanoshells, single layer of Au- Ag nanoshells on glass. The deposition of Au and oxidation of Ag solids lead to the formation of Au- Ag nanoshells with interior cavity. 2. 2.1 Synthesis of Ag nanoparticles In a typical synthesis of Ag nanoparticles. (Na 2 CO 3 ) in trifluoroacetic acid, followed by drying in oven at 80 o C. 2. 1 .2 Synthesis of NaYF 4 :20 %Yb ,2% Er UC nanoshells In the synthesis of NaYF 4 :20 %Yb ,2% Er nanoshells, 20 mL of