Determining the Size and Shape of Gold Nanoparticles

Transcription

1 Determining the Size and Shape of Gold Nanoparticles Carthage College 11/17/2011

2 Abstract: Gold nanoparticles (GNPs) are studied because they have unique optical properties making them very useful for things such as diagnostics. GNPs were synthesized using cetyltrimethylammonium bromide (CTAB) to create icosahedral shapes and were then studied based on their Surface Enhanced Raman Scattering (SERS). Various imaging techniques were used in order to better understand the structure of GNPs and to prove a high yield for the controlled synthesis. Through the work by Haiss, et al. spherical gold nanoparticles were also synthesized and size was determined through UV-Visible spectroscopy (UV-Vis) and surface plasmon resonance (SPR). In another study by Kwon, et al. transmission electron microscope (TEM), scanning electron microscope (SEM), X-ray diffraction (XRD), and UV-vis were used to study the GNPs synthesized to determine the size and shape. Equations were developed based on the diameter of the particle through SPR and UV-vis spectra data which allowed one to be able to quickly and easily calculate size and concentration of GNPs. In conclusion, GNPs were synthesized using two different methods to produce spherical and icosahedral shapes and then were studied to determine size and their SERS properties. Introduction: Gold nanoparticles (GNPs) are the most stable metal nanoparticle and their unique optical and electrical properties make many researchers believe that they will be among the key materials and building blocks for nano-materials in the 21 st century 1. Determining the shape and diameter of nanoparticles is important because their uses in 1 of 25

3 optical 2, electronic, magnetic and catalytic applications are dependent upon shape and size 1. Nanoparticles can be prepared in numerous shapes such as rods, prisms, and wires 2, but high yields have never resulted from these syntheses. This thesis will examine a new high-yield synthesis of icosahedral GNPs and will present a study that characterizes the size of spherical GNPs. In the article by Kwon, et al., cetyltrimethylammonium bromide (CTAB) was used to control particle aggregation in a icosahedral GNPs synthesis 3. CTAB stops the growing process because it can act as a capping agent. The capping between the CTAB molecules and GNPs occur due to electrostatic interactions. If the GNPs were not capped then they could fuse into larger particles because GNPs experience dispersion interactions which increase their tendency to fuse. CTAB counteracts this fusing process by overcoming the electrostatic repulsion. Therefore, once the driving forces for growth by GNPs and the CTAB counteracting interactions are in equilibrium, the GNPs stop growing 1. In the article by Kwon, et al. Gold particles of 3.5nm size were used as seeds to grow larger nanoparticles with CTAB. A controlled synthesis was then preformed to produce GNPs of 10 to 90nm in size. Five synthesized samples were labeled A-E and had increasing particle size. Each of the five solutions contained the following: 9.0 ml of growth solution containing 2.5x10-4 M HAuCl 4, 0.10 M CTAB solutions and 50 µl of 0.10 M ascorbic acid. Of the seed solution 1.0 ml was mixed with solution A. Then 1.0 ml was taken from solution A and added to solution B. Again 1.0 ml of solution B was added to solution C and the same process was used to solutions D, and E. The 2 of 25

4 concentrations of the GNPs were calculated and are listed in Table 1. The five different samples were then studied using microscopy and spectroscopy 3. To examine the five samples, transmission electron microscopy (TEM) was used to capture images of the GNPs. In TEM, an electron beam interacts with and is transmitted through a thin sample, and then projected onto a florescent screen. An image of the sample is thus produced and the brighter regions of the image represent areas where more electrons have passed through the sample; whereas the darker regions represent areas where fewer electrons have been passed through due to higher sample density. TEM was also used because images are able to be magnified up to 500,000 times thus producing a high resolution, highly magnified, image. 4,5, 6 A scanning electron microscope (SEM) was also used to study the synthesized GNPs. In SEM, a set of coils moves the electron beam across a sample in a two dimensional grid. When the electron beam across over the sample, different interactions occur. Some of the electrons from the surface material kick of their electrons by the beam thus producing secondary electrons. These secondary electrons can then be detected by the secondary electron detector on a SEM. An image is then produced at the surface of the sample and is projected. Similarly to TEM, SEM images can be magnified up to 100,000 times while maintaining a high resolution. 5,4,6 3 of 25

5 Both TEM and SEM are electron microscopes and require an electron source. The most common electron source used is a tungsten filament. When the filament is heated, electrons are produced and are attracted by the anode where they are pushed down the column of the apparatus to interact with the sample. 6 Although SEM and TEM both use electron beams to produce images of particles, the techniques have a few important differences. For example, in TEM an electron beam passes through a thin sample whereas in SEM the electron beam scans the entire surface of the sample. Also, in TEM the samples are very thin, and in SEM the sample can be of any thickness. Lastly, TEM images are shown on fluorescent screens and SEM images are shown on television monitors. X-Ray diffraction (XRD) was also used to also examine the synthesized GNPs. In XRD, x-rays that are produced by the x-ray beam get scattered by the atoms in the sample. Waves are then scattered spherically from the atoms which causes the intensity of the scattered radiation to show minimum and maximums in different directions. XRD allows a better visual of the structure of the observed atoms because it shows axes, shape, size and position of the atoms 7. In the article of Kwon, et al., XRD was used to show that particle growth occurs through adding more gold atoms down the preferential plane of the five solutions 3. Surface-enhanced Raman scattering (SERS) is a more advanced spectroscopic technique which is a form of Raman spectroscopy. Raman scattering is a type of molecular vibration that can occur between light and molecules. If the energy of light is not enough to excite the molecules from the ground state to the lowest electronic state, the molecule is then instead excited to a virtual state between the two. SERS was 4 of 25

6 discovered by accident when a researcher was trying to do Raman scattering and produced a Raman scattering more intense than expected. This strong signal was then coined SERS. Therefore, SERS is a technique that enhancing Raman scattering by molecules adsorbed on a rough surface. SERS can occur when molecules are brought to the surface of metals and can be observed from silver, gold and copper. Also, if metal nanoparticles are used in SERS, the particle size needs to be from nm. 3,8 Surface plasmon resonance (SPR) was used and involved light being directed onto a thin surface which contains gold. Some of the lights electric field can leak onto the surface if the angle of light causes it to be reflected. The electric field excites the surface plasmon waves in the metal. A surface plasmon is an electron wave that travels along the surface of the gold film. The wavelength of this plasmon wave is then detected in the SPR experiment. 9,10 Ultraviolet-visible spectroscopy (UV-Vis) involves the absorption of light by molecules in ultraviolet-visible region. The absorption in the visible range directly corresponds to the color of the chemicals involved. UV-Vis can be used to determine the concentration of the solution based on the absorbance. UV-Vis uses a spectrophotometer which measures the intensity of light that passes through a given sample (I). The instrument then compares the measurement to the intensity of light before it passes through the sample (I 0 ). A percent transmittance is then determined by using I/I 0. Next, from the percent transmittance, an absorbance can be calculated by using Equation 1. A= -log (%Transmittance / 100%) (1) 5 of 25

7 Uv-Vis thus allows a concentration and absorbance to be calculated depending on the wavelength of the particle or solution studied 11. In a second study by Haiss, et al. UV-Vis and TEM were used to determine the size of spherical gold nanoparticles. GNPs in aqueous solutions with diameters of three to 120 nm were studied. Through UV-Vis spectroscopy and SPR, particle diameters were determined which are important for depending on the application of GNPs. Equations are developed that correlate UV-vis absorbance and SPR to particle size thus yielding a simple and quick method to determine size. 11 In the Haiss, et al. study Gold hydrosols were studied; a hydrosol is a colloidal system where the dispersion medium is water. The phase for this dispersion can be a solid, gas or another liquid 12. A colloidal system is a substance which is dispersed throughout another substance 13. Michael Faraday a famous chemists known for contributing to electrochemistry and electromagnetism first noted how to form red solutions when a reduction of chloroaurate (AuCl 4 - ) occurred 1. In order to synthesis the gold hydrosols used in the article by Haiss, et al. HAuCl 4 was used along with water. GNPs seeds were synthesized using citrate to reduce Au 3+ and had an approximate diameter of 17-20nm. The synthesized GNP seeds were then used as seed particles for the synthesis of GNPs larger than 20nm. This method allowed Au 3+ to be reduced on the surface. Gold needs to be reduced on the surface of GNPs because it increases their diameter depending on the side of the seed particle and the amount of gold reduced. The gold was then studied using UV-Vis spectroscopy in the wavelength range of 350 to 800 nm of 25

8 The total amount of gold used for synthesis, measured particle diameter, and volume of synthesized solutions were all used to then calculate the density of particles. Commercial GNPs were also used for comparison with the synthesized GNPs. The manufactures of the commercial GNPs provided the size and density of these GNPs. Through UV-vis and TEM, the synthesized and manufactured GNPs were compared and size and concentration were determined 11. In conclusion, gold nanoparticles were synthesized via two methods to create two different shaped particles. Spherical gold nanoparticles were synthesized and the size was characterized. Icosahedral particles were also synthesized in order to study their SERS properties. Other techniques such as TEM, SEM, XRD, UV-Vis were used to image the particles and to determine size and shape of GNPs. Therefore, through these two studies gold nanoparticles were synthesized and their size, and shape we characterized. Results and Discussion: Gold seeds were used in a synthesis to create gold nanoparticles with diameters ranging from 10 to 90 nm. The synthesis also used cetyltrimethylammonium bromide (CTAB) and ascorbic acid. The starting oxidation state of gold in the synthesis was Au 3+ and the final oxidation state was Au 0. As a result, a reducing agent was needed in order to first reduce gold to Au + from Au 3+. The reducing agent used in this synthesis was ascorbic acid. It allowed larger sized particles to be grown with the growth seeds 3. The reduction of gold was confirmed because the color changed when ascorbic acid was added. For example, before CTAB was added the solution color was orange and 7 of 25

9 once ascorbic acid was added, the orange color disappeared. Gold was not reduced to the Au 0 because ascorbic acid is too weak to reduce the gold further because it is a weak acid, with a pka of , 15 Gold seeds were also used in the synthesis because they act as nucleation centers which allow the gold to be reduced. The particle size of the gold seeds were around 3.5 nm 15. The gold seed particles help the synthesis to reduce Au + to Au 0 because they act as nucleation centers. Growth solution containing HAuCl 4 and CTAB was added to ascorbic acid and in labeled vials a-e. Each of the five solutions contained the following: 9.0 ml of growth solution containing 2.5x10-4 M HAuCl 4, 0.10 M CTAB solutions and 50 µl of 0.10 M ascorbic acid. One millimeter of the seed solution was mixed with solution A. Then 1.0 ml was taken from solution A and added to solution B. Again 1.0 ml of solution B was added to solution C and the same process was used for solutions D, and E. Therefore, after the reducing agents had been added gold atoms form in the solution and particles begin to form. 3 This nucleation process allows the remaining gold atoms to attach to these nucleation sites. As a result of the reducing agent and nucleation process, the growth of large nanoparticles was achieved. In this experiment, five different solutions labeled a-e were prepared of varying sizes of gold nanoparticles. Imaging measurements were used to determine the particle size of each solution of gold nanoparticles. Each solution a-e increased in particle size as shown through Figure 1 3. TEM images were obtained for the five solutions. TEM uses electrons as the light source to produce images of the particles examined 5,4. Figure 1 shows the TEM images of the CTAB-stabilized gold nanoparticles from solutions a-e. As seen from Figure 1, hexagonal shapes are seen in all solutions. 8 of 25

10 However, in solutions a and e hexagonal shapes are not as visible due to image quality. Particles in solution a are too small to determine a shape and particles in solution e are too large to show the hexagonal shape. Solutions b- d have good image quality and show recognizable hexagonal shapes 3. Gold particles that are icosahedral may show hexagonal shapes under TEM imagining because the shape of fine gold seeds in CTAB solutions is faceted with (111) faces. The differences between an icosahedron with (111) faces and a cube with (100) faces is illustrated in Figure Also, lower CTAB and higher ascorbic acid concentrations favor faster formation and deposition of neutral gold onto the (111) faces. This leads to the disappearance and formation of (100) faces and as a result produces cubic instead of spherical shapes. If the CTAB concentration was similar or slightly higher and the ascorbic acid concentration was slightly lower, then there would be a truncated octahedral which would contain both (100) and (111) faces 17. Therefore, 9 of 25

11 solutions a and e are not as visible as compared to b-d because TEM only gives a projected image of objects. Next, from the TEM measurements, nanoparticles diameters were measured for CTAB gold nanoparticles solutions a-e. Table 2, shows the mean particle diameters in nm for the 5 solutions 3. Table 2 and Figure 1, both show that the particle size increases greatly from solutions a-e using. Therefore TEM was successfully used to study the shape and diameter of the CTAB-stabilized gold nanoparticles. Scanning Electron Microscopy, (SEM) was also used in order to further examine the five solutions and see the definite structure of gold nanoparticles. In SEM, a set of coils moves the electron beam across a sample. When the electron beam moved across the sample, different interactions occurred. Some of the electrons from the surface material kick of their electrons by the beam thus producing secondary electrons and a image of the sample. 5,4 Solution D was analyzed with TEM and further used to show the shape of the gold nanoparticles. The SEM image of solution D is displayed in 10 of 25

12 Figure 3 3. The image shows that the nanoparticles have an icosahedral shape. Also, in Figure 3 the inset shows all of the (111) facets of an icosahedral GNPs. Solution D was then examined again using TEM to obtain a high-resolution image for better understanding the shape of the particles. Solution D was used because it offered the best images of the hexagonal shape which is the most useful for studying the gold nanoparticles icosahedral shape. Figure 4 shows the TEM images of solution D 3. Images, a and b are of solution D where b is an enlargement of a. The lines drawn on image b represent the two boundaries which intersect the two neighboring facets. To further confirm the icosahedron shape, the d-spacing was found to be 2.36 Å which is consistent with the (111) planes of face 11 of 25

13 centered cubic gold 3. D-spacing is important in the identification process. For example, each particle has a known value for d-spacing and thus when a d-spacing is known it can help establish which type of facets are observed 18. Through these further studies of solution D, it was concluded that the shape of the GNPs was icosahedron by using TEM and SEM. X-Ray diffraction (XRD) was also used to characterize the nanoparticles. This experiment involves scattering x- rays through specifically placed atoms of crystals. In this study XRD was used to show that particle growth proceeds by adding more gold atoms on the preferential planes for each solution. Figure 5 shows the XRD measurements for solutions a-e 3.Three peaks were obtained from each solution. As seen in Figure 5, solution E shows the largest peak for the (111) planes as expected because sample E has the largest particles. As a result, the particles grow when more gold atoms are added into the solutions causing an increase in the XRD patterns. The three peaks were assigned to diffraction peaks of gold metal of (111), (200), (220). The work by Hyunjoon, et al. also 12 of 25

14 produced XRD patterns for Pt nanoparticles at facets of (111), (200) and (220) 19. Equation 1 was used to calculate the intensities ratios of the 5 solutions 3. (2) Next, by examining Figure 5 and using Equation 2, the intensity ratios of the five solutions were calculated and are shown below in Table 2. Although, solution A was studied and shows a XRD pattern, the intensity could not be obtained due to poor spectral quality. Table 2 and Figure 5, clearly show that intensity ratios decrease as particle size increases. The peak for the (111) is always the largest peak compared to the (200) and (220) peaks because the ratio is less than one. The (220) peak is the smallest in most samples except for solution e because e has the GNPs size 3. From these ratios it was concluded that the gold nanoparticles are primarily composed of (111) facets. In other words, the five solutions have the highest intensities of the (111) peak and thus have an icosahedron shape. Therefore, the XRD, SEM and TEM results all point to gold nanoparticles with an icosahedral shape. Next, in order to further study gold nanoparticles, Ultraviolet-visible spectroscopy (UV-Vis) was used to determine the wavelength maximum of the five solutions. UV-vis 13 of 25

15 was used because gold nanoparticles have strong plasmon resonance absorption which is dependent on size and shape of the particles 3. Plasmon resonance absorption occurs because of d-d band transitions. Plasmon excitation occurs with the whole particle jumping through transition bands. Also, the resonance frequency can be affected by four factors: density of electrons, electron mass, mass and size of the charge. The UV-vis spectra of the 5 solutions were then compared to the gold seed used in the growth system. The literature value for Plasmon bands is usually a between 520 and 530 nm for spherical gold nanoparticles 3. Figure 6 shows the visible spectra of the five solutions (a-e) and the gold seed. From the data in Figure 5, Table 3 constructed to show the wavelength maxima for solutions a-e. The data shows that for solutions a-e, 14 of 25

16 wavelength increases, steadily. The wavelengths increase with each sample because the particle size also increases. Larger particles show plasmon absorbance at longer wavelengths. Finally, Surface Enhanced Raman Scattering, (SERS) spectra were used to study gold nanoparticles and investigate further the spherical and icosahedral shapes 3. SERS enhances the Raman signal from Raman-active molecules that have been absorbed on metal surfaces. Spherical gold nanoparticles were compared to the icosahedral gold nanoparticles through SERS. Noble metallic nanostructures exhibit SERS in which the scattering cross-sections are dramatically enhanced for molecules adsorbed on. Figure 7 3, shows the spectra of two gold nanoparticle shapes under three different concentrations. The average particle size for the spherical gold nanoparticles was 3.5 nm, the same as the icosahedral. Figure 6a shows the spectrum of 1 x 10-4 M 4- nitrobenzenethiol (4-NBT), Figure6b shows the spectrum of 1 x 10-4 M 2- mercaptopyridine (2-MP), and lastly Figure 6c shows the spectrum of 1 x 10-6 M 15 of 25

17 rhodamine 6G (R6G). Therefore, the three spectras in Figure 6 are of solution c and the three organic molecules used. Not only is the size important but the morphology or shape of the particles is also important because it has a great effect on SERS activities of organic molecules. In each of the three spectra in Figure 7, the upper line refers to the icosahedral particles and the lower line refers to the spherical particles. The icosahedral particles used in the SERS experiment were from solution c. Particles from solution c were used because they produced the strongest Raman shift for the SERS spectrum of 1 x 10-4 M 4-nitrobenzenethiol (4-NBT). In all the spectra, the icosahedral particles show intensities nearly four times that of the spherical particles. Icosahedral particles give higher intensities because they have very well-defined edges and corners and thus sharper features as compared to spherical particles. The detailed shape difference between icosahedral and spherical particles could be because of the larger localized field enhancement 3. Therefore, the SERS studies showed the intensities for two different shaped nanoparticles of gold. It was concluded that the icosahedral shapes gives stronger signals for intensities as compared to spherical gold nanoparticles. In conclusion, TEM, XRD, SEM, SERS, and UV-Vis spectroscopy techniques were used to study GNPs. In all experiments the nanoparticles synthesized were shown to have an icosahedral shape. Also, spherical particles were compared to the icosahedral particles using SERS. This study is related to the work by Haiss, et al. which deals with spherical GNPs. Haiss, et al. determined the size and concentration of spherical GNPs. This work also used UV-Vis and TEM to characterize the GNPs. 16 of 25

18 Gold nanoparticles (GNPs) can be used in a number of applications due to their optical properties. The applications include diagnostics, therapeutics, catalysis, and optical sensing 1. Wolfgang, et al. studied the optical properties of spherical gold nanoparticles, ranging in size from 3 to 120 nanometers 11. Through this work UV- Visible spectroscopy allowed the researchers to calculate particle size and to determine particle concentration. The Haiss, et al. study investigated particles with diameters ranging from three to 120 nanometers 11. Transmission electron microscopy (TEM) was used as in the previous study by Kwon, et al., to record images of gold particles and the gold hydrosols. In a hydrosol particles are uniformly dispensed in water 12. In this study, TEM was also used to image at least 100 particles from each particle size group. The size distribution of gold nanoparticles from the TEM images can be seen in Figure 8 11.The histogram was generated from 617 measurements of particle size 11. The inset shows a representative TEM image of the spherical gold nanoparticles. From the data in the histogram, the average diameter of spherical gold nanoparticles was found to be 60±9 nm. 17 of 25

19 Spherical GNPs were also studied with surface plasmon resonance or SPR. In the SPR experiment, light is directed onto a thin surface containing gold. If the angle of light causes it to all be reflected, some of the light s electric field leaks onto the surface. This electric field can then excite surface plasmon waves in the metal. A surface plasmon is an electron wave that travels across the surface of the gold film. The wavelength of this plasmon wave is detected in the SPR experiment. The wavelength has been shown to depend on the size of the nanoparticles on the surface. Previous work has shown that GNPs produce surface plasmon waves with wavelengths of 520 nm 9,10. Figure 9 shows a diagram of the surface plasmon resonance (SPR) experiment. 18 of 25

20 The overall goal of this experiment was to correlate the size of GNPs to the wavelength of the surface plasmon wave. Therefore, the wavelength of SPR (λ spr ) was examined as a function of the diameter of the particles. The results are shown in Figure The calculated wavelengths of surface plasmon peaks are shown as circles and the experimental data is shown as triangles. The upward triangles represent particles that were synthesized in house and the downward triangles represent the commercial gold nanoparticles 11. The error bars in this figure are horizontal and show the errors in the diameters of the particles used. The error bars are only shows for the in-house synthesized particles. As seen in the figure, the calculated and experimental wavelengths are in perfect agreement with one another. Wavelengths vs. diameters data for particles that were larger than 25nm were fit to the exponential function shown in Equation (3) 11. λ spr= λ o + L 1 exp (L 2 d) (3) 19 of 25

21 λ spr was the surface plasmon resonance wavelength and d was the diameter. The fit to the data in Figure 10 gave values for the free parameters to be λ 0 = 512, L 1 = 6.53, and L 2 = The error in the fit was three percent. Next, with the three values established, Equation (3) was rearranged to solve for d. This expression is shown in Equation (4). With the relationship between λ spr and d now established, Equation (4) can be used to calculate nanoparticles size (d) from the measurements of SPR wavelengths. λ λ (4) However, the equation cannot be used for particles smaller than 25nm because the experimentally observed wavelength is lower than what would be expected 11. Recall that the SPR wavelength for spherical GNPs is usually around 540 nm 20 and this experiment had a range of nm 11. However, when particles were smaller than 25 nm, the wavelength of SPR was smaller than 520 nm. The wavelength may be smaller for particles smaller than 25 nm because of the increase of the ratio of surface atoms to bulk atoms for small particle diameters 11. Since the data for GNPs smaller than 35 nm did not show an experimental dependence of λ spr on d, the size cannot be estimated by Equation (3). Therefore, UVvis spectroscopy had to be used to incorporate particles that are smaller than 35 nm into the study. UV-Vis spectra were collected for GNPs as a function of size. Data plotted on Figure 11 (a) of 25

22 Figure 11 (a) shows a plot of the ratio of the absorbance at two wavelengths vs. particle diameter. 11 For example, the circles in the image represent the ratio of the absorbance at the wavelength of SPR wave to the absorbance at 450. Unlike the square, diamonds and triangles, these data points increase at a steady rate with increasing diameter. Therefore the ratio of the absorbance at the SPR wavelength to the absorbance at 450 nm may be useful in measuring particle sizes less than 35 nm. Since none of the lines have a strong linearity, Figure 11 (a) is not very useful because only one line out of the five could be used for further analysis of the ratio of absorbencies to diameters. As a result, the data represented by the circles in Figure 11 (a) was used to construct, Figure 11 (b) 11. In Figure 11 (b) the absorbance ratio is plotted vs. the natural log of the GNPs diameter. Therefore, Figure 11 (b) is more useful in showing agreement between theoretical and experimental results when the absorbance ratios are in the wavelength 21 of 25

23 region below 600 nm. The particle diameters used were in the size range of 5 to 80 nm. The circles in the graph are the theoretical data points produced and can be fitted to a line with a R 2 value of The triangles in the graph again represent the experimentally in-house and commercial GNPs. Since a strong linear correlation was produced in the graph the ratio A spr /A 450 can be used to calculate the particle diameter with Equation (5). 11 (5) In the equation, B 1 is the inverse slope, and B 2 =B 0 /m where B 0 is the intercept. The values obtained for B1 and B2 are shown in Table 4. The equation had an error of 18% when the theoretical data for B1 and B2 were used to calculate particle diameter. However, if the experimental values for B1 and B2 were used, the error in determining particle size was 11%. Table 4 shows a comparison of experimental and theoretical data for B1 and B2. 11 As a result, Equation (5) can be used to determine the particle diameter of particles between 5 to 80 nm. In conclusion, the particle diameters can be determined through experimental techniques. The best technique to use depends on the size of the particles. For example Equation (3) from SPR can be used to calculate particle diameters for particles 22 of 25

24 that are between 35 to 100 nm. Equation (4) from UV-Vis spectroscopy can be used to calculate diameters of the particles when the absorbance ratio is known. Therefore, these experiments offered efficient methods to calculate particle diameter for spherical GNPs. If scientists are thus working with gold hydrosols, a quick method for size determination can be applied due to this experiment because it offers researchers a way to determine the size and concentration from either the SPR wavelength or UV-Vis spectrum. Conclusion: This thesis has shown the importance of gold nanoparticles because of their size and shape. The shape and size of GNPs is important for their uses in optical, electronic, magnetic and catalytic properties. Therefore, through the work by Kwon, et al., GNPs of icosahedral shape were synthesized and studied. The GNPs were studied through TEM, SEM, XRD, UV-Vis to focus on the size of each particle synthesized and their icosahedral properties. The icosahedral particles were also studied to examine their surface-enhanced Raman scattering properties. In this thesis spherical GNPs were also studied through the work of Haiss, et al. to determine the size of spherical particles. The particles were synthesized and studied through UV-Vis, and TEM to develop equations to calculate the concentration. In essence, icosahedral and spherical GNPs were synthesized and studied to determine the size, shape, and their surface-enhanced Raman scattering properties. Further research could be done to see if the GNP shape affected the correlation observed. Since GNPs can form numerous shapes; such as prisms and rods, can 23 of 25

25 equation to calculate the diameters of these particles can be developed. Another idea to focus on for further research could be which shape is better for different applications. For example, each shape of GNPs have different physical properties thus making it useful to determine which shape is better in areas such as diagnostics, therapeutics, catalysis, optical sensing, and in further nanotechnology. Thus, a study could be conducted to learn how shape affects the application GNPs used. References: 1. Daniel, M.-C.; Astruc, D., Gold Nanoparticles: Assembly, Supramolecular Chemistry, Quantum- Size-Related Properties, and Applications toward Biology, Catalysis, and Nanotechnology. Chemical Reviews 2003, 104 (1), Jain, P. K.; Lee, K. S.; El-Sayed, I. H.; El-Sayed, M. A., Calculated Absorption and Scattering Properties of Gold Nanoparticles of Different Size, Shape, and Composition: Applications in Biological Imaging and Biomedicine. The Journal of Physical Chemistry B 2006, 110 (14), Kwon, K.; Lee, K. Y.; Lee, Y. W.; Kim, M.; Heo, J.; Ahn, S. J.; Han, S. W., Controlled Synthesis of Icosahedral Gold Nanoparticles and Their Surface-Enhanced Raman Scattering Property. The Journal of Physical Chemistry C 2006, 111 (3), Wang, Z. L., Transmission Electron Microscopy of Shape-Controlled Nanocrystals and Their Assemblies. The Journal of Physical Chemistry B 2000, 104 (6), electron microscopy. In Dictionary of Microbiology & Molecular Biology, Wiley: Sewell, G. R. D. a. B. T. Electron Science Tutor wav/microscope.htm#differences. 7. X-ray diffraction. In McGraw-Hill Concise Encyclopedia of Science and Technology, McGraw-Hill: Raman effect. In McGraw-Hill Concise Encyclopedia of Science and Technology, McGraw-Hill: Surface Plasmon Resonance. In Encyclopedia of the Human Genome, Wiley: Ghosh, S. K.; Pal, T., Interparticle Coupling Effect on the Surface Plasmon Resonance of Gold Nanoparticles: From Theory to Applications. Chemical Reviews 2007, 107 (11), Haiss, W.; Thanh, N. T. K.; Aveyard, J.; Fernig, D. G., Determination of Size and Concentration of Gold Nanoparticles from UV Vis Spectra. Analytical Chemistry 2007, 79 (11), hydrosol. In McGraw-Hill Dictionary of Scientific and Technical Terms, McGraw-Hill: Colloidal crystals. In McGraw-Hill Concise Encyclopedia of Science and Technology, McGraw-Hill: Jana, N. R.; Gearheart, L.; Murphy, C. J., Seeding Growth for Size Control of 5 40 nm Diameter Gold Nanoparticles. Langmuir 2001, 17 (22), Jana, N. R.; Gearheart, L.; Murphy, C. J., Wet Chemical Synthesis of High Aspect Ratio Cylindrical Gold Nanorods. The Journal of Physical Chemistry B 2001, 105 (19), Zhang, Y.; Grass, M. E.; Huang, W.; Somorjai, G. A., Seedless Polyol Synthesis and CO Oxidation Activity of Monodisperse (111)- and (100)-Oriented Rhodium Nanocrystals in Sub-10 nm Sizes. Langmuir 2010, 26 (21), of 25

26 17. Sau, T. K.; Murphy, C. J., Room Temperature, High-Yield Synthesis of Multiple Shapes of Gold Nanoparticles in Aqueous Solution. Journal of the American Chemical Society 2004, 126 (28), Pozun, Z. D.; Tran, K.; Shi, A.; Smith, R. H.; Henkelman, G., Why Silver Nanoparticles Are Effective for Olefin/Paraffin Separations. The Journal of Physical Chemistry C 2011, 115 (5), Song, H.; Kim, F.; Connor, S.; Somorjai, G. A.; Yang, P., Pt Nanocrystals: Shape Control and Langmuir Blodgett Monolayer Formation. The Journal of Physical Chemistry B 2004, 109 (1), Norman, T. J.; Grant, C. D.; Magana, D.; Zhang, J. Z.; Liu, J.; Cao, D.; Bridges, F.; Van Buuren, A., Near Infrared Optical Absorption of Gold Nanoparticle Aggregates. The Journal of Physical Chemistry B 2002, 106 (28), of 25