Concentration of Au Nanoparticles in Toluene as a function of Temperature
by Nels Jacob Hotvedt
Supervisor: Dr. Chris Sorensen
Kansas State University Physics Department REU Program, sponsored by NSF
This program is funded by the National Science Foundation through grant number PHY-0851599.
Welcome to my webpage. This page summarizes my experience doing research for the Summer of 2010 at Kansas State University in the lab of Chris Sorensen. My research this summer is an extension of a former graduate student named Hao Yan, where I am researching how the absorbency of a gold nanoparticle colloidal solution in toluene changes with temperature.
Below, I describe the Project Goals, my Research Strategy, my Research Progress. Scroll all the way down to learn more About Me. Finally, I've included some Useful Links.
Summary Statement: This particular project is all about studying the equilibrium line produced from gold nanoparticles suspended in toluene. As gold nanoparticles are cooled within the solution, they will aggregate and form a precipitate, reducing the concentration of particles in the solution. The equilibrium line that’s mentioned is the line of equilibrium of the formed precipitate and suspended particles. This equilibrium line can be found by analyzing the absorbency of light of the suspended particles at different temperatures. According to Beer’s Law, concentration is linearly correlated with absorption. With this in mind, a concentration vs. temperature graph can be produced by analyzing the absorbency of a sample at different temperatures and translating that absorbency into concentration.
Project Goals: My goal by the end of this summer is to create an equilibrium line that represents the equilibrium of the suspended Au nanoparticles in toluene (i.e. the supernatant) and aggregated Au nanoparticles (i.e. the precipitate)
Research Strategy: I am reusing a temperature controlled chamber made of aluminum that’s lined with foam insulation to ensure that the Au nanoparticle sample is at a controlled temperature. A temperature controlled bath pumps antifreeze liquid through a series of coiled pipes inside the chamber to manage temperature. Since the decrease of absorption comes from the nanoparticles aggregating due to cold temperatures, the sample must be spun inside a centrifuge which is also placed inside the temperature chamber. Also inside this chamber is a spectroscope cell which was originally used to hold 1x1 cm cuvettes for spectroscopy. Recently I have been using a modified holder to hold in capillary slides (instead of standard cuvettes) which are 0.3 mm in thickness, to hold in very concentrated samples. Once a sample is placed inside the cuvette holder, a beam of light generated from an UV/Visible Light spectroscope is shot through the sample and sent to a computer where the absorption is analyzed. The absorption of these gold nanoparticle solutions is calculated by looking at the absorption level of the plasmon peak. A plasmon is a quantized unit of electron vibration. In the case of gold nanoparticles (which are ~5 nm in diameter), wavelengths of light around 530 nm will resonate the electrons inside the nanoparticles and absorb their energy creating a plasmon peak that shows up under the spectroscope analysis.
Research Progress: The early weeks of this project merely consisted of practice with using the temperature chamber, the centrifuge, and the bath cooler. The data from these first few experiments allows me to get a time scale of how fast I can cool or heat a sample inside the chamber. The maximum cooling rate, which includes the use of an additional cooling unit (whom I’ve appropriately titled “Ice Princess”) turns out to be 0.5 C/min. And the maximum heating rate, with the help of an immersion heater named “Emerson”, turns out to be around 0.5 to 1.0 C/min.
The next few weeks involved practice with the new spectroscope obtained from the chemistry department. This allowed me to get familiar with the spectroscope, how it handles too little absorption, too much absorption, and heating constraints. Samples that were leftover from Hao Yan’s research were used to help me get familiar with the future “fresh” samples I would get later. I took an old sample and I made six dilutions of the sample with toluene with ratios of 1:10, 1:20, 1:30, 1:40, 1:50, and 1:60. I placed each sample inside the spectroscope and analyzed the difference in absorption of the plasmon peak to the background level. When comparing the concentration to the level of absorption it was confirmed that the absorption is indeed linearly correlated with concentration, as remembered by Beer’s Law.
Figure
1: Graph showing absorption versus relative dilution of a leftover sample from
Hao’s research.
Recently, I have been given two fresh samples of gold nanoparticles from the chemistry department. Sample A was used in an experiment with the very thin capillary slides (0.3 mm), and was used to see a correlation between the absorption and temperature. Sample B was more dilute than the first sample, so that meant its absorbency spectrum would have to be analyzed using the 5.0 mm path length cuvettes. Sample B, when first obtained, had many aggregated particles settling down at the bottom. To solve this problem, the whole sample was sonicated and heated to reduce the size of the aggregated particles in the precipitate. Below are the initial absorbency graphs obtained from the absorbency experiments.
Figure 2: Graph showing measured
absorption versus temperature in Celsius for Sample A measured in a 0.3 mm path
length capillary slide. Notice the leveling off of absorption at ~15 degrees,
implying that the sample has run out of precipitate to be suspended within the
supernatant.
Figure 3: Graph showing measured
absorption versus temperature in Celsius for Sample B measured in a 5.0 mm path
length cuvette. Notice that there is a “kink” centering on room temperature (23
degrees).
While the data from Sample A provided much evidence of a linear equilibrium
line, Sample B had to be repeated to see whether or not the “kink” was either
an actual phenomenon or just an artifact. The data obtained around room
temperature was obtained straight out of Sample B’s jar. Seeing as how it’s had
a very long time to come to equilibrium at room temperature, something was not
lining up. However, I had left were the centrifuged samples of Sample B
leftover from either additional heating or cooling. Those samples, while left
at room temperature to level off, neither the size of their aggregated
particles, or the color seemed to change. It was concluded that while these
samples had varying concentrations, they were still spawned samples from
Sample B. These samples were mixed together and reformed Sample B “Centrifuge”
with an unknown amount of precipitate. The new theory was that this sample must
be given a forced amount of heating or cooling in order to spawn a change in
the absorbency, so the recombined Sample B was heated to 42 C and took a
measurement every ten degrees or so.
Figure 4: Graph showing the absorption of Sample B versus temperature for both the original Sample B, and the recombined Sample B. Here the “kink” has been removed. The forced cooling starting from 42 degrees indicates that the “kink” from the original Sample B was probably due to the fact that the kinetics of the nanoparticles were never working or forced.
Figure 5: Graph showing the absorption of Sample A versus temperature. The data where the absorbency leveled off was removed to help analyze the slope of the equilibrium line.
Now it was time to use Hao Yan and
Ben Scott’s data from the previous year to help turn these graphs into
concentration versus temperature. According to Ben Scott’s data, he obtained a
linear correlation between concentration and absorption for samples being
analyzed with a 10 mm path length. To use Ben Scott’s data properly, the
absorbency data was multiplied by 10 mm and then divided by either 0.3 or 5.0
mm depending whether the initial data came from Sample A or B. The data was
then converted with Ben Scott’s correlation and is shown in the graph below.
Figure 6: Graph showing concentration versus temperature. Blue is for
Sample A, Red is for Sample B.
About Me: I’m an undergraduate student majoring in physics at Kansas State University. Physics is one of my favorite passions, and while my recent research has been centering on condensed matter physics, my primary interest is around high energy physics. However, I don’t think that my research is at all a setback because this research experience has been invaluable to me in the sense that it’s giving me a firm grasp on how the scientific method is performed outside of classroom based labs. I am also a very huge nerd who’s also been attempting to combine the aspects of Japanese animation with the four fundamental forces of nature with very much success.
Useful Links:
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