Solutions as Suspensions:
Solubility of Nanoparticles
Haley Buckner
Supervisor: Christopher M. Sorensen, University Distinguished Professor
Kansas State University Physics Department REU Program
This program is funded by the National Science Foundation through grant number PHYS-1461251. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
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Due to modern synthetic chemistry, it is now possible to produce nanoparticles with a high degree of uniformity1. When the nanoparticles are surface ligated with organic compounds, they are stable against irreversible aggregation. Nanoparticle colloidal suspensions can act as solutions with reversible, temperature dependent solubility. So, a suspension of nanoparticle monomers can be in equilibrium with precipitated aggregates of nanoparticles.
Like a solution, the nanoparticle suspension has a phase diagram with one- and two-phase regions. In the two phase region, the supernatant is the mixture of nanoparticle monomers in the solvent. Aggregates of nanoparticles settle out under gravity to form the precipitate. Specifically for this experiment, we used gold nanoparticles ligated with oleylamine to further investigate the solubility properties and determine an enthalpy of dissolution.
NOTE: We refer to dissolution as the breaking apart of aggregates into nanoparticle monomers, not individual gold ions.
Figure 1. Schematic phase diagram of nanoparticles in solution
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The gold nanoparticles used in this experiment were synthesized using the inverse micelle method to make polydisperse gold nanoparticles. We then coated those particles with oleylamine. Finally, we used to digestive ripening, refluxing with excess ligand, to try to make the particles monodisperse. After digestive ripening, samples of this solution were placed in glass ampules and flame sealed. The ampules have an internal optical path length of 0.8 mm.
When the sample is in the two-phase region the nanoparticle monomer solution is in equilibrium with the precipitated aggregates of nanoparticle monomers. Oleylamine ligated gold nanoparticle monomers in solution have a plasmon peak near 524 nm. The peak is used to measure monomer concentration in the supernatant. From the Beer-Lambert law, the nanoparticle concentration of the supernatant is proportional to the absorbance of the supernatant. We measure the absorbance of the sample using UV-Vis spectroscopy and then convert into concentration, as a mole fraction.
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In the two-phase region, cooling will cause gold nanoparticle monomers to aggregate as a precipitate. This precipitate will take a long time to settle out of the supernatant due to gravity, but isolation of the supernatant is necessary to take the absorbance measurement. So, in order to speed up the settling process, the experimental apparatus includes a modified commercial centrifuge with temperature control to allow for temperature regulation of the nanoparticle sample. The centrifuge rotor doubled as the sample holder for the UV-Vis absorbance measurement, reducing the handling of the glass ampule.
To begin, we reset the system by submerging the sample in an ultrasonic bath to disperse the aggregates throughout the supernatant. Then, the sample is placed in the centrifuge holder at constant temperature. To ensure the system is in equilibrium, the sample is left for 90 minutes. After waiting, the sample is centrifuged at 3300g for 12 minutes spin all the aggregates out of the supernatant. The sample is flipped after spinning to remove the precipitate from the supernatant, thus isolating it. We use the absorbance at 850 nm as the baseline. The Ocean Optics spectrometer has a resolution ceiling at about 2.6 absorbance units. For samples with high concentration or at high temperatures, the absorbance may surpass this ceiling at the peak wavelength, 524 nm. Since the ratio of the absorbance at 524 nm to the absorbance at 600 nm is constant, we used 600 nm as the actual absorbance. The entire process is repeated for -8°C to 24°C.
Final Presentation: Click here to download my presentation in powerpoint.
Lectures:
Professor Kristan Corwin Energy Scales and Optical Frequency Combs
Professor Tim Bolton High Energy/Particle Physics: Quantum Field Theory
Assoc. Professor Brian Washburn Electronics Class
Dist. Professor Chris Sorensen Light Scattering
Assoc. Professor Vinod Kumarappan Laser-induced Alignment of Molecules
Dist. Professor Bharat Ratra A 75 min Review of 95% of the Contents of the Current Universe
Assoc. Professor Glenn Horton-Smith Neutrinos
Asst. Professor Eleanor Sayre Physics Education Research
Asst. Professor Daniel Rolles Free Electron Lasers
Assoc. Professor Jeremy Schmit Biological Physics
I attend West Chester University of Pennsylvania as a Physics and Industrial Mathematics double major in the Honors College. At WCU, I am a general physics tutor at the Learning and Resource Center as well as a research assistant to Dr. Matthew Waite. With Dr. Waite, I am growing Aluminum Zinc Oxide thin films, using DC magnetron sputtering and investigating the resitivity. I am also a general physics tutor at the Learning Assistance and Resource Center at West Chester University. Outside of school I enjoy playing soccer and reading.
This experience has given me valuable research experience in which I developed my abilities to plan and carry out research, learned how to cope with frustration, and practiced presenting my research to others. On top of it all, I was able to spend 10 weeks with peers who love physics and excel at it. I highly recommend the REU experience to anyone interested in pursuing a graduate degree in physics.
What we observe is not nature itself, but nature exposed to our method of questioning. Werner Heisenberg
I would also like to thank Kansas State University and the Physics Department for hosting me this summer. Thank you to Dr. Sorensen, Jeff Powell, and Jessica Changstrom for their many efforts working with me on this project.
References:
1. Lohman, B. C., Powell, J. A., Cingarapu, S., Aakeroy, C. B., Chakrabarti, A., Klabunde, K. J., Sorensen, C. M. (2012). Solubility of gold nanoparticles as a function of ligand shell and alkane solvent. Physical Chemistry Chemical Physics, 14(18), 6509. doi:10.1039/c2cp40645d
2. Powell, J. A., Schwieters, R. M., Bayliff, K. W., Herman, E. N., Hotvedt, N. J., Changstrom, J. R., Sorensen, C. M. (2016). Temperature dependent solubility of gold nanoparticle suspension/solutions. RSC Adv., 6(74), 7063870643. doi:10.1039/c6ra15822f
3. Prausnitz, J. M., Lichtenthaler, R. N., Gomez de Azevedo, E., Molecular Thermodynamics of Fluid-Phase Equilibria, Prentice Hall, Englewood Cliffs, NJ, 1986.
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