Amino acid interactions with varying geometry gold nanoparticles Hailey Cramer Mentored by Dr. Shashi Karna

1. Amino acid interactions with varying geometry gold nanoparticles Hailey Cramer Mentored by Dr. Shashi Karna INTRODUCTION RESULTS CONCLUSIONS To develop the potential biomedical applications of gold nanoparticles (AuNPs), it is necessary to understand and quantify their interactions with biomolecules. This involves investigating the properties of AuNPs, the factors that affect the binding of AuNPs with biomolecules, and the characterization methods that can be used to determine where and why binding may occur. AuNPs are the most stable of the metal NPs. They range from 5-100 nm in diameter and consist of a few atoms that form excitons. These excitons exhibit quantum confinement, thus changing the NP’s properties and behavior drastically from the bulk material. The AuNPs have atom-like discrete energy levels, a high surface-to-volume ratio, and a broad absorbance and fluorescence range depending on particle size, particle shape, and interparticle distance. These qualities allow for the optical, electrical, magnetic, and chemical properties of AuNPs to be controlled. AuNPs have been of recent interest to the field of biotechnology because of their ability to bind with organic molecules. However, the interactions between organic molecules and NPs are poorly understood because it is difficult to document how molecules will arrange on the NP surface. Therefore, little is known about AuNP-amino acid hybrid formation and whether AuNPs could change an amino acid’s function in a protein. In this experiment, the interactions between AuNPs and the amino acid tryptophan were studied to determine if binding occurred as hypothesized. Through this experiment, it cannot yet be determined if binding occurred between AuNPs and tryptophan. However, it was found that varying shaped AuNPs with an average size of 8-9 nm can be successfully synthesized through a citrate reduction method. These AuNPs were found to have absorbance peaks at 220 nm and 521 nm which are supported by literature (Sethi, 2009). These particles could have future applications in alternative energy, biotechnology, biosensors, and biomedicine (Sun, 2009). When the AuNPs were combined with a 1 mM solution of tryptophan in a 1:1 ratio, no obvious shift in absorbance was observed. The tryptophan solution had a significantly stronger concentration compared to the AuNP solution, so it’s peaks were more intense. This difference in molarity may have caused the 521 nm AuNP absorbance peak to appear less intense in Graph 3. If binding had occurred, a shift in absorbance would have been observed, but Graph 3 shows no evidence of a shift and only shows each solution’s individual absorbance peaks. In the future, the solutions of AuNP and tryptophan could be combined at different ratios to determine which would allow for the greatest binding. They could also be combined with the addition of a buffer. Further analysis of AuNP-tryptophan hybrid formation should be done through alternative characterization methods such as fluorescence spectroscopy, HRTEM, Raman spectroscopy, or dynamic light scattering. Ultimately, other amino acids with different structures should be combined with AuNPs to determine if binding occurs. If binding occurs, the results can be used to further understand the fundamental interactions between NPs and biomolecules and allow for the use of AuNP-amino acid hybrids in biomedical and biotechnology applications. a) b) Figure 1: HRTEM images of AuNPs. a) 5 nm scale b) 2 nm scale. AuNPs are polycrystalline, varying shapes, and range in size from 6-12 nm with a high proportion at 8-9 nm. Some particles are individual and others are connected. Graph 1: Absorbance and emission of tryptophan. Absorbance peak at 296 nm and broad emission peak at 357 nm. MATERIALS AND METHODS AuNPs were synthesized through a citrate reduction method. A solution of 22 mg tetrachloroauric [III] acid trihydrate (HAuCl4 · 3H2O) in 200 mL distilled de-ionized water was heated to boiling. Once boiling, 4 mL 1% (w/v) trisodium citrate dihydrate (Na3C6H5O7· 2H2O) was added and stirred constantly. A change of color from yellow to dark red occurred after about 15 minutes, at which time the solution was removed, cooled, and centrifuged to remove impurities. Experimental characterization of the AuNPs was performed to determine their morphology and optical properties. This was done through the use of ultraviolet-visible spectroscopy (UV-Vis), fluorescence spectroscopy, atomic force microscopy (AFM), energy dispersive x-ray spectroscopy (EDS), and high resolution tunneling electron microscopy (HRTEM). The amino acid tryptophan was chosen for study because of its ability to fluoresce. A 1mM tryptophan solution and the AuNP-tryptophan hybrid solution were characterized through UV-Vis and fluorescence spectroscopy. REFERENCES Sethi, M., & Knecht, M. (2009). Experimental studies on the interactions between Au nanoparticles and amino acids: bio-based formation of branched linear chains. Applied Materials and Interfaces, 6(1), 1270-1278. Sun, K., Asudev, M., Jung, H., Yang, J., Kar, A., Li, Y., ... Dutta, M. (2009). Applications of colloidal quantum dots. Microelectronics Journal, 40, 644-649. ACKNOWLEDGEMENTS Special thanks to Dr. Mark Griep, Ms. Patricia Johnson, Ms. Molly Karna, and Mr. Gareth Davis for assistance with this project. Graph 2: Absorbance of AuNP-tryptophan hybrid. AuNP and 1 mM tryptophan solutions were combined in a 1:1 ratio. Absorbance peaks observed at 220 nm, 296 nm, and 521 nm.