Amino acid interactions with varying geometry gold nanoparticles
Mentored by Dr. Shashi Karna
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 5100 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 AuNPamino 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.
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
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.
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.
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.
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.
Special thanks to Dr. Mark Griep, Ms. Patricia Johnson, Ms. Molly
Karna, and Mr. Gareth Davis for assistance with this project.