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SSTP EE
1. Chemical Synthesis of Plasmonic Noble Metal Nanoparticles
Research Question: Can I use different parameters to control the shape of and size of
nanoparticles?
Norah Mubarak
Area of Study: Chemistry
July 2014
Word Count: 3,023
2. Abstract
Nanotechnology holds significant promise for developments in numerous areas such as
biomedicine, drug therapy, cancer research and solar energy. Although promising, shape
controlled synthesis of nanostructures is important, as their shape and size dictate the eventual
applications. In an attempt to improve the efficiency of the chemical synthesis of gold
nanocubes, I used a two-step seed-mediated growth method. I created a growth solution to
reduce the seeds oxidation number from a positive three to a positive one. Following that I
created a seed solution that was mixed into the growth solution which reduced the gold seeds
from a positive one oxidation to a zero oxidation thus creating the nanocrystals. The shape of the
nanocrystal desired however, is dictated by another factor in which I later found to be the
ascorbic acid (reducing agent) concentration that is entered into the growth solution. I
hypothesized that by controlling the seed structure I could obtain different shapes of
nanocrystals. I later found that by manipulating the aging time of the seeds and by using different
concentrations of reducing agent (ascorbic acid) I could obtain different nanoparticle shapes
(Figure 1). My results support our initial hypothesis, proving that shape-controlled synthesis of
metal nanoparticles is possible via a simple wet-chemistry method.
Word Count: 206
4. Introduction
If the size and shape of a nanoparticle is controlled, then the appropriate nanoparticle for
each different application could be developed as desired. Size and shape of a nanoparticle are
determined by multiple factors that were researched such as age time of the gold seeds, and
growth solution reducing agent concentration. My research question is as follows:
“Can a specific factor or factors control seed mediated chemical synthesis and control the shape
and size of the synthesized nanoparticle?”
One application of this research is to assist cancer therapy and help the precision of
drug delivery. In an attempt to improve the efficiency I used shape and size of nanostructures to
dictate what applications I can use it for. Specifically, photodynamic cancer therapy is the
destruction of the cancer cells by laser oxygen which is cytotoxic, the degree in which something
is toxic to living cells. To avoid the side effect of the remaining dye molecules migrating to the
skin and the eyes and making the patient sensitive to daylight exposure is where the nanoparticle
becomes important. So, as an example of the importance, “The kinetics of nanoparticle
biodegradation is an important factor that can control how and where a drug is released,
impacting treatment efficiency as well as potential toxicity to nontarget tissues from nanoparticle
exposure. If nanoparticles given to a patient release a drug before particles can ever get to target
tissue, then we get high toxicity and low effect. Conversely, if particles are drawn to a tissue but
don’t release the drug until long afterward, then we also don’t get the therapeutic effect.”
(Reineke, 2013)
5. The reason gold nanoparticles are used is because they are usually more stable than
silver, when dispersed in aqueous solution. The gold nanoparticle surface unlike that of silver
can be functionalized with a variety of chemical and biological molecules. Plasmon resonance
wavelengths of gold nanoparticles in different shapes and sizes are on average longer than that of
silver nanoparticles. Why the Plasmon resonance of the type of nanoparticle is important is
because depending on how much light it can absorb and its wavelengths, it can be a critical
factor in the nanoparticle synthesis or the use of the nanoparticle after.
Gold nanoparticles are most widely used for applications in both biology and biomedical
engineering due to their unique optical properties. Optical properties and the surfactant CTAB
are used to control shape and size or so it is believed. Optical properties are the result of the
phenomenon called Localized Surface Plasmon Resonance (LSPR). These properties are shown
by the interaction of light with electrons on the gold nanoparticle surface. The particular
wavelength of light where this occurs is strongly dependent on the gold nanoparticle size, shape,
and surface structure. “Oscillating electric fields of a light ray propagating near a colloidal
nanoparticle interact with the free electrons causing a concerted oscillation of electron charge
that is in resonance with the frequency of visible light. These resonant oscillations are known as
surface Plasmon’s.”(Sigmund-Aldrich). In particular, by varying specific parameters I will
synthesize gold nanocubes. This process has been nearly perfected for other metals such as silver
however for gold there is room for improvement.
A dramatic ‘parameter’ that can be used is the way it is synthesized, the two most
common ways to synthesizing nanoparticles, chemical synthesis and photo-induced synthesis.
Chemical synthesis is the more widely used way to synthesize nanoparticles, however to improve
this process the highest yield possible for the researcher should be used and the researcher should
6. try to exceed it by varying the parameters. The difference between chemical synthesis and photo-
induced synthesis is in photo induced synthesis the nanoparticles are synthesized using a halogen
light filter on a halogen lamp that toils with the LSPR of the metal to then continue with
synthesis. In chemical synthesis the nanoparticles are synthesized in a growth solution using
various chemicals and reducing agents to reduce the oxidation of the gold seeds and therefore
synthesize them into the desired shape or size.
Researchers use these processes to try and learn to control the size and shape of the
nanoparticles because learning to do this can help control the yield. The reason for this research
is vaguely because beginning as small as a nanoparticle will lead to achieving the biggest goal.
In my experiment I used chemical mediated synthesis to grow nanoparticles using various
reducing agents, techniques, and apparatuses to synthesize the shape I desired.
Background Information:
A nanoparticle ,a microscopic particle between 1-100 nanometers, that does not consist of
constant physical properties and is an effective medium between bulk materials and
molecular/atomic structures for expanding science technology research.(Sciencedaily.com,2013)
The role of the sodium borohydride in the synthesis is to reduce the gold seeds from a +3
oxidation to +1 oxidation. This reduction takes place in the Cetyltrimethylammonium bromide
(CTABr) concentrated seed solution. The role of the ascorbic acid in the synthesis is to reduce
the gold seeds from a +1 oxidation to a 0 oxidation. This final reduction takes place in the
growth solution where the seeds also synthesize into the desired nanoparticle shape.
A surface active agent (Surfactant) has its name because they are active at different
interfaces (surfaces). Surfactants are used in a multitude of different applications and may be
7. called something different depending on the application/field of study. They are used to lower
the surface tension and protect the nanoparticle from growing a different way than you want it to.
Different surfactants and different surfactant concentrations help control the growth of the
nanoparticles to a specific size or shape.
In using chemical synthesis, one can observe firsthand the color changes that must occur
throughout the process of the experiment for the metal seeds to synthesize correct/accurately.
These color changes occur when the solutions are mixed together or a reducing agent is added
for example because when it is added it reduces other ‘species’ in the solution.
Proposed Growth Model:
(Figure 1)
Xia, Y., Xiong, Y., Lim, B., &Skrabalak, S. E. Angew. Chem. Int. Ed., 2009, 48, 60-103.
8. Figure 1 above shows how the synthesis of nanoparticles takes place. It starts with the
single crystal, in this case gold crystal, and shows the shape it is further synthesized. (This
diagram is an overview of how synthesis generally takes place). However in the diagram, it does
not show how the crystal gets from a single crystalline structure to the desired nanoparticle
shape. In between the single crystal and the desired shape is the process of reduction from a +3
oxidation to a +1 then down to a 0 oxidation number of gold by the time the gold seeds are
entered into the ‘growth solution’. The crystal structure is now a cuboctahedron and for it to
synthesize into the desired shape (shown in this figure as octahedron and cube) it depends on the
parameters used. The synthesis is controlled by many factors however, including the ascorbic
acid concentration, surfactant concentration/type, and the incubation time of the gold seeds in the
seed solution. What happens is, the surfactant (in this experiment Cetyltrimethylammonium
bromide, CTABr) “guides” the gold precursor in the growth solution to grow on specific faces
<111><110><100>. For example, looking at the cuboctahedron in the middle of the diagram you
can see gold <111> and green <100> faces and the gold precursor in the growth solution will
grow off of the cuboctahedron faces like vectors, if you will. And will grow in that specific
direction depending on the different parameters that one will test. As the specific faces continue
to grow they will eventually become one of the two shapes shown in figure 1. The octagonal rod
and bar in the diagram are what would have been the outcome if I furthered my research and
continued to synthesize the particles. In future research, I will attempt to synthesize gold
nanorods and continue my research and test to see if the rods can be altered using the same or
different parameters.
9. VARIABLES:
Control variables:
Amount of nanopure water
Amount of gold precursor
Amount of surfactant (CTABr)
Surfactant concentration (CTABr)
Temperature of water bath
Amount of sodium borohydride (reducing agent; used in creating gold seeds)
Apparatus/Materials:
Required for experiment
Water bath
Centrifuge tubes (50mL & 1mL)
Centrifuge
Scale
Weighing paper
Spatula
Beaker
Graduated cylinder
Crushed Ice
Micropipettes (100μ-1000μL &
1μL-10μL)
Safety Gloves
Safety goggles
Chemicals required for
experiment
Totalquantity required for
experiment
Ascorbic Acid 15.2 mL
Sodium Borohydride (NaBH4) 0.3 mL
Gold Chloride (HAuCl4) 0.925 mL
Cetyltrimethylammonium
bromide (CTABr) 10.15 mL
10. Nanopure water ≈160 mL
Methods:
Making Solutions:
Several solutions were created before beginning the seed mediated synthesis. The first
solution that was created was the sodium borohydride dilution solution. I used 0.38g of stock
sodium borohydride granules and added it to a centrifuge tube that was in a beaker of ice and
contained 10mL of nanopure water although the solution was not capped completely because of
sodium borohydride’s reactive properties. However, the desired solution must be 0.01 Molar and
the solution created was a 0.1 Molar. The next solution that was created used 1mL of the 0.1
Molar solution of sodium borohydride and was added to a centrifuge tube that was inside a
beaker of ice and contained 9mL of nanopure water by means of micro pipette. This created the
desired 0.01 Molar solution by diluting the first solution 10 times. This solution must be made
the day the synthesis will take place.
The second set of solution prepared were the growth solutions. These solutions are all the
same and there is more than one to represent the different age time of the seeds. The four growth
solutions represent 15 minutes, 30 minutes, 45 minutes, and an hour. To create these solutions I
used a stock solution of the surfactant CTABr (0.1M, 6.4 mL) that was been held at room
temperature in a water bath, HAuCl4 (0.01 M, 0.8mL) ,Nanopure water (32mL), and ascorbic
acid (.1M, 3.8mL). Each solution began with 32mL of nanopure water in a centrifuge tube and
the following chemicals were entered by means of micro pipette in the order of CTABr, HAuCl4,
and five minutes before entering the seed solution is when the ascorbic acid is added. The most
11. significant change to look for in the growth solutions is to make sure the color is changed for
yellowish/gold to colorless once the ascorbic acid is added. If the color change does not occur,
one must restart because it means that reduction from a +1 oxidation to a 0 oxidation number
will not occur when the gold seeds are added.
The third solution that was created is the seed solution. This solution was created by
using the following: Fresh NaBH4 (sodium borohydride) (.01M, .3mL) , HAuCl4 (.01m,
.125mL), and CTABr (.1M, 3.75mL). These chemicals were added into a centrifuge tube
containing the CTABr amount (3.75mL) in the following order: HAuCl4 thenNaBH4. After
adding the reducing agent (NaBH4) a color change should occur in the centrifuge tube from
yellowish/gold to brownish/gray. If the color change does not occur the seeds cannot be used for
synthesis. The solution was then capped quickly and tightly then is inverted for 2 minutes then
placed into a water bath at 30.0°C.
Experiment:
1) After seed solution is placed into water bath, begin timer.
2) After 10 minutes have elapsed, add 3.8mL of ascorbic acid into first growth solution and
observe color change.
3) Next I removed 100μL of seed solution and added it to a centrifuge tube that contains
900μL of nanopure water.
4) Once the timer has reached 15 minutes: using a micro pipette take out 200 μL of seed
solution and insert it rapidly into the growth solution. The solution was capped quickly
and tightly then inverted for 10 seconds. The tube was then placed in a water bath
overnight at 30.0 ° C.
12. 5) Repeat for the different times (30 minutes, 45 minutes, and one hour after the seeds have
entered the bath).
6) The next day a color change should be observed (colorless to reddish/purple).
Creating SEM (Scanning electron microscope) imaging samples:
1) Using a micropipette: Enter 1000μL (1mL) of growth solution into 1mL centrifuge tube.
2) Centrifuge at 10,000 rpm for 10 minutes.
3) In the bottom of the tube there should be a pellet. Pour out as much surfactant as possible
without pouring out the pellet.
4) Fill centrifuge tube back up to 1mL with nanopure water.
5) Centrifuge again for 10 minutes at 10,000 rpm.
6) Again there should be a pellet at the bottom of the tube. Pour out as much
surfactant/water as possible without pouring out pellet.
7) Gather, rinse and score a silicon plate strip with a designated number to identify which
growth solution was used.
8) Using a micropipette, pipette out 7μL of the solution that was just centrifuged and drop
onto silicon strip.
9) Repeat with other growth solutions.
10) Once completed with other growth solution place silicon strip(s) in a Petri-dish and place
in a desiccator until taken to imaging.
11) Send to imaging facility with SEM. Analyze the results.
13. Evidence/Results:
Figure 2.
Figure2 . Representative SEM images of plasmonic gold nanoparticles. (A) Nanocubes (B) Nano
octahedral (C)Pseudo-spherical particles: the ones that are composed are strange facets, no well-
defined structure. Bi pyramids and truncated bi pyramids are also seen. (D) Bi-pyramids and
truncated bi-pyramids. In Figure 2, A and B are a representation of an outcome of high yields. C
and D are representations of an outcome of different shapes.
C
BA
D
14. Conclusion/Discussion:
Conclusions:
This experiment has shown both signs of success and weakness. However, in terms of
results, I have several interesting observations to report.
I was able to conclude that shape-controlled syntheses of gold nanocrystals are easily controlled
through careful choice of seed incubation time and reducing agent concentration (ascorbic acid).
Also, that careful manipulations of ascorbic acid concentration yields cubic and octahedral
nanocrystal shapes in high (>80%) yield.
Discussion:
Applications in many different fields such as biomedical engineering, solar energy, and
along with used to help catalyze reactions. The shapes were synthesized using a seed-mediated
method, where the changes of shape were induced by changing a few parameters such as the
ascorbic acid concentration (reducing agent), age time of the seeds, and optimum washing
conditions for SEM imaging (how many times sample was washed. The first shape synthesized
was the nanocube (synthesized in a sodium borohydride solution along with CTAB, and gold
chloride). Such monodisperisty, which is a polymer system that is homogeneous in molecular
weight, that is, it does not have a distribution of different molecular-weight chains within the
total mass (McGraw-Hill,2003 ),(Figure 2 A) and single crystalline is very important. The best
results shown were in between 30-45 minutes of the CTABr concentrated seed solution. Gold
octahedrons were obtained by changing the ascorbic acid concentration (Figure 2 B). This shape
was synthesized under the same conditions, and the best results came from the 30 minute aged
15. CTABr concentrated growth solution. Bi-pyramids and truncated bi pyramids are formed when a
synthesis does not go to completion to the shape desired (Figure 2 C&D).
There are many factors that can affect the chemical synthesis such as speed the solution
is added, and the way it is entered (drops, all at once, etc.). The chemical synthesis takes place in
the growth solution where the ascorbic acid has reduced the gold precursor from a +3 oxidation
to a +1. Once the gold Nano seeds that are in the seed solution are entered into the growth
solution, the gold cations in the growth solution bombard the gold seed and begin to grow in
different directions creating the optimum shape desired. The concentration of the surfactant will
decide in which direction the seed will grow, in this experiment I used a 0.1 M CTABr.
Various nano particles with simple procedures were formed simply by changing the
concentration of the reducing agent (ascorbic acid) in the CTABr growth solution. Assuming the
number of nano particles before and after the seed mediated growth reaction stayed constant then
the synthesis would have been able to go to completion. The reason a reluctant such as ascorbic
acid helps tailor the final (found in CTAB growth solution) shape is because it pushes cations
onto a specific surface. The other important reducing agent, sodium borohydride, (found in
CTABr concentrated gold seed solution) completes the oxidation from a +1 to a 0 charge. The
role of the reductant is to push the atoms on to the surface of the seed. Because if something is
forced onto it and grows from every corner than you will just get bigger particles if it grows from
every side and it will result in a bigger nanospheres. The point of the growth solution is to create
an environment to allow seeds to grow because being able to control the evolution of seeds is the
point of separate solutions.
As seen in the figures above you get a distribution of structures at time although we try to
16. create an environment to lower their surface energy so we can tailor the final shape of the
particle. We get a distribution of crystalline structures and when added to the growth solution
metal becomes deposited onto it.
Improvements:
Some improvements that could be made are changing some parameters in the experiment
to see if the different outcomes are better than the one I was initially trying to reach which was to
learn to control the shape and size.. Such parameters that I would change are: using different
concentrations of surfactant to see if it affected the synthesis of the nanoparticles and how. As
well as, I would like to try to use different surfactants in the synthesis to see how it affects the
nanoparticle outcome seeing that the surfactant does play a tremendous role in the outcome of
the nanoparticle shape.. So forth, I would like to experiment with photo-induced synthesis and
compare the results of the nanoparticles to those synthesized using chemical synthesis.
17. Bibliography
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