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Cheung, Justin
Gold Nanoparticles: Efficient Synthesis of Catalytically Active Nanoparticles using a One-Pot Method
Cheung, Justin
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ABSTRACT
Gold nanoparticles have recently come to prominence due to increased demand for
nanoscale technologies. Nanoparticles of different shapes have unique capabilities. Current
methods for synthesizing variably shaped gold nanoparticles require time intensive, multi-step
procedures. In this study, a single-step, one-pot approach to non-spherical gold nanoparticle
synthesis is developed using poly(glycidyl methacrylate) (PGMA) microspheres as the novel
reductant in the synthesis process. PGMA’s reactive functional groups and slow reducing
capabilities made it a promising method for single step nanoparticle synthesis. Two reaction
parameters (chloroauric acid concentration and reaction temperature) were optimized for the
PGMA induced gold nanoparticle synthesis (1mM, 90°C-110°C). Transmission electron
microscopy and UV-VIS spectroscopy conducted on timed extractions of the PGMA/gold
nanoparticle solutions showed evidence of morphological evolution (aggregations → mixture of
non-spherical shapes → spheres) and increased particle size over time. Catalysis tests on the
nanoparticles found that aggregates and spherical particles had the strongest catalytic properties.
These results demonstrate the effectiveness of PGMA as a reductant in the one-pot synthesis of
catalytically active gold nanoparticles. Furthermore, this process represents a 60%-90%
reduction in synthesis time compared to conventional multi-step procedures for non-spherical
gold nanoparticle production. The effectiveness of PGMA in this study, along with the speed of
the one-pot process enables more efficient synthesis of gold nanoparticles, with the potential to
help facilitate their production and implementation in emerging industrial and medical
applications such as drug delivery, biodetection, and catalysis.
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I. Introduction
Gold nanoparticles and their various applications have come to prominence in recent years.
Many studies have been conducted on the properties and the industrial and medical capabilities
of gold nanoparticles, underscoring the versatility of their uses [1]. Specifically, gold
nanoparticles have great potential to impact fields such as drug delivery, biodetection, catalysis,
and environmental engineering [1-5]. Variations in gold nanoparticle shape have been found to
influence their properties and their applications [6]. This makes non-spherically shaped gold
nanoparticles of particular interest. Just as different protein shapes serve different functions,
differently shaped gold nanoparticles also have different purposes. For example, gold nanorods
have a large surface area, facilitating and enhancing their capabilities in drug delivery systems
[7]. Polygonal and branched nanostructures show potential use as biosensors due to their
enhanced Plasmon resonance capabilities [8-9]. The unique optical properties of gold
nanoparticles make them useful in detection of tumors, characteristics that can be enhanced or
diminished by the shape of the gold nanoparticle [4]. Gold nanoparticles have also been found to
have strong catalytic properties, or the ability to speed up reactions, due to their highly reactive
characteristics, though this has been found to vary with their shape and size [5]. This makes them
particularly valuable in industries reliant on catalysts such as pollution control, fuel cell
production, and bulk synthesis [10] since nanoparticles hold an advantage over conventional
catalysts due to their greatly enhanced reactivity [5].
The utility of gold nanoparticles outlined above underscores the importance of developing
methods that are capable of synthesizing gold nanoparticles of various shapes (particularly non-
spherical ones), a task that is often time consuming and expensive with current methods. The
main purpose of this experiment was to develop a time efficient approach towards the synthesis
of non-spherical gold nanoparticles. The uniqueness of this investigation lies in its use of a novel
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Synthesis of reducing agent
Addition of HAuCl4
Addition of different reductant and shape templating surfactant
Change of reaction conditions/addition of more gold ions
Formation of Seed Particles
Add reducing agent to HAuCl4
One-Pot M
ethod Conventional Approach
polymer reductant towards the efficient synthesis of variably shaped gold nanoparticles.
Furthermore, the secondary goals of this investigation involved (1) analyzing the novel reducing
agent’s impact on gold nanoparticle evolution and mechanism of formation over time and (2)
assessing the catalytic properties of the newly synthesized nanoparticles to demonstrate their
applicability to industrial processes.
Gold nanoparticles are generally synthesized through a “bottom up” method by the reduction
of gold ions present in the chemical compound HAuCl4 [11]. Through varying the reaction
conditions for synthesis, gold nanoparticles can be formed into various shapes and sizes [6].
Reaction temperatures, reductant identity,
length of reaction, and
concentration/volume of the reagents
that partake in the synthesis have been
found to influence the morphology of the
formed gold nanoparticles [11]. While the
majority of previous studies have used
multistep approaches to synthesize
gold nanoparticles into different shapes [8,
11-13], this study focused on the
application of the time efficient one-pot
synthesis method. A comparison of these
approaches is shown in Fig. 1. Multistep procedures involve the initial synthesis of small
spherical gold seed particles followed by the addition of a different reductant or shape-
templating surfactant to induce the formation of a specific gold nanoparticle shape [8]. However,
Figure 1-Comparison between the one-pot and conventional approach methods for synthesizing gold nanoparticles. Note the fewer number of steps associated with the one-pot method.
Variously Shaped Gold Nanoparticles
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these multistep approaches are time consuming [14]. According to previously conducted studies,
traditional multi-step methods for synthesizing non-spherically shaped gold nanoparticles
generally take from 4.5 hours [13] to 25 hours [9] to produce non-spherical shapes, depending on
the method used. The one-pot approach simplifies the nanoparticle synthesis process, requiring
only a single reaction chamber and reducing agent without the need for seeds [15]. In
comparison to multistep approaches, the one-pot approach increases the speed of synthesis of the
production process, making it promising for high-volume industrial reactions [16]. The method
does, however, give up some of the shape control associated with conventional multistep
methods. In this paper a one-pot method was used as an alternative method for the efficient
synthesis of gold nanoparticles.
Polymers are collective chains of single molecules known as monomers. In many instances
polymers contain functional groups, allowing them to be chemically reactive [17-18]. In this
experiment a microsphere shaped polymer known as
PGMA or poly(glycidyl methacrylate), shown in Fig.
2, was used as the novel reducing agent to induce the
synthesis of gold nanoparticles from HAuCl4. PGMA
has multiple applications in protein separation,
enzyme immobilization, and liquid chromatography
[19]. However, it has never been used as a reducing
agent in the synthesis of gold nanoparticles. PGMA is useful because it is highly reactive and
multifunctional due to the presence of epoxy groups on its surface [19], a characteristic which
also makes it an effective reducing agent. Since slow reductants are favored for the formation of
non-spherical gold nanoparticles [8] and PGMA has slow reducing characteristics [15], the
Figure 2-Poly(glycidyl methacrylate) molecule (PGMA). The oxygen atom containing the lone pair is the epoxy group that gives the PGMA its reducing ability.
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polymer could be potentially useful in the synthesis of non-spherically shaped gold
nanoparticles. The epoxy groups gain their reducing capabilities when potassium persulfate is
added to initiate polymerization. The sulfate ions attack the PGMA’s epoxy functional groups in
a ring opening reaction. The epoxy groups are broken into hydroxyl groups which then oxidize to
aldehydes. The process of this reaction is illustrated in Fig. 3.
O + S OO-
O-
O-2 OH
OH
CHO
CHOR
Aldehyde groups are known for their reducing capabilities. As such, one of the goals of this
project was to determine whether the PGMA’s aldehyde functional groups could reduce gold in
HAuCl4 into usable nanoparticles. Eq. (1), (where ‘R’ represents the nonreactive portions of the
PGMA) illustrates how the aldehyde functional groups could act as effective reductants in
reducing the Au (III) ions to neutral Au atoms, oxidizing into carboxylic acid groups in the
process.
3𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅 + 3𝑅𝑅2𝑅𝑅 + 2𝐴𝐴𝑢𝑢3+ → 3𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅 + 6𝑅𝑅+ + 2𝐴𝐴𝑢𝑢 Eq. (1)
The majority of past studies on shape-controlled synthesis of gold nanoparticles have focused
on using multistep approaches to revise conventional methods geared towards the production of
a single gold nanoparticle shape [7-9, 11-13]. Only a few studies have ever investigated the
morphological evolution of gold nanoparticles over a specific reaction parameter or parameters
[5] and none of these have used a one-pot synthesis approach. As such, this study sought to
develop an approach to production of gold nanoparticles through the use of a novel polymer
Figure 3-The sulfate ions from the potassium persulfate initiator cause the PGMA’s epoxy groups to break into hydroxyl groups. The hydroxyl groups are then oxidized to aldehydes, giving the PGMA reducing properties.
Ring Opening Reaction Hydroxy-Aldehyde Oxidation
Epoxy Group Sulfate Ion Hydroxyl Group Aldehyde Group
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microsphere reductant in a one-pot method, with a subsequent study of shape control possible in
this method. The morphological evolution and formation mechanism of the synthesized gold
nanoparticles was assessed using serial extractions from the reacting solutions at timed intervals
to investigate the reducing and shape-controlling capabilities of the novel PGMA reductant.
Additionally, the catalytic properties of the synthesized gold nanoparticles were tested to
determine whether the PGMA synthesized gold nanoparticles might be applicable in an industrial
situation. This investigation is set apart from previous studies in this field through the use of the
novel PGMA reductant to synthesize nanoparticles in a one-pot method and using serial
extractions at timed intervals to study PGMA’s effectiveness. These modifications to
conventional methods of gold nanoparticle synthesis enhance the possibility that shape-tailored,
catalytically active gold nanoparticles could be produced more efficiently while remaining
applicable in many areas including health sciences and industrial processes.
II. Methodology
For this investigation, gold (III) was reduced from chloroauric acid (HAuCl4) into neutral
gold atoms to form
gold nanoparticles.
During this process,
the gold (III) present
in the chloroauric acid
is first reduced to a
neutral state through
chemical reduction
(Fig. 4, Step 1). Next, Figure 4-Diagram illustrating the process resulting in the formation of a gold nanoparticle. Step 1: Reduction of gold ions by a reducing agent. Step 2: Nucleation of gold atoms. Step 3: Gold nanoparticle formation. Note that the reducing agent in this investigation is the PGMA microsphere.
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the newly reduced gold atoms aggregate onto one another into a gold nucleus (Fig. 4, Step 2).
Finally, additional aggregation of gold atoms onto the nucleus results in the formation of a gold
nanoparticle (Fig. 4, Step 3).
i. Optimization of One-Pot Reaction Conditions for Gold Nanoparticle Synthesis
A series of trials were conducted to determine the best reaction conditions for synthesizing
gold nanoparticles using the PGMA reductant. During these trials, reaction temperature and
HAuCl4 solution concentration were varied. Due
to the quantum size effect and Plasmon
resonance, theories that distinguish the properties
of bulk and nano gold, gold nanoparticles’ small
size results in their ruby-red color appearance [2,
20-21]. As such, the presence of a red color in the
resulting solutions was used to initially determine
if the gold nanoparticles were synthesized. Both
the synthesis of the PGMA and the gold
nanoparticles took place in the same reaction
apparatus shown in Fig. 6.
PGMA microspheres were first synthesized
through an emulsion polymerization method. The exact parameters for synthesis of the PGMA
Figure 6-Schematic of apparatus used for the one-pot synthesis of PGMA and the gold nanoparticles.
Con
dens
er
Dropping Funnel
Round Bottom Flask
Silicone Oil Bath
Nitroge
n Purg
e
Thermometer
Synthesis reaction parameter
optimization
Transmission electron microscopy
imaging and analysis
UV-VIS spectroscopy
analysis of growth mechanism
Testing catalytic properties through
kinetics trials
Figure 5-Diagram of the different stages of the investigation/experimentation with the gold nanoparticles: 1) Parameter optimization for nanoparticle synthesis 2) TEM imaging of nanoparticles 3) UV-VIS analysis of nanoparticle formation mechanism 4) Testing catalytic properties of nanoparticles.
Addition of: • GMA Monomer • KPS Initiator • HAuCl4 Solution
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were based on those set by Liu et al., 2013 [21]. First the monomer, glycidyl methacrylate
(GMA), was purified to remove any polymerization inhibitors. 2.5 mL of purified GMA and 25
mL of deionized water were mixed at 1200 rpm and purged with nitrogen gas for 15 min. The
solution was then raised to 90°C in a temperature controlled silicone oil bath and 2.0 mL of
potassium persulfate initiator was added dropwise to begin the polymerization process.
Potassium ions attacked the vinyl group double bond on the GMA molecules, initiating the
chain reaction forming the polymer microspheres, shown in Fig. 7. After an hour of reaction at
90°C and 1200 rpm stirring, the solution gained a white milky appearance indicating the
formation of the PGMA microspheres (250 nm diameter).
Following the PGMA synthesis, 15 mL of HAuCl4 solution was added directly into the
reaction chamber. Depending on the trial, the concentration of HAuCl4 solution ranged from
1mM to 5mM. Furthermore, the reaction temperature (temperature of the silicone oil) was
changed with each trial (50°C, 60°C, 70°C, 80°C, 90°C, 100°C, 110°C, and 150°C) prior to the
chloroauric acid addition. These temperature and concentration parameters, were investigated to
determine what combination best induced the synthesis of the gold nanoparticles. Immediately
after the addition of HAuCl4, a set of 14 serial extractions from the solution at timed intervals
were conducted (0.5 min, 1 min, 1.5 min, 2 min, 3 min, 4 min, 5 min, 10 min, 15 min, 20 min, 30
min, 40 min, 50 min, 60 min). The timed extraction samples were placed in ice to halt the
reaction. One hour after the HAuCl4 was added, the heat and stirring was halted, stopping the
Figure 7-Polymerization of GMA initiated by the potassium ion attack on the GMA’s vinyl group.
K+
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reaction. Based on all combinations of reaction temperatures and HAuCl4 concentrations tested,
the parameters that produced solutions with a noticeably red colored supernatant were
determined to be conducive for the synthesis of gold nanoparticles using PGMA.
ii. Transmission Electron Microscopy Imaging and Analysis of Gold Nanoparticles
In order to image the gold nanoparticles and the PGMA microspheres, a transmission
electron microscope, or TEM was used. The TEM provided nanometer scale, high resolution
imaging that allowed for the analysis of both the shape and size of the synthesized nanoparticles.
TEM imaging was conducted on the timed serial extraction solutions to investigate the shapes
and morphological evolution of the nanoparticles. TEM slides were prepared by depositing small
amounts of the solution to be imaged onto standard carbon-coated mesh copper grids. After
imaging the various solutions, the Nanomeasure program was used to quantitatively assess the
size and shape of the gold nanoparticles.
iii. UV-VIS Analysis of Gold Nanoparticle Growth Mechanism
To assess the evolution of gold nanoparticle formation over time, a UV-VIS spectrometer
was used on the extracted sample. Since gold nanoparticles absorb in the 520 nm range, the
absorbance peaks, within the range of 515 nm to 540 nm, for the supernatants of each of the
timed extraction samples was measured. The wavelengths of the peak absorbance values as well
as the peak absorbance values themselves were analyzed with relation to their time of extraction
to model the formation mechanism of the gold nanoparticles throughout the reaction.
iv. Assessment of Catalytic Properties of Gold Nanoparticles
The catalytic properties of the PGMA synthesized gold nanoparticles were examined using a
UV-VIS spectrometer in kinetics mode, used to measure the progression of a reaction in real
time. A model reaction where 4-nitrophenol was reduced to 4-aminophenol by sodium
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borohydride was used to assess the catalytic properties of the gold nanoparticles. Initially, 1.25
mL of 0.1 mM 4-Nitrophenol was mixed with 0.5 mL of 0.1 M sodium borohydride in a quartz
cuvette. Once the reaction began, 200 μL of the supernatant (containing the gold nanoparticles)
of a specific solution was added in. A UV-VIS tracked the 400 nm wavelength, corresponding to
the concentration of 4-nitrophenol, throughout the reaction. The effectiveness of the gold
nanoparticle catalyst was determined based on the time it took for the 4-nitrophenol to be
completely reduced, a standard procedure when assessing the catalytic reactivity of any material.
The reaction times between different serial timed extraction solutions were compared with one
another as well as to a control reaction (in which no catalyst was added) to assess the catalytic
effectiveness of the PGMA synthesized gold nanoparticles.
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III. Results and Discussion
i. Optimization of One-Pot Reaction Conditions for Gold Nanoparticle Synthesis
Temperature Variation Trials: 1mM Concentration
5mM Concentration
Figure 8-Upper: Photo images of 1mM solutions synthesized at various temperatures. As the temperature increased, the color turned closer to a ruby red. Due to the quantum size effect and Plasmon resonance, gold nanoparticles’ small size results in their ruby-red color appearance [2, 20-21]. The solutions at 90°C, 100°C, 110°C, were found to have the most promising and distinct red colored supernatant indicating successful gold nanoparticle synthesis. Lower temperature solutions produced a purple color with less defined supernatant.
Lower: Photo images of 90°C, 5mM concentration solutions are shown. Note that solution contained no supernatant (Shown in Left Image) but rather had large precipitates indicating large clumps of gold (Shown in Right Image), rather than gold nanoparticles had formed.
90°C 5mM
90°C 1mM 100°C 1mM 110°C 1mM
110°C 1mM
50°C 1mM 60°C 1mM 70°C 1mM 80°C 1mM
80°C 1mM
• No supernatant • Lacks ruby
color
• Distinct layers (Top: Au NPs Bot.: PGMA
• Distinct ruby color
• No supernatant • Gold aggregations
formed
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Solutions of gold nanoparticles with PGMA were synthesized under a variety of temperatures
and chloroauric acid concentrations. Photo images of these solutions are shown in Fig. 8. Out of
all the solutions that were synthesized, the high temperature (90°C-110°C), 1 mM solutions were
the most promising in terms of gold nanoparticle formation. These solutions contained a
distinctly red supernatant (Fig. 8) indicating gold nanoparticles had successfully formed (with
the white colored deposit being the PGMA). Conversely, lower temperature (50°C-80°C), 1 mM
solutions had a purplish color, indicating that few gold nanoparticles had formed. This was likely
due to the reaction temperatures preventing the reduction of HAuCl4 in sufficient amounts.
Higher temperatures (Up to 150°C) were tested, however, at these high temperatures, the PGMA
would rapidly degenerate making it impossible for any gold nanoparticles to be formed. This fact
made 110°C the highest temperature that could be used. Solutions synthesized using 5 mM
HAuCl4 solution failed to show any promising results. There was no supernatant present, but
rather, large amounts of precipitate that indicated large clumps of bulk gold were forming instead
of gold nanoparticles.
With this information, high resolution TEM imaging was performed on the timed serial
extraction solutions that were synthesized under the “optimal” conditions. The images in Fig. 9
are from the 90°C, 1 mM solution, findings that can be generalized to all of the optimal reaction
parameters solutions. In the TEM images, the dark black spots/shapes are the gold nanoparticles.
The mesh-looking structure in the background is the copper grid slide and the large light gray
spheres are the PGMA.
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ii. Transmission Electron Microscopy Imaging and Analysis of Gold Nanoparticles
Extraction Time
TEM Images
0.5 Min Note: -Large gold nanoparticle aggregates -Undefined shape -Average Sphere Size: 11.65 nm SD: 4.84 nm
2 Min
Note: -Aggregations broken up -Versatile Geometric Shapes -Average Sphere Size: 14.07 nm SD: 10.30 nm
5 Min
Note: -Aggregations broken up -Versatile Geometric Shapes -Average Sphere Size: 27.13 nm SD: 13.47 nm
Figure 9-TEM Images of the timed extraction samples of an optimal condition solution (90°C, 1mM). TEM images show evident gold nanoparticle evolution over the progression of the reaction (starting from top left going counter clockwise). At the 0.5 min extraction, there are large aggregations of gold nanoparticles with undefined shapes (determined by looking at images under higher magnification). By the 2 and 5 min extractions, these clumps have broken up into a large variety of shaped gold nanoparticles including rods, triangles, hexagons, pentagons, and spheres. From fifteen minutes to the reaction’s completion, the gold nanoparticles evolve into spheres without any trace of non-spherical shapes evident in the earlier time extractions. In each of these images, the copper mesh slide grid is visible (see label in 15 min image).
Spherical Nanoparticles
Large Aggregations
Non-Spherical Shapes
Spherical Nanoparticles
Non-Spherical Shapes
Spherical Nanoparticles
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As illustrated by the TEM images, the PGMA microspheres succeeded in synthesizing gold
nanoparticles. Interestingly, at the 0.5 min stage of the reaction, the majority of gold
nanoparticles appeared as clumped aggregations lacking any defining shape. However, by the 2-
5 min stages, the aggregated clumps of gold nanoparticles evolved into gold nanoparticles with
large amounts of shape versatility as well as spheres. It is likely that the initial aggregations of
gold broke apart into the well-defined shapes seen moments later in the reaction. These shapes
included nanorods, triangles, spheres, hexagons, pentagons, diamonds, and trapezoids. The
versatility of the shapes that were synthesized is a property of the PGMA that has not been seen
before in other reducing agents. The fact that PGMA works as a slow reductant [14] may account
for the shape versatility in the initial stages of the reaction. According to published research, fast
reductants often inhibit non-spherical shape formation due to the speed at which the neutral gold
atoms are being synthesized whereas slower reductants have been more favorable towards non-
spherical shape formation [6].The later stage TEM images from 15 min to 60 min showed that
particle shapes converged towards a spherical morphology, explaining the lack of any particle
shape versatility by the late stages of the reaction.
Nanomeasure analysis of the TEM images showed that spheres, triangles, nanorods were the
most common shapes to occur. However, triangles and nanorods were only present for small
portions of time (2-5 min stages) whereas spheres were imaged in every timed extraction
solution, indicating their presence throughout the reaction. This made tracking the evolution of
spherical particle diameter an accurate method for modeling the reaction growth mechanism.
Table. 1and Fig. 10 illustrate the increase in both mean size and standard deviation of only the
spherical nanoparticles’ diameters as the reaction progressed. The increase in size was likely due
to the continuous synthesis of neutral gold atoms that aggregated onto the already existing
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particles (accounting for increase in mean diameter) as well as forming new, smaller gold
nanoparticles themselves (accounting for the increase in variance).
Reaction Time (min) Mean (nm) SD (nm) 0.5 (n=23) 11.65 4.84 2 (n=49) 14.07 10.30 5 (n=20) 27.13 13.47
15 (n=73) 29.97 14.02 60 (n=33) 31.75 17.72
Table 1-Mean and standard deviation of spherical nanoparticle diameters as a function of time as determined through analysis of the TEM images. Data indicates that mean spherical particle diameter increased throughout the reaction, though mainly within the first 15 minutes. Furthermore, as the reaction progressed, the standard deviation of the particle distributions increased. These results can be seen visually in Fig. 9.
Figure 10-Graph of mean spherical particle diameter and average solution supernatant peak absorbance against reaction time. Trend in particle size shows an overall growth in average diameter over time. Serial extraction peak absorbance over time shows an initial decrease in absorbance, possibly due to the aggregations breaking up and some of the gold dissolving into solution. This is followed by an overall increase in absorbance correlating to growth in particle size. The absorbance appears to increase then decrease in the final 40 min of the reaction, indicating a highly variable process.
Note that 1 absorbance unit (AU) corresponds to 90% absorbance, 0.75 AU corresponds to 82% absorbance, 0.50 AU corresponds to 68% absorbance, 0.25 AU corresponds to 44% absorbance, and 0.1 AU corresponds to 21% absorbance.
0
0.2
0.4
0.6
0.8
1
1.2
0
5
10
15
20
25
30
35
0 10 20 30 40 50 60
Abso
rban
ce (A
U)
Part
icle
Dia
met
er (n
m)
Reaction Time (Min)
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iii. UV-VIS Analysis of Gold Nanoparticle Growth Mechanism
UV-VIS absorbance analysis of the supernatants of the timed extractions of the optimal
condition solutions provides a model of gold nanoparticle growth. The absorption at the 520 nm
wavelength range (515 nm-540 nm; wavelength for gold nanoparticle absorbance) was measured
for each of the solution supernatants from the timed extractions. Fig. 10 illustrates the averaged
peak absorbances for each of the timed extractions under optimal conditions (90°C, 100°C,
110°C, 1mM), where each timed extraction is represented by a point on the graph.
The high initial peak absorbance at 0.5 min signifies high concentrations of synthesized gold
present in the initial aggregations imaged at the 0.5 min extraction (Fig. 9). However, there is a
sudden drop in absorbance in the moments after followed by an increase then decrease, hinting at
a highly variable pattern. This may be due to the fact that as more gold atoms are being
introduced into solution (due to the constant PGMA reduction), the gold nanoparticles become
too large to remain suspended in the supernatant and are therefore falling out of solution. This
process repeats itself as new gold particles are synthesized, increasing the concentration of gold
in the supernatant before falling out of solution again when the particles become too large.
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iv. Assessment of Catalytic Properties of Gold Nanoparticles
The final portion of this experiment involved analyzing the catalytic properties of the gold
nanoparticles by adding them into a model reaction of 4-nitrophenol reduction by sodium
borohydride to 4-aminophenol. For this reaction, the gold nanoparticles served as the catalyst by
helping to speed the electron transfers taking place, illustrated in Fig. 11.
Fig. 12 and Fig. 13 show the UV-VIS kinetics that were run on 4-nitrophenol to 4-
aminophenol model reactions with 200 µL of different gold nanoparticle time extractions added
to them as catalysts. The 400 nm wavelength (Fig. 12) absorbance corresponds to the
concentration of 4-nitrophenol left. When this absorbance no longer decreased, the reaction was
determined to have gone to completion. Additionally, the 300 nm wavelength (Fig. 13)
corresponds to the 4-aminophenol concentration in the reaction. The 300 nm wavelength
measurement was done to show that 4-aminophenol was being synthesized. The absorbance at
the 300 nm wavelength is not an accurate measure of 4-aminophenol concentration since the
majority of newly synthesized 4-aminophenol remains attached to the gold nanoparticles’ active
sites and cannot be detected by the UV-VIS absorbance.
Figure 11-An illustration of the model reaction used to test the catalytic properties of the gold nanoparticles. During the reduction of 4-nitrophenol to 4-aminophenol through the use of sodium borohydride, the gold nanoparticles aid in the transfer of electrons, helping to speed up the reaction [22].
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Figure 12- UV-VIS Kinetic analysis of the catalytic properties of gold nanoparticles extracted at different times during the synthesis reaction.
Left: Compared to the control group (no catalyst), the gold nanoparticle catalytic properties were best in the aggregations (0.5 min) while nonexistent in the non-spherical gold nanoparticles (2 minutes). Note that for the control and 2 min samples, only the first and last data points are shown to illustrate no change in the absorbance and therefore, no reaction. Right (Below): The more mature, spherical gold nanoparticles extracted from later time points in the reaction were effective catalysts. All nanoparticles extracted from 15 to 60 min performed equally effectively, driving the reaction to completion in 600 sec. The graph shown in the figure is representative of all model reaction results that used 15-60 min gold nanoparticle extractions.
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Figure 13: The concentration of 4-aminophenol (300 nm) is shown to be increasing during a successful model reaction. Note that the 300nm increase shown is representative of results in other successful model reactions though it is not an accurate measurement of 4-aminophenol concentration.
The data from the UV-VIS Kinetic
analysis indicates that the gold
nanoparticles extracted at various times
throughout the reaction have varying
catalytic properties. The gold
nanoparticles extracted in the initial 0.5
min of the reaction showed the strongest
catalytic properties, catalyzing the
reaction to completion in approximately
400 sec. However, the gold nanoparticles
extracted at 2 min showed no catalytic
properties when compared to the control.
The gold nanoparticles extracted from 15 to 60 min into the reaction also showed strong catalytic
properties, completing the model reaction in approximately 600 sec with little variation. The
spherically shaped gold nanoparticles found at time intervals from 15 min to 60 min as well as
the aggregations found at 0.5 min likely had the most active sites present on their surfaces,
accounting for the resulting fast reaction times. Conversely, the non-spherically shaped
nanoparticles that were found at the two minute interval most likely lacked sufficient numbers of
active sites to effectively catalyze the reaction. The results of these tests show that small amounts
of PGMA synthesized gold nanoparticles under specific conditions have the potential to act as
strong catalysts in much larger, volume ratio-wise, reactions.
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IV. Conclusions
One of the primary goals of this experiment was to determine how effective PGMA
microspheres are as a reductant to synthesize gold nanoparticles from HAuCl4 in a one-pot
reaction. Through repeated trials under various conditions, PGMA resulted in gold nanoparticles
in a wide variety of shapes and sizes. Non-spherical shapes under specific reaction parameters as
well as the morphological evolution over time in terms of nanoparticle size and shape is a unique
attribute of the PGMA that has not been seen in previous one-pot reductants. It was found that an
increase in reaction time caused the gold nanoparticles to increase in average size and adopt a
spherical shape. The results of this portion of the experiment indicated that the use of PGMA in a
one-pot synthesis reaction, through careful control of the reaction parameters, can replace more
time intensive multistep approaches [14] to nanoparticle shape-controlled synthesis.
Furthermore, some of the PGMA synthesized gold nanoparticles demonstrated strong catalytic
properties. Nanoparticles extracted at the 0.5 min and 15-60 min enabled the model reaction (4-
nitrophenol→4-aminophenol) which, without a catalyst, does not occur at all. The catalytic
activity of the nanoparticles at those specific timed extractions was likely related to the increased
number of active sites on the spherical and aggregated particles. These findings underscore the
effectiveness of PGMA as a novel one-pot reductant for nanoparticle synthesis as well as the
applicability of the newly synthesized gold nanoparticles.
According to previously conducted studies, traditional multi-step methods for synthesizing
non-spherically shaped gold nanoparticles have been found in general to take anywhere from 4.5
hours [13] to 25 hours [9] to produce non-spherical shapes, depending on the method used. By
comparison, the entire process for synthesizing PGMA microspheres and reducing HAuCl4 was
completed in approximately 2.5 hours. Furthermore, since the non-spherically shaped gold
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nanoparticles generally appeared at the early stages of the PGMA reduction cycle, the time for
the entire process to synthesize non-spherically shaped gold nanoparticle only took
approximately 1.5 hours. So this PGMA one-pot methodology has the potential to reduce gold
nanoparticle fabrication time from 60% to 90%, an advantage that results in economic savings at
the production level as well.
The versatility of the early stage particles illustrates a unique feature of the PGMA reductant.
In these trials, chloroauric acid concentration and reaction temperature were varied. Given that
PGMA has been shown as a feasible option for gold nanoparticle synthesis, additional
parameters of the one-pot synthesis reaction must be varied and optimized to improve the
monodispersity, or shape uniformity, of the gold nanoparticle products. Maximizing the
monodispersity of the non-spherical shapes may improve this method’s applicability towards
synthesizing shape-controlled nanoparticles. Another area of potential future research would be
to apply the PGMA synthesized gold nanoparticles as catalysts in industrial chemical reactions
rather than model ones. Furthermore, if the PGMA synthesized gold nanoparticles are to be used
in industry, it will be necessary to significantly increase the volume of the PGMA/Au NP
solution which in these is experiments is only 45 mL. PGMA could eventually be applied
towards the synthesis of other metallic nanoparticles such as silver which also require reducing
agents for synthesis. Finally, a unique characteristic of PGMA is that it is a nontoxic polymer
[15]. As such, future research might entail attaching newly synthesized gold nanoparticles loaded
with medication onto the PGMA microspheres to be used for the delivery of drugs in high
concentration to a particular area. In lieu of growing demands to implement nanotechnology on
an industrial level in fields such as industry and health, the results of this study can potentially be
applied towards improving the time-efficient synthesis of usable gold nanoparticles.
Cheung, Justin
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