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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