1. Reduction of Gold Dendrimer-Encapsulated Nanoparticles
From 4-Nitrophenolto 4-Aminophenol
Shripal R. Shah†*, Stacia E. Rodenbusch†, Keith J. Stevenson†
†Department of Chemistry and Biochemistry, University of Texas, 1 University Station, Austin,TX 78712
*Corresponding author: provide the the e-mail address of the authorto whom correspondence should be addressed
KEYWORDS: Langmuir-Hinshelwood model, 4-nitrophenol,Dendrimer-Encapsulated Nanomaterials (DENs)
ABSTRACT: Dendrimer-Encapsulated Nanoparticles were used to reduce 4-nitrophenol at 40°C to see if the
reaction rates increasedas theconcentration of4-nitrophenol decreased. Stock solution such as NaBH4, NaOH,
pH12 water, and 4-nitrophenolwere used with the addition ofHAuCl4 and G4-NH2. Five cuvette conditions were
created with the solutions pH12 water, nanopure water, 4-nitrophenol, NaBH4, and DENs. The amount of 4-
nitrophenolincreased with each newcuvette where as the amountofpH12water decreased.The results fromfull
spectrumkinetics in which the rate constants,measuredthrough an Agilentspectrophotometer, showed no linear
relationships. For 4 M, the rate was 0.16 ± 0.14 sec-1, 3M was 0.11 ± 0.02 sec-1, 2.4 M was 0.12 ± 0.06 sec-1, 2
M was 0.18 ± 0.08 sec-1,and 1.7 M was 0.18 ± 0.04 sec-1.The rate constants increasedas the concentration of4-
nitrophenol in the cuvette decreased, with the disregard of the first cuvette. Possible error could include the
addition of NaBH4 before data collection started.
Introduction
Conversion of 4NP to 4-AP
One of the most widely used model reactions in the evaluation of DENs and other nanoparticles
as catalysts is the conversion of 4-nitrophenol (NP) to 4-aminophenol (4-AMP) by sodium
borohydride. This conversion is exhibited by equation one. This equation just shows 4-NP is
going to be used by the reaction in which as the amount of 4-NP goes down, the amount of 4-AP
will increase because it’s a simple conversion.
(1) 4-NP  4-AP
This is considered to be a model reaction because there are no byproducts formed.1
Reduction of Gold DENs

2. 2
Dendrimer-encapsulated nanoparticles (DENs) makes good catalysts for the reduction of p-
nitrophenol. The dendrimer used in this experiment was G4-NH2 PAMAM dendrimer with Au55
metal and reduced with NaBH4. Equation two shows the reaction occurring. The full scheme is
shown in Figure 1.
(2) G4-NH2 + 55 Au3+  G4-NH2(Au55)
Langmuir Hinshelwood Model
The goal of this experiment is to figure out the mechanism of p-nitrophenol reduction. One
possible model for the reaction mechanism is the Langmuir-Hinshelwood model, which states
that both reactants in a bi-molecular reaction must be adsorbed onto the catalyst surface for the
reaction to occur.1-5 It has already been shown that the Langmuir-Hinshelwood model is a good
model for catalysis by other types of gold nanoparticles, and for copper, silver, gold, and
palladium DENs.
Adaptation of past Research-Bingwa and Mejiboom
Bingwa and Mejiboom through past experiments presented a new model through which they
tested DENs in different temperatures to see if the rate constants increase as the temperature
decreases with the use of palladium DENs.2 Their results showed that the higher the temperature,
the higher the rate constants. In this experiment, this model was changed in which the palladium
DENs were replaced with G4-NH2 (Au55) DENs. The DENs were only measured at one
temperature, 40°C, to see if the rate constants of gold DENs were higher than the palladium
DENs that were tested in the Bingwa and Mejiboom article.
Calculation of rate constants
Rate constants for each reaction were calculated due to the fact that this was a first order
NaBH4

3. 3
reaction. A first-order reaction can be determined by plotting a graph of ln[A] vs. time t:
a straight line is produced with slope –k. Therefore, this was calculated by taking natural log of
the data collected at 405nm (the amount of 4-NP) subtracted by 600nm (background subtraction)
to find each rate constant. The rate constants for all cuvettes were averaged together. This
calculation can be seen in equation 3.
(3) ln(“405nm” - “600nm”)
Materials & Methods
Materials
The materials needed in this experiment were mainly stock solutions. First, 0.0378g from Alfa
Aesar of NaBH4 (98% pure) was measured out. It was dissolved in 5 mL of nanopure water to
make 0.2M NaBH4. Second, two g of Fischer Scientific NaOH (97% pure) were massed out
using a balance in a 50mL plastic vial. Nanopure water was added to filled to the 50mL mark.
Next, pH12 water was made from 0.1M NaOH by pipetting 5mL of it into a 50mL plastic vial.
Finally, 600m p-NP was made by dissolve 0.0083g in 10ml pH12 water to get 6mM. Then,
0.5ml of 6mM solution was added to 4.5ml pH 12 water to get 600 M. HAuCl4 (49% pure)
from Sigma Aldrich was also used. The Agilent 8453 Spectrophotometer was used to record
data. An Ohaus analytical balance was used to measure the weight of all the chemicals used in
the experiment. PAMAM G4-NH2 dendrimer (10 wt. % in methanol) was used from Sigma to
catalyze the reaction. 4 nitrophenol (99% pure) from Acros was used in the experiment. To carry
out the experiment, a thermometer was needed to measure the temperature of the cuvettes. A hot
plate was used to heat up the cuvettes to 40°C, the temperature that was being tested. The
cuvettes were placed in a petri dish filled with sand, since sand is a good conductor of heat.

4. 4
Methods
Past students, Tiffany Kim and Erik Zavala with the help of mentor Stacia Rodenbusch created
the methods in this experiment. The methods can be referenced to the 2009 Crooks article as the
source of origin for the ideas. Stock solutions such as 600 m 4p-NP, 0.2M NaBH4, PH 12
water, 2m DENs were created first.3 To create the DENs, 3.4L of 10% wt. PAMAM G4-NH2
dendrimer solution was pipetted into a 20 ml glass vial. Then, due to the dendrimer solution
being mostly methanol, the solution was set the vial in the hood for 10 minutes to let the
methanol evaporate. After, 9.934mL of nanopure water was added to the vial. Next, 1 ml of this
dendrimer solution was transferred in to a new, clean, small-volume UV-plastic cuvette and a
full UV-Vis spectrum was taken. 11L of 0.1 M HAuCl4 was added to the vial containing the
dendrimer solution. To allow the Au3+ ions to associate with the dendrimer, the vial was capped
and left to mix on the nutator for 20 minutes. Finally, 55L of 0.2 M NaBH4 solution (made in
0.3 M NaOH) was added to reduce the Au3+ to Au0, making a dendrimer-encapsulated
nanoparticle. Then, five cuvettes were filled according to conditions in Table 1. Each cuvette
was placed in a petri dish filled with sand. The sand heated up the cuvettes, with 4-nitrophenol,
pH12 water, and nanopure water, to 40°C, the temperature that was being tested at. A
thermometer was placed both in the sand and the cuvette for accuracy. Once, the temperature of
the cuvette reached 40°C, it was placed in the Agilent 8453 spectrophotometer. The Agilent 8453
spectrophotometer was used for kinetics in which it was set to 40 seconds, 0.5-second intervals.
All spectra was chosen and set to read at 405nm (4np), 315nm (4-ap), 240nm (possible second
product), 600nm (background subtraction). In cuvette one, 200 L pH 12 water, 1970 mL of
nanopure water, and 150 L of 4-NP were added. Next, 600 L of 0.2M NaBH4 was added.
Once the Agilent 8453 spectrophotometer started recording data, 240 L of 2 M Au DENs

5. 5
were added. Cuvettes two through five followed the same procedures but they followed their
respective amounts of pH 12 water, nanopure water, and 4-NP according to Table 1. All of the
data was recorded through the Agilent 8453 spectrophotometer and transferred to LoggerPro to
calculate the rate constants of the reactions using equation three since it is a first order reaction.
Results & Discussion
In this experiment, reduction of 4-NP to 4-AMP took place by following the Langmuir-
Hinshelwood model. Stock solution such as NaBH4, NaOH, pH12 water, and 4-nitrophenol were
used in this experiment with the addition of HAuCl4 and G4-NH2. The Crooks example was
followed for the methods in which five cuvette conditions were formed.3 Different amounts of
each stock solution were placed into the cuvettes according to Table 1 and results were
measured. Since this experiment was first-order, the rate constant was calculated for each
cuvette. The spectra was chosen to record data at 405nm (4np), 315nm (4-ap), 240nm (possible
second product), 600nm (background subtraction) in which the natural log of “405nm” minus
“600nm” (ln(405nm-600nm)) to find the rate constant of the reaction. An example of the rate
constant can be referred to in Figure 1. The slope of the graph is equaled to negative k. Full
spectrum kinetics’ were taken for each cuvette. An example of this can be referred to in Figure 2.
In Figure 2, as the 405nm peak goes down, the one at around 300nm goes up which according to
Feng 2009 is a pattern where p-NP goes down (being used up), p-aminophenol goes up, in which
the assay works.6 The results were that for concentration of 4 M 4-NP, the rate was 0.16 ± 0.14,
3 M was 0.11 ± 0.02 sec-1, 2.4 M was 0.12 ± 0.06 sec-1, 2 M was 0.18 ± 0.08 sec-1, and 1.7 M
was 0.18 ± 0.04 sec-1. These rate constants (k) were calculated by measuring the slope of the
graph in which it equaled – k. The graph was derived by converting the data from the trials using
equation 3. An example of this can be referred to in Figure 3. This is a different from the

6. 6
expected results since it was expected that once the amount of 4-NP was increased, the rate
constant would decrease. The results told a different story though in which the rate constants
increased as the amount of 4-NP in the cuvette increased. This trend of data can be seen in
Figure 3. The margin of error, shown by the standard deviation, is quite high in my opinion.
Even though for cuvettes 2-5, the margin of error seems low, it is large enough to make an
impact of the results of this data. This trend disregards the first cuvette from the trials due to the
fact that improper procedures could’ve resulted in the high rate constant with a smaller 4-NP
amount. These results could also be due to the age of DENs in which the amount of time they
were left out in the open could have changed the outcome of this experiment resulting into the
high margin of error. Also, the addition of NaBH4 right before recording data could have caused
an additional reaction to happen and not be recorded. This trend that was found is very interested
in which it is different from the expected trend that was to be found in Crooks and Mejiboom
papers. It is definitely worth to explore more into why this was the case. Compared to the
Bingwa and Mejiboom article, the rate constants found are significantly higher for gold DENs
compared to palladium DENs. Their rate constant for Pd55 was 1.83 ± 0.5 x 10-6 sec-1. Even
though they only had the average for one cuvette condition, this outcome could be a result of the
different properties of the two metals, such as weight. Future directions for this project would be
to control the amount of time the DENs are sitting out in the open and to try put NaBH4 at the
same time as the DENs. Lastly, more trials need to be executed to confirm this new trend of data,
which might lead to a new assertion.
Figures

7. 7
G4-NH2 Dendrimer
Figure 1. This is an example of the Dendrimer scheme that is being focused on in this experiment. G4-
NH2 dendrimer is being reduced with NaBH4.
G4-NH2 Dendrimer
G4-NH2(Au55)+2
Complex
4-nitrophenol Reduction
G4-NH2(Au55)
DENs

8. 8
Figure 2. This is an example of full spectrum kinetics. There is a peak at 405nm and 315nm. As the
405nm goes down, the one at around 300nm goes up.
Full Spectrum Kinetics
Wavelength (nm)
Absorbance(AU)

9. 9
Figure 3. This is an example of a sample rate constant (k) derived from the experiment. The slope of this
graph equals –k.
ln(“405nm-”600nm)
Wavelength (nm)
Rate Constant Example

10. 10
Figure 4. This represents the weak linear trend between the concentration of 4-NP (M) and the rate
constant (k).
0
0.05
0.1
0.15
0.2
0.25
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
4 NP vs Rate Constant
4NP concentration (M)
Rateconstant(k)

11. 11
Tables
Cuvette
Conditions
600m p-NP
(l)
0.2 NaBH4
(l)
PH 12 water
(l)
Nanopure
water (l)
2um DEN
(l)
1 150 600 200 1970 240
2 200 600 150 1970 240
3 250 600 100 1970 240
4 300 600 50 1970 240
5 350 600 0 1970 240
Table 1. Cuvette conditions that were prepared for the experiment.

12. 12
Concentration of 4-NP (M) Rate Constant, k, (sec-1)
4
0.16 ± 0.14
3
0.11 ± 0.02
2.4
0.12 ± 0.06
2
0.18± 0.08
1.7
0.18 ± 0.04
Table 2. These are the rate constants for the reduction of 4-NP that were derived in the experiment
through ln(405nm-600nm)

13. 13
References
1. Bingwa, N., & Meijboom, R. (2015). Evaluation of catalytic activity of Ag and Au
dendrimer-encapsulated nanoparticles in the reduction of 4-nitrophenol. Journal of
Molecular Catalysis A: Chemical, 396, 1–7.
2. Bingwa, N., & Meijboom, R. (2014). Kinetic Evaluation of Dendrimer-Encapsulated
Palladium Nanoparticles in the 4-Nitrophenol Reduction Reaction. The Journal of
Physical Chemistry C, 118(34), 19849–19858.
3. Crooks, R. M., Zhao, M., Sun, L. I., Chechik, V. & Yeung, L. E. E. K. Dendrimer-
Encapsulated Metal Nanoparticles : Synthesis , Characterization , and Applications to
Catalysis. 34, 181–190 (2001).
4. Nemanashi, M., & Meijboom, R. (2013). Synthesis and characterization of Cu, Ag and
Au dendrimer-encapsulated nanoparticles and their application in the reduction of 4-
nitrophenol to 4-aminophenol. J Colloid Interface Sci, 389(1), 260–267
5. Noh, J.-H., & Meijboom, R. (2015). Synthesis and catalytic evaluation of dendrimer-
templated and reverse microemulsion Pd and Pt nanoparticles in the reduction of 4-
nitrophenol: The effect of size and synthetic methodologies. Applied Catalysis A:
General, 497, 107–120
6. Feng, Z. V., Lyon, J. L., Croley, J. S., Crooks, R. M., Vanden Bout, D. a., & Stevenson,
K. J. (2009). Synthesis and catalytic evaluation of dendrimer-encapsulated Cu
nanoparticles. An undergraduate expirement catalytic nanomaterials. Journal of Chemical
Education, 86(3), 368–372. doi:10.1021/ed086p368