Synthesis of narrowly dispersed silver and gold nanoparticles and their catalytic evaluation for morin oxidation

Article abstract of DOI:10.1016/j.apcata.2015.10.019

We present a study on the synthesis of narrowly dispersed silver and gold nanoparticles using generation five amino-terminated poly(amidoamine) dendrimers as a template. UV-vis spectrophotometry was performed to monitor the synthesis process and to characterize the metal nanoparticles. Infrared (IR) spectroscopy, and transmission electron microscopy (TEM) were also used to characterize the metal nanoparticles. A catalytic oxidation of morin (quercetin) in the presence of hydrogen peroxide was performed as a model reaction to evaluate the metal nanoparticle activities. Ultra high performance liquid chromatography analysis was performed to identify the reaction products. The kinetic data obtained were modeled to the Langmuir-Hinshelwood equation. The encapsulated silver and gold nanoparticles show high activity which confirm the passive interaction with the dendrimer.

Full text of DOI:10.1016/j.apcata.2015.10.019

Accepted Manuscript
Title: Synthesis of narrowly dispersed silver and gold
nanoparticles and their catalytic evaluation for morin oxidation
Author: Ali Kabeya Ilunga Reinout Meijboom
PII:
S0926-860X(15)30196-4
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2015.10.019
APCATA 15595
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
30-6-2015
7-10-2015
14-10-2015
Please cite this article as: Ali Kabeya Ilunga, Reinout Meijboom, Synthesis of
narrowly dispersed silver and gold nanoparticles and their catalytic evaluation for morin
oxidation, AppliedCatalysisA, Generalhttp://dx.doi.org/10.1016/j.apcata.2015.10.019
This is a PDF file of an unedited manuscript that has been accepted for publication.
As a service to our customers we are providing this early version of the manuscript.
The manuscript will undergo copyediting, typesetting, and review of the resulting proof
before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that
apply to the journal pertain.

Synthesis of narrowly dispersed silver and gold nanoparticles and
their catalytic evaluation for morin oxidation
Ali Kabeya Ilunga and Reinout Meijboom
Department of Chemistry, University of Johannesburg, PO Box 524, Auckland Park 2006,
Johannesburg, South Africa. Tel: +27(0)11 559 2367; fax.: +27(0)115592819; e-mail:
rmeijboom@uj.ac.za
Dedicated to Professor Cedric Holzapfel on the occasion of his 80th birthday
1

Graphical abstract
2

HIGHLIGHTS





Dendrimer encapsulated Ag and Au nanoparticle were synthesized in aqueous solution.
Oxidation of morin was performed as a catalytic model reaction.
The catalytic oxidation was modeled to the Langmuir-Hinshelwood equation.
Effect of temperature and concentration were investigated.
Kinetic and thermodynamic parameters were determined.
3

Abstract
We present a study on the synthesis of narrowly dispersed silver and gold nanoparticles using
generation five amino-terminated poly(amidoamine) dendrimers as a template. UV-vis
spectrophotometry was performed to monitor the synthesis process and to characterize the metal
nanoparticles. Infrared (IR) spectroscopy, and transmission electron microscopy (TEM) were
also used to characterize the metal nanoparticles. A catalytic oxidation of morin (quercetin) in
the presence of hydrogen peroxide was performed as a model reaction to evaluate the metal
nanoparticle activities. Ultra high performance liquid chromatography analysis was performed to
identify the reaction products. The kinetic data obtained were modeled to the Langmuir-
Hinshelwood equation. The encapsulated silver and gold nanoparticles show high activity which
confirm the passive interaction with the dendrimer.
Keywords: Dendrimer; Langmuir-Hinshelwood; Oxidation; Silver nanoparticles; Gold
nanoparticles; Morin.
4

Introduction
Since the commercialization of poly(amidoamine) (PAMAM) dendrimers, their use in biology,
medicine, and chemistry, particularly catalysis, are increasing considerably. A number of key
features make PAMAM dendrimers interesting; notably the well-defined molecular composition,
size, and shapes. Their high density of nitrogen ligands along with the potential of interaction
with metal ions allows the use of dendrimers as templates for the synthesis of metal
nanoparticles [1, 2]. The use of metal nanoparticles increased due to the fact that they show
electronic, optical, catalytic, and biological activity different from their bulk materials [3, 4]. The
ability of metal nanoparticles to lower the activation energy of a reaction, and thus increase the
rate of reaction and yield desired products, could be attributed to their high surface-area to
volume ratio, amongst a number of other explanations. Metal nanoparticles as catalysts result in
higher turnover frequencies, per weight, as well as a potential increase in selectivity [5-8].
Around 15% of dyes is lost in the textile industry during the dyeing process and is released into
the wastewater. The major techniques of removing these dyes in wastewater are oxidation,
adsorption, and flocculation or precipitation. Those approaches are used in bleaching processes
and the chemical oxidation applied in organic dye degradation can be performed as pre- or post-
treatment [9-11]. Hydrogen peroxide, a high potential oxidizing agent, generates radicals which
leads to degradation of organic dyes [12, 13].
Morin, a bioflavonoid dye present in tea, fruits, and vegetables, is used in here as model
compound to evaluate the catalytic activity of silver and gold nanoparticles in an oxidation
process using hydrogen peroxide as oxidizing agent [14].
The synthesis of silver and gold nanoparticles encapsulated into dendrimers is well known and
their catalytic activities were reported in many reaction types. It was established that the catalytic
activities described was related to their surface area and concentration [15-17].
Here, we report the synthesis of NH terminated generation five dendrimer (PAMAM)
2
encapsulated silver and gold nanoparticles. The characterization of these metal nanoparticles
were performed by UV-vis spectroscopy, Fourier transform infrared spectroscopy, and high-
5

resolution transmission electron microscopy. The balance between metal ions and dendrimer or
template was established by spectrophotometric titration monitored via UV-vis spectroscopy.
A full kinetic evaluation of silver and gold nanoparticles was performed on the oxidation of
morin by hydrogen peroxide. The binding energy of the reactants onto the nanoparticle surfaces
was determined. Thus, modeling the kinetic data to the Langmuir-Hinshelwood mechanism is
described. [18, 19]. To the best of our knowledge, this is the first full kinetic elucidation of a
liquid phase oxidation reaction catalyzed by Ag and Au nanoparticles.
Experimental
Materials and instruments
All chemicals were commercially available and used without further purification. Fifth
generation amine-terminated poly(amido-amine) dendrimer, obtained as 5 wt.% solution in
methanol was purchased from Sigma-Aldrich. The volatiles were removed in vacuo at room
temperature. Morin hydrate (C13
chloroauric acid (HAuCl ·3H O, ≥99.9%), and sodium borohydride (NaBH
purchased from Sigma-Aldrich. Sodium hydroxide pellets (NaOH, ≥99.86%), hydrochloric acid
0% (HCl, ≥31.79%), and hydrogen peroxide 30% (H , 30-35%) were purchased from
Promark Chemicals. Sodium hydrogen carbonate (NaHCO , ≥99.86%), and anhydrous sodium
carbonate (Na CO , ≥99.5 %) were obtained from Merck Laboratories. Deionized water was
H
10
O
7
H
2
O, ≥98.0%), silver nitrate (AgNO
3
, ≥99.0%),
4
2
4
, ≥98.0 %) were
3
2
O
2
3
2
3
acquired from an in house Milli-Q system (18.2 MΩ·cm) was used as solvent in all solutions
prepared. All chemicals used in this work were of analytical-reagent grade.
Micropipettes from Physiocare concepts (Eppendorf Research Plus) were used to transfer all
aqueous solutions. An Orion pH meter model 420A was applied to determine the solution pH at
room temperature. Atomic Absorption Spectrometry (Perkin Elmer Precisely, A. Analyst 400,
made in Germany) was used to determine the metal ion and nanoparticle concentration. UV-vis
(Shimadzu UV-1800 spectrophotometer) was applied to monitor the synthesis process and the
reaction, and characterize the nanoparticle synthesis. FT-IR spectroscopy was performed on a
Bruker Tensor 27. HR-TEM analysis was performed on a JOEL JEM-2100F electron microscope
with accelerating voltage of 200 kV using a copper grid. The average size of the nanoparticles
6

was determined using Image-J software [20]. All catalytic data were recorded and fitted using
Kinetic Studio and Origin Pro 8.5 graphing and modeling software [21, 22]. An Ultra High
Performance Liquid Chromatography UHPLC (Nexera Series (8030) Triple Quadrupole
Instrument) from Shimadzu was used to identify the reaction products.
Synthesis of nanoparticles
The interaction of tertiary amino groups located inside the dendrimer with metal ion is an
important aspect in the synthesis of dendrimer encapsulated nanoparticles. Spectrophotometric
titration reported in previous work was conducted to determine the loading capacity of the
dendrimer [23, 24]. Aliquots of 0.5 mM of both salt solutions were added successively into 3 mL
-5
of dendrimer solution (10 µM, 310 mmol), to investigate the interaction between the inner
amino group and metal ion. The titrations were performed in triplicate.
The synthesis of a colloidal solution of dendrimer encapsulated silver and gold nanoparticles
-4
were adapted from the literature [25, 26]. Two solutions of 5 µM dendrimer in 50 mL (2.510
mmol), acidified with hydrochloric acid (0.2 M, pH~2), were prepared as template for each metal
-2
nanoparticle. Exactly, 231 µL of silver nitrate (1.08 M, 2.510 mmol) or 306 µL of chloroauric
-2
acid (0.08 M, 2.510 mmol) were added to each dendrimer solution. The metal nanoparticle
formation from the individual metal ions was performed by a chemical reducing process. An
aqueous solution of 1 M sodium borohydride (250 µL, 0.25 mmol) was added to obtain silver
nanoparticles. Aqueous sodium hydroxide (0.3 M) was used as solvent in the preparation of 1 M
sodium borohydride (250 µL, 0.25 mmol), used as reducing agent for the synthesis of gold
nanoparticles. All the catalytic studies were performed using the same stock solution.
Catalytic oxidation of morin
-6
A set volume of catalyst, 300 µL of AgDENs (0.49 µM, 1.4710 mmol) or 400 µL of AuDENs
-6
(
0.62 µM, 1.8610 mmol), was used to evaluate the catalytic oxidation of morin. A UV-vis
spectrophotometer was used to monitor the process by recording the absorbance at λ 410 nm, the
maximum morin absorbance, for three hours. Exactly, 180 µL of hydrogen peroxide (24 mM,
-
3
4
0
.3210 mmol) was used as oxidant. A volume of 300 µL of carbonate buffer solution (50 mM,
.15 mmol) was used to assure the stability of hydrogen peroxide by maintaining the pH at 10
7

[
27]. A freshly prepared solution of 1 mM morin was diluted to 0.05-0.15 mM for all
evaluations. A required volume of deionized water was added unto each cuvette to the total of 3
mL. At least 15 minutes were allocated for temperature equilibration of the reactants into the
UV-Vis compartments and morin solution was added last. For the uncatalyzed reaction, the
nanoparticle solution was replaced with a dendrimer solution of the same concentration. The
stability of the colloidal catalysts was investigated by three recycle runs of 5 mL of both silver
-6
-6
and gold nanoparticles, (0.49 µM, 2.4510 mmol) and (0.62 µM, 3.1010 mmol) respectively.
The catalyst solutions were dialyzed against 50 mL of aqueous solution of all reactants (morin
-3
(
0.05 mM, 2.510 mmol), hydrogen peroxide (24 mM, 1.2 mmol), and carbonate buffer (50
mM, 2.5 mmol)) at room temperature.
Kinetic analysis
Previous work reported the use of dendrimers as template in synthesis of metal nanoparticles,
and the catalytic activity related to the metal nanoparticles was demonstrated, as well as the fact
that the dendrimer does not present a diffusion barrier to the diffusion of reactant to the metal
nanoparticle surface [28]. To evaluate catalytic activity of silver and gold nanoparticles, the
oxidation of morin has been used as a model reaction. The catalytic oxidation of morin was
monitored using time resolved UV-vis spectroscopy by recording the relative decrease of
adsorption at λ 410 nm which is directly proportional to the concentration. The kinetic analysis
of the catalytic oxidation reaction is mathematically described by equation 1.
Where, kobs is the observed rate constant of the reaction, and n, m, and l are order of the reaction.
By using a large concentration of the hydrogen peroxide compared to the morin, we assume that
the concentration of hydrogen peroxide remains constant. It is also assumed that the catalyst
th
concentration remains constant. Therefore, the reaction appeared to be pseudo-n order (n=1)
[
29-31] with the absorbance versus time, as measured at λ 410 nm, being perfectly described by
equation 2.
8

The basic buffer system has been used to assure the adsorption of morin on the catalyst surface
and to maintain stable hydrogen peroxide while the reaction is proceeding. A study of acid and
alkaline buffer systems on morin oxidation catalyzed by manganese oxide nanoparticles resulted
to the same observed rate constants [32]. Therefore, the carbonate buffer system was applied to
maintain the solution pH at 10. The equilibrium between hydrogen carbonate and hydrogen
peroxide forms the peroxocarbonate ion which is more active than hydrogen peroxide [33, 34].
The formation of the peroxocarbonate ion results in a small increase of observed rate constant.
The adsorption of the reactant on the catalyst surface is necessary for the reaction. The active
surface area may be calculated by considering the average diameter of the nanoparticles based on
the assumption that they are spherical. The catalyst concentration is directly proportional to the
observed rate constant. That may be explained by the fact that the kobs is directly related to both
the concentration and total surface area of the nanoparticles. Thus equation 1 could be expressed
as equation 3.
Where k, and S are surface area normalized rate constant per unit volume of solution, and total
surface area of the system, respectively.
A main prerequisite of the above analysis is that the rate-determining step is the reaction on the
catalyst surface. This means that the diffusion of the reactants to and from the nanoparticle
surface should be much faster than the chemical reaction on the surface. Diffusion limitation of
the process can be ruled out by calculating the second Damköhler number (DaII), which assesses
the competition between the chemical reaction and mass transport (equation 4) [28, 35, 36].
Where kobs is the observed rate constant, [Morin] is the morin concentration, n is the reaction
order (here, pseudo-first order), β is the mass transport coefficient, and ɑ is the total area of the
interface. The mass transfer coefficient is obtained by the diffusion coefficient divided by
characteristic length scale, δ, over which mass transfer occurs [37]. For estimation of β, it is
expedient to use the diameter of the generation five amino terminated PAMAM dendrimer as the
9

-10
2
magnitude (δ = 5.4 nm) [38], and the diffusion coefficient reported for morin is 5.4510 m /s
39]. Hence, β is given as 0.101 m/s. The observed rate constant for silver and gold
[
-4
-1
-4 -1
nanoparticles, 1.5110 s and 1.5310 s , respectively, which were derived from 0.1 mM of
-3
-2
morin (3 mL, 310 mmol) and 24 mM (3 mL, 7.210 mmol) of hydrogen peroxide at 25 °C
-1
-1
(
pH=10). The value calculated for the total area interface is 315.87 m and 277.83 m for silver
-7
-7
and gold nanoparticles, respectively. The estimated values for DaII were 4.7410 and 5.4910
for silver and gold nanoparticles, respectively. Those values are far below unity, indicating that
the oxidation of morin is conducted in the kinetic domain.
The liquid phase chromatographic analysis was conducted in order to identify the product
-3
formation pathway. Exactly, 0.1 mM of morin (110 mmol), 50 mM of carbonate buffer
-2
-2
(
2.510 mmol), 30 mM of hydrogen peroxide (1.510 mmol), and 0.62 µM of gold
-7
nanoparticles (0.3110 mmol) were mixed and the mixture was diluted to 50 mL with
deionized water. After 45 minutes, a mixture was sampled for investigation. The UHPLC trace at
7
.567 tR (MS heat block temperature 300 °C) shows the intermediate product (2-(2,4-
dihydroxyphenyl)-2,5,7-trihydroxychroman-3,4-dione) which decomposes to 2,4-
dihydroxybenzoic acid (β-resorcyclic acid), and 2,4,6-trihydroxybenzoic acid (phloroglucinol
carboxylic acid). These benzoic acids can undergo further oxidation processes [40-42]. Figure 1
shows the MS spectrum and oxidation pathway of morin oxidation. However, the first stage of
the morin oxidation (λ 410 nm) is of interest for our kinetic study.
The kinetic analysis was investigated in terms of the Langmuir-Hinshelwood model, which
consist of the adsorption of the reactant to the catalyst surface in order to form the active species.
The catalyst surface presents the active sites, which could occupy one reactant and there is no
interaction between adsorbate species on adjacent sites [43-45]. Figure 2 displays the essential
assumption of this model. A competitive adsorption takes place between the reactants to occupy
the active sites. Therefore, the rate-determining step of the catalytic oxidation occurs on the
surface of the metal nanoparticles. The process has been investigated in detail by measuring two
different sets of reactions. By maintaining constant concentration of hydrogen peroxide, kobs was
measured while varying the concentration of morin. Inversely, morin concentration was kept
constant while the concentration of hydrogen peroxide was varied.
1
0

The interaction of the reagents with the surface of the metal nanoparticles can be expressed in
terms of a Langmuir adsorption isotherm [46]. The adsorption of the reactants modeled in terms
of the Langmuir-Hinshelwood mechanism can be expressed mathematically by
Where
and
are the surface coverage of the nanoparticles by the reactants, morin and
hydrogen peroxide, respectively. The surface coverage of the metal nanoparticles is given by
Here, KMorin, [Morin]; and KH2O2, [H
2
2
O ] are the adsorption equilibrium constant and
concentration of each reactant, morin and hydrogen peroxide, respectively. Sips exponents n and
m (not order) relate to the heterogeneity of the sorbent [47]. Those express the adsorption
isotherm which is reduced to a linear adsorption isotherm when both exponents equal to unity
[
47].
Thus equation 1 can be rewritten as equation 8.
And thus, kobs can be written as equation 9.
Equation 9 demonstrates that the observed rate constant, kobs, is related to the rate constant on the
metal nanoparticles’ surface; k, a normalized surface area of particles; S, and the concentration
and equilibrium constant of each reactant, morin and hydrogen peroxide.
The dependency of the rate constant k and kobs on the temperature, T, can be expressed through
the Arrhenius equation (10)
1
1

Where, k
o
is the frequency factor of the surface reaction and E describes the activation energy
A
of the surface reaction. The Erying equation was used to determine the thermodynamic
parameters (enthalpy and entropy) on the catalyst surface and for the reaction process (both the
kinetic rate on the surface and the observed rate constant) [48]. The Eyring equation is written in
equation 11.
#
#
Where the rate constant of the reaction, k , related to temperature, ΔH and ΔS are the enthalpy
n
and entropy of activation of process, R is the universal gas constant.
The dependency of the equilibrium constant of both reactants on temperature can be evaluated
from the application of Van’t Hoff’s equation. This application leads to obtaining the enthalpy
and the entropy of both reactants of the process. Van’t Hoff’s equation is given in equation 12,
#
where ΔG is the Gibbs energy of activation of the reactants.
Results and Discussion
The spectrophotometric titration performed highlights that the internal amino group interacts
with silver and gold cations by coordination. Table 1 presents the value of number internal amino
groups, and the equilibrium constant established, figure 3 shows the absorbance for each
interaction and the data plotted using the binding isotherm equation reported previously [24].
The synthesis of metal nanoparticles encapsulated in dendrimers can be subdivided at three steps.
All these steps can be monitored by UV-vis spectrophotometry. Figure 4 shows the spectra for
the silver and gold nanoparticle synthesis. Initially, the spectrum of dendrimer solution was
recorded, no absorbance peaks were detected. The second step is the interaction of inner amino
group with metal ions. The UV-vis spectra illustrate this interaction by showing the absorbance
at λ 290 nm and λ 315 nm for silver and gold ions respectively. A final step, is the formation of
encapsulated metal nanoparticles using a chemical reducing agent. The formation of metal
nanoparticles was detected by UV/vis spectroscopy by recording the maximum absorbance at λ
1
2

4
20 nm and λ 519 nm for silver and gold nanoparticles, respectively. The shift of colorless
solution to light yellow indicates the formation of silver nanoparticles. Similarly, a shift of light
yellow solution to red solution indicate gold nanoparticle formation. The molar ratio of
dendrimer-metal nanoparticles synthesized was 100 equivalents of metal ions. The purification
of the nanoparticles was performed by dialyzing against deionized water overnight [49, 50]. The
concentration of the purified nanoparticles was determined by atomic absorption spectroscopy
analysis as 4.893 ± 0.034 and 4.652 ± 0.081 µM of silver and gold nanoparticles, respectively.
FTIR analysis was conducted to assess the interaction of metal cations with internal amino
groups [51]. The coordination of silver ions with tertiary amine groups was ascertained by shifts
-1
-1
of adsorption bands observed from ν 1368-1457 cm to ν 1363-1443 cm , whereas gold ion
-1
-1
-1
coordination was detected from ν 1355-1373 cm to ν 1372-1395 cm and from ν 1127 cm to ν
-
1
-1
-1
1
1
137 cm . The carbonyl group interaction was observed from ν 1616 cm to ν 1625 cm , and ν
-
1
-1
590 cm to ν 1614 cm for silver ion and gold ion respectively (see supporting information,
figures S1).
Three trials were performed for each titration. The binding isotherm parameters were determined
by fitting the data of each trial individually. The number of internal amino group of dendrimer
(N), and the equilibrium constant (K’) were determined. Equilibrium constant values are in units
-1
of 10 µM dendrimer. The average of three trials ± the standard deviation is obtained for each
data set.
High-resolution transmission electron microscopy analysis was performed to determine the size
distributions of both nanoparticles, EDX analysis (on copper gird) was also performed to confirm
the metal nanoparticle presence in colloidal solution. Figure 5 presents the micrograph obtained
for each catalyst, and the average size was determined using Image J software. The average size
calculated for silver and gold nanoparticles was 3.8 ± 0.5 and 2.8 ± 0.4 nm, respectively. A
minor amount of chlorine, possibly from the HCl used during the first step of the synthesis, was
detected in both catalyst solutions, and copper detected was from the copper grid (see supporting
information, figure S2).
1
3

The specific surface area for the catalytic reaction involving both catalysts was calculated using
the size distribution. The primary amine group, located on the periphery of the dendrimer,
controls the solubility and the diffusion of compounds into the dendrimer. Those attributes allow
the use of dendrimers as template for nanoparticles. A low concentration of catalyst was
considered in each catalytic run which prevented Ostwald ripening and maintained the catalytic
activity of total surface area, and additionally increased the stabilization provided by the
dendritic structure.
The UV-vis spectra, figure 6, illustrate the catalytic oxidation of morin by describing the
absorbance decrease at λ 410 nm and the kinetics of oxidation of morin by recording the relative
adsorption at λ 410 nm as a function of time. The rate for the catalyzed and the uncatalyzed
reaction demonstrated that the oxidation without any catalyst does not proceed in the time frame
of the experiment. To evaluate the molecular oxygen interference during the catalytic process, air
was used as an oxidizing agent. The UV-vis spectra of the catalytic oxidation of morin by air
performed for 2 h 50’ show no significant decrease of absorbance at λ 410 nm (see supporting
information figure S3).
The evaluation of catalyst stability during the course of reaction was determined using three
consecutive runs performed on the gold nanoparticle catalyst and demonstrated that the metal
nanoparticles remain active after the first run. Note that these runs were performed inside a
dialysis membrane, and thus the rates are significantly lower than during kinetic runs. The
catalyst located inside a dialysis bag did not undergo additional purification for the next run. The
-4
-1
-4 -1
observed rate constant calculated was (2.27 ± 0.05)10 s , (1.72 ± 0.04)10 s , and (1.38 ±
-4
-1
0
.04)10 s for the first, second, and third run respectively (Figure 7). In addition, no metal
was observed outside the membrane bag, as determined by AAS. The observed rate constant is
the slope obtained by constructing the plot of relative absorption recorded at λ 410 nm as
function of time using equation 2. The atomic adsorption on the solution outside the bag showed
no appreciable gold concentration. We therefore conclude that leaching and/or Ostwald ripening
does not occur during catalysis and the catalysts remained stable.
1
4

The linear relationship between kobs and catalyst concentration indicates that the catalytic process
occurs on the surface of the nanoparticles and depends on the total surface area of the
nanoparticles. This observation assesses the reliability of the rate constant data and proves that
the metal nanoparticles play the role of a true catalyst in the catalytic oxidation of morin. The
dependency of observed rate constant on the catalyst concentration was evaluated at 25 °C with
-4
an aqueous solution of 0.1 mM of morin (3 mL, 310 mmol), 24 mM of hydrogen peroxide (3
-2
mL, 7.210 mmol), and 50 mM of carbonate buffer (3 mL, 0.15 mmol). Figure 8 shows the
linear relationship between the observed rate constant and the catalyst concentration. A diffusion
limitation zone was detected in case of silver nanoparticles, whereas no diffusion limitation zone
was observed for gold nanoparticles. These experiments also support the conclusion drawn from
the calculated values for the second Damköhler number, namely that the following kinetic
investigation takes place in the kinetic domain, for the chosen catalyst and substrate
concentrations, and that there is no interference from diffusion.
Two sets of experiments were performed at three different temperatures for the kinetic
investigation. First, the influence of morin concentration was evaluated by varying the
concentration from 0.05 mM to 0.125 mM at two constant concentrations of hydrogen peroxide
(24 mM and 48 mM). In the second set of experiments, the effect of hydrogen peroxide
concentration was analyzed, varying the hydrogen peroxide concentration (from 4-60 mM) at
constant concentration of morin (0.05 mM and 0.1 mM). The observed rate constant as function
of the concentration of either morin (a, b) or hydrogen peroxide (c, d) is shown in figures 9 and
1
0 for each catalyst involved. The data points were fitted using the Langmuir-Hinshelwood
model (eq. 9). The determined parameters are summarized in Table 2. We found that an increase
of morin concentration leads to a decrease in the observed rate constant, whereas an increase of
hydrogen peroxide results in an increase of the observed rate constant, which levels off at higher
concentrations. Those observations can be explained by assuming that both reactants compete for
the same site on the catalyst, and only the absorbed species participate in the reaction. If most of
the active sites are occupied by one of the reactants, the observed rate constant will decrease.
The accuracy of catalytic oxidation of morin modeled by the Langmuir-Hinshelwood mechanism
was assessed in more detail by plotting kobs·[Morin] versus the product of θMorin·θH2O2 according
to equation 9. The slope of the straight lines provides the product of rate constant and the
1
5

normalized surface area of the metal nanoparticles involved. Given the error propagation through
the calculations of both adsorption isotherms leading to errors in and , respectively,
The kinetic analysis at different temperatures leads to an investigation of activation energy on
catalyst surface and of the reaction. The adsorption constant for morin onto the catalyst surface is
related on each catalyst involved. The dependency of the adsorption constant of morin onto the
silver nanoparticle surface is inversely proportional to the temperature. The adsorption constant
of morin onto the gold nanoparticle surface shows a direct proportional relationship. The
adsorption constant of hydrogen peroxide for both catalysts is directly proportional to the
temperature. The rate constant and thermodynamic constant of morin onto manganese oxide
nanoparticles surface reported by Ballauff group show the same thermal dependency as the silver
nanoparticle. Table 3 summarizes the kinetic parameters, and the thermodynamic parameters
calculated for the reactants.
The uncatalyzed oxidation of morin by hydrogen peroxide in carbonate buffer solution describes
the activity due to formation of the peroxocarbonate ion, the activation energy calculated using
equation 10 is 73.41 kJ/mol. The activation energy obtained for the catalyzed process, as
determined from the observed rate constants is much higher than the activation energy obtained
with the rate constant on the catalysts surfaces (table 3). That can be explained by the fact that
the observed rate constant is related to the rate constant on the catalyst surface, and the
adsorption constant of all reactants. Figure 12 shows the Arrhenius plots, the observed rate
constant versus temperature (A), and the rate constant on catalyst surface versus temperature (B).
The thermodynamic parameters were calculated through the relationship between rate constant
and thermodynamic parameters established by utilization of the Erying equation. The enthalpy in
the uncatalyzed process is higher than the catalyzed process.
The enthalpy and entropy of the reactions were determined through equation 12. The adsorption
enthalpies of morin and hydrogen peroxide on silver nanoparticles describes an exothermic and
endothermic process for each of the reactants. The adsorption enthalpies on gold nanoparticles
for both reactants are endothermic. Figure 13 shows the graph of adsorption constant versus
temperature of each reactant constructed using the Van’t Hoff’s equation. The hydrogen
1
6

peroxide adsorption on the surface of the metal catalyst is a complicated process that involves
several steps, the thermodynamic parameters determined therefore need further analysis.
Conclusions
This study consists of the evaluation of catalytic activity of silver and gold nanoparticles
encapsulated into dendrimer cavities. The catalytic oxidation of morin by hydrogen peroxide in
aqueous solution was performed as a model reaction and the analysis of the kinetic data suggests
that the rate-determining step occurs on the catalyst surface. The use of dendrimers as a template
for the synthesis of monodisperse metal nanoparticles did not exhibit a diffusion barrier for the
reactant to adsorb onto the catalyst surface. The Langmuir-Hinshelwood model was evaluated to
understand the adsorption of reactants onto the catalyst surface. The surface reaction and
adsorption process of reactants are described by the kinetic constant, and thermodynamic
adsorption constants for both reactants. The adsorption process of morin onto the metal
nanoparticle surface is related to the type of nanoparticles involved. The encapsulated silver
-1
nanoparticles (-27.54 kJ·mol ) show a similar thermal adsorption process (exothermic) as
-1
manganese oxide nanoparticles (-19.8 kJ·mol ), whereas the adsorption of morin onto
encapsulated gold nanoparticle surface is an endothermic process. The lower activation energy of
the surface rate k, by both encapsulated metal nanoparticles, compared to the manganese oxide
-1
nanoparticles (45.8 kJ·mol ) [32], indicated the passivity of the dendrimer and high activity of
the encapsulated nanoparticles.
Acknowledgements
This work was supported by the National Research Foundation of South Africa {Grant specific
unique reference number (UID) 85386}. We acknowledge the University of Johannesburg for
funding as well as Mr. D. Harris and Dr. R. Meyer from Shimadzu South Africa for the use of
their equipment.
1
7

References
[
1]
B. Klajniert, M. Bryszewska, Dendrimers: Properties and applications, Acta Biochim.
Pol., 48 (2001) 199-208.
[
[
2]
3]
P. Holister, C.R. Vas, T. Harper, Dendrimers, in, Cientifica, 2003, pp. 15.
A. Tavakoli, M. Sohrabi, A. Kargari, A review of methods for synthesis of
nanostructured metals with emphasis on iron compounds, Chem. Pap., 61 (2007) 151-
1
70.
[
[
4]
5]
D. Astruc, Transition metal nanoparticles in catalysis, in: Wiley-VCH (Ed.), California,
USA, 2008, pp. 1-45.
D. Astruc, F. Lu, J.R. Aranzaes, Nanoparticles as recyclable catalysts: The frontier
between homogeneous and heterogeneous catalysis, Angew. Chem. Int. Ed., 44 (2005)
7
852-7872.
[
6]
S. Eustis, M.A. El-Sayed, Why gold nanoparticles are more precious than pretty gold:
Noble metal surface plasmon resonance and its enhancement of the radiative and
nonradiative properties of nanocrystals of different shapes, Chem. Soc. Rev., 35 (2006)
2
09-217.
E. Roduner, Size matters: why nanomaterials are different, Chem. Soc. Rev., 35 (2006)
83-592.
[
[
[
[
[
7]
8]
9]
5
S. Horikoshi, N. Serpone, General features of microwave chemistry, in: Microwaves in
Nanoparticle Synthesis, Wiley-VCH Verlag, New York, USA, 2013, pp. 25-37.
P. Pandit, S. Basu, Removal of ionic dyes from water by solvent extraction using reverse
micelles, Environ. Sci. Technol., 38 (2004) 2435-2442.
10] M. Bernal, R. Romero, G. Roa, C. Barrera-Diaz, Ozonation of indigo carmine catalyzed
with Fe-pillared clay, Int. J. Photoenerg., 2013 (2013) 7.
11] T.H. Han, M.M. Khan, S. Kalathil, J. Lee, M.H. Cho, Simultaneous enhancement of
methylene blue degradation and power generation in a microbial fuel cell by gold
nanoparticles, Ind. Eng. Chem. Res., 52 (2013) 8174-8181.
[
12] A.L.N. Mota, L.F. Albuquerque, L.T.C. Belatrane, O. Chiavone-Fiho, A. Machulek,
C.A.O. Nascimento, Advance oxidation processes and their application in the petroleum
industry: A review, Brazalian J. Pet. Gas., 2 (2008) 122-142.
1
8

[
[
13] M.A. Quiroz, E.R. Banadala, C.A. Martinez-Huitle, Advance oxidation processes (AOPs)
for removal of pesticides from aqueous media, InTech, Brazil, 2011.
14] A.Y. Waddad, S. Abbad, F. Yu, W.L.L. Munyendo, J. Wang, H. Lv, J. Zhou,
Formulation, characterization and pharmacokinetics of Morin hydrate niosomes prepared
from various non-ionic surfactants, Int. J. Pharma., 456 (2013) 446-458.
[
15] H. Lang, S. Maldonado, K.J. Stevenson, B.D. Chandler, Synthesis and characterization of
dendrimer templated supported bimetallic Pt−Au nanoparticles, J. Am. Chem. Soc., 126
(2004) 12949-12956.
[
[
16] R.W. Scott, O.M. Wilson, S.K. Oh, E.A. Kenik, R.M. Crooks, Bimetallic palladium-gold
dendrimer-encapsulated catalysts, J. Am. Chem. Soc., 126 (2004) 15583-15591.
17] M. Nemanashi, R. Meijboom, 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 (2013) 260-267.
[
[
18] I. Langmuir, A chemically active modification of hydrogen., J. Am. Chem. Soc., 34
(1912) 1310-1325.
19] I. Langmuir, The adsorption of gases on plane surfaces of glass, mica and platinum, J.
Am. Chem. Soc., 40 (1918) 1361-1403.
[
[
[
20] W. Rasband, ImageJ, 1.47v, National Institute of Health, (2013).
21] Kinetic Studio, 2.0.8.14953, TGK Scientific Limited, (2010).
22] Origin Pro 8.5.0. SR1, OriginLab Corporation, One Roundhouse Plaza Northampton,
(2010).
[
[
[
23] M.T. Carter, M. Rodriguez, A.J. Bard, Voltammetric studies of the interaction of metal
chelates with DNA. 2. Tris-chelated complexes of cobalt (III) and iron (II) with 1,10-
phenanthroline and 2,2'-bipyridine, J. Am. Chem. Soc., 111 (1989) 8901-8911.
24] K.A. Marvin, J.A. Johnson, S.E. Rodenbusch, L. Gong, D.A. Vanden Bout, K.J.
Stevenson, Spectrophotometric titration of bimetallic metal cation binding in
polyamido(amine) dendrimer templates, Anal. Chem., 84 (2012) 5154-5158.
25] Y.-G. Kim, S.-K. Oh, R.M. Crooks, Preparation and characterization of 1−2 nm
dendrimer-encapsulated gold nanoparticles having very narrow size distributions, Chem.
Mater., 16 (2004) 167-172.
1
9

[
26] D.F. Yancey, S.T. Chill, L. Zhang, A.I. Frenkel, G. Henkelman, R.M. Crooks, A
theoretical and experimental examination of systematic ligand-induced disorder in Au
dendrimer-encapsulated nanoparticles, Chem. Sci., 4 (2013) 2912-2921.
[
[
27] G. Czapski, S.V. Lymar, H.A. Schwarz, Acidity of the Carbonate Radical, J. Phys. Chem.
A, 103 (1999) 3447-3450.
28] N.C. Antonels, R. Meijboom, Preparation of well-defined dendrimer encapsulated
ruthenium nanoparticles and their evaluation in the reduction of 4-nitrophenol according
to the Langmuir–Hinshelwood approach, Langmuir, 29 (2013) 13433-13442.
29] S.-A. Ong, E. Toorisaka, M. Hirata, T. Hano, Biodegradation of redox dye methylene
blue by up-flow anaerobic sludge blanket reactor, J. Hazard. Mater., 124 (2005) 88-94.
30] A.A. Osunlaja, S.O. Idris, J.F. Iyun, Kinetics and mechanism of the methylene blue-
permanganate ion reaction in acidic medium, Appl. Sci. Res., 4 (2012) 772-780.
31] M. Salehi, H. Hashemipour, M. Mirzaee, Experimental study of influencing factors and
[
[
[
kinetics in catalytic removal of methylene blue with TiO
2
nanoparticles, Am. J. Environ.
Eng., 2 (2012) 1-7.
[
[
32] F. Polzer, S. Wunder, Y. Lu, M. Ballauff, Oxidation of an organic dye catalyzed by
MnOx nanoparticles, J. Catal., 289 (2012) 80-87.
II
33] E. Ember, H.A. Gazzaz, S. Rothbart, R. Puchta, R. van Eldik, Mn —A fascinating
oxidation catalyst: Mechanistic insight into the catalyzed oxidative degradation of
organic dyes by H
2
2
O , Appl. Catal. B: Environ., 95 (2010) 179-191.
[
34] S. Rothbart, E. Ember, R. van Eldik, Comparative study of the catalytic activity of
II
III/IV(µ-O)
[
Mn (bpy)
2
Cl
2
] and [Mn
2
2
(bpy)
4
](ClO
4
)
3
in the H
2
2
O induced oxidation of
organic dyes in carbonate buffered aqueous solution, Dalton Trans., 39 (2010) 3264-
272.
3
[
[
[
35] S. Wunder, F. Polzer, Y. Lu, Y. Mei, M. Ballauff, Kinetic analysis of catalytic reduction
of 4-nitrophenol by metallic nanoparticles immobilized in spherical polyelectrolyte
brushes, J. Phys. Chem. C, 114 (2010) 8814-8820.
36] J.A. Johnson, J.J. Makis, K.A. Marvin, S.E. Rodenbusch, K.J. Stevenson, Size-dependent
hydrogenation of p-nitrophenol with Pd nanoparticles synthesized with
poly(amido)amine dendrimer templates, J. Phys. Chem. C, 117 (2013) 22644-22651.
37] M.A. Vannie, Kinetics of catalytic reactions, Springer, New York, 2005.
2
0

[
[
[
38] Y. Niu, R.M. Crooks, Dendrimer-encapsulated metal nanoparticles and their applications
to catalysis, Catal. Rev., 6 (2003) 1049-1059.
39] A. Masek, E. Chrzescijanska, M. Zaborski, Electrooxidation of morin hydrate at a Pt
electrode studied by cyclic voltammetry, Food Chem., 148 (2014) 18-23.
40] M.P. Colombini, A. Andreotti, C. Baraldi, I. Degano, J.J. Łucejko, Colour fading in
textiles: A model study on the decomposition of natural dyes, Microchem. J., 85 (2007)
1
74-182.
[
41] A. Zhou, O.A. Sadik, Comparative analysis of quercetin oxidation by electrochemical,
enzymatic, autoxidation, and free radical generation techniques: A mechanistic study, J.
Agric. Food Chem., 56 (2008) 12081-12091.
[
[
42] A. Osman, D. Makris, Oxidation of morin (2', 3, 4', 5, 7-Pentahydroxylavone) with a
peroxidase homogenate from onion, Int. Food Res. J., 18 (2011).
43] S. Kundu, A.K. Gupta, Arsenic adsorption onto iron oxide-coated cement (IOCC):
Regression analysis of equilibrium data with several isotherm models and their
optimization, Chem. Eng. J., 122 (2006) 93-106.
[
[
[
44] K. Vijayaraghavan, T.V.N. Padmesh, K. Palanivelu, M. Velan, Biosorption on nickel (II)
ions onto Saragassum wightii: application of two-parameter and their parameter isotherm
models, J. Hazard. Mater. B, 133 (2006) 304-308.
45] A.B. Pérez-Marín, V.M. Zapata, J.F. Ortuño, M. Aguilar, J. Sáez, M. Lloréns, Removal
of cadmium from aqueous solutions by adsorption onto orange waste, J. Hazard. Mater.,
1
39 (2007) 122-131.
46] C. Rill, Z.I. Kolar, G. Kickelbick, H.T. Wolterbeek, J.A. Peters, Kinetics and
1
60
thermodynamics of adsorption on hydroxyapatite of the [ Tb]terbium complexes of the
bone-targeting ligands DOTP and BPPED, Langmuir, 25 (2009) 2294-2301.
[
[
[
47] D.G. Kinniburgh, General purpose adsorption isotherms, Environ. Sci. Technol., 20
(1986) 895-904.
48] H. Eyring, The activated complex and the absolute rate of chemical reactions, Chem.
Rev., 17 (1935) 65-77.
49] F.M. Fatin, A. Kazazz, A simple method for synthesis, purification and concentration
stabilized goldnanoparticles, J. Eng. Res. Appl., 6 (2013) 21-30.
2
1

[
[
50] B. Harper, F. Sinche, R.H. Wu, M. Gowrishankar, G. Marquart, M. Mackiewiz, S.L.
Harper, The impact of surface ligands and synthesis methods on the toxicity of
glutathione-coated gold nanoparticles, Nano Mater., 4 (2014) 355-371.
51] B. Stuart, Infrared spectroscopy: Fundamentals and applications, in, John Wiley & Sons,
UK, 2004, pp. 1-221.
2
2

Figure 1: (A) The MS spectrum of reaction intermediate in catalytic oxidation of morin, and (B)
the morin oxidation pathway as suggested by Colombini and coworkers [40].
2
3

Figure 2. Langmuir-Hinshelwood mechanism of the catalytic oxidation of morin by hydrogen
peroxide on the surface of the encapsulated metal nanoparticles. Hydrogen peroxide reacts on the
catalyst surface to form the adsorbed active species. Concomitantly, morin adsorbs onto
unoccupied sites of the nanoparticles surface. The adsorption/desorption of both reagents on the
metal nanoparticle surface is assumed to be a fast process.
2
4

Figure 3. Spectrophotometry titration of metal ions into dendrimer. UV/vis spectra of silver ions
A), and gold ions (B) titrated into G5-PAMAM dendrimer. Best fit using a binding isotherm
(
equation of silver ions (C) and gold ions (D) interacting with internal amino functionalities. The
data points represent the average of three independent runs.
2
5

Figure 4. UV/vis spectra of dendrimer encapsulated nanoparticle syntheses. Figure A presents
encapsulated silver nanoparticle and figure B encapsulated gold nanoparticle syntheses. (1) The
dendrimer solution spectrum. (2) Spectra of the metal precursor’s solution. (3) Absorbance of
interaction of internal amino groups versus metal ions in each case. (4) Metal encapsulated
nanoparticles.
2
6

Figure 5. HRTEM micrograph of dendrimer encapsulated metal nanoparticles and the
corresponding histogram for the size distribution. (A), and (B) silver nanoparticles micrograph
and their average diameter (3.8 ± 0.5 nm) obtained by counting ≥140 particles. (C), and (D)
presents the micrograph and average diameter (2.8 ± 0.4 nm) of gold nanoparticles by counting
≥
150 particles.
2
7

Figure 6. The UV-vis spectra of catalytic oxidation of morin (0.1 mM) by hydrogen peroxide (24
mM) with pH stabilized at 10 using carbonate buffer (50 mM) using gold nanoparticle (0.62 µM)
as catalyst at 25 °C. The spectrum shows a decrease in characteristic absorption maximum at λ
4
10 nm of morin with time (A). The relative adsorption recorded at 410 nm as function of the
reaction time (B).
2
8

Figure 7. Stability of metal nanoparticles evaluated using oxidation of morin at room
temperature. The evaluation performed on three recycle runs of catalyst contained in a dialysis
bag.
2
9