Dipyrromethane as a new organic reagent for the synthesis of gold nanoparticles: Preparation and application

Article abstract of DOI:10.1007/s13738-013-0253-1

Condensation reaction of several ketones with pyrrole in the presence of ferric hydrogen sulfate as a green homogenous acidic catalyst furnished the corresponding pure dipyrromethanes in good yields. Gold nanoparticles were produced through reduction of H

Full text of DOI:10.1007/s13738-013-0253-1

J IRAN CHEM SOC
DOI 10.1007/s13738-013-0253-1
ORIGINAL PAPER
Dipyrromethane as a new organic reagent for the synthesis of gold
nanoparticles: preparation and application
H. Eshghi
M. Bakavoli
•
M. Rahimizadeh
•
N. Attaran
•
Received: 4 December 2012 / Accepted: 17 March 2013
Ó Iranian Chemical Society 2013
Abstract Condensation reaction of several ketones with
pyrrole in the presence of ferric hydrogen sulfate as a green
homogenous acidic catalyst furnished the corresponding
pure dipyrromethanes in good yields. Gold nanoparticles
plasmon resonance, potential applications in medical
diagnostics, imaging, and therapeutic treatments, applica-
tion as scaffolds for molecular recognition of elements and
molecules [4], in optoelectronics and data storage [5], and
in nanotechnology with molecular switches [6]. Gold
nanoparticles can be synthesized in a range of shapes and
sizes such as nanorods [7], nanotriangles [8], nanocubes
[9], nanocages [10], nanonetworks [11–13], nanoplates
[14], and nanowires [11]. Typically, gold nanoparticles are
generally obtained by chemical reduction of tetrachloro-
auric acid using sodium borohydride [15, 16], sodium cit-
rate, sodium ascorbate [17], sodium oxalate [18], hydrazine
[19], hydroquinone [20], cysteine [21] or ionic liquids [22].
It has been noted that the variation of particle growth
medium plays a profound role in controlling the size and
colloidal stability. However, an alternative way for the
were produced through reduction of HAuCl with substi-
4
tuted dipyrromethanes as new reducing agents at room
temperature with the exclusion of any capping agent or
surfactant. Gold nanoparticles were characterized by
transmission electron microscopy, scanning electron
microscopy, XRD and UV–visible absorption spectro-
scopic measurements. It is proposed that in situ formed
oxidative products of dipyrromethane, such as polydipyr-
romethane could serve effectively as a capping agent to
preferably adsorb the {111} facets of gold crystals during
the reduction process, which leads to the formation of gold
nanoparticles.
-
formation of gold nanoparticles from AuCl4 is to choose
Keywords Gold ꢀ Nanoparticles ꢀ Dipyrromethanes ꢀ
Ferric hydrogen sulfate ꢀ Green conditions
an efficient reducing as well as a capping agent so that the
relative rates between nucleation and growth are well
controlled.
On the other hand, dipyrromethanes are crucial inter-
mediates for the synthesis of substituted porphyrins,
expanded porphyrins, porphyrin analogs (dipyrrins, calix-
pyrroles, chlorins, corroles) and polypyrrolic polymers that
are useful in chemistry, physics, biology, medicine and
engineering. Dipyrromethanes are also widely used as
ligands in organometallic synthesis and catalysis [23–25].
Therefore, the growing interest on these molecules led
synthetic chemists to find new routs in order to improve
reaction conditions. Most of the methods have reported the
condensation of an aldehyde or ketone and pyrrole using
various combinations of acids and solvents, such as p-tol-
uenesulfonic acid (in MeOH or toluene) [26, 27], acetic
acid (in DMF or THF) [28, 29], trifluoroacetic acid (in
TFA) [30], TiCl (in CH Cl ) [31], SnCl (in CH Cl ) [32],
Introduction
During the past decades, metal nanostructures have
attracted extensive interests because of the different per-
formances from their bulk counterparts and also their
unique optical, electronic, photonics, magnetic, biologic,
catalytic properties [1–3]. With regard to noble metal
nanoparticles, gold nanoparticles (Au-NPs) have been a
major field of study, because of their strong surface
H. Eshghi (&) ꢀ M. Rahimizadeh ꢀ N. Attaran ꢀ M. Bakavoli
Department of Chemistry, School of Sciences, Ferdowsi
University of Mashhad, 91775-1436 Mashhad, Iran
e-mail: heshghi@um.ac.ir; heshghi@ferdowsi.um.ac.ir
4
2
2
4
2
2
123

J IRAN CHEM SOC
BF ꢀetherate (in CH Cl ) [33], aqueous HCl (in THF) [34]
colloidal solution onto a carbon-coated copper grid and
allowing it to dry in air naturally. Based on the TEM
images we determined the size distributions of the final
product by counting at least 300 particles. Scanning elec-
tron microscopy (SEM) images were recorded by using a
LEO-1530VP field-emission scanning electron microscope
with an accelerating voltage of 5.0 kV and the samples
were deposited on silicon substrates, which were also used
for elemental analysis conducted by energy-dispersive
X-ray spectrograph (EDX) attached to the SEM. Powder
X-ray diffraction pattern (XRD, D/Max-RA, Cu Kk,
40 kV, 30 mA) was taken from the sample prepared by
dropping 200 ll of the products dispersed in water on a
silicon substrate and allowing the solvent to evaporate
spontaneously at ambient temperature.
3
2
2
and resins [35], etc. However, among the above catalysts,
some are expensive, some are inconvenient to use and
some gave a low yield of reaction. Also these methods
usually involve a final workup that requires, at least, the
distillation of the excess pyrrole. On the other hand,
chemists have great interest in developing an efficient way
for the preparation of organic compounds by using envi-
ronmentally feasible reagents and solvents. Recently, ferric
hydrogen sulfate is reported to be an environmentally
friendly and heterogeneous acidic catalyst. Ferric hydrogen
sulfate is an inexpensive, non-toxic, stable, and efficient
catalyst for various transformations [36] and can be easily
prepared from commercially available starting materials
[
37].
In this study, we have used a catalytic amount of ferric
hydrogen sulfate to condense pyrrole with ketones in the
absence of any organic solvent, which provides an efficient
method to prepare different substituted dipyrromethanes in
aqueous and green conditions and then we have developed
a facile route for the synthesis of gold nanoparticles by
simply adding a typical dipyrromethane as a new and
effective reducing agent to an aqueous solution of HAuCl4
at room temperature with the exclusion of any other cap-
ping agent or surfactant. To the best of our knowledge, the
use of dipyrromethane as an organic reducing reagent in
the synthesis of gold nanoparticles under mild conditions
has not been reported in the literature.
Synthesis of substituted dipyrromethanes
In a typical experiment, pyrrole (0.25 ml, 2 mmol) was
added to a suspension of 3-pentanone (0.18 ml, 1 mmol) in
water (5 ml), followed by slow addition of Fe(HSO4)3
(36 mg, 0.1 mmol). After refluxing for 50 min, the reaction
mixture was extracted with chloroform (2 9 25 ml) and
washed with water (2 9 25 ml). The organic phase was
dried over anhydrous Na SO and the solvent was evapo-
2
4
rated under high vacuum. The brown residue was relatively
pure, but it can be further purified by recrystallization with
ethanol:water (20:1).
0
Meso-dimethyl-2,2 -dipyrromethane 1: yield 71 %, mp:
1
Experimental
56–58 °C [38]. HNMR (solvent: CDCl ; internal standard:
3
TMS): d = 1.67 (6H, s, CH -C), 6.08 (2H, m, pyrrole-H),
3
Materials and physical methods
6.20 (2H, m, pyrrole-H), 6.64 (2H, m, pyrrole-H), 7.88
-
2H, bs, NH). FT-IR (cm ; group): (3,380, N–H).
1
(
0
Gold chloride trihydrate (HAuCl ꢀ3H O) was purchased
Meso-diethyl-2,2 -dipyrromethane 2: yield 68 %, mp:
1
4
2
from Aldrich and used without purification. Other chemical
compounds used in this research were purchased from
Merck. All aqueous solutions were prepared from Milli-
pore water (resistivity, 18 MX; Millipore Systems). Melt-
ing points were recorded on an Electrothermal type 9100
melting point apparatus. The 1H NMR (100 MHz) spectra
were recorded on a Bruker AC100 spectrometer. Elemental
analysis was obtained on a Thermo Finnigan Flash EA
microanalyzer. The FT-IR spectra were obtained on an
Avatar 370 FT-IR Thermo Nicolet spectrometer. UV–vis-
ible (UV–vis) absorption spectroscopic measurements were
recorded on a UV–vis spectrometer, Shimadzu UV1700,
using quartz cells of 1 cm path length. Transmission
electron microscopic (TEM) images of the nanoparticles
were taken with a LEO 912AB instrument operated at an
accelerating voltage of 120 kV with line resolution of
108–110 °C [39]. HNMR (solvent: CDCl ; internal stan-
3
dard: TMS): d = 0.68 (6H, t, J = 7.2 Hz, CH -CH -C),
3
2
1.92 (4H, q, J = 7.2 Hz, CH -CH -C), 6.11 (4H, m, pyr-
2
3
role-H), 6.61 (2H, m, pyrrole-H), 7.65 (2H, bs, NH). FT-IR
-
(cm ; group): (3,371, N–H).
1
0
Meso-ethyl-meso-methyl-2,2 -dipyrromethane 3: yield
1
57 %, mp: 103–105 °C. HNMR (solvent: CDCl ; internal
3
standard: TMS): d = 0.81 (3H, t, J = 7.1 Hz, CH -CH -
3
2
C), 1.52 (3H, s, CH -C), 1.98 (2H, q, J = 6.9 Hz, CH -
3
3
CH -C), 5.98 (2H, m, pyrrole-H), 6.14 (2H, m, pyrrole-H),
2
-
1
6.67 (2H, m, pyrrole-H), 7.75 (2H, bs, NH). FT-IR (cm
group): (3,388, N–H).
;
0
Meso-diisopropyl-2,2 -dipyrromethane 4: yield 55 %,
1
mp: 112–114 °C. HNMR (solvent: CDCl ; internal stan-
3
dard: TMS): d = 1.57 (12H, d, J = 7.4 Hz, (CH ) -CH-
3
2
C), 1.11–1.30 (2H, m, (CH ) -CH-C), 5.92 (2H, m, pyr-
3
2
0
.3 nm at room temperature. The samples for TEM mea-
role-H), 6.20 (2H, m, pyrrole-H), 6.62 (2H, m, pyrrole-H),
-
7.82 (2H, bs, NH). FT-IR (cm ; group): (3,400, N–H).
1
surements were prepared by placing a droplet of the
1
23

J IRAN CHEM SOC
Cyclopentyldipyrrole 5: yield 69 %, mp: 117–119 °C.
nanoparticles surface by washing several times using ace-
tonitrile and dichloromethane.
1
HNMR (solvent: CDCl ; internal standard: TMS):
3
d = 1.74 (4H, m, –CH -C), 2.20 (4H, m, –CH -C), 5.98
2
2
(
2H, m, pyrrole-H), 6.06 (2H, m, pyrrole-H), 6.60 (2H, m,
-
pyrrole-H), 7.78 (1H, bs, NH). FT-IR (cm ; group):
1
Results and discussion
(
[
3,378, N–H).
Cyclohexyldipyrrole 6: yield 67 %, mp: 106–108 °C
Preparation of substituted dipyrromethanes
1
40]. HNMR (solvent: CDCl ; internal standard: TMS):
3
d = 1.50 (6H, m, –CH -C), 2.20 (4H, m, –CH -C), 6.10
The reaction of pyrrole with different ketones in water in
the presence of a catalytic amount of Fe(HSO ) under
2
2
(
2H, m, pyrrole-H), 6.22 (2H, m, pyrrole-H), 6.63 (2H, m,
4
3
-
pyrrole-H), 7.78 (1H, bs, NH). FT-IR (cm ; group):
1
reflux condition gave mainly the desired dipyrromethane
with 50–70 % yields (Scheme 1).
(
3,400, N–H).
-Hydroxycyclohexyldipyrrole 7: yield 63 %, mp:
2
To obtain the best reaction conditions for dipyrrome-
thanes synthesis, the effects of different parameters like
solvent and catalyst molar ratio were investigated on the
reaction of pyrrole and acetophenone as a model reaction.
At first, the reaction was carried out in several solvents
(H O, EtOH, CHCl , CH Cl and C H ). As shown in
1
1
06–108 °C. HNMR (solvent: CDCl ; internal standard:
3
TMS): d = 1.60 (9H, m, –CH -C), 2.30 (2H, m, –CH -C),
2
2
2
.71 (1H, m, –CH -C), 4.09 (1H, bs, cyclohexyl-OH), 6.07
2
(
2H, m, pyrrole-H), 6.18 (2H, m, pyrrole-H), 6.74 (2H, m,
-
pyrrole-H), 7.80 (1H, bs, NH). FT-IR (cm ; group):
1
2
3
2
2
6 6
(
3,408, N–H).
Table 1, H O is the best solvent for dipyrromethane
2
0
Meso-methyl-meso-phenyl-2,2 -dipyrromethane 8: yield
1
0 %, mp: 103–105 °C [39]. HNMR (solvent: CDCl ;
synthesis.
7
The yield of dipyrromethane as a function of catalyst
concentration is also shown in Table 1. The maximum
yield of 8 is observed when 10 mol % molar ratio of cat-
alyst is used. The yield decreases remarkably at higher
concentration of catalyst (Table 1).
3
internal standard: TMS): d = 2.07 (3H, s, CH -C), 6.01
3
(
2H, m, pyrrole-H), 6.21 (2H, m, pyrrole-H), 6.75 (2H, m,
pyrrole-H), 7.27 (5H, m, PhH), 7.80 (2H, bs, NH). FT-IR
-
1
(
cm ; group): (3,421, N–H).
Meso-methyl-meso-(4-methylphenyl)-2,2 -dipyrrome-
0
In our synthetic endeavor, we found out that pyrrole and
ketones undergo condensation reaction in an aqueous
1
thane 9: yield 61 %, mp: 119–120 °C. H NMR (solvent:
CDCl ; internal standard: TMS): d = 2.04 (3H, s, CH -C),
medium by the drop-wise addition of Fe(HSO ) solution
4 3
3
3
2
.48 (3H, s, H C-Ar), 5.89 (2H, m, pyrrole-H), 6.26 (2H,
3
at reflux condition. Different substituted dipyrromethanes
are prepared in 50–70 % yields by this green method in
aqueous conditions (Table 2).
m, pyrrole-H), 6.70 (2H, m, pyrrole-H), 7.09 (4H, m, PhH),
7
-
1
.86 (1H, bs, NH). FT-IR (cm ; group): (3,410, N–H).
0
Meso-methyl-meso-(4-methoxybenzyl)-2,2 -dipyrrom-
1
It is proposed that the reaction occurs at the interface
between the pyrrole and the aqueous ketone solution and
the release of the product from the aqueous layer takes the
reaction to completion and also prevents the reactions to
proceed further [39].
ethane 10: yield 51 %, mp: 117–118 °C. HNMR (solvent:
CDCl ; internal standard: TMS): d = 2.06 (3H, s, CH -C),
3
3
3
.64 (2H, s, Ar-CH -C), 3.70 (3H, s, H CO-Ar), 5.62 (2H,
2 3
m, pyrrole-H), 5.89 (2H, m, pyrrole-H), 6.61 (2H, m,
m
pyrrole-H), 6.86 (2H, d, J = 6.4, PhH ), 7.14 (2H, d,
o
-1
J = 6.5, PhH ), 7.91(2H, bs, NH). FT-IR (cm ; group):
3,410, N–H).
Preparation and characterization of gold nanoparticles
(
There are many reports on the synthesis of gold nanopar-
ticles in the presence of pyrrole and aniline [41–43]. When
these monomer compounds are added to the solution, each
nuclei serves as a catalytic reaction centre to enhance the
Synthesis of gold nanoparticles
-
The synthesis of gold nanoparticles was achieved by the
reaction between AuCl4 ions and the monomers. These
chemical reduction of HAuCl in water medium with di-
4
results suggested that autopolymerization of the monomers
occurred on gold nanoparticles [41]. In other words,
pyrromethane without using any stabilizer. In a typical
-
AuCl4 ions are reduced to form gold atoms and the
experiments, HAuCl ꢀ3H O (0.03 g, 0.08 mmol) was dis-
4
2
solved in deionized H O (3 ml) and meso-methyl-meso-
2
0
phenyl-2,2 -dipyrromethane 8 (0.047 g, 0.2 mmol) was
slowly added. The suspension was magnetically stirred for
R1
R2
H
Fe(HSO ) / H O
4 3
2
O
N
1
h. Finally, the gold nanoparticles were separated from
R1
R2
50 min, reflux
NH HN
water by precipitation under centrifuge at 10,000 rpm. The
desorbed organic monomer were removed from Au
Scheme 1 Preparation of different substituted dipyrromethanes
123

J IRAN CHEM SOC
0
Table 1 Synthesis of meso-methyl-meso-phenyl-2,2 -dipyrrome-
thane (8) using various conditions
Table 2 Reactions of ketones with pyrrole to give substituted dipyr-
romethanes under green aqueous conditions
a
yeild(%)a
Entry
Solvent
EtOH
Catalyst:ketone
Yield (%)
Ketone
O
Product (number)
H C CH3
3
1
2
3
4
5
6
7
8
0.1:1
0.1:1
0.1:1
0.1:1
0.1:1
10:1
1:1
60 %
40 %
20 %
5 %
(
1)
71%
CH
Cl
2 2
H
3
C
CH
3
NH HN
CHCl
3
O
H CH C CH CH
3
2
2
3
6
C H
6
H CH C
CH CH
3
2
2
3
(2)
(3)
68%
57%
NH HN
2
H O
2
H O
2
H O
2
H O
70 %
b
–
O
H CH C CH3
3
2
H CH C
CH3
3
2
5 %
NH HN
O
O
0.5:1
40 %
(H C) HC CH(CH3)2
3
2
H C
CH3
CH3
3
HC
CH
The reactions were performed with 1 mmol of acetophenone and
mmol pyrrole in solvent (5 ml) in the presence of (10 mol %)
Fe(HSO ) under reflux condition
(4)
55%
69%
H C
NH HN
NH HN
3
2
4
a
Isolated yields
(
(
5)
6)
b
TLC did not show dipyrromethane formation
O
organic compound oxidized simultaneously to produce
polymers in situ.
67%
63%
NH HN
O
On the other hand, dipyrromethanes are important pre-
cursors for the synthesis of meso-substituted porphyrins,
corroles, and related compounds such as calixpyrroles
macrocycles. In addition, for decades, a number of methods
have been developed for the synthesis and purification of
dipyrromethanes along with their oligomeric or polymeric
by-products. So, these results demonstrate that polymeri-
zation of dipyrromethanes can occur in suitable conditions.
Also, it is mentioned that there are some studies on elec-
trochemical polymerization of dipyrromethanes and lim-
ited reports about their electrochromic properties [44].
So, in this present study, we introduce a facile route to
synthesize gold nanoparticles using dipyrromethanes as
reducing agents without the presence of any stabilizing or
capping agents in large quantities, for the first time. For this
purpose, we examined the application of three different
OH
OH
(
7)
NH
HN
O
CH
3
CH3
(
8)
70%
NH HN
H C
3
O
CH
3
CH3
(9)
61%
51%
H C
3
NH HN
H CO
3
CH3
O
CH
3
(10)
H
CO
3
NH HN
The reactions were carried out in 1 mmol of acetophenone, 2 mmol
0
4 3
pyrrole and Fe(HSO ) (10 mol %) in water at reflux conditions
substituted dipyrromethanes, meso-dimethyl-2,2 -dipyr-
0
a
Islolated yields
romethane 1, meso-ethyl-meso-methyl-2,2 -dipyrromethane
0
3
and meso-methyl-meso-phenyl-2,2 -dipyrromethane 8, in
the synthesis of gold nanoparticles. Gold nanoparticles
exhibit strong plasmon resonance absorption that its kmax
depends on the particle size and the shape [45–47]. The
characteristic UV–vis absorption spectra of the Au nano-
particles prepared by this method are shown in Fig. 1. The
plasmon band of gold nanoparticles synthesized by dipyr-
romethanes 1, 3 and 8 is observed at 525, 542 and 535 nm,
respectively. These results indicated that the reduction of
intense absorption peak at 517 nm is generally attributed to
the surface plasmon excitation of small spherical gold
particles [49]. When the GNP size increases, the maximum
extinction of surface plasmon band (SPB) is red shifted
slightly in the visible region which may be attributed to the
surface plasmon oscillation of free electrons. However, it
can be affected by four factors: density of electrons, elec-
tron mass, mass and size of the charged nanoparticles.
On the other hand, when the shape of the nanoparticles
changes from nanosphere to nanorods, the surface plasmon
absorption spectrum also changes [50]. The surface plas-
mon absorption of gold nanorods have two bands: a strong
long wavelength band in the near infrared region due to the
-
AuCl4 occurs by these three substituted dipyrromethanes
to produce the gold nanoparticles.
The peak intensity and position of the surface plasmon
absorption band are dependent on the size and shape of the
metal nanoparticles, the dielectric constant of the metals as
well as the medium surrounding the particles [48]. An
1
23

J IRAN CHEM SOC
Fig. 1 UV–vis spectra of the as-prepared gold nanoparticles dis-
persed in water in the presence of dipyrromethanes 1, 3 and 8
Fig. 3 Typical SEM images of the resulting precipitates of gold
nanoparticles
longitudinal oscillation of electrons and a weak short
wavelength band in the visible region around 520 nm due
to the transverse electronic oscillation.
been reported about the preparation of gold nanoparticles
in the presence of dipyrromethane system.
The EDX spectrum determined from these purified
nanoparticles shows strong peaks due to gold element.
Weak peaks ascribed to carbon are also observed (Fig. 4),
which reveals these gold nanoparticles are covered with
oxidized dipyrromethane 8.
For further characterization of these prepared gold
nanoparticles, we chose the product obtained from reagent
8
.
Figure 2 shows the TEM image of gold nanoparticles
prepared by this method in the presence of meso-methyl-
0
In order to confirm the formation of a crystal phase and
calculate the mean size of the nanoparticles, X-ray dif-
fraction (XRD) analysis was performed (Fig. 5). Figure 5
presented an XRD pattern of the gold nanoparticles after
centrifugation. The labeled diffraction peaks appearing at
meso-phenyl-2,2 -dipyrromethane 8. The micrograph
clearly indicates the formation of nanoparticles with an
average diameter of 30 nm. The particles do not get self-
assembled because the polymeric compound obtained from
the oxidation of compound 8 acts as a capping agent.
Figure 3 shows typical SEM images of the resulting
nanoparticles. The image indicates that the precipitates
consist of a mass of plates coexisted with spherical parti-
cles. To the best of our knowledge, no previous work has
2
h = 38.35°, 44.58°, 64.74°, 77.75° and 81.87° corre-
sponding to the (111), (200), (220), (311) and (222) planes
of the standard cubic phase of Au, respectively, are similar
to pure gold nanoparticles. The broad peak at about 28° can
be associated with the amorphous form of the polymer
which surrounded the gold nanoparticles [41, 51].
An overwhelming intensity diffraction peak located at
38.35 is attributed to the {111} facets of face-centered
cubic (fcc) gold crystal (JCPDS no. 4-0783) while dif-
fraction peaks of other four facets are weak. This confirms
that the nanoparticles were mainly dominated by {111}
facets, which were oriented parallel to the supporting sur-
face preferentially.
Fig. 2 Typical TEM image of individual gold nanoparticles in water
medium. The scale bar is 50 nm
Fig. 4 The EDX pattern of the purified gold nanoparticles
123

J IRAN CHEM SOC
Fig. 5 The XRD pattern of the
gold nanoparticles after
centrifugation
The crystallite size of gold nanoparticles (s) was
deduced from XRD line broadening using the Scherrer
equation:
s ¼ Kk=b cos h
where K is a constant (0.94), k is the X-ray wavelength (Cu
˚
Kk, k = 1.54184 A), b is the line broadening at half the
maximum intensity (FWHM) in radians, and h is the half
diffraction angle of the centroid of the peak in degrees.
Contributions to broadening due to the non-uniform stress
were neglected and the XRD instrumental line width was
subtracted. The average size of the gold nanoparticles
ranged from 30.1, 27.8 and 28.8 nm (mean 28.9 nm) for
the (111), (200) and (220) planes, respectively, and hence
was in good agreement with the UV–vis spectroscopy and
TEM measurements. In other words, all obtained results
are confirming and complement each other. The XRD data
suggests that the nanoparticles are polycrystalline in nature
with a predominant (111) Bragg reflection (Scheme 2).
According to thermodynamic points, gold atoms should
nucleate and grow in a solution phase to form sphere like
cuboctahedrons bounded by a mix of {111} and {100}
facets. If the nucleation and growth of gold nanoparticles
were subjected to kinetic control, gold atoms would pref-
erentially add to facets of the seeds with higher surface
energies. In the meantime, the oxidative products of di-
pyrromethane, including its polymers or oligomers,
inclined to adsorbing on the lowest energy {111} facets of
the particles and compress the growing rate of these facets.
Then, as a result, other facets adsorbed with fewer mole-
cules of oxidized dipyrromethane grew more quickly than
Scheme 2 Schematic formation of gold nanoparticles with a pre-
dominant (111) Bragg reflection using dipyrromethane 8
rather than plates. Excessive quantity of dipyrromethane
-
led to a high reduction rate of AuCl
ions, leading to
4
thermodynamic control over both nucleation and growth.
This leads to a heavy coverage of oxidized dipyrromethane
on the surfaces of the particles and results in products
dominated by spherical particles due to the isotropic
growth of all facets [52].
Conclusions
In conclusion, we demonstrate a facile route to synthesize
single-crystal, polygonal gold nanoparticles bounded
mainly by {111} facets in aqueous solution using dipyr-
romethane as the reducing agent at room temperature
without the presence of any stabilizing or capping agents in
large quantities. During the synthetic process, the dipyr-
romethane acted as reducing agent and their oxidative
products worked as capping agents to control the growing
rate of different facets of the initial gold nuclei leading to
the formation of the gold nanoparticles bound primarily by
{111} facets. In this study, dipyrromethanes have been
{
{
111} facets and polygonal gold plates bound primarily by
111} facets were preferentially generated. However,
insufficient quantity of dipyrromethane also decreased the
production of oxidized dipyrromethane, so that cannot
effectively work as a capping agent for special facets. This
finally resulted in dominating big quasi-spherical particles
1
23

J IRAN CHEM SOC
prepared from pyrrole and ketones in water and in the
presence of ferric hydrogen sulfate, which is an inexpen-
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