The room temperature formation of gold nanoparticles from the reaction of cyclohexanone and auric acid; A transition from dendritic particles to compact shapes and nanoplates

Article abstract of DOI:10.1039/c3ta11546a

A new straightforward method for the synthesis of gold nanoparticles from addition of cyclohexanone to aqueous solutions of auric acid at room temperature is presented. By understanding this process we have discovered a new organic chemistry transformation reaction for converting cyclic ketones to α-chloro ketones and a mechanism for the nanoparticle formation. Contrary to conventional gold nanoparticle syntheses, the reaction self- initiates at room temperature and forms an increasingly red solution over ≈60 minutes. By studying the gold colloid's formation using transmission electron microscopy it was observed that large dendritic (63 ± 21 nm diameter) structures made of clustered particles (6 ± 1 nm) were initially formed. These dendritic particles then compacted into an array of denser shapes that slowly increase in size until the reaction is complete. The most prominent shapes observed were spheres (43 ± 7 nm); other shapes included dodecahedra (39 ± 10 nm) triangular (≈50 nm in height) and hexagonal (≈70 nm wide) nanoplates. The solution was stable to precipitation for over 3 months. During this period the nanoplate structures substantially increased in size (triangular ≈ 250 nm, hexagonal ≈ 320 nm) whereas other structures showed no further growth. X-ray diffraction studies demonstrated that the gold nanoparticles were crystalline. The formation of the 2-chlorocyclohexanone by-product was observed in solution phase ¹H & ¹³C NMR, gas phase chromatography and IR spectroscopy. A mechanism is presented to account for this by-product and the reduction of auric acid to gold. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ta11546a

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Materials Chemistry A
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PAPER
The room temperature formation of gold nanoparticles
from the reaction of cyclohexanone and auric acid; a
transition from dendritic particles to compact shapes
and nanoplates†
Cite this: J. Mater. Chem. A, 2013, 1,
7351
a
b
a
a
*
Madeeha A. Uppal, Andreas Kafizas, Michael B. Ewing and Ivan P. Parkin
A new straightforward method for the synthesis of gold nanoparticles from addition of cyclohexanone to
aqueous solutions of auric acid at room temperature is presented. By understanding this process we have
discovered a new organic chemistry transformation reaction for converting cyclic ketones to a-chloro
ketones and a mechanism for the nanoparticle formation. Contrary to conventional gold nanoparticle
syntheses, the reaction “self-initiates” at room temperature and forms an increasingly red solution over
z60 minutes. By studying the gold colloid's formation using transmission electron microscopy it was
observed that large dendritic (63 ꢀ 21 nm diameter) structures made of clustered particles (6 ꢀ 1 nm)
were initially formed. These dendritic particles then compacted into an array of denser shapes that slowly
increase in size until the reaction is complete. The most prominent shapes observed were spheres (43 ꢀ 7
nm); other shapes included dodecahedra (39 ꢀ 10 nm) triangular (z50 nm in height) and hexagonal
(z70 nm wide) nanoplates. The solution was stable to precipitation for over 3 months. During this
period the nanoplate structures substantially increased in size (triangular z 250 nm, hexagonal z 320
nm) whereas other structures showed no further growth. X-ray diffraction studies demonstrated that the
gold nanoparticles were crystalline. The formation of the 2-chlorocyclohexanone by-product was
observed in solution phase 1H & 13C NMR, gas phase chromatography and IR spectroscopy. A mechanism
is presented to account for this by-product and the reduction of auric acid to gold.
Received 18th April 2013
Accepted 9th May 2013
DOI: 10.1039/c3ta11546a
www.rsc.org/MaterialsA
used to investigate the biological toxicity of gold nanoparticles
in zebrash embryos17 and mammalian cells18 but are yet to be
Introduction
Gold nanoparticle colloids have received a great deal of interest
because of their intense and size/shape tuneable surface plas-
mon resonance (SPR) properties.1–3 Gold nanoparticles are used
in a wide sector of applications as optical enhancers in
composite semiconductor lms,4–7 bactericides,8,9 biological
markers,10 DNA and single molecule detectors11,12 and in cata-
lysts.13 The most common method for preparing gold colloids is
based on the thermal reduction of aqueous auric acid with tri-
citrate, devised by Turkevich et al.;14,15 where solutions are
typically heated to 100 ꢁC for the reaction to initiate. Gold
nanoparticles have been synthesized by heating auric acid
solutions with a variety of fruit extracts.16 These fruit extracts
contained several ketone functional groups and it may be that
these functional groups mediate the formation of multi-shaped
nanoparticles. Such multi-shaped gold nanoparticles have been
applied in a functional material. Silver nanoparticles of
different shapes and sizes have also been found to induce multi-
colour photochromism when photo-chemically loaded onto
TiO2 thin-lms.19
The use of ketones in precious metal nanoparticle synthesis
has been reported – although all routes involve signicant
heating and no mechanism for their formation has been
proposed. Truncated gold nanocubes could be produced using
an electrochemical approach and an acetone injection stage.20,21
Dendritic nanoparticle structures comprised of Pd have been
synthesized by reaction of the Pd acetate salt mixed with
ethylene glycol and KOH in a methyl isobutyl ketone solvent
initiated by microwave heating.22 This formed stable z50 nm
wide dendritic structures. The formation of large 2D triangular
and hexagonal nanoplates has also been previously
observed.23–25 By mixing gold seeds with a mixture of CTAB, PEG
and the ketone polymer PVP a slow room temperature reaction
over 8 hours produced platelet-type particles that were typically
z5 nm thick and z200 to 500 nm wide. It was also shown that
microwave heating (85 ꢁC) of aqueous solutions of auric acid
with long-chained PVPs yielded similar structures.26 Mixing
aDepartment of Chemistry, University College London, 20 Gordon Street, London,
WC1H 0AJ, UK. E-mail: i.p.parkin@ucl.ac.uk; Fax: +44 (0)20 7679 7463
bDepartment of Chemistry, Imperial College London, South Kensington Campus,
London SW7 2AZ, UK
† Electronic supplementary information (ESI) available: Zoomed-in 13C NMR
spectra identify by-products in the reaction. See DOI: 10.1039/c3ta11546a
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aqueous auric acid solutions with a lemongrass plant extract temperature on the reaction was investigated by mixing auric
(Cymbopogon exuosus) at room temperature produced nano- acid (2.6 mg, 0.28 mM) in distilled water (28.5 ml) with cyclo-
ꢁ
plate mixtures with lengths on the micrometer scale, where the hexanone (1.5 ml, 1.4 g, 0.48 M) at z20, 30, 40, 50 and 60 C.
directional growth was related largely to the ketone groups Aer rst heating the aqueous auric acid solution to the desired
present in the extract.27
temperature, the cyclohexanone reducing agent was rapidly
To our knowledge, the use of cyclohexanone in initiating a injected, causing a thorough mixing of the solutions. The rate of
self-propagating gold nanoparticle formation reaction has not consumption of auric acid in the reaction was determined by
previously been reported. Nevertheless, the use cyclohexanone– titrating solution aliquots during the nanoparticle formation
water–isopropanol mixtures in synthesizing a range of copper reaction.35 This was achieved by scaling the reaction up by a
based nanoparticles28 and cyclohexanone as solvent and capping factor of 5 and taking 2 ml aliquots out every 5 minutes for
agent for forming gold nanoparticles through laser ablation of a analysis. The nanoparticle formation reaction was quenched as
submerged gold target.29 It was also shown that aryl ketones can soon as the aliquots were extracted by addition of excess KI
photocatalytically reduce solutions containing Au3+ to Au0.30 The (0.02 g). This caused all auric acid present within the aliquot to
growth mechanisms for traditional gold nanoparticle formation be consumed to AuI, forming an I2 side-product. The level of I2
methods have come to be somewhat understood through in situ formed, and thus the amount of auric acid originally present,
monitoring methods such as UV-visible spectroscopy, dynamic was measured by titration against K2S2O3 (1 mM) using potato
light scattering, electrode potential measurements or through starch indicator (0.02 g) where the disappearance of blue-black
the extraction and transition electron microscopy analysis of colour indicated completion.
aliquots at set times during the reaction.31,32
The size and shape of nanoparticles formed at room
We show that cyclohexanone reacts with auric acid and temperature by rapid mixing of the solution were investigated
causes a “self-initiated” room temperature nanoparticle forma- using transmission electron microscopy (TEM) using a Jeol
tion reaction to commence. During this reaction the particles 4000EX microscope. Solution aliquots were taken out from the
undergo several shape changes. Initially, large dendritic struc- nanoparticle reaction aer 5, 20 and 65 minutes of reaction and
tures composed of smaller aggregates form that later compact placed onto lacey carbon-coated copper grids and allowed to air
primarily into spherical particles and a fraction form alternative dry before being investigated by TEM.
3D structures such as triangular prisms and dodecahedrons and
Nanoparticles solutions formed at room temperature by
large at 2D hexagonal and triangular nanoplates. We also rapid mixing were freeze dried to remove the aqueous solvent
provide an insight into the underpinning mechanism showing and any excess cyclohexanone to form a nanoparticle powder.
that the cyclohexanone acts as the reducing agent; reducing Au3+ The powder was analyzed by X-ray diffraction (XRD) using a
in auric acid to Au1+ (that later disproportionates to Au0) and micro-focus Bruker GADDS powder X-ray diffractometer with a
˚
being converted to 2-chlorocyclohexanone in this process. This monochromated CuKa (1.5406 A) source and CCD area detector.
new method of multi-shaped nanoparticle synthesis is repro- Aer mixing the powder within a pressed KBr disc, the infrared
ducible and amongst the simplest reported33,34 in that it involves (IR) spectrum of the solid was taken using a PerkinElmer RX-I
no heating stage, occurs at room temperature and just requires instrument.
one stage of reagent mixing. Furthermore, the nanoparticles
Nanoparticle solutions were centrifuged on a Beckman
formed are stable against precipitation for more than 3 months Coulter Avanti Centrifuge J-26 XP at 20 000 rpm (z48 000g)
stored at room temperature.
causing the gold particles to rest at the bottom of the solution.
The liquid layer above was removed and passed through a gas
chromatograph (Clarus 500, PerkinElmer) equipped with ame
ionization detector and 30 m capillary column (Ellite-1, cross-
Experimental section
All reagents were purchased from Sigma-Aldrich UK. All glass- bond 100% dimethyl polysiloxane) and compared with stan-
ware was washed with aqua regia and rinsed with copious dards to elucidate the reaction by-products. The gases released
amounts of de-ionized water before use. All reactions were from the nanoparticle reaction were analyzed by IR spectros-
performed in beakers.
copy and compared with the pure gas phase spectra of cyclo-
The nanoparticle solution was made by mixing auric acid hexanone and 2-chlorocyclohexanone in a sealed gas cell under
(HAuCl4$3H2O – 0.85 mg, 0.28 mM) in distilled water (9.5 ml) vacuum. Three samples were examined by both 1H and 13C
with cyclohexanone ((CH2)5CO – 0.50 ml, 0.47 g, 0.48 M). This nuclear magnetic resonance (NMR) spectroscopy using a Bruker
solution did not require thermal activation (or any alternative AV600 (600 MHz) instrument: (i) a gold nanoparticle solution
energy source) and “self-initiated” the uniform formation of (diluted by a factor of 100) formed from the reaction of auric
gold nanoparticles throughout the solution when conducted acid and cyclohexanone in D2O at room temperature, (ii)
under room light or in a dark cupboard. The reaction was cyclohexanone in D2O and (iii) 2-chlorocyclohexanone in D2O.
accompanied by a darkening of the solution. A further experi-
ment was conducted where the cyclohexanone was injected
gently on top of the aqueous auric acid solution to form a
Results and discussion
separate organic layer on top of the aqueous. In this case, the The self-initiated formation of gold nanoparticles was induced
nanoparticle formation reaction occurred only at the emulsion by reacting a thousand fold excess of cyclohexanone with
interface as judged by the change in colour. The effect of aqueous solutions of auric acid at room temperature. Several
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photographs were taken at set time intervals during the self-
initiated nanoparticle formation reaction, as displayed in Fig. 1.
Two reactions were studied; (i) the rapid mixing of cyclohexa-
none with auric acid and (ii) the layering of cyclohexanone on
top of the aqueous auric acid layer. In the case of thorough pre-
mixing of the cyclohexanone reducing agent with the auric acid
solution (Fig. 1(a)) no colour change was observed until aer
z10 minutes when the solution turned slightly pink. This
coloration became more intense as the reaction proceeded.
Aer z40 minutes of reaction the solution appeared purple and
was more strongly coloured. Aer z60 minutes of reaction, the
solution did not further deepen in colour, signifying the end of
the gold nanoparticle formation reaction. When the solution
was viewed off-angle, a two-tone effect was revealed where the
solution showed a more orange tone when viewed at high angles
and a more purple tone when looked at directly.
In the case of gentle layering of cyclohexanone at the surface
of the aqueous auric acid solution, an emulsion layer was
formed in which the colour change and hence nanoparticles
formation occurred solely at the cyclohexanone–water interface.
Fig. 1(b) shows that just aer addition of the reducing agent
(0 min), two separate immiscible layers can be observed.
Approximately 20 minutes into the reaction a pink layer began
to form within the aqueous layer at the interface. As the reaction
progressed, the pink layer deepened in colour and penetrated
Fig. 2 UV-visible spectrum from 300 to 900 nm monitoring the growth of gold
nanoparticles in situ during their self-initiated formation reaction from an
aqueous auric acid solution and cyclohexanone. Each spectrum was taken
approximately every 70 seconds and the plasmon band appeared z10 minutes
after the addition of cyclohexanone. The spectra are shown as recorded and no
shift was applied to the baseline – this naturally shifted upwards with the growth
of the SPR band.
deeper into the aqueous layer as the nanoparticles diffused. of Au nanoparticles. The UV-visible spectrum was taken every
Aer 60 minutes of reaction, almost half of the aqueous layer z70 seconds until the SPR band stopped increasing. Approxi-
had turned pink. On comparing the two reactions it is clear that mately 10 minutes aer the cyclohexanone was added a SPR
nanoparticle formation is more rapid in the solution that was band started to appear. This continued to increase at a relative
thoroughly pre-mixed. However, one can also infer the impor- steady rate until aer z65 minutes aer cyclohexanone addi-
tance of the partial miscibility of the cyclohexanone on the tion, when the SPR band suddenly stopped increasing in
mechanism of nanoparticle formation.
magnitude. The overall baseline also increased its absorbance
The nanoparticle formation reaction, where the cyclohexa- during the process – this stopped its upward shi at the same
none initiator was well-mixed, was also followed using UV- time as the SPR band showed no further increase in magnitude.
visible spectroscopy over the 190–1100 nm range, as shown in
The centre (nm) and height (absorbance) of each progres-
Fig. 2. The spectra showed the formation of a band at 540– sively growing SPR band was extracted and plotted against
575 nm corresponding to the surface plasmon resonance (SPR) reaction time (minutes), as shown in Fig. 3. As was seen in UV-
visible absorbance spectra, an SPR band (543 nm, 0.01 absor-
bance units) rst appeared 10 minutes aer the addition of
cyclohexanone. The SPR band then grew in height at a relatively
steady rate of roughly 0.012 absorbance units per minute. The
centre of the SPR also shied to longer wavelengths. These two
properties continued to increase until exactly 64 minutes aer
the addition of cyclohexanone, where they both reached a
plateau. From 64 to 75 minutes reaction time both the SPR
centre and height did not change signicantly. Over the reac-
tion period the SPR height increased from 0 to 0.61 absorbance
units. In addition, the SPR centre shied from 543 to 576 nm.
The solutions were highly stable to precipitation and altered
little aer being stored for 3 months; although the (SPR centre
decreased to 563 nm and the absorbance dropped to 0.59).
It is well known that an increase in SPR centre wavelength
signies an increase in average nanoparticle size.15 Using the
relationship derived by Haiss et al.,36 the increase in SPR centre
from 543 to 576 nm signies an average increase in nano-
Fig. 1 A series of images of the same solution taken at time intervals of 0, 10, 20,
30, 40, 50 and 60 minutes reaction time (from left to right) in the self-initiated gold
particle diameter of z34 nm, assuming the particles are
predominantly spherical.
nanoparticleformationreactionofauric acid solution withcyclohexanone that was
(a) shaken into the solution or (b) layered gently on top of the aqueous layer.
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band formation, indicating the formation of gold nanoparticles,
terminated aer z65 minutes of reaction, whereas our titra-
tions showed that the auric acid was depleted aer 45 minutes.
This showed that there was a 20-minute lag between the
complete reduction of gold ions and their subsequent nucle-
ation to form nanoparticles with detectable surface plasmon
resonance band.
The gold nanoparticle formation reaction was repeated with
solution aliquots being extracted 5, 20 and 65 minutes aer the
addition of cyclohexanone and placed on lacey carbon-coated
copper grids for transmission electron microscopy (TEM)
analysis. Representative TEM images are shown in Fig. 5.
Fig. 5(a) shows that aer 5 minutes of reaction dendritic
structures had formed, which ranged in size from 20 to 93 nm in
diameter. The average diameter and standard deviation of these
dendritic structures was 63 ꢀ 21 nm. On closer inspection it can
be seen that these dendritic structures are composed of clusters
of smaller particles. These particles were predominantly
spherical and ranged in size from 4.0 to 10 nm. The average
diameter and standard deviation of these particles of which the
dendritic structures were composed was 6 ꢀ 1 nm. Fig. 5(b)
shows a less magnied image of the same sample. Some well-
formed gold nanoparticles had formed alongside the dendritic
structures. Mainly spherical shapes (11 ꢀ 7 nm) were observed,
however dodecahedral (21 ꢀ 2 nm) and rod-like (33 ꢀ 17 nm in
length) gold nanoparticle shapes were also present. Intrigu-
ingly, large nanoplates (at structures) were also observed that
were mainly triangular in shape (29 ꢀ 12 nm in height). Fig. 5(c)
shows that aer 20 minutes of reaction these large dendritic
structures had disappeared and only more compact looking and
more regularly shaped gold nanoparticles were present. Spher-
ical nanoparticles were still the most abundant shape, but had
more than doubled in average size from 11 ꢀ 7 nm aer 5
minutes of reaction to 25 ꢀ 7 nm aer 20 minutes of reaction.
Dodecahedral (28 ꢀ 5 nm), rod-like (49 ꢀ 23 nm in length) and
triangular nanoplate (29 ꢀ 5 nm in height) structures were also
still present. However, some extra shapes, not previously
observed aer 5 minutes of reaction were now observed, such as
decahedra (31 ꢀ 4 nm), trigonal bipyramids (35 ꢀ 6 nm) and
triangular prisms (30 ꢀ 5 nm). Fig. 5(d) shows the nal state of
the nanoparticles aer they had been allowed to react to
completion (65 minutes total reaction time). No new shapes
were observed and all of the various kinds of nanostructure
observed aer 20 minutes of reaction were still present, except
for triangular prisms. The diameter/length of each type of shape
had increased on average by z20 nm. Spheres were still the
most predominant shape, increasing in size from 25 ꢀ 6.4 nm
aer 20 minutes of reaction to 43 ꢀ 7 nm aer 65 minutes of
reaction. Colloidal solutions were stable to precipitation upon
prolonged storage (at least 3 months). A TEM image size anal-
ysis of such a solution aer 3 months of storage demonstrated
how little the predominantly spherical nanoparticles changed
in size (50 ꢀ 10 nm). However, the primarily at nanoplate
structures showed an almost 5-fold increased in size (Fig. 5(e)
and (f)). These nanoplates were similar in size and shape to
those formed by Tsuji et al. from the microwave heating (85 ꢁC)
Fig. 3 A plot of the centre (nm) and height (absorbance) of the progressively
growing surface plasmon resonance band extracted from UV-visible spectra
monitoring the growth of gold nanoparticles in situ during their self-initiated
formation reaction from auric acid solution and cyclohexanone.
The rate of consumption of auric acid in the reaction was
determined by titrating solution aliquots during the nano-
particle formation reaction.35 The level of auric acid was found
to decrease linearly over the time period (Fig. 4); decreasing
from 0.26 mM at the start of the reaction (0 minutes) at a rate of
6.2 mM minꢂ1 to there being no presence of auric acid aer 45
minutes of reaction. This was in contrast with nanoparticles
formed by the reaction of auric acid and citric acid (the Tur-
kevich method), where during the reaction a sharp change in
auric acid concentration is observed at a critical point.31 From
our spectroscopic studies it was observed that surface plasmon
Fig. 4 Auric acid (mM) depletion in the self-initiated gold nanoparticle forma-
tion reaction of auric acid (0.28 mM) with cyclohexanone (0.48 M). This was
measured by quenching the reaction in sample aliquots with iodide salt and
measuring the level of I2 side-product by titration with thiosulfate using starch
indicator. The decrease in auric acid concentration was strongly linear (r2 ¼ 0.99). of aqueous solutions of auric acid with long-chained PVPs.26
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Fig. 5 Transmission electron microscopy images of solution aliquots taken from the self-initiated gold nanoparticle formation reaction of auric acid solution and
cyclohexanone after (a and b) 5 minutes, (c) 20 minutes and (d) 65 minutes of reaction. After 3 months storage larger (e) hexagonal and (f) triangular nanoplate
structures were also present.
The distribution of nanoparticle sizes and shapes are
A gold nanoparticle solution that had been allowed to react
summarized in the histograms shown in Fig. 6. The histogram to completion (>65 minutes) was freeze dried to form a powder
plots for the nanostructures observed aer 20 and 65 minutes of for XRD analysis. The pattern, shown in Fig. 7, was phase
reaction and 3 months storage were skewed distributions. The equivalent to that of bulk gold showing primitive face-centred
37
˚
ꢀ
histogram aer 5 minutes reaction was very much skewed to the cubic (FCC) symmetry (Fm3m, a ¼ 4.0781 A). Through tting
le due to the large number of minute spherical particles that the pattern to a Le Bail rened model, a slightly larger unit cell
˚
were present within the initial large dendritic structures. The was observed, where a ¼ 4.1031 A. This equated to an average
overall average diameter of all nanostructures increased during unit cell expansion of 1.5% compared with bulk gold. In
the reaction period from z15 nm aer 5 minutes, to z25 nm measuring the relative heights of each diffraction peak, slight
aer 20 minutes, to z40 nm aer 65 minutes of reaction. Aer preferred growth was observed in the (200) plane. Through
3 months in storage the average nanoparticle size did not applying the Scherrer equation to each diffraction peak, an
increase greatly (z40 nm) and signied that the nanoparticle average crystallite size of 14 nm with a standard deviation of
formation reaction was primarily over aer 65 minutes of 1.4 nm was observed. This indicates that the particles observed
reaction. Although a multitude of 3D shapes and at nanoplate by TEM with average size ca. 40 nm are in fact on average
structures were observed, spherically shaped nanoparticles composed of multiple crystal grains.
were present in majority (70% presence on average) throughout
The freeze dried powder was also investigated using IR
the reaction. For the duration of the reaction it was observed spectroscopy within a pressed KBr disc. Cyclohexanone solution
that the SPR band centre shied from 543 to 576 nm (Fig. 3), was investigated as a standard for comparison. The presence of
denoting an average increase in size of z34 nm.36 From TEM no organic compound other than cyclohexanone was observed
studies, a similar size change was observed for the spherical in both spectra. This indicated that cyclohexanone not only acts
particles that were present in majority, increasing in average as a reducing agent in the self-initiated formation of gold
size from 11 ꢀ 7 nm aer 5 minutes of reaction to 43 ꢀ 7 nm nanoparticles from auric acid, but also seems to play a surfac-
aer 65 minutes of reaction; a 32 nm average size increase, tant-type role for stabilizing these particles. In addition, the
hence correlating well with the SPR shi.
carbonyl C]O dimer bond stretch did not within error shi in
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Fig. 6 Histogram plots of nanoparticle size (nm) and count for the various nanostructures observed after (a) 5 minutes, (b) 20 minutes and (c) 65 minutes of reaction
from the self-initiated gold nanoparticle formation reaction of auric acid solution and cyclohexanone and (d) after 3 months storage.
the gold nanoparticle powder (1714 cmꢂ1) compared with that place at room temperature, unlike typical Au nanoparticle
of pure cyclohexanone (1715 cmꢂ1). In Sarkosh et al.'s IR study syntheses, without the need for any initiation i.e. thermal acti-
of gold nanoparticles formed by ablating a gold target vation, sonication, photo-activation, etc. Three components were
submerged in cyclohexanone solution an z24 cmꢂ1 decrease in present in the solution; water, auric acid and cyclohexanone.
this C]O stretch was observed and was attributed to the
The reaction was repeated in D2O so that any by-products
capping of cyclohexanone molecules to the gold nanoparticles could be examined using 1H and 13C NMR. In the 13C NMR
through their C]O bond at oxygen. In comparison, this indi- spectrum, several peaks not relating to the presence of the
cates that either cyclohexanone did not preferentially bind excess reducing agent, cyclohexanone [13C NMR (600 MHz, D2O)
through this functional group to gold in the colloid reported in d (ppm) 25.1, 27.2, 41.8, 128.2, 209.2 {ESI 1†}], were observed.
this paper or that a number of cyclohexanone layers were These peaks [13C NMR (600 MHz, D2O) d 22.7, 23.9, 33.8, 36.8,
present, stacking randomly around each nanoparticle.
39.2, 41.2, 65.2, 67.0, 95.2, 210.2, 216.3 {ESI 2†}] well-matched a
pure spectrum of 2-chlorocyclohexanone [13C NMR (600 MHz,
D2O) d (ppm) 22.6, 24.7, 27.7, 30.9, 33.8, 37.4, 39.1, 41.2, 65.9,
67.0, 94.9, 210.1, 216.0 {ESI 3†}].
Mechanistic studies
Gold nanoparticles were formed by simply mixing cyclohexa-
The two missing peaks at d (ppm) 27.7 and 30.9 of 2-
none with an aqueous solution of auric acid. The reaction took chlorocyclohexanone were masked by the large presence of
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Fig. 8 IR spectra of the gas phase above the solution taken during the reaction
of auric acid (0.28 mM) with cyclohexanone (0.48 M). Gas phase measurements of
2-chlorocyclohexanone and cyclohexanone are shown for reference.
Fig. 7 X-ray diffraction pattern of a colloidal powder suspension. The powder
was acquired through freeze-drying a gold nanoparticle solution, formed from
the self-initiated reaction of auric acid solution and cyclohexanone that was
allowed to react to completion (>65 minutes).
The tautomerism of secondary ketones such as cyclohexa-
none is well known, forming an enol functional group with
greater capacity for oxidation.38 The presence of this tautomer
was indicated in this study as aqueous solutions of cyclohexa-
none discoloured bromine water (indicating an alkene group
was present) and turned an acidied solution of K2Cr2O7 from
orange to green upon slight heating (primary/secondary
hydroxyl group present). In fact, a slight difference in the UV-
visible absorption spectrum of aqueous cyclohexanone
compared with pure was observed, when the peak at 913 nm in
pure cyclohexanone disappeared in the aqueous mixture.
With the by-product and keto–enol chemistry in mind, a
mechanism by which Au3+ could be reduced to Au0 is presented
(Scheme 1). The mechanism was based in part on the similar
chemistry observed between the pentavalent bismuth salt of
Ph3Bi(V)Cl2 with cyclohexanone that leads to chlorine addition
at the a position on cyclohexanone in a 2eꢂ process forming 2-
phenylcyclohexanone.39 Furthermore, Kochi demonstrated the
reaction of CuCl2 with acetone in forming a-chloro ketones,
again a 2eꢂ transfer process.40 To our knowledge, the compa-
rable reaction of auric acid (HAuCl4) and cyclohexanone has not
been reported.
cyclohexanone in this region. Additional peaks were observed
but not identied. A similar case was observed in 1H NMR,
where both cyclohexanone [1H NMR (600 MHz, D2O) d (ppm)
1.18 (2H, dddd), 1.36 (4H, dddd), 1.98 (4H, dd)] and 2-chlor-
ocyclohexanone [1H NMR (600 MHz, D2O) d (ppm) 1.38 (1H, dtt),
1.51 (1H, dtt), 1.63 (1H, ddddd), 1.73 (1H, ddddd), 1.82 (1H,
dddd), 1.97 (1H, dddd), 2.57 (1H, ddd), 2.63 (1H, ddd), 4.05 (1H,
dd)] environments were observed. The majority by-product was
thus identied as 2-chlorocyclohexanone, which was shown to
be identical to literature spectra and a dilute solution of the
reagent. This was supported by gas chromatography measure-
ments. Gas chromatography of the nanoparticle solution
(containing the by-product) and clean measurements of pure 2-
chlorocyclohexanone was performed. Two matching peaks at
the same retention time of 17.9 minutes were observed. Further
conrmation of 2-chlorocyclohexanone being formed was
found by IR spectroscopy. Measurements of the gas phase taken
during the nanoparticle formation reaction in a gas-tight cell
showed the progressive release of 2-chlorocyclohexanone
(Fig. 7). A maximum concentration of 2-chlorocyclohexanone
was reached aer approximately 30 minutes, in line with titra-
tions that indicated the complete reduction of auric acid in a
similar time-frame (Fig. 8). Although cyclohexanone possesses a
higher vapor pressure (4.5 mmHg, 25 ꢁC) than 2-chlor-
ocyclohexanone (0.28 mmHg, 25 ꢁC), the far poorer solubility of
2-chlorocyclohexanone in water (0.02 g lꢂ1) compared with
cyclohexanone (87 g lꢂ1) was the over-riding factor that led to its
existence in the gas phase. Assuming the 1 : 1 formation of 2-
chlorocyclohexanone from auric acid, 0.32 mg would have
formed during the reaction; 0.16 mg over the solubility limit in
10 ml of water and encouraging vaporization. On the other
hand, when mixed thoroughly, cyclohexanone (0.47 g) was fully
soluble, being 0.4 g under the solubility limit.
Auric acid can release HCl freely in solution, leaving AuCl3
(Scheme 1a). The acid can tautomerise cyclohexanone to the
enol catalytically (Scheme 1b). Trigonal planar AuCl3 is then
nucleophilically attacked by the lone pair on the enol leading to
the SN2 release of Clꢂ (Scheme 1c). The locality of the Cl–Au–O
bond to the carbon a position induces a cyclic nucleophilic
attack starting from the alkene bond. This leads to the reduc-
tion of gold from Au3+ (AuCl3) to Au1+ (AuCl2ꢂ) and the forma-
tion of 2-chlorocyclohexanone.
As the reduction of a metal chloride by a ketoꢂne is a 2eꢂ
transfer process,39,40 the direct reduction of AuCl2 to Au0 by
cyclohexanoneꢂwas not possible. Therefore, the disproportion-
ation of AuCl2 to Au0 was the only viable route to Au0:
ꢂ
ꢂ
3AuCl2 / 2Au0 + AuCl4 + 2Clꢂ
(1)
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Journal of Materials Chemistry A
Paper
Scheme 1 The mechanism for self-initiated gold nanoparticle growth from auric acid and cyclohexanone.
´
´
According to Gammons et al., the disproportionation of Turkevich synthesis, Rodrıguez-Gonzalez et al. found that
ꢂ
AuCl2 to Au0 takes place at room temperature.41 However, thꢂe chloride ions primarily surround the large gold structures that
rate of disproportionation increases rapidly with AuCl2
initially form. However, when the AuCl4ꢂ is near consumed, the
concentration and temperature. Moreover, the reaction is redox potential reduces allowing citrate ions to replace the
catalyzed in the presence of Au foil. The fact that Au0 catalyzes surrounding chloride. This induces peptization as the gold
the disproportionation of AuCl2 to Au0 shows that particle surface is made more electrically neutral. We propose a similar
ꢂ
growth on Au0 seed sites will be favoured; thus leading to the phenomenon for the reaction of auric acid and cyclohexanone.
nucleation and growth of nanoparticles through the continued We also suggest an extended Ostwald ripening43 process occurs
absorption and disproportionation of AuCl2ꢂ. The steps can be over the 3 month storage period, causing the gold nanoplates to
summarized as follows:
dramatically increase in size. We believe that the phenomenon
is observed in the 2D shapes formed as opposed to the 3D
shapes formed as growth in just a single plane facilitates
signicant increases in particle size.
ꢂ3C5H9COCl
3C5H10CO þ 3HAuCl4
3Hþ þ 3HCl
ƒƒƒƒƒƒƒƒƒ!
ꢂ
þ3AuCl2 /2Au0 þ HAuCl4 þ 5HCl
(2a)
Conclusions
and the overall reaction as:
3C5H10CO þ 2HAuCl4
A new and simple method for growing multi-shaped colloidal
gold is presented. By adding cyclohexanone to aqueous solu-
tions of auric acid, a self-initiating and self-propagating room
temperature reaction occurs; resulting in the formation of gold
nanoparticles. Such room temperature self-initiating nano-
particle synthesis reactions are different to traditional methods,
such as the Turkevich method, where solutions require boiling
temperatures to react. The formation proceeded in majority via
large dendritic structures that collapsed to form more dense
particles with a variety of shapes though most prominently
spheres. The particles were highly crystalline and were stabi-
lized by the cyclohexanone reducing agent, dually acting as a
surfactant. Through identifying the 2-chlorocyclohexanone by-
product formed in this nanoparticle synthesis a mechanism by
which Au3+ was reduced to Au0 was proposed. Furthermore, a
new method of converting cyclic ketones to a-chloro ketones
was discovered. This new synthetic route to colloidal gold is
straightforward and fast, forming nanoparticles with a range of
ꢂ3C5H9COCl
2Au0 þ 5HCl
(2b)
ƒƒƒƒƒƒƒƒƒƒ!
Our TEM studies showed that large dendritic structures
initially formed 5 minutes into the nanoparticle formation
reaction. These structures then collapsed into smaller and
denser particles with a variety of shapes. A similar growth
mechanism has been observed in the Turkevich reaction (auric
acid and tri-sodium citrate) between 70 and 100 C.31 Initially,
ꢁ
large particles formed that were z100 nm in diameter that then
collapsed over the course of the reaction into predominantly
spherical shaped nanoparticles that were z20 nm in size. The
similar nanoparticle growth mechanism observed in this study
demonstrates the similar behaviour of the surfactants. Never-
theless, the observed phenomenon of particle shrinkage is akin
to an inverse-Ostwald ripening process.42 This phenomenon
´
´
was explained by Rodrıguez-Gonzalez et al. in terms of the
specic adsorption of steric/electrosteric layers.32 Assessing the
7358 | J. Mater. Chem. A, 2013, 1, 7351–7359
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Journal of Materials Chemistry A
shapes. The resulting nanoparticle solution is stable to precip- 18 B. D. Chithrani, A. A. Ghazani and W. C. W. Chan, Nano Lett.,
itation for more than 3 months at room temperature. Surpris- 2006, 6, 662.
ingly the at nanoparticle sheets seem to grow over the 3 month 19 Y. Ohko, T. Tatsuma, T. Fujii, K. Naoi, C. Niwa, Y. Kubota
period whereas the other nanoparticle shapes remain invariant
aer ca. 1 h of reaction.
and A. Fujishima, Nat. Mater., 2003, 2, 29.
20 C. Huang, P. Chiu, Y. Wang, W. R. Chen and T. H. Meen, J.
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Acknowledgements
22 X. Tong, Y. Zhao, T. Huang, H. Liu and K. Y. Liew, Appl. Surf.
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23 B. K. Jena and C. R. Raj, Chem. Mater., 2008, 20, 3546.
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IPP wishes to thank the EPSRC for funding. Dr Daniel Ortega is
thanked for his help with TEM studies and Dr Abil Aliev is
thanked for his help with NMR studies. Prof. William Mother-
well and Dr Matthew Powner are thanked for help with the
mechanism.
26 M. Tsuji, M. Hashimoto, Y. Nishizawa, M. Kubokawa and
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