Identification of a source of size polydispersity and its solution in Brust-Schiffrin metal nanoparticle synthesis

Article abstract of DOI:10.1039/c1cc11642h

The co-presence of thiol vs. disulfide in the well-known Brust-Schiffrin two-phase synthesis has been identified as a source of size polydispersity in nanoparticles synthesized and a procedure has been proposed to address this long outstanding issue.

Full text of DOI:10.1039/c1cc11642h

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COMMUNICATION
Identification of a source of size polydispersity and its solution in
Brust–Schiffrin metal nanoparticle synthesisw
Ying Li, Oksana Zaluzhna and YuYe J. Tong*
Received 22nd March 2011, Accepted 12th April 2011
DOI: 10.1039/c1cc11642h
The co-presence of thiol vs. disulfide in the well-known
Brust–Schiffrin two-phase synthesis has been identified as a source
of size polydispersity in nanoparticles synthesized and a procedure
has been proposed to address this long outstanding issue.
Our own recent work16 has not only confirmed Goulet and
Lennox’s findings but also shown that the BSM is in principle
an inverse-micelles based synthesis. We have further clarified
and differentiated the reaction conditions that lead to either
the [TOA]+[M(I)X4]ꢀ complex or this complex mixed with
polymeric (AuSR)n species as reaction intermediates in the
BSM.17 These progresses put us in a position to address the
aforementioned polydispersity issue.
Since the seminal 1994 paper by Brust and co-authors on
synthesizing alkanethiolate-protected gold (Au) nanoparticles
(NPs),1 this synthetic method, abbreviated here as BSM
(Brust–Schiffrin method) for simplicity, has probably become
the most widely used method for synthesizing o5 nm metal
NPs2 stabilized by thiolates3–9 in general and by other organo-
chalcogenates10–13 in particular. Although thiols are originally
used for NP synthesis in the BSM, the thiol–gold bond is
commonly described as a surface bound thiolate.14 After
Whetten’s group achieved the synthesis of o2 nm Au NPs,3
a thiol to Au molar ratio of 3 : 1 has become a routine reaction
condition of the BSM.9 However, polydispersity in the synthe-
sized Au NPs has been an outstanding issue.9 Consequently,
laborious fractionalization3 has been frequently employed to
achieve homogeneously distributed Au NPs and for the same
purpose a novel size-focusing method was developed recently.9
Despite its clear practical importance, few pertinent studies are
available in the literature to shed light on the (molecular) cause
of the polydispersity simply because detailed mechanistic
information on the BSM has been too sketchy to enable
well-targeted research.
Generally speaking, there are two main variants of the
BSM. The first is the original BSM in which the organic phase
is separated out after the phase transfer of the metal (Au) ions
and thiols are added to the separated organic phase. The
second is the Whetten adaptation18 of the original BSM in
which no separation of the organic phase is carried out and
thiols are added to the mixed aqueous and organic phases. At
a thiol/Au ratio of 3 : 1, the latter leads to a mixture of the
[TOA]+[M(I)X2]ꢀ complex, polymeric (AuSR)n species, residual
thiols, and reduction-generated disulfide.17 Although the
mixed presence of the [TOA]+[M(I)X2]ꢀ complex and polymeric
(AuSR)n species was identified as a source of polydispersity,17
it is still unclear what is the effect of the co-presence of thiol
and disulfide.
Numerous studies have observed that both thiols and
disulfides can form self-assembled monolayers (SAMs) on
the bulk gold surface.19–21 It has been demonstrated that
disulfide had a much slower SAM formation kinetics than
thiols.20 On the other hand, disulfide also showed a similar
activity in the ligand-exchange reaction of thiolate-protected
Au NPs as thiol.22 These observations indicate that the action
by thiol and disulfide are highly reaction-environment dependent.
Now our improved mechanistic understanding of the BSM,
particularly the observation that the Au–S bonds do not form
until the last reduction step by NaBH4 in the original BSM,16
enables us to design experiments to address the source of
polydispersity specifically and this communication reports the
results of such a study.
A recent thought-provoking work by Goulet and Lennox led
to a rethinking of the long-held belief.15 Specifically, instead of
confirming the long-held belief that polymeric (AuSR)n species
were the intermediate Au(I)-ion precursors, their results have
shown that tetraoctyl-ammonium metal(I) halide complex i.e.,
[TOA]+[M(I)X2]ꢀ, was actually the Au(I)-ion precursor in the
BSM, which was generated by reducing the [TOA]+[M(III)X4]ꢀ
complex with thiol:
[TOA][M(III)X4] + 2RSH - [TOA][M(I)X2] + RSSR + 2HX
(1)
For a thiol/Au ratio of 3 : 1, reaction (1) predicts that one
equivalents of RSH will not be consumed. This inference has
been confirmed by the 1H NMR spectra of the intermediatesz
in the BSM, where the peaks indicated the existence of thiol
(dH (300 MHz; C6D6; C6D6) 2.17 (2H, m, CH2S)), disulfide
(dH (300 MHz; C6D6; C6D6) 2.58 (4H, t, CH2SSCH2)), and
the [TOA][AuBr2] complex (dH (300 MHz; C6D6; C6D6) 3.11
(8H, br, N+CH2)) shown in Fig. S1 (see the ESIw). In order to
Department of Chemistry, Georgetown University,
37th and ‘‘O’’ Streets NW, Washington DC, 20057, USA.
E-mail: yyt@georgetown.edu; Fax: +1 202 6875591;
Tel: +1 202 6875872
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c1cc11642h
c
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Chem. Commun., 2011, 47, 6033–6035 6033

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distinguish the effect of thiolate precursors (i.e. thiol vs.
disulfide), revised BSM procedures were adapted in which
the synthesized [TOA][Au(I)Br2] or [TOA][Au(III)Br4] complex
was used as the starting Au source.
of the approaching Au substrate and a lower activation entropy
of adsorption for thiol,20 although the bond dissociation energy
of RS–H is higher than that of RS–SR.24,25 Similar preference
for thiol during the formation of thiolate-protected Au NPs to
that in the thiolate SAM on bulk gold strongly supports our
previously reported BSM mechanism, in which thiolate bonding
to Au occurs after Au(0) nuclei are formed.16 When only disulfide
was used, even larger and less homogeneous (2.58 ꢁ 0.33 nm) Au
NPs were produced, indicating that the presence of disulfide led
to wider size dispersity in the synthesized Au NPs.
We first discuss the results of using [TOA][AuBr2] as a
starting material. Specifically, 0.025 mmol [TOA][AuBr2] was
dissolved in 10 mL toluene and mixed with 0.21 mL H2O.
Then 0.075 mmol of dodecanethiol, 0.0375 mmol didodecyl
disulfide, or a mixture of 0.025 mmol of dodecanethiol and
0.025 mmol of didodecyl disulfide was added to the solution.
After the colorless solution was stirred for 1 h, a fresh NaBH4
aqueous solution (0.25 mmol NaBH4 in 0.5 mL H2O) was
poured in and stirred for another 3 h, leading to a dark brown
solution. The NPs could be collected in the solid state with
ethanol.
Remarkably, opposite ligand effect was observed when no
water was added before the reduction with NaBH4. In this case
in which the step of adding 0.21 mL water was omitted, using
didodecyl disulfide made smaller and more homogeneous Au
NPs (1.93 ꢁ 0.36 nm) than using thiol did (2.81 ꢁ 0.42 nm), as
shown by the TEM images in Fig. S3 (ESIw). The absence of
H2O precludes an excellent receiving medium for accepting
hydrophilic protons that would be generated from the breakage
of thiol’s S–H bond and therefore increases substantially the
reaction barrier.17 Yet no such barrier is expected to exist for
the breakage of the disulfide (RS–SR) bond and becomes
easier to be broken. Consequently, using disulfide produces
better results than using thiol in a water deprived situation.
We now discuss the results of using synthesized
[TOA][Au(III)Br4] as the starting material. In a typical BSM
synthesis with the organic phase separated (the original
BSM),1 the [TOA][Au(III)Br4] complex is formed after the
phase transfer with TOAB but before the addition of
thiols.15,16 For the purpose of comparison, Fig. 2(a) shows
the results of a normal BSM synthesis (0.025 mmol of HAuCl4
aqueous solution and 0.025 mm of TOAB toluene solution)
with a thiol/Au ratio of 3 : 1. Au NPs of 1.70 ꢁ 0.22 nm were
obtained, which is the typical result of such synthesis (see the
ESIw). If 1.5 equivalents of disulfide was used in lieu of the
thiol above, Au NPs of 2.24 ꢁ 0.28 nm were obtained, as
shown in Fig. 2(b). Notice that in the former case, reduction of
[TOA][Au(III)Br4] to [TOA][Au(I)Br2] took place after the
addition of thiols that led to a mixture of residual thiol and
reduction-generated disulfide (reaction (1)). In the latter case,
no reduction of [TOA][Au(III)Br4] happened after the addition
of disulfide. When the synthesized [TOA][Au(III)Br4] was used
The IR spectra of the Au NPs protected by the ligands
originated from thiol and disulfide, respectively, are shown in
Fig. S2 (ESIw). Both samples showed similar IR patterns
(nmax/cmꢀ1 2954 (CH3), 2920 and 2850 (CH2), 721 (CH2 and
S–Ctrans)), in agreement with the literature observations.5,23
The absence of vibrational bands at 2575 cmꢀ1 of n(S–H) and
at 575 cmꢀ1 of n(S–S) indicates the breakage of the S–H bond
of dodecanethiol or the S–S bond of the didodecyl disulfide
had occurred during the formation of BSM metal NPs. In
other words, thiolate-protected Au NPs were formed in both
cases no matter if thiol or disulfide was added as the ligand
precursor. This is similar to the absorption of thiol or disulfide
on bulk gold substrate where both thiol and disulfide were
observed to produce thiolate-like species when being self-
assembled.20
The TEM images and corresponding surface plasmon
resonance (SPR) spectra of the thiolate-protected Au NPs origi-
nated from the thiol (1.36 ꢁ 0.19 nm), the mixture of thiol and
disulfide (1.68 ꢁ 0.21 nm), and the disulfide (2.58 ꢁ 0.33 nm),
respectively, are shown in Fig. 1. They clearly show that using
thiol made smaller and more homogeneous thiolate-protected Au
NPs than using disulfide did. This observation is more consistent
with the observation of thiol exhibiting stronger preference than
disulfide in forming SAMs on bulk Au substrate20 than with the
one of disulfide showing similar exchange reaction activity as
thiol.22 The former was explained by the smaller steric hindrance
Fig.
2 TEM images with corresponding size distributions and
UV-visible spectra of the Au nanoparticles formed by a typical BSM
from the organic layer of 0.025 mmol of HAuCl4 aqueous solution
and 0.025 mmol of TOAB toluene solution (10 mL) together with
(a) 3 equiv. of dodecanethiol and (b) 1.5 equiv. of didodecyl disulfide,
formed from a mixture of [TOA][Au(III)Br4] toluene solution and
0.21 mL water together with (c) 3 equiv. of dodecanethiol and
(d) 1.5 equiv. of didodecyl disulfide. (Au : S = 1 : 3).
Fig.
1 TEM images with corresponding size distributions and
UV-visible spectra of the Au nanoparticles formed from a mixture
of [TOA][AuBr2] toluene solution and water with (a) 3 equiv. of
dodecanethiol, (b) a mixture of 1 equiv. of dodecanethiol and 1 equiv.
of didodecyl disulfide, (c) 1.5 equiv. of didodecyl disulfide. (Au : S = 1 : 3).
c
6034 Chem. Commun., 2011, 47, 6033–6035
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directly as the starting material to synthesize NPs with thiol and
disulfide, respectively, the phase transfer step was no longer
needed. The reaction medium was thus a 10 mL toluene
solution of the [TOA][Au(III)Br4] complex plus 0.021 mL H2O
with the amount of ligands (thiol or disulfide) giving a S/Au
ratio of 3 : 1. TEM images and corresponding SPR spectra of
the resulting Au NPs are shown in Fig. 2(c) for thiol (1.50 ꢁ
0.19 nm) and (d) for disulfide (2.14 ꢁ 0.43 nm). While Fig. 2(b)
and (d) show the same results which are somewhat expected
because they were basically the same synthesis, using thiol as the
sole source of ligand improved the quality of Au NP synthesis.
In summary, the recently improved mechanistic understanding
of the BSM15–17 enabled us to have designed some specifically
targeted experiments to address the size polydispersity problem
frequently observed in the traditional BSM syntheses. We were
able to identify that the co-presence of the residual thiol and
reduction-generated disulfide as found in typical BSM syntheses
is a source of size polydispersity observed. We also found that
in the presence of H2O, thiol was a better ligand than disulfide
for making smaller and more homogenous Au NPs but in a
water deprived situation bisulfide was better. Based on the
observations discussed above, we believe that the organic
(toluene or benzene) solution of synthesized [TOA][AuBr2]
and thiols plus a small amount of water consist the optimal
reaction medium before the addition of NaBH4 for a BSM
synthesis that minimized the size and size polydispersity from
1.70 ꢁ 0.22 nm of a typical BSM to 1.36 ꢁ 0.19 nm.
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This work is supported by grants from the National Science
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Notes and references
z The intermediate solutions monitored with 1H NMR spectroscopy
were prepared as follows: a hydrogen tetra-chloroaurate (HAuCl4,
0.025 mmol) aqueous solution (0.21 mL) was mixed with a TOAB
(0.050 mmol) C6D6 solution (0.8 mL) and stirred until the color of the
aqueous phase disappeared. The bottom colorless layer was then
discarded. 1, 2, or 3 equiv. of dodecanethiol (C12H25SH) was added
to the separated wine-red C6D6 layer. After the solution was stirred for
1
1 h, H NMR spectra were obtained.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 6033–6035 6035