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COMMUNICATION LouisNadjo et al.One-stepsynthesisandstabilizationofgoldnanoparticlesinwaterwiththesimpleoxothiometalateNa2[Mo3(μ3-S)(μ-S)3(Hnta)3]

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COMMUNICATION www.rsc.org/materials | Journal of Materials Chemistry

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One-step synthesis and stabilization of gold nanoparticles in water with thesimple oxothiometalate Na2[Mo3(m3-S)(m-S)3(Hnta)3]†

Bineta Keita,a Rosa Ngo Biboum,a Israel Martyr Mbomekalle,b Sebastien Floquet,b Corine Simonnet-Jegat,b

Emmanuel Cadot,b Frederic Miserque,c Patrick Berthetd and Louis Nadjo*a

Received 27th March 2008, Accepted 24th April 2008

First published as an Advance Article on the web 8th May 2008

DOI: 10.1039/b805224g

Na2[Mo3(m3-S)(m-S)3(Hnta)3] serves both as a reducing and

a capping agent in the synthesis of Au nanoparticles in water at room

temperature in a ‘‘fully green chemistry-type process’’, thus avoiding

the usual two-step preparation of thiolate-monolayer-coated gold

nanoparticles. The nanoparticles were characterized by TEM,

DLS, UV-vis spectroscopy, XPS and XRD analyses and cyclic

voltammetry.

Fig. 1 Structural representation of [Mo3S4(Hnta)3]2� (from ref. 17).

The impetus for the study of gold nanoparticles (Au-NPs) is due to

the fact they play important realized or potential roles in different

areas of science, including nanoelectronics, nonlinear optics, oxida-

tion catalysis, electrochemical applications and biological labelling

among others.1–5 The toluene phase separation of alkanethiolate-

monolayer-coated gold nanoparticles realized by Brust et al.6 opened

the way toward versatile precursors for the fabrication of nanoscale

systems.7 Organic alkylthiols are commonly used, with the conse-

quence that Au colloids stabilized by these capping agents are mainly

soluble in organic solvents. Alternatively, methods were also reported

for the synthesis of Au nanoparticles in aqueous media, with various

stabilizing agents such as citrate,8 polymers,6,9 and oligonucleotides10

and even an organically modified polyoxometalate (POM).11 Most of

these syntheses are realized with a reduction reagent which is different

from the stabilizing agent. Even in the case of Au nanoparticles

capping with tungsten tetrathiometalate [WS4]2� or oxothiometalate

[WS2O2]2�, the reduction of HAuCl4 was first achieved by NaBH4.

12

It is worth noting the recent paper describing the straightforward

reduction of HAuCl4 by sodium sulfide and giving experimental

arguments to solve the controversy over the structure of the resulting

product.13

We now report on the unprecedented one-step synthesis and

stabilization of Au-NPs with the simple oxothiometalate [Mo3(m3-S)

aLaboratoire de Chimie Physique, UMR 8000, Universite de Paris-Sud 11,bat. 349, 91405 Orsay Cedex, France. E-mail: louis.nadjo@lcp.u-psud.frbInstitut Lavoisier de Versailles, UMR 8180, Universite de Versailles, 45avenue des Etats Unis, 78035 Versailles, FrancecLaboratoire de Reactivite des Surfaces et Interfaces/ CEA-Saclay DEN/DANS/DPC/SCP, bat. 391, 91191 Gif-sur-Yvette Cedex, FrancedLaboratoire de Physicochimie de l’Etat Solide, ICMMO, UMR 8648,Universite Paris Sud 11, bat. 410, 91405 Orsay Cedex, France

† Electronic supplementary information (ESI) available: Experimentaldetails including the pH of the mother mixtures, evolution of Au0 SPRspectra as a function of time, DLS measurement of particle size,superposition of the UV-visible spectra of the starting oxothiometalate,the mother mixture of HAuCl4 and Na2[Mo3S4(Hnta)3] after reactioncompletion and of the nanoparticles after centrifugation and washing,TEM image for g ¼ 0.5 and cyclic voltammograms. See DOI:10.1039/b805224g

3196 | J. Mater. Chem., 2008, 18, 3196–3199

(m-S)3(Hnta)3]2� (Fig. 1) (Hnta2� is the nitrilotriacetate ligand) in water

at room temperature without any catalyst. The simultaneous presence

of MoIV, sulfide and Hnta2� in this anion is anticipated to offer both

a rich redox chemistry for reducing HAuCl4 and strong stabilizing

properties toward the resulting Au-NPs, in green chemistry

conditions.14–16 To our knowledge, such single reagent preparation of

thiolate-monolayer-coated gold nanoparticles has not been described

previously.

Na2[Mo3(m3-S)(m-S)3(Hnta)3] was synthesised according to litera-

ture procedures17 and checked by routine methods (UV-visible,

FT-IR, NMR). Other experimental details, including nanoparticle

synthesis conditions, are reported in the ESI.†

The formal potential of the first redox process of Na2[Mo3(m3-S)

(m-S)3(Hnta)3] is around E00 ¼ �0.663 V vs. SCE and is pH-

independent.18 We have checked, for several hours, by UV-visible

spectroscopy, that this oxothiometalate is very stable in pure Millipore

water, in full agreement with literature indications.18 These charac-

teristics guarantee that the selected oxthiometalate should be suitable

for reducing HAuCl4. All the Au-NP syntheses were performed in

pure Millipore water. The incentive for choosing this medium is to

generate as small particles as possible by keeping the ionic strength

low.19 In a typical experiment, a mixture containing 0.5 mM of

HAuCl4 and 0.5 mM of Na2[Mo3(m3-S)(m-S)3(Hnta)3] was assembled

in water. To the 0.5 mM solution of Na2[Mo3(m3-S)(m-S)3(Hnta)3], an

aliquot of the appropriate mother HAuCl4 solution was added in

order to avoid any important dilution. In this example, the excess

parameter g ¼ [HAuCl4]/[oxothiometalate] is equal to 1. It is worth

noting that this g value guarantees that there is more than the

theoretically necessary amount of reductant to reduce all the AuIII ions

in the starting solution. The UV-visible characteristics of the mixture

were monitored as a function of time with a diode array HP 8453

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spectrophotometer. Immediate detection and gradual increase of the

surface plasmon resonance (SPR) spectrum of Au0 nanoparticles with

a maximum around 526 nm during a period from 14 s to 2 min after

solution mixing indicate the fairly fast kinetics of nanoparticle

formation (ESI† Fig. S1). For longer durations than 2 min, the

evolution of the UV-visible spectrum of the mixture becomes negli-

gible. After completion of the reaction, the mixture was centrifuged,

redispersed and washed at least twice with Millipore water and, finally,

samples were prepared for TEM imaging, XRD and XPS analysis.

Fig. 2A shows the TEM image of the NPs deposited on a grid. Most

nanoparticles are spherically shaped. Visually, the NPs appear

monodisperse. A histogram established from the diameter measure-

ments of 128 NPs confirms this impression with a majority of the

population around 9.5 � 0.5 nm. Such estimate was confirmed by

direct DLS measurements of the synthesis mother solution (ESI†

Fig. S2). Detailed analysis of the complementary insights which could

be derived from further comparison of the results of TEM and DLS

size measurements is beyond the scope of this work. The obtained

colloidal solution is very stable and does not show any precipitate after

more than half a year without adding any organic stabilizer,

a complementary indication that the oxothiometalate serves both as

a reductant and an efficient stabilizer. Such stabilization efficiency

Fig. 2 A) TEM images of Au0 nanoparticles synthesized by the reduction of

[HAuCl4]/[oxothiometalate] was 1. The initial concentration of the oxothiomet

XPS spectrum. C) XPS analysis: deconvolution of the Mo core 3d level XPS sp

This journal is ª The Royal Society of Chemistry 2008

follows the prediction that the oxothiometalate coordination to gold

surface should be strong due to the multidentate nature of this capping

agent. Fig. 2 (B–D) also shows the XPS analysis of two main elements

in the starting solution and the XRD pattern. Confirmation of the

synthesis of Au0 NPs was assessed throughXPSanalysis of the sample.

It indicates the presence of gold in two valence states (Fig. 2B): metallic

Au0 (located at 84.0� 0.3 eV and 87.7� 0.3 eV for 4f7/2 and 4f5/2 levels

respectively) and AuIII (85.8� 0.3 eV and 89.2� 0.3 eV respectively for

4f7/2 and 4f5/2 levels). However, the amount of AuIII is low and metallic

Au0 is by far the most abundant species in the mixture, thus confirming

the effectiveness of the reduction process. Fig. 2C reveals the presence

of the constituent elements of the initial oxothiometalate, despite the

washing of the sample. Molybdenum is present in valence states IV, V,

and VI, with the IV state representing the largest amount. It is rewarding

that the 2s level of sulfur could be detected in a close energy range.

Taking into account the Scofield sensitivity factors, the relative atomic

composition of the analyzed deposit is 28.4% Mo and 71.6% Au even

though such values must mainly be considered as indicative. The

oxothiometalate appears to serve both as a reductant of the metallic

salt and the capping agent of the resulting NPs. This remark will

receive complementary support in the following. The capping of the

AuNPs by the oxothiometalate is in complete agreement with

HAuCl4 with Na2[Mo3(m3-S)(m-S)3(Hnta)3]. The concentration ratio g ¼alate was 0.5 mM. B) XPS analysis: deconvolution of the gold core 4f level

ectrum and the sulfur 2s level. D) XRD analysis of the Au0 nanoparticles.

J. Mater. Chem., 2008, 18, 3196–3199 | 3197

Fig. 3 Evolution, as a function of time, of the UV-visible spectra of the

mother mixtures of HAuCl4 and Na2[Mo3(m3-S)(m-S)3(Hnta)3] with two

values of g ¼ [HAuCl4]/[oxothiometalate]. A) g ¼ 2. The evolution was

followed over a total of 24 h. In the direction of the arrow, spectra were

collected over 7 h, the first one 5 min after mixing of solutions, the second

and the third ones after 15 min each and the five subsequent spectra every

other hour. The last spectrum in the direction of the arrow was recorded

after 24 h, and can hardly be distinguished from five supplementary

spectra run every other hour after 28 h standing. B) g ¼ 3. The evolution

was followed over a total of 9 h. The dotted line spectrum is that of

Na2[Mo3(m3-S)(m-S)3(Hnta)3] alone. The other spectra in the direction of

the arrow correspond to the NP evolution during the first 2 h, and after

3 h and more, respectively.

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expectation due to the presence of both sulfur and molybdenum in

Na2[Mo3(m3-S)(m-S)3(Hnta)3]. Such conclusions were also reached

in the study of self-assembled monolayers of [Mo3S4]4+ clusters on an

Au(111) surface.20 XRD analysis allowed for further characterization

of the NPs by determining their crystaldirection.The X-ray diffraction

pattern recorded from a dry powder of Au0 nanoparticles fixed

on adhesive double sided tape (Fig. 2D) clearly shows the (111), (200),

(220) and (311) Bragg reflections of face-centered cubic (fcc) of gold in

agreement with the expectation of peaks at 38.1� (111), 44.4� (200),

64.7� (220) and 77.5� (311). In addition, the intensity ratio of the {111}

to the {200} diffraction peaks is larger than the 1.9 value of the

standard diffraction of gold powders, a feature suggesting that the

nanocrystallites grow with the surfaces terminated by the lowest

energy {111} facets.21Evaluation of the average diameter of NPs in the

h111i direction through the Scherrer formula applied to the (111)

Bragg reflection gives a value of 10� 0.5 nm, in agreement with direct

measurements from the TEM image. Finally, particle size measure-

ments by three different techniques (TEM, DLS and XRD) converge

to very close evaluations.

As a step toward a further understanding of the reduction of

HAuCl4 by Na2[Mo3(m3-S)(m-S)3(Hnta)3], several complementary

experiments were undertaken, mainly based on the monitoring of

UV-visible spectra as a function of various parameters. The excess

parameter g appeared to display a major influence on the HAuCl4/

Na2[Mo3(m3-S)(m-S)3(Hnta)3] system, with two domains corres-

ponding roughly to g# 2 and g$ 3. The initial concentration of the

oxothiometalate is fixed at 0.5 mM in the mixtures. Whatever the g

domain, the spectrum of the washed NPs persistently shows

a shoulder around 242 nm and another one around 371 nm, thus

indicating the presence of the oxothiometalate as a capping agent for

NPs (ESI† Fig. S3). The TEM image (ESI† Fig. S4) obtained with g

¼ 0.5 shows a good monodispersity of the NPs with an average

diameter around 6 � 0.5 nm. For g < 2, the maximum absorption

SPR peak in intensity and wavelength location, for each spectrum,

remain constant and no precipitation of the NPs was observed over

several months. In contrast, Fig. 3A illustrates the slightly different

behaviour observed for g ¼ 2. After 24 h, the peak had shifted from

526 nm to 561 nm and the broadness had varied from 90 nm to

151 nm. However, no precipitate was observed. In contrast,

a precipitate was observed after a few hours for g$ 3. Fig. 3B shows

the evolution of UV-visible spectra before precipitation. After three

hours standing, a new band emerges around 661 nm and grows at the

expense of the band at 526 nm. The first step is likely the formation of

monodisperse small particles with subsequent aggregation, inducing

heterogeneity in size and shape. Very recently, an analogous set of

curves was observed for the UV-visible absorption spectra of citrate

prepared Au0 NPs after benzyl mercaptan addition; the phenomenon

was explained by gradual aggregation of nanoparticles and was

mathematically modeled.22 As the increase of the aggregation process

appeared to be monitored mainly by an increasing of the g values,

NP growing might simply be favored by an insufficient amount of

oxothiometalate to completely prevent aggregation. Direct study of

the influence of the starting concentration of the oxothiometalate was

carried out, with g values #1 selected to avoid fast precipitation of

the NPs (vide supra). In such conditions, the UV-visible spectra of the

mixtures show little variation from one mixture to the next, except for

their intensities. The differences were assessed through the shapes and

sizes of the NPs obtained in TEM observations. Three initial

concentrations C0 were studied (0.2 mM, 0.5 mM and 1 mM) with g

3198 | J. Mater. Chem., 2008, 18, 3196–3199

values ranging from 0.5 to 4. In short, it turned out that g governs the

dispersion of shapes and sizes. In the range of concentrations used

here, the best choice is C0 # 0.5 mM in the presence of an excess of

oxothiometalate (g # 1) to minimize shape variation and to obtain

a narrow size distribution in the synthesis of Au0 NPs. For example,

fairly monodisperse spherical NPs (about 10 nm in diameter) were

observed using C0 ¼ 0.2 mM and 0.5 mM in oxothiometalate and

g ¼ 0.5 or g ¼ 1.0. Outside this g domain, the dispersion of sizes is

larger. For instance, sizes increase from 5 to 54 nm when g varies

from 2 to 4 with C0 ¼ 0.5 mM. All together, these observations

support the conclusion that for such a system, an excess of

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oxothiometalate is required to act both as a reductant of the AuIII salt

and an efficient capping agent of the resulting Au0 NPs.

The NPs were further characterized by cyclic voltammetry (CV).

Typical results are described in the ESI.† In short, the cyclic

voltammetry response in 0.5 M H2SO4 solution (pH 0.30) for an

electrode modified with the present Au0 NPs (ESI† Fig. S5A) shows

a broad composite oxidation peak at 1.338 V vs. SCE associated with

a sharp reduction located at 0.882 V featuring a fingerprint of these

NPs on the electrode surface. We also wondered whether the

oxothiometalate used for NP synthesis could be detected, at least

partly, on this electrode. For this purpose, the potential domain

where such observation is expected on gold (between �0.1 V and

+1.1 V vs. SCE) was focused on and current intensities expanded.

Comparison of the CV patterns recorded with the modified electrode

in pure 0.5 M H2SO4 electrolyte and with a polished glassy carbon

electrode soaking in a solution of the oxothiometalate freely diffusing

in the same medium (ESI† Fig. S5B) confirms the expectations. This

observation is supported by the conclusions of both UV-visible

spectra and XPS analysis of centrifuged and washed Au NPs

obtained in the present experiments.

In summary, small and stabilized Au0 NPs are obtained, in one

step, in water at room temperature, by reducing HAuCl4 with

Na2[Mo3(m3-S)(m-S)3(Hnta)3]. The oxothiometalate appears to carry

simultaneously built-in reduction capabilities, with subsequent

adsorbability and covalent attachment properties on the resulting

NPs. In short, it can be considered that the selection of water, one of

the best environmentally acceptable solvents, of an eco-friendly

reducing agent and a benign capping agent realize the mild conditions

of a fully ‘‘green chemistry-type’’ process. Such Au-oxothiometalate

composite nanostructures functionalized with Mo-containing moie-

ties are expected to have favourable photo-, electronic, analytical and

catalytic properties. The pseudocuboidal {Mo3S4}4+ core exhibits rich

redox properties with the unique ability to incorporate about 22

various transition and post-transition metals (M0) to give a series of

heterometallic complex derivatives containing the cuboidal

{Mo3M0S4} core.23 To our knowledge, the incorporation of Au has

not been described in this series. Such systems thus appear good

candidates for the rational synthesis of mixed metal Au-NP based

nanostructures.

Acknowledgements

This work was supported by the CNRS (UMR 8000, 8180 and 8648),

the Universite Paris-Sud 11, the Universite Versailles Saint-Quentin,

the CEA-Saclay (Laboratoire de Reactivite des Surfaces et

This journal is ª The Royal Society of Chemistry 2008

Interfaces). The help of Dr Luis Brudna Holzle with preliminary

TEM observations is gratefully acknowledged. Lynda Ould-Bouali

participated in the synthesis of the oxothiometalate used in this work.

Mrs Daniele Jaillard (UMR 8080, CNRS, Orsay) is thanked for

continuous help during TEM analysis. Mr Nicolas Buton (Malvern

Instruments, Orsay) is thanked for running DLS experiments.

Notes and references

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