On the Synthesis of Au Nanoparticles Using EDTA as a Reducing Agent

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On the Synthesis of Au Nanoparticles Using EDTA as a Reducing Agent Helene Dozol, Guillaume Meriguet, Bernard Ancian, Valerie Cabuil, Haolan Xu, Dayang Wang, and Ali Abou-Hassan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp4067789 • Publication Date (Web): 16 Sep 2013 Downloaded from http://pubs.acs.org on September 19, 2013

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

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On the Synthesis of Au Nanoparticles Using EDTA

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as a Reducing Agent

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Hélène Dozol,† Guillaume Mériguet,† Bernard Ancian,† Valérie Cabuil,† Haolan Xu,†† Dayang

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Wang†† and Ali Abou-Hassan*,†.

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†

Laboratoire de Physicochimie des Electrolytes Colloïdes et Sciences Analytiques, PECSA,

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UMR 7195, Université Pierre et Marie Curie, Paris VI, 4 place Jussieu, 75252 Paris Cedex 05,

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France

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††

Ian Wark Research Institute, University of South Australia, Adelaide, SA 5095, Australia

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ACS Paragon Plus Environment

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ABSTRACT

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The synthesis of gold nanoparticles (Au NPs) from HAuCl 4 by using EDTA as a reducing

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agent is investigated for the first time by using UV-Vis spectroscopy, transmission electron

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microscopy (TEM), and dynamic light scattering (DLS). We show that the size of resulting Au

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NPs is dictated by the reactivity of ionic Au(III) species and the charge density of EDTA

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molecules with pH. Moreover by varying the pH of the reaction media, the size of Au NPs can

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be tuned from 25 to 100 nm. Investigation of the nucleation and growth of Au NPs by time

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resolved UV-Vis and TEM revealed the presence of nanowires that progressively increases in

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size at the same time the connected network is fragmented into small segments before the final

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spherical particles are formed. The identification of reaction intermediates and final products

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resulting from the reduction of Au(III) by EDTA was possible by recording several 1D- and 2D-

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Nuclear Magnetic Resonance (NMR) spectra. Our results show that Au(III) reduction to Au(0)

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NPs by EDTA is accompanied by decarbonylation of EDTA and the formation of formaldehyde

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which, depending on pH, interferes in the reduction process and contributes to the synthesis.

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Beside the immense flexibility that can be provided by EDTA for further conjugation with

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functional molecules to diversify the surface functionality and in turn broaden the application,

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many synthetic routes concerned with the elaboration of nanoshells and core-shell metal

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nanoparticles use a large excess of the toxic formaldehyde as a reducing agent. Given the interest

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in biomedical applications of such nanomaterials and the important role of formaldehyde as a

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reducing agent in the synthesis process, using EDTA as a reducing agent can be a good starting

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point for minimizing or replacing formaldehyde use in the synthesis.

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KEYWORDS: DLS – NMR – nucleation - growth


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INTRODUCTION

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EDTA – ethylenediaminetetraacetic acid – is one of the most commonly-used multidentate

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chelating ligands.1,2 Thanks to strong complexation with transition metal ions, it has been widely

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utilized in numerous industrial processes, such as metal ion extraction and separation, scale

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removal from decontamination equipment exposed to radioactive materials, chelation therapy,

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anticoagulation, and many others.3-6 However, many EDTA complexes are not stable. It has been

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well documented that EDTA can be readily oxidized by for instance Ce(IV), Mn(III), Cr(VI),

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and Pb(IV) ions at room temperature and Fe(III) ions at high temperature.5,7-10 This oxidation is

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accompanied by simultaneous reduction of metallic ions.11,4,5,7-10 EDTA oxidation by Au(III) has

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been also reported at different temperatures. This has encouraged many groups to harness EDTA

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to reduce HAuCl 4 to Au nanoparticles (NPs) in water especially by taking into account the

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exceeding instability of EDTA/Au(III) complexes.

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first chemical synthesis of gold and silver sols using EDTA as a reducing agent. The effect of the

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concentration of Au(III) precursor during the synthesis was studied using UV-Vis. Very recently,

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Wang et al.13 and Yang et al.11 described the preparation of a series of novel shaped gold

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microcrystals by simply mixing HAuCl 4 with disodium salt of ethylenediaminetetraacetic acid

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(Na 2 EDTA) for applications in high surface-enhanced Raman scattering (SERS). Under the

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different reaction temperatures, spinous structures, multipod microspheres (coral shaped), and

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rough surfaced microspheres were obtained. Guo et al.14 described a direct facile approach to the

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fabrication of chitosan impregnated with EDTA followed by HAuCl 4 and demonstrated the

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formation of gold hybrid nanospheres due to the reduction of Au(III) by EDTA. Following the

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same method, Bonggotgetsakul et al.15 reported the preparation of gold nanoparticle monolayer

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on the surface of a polymer inclusion membrane using EDTA as the reducing agent. Parameters

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In 1963, Fabrikanos et al.12 reported the


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such as EDTA concentration, pH, reduction time, temperature, shaking rate were optimized to

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obtain a uniform monolayer of Au NPs on the surface of the membrane. If different studies have

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reported on the use of EDTA as an effective reducing agent for the synthesis of gold

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nanoparticles, surprisingly to the best of our knowledge, no studies have tried to understand how

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and why EDTA can reduce Au(III) to Au NPs. Given the interest in medical applications of these

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nanomaterials, it is particularly concerning to identify the different molecules that can be formed

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during the reduction of Au(III).16,17 Herein we report for the first time a detailed physico-

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chemical and NMR studies in order to understand the synthesis of gold nanoparticles via EDTA

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oxidation. The identification of the reaction intermediates during the reduction was made

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possible by the use of 1H,

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effectively reduce Au(III) to Au NPs, however depending on the pH other product species may

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also play a role in the nucleation and growth mechanism of Au NPs. To our best knowledge, the

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present work should be the first study that aims to understand and control the synthesis of gold

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nanoparticles by using EDTA as a reducing agent.

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C, and multinuclear NMR studies. We show that EDTA can

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EXPERIMENTAL SECTION

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Hydrogentetrachloroaurate(III) hydrate (99.9%, Au >49%, min, CAS: 27988-77-8 ), disodium

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ethylenediaminetetraacetate (99 –101%, CAS: 6381-92-6), sodium hydroxide, deuterium oxide

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(for NMR studies) were purchased from Aldrich, and were used as received. Prior to performing

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nanoparticle synthesis protocols, the glassware and stir bars were cleaned with aqua regia (3:1

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v/v HCl (37%): HNO 3 (65%)) solutions (caution: aqua regia solutions are dangerous and should

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be used with extreme care; never store these solutions in closed containers) and then rinsed


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thoroughly with H 2 O before use. The water in all experiments was prepared in a three-stage

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Millipore Milli-Q Plus 185 purification system and had a resistivity higher than 18.2 MΩ.cm.

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Syntheses of the NPs: typically, 0.5 mL of the aqueous solution of HAuCl 4 (1 wt%) were well

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mixed into 0.2 mL of disodium ethylenediaminetetraacetate (Na 2 H 2 EDTA, 1 wt%) at room

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temperature, followed by an adjustment of the pH of the mixture solution from 4.5 to 10.3 with

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aqueous solution of NaOH (0.1M). The molar ratio of HAuCl 4 to EDTA was fixed at 2.8 in the

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current work. The volume of the mixture solutions was adjusted to 10 mL with water, followed

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by 1 min vigorous mixing. Note that the mixture solution remained yellowish during mixing.

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Subsequently, the resulting mixture was very rapidly poured into 50 mL of boiling water under

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vigorous stirring, followed by further 2 h boiling before cooling down to room temperature. Note

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that prior to addition, the water was kept boiling for 10 min.

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NMR spectra were recorded by using a 1H/13C/15N TXI and a 1H/X BBO broadband probe on a

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Bruker Avance DRX 500 NMR spectrometer operating at 499.62 MHz for 1H. The temperature

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was set to 298.0 ± 0.1 K thanks to a BCU unit and a BVT controller. All chemical shifts are

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measured from the signal of internal residual protons of HOD (semiheavy water) in D 2 O at 4.71

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ppm. Diffusion NMR experiments were performed with a 57.0 G cm-1 gradient coil and the BPP-

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STE-LED sequence.18 Fresh stock solutions of HAuCl 4 (1% wt) and Na 2 EDTA (1% wt) in D 2 O

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were used in this study. NMR samples were prepared by mixing HAuCl 4 and the EDTA in a

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molar ratio Au(III)/EDTA = 2.8. The final concentrations of AuCl 4 - and EDTA respectively

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were 0.73 mM and 0.26 mM. The pH of the mixture was 2.8.

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UV-vis absorption spectra were recorded with an Agilent 8453 UV-vis spectrophotometer.

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Transmission electron microscopy (TEM) images were obtained with a Zeiss EM 912 Omega

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microscope at an acceleration voltage of 120 kV. Dynamic light scattering (DLS) experiments


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were performed at room temperature and at a fixed angle of 173° on a Malvern Zetasizer Nano

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ZS equipped with 50 mW 533 nm laser and a digital autocorrelator. Prior to DLS measurement,

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the aqueous dispersions of as-prepared Au NPs were filtered through 0.2 μm polycarbonate

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membrane.

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RESULTS AND DISCUSSIONS

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Figure 1. a) UV-visible absorption spectra of Au NPs obtained via EDTA reduction of HAuCl 4

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at pH 4.5 (a), 5.3 (b), 6.3 (c), 7.2 (d), 8.4 (e), 9.1 (f), and 10.3 (g). The inset shows the photos of

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the corresponding NP solutions; b) Temporal evolution of the UV-vis absorption spectra of the

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reduction media during EDTA reduction of HAuCl 4 at pH 9.1 and at 100°C; c) Summary of λ max


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(triangles) and dynamic light scattering averaged particle diameter (circles) as a function of time

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for the same sample studied in b.

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Figure 1.a shows the UV-Vis absorption spectra of the synthesized Au suspensions at different

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pH. A clear blue shift of the surface plasmon resonance (SPR, λ max ) band of the resulting Au

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NPs with an increase of the pH of the EDTA/HAuCl 4 mixed solutions (pH EDTA/HAuCl4 ) is

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observed. The SPR band blue shift is clearly indicated also by the color variation of the NP

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suspensions from blue to purple, red, and orange with the pH EDTA/HAuCl4 increases from 4.5 to

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9.1. However when the pH EDTA/HAuCl4 is above 10.3, the resulting Au NPs show a weak SPR

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band centered at 525 nm, corresponding to the red color of the suspension.

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We observed that the reduction of Au(III) in boiling water and in acidic medium was complete in

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less than 30s. In alkaline medium such as for pH = 9.1 the reaction is slower which allowed us to

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monitor the synthesis by time resolved UV-Vis and by DLS. When the alkalinized mixture of

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auric acid and EDTA solution was injected in the boiling water at this pH, the reacting solution

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progressively went from clear to purple and then red orange after ≈7 minutes indicating the

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formation of 18 to 20 nm diameter gold NPs. Aliquots were sampled during the synthesis from

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the reacting solution, quenched in an ice bath and the samples were analyzed by UV-Vis (Figure

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1b) and the average particle diameter by DLS (Figure 1c). As shown in Figures 1b and 1c, the

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SPR band (λ max ) initially lies around 570 nm giving a purple appearance of the solution, but over

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time it shifts smoothly to a constant value of 517 nm where the solution is red orange. These

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results are in agreement with the measurements of the average hydrodynamic diameter of the

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particles in the suspension by DLS as a function of time (Figure 1c). A monotonic decrease in

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particle size is observed: the diameter of the first particles is 10-15 times the diameter of the final

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particles. Our observations obtained from UV-Vis (and DLS) are similar to those described for


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Au(III) reduction by citrate i.e. both the position and intensity of the SPR band change with the

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reaction time which may suggest a similar mechanism in the reduction process. Such a behavior

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has been rationalized for the reduction of Au(III) by citrate in terms of the aggregative growth of

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Au NPs accompanied by a progressive decrease of the aggregates in size.19-22

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Transmission electron microscopy (TEM) was used to precisely analyze the size, size

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distribution, and shape of the resulting Au NPs synthesized at different pH. As shown in Figure

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2, the pH EDTA/HAuCl4 increase causes not only a reduction of the size of the Au NPs obtained but

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also an alteration of the NP shape. When the pH of the EDTA/HAuCl 4 mixture solution is less

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than 6.3, irregular Au NPs with the denticulate surfaces are obtained with a size of 100 ± 8 nm

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(Figure 2a and b). Elongated NPs with the major axis length of 38 ± 5 nm are obtained at

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pH EDTA/HAuCl4 of 6.3 (Figures 2c and d).

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Figure 2. TEM images of Au NPs prepared via EDTA reduction of HAuCl 4 at pH EDTA/HAuCl4 of

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4.5 (a,b), 6.3 (c,d), 7.2 (e), 9.1 (f) and 10.3 (g).


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When the pH EDTA/HAuCl4 is above 6.3, the shape of the resulting NPs becomes more regular with

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a size of 29 ± 2 nm at pH EDTA/HAuCl4 7.2 (Figure 2e) and 26 ± 2 nm at pH EDTA/HAuCl4 9.1 (Figures

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2f). High resolution TEM, selected area electron diffraction patterns, and X-ray diffraction

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patterns demonstrate that the resulting Au NPs are of polycrystalline and face-centered-cubic

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structure (Supporting Information, Figure S1). The plots of the NP hydrodynamic size and their

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SPR absorption maxima as a function of pH EDTA/HAuCl4 (Supporting Information, Figure S2a)

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follow the same master curve and show an inflection point at pH EDTA/HAuCl4 6.3, thus suggesting

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that the size reduction of the resulting NP with the pH EDTA/HAuCl4 increase is the major cause of

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the blue-shift of the NP SPR bands.

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It has been found that EDTA was able to effectively reduce HAuCl 4 to Au NPs at room

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temperature, which provided the advantage to assess the growth kinetics of Au NPs by

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monitoring the temporal evolution of the NP SPR band by means of in situ absorption

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spectroscopy (Figure 3) and the NP shape and morphology by ex situ time resolved TEM (Figure

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4).

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Figure 3. a) Plot of the intensity of the SPR absorption maxima of the Au NPs obtained via

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EDTA reduction of HAuCl 4 at room temperature and different pH versus the reaction time; b)


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Temporal evolution of the UV-Vis absorption spectra of the reduction media during EDTA

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reduction of HAuCl 4 at pH 7.2 and at room temperature.

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Figure 3a shows the evolution of the intensity of the SPR absorption maxima of Au NPs during

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the reaction at room temperature and different pH with time. It is seen that the kinetic curve

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exhibits in general three different ranges. The first range corresponds to an incubation period.

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The second range is connected with a fast increase of the number of gold particles in the

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solution. The third, which is the last one and depending on pH, is characterized by the slight

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decrease of absorbance, which is probably connected with the formation of bigger agglomerates

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and their sedimentation. It also clearly shows that the intensity increase kinetics of the SPR band

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of the resulting NPs is strongly correlated with the pH EDTA/HAuCl4 ; lowering the pH EDTA/HAuCl4

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significantly accelerates the intensity increase of the SPR band. For pH < 5.3 the reaction

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proceeds very quickly; it was complete in less than 10 min and the nanoparticles aggregated at

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the end of the reaction. For pH > 6.3 the reaction is slower; the kinetic of reduction decreased

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when the pH increased and the final nanoparticles were very stable to aggregation. For the

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reasons mentioned above, time resolved in situ UV-Vis absorption spectroscopy was used to

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study the mixture at different reaction stages when the pH was around 7.2 (Figure 3.b). As it can

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be seen in less than 1 min, the peak of AuCl 4 - usually present at about 320 nm disappeared and

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there was no absorption from 500 to 800 nm, thus the yellow solution turned colorless. A flat

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absorption pattern, which is assigned in the literature,20,23,24 to 2-D gold nanowire network

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formation appeared during the early reaction process, that is, at 4 and 64 min. However, before

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that the solution turned red orange, the flat spectrum disappeared at around 64 min and the

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plasmon band blue shifted (90 min). As shown respectively, from Figures 1b and 3b the time

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necessary to observe the formation of gold NPs decreased at 100°C compared to the one at room


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temperature. TEM images analysis showed a decrease in the average size of NPs when the

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temperature decreases. For instance when the synthesis were studied at pH around 4.5, we

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observed an increase of the NP diameter from 140 nm at 100°C to 207 nm at 60°C and 280 nm at

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room temperature. This is in agreement with UV-Vis data and with what would be predicted

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since it has been observed that a decrease in the temperature of the synthesis overall yields larger

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NPs.11,13,25

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Temporal evolution of the size and shape of the gold nanocrystals during the reaction was

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acquired using TEM, at pH = 7.2 and room temperature and compared to the results from UV-

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Vis (Figure 3).

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Figure 4. Temporal size/shape evolution (a-f) of gold nanocrystals during EDTA reduction of

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HAuCl 4 at pH 7.2 at room temperature.

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For the first step of the synthesis (Figure 4a) large area of randomly cross-linked Au

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networks can be easily observed. These results are in agreement with the flat absorption pattern

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observed in early stage of the reaction by UV-Vis. The average diameter of the cross-linked

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nanowires was 27±3 nm. The nanowires were formed by interconnection of nodule like

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structures as can be deduced from the high magnification image (inset of Figure 4a). After 24

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min the nanowires coagulated and larger nodules with an average diameter of 51±9 nm (inset

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Figure 4b) could be observed. Small spherical nanoparticles with an average diameter of 5±2 nm

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(red arrows on inset of Figure 4b) surrounding the dense nodules were also detected. With time

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the wires disappeared and individual particles with an average diameter of 30±7 nm were only

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observed on the TEM grids (Figure 4c). For a reaction time of 74 min, the large nanoparticles

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shrank into smaller and branched gold nanostructures (Figure 4d) that later divided into smaller

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particles (Figure 4e) with a high contrast in some parts of the structures (inset Figure 4e). After

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90 min, individual quasi-spherical and crystalline gold nanoparticles with an average diameter of

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19±2 nm, were mostly present, which agreed with the SPR observed around 522 nm by UV-Vis.

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The TEM results clearly demonstrate that the nucleation and growth of Au NPs using EDTA as a

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reducing agent do not follow the well-known LaMer nucleation-growth theory where nuclei are

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formed by collision of Au atoms.26 Similar aggregation and evolution steps were reported at the

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early stages of the synthesis of Au NPs by using citrate as a reducing agent in the Turkevich

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method,20,24,27 As initially proposed by Turkevich and later supported by others,19,25,28-30

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macromolecular complexes between dicarboxyacetone and Au(I) are formed which

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consecutively coagulate into colloidal stable precursor particles, in which a number of Au nuclei


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are formed by disproportionation of Au(I). Our results may suggest a similar mechanism of

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reduction with EDTA, i.e formation of macromolecular complexes of Au(I) that evolve

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consecutively to Au NPs.

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Less is known on the electrochemical properties of EDTA and there are only a few reports. 3,6 At

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high temperature the rapid evolution of the reaction during the first minutes of the synthesis was

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incompatible with the NMR time scale required to accumulate spectra with a good signal/noise

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ratio. In order to get more insights into the synthesis of gold NPs via EDTA oxidation and

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establish a possible mechanism for the reduction of Au(III) to Au NPs, deuterated solutions of

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EDTA and HAuCl 4 were mixed at room temperature. The sample was divided into two and

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monitored using in situ time resolved UV-Vis (Figure S3) and in situ time resolved 1H-NMR

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(Figure S4). Evolution of the UV-Vis spectra during the synthesis showed a slight increase of the

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absorption only 100 min after the beginning of the reaction. Morever, both UV-visible spectra

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and atomic absorption displayed no evolution after 15 h (900 min), showing that all Au (III) was

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consumed.

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In a first step, we have identified the different chemical species present at the end of the reaction,

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which allowed us in a second step to identify the other species effectively involved during the

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reduction of Au(III) to Au NPs. The reaction products obtained at the end of the reduction of

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tetrachloroaurate ion AuCl 4 - by EDTA were elucidated by a series of 1D-1H and -13C NMR

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experiments, by standard 2D-Correlation Spectroscopy (COSY), Nuclear Overhauser Effect

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Spectroscopy (NOESY), Heteronuclear Single Quantum Coherence (HSQC), Heteronuclear

22

Multiple Bond Correlation (HMBC) correlation spectra (See SI), and by diffusion experiments.

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According to the DOSY spectrum (see Figure 5 and inset) which was recorded using the BPP-

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LED-STE pulse sequence,18 four main compounds can be isolated, apart from the remaining


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EDTA (EDTA 1 and EDTA 2). An approximate power law relates the self-diffusion coefficient

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D with the inverse square root of the molecular mass, as reported for a large variety of

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compounds.31 From this law, molar masses around 210-220 (A), 170-180 (B), 90-100 (C) and

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45-50 (D) can be postulated. From

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spectrum (Figure S5, ESI), the C and D signals are readily assigned respectively to glyoxal

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monomer hydrate (G) or ethane-1,1,2,2-tetrol ((OH) 2 CHCH(OH) 2 Mw = 94 g mol-1) and formol

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hydrate or methanediol (F) (CH 2 (OH) 2 Mw = 48 g mol-1). A small amount of formic acid is also

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observed (δ1H = 8.15 ppm, δ13C = 165.6 ppm, D = 1.2 x 10-9 m2 s-1).

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C-DEPT, 1H-spectra and the 1H-13C-HSQC correlation

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Scheme 1. Suggested reaction scheme for EDTA oxidation by Au(III). EDTA: EthyleneDiamine

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Tetraacetic Acid; ED3A: EthyleneDiamine Triacetic Acid; S-EDDA: EthyleneDiamine Diacetic

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Acid; A: 2-[4-(carboxymethyl)-3-oxopiperazin-1-yl] acetic acid; EDA: EthyleneDiamine; G:

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Glyoxal; F: Formol hydrate.* Species not present at the end of the reaction. Colloidal gold

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nanoparticles are deposited at the bottom of the NMR tube.

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Now going from the high to the low fields, the proton spectrum (inset Figure 5) shows four

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distinct triplets (J ~ 7 Hz): a well-resolved one at 3.52 ppm (1), two overlapping at 3.70 (2) and

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3.71 ppm (3), and a last well-resolved triplet at 3.77 ppm (4). A COSY experiment (Supporting

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Information, Figure S6) reveals that the 3.77 ppm and 3.71 ppm triplets result from two adjacent

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methylene protons while the 3.52 ppm and 3.70 ppm triplets can be assigned to two other

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adjacent methylene protons. In addition, the DEPT spectrum indicates that all signals labeled 1,

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2, 3, 4 and a, b, c, d, e from high to low field are methylene protons. Finally, the diffusion

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experiment (Figure 5) shows that the protons 1, 2, a, and d belong to B while the protons 3, 4, b,

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c, and e are constituent parts of another distinct compound A. The NOESY and the 1H-13C-

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HMBC (Supporting Information, Figure S7 and S8) give additional evidence for the

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identification of the two compounds A and B.

13 14

Figure 5. 1H NMR spectrum and corresponding diffusion coefficients (FA: formic acid, G:

15

glyoxal, F: formol hydrate, HOD: Deuterium Hydrogen Oxide (semiheavy water) A: 2-[4-


15


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Page 16 of 28

2

(carboxymethyl)-3-oxopiperazin-1-yl] acetic acid, B: 2-(2-oxopiperazin-1-yl)acetic acid). *

3

Peaks disappeared in the final spectrum. 1D- and 2D-NMR spectra recorded at the end of the

4

synthesis and one-month later showed only minor modifications related to the intensity of the

5

signals (the corresponding peaks are marked with asterisks).

6

For instance, in A, strong to medium HMBC correlation between protons 4, c and e and a carbon

7

at 163.4 ppm indicates a carbonyl fragment in an oxopiperazine cycle. In contrast, the same

8

strong HMBC correlation between protons b and the carbon at 168.2 ppm in the same molecule

9

compound A can only result of a geminal 2J CH hydrogen coupling of a carboxylic carbon. Such

10

an assignment is also clearly corroborated by strong 3, b and 4, e and b, c NOESY dipole-dipole

11

interaction. All the relevant 1H-1H and 1H-13C cross-correlations are reported in Table 1.

12

The overall data are then consistent with the formation of A (2-[4-(carboxymethyl)-3-

13

oxopiperazin-1-yl] acetic acid) and B (2-(2-oxopiperazin-1-yl) acetic acid). Finally, from the

14

proton integration a tentative estimation of the respective amount of all the reactions products

15

can be done: EDTA~10%, A~B~G~10%, F~60% and a small amount of formic acid (FA < 3%).

16

The chemical shifts and the cross correlations are collected in table 1.

17

Table 1. Chemical shifts and cross correlations. Chemical shifts (ppm)a

Cross correlationsa

Atom number

δ(1H)

Integration δ(13C) (1H) HSQC

1

3.56

1.01

2

3.70

44.7

3

3.71

48.2

4

3.77

1.12

δ(13CO) HMBC

40.0

43.9

164.2 (sm)

163.4 (sm)


Others δ(13CO) HMBC

δ(1H) NOESY

a, 2

2, a

1, d

1, d

4, b, c

4 (br), b, c

3, e

3 (br), e

16

Page 17 of 28

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2


a

a

3.93

1.08

44.3

164.2 (st)

1

1

b

4.03

1.12

56.8

168.2 (st)

c

c

c

4.10

1.07

52.5

163.4 (st)

3

3, b

d

4.19

1.10

48.6

164.2 (m), 2 171.7 (st)

2

e

4.20

1.06

48.2

163.4 (m), 4 171.6 (st)

4

EDTA 1

3.38

0.87

51.0

EDTA 2

EDTA 2

3.90

1.96

55.3

EDTA 1

F

4.74

6.64

81.7

G

5.27

1.03

86.1

FA

8.15

0.45

165.6

sm = small; st = strong; m = medium; br = broad.

3

4 5 6

Figure 6. Evolution of the NMR intensities of the different species during the synthesis of the

7

NPs. For figure a), values for EDTA should be read on the right axis. S-EDDA:

8

EthyleneDiamine Diacetic Acid; A: 2-[4-(carboxymethyl)-3-oxopiperazin-1-yl] acetic acid.


17


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Page 18 of 28

2

In addition, the temporal progression of 1H-RMN spectra during the synthesis (Figure S4)

3

allowed to plot the evolution of the different species, in order to further investigate the

4

mechanism of oxidation of EDTA by Au(III) and confirm our hypothesis on the mechanism

5

proposed in scheme 1. Figure 6a and 6b, shows respectively the evolution of the species for short

6

and long time during the synthesis. As can be seen for the first stage of the synthesis (time < 100

7

min) 40% of EDTA was consumed, whereas no formation of NPs was evidenced by UV-Vis for

8

the same period of time (Figure S3). We believe that this is due to the establishment at this first

9

stage of the synthesis of Au(III)-EDTA complexes (denoted AuEDTA complex in Figure 6) by

10

interaction of EDTA with Au(III) square planar complex which evolves or rearranges into

11

another intermediate. Later, EDTA in this complex is oxidized very quickly to ethylendiamine

12

diacetic acid (S-EDDA) with elimination of formol. The presence of glyoxal and 2-[4-

13

(carboxymethyl)-3-oxopiperazin-1-yl] acetic acid (A) support the idea of fast formation and

14

consumption of (2-[carboxymethyl-[2-(carboxymethylamino)ethyl]amino]acetic acid) ED3A and

15

ethylenediamine (EDA). Indeed Johnson et al. have studied the electrochemical oxidation of

16

EDTA on Pt electrodes but they only speculated the distinct intermediates.3 On the other hand

17

Doran et al. have tried to identify diaminocarboxylic acids and related compounds by rapid paper

18

electrophoresis.32 It should be stressed that the intermediate A, identified in the present study has

19

only been postulated by Doran. Furthermore and according to Doran ED3A (2-[carboxymethyl-

20

[2-(carboxymethylamino)ethyl]amino]acetic acid) cannot be detected by paper electrophoresis in

21

solution since it is converted too rapidly into A. As for ethylenediammine (EDA), apart from its

22

small amount in solutions, it forms strong complexes with paramagnetic Au(III) ions, which may

23

hinder its detection by NMR.33,34 A similar mechanism involving Au(III) ions for the oxidation of

24

glycine has been suggested by Zou et al.35.


18

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2

NMR studies (scheme 1) support the hypothesis of the ability of EDTA to act as a reducing agent

3

toward Au(III). They also show that formaldehyde, a known reducing agent, is a by-product of

4

the reaction, that may take part in the nucleation and growth processes. In a first approximation if

5

formaldehyde is excluded as a reducing agent, twelve electrons can be released after complete

6

oxidation of EDTA as in scheme 1. It is thus expected that one molecule of EDTA will be able to

7

reduce four Au(III) to Au(0). Atomic absorption spectroscopy of the supernatant after

8

centrifugation of the NPs at the end of the NMR experiments showed the absence of ionic gold

9

species indicating that the reaction was complete for gold. However, EDTA is not consumed

10

completely at the end of the synthesis and is the limiting reactant (initial molar ratio of

11

Au(III)/EDTA = 2.8 instead of 4). EDTA can not be the only responsible for the reduction of

12

Au(III) and other species such as formaldehyde contribute to the formation of gold nanoparticles.

13

In addition, the UV-Vis spectroscopy sustains the idea that the reduction of Au(III) proceed via

14

Au(I) and macromolecular complexes are formed during the formation of Au NPs as shown in

15

Figure 3b.

16

Goia and Matijevic,36,37 have demonstrated that AuCl 4 - ions can be hydrolyzed into AuCl 3 (OH)-,

17

AuCl 2 (OH) 2 -, AuCl(OH) 3 -, and Au(OH) 4 - with the pH increase in aqueous solution, and their

18

reactive activity decreases in the following sequence: AuCl 4 - > AuCl 3 (OH)- > AuCl 2 (OH) 2 - >

19

AuCl(OH) 3 - > Au(OH) 4 -. Giving the increase in reactivity of auric species, the different states of

20

protonation and stabilization of EDTA with pH, and the results from UV-Vis, TEM, and NMR, a

21

scheme with the different reactions that can occur during the nucleation and growth of Au NPs

22

using EDTA as a reducing agent can be postulated.

23

The first step is the reduction of Au(III) to Au(I) together with an oxidative decarbonylation of

24

EDTA presumably followed by the formation of macromolecular complexes by coordination of


19


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Page 20 of 28

2

gold species and ligands coming from the oxidation of EDTA (see scheme 1.) Macromolecular

3

complexes can consecutively coagulate into colloidal stable precursor particles, in which a

4

number of Au nuclei are formed by disproportionation of AuCl. The auric ions resulting from the

5

disproportionation are then reduced by the intermediate or S-EDDA, leading to the secondary

6

nucleation and in turn broadening the size distribution of Au NPs.

7

Formaldehyde, which is a by-product of the different decarbonylation reactions, is a strong

8

reducing agent that can induce a third nucleation during the synthesis of Au NPs. However, the

9

ability of formaldehyde to reduce Au(III) will be dependant on a concentration threshold in the

10

solution that is kinetically limited by its production generated by decarbonylation of EDTA.

11

Moreover and as documented, the reducing ability of formaldehyde depends on pH.38 In acidic

12

solutions formaldehyde exists mostly as formol hydrate HO-CH 2 -OH that limits its ability as a

13

reducing agent and thus its contribution to the nucleation and growth of gold nanoparticles

14

during the reduction by EDTA. These results are consistent with the high proportion of formol

15

(compound F, scheme 1) detected by NMR in the solution at the end of the reaction. Thus at low

16

pH, the nucleation and growth of Au NPs is governed by decarbonylation reactions of EDTA and

17

disproportionation of Au(I) and the formation of Au NPs is fast due to the presence of high

18

reactive auric species. Since there are only HAuCl 4 and EDTA present in the reaction solution

19

and the oxidation of EDTA is dominated by decomposition of its carboxyl groups one by one to

20

CO 2 , Au NPs should be stabilized predominantly by the excess of EDTA. To stabilize Au NPs,

21

we hypothesize that EDTA anchors either or both of its two tertiary amine groups, on the

22

surfaces of Au NPs while exposing its four-carboxyl groups to the surrounding media. A similar

23

binding mechanism can also occur during the synthesis with S-EDDA, but is less likely for the

24

cyclic products A and B due to steric hindrance of the cycle. Because the four carboxyl groups of


20

Page 21 of 28

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2

EDTA have pKa values of 2.0, 2.7, 6.2 and 10.3 respectively all of them remain deprotonated

3

(negatively charged) in pH > 3, thus rendering the surfaces of the resulting Au NPs highly

4

negatively charged. This is in line with the fact that all of the resulting NPs have large negative

5

zeta-potential, which decreases from -30 mV to -65 mV with increase of pH EDTA/HAuCl4

6

(Supporting Information, Figure S2b). Since the pKa values of the two tertiary amine groups of

7

EDTA are 6.1 and 10.4, the two-amine groups are protonated at pH < 6.1. It should be plausible

8

that the bonding affinity of the protonated amine group to the surfaces of Au NPs is rather weak,

9

so EDTA is less efficient to stabilize Au NPs. In this scenario, we expect fast reduction of

10

HAuCl 4 to tiny crystallites at pH EDTA/HAuCl4 < 6.1 and, at the same time, fast agglomeration of

11

these crystallites, which can usually result in large polycrystalline NPs with uniform sizes.36,37

12

Although Ostwald ripening (during boiling of the reaction media) efficiently transferred the

13

aggregates of tiny NPs to large polycrystalline NPs (Supporting Information, Figure S9), their

14

denticulate surface morphology gave a strong implication of agglomeration of tiny NPs (Figure

15

2b).

16

In alkaline medium when the pH > 6.3, formaldehyde is active and can participate to the

17

reduction of Au(III) to Au(I). Thus a fast reduction of Au(III) to Au NPs is expected to occur for

18

pH > 6.3. However as observed from the kinetic analysis of the UV-Vis spectrum during the

19

formation of Au NPs at different pH (Figure 3a), the reduction process is slow in alkaline

20

medium and the size of the final nanoparticles decreases. These results may be explained i) by a

21

slow production of formaldehyde that is kinetically limited by a slow decarbonylation of EDTA

22

due to the presence of low reactive auric Au(OH) 4 - ions, ii) by a concentration threshold of

23

formaldehyde that has to be reached in order to allow its participation to the reduction process,

24

and iii) the high colloidal stability of the formed nanoparticles in an early stage of nucleation and


21


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Page 22 of 28

2

growth due the high charge of EDTA molecules anchored on the surface of the NPs which avoid

3

their aggregation to larger structures. It is also important to note that dispersions of Au NPs

4

synthesized in alkaline medium were stable for more than 4 years compared to 1 year in acidic

5

medium (pH < 4). The polycrystalline character of the NPs obtained at pH EDTA/HAuCl4 from 6.3 to

6

9.1 underlines that they are formed as a result of agglomeration of tiny crystallites. Since this

7

pH EDTA/HAuCl4 range is in between the two pKa values of the tertiary amine groups of EDTA, one

8

of the two tertiary amine groups is not ionized and can strongly bind to the Au NP surface, thus

9

largely improving the stabilization efficiency of EDTA to prevent agglomeration of tiny

10

crystallites upon EDTA reduction of HAuCl 4 . According to Goia and Matijevic,36,37 on the other

11

hand, the increase of pH EDTA/HAuCl4 from 6.3 to 7.2, 8.4, and 9.1 transformed the more active

12

AuCl 4 - ions to less active AuCl 3 (OH)-, AuCl 2 (OH) 2 -, and AuCl(OH) 3 - ions. Lowering the

13

activity of auric ions obviously slows down production of Au NPs as can be seen from UV-vis

14

(Figure 3a). As such, the numbers of tiny crystallites formed upon EDTA reduction of HAuCl 4

15

are considerably reduced. Taken together, the aggregates of the crystallites and the NPs derived

16

thereof are expected smaller. Although the dominant auric precursor should be the least active

17

Au(OH) 4 - ions at pH EDTA/HAuCl4 > 10.3, the resulting Au NPs are slightly larger than those

18

obtained at pH EDTA/HAuCl4 9.1 (Figure 2f and 2g). Owing to their pKa values, both two tertiary

19

amine groups of EDTA tend to attach on the Au NPs surfaces at pH EDTA/HAuCl4 > 10.3. Since it

20

consumes more space to anchor two amine groups than one, the density of EDTA anchored on

21

the NPs at pH EDTA/HAuCl4 > 10.3 should be slightly lower than at pH EDTA/HAuCl4 9.1, which is also

22

indicated by the increase of the zeta-potential of the resulting Au NPs (Supporting Information,

23

Figure S2b). As such, EDTA stabilization at pH EDTA/HAuCl4 > 10.3 is less efficient than at

24

pH EDTA/HAuCl4 9.1, thus leading to slightly larger Au NPs.


22

Page 23 of 28

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2

In summary we have investigated for the first time the synthesis of gold nanoparticles

3

from HAuCl 4 by using EDTA as a reducing agent by using different techniques such as UV-Vis,

4

TEM, DLS. The size of the resulting Au NPs can be readily tuned from 25 nm to 100 nm by

5

varying the pH of the reaction media. Time resolved TEM revealed the presence of nanowires

6

that progressively increases in size at the same time they fragmented into small segments before

7

the final spherical particles are formed.

8

intermediates and the final products resulting from the reduction of Au(III) by EDTA. In view of

9

the NMR results, the formation of colloidal gold NPs was discussed and a scheme was drawn to

10

explain the role of EDTA as a reducing agent in the nucleation and growth of Au NPs. Our

11

results showed that Au(III) reduction to Au NPs by EDTA is accompanied by formaldehyde

12

production that may interfere in the reduction process and contribute to the nucleation and

13

growth of colloidal gold nanoparticles depending on the pH. Beside the mechanistic aspect,

14

many synthetic routes concerned with the elaboration of nanoshells and core-shell metal

15

nanoparticles use a large excess of the toxic formaldehyde as a reducing agent.39,40 Given the

16

interest in biomedical applications of these metallic nanomaterials, and the important role of

17

formaldehyde as a reducing agent to obtain these nanomaterials, using EDTA as a reducing agent

18

can be a good starting point for minimizing or replacing formaldehyde use in the synthesis.

19

Furthermore, the carboxyl groups of the EDTA coating on as-prepared Au NPs also provide

20

immense flexibility for further conjugation with functional molecules to diversify the surface

21

functionality and in turn broaden the application spectrum of the Au NPs. All these features

22

make EDTA a unique reducing agent as compared with currently available ones.

NMR studies were used to identify the reaction

23 24


23


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2

ASSOCIATED CONTENT

3

Supporting Information. Additional figures, TEM, HRTEM images, dynamic light scattering,

4

time resolved UV-vis spectroscopy, time resolved 1H-RMN spectra electron diffraction pattern

5

and XRD. Detailed experimental procedures and NMR (HMBC, HMC, HSQC, COSY and,

6

NOESY), spectra are available free of charge via the Internet at http://pubs.acs.org.

7

AUTHOR INFORMATION

8

Corresponding Author

9

Page 24 of 28

*E-mail: [email protected]. Phone: (+33) 1-44-27-31-74. Fax: (+33) 1-44-27-36-75.

10

ACKNOWLEDGMENT

11

We thank Isabelle Correia, Émeric Micelet, and Olivier Lequin from Laboratoire des

12

Biomolécules (UPMC) for their help with the NMR measurements.

13

ABBREVIATIONS

14

EDTA, EthyleneDiamineTetraAcetic acid; ED3A, EthyleneDiamine Triacetic Acid or 2-

15

[carboxymethyl-[2-(carboxymethylamino)ethyl]amino]acetic acid; EDA, EthyleneDiamine; NPs,

16

nanoparticles; S-EDDA: EthyleneDiamine Diacetic Acid.

17 18

REFERENCES

19 20 21 22 23 24 25 26 27

(1) Field, T. B.; Mcbryde, W. A. E. Apparent Stability-Constants of Proton and Metal-Ion Complexes of Glycine, Iminodiacetic Acid, Nitrilotriacetic Acid, and Triethylenetetramine in Aqueous Methanol. Canad. J. Chem. 1981, 59, 555-558. (2) Danil de Namor, A. F.; Alfredo, P.; Tanaka, D. Thermodynamics of Protonation and Complexation of EDTA Derivatives and Metal Cations in Water. J. Chem. Soc., Faraday. Trans. 1998, 94, 3105-3110. (3) Johnson, J. W.; James, W. J.; Jiang, H. W.; Hanna, S. B. Anodic-Oxidation of Ethylenediaminetetraacetic Acid on Pt in Acid Sulfate Solutions. J. Electrochem. Soc. 1972, 119, 574-&.


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TABLE OF CONTENTS

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On the Synthesis of Au Nanoparticles Using EDTA as a Reducing Agent

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