Polymer inclusion membranes as substrates for controlled in-situ gold nanoparticle synthesis

Reactive and Functional Polymers 130 (2018) 81–89

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Polymer inclusion membranes as substrates for controlled in-situ gold nanoparticle synthesis Colin Spechta, Robert W. Cattralla, Tony G. Spassovb, Maya I. Spassovab, Spas D. Koleva, a b

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School of Chemistry, The University of Melbourne, Victoria 3010, Australia Sofia University “St. Kl.Ohridski”, Faculty of Chemistry and Pharmacy, 1 James Bourchier Blvd., 1164 Sofia, Bulgaria

A R T I C LE I N FO

A B S T R A C T

Keywords: Polymer inclusion membrane (PIM) Gold nanoparticles Aliquat 336

Poly(vinyl chloride) (PVC)-based polymer inclusion membranes (PIMs) containing the commercial anionic extractant Aliquat 336 and in some cases also 1-dodecanol as plasticizer were used for the fabrication of PIM surface-confined Au nanoparticles (NPs) by reduction of Au(III), extracted into the membranes as the [AuCl4]− complex. The experimental conditions controlling Au NP size and distribution were studied using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The former technique showed unique NP distributions dependent on the reducing agent used while XRD data were found to be consistent with those obtained by wideangle X-ray scattering (WAXS) and revealed that the Au crystallite size decreased when the reduction temperature, reduction time, or reducing agent strength were increased. Conditions for producing PIM supported Au NPs that could be appropriate for chemical sensing or catalytic applications and with an acceptable thermal stability, based on thermogravimetric analysis (TGA) measurements, were established. Loaded to saturation with Au(III) PVC-based PIMs containing 20 wt% Aliquat 336 and 10 wt% 1-dodecanol were found to be suitable for producing dense Au NP layers which could be appropriate for catalytic applications. Partially loaded with Au(III) PVC-based PIMs without a plasticizer and containing 30 wt% Aliquat 336 allowed the fabrication of discrete disperse Au NPs on the membrane surface which could be expected to be suitable for sensing applications. This study demonstrates that PIMs are attractive low-cost substrates for the synthesis and immobilization of Au NPs of controlled size, density and shape which can potentially be used in catalytic and chemical sensing applications.

1. Introduction Metal nanoparticles (MNPs) have been a particular area of interest within the field of nanoscience with great potential for applications such as chemical sensing or catalysis [1, 2]. For chemical sensing applications, the unique properties exhibited by MNPs differ from those of the bulk material and can be highly sensitive to the surrounding environment [3]. For catalytic applications, MNP catalysts show superior reaction selectivity and surface area to volume ratios in comparison to traditional bulk metal catalysts [4, 5]. One of the challenges in using MNPs for practical applications is immobilizing them on a suitable substrate. Of the common substrates, polymer based supports are convenient, easily available and can often improve NP stability by eliminating problems associated with NP agglomeration [6, 7]. Polymer supported MNPs can be produced ex situ by use of colloidal techniques followed by subsequent deposition onto polymer supports [8]. This approach has the advantage of direct

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Corresponding author. E-mail address: [email protected] (S.D. Kolev).

https://doi.org/10.1016/j.reactfunctpolym.2018.06.005 Received 11 December 2017; Received in revised form 4 June 2018; Accepted 13 June 2018 1381-5148/ © 2018 Elsevier B.V. All rights reserved.

application of the extensive knowledge on solution-based NP synthesis already available in the literature. However, in-situ synthesis of MNPs, i.e., synthesizing directly on the substrate, simplifies the process and allows synthesis of MNPs on a range of surfaces that can be coated with the polymer substrate. Towards this end some relatively recent research on the use of polymer inclusion membranes (PIMs) for the synthesis of Au NPs is of particular interest [9–11]. PIMs are hydrophobic films and separation based on these membranes has emerged as an attractive alternative to conventional solvent extraction or separation involving supported liquid membranes [12, 13]. PIMs have been used for the selective extraction of chemical species such as metal ions from aqueous solutions [12–16]. These membranes consist of a base polymer, an extractant (often referred to as carrier) and a plasticizer or modifier as needed. As metal ions can be readily extracted and then subsequently reduced to synthesize MNPs, PIMs can be used for in-situ synthesis of MNPs. The utilization of PIMs for such applications has only recently been explored, with initial work

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solutions. High molecular weight PVC (Sigma-Aldrich) was used as the basepolymer for membrane casting. Aliquat 336 (a mixture of quaternary alkylammonium chlorides, Sigma-Aldrich) with ≥88.5% purity due to the presence of octanol and dodecanol was used for extracting the [AuCl4]− complex into the PIMs studied. 1-Dodecanol (Sigma-Aldrich, ≥98% purity) was used as a plasticizer for some of the PIMs. Tetrahydrofuran (THF) (VWR, ≥99.7% purity) was used for preparing the PIM casting solutions. Gold(III) chloride trihydrate (Sigma-Aldrich, ≥99.9% purity) stock solutions were used for the preparation of the Au(III) feed solutions after appropriate dilution. Nitric acid (70 wt%, Ajax Chemicals) was used in the preparation of Au(III) feed solutions while NaOH (pellets, Chem-supply, ≥97% purity) was used for adjusting the pH of ethylenediaminetetraacetic acid (EDTA) reduction solutions. The following reducing agents were used in this study: disodium salt of EDTA (Chem-supply, ≥97% purity), tri‑sodium citrate dihydrate (VWR, ≥99% purity), sodium borohydride (Scharlau, ≥98% purity), oxalic acid (BDH chemicals, ≥99.5% purity), L-ascorbic acid (SigmaAldrich, ≥99% purity), citric acid (Chem-supply, ≥99% purity), hydrazinium sulfate (Ajax Chemicals, ≥98% purity), hydroxylammonium sulfate (Ajax Chemicals, ≥98% purity), and triethylamine (SigmaAldrich, ≥99% purity).

by Kumar et al. [9] using cellulose triacetate (CTA)-based PIMs. This study was followed by more extensive research by Bonggotgetsakul et al. [10, 17, 18] using poly(vinyl chloride) (PVC)-based PIMs for the synthesis of Au, Ag or Pd NPs predominantly on the membrane surface. This was achieved by selecting reductants which could not be readily extracted into the corresponding membranes from the reductants' solutions. In this case the extracted into the membranes metal cations (Au (III), Ag(I) or Pd(II)) could be reduced only on the corresponding membranes' surfaces where they formed Au, Ag or Pd NPs. The concentration gradient within the membranes, generated in this way, induced diffusional mass transport of metal cations from the interior of the membranes towards their surfaces. The effect of important experimental conditions such as the loading time, determining the extent of membrane loading with metal ions, and the reduction time during which the membrane was exposed to the reductant's solution were investigated. Most recently work by Mora-Tamez et al. [11] explored the use of several different types of membranes including CTA-based PIMs for simultaneous extraction and in-situ reduction to produce supported Au NPs. This work also demonstrated that these Au NPs were catalytically active in the reduction of 4-nitrophenol. As cheap, optically transparent, easily handled, simple to produce, plastic-like solids that can concentrate large quantities of metal ions, PIMs have significant potential for surface synthesis and immobilization of MNPs. The Au NP in-situ synthesis occurs entirely on or within the PIM and variations in membrane composition, metal ion loading, and subsequent reduction conditions influence the morphology and size of the supported NPs produced. In order to use this system for practical applications it is necessary to understand the effects of these variations throughout the synthesis process. In the research of Bonggotgetsakul et al. [10, 17, 18] the role of loading temperature, loading time, reduction temperature, reduction time, and reducing agent were preliminarily explored by examining the resulting Au, Ag and Pd NP morphology using scanning electron microscopy (SEM). However, as an initial exploratory approach working within the limitations of SEM for evaluation of particle size, the research mentioned above was unable to establish the appropriate experimental conditions for producing Au NP coated PIMs tailored towards chemical sensing or catalytic applications. Instead past work was only focused on producing surface bound NPs. Furthermore, previous work did not address the correlation between trends in Au NP synthesis on PIMs and Au NP synthesis by traditional solution-based methods. The purpose of this study was to gain a better understanding how various experimental conditions control the process of Au NP synthesis on PVC-based PIMs by the use of a suite of surface characterization techniques providing complementary information. SEM was heavily utilized for qualitative estimation of Au NP size, as well as evaluation of Au NPs distribution and shape. However, in this study, unlike in previous research, SEM results were complemented with atomic force microscopy (AFM) and X-ray powder diffraction (XRD) analysis, thus significantly improving the reliability of the conclusions made. AFM was explored for comparison to other techniques and showed good agreement with SEM results but was ineffective for dense Au NP layers and rough surfaces. Use of XRD allowed a more systematic evaluation of Au crystallite size across different conditions. As has been noted elsewhere [19] multiple characterization techniques should be utilized in order to understand systems of this complexity. By outlining both the key synthesis parameters and their relative effects on Au crystallite size this study serves as a starting point for selection of appropriate PIM supported Au NPs synthesis conditions towards future applications.

2.2. PIM manufacturing PIMs consisting of 70 wt% PVC, 20 wt% Aliquat 336 and 10 wt% 1dodecanol or 70 wt% PVC and 30 wt% Aliquat 336 were prepared by a casting method reported in a previous study [10]. In this method the membrane components were dissolved in a small volume (e.g., 8–10 mL) of THF for the preparation of the PIM casting solutions. Each PIM casting solution was poured into a glass ring with a diameter of 7.5 cm which was positioned on a flat glass plate. The ring was covered with a watch glass to allow gradual evaporation of THF over 24 h. The PIMs fabricated by this method were optically transparent and homogenous as reported in previous studies (i.e., [10, 20] for PIMs containing both Aliquat 336 and 1-dodecanol and [21] for PIMs containing only Aliquat 336) and had an average thickness of 30 ± 5 μm. Circular sections of 2.5 cm in diameter and an average mass of 30 mg were cut from the PIMs and used in the experiments. 2.3. Au NP synthesis Au NPs were synthesized by the procedure proposed by Bonggotgetsakul et al. [10] which involved extraction of Au(III) into a PIM for a predetermined period of time (loading time) while stirring the feed solution. The feed solution used for 2.5 cm diameter PIM circular sections was 50 mL and consisted of 0.5 mol L−1 HCl and Au(III), with a concentration of 100 mg L−1 in most experiments. This was followed by immersion of the 2.5 cm in diameter circular section loaded with Au(III) into a 50 mL of a mechanically stirred solution containing a suitable reductant (usually in concentration of 0.1 mol L−1) capable of reducing Au(III) to metallic Au for a predetermined period of time (reduction time). Table 1 lists all the loading and reduction conditions used in the present study. It should be noted that this table does not present the conditions for the individual experiments. 2.4. Instrumentation All SEM images included in this work were obtained using an FEI Quanta 200 microscope. Secondary electron imaging mode under high vacuum with an Everhart-Thornley detector was used for all NP samples. Due to the presence of a surface coating of Au NPs, conductive sputter coating of samples prior to imaging was unnecessary, with the exception of images of fresh PIMs. Size estimation by SEM was done by manual measurement using ImageJ software with BioFormats plugin of

2. Experimental 2.1. Reagents All reagents were used as received. Deionized water (18.2 MΩ cm, Synergy 185, Millipore) was used for the preparation of all aqueous 82

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3. Results and discussion

Table 1 Loading and reduction conditions for PIM containing 70 wt% PVC, 20 wt% Aliquat 336 and 10 wt% 1-dodecanol or 70 wt% PVC and 30 wt% Aliquat 336. Au(III) loading conditions

The experimental parameters are discussed in the order of the magnitude of their influence on Au NP size.

Reduction conditions

Au(III) loading time (min)

Au(III) concentration in feed solution (mg L−1)

Reduction temperature (°C)

Reduction time (min)

5 10 60 120 240 360 1440

5 10 20 50 100

15 20 25 30

60 120 240 360 1440

3.1. Trends in Au NP size While SEM provides an invaluable insight into the changes in morphology of the Au NPs as synthesized on a PIM surface, processes such as clustering, agglomeration and surface roughness limit the reliability of SEM in consistently evaluating the size of the individual Au crystallites. Therefore, powder XRD peak broadening was used to evaluate how the changes under particular synthesis conditions influenced the Au crystallite size and to determine the resulting trends. The absolute size values derived from XRD measurements are not intended as independent quantitative measures of the Au NP size, however, in conjunction with SEM measurements they allow both a reliable qualitative Au NP size estimation and the formulation of guidelines for the selection of suitable experimental conditions for producing Au NPs that might be appropriate for a desired application such as chemical sensing or catalysis. A comparison of size values obtained with XRD, SEM, and AFM can be seen in Table 2. While SEM, AFM and XRD show reasonable agreement at low Au particle density, as that density increases SEM resolution limits the ability to distinguish between particles and individual crystallites while AFM is confounded by the increasing roughness and multiple surface layers. As XRD provides consistent bulk analysis of the crystallite size rather than particle size, it is not influenced by increasing Au NP density. XRD crystallite size estimates were consistently lower than either SEM or AFM estimates of particle size even in cases of low Au surface density. This is expected as analysis based on the Scherrer equation provides a lower bound in crystallite size, appropriate for qualitative evaluation of trends across conditions. The crystallite peak broadening values obtained by a laboratory XRD instrument were also compared to those obtained from a synchrotron WAXS beamline to investigate the magnitude of instrument contributions to the XRD-based crystallite size estimates. As can be seen in Table 3, the comparison showed good agreement between synchrotron WAXS and laboratory XRD, with the laboratory XRD crystallite size estimates generally 10–20% lower than the synchrotron WAXS crystallite size estimates due to the instrument contributions to peak broadening. This is consistent with the previously noted differences between XRD crystallite size estimates and SEM or AFM size estimates at low Au surface density.

30 randomly chosen particles per image for several images per estimate. X-ray powder diffraction (XRD 3000, Seifert) using a Co-Ka radiation (wavelength of 1.7889 Å) was applied for the microstructural characterization of the PIM supported Au NPs. Peak analysis was performed using Lorenztian fitting of the diffraction pattern with IgorPro software (Wavemetrics). The integral breadth peak value was then used to qualitatively assess average Au NP crystallite size and compare trends in NP size produced under varying conditions with application of the Scherrer equation [22, 23]. Thermogravimetric analysis was performed by using Perkin Elmer Diamond TG/DTA with a scanning rate of 10 K/min. Dry nitrogen was used as the purge gas at a flow rate of 20 mL min−1. Membrane samples were dried and finely cut with a scalpel for packing in the TGA sample pan. Typical sample mass per run was 20–30 mg. Visible spectra of membranes over the course of reduction were obtained using an UV/Vis spectrophotometer (Libra S12, Biochrom) in absorbance wave-scan mode. Quartz cuvettes containing Au(III) loaded membranes in a reductant's solution were scanned hourly over the course of 24 h. AFM measurements were made on a Cypher S high resolution AFM microscope. Both tapping mode and contact mode were utilized with results presented in this study from contact mode measurements. Mapping area of 20 × 20 μm was acquired over an 8 min scan time. Size estimates from AFM were based on the use of instrumental automated particle analysis software using the average z directional height of variation in the particle surface. All wide-angle X-ray scattering (WAXS) experiments were carried out at the SAXS/WAXS beamline of the Australian Synchrotron using a photon energy of 20 keV with pinhole geometry and offset Pilatus 1 M detector. Scattering measurements were performed on free-standing membranes with primary beam dimensions of the sample of 250 μm × 150 μm and an exposure time of 5 s. Absolute intensity calibration was carried out using a glassy carbon standard and q-calibration was performed using a silver behenate standard. Initial data processing including q-calibration, intensity calibration, normalization to beamstop intensity, averaging and background subtraction were carried out using Scatterbrain software. Analysis of the WAXS pattern was performed using the Irena package of Igor Pro software, with automated conversion of q data to theta range and Lorentzian peak fitting. As with the XRD data, the integral breadth peak value from the Lorentzian peak fitting was then used to estimate the Au NP crystallite size with application of the Scherrer equation for comparison to XRD values. Oxidation-reduction potential (ORP) measurements were performed using an Ionode IJ 64 ORP electrode connected to TPS smartCHEM-lab analyzer. All reductants were dissolve in deionized water to produce solutions of 0.1 mol L−1 concentration and the reduction potential measurements were performed immediately following solution preparation.

3.2. Reduction time Reduction time is one parameter which has a dramatic effect upon both the size and morphology of the resulting PIM supported Au NPs. In order to better understand the process of Au NP formation and their growth on PIMs, varying reduction times in both 0.1 M EDTA and NaBH4 solutions were evaluated. It has been previously concluded that EDTA reduction time does not affect particle size [10], however, Table 2 Comparison of size estimates of AuNPs fabricated on PVC-based PIMs after varying reduction times as obtained by XRD, SEM and AFM. PIMs (20 wt% Aliquat 336 and 10 wt% 1-dodecanol) were loaded for 24 h in 100 mg L−1 Au (III), 0.5 M HNO3 feed solution and reduced for 24 h in 0.1 M EDTA solution. Reduction time (min)

60 120 1560

83

Size estimate (nm) XRD

SEM

AFM

33.1 30.5 16.4

43.8 ± 9.4 60.5 ± 21.1 94.3 ± 24.1

43.0 ± 8.4 77.9 ± 20.0 104 ± 23

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Table 3 Comparison of crystallite size estimates of AuNPs fabricated on PVC-based PIMs after varying loading times as obtained by laboratory XRD and synchrotron WAXS. PIMs (20 wt% Aliquat 336 and 10 wt% dodecanol) were loaded for 60, 120, 240 or 1560 min in 100 mg L−1 Au(III), 0.5 M HNO3 feed solution and reduced for 24 h in 0.1 M EDTA solution. Load time (min)

Laboratory XRD

Synchrotron WAXS

60 120 240 1560

13.3 15.8 16.9 16.4

14.2 17.5 19.2 19.9

Fig. 2. Visible absorbance spectra of a PVC-based PIM loaded with Au(III) and immersed in a 0.1 M EDTA reduction solution at different reduction times. Remaining experimental conditions as in Fig. 1.

Due to the slow reduction rate of Au(III) by EDTA, insufficient Au crystal formation at short reduction times limited the reliability of the measurements at reduction times under 1 h. To compare these results with a more traditional and stronger reducing agent that rapidly formed Au NPs, Au(III) loaded PIMs were immersed in 0.1 M NaBH4 solutions for varying reduction times and Au NP size measurements using XRD were conducted. As can be seen in Fig. 3 these results show a similar trend in crystallite size, though significantly more pronounced than in the case of EDTA. As was the case with EDTA reduction, the decrease in crystallite size over the course of reduction was indicative of new Au crystal formation throughout the course of reduction. The NaBH4 results differ from those obtained by typical solutionbased methods in which NaBH4 is used as the reducing agent as these ex-situ methods show an increase in particle size at longer reduction times [25, 26]. Two possible reasons are most likely responsible for this difference and both relate to the increasing density of the Au particles on the PIM surface. This increasing density may result in spatial limitation of crystal growth, preventing later crystal growth from occurring to the same degree as during the initial nucleation phase. Alternatively, the increasing density of Au NPs may help promote nucleation of new crystals to a greater degree than the growth of existing ones, something that has been noted with the addition of a strong reducing agent in solution-based synthesis [27].

Fig. 1. Effect of the reduction time in 0.1 M EDTA solution at 25 °C on XRD estimated crystallite size of AuNPs fabricated on a PVC-based PIMs containing 20 wt% Aliquat 336, 10 wt% 1-dodecanol which was loaded for 24 h in 100 mg L−1 Au(III) solution containing 0.5 HNO3.

average crystallite size, defined as the effective crystallite diameter estimated by the size of domains coherently scattering X-rays, as seen in Fig. 1, shows a clear trend of decreasing crystallite size with increasing EDTA reduction time. For both EDTA and NaBH4 reduction experiments PIMs composed of 70 wt% PVC, 20 wt% Aliquat 336 and 10 wt% 1dodecanol were loaded for 24 h in 100 mg L−1 Au(III) solutions containing 0.5 M HNO3 prior to reduction. These results correlate well with those of solution-based Au NP synthesis involving EDTA reduction of Au(III) [24]. Furthermore, this suggests that nucleation of new Au NPs must occur throughout the process of reduction as only growth of existing seeds would not decrease average crystallite size. This is likely a result of the complexity of the mechanism by which EDTA acts as a reducing agent. In the work by Dozol et al. [24] it was suggested that the mechanism was multistep including formation of Au(I) and its subsequent reduction to Au(0) with the oxidation of EDTA generating products that included formaldehyde, a stronger reducing agent than EDTA itself. As such, initial Au(III) reduction would be controlled by EDTA (which was in significant excess), with contribution from formaldehyde as sufficient concentration was reached in the reduction solution. Stronger reducing agents typically produce smaller nanoparticles, thus increase in the reduction strength of the solution over the course of reduction could explain the decrease in NP size reflected in the XRD results. A simple method for evaluating the trend in particle size over the course of reduction was based on utilizing the characteristic surface plasmon resonance (SPR) peak of the Au NPs. By continuous monitoring of the SPR absorbance around 540 nm a clear blue shift was observed over the course of reduction (Fig. 2), corresponding to increasingly red Au NPs characteristic of decreasing particle size. As PIMs were optically transparent this monitoring could be performed directly on an Au(III) loaded PIM that was immersed in the reductant's solution.

Fig. 3. Effect of NaBH4 reduction time on crystallite size of AuNPs fabricated on the surface of PVC-based PIMs. Remaining experimental conditions as in Fig. 1. 84

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Fig. 5. Effect of loading time in 100 mg L−1 Au(III) solutions on crystallite size of AuNPs fabricated on the surface of PVC-based PIMs with (♦) and without (●) 1-dodecanol as plasticizer. Plasticized PVC-based PIMs contained 20 wt% Aliquat 336 and 10 wt% 1-dodecanol. Non-plasticized PVC-based PIMs contained only 30% wt% Aliquat 336 and no 1-dodecanol. Remaining experimental conditions as in Fig. 1.

Fig. 4. Effect of reduction temperature on crystallite size of AuNPs fabricated on the surface of PVC-based PIMs. Remaining experimental conditions as in Fig. 1.

3.3. Reduction temperature The effect of reduction temperature on the resulting PIM supported Au NPs was explored from 10 °C to 35 °C but below 15 °C the results were found to be unreliable because of the very low XRD signals obtained due to insignificant reduction of Au(III). Above 30 °C the reduction process was not repeatable. Therefore, more detailed studies were conducted in the 15 °C–30 °C temperature rage, and the XRD results were compared to those obtained by SEM. Reduction temperature had a particularly significant effect on the process of NP formation as it altered the rate of diffusion of the Au(III) from the membrane interior towards its surface in addition to the reduction reaction rate. The XRD results (Fig. 4) confirmed a general trend of decreasing crystallite size with increasing reduction temperature, which is in agreement with solution-based Au NP synthesis results [28, 29]. With the exclusion of some catalytic applications which might favor a larger Au NP size [30, 31], the higher reduction temperature is favorable for both sensing and most catalytic applications. Most catalytic applications favor a dense layer of small NPs in order to maximize the reactive surface area [32–35], something effectively accomplished with higher temperature reduction. While lower reduction temperatures produced incomplete surface coverage, the clustering and agglomeration of the Au NPs produced were expected to limit potential sensing applications which would be favored by the presence of discrete small NPs.

maximum membrane loading was obtained within the first 5 min of extraction, while due to the higher viscosity of Aliquat 336 (membrane liquid phase of the non-plasticized PIM) compared to the mixture of Aliquat 336 and 1-dodecanol (membrane liquid phase of the plasticized PIM), the non-plasticized PIMs extracted only 20% of the maximum membrane loading after around 60 min of extraction. Despite the different diffusion coefficients due to the different viscosities of the membrane liquid phases of the plasticized and non-plasticized PIMs, the XRD analysis (Fig. 5) did not show any significant loading time or membrane composition effects on the resulting Au crystallite size for both membrane compositions. This unexpected result was probably due to the fact that Au(III) was extracted into a network of nanometer-size channels filled with the membrane liquid phase [36]. In the case of the lower viscosity membrane liquid phase containing Aliquat 336 and 1-dodecanol, the initial fast transport of Au(III) towards the plasticized PIM surface resulted in partially blocking of the surface outlets of these channels by the rapidly formed Au NPs. This process would have resulted in slowing down the long-term transport rate of Au(III) towards the PIM surface to the level established in the non-plasticized PIMs. Au(III) loading can also be controlled by the concentration of the Au (III) feed solution rather than by the duration of loading. The effect of varying Au(III) feed concentration on Au crystallite size determined by XRD measurements (Fig. 6) was found to be insignificant. This result is consistent with the finding that the rate of PIM diffusion is critical to crystallite size and therefore the membrane loading, which does not influence the diffusion rate, should also not affect the crystallite size. However, it was of interest to establish if different combinations of feed solution concentration and loading time that produced similar in value total membrane loading but different Au(III) distribution within the PIM would also result in a similar Au NP coating. Two extreme loading conditions were compared. Under the former conditions a PIM (30 wt% Aliquat 336 and no 1-dodecanol) was loaded for 24 h in a 10 mg Au(III) L−1 feed solution resulting in the extraction of 0.50 mg Au(III) while under the latter conditions 0.44 mg Au(III) were extracted into a PIM of the same composition during a loading time of only 5 min but from a feed solution containing 100 mg Au(III) L−1. After reduction in 0.1 M EDTA solution, the PIM loaded for 24 h acquired a blue color which was darker on the edges (Fig. 7a). When the loading time was 5 min, the PIM was a reddish-purple color and showed a more even Au NP distribution (Fig. 7b). The dark blue color of the former PIM was caused by significant clustering and agglomeration of the Au NPs

3.4. Loading time and feed solution concentration Due to the high Au(III) loading achieved using the 70 wt% PVC, 20 wt% Aliquat 336 and 10 wt% 1-dodecanol PIM composition, the resulting Au NPs after reduction formed a dense layer on the membrane surface rather than discrete, disperse NPs. In order to produce surface coatings for potential sensing applications, efficient control of Au(III) loading is a key parameter. As the ratio of Au(III) ion concentration to reducing agent concentration had been shown to affect the size of the resulting NPs in solution-based synthesis methods, the influence of the ratio between the Au(III) concentration on the membrane surface and the solution concentration of the reductant used was investigated. For this purpose, plasticized (i.e., containing 1-dodecanol) and non-plasticized Aliquat 336/PVC-based PIMs with different Au(III) loadings were reduced and the corresponding Au NP surface coatings were evaluated by XRD. It can be expected that by varying the PIM composition it would be possible to affect the Au(III) diffusion rate from the PIM interior towards its surface. The plasticized PIMs were found to extract Au (III) rapidly in the initial stages of the extraction process, i.e., 20% of 85

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Table 4 Comparison of Au crystallite sizes estimated by XRD corresponding to various 0.1 M reduction solutions with experimentally measured reduction potential values. All PIMs were loaded for 24 h in 100 mg L−1 Au(III), 0.5 M HNO3 feed solution prior to reduction. Reducing agent

Crystallite size (nm)

Reduction potential (mV)

NaBH4 Triethylamine Tri-sodium citrate L-ascorbic acid Oxalic acid EDTA Hydrazinium sulfate Hydroxylamine sulfate Citric acid

4.0 11.4 16.5 20.7 21.8 25.2 26.0 30.0 163.

−535 −13.5 385 349 604 519 382 328 651

solution interface corresponding to a decrease in resulting Au crystallite size. In all cases the reduction solution used was in excess to the amount of Au(III) available for reduction.

Fig. 6. Loading concentration versus crystallite size of AuNPs fabricated on the surface of PVC-based PIMs (20 wt% Aliquat 336, 10 wt% 1-dodecanol) with Au (III) loading for 24 h at different Au(III) feed solution concentrations. Reduction conditions as in Fig. 1.

3.6. Surface distribution and particle shape resulting in the formation of larger particles as shown in the corresponding SEM image (Fig. 7c) which was not observed when the loading time was 5 min (Fig. 7d). The clustering and agglomeration in the case of a long loading time could be explained by the fact that the extended loading allowed Au(III) to be uniformly distributed throughout the PIM while in the case of the much shorter loading time the same amount of Au(III) was only distributed within the PIM subsurface layers. Because of this uniform distribution, the rate of delivery of Au(III) to the membrane surface in the former case was significantly lower than in the latter case, thus providing sufficient time for Au NP clustering and agglomeration. These results indicate that by limiting the Au(III) loading in the interior of the PIM it is possible to produce smaller supported Au NPs which might be potentially suitable for sensing applications.

Beyond controlling size, a key aspect of producing PIM supported Au NPs for a desired application is their distribution across the membrane. Cross-sectioning of membranes consistently showed negligible quantities of Au NPs in the membrane interior with any of the conditions or reducing agents explored in this study. It should be noted that in the studies by Kumar et al. [9] and Mora-Tamez et al. [11] using CTA-based PIMs Au NPs were also produced in the membrane interior. The latter research demonstrated that CTA-based PIMs with the appropriate plasticizer and extractant resulted in reduction of Au(III) inside the membrane during the extraction step. However, similar membranes utilizing PVC rather than CTA in the present study did not show reduction during the extraction process. While variations in Au NP distribution throughout the membrane interior were not observed in the current study, significant differences in the distribution across the membrane surface were observed as a result of the composition of the reduction solution used. SEM imaging of the membrane surface for several reducing agents demonstrated the significant effect of the reducing agent on AuNP distribution (Fig. 8). The reducing agent can often play a significant role in stabilizing growing nanoparticles by acting as a stabilizing agent limiting nanoparticle agglomeration [42, 43]. Relatively uniform distribution was observed with a hydrophilic reducing agent such as EDTA (Fig. 8a). The mobility of Au NPs stabilized with less hydrophilic reducing agents is expected to be limited, thus hindering dispersion across the membrane surface. In this case one would expect limited surface coverage, with either the formation of very large crystallites (Fig. 8b) or large networks of small crystals (Fig. 8c and d). Similarly, different reducing agents can result in different Au particle shapes by preferentially stabilizing specific crystal facets during

3.5. Reducing agent and its concentration A wide range of reducing agents can be effectively used for producing Au NPs on the surface of PIMs loaded with Au(III) with significant differences in their morphology, size and distribution. The most frequently used reducing agents [3, 10, 37, 38] were tested and the resulting supported Au NPs were analyzed by XRD to determine the range of crystallite sizes produced. In solution-based synthesis methods it has been found that increasing the strength of the reducing agent reduces the size of the synthesized NPs due to an increased rate of reduction [39–41]. A comparison of the experimentally measured reduction potential of each reduction solution showed that this general trend was also observed for PIM-supported Au NPs (Table 4). This is consistent with increasing the rate of reduction at the membrane/

Fig. 7. Photographs and SEM images of non-plasticized PVC-based PIMs loaded with 0.50 mg Au(III) over 24 h in a 10 mg L−1 Au(III) feed solution (a, c) and with 0.44 mg Au(III) over 5 min in a 100 mg L−1 Au(III) feed solution containing 0.5 M HNO3 (b, d). Reduction conditions as in Fig. 1. 86

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Fig. 8. SEM images of Au NPs on the surface of PVC-based PIMs reduced in a) 0.1 M EDTA, b) 0.1 M hydroxylamine sulfate, c) 0.1 M hydrazinium sulfate, and d) 0.1 M trioctylamine. Remaining experimental conditions as in Fig. 1.

Fig. 9. SEM images of Au NPs on the surface of PVC-based PIMs reduced in a) 0.1 M ascorbic acid, b) 0.1 M citric acid, c) 0.1 trisodium citrate, and d) 0.1 M disodium oxalate. Remaining experimental conditions as in Fig. 1.

PVC-based PIMs of the same composition (20 wt% Aliquat 336 and 10 wt% 1-dodecanol) retained their plasticity throughout liquid nitrogen immersion. The thermal stability of a fresh plasticized PIM was compared with that of a PIM with the same composition but coated with Au NPs using TGA and the corresponding TGA curves are shown in Fig. 11. The TGA curve of the fresh plasticized PIMs showed significant mass loss between 150 °C and 250 °C corresponding to decomposition of PVC due to thermal dehydrochlorination [49] as well as loss of both plasticizer and carrier. After coating with Au NPs, the same PIMs retained their shape and size, though loss of flexibility was observed due to the loss of the volatile components and some polyene formation. Comparison of the TGA results after Au coating (Fig. 11) shows a moderate decrease in the mass loss of the PIM within the primary onset range of thermal dehydrochlorination [49], particularly within the range of 250 °C to 300 °C. The improved mechanical stability in the presence of Au NPs could be the result of stabilization of the polymer structure due to restriction of access to the polymer chains thus limiting the propagation of dehydrochlorination, which has been shown to follow a zip type mechanism [50]. While the TGA mass loss difference did not suggest that the Au coating completely protected the PIM, some reduction in the amount of dehydrochlorination was observed.

particle growth. This is a commonly utilized approach for producing metal NPs of a specific shape in solution-based synthesis [44–46]. As can be seen in Fig. 9, this effect was also observed in PIM-based synthesis with variation of the reducing agent resulting in the production of small cubes (Fig. 9a), large triangles visibly growing with agglomeration of smaller particles forming the next layer (Fig. 9b), small wires (Fig. 9c), and large dodecahedrons (Fig. 9d). While not explored in detail in the present study, the resulting shape of the PIM supported Au NPs is an important factor to consider for Au NP optimization for potential specific catalytic applications as a particle shape can significantly affect catalytic activity [47, 48]. 3.7. Au NP density The density of the PIM supported Au NPs is an additional factor to consider in optimizing conditions towards a potential application. As mentioned previously, control of Au(III) loading was effective for limiting Au NP density, with non-plasticized PVC-based PIMs (70 wt% PVC and 30 wt% Aliquat 336) being more easily utilized towards this end due to their slower extraction rates. For achieving disperse discrete Au NPs on the PIM surface, PIMs with this composition was loaded for 5 min and subsequently reduced to produce surface coatings of 0.041 mg Au cm−2. Membranes with discrete Au NPs showed reddishpurple color which was characteristic of individual Au NPs. As loading was increased and the membrane surface became a continuous, dense, conductive layer of Au NPs, this color changed to the color of bulk metallic gold. Photographic images highlight the color change at different Au NP surface coverage (Fig. 10), while SEM images show the corresponding Au NPs on the membranes surface.

4. Conclusions The results obtained in the current study clarify the effect of important Au NP fabrication parameters such as reduction time and temperature and reduction potential of the reducing agent on Au NP characteristics such as crystallite size, density and distribution on the PIM surface. It was established that by increasing the rate of Au(III) reduction at the membrane/solution interface via using stronger reducing agents, higher temperatures or longer reduction times, the crystallite size decreased. This effect is in agreement with results reported for solutionbased methods thus suggesting that the vast amount of knowledge regarding solution-based Au NP synthesis is a valuable source of

3.8. Effect of Au NPs on PIM thermal and mechanical properties PVC-based PIMs coated with Au NPs showed retention of their mechanical properties within a wider range of temperatures. Fresh PIMs could be easily fractured in liquid nitrogen as this was well below their glass transition temperature. However, after coating with Au NPs, 87

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Fig. 10. Photographic and SEM images of a fresh PIM (a,d); a non-plasticized PVC PIM loaded for 5 min in 100 mg L−1 Au(III) (b,e) and a plasticized PVC PIM loaded for 24 h in 100 mg L−1 Au(III) containing 0.5 M HNO3 (c, f). Remaining experimental conditions as in Fig. 1.

with several Au NP layers in a single loading with Au(III), followed by reduction with EDTA. For potential catalytic applications, this high density would be ideal for achieving maximum NP surface area. For potential sensing applications, control of the loading time of non-plasticized PVC-based PIMs could allow surface coverage with individual non-clustered Au NPs. The increased thermal stability of Au NP coated PIMs could be of significant benefit in potential catalytic applications where the stability of a catalytic reactor through numerous cycles at elevated temperatures would be of primary concern. In future research it would be of interest to evaluate the catalytic activity of Au NP surfaces on PIMs in model reactions, such as liquidphase oxidation reactions and the potential of PIMs coated with AuNPs of controlled size and density for sensing applications. On the basis of the above mentioned it can be concluded that a PIM coated with Au NPs represents a unique system for the synthesis and immobilization of Au NPs, with advantages including simplicity of fabrication, low price, high flexibility, thermal stability, and optical transparency.

Fig. 11. TGA curves showing the thermal decomposition of plasticized PVC PIMs containing 70 wt% PVC, 20 wt% Aliquat 336 and 10 wt% 1-dodecanol and without Au NP coating. with

Acknowledgements

information for providing guidelines for PIM-based NP synthesis. It was found that distribution of Au NPs on the surface of PIMs showed significant dependence on the reducing agent used. Reducing agents can play a key role in stabilizing Au NPs and are often a critical parameter in controlling the NP shape in solution-based synthesis methods. As the PIM-based synthesis occurs at the interface between the hydrophobic membrane and an aqueous solution, reducing agents stabilizing the Au NPs will also have significant control over the mobility of Au NPs as they are being formed. In this research, EDTA showed consistent results in producing highly spherical, homogenous Au NPs across the membrane surface. Density of Au NPs on the PIM surface can be easily controlled by limiting the amount of Au(III) extracted prior to reduction. At maximum loading, PIMs containing 20 wt% Aliquat 336 could be coated

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