Separation and recycling of nanoparticles using cloud

Journal of Colloid and Interface Science 363 (2011) 490–496

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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis

Separation and recycling of nanoparticles using cloud point extraction with non-ionic surfactant mixtures Muhammad Faizan Nazar a,e, Syed Sakhawat Shah b, Julian Eastoe c,⇑, Asad Muhammad Khan d, Afzal Shah a a

Department of Chemistry, Quaid-i-Azam University, 45320 Islamabad, Pakistan Department of Chemistry, Vice Chancellor Hazara University, Mansehra, NWFP, Pakistan School of Chemistry, University of Bristol, Bristol, BS8 1ST, UK d Department of Chemistry, Forman Christian College (A Chartered University), 54600 Lahore, Pakistan e Department of Chemistry, University of Sargodha, 40100 Sargodha, Pakistan b c

a r t i c l e

i n f o

Article history: Received 3 May 2011 Accepted 18 July 2011 Available online 29 July 2011 Keywords: Non-ionic surfactants Nanoparticles Liquid–liquid phase separation Cloud point extraction

a b s t r a c t A viable cost-effective approach employing mixtures of non-ionic surfactants Triton X-114/Triton X-100 (TX-114/TX-100), and subsequent cloud point extraction (CPE), has been utilized to concentrate and recycle inorganic nanoparticles (NPs) in aqueous media. Gold Au- and palladium Pd-NPs have been pre-synthesized in aqueous phases and stabilized by sodium 2-mercaptoethanesulfonate (MES) ligands, then dispersed in aqueous non-ionic surfactant mixtures. Heating the NP-micellar systems induced cloud point phase separations, resulting in concentration of the NPs in lower phases after the transition. For the Au-NPs UV/vis absorption has been used to quantify the recovery and recycle efficiency after five repeated CPE cycles. Transmission electron microscopy (TEM) was used to investigate NP size, shape, and stability. The results showed that NPs are preserved after the recovery processes, but highlight a potential limitation, in that further particle growth can occur in the condensed phases. Ó 2011 Elsevier Inc. All rights reserved.

1. Introduction Owing to chemical versatility, nanoparticles (NPs) are recognized as model systems of choice for exploring self-assembly, bioassays, catalysis, electron-transfer, phase-transfer, and crystal growth phenomena [1]. These colloidal fluids also have extensive applications in evolving nanoparticle technologies. Over recent years, the study and applications of metal nanoparticles has emerged as an important sub discipline of colloid and surface sciences. Of course, if the NPs are made from high value precursors such as precious metals, then processing, recovery and recycling impacts on the economics of any potential NP application. Today oxidation catalysis is envisaged as a major potential use of gold and other inorganic NPs, hence it now becomes important to new explore fast, cheap (green) and effective methods to separate, purify and recover them from the supporting reaction medium and un-reacted background ‘‘reaction debris’’ [2]. Aqueous solutions of non-ionic surfactant micelles exhibit thermoreversible phase separation phenomena on heating/cooling through a cloud point (Tc). This limiting temperature Tc can be ⇑ Corresponding author. E-mail addresses: [email protected] (M.F. Nazar), sakhawat_shah@yahoo. com (S.S. Shah), [email protected] (J. Eastoe), [email protected] (A.M. Khan), [email protected] (A. Shah). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.07.070

thought of as the upper stability temperature for the dilute dispersed micellar phase (L1). On approaching Tc from lower temperatures intermicellar interactions increase, the longer correlations now are commensurate with the wavelength of visible light, and the once transparent micellar dispersions scatter light efficiently (clouding). Above Tc attractions dominate resulting in macroscopic separation of dilute and concentrated surfactant phases. Over recent years, significant advances have been made in applying such cloud point phenomena in extraction and separation science. As such cloud point extraction (CPE) has been successfully used for recovery of different species (organic compounds, inorganic metal ions, biological analytes). For example, Watanabe et al. [3] illustrated the extraction of soluble metallic (Zn(II)) chelates from aqueous solutions by CPE using polyoxyethylene-pnonylphenylether (PONPE-7.5) as the surfactant extractor. Following on from that CPE was shown to be effective with different metal ions, biomaterials and organic compounds [4–12]. Compared to other conventional approaches CPE presents an attractive method for extraction having advantages over the traditional liquid–liquid approach involving non-structured molecular solvents [13–15]. These benefits are because high extraction efficiencies and pre-concentration factors can be achieved, and CPE is water based, avoiding the use of expensive and toxic solvents. In addition, the presence of surfactants can reduce extractant losses [16,17]. The phase behavior of certain non-ionic surfactants

M.F. Nazar et al. / Journal of Colloid and Interface Science 363 (2011) 490–496

in aqueous solutions can readily yield a surfactant-rich phase of small volume above Tc, so the process is recognized as simple, cheap, highly efficient, and less environmentally hazardous compared to those employing organic solvents [18]. In terms of NP separation strategies, Jiang et al. recently introduced an interesting approach to recovery of various nanoparticles (e.g., CdS, ZnS, Fe3O4, TiO2, Ag, and Au) by exploiting CPE [19]. The systems were heated to induce post Tc separations, resulting in preferential incorporation of the added and dispersed NPs into the surfactant-rich lower phases. Significantly, the nanoparticles were hardly affected, showing no aggregation after two months, and no changes in particle dimensions and shape [19]. Following on, Jiang et al. utilized the non-ionic TX-114 in CPE for analysis and removal of trace silver nanoparticles and nanosized copper oxide respectively, from environmental wastewater [20,21]. The objective of this work is to explore the generality of the latter method [19], but now with mixed non-ionic surfactants. The micellar solutions used here are based on those employed previously [19], but with a new formulation variable which improves upon NP recovery efficiency. The advantage of using surfactant mixtures, demonstrated here, is an ability to now tune the absolute value of Tc meaning it can be brought down (up) to an experimentally convenient range, for example just above room temperature. The additional close control over Tc then permits optimization of the CPE process, especially in terms of NP recovery efficiency, and to minimize thermal losses incurred if using surfactants with high Tc values. The extraction performance of this mixed TX-114/TX-100 system is compared with the single surfactant TX-114 (basic molecular structure of Triton X series is shown in Scheme 1). The NPs were pre-synthesized [22], separated using an anti-solvent method outlined previously [23], then incorporated into the micellar systems, after which the NPs were re-recovered by CPE. Both strengths and weaknesses of the approach are highlighted, with the aim to stimulate further research into CPE parameter optimization. It has recently become recognized [see recent review [23]] that NPs can be dispersed in a range of ‘‘colloidal fluids’’, such as microemulsions, microemulsion/polymer mixtures as well as pure polymer and micellar solutions [19–21], instead of using normal chaotic, non-structured molecular solvents (water, alkanes). Interestingly, use of colloidal solvents provides new opportunities for manipulation of NP phase behavior and stability, which are not available in molecular solvents. For example, inorganic NPs added to microemulsions can be partitioned and recovered by exploiting phase transitions of the background microemulsion ‘‘solvent’’ itself [24]. The use here of aqueous micellar solutions, with mixed surfactants in combination with added NPs, extends the range of different ‘‘colloidal solvents’’ which are now known to be appropriate for stability control, partitioning and recovery of NPs. 2. Materials and methods 2.1. Chemicals Gold(III) chloride trihydrate (HAuCl4.3H2O, 99.99%,) was purchased from Sigma. Cetyltrimethylammonium chloride (CTACl,

Scheme 1. Basic molecular structure of Triton X series, ‘n’ is an average ethylene oxide chain length.

491

95+%, Alfa-Aesar) and propanol (propan-1-ol, 99.5%, BDH Limited) were purchased and used without further purification. Sodium hexachloropalladate (IV) tetrahydrate (98%) and sodium borohydride (NaBH4, 99%) were purchased from Sigma. Sodium 2-mercaptoethanesulfonate (MES, >98%, Sigma) and tetrahydrofuran (99.5%, Fisher Scientific) were purchased and used without further purification. H2O was of ultra-high purity (Elga Maxima Purelab system, 18 MX cm). 2.2. Characterization techniques UV–vis absorption spectra of the NP-containing systems were recorded using a Thermo Evolution 300 spectrometer with Vision Pro software. Transmission electron microscopy (TEM) was carried out with a JEOL JEM 1200 EX Mk. 2 microscope operating at 120 kV attached with a digital camera. The size distributions of the nanoparticles were estimated using ImageJ software: at least 400 particles were counted. 2.3. Synthesis and extraction of nanoparticles (NPs) The processes and procedures described in this section have been repeated by different operators, at least three times, using freshly made solutions and formulations, yielding reproducible results. The overall synthesis and extraction procedure of nanoparticles is outlined in Scheme 2. 2.3.1. Pre-synthesized MES-stabilized nanoparticles in aqueous phases First of all anionic MES-stabilized nanoparticles were prepared by reduction of MES containing metal salt solutions with NaBH4, followed by nanoparticle separation by THF as anti-solvent. A 2.0 mL of aqueous solutions of HAuCl4.3H2O (1 wt.%) and Na2PdCl6.4H2O (1 wt.%) were taken independently and were diluted to 180 mL and 100 mL respectively with water. Then to these 20 mL (for gold) and 100 mL (for palladium) of an aqueous MES solution (1 mmol dm 3) was added, and the mixture was stirred for 10 min (magnetic stirrer/flea). After that freshly prepared NaBH4 in 0.5 mL of water (0.03 g for gold and 0.008 g for palladium) was slowly added drop wise to the respective mixtures, resulting in the formation of gold Au-MES-NPs and palladium PdMES-NPs nanoparticles respectively. The appearance of the reaction mixtures changed pale yellow to strong golden brown (for Au-MES-NPs), and strong golden brown to dark brown (for PdMES-NPs). After all the NaBH4 solution was added, the reaction mixtures were stirred overnight. To separate the MES-stabilized NPs from these aqueous mixtures the anti-solvent procedure employed previously was used [23]. An aliquot (5.0 mL) of the aqueous MES-stabilized nanoparticle dispersion was decanted and 9.0 mL of the NP anti-solvent tetrahydrofuran (THF) was added. After manual shaking the mixtures were left to settle for two days. The upper clear liquid phases (majority of the systems) were removed carefully by pipette, taking care not to disturb the lower separated NP residues. Next the NP ‘‘precipitates’’ were dried in an oven, and then re-dispersed into 5.0 mL of non-ionic-micellar solution. 2.3.2. Extraction of NPs by cloud point procedure Cloud point extraction methodology was employed to separate and recover the pre-synthesized gold and palladium particles: the steps involved are shown in Scheme 3. To study the extraction of Au-MES-NPs with non-ionic surfactants, 5 mL of TX-114 (0.2%, w/v) solution was mixed with TX114–TX-100 at different compositions, then added to separated Au-NPs in a centrifuge tube. The systems were mixed and incubated at 35–45 °C (depending of exact mixture composition) in water bath for 30 min. After this 0.2 mL of NaCl (0.10 mol dm 3)

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Scheme 2. Synthesis and extraction of nanoparticles.

was added to maintain the ionic strength, and then the system was centrifuged at 2000 rpm for 15 min to facilitate the phase separation. The absorbance of surfactant rich phase (0.2 mL) was recorded after diluting with 50% (v/v) methanol (0.4 mL) and levelled 5 mL with water. 3. Results and discussion 3.1. The cloud point of TX-114/TX-100 mixtures Fig. 1 demonstrates the variation in cloud point temperature of mixed TX-114/TX-100 micellar systems (total concentration 1.0 wt.%) at various surfactant concentrations. The values of cloud

point temperatures and concentrations of surfactants are reported in Table S1 (Supporting information). The hydrophobic groups of these two surfactants are identical, but TX-100 comprises 2.0 more hydrophilic ethylene oxide units per molecule than TX-114. Therefore, as would be expected Tc for pure TX-100 is higher than for the more hydrophobic TX-114. Hence, by blending it is possible to control Tc of the mixture so that the critical operation temperature for CPE can be optimized for the conditions of interest [25,26]. A slight decrease in Tc on addition of Au-NPs was noted, but this is essentially within the error of measurement (ocular determination of Tc). The ability to control Tc by mixture composition was used to optimize conditions and efficiencies of NP recovery using CPE, as outlined below. 3.2. Extraction of Au-MES-NPs by different micellar systems

Supernatant of Nanoparticles Addition of surfactant or surfactant mixture

Fig. 2 shows UV–vis absorption spectra of Au-MES-NPs in micellar systems before (inset) and after separation. The UV spectrum 75 CP

Incubate at a suitable temperature

CP (NPs)

Addition of appropriate amount of NaCl Centrifugation

Decant off the surfactant-depleted phase

Surfactant-rich phase

Cloud Point / ºC

60

45

30

15

0 0

Dilution for further analysis Scheme 3. Steps involved in cloud point extraction.

15

30

45

60

75

90

105

Triton X-100 / %wt Fig. 1. The cloud point curve of mixed TX-114/TX-100 micellar solutions with and without added Au-NPs.

493

Absorbance


0.4

Absorbance

0.3

Table 1 Effect of TX-114/TX-100 mixture composition on extraction efficiency of Au-NPs (surfactant-rich phase). Cloud points of these surfactants mixtures are given in Fig. 1.

0.6 0.4

System

0 350

TX-114 TX-114 + TX-100

500

650

800

Wavelength / nm

0.2 TX114 TX114(95%)+TX100(5%) TX114(90%)+TX100(10%)

0.1

TX114(85%)+TX100(15%) TX114(80%)+TX100(20%) TX114(70%)+TX100(30%) TX114(60%)+TX100(40%) TX114(50%)+TX100(50%)

0 350

425

500

Absa(541nm)

0.2

575

650

725

800

Wavelength / nm Fig. 2. UV–vis spectra of Au-MES-NPs in the surfactant-rich phase after extraction. Inset: Au-MES-NPs in homogeneous surfactant solution before extraction.

before separation (inset) shows the plasmon band centred on 527 nm characteristic of Au-NPs [27]. Comparison of the plasmon resonance spectra indicated kmax = 527 nm before separation and 541 nm after CPE, with no significant absorbance beyond 600 nm seen, suggesting lack of any significant aggregation or size changes of the Au-MES-NPs. The near disappearance of the absorption peak at 541 nm for the upper surfactant-depleted phase (Fig. S1; Supporting Information) indicates most of the Au-MES-NPs were transferred into the surfactant-rich (lower) phase. The absolute values for the absorbance, alongside calculated concentrations for the surfactant rich lower phase are shown in Table 1 given the known concentration of the Au-MES-NPs in the reverse micelle samples (assuming 100% conversion Au3+ to Au0 and Beer–Lambert behavior). Hence, it can be calculated that approximately 52% by mass of the Au-MES-NPs from the original Au-NP-containing micellar system partitions into the bottom phase. Amongst all the surfactant systems mentioned above, TX-114 (80%) + TX-100 (20%) has greater extracting efficiency of Au-MES-NPs (52%), so this system was used for further studies. These Au-NPs are negatively charged by virtue of the stabilizing MES shell [28], so Au-MES-NPs are stable dispersions in the aqueous phase due to Coulomb repulsion. Heating above the cloud point temperature will transform the composites from hydrophilic to hydrophobic, and hence after centrifugation of the bi-phasic CPE systems it can be expected that surfactant-rich NP containing phases would be recovered. For the quantitative extraction of Au-MES-NPs, 5 mL of optimized system (TX-114 (80%) + TX-100 (20%)) was incubated at 40 °C in water bath for 30 min. After the addition of 0.2 mL of NaCl (0.10 mol dm 3) the system was centrifuged at 2000 rpm for 15 min. The absorbance of surfactant rich phase (0.12 mL) was recorded after diluting with 50% (v/v) methanol (0.4 mL) and levelled 5 mL with water. At higher concentrations of TX-100 (>20%), the extraction efficiency decreases. This may be due to the increase in the volume of the surfactant-rich phase (0.2 mL) which probably leads to the loss of the composites and minimizes the extractability. Moreover, the increase in viscosity of the surfactant-rich phase also leads to poor sensitivity [29]. 3.2.1. Transmission electron microscopy Fig. 3 shows the visual appearance and TEM images of an example micellar system containing Au-MES-NPs before (Fig. 3i) and

TX-114 (after separation) TX-114 (95%) + TX100 (5%) TX-114 (90%) + TX100 (10%) TX-114 (85%) + TX100 (15%) TX-114 (80%) + TX100 (20%) TX-114 (70%) + TX100 (30%) TX-114 (60%) + TX100 (40%) TX-114 (50%) + TX100 (50%)

[Au-NPs] (mmol dm

3

)

[Au-NPs] (mg dm 3)

Extraction efficiency (%)

0.251

2.34

461

42.7

0.262

2.44

481

44.5

0.272

2.54

500

46.4

0.285

2.66

524

48.5

0.304

2.83

558

51.6

0.301

2.81

554

51.3

0.295

2.75

542

50.2

0.281

2.62

516

47.8

a Absorbance of initial sample (before separation) was 0.588 at 527 nm and concentration was 5.48 mmol dm 3.

after (Fig. 3ii) cloud point extraction respectively. Both TEM images are consistent with a weakly polydisperse collection of spherical particles. However, more quantitative image analysis (Fig. 3iii) reveals notable variations in size distribution between the two samples. The TEM images indicate an average particle size of 6.4 nm before extraction (Fig. 3i) but 17.0 nm after extraction (Fig. 3ii), with standard deviations 1.9 nm and 2.3 nm respectively. The increase in size after extraction compared with the original system may be due to the further growth of nanoparticles caused by the change in solvent environment and/or that the separation step might be size selective. At present it is not possible to distinguish between the two owing to limitations (time resolution) of the TEM method. 3.3. Recycling of Au-MES-NPs the TX-114 (80%) + TX-100 (20%) system Further explorations of the Au-NP extraction efficiency with the optimized system TX-114 (80%) + TX-100 (20%) were made, and the concept of NP re-processing and recycling was assessed. In a typical procedure, Au-MES-NPs were recovered from 5 mL portions of the optimized micellar system by CPE. The lower phase was then recovered and re-dispersed in a fresh aqueous phase containing 0.4 mL of 50% (v/v) methanol. This separation and re-dispersion procedure was repeated at least five times; however, 0.2 mL of blank mixed micellar solution and 0.10 mol dm 3 NaCl (0.2 mL) was also added each time to the NP dispersion/micellar mixture to preserve the surfactant concentrations and to achieve maximum NP separation. UV/vis spectroscopy and TEM were utilized to follow the possible changes in NP size, shape, and stability. Fig. 4 shows the UV–vis absorption spectra of Au-MES-NPs before (initial sample) and after five successive extractions. The absolute values for reuse experiments are shown in Table 2. This procedure resulted in 35% recovery of the NPs after a fifth cycle. Importantly, no changes in UV/vis spectra were observed after repeated recovery/re-dispersion, suggesting that the recycling process had no notable effect on the NP structure or stability.

494


Fig. 3. Visual appearance and TEM images of Au-MES-NPs in micellar solution (i) before cloud point extraction; (ii) after cloud point extraction. Scale bars 50 nm. (iii) Particle size distributions for Au-MES-NPs obtained from analyses of TEM images from (i) and (ii). In both cases the distributions were based on a sample size of 400 imaged Au-MESNPs.

0.7 Before Separation Surfactant Rich (Cycle 1) Cycle 2 Cycle 3 Cycle 4 Cycle 5 Water Rich (Cycle 5)

0.6

Absorbance

0.5

Table 2 Efficiencies of Au-NP-containing systems estimated by analysis of UV spectra after five successive extractions. Sample

Cycle Cycle Cycle Cycle Cycle

0.4 0.3 0.2

1 2 3 4 5

Absa(541nm) (surfactant-rich phase)

[Au-NPs] (mmol dm

0.304 0.278 0.251 0.226 0.195

2.83 2.59 2.34 2.11 1.89

3

)

[Au-NPs] (mg dm 3)

Extraction efficiency (%)

558 510 461 416 372

51.6 47.3 42.7 38.5 34.5

a Absorbance of initial sample (before separation) was 0.588 at 527 nm and concentration was 5.48 mM.

0.1 0 350

425

500

575

650

725

800

Wavelength / nm Fig. 4. UV–vis spectra of recycled Au-NPs using the optimized micellar system before and after five successive extractions.

Concerning particle sizes, the recovered Au-MES-NPs retained their dimensions as confirmed by TEM. Fig. S2 (Supporting Information) shows the shape and size of Au-MES-NPs extracted after the fifth cycle. Fig. S3 (Supporting Information) shows the particle-size distribution of Au-MES-NPs relating to the total number of nanoparticles. The average particle size calculated is 17.5 nm with 2.4 nm as standard deviation. 3.4. Extraction of Pd-MES-NPs by selected micellar system Same the cloud point extraction process was applied to anionic Pd-MES-NPs, which had been added to the optimized micellar sys-

tem (TX-114 (80%) + TX-100 (20%)). The recoveries of Pd-MES-NPs were quantitatively estimated by gravimetric analyses, giving 50%. Fig. 5 shows the visual appearance and TEM images of an example micellar system containing Pd-MES-NPs before (Fig. 5i) and after (Fig. 5ii) cloud point extraction respectively. TEM images and derived particle size distributions show that the Pd-MES-NPs retain their structural integrity both before and after extraction. However, more quantitative analysis of the images (Fig. 5iii) reveals considerable variations in size distribution of the two samples. The TEM images indicate an average particle size of 7.1 nm before extraction (Fig. 5i) but 14.3 nm after extraction (Fig. 5ii), with standard deviations 2.1 nm and 2.3 nm respectively. The increase in size after extraction compared with the original system may be due to the further growth of nanoparticles caused by the change in the environment (micelle to reverse micelle), and/or that the separation step might be size selective.


495

Fig. 5. Visual appearance and TEM image of Pd-MES-NPs in micellar solutions (i) before cloud point extraction; (ii) after cloud point extraction. Scale bars 50 nm. (iii) Particle size distributions for Pd-MES-NPs obtained from analyses of TEM images from (i) and (ii). In both cases the distributions were based on a sample size of 400 imaged Pd-MESNPs.

4. Conclusions This research extends recently published work [19], now importantly demonstrating the application of mixed non-ionic micellar systems, here TX-114/TX-100, to extract and recycle different kinds of inorganic nanoparticles. By using blended surfactant mixtures, instead of a single surfactant component [19], it now becomes possible to fine tune the cloud point Tc, bringing it into an experimentally convenient range. This has considerable advantages for process optimization, allowing for fast and easy cycling between single phase and separated bi-phasic conditions, and minimizing the need for time lags essential for thermal equilibration with high Tc surfactants. For Au-NPs a maximum recovery of 52% was obtained using a mixture composition giving Tc 30 °C (i.e. just over room temperature), pointing to a limitation of this specific method. During phase transfer the dimensions and stability of the nanoparticles in the surfactant-rich phase were broadly preserved, but there was evidence for growth of the NPs compared to the original size, after the temperature-induced extractions. This is another apparent weakness of NP–CPE, at least as applied to the NPs in this study. The observed size population shift might be down to a combination of effects, changes in solvent environment (dilute to concentrated surfactant phase), or possibly even size selectivity of the extraction process (although that is difficult to confirm at present). Interestingly, it was shown that NP–CPE recovery can be repeated, allowing for recycling of the dispersion/ extraction process. Of course mass losses were noted after each subsequent CPE cycle, but in potential this limitation too could be overcome with further research into to surfactant mixtures having phase behavior optimized for this process. Therefore, this method shows certain potential benefits for dispersion, storage, application, recovery and re-use of high value NPs, especially those made from expensive and rare noble metals such

as gold and palladium. On the other hand, limitations are apparent, which may be overcome by further research to optimize non-ionic surfactant mixtures to generate stronger thermodynamic advantages (i.e. even greater partitioning by virtue of a more favorable phase diagram, with greater bias to the tie-lines). This type of approach may also be potentially applied for removal of nano-wastes in future industrial processes involving nanoparticles as well as catalysts [30]. The advantages of this neat, low-energy recovery method over other currently accessible purification routes for NPs is that it is cost-effective, requiring only (relatively) cheap and commercially available and inexpensive surfactants. Hence, the use of colloidal self-assembly fluids as dispersion media for NPs, instead of normal unstructured molecular solvents, may lead to new developments in the ever evolving field of nanoparticle technology [27,28]. Acknowledgments The authors acknowledge University of Bristol UK and Higher Education Commission of Pakistan for financial support. Appendix A. Supplementary data Details of cloud point values and particle size distributions (TEM) are available in supporting information. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jcis.2011.07.070. References [1] R. Sardar, A.M. Funston, P. Mulvaney, R.W. Murray, Langmuir 25 (2009) 13840. [2] J.A. Dahl, B.L.S. Maddux, J.E. Hutchison, Chem. Rev. 107 (2007) 2228. [3] H. Watanabe, H. Tanaka, Talanta 25 (1978) 585.

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