Comparison of Two Synthesis Routes to Obtain Gold Nanoparticles in

ARTICLE pubs.acs.org/JPCC

Comparison of Two Synthesis Routes to Obtain Gold Nanoparticles in Polyimide Katrien Vanherck,† Thierry Verbiest,‡ and Ivo Vankelecom*,† † ‡

K.U. Leuven, Centre for Surface Chemistry and Catalysis, Kasteelpark Arenberg 23, 3001 Heverlee, Belgium K.U. Leuven, Molecular and Nanomaterials, Celestijnenlaan 200D, 3001 Heverlee, Belgium ABSTRACT: Gold nanoparticle containing polymer materials find applications in catalysis, facilitated transport, sensing, and separations. In this study, two routes to obtain stable gold nanoparticles in a polymer matrix, namely, in situ chemical reduction of a gold salt and the use of preformed poly(vinylpyrrolidone) protected gold nanoparticles, were followed to prepare gold containing polyimide hybrid membranes. The influence of the synthesis method on the nanoparticle size, dispersion, and surface plasmon behavior was investigated by transmission electron microscopy, UVvis spectroscopy, and diffuse reflectance spectroscopy. Significant differences were found concerning the dispersion and aggregation of the nanoparticles. The influence of the synthesis method on the membrane structure and performance was also studied by scanning electron microscopy and in filtrations of dye solutions in ethanol and isopropanol. The filtrations were repeated while the gold nanoparticles were plasmonically heated by a green Argon ion laser beam, resulting in localized heating of the membrane and increased fluxes.

’ INTRODUCTION To obtain stable metal nanoparticles (NP) in a solid polymeric matrix, two routes are commonly followed. First, the NPs can be presynthesized in a solvent that is then used to prepare the polymer matrix. In this case, the NPs are usually protected by a ligand to avoid their aggregation. Second, the NPs can be formed in situ, by (photo)chemical reduction inside the solid matrix. These two methods to prepare NPpolymer composites have been studied for a variety of polymer matrices and gold nanoparticles (GNPs).117 Overall, the first method has been shown to allow a better control of the size of the NPs while the second method strongly reduces the incidence of NP aggregation.4,6 Such problems with aggregation during the incorporation of preformed nanoparticles into a solid matrix may be avoided by surface-modifying the nanoparticles with a suitable agent.4,6,15 A direct comparison of the two methods has so far only been done by Dammer et al. for GNPs synthesized in poly([2-methoxy5-(2-ethylhexyloxy)-1,4-phenylene]vinylene).13 However, polymer degradation occurring in the case of in situ reduction, through oxidation by the GNP precursor (H[AuCl4]/tetraoctylammonium bromide/tetraoctylammonium bromide (TOAB) complex), did not allow for a proper comparison. Polymeric membranes containing gold nanoparticles have been prepared for various applications, such as (electro)catalysis,8,1719 facilitated transport,14,20 protein separation,21 and sensing,2224 and have potential applications in other areas such as drug delivery. When preparing GNPs inside a polymer membrane matrix, it can be expected that both methods will have a different influence on the nanoparticle size and dispersion in the membrane but also on the membrane structure and hence the membrane performance. To our knowledge, no comparison r 2011 American Chemical Society

between the two methods has been made for a nanofiltration membrane. Solvent resistant nanofiltration (SRNF) involves the separation of an organic mixture down to a molecular level by simply applying a pressure gradient over a membrane.25 It has some important advantages compared to other industrial separation processes, such as its energy and waste efficiency. To turn SRNF into a viable industrial process, excellent membranes should become available, combining chemical, mechanical, and thermal stability with good rejections and sufficiently high fluxes. However, most commercially available membranes for SRNF combine high rejections for low molecular weight (MW) compounds with low fluxes. Recently, we have studied the effects of plasmonic heating of GNPs incorporated into nanofiltration membranes on the membrane performance, showing an overall increase of the membrane permeability without affecting its rejection of a low MW dye.26,27 Plasmonic heating is a method more commonly employed in imaging and sensing, drug release, and biomedicine (tumor destruction).6,2833 In most membrane processes, developing a membrane with a higher selectivity is coupled to a loss in permeability and visa versa. Photothermal heating of GNP containing membranes is thus of high interest as a potential route to overcome this traditional flux-selectivity trade-off. In this paper, two common synthesis routes were used to obtain composite GNPpolyimide phase inversion membranes with varying gold content. Polyimide (PI) is a well-known polymer Received: July 29, 2011 Revised: November 17, 2011 Published: November 23, 2011 115

dx.doi.org/10.1021/jp207244y | J. Phys. Chem. C 2012, 116, 115–125

The Journal of Physical Chemistry C

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Table 1. Membrane Compositions (Weight in g in the Casting Solution) for Reference and GNP Containing Membranes Prepared by Two Methods (PRE and ISR) HAuCl4 3 3H2O [g]

membrane

method

matrimid [g]

DMA [g]

THF [g]

ISR-0

ISR

2.2

4.5

3.3

0

0

0

ISR-1 ISR-2

ISR ISR

2.2 2.2

4.5 4.5

3.3 3.3

0.044 0.088

0 0

0 0

ISR-3

ISR

2.2

4.5

3.3

0.132

0

0

ISR-4

ISR

2.2

4.5

3.3

0.176

0

0

PRE-0

PRE

2.2

4.5

3.3

0

0.268

0

PRE-1

PRE

2.2

4.5

3.3

0.044

0.134

0.02

PRE-2

PRE

2.2

4.5

3.3

0.088

0.268

0.04

PRE-3

PRE

2.2

4.5

3.3

0.132

0.45

0.06

PRE-4

PRE

2.2

4.5

3.3

0.176

0.536

0.08

for producing SRNF membranes.25 PI membranes containing GNPs have been prepared by adding presynthesized PVPprotected GNPs by Mertens et al.,8 but they have never before been prepared by in situ reduction of the GNPs. It can be anticipated that the two different NP incorporation strategies to be studied will also strongly influence the surface plasmon resonance behavior of the GNPs and thus further change the membrane performance. The polymer composites were characterized by UVvis spectroscopy and diffuse reflection spectroscopy (DRS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The photothermal effect of the selected GNPs on the temperature and flux behavior of the membranes was compared by irradiating the membrane with continuous green laser light during solvent filtrations.

PVP [g]

NaBH4 [g]

was obtained. The solutions were then allowed to stand until air bubbles had disappeared and were cast onto a nonwoven support material that had been saturated with DMA. An automated casting knife (250 μm slid) was used, and the resulting polymer films were immediately immersed into a water bath. The reference membrane was yellow, and the membranes containing GNPs were light pink to red-brown in color. The membranes were then stored in an IPA bath for 3 h and transferred to an IPA/glycerol bath (volume ratio 60:40) for three days, before being dried in an oven at 60 °C. The membranes will further be referred to as PRE-0, PRE-1, PRE-2, PRE-3, and PRE-4, respectively corresponding with the membrane containing 0, 1.0, 2.0, 3.0, and 4.0 wt % GNPs. For the in situ chemical reduction (ISR) method, based on Huang et al.,21 HAuCl4 3 3H2O was added to a PI solution prepared in a mixture of DMA and THF to obtain casting solutions with gold to polymer weight ratios of 1.0, 2.0, 3.0, and 4.0 wt %. A similar polymer solution PI was prepared without HAuCl4 3 3 H2O, as a reference. The exact membrane compositions are given in Table 1. The solutions were stirred until homogeneous and cast onto the nonwoven support material. After a solvent evaporation step (30s), they were immersed into a water coagulation bath. After the immersion, the membranes were moved immediately into a solution of NaBH4 in water to reduce the gold to nanoparticles, upon which the membrane color turned from yellow to dark red. The membranes were further kept in IPA and IPA:glycerol and then dried as in Method A. The membranes will further be referred to as ISR-0, ISR-1, ISR-2, ISR-3 and ISR-4, respectively corresponding with the membrane containing 0, 1.0, 2.0, 3.0, and 4.0 wt % GNPs. Membrane Characterization. Diffuse reflectance spectra (DRS) were taken of the membrane surfaces by a PerkinElmer Lambda 40 spectrophotometer with deuterium and wolfram lamps. A piece of each membrane was redissolved in DMA, and these GNP solutions were characterized by a PerkinElmer UVvis spectrophotometer. Membrane pieces were immersed and broken in liquid nitrogen. The cross sections were studied with a Philips XL 30 FEG SEM, a semi-in-lens type SEM with a cold field-emission electron source. All SEM samples were first coated with a 1.52 nm Au layer to reduce sample charging under the electron beam using a Cressington HR208 high resultion sputter coater. To study the size of the GNPs in the membranes, the cross sections were examined by TEM. The membranes were dried and then embedded into Araldite resin. Semithin sections for light microscopy with a thickness of 5 μm were made with a Reichert Ultracut E microtome. Finally, cubic samples of

’ EXPERIMENTAL SECTION Materials. Matrimid9725 PI was obtained from Huntsman (Switzerland). The polyethylene/polypropylene nonwoven fabric Novatexx 2471 was kindly provided by Freudenberg (Germany). Hydrogen tetrachloroaurate(III) trihydrate(HAuCl4 3 3H2O) and sodium borohydrid (NaBH4, >98.5%) were obtained from SigmaAldrich. Poly(vinylpyrrolidone) (10 000 g mol1), N,N0 -dimethylacetamide (99.5%, DMA), tetrahydrofuran (99.5%, THF), isopropanol (99.5%, IPA), and absolute ethanol (EtOH) were obtained from Acros. All used water was desionized. Membrane Synthesis. Membranes were synthesized according to two different methods. For the incorporation of presynthesized GNPs (PRE), PVP-protected GNPs were prepared in DMA similar to the synthesis methods described by Teranishi et al.58 and Mertens et al.8 Solutions of HAuCl4 3 3H2O (0.05, 0.1, and 0.2 mmol) and an amount of PVP (molar ratio monomeric units of PVP/gold = 12) were prepared in DMA (6 g). Then, a freshly prepared NaBH4 solution in DMA (2 g) was added under vigorous stirring (molar ratio NaBH4/gold = 5), and immediately, a color change from yellow to dark red occurred in the solution, indicating the reduction of gold into nanoparticles. The solution was characterized by a PerkinElmer UVvis spectrophotometer, and the typical dark red color showed as a large peak at 530 nm, corresponding to the plasmon absorbance band of the GNPs.58 PI was added to the GNP solutions in DMA resulting in four casting solutions with different gold/PI weight ratios (1.0, 2.0, 3.0, and 4.0%). A similar PVP containing polymer solution PI in DMA and THF without GNPs was prepared as a reference. The compositions are given in Table 1. The solutions were stirred at room temperature until a homogeneous mixture 116

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Figure 2. Chemical structures of dye rose bengal (1017 Da) and methyl orange (327 Da).

The irradiation improvement factor (IIF) is calculated as the percentual increase in permeance or rejection when the membrane is irradiated, as follows:

Figure 1. Schematic representation of a dead-end filtration cell equipped for laser irradiation of the GNP containing membrane during separations.

about 1 mm side were obtained. Double stained 70 nm thin sections were examined in a Zeiss EM900 electron microscope. Chemicals and procedures for sample treatments were obtained from the Laboratory for Entomology of the K.U. Leuven, Leuven, Belgium. The particle size distributions of the GNPs were measured from the TEM pictures using ImageJ software (Image Processing and Analysis in Java59). Dead-End Filtrations. Dead-end membrane filtrations were carried out in a specially made glass filtration cell (Figure 1). A transparent glass window was built in the top to allow a laser beam to pass and illuminate 40% of the active membrane surface (0.001736 m2). For each filtration, a membrane was mounted in the cell and sealed off with a Viton O-ring. In some filtrations, a sealing flat plate was used to reduce the active membrane surface to equal the illuminated part. Before each filtration, the membranes were immersed in isopropanol for at least one day. Filtrations were carried out with dilute ethanol and isopropanol based methyl orange (MO, 327 Da, 35 μM) and rose bengal solutions (RB, 1017 Da, 17 μM) at 5 bar with and without laser irradiation. The chemical structures of the dyes are given in Figure 2. A continuous green argon laser beam (514 nm) was used to illuminate the membrane. The laser intensity was measured as the laser power divided by the illuminated surface, also calculating the minor loss of intensity in the laser pathway. The laser was set at an intensity of 0.2 W/cm2. Permeances were calculated as the amount of solvent (V) that passed through the membrane per unit of time (t), membrane surface (A), and applied pressure (ΔP) so that Permeance ¼ V 3 t 1 3 A1 3 ΔP1

ð1Þ

ð3Þ

IIFR ¼ 100 3 ðRL RC Þ 3 RC 1

ð4Þ

where PC and RC are the conventual permeance and rejection (measured without laser irradiation) and PL and RL are the permeance and rejection measured when the membrane is irradiated.

’ RESULTS AND DISCUSSION Since the size, distribution, aggregation, and dielectric environment all have a strong influence on the surface plasmon resonance behavior of the GNPs,29,3440 the GNPs inside the PI membranes were thoroughly characterized. In the ISRmembranes, the GNPs are formed inside the solid membrane matrix by chemical reduction of a gold salt, wherein the membrane polymer itself acts as a stabilizer. In the PRE-membranes, PVP-stabilized GNPs are present in the polymer solution before the membrane is cast and solidified by phase inversion. The preformed GNPs may have an influence on the membrane structure, as it has been previously shown that adding (nano)particles to a polymer solution can cause significant changes in the resulting phase inversion membrane structure.4145 However, the addition of salt to a membrane casting solution may also influence the membrane morphology. For example, Park et al. have shown that polyetherimide (PEI) membranes containing ZnCl2 have thicker and denser top layers.46 It has been shown for lithium salts in poly(vinylidene fluoride) (PVDF) that the addition of the salts increases the viscosity of the casting solution and affects the phase inversion process.4749 Similar effects may be found for the addition of HAuCl4 3 3H2O, where the salt will later be reduced to GNPs. Influence of Gold Content and Synthesis Method on the Polyimide Membrane Morphology. The cross sections of

where V is the collected permeate volume in a time t, A is the active membrane surface area, and ΔP is the applied pressure. Rejections were calculated as the percentage of the feed concentration that was retained: Rejection ¼ 100½1 ðCp 3 Cf 1 Þ

IIFP ¼ 100 3 ðPL PC Þ 3 PC 1

the upper part of the reference membranes PRE-0 and ISR-0 are given in Figure 3. Both membranes have an asymmetric structure and show a densification of the matrix toward the upper part of the cross section, which is typical for an asymmetric membrane prepared by phase inversion. Larger pores are visible in the substructure of PRE-0, probably due to the presence of PVP in the membrane casting solution. PVP increases the viscosity of a polymer solution, and it can generally be used as a pore former.5055 Since the PVP-protected GNPs are synthesized in a DMA solution containing an excess amount of PVP to ensure the NP stability,

ð2Þ

where Cp is the permeate concentration and Cf is the feed concentration of the dye. All permeances and rejections shown are averages of three measurements with a standard deviation below 10%. When necessary, measurements were repeated more than three times, to obtain a standard deviation below 10%. 117



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Figure 3. SEM pictures of ISR-0 and PRE-0 cross sections, magnified at 20000.

Figure 4. SEM pictures magnified at 20000 of the cross sections of membranes ISR-1, ISR-2, ISR-3, and ISR-4.

it can be expected that a similar porous structure will be found in the other PRE-membranes (see further below). SEM Pictures for ISR-Membranes. The cross sections of ISR-1 to ISR-4 are given in Figure 4. The membranes containing increasing weight percent of GNPs have rather similar structures as the reference membrane, although the roughness of the cross section increases. Some effects of salt addition to the casting solution on the membrane morphology have been reported in literature for PEI and PVDF membranes.46,49 For these ISR membranes, there seems to have been no large influence of the addition of the chloroauric acid to the polymer solution on the membrane morphology. However, the resolution of SEM is not high enough to fully characterize the structure. SEM Pictures for PRE-Membranes. The PRE-membranes (Figure 5), cast from a solution containing PVP-protected GNPs, have a structure that is clearly different from the ISR-membranes. The pictures of PRE-1 and PRE-2 show a porous substructure similar to the reference membrane PRE-0. For PRE-3 and PRE-4, the pores reach almost to the very top of the membrane. Since the

GNP content of PRE-3 and PRE-4 is higher, the excess amount of PVP will be higher as well, which can explain these more porous structures. PVP is known to increase the membrane porosity, as it may leach from the membrane during its immersion in water, the final step in the phase inversion synthesis process. The cross sections are a lot smoother than those obtained for the ISR membranes. Since the resolution of SEM is not high enough, TEM pictures were made of the top layer and substructure of the cross sections to gain information on the size and dispersion of the GNPs in the membranes. Influence of the Synthesis Method on the GNP Properties. An important parameter of GNPs for purposes such as sensing and photothermal heating is the surface plasmon resonance wavelength. A surface plasmon is a collective movement of the outer band electrons circling a GNP. This electron gas moves at a certain wavelength, and when light of this same wavelength is aimed at the nanoparticle, it is strongly absorbed and turned into thermal energy. The wavelength at which surface plasmon resonance occurs is strongly dependent on the size and shape of 118



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Figure 5. SEM pictures magnified at 20000 of the cross sections of membranes PRE-1, PRE-2, PRE-3, and PRE-4.

This indicated that, regardless the gold concentration, the size and dispersion of GNPs in the membrane top layer were similar. The wavelength found by UVvis spectroscopy increased from 1 to 3 wt % gold and stabilized further. A higher SPR wavelength may indicate a larger GNP size. Alternatively, since the GNPs were formed in a solid membrane matrix, the rise in gold concentration may have resulted in a stronger aggregation of the GNPs. For the PRE membranes, both the DRS and the UVvis wavelength are clearly increasing for increasing gold concentration. This indicates that, both in the top and sublayer, larger GNPs may have been formed at higher gold concentrations. It may also indicate that the GNPs have been insufficiently stabilized at the higher concentrations in the DMA solution during the membrane synthesis. This would lead to an aggregation of GNPs already in the membrane casting solution. Since the DRS and UVvis data are purely indicative and provide no real data on the size and dispersion of the GNPs in the PI membranes, the membrane cross sections were also investigated by TEM. Transmission Electron Microscopy. The TEM pictures of the cross sections of the skin layers and the porous substructures of the membranes are given in Figures 69. In Figures 6 and 8, the top of the membrane is shown, with the skin layer slowly blending into the substructure toward the bottom of the photo. For both methods, the mean particle size is around 35 nm, depending on the gold content of the membrane. There is one clear difference between the IRS and PRE membranes, namely, in the aggregation of the GNPs. In the ISR membranes, hardly any aggregation of GNPs is visible neither in the top layer nor in the substructure, at any concentration of gold. In the PRE membranes, clustering of GNPs occurs both in the skin layer and the substructure. The aggregation is moderate at the lowest GNP

Table 2. Maximum Absorption Wavelengths Obtained in DRS and UVVis Spectroscopy for GNP Containing PI Membranes Prepared by PRE and ISR Methods membrane

DRS wavelength [nm]

UVvis wavelength [nm]

ISR-1

556

556

ISR-2

556

556

ISR-3

555

555

ISR-4

562

562

PRE-1

533

533

PRE-2

545

545

PRE-3

552

552

PRE-4

558

558

the GNPs and on the dielectric environment.29,3537,56,57 For GNPs, the resonance wavelength will generally be at 520 nm or higher. The two preparation methods have a different influence on the GNP size and dispersion in the membrane and will thus affect the SPR behavior differently. Spectroscopy Measurements. To estimate the dispersion and aggregation of the GNPs in the membrane, DRS and UVvis spectra were obtained. Since DRS is a surface characterization technique, these measurements will give information solely on the GNPs found in the top layer of the membrane, near the surface. To obtain information on the GNPs found in the entire membrane, a piece of each membrane was redissolved in DMA and analyzed by UVvis spectroscopy. The wavelength where the maximal absorbance is found is given in Table 2. For the ISR membranes, the SPR wavelength found by DRS was stable for 13 wt % gold and increased slightly for 4 wt % gold. 119



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Figure 6. TEM pictures and particle size distributions of the GNPs in the skin layer of ISR membranes containing 14 wt % GNPs.

Figure 7. TEM pictures and particle size distributions of the GNPs in the porous substructure of ISR membranes containing 14 wt % GNPs.

concentration but aggregates at the higher concentrations. In membranes PRE-3 and PRE-4, the GNP clusters are dominant, and there are hardly any well-dispersed particles visible. These results are in accordance with literature, also indicating that the in situ synthesis methods often lead to a better dispersion and less aggregation of the GNPs compared to the use of presynthesized GNPs.4,6 For the ISR membranes, the amount of GNPs in the top layer was higher than in the substructure. This is probably due to size restrictions in the denser top layer, where the GNPs would remain smaller and did not have the chance to grow closely together. In the more porous substructure, the GNPs have room to grow larger. Since gold nanoparticles are visibly present in the entire cross section of the membrane, it may be assumed that the NaBH4 reducing agent was able to penetrate into the entire bulk of the membrane. These TEM data provide an explanation for the spectroscopic data mentioned above. In the skin layer of ISR membranes, the mean particle diameter is 3 nm for ISR-1 to ISR-3, rising to 5 nm in ISR-4. The DRS data, giving information on the membrane surface and thus mostly on the skin layer, clearly reflect this; the SPR wavelengths remains stable for ISR-1 to ISR-3, slightly rising for ISR-4. In the substructure of ISR membranes, the 3 nm particles are still present, but many larger particles are also visible. Since the UVvis data were taken for redissolved membranes, these larger particles are also taken into account. The amount of larger particles rises at higher gold concentration, and this is reflected in the rise of the SPR wavelength for the higher gold concentrations. The systematically higher wavelengths observed

in DRS compared to the UVvis data may be due to the difference in environment: the solid PI versus the DMA solution. The UVvis data seem to better reflect the size range of the GNPs, since 35 nm GNPs in solution have been indicated to have wavelengths around 530 nm.18,58 SPR wavelengths obtained for membranes containing increasing gold concentrations are higher, especially for PRE-3 and PRE-4. This is caused by the increasing aggregation of the GNPs that is abundantly clear on the TEM pictures. The TEM pictures also indicate that the lower DRS wavelengths obtained for PRE compared to ISR membranes may not be interpreted as an indication of smaller GNPs, since the mean particle size is 3 nm in both cases. However, this difference in wavelength more probably reflects the difference in immediate environment of the GNPs, that are protected by PVP in the PRE membranes and by PI in the ISR membranes. Overall, the TEM pictures prove that higher gold contents in the membranes lead to broader particle size distributions but that the mean particle size remains constant up to 3 wt % gold, regardless the incorporation method. It also shows that the presynthesized GNPs are more prone to aggregation, as was expected from literature.4,6 During the PRE membrane synthesis, there are many steps in which this aggregation may occur, for example, while adding the polymer to the GNP solution, or during the casting and solidification of the membrane. These TEM images indicate that, even though PVP blends well with PI, it did not improve the dispersion of GNPs in the PI membranes. It is possible that, while the PI was added to the GNP solution, the increasing viscosity resulted in an entanglement of the GNPs between the 120



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(520 nm) visible in the top 500 nm of the membrane PRE-1 and larger pores (50100 nm) in PRE-2 to PRE-4. In the ISR membranes, a denser top layer is seen, which may partially be a result of the higher viscosity in the casting solution, induced by the addition of the gold salt. A higher viscosity in the casting solution will generally lead to a denser membrane top layer due to a delayed demixing in the phase inversion synthesis process.46 Influence of Gold on Membrane Filtration Performance. Due to the changes in membrane morphology, the gold content should also have an influence on the membrane performance, even in absence of laser irradiation. This was studied by carrying out IPA and ethanol filtrations with dyes rose bengal (1017 Da) and methyl orange (324 Da). The permeance and rejection for the ISR membranes and the PRE membranes are given in Figure 10. For the ISR membranes, an overall slight increase in membrane permeance was found for higher gold contents. In IPA, the rejection of both dyes was higher than 95%, and the rejection did not depend on the gold content of the membrane. In ethanol, however, the permeances were higher than in IPA and the rejection lowered somewhat. For the PRE membranes, the permeance depends strongly on the gold content in the membrane, showing an overall decrease at increasing gold content. This seems to contradict the increasing porosity clearly seen on the TEM pictures in the top layer. An explanation may be that the very thin (∼nm) skin layer of these membranes is still very dense, thus resulting in such a low permeance. It is commonly supposed that this skin layer has the largest influence on the membrane separation performance. However, it is expected to be only a couple of nanometers thick and cannot be differentiated on the SEM or TEM pictures. Similar to the ISR membranes, there was no strong influence on the rejection in the case of the isopropanol filtrations for the PRE membranes. At the lower gold content, the permeance was higher for PRE membranes than for ISR membranes, which is in accordance with the TEM pictures showing a higher porosity in the skin layer for the PRE membranes. At the higher gold contents, ISR-3 and ISR-4 had higher permeances compared to PRE-3 and PRE-4, which again seems to contradict the very high porosity seen on TEM pictures for the latter. For both methods, it is clear that the incorporation of GNPs and the method used to do so has a strong influence on the membrane structure and performance. The method of incorporation also has a clear effect on the GNP size and dispersion in the membrane matrix and on their SPR wavelength. Effect of Light-Induced Local Photothermal Heating of Membrane on Filtration Behavior. The effect of plasmonic heating of the GNPs in the membranes on the membrane performance was finally tested as a possible application for these membranes. Dead-end filtrations of methyl orange in ethanol were repeated for the PRE and ISR membranes under laser irradiation. The laser irradiation of the GNPs induces plasmonic heating inside the membrane matrix. As our previous studies has indicated, this local heating of the membrane can have a positive effect on the membrane permeance without affecting the membrane selectivity.26,27 The performance under laser irradiation was compared to the original performance of the membranes in Figure 11. For both the ISR and the PRE membranes, the IRR in permeance induced by plasmonic heating increased at higher gold contents. The absolute differences in permeance are similar for

Figure 8. TEM pictures and particle size distributions of the GNPs in the skin layers of PRE membranes containing 14 wt % GNPs.

Figure 9. TEM pictures of the porous substructure of PRE membranes containing 14 wt % GNPs. No accurate size distributions of the GNPs could be measured due to the strong aggregation.

PI chains, preventing a good dispersion of the GNPs in the casting solution. The effect of PVP as a pore-former on the membrane structure was also visualized on the TEM pictures. There are small pores 121



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Figure 10. Isopropanol and ethanol permeance and rejection of dyes rose bengal and methyl orange for membranes prepared by ISR (A) and PRE (B).

Figure 12. Temperature increase upon laser irradiation for a PI reference membrane and GNP containing ISR and PRE PI membranes wetted by ethanol (ambient temperature 20 °C).

Figure 11. Irradiation improvement factors for permeance and rejection of ethanol + methyl orange mixtures obtained by laser irradiation for PRE and ISR membranes.

followed by the increasing percentual difference in permeance. However, for both methods, the temperature stabilizes at 3 wt % of gold, which is not reflected in the filtration results. Higher temperatures are obtained for the ISR membranes compared to the PRE membranes, which is probably due to the problems with aggregation in the PRE membranes, diminishing the photothermal effect of the GNPs. However, this difference in temperature is not reflected in the filtration data. It should be kept in mind that the heating experiments were carried out in a static system, while during the filtrations there is heat dissipation to a flowing solvent stream involved, leading to a more complicated mass and heat transfer process. During filtrations, the heat produced by the

both methods. The rejection is in neither case affected by the laser irradiation, and the differences fall within the expected experimental error. This also indicates that there was no unwanted influence of the laser heating on the membrane material, such as melting. In previous works, it was already shown that no defects were induced by heating the GNP containing membranes at low laser intensities, since the membrane performance returned to the original state after turning off the laser.26,27 When these data are compared to the measured temperature increase in the ethanol-wetted membrane under laser irradiation (Figure 12), it is clear that the rising temperature trend is 122



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Figure 13. Combined data taken from this study ()), reference 26 () and reference 60 (() comparing the conventional and laser-irradiated permeance and rejection for different GNP containing membranes in filtrations of IPA and ethanol dye solutions.

GNPs inside the membrane is dissipated in the medium, including the permeating solvent. It was shown previously that the permeating solvent may have a cooling effect, the extent of which depends on the intrinsic permeability of the membrane.27 Since the ISR membranes at the higher gold contents have a higher intrinsic ethanol flux than the PRE membranes (see Figure 10), the solvent cooling effect will be stronger. If the solvent cooling effect is large enough, it may result in a lower IIF (Figure 11) even though these membranes reached higher temperatures under static conditions. To fully comprehend the mechanism involving the flux increase by plasmonic heating, more data should still be collected. The continuous green argon laser beam used in these experiments emits light at wavelength of 514 nm, which is lower than the actual surface plasmon wavelength of the GNPs (see Table 1) but close enough to expect a heating effect. Also, the illuminated membrane surface was only 40% of the active membrane surface. Due to both these parameters, the experiments were carried out at a suboptimal level, and even stronger increases in permeance can be expected when the laser is at the exact SPR wavelength and when the membrane is illuminated entirely. Also, some losses in laser intensity are expected when the laser beam travels through the dye feeds, for example, due to reflection by impurities. In upscaled filtration units, the light should be transferred to the membrane more efficiently, for example, by use of optical fibers incorporated into the membrane support. In Figure 13, the new data are combined with the previously obtained data.26,27 The added dashed line indicates the 1:1 data, where the irradiated permeance and rejection are equal to the nonirradiated results. It is clear that, while the rejection data fluctuate around this 1:1 line, the permeance data are consistently above this line. This confirms that overall rejections are not significantly influenced by the irradiation while permeances are always increased. To increase fluxes of a given membrane without lowering its selectivity is a highly desired but rarely found effect in membrane technology.27

’ CONCLUSIONS Two methods to prepare GNP containing polymeric solids were compared, namely, the incorporation of preformed PVPprotected GNPs into a PI membrane and the in situ synthesis of GNPs inside a PI membrane matrix. In both cases, GNPs are obtained with an average size of 3 nm in the top layer of the membrane. However, there is a clear difference in the membrane behavior and the GNP distribution. When preformed GNPs are used, the excess of PVP in the casting solution induces a higher

porosity in the membrane, and the GNPs are more prone to aggregation. It is possible that PVP is not the optimal GNP stabilizer during the specific PI membrane synthesis procedure reported here. When the GNPs are synthesized in situ, the GNPs are dispersed very well, with smaller nanoparticles formed in the dense top layer of the membrane and larger nanoparticles in the porous sub layer, where more space is available. The better dispersion also resulted in a stronger heating of the composite material upon laser irradiation. The permeance of GNP containing PI membranes in SRNF could thus be increased by plasmonic heating of the GNPs in the membrane by means of a green argon ion laser. Higher IIFs were found for higher gold contents, regardless the synthesis method. The aggregation of the preformed PVP-protected GNPs in the membrane unexpectedly did not have a large influence on the photothermal filtration behavior of the membranes. These data further confirm that localized photothermal heating of a membrane during a filtration process can significantly enhance the separation, by inducing an increased permeance without lowering rejections, a most remarkable combination. The GNP containing PI membranes may also be used for other applications, such as combined catalysis and membrane separation processes.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; fax: 32 1632 1998; phone: 32 1632 1549.

’ ACKNOWLEDGMENT K.V. acknowledges the Fund of Scientific Research Flanders (FWO-Vlaanderen) for financial support as a research assistant. This research was done in the framework of an I.A.P.-PAI grant (IAP 6/27) sponsored by the Belgian Federal Government, of a GOA grant from K.U. Leuven and of long-term structural fundingMethusalem funding by the Flemish Government. Professor J. Billen of the Laboratory for Entomology of K.U. Leuven, Leuven, Belgium, is kindly acknowledged for assisting with TEM measurements. ’ REFERENCES (1) Porel, S.; Venkatram, N.; Rao, D. N.; Radhakrishnan, T. P. In situ synthesis of metal nanoparticles in polymer matrix and their optical limiting applications. J. Nanosci. Nanotechnol. 2007, 7 (6), 1887–1892. 123



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