0000000000000000000000
Trang 1Published: November 23, 2011
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
’ 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
nano-particles (GNPs).117Overall, thefirst 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.4,6,15A
direct comparison of the two methods has so far only been done
by Dammer et al for GNPs synthesized in
poly([2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene]vinylene).13However,
poly-mer degradation occurring in the case of in situ reduction,
through oxidation by the GNP precursor (H[AuCl4
]/tetraocty-lammonium bromide/tetraocty]/tetraocty-lammonium 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,2224and 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
between the two methods has been made for a nanofiltration membrane
Solvent resistant nanofiltration (SRNF) involves the separa-tion 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 How-ever, most commercially available membranes for SRNF com-bine high rejections for low molecular weight (MW) compounds with lowfluxes
Recently, we have studied the effects of plasmonic heating of GNPs incorporated into nanofiltration membranes on the mem-brane performance, showing an overall increase of the memmem-brane 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).6,2833In most membrane processes, developing a membrane with a higher selectivity is coupled to a loss in perme-ability and visa versa Photothermal heating of GNP containing membranes is thus of high interest as a potential route to over-come this traditionalflux-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
ABSTRACT:Gold nanoparticle containing polymer materialsfind 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 Thefiltrations were repeated while the gold nanoparticles were plasmonically heated by a green Argon ion laser beam, resulting in localized heating of the membrane and increasedfluxes
Trang 2for producing SRNF membranes.25 PI membranes containing
GNPs have been prepared by adding presynthesized
PVP-protected GNPs by Mertens et al.,8but 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
char-acterized by UVvis spectroscopy and diffuse reflection
spec-troscopy (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 solventfiltrations
’ 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(HAuCl43 3H2O) and
sodium borohydrid (NaBH4, >98.5%) were obtained from
Sigma-Aldrich Poly(vinylpyrrolidone) (10 000 g mol1), N,N0
-dimethyl-acetamide (99.5%, DMA), tetrahydrofuran (99.5%, THF),
iso-propanol (99.5%, IPA), and absolute ethanol (EtOH) were
obtained from Acros All used water was desionized
Membrane Synthesis.Membranes were synthesized
accord-ing to two different methods For the incorporation of
pre-synthesized GNPs (PRE), PVP-protected GNPs were prepared
in DMA similar to the synthesis methods described by Teranishi
et al.58and Mertens et al.8Solutions of HAuCl43 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 NaBH4solution in DMA (2 g) was added under
vigorous stirring (molar ratio NaBH4/gold = 5), and
immedi-ately, 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
spectro-photometer, and the typical dark red color showed as a large peak
at 530 nm, corresponding to the plasmon absorbance band of the
GNPs.58PI 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
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 (250μm 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 correspond-ing with the membrane containcorrespond-ing 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.,21HAuCl43 3H2O was added to a PI solution pre-pared 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 HAuCl43 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 evapo-ration step (30s), they were immersed into a water coagulation bath After the immersion, the membranes were moved imme-diately into a solution of NaBH4in 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 branes, the cross sections were examined by TEM The mem-branes were dried and then embedded into Araldite resin Semithin sections for light microscopy with a thickness of 5μm were made with a Reichert Ultracut E microtome Finally, cubic samples of
Table 1 Membrane Compositions (Weight in g in the Casting Solution) for Reference and GNP Containing Membranes Prepared by Two Methods (PRE and ISR)
Trang 3about 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
mem-branes 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, 17μM) 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
calculat-ing 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 t1
where V is the collected permeate volume in a time t, A is the
active membrane surface area, andΔP is the applied pressure
Rejections were calculated as the percentage of the feed
con-centration that was retained:
Rejection ¼ 100½1 ðCp3 Cf 1Þ 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%
The irradiation improvement factor (IIF) is calculated as the percentual increase in permeance or rejection when the mem-brane is irradiated, as follows:
IIFP ¼ 100 3 ðPL PCÞ 3 PC 1 ð3Þ
IIFR ¼ 100 3 ðRL RCÞ 3 RC 1 ð4Þ where PCand RC are the conventual permeance and rejection (measured without laser irradiation) and PLand RLare the per-meance and rejection measured when the membrane is irradiated
’ RESULTS AND DISCUSSION
Since the size, distribution, aggregation, and dielectric envi-ronment all have a strong influence on the surface plasmon resonance behavior of the GNPs,29,3440the GNPs inside the PI membranes were thoroughly characterized In the ISR-membranes, the GNPs are formed inside the solid membrane matrix by chemical reduction of a gold salt, wherein the mem-brane polymer itself acts as a stabilizer In the PRE-memmem-branes, 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 How-ever, 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 ZnCl2have thicker and denser top layers.46It has been shown for lithium salts in poly(vinylidenefluoride) (PVDF) that the addition of the salts increases the viscosity of the casting solution and affects the phase inversion process.4749Similar effects may
be found for the addition of HAuCl43 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 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 sub-structure of PRE-0, probably due to the presence of PVP in the membrane casting solution PVP increases the viscosity of a poly-mer solution, and it can generally be used as a pore forpoly-mer.5055 Since the PVP-protected GNPs are synthesized in a DMA solution containing an excess amount of PVP to ensure the NP stability,
Figure 1 Schematic representation of a dead-end filtration cell
equipped for laser irradiation of the GNP containing membrane during
separations
Figure 2 Chemical structures of dye rose bengal (1017 Da) and methyl orange (327 Da)
Trang 4it 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 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
Trang 5the GNPs and on the dielectric environment.29,3537,56,57For
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
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 con-centration 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 concentra-tion 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 mem-branes, clustering of GNPs occurs both in the skin layer and the substructure The aggregation is moderate at the lowest GNP
Figure 5 SEM pictures magnified at 20000 of the cross sections of membranes PRE-1, PRE-2, PRE-3, and PRE-4
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] UV vis wavelength [nm]
Trang 6concentration 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
to-gether 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
NaBH4reducing 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 dif-ference 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 increas-ing 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 indi-cation of smaller GNPs, since the mean particle size is 3 nm in both cases However, this difference in wavelength more prob-ably 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, regard-less the incorporation method It also shows that the presyn-thesized 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 im-prove 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 Figure 6 TEM pictures and particle size distributions of the GNPs in
the skin layer of ISR membranes containing 14 wt % GNPs Figure 7.the porous substructure of ISR membranes containing 14 wt % GNPs.TEM pictures and particle size distributions of the GNPs in
Trang 7PI 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
(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 mem-brane 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 rejec-tion 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 separa-tion 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 per-formance was finally tested as a possible application for these membranes
Dead-end filtrations of methyl orange in ethanol were re-peated 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 indi-cated, this local heating of the membrane can have a positive effect on the membrane permeance without affecting the mem-brane 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 per-meance induced by plasmonic heating increased at higher gold contents The absolute differences in permeance are similar for
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
Figure 8 TEM pictures and particle size distributions of the GNPs in
the skin layers of PRE membranes containing 14 wt % GNPs
Trang 8both 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
un-wanted 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
re-turned 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
irradia-tion (Figure 12), it is clear that the rising temperature trend is
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 photother-mal 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 thefiltrations there is heat dissipation to a flowing sol-vent stream involved, leading to a more complicated mass and heat transfer process Duringfiltrations, the heat produced by the
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 11 Irradiation improvement factors for permeance and
rejec-tion of ethanol + methyl orange mixtures obtained by laser irradiarejec-tion for
PRE and ISR membranes
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)
Trang 9GNPs inside the membrane is dissipated in the medium,
includ-ing the permeatinclud-ing 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.27Since
the ISR membranes at the higher gold contents have a higher
intrinsic ethanolflux 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 theflux
increase by plasmonic heating, more data should still be collected
The continuous green argon laser beam used in these
experi-ments 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
impu-rities In upscaledfiltration 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.26,27The 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 datafluctuate
around this 1:1 line, the permeance data are consistently above this
line This confirms that overall rejections are not significantly
influ-enced by the irradiation while permeances are always increased To
increasefluxes 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
PVP-protected 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 photothermalfiltration behavior of the membranes These data further confirm that localized photothermal heating of a mem-brane during afiltration process can significantly enhance the separation, by inducing an increased permeance without low-ering rejections, a most remarkable combination The GNP con-taining PI membranes may also be used for other applications, such as combined catalysis and membrane separation processes
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: ivo.vankelecom@biw.kuleuven.be; fax: 32 1632 1998; phone: 32 1632 1549
’ ACKNOWLEDGMENT
K.V acknowledges the Fund of Scientific Research Flanders (FWO-Vlaanderen) forfinancial 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
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