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Extractant Assisted Synthesis of Polymer Stabilized Platinum andPalladium Metal Nanoparticles for Sensor ApplicationsDmitri N. Muravieva; Maria Isabel Pividorya; José Luis Montañez Sotoa; Salvador Alegreta

a Grup de Sensors i Biosensors, Departament de Química, Universitat Autònoma de Barcelona,Bellaterra, Barcelona, Spain

To cite this Article Muraviev, Dmitri N. , Pividory, Maria Isabel , Soto, José Luis Montañez and Alegret, Salvador(2006)'Extractant Assisted Synthesis of Polymer Stabilized Platinum and Palladium Metal Nanoparticles for SensorApplications', Solvent Extraction and Ion Exchange, 24: 5, 731 — 745To link to this Article: DOI: 10.1080/07366290600851588URL: http://dx.doi.org/10.1080/07366290600851588

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Extractant Assisted Synthesis of PolymerStabilized Platinum and Palladium MetalNanoparticles for Sensor Applications

Dmitri N. Muraviev, Maria Isabel Pividory,

Jose Luis Montanez Soto, and Salvador Alegret

Grup de Sensors i Biosensors, Departament de Quımica, Universitat

Autonoma de Barcelona, Bellaterra, Barcelona, Spain

Abstract: Extractant-assisted synthesis of platinum and palladium polymer-stabilized

metal nanoparticles (PSMNP) was carried out for the first time. The synthesis included

the following sequential steps: a) loading of extractant (tributyl-phosphine oxide,

TBPO) with the desired metal ion; b) preparation of a membrane “cocktail” by

mixing a metal-containing extractant, solution of the polymer (PVC or polysulfone)

and plastisizer; c) membrane deposition and metal reduction inside the membrane

(intermatrix synthesis of PSMNP) by using either a chemical or an electrochemical

reduction technique. The electro conductivity of the resulting polymer-metal nanocom-

posite membrane appeared by several orders of magnitude higher than that of the

metal-free polymer. The mass-exchange properties of PSMNP-containing

membranes were shown to depend on both the type of the polymer and the

membrane deposition technique.

Keywords: Polymeric membrane, extractant, metal nanoparticles, sensors

INTRODUCTION

Nanometer-sized metal particles or Metal NanoParticles (MNP) are objects of

great interest in modern chemical research[1–5] due to their unique physical

and chemical properties (e.g., electrical, magnetic, optical, ionization

Received 22 June 2005, Accepted 31 October 2005

Address correspondence to Dmitri N. Muraviev, Grup de Sensors i Biosensors,

Departament de Quımica, Universitat Autonoma de Barcelona, 08193 Bellaterra,

Barcelona, Spain. E-mail: dimitri.muraviev@uab.es

Solvent Extraction and Ion Exchange, 24: 731–745, 2006

Copyright # Taylor & Francis Group, LLC

ISSN 0736-6299 print/1532-2262 online

DOI: 10.1080/07366290600851588

731

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potentials, etc.), which are distinct from both those of the bulk metal and those

of isolated atoms and molecules. MNP can be so fragile and unstable that if

their surfaces touch, they will fuse together, losing their special shape and

properties. The development of the polymer-stabilized MNP (PSMNP) is

one of the most promising solutions to the MNP stability problem, and as

such, they attract great attention of scientists and technologists.[6]

The areas of practical applications of metal-polymer nanocomposites are

very wide. They find applications as electroconductive pastes and glues,

special coatings, paints and varnishes, rheomagnetic fluids, antifrictional

polymeric coatings, construction materials in aviation and space technology,

catalysis and many others.[7–16] The PSMNP-containing polymeric

membranes are used as electroconductive and optical materials, supported

catalysts, and in some other fields. The PSMNP do not yet find a wide use

in the field of sensors and biosensors although in some recent publications

one can find a clear anticipation of their applicability in different sensing

systems.[17–21]

The general methods for the preparation of the PSMNP can be divided

into two main groups,[16,22–25] namely, the physical, and the chemical one.

An example of physical methods is the cryochemical deposition of metals

on polymeric supports. The chemical methods include, for example, a

thermal decomposition of precursor compounds in polymer, the formation

of the PSMNP at the polymerization stage, and an intermatrix synthesis of

the PSMNP.

The intermatrix synthesis of the PSMNP can be based on the use of solid-

phase-incorporated-reagents, which permit to immobilize metal ions or metal

complexes in non-functionalized polymeric matrix followed by metal

reduction (intermatrix synthesis stage). The solid-phase-incorporated-

reagents (SPHINER) are known as extraction chromatographic materials

(ECM),[26–29] solvent impregnated resins (SIR),[30–36] and intercalates

(IC).[37–39] Only some of the SPHINER systems (namely IC) start to be

applied for the preparation of MNP.[40,41]

The solid supports in the majority of SPHINER systems represent pre-

shaped matrices (organic or inorganic) of different physical form (mainly

granulated or powdered). The polymeric supports in a majority of

SPHINER systems are insoluble (in organic solvent) crosslinked polymers.

This makes it hardly possible to change the physical form of the support

which sometimes is required in the design of sensors. This problem can be

successfully solved by using non-crosslinked polymers, which are soluble in

organic solvents and insoluble in water, as supports for the SPHINER and

therefore for the PSMNP produced in this way. The polymers of this type

such as, for example, poly(vinyl chloride), PVC, are used as both matrices

for the synthesis of PSMNP[42] and for enzyme immobilization in biosensors

of different types.[43–45] The SPHINER to be used for the intermatrix

synthesis of the PSMNP can be extractants selective to the desired metal

ions. To the knowledge of the authors the use of the SPHINER systems of

D. N. Muraviev et al.732

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this type for the intermatrix synthesis of PSMNP has never been described in

the literature. [See, for example, Ref. 1–6]

In this paper for the first time we demonstrate the applicability of the

SPHINER approach to synthesize Pt and Pd MNP inside inert (non-functiona-

lized) polymer matrices such as, PVC and polysulfone (PS). The PSMNP-

based membranes have been shown to substantially increase (by several

orders of magnitude) their electric conductivity in comparison with that of the

metal-free membranes. The mass-transfer properties of PSMNP-containing

membranes have been shown to depend on the membrane deposition technique.

EXPERIMENTAL

Chemicals

Metal salts (PtCl4, PdCl2, KCl, and K4[Fe(CN)6], all obtained from Aldrich,

Germany), acids, and organic solvents, xylene and tetrahydrofurane

(Panreac, S.A. Spain) of p.a. grade were used as received. The polymers

PS, Ultrason S 3010 natur (BASF, Germany), and PVC (Fluka), the extractant,

tributyl-phosphine oxide (TBPO), and the plastisizer, o-nitrophenyl-octyl-

ether, (Sigma, Germany) were also used without any pretreatment. MiliQ

water was used in all of the experiments that were carried out.

Methods

The experimental procedure for the preparation of the PSMNP-based electro-

des included the following sequential steps:

1. loading of the extractant with metal ion (Pt or Pd);

2. preparation of the membrane “cocktail” by mixing the polymer solution

with the plasticizer and the extractant preloaded with the desired metal

ion;

3. deposition of the membrane on the surface of the graphite-epoxy

electrode (GEC) and

4. electrochemical reduction of the metal inside the membrane (intermatrix

synthesis of PSMNP) followed by characterization of the modified GEC

electrode by cyclic voltammetry.

The loading of the extractant with the desired metal ion was carried out by

vigorous agitation of 5 ml of 0.5 M TBPO solution in xylene and 10 ml of

0.01 M Pt(IV) or Pd(II) chloride solutions in 1 M HNO3 for 8 hours.[46]

This time has been shown to be sufficient for a 100% extraction of both Pt

and Pd by TBPO. Then the organic phase was separated from the aqueous

one and xylene was evaporated on a rotor evaporator followed by dissolution

of the residue in 1 ml of THF. In the case of the PVC-based membranes, the

Synthesizing PSMNP for Sensor Applications 733

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membrane cocktail was composed of 33% of PVC, 67% of the plasticizer, and

0.1 ml of solution of TBPO in THF. All cocktail components were dissolved

in THF (0.05 ml of THF per 1 mg of PVC). In the case of PS-based

membranes, the membrane cocktail contained a 5% PS solution in THF and

the solution of TBPO in THF (0.1 ml of TBPO solution per 0.5 ml PS

solution). An aliquot of the membrane cocktail was deposited on the surface

of the GEC electrodes, which were prepared as described elsewhere.[47] The

membrane deposition was carried out by using either drying (PVC and PS)

or phase inversion (PS) techniques.[48–50] In the last case either water or a

DMF-H2O mixture (DMF:H2O ¼ 2:1) were used as non-solvents to form

the PS membrane. The reduction of the metals inside the membranes was

made by applying a constant negative potential (21.6 V for Pt) during 10

minutes. PS and PVC membrane samples were also prepared by using a con-

ventional casting on a Teflon support followed by a chemical reduction with

either a 50% aqueous formaldehyde solution or 0.1 M NaBH4 in 1:1 water-

ethanol mixture. After the metal reduction the membranes were dissolved in

THF to prepare a PSMNP “ink,” which was characterized by a transmission

electron microscopy (TEM) to determine the size and structure of metal nano-

particles. The PSMNP-modified GEC electrodes were characterized by cyclic

voltammetry by using an Autolab PGSTAT 10 potentiostat-galvanostat (Eco

Chemie) supplied with an auxiliary platinum electrode 52–671 (Crison) and

a reference Ag/AgCl electrode ORION 900200. The cyclic voltammetry

tests were carried out to evaluate electrochemical and mass-transfer character-

istics of PSMNP-modified GEC electrodes by using a model Fe(II) , Fe(III)

redox system.

RESULTS AND DISCUSSION

The proposed extractant-assisted synthesis of PSMNP refers to the chemical

methods of preparation of polymer-metal nanocomposites. These methods

are based, for example, on decomposition and/or reduction of metal ions or

compounds inside polymeric matrices (inert or functionalized), polymeriz-

ation of metal-containing monomers followed by metal reduction, and some

other techniques.[22–24] In comparison with known chemical methods the

proposed technique has several features. Indeed, this technique is based on

the use of soluble polymers and extractants pre-loaded with the desired

metal ions. The necessary condition in this case is the miscibility of the

polymer and extractant solutions. To fulfill this requirement one usually

needs to change the extractant solvent after the loading with metal ions, as

in many instances the favorable extraction conditions are based on the use

of solvents, in which the polymers are not soluble (e.g., xylene in our case).

This condition is of particular importance when using the phase inversion

technique to manufacture polymeric membranes as in this case water

miscible organic solvents are usually used to prepare the membrane casting

D. N. Muraviev et al.734

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solution. After metal reduction the polymer-immobilized extractant appears to

be “metal free” and can be used for the second metal loading-reduction cycle

to accumulate more MNP in the polymer. Finally, the extractant can be either

left inside the polymer matrix or removed from the polymer by its re-extrac-

tion. In the first case it may serve as a plasticizing agent.[43] All the above

features permit to distinguish the proposed technique as an independent

method, which has certain advantages in comparison with the known ones.

The interpolymer synthesis of PSMNP proceeds, in fact, during the metal

reduction stage either in the course of the treatment of the pre-shaped

membrane with the reducing agent (chemical or ex situ reduction) or when

the metals are reduced electrochemically (in situ reduction) inside the

membrane deposited on the GEC electrode. The chemical reduction is

accompanied by visual changes of the membrane color from slightly yellow

(associated with the yellowish color of Pt-TBPO or Pd-TBPO solution) to

dark brown which indicates the formation of MNP inside the matrix. In

case of the use of a formaldehyde solution as the reducing agent, the metal

reduction occurs far slower than when using sodium borohydride. The

reduction of metals in both cases proceeds by the following reaction schemes:

PtCl4 þ 2HCHOþ 2H2O �! Pto þ 2HCOOHþ 4HCl ð1Þ

PdCl2 þ HCHOþ H2O �! Pdo þ HCOOHþ 2HCl ð2Þ

2PtCl4 þ NaBH4 þ 2H2O �! 2Pto þ HBO2 þ 7HClþ NaCl ð3Þ

4PdCl2 þ NaBH4 þ 2H2O �! 4Pdo þ HBO2 þ 7HClþ NaCl ð4Þ

Quite obviously the higher reduction power of NaBH4 in comparison with

HCHO results in a faster metal reduction inside the polymer matrix.

Another reason for faster metal reduction can be associated with different

diffusivities of formaldehyde and molecular hydrogen (essential reducing

agent in case of sodium borohydride: NaBH4þ 2H2O ! 4H2þNaBO2)

into the polymeric membrane, where reactions 1–4 take place.

Figure 1 demonstrates the results of MNP synthesis with and without

stabilization. Fig. 1a shows the TEM images of Pt-MNP synthesized inside

the PVC matrix by using the SPHINER technique after reduction with an

aqueous formaldehyde solution. As it is seen, the size of the platinum nanopar-

ticles does not exceed 5–7 nm. The particles are well separated from each

other due to stabilization with PVC chains inside the polymeric matrix,

which prevents them from aggregating. Similar images were obtained for

Pt-PVC membranes, which were kept under ambient laboratory conditions

for 3 and 6 months. This testifies to the high stabilizing effect of the

polymer matrix towards PSMNP.

Figures 1b and 1c illustrate the case of Pt-MNP formation outside the

PVC matrix, i.e. in the solution of the reducing agent (formaldehyde),

which occurs in the course of the same experiment due to the partial extraction

of platinum ions from the polymer surface by a formaldehyde solution. As is

seen in Fig. 1b, in the absence of the stabilizing agent, the size of Pt-MNP

Synthesizing PSMNP for Sensor Applications 735

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exceeds by almost one order of magnitude that of Pt-PSMNP (approx. 40–

50 nm vs 5–7 nm). Moreover, the non-stabilized Pt-MNPs tend to

aggregate in the solution phase so that they form particle “dimers,”

“trimers,” and finally big particle clusters of micrometric size (see Fig. 1c).

This effect can be ascribed to the Oswald ripening,[51] known to be the

mechanism of particles’ growth where small particles dissolve, and are

consumed by larger particles. As a result the average nanoparticle size

increases and the particle concentration decreases. In the case of PSMNP

this effect is not observed.

The mechanism of MNP stabilization with polymers can be explained by

the substantial increase of viscosity of the immobilizing media (the polymer

Figure 1. Formation of Pt-MNP with (a) and without (b, c) stabilization. Conditions:

(a) synthesis of Pt-MNP by SPHINER technique (see text) inside PVC matrix;

(b) formation of big Pt-MNP in aqueous solution of reducing agent (formaldehyde);

(c) further aggregation of Pt-MNP in formaldehyde solution.

D. N. Muraviev et al.736

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matrix). Indeed, as follows from the Smoluchowsky equation, the rate

constant of particle coagulation, kc, is inversely proportional to the viscosity

of the media, h, i.e.

kc ¼4kT

hð5Þ

Here k stands for the Boltzman constant, and T is the temperature. A similar

conclusion follows from the Stokes-Einstein equation, which permits to

determine the diffusion coefficient of a spherical particle of radius r in a

viscous medium:

D ¼kT

6phrð6Þ

Nevertheless, it has been shown by Cole et al.[52] that the mobility of gold

MNP in poly(t-butyl acrylate)/gold composite decreases by 2 or 3 orders of

magnitude compared with that predicted by the Stokes-Einstein equation.

The authors ascribe this discrepancy to strong bridging interactions between

Au-MNPs and the chain segments of the stabilizing polymeric matrix. As a

result, the mobility of nanoparticles inside the polymer substantially

decreases and the matrix, in turn, appears to be somewhat “crosslinked” so

that the effective viscosity of the polymer increases by a factor of �4.[53]

On the other hand, the stabilization effect can also be ascribed to a substantial

decrease of the energy of particle-particle interaction in the PSMNP systems

(e.g., in polymer-MNP composites) versus non-stabilized MNP dispersions.

The potential energy of the attraction (Ur) between two spherical particles

of radius r can be described by the following simplified expression:

Ur �Ar

12loat r � lo ð7Þ

where A is the effective Hamaker constant having the dimensions of energy,

and lo is the minimum distance between particle surfaces. The value of the

Hamaker constant A is known to be close to kT for the polymer particles,

while for the metal dispersions it is far higher.[22,23]

Figures 2 and 3 show high resolution TEM images of Pt- and Pd-PSMNP,

respectively, obtained by using the SPHINER technique inside a PVC stabiliz-

ing matrix. As it is seen both Pt and Pd form nanoparticles with an average size

of 6–7 nm, although the formation of larger MNP clusters is also observed in

some instances (see Fig. 2a). As it is also seen, both Pt- and Pd-MNP are

highly crystalline and give clear diffraction patterns (see Fig. 2c and 3b),

which can be used to both estimate the parameters of their crystalline

structure and produce inverse FFT images to confirm that the diffraction

pattern belongs to a certain particle (see Fig. 3c).

The PSMNP-ink obtained after the ex situ (chemical) reduction of metals

inside the polymer can be used to deposit PSMNP-containing membranes on

Synthesizing PSMNP for Sensor Applications 737

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Figure 2. High resolution TEM (HRTEM) images of PVC immobilized Pt-PSNMP

(a, b) and (c) diffraction pattern of Pt nanoparticles shown in (b).

Figure 3. (a) HRTEM images of PVC immobilized Pd-PSNMP, (b) diffraction

pattern of Pd nanoparticles shown in (a), and (c) inverse FFT image of Pd particle

shown in (b).

D. N. Muraviev et al.738

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the surface of the GEC electrodes to study the performance of the PDMNP-

modified electrodes by using, for example, cyclic voltammetry. The same

can be done by using membrane cocktails containing unreduced metal ions,

in which reduction is done in situ, i.e., after deposition of the membrane on

the surface of the GEC electrode followed by exposition of the modified

electrode at a negative potential corresponding to the reduction potential of

a given metal. In both cases the GEC electrodes serve as a conventional

tool for studying the electrochemical and mass-transfer properties of

polymer-MNP composites of different types. The results of comparative

cyclic voltammetric studies of Pt-MNP-poly(vinyl chloride) (PVC) and

Pt-MNP-polysulfone (PS) composite membranes deposited on the GEC

electrodes by different techniques are shown in Figs. 4, 5, and 6.

Figure 4 shows a comparison of the performance of the GEC electrodes

modified with a metal-free PVC membrane (see curve 3) and electrodes

modified with Pt-PSMNP-PVC-composite membranes (see curves 1 and 2).

In all cases the membranes were formed on the electrode surface by a

dropwise deposition of the corresponding membrane “cocktail” containing

either a platinum-free extractant (tributyl-phosphine oxide) or an extractant

pre-loaded with platinum ions followed by drying the membrane at 408C. Inall cases the Pt-PSMNP inside the polymer matrix were formed by in situ

electrochemical reduction of the platinum. The metal reduction was carried

out by exposing the modified GEC electrode at 21.6 V for 10 min in

Figure 4. Cyclic voltammetry of GEC sensors modified with PVC-Pt-MNP (curves 1

and 2) and MNP-free PVC (curve 3) membranes. Conditions: membranes formation by

drying, scan rate 3 mV/s, K4Fe(CN)6 5 mM, acetate buffer pH 5, KCl 0.1 M. Electro-

chemical Pt reduction during 10 minutes at 21.6 V.

Synthesizing PSMNP for Sensor Applications 739

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acetate buffer. As can be seen, the presence of Pt-MNP inside the PVC matrix

dramatically changes the membrane electroconductivity so that it increases by

several orders of magnitude. At the same time, the mass-transfer properties of

the membrane appear to be quite poor, what follows from the absence of Fe(II)

oxidation and Fe(III) reduction peaks of the analyte under study

(K4[Fe(CN)6] , K3[Fe(CN)6]).

The situation improves when moving from PVC-Pt to polysulfone (PS)-Pt

membranes. This clearly follows from the comparison of curves b and c in

Fig. 5. Nevertheless, the mass-transfer characteristics of the sensor still

remain unsatisfactory. The desired result is achieved when changing the

membrane deposition technique from drying to the phase inversion. Indeed,

as follows from the comparison of results shown in Fig. 6, the shape of vol-

tammograms of the GEC-PS-Pt sensors with membranes obtained by

different methods differs dramatically from each other. Moreover, the use of

a more active non-solvent (H2O instead of DMF-H2O mixture) allows for

further improvement of mass-transfer characteristics of membranes, which

is clearly seen in Fig. 6b. The first (and most likely the main) reason for

such a remarkable difference in the sensors performance is attributed to the

different morphology of a PS-membrane obtained by using the drying and

the phase inversion techniques. Unlike the first technique, the second one

permits to obtain PS-membranes with a highly developed macroporous

structure resulting in far higher rates of mass transfer.[33,49] The second

Figure 5. Comparison of cyclic voltammetry of GEC sensors modified with PVC (a),

PVC-Pt_MNP (b) and PS-Pt (c). Conditions: membranes formation by drying; scan rate

3 mV/s, K4[Fe(CN)6] 5 mM in acetate buffer pH 5, KCl 0.1 M. Electrochemical Pt

reduction for 10 min. at 21.6 V.

D. N. Muraviev et al.740

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Figure 6. Comparison of cyclic voltammetry of GEC sensors modified with PS-Pt-

MNP prepared by using different techniques: (a) Pt-free PS, conventional drying (1);

phase inversion with DMF-H2O mixture; (b) phase inversion with DMF-H2O mixture

(2) (1) and H2O (2).

Synthesizing PSMNP for Sensor Applications 741

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reason is associated with a higher hydrophilicity of the PS matrix in compari-

son with PVC. This conclusion clearly follows from the structure of repeated

units of PS chains shown in Scheme 1 and also from the values of water

contact angles (u) reported by Palmers for PVC (u ¼ 908) and PS

(u ¼ 76.68) (Scheme 1).[54]

In conclusion, we would like to emphasize that the results presented in

this work represent the first successful attempt to use an extractant assisted

intermatrix synthesis to produce membranes containing polymer stabilized

metal nanoparticles.

CONCLUSIONS

From the results obtained in this study the following conclusions can be

derived:

1. The intermatrix synthesis of PSMNP can be successfully carried out by

using metal-selective extractants as carriers for immobilization of metal

ions inside a polymer matrix prior to their reduction.

2. The reduction of metals inside the polymeric membrane (intermatrix

synthesis of PSMNP) can be carried out after membrane formation by

using either chemical or electrochemical techniques. In both cases the

size of Pt- or Pd-MNP appears to be 5–7 nm.

3. The electrical conductivity of the PSMNP-containing PVC or PS

membranes increases by several orders of magnitude in comparison

with metal-free membranes.

4. The mass-transfer characteristics of PSMNP-modified electrodes depend

on both the type of the polymer and the membrane deposition technique

applied. In the case of PSMNP-PS system the membranes deposited by

using the phase inversion technique are characterized by far better

mass-transfer parameters than those formed by conventional drying.

ACKNOWLEDGEMENTS

This work was supported by the research grant BIO2003-06087 from the

Ministry of Science and Technology of Spain, which is also acknowledged

with thanks for the financial support of Dmitri N. Muraviev within

Scheme 1. Molecular structure of repeated units of polysulfone.

D. N. Muraviev et al.742

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the Program Ramon y Cajal. The Electron Microscopy Service of the

Autonomous University of Barcelona is acknowledged for their assistance.

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