Nanoparticle composites

Phase-Inversion Method for Incorporation of MetalNanoparticles into Carbon-Nanotube/Polymer Composites**

Samuel Sanchez, Esteve Fabregas, Hideo Iwai, and Martin Pumera*

Colloidal metal and semiconductor nanoparticles (NPs) and

quantum dots are widely recognized as potential functional

elements in carbon nanotube (CNT)/polymeric devices due to

the wide range of their interesting optical, electronic,

magnetic, and electrochemical properties.[1] There is great

interest in coupling of CNTs and polymers with metallic or

semiconducting NPs and quantum dots.[2] There are two

categories of method for preparing NP/CNT composites with

desirable compositions. The first type is a one-step method,

such as electrospinning;[3] the second type is a two-step

preparation method involving chemical or electrochemical

synthesis and deposition of NPs and polymer.[4–7] For

example, Chen et al. and Kong et al. have reported the

synthesis of Fe3O4 NPs on CNTs followed by spontaneous

precipitation of polyaniline on the NP/CNT hybrid.[4,5] Wang

et al. electrochemically deposited polyaniline on CNTs

and subsequently electrochemically deposited gold NPs

(AuNPs).[6] Carillo et al. incorporated AuNPs in poly-

ethyleneimine/poly(acrylic acid)-coated CNTs by using the

layer-by-layer technique.[7] The methods described above

employ water-soluble polymers or precursors for NP/CNT/

polymer hybrid preparation.

Herein, we describe a facile and general way of

incorporating NPs into CNT/polymer composites by a phase

inversion method. In this one-step method, CNTs and water-

insoluble polymer are dissolved in an organic solvent that

is miscible with water. The suspension of CNTs in such solvent

is stabilized by wrapping the CNTs with polymeric chains.

[�] Dr. M. Pumera, Dr. S. Sanchez

Biomaterials Center and International Center for Materials

Nanoarchitectonics

National Institute for Materials Science

1-1 Namiki, Tsukuba 305-0044 (Japan)

E-mail: pumera.martin@nims.go.jp

Dr. S. Sanchez, Prof. E. Fabregas

Group of Sensors and Biosensors, Department of Chemistry

Autonomous University of Barcelona (Spain)

Dr. H. Iwai

Materials Analysis Section

National Institute for Materials Science

1-2-1 Sengen, Tsukuba (Japan)

[��] This research was supported by the Japanese Ministry for Edu-cation, Culture, Sports, Science, and Technology (MEXT) throughthe MANA program (M.P.), by the research program CTQ2006-15681-C0 from the Spanish Ministry of Education and Science(S.S. and E.F.), and by a BE-2007 grant from the Generalitat ofCatalonia, Spain (S.S.).

DOI: 10.1002/smll.200801482

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After dispensing the CNT/polymer suspension onto the desired

surface, an aqueous solution containing water-solublemetal NPs

is deposited on the CNT/polymer suspension. Consequently, the

solvent phase is exchanged (i.e., N,N-dimethylformamide

(DMF) for water), the CNT composite wrapped with water-

insoluble polymer precipitates, and metal NPs present in the

aqueous phase are incorporated into the composite.

We illustrate the composite preparation process in

Scheme 1. The CNTs were first functionalized by refluxing

in nitric acid (2 M) at room temperature for 24 h, to give surface

carboxyl groups at the defect sites of the outer graphene layer

of the nanotubes and to remove residual metal-catalyst

impurities.[8–11] Subsequently, the carboxylic acid-functionalized

multi-wall CNTs were washed with distilled water, dried, and

dispersed in DMF (2.5mg mL�1) containing polysulfone (PSf;

85mg mL�1). The mixture was deposited onto the required

substrate. When an aqueous solution of colloidal gold was added

to the polymer composite suspension, the AuNP/CNT/PSf

composite started to coagulate. The AuNPs were immobilized

into the PSf that coated the CNTs.

Figure 1 shows typical scanning electron microscopy

(SEM) images of soft AuNP/CNT/PSf composite at lower

(A) and higher (B) resolution. The images demonstrate that

AuNPs (represented by bright dots in the SEM image) are well

dispersed in the polymer matrix. It is also clear from Figure 1B

that the PSf coaxially wraps the CNTs and that all CNTs are

encapsulated by AuNP-doped polymer. This was further

confirmed by transmission electron microscopy (TEM).

To investigate the presence and localization of AuNPs in

the CNT/PSf hybrid material, a Z-contrast scanning TEM

(STEM) study of AuNP/CNT/PSf composite was performed

with a high-angle annular dark-field (HAADF) detector.

Figure 2 shows HAADF-STEM images at different magni-

fications, in which I�Z2 where I is intensity and Z is atomic

number. The higher intensities correspond to heavier

elements. In Figure 2, all of the components of the AuNP/

CNT/PSf composites can be observed, such as high-contrast

gold spherical particles and weakly contrasted CNTs wrapped

in PSf. HAADF-STEM confirms that the CNTs are com-

pletely wrapped by PSf and that the AuNPs are scattered in

the polymer and do not cluster on the CNT surface.

The elemental composition of the AuNP/CNT/PSf com-

posite was analyzed by SEM/energy-dispersive X-ray (EDX)

spectroscopy. SEM/EDX analysis of the section of AuNP/

CNT/PSf composite that is depicted in Figure 1B is shown in

Figure 3. The EDX spectrum reveals strongly the presence of

sulfur, which originates from the PSf matrix, and gold from the

AuNPs, as indicated by the characteristic elemental emissions

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Scheme 1. Preparation process of the AuNP/CNT/polysulfone (PSf) composite. A) Deposition

of CNT/PSf suspension on the substrate. B) Addition of an aqueous solution of AuNPs, and

subsequent coagulation of the PSf and incorporation of AuNPs by the phase inversion

method. C) Formation of AuNP/CNT/PSf composite.

796

of the S Ka and AuMz, AuMa, Au La, and Au Lb lines. Note

that Ni Ka originates from residual metal-catalyst NPs

intercalated in the CNTs.[12,13] Quantitative analysis of the

EDX spectra reveals that the atomic percentage of carbon is

90.47% (83.52 wt%), of oxygen is 8.31% (10.22 wt%), of sulfur

is 0.96% (2.38 wt%), and of gold is 0.26% (3.88 wt%). The

standard deviation of AuNP loading in the composite is 0.022

at% (n¼ 12).

To gain insight into the structure of the AuNP/CNT/PSf

composite, we employedX-ray photoelectron spectroscopy (XPS)

tomeasure the elemental composition of theCNT/PSf andAuNP/

CNT/PSf composites (Figure 4). As expected, strong signals from

Figure 1. SEM images of AuNP/CNT/PSf composite at A) 10 000� and

B) 40 000�magnification. Figure 2. HAADF-STE

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carbon (C 1s), oxygen (O1s), sulfur (S 2p), and

impurities (silicon, sodium, chlorine) were

observed for both CNT/PSf (Figure 4A) and

AuNP/CNT/PSf (Figure 4B) composites. The

weak signal of the Au 4f peak was observed in

the wide spectrum for the AuNP/CNT/PSf

composite, as shown in Figure 4B. The atomic

concentration of gold was evaluated to be

0.01%.To confirm thepeak energies forAu4f,

a detailed scan was performed with X-ray

scanning at 100W to improve the signal-to-

noise ratio (see inset of Figure 4B). The peak

energies ofAu4f7/2 andAu4f5/2were assigned

as 84.0 and 87.7eV, respectively, which

corresponded to the gold metallic state. The atomic concentration

obtained from XPS measurements is significantly lower than

that of the EDX measurements. The inelastic mean free path of

Au 4f is less than 2nm, so the intensity ofAu4fwill be decreased if

the particles are located underneath the top surface. The intensity

can also be decreased by the sample surface topography; a

reasonable explanation of the XPS data is that AuNPs are

completely integrated and wrapped within PSf. The conclusion

from the XPS results is consistent with the HAADF-STEM

observations.

Electrochemical impedance spectroscopy was used for

measuring the changes in capacitance and resistance to

heterogeneous electron transfer of AuNP/CNT/PSf and

CNT/PSf composites. The corresponding Nyquist impedance

plots are shown in Figure 5A. It is clear from the graphs that

AuNP/CNT/PSf composite (gray line, circles) demonstrates

lower resistance to heterogeneous electron transfer and also

higher capacitance than the CNT/PSf composite (black line,

squares), which is beneficial for several electrochemical

applications, including sensing/biosensing and battery applica-

tions. The corresponding values of resistance to heteroge-

neous electron transfer (Rp) were found to be 21.6 and 34.9 kV

for the AuNP/CNT/PSf and CNT/PSf composites, respec-

tively, and the capacitances were 275.2 and 244.0mF,

respectively.

Cyclic voltammetry of ferricyanide (10mM; Figure 5B)

shows that the oxidation/reduction signal of the ferrous/ferric

M images of AuNP/CNT/PSf composite.

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Figure 3. EDX spectrum of AuNP/CNT/PSf composite acquired in the

SEM/EDX configuration.

couple is about two times higher at the AuNP/CNT/PSf

composite electrode than on the CNT/PSf composite elec-

trode. The magnitude of the anodic current is 1.40mA (at

Epa¼ 250mV) and 0.66mA (at Epa¼ 215mV) for AuNP/

CNT/PSf and CNT/PSf composites, respectively; the cathodic

current is 1.57mA (at Epc¼ 80mV) and 0.82mA (at Epc¼111mV) for AuNP/CNT/PSf and CNT/PSf composite electro-

des, respectively (all potentials are uncorrected from ohmic

Figure 4. X-ray photoelectron spectra of A) CNT/PSf composite and B) A

composite. Expanded spectra in the 200 to �20 eV binding energy region a

insets in the expanded spectra are detailed scan spectra of Au 4f.

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drop). It is also possible to note a higher background current of

the AuNP/CNT/PSf composite electrode than of the CNT/PSf

composite electrode, which is due to the higher capacitance of

the AuNP/CNT/PSf composite and is consistent with the

electrochemical impedance measurements discussed above.

The relative standard deviations (RSDs) of the voltammetric

experiments using different electrodes (n¼ 12) are

DEpp¼ 7.1% and Epa¼ 9.3%. Such RSDs are comparable

with those of screen-printed electrodes fabricated by different

methods.[14,15]

To evaluate the practical suitability of AuNP/CNT/PSf

composites for biosensing applications, we coupled the

enzyme horseradish peroxidase (HRP) within the AuNP/

CNT/PSf composite. Figure 6 shows the chronoamperometric

response of HRP/CNT/PSf- and HRP/AuNP/CNT/PSf-based

biosensors in the electrocatalytic reduction of H2O2 to H2O at

�0.2V using hydroquinone as mediator. The amperometric

response of the HRP/AuNP/CNT/PSf biosensor to H2O2 is

very fast (within 1 s) and it exhibits a current nine times

higher than that of the HRP/CNT/PSf biosensor. The higher

response of the AuNP-modified composites is expected to

result from the larger conductive area provided by the AuNPs,

which improves the electron transfer and increases the

reduction current for hydrogen peroxide. In the context of

electrochemical experiments, it is important to mention that

even a small number of electroactive NPs at very small (under

uNP/CNT/PSf

re also shown. The

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1%) random array coverage may exhibit

behavior analogous to that of the corre-

sponding macroelectrode due to heavily

overlapping diffusion layers, as was

shown theoretically and experimentally

for voltammetry at micro/nanoelectrode

arrays.[16–19] Importantly, the typical thick-

ness of thediffusion layer (in the framework

of conditions used in our voltammetric

experiments) is about 54mm. According to

the TEM and SEM images, the distance

between AuNPs (which behave as gold

nanoelectrodes) is on the order of tens to

hundreds of nanometers and their diffusion

layers are indeed heavily overlapped.

Therefore, AuNPs embedded in CNT/PSf

composite are responsible for the enhanced

electrochemical properties of AuNP/CNT/

PSf composites.

The electrodes are chemically stable in

aqueous media. However, note that at

potentials of 1.1V and higher (vs. Ag/

AgCl) in strongly acidic media the AuNPs

undergo electrochemical oxidation and

dissolution.[20]

In conclusion, we have developed a

simple, fast, and general method for facile

incorporation of water-soluble metal NPs

in CNT/polymer composites. The AuNP/

CNT/PSf composites were successfully

prepared by the phase inversion method,

which involved immobilization of the NPs

and coagulation of the soft composite at the

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Figure 5. A) Nyquist impedance plots of a) CNT/PSf composite elec-

trodes and b) AuNP/CNT/PSf composite electrodes for a 10 mM ferri-

cyanide solution. The supporting electrolyte was 0.1 M KCl. Inset: the

Randles equivalent circuit. B) Cyclic voltammograms resulting from the

electrochemical properties of 10 mM ferricyanide at a) CNT/PSf com-

posite electrodes and b) AuNP/CNT/PSf composite electrodes. Also

shown are the corresponding cyclic voltammograms of the background

electrolyte (c and d for CNT/PSf and AuNP/CNT/PSf composite elec-

trodes, respectively). Conditions: scan rate, 100 mV s�1; phosphate-

buffered saline (PBS), pH 7.0.

Figure 6. Amperometric response of a) HRP/CNT/PSf and b) HRP/

AuNP/CNT/PSf composite-based biosensors to 2.4 mM hydrogen per-

oxide at a potential of �0.2 V. Conditions: PBS buffer, pH 7.0; con-

centration of hydroquinone, 1.8 mM.

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same time. The AuNPs were dispersed in a polymer coating of

the CNTs. The phase inversion incorporation technique is

easy, very fast (about 5min for the whole process), and

versatile. It also allows incorporation of biomolecules in the

composite. The electrochemical impedance spectroscopic and

voltammetric data clearly prove that AuNPs enhance the

CNT/PSf composite and that they bring advantageous

properties, which should be greatly beneficial in the areas

of highly sensitive sensors and biosensors and for battery/

energy-storage applications. The phase inversion method can

incorporate water-soluble metallic and semiconductor NPs

and quantum dots into the CNT/polymer matrix in a one-step

process. In principle, this process is applicable to any polymer

that is soluble in water-miscible organic solvent. The simplicity

of the method will contribute widely to various fields of

application.

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Experimental Section

Apparatus: An S-4800 field-emission scanning electron

microscope (Hitachi) was employed to obtain SEM images. A

JEM 2100F field-emission transmission electron microscope (JEOL)

was used to obtain TEM images in the STEM mode (spot size,

0.4 nm; 200 kV; HAADF). All voltammetric and electrochemical

impedance spectroscopy experiments were performed using an

electrochemical analyzer Autolab 302 (Ecochemie, The Nether-

lands). Electrochemical experiments were carried out in a 5-mL

voltammetric cell at room temperature (25 -C) using a three-

electrode configuration. A platinum electrode served as an

auxiliary electrode and an Ag/AgCl electrode as reference

electrode. X-ray photoelectron spectra were obtained with a PHI

Quantera SXM (ULVAC-PHI) spectrometer, which was equipped

with monochromatic Al Ka X-rays. The analysis area was defined

by the X-ray beam. The beam was 100mm in diameter at a beam

energy of 15 kV/25 W for wide-scan spectra and 1.5T0.1 mm2

scanned beam at a beam energy of 20 kV/100 W for detailed scan

spectra; the photoelectron take-off angle was 45-. The binding

energy was calibrated to the Au 4f7/2 line of 83.94 eV and a pass

energy of 26 eV was used. Wide-scan spectra were obtained at a

pass energy of 280 eV with 0.5 eV steps, and detailed scan

spectra for Au 4f were obtained at pass energy 26 eV with 0.1 eV

steps.

Preparation of AuNP/CNT/PSf composites: CNTs (outer di-

ameter 30–50 nm, length 0.5–200mm) were first purified by

stirring in 2 M nitric acid for 24 h.[21] Subsequently, purified CNTs

were dispersed in DMF at a concentration of 5 mg mLS1, and the

suspension was then placed in an ultrasonic bath for 10 min. PSf

(BASF Ultrasons S 3010 natur) was dissolved separately in DMF

(87 mg PSf mLS1) and both solutions were mixed together in a

ratio of 1:1 and subsequently sonicated for 30 min to achieve a

stable suspension. The final CNT concentration was 2.5 mg mLS1.

Immediately after deposition of CNT/PSf (15mL) on the required

substrate, the colloidal gold solution (15mL; 20 nm in diameter,

citrate stabilized) was dropped onto the CNT/PSf. The CNT/PSf

suspension started instantly to precipitate due to the insolubility

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of PSf in water. To ensure the complete coagulation of the

composite and completely remove the DMF, distilled water (30mL)

was dropped onto the modified nanocomposite. For control

experiments (preparation of CNT/PSf composites), distilled water

was used instead of colloidal gold solution.

Preparation of AuNP/CNT/PSf composites on screen-printed

electrodes: The working electrodes were fabricated by screen

printing using a Dek248 semiautomatic system. The squeegees

used were of the soft polymer type, and the pressure applied

during the printing process was set to 7 kg cmS2. A double-sweep

process was programmed at a speed of 20 mm sS1. AuNP/CNT/

PSf composite was deposited on the surface of the screen-printed

electrode by the above-described procedure. Acheson carbon ink

(Electrodag 400B, Acheson Colloids Co.), conductive silver ink

(Electrodag 6037 SS), and insulating ink (Minico M 7000) were

used for the preparation of screen-printed electrodes.

Preparation of HRP/AuNP/CNT/PSf biosensors: AuNP colloidal

solution was mixed with HRP aqueous solution (25 IU mLS1) for

10 min under stirring in a 1:5 ratio. After preparation of the CNT/

PSf suspension and its deposition onto the screen-printed

electrodes as described above, the electrode was immersed in

the HRP/AuNP solution. In this step, the polymer coagulated at the

same time as the AuNPs and HRP were incorporated into the

matrix. After 5 min of phase inversion, the HRP/AuNP/CNT/PSf

biosensor was rinsed with distilled water for 1 min.

Characterization of composites: Cyclic voltammetric experi-

ments were carried out at a scan rate of 100 mV sS1 with PBS

(50 mM phosphate, 100 mM KCl, pH 7.0). Electrochemical impe-

dance spectroscopy was performed in 0.1 M KCl. Fe(CN)63S/4S was

used as electrochemical probe at a concentration of 10 mM. For

TEM measurements, AuNP/CNT/PSf composites were cut by

microtome and then transferred to a TEM grid. For SEM and XPS

measurements, the AuNP/CNT/polymer composite was placed

onto a polycarbonate substrate. EDX spectra were obtained in the

SEM/EDX configuration.

Keywords:carbon nanotubes . composites . nanoparticles . phaseinversion . polymers

small 2009, 5, No. 7, 795–799 � 2009 Wiley-VCH Verlag Gmb

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H & Co. KGaA, Weinheim

Received: October 7, 2008Published online: December 23, 2008

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