Fe3O4-PPy核壳结构

Fe3O4/Polypyrrole/Au Nanocomposites with Core/Shell/Shell Structure: Synthesis,Characterization,and Their Electrochemical Properties Hui Zhang,Xue Zhong,Jing-Juan Xu,*and Hong-Yuan Chen

Key Laboratory of Analytical Chemistry for Life Science(MOE),Chemistry and Chemical Engineering,

Nanjing Uni V ersity,Nanjing210093,China

Recei V ed September3,2008.Re V ised Manuscript Recei V ed October5,2008

Uniform Fe3O4nanospheres with a diameter of100nm were rapidly prepared using a microwave solvothermal method.Then Fe3O4/polypyrrole(PPy)composite nanospheres with well-de?ned core/shell structures were obtained through chemical oxidative polymerization of pyrrole in the presence of Fe3O4;the average thickness of the coating shell was about25nm.Furthermore,by means of electrostatic interactions,plentiful gold nanoparticles with a diameter of15nm were assembled on the surface of Fe3O4/PPy to get Fe3O4/PPy/Au core/shell/shell structure.The morphology, structure,and composition of the products were characterized by transmission electronic microscopy(TEM),scanning electronic microscopy(SEM),X-ray powder diffraction(XRD),and Fourier transform infrared(FT-IR)spectroscopy. The resultant nanocomposites not only have the magnetism of Fe3O4nanoparticles that make the nanocomposites easily controlled by an external magnetic?eld but also have the good conductivity and excellent electrochemical and catalytic properties of PPy and Au nanoparticles.Furthermore,the nanocomposites showed excellent electrocatalytic activities to biospecies such as ascorbic acid(AA).

Introduction

In recent years,magnetic nanomaterials stand out for their novel properties.1The most widely studied superparamagnetic nanoparticles(NPs)is magnetite(Fe3O4),which offers a large variety of applications,such as information storage,2color imaging,3microwave absorption,4medical diagnosis,5cell separation,6and so on.However,naked Fe3O4NPs are very sensitive to oxidation for their high chemical reactivities and prone to aggregate for their large ratio of surface area to volume and thus result in poor magnetism and dispersibility.1,7-10Those defects limit their further applications.One of the main approaches to overcome these limitations is to protect naked magnetic NPs with polymers8-18or inorganic shell such as metals18-24or oxides.18,25,26These protective layers not only prevent the Fe3O4 NPs from aggregating but also provide a useful platform for further functionalization.18Among these coating materials,gold NPs have received more attention for their unique electrical and optical properties.Meanwhile,their facile and robust interaction with thiolated organic molecules could create multifunctional groups for further conjugation.As one of the bifunctional nanomaterials,gold-coated Fe3O4core/shell NPs are extremely interesting for magnetic,optical,biomedical,electronic,catalytic, and biosensor applications.To date,several methods for the synthesis of Fe3O4/Au core/shell NPs have been reported,for example,reduction of Au(OOCCH3)3in the presence of the presynthesized Fe3O4,22reducing Au3+onto the surface of Fe3O4 via iterative hydroxylamine seeding,23and coating Au on the surface of Fe3O4NPs by reducing HAuCl4in a chloroform solution of oleylamine and then mixing them with sodium citrate and cetyltrimethylammonium bromide(CTAB).24

Recently,three-component magnetic nanocomposites have been paid much attention.Stoeva et al.27assembled magnetic NPs into a core/shell/shell structure with a silica core with Fe3O4 and gold as the inner and outer shells,respectively.These core/ shell/shell magnetic NPs have excellent DNA binding properties as well as magnetic properties.Wang et al.28reported the synthesis of Fe3O4@PAH@Au multifunctional NPs,which display both magnetism and near-infrared(NIR)absorption.Yu et al.29 prepared Fe3O4@Au/polyaniline(PANI)nanocomposites by in

*To whom correspondence should be addressed.E-mail:xujj@https://www.sodocs.net/doc/c414301092.html,.

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13748Langmuir2008,24,13748-13752

10.1021/la8028935CCC:$40.75 2008American Chemical Society

Published on Web11/08/2008

situ polymerization in the presence of mercaptocarboxylic acid with integrated and stable optical,electrical,and magnetic properties.These species of nanocomposites exhibited promising application in many areas.In this paper,a new kind of three-component magnetic nanocomposite,Fe3O4/PPy/Au,was syn-thesized.First,uniform Fe3O4nanospheres with a diameter of 100nm were rapidly prepared via the microwave solvothermal method.Then,Fe3O4/polypyrrole(PPy)composite nanospheres with well-de?ned core/shell structures were obtained through oxidative polymerization of pyrrole.Finally,by means of electrostatic interactions between PPy coating and citrate-stabilized Au NPs,plentiful gold NPs with a diameter of15nm were assembled on the surface of Fe3O4/PPy.As a result,a new type of three-component Fe3O4/PPy/Au nanocomposites with core/shell/shell structure was successfully obtained.It is well known that PPy,one of the most important conducting polymers, has been extensively explored and used in many areas such as electrochromic devices,secondary batteries,catalysis,and biosensors for its good electrical conductivity,reversible redox property,good environmental stability,and virtue of easy preparation by both chemical and electrochemical ap-proaches.30-32Therefore,compared with the Fe3O4/Au nano-compsites,the resultant three-component nanocomposites had better electrical conductivity and catalytic properties.Furthermore, the plentiful gold NPs adsorbed on the surface of Fe3O4/PPy exhibit highly active surface areas.Those would greatly enhance the potential application of such a multifunctional nanomaterial. By means of an external magnetic?eld,the magnetic nano-composites could be easily immobilized on the electrode surface to get Fe3O4/PPy/Au nanocomposites modi?ed electrode.In order to prove their practicability and great potential in electrochemical biosensors,the electrochemical properties and electrocatalytic activity of the Fe3O4/PPy/Au nanocomposites modi?ed electrode were investigated.The results indicated that the Fe3O4/PPy/Au nanocomposite exhibited excellent electrocatalytic properties for ascorbic acid(AA).Due to the good magnetism and biocom-patibility and ease of functionalization,the nanocomposites might be used to construct biosensors and the biosensor fabrication process could be greatly simpli?ed.

Experimental Section

Reagents.Pyrrole monomer,iron(III)chloride hexahydrate (FeCl3·6H2O),sodium acetate(NaAc),ethylene glycol(EG),sodium dodecylbenzenesulfonate-6(NaDBS),and hydrogen tetrachloroaurate hydrate(HAuCl4·4H2O)were purchased from Shanghai Chemical Reagent Co.Ltd.(China).Poly(ethylene glycol)-block-poly(pro-pylene glycol)-block-poly(ethylene glycol)(P123)was purchased from Sigma.Pyrrole monomer was distilled under reduced pressure, and other reagents were used as received without further treatment.

A phosphate buffer solution(PBS)(25.0mM)was prepared by mixing solutions of Na2HPO4and NaH2PO4.The pH was adjusted with H3PO4or NaOH.Other chemicals were of analytical grade.All solutions were prepared with twice distilled water. Preparation of Fe3O4NPs.Brie?y,P123(0.80g)was dissolved in ethylene glycol(40.0mL)followed by addition of FeCl3·6H2O (1.0g)and NaAc(3.6g)to the mixture and stirring vigorously until being completely dissolved and then transferred into a digestion vessel(80mL capacity).The mixed solution was treated at210°C for30min in a microwave-assisted solvothermal reaction system MARS-5(CEM);the operating power used was150W.The precipitated products were collected and removed from the solution by applying an external magnetic?eld and washed with ethanol and water several times.Finally,Fe3O4NPs were obtained by drying in vacuum for24h.

Preparation of Fe3O4/PPy Nanocomposites.Fe3O4/PPy nano-composites were prepared according to ref16with slight modi?cation. Brie?y,twice distilled water(50.0mL)was placed in a100mL three-necked,round-bottomed?ask equipped with a mechanical stirrer and deaerated by bubbling highly pure nitrogen for20min followed by addition of NaDBS(0.066g)and stirring until dissolved. Fe3O4(0.025g)was added and stirred for15min;then0.50mL of pyrrole was added and stirred for1h.Polymerization was started by adding FeCl3(0.28g dissolved in5.0mL of water and added dropwise to the?ask)and allowed to proceed for12h at room temperature under stirring.The products were collected and washed with ethanol and water until the supernatant solution was neutral. Preparation of Fe3O4/PPy/Au Nanocomposites.Au colloids with an average diameter of15nm were prepared according to the reported method via deoxidizing HAuC14aqueous solution with sodium citric acid.33Fe3O4/PPy nanocomposites solution was added to the Au colloid under stirring.Au NPs could be electrostatically attracted on the surface of Fe3O4/PPy nanocomposites,leading to formation of Fe3O4/PPy/Au nanocomposites.With the increase of reaction time,the wine red solution turned colorless;adding Au colloids until the color no longer changed,and the resultant product was removed from the solution by applying an external magnetic ?eld and dried.

TEM and SEM Characterization.Morphologies of Fe3O4, Fe3O4/PPy,and Fe3O4/PPy/Au NPs were studied on FEI Tecnai-12 transmission electron microscope using an accelerating voltage of 120kV and a FEI Sirion200?eld-emission scanning electron microscope.

XRD Analysis.X-ray diffraction(XRD)analyses of Fe3O4,PPy, Fe3O4/PPy,and Fe3O4/PPy/Au NPs were carried out on a Philips X′pert Pro X-ray diffractometer(Cu K R radiation,λ)0.15418nm). FT-IR Analysis.FT-IR spectroscopic measurements of PPy, Fe3O4/PPy,and Fe3O4/PPy/Au NPs were performed on a Bruker model VECTOR22Fourier transform spectrometer using KBr pressed disks.

Electrochemical Measurements.Electrochemical experiments were performed on an Autolab PGSTAT-30potentiostat/galvanostat (Eco Chemie BV,Utrecht,The Netherlands).The bare glassy carbon slice was used as a working electrode;it was polished with1.0,0.3, and0.05μm alumina slurry followed by rinsing thoroughly with twice distilled water and then allowed to dry at room temperature. An electrochemical cell was made up of a clean glassy carbon slice immobilized at the bottom of the cell(0.2cm2area exposed to the electrolyte solution)as a working electrode,a platinum wire as an auxiliary electrode,and a saturated calomel electrode(SCE)as a reference electrode.

Fe3O4,Fe3O4/PPy,and Fe3O4/PPy/Au NPs were dispersed in water to form a0.5mg/mL solution and ultrasonicated for30min;20μL of colloidal solution was dispersed in PBS in the electrolytic cell.

A piece of NdFe

B magnet was used to move the magnetic NPs.

Results and Discussion

The synthesis procedure and possible mechanisms for forma-tion of Fe3O4,Fe3O4/PPy,and Fe3O4/PPy/Au NPs are shown in Scheme1.EG is a good reducing agent and has been widely used in the polyol process to prepare metal or metal oxide NPs.The Fe3O4spheres were prepared via the reduction reactions between FeCl3and EG under microwave-assisted solvothermal condition. NaAc was added as an assistant in the reduction reaction and also to increase the electrostatic stabilization of magnetite.34 P123was added as a surfactant to prevent magnetite agglomera-tion https://www.sodocs.net/doc/c414301092.html,pared with the conventional solvothermal method,

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the microwave-assisted solvothermal method has several ad-vantages:?rst,microwave radiation offers a more homogeneous reaction and faster kinetic process.Utilization of microwave radiation saves the reaction time and lowers the energy and costs.35Second,the fast nucleation speed makes it possible to produce more uniform and ?ne NPs (100nm)compared with conventional solvothermal methods (200-800nm).34To the best of our knowledge,there are no reports on the preparation of Fe 3O 4NPs with good uniformity by this technique.

The resultant magnetite nanospheres were treated with FeCl 3solution;because of the high amount of dangling bonds on the Fe 3O 4NPs’surfaces and the common ion effect,lattice ion Fe 3+were adsorbed onto the surface of Fe 3O 4,and the positively charged shells help to prevent their aggregation through electrostatic repulsion.15,16Meanwhile,Fe 3+was employed as an oxidizing agent to polymerize pyrrole monomers to synthesize PPy shell.

Since PPy is electropositive and the surfaces of citrate-stabilized Au NPs are electronegative,when the nanocomposites were introduced into the Au colloid solution,Au-decorated Fe 3O 4/PPy composite spheres were formed by an electrostatic effect.By combining the microwave solvothermal method,chemical oxidative polymerization,and electrostatic self-assembly,a typical core/shell/shell structure was successfully prepared with a magnetite core,PPy and gold as the inner and outer shells,respectively.

Figure 1shows the typical TEM and SEM images of Fe 3O 4,Fe 3O 4/PPy,and Fe 3O 4/PPy/Au NPs.The obtained Fe 3O 4NPs showed uniform spherical morphology (Figure 1a).The inset in the Figure 1a clearly shows the diameter of Fe 3O 4is about 100nm.As shown in Figure 1b,Fe 3O 4/PPy composite spheres with well-de?ned core/shell structures were successfully synthesized,the Fe 3O 4core was well wrapped by the coating layer,and the average thickness of the coating PPy shell was about 25nm.It was found that there exists a clear interface between the PPy shell and the Fe 3O 4core,indicating tight encapsulation.The SEM image showed that the Fe 3O 4and Fe 3O 4/PPy spheres were spherical and very uniform in both size and shape (Figure 1a ′and 1b ′).Since the Fe 3O 4NPs in our system were a nearly homogeneous spherical shape with a bigger size than the previous reports,aggregation of Fe 3O 4cores could be overcome and a more uniform core/shell structure could be obtained.13-17The outside particles in Figure 1c and the corresponding spots in Figure 1c ′were Au NPs,which con?rmed that Au NPs have been successfully attached onto the surface of Fe 3O 4/PPy.The concentration of FeCl 3played an important role in the formation of uniform core/shell structure.As shown in Figure 2a,when the FeCl 3amount was low (0.10g),many colloidal Fe 3O 4NPs aggregated together for the Fe 3+shell is too de?cient to prevent the Fe 3O 4NPs aggregation.In addition,insuf?cient Fe 3+cation could not oxidize enough pyrrole to enwrap the Fe 3O 4core completely (Figure 2a).With the FeCl 3concentration increased,more Fe 3+would be adsorbed on the surface of Fe 3O 4until the surface charges was near the maximum;these adsorbed

Fe 3+cations were favorable to prevent aggregation of Fe 3O 4NPs and induce formation of PPy shell with uniform density and thickness (Figure 2b).However,continued increase of the FeCl 3amount would lead to an increase of the acidity of the solution and result in partial dissolution of Fe 3O 4cores;the super?uous Fe 3+oxidizes excess pyrrole to PPy,as shown in Figure 2c.Figure 3illustrates the XRD pattern of the synthesized pure PPy,pure Fe 3O 4,Fe 3O 4/PPy,and Fe 3O 4/PPy/Au NPs.Pure PPy is amorphous;there was only a broad peak that appears at a 2θvalue of 20-30°.36In the case of Fe 3O 4NPs,seven major peaks at about 30.3°,35.6°,43.2°,53.4°,57.2°,62.7°,and 74.3°were observed and could be assigned to diffraction from the (220),(311),(400),(422),(511),and (440)planes of the face-centered cubic (fcc)lattice of Fe 3O 4(JCPDS card no.79-0418),respectively.Except the broad peak of PPy,the peaks of Fe 3O 4were also observed in Fe 3O 4/PPy nanocomposites (Figure 3c),revealing that the surface-modi?ed Fe 3O 4NPs did not change their phases,while for the Fe 3O 4/PPy/Au nanocomposite (Figure 3d),besides the PPy and Fe 3O 4peaks,four additional peaks at 38.4°,44.5°,65.1°,and 78°representing Bragg re?ections from (111),(200),(220),and (311)planes of Au were also observed (JCPDS card no.04-0784).It indicated that the Au NPs do exist in the Fe 3O 4/PPy/Au nanocomposites.

Figure 4gives the FT-IR spectra of the prepared pure PPy,Fe 3O 4/PPy,and Fe 3O 4/PPy/Au NPs.As shown in Figure 4a,the

(35)Chung,C.C.;Chung,T.W.;Yang,T.C.K.Ind.Eng.Chem.Res.2008,47,2301.

Scheme 1.Preparation Process of Fe 3O 4/PPy/Au

Nanocomposites

Figure 1.(a -c)TEM and (a ′-c ′)SEM images of (a,a ′)Fe 3O 4,(b,b ′)Fe 3O 4/PPy,and (c,c ′)Fe 3O 4/PPy/Au NPs.The inset in a is the high-magni?cation TEM image of Fe 3O 4.

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Zhang et al.

absorptions centered at 543and 1458cm -1correspond to the pyrrole rings vibration;the bands at 1296,1170,and 1088cm -1and 890and 783cm -1were assigned to d C -H in-plane vibration and out-of-plane vibration,respectively.13As shown in Figure 4b,incorporation of Fe 3O 4led to some peaks of PPy shifted to high wavenumber.13When Fe 3O 4/PPy was combined with Au,the peaks of PPy shifted to low wavenumber,which are directly related to the differences in electron density.As the electron density donator,Au makes the electron cloud of PPy more delocalized.A similar change in the FT-IR absorption spectra of polyaniline was observed after combining with Au.37Overall,these FT-IR spectra provided supportive evidence that Fe 3O 4/PPy and Fe 3O 4/PPy/Au nanocomposites have been successfully prepared.

It is well known that PPy and Au NPs bear good electrochemical behaviors and have been widely applied in the ?eld of electrocatalysis and electroanalysis.Herein,we used an external magnetic ?eld to immobilize the magnetic nanocomposites on the surface of the glassy carbon electrode (GCE)and control the electrochemical process.38-40As shown in the inset of Figure 5,In the absence of an applied magnetic ?eld,the nanocomposites were dispersed in water completely;after a piece of magnet was

applied near the side wall of the glass vial,the black particles were attracted toward the magnet within 30s,demonstrating the excellent magnetic separation ability of our product,which makes them easy to be immobilized on the electrode.

When the magnet was positioned below the glassy carbon electrode,Fe 3O 4/PPy/Au nanocomposits were attracted to the electrode and the redox transformation occurred;a pair of well-de?ned redox peaks at E pc )-0.67V and E pa )-0.37(vs SCE)were observed (Figure 5a)corresponding to the redox reactions of PPy;41-43when the magnet was above the electrochemical cell,Fe 3O 4/PPy/Au nanocomposites would retract from the electrode surface and block the electrical contact between them.As shown in Figure 5b,no redox peak occurred at the potential range of 0-1.0V.

Ascorbic acid (AA)is an important biological molecule;it exists in the extracellular ?uid of the central nervous system and serum.Here we used it as a model to test the practical applications of the proposed modi?ed electrode.The CV behaviors of AA at bare GCE and GCE modi?ed by Fe 3O 4,Fe 3O 4/PPy,and Fe 3O 4/PPy/Au are shown in Figure 6.At bare GCE,AA gives a broad and weak oxidation peak at 0.6V (curve a).The onset potential for AA oxidation at the Fe 3O 4-modi?ed GCE is 0.2V (curve b),the oxidation peak was not obvious,and the oxidation current was smaller than that at the bare GCE.It was because the Fe 3O 4?lm was obstructive to the electron transfer.At the Fe 3O 4/PPy and Fe 3O 4/PPy/Au-modi?ed GCE a sharp and well-de?ned oxidation peak was observed at +0.1V (vs SCE),which was 500mV more negative than that observed at the bare GCE.The

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.

Figure 2.TEM images of Fe 3O 4/PPy nanocomposites with different amounts of FeCl 3:(a)0.10,(b)0.28,and (c)0.50

g.

Figure 3.XRD patterns of (a)PPy,(b)Fe 3O 4,(c)Fe 3O 4/PPy,and (d)Fe 3O 4/PPy/Au

NPs.

Figure 4.FT-IR spectra of (a)PPy,(b)Fe 3O 4/PPy,and (c)Fe 3O 4/PPy/Au

NPs.

Figure 5.Cyclic voltammograms of (a)Fe 3O 4/PPy/Au nanocomposite-modi?ed GCE and (b)nanocomposites retracted from the GCE surface (bare GCE)in pH 7.0PBS.Scan rate:100mV/s.(Inset)Photographs of the Fe 3O 4/PPy/Au nanocomposites colloidal solution (c)before and (d)after magnetic separation by an external magnetic ?eld.

Fe 3O 4/Polypyrrole/Au Nanocomposites Langmuir,Vol.24,No.23,200813751

electrocatalytic anodic current in the Fe 3O 4/PPy/Au system was about 3-fold higher than that in the Fe 3O 4/PPy system,implying that Au NPs can act as good catalysts for oxidation of AA.In addition,in the -1.0to -0.3V potential range we ?nd that the redox process of PPy at Fe 3O 4/PPy/Au nanocomposites modi?ed GCE had more positive cathodic peak potentials and the peak potential difference was smaller than that of Fe 3O 4/PPy nano-composites modi?ed GCE;this was because incorporation of Au NPs into the PPy matrix could increase the conductivity of the ?lm and enhance the reversibility of the redox reaction of PPy for higher electrochemical activity.44Additional experiments were done to test the nanocomposites’stability.It was found that

no obvious changes for the catalytic peak current were found for Fe 3O 4/PPy/Au-modi?ed GCEs after the Fe 3O 4/PPy/Au NPs colloidal solution was ultrasonicated for 1h or stored in air for 24h,which re?ects the good stability of the nanocomposites.

Conclusions

In this paper,Fe 3O 4/PPy/Au nanocomposites with core/shell/shell structure have been successfully prepared,which embedded a magnetite core,enwrapped a PPy layer in the inner shell,and assembled plentiful Au NPs in the outer shell.The as-prepared multifunctional nanocomposites combine the excellent magnetic properties of Fe 3O 4and the special properties of PPy and Au.By means of an external magnetic ?eld,the magnetic nano-composites can be attracted to the surfaces of electrodes,which simplify the tedious preparation procedure of the conventional modi?ed electrode.The Fe 3O 4/PPy/Au nanocomposites modi?ed electrode exhibited excellent electrocatalytic activities to ascorbic acid (AA).The nanocomposites may have potential application in biological separation,enzyme immobilization,radiation absorption,and as biosensors.

Acknowledgment.Financial support from the National Natural Science Foundation (20890020,20635002,and 20775033),National Natural Science Funds for Creative Research Groups (20521503),973Program (2007CB936404),and program for New Century Excellent Talents in University (NCET)of China are gratefully acknowledged.

LA8028935

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.

Figure 6.Cyclic voltammograms of (a)bare GCE,(b)Fe 3O 4,(c)Fe 3O 4/PPy,and (d)Fe 3O 4/PPy/Au NPs modi?ed GCE in pH 7.0PBS in the presence of 5.0mM AA.Scan rate:100mV/s.

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