Ag-Coated Fe 3 O 4 @SiO 2 文献

Ag-Coated Fe 3O 4@SiO 2Three-Ply Composite Microspheres:Synthesis,Characterization,and Application in Detecting Melamine with Their Surface-Enhanced Raman Scattering

Haibo Hu,Zhenghua Wang,*Ling Pan,Suping Zhao,and Shiyu Zhu

Anhui Key Laboratory of Functional Molecular Solids,College of Chemistry and Materials Science,Anhui Normal Uni V ersity,Wuhu 241000,People’s Republic of China

Recei V ed:January 6,2010;Re V ised Manuscript Recei V ed:March 24,2010

Ag nanoparticles with average sizes of 20nm were well-dispersed on the surfaces of Fe 3O 4@SiO 2composite microspheres through a simple wet-chemical method employing the Ag-mirror reaction.The as-synthesized Ag-coated Fe 3O 4@SiO 2three-ply composite microspheres are monodisperse and bifunctional,with ferro-magnetic and surface-enhanced Raman scattering (SERS)properties.The products were characterized by means of X-ray diffraction (XRD),scanning electron microscopy (SEM),transmission electron microscopy (TEM),and energy dispersive X-ray analysis (EDX).SERS signals of typical analytes such as rhodamine 6G (Rh 6G)were observed on Ag nanoparticles from the Ag-coated Fe 3O 4@SiO 2microspheres,even though the concentration of the analyte was as low as 1×10-15M (Rh 6G).The Ag-coated Fe 3O 4@SiO 2microspheres were applied to detecting melamine,and strong SERS signals were obtained with melamine concentration of 1×10-6M.This work may provide a potential and unique technique to detect melamine.

1.Introduction

In recent years,much attention has been attracted by the fabrication of magnetic nanoparticles and nanocomposites because they have both fundamental and practical values due to their potential applications in areas such as ferro?uids,medical imaging,drug targeting and delivery,cancer therapy,separations,and catalysis.1-5Especially,magnetic nanocom-posites which contain two or more different nanoscale func-tionalities are of special interest.Due to their particular structure and interface interactions,these magnetic nanocomposites can exhibit novel physical,chemical,and biologic properties that will be essential for future technological applications.The magnetic nanocomposites with core -shell structures are well-known composite systems that have shown multiple properties such as adsorptive,optical,magnetic,antibacterial,and catalytic properties compared to their individual single-component materials.6-9For example,Maceira et al.have synthesized semiconductor-coated magnetic silica spheres with both optical and magnetic properties.6Liu and co-workers have shown that Chitosan modi?ed magnetite nanoparticles can remove above 85%heavy metal ions such as Pb 2+,Cu 2+,and Cd 2+from wastewater.7Wang et al.reported that Ag immobilized magnetic nanoparticles could easily kill Escherichia coli (gram-negative bacteria),Staphylococcus epidermidis (gram-positive bacteria),and Bacillus subtilis (spore bacteria)and could easily be removed from water with a magnetic ?eld to avoid contamina-tion of surroundings because they have superparamagnetic and antibacterial properties.8Therefore,the synthesis of multifarious magnetic nanocomposites with multiple properties attracts more and more attentions.

Since surface-enhanced Raman spectroscopy (SERS),which can enhance Raman signal intensity by up to 6orders of magnitude depending on the preparation of the nanometer sized metallic structures (typically Ag and Au),10,11was discovered by Fleischmann and Hendra,12it arouses intense interest because

it has wide potential application in many ?elds.13,14Various synthesis methods of such metals have been reported which include vapor deposition,15chemical reactions,16and ion-beam-sputtered technique.17However,more work is still needed to simplify the synthesis method and to improve the sensitivity.In this work,a sort of three-ply composite microsphere was prepared by coating Ag nanoparticles on the surfaces of Fe 3O 4@SiO 2composite microspheres through the well-known Ag-mirror reaction.The prepared Ag-coated Fe 3O 4@SiO 2microspheres possess both ferromagnetic and SERS properties.On the one hand,they show obvious ferromagnetic property at room temperature and can be easily magnetized by an external magnetic ?eld.On the other hand,they can serve as an ef?cient SERS substrate.The typical analyte Rhodamine 6G (Rh 6G)which has been full-characterized by SERS was selected as the model to validate the effect of this substrate;the result indicates that this substrate is ultrasensitive.Furthermore,this SERS substrate was applied to detect melamine and the SERS signals of melamine are strong even though the concentration of melamine is as low as 1×10-6M.2.Experimental Section

All reagents were commercially available from Shanghai Chemical Regents Co.with analytical grade and were used without further puri?cation.

2.1.Monodisperse Fe 3O 4Microspheres Synthesis.The synthesis was carried out according to a previous report with a little modi?cation.19In a typical procedure,1.35g of FeCl 3·6H 2O was dissolved in 40mL of ethylene glycol to form a clear solution,then 1.0g of polyethylene glycol 20000and

3.6g of NaAc ·3H 2O were added.The mixture was stirred until the reactants were fully dissolved.After that,the mixture was transferred into a Te?on-lined autoclave with a capacity of 50mL and heated at 200°C for 8h.The products were collected and rinsed with deionized water and ethanol several times each,then dried under vacuum at 60°C for 6h for further use.

*To whom correspondence should be addressed.Phone:+86-553-3869303.Fax:+86-553-3869302.E-mail:zhwang@https://www.360docs.net/doc/4f1883714.html,.

J.Phys.Chem.C 2010,114,7738–7742

773810.1021/jp100141c 2010American Chemical Society

Published on Web 04/14/2010

2.2.Magnetic Fe 3O 4@SiO 2Composite Microspheres Syn-thesis.The synthesis was carried out according to the noted Sto ¨ber method with a little modi?cation.20Typically,0.2g of prepared Fe 3O 4microspheres was dispersed in a mixture of 20mL of ethanol and 4mL of deionized water by ultrasonication for about 10min.Then under continuous mechanical stirring,1mL of ammonia solution (25%)and 0.8mL of tetraethyl orthosilicate (TEOS)were consecutively added to the mixture.The reaction was allowed to proceed at room temperature for 3h under continuous mechanical stirring.The resulting products were collected and washed,and then dried under vacuum at 60°C for 3h for further use.

2.3.Monodisperse Ag-Coated Fe 3O 4@SiO 2Composite Microspheres Synthesis.First,0.05g of Fe 3O 4@SiO 2com-posite microspheres was dispersed into 30mL of a 0.1M Ag(NH 3)2+solution and the solution was stirred by a mechanical stirrer for 0.5h to ensure suf?cient adsorption of Ag(NH 3)2+by the Fe 3O 4@SiO 2composite microspheres.Then,the micro-spheres were collected and washed with deionized water two times.Next,the microspheres were dispersed into 30mL of a 0.5M glucose solution.The solution was heated with a water bath at 50°C for 1h.During the heating process,the solution was also stirred by a mechanical stirrer.The ?nal products were collected and washed,then dried under vacuum at 60°C for 3h.The whole synthesis procedure is shown in Figure 1.2.4.Preparation of SERS Substrate.A piece of copper foil (8mm ×8mm)was washed by ultrasonication in ethanol and acetone one time each.Then a small copper box without a cover (5mm ×5mm ×1mm)was fabricated by tailoring the copper foil.The Ag-coated Fe 3O 4@SiO 2composite microspheres (about 0.005g)were put into the copper box and pressed.

2.5.Characterization Methods.The structure and morphol-ogy of samples were characterized by X-ray diffraction (XRD;XRD-6000),scanning electron microscopy (SEM;Hitachi S-4800),and transmission electron microscopy (TEM;JEOL-2010with energy dispersive X-ray analysis (EDXA)system).Infrared (IR)spectra of the samples were obtained on a Vectortm 22Fourier transform infrared (FT-IR)spectrometer (Bruke,Germany).The magnetic properties of the samples were investigated by using a vibrating sample magnetometer (VSM)with an applied ?eld between -5000and 5000Oe at room temperature (BHV-55,Riken,Japan).Rh 6G and melamine were detected via SERS with a Labram-HR confocal laser micro-Raman spectrometer equipped with an argon ion laser with excitation of 514.5nm.An air-cooled CCD was used as the detector,the accumulation time was 20s,and the incident power was 3mW.The spot size of the laser was 1μm in diameter,using a 50×objective.

3.Results and Discussion

3.1.Characterization of the Obtained Samples.The phase and purity of the as-obtained samples were examined by XRD.Figure 2a shows a typical XRD pattern of the obtained Fe 3O 4sample;all of the diffraction peaks could be readily indexed to the orthorhombic phase of Fe 3O 4(JCPDS card No.75-1609),which matched well with the result of Li et al.19Figure 2b is a typical XRD pattern of the Fe 3O 4@SiO 2sample,which shows almost the same feature as that shown in Figure 2a:no diffraction peaks corresponding to SiO 2were observed because the prepared SiO 2is amorphous.Figure 2c shows a typical XRD pattern of the Ag-coated Fe 3O 4@SiO 2sample.In addition to the diffraction peaks that correspond to Fe 3O 4,there also exist three other diffraction peaks (labled with the symbol #).These diffraction peaks could be easily indexed to the cubic phase of Ag (JCPDS card No.04-0783).

The samples were also characterized by IR.Figure 3a shows the IR spectrum of Fe 3O 4microspheres,in which only a peak at 582cm -1can be seen.This peak is ascribed to the Fe -O stretching vibration.21Figure 3b shows the IR spectrum of the Fe 3O 4@SiO 2sample.In addition to the peak at 582cm -1ascribed to Fe -O stretching vibration,there is a new strong band around 1093cm -1that originates from the Si -O bond of silica.21This result indicates that SiO 2is immobilized on the surfaces of Fe 3O 4microspheres.Figure 3c shows the IR spectrum of the Ag-coated Fe 3O 4@SiO 2sample.Because Ag nanoparticles do not have absorption in the infrared region,the IR spectrum of the Ag-coated Fe 3O 4@SiO 2sample is almost the same as that of the Fe 3O 4@SiO 2sample.

The size and shape of the samples were examined by SEM and TEM.Panels a and b of Figure 4show the SEM images of the Fe 3O 4sample,from which it can be clearly seen that the sample is composed of many nearly monodisperse spherical particles with a diameter of about 400nm.Furthermore,

Figure

Figure 1.Synthesis route of Ag-coated Fe 3O 4@SiO 2composite

microspheres.

Figure 2.Typical XRD patterns of (a)Fe 3O 4microspheres,(b)Fe 3O 4@SiO 2composite microspheres,and (c)Ag-coated Fe 3O 4@SiO 2composite microspheres.

Ag-Coated Fe 3O 4@SiO 2Three-Ply Composite Microspheres J.Phys.Chem.C,Vol.114,No.17,20107739

4b indicates that the magnetic microspheres are composed of many smaller particles and their surfaces are not smooth.SEM images of the Fe 3O 4@SiO 2sample are shown in Figure 4c,d,which indicate that the Fe 3O 4@SiO 2composite microspheres have a smooth silica shell.Panels e and f of Figure 4show the SEM images of the Ag-coated Fe 3O 4@SiO 2sample.Many little Ag nanoparticles with a diameter of about 20nm are adhered to the surfaces of the Fe 3O 4@SiO 2microspheres.

The TEM images (Figure 5a -c)further con?rm the above results.It can be clearly seen from Figure 5a that the Fe 3O 4@SiO 2composite microspheres have the core -shell structure.TEM images of Figure 5b,c show distinctly that the Ag-coated Fe 3O 4@SiO 2composite microspheres have three-ply structure and the outermost layer is composed of many tiny Ag nano-particles.Figure 5d is the EDX spectrum of the Ag-coated Fe 3O 4@SiO 2composite microspheres,in which Fe,O,Si,and Ag are all present.The C and Cu were derived from the carbon-coated copper TEM grid.

Figure 6shows the magnetic hysteresis loops of the as-obtained Fe 3O 4microspheres,Fe 3O 4@SiO 2composite micro-

spheres,and Ag-coated Fe 3O 4@SiO 2composite microspheres.All of them show ferromagnetic behavior at room temperature.The magnetic saturation (Ms )values of the Fe 3O 4microspheres,Fe 3O 4@SiO 2composite microspheres,and Ag-coated Fe 3O 4@SiO 2composite microspheres are 80.0,36.1,and 34.5emu ·g -1,respectively.It is noticed that the Ms value of Fe 3O 4micro-spheres is higher than that of Fe 3O 4@SiO 2composite micro-spheres and Ag-coated Fe 3O 4@SiO 2composite microspheres,because in the Fe 3O 4@SiO 2composite microspheres and Ag-coated Fe 3O 4@SiO 2composite microspheres the Fe 3O 4is coated with a layer of amorphous SiO 2.The small decrease of the Ms value of Ag-coated Fe 3O 4@SiO 2composite microspheres compared to that of Fe 3O 4@SiO 2composite microspheres can be attributed to the slight increase of size and mass due to the adherence of Ag nanoparticles on the surface of the magnetic composites.Such an excellent magnetic property means that all of the prepared samples have strong magnetic responsivity and can be separated easily from solution with the help of an external magnetic

force.

Figure 3.IR spectra of (a)Fe 3O 4microspheres,(b)Fe 3O 4@SiO 2composite microspheres,and (c)Ag-coated Fe 3O 4@SiO 2composite

microspheres.

Figure 4.(a,b)SEM images of Fe 3O 4microspheres,(c,d)SEM images of Fe 3O 4@SiO 2composite microspheres,and (e,f)SEM images of Ag-coated Fe 3O 4@SiO 2composite

microspheres.

Figure 5.(a)TEM image of Fe 3O 4@SiO 2composite microspheres,(b,c)TEM images of Ag-coated Fe 3O 4@SiO 2composite microspheres,and(d)EDXspectrumofAg-coatedFe 3O 4@SiO 2

compositemicrospheres.

Figure 6.Magnetic hysteresis loops of (a)Fe 3O 4microspheres,(b)Fe 3O 4@SiO 2composite microspheres,and (c)Ag-coated Fe 3O 4@SiO 2composite microspheres.

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3.2.Effect of Ag(NH 3)2+Concentration on the Formation of Ag-Coated Fe 3O 4@SiO 2Composite Microspheres.It must be noticed that the concentration of Ag(NH 3)2+solution plays an important role in the formation of Ag-coated Fe 3O 4@SiO 2composite microspheres.Along with the concentration of Ag(NH 3)2+solution decreases,the amount of Ag nanoparticles is decreased and their distribution becomes nonuniform on the surfaces of Ag-coated Fe 3O 4@SiO 2composite microspheres.This result can be obtained from the comparison of the sample prepared with Ag(NH 3)2+concentration of 0.1M (Figure 7a)and the sample prepared with Ag(NH 3)2+concentration of 0.01M (Figure 7b).Higher concentrations of Ag(NH 3)2+proved to be crucial for augmenting the yield of the Ag nanoparticles and their uniformity.

3.3.Application of Ag-Coated Fe 3O 4@SiO 2Composite Microspheres in SERS.The prepared Ag-coated Fe 3O 4@SiO 2SERS substrate was ?rst used to detect the typical SERS active analyte such as Rh 6G to test its effect.Figure 8shows the SERS spectrum obtained by adding 25μL of Rh 6G ethanol solution with a concentration of 1×10-15M on the substrate.Four peaks exist in this spectrum and these peaks are charac-teristic peaks of Rh 6G.16,18,22The peak at 1188cm -1is associated with C -C stretching vibrations,while the peaks at 1303,1356,and 1575cm -1are associated with aromatic C -C stretching vibrations.23,24Although the concentration of Rh 6G is very low,the signals of the SERS are still strong.These results indicate that this substrate for SERS detection is effective and ultrasensitive.

It is reported that there are two major mechanisms that contribute to the enhancement effect:16,18one is the electro-magnetic effect associated with large local ?elds due to resonances occurring in the microstructures on the metal surface,the other is the chemical effect involving a scattering process associated with chemical interaction between the molecule and the metal surface.The enhancement due to the former is believed

to be a few orders of magnitude more than the latter.According to Garcia-Vidal and Pendry’s model,25the smaller metallic particles give higher enhancement.But when the particle size is less than 15nm,the enhancement saturates because the separation between metal nanoparticles is equal to their diameter.In this work,the average size of the Ag nanoparticles coated on Fe 3O 4@SiO 2microspheres is close to this size,which may be responsible for the ultrasensitivity of the Ag-coated Fe 3O 4@SiO 2SERS substrate.

Melamine (2,4,6-triamino-1,3,5-triazine)is a nitrogen-rich chemical commonly used to produce melamine resin,a synthetic heat-tolerant polymer.Although melamine is not inherently a carcinogen and toxic chemical,illegal large-dose adulteration in routine dairy products can result in urinary calculi,acute renal failure,and even infant death.Thus developing an accurate and rapid on-site melamine screen method is a very important and signi?cant task.Herein,we used Ag-coated Fe 3O 4@SiO 2composite microspheres as a SERS substrate to detect melamine.Figure 9shows the SERS spectra obtained by adding 25μL of melamine methanol solution with different concentrations on the substrate.The SERS spectrum of the 1×10-6M melamine methanol solution (Figure 9a)clearly shows the peaks at 686and 582cm -1.26The most prominent peak around 686cm -1is assigned to the ring breathing mode II and involves in-plane deformation of the triazine ring in melamine molecules.27Spectra b -d in Figure 9show the SERS spectra of the melamine methanol solution with concentrations of 1×10-7,1×10-8,and 1×10-9M,respectively.In these spectra only the weak peak at 686cm -1can be seen.These results demonstrate that melamine can be well detected with the Ag-coated Fe 3O 4@SiO 2SERS substrate in concentrations as low as 1×10-6M.4.Conclusions

In summary,bifunctional and monodisperse Ag-coated Fe 3O 4@SiO 2composite microspheres with both ferromagnetic and SERS properties have been successfully synthesized through a simple solution-phase method employing the Ag-mirror reaction.The Ag-coated Fe 3O 4@SiO 2composite microspheres can be used as a SERS substrate to detect melamine with a high sensitivity even when the concentration is as low as 1×10-6M.This method is effective and convenient for detecting melamine at very low concentration and with a short detection time.

Acknowledgment.Financial support form the National Natural Science Foundation of China (20701001)and

Anhui

Figure 7.SEM images of Ag-coated Fe 3O 4@SiO 2composite micro-spheres obtained with different concentrations of Ag(NH 3)2+:(a)0.1M and (b)0.01

M.

Figure 8.SERS spectrum of a 1×10-15M R6G ethanol solution adsorbed on Ag nanoparticles of the Ag-coated Fe 3O 4@SiO 2composite

microspheres.

Figure 9.SERS spectra of different concentrations of melamine methanol solutions adsorbed on Ag nanoparticles of the Ag-coated Fe 3O 4@SiO 2composite microspheres:(a)1×10-6M,(b)1×10-7M,(c)1×10-8M,and (d)1×10-9M.

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Key Laboratory of Controllable Chemistry Reaction&Material Chemical Engineering is gratefully acknowledged. References and Notes

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