Thin Solid Films 546 (2013) 255–258

Contents lists available at ScienceDirect

Thin Solid Films

j ourna l homepage: www.e lsev ie r .com/ locate / ts f

Synthesis and cysteamine functionalization of CoFe/Au/CoFe nanowires

T.S. Ramulu a, R. Venu a, B. Sinha a, B. Lim a, S.S. Yoon b, C.G. Kim a,⁎a Department of Materials Science and Engineering, Chungnam National University, Daejeon 305-764, South Koreab Department of Physics, Andong National University, Andong 760-749, South Korea

⁎ Corresponding author. Tel.: +82 42 821 6632; fax:E-mail address: cgkim@cnu.ac.kr (C.G. Kim).

0040-6090/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.tsf.2013.04.080

a b s t r a c t

a r t i c l e i n f o

Available online 1 May 2013

Keywords:CysteamineElectrodepositionHigh magnetic momentNanowires

In the present article, we have synthesized CoFe/Au/CoFe multisegment nanowires by electrochemical deposi-tion technique and selectively functionalized the surface of Au segmentwith cysteamine for biological and chem-ical applications. Themorphological structurewas examined by scanning electronmicroscope and found that thenanowires are around 4 μm in length. The magnetic property of the nanowires was measured by vibrating sam-ple magnetometer. We have compared the magnetic properties of CoFe/Au/CoFe multisegment nanowires withCoFe nanowires and found that there is a decrease in magnetic moment for multisegment nanowires. The Ausegment of the fabricated multisegment nanowires is selectively functionalized with cysteamine. The averagecomposition and the surface functionalization of nanowires were confirmed through electron dispersive spec-troscopy and X-ray photoelectron spectroscopy.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Surface modification of nanostructure materials with chemicaland biological entities has a great interest due to their applications to-wards biosensing, bio-analysis and bioelectronics [1–3]. Even though,the surface modification of gold surface with DNA achieved throughthiol linker [4–6], there is still a great difficult task for the immobiliza-tion of protein on the Au surface directly. For the specific immobiliza-tion of different protein, there is a need for the surface modification ofAu by different molecular linkers. Among the different linkers cyste-amine is the suitable candidate for the surface modification of theAu nanostructure materials [7,8]. The terminal thiol of the cysteamineis attached to the Au surface whereas the amine terminal group iseasily bound to the arm of the protein. Hence, there has been agreat interest in the surface modification of Au nanostructure mate-rials through cysteamine in the field of biosensing and bio-devices[9,10]. Among the nanostructure materials, nanowires are more use-ful for biological application due to its high aspect ratio and surfacearea [11,12]. Especially, multisegment nanowires with magnetic andnon-magnetic Au segment are more interesting in the manipulationand detection of biomolecules through magnetic methods [13,14].

The synthesis technique is more important for the fabrication ofmultisegment nanowires with uniform and controlled growth. Thereare few synthesis techniques available for nanowires such as chemicalvapor deposition, hydrothermal method, lithographymethod and tem-plate assisted electrodeposition method [15–19]. In these methods, thetemplate assisting electrochemical method is the most suitable

+82 42 822 6272.

rights reserved.

technique for the synthesis of multisegment nanowires with controlledgrowth and aspect ratio [20].

Magnetic labels with high magnetic moments are desired to pro-duce large signals for bio-magnetic sensing, magnetic manipulationin solutions and advance magnetic resonance-imaging [21]. CoFealloy has the superior magnetic property which is an ideal magneticlabel for these applications [22]. Even though there are some reportson the synthesis of CoFe nanowires, there are no reports on the cyste-amine modification of multisegment nanowires with CoFe as a mag-netic segment.

In this paper, we have reported the electrochemical synthesis of CoFe/Au/CoFe multisegment nanowires through track-etched polycarbonatemembrane with a nominal pore diameter of 50 nm. After synthesis ofthese nanowires, we have funtionalized the surface of the Au segmentthrough cysteamine for biological applications. The synthesized and sur-face modified multisegment nanowires are characterized through differ-ent characterization techniques such as field emission scanning electronmicroscopy (FE-SEM), energy-dispersive spectroscopy (EDS), vibratingsample magnetometry (VSM), Fourier transform infrared spectroscopy(FT-IR) and X-ray photoelectron spectroscopy (XPS).

2. Experimental details

2.1. Synthesis of multisegment nanowires

CoFe/Au/CoFe nanowireswere electrochemically deposited in track-etched polycarbonate membrane with a nominal pore diameter of50 nm. One side of the membrane was sputtering with 200 nm of me-tallic Au to form a thin layer for electric conduction, which served as aworking electrode. The room-temperature electrolyte bath for theCoFe contains CoSO47H2O 60 g/L, FeSO47H2O 6 g/L and H3BO3 25 g/L

Fig. 1. Schematic diagram of the synthesis of multisegment nanowires. (WE—workingelectrode, RE—reference electrode, CE—counter electrode).

256 T.S. Ramulu et al. / Thin Solid Films 546 (2013) 255–258

(the above-mentioned chemicals are purchased from Sigma-Aldrich)and the electrolyte bath for the Au contains gold plating solution(SME) respectively. The pH for the deposition of CoFe was adjusted toaround 3 by using 1 N NaOH.

The deposition of the nanowires was conducted potentiostaticallyby a three electrode configuration with a platinum sheet and Ag/AgClas the counter and reference electrodes, respectively. The schematicrepresentation of synthesis of multisegmented nanowires with experi-mental setup were shown in Fig. 1. The sequential electrodepositiontechnique was employed for the synthesis of CoFe/Au/CoFe nanowireswithin the pores of a polycarbonatemembrane by changing the electro-lyte in the deposition cell. The deposition of both CoFe and Au segmentswas carried out at a constant potential of−1.0 V. The deposition condi-tion parameters such as pH, temperature and growth were optimizedfor each segment prior to the synthesis of multisegmented nanowires.The multisegmented nanowires were released by dissolving the depos-ited membrane in dichloromethane for a few minutes. The dissolvednanowires in dichloromethane were washed several times with dis-tilled water and finally stored in isopropyl alcohol.

2.2. Cysteamine functionalization of nanowires

The stored CoFe/Au/CoFe nanowires were washed several timeswith distilled water and dried at 100 °C. The multisegment nanowireswere functionalized with cysteamine by treating 100 μg of it with1 mL of 2 mM cysteamine hydrochloride in ethanol solution. Thecysteamine-nanowire solution was put in an ultrasonic bath for a fewminutes to avoid the aggregation of nanowires and expose all surfaceareas of individual nanowires to the cysteamine solution. By keepingthe cysteamine-nanowire solution for 12 h at room temperatureyielded cysteamine functionalized nanowires, which were separated

Fig. 2. Backscattering SEM image (a) and EDS spectrum (b) o

and purified by repeated magnetic purification method—using a smallmagnetic field.

2.3. Characterization techniques

The structural growth, morphology and multilayer structure ofnanowires were observed by FE-SEM (Nova-230) with an operatingvoltage of 10 kV. The EDS coupled with the FE-SEM was used forthe analysis of elemental composition. The operating voltage andprobe current for the measurement EDS system was 20 kV and1.5 nA. The as-deposited nanowires within the polycarbonate mem-brane were used for the measurement of magnetic properties by aVSM (Lake-Shore 7400) at room temperature with an applied fieldrange of 800 kA m−1 to −800 kA m−1. Cysteamine functionalizedmultisegment nanowires were characterized through EDS and FT-IR(Brucher Optic GmbH,Germany). The XPS measurements wereperformed using X-ray photoelectron spectrometer (Multilab 2000,Thermo scientific) equipped with an alpha 110 hemisphericalelectron energy analyzer and an Al Kα X-ray twin anode source(hυ—1486 eV, power—200 W, energy range 0–15 kV). The passenergy was set at 50 eV for general survey scan and 20 eV for highresolution scans. The XPS survey was done from binding energy 0 to1100 eV.

3. Results and discussion

3.1. Synthesis and characterization of multisegment nanowires

Initially, we optimized the growth rate and electrodeposition con-dition parameters such as temperature, pH and concentration of elec-trolyte, applied voltage etc. for the estimation of the length of CoFeand Au single segment separately with respect to time. The structuralmorphology of the multisegment nanowires was illustrated by scan-ning electron microscope. For clear visualization of both segmentswe employed backscattering electron detector. The physical sourceof the electron contrast in backscattered SEM images is the differencein atomic numbers, which results in the electron dispersion ability ofthe material [23,24].

Fig. 2a shows the backscattering image of CoFe/Au/CoFemultisegment nanowires. The black contrast region is correspondedto CoFe and the white region reveals the Au segment. The further con-firmation was done by analyzing the elemental composition of eachsegment through EDS measurement (data not shown). The averagelength of the nanowires observed to be around 4 μm, in which themiddle Au segment approximately 3 μm and both the terminal CoFesegments are 0.5 μm in each. The single nanowire image is shownas an inset in Fig. 2a.

The elemental composition of CoFe/Au/CoFe nanowires was stud-ied by EDS which is coupled with the FE-SEM. Fig. 2b shows the EDSspectrum of CoFe/Au/CoFe nanowires which confirms the presence of

f CoFe/Au/CoFe nanowires. Inset shows single nanowire.

Fig. 3. Energy-dispersive X-ray spectroscopy images of CoFe/Au/CoFe nanowires (a) SEM image (b) map of Co concentration (c) map of Fe concentration (d) map of Auconcentration.

257T.S. Ramulu et al. / Thin Solid Films 546 (2013) 255–258

the gold (Au), cobalt (Co) and iron (Fe) elements. The average ele-mental composition of cobalt and iron was found to be approximately70% and 30%, respectively. It is seen from Fig. 2b that, the nanowireshave no other impurity materials, which indicate the magnetic andchemical properties shown by the nanowires are completely pro-duced from the cobalt and iron elements. Fig. 3 shows theenergy-dispersive X-ray spectroscopy images of CoFe/Au/CoFenanowires, indicating the spatial distribution of elements Au, Co andFe. The signal intensity of Co and Fe is very weak in comparisonwith the Au, which may be due to the short length of CoFe segmentthan Au segment.

The magnetic moment of CoFe/Au/CoFe multisegment nanowiresand CoFe single segment nanowires embedded in a polycarbonatemembrane with an equal area were measured at room temperaturewith the external magnetic field applied parallel to the nanowire'slong axis. The hysteresis loops for both CoFe and CoFe/Au/CoFe

Fig. 4. Magnetic properties of CoFe and CoFe/Au/CoFe nanowires measured at theapplied magnetic field parallel to the nanowire axis.

nanowires are compared in Fig. 4. It is clear from Fig. 4 that singlesegment CoFe nanowires show 5.4 times higher saturation magne-tization than the saturation magnetization of CoFe multisegmentnanowires for same wire length. This is due to the incorporation ofthe non-magnetic segment between two magnetic CoFe segment inthe nanowire vicinity. Even though, the magnetic moment of CoFe/Au/CoFe decreases, it is sufficient to be attracted by the external mag-netic field. It also reveals that the coercivity and the remanence ofCoFe/Au/CoFe nanowires are smaller than a single segment of CoFenanowires. This may be due to the length of CoFe segment in CoFe/Au/CoFe nanowire is small compared to that of the CoFe single segmentnanowire [25].

3.2. Surface modification of the Au segment of CoFe/Au/CoFe nanowires

The confirmation of amine functionalization on multisegmentnanowires was studied by EDS. Fig. 5a shows the EDS spectrum ofcysteamine functionalized multisegment nanowires. The presence ofnitrogen and sulfur element indicates the immobilization of cyste-amine molecule. The peak intensity of nitrogen and sulfur is smallerthan other elements, because of the lower atomic percentage of thenitrogen and sulfur present in the nanowire vicinity compared toother elements.

Fig 5b shows the FT-IR spectrum of cysteamine functionalizedmultisegment nanowires. The FT-IR signal for cysteamine on the Ausegment of CoFe/Au/CoFe multisegment nanowires is very weak.However, the presence of amine on the nanowires was observed bythe overlapped vibration band from 1510 to 1595 cm−1, which corre-sponds to the deformation vibration of (δNH of NH2) and the N\Hbending mode [26]. The observed peaks at 1450 cm−1 and1400 cm−1 are C\H bending mode of cysteamine and Au metalpeak, respectively.

The further confirmation of cysteamine functionalization onmultisegment nanowires was obtained from the XPS measurement.The C (1s), S (2p), N (1s) Co (2p), Fe(2p) and Au (4f) XPS spectra

Fig. 5. Cysteamine functionalized nanowires (a) EDS spectrum (b) FT-IR spectrum.

Fig. 6. XPS survey spectra of cysteamine functionalized CoFe/Au/CoFe nanowires.

258 T.S. Ramulu et al. / Thin Solid Films 546 (2013) 255–258

for the cysteamine modified multisegment nanowires are shown inFig. 6. The presence of C 1s, N 1s and S 2p with binding energiesaround 401.2 eV, 288.6 eV and 161.5 eV respectively, correspondsto the cysteamine. The results are in good agreement with the previ-ously reported for free amine groups in close proximity to sulfur andfor cysteamine on gold [27–30]. In addition, the binding energiesaround 714.4 eV, 782.6 eV and 84.2 eV are related to Fe 2p, Co 2pand Au 4f respectively [31,32].

4. Conclusion

We have synthesized CoFe/Au/CoFe multisegment nanowires viaelectrochemicalmethod by using polycarbonatemembrane as a templatewith a pore diameter of 50 nm. The physical and chemical properties ofnanowires were characterized by different characterization techniquessuch as SEM, EDS and VSM. The total observed length of the nanowiresis found to be around 4 μm. The magnetic properties like magnetization,coercivity and remanence of CoFe/Au/CoFe are found to be smaller incomparison to the single segment of CoFe nanowires which indicatesthe incorporation of non-magneticmaterial. The fabricatedmultisegmentnanowires are functionalized with cysteamine, which is confirmed byEDS, XPS and FT-IR spectroscopy. This surface functionalizedmultisegment nanowires can be used further in biological application.

Acknowledgment

This research was supported by WCU (World Class University)program through the National Research Foundation of Korea fundedby the Ministry of Education, Science and Technology (R32-20026).

References

[1] H.Y. Park, M.J. Schadt, L. Wang, I.S. Lim, P.N. Njoki, S.H. Kim, M.Y. Jang, J. Luo, C.J.Zhong, Langmuir 23 (2007) 9050.

[2] H. Hu, Y. Ni, V. Montana, R.C. Haddon, V. Parpura, Nano Lett. 4 (2004) 507.[3] A.J. Haes, W.P. Hall, L. Chang, W.L. Klein, R.P. Van Duyne, Nano Lett. 4 (2004)

1029.[4] F. Long, C. Gu, A.Z. Gu, H. Shi, Anal. Chem. 84 (2012) 3646.[5] C. Boozer, J. Ladd, S. Chen, S. Jiang, Anal. Chem. 78 (2006) 1515.[6] T.S. Ramulu, R. Venu, B. Sinha, S.S. Yoon, C.G. Kim, Int. J. Electrochem. Sci. 7 (2012) 7762.[7] G. Chang, Y. Luo, W. Lu, J. Hu, F. Liao, X. Sun, Thin Solid Films 519 (2011) 6130.[8] R.K. Shervedani, M. Bagherzadeh, S.A. Mozaffari, Sens. Actuators B 115 (2006)

614.[9] M. Adabi, R. Saber, M. Adabi, S. Sarkar, Microchim. Acta 172 (2011) 83.

[10] P. Du, H. Li, Z. Mei, S. Liu, Bioelectrochemistry 75 (2009) 37.[11] G.J. Zhang, J.H. Chua, R.E. Chee, A. Agarwal, S.M. Wong, K.D. Buddharaju, N.

Balasubramanian, Biosens. Bioelectron. 23 (2008) 1701.[12] T.S. Ramulu, R. Venu, B. Sinha, B. Lim, S.J. Jeon, S.S. Yoon, C.G. Kim, Biosens.

Bioelectron. 40 (2013) 258.[13] H. Wang, X. Wang, X. Zhang, X. Qin, Z. Zhao, Z. Miao, N. Huang, Q. Chen, Biosens.

Bioelectron. 25 (2009) 142.[14] A.K. Salem, P.C. Searson, K.W. Leong, Nat. Mater. 2 (2003) 668.[15] K.B. Lee, S. Park, C.A. Mirkin, Angew. Chem. Int. Ed. 43 (2004) 3048.[16] Y. Ye, L. Dai, T. Sun, L.P. You, R. Zhu, J.Y. Gao, R.M. Peng, D.P. Yu, G.G. Qin, J. Appl.

Phys. 108 (2010) 044301.[17] G. Tai, W. Guo, Z. Zhang, Cryst. Growth Des. 8 (2008) 2906.[18] L. Gangloff, E. Minoux, K.B.K. Teo, P. Vincent, V.T. Semet, V.T. Binh, M.H. Yang,

I.Y.Y. Bu, R.G. Lacerda, G. Pirio, J.P. Schnell, D. Pribat, D.G. Hasko, G.A.J.Amaratunga, W.I. Milne, P. Legagneux, Nano Lett. 4 (2004) 1575.

[19] T.S. Ramulu, R. Venu, S. Anandakumar, V. Sudha Rani, S.S. Yoon, C.G. Kim, ThinSolid Films 520 (2012) 5508.

[20] S.R. Nicewarner-Pena, R.G. Freeman, B.D. Reiss, L. He, D.J. Pena, I.D. Walton, R.Cromer, C.D. Keating, M.J. Natan, Science 294 (2001) 137.

[21] W.S. Seo, J.H. Lee, X. Sun, Y. Suzuki, D. Mann, Z. Liu, M. Terashima, P.C. Yang, M.V.mcconnell, D.G. Nishimura, H. Dai, Nat. Mater. 5 (2006) 971.

[22] B. Srinivasan, Y. Li, Y. Jing, Y.H. Xu, X. Yao, C. Xing, J.P. Wang, Angew. Chem. Int. Ed.48 (2009) 2764.

[23] Y. Zhou, A. Hu, Open Surf. Sci. J. 3 (2011) 32.[24] Y.G. Guo, L.J. Wan, C.F. Zhu, D.L. Yang, D.M. Chen, C.L. Bai, Chem. Mater. 15 (2003)

664.[25] J. Escrig, R. Lavın, J.L. Palma, J.C. Denardin, D. Altbir, A. Cortes, H. Gomez, Nano-

technology 19 (2008) 075713.[26] A. Rai, C.C. Perry, Langmuir 26 (2010) 4152.[27] R. Stine, D.Y. Petrovykh, J. Electron. Spectrosc. 172 (2009) 42.[28] R. Srinivasan, R.A. Walton, Inorg. Chim. Acta 25 (1977) L85.[29] T.H. Lee, J.W. Rabalais, J. Electron Spectrosc. Relat. Phenom. 11 (1977) 123.[30] K.I. Ozoemena, N.S. Mathebula, J. Pillay, G. Toschi, J.A. Verschoor, Phys. Chem.

Chem. Phys. 12 (2010) 345.[31] J. Sun, Y. Li, X. Liu, Q. Yang, J. Liu, X. Sun, D.G. Evans, X. Duan, Chem. Commun.

48 (2012) 3379.[32] Y. Mikhlin, M. Likhatski, Y. Tomashevich, A. Romanchenko, S. Erenburg, S.

Trubina, J. Electron Spectrosc. Relat. Phenom. 177 (2010) 24.