Microelectronic Engineering 115 (2014) 2–5

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Microelectronic Engineering

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High-fidelity replica molding for large-area PMMA 3D nanostructureswith high performance surface-enhanced Raman scattering andhydrophobicity

0167-9317/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.mee.2013.10.017

⇑ Corresponding authors. Tel./fax: +86 02166137197 (J. Wang). Tel.: +8602164101616 (W. Zhou).

E-mail addresses: wangjinhe@gmail.com (J. Wang), zhouweimin@snpc.org.cn(W. Zhou).

Jinhe Wang a,⇑, Weimin Zhou b,⇑, Jing Zhang b, Mingjin Yang a, Chentao Ji a, Xuexiang Shao b, Liyi Shi a

a Nano-Science & Technology Research Center, Shanghai University, Shanghai 200444, Chinab Key Lab of Nanotechnology Promotion Center (SNPC), Shanghai 200237, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 20 August 2013Received in revised form 8 October 2013Accepted 24 October 2013Available online 6 November 2013

Keywords:PMMA 3D nanostructuresReplica moldingSERSHydrophobicity

Nanogaps or sharp protrusions has been proved useful for surface enhanced Raman scatting (SERS) in sin-gle-molecule sensing and a low cost method for fabricating SERS active substrate with these naonstruc-tures is very important for practical applications of SERS in clinical or biological area. In this work, PMMA3D nanostructures with sub-20 nm ordered nanogap network and sharp tips are fabricated by high-fidelityreplica molding. The morphology, water repellency property and Raman enhancement effect of the 3Dnanostructures are characterized by SEM, water contact angle and SERS. The results show that the 3Dnanostructure of rocket-like array has higher SERS enhancement factor (EF) than the 3D nanostructureof nano-lawn due to the ordered nanogaps, and both of them change the surface wettability of PMMAfrom hydrophilic to hydrophobic, especially for the 3D structure of nano-lawn.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Biochip is a miniaturized standard biochemical laboratoryprocesses which has potential to revolutionize many life scienceapplications and related areas, including clinical, biological, andchemical applications [1–4]. Microfluidic channels and surface-enhanced Raman scatting (SERS) active areas are two importantparts for a biochip based on SERS [5]. Controlling the wettabilityof the microfluidic channel surface is highly desirable as it couldbe employed to tailor the flow characteristics in the channels [6],and highly ordered nanostructures are expected for SERS becauseof its high sensitivity, uniformity and repeatability [7–11], espe-cially nanostructures with sharp protrusions or nanogaps whichare essential to excite the localized surface plasmons and provideseveral orders of magnitude larger enhancement factor (EF) thanthose in other places [12–17]. In addition, from the applicationconsideration, low cost and disposability are required and goodoptical transmission of visible wavelength is favored for combiningsome optical observation instruments [18].

Poly(methylmethacrylate) (PMMA) has good biocompatibility,mechanical stability, high optical transmission in the visiblewavelengths and low auto-fluorescence, which make itself the

first choice of biochip substrate materials [2,19,20]. However,fabricating PMMA micro/nanostructures are usually rely on pho-tolithography, electron beam lithography, femtosecond laserablation or hot embossing lithography [21,22], which are time-consuming and high cost. Replica molding is an alternative choicefor fabricating large area polymer micro/nanostructures with lowcost [23–25]. Replica molding materials are usually thermosetelastomers, and poly(dimethylsiloxane) (PDMS) is the most com-mon one due to its low cost and flexibility [26,27]. However,PDMS has poor mechanical properties and lack of replicationfidelity at sub-micrometer resolutions [27]. What is more, mostorganic solvents cause PDMS to swell severely, hindering its useas a stamp for replicating a PMMA copy. So preparing large areaPMMA micro/nanostructures with high fidelity and low cost isstill a challenge [10].

In this work, large area 3D nanostructures of PMMA rocket-likearray and PMMA nano-lawn are prepared with low cost by a sim-ple and flexible double-layer replica molding method. Throughcontrolling spin-coating and peel-off conditions, one inch PMMArocket-like array with sub-20 nm nanogaps, nanotips and nano-lawn with near 1 lm length of nanorods can be fabricated easily,and the SERS effect and wettability of these 3D nanostructuresare tested. The results show that double-layer replica molding pro-vides a low cost way for large-scale production of PMMA nano-structures, which is a promising low cost method for highperformance SERS active substrate fabrication.

Fig. 1. Nanostructure of AAO master.

Fig. 2. PMMA rocket-like array 3D nanostructure.

J. Wang et al. / Microelectronic Engineering 115 (2014) 2–5 3

2. Experimental section

2.1. Materials

PMMA (CAS No.: 9011-14-7, average MW 350,000) waspurchased from Alfa Aesar (Tianjin, China) Company. Epoxy resinadhesive was obtained from Shenzhen Huasheng TechnologyCompany, China. Porous anodic alumina (AAO) with nanoholesdiameters of 50–100 nm was purchased from Hefei Puyuan Nano-technology Company, China. Rhodamine 6G (R6G), 1H,1H,2H,2H-Perfluorodecyltrichlorosilane (FDTS) and toluene were obtainedfrom Sigma–Aldrich.

2.2. Double-layer replica molding process

Double-layer replica molding consists mainly there steps, thefirst step is spin-coating PMMA solution on a AAO master andforming a very thin layer of PMMA with an inverse nanostructurewith that of the inorganic master. The second step is casting theepoxy resin layer. The third step is peel the double layer replicaoff from the master. The rotate speed is 2000 rpm for both of thesetwo nanostructures fabrications. After the PMMA spin-coating, theAAO master carrying the thin PMMA layer are removed to a vac-uum oven to wipe off the remain toluene solvent. The epoxy resinis casted and cured at 120 �C for 30 min. Then, the double-layerreplica with inverse nanostructures is peeled off from the AAOmaster slowly. The SERS substrates are obtained by vacuum evap-oration of 5 nm Au film on the PMMA nanostructures.

Before these three steps, some preparations need to be donefirst. AAO master needs an self-assembled anti-adhesive FDTSmonolayer by vapor phase deposition for demolding convenience.PMMA is dissolved in toluene at 40 �C and electromagnetic stirringfor 3 h. The concentration of the PMMA toluene solution is 150 mg/ml for the rocket-like array and 50 mg/ml for the nano-lawns. ThePMMA solution is dripped in different ways during the fabricationsof rocket-like array and nano-lawns. For the rocket-like array, thePMMA solution is dripped after the rotation of AAO master andpeeled off at room temperature, while for the nanorods, the PMMAsolution is dripped before the rotation of AAO master and peeledoff at the curing temperature.

2.3. Characterization

The morphologies of the AAO master, rocket-like array andnano-lawn are characterized by FE-SEM (Hitachi S-4800). The con-tact angle of water droplet is measured using a contact angle ana-lyzer before the Au film evaporation. In the SERS tests, rhodamine6G (R6G) was adsorbed on SERS active substrate or glass slide byplacing a drop of R6G water solution and letting it dry in air.Raman measurements were carried out using 514 nm argon ionlaser with a output power of 25 milli-watt (mW), a spot size of2 lm in diameter and collection time of 20 s.

3. Results and discussion

Fig. 1 shows the morphology of one of the AAO master with ananohole array of diameter about 50 nm and center-to-centerdistance about 160 nm. Each nanohole has a hexagonal funnel ori-fice which is ordered in a range of 6–10 units. Fig. 2 is the PMMArocket-like array 3D nanostructure fabricated by the double-layerreplica molding method. From the insert image on the right cornerwe can see that the rocket-like array are distributed uniformly overlarge area. Due to the good penetration of the PMMA solution andthe high modulus of PMMA, the hexagonal funnel orifice of theAAO are transferred to the PMMA layer with high fidelity and

forms nanogaps with figure size sub-20 nm and rocket-like nano-rods with sharp tips. The diameter and height of these rocket-likenanorods is about 50 nm and 150 nm, respectively. The nanogapsform a big network which contains many connected hexagonalnanogap rings. The aspect ratio of the nanorods is about 3:1.Fig. 3 gives the schematic illustration of the rocket-like array.

Fig. 4 is the PMMA nano-lawn which fabricated by the samemethod but different spin-coating condition and peel-off tempera-ture. Compared with Fig. 2, the hexagonal tables under the nano-rods are disappeared, which is result from the stretching duringthe hot peel-off process. The diameter and length of the nanorodsis about 80 nm and a wide range distribution from 150 to 1000 nm,respectively. Some long nanorods have an aspect ratio of 12:1,which make these nanorods hardly to remain straight and theirtips touch each other, forming like a nano-lawn. The AAO masterafter the peel-off process is also characterized and many brokenPMMA nanorods are on the AAO master surface (Fig. 5), whichillustrates that the difference of the length of the nanorods is re-sulted from the broken of these nanorods during the hot peel-offprocess.

Water repellency has recently received significant attentionbecause of its importance in many biological processes and techno-logical applications. For example, microfluidic channels, cell motil-ity controlling, self-cleaning [28,29] and single molecule detection[30–32]. For comparison, plane PMMA surface was prepared byspin-coating PMMA solution on a cleaned glass slide and then wipeoff the remain toluene solvent. Fig. 6 gives the water contact angles

Fig. 3. Schematic illustration of the rocket-like array.

Fig. 4. PMMA nano-lawn 3D nanostructure.

Fig. 5. The surface of the AAO master after the nano-lawn fabrication.

ig. 6. Water contact angle of different PMMA substrates: (a) Bulk PMMA; (b)MMA rocket-like array substrate and (c) PMMA nano-lawn.

Fig. 7. Raman spectrum of R6G on different PMMA substrates.

4 J. Wang et al. / Microelectronic Engineering 115 (2014) 2–5

of plane, rocket-like array and nano-lawn of PMMA. Fig. 6a is thecontact angle of plane PMMA (71.2�), which shows bulk PMMA ishydrophilic. The PMMA rocket-like array has a water contact angleof 97.2� (Fig. 6b) and the PMMA nano-lawn has a contact angle of131.3� (Fig. 6c), meaning that the PMMA surface wettability hasbeen changed from hydrophilic to hydrophobic. Especially, forthe PMMA nano-lawn, the composite structure of long nanorodsand shot nanorods gives high performance of water repellency ofthe nano-lawn [33]. This composite structure provides an alterna-tive choice for artificial superhydrophobic surface.

Fig. 7 is the Raman spectrum of R6G on different substrates.Figs. 7a and 7b is the SERS of R6G water solution with 10 nMand 1 lM on PMMA rocket-like array substrate and PMMA nano-lawn substrate, respectively. The ordinary Raman spectrum ofR6G with 0.1 M on glass slide (Fig. 7c) and SERS of R6G with1 lM on plane PMMA deposited Au film (Fig. 7d) are also testedfor comparison. From this figure we can see that the enhancementseffect of the rocket-like array and the nano-lawn substrates areobvious. The EF can be calculated by the following expression [10],

EF ¼ ISERS=NSERS

Ibulk=Nbulkð1Þ

where ISERS and Ibulk is the Raman intensity of SERS substrate andthat of non-enhanced scatting of bulk sample at the same band(1649 cm�1 was used in this work), respectively; NSERS and Nbulk

FP

is the number of excited molecules on the SERS substrate and on thebulk sample, respectively. Because the volumes of the R6G solutiondrops and the covered areas of these substrates are almost thesame, so the areal density of the R6G molecular is in direct propor-tion to the concentration. In addition, the laser spot size, the laseroutput power and the collection time are the same during all theseRaman tests. So the EF can be simplified as:

J. Wang et al. / Microelectronic Engineering 115 (2014) 2–5 5

EF ¼ ISERS=CSERS

Ibulk=Cbulkð2Þ

where the CSERS and the Cbulk is the concentrations of R6G of theSERS substrate used and the concentration of R6G of the bulksample used. Using Eq. (2), the EFs of the rocket-like array andthe nano-lawn substrates is 3 � 108 and 1 � 105. Considering therocket-like array has less specific surface area than the nano-lawn,the higher EF of the rocket-like array substrate is attributed to theordered nanogaps’ near-field coupling which excites the localizedsurface plasmons [34,35].

4. Conclusions

A simple and low cost replica molding method using PMMA asthe replica material directly for fabricating large-area PMMA 3Dnanostructure is introduced in this work. Rocket-like array andnano-lawn can be fabricated by the same method through chang-ing the spin-coating condition and peel-off temperature. Due to thehigh modulus of PMMA, this method has high fidelity andsub-20 nm nanogap network has been formed in the 3D rocket-likearray substrate, which endows the substrate with higher EF of SERSafter Au deposition than that of the nano-lawn substrate. Boththese two kinds 3D nanostructures change the surface wettabilityof PMMA from hydrophilic to hydrophobic and the nano-lawnsubstrate has a water contact angle of 131.3� resulted from thecomposite structure of long nanorods and shot nanorods. Thiscomposite structure provides an alternative choice for artificialsuperhydrophobic surface.

Acknowledgments

We gratefully acknowledge financial support from the ShanghaiYoung Teachers Subsidy Scheme of 2012, the Innovation Fund ofShanghai University (sdcx2012035), Natural Science Foundationof Shanghai (11ZR1432100) and Professional and Technical ServicePlatform of Design and Manufacture for Advanced CompositeMaterials, Shanghai (13DZ2292100). Dr. Bo Lu and Hongrui Li areacknowledged for SERS and water contact angle measurements.

References

[1] K.Y. Hwang, J.H. Kim, K.Y. Suh, J.S. Ko, N. Huh, Sens. Actuators B 155 (2011)422–429.

[2] B. Altmann, T. Steinberg, S. Giselbrecht, E. Gottwald, P. Tomakidi, M. Bachle-Haas, R.J. Kohal, Biomaterials 32 (2011) 8947–8956.

[3] Z.S. Li, W.D. Ruan, W. Song, X.X. Xue, Z. Mao, W. Ji, B. Zhao, Spectrochim. ActaPart A 82 (2011) 456–460.

[4] T. Bhuvana, G.U. Kulkarni, Small 4 (2008) 670–676.

[5] K.K. Strelau, R. Kretschmer, R. Moller, W. Fritzsche, J. Popp, Anal. Bioanal.Chem. 396 (2010) 1381–1384.

[6] C. De Marco, S.M. Eaton, M. Levi, G. Cerullo, S. Turri, R. Osellame, Langmuir 27(2011) 8391–8395.

[7] B. Cui, L. Clime, K. Li, T. Veres, Nanotechnology 19 (2008) 145302(1–6).[8] S.W. Lee, K.S. Lee, J. Ahn, J.J. Lee, M.G. Kim, Y.B. Shin, ACS Nano 5 (2011) 897–

904.[9] R. Alvarez-Puebla, B. Cui, J.P. Bravo-Vasquez, T. Veres, H. Fenniri, J. Phys. Chem.

C 111 (2007) 6720–6723.[10] W.Y. Chang, K.H. Lin, J.T. Wu, S.Y. Yang, K.L. Lee, P.K. Wei, J. Micromech.

Microeng. 21 (2011) 035023(1–5).[11] D.K. Lim, K.S. Jeon, J.H. Hwang, H. Kim, S. Kwon, Y.D. Suh, J.M. Nam, Nat.

Nanotechnol. 6 (2011) 452–460.[12] F.J. GarciaVidal, J.B. Pendry, Phys. Rev. Lett. 77 (1996) 1163–1166.[13] M. Futamata, Y. Maruyama, M. Ishikawa, J. Phys. Chem. B 107 (2003) 7607–

7617.[14] W. Wu, M. Hu, F.S. Ou, Z.Y. Li, R.S. Williams, Nanotechnology 21 (2010)

255502(1–6).[15] C. Qian, C. Ni, W.X. Yu, W.G. Wu, H.Y. Mao, Y.F. Wang, J. Xu, Small 7 (2011)

1801–1806.[16] C.F. Tian, Z. Liu, J.H. Jin, S. Lebedkin, C. Huang, H.J. You, R. Liu, L.Q. Wang, X.P.

Song, B.J. Ding, M. Barczewski, T. Schimmel, J.X. Fang, Nanotechnology 23(2012) 165604(1–7).

[17] F. De Angelis, M. Malerba, M. Patrini, E. Miele, G. Das, A. Toma, R.P. Zaccaria, E.Di Fabrizio, Nano Lett. 13 (2013) 3553–3558.

[18] J.P. Singh, T.E. Lanier, H. Zhu, W.M. Dennis, R.A. Tripp, Y.P. Zhao, J. Phys. Chem.C 116 (2012) 20550–20557.

[19] A. Singh, W. Pfleging, M. Beiser, C.K. Malek, Microsyst. Technol. 19 (2013) 445–453.

[20] D. Sabourin, J. Petersen, D. Snakenborg, M. Brivio, H. Gudnadson, A. Wolff, M.Dufva, Biomed. Microdevices 12 (2010) 673–681.

[21] C.Y. Lee, T.C. Chang, S.C. Wang, C.W. Chien, C.W. Cheng, Biomicrofluidics 4(2010) 046502(1–5).

[22] U. Huebner, K. Weber, D. Cialla, R. Haehle, H. Schneidewind, M. Zeisberger, R.Mattheis, H.G. Meyer, J. Popp, Microelectron. Eng. 98 (2012) 444–447.

[23] S. Park, P. van Rijn, A. Boker, Macromol. Rapid Commun. 33 (2012) 1300–1303.[24] X.H. Dai, X.D. Miao, G.C. Shao, W.J. Wang, Microsyst. Technol. 19 (2013) 403–

407.[25] B.D. Gates, Q.B. Xu, M. Stewart, D. Ryan, C.G. Willson, G.M. Whitesides, Chem.

Rev. 105 (2005) 1171–1196.[26] D.W. Inglis, Biomicrofluidics 4 (2010) 026504(1–8).[27] M.T. Demko, J.C. Cheng, A.P. Pisano, ACS Nano 6 (2012) 6890–6896.[28] H.Y. Erbil, A.L. Demirel, Y. Avci, O. Mert, Science 299 (2003) 1377–1380.[29] P.N. Manoudis, I. Karapanagiotis, A. Tsakalof, I. Zuburtikudis, C. Panayiotou,

Langmuir 24 (2008) 11225–11232.[30] F. De Angelis, F. Gentile, F. Mecarini, G. Das, M. Moretti, P. Candeloro, M.L.

Coluccio, G. Cojoc, A. Accardo, C. Liberale, R.P. Zaccaria, G. Perozziello, L.Tirinato, A. Toma, G. Cuda, R. Cingolani, E. Di Fabrizio, Nat. Photonics 5 (2011)683–688.

[31] F. Gentile, M.L. Coluccio, E. Rondanina, S. Santoriello, D. Di Mascolo, A. Accardo,M. Francardi, F. De Angelis, P. Candeloro, E. Di Fabrizio, Microelectron. Eng. 111(2013) 272–276.

[32] F. Gentile, M.L. Coluccio, A. Accardo, G. Marinaro, E. Rondanina, S. Santoriello,S. Marras, G. Das, L. Tirinato, G. Perozziello, F. De Angelis, C. Dorigoni, P.Candeloro, E. Di Fabrizio, Microelectron. Eng. 97 (2012) 349–352.

[33] E. Celia, T. Darmanin, E.T. de Givenchy, S. Amigoni, F. Guittard, J. ColloidInterface Sci. 402 (2013) 1–18.

[34] S.S. Acimovic, M.P. Kreuzer, M.U. Gonzalez, R. Quidant, ACS Nano 3 (2009)1231–1237.

[35] K. Hoflich, M. Becker, G. Leuchs, S. Christiansen, Nanotechnology 23 (2012)185303(1–7).