Electrochemistry Communications 13 (2011) 934–937

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Electrochemistry Communications

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Porous anodic alumina: Amphiphilic and magnetically guidable micro-rafts

Himendra Jha, Yan-Yan Song 1, Min Yang, Patrik Schmuki ⁎Department of Materials Science WW-4, LKO, University of Erlangen-Nuremberg, Martenstrasse 7, 91058 Erlangen, Germany

⁎ Corresponding author. Tel.: +49 9131 85 275 75; fE-mail address: schmuki@ww.uni-erlangen.de (P. Sc

1 Current Address: College of Sciences, Northeastern110004, China.

1388-2481/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.elecom.2011.06.004

a b s t r a c t

a r t i c l e i n f o

Article history:Received 16 May 2011Received in revised form 1 June 2011Accepted 2 June 2011Available online 12 June 2011

Keywords:Electrochemical anodizationPorous anodic aluminaSurface modificationMagnetic guiding

The present work illustrates the fabrication of wettability-sensitive, magnetically guidable micro-chips(“rafts”) based on anodic porous alumina. First, porous anodic alumina is grown with in photo-lithographically defined squares (400 μm×400 μm), then magnetic nanoparticles are embedded in theporous structure by sucking a ferro-fluid into the nanopores with the help of a permanent magnet. After pore-sealing, the top surface of the alumina patterns is grafted with a self assembled monolayer of octadecylsilane(ODS). This makes the top surface super-hydrophobic while the bottom layer (i.e. barrier layer exposed afterselective removal) remains hydrophilic. The resulting 400×400 μm2 micro-rafts can be easily guidedmagnetically along amphiphilic interfaces. The principles outlined in this work may provide a basis formanyfold applications for example, in payload delivery or -harvesting devices.

ax: +49 9131 85 275 82.hmuki).University, Box 332, Shenyang

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© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Porous anodic alumina has extensively been used as a template forthe synthesis of various nanomaterials for more than a decade [1–3]. Itfound applications in integrated circuits [4], micro-wiring boards [5],chemical sensing [6,7] and various components of micro-devices inelectronics [8], medicine [9,10] or military [11]. Recently, porousanodic alumina was used to synthesize highly uniform aluminumoxy-hydroxide nanorods and was successfully employed to encapsu-late nanoparticles for potential biomedical applications [12,13].

Over the past years, modified porous materials have attracted alsoincreasing attention as microstructures that have self-aligning (self-orientation) properties, for example the ability to recognize wettabilitydifferences (“smart dust” [14]), or can be guided or harvested by remoteforces. Such structures are of particular interest in various targetedpayload delivery based chemical or medical applications [15,16]. Whilethe specific recognition part can be based on many different selectiveinteractions, the guiding system is often based on a magneticcomponent, because of the ease of control and manipulation [17–19].

In the present work we use anodic porous alumina to achieve acombined multifunctional platform (micro-raft) with amphiphilicself-aligning and magnetic guiding properties.

2. Experimental

The fabrication steps of these multifunctional micro-rafts areoutlined in Fig. 1. Aluminum sheets with 125 μm thickness (99.999%;

Advent metals) were cut into 5 cm×5 cm pieces and ultrasonicallycleaned. The pieces were then patterned using photolithography. Aphoto-resist (S1813™ G2, Rohm and Haas Electronic Materials) wasspin coated, baked at 120 °C for 2 min and patterned with a photo-mask to create 400 μm×400 μm openings using a developer (351Developer, Rohm and Haas Electronic Materials). The patterned sheetwas then heated to 250 °C for 5 min to obtain patterns of carbonizedresist. In the process it is crucial to transform the patterned photo-resist to a “carboneous layer” as such patterns remain well definedand stable on the surface, even during several hours of anodization.The patterned aluminum sheets were then anodized in 0.22 M oxalicacid solution, for 2 h at 50 V at room temperature.

After anodization, the specimens were immersed in the same0.22 M oxalic acid solution at 35 °C for 90 min to widen the pore-diameter. Magnetic nanoparticles (ferro-fluid, MSG W11, FerroTechUSA) were then filled inside the pores by placing a permanentmagnet(NdFeB cylinder, 1.4 T, Neotex) underneath the specimen as shown inFig. 1b. Such filled specimens were then dried in an oven for 3 h at50 °C. Afterwards, the specimens were subjected to a hydrothermaltreatment in boiling, doubly distilled water for 1 h at atmosphericpressure. This closes the pores and forms a hydroxide layer on thesurface, as shown in Fig. 1c. This hydroxide layer was then madehydrophobic by attaching a monolayer of octadecylsilane (ODS). Forthis the cleaned specimens were immersed in a 5 mM ODS in toluene,and kept at 70 °C for 24 h. After coating, the specimenswerewashed inacetone–ethanol–water. Finally, the remaining aluminum substrateunderneath the patterned oxide surfacewas selectively removedusinga solution of 1.7 g of CuCl2 in 50 ml of HCl (38%) at room temperature.The solution was dropped on the exposed aluminum surface (i.e. backside of the specimen) and the reaction was continued until all themetallic aluminum was dissolved. After aluminum dissolution themicro-structures were separated, as shown in Fig. 1e.

Fig. 1. Schematics of anodic alumina “micro-raft” fabrication (not to scale). [a] Formation of porous anodic alumina on patterned aluminum substrate; [b] magnetic nanoparticlefilling; [c] hydrothermal pore-sealing of porous anodic alumina; [d] ODS monolayer coating on the whole surface; and [e] dissolution of aluminum substrate to expose the barrieroxide layer of anodic alumina.

935H. Jha et al. / Electrochemistry Communications 13 (2011) 934–937

The morphological characterization was carried out with a FE-SEM(Hitachi,Hitachi S4800).Optical imageswere takenbyacommercialdigitalcamera. The chemical composition of the surface was analyzed by XPS.

3. Results and discussion

Fig. 2 shows the micro-patterns (400×400 μm2) of porous aluminathat were grown on the aluminum surface after photolithographic

Fig. 2. FE-SEM images of porous anodic alumina before and after magnetic filling. [a] Patterformed in the anodized part; [c] surface after magnetic nanoparticle filling; and [d] cross-s

patterning and anodization. The layer was fabricated to a thickness ofapprox. 32 μm. A magnified view of porous anodic alumina formed onexposed area is shown in Fig. 2b (after pore-widening). It can be seenthat the porous oxide surface is uniform and has defined openings ofapprox. 80 nm in diameter.

Fig. 2c shows the surface of the porous oxide layer after magneticnanoparticle filling. From Fig. 2c and d, it can be seen that the methodof filling is very effective and most of the nanopores are packed with

ned aluminum substrate after anodizing; [b] magnified view of porous anodic aluminaection of the magnetic nanoparticle filled porous alumina near the barrier layer.

936 H. Jha et al. / Electrochemistry Communications 13 (2011) 934–937

particles down to the bottom of the porous oxide layer (Fig. 2d)(although some loss occurs during sample preparation for SEM).

The filled porous oxide layer was then treated in boiling water toclose the pores. After the treatment, two distinct changes can beobserved in the film, (i) the pores are plugged with aluminum oxy-hydroxide over the entire length, and (ii) a highly crystalline acicularhydroxide layer is formed on the entire surface of the film [20] asshown in Fig. 3a. Because of the roughness and hydroxide termina-tion, these surfaces show a super-hydrophilic water wetting behavior.In order to make the layer hydrophobic, an octadecylsilane monolayerwas grafted on the surface which turned the wetting characteristics tosuper-hydrophobic as shown in Fig. 3b (the resulting water contact-angle is about 165°).

After the aluminum removal the barrier oxide of the film isexposed as shown in Fig. 3c. The water-contact angle on the barrieroxide surface is measured as about 43°, as shown in Fig. 3d, i.e. thesurface is hydrophilic. In other words, the obtained free standingoxide film has bipolar wetting properties with a super-hydrophobictop surface and a hydrophilic bottom oxide.

XPS analysis is used to confirm the presence of ODS monolayer onthe surface. Fig. 3e shows the C (1 s) peak at around a binding energyof 284.7 eV before and after attachment. It can clearly be seen that thecarbon peak increased strongly after the ODS coating. Furthermore,the specimens were analyzed by XRD, as shown in Fig. 3f. The spectra

Fig. 3. [a] FE-SEM images of porous anodic alumina surface after hydrothermal treatment a(scale bar 1 μm); [b] water-contact angle measurement on the monolayer grafted surface (aluminum substrate; [d] water-contact angle measurement on the barrier oxide of the anodand after coating; [f] XRD spectra of the specimen after hydrothermal treatment showinhydrothermal treatment (Al peak is from substrate).

clearly indicate the presence of the magnetic nanoparticles (Fe3O4)and aluminum oxy-hydroxide.

To test the guiding and wetting properties of the free standingoxide squares (400×400 μm2) several tests were performed. Fig. 4illustrates how these “micro-rafts” align themselves at bipolarinterfaces and are magnetically guidable. Fig. 4a–c demonstrates theimportance of the ODC monolayer on the top surface. If magneticguiding is performed with specimens without a hydrophobicmonolayer, they easily sink in water (as shown in Fig. 4a) as boththe top and bottom surfaces are hydrophilic. In contrary, thespecimens with the monolayer on the top do not sink even afterapplication of a magnetic field along the “sinking direction” (Fig. 4a).This illustrates the strong stabilizing effect of the hydrophilic–super-hydrophobic interfacial forces at these “bipolar” surfaces.

Furthermore, the micro-rafts can be guided on the water surfacevery effectively, as shown in Fig. 4b and c. In Fig. 4b two boats are atthe center of a petri-dish half filled with water, and a permanentmagnet is placed at the edge from down side [a corresponding moviefile is provided at [21]]. Upon placing a magnet, immediately themicro-rafts swiftly move towards the magnet as shown in Fig. 4c (therafts are inside the circle). Because of the hydrophilic–hydrophobicdual surfaces, the magnetic rafts can also be manipulated at theinterface of polar (water) and non-polar (hexane) solutions. Fig. 4dshows a condition where the micro-rafts are magnetically

nd ODS monolayer grafting, the inset figure shows an image before monolayer grafting165°). [c] Bottom part of the porous anodic alumina (barrier layer side) after removingic alumina (43°). [e] XPS analysis of the ODS monolayer coated surface, C1s peak beforeg presence of iron-oxide nanoparticles, and aluminum oxy-hydroxide formed during

Fig. 4. Optical photographs of the guiding themagnetic micro-rafts (circled in figures) on water surface with an external permanentmagnet. [a, c] Onemonolayer coated and twonon-coated specimens experiments (the non-coated sinks down to the magnet); [b] and [c] top view of magnetic manipulation, the micro-rafts are quickly attracted towardsmagnet; [d] guiding themicro-rafts atwater-hexane interface [arrow on sides indicates the interface]. [Note:movie clips of all themanipulation can be seen onweb site: http://www.lko.unierlangen.de/Download/video_paper_Jha/index.htm].

937H. Jha et al. / Electrochemistry Communications 13 (2011) 934–937

manipulated at the interface of hexane (top) and water (bottom), aseasily as in the case of the water-air surface.

4. Conclusions

A microfabrication method for magnetic amphiphilic micro-raftsbased on porous anodic alumina is demonstrated. Photolithograph-ically defined anodization combined with magnetic nanoparticlefilling, pore-sealing and monolayer deposition techniques allows tofabricate magnetic micro-rafts that provide different wettabilityproperties on the top (super-hydrophobic) and bottom (hydrophilic)surfaces. Such dual wetting properties make it possible to establish anextremely high floating capability of the micro-rafts while magnet-ically guiding. The principle shown in this work is a parallel, sizescalable process with the potential for much wider use in guiding,recognition and harvesting applications. Furthermore, the side-selective attachment of silanes may also be used for furtherapplications where a specific functionality is desired to be transportedsuch as in drug delivery or bio-recognition devices.

Acknowledgment

We would like to acknowledge financial support from theAlexander von Humboldt Foundation (for H.J. and Y.Y.S.) and theGerman Research Foundation (DFG), and Prof. Dr. SannakaisaVirtanen for valuable help.

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