07) 2247–2250www.elsevier.com/locate/matlet

Materials Letters 61 (20

Fouling study of silicon oxide pores exposed to tap water

Joakim Nilsson, William L. Bourcier, Jonathan R.I. Lee, Sonia E. Létant ⁎

Chemistry and Materials Science Directorate, Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550, USA

Received 27 June 2006; accepted 28 August 2006Available online 15 September 2006

Abstract

We report on the fouling of Focused Ion Beam (FIB)-fabricated silicon oxide nanopores after exposure to tap water for two weeks. Poreclogging was monitored by Scanning Electron Microscopy (SEM) on both bare silicon oxide and chemically functionalized nanopores. Whilefouling occurred on hydrophilic silicon oxide pore walls, the hydrophobic nature of alkane chains prevented clogging on the chemicallyfunctionalized pore walls. These results have implications for nanopore sensing platform design.© 2006 Elsevier B.V. All rights reserved.

Keywords: Sensors; Ion beam technology; Surfaces

1. Introduction

We recently developed a method to fabricate and functiona-lize silicon nanopores with single stranded DNA using acombination of FIB drilling and ion beam-assisted oxidedeposition [1]. Nanopore-based bio-sensing has attractedinterest in the last few years and several platforms have beenfabricated and tested in clean laboratory conditions [2–8].However, as the design of these platforms becomes more andmore refined and moves toward real-life applications, there is agrowing need for fouling studies to be performed.Water analysisand surveillance are likely applications for nanopore-basedsensors and it is therefore relevant to start the study in thismedium. In this communication, we report on the fouling of FIB-fabricated silicon oxide nanopores after exposure to tap water fortwo weeks. Pore clogging was monitored by scanning electronmicroscopy (SEM) on both bare silicon oxide and chemicallyfunctionalized nanopores. While fouling occurred on hydro-philic silicon oxide pore walls, the hydrophobic nature of thealkane chains prevented clogging on the chemically functiona-lized pore walls. The results have implications for futurenanopore sensing platform design.

⁎ Corresponding author. Tel.: +1 925 423 9885; fax: +1 925 422 2041.E-mail address: letant1@llnl.gov (S.E. Létant).

0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2006.08.062

2. Experimental

The pore fabrication and functionalization procedure hasbeen described in detail elsewhere [1]. It is a two-step process inwhich the pores are first drilled and then locally oxidized usinga mixture of tetraethylorthosilicate (TEOS) gas and H2O vapor,and the Ga+ ion beam of the FIB [9–11]. Un-doped silicon onInsulator (SOI) wafers with a device layer thickness of 700 nmand a handle thickness of 300 μm were coated with 100 nm ofsilicon nitride, patterned, and etched (DRIE) in order to obtainindividual 100×100 μm silicon membranes. Temporarychromium locator lines were fabricated on the top of the devicein order to locate the membrane area in the FIB. These lineswere dissolved in a selective chromium etch bath after drilling(CR-7S from Cyantek Corporation). We used an FEI Strata dualbeam FIB for both imaging and fabrication of the devices. TheGa+ ion beam allows ion beam-assisted drilling and depositionwhile the electron beam allows in situ characterization such asscanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDXS). Varying the Ga+ ion beam currentfrom 10 to 50 pA and adjusting the drilling time from less than60 s to several minutes on a 700 nm thick membrane lead topore diameters ranging from less than 100 nm up to one micron.After the initial pore drilling, a mixture of tetraethylorthosilicate(TEOS) gas and H2O vapor was allowed into the chamberthrough a fine needle. The ion beam, with a 10 pA beam current,was then focused on the area where deposition was desired.

Fig. 1. Overview of the surface chemistry employed on the silicon pore platform. A ring of silicon oxide is deposited around the FIB-drilled silicon pore. Thiol-terminated linkers (see structure on the right) are then covalently anchored on the silicon oxide via silane chemistry.

Fig. 2. High resolution sulfur XPS scans performed on a) bare silicon oxide, andb) linker functionalized silicon oxide. The silicon resonance corresponding tosilicon bound on silicon oxide appears at 154 eVon both samples while the S(2p)resonance at 163.8 eVonly appears on the functionalized sample.

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FIB-induced deposition takes place through the decompositionof the precursor gas via interactions with the ion beam. Thepressure in the sample chamber was below 10−6 mbar whiledrilling and about 2.5 10−5 mbar (1 mbar=100 Pa) during oxidedeposition.

Chemical functionalization of the silicon oxide is then carriedout via thiol-terminated alkane chains that are attached to thepore walls via silane chemistry (see Fig. 1). Samples were loadedinto a glove box under a nitrogen atmosphere with a relativehumidity around 15–20%. Inside the glove box, the sampleswere immersed in a solution of 0.1 mL 3-mercaptopropyltri-methoxysilane (MPTS, Gelest), and 10 mL toluene (anhydrous,99.8%, Sigma-Aldrich) for 4 h. They were then rinsed in three10 mL aliquots of toluene followed by two 10 mL aliquots ofdimethylformamide (DMF), (anhydrous, 99.8%, Sigma-Aldrich). After each rinse, the samples were dried with nitrogen.

Contact angle measurements were performed on the baresilicon oxide surfaces as well as on the linker-terminatedsurfaces with a Kruss™ contact angle measurement system. Thedata confirmed that while the silicon oxide surfaces arehydrophilic (30°±2° before Piranha cleaning and 20°±2° aftercleaning), the alkanethiol modified surfaces are hydrophobic(56°±2°).

3. Results and discussion

In order to confirm the presence of thiol-terminated linkers, wecharacterized the functionalization process on flat TEOS surfaces(1×1 cm2) using X-ray photoemission spectroscopy (XPS). All XPSmeasurements were conducted on BL 8.2 of the Stanford SynchrotronRadiation Laboratory (SSRL). Fig. 2a provides evidence that no sulfurwas present on the bare silicon oxide samples. In contrast, theappearance of an S(2p) peak at 163.8 eV (Fig. 2b) indicates that sulfuris present on the surface of the functionalized samples, which confirmsthat thiol-terminated linkers are anchored on the functionalized siliconoxide surfaces.

Arrays of through pores with various diameters (100 nm–1 μm)were fabricated on 700 nm thick silicon membranes using ion beamdrilling followed by ion beam-assisted silicon oxide deposition. Thesamples were imaged by SEM immediately after fabrication, immersedin tap water for two weeks, and then re-imaged by SEM. Fig. 3 showsimages of the same sample before (Fig. 3a) and after (Fig. 3b, c, and d)exposure to tap water. The pores on the bottom row were drilled andthen locally oxidized with TEOS gas and water vapor using the Ga+ ion

beam spot of the FIB, while the pores on the top rowwere left intact afterdrilling. Although not oxidized by ion beam-assisted oxide deposition,the pores on the top row are covered by a layer of native silicon oxide.Fig. 3b shows that after tap water exposure for two weeks, both series ofpores are completely obstructed (see zoom on the plugs shown in Fig. 3cand d). Although no plugs could be observed by SEM at the entrance of

Fig. 3. An array of pores is shown immediately after fabrication (a), and after exposure to tap water for 2 weeks (b). Enlarged images of the deposits formed in the largerpores are shown in (c) for the pores with native oxide, and in (d) for the pores with grown oxide rings.

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the smallest pores, deposits can still reside deeper in the channels.Additionally, capillary forces can also be responsible for plug lift-off onthese smaller features upon drying.

In a second experiment, an array of pores was fabricated using thesame conditions as the one described above. In addition, the pores werechemically functionalized with alkanethiol linkers, as previouslydescribed in this text. After the reference image (Fig. 4a) was captured,the sample was exposed to a solution of 3-mercaptopropyltrimethox-ysilane in toluene for 4 h, and then rinsed, dried, and immersed in tapwater for two weeks. The sample was then removed from the tap waterand allowed to dry before the second image (Fig. 4b) was captured.Functionalized pores showed no trace of clogging, in contrast to un-functionalized pores which became completely obstructed after beingimmersed in tap water for two weeks. We attribute these results to thehydrophobic nature of the alkane linker, which dramatically reducedfouling of the pores.

Both open and obstructed pores were characterized with energydispersive X-ray spectroscopy (EDXS) in order to determine the natureof the deposits formed inside the pores that were not functionalized.Although no atomic composition can be derived from the spectrarecorded on uneven surfaces, this technique allows to determine theelements present at specific locations on the sample. Fig. 5a shows aspectrum acquired on the deposits formed in an un-functionalized poreafter two weeks in tap water (see pores shown in Fig. 3b). Fig. 5b showsa spectrum acquired on a functionalized silicon oxide pore whichremained open after immersion in tap water (see pores shown inFig. 4b). The EDXS spectrum recorded on the oxide ring (Fig. 4b) of the

Fig. 4. An array of FIB-fabricated pores is presented as prepared in (a), and

functionalized pore shows the presence of silicon, oxygen, gallium, andcarbon. Silicon and oxygen come from the silicon oxide layer, asexpected. The gallium peak is due to gallium ions implanted in thedevice during ion beam drilling and ion beam-assisted oxide deposition.Finally, carbon comes from the alkyl chains anchored on the siliconoxide pore. Sulfur cannot be detected by EDS due to its relatively lowabundance and cross-section. No additional unexpected element wasobserved, confirming the open and clean nature of the functionalizedpores after water immersion. The EDXS spectrum recorded on the plugof an obstructed pore shows that the plug has a high carbon and/orcalcium content, and that it also contains oxygen, and traces ofaluminum. The silicon peak most likely originates from the pore walls.

Compositional data supplied by the Zone 7 water agency for the tapwater used in our experiments shows that several minerals are at or nearsaturation and are therefore likely candidates for the solids we observeoccupying the pores [12]. The minerals include CaCO3, BaCO3, SiO2

and a Ca–Mg clay phase. The solubility calculations were carried outusing the GWB (Geochemist's WorkBench) software [13]. Althoughour experiments with tap water were covered, even a small amount ofevaporation or equilibration with carbon dioxide in the atmospherecould lead to precipitation of one or more of these phases. The fact thatcarbon is enhanced in the EDXS spectra of the samples with fouledpores suggests the phase is a carbonate phase, probably containingcalcium and/or barium. The EDXS data does not indicate the presenceof barium, however the calcium and carbon peaks are superimposed (theenergy resolution of EDXS is about 0.15 keV, which does not allowseparation of the carbon and the calcium peaks). Carbonate phases such

after functionalization and immersion in tap water for two weeks in (b).

Fig. 5. EDXS data acquired on: (a) the deposits inside a pore that was not functionalized and (b) a functionalized pore, after two weeks of immersion in tap water. Thespectrum shown in (a) was acquired on a plugged pore (see Fig. 3b) and the spectrum shown in (b) was acquired on an open pore (see Fig. 4b).

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as these are very common scaling materials in water treatment plants.The slightly increased presence of silicon in the EDXS spectrum of theplugged sample also suggests some silica precipitation, although themorphology of the deposits and the very slow known rates ofprecipitation of silica under our experimental conditions make thisseem unlikely.

4. Conclusion

In summary, we studied fouling of bare and functionalizedsilicon oxide pores immersed in tap water by SEM and EDXS.Bare hydrophilic silicon oxide pores were completely obstructedby large in-organic plugs after two weeks of immersion, whilethe hydrophobic nature of the alkane chains prevented cloggingon the chemically functionalized pores. Although further studiesare needed for other media such as body fluids, these first resultsillustrate how adequate surface coatings might be used toprevent fouling of membranes and single pores for futuresensing applications.

Acknowledgments

This work was performed under the auspices of the U.S.Department of Energy by University of California LawrenceLivermore National Laboratory under contract W-7405-Eng-48.It was funded by a Laboratory Directed Research andDevelopment grant (LDRD-ER # 03-ERD-013). We are alsograteful to the staff of the National Center for Electron

Microscopy (NCEM) at Lawrence Berkeley National Labora-tory, especially Andrew Minor, for the valuable help with theFIB. NCEM is supported by the U.S. Department of Energyunder Contract # DE-AC02-05CH11231. We would also like tothank the SSRL staff, particularly Dan Brehmer. SSRL issupported by the U.S. Department of Energy under Contract #DE-AC03-76SF00515. Finally, we are also grateful to ChristianBjörnerhag for the help he provided with the design of Fig. 1.

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