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Sensors and Actuators B 137 (2009) 76–82

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journa l homepage: www.e lsev ier .com/ locate /snb

orous anodic alumina for the adsorption of volatile organic compounds

omenico Mombelloa, Nello Li Pirab, Luca Belforteb, Pietro Perlob, Gianfranco Innocentib,imone Bossia, Massimo E. Maffei a,∗

Plant Physiology Unit, Department Plant Biology and Centre of Excellence CEBIOVEM, University of Turin, Via Quarello 11/A, 10135 Turin, ItalyFIAT Research Centre, Micro & Nanotechonolgies Department, Str. Torino 50, 10043 Orbassano, Italy

r t i c l e i n f o

rticle history:eceived 11 August 2008eceived in revised form4 November 2008

a b s t r a c t

A multi-step anodization and leaching process was employed to produce three-dimensional nanometerscale structured alumina plates, used to adsorb volatile organic compounds (VOCs) dissolved in liquidsand present in a gas phase. Nanostructured porous anodic alumina (PAA) plates were observed by meansof atomic force microscopy (AFM) and scanning electron microscopy (SEM). After exposure to VOCs,

ccepted 19 November 2008vailable online 9 December 2008

eywords:orous anodic aluminaolatile organic compoundstomic force microscopy

PAA was analysed by gaschromatography–mass spectrometry after cryo-desorption through a thermaldesorption unit. A direct comparison between PAA and other VOC adsorbing/sorpting systems, such assolid phase microextraction (SPME) and stir bar sorptive extraction (SBSE), was performed. PAA proved tobe a suitable and inexpensive material for the adsorption of VOCs with adsorbing properties comparableto the more expensive SPME and SBSE.

© 2008 Elsevier B.V. All rights reserved.

as chromatography–mass spectrometry

. Introduction

Porous anodic alumina (PAA) has a huge number of nano-cale holes whose cell diameter (pore to pore) can be controlledrom about 10 nm when grown in the anodization solutions ofxalic and/or sulphuric or phosphoric acids. PAA has been used forecades as protection and hard coatings or adhesive layers [1,2]. Inecent years these films, that have good resistance to corrosion andre suitable for applications related to structural materials becausef their thick and dense porous structure, have been widely used as aost material for many applications including quantum-dot arrays,agnetic materials, and photocatalysts [3–7] and for the fabrica-

ion of nanostructured arrays of metals [8], semiconductors [9] andonducting polymers [10]. PAA is also a promising candidate for theabrication of functional electrodes [11] and future sensors [12,13].

Despite all of these applications, little is known about the poten-ial of PAA as an adsorbing system for volatile organic compoundsVOC) produced either by natural sources (such as plant and animalmissions) or as a consequence of flavour application for indoor andutdoor uses. Recently, aluminium foams were studied as struc-

ured supports for the elimination of VOCs [14], but nothing haseen reported on the adsorbing properties of PAA with regardso VOCs. So far, most of the developments and/or applications ofample preparation methods for analysis of VOC, mainly in air and

∗ Corresponding author. Tel.: +39 0116705967; fax: +39 0112365967.E-mail address: massimo.maffei@unito.it (M.E. Maffei).

925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2008.11.046

water matrices, have been based on well-established airborne VOCenrichment techniques as well as the implementation of advancedcooling systems in cryogenic trapping for subsequent analysis.Among the most recent techniques are solid phase dynamic extrac-tion (SPDE) and solid phase microextraction (SPME) [15]. In bothcases, polydimethylsioxane (PDMS) fiber ensures that extractedVOCs remain on the fiber until they are thermally desorbed. Basedon the same principle, stir bar sorptive extraction (SBSE) has beendeveloped more recently by Baltussen et al. [16]. Its design includesa central magnet permitting either stirring of the sample to extractcompounds or its suspension into the headspace through mag-netic force [17]. With detection limits 10–25 times lower thanSPME [18], SBSE is typically used for trace and ultra-trace anal-yses. Additionally, because of the much larger fiber surface areathan in SPME, competition among analytes at the fiber surface isstrongly reduced, making quantification feasible even with a lim-ited knowledge of the matrix [18]. The drawbacks of both SPME andSBSE techniques are the relatively high costs and the matrix bleedthat occurs during desorption. The latter often generates a widenumber of peaks mainly made of dimethylxyloxane oxides [18 andreferences therein].

In this work we used PAA as an adsorbing material for VOCspresent in the air and in liquids and we compared the efficiency

of PAA with SPME and SBSE. PAA was desorbed by using a thermaldesorption unit (TDU) and the desorbed VOCs were analyzed bygas chromatography–mass spectrometry (GC–MS). The results ofthis work show interesting applications of PAA for the adsorptionof VOC in both air and liquid phases.

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. Experimental

.1. PAA preparation

Aluminium foils (99.99% purity, 0.1 mm thickness; purchasedrom Sigma and cut in plates of 40 mm × 20 mm) were mechani-ally polished using sandpapers of different grains and diamondaste on velvets and then electropolished (1.5 A for 30 min in aolution of perchloric acid:ethanol 1:4) in order to reduce surfaceicro-roughness. Anodization was performed in a beaker filledith a solution of 0.4 M phosphoric acid and the solution was kept

t a temperature of 0 ◦C under a constant current of 1.2 mA cm−2

or 10–90 min. A chemical etching was carried out by dipping theAA in a mixture of 0.5 M CrO3 and 0.5 M H3PO4 at 60 ◦C for 3 h.hen a second anodization done at the above conditions was per-ormed for 1 h. The electrochemical process leads to membranesith suitable features that enhance adsorption of VOCs. By chang-

ng the electrochemical parameters we were able to control theorphology of pores (size, shape and aspect ratio). The quality of

AA samples was then analyzed by using atomic force microscopyAFM-Veeco Dimension 3100-Nanoscope IV), and coupled focusedon beam microscopy (FIB) and scanning electron microscopy (SEM)

FEI QUANTA 3D). High aspect ratio Antimony (n)-doped Si probestip high 10–15 �m, 42 N m−1, 320 kHz) and standard silicon probestip high 10–15 �m, 40 N m−1, 300 kHz) were used for non-contactFM (tapping mode) examination of surface and pore shape andimension. In order to investigate the section and the aspect ratio

ig. 1. (A and B) SEM pictures of PAA samples obtained by phosphoric acid. (A) SurfaceFM of PAA samples obtained by oxalic acid: (C) particular of pore disposition. X-range = 3luminium foil structure. X-range = 359 nm, y-range = 359 nm, z-range = 47.9 nm.

tuators B 137 (2009) 76–82 77

of PAA samples, high precision milling was performed by FIB. Then,SEM metrology fulfilled a precise morphological analysis of surfaceand milled sections.

Aluminium and glass slides supports were also made with thesame dimensions of PAA plates, polished as above and used as con-trol substrates.

Before all experiments, aluminium plates, glass slides and PAAsamples were cleaned with a Soxhlet apparatus by using acetone.

2.2. Sampling and analytical methods

Three types of experiments were performed by using PAA, glassand aluminium supports, SPME (Sigma) and SBSE (Twister, Gers-tel): (1) static headspace (SH), (2) dynamic headspace (DH), and (3)liquid extraction (LE). SPME and SBSE were pre-conditioned accord-ing to the manufacturer instructions. All samplings were performedseveral times using VOCs from peppermint essential oil, whosecomposition has been well documented [19,20], purchased fromMaraschi & Quirici, Italy.

For SH sampling, a 200 ml beaker containing peppermint oil wasplaced in a sealed glass container: the total volume of the systemwas 1280 ml. A platinum grid was placed at 9 cm from the upper

edge and was used to hold PAA, aluminium, glass, SPME and SBSE.The whole system was kept at a temperature of 50 ◦C. SH experi-ments were carried out by using 1 ml peppermint oil. The extractionlasted for 2 h. For each extraction at least three repetitions weredone. After each experimental session, the system was cleaned

and (B) cross-section by FIB after Pt local deposition. Scale bars = 1 �m. (C and D)�m, y-range = 3 �m, z-range = 105.6 nm; (D) sub-micrometer domain due to initial

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sing a perfume-less soap (Micronova LoNa Soap) and differentrganic solvents in order to remove all volatiles.

For DH sampling, an 800 ml desiccator was used and PAA, alu-inium, glass, SPME and SBSE were placed onto a platinum grid

anging inside the desiccator. A two-way Erlenmeyer flask contain-ng the peppermint oil (1 ml) was connect by a glass tube to theesiccator on the one side and on the other to a tank of N2, whichas fluxed at 30 ml min−1 for 30 min. At least three samplings wereerformed and each time the system was cleaned as above.

For LE experiments, PAA, aluminium, glass, SPME and SBSE wereipped into a sealed Erlenmeyer flask containing 1 l distilled waterdded with 5 ml of peppermint oil dissolved in methanol (40 �leppermint oil in 20 ml methanol). The Erlenmeyer flask was placedn a magnetic stirrer to stir the solution. At least three repetitions

ig. 2. VOC adsorbing ability of PAA with respect to aluminium, glass, SPME and SBSE afthromatographic profiles after thermal desorption. Note the difference in the ion scale foable 1.

tuators B 137 (2009) 76–82

were done for each experiment that lasted 1 h; after each experi-ment the system was cleaned as above.

PAA, aluminium, glass and SBSE were desorpted by using a ther-mal desorption unit (TDU-Gerstel) connected to a Gerstel CIS3 cry-ofocusing system, which uses liquid CO2 as the cooling agent. SPMEwas directly injected into the gas chromatograph–mass spectrom-eter (GC–MS) injector following the manufacturer instructions.GC–MS of peppermint essential oil and desorption of samples wasperformed as detailed elsewhere [18,21].

3. Results and discussion

Phosphoric acid PAA samples showed a high vertically growthof pores and the surface was quite regular, with regular pat-

er static exposure to peppermint essential oil. The figure shows representative gasr aluminium and glass and for SBSE. Numbers correspond to compounds listed in

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erns of holes depending on operating electrolytes (Fig. 1A and). PAA surface also showed a high regularity in pore shape andimension, but also sub-micrometer domains due to the initial alu-inium foils structure as revealed by AFM analysis (Fig. 1C and).

PAA plates were then exposed to peppermint oil VOCs, whosehemical composition is described in Table 1. As controls, pureluminium plates and glass slides of the same dimension of PAA

ere used and exposed to the same VOCs. In order to compare the

esults obtained from PAA, SPME and SBSE were also used in allxperiments.

Fig. 2 shows the GC–MS profiles after SH sampling. Aluminiumnd glass supports show a low ability to adsorb VOCs, whereas

ig. 3. VOC adsorbing ability of PAA with respect to aluminium, glass, SPME and SBSE afgure shows representative gas chromatographic profiles after thermal desorption. Numb

tuators B 137 (2009) 76–82 79

PAA shows a comparable ability with respect to SPME and SBSE,with particular reference to the main peppermint oil compounds(menthol, menthone, menthofuran and 1,8-cineole). PAA shows agreater adsorption of higher molecular weight unidentified com-pounds which are not adsorbed by both SPME and SBSE (Fig. 2).Moreover, PAA does not show carry-over effects after desorption,whereas several peaks in both SPME and SBSE are the result of thebleeding of PDMS. The PAA plate reveals to be a good detecting

system for VOCs and also to be reproducible with low variationsbetween technical replicates. In order to show VOC peaks, scalesof ion intensity in Fig. 2 are lower for aluminium and glass plates,comparable between PAA and SPME, and higher in SBSE (due to thehigher sorption capacity).

ter dynamic exposure to peppermint essential oil. All ion scales are the same. Theers correspond to compounds listed in Table 1.

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In DH experiments, PAA shows the lowest ability to adsorb VOCsven when compared to the controls aluminium and glass (Fig. 3).he latter two systems appear to behave much better and with andsorbing ability similar to SPME. In this comparison SBSE has oncegain the highest ability to adsorb VOCs. Ion scales of Fig. 3 are allhe same.

In LE experiments, PAA shows comparable qualitative abilityo adsorb VOCs from the solution with respect to SPME and SBSEFig. 4). However, the ion scale for SBSE is higher, indicating a major

orption capacity.

The results of this investigation show that PAA is a suitable mate-ial for the adsorption of VOCs, with high performances in staticeadspace analyses and when used to detect VOCs dissolved in

ig. 4. VOC adsorbing ability of PAA with respect to aluminium, glass, SPME and SBSE afhe figure shows representative gas chromatographic profiles after thermal desorption. N

tuators B 137 (2009) 76–82

water. Alumina adsorption properties in water are known since it isused in the processing of wastewater. However, AAO commerciallyavailable show irregular porosity with different shape and size ofpores. The electrochemical process used in this work allowed thefabrication of PAA membranes made of regular pores that enhancedVOCs adsorption. Hydroxyl groups formed on alumina surface in anaqueous environment act like chemical traps for analytes. In thiscontext, in our LE experiments with PAA, both chemsorption andadsorption on a porous surface appeared to occur. One of the most

evident advantages of PAA is the lack of artefacts during desorptioninto the GC–MS injector. During desorption of both SPME and SBSEdegradation fragments of the PDMS sorbent are frequently gener-ated and are made of compounds with characteristic silicone mass

ter liquid exposure to peppermint essential oil. Note the higher ion scale for SBSE.umbers correspond to compounds listed in Table 1.

D. Mombello et al. / Sensors and Ac

Table 1Chemical composition of peppermint (Mentha × piperita) used as a source of VOC.R.T. = retention time; K.I. = Kováts index.

Compound R.T. K.I. Relative percentage

(1) �-Pinene 6.63 939 0.74(2) Sabinene 8.53 975 0.42(3) �-Pinene 8.67 979 1.05(4) Myrcene 9.59 991 0.17(5) 3-Octanol 9.85 991 0.24(6) �-Phellandrene 10.29 1003 0.01(7) �-Terpinene 11.11 1017 0.33(8) o-Cymene 11.65 1026 0.36(9) Limonene 11.94 1029 1.99(10) 1,8-Cineole 12.07 1031 5.07(11) cis-�-Ocimene 12.81 1037 0.18(12) trans-�-Ocimene 13.57 1050 0.05(13) �-Terpinene 14.25 1060 0.54(14) cis-Sabinene hydrate 14.85 1070 0.69(15) Terpinolene 16.69 1089 0.15(16) Linalool 17.95 1097 0.20(17) Isopulegol 21.91 1150 0.09(18) Menthone 22.84 1153 18.62(19) Menthofuran 23.80 1164 6.42(20) Neomenthol 24.01 1166 3.45(21) Menthol 25.10 1172 44.28(22) Terpinene-4-ol 25.31 1177 1.35(23) Isomenthol 25.89 1183 0.75(24) Neoisomenthol 26.61 1187 0.12(25) �-Terpineol 26.75 1189 0.22(26) Pulegone 31.98 1237 1.41(27) Piperitone 33.63 1253 0.48(28) Neomenthol acetate 36.67 1274 0.23(29) Menthyl acetate 38.94 1295 5.49(30) Isomenthyl acetate 40.27 1305 0.25(31) �-Bourbonene 48.54 1388 0.44(((

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ragments [16] that may overlap or mask compounds present in theamples.

. Conclusions

PAA applications are growing and extending to several diverseelds of application. We showed that PAA has adsorbing propertiesimilar to PDMS, which is quite expensive and that releases columnleed components during thermal desorption. Although the use ofAA electrodes as support for amperometric sensors is unexploredue to high resistance of alumina [1], it would be an interestinghallenge to exploit the VOC adsorbing properties of PAA. Furtherevelopments of the PAA may lead to the construction of novel

ow-cost, high-sensitive and reliable VOCs adsorbing supports assubstitute for PDMS matrices.

cknowledgements

This work was supported by a grant from the FIAT Researchentre and by the Centre of Excellence for Plant Biosensing.

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Biographies

Domenico Mombello graduated in biology on October 2004 at the University ofTurin, where he also obtained his Ph.D. in plant and environmental biosensing onJanuary 2008 discussing the use of PAA in gas chromatography with a grant from theFIAT Research Centre. Since January 2008 he is post-doc in the Polytechnic Schoolof Turin.

Nello Li Pira graduated in physics at the University of Turin in 2000. He has workedat FIAT Research Centre since 2002 and currently he is responsible for “Nanomate-rials and Nanomanufacturing” laboratory. His current interest is the developmentof various techniques for evaporation and synthesis of metallic and non-metallicnanostructured materials and the engineering and fabrication of optical systemsand light-emitting devices. Other interests are inorganic nanostructured display andnanoparticles deposition for catalytic sensors applications.

Luca Belforte graduated in physics at the University of Turin in 2003 and he hasworked at FIAT Research Centre since 2005. His activities focus on new technologicalapproaches in manufacturing of gas sensors, thermoelectric systems for energy sav-ing and recovery. Moreover, he is experienced in focus ion beam patterning, E-beamlithography, photonic crystals, and thin film deposition.

Pietro Perlo graduated in physics at the University of Turin in 1980. He has worked atFIAT Research Centre since 1981 and he was involved in the opto-mechanical design

of laser robots, area on which he is author of a selected paper for the milestoneSPIE series. He led the first world-wide commercial introduction of diffractive andmicro-optics into the automotive, motorcycles, general lighting and IR systems forintrusion alarm. He co-ordinates national and European projects in different areas:future and emerging technologies, IST, growth. He is member of the Committee forthe orientation of the nanotechnology programs of the European Community.

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ianfranco Innocenti graduated in physics at the University “La Sapienza” in Rome.e is responsible for the Micro and Nanotechnology Department at the FIAT Researchentre. His interest is in the study of microcavities, that are considered the basis

or the OLED systems, developing electro-optical systems for automotive applica-ion (anti-collision systems, vehicle dynamic control) and on new technological

pproaches in manufacturing of electro-optical sensors (optical fibres), actuationystems (piezoelectrical materials, SMA, magnetorheological fluids) and their inte-ration inside miniaturized configuration with control systems.

imone Bossi graduated in biological science at the University of Turin in 1998 andbtained his Ph.D. degree in biochemistry in 2003 from the same University. His is

tuators B 137 (2009) 76–82

technician in the Department of Plant Biology of the University of Turin. His currentinterest is in GC–MS of VOCs.

Massimo E. Maffei graduated in plant biology in the University of Turin, where hefirst became assistant professor, then associate professor and, since 2000, full profes-

sor of plant physiology. He is the coordinator of the Ministerial Center of Excellence inPlant and Microbial Biosensing-CEBIOVEM and editor in chief of the Journal of PlantInteractions. His current interest is the study of early signals in plant–biotrophs inter-action which is accomplished by using confocal microscopy, electrophysiology andmolecular biology techniques as well as the development of new sensing materialsfor the detection of volatile organic compounds.