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Sensors and Actuators B 132 (2008) 107–115

Optical gas sensing of TiO2 and TiO2/Au nanocomposite thin films

M.G. Manera a,b,∗, J. Spadavecchia a, D. Buso c, C. de Julian Fernandez d,1, G. Mattei d,A. Martucci c, P. Mulvaney e, J. Perez-Juste f, R. Rella a, L. Vasanelli g, P. Mazzoldi d

a CNR-IMM Lecce, University Campus, via Monteroni, 73100 Lecce, Italyb ISUFI - University of Lecce, viale Gallipoli, 73100 Lecce, Italy

c INSTM - Department of Mechanical Engineering, University of Padova, via Marzolo 9, 35131 Padova, Italyd Department of Physics, University of Padova, via Marzolo 8, 35131 Padova, Italy

e School of Chemistry, University of Melbourne, Parkville, Vic. 3010, Australiaf Department of Chemical Physics, University of Vigo, 36310 Vigo, Spain

g Innovation Engineering Department, University of Lecce, via per Arnesano, 73100 Lecce, Italy

Received 22 October 2007; received in revised form 9 January 2008; accepted 9 January 2008Available online 17 January 2008

bstract

The optical gas sensing properties of virgin TiO2 thin films and with inclusion of dot- and rod-shaped gold nanoparticles (NPs) prepared by aol–gel method and annealed at 200 ◦C have been investigated. The obtained nanostructures have been analysed from both optical and morphologicaloints of view. In particular, the prepared films show interesting dynamic optical absorption sensing responses towards different kinds of alcohol

apours in the spectral range corresponding to the surface plasmon resonance (SPR) peak of the gold nanoparticles. Total attenuation surfacelasmon resonance measurements in controlled atmosphere demonstrate a sensing activity due to variation of the thickness and the real part of thective sensing layer refractive index.

2008 Elsevier B.V. All rights reserved.

in film

[pncpaatoi

eywords: Surface plasmon resonance; Optical gas sensors; Nanostructure; Th

. Introduction

Nanocomposite films consisting of small metal particles inhe range of a few to several nanometers embedded in metalxides find application in catalysis, photocatalysis, sensors andovel optoelectronic devices [1,2]. Typically these nanocompos-tes consist of noble metal nanocrystals dispersed into a metalxide matrix created through sputter coating or sol–gel pro-essing. TiO2 is one of the most commonly used functionallyctive matrices in optoelectronics because of its chemical sta-

ility, high refractive index, and high dielectric constant [3,4].articles of noble metals incorporated into a TiO2 matrix can

mprove its catalytic properties as well as its sensing features

∗ Corresponding author at: CNR-IMM Lecce, University Campus, via Mon-eroni, 73100 Lecce, Italy. Tel.: +39 0832 422531; fax: +39 0832 422552.

E-mail address: mariagrazia.manera@le.imm.cnr.it (M.G. Manera).1 Present address: Laboratory of Molecular Magnetism, INSTM, Depart-ent of Chemistry, University of Florence, via della Lastruccia 3, 50019 Sestoiorentino (FI), Italy.

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5]. Moreover, the studies on the optical activity, reflectivityroperties, photo-sensitization and thermal stability with goldanocrystal-doped TiO2 films are growing [6]. Despite thehemical inertness of bulk gold, Au nanoparticles (NPs) sup-orted on metal oxide surfaces show surprisingly high chemicalctivity [7–9]. One method for exploring the chemical sensorctivity of such nanocomposites is through the monitoring ofhe optical changes of the films during catalysis or sensing. Theptical properties of nanocrystal-doped thin films are dominatedn the visible range by the surface plasmon resonance (SPR) ofhe noble metal nanoparticles.

The advantages of opto-chemical sensors over conventionalesistive sensors are the higher resistivity to electromagneticoise, compatibility with optical fibres and multi-gas detec-ion using differences in the intensity, wavelength, phase andolarization of the output light signals.

In this work, the optical gas sensing properties of TiO2 thinlms doped with gold nanoparticles prepared by the sol–gelethod were investigated. Chemical methods, such as sol–gel

rocess, provide an attractive route for the preparation of

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08 M.G. Manera et al. / Sensors a

ulticomponent oxide materials assuring homogeneity in theeposition of the film on the substrate. The nanocomposite thinlms were investigated from both optical and morphologicaloints of view. Optical gas sensing tests were carried out inhe presence of methanol, ethanol and isopropanol vapours.ptical investigations in air and in controlled atmosphere were

arried out using the surface plasmon resonance technique inhe Kretschmann configuration. It is well documented that thisechnique is extremely useful for monitoring in situ chemicalnd bio-chemical processes at metal interfaces [10,11] and foretecting small changes in the dielectric constant near a metalurface, thus resulting a valuable tool for surface investigation.

oreover, improvements in the sensitivity of the SPR sensor arexpected for TiO2 films containing Au NPs; indeed, the presencef Au NPs has been found to result in a larger plasmon anglehift and change in reflectivity [12], which enhances the detec-ion sensitivity of the SPR device [13]. The enhanced sensitivityas been attributed to the interaction between the localized sur-ace plasmons (LSPs) of the Au NPs and the propagating surfacelasmons (PSPs) of the Au substrate [14].

. Experimental

For the synthesis of TiO2 films a sol using titanium isopropox-de (Tiso), isopropanol (IPA) and acetic acid (AcOH) with a

olar ratio Tiso:AcOH:IPA = 1:6:1.3 was prepared. For the syn-hesis of Au–TiO2 nanocomposite films, first of all, a colloidalolution containing Au spherical nanoparticles was prepared andurther mixed with the previously described sol. Colloidal goldas synthesized by reducing HAuCl4 with tris-sodium citrate

n water [15] and then dispersed in ethanol by using poly(N-inylpyrrolidone) as a stabilizer. The gold solution and the TiO2ol were mixed in order to get a molar ratio Au/Ti = 2%. Theixed sol was spin coated in air (RH = 35%) at 3000 rpm for

5 s onto fused silica slides (Herasil I) for optical absorptioneasurements (∼150 nm in thickness). For SPR measurements,

he same solution was spin coated at 5000 rpm (obtained thick-ess ∼30–40 nm) onto a 50 nm gold film preliminary evaporatednto suitable substrates (Corning glass 7059) after flash evap-ration of chromium (∼2 nm) for adhesion purposes. After theeposition, the samples were annealed in air at a temperaturef 200 ◦C for 20 min. The thin film thickness was measuredsing a profilometer (Alpha Step 200 Tencor Instruments) on atep made by scratching the films after the deposition. The sur-ace topography of the TiO2 thin films deposited onto Au/glassubstrates intended for SPR measurements was inspected bytomic force microscopy (AFM) in contact mode by using anXPLORER–VEECO system equipped with a Si3N4 pyramidal

ip. For each sample, images were recorded from different areasf the sample surface, at different scan sizes, in order to checkhe lateral uniformity of the TiO2 thin films. Grazing incidence-ray diffraction (GIXRD) measurements were realized usingCu K� radiation and different incidence angles between 0.22◦

nd 1◦.

Transmission electron microscopy (TEM) measurementsere made using a microscope working at 200 kV by depositingdrop of the colloid onto a carbon-coated Cu grid and allowing

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tuators B 132 (2008) 107–115

he evaporation of the solvent. Optical absorption measurementsn air were realized using a JASCO V-570 spectrophotometer inhe UV–VIS–NIR range. Optical sensing tests were performedy using a filtered light source (AVANTES tungsten–halogenamp) with the help of an optical fibre. The absorption spec-ra (in 300–700 nm range) were collected and analysed usingcommercial spectrophotometer AVANTES model MC 2000.specific experimental set-up was realized in order to acquire

imultaneously the optical responses of the sensing layer to gasesnd/or vapours. The set-up enables the dynamic responses to becquired in different fixed spectral ranges where the absorptionesponse shows a maximum. All the measurements were car-ied out at room temperature and at normal incidence of theight beam.

SPR characterization of the TiO2 and TiO2/Au nanocompos-te thin films was performed by using a home-made experimentalet-up assembled in the Kretschmann prism configuration [16].he free face of the glass substrate was brought into opticalontact with the prism (refractive index n = 1.515, determinedy Brewster angle measurement) using a thin layer of an index-atching fluid (n = 1.517). The prism/sample combination was

laced on a θ–2θ rotation table driven by a microprocessor-ontrolled stepping motor (with a resolution of 0.01◦). Surfacelasmon excitation was achieved by directing a 1 mW p-olarised (i.e. polarised parallel to the plane of incidence) He–Neaser (632.8 nm) onto the prism/sample interface and measuringhe intensity variation of the reflected light as a function of thencident angle. A suitable teflon test chamber was used in order toecord the reflectivity in the presence of different analytes mixedith dry air, and to evaluate the sensing activity of the deposited

ayers from the variations in the SPR curve. The dynamic sens-ng measurements were performed by keeping the incident anglef the laser beam at a fixed value during the exposure to eachnalyte.

. Results

.1. Structural and morphological characterization

The shape and size of the gold nanoparticles were investigatedy TEM on the precursor colloid. TEM images are reported inig. 1 for colloids A and B. Colloid A is composed of nearlypherical NPs having an average particle size of 14 nm with atandard deviation of 10%. The NPs obtained using this colloidill be referred as “dots”. Colloid B is composed of mainly

ylindrical NPs (here called “rods”) with minimum length ofround 7 nm and a mean aspect ratio between 4:1 and 1:1.

The GIXRD pattern of the deposited films (data not pre-ented) shows diffraction peaks corresponding to the goldanoparticles on the film and also one very broad band centeredround 2θ = 26◦ that corresponds to TiO2 with an amorphousr nanocrystalline structure. Our recent study has revealed thatiO2 films crystallize in anatase phase at temperatures higher

han 400 ◦C.In Fig. 2 topographic AFM images (a) of the TiO2 film and

b) of TiO2 films prepared with inclusions of Au dots (b) are pre-ented. The AFM images reveal smooth surfaces with a slightly

M.G. Manera et al. / Sensors and Actuators B 132 (2008) 107–115 109

Fig. 1. TEM images of dot- and rod-shaped Au nanoparticles.

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3SPR curves of TiO2 thin films with and without Au NPs have

been analysed in order to extract the optical parameters (n, k andd) of the sensing layers (where n and k are the real and imaginarypart of complex refractive index of the film, respectively, and d is

ig. 2. AFM 3D view images of the TiO2 thin films of (a) TiO2 thin films depo

igher root-mean-square (RMS) roughness for the TiO2 samplesith inclusion of Au dots (1.1 ± 0.1 nm) compared to the bareiO2 layers (0.5 ± 0.1 nm). Some randomly distributed struc-

ures have been found in the TiO2 sample with inclusions of Auods (RMS = 2.1 ± 0.1 nm); this slight difference in morphol-gy and roughness can likely be ascribed to the higher thicknessbtained for this sample with respect to the above mentionedlms or to the presence of the cylindrical NPs randomly dis-

ributed in the matrix, which produce more evident features thanhose produced by a spherical NPs distribution.

.2. Optical properties

.2.1. Optical absorptionIn Fig. 3 the absorption spectra obtained for the bare TiO2

lms and those with Au dots and rods NPs are presented. In thesepectra, a blue-shift can be observed in the absorption edge asompared to the TiO2 bulk (Eg ∼= 387 nm) due to the nanostruc-ured nature of the film [17]. In the nanocomposite film, the maineatures of TiO2 film are recognizable from the spectra, as wells the presence of metal NPs in the TiO2 matrix. In particular, inhe sample with Au dot NPs a broad maximum centered around70 nm is observed, which is a typical surface plasmon mode ofolloidal gold. Due to the high refractive index of titania, the SPesonance is red-shifted from its value of 520 nm in water [9].

n the sample with Au rods, two broad maxima are observedround 550 and 790 nm. These maxima are due to the trans-erse and longitudinal surface plasmon resonances of the rods18] as predicted by Mie theory. The transverse mode is close

Ffis

onto Au/glass substrate for SPR measurement and (b) Au/TiO2 dot thin films.

o the surface plasmon band of the spherical particles, while theesonance of the longitudinal mode is red-shifted and dependstrongly on the rod aspect ratio (defined as the length-to-widthatio of the rod) [19].

.2.2. Surface plasmon resonance

ig. 3. Optical absorption spectra of TiO2 and TiO2/Au nanocomposite thinlms. The distinctive absorption peak due to SPR effect of spherical and rod-haped Au NPs are clearly evident.

110 M.G. Manera et al. / Sensors and Ac

Fig. 4. SPR curve relative to TiO2 and Au/TiO2 nanocomposite thin filmsdcb

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Ttresents a columnar microstructure. Eq. (1) has been validatedfor gold NPs-doped silica sol–gel films for gold particle vol-ume fractions up to ϕ ∼ 0.5 [23]. The analysed nanocompositeTiO2–Au thin films are assumed to be composed of 98 vol%

eposited onto Au/glass substrates, compared with the bare gold substrate SPRurve. Results of the fitting procedure using Fresnel’s equations are representedy a continuous line.

he thickness of the layer) and to measure their variations duringxposure to analytes. The SPR data have been analysed usingtandard fitting models based on Fresnel theory [20]. The opticalarameters of the TiO2-based thin layers have been determinedequentially, after a preliminary determination of the bare goldlm optical parameters. The details of the calculation have beenescribed in Ref. [21]. In Fig. 4 SPR curves relative to the bareiO2 and TiO2/Au composite films are reported together with theorresponding SPR curve of the Au film. The internal incidentngle θspr of the TiO2 thin film exhibits a shift (∼3.3◦) towardsarger angles than θspr for the bare Au/glass. This difference isue to the change in the refractive index when the Au surfaces covered by the TiO2 thin film. The magnitude of the shift inhe SPR angle typically depends on both the thickness and theefractive index n of the deposited titania layer. For calculationurposes, refractive indices of 1.0 and 1.515 for the ambientedium and the prism were assumed. In Fig. 4 results of the

umerical fitting procedure on the TiO2 thin films are reportedoo. They are summarized in Table 1. The quality of the adoptedtting procedure is demonstrated by the figures themselves and

able 1ummary of the optical parameters of the TiO2 thin films calculated after the fitf the experimental SPR curves by using the Fresnel’s theory at a wavelengthf 633 nm

n k d (nm)

iO2 1.80 0 13.6iO2/Au dot 1.92 0.1 23.0iO2/Au rod 1.68 0.02 45.0

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tuators B 132 (2008) 107–115

y the low values of the minimum RMS errors obtained (∼1).ig. 4 shows also the change in the SPR curves corresponding

o TiO2 thin films containing Au dots and Au rods. The shift inhe minimum in reflectance towards larger angles with respecto the bare TiO2 layer is evident as well as a broadening of theeak absorption width compared to the unmodified film. Thesehanges are due to the changes in refractive index and thicknessf the modified film. A Maxwell–Garnett theory was used toodel the structure of the TiO2 films modified with the inclusion

f Au NPs.The Maxwell–Garnett theory is an Effective Medium Theory

EMA) with which the complex dielectric function of the com-osite material can be calculated as a weighted average dielectriconstant of two components. The composite dielectric functionnly depends on the volume fraction and shape of the metal par-icles [22]. The general equation that describes the EMA models given by:

εt − εh

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εj − Yεh(1)

here εt and εh are the dielectric functions of the EMA (totalystem) and the host materials, respectively, m is the number ofaterials included in the basic material matrix, εj is the dielectric

onstant of the included material, in our case Au, fj is the volumeraction of the material included and Y is a screening factorsed to describe the microstructure of the mixed materials. Thecreening factor describes the shape of microstructure throughhe equation:

+ 1 = 1

screening factor. (2)

he screening factor of 1/3 describes a spherical microstructure,he screening factor of 1 describes a flat disc, and a 0 value rep-

ig. 5. Optical absorption spectra of the TiO2 thin film with Au dots NPs inclu-ion in the presence of dry air and with 6000 ppm of methanol vapours. Inset:ptical absorption difference calculated taking into account both spectra.

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brmclanism giving rise to the optical signal changes. In order toinvestigate changes in the real part of the film dielectric func-tion, SPR measurements in the Kretschmann configuration havebeen carried out on the virgin TiO2 and nanocomposite Au–TiO2

Table 2Summary of the optical sensing features of TiO2 and TiO2/Au dot nanocom-posite thin films towards different VOC’s ananlyte by using optical absorptionmeasurements

Vapours (6000 ppm) Optical absorption response (%)

TiO TiO + Au dots

Fma

M.G. Manera et al. / Sensors a

f TiO2 and 2 vol%. of gold NPs, the shape being accountedor by the value of the screening factor. The choice of the two-omponent volume fraction is related to the atomic ratio Ti:Auoming from the synthesis process.

.3. Sensing properties

.3.1. Optical absorption propertiesIn this section, the change in the optical properties of the

are TiO2 thin films and with inclusion of Au NPs are goingo be analysed. Such changes are due to the physisorption orhemisorption of a particular analyte interacting with the sur-ace. First, in Fig. 5 the optical absorption spectra of the TiO2lm doped with Au dot NPs in the presence of dry air and of000 ppm of methanol vapours are presented. The interactionith the analyte produces an increase in the optical absorption

n the spectral range between 300 and 600 nm. This is betterbserved in the inset of Fig. 5 where the relative differencesn the optical absorption are shown. A maximum change inbsorption is observed around 470 nm. In Fig. 6 the dynamicesponses are schematized for the pure TiO2 films and for theiO2 containing Au dot NPs, in the presence and absence ofifferent vapours and gases. The change in the integral area�I) calculated under the absorption curve in the range from00 to 600 nm is plotted. As shown, in all cases the interac-ion with organic vapour molecules produces an increase inhe optical absorption signal with respect to that recorded inry air. The signal saturates at higher analyte concentrations,ossibly due to saturation of the available nanoparticle adsorp-

ion sites or to the filling of the available pore space within thelm. From the response curves shown in Fig. 6, the reversibil-

ty of the adsorption and desorption steps are clearly evident.n Table 2 the percentage variations in the integrated absorp-

MEI

ig. 6. Dynamic sensor optical absorption curves representing the change of the inteasured in the presence of dry air and spaced out repeatedly with different analytes

nd 50 ppm of NH3 for the TiO2 films and of the TiO2 films with Au dots.

tuators B 132 (2008) 107–115 111

ion signal are summarized when both TiO2 and Au dot-TiO2anocomposite thin films are exposed to methanol, ethanol andsopropanol vapours at a concentration of 6000 ppm. As onean see, TiO2 thin film responses to methanol and isopropanolapours are similar and higher than the corresponding responseo ethanol vapours. The optical sensing profiles change when Auanoparticles are incorporated into the TiO2 matrix: in general,he responses are similar for all the investigated vapours andigger than those recorded for TiO2 thin films. The biggest dif-erence is for isopropanol vapours. Significantly, NH3 gas whichoes not induce a response with the bare TiO2 films is detectedsing a gold doped titania film.

If the analyte alters the matrix refractive index, then there wille also a shift in the position of the SPR band, which could giveise to the observed signals. Because the absorbance measure-ents only allow the monitoring of changes in the absorption

oefficient, and not in the refractive index n of the investigatedayer, it is not possible to unambiguously determine the mech-

2 2

ethanol 3.6 4.1thanol 1.5 4.3

sopropanol 3 5.3

egrated intensity (�I) under the absorption curve in the range of 300–600 nm: vapours of methanol (6000 ppm), ethanol (6000 ppm), isopropanol (600 ppm)

112 M.G. Manera et al. / Sensors and Actuators B 132 (2008) 107–115

Fig. 7. SPR reflectivity curves relative to the TiO2 film with Au dots NPs indry air and in the presence of saturated methanol vapours showing the shift inrcv

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Table 3Percentage variations in the real part of the refractive index and in the thickness ofTiO2 and TiO2/nanocomposite investigated films from fitting calculation usingFresnel’s equations after the exposure to organic vapours

MetOH EtOH IsopropylOH

�n/n(%)

�d/d(%)

�n/n(%)

�d/d(%)

�n/n(%)

�d/d(%)

TiO2 11 28 7.2 19 5.6 7++

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esonance minimum upon interaction with the alcohol vapours. Inset: Dynamicalurve obtained at a fixed angle upon the injection of 6000 ppm of methanolapours into the test chamber.

hin films in dry air and in the presence of the same analytesnvestigated above.

.3.2. SPR measurementsSPR measurements in Kretschmann configuration can give

s more information about the interaction between the analyteapours and the nanocomposite film owing to the fact that thehape of the SPR curve is dependent either on the real part ofefractive index n and/or the film thickness d [20]. In Fig. 7,he SPR reflectivity curves relative to TiO2 films with Au dots

Ps in dry air and in the presence of saturated methanol vapours

re reported. The interaction of the sensing layer with the alco-ol vapours produced a shift of the angular minimum towardsarger angles, which can be ascribed to the adsorption process of

Nmet

ig. 8. Calibration curves reporting the response �R/R = (Rgas − Rair)/Rair to vapoursthanol, (b) methanol, (c) isopropanol.

Au dots 14.1 18 6.1 8 5.0 4Au rods 5.6 22 5.6 11 5.8 25

lcohol vapours onto the surface and probably into the poroustructure of the sensing layer and consequently to a change ints refractive index and/or thickness. The dynamic response ofhe same nanocomposite thin film in the presence of 6000 ppmf methanol vapours is reported in the inset. Dynamic dataere recorded at a fixed angle of incidence, corresponding to

he maximum value of the slope of the SPR curve relative tohe TiO2–Au/glass structure. The responses towards the testedapours are stable, reproducible and reversible. A fitting pro-edure based on the Fresnel theory allows us to extract theptical parameters of all the investigated layers in the presencef different analytes and to evaluate the changes in their opticalarameters. The results of the fitting procedure are summarizedn Table 3. The interaction between the analyte molecules andhe sensing layer produces a variation n, in the real part of thelm refractive index, and in d, the film thickness, the effect beingore evident in TiO2/Au dot thin films interacting with methanol

apours. However, in all cases the change in n and d can be jus-ified by assuming that the interaction with vapour molecules isue first to a superficial adsorption onto the sensing layer andhen to their accumulation in the porous matrix of the film, thusroducing a change in the density of the investigated structure.his result can be attributed to the fact that the role of the Au

Ps, in the whole interaction mechanism, is passive. This isore evident when analysing the changes in the optical param-

ters of the TiO2/Au rod NPs thin film: for this sample onlyhe sensing test with isopropanol vapours reported higher vari-

exposure as a function of their concentration relative to different alcohols: (a)

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tions in n and d with respect to the virgin TiO2 or TiO2/Au dothin films. This can be considered a further demonstration of theecondary role played by the Au NPs in the sensing behaviour.ll TiO2 and TiO2/Au nanocomposite thin films were tested

t room temperature after repeated exposure to the alcoholsapours in different concentrations. The corresponding cali-ration curves, reporting the response �R/R = (Rgas − Rair)/Rairo vapours exposure as a function of their concentration, arehown in Fig. 8. An approximately linear behaviour with theoncentration is evidenced for all samples. Neither in this casehere is an evident trend showing the improvement in the sen-or response due to the presence of Au NPs except for theiO2/Au dot nanocomposite thin films interacting with methanolapours.

The current results clearly indicate that metal NPs-dopedetal oxide films offer a real opportunity for realizing direct

ptical signal transduction in chemical sensors with rapid andensitive responses to common gases. However, the actualechanism is not completely clear. Absorption measurements

ndicate that there are absorption changes around the localizedurface plasmon resonance peak; however, SPR measurementsn the Kretschmann configuration suggest that these LSP shiftsre due to changes in the real part of the matrix refractive indexr thickness due to the diffusion and adsorption of the analyteolecules. A change in the dielectric function of the titania hostatrix will cause a shift in the SPR wavelength. Thus the par-

icles may simply act as optical transducers for the physicalhanges of the matrix properties. Even in this passive mode,owever, these composite films constitute a very simple andffective gas-sensing element.

. Conclusions

The optical gas sensing properties of TiO2 and TiO2/Auanocomposite thin films deposited by sol–gel technique haveeen studied using optical absorption and surface plasmon res-nance techniques in the presence of different concentrationsf alcohol vapours such as ethanol, methanol, isopropanol andases such as NH3. Exposure of the nanocomposite films pro-uced an increase in the optical absorption in the spectral rangeetween 300 and 600 nm; the responses are similar for all inves-igated vapours and higher than those recorded for TiO2 thinlms. The biggest difference is found for isopropanol vapours.he same positive trend was not found using the SPR technique

n the Kretschmann configuration, except for the TiO2/Au dotanocomposite thin films interacting with methanol vapours.owever, this technique has allowed the determination of theptical parameters of the sensing layers and their variation uponhe interaction with alcohol vapours: only an increase in the realart of the refractive index and in the thickness of the inves-igated layers could be extracted from the fitting procedure,ue to the interaction with alcohol vapours. Further investi-ations are needed in order to relate the sensing features of

he two different techniques and to make the localized sur-ace plasmon oscillation of Au NPs the predominant effectn the interaction mechanism. The results show that the goldPs simply act as optical transducers for the physical changes

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tuators B 132 (2008) 107–115 113

f the matrix properties. Hence, the nanoparticle shape doesot play a significant role in these sensors. However, the rodptical properties may be used to shift the optical responsento regions of maximum detector sensitivity and even into theIR.

cknowledgment

This work was supported by the Italian FIRB projectICROPOLYS.The authors are grateful to F. Casino for technical assistance.

lessandro Martucci thanks the Universities of Melbourne andadova for support through the University academic exchangerogram.

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16] E. Kretschmann, Die Bestimmung optisher Konstanten von Metallen durchAnregung von Oberflachenplasmaschwingungen, Z. Phys. 241 (1971)313–324.

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17] P. Chysicopoulou, D. Davazoglou, C. Trapalis, G. Korda, Optical propertiesof very thin (<100 nm) sol–gel TiO2 films, Thin Solid Films 323 (1998)188–193.

18] J. Perez-Juste, I. Pastoriza-Santos, L.M. Liz-Marzan, P. Mulvaney, Goldnanorods: synthesis, characterization and applications, Coord. Chem. Rev.249 (2005) 1870–1901.

19] S. Link, M.A. El-Sayed, Spectral properties and relaxation dynamics ofsurface plasmon electronic oscillations in gold and silver nanodots andnanorods, J. Phys. Chem. B 103 (1999) 8410–8426.

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iographies

aria Grazia Manera obtained her degree in physic from the University ofecce, Italy in 2003. She received the PhD degree in “Material and news tech-ologies” in the same University. She is involved in the study of the opticalroperties and their variation of sensing organic and inorganic materials by usingptical transduction methodologies. Her research interests concern optical andorphological investigations on organic and inorganic materials for optochem-

cal sensing and biosensing applications exploiting surface plasmon resonanceransduction methodologies.

olanda Spadavecchia obtained her degree in pharmaceutical chemistry andechnology from the University of Bari, Italy in 2000 following postgraduatetudies at the University of Lecce on the synthesis of phthalocyanines fromatural products. Since 2001 she is a PhD student in material engineering in thengineering Faculty of the University of Lecce. Her research interests concern

he synthesis of organic compounds, sensor applications and characterization,NA-based sensors. Now, her current activity focuses on surface biomolecular

nteractions as characterized by surface plasmons resonance imaging systems,or dynamic biochip applications. Actually, she works at the Laboratoire Charlesabry of the Optic Institute, in Orsay, France. She is co-author of about 40ublications.

ario Buso graduated in chemical engineering at Padova University in 2003.e then joined the Department of Mechanics–Materials sector of the same Uni-ersity as a post-graduate fellow. In 2005 he got a PhD position in the Doctoralchool of Materials Science and Engineering in Padova. His works is focusedn development of strategies to synthesize sol–gel layers containing metal andemiconductor nanoparticles with highly controlled morphology for both gasensing applications and integrated photonics. At the present time he appears inwelve publications in international journals.

esar de Julian Fernandez received the PhD degree in physics from the Univer-idad Autonoma de Madrid, Spain, in 1995 working on the magnetic propertiesf nanostructured materials. He made post-docs in the Laboratoire L. Neel inrenoble (France) and in the Department of Physics at the University of Padova,

taly. From 2007 he has post-doc position in the Department of Chemistry athe University of Florence—INSTM, Italy. His research concerns mainly mag-etic, optical, magneto-optical and structural investigations on nanostructuredaterials, with emphasis on metal–alloy and oxide nanoparticulated materials.oreover he is interested in the research on nanostructured materials for plasmon

nd magneto-plasmon based optical applications.

iovanni Mattei PhD in physics from the University of Padova, Italy. He hasresently a permanent position as associate professor at the Physics Depart-ent of the University of Padova (Italy), teaching physics and physics of

o

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anostructured materials. Among his research topics: (i) synthesis of nanopar-icles embedded in dielectric and polymeric matrices for non-linear optics,

agnetisms, catalysis and sensors; (ii) calculation of the optical properties ofanoclusters. He is responsible for national and European projects. He is authorf more than 100 publications on international refereed journals and varioushapters on monographic books.

lessandro Martucci graduated in Physics at the University of Padova and in997 he got the PhD degree in materials science and engineering at the sameniversity. From 1999 he holds a faculty position at Padova University and he

s associate professor teaching materials science and engineering at the Facultyf Engineering. His main research activity is devoted to nanoparticles dopedol–gel materials for photonics and gas sensing applications. He is responsi-le of national and international research projects and holds more than 100nternational publications.

aul Mulvaney is currently professor of chemistry and a Federation Fellowt the University of Melbourne, at which he also received his BSc and PhDegrees. He worked as a postdoctoral researcher at the Hahn-Meitner-Instituten Berlin with Professor Arnim Henglein from 1989 to 1992. He was recipi-nt of an ARC QEII Research Fellowship in 1993, the David Syme Prize in999 and a Humboldt Foundation Award in 2000 and 2005. He is a mem-er of the Editorial Boards of Advanced Functional Materials and Physicalhemistry Chemical Physics. His current interests involve the application ofanosized semiconductor particles for electroluminescence, the use of col-oidal metals and quantum dots as biochemical markers, and in optical devices,lectrochemistry of colloids, effects of double layers on redox reactions atarticle surfaces and the use of atomic force microscopy to measure surfaceorces.

orge Perez-Juste was born in Vigo, Spain, in 1972. He obtained a chemistryegree from the University of Santiago de Compostela (Spain) in 1995. Hebtained a PhD degree in chemistry from the University of Vigo (under super-ision of Professors Pablo Herves and Luis Garcıa-Rıo) in 1999. He worked aspostdoctoral fellow at UCSC in 2000 with Professor Bernasconi and at Mel-ourne University (Australia) in 2002 and 2003 with Professor Paul Mulvaney.e currently holds a Ramon y Cajal research position at the University of Vigo

working with Professor Luis Liz-Marzan)

oberto Rella, physicist, senior researcher, received his degree in physics in985 from the University of Lecce. His initial research activity, developed athe Department of Materials Science of Lecce University, was focused on therowth of semiconductor materials both in the form of bulk and thin film and theirptical and electrical characterization for application in optoelectronic devices.ctually, he works as a senior researcher at the Institute for Microelectronic andicrosystems of the Council National of Research in Lecce and his research

nterests include molecular electronics, self assembly and structure of self assem-led systems, chemical and biochemical sensors, conducting polymers. He isnvolved in different national and international projects regarding the develop-ent of resistive opto- and bio-chemical sensors based on organic, inorganic

nd opportunely designed hybrid sensing layers for the analysis of liquids andases.

orenzo Vasanelli was born in 1947. After the degree in physics, he was athe Department of Physics of Bari as a lecturer and successively as an asso-iate professor at Experimental Physics. In 1987 he became full professor ofolid-state physics at the University of Lecce and then Director of the Materi-ls Science Department of this University. In 1994 he became director of thenstitute for the study of new materials for Electronics of CNR (IME), locatedn Lecce. His research activity was initially devoted to transport and photoelec-ronic properties of layered III–VI semiconductor compounds. His interest wasuccessively devoted to structural and electrical properties thin semiconductinglms prepared by sputtering and their applications (solar cells, nuclear detec-

or, sensors). He has been also involved in some researches about GaAs-basedevices. Actually he is director of the innovation engineering at the University

f Salento.

aolo Mazzoldi is full professor of physics at the Faculty of Engineering, Padovaniversity from 1975. Teaching specialized PhD courses. He is component and

eferee of several national and international commissions in the field of materials

nd Ac

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nd nanotechnologies. Scientific responsible for the development of three labo-

atories in the field of material technologies at IRST Trento, CNRSM-Mesagne,anofab-Venice. He is a European Community consultant for cooperation pro-rams in material research and technology. He has been invited to serve asession chairman in several national and international conferences. He pub-ished more than 340 papers in international journals with referee and specialized

ianna

tuators B 132 (2008) 107–115 115

ooks. His research activity, mainly experimental, has encompassed several top-

cs in the field of material science and physics, both from a fundamental and anpplied point of view and actually is mainly focused in physics and chemistry ofanoclusters for applications in non-linear optics and magnetism, synthesis ofanocomposites, by using different techniques, ferroelectric materials, sensingnd optical devices.