Thin Solid Films 519 (2010) 487–493

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Thin Solid Films

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Development and characterization of fluorine tin oxide electrodes modified withhigh area porous thin films containing gold nanoparticles

Carmen Quintana a,⁎, Pedro Atienzar b, Gerolamo Budroni b, Laura Mora a, Lucas Hernández a,Hermenegildo García b, Avelino Corma b

a Dpto. Química Analítica y Análisis Instrumental. Facultad de Ciencias. Universidad Autónoma de Madrid. Cantoblanco. 28049-Madrid, Spainb Instituto de Tecnología Química de Valencia, UPV-CSIC, Universidad Politécnica de Valencia, Av. de los Naranjos s/n, 46022-Valencia, Spain

⁎ Corresponding author. Tel.: +34 914977626; fax: +E-mail address: carmen.quintana@uam.es (C. Quinta

0040-6090/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.tsf.2010.07.126

a b s t r a c t

a r t i c l e i n f o

Article history:Received 7 October 2009Received in revised form 30 July 2010Accepted 30 July 2010Available online 6 August 2010

Keywords:Fluorine doped tin oxideElectrodesMetal oxidePorosityGold nanoparticles

Different electrode materials are prepared using fluoride doped tin oxide (FTO) electrodes modified with higharea porous thin films of metal oxides containing gold nanoparticles. Three different metal oxides (TiO2, MgOand SnO2) have been assayed to this end. The effect of themetal oxide nature and gold loading on the structureand performance of the modified electrodes was examined by Scanning Electron Microscopy, TransmissionElectron Microscopy, X-Ray Diffraction (XRD), Diffuse Reflectance Spectroscopy and electrochemicaltechniques. XRD measurements reveal that MgO electrodes present the smallest gold nanoparticles afterthe sintering step however, the electrochemical response of these electrodes shows important problems ofmass transport derived from the high porosity of these materials (Brunauer Emmett Teller area of 125 m2/g).The excellent sintering properties of titania nanoparticles result in robust films attached to the FTO electrodeswhich allow more reliable and reproducible results from an electroanalytical point of view.

34 914974931.na).

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© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Thin films containing metal nanoparticles and semiconductorparticles have attracted considerable attention due to their vastpotential applications in different fields including catalysis, optics,microelectronics and chemical and biological sensors [1]. In electro-chemistry, metal nanoparticles, especially gold nanoparticles, havefound a wide range of applications in electroanalysis [2], bioelectroa-nalysis [3] and electrocatalysis [4]. Different fabrication methods of thiskindoffilms suchas sputtering [5] or sol–gel [6] havebeen reported. Theenhancement of mass transport, catalysis, high effective surface areaand the control of the electrode microenvironment can be noted assome advantages derived from the use of gold nanoparticles overmacroelectrodes for electroanalysis [1]. A largenumberof approaches tosupporting gold nanoparticles have been reported in literature. Theyhave been supported either on the electrode sufaces [7] or on otherconducting or semiconducting supports [4,8,9]. Gold nanoparticles havebeen used to modify indium tin oxide (ITO) electrodes leading tosatisfactory simultaneous electrochemical detection of biomoleculessuch as guanosine and guanosine-5´-triphosphate in human bloodsamples at μM level [7] as well as mediators in an amperometricbiosensor for H2O2 determination [8]. Gao presents a high sensitive

method (limit of detection=5.0 nM) for guanine determination with aredox polymer modified ITO electrode that results in a reduction of theoverpotential of guanine oxidation by asmuch as 550 mV [9]. Also, goldnanoparticles have been embedded in “sponge-like” silicamaterials andmesoporous silica films which have demonstrated the advantages ofthese high surface hybrid materials as precursors of highly activecatalysts and as electrochemical sensors [10,11]. Scanning electronmicroscopy (SEM), transmission electron microscopy (TEM) or X-raydiffraction (XRD) techniques have been widely employed for thecharacterization and study of these materials [12]. Usually, thepreparation of metallic nanoparticles within mesoporous thin filmscan be simply achieved by aqueous impregnation methods. Otherstrategies developed to this end include functionalization of themesoporous silica with charged groups and a postsynthesis graftingprocess [13]. Generally, the development of thick, highly-porous TiO2

films have been focused on the application of these materials to dye-sensitized solar cells [14,15] and on the study of the texture andstructure of those films [16,17] however, very few analytical applica-tions of these materials have been found [18].

The development of modified electrodes is a strategy widelyemployed in order to increase the selectivity. However, sometimesthe incorporation of a modifier on the electrode surface can alsoproduce a decrease in the sensitivity of the analytical determinations.The work reported herein focuses on the modification of fluoridedoped tin oxide (FTO) electrodes with high area porous thin films ofmetal oxides (nanometre size) containing gold nanoparticles with theobjective of compensate the decrease in detection limits produced

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from some modification strategies. The modification procedurefollowed includes a thermal treatment to sinter the films to get aceramic hybrid structure (instead a layered structure) which cannotbe put up with other conventional electrodes (i.e. glassy carbon).Three different materials (MgO, TiO2 and SnO2) have been assayed tosupport and embed the gold nanoparticles. These electrode materialshave been characterized by SEM, TEM, XRD, diffuse reflectance opticalspectroscopy (DRS), cyclic voltammetry (CV) and electrochemicalimpedance spectroscopy (EIS). Morphology, nanoparticle size distri-bution and electrochemical performance of these films are hereinstudied. Application in electroanalytical chemistry of these electrodeshas also been demonstrated in studies of voltammetry.

2. Experimental procedure

2.1. Reagents and equipment

Chemical reagents were obtained as follows: Titanium (IV) oxidenanopowder was supplied by Degussa (P-25, 99.7%, 80% anatase, 20%rutile), ethyl cellulose and terpineol were purchased from AldrichChemical Co. (Milwaukee, USA), magnesium oxide nanopowder, tinoxide nanopowder and Guanosine 5´-monophosphate (GMP) weresupplied from Sigma Chemical Co. (St. Louis, USA). 4-amine-2-mercaptopytimidine (Thyocitosine, TC) from Across Organic. Trans-parent conducting oxide coated glass (TCO22-15, 2.2 mm thicksodalime glass coated on one side with a fluorine doped tin oxide(SnO2:F) layer ,“FTO” glass, ~15 ohm/square) was commerciallyavailable from Solaronix (Aubonne, Switzerland). All other chemicalswere of at least reagent grade quality and were used as received.Water was purified with a Millipore Milli-Q-System. All solutionswere prepared just prior to use.

2.2. Preparation of the nanoparticles

Gold nanoparticles were prepared following the Brust method[19]. In detail, 0.45 mmol of HAuCl4 were dissolved in 15 ml ofpurified water and 1 mmol of tetraoctylammonium bromide wasdissolved in 40 ml of toluene. The two solutions were mixed andvigorously stirred. Then, the stabilizer (dodecanethiol, 0.45 mmol)was added to the mixture and subsequently, AuCl4− was reduced byadding dropwise a solution of 5 mmol of NaBH4 in water (12 ml).The organic phase turned from orange to intense brown indicatingthe formation of gold nanoparticles.

After about 30 min of further stirring the toluene phase wasseparated from the aqueous phase and concentrated up a volume ofabout 2 ml before to be diluted with 400 ml of ethanol. The solutionwas kept for at least 2 h at −22 °C and the precipitate was thenfiltered with 0.45 μm Whatman Nylon membrane and washed withfresh ethanol. Finally the nanoparticles were resuspended in

Fig. 1. TEM image of the gold nanoparticles and partic

chloroform and the colloidal solution used to characterization andto impregnation.

The average gold particle size was measured from TEM. Fig. 1presents a representative image of the nanoparticles prepared and astatistical analysis based on it showing a narrow size distribution withan average size of 1.8 nm.

2.3. Preparation of the FTO/metal oxide film/gold nanoparticles electrodes

The procedure followed to prepare the films on the FTO electrodeswas essentially the same previously reported by Atienzar et al. [15].Briefly described, each metal oxide (100 mg)—containing goldnanoparticles (2.5%, 5% and 10% w/w, average size 1.8 nm) isdispersed in ethyl cellulose:terpineol:acetone mixture by constantstirring for ca. 2 days. Once the acetone solvent was evaporated atroom temperature, a paste-like of metal oxide was obtained. FTOsheets were cut in 2.5×1.5 cm glasses and exhaustively washed withethanol and acetone before manipulation. The paste was spread onthe transparent FTO electrode between two parallel strips of adhesivetape leading to films of 1.5×1.5 cm using the doctor blade technique.Next, the film on the glass was sintered at 450 °C for 90 min.

2.4. Characterization

The materials were characterized by SEM (JEOL JSM-6300Scanning Microscope with an operating voltage of 20 kV), TEM(Philips CM300 FEG systemwith an operating voltage of 100 kV), XRD(Philips X´Pert diffractometer equipped with a graphite mono-chromator, operating at 40 kV and 45 mA and employing nickel-filtered CuKα radiation λ=0.1542 nm) and diffuse reflectance opticalspectroscopy (CARY 5G UV–Vis–NIR spectrophotometer having anintegrating sphere). Surface area was determined by isothermalnitrogen adsorption isotherms (Micromeritics ASAP 2000) applyingthe Brunauer Emmett Teller (BET) algorithm.

Electrochemical measurements were recorded at room tempera-ture using an Amel 7050 potentiostat with a standard three electrodecell. CV experiments were carried out at a scan rate of 100 mV/s. Theelectrodes developed served as working electrodes. A large-areacoiled platinum wire was employed as a counter electrode. Allpotentials are reported against an Ag/AgCl/KCl 3 M.

The impedance spectra were recorded with a Frequency ResponseAnalyzer Model 7200 from Amel Instruments coupled with thepotentiostat. Impedance measurements were performed in 0.1 MNaCl with 10−4 M K3[Fe(CN)6] as probe and at bias potential of 0.4 Vin a frequency range of 0.1 Hz to 1.0 MHz (20 points per decade) using50 mV amplitude of sinusoidal voltage. The experimental data wereplotted in the form of Nyquist plots and stimulated using electronicequivalent circuits.

les size distribution calculated over 450 particles.

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3. Results and discussion

3.1. Characterization of gold nanoparticles/metal oxides films/FTOelectrodes

3.1.1. SEM characterizationThemodified FTO electrodes were firstly examined using SEM. The

images obtained at high and low magnification (Fig. 2) clearly showthe high spongy texture for all the films on the FTO surface. Themeso-/macro-porosity of the metal oxide film is a direct consequenceof the sintering of nanostructured particles in the thermal treatmentat 450 °C. The appearance of open porous structure with spherical-shaped grains is very similar to that reported by Karuppuchamy et al.[20] for electrosynthesized and sintered TiO2 thin films, who observedthat each grain is made up with an aggregate of very small crystallites.From the SEM photographs, it was calculated the thickness of all thefilms in the range from 6.0 to 7.5 μm.

3.1.2. TEM characterizationTEM was employed to study the distribution and morphology of

gold nanoparticles in the nanostructured films.After the deposition of the nanoparticles on the support by

impregnation of the metal oxide nanoparticles with the gold colloidal

Fig. 2. SEM photographs of the different oxide thin films assayed on th

solution and, in order to eliminate the organic ligand and promoteparticle sintering, the supported metal materials were calcined at450 °C for 90 min. At this temperature, the dodecanethiol thatstabilize the nanoparticles is completely removed and metal oxidenanoparticles become connected by covalent bonds (sintering). Anundesirable process occurring also in the thermal treatment is thegrowth of gold particle size. Thus, the temperature of the thermaltreatment has to be carefully controlled and kept to the minimumpossible value to ensure complete ligand decomposition and sinter-ing, while keeping at a reasonable extent gold particle growth. TEMmicrographs recorded from MgO materials allows a clear identifica-tion of gold nanoparticles as black dots well separated from each otherand distributed randomly in the MgO support (Fig. 3) with particlessize between 5 and 22 nm. However, the low contrast observedbetween the gold nanoparticles and the metal oxide supports makesambiguous the safe measurement of the particle size in the case ofSnO2 and TiO2.

3.1.3. XRD characterizationTo overcome limitations of TEM characterization, we preferred

estimate the average particle size by the broadening of the XRDsignals using the Scherrer equation. To this end, each composite film,different metal oxide supports and different Au percentages were

e FTO electrodes containing gold nanoparticles at 5% loading level.

Fig. 3. TEM images MgO at 5% gold loading level.

Table 1Average diameter of nanoparticles calculed from XRD data.

Au loading(%)

MgO TiO2 SnO2

2θ=38.2 2θ=44.4 2θ=38.2 2θ=44.4 2θ=38.2 2θ=44.4

2.5 12.4 13.2 14.9a 14.8 18.6a –

5 12.5 15.3 19.2 a 16.6 21.7a 34.410 13.6 16.4 27.2a 27.5 32a 36.3

a Calculated after subtraction of the support signal.

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scraped off from FTO electrode surface and, the resulting powderssubmitted to XRD. Fig. 4 show a selection of the diffractogramsobtained for the samples.

In all cases, it was possible to observe the diffraction peaks at2θ=38.2° assigned to (111) planes of face-centered cubic structure ofcrystalline gold nanoparticles [21]. As expected, characteristic Audiffraction peak intensities (at 2θ=38.2°, 44.4° and 64.6°) increasewith gold loading in all the cases assayed. On the other hand, thebroadening of the signal provides information of the crystalline grainsize and in our case of the particle size. In fact, large particles producesharp peaks while small particles (smaller than 100 nm) show abroadening from which the particle size can be calculated [22]. As canbe observed in Fig. 4d, where a comparison among the three gold-containing materials is shown, the widest signal is produced whenMgO is employed to embed the gold nanoparticles, which indicatesthe smallest particle size. Table 1 collects the average diameter of thenanoparticles calculated with the Scherrer equation. At low loadingthe sintering leads to particles between 12 and 19 nm while at higher

Fig. 4.Wide-angle XRD patterns of gold loaded composite films from the different materialsTiO2. (b) MgO. (c) SnO2. (d) Compared 2θ=38.2° diffraction peaks of the three supporting

loading, the sintering occur more extensively in the case of Au–SnO2

and Au–TiO2. Interestingly, not only the gold nanoparticles supportedon MgO were the smallest (12–13 nm) but, differently from TiO2 andSnO2, the sintering appeared to be only slightly dependent from theloading. For gold contents on MgO of 10% particles of 14–16 nm wereformed.

3.1.4. Diffuse reflectance optical characterizationDiffuse reflectance UV–Vis spectra were recorded with the aim of

study the differences observed in the colours of the different materials(Fig. 5a).

It is well known that the colour of gold nanoparticles is a reflectionof its size. Thus, although the average size of initial gold nanoparticleswas 1.8 nm, during electrode preparation and, particularly during thesintering at 450 °C of the particles to promote electrical conductiveamong the nanoparticles and the FTO electrode, the size of thenanoparticles grows in a different extent depending on the metaloxide support. Gu et al. [12] reported an increase on the intensity ofthe surface plasmon resonance absorption peak and backgrounds ofgold nanoparticles encapsulated in mesoporous silica films as afunction of gold loading level. This was explained according to Mie'sscattering theory [23,24]. Herein we have observed the same effect forMgO (Fig. 5b showing different incorporated gold loading). In the case

under investigation without and with gold nanoparticles at different loading levels. (a)materials at 10% gold loading.

Fig. 5. (a) Images of the different modified FTO electrodes developed. Diffusereflectance UV–vis spectra of (b) MgO materials at different gold loading and (c) thecomparison of the different supports.

Fig. 6. Cyclic voltammograms recorded from the modified FTO electrodes with (a) TiO2,

SnO2 and MgO films containing gold nanoparticles at 5% gold loading. (b) 1.0 mM Fe(CN)63−/Fe(CN)64− solutions with unmodified FTO electrode and modified with thedifferent materials assayed at 5% gold loading. 0.1 M H2SO4; Vb=100 mV/s.

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of gold supported on titania or tin oxide the influence of theoverlapping gold particle growth as a function of loading is reflectedin a deviation of this behaviour. Thus, the intensity of the surfaceplasmon is not proportional to the gold loading due to significantvariations in the dimensions of the gold nanoparticles from sample tosample. In addition to variations of the surface plasmon intensity, thepeak shift towards red with increasing particle size, as it is observedfrom the comparison among the three materials under investigation(Fig. 5c).

3.1.5. Electrochemical characterizationIn order to employ the developed materials as electrodes, their

electrochemical response was checked by CV and square wavevoltammetry (SWV) measurements.

First, the cyclic voltammograms of the electrodes modified withthe different metal oxide assayed without gold nanoparticles were

recorded in order to study their electrochemical response andelectroactivity limits. From the results (data not shown) it is observedthat the three materials showed an electroactivity potential rangebetween −0.2 and 1.5 V for voltamperometric working purposes. Inthe case of SnO2 and MgO materials, their redox properties arereflected in cathodic waves at more negative values (c.a. −0.5 V).However, once the gold nanoparticles are presented in the films, thecyclic voltammograms recorded in both, 0.1 M sulphuric media(Fig. 6a) and in 10−4 M Ferro/Ferricyanide solution in 0.1 M sulphuricacid (Fig. 6b), showed great differences respect to materials withoutgold and depending on the metal oxide employed. It is assumed thatthe modification procedure carried out leads to a hybrid film of goldnanoparticles and oxide metal where the electrochemical processesoccur.

With respect to SnO2 electrodes, different phenomena areobserved: some oxidation and reduction waves probably due toredox properties of the support are recorded leading to a decrease inthe electroactivity limits showed without gold nanoparticles. Inaddition an increase in the capacitive current is recorded (Fig. 6).Moreover, as consequence of the great nanoparticles growth duringthe sintering step, the electrodes developed with this oxide show thelowest electroactive gold surface respect to TiO2 and MgO electrodes(Fig. 6a).

In contrast with this behaviour, the electrodes prepared with MgOshow the highest electroactive gold surface clearly in relationwith thegold nanoparticle size of these materials (see XRD discussion.) As itwas stated before, these are thematerials less affected by the sinteringstep and therefore, with the lowest gold nanoparticule size. Therefore,the currents recorded for Ferro/Ferri solution are the highest incomparison with those obtained with SnO2 and TiO2 materials

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(Fig. 6b). Despite of these promising results, the modified electrodesprepared with MgO show some problems related to their chemicalstability at acid media conditions as well as a great influence of masstransport problems as it can be deduced from the Ferro/Ferrivoltammograms depicted in Fig. 6b.

These diffusive limitations could be attributed to the high porosityof MgO (BET area values of 125 m2/g in contrast with 50 and 40 m2/gcalculated for titania and tin oxides respectively) that could difficultthe mass transport step. These diffusion contributions are alsoobserved in the EIS experiments carried out (Fig. 7a).

EIS is a powerful tool for the study variations on the interfacialproperties on modified electrodes. Nyquist plots for the bare andmodified with MgO (2.5% gold loading) FTO electrodes show anegligible semicircle which corresponds with an electron-transferprocess and a large straight line at a wide range of frequenciescorresponding to a diffusion limiting step. However, we can observethe typical shape of a Faradaic impedance spectrum including asemicircular region on the Zreal-axis (Z´) followed by straight line in

Fig. 7. Nyquist plots of impedance spectra of FTO/oxide metal films containing goldnanoparticles electrodes (2.5% Au loading) in 0.1 M NaCl containing 1.0 mM Fe (CN)63-/Fe(CN)64− solutions.

the impedance spectra of the FTO electrode modified with TiO2

(Fig. 7b). As it was stated before, the semicircular region at highfrequencies corresponds to the electron transference limited processwhile the linear part of the spectrum, observed at lower frequencies, isassociated with the diffusion limited process. The semicircular regiondiameter observed in the case of SnO2 based electrodes (the highestone) (Fig. 7c) represents the high resistance to the charge transferpresented by these electrodes that could be produced as a result ofweak contact between the support and the gold particles.

From all the experiments carried out, it can be conclude that theoxide metal nature plays the most important role in the finalbehaviour of the electrodes as they are the responsible for the finalnanoparticle sizes and provide higher or lower hybrid electroactivesurfaces.

Titania oxide based electrodes showed the best results foranalytical purposes among all the materials developed. On the onehand, the currents recorded from Ferro/Ferri solutions working withTiO2 (2.5%) gold loading are similar to those observed with MgO atsame gold loading (that is higher than the rest). These results can easybe rationalized from taking into account the results obtained in theXRD experiments that showed that both electrodes (TiO2 and MgO at2.5 gold loading) keep the smallest nanoparticle size with a verysimilar values (14.8 nm and 13.2 nm for TiO2 and MgO respectively)after the sintering step. On the other hand, the data depicted in Table 2show the lowest ΔEa–c values and Ipa/Ipc ratios close to unit for titaniabased electrodes irrespective of the initial gold loading. These resultscould be explained considering the semiconducting behaviour of TiO2

that act as electrical conductor for negative potentials due to electrontransport through the conduction band. Finally, the excellentsintering properties of titania nanoparticles result in robust filmsattached to the FTO electrodes which allow more reliable andreproducible results (Relative standard deviation of 4.3 in averagefor n=5 ).

For all these reasons (best chemical and physical properties) TiO2

electrodes with 2.5% gold loading were used in order to test theirbehaviour as working electrode in an electrochemical application. Theloading of gold nanoparticles in these materials allows subsequentmodification with biological molecules containing amine or sulphidegroup. We chose the nucleotide Thyocitosine (TC) like modifier todetect guanosine 5′-monophosphate (GMP) because these two arecomplementary nucleotides and that allows obtaining a selectivityresponse. Treatmentwith TCwas carried out cover the electrode surfacewith 1 ml of TC 10−2 M solution during 20 s and rinsed with waterbefore used. Electrodes were submerged in GMP 10−4 M solution(supporting electrolyte contained 0.1 M phosphate buffer, pH 4.0) andsquare wave voltammograms were recorded. Measurements wereobtained over 0.5 to 1.4 V and the voltammograms obtained showed anexcellent selectivity and sensitive response for GMP (Fig. 8).

We observed a low current in background voltammograms and ahigher peak current for GMP when modifier electrodes with TC wereused.

We showed this wonderful opportunity to use these electrodes inelectroanalitycal applications, which will be further investigated inthe future in our labs.

Table 2Evolution of the characteristic cyclic voltammetric Fe (CN)63−/4− parameters as resultsof the different FTO modifications.

Au loading(%)

MgO TiO2 SnO2

Ipa/Ipc ΔEa–c (mV) Ipa/Ipc ΔEa–c (mV) Ipa/Ipc ΔEa–c (mV)

– 0.82 215 0.96 131 0.97 1802.5 0.84 161 1.00 98 0.74 1165 0.70 182 0.89 102 0.72 10810 0.74 165 0.94 100 0.78 113

Fig. 8. Square wave voltammograms recorded from modified FTO electrodes with (1)TiO2 films and (2) TiO2 films treated with TC, in (A) phosphate buffer solution pH 4.0and (B) GMP 10-4 M in phosphate buffer solution pH 4.0. TiO2 films containing 2.5% goldloading. Step potential 4 mV, amplitude 25 mV and frequency 15 Hz.

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4. Conclusions

With the aim of developing electrodes with high surface area andhigh response intensity, high area porous thin films of metal oxidescontaining gold nanoparticles have been developed to modify FTOelectrodes. Three different metal oxides were assayed as supports forgold nanoparticles embedding: Titanium oxide, magnesium oxide andtin oxide. A surface area value about 50 m2/g obtained for sinteredtitania by BET measurements, agree with the results previouslyreported (Atienzar et al. [15]) for these nanostructured films. Similarresults were obtained for sintered tin oxide (40 m2/g). In contrast,magnesium oxide exhibits significantly larger surface area (about125 m2/g). This different nature of the supports lead to a differentbehaviour of these materials to embed gold nanoparticles as it wasshown by the characterization of these electrodes by differenttechniques such SEM, TEM or XRD. The consequence of the differentgrowth of the gold nanoparticles during the sintering step is adifferent electrochemical behaviour of the modified electrodes

developed. Among all the electrodes designed, TiO2, 2.5% goldnanoparticles loading, results the most promising one in order to beused to electroanalytical proposes as it was shown in the square wavevoltammograms recorded for guanosine 5′-monophosphate.

Acknowledgements

Authors thank to CCG07-UAM/PPQ-1645, MAT 2006-14274-C02-01and CTQ06-06859 for financial support.

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