Sensors and Actuators B 96 (2003) 399–406

Grafting of anion exchanging groups on SiO2/Si structuresfor anion detection in waters

H. Touzia, N. Saklyb, R. Kalfata, H. Sfihic, N. Jaffrezic-Renaultd,∗,M.B. Rammahb, H. Zarrouka

a UR Physico-Chimie des Matériaux Solides, Faculté des Sciences de Tunis, 1060 Tunis, Tunisiab Laboratoire de Physique et Chimie des Interfaces, Faculté des Sciences de Monastir, 5000 Monastir, Tunisia

c Laboratoire de Physique Quantique, CNRS FRE 2312 ESPCI, 10 rue Vauquelin,75005 Paris, Franced Ingénierie et Fonctionnalisation des Surfaces, UMR CNRS 5621, Ecole Centrale de Lyon, F-69134 Ecully Cedex, France

Received 15 February 2003; received in revised form 29 May 2003; accepted 16 June 2003

Abstract

Here, we report results of anion exchanger film, based on quaternary ammonium, obtained by grafting EPTMAC (glycidyltrimethyl-ammonium chloride) on silica transducer using AEAPTS (3-(2-aminoethyl-amino)propyltrimethoxysilane) coupling agent. The graftingproduct was characterized by1H, 13C, 29Si solid state NMR spectroscopy. Anion exchange on grafted SiO2/Si structures was monitored byimpedance measurements: studying the flat band potential shift and the variation of capacitance in accumulation mode versus concentrationof anionic species in solution.

The grafted membrane exhibits a good affinity for different anions like CO32−, F−, I−, dye-SO3

− with a detection limit of 10−7 M.Applying site-binding model, the affinity constant of the anion species towards quaternary ammonium group is found to be 107 M−1. Thesize of the anion limits the sensitivity of the grafted SiO2/Si structures.© 2003 Elsevier B.V. All rights reserved.

Keywords:Ion exchanger; Silica gel; Solid state NMR; Impedance

1. Introduction

Water treatment for the industrial, domestic or foodfabrication uses is certainly the oldest application of ionexchangers. The most usual impurity of waste water is anexcessive content of mineral or organic matter (i.e. dyes andsurfactants) that do not undergo biochemical degradation.These impurities can be removed by chemical processing,precipitation or by ion exchange. Therefore, a variety ofion exchangers were elaborated in order to be useful inwater purification[1–5]. The current tendency is the devel-opment of simple analytical methods in order to monitor inreal time the impurity content in water during their elimi-nation by ion-exchange process. These methods are basedon ion-exchange membranes associated with an electrictransducer.

In the recent years a great attention is attracted toward thedevelopment of new sensing electrodes for the detection ofionic compounds in water[6–9]. Several works have beencarried out on the potentiometric and impedimetric prop-

∗ Corresponding author. Tel.:+33-472186243; fax:+33-478331140.E-mail address:nicole.jaffrezic@ec-lyon.fr (N. Jaffrezic-Renault).

erties of some ion exchanging membranes like plasticizedpoly(vinyl chloride) containing different quaternary ammo-nium salts for anion exchange[10] or doped with KBPh4and some proportion of valinomycin for K+ exchange[11].

Gagneux et al.[12] have used cellulose fibres modifiedby cationic species as adsorbents of anionic character dyein aqueous medium. The cellulose was modified by graftingglycidyltrimethyl-ammonium chloride (EPTMAC). The am-monium terminal group presents a cationic site that favoursthe adsorption of dyes. Such adsorption involves an ion ex-change between Cl− and SO3

− containing dye[13].In this work we bring out the ion-exchange phenomenon

by electrochemical impedance measurements. As a work-ing electrode, we used silicon–silica slide with EPTMACmolecules anchored to its surface. Adherence problemsbetween EPTMAC and silica surface leads us to use3-(2-aminoethyl-amino)propyltrimethoxysilane (AEAPTS)on the first stage and a classical glycidyl ether reactionwith the amino group[14]. The preliminary chemicalgrafting was optimized using a silica gel support insteadof silicon–silica slide. The silica gel matrix is suitable tocommon characterization techniques.

0925-4005/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0925-4005(03)00578-1

400 H. Touzi et al. / Sensors and Actuators B 96 (2003) 399–406

AEAPTS belongs to a broad class of surface-modifying,silylating chemicals used primarily as adhesion promoters incomposite materials, particularly for treating the surface ofmetals and glasses to achieve enhanced bond strength withpolymeric material[15–18].

UV-vis technique was used in order to follow the ex-change phenomenon taking place between functionalizedsilica gel and acid blue dye (AB25). The modified silica gelwas characterized by UV-vis and1H, 13C and 29Si solidstate NMR spectroscopies.

2. Experimental

2.1. Reagents

The coupling agent AEAPTS and the EPTMAC were pur-chased from Fluka. To avoid polymerization, the AEAPTSwas distilled before use and stored under vacuum. Tolueneand dimethylformamide (DMF) were purchased from Pro-labo and distilled and stored under dry conditions.

Silica gel Kieselgel 60 PF254 (particle size 0.63–0.20 mm)was purchased from Merck. The acid blue dye (AB25) waspurchased from Aldrich (Scheme 1).

2.2. Si/SiO2 substrates

The silicon/silica substrates were purchased from Micro-electronic Institute, Neuchatel University, Switzerland. Itconsists of p-type silicon wafer of 6� cm resistivity withcrystal orientation〈1 0 0〉. A thin layer of silica is gener-ated on the surface of silicon plates by thermal oxidation.The thickness of the thermal silica layer was approximately50 nm. The ohmic contact was realized with gold layer of300 nm thickness. The silica surface was treated with sul-fochromic solution and washed with bidistilled water to cre-ate reactive silanol sites. The adsorbed water molecules wereremoved by drying at 150◦C during 1 h.

2.3. Functionalization of silica surface

2.3.1. Attachment of AEAPTSSilica gel was used as model for the study of chemi-

cal grafting reactions. The first attempts to directly adsorb

O

O

NH2

SO3Na

HN

Scheme 1. Chemical structure of AB25 dye.

EPTMAC on the silica surface of the substrate have failedowing to the very weak adhesion of EPTMAC molecules.Therefore, the silica surface was first modified by AEAPTStreatment.

The functionalization was primarily done on silica gel sur-face following the same procedure as given by Haan et al.[19]. Silica gel surface is rich in silanol sites and presentsquite similar surface structure to silica slides substrate ofthe sensing device. More, the modified silica powder canbe easily characterized using simple technical methods. Thepresence of alkoxysilane groups in AEAPTS enables con-densation reactions with the surface silanol groups givingrise to chemical bond formation.

A sample of Silica gel of 5 g was immersed in 100 ml ofAEAPTS/toluene (1%, v/v) solution for 2 h under stirring atambient temperature. Toluene solvent was chosen in orderto avoid the flip-flop phenomena which take place in aque-ous solutions[20]. This phenomenon occurs due to highaffinity of AEAPTS ammonium end groups (NH3

+) to sil-ica silanols.

2.3.2. Grafting of EPTMACThe modified silica gel was washed three times with

purified toluene and then dried under vacuum at 110◦Cduring 16 h to remove adsorbed organics[21]. Epoxy-terminated-EPTMAC functionality easily reacts with amineend group of AEAPTS residues bonded to silica[22–24].This reaction takes place by mixing the AEAPTS modi-fied silica gel with EPTMAC/DMF solution (2%, v/v) at45◦C during 6 h. Afterwards, the silica gel was separatedfrom the reaction medium and carefully washed with DMFto remove the unreacted EPTMAC molecules.Scheme 2shows the chemical reactions corresponding to the graftingof the coupling agent (AEAPTS) and the final modificationintroducing the sensitive element (EPTMAC).

The same procedure was used for the functionalization ofsilicon–silica slide surface.

2.4. Spectroscopy

2.4.1. UV-visThe AB25 dye greatly absorbs in visible region (λmax =

600 nm). It can be used for diagnostic purposes in the mod-ification process and also as a probe to prove the exchangephenomena taking place on the surface of grafted silica gel.Different amounts of grafted silica gel (10, 30, 50, 70 and90 mg) were stirred for 30 min in 30 ml of acid blue solution(10−3 M). The UV-vis absorbance of different filtrates wasrecorded.

2.4.2. Solid state NMRThe1H MAS, 29Si and13C CP-MAS NMR spectra were

recorded at 500.13, 99.35 and 125.77 MHz, respectively, onBruker ASX 500 spectrometer operating in a static field of11.7 T. The spinning frequency was 6 kHz for29Si and13C,and 13 kHz for1H NMR. The recycle delay was 5 s for all the

H. Touzi et al. / Sensors and Actuators B 96 (2003) 399–406 401

at 110˚ C

OH

OH

OH

CH3O Si

OCH3

OCH3

CH2 CH2 CH2 NH CH2 CH2 NH2+

OH

O

OH

Si CaH2 CbH2 CcH2 NH CdH2 CeH2 NH2

1/ Toluene

2/ Curing under vacuum

X

X

X =

OCH3

OH

O Si

X

X

NH2CH2NH CH2CH2CH2CH2Si

OH

O

OH

b/ Second stage :

a/ First stage :

CfH2 CgH CH2 N+

+

CH3

CH3

CH3

Cl-

O

OH

OH

Si CH2 CH2 CH2 NH CH2 CH2 NH

X

X

Cf 'H2 Cg'H CH2

Cl-

CH3

CH3

CH3

N+

OH

O

45˚C

DMF

Scheme 2. Grafting reactions of AEAPTS on silica gel (a/first stage) and of EPTMAC on AEAPTS/silica gel system (b/second stage).

measurements. The contact time was 5 ms for29Si and13CCP-MAS. Chemical shifts were referenced to the externaltetramethylsilane (TMS). The number of scans (NS) is givenin the figures.

2.5. AC capacitance

The AC capacitance measurements on grafted SiO2/Sifield effect structures were performed in an electrochemi-cal cell using a potentiostatic three electrodes set up. Sat-urated calomel electrode (SCE) was used as reference andplatinum as counter electrode. Electrical measurements wereperformed using impedance spectrometer (VoltaLab 40 Ra-diometer Analytical SA, Villeurbanne, France) working atalternating voltage of 10 mV and 50 kHz frequency. Theanion exchange measurements were done for sodium io-dide, fluoride and carbonate. The ionic strength was main-tained constant using sodium acetate 0.01 M and the mediumpH was fixed at 3.9 using acetic acid buffer. The deter-mination of the electrochemical impedance resulted in thecapacitance versus voltage (C–V) curves. The ionic con-centration was correlated with the flat band potential shift�Vfb calculated on theC–V curves[25]. The electrical re-sponse gives the slope (of the linear part) of the�Vfb ver-sus pX [26], where pX is the cologarithm of the X ionconcentration.

3. Results and discussion

3.1. Grafted silica gel

3.1.1. UV-vis spectroscopyThe absorbance of AB25 versus different mass of grafted

silica gel is reported inFig. 1. The absorbance decreases sig-nificantly for solutions treated with larger amount of graftedsilica gel. This result can be explained as an increase ofthe retention of the AB25 dye molecules on the grafted sil-ica. This retention effect was not observed with pure silicagel.

3.1.2. Solid state NMRThe 29Si CP-MAS NMR spectra of silica gel, the sil-

ica gel/AEAPTS and the silica gel/AEAPTS/EPTMACsystems are given inFig. 2. All spectra exhibit two mainresonance lines at−101 and−110 ppm assigned to thesingle silanol groups (QOH

3 ) present at the gel surface andto the silica backbone (Q4 units), respectively. The line ofa very low intensity appearing at−92 ppm in the silicagel spectrum (Fig. 2a) is assigned to the geminal silanolgroups (QOH

2 ) (Scheme 3). Comparison of the relative in-tensity of the lines at−101 and−110 ppm in each spectrumclearly indicates that the silica gel/AEAPTS and the silicagel/AEAPTS/EPTMAC systems contains qualitatively less

402 H. Touzi et al. / Sensors and Actuators B 96 (2003) 399–406

0 20 40 60 80 1000,0

0,5

1,0

1,5

2,0

2,5

3,0

Abs

Mass of grafted silica gel [mg ]

Fig. 1. Absorbance of AB25 versus different mass of grafted silica gel.

QOH3 sites than the silica gel, relatively to Q4 sites content.

This is a result of grafting of AEAPTS at the silica surface.In addition to the two main resonance lines, the spec-

tra of the silica gel/AEAPTS (Fig. 2b) and of the silicagel/AEAPTS/EPTMAC (Fig. 2c) systems exhibit two otherlines of relatively weak intensities at−58 and−66 ppm.These lines, assigned to T2 and T3 units reveal the presence

-140 -120 -100 -80 -60 -40

(ppm/TMS)

(a)

(b)

(c)

Fig. 2. 29Si CP-MAS NMR spectra of silica gel (a), silica gel/AEAPTS(b) and silica gel/AEAPTS/EPTMAC (c). NS= 1024 (a), 1785 (b) and4096 (c).

of AEAPTS moieties attached to the surface of silica gel.The T3 sites are obviously due to the polycondensation ofAEAPTS-methoxy groups after hydrolysis. The presence ofT2 units suggests the existence of non-hydrolysed methoxygroups TOR

2 and/or non-condensed silanol groups TOH2

(Scheme 3). Furthermore, the similarity of29Si CP-MASNMR spectra of the silica gel/AEAPTS (Fig. 2b) and of thesilica gel/AEAPTS/EPTMAC (Fig. 2c) systems indicatesthat the surface structure obtained after AEAPTS is notaltered by the fixation of EPTMAC moieties.

The13C CP-MAS NMR spectra qualitatively confirm theresults obtained by29Si CP-MAS. Indeed, the presence in13C CP-MAS NMR spectra of the AEAPTS-hydrocarbon

Si

O

OO

O

Q4 : _ 110 ppm

O

O OH

O

Si

Q3OH : _ 101 ppm

_92 ppm Q2

OH :

Si

OH

OHO

O

O

O R

O

Si

_ 66 ppm T3 :

O

O R

OCH3

Si

O

O R

OH

Si

_ 58 ppm T2OCH3 : T2

OH :_ 58 ppm

R = AEAPTS or AEAPTS / EPTMAC

Scheme 3.29Si CP-MAS NMR chemical shifts (ppm) and assignments.

H. Touzi et al. / Sensors and Actuators B 96 (2003) 399–406 403

peaks at 10, 21, 42, 48 and 51 ppm show unambiguouslythe grafting of the coupling agent (AEAPTS) has occurred.These peaks were assigned to the CH2 groups in (a)–(d)positions (Scheme 2, first stage), respectively[27]. Thesedifferent peaks become much broader in the case of theEPTMAC/AEAPTS/silica system. The peak broadening isdue to a distribution of chemical shift arising from the newCH2 and to the CH3 groups carried by EPTMAC moieties(Scheme 2, second stage) and also from the changes in thechemical shift of the previous CH2 (a–d positions) inducedby the grafting of EPTMAC. In addition, the spectrum of thesilica gel/AEAPTS/EPTMAC system shows an additionalpeak at 55 ppm and a broad shoulder centred∼65 ppm. Itwas attributed to N+(CH3)3Cl− of EPTMAC end group[28] and the shoulder arises from the epoxy CH2 andCHOH groups in f′and g′ positions appearing after graftingof EPTMAC on AEAPTS-amine end groups (Scheme 2,second stage). Before grafting, the chemical shift of theseCH2 and CH groups, which correspond to f and g positions(Scheme 2, second stage) is∼43, 50 ppm[29].

The broad peaks appearing in the 160–200 ppm range areassigned to the carbonyl of ammonium carbamate groups(–NHCO2

−+H3N–) formed after reaction between gaseouscarbon dioxide (from air) and aminosilane coupling agentsAEAPTS as shown below:

2(–CH2CH2NH2) + CO2

→ (–CH2CH2NHCO2−+NH3CH2CH2–)

One amino group is consumed in the formation of the com-paratively unstable carbamic acid, which quickly reacts withanother amine to form the more stable alkylammonium car-bamate salt[30].

The1H MAS NMR spectra of the silica gel/AEAPTS andof the silica gel/AEAPTS/EPTMAC systems are shown inFig. 3. The spectrum of the former (Fig. 3a) exhibit fourpeaks at∼2.1, ∼3.7, ∼5.4 and∼7.1 ppm. The first peak(2.1 ppm) is assigned both to the silanol groups present at

(b)

(a)

Fig. 3. 1H MAS NMR of silica gel/AEAPTS (a) and silica gel/AEAPTS/EPTMAC (b).

the silica surface and to those formed after hydrolysis of themethoxy groups (–OCH3) of AEAPTS. The 3.7 ppm peakis assigned to the CH2 groups of the hydrocarbon chainsof AEAPTS. The peak at∼5.4 ppm is attributed to watermolecules adsorbed during the storage of products, and thatat∼7.1 ppm, which reflects presence of the primary (–NH2)and the secondary (–NH–) amino groups of AEAPTS. Thelater broad peak is shifted to 8.2 ppm in the spectrum of thesilica gel/AEAPTS/EPTMAC system and becomes narrow(Fig. 3b). This could be explained by a significant decreaseof the proportion of the secondary amine NH2 groups, ascompared to the grafted EPTMAC. In addition, each of thepeaks at 2.1 ppm and 3.7 ppm presents a shoulder at 1.6 and3.2 ppm in the spectrum of the silica gel/AEAPTS/EPTMACsystem. The appearance of those shoulders is accompaniedby an increase of the relative intensity of the peak at 3.7 ppm,and by intensity of the peak at 2.1 ppm. Concerning theshoulders, we assign them to the methyl groups (3.2 ppm) ofEPTMAC and to the OH groups of CHOH (1.6 ppm) appear-ing in the hydrocarbon chains after the reaction of EPTMAC(Scheme 2, second stage). The appearance of this later peakconfirms therefore the grafting of EPTMAC on AEAPTS, inaccordance with13C CP-MAS NMR results. The increaseof the relative intensity of the peak is due to the added CH2of EPTMAC. The relative decrease of the intensity of SiOHpeak (2.1 ppm) can be only explained by a decrease of thenumber of the OH groups formed after the hydrolysis ofthe methoxy groups (–OCH3) of AEAPTS, which probablyleads to the formation of siloxane bridges between AEAPTSchains. The formation of these bridges would be favouredby the experimental conditions (temperature, solvent) usedfor grafting of EPTMAC, which are different from that usedin grafting AEAPTS.

3.2. Electrical tests on grafted SiO2/Si

As previously indicated, EPTMAC does not adsorb on thesilica surface make necessary the use of a coupling agent(AEAPTS). The coupling agent AEAPTS is bearing a pri-mary and secondary amine function –NH2 and –NH–. Thesefunctions can give specific interactions with ions of the elec-trolyte. These interactions were evaluated by measurementsof the effect of concentrations of some ions on the flat bandpotential of AEAPTS grafted SiO2/Si structures. The pH ofthe test solution is fixed at 3.9 in order to stabilize the proto-nated form of the two amine functions. The measurementswere carried out for different anions CO3

2−, F−, I− anddye-SO3

−. The variation of the flat band potential shift andof the capacitance value in accumulation regime versus theconcentration of the added ions (expressed as cologarithmpX) is shown inFig. 4a and b. Both figures were extractedfrom the electrochemical capacitance curves (C–V) on theEIS structures. The variations of the flat band potential ver-sus pX (Fig. 4a) are of sigmoidal shape and can be decom-posed into three regimes: a low concentration regime wherethe flat band was nearly constant, a medium concentration

404 H. Touzi et al. / Sensors and Actuators B 96 (2003) 399–406

2 3 4 5 6 7 8

0,00

0,01

0,02

0,03

0,04

0,05

∆Vfb

(V

olt)

p [ x ]

2 3 4 5 6 7 8

0,00

0,05

0,10

0,15

∆C (

nF)

p [ x ]

(a)

(b)

Fig. 4. (a) Response of AEAPTS membrane for CO32− (�), F− (�),

I−(�) and dye-SO3− (�) anions at pH= 3.9. (b) Capacitance accu-mulation on AEAPTS membrane for CO3

2− (�), F− (�), I− (�) anddye-SO3

− (�) anions at pH= 3.9. Test solution sodium acetate of 0.01 M.

regime where the effect of ion exchange was detected anda high concentration regime where there is no longer varia-tion. The detection range is between lower and upper limitsof the medium concentration range where the flat band po-tential varies linearly as a function of pX. The lower limitcorresponds to the detection limit and the upper limit to thesaturation. The detection range for each X− anion and theslope of the�Vfb = f(pX) curve are given inTable 1. Forall X− anions, the detection range is very narrow and thedetection limit is between 10−4 and 10−5 M. All sensitivi-ties are subnernstian. The lower sensitivity corresponds tothe divalent anions CO32−.

When EPTMAC is grafted on the coupling agentAEAPTS, a different behaviour versus anionic exchange ofthe grafted SiO2/Si structure is obtained. The measurements

Table 1Linear regime of the flat band potential shift versus−log[Xn−] (experi-mental slope and range for anion detection)

Anion Slope(mV/dec)

Theoreticalslope (mV/dec)

Linearity range(M)

CO32− 16 29 10−3 to 10−5.5

F− 30 59 10−3.5 to 10−4.0

I− 22 59 10−3.5 to 10−5

Dye-SO3− 40 59 10−4.0 to 10−4.5

2 3 4 5 6 7 8-0,04

-0,03

-0,02

-0,01

0,00

0,01

0,02

0,03

0,04

0,05

∆C (

nF)

p [ X ]

2 3 4 5 6 7 8-0,01

0,00

0,01

0,02

0,03

0,04

0,05

0,06

0,07

0,08

∆Vfb

(V

)

p [ X ](a)

(b)

Fig. 5. (a) Response of AEAPTS/EPTMAC membrane for CO32− (�),

F− (�), I− (�) and dye-SO3− (�) anions at pH= 3.9. (b) Capacitanceaccumulation on AEAPTS/EPTMAC membrane for CO3

2− (�), F− (�),I− (�) and dye-SO3− (�) anions at pH= 3.9. Test solution sodiumacetate of 0.01 M.

were carried out for the different anions CO32−, F−, I−

and dye-SO3−. The variation of the flat band potential shiftand of the capacitance value in accumulation regime versusthe cologarithm (pX) of the concentration of the added Xions is shown inFig. 5a and b. The variations of the flatband potential versus pX (Fig. 5a) are of sigmoidal shapeand can be divided on three regimes: a low concentrationregime, where the flat band was nearly constant, a mediumconcentration regime where the effect of ion exchangewas detected and a high concentration regime where thereis a lower sensitivity. The two detection ranges for eachX− anion and the two slopes of the curve�Vfb = f(pX)

are presented inTable 2. For all X− anions, the detectionlimit is around 10−7 M. The relation between�Vfb and pXdeduced from the site-binding model is[31]

pX = q�Vfb

2.3kT− log

(qNS

�VfbCS− 1

)− pKX

NS is the number of grafted EPTMAC groups,KX the affinityconstant of the X− anions with the grafted EPTMAC group,q the elementary electrical charge,k the Boltzmann constantandT the absolute temperature.

In the detection limit (�Vfb = 0), the above relation givespX = –pKX. Affinity constant of X− anion for the grafted

H. Touzi et al. / Sensors and Actuators B 96 (2003) 399–406 405

Table 2Two linear regimes of the flat band potential shift versus−log[Xn−] (experimental slope and range for anion detection)

Anion First slope (mV/dec) Linearity range (M) Second slope (mV/dec) Linearity range (M)

CO32− 33 10−5.5 to 10−7 6 10−3 to 10−5.5

F− 40 10−5.5 to 10−7 10 10−3.5 to 10−5.5

I− 25 10−4.5 to 10−6 12 10−3.5 to 10−4.5

Dye-SO3− 10 10−5.5 to 10−7 6 10−3 to 10−5.5

EPTMAC group is of the order of 107 M−1 whereas affin-ity constant for AEAPTS coupling agent is of the order of104–105 M−1.

The detection sensitivity seems to depend on the size ofmonovalent ion. Indeed, as the size (S) of monovalent ionincreases (SF− < SI− < Sdye-SO3

− ), the detection sensitivitydecreases (cf.Table 2). From the previous relation, it appearsthat the slope of the curve�Vfb = f(pX) is higher whenthe number of exchangeable grafted sites is higher. In thiscase, the number of exchangeable sites decrease when thesize of the monovalent ion increases which shows that theanion exchange is limited by steric hindrance.

The variation of the capacitance in accumulation regimeof the grafted SiO2/Si structures according to the concen-trations of anions CO32−, F−, I− and dye-SO3−, is shownin Figs. 4b and 5b. The capacitance in accumulation regimestarts to increase at low anion concentration which trans-lates an increase of dielectric constant (ε) of the film dur-ing adsorption of the ions, due to a higher polarisabilityof counter ions, as already observed in[32] when poly-THF was firstly equilibrated in sodium acetate solution. ForCO3

2− and dye SO3− ions, a decrease for higher concen-trations can be explained by an increase in the thicknessof the grafted layer with the accumulation of anions. Fora large dye-SO3−, the capacitance decreases after an in-crease, this behaviour can be explained by an increase ofthe film thickness with the adsorption of the organic-SO3

−anions.

4. Conclusion

In this paper, we report characteristics of anion exchangerfilm obtained by grafting EPTMAC on silica transducer us-ing coupling agent AEAPTS. This membrane shows a goodaffinity for different anions with a detection limit of 10−7 M.The size of the anion limits the sensitivity of the graftedSiO2/Si structures. This anionic detection procedure will betransferred to ISFET structures.

Acknowledgements

This work was financially supported in the frameworkof Monastir-Rhone-Alpes MIRA Research Program andCMCU Research Program.

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Biographies

Hassen Touziwas born in 1974 in Monastir (Tunisia). He is a graduatestudent in Department of Chemistry of University of El Manar in Tunis(Tunisia).

Nawfel Sakly, born in 1972 in Métlaoui-Gafsa (Tunisia), received anMSc in solid-state physics from University of Tunis (Tunisia) in 1999.Presently, he is preparing a thesis on the anionic exchange supportedmembranes.

Rafik Kalfat received his doctorate of materials science in 1985 fromUniversity of Science and Technology of Languedoc (France). Hejoined the Laboratory of Physical Chemistry of Solid Materials of theScience University of Tunis (Tunisia). He received his PhD degreein Chemistry of Materials from Science University of Tunis in 2001.He is now a Maıtre de Conferences in Science University of Bizerte(Tunisia). His current research interest is in the preparation of organ-ically modified polysiloxane gels based on sol–gel method for sensorapplications.

Hocine Sfihi is associate professor at University Paris-Nord and se-nior researcher at the Quantum Physics Laboratory at the EcoleSupérieure de Physique et de Chimie Industrielles de la Ville de Paris(ESPCI).

Nicole Jaffrezic-Renaultreceived her engineering degree from the EcoleNationale Superieure de Chimie, Paris, in 1971 and her Doctorat d’Etatès Sciences Physiques from the University of Paris in 1976. Since 1971she has been a research worker at the Centre National de la RechercheScientifique. In 1984 she joined the Laboratory of Interfacial Physico-chemistry of the Ecole Centrale de Lyon, France, where she is in chargeof the chemical sensor group. She is now the co-director of the Labo-ratory of Engineering and Functionalization of Surfaces, UMR CNRS5621. Since 1997, she is the president of the Chemical MicrosensorClub in France. Her research activities concern the preparation and thephysicochemical characterization of the recognition part of electrochem-ical and optical sensors.

Mohamed el Baker Rammahreceived his engineering degree fromthe Ecole Nationale Superieure de Chimie-Besançon in 1972, hisDocteur-Ingénieur in Chemistry from University of Franche-Comtéin 1975 and his Doctorat d’Etat ès Sciences Physiques from theUniversity of Franche-Comté in 1983. In 1984 he joined the Labora-tory of Organic Heterocycle Chemistry of the Faculty of Sciences ofMonastir-Tunisia, where he is in charge of the organic chemical synthe-sis and structural group. He is now the co-director of the Laboratory ofPhysico-Chemistry of Interfaces, FSMonastir. His research activities con-cern the preparation and the physicochemical characterization of organicsensors.

Hedi Zarrouk received his Doctorat d’Etat ès Sciences in 1982 fromthe Faculty of Science of Tunis (Tunisia). He is professor of chemistryand head of the Laboratory of Physical Chemistry of Solid Materialsin the same institution. In addition, since 1996, he is the general di-rector of the National Institute for Research and Physical ChemicalAnalysis. His research interests include catalysis, ceramics, inorganicand organic–inorganic polymeric materials based on sol–gel methods forelectrical and sensor applications.