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Talanta 52 (2000) 495–507

Grafting of phosphonate groups on the silica surface for theelaboration of ion-sensitive field-effect transistors

Zoubida Elbhiri a,b, Yves Chevalier a,*, Jean-Marc Chovelon b,c,Nicole Jaffrezic-Renault b

a Laboratoire des Materiaux Organiques a Proprietes Specifiques, UMR 5041 CNRS -Uni 6ersite de Sa6oie, BP 24 ,

69390 Vernaison, Franceb Ingenierie et Fonctionnalisation des Surfaces, IFoS , E ´ cole centrale de Lyon, BP 163 , E ´ cully, France

c Laboratoire d ’ Application de la Chimie a l ’ En6ironnement, UMR 5634 CNRS -Uni 6ersite Claude Bernard Lyon 1,43 Boule6ard du 11 No6embre 1918 , 69622 Villeurbanne Cedex, France

Received 23 November 1999; received in revised form 27 March 2000; accepted 27 March 2000

Abstract

Ion-sensitive field-effect transistors (ISFETs) sensitive to Ca2+ ions could be elaborated by means of a new grafting

process of the phosphonate group at the surface of the silica gate of FETs. A grafting process involving only one

chemical reaction step at the surface afforded a significant improvement of the ISFET properties. The sensitivity of

the ISFET towards Ca

2+

ions at pH 10 was quasi-linear in the concentration range from 10

−1

to 10

−3

M, and theslope was 10 mV pCa−1. The site-binding model works well in predicting the experimental data, giving the

complexation constant of 102.7 and a low value of the grafting density. The origin of the poor response of ISFETs

sensitized by means of a multistep grafting process was investigated on silica powders of high specific area: the

cleavage of the organic grafts at the Si O Si bonds occurring at each step could be disclosed by means of elemental

analyses, infrared, and cross-polarization and magic angle spinning nuclear magnetic resonance of the grafts. © 2000

Elsevier Science B.V. All rights reserved.

Keywords: ISFET; Phosphonate; Calcium; Chemical grafting

www.elsevier.com /locate/talanta

1. Introduction

Calcium-sensitive electrochemical devices can

be elaborated by the immobilization of the phos-

phonate group at interfaces where an electro-

chemical potential can be measured. The

molecules bearing the phosphonate group arestrong complexing agents towards alkaline earth

and transition metal ions. This chemical function,

when immobilized at an interface, brings about

sensitivity to such ions. This is the basic principle

of electrochemical devices such as ion-selective

electrodes (ISEs) and ion-sensitive field-effect

transistors (ISFETs). The best example is the

* Corresponding author. Tel.: +33-4-78022271; fax: +33-

4-78027187.

E -mail address: [email protected] (Y. Chevalier)

0039-9140/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.

PII: S 0 0 3 9 - 9 1 4 0 ( 0 0 ) 0 0 3 9 5 - 7

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Z . Elbhiri et al . / Talanta 52 (2000) 495–507 496

elaboration of membranes of ISEs by means of

the immobilization of organic molecules contain-

ing the phosphate or phosphonate group, into

thick polymer films [1,2]. For the application to

analysis, the organophosphonates are preferred to

organophosphates because of their better chemi-

cal stability with respect to hydrolysis. The

molecules known as ‘ionophores’ contain a large

hydrophobic alkyl chain, which ensures their solu-bility into the non-polar polymer film and a very

low solubility in water. The main problem is the

slow leaking of the ionophore into the aqueous

solutions during the ISE utilization; this phe-

nomenon is always present even when the

ionophore has a very low solubility in water. The

polymer film of ISEs is also a membrane separat-

ing the reference solution inside the electrode and

the solution to be analyzed. The need for a high

enough mechanical strength of the unsupported

polymer membrane is a severe limitation in thechoice of the polymer. The technological require-

ments are quite different for the ISFETs.

The surface of the grid of a field effect transis-

tor (FET) is made of a dielectric layer of silicon

oxide or silicon nitride. These materials bear sur-

face groups with acido-basic properties so that

H+ ions are potential determining ions. On the

contrary, alkaline-earth ions such as calcium are

not potential determining ions and the immobi-

lization of complexing groups is necessary for

making this device an ISFET. The immobilizationinto a thin polymer film deposited at the FET

surface was the first technique [3] derived from the

ISE membrane technology. Calcium-sensitive IS-

FETs could be designed in this way by the immo-

bilization of alkyl-phosphonates or phosphates

into a polymer (most often plasticized PVC) film

[4–8]. But this method of immobilization suffers

from serious drawbacks such as the ionophore

leaking into the solution and the poor adhesion of

the polymer film to the silica surface [9]. In con-

trast with ISEs, the hydrophobic character andthe mechanical strength of the polymer film are

no longer absolute requirements for ISFET tech-

nology. As an example, the immobilization of

phosphonate molecules into a water-swollen gel

was studied as an efficient alternative [10]. The

leaking into water was avoided by the electrostatic

interaction between the negative phosphonate

groups and the cationic cross-links of the gel.

Another alternative that was studied in the

present work consisted of the direct grafting of

the complexing molecule (phosphonate) onto the

silica surface of the transducer. This method

could be successfully applied to the detection of

Ag+ or K+ ions using neutral complexing

molecules grafted on the silica grid of ISFETs[11,12]. An attempt towards the detection of

Ca2+ ions with grafted phosphonate groups re-

sulted in a poor sensitivity [13]; the slope of the

electrical response (potential versus log[Ca2+])

was only 7 mV per decade at pH 10, and the

detection limit was quite high (10−3 M). The poor

performances were ascribed to the low yield of the

chemical grafting procedure involving several

steps at the FET surface.

The first aim of the present work was to im-

prove the sensitivity of calcium-sensitive ISFETsby the use of a more efficient one-step grafting

procedure. The second part was devoted to a

detailed study of the grafting process as used in

our previous works. This paper is constructed as

follows. First, the two grafting processes, the mul-

tistep and the one-step, are described and the

responses of the ISFETs are compared. The

fitting of the site-binding model to the data indi-

cates low grafting densities. Second, a study of the

multistep grafting process, step by step, at the

surface of a silica powder of high specific areasheds light on the superiority of the one-step

process. Finally, the ability of the phosphonate

groups for the complexation of calcium ions at

the silica surface was checked by means of elec-

trophoretic measurements.

2. Experimental

2 .1. Elaboration of the ISFETs by the multistep

process

The reagents used, the synthesis of sodium

dimethylaminopropylphosphonate and the differ-

ent chemical steps carried out at the surface of the

FETs have already been described in previous

work [13].

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2 .2 . Synthesis of the silylated phosphonate

molecules

2 .2 .1. Synthesis of the diethylallylphosphonate

Stoichiometric amounts of allyl bromide and

triethylphosphite were heated at 80°C under a dry

argon gas flow for 12 h. Diethylallylphosphonate

was recovered with 90% yield by distillation [14].

1H-NMR in DMSOd 6 : 1.22 (t, 6H); 2.64 (dd,2H); 3.99 (quint, 4H); 5.2 (m, 1H); 5.7 (m, 2H).

2 .2 .2 . Synthesis of the silylated

diethylallylphosphonate molecules

A mixture of 20 mmol diethylallylphosphonate

and 25 mmol chlorodimethylsilane containing 1%

Wilkinson catalyst (tris(triphenylphosphine)-

chlororhodium) were stirred for 24 h at room

temperature under a dry argon atmosphere. The

excess chlorodimethylsilane was then removed un-

der reduced pressure. The 1H-NMR study of theliquid indicates complete conversion of the diethy-

lallylphosphonate (the lines at 5.2 and 5.7 ppm

have disappeared), and was analyzed as a mixture

of (A), (B) and (C) with the mole fractions 0.17,

0.65 and 0.18, respectively.1H-NMR in CDCl3 (ppm from TMS): (A) 0.29

(s, 6H); 0.7 (m, 2H); 1.275 (t, 6H); 1.6–2.1 (m,

4H); 4.1 (m, 4H); (B) 0.26 (s, 6H); 0.7 (m, 2H);

1.275 (t, 6H); 1.6–2.1 (m, 4H); 4.1 (m, 4H); (C)

0.29 (s, 6H); 0.7 (m, 1H); 1.13 (d, 3H); 1.275 (t,

6H); 1.6–2.1 (m, 2H); 4.1 (m, 4H).

2 .3 . Grafting of the FETs

2 .3 .1. Hydroxylation of the silica surfaces

Since the silica surface of the FETs has been

obtained by means of a direct oxidation of silicon

by oxygen, it is not reactive towards organosi-

lanes. A hydrolysis of the surface disiloxane

bridges into silanol groups was carried out in

sulfochromic acid for 15 min prior to the grafting

reaction [15]. This treatment also allows efficientcleansing of the surface. The surfaces were thor-

oughly rinsed with pure water and dried under

vacuum at 140°C for 2 h. This moderate drying

temperature allows the removal of adsorbed water

but retains the surface silanol groups for the

grafting reaction.

2 .3 .2 . Grafting of the silylated phosphonates onto

the silica grids of FETs

For the grafting of the phosphonates, a drop of

mixture of silylphosphonates was directly de-

posited on the silica surface of the ISFET grid

and the reaction was performed at 70°C for 48 h

in sealed thick glass tubes. No solvent was used.

This procedure differs from the two-step proce-

dure (‘impregnation’ and ‘condensation’) used inour previous studies [11,12,15]. The electrical con-

tacts have been protected by means of a varnish

layer during the grafting with chlorosilanes in

order to prevent the corrosion by the hydrogen

chloride evolved during the grafting reaction. The

grafted ISFETs were rinsed with tetrahydrofuran

(THF) several times in order to remove the excess

silane.

2 .3 .3 . Deprotection of the phosphonate group at

the silica surfaceThe ethyl esters were cleaved with bro-

motrimethylsilane [16]. Thus, a drop of bro-

motrimethylsilane deposited on the grid of the

ISFETs was heated at 60°C for 3 h. The very

labile trimethylsilyl esters formed in these condi-

tions were hydrolyzed with water at room

temperature.

2 .4 . Measurements of the ISFET response

The FETs produced at the Centre Interuniversi-taire de MicroElectronique of Grenoble (France)

were the same as previously [13]. The potential

variation induced by the presence of increasing

concentrations of Ca2+ ions was measured for the

ISFET. The measurements were carried out at

constant current with the source follower elec-

tronic circuit and in a differential mode, where the

potential difference between a measurement IS-

FET and a reference FET were measured [17].

The measurement ISFET was sensitized by the

grafted phosphonate groups as already described.The reference FET of the same type has not been

grafted and could not bind Ca2+ ions, its surface

was bare silica. The difference between these two

potentials DV was made in a differential amplifier.

This mode of measurement allows an efficient

compensation of most of the drifts. The sensitivity

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of the silica surface to pH variations is also

partially compensated in this mode. The measure-

ments were carried out at pH 10 (Tris buffer) and

in 0.5 M KCl as a background electrolyte.

2 .5 . Multistep grafting of the silica powder

The reactions on the silica powders were essen-tially identical with those carried out on the IS-

FET grid described previously [13]. The silica

surface was hydrolyzed in boiling water for 3 days

and the silica powder was dried at 140°C under

vacuum for 2 h. The ‘impregnation’ step of the

grafting of (3-chloropropyl)dimethylchlorosilane

consisted of the immersion of the silica powder

into a solution of the silane in isopentane, the

isopentane was then evaporated at −30°C under

reduced pressure and the second step of ‘conden-

sation’ was carried out by heating the silica pow-

der with the silane adsorbed at its surface at 70°C

for 48 h. Two successive extractions of 24 h with

THF in a Soxlhet apparatus were performed for

the complete removal of unreacted molecules. The

powders were dried at 100°C under vacuum for 2

days. For the next two steps, leading to the com-

pounds G2 and G3 shown in Fig. 1, the silica

powders were dipped into the reaction medium as

in the case of ISFETs, and were washed with

THF in a Soxlhet apparatus and dried under

vacuum.

2 .6 . Analysis of grafted silica powders by

spectroscopic techniques

IR spectra of grafted silica powders were

recorded by the diffuse reflectance technique with

a Nicolet 20SX FTIR spectrometer equipped for

diffuse reflectance infrared Fourier transform

(DRIFT) spectroscopy. The diffuse reflectance

data were expressed in Kubelka–Munk units: K /

S =(1−R

)2/2R

.13C- and 29Si-NMR spectra were measured by

cross-polarization and magic angle spinning (CP/

MAS). Thus, the magnetic polarization of the

protons was transferred to the 13C or 29Si nucleibecause of their dipolar coupling. Magic angle

spinning allowed a narrowing of the NMR lines

by averaging out the dipolar interactions, so that

high-resolution spectra could be obtained with

solid samples. The advantage of this technique is

that the nuclei such as the 29Si of bulk silica,

which are not close enough to protons, are not

observed because their dipolar coupling is too

weak. This technique allows the observation of

surface species with a good sensitivity. A Bruker

AC200 spectrometer equipped with a rotor forMAS allowed the observation of the 13C and 29Si

nuclei at 50.3 and 39.8 MHz, respectively. For

each sample containing about 500 mg silica pow-

der, 2000–4000 scans were accumulated; the rotor

spinning rate was 4 kHz, and the contact times

were 2 and 7 ms for 13C and 29Si observations,

respectively [18].

2 .7 . Electrophoresis

Electrophoretic mobilities were determined witha Rank-Brothers Mark2 device equipped with two

platinum electrodes in a flat cell. The rates of the

particles were measured by their direct observa-

tions at one of the stationary planes with a cam-

era. Consistent values of the velocities could be

obtained by averaging ten measurements. TheFig. 1. Multistep grafting process.

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Fig. 2. Target silylated protected phosphonate molecule.

described in the literature, which differ by their

type of leaving group and their functionality [19].

The most common leaving groups are alkoxysi-

lanes (methoxysilanes or ethoxysilanes), chlorosi-

lanes or (dimethylamino)silanes [20– 22]. This

short list is not intended to be exhaustive. The

functionality is the number of leaving groups

borne by the silicon atom such as for trichlorosi-

lanes, dichloromethylsilanes and chlorodimethyl-silanes in the chloro leaving group series. Our

choice was led by two important requirements

[23]: the grafted layers have to be monomolecular

and dense. The trouble with the use of multifunc-

tional silanes is the polycondensation at the sur-

face in the presence of traces of water, which leads

to thick and ill-defined grafted layers. The choice

of a monofunctional silane avoids this problem,

but monofunctional silanes are much less reactive

than multifunctional ones [24]. The chloro leaving

group was then chosen among the most reactiveones.

There is a chemical incompatibility with the

presence of the chlorosilane group and a phos-

phonic acid or phosphonate salt on the same

molecule. The phosphonate group has then to be

protected with a group that should be easy to

cleave after the grafting process on the silica

surface. The ethyl ester can be cleaved smoothly

by the reaction of bromotrimethylsilane and sub-

sequent hydrolysis by water at room temperature

[16]. Thus, the selected target molecule was di-ethyl - 3 - (chlorodimethylsilyl)propylphosphonate

(A) (Fig. 2).

The synthesis of A was carried out in two steps:

the synthesis of diethyl(allylphosphonate) and the

hydrosilylation of the double bond by

chlorodimethylsilane. In the course of the hy-

drosilylation, an intramolecular side reaction was

observed where the chlorosilyl group cleaved the

ethyl ester group at the phosphonate end of the

molecule. The formation of a cyclic silyl phospho-

nolactone resulted as shown in the reactionscheme of Fig. 3. The hydrosilylation in the b-po-

sition instead of the usual terminal attack in the

a-position also took place to a small extent with

the Wilkinson catalyst, where the silicon atom

bound to the carbon C2 instead of the C3. The

regioselectivity of the hydrosilylation reaction is

electrophoretic mobilities U E were calculated as

the ratio of the particle mean velocity to the

electric field. Grafted silica particles were dis-

persed at 0.02 g l−1 i n a 5×10−3 M NaCl

solution as a background electrolyte for the mea-

surements; the pH was varied by additions of

concentrated HCl or NaOH solutions.

3. Results and discussion

3 .1. Synthesis of the ready -to- graft molecules and

grafting processes on the FET surface

3 .1.1. Multistep grafting process

The multistep grafting process, which has been

described previously [13], is shown in Fig. 1. Since

this process involved several chemical reactions

carried out for long times at the surface of the

silica transducer, a more direct grafting scheme is

introduced in the present work. Because it should

reduce the opportunities for side reactions, it wasexpected that a simpler grafting process would

afford a higher grafting density and a cleaner

surface chemistry. A ready-to-graft molecule was

designed as follows, allowing a one-step grafting

process, similar to the grafting of nitrile groups or

crown ethers previously reported [11,12].

3 .1.2 . Synthesis of the ready-to- graft

phosphonates

A one-step grafting process required the synthe-

sis of a multifunctional molecule containing aphosphonate group at one end and a reactive

centre towards the silica surface at the other end

of the molecule. The function that allowed the

grafting onto the silanol groups of the silica sur-

face was the monochlorodimethylsilane group.

There is quite a large variety of reactive silanes

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Fig. 3. Reaction scheme of the silylated phosphonates ( A), (B)

and (C).

3 .1.3 . Grafting of the silylated phosphonates

The grafting reaction of chlorosilanes on sur-

face silanol groups implies the nucleophilic attack

at the silicon atom of the chlorosilane. The reac-

tion releases the leaving group as Cl−, a n d a

siloxane bridge is formed between the surface and

the graft. The cyclic silyl ester (B) can react in the

same way with the phosphonate as the leaving

group. Because of the cyclic nature of thismolecule, the ring-opening reaction yields a phos-

phonate group at the free end of the graft (Fig. 4).

The reactivity of the phosphonic silyl ester to-

wards the silanol group is unknown. As a conse-

quence, it cannot be clearly established whether

the cyclic ester (B) contributes to the grafting

process. The smooth deprotection of the ethyl

esters was performed by bromotrimethylsilane fol-

lowed by hydrolysis by water at room tempera-

ture [16]. All the surface reactions are summarized

in Fig. 4.

3 .2 . Electrical response of the ISFETs towards

calcium ions

The potential variation induced by the presence

of Ca2+ ions was measured for the ISFETs in the

differential mode. The measurements were carried

out in 0.5 M KCl background electrolyte and at

pH 10, where the phosphonate groups at the

known to depend on the catalyst and substituents

at the silicon atom in quite a disconcerting man-ner [25]. A mixture of different silylated phospho-

nates was obtained with the relative abundances

given in Fig. 3. The mixture was used without

purification for the grafting of the silica surfaces.

It contains the cyclic silyl ester (B) as the major

component, but it is difficult to know which com-

pound actually reacts with the surface silanols

during the grafting process because the excesses

used are so large.

Fig. 4. Different grafting reactions occurring at the silica surface with the silylated phosphonates (A) and (B). The grafting reaction

of (C) is similar to that of (A).

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Fig. 5. Response to the Ca2+ ions of the ISFET in differential

mode at pH 10. , ISFET grafted according to the one-step

process, and theoretical prediction (solid line) according to the

site-binding model (see text); , ISFET grafted according to

the multistep process, the solid line is a guide for the eyes only.

Si−(CH2)3−PO32−+CaS

2+X

Si−(CH2)3−PO3−Ca (1)

where the concentrations of the different species

at the surface are related according to

K =[Si−(CH2)3−PO3−Ca]

[Si−(CH2)3−PO32−][CaS

2+](2)

The concentrations of ionic species in the elec-trical double layer were expressed as a function of

the bulk concentrations according to the Boltz-

mann statistics, giving for the concentration of

Ca2+ ions at the surface

[CaS2+]= [Ca2+]exp− [(2e)/kT ] (3)

where is the electrical potential at the silica/elec-

trolyte interface as modified by the electrical

charge of the Ca2+ ions bound to the phospho-

nate surface groups. This leads to the following

expression relating the concentration of Ca2+ions in the bulk solution to the electrical potential

at the silica surface , the surface density of

phosphonate binding sites N S and the Stern ca-

pacity C S [17,29]:

log[Ca2+]=2e

2.3kT − log

2eN S

C S−1

− log(K )

(4)

For the ISFET grafted according to the one-

step process, the best fit to the experimental data

(Fig. 5) was achieved with log(K )=2.7 and N S/C S=8.5×1016 SI units. The fit was quite good,

showing the validity of the electrostatic model and

of the assumed complexation equilibrium at the

surface with a 1:1 stoichiometry. Taking a value

within the accepted range for the Stern capacity

as C S=20 mF cm−2, the surface density of phos-

phonate groups is N S=1.7×1016 m−2, corre-

sponding to nS=0.03 mmol m−2. This value of

the coverage is lower by a factor of 100 with

respect to the range usually found for the grafting

of chlorosilanes (3 mmol m−2). This low graft-ing density found in the model might be the

reason why the sensitivity for Ca2+ ions was so

low. But the model may also underestimate the

grafting density. The value of the complexation

constant K =102.7 was found in the expected

range, and compares well with the literature val-

surface were in their fully ionized form ( PO32−)

of the largest complexing ability. For concentra-tions of Ca2+ between 10−3 and 10−1 M, the

electrical response as a function of log[Ca2+] was

quasi-linear with a slope of 10 mV pCa−1, lower

by a factor of 3 than Nernst law (Fig. 5). This

response was larger (of higher slope) than for

ISFETs grafted with the multistep process, but

the main improvement of the single-step process

was the much larger concentration domain of

sensitivity.

The response time was very short (B1 s), the

drift was minimized by the differential measure-ment mode (0.01 mV h−1) and the lifetime was

larger than 2 months because of the chemical

grafting, as was already found in previous work

[11– 13]. These are the characteristics of the

present process where the complexing molecules

are directly grafted onto the silica gate of the

FET.

3 .2 .1. Prediction of the site-binding model

The potential variation caused by the complex-

ation of the Ca2+ ions at the silica surface wascalculated according to the ‘site-binding model’

[26,27] as modified for chemically grafted ISFETs

[17,28].

The complexation of the Ca2+ ions by the

phosphonate groups at the ISFET surface was

described by the simple 1:1 equilibrium as follows

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ues dealing with bulk aqueous solutions [30,31].

For the ISFET grafted by the multistep grafting

process, it was not possible to fit the site-binding

model to the experimental data. Deviations from

the simple complexation equilibrium (Eq. (1))

have non-negligible effects when the signal is so

weak.

As a summary, the one-step grafting process

gave a better sensitivity of the ISFET, but theresults are not satisfactory since the response is

only one-third that of the Nernst law. It should be

noticed that the response of ISFET grafted di-

rectly onto the silica gate never reaches the Nernst

law [11,12]. Only ISFETs covered with a thick

polymer layer where the complexing molecules

(ionophore) are immobilized have Nernstian re-

sponse [3–8].

In order to understand what was going wrong

in the multistep grafting process, a careful analy-

sis of each step has been carried out. This ispresented in the following.

3 .3 . Step -by-step analysis of the multistep

grafting process

The analysis of the reaction products at the

silica surface of ISFETs is difficult because of the

small area (10 000 mm2) of the grafted surface.

The multistep grafting procedure described previ-

ously was applied to silica powders of large spe-

cific area in order to allow quantitative chemical

and spectroscopic analyses of the grafted

molecules. Thus, fumed silica was selected as the

most similar to the thermal silica of the FETs.

Aerosil 200 from Degussa having a specific area

of 200 m2 g−1 was used, allowing direct elemental

analyses of the organic grafts for the carbon,

nitrogen, phosphorus and halogen elements, in-

frared spectroscopy by means of diffuse reflec-

tance (DRIFT) and solid-state NMR of the 13C

and 29Si nuclei by means of CP-MAS.

The multistep process as described in Fig. 1 was

analyzed at each step in order to find out the

precise origin of the failure in the overall grafting

process.

The same reactions as for the ISFETs were

carried out at the surface of the fumed silica, and

the systematic chemical analyses are discussed inthe following. The preparation of the silica sur-

face was the same as for the thermal silica of the

FET grid. Although it is known that the surface

of fumed silica is covered with a high density of

silanol groups, the hydrolysis and subsequent dry-

ing of the silica powder surface was performed in

the same way as for the ISFETs. The grafting of

the (3-chloropropyl)dimethylchlorosilane was car-

ried out in two steps as for the ISFETs. First, an

‘impregnation’ step consisted of the immersion of

the silica powder into a solution of the silane inisopentane, followed by the evaporation of the

isopentane. A layer of pure silane was left after

the isopentane was evaporated under reduced

pressure. The second step of ‘condensation’ was

carried out by heating the silica powder with the

silane adsorbed at its surface at 70°C for 48 h.

The main difference with the ISFETs was the

rinsing procedure. While it was quite easy to

remove the unreacted materials from the flat sur-

face of the ISFETs by simple washing in THF, a

more efficient extraction was necessary for the

powders because liquids were trapped by capillary

condensation in small pores. Two successive ex-

tractions with THF in a Soxlhet apparatus were

necessary for the complete removal of unreacted

silane molecules. The efficiency of the Soxlhet

extraction could be checked with samples after the

impregnation step where the silane was only

adsorbed.

Table 1

Elemental analyses of the grafted silica powders in the multistep grafting process

P (%)Compound C (%) H (%) Cl (%) I (%) N (%)

0.90 2.07G1 4.05

G2 :0 2.833.93 0.84

0.53 :0 :0 0.10:02.28G3

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Fig. 6. Diffuse reflectance IR spectra of the bare silica (bottom

data) and the chloropropyl grafted silica G1 (top data shifted

upwards by 0.2 Kubelka–Munk units).

excess was ascribed to the hydrogen atoms of the

residual Si OH groups. The C H stretching ab-

sorption bands of the grafts could be observed at

2900 cm−1 in the IR spectrum (Fig. 6). The C Cl

absorption bands around 700– 750 cm−1 could

not be observed with confidence because there are

too many different absorption bands in this

range. The intensity of the OH absorption band

at 3000 cm−1 from the surface Si OH groups of silica was significantly lowered because of the

grafting on these groups. The CP-MAS 13C-NMR

spectrum showed four lines at −2.6, 15.0, 26.9

and 47.1 ppm from TMS, as expected from the

chemical structure of the grafts (Fig. 7). Finally,

the CP-MAS 29Si-NMR spectrum (Fig. 8a)

showed three lines corresponding to the 29Si nuclei

magnetically coupled with neighbouring protons

and involved in the structural units, referred to as

Si O Si(CH3)2CH2 (13 ppm), Q3 (O3Si OH,

−100 ppm) and Q4 (SiO4, −110 ppm) [33–35].The line of the 29Si nuclei in Q4 units remained of

moderate intensity despite of the very large abun-

dance of Q4 inside bulk silica because such struc-

tural units are too far from the surface for an

efficient polarization transfer from the protons of

the grafts and Si OH groups. In summary, the

first step leading to G1 was successful and the

grafting density was high.

The chloro group was substituted for the iodo

group by the reaction with sodium iodide in

refluxing methanol in the second step. The ele-mental analysis of the grafted silica showed the

complete replacement of chlorine by iodine as

expected. The carbon-to-iodine weight ratio was

higher than expected according to the chemical

structure G2. The carbon excess found in the

elemental analyses was caused by residual

methanol, which was strongly adsorbed at the

silica surface and which could not be evaporated

during the drying. Indeed, the quasi-irreversible

adsorption of methanol was observed with a bare

silica powder sample (not grafted): after the im-mersion of the silica powder in boiling methanol

for 24 h, the removal of the methanol by means of

extraction with THF in the Soxlhet apparatus

could not reach completion, even after repeated

cycles of 24 h where the THF was renewed. The

use of methanol as the reaction solvent was dic-

Fig. 7. CP-MAS 13C-NMR spectrum of silica G1.

The grafting of the chlorosilane in the first step

was successful, the grafted amount per unit area

as calculated from the elemental analysis (Table 1)

of carbon or chlorine was in the same range as

reported in the literature [20,32,33]. According to

the specific area of 200 m2 g−1, the experimental

chlorine analysis of 2.07 wt.% corresponds to a

grafting density of 3 mmol m−2. Within the exper-

imental errors, the elemental analyses of carbonand chlorine were in agreement with the chemical

structure G1; the experimental carbon-to-chlorine

weight ratio was 1.96, against the theoretical

value of 2.03. The hydrogen analysis was in slight

excess as compared with the value expected on the

basis of the carbon and chlorine analyses; this

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tated by the solubility of the reagents; especially,

dimethylaminopropylphosphonate is soluble in

methanol and water only. The carbon elemental

analyses were then discarded for the samples that

have been immersed in methanol (G2 and G3).

The iodine content of G2 (mol m−2) was lower

than the chlorine content of G1 by a factor of 2.6.

This was ascribed to a cleavage of part of the

grafts during the reaction. The IR spectra of G2

remained essentially unchanged as compared with

G1, but this is not a quantitative analysis. The

yield of this step is then 40% because of the

cleavage of the grafts.

The final step was the quaternarization of the

tertiary amine of disodium dimethylaminopropy-

lphosphonate in refluxing methanol. The elemen-

tal analysis of G3 showed chlorine and iodine

contents lower than the limits of detection and the

presence of the phosphorus element. The nitrogen

content was lower than the detection limit of the

conventional nitrogen analysis. The sole phospho-

rus analysis could be used for the determination

of the yield of the third step. The very low phos-

phorus content of 1000 ppm corresponds to a

yield of 15% for the third step. This value is

consistent with the low nitrogen analysis. The

concomitant decrease of carbon and hydrogen

contents strongly suggests that a cleavage of the

grafts took place during the reaction. A loss of

grafting density was clearly observed at each step.

The reduction of the number of steps and reaction

time would be a benefit. Indeed, if the final two

steps were carried out in one pot (in a single step

where sodium iodide and the aminophosphonate

were reacted simultaneously), the phosphorus

analysis was higher (0.43%) but the overall yield

was still very low. The IR absorption bands of thephosphonate group at 980 and 1000 cm−1 could

not be observed because the strong absorption

bands of the silica hid them. The CP-MAS NMR

analyses confirm the loss of grafting density dur-

ing the successive reaction steps. Thus, the obser-

vation of the grafts by means of 13C-NMR was

unsuccessful because of the low signal-to-noise

ratio. A single line could be observed at 48.7 ppm,

which was ascribed to adsorbed methanol. The29Si-NMR spectrum (Fig. 8b) clearly showed that

the cleavage of the grafts took place at the

Si O Si bond. Indeed, the line at 13 ppm corre-

sponding to the silicon atom of the graft was very

weak for G3 as compared with that of G1.

As a conclusion, the grafting process where

several reactions are carried out successively at

the surface leads to low grafting densities because

of the cleavage of the grafts that occurs at each

step. An efficient grafting process should avoid

Fig. 8. CP-MAS 29Si-NMR spectra of silica G1 (a) and G3 (b).

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Fig. 9. Influence of the presence of Ca2+ ions on the elec-

trophoretic mobilities of the grafted silica powders as a func-

tion of pH. The n potentials calculated according to Eq. (5) are

given on the right-hand y axis.

ionic strength ensured by a 0.5 M NaCl back-

ground electrolyte (Fig. 9).

For the silica powders immersed in the calcium-

free electrolyte solution, the electrophoretic mo-

bility was of negative sign because the silica

surface groups and the phosphonate grafts were

both negatively charged. The electrophoretic mo-

bility, and thus the surface charge, decreased

when the pH decreased because of the protona-tion of the Si O− and phosphonate groups, and

reached zero when the surface was electrically

neutral. The isoelectrical point was found around

pH 3, which was larger than the isoelectrical point

of bare silica (pH 2) because of the presence of the

ammoniopropylphosphonate groups. This shift of

the isoelectrical point (and of the whole U E versus

pH curve) as compared with that of bare silica

shows that significant amounts of phosphonate

groups were grafted at the silica surface. Although

the coverage of grafts could not be estimatedfrom this data, it was large enough for inducing

an electrical effect. When a concentrated solution

of CaCl2 was added so that the Ca2+ concentra-

tion was 10−3 M, the whole U E versus pH curve

shifted towards the positive mobility values. This

clearly showed the decrease of the negative charge

of the surface by the adsorption of Ca2+ ions. It

appears from the present electrophoretic experi-

ments that the complexation of the Ca2+ ions by

the phosphonate groups at the surface of the

grafted silica was large enough for inducing asignificant electrical effect. This was observed de-

spite of the low coverage of grafted phosphonate

groups.

The influence of the Ca2+ ions was to shift the

electrical surface potential towards positive val-

ues. An order of magnitude of the surface poten-

tial shift can be obtained by looking at the

electrophoretic potential (n). The electrophoretic

mobilities can be converted into n potentials by

means of the Smoluchowski equation modified

according to Henry [36]

U E=2m 0mn

3pf 1(sa):

m 0mn

p(5)

where m and p are the dielectric constant and the

viscosity of water, respectively (m =80, p=0.9

mPa s), and f 1(sa) is the Henry correction factor.

such multiple steps and long reaction times in

protic solvents. The hypothesis of low grafting

density that was put forward in our previous

work [13] was correct, and its origin has been

elucidated.

4. The sensitivity of surfaces covered with

phosphonate groups towards calcium ions

In spite of the low grafting density, the ISFETs

had a measurable sensitivity towards calcium

ions. The sensitivity of ISFETs towards ions is

related to the variation of the electrical surfacepotential when ions are adsorbed at the silica grid

surface. The surface potential was thus measured

at the surface of the grafted silica powder by

means of electrophoresis. The electrophoretic mo-

bility of particles as their suspension in water is

related to the electrical potential at the shear

plane. This plane is close to the surface of the

particles but slightly apart from the true water–

silica interface. Because of the small size of cal-

cium ions, their presence at the surface cannot

affect significantly the location of the shear plane.Thus, any variation of electrophoretic mobility

induced by the presence of calcium ions is due to

a variation of the electrical potential better than

to a shift of the shear plane. These measurements

were performed without and in the presence of

calcium ions as a function of pH, and at constant

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In the present case where the background elec-

trolyte concentration is quite high, the Debye

length s−1 is small (4.3 nm for 5×10−3 M

NaCl) and the radii of the particles that can be

observed are larger than the resolution of the

optical microscope. Then, f 1(sa):3/2, close to its

asymptotic limit at large sa. The values of the n

potentials are given on the right-hand axis scale of

Fig. 9.The shift of n potential induced by the presence

of the Ca2+ ions was larger at large pH values.

The effect of the presence of Ca2+ ions was rather

insignificant for pHB4. This dependence on the

pH was quite similar to that observed for the

response of the ISFETs [13], as expected. This is

ascribed to the larger complexing power of the

phosphonate groups in basic media. The complex-

ation of alkaline earth ions by the fully ionized

phosphonate group ( PO32−) present at high pH is

stronger than that of the hydrogenphosphonate( PO3H−) and phosphonic acid ( PO3H2) groups,

as already discussed [13].

5. Conclusions

The grafting of phosphonate groups at the sur-

face of the silica gate of FETs could be achieved.

A grafting process where the number of chemical

reactions carried out at the surface is a minimum

has to be preferred. Indeed, in a process whereseveral reactions of the SN2 type were performed

on the surface, a dramatic loss of grafting density

was observed at each step. The loss of the grafts

occurred because of the cleavage of the Si O Si

bonds. The grafting process in one step improves

the sensitivity of the ISFET towards Ca2+ ions:

the observed slope was higher than in our previ-

ous work and the domain where the ISFET was

operative was shifted considerably towards lower

concentrations of Ca2+. The site-binding model

works well in predicting the experimental datasince the complexation constant used in the appli-

cation of the model was found in a realistic range.

The other interesting conclusion from this model

was a low grafting density, which could be the

main origin of the moderate slope of the DV

versus log[Ca2+] data. This reveals the difficulties

for the grafting of the thermal silica of the FET

insulator as compared with the precipitation silica

or fumed silica used for liquid chromatography

purposes.

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