Polyethyleneimine as a pH sensitive film for potentiometric transducers

Materials Science and Engineering C 14 Ž2001. 47–53 www.elsevier.comrlocatermsec

Polyethyleneimine as a pH sensitive film for potentiometric transducers Karine Reybier a,) , Soufiane Zairi b, Nicole Jaffrezic-Renault b, Guillaume Herlem a , Albert Trokourey c , Bernard Fahys a a

Laboratoire de Chimie des Materiaux et Interfaces, UFR Sciences and Techniques La Bouloie, 16 route de Gray, 25030 Besanc¸on Cedex, France ´ b Laboratoire d’Ingenierie et de Fonctionnalisation des Surfaces, UMR-CNRS ECL-Lyon, BP 163, 69131 Ecully Cedex, France ´ c Laboratoire de Chimie-Physique, UniÕersite´ de Cocody, UFR Sciences des Structure de la Matiere ` et de la Technologie, 22 BP 582, Abidjan 22, IÕory Coast Received 7 March 2000; received in revised form 3 May 2001; accepted 11 May 2001

Abstract The anodic oxidation of ethylenediamine ŽEDA. leads to the formation of polyethyleneimine ŽPEI. as a pH sensitive thin film on conducting and semi-conducting electrode surfaces. The passivated electrodes exhibit a sub-nerstian response which depends strongly on the electrode material. In particular, the PEI-coated silicon electrodes present a linear pH sensitivity close to 50 mV per pH unit over a large pH range. This response could be fitted with the site-binding model applied to the acido-basic equilibrium of amino groups present at PEIrelectrolyte interface. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Polyethyleneimine; Potentiometric sensor; pH sensitivity

1. Introduction Conventional pH electrodes are commonly based on a pH sensitive glass membrane. An alternative to the use of a glass bulb is a structure containing a metal oxide which acts as the active component. pH response has been observed for certain types of electrically semiconducting oxides, in particular iridium, platinum and ruthenium dioxides w1,2x. Metallic oxide-based electrodes have some advantages over glass electrodes; they are easier to miniaturize. However, they are toxic. Recently, the use of macromolecules appeared to be of considerable interest in microsystems and microsensors. The particular features of these materials are their low cost of fabrication and the extreme ease of deposition as thin film w3,4x. It is the case w5,6x or conductive for macromolecules as calixŽn.arenes ` polymers such as polypyrrole or polyaniline w7x, which have been intensively studied as sensitive membrane for chemical microsensors over the last decade. Another polymer used widely in sensors is polyethyleneimine ŽPEI. w8,9x. Most applications using polyethyleneimine are determined by its surface activity and its ability to form complexes with anionic polyelectrodes w10x

)

Corresponding author. Tel.: q33-3-81666538; fax: q33-3-81666288. E-mail address: [email protected] ŽK. Reybier..

and with metal ions w11x. PEI has been widely used in a number of industrial immobilizing biosystems w12x for its high concentration of amino groups. Branched polyethyleneimines are generally synthesized as a special case of ring-opening polymerization of aziridine via a nucleophilic addition mechanism. Recently, one of us showed that anodic oxidation of ethylenediamine ŽEDA. leads to the formation of linear polyethyleneimine coatings as thin film at the electrode surface w13,14x in one step w15x. The aim of this paper is to demonstrate that a potentiometric pH response is obtained for different electrode materials electrochemically modified by PEI through anodic oxidation of ethylenediamine, since amine sites of polymer are available for protonation w16x. A variety of materials is modified, the potentiometric response is studied for these materials and fitted with site-binding model.

2. Experimental 2.1. Material and electrode preparation All chemicals used were obtained from Aldrich Žpurest quality available, purity) 99%.. The amines and the salts were used without any further purification and the electrolyte solutions were prepared in a glove box under a dehydrated argon atmosphere, where electrochemical stud-

0928-4931r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 8 - 4 9 3 1 Ž 0 1 . 0 0 3 1 1 - 3

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K. Reybier et al.r Materials Science and Engineering C 14 (2001) 47–53

Fig. 1. Voltammogram at a glassy carbon electrode of EDA charged with LiSO 3 CF3 0.1 M. Scan rate: 10 mVrs. Room temperature.

ies were performed. The voltammograms were carried out on an AUTOLAB PGSTAT 20 potentiostat–galvanostat from Ecochemie, using a three-electrode set-up. The counter-electrode was platinum and the reference electrode was a silver wire. The reference electrode Žnoted SRE. was placed in a separated compartment to avoid diffusion of Ag ions w17x. All experiments were carried out at room temperature Ž293 K.. The solutions were purged by ultrahigh purity argon. Before deposition of PEI, the electrodes were pretreated: glassy carbon and gold surfaces were polished with alumina, then sonicated in pure alcohol for 10 min w18x. Alumina used for polishing was 2–3 mm in diameter obtained from ESCIL, France. The semiconducting material used in this study was a p-type Si ²100: Žresistivity; 1.5–5 V cm.. In this case, two treatments have been considered. Ža. Si ²100: samples were cleaned with acetone and then chemically etched in 2% HF in 0.1 M H 2 SO4 for 1 min and then dipped in a 40% NH 4 F solution for 6 min w19x, in order to prepare atomically flat surfaces terminated by a layer of H w20x Želectrode .. Žb. Si ²100: samples were etched with the classical sulphochromic mixture for 40 s and rinsed with pure ethanol to generate Si–OH on the surface w21x Želectrode .. The surfaces were then dried at 45 8C under an argon stream.

ance of an anodic irreversible plate of current after the oxidation peak, then a downward shift in the current and disappearance of the oxidation peak for the remaining scans w22x as shown for glassy carbon in ŽFig. 1.. This passivation effect is in agreement with a total dense coating of the electrode surface. Finally, the electrode was rinsed with pure acetone and dried at 45 8C under an argon stream. The EDA electropolymerization process obtained on silicon surface is very specific, as shown on the corresponding cyclic voltammograms plotted in ŽFig. 2.. A sharp EDA oxidation peak appears clearly at 2.1 V vs. SRE, and then the passivation effect appears on silicon electrodes as well as on conducting electrodes. The electrode was dried for 1 day before testing their potentiometric pH response. The total thickness of PEI passivating layers, determined by total reflection spectrum FTIR analysis was approximately 30 mm w9x. However, analysis performed by spectroscopic ellipsometry on Si ²100: demonstrate that after rinsing with water, the coating become uniform with a thickness of 2 and 3 nm w23x. Indeed, the polymer fragments that are not strongly attached to the electrode surface may be eliminated, which gives rise to a uniform coverage, which was not the case before rinsing. 2.3. Measurements The response of all the modified electrodes in aqueous buffer solutions was measured using a high impedance voltmeter ŽTacussel millivoltmeter.. Testing were performed using the device presented in ŽFig. 3.. The potential was determined vs. a Saturated Calomel Electrode ŽSCE. ŽRadiometer.. The buffer solutions obtained from Prolabo ŽpH s 1, 4, 7, 8, 9.23 and 10. had an identical ionic strength. The pH measurements were performed

2.2. PassiÕation process All the substrates Žglassy carbon, gold, silicon., when anodically biased in ethylenediamine charged with Li SO 3 CF3 , are passivated by a coating. Many observations performed on the passivating film, such as SEM, Raman and IR-ATR spectroscopies, demonstrate that it has a structure similar to polyethyleneimine w13x. For each electrode, the cyclic voltammograms obtained during electrode preparation present characteristics of passivation: appear-

Fig. 2. Voltammogram at a Si ²100: p-type electrode treated in HF of EDA charged with LiSO 3 CF3 0.1 M. Scan rate: 20 mVrs. Room temperature.

K. Reybier et al.r Materials Science and Engineering C 14 (2001) 47–53

Fig. 3. Scheme of the potentiometric pH electrode device.

using a SCHOTT pH-meter CG840. All measurements were carried out at room temperature. At each change of buffer, the electrode was rinsed in deionized water. The set-up consisted of a three-electrode cell w24x connected to Voltalab40 ŽRadiometer Analytical .. Capacity measurements were carried out with a working electrode in silicon p modified by electrochemical oxidation of EDA, after treatment in sulphochromic acid Želectrode .. A slope of tension was applied to the working electrode, and a modulation of 5 mV amplitude and 10 kHz frequency was superimposed. A solution of ammonium acetate 0,5 M was used as buffer solution pH s 7 in which were added increasing quantities of hydrochloric acid. The sensor response was quantified through flat band potential VFB shift, which was given starting from C Ž V . curve at 60% of the value of the accumulation capacity. Measurements were performed at 298 K. All experiments on silicon were performed in the dark.

3. Results and discussion 3.1. Potentiometric measurements Õs. pH for different passiÕated electrodes 3.1.1. Conducting materials 3.1.1.1. Glassy carbon. Testing was carried out in seven different pH buffers. Glassy carbon electrode was previously tested as a pH sensor without any coating at the surface; the potentiometric response vs. pH is linear and the slope is close to 5 mVrpH Žcf. Table 1.. For each measurement, potential of the coated electrode was allowed to reach equilibrium, equilibration time was found to be approximately 10 min. For a pH range 4–10, the response of the electrode vs. pH was linear. For pH less than 4, potential remained stable that could be interpreted in terms of a saturation effect, the proton concentration

49

being too high compared to the amine sites of polymer available for protonation in the polymer. The slope of the pH curve of the modified electrode is about 30 mV per pH unit Žcf. Table 1. compared to 59 mV per pH unit for a glass microelectrode w25x. This sensitivity is relatively weak compared to sensitivity recorded with ruthenium oxide Ž40–50 mVr pH. w26,27x but close to those obtained with other oxides such as Pd–O Ž15–20 mVrpH. or TiO 2 Ž10–30 mVrpH.. Furthermore, these values are lower than sensitivity obtained in the case of polymeric matrix such as polypyrrole deposited on GC Ž40 mVrpH. w28x. In order to study the ageing of this GC modified electrode, several tests have been performed with the electrode Žcf. Table 1.. Between each series of measurements, the electrode was dried and kept 1 day under atmospheric pressure. The electrode response decreases from 35.7 to 25.6 mV per pH when tested five times. These results suggest that the surface of the electrode evolves during measurements in aqueous buffers. 3.1.1.2. Gold. The passivation process takes place on gold with an EDA oxidation potential equal to 2 V vs. SRE. As for glassy carbon, the gold electrode was dried for 1 day before measurements. Equilibration time is longer than for glassy carbon, the passivated electrode was allowed to equilibrate for 15 min before the open circuit potential was measured. A potentiometric response was obtained for a bare gold electrode Žabout 6 mvrpH. and experimental points were scattered Ž R s y0.96.. For passivated gold electrode, potential presents a linear variation for a pH interval from 3 to 9. Sensitivity of the electrode increases from one test to the other from 9 to 12.2 mVrpH Žcf. Table 2.. From the 3rd test, response of the electrode seems to become stable. Lifetime of passivated gold electrode is slightly longer Žseven tests. than that of passivated glassy carbon electrodes Žfive tests.. Another gold electrodes as well as carbon electrodes was passivated and tested as pH sensors in order to study reproducibility. Potential values obtained in this case were relatively close to those obtained for the first electrode Ž10%.. 3.1.2. Semiconducting materials 3.1.2.1. Potentiometric measurements. Measurements carried out with H terminated silicon electrode in buffer

Table 1 Potentiometric pH response of a PEI-coated glassy carbon electrode

Table 2 Potentiometric pH response of a PEI-coated gold electrode

GC electrode

Au electrode

Bare electrode

PEI test 1

PEI test 2

PEI test 3

PEI test 4

PEI test 5

Slope of potential–pH curve ŽmVrpH.

y6

y9.9

y9

y12

y12.8

y12.2

Bare PEI electrode test 1

Slope of y5 potential–pH curve ŽmVrpH.

PEI test 2

PEI test 3

PEI test 4

PEI test 5

y35.7 y34.1 y33.6 y31.1 y25.6

K. Reybier et al.r Materials Science and Engineering C 14 (2001) 47–53

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solutions revealed a quasi linear potential response with a fast response time Ž5–7 min. compared to passivated conducting electrodes. Furthermore, potential response is linear for a large range of pH including very acidic pH ŽpH s 1–11.. Sensitivity varies irregularly with tests from 12 to 21 mV vs. SCE per pH unit Žcf. Table 3.. A polymer-free electrode was tested, potential response in this case is very low and not linear Žabout 10 mVrpH.. After 3 days, sensitivity for pH decreased to the level obtained with a bare surface. For this reason, another way of preparing was considered using sulphochromic acid Želectrode .. A bare Si–OH electrode tested for its pH sensitivity reveals a quasi linear behaviour due to the surface nature, with a low sensitivity: 15 mVrpH unit. After passivation by PEI and several hours of drying under argon atmosphere, the electrode was tested as a pH sensor. The electrode was allowed to equilibrate for 8 min before the open circuit potential was measured. The electrode exhibits a linear response even for high pH after few days. Results are very interesting since, as shown in Table 4, except for the two first tests, pH response is high and increases along the different tests from 40 to 50 mV per pH unit which is close to the Nerstian response. Furthermore, lifetime of the electrode is increased compared to the Si–H electrode, since after 6 days the electrode is still active. This difference of behaviour between Si–H and Si–OH electrodes modified by PEI could be interpreted by the well-known strong adsorption of the amine groups on the Si–OH surface. 3.1.2.2. Capacity measurements. Capacity measurements were performed to correlate capacity and Hq ion adsorption on the polymer for the SirPEIrelectrolyte structure. Indeed, taking into account the electrically blocking character of these structures, C Ž V . measurements will exhibit their sensitivity for Hq ions present in the electrolyte w23x. Fig. 4 presents sensor response for Siliconr PEI r electrolyte structure. Capacity measurements reveal a shift of the initial curve ŽpH s 7. and consequently a shift of the flat band potential when the proton concentration increases. This shift is the expression of the field effect, which takes place in the depletion layer at silicon surface when amine groups in the polymer are protonated. DVFB is equal to the variation of c 0 potential at the PEIrelectrolyte interface introduced in Eq. Ž5. w30x Žcf. Section 3.2.. Fig. 5 presents experimental results DV FB vs. pH, the

Table 4 Potentiometric pH response of a PEI-coated Si–OH ²100: p-type Želectrode . Si–OH electrode Bare PEI electrode test 1 Slope of potential–pH curve ŽmVrpH.

y15.5

PEI test 2

PEI test 3

PEI test 4

PEI test 5

y10.5 y10.5 y33.6 y42.2 y48.6

mean sensitivity is found to be 45 mV per pH unit. This intrinsic sensitivity is higher than that of thermal silica which has a sensitivity lower than 16 mVrunity of pH w29x. From the structure capacity in the accumulation mode, it is possible to deduce the thickness of the PEI layer, at pH 7, with a dielectric constant of 3 w19x. The thickness was found equal to 2.6 nm, which is in good agreement with the value obtained by ellipsometric measurements. When pH becomes more acidic, the capacity in accumulation mode decreases, which means that a change in dielectric constant and in thickness of PEI film occurs, due to the protonation equilibrium of amine groups in polymer structure. This effect can explain the rapid ageing of modified electrodes in acidic media Žcf. Section 3.3.. 3.2. Theoretical approach PEI polymer, of general formula Ž –NH–CH 2 –CH 2 – . n contains amine groups Žsurface concentration per cm2 : Ns ., schematized as ) N–H, which are involved in the acidobasic equilibrium, according to the site-binding theory w30x ŽEq. Ž1..: q ) N y H q Hq s l) N y H 2

KH s

) N y Hq 2

w ) N y H x Hqs Ž 1.

which leads to a variation of the surface charge s 0 of the polymer when concentration of wHqx in the solution varies.

Table 3 Potentiometric pH response of a PEI-coated Si–H ²100: p-type Želectrode . Si–H electrode

Bare PEI electrode test 1

Slope of y10 potential–pH curve ŽmVrpH.

PEI test 2

PEI test 3

PEI test 4

PEI test 5

y21.5 y17.7 y12.7 y31.1 y19.0 Fig. 4. Capacity measurements performed for a Si ²100: p-type electrode for a modulation frequency equal to 10 kHz.

K. Reybier et al.r Materials Science and Engineering C 14 (2001) 47–53

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Experimental points DV FB –pH were fitted with expression Ž5., DV FB being equal to c 0 . Simplex minimization method was used. The best fit is represented in Fig. 5 as a full line. Ns was found equal to 5.2 = 10 13 cmy2 and p K H is y6.5. This value of p K H is closer to the value for tertiary amines such as triethanolamine Žp K H s y7.8. than to the value for primary amines Žp K H ; y10.. The fitting of potential–pH experimental points obtained for glassy carbon and gold was performed, keeping constant the p K H value. Ns , for glassy carbon, is found equal to 1.7 = 10 13 cmy2 and for gold 5.4 = 10 12 cmy2 . The PEI film appears to be less dense on the conducting materials than on Si–OH electrode. Fig. 5. Shift of the flat band potential DVFB Ž ( c 0 . vs. pH of a PEI-coated Si ²100: p-type electrode.

Concentration wHqx in the solution is related to the surface x Ž Ž .. concentration wHq s by Eq. 2 : q Hq s s w H x exp

ž

yqC 0

kT

/

Ns s ) N y Hq 2 q w) N y Hx

s0 s q

Ž 3. Ž 4.

.

Combining the formulae, relationship between C 0 and pH can be treated theoretically as a Nernstian law: pH s y

qC 0 2.3 k T

q log

ž

qNs

C 0 Cs

In order to determine alterations in the PEI coating under chemical exposure to buffer solutions, SEM imaging and voltamperograms were performed on PEI modified surfaces after potentiometric measurements in buffer solutions.

Ž 2.

where C 0 is the potential drop in the double layer at the PEIrelectrolyte interface. C 0 is related to the surface charge s 0 by: C 0 s Ž s 0 .rŽ Cs . where Cs is the capacity of the double layer. Amine site concentration at the electrode surface, and surface charge can be expressed as: ) N y Hq 2

3.3. Stability of PEI coating

/

y1 ypKH .

Ž 5.

Variation of C 0 can be measured through potentiometric response of PEI-modified electrode. Sensitivity depends on two factors: Ns and p K H .

3.3.1. Glassy carbon SEM imaging was firstly performed on a freshly passivated GC electrode and then on the same electrode after being used for 5 days of potentiometric measurements. Fig. 6a presents the SEM image of a bare electrode surface and Fig. 6b and c presents the same electrode freshly coated with PEI and after 5 days in buffer solutions, respectively. Fig. 6b highlights a specific clover structure of the polymer: in some spots there are clover like aggregates. On the contrary to Fig. 6b, Fig. 6c shows a heterogeneous surface with free and passivated areas. From these observations, it appears that PEI coating is altered during measurements. In order to study the influence of acidic or basic pH on the PEI damaging process, GC electrodes were placed in buffer solutions pH s 1 or pH s 10. At different time intervals, one of the electrodes was taken out of the solutions, rinsed with acetone, dried at 45 8C under Ar

Fig. 6. Ža. SEM image of the surface of a free PEI-coated glassy carbon Ž=10 4 .. Žb. SEM image of the surface of a freshly PEI-coated glassy carbon Ž=10 4 .. Žc. SEM image of 5-day-old PEI-coated glassy carbon Ž=10 4 ..

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K. Reybier et al.r Materials Science and Engineering C 14 (2001) 47–53

Fig. 7. Voltammogram of EDA charged with LiSO 3 CF3 0.1 M at a PEI-coated glassy carbon electrode after 5, 10 and 15 min in buffer solution pH s1. Scan rate: 10 mVrs. Room temperature.

stream and oxidation of pure EDA was then performed on these altered surfaces. The shape of voltamperograms allows to qualitatively appreciate the residual passivation of surfaces. In the case of pH s 10, PEI coating was altered after 30 h. Fig. 7 presents voltamperograms recorded after an immersion time of 5, 10 and 15 min, respectively, in acidic buffer solution ŽpH s 1.. Those experiments reveal that only a few minutes are sufficient to damage the coating. Indeed, after 10 min, EDA oxidation peak appears as if the electrode was no longer inhibited whereas the electrode immersed for 5 min remains inhibited. Furthermore, as shown in Fig. 7, potential of EDA oxidation is lower for aged surfaces than those passivated for the first time. 3.3.2. Silicon For silicon pre-treated with HF Želectrode ., voltamperograms recorded after immersion in buffer solution

Fig. 8. Voltammogram of EDA charged with LiSO 3 CF3 0.1 M at a PEI-coated Si ²100: p-type electrode Želectrode . after 2, 4 and 10 min in buffer solutions pH s1. Scan rate: 20 mVrs. Room temperature.

Fig. 9. Voltammogram of EDA charged with LiSO 3 CF3 0.1 M at a PEI-coated Si ²100: p-type electrode Želectrode . after 1, 5 and 10 min in buffer solutions pH s1. Scan rate: 20 mVrs. Room temperature.

pH s 1, reveal that after 4 min, the coating seems altered. ŽFig. 8.. EDA oxidation peak is then located at 3.75 V instead of 2.2 V for bare Si–H surface. This observation undoubtedly states that silicon surface has not completely been regenerated Žbare silicon. during immersion in acidic solutions. For silicon pre-treated with sulphochromic acid Želectrode ., the coating is altered after 1 min of immersion in buffer solution pH s 1. Corresponding voltammograms are presented in ŽFig. 9. for 1, 5 and 10 min of immersion. As for Si treated with HF, a shift in potential Ž DV s 0.5 V. is observed for the EDA oxidation peak, showing an incomplete regenerated surface. In each case Želectrode or ., the polymer is altered after 36 h of immersion in basic buffer solution ŽpH s 10..

4. Conclusion This study demonstrates that a film of PEI is pH sensitive and its behaviour Žsensitivity, lifetime. largely depends on the nature of the PEI passivated materials: conducting materials such as glassy carbon, gold and semi-conducting materials, such as pre-treated Si. All PEI-modified electrodes have the advantage of a potentiometric response which varies linearly with pH. The PEImodified hydroxylated silicon electrode shows a linear response close to 50 mV per pH unit. This response could be fitted with the site-binding model applied to the acidobasic equilibrium of amino groups present at PEIrelectrolyte interface. These results suggest that these passivated materials may have a use as potentiometric pH sensor. However, we have also shown how the coating is damaged when it is immersed in very acidic solutions. Further study is planned to investigate improvements in the resistance of polymer coating by pre-treatment of the initial surfaces.

K. Reybier et al.r Materials Science and Engineering C 14 (2001) 47–53

Acknowledgements The authors are grateful for the support and valuable discussion from Dr. J.P. Malugani and Dr. M. Herlem and for the calculations from Dr. H. Jaffrezic.

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