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Thin films of chalcogenide glass as sensitive membranes for the detection of mercuric ions in solution
S@JRS
mm B
CHEMICAL
Sensors and Actuators
B 244
(1995) 296-299
Thin films of chalcogenide glass as sensitive membranes for the detection of mercuric ions in solution A. Guessous, Labomtoti
P. Papet, J. Sarradin,
M. Ribes
de Physicochimie des Mat&iuw; Solids, CC003 (IRA CNRS 00407, Universitb de Montpeliier II, 34095 Montpellier Ceder 5, France
Abstract Thin vitreous films elaborated by microelectronic techniques can be used to build up sensitive microsystems in order to detect ionic species (HP”‘) in aqueous solution. Characteristics of sensing systems based on thin films of chalcogenide glasses are presented. The microsensors exhibit quasi-Nernstian potentiometric response over five orders of magnitude with a limit of detection below 10m6 mol I-‘. In the presence of interfering heavy-metal ions (Pb*+, Cd’+) the sensing systems are very selective, since low selectivity coefficients are obtained versus those ionic species. Keyword; Chalcogenide glass; Electrochemical sensors; Mercury sensors; Thin films
The present work is intended to provide some experimental results concerning a suitable Hg*’ ion microsensing system.
1. Introduction
Electrochemical
sensors
based on ion-sensitive
mem-
have in recent years become a focus of attention regarding the increasing demand from a wide application field (medicine, industry, environment, monitoring, etc.). The miniaturization and VLSI of electronic components has induced laboratory studies on all-solid-state microdevices such as microsensors (ISEs). The sensing microsystem should be optimized in two ways: (a) Sensitivity. In order to detect small quantities of interesting ionic species present in the solution, a thin film with a high sensitivity is needed. (b) Selectivity. The microsystem has to be selective to one ionic component in the midst of many interfering substances that may be present at high concentration. Due to their specific ionic or mixed (ionic t electronic) conductivity and their stability with regard to aqueous solution, glasses seem to be suitable materials for this kind of application [l-6]. The sensitive membrane selected for the detection of Hg’+ ion in aqueous solution is a chalcogenide glass, because on one hand good results with bulk chalcogenide glasses as membrane materials for conventional heavy-metal ion-selective electrodes have already been carried out [3], and on the other hand these materials are chemically stable in acidic solution (pH<2). branes
09254h35i95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDI 0925.4005(94)01493-2
2. Experimental A doped chalcogenide glass (5 HgTe-95 Ge0.2Te0,3SeO,J was prepared from high-purity elements by the melt-quenching method. Thin glass films as sensitive membranes were obtained by vacuum evaporation onto a chromium sputtered glass substrate. Even though the sensor voltage is sensitive to electrical and chemical disturbances, some accurate measurements were obtained in carefully controlled experimental conditions. This sensing thin film is a mixed conductor and any change in the Hg” concentration of the solution is converted into a membrane electrostatic potential change. For this system the additional electronic conductivity does not have any detrimental effect. Indeed, the existence of a partial electronic conductivity in the membrane ensures a good anchoring of the electronic Fermi level at the membrane/substrate interface and consequently of the ionic Fermi level. So, the distance between the two Fermi levels is fixed 171.The vitreous structure and electrical properties of the thin films were investigated using X-ray diffractometer and spec-
A. Guessous et al. / Semors and Actuators B 24-25 (1995) 296-299
troscopy impedance techniques. The thickness and morphology of the thin films were observed by means of an SEM. Potentiometric measurements were performed on the microsensors dipped in stirred acidic aqueous solutions (pH=2) containing mercmy ions at various concentrations (in the 10-‘-lo-’ mol 1-l range).
3. Results and discussion Fig. 1 shows an SEM micrograph of a thin film of the Hg” ion-sensitive membrane. The thickness of the layer is close to 1.5 ,um and appears to be pinhole free. We can observe in Fig. 2 the X-ray patterns of vacuum-deposited thin films obtained from the Hg-Ge-Te-Se bulk glass. No crystallization peak appears before any annealing treatment (Fig. 2(a)). The partial crystallization observed after an annealing treatment performed at 100 “C (Fig. 2(b)) is confirmed by
Fig. 1. SEM cross-sectional view of a Hg-Ge-TeSe
197
the d-c. conductivity measurements (Fig. 3). Before crystallization, the conductivity of the thin film is very close to the bulk value (10e6 S cm-‘), indicating a similar composition of the thin film and the bulk material. The change of d.c. conductivity near 100 “C corresponds to the appearance of the crystallization peak previously checked in Fig. 2(b) and attributed to HgTe. At this temperature, the membrane become homogeneous and after partial crystallization, the conductivity of the thin film decreases drastically. So, using these sensitive membranes at temperatures up to 100 “C may cause an increase of the internal resistance and modify the response time and the sensitivity of the microsensor. Regarding the electrochemical behaviour, Fig. 4 shows the potentiometric response versus time when the ionic concentration of mercury is changed. As we can observe, the mercmy microsensor has a fast response time, which indicates that the thermodynamic equilibrium between the sensing membrane and the tested solution is quickly reached. However, this rate is higher when the change in concentration is from 10-l to low3 mol 1-l. Potentiometric measurements obtained with the mercury microsensor are presented in Fig. 5. A quasiNemstian behaviour is observed in the concentration range 1O-2-1O-6 mol 1-l with a slope = 24 mV/decade (29 mV/decade for the Nernst law). As soon as the Nemstian potential activity relationship cannot be applied due to a too small activity of Hg’+ ions in solution, the detection limit is reached. Experimentally this limit
thin film.
2
24
20
32
I&
(KP
Fig. 3. Direct current conductivity vs. lMKl/T for a Hg-Ge-T&e thin film.
1
‘"0
40
60
120
160
200
Time (S)
Fig. 2. X-ray diffraction pattern of Hg-Ge-TeSe thin film: (a) no annealing; (b) annealed at 100 “C; (c) annealed at 350 “C.
Fig. 4. Sensing microsystem potentiometric response vs. time in Hg’ concentration range 10-‘-10-3 mol I-’ (pH=2).
A. GuessourEdal. I Sensors and Actuators B 24-25 (1995) 296-299
298
240 190 ,65L_ 2
3
4
5
6
7
1
P H&+
Fig. 5. Hg2+ mnresponse of the sensing system measured with test solutions at constant pH=Z.
Fig. 7. Cd”
_ 2
KXIconcentration
_
~ 3
~~~ 4
J 5 pcdz+6
effect on the sensing system.
4. Conclusions 270
I
I
265 n
260
5 E
255
w \
250
"
245 240
\
s-_a._v
_a
L 1
2
Fig. 6. Pb ‘+ ion concentration
3
4
5
6 p PtP7
effect on the sensing system.
is close to 3 X 10e6 mol 1-l for our microsystem. In practice, an ideally sensitive membrane behaviour, such as a quasi-Nernstian one, can most often not be reached. The preliminary results have shown that a stable potential of the sensor was obtained during the first five to seven days. Heavy-metal ions such as Pb*’ and Cd’* are often associated with Hg’.C ions in polluted water; so it is important to know the additional contributions to the total measured activity that result from the presence of interfering species i in the sample solution. In order to determine the selectivity coefficients KHg2+,i(for an interfering ion i) that give a full specification of the potentiometrically observable ion selectivity of a sensing membrane, we used the Nikolskii equation [8] and the fixed interference method. Measurements were performed with a constant Hg*’ ion concentration (i.e., [HgZ’]=10-5 mol 1-i) while the interfering ion concentration increased from 10m6 to 10-l mol 1-l. Figs. 6 and 7 present the responses versus Pb” and Cd’+ ion concentrations. The selectivity is not ideal since the microsensor responds to high Hg*’ and Pb” ions concentrations. However, such concentrations are rarely observed in waste water. According to the Nikolskii equation, the values of KHg2+,pbz+ and KHgZ+,CB+(selectivity coefficients) are 6.3 X 10e4 and 7.3 X 10m4, respectively. These values seem interesting, since the lower the coefficient, the smaller is the influence of the interfering ion.
The interest of all-solid-state microsensors built up with vitreous thin films has been demonstrated. Thin films obtained by vacuum evaporation exhibit high selectivity and good sensitivity in the Hg2+ concentration range 10~‘-10-6 mol 1-l. Further investigations are currently in progress regarding the reproducibility and stability of such microdevices, as the absolute value of the response may change with time. In steady-state conditions, responses in the concentration range where the Nernst law is valid can be obtained with an accuracy close to 5%.
References N. Tohge and M. Tanaka, Chalcogenide glass electrodes sensitive to heavy metal ions, J. Non-Cry$folline Solids, 80 (1986) 550. 121 CT. Baker and I. Trachtenberg, Ion-selective electrochemical sensors - Fe3+, Cu*+, 1. Elecfmchem Sm., 118 (1971) 571. [31 Y.G. Vlasov and Y.A. Tarantov, Development of ISFET using glassy solid electrolytes, in T. Seiyama (ed.), Chemical Sensor Technolo~, Elsevier, Amsterdam/Kodansha, Tokyo, 1989. c41 _I. Koryta and K. Stulik, Ion Sekcrive Electrodes, Cambridge University Press, Cambridge, 2nd edn., 1983. [51 A.V. Legin, E.A. Bychkov and Y.G. Vlasov, Thin-layer chemical sensors based on chemically deposited and modified chalcogenide glasses, Sensors and Acluafors 8, 15-16 (1993) 184. 161 R. Crew, J. Sarradin and M. Ribes, Thin films of ionic and mixed conductive glasses: their use in microdevices, Solid Slate lonics, 53-56 (1992) 641. [71M. Kleitz, J.F. Million-Bmdaz and P. Fabry, New compounds for ISFET’s, Solid State lonics, 22 (1987) 295. PI B.P. Nickolskii, Theory of the glass electrode,dcfa Physicochimi USSrz 7 (1937) 597.
PI
Biographies
A. Guessous was born in 1966 and is presently working at Montpellier University on thin-film glasses used in microdevices. She earned a Ph.D. degree in July 1994. P. Pupet was born in 1959. He obtained a diploma in engineering from the ENSCI (Limoges) and a Ph.D. from Limoges University. He spent a year as a post-
A. Gwssow et al. I Semors and Actuators B 24-25 (1995) 296-299
doctoral fellow at Penn State University studying ferroelectric materials. He is presently an assistant professor at Montpellier University. J. Sumdin was born in 1942. He obtained a diploma in engineering in 1966. He spent a year at Case Western Reserve University in Professor C.C. Liu’s laboratory.
His research activities concern microbatteries crosensors.
299
and mi-
M. Ribes was born in 1941. He is presently professor at Montpellier University and director of the Laboratory of Solid State Chemistry. His field of expertise concerns the chemistry of solids and glasses.
mm B
CHEMICAL
Sensors and Actuators
B 244
(1995) 296-299
Thin films of chalcogenide glass as sensitive membranes for the detection of mercuric ions in solution A. Guessous, Labomtoti
P. Papet, J. Sarradin,
M. Ribes
de Physicochimie des Mat&iuw; Solids, CC003 (IRA CNRS 00407, Universitb de Montpeliier II, 34095 Montpellier Ceder 5, France
Abstract Thin vitreous films elaborated by microelectronic techniques can be used to build up sensitive microsystems in order to detect ionic species (HP”‘) in aqueous solution. Characteristics of sensing systems based on thin films of chalcogenide glasses are presented. The microsensors exhibit quasi-Nernstian potentiometric response over five orders of magnitude with a limit of detection below 10m6 mol I-‘. In the presence of interfering heavy-metal ions (Pb*+, Cd’+) the sensing systems are very selective, since low selectivity coefficients are obtained versus those ionic species. Keyword; Chalcogenide glass; Electrochemical sensors; Mercury sensors; Thin films
The present work is intended to provide some experimental results concerning a suitable Hg*’ ion microsensing system.
1. Introduction
Electrochemical
sensors
based on ion-sensitive
mem-
have in recent years become a focus of attention regarding the increasing demand from a wide application field (medicine, industry, environment, monitoring, etc.). The miniaturization and VLSI of electronic components has induced laboratory studies on all-solid-state microdevices such as microsensors (ISEs). The sensing microsystem should be optimized in two ways: (a) Sensitivity. In order to detect small quantities of interesting ionic species present in the solution, a thin film with a high sensitivity is needed. (b) Selectivity. The microsystem has to be selective to one ionic component in the midst of many interfering substances that may be present at high concentration. Due to their specific ionic or mixed (ionic t electronic) conductivity and their stability with regard to aqueous solution, glasses seem to be suitable materials for this kind of application [l-6]. The sensitive membrane selected for the detection of Hg’+ ion in aqueous solution is a chalcogenide glass, because on one hand good results with bulk chalcogenide glasses as membrane materials for conventional heavy-metal ion-selective electrodes have already been carried out [3], and on the other hand these materials are chemically stable in acidic solution (pH<2). branes
09254h35i95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDI 0925.4005(94)01493-2
2. Experimental A doped chalcogenide glass (5 HgTe-95 Ge0.2Te0,3SeO,J was prepared from high-purity elements by the melt-quenching method. Thin glass films as sensitive membranes were obtained by vacuum evaporation onto a chromium sputtered glass substrate. Even though the sensor voltage is sensitive to electrical and chemical disturbances, some accurate measurements were obtained in carefully controlled experimental conditions. This sensing thin film is a mixed conductor and any change in the Hg” concentration of the solution is converted into a membrane electrostatic potential change. For this system the additional electronic conductivity does not have any detrimental effect. Indeed, the existence of a partial electronic conductivity in the membrane ensures a good anchoring of the electronic Fermi level at the membrane/substrate interface and consequently of the ionic Fermi level. So, the distance between the two Fermi levels is fixed 171.The vitreous structure and electrical properties of the thin films were investigated using X-ray diffractometer and spec-
A. Guessous et al. / Semors and Actuators B 24-25 (1995) 296-299
troscopy impedance techniques. The thickness and morphology of the thin films were observed by means of an SEM. Potentiometric measurements were performed on the microsensors dipped in stirred acidic aqueous solutions (pH=2) containing mercmy ions at various concentrations (in the 10-‘-lo-’ mol 1-l range).
3. Results and discussion Fig. 1 shows an SEM micrograph of a thin film of the Hg” ion-sensitive membrane. The thickness of the layer is close to 1.5 ,um and appears to be pinhole free. We can observe in Fig. 2 the X-ray patterns of vacuum-deposited thin films obtained from the Hg-Ge-Te-Se bulk glass. No crystallization peak appears before any annealing treatment (Fig. 2(a)). The partial crystallization observed after an annealing treatment performed at 100 “C (Fig. 2(b)) is confirmed by
Fig. 1. SEM cross-sectional view of a Hg-Ge-TeSe
197
the d-c. conductivity measurements (Fig. 3). Before crystallization, the conductivity of the thin film is very close to the bulk value (10e6 S cm-‘), indicating a similar composition of the thin film and the bulk material. The change of d.c. conductivity near 100 “C corresponds to the appearance of the crystallization peak previously checked in Fig. 2(b) and attributed to HgTe. At this temperature, the membrane become homogeneous and after partial crystallization, the conductivity of the thin film decreases drastically. So, using these sensitive membranes at temperatures up to 100 “C may cause an increase of the internal resistance and modify the response time and the sensitivity of the microsensor. Regarding the electrochemical behaviour, Fig. 4 shows the potentiometric response versus time when the ionic concentration of mercury is changed. As we can observe, the mercmy microsensor has a fast response time, which indicates that the thermodynamic equilibrium between the sensing membrane and the tested solution is quickly reached. However, this rate is higher when the change in concentration is from 10-l to low3 mol 1-l. Potentiometric measurements obtained with the mercury microsensor are presented in Fig. 5. A quasiNemstian behaviour is observed in the concentration range 1O-2-1O-6 mol 1-l with a slope = 24 mV/decade (29 mV/decade for the Nernst law). As soon as the Nemstian potential activity relationship cannot be applied due to a too small activity of Hg’+ ions in solution, the detection limit is reached. Experimentally this limit
thin film.
2
24
20
32
I&
(KP
Fig. 3. Direct current conductivity vs. lMKl/T for a Hg-Ge-T&e thin film.
1
‘"0
40
60
120
160
200
Time (S)
Fig. 2. X-ray diffraction pattern of Hg-Ge-TeSe thin film: (a) no annealing; (b) annealed at 100 “C; (c) annealed at 350 “C.
Fig. 4. Sensing microsystem potentiometric response vs. time in Hg’ concentration range 10-‘-10-3 mol I-’ (pH=2).
A. GuessourEdal. I Sensors and Actuators B 24-25 (1995) 296-299
298
240 190 ,65L_ 2
3
4
5
6
7
1
P H&+
Fig. 5. Hg2+ mnresponse of the sensing system measured with test solutions at constant pH=Z.
Fig. 7. Cd”
_ 2
KXIconcentration
_
~ 3
~~~ 4
J 5 pcdz+6
effect on the sensing system.
4. Conclusions 270
I
I
265 n
260
5 E
255
w \
250
"
245 240
\
s-_a._v
_a
L 1
2
Fig. 6. Pb ‘+ ion concentration
3
4
5
6 p PtP7
effect on the sensing system.
is close to 3 X 10e6 mol 1-l for our microsystem. In practice, an ideally sensitive membrane behaviour, such as a quasi-Nernstian one, can most often not be reached. The preliminary results have shown that a stable potential of the sensor was obtained during the first five to seven days. Heavy-metal ions such as Pb*’ and Cd’* are often associated with Hg’.C ions in polluted water; so it is important to know the additional contributions to the total measured activity that result from the presence of interfering species i in the sample solution. In order to determine the selectivity coefficients KHg2+,i(for an interfering ion i) that give a full specification of the potentiometrically observable ion selectivity of a sensing membrane, we used the Nikolskii equation [8] and the fixed interference method. Measurements were performed with a constant Hg*’ ion concentration (i.e., [HgZ’]=10-5 mol 1-i) while the interfering ion concentration increased from 10m6 to 10-l mol 1-l. Figs. 6 and 7 present the responses versus Pb” and Cd’+ ion concentrations. The selectivity is not ideal since the microsensor responds to high Hg*’ and Pb” ions concentrations. However, such concentrations are rarely observed in waste water. According to the Nikolskii equation, the values of KHg2+,pbz+ and KHgZ+,CB+(selectivity coefficients) are 6.3 X 10e4 and 7.3 X 10m4, respectively. These values seem interesting, since the lower the coefficient, the smaller is the influence of the interfering ion.
The interest of all-solid-state microsensors built up with vitreous thin films has been demonstrated. Thin films obtained by vacuum evaporation exhibit high selectivity and good sensitivity in the Hg2+ concentration range 10~‘-10-6 mol 1-l. Further investigations are currently in progress regarding the reproducibility and stability of such microdevices, as the absolute value of the response may change with time. In steady-state conditions, responses in the concentration range where the Nernst law is valid can be obtained with an accuracy close to 5%.
References N. Tohge and M. Tanaka, Chalcogenide glass electrodes sensitive to heavy metal ions, J. Non-Cry$folline Solids, 80 (1986) 550. 121 CT. Baker and I. Trachtenberg, Ion-selective electrochemical sensors - Fe3+, Cu*+, 1. Elecfmchem Sm., 118 (1971) 571. [31 Y.G. Vlasov and Y.A. Tarantov, Development of ISFET using glassy solid electrolytes, in T. Seiyama (ed.), Chemical Sensor Technolo~, Elsevier, Amsterdam/Kodansha, Tokyo, 1989. c41 _I. Koryta and K. Stulik, Ion Sekcrive Electrodes, Cambridge University Press, Cambridge, 2nd edn., 1983. [51 A.V. Legin, E.A. Bychkov and Y.G. Vlasov, Thin-layer chemical sensors based on chemically deposited and modified chalcogenide glasses, Sensors and Acluafors 8, 15-16 (1993) 184. 161 R. Crew, J. Sarradin and M. Ribes, Thin films of ionic and mixed conductive glasses: their use in microdevices, Solid Slate lonics, 53-56 (1992) 641. [71M. Kleitz, J.F. Million-Bmdaz and P. Fabry, New compounds for ISFET’s, Solid State lonics, 22 (1987) 295. PI B.P. Nickolskii, Theory of the glass electrode,dcfa Physicochimi USSrz 7 (1937) 597.
PI
Biographies
A. Guessous was born in 1966 and is presently working at Montpellier University on thin-film glasses used in microdevices. She earned a Ph.D. degree in July 1994. P. Pupet was born in 1959. He obtained a diploma in engineering from the ENSCI (Limoges) and a Ph.D. from Limoges University. He spent a year as a post-
A. Gwssow et al. I Semors and Actuators B 24-25 (1995) 296-299
doctoral fellow at Penn State University studying ferroelectric materials. He is presently an assistant professor at Montpellier University. J. Sumdin was born in 1942. He obtained a diploma in engineering in 1966. He spent a year at Case Western Reserve University in Professor C.C. Liu’s laboratory.
His research activities concern microbatteries crosensors.
299
and mi-
M. Ribes was born in 1941. He is presently professor at Montpellier University and director of the Laboratory of Solid State Chemistry. His field of expertise concerns the chemistry of solids and glasses.