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Piezoelectric admittance-based sensing of electrolyte solutions by montmorillonite clay film-coated quartz-crystal oscillators
372
Sensors and Actuators B, 13-14 (1993) 372-375
Piezoelectric admittance-based sensing of electrolyte solutions by montmorillonite clay film-coated quartz-crystal oscillators Noboru Oyama*, Kazutake Takada, Tetsu Tatsuma and Katsuhiko Naoi Departmentof Applied Chembby, Faculty of Technology, Tokyo Uniwrsi~ of Agr&xltureand Technology 2-24-M Naka-machi Koganei, Tokyo 184 (Japan)
Takeyoshi Okajima and Takeo Ohsaka Depatint of Electronic Chew&y, Intem%ciplinalyGmduate School of Science and Engineering Tokyo In.&ute of Technob, 4259 Nagatsuta,Midori-ku, Yokohama 227 (Japan)
Abstract admittance of quartz-crystal oscillators coated with a montmorillonite clay film or a clay/ poly(vinyl alcohol) (PVA) composite film in contact with an aqueous electrolyte solution has been measured and the possibility of applying the technique to chemical sensing examined. The resistance of the equivalent circuit, which has been conventionally accepted for a piezoelectric quark crystal, of the clay film-coated oscillator evaluated from the admittance measurement increases with decreasing Na,SO, concentration, though the dependency is not linear. This change in resistance may be ascribed to changes in the density end/or the viscosity of the clay film, which result from the structural change of the film induced by the cation. The sensitivity is suppressed by composition of the clay film with PVA. The clay/PVA composite film-coated oscillator exhibits lower resistance for a cation with higher valence; the oscillator can measure the equivalent concentration or ionic strength of a The piezoelectric
cation.
Intruduction Quartz-crystal oscillators have been used as microgravimetric sensors in both gaseous and liquid media [l]. Mass loading onto the surface of a quartz-crystal oscillator causes a decrease in the resonance frequency of the oscillator and the frequency change is proportional to the mass change. In order to understand more essentially not only the mass change but also the surface chemical and physical processes on a quartz-crystal oscillator, it is preferable to study the resonance properties of the system of interest on the basis of the electromechanical equivalent circuit model for a piezoelectric quartz-crystal oscillator. The electrical equivalent circuit that has been accepted generally [2] is shown in Fii. 1. The capacitance C, represents the mechanical elasticity of the vibrating body, the inductance L1 is a measure of the vibrating mass, the resistance R, corresponds to the loss in mechanical energy dissipated to the surrounding medium and the supporting structures and the parallel capacitance C, originates from the electrodes and the stray capacitance to the supporting structure. *Author to whom correspondence
09254005/93/$6.00
should be addressed.
CO
Fig. 1. Electrical equivalent circuit for a piezoelectric quartz crystal oscillator in vacuum, in air or in solutions. See the text for the physical meanings of RI, C1, L, and C,.
In recent papers, we described the characteristics of montmorillonite clay films [3-S], and a change in the structure of the clay film was observed by the use of quartz-crystal oscillators. The swelling and shrinking processes of the clay film in contact with an aqueous Na,SO, solution, induced by the change in the salt concentration, were found to affect the R1 parameter of the equivalent circuit. In the present work, this is applied to the sensing of an electrolyte and the possibility of applying the piezoelectric admittance measurement to chemical sensing is examined. Experimental
Biplanar, circular AT-cut quartz-crystal plates (Toy0 Kurafuto, Japan) with 13 mm diameter, the normal
0 1993 -Else&r
Sequoia. All rights reserved
373
fundamental frequency of which is 5 MHz, were coated successively with Cr (5 2 nm thick) as an adhesion layer and then with Au (~300 nm) by vacuum deposition. An asymmetric keyhole electrode arrangement was used, in which the circular electrode areas were 0.64 cm2 (front side) and 0.28 cm2 (back side). The quartz-crystal oscillator thus prepared is hereafter referred to as an AulQC oscillator. An aqueous colloidal suspension of 0.5 wt.% sodium montmorillonite clay and of 0.5 wt.% clay and 0.2 wt.% poly(viny1 alcohol) (PVA) were prepared as described previously [3]. A 3.6 ~1 aliquot of the clay (or clay/ PVA) suspension was cast on the front side of the Au/ QC oscillator, and was dried in vacua at room temperature for 30 min, unless otherwise noted. The thickness of the clay film was about 0.2 pm in the dry state. The admittance measurements of the bare, the clay film-coated and the clay/PVA composite film-coated Au/QC oscillator systems were performed in a grounded Faraday cage using a 4192A-LF impedance analyser (Yokogawa Hewlett-Packard, Japan) connected to a PC-9801 (NEC, Japan) personal computer via a GPIB interface.
Piewelectric admittance of the oscillators in contact with Na,SO, solutions The correlation between susceptance B and conductance G for the equivalent circuit (Fig. 1) is formulated as follows [2]: (G-1/2B,)2+(B-oC,)2=(1/2B,)2
(1)
where w is the angular frequency. Thus a B-G curve is anticipated to be a circle as long as the system can be modelled by the equivalent circuit. Figures 2 and 3 show the typical B-G curves for a bare and a clay film-coated Au/QC oscillator, respectively. The resistance RI of the equivalent circuit, which is the reciprocal of the diameter of a B-G curve (see eqn. (l)), for the clay-coated oscillator decreases with increasing con-
Results and discussion Piez&lectric admittance of the oscillators in contact with dried or moisturized air Table 1 lists the peak frequency (resonance frequency) change (Af,,), the peak height (G,,), and the peak width at half height (AfW,,,.,) of conductance spectra for a bare and the clay film-coated Au/QC oscillator in contact with dried or moisture-saturated air [4]. Both the clay film coating and the humidity cause almost no changes in G,, or in Afuhh; this may reflect that the clay film, as well as Au, is rigid in air, whether it is dried or moisturized. A viscoelastic fihn or liquid in contact with a quartz crystal causes the lowering and broadening of the conductance spectra. Thus, the decrease in the peak frequency observed here is chiefly ascribed to mass loading onto the oscillator, that is the clay film coating, and absorption and/or adsorption of water into (onto) the fihn.
I
-20
2
I
Conductance
3
4
(mS)
Fig. 2 Correlations between susceptance and conductance for a bare Au/QC oscillator in contact with (A) 0.01, (B) 0.35 and (C) 1.0 M aqueous solutions of NarSO,.
TABLE 1. Conductance spectral data for Au/QC oscillators with and without montmorillonite clay tilm coating in contact with dried or moisturized air QCM Bare Clay-coated Clay-coated
Air Dried Dried Moisture-saturated
G mn
4P O(std.) -2940 -3822
3.54 3.73 3.83
4& 109 112 109
0.5
1.0
Conductance
1.5
,
(mS)
Fig. 3. Correlations between susceptance and conductance for the clay Cbn-coated Au/QC oscillator in contact with (A) 0.01, (B) 0.35 and (C) 1.0 M aqueous solutions of Na$O,.
374
c
1.0
E
”
E
3E
A
B
: : 1
0.5
0
Fig. 4. Schematic illustration of proposed microstructures for the clay tilm in contact with N+SO, aqueous solutions of (A) high (e.g., 1 M) and (B) low (e.g., O,Ol M) concentrations. -o.50
centration of Na,SO,, though the concentration dependence is not linear. On the other hand, the change in R1 for a bare Au/QC oscillator is much smaller than that for a clay-coated one. No significant changes in L,, C, and C, were observed for both oscillators. It is well known that the structure of a clay film changes with electrolyte concentration. According to Lee and Fitch [6], the edge-to-face structure of a clay film in lower electrolyte concentrations is converted to the face-to-face structure by increasing the electrolyte concentration (Fig. 4). Such a change in the structure is expected to cause changes in the density and the viscosity of the clay film. When a quartz crystal oscillator is in contact with a Newtonian fluid, R1 is reportedly proportional to (p#” [2], where p and u are the density and the viscosity of the fluid. Though the clay film could not be treated as a Newtonian fluid, the observed change in R, most probably reflects the structure change of the film. Next the clay/PVA composite film-coated oscillator is examined. The change in R, induced by the change in Na,SO, concentration for the clay/PVA film-coated oscillator was more significant than that for a bare oscillator, but was less significant than that for the clay-coated one [5]. PVA in the clay/PVA composite film matrix is speculated to be strongly adsorbed on the surfaces of the clay platelets, mainly because of hydrogen bonds between the hydroxyl groups of PVA and the oxygens of the silicate of the clay platelets. Thus, the motion of the clay platelets is limited by PVA, so that the structural change with electrolyte concentration is suppressed by the introduction of PVA. Cations with higher valences Figure 5 depicts typical B-G curves for a clay/PVA composite film-coated Au/QC oscillator (2.0 ~1 aliquot of the suspension was cast) in contact with a 0.2 M aqueous solution of NaCl (pH 7), MgCl? (pH 3) or Ah& (pH 3). A clay film-coated oscillator without PVA was not used here because the structure change of the
0.5
1.0
Conductance
1.5
i
tl
(mS)
Fig. 5. Correlations between susceptance and conductance for the clay/PVA composite film-coated Au/QC oscillator in contact with 0.2 M (A) NaCl (pH 7), (B) MgCI, (pH 3) and (C) AQ (pH 3) aqueous solutions.
film is too large to hold a stable oscillation. As shown in Fig. 5, the resistance R, increases with decreasing equivalent concentration. Such a large change in R, in spite of the composition with PVA may rule out the possibility that the change is caused by the anion, and may imply that the change depends on the ionic strength rather than the equivalent concentration. The clay platelet is anionic, so that the shrinking behaviour with increasing electrolyte concentration may be ascribable to cations and the shrinking may be stronger for cations with higher valences.
A montmorillonite clay film-coated Au/QC oscillator can be used as a piezoelectric admittance-based sensor for electrolyte solutions, though the concentration dependence of RI is not linear. The sensitivity of the sensor can be suppressed by composition of the clay film with PVA. These oscillators may be applied to a sensor monitoring the change in equivalent concentration or ionic strength of a cation in electrolyte solutions.
Acknowledgement This work is supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan (No. 04555194, for N. Oyama).
375
References M. R. Deakin and D. A. Buttry, Ehxtrochemical applications of the quarts crystal microbalance, And Chem., 61 (1989) 1147A-1154A. H. Muramatsu, E. Tamiya and I. Karube, Computation of equivalent circuit parameters of quartz crystal in contact with liquids and study of liquid properties, Anal Chem., 60 (1986) 21422146. T. Okajima, T. Ohsaka and N. Oyama, Electrode kinetics of [Ru(NH&]~+~+ complex axdined in montmorillonite clay coatings on graphite electrodes. J. EIcctrwnuL Chem., 315 (1991) 175-189.
4 T. Okajima, H. Sakurai, N. Oyama, K Tokuda and T. Obsaka, Electrical equivalent circuit parameters for montmorihonite clay fihn-coated quarts crystal oscillators in contact with air, moisture and electrolyte solutions, E%cbochim Actu in press. 5 T. Okajima, H. Sakurai, N. Oyama, R. Tokuda and T. Ohsaka, Piwoclectric admittance measurements of montmorillonite/ poly(vinyl alcohol) composite film-coated quartz crystal oscillators, Bull. Chem Sot. Jp, 65 (1992) 1884-1890. 6 S. A. Lee and A. Fitch, Conductivity of clay-modified electrodes: alkali metal cation hydration and film preparation effects, J. Phys. Chem., 94 (1990) 4998-5OW
Sensors and Actuators B, 13-14 (1993) 372-375
Piezoelectric admittance-based sensing of electrolyte solutions by montmorillonite clay film-coated quartz-crystal oscillators Noboru Oyama*, Kazutake Takada, Tetsu Tatsuma and Katsuhiko Naoi Departmentof Applied Chembby, Faculty of Technology, Tokyo Uniwrsi~ of Agr&xltureand Technology 2-24-M Naka-machi Koganei, Tokyo 184 (Japan)
Takeyoshi Okajima and Takeo Ohsaka Depatint of Electronic Chew&y, Intem%ciplinalyGmduate School of Science and Engineering Tokyo In.&ute of Technob, 4259 Nagatsuta,Midori-ku, Yokohama 227 (Japan)
Abstract admittance of quartz-crystal oscillators coated with a montmorillonite clay film or a clay/ poly(vinyl alcohol) (PVA) composite film in contact with an aqueous electrolyte solution has been measured and the possibility of applying the technique to chemical sensing examined. The resistance of the equivalent circuit, which has been conventionally accepted for a piezoelectric quark crystal, of the clay film-coated oscillator evaluated from the admittance measurement increases with decreasing Na,SO, concentration, though the dependency is not linear. This change in resistance may be ascribed to changes in the density end/or the viscosity of the clay film, which result from the structural change of the film induced by the cation. The sensitivity is suppressed by composition of the clay film with PVA. The clay/PVA composite film-coated oscillator exhibits lower resistance for a cation with higher valence; the oscillator can measure the equivalent concentration or ionic strength of a The piezoelectric
cation.
Intruduction Quartz-crystal oscillators have been used as microgravimetric sensors in both gaseous and liquid media [l]. Mass loading onto the surface of a quartz-crystal oscillator causes a decrease in the resonance frequency of the oscillator and the frequency change is proportional to the mass change. In order to understand more essentially not only the mass change but also the surface chemical and physical processes on a quartz-crystal oscillator, it is preferable to study the resonance properties of the system of interest on the basis of the electromechanical equivalent circuit model for a piezoelectric quartz-crystal oscillator. The electrical equivalent circuit that has been accepted generally [2] is shown in Fii. 1. The capacitance C, represents the mechanical elasticity of the vibrating body, the inductance L1 is a measure of the vibrating mass, the resistance R, corresponds to the loss in mechanical energy dissipated to the surrounding medium and the supporting structures and the parallel capacitance C, originates from the electrodes and the stray capacitance to the supporting structure. *Author to whom correspondence
09254005/93/$6.00
should be addressed.
CO
Fig. 1. Electrical equivalent circuit for a piezoelectric quartz crystal oscillator in vacuum, in air or in solutions. See the text for the physical meanings of RI, C1, L, and C,.
In recent papers, we described the characteristics of montmorillonite clay films [3-S], and a change in the structure of the clay film was observed by the use of quartz-crystal oscillators. The swelling and shrinking processes of the clay film in contact with an aqueous Na,SO, solution, induced by the change in the salt concentration, were found to affect the R1 parameter of the equivalent circuit. In the present work, this is applied to the sensing of an electrolyte and the possibility of applying the piezoelectric admittance measurement to chemical sensing is examined. Experimental
Biplanar, circular AT-cut quartz-crystal plates (Toy0 Kurafuto, Japan) with 13 mm diameter, the normal
0 1993 -Else&r
Sequoia. All rights reserved
373
fundamental frequency of which is 5 MHz, were coated successively with Cr (5 2 nm thick) as an adhesion layer and then with Au (~300 nm) by vacuum deposition. An asymmetric keyhole electrode arrangement was used, in which the circular electrode areas were 0.64 cm2 (front side) and 0.28 cm2 (back side). The quartz-crystal oscillator thus prepared is hereafter referred to as an AulQC oscillator. An aqueous colloidal suspension of 0.5 wt.% sodium montmorillonite clay and of 0.5 wt.% clay and 0.2 wt.% poly(viny1 alcohol) (PVA) were prepared as described previously [3]. A 3.6 ~1 aliquot of the clay (or clay/ PVA) suspension was cast on the front side of the Au/ QC oscillator, and was dried in vacua at room temperature for 30 min, unless otherwise noted. The thickness of the clay film was about 0.2 pm in the dry state. The admittance measurements of the bare, the clay film-coated and the clay/PVA composite film-coated Au/QC oscillator systems were performed in a grounded Faraday cage using a 4192A-LF impedance analyser (Yokogawa Hewlett-Packard, Japan) connected to a PC-9801 (NEC, Japan) personal computer via a GPIB interface.
Piewelectric admittance of the oscillators in contact with Na,SO, solutions The correlation between susceptance B and conductance G for the equivalent circuit (Fig. 1) is formulated as follows [2]: (G-1/2B,)2+(B-oC,)2=(1/2B,)2
(1)
where w is the angular frequency. Thus a B-G curve is anticipated to be a circle as long as the system can be modelled by the equivalent circuit. Figures 2 and 3 show the typical B-G curves for a bare and a clay film-coated Au/QC oscillator, respectively. The resistance RI of the equivalent circuit, which is the reciprocal of the diameter of a B-G curve (see eqn. (l)), for the clay-coated oscillator decreases with increasing con-
Results and discussion Piez&lectric admittance of the oscillators in contact with dried or moisturized air Table 1 lists the peak frequency (resonance frequency) change (Af,,), the peak height (G,,), and the peak width at half height (AfW,,,.,) of conductance spectra for a bare and the clay film-coated Au/QC oscillator in contact with dried or moisture-saturated air [4]. Both the clay film coating and the humidity cause almost no changes in G,, or in Afuhh; this may reflect that the clay film, as well as Au, is rigid in air, whether it is dried or moisturized. A viscoelastic fihn or liquid in contact with a quartz crystal causes the lowering and broadening of the conductance spectra. Thus, the decrease in the peak frequency observed here is chiefly ascribed to mass loading onto the oscillator, that is the clay film coating, and absorption and/or adsorption of water into (onto) the fihn.
I
-20
2
I
Conductance
3
4
(mS)
Fig. 2 Correlations between susceptance and conductance for a bare Au/QC oscillator in contact with (A) 0.01, (B) 0.35 and (C) 1.0 M aqueous solutions of NarSO,.
TABLE 1. Conductance spectral data for Au/QC oscillators with and without montmorillonite clay tilm coating in contact with dried or moisturized air QCM Bare Clay-coated Clay-coated
Air Dried Dried Moisture-saturated
G mn
4P O(std.) -2940 -3822
3.54 3.73 3.83
4& 109 112 109
0.5
1.0
Conductance
1.5
,
(mS)
Fig. 3. Correlations between susceptance and conductance for the clay Cbn-coated Au/QC oscillator in contact with (A) 0.01, (B) 0.35 and (C) 1.0 M aqueous solutions of Na$O,.
374
c
1.0
E
”
E
3E
A
B
: : 1
0.5
0
Fig. 4. Schematic illustration of proposed microstructures for the clay tilm in contact with N+SO, aqueous solutions of (A) high (e.g., 1 M) and (B) low (e.g., O,Ol M) concentrations. -o.50
centration of Na,SO,, though the concentration dependence is not linear. On the other hand, the change in R1 for a bare Au/QC oscillator is much smaller than that for a clay-coated one. No significant changes in L,, C, and C, were observed for both oscillators. It is well known that the structure of a clay film changes with electrolyte concentration. According to Lee and Fitch [6], the edge-to-face structure of a clay film in lower electrolyte concentrations is converted to the face-to-face structure by increasing the electrolyte concentration (Fig. 4). Such a change in the structure is expected to cause changes in the density and the viscosity of the clay film. When a quartz crystal oscillator is in contact with a Newtonian fluid, R1 is reportedly proportional to (p#” [2], where p and u are the density and the viscosity of the fluid. Though the clay film could not be treated as a Newtonian fluid, the observed change in R, most probably reflects the structure change of the film. Next the clay/PVA composite film-coated oscillator is examined. The change in R, induced by the change in Na,SO, concentration for the clay/PVA film-coated oscillator was more significant than that for a bare oscillator, but was less significant than that for the clay-coated one [5]. PVA in the clay/PVA composite film matrix is speculated to be strongly adsorbed on the surfaces of the clay platelets, mainly because of hydrogen bonds between the hydroxyl groups of PVA and the oxygens of the silicate of the clay platelets. Thus, the motion of the clay platelets is limited by PVA, so that the structural change with electrolyte concentration is suppressed by the introduction of PVA. Cations with higher valences Figure 5 depicts typical B-G curves for a clay/PVA composite film-coated Au/QC oscillator (2.0 ~1 aliquot of the suspension was cast) in contact with a 0.2 M aqueous solution of NaCl (pH 7), MgCl? (pH 3) or Ah& (pH 3). A clay film-coated oscillator without PVA was not used here because the structure change of the
0.5
1.0
Conductance
1.5
i
tl
(mS)
Fig. 5. Correlations between susceptance and conductance for the clay/PVA composite film-coated Au/QC oscillator in contact with 0.2 M (A) NaCl (pH 7), (B) MgCI, (pH 3) and (C) AQ (pH 3) aqueous solutions.
film is too large to hold a stable oscillation. As shown in Fig. 5, the resistance R, increases with decreasing equivalent concentration. Such a large change in R, in spite of the composition with PVA may rule out the possibility that the change is caused by the anion, and may imply that the change depends on the ionic strength rather than the equivalent concentration. The clay platelet is anionic, so that the shrinking behaviour with increasing electrolyte concentration may be ascribable to cations and the shrinking may be stronger for cations with higher valences.
A montmorillonite clay film-coated Au/QC oscillator can be used as a piezoelectric admittance-based sensor for electrolyte solutions, though the concentration dependence of RI is not linear. The sensitivity of the sensor can be suppressed by composition of the clay film with PVA. These oscillators may be applied to a sensor monitoring the change in equivalent concentration or ionic strength of a cation in electrolyte solutions.
Acknowledgement This work is supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan (No. 04555194, for N. Oyama).
375
References M. R. Deakin and D. A. Buttry, Ehxtrochemical applications of the quarts crystal microbalance, And Chem., 61 (1989) 1147A-1154A. H. Muramatsu, E. Tamiya and I. Karube, Computation of equivalent circuit parameters of quartz crystal in contact with liquids and study of liquid properties, Anal Chem., 60 (1986) 21422146. T. Okajima, T. Ohsaka and N. Oyama, Electrode kinetics of [Ru(NH&]~+~+ complex axdined in montmorillonite clay coatings on graphite electrodes. J. EIcctrwnuL Chem., 315 (1991) 175-189.
4 T. Okajima, H. Sakurai, N. Oyama, K Tokuda and T. Obsaka, Electrical equivalent circuit parameters for montmorihonite clay fihn-coated quarts crystal oscillators in contact with air, moisture and electrolyte solutions, E%cbochim Actu in press. 5 T. Okajima, H. Sakurai, N. Oyama, R. Tokuda and T. Ohsaka, Piwoclectric admittance measurements of montmorillonite/ poly(vinyl alcohol) composite film-coated quartz crystal oscillators, Bull. Chem Sot. Jp, 65 (1992) 1884-1890. 6 S. A. Lee and A. Fitch, Conductivity of clay-modified electrodes: alkali metal cation hydration and film preparation effects, J. Phys. Chem., 94 (1990) 4998-5OW