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Disposable ion-selective electrodes
ELSEVIER
Sensors and Actuators B 24-25 (1995) 721-723
Disposable
ion-selective
electrodes
Michael Borchardt, Christa Dumschat, Karl Cammann, Meinhard Knoll In.Mut j?ir Chemo- und
Biow~~~rik,Menddstmsse7, D-48149Miurtter.Germany
Abstract In this paper we describe a low-cost ion-selective electrode in double matrix membrane technology. This sensor is prepared from filter paper with an evaporated silver conducting line on one side. For insulation the sensor is laminated with a preperforated heat-sealing film. A coated film sensor in double matrix membrane technology is produced by filling one hole with a defined volume of an ion-selective polymer matrix membrane cocktail. The double matrix membrane (DhIM) is formed by the polymer matrix membrane and the additional microfibre matrix (MFM) of the filter paper. Such sensors can be produced in a batch process. Therefore mass-production of low-cost sensors is possible. The response behaviour of the low-cost ion-
selective electrode is comparable to that of conventional macro ion-selective electrodes. Keywordr: Ion-selective electrodes
1. Introduction
Ion-selective electrodes (ISEs) are widely used in analytical chemistry [l]. They are powerful analytical tools for the determination of the activity of many ions. For some applications, e.g., in clinical chemistry and in environmental analysis, low-cost disposable electrodes have some advantages, e.g., small sample volume and no cleaning operation necessary. Therefore we developed a new concept for producing ISEs in a batch process [2]. The ion-selective polymeric membrane is formed in the volume of an electrochemical inert microfibre matrix (MFM) by a casting technique. There it is homogeneously distributed by capillary forces and after the evaporation of the solvent the double matrix is obtained.The key innovation is to use the homogeneity of the inert matrix to produce almost identical ionselective membranes. Thin silver dots can be formed on the MFM by vacuum evaporation, sputtering or screen-printing before membrane deposition. Thus
micm fibra ma*ix
insulator/’ (heat sealing film)
iow3ebctive membrane
0925-4005/95/$09.50
Q 1995 Elsevkr Science S.A. All rights
2. Experimental 2.1. Sensor preparation Fig. 1 shows a schematic view of the low-cost ionselective electrode. The electrodes were produced in a batch process as shown for one electrode in Fig. 2. To prepare the silver contact and the strip conductor, silver was evaporated (Univex 300 vacuum coater, Leybold) on the rear side of a sheet of filter paper (Schleicher & Schuell, 58g3 Blue ribbon ashless, ref. no. 300 211), which was then cut into strips (40 mmx
_-l..-___-.-l ---._. ---/ _
Fig. 1. Schematic view of the lowcost ion-selective electrode.
SSDI 0925-4005(94)01454-P
‘coated-film’ electrodes with very well-adhering double matrix membranes can be prepared. We have described such double matrix membranes for applications in an electrode body with a liquid internal junction and as coated-film electrodes [3,4]. Here we describe the development of such membranes into disposable ISEs.
reserved
Fig. 2. Production of the electrode.
flltsr
paper (stlvar deposit&l
722
M. Borchardt et aI. / Semors and Achcators B 24-2s
4 mm strip size). In the next step heat-sealing films (150 pm thick, Team Codor, DUBO-Schweitzer GmbH) were perforated (holes with a diameter of 3 mm) and cut into suitable sizes. The heat-sealing films consist of polyethylene and polyester. If they are heated under pressure together with paper in the middle, both materials are linked very stably. Ten strips were laminated together at 125 “C between two heat-sealing films for insulation. By that procedure paper strips are completely wrapped up with the heat-sealing film except one hole for the membrane and the contact area on the other side. In the areas without paper between the two heatsealing films, the fihns are linked together veIy stably, so after cutting the side encapsulation is ensured. The 5 mm long silver pads on the back of the MFM were electrically contacted through an opening in the lower film. The following membrane cocktail was used (wt.%): 32.8% PVC, 2% valinomycin, 0.5% potassium tetrakis(C chlorophenyl) borate, 64.7% bis(2-ethylhexyl) sebacate (DOS). 400 mg of membrane components were weighed in a 5 ml glass bottle and then 1.5 ml THF and 0.5 ml cyclohexanone were added. 10 ~1 of the membrane cocktail were placed in each hole. To prevent an overflow of substance, four dispensing steps at intervals of 5 min were automatically carried out by the Asymtek Automove 402 Fluid Dispenser. Before the measurement was started and the electrodes were separated, the solvents were allowed to evaporate overnight. 2.2. Measurements The performance of the electrodes was judged by the slope, the detection limit, various selectivity coefficients and the reproducibility of the electrode preparation. All measurements were carried out with a pH meter (Microprocessor pH Meter pH 30OO/Multiplex 3000, WTW Weilheim, Germany) and an AglAgCl reference electrode from Orion (model 90-02) at room temperature. A 0.1 M sodium formate solution was filled into the outer chamber of the reference electrode. The calibration curves were obtained by applying the addition method in which known amounts of the measured ion (KCl) were added to the sample solution from low to high concentrations. The selectivity coefficients were evaluated by the fixed interference method according to IUPAC recommendations [5].
3. Results and discussion The electrochemical response behaviour of a potassium-selective sensor versus a commercial Ag/AgCl electrode is shown in Fig. 3 and the calibration graph in Fig. 4. As determined from Fig. 4, the detection limit is 10e6.” mol 1-l and the slope 58 mV decade-‘.
(1995) 721-723
10
0
30 tlmin4’
20
xl
60
70
Fig. 3. Response behaviourof a K+-selective electrode (concentration steps of one decade from lo-’ to 10m2 mol I-’ KCI).
N=4
loo
““I”“‘.
-8
-7
-6
.
0 ” I( . . ‘I”. -5 _pK4 -3
”
-2
Fig. 4. Calibration graph of a K+-selective electrode. 560
m 450
i
Pa
6 m- . zo-
Fig. 5. Calibration curves of ten K+-selective electrodes (in 0.1 M NaCl solution).
The selectivity coefficient log QYNa is -4.3 (in 0.1 M NaCl solution). Fig. 5 shows the calibration curves of ten potassium-selective electrodes (a whole batch of simultaneously produced electrodes without selection). The statistical data of that measurement are given in Table 1. The lifetime of the electrodes (continuous immersion in electrolyte solution) was in the range of a few days.
hf. Borchordi et al. I Sensors and Actuators B 24-25 (1995) 721-723 Table 1 Reproducibility of ten potassium electrodes (lo-‘-lo-* mol 1-y
in the linear range
-4
-3
-2
-1
C [mv] D [mv] E [mv] F [mv] G [mV) H [mv] I [mV) J [mV) K [mv]
347.8 349.8 349.8 350.3 349.1 352.6 350.3 349.5 349.0 346.4
395.5 397.7 397.6 297.3 397.1 400.3 398.6 397.4 397.5 394.3
450 452.4 452.4 452.4 451.8 455.4 453.3 452.3 452.3 449.5
499.8 504.6 504.4 504.6 504.4 507.8 505.4 584.3 503.7 501.5
Average potential [mv] Standard deviation [mv]
349.4 1.6
397.3 1.6
452.1 1.6
504.0 2.1
-PK Sensor B [mv]
Sensor Sensor Sensor Sensor Sensor Sensor Sensor Sensor Sensor
The results show that the electrochemical response behaviour of the disposable electrodes is comparable with that of a conventional ISE using the same membrane cocktail. The main advantage of the new electrode configuration is the technology of electrode preparation. High productivity can be achieved by a simple batch technology. In addition, only cheap mass products were used. As demonstrated in Fig. 5 and Table 1, the double matrix membrane technology allows the preparation of ISEs with very similar response characteristics, so it should be possible to calibrate only a few electrodes from a batch. This is the precondition for a quick,
723
accurate and reliable on-site test using an ISE. In combination with a disposable reference electrode, it is possible to produce a low-cost disposable test strip [6,7]. The previous results (Table 1) for these handmade electrodes should be improved by optimizing the sensor preparation.
Acknowledgement We are grateful to the Ministerium fiir Wissenschaft turd Forschung, Nordrhein-Westfalen, Germany, for financial support.
References 311 K. Cammann, Working with Ion-selective Electrodes, Springer, New York, 1979. [21 M. Knoll, Gemtan Patent No. P4137261.1 (13 Nov., 1991). [31 M. Knoll, K. Cammann, C. Dumschat, M. Borchardt and G. H&g, Microfibre matrix-supported ion-selective PVC-membranes, Sensors and Achmom B, 20 (1994) 1. t41 C. Dumschat, M. Borchardt, C. Diekmann, J. Hepke and M. Knoll, Double matrix membranes for potentiometric cation sensitive sensors, Fresenius’Z. Anal. Chem, 348 (1994) 553. [51 Recommendations for Nomenclature of Ion-Selective Electrodes, Pure AppZ. Chem., 48 (1976) 129. [61 C. Dumschat, M. Borchardt, C. Diekmann, K. Cammann and M. Knoll, Potentiometric test strip, Semors and Actuators 8, 24-25 (1995) 279-281. [71 C. Diekmann, C. Dumschat, K. Cammann and M. Knoll, Disposable reference electrode, Sensors and Actuators 8, 24-25 (1995) 276-278.
Sensors and Actuators B 24-25 (1995) 721-723
Disposable
ion-selective
electrodes
Michael Borchardt, Christa Dumschat, Karl Cammann, Meinhard Knoll In.Mut j?ir Chemo- und
Biow~~~rik,Menddstmsse7, D-48149Miurtter.Germany
Abstract In this paper we describe a low-cost ion-selective electrode in double matrix membrane technology. This sensor is prepared from filter paper with an evaporated silver conducting line on one side. For insulation the sensor is laminated with a preperforated heat-sealing film. A coated film sensor in double matrix membrane technology is produced by filling one hole with a defined volume of an ion-selective polymer matrix membrane cocktail. The double matrix membrane (DhIM) is formed by the polymer matrix membrane and the additional microfibre matrix (MFM) of the filter paper. Such sensors can be produced in a batch process. Therefore mass-production of low-cost sensors is possible. The response behaviour of the low-cost ion-
selective electrode is comparable to that of conventional macro ion-selective electrodes. Keywordr: Ion-selective electrodes
1. Introduction
Ion-selective electrodes (ISEs) are widely used in analytical chemistry [l]. They are powerful analytical tools for the determination of the activity of many ions. For some applications, e.g., in clinical chemistry and in environmental analysis, low-cost disposable electrodes have some advantages, e.g., small sample volume and no cleaning operation necessary. Therefore we developed a new concept for producing ISEs in a batch process [2]. The ion-selective polymeric membrane is formed in the volume of an electrochemical inert microfibre matrix (MFM) by a casting technique. There it is homogeneously distributed by capillary forces and after the evaporation of the solvent the double matrix is obtained.The key innovation is to use the homogeneity of the inert matrix to produce almost identical ionselective membranes. Thin silver dots can be formed on the MFM by vacuum evaporation, sputtering or screen-printing before membrane deposition. Thus
micm fibra ma*ix
insulator/’ (heat sealing film)
iow3ebctive membrane
0925-4005/95/$09.50
Q 1995 Elsevkr Science S.A. All rights
2. Experimental 2.1. Sensor preparation Fig. 1 shows a schematic view of the low-cost ionselective electrode. The electrodes were produced in a batch process as shown for one electrode in Fig. 2. To prepare the silver contact and the strip conductor, silver was evaporated (Univex 300 vacuum coater, Leybold) on the rear side of a sheet of filter paper (Schleicher & Schuell, 58g3 Blue ribbon ashless, ref. no. 300 211), which was then cut into strips (40 mmx
_-l..-___-.-l ---._. ---/ _
Fig. 1. Schematic view of the lowcost ion-selective electrode.
SSDI 0925-4005(94)01454-P
‘coated-film’ electrodes with very well-adhering double matrix membranes can be prepared. We have described such double matrix membranes for applications in an electrode body with a liquid internal junction and as coated-film electrodes [3,4]. Here we describe the development of such membranes into disposable ISEs.
reserved
Fig. 2. Production of the electrode.
flltsr
paper (stlvar deposit&l
722
M. Borchardt et aI. / Semors and Achcators B 24-2s
4 mm strip size). In the next step heat-sealing films (150 pm thick, Team Codor, DUBO-Schweitzer GmbH) were perforated (holes with a diameter of 3 mm) and cut into suitable sizes. The heat-sealing films consist of polyethylene and polyester. If they are heated under pressure together with paper in the middle, both materials are linked very stably. Ten strips were laminated together at 125 “C between two heat-sealing films for insulation. By that procedure paper strips are completely wrapped up with the heat-sealing film except one hole for the membrane and the contact area on the other side. In the areas without paper between the two heatsealing films, the fihns are linked together veIy stably, so after cutting the side encapsulation is ensured. The 5 mm long silver pads on the back of the MFM were electrically contacted through an opening in the lower film. The following membrane cocktail was used (wt.%): 32.8% PVC, 2% valinomycin, 0.5% potassium tetrakis(C chlorophenyl) borate, 64.7% bis(2-ethylhexyl) sebacate (DOS). 400 mg of membrane components were weighed in a 5 ml glass bottle and then 1.5 ml THF and 0.5 ml cyclohexanone were added. 10 ~1 of the membrane cocktail were placed in each hole. To prevent an overflow of substance, four dispensing steps at intervals of 5 min were automatically carried out by the Asymtek Automove 402 Fluid Dispenser. Before the measurement was started and the electrodes were separated, the solvents were allowed to evaporate overnight. 2.2. Measurements The performance of the electrodes was judged by the slope, the detection limit, various selectivity coefficients and the reproducibility of the electrode preparation. All measurements were carried out with a pH meter (Microprocessor pH Meter pH 30OO/Multiplex 3000, WTW Weilheim, Germany) and an AglAgCl reference electrode from Orion (model 90-02) at room temperature. A 0.1 M sodium formate solution was filled into the outer chamber of the reference electrode. The calibration curves were obtained by applying the addition method in which known amounts of the measured ion (KCl) were added to the sample solution from low to high concentrations. The selectivity coefficients were evaluated by the fixed interference method according to IUPAC recommendations [5].
3. Results and discussion The electrochemical response behaviour of a potassium-selective sensor versus a commercial Ag/AgCl electrode is shown in Fig. 3 and the calibration graph in Fig. 4. As determined from Fig. 4, the detection limit is 10e6.” mol 1-l and the slope 58 mV decade-‘.
(1995) 721-723
10
0
30 tlmin4’
20
xl
60
70
Fig. 3. Response behaviourof a K+-selective electrode (concentration steps of one decade from lo-’ to 10m2 mol I-’ KCI).
N=4
loo
““I”“‘.
-8
-7
-6
.
0 ” I( . . ‘I”. -5 _pK4 -3
”
-2
Fig. 4. Calibration graph of a K+-selective electrode. 560
m 450
i
Pa
6 m- . zo-
Fig. 5. Calibration curves of ten K+-selective electrodes (in 0.1 M NaCl solution).
The selectivity coefficient log QYNa is -4.3 (in 0.1 M NaCl solution). Fig. 5 shows the calibration curves of ten potassium-selective electrodes (a whole batch of simultaneously produced electrodes without selection). The statistical data of that measurement are given in Table 1. The lifetime of the electrodes (continuous immersion in electrolyte solution) was in the range of a few days.
hf. Borchordi et al. I Sensors and Actuators B 24-25 (1995) 721-723 Table 1 Reproducibility of ten potassium electrodes (lo-‘-lo-* mol 1-y
in the linear range
-4
-3
-2
-1
C [mv] D [mv] E [mv] F [mv] G [mV) H [mv] I [mV) J [mV) K [mv]
347.8 349.8 349.8 350.3 349.1 352.6 350.3 349.5 349.0 346.4
395.5 397.7 397.6 297.3 397.1 400.3 398.6 397.4 397.5 394.3
450 452.4 452.4 452.4 451.8 455.4 453.3 452.3 452.3 449.5
499.8 504.6 504.4 504.6 504.4 507.8 505.4 584.3 503.7 501.5
Average potential [mv] Standard deviation [mv]
349.4 1.6
397.3 1.6
452.1 1.6
504.0 2.1
-PK Sensor B [mv]
Sensor Sensor Sensor Sensor Sensor Sensor Sensor Sensor Sensor
The results show that the electrochemical response behaviour of the disposable electrodes is comparable with that of a conventional ISE using the same membrane cocktail. The main advantage of the new electrode configuration is the technology of electrode preparation. High productivity can be achieved by a simple batch technology. In addition, only cheap mass products were used. As demonstrated in Fig. 5 and Table 1, the double matrix membrane technology allows the preparation of ISEs with very similar response characteristics, so it should be possible to calibrate only a few electrodes from a batch. This is the precondition for a quick,
723
accurate and reliable on-site test using an ISE. In combination with a disposable reference electrode, it is possible to produce a low-cost disposable test strip [6,7]. The previous results (Table 1) for these handmade electrodes should be improved by optimizing the sensor preparation.
Acknowledgement We are grateful to the Ministerium fiir Wissenschaft turd Forschung, Nordrhein-Westfalen, Germany, for financial support.
References 311 K. Cammann, Working with Ion-selective Electrodes, Springer, New York, 1979. [21 M. Knoll, Gemtan Patent No. P4137261.1 (13 Nov., 1991). [31 M. Knoll, K. Cammann, C. Dumschat, M. Borchardt and G. H&g, Microfibre matrix-supported ion-selective PVC-membranes, Sensors and Achmom B, 20 (1994) 1. t41 C. Dumschat, M. Borchardt, C. Diekmann, J. Hepke and M. Knoll, Double matrix membranes for potentiometric cation sensitive sensors, Fresenius’Z. Anal. Chem, 348 (1994) 553. [51 Recommendations for Nomenclature of Ion-Selective Electrodes, Pure AppZ. Chem., 48 (1976) 129. [61 C. Dumschat, M. Borchardt, C. Diekmann, K. Cammann and M. Knoll, Potentiometric test strip, Semors and Actuators 8, 24-25 (1995) 279-281. [71 C. Diekmann, C. Dumschat, K. Cammann and M. Knoll, Disposable reference electrode, Sensors and Actuators 8, 24-25 (1995) 276-278.