Montevideo, 27-29 de setiembre de 2006 IBERSENSOR 2006
ISBN: 9974-0-0337-7 1/4
LTCC microsystem for the detection of lead in natural water.
Integration of a PVC-based ion selective membrane
E.Arasa Puig, N. Ibáñez-García, J. Alonso-Chamarro (*)
Analytical Chemistry Department, GSB, Universitat Autònoma de Barcelona, Spain e-mail: [email protected]
Phone number: +34-93 581 18 36 Fax number: +34-93 581 23 79 Abstract
In this work a new configuration of batch electrodes based on LTCC (Low Temperature Co-fired Ceramics) technology aimed at the detection of lead in water were constructed. Different configuration devices and transductors were tested. The best results were obtained when copper was deposited onto the silver screen- printed conductive pastes acting as a membrane conductive inner support. Using the optimized experimental conditions, a microflow injection system for the detection of lead was fabricated.
Key words: LTCC, lead, copper electrodeposition, microflow system
Introduction
During the last years, the mandatory control of environmental pollutants like heavy metals in natural waters has generated an increasing interest in the development of novel sensors for these compounds.
The extended industrial use of lead and its serious hazardous effects on human health, which involves neurological alterations, nephrotoxicity, anaemia and kidney cancer [1], have converted this heavy metal in object of great social concern.
Lead was used for the fabrication of pipes, until lead from their lixiviation was found in water. Although pipes are now made of PVC, there are still houses with lead pipes. More restrictive regulations have been approved at this respect. Water Distribution Companies are bound by these regulations to guarantee that lead level in water in European houses will be less than 10 ppb in 2013 (98/83/CE). Due to this fact, new low size and portable analytical instrumentation capable of providing rapidly accurate and in situ information at a low cost is demanded in order to enforce the law.
The use of LTCC (Low Temperature Co-fired Ceramic) technology for the fabrication of microflow systems assembles miniaturization advantages (reduction of sample/reagent consumption, mass production and portability) with those provided by continuous flow techniques (simplicity, versatility, connectivity, speed of analysis, sample pre-treatment, etc.).
In this work potenciometric sensors based on tetrabenzyl pyrophosphate (TBPP) as ionophore for the determination of lead have been used.
Previous to the integration in the LTCC system, they were evaluated in batch conditions using conventional epoxy-graphite electrodes, showing good response characteristics. In order to study the compatibility between the sensing membranes and the materials commonly used in the LTCC technology, preliminary batch experiments were performed. Afterwards a miniaturized analyzer that integrates these membranes was also fabricated using LTCC technology.
Experimental section
Connection to the external set-up
Membrane-conductor contact area Conductive paste 1.5 cm
0.5 cm
Figure 1. Up: picture closed device. Down: Schematic view of the different layer.
Montevideo, 27-29 de setiembre de 2006 IBERSENSOR 2006
E (mV/dec)
0 10 20 30
26 m 11.
DuPont and Heraeus green tapes ceramics and screen-printing pastes were used in order to study the behaviour of the ion-selective membranes once deposited onto them. It is known that green tapes and screen-printing pastes contain heavy metals which could contaminate the membrane. This fact will be also studied.
Furthermore, several devices with different membrane-conductor contact areas were also tested.
Basically, two devices configuration were tested: the so-called opened and the closed one (figure 1 and).
In the first ones DuPont dielectric paste was used to insulate the conductive path from water.
Finally, the performance of other transductor materials deposited onto the conductive paste, such as screen-printed epoxy-graphite and electrodeposited copper was evaluated.
Results and discussion
All the electrodes were calibrated using the multiple standard addition procedure. Activities were calculated according to the Debye-Hückel procedure
[2]. m2 c 6 mm2 c 2.9 mm2c 26 mm2o 11.6 mm2 o 2.9 mm2 o
For all the experiments performed, the sensor cocktail contained tetrabenzyl pyrophosphate (TBPP) as ionophore. The cocktail was poured into the transducer membrane region and then, the volatile solvent was evaporated at room temperature.
The electrode was stored dry when not is in used.
All the measurements were made at room temperature (25ºC).
It was observed the importance of avoiding an incorrect insulation of the conductive pastes because that caused mixed potentials and irreproducibility in the results obtained.
In all cases, the slope obtained in batch conditions was between 20-23 mV/dec, except in the case of the paste Heraeus “Via Fill”, onto which it was not possible to solder the connection cable to the external set-up (see table 1).
Table 1. Slopes and detection limit (LD) using different screen printing conductive pastes with DuPont green tapes.
Heraeus DuPont Type of
paste Slope (mV/dec)
LD (ppm)
Slope (mV/dec)
LD (ppm) Internal
Layer
23 ± 2 (n= 9)
2 ± 2 (n=9)
22 ± 3 (n= 7)
5 ± 2 (n= 7) Solderable 22± 4
(n= 6)
3 ± 6 (n= 6)
20 ± 3 (n= 7)
2 ± 2 (n= 7) Via Fill
*** ***
20± 3 (n= 9)
1 ± 0.9 (n= 9)
A slope of 23 ± 5 mV/dec and a detection limit (LD) of 3 ± 1 ppm (n=5) were obtained with green tapes and solderable screen printing pastes from Heraeus.
The results obtained with Heraeus green tapes (free from heavy metals) were similar to those obtained by the DuPont ones. After 15 days of periodic
calibrations there were no evidences that membrane contamination had occurred.
In order to evaluate the possible influence of the device configuration in the membrane response two configuration devices with different membrane- conductor contact area were tested: 26, 11.6 and 2.9 mm2. As it can see in figure 2 the calibration slopes obtained were about 20mV/dec in all cases. No evidences of the influence of configuration device in these results were found. The detection limit was about 2 ppm in all LTCC batch devices tested.
Figure 2. Slopes obtained with closed and opened LTCC batch devices. Membrane-conductor contact area tested: 26, 11.6, 2.9
mm2. c=closed; o= opened.
Smaller membrane-conductor contact areas, closer to those normally used in a FIA microsystems, (1.7, 0.9 and 0.2 mm2) were also evaluated, As expected, the results were independent of the membrane-conductor contact area.
As it can be seen slopes obtained in all cases were slower than that predicted by Nernst law for divalent cations. Some batch LTCC electrodes were conditioned overnight in 10-3M Pb(NO3)2 solution obtaining the same results that in unconditioned membranes rejecting a problem in the membrane conditioning process.
In order to improve the results, devices with thicker paths of screen-printed conductive pastes were constructed. Since no improvement in membrane response was obtained, we decide to test different transductor materials deposited onto the silver screen-printed paths.
In first place, we deposited a mixture of epoxy- graphite onto the silver screen-printed conductive paths [3]. Once epoxy-graphite was cured the membrane was deposited onto it.
As it can be seen in figure 3, with an epoxy-graphite mixture as transductor material slopes obtained were about 15 mV/dec, far enough from the theoretical slope of 29.58 mV/dec predicted by Nernst law. The detection limit in all cases was about 3 ppm of lead.
ISBN: 9974-0-0337-7 2/4
Montevideo, 27-29 de setiembre de 2006 IBERSENSOR 2006
ISBN: 9974-0-0337-7 3/4
Figure 3. Slopes obtained with LTCC batch device when epoxy- graphite (1: 1) was used as transductor material.
Apart from silver other metals, like platinum, gold or copper had been reported as transducer materials [4].
In this case, copper were electrodeposited onto silver screen-printed paste. It is known that the morphology of the electrodeposited copper is an important fact in membrane response [5,6] As a first approach, the selection of potential applied and deposition time had to be done. Potential and times between 0.1 and 3V and 15 and 60 minutes, respectively, were tested.
Figure 4. Right: opened device using silver screen-printing pastes. Left: openend device after copper electrodeposition onto
silver screen-printed paste.
As it can be seen in table 2, the best results, closer to the ones obtained with a conventional epoxy- graphite electrode, were obtained with a potential electrodeposition of 3V during 15 minutes.
Table 2. Slope and detection limit obtained using different electrodeposition conditions.
Electrodeposition Potential (V)
Electrodeposition Time (min)
Slope (mV/dec)
Detection limit (ppm)
0.1 15
21 ± 5 (n= 4)
1 ± 0.6 (n= 4)
0.1 60
31 ± 6 (n= 4)
1.± 2 (n= 4)
3 15
34 ± 1.
(n= 4)
0.9 ± 0.6 (n= 4)
The higher the potential or the electrodeposition time, the higher the surface roughness and the compactness of the copper layer obtained. This surface morphology is supposed to increased membrane adhesion obtaining best results.
device
0 1 2 3 4
E (mV/dec)
0 5 10 15 20 25
D
Figure 5. SEM pictures. A: screen-printed silver paste. B:
copper electrodeposition 0.1V 15 minutes. C: copper electrodeposition 0.1V 60 minutes. C: copper electrodeposition
3V 15 min
The copper electrodeposition, not only improved the sensibility, but also the detection limit. As it can be seen in figure 6 the detection limits obtained with copper electrodeposition were comparable to those obtained with conventional epoxy-graphite electrodes.
ppm Pb2+
0 1 2 3 4
silver-based screen-printed epoxy-graphite mixture copper electrodeposition conventional epoxy-graphite electrode
Figure 6. Detection limit obtained with batch LTCC devices and conventional epoxy-graphite.
As it has been said before, the use of the LTCC technology for the fabrication of microflow systems assembles miniaturization advantatges with those provided by continuous flow techniques. In order to implement the evaluated membrane deposited onto the copper layer (electrodeposited over the silver conductive layer following the experimental conditions already optimized), the LTCC microsystem showed in figure 7 was designed, constructed and tested. The miniaturized device also
Montevideo, 27-29 de setiembre de 2006 IBERSENSOR 2006
included the reference electrode, previously tested in other LTCC systems [7].
a
b
c
d e f g
h
i d1
d2 a
b
c
d e f g
h
i a
b
c
d e f g
h
i a
b
c
d e f g
h
i d1
d2
Figure 7. LTCC device picture; a: carrier solution inlet; b:
water/sample solution inlet; c: Cl- solution inlet; d: ground connection; e: reference electrode; f: ground connection; g: ISE
connection; h: copper conductor path where membrane is deposited; i: waste. d1: 5.3 cm; d2: 3.9 cm. Black line: hydraulic
circuit.
Preliminary results showed a slope about 30 mV/dec comparable to that obtained in a conventional FIA system. However, the detection limits that presumably could be obtained will be far enough from that required by law regulation in a next future and that what is possible to find in natural water.
Due to these facts, a preconcentration step is needed.
The next step will be the implementation of electrolytic preconcentration followed by lead redissolution in a small volume to finally be detected by the ion-selective electrode in a device similar to the one showed in figure 7.
References
[1] http://www.pangea.org (3/3/2004)
[2] S. Kamata et al, Copper (II)-selective electrode using thiuram disulfide neutral carrier. Anal. Chem. 60 (1988) 29-43.
[3] J. Alonso “Diseño y construcción de detectores potenciométricos para FIA. Aplicación al análisis multiparamétrico en FIA”. Doctoral Thesis. UAB, Bellaterra (1987).
[4] K.B. Male et al, Electrochemical detection of carbohidratos using copper nanoparticles and carbon nanotubes. Analytica Chimica Acta, 516 (2004) 35-41.
[5] C.M. Ghimbeu et al, Preparation of SnO2 and Cu- doped SnO2 thin films using electrostatic spray elctrodeposition. Journal of the European Ceramic Society (2006), in press.
[6] R.S. Niranjan et al, High H2S-sensitive copper-doped tin oxid thin film. Materials Chemistry and Physics, 80 (2003) 250-256.
[7] N. Ibanez-Garcia et al, Continuous flow analytical microsystems based on LTCC technology. Integrated potentiometric detection based on solvent polymeric ion- selective electrodes. Analytical Chemistry (2006), 78 (9), 2985-2992.
ISBN: 9974-0-0337-7 4/4
