Miniaturized chemical sensors in monitoring of water
M. Chudy, W.Wróblewski, A. Dybko, Z Brzózka
Warsaw University of Technology, Department of Analytical Chemistry,
Noakowskiego 3, 00-664 Warsaw, Poland, ph. + 48 22 660 5427, fax. + 48 22 660 561,
e-mail: brzozka@ch.pw.edu.pl
Keywords: CHEMFET, Potentiometric sensors, Fibre optic sensors, Flow cell, Multi-ion analysis.
Introduction
In Poland it becomes popular to drink Oligocene water from natural deep intakes,
especially in big cities where water purification plants are not efficient and obsolete. It is
obvious that quality of water from new intakes should be continuously controlled both
biologically and chemically. Concentration of the ions to be monitored can determine the taste
and the quality of the water. Maximum concentrations of ions in potable water according to
Polish law are as follows: 200 mg/dm3, 300 mg/dm3 and 10 mg/dm3 for Na+, Cl- and NO3respectively. The special recommendations for water hardness levels indirect define the limits
of calcium content in waters. High concentration of chloride in drinking water might have
negative influence on people with heart disease. These ions even at the level of 250 mg/dm3
can cause the salty taste of waters. The dieticians recommend drinking water in which Na+
ions concentration is not higher than 100 mg/dm3 [1]. High concentration of sodium in natural
water can indicate its high level of pollution especially when water reservoirs are placed near
industrial plants (pulp and paper, chemical, food-processing or glass-works). Infants can
suffer from cyanosis disease after long-time drinking of water which consists of nitrates over
10 mg/dm3. Overdose of nitrate fertilisers can be one of the reason of lakes eutrophication so
an investigation of nitrate fluctuations in natural water can be used for the primary biomass
growth observations. Calcium ions content in natural water is limited of natural CaCO3 and
CaSO4 solubility. It has not special toxic influence on a human body but too hard water is less
useful both in households and for industrial purposes. All that information pointed that these
simple ions’ monitoring seems to be as important as other toxic specious determination.
Miniaturized chemical sensors
Monitoring of water composition is an important field of research and seems to be much
more interesting when several components can be measured simultaneously. The probes
designed for monitoring systems use a wide range of sensors. The chemical sensors are based
on potentiometric, amperometric and spectroscopic principles. Many of the systems are
specialized to specific requirements allowing monitor only one pollutant (for example: pH,
redox potential, turbidity, Biological Oxygen Demand - BOD).
Chemically modified Field Effect Transistors called CHEMFETs allowed to construct
microsized sensors and thus miniaturising monitoring systems – figure 1. Using the modern
semiconductor technology such microsensors can be cheap and mass produced. By depositing
different ion-sensitive polymer membranes one can obtain sensitivity for chosen target
analyte. CHEMFET's performance is similar to that obtained for other classical chemical
sensors, the response time can be diminished.
5
m
m
2
10
m
m
Figure 1. Photographs of CHEMFETs
Figure 2. Photography of fibre optic sensor
Nowadays ground waters have only been controlled in time- and space-limited intervals
and the various sampling have been accompanied by expensive analysis. This procedure gave
no information about possible fluctuations of concentrations of the measured components
occurring in the meantime. This problem can be solved by the use of the flow system based on
the versatile head with optional configurations of various chemical sensors for multiparameter
waters monitoring.
Multiparameter analysis using chemical sensors is a very useful analytical technique
because of avoiding laborious and time-consuming preliminary sample treatment. Chemical
sensors for such a purpose should be carefully designed and tested under similar conditions to
those, which could appear during analysis of real samples. Continuous measurements of the
analyte are possible if the sensors are mounted in a flow configuration. The calibration
procedure in such systems is quite simple allowing fast determination of the chemical species.
Moreover, flow systems improve the effectiveness and some performances of used sensors
e.g. the response time and the selectivity [2-4]. The application of chemical microsensor based
on a field effect transistor (FET) or fiber optic (figure 2) as a transducer gives the possibility
of miniaturisation of whole measuring system. Up to now a few flow devices were described
in the literature [4-7]. Most of published flow-cells were strictly dedicated to one type of the
sensors e.g. (ISFETs, solid state electrodes, amperometric sensors) which limited their
applications. We have designed and developed our own flow measurement system with
versatile head with multiparameter ability based on different types of sensors – figure 3.
SENSOR amplifier
reference
electrode
inlet
peristaltic
pump
METROHM
Dosimat
outlet
sample inlet
Figure 3. Flow set-up for measurements of various sensors.
Preliminary tests evaluating properties of the head were published elsewhere [8,9]. For
the sake of application of chemical sensors, the designed earlier system is quite flexible. As
exemplary sensors the Na+, Ca2+, NO3- - selective back side contacts CHEMFETs and Cl- selective solid state electrodes were mounted in the flow head and used for these ions
determinations – figure 4.
 E [V]
Na
cal 1
0,20
cal 2
sample
sample
cal 3
 E [V]
+
Ca
cal 2
cal 1
0,12
sample
2+
cal 3
sample
sample
sample
0,10
0,15
0,08
0,06
0,10
0,04
0,05
0,02
0,00
0,00
0
20
40
60
80
100
120
140
160
0
20
40
60
TIME [min]
 E [V]
0,25
80
100
120
140
160
TIME[min]
 E [V]
-
NO3
-
Cl
0,25
Cal 3
Cal 2
Cal 1
0,20
0,20
0,15
0,10
0,15
sample
0,05
sample
sample
0,10
0,00
cal 1
sample
cal 3
cal 2
sample
sample
0,05
-0,05
0
20
40
60
80
100
TIME[min]
120
140
160
0
20
40
60
80
100
120
140
160
TIME [min]
Fig. 4. Responses of the Na+, Ca2+, NO3- and Cl- sensors (three-step calibration and sample
measurements).
The chosen ions are mostly responsible for some important features of natural waters (salty
taste, conductivity and hardness).
References
1. S.D. Faust, O.M. Aly, Chemistry of natural waters, An Arbour Science, USA 1981
2. K. Cammann, Instrum. Forsch. 9 (1982) 1.
3. L. Ilcheva, K. Cammann, Flow Injection Analysis of Chloride in Tap and Seawage Water using
Ion-Selective Electrode Detection Fresenius Z. Anal. Chem., 322, (1985), 322-326.
4. P.D. van der Wal, E.R.J. Sudhölter, D.N. Reinhoudt, Design and properties of a flow-injection cell
using potassium-selective ion-sensitive field-effect transistors as detection elements, Anal. Chim.
Acta, 245, (1991), 159-166.
5. K. Cammann, Continuous pollution control by chemical sensors, Sensors and Actuators B 6 (1992)
19-24.
6. C. Jimenez, I. Marques, J. Bartroli, Continuous-Flow System for On-Line Water Monitoring Using
Back-Side Contact ISFET-Based Sensors Anal. Chem. 68 (1996) 3801-3807.
7. H.H. van den Vlekkert, U.H. Verkerk, P.D. van der Wal, A. van Wingerden, D.N. Reinhoudt, J.R.
Haak, G.W.N. Honig, H.A.J. Holterman, Multi-ion sensing device for horticultural application
based upon chemical modification and special packaging of ISFETs, Sensors and Actuators B 6
(1992), 34.
8. A. Dybko, M. Chudy, W. Wróblewski, J.;Maciejewski, J. Berliński, Z. Brzózka, Flow device for
multi parameter chemical analysis and comparable measurements of chemical sensors, Polish Patent
Application 1999, P.333 615.
9. M. Chudy, A. Dybko, W. Wróblewski, Z. Brzózka, Novel head for testing and measurements of
chemical microsensors, Anal. Chim. Acta 429, (2001), 347-355.