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Detection of ionic compounds in water with a new poly-carbon acid coated quartz crystal resonator
Sensors and Actuators B 65 Ž2000. 273–276 www.elsevier.nlrlocatersensorb
Detection of ionic compounds in water with a new poly-carbon acid coated quartz crystal resonator R. Borngraber, J. Hartmann, R. Lucklum ) , S. Rosler, P. Hauptmann ¨ ¨ Institute of Measurement Technology and Electronics, Otto-Õon-Guericke-UniÕersity, Magdeburg, P.O. Box 4120, D-39016 Magdeburg, Germany Accepted 4 August 1999
Abstract A new hydrophilic poly-carbon acid ŽPCA. coated quartz crystal resonator is able to detect ionic compounds, for example, organic and inorganic acids and amines in drinking water. The mass change is induced by an intra- or intermolecular ion–proton exchange or by strong acid–base interactions. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Ionic compounds; Poly-carbon acid; Quartz
1. Introduction The continuous control of the quality of several kinds of water is one of the important task of the environmental analysis. Today, the automatic water analysis to detect small amounts of toxic substances and pollutants is very costly and time-consuming. We have shown recently that quartz-crystal-microbalance-sensors coated with chemically sensitive layers are promising devices for the detection of neutral organic pollutants in drinking water w1,2x. The mass increase on the quartz sensor surface through specific interactions between the coated solid layer and the analyte in water produces a shift in the resonant frequency of the oscillating quartz disc. The application of coated crystals for sorption of ionic species w3–5x and amines w6x from solutions have been reported in some publications. We synthesised a new poly-carbon acid ŽPCA. with a rigid polymer skeleton and polar side groups with the capability to interact with ionic and polar analytes via hydrogen bonding or salt formation. We found that our new polymer gives high sensor signals towards organic and inorganic acids in drinking water due to a mass decrease process on the sensor surface. We assume a reversible ion exchange process and report about the behaviour of these polymers towards Na, Ca and Cu and Fe cations in pure water. Also, the regener)
Corresponding author. Tel.: q49-391-67-1-8309; fax: q49-391-67-12609
ation step of these ion loaded polymers will be shown. The behaviour of PCA-films towards neutral analytes Žchlorinated and aromatic hydrocarbons and pesticides. will be discussed.
2. Experimental 2.1. Sensor coatings The PCA was synthesised by polymer-analogous reaction of styrene–maleic anhydride copolymer Ž1:1. with n-butylamine in tetrahydrofuran at room temperature. We obtained a mixture of copolymeric mono- and dicarbon acid ŽPCA Ia r IIa., which is insoluble in water and soluble in alcohols w7x. We used a copolymer of methacrylic acid and methylmethacrylate ŽFLUKA. as reference material. Solutions of 0.05% by mass of the acidic polymers in methanol were applied by airbrush technique onto the quartz surface. 2.2. Experimental set-up The experimental set-up is shown in Fig. 1 and described in detail in Ref. w1x. The test mixture Žsalt solutions and diluted acids. and a reference Ždeionized or drinking water. are pumped alternately through a thermostat and a low-pass into the measuring cell which contains eight quartz sensors. The quartz crystal is in contact with the liquid one side only. The flow rate of the liquid was kept
0925-4005r00r$ - see front matterq 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 9 9 . 0 0 3 0 4 - 4
274
R. Borngraber ¨ et al.r Sensors and Actuators B 65 (2000) 273–276
Scheme 1. Principle of intramolecular proton–metal ion exchange in the rIIa ŽR s n-butylamido.. polymer layer Iar Fig. 1. Experimental set-up for the measurements of ionic and polar analytes in deionized and drinking water.
constant at 3 lrh. Each quartz is held in place by a glass ceramic block. The quartzes are sealed with silicone glue ŽRS Components.. The sensor array is maintained to within 0.1 K of a pre-set value by the flowing measurand. We have used AT-cut quartz discs with gold electrodes and a resonance frequency of 10 MHz for our investigations.
3. Results and discussion After coating the quartz sensor with the copolymeric mono- and dicarbon acid ŽIa r IIa., we tested the layers by addition of organic and inorganic acids in drinking water. High positive frequency shifts can be observed, indicating a mass decrease at the sensor surface. After addition of drinking water the resonant frequency recovers. Fig. 2 shows positive frequency shifts by addition of 250 vpm hydrochloric acid in drinking water using a thick layer Ža: D f s q3060 Hz. and a thin layer Žb: D f s q1570 Hz. of PCA I r II. The sensor response is reversible, if drinking water is used as reference liquid during the washing cycle. By contrast, a significant irreversible contribution was
found in case of deionized water as reference liquid. In comparison with PCA, the copolymeric methacrylic acid Žc. is not stable enough in water. It reacts not sensitive after addition of acid Ž D f s q150 Hz. and shows no reversibility after treatment with both drinking water and deionized water. We assume an intramolecular or intermolecular ion– proton exchange at the poly-carbon acid PCA Ia r IIa coated quartz sensor. In the first step, the polymer PCA layer absorbs metal ions from drinking water to form the metal salts Ib r IIb ŽScheme 1.. If the sensor is exposed to diluted acids, the resonant frequency increases indicating a loss in the surface mass. The metal ions are desorped and replaced by the lighter protons to get the starting acidic polymer PCA Ia r IIa. The frequency shifts are dependent on the acid concentration Žfor hydrochloric acid in the range of 10–150 vpm. and the layer thickness Ž1–10 kHz. w2x. A viscoelastic effect due to an ionic crosslinking reaction or swelling processes as we reported previously for some other coating materials w8x could be excluded by impedance measurements w2x. Also, the influence of conductivity of the solutions is not important, because the uncoated bare quartz sensor, which acts as reference, shows only very small frequency shifts.
Fig. 2. Sensor responses of quartz crystal resonators coated with acidic polymers after exposure of 250 vpm hydrochloric acid in drinking water and after different washing cycles. Ža. 345 nm PCA I r II, Žb. 180 nm PCA I r II and Žc. 180 nm poly-methacrylic acid.
R. Borngraber ¨ et al.r Sensors and Actuators B 65 (2000) 273–276
275
Fig. 3. Ion–proton exchange of polymer PCA I r II in deionized water Ža.: reversible sensor response by loading with calcium cations and unloading with hydrochloric acid. As comparable layer we used poly-methacrylic acid Žb..
3.1. Measurements in deionized water To check our thesis, we repeated the experiment with deionized water ŽFig. 3.. In the first step, we activated the layer with diluted acid Ž250 vpm hydrochloric acid. to get the acidic polymer Ia r IIa. After washing the layer with deionized water a solution of calcium chloride Ž50 ppm. was added to accumulate calcium via salt forming at the carboxylate groups Žsee structure IIb.. The reversible regeneration was possible via the diluted acid again. The exchange process can be performed as well with higher calcium chloride concentrations. Fig. 4 summarises the ion sensitivities of PCA I r II towards calcium ions in dependence on the cation concentration and the layer thickness.
Fig. 4. Dependence of the ion sensitivities of PCA to Ca2q in deionized water on the Ca2q concentration and the PCA-layer thickness.
The PCA coating responses to other metal cations to form Ib r IIb in a similar way. We found the following tendency of negative frequency shifts in deionized water: Naq- Ca2qq f Cu2q. The number of positive charges and the affinity to salt formations are the important parameters for the ion sensitivity. It is possible to detect also heavy metal ions in deionized water reversible and with high sensitivity Že.g., 20 ppm copper salt with a frequency shift of 650 Hz using a PCA layer of 400 nm thickness..
Fig. 5. Principle component analysis of the sensor data of the quartz crystal resonator array using five sensitive sensor layers: cis-1,4-polybutadiene, polymethylmethacrylate, amyl-calixw8xarene, polyŽethylene– rII. propylene–styrene.q5% acrylic acid and polymer PCA Ir
276
R. Borngraber ¨ et al.r Sensors and Actuators B 65 (2000) 273–276
Scheme 2. Strong interaction between the acidic PCA Ir rII Žhere part Ia. and the base amino-triazol.
Only Fe 3q cations gave unexpected positive frequency shifts after loading the PCA-layer with Naq. IronŽIII.chloride is a strong Lewis acid and it is able to form complexes with sodium ions ŽNaFeCl 4 .. As a result of this complexation, we assume the same acidic form Ia r IIa like after treatment with diluted acids. The poly-methacrylic acid layer shows a similar behaviour like the polymer Ia r IIa, but the sensitivity towards metal ions is much lower Žsee Fig. 3.. One reason is the fact, that the poly-methacrylic acid has only monocarbon acid-units. 3.2. Measurement of pollutants in drinking water using a quartz crystal resonator array We tested different hydrophilic and hydrophobic coatings on a quartz-crystal-resonator-array in drinking water Žsee Fig. 1. towards hydrochloric acid and neutral chlorinated and aromatic hydrocarbons as well as two mixtures of chlorinated hydrocarbons with and without hydrochloric acid. We used polybutadiene, polymethylmethacrylate, amyl-calixw8xarene as hydrophobic layers, PEPSA wpolyŽethylene–propylene–styrene. with 5% acrylic acidx and PCA I r II as a hydrophilic layer. The hydrophilic polymers gave positive frequency shifts towards acids or mixtures of hydrocarbons and acids w2x. By contrast, the sensor responses of these materials to chlorinated and aromatic hydrocarbons in drinking water were negligible small Ž10–50 Hz.. On the other hand, amyl-calixw8xarene is very sensitive towards neutral lipophilic analytes Ž150–850 Hz., but it does not interact with acids Ž20 Hz.. After the measurements, the resulting signal pattern were analyzed with a principle component analysis ŽFig. 5.. The experimental data can be separated into four analyte Žobject. groups: group I with samples containing hydrochloric acid, group II with chlorinated hydrocarbons and the 2-chloroethyl-phosphonic acid, group III with aromatics and trichloroethylene and group IV with the polar analyte, the 3-amino-1,2,4-triazol used as pesticide. Mixture 1 belongs clearly to group III whereas mixture 2 and hydrochloric acid form group I. In the case of 2-chlorethyl-phosphonic
acid, the chemical properties are more similar to chlorinated organics ŽII. than to inorganic acids ŽI.. The selectivity of the array used towards amino-triazol is based on the high sensitivity of PCA Ia r IIa via the strong acid–base interaction between the polymeric acidgroups and the amino-group of the pesticide ŽScheme 2.. We got quasi-irreversible frequency shifts due to a partially salt formation with high frequency shifts. It is possible to detect 1 ppm of amino-triazol in drinking water Ž D f s y38 Hz.. The hydrophobic polymers are not sensitive towards the base.
4. Conclusions We have shown that the new synthesized hydrophilic polymer PCA can be used as a stable sensor layer to detect ionic and polar compounds. Via a proton–metal ion exchange, a reversible sensor response is possible in drinking water and deionized water. PCA I r II is able to detect acids in drinking water due to an exchange process of metal ions absorbed from the reference and the lighter ion protons Žmass decrease.. With this layer also a very sensitive and selective detection of amino-triazol in drinking water is possible. PCA broadens the range of chemical layers for the detection of toxic heavy metal salts in pure water and opens the application as chemosensor for monitoring of quality of pure water.
References w1x S. Rosler, R. Lucklum, R. Borngraber, J. Hartmann, P. Hauptmann, ¨ ¨ Sensor system for the detection of organic pollutants in water by thickness shear mode resonators, Sensors and Actuators B 48 Ž1998. 416–425. w2x S. Rosler, Quarzresonator-Sensoren zum Nachweis chemischer Sub¨ stanzen in Wasser, PhD, Otto-von-Guericke-University of Magdeburg, 1997. w3x T. Nomura, M. Sakai, Behaviour in solution of piezoelectric quartz crystals coated with polyŽvinylpyridines., Anal. Chim. Acta 183 Ž1986. 301–305. w4x T.C. Hunter, G.J. Price, Selective piezoelectric sensors using polymer reagents, Analyst 120 Ž1995. 161–165. w5x Y.S. Jane, J.S. Shil, Application of crown ether coated piezoelectric crystal as a detector for the ion chromatography, Analyst 120 Ž1995. 5–17. w6x X.C. Zhou, S.C. Ng, H.S.O. Chan, S.F.Y. Li, Detection of organic amines in liquid with chemically coated quartz crystal microbalance devices, Sensors and Actuators B 42 Ž1997. 137–144. w7x J. Hartmann, Synthesis and structural analysis, to be published. w8x R. Lucklum, S. Rosler, J. Hartmann, P. Hauptmann, On-line-detection ¨ of organic pollutants in water by thickness shear mode resonators, Sensors and Actuators B 35–36 Ž1996. 103–111.
Detection of ionic compounds in water with a new poly-carbon acid coated quartz crystal resonator R. Borngraber, J. Hartmann, R. Lucklum ) , S. Rosler, P. Hauptmann ¨ ¨ Institute of Measurement Technology and Electronics, Otto-Õon-Guericke-UniÕersity, Magdeburg, P.O. Box 4120, D-39016 Magdeburg, Germany Accepted 4 August 1999
Abstract A new hydrophilic poly-carbon acid ŽPCA. coated quartz crystal resonator is able to detect ionic compounds, for example, organic and inorganic acids and amines in drinking water. The mass change is induced by an intra- or intermolecular ion–proton exchange or by strong acid–base interactions. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Ionic compounds; Poly-carbon acid; Quartz
1. Introduction The continuous control of the quality of several kinds of water is one of the important task of the environmental analysis. Today, the automatic water analysis to detect small amounts of toxic substances and pollutants is very costly and time-consuming. We have shown recently that quartz-crystal-microbalance-sensors coated with chemically sensitive layers are promising devices for the detection of neutral organic pollutants in drinking water w1,2x. The mass increase on the quartz sensor surface through specific interactions between the coated solid layer and the analyte in water produces a shift in the resonant frequency of the oscillating quartz disc. The application of coated crystals for sorption of ionic species w3–5x and amines w6x from solutions have been reported in some publications. We synthesised a new poly-carbon acid ŽPCA. with a rigid polymer skeleton and polar side groups with the capability to interact with ionic and polar analytes via hydrogen bonding or salt formation. We found that our new polymer gives high sensor signals towards organic and inorganic acids in drinking water due to a mass decrease process on the sensor surface. We assume a reversible ion exchange process and report about the behaviour of these polymers towards Na, Ca and Cu and Fe cations in pure water. Also, the regener)
Corresponding author. Tel.: q49-391-67-1-8309; fax: q49-391-67-12609
ation step of these ion loaded polymers will be shown. The behaviour of PCA-films towards neutral analytes Žchlorinated and aromatic hydrocarbons and pesticides. will be discussed.
2. Experimental 2.1. Sensor coatings The PCA was synthesised by polymer-analogous reaction of styrene–maleic anhydride copolymer Ž1:1. with n-butylamine in tetrahydrofuran at room temperature. We obtained a mixture of copolymeric mono- and dicarbon acid ŽPCA Ia r IIa., which is insoluble in water and soluble in alcohols w7x. We used a copolymer of methacrylic acid and methylmethacrylate ŽFLUKA. as reference material. Solutions of 0.05% by mass of the acidic polymers in methanol were applied by airbrush technique onto the quartz surface. 2.2. Experimental set-up The experimental set-up is shown in Fig. 1 and described in detail in Ref. w1x. The test mixture Žsalt solutions and diluted acids. and a reference Ždeionized or drinking water. are pumped alternately through a thermostat and a low-pass into the measuring cell which contains eight quartz sensors. The quartz crystal is in contact with the liquid one side only. The flow rate of the liquid was kept
0925-4005r00r$ - see front matterq 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 9 9 . 0 0 3 0 4 - 4
274
R. Borngraber ¨ et al.r Sensors and Actuators B 65 (2000) 273–276
Scheme 1. Principle of intramolecular proton–metal ion exchange in the rIIa ŽR s n-butylamido.. polymer layer Iar Fig. 1. Experimental set-up for the measurements of ionic and polar analytes in deionized and drinking water.
constant at 3 lrh. Each quartz is held in place by a glass ceramic block. The quartzes are sealed with silicone glue ŽRS Components.. The sensor array is maintained to within 0.1 K of a pre-set value by the flowing measurand. We have used AT-cut quartz discs with gold electrodes and a resonance frequency of 10 MHz for our investigations.
3. Results and discussion After coating the quartz sensor with the copolymeric mono- and dicarbon acid ŽIa r IIa., we tested the layers by addition of organic and inorganic acids in drinking water. High positive frequency shifts can be observed, indicating a mass decrease at the sensor surface. After addition of drinking water the resonant frequency recovers. Fig. 2 shows positive frequency shifts by addition of 250 vpm hydrochloric acid in drinking water using a thick layer Ža: D f s q3060 Hz. and a thin layer Žb: D f s q1570 Hz. of PCA I r II. The sensor response is reversible, if drinking water is used as reference liquid during the washing cycle. By contrast, a significant irreversible contribution was
found in case of deionized water as reference liquid. In comparison with PCA, the copolymeric methacrylic acid Žc. is not stable enough in water. It reacts not sensitive after addition of acid Ž D f s q150 Hz. and shows no reversibility after treatment with both drinking water and deionized water. We assume an intramolecular or intermolecular ion– proton exchange at the poly-carbon acid PCA Ia r IIa coated quartz sensor. In the first step, the polymer PCA layer absorbs metal ions from drinking water to form the metal salts Ib r IIb ŽScheme 1.. If the sensor is exposed to diluted acids, the resonant frequency increases indicating a loss in the surface mass. The metal ions are desorped and replaced by the lighter protons to get the starting acidic polymer PCA Ia r IIa. The frequency shifts are dependent on the acid concentration Žfor hydrochloric acid in the range of 10–150 vpm. and the layer thickness Ž1–10 kHz. w2x. A viscoelastic effect due to an ionic crosslinking reaction or swelling processes as we reported previously for some other coating materials w8x could be excluded by impedance measurements w2x. Also, the influence of conductivity of the solutions is not important, because the uncoated bare quartz sensor, which acts as reference, shows only very small frequency shifts.
Fig. 2. Sensor responses of quartz crystal resonators coated with acidic polymers after exposure of 250 vpm hydrochloric acid in drinking water and after different washing cycles. Ža. 345 nm PCA I r II, Žb. 180 nm PCA I r II and Žc. 180 nm poly-methacrylic acid.
R. Borngraber ¨ et al.r Sensors and Actuators B 65 (2000) 273–276
275
Fig. 3. Ion–proton exchange of polymer PCA I r II in deionized water Ža.: reversible sensor response by loading with calcium cations and unloading with hydrochloric acid. As comparable layer we used poly-methacrylic acid Žb..
3.1. Measurements in deionized water To check our thesis, we repeated the experiment with deionized water ŽFig. 3.. In the first step, we activated the layer with diluted acid Ž250 vpm hydrochloric acid. to get the acidic polymer Ia r IIa. After washing the layer with deionized water a solution of calcium chloride Ž50 ppm. was added to accumulate calcium via salt forming at the carboxylate groups Žsee structure IIb.. The reversible regeneration was possible via the diluted acid again. The exchange process can be performed as well with higher calcium chloride concentrations. Fig. 4 summarises the ion sensitivities of PCA I r II towards calcium ions in dependence on the cation concentration and the layer thickness.
Fig. 4. Dependence of the ion sensitivities of PCA to Ca2q in deionized water on the Ca2q concentration and the PCA-layer thickness.
The PCA coating responses to other metal cations to form Ib r IIb in a similar way. We found the following tendency of negative frequency shifts in deionized water: Naq- Ca2qq f Cu2q. The number of positive charges and the affinity to salt formations are the important parameters for the ion sensitivity. It is possible to detect also heavy metal ions in deionized water reversible and with high sensitivity Že.g., 20 ppm copper salt with a frequency shift of 650 Hz using a PCA layer of 400 nm thickness..
Fig. 5. Principle component analysis of the sensor data of the quartz crystal resonator array using five sensitive sensor layers: cis-1,4-polybutadiene, polymethylmethacrylate, amyl-calixw8xarene, polyŽethylene– rII. propylene–styrene.q5% acrylic acid and polymer PCA Ir
276
R. Borngraber ¨ et al.r Sensors and Actuators B 65 (2000) 273–276
Scheme 2. Strong interaction between the acidic PCA Ir rII Žhere part Ia. and the base amino-triazol.
Only Fe 3q cations gave unexpected positive frequency shifts after loading the PCA-layer with Naq. IronŽIII.chloride is a strong Lewis acid and it is able to form complexes with sodium ions ŽNaFeCl 4 .. As a result of this complexation, we assume the same acidic form Ia r IIa like after treatment with diluted acids. The poly-methacrylic acid layer shows a similar behaviour like the polymer Ia r IIa, but the sensitivity towards metal ions is much lower Žsee Fig. 3.. One reason is the fact, that the poly-methacrylic acid has only monocarbon acid-units. 3.2. Measurement of pollutants in drinking water using a quartz crystal resonator array We tested different hydrophilic and hydrophobic coatings on a quartz-crystal-resonator-array in drinking water Žsee Fig. 1. towards hydrochloric acid and neutral chlorinated and aromatic hydrocarbons as well as two mixtures of chlorinated hydrocarbons with and without hydrochloric acid. We used polybutadiene, polymethylmethacrylate, amyl-calixw8xarene as hydrophobic layers, PEPSA wpolyŽethylene–propylene–styrene. with 5% acrylic acidx and PCA I r II as a hydrophilic layer. The hydrophilic polymers gave positive frequency shifts towards acids or mixtures of hydrocarbons and acids w2x. By contrast, the sensor responses of these materials to chlorinated and aromatic hydrocarbons in drinking water were negligible small Ž10–50 Hz.. On the other hand, amyl-calixw8xarene is very sensitive towards neutral lipophilic analytes Ž150–850 Hz., but it does not interact with acids Ž20 Hz.. After the measurements, the resulting signal pattern were analyzed with a principle component analysis ŽFig. 5.. The experimental data can be separated into four analyte Žobject. groups: group I with samples containing hydrochloric acid, group II with chlorinated hydrocarbons and the 2-chloroethyl-phosphonic acid, group III with aromatics and trichloroethylene and group IV with the polar analyte, the 3-amino-1,2,4-triazol used as pesticide. Mixture 1 belongs clearly to group III whereas mixture 2 and hydrochloric acid form group I. In the case of 2-chlorethyl-phosphonic
acid, the chemical properties are more similar to chlorinated organics ŽII. than to inorganic acids ŽI.. The selectivity of the array used towards amino-triazol is based on the high sensitivity of PCA Ia r IIa via the strong acid–base interaction between the polymeric acidgroups and the amino-group of the pesticide ŽScheme 2.. We got quasi-irreversible frequency shifts due to a partially salt formation with high frequency shifts. It is possible to detect 1 ppm of amino-triazol in drinking water Ž D f s y38 Hz.. The hydrophobic polymers are not sensitive towards the base.
4. Conclusions We have shown that the new synthesized hydrophilic polymer PCA can be used as a stable sensor layer to detect ionic and polar compounds. Via a proton–metal ion exchange, a reversible sensor response is possible in drinking water and deionized water. PCA I r II is able to detect acids in drinking water due to an exchange process of metal ions absorbed from the reference and the lighter ion protons Žmass decrease.. With this layer also a very sensitive and selective detection of amino-triazol in drinking water is possible. PCA broadens the range of chemical layers for the detection of toxic heavy metal salts in pure water and opens the application as chemosensor for monitoring of quality of pure water.
References w1x S. Rosler, R. Lucklum, R. Borngraber, J. Hartmann, P. Hauptmann, ¨ ¨ Sensor system for the detection of organic pollutants in water by thickness shear mode resonators, Sensors and Actuators B 48 Ž1998. 416–425. w2x S. Rosler, Quarzresonator-Sensoren zum Nachweis chemischer Sub¨ stanzen in Wasser, PhD, Otto-von-Guericke-University of Magdeburg, 1997. w3x T. Nomura, M. Sakai, Behaviour in solution of piezoelectric quartz crystals coated with polyŽvinylpyridines., Anal. Chim. Acta 183 Ž1986. 301–305. w4x T.C. Hunter, G.J. Price, Selective piezoelectric sensors using polymer reagents, Analyst 120 Ž1995. 161–165. w5x Y.S. Jane, J.S. Shil, Application of crown ether coated piezoelectric crystal as a detector for the ion chromatography, Analyst 120 Ž1995. 5–17. w6x X.C. Zhou, S.C. Ng, H.S.O. Chan, S.F.Y. Li, Detection of organic amines in liquid with chemically coated quartz crystal microbalance devices, Sensors and Actuators B 42 Ž1997. 137–144. w7x J. Hartmann, Synthesis and structural analysis, to be published. w8x R. Lucklum, S. Rosler, J. Hartmann, P. Hauptmann, On-line-detection ¨ of organic pollutants in water by thickness shear mode resonators, Sensors and Actuators B 35–36 Ž1996. 103–111.