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Synthesis and evaluation of fluoropolyol isomers as saw microsensor coatings: role of humidity and temperature
Sensors and Actuators B 49 (1998) 139 – 145
Synthesis and evaluation of fluoropolyol isomers as saw microsensor coatings: role of humidity and temperature Dominique Rebie`re a,*, Corinne De´jous a, Jacques Pistre´ a, Jean-Franc¸ois Lipskier b, Roger Planade c b
a Laboratoire IXL, Uni6ersite´ Bordeaux I, 351 cours de la libe´ration, F 33405 Talence, Cedex, France Laboratoire Central de Recherche, Thomson CSF, Domaine de corbe6ille, F 91404 Orsay, Cedex, France c ´ tudes du Bouchet (DCE/DGA), Le Bouchet BP 3, F 91710 Vert-Le-Petit, France Centre d’E
Abstract A surface acoustic wave (SAW) organophosphorus gas detector has been designed, fabricated and tested. The gas detector consists of a dual delay line fabricated on a single quartz substrate. Each delay line is connected into the feedback path of a radio-frequency amplifier, to realize a SAW oscillator. The propagation path of one delay line is coated with fluoropolyol (FPOL). These polymers offer an interesting way to detect organophosphorus compounds like GB at low concentration levels. The absorption of vapors induces phase variations due to mass loading and stress effects. These variations result in corresponding frequency shifts. In this work, we have synthesised the four FPOL isomer combinations separately and characterized these materials by physico–chemical analysis. A series of SAW sensors have been coated with these materials and experimental results as a function of vapor concentration are presented. The influence of coating thickness, temperature and humidity are examined. Results showed on one hand that frequency variations are linear with GB gas concentrations from 0.5 to 10 ppm and on the other hand that sensitivity to organophosphorus compounds is two times greater in wet atmospheres (RH = 60%) than in dry air. The sensitivity was also better at a working temperature close to the glass transition point (TG) of the polymer. Above TG, the modification of the polymer structure induced a great radiation of the acoustic energy in the coating. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Chemical gas sensors; Surface acoustic waves; Organophosphorus compounds
1. Introduction SAW (surface acoustic wave) sensors are generally used for gas concentration measurement. They rely on acoustic waves whose propagation is influenced by gases selectively absorbed in specific layers deposited on the sensor surface [1,2]. These SAW sensor technologies, compatible with planar silicon technology, allow the miniaturization of sensor instrumentation to the point of portability with low cost and potential for real time feedback of analyte information. A further advantage of these devices is their potential to be adapted to a variety of gas detection problems by the selection of specific coatings. In the literature, several studies have been published dealing with the development of acoustical sensors * Corresponding author. Tel.: +335 56 846540; fax: + 335 56 371545; e-mail: [email protected] 0925-4005/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0925-4005(98)00143-9
(SAW and BAW devices) for organophosphorus compounds. The sensitivity of the SAW chemosensor to a specific vapor analyte is highly dependent on the choice of the polymeric overlay. A suitable chemical interface is determined by several physicochemical properties. The coating-vapor affinity and the sorption mechanisms (reversibility and rate) are the main parameters. The affinity between the coating and the vapor is represented by the partition coefficient and the rate of vapor transport into the polymer by the diffusion coefficient. The aim of this paper is to report on the results obtained with SAW devices coated with a soft polymeric material referred to as fluoropolyol (FPOL), which has demonstrated its ability, in terms of sensitivity and selectivity, to detect organophosphorus compounds [3–10]. Indeed, a solvation equation permits the quantification of interactions between vapors and polymers, and Abraham et al. have reported solubility
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high gain (28 dB) with a typical noise figure of 2.7 dB. The gain loop condition is achieved by a voltage attenuator (Macom AT-101). A 20 dB directional coupler (Macom CH-140) allows the measurement of the oscillator frequency output with a Hewlett-Packard electronic counter (HP 5334 A) equipped with a highstability time-base option. The short-term stability was measured at 25°C with a gate time equal to 1 s. The frequency variations of SAW oscillator are within 4 Hz.
2.2. Gas dilution bench
Fig. 1. SAW interchangeable element.
values at several temperatures for fluropolyol materials. These elements showed the interesting potentialities offered by FPOL coatings to detect organophosphorus vapors. In previous papers [14,15], preliminary results have allowed us to confirm that FPOL material, compared to other coatings, gives some of the highest observed responses.
2. Test bench
2.1. SAW sensor SAW delay-line oscillators were used. The interdigital transducers (aluminium thin films) were oriented on ST-quartz substrates to obtain an X-direction propagation. The periodicities of two types of transducer design were fixed to 40 and 32 mm, so that the synchronous frequencies were close to 79 and 98 MHz, respectively. SAW devices were bounded onto a ceramic plate (Al2O3), which supported a thick screen-printed resistor to achieve substrate heating. The acoustic path of the delay-lines were coated with specific polymer coatings. This assembly formed an interchangeable element (Fig. 1). By placing the delay-line into the feedback path of an amplifier, which resulted in an oscillator, the change in acoustic wave velocity induced by gas sorption was subsequently converted into a frequency variation. The airtight detection cell as well as the computer controlled bench have been described elsewhere [2]. Our electronic configuration was developed with broadband amplifiers (Macom AMC-182) which produce a
The measurement bench stands at the Centre d’Etudes du Bouchet where gas standard atmospheres of liquid or gas pollutants can be obtained. This bench can generate gas mixtures from either diffusion tubes, or permeation tubes or industrial cylinders. It is composed of: Sources of carrier gas: this reference gas can be ultra zero dry air, ultra zero wet air (known hygrometry), or ultra pure nitrogen. It is also used for the dilution of studied pollutants. Industrial gas mixtures, directly introduced ahead of the bench at a concentration between 1000 and 10000 ppm. Specific gas mixtures, obtained from diffusion or permeation tubes, are carried out and calibrated in the same laboratory and maintained at constant temperature and flow in a permeameter. The two-stages system of dilution and the performant flow mass meters can generate gas atmospheres at a constant flow in the measuring cell, with programmable concentrations of one, two, or three pollutants, at a controlled relative hygrometry (0–90% RH). The dilution range is continued between 1.1 and 62000 times. The vapour of GB, purchased by the Centre d’Etudes du Bouchet, was generated from a liquid source in the permeation tube system. By using dry air, temperature and flow regulations permit the control of GB concentrations.
3. Fluoropolyol coating For fast vapor sorption, it is necessary to choose a polymer with a high permeability. To obtain a rubbery polymer, it is necessary to operate at a temperature above the glass transition temperature (TG) and to
Fig. 2. Fluoropolyol (FPOL) pre-polymer structure.
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Fig. 3. Synthesis of diglycidylethers 1,3-(6) and 1,4-(7).
choose polymer coatings with low TG values. The interaction between vapor and polymer coating is similar to gas – liquid thermodynamic equilibrium. The polymer sorbs and concentrates vapor molecules. This sorption induces a film mass variation and viscoelastic perturbation of the polymer coating. The partition coefficient K represents the ratio between the concentration of the solute in the polymer phase CS and the concentration of the vapor in the gas phase CV. The value of the partition coefficient K allows for the to selection of an optimum coating in terms of sensitivity and selectivity. Using the linear solvation energy relationship (LSER), the calculated log K values [9] brings to light the potentialities of FPOL coatings to detect organophophorus compounds. At a temperature T= 25°C, solute vapor solvation parameters, with dimethylmethylphosphonate (DMMP: organophosphorus compound), obtain a typical value (log K) equal to 6.4. In comparison, with H20 and ethanol, log K is equal to 2.3 and 3.0, respectively. As a result, FPOL appears as an interesting coating in terms of sensitivity and selectivity. FPOL has been described as an oligomeric pre-polymer with the formula represented by the Fig. 2. The FPOL incorporates CF3 groups at regular intervals along a polymer backbone. In 1976, Field [11] described succintly the preparation of several fluorinated diols and their condensation with epichlorohydrin.
Considering the synthetic approach described in Field’s paper, one is led to assume that the polymer actually consists in a statistical combination of 1,3-(90%) and 1,4-(10%) substituted aromatic as well as cis-(5%) and trans-(95%) substituted ethylenic repeat units1. In this work, we have devised an improved procedure which allowed us to separately prepare the four isomer combinations: (1,4-trans), (1,4-cis), (1,3-trans), and (1,3-cis), taking advantage of the commercial availability of the two fluorinated aromatic diols 1,3-bis(2-hydroxyhexafluoroisopropyl)benzene (26 ) and 1,4-bis(2-hydroxyhexafluoroisopropyl) benzene (36 ) (Fig. 3). In Field’s synthetic approach, the diols were reacted with an excess of epichlorohydrin in alkaline conditions to produce directly diglycidylethers 66 and 76 , following a procedure first devised by Kelly et al. [12]. However, due to their high acidity, the reactivity of diols 2 and 3 to epichlorohydrin is much higher than that of the products used by Kelly et al., and inevitably leads to a partial polycondensation of the reactants. Theoretically, the separation of desired diglycidylethers 66 and 76 is possible by fractionated recrystalization from methanol. In practice, however, more than six recrystalisations at − 20°C are necessary to obtain 66 and 76 with
1
Due to steric hindrance, the 1.2-substitution of the aromatic units is most unlikely.
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Fig. 4. Preparation of olefinic diols-trans (9) and -cis(8).
sufficient purity. In such conditions, the synthesis yield does not exceed 15%. Considering the relatively high price of the two precursors, we found that the stepwise approach summarised in Fig. 3 was preferable. Diols 26 and 36 were thus refluxed with allyl bromide in acetone in the presence of potassium carbonate, to yield the dienes 46 and 56 , respectively. These were then oxidized to 66 and 76 with meta-chloroperbenzoic acid. In order to avoid all chances of ring opening and condensation of the epoxides, we did not perform the usual washing with an alkaline solution to remove the excess peroxyacid and its by-products was omitted. Instead, products 66 and 76 were purified by column chromatography over silica gel. In these conditions, 66 and 76 were obtained as analytically pure products with 68 and 72% yield, respectively. Olefinic diols 86 and 96 were prepared by reacting hexafluoroacetone with propene (Fig. 4), following a procedure described by Urry et al. [13]. Caution: hexafluoroacetone is a highly toxic, corrosive gas which should be handled with due care! Due to steric hindrance, only a small proportion of cis-diol 86 is obtained as a white crystaline product which is recovered and purified by recrystalization from chloroform (alternatively, 86 can be sublimed at 80°C at atm. pressure). Trans-diol 96 is obtained as a liquid contaminated by
Fig. 5. Frequency variations versus time, with GB gas concentration as a parameter, at 40°C.
the side-product mono-alcohol 10. Diol 96 is easily purified by distillation at reduced pressure (90°C at 30 mm Hg). The structure and purity of all products 26 –10 was checked by elemental analysis, FTIR, 1HNMR, 13C NMR and 19F NMR spectrometry. Analytical characteristics and synthetic details are available on request from the authors2. The four polymers designed as (1,4-trans), (1,4-cis), (1,3-trans), and (1,3-cis) were prepared by condensation of 66 or 76 with 86 or 96 (four combinations). The reactants were mixed in stoichiometric proportions and heated to 70°C in the presence of a catalytic amount of tributylamine, without any solvent. The polymers were recovered as visqueous liquids which on cooling turned into vitreous, transparent solids with a slight amber taint. Each polymer was thoroughly characterized by elemental analysis, FTIR, 1HNMR, 13C NMR and 19F NMR spectrometry (from solutions in methanol or chloroform), gel permeation chromatography (in THF), thermogravimetric analysis and differential enthalpic analysis, and were compared to a sample of original FPOL material. The results confirm that the latter mainly contains the 1,3-aromatic and trans-ethylenic repeat units, as may be expected from the respective yields of the Field’s synthetic procedure. On the other hand, the different synthetic approaches are reflected in the molecular mass distributions of the original FPOL material and of the pure 1,3-trans material: the original FPOL contains a significant proportion of low-mass oligomers which may have a plastifying effect. This is also reflected in the significantly lower glass-transition temperature of the original with respects to the four separate isomer combinations. Indeed, the glass transition temperature TG of the original FPOL is 16°C, while the TG values of the isomer combinations are evaluated at 35°C (1,3-trans), 41°C (1,4-trans), and 45°C (1,4-cis). Dilute solutions of the coating materials were prepared in methanol. These solutions generate a finely 2
Requests should be addressed to Jean-Franc¸ois Lipskier.
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Table 1 Response time versus temperature and GB concentration. Sarin (GB) (ppm)
Temperature (°C)
Response time
Frequency shifts (Hz)
1 2.5 1 2.5
30 30 40 40
40 min 55 min 6 min 20 s 4 min 20 s
190 450 125 310
Quartz ST-98 MHz-FPOL 1,3-trans-Frequency shift due to FPOL coating thickness 340 kHz.
dispersed aerosol, which was deposited by spray technique on the acoustic path of the SAW devices. 4. Experimental results and discussion The responses of the SAW sensor to GB concentrations ranging between 0.5 and 10 ppm were measured. The SAW change in frequency as a function of time was measured for several GB gas concentrations, at various temperatures. No significant difference appeared between the devices at 98 and 79 MHz (with a small advantage for the 98 MHz device). Therefore, we only present results obtained with the device at 98 MHz. Furthermore, tests have been undertaken with three isomeric forms of FPOL: we have observed that the 1,3-trans form was the most sensitive one, with a factor 1.3 when comparing frequency shifts of one device covered with the isomeric form 1,3-trans and frequency shifts of similar devices with 1,4-cis or 1,4trans forms [16].
In Table 1, values of the response time (90% of the steady-state) are reported. At 40°C, it took several minutes to reach the steady-state. At 30°C, it was even longer. This time is too long for the detection of organophosphorus vapors. However, the presence of this pollutant can be seen from the slope in the curve, only after a few seconds. This explains that the smallest measurable concentration was equal to 0.5 ppm. Fig. 6 represents the influence of the film thickness on the frequency shift due to the pollutant (for three pollutant concentrations). The linear variation is compatible with the theoretical proportionality between the frequency shift and the mass deposited on the quartz. However, the film thickness was limited by several parameters. In particular, above a few hundreds kilohertz, the delay-line insertion loss increased dramatically, and the filter rejection level became too weak to allow stable oscillations. In Fig. 7, frequency shifts as a function of the pollutant concentration, for three operating temperatures, depicts a linear evolution according to mass loading effect.
4.1. Influence of gas concentration Fig. 5 illustrates the response to GB at various concentrations in dry air, at 40°C, with the isomeric form 1,3-trans. These results showed a reversible sorption phenomenon. The frequency shift in response to 2.5 ppm of the pollutant was 310 Hz. Furthermore, systematic measurements were carried out, where good reproducibility was observed. Therefore, with a shortterm stability of a few hertz, we should detect concentrations as low as 0.1 ppm.
Fig. 6. Frequency variations (Hz) versus coating thickness (kHz) with GB gas concentration as parameter.
4.2. Influence of temperature The steady-state change in frequency was measured for several GB concentrations, at various device temperatures (Fig. 8). Due to the long response times at 30°C (Table 1) and a shorter sample duration (900 s), reported values have been estimated by extrapolation of the frequency curve. These results, obtained with the 1,3-trans isomer, show that the best working temperature is near the
Fig. 7. Frequency shifts versus GB gas concentration, for three operating temperatures.
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4.3. Influence of relati6e humidity Results (Fig. 9) showed that the sensitivity to organophosphorus compounds (GB) was two times greater in wet atmospheres (RH 60%) than in dry air, at low concentration levels ranging from 0.5 to 2.5 ppm. At 10 ppm, the influence of humidity is limited to 1.5. This increased sensitivity in wet atmospheres shows that the swelling of the sorbent polymer coating plays a major role in the sensor response.
Fig. 8. Frequency variations in steady-state versus temperature, with GB gas concentration as a parameter.
glass transition point (TG) of the polymer, which is between 30 and 35°C for this form. For each isomeric form of FPOL, the best results were obtained when the working temperature was near the TG of the corresponding isomer. The TG point is determined by the transition between the glassy and the elastomeric regions. In the glassy region, the polymers chains are locked into the lowest energy conformations and free rotation around the polymer backbone is not allowed. In the opposite case, elastomeric region offers a sufficient energy level to permit rotational freedom. The sensitivity is due to both mass and viscoelastic effects [10]. At this temperature, vapor adsorption might modify more easily the polymer viscoelastic coefficients, resulting in an improved sensitivity. Above TG, higher temperatures lead to lower sensitivity due to lower chemical activity. Furthermore, electrical characterizations with a network analyzer showed that a great radiation of the acoustic energy in the coating appeared above TG, due to viscoelastic effects. Above 45°C, the induced attenuation was not compatible with an oscillator configuration.
Fig. 9. Frequency variation versus time, at 40°C, influence of relative humidity.
5. Conclusions A SAW gas sensor, which utilizes a selectively sorbent film of FPOL, is shown to be a very sensitive and a selective detector of organophosphorus vapors. Frequency responses are linear with GB gas concentration ranging from 0.5 to 10 ppm. The best isomer seems to be the 1,3-trans form. The sensitivity to organophosphorus compounds was two times greater in wet atmospheres (RH 60%) than in dry air. The sensitivity was also better at a working temperature close to the glass transition point (TG) of the polymer. Moreover, if we consider the response time parameter, a working temperature close to 40°C appears as an optimum.
Acknowledgements This work was supported by the French Ministry of Defence, through a specific program from the Centre d’Etudes du Bouchet (CEB-DCE-DGA).
References [1] M.S. Nieuwenhuizen, A. Venema, Surface acoustic wave chemical sensors, Sensors and Materials 5 (1989) 261 – 300. [2] D. Rebie`re, Capteurs a` ondes acoustiques de surface, Application a` la de´tection des gaz, Thesis 826, 1992, University Bordeaux I. [3] D.S. Ballantine Jr, S.L. Rose, J.W. Grate, H. Wohltjen, Correlation of surface acoustic wave device coating responses with solubility properties and chemical structure using pattern recognition, Anal. Chem. 58 (14) (1986) 3058 – 3066. [4] D. Ballantine, A. Snow, M. Klusty, G. Chingas, H. Wohltjen, Naval Research Laboratory Memorandum Report, 1986, NRLML-5865. [5] J.W. Grate, A. Snow, D.S. Ballantine Jr, H. Wohltjen, M.H. Abraham, R.A. McGill, P. Sasson, Determination of partition coefficients from surface acoustic wave vapor sensor responses and correlation with gas – liquid chromatographic partition coefficients, Anal. Chem. 60 (9) (1988) 869 – 875. [6] S.L. Rose-Pehrsson, J.W. Grate, D.S. Ballantine Jr, P.C. Jurs, Detection of hazardous vapors including mixtures using pattern recognition analysis of responses from surface acoustic wave devices, Anal. Chem. 60 (24) (1988) 2801 – 2811.
D. Rebie`re et al. / Sensors and Actuators B 49 (1998) 139–145 [7] J.W. Grate, R.A. McGill, M.H. Abraham, Chemically selective polymer coatings for acoustic vapor sensors and array, IEEE Ultrason. Symp. Proc. 1 (1992) 175–279. [8] J.W. Grate, S.L. Rose-Pehrsson, D.L. Venezky, M. Klusty, H. Wohltjen, Smart sensor system for trace organophosphorus and organosulfur vapor detection employing a temperature-controlled array of surface acoustic wave sensors, automated sample preconcentration, and pattern recognition, Anal. Chem. 65 (14) (1993) 1868 – 1881. [9] R.A. McGill, M.H. Abraham, J.W. Grate, Choosing polymer coatings for chemical sensors, Chemtech 24 (9) (1994) 27 – 37. [10] J.W. Grate, S.J. Patrash, M.H. Abraham, Method for estimating polymer-coated acoustic wave vapor sensor responses, Anal. Chem. 67 (13) (1995) 2162–2169. [11] D.E. Field, Fluorinated polyepoxy and polyurethane coatings, J. Paint Technol. 48 (615) (1976) 43–47. [12] P.B. Kelly, A.J. Landua, C.D. Marshall, Relationship of molecular structure and resin performance for a series of diglycidyl ethers, J. Appl. Polym. Sci. 6 (22) (1962) 425–432. [13] W.H. Urry, J.H. Niu, L.G. Lundsted, Multiple muticenter reactions of perfluoro ketones with olefines, J. Org. Chem. 33 (6) (1968) 2302 – 2310. [14] C. De´jous, D. Rebie`re, J. Pistre´, C. Tiret, R. Planade, A surface acoustic wave gas sensor: detection of organophosphorus compounds, Sensors and Actuators B 24 (1–3) (1995) 58–61. [15] D. Rebie`re, C. De´jous, J. Pistre´, R. Planade, J.F. Lipskier, P. Robin, Surface acoustic wave detection of organophosphorus compounds with fluoropolyol coatings, Proc. Eurosensors X, vol. 1, Leuven (B), 8–11 September 1996, pp. 67–70. [16] to be proposed at C2I 98, Cachan, Paris, France, 18–19 November 1998.
Biographies Dominique Rebie`re received the Maıˆtrise d’E´lectronique-E´lectrotechnique-Automatique, the Diploˆme d’E´ tudes Approfondies in electronics and a Ph.D. from Bordeaux I University, France in 1987, 1988 and 1992, respectively. He has been involved in research on surface acoustic wave sensors since 1989 at Bordeaux I University, IXL Microelectronic Laboratory, and is Maıˆtre de Confe´rences at Bordeaux I University in electronic engineering. Corinne De´jous studied electronic engineering and ´ cole Nareceived the Diploˆme d’Inge´nieur from the E ´ tionale Supe´rieure d’Electronique et de Radioe´lectricite´ de Bordeaux (ENSERB) in 1991. She received a Ph.D. in 1994, and is now Maıˆtre de Confe´rences at Bordeaux
.
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I University. Since 1991, she has been working at the IXL Microelectronic Laboratory, where her main interest is acoustic waves used in chemical sensors. Jacques Pistre´ received an M.Eng. degree in electronics from Bordeaux University, France in 1968, and a The`se d’E´tat degree in ionic conductivity of thin films in 1979. Since joining the IXL Laboratory (formerly ´ lectronique Applique´e), he has worked Laboratoire d’E in several areas of thick-film microelectronics, including applications to microwave circuits and sensors. He was sent on secondment for 18 months to the French company Thomson, working in the division of Radars, Countermeasures, Missiles. In 1987, he returned to the IXL Laboratory where he is in charge of the sensors group. This group is mainly involved in gas sensors, using either elastic waves in solids or microwave devices. He is a professor at ENSERB, a French ‘Grande E´cole’, where he teaches electronic systems and is responsible for international relations. Jean-Franc¸ois Lipskier graduated as an Engineer ´ cole Nationale Supe´rieure de Chimie de from the E Paris and holds a D.Sc. in Physical Chemistry from the University of Paris XI. Specializing in the Chemical Physics of organic materials, J.-F. Lipskier worked for ´ tudes de three years both in France at the Centre d’E ´ Saclay (Commissariat a` l’Energie Atomique) and in Canada at the Department of Nuclear Medicine and Radiobiology of the University of Sherbrooke, on the study of photoinduced electron and energy transfers in supramolecular assemblies by ultra-fast spectroscopy. In 1991, he joined the Central Research Laboratory of Thomson-CSF and soon started studying chemical sensors. He presently manages several research projects in this field. Roger Planade obtained his engineering degree from CNAM in 1975. Responsible for the Laboratoire de ´ tudes de De´fense Physico-Chimie of the De´partement E ´ tudes du Bouchet since 1980. He has been in Centre d’E responsible for the Miniaturized Gas Analyser program since 1990. The aim of this program is a local sensing system including different technologies (semiconducting oxide, surface acoustic waves, …).
Synthesis and evaluation of fluoropolyol isomers as saw microsensor coatings: role of humidity and temperature Dominique Rebie`re a,*, Corinne De´jous a, Jacques Pistre´ a, Jean-Franc¸ois Lipskier b, Roger Planade c b
a Laboratoire IXL, Uni6ersite´ Bordeaux I, 351 cours de la libe´ration, F 33405 Talence, Cedex, France Laboratoire Central de Recherche, Thomson CSF, Domaine de corbe6ille, F 91404 Orsay, Cedex, France c ´ tudes du Bouchet (DCE/DGA), Le Bouchet BP 3, F 91710 Vert-Le-Petit, France Centre d’E
Abstract A surface acoustic wave (SAW) organophosphorus gas detector has been designed, fabricated and tested. The gas detector consists of a dual delay line fabricated on a single quartz substrate. Each delay line is connected into the feedback path of a radio-frequency amplifier, to realize a SAW oscillator. The propagation path of one delay line is coated with fluoropolyol (FPOL). These polymers offer an interesting way to detect organophosphorus compounds like GB at low concentration levels. The absorption of vapors induces phase variations due to mass loading and stress effects. These variations result in corresponding frequency shifts. In this work, we have synthesised the four FPOL isomer combinations separately and characterized these materials by physico–chemical analysis. A series of SAW sensors have been coated with these materials and experimental results as a function of vapor concentration are presented. The influence of coating thickness, temperature and humidity are examined. Results showed on one hand that frequency variations are linear with GB gas concentrations from 0.5 to 10 ppm and on the other hand that sensitivity to organophosphorus compounds is two times greater in wet atmospheres (RH = 60%) than in dry air. The sensitivity was also better at a working temperature close to the glass transition point (TG) of the polymer. Above TG, the modification of the polymer structure induced a great radiation of the acoustic energy in the coating. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Chemical gas sensors; Surface acoustic waves; Organophosphorus compounds
1. Introduction SAW (surface acoustic wave) sensors are generally used for gas concentration measurement. They rely on acoustic waves whose propagation is influenced by gases selectively absorbed in specific layers deposited on the sensor surface [1,2]. These SAW sensor technologies, compatible with planar silicon technology, allow the miniaturization of sensor instrumentation to the point of portability with low cost and potential for real time feedback of analyte information. A further advantage of these devices is their potential to be adapted to a variety of gas detection problems by the selection of specific coatings. In the literature, several studies have been published dealing with the development of acoustical sensors * Corresponding author. Tel.: +335 56 846540; fax: + 335 56 371545; e-mail: [email protected] 0925-4005/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0925-4005(98)00143-9
(SAW and BAW devices) for organophosphorus compounds. The sensitivity of the SAW chemosensor to a specific vapor analyte is highly dependent on the choice of the polymeric overlay. A suitable chemical interface is determined by several physicochemical properties. The coating-vapor affinity and the sorption mechanisms (reversibility and rate) are the main parameters. The affinity between the coating and the vapor is represented by the partition coefficient and the rate of vapor transport into the polymer by the diffusion coefficient. The aim of this paper is to report on the results obtained with SAW devices coated with a soft polymeric material referred to as fluoropolyol (FPOL), which has demonstrated its ability, in terms of sensitivity and selectivity, to detect organophosphorus compounds [3–10]. Indeed, a solvation equation permits the quantification of interactions between vapors and polymers, and Abraham et al. have reported solubility
D. Rebie`re et al. / Sensors and Actuators B 49 (1998) 139–145
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high gain (28 dB) with a typical noise figure of 2.7 dB. The gain loop condition is achieved by a voltage attenuator (Macom AT-101). A 20 dB directional coupler (Macom CH-140) allows the measurement of the oscillator frequency output with a Hewlett-Packard electronic counter (HP 5334 A) equipped with a highstability time-base option. The short-term stability was measured at 25°C with a gate time equal to 1 s. The frequency variations of SAW oscillator are within 4 Hz.
2.2. Gas dilution bench
Fig. 1. SAW interchangeable element.
values at several temperatures for fluropolyol materials. These elements showed the interesting potentialities offered by FPOL coatings to detect organophosphorus vapors. In previous papers [14,15], preliminary results have allowed us to confirm that FPOL material, compared to other coatings, gives some of the highest observed responses.
2. Test bench
2.1. SAW sensor SAW delay-line oscillators were used. The interdigital transducers (aluminium thin films) were oriented on ST-quartz substrates to obtain an X-direction propagation. The periodicities of two types of transducer design were fixed to 40 and 32 mm, so that the synchronous frequencies were close to 79 and 98 MHz, respectively. SAW devices were bounded onto a ceramic plate (Al2O3), which supported a thick screen-printed resistor to achieve substrate heating. The acoustic path of the delay-lines were coated with specific polymer coatings. This assembly formed an interchangeable element (Fig. 1). By placing the delay-line into the feedback path of an amplifier, which resulted in an oscillator, the change in acoustic wave velocity induced by gas sorption was subsequently converted into a frequency variation. The airtight detection cell as well as the computer controlled bench have been described elsewhere [2]. Our electronic configuration was developed with broadband amplifiers (Macom AMC-182) which produce a
The measurement bench stands at the Centre d’Etudes du Bouchet where gas standard atmospheres of liquid or gas pollutants can be obtained. This bench can generate gas mixtures from either diffusion tubes, or permeation tubes or industrial cylinders. It is composed of: Sources of carrier gas: this reference gas can be ultra zero dry air, ultra zero wet air (known hygrometry), or ultra pure nitrogen. It is also used for the dilution of studied pollutants. Industrial gas mixtures, directly introduced ahead of the bench at a concentration between 1000 and 10000 ppm. Specific gas mixtures, obtained from diffusion or permeation tubes, are carried out and calibrated in the same laboratory and maintained at constant temperature and flow in a permeameter. The two-stages system of dilution and the performant flow mass meters can generate gas atmospheres at a constant flow in the measuring cell, with programmable concentrations of one, two, or three pollutants, at a controlled relative hygrometry (0–90% RH). The dilution range is continued between 1.1 and 62000 times. The vapour of GB, purchased by the Centre d’Etudes du Bouchet, was generated from a liquid source in the permeation tube system. By using dry air, temperature and flow regulations permit the control of GB concentrations.
3. Fluoropolyol coating For fast vapor sorption, it is necessary to choose a polymer with a high permeability. To obtain a rubbery polymer, it is necessary to operate at a temperature above the glass transition temperature (TG) and to
Fig. 2. Fluoropolyol (FPOL) pre-polymer structure.
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Fig. 3. Synthesis of diglycidylethers 1,3-(6) and 1,4-(7).
choose polymer coatings with low TG values. The interaction between vapor and polymer coating is similar to gas – liquid thermodynamic equilibrium. The polymer sorbs and concentrates vapor molecules. This sorption induces a film mass variation and viscoelastic perturbation of the polymer coating. The partition coefficient K represents the ratio between the concentration of the solute in the polymer phase CS and the concentration of the vapor in the gas phase CV. The value of the partition coefficient K allows for the to selection of an optimum coating in terms of sensitivity and selectivity. Using the linear solvation energy relationship (LSER), the calculated log K values [9] brings to light the potentialities of FPOL coatings to detect organophophorus compounds. At a temperature T= 25°C, solute vapor solvation parameters, with dimethylmethylphosphonate (DMMP: organophosphorus compound), obtain a typical value (log K) equal to 6.4. In comparison, with H20 and ethanol, log K is equal to 2.3 and 3.0, respectively. As a result, FPOL appears as an interesting coating in terms of sensitivity and selectivity. FPOL has been described as an oligomeric pre-polymer with the formula represented by the Fig. 2. The FPOL incorporates CF3 groups at regular intervals along a polymer backbone. In 1976, Field [11] described succintly the preparation of several fluorinated diols and their condensation with epichlorohydrin.
Considering the synthetic approach described in Field’s paper, one is led to assume that the polymer actually consists in a statistical combination of 1,3-(90%) and 1,4-(10%) substituted aromatic as well as cis-(5%) and trans-(95%) substituted ethylenic repeat units1. In this work, we have devised an improved procedure which allowed us to separately prepare the four isomer combinations: (1,4-trans), (1,4-cis), (1,3-trans), and (1,3-cis), taking advantage of the commercial availability of the two fluorinated aromatic diols 1,3-bis(2-hydroxyhexafluoroisopropyl)benzene (26 ) and 1,4-bis(2-hydroxyhexafluoroisopropyl) benzene (36 ) (Fig. 3). In Field’s synthetic approach, the diols were reacted with an excess of epichlorohydrin in alkaline conditions to produce directly diglycidylethers 66 and 76 , following a procedure first devised by Kelly et al. [12]. However, due to their high acidity, the reactivity of diols 2 and 3 to epichlorohydrin is much higher than that of the products used by Kelly et al., and inevitably leads to a partial polycondensation of the reactants. Theoretically, the separation of desired diglycidylethers 66 and 76 is possible by fractionated recrystalization from methanol. In practice, however, more than six recrystalisations at − 20°C are necessary to obtain 66 and 76 with
1
Due to steric hindrance, the 1.2-substitution of the aromatic units is most unlikely.
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Fig. 4. Preparation of olefinic diols-trans (9) and -cis(8).
sufficient purity. In such conditions, the synthesis yield does not exceed 15%. Considering the relatively high price of the two precursors, we found that the stepwise approach summarised in Fig. 3 was preferable. Diols 26 and 36 were thus refluxed with allyl bromide in acetone in the presence of potassium carbonate, to yield the dienes 46 and 56 , respectively. These were then oxidized to 66 and 76 with meta-chloroperbenzoic acid. In order to avoid all chances of ring opening and condensation of the epoxides, we did not perform the usual washing with an alkaline solution to remove the excess peroxyacid and its by-products was omitted. Instead, products 66 and 76 were purified by column chromatography over silica gel. In these conditions, 66 and 76 were obtained as analytically pure products with 68 and 72% yield, respectively. Olefinic diols 86 and 96 were prepared by reacting hexafluoroacetone with propene (Fig. 4), following a procedure described by Urry et al. [13]. Caution: hexafluoroacetone is a highly toxic, corrosive gas which should be handled with due care! Due to steric hindrance, only a small proportion of cis-diol 86 is obtained as a white crystaline product which is recovered and purified by recrystalization from chloroform (alternatively, 86 can be sublimed at 80°C at atm. pressure). Trans-diol 96 is obtained as a liquid contaminated by
Fig. 5. Frequency variations versus time, with GB gas concentration as a parameter, at 40°C.
the side-product mono-alcohol 10. Diol 96 is easily purified by distillation at reduced pressure (90°C at 30 mm Hg). The structure and purity of all products 26 –10 was checked by elemental analysis, FTIR, 1HNMR, 13C NMR and 19F NMR spectrometry. Analytical characteristics and synthetic details are available on request from the authors2. The four polymers designed as (1,4-trans), (1,4-cis), (1,3-trans), and (1,3-cis) were prepared by condensation of 66 or 76 with 86 or 96 (four combinations). The reactants were mixed in stoichiometric proportions and heated to 70°C in the presence of a catalytic amount of tributylamine, without any solvent. The polymers were recovered as visqueous liquids which on cooling turned into vitreous, transparent solids with a slight amber taint. Each polymer was thoroughly characterized by elemental analysis, FTIR, 1HNMR, 13C NMR and 19F NMR spectrometry (from solutions in methanol or chloroform), gel permeation chromatography (in THF), thermogravimetric analysis and differential enthalpic analysis, and were compared to a sample of original FPOL material. The results confirm that the latter mainly contains the 1,3-aromatic and trans-ethylenic repeat units, as may be expected from the respective yields of the Field’s synthetic procedure. On the other hand, the different synthetic approaches are reflected in the molecular mass distributions of the original FPOL material and of the pure 1,3-trans material: the original FPOL contains a significant proportion of low-mass oligomers which may have a plastifying effect. This is also reflected in the significantly lower glass-transition temperature of the original with respects to the four separate isomer combinations. Indeed, the glass transition temperature TG of the original FPOL is 16°C, while the TG values of the isomer combinations are evaluated at 35°C (1,3-trans), 41°C (1,4-trans), and 45°C (1,4-cis). Dilute solutions of the coating materials were prepared in methanol. These solutions generate a finely 2
Requests should be addressed to Jean-Franc¸ois Lipskier.
D. Rebie`re et al. / Sensors and Actuators B 49 (1998) 139–145
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Table 1 Response time versus temperature and GB concentration. Sarin (GB) (ppm)
Temperature (°C)
Response time
Frequency shifts (Hz)
1 2.5 1 2.5
30 30 40 40
40 min 55 min 6 min 20 s 4 min 20 s
190 450 125 310
Quartz ST-98 MHz-FPOL 1,3-trans-Frequency shift due to FPOL coating thickness 340 kHz.
dispersed aerosol, which was deposited by spray technique on the acoustic path of the SAW devices. 4. Experimental results and discussion The responses of the SAW sensor to GB concentrations ranging between 0.5 and 10 ppm were measured. The SAW change in frequency as a function of time was measured for several GB gas concentrations, at various temperatures. No significant difference appeared between the devices at 98 and 79 MHz (with a small advantage for the 98 MHz device). Therefore, we only present results obtained with the device at 98 MHz. Furthermore, tests have been undertaken with three isomeric forms of FPOL: we have observed that the 1,3-trans form was the most sensitive one, with a factor 1.3 when comparing frequency shifts of one device covered with the isomeric form 1,3-trans and frequency shifts of similar devices with 1,4-cis or 1,4trans forms [16].
In Table 1, values of the response time (90% of the steady-state) are reported. At 40°C, it took several minutes to reach the steady-state. At 30°C, it was even longer. This time is too long for the detection of organophosphorus vapors. However, the presence of this pollutant can be seen from the slope in the curve, only after a few seconds. This explains that the smallest measurable concentration was equal to 0.5 ppm. Fig. 6 represents the influence of the film thickness on the frequency shift due to the pollutant (for three pollutant concentrations). The linear variation is compatible with the theoretical proportionality between the frequency shift and the mass deposited on the quartz. However, the film thickness was limited by several parameters. In particular, above a few hundreds kilohertz, the delay-line insertion loss increased dramatically, and the filter rejection level became too weak to allow stable oscillations. In Fig. 7, frequency shifts as a function of the pollutant concentration, for three operating temperatures, depicts a linear evolution according to mass loading effect.
4.1. Influence of gas concentration Fig. 5 illustrates the response to GB at various concentrations in dry air, at 40°C, with the isomeric form 1,3-trans. These results showed a reversible sorption phenomenon. The frequency shift in response to 2.5 ppm of the pollutant was 310 Hz. Furthermore, systematic measurements were carried out, where good reproducibility was observed. Therefore, with a shortterm stability of a few hertz, we should detect concentrations as low as 0.1 ppm.
Fig. 6. Frequency variations (Hz) versus coating thickness (kHz) with GB gas concentration as parameter.
4.2. Influence of temperature The steady-state change in frequency was measured for several GB concentrations, at various device temperatures (Fig. 8). Due to the long response times at 30°C (Table 1) and a shorter sample duration (900 s), reported values have been estimated by extrapolation of the frequency curve. These results, obtained with the 1,3-trans isomer, show that the best working temperature is near the
Fig. 7. Frequency shifts versus GB gas concentration, for three operating temperatures.
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4.3. Influence of relati6e humidity Results (Fig. 9) showed that the sensitivity to organophosphorus compounds (GB) was two times greater in wet atmospheres (RH 60%) than in dry air, at low concentration levels ranging from 0.5 to 2.5 ppm. At 10 ppm, the influence of humidity is limited to 1.5. This increased sensitivity in wet atmospheres shows that the swelling of the sorbent polymer coating plays a major role in the sensor response.
Fig. 8. Frequency variations in steady-state versus temperature, with GB gas concentration as a parameter.
glass transition point (TG) of the polymer, which is between 30 and 35°C for this form. For each isomeric form of FPOL, the best results were obtained when the working temperature was near the TG of the corresponding isomer. The TG point is determined by the transition between the glassy and the elastomeric regions. In the glassy region, the polymers chains are locked into the lowest energy conformations and free rotation around the polymer backbone is not allowed. In the opposite case, elastomeric region offers a sufficient energy level to permit rotational freedom. The sensitivity is due to both mass and viscoelastic effects [10]. At this temperature, vapor adsorption might modify more easily the polymer viscoelastic coefficients, resulting in an improved sensitivity. Above TG, higher temperatures lead to lower sensitivity due to lower chemical activity. Furthermore, electrical characterizations with a network analyzer showed that a great radiation of the acoustic energy in the coating appeared above TG, due to viscoelastic effects. Above 45°C, the induced attenuation was not compatible with an oscillator configuration.
Fig. 9. Frequency variation versus time, at 40°C, influence of relative humidity.
5. Conclusions A SAW gas sensor, which utilizes a selectively sorbent film of FPOL, is shown to be a very sensitive and a selective detector of organophosphorus vapors. Frequency responses are linear with GB gas concentration ranging from 0.5 to 10 ppm. The best isomer seems to be the 1,3-trans form. The sensitivity to organophosphorus compounds was two times greater in wet atmospheres (RH 60%) than in dry air. The sensitivity was also better at a working temperature close to the glass transition point (TG) of the polymer. Moreover, if we consider the response time parameter, a working temperature close to 40°C appears as an optimum.
Acknowledgements This work was supported by the French Ministry of Defence, through a specific program from the Centre d’Etudes du Bouchet (CEB-DCE-DGA).
References [1] M.S. Nieuwenhuizen, A. Venema, Surface acoustic wave chemical sensors, Sensors and Materials 5 (1989) 261 – 300. [2] D. Rebie`re, Capteurs a` ondes acoustiques de surface, Application a` la de´tection des gaz, Thesis 826, 1992, University Bordeaux I. [3] D.S. Ballantine Jr, S.L. Rose, J.W. Grate, H. Wohltjen, Correlation of surface acoustic wave device coating responses with solubility properties and chemical structure using pattern recognition, Anal. Chem. 58 (14) (1986) 3058 – 3066. [4] D. Ballantine, A. Snow, M. Klusty, G. Chingas, H. Wohltjen, Naval Research Laboratory Memorandum Report, 1986, NRLML-5865. [5] J.W. Grate, A. Snow, D.S. Ballantine Jr, H. Wohltjen, M.H. Abraham, R.A. McGill, P. Sasson, Determination of partition coefficients from surface acoustic wave vapor sensor responses and correlation with gas – liquid chromatographic partition coefficients, Anal. Chem. 60 (9) (1988) 869 – 875. [6] S.L. Rose-Pehrsson, J.W. Grate, D.S. Ballantine Jr, P.C. Jurs, Detection of hazardous vapors including mixtures using pattern recognition analysis of responses from surface acoustic wave devices, Anal. Chem. 60 (24) (1988) 2801 – 2811.
D. Rebie`re et al. / Sensors and Actuators B 49 (1998) 139–145 [7] J.W. Grate, R.A. McGill, M.H. Abraham, Chemically selective polymer coatings for acoustic vapor sensors and array, IEEE Ultrason. Symp. Proc. 1 (1992) 175–279. [8] J.W. Grate, S.L. Rose-Pehrsson, D.L. Venezky, M. Klusty, H. Wohltjen, Smart sensor system for trace organophosphorus and organosulfur vapor detection employing a temperature-controlled array of surface acoustic wave sensors, automated sample preconcentration, and pattern recognition, Anal. Chem. 65 (14) (1993) 1868 – 1881. [9] R.A. McGill, M.H. Abraham, J.W. Grate, Choosing polymer coatings for chemical sensors, Chemtech 24 (9) (1994) 27 – 37. [10] J.W. Grate, S.J. Patrash, M.H. Abraham, Method for estimating polymer-coated acoustic wave vapor sensor responses, Anal. Chem. 67 (13) (1995) 2162–2169. [11] D.E. Field, Fluorinated polyepoxy and polyurethane coatings, J. Paint Technol. 48 (615) (1976) 43–47. [12] P.B. Kelly, A.J. Landua, C.D. Marshall, Relationship of molecular structure and resin performance for a series of diglycidyl ethers, J. Appl. Polym. Sci. 6 (22) (1962) 425–432. [13] W.H. Urry, J.H. Niu, L.G. Lundsted, Multiple muticenter reactions of perfluoro ketones with olefines, J. Org. Chem. 33 (6) (1968) 2302 – 2310. [14] C. De´jous, D. Rebie`re, J. Pistre´, C. Tiret, R. Planade, A surface acoustic wave gas sensor: detection of organophosphorus compounds, Sensors and Actuators B 24 (1–3) (1995) 58–61. [15] D. Rebie`re, C. De´jous, J. Pistre´, R. Planade, J.F. Lipskier, P. Robin, Surface acoustic wave detection of organophosphorus compounds with fluoropolyol coatings, Proc. Eurosensors X, vol. 1, Leuven (B), 8–11 September 1996, pp. 67–70. [16] to be proposed at C2I 98, Cachan, Paris, France, 18–19 November 1998.
Biographies Dominique Rebie`re received the Maıˆtrise d’E´lectronique-E´lectrotechnique-Automatique, the Diploˆme d’E´ tudes Approfondies in electronics and a Ph.D. from Bordeaux I University, France in 1987, 1988 and 1992, respectively. He has been involved in research on surface acoustic wave sensors since 1989 at Bordeaux I University, IXL Microelectronic Laboratory, and is Maıˆtre de Confe´rences at Bordeaux I University in electronic engineering. Corinne De´jous studied electronic engineering and ´ cole Nareceived the Diploˆme d’Inge´nieur from the E ´ tionale Supe´rieure d’Electronique et de Radioe´lectricite´ de Bordeaux (ENSERB) in 1991. She received a Ph.D. in 1994, and is now Maıˆtre de Confe´rences at Bordeaux
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I University. Since 1991, she has been working at the IXL Microelectronic Laboratory, where her main interest is acoustic waves used in chemical sensors. Jacques Pistre´ received an M.Eng. degree in electronics from Bordeaux University, France in 1968, and a The`se d’E´tat degree in ionic conductivity of thin films in 1979. Since joining the IXL Laboratory (formerly ´ lectronique Applique´e), he has worked Laboratoire d’E in several areas of thick-film microelectronics, including applications to microwave circuits and sensors. He was sent on secondment for 18 months to the French company Thomson, working in the division of Radars, Countermeasures, Missiles. In 1987, he returned to the IXL Laboratory where he is in charge of the sensors group. This group is mainly involved in gas sensors, using either elastic waves in solids or microwave devices. He is a professor at ENSERB, a French ‘Grande E´cole’, where he teaches electronic systems and is responsible for international relations. Jean-Franc¸ois Lipskier graduated as an Engineer ´ cole Nationale Supe´rieure de Chimie de from the E Paris and holds a D.Sc. in Physical Chemistry from the University of Paris XI. Specializing in the Chemical Physics of organic materials, J.-F. Lipskier worked for ´ tudes de three years both in France at the Centre d’E ´ Saclay (Commissariat a` l’Energie Atomique) and in Canada at the Department of Nuclear Medicine and Radiobiology of the University of Sherbrooke, on the study of photoinduced electron and energy transfers in supramolecular assemblies by ultra-fast spectroscopy. In 1991, he joined the Central Research Laboratory of Thomson-CSF and soon started studying chemical sensors. He presently manages several research projects in this field. Roger Planade obtained his engineering degree from CNAM in 1975. Responsible for the Laboratoire de ´ tudes de De´fense Physico-Chimie of the De´partement E ´ tudes du Bouchet since 1980. He has been in Centre d’E responsible for the Miniaturized Gas Analyser program since 1990. The aim of this program is a local sensing system including different technologies (semiconducting oxide, surface acoustic waves, …).