Water sorption isotherms on aminopropyltriethoxysilane coated surface acoustic wave sensors

Talmra, Vol. 39, No. 11, pp. 1505-I 509, 1992 Printed in Great Britain. All rights reserved

0039-9140/92 $5.00 + 0.00 Pergamon Press Ltd

WATER SORPTION ISOTHERMS ON AMINOPROPYLTRIETHOXYSILANE COATED SURFACE ACOUSTIC WAVE SENSORS JULIE T. WOOD and JOHN F. ALDER* Department of Instrumentation

and Analytical Science, UMIST, P.O. Box 88, Manchester M60 lQD, UK (Received 1992. Accepted 14 May 1992)

Summary-Water sorption isotherms were obtained on surface acoustic wave sensors (SAWS) coated with aminopropyltriethyoxysilane (APTES), and on uncoated SAWS of which the substrate material was polished ST-quartz The isotherms were obtained at 25”, 30” and 40” over the range 1430% relative humidity (RH). The isotherms exhibit BDDT type III characteristics typical of weak gas-solid interaction. The isotherms showed good fit to quadratic equations relating frequency change on exposure to humid air with relative humidity. There was no significant hysteresis in the isotherms when the SAWS was taken through a cycle of relative humidity at any of the three temperatures employed. These results are similar to those obtained in earlier work on FPOL and polyvinylpyrollidone coated SAWS. They demonstrate that a correction algorithm based on a quadratic equation should be possible to overcome water vapour response of coated SAWS.

Surface acoustic wave sensors (SAWS) have been employed widely over the last decade and are still the subject of active research. In this laboratory, and previously in the analytical chemistry laboratories at Imperial College under the direction of Professor T. S. West, we have carried out extensive studies on the interaction of gaseous species with coated quartz bulkwave piezoelectric crystals (PZX)‘** and latterly their more recent SAWS counterparts.3 All sensors that rely upon reversible sorption of gas onto their surface as a prerequisite to the sensing process, will exhibit a response which is closely related to the sorption isotherm of the gas over the surface. It may not be exactly the same, because of non-linear response in the relationship between partial pressure of the gas (p) over the surface, mass of material sorbed (m) and the response function of the sensor R(m.p). In the work ongoing in this laboratory a series of experiments is being carried out to determine the nature of the response isotherms of SAWS coated with materials which exhibit selectivity towards target species, but also, and almost inevitably, exhibit response to water vapour. Previous work has specifically addressed the sorption of water vapour onto polyvinylpyrrolidone and F-POL (a fluorinated

*Author for correspondence.

polyether-polyol),4 which are known to respond to organophosphonate compoundss-’ and it was demonstrated that the isotherms for water resembled the BDDT type III, characteristic of weak gas-solid interactions, and could be modelled using a quadratic equation relating SAWS frequency change to the relative humidity of the challenge gas. The present work studied the response of aminopropyltriethyoxysilane (APTES) coated SAWS, which is known to sorb nitrobenzene,’ to water vapour. This was the preliminary step in a continuing programme to study the interaction between the surface of coated SAWS and a two-component vapour system, nitrobenzene and water, in air. EXPERIMENTAL

The experimental rig used to generate the humidified atmospheres was in all respects identical to that described by Fox and Alder.4 The SAWS employed were manufactured for the authors by Plessey (Towcester, U.K.) and comprised a pair of 70 MHz delay lines on a quartz substrate. The SAWS and its associated oscillator have been described elsewhere.g The output of each SAWS controlled oscillator was to a Philips PM6673 frequency counter (Philips, Cambridge, U.K.) with digital-analogue converter output to a strip chart recorder.

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JULIET. WOODand JOHNF. ALDER

1504 2ooo

gJoo

a

0

IO

20

50 60 70 30 40 f%) R&dive Humidity

90

mRemive

Fig, I. Sorption isotherm at 25” onto AFTES coated (side 1 -a--) and notionally uncoated (side 2 -_O-) SAWS. It can be seen that there is no significant difference between the responses of the two sides, almost certainly an indication that the coating had crept from side 1 to side 2. Error bars are standard deviation.

The rig ran continuously throughout the period of the experiments. Relative humidity could be maintained to within +0.5% and temperature within f0.5” over a period of 9 months. Equilibrium of the SAWS to switched dry-wet air stream was of the order 15-100 min, depending on the actual change imposed. Coating of the SAWS with APTES was achieved using a fine (00000 grade) paintbrush (available from model makers shops). A 20% solution of APTES in toluene was brushed on to the area in between the electrodes on one side of the SAWS only. The procedure employed was to first wash the SAWS free of grease using methylene chloride in a clean glass beaker and then allow it to dry in the oven at 60”, and cool in a desiccator. It was then placed in the oscillator circuit and maintained in the test cell at 25” and 1% relative humidity (RH) (-Lf:0.5% RH) as read on a Vaisala HMP123 humidity meter (Vaisala, Cambridge, U.K.). When the frequency of the SAWS had become stable, the side of SAWS exhibiting the lower frequency was coated. This caused the frequency difference between the coated and uncoated sides to separate further, thus minimising the problem of the oscillators associated with the two sides, locking

Hunmy

Fig. 3. Sorption isotherm at 40”; see legend to Fig_ 1.

together. The coated SAWS was then placed in an oven at 45 f 2” overnight to evaporate volatile materials and allow the coating to relax. The SAWS was then brought to ~~~b~~ in the last cell at 25” 1% RH and the new f?equenties noted. The resonant frequency of the uncoated side was virtually unchanged ( f 100 Hz) whereas the frequency of the coated side had reduced. On one occasion, for example, this was - 5.39 kHz which corresponded to a coating of approximately 21 & 3 molecular layers, calculated from the equation

where k, and k2 are the material constants for quartz,“*” f, is the unperturbed frequency, m is the mass of coating and A is the surface area over which the coating was applied. Much of the work reported here was carried out with the 21 layer coated SAWS. The coated SAWS was m~ntained in the test chambefl in a stream of air maintained at different relative humidities and one of the temperatures 25,30 or 40” (~0.5’). The flow rate chosen was 410 cm3/min which was shown to give reasonable response time between the humid and dry air flows ~approximately 1 min on switchover to 95% change of frequency). 2ow

r

2000

B ;jroca (961Rebtive 0

IO

20

30

40

SO

00

x,

SO

90

(43 Relative Humidity

Fig. 2. Sorption isotherm at 30”; see legend to Fig. 1.

Humidity

Fig. 4. Sorption isotherm at 25” onto ARES coated side 1 (-D-) of the same SAWS as in Fig. 1, (same data) compared with sorption isotherm onto a clean, uncoated SAWS at the same temperature, (-0--).

Water sorption isotherms

0

IO

20

30

40

a0

(%I IWAive

a0

70

60

SC

too

0

IO

20

Fig. 5. Sorption isotherm at 30” onto ARTES coated side 1 (-_O-) of the same SAWS as Fig. 2, (same data) compared with sorption isotherm onto a clean, uncoated SAWS at the same temperature, (-_O-).

The changes in relative humidity took longer to stabilize (15 min typically). RH could be maintained to within about 0.5% RH. The whole system was maintained in this regime for several months during the course of the experiments. The sorption isotherms were obtained by first stabilizing the SAWS in a dry (1% RH) atmosphere at the chosen temperature and noting the frequency and the frequency stability. The flow was then switched to the humid air at the same temperature and allowed to stabilize again. These cycles were then repeated, each taking about 15 min. The RH was then changed and the cycles repeated to obtain the data points on the isotherms. Each isotherm took between one and three weeks to obtain. RESULTS AND DISCUSSION

It was noted that the frequency stability degraded as the temperature of the SAWS increased from 25 to 40”, being typically +2 Hz p-p at 25” degrading to f 10 Hz p-p at 40” for the uncoated, and & 15 Hz for the coated side, in dry air. In humid air the problem was worse, for example, the SAWS exposed to 80% RH at

-i-

Fig. 6. Sorption isotherm at 40” onto AFTES coated side 1 (-_O--) of the same SAWS as Fig. 3, (same data) compared with sorption isotherm onto a clean, uncoated SAWS at the same temperature, (-_O-).

30

40

so

60

70

a0

90

loo

(%I Relative Humidity

Humidity

Fig. 7. APTES coated SAWS (side 1 as in previous figures) exposed to relative humidity cycle at 25”. Isotherms drawn (-_O-) for ascending RH: (-_O-) for descending RH.

40” for 2 hr exhibited noise of about f 15 Hz p-p, rendering it virtually unusable as a sensor. The authors are not sure what causes this effect, but it is at least in part influenced by the presence of the coating, as the noise is greater on the coated side of the sensor. Isotherms were initially obtained using the dual SAWS described above, one side coated with APTES and the other side notionally uncoated. Figures l-3 show the isotherms for 25”, 30” and 40” respectively. One notes however, that the two sides of the SAWS behaved in a very similar manner, with hardly any significant difference between them. This was not thought to be a true result. The isotherms were obtained for another SAWS device which had been thoroughly cleaned with methylene chloride, and indeed had never been coated with APTES. The isotherms at the three temperatures are given in Figs 4-6, for the uncoated, second SAWS, and the truly coated side of the first SAWS. The initial frequencies [f,x) of the two SAWS oscillators were very similar. One can clearly see a difference between the sorption isotherms onto the uncoated SAWS and the coated device in these figures. This indicates that some of the coating had crept from the coated, to the uncoated side of the dual SAWS. One would expect to detect an APTES monolayer

0

IO

20

30 40 w so 70 (%I Relatiw Humidity

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Fig. 8. Conditions as for Fig. 7, at 30”.

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JULIET. WOODand JOHNF. ALDER

coverage on a 70 MHz SAWS as it represents about a 240 Hz frequency shift. Irreproducibility in manipulating and fitting the SAWS in its socket however, somewhat blurs these small changes in frequency, and a partial monolayer could readily be missed using this method of diagnosis. The water sorption isotherms onto the uncoated SAWS were similar in shape and magnitude to those obtained in previous studies in this laboratory, using 158 MHz devices.4 The fact that creep was not evident using the FPOL and polyvinylpyrrolidone coatings in the previous work4 was possibly due to the much higher molecular weight and viscosity/lower mobility of the polymers compared to the lower molecular weight APTES. As was found in previous studies,4 the sorption isotherms resemble the BDDT type III behaviour12 which are characteristic of weak gas-solid interactions, particularly where the sorbed gas is polar and acts as sites for stronger polar-polar sorption of further water molecules. One would expect therefore a quadratic relationship between the frequency change on exposure of the SAWS to water vapour, and the relative humidity. Using a curve fitting routine “Techcad”, quadratic functions of the form y = ax2 + bx + c were fitted to the data points obtained, and the best fit curves are shown as the solid lines in the Figs l-6. It must be emphasized that these are purely empirical fits and although they show good regression analysis coefficients to the measured data, they have no detailed quantitative relationship to the underlying isotherm theory-i2 Table 1 shows the equation coefficients and the regression coefficient for the Figs 4-6. The effect of cycling the coated SAWS through a range of relative humidity was essayed at 25”, 30” and 40”. Quadratic functions were fitted to the data in ascending and descending order and the data shown in Figs 7-9. From the data it can be

Table 1. Table of water sorption isotherm equation coefficients. Fitting the isotherm data to an equation of the form y = ax 2 + bx + c where y represents frequency change (AflHz) and x represents relative humidity (% RH) Data from Figure Coating 4 4 5 5 6 6

APTES Uncoated AIYTES Uncoated APTES Uncoated

Equation coefficients Temp (“C) 25 25 30 30 40 40

(c)

(b)

9.6 5.74 68.3 -3.21 65.3 11.95 71.0 -2.31 -41.0 11.24 0.79 1.66

(a)

Regression coefficient

0.24 0.10 0.18 0.10 0.23 0.09

0.978 0.996 0.998 0.995 0.997 0.989

seen that there is no significant hysteresis in the cycle. CONCLUSIONS

This work has shown that SAWS coated with APTES demonstrate sorption isotherms for water vapour which can be represented by quadratic equations which fit well over the range l-70% RH at 25-30” and up to 50% RH at 40”. At higher temperatures and higher RH the frequency response becomes increasingly noisy for reasons which are at present not clear. Hysteresis in the adsorption and desorption curves is almost indiscernible at 25, 30 or 40” with the coating employed (21+ 3 monolayers). These results indicate that it should be possible to partially correct the response of an APTES coated SAWS for water vapour adsorption up to 30” and 70% RH by data processing. There is bound to be a further effect due to competition between water vapour and competing target analytes for active sites, which will further complicate the response characteristics of coated SAWS. Current work in this laboratory is investigating the response of APTES coated SAWS to nitrobenzene, in atmospheres of varying relative humidity and temperature. Acknowledgements-We are grateful to the Plessey Company for the gift of the 70 MHz SAWS.

REFERENCES

W&lRetctivs HumMity

Fig. 9. Conditions as for Fig. 7, at 40”.

1. J. F. Alder and J. J. McCallum, Analyst, 1983, 108, 1169. 2. J. J. McCallum, Analyst, 1989, 14, 1173. 3. C. G. Fox and J. F. Alder, ibid., 114, 1989, 997. 4. I&m, Anal. Chim. Acta, 1991, 248, 337. 5. J. W. Grate, A. Snow, D. S. Ballantine, H. Wohltjen, M. H. Abraham, R. A. McGill and P. Sasson, Anal. Chem., 1988, 60, 869.

Water sorption isotherms 6. D. S. Ballantine, S. L. Rose, J. W. Grate and H. Wohitjen, ibid., 1986, 58, 3058. 7. S. L. Rose-Pehrsson, J. W. Grate, D. S. Ballantine and P. C. Jurs, ibid., 1988,60, 2801. 8. W. M. Heokl, F. M. Marassi, K. M. R. Kallery,

D. C. Stone and M. Thompson, ibid., 1990, 62, 32. 9. J. F. Alder, C. G. Fox, A. R. M. Przybylko,

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N. D. Rez8ui and R. D. Snook, Analyst, 1989, 114, 1163.

10. H. Wohltjen, Sensors Actuators, 1984, 5, 307. 11. B. A. Auld, Acoustic Fiela!~and Waves in Soliak, Vol. 2, Chap. 13. Wiley Interscience, New York, 1973. 12. S. J. Gregg and K. S. W. Sing, Adsorption, Surface Atea uttd Porosity, 2nd Edn., p. 249. Academic Press, New York, 1982.