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A NOVEL CHEMICAL SENSOR USING CH3Si(OCH3)3 SOL-GEL THIN FILM COATED QUARTZ-RESONATOR MICROBALANCE

Hidehito Nanto, Shiro Tsubakino, Masaaki Habara, Kouichi Kondo, Takashi Morita Yoshiteru Douguchi*, Hiroyuki Nakazumi** and Ruth I.Waite***

Electron Device System Research La'boratory, Kanazawa Institute of Technology, 7-1 Oogigaoka, Nonoichimachi, Ishikawa 921 Japan

*Industrial Research Institute of Ishikawa, 1 Tomizu-machi, Kanazawa 920-02 Japan **Dept. of Appl. Mater. Sci., College of Enginnering, University of Osaka Prefecture, Sakai, Osaka 593 Japan

***Rose-Hulman Institute of Technology, 5500 Wabash Avenue, Terre Haute, IN 4780 USA

INTRODUCTION

A quartz crystal microbalance is known to provide a very sensitive mass-measuring devices in nanogram levels, because of the resonance frequency changes upon the deposition of a given mass on the electrode. Syn- thetic polymer-coated quartz crystal resonator have been studied as sensors for various gases, since a quartz crystal resonator coated with a sensing membrane works as a chemical sensor. Recently con,siderable interest has arisen on the use of arrays of quartz crystal res- onator gas sensors in conjunction with an associated pattern recognition technique for the identification of odors, fragrances and aromas [l-71. Recently, we have reported that a novel aroma sensing imethod using the neural network pattern recogniton analysis of transient responses measured with epoxy-coatled quartz crystal resonator gas sensors, is useful for the discrimination among aromas from different kinds of dripped coffee or wine [8,9]. The new point of the method is to detect the aroma which is generated by vaporization of a constant amount of coffee or wine using heater. Another new point is to use parameters, which characterize the tran- sient responses of the sensors, for the pattern recogni- tion. In this paper, we demonstrate a inovel alcohol sen- sor system using epoxy-film-coated, sol-gel-film-coated or lacquer-film-coated quatrz crystal resonator gas sen- sor together with the principal component analysis or neural network analysis to discriminate among differ- ent kinds of alcohols.

EXPERIMENTAL

Commercially-available AT-cut, 9 [MHz] quartz-reso- nator with silver electrodes coated with CH3Si(OCH3)3 sol-gel-film as a sensing membrane was used as a sen- sor. For sol-gel-film preparation, coating solutions were prepared by dissolving 1.8-5.0 [g] of the appropriated methyltrimethoxy-sillane in 10 [g] of reagent grade etha- nol and adding 1.04 [g] of distilled water containing a few drop of acid (30/,uk'], 30[%] HN03). The solution

was stilled for 2-5 [h] at room temperature (RT) for hydrolysis and polycondensation of alkoxysilanes. A transparent coating layer on a quartz-resonator was formed by the dipping-withdrawing method (rate of drawing: 1 [mmss-']). Heating this coating layer for 15 [min] a t lOO[Oc] formed a hard gel film, The frequency of the vibrating quartz resonator gas sensor, which is decreased with increasing the adsorbed gas amount on the sensor surface, was measured with a conventional frequency counter attached to a personal computer. Transient responses of the quartz resonator gas sen- sor for aromas from ethanol solutions with different ethanol concentrations (O[%] , 25[%], 50[%], 75[%] and loo[%]) or five kinds of alcohols were measured using a sensing chamber [9]. A constant amount (30[p l l ) of ethanol solution or alcohol was injected into the sens- ing chamber and vaporized on a heater (50["c]).

RESULTS AND DISCUSSION

Figure 1 shows typical transient response curves of sol-gel-film-coated sensor for exposure to each aroma from ethanol solutions with different ethanol concen- trations. It can be seen that the response curves de- pend upon the concentration of ethanol. It should be noted that the time at the maximum frequency change strongly depends on the ethanol concentration. Figure 2 shows typical transient curves of the sol-gel- film-coated senior for each aroma from several alcohols with different ethanol concentrations, such as Japanese Sake (ethanol conc.:15[%]), Shoutyuu (25[%]), Cognac (45[%]), Vodka (50[%]) and Vodka (95[%]). It can be seen that the time at the maximum frequency change also becomes longer with decreasing the ethanol con- centration. In order to discriminate aromas using prin- cipal component or neural network pattern recognition analysis, we defined three parameters such as T,,, (time at the maximum frequency change), Gt-2 (gra- dient of the transient response curve a b (Tmat-2) min) and Gt+z (gradient at (TmaX+2) min), which charac-

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The 8th International Conference on Solid-State Sensors ancl Actuators, and Eurosensors IX. Stockholm, Sweden, June 25-29, 1995 765

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terize the transient response curve of the sensor, as shown in Fig.3. The result of principal component analysis for transient response curves measured for a r e mas from ethanol solutions with different concentra- tions as shown in Fig.1, reduced to the first two compo- nents for the visualization of three-dimensional data, is given in Fig.4. It is clear that responses for individual aromas tend to cluster in discrete sections of space with

2000 I I I I I 1 , I , I , , , ,

Ethylalcohol Conc. r

77

3 1000

500

0 0 5 10 15

TIME [min]

Fig.1 Typical transient response curves of CH3Si(OCH3)3 sol-gel-film-coated sensor for each aroma from ethanol solutions with diflerent ethanol concentrations.

2000

- N 3 r io00 4

0 0 5 10 15

TIME [mln]

Fig.2 Typical transient response curves of sol-gel-film- coated quartz resonator gas sensor for each aroma from several kinds of alcohols such as Japanese Sake (ethanol conc.:l5[%]), Shoutyuu (25[%]), Cognac (45[%]), Vodka (50[%]) and Vodka (95[%11*

well defined boundaries. A plot of the first two princi- pal components for tarnsient response curves measured for aromas from several alcohols, is shown in Fig.5, where each group of alcohols is completely separated. These results suggest that it is possible to discriminate among aromas from ethanol solutions with different concentrations and from several kinds of alcohols with different concentrations, using neural network analysis. The neural network used had a three-layers structure made up of three input, five hidden and five output units. The back propagation algorithm was applied

I I I

I I I I

I I I I

Tnax-2 Tmax Tmaxt.2 TIME [mln]

Fig3 Typical transient response curve. Three param- eters such as T,,, (the time a t maximum fre- quency change), Gt-2 ( t h e gradient at Tmaz-2 [min]) and Gtf2 (the gradient at TmaZ $2 [min]) were defined for pattern recognition analysis as shown in the figure.

-3.353 1.622 6.597

PRINCIPAL COMPONENT,X2

f 0: a.

Fig.4 Principal component plots using results for aro- mas from ethanol solutions with diflerent ethanol concentrations.

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Toyama University of International Studies, Profes- sor E. Tamiya of Japan Advanced Institute of Sci- ence and Technology (Hokuriku) for fruitful discussion, S.Kitagawa, N.Oobayashi and A.Osada for their excel- lent technical assistance in the experiments. This work was partly supported by the Hokuriku Kasseika Center Foundation.

REFERENCES

E 2.660 6.402 10.143 a Q PRINCIPAL COMPONIENT,XZ

Fig.5 Principal component plots using results for aro- mas from seveml kinds of alcohols.

as the learning rule. The recognition probability was defined as the ratio of the number of right answers to that of the total trials. Consequently, the recogni- tion probability of the neural network analysis for data from transient response curves measured using sol-gel- film-coated sensor for ethanol solutions is loo[%] for 50 trials. The probability for data from transient re- sponse curves measured for several kinds of alcohols is loo[%] for 50 trials.

SUMMARY

[l] J.W.Gardner and P.N.Bartlet, Sensors and Actua- tors, B18&19, pp.211-220, 1994.

[2] J.W.Gardner, Sensors and Actuators, B4, pp.109-

[3] R.Muller, Sensors and Actuators, B4, pp.35-39,

115, 1991.

1991.

[4] W.P. Carey, K.R.Beebe and B.R.Kowalski, Anal. Chem., 58, pp.149-153, 1986.

[5] T.Nakamoto, K.Fukunishi and T.Moriizumi, Sen- sors and Actuators, B1, pp.473-476, 1990.

[6] T.Nakamoto, A.Fukuda and T.Moriizumi, Sensors and Actuators, B10, pp.85-90, 1993.

[7] H.Nanto, S.Tsubakino, T.Kawai, M.Ikeda, S.Kitagawa and M.Habara, J . Materials Sci., 29, pp.6529-6532, 1994.

[8] H.Nanto, T.Kawai, HSokooshi and T.Usuda, Sen- sors and Actuators, B13&14, pp.718-720, 1993.

[9] H.Nanto, T.Kawai, H.Sokooshi and T.Usuda, Tech-

Discrimination among aromas from ethanol solution with different ethanol concetrations or among several

nical Digest of The 11th Sensor Symp., Tokyo, Japan, pp.225-228, 1992.

kinds of alcohols was successfully demonstrated us- ing a novel aroma sensing method with the following features. (1) The aroma which is detected using the quartz resonator gas sensor coated with epoxy-film, lacquer-film or sol-gel film is genera1,ed by vaporizing the ethanol solutions or several kinds of alcohols using heater (50["C]). (2) Three parameters which charac- terize the transient response curves are used for the principal component or neural network analysis. Since the sensing method proposed here is very simple, this new sensing method is one of the most attractive can- didates for aroma sensing and identification.

ACKN0WLEDGEM:ENT

The authos wish to thank Professor T.Oyabu of

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