Sensors and Actuators B, 20 (1994) M-249

SAW gas sensor with copper tetrasulphonated phthalocyanine film

Chen Yuquan, Zhang Wuming, Li Guang Depariment of Scienti$c Inshumentatbn, Zhejiang VniversiQ 310027 Hangzhou (China)

(Received December 30, 1993; in revised form July 14, 1994, accepted July 18, 1994)

Abstract

241

A surface acoustic wave (SAW) piezoelectric device with a dual-path SAW delay line oscillator configuration has been developed to detect nitrogen dioxide (NO,) gas. One delay line is coated with a Langmuir-Blodgett (LB) film of copper tetrasulphonated phthalocyanine (CuTsPc), while the other is uncoated. NOa gas can be selectively absorbed by the CuTsPc LB tilm and then alters the SAW characteristics, which in turn causes the oscillation frequency to change. The concentration of NOa can be detected by measuring the relative change in the frequency of the two oscillators. The CuTsPc LB film-mated SAW device has been demonstrated to be sensitive to low concentrations of NO1 gas. There is a linear relationship between NOa concentration and frequency change under 12 ppm NOa concentration. The sensitivity of the device is ahout 128 Hz/ppm NOa. The response rime is 10 mm, and the recovery time is 40 mm. It can be applied as a small, inexpensive and sensitive NO, gas sensor.

1. introduction

Since the first work relating to the use of surface acoustic wave (SAW) devices as gas sensors was done by Wohltjen and Dessy [l], SAW sensors sensitive to CO, NOu NH,, etc., have become attractive for chemical applications because of their low cost, small sire, good performance and their compatibility with planar in- tegrated circuit (IC) technology. The sensitivity of a SAW gas sensor is dependent on the selectively ab- sorbent coating tilm, which is sensitive to a certain gas, on the delay line. The response and recovery of the absorption of a Langmuir-Blodgett (LB) film is the- oretically faster than that of other thin films because of the thinness and the more ordered structure of the LB film. It is known that CuTsPc is sensitive to the presence of nitrogen dioxide (NO,). In this paper, we report on a SAW device coated with an LB film of CuTsPc as an NO* gas sensor.

2. Sensor design

The sensor is based on a SAW device with dual delay lines. One is coated by a CuTsPc LB film acting as a sensing path, while the other is uncoated and is used for temperature compensation, as shown in Fig. 1. Both delay lines and an external circuit compose an oscillator. The CuTsPc LB film can sensitively absorb a particular gas, NOz. The NO, absorbed by the LB

,CuTsPc LB film

Fig. 1. SAW gas sensor and measuring system.

film will alter the SAW characteristics, causing the oscillation frequency to change. The relative change in the frequency of the two oscillators represents the concentration of the NO, gas. We have studied the factors that will affect the sensitivity of the sensor and found that

where f is the frequency shift, f. is the undisturbed oscillation frequency, rr = 1-2, h is the thickness of the sensing film, K is the electromechanical coupling coef- ficient, p is the mass density of the overlying film material and u is the conductivity. All of these con- siderations result in a set of scaling laws that can offer guidance in predicting the ultimate design criteria. The other static and dynamic characteristics, such as

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248

linearity, response time and recovery time, were also considered.

A 131” Y-XLiNbO, crystal was chosen as the substrate for fabricating the SAW device because of its large electromechanical coupling coefficient. The SAW in- terdigital transducer (IDT) is designed in a split-finger pattern, with equal electrode and spacing widths of 8 brn and a total period of 64 pm. Its base frequency is 61.3 MHz. The third-harmonic frequency, 183.9 MHz, is excited by the external circuits and is used as the operating frequency. This design is used to improve the sensitivity of the sensors without increasing the technological difficulty. But the frequency band widening effect caused by the third-harmonic oscillation should be considered.

The CuTsPc film used in this study was deposited with the LB film-transfer technique [2]. Fig. 2 shows the LB film apparatus. In order to make the CuTsPc molecule compatible with the LB method, a chloroform solution of a compound of octadecylamine (C,,NI-&) and methylarachidate (AME) was prepared as the assisting material. The mole ratio C,,NH,:AME is 1:4, and the percentage mole concentration is 10 PM. A monolayer is first formed by spreading the assisting material on the 10 PM CuTsPc aqueous solution at pH 4.0. Then the monolayer is transferred (Z-type transfer) onto one of the delay lines of the SAW device at a film pressure of 35 mN/m. The SAW device was removed for drying upon monolayer transfer and was then immersed in a saturated aqueous BaCI, solution for a period of time. The BaZC ions replace Na+ ions bound to the sulphonate ion and also reduce the interaction between the ammonium head groups of the lipid and the PC titration, thereby making the film more densely packed and insoluble in aqueous solutions. Finally, the SAW device was washed in chloroform to remove the lipid layer, and a CuTsPc lipid-free mono- layer firmly bound to the surface of the delay line was obtained. Multilayers of CuTsPc were built up by rep- etition of this procedure. In this study, we built up a five-layer CuTsPc LB film 3.56 nm thick. Fig. 3 is an SEM micrograph of such a film. The size of the delay line for the CuTsPc material is 2.0 mmX 2.5 mm, as shown in Fig. 4, and the total size of the SAW sensor chip is 5 mm X 10 mm. A backside heater and a feedback

Fig. 2. Diagram of LB film balance apparatus.

Fig. 3. SEM micrograph of five-layer CuTsPc LB film.

circuit are used to control the temperature stabilization of the sensor device in order to eliminate the influence of temperature and let the sensor reach the optimum working state. The controlled working temperature is 50 “C.

3. Experimental results and discussion

The experimental measurement system was composed of a gas-mixing system, a SAW device covered with CuTsPc LB film, an external electronic measuring circuit and a frequency counter. While the SAW delay line with the LB film acted as a sensing path, the other one without the LB film was used to compensate the influence of temperature changes. The base frequency of the SAW split-finger pattern IDTs is 61.3 MHz. A group of IDTs, a delay line, a triple-harmonic oscillating circuit and an r.f. amplifier compose an independent SAW oscillator. The triple-harmonic oscillating circuit included in the external circuits excites the base fre- quency of 61.3 MHz to 183.9 MHz as an operating frequency, as shown in Fig. 1. It is used to improve the measuring sensitivity without improving the SAW device. The design of the split-finger pattern is a useful way to reduce the reflection at the edge of the interdigital

249

Af(Hz]

1500

1000

500

0

NO2 concentrstion(ppn I I , I 5 10 15 20

Fig. 5. Response of the SAW gas sensor as a function of NO, concentration.

transducer while the triple-harmonic wave is excited. The frequency counter is used for measuring the fre- quency change between the sensing SAW oscillator and the compensating SAW oscillator.

NO, mixed with N, was used as test gas. The NO2 gas concentration intluenced the frequency output. The greater the NO2 concentration, the larger is the fre- quency shift. The responses of the sensor to NO2 concentrations between 0 and 20 ppm are shown in Fig. 5. It is clear that there is a linear relationship below 12 ppm and the sensor has a sensitivity of 128 Hx/ppm NO, gas. Even though the frequency shift increases with the increase of the NOz concentration when it is greater than 12 ppm, the response becomes non-linear. The upper limit of the sensor’s range was found to be in the proximity of 25 ppm. It is found that sufficient NO, absorbed on the CuTsPc LB film would damage the film irreversibly. The response and recovery times are 10 and 40 min, respectively at 25

ppm. CO was also tested as an interference gas. The

sensitivity of the sensor to CO is 3.5 Hx/ppm.

4. Conclusions

A surface acoustic wave device coated with a copper tetrasulphonated phthalocyanine Langmuir-Blodgett film has been demonstrated to be sensitive to low concentrations of NO* gas. It could be applied as a small, inexpensive and sensitive NO, gas sensor. The design of the SAW interdigital transducers in a split- linger pattern and the use of the triple-harmonic fre- quency as the working frequency are suitable for NO,

gas measuring without increasing the technological dif- ficulty.

Acknowledgement

This project was supported by the National Biomedical Transducers Key Laboratory of China and the Zhejiang Science Foundation.

References

H. Wohltjen and R. Dessy, Surface acoustic wave probe for chemical analysis, AMZ. C&m., 51 (1979) 1458-1478. K. Tian, M. Gong, Y. Bai, Y. Zhao and T. Li, Formation and conductivity of lipid-free multilayer *terns of copper phthalocyanine, l?ain Solid FiLns, 159 (1988) 239-242. S. Baker, G.G. Roberts and MC. Petty, Phtbalocyanine Langmuir Blodgett-film gas detector, IEE Rot., 130 (1983) 260-263. P. Kblebarov, A.I. Stoyanova and D.P.I. Topalova, Surface acoustic wave gas sensors, Sensors and Actuators B, 8 (1992) 33110.

Biographies

Chen Yuquon is an associate professor and director of the National Biomedical Sensor Key Laboratory of China in Zhejiang University. During 1985-1987, as a visiting scholar, he studied and was engaged in research on microelectronics and solid-state sensors at Case Western Reserve University, USA. Currently, his re- search involves chemical and biomedical sensors and other solid-state sensors in environmental status sensing and life sciences.

Zhang Wuming was born on September 4, 1962. He got his bachelor’s and master’s degrees in 1984 and 1987, respectively, at Specialization of Biomedical En- gineering and Instrumentation, Department of Scientific Instrumentation, Zhejiang University, China. From 1987 to now, he teaches and does research at the Biomedical Engineering Institute, Zhejiang University and the Na- tional Biomedical Transducers Key Laboratory of China. His interest focuses on electronic system design, digital signal processing, sensor and sensor signal processing and artificial neural networks.

Li Guang obtained B.S. and MS. degrees in 1987 and 1991, respectively, and now works in the National Biomedical Sensor Key Laboratory of China as an assistant professor. His research involves chemical and biomedical sensors and instrumentation.