Surface Acoustic Wave Chemical Microsensors and Sensor Arrays for

Chapter 9

Surface Acoustic Wave Chemical Microsensors and Sensor Arrays for Industrial Process Control and Pollution Prevention

Downloaded by STONY BROOK UNIV SUNY on May 19, 2018 | https://pubs.acs.org Publication Date: October 6, 1992 | doi: 10.1021/bk-1992-0508.ch009

H. Wohltjen, N. L. Jarvis, and J. R. Lint Microsensor Systems, Inc., 6800 Versar Center, Springfield, VA 22151

Surface Acoustic Wave (SAW) microsensors have many unique c h a r a c t e r i s t i c s that make them ideal f o r monitoring gases and vapors associated with industrial chemical processes. Individual SAW devices are very small, rugged, inexpensive, and r e l i a b l e . They can operate over a range of temperatures up to 50°C and provide electronic signals that can be readily integrated into computer networks for real time process control. They are sensitive to ppm levels of many chemical vapors, respond i n seconds to changes i n vapor concentration, and can be made s e l e c t i v e for many d i f f e r e n t compounds or classes of compounds. The state of development of SAW technology and i t s application to process control for p o l l u t i o n prevention w i l l be discussed. The use of arrays of SAW sensors for industrial monitoring w i l l be emphasized.

I t i s accepted t h a t t h e best approach t o environmental p r o t e c t i o n i s t o prevent p o l l u t i o n a t i t ' s source, r a t h e r than r e p a i r environmental damage a f t e r the f a c t . Thus the Environmental P r o t e c t i o n Agency i s encouraging i n d u s t r y and the s c i e n t i f i c community t o explore new approaches f o r the m o n i t o r i n g and c o n t r o l of i n d u s t r i a l p r o c e s s e s . A new technology t h a t shows c o n s i d e r a b l e p o t e n t i a l f o r t h i s a p p l i c a t i o n i s t h a t of chemical microsensors. Chemical microsensors i n c l u d e a v a r i e t y of c h e m i c a l l y r e s p o n s i v e d e v i c e s , i n c l u d i n g s u r f a c e a c o u s t i c wave (SAW) d e v i c e s , organic and inorganic semiconductors, micro-electrochemical c e l l s , chemfets, and o t h e r d e v i c e s based on t h e use of s o l i d - s t a t e e l e c t r o n i c m i c r o f a b r i c a t i o n technology. Even 0097-6156/92/0508-0086$06.00/0 © 1992 American Chemical Society

Breen and Dellarco; Pollution Prevention in Industrial Processes ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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though each type of microsensor has i t s unique advantages and disadvantages, they a l l a r e very small, inexpensive, rugged, and have low power consumption. In a d d i t i o n t o o f f e r i n g new a n a l y t i c a l c a p a b i l i t i e s , chemical microsensors are a l s o i n the vanguard of attempts t o reduce the cost of chemical i n f o r m a t i o n . Thus chemical microsensors should f i n d i n c r e a s i n g a p p l i c a t i o n as monitors of i n d u s t r i a l p r o c e s s e s where t h e e x c e s s i v e s i z e , c o s t and power consumption of t r a d i t i o n a l sensors preclude t h e i r use. One of the most promising of the chemical microsensors f o r chemical vapor monitoring i s the Surface A c o u s t i c Wave (SAW) device, which has been the focus of growing i n t e r e s t since the f i r s t s t u d i e s r e p o r t e d i n 1979 (1-14). The SAW device o f f e r s a simple, d i r e c t , and s e n s i t i v e method f o r probing the composition of organic vapors. SAW S e n s o r

Technology

SAW Sensor O p e r a t i n g P r i n c i p l e s . SAW d e v i c e s a r e mechanically resonant s t r u c t u r e s whose resonance frequency i s perturbed by the mass or e l a s t i c p r o p e r t i e s of materials i n contact with the device surface. Rayleigh surface waves can be g e n e r a t e d on v e r y s m a l l p o l i s h e d c h i p s of p i e z o e l e c t r i c m a t e r i a l s (e.g. quartz) on which an i n t e r d i g i t a l electrode array i s l i t h o g r a p h i c a l l y patterned (Figure 1) . When the e l e c t r o d e i s e x c i t e d w i t h a r a d i o frequency voltage, a wave i s generated that t r a v e l s across the d e v i c e s u r f a c e u n t i l i t i s " r e c e i v e d " by a second electrode. T h i s R a y l e i g h wave has most of i t s energy constrained t o the surface of the device and thus i n t e r a c t s very s t r o n g l y with any m a t e r i a l that i s i n contact with the surface. Changes i n mass o r mechanical modulus of a surface coating a p p l i e d t o the device produce corresponding changes i n wave v e l o c i t y . The most common c o n f i g u r a t i o n f o r a SAW vapor/gas sensor i s t h a t of a d e l a y l i n e o s c i l l a t o r . In t h i s c o n f i g u r a t i o n the device resonates at a frequency determined by the wave v e l o c i t y and the e l e c t r o d e spacing. If the mass of the coating i s a l t e r e d , the r e s u l t i n g change i n wave v e l o c i t y can be measured as a s h i f t i n resonant frequency. SAW vapor sensors a r e s i m i l a r t o b u l k wave p i e z o e l e c t r i c c r y s t a l s e n s o r s , except t h e y have t h e d i s t i n c t advantages of s u b s t a n t i a l l y h i g h e r s e n s i t i v i t y , smaller s i z e , greater ease of coating, uniform surface mass s e n s i t i v i t y , and improved ruggedness. P r a c t i c a l SAW sensors c u r r e n t l y have a c t i v e surface areas of a few square m i l l i m e t e r s and resonance f r e q u e n c i e s i n t h e range of hundreds of MHz. However, SAW devices having t o t a l surface areas s i g n i f i c a n t l y l e s s than a square m i l l i m e t e r a r e p o s s i b l e using modern microlithography. SAW Sensor and Support E l e c t r o n i c s . Most of t h e SAW vapor sensors r e p o r t e d i n the l i t e r a t u r e employ two delay l i n e o s c i l l a t o r s f a b r i c a t e d s i d e by s i d e on the same chip, as shown i n F i g u r e 1, with one delay l i n e used t o monitor



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the s e l e c t e d chemical vapor and the o t h e r t o a c t as a reference t o compensate f o r changes i n ambient temperature and p r e s s u r e . The dual SAW d e v i c e s can be mounted i n a v a r i e t y of e l e c t r o n i c packages?, depending upon the intended application. Tne 15 8 MHz SAW device shown i n F i g u r e 2 i s bonded to a standard 1.5 cm diameter c i r c u l a r package that can be "plugged i n " t o a r e c e i v i n g f i x t u r e on a c i r c u i t board and i s t h e r e f o r e e a s i l y r e p l a c e a b l e . The s i z e of the sensor package i s determined by the s i z e of the SAW sensor, thus a much smaller 300 MHz dual SAW sensor c o u l d be mounted i n a package only h a l f the diameter. The SAW sensor, as shown i n F i g u r e 2, can be exposed d i r e c t l y to the i n d u s t r i a l process environment, r e l y i n g on d i f f u s i o n t o t r a n s p o r t the v a r i o u s vapors t o the sensor surface. A l t e r n a t i v e l y , the sensor packages can be covered w i t h c l o s e l y f i t t e d l i d s , w i t h vapor i n l e t and o u t l e t f i t t i n g s , so t h a t vapor samples can be a c t i v e l y moved passed the sensor s u r f a c e s by a s m a l l pump, f o r more accurate, c o n t r o l l e d sampling. A SAW sensor package with a s e a l e d l i d i s an extremely rugged d e v i c e and can e a s i l y w i t h s t a n d the shock and v i b r a t i o n a s s o c i a t e d w i t h most f i e l d or i n d u s t r i a l monitoring a p p l i c a t i o n s . A t y p i c a l SAW Vapor Sensor Module w i l l g e n e r a l l y c o n t a i n a d u a l SAW sensor and a l l n e c e s s a r y support e l e c t r o n i c s . A l l components of a t y p i c a l 158 MHz SAW Vapor Sensor w i l l e a s i l y f i t on a c a r d approx. 3" χ 2" χ 0.5". This card i s e s s e n t i a l l y a complete chemical vapor monitor, i t r e q u i r e s only power to operate and provides a frequency d i f f e r e n c e , Af, s i g n a l p r o p o r t i o n a l t o the challenge vapor concen-tration. The Δί can be e a s i l y measured w i t h a frequency counter. Even though a c i r c u i t card of t h i s s i z e i s c o n v e n i e n t l y s m a l l f o r many a p p l i c a t i o n s , i t can be f u r t h e r m i n i a t u r i z e d by a r r a n g i n g the d i s c r e t e components i n a more compact manner. Obviously a very large reduction i n s i z e could be achieved i f the sensor and module were re designed using hybrid microelectronic fabrication technology. By t h i s approach the s i z e of the e n t i r e module could be reduced to that of a d i g i t a l watch. However, t h i s would r e q u i r e an expensive e n g i n e e r i n g e f f o r t . Thus the s i z e of a current SAW Vapor Sensor system i s convenient f o r many i n d u s t r i a l and f i e l d a p p l i c a t i o n s , y e t t h e r e i s c o n s i d e r a b l e p o t e n t i a l f o r system m i n i a t u r i z a t i o n t o meet new and more demanding s i z e and conformation requirements. Role of S e l e c t i v e Coatings f o r SAW Devices. The o p e r a t i n g frequency of a SAW d e v i c e i s v e r y s e n s i t i v e t o changes i n p h y s i c a l p r o p e r t i e s of the surrounding medium, e.g., d e n s i t y , e l a s t i c modulus, and v i s c o s i t y . However, by themselves they a r e not i n h e r e n t l y s e n s i t i v e t o the chemical p r o p e r t i e s of the surrounding medium. Thus a chemically s e l e c t i v e t h i n f i l m must be a p p l i e d to a device surface (Figure 3) i n order f o r i t t o e x h i b i t s e l e c t i v i t y as well as s e n s i t i v i t y to chemical vapors. The coatings my e i t h e r adsorb or absorb s p e c i f i c vapors, and they may i n t e r a c t r e v e r s i b l y or i r r e v e r s i b l y , depending upon t h e i r


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CHEMICALLY SENSITIVE COATING 'Ν) 1 (ELECTRODE COATING NOT SH OWN)

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Figure 1. Operation of SAW chemical vapor sensor.

Figure 2. 158 MHz dual SAW device in circular package. TO-8 Header/0.600" diameter, pin package (Part No. SD-158-A).



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chemistry. There are two approaches to the development of c h e m i c a l l y s e l e c t i v e SAW coatings f o r i n d u s t r i a l l y important vapors: (1) coatings that w i l l s e l e c t i v e l y and r e v e r s i b l y absorb a s e l e c t e d vapor or gas by matching " s o l u b i l i t y " c h a r a c t e r i s t i c s (and thus be a b l e to c o n t i n uously monitor the concentration of a s e l e c t e d vapor); and (2) coatings that react chemically and i r r e v e r s i b l y with a s e l e c t e d vapor or gas. The i r r e v e r s i b l e coatings are most s u i t a b l e f o r dosimeter or cumulative exposure a p p l i c a t i o n s . SAW s e l e c t i v i t i e s i n excess of 1,000 to 1 f o r c e r t a i n t o x i c chemical agents have been demonstrated u s i n g the " s o l u b i l i t y " approach. Greater s e l e c t i v i t i e s should be p o s s i b l e using chemically r e a c t i v e coating/vapor (gas) combinations. Mass S e n s i t i v i t y of SAW Devices. A 158 MHz SAW o s c i l l a t o r having an a c t i v e area of 8 mm w i l l give a frequency s h i f t of about 365 Hz when perturbed by a surface mass change of 1 nanogram. T h i s s e n s i t i v i t y i s p r e d i c t e d t h e o r e t i c a l l y and has been confirmed experimentally using successive d e p o s i t i o n s of Langmuir-Blodgett monolayers of known dimensions and mass. The same d e v i c e e x h i b i t s a t y p i c a l frequency "noise" of l e s s than 15 Hz RMS over a 1 second measurement i n t e r v a l ( i . e . 1 p a r t i n 1 0 ) . Thus, the 1 nanogram mass change gives a s i g n a l to noise r a t i o of about 23 to 1. I t has r e c e n t l y been shown that SAW resonator devices have a d i s t i n c t advantage over SAW o s c i l l a t o r s as vapor sensors (15, 16). At a" g i v e n frequency (e.g., 158 MHz) both w i l l have the same s e n s i t i v i t y ; however, a SAW o s c i l l a t o r w i l l t y p i c a l l y e x h i b i t a frequency n o i s e of about 15 Hz RMS over a one second measurement i n t e r v a l , whereas a SAW resonator w i l l e x h i b i t no more than 2 Hz RMS noise. Thus a SAW o s c i l l a t o r w i l l p r o v i d e a s i g n a l to n o i s e of about 23 t o l f o r a nanogram mass change, but a resonator w i l l p r o v i d e a s i g n a l to n o i s e r a t i o c l o s e r to 180 to l . The SAW frequency change, At, produced by a change i n mass w i l l i n c r e a s e w i t h the square of the unperturbed resonant f r e q u e n c y of the SAW device, f (D . Thus i n c r e a s e s i n s e n s i t i v i t y can be a c h i e v e d by u s i n g SAW devices of h i g h e r frequency. For example, i n c r e a s i n g the operating frequency from 158 MHz to approximately 300 MHz w i l l i n c r e a s e the s e n s i t i v i t y by about a f a c t o r of 2. By u s i n g 300 MHz SAW r e s o n a t o r d e v i c e s and new, high s p e c i f i c i t y c o a t i n g s , i t s h o u l d be s t r a i g h t f o r w a r d t o detect sub-nanogram q u a n t i t i e s of many vapors as they react with s u i t a b l y coated SAW devices. 2

7

Q

P e r f o r m a n c e o f SAW

Devices

as V a p o r

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E f f e c t of Temperature. SAW d e v i c e s are i n h e r e n t l y v e r y s e n s i t i v e t o temperature, changing many Hertz per degree, depending on the resonance frequency. In many a p p l i c a t i o n s the temperature dependence of the d e v i c e (as w e l l as p r e s s u r e dependence) can be s u f f i c i e n t l y compensated by



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u s i n g a r e f e r e n c e SAW d e v i c e . Temperature d r i f t of SAW d e v i c e s can a l s o be c o n t r o l l e d by h o l d i n g them at a constant temperature, f o r example, w i t h a simple s o l i d s t a t e t h e r m o e l e c t r i c device. However, there i s an e f f e c t of temperature than cannot be compensated and that i s the e f f e c t of temperature on the p a r t i t i o n of a vapor between the gas phase and the absorbed phase i n a c h e m i c a l l y s e l e c t i v e SAW c o a t i n g . As the temperature i n c r e a s e s , p r o p o r t i o n a t e l y l e s s vapor w i l l be absorbed i n the coating and the observed mass l o a d i n g w i l l be l e s s , f o r a g i v e n c o n c e n t r a t i o n . I t was observed f o r dimethylmethylphosphonate vapor absorbing i n a f l u o r i n a t e d p o l y o l coating on a 158 MHz SAW device that as the temperature increased from 23°C to 42°C the observed frequency s h i f t decreased by a f a c t o r of four. T h i s e f f e c t of temperature can have an impact on the p o t e n t i a l use of the present s t a t e - o f - t h e - a r t SAW devices in i n d u s t r i a l processes. w i t h the c o a t i n g m a t e r i a l s i n v e s t i g a t e d t o date SAW d e v i c e s w i l l be l i m i t e d t o a p p l i c a t i o n s where the ambient temperatures are below about 50-80°C, depending upon t h e s p e c i f i c v a p o r / c o a t i n g combination and the c o n c e n t r a t i o n t o be monitored. In p r a c t i c e , however, t h i s l i m i t a t i o n may often be overcome by drawing vapor from the h i g h temperature environment and c o o l i n g i t a p p r o p r i a t e l y b e f o r e measurement. At these temperatures i t should be p o s s i b l e i n many a p p l i c a t i o n s t o p l a c e the sensors d i r e c t l y o n - l i n e . Future r e s e a r c h may provide coatings that w i l l allow the SAW sensors to operate at the higher temperatures of many i n d u s t r i a l a p p l i c a t i o n s . One other f a c t o r to consider i n the use of SAW sensors at h i g h e r temperatures i s the performance of the o t h e r e l e c - t r o n i c components. The current SAW e l e c t r o n i c s module was designed to operate at r e l a t i v e l y modest temperatures. I d e a l l y i t would be d e s i r a b l e t o mount and operate the SAW sensors on extended leads, so that the e l e c t r o n i c s support package could be h e l d at a lower temperature. This w i l l require a c a r e f u l re-design of the SAW e l e c t r o n i c system. Response Time of Coated SAW Devices. SAW devices respond r a p i d l y to changes i n vapor concentration, as shown i n Figure 4 f o r the absorption dimethylmethylphosphonate by a FPOL coated 158 MHz device. I t i s seen t h a t at a c o n c e n t r a t i o n of 1.0 ppm t h e SAW sensors respond immediately t o the DMMP vapor . I t should be noted that the e q u i l i b r a t i o n (response) time i n t h i s case includes the time r e q u i r e d f o r the vapor i n l e t l i n e s and dead volume w i t h i n the sensor package t o e q u i l i b r a t e , as w e l l as the SAW coating. The a c t u a l response time of the sensor i t s e l f i s therefore very rapid, on the order of a few seconds. The time r e q u i r e d f o r desorption of the vapor from the coating (when the vapor c h a l l e n g e i s removed) i s o n l y s l i g h t l y longer. The r a p i d response time of SAW sensors i s an obvious advantage i n the r e a l time m o n i t o r i n g of many i n d u s t r i a l processes.




ADSORPTION

ABSORPTION

Selective Coating SAW Device

SAW Device

Dynamic Adsorption Concept

Solution Concept

Figure 3. Vapor/coating interactions.

194000

%

188000

CO

186000 200

400 Time

600

800

1000

(seconds)

Figure 4. Response ofFPOL coated SAW sensor to 1 ppm DMMP.



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S e l e c t i v i t y o f SAW C o a t i n g s . As i n d i c a t e d above, SAW coatings are generally selected f o r t h e i r p r e f e r e n t i a l i n t e r a c t i o n w i t h a s p e c i f i c chemical vapor o r c l a s s of chemical vapors. An example of t h e s e l e c t i v i t y of one coating f o r d i f f e r e n t chemical vapors i s given i n Figure 5. F i g u r e 5 shows t h e response of a FPOL coated sensor t o a v a r i e t y of vapors i n a d d i t i o n t o a s p e c i f i c vapor i t was designed t o d e t e c t , dimethylacetamide (DMAC) (10) . The sensor e x h i b i t s an order of magnitude g r e a t e r s e n s i t i v i t y to DMAC than t o any other vapor t e s t e d . For example, the s e l e c t i v i t y r a t i o f o r DMAC vs tributylphosphate (the second l a r g e s t response) i s (358/11.9), o r 30, while the s e l e c t i v i t y r a t i o f o r DMAC vs water vapor i s 4,47 5. An example of a SAW c o a t i n g c a r e f u l l y designed t o respond i r r e v e r s i b l y t o a s i n g l e vapor i s p o l y ( e t h y l e n e maleate) (PEM) w h i c h reacts specifically with c y c l o p e n t a d i e n e t o form a D i e l s - A l d e r adduct. The PEM coated SAW d e v i c e was a l s o exposed t o a v a r i e t y of other vapors t o f u r t h e r demonstrate the s e l e c t i v i t y of t h i s f i l m (6). A l i q u o t s measuring 2,000 ppm of each vapor were prepared by a d d i t i o n of the v o l a t i l e l i q u i d i n t o a sample f l a s k c o n t a i n i n g the SAW sensor. The SAW frequency s h i f t was recorded as a f u n c t i o n of time. The f l a s k was then f l u s h e d w i t h c l e a n a i r t o remove t h e vapor sample. A l l samples i n v e s t i g a t e d except c y c l o p e n t a d i e n e gave relat i v e l y small i n i t i a l responses, which l e v e l e d o f f r a p i d l y a f t e r i n j e c t i o n and returned t o b a s e l i n e as the vapor was replaced with clean a i r . Of t h e vapors s t u d i e d , o n l y cyclopentadiene gave a large, rapid, i r r e v e r s i b l e response. The response of t h e PEM c o a t e d SAW s e n s o r was r e v e r s i b l e w i t h a l l vapors except t h e c y c l o p e n t a d i e n e , which gave a l a r g e n o n - r e v e r s i b l e response due t o a chemical r e a c t i o n with t h e f i l m , forming t h e D i e l s - A l d e r adduct. The behavior of t h e two c o a t i n g s , FPOL and PEM, demonstrate the trade-off between vapor s e l e c t i v i t y and SAW sensor r e v e r s i b i l i t y . In general SAW sensors that respond r e v e r s i b l y w i l l have coatings that i n t e r a c t p h y s i c a l l y with the vapors (e.g., t h e p h y s i c a l f o r c e s of a t t r a c t i o n a s s o c i a t e d with s o l u b i l i t y ) . SAW sensors that a r e h i g h l y s e l e c t i v e , on the other hand, r e l y on coatings that r e a c t chemically with the vapor, and thus are not r e v e r s i b l e . This shows the p o t e n t i a l f o r developing coatings f o r a range of a p p l i c a t i o n s , e.g., coatings that a r e r e v e r s i b l e and have s e l e c t i v i t y r a t i o s up t o 1,000 t o 1 o r so f o r s p e c i f i c vapors, as w e l l as coatings that are i r r e v e r s i b l e and have s e l e c t i v i t i e s i n excess of 10,000 t o 1. A l e v e l of s e l e c t i v i t y of 1,000 t o 1 w i l l be u s e f u l f o r many a p p l i c a t i o n s , e s p e c i a l l y i n processes where t h e r e a r e a l i m i t e d number of known vapors and t h e vapor t o be monitored i s present i n equal o r higher c o n c e n t r a t i o n than p o t e n t i a l i n t e r f e r e n t s . However, there w i l l be many other a p p l i c a t i o n s i n which much h i g h e r s e l e c t i v i t y i s r e q u i r e d as w e l l as r e v e r s i b i l i t y . There a r e s e v e r a l techniques that can be used t o enhance s e l e c t i v i t y of SAW sensors, besides changing the coating chemistry.




Figure 5. Vapor response of290 MHz SAW device.


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o f SAW

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Sensors

SAW Sensor A r r a y s . I t can be i n f e r r e d from the above d i s c u s s i o n t h a t h i g h s e l e c t i v i t y f o r many a n a l y t e s i s d i f f i c u l t to achieve without the c o s t l y and time consuming development of c o a t i n g s s p e c i f i c f o r those a n a l y t e s . In a d d i t i o n , many a p p l i c a t i o n s demand that a number of vapors be detected or monitored simultaneously. SAW sensor arrays have t h e r e f o r e been developed t o meet the more complex vapor m o n i t o r i n g a p p l i c a t i o n s . In the s e n s o r a r r a y approach a number of s e n s o r s , each w i t h a d i f f e r e n t chemically s e l e c t i v e coating, are exposed simultaneously t o the a n a l y t e vapors. Because of t h e i r d i f f e r e n t c o a t i n g chemistries, each sensor w i l l g i v e a d i f f e r e n t response t o the vapors p r e s e n t . The r e s u l t i n g p a t t e r n of responses from the a r r a y of sensors can then be a n a l y z e d u s i n g a v a r i e t y of p a t t e r n r e c o g n i t i o n a l g o r i t h m s . P a t t e r n s of known vapor mixtures can normalized to eliminate the e f f e c t of vapor c o n c e n t r a t i o n and can be s t o r e d i n a computer l i b r a r y f o r use i n a n a l y z i n g vapors a s s o c i a t e d w i t h an i n d u s t r i a l process. A key advantage of a sensor a r r a y i s that i t can u s u a l l y detect and monitor a number of vapors f a r g r e a t e r than the number of sensors i n the a r r a y . In a d d i t i o n , s i n c e the a r r a y responds t o a l a r g e number of vapors, the i d e n t i f i c a t i o n of new vapors can o f t e n be accomplished with the p a t t e r n r e c o g n i t i o n software. A SAW a r r a y sensor system i s by n e c e s s i t y more complex than a s i n g l e sensor system. I t w i l l not o n l y r e q u i r e a d d i t i o n a l sensors and e l e c t r o n i c s , but i n order t o expose a l l sensors simultaneously t o the same vapor composition, and t o p e r i o d i c a l l y e s t a b l i s h a c l e a n a i r b a s e l i n e , a number of a d d i t i o n a l system components w i l l be r e q u i r e d . These w i l l i n c l u d e one or two onboard pumps, a charcoal or mole s i e v e t r a p , v a l v e s , and a microcomputer f o r data a c q u i s i t i o n and system operation. An example of the response patterns obtained from a 4 SAW sensor a r r a y are shown i n Figure 6. Four 158 MHz SAW sensors were coated w i t h f i l m s of p o l y ( e t h y l e n e i m i n e ) , f l u o r o p o l y o l , e t h y l c e l l u l o s e , and TENAX GC. The sensor array was then exposed to a v a r i e t y of vapors. Each sensor responded to the several vapors to a d i f f e r e n t extent. The magnitude of the s i g n a l o b t a i n e d from each sensor ( i . e . , channel A, B, C, and D) i s p l o t t e d i n histogram form. The r e s u l t i n g response p a t t e r n s a r e s i g n i f i c a n t l y d i f f e r e n t from each other, thereby p e r m i t t i n g easy " f i n g e r p r i n t i n g " of the vapor a n a l y t e s . As expected, c h e m i c a l l y s i m i l a r compounds produce patterns that are s i m i l a r but most o f t e n d i f f e r e n t enough t o permit i d e n t i f i c a t i o n by the computer (e.g. methanol and p r o p a n o l ) . The a r r a y w i l l respond t o m i x t u r e s of vapors and can " f i n g e r p r i n t " them, but determining the i n d i v i d u a l components of complex mixtures q u a n t i t a t i v e l y i s beyond the scope of the present state-ofthe-art SAW microsensor technology.




METHANOL

2-PR0PAN0L

CHLOROFORM

DICHLOROETHANE

F R 256 "51%-

Ε Q S 128 H I F

τ (KHz)

o

24%B~~ 12%

•

fl~H

13%

I I I

H—I—I Α Β Ο

Κ

D

NAIL POLISH REMOVER

(KHz)

Figure 6. Four SAW array patterns for typical chemical vapors.


9. WOHLTJEN ET AL.


97


In another experiment, 4 SAW sensors w i t h d i f f e r e n t surface coatings were exposed s e q u e n t i a l l y to room a i r and a s e r i e s of smokes, i n c l u d i n g smoke from c o t t o n , c i g a r e t t e s , PVC, and motor o i l . The sensor a r r a y gave d i s t i n c t l y d i f f e r e n t : response p a t t e r n s f o r each smoke. Thus SAW sensor a r r a y s have the p o t e n t i a l t o i d e n t i f y the source of a smoke as well as detect i t , provided i t i s from a s i n g l e source, which i s v e r y o f t e n the case i n the i n i t i a l stage of a f i r e . The development of s o p h i s t i c a t e d p a t t e r n r e c o g n i t i o n algorithms to analyze simple mixtures i s c u r r e n t l y an area of a c t i v e research (12,13). SAW/GC Sensor Systems. Another approach t o a c h i e v i n g enhanced s e l e c t i v i t y f o r many analytes i s t o i n t e r f a c e the SAW sensor with a simple GC column and s e l e c t the s p e c i f i c components t o be monitored based on column r e t e n t i o n time. A commercially a v a i l a b l e instrument, the MSI-301 Organic Vapor Monitor, has been developed based on t h i s p r i n c i p l e . A i r samples t o be analyzed can be drawn i n t o a sample loop with a small pump, or by s y r i n g e i n j e c t i o n . The trapped vapor sample i s then i n j e c t e d i n t o the GC column. Sample i n j e c t i o n , o p e r a t i o n of the GC column, and data a n a l y s i s are c o n t r o l l e d by an on-board microcomputer. Once the chromatogram i s completed, the microcomputer determines r e t e n t i o n times and b a s e l i n e c o r r e c t e d h e i g h t s f o r a l l peaks. Peak heights can be converted to concentrations by refernce to c a l i b r a t i o n tables stored i n memory. Instruments such as the MSI-301 are i d e a l l y s u i t e d f o r c o n t i n u o u s , unattended m o n i t o r i n g of the workplace, i n d u s t r i a l and environmental s i t e s f o r the presence of organic vapors. They a r e a l s o w e l l s u i t e d f o r the continuous monitoring of i n d u s t r i a l chemical processes f o r q u a l i t y c o n t r o l and f o r r e d u c i n g and/or d e t e c t i n g and monitoring the a c c i d e n t a l r e l e a s e of hazardous organic vapors i n t o the environment. SAW

Sensors

f o r Dosimeter

Applications

There are a p p l i c a t i o n s i n i n d u s t r y where i t i s necessary to determine the cumulative c o n c e n t r a t i o n of a given chemical o c c u r i n g (or a c c i d e n t l y released) at a s p e c i f i c s i t e . As i n d i c a t e d above, the primary c h a l l e n g e f o r f o r such an a p p l i c a t i o n would be the development of c o a t i n g s f o r the SAW d e v i c e s t h a t are s e n s i t i v e and i r r e v e r s i b l e t o the s p e c i f i c t o x i c or hazardous vapors and w i l l g i v e an accurate, cumulative, r e a l - t i m e measurement of the vapor being monitored. I t i s a l s o a c h a l l e n g e t o develop SAW c o a t i n g s t h a t w i l l respond t o i n o r g a n i c or f i x e d t o x i c gases, s i m i l a r t o the rersponse of PEM coatings to c y l c o p e n t a d i e n e . In a r e c e n t study i r r e v e r s i b l e SAW c o a t i n g s were proposed f o r m o n i t o r i n g a number of such gases i n c l u d i n g NH and HC1 (14). The coatings evaluated f o r NH and HC1 were C o C l , and polyvinylpyridine, respectively. R e s u l t s of the exposure of c o a t e d SAW sensors to these gases are given i n the F i g . 7 (a) and (b). 3

3

2


98


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>i U

A

« Φ

, Ν -

"ι

Φ

U h

Κ

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140000

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• —

.

130000

200

400

600 Time

800

1000

1200

1400

(seconds)

230000

>i ϋ α

229000 "

Φ

%

228000 "

Φ

u ι*

227000 CQ

226000 200

400 Time

600

800

1000

(seconds)

Figure 7. Frequency shift vs. time for exposure of CoCl coated SAW devices to 20 ppm NH for 20 sec (a) and PVP coated SAW devices to 20 ppm HClfor 20 sec (b). 2


3

9. WOHLTJEN ET AL.


99


The p l o t s of SAW frequency, At, with time i n each case show a stepwise i n c r e a s e i n At w i t h each exposure of the c o a t i n g t o 2 0 ppm of the s p e c i f i c vapor f o r 2 0 seconds. The s e n s i t i v i t y of each c o a t i n g was b e t t e r than 1 Hz/ppm/sec, s u f f i c i e n t i n every case t o detect and monitor these gases a t c o n c e n t r a t i o n s w e l l below t h e i r OSHA t o x i c exposure l i m i t s . Thus SAW sensor do indeed have p o t e n t i a l as dosimeters f o r a v a r i e t y of t o x i c gases i n the environs of an i n d u s t r i a l o p e r a t i o n . The SAW sensors and much of the support e l e c t r o n i c s and hardware are r e a d i l y a v a i l a b l e to develop t h i s technology. SAW Sensors Processes

f o r Vapor

Monitoring

of

Industrial

S o l i d - s t a t e SAW chemical microsensors have many p h y s i c a l and performance c h a r a c t e r i s t i c s that make them i d e a l as i n s i t u sensors f o r t h e m o n i t o r i n g and c o n t r o l of many i n d u s t r i a l processes. Such sensors c o u l d be used t o optimize chemical processes as w e l l as c o n t r o l the r e l e a s e of hazardous chemicals t o the environment. They a r e very s m a l l , rugged, and i n e x p e n s i v e d e v i c e s , and can be s e n s i t i v e t o ppm (or lower) l e v e l s of many organic vapors of i n d u s t r i a l importance, such as benzene, ethylene oxide, halogenated hydrocarbons, etc. In a d d i t i o n , i n d i v i d u a l SAW sensors can be made h i g h l y s e l e c t i v e f o r many compounds o r c l a s s e s of compounds by the s e l e c t i o n o r s y n t h e s i s of a p p r o p r i a t e c o a t i n g m a t e r i a l s f o r the SAW sensors. The present SAW sensors can operate a t temperatures t o 50°C o r so, and they p r o v i d e an e l e c t r o n i c s i g n a l t h a t can be r e a d i l y i n t e g r a t e d i n t o computer networks f o r r e a l time monitoring a t m u l t i p l e process s i t e s and data f o r process control. S i n g l e SAW sensors can be v e r y u s e f u l f o r monitoring the c o n c e n t r a t i o n of a s i n g l e chemical vapor, o r c l a s s of vapors, i n an environment where there are few i n t e r f e r e n t s , however i n i n d u s t r i a l processes t h e r e a r e l i k e l y t o be c o n s i d e r a b l y more complex chemical environments. The use of m u l t i p l e sensor a r r a y s would be a t r u l y novel approach to the r e a l time monitoring of complex i n d u s t r i a l chemical processes, i n c l u d i n g f e e d stocks and p o t e n t i a l p o l l u t i o n sources. An a r r a y of sensors, each with a d i f f e r e n t vapor s e n s i t i v e c o a t i n g , would p r o v i d e chemical " f i n g e r p r i n t " p a t t e r n s that c o u l d be used as t h e b a s i s f o r i d e n t i f y i n g the chemical vapors i n a mixture as w e l l as t h e i r r e l a t i v e concentrations. Thus f o r a given a p p l i c a t i o n a SAW sensor array could be "trained" using a simple p a t t e r n r e c o g n i t i o n algorithm and a computer, t o i d e n t i f y and monitor s p e c i f i c vapor combinations. A SAW a r r a y c o u l d determine when the combination of chemicals i n an i n d u s t r i a l p r o c e s s a r e w i t h i n p r e - s e t l i m i t s and p r o v i d e a s i g n a l whenever those l i m i t s a r e exceeded. S i g n a l s c o u l d a l s o be p r o v i d e d t o a p r o c e s s c o n t r o l computer network so t h a t necessary concentration adjustments could be made t o keep the process chemicals w i t h i n the desired concentration l i m i t s .



100


A s i g n i f i c a n t advantage of an a r r a y system i s that i t can i n f a c t e a s i l y i d e n t i f y a number of chemical vapors i n excess of the number of sensors i n the a r r a y . Thus a SAW a r r a y c o u l d d e t e c t t h e appearance of an unknown o r undesirable chemical i n an i n d u s t r i a l process, even though i t may not be able to s p e c i f i c a l l y i d e n t i f y i t . From Figure 8 i t i s apparent that SAW sensors and SAW Sensor Arrays can be used t o monitor chemical vapors i n various steps and l o c a t i o n s i n an i n d u s t r i a l process. They can monitor chemical vapors a s s o c i a t e d with feed stocks, or reactants, being introduced i n t o an i n d u s t r i a l process, as w e l l as monitor the process i t s e l f . SAW sensors c o u l d a l s o monitor the products of a process f o r q u a l i t y c o n t r o l and f o r the presence of p o t e n t i a l environmental p o l l u t a n t s . SAW sensors systems could r e a d i l y be engineered t o monitor s t a c k gases as w e l l , p r o v i d e d gas samples can be s u f f i c i e n t l y reduced i n temperature. SAW instrumentation i s c u r r e n t l y a v a i l a b l e f o r monitoring the e f f i c i e n c y (and/or breakthrough) of a i r p u r i f i c a t i o n ( f i l t e r ) systems used t o remove hazardous m a t e r i a l s from a i r surrounding i n d u s t r i a l processes. F i n a l l y , there are now commercially a v a i l a b l e SAW based c h e m i c a l vapor monitors designed e s p e c i a l l y f o r t h e c o n t i n u o u s , l o n g term perimeter monitoring of i n d u s t r i a l s i t e s . The output s i g n a l s from SAW sensors and sensor arrays used t o monitor any o r a l l a s p e c t s of an i n d u s t r i a l process, can be r e a d i l y i n t e g r a t e d i n t o a computer network t o p r o v i d e r e a l time i n f o r m a t i o n on the performance o r s t a t u s of the o p e r a t i o n . Such i n f o r m a t i o n can p r o v i d e information t o plant operators f o r the e f f e c t i v e automation of i n d u s t i r a l process and a t the same time assure t h a t p o l l u t i o n i s being prevented at the source. SAW sensors and SAW based vapor monitors, as described above, a r e now c o m m e r c i a l l y a v a i l a b l e and a r e b e i n g e v a l u a t e d f o r a number of i n d u s t r i a l applications; however, the d e t a i l s of t h e i r a p p l i c a t i o n and performance tend t o be c l o s e l y h e l d p r o p r i e t a r y i n f o r m a t i o n by the companies i n v o l v e d . Because t h e technology i s so new, t h e r e a r e s t i l l some t e c h n i c a l l i m i t a t i o n s t h a t must be addressed. F i r s t , the use of SAW sensors as chemical vapor monitors i s dependent upon the a v a i l a b i l i t y of c h e m i c a l l y s p e c i f i c coatings f o r the intended a p p l i c a t i o n . As new and more e f f e c t i v e coatings become a v a i l a b l e through research and development, SAW d e v i c e s w i l l f i n d a wider range of a p p l i c a t i o n s . Second, SAW sensors are p r e s e n t l y l i m i t e d t o monitoring a p p l i c a t i o n s a t temperatures below about 50°C. At higher temperatures the present organic based coatings lose s e n s i t i v i t y and, depending upon t h e i r composition, can become c h e m i c a l l y u n s t a b l e . I n a d d i t i o n , new s u r f a c e a c o u s t i c wave sensor and support e l e c t r o n i c designs may be required f o r higher temperature use.



tficrosensor Perimeter tlonitor

tficrosensor f o r / Reactant C o n t r o l

Keactants

Coapnter f o r Automated P r o c e s s Control

tficrosensor f o r Process O p t i a i z a t r i o n

INDUSTRIAL PROCESS \

FILTER

Clean Air

Hicrosensor Periaeter tlonitor

Bicrosensor f o r Quality Control

-> P r o d u c t s

tiicrosensors f o r Filter Efficiency

Figure 8. Vapor monitoring of industrial processes.

tficrosensor f o r Environmental — Hondtoring

Stack Gases


102


Literature Cited 1. 2. 3. 4.


5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

15. 16.

Wohltjen, H . ; Dessy, R. E. Anal. Chem. 1979, 5 (9), pp.1458-1475 Bryant, Α . ; Lee, D. L.; Vetelino, J . F. Proc. IEEE Ultrason. Symp. 1981, pp. 171-174 Chung, C. T . ; White, R. M . ; Bernstein, J . J. 1982, Electron. Dev. L e t t . , EDL-3 (6), pp. 145-148 D'Amico, Α . ; Palma, Α . ; Verona, Ε. Sensors and Actuators 1984, pp. 307-325 Wohltjen, H . ; Sensors and Actuators 1984, pp. 307-324 Snow A.; Wohltjen H. Anal. Chem., 1984, 56 (8), pp. 1411-1416 Martin, S. J.; Schweizer, K. S.; Schwartz, S. S.; Gunshor, R. L. Proc. IEEE Ultrason. Symp., 1984 Barendsz, A. W.; et al.Proc IEEE Ultrason. Symp., 1984. Wohltjen, H . ; Snow, A. W.; Barger, W. R.; Ballantine, D. S.IEEE Trans. on Ultrasonics, Ferroelectrics, and Frequency Control 1987, UFFC-34 (2), pp. 172-178 Wohltjen, H . ; Ballantine, D. S. Jr.; Jarvis, N. L . ACS Symposium Series No. 403 1989, pp. 157-175 Ballantine, D. L . Jr.; Wohltjen, H. Anal. Chem. 1989, 61 (11) pp.704-713. Ballantine, D. S. Jr.; Rose, S. L.; Grate, J . W.; Wohltjen, H. Anal. Chem. 1986, 58, pp. 3058-3066 Rose-Pehrsson, S. L.; Grate, J . W.; Ballantine, D. S. Jr.; Jurs, P. C. Anal. Chem. 1988, 60, pp. 2801-2811. Wohltjen, H . ; Jarvis, N. L.; Lint, J . R. Proceedings of the Second International Symposium on "Field Screening Methods for Hazardous Wastes and Toxic Chemicals", Las Vegas, Nevada, Feb 12 -14, 1991, pp. 73-84 Bowers, W. D.; Chuan, R. L.; Duong, T. M. Rev. Sci. Instrum.1991, 62, pp.1624-1629 Grate, J . W.; Klusty, M . ; Naval Research Laboratory Memorandum Report 6829, 1991

RECEIVED May 27, 1992


Surface Acoustic Wave Chemical Microsensors and Sensor Arrays for

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