Chemical Sensors and Microinstrumentation - ACS Publications

Chapter 13

Liquid-Phase Sensors Based on Acoustic Plate Mode Devices 1

A. J. Ricco, S. J. Martin, T. M. Niemczyk , and G. C. Frye

Downloaded by CORNELL UNIV on July 28, 2016 | http://pubs.acs.org Publication Date: August 24, 1989 | doi: 10.1021/bk-1989-0403.ch013

Sandia National Laboratories, Albuquerque, NM 87185

The response of piezoelectric devices propagating shear horizontal acoustic plate modes (SH-APMs) has been modeled and experimentally characterized for variations in surface mass, liquid rheological properties, and solution dielectric coefficient and electrical conductivity. The nature of the SH-APM and its propagation characteristics are outlined and used to describe a range of interactions at the solid/liquid interface. Sensitivity to sub-monolayer mass changes is demonstrated and a Cu sensor is described. The APM device is compared to the surface acoustic wave device and the quartz crystal microbalance for liquid sensing applications. 2+

Chemical sensors based on acoustic wave (AW) devices have been studied for a number of sensing applications, the majority of which fall in the category of gas and vapor detection (1-8). Recently, the use of these sensors in liquid environments has been explored (9-13). AW sensors utilize various types of acoustic waves, including the surface acoustic wave (SAW), the shear-horizontal acoustic plate mode (SH-APM) (10-13), and the Lamb wave (also a plate mode) (3.14). Even though most studies of these piezoelectric sensors have centered on SAW devices (1.2.4-8), differences in the propagation characteristics of the various acoustic modes make some better suited than others for a given sensing application. An interdigital transducer on the surface of a piezoelectric material can excite and detect waves which propagate along the surface (e.g. the SAW) or through the bulk (e.g. the Lamb wave and the SH-APM) of the substrate. AW sensors typically include an input transducer to generate the wave, an interaction region in which the propagating wave is affected by its environment, and an output transducer to detect the wave. Thus, unlike the quartz crystal microbal3

Current address: Department of Chemistry, University of New Mexico, Albuquerque, NM 87131 0097-6156^/0403-0191$06.00y0 © 1989 American Chemical Society

Murray et al.; Chemical Sensors and Microinstrumentation ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


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CHEMICAL SENSORS AND

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ance (QCM) (15-20), AW sensors are two-port devices i n which the mode propagates many wavelengths between input and output transducers. This d i s t i n c t i o n , combined with higher operating frequencies, make AW devices s i g n i f i c a n t l y more s e n s i t i v e than the QCM to changes i n surface mass. I n addition, those AW devices which u t i l i z e bulk modes have the considerable advantage that a l l e l e c t r i c a l connections can be made to the face of the c r y s t a l not immersed i n solution. In this chapter, we describe the SH-APM and i t s propagation c h a r a c t e r i s t i c s , pointing out the advantages of this device f o r l i q u i d sensing. Experimental characterizations of the APM's response to surface mass changes and to the viscous and acoustoelectric coup l i n g which occurs between the device and the contacting s o l u t i o n are presented as w e l l . The d e t a i l e d mathematical derivations necessary to f u l l y describe viscous coupling and mass s e n s i t i v i t y , which are beyond the scope of t h i s chapter, w i l l be published elsewhere (Martin, S. J . ; Ricco, A. J . ; Niemczyk, T. M.; Frye, G. C., Sensors & Actuators, submitted for p u b l i c a t i o n ) . An important a p p l i c a t i o n of the APM device i n the area of chemical sensing i s the i n d i c a t i o n of the presence, and i n optimal cases the concentration, of s p e c i f i c dissolved species. We have reported the use of the APM device to monitor electrodeposition (21), e l e c t r o l e s s deposition ( 1 3 ) , and the corrosion of metal films (21) · In these applications, the device responds n o n - s p e c i f i c a l l y to the presence of mass accumulated on (or removed from) the surface. Although such applications c l e a r l y demonstrate the use of the APM device to monitor surface mass changes while i n d i r e c t contact with solution, they are not examples of s p e c i f i c chemical sensors. Through suitable chemical modification of the surface, the APM device can be s e n s i t i z e d to the presence of species i n solution. The general approach we are taking i s to d e r i v a t i z e the quartz surface of the APM device with molecules well known as ligands f o r metal ions i n solution. The ligands are covalently attached to the surface via siloxane bonds using a reagent which has the ligand of interest attached to the a l k y l group of a t r i a l k o x y s i l y l a l k y l moiety (23-25). Ethylenediamine (en)» a bidentate ligand which forms complexes with many of the t r a n s i t i o n metals (26) including copper ( 2 7 ) , has format i o n constants such that two ens w i l l bind each Cu * ion ( 2 8 ) . In addition, the nitrogens which bind the metal are s u f f i c i e n t l y basic that addition of a c i d protonates them, freeing the Gu and allowing a check on the r e v e r s i b i l i t y of the reaction. Thus, the a p p l i c a t i o n of the APM to chemical sensing i s demonstrated i n the form of a sensor for aqueous Cu . 2

2+

2+

Propagation and Interactions of Acoustic Plate Modes Fundamentals. In an AW device, an a l t e r n a t i n g voltage applied to an i n t e r d i g i t a l transducer on a p i e z o e l e c t r i c substrate generates an a l ternating s t r a i n f i e l d , which i n turn launches an acoustic wave (2.9) * Depending on the frequency of the a l t e r n a t i n g voltage, the physical properties of the substrate, and the o r i e n t a t i o n of the transducers on the substrate, the AW energy may be confined l a r g e l y to within one wavelength of the surface, as i n the case of the SAW, or i t may propagate through the bulk of the substrate, as i n the case of the APM. Regardless of the path i t follows, an acoustic mode having s i g n i f i cant amplitude at the substrate surface can have i t s propagation



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Sensors Using Acoustic Plaie Modes

c h a r a c t e r i s t i c s a l t e r e d by changes i n t h e n a t u r e and/or q u a n t i t y o f a m a t e r i a l i n i n t i m a t e c o n t a c t w i t h t h e d e v i c e s u r f a c e . The cumula t i v e e f f e c t s o f such i n t e r a c t i o n s o v e r t h e wave p r o p a g a t i o n p a t h a r e changes i n AW a m p l i t u d e and phase d e l a y a t t h e o u t p u t t r a n s d u c e r . The SH-APM s e n s o r s d i s c u s s e d i n t h i s c h a p t e r u t i l i z e t h i n , crys t a l l i n e , ST-cut q u a r t z p l a t e s w h i c h a c t as a c o u s t i c waveguides, con f i n i n g wave energy between t h e upper and lower s u r f a c e s o f t h e p l a t e as t h e wave p r o p a g a t e s between i n p u t and o u t p u t t r a n s d u c e r s . Though n o t s t r i c t l y v a l i d f o r a s i n g l e c r y s t a l , treatment o f t h e s u b s t r a t e as an i s o t r o p i c medium g r e a t l y s i m p l i f i e s c a l c u l a t i o n s and l e a d s t o r e a s o n a b l e p r e d i c t i o n s o f wave p r o p a g a t i o n c h a r a c t e r i s t i c s and p e r t u r b a t i o n s thereon. A s c h e m a t i c r e p r e s e n t a t i o n o f an a c o u s t i c wave d e v i c e w i t h a p r o p a g a t i n g SH-APM i s shown i n F i g u r e l a . F o r s i m p l i c i t y , o n l y one p a i r o f i n t e r d i g i t a t e d f i n g e r s i s shown f o r each t r a n s d u c e r ; a c t u a l t r a n s d u c e r s t y p i c a l l y c o n s i s t o f 25-75 such p a i r s . Note t h a t t h e w a v e l e n g t h (λ) o f t h e APM as i t p r o p a g a t e s between i n p u t and o u t p u t t r a n s d u c e r s i s t w i c e t h e c e n t e r - t o - c e n t e r s p a c i n g ( l a b e l e d d/2) o f t h e t r a n s d u c e r f i n g e r s . Together w i t h t h e propaga t i o n v e l o c i t y ( v ) o f t h e APM, t h e o v e r a l l t r a n s d u c e r p e r i o d i c i t y (d) d e t e r m i n e s t h e f r e q u e n c y ( f ) a t w h i c h a g i v e n mode p r o p a g a t e s : f„ - v /d. An SH p l a t e mode may be thought o f as a s u p e r p o s i t i o n o f SH p l a n e waves m u l t i p l y r e f l e c t e d a t some a n g l e between t h e upper and lower f a c e s o f t h e q u a r t z p l a t e . The s u b s t r a t e f a c e s impose a trans v e r s e resonance c o n d i t i o n w h i c h r e s u l t s i n each APM h a v i n g d i s p l a c e ment maxima a t t h e s u r f a c e s . F i g u r e l b shows a c r o s s s e c t i o n a l v i e w (normal t o t h e p r o p a g a t i o n d i r e c t i o n o f F i g u r e l a ) o f t h e p a r t i c l e d i s p l a c e m e n t f o r t h e f o u r l o w e s t - o r d e r SH modes, w i t h mode i n d i c e s η - 0 t h r o u g h 3 r e p r e s e n t i n g t h e number o f d i s p l a c e m e n t nodes between substrate faces. As i l l u s t r a t e d by F i g u r e l b , each mode has e q u a l d i s p l a c e m e n t on b o t h s u r f a c e s o f t h e APM d e v i c e , a l l o w i n g t h e use o f e i t h e r s u r f a c e f o r l i q u i d s e n s i n g . Each o f these a c o u s t i c modes has a s l i g h t l y d i f f e r e n t u n p e r t u r b e d v e l o c i t y ; t h e f r e q u e n c y o f most e f f i c i e n t c o u p l i n g between t r a n s d u c e r and s u b s t r a t e f o r t h e n mode may be w r i t t e n : n

n

n

t h

(1) where b i s t h e s u b s t r a t e t h i c k n e s s . Thus, t h i n n i n g t h e s u b s t r a t e i n c r e a s e s t h e f r e q u e n c y s p a c i n g between modes. Because t h e e f f e c t o f e x t e r n a l p e r t u r b a t i o n s ( i . e . causes o f s e n s o r r e s p o n s e ) on t h e o s c i l l a t i o n f r e q u e n c y o f a g i v e n mode may depend on t h e d i s p l a c e m e n t d i s t r i b u t i o n f o r t h a t mode, i n t e r p r e t a t i o n o f APM sensor r e s u l t s i s s i m p l i f i e d by e x c i t i n g o n l y a s i n g l e mode. T h i s i s a c c o m p l i s h e d by u s i n g a t r a n s d u c e r w i t h a b a n d w i d t h w h i c h i s l e s s t h a n t h e frequency s e p a r a t i o n between modes. The t r a n s d u c e r b a n d w i d t h d e c r e a s e s as t h e number o f f i n g e r p a i r s , N, i n c r e a s e s , w h i l e t h e f r e q u e n c y s e p a r a t i o n between modes i n c r e a s e s i n p r o p o r t i o n t o ( d / b ) . Thus, i n c r e a s i n g t h e v a l u e o f t h e f a c t o r 2

R - N(d/b)

2

(2)

enhances separation between adjacent modes. Mass S e n s i t i v i t y . Extreme s e n s i t i v i t y t o changes i n s u r f a c e mass s e t AW d e v i c e s a p a r t from a l l o t h e r types o f s e n s o r s . When mass i s bound



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CHEMICAL SENSORS AND MICROINSTRUMENTATION

CROSS-SECTIONAL DISPLACEMENT

Figure I. a ) Schematic o f a n APM d e v i c e showing t h e shear h o r i z o n t a l d i s p l a c e m e n t o f t h e mode as i t p r o p a g a t e s between i n p u t and o u t p u t t r a n s d u c e r s . Each t r a n s d u c e r c o n s i s t s o f 50 75 i n t e r d i g i t a t e d e l e c t r o d e p a i r s , one o f w h i c h i s shown. b) C r o s s - s e c t i o n a l displacement p r o f i l e s f o r the four lowest order s h e a r h o r i z o n t a l (SH) p l a t e modes, η - 0 through 3. The η - 1 mode i s d e p i c t e d i n t h e d e v i c e o f p a r t a ) .


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Setisors Using Acoustic Plate Modes

s t r o n g l y t o t h e s u r f a c e o f an APM d e v i c e , i t o s c i l l a t e s synchronously w i t h t h e q u a r t z s u r f a c e under t h e i n f l u e n c e o f t h e p a s s i n g p l a t e mode. The e x t e n t t o w h i c h m e c h a n i c a l s u r f a c e p e r t u r b a t i o n s i n f l u e n c e APM p r o p a g a t i o n i s p r o p o r t i o n a l t o t h e r a t i o o f s u r f a c e p a r t i c l e v e l o c i t y s q u a r e d t o a c o u s t i c power f l o w t h r o u g h t h e s u b s t r a t e . This makes t h e η - 0 mode, w h i c h has s m a l l e r s u r f a c e d i s p l a c e m e n t (and hence p a r t i c l e v e l o c i t y ) r e l a t i v e t o t h e h i g h e r o r d e r modes, l e s s s e n s i t i v e t o mass changes. A s i m p l e and h i g h l y a c c u r a t e way t o measure wave v e l o c i t y i s t o u t i l i z e t h e APM d e v i c e as t h e feedback element o f an o s c i l l a t o r l o o p ( v i d e infra). For the n mode, t h e f r e q u e n c y s h i f t A f caused by a change i n s u r f a c e mass/area o f magni tude p i s a p p r o x i m a t e d by: t h

n

s


(3) i n w h i c h c i s t h e frequency s e n s i t i v i t y t o s u r f a c e mass, ρ i s t h e d e n s i t y o f t h e q u a r t z s u b s t r a t e , u • H and i/ « 1 f o r η > 1. Equa t i o n 3 p r e d i c t s t h a t frequency w i l l d e c r e a s e l i n e a r l y w i t h accumulat ed mass d e n s i t y . A l s o , s e n s i t i v i t y i s p r e d i c t e d t o depend i n v e r s e l y on p l a t e t h i c k n e s s . As w i t h Lamb waves ( 3 . 1 4 ) . changes i n e l a s t i c p r o p e r t i e s o f a s u r f a c e l a y e r c a n i n f l u e n c e p l a t e mode o s c i l l a t o r f r e q u e n c y as w e l l , b u t t h i s e f f e c t i s o f t e n n e g l i g i b l e . I t should be p o i n t e d o u t t h a t E q u a t i o n 3 w i l l a c c u r a t e l y r e l a t e frequency s h i f t to mass changes o n l y when no s i z a b l e changes i n v a r i o u s p h y s i c a l proper t i e s o f t h e s o l u t i o n ( d e s c r i b e d below) accompany t h e mass change. f

Q

t

n

L i q u i d E n t r a i n m e n t . Because o f i t s s h e a r - h o r i z o n t a l s u r f a c e p a r t i c l e d i s p l a c e m e n t ( i n t h e p l a n e o f t h e d e v i c e s u r f a c e ) , t h e SH-APM propa g a t e s i n c o n t a c t w i t h l i q u i d s w i t h o u t e x c e s s i v e a t t e n u a t i o n . The i n p l a n e o s c i l l a t i o n o f t h e q u a r t z s u r f a c e c o n t a c t i n g t h e l i q u i d does l e a d , however, t o e n t r a i n m e n t o f a t h i n l a y e r o f l i q u i d near t h e i n terface (11). T h i s v i s c o u s c o u p l i n g o f l i q u i d t o t h e APM has two e f f e c t s : (1) t o a l t e r the p r o p a g a t i o n c h a r a c t e r i s t i c s ( v e l o c i t y and a t t e n u a t i o n ) o f t h e APM, and (2) t o a l t e r t h e t r a n s d u c t i o n e f f i c i e n c y f o r e x c i t a t i o n and d e t e c t i o n o f APMs. By c o n f i n i n g t h e l i q u i d to the r e g i o n between t r a n s d u c e r s , p r o p a g a t i o n e f f e c t s a r e measured w i t h o u t the i n f l u e n c e o f t r a n s d u c t i o n e f f e c t s ; t h i s i s n e c e s s a r y even i f t h e s i d e o f t h e d e v i c e o p p o s i t e t h e t r a n s d u c e r s i s used f o r measurement. C o n t i n u i t y o f p a r t i c l e displacement across the s o l i d / l i q u i d i n t e r f a c e r e q u i r e s t h a t t h e s u r f a c e d i s p l a c e m e n t o f t h e APM g e n e r a t e motion i n the l i q u i d . S o l u t i o n o f the Navier-Stokes equation i n the l i q u i d , s u b j e c t t o t h i s n o n - s l i p boundary c o n d i t i o n a t t h e s o l i d / l i q u i d i n t e r f a c e , i n d i c a t e s t h a t t h e l i q u i d undergoes a shear m o t i o n w h i c h decays r a p i d l y w i t h d i s t a n c e from t h e s u r f a c e ( 1 1 ) . F o r an a n g u l a r f r e q u e n c y o f o s c i l l a t i o n ω , t h e v e l o c i t y f i e l d decay l e n g t h δ o f t h e l i q u i d e n t r a i n e d by t h e p l a t e mode i s a p p r o x i m a t e d by: η

δ « Jin/p

(4)

i n w h i c h p and η a r e l i q u i d d e n s i t y and shear v i s c o s i t y , r e s p e c t i v e ly. Because t h e APM d e v i c e o p e r a t e s a t h i g h f r e q u e n c y , t h e c o u p l e d l i q u i d l a y e r i s v e r y t h i n : about 50 nm i n w a t e r a t 158 MHz. The h i g h f r e q u e n c y o f t h e APM n e c e s s i t a t e s c o n s i d e r a t i o n o f v i s c o e l a s t i c response by t h e l i q u i d . M o d e l i n g s i m p l e l i q u i d s as M a x w e l l i a n f l u i d s w i t h a s i n g l e r e l a x a t i o n t i m e , r , g i v e s good agree ment w i t h e x p e r i m e n t a l d a t a . When t h e M a x w e l l i a n f l u i d i s d r i v e n i n L


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C H E M I C A L SENSORS AND MICROINSTRUMENTATION

o s c i l l a t o r y flow with ω τ « 1, i t responds as a Newtonian ( i d e a l ) f l u i d c h a r a c t e r i z e d by t h e s h e a r v i s c o s i t y η. F o r w r » 1, t h e o s c i l l a t i o n r a t e exceeds t h e r a t e o f m o l e c u l a r m o t i o n i n t h e l i q u i d and energy c e a s e s t o be d i s s i p a t e d i n v i s c o u s f l o w , b e i n g s t o r e d e l a s t i c a l l y i n s t e a d (12). C o n s e q u e n t l y , when d r i v e n a t h i g h frequen c i e s , a M a x w e l l i a n f l u i d behaves as an amorphous s o l i d w i t h s h e a r p r o p e r t i e s c h a r a c t e r i z e d b y a s h e a r modulus μ. The r e l a x a t i o n time a s s o c i a t e d w i t h t h e t r a n s i t i o n from v i s c o u s t o e l a s t i c b e h a v i o r i n a M a x w e l l i a n l i q u i d i s r e l a t e d t o t h e s e parameters by r - η/μ ( 3 0 ) . V i s c o u s c o u p l i n g o f an APM t o an a d j a c e n t l i q u i d r e s u l t s i n both a t t e n u a t i o n (o^) o f t h e modes and a change i n p r o p a g a t i o n v e l o c i t y (Δν ). These c a n be e s t i m a t e d from a p e r t u r b a t i o n a n a l y s i s and a r e g i v e n by: η

n


η

5£E*H R*\—JM

«II and

ËIXL « .

where

7„

2

-

fa

I

a

1

(5a)

| _! _|

- S?*)

T

(5b)

5

+

Here, Re and l i s denote t h e r e a l and i m a g i n a r y p a r t s o f t h e q u a n t i t y i n brackets, respectively, c i s t h e mass s e n s i t i v i t y o f p l a t e mode v e l o c i t y , and β i s t h e APM wavenumber: β - [ ( ω / ν ) - (ηπ/b) ]^. The v e l o c i t y s h i f t a r i s e s from mass l o a d i n g by t h e e n t r a i n e d l i q u i d l a y e r , e x p l a i n i n g t h e dependence o f t h i s e f f e c t on c ; a t t e n u a t i o n a r i s e s from power d i s s i p a t i o n i n t h e l i q u i d . v

2

η

η

η

2

η

vn

A c o u s t o e l e c t r i c Coupling. P r o p a g a t i o n o f an APM t h r o u g h a piezoelec t r i c waveguide g e n e r a t e s a l a y e r o f bound charge a t t h e d e v i c e s u r f a c e , and t h e evanescent e l e c t r i c f i e l d a s s o c i a t e d w i t h t h i s charge e x t e n d s i n t o t h e a d j a c e n t l i q u i d , c o u p l i n g t o i o n s and d i p o l e s i n s o l u t i o n (11). T h i s i n t e r a c t i o n i s analogous t o t h a t o b s e r v e d between p i e z o e l e c t r i c waves and charge c a r r i e r s i n s e m i c o n d u c t o r s ( 3 2 ) , w h i c h has been e x p l o i t e d t o c o n s t r u c t a SAW-based N 0 s e n s o r (1). I o n c o u p l i n g decays e x p o n e n t i a l l y w i t h d i s t a n c e from t h e s o l i d / l i q u i d i n t e r f a c e , extending s e v e r a l microns i n t o the l i q u i d (decay l e n g t h * λ/2π; t h e double l a y e r c a p a c i t a n c e behaves as a s h o r t c i r c u i t a t t h e t y p i c a l APM f r e q u e n c i e s o f > 100 MHz). I o n , d i p o l e , and i n d u c e d d i p o l e m o t i o n r e s u l t i n g from t h i s a c o u s t o e l e c t r i c c o u p l i n g l e a d t o p e r t u r b a t i o n s i n p l a t e mode v e l o c i t y and a t t e n u a t i o n , which a r e r e l a t e d t o the s o l u t i o n c o n d u c t i v i t y σ by (31): 2

.

Κ + «.]

2n . 5! ii*-±_i°] k 2 U. + «J

°1

,

+ c,) +V) Î t y p i c a l l y , η - 1. T h i s resonance c o n d i t i o n r e s t r i c t s the o p e r a t i o n o f the QCM t o lower f r e q u e n c i e s , because t h e r e a r e l i m i t a t i o n s on how t h i n the s u b s t r a t e can be made. The v a r i a t i o n i n e l e c t r i c a l i n p u t impedance w h i c h o c c u r s a t t h i s r e s o n a n t f r e q u e n c y c a n be used to con t r o l an o s c i l l a t o r c i r c u i t ; c i r c u i t r y i s c o m m e r c i a l l y a v a i l a b l e t o i n s t r u m e n t the s i n g l e - p o r t QCM. I n g e n e r a l , however, two-port d e v i c e s such as the SAW and APM a r e more e a s i l y i n s t r u m e n t e d as o s c i l l a t o r s , p a r t i c u l a r l y at high frequencies (34). I n s t r u m e n t a t i o n o f APM Sensors. Two i n s t r u m e n t a t i o n arrangements were u t i l i z e d i n t h i s s t u d y . I n oscillator measurements, the d e v i c e i s u s e d as the f r e q u e n c y c o n t r o l element o f an o s c i l l a t o r c i r c u i t ; p e r t u r b a t i o n s i n o s c i l l a t i o n frequency A f / f are monitored i n r e sponse t o changes i n l i q u i d p r o p e r t i e s o r s u r f a c e p e r t u r b a t i o n s . The APM d e v i c e i s u t i l i z e d as the f r e q u e n c y c o n t r o l element o f an o s c i l l a t o r c i r c u i t by s e r v i n g as an a m p l i f i e r feedback p a t h . The elements o f a s i m p l e o s c i l l a t o r l o o p a r e shown i n F i g u r e 2a. To form a s t a b l e o s c i l l a t o r , a s i g n a l t r a v e r s i n g the c l o s e d l o o p must r e t u r n t o i t s s t a r t i n g p o i n t (1) h a v i n g e q u a l a m p l i t u d e and (2) b e i n g phase s h i f t e d by a m u l t i p l e o f 2ir r a d i a n s ; the l a t t e r r e q u i r e m e n t i s f a c i l i t a t e d by a t u n a b l e phase s h i f t e r . The bandpass f i l t e r p r e v e n t s o s c i l l a t i o n a t o t h e r f r e q u e n c i e s , such as t h a t c h a r a c t e r i s t i c o f the SAW. For o s c i l l a t o r measurements, the t r a n s d u c e r r e g i o n s h o u l d be p e r t u r b e d a l o n g w i t h the i n t e r v e n i n g p r o p a g a t i o n p a t h . When l i q u i d c o n t a c t s the s u r f a c e o f the s u b s t r a t e w i t h o u t t r a n s d u c e r s , c e l l d e s i g n s h o u l d be such t h a t l i q u i d i s p r e s e n t on the r e g i o n o f the c r y s t a l o p p o s i t e the t r a n s d u c e r s . As the o s c i l l a t i o n f r e q u e n c y changes, the transduc e r s w i l l t h e n " s e l f - t u n e " t o the c h a n g i n g f r e q u e n c y , p r e s e r v i n g the l i n e a r i t y o f response t o v a r i o u s p e r t u r b a t i o n s . n

n


197


Figure 2. a) I n s t r u m e n t a t i o n f o r t h e APM o s c i l l a t o r c o n f i g u r a t i o n (M - impedance m a t c h i n g n e t w o r k ) . b) I n s t r u m e n t a t i o n u s e d t o measure changes i n a m p l i t u d e and phase between t r a n s d u c e r s f o r an i n p u t s i g n a l o f f i x e d f r e q u e n c y and a m p l i t u d e (T — 50 Ο t e r m i n a t i o n ) . c ) E x p l o d e d v i e w o f APM d e v i c e mounted i n a m e t a l f l a t pack and f i t t e d w i t h a t e f l o n f l o w - t h r o u g h c e l l t o c o n t a i n l i q u i d i n c o n t a c t w i t h t h e s i d e o f the d e v i c e o p p o s i t e t h e t r a n s d u c e r s ,


53 ^ η 2| ζ Q 25


13. RICCO ET AL.

199

Sensors Using Acoustic Plate Modes

I n propagation measurements, a s i g n a l o f f i x e d f r e q u e n c y and a m p l i t u d e i s i n p u t t o t h e d e v i c e u s i n g an e x t e r n a l o s c i l l a t o r , w h i l e a v e c t o r v o l t m e t e r m o n i t o r s changes i n APM a m p l i t u d e and phase be tween t r a n s d u c e r s , as i l l u s t r a t e d i n F i g u r e 2b. These measurements a l l o w d e t e r m i n a t i o n o f t h e p e r t u r b a t i o n i n APM phase v e l o c i t y and attenuation. A l t h o u g h l e s s s e n s i t i v e t h a n t h e o s c i l l a t o r measure ment, t h i s t e c h n i q u e i s n o t s u b j e c t t o "mode h o p p i n g " (sudden jumps from one mode t o a n o t h e r ) w h i c h c a n a f f l i c t an APM o s c i l l a t o r i n t h e event o f r a p i d e n v i r o n m e n t a l p e r t u r b a t i o n s , e.g. t h e t r a n s i t i o n from a d r y s u r f a c e t o one c o v e r e d w i t h l i q u i d . D u r i n g p r o p a g a t i o n mea surements , i t i s i m p o r t a n t t o i s o l a t e t h e t r a n s d u c e r s , i n c l u d i n g t h e r e g i o n o p p o s i t e them, from t h e p e r t u r b a t i o n ( M a r t i n , S. J . ; R i c c o , A. J . ; Niemczyk, T. M.; F r y e , G. C., Sensors & Actuators, submitted for p u b l i c a t i o n ) . For a n o n - d i s p e r s i v e AW (e.g. a SAW), f r e q u e n c y and v e l o c i t y p e r t u r b a t i o n s a r e e q u a l i n magnitude. As n o t e d by White ( 1 4 ) , how e v e r , t h e s e n s i t i v i t y o f A f / f and Δ ν / ν t o a p e r t u r b a t i o n a r e n o t n e c e s s a r i l y e q u a l i n a system i n v o l v i n g d i s p e r s i v e modes, such as the APM o r Lamb wave, because t h e phase v e l o c i t y o f each mode depends on frequency. S i n c e SH modes a r e d i s p e r s i v e , t h e f o l l o w i n g r e l a t i o n ship holds: n

n

η

η

Δν

(7) η where v and v a r e t h e phase and group v e l o c i t i e s , r e s p e c t i v e l y , for the n SH-APM. F o r p r o p a g a t i n g SH modes, v < v , so t h a t f r a c t i o n a l f r e q u e n c y p e r t u r b a t i o n s a r e l e s s than f r a c t i o n a l v e l o c i t y p e r t u r b a t i o n s ( F o r Lamb waves, t h e c o n v e r s e h o l d s : v Sv ) . T y p i c a l l y , v / v - 0.6 - 0.7 f o r o u r d e v i c e s . n

g n

t h

g n

n

n

g n

Experimental

g n

n

Methods and M a t e r i a l s

D e v i c e F a b r i c a t i o n and I n s t r u m e n t a t i o n . APM d e v i c e s were d e s i g n e d a t S a n d i a N a t i o n a l L a b o r a t o r i e s and f a b r i c a t e d b y C r y s t a l Technolo g i e s , I n c . , P a l o A l t o , CA and Sawtek, I n c . , O r l a n d o , FL. The ST-cut o f q u a r t z was used, w i t h p r o p a g a t i o n a l o n g t h e x - d i r e c t i o n o f t h e c r y s t a l ; s u b s t r a t e s measured 22.9 χ 7.6 χ 0.5 mm t h i c k b e f o r e lapping. T r a n s d u c e r f i n g e r dimensions s c a l e w i t h p e r i o d i c i t y : f i n g e r w i d t h and s e p a r a t i o n a r e b o t h d/4; f i n g e r l e n g t h i s 50d. Two d i f f e r e n t t r a n s d u c e r p e r i o d i c i t i e s were examined. The f i r s t d e v i c e , d e s i g n e d w i t h d - 32 /xm t o propagate t h e SH-APM a t 158 MHz, has a c e n t e r - t o - c e n t e r s e p a r a t i o n between t r a n s d u c e r s o f 7.36 mm. Each t r a n s d u c e r i s composed o f 50 f i n g e r - p a i r s , p h o t o l i t h o g r a p h i c a l l y d e f i n e d from 200 nm-thick Au-on-Cr m e t a l l i z a t i o n . The second device, d e s i g n e d w i t h d - 50 μπι t o propagate t h e SH-APM a t 104 MHz, has t r a n s d u c e r s c o m p r i s e d o f 75 f i n g e r - p a i r s d e f i n e d from 100 nm t h i c k A l metallization. C e n t e r - t o - c e n t e r s e p a r a t i o n between t r a n s d u c e r s i s 7.28 mm. The u n m e t a l l i z e d s i d e o f a l l d e v i c e s was l a p p e d t o o b t a i n the d e s i r e d p l a t e t h i c k n e s s , t h e n p o l i s h e d so t h a t t h e upper and lower f a c e s were p a r a l l e l and o p t i c a l l y smooth. Each d e v i c e was mounted i n a 25.5 mm χ 12.7 mm g o l d - p l a t e d s t e e l f l a t p a c k ( I s o t r o n i c s ) w i t h a 20.5 mm χ 3.7 mm o p e n i n g , a l l o w i n g l i q u i d t o c o n t a c t t h e u n m e t a l l i z e d s i d e o f t h e d e v i c e as shown i n e x p l o d e d v i e w by F i g u r e 2c. The u n m e t a l l i z e d f a c e o f t h e d e v i c e was bonded ( i n t h e r e g i o n s u r r o u n d i n g t h e a c o u s t i c wave p a t h ) t o t h e



200


o p e n i n g i n t h e f l a t p a c k u s i n g a bead o f RTV e l a s t o m e r ; t h e d e v i c e thus c o n t a c t s l i q u i d o n l y i n the APM p r o p a g a t i o n p a t h . Electrical c o n t a c t was made between t r a n s d u c e r b o n d i n g pads and f l a t p a c k f e e d throughs b y 76-/im d i a m e t e r Au o r A l l e a d s a t t a c h e d w i t h an u l t r a s o n i c bonder (Westbond). The f l a t p a c k was mounted i n a b r a s s t e s t f i x t u r e c o n t a i n i n g impedance m a t c h i n g networks ( I n t e g r a t e d C h e m i c a l S e n s o r s , Inc., Newton, MA). W i r e s were s o l d e r e d t o the f l a t p a c k feed-throughs to make c o n t a c t t o the m a t c h i n g networks i n the body o f the f i x t u r e below. L i q u i d was h e l d i n c o n t a c t w i t h the s e n s i n g s u r f a c e b y a t e f l o n c e l l s e a l e d by compression t o t h e metal f l a t p a c k . An open c e l l , h a v i n g an approximate i n t e r n a l volume o f 1 m l , was used f o r some e x p e r i m e n t s , w i t h l i q u i d s b e i n g added and withdrawn by means o f a p i p e t . The c l o s e d c e l l shown i n F i g u r e 2c was used i n o t h e r e x p e r i ments w i t h the l i q u i d b e i n g drawn through by a p e r i s t a l t i c pump. I n s t r u m e n t a t i o n f o r t h e o s c i l l a t o r measurements, a r r a n g e d as shown i n F i g u r e 2a, i n c l u d e d two cascaded wide-band a m p l i f i e r s (Hew l e t t - P a c k a r d 8447D), a band-pass f i l t e r (K&L Microwave 5BT-95/1905N) , two v a r i a b l e a t t e n u a t o r s i n s e r i e s ( H e w l e t t - P a c k a r d 8494A and 8495A), a t u n a b l e phase s h i f t e r (Merrimac PSL-4-160B o r PSL-4-100B), a 10 dB d i r e c t i o n a l c o u p l e r (Anzac DCG-10-4), a f r e q u e n c y counter ( H e w l e t t - P a c k a r d 5384A), and a computer f o r d a t a a c q u i s i t i o n (Hew l e t t - P a c k a r d 9816). For the p r o p a g a t i o n measurements, a s y n t h e s i z e d o s c i l l a t o r (Hewlett-Packard 8656A) was used a l o n g w i t h a v e c t o r v o l t m e t e r ( H e w l e t t - P a c k a r d 8405A) and t h e HP 9816 computer i n t h e arrangement o f F i g u r e 2b. Chemicals. Surface D e r i v a t i z a t i o n . and M i s c e l l a n e o u s Apparatus. Water was doubly d i s t i l l e d . E l e c t r o l y t e s and s o l v e n t s were commer c i a l l y a v a i l a b l e reagent grade, used as r e c e i v e d . S o l u t i o n conduc t i v i t i e s were measured a t 1 kHz u s i n g a 1 cm p a t h l e n g t h c o n d u c t i v i t y c e l l (YSI Models 32 and 3445). V i s c o s i t y s t a n d a r d s were p r e p a r e d from w a t e r and reagent-grade g l y c e r o l . D e r i v a t i z a t i o n o f APM d e v i c e s u r f a c e s was a c c o m p l i s h e d by immersing t h e d e v i c e ( c l e a n e d by r i n s i n g i n t r i c h l o r o e t h y l e n e , acetone, 2 - p r o p a n o l , and w a t e r , then s o a k i n g i n cone. HN0 ) i n a 2% (v/v) s o l u t i o n o f #-2-aminoethyl-3a m i n o p r o p y l t r i m e t h o x y s i l a n e ( P e t r a r c h Systems, I n c . ; d i s t i l l e d under reduced p r e s s u r e p r i o r t o use) i n r e f l u x i n g d r y t o l u e n e f o r 1 h. The d e v i c e was then r i n s e d w i t h t o l u e n e and annealed a t 150 C f o r 1 h. S i l v e r s h o t ( A l f a , 99.999%) was t h e r m a l l y e v a p o r a t e d from a r e s i s t i v e l y h e a t e d t u n g s t e n b a s k e t a t a r a t e o f 0.6 - 0.8 A/s; base pressure i n t h e cryo-pumped vacuum system was 2 χ 1 0 " T o r r . E v a p o r a t i o n was m o n i t o r e d w i t h an I n f i c o n XTC q u a r t z c r y s t a l m i c r o b a l a n c e o p e r a t i n g a t 6 MHz, 3

8

R e s u l t s and D i s c u s s i o n Mode R e s o l u t i o n . By u s i n g t h r e e d e v i c e s w i t h d i f f e r e n t s u b s t r a t e t h i c k n e s s e s , t r a n s d u c e r p e r i o d i c i t i e s , and numbers o f t r a n s d u c e r f i n g e r p a i r s , a n e s t i m a t e o f the minimum r e q u i r e m e n t s f o r r e s o l u t i o n o f a d j a c e n t a c o u s t i c p l a t e modes was o b t a i n e d . Modes were e s s e n t i a l l y u n r e s o l v a b l e u s i n g Device 1 (158 MHz), w h i c h h a d a 191-μπι t h i c k s u b s t r a t e , 32-μπι t r a n s d u c e r p e r i o d i c i t y , and 50 f i n g e r p a i r s (R 1.4 i n E q u a t i o n 2) . Device 2, a l s o o p e r a t i n g a t 158 MHz, h a d b 152 /im, d - 32 μιη, and Ν - 50 (R - 2.2) , and r e s o l v e d modes w e l l .


13.


MCCOETAL.

201


Mode resolution was best with Device 3 (104 MHz), having b - 203 /im, d - 50 /im, and Ν - 75 (R - 4.5). Mass S e n s i t i v i t y . Calibration. The mass s e n s i t i v i t y of an APM device was c a l i b r a t e d by vacuum deposition of s i l v e r onto the (unmet a l l i z e d ) quartz surface. Device 3 was incorporated i n an o s c i l l a t o r loop (Figure 2a) and the frequency monitored during thermal evapora t i o n of a Ag f i l m onto the substrate. The frequency s h i f t was com pared to the f i l m thickness measured by the commercial QCM. Heating of the device was negligible: after removing power from the evapora t i o n filament, no s i g n i f i c a n t s h i f t i n frequency was observed as the APM device cooled. The APM frequency s h i f t i s p l o t t e d vs. the mass density of deposited s i l v e r i n Figure 3a for modes 0 through 3. As expected (Equation 3), the device i s approximately twice as sensitive when higher order modes are excited as f o r the η - 0 mode. The mass s e n s i t i v i t y measured for Device 3 i s 9.5 cm /g (0.99 Hz-cm /ng) f o r the η - 0 mode and 19,4 cm /g (2.0 Hz-cm /ng) f o r the average of the next three higher-order modes. The corresponding s e n s i t i v i t i e s estimated from Equation 3, 9.3 cm /g and 18.6 cm /g, are i n excel lent agreement. The accuracy with which the mass s e n s i t i v i t y i n vacuo r e f l e c t s that when the device i s i n contact with l i q u i d was investigated with an etching experiment. After evaporating an 80.5 nm-thick s i l v e r f i l m onto Device 3, i t was incorporated i n an o s c i l l a t o r c i r c u i t and an open c e l l on the device surface was f i l l e d with 1.0 ml of water. A f t e r stable o s c i l l a t i o n of the η - 1 mode was achieved, 0.1 ml of a 2:2:1 H 0:H S0 :HN0 et chant was added to the c e l l . The 1470 ppm frequency s h i f t measured as the f i l m dissolved over a 15 min period y i e l d s a mass s e n s i t i v i t y of 17.4 cm /g, approximately 6% less than the value measured i n vacuum. A similar vacuum deposition experiment was carried out using Device 2. In this case, the measured mass s e n s i t i v i t i e s were 31 cm /g (4.9 Hz-cm /ng) for η - 0 and 65 ± 4 cm /g (10 Hz-cm /ng) f o r the average of η - 1, 2, and 3. The s e n s i t i v i t y of t h i s device, which i s within a factor of 2 of that f o r a 97 MHz SAW device (130 cm /g or 13 Hz-cm /ng), i s s i g n i f i c a n t l y larger than the s e n s i t i v i t y calculated from Equation 3. This i s probably because plate modes begin to couple with surface modes, enhancing surface p a r t i c l e velo c i t y and thus mass s e n s i t i v i t y , when b/λ exceeds approximately f i v e 2

2

2

2

2

2

2

4

2

3

2

2

2

2

2

2

2

(11) .

Comparison of APM to Related Devices. The APM device has s i g n i f i c a n t l y higher mass resolution than the QCM, i n terms of frequency s h i f t for a given change i n mass/unit area, as a r e s u l t of the APM's higher operating frequency. The commercial (6 MHz) QCM used to monitor Ag deposition has a mass s e n s i t i v i t y of 14 cm /g (0.084 Hz-cm /ng) (15), while the highest APM device s e n s i t i v i t y i s 65 cm /g (10 Hz-cm /ng) f o r Device 2. With a measured short-term frequency s t a b i l i t y of 5 Hz, APM Device 2 has a minimum detectable mass l i m i t of approximately 0.5 ng/cm . We estimate the commercial QCM to have short-term frequency s t a b i l i t y of approximately 2 Hz, corresponding to a 24 ng/cm mass change. As mentioned above, the mass s e n s i t i v i t y of a 97 MHz SAW device exceeds that of the best APM s e n s i t i v i t y by a factor of two. However, because a 97 MHz Rayleigh wave propagating on ST-quartz suffers over 45 dB of increased attenuation when water covers j u s t 3 2

2

2

2

2

2


202



Ο

100

200

300

TIME

400

500

ΒΟΟ

700

800

(seconde)

Figure 3. a ) S h i f t i n o s c i l l a t i o n f r e q u e n c y o f APM D e v i c e 3 as s i l v e r i s d e p o s i t e d onto i t i n vacuo. Symbols a r e d a t a and l i n e s are l i n e a r least-squares f i t s , b ) APM s e n s o r response d u r i n g t h e b i n d i n g and r e l e a s e o f aqueous Cu i o n s ( [Cu ] - 0.25 mM) b y the e t h y l e n e d i a m i n e - d e r i v a t i z e d s u r f a c e . HC1 was added t o g i v e a pH n e a r 3» removing t h e bound Cu . 2+

2+

2+


13.

WCCOETAL.

203


mm of the acoustic path (13) » such a device i s d i f f i c u l t to use for l i q u i d sensing. In contrast, less than 8 dB of attenuation typically r e s u l t s for the SH-APM when l i q u i d i s added to a dry device surface (13), leaving ample signal with which to make measurements. Detection of Aqueous Cu . The high mass s e n s i t i v i t y of the APM device suggests that chemical sensors can be developed u t i l i z i n g i n teractions which lead to mass changes as small as a f r a c t i o n of a monolayer. D e r i v a t i z a t i o n of the quartz APM device surface with the en-based ligand, under the reaction conditions given i n the Experi mental Methods and Materials section, should r e s u l t i n surface cover ages of between one and several molecular layers (.23); the surfaceimmobilized ligand w i l l be denoted e n . An open l i q u i d c e l l affixed to the device was f i l l e d with 0.1 Μ KN0 ; this solution maintains ionic conductivity at a constant l e v e l , eliminating acoustoelectric effects. The device was included i n the o s c i l l a t o r loop of Figure 2a. After achieving stable o s c i l l a t i o n , 0.25 ml of 1 mM CuS0 i n 0.1 Μ KN0 were added to the c e l l , r e s u l t i n g i n a f i n a l C u concentra t i o n of approximately 0.25 mM. The data of Figure 3b show that aq ueous Cu binds readily and reversibly to the en -dérivâtized APM device surface. A frequency s h i f t of -13 ppm occurred over a period of roughly 400 sec following addition of Cu to the c e l l ; this shift corresponds to a bound ion density of about 3 χ 10 /cm . Based on additional experiments, e s s e n t i a l l y a l l the available binding s i t e s are occupied at this concentration, so the coverage of e n i s thus about 6 χ 10 /cm . To learn how much of the C u binding might be attributable to the quartz surface i t s e l f , a control experiment was performed using an underivatized device. In this case, C u was added to give a f i n a l concentration of approximately 100 μΜ; the r e s u l t was a frequency s h i f t of roughly -2 ppm, about s i x times smaller than the s h i f t recorded for the en -treated surface. To examine the r e v e r s i b i l i t y of the C u binding, 0.25 ml of 10 mM HC1 i n 0.1 Μ KN0 were added to the c e l l , giving a f i n a l pH near 3 and protonating the Ν atoms of the e n . The data of Figure 3b show the return of the o s c i l l a t i o n frequency to near i t s i n i t i a l value, i n d i c a t i n g nearly complete release of the bound C u ions as a r e s u l t of the acid addition. The binding and release of C u were found to be repeatable with this device, indicating that the derivat i z e d APM device functions as a reversible detector of low concen trations of C u i n solution. 2+

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Viscous Coupling. APM v e l o c i t y s h i f t s and attenuation a r i s i n g from l i q u i d entrainment were measured for Device 1. The changes i n propa gation c h a r a c t e r i s t i c s were measured while glycerol/water mixtures having v i s c o s i t i e s between 1 and 62 centipoise (cP) were placed i n an open c e l l which confined the l i q u i d to the region between trans ducers . The device and c e l l were maintained at 20 °C while α and Δ ν / ν were measured using the instrumentation of Figure 2b; the mea sured v a r i a t i o n s are p l o t t e d vs. frj i n Figure 4a and b. I t should be noted that although the density of the solution increases by approximately 20% over the range of v i s c o s i t i e s shown i n Figure 4, the e f f e c t of this change i s n e g l i g i b l e compared to the e f f e c t of changing v i s c o s i t y . At low values of v i s c o s i t y , the l i q u i d behaves as a Newtonian f l u i d with Δ ν / ν and a proportional to frj. For v i s c o s i t i e s exceeding about 10 cP, relaxation times become comparable to the wave period (6 ns) and v i s c o e l a s t i c behavior results (11). η

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Figure 4. Perturbations i n a ) plate mode v e l o c i t y and b) attenuat i o n due to l i q u i d entrainment by the APM device surface. Data are f o r glycerol/water mixtures which only contact the surface between transducers. The s o l i d l i n e s are calculated from a v i s c o e l a s t i c model f o r the l i q u i d ; the dashed l i n e s are calculated using a Newtonian model. c) Perturbation i n o s c i l l a t o r frequency due to acoustoelectric interactions between APM and ions and dipoles i n solution. S o l i d l i n e s are calculated using b e s t - f i t values f o r the d i e l e c t r i c c o e f f i c i e n t of each solvent.


13. RICCO ET A L

205


The perturbation theory estimates are i n good agreement with the data over the v i s c o s i t y range tested, as shown by the s o l i d l i n e s calculated from Equations 5 using the experimentally determined value of c f o r t h i s device. A shear modulus μ - 3.1 χ 10 dyne/cm was found to give the simultaneous best f i t of Equations 5 to the experi mental data shown i n Figure 4a and b. Interestingly, t h i s modulus implies a l i q u i d relaxation time which agrees well with measured d i e l e c t r i c r e l a x a t i o n times (11). The dashed l i n e s are the predic tions when v i s c o e l a s t i c e f f e c t s are neglected and the l i q u i d i s treated as a Newtonian f l u i d , i . e . , μ - » i n Equations 5. 8

2


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Acoustoelectric Coupling. The acoustoelectric i n t e r a c t i o n between plate modes and ions was investigated by measuring the wave v e l o c i t y s h i f t and attenuation as a function of the ionic conductivity of a s o l u t i o n contacting Device 1. The s a l t (KN0 or L i C l ) concentration of a solution pumped across the device was varied using the closed c e l l configuration of Figure 2c while monitoring solution conductiv ity. To determine the e f f e c t of the solution d i e l e c t r i c constant, t h i s experiment was repeated using solvents with widely varying d i e l e c t r i c constants. The v a r i a t i o n i n APM v e l o c i t y as a function of solution conduc t i v i t y σ i s shown i n Figure 4c f o r four solvents having € /€ r a t i o s of 79.3 (H 0), 52.5 (H 0/ethanol), 33.0 (methanol), and 24.6 (ethanol) . The behavior of KN0 and L i C l i n water are indistinguishable, arguing against s p e c i f i c ion e f f e c t s . The highest conductivity shown for KN0 i n H 0 corresponds to a concentration of about 0.25 M. Changes i n density and v i s c o s i t y over the range of c o n d u c t i v i t i e s examined are i n s i g n i f i c a n t on the scale of Figure 4c. As Figure 4c shows, the d i e l e c t r i c constant of the solvent s i g n i f i c a n t l y affects both the rate at which v e l o c i t y changes with conductivity and also the o v e r a l l magnitude of the change. Attenuation of the APM due to ionic conductivity i s too small to measure with p r e c i s i o n using quartz devices. The s o l i d l i n e s i n Figure 4c are v e l o c i t y s h i f t s calculated from Equation 6a using a single value of K and the bestf i t value of the d i e l e c t r i c constant f o r each solvent; € /€ r a t i o s of 74, 53, 36, and 21 give the curves shown f o r H 0, ethanol/H 0, methanol, and ethanol, respectively. The K of 3.2 χ 10"* was obtained from a least-squares f i t of Equation 6a simultaneously to a l l four sets of data. Independent measurements of K during the vacuum evaporation of a metal f i l m give values i n the range 2.23.1 χ 10'*, suggesting the b e s t - f i t value i s reasonable (31). In many sensor applications, response r e s u l t i n g from conductiv i t y v a r i a t i o n s are undesirable ; they can be eliminated using several methods : (1) b u f f e r i n g solution ionic concentration; (2) depositing a conductive metal layer on the quartz surface to decouple ions i n solution from the APM; or (3) adjusting solution conductivity to a value outside the range over which i t s e f f e c t s are important. 3

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Conclusions In comparison to s i m i l a r devices, the SH-APM o f f e r s advantages for sensing species i n solution. These include considerably less attenuation of the propagating wave i n comparison to the SAW, and enhanced s e n s i t i v i t y and the complete i s o l a t i o n of transducers from the solution i n comparison to the QCM. The APM interacts with


206

CHEMICAL SENSORS AND

MICROINSTRUMENTATION

s p e c i e s i n s o l u t i o n and a t the s o l i d / l i q u i d i n t e r f a c e via s e v e r a l mechanisms, i n c l u d i n g changes i n s u r f a c e mass and m e c h a n i c a l p r o p e r t i e s , v i s c o u s c o u p l i n g t o the a d j a c e n t l a y e r o f s o l u t i o n , and acous t o e l e c t r i c c o u p l i n g between the APM's e v a n e s c e n t e l e c t r i c f i e l d and i o n s and d i p o l e s i n s o l u t i o n . Knowledge o f t h e s e i n t e r a c t i o n s a l l o w s the d e t e r m i n a t i o n o f sub-monolayer mass changes (0.5 ng/cm ) at the s o l i d / l i q u i d i n t e r f a c e ; combination o f t h i s s e n s i t i v i t y w i t h surface d e r i v a t i z a t i o n techniques leads to chemical sensors f o r dissolved species. 2


Acknowledgments The a u t h o r s a r e g r a t e f u l t o P. A. T a y l o r , V. M. H i e t a l a , and L. Romero o f S a n d i a N a t i o n a l L a b o r a t o r i e s f o r h e l p f u l d i s c u s s i o n s and to B. J . Lammie o f S a n d i a N a t i o n a l L a b o r a t o r i e s and I . A d h i h e t t y o f the U n i v e r s i t y o f New Mexico Department o f C h e m i s t r y f o r v a l u a b l e techni cal assistance. T h i s work was s u p p o r t e d by the U.S. Department o f Energy under c o n t r a c t no. DE-AC04-76DP00789.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

Wohltjen, H; Dessy, R. Anal. Chem. 1979, 51, 1458. Wohltjen, H. Sensors and Actuators 1984, 5, 307. Zellers, E. T.; White, R. M.; Wenzel, S. W. Sensors and Actua tors 1988, 14, 35. Martin, S. J.; Schweizer, K. S.; Schwartz, S. S.; Gunshor, R. L. Proc. 1984 IEEE Ultrasonics Symp., 1984, p 207. Ricco, A. J.; Martin, S. J.; Zipperian, T. E. Sensors and Actuators 1985, 8, 319. D'Amico, A.; Palma, A.; Verona, E. Sensors and Actuators 1982, 3, 31. Venema, A.; et al. IEEE Trans. Ultrasonics, Ferroelectrics, and Freq. Contr. 1987, UFFC-34, 148. Bryant, A.; Lee, D. L.; Vetelino, J. F. Proc. 1981 IEEE Ultrasonics Symp., 1981, p 171. Roederer, J. E.; Bastiaans, G. J. Anal. Chem. 1983, 55, 2333. Martin, S. J.; Ricco, A. J. Proc. Int. Electron Devices Mtg., 1987, p 290. Ricco, A. J.; Martin, S. J. Appl. Phys. Lett. 1987, 50, 1474. Hou, J.; van de Vaart, H. Proc. 1987 IEEE Ultrasonics Symp., 1987, p 573. Ricco, A. J.; Martin, S. J. Proc. Symp. on Electroless Dep. of Metals and Alloys, 1988, Vol. 88-12, p 142. White, R. M.; Wicher, P. J.; Wenzel, S. W.; Zellers, Ε. T. IEEE Trans. Ultrasonics, Ferroelectrics, Freq. Contr. 1987, UFFC-34, 162. Sauerbrey, G. Z. Phys. 1959, 155, 206. Thompson, M.; Dhaliwahl, G. K.; Arthur, C. L.; Calabrese, G. C. IEEE Trans. Ultrasonics, Ferroelectrics, Freq. Contr. 1987, UFFC-34, 128. Carey, W. P.; Beebe, K. R.; Kowalski, B. R. Anal. Chem. 1987, 59, 1529. Nomura, T.; Okuhara, M. Anal. Chim. Acta 1982, 142, 281. Bruckenstein, S.; Shay, M. J. Electroanal. Chem. 1985, 188, 131.


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20. Kanazawa; Κ. K.; Gordon II, J. G. Anal. Chem. 1985, 57, 1770. 21. Ricco, A. J.; Martin, S. J. Ext. Abstr. 171 Electrochem. Soc. Mtg., 1987, Vol. 87-1, p 501. 22. Martin, S. J.; Ricco, A. J.; Sorensen, N. R. Ext. Abstr. 171 Electrochem. Soc. Mtg., 1987, Vol. 87-1, p 50. 23. Arkles, B. Chemtech 1977, 7, 766. 24. Leyden, D. E.; Collins, W. Silylated Surfaces; Gordon and Breach Science: New York, 1980. 25. Bookbinder, D. C.; Wrighton, M. S. J. Electrochem. Soc. 1983, 130, 1080. 26. Cotton, F. Α.; Wilkinson, G. Advanced Inorganic Chemistry, 4th Edn.; John Wiley & Sons: New York, 1980; Chapters 3, 21, 22. 27. Hathaway, B. J.; Billing, D. E. Coord. Chem. Rev. 1970, 5, 143. 28. McIntyre Jr., G. H.; Block, B. P.; Fernelius, W. C. J. Am. Chem. Soc. 1959, 81, 529. 29. Auld, B. A. Acoustic Waves and Fields in Solids; John Wiley & Sons: New York, 1973; Vol. 2. 30. Matheson, A. J. Molecular Acoustics; John Wiley & Sons: New York, 1971; pp. 82-83. 31. Niemczyk, T. M.; Martin, S. J.; Frye, G. C.; Ricco, A. J. J. Appl. Phys. 1988, 64, 5002. 32. Gunshor, R. L. Sol.-State Electron. 1975, 18, 1089. 33. Calabrese, G. S.; Wohltjen, H.; Roy, M. K. Anal. Chem. 1987, 59, 833. 34. Parker, T. E.; Montress, G. K. IEEE Trans. Ultrasonics, Fer roelectrics, Freq. Contr. 1988, UFFC-35, 342. st


st

RECEIVED March 9, 1989


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