Chemical Sensors and Microinstrumentation - American Chemical

Chapter 1

Chemical Sensors and Microinstrumentation An

Overview

Downloaded by WESTERN MICHIGAN UNIV on August 6, 2016 | http://pubs.acs.org Publication Date: August 24, 1989 | doi: 10.1021/bk-1989-0403.ch001

Royce W. Murray Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, NC 27599-3290 A chemical sensor is a measurement system designed to exhibit an experimental response relatable to the quantity of a sample chemical species or class of chemical species. It may be designed to respond to the target species as present i n a gaseous, liquid, or solid sample of indeterminate volume, or to a target contained within a defined space such as within a small volume, on the surface of the sample, or on a microscopic part of the surface of the sample. Also known as a chemical transducer, the chemical sensor can be based on any characteristic of the sample species that serves to distinguish i t from i t s matrix, including i t s chemical reactivity, optical or electrical properties, or mass. The student of analytical chemistry w i l l recognize that these statements encompass the field traditionally known as instrumental analysis. The more recent popularity of the descriptor chemical sensor subtlely reflects the quite important fact that the chemical/instrument interface (i.e., the sensor) is today more commonly the limiting aspect of the chemical analysis capability than is the instrument i t s e l f . The range o f sample c h a r a c t e r i s t i c s and manner o f t h e i r d e t e c t i o n , i s much l a r g e r than c a n be r e a l i s t i c a l l y addressed i n the space o f a s i n g l e c h a p t e r . We w i l l c o n f i n e t h i s c h a p t e r m a i n l y t o the c h e m i c a l s e n s o r r e s e a r c h areas d i s c u s s e d i n o t h e r c h a p t e r s i n t h i s volume, d i v i d i n g them i n t o e l e c t r i c a l , o p t i c a l , and mass and t h e r m a l measurements. Our f o c u s w i l l f u r t h e r m o r e be on t h e g e n e r i c c h e m i c a l and p h y s i c a l phenomena upon w h i c h such measurements c a n be based, as opposed t o t h e a l t e r n a t i v e o r g a n i z a t i o n t h a t would address c h e m i c a l s e n s o r s i n the c o n t e x t of t h e i r a p p l i c a t i o n ( i . e , auto exhaust sensor, clinical d i a g n o s t i c sensor, environmental sensor) o r o f the k i n d s o f samples d e t e c t e d ( i . e , CO s e n s o r s , h u m i d i t y s e n s o r , b i o s e n s o r , e t c . ) , as used i n a p r e v i o u s ACS Symposium S e r i e s volume on Chemical Sensors (D. S c h u e t z l e , R. Hammerle, Eds., ACS Sympos. Ser. 309, 1986). 0097-^6156/89/0403-OOOlS06.00yO © 1989 American Chemical Society

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


2

CHEMICAL SENSORS AND

MICROINSTRUMENTATION

Throughout c u r r e n t c h e m i c a l sensor d e s i g n r e s e a r c h , one can r e a d i l y see the impact o f new c h e m i c a l m a t e r i a l s , o f m i c r o f a b r i c a t i o n t e c h n o l o g y t h a t arranges c h e m i c a l m a t e r i a l s i n m i n i a t u r i z e d forms, and o f m i c r o e l e c t r o n i c s and communications d e v i c e s and t e c h n o l o g y . M i c r o e l e c t r o n i c s t e c h n o l o g y has g i v e n us the s c i e n c e o f m i c r o l i t h o g r a p h y , w h i c h i n p a r t i c u l a r a l l o w s the s p a t i a l l y d e f i n e d a r r a n g i n g o f e l e c t r i c a l l y sensitive elements and c o n t a c t s t o our c h e m i c a l systems on s c a l e s o f a few m i c r o n s . P a t t e r n s o f metal f i l m s t h a t d e t e c t s u r f a c e a c o u s t i c a l waves (1,2), m o l e c u l a r c o n d u c t i v i t y ( 3 ) , and e l e c t r o c h e m i c a l charge t r a n s f e r s (4-6) a r e p r o d u c t s o f the a n a l y t i c a l c h e m i s t s ' use o f m i c r o l i t h o g r a p h i c s c i e n c e . Telecommunications t e c h n o l o g y has g i v e n us o p t i c a l f i b e r s , w h i c h y i e l d new d i m e n s i o n s o f c o n t a c t i n g o p t i c a l l y s e n s i t i v e c h e m i c a l systems w i t h sample media ( 7 , 8 ) . The s c i e n c e o f polymer composites has l e d t o the a v a i l a b i l i t y o f c j | . 5 m i c r o n r a d i u s c a r b o n f i b e r s w h i c h have found use as m i c r o e l e c t r o d e s (9) i n i n v i v o s e n s i n g of neurotransmitters. T h i s i s o f c o u r s e an o l d s t o r y ( 1 0 ) ; measurement s c i e n c e has t r a d i t i o n a l l y b e n e f i t e d from and r e c o n t r i b u t e d t o t e c h n o l o g i c a l developments and p r o d u c t s t h a t a t t h e i r r o o t s were d r i v e n by consumer o r m i l i t a r y economics and goals. Dealing with molecules, chemists are also good at m i n i a t u r i z i n g the p h y s i c a l dimensions o f m a t e r i a l s they work with. T h i s has been p a r t i c u l a r l y e v i d e n t i n e x p l o i t a t i o n o f t h i n p o l y m e r i c f i l m s i n m o d i f i e d e l e c t r o d e s (11,12), q u a r t z c r y s t a l microbalances (13,14), and s u r f a c e a c o u s t i c a l wave (1,2), o p t i c a l f i b e r ( 7 , 8 ) , and p o t e n t i o m e t r i c (15,16) c h e m i c a l sensors. S m a l l polymer f i l m t h i c k n e s s e s a r e especially s i g n i f i c a n t i n m a n i p u l a t i n g the response times and s e l e c t i v i t i e s of chemical sensors v i a t r a n s p o r t r a t e e f f e c t s . Combining c h e m i c a l r e a g e n t s w i t h u l t r a t h i n polymer f i l m s (and w i t h u l t r a t h i n o r g a n i z e d m o l e c u l a r f i l m s ) has become an enormously p r o f i t a b l e t a c t i c i n s e n s o r d e s i g n , as w e l l as a s t i m u l u s f o r a n a l y t i c a l c h e m i s t s t o draw upon s y n t h e t i c a s p e c t s o f polymer chemistry, CHEMICAL SENSÛES BASED ON ELECTRICAL PHENOMENA The most i m p o r t a n t e l e c t r i c a l phenomena i n c h e m i c a l s e n s i n g are e l e c t r i c a l c o n d u c t i v i t y , i n t e r f a c i a l p o t e n t i a l s ( i . e . , p o t e n t i o m e t r y ) , and e l e c t r o c h e m i c a l r e a c t i o n c u r r e n t s ( i . e , f a r a d a i c o r amperometric methods). A l l depend on charge exchange and t r a n s p o r t p r o c e s s e s i n some manner, and a l l have been a c t i v e r e s e a r c h areas i n r e c e n t y e a r s . Conductivity, Electrical c o n d u c t i v i t y measurements are appealingly simple and s e n s i t i v e , and r e f l e c t the charge c a r r y i n g a b i l i t y ( c a r r i e r p o p u l a t i o n and m o b i l i t y ) o f a t e s t medium i n t e r p o s e d between two e l e c t r o d e s b i a s e d by DC o r AC potentials. The key f o r c h e m i c a l s e n s o r i n g i s t o d e s i g n some element o f c h e m i c a l s e l e c t i v i t y i n t o the v a l u e s o f c a r r i e r p o p u l a t i o n o r m o b i l i t y . Measurements i n l i q u i d i o n i c s o l u t i o n s ( t h e c l a s s i c a l method o f c o n d u c t i m e t r y ) have l i t t l e i n t r i n s i c



1.

3

Chemical Sensors and Microinstrumentation

MURRAY

chemical s e l e c t i v i t y and are mainly useful as detectors of ionic populations a f t e r a chromatographic separation. S o l i d state conductances can have greater significance for chemical sensing i f a c a r r i e r generation process can be t i e d to a chemical process s p e c i f i c to a target chemical. For example, binding of a target chemical from i t s vapor into a t h i n molecular f i l m , i n a chemically selective way that produces an increase i n charge c a r r i e r s , i s a useful design goal. Barger et a l (3) have f o r example reported how the conductivity of a thin phthalocyanine f i l m i s enhanced by traces of axially-coordinating organic vapor contacting the f i l m . This work was done with a l i t h o g r a p h i c a l l y defined conductance c e l l consisting of i n t e r d i g i t a t e d metal fingers, the phthalocyanine f i l m being deposited over the metal finger array and i n the insulating gaps i n between the fingers. Potentiometry. Chemical sensors based on measurements of i n t e r f a c i a l electrochemical potentials (potentiometry) mainly involve p o t e n t i a l differences that e x i s t across membranes that separate two solutions. The o r i g i n of the membrane p o t e n t i a l rests on binding of a charged species at the membrane surface and/or transport of the species through i t . I f the binding and transport are selective to a given charged species, then membrane p o t e n t i a l varies with (RT/nF) In[ a /a ] where a and a are the concentrations on the sample and reference sides of the membrane, respectively. I f the membrane i s not e n t i r e l y s e l e c t i v e , t h i s r e l a t i o n i s changed (15) to x

E

c e l l

- constant + (0.059/z) l o g [ (

a i

+ b

Za/zb

2

K

x

a(b

)/a ] 2

where b i s the a c t i v i t y of the i n t e r f e r i n g sample species and K i s the potentiometric s e l e c t i v i t y c o e f f i c i e n t . K i s the usual way to express the i d e a l i t y of the membrane s e l e c t i v i t y and i s zero f o r t o t a l s e l e c t i v i t y toward a., Clearly potentiometric sensor design i s substantially based on chemical design of membranes that minimize K coefficients. The potentiometric l i t e r a t u r e (15,17) i s substantial i n the areas of H , a l k a l i metal ion, Ca , and F~ sensors and has involved membranes prepared from glasses, c r y s t a l l i n e materials, composites, and functionalized polymers. The s i t u a t i o n i s less s a t i s f a c t o r y f o r potentiometric sensors for anions and for more complex molecular ions, and some current work has been directed at the problems of designing chemical binding s p e c i f i c i t y into polymer films. For metal-coordinating ionic bases, the metalloporphyrins (16,18) o f f e r a r i c h coordination chemistry that i s manipulable by choice of metal and porphyrinic framework. The p r i n c i p l e s of host-guest (19) molecular design are also p o t e n t i a l l y important i n this area. These e f f o r t s , as do most involving designed molecular s p e c i f i c i t y , carry a substantial chemical synthetic involvement, or collaboration with synthetic chemists (a useful moral). Faradaic Methods. Chemical sensors based on electrochemical reactions tend to be more complex than potentiometric sensors because t h e i r operation depends on a greater d i v e r s i t y of ab

ab

ab

+

2+


2


4

CHEMICAL SENSORS AND

MICROINSTRUMENTATION

k i n e t i c e v e n t s : heterogeneous e l e c t r o n t r a n s f e r k i n e t i c s , mass t r a n s p o r t k i n e t i c s , and the k i n e t i c s o f c h e m i c a l reactions c o u p l e d t o the heterogeneous e l e c t r o d e r e a c t i o n . A t the same t i m e , the s e n s i t i v i t y o f k i n e t i c s t o m o l e c u l a r c h a r a c t e r and the u t i l i t y of c o n t r o l l i n g r e a c t i o n rates with applied electrode p o t e n t i a l s c r e a t e s a u s e f u l scope o f sample s e l e c t i v i t y . A v a l u e o f o x i d a t i o n o r r e d u c t i o n c u r r e n t r e l a t a b l e t o the sample c o n c e n t r a t i o n i s the u s u a l f a r a d a i c method o u t p u t . A s u b s t a n t i a l p o r t i o n o f r e c e n t e l e c t r o c h e m i c a l s e n s o r work has been d i r e c t e d a t measurements o f ( o r i n ) b i o l o g i c a l systems. The a c t i v e r e s e a r c h a r e a s i n c l u d e d e t e c t i o n o f n e u r o t r a n s m i t t e r s (catecholamines) and of reactions i n v o l v i n g enzymes and a n t i b o d i e s . The l a t t e r c l a s s e s o f s t u d y seek t o take advantage o f the sharp c h e m i c a l s e l e c t i v i t i e s t h a t n a t u r a l systems have evolved. S e v e r a l ways t h a t enzymes have been i n c o r p o r a t e d into e l e c t r o d e r e a c t i o n schemes are i l l u s t r a t e d i n F i g . 1. The c l a s s i c a l method ( F i g . 1A) uses a s o l u t i o n o f sample s u b s t r a t e , enzyme, and enzyme c o f a c t o r (mediator o x i d a n t o r r e d u c t a n t ) and d e t e c t s w i t h the e l e c t r o d e , a p r o d u c t o f the enzyme r e a c t i o n ( e i t h e r t h a t o f the s u b s t r a t e o r o f the m e d i a t o r ) . For the i m p o r t a n t case o f g l u c o s e d e t e r m i n a t i o n , the H 0 p r o d u c t o f the a e r o b i c t u r n o v e r o f the enzyme can be d e t e c t e d by i t s o x i d a t i o n c u r r e n t a t a P t e l e c t r o d e , o r the consumption o f d i o x y g e n can be d e t e c t e d through d i m i n u t i o n o f i t s r e d u c t i o n c u r r e n t ( 2 0 ) . Most enzymes do not r e a d i l y undergo d i r e c t e l e c t r o n t r a n s f e r s w i t h c o n d u c t i n g e l e c t r o d e s u r f a c e s ( F i g . I B ) ; the enzyme a c t i v e s i t e i s e i t h e r b u r i e d and i n a c c e s s i b l e t o a m a c r o s c o p i c a l l y huge e l e c t r o d e s u r f a c e , o r the enzyme may be s t e e r e d by a d s o r p t i v e o r e l e c t r o s t a t i c f o r c e s i n t o c o n t a c t i n g the e l e c t r o d e a t a r e g i o n on i t s o u t e r s u r f a c e u n f a v o r a b l e f o r e l e c t r o n t r a n s f e r . A d d i n g f a s t e l e c t r o n t r a n s f e r m e d i a t o r s (21,22) can be an e f f e c t i v e way t o d e a l w i t h the a c t i v e s i t e a c c e s s i b i l i t y problem ( F i g . 1C). I n the determination of glucose (22) f o r example, the e l e c t r o c h e m i c a l o x i d a t i o n of ferrocene generates f e r r i c e n i u m , an o x i d a n t w h i c h r e a c t s w i t h g l u c o s e o x i d a s e t o produce the a c t i v e form t h a t consumes g l u c o s e and s i m u l t a n e o u s l y r e g e n e r a t e s the ferrocene. The c a t a l y t i c regeneration enhances the e l e c t r o d e c u r r e n t which i s i n t u r n r e l a t e d t o the glucose concentration. A recent innovative step(23) i s to chemically b i n d the m e d i a t o r t o the enzyme i n a way a l l o w i n g the m e d i a t o r t o r e a c t d i r e c t l y w i t h b o t h the e l e c t r o d e and the enzyme a c t i v e s i t e ( F i g . I D ) . W h i l e t h i s approach r e q u i r e s d e v e l o p i n g and c h a r a c t e r i z i n g the mediator-enzyme attachment c h e m i s t r y , i t a p p a r e n t l y can p r o v i d e an e f f i c i e n t e l e c t r o n c o u p l i n g between e l e c t r o d e and enzyme t u r n o v e r o f s u b s t r a t e . F o u r t h l y , a u s e f u l v a r i a n t t o a l l o f the above schemes i s t o a t t a c h the enzyme d i r e c t l y t o the e l e c t r o d e s u r f a c e . Recent work i n t h i s d i r e c t i o n (24,25) aims a t i m p r o v i n g the attachment s t a b i l i t y and the k i n e t i c s and s e l e c t i v i t y o f t r a n s p o r t o f charge between e l e c t r o d e and enzyme and o f s u b s t r a t e between sample s o l u t i o n and i m m o b i l i z e d enzyme. F i n a l l y , i t i s i m p o r t a n t t o u n d e r s t a n d 2

2


MURRAY

Chemical Sensors and Microinstrumentation

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Chemical Sensors and Microinstrumentation - American Chemical

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