Chapter 5
Electrochemical Sensors Based on Polymer Films Immobilized by Gamma Irradiation Louis A. Coury, Jr., Christos Galiatsatos, Edward W. Huber, Sandra Sponaugle, and William R. Heineman 1
Edison Sensor Technology Center, Biomedical Chemistry Research Center, and Department of Chemistry, University of Cincinnati, Cincinnati, OH 45221-0172 Gamma irradiation is used as a means of immobilizing water-soluble polymers on spectroscopic graphite electrodes for the purpose of imparting certain selectivity characteristics to electrochemical sensors. Poly(N-vinylpyrrolidone) enables catecholamine neurotransmitters to be determined in the presence of a 10fold excess of ascorbate by concentrating the catecholamine at the electrode surface while discriminating against ascorbate. This polymer also imparts resistance to electrode fouling from oxidation products generated during the detection of various phenols. Glucose oxidase can be immobilized in poly(vinyl alcohol) by irradiation to form a long-lived glucose sensor with rapid response time. The development of electrochemical sensors for use in biological systems has been of interest for many years Q). Since samples of biological origin are complex, electrochemical sensors must demonstrate a high degree of selectivity for successful monitoring of the analyte of interest without interference. à typical strategy in sensor development to gain this selectivity is electrode modification, which involves the immobilization of a species at the surface of the electrode. Electrode modification can alter the microenvironment of the electron-transfer reactions that occur at the electrode surface, thus leading to the fabrication of electrodes with unique analytical properties. The modification of electrode surfaces with polymer films has received considerable attention because of many advantageous properties of polymers (2_,3>) · Polymer films are chemically stable, provide diffusional barriers that can lead to selectivity based on size or charge exclusion properties, provide a means of preconcentrating analytes by ionic or other complexation interactions, and are a convenient matrix for the immobilization of other reagents, such as enzymes. Coating electrode surfaces with polymer films takes advantage of these properties. Complexation of a specific address correspondence to this author. 0097-^156/89/Ο403-Ό078$06.00/Ο © 1989 American Chemical Society
5. COURY ET AL.
Electrochemical Sensors Based on Polymer Films79
a n a l y t e ( s ) by a polymer can i n c r e a s e the e f f e c t i v e c o n c e n t r a t i o n and hence the a n a l y t i c a l s i g n a l due t o the s p e c i e s o f i n t e r e s t a t the e l e c t r o d e s u r f a c e w h i l e the s i g n a l due to i n t e r f e r e n t s t h a t do not i n t e r a c t w i t h the polymer i s a t t e n u a t e d by a c o m b i n a t i o n of d e c r e a s e d c o n c e n t r a t i o n near the e l e c t r o d e s u r f a c e and hindered mass t r a n s p o r t t h r o u g h the polymer. The coupling of these p r o p e r t i e s w i t h the v e r y s e n s i t i v e i n s t r u m e n t a l methods a v a i l a b l e i n e l e c t r o c h e m i s t r y r e s u l t s i n sensors w i t h h i g h s e l e c t i v i t y and sensitivity. Electrochemical biosensors based on enzymes o r e n z y m a t i c s u b s t r a t e s i m m o b i l i z e d w i t h i n the polymer f i l m a t the e l e c t r o d e s u r f a c e g a i n t h e i r s e l e c t i v i t y and s e n s i t i v i t y from v e r y s p e c i f i c enzyme-substrate r e a c t i o n s t h a t can be m o n i t o r e d e l e e t r o chemically. Fundamental t o p r o g r e s s i n expanding the a p p l i c a t i o n s and reliability of electrochemical-based sensors is the availability and understanding of m o d i f i c a t i o n s t r a t e g i e s f o r imparting the requisite selectivity and sensitivity to these devices. S t r a t e g i e s f o r the polymer m o d i f i c a t i o n o f e l e c t r o d e s have been based on d i p c o a t i n g o r s p i n c a s t i n g polymers o n t o the e l e c t r o d e from v o l a t i l e s o l v e n t s , a d s o r p t i o n o r p r e c i p i t a t i o n from solution, chemical coupling to reactive surface groups, i n s o l u b i l i z a t i o n by d e g r a d a t i o n or the f o r m a t i o n o f c r o s s l i n k s , and polymerization. The polymer adheres t o the e l e c t r o d e s u r f a c e by some c o m b i n a t i o n o f a d s o r p t i o n , c o v a l e n t b o n d i n g , p h y s i c a l e n t r a p ment, and low s o l u b i l i t y i n the c o n t a c t i n g s o l v e n t . Enzymes o r o t h e r r e a g e n t s / c a t a l y s t s a r e i m m o b i l i z e d i n t o the polymer f i l m s by p h y s i c a l entrapment o r by bond f o r m a t i o n as a r e s u l t o f the m o d i f i c a t i o n t e c h n i q u e used. Once p r e p a r e d , p o l y m e r - m o d i f i e d e l e c t r o d e s are t r a n s f e r r e d t o an e l e c t r o l y t e / s o l v e n t medium f o r e l e c t r o c h e m i c a l s t u d y . The solvent permeates and swells the polymer, decreasing ohmic r e s i s t a n c e and a l l o w i n g p e r m e a t i o n o f s o l u t i o n s p e c i e s . F o r some polymers t h i s s w e l l i n g has l e d to u n s a t i s f a c t o r y r e s u l t s because of film dissolution. P r o c e d u r e s t o overcome t h i s i n s t a b i l i t y i n s w e l l i n g s o l v e n t s o f t e n i n v o l v e the f o r m a t i o n o f c r o s s - l i n k s . As an example, polymer f i l m s have been s t a b i l i z e d by i n s i t u c r o s s l i n k i n g using organosilane condensation (4), divinylbenzene (5)» t r i a l l y l t r i m e l l i t a t e (6^), and h e a t c u r i n g (7). We have i n v e s t i g a t e d the i m m o b i l i z a t i o n o f water s o l u b l e polymers by exposure t o gamma r a d i a t i o n ( 8 - 1 0 ) . This chapter d e s c r i b e s some o f our work i n t h i s a r e a as i t r e l a t e s t o the development of e l e c t r o c h e m i c a l s e n s o r s , e s p e c i a l l y f o r use in biological systems. The article focuses on two polymers: p o l y ( N - v i n y l p y r r o l i d o n e ) and p o l y ( v i n y l a l c o h o l ) . Gamma R a d i a t i o n I m m o b i l i z a t i o n o f Polymer F i l m s Gamma r a y s are h i g h l y e n e r g e t i c photons t h a t are e f f i c i e n t i n the generation of radicals i n polymer s o l u t i o n s and films (11). R a d i c a l s i t e s on d i f f e r e n t polymer c h a i n s can r e a c t t o form new bonds, or cross-links, and thereby drastically increase the m o l e c u l a r weight o f the m a c r o m o l e c u l a r s p e c i e s . Under c e r t a i n cond i t i o n s , a l l of the polymer c h a i n s i n a g i v e n sample may become i n t e r c o n n e c t e d , forming a polymer network t h a t i s i n s o l u b l e i n a
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g i v e n s o l v e n t , b u t s t i l l r e t a i n s t h e a b i l i t y t o s w e l l when immersed i n t h a t s o l v e n t (12,)· Gamma i r r a d i a t i o n may a l s o r e s u l t i n f u r t h e r p o l y m e r i z a t i o n o f t h e polymer w i t h monomer s p e c i e s p r e s e n t , o r i n main c h a i n d e g r a d a t i o n o f t h e polymer ( 1 3 ) , e i t h e r o f which may r e s u l t i n i n s o l u b i l i z a t i o n o f t h e polymer. Gamma i r r a d i a t i o n has s e v e r a l advantageous f e a t u r e s as a method o f i m m o b i l i z i n g polymers on e l e c t r o d e s u r f a c e s . It isa simple, r e l a t i v e l y inexpensive procedure. The r e s u l t i n g i n s o l u b l e g e l s t h a t a r e formed s w e l l i n s o l u t i o n p e r m i t t i n g a c c e s s f o r solvent, supporting e l e c t r o l y t e and a n a l y t e t o the electrode s u r f a c e (8,14). V a r y i n g t h e r a d i a t i o n dosage o r monomer c o n t e n t o f the f i l m c a s t on t h e e l e c t r o d e c a n be used t o c o n t r o l t h e degree o f s w e l l i n g o f t h e polymer when i t i s p l a c e d i n solvent postirradiation (J3). This allows f o r c o n t r o l o f the r e l a t i v e p e r m e a b i l i t y o f t h e r e s u l t i n g s w o l l e n f i l m (8,160. Gamma r a y s a r e h i g h l y p e n e t r a t i n g compared w i t h e l e c t r o n beam o r u l t r a v i o l e t (UV) r a d i a t i o n , hence they c a n be used t o cause g e l f o r m a t i o n i n polymer s o l u t i o n s (12) and t h i c k polymer f i l m s ( 8 ) . No a d d i t i v e s such as i n i t i a t o r s o r s e n s i t i z e r s need t o be added t o t h e polymer b e f o r e gamma i r r a d i a t i o n ( Γ7), u n l i k e most UV i r r a d i a t i o n p r o t o c o l s . R a d i a t i o n induced g r a f t i n g o f d i s s i m i l a r polymers has been used t o create novel blood-compatible hydrogels f o r biomedical a p p l i c a t i o n s (18). Because t h i s type o f polymer c o n t a i n s a h i g h p e r c e n t a g e by volume o f water when s w o l l e n (and no r e s i d u a l i n i t i a t o r s o r c r o s s l i n k i n g a g e n t s ) , such h y d r o g e l s would be e x p e c t e d t o be w e l l - s u i t e d f o r use i n b i o s e n s o r development. F u r t h e r m o r e , gamma r a d i a t i o n has been used t o s t e r i l i z e s u r g i c a l i n s t r u m e n t s and p h a r m a c e u t i c a l p r e p a r a t i o n s ( 1 9 ) , hence t h e s u b j e c t i o n o f polymer c o a t e d d e v i c e s t o gamma i r r a d i a t i o n w i l l a l s o s t e r i l i z e t h e d e v i c e i f t h e dosage is s u f f i c i e n t l y high. T h i s i s p o t e n t i a l l y advantageous f o r t h e f a b r i c a t i o n o f sensors f o r m e d i c a l use and f e r m e n t a t i o n m o n i t o r i n g . Some d i s a d v a n t a g e s o f t h e gamma i r r a d i a t i o n method are the r e l a t i v e l y poor c o n t r o l o v e r some f r e e r a d i c a l r e a c t i o n s , t h e need f o r a c c e s s t o a gamma source such as ^ C o , $ p o t e n t i a l r a d i a t i o n damage t o components o f t h e d e v i c e . a n
