Biosensors and Chemical Sensors - American Chemical Society

Chapter 10

Permselective Coatings for Amperometric Biosensing

Downloaded by EAST CAROLINA UNIV on November 3, 2016 | http://pubs.acs.org Publication Date: April 23, 1992 | doi: 10.1021/bk-1992-0487.ch010

Joseph Wang Department of Chemistry, New Mexico State University, Las Cruces, NM 88003

Access to the surface of amperometric biosensors can be controlled by coverage with an appropriate permselective film. Such coatings effectively exclude coexisting interferences, and thus greatly improve the selectivity and stability. Discriminative films based on different transport properties are described. Structural factors and fundamental interactions that govern the transport through such films are discussed. Future prospects are examined. Amperometric biosensors satisfy many of the requirements for clinical assays, environmental monitoring or process control. Such sensors offer excellent sensitivity, fast response, selectivity toward electroactive species, miniaturization and low cost. However, there are still problems of stability and selectivity associated with the utility of amperometric devices. Despite the inherent specificity accrued from the biological recognition process, co-existing electroactive species may result in overlapping current signals. In addition, adsorption of surface-active macromolecules can cause a gradual fouling of the sensing surface. One promising avenue to impart higher selectivity and stability to amperometric sensors is to cover the sensing surface with an appropriate permselective coating. The selective permeation and protection offered by such films can thus greatly promote routine applications of biosensing devices. The following sections examine the requirements, utility, and possibilities of using permselective films for electrochemical biosensing. The physical and chemical features that determine the permeability of these coatings are also discussed. While the concept is presented in the context of amperometric measurements, it could be easily extended to other (nonelectrochemical) sensing schemes. Discriminative Films Permselective films greatly enhance the selectivity and stability of amperometric probes by rejecting from the surface undesired (interfering) constituents, while allowing transport of the target analyte (Figure 1). An effective separation step is thus performed in situ on the sensing surface. Different avenues to control the access to the surface, based on various discriminative coatings, have been explored (Table I). Often, the coverage of the surface with the permselective film is used for a 0097-6156/92/0487-0125$06.00/0 © 1992 American Chemical Society

Edelman and Wang; Biosensors and Chemical Sensors ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

BIOSENSORS AND CHEMICAL SENSORS

126

Table I. Discriminative coatings for amperometric biosensors


Transport mechanism

Permselective Film

Ref.

Size exclusion

Cellulose acetate Base-hydrolyzed cellulose acetate Phase-inversion cellulose acetate Polyaniline, Polypyrrole Polyphenol Gamma radiated poly(acrylonitrile)

1 2 3 4 5 6

Charge exclusion

Nafion Poly(vinylpyridine) Poly(ester-sulfonic acid)

7,8 9 10, 11

Polarity

Phospholipid

12, 13

Mixed control

Cellulose acetate - Nafion Cellulose aœtate-poly(vinylpyridine)

14 15

simultaneous entrapment of the biocomponent (and its cofactor), as well as of another "active" moiety (e.g. electrocatalyst). The polymerization/casting conditions (e.g. pH, solvent) should be compatible with the requirements for the enzyme activity/stability. Attention should be given to possible changes in the film permeability in the presence of these immobilized reagents. In designing effective membrane barriers, one should attempt to maintain a fast and sensitive response for the analyte through a facile transport of this target species. As such, the optimal film thickness represents a compromise between effective rejection of interferences, and high sensitivity and speed. The transport of analytes and interferents through a given coating is determined by their diffusion coefficients in the film and their distribution ratios (between the film and solution). The improved selectivity is thus being achieved by taking advantage of analyte properties such as charge, size, polarity or shape. Size exclusion films are particularly attractive in connection with oxidase-based enzyme electrodes. Such films greatly facilitate the anodic detection of the small hydrogen peroxide product Cellulose-acetate (CA) modification has been used to build size-exclusion selectivity into amperometric sensors. Sittampalam and Wilson (1) illustrated the utility of C A coatings for minimizing surface fouling by proteins adsorption during the detection of hydrogen peroxide. Wang and Hutchins (2) demonstrated that base hydrolysis of C A coatings opens up their pores (through hydrolytic removal of acetate functionalities). Hence, different permeabilities were obtained by changing the hydrolysis time (Figure 2). Efforts to further extend the molecular-weight cut-off of porous C A films (up to 1500 daltons), based on a phase inversion method, were described by Kuhn et al (3). Selectivity based on molecular size can be achieved also through gamma radiation cross-linking of certain films (6). The permeability may be affected by variations in the radiation dose; response times are relatively long. The gamma irradiation scheme was used also for immobilizing enzymes (e.g. lactate oxidase) in polymeric layers (16). Of particular interest is the ability to control and manipulate the permeability of size-exclusion films to meet specific biosensing needs. Electropolymerization processes are particularly suitable for creating controllable size-exclusion films (4).



10. WANG


127

Figure 1. A permselective coating for amperometric sensing (A, analyte; P, product; Int., interfèrent).

1600

I 0

ί-

ι 20

40

Hydrolysis

time, min

Figure 2. Dependence of the current response on the hydrolysis time of the C A film for phenol (a), acetaminophen (b), estriol (c), N A D H (d), and potassium ferrocyanide (e). Flow injection amperometric operation. (Reproduced from ref. 2. Copyright 1985 American Chemical Society.)



128


This can be accomplished by controlling the amount of charge consumed during the anodization process (through the use of different polymerization times or monomer concentrations). The electropolymerization process has been very useful for a simultaneous physical entrapment of enzymes (17). In particular, glucose sensors based on electropolymerized 1,2-diaminobenzene couple an effective permselective response (Figure 3), with a very fast (Is!) response and thermal stabilization of the immobilized enzyme (18,19). Note also (from Figure 3) the protection of the surface from foulants present in the serum solution. Extension of the dynamic linear range (through control of the film thickness) is another attractive advantage. Polypyrrole/ glucose-oxidase systems have received considerable attention (17,20), with additional advantages attained through the coimmobilization of redox mediators (e.g. ferrocene, quinone) (21) or electrocatalytic centers (e.g. platinum microparticles) (22). Other enzymes, e.g. cholesterol oxidase, have also been successfully incorporated within polypyrrole films (23). Other electropolymerized films, useful for one-step enzyme entrapments, include poly-N-methylpyrrole, polyacetylene, polyaniline or polyphenol. Discriminative properties based on solute charge can offer additional selectivity improvements. As a result of electrostatic interactions, charged coatings offer a facile transport of oppositely charged ionic species while excluding co-ionic interferences. In particular, the common anionic interferences, ascorbic and uric acids, have been excluded by negatively-charged perfluorinated (Nafion) and polyester (Eastman Kodak AQ) ionomeric films (7,8,10,11,24). Effective prevention of surface fouling and immobilization of glucose oxidase is also offered by these sulfonated coatings (2526). The dispersion of the A Q polymers in aqueous media is advantageous for the enzyme casting task. Because of their hydrophobic character, Nafion and A Q ionomers possess large ion-exchange affinity for organic cations relative to simple inorganic ones. Positively charged coatings, such as the cationic polyvinylpyridine (PVP), can be used to repel cationic interferences (9). The charge (and hence the permeability) of these polyelectrolytes are strongly dependent upon the solution pH. Analogous improvements in the selectivity can be obtained for ionomer-film coated potentiometric enzyme electrodes (27). In particular, the rejection of endogeneous ammonium and potassium ions greatly facilitated the use of enzyme electrodes based on detection of liberated ammonium ions. Permselective films based on other transport properties should be suitable for amperometric biosensing. In particular, cast layers of lipids can facilitate the detection of hydrophobic compounds (at the underlying electrode) through the rejection of hydrophilic interferences (Figure 4) (12,13). Depending on the charge of the lipid (presence of anionic phosphate groups) electrostatic interactions can also contribute to the overall permeation. Various approaches to increase the mechanical stability of lipid coatings have been explored (13). The selectivity improvements were illustrated for lipid modified glucose electrodes, based on optimizing the amount of the asolectin lipid in the enzyme layer (28). In addition to ascorbic and uric acids, reduced interferences was reported for acetaminophen, tyrosine and glutathione. Loading of the lipid film with redox active lipophilic substances or the use of alkylthiol monolayer assemblies may offer further advantages. Additional improvements can be achieved through the use of multilayers (based on different overlaid films). Such combination of the properties of different films has been documented with bilayers of Nation/CA (14) and Nafion/collagen (29). The former allows selective measurements of the neurotransmitter dopamine in the presence of the slightly larger epinephrine and the anionic ascorbic acid (Figure 5). In addition to bilayers, mixed (composite) films, such as PVP/CA (75) or polypyrrole/Eastman Kodak A Q (30) layers can offer additional permselectivity advantages, such composites exhibit properties superior to those of their individual components. Also promising are sensor arrays, based on electrodes coated with



10. WANG


a

200 Secs

a

TIME

129

a

(SECS)

Figure 3. Current response of a poly(l,2-diaminobenzene)-coated electrode to hydrogen peroxide (a), ascorbic acid (b), uric acid (c), cysteine (d), and control human serum (e). Flow injection amperometric operation. (Reproduced from ref. 18. Copyright 1990 American Chemical Society.)

JD Ε CL

COMPOUND Figure 4. Permeability of a phospholipid/cholesterol film for different solutes: ascorbic acid (1), tyrosine (2), uric acid (3), acetaminophen (4), cysteine (5), desipramine (6), perphenazine (7), trimipramine (8), promethazine (9), and chlorpromazine (10). (Adapted from ref. 13.)



130


Figure 5. A Nafion/cellulose acetate bilayer coated electrode. (Reproduced from ref. 14. Copyright 1986 American Chemical Society.)


10. WANG


131

different permselective films (i.e. tuned toward different analytes), and operated in connection with statistical pattern recognition methods (31).


Future Prospects The above discussion illustrates the biosensing opportunities provided by permselective coatings. Besides electrochemical transducers, such films can greatly benefit mass, optical or thermal sensing devices. A redox polymer may be coupled to the permselective film (in a mixed or multilayer configurations) to establish electrical communication between the underlying electrode and the enzyme. Recent work by Karube's group (32) has illustrated that an efficient direct electron transfer can be obtained between the active site of glucose oxidase, entrapped with an electropolymerized layer of N-methylpolypyrrole film on gold, and the electrode. Immobilization of other bioreagents (e.g. antibodies) within permselective films is also envisioned. Further improvements will be achieved by understanding the exact mechanism by which solutes permeate through coatings (33), and by engineering novel surface microstructures with unique environments for molecular interactions and consequently molecular recognition. One promising avenue is to produce defects within monolayer coatings that recognize target molecules based on their characteristic shape. In addition to a template approach, it is possible to functionalize the polymeric network for effective molecular recognition and permselective response. Highly stable inorganic (filtering) layers may offer additional advantages. The unique structural characteristics of zeolites and clays hold a great promise in this direction. Further insights into the structural factors and fundamental interactions that govern the transport through permselective films can be gained through high-resolution surface techniques (e.g. scanning tunneling microscopy)(54). These developments should be coupled to improved reproducibility and long-term stability of the coatings. As our ability to precisely manipulate the surface microstructures (and hence to transport properties) continues to grow, one can expect new and powerful biosensing opportunities. Acknowledgments The author gratefully acknowledges the financial support of the Petroleum Research Fund, administrated by the American Chemical Society. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

Sittampalam, G.; Wilson, G.S. Anal. Chem. 1983, 55, 1608. Wang, J.; Hutchins, L.D. Anal. Chem. 1985, 57, 1536. Kuhn, L.S.; Weber, S.G.; Ismail, K.Z. Anal. Chem. 1989, 61, 303. Wang, J.; Chen, S.P.; Lin. M.S. J. Electroanal. Chem. 1989, 273, 231. Ohsaka, T.; Hirokawa, T.; Miyamoto, H.; Oyama, N. Anal. Chem. 1987, 59, 1758. De Castro E.S.; Huber, E.W.; Villarroel, D.; Galiatsatos, C.; Mark. J.E.; Heineman, W.R.; Murray, P.T. Anal. Chem. 1987, 59, 134. Szentirmay, M.N.; Martin, C.R. Anal. Chem. 1984, 56, 1898. Wang, J.; Tuzhi, P.; Golden, T. Anal. Chim. Acta 1984, 194, 129. Wang, J.; Golden, T.; Tuzhi, P. Anal. Chem. 1987, 59, 740. Wang, J.; Golden, T. Anal. Chem. 1989, 61, 1397. Fortier, G.; Beliveau, R.; Leblond, E.; Belanger, D. Anal. Lett. 1990, 23, 1607. Garcia, O.J.; Quintela, P.A.; Kaifer, A.E. Anal. Chem. 1989, 61, 979.


132



13. 14. 15. 16.

Wang. J.; Lu, Ζ. Anal. Chem. 1990, 62, 826. Wang, J.; Tuzhi, P. Anal. Chem. 1986, 58, 3257. Wang, J.; Tuzhi, P. J. Electrochem. Soc. 1987, 134, 586. Hajizadeh, K.; Halsall, H.B.; Heineman, W.R. Anal. Chim. Acta 1991, 243, 23. 17. Umana, M.; Waller, J. Anal. Chem. 1986, 58, 2979. 18. Sasso, S.V.; Pierce, R.J.; Walla, R.; Yacynych, A. Anal. Chem. 1990, 62, 1111. 19. Malitesta, C.; Palmisano, F.; Torsi, L.; Zambonin, P.G. Anal. Chem. 1990, 62, 2735. 20. Foulds, N.C.; Lowe, C.R. J. Chem. Soc. Faraday Trans. 1986, 82, 1259. 21. Kajiya, Y.; Sugai, H.; Iwakura, C.; Yoneyama, H. Anal. Chem. 1991, 63, 49. 22. Belanger, D.; Brassard, E.; Fortier, G. Anal. Chim. Acta 1990, 228, 311. 23. Kajiya, Y.; Tsuda, R.; Yoneyama, H. J. Electroanal. Chem. 1991, 301, 155. 24. Gorton, L.; Karan, H.I.; Hale, P.D.; Inagaski, Y.; Okamoto, Y.; Skotheim, T.A. Anal. Chim. Acta 1990, 223, 23. 25. Harrison, D.J.; Turner, R.F.B.; Baltes, H.P. Anal. Chem. 1988, 60, 2002. 26. Wang, J.; Leech, D.; Ozsoz, M.; Martinez, S.; Smyth, M.R. Anal. Chim. Acta 1991, 245, 139. 27. Rosario, S.A.; Cha, G.S.; Meyerhoff, M.E.; Trojanowicz, M . Anal. Chem. 1990, 62, 2418. 28. Amine, Α.; Kauffmann, J.M.; Patriarche, G.J.; Guilbault, G.G. Anal. Lett. 1989, 22, 2403. 29. Bindra, D.S.; Wilson, G.S. Anal. Chem. 1989, 61, 2566. 30. Wang, J.; Sun, Z. .; Lu, Ζ. J. Electroanal. Chem., in press. 31. Wang. J.; Rayson, G.; Lu, Z.; Wu, H. Anal. Chem. 1990, 62, 1924. 32. de Taxis du Poet, P.; Miyamoto, S.; Mukarami, T.; Kimura, J.; Karube, I. Anal. Chim. Acta 1990, 235, 255. 33. Saveant, J.M. J. Electroanal. Chem. 1991, 302, 91. 34. Yaniv, D.R.; McCormick, L.; Wang, J.; Naser, N. J. Electroanal. Chem., in press. RECEIVED November 5, 1991


Biosensors and Chemical Sensors - American Chemical Society

Recommend Documents