Chapter 14
Viologen Derivative Containing Polysiloxane as an Electron-Transfer Mediator in Amperometric Glucose Sensors 1
2,3
2,4
Hiroko I. Karan , Hsing Lin Lan , and Yoshiyuki Okamoto
Downloaded by UNIV OF ARIZONA on May 12, 2017 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/bk-1994-0556.ch014
1
Department of Physical Sciences and Computer Science, Medgar Evers College, City University of New York, Brooklyn, NY 11225 Department of Chemistry and Polymer Research Institute, Polytechnic University, 6 Metrotech Center, Brooklyn, NY 11201
2
Viologen derivatives which have higher oxidation potentials than that of FAD can mediate electrons efficiently from the F A D centers of glucose oxidase to the electrode surface. The oxidation potential of viologen derivatives are lower than those of ferrocene derivatives or quinones and hence sensors can operate at potentials where the oxidation of common interferents in biological fluid are minimized. Since viologen derivative are water soluble, insoluble viologen -containingsiloxane polymer was prepared for this study to prevent the mediator diffusing away from the electrode surface into the bulk media. Sensors constructed from this polymer and glucose oxidase efficiently mediated electron transfer and showed linear response in clinically relevant ranges of glucose concentrations. Amperometric glucose sensors based on glucose oxidase and non-physiological redox mediators use the following mechanism to shuttle electrons between the reduced flavin adenine dinucleotide center of the enzyme (FADH2) and the electrode: glucose + GO (FAD) GO(FADH ) 2
+ 2M
> gluconolactone + >
o x
2*4^
_> 2 Μ
ο χ
GO (FAD) + 2e-
GCKFADH2)
+ 2M d re
+
2H+
(at the electrode)
GO (FAD) and GO (FADH ) represent the oxidized and reduced forms of glucose oxidase, respectively. The mediator M ^ / M ^ is assumed to be a one-electron 2
3
This chapter is based on the Ph.D. thesis of Hsing Lin Lan, written in 1992. Current address: Institute of Biomedical Engineering, National Yang Ming Medical College, Taipei 11221, Taiwan Corresponding author
4
0097-6156/94/0556-0169$08.00/0 © 1994 American Chemical Society
Usmani and Akmal; Diagnostic Biosensor Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
Downloaded by UNIV OF ARIZONA on May 12, 2017 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/bk-1994-0556.ch014
170
DIAGNOSTIC BIOSENSOR POLYMERS
couple. Sensors based on ferrocene/ferricinium redox couple (7-5 ), various quinones (4, 5 ) , tetratWafulvalene-tetracyancquinodimethane (6 - 8), tungsten complexes (9) and rhutenium complexes (10) have recently been reported. These mediators have fairly high oxidation potentials (e.g., +100 to +400 mV vs. the saturated calomel electrode (SCE) ), however, and must be operated at potentials where several common interferents in biological fluids, such as ascorbic acid, uric acid and acetaminophen, are directly oxidized at the electrode (11, 12). This additional response current can make it difficult to accurately determine the glucose concentration in a sample. To minimize this interfèrent effect several research groups have modified the surface of the glucose sensors with anionic polymer coating (75, 14 ), which can partially exclude negatively charged as cor bate and urate ions from the electrode surface. More recently, some research groups preoxidized the samples by applying peroxidase films to the electrode surfaces to eliminate interferents (75 ). We have previously reported an effective method to avoid these elecrtrooxidizable consituents by using viologen (4,4'-bipyridyl) derivatives as mediators (76, 7 7 ). These mediators can be synthetically tailored to lower their oxidation potentials sufficiently to eleminate the electrooxidation of interferents. However, viologen derivatives are water soluble and sensors based on these mediators suffer the inherent drawback that soluble mediating species diffuse away from the electrode surface into the bulk solution. This precludes the use of these sensors for on line systems or in vivo systems. Recently, studies have included systems in which mediating redox moieties are covalently attached to polymers such as polypyrrole (18-20 ), poly(vinylpyridine) (21-23 ), and polysiloxane (24-27 ). Here, we have synthesized a viologen- containing siloxane polymer, by covalently attaching a viologen derivative to siloxane copolymer. The electrochemical properties of the polymer were investigated and comparison of electron mediation efficiency among previously reported viologen derivatives and the viologen-containing polysiloxane was made.
Experimental Materials Methylhydrc(55%)-dimethyl(45%)siloxane copolymer was obtained from Petrarch Systems (Bristol, P A ) . 4-Nitrobenzyl chloride, 4,4'-bipyridyl and chloromethyl-styrene were obtained from Aldrich (Milwaukee, WI). Graphite powder (product No. 50870) and paraffin oil (product No. 76235) were obtained from Fluka (Ronkonkoma, N Y ) . Glucose oxidase (E.C. 1.1.3.4, type VII, from Aspergillus Niger) and glucose were obtained from Sigma (St. Louis, MO). Glucose solutions were prepared by dissolving appropriate amounts in 0.1M phosphate- 0.1M KC1 buffer (FH 7.0); the glucose was allowed to reach mutarotational equilibrium before use (ca. 24 hr). A l l other chemicals were of reagent grade and were used as received. Synthesis of a viologen-containing polymer N-4-Nitrobenzylviologen was prepared by reacting 1:1 molar ratio of 4,4'bipyridyl and p-nitrobenzyl chloride in acetonitrile; the solution was refluxed for 40 hr in a nitrogen atmosphere. The resulting precipitate was removed by filtration, purified by recrystallization from methanol-acetonitrile and dried under
Usmani and Akmal; Diagnostic Biosensor Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
14. KARAN ET AL.
Viologen Derivative Containing Polysiloxane
171
vacuum at 60 °C. The viologen-containing polysiloxane was prepared by reacting p-chloromethylstyrene (7.6g, 0.05 mole) with methylhydro(55%)-dimethyl(45%)siloxane copolymer (6.1g) in the presence of H 2 P t C l 6 and trace amount of hydroquinone in T H F under nitrogen atmosphere at reflux temperature. After the hydrosilation reaction, the resulting viscous polymer (2.2g) was reacted with 1.2g of N-^nitrobenzylviologen in 150 mL DMSO at 90 C for 24 hr (Scheme I). The polymer obtained was precipitated by pouring the reaction mixture into chloroform and then purified by washing with water. The monomer and polymer synthesized were characterized by IR and N M R measurements.
Downloaded by UNIV OF ARIZONA on May 12, 2017 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/bk-1994-0556.ch014
e
Electrode construction A modified carbon paste for the sensors was made by thoroughly mixing 50 mg of graphite powder with a measured amount of the viologen-containing polymer, the molar amount of viologen moiety was the same for all electrodes (36 μτηοΐ of viologen per gram of graphite powder). 5 mg of glucose oxidase and 10 /
