Research Article  Open Access
Analytical Expressions of Concentrations of Substrate and Hydroquinone in an Amperometric Glucose Biosensor
Abstract
The theoretical model for an amperometric glucose biosensor is discussed. In this model glucose oxidase enzyme is immobilized in conducting polypyrrole. This model contains a nonlinear term related to enzyme reaction kinetics. He’s homotopy perturbation method is used to find the approximate analytical solutions of coupled nonlinear reaction diffusion equations. A closedform expression of substrate and mediator concentration under nonsteadystate conditions is obtained. A comparison of the analytical approximation and numerical simulation is also presented. An agreement between analytical expressions and numerical results is observed.
1. Introduction
Since the second half of the last century, numerous efforts have been devoted to the development of insoluble immobilized enzymes for a variety of applications [1]. These applications can clearly benefit from use of the immobilized enzymes rather than the soluble counterparts, for instance as reusable heterogeneous biocatalysts, with the aim of reducing production costs by efficient recycling and control of the process [2], as stable and reusable devices for analytical and medical applications [3–9], as selective adsorbents for purification of proteins and enzymes [10], as fundamental tools for solidphase protein chemistry [11, 12], and as effective microdevices for controlled release of protein drugs [13].
Immobilized enzymes are becoming increasingly popular as reusable, selective analytical chemical reagents in solidphase flowthrough reactors, as membranes in sensors, and as films in dry reagent kits. The attractions of immobilized enzymes from an analytical standpoint are primarily their reusability, and hence cost saving, and the greater efficiency and control of their catalytic activity [14] (e.g., potentially longer halflives, predictable decay rates and more efficient multistep reactions).
The immobilization of enzymes in conducting polymer [15] during electropolymerization step has proved to be well suited to the preparation of biosensors [16–19]. This method is simple and easy to control. Another important advantage of this immobilization technique is the possibility of entrapping the mediator in the polymer as a dopant anion [20–23] or by covalent fixation on the pyrrole monomer [24]. Bartlett and Whitaker [25] have already presented a theoretical model for an amperometric polypyrrole + glucose oxidase (PPY + GOD) electrode. In their model PPY was considered as an insulating polymer, and the reduced mediator H_{2}O_{2} was oxidized at the metallic surface after diffusion in the polymer. Marchesiello and Geniès [26] obtained the analytical expressions of concentrations of substrate and benzoquinone for low substrate and high benzoquinone concentration compared with respective Michaelis constants.
To my knowledge no rigorous analytical expressions of concentrations of substrate and mediator of ampereometric glucose under nonsteadystate conditions for all values parameters , and have been reported. These parameters are defined in (6). The purpose of this communication is to derive approximate analytical expressions for the nonsteadystate concentrations and current using Homotopy perturbation method.
2. Mathematical Formulation and Analysis of the Problems
2.1. Mathematical Formulation
Building upon earlier work, Marchesiello and Geniès [26] presented a concise discussion and derivation of mass transport nonlinear equations in glucose biosensor, which is summarized briefly below. Figure 1 is a schematic representation of the PPY + GOD electrode working as a glucose sensor. The different steps which lead to the electrocatalytic current are as follows. (i) Glucose and benzoquinone diffusion from bulk of the solution to the electrode, (ii) glucose and benzoquinone diffusion into the conducting polymer, and (iii) the enzymatic reaction between glucose and benzoquinone: (iv) The oxidation of hydroquinone: .
As shown in Figure 1, this reaction occurs on the surface of the platinum electrode after diffusion in the polymer and in the conducting polymer itself. The rate of the enzymatic reaction between glucose and benzoquinone follows a pingpong type mechanism [27] where is the turnover number for GOD, and are the Michaelis constants for glucose and benzoquinone, respectively, and , , and are the enzyme, benzoquinone, and glucose concentrations in the film. The third term is the oxidation term. The rate of the electrochemical reaction in the conducting polymer can be written as , where is the chargetransfer constant for the oxidation of in the conducting polymer. It depends on the electrode potential. is the PPY specific surface and it was estimated to be in the range 5–50 for a PPY electrode deposited film [28]. The differential equations describing the concentrations of and at steady state are as follows [26]: These nonlinear reaction/diffusion equations are solved for the following boundary conditions. The boundary conditions are as follows [27]: The total electrocatalytic current is the sum of two currents. The current For oxidation on the platinum electrode, and where is the film thickness of the reaction layer. We assume that there is no concentration polarization of and in the solution and that the partition coefficients are equal to unity. is the concentration of glucose in the bulk of the solution.
2.2. Normalized Form
By introducing the following set of nondimensional variables,
The dimensionless parameters compares the enzymatic reaction rate with the substrate diffusion in the polymer. The dimensionless parameter compares the oxidation rate in the conducting polymer with diffusion in the polymer. For electrochemically inert polymer, has low value and for electrochemically active polymer, has high value. The parameter is a dimensionless constant.
The coupled reaction/diffusion equations (2) take the following normalized form: where and are the dimensionless concentration of and .
The transformed boundary conditions are The dimensionless form of the current is given by
2.3. Analytical Expressions of Concentrations of Substrate and the Mediator under SteadyState Condition
In recent days, Homotopy perturbation method (HPM) is often employed to solve several analytical problems. In addition, several groups demonstrated the efficiency and suitability of the HPM for solving nonlinear equations and other electrochemical problems [29–32]. Mousa and Ragab [33] used HPM to solve the Lighthill equation, the Duffing equation [34], and the Blasius equation [35]. HPM has also been used to solve nonlinear boundary value problems [33], integral equation [36–38], EmdenFlower type equations [39], and several other problems. Detailed derivation of the concentrations using Homotopy perturbation method is described in Appendix 5. As a result, we can obtain the dimensionless concentrations of substrate and mediator as follows: The dimensionless current and are obtained using (9) and (11): The electrocatalytic current is the sum of two currents and
2.4. FirstOrder Catalytic Kinetics
In this case, or . Now (7) reduces to the following forms: The solutions of (15) are given as follows:
2.5. ZeroOrder Catalytic Kinetics
In this case, or . Now (7) reduce to the following forms: The solution of (21) is given as follows: The current is given by
3. Numerical Simulation
The nonlinear differential equations (7) are also solved numerically. The function pdepe in Matlab/Scilab software is used to solve boundary value problems (BVPs) for partial differential equations. The numerical solution is compared with our analytical results. The Matlab/Scilab program is also given in Table 1. Upon comparison it is evident that both results gives satisfactory agreement.

4. Discussion
Figures 2(a)–2(c) represents the normalized nonsteady state concentration of substrate versus dimensionless distance , for different values of dimensionless reaction diffusion parameter and . From this figure, it is evident that the concentration of substrate increases when decreases and increases. From this Figures 1(a)–1(c), it is evident that the value of concentration is uniform when .
(a)
(b)
(c)
Figures 3(a)–3(c) shows the concentration of mediator versus dimensionless distance , for various values of dimensionless diffusion parameter , and . From this figure, it is inferred that the value of the concentration of mediator decreases abruptly when decreases and and increases. Also the concentration of mediator reaches the maximum value at the middle of membrane for all values of , , and .
(a)
(b)
(c)
The normalized steady state current and as the function of the dimensionless parameter is given in Figures 4(a)–4(d). From this figures, it is obvious that the values of the current decreases slowly and reaches constant value when . Also values of the current decreases when increases.
(a)
(b)
(c)
(d)
In Figures 5(a)5(b), profile of the normalized current is shown. The current were computed for various values of , , and . The curves are plotted using (12). The value of the current increases when increases. The normalized nonsteadystate current for diverse values of dimensionless diffusion parameter is plotted in Figures 6(a)–6(c).
(a)
(b)
(a)
(b)
(c)
(a)
(b)
(c)
5. Conclusions
The steady state nonlinear reaction/diffusion equations in an amperometric glucose sensor have been solved analytically. An approximate analytical expression for the concentrations and current for an amperometric glucose sensor are obtained by using the Homotopy perturbation method. The primary result of this work is simple approximate calculation of concentration profiles and current for all values of fundamental parameters and . The analytical results obtained can be used for the optimization of the design of glucose sensor.
Appendix
Approximate Analytical Expression of the Normalized Concentration of Substrate and H_{2}Q
Using Homotopy perturbation method, we construct a homotopy for (7) as follows: The approximate solution of (A.1) is given by Substituting (A.3) into (A.1) and comparing the coefficients of like powers of : The boundary conditions for the above equations are Using this boundary conditions the solution of (A.4) becomes as follows: Adding (A.6) we get (10) in the text
The approximate solution of (A.2) is given by Substituting (A.3) and (A.2) and arranging the coefficients of like powers of , we have The boundary conditions for the above equations are as follows Upon solving (A.7) and (A.8), and using this boundary condition, we get Adding (A.11) we get, (11) in the text.
Acknowledgemnts
This work was supported by the University Grants Commission (F. No. 3958/2010(SR)), New Delhi, India and Council of Scientific and Industrial Research (CSIR No.: 01(2442)/10/EMRII), New Delhi, India. The authors are thankful to Dr. R. Murali, The Principal, The Madura College, Madurai and The Secretary, Madura College Board, Madurai for their encouragement.
Supplementary Materials
Matlab/Scilab program to find the numerical solution of Eqn. (7)
References
 H. Silman and E. Katchalski, “Waterinsoluble derivatives of enzymes, antigens and antibodies,” Annual Review of Biochemistry, vol. 35, pp. 837–877, 1966. View at: Google Scholar
 E. J. Vandamme, “Peptide antibiotic production through immobilized biocatalyst technology,” Enzyme and Microbial Technology, vol. 5, no. 6, pp. 403–416, 1983. View at: Google Scholar
 B. Schulze and M. G. Wubbolts, “Biocatalysis for industrial production of fine chemicals,” Current Opinion in Biotechnology, vol. 10, pp. 609–615, 1999. View at: Google Scholar
 L. C. Clark and C. Lyons, “Electrode systems for continuous monitoring in cardiovascular surgery,” Annals of the New York Academy of Sciences, vol. 102, pp. 29–45, 1962. View at: Google Scholar
 D. H. Campbell, F. L. Luescher, and L. S. Lerman, “Immologic adsorbents. I. Isolation of antibody by means of a celluloseprotein antigen,” Proceedings of the National Academy of Sciences, vol. 37, pp. 575–578, 1951. View at: Google Scholar
 S. Watanabe, Y. Shimizu, T. Teramatsu, T. Murachi, and T. Hino, “Application of immobilized enzymes for biomaterials used in surgery,” Methods in Enzymology, vol. 137, pp. 545–551, 1988. View at: Google Scholar
 T. M. S. Chang, Medical Application of Immobilised Enzymes and Proteins, vol. 12, Plenum Press, New York, NY, USA, 1977.
 M. D. Klein and R. Langer, “Immobilised enzymes in clinical medicines: an emerging approaches to new drug therapies,” Trends in Biotechnology, vol. 4, pp. 179–186, 1986. View at: Google Scholar
 L. J. Kircka and G. H. G. Thorpe, “Immobilised enzymes in analysis,” Trends in Biotechnology, vol. 4, pp. 253–258, 1986. View at: Google Scholar
 B. R. Dunlap, Immobilised Chemicals and Affinity Chromatography, Plenum Press, New York, NY, USA, 1974.
 G. F. Bickerstaff, “Application of immobilised enzymes to fundamental studies on enzyme structure and function,” in Topics in Enzyme and Fermentation Biotechnology, A. Wiseman, Ed., pp. 162–201, Ellis Horwood, Chichester, UK, 1984. View at: Google Scholar
 K. Martinek and V. V. Mozhaev, “Immobilization of enzymes: an approach to fundamental studies in biochemistry,” Advances in Enzymology and Related Areas of Molecular Biology, vol. 57, pp. 179–249, 1985. View at: Google Scholar
 C. Cristallini, L. Lazzeri, M. G. Cascone, G. Polacco, D. Lupinacci, and N. Barbani, “Enzymebased bioartificial polymeric materials: the αamylasepoly(vinyl alcohol) system,” Polymer International, vol. 44, no. 4, pp. 510–516, 1997. View at: Google Scholar
 P. J. Worsfold, “Classification and chemical characteristics of immobilized enzymes,” Pure and Applied Chemistry, vol. 67, no. 4, pp. 597–600, 1995. View at: Google Scholar
 N. C. Foulds and C. R. Lowe, “Enzyme entrapment in electrically conducting polymers. Immobilisation of glucose oxidase in polypyrrole and its application in amperometric glucose sensors,” Journal of the Chemical Society, Faraday Transactions 1, vol. 82, no. 4, pp. 1259–1264, 1986. View at: Publisher Site  Google Scholar
 P. N. Bartlett and R. G. Whitaker, “Electrochemical immobilisation of enzymes. Part II. Glucose oxidase immobilised in polyNmethylpyrrole,” Journal of Electroanalytical Chemistry, vol. 224, no. 12, pp. 37–48, 1987. View at: Google Scholar
 D. Bélanger, J. Nadreau, and G. Fortier, “Electrochemistry of the polypyrrole glucose oxidase electrode,” Journal of Electroanalytical Chemistry, vol. 274, no. 12, pp. 143–155, 1989. View at: Google Scholar
 P. Janda and J. Weber, “Quinonemediated glucose oxidase electrode with the enzyme immobilized in polypyrrole,” Journal of Electroanalytical Chemistry, vol. 300, no. 12, pp. 119–127, 1991. View at: Google Scholar
 M. Marchesiello and E. M. Geniés, “Glucose sensor: polypyrroleglucose oxidase elecrtode in the presence of pbenzoquinone,” Electrochimica Acta, vol. 37, no. 11, pp. 1987–1992, 1992. View at: Google Scholar
 Y. Kajiya, H. Sugai, C. Iwakura, and H. Yoneyama, “Glucose sensitivity of polypyrrole films containing immobilized glucose oxidase and hydroquinonesulfonate lons,” Analytical Chemistry, vol. 63, no. 1, pp. 49–54, 1991. View at: Google Scholar
 P. N. Bartlett, Z. Ah, and V. E. Field, “Electrochemical immobilisation of enzymes,” Journal of the Chemical Society, Faraday Transactions, vol. 88, no. 18, p. 2677, 1992. View at: Google Scholar
 Y. Kajiya, R. Tsuda, and H. Yoneyama, “Conferment of cholesterol sensitivity on polypyrrole films by immobilization of cholesterol oxidase and ferrocenecarboxylate ions,” Journal of Electroanalytical Chemistry, vol. 301, no. 12, pp. 155–164, 1991. View at: Google Scholar
 Y. Kajiya, H. Matsumoto, and H. Yoneyama, “Glucose sensitivity of poly(pyrrole) films containing immobilized glucose dehydrogenase, nicotinamide adenine dinucleotide, and βnaphthoquinonesulphonate ions,” Journal of Electroanalytical Chemistry, vol. 319, no. 12, pp. 185–194, 1991. View at: Google Scholar
 N. C. Foulds and C. R. Lowe, “Immobilization of glucose oxidase in ferrocenemodified pyrrole polymers,” Analytical Chemistry, vol. 60, no. 22, pp. 2473–2478, 1988. View at: Google Scholar
 P. N. Bartlett and R. G. Whitaker, “Electrochemical immobilisation of enzymes: part II. Glucose oxidase immobilised in polyNmethylpyrrole,” Journal of Electroanalytical Chemistry, vol. 224, no. 12, pp. 37–48, 1987. View at: Google Scholar
 M. Marchesiello and E. Geniès, “A theoretical model for an amperometric glucose sensor using polypyrrole as the immobilization matrix,” Journal of Electroanalytical Chemistry, vol. 358, no. 12, pp. 35–48, 1993. View at: Google Scholar
 C. Bourdillon, C. Hervagault, and D. Thomas, “Increase in operational stability of immobilized glucose oxidase by the use of an artificial cosubstrate,” Biotechnology and Bioengineering, vol. 27, no. 11, pp. 1619–1622, 1985. View at: Google Scholar
 H. Naarmann, W.R. Salaneck, D.T. Clark, and E. J. Samuelsen, “Science and applications of conducting polymers,” in Proceedings of the 6th Europhysics Industrial Workshop, Adam Hiler, Lofthus, Norway, May 1990. View at: Google Scholar
 E. M. Geniès, M. Marchesiello, and G. Bidan, “Preparation and properties of polypyrrole made in the presence of biological buffers,” Electrochimica Acta, vol. 37, no. 6, pp. 1015–1020, 1992. View at: Google Scholar
 Q. K. Ghori, M. Ahmed, and A. M. Siddiqui, “Application of homotopy perturbation method to squeezing flow of a newtonian fluid,” International Journal of Nonlinear Sciences and Numerical Simulation, vol. 8, no. 2, pp. 179–184, 2007. View at: Google Scholar
 T. Öziş and A. Yildirim, “A comparative study of he's homotopy perturbation method for determining frequencyamplitude relation of a nonlinear oscillator with discontinuities,” International Journal of Nonlinear Sciences and Numerical Simulation, vol. 8, no. 2, pp. 243–248, 2007. View at: Google Scholar
 S. J. Li and Y. X. Liu, “An improved approach to nonlinear dynamical system identification using PID neural networks,” International Journal of Nonlinear Sciences and Numerical Simulation, vol. 7, no. 2, pp. 177–182, 2006. View at: Google Scholar
 M. M. Mousa and S. F. Ragab, “Application of the homotopy perturbation method to linear and nonlinear schrödinger equations,” Z. Naturforsch, vol. 63a, pp. 140–144, 2008. View at: Google Scholar
 J. H. He, “Homotopy perturbation technique,” Computer Methods in Applied Mechanics and Engineering, vol. 178, no. 34, pp. 257–262, 1999. View at: Google Scholar
 J. H. He, “Homotopy perturbation method: a new nonlinear analytical technique,” Applied Mathematics and Computation, vol. 135, no. 1, pp. 73–79, 2003. View at: Publisher Site  Google Scholar
 J. H. He, “A simple perturbation approach to Blasius equation,” Applied Mathematics and Computation, vol. 140, no. 23, pp. 217–222, 2003. View at: Publisher Site  Google Scholar
 J. H. He, “Homotopy perturbation method for solving boundary value problems,” Physics Letters A, vol. 350, no. 12, pp. 87–88, 2006. View at: Publisher Site  Google Scholar
 M. Ghasemi, M. Tavassoli kajani, and E. Babolian, “Numerical solutions of the nonlinear integrodifferential equations: waveletGalerkin method and homotopy perturbation method,” Applied Mathematics and Computation, vol. 188, no. 1, pp. 450–455, 2007. View at: Publisher Site  Google Scholar
 Z. Odibat and S. Momani, “A reliable treatment of homotopy perturbation method for KleinGordon equations,” Physics Letters A, vol. 365, no. 56, pp. 351–357, 2007. View at: Publisher Site  Google Scholar
Copyright
Copyright © 2013 M. Uma Maheswari and L. Rajendran. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.