BIOTECHNOLOGY TECHNIQUES Volme 9 No.4 (April 1995) pp.277-282 Received as revised 23rd February

THE EFFECT OF CROSSLINK DENSITY ON PERMEABILITY IN BIOSENSORS:

AN UNSTEADY-STATE APPROACH

Mehmet Mutlu* and Selma Mutlu

Hacettepe University, Food* and Chemical Engineering Departments, Beytepe Campus, 06532 Ankara, TijRKiY E

Summary

The influence of cross-linker density (2%, 5% and 10% of glutaraldehyde) on the permeability, P, of the enzyme layer (267.7, 203.5 and 146.0 pm/s respectively for 1251- cortisol-histamine conjugate, CHC (MW: 651) as tracer) in Clark enzyme electrode is reported. A diffusion chamber technique based on unsteady-state analysis for the estimation of permeabilities of radiolabelled molecules through enzyme layer is performed.

Introduction

Sandwich type amperometric enzyme electrodes are amongst the most widely studied electrodes for diagnostic medicine. A typical sandwich type enzyme electrode consists of three major layers, i.e., outer membrane, enzyme layer and inner membrane. The outer membrane, where the analyte diffuses through should have diffusion-limiting property which provides a means of extending linear range through their reduction in local substrate concentration while maintaining sufficient oxygen for the enzymatic reaction. In the middle, the enzyme layer accepts the analyte and necessary oxygen to achieve the enzyme-substrate reaction and to produce electrochemically detectable species. The immobilization of the enzymes is generally maintained by gel-entrapment. Bifunctional (especially glutaraldehyde) or multifunctional reagents that induced inter molecular crosslinking can be used to immobilize enzymes. The inner membrane with pennselective property prevents the interference of the other electrochemical species on electrode (Vadgama, 1986; T umer et al., 1987; Mutlu et al.. 1994). The general mechanism and the function of the layers in an enzyme electrode is given in Figure I.

Sensitivity, response time, linearity and selectivity can be defined as basic characteristics of biosensors. The first two modes i.e. sensitivity and response time, are particularly influenced by mass transfer limitations by the permeability and the thickness of the membrane. For the response time, transient current in the sensor response depends not only on kinetics parameters of the electrochemical reaction (Mutlu el al., 1991) but also essential on diffusion conditions. Therefore, the determination of permeability

277

coefficients to define the transport of product and substrate species through membranes consisting of enzyme layers is an important area of interest in biosensor development. Several studies for the determination of diffusion and partition coefficients with stationary and rotating disc electrodes (Maresse et al., 1987) as well as diffusion chamber methods (Hannour and Stephanapulous, 1986; Desai et al., 1992; Koochaki et al., 1993) have been previously described.

7 silver &bode

Platinum anode

I

Blood interferences

Red blood cells

Inner membrane Outer membrane

I Immobilized

glucose oxidase

Figure 1. Functions of membrane layers in a sandwich type enzyme electrode.

In this study, we report, simplified measurement of enzyme layer permeabilities for layers formed with different concentrations of crosslinking agents which causes a series of diffusion barriers for substrate and products.

Principal of permeability coefficient measurement

A conventional diffusion chamber was used to follow the mass transport of radiolabelled model solutes across membrane laminates (Fig. 2). An unsteady state mass balance between two chambers is utilized. This unsteady approach requires that both volumes of the chambers be known and maintained constant. The unsteady state mass balance for chamber A is:

vA-$-- - dCA- PA(Cu-CA)

Where, VA : the volume of chamber A, (cm3) P : permeability, (cm/s) A : effective mass transfer area (cm2) CA and C, : concentration of tracer in chamber A and B at t (g/cm3) t : time, s

278

Assuming a closed system where no mass enters or leaves, and m is constant;

V,C,-V,C,=m (2)

Where, m is total amount of tracer (g). Integration of equation (I) after substitution with (2) gives;

(3)

where CA0 and CnO are the initial concentrations.

This equation was used to calculate the permeabilities of tracers by measuring the concentration of tracer at compartment B at different times,

Motor Drive

Figure 2. Diffusion Chamber

MATERIALS AND METHODS

Reagents

Polycarbonate (PC) membranes with nominal pore size 0.05 pm (rated by manufacturer) were supplied by Poretics (USA). The enzyme, i.e .) glucose oxidase (E.C. 1. I-3.4. from Aspergillus niger: 292 IU rng-l protein) was purchased from Sigma (UK}. Glutaraldehyde (25% aqueous solution), bovine serum albumin (fraction V), solvents, buffer components and other standard reagents were obtained from Sigma, UK. Nai2-51 which is used to label the model molecules were obtained from Amersham, WK. Cortisol- histamine conjugate, CHC, (MW: 651) were radiolabelled with f251 by the method described in literature ( l*sI-CHC) (Atkins and Richards, 1967). Nal251 (MW: 148.5) is directly used to be the second tracer in the permeability experiments.

Preparation of Membrane/Enzyme/Membrane Laminates

The enzyme layer was prepared by crosslinking of glucose oxidase (GOD) with glutaraldehyde, in which GOD was diluted (co-crosslinked) by mixing with bovine serum albumin (BSA). In a typical procedure 30 mg of GOD was mixed with a 200 mg BSA in 1 ml of buffer solution for each enzyme layer. A 6 ~1 volume of this mixtures in three different combinations was then rapidly mixed with 3 ~1 of glutaraldehyde (2%, 5% and lo%, v/v) in a Gilson pipette tip and transferred between two polycarbonate (PC) membranes. This sandwich was then compressed between two glass slides and hold under hand pressure for 5 minutes. The glass slides were prised apart and the membrane/enzyme/membrane laminate was left to dry at room temperature for a further 5 minutes, washed with buffer and then placed into the diffusion chamber.

Methodology

Solute mass transfer measurements across PC/PC membranes or PC/immobilized enzyme/PC laminates to assess their permeability were performed at 201tl “C in a classical diffusion chamber apparatus consisting of two chambers (Fig 2). Both chambers had a volume of 170 ml and were separated by two stainless steel discs and two sealing rubber O-rings clamped together to hold the membrane of interest with a cross sectional area available for mass transport of 7.07 cm2. The solute of interest was added to one chamber and mass transfer was than determined by measuring solute concentrations in both chambers at periodic intervals. The concentrations of the tracers were determined by a gamma scintillation counter (Berthold, BF 5300, Germany).

Results and Discussion

The mass transfer resistance of individual membranes in the laminate could be summed in series, so the actual permeability, P, of the enzyme layer was calculated from the equation;

1 1 I h+zyme Layer = PSandwch

_- ~UPC

(4)

The actual permeabilities of different types of laminates are given in Table 1.

In the first part of the permeability calculations, the mass transfer data of two polycarbonate membranes (PC+PC) were evaluated in the absence of enzyme layer. The results are displayed in the first part of the Table 1. Than, the total permeability of the sandwich type electrode membrane was calculated by utilizing the equation 3. in the presence of enzyme layer. The absolute permeability of the enzyme layer formed with different concentrations of cross-linking agent was calculated by using the equation 4. The total results are given in the Table 1.

Table 1. Permeabilities of different tracers in enzyme layer formed with various

crosslinking agent concentrations

COMPOSITE Permeability, P, (pm/s)

125LCHC 125I-Na

Polycarbonate (PC) + Polycarbonate (PC) 76.5k2.6 77.3*1.8

PC + Enzyme layer (2% GIutaraldehyde) + PC

PC + Enzyme layer (5% Glutaraldehyde) + PC

PC +Enzyme layer (10% Glutaraldehyde)+ PC

59.51t1.2 6O.lk2.0

55.6~t2.3 57.1a3.1

50.2~~0.8 51.3*1.4

Enzyme layer with 2% Glutaraidehyde

Enzyme layer with 5% Glutaraldehyde

Enzyme layer with 10% Glutaraldehyde

267.7k21.1 270.11t19.6

203.5k18.9 218.5+21.1

1460~10.4 152.5&l 2.8

Table 1 clearly shows that, increase in crosslinking agent concentration decreases the

permeability of the enzyme layer as well as that of the composite. The difference in the

molecular weights of the tracers that we used in this series of experiments has not

significantly affected the permeability.

It is also to be noted that, the mass transfer and kinetic limitations in the enzyme layer

where the enzymatic reaction occurs in a biosensor, is directly affected the electrodes’

performance, in terms of response time and sensitivity. The increase in the crosslink

density of the enzyme layer in electrode, decreases the permeability of the substrate to be

reacted with enzyme as well as decreases the permeability of the product (electroactive

compound) which reacts on the electrode to create the response of the electrode (Fig I .).

In this study, in case of utilizing the t2sl-CHC as model molecule, to increase the cross

linker concentration for 5 times (from 2% to 10%) causes a decrease about 15% in the

total permeability of the laminate, and approximately 46% decrease in the particular

permeability of the enzyme layer.

It can be concluded that, the studies to explain the hindered mechanism of transport

through such membranes carries a vital importance to improve the performance of the

enzyme electrodes.

281

Acknowledgement

Dr. Mehmet Mutlu gratefully acknowledges technical and financial support from International Atomic Energy Agency (IAEA) (Grant No: C6rTUR/88 13).

References

1. Vadgama, P., (1986), In: Clinical Biochemistry Nearer the Patient II; Marks, V. and Alberti, K.G.M.M. Eds.; p. 68-85, Bailliere Tindall.

2. Turner, A.P.F., Karube, I. and Wilson, G. S., (1987), Biosensors: Fundamentals and Applications, Oxford University Press.

3. Mutlu, S., Mutlu, M., Vadgama, P., Pigkin, E., (1994), American Chemical Society (ACS) Symposium Series, 5 5 6, 7 l-83.

4. Mutlu, M., Mutlu, S., Vadgama, P., (1991), In: 1991 IChemE Research Event; Kenny, C.N. Darton, R. Davidson, P.J. Seaton, N.A. and Stitt E.H. Eds.; Institute of Chemical Engineers, Rugby, U.K.pp. 233-234.

5. Maresse, CA., Miyawaki, 0. and Wingard, L.B., (1987), Ad. Chem., 59, 248- 253.

6. Hannour, B.J.M. and Stephanapulous G., (1986), Biozech. Bioeng., 28, 829-735. 7. Desai, M.A., Mutlu, M., Vadgama, P., (1992), Experientia , 48, 22-26. 8. Koochaki, Z., Mutlu, M., Vadgama, P., (1993), 1. of Membrane Science , 7 6

(2+3), 261-268. 9. Atkins, H.L., Richards, P., (1%7), United States Atomic Energy Committee

Publication, BNL- 1183 1.

282