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Polymer International 47 (1998) 331È334
DC Conduction Studies in Amylase
Vinay Mishra, S. C. Thomas & R. Nath*
Department of Physics, Dr H. S. Gour Vishwavidyalaya, Sagar (MP), India
(Received 18 June 1997 ; revised version received 5 January 1998 ; accepted 16 April 1998)
Abstract : DC conductivity measurements have been carried out on an enzyme
amylase (EC 3.2.2.1) in solid pellet form, provided with silver-coated electrodes.
The results are presented in the form of currentÈvoltage characteristics for the
temperature range 300È360 K. The analysis of the results obtained by plotting
the temperature dependence of current density, PooleÈFrenkel plots, Schottky
plots, Richardson plots and Arrhenius plots shows that the SchottkyÈRichardson
mechanism is responsible for the observed d.c. conduction in amylase. Furthermore, the presence of Schottky barriers at the metal electrodeÈinsulator interface
suggests that, by using silver electrodes, satisfactory contacts can be achieved for
electrical investigations on biomaterials. ( 1998 Society of Chemical Industry
Polym. Int. 47, 331È334 (1998)
Key words : d.c. conduction ; IÈV characteristics ; SchottkyÈRichardson mechanism ; PooleÈFrenkel mechanism
conduction and dielectric studies on amylase (EC
3.2.2.1).5,6 In the present paper, the results of d.c. conductivity measurements are reported.
INTRODUCTION
The electrical properties of polymeric systems have been
extensively investigated over the last few decades.
However, it is only in recent years that attention
focused on the study of electrical properties of biopolymers because these have been found to play a key
role in the action of biopolymers in many biological
phenomena.1 The study of the electrical properties of
enzymes, biocatalysts, is of special interest as it may be
helpful in understanding their mechanisms of action,
which are still not clearly known.2 The study of electrical conduction is of considerable signiÐcance from two
major points of view : Ðrstly for its own sake, because
charge-transfer characteristics are of fundamental interest ; secondly for the information such studies can
provide on the nature of electrical contacts, which may
have a great inÑuence on the measured electrical
properties.3,4 A systematic study of the electrical
properties of enzymes has been undertaken in our
laboratories and we have reported the results of a.c.
EXPERIMENTAL
The enzyme used in the present investigation was procured from Central Drug House (CDH), India, in
powder form. The purity of the material was established
by thin layer chromatography and column chromatography. The d.c. conductivity measurements were carried
out on amylase in the form of compressed pellets form
(diameter 1 cm, thickness 0É5 mm) coated with silver
electrodes. The Aplab medium voltage d.c. power
supply (model no. 7332 ; range up to 600 V) was used to
apply the desired d.c. electric Ðeld, and the steady-state
current was measured using a Keithley electrometer
(model no. 610C). Measurements were made at thermostatically controlled temperatures in the range 300È
360 K. A d.c. Ðeld in the range 0É9È6É6 kV cm~1 was
applied and the corresponding value of the steady-state
current was recorded at each Ðxed temperature.
* To whom all correspondence should be addressed.
Contract/grant sponsor : Council of ScientiÐc and Industrial
Research, India.
331
( 1998 Society of Chemical Industry. Polymer International 0959È8103/98/$17.50
Printed in Great Britain
V . Mishra, S. C. T homas, R. Nath
332
RESULTS AND DISCUSSION
The currentÈvoltage characteristics of amylase are
shown in Fig. 1 in the form of logÈlog plots of current
density (J) and Ðeld (E). As is evident from the Ðgure,
the current density does not follow a power law
J \ KEm, where K and m are constants (for ohmic conduction m \ 1 and for space-charge-limited currents
m \ 2). Thus the observed behaviour of currentÈvoltage
characteristics rules out the possibility of both ohmic
and space-charge-limited conduction.7,8 The temperature dependence of current density, as presented in
the form of ln J versus temperature plots (Fig. 2) shows
that ln J increases linearly with temperature. The strong
temperature dependence rules out the applicability of a
tunnelling mechanism for the observed conduction, and
suggests that a thermally-activated process may be
operative in the present case.8,9 Furthermore, straight
lines with a single slope are observed for all the Ðelds,
indicating the absence of any thermodynamic transition
in the temperature range studied.9
The other likely processes which may be operative in
the present case are the Ðeld-enhanced emission of
charge carriers from localized coloumbic traps, i.e. the
PooleÈFrenkel mechanism, or the thermionic emission
of charge carriers over the metalÈinsulator interface
barrier, i.e. the SchottkyÈRichardson mechanism.8h12
In the case of the measurements on compressed
pellets, which may contain traps, the thermal excitation
of trapped electrons into the conduction band of insulator via Ðeld-assisted lowering of trap depth (PooleÈ
Frenkel e†ect11) may profoundly a†ect the conduction
process. The PooleÈFrenkel mechanism predicts a Ðeld
dependence for the conductivity expressed as :
p \ p exp(b E1@2/2kT )
0
PF
and is characterized by the linearity of ln p versus E1@2
plots, i.e. PooleÈFrenkel plots. In the case of amylase,
the contribution of this mechanism is not very signiÐcant, as is evident from the PooleÈFrenkel plots (Fig. 3) ;
ln p does not show a linear increase with E1@2, as predicted by the expression for conductivity.
In the case of the SchottkyÈRichardson mechanism,11,12 thermal activation of electrons over the
metal-insulator interface barrier takes place with the
added e†ect that the applied electric Ðeld reduces the
height of the barrier. The currentÈvoltage relationship
for the SchottkyÈRichardson mechanism is expressed
as :
J \ AT 2 expM([/ ] b E1@2)/kT N
S
SR
where A is a constant, T is the absolute temperature, /
S
is the metalÈinsulator potential barrier, k is BoltzmannÏs
constant, E is the applied electric Ðeld and b is the
SR
Schottky Ðeld lowering constant which is given by :
Fig. 1. CurrentÈvoltage characteristics.
Fig. 2. Temperature dependence of current density.
b \ (e3/4nee d)1@2
SR
0
where e is the electronic charge, e is the dielectric constant, e is the permittivity of free space and d is the
0
Fig. 3. PooleÈFrenkel plots.
POLYMER INTERNATIONAL VOL. 47, NO. 3, 1998
DC conduction in amylase
thickness of the sample. It is clear that ln J is a linear
function of the square root of the Ðeld strength ; results
plotted with the axes marked in this way are referred to
as Schottky plots, and linear positive slopes of Schottky
plots generally characterize the SchottkyÈRichardson
mechanism.11
Schottky plots for the case of amylase are shown in
Fig. 4 from which it is evident that ln J is a linear function of the square root of the Ðeld strength ; linear positive slopes of Schottky plots are observed, indicating the
applicability of the SchottkyÈRichardson mechanism.
Another characteristic feature of the SchottkyRichardson mechanism is the linearity of Richardson
plots, i.e. the ln(J/T 2) versus (1/kT ) plots. The observed
Richardson plots (Fig. 5) are straight lines, in agreement
with the SchottkyÈRichardson mechanism. The e†ective
metalÈinsulator potential barrier has also been computed and is found to be in the neighbourhood of
0É45 eV (Table 1) for all the applied Ðelds.
Fig. 4. Schottky plots.
Fig. 5. Richardson plots.
POLYMER INTERNATIONAL VOL. 47, NO. 3, 1998
333
TABLE 1. Comparison of experimentally calculated
potential barrier and the corresponding activation
energy
Applied
field
(kV cmÉ1)
Effective
potential
barrier
(eV)
Activation
energy
(eV)
0·9
1·8
2·8
3·7
4·7
5·6
6·6
0·46
0·46
0·45
0·44
0·43
0·41
0·41
0·67
0·66
0·65
0·65
0·65
0·64
0·64
The activation energy for the conduction process is
computed with the aid of Arrhenius equation :
p \ p expM[E/kT N
0
by plotting ln p as a function of reciprocal absolute
temperature (Fig. 6) ; it is found to be in the neighbourhood of 0É60 eV (Table 1). The activation energy values
are of the same order as the experimentally determined
metalÈinsulator e†ective potential barriers ; this supports
the applicability of the SchottkyÈRichardson mechanism for conduction in amylase. The fairly low value of
activation energy rules out the possibility of charge carriers being ionic in nature.11 However, the possibility of
trapping of charge carriers in shallow traps and their
subsequent release by the PooleÈFrenkel mechanism,
cannot be ruled out, because the activation energy is
slightly higher than the e†ective metalÈinsulator potential barrier.
The results of the present investigation indicate that
silver electrodes provide satisfactory electrical contacts
for the study of electrical properties of biopolymers.
The formation of Schottky barriers12 at the metalÈ
insulator interface rules out the possibility of free injection of charge carriers in the low temperature range
Fig. 6. Arrhenius plots.
V . Mishra, S. C. T homas, R. Nath
334
(80È300 K) where the measurements are generally made
with biopolymers,1 and therefore the inherent electrical
characteristics of biopolymers can be studied.
tal facilities in his laboratory, and to the Council of
ScientiÐc and Industrial Research, India for the Ðnancial assistance to one of them (V.M.).
CONCLUSIONS
REFERENCES
The results of d.c. conductivity measurements on
amylase, as analysed in terms of di†erent possible
mechanisms, suggest that the SchottkyÈRichardson
mechanism is primarily responsible for the observed
conduction in the temperature range studied. Furthermore, the presence of Schottky barriers at the metal
electrodeÈinsulator interface suggests that by using
silver electrodes, satisfactory, non-injecting contacts can
be achieved for further electrical investigations on biomaterials.
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6.
2 Fox, M. A. & Whitesell, J. K., Organic Chemistry, Jones & Bertlett,
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(1996) 553.
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ACKNOWLEDGEMENTS
The authors are grateful to Prof. T.C. Goel, Indian
Institute of Technology, Delhi, for providing experimen-
POLYMER INTERNATIONAL VOL. 47, NO. 3, 1998
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