Biomedical Engineering Reference
In-Depth Information
- In some cases,thermal biosensors work without complicated and interference-
prone multi-enzyme systems,e.g.,disaccharide analysis.
- Thermal biosensors have found multiple applications.
In this article,we would like to review the principles and applications ofthermal
biosensors.Especially,the newest results of ET research and trends in hardware
developments are pointed out. Thermal biosensors will probably have a pro-
mising future in biotechnology.
2
Thermal Biosensors: Principles and State of the Art
Nearly all biochemical reactions are of exothermic character,i.e.,an enzymatic
conversion of a substrate is accompanied by heat production. The first law of
thermodynamics decribes a proportional relationship between the heat produ-
ced and amount of molar enthalpy:
Q = -
n
p
SD
H
Due to heat production,a local temperature shift
D
T is observed that depends
on the heat capacity C
s
of the surrounding system:
Q n
p
SD
H
D
T =
= -
5
931
C
s
C
s
Enzymatic reaction enthalpies are in the range of -10 to -100 kJ/mol and cause
local temperature changes of a few mK. In the last twenty years, many ex-
periments were described in literature using thermistors for measuring in the
mK range.Thermistors are ceramic semiconductor resistances with high Ohm
data and strong negative temperature coefficient (between -3 and -6%/K).
Therefore, thermistors are called NTC-resistances (
n
egative
t
emperature co-
efficient). The universal detection principle to link a nonspecific detection of
heat with a highly specific enzymatic reaction rose in a number of different
applications.
In the initial experiments, very simple devices were used. Partly, they had
unfavourable response-times, a complicated thermostating, small sample fre-
quencies and an irregular baseline.The thermistor was fixed at the tip of a flow
through coil (cartridge or tube with immobilized enzyme).The enzymatic reac-
tion takes places in the coil and is accompanied by heat production.The flowing
medium transports the resulting temperature gradient to the fixed thermistor
that detects the local temperature change.Therefore,the thermistor data are lin-
ked with the enzymatic conversion. Several authors have described arrange-
ments of this kind (Mosbach and Danielsson, 1974; Cannings and Carr, 1975;
Schmidt et al.,1976).
At the University of Lund in Sweden,Mosbach and Danielsson (1981) devel-
oped the first operational ET systems. These are still being supplied in small
series by Thermometric Co, Järfalla, Sweden or ABT, Lund. Figure 1 schemati-
cally shows the ET setup. It consists of an external aluminium cylinder that is
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