Thermal Biosensors in Biotechnology - Advances in Biochemical Engineering Biotechnology - page 37

Fig.1. Principle set-up of an enzyme thermistor

thermostated via a proportional controller at physiological temperatures (25,30
or 37°C

0.01°C).An inner aluminium cylinder contains two fastenings for a
measuring and reference thermistor.A box filled with polyurethane foam insu-
lates the aluminum cylinders. Samples are injected via the FIA-principle and
pumped to the ET.Here,the aluminium cylinder thermostats the buffer stream
that flows through thin-faced steel tubes (0.8 mm inner diameter).The tubes are
connected with gold capillaries (good heat exchange) with fixed thermistors
(type: GB42JM65, 16 k

±

at 25°C; Fenwal Electronics, Framingham, MA, USA)
and interchangeable columns containing the immobilized enzyme. After an
enzymatic conversion, the heated sample flows through the gold capillary and
reduces thermistor resistance.A Wheatstone bridge registers the signal, and a
chopper stabilized amplifier (MP221,Analogic Corp.) indicates a voltage. The
ET registers about 80% of the heat produced.In order to minimize the effect of
mixing enthalpies,a two channel version is obvious:one channel with immobi-
lized biocatalyst,and a second reference channel with an inactive column.
Sauerbrei (1988) developed a multi channel calorimeter for the determina-
tion of up to three different analytes.A two point controller grossly thermostats
the aluminium cylinder to a desired temperature, and a PI controller ensures
fine tuning.A multiple bridge is connected with an amplifier and an 8-bit A/D
board, and a microprocessor takes care of data acquisition and analysis (peak
height and area).
Based on experiences with the Lund-ET,Hundeck et al.(1992) continued the
development of multi channel ET and constructed a stand-alone version for the
simultaneous determination of up to four analytes (Fig. 2). Here, thermistors

W