Thermal Biosensors in Biotechnology - Advances in Biochemical Engineering Biotechnology - page 60

industrial application. Thus, Urban et al. (1991) developed a new thermal bio-
sensor using thin-film thermistor arrays and immobilized enzymes.The minia-
turized thermistor arrays were produced on glass substrates and exhibit a
temperature dependence of conductivity of 2%/K, a temperature resolution
of 0.1 mK and a response-time of 3ms. The experimental set-up for glucose
monitoring consists ofa 3 5-cm Plexiglas block containing a flow through

channel,a column with immobilized glucose oxidase and two thermistor arrays,
inserted in a Peltier-thermostated aluminium block. The whole device has
dimensions of 25

¥ 2.5 ¥

25 cm,and the thermostat has a stability of 1mK.Thus,
miniaturization has to be improved.Nevertheless,a very interesting point ofthis
study is the high reproducibility ofthe thermistor characteristics due to the high
level of development of thin-film technology.

¥ 25 ¥

5.2
Miniaturized Enzyme Thermistors

Xie et al.(1992) have developed very promising thermal biosensors produced on
silicon wafers (Fig. 20). Here, a small reactor cell (5

¥ 1 ¥ 0.014 mm) was pre-

pared on a silicon chip (14 0.4 cm) with minimal heat capacity. The cell

consists of 33 parallel channels and has a total volume of 0.02

¥ 6 ¥

l. Microbead
thermistors were fixed on gold tubes at the inlet and at the outlet of the silicon
chip.The immobilization of enzymes are performed via glutaraldehyde activa-
tion of the silicon chip.Enzyme solutions are layered over the channel section
and dried at room temperature. The whole preparation has a glass cover
attached by a thin layer of silicon rubber glue.Due to the very small reactor cell
volume,the residence time of samples in the microchannels is short.Thus,heat
leakage from the cell is reduced.Nevertheless,the sensor has to be placed in an
aluminium box filled with polyurethane foam. Moreover, huge Wheatstone-
bridge equipment is necessary. In comparison to the commonly used ET, the

m

Fig.20. Miniaturized ET-chip