Biosensors - Principles of Biomedical Engineering - page 378

the gas phase using formaldehyde dehydrogenase with no interference from other
aldehydes or alcohols.
An advantage of acoustic techniques is the detection, in real time, of binding
reactions of chemical compounds (in a gaseous form or dissolved in solution) with
the solid surface of the crystal. This feature allows kinetic evaluation of affinity
interactions (typically between antibodies and antigens). In addition, the cost of
the apparatus is low. Limitations for this transduction method involve format and
calibration requirements. Each crystal should be calibrated because its frequency
depends on the crystal geometry and the immobilization technique used to coat
the surface (generally gold-plated quartz) with the antigen or antibody. The main
source of variability is the uniformity of the protein immobilized on the surface.

9.4.4 Other Transducers
The calorimetric approach to signal transduction exploits thermal changes in a
solution. Calorimetry is a general detection scheme and is a convenient method for
sensing any chemical, biochemical, or physical reaction. Some enzyme-mediated
reactions produce heat during the conversion of the substrate to a product. Re-
leased heat changes the temperature of the solution, which can be monitored using
thermistors. Miniaturized thermistors can also be formed. The sensitivity (10 −4 M)
and range (10 −4 -10 −2 M) of thermistor biosensors are low for the majority of ap-
plications relative to other transduction methods.
An alternative approach for sensing temperature and/or stress is based on the
micromachined cantilever. Microcantilevers are micromechanical devices with di-
mensions on the order of a 100- μ m length with 30-40 μ m in width and less than 1

m in thickness (Figure 9.13). The deflection of the cantilever is measured precisely
and exploited for sensing temperature by coating one side of the cantilever with
a metal layer to create a bimetallic beam. The different coefficients of thermal ex-
pansion associated with each layer cause the cantilever to deflect with temperature
change. Considering the low mass of a micromachined cantilever and the sensitivity
with which deflection can be measured, the temperature sensitivity is on the order
of microdegrees and the energy sensitivity is on the order of 10 −12 Joules. The small
thickness of the cantilever also enables sensitive responses to mechanical stress.
Hence, the distinction between thermally and mechanically induced stresses can be
deciphered.
Microcantilevers can also be excited into resonance by a number of means in-
cluding the ambient thermal motion. The resonance frequency of a microcantilever

μ

Bacteria

Protein

(b)
Figure 9.13 Microcantilever technology: (a) with immobilized protein for a specifi c bacterium and
(b) bending after adsorption of bacteria to the protein.

(a)