Calorimetric sensors are based on measurement of the heat produced by the molecular recognition reaction and the amount of heat produced is correlated to the reactant concentration. The principle of the calorimetric sensors is the determination of the presence or concentration of chemical species by the measurement of the enthalpy change produced by any chemical reaction or physisorption process that releases or absorbs heat. We talk about exothermic reactions if heat is generated and endothermic reactions if it is absorbed.
What is a Calorimetric Sensors?
Calorimetric sensors have been described for enzyme reactions for detecting glucoses, urea, gases, etc. The thermal conductivity gas sensor has long been employed as a detector in gas chromatography (GC) and works on the basis of a heated W-Re wire filament to measure relatively high concentrations in gases.
Calorimetric sensors or chemoresistors can be classified in low-temperature chemoresistors and high-temperature chemoresistors.
Low-temperature chemoresistors consist of chemically sensitive layers applied over interdigitated electrodes on an insulating substrate. Examples of the chemically sensitive layers are: metal phtalocyanines, conducting polymers such as poly(pyrrol) and poly(aniline). These types of chemoresistors are used in the detection of ethanol, methanol and other organic volatile molecules.
High-temperature chemoresistors consist of micromachined semiconductor hotplates with a sensing film on a thermally insulated inorganic membrane. ZnO, InO, GaO, SnO are usually employed as sensitive materials. Gaseous electron donors (H) or electron acceptors (NOx) adsorb on metal oxides and form surface states, which can exchange electrons with the semiconductor.
Calorimetric Sensors Classification
Another way to classify the calorimetric sensors is taking into account the different ways of transducing heat variations :
Catalytic sensors or pellistors, which use a platinum coil used as heater and temperature sensors (resistance thermometer) and contain a catalyst to enhance a combustion process. The heated catalyst permits gas oxidation at reduced temperatures and at concentrations below the lower explosive limit. Some of the applications are: monitoring/detection of flammable gas hazards, CH4 (methane), H2, C3H8 (propane), CO and organic volatiles.
Thermistor based sensors, these devices detect with high accuracy changes in the electrical resistance that result from temperature changes. If a linear relationship between resistance and temperature is assumed (i.e. a first-order approximation), the following expression results:
ΔR = kΔT
where ΔR is the change in resistance, ΔT, the change in temperature and k a first-order temperature coefficient of resistance. Thermistors can be classified in two types depending on the sign of k. If k is positive, the resistance increases with increasing temperature, and the device is called a positive temperature coefficient (PTC) thermistor. If k is negative, the resistance decreases with increasing temperature, and the device is called a negative temperature coefficient (NTC) thermistor. These sensors use composite oxides. Negative Temperature Coefficient (NTC) Thermistors use sintered metal oxides like titanium oxide and Positive Temperature Coefficient (PTC) Thermistors use Ba/Pb titanate.
Pyroelectric sensors, which are based on the phenomenon of pyroelectricity, the ability of certain materials to generate an electrical potential when they are heated or cooled (gallium nitride (GaN), cesium nitrate (CsNO3), polyvinyl fluorides, derivatives of phenylpyrazine, cobalt phthalocyanine, Lithium tantalate (LiTaO3), etc.).
Seebeck-effect based sensors, which transform temperature differences directly into electricity.
Thermal-flow based sensors and bimorph effect cantilevers.