Metal-oxide sensors (example CO gas for SnO 2 based sensor)

1. Different metal oxides can be used, i.e. ZnO, SnO2, In2O3, TiO2, Ga2O3, WO3 Conductivity of the oxide can be written as: Metal-oxide sensors(example CO gas for SnO2 based sensor) • 0 is the conductivity of the tin oxide at 300C, without CO present • P is the concentration of the CO gas in ppm (parts per million), • k is a sensitivity coefficient (determined experimentally for various oxides) • m is an experimental value - about 0.5 for tin oxide. • Conductivity increases with increase in concentration • Resistance is proportional to the inverse of conductivity so that it may be written as • a is a constant defined by the material and construction and • a an experimental quantity for the gas. • P is the concentration.

2. The response is exponential (linear on the log scale) A transfer function of the type shown earlier must be defined for each gas and each type of oxide. SnO2 based sensors as well as ZnO sensors can also be used to sense CO2, toluene, benzene, ether, ethyl alcohol and propane with excellent sensitivity (1-50ppm). The mechanism for sensing of different metal oxides, but presence of oxide plays the critical role Metal oxide sensor generally need to be heated to get the reaction started. Usually few hundred degrees is sufficient. They can be easily multiplexed to perform mixture analysis and multi-parameter sensing Their greatest disadvantage is cross sensitivity To avoid cross-sensitivity temperature and compositional variation can be used. Metal-oxide sensors

3. A variation of the structure above is shown below It consists of an SnO2 layer on a ferrite substrate. The heater here is provided by a thick layer of RuO2, fed through two gold contacts. The resistance of the very thin SnO2 (less than about 0.5 m) is measured between two gold contacts. This sensor, which operates as described previously is sensitive to ethanol and carbon monoxide Metal-oxide sensors - Variations

4. Electrochemistry based sensor: Oxygen sensing Oxygen G. Koley, J. Liu, M. W. Nomani, M. Yim, X. Wen, T. Y. Hsia, “Miniaturized implantable pressure and oxygen sensors based on polydimethylsiloxane thin films”, Mater. Sci. Eng. C 29, 685 (2009)

5. …Continued Best sensitivity of 2.98 µA for 1% change of air content in surrounding media Noise limited resolution of ~6. 18 ppm (parts-per-million) oxygen Although amperometric sensors can be easily miniaturized, they are not very selective, hence potentiometric sensing is necessary to provide another complementary signal, and offer unique detection capability

6. Microcantilever based sensors • Advantages • Fast response • High sensitivity (force) • High resolution • Low power consumption Disadvantage Lack of selectivity without a sensing layer Microcantilever sensor array (From NSF website) Microcantilever based sensors were first demonstrated by Dr. T. Thundat at Oak Ridge National Laboratory in the early 1990s

7. Principle of microcantilever sensors Sensing based on mass changes Sensing based on stress changes Deflection due to adsorption of chemicals on functionalized surface k: spring constant m: mass Sensing layer ∆f Waggoner et al, Lab Chip 7, 1238 (2007) Ilic et al, Appl. Phys. Lett., 85, 2604 (2004) In general, microcantilevers are coated with sensing layers to perform sensing

8. Potentiometricsensing Sensing based on coated microcantilever is a bad idea! Substrate can be coated with functionalized layer Electrical signal is monitored (capacitive force): based on changes in surface work function, working in non-contact mode • Drawbacks(traditional microcantilever sensing): • Microcantilever can not be replaced easily for different chemicals • Less sensitive and degrades easily • Uniform coating is difficult