Slide #

Metal-oxide sensors(example CO gas for SnO2 based sensor)

• Different metal oxides can be used, i.e. ZnO, SnO2, In2O3, TiO2, Ga2O3, WO3 • Conductivity of the oxide can be written as:

0 is the conductivity of the tin oxide at 300C, 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.

= 0 + kPm

• 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

an experimental quantity for the gas.

P is the concentration.

R = aP

Slide #

Metal-oxide sensors

• 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.

Slide #

Metal-oxide sensors - Variations• 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

Slide #4

Electrochemistry based sensor:Oxygen sensing

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)

Oxygen

Slide #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

Slide #6

• Advantages– Fast response– High sensitivity (force)– High resolution– Low power consumption

Microcantilever based sensors

Microcantilever sensor array

(From NSF website)

Disadvantage

Lack of selectivity without a sensing layer

Microcantilever based sensors were first demonstrated by Dr. T.

Thundat at Oak Ridge National Laboratory in the early 1990s

Slide #

Principle of microcantilever sensors

7

Sensing based on stress changes Sensing based on mass changes

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

Deflection due to adsorption of chemicals on functionalized surface

Sensing layer

∆f

m

kf

32

21

1

k: spring constant

m: mass

Slide #8

Sensing based on coated microcantilever is a bad idea!

Potentiometric sensing

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): 1.Microcantilever can not be replaced easily for different chemicals2.Less sensitive and degrades easily 3.Uniform coating is difficult