Organic humidity sensors

Humidity sensors consist of two major types, i.e., one which measures relative humidity and the other which detects dew condensation. The sensors for relative humidity made of ceramic or organic polymers are produced in amounts exceeding one million. They are applied to controling systems in air conditioning, micro wave ovens, and so on... 

L. Sabbatini, and P. G. Zambonin, NTCDA organic thin-film transistor as humidity sensor weaknesses and strengths , Sensors And Actuators B -Chemical 77, 7 (2001). 

Suitably modified fiber optic sensors can also be used for detecting gas vapors, humidity, ions, and organic compounds. Fiber inclusions that show length variation were used to develop humidity sensors, whereas ion-responsive lipid bilayers formed the basis for the detection of inorganic ions. Immobilized neutral and ionic crown ethers in polymeric membranes were designed as sensors for determination of barium and copper (Wolfbeis 2000). 

The range of sample characteristics and manner of their detection, is much larger than can be realistically addressed in the space of a single chapter. We will confine this chapter mainly to the chemical sensor research areas discussed in other chapters in this volume, dividing them into electrical, optical, and mass and thermal measurements. Our focus will furthermore be on the generic chemical and physical phenomena upon which such measurements can be based, as opposed to the alternative organization that would address chemical sensors in the context of their application (i.e, auto exhaust sensor, clinical diagnostic sensor, environmental sensor) or of the kinds of samples detected (i.e, CO sensors, humidity sensor, biosensor, etc.), as used in a previous ACS Symposium Series volume on Chemical Sensors (D. Schuetzle, R. Hammerle, Eds., ACS Sympos. Ser. 309, 1986). 

Ralston ARK, Klein CF, Thoma PE, Denton DD (1996) A model for the relative environmental stability of a series of polyimide capadtance humidity sensors. Sens Actuators B 34 343—348 eboun J, Hamacek A (2008) Organic materials for humidity sensors. In Proceedings of IEEE sensor conference, 26-29 October, Lecce, Italy, G003, pp 473 77... 

Conductometric relies on the measurement of either conductivity or resistivity. This type of sensor comprises a capacitor that changes its capacitance when exposed to the desired analyte. The capacitance of the sensor changes due to a selectively absorbing material such as polymers or other insulators. These absorbing materials serve as the dielectric layer of the capacitor and their permittivity changes with exposure to the analyte. These sensors are commonly used to detect humidity as well as carbon dioxide and volatile organic compounds (Patel et al., 2003). In the humidity sensor case, the dielectric layer comprises a water-sensitive polymer (Lazarus and Fedder, 2011). 

When it is important to control the water activity in a reactor, a water activity sensor is quite useful. The sensor should ideally measure the water activity in the liquid reaction medium. However, the sensors available are designed for gas phase measurements, and, provided there is effective enough equilibration between the liquid and gaseous phases, they can be used to control the water activity in the reactor. If the measured water activity is above the set point, drying is initiated, for example, by passing dry air through the reactor. On the other hand, if the water activity is too low, water can be added, either as liquid water or as humid air. Automatically controlled systems of this kind have been successfully used to monitor and control enzymatic reactions in organic media [13, 14]. 

The time has come for the sensors to be taken from the well-controlled environment of the research laboratory and applied to real conditions. This means that the sensors must be robust and able to stand rough conditions in terms of, for example, humidity, temperature, and exposure to organic solvents. When applying the sensors in direct contact with biological fluids biocompatibility will be a major problem. 

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