1. (Bio)Chemical Sensors
A (bio)chemical sensor is a device which is capable of converting a chemical (or biological) quantity into an electrical signal.
Interaction of a chemical species (X) with the sensing material (M) may often be described as
The selectivity of a chemical sensor depends on the nature of the reaction mechanism.

2. Principles, Measurands and Typical Examples of (Bio)Chemical Sensors
Typical Sensor
Conductometric
Resistance/conductance
Tin oxide gas sensor
Potnetiometric
Voltage/emf
Ion selective FET for pH
Capacitive
Capacitance/charge
Polymeric humidity sensor
Amperometric
Current
Electrochemical cell
Calorimetric
Heat/temperature
Pellistor gas sensor
Gravimetric
Mass
Piezoelectric or SAW sensors
Optical
Path length/absorption
Infra-red detector for methane gas
Resonant
Frequency
Surface plasmon
Fluorescent
Intensity
Fiber-optic

3. Common Materials for (Bio)Chemical Sensors
Active Material
Examples
Sensing Principles
Measurand
Thin oxide layer
SnO2, ZnO
Surface conductance
Combustible gases
Thick porous oxide layers
SnO2, ZnO, TiO2
Bulk conductance
Catalytic metals
Pd-TiO2, Pd-MOS, Pd-MOSFET
Surface potential, threshold voltage
H2, CO, NH3
Ion-selective devices
AgCl, AgBr
Electrochemical potential
Cl, Br
Catalytic coating
ThO2/Al2O3
Heat of combustion
H2, CH4
Organic films
Substituted phthalocyanines
NOx
Conducting polymers
Poly(pyrrole)
Bulk conductance/mass
Polar compounds, NH3

4. Chemoresistors
The simplest type of chemical sensors is the chemoresistor in which a change in the electrical conductivity of a chemically-sensitive layer is measured.
A schematic structure of a chemoresistive gas sensor

5. Chemocapacitors
It is possible to measure the gas sensitivity of the dielectric constant of a film using a parallel-electrodes arrangement.
Structure
Typical response

6. Chemodiodes Structure
Response of an organic chemodiode in sensing Nox and NH3

7. Chemotransistors
A MOSFET hydrogen sensor exhibits a shift in threshold voltage, DVT.
Structure of Pd-gate gas sensitive MOSFET
Response of a chemotransistor