(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.

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

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

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

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

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

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