Sensors can be classified according to their operating principle [15],
for instance (i) metal-oxide semiconductor sensors (MOS) based on
conductivity variation, (ii) amperometric sensors based on solid or
liquid electrolytes, or (iii) optical sensors using fluorescence or
absorption of light. Specific sensors can provide accurate information
regarding a target compound, whereas non-specific sensors
give a global response to one or several families of chemical species