The role of porphyrins in life processes is well known. Iron complexes for instance are prostetic groups of important proteins such as hemoglobine and cytochromes. On the other hand, reduced porphyrins, as magnesium chlorin complexes, are fundamental in the photosynthetic processes. Such richness of properties is particularly attractive for chemical sensing. The molecular framework of porphyrins is a unique site for a wide range of interaction mechanisms useful for analyte binding. the interactions span from Van der Waals forces to hydrogen bond, to π interactions and to the coordination to the central metal ion.

The concurrence of all these mechanisms makes difficult the development of selective sensors, even if selectivity can in some cases found or improved by a suitable design of the sensor. On the other hand, the sensitivity of porphyrins can be oriented, through a proper molecular design, towards different classes of analytes. This control is of great value for the design of sensor arrays where several sensors non-selective but different among them are considered together.

We have been interested since about two decades to the use porphyrins to develop sensors for volatile compounds and gases. The physical properties of porphyrins restrict the transduction mechanisms that can be exploited to create an electric signal from the porphyrin-analyte interaction. It is important to recall that a chemical sensor actually measures a physical quantity of the sensing material that is affected by the interaction with the analyte.

We developed sensors measuring the mass, the optical absorbance, the surface potential and the charge transferred either to or from an inorganic semiconductor.

The sensing properties in all these devices can be related to the properties of the individual porphyrins, for instance the Hard and Soft Acids and Bases (HSAB) Pearson’s rule can be invoked to elucidate the sensitivity of porphyrins coated quartz microbalance sensors with regards to amines and alcohols.

In this paper, we will provide a rationale for the development of porphyrins based gas sensors discussing the pros and the cons of each transduction principle. A review of the applications will also be illustrated with a special emphasis on medical diagnosis through the analysis of volatile compounds.