The interplay between the metal ion, the aromatic ring and the peripheral compounds establishes unique selectivity patterns which are fundamental elements for sensor array design and development.
Porphyrin films can be adequately applied to inorganic surfaces making possible the preparation of different kinds of chemical sensors. Among them mass transducers, such as Quartz Microbalances (QMBs), have been found particularly suitable for practical applications. Indeed, in QMBs the moderate sensitivity of the transducer avoids unpleasant effects of signal saturation that could be observed with devices with largest sensitivity to the amount of absorbed molecules.
Among the various applications, porphyrins sensor arrays have been found particularly efficient in the detection of spoilage processes in food, this because of the natural sensitivity of metalloporphyrins to electron donor species, such as amines, which are abundantly found in degraded food.
The most intriguing application of porphyrin sensors is their use for medical diagnosis, or better to the identification of pathologies in human samples. To this regard, these sensors have been applied to the measure of volatile compounds released by various body compartments such as the skin, the breath, and the urines.
In breath, the identification of lung cancer has been carried out since 2003 and it has been demonstrated in several successive experiments. The detection of Skin cancer and bladder cancer were also being demonstrated measuring the volatile emission from skin and urines.
In case of skin, this is one of the few direct measurement of volatile compounds released by the tumor mass. In breath analysis, the detection is rather mediated by the air/blood exchange in the lungs of volatile metabolites excreted in the blood stream.
Another experimental line is concerned with cell cultures. Here results about the identification of cancer cells or, more recently, the identification of the steps of stem cells differentiation, has been demonstrated. Finally, it has to be mentioned the use of sensors to study the evolution of disease in animal models. To this regard, xenografted tumors and malaria models have been studied.
All these results evidence an important capability of porphyrin sensor array to capture the differences in volatile compounds patterns elicited by pathologies.
Similar results have been obtained, separately, even by other sensors technologies, however porphyrin sensors show a unique character of universal approach to the identification of pathologies.
A suggestive explanation of this behavior is based on the role of oxidative stress to the production of cancer related volatile metabolites and in particular on the role played by natural porphyrins in the oxidative stress promoted by the Cytochrome p450. In practice, a porphyrin may be active in the synthesis of the relevant compounds and other porphyrins are used to detect them.
In this paper, a rationale for the development of porphyrin sensors for medical diagnosis is presented together with a thorough discussion about the evidences collected so far.