Integration of Porphyrinoids Based Gas Sensor Arrays with Direct Injection Mass Spectrometry
Authors: Capuano Rosamaria , Khomenko Iuliia , Paolesse Roberto , Biasioli Franco , Di Natale Corrado 
Published: May 2020

Porphyrins have been used in the last two decades to prepare sensor arrays for a variety of applications [1]. The wide chemical interactivity of porphyrins sustains these applications making possible the measurement of complex patterns of volatile compounds. 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 several applications. QMBs do not select the different interaction mechanisms, thus this transducer offer the unique chance to investigate the whole bouquet of interactions established between the volatile compounds and the sensitive layers.
The combination of the intrinsic porphyrins sensitivity and the properties of QMB transducer enables these sensor array to discriminate among subtle changes in patterns of volatile compounds produced in living organisms. In-vivo, these sensors have been applied to the measure of volatile compounds released by various body compartments such as skin, breath, and urines while in vitro they have been used to detect the volatile compounds of cell cultures.
However, in spite of the positive results, gas sensor arrays are always characterized by the lack of information about the nature of the sensed compounds. So, without this information it may be complicated to provide convincing explanations about the origin of the achieved results. Furthermore, gas sensors results do not shed light into the physiological mechanisms that link the volatile compounds to the disease and its progression.
This problem is well known in the field, and gas chromatography-mass spectrometry is the typical companion of sensor arrays investigation. However, these instruments cannot work on exactly the same samples. Consequently, due the lability of volatile compounds the comparison between instrument is far to be straightforward. On the other hand, several mass spectrometer techniques may work in real-time, and then they are virtually compatible to be operated in series to a gas sensor array enabling the simultaneous measurement of the very same samples. Among these techniques, Proton Transfer Reaction - Mass Spectrometer (PTR-MS) is the most suitable in terms of rapidity of measurements. In addition, the moderate molecular fragmentation results in a more friendly interpretation of the results [2]
Recently, we applied the combination of porphyrins coated QMB sensor array and PTR-MS to the analysis of volatile compounds released by red blood cells infected by Plasmodium falciparum at different stage of development [3]. Plasmodium falciparum is the most lethal parasite related to malaria, a diseases that kill about half millions of people per year in the world. The gas sensor array identifies the cells infected with the transmissible stage of the parasite, as well the PTR-MS. The analysis of PTR-MS data shown that an exceptional emission of hexanal is observed in these cells. So at the same time sensors demonstrated the identification of infected cells, PTR-MS provided the explanation of the detection, and it provides the direction for the further optimization of sensors for the identification of malaria during the stages of maximum infectiveness.
This study introduces a general methodology for sensors selection and optimization in order to improve the performance of gas sensor array in specific applications.
References
[1] Paolesse R, Nardis S, Monti D, Stefanelli M, Di Natale C. Porphyrinoids for chemical sensor applications, Chem Rev, 2017;117: 2517-2583
[2] Biasioli, F., Gasperi, F., Yeretzian, C., Märk, T. PTR-MS monitoring of VOCs and BVOCs in food science and technology. Trends in Analytical Chemistry, 2011; 30, 968-977
[3] Capuano, R., Khomenko, I., Grasso, F., Messina, V., Olivieri, A., Cappellin, L., Paolesse, R., Catini, A., Ponzi, M., Biasioli, F., Di Natale, C. Simultaneous Proton Transfer Reaction-Mass Spectrometry and electronic nose study of the volatile compounds released by Plasmodium falciparum infected red blood cells in vitro, Scientific Reports, 2019; 9, 1-10