Porphyrin-based chemical sensors

Myriad metalloproteins bind iron-protoporphyrin IX, known as heme (Fig. 15). Heme protein properties are determined by a variety of factors within the inner coordination sphere and without. These include chemical modifications to the porphyrin macrocycle, different axial ligation, perturbations to conformation, and protein dynamics surrounding the cofactor. Because of the extensive proliferation of heme proteins, we will limit ourselves to a small subset. These will include the cytochromes c, myoglobins, heme oxygenases and peroxidases, and a heme-based chemical sensor. 

Quite a few phthalocyanine derivatives have been reported to date with regard to the conductivity in forms of LB films [222-231], some of them aimed at constructing chemical sensors [207,232-234]. Voltage controlled negative resistance [235] as well as electro-chromism [236] have been reported. Porphyrines were also used for the construction of conductive LB films [237,238] and applied to porphyrin based gas sensors [239-241]. 

Excellent recent reviews on graphene-based electrochemical sensors [113] and on analytical devices in biomolecules detection and cancer diagnostics [114] appeared recently, where the superior electrochemical sensing performances of graphene-based electrodes toward the detection of various biomolecules and chemicals have been demonstrated. Also in this field, porphyrin-functionalized graphene hybrids resulted in interesting analytical applications, as will be discussed next. 

The previously mentioned quantities are completely general, and their importance holds for any kind of sensor. For chemical sensors an additional parameter of great importance is the selectivity. The selectivity defines the capability of a sensor to be sensitive only to one quantity rejecting all the others. In case of physical sensors, the number of quantities is limited to a dozen and the selectivity can be achieved in many practical applications. For chemical sensors, it is important to consider that the number of chemical compounds is of millions and that the structural differences among them may be extremely subtle. With these conditions the selectivity of chemical sensor can be obtained only in very limited conditions. Lack of selectivity means that the sensor responds with comparable intensity to different species and with such a sensor it is not possible to deduce any reliable information about the chemical composition of the measured sample. Selectivity is a straightforward requisite for analytical systems where sensors and its related measurement technique are addressed to the detection of individual compounds. As mentioned in the previous section, selectivity is not found in olfactory receptors. As a consequence, artificial olfaction systems are not based on individual selective sensors, but on sensors whose selectivity can be oriented towards molecular families, or better, towards interaction mechanisms. Figure 22.5 shows a typical selectivity map related to an array of quartz microbalances (see next section) coated with different metalloporphyrins based on the same macrocycle (tetraphenyl-porphyrin) but with different metal atoms. Figure 22.5 depicts well the concept of combinatorial selectivity, namely each compounds is identified by a unique sensitivity pattern that makes possible the identification. 

In contra.st to their interaction with applied electric, magnetic or electromagnetic fields, porphyrins and metalloporphyrins can also interact with other chemical species. One might view such interactions as chemo-responsive rather than field-responsive. The development of chemo-responsive materials based on porphyrins, however, is somewhat less advanced. One example of such applications is that porphyrin solids, being highly porous, are involved in the current development of molecularly based molecular sieves or shape-selective solid catalysts. Porphyrins and metalloporphyrins have also been examined for a variety of sensor applications, further proving their importance as a class of chemo-responsive materials. 

---