Porphyrin assemblies as chemical sensors

A simple 'non-covalent' approach was employed to obtain sensitive and specific sensors for pH, DNA and metal ions, by 'mixing' commercially available chemicals such as cationic or anionic water-soluble porphyrins and polypeptides. For example, under the appropriate conditions, the supramolecular complex formed between the anionic meso-tetrakis(4-sulfonatophenyl)porphine (H2TPPS) and the protonated form of poly-Lysine can behave as pH-sensor. In fact, H2TPPS in the pH range 5.5-12 exists in a monomeric form, and its fluorescence is not pH-dependent. However, at low pH values (≤ 7), the protonated poly-Lysine promotes porphyrins binding and self-aggregation with consequent strong quenching of their fluorescence, while at pH values higher than 9-10, the porphyrins exist in solution essentially as free monomers and are characterized by an intense fluorescence emission. As a consequence, the H2TPPS fluorescence intensity versus pH behavior shows a sigmoidal profile. Interestingly, the molecular recognition processes leading to the formation of these aggregates can be also modulated by using matrices of different nature and length as well as employing porphyrins containing different central metal ions with particular coordination geometries. In such a way we have been able to develop a whole family of sensors covering a wide range of pH. These supramolecular aggregates can also be employed as sensors for DNA. In fact, the addition of DNA (which is a poly-anion) to a preformed H2TPPS/poly-Lysine system (pH ≤ 7), causes a displacement of the porphyrin bound to the poly-cationic matrix with consequent increase in the fluorescence intensity of the solution. Therefore, since the variation in fluorescence emission is linearly related to the concentration of DNA added, we have employed such supramolecular system to develop a simple and rapid method for the quantitative determination of DNA in solution. Finally, a remarkable acceleration of the insertion of copper(II) and zinc(II) in cationic porphyrins is observed when these porphyrins are monodispersed on the surface of negatively charged matrices, such as anionic poly-Glutamate. Such peculiarity allowed us to develop a specific fluorescent sensor for both metal ions capable of detecting their presence even at very low concentrations in the nanomolar range.