Porphyrin assemblies as chemical sensors

A simple "non-covalent' approach was employed to obtain sensitive and speci fic sensors for pH, DNA and metal ions, by 'mixing' commercially available chemicals such as cationic or anionic water-soluble porphyrins and polypept ides. For example, under the appropriate conditions, the supramolecular com plex formed between the anionic meso-tetrakis(4-sulfonatophenyl)porphine (H 2TPPS) and the protonated form of poly-lysine can behave as pH-sensor. In f act, H2TPPS in the pH range 5.5-12 exists in a monomeric form, and its fluo rescence is not pi-I-dependent. However, at low pH values (less than or equ al to 7), the protonated poly-lysine promotes porphyrins binding and self-a ggregation. with consequent strong quenching of their fluorescence, while a t 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 sho ws 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 co ntaining different central metal ions with particular coordination geometri es. In such a way we have been able to develop a whole family of sensors co vering a wide range of pH. These supramolecular aggregates can also be empl oyed as sensors for DNA. In fact, the addition of DNA (which is a poly-anio n) to a preformed H2TPPS/poly-Lysine system (FH less than or equal to 7), c auses a displacement of the porphyrin bound to the poly-cationic matrix wit h consequent increase in the fluorescence intensity of the solution. Theref ore, since the variation in fluorescence emission is linearly related to th e concentration of DNA added, we have employed such supramolecular system t o develop a simple and rapid method for the quantitative determination of D NA in solution. Finally, a remarkable acceleration of the insertion of copp er(II) and zinc(II) in cationic porphyrins is observed when these porphyrin s are monodispersed on the surface of negatively charged matrices, such as anionic poly-Glutamate. Such peculiarity allowed us to develop a specific f luorescent sensor for both metal ions capable of detecting their presence e ven at very low concentrations in the nanomolar range. (C) 1999 Elsevier Sc ience S.A. All rights reserved.