Polypyrrole: a reactive and functional conductive polymer for the selective electrochemical detection of heavy metals in water

The last two decades have witnessed attractive, innovative aspects of conductive polymers (CPs) in monitoring environmental pollution. In this regard, CP-based electrode materials were designed for the selective recognition of heavy metal ions in the environment (e.g. waste, river or tap water) or in simulated polluted environmental samples. In this review, the emphasis is on polypyrrole (PPy), an interesting electrosensing electrode material for heavy metals due to its facile preparation, versatile chemistry and physicochemical features. Indeed, health issues raised by metal ion pollutants require an urgent holistic approach for environmental problem solving. In this review, we will summarize the existing knowledge on the use of PPy as electrode material for the detection of heavy metals. We will report strategies of preparation of polypyrrole that exhibit selectivity towards heavy metal ions: (i) choice of dopant, (ii) functionalization of polymer backbone by chelatant groups, and (iii) preparation of ion imprinted polypyrrole. It is clear from this review that dopants could act as chelatant of metal ions and increase the selectivity. Such improvement could also be achieved by copolymerization of pyrrole with pyrrole-bearing chelatant groups (e.g. EDTA-like) or finally by the imprinting technique. The latter imparts artificial receptor sites for the recognition of metal ions combining the shape of the receptor site within the polypyrrole matrix that fit in well with the size of the metal ion, on the one hand, and the receptor site-ion interactions, on the other hand. Regardless, the method employed to design polypyrrole sensing layers for heavy metal nanostructuration seems to definitely improve the sensitivity of polypyrrole-based sensor devices. The review finishes by concluding remarks and indication of possible challenging new directions exploring polypyrrole in tracking occurrence of heavy metal ions in the environment.