Sensor applications of polypyrrole for oxynitrogen analytes: a DFT study

Density functional theory calculations are performed to evaluate the sensing ability of polypyrrole for oxynitrogen analytes. Interaction energies of PPy-X (X = NO₂^-, NO₂, and NO) are calculated at B3LYP-CP/6-31G(d) and B3LYP/6-31G(d) levels of theory and compared with the high level calibrated method (M05-2X/aug-cc-pVDZ). B3LYP-CP/6-31G(d) gives the best correlation with the high level calibrated method compared to B3LYP/6-31G(d). Interaction of oligopyrrole with analytes shows a significant effect on the geometric and electronic properties; the conjugation is increased in the pyrrole oligomers and movement of charge is increased over the polymeric backbone. The charge is transferred from analytes to pyrrole oligomers (except nPy-NO₂), and a more pronounced effect of charge transfer is observed in the case of nitrite ion (NO₂^-) compared to NO. In nPy-NO₂, the charge is transferred from polymer to analyte. This transfer of charge indicates the n-type doping effect of analytes. The HOMO-LUMO gap decreases after interaction with analytes, which results in a drop of resistance (conductivity increases). These theoretical outcomes are consistent with the experimental results; polypyrrole has more sensing ability toward the nitrite anion (NO₂^-). Graphical abstract High sensitivity of polypyrrole towards NO over NO₂ and NO₂.