Poly(indole-5-carboxylic acid) Synthesized by Acid-Assisted Polymerization as an Innovative Material for Detecting Cationic Surfactants and Heavy-Metal Ions

Poly(indole-5-carboxylic acid) [poly(IN5CA)] was prepared by acid-assisted oxidative polymerization of the indole-5-carboxylic acid monomer in concentrated formic acid, using ammonium peroxodisulfate as the oxidant, at three monomer-to-oxidant ratios, and evaluated as a proof-of-concept sensing platform. MALDI-TOF mass spectrometry showed a dominant peak consistent with a cyclic trimer, together with a minor series of higher-mass peaks spaced at single monomer-unit intervals, indicating that a small fraction of the material extends as linear oligomers. Raman and ATR-FTIR spectroscopy confirmed oxidized conjugated segments. At the highest oxidant loading (1:1), ATR-FTIR revealed strong carboxylate (COO–) bands indicative of an additional, oxidant-induced transformation of the carboxylic acid groups; the intermediate 2:1 ratio was therefore selected, as it yielded oxidized conjugated oligomers without this transformation. X-ray diffraction showed the material to be predominantly amorphous, with a minor crystalline component attributed to a residual sulfate-based salt derived from the oxidant. The selected formulation remained stable within its UV–vis spectral envelope over 21 days, while fluorescence intensity showed a modest, nonmonotonic variation over the same period. Poly(IN5CA), immobilized as a drop-cast film on glassy-carbon electrodes, showed a concentration-dependent potentiometric response to the cationic surfactants CTAB and CHX, attributed to neutralization of the film’s negative surface charge. The CTAB slope (−21.2 mV/decade) was markedly sub-Nernstian for a monovalent cation, whereas the CHX slope (−29.2 mV/decade) closely approached the theoretical value expected for an effectively divalent cation. A potentiometric response to the heavy metal ions tested was not reproducible under the present conditions. In solution, the free polymer showed a fluorometric response to both surfactants and to Pb2+, Co2+, Ni2+, Fe2+, and Cu2+, with the direction and magnitude of the response varying by ion. A preliminary paper-based assay further confirmed that CTAB- and CHX-induced fluorescence changes remain detectable when poly(IN5CA) is immobilized on a solid support, though with a concentration dependence distinct from that observed in solution. This pattern of divergent, ion-specific responses across both transduction modes is consistent with a chemically interpretable fingerprint rather than selective behavior. Although drift correction, selectivity, reversibility, and validation in real matrices remain necessary, the results identify poly(IN5CA) as a promising low-cost material for sensor arrays, wastewater surveillance, disinfectant residue monitoring, and adaptive process control.