An assay method has been developed to detect oxalic acid from other structurally similar dicarboxylic acids using a cationic polythiophene derivative (PFT). Upon adding oxalic acid, the PFT can be cross-linked through the bifunctional diacids of oxalic acid to form tense interpolymer pi center dot center dot stacking aggregation, which results in the fluorescence self-quenching of the PFT. Upon adding other dicarboxylic acids with a longer tether, loose interpolymer pi-stacking aggregation forms and the PFT fluorescence is less quenched. The fluorescence analysis was carried out in solution for the aggregates prior to precipitation. As a result of the short tether of oxalic acid, the unique fluorescent response from the PFT/oxalic acid assembly can be used to detect oxalic acid.