The structural design of polyacrylonitrile fibre-based colorimetric sensors and their synergistic interaction mechanism for Cu2+ detection

Excessive content of Cu 2+ in water is not allowed since it is pathogenic to human beings and most aquatic animals. In this report, to reveal the general colorimetric recognition mechanism of solid-based sensors, containing soft acid coordination ligand types (–COOH, –C=O, –S–) for Cu 2+ , a series of polyacrylonitrile fibre-based colorimetric sensors (PAN-TEPA-X) with regular precise coordination structures was designed and prepared, where axially symmetric carboxylic acid molecules (X, (CH2) m S n (COOH)2, 3 <  m  < 7, n  < 3) were grafted onto aminated polyacrylonitrile (PAN-TEPA), respectively. For the PAN-TEPA-X, the limit detection of Cu 2+ decreased from 1 × 10 –5 to 1 × 10 –6  mol/L and to 1 × 10 −7  mol/L with the increase in the number ( n ) of S coordination atoms in (CH 2 ) m S n (COOH) 2 ; thus, –S– was a significant ligand type for improving the colorimetric limit of detection of PAN-TEPA-X for Cu 2+ . The selective recognition mechanism of PAN-TEPA-X for Cu 2+ was unified and revealed that a portion of the Cu 2+ was reduced to Cu 1 species, and then, the Cu 2 /Cu 1 species were chelated and enriched on the PAN-TEPA-X fibres through a tetradentate ligand reaction (arranged “tail to tail” and “shoulder to shoulder”) between the coordination ligand types (–COOH, –C=O, and –S–) of PAN-TEPA-X and Cu 2 /Cu 1 according to the theory of hard and soft acids and bases.