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 Cu2+, 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 x 10(-5)to 1 x 10(-6) mol/L and to 1 x 10(-7) mol/L with the increase in the number (n) of S coordination atoms in (CH2)(m)S-n(COOH)(2); thus, -S- was a significant ligand type for improving the colorimetric limit of detection of PAN-TEPA-X for Cu2+. 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.