A self-assembled metallogel 1-Co was prepared using 5-methyltetrazole (1) and Co2+. Furthermore, the visual-size discrimination of 2-picolinic acid (2-PA) out of its two positional isomers and 12 other analogues has been achieved by the selective collapse of 1-Co, in which a chelating five-membered ring structure was formed between 2-PA and Co2+ by competitive bindings.