More importantly, the absorption changes were clearly visible to the naked eye. The colorless solution of the sensor 1 immediately turned to pink upon addition of Ni2+ ion, even at 0.5 μM concentration (Fig. 2b), which means that the limit of Ni2+ ion by naked eye could be as low as 0.5 μM, lower than the limit of 0.07 mg/L (~ 1.2 μM) defined by the American Environmental Protection Agency (EPA) [14].