hB-Im by Cu2 þ ion is a static quenching mode due to the formation of non- fluorescent complex in the ground state. In the case of a static quenching, the Stern–Volmer plot is linear, and the quenching rate constants decrease with elevated temperature. As for a dynamic quenching, the Stern–Volmer plot is linear whereas the variation of quenching rate constants is in parallel with tempera- ture changes [42,43]