Fig. 7. The absorbance ratio of A_640/520 in the presence of transition metal ions at 0.1 mM before and after adding 1.5 μM Cys at pH 3.5 (a). The photograph of AuNPs solution before (b) and after adding Cys (c) with various cations (0.1 mM): no cation, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺.