Fig. 6. The absorbance ratio of A_640/520 in the presence of transition metal ions at 1.0 mM before and after adding 1.5 μM Cys at pH 5 (a). The photograph of AuNPs solution before (b) and after adding Cys (c) with various cations (1.0 mM): no cation, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺.