We fabricated synthetic double-crossover (DX) DNA lattices and natural salmon DNA (SDNA) thin films, doped with 3 combinations of double divalent metal ions (M²⁺)-doped groups (Co²⁺-Ni²⁺, Cu²⁺-Co²⁺, and Cu²⁺-Ni²⁺) and single combination of a triple M²⁺-doped group (Cu²⁺-Ni²⁺-Co²⁺) at various concentrations of M²⁺ ([M²⁺]). We evaluated the optimum concentration of M²⁺ ([M²⁺]_O) (the phase of M²⁺-doped DX DNA lattices changed from crystalline (up to ([M²⁺]_O) to amorphous (above [M²⁺]_O)) and measured the current, absorbance, and photoluminescent characteristics of multiple M²⁺-doped SDNA thin films. Phase transitions (visualized in phase diagrams theoretically as well as experimentally) from crystalline to amorphous for double (Co²⁺-Ni²⁺, Cu²⁺-Co²⁺, and Cu²⁺-Ni²⁺) and triple (Cu²⁺-Ni²⁺-Co²⁺) dopings occurred between 0.8 mM and 1.0 mM of Ni²⁺ at a fixed 0.5 mM of Co²⁺, between 0.6 mM and 0.8 mM of Co²⁺ at a fixed 3.0 mM of Cu²⁺, between 0.6 mM and 0.8 mM of Ni²⁺ at a fixed 3.0 mM of Cu²⁺, and between 0.6 mM and 0.8 mM of Co²⁺ at fixed 2.0 mM of Cu²⁺ and 0.8 mM of Ni²⁺, respectively. The overall behavior of the current and photoluminescence showed increments as increasing [M²⁺] up to [M²⁺]_O, then decrements with further increasing [M²⁺]. On the other hand, absorbance at 260 nm showed the opposite behavior. Multiple M²⁺-doped DNA thin films can be used in specific devices and sensors with enhanced optoelectric characteristics and tunable multi-functionalities.

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