Logical regulation of the enzyme-like activity of gold nanoparticles by using heavy metal ions

In this study we employed self-deposition and competitive or synergistic interactions between metal ions and gold nanoparticles (Au NPs) to develop OR, AND, INHIBIT, and XOR logic gates through regulation of the enzyme-like activity of Au NPs. In the presence of various metal ions (Ag⁺, Bi ³⁺, Pb²⁺, Pt⁴⁺, and Hg²⁺), we found that Au NPs (13 nm) exhibited peroxidase-, oxidase-, or catalase-like activity. After Ag⁺, Bi³⁺, or Pb²⁺ ions had been deposited on the Au NPs, the particles displayed strong peroxidase-like activity; on the other hand, they exhibited strong oxidase- and catalase-like activities after reactions with Ag⁺/Hg²⁺ and Hg ²⁺/Bi³⁺ ions, respectively. The catalytic activities of these Au NPs arose mainly from the various oxidation states of the surface metal atoms/ions. Taking advantage of this behavior, we constructed multiplex logic operations - OR, AND, INHIBIT, and XOR logic gates - through regulation of the enzyme-like activity after the introduction of metal ions into the Au NP solution. When we deposited Hg²⁺ and/or Bi³⁺ ions onto the Au NPs, the catalase-like activities of the Au NPs were strongly enhanced (>100-fold). Therefore, we could construct an OR logic gate by using Hg ²⁺/Bi³⁺ as inputs and the catalase-like activity of the Au NPs as the output. Likewise, we constructed an AND logic gate by using Pt ⁴⁺ and Hg²⁺ as inputs and the oxidase-like activity of the Au NPs as the output; the co-deposition of Pt and Hg atoms/ions on the Au NPs was responsible for this oxidase-like activity. Competition between Pb ²⁺ and Hg²⁺ ions for the Au NPs allowed us to develop an INHIBIT logic gate - using Pb²⁺ and Hg²⁺ as inputs and the peroxidase-like activity of the Au NPs as the output. Finally, regulation of the peroxidase-like activity of the Au NPs through the two inputs Ag⁺ and Bi³⁺ enabled us to construct an XOR logic gate.