Logic control of enzyme-like gold nanoparticles for selective detection of lead and mercury ions

Functional logic gates based on lead ions (Pb²⁺) and mercury ions (Hg²⁺) that induce peroxidase-like activities in gold nanoparticles (Au NPs) in the presence of platinum (Pt⁴⁺) and bismuth ions (Bi³⁺) are presented. The "AND" logic gate is constructed using Pt⁴⁺/Pb²⁺ as the input and the peroxidase-like activity of the Au NPs as the output; this logic gate is denoted as "Pt⁴⁺/Pb²⁺(AND)-Au NP_POX". When Pt⁴⁺ and Pb²⁺ coexist, strong metallophilic interactions (between Pt and Pb atoms/ions) and aurophilic interactions (between Au and Pb/Pt atoms/ions) result in significant increases in the deposition of Pt and Pb atoms/ions onto the Au NPs, leading to enhanced peroxidase-like activity. The "INHIBIT" logic gate is fabricated by using Bi³⁺ and Hg²⁺ as the input and the peroxidase-like activity of the Au NPs as the output; this logic gate is denoted as "Bi³⁺/Hg ²⁺(INHIBIT)-Au NP_POX". High peroxidase-like activity of Au NPs in the presence of Bi³⁺ is a result of the various valence (oxidation) states of Bi³⁺ and Au (Au⁺/Au⁰) atoms on the nanoparticle's surface. When Bi³⁺ and Hg²⁺ coexist, strong Hg-Au amalgamation results in a large decrease in the peroxidase-like activity of the Au NPs. These two probes (Pt⁴⁺/ Pb²⁺(AND)-Au NP_POX and Bi³⁺/Hg ²⁺(INHIBIT)-Au NP_POX) allow selective detection of Pb ²⁺ and Hg²⁺ down to nanomolar quantities. The practicality of these two probes has been validated by analysis of Pb²⁺ and Hg²⁺ in environmental water samples (tap water, river water, and lake water). In addition, an integrated logic circuit based on the color change (formation of reddish resorufin product) and generation of O₂ bubbles from these two probes has been constructed, allowing visual detection of Pb²⁺ and Hg²⁺ in aqueous solution.