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 pt(4+)/Pb2+(AND)-Au NPPOX. 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+/Hg2+(INHIBIT)-Au NPPOX. 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+/Au-0) 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 NPPOX and Bi3+/He2+(INHIBIT)-Au NPPOX) allow selective detection of Pb2+ 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 O-2 bubbles from these two probes has been constructed, allowing visual detection of Pb2+ and Hg2+ in aqueous solution.