Glucometer-based quantitative determination of Hg(II) using gold particle encapsulated invertase and strong thymine-Hg(II)-thymine interaction for signal amplification

We report on a simple and sensitive protocol for quantitative assay of Hg(II) ions. The process involves the following steps: A thymine (T)-rich single-stranded DNA(1) is used to modify the surface of a well of a microplate by the use of biotin-avidin interface chemistry. In parallel, clusters of gold microspheres with a core containing the enzyme invertase were prepared and modified, via thiol chemistry, with a second T-rich oligomer (DNA(2)). If the gold clusters incorporating invertase and carrying DNA(2) are added to the DNA(1) immobilized in the wells of the microplate, no interaction will occur and the gold cluster will be removed in the subsequent washing step. If, however, Hg(II) is present in the sample, the DNA on the gold clusters incorporating the enzyme invertase will bind to the DNA in the wells due to the formation of strong T-Hg(II)-T links between the two DNA strands. The encapsulated invertase will not be removed in the following washing step. Sucrose is added in the next step along with invertase which will hydrolyze it to form glucose and fructose. The quantity of glucose formed increases with the quantity of Hg(II) ions present in the sample. The glucose generated is then quantified by using a commercial personal glucometer. Under optimal conditions, the signal for glucose increases with Hg(II) concentration in the range from 0.05 to 80 nM, and the detection limit is as low as 10 pM. The assay has good repeatability and shows an intermediate precision of down to 7.5 %. The method is highly specific for Hg(II) over other metal ions. It was applied to the determination of Hg(II) in naturally contaminated sewage and in spiked samples of drinking water. This approach has a wide scope of application in that it may be extended to numerous other kinds of nanoparticles, oligomer interactions, enzymes and ions.