An exonuclease III-aided “turn-on” fluorescence assay for mercury ions based on graphene oxide and metal-mediated “molecular beacon”

Abstract

A novel fluorescence “turn-on” strategy, which is based on the formation of Hg²⁺-mediated molecular-beacons (MBs), the preferable cleavage capacity of exonuclease III to double-stranded DNA compared with single-stranded one, and the remarkable difference in the binding ability of graphene oxide (GO) with single-stranded DNA and the mononucleotides, is designed for Hg²⁺ assay. The Hg²⁺-mediated base pairs facilitate the dye labeled MBs to fold into a hairpin structure, which is more likely to be digested by exonuclease III, and an obvious increase in the fluorescence intensity is observed after incubating with GO due to the weak affinity of the product-mononucleotides to GO. A fluorescent “turn-on” method based on graphene oxide and exonuclease III was designed for Hg²⁺ assay. The introduction of GO greatly increases the signal-to-background ratio, and the sensitivity is significantly improved due to the amplified capability of exonuclease III. Under the optimal conditions, Hg²⁺ is specifically and sensitively detected with a detection limit of 0.83 nM. Compared with the reported Hg²⁺ assay methods, the proposed strategy is simple, cost effective and selective, which might provide a new platform for developing a sensitive Hg²⁺ biosensor. Mercury level in the blood is an important indicator of mercury poisoning in clinical study. To testify the possibility of the use of this method for the assay of Hg²⁺ in real samples, detection of Hg²⁺ in 1% human serum was investigated and satisfactory results were obtained, which suggests that this method has great potential for bioanalysis.