DNA-biosensor for Hg2+ determination based on polythymine-methylene blue modified gold surface and electrochemical transduction

Mercury is one of the most worrisome environmental contaminant; among its various forms, the EPA (Environmental Protection Agency) has set the limit at 10 nmol L-1 concentration of mercury (II) in drinking water [1]. In recent years, it was demonstrated that the determination of Hg2+ can be employed with an excellent degree of selectivity by the use of DNA biosensors through the formation of the complex Thymine-Hg-Thymine (T-Hg-T): in fact, Hg2+ tends to bind two thymines, generating a T-Hg-T complex with a formation constant higher than that one of the coupling Adenine-Thymine, which can be employed for a selective, fast and cost-effective Hg2+ detection [2,3] In this work we have developed a new electrochemical DNA biosensor for the selective determination of the ion Hg2+ by the use of a gold electrode where polythymine modified in 3’ position with Methylene Blue as redox probe, was immobilized. The presence of the Hg2+ in solution leads to formation of the complex T-Hg-T thus causing the “hairpin-like” folding of oligonucleotide, leading to an improved electronic exchange of methylene blue with the gold electrode and thus to an increase of the faradic current which is detected by means of square wave voltammetry (SWV). The biosensor displayed to be sensitive to Hg2+ concentration in the range 5-2000 nmol L-1 with a LOD of 2 nmol L-1. [1] Xiaoru Zhang, Ying Li, Haoran Su, Shusheng Zhang, Biosensors and Bioelectronics 25 (2010) 1338–1343 [2] Zhiqiang Zhu, Yuanyuan Su, Jiang Li, Di Li, Jiong Zhang, Shiping Song, Yun Zhao, Genxi Li, Chunhai Fan, Analytical Chemistry 81 (2009) 7660–7666 [3] Si-Jia Liu, Hua-Gui Nie, Jian-Hui Jiang, Guo-Li Shen, Ru-Qin Yu, Analytical Chemistry 81 (2009) 5724–5730