Label-free biosensing of mercury(II) in milk using an aptamer-gated graphene field-effect transistor

Due to the environmental pollution by the growing urban, agricultural and industrial disposals, dairy products are susceptible to heavy metal pollution in the production process. As one of the most toxic heavy metals, mercury cause serious damage to the environment and even public health. Effective monitoring of Hg(II) in large number of diary samples requires the development of rapid assaying methods. In this research work, we report the preparation of a label-free biosensor for Hg(II) using an aptamer-gated graphene field-effect transistor (Apt-SGGT) with the gate electrode modified by a structure-switching aptamer against Hg(II) and blocked with bovine serum albumin. The aptamer immobilized on the gate electrode acts as both a receptor and a reporter for Hg(II) by specifically recognizing Hg(II) through stable T-Hg2+-T coordination complex, triggering the folding of the single-stranded aptamer into a duplex structure. The conformational change results in the redistribution of the surface charge on the gate electrode, and produces a large channel current response by the inherent field-effect amplification. This sensing mechanism can effectively enhance the selectivity of the SGGT-based biosensor to mercury ions by significantly reducing the unspecific responses of other interfering cations. The detection limit of Hg(II) by Apt-SGGT sensor was as low as 5 nM, and the linear detection range was from 5 to 200 nM. The sensor can realize the detection of mercury ion residues in milk with a high recovery rate, and has a good application prospect in the detection and analysis of heavy metal residues in complex systems.