AbstractThe feasibility of designing a highly selective and sensitive label‐free cost‐effective biosensor to determine Hg2+ ions using gold electrodes functionalized with short‐chain, thymine‐rich, ss‐DNA oligonucleotides wrapped MWNTs is examined. The potassium ferricyanide was used as a redox marker for analytical signal generation. Biosensor response was based on the difference in the electrochemical signal before and after subjecting it to a sample containing Hg2+ ions. The lowest detection limit of 1 femtomolar (fM) of mercury ions was observed in the aqueous medium, together with good selectivity against common metal cations. The method developed was successfully applied for the determination of Hg2+ ions in environmental waste water samples.