Biosensors employing DNA oligonucleotide probes and electrochemical readout have become valuable tools of growing importance in environmental analysis. DNA-based electrochemical aptasensors owe their broad applicability to the unique sensing properties of single-stranded oligonucleotides. Their appropriate design can result in DNA probes capable of achieving selective interactions with different analytes, such as heavy metal ions. Among these, the determination of Hg²⁺ in drinking water is still a pressing challenge due to the fact that high mercury concentrations are very harmful for human health, often leading to death.

The most common Hg²⁺-sensing mechanism utilizes mercury(II)'s affinity to thymine nucleobases. Various mechanisms of measurable signal generation triggered by ion–aptamer binding are known. The most common ones include the following: (i) Hg²⁺ binding by aptamers composed of thymines, where thymine bridges (T-Hg²⁺-T) are formed between two neighbouring DNA strands, (ii) a cooperative mechanism in which, apart from thymines, other nucleotides also support the adoption of a specific secondary DNA form (usually a 'hairpin’ or 'sandwich’ conformation), (iii) a competitive mechanism in which DNA with an initial 'hairpin' structure undergoes a conformational change under the influence of mercury(II) ion through its dissociation or by Hg²⁺-dependent hampering of DNA–DNA duplex formation. The selection of the signal generation mechanism is crucial for the design of aptasensors and their working parameters.

The research presented here describes comparative studies on different DNA aptamer sequences for Hg²⁺ detection according to different mechanisms. They include the formation of intra- and intermolecular T-Hg²⁺-T bridges. The thermodynamic stability of such probe–analyte complexes is compared to classical dsDNA duplexes. For this purpose, we use label-free methods, i.e. SPR or QCM, to measure the kinetics of ion–aptamer interactions. The final step is the application of the selected DNA-based receptor for the biofunctionalization of magneto-catalytic nanoparticles. Their use should further improve the performance of DNA biosensors.