A schematic representation of the aptamer–antibody sandwich assay.

Application of aptamers as the sensing layers in biosensors offers several advantages for the detection of various analytes. Herein, an electrochemical label - free biosensor for dopamine (DA) was developed with the use of 57-mer DNA aptamer. The proposed aptasensor operates via specific interaction between dopamine and aptamer sequence tethered to the gold disk electrode via thiol group. DA concentration was determined without use of any external redox indicator and the dopamine oxidation current serves as the analytical signal. Electrodes modified with DNA aptamer show selectivity toward dopamine within micromolar range with the lower detection limit of 26.8 μmol·L⁻¹. The developed assay exhibits minor response in the presence of various interferents and its utility was tested in sample of diluted serum. Furthermore, the performance of proposed sensor was improved with LOD of 3.36 μmol·L⁻¹ and the range of linear response from 5 to 75 μmol·L⁻¹ by utilization of DNA aptamer/AuNP/reduced graphene oxide - modified glassy carbon electrode.

Protein biomarkers of cancer allow a dramatic improvement in cancer diagnostics as compared to the traditional histological characterisation of tumours by enabling a non-invasive analysis of cancer development and treatment. Here, an electrochemical label-free assay for urokinase plasminogen activator (uPA), a universal biomarker of several cancers, has been developed based on the recently selected uPA-specific fluorinated RNA aptamer, tethered to a gold electrode via a phosphorothioated dA tag, and soluble redox indicators. The binding properties of the uPA–aptamer couple and interference from the non-specific adsorption of bovine serum albumin (BSA) were modulated by the electrode surface charge. A nM uPA electroanalysis at positively charged surfaces, complicated by the competitive adsorption of BSA, was tuned to the pM uPA analysis at negative surface charges of the electrode, being improved in the presence of negatively charged BSA. The aptamer affinity for uPA displayed via the binding/dissociation constant relationship correspondingly increased, ca. three orders of magnitude, from 0.441 to 367. Under optimal conditions, the aptasensor allowed 10⁻¹²–10⁻⁹ M uPA analysis, also in serum, being practically useful for clinical applications. The proposed strategy for optimization of the electrochemical protein sensing is of particular importance for the assessment and optimization of in vivo protein ligand binding by surface-tethered aptamers.