In this work, we report on the development of an electrochemical biosensor for high selectivity and rapid detection of Hg2+ and Pb2+ ions using DNA-based specific aptamer probes labeled with ferrocene (or methylene blue) and thiol groups at their 5′ and 3′ termini, respectively. Aptamers were immobilized onto the surface of screen-printed gold electrodes via the SH (thiol) groups, and then cyclic voltammetry and impedance spectra measurements were performed in buffer solutions with the addition of HgCl2 and PbCl2 salts at different concentrations. Changes in 3D conformation of aptamers, caused by binding their respective targets, e.g., Hg2+ and Pb2+ ions, were accompanied by an increase in the electron transfer between the redox label and the electrode. Accordingly, the presence of the above ions can be detected electrochemically. The detection of Hg2+ and Pb2+ ions in a wide range of concentrations as low as 0.1 ng/mL (or 0.1 ppb) was achieved. The study of the kinetics of aptamer/heavy metal ions binding gave the values of the affinity constants of approximately 9.10−7 mol, which proved the high specificity of the aptamers used.

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This work presents a proof of concept of a novel, simple, and sensitive method of detection of dopamine, a neurotransmitter within the human brain. We propose a simple electrochemical method for the detection of dopamine using a dopamine-specific aptamer labeled with an electrochemically active ferrocene tag. Aptamers immobilized on the surface of gold screen-printed gold electrodes via thiol groups can change their secondary structure by wrapping around the target molecule. As a result, the ferrocene labels move closer to the electrode surface and subsequently increase the electron transfer. The cyclic voltammograms and impedance spectra recorded on electrodes in buffer solutions containing different concentration of dopamine showed, respectively, the increase in both the anodic and cathodic currents and decrease in the double layer resistance upon increasing the concentration of dopamine from 0.1 to 10 nM L −1 . The high affinity of aptamer-dopamine binding (KD ≈ 5 nM) was found by the analysis of the binding kinetics. The occurrence of aptamer-dopamine binding was directly confirmed with spectroscopic ellipsometry measurements.Chemosensors 2020, 8, 28 2 of 11 done on the electrochemical detection of dopamine and other neurotransmitters [2,3]. Dopamine can be very easily detected in an aqueous solution by electrochemical methods such as cyclic voltammetry since dopamine undergoes electrochemical oxidation. However, a major issue is the selectivity since complex matrices such as blood contain a range of other oxidizable compounds such as urate or ascorbate which would also generate signals, leading to inaccurate readings. Selectivity towards dopamine is therefore required.A wide range of electrode materials have been proposed to increase the selectivity of dopamine detection [2,3]. A few recent examples include the use of materials such as Nafion combined with graphite [4] or multi-walled carbon nanotubes [5] to improve the selectivity towards dopamine. Other researchers have utilized graphene-modified screen-printed electrodes [6] or graphene aerogels [7] as substrates for the selective determination of dopamine. Composites of carbon nanotubes and graphene oxide could be used to determine dopamine, nitrate, ascorbate, and urate [8]. Metal nanoparticles have also been utilized, for example palladium nanoparticles combined with graphene [9], which were used as a base of electrochemical determination of dopamine whereas a gold nanoparticle/DNA/polymer composite could be used for the simultaneous determination of dopamine, uric acid, guanine, and adenine [10]. Composites of gold nanoparticles with conducting polymers have also been used for the selective determination of dopamine [11]. Boron-doped carbon nanotubes could be used to determine levels of dopamine and ephedrine in the presence of urate [12]. Composites of graphene oxide with Bi 2 S 3 nanorods were used for the electrochemical determination of dopamine [13], and arrays of cylindrical gold nanoelectrodes could be used for both the detection of dopamine an...