Surface state of the dopamine RNA aptamer affects specific recognition and binding of dopamine by the aptamer-modified electrodes

Specific monitoring of dopamine, in the presence of structurally related neurotransmitters, is critical for diagnosis, treatment and mechanistic understanding of a variety of human neuropathologies, but nevertheless the proper tools are scarce. Recently, an electrochemical aptasensor for specific analysis of dopamine, exploiting dopamine biorecognition by the RNA aptamer electrostatically adsorbed onto a cysteamine-modified electrode, has been reported (Analytical Chemistry 85 (2013) 121). However it was not clear which way dopamine biorecognition and binding by such aptamer layers proceed and if they can be improved. Here, we show that the aptamer surface state, in particular the aptamer surface density, in a bell-shaped manner affects the dopamine binding, being maximal for the 3.5 +/- 0.3 pmol cm(-2) monolayer coverage of the aptamer molecules lying flat on the surface. Therewith, the aptamer affinity for dopamine increases one order of magnitude due to electrostatically regulated immobilization, with the aptamer-dopamine dissociation constant of 0.12 +/- 0.01 mu M versus 1.6 +/- 0.17 mu M shown in solution. Under optimal conditions, 0.1-2 mu M dopamine was specifically and 85.4 nA mu M-1 cm(-2) sensitively detected, with no interference from structurally related catecholamines. The results allow improvement of the robustness of dopamine monitoring by aptamer-modified electrodes in biological systems, within the 0.01-1 mu M dopamine fluctuation range.