Sensitive Electrochemical Detection of Microcystin-LR in Water Samples Via Target-Induced Displacement of Aptamer Associated [Ru(NH3)6]3.

In this study, we demonstrate the successful development of an electrochemical aptamer-based sensor for point-of-use detection and quantification of the highly potent microcystin-LR (MC-LR) in water. The sensor uses hexaammineruthenium(III) chloride ([Ru(NH3)6]3+) as redox mediator, because of the ability of the positively charged (3+) molecule to associate with the phosphate backbone of the nucleic acids. We quantitatively measure the target-induced displacement of aptamer associated, or surface confined, [Ru(NH3)6]3+ in the presence of MC-LR. Upon the addition of MC-LR in the water, surface-confined [Ru(NH3)6]3+ dissociates, resulting in less faradaic current from the reduction of [Ru(NH3)6]3+ to [Ru(NH3)6]2+ Sensing surfaces of highly packed immobilized aptamers were capable of recording decreasing square wave voltammetry (SWV) signals after the addition of MC-LR in buffer. As a result, SWV recorded substantial signal suppression within 15 min of target incubation. The sensor showed a calculated limit of detection (LOD) of 9.2 pM in buffer. The effects of interferents were minimal, except when high concentrations of natural organic matter (NOM) were present. Also, the sensor performed well in drinking water samples. These results indicate a sensor with potential for fast and specific quantitative determination of MC-LR in drinking water samples. A common challenge when developing electrochemical, aptamer-based sensors is the need to optimize the nucleic acid aptamer in order to achieve sensitive signaling. This is particularly important when an aptamer experiences only a small or localized conformational change that provides only a limited electrochemical signal change. This study suggests a strategy to overcome that challenge through the use of a nucleic acid-associated redox label.