Template-independent, in situ grown DNA nanotail enabling label-free femtomolar chronocoulometric detection of nucleic acids.

A routine electrochemical DNA (E-DNA) sensor requires either an exquisite design of conformation-switchable recognition probe that is critical to facilitate electron transfer at a sensing interface, or a template-dependent DNA amplification, which often involves designing prone-to-false "sticky ends" and labeling redox tags at one end of the signal probes. Here we report an in situ grown DNA nanotail (IGT)-mediated straightforward and template-free signal amplification strategy for highly sensitive and sequence-specific DNA detection. This novel electrochemical IGT (E-IGT) DNA sensor can quantify target nucleic acids in a label-free manner because the electrochemical signals are generated by chronocoulometric interrogation of redox [Ru(NH3)6](3+) that electrostatically and quantitatively binds to the negatively charged phosphate moieties in the electrode surface-attached DNA. By introduction of terminal deoxynucleoside transferase (TdT) to this sensor design, both the sensitivity and selectivity have been significantly enhanced. This DNA sensor achieves an impressive detection limit of 20 fM for a DNA sequence with 22 nucleotides, which is lower than that of an analogous optical DNA sensor by 2 orders of magnitude. More importantly, it exhibits excellent selectivity against even a single-base mismatched sequence. In addition, this novel DNA sensor presents reliable reusability and is capable of measuring target DNA in complex matrixes, such as undiluted human serum, with minimal interference. These advantages make our E-IGT sensor a promising contender in the E-DNA sensor family for medical diagnostics.