Ultrasensitive and reusable electrochemical aptasensor for detection of tryptophan using of [Fe(bpy) 3 ](p-CH 3 C 6 H 4 SO 2 ) 2 as an electroactive indicator.

In this paper, we report the application of a reusable electrochemical aptasensor for detection of tryptophan by using [Fe(bpy)3 ](p-CH3 C6 H4 SO2 )2 as an electroactive indicator and based on the target-compelled aptamer displacement. The aptasensor fabricated by self-assembling the thiolated probe on the surface of graphite screen-printed electrode modified with gold nanoparticles/multiwalled carbon nanotubes and chitosan nanocomposite (AuNPs/MWCNTs-Chit/SPE). Afterward, Trp aptamer (Apt) immobilized on the modified electrode surface through hybridization. In the absence of Trp, a sharp peak of [Fe(bpy)3 ](p-CH3 C6 H4 SO2 )2 can be observed in differential pulse voltammetry (DPV) study. The introduction of Trp led to the formation of aptamer-Trp complex and dissociation of the aptamer from the DNA-Apt duplex on the electrode surface into the solution and decreases the peak current intensity of electroactive indicator. This is because, [Fe(bpy)3 ](p-CH3 C6 H4 SO2 )2 tends to bind to the two strands DNA. Therefore, the peak current of [Fe(bpy)3 ](p-CH3 C6 H4 SO2 )2 linearly decreased with increasing the concentration of Trp over a range of 3.0 nM- 100 μM. The detection limit (3 σ) was 1.0 nM. In addition, we examined the selectivity of the constructed biosensor for tyrosine, histidine, arginine, lysine, valine and methionine that belonged to the amino acid family. The obtained results showed that the fabricated sensor had a good selectivity for Trp against the other examined amino acids. Also, the potential applicability of the aptasensor was investigated by detecting the Trp in a complex media such as human blood plasma spiked with Trp.