Transduction of interaction between trace tryptophan and surface-confined chromium salen using impedance spectroscopy. A sensing device that works based on highly selective inhibition of mediator's Faradaic process.

It is known that tryptophan (Trp) tends to be electropolymerized over a potential window that is mostly applied for voltammetric determination of Trp. Furthermore, over the applied potential range, most of Trp-sensors suffer from some possible interferences. Knowing these challenges motivated us to establish a novel sensing device able to monitor Trp before approaching the mentioned potential range, decreasing memory-effects and some sources of non-linearity during electrochemical measurements. However, considering its molecular structure, it is unrealistic to expect tryptophan oxidation in negative potentials. This work reports the application of a surface-confined chromium-salen complex that delivered a redox couple attributed to Cr(II)/Cr(III) at a low positive potentials. The recorded cyclic voltammograms (CVs) clarified that Trp has a high inhibitory activity toward the chromium oxidation peak in the alkaline medium at a low potential of -0.18 V vs. Ag/AgCl. The positive shift of the anodic peak potential of redox reaction disclosed that Trp drastically influenced the capability of the interfacial charge transfer of the modified electrode, the features that was exploited in establishing a sensitive Trp detection method using electrochemical impedance spectroscopy. The proposed sensor was successfully employed to determine Trp over a concentration range of 4.0-60.0 nM with the calculated detection limit of 0.78 nM.