Electrochemical chiral sensing of tryptophan enantiomers by using 3D nitrogen-doped reduced graphene oxide and self-assembled polysaccharides

A chiral sensor is described for the enantioselective recognition of L- and D-tryptophan (Trp). The sensor is based on the use of (a) Cu(II) ions coordinated with beta-cyclodextrin (Cu-beta-CD) that was self-assembled with carboxymethyl cellulose (CD-Cu-CMC) as a chiral selector, (b) of N-doped reduced graphene oxide (N-rGO) as substrate materials, and (c) of differential pulse voltammetry that was used for enantiorecognition. The 3D N-rGO was prepared by using reduced graphene oxide and pyrrole as the starting materials. Electrostatic interaction occurs between the carboxy groups of CMC and Cu(II) ions in Cu-beta-CD. The FT-IR, SEM, XRD and XPS techniques showed that 3D N-rGO and the CD-Cu-CMC composite were successfully synthesized. The 3D N-rGO enabled the immobilization of the chiral selector (CD-Cu-CMC) and improves the active areas. A glassy carbon electrode was modified with N-rGO/CD-Cu-CMC and then showed a stronger electrochemical signal for L-Trp than for D-Trp, typically at a working potential of around 0.78V (vs. SCE). UV-vis spectroscopy proved that CD-Cu-CMC has a higher affinity for D-Trp. The enantioselectivity for D-Trp over L-Trp is 4.72. The modified electrode had a limit of detection of 0.063 mu M and 0.0035 mu M for L-Trp and D-Trp, respectively, with a linear response range of 0.01mM to 5mM. The sensor was used to detect Trp (D- or L-Trp) in spiked real human urine and human serum protein samples.