Reusable potentiometric screen-printed sensor and label-free aptasensor with pseudo-reference electrode for determination of tryptophan in the presence of tyrosine

Analysis of l-tryptophan (Trp) in biological samples has great importance for biomedical studies. Amino acid tyrosine (Tyr) that usually coexist with Trp in biological fluids can significantly interfere with reliable determination of Trp. In the current study, we demonstrate the development of two ultra-sensitive electrochemical sensor and label-free aptasensor for selective analysis of Trp in biological samples (i.e., cow's milk and human plasma, saliva and urine samples). In addition, without using AgCl/KCl, an Ag pseudo-reference screen printed electrode (Ag-PR-SPE) was exploited as a reference electrode. To prepare the engineered Trp sensor/aptasensor, a gold SPE was first modified with multiwall carbon nanotube (MWCNT-AuSPE) and then armed with Trp aptamer molecules (Apt-MWCNT-AuSPE). The prepared sensors were characterized using constant current-potentiometric stripping analysis (CC-PSA) and electrochemical impedance spectroscopy (EIS). The MWCNT-AuSPE and Apt-MWCNT-AuSPE were compared with respect to the linear detection range, limit of detection (LOD), accuracy, precision, repeatability. MWCNT-AuSPE and Apt-MWCNT-AuSPE demonstrate fast near-Nernstian response for PSA of Trp over the concentration ranging from 1.0×10-9 to 2.0×10-4 mol L-1 and 1.0×10-11 to 1.0×10-4 mol L-1 with detection limits of 3.6×10-10 mol L-1 and 4.9×10-12 mol L-1, respectively. Common interfering species present in the biological fluids (i.e., tyrosine, uric acid, ascorbic acid) showed no effects on the determination of Trp using CC-PSA. MWCNT-AuSPE and Apt-MWCNT-AuSPE represented well reproducibility and great precision with relative standard deviation (RSD) of 2.9% and 5.3% respectively. In comparison with the MWCNT-AuSPE, Apt-MWCNT-AuSPE provided higher sensitivity, selectivity and accuracy of Trp detection in real samples. Based on these findings, we propose the developed Apt-MWCNT-AuSPE as a simple detection method for analysis of Trp in biological samples.