Direct detection of tryptophan for rapid diagnosis of cancer cell metastasis competence by an ultra-sensitive and highly selective electrochemical biosensor

The detection of l-tryptophan (Trp) in the extracellular matrix (ECM) of solid tumors is important, particularly in metastatic tumors, which catabolize Trp to kynurenine to escape from host immune system-mediated recognition. The presence of a co-existing amino acid such as l-tyrosine (Tyr) in the ECM routinely interferes with the detection of Trp. The current study demonstrates the development of aptamer-assisted ultra-sensitive and label free biosensor (aptasensor) based on the constant current-potentiometric striping analysis (CC-PSA) technique used for quantitative Trp analysis. To prepare the aptasensor, a gold electrode was first decorated with carboxylated multiwall carbon nanotubes (MWCNTs) and then armed with Trp aptamer molecules (Apt). The engineered aptasensor was characterized electrochemically by cyclic voltammetry (CV), linear sweep voltammetry (LSV), and CC-PSA. For this biosensor, the limit of detection (LOD) was found to be 6.4 × 10-11 M (S/N = 3) and two linear detection ranges (i.e. 1.0 × 10-10 to 1.0 × 10-5 and 1.0 × 10-5 to 3.0 × 10-4 M) were observed in the calibration graph. For proof-of-technology, the aptasensor was used for the detection of Trp in biological samples such as cow's milk and human blood serum, saliva, and urine samples. Taking a good facet of the proposed aptasensor into account, it was implemented for the detection of the Trp consumption rate in various human cancer cell lines such as HepG2 (hepatocarcinoma), 1321NI (astrocytoma), Calu-6 (lung carcinoma), NCI-H1299 (lung carcinoma), and HT29 (colorectal carcinoma).