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Herein, a flow injection amperometric aptasensor was developed for the determination of adenocarcinoma gastric cell (AGS) cancer cells in presence of hydrogen peroxide (H 2 O 2 ). For this purpose, the aptamer/gold-core and silver-shell (Au@Ag) nanoparticles were synthesized as the secondary labeled aptamers using thiolated cytosine-rich aptamer. First, the thiolated aptamer was self-assembled to the gold nanoparticles (AuNPs) surface through S-AuNPs bonds. Then, this cytosine-rich aptamer/AuNPs were added to the Ag + solution to form (C-Ag + -C) n complexes. The adsorbed Ag + ions on the aptamer were then reduced to Ag shell on the surface of AuNPs core by thyme leaves extract as a green reducing agent. The surface of screen printed electrode (SPE) was also modified with multiwall carbon nanotube decorated with gold nanoparticles (MWCNT-Au nano ) as a nano-platform to increase the surface area to immobilize primary thiolated non-cytosine-rich aptamer and improve electron transfer property of electrode. The MWCNT-Au nano , AuNPs and aptamer AGS -Au@Ag nanoparticles were characterized using transmission electron microscopy (TEM) and energy dispersive X-ray (EDX). Cyclic voltammetry and electrochemical impedance spectroscopy (EIS) were used to confirm the stepwise changes in the electrochemical surface properties of the electrode. The determination processes were performed in a flow injection microchannel device under the optimized conditions. The selective interactions between primary aptamer/AGS/secondary aptamer-Au@Ag nanoparticles led to increase in the response of aptasensor to AGS cancer cell in presence of 0.1 mM H 2 O 2 because of the electrocatalytic activity of Au@Ag nanoparticles toward the reduction of H 2 O 2 . The proposed sandwich-type electrochemical aptasensor exhibited a good response to the logarithmic value of AGS cancer cells in the range of 1 × 10 1 to 5 × 10 5 cells mL −1 with a low limit of detection of 6 cells mL −1 in the flow injection amperometric determination process. The target-aptamer dissociation constant (k d ) was also calculated based on the change in current to be 104 cells mL −1 . The fabricated sandwich-type electrochemical aptasensor was applied for the determination of AGS cancer cell in the human serum sample. The proposed sandwich-type aptasensor exhibited good linear range responsibility, selectivity, and stability. [ABSTRACT FROM AUTHOR]