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Biosensors fabricated with nanomaterials promise faster, cheaper, and more efficient alternatives to traditional, often bulky devices for early cancer diagnosis. In this study, we fabricated a thin film sensing unit on interdigitated gold electrodes combining polyethyleneimine and carbon nanotubes in a layer by layer fashion, onto which antibodies anti-CA19-9 were adsorbed with a supporting layer of N-hydroxysuccinimide and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide solution. By use of impedance spectroscopy, the pancreatic cancer biomarker CA19-9 was detected in a buffer with limit of detection of 0.35 U/mL. This high sensitivity allowed for distinction between samples of blood serum from patients with distinct probabilities to develop pancreatic cancer. The selectivity of the biosensor was confirmed in subsidiary experiments with HT-29 and SW-620 cell lines and possible interferents, e.g., p53 protein, ascorbic acid, and glucose, where significant changes in capacitance could only be measured with HT-29 that contained the CA19-9 biomarker. Chemisorption of CA19-9 molecules onto the layer of anti-CA19-9 antibodies was the mechanism responsible for sensing while electrostatic interactions drove the adsorption of carbon nanotubes, according to polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS). The adsorption behavior was successfully described by the Langmuir-Freundlich isotherm.