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Additional file 1: Table S1. Clinical characteristics of individuals in the detection of cathepsin B. Table S2. The yield of biopanning for cathepsin B. Table S3. Screening results of biopanning for identifying cathepsin B specific peptides via phage display. Table S4. The synthetic peptides for detection of cathepsin B. Table S5. Analysis of binding site of CTSB BP3 peptide for cathepsin B using molecular docking. Table S6. The electroactive surface area of bare gold and AuNPs–MXene fabricated electrodes. Table S7. Detection capability of developed sensors at different concentrations of cathepsin B spiked in human plasma. Table S8. Comparison of the analytical performance for the detection of cathepsin B. Table S9. Analytical results between our developed sensor and the reference ELISA with Crohn’s patient samples. Figure S1. The validation assay for biotinylated cathepsin B protein. (a) The biotin-labeled BSA (250 nM = 16.5 μg/mL) and CTSB (250 nM = 6.875 μg/mL) were immobilized on streptavidin coated plate, and it was confirmed using HRP conjugated streptavidin and ABTS at 405 nm. (b) To quantitate labelled biotin, the biotinylated cathepsin B was added to the mixture of HABA and avidin and biotinylated HRP and BSA were used as control. (c) Biotin ratio was calculated from biotin quantitation results, which biotin ratio for cathepsin B was about 3.745. Figure S2. The responses of electrochemical measurements of phage-on-a sensor. (a–c) CV, EIS and SWV responses for preparation steps of phage sensor (bare Au, Au@1 mM MUA, Au@ 400 mM/100 mM EDC/NHS, Au@4-1 phage). It was conducted at 1 M KNO3 with 2.5 mM [Fe(CN)6]3−/4−. (d) X-ray photoelectron spectroscopy (XPS) spectra of bare Au, Au@MUA-EDC/NHS, Au@4-1 phage. Figure S3. The responses of electrochemical measurements of peptide-on-a sensor. (a–c) CV, EIS, and SWV responses for preparation steps of phage sensor (bare Au, Au@1 mM MUA, Au@ 400 mM/100 mM EDC/NHS, Au@streptavidin, Au@CTSB BP3 peptide). It was conducted at 1 M KNO3 with 2.5 mM [Fe(CN)6]3−/4−. (d) X-ray photoelectron spectroscopy (XPS) spectra of bare Au, Au@MUA-EDC/NHS, Au@streptavidin, Au@CTSB BP3 peptide. Figure S4. Effect of peptide concentration for peptide-on-a sensor. (a) The current responses of different CTSB BP3 peptide concentrations (5–100 μM). (b) The current changes of peptide-on-a sensor in different CTSB BP3 peptide concentrations for cathepsin B. Figure S5. Morphological characterization of AuNPs-MXene composite; SEM images under (a) low and (b) high resolution; TEM images under (c) low and (d) high resolution. Figure S6. (a) TEM image and (b) size distribution graph of pre-synthesized AuNPs. (c) TEM image and (d) SEM image of Ti3C2F MXene. Figure S7. (a) EDS spectrum of AuNPs-MXene composites. (b-e) Elemental mapping images obtained from EDS for elements Ti, C, F and Au, respectively. Figure S8. (a) X-ray diffraction spectra and (b) FT-IR spectra of AuNPs-MXene composites (red line) and MXene (black line). Figure S9. Optimization of AuNP-MXene concentration and comparison of the effect of scan rate on gold electrode. (a) Chronoamperometric response of the sensor with different concentration of AuNP-MXene. (b) Square wave voltammetry analysis of the sensor with different concentration of AuNP-Mxene. Figure S10. Effect of peptide concentration for peptide-on-a-AuNPs/MXene sensor. (a) The current responses of different CTSB BP3 peptide concentrations (5–100 μM). (b) The current changes of peptide-on-a-AuNPs/MXene sensor in different CTSB BP3 peptide concentrations for cathepsin B. Figure S11. Comparison of standard curves of the developed sensors: a) Phage-on-a-sensor, b) Peptide-on-a-sensor. The change of current was measured by SWV with 1 M KNO3 containing 2.5 mM [Fe(CN)6]3−/4−. In phage-on-a-sensor, the titer of phages was 1 × 1012 PFU/mL, while the concentration of peptide used in peptide-on-a-sensor was 50 μM. All measurements were done in triplicate, and error bars represent standard deviations. Figure S12. Comparison of stability of the developed sensor. (a) Phage on a sensor, (b) peptide on a sensor, (c) peptide on a AuNPs/MXene sensor.