Fig. 1. (Color online) Schematic diagram illustrating the fabrication of the paper-based cTnI immunosensor and their usage in detection of cTnI.

Fig. 2. (Color online) Characterizations of MXene. (a) Scheme showing the crystal structure of the MXene and a photograph of the colloid solution of the MXene. (b) TEM image of the MXene. (c) XRD patterns of the etched MXene (black line) and original MAX phase (red line). (d) AFM image of the Ti₃C₂ MXene. The insert is the line profile of the white line in (d).

Fig. 3. (Color online) Surface modifications of the electrodes. (a) EIS of the modified electrodes measured in a mixed solution of KCl (0.1 mol/L) and K₃[Fe(CN)₆] (0.005 mol/L). (b) CV of the modified electrodes, measured at a scan rate of 50 mV/s in a mixed solution of KCl (0.1 mol/L) and K₃[Fe(CN)₆] (0.005 mol/L).

Fig. 4. (Color online) Optimization of the immunosensor. (a) The effect of pH value of PBS (5.5, 6, 6.5, 7, 7.4, 8, 8.5, and 9) on the current response of the immunosensor. (b) The effect of the immobilization period (10, 20, 30, 40, 50, and 60 min) of antibodies on the current response of the immunosensor, error bar = standard deviation (n = 5). (c) The effect of the washing times (1, 2, 3, 4, and 5) on the current response of the immunosensor.

Fig. 5. (Color online) Electrochemical detection of cTnI. (a) DPV responses of the immunosensor to cTnI at concentrations of 0–100 ng/mL. (b) Calibration curve toward different concentrations of cTnI, error bar = standard deviation (n = 7).

Fig. 6. (Color online) Characterization of the repeatability and selectivity of the immunosensor. (a) The selectivity to cTnI against non-target protein molecules: BSA, AFP, CEA, GOx, NSE, HCG, and Blank. (b) Reproducibility of the proposed immunosensors in the detection of cTnI.

Table 1. Performance comparisons of different cTnI biosensors.