Fig 2: Detection of CEA by the metasurface biosensor and ELISA. (a) CEA detection curve in a low concentration range down to 8 pg/mL, measured by the metasurface biosensor. Measured data are shown using orange closed circles with error bars. Dashed curve is a fitted curve by the Hill equation (Equation (1)). (b) Detection results for three CEAs (black closed circle: ab264604, Abcam, blue triangle: 15-N-100, Alpha Diagnostic, green open circle: ab742, Abcam) by ELISA, which was conducted using a commercial kit. The vertical axis represents optical density at 450 nm. Three horizontal bars indicate zero levels for the three CEAs; the colors match up the measured data.

Fig 3: CEA detection. (a) Schematic of incubation and immobilization of the CEA sandwich bodies on the metasurface of a periodic array of Si nanopellets. (b) Detection of CEA in the sample diluent buffer. Measured FL intensities are shown using orange closed circles with error bars. Dashed curves denote fitted curves by the Hill equation (Equation (1)). 3s level from the zero level is indicated by a horizontal bar; the crossing point with the Hill curve means the limit of detection, indicating 0.002 ng/mL (arrow). The inset magnifies the detection curve around the zero concentration on a linear scale. (c) FL signals from CEA in the human serum. Horizontal bar indicates zero-signal level. Arrow indicates the clinical criterion of 5 ng/mL.