Fig. 1. Schematic illustration of the fabrication of the three proposed biosensors.

Fig. 2. (a) Photomicrograph of a unit cell of the microstructure. Inset: picture of the sample. (b) Schematic of the unit cell, which consists of D and C shapes. The geometric parameters for D shape are R=50  μm and r=40  μm and for C shape are r1=20  μm and d=6  μm. The periodicity is 130 μm. (c) Raman spectrum of graphene. Inset: schematic of the graphene sample. (d) X-ray diffraction (XRD) pattern of the g-C3N4 film. Inset: representative scanning electron microscopy image of g-C3N4.

Fig. 3. Left: schematic illustration of three biosensors. Right: simulated and experimental transmission spectrum of the three samples, (a) MS@CN sample, (b) MS@Gr sample, and (c) MS@HTJ sample.

Fig. 4. Simulated and experimental transmission spectrum of the three samples, (a) MS@CN, (b) MS@Gr, and (c) MS@HTJ, respectively.

Fig. 5. Schematic of the energy band structure under different casein concentrations: (a) g-C3N4, (b) graphene, and (c) heterojunction.

Fig. 6. Dependence of (a) frequency and (b) transmission values on protein concentration increasing from 0 to 1.56 ng/mL. Dependence of (c) frequency and (d) transmission difference values on protein concentration increasing from 0 to 1.56 ng/mL.

Fig. 7. Plots of energy bands along the vertical direction from the front surface to the rear surface of (a) g-C3N4, (d) graphene, and (g) g-C3N4-graphene heterojunction as a function of the positive fixed charges density (Qf) at the surface; activation energy (Ea) near the front surface of (b) g-C3N4, (e) graphene, and (h) C3N4-graphene heterojunction as a function of the positive fixed charge density (Qf) at the surface; electron concentration mapping for Qf=+1012 cm−2 of (c) g-C3N4, (f) graphene, and (i) g-C3N4-graphene heterojunction.

Fig. 8. Phase difference between the bare sensor (C0) and each CC tested for the (a), (d) MS@CN sample, (b), (e) MS@Gr sample, and (c), (f) MS@HTJ sample.