Fig. 1. Preparation process of G/HMM/D-POF.

Fig. 2. Schematic of an experimental setup based on the G/HMM/D-POF sensor.

Fig. 3. Simulation results. (a) Real and imaginary parts of the in-plane εx and out-of-plane εz permittivity components for an n*Au/Al2O3 structure with n=3. (b) Critical wavelength for n*Au/Al2O3 structures as a function of n value. (c) Simulation setup of the structure n*Au/Al2O3 (n=2–5). (d) Normalized transmission spectra of Au/D-POF and n*Au/Al2O3 (n=2–5) for RI increasing from 1.340 to 1.352, respectively. (e) SPR peak wavelengths at the RI of 1.340 and their redshifts at RIs varying from 1.340 to 1.352 for Au/D-POF and n*Au/Al2O3 (n=2–5). (f) Electric field distributions within Au/D-POF and n*Au/Al2O3 (n=2–5) structures at the resonance wavelengths for the RI of 1.340. (g) NormE values for Au/D-POF and n*Au/Al2O3 (n=2–5).

Fig. 4. (a) Optical graph of Au/D-POF and n*Au/Al2O3 (n=2–5). (b) Top SEM image of 3*Au/Al2O3. (c) SEM cross-sectional image of a 50-nm-thick gold structure. (d) SEM cross-sectional image of 3*Au/Al2O3. (e) EDS spectra of 3*Au/Al2O3. (f)–(h) EDS element distribution maps from (f) Al, (g) O, (h) Au on 3*Au/Al2O3.

Fig. 5. (a)–(e) Normalized transmission spectra of Au/D-POF and n*Au/Al2O3 (n=2–5) in the ethanol solution with RIs ranging from 1.340 to 1.352, respectively. (f) SPR peak wavelengths for Au/D-POF and n*Au/Al2O3 (n=2–5) in the ethanol solution at the RI (1.340) and the redshifts caused by increasing RI (from 1.340 to 1.352).

Fig. 6. (a) Front SEM image of G/HMM/D-POF in the sensing area. (b) Raman spectra of graphene oxide and chemically produced graphene. The inset shows the 2905  cm−1 peak position distribution within a scanned area of 20 μm×20  μm. (c) Normalized transmission spectra of G/HMM/D-POF in the ethanol solutions with RIs from 1.340 to 1.352. (d) Resonance wavelength redshift of G/HMM/D-POF as a function of RI. (e) Normalized transmission spectra of G/HMM/D-POF at the RI of 1.352, recorded in a cycling mode (up to 10 cycles). (f) Typical response-recovery characteristic curve of G/HMM/D-POF in ethanol solution (RI of 1.352) at a resonance wavelength of 648 nm.

Fig. 7. (a) Normalized transmission spectra of the G/HMM/D-POF sensor during DNA modification. (b) Normalized transmission spectra of G/HMM/D-POF in t_DNA solutions with concentrations varying from 10 pM to 100 nM after being sufficiently complementary. (c) Redshift and SPR peak wavelength versus t_DNA concentration.

Fig. 8. (a) Normalized transmission spectra before and after adding mis_DNA. (b) Timing detection curve for t_DNA solutions with concentrations from 10 pM to 100 nM. (c) Normalized transmission spectra of G/HMM/D-POF in the 0.1 nM t_DNA solution during the binding process. (d) Normalized transmission spectra of G/HMM/D-POF in the 0.1 nM t_DNA solution during the dissociation. (e) Real-time redshift for the binding process and dissociation in the 0.1 nM t_DNA solution. (f) Real-time SPR wavelength shift for G/HMM/D-POF in mis_DNA and t_DNA solutions with concentrations from 10 pM to 100 nM.