Fig. 1. (a) Schematic diagram of the MCR. (b) Adsorption between BP and Pb2+.

Fig. 2. (a) Schematic diagram of MCR fabrication process. (b) Schematic diagram of BP deposition and the Pb2+ detection process. The inset in (b) shows the scanning electron microscopy image of the BP nanosheets.

Fig. 3. (a) Spectral response of sensor without BP functionalized to varying Pb2+ concentration. (b) Sensitivity curve for refractive index for a non-functionalized sensor without BP deposition.

Fig. 4. (a) Transmission spectra in the BP deposition. (b) Response time curve for the deposition of BP on the MCR.

Fig. 5. Spectral evolution before and after several cycles of BP deposition.

Fig. 6. (a) Resonance wavelength change against the number of deposition cycles. (b) Extinction ratio change against the number of deposition cycles.

Fig. 7. (a) Transmission spectra of the BP-MCR sensor with different concentrations of Pb2+. (b) Sensitivity comparison between non-functionalized and BP functionalized sensors.

Fig. 8. Langmuir mode of the resonance wavelength shift with concentration of Pb2+.

Fig. 9. Response time curves with the BP-MCR sensor immersed in Pb2+ concentrations of 0.1, 10, and 100 ppb.

Fig. 10. (a) Transmission spectra of the BP functionalized sensor at different temperatures. (b) Linear fitting of the temperature response of the BP functionalized sensor. The points are experimental data, and the line is the linear fit of experimental data to temperature.

Fig. 11. Resonance wavelength shift of the BP-MCR sensor with different pH values of the solution.

Fig. 12. Resonance wavelength shifts of the BP functionalized sensor for Pb2+ solutions after different durations.