Heavy metal ions have drawn enough attention due to their harm to both humans and environments, and there has been an urgent need for stable and quick detection of them. Besides, the manganese ion is one of the most abundant ions in urban water systems, which needs to be monitored carefully. Hereby, a carbon dots (CDs) (the microwave method) based surface plasmon resonance (SPR) Mn²⁺ sensor has been proposed and discussed, which could achieve high detection sensitivity of 6.383 nm/lg(ppb) in the range of 0 ppb–200 ppb on manganese ions, with the detection limit of 0.3462 ppb. The proposed sensor also possesses high stability upon time and temperature, and it also has great ion selectivity against 9 other ions. Practical usage scenarios have been tested on the human serum, tap water, lake water, and river water, which confirm that the proposed sensor holds great potential for both blood tests and environmental monitoring.

In this study, we design a refractive index (RI) sensor using a novel cadmium telluride photonic crystal fiber (TPCF). Based on four-wave mixing (FWM), the changes in RI can be accurately detected, and RI sensing in the mid-infrared region (MIR) can be achieved by detecting wavelength shifts in the Stokes and anti-Stokes spectra caused by the changes in RI of the liquid to be measured. When the pump wavelength of FWM lies in the normal and abnormal dispersion regions of the TPCF, the RI response of the idler frequency wave and the signal wave are analyzed by numerical simulation methods. The simulation results show that the RI sensitivity of the sensor can be as high as 7692nm/RIU with a linearity is up to 99.9% at the pump wavelength of 3380nm. To our knowledge, the RI sensing sensitivity of the MIR is presented for the first time in this study by using FWM in the non-silicon PCF.