Fig. 3
From: Label-free upconversion nanosensor for water safety monitoring of permanganate and dichromate ions
(a) TEM image and (b) FT-IR transmittance spectra of UCNPs@SiO2, with later in comparison with OA-UCNPs; (c) Zeta potential of UCNPs@SiO2, and (d) UCL spectra of UCNPs@SiO2 (1.0 mg/mL), UCNPs@SiO2 (1.0 mg/mL) with presence of MnO4− (500 µM) or Cr2O72− (1000 µM) in aqueous solution under 980 nm laser excitation