Fig. 4. (a) Fourier transform infrared spectra of the PEI, bare and PEI modified NaYF4:Yb3+/Er3+/Zn2+ nanoparticles, (b) the distribution of copper and chloride ions on single particle fluorescent probe after the addition of 1 μM CuCl2 solution to the particle, (c) emission spectra and corresponding microscopic images of the single particle probe before and after adding copper ion solution on it, and the scale bars are 2 μm.
