Fig. 3. (a) Upconversion emission spectra of UCNP, UCNP-PAA and UCNP–BPNS, with the visible absorption spectrum of BPNS (blue line). The inset shows the magnified upconversion emission spectrum of UCNP–BPNS nanocomposite. (b) The bright-field photographs of UCNP-PAA and UCNP–BPNS in H₂O (from left to right) and (c) their corresponding upconversion emission photographs when being excited with 980 nm laser (0.5 W cm⁻²), respectively.