Fig. 3. FRET-enhanced upconversion in DSPE-UCNPs. (a) TEM images of DSPE-Sulfo-RB-CSS25-NaLuF4 dispersed in ethanol. (b) Emission spectra of DSPE-Sulfo-RB-CSS25-NaLuF4 in ethanol with the sulfo-rhodamine B concentration varied from 0 to 19.33 μM. (c) Emission spectra of DSPE-RB-CSS25-NaLuF4 in ethanol with the sulfo-rhodamine B concentration varied from 0 to 14.67 μM, (inset) the molecular structure of rhodamine B (RB), and the emission intensity at 654 nm of DSPE-CSS25-NaLuF4 (the red emission peak of Er3+) in (b) and (c) was normalized to unity, respectively. Note: the gradient arrows indicate the trend of the spectral peak as a function of concentration. (d) The UCL spectra of DSPE-UCNPs (blue), DSPE-Sulfo-RB-CSS25-NaLuF4 (purple, 18.00 μM sulfo-rhodamine B) and DSPE-RB-CSS25-NaLuF4 (green, 14.67 μM rhodamine B) upon excitation at 980 nm (63 W cm−2 irradiance), and the emission intensity at 587 nm of DSPE-Sulfo-RB-CSS25-NaLuF4 (the emission peak of Sulfo-RB) was normalized to unity, respectively. (e) The intensity of the emission peak of sulfo-rhodamine B at 585 nm and the green emission peak of Er3+ in the DSPE-Sulfo-RB-UCNP system (ethanol) with the concentration of sulfo-rhodamine B varied from 0 to 19.33 μM. (f) The corresponding I585/I541 and I577/I541 ratio versus the sulfo-rhodamine B concentration of DSPE-Sulfo-RB-UCNPs and rhodamine B concentration of DSPE-RB-UCNPs, respectively.
