Characterization of csUCNPs/EBT Nanocomposites

Fig. 4.3 TEM images of (a) OA-UCNPs, (b) ligand-free UCNPs, (c) OA-csUCNPs, (d) ligand-free csUCNPs, and (e) PEI-csUCNPs. Insets: corresponding size histogram. Scale bars: 100 nm. (f) HR-TEM image of OA-csUCNPs. Scale bar: 10 nm.

Hydrophobic core OA-UCNPs are first prepared by employing OA as the ligand via the high-temperature coprecipitation method^[13]. TEM image exhibits that the core OA-UCNPs have a uniform hexagonal morphology, and the average diameter is measured to be about 35 nm (Fig. 4.3a). OA-csUCNPs core-shell-structured upconversion NPs are prepared by coating a similar material on the core OA-UCNPs, the average diameter of hexagonal-shaped OA-csUCNPs increases to approximately 43 nm with maintained uniformity, suggesting the successful growth of the shell on the core OA-UCNPs (Fig. 4.3c). Moreover, bare OA-UCNPs and csOA-UCNPs can be easily obtained by acid treatment of corresponding OA-capped upconversion NPs via the vortexing method, and TEM images show that the particle size and morphology remain unchanged after the ligand removal from the surface of NPs (Fig. 4.3b, d). Additionally, bare csUCNPs can be further functionalized by water-soluble PEI molecules via

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secondary surface modification, and the TEM image of obtained PEI-csUCNPs shows unchanged size and morphology compared with ligand-free csUCNPs (Fig. 4.3e).

The crystal type and phase purity of core-only and core-shell upconversion NPs are examined by XRD. The XRD patterns of the core-only UCNPs and core-shell-structured csUCNPs (Fig. 4.4a), which are in excellent agreement with the pure hexagonal phase (JCPDS file number 28-1192), demonstrate high crystallinity with well-defined diffraction peaks. In addition, the lattice fringes on the individual core-shell upconversion are clearly distinguished in the HR-TEM image (Fig. 4.3f), confirming the high crystallinity of the prepared OA-csUCNPs. The distance between the lattice fringes is measured to be about 0.52 nm, corresponding to the d-spacing for the (100) lattice planes of the hexagonal NaYF4 structure.

Fig. 4.4 (a) XRD patterns of OA-UCNPs, OA-csUCNPs, and the standard data of hexagonal NaYF⁴ (JCPDS No. 28-1192). (b) FT-IR spectra of OA-, ligand-free, and PEI-csUCNPs.

The high-quality OA-csUCNPs obtained by the high-temperature coprecipitation method are prone to disperse in nonpolar solvents due to the presence of oleate ligand on the surface. However, the ligand can be efficiently removed by the vortexing method.

Ligand removal via acid treatment and further PEI molecules functionalization via secondary surface modification are confirmed by FT-IR measurement (Fig. 4.4b). The transmission bands of as-synthesized OA-csUCNPs at 2923 and 2854 cm^-1 are attributed to asymmetric and symmetric stretching vibrations of methylene (-CH2-) groups in the long alkyl chain. A weak peak at 3008 cm^-1, assigned to the =C-H stretching vibration, can be clearly observed in the spectrum. Moreover, two peaks centered at 1561 and 1460 cm^-1 can be assigned to the asymmetric and symmetric stretching vibrations of the carboxylate group. These characteristic peaks validate the

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presence of oleate ligand on the surface of OA-csUCNPs. Nevertheless, all the characteristic peaks disappeared after the acid treatment, except the broad band centered at around 3420 cm^-1, assigning to the solvated water molecules, validate the success in ligand removal and the hydrophilic nature of the obtained ligand-free csUCNPs. After the modification of bare csUCNPs by PEI, two bands centered at 2930 and 2854 cm^-1 are shown in the spectrum, which can be attributed to the asymmetric and symmetric stretching vibrations of the C-H bond, respectively. A weak peak at 1167 cm^-1 is attributed to the stretching vibrations of the C-N bond, and a strong transition band centered at 1545 cm^-1 is observed, which can be attributed to the N-H bending mode of the amino group. Accordingly, the FT-IR results verify the success in ligand removal of OA-csUCNPs and further attachment of PEI on bare csUCNPs.

Fig. 4.5 UCL spectra of OA-UCNPS and OA-csUCNPs dispersed in cyclohexane, ligand-free UCNPs, ligand-free csUCNPs, and PEI-csUCNPs dispersed in water (the concentrations of UCNPs and csUCNPs are fixed at 1 mg/mL).

The UCL is the most significant feature of upconversion NPs, and the UCL emission spectra of hydrophobic OA- and ligand-free upconversion NPs under 980 nm laser excitation are shown in Fig. 4.5. OA-UCNPs present a relatively low UCL emission intensity due to the energy loss by large surface defects, and a remarkable loss of green and red emissions in intensity occurs in aqueous solution after the transition of OA-UCNPs to ligand-free ones via ligand exfoliation by FA, which is attenuated by water molecules^{[14, 15]}. In order to enhance the UCL intensity of upconversion NPs, especially hydrophilic ones, core-shell-structured OA-csUCNPs are synthesized by coating a similar material on core OA-UCNPs, and bare csUCNPs are obtained by FA treatment via the vortexing method. The UCL intensity of OA-csUCNPs enhances by

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a factor of two compared with the core-only OA-UCNPs in cyclohexane, while the UCL intensity of bare csUCNPs is about 4.5 times larger than ligand-free core-only UCNPs in aqueous solution, owing to the passivated surface defects by the shell layer.

Moreover, nearly identical UCL intensity is observed in PEI-csUCNPs in the aqueous solution compared with bare csUCNPs.

Fig. 4.6 (a) UV-vis absorption spectra of EBT with different concentrations in BRB solution (100 mM, pH 3). The red curve represents the absorption data of csUCNPs/EBT nanoprobes (0.5 mg/mL csUCNPs).

(b) The plot of absorbance intensities at 508 nm as a function of EBT concentration. The red dot represents the absorption intensity of csUCNPs/EBT at 508 nm.

After the treatment of OA-csUCNPs with FA and further surface modification by PEI, positively charged PEI-csUCNPs are obtained, and the EBT dye can easily assemble on the surface via electrostatic interactions. The amount of EBT on the surface of csUCNPs/EBT nanocomposites is determined by detailed spectrophotometric titration spectra. The absorption spectrum of EBT shows a maximum absorption peak at 508 nm, and the absorption intensity increases with the increasing concentration of the EBT (Fig. 4.6a). A linear relationship is shown between the EBT concentrations and absorption intensities at 508 nm. The amount of EBT on the surface of 0.5 mg/mL csUCNPs/EBT is calculated to be about 187 µM (Fig. 4.6b).