Chapter 3: Synthesis of Hydrophilic Upconversion Nanocrystals: A Two-Step
3.3.2 Surface Modification of Ligand-Free UCNPs
The treatment of OA-UCNPs with FA makes the ligand-free UCNPs dispersible in water and positively charged bare UCNPs generate, favoring the attachment of water-soluble capping molecules by sequential surface functionalization. PAA, PVP, and PEI are applied as water-soluble capping molecules to modify the bare UCNPs. Different water-soluble capping molecules modified UCNPs are characterized by TEM, FT-IR, TGA, and UCL measurements.
The success in secondary surface modification by water-soluble polymers is first confirmed by FT-IR (Fig. 3.12d). All the transmission bands of polymers modified UCNPs show peaks centered around 2920 and 2856 cm-1 in the FT-IR spectrum, which are attributed to asymmetric and symmetric stretching vibrations of methylene (-CH2-)
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groups in polymers. Moreover, two typical peaks centered at 1553 and 1458 cm-1, assigning to the asymmetric and symmetric stretching vibrations of the carboxylate group, confirm the presence of PAA on the surface. The peak centered at 1652 cm-1 can be assigned to the stretching vibration of the carbonyl (C=O) group, and the other two peaks centered at 1210 cm-1 and 1155 cm-1 can be attributed to the stretching vibrations of the C-N group, all these typical transition bands validate the presence of PVP on the surface. The strong transition band centered at 1560 cm–1 can be attributed to the N-H bending mode of the amino group (-NH2) and another transition peak at 1170 cm-1 can be attributed to the stretching vibration of the C-N bond, these two characteristic peaks verify the presence of PEI on the surface. Additionally, all the transition bands of polymers modified UNCPs in the FT-IR spectrum show a broad peak centered at around 3500 cm-1, which is due to the adsorbed water molecules, proving the hydrophilic nature of PAA-, PVP-, and PEI-UCNPs. After the transfer of polymer-modified UCNPs to the aqueous solution, the ζ-potential of PAA-, PVP-, and PEI-UCNPs are determined to be -20.6 mV, 26.9 mV, and 30.6 mV at pH 6, respectively, indicating stable hydrophilic colloidal NPs formed in water after surface modification of bare UCNPs by water-soluble polymers. TGA is then conducted to evaluate the amount of the polymers attached on the surface-modified UCNPs. As shown in Fig. 3.12e, the total weight loss of PAA-, PVP- and PEI-UCNPs above 200 ºC are about 5.8%, 3.5%, and 5.1%, respectively.
Fig. 3.12 TEM images of (a) PAA-UCNPs, (b) PVP-UCNPs, and (c) PEI-UCNPs. Scale bars: 100 nm. (d) FT-IR, (e) TGA, and (f) UCL of surface-modified UCNPs.
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Compared with ligand-free UCNPs, TEM images (Fig. 3.12a-c) confirm the maintained shape and size after surface functionalization by different water-soluble polymers, and there are no obvious aggregations after modification with different capping agents, validating the versatility of bare UCNPs and providing more possibilities for further functionalization. Moreover, there are no apparent differences in the position and intensity of UCL emissions among bare, PAA-, PVP-, and PEI-modified UCNPs, as shown in Fig. 3.12f.
3.4 Conclusion
In summary, by introducing biphasic and single solvent systems, we have developed an efficient synthetic strategy for transferring hydrophobic OA-UCNPs to ligand-free ones using short-chain acids as stripping agents by a simple vortexing method. A wide range of polar solvents can be applied to form biphasic interfaces with cyclohexane, facilitating in-solution ligand exfoliation and allowing the transfer of bare UCNPs from the hydrophobic phase to the polar phase. Moreover, short-chain acids can be added directly as well to remove the surface ligand when cyclohexane, chloroform, or toluene is adopted as the dispersant for OA-UCNPs. The here described method is simply performed at ambient conditions on the second time scales without affecting the size, shape, and phase of the crystals. Additionally, the here developed method can be applied to HCl-sensitive materials as well. More importantly, ligand-free UCNPs are readily functionalized by sequential surface functionalization with water-soluble capping molecules, proving the versatility of obtained bare UCNPs. It is rational to believe that this facile approach can be applied to other oleate-stabilized NPs, allowing for the effective and efficient hydrophobic-to-hydrophilic transition.
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