Preparation of SiUNPs, AUNPs and pAUNPs:

UCNPs with a chemical composition of NaYF₄:Yb/Tm were synthesized by a coprecipitation method. In a typical synthesis of blue-emitting UCNPs, 0.278 mmol of Y(CH₃CO₂)₃, 0.12 mmol of Yb(CH₃CO₂)₃ and 0.002 mmol of Tm(CH₃CO₂)₃ lanthanide precursors were mixed with 3 mL of oleic acid and 7 mL of 1-octadecene and heated at 150°C for 1.5 h to remove any moisture content. After cooling the reaction solution to 50 °C, a methanol solution containing 1 mmol of NaOH and 1.6 mmol of NH₄F was quickly added and the reaction mixture was further stirred at 50°C for another 0.5 h. After that, the reaction was heated to 100°C under vacuum for 0.5 h to remove any low boiling point solvent. After three times of N₂ purge at 100°C, the reaction was heated to 280°C and maintained at this temperature for 2 h to allow nanoparticle growth. Once the reaction was cooled to room temperature, UCNP products were purified twice by ethanol washing under centrifugation at 6,000 rpm for 10 min, followed by ethanol re-dispersion. Purified UCNPs were dispersed into 4 mL of cyclohexane. For synthesis of UCNPs with a chemical composition of NaYF₄:Yb/Er, experimental procedure was the same as that for NaYF₄:Yb/Tm except that 0.272 mmol of Y(CH₃CO₂)₃, 0.12 mmol of Yb(CH₃CO₂)₃ and 0.008 mmol of Er(CH₃CO₂)₃ were used as lanthanide precursors.

To suppress surface quenching of luminescence, an optically-inert shell layer of NaYF₄ was grown epitaxially onto the as-synthesized UCNPs. Specifically, 0.4 mmol of Y(CH₃CO₂)₃ were mixed with 3 mL of oleic acid and 7 mL of 1-octadecene and heated at 150°C for 1.5 h. After cooling to 80 °C, all UCNP products from the previous step (in 4 mL of cyclohexane) were added to the reaction mixture. The solution was further stirred at 80°C for 1 h to evaporate the cyclohexane content. Subsequently, a methanol solution containing 1 mmol of NaOH and 1.6 mmol of NH₄F was added to the reaction solution at 50°C. After 0.5 h of continuous stirring at 50°C, the reaction was heated to 100 °C under vacuum for 0.5 h. After three rounds of N₂ purging at 100°C, the reaction temperature was ramped to 280 °C and maintained at 280 °C for 2 h. Once the reaction was cooled down to room temperature, core-shell UCNP products were washed twice with ethanol and dispersed into 4 mL of cyclohexane.

The as-synthesized UCNPs were transferred into an aqueous phase through an acid-induced ligand removal process. 4 mL of the cyclohexane was added with 4 mL of ethanol and centrifuged at 14,800 rpm for 5 min. The pellet was re-dispersed with a mixture of 4 mL of ethanol and 4 mL of HCl aqueous solution (2 M) by sonication. The acid-treated UCNPs were thereafter washed three times with ethanol to remove excess acid content. The ligand-free UCNPs were dispersed into 4 mL of DI H₂O for subsequent surface modification.

To obtain SiUNPs, 4 mL of the ligand-free UCNPs were slowly added to 18 mL of polyvinylpyrrolidone aqueous solution (50 mg/mL). The solution was sonicated for 20 min and subsequently stirred for 1 h. After adding 80 mL of ethanol, the reaction mixture was further sonicated for 20 min and stirred for 2 h. 3.2 mL of aqueous ammonia were added to adjust the solution pH, followed by 20 min sonication. Subsequently, 80 µL of tetraethyl orthosilicate were added to the solution to initiate silica growth on the surface of UCNPs. The reaction solution was kept stirring for 12 h, and SiUNPs were purified by three rounds of ethanol washing and dispersed in DI H₂O. The final concentration of SiUNPs were 200 mg/mL.

To prepare AUNPs, 50 mg silica coated UCNPs were washed with DI water two times and then dispersed in 20 mL ethanol/water mixture (9/1). The reaction mixture was further sonicated for 20 min and stirred for 2 h. 0.8 mL of ammonium was added to adjust the solution pH, followed by 20 min sonication. Subsequently, 200 μL TPEBTPy-Si in THF (2 mg/mL) were added into the solution to coat on the surface of UCNPs through a condensation reaction. The reaction solution was kept stirring for 1 h and then were purified by three rounds of ethanol washing and dispersed in DI H₂O. For further PEG modification, mPEG₂₀₀₀-Silane were dissolved in THF (5 mg/mL) and 200 μL mPEG₂₀₀₀-Silane solution were added into the reaction solution containing AUNPs. The reaction solution was continued to stir for 2 h, and the final products were purified by three rounds of ethanol washing and dispersed in DI H₂O.