1 SUPPLEMENTARY INFORMATION Amphiphilic Coatings for Protection of Upconverting Nanoparticles against Dissolution in Aqueous Media Olivija Plohl, a,b Slavko Kralj, a Boris Majaron, c Eleonore Fröhlich, d Maja Ponikvar‐Svet, e Darko Makovec, a and Darja Lisjak* a a Department for Materials Synthesis, Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia b Jožef Stefan, International Postgraduate School, Jamova 39, SI-1000 Ljubljana, Slovenia c Department of Complex Matter, Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia d Medical University of Graz, Center for Medical Research, Stifingtalstr. 24, 8010 Graz, Austria e Department of Inorganic Chemistry and Technology, Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia Synthesis details Synthesis of UCNPs@OA ‐NaYF 4 UCNPs, co‐doped with 3.33 at.% of Yb 3+ and 0.33 at.% of Tm 3+ (NaY 0.78 Yb 0.20 Tm 0.02 F 4 ), were synthesized with thermal decomposition, using a modified procedure from ref. 1 . A total of 2 mmol of the LnCl 3 reagents in a stoichiometric ratio of Y:Yb:Tm = 0.78:0.20:0.02 were mixed with 12 ml OA and 30 ml ODE and heated to 156 °C for 30 min, when a yellow transparent solution formed. After cooling the solution to 70 °C the solution of NH 4 F (8 mmol) and NaOH (5 mmol) in 10 ml of methanol was slowly added. The reaction mixture was stirred at 50 °C for 40 min to evaporate the methanol. After that the mixture was heated to 300 °C in an Ar atmosphere for 1.5 h and finally cooled naturally to room temperature. The as‐synthesized UCNPs were collected with the addition of acetone and centrifuging (3000 rcf for 5 min, Eppendorf Centrifuge 5804). They were washed with ethanol and water, and dispersed in cyclohexane or chloroform. Formation of coatings UCNP@TPGS were synthesized in a similar way as described previously 2 by applying some modifications. TPGS ( 100 mg) was immersed in cyclohexane (10 ml) and stirred for 1 h. After the addition of the as‐ synthesized UCNPs@OA (20‐30 mg), the mixture was stirred for another 0.5 h and sonicated for 20 min. After this, 20 ml of water was added to the transparent solution, sonicated for additional 20 min and a white water‐in‐oil emulsion was formed. The phases were separated by centrifuging (3000 rcf for 5 min) and decanting of the oil phase. The water phase was centrifuged again and the remaining cyclohexane was evaporated at 70 °C for 15 min, resulting in the pure water dispersion of UCNPs@TPGS. The synthesis of UCNPs@PMAO‐BHMT was based on Ref. 3 . A stable dispersion of the as‐synthesized UCNPs@OA in chloroform (12 mg/ml UCNPs) was mixed with a solution of PMAO (0.137 M) in chloroform. The ratio of the PMAO monomer units per UCNPs surface (in nm 2 ) was 300. The BHMT Electronic Supplementary Material (ESI) for Dalton Transactions. This journal is © The Royal Society of Chemistry 2017