Cyclohexane

The sb-UCNPs were synthesized by a reversed-phase microemulsion method. In a typical synthesis of single red-emission sb-UCNPs, a mixing solution of 0.5 ml of surfactant CO-520 (purchased from Sigma Aldrich), 16 ml of cyclohexane (purchased from Sinopharm Chemical Reagents Co. Ltd) and 1 ml of 0.1 Mmol α-NaYbF₄:10% Er@NaYF₄ solution in cyclohexane was vigorously stirred for 15 min. Next, 2.1 ml of CO-520 and 0.2 ml of NH₃·₂O (30 wt%, purchased from Beijing Chemical Reagents Co. Ltd) were added to the solution, and the container was sealed and sonicated for 15 min until a transparent reversed-phase microemulsion was formed. Then, 0.1 ml of TEOS (purchased from Sigma Aldrich) was added to the solution. After vigorous stirring for 2 h, 0.1 ml of (3-Aminopropyl) triethoxysilane (APTES) (purchased from Sigma Aldrich)-modified rhodamine B isothiocyanate (purchased from Sigma Aldrich) solution (5 mg ml⁻¹) was added. After continuous stirring at room temperature at 750 r.p.m. for 2 days, the products were precipitated by adding ethanol. The collected precipitates were washed twice with ethanol and dispersed in ethanol for further use. The blue and green emission sb-UCNPs were synthesized in a similar manner using nickel (II) phthalocyanine-tetrasulfonic acid tetrasodium (5 mg ml⁻¹, purchased from Sigma Aldrich) doping dyes.