Figure 3
(a) Schematic representation of the synthesis of Nile Red (NR) loaded PSNPs, where the pH-inert reference dye is homogeneously distributed within the particle core; (i) synthesis of pristine PSNP-COOH, first step: SDS, PPS, MilliQ-H2O, 70 °C, 1 h, second step: AA, MilliQ-H2O, 70 °C, 3 h, (ii) NR loading by swelling procedure, NR, THF, room temperature (r.t.), 30 min; (b) SiO2-NP synthesis and staining with a self-made pH-inert rhodamine B (RhB)-silane derivative, (iii) cyclohexane/MilliQ-water, l-arginine, 60 °C, 20 h (1 × seed growing, 2 × regrowth steps); (iv) first step: RhB-APTES, cyclohexane/MilliQ-water, l-arginine, 60 °C, 20 h, second step: APTES, EtOH, Ar, r.t., 20 h, third step: succinic anhydride, DMF, Ar, 40 °C, 20 h. The reference dye is located in the outer silica shell surrounding the unstained silica core. The final NR- and RhB-loaded PSNPs and SiO2-NPs bear carboxylic acid groups on the particle surface introduced in one step (PSNP-COOH) and two step grafting reactions (SiO2-NP-COOH). Surface modification of both types of carboxylated NPs with optical probe 3 (green star) using EDC/NHS coupling chemistry yielded the green–red emissive ratiometric nanosensors PSNP-NR-3 and SiO2-RhB-3.
