S -Nitrosothiols (SNO) as light-responsive molecular activators for post-synthesis fluorescence augmentation in fluorophore-loaded nanospheres

Dye-loaded, fluorescent nanoparticles (FNPs) have been extensively studied as promising imaging or probing agents for therapeutic and biomedical applications, because of improved photostability and reduced cytotoxicity (compared with free dyes). The synthesis of FNPs often involves entrapment of fluorescent dye molecules into nanostructures, which, however, would encounter a problem associated with the formation of molecular aggregates within the confined matrix space. The formation of nonfluorescent aggregates seems unavoidable for some conventional fluorescent dyes, thereby leading to fluorescent quenching. The problem is well-recognized, but frequently ignored; and FNPs are usually applied as prepared without addressing it either during or after the synthesis of FNPs. The ignorance may be due to the difficulty in altering post-synthetically the intraparticulate molecular arrangements and interactions. Herein, we describe how light-responsive S-nitrosothiol (SNO) can be engineered into fluorophore-loaded silica nanoparticles for efficient dye-loading and post-synthesis fluorescence augmentation. Silica nanoparticles loaded with various fluorescent molecules were prepared using one-pot, simultaneous, acid-catalyzed sol-gel condensation and nitrosation of a single mercaptosilane source, 3-mercaptopropyl trimethoxysilane (MPTMS), followed by nanoprecipitation. We first show how doxorubicin-loaded silica NPs respond to stepwise visible-light exposure and exhibit ON/OFF fluorescent response. We then demonstrate that rhodamine 6G (R6G) can be stably incorporated into SNO-enriched silica nanostructure with negligible payload leakage (<0.2%) and ∼1000-fold reduction in cytotoxicity (compared with free R6G). Remarkably, visible light irradiation leads to ∼100-fold increase in fluorescent intensity. Thus, for the first time, SNO is proposed as a light-responsive entity for post-synthesis fluorescence intensification in FNPs.