Silica nanoparticles with continuously tunable sizes: Synthesis and size effects on cellular contrast Imaging

Controlling the size of silica nanoparticles (NPs) on a continuously variable scale was achieved by systematically varying the organic solvent(s) used in water-in-oil microemulsion synthesis. A number of individual as well as binary solvent mixtures were investigated for tuning silica NP size. The results demonstrated that the size of a silica NP was continuously tunable over as range of 20-100 nm by varying the alkane chain length of the organic solvent(s) being used. A simple physical model was proposed to describe the size effect and identify the principle factors needed for precisely controlling the size of a silica NP. In the model, the alkane chain length(s) of the organic solvent(s) primarily determined the average size of a silica NP when other synthetic conditions were fixed, whereas variance in size was affected by water droplet percolation with the precursor microemulsion. The significance and utility of these tunable silica NPs was evaluated for bioimaging applications. Specifically, dye-doped silica NPs of variable but precise size were used for in vitro contrast imaging of cells. The results demonstrated that precise control of silica NPs size can be used to reduce cytotoxicity, optimize luminescence signal intensity, and selectively discriminate between structures both inside and outside of cellular membranes.