Zinc Oxide @ Silica Core/Shell Microspheres for Single-Molecule Force Microscopy in Aqueous and Nonaqueous Solvents

Optical tweezers provide a platform for both manipulating and probing the chemistry of a single polymer molecule tethered between dielectric microspheres. It has been challenging to adapt this technology to organic solvents, in part due to the limited availability of optically trappable materials possessing the necessary diameter and refractive index contrast. Here we report on the development of broadly accessible optical trapping in aqueous and organic solvents that utilizes zinc oxide-silica core/shell microspheres (beads). The addition of a silica shell allows otherwise highly scattering zinc oxide nanoparticles to be stably trapped and readily functionalized. Trapping was observed in water, chloroform, tetrahydrofuran, and ethyl acetate. We demonstrate how these beads can be used to measure the force-extension curves of DNA and poly(methyl methacrylate) respectively utilizing antibody/antigen complementation or strain-promoted azide/alkyne cycloaddition to form linkages in situ. In the latter, a strong, contiguous chain of covalent bonds is formed between the microspheres; therefore, UV bond photolysis was used to count the number of rupture steps and control for single-molecule link formation. In addition to being trappable in many solvents, ZnO@SiO2 core/shell beads can be used as solid support during harsh synthetic conditions and can be readily prepared in the presence of atmospheric oxygen.