A noncovalent, fluoroalkyl coating monomer for phosphonate-covered nanoparticles

Gadolinium-containing phosphonate-coated gold nanoparticles were prepared and then non-covalently coated with an amphiphilic fluorous monomer. The monomer spontaneously self-assembles into a non-covalent monolayer shell around the particle. The binding of the shell utilizes a guanidinium-phosphonate interaction analogous to the one exploited by the Wender molecular transporter system. Particle-shell binding was characterized by a 27% decrease in ¹⁹F T₁ of the fluorous shell upon exposure to the paramagnetic gadolinium in the particle and a corresponding increase in hydrodynamic diameter from 3 nm to 4 nm. Interestingly, a much smaller modulation of ¹⁹F T₁ is observed when the shell monomer is treated with a phosphonate-free particle. By contrast, the phosphonate-free particle is a much more relaxive ¹H T₁ agent for water. Together, these observations show that the fluoroalkylguanidinium shell binds selectively to the phosphonate-covered particle. The system's relaxivity and selectivity give it potential for use in ¹⁹F based nanotheranostic agents.