Mesoporous Silica Nanoparticles Modified inside and out for ON:OFF pH-Modulated Cargo Release

Highly efficient pH-modulated cargo release was achieved with a new hybrid nanocarrier composed of a mesoporous silica core with functionalized pores and a grafted pH-responsive crosslinked polymer shell of 2-(diisopropylamino)ethyl methacrylate (pKa ≈ 6.5). The retention/release performance of the system was optimized by a novel approach using selective functionalization of the silica pores to tune the carrier-cargo interaction and by tunning the amount of grafted polymer. The system features excellent retention of cationic cargo at low pH and a burst release at higher pH. This results from the expanded-collapsed conformation transition of the pH-responsive polymer shell and the simultaneous change in the interaction between the cargo and the polymer shell and the modified pore walls. At low pH, the electrostatic interaction of the cationic cargo with the protonated amine groups of the extended polymer shell retains the cargo, resulting in very low leakage (OFF state). At high pH, the electrostatic interaction with the cargo is lost (due to deprotonation of the polymer amine groups), and the polymer shell collapses, squeezing out the cargo in a burst release (ON state). Pore functionalization in combination with the stimuli-responsive polymer shell is a very promising strategy to design high-performance ON:OFF smart hybrid nanocarriers for stimuli-actuated cargo release, with great potential for application in the controlled release of drugs and other biologically active agents.

Castanheira E, P. C. Alves S, Baleizão C, Farinha JP. Grafting with RAFT-gRAFT Strategies to Prepare Hybrid Nanocarriers with Core-shell Architecture

Stimuli-responsive polymer materials are used in smart nanocarriers to provide the stimuli-actuated mechanical and chemical changes that modulate cargo delivery. To take full advantage of the potential of stimuli-responsive polymers for controlled delivery applications, these have been grafted to the surface of mesoporous silica particles (MSNs), which are mechanically robust, have very large surface areas and available pore volumes, uniform and tunable pore sizes and a large diversity of surface functionalization options. Here, we explore the impact of different RAFT-based grafting strategies on the amount of a pH-responsive polymer incorporated in the shell of MSNs. Using a "grafting to" (gRAFT-to) approach we studied the effect of polymer chain size on the amount of polymer in the shell. This was compared with the results obtained with a "grafting from" (gRAFT-from) approach, which yield slightly better polymer incorporation values. These two traditional grafting methods yield relatively limited amounts of polymer incorporation, due to steric hindrance between free chains in "grafting to" and to termination reactions between growing chains in "grafting from." To increase the amount of polymer in the nanocarrier shell, we developed two strategies to improve the "grafting from" process. In the first, we added a cross-linking agent (gRAFT-cross) to limit the mobility of the growing polymer and thus decrease termination reactions at the MSN surface. On the second, we tested a hybrid grafting process (gRAFT-hybrid) where we added MSNs functionalized with chain transfer agent to the reaction media containing monomer and growing free polymer chains. Our results show that both modifications yield a significative increase in the amount of grafted polymer.