pH-controlled release of substrates from mesoporous SiO₂ nanoparticles gated by metal ion-dependent DNAzymes

The pH-controlled release of substrates from mesoporous SiO₂ nanoparticles, MP-SiO₂ NPs, is demonstrated by capping the pores with the Mg²⁺- or UO₂ ²⁺-dependent DNAzyme sequences and unlocking of the pores with Mg²⁺ ions or UO₂ ²⁺ ions at appropriate pH values. While the Mg²⁺-dependent DNAzyme reveals high activity at pH = 7.2, moderate activity at pH = 6.0, and it lacks activity at pH = 5.2, the UO₂ ²⁺-dependent DNAzyme reveals high activity at pH = 5.2, moderate activity at pH = 6.0, and it is catalytically inactive at pH = 7.2. Accordingly, the MP-SiO₂ NPs were loaded with methylene blue, MB⁺, or thionine, Th⁺, and locked in the pores by the Mg²⁺- and UO₂ ²⁺-dependent DNAzyme sequences, respectively. The pH-programmed release of MB⁺ or Th⁺ from the loaded NPs proceeds, in the presence of Mg²⁺ ions or UO₂ ²⁺ ions, at pH = 7.2 and pH = 5.2, using the Mg²⁺- and UO₂ ²⁺-dependent DNAzyme as catalysts that cleave the protecting caps and unlock the pores, respectively. At pH = 6.0 the MB⁺- and Th⁺-loaded NPs are concomitantly unlocked by the two DNAzymes. The unlocking processes are selective and other metal ions do not stimulate the release processes.