Fig. 1. Mechanism of IEDDA chemistry and tetrazine triggered siRNA activation using bio-orthogonal chemistry. (a) The underlying mechanism of the IEDDA chemistry involves a click step, which is followed by the spontaneous tautomerization and release of the payload attached at the allylic position of TCO; (b) siRNA is immobilized to the nanoparticle surface through the heterobifunctional TCO. The TCO-facilitated immobilization completely shuts down the activity of siRNA which is activated only upon addition of tetrazine, a chemical stimulus highly specific to TCO.