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Periodic Mesoporous Organosilica Nanoparticles (PMONPs) are nanoparticles of high interest for nanomedicine applications. These nanoparticles are not composed of silica (SiO₂). They belong to hybrid organic&#8722;inorganic systems. We considered using these nanoparticles for CO₂ release as a contrast agent for High Intensity Focused Ultrasounds (HIFU). Three molecules (<b>P1</b>&#8722;<b>P3</b>) possessing two to four triethoxysilyl groups were synthesized through click chemistry. These molecules possess a tert-butoxycarbonyl (BOC) group whose cleavage in water at 90&#8722;100 &#176;C releases CO₂. Bis(triethoxysilyl)ethylene <b>E</b> was mixed with the molecules <b>Pn</b> (or not for <b>P3</b>) at a proportion of 90/10 to 75/25, and the polymerization triggered by the sol-gel procedure led to PMONPs. PMONPs were characterized by different techniques, and nanorods of 200&#8722;300 nm were obtained. These nanorods were porous at a proportion of 90/10, but non-porous at 75/25. Alternatively, molecules <b>P3</b> alone led to mesoporous nanoparticles of 100 nm diameter. The BOC group was stable, but it was cleaved at pH 1 in boiling water. Molecules possessing a BOC group were successfully used for the preparation of nanoparticles for CO₂ release. The BOC group was stable and we did not observe release of CO₂ under HIFU at lysosomal pH of 5.5. The pH needed to be adjusted to 1 in boiling water to cleave the BOC group. Nevertheless, the concept is interesting for HIFU theranostic agents.