Angew.Chem.Int.Ed.http://dx.doi.org/10.1002/anie.201410778(2015)

Encapsulation and triggered release of drugs or other small molecules is one method to ensure that said molecules are delivered at the correct time, location and in the right amounts. Generally, these strategies involve bespoke syntheses of capsules or containers that have responsive functionalities embedded in them — when applying the relevant environmental trigger, the capsules degrade and release their cargo. Now, Andreas Herrmann, Damien Berthier and colleagues from Firmenich, a perfumery and flavour chemical company, have adopted a different, yet complementary, approach to small molecule release.

Instead of tailoring the capsule with photoresponsive functionalities, they used 'off-the-shelf' non-responsive polyurea-based hollow microcapsules. These microcapsules simply burst if an overpressure of gas is built up inside. To achieve this, the team modified their intended cargo — volatile fragrance molecules containing ketones or aldehydes — to contain a 2-oxoacetate functionality. Visible light degrades 2-oxoacetates creating ketones and aldehydes, and also releasing a molar equivalent of CO or CO2. Since the microcapsules are gas permeable to a certain degree, some optimization of the precise 'profragrant' 2-oxoacetate species was required, in order to ensure that degradation (and thus generation of overpressure) was much faster than the diffusion of the gas out of the microcapsule. They found that the thickness of the capsule wall in comparison to the amount of gas contained within was also an important parameter to control.

Credit: © 2015 WILEY

Using optical microscopy, they observed gas bubbles forming inside the microcapsules (black arrow), as well as a 'burst release' of the cargo (white arrow) on rupture. Because 2-oxoacetates degrade into two separate aldehyde- or ketone-containing molecules, potentially two different fragrance molecules can be released at once. Additionally, spectator fragrance molecules can also be co-encapsulated, either with a non-functional oxoacetate that serves as the gas source only, or with a profragrant oxoacetate that both supplies the overpressure and releases additional fragrant aldehydes or ketones. CH