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Angewandte
Communications
the efficiency of the delivery system. We thus prepared
microcapsules containing different quantities of photolabile
2-oxoacetate 2 and topped the amount up to 100 wt% with
Romascone. Besides microcapsule C (containing 50 wt% of
2), we investigated microcapsules E (with 80 wt% of 2) and F
(using 25 wt% of 2). The quantity of capsules irradiated was
adjusted to release the same total amount of fragrance in all
cases. Figure 2a indicates the average headspace concentra-
Figure 3. Optical microscopy images for the photoirradiation of micro-
capsules E containing 80 wt% of photolabile 2-oxoacetate 2 and
20 wt% of Romascone as the fragrance to be released. Image a) was
taken before irradiation, images b) to f) at 5 s intervals after switching
on the UVA light source. The black arrows in d) and e) show the
formation of gas bubbles inside two of the capsules. The white arrows
in (e) demonstrate the cleavage of a capsule’s shell and the leakage of
the oil phase out of the capsule. The video from which these images
have been taken is part of the Supporting Information.
Figure 2. Dynamic headspace concentrations of Romascone
a) released from microcapsules C, E, and F as a function of the
amount of encapsulated 2-oxoacetate 2 and b) as a function of the
shell/core ratio of microcapsules C, G, and H. The reported headspace
concentrations correspond to the peak maxima observed after 10–
30 min of photoirradiation.
release from the microcapsules. Figure 3 shows a series of
optical microscopy images that were recorded during the
irradiation of microcapsules E, which were exposed to the
UVA light of the microscope. Figure 3a shows the capsules
before irradiation, and the other images were taken at
intervals of 5 s after switching on the lamp (see also the
video in the Supporting Information). After 15–20 s of
exposure to the light, a gas bubble forms inside some of the
capsules (black arrows in Figure 3d,e). Figure 3e even shows
the spontaneous rupture of the shell in one of the capsules
followed by the leakage of the oil phase out of the capsule
(white arrows). The almost instantaneous release of the
fragrance under the microscope is a consequence of the light
of the microscope being much more focused and thus more
intense than the diffuse UVA light emitted from the lamp
used in the previous experiments.
The encapsulation of photolabile 2-oxoacetates into
suitably designed polyurea core–shell microcapsules is
a simple, inexpensive, and highly efficient way to control
the light-induced release of bioactive compounds. On expo-
sure to natural daylight, encapsulated 2-oxoacetates degrade
to form CO and CO2 at rates that are sufficient to generate an
overpressure of gas to expand or break the capsule wall and
thus liberate the entrapped compounds. Headspace analysis
demonstrated both the simultaneous formation of the gas to
cleave the capsule and the release of an encapsulated
bioactive compound as a direct consequence of UVA
irradiation. The formation of gas bubbles inside the capsules,
the cleavage of the capsule wall and the leakage of the oil
phase could be followed by optical microscopy.
tions of Romascone measured at the peak maxima after 10–
30 min of UVA irradiation as a function of the amount of 2-
oxoacetate 2 encapsulated.
As almost no fragrance was released from microcapsules
F, 25 wt% of oxoacetate 2 seemed to be insufficient to release
the fragrance under the given conditions. The highest amount
of Romascone was released from microcapsule C with
50 wt% of 2, while microcapsule E, containing 80 wt% of 2,
released much less fragrance. This result seems surprising at
first; it might, however, be explained by the fact that the
composition of the oil phase influences the droplet size of the
emulsion and thus the average size of the microcapsules. With
an average diameter of 22.6 mm, capsule E is almost twice as
big as capsule C (11.6 mm) and thus has a considerably thicker
capsule wall, which is more difficult to rupture.
The thickness of the wall with respect to the size of the
capsules,[24] has an important impact on the overall efficiency
of the delivery system. Microcapsules A–F were all prepared
with a constant shell/core ratio of 0.19. We thus decided to
vary the shell thickness and prepared two more microcapsules
with the same core composition as that of microcapsule C, but
with shell/core ratios of 0.13 (microcapsule G, thinner shell)
and 0.25 (microcapsule H, thicker shell). The peak maxima of
the Romascone headspace concentrations measured after 10–
30 min of UVA irradiation showed that microcapsules G were
the most efficient capsules tested (Figure 2b), and that the
amount of Romascone released into the headspace decreased
with an increasing shell/core ratio, if all other parameters
were left constant.
Our strategy could probably be applied to a broad variety
of structures other than the 2-oxoacetates reported in this
work, if they are able to generate a gas on exposure to light.
Our approach is generally applicable to different types of
Finally, the intensity of the irradiating light influences the
rate of degradation of the oxoacetates[4] and is thus another
important parameter to impact the efficiency of fragrance
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
These are not the final page numbers!