Local Light-Controlled Generation of Calcium Carbonate and Barium Carbonate Biomorphs via Photochemical Stimulation

Article abstract of DOI:10.1002/chem.202102321

Photochemical activation is proposed as a general method for controlling the crystallization of sparingly soluble carbonates in space and time. The photogeneration of carbonate in an alkaline environment is achieved upon photo-decarboxylation of an organic precursor by using a conventional 365 nm UV LED. Local irradiation was conducted focusing the LED light on a 300 μm radius spot on a closed glass crystallization cell. The precursor solution was optimized to avoid the precipitation of the photoreaction organic byproducts and prevent photo-induced pH changes to achieve the formation of calcium carbonate only in the corresponding irradiated area. The crystallization was monitored in real-time by time-lapse imaging. The method is also shown to work in gels. Similarly, it was also shown to photo-activate locally the formation of barium carbonate biomorphs. In the last case, the morphology of these biomimetic structures was tuned by changing the irradiation intensity.

Full text of DOI:10.1002/chem.202102321

Communication
doi.org/10.1002/chem.202102321
Chemistry—A European Journal
Local Light-Controlled Generation of Calcium Carbonate
and Barium Carbonate Biomorphs via Photochemical
Stimulation
Arianna Menichetti,[a] Alexandra Mavridi-Printezi,[a] Giuseppe Falini,[a] Patricia Besirske,[b]
Juan Manuel García-Ruiz,*[c] Helmut Cölfen,*[b] and Marco Montalti*[a]
phy at high speed and with sub-μm resolution.[1] The main
advantage of this approach is that the process can be easily
Abstract: Photochemical activation is proposed as a general
method for controlling the crystallization of sparingly
controlled in space and time since the photo-reaction can be
soluble carbonates in space and time. The photogeneration
activated only where the light is focused and when the
of carbonate in an alkaline environment is achieved upon
luminous stimulus is switched on.[2] Even though many
photo-decarboxylation of an organic precursor by using a
technologically and scientifically important materials are in-
conventional 365 nm UV LED. Local irradiation was con-
organic, photo-controlled reactions have been mostly used to
ducted focusing the LED light on a 300 μm radius spot on a
closed glass crystallization cell. The precursor solution was
produce organic polymeric materials.[3]
Focused laser light has been previously used to induce
optimized to avoid the precipitation of the photoreaction
nucleation of supersaturated metastable systems without
organic byproducts and prevent photo-induced pH changes
involving photochemical reactions.[4] In this work, we demon-
to achieve the formation of calcium carbonate only in the
strate, for the first time, that light-controlled chemical reactions
corresponding irradiated area. The crystallization was
can be used to locally precipitate calcium carbonate (CaCO3),
monitored in real-time by time-lapse imaging. The method
one of the most important inorganic chemical products with
is also shown to work in gels. Similarly, it was also shown to
many implications in biological, environmental, industrial, and
photo-activate locally the formation of barium carbonate
technological processes.[5] Our precipitation technique works
biomorphs. In the last case, the morphology of these
well for other alkaline-earth carbonates and has been successful
biomimetic structures was tuned by changing the irradi-
to promote the synthesis of biomorphs of witherite (BaCO3)
ation intensity.
within local microvolumes of the solution.
Biomorphs are silica-carbonate curved architectures with
hierarchical textures much reminiscent of structural principles
found in natural biominerals.[6] They form amazing biomimetic
Photo-activated reactions for the production of solid materials
from liquid precursors are at the basis of the most advanced
technologies for micro- and nano-fabrication. For photopolyme-
rization, 3D structuring is possible using two-photon lithogra-
structures easily to identify morphologically, thus offering a
dramatic demonstration of the efficiency of our method. Finally,
we also demonstrate that our light-induced precipitation
technique also works within agarose gels, allowing the control
of the nucleation of metal carbonate crystals in small local
volumes where they will grow. This technique opens a gate to
the design of three-dimensional inorganic structures by draw-
ing with light.[7]
[a] A. Menichetti, A. Mavridi-Printezi, Prof. Dr. G. Falini, Prof. Dr. M. Montalti
Department of Chemistry “Giacomo Ciamician”
University of Bologna
Via Selmi 2, 40126 Bologna (Italy)
E-mail: marco.montalti2@unibo.it
The chemical basis of the method is simple and clear. A
molecular photo generator of CO2 is irradiated within an
alkaline solution of CaCl2. The CO2 thus generated converts
spontaneously at high pH to bicarbonate and carbonate, thus
provoking the precipitation of CaCO3 within the area of interest
selected by local irradiation with light of a given wavelength.
We have chosen the Ketoprofen anion (KÀ ) as the photoactive
molecule in our experiments. This water-soluble species is
known to undergo photo-decarboxylation, generating in a
controlled way bicarbonate that in an alkaline environment is
converted into carbonate triggering the precipitation of metal
ions as sparingly soluble metal carbonates. The implementation
of this idea needs to solve several obstacles that we discuss
below.
[b] P. Besirske, Prof. Dr. H. Cölfen
Physical Chemistry
University of Konstanz
Universitätsstraße 10, 78457 Konstanz (Germany)
E-mail: helmut.coelfen@uni-konstanz.de
[c] Prof. Dr. J. M. García-Ruiz
Laboratorio de Estudios Cristalográficos
Instituto Andaluz de Ciencias de la Tierra
CSIC-Universidad de Granada
Av. De las Palmeras 4, 18151Armilla, Granada (Spain)
E-mail: juanmanuel.garcia@csic.es
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/chem.202102321
© 2021 The Authors. Published by Wiley-VCH GmbH. This is an open access
article under the terms of the Creative Commons Attribution License, which
permits use, distribution and reproduction in any medium, provided the
original work is properly cited.
The light-activated degradation of KÀ anion is schematized
in Figure 1. Regarding the mechanism, the absorption of one
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Communication
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Chemistry—A European Journal
Figure 1. Left: Schematic illustration of the irradiation cell, which is made by
two slides of glass (1) separated by a rubber spacer (2) 0.1 mm thick. The cell
was filled with the crystallization solution containing KÀ (3) and then sealed.
UV light of a LED at 365 nm was focused on a 300 μm radius spot (4) in the
solution with a microscope objective. A white light illuminator (5) was used
to detect the time-lapsed images of a large area of solution (6) during the
UV local illumination. Right: the reaction of photodecarboxylation oÀf KÀ
which leads to the formation of the photoproduct, PP, and of HCO3
.
UV photon leads to the decarboxylation of KÀ forming a
carbanion which is hence protonated by water. If the photo-
reaction is performed in a closed system (as the crystallization
cell shown in Figure 1), the net process will form a water-
insoluble photoproduct PP and the bicarbonate anion. Indeed,
the local photo-precipitation of PP was demonstrated using an
inverted microscope for focusing UV light, as shown in Figure 1.
As discussed in detail in the Supporting Information the
Figure 2. a–d) Transmission images of a solution containing KÀ (20 mM),
NH4OH/NH3 buffer (40 mM, pH 8.9), Triton X-100 (0.3 mM) and CaCl2 (20 mM)
in a 0.1 mm thick glass cell irradiated with a circular spot of UV light
(365 nm, 1.3 mW/mm2, shown as a white circle) for 0 s (a), 400 s (b), 800 s (c)
and 1200 s (d). Transmittance image after 1200 s irradiation at lower and
higher magnification are shown in e) and f), respectively. The green square
area shown in b, c, and d was digitally enlarged g, h and i. In b-d, the
simultaneous growth of rhombohedral crystals (as the one labeled 1) and of
more spherical crystals (as the one labeled 2) can be followed.
À
formation of HCO3 was demonstrated by measuring pH
decrease of alkaline KÀ solutions during irradiation. We would
like to underline that local PP photo-precipitation and pH drop
represent severe drawbacks for carbonate photo-precipitation
since: i) the PP precipitate contaminates the photo-produced
carbonate and ii) pH drop leads to carbonate dissolution.[8]
These obstacles were overcome by adding to the solution a
Triton X that incorporated and solubilized the PP forming
micelles and by using a NH4OH/NH3 buffer, respectively.
Next, the local calcium carbonate photo-precipitation was
investigated with an inverted optical microscope using the
optimized solution containing KÀ (20 mM), NH4OH/NH3 buffer
(40 mM), and Triton X-100 (0.3 mM) after addition of CaCl2
(20 mM). The experiment was performed upon spot irradiation
with a 365 nm diode in the crystallization cell as schematized in
Figure 1.
mation of crystals in the proximity of the irradiation spot. Tiny
crystals formation could be detected after 400 s irradiation
(Figure 2b). These crystals were observed to grow in time after
800 s (Figure 2c) and after 1200 s (Figure 2d). Note that
crystallization occurs mainly in the corresponding irradiated
area and that the crystals cover about 5% of the total surface.
However, some crystals were formed slightly outside this
area, in a band about 100 μm thick, because of the diffusion of
À
the photoproduced HCO3 out of the light spot. This local
precipitation confined close to the illumination spot was
confirmed by observing the exposed areas with a lower
magnification objective of 4X to expand the field of view. As
shown in Figure 2e, no crystals are observed at a distance
farther than 100 μm from the illumination spot. To identify the
polymorphs of calcium carbonate by the morphology of the
crystals, the green square area of Figure 2b–d was digitally
enlarged (Figure 2g–i). In the enlarged Figures 2g, 2 h, and 2i
Time-lapsed acquisition (0.1 frame/s) for 1200 seconds in
transmission mode with a 20 MPX CMOS camera allowed the
formation of CaCO3 to be followed during local (300 μm radius)
irradiation using a 10X magnification objective.
As shown in Figure 2a, no crystals could be observed in the
detected area before irradiation. Figure 3b–c shows the for-
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was confirmed by the IR spectra shown in Figure 3f. The
decreased diffusion in the gel can be deduced from Figure 3b.
Crystals are found in an area of only 70 μm around the
irradiation spot, although the irradiation time of 30 min was 1.5
times longer than the one in Figure 2e.
To demonstrate the general application of our method for
space and time-controlled photo-stimulated production of
carbonate, we investigated its applicability to the photogenera-
tion of biomorphs. Silica/carbonate biomorphs are fascinating
inorganic-inorganic composite nanocrystalline structures that
emulate the textures and shape of living organisms.[8] This
property facilitates their identification, and therefore they are
perfect markers of carbonate precipitation. Silica biomorphs
have been reported to form in silicate-rich alkaline solutions
containing alkaline-earth metals, like Ba2+ or Sr2+
, upon
exposure to atmospheric CO2.[9] Therefore, in this case, KÀ
(5 mM) was added to a solution containing sodium silicate
(30 mM), BaCl2 (5 mM), and NaOH for final pH=11.7. This
solution, kept under nitrogen flow, was used as a precursor for
the photogeneration of the biomorphs. For the local photo-
generation of the biomorphs, we used the closed glass cell,
shown in Figure 1 and described above.
After filling the cell with the biomorph precursor solution,
time-lapsed transmission images were acquired at the inverted
microscope under local illumination. In the first experiment, the
UV light irradiance was the same as the one previously used for
the calcium carbonate photogeneration (1.3 mW/mm2). The
time-lapsed image sequence showed the growth of the
biomorphs after 9000 s only in the UV illuminated spot. The
final structure of the biomorphs shown in Figure 4a (magnified
with a 60X objective in the inset of Figure 4a) shows the typical
Figure 3. a) Photograph showing the transparency of the agarose gel used
for the photo-crystallization. b) Transmission image of the gel loaded with
CaCl2 20 mM, covered with a 20 mM solution of KÀ and irradiated with a UV
light spot (white circle) for 30 minutes. c) and d) are the SEM image and the
EDX image of the crystals obtained after irradiation of the gel after drying. e)
EDX spectrum of the area shown in d). f) IR spectrum of the crystals obtained
by photo-precipitation after washing with EtOH and drying.
(corresponding to 400 s, 800 s, and 1200 s irradiation, respec-
tively), it is possible to follow the simultaneous growth of
rhombohedral crystals (morphology typical of calcite), like the
one labeled as 1. In addition, some spherical crystals labeled as
2 are also observed, which is characteristic of vaterite, the
hexagonal polymorph of calcium carbonate.
These results demonstrated that the photoproduction of
CaCO3 can be triggered within a few hundred micrometer
region of a water solution in a controlled way. Nevertheless, for
the actual application of this approach, the immobilization of
the photogenerated crystals in
a matrix is undoubtedly
advantageous, as it will avoid sedimentation of the formed
crystals and decrease diffusion of the formed bicarbonate, thus
increasing the resolution of the technique. Therefore, we have
performed similar experiments within an agarose hydrogel
(modified with sucrose for refractive index matching to increase
the transparency) that works as a crystallization substrate[5c]
.
The hydrogel is shown in Figure 3a. It was prepared as
described in the Supporting Information, having a final thick-
ness of 1 mm, loaded with calcium ions, covered with a solution
of KÀ (20 mM), and irradiated with the focused UV light spot.
After 30 minutes of irradiation, a spot of calcium carbonate
crystals could be detected on the transmission image (Fig-
ure 3b). To characterize the grown crystals, the gel was dried in
an argon atmosphere and analyzed by SEM (Figure 3c) and EDX
(Figure 3d). As in solution experiments, calcium carbonate
crystallizes mainly as calcite, with fewer vaterite crystals also
forming. The concomitant precipitation of calcite and vaterite
Figure 4. a) Transmission image of BaCO3 biomorphs obtained after 900 s
irradiation (UV spot 365 nm, 1.3 mW/mm2 is shown in red). Inset shows the
morphology of the biomorphs. b) Transmission image of biomorphs
obtained after 150 minutes irradiation (UV spot 365 nm, 0.1 mW/mm2 is
shown in red). Inset shows the morphology of the biomorphs. c1–c8)
Transmission images of the biomorph labeled as 1 in b after 1000 s, 2000 s,
3000 s, 4000 s, 5000 s, 6000 s, 7000 s, 8000 s, and 9000 s, respectively.
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dual lobe structure of barium carbonate biomorphs, a fake
mitosis-like morphology, which is the end-term of the fractal
formation route. Indeed, the biomorphs obtained under these
irradiation conditions are numerous (per surface area) and quite
monodisperse in size. However, they are relatively small. This is
consistent with fast nucleation induced by high supersaturation,
which is produced by the rapid generation of carbonate from
KÀ photodegradation under intense illumination. Hence, we
investigated the effect of a weaker irradiation intensity by
repeating the experiment at an irradiance 0.1 mW/mm2. As
expected, the sequence of time-lapse images of this experiment
at a slower rate of photogeneration of carbonate revealed that
the nucleation density of biomorphs (number per surface unit)
is smaller. Consequently, the biomorphs reached a considerably
larger size at the end of the irradiation. As shown at the inset of
crystallization with a μm sized focused laser beam could, for
example, improve the resolution of the recently introduced “Ion
Exchange Lithography”,[11] which relies on PbCO3 transformed
to perovskites with ion exchange reactions by inks, which
would not need to be printed anymore with the associated
limitations.
Finally, our approach is in principle transferable to other
photo-labile molecules or pH-dependent reactions and even
the application of several photolabile molecules with different
chromophores at the same time opening a whole range of
photo-induced (nano)crystals like semiconductors or metals for
2D or even 3D writing with light towards miniaturized
electronics or other applications.
Figure 4b (upon magnification at 60X), these biomorphs also Acknowledgements
showed a different, more complex morphology when compared
to the ones obtained at higher irradiance. By expanding the
image sequence acquired at 10X magnification, it was also
possible to follow, in real-time, the growth of an individual
biomorph during the irradiation: the different steps of the
development are shown in Figures 4c1–c8.
In conclusion, we have demonstrated that the photo-
induced decarboxylation of a molecule like KÀ can be exploited
to produce HCO3À locally and hence, in an alkaline environment
CO32À . The local production of carbonate induces crystallization
of carbonate-based structures like calcite or biomorphs. Addi-
tionally, as shown in Figure 4a and b, in the case of biomorphs,
The authors thank the European Research Council under the
European Union’s Seventh Framework Program (FP7/2007-
°
2013)/ERC grant agreement n 340863 and ERC PoC LACRYS
(837874) as well as Junta de Andalucía for financing the project
P18-FR-5008. HC thanks the Deutsche Forschungsgemeinschaft
DFG for financial support of the work on Biomorphs (CO 194/
28-1). M.M., A.M., and A.M.-P. thank MIUR, (PRIN 2017)
2017E44A9P. Open access funding enabled and organized by
Projekt DEAL.
by tuning the irradiation intensity, it is also possible to control, Conflict of Interest
for the first time with a photochemical method, the morphol-
ogy of the resulting structures. This will allow for the writing of
different biomorph structures in the same experiment by
varying the irradiation time at different irradiation spots. Also,
variation of the irradiation time in subsequent growth cycles
might produce carbonate/silica structures with several coexist-
ing textures as was revealed for temperature variations[10] but
now with control of the biomorph nucleation site.
The authors declare no conflict of interest.
Keywords: photo-induced crystallization · calcium carbonate ·
biomorphs · ketoprofen · photochemistry
Although substantial localization of the crystallization was
demonstrated, diffusion of the photoproduced species out of
the irradiation spot partially limited the space resolution.
However, we would like to stress that the study presented here
was aimed to demonstrate the feasibility and the versatility of a
new photochemical strategy for photo-induced crystallization
that we believe can be easily extended from carbonate to a
wide variety of inorganic materials.
Hence, our approach has significant potential for future
applications, and the resolution of the written structures can be
enhanced using an appropriate UV laser with corresponding
focusing optics rather than the here applied LED. This would
enable 2D photolithographic crystallization with precise writing
of crystallographic structures just limited by the size of the laser
spot and the size of the growing crystal if illumination times
can be optimized to be short enough that bicarbonate diffusion
is not relevant. For our example in Figure 2, reduction of the
illumination time from 1200 s to 1 s by a sufficiently high light
intensity would reduce diffusion by a factor of 35 since diffusion
scales with the square root of time. Photo-induced patterned
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Manuscript received: June 28, 2021
Accepted manuscript online: July 8, 2021
Version of record online: ■■■, ■■■■
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COMMUNICATION
Local irradiation of a liquid precursor
leads to the space- and time-con-
trolled photogeneration of calcium
carbonate crystals and biomorphs.
The method is based on the photode-
carboxylation of an organic precursor,
and it was optimized to avoid precipi-
tation of organic matter and pH
A. Menichetti, A. Mavridi-Printezi,
Prof. Dr. G. Falini, P. Besirske,
Prof. Dr. J. M. García-Ruiz*, Prof. Dr. H.
Cölfen*, Prof. Dr. M. Montalti*
1 – 6
Local Light-Controlled Generation of
Calcium Carbonate and Barium
Carbonate Biomorphs via Photo-
chemical Stimulation
changes. By tuning the light intensity,
the morphology of the resulting
biomorphs can be controlled.