Light-switchable anchors on magnetized biomorphic microcarriers

Article abstract of DOI:10.1039/c9tb02955a

Microcarriers with the ability to release and catch substances are highly desired metamaterials and difficult to obtain. Herein, we report a straightforward strategy to synthesize these materials by combining silica-biomorphs with mesocrystals. An easy access to microcarrier hulls with covalently bound spiropyrans as light-switchable anchor points is presented.

Full text of DOI:10.1039/c9tb02955a

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Materials Chemistry B
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COMMUNICATION
Light-switchable anchors on magnetized
biomorphic microcarriers
ab
Cite this:DOI: 10.1039/c9tb02955a
c
a
ac
Julian Opel,† Lisa-Catherine Rosenbaum, † Julian Brunner, Anne Staiger,
a
d
c
a
Received 29th December 2019,
Accepted 24th April 2020
Ramon Zimmermanns, Matthias Kellermeier, Tanja Gaich, Helmut Colfen
*
¨
b
and Juan-Manuel Garcıa-Ruiz*
´
DOI: 10.1039/c9tb02955a
rsc.li/materials-b
Microcarriers with the ability to release and catch substances are
highly desired metamaterials and difficult to obtain. Herein, we
report a straightforward strategy to synthesize these materials by
combining silica-biomorphs with mesocrystals. An easy access
to microcarrier hulls with covalently bound spiropyrans as light-
switchable anchor points is presented.
Introduction
Fig. 1 Scanning electron micrographs of a biomorph worm (left) and a
biomorph helix (right) decorated with magnetite mesocrystals (highlighted
in red) at the globular apex of the structures.
The so-called silica-biomorphs are purely inorganic composite
1
–3
materials that exhibit outstanding shapes. They are prepared
in alkaline, silica-rich solutions containing earth-alkaline metal
2+
2+
2+ 4,5
ions, such as Ba , Sr and Ca . Their formation mechanism is
assumed to be based on autocatalytic co-precipitation of earth- non-classical pathway via the oriented assembly of non-
1
3,14
alkaline carbonates and silica linked to a local pH cycling induced spherical nanocrystals into superlattices.
6–8
by alternating silica and carbonate precipitation. A huge variety
In order to create further functionality, the structures
of self-assembled carbonate nanorods are attractive for several must be equipped with an additional responding unit that
applications. The elongated architecture of worm-like and allows addressing of a switchable anchor point for controlled
helicoidal shaped biomorphs renders them with the potential to compound load/release. Previous studies employed post-
9
9,15
be used as microcarrier hulls. Magnetic nanoparticles like mag- functionalization by the use of silane coupling chemistry.
netite nanocubes are used as responding units, which are needed The incorporation of light-switchable molecules into various
The benefit systems and the development of so-called dynamic materials
of this system is the ability to attach mesocrystals at specific came into the focus of attention during the last decades. Not
10,11
for controlled movement through various media.
9
,12
16
sides of the biomorph (cf. Fig. 1).
Mesocrystals form in a only the synthesis of materials for data storage, electronic
1
7
18
devices or sensors but also the biological applications of
photo-responsive compounds, e.g. in photopharmacology,
1
9–21
a
Physical Chemistry, University of Konstanz, Universit a¨ tsstrasse 10,
D-78457 Konstanz, Germany. E-mail: helmut.coelfen@uni-konstanz.de;
Fax: +49 7531 88 3139; Tel: +49 7531 88 4063
Laboratorio de Estudios Cristalogr a´ ficos, Instituto Andaluz de Ciencias de la
Tierra (CSIC-UGR), Avenida de las Palmeras No. 4, E-18100 Armilla, Granada,
Spain. E-mail: juanmanuel.garcia@csic.es; Fax: +34 958 552620;
Tel: +34 958 230000
have been investigated. Photo-switchable molecules show rever-
sible transformation between at least two different thermo-
dynamically stable isomers induced by irradiation with light of
a particular wavelength. Among the wide range of photo-switches,
spiropyrans achieved a privileged status since they enable the
production of materials that are responsive to multiple external
stimuli in an orthogonal fashion. The two isomeric structures of a
spiropyran show vastly different properties, which results in
an isomerization process that is not only induced by irradiation
with light but also by several other external stimuli including
b
c
Organic Chemistry, University of Konstanz, Universit a¨ tsstrasse 10,
D-78457 Konstanz, Germany. E-mail: Tanja.Gaich@uni-konstanz.de
Material Physics, BASF SE, GMC/O – G201, Carl-Bosch-Strasse 38,
D-67056 Ludwigshafen, Germany. E-mail: matthias.kellermeier@basf.com;
Fax: +49 621 66 43388; Tel: +49 621 60 43388
d
22
†
JO and LCR wrote the paper and have contributed equally to the work.
temperature, solvent and pH-value. In addition, the covalent
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Fischer Scientific Orbitrap Velos Pro. Scanning electron micro-
graphs (SEM) were recorded on a Hitachi table-top SEM TM3000
with an acceleration voltage of 15 kV. Laser scanning micrographs
(
LSM) were recorded on a Zeiss LSM 700 with a 63ꢀ/1.40 Plan
Apochromat (Oil) objective and with laser diodes (405, 488, 555
and 637 nm). The images were recorded in fluorescence and
transmission channel on two photomultipliers.
Fig. 2 Typical structural formula of a spiroindolinopyran.
attachment of spiropyran units to a solid support results
in numerous advantages compared to the non-immobilized
Synthesis of magnetite mesocrystal silica-biomorph composites
2
3,24
The formation of silica biomorphs and the needed functiona-
counterparts, including an improved fatigue resistance,
and the possibility to obtain biocompatible photoresponsive
9
lization for mesocrystal attachment is described elsewhere.
2
5,26
The analysis of the structures was performed by IR-spectroscopy,
light- and scanning electron microscopy.
materials.
The incorporation of silane-containing switchable molecules like
22,27
spiropyrans is a desired method to obtain new metamaterials.
Synthesis of compound 6
The typical structural formula of a spiroindolinopyran (SP) is
shown in Fig. 2, consisting of an indoline and a chromene
moiety, which are connected via a spiro-center. The isomeric
merocyanine (MC) is accessible by heterolytic cleavage of the
Compound 6 was synthesized in two steps from commercially
available 2,3,3-trimethylindolenine by
procedure. The obtained spectroscopic data are consistent
with those reported previously.
a literature-known
3
8
C
spiro–O bond induced by irradiation with UV-light. The popu-
1
H-NMR (400 MHz, DMSO): d [ppm] = 12.24 (bs, 1H), 8.24
lation of the excited state is followed by an intersystem crossing
process to the corresponding triplet state in which the ring
3
3
0
(
(
1
(
d, J
= 2.8 Hz, 1H), 8.03 (dd, J
= 2.8 , 9.0 Hz, 1H), 7.24
HH
HH
3
3
2
8
d, JHH = 10.3 Hz, 1H), 7.17–7.13 (m, 2H), 6.88 (d, JHH = 9.0 Hz,
opening takes place. Internal rotation in the ground state to
the cis- and trans-isomers is in most cases followed by a thermal
isomerization to the trans-isomer. The open-ring isomer exists
as either zwitterionic (2) or quinoidal species (3).
3
3
H), 6.83 (t, JHH = 7.5 Hz, 1H), 6.69 (d, JHH = 7.8 Hz, 1H), 6.03
3
d, JHH = 10.3 Hz, 1H), 3.57–3.39 (m, 2H), 2.64–2.44 (m, 2H),
1.21 (s, 3H), 1.10 (s, 3H).
Merocyanine often shows strong fluorescence and exhibits a
Synthesis of compound 7
2
9
remarkably higher affinity to different chemical structures,
a
Compound 6 (2.30 g, 6.05 mmol, 1.0 eq.), EDCꢁHCl (1.28 g,
property that is caused by the additional free oxygen acting as a
donor function. The zwitterionic merocyanine has already been
6
.66 mmol, 1.1 eq.) and DMAP (0.74 g, 6.05 mmol, 1.0 eq.)
3
0–32
2 2
were dissolved in CH Cl (20 mL) and stirred for 10 minutes.
described as a ligand for (divalent) metal ions
whereas the
(
3-Aminopropyl)triethoxysilane (1.41 g, 6.35 mmol, 1.05 eq.)
neutral spiropyran is a rather unfavorable donor. Utilizing
the huge differences in the properties of the isomers, multi-
was added dropwise and the reaction was stirred for 20 hours at
room temperature. The solvent was removed under reduced
pressure and the crude product was purified by flash chroma-
tography on silica gel (2 : 1 ethyl acetate/hexane). Compound 7
was obtained as a red solid (2.71 g, 4.64 mmol, 77% yield).
3
3
34
fluorescent hybrid particles, traps for ions and small mole-
3
5
36
cules are prepared, the hydrophobicity of solid surfaces can be
37
tuned reversibly, and the electrochemical properties of a surface
are controlled. Reverse isomerization to the initial spiro-compound
is possible by either thermal relaxation or irradiation with visible
light. These properties made the spiropyran–merocyanine system
to be the focus of our interest with regard to the design of a
magnetized biomorphic microcarrier system.
1
3
H-NMR (400 MHz, DMSO): d [ppm] = 8.20 (d, J = 2.8 Hz,
HH
3
3
1
7
9
1
3
H), 7.99 (dd, JHH = 2.8, 8.9 Hz, 1H), 7.86 (t, JHH = 5.5 Hz, 1H),
.18 (d, JHH = 10.4 Hz, 1H), 7.14–7.09 (m, 2H), 6.85 (d, JHH
.0 Hz, 1H), 6.78 (dt, JHH = 7.4, 0.6 Hz, 1H), 6.65 (d, JHH = 7.7 Hz,
H), 5.97 (d, JHH = 10.4 Hz, 1H), 3.70 (q, JHH = 7.0Hz, 6H), 3.49–
3
3
=
3
3
3
3
.29 (m, 2H), 3.01–2.87 (m, 2H), 2.43–2.27 (m, 2H), 1.41–1.33
3
(m, 2H), 1.18 (s, 3H), 1.12 (t, J
= 7.0 Hz, 9H), 1.07 (s, 3H),
HH
Experimental
Materials and methods
0
.49–0.45 (m, 2H).
13
C-NMR (101 MHz, DMSO): d [ppm] = 170.1, 159.2, 146.4, 140.5,
The purchased ethyl acetate and hexane for column chromato- 135.6, 127.9, 127.5, 125.6, 122.7, 121.9, 121.6, 119.1, 118.9, 115.4,
graphy were of technical grade and distilled before usage. 106.7, 106.6, 57.6, 52.4, 41.3, 39.5, 34.9, 25.5, 22.7, 19.4, 18.2, 7.4.
ꢂ1
All other chemicals were purchased from Sigma-Aldrich and
IR (ATR): ˜v [cm ] = 3307, 2973, 2928, 1740, 1639, 1611,
TCI and used without further purification. NMR spectra were 1511, 1479, 1334, 1273, 1075, 951, 919, 808, 787, 748.
+
recorded on a Bruker Avance III 400. Chemical shifts are
42 3 7
HRMS: m/z calculated for C30H N O Si : 584.2787; found:
referred relative to the solvent residual peaks. Data are reported 584.2785.
as: chemical shift, multiplicity, coupling constant J, and inte-
gration. Infrared spectra (IR) of thin films were recorded on a
PerkinElmer Spectrum 100 spectrometer with an ATR-unit.
Functionalization of mesocrystal-biomorph composites with
compound 7
UV/Vis spectra were recorded on a Cary 50 spectrometer. A 1% solution of compound 7 in a mixture of EtOH:water (95 : 5)
High-resolution mass spectra (HRMS) were recorded on a was prepared. 10 mg of mesocrystal biomorph composites were
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9
incubated in 2 mL of the solution overnight. After reaction, the additional witherite attach to the structures. A thicker silica
material was centrifuged and washed several times with milliQ layer around the structures is desirable to improve the stability
water. The structures were dried in a vacuum oven at 40 1C and once the inner core dissolves for an improved carrier capacity
reduced pressure.
of the MCBCs. Selective secondary precipitation of witherite
crystals around the tips of the structures is desired to induce
the heterogeneous magnetite mesocrystal formation. The
obtained solid from the biomorph synthesis was rinsed several
times with deionized water and dried. The biomorph powder
Results and discussion
The synthesis of a silane-functionalized spiropyran is desired mainly consisted of worm-like braids and helicoidal structures,
for a carrier system based on mesocrystal-biomorph compo- decorated with witherite crystals on the former globular tips of
sites (MCBCs). Spiropyrans for the desired silica-modification the biomorphs. The structures were transferred into a magne-
2
7
were already published in the literature, but their accessibility tite nanocube solution in THF containing a certain amount of
and feasibility were improved in this approach. Based on a oleic acid. The biomorphs act as substrates for the mesocrystal
literature-known synthesis of a spiropyran bearing a carboxylic formation. Mesocrystals were formed through gas phase diffu-
3
9
acid group, the desired compound was accessible by peptide sion of the anti-solvent (ethanol) into the magnetite nanocube
coupling with 3-aminopropyl triethoxysilane. The reaction solution. The particle and the oleic acid concentrations were
scheme is shown in Fig. 3A. Compound 7 was subsequently accurately chosen in order to generate mesocrystals in the range
immobilized on silica-biomorphs. The attachment of func- of the tip size (cf. Fig. 1). The decreased nanocube concentration
tional silanes to various biomorphs was already proven and leads to smaller mesocrystals and a lower total magnetite loading
allowed the desired functionalization of the particle surface of the structures. The result is a different responding behaviour
1
5
with spiropyrans. The scheme of this procedure is shown in the magnetic field. The decreased oleic acid concentration
in Fig. 3B. results in an increased number of smaller mesocrystals. The total
Silica-biomorphs were precipitated from barium-containing magnetite loading remained constant. The obtained MCBCs
alkaline silica sol through continued CO diffusion into the sol. allow a further post-functionalization treatment with synthesized
9
2
The structures remained in the mother sol for 16 h to reach the compound 7 by incubation of the structures in a 1 wt% solution
secondary precipitation stage, where an outer silica shell and in ethanol/water (95 : 5) for several hours. The generated
Fig. 3 (A) Synthesis route of the silane functionalized spiropyran (target compound 7). (B) Formation scheme of the light switchable MCBCs via the
selective formation of magnetite mesocrystals around silica-biomorphs tips and the post functionalization of their silica shell with compound 7 and its
ability of light induced switching on the MCBC surface which induces the glowing (bright green).
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functionalized silica layer can be seen in Fig. 3B as a blue layer 555 nm laser, which was additionally chosen in other experi-
in the SP form. The UV-irradiation of the material induces the ments to turn off the fluorescence while switching the MC back
spiro-compound to switch to the MC form, which is indicated to the SP form. To generate a high amount of fluorescent MCs on
by the glowing green colored layer (Fig. 3B). As previously the surface, the structures were ‘‘bleached’’ in a region of interest
mentioned in the literature, the vastly different properties of (ROI) with a 405 nm laser (20% laser power, 4 scans). The result is
the two isomers can be further used for selective catch and shown in Fig. 4B. The bleaching of the ROI occurred after the first
2
7
release of compounds.
and every third following cycle, indicated by the bluish bars. The
To prove the postulated scheme of the formation of light measured data originated from a programmed times series.
switchable MCBCs, laser scanning microscopy (LSM) was used. A continuous decrease of the fluorescence intensity in the 3 steps
A selective switching of the SP by using the bleach mode was after excitation to a level lower than before the bleaching explains
induced with a different wavelength. Therefore, the excitation the ongoing decrease of the overall fluorescence intensity. The
and emission spectra of compound 7 were recorded and are effect becomes even clearer by considering several cycles.
shown in Fig. 4A. The black spectrum shows the excitation An example is shown in Fig. 4C, where 72 cycles are recorded
spectrum of the SP-form detecting the intensity of fluorescence in the same area. To visualize this effect, Fig. 4D shows a
at 625 nm while the excitation wavelength changes. The spec- functionalized MCBC in its OFF state, before the frame was
trum has a maximum at 555 nm and a shoulder at 520 nm. illuminated with 405 nm light. Fig. 4E shows the activated
In addition, several emission spectra were recorded to elaborate structure after excitation. It has to be mentioned that the pictures
the obtained fluorescence as the subject of the excitation show a lower end of a functionalized biomorph worm, and the
wavelength. Using an excitation wavelength of 555 nm, the lowest part is in focus. The pin hole cuts off the fluorescent light
highest fluorescence intensity was obtained (green spectrum), emitted from the upper part, allowing for an improved image
while an excitation wavelength of 490 nm exhibited lower quality and spatial resolution. Therefore, LSM seems to be an
fluorescence yields (blue spectrum). Nevertheless, the excitation appropriate method to prove the incorporation of the functional
wavelength of 490 nm is the most important result due to the silane on the structures. This has also been shown in previous
15
equipped analysing lasers on the LSM. For the fluorescence studies and demonstrates the suitability of spiropyrans as
experiments on the LSM, the imaging was performed with a switchable anchor points on biomorph-based microcarriers.
488 nm laser (1% laser power, pin hole: 1 a.u.). Imaging the
structures with visible light continuously decreased the obtained
fluorescence. The degradation of fluorescence is worse with the
Conclusions
This work highlights a rationally designed pathway to generate
easily accessible microcarriers by utilising the silane chemistry
toolbox linked to light switchable spiropyrans and the excep-
tionally shaped ultrastructures called silica-biomorphs. On top
a magnetite mesocrystal is used as a responding unit, which
results in a full functional metamaterial extending the frontiers in
all three used fields (biomorphs, mesocrystals and spiropyrans).
The materials described here have potential in a number of
applications. Since it is already known from the literature that
30–32
the merocyanine form can bind metal cations,
the micro-
carriers could be used for the transport of metal ions by a magnetic
field. Ion release by switching to the spiropyran form would allow
reactions of the released cations like catalysis, mineralization,
complexation, etc. One could also think of binding polar nano-
particles to the merocyanine form, which can be released by light
at the location of interest. Also, polar drugs could be bound,
transported and released. If the spiropyran form could be used for
binding of unipolar species (drugs, molecules, etc.), release by light
switching to the merocyanine form would enhance the application
Fig. 4 (A) Excitation (black) and emission spectra of the synthesized spectrum of the microcarriers a lot. Future studies will show
compound 7 in ethanol. The vertical coloured lines correspond to the
excitation wavelength (blue: 490 nm; red: 520 nm; green: 555 nm). (B) ROI
exciting applications, which will become possible with the meso-
crystal functionalized biomorph microcarriers.
(region of interest) intensity measurement over twelve scanning cycles on
the LSM. Bleaching of the region (405 nm; 20% laser power; 4 scans; after
the 1st and every following 3rd image) is indicated as bluish areas.
Authors contributions
(
C) Long-term cycling at the ROI over 72 cycles with the same settings
chosen for B. (D) LSM image (excitation wavelength: 488 nm, 1% laser
power) in false colours of a SP functionalized structure in the OFF (SP)
JO and HC conceived the project. LCR and AS synthesized and
state. (E) LSM image of a SP functionalized structure in the ON (MC) state. analysed compound 7 under supervision of TG. JO, JB and RZ
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designed the magnetized biomorphs. JO and AS functionalized the 16 M. H. Sharifian, A. R. Mahdavian and H. Salehi-Mobarakeh,
biomorphs with compound 7. JO, AS and RZ performed the
Langmuir, 2017, 33, 8023–8031.
microscope experiments. The results were discussed by all authors. 17 D. Kim, H. Jeong, H. Lee, W.-T. Hwang, J. Wolf, E. Scheer,
T. Huhn, H. Jeong and T. Lee, Adv. Mater, 2014, 26,
3968–3973.
Conflicts of interest
1
8 S. Heng, P. Reineck, A. K. Vidanapathirana, B. J. Pullen,
D. W. Drumm, L. J. Ritter, N. Schwarz, C. S. Bonder, P. J.
Psaltis, J. G. Thompson, B. C. Gibson, S. J. Nicholls and
A. D. Abell, ACS Omega, 2017, 2, 6201–6210.
The authors declare that there is no conflict of interests
regarding the publication of this communication.
1
2
2
9 W. A. Velema, W. Szymanski and B. L. Feringa, J. Am. Chem.
Soc., 2014, 136, 2178–2191.
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