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Photoswitchable Dissipative Two-Dimensional Colloidal
Crystals
Jacopo Vialetto, Manos Anyfantakis, Sergii Rudiuk, Mathieu Morel, Damien
Baigl
To cite this version:
Jacopo Vialetto, Manos Anyfantakis, Sergii Rudiuk, Mathieu Morel, Damien Baigl. Photoswitchable
Dissipative Two-Dimensional Colloidal Crystals. Angewandte Chemie International Edition, Wiley-
VCH Verlag, 2019, 58 (27), pp.9145-9149. �10.1002/anie.201904093�. �hal-02363436�
German Edition: DOI: 10.1002/ange.201904093
Self-Assembly
International Edition: DOI: 10.1002/anie.201904093
Photoswitchable Dissipative Two-Dimensional Colloidal Crystals
Jacopo Vialetto, Manos Anyfantakis,* Sergii Rudiuk, Mathieu Morel, and Damien Baigl*
Abstract: Control over particle interactions and organization
at liquid interfaces is of great importance both for fundamental
studies and practical applications. Rendering these systems
stimulus-responsive is thus a desired challenge both for
investigating dynamic phenomena and realizing reconfigura-
ble materials. Here, we describe the first reversible photo-
control of two-dimensional colloidal crystallization at the air/
water interface, where millimeter-sized assemblies of micro-
particles can be actuated through the dynamic adsorption/
desorption behavior of a photosensitive surfactant added to the
suspension. This allows us to dynamically switch the particle
organization between a highly crystalline (under light) and
a disordered (in the dark) phase with a fast response time
(crystallization in 10 s, disassembly in 1 min). These
results evidence a new kind of dissipative system where the
crystalline state can be maintained only upon energy supply.
Precise regulation of the interactions governing the assem-
bly of colloidal particles at fluid interfaces is the subject of
vivid interest in the scientific community, either for the design
of novel materials
[1–3]
or for studies of more fundamental
phenomena.
[4,5]
Specifically, while reconfigurable structures
are sought for the development of innovative devices,
[6]
dissipative systems are also studied to understand how
matter organizes itself.
[7]
Devising a system that can dynam-
ically respond in a dissipative fashion to an external stim-
ulation is thus a desired and demanding task
[8]
giving rise to
reconfigurable and emergent functions.
[9–12]
Examples of
dissipative particle organizations at liquid interfaces have
been reported, but only using chemical
[13,14]
or magnetic
control.
[15–17]
Alternatively, light appears as a valuable stim-
ulus for the study of dynamic assembly.
[7]
However, light
stimulation has never been used to dynamically trigger the
formation of extended 2D-ordered colloidal structures at
liquid interfaces. Optical regulation of particle organization
has been reported in the bulk (that is, in suspension) or on
solid substrates. For instance, aggregation was triggered both
with photoresponsive
[18]
and non-photoresponsive parti-
cles.
[19–21]
Light was also shown to control the crystallization
of colloids, either with active particles,
[22,23]
photoresponsive
particles,
[10]
light-sensitive substrates,
[24,25]
or non-responsive
colloids.
[26,27]
To our knowledge, the only report on the
photocontrol of colloidal crystallization at a liquid interface
was recently achieved using optical trapping at the interface
of an oil-in-water drop and led to ordered, yet very small,
assemblies.
[28]
Here, we describe the first system where
extended 2D colloidal crystallization is induced on-demand
using light as an input. The system is composed of inherently
passive anionic microparticles mixed with small amounts of
a cationic photosensitive surfactant. The light-induced
dynamic adsorption/desorption behavior
[29]
of the surfactant
from the air/water interface allows us to tune the particle–
particle interaction with light. As a result, mm-sized colloidal
assemblies are photoreversibly switched between a disordered
and a highly crystalline state. We analyze both structural and
dynamic features of the colloidal assemblies in response to
different irradiation profiles, and perform cycles of crystal-
lization/disassembly to assess the reversibility of the process.
Strikingly, our experiments demonstrate the dissipative, out-
of-equilibrium nature of the ordered colloidal assemblies,
since the crystals are only formed when continuous energy
supply is provided by light irradiation, whereas energy
removal leads to a rapid disassembly of the colloidal structure.
We recently showed that micromolar amounts of conven-
tional cationic surfactants (for example, dodecyltrimethylam-
monium bromide, DTAB) induced the adsorption of anionic
particles by decreasing the adsorption barrier at the air/water
interface.
[30]
At such low surfactant concentrations (CMC/
1000–CMC/100, where CMC is the critical micellar concen-
tration), the particles adsorbed with a low contact angle
( 30
8
) and remained highly charged, forming disordered
assemblies or polycrystalline patches in a particle- and
surfactant-concentration-dependent manner. Here, to
explore the possibility to dynamically switch particle assem-
blies at the air/water interface at constant composition, we
used anionic polystyrene particles (diameter 5.1 mm) at
a fixed concentration (0.01 mgmL
1
) and 10 mm of the
AzoTAB photosensitive surfactant
[31–33]
(Figure 1A).
AzoTAB is a cationic surfactant with an azobenzene moiety
in its hydrophobic tail, which can isomerize from trans to cis
upon suitable light irradiation (Figure 1B). Note that 10 mm of
AzoTAB is about three orders of magnitude lower than its
CMC (12.6 mm and 14.6 mm for the trans and cis isomers,
respectively
[34]
), resulting in a surface tension comparable to
that of pure water, regardless of irradiation conditions
(Supporting Information, Figure S1). The particles were
brought to the air/water interface of a suspension in
a cylindrical cell by flipping it up and down using our
previously described protocol
[30]
(Figure 1 C, top panel). We
found that, similarly to the case of non-photosensitive
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[*] Dr. J. Vialetto, Dr. M. Anyfantakis, Dr. S. Rudiuk, Dr. M. Morel,
Prof. Dr. D. Baigl
PASTEUR, Department of Chemistry, cole Normale Suprieure
PSL University, Sorbonne Universit, CNRS
75005 Paris (France)
E-mail: anyfas.com@gmail.com
damien.baigl@ens.fr
Dr. M. Anyfantakis
Physics & Materials Science Research Unit
University of Luxembourg
162a Avenue de la Faiencerie, Luxembourg 1511 (Luxembourg)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201904093.
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surfactants, almost all particles adsorbed at the air/water
interface and accumulated at the center of the cylindrical well
containing the sample due to the slightly concave shape of the
meniscus. In the absence of light irradiation, the particles
formed a disordered structure containing voids (Figure 1C-i).
Irradiation of the sample with UV light (365 nm, 8.1 Wm
2
)
promoted fast trans-to-cis isomerization of the AzoTAB
molecules. By combining absorption and NMR spectros-
copies, we measured the isomeric composition in the bulk,
both in the dark and under UV irradiation. We found that the
system was initially composed of 100% trans molecules and
reached, after about one minute of irradiation, a photosta-
tionary state composed of 5% trans and 95% cis (Figures 1B,
S2, and S3). Strikingly, irradiation of the whole particle patch
resulted in a fast and dramatic change in the organization of
the particles: in a few seconds, the whole amorphous patch
evolved into a mm-sized 2D crystal that remained crystalline
upon irradiation (Figure 1C-ii and Movie S1). The transition
occurred through the formation of small crystallites, which, by
local packing, induced a global confinement toward the patch
center, followed by rearrangements and formation of large
monocrystalline domains. To our
knowledge, this is the first time that
such a dynamic photoresponse of
a large colloidal assembly at the air/
water interface is reported. A con-
trol experiment was performed with
the non-photosensitive cationic sur-
factant DTAB having a CMC
(13.4 mm
[20]
) close to that of
AzoTAB. In that case, the structure
of the adsorbed particles did not
change upon extensive irradiation
with UV light (Figure S4), showing
that AzoTAB photoisomerization
was instrumental in controlling the
colloidal organization.
We then characterized the kinet-
ics of the UV-induced disorder-to-
order transition by computing the
radial distribution function (RDF)
from images of the colloidal patch as
a function of the UV irradiation time
(Figure 2). Several peaks appeared
in the RDF during the first seconds
of irradiation and, after about 10 s,
the structure reached a stable state
composed of well-defined peaks up
to r/D = 7 (where r and D are the
center-to-center interparticle dis-
tance and the particle diameter,
respectively) showing long-range
hexagonal packing (Figure 2B).
Interestingly, although this structure
was maintained upon prolonged UV
exposure (20 min, Figure 2A-II,III),
switching off the irradiation resulted
in a transition back to the disordered
state. The crystalline structure pro-
gressively lost its order, reaching a state similar to the initial
one, that is, composed of particles in close proximity but
without any long-range organization (Figure 2A-IV). This
transition took about 5 min (Figure 2 C), as evidenced by the
vanishing of most of the peaks in the RDFs. These results
demonstrate not only the photoreversible nature of the
particle organization in our system, but also emphasize the
surprising behavior that, after light removal, particles switch
from a highly ordered to a disordered state in an isothermal
manner. To gain a better insight into the underlying mech-
anisms of these dynamic processes, the response dynamics of
the particle organization are compared to the isomerization
characteristics of AzoTAB. UV-induced enrichment in cis-
AzoTAB was very fast, with the bulk composition rapidly
increasing from 0% to 50 % within 10 s (Figure S5). After
60 s, this value reached 95% and stayed approximately
constant upon extended irradiation (Figure 1B). The transi-
tion time from disordered to crystalline particle assembly at
the air/water interface thus followed similar kinetics. In
contrast, when the light was switched off, the transition from
the ordered to the disordered phase was achieved in a few
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Figure 1. Light-induced two-dimensional (2D) colloidal crystallization at the air/water interface.
A) Components of the system: the cationic photosensitive surfactant AzoTAB (10 mm), which, upon
suitable light irradiation, converts from the trans (less polar) to the cis (more polar) isomer; and
anionic polystyrene microparticles (5.1 mm diameter, 0.01 mgmL
1
). B) Percentage of the cis-AzoTAB
isomer in solution before, during, and after either UV (violet points) or blue (blue points) light
irradiation. C) Top: set-up for particle adsorption at the air/water interface and light irradiation. A
mixture of particles and AzoTAB in water is turned upside down for two hours to let particles
sediment toward the interface. The sample cell is then flipped back and left overnight for adsorbed
particles to accumulate at the center of the interface. Bottom: transmission microscopy images
before (left) and after (right) UV irradiation for 30 s. UV and blue light irradiation was performed at
365 nm (8.1 Wm
2
) and 440 nm (63 Wm
2
), respectively. Scale bar: 100 mm.
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minutes, while the AzoTAB cis/trans composition was not
affected in this time scale (Figure 1 B). We thus conclude that
the colloidal structure was not controlled by the relative
composition of cis/trans AzoTAB isomers in the bulk, instead
we propose that it was mediated by the adsorption/desorption
dynamics of AzoTAB. Upon UV-induced trans-to-cis isomer-
ization, AzoTAB became more polar
[35,36]
and desorbed from
the air/water interface in an analogous way to what was
reported for similar photosensitive surfactants.
[29]
This light-
induced desorption of surfactants could induce a Marangoni
stress toward the irradiated area,
[31,33, 37–39]
increasing particle
confinement and promoting their crystallization. When
irradiation smaller than the particle patch was used, some
particle motion towards the light was observed &&do you
mean: towards the irradiated area?&& and crystallization
occurred on a larger area than the spot size (Figure S6),
confirming that Marangoni-induced confinement could con-
tribute to particle crystallization. However, our regular
experiments were systematically performed with an irradi-
ation area ( 4 mm) larger than the particle assembly
( 1 mm). Additionally, the small amount of AzoTAB in
the system resulted in an undetectable surface-tension change
(Figure S1). Aside from Marangoni
stress, additional contributing mecha-
nisms have thus to be considered. We
propose that, upon AzoTAB desorption
from the air/water interface, the screen-
ing between like-charged particles by the
cationic surfactants decreased while par-
ticles remained adsorbed because of the
too high energy of detachment.
[40]
Sim-
ilarly, the desorption of AzoTAB from
the particle surface
[32]
could contribute to
this increase of electrostatic repulsion.
Overall, the increased electrostatic
repulsion between particles added to
their confinement caused by eventual
Marangoni stress and the collective
deformation of the interface
[41]
resulted
in the formation of a highly crystalline
structure under UV irradiation. As soon
as the irradiation was removed, trans
molecules coming from the bulk could
adsorb to repopulate the interface. The
transition to the disordered state pro-
ceeded through particle diffusion initi-
ated at the location of the few defects
(vacancies and grain boundaries) in the
crystalline structure (Movie S2), which
prevented the system to be kinetically
trapped in a metastable state. This
resulted in a rapid evolution of the
particle assembly back to the disordered
state, which was the equilibrium state of
our system. Therefore, the ordered struc-
ture was maintained only under contin-
uous UV irradiation (Figure 2A) in
order to keep AzoTAB desorbing from
the interface. Consequently, the 2D crys-
tals reported here were not a minimum in the free energy
landscape of the system. On the contrary, they constituted an
example of an out-of-equilibrium self-assembled structure
where the structural complexity was obtained by reaching
a dissipative state through the continuous consumption of
energy necessary to maintain surfactant depletion from the
air/water interface and from the particle surface.
To gain more information about these dissipative colloidal
crystals, we investigated whether they could be obtained with
a less efficient bulk photoconversion process. To this end, we
irradiated the sample with blue light (440 nm, 63 W m
2
),
leading to a photostationary state reached within one minute
and composed of 55% trans and 45% cis (Figures 1 B, S2, S5,
and S7). Although this photostationary state was significantly
different from the one obtained after UV irradiation, the
response of the colloidal assembly at the air/water interface
was strikingly similar. Blue irradiation induced a fast disorder-
to-order transition and the formation of extended, highly
crystalline colloidal assemblies (Figure S8 and Movie S3).
Crystals were maintained upon blue irradiation and switched
back to the disordered phase as soon as light was switched off
(Figure S8). The structural properties of the light-induced
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Figure 2. Temporal evolution of the structural properties of the colloidal assembly upon
application and subsequent removal of a light stimulus. A) Typical transmission image of the
center of a colloidal assembly before irradiation (I), after continuous UV irradiation for 30 s
(II) and 20 min (III), and 10 min after switching off the UV stimulus (IV). Scale bar: 50 mm.
B) Radial distribution functions (RDFs) before (black curve) and after exposure to UV
irradiation for 2, 4, 6, 10, 30, and 1200 s as a function of the ratio between the distance from
a reference particle, r, and the particle diameter, D. The curves are vertically shifted for clarity.
C) RDFs after switching off the UV stimulus, at 0, 30, 60, 120, 180, 300, and 600 s under dark
conditions. D) Height of the twelfth peak in the RDF curves as a function of irradiation time
for samples irradiated with UV (violet circles) or blue light (blue triangles). E) Height of the
twelfth peak in the RDF curves as a function of time under dark after switching off the light
stimulus for samples exposed to 20 min of UV (violet circles) or blue irradiation (blue
triangles). UV and blue irradiation were applied at 365 nm (8.1 W·m
2
) and 440 nm
(63 Wm
2
), respectively.
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colloidal assemblies were very similar for UV and blue
irradiation, but a few differences could be noticed in the
transition kinetics. Figure 2 D,E shows the height of the
twelfth peak in the RDFs curves (r/D 5.8, long-range
crystalline order signature) as a function of the irradiation
time. Light-induced crystallization was achieved in around
10 s for both irradiation wavelengths (Figure 2D), but
relaxation in the dark was faster after blue irradiation
( 30 s) than after UV irradiation ( 350 s, Figure 2E).
These results first confirmed that our system was not
controlled by the bulk composition of AzoTAB under light
irradiation, since two different photostationary states led to
very similar structures. We thus propose that it was the
continuous light-induced desorption of AzoTAB that main-
tained the system out of equilibrium and allowed its
crystallization. Interestingly, the system was shown to be
history-dependent &&what do you mean with history-
dependent? a type of memory effect? please clarify&&
with two different kinetics under dark conditions as a function
of the previously applied light wavelength. This can be
explained by the different composition of the bulk, which was
richer in trans-AzoTAB after blue irradiation (Figure 2B),
leading to a faster kinetics of re-adsorption.
To assess the reversibility of the system, we applied
successive light-on/off cycles on the colloidal assembly (Fig-
ure 3 A), choosing blue light as the stimulus because of the
faster response of the system after light removal. One minute
of irradiation efficiently crystallized the whole particle
assembly (Figure 3B, blue curves), whereas 2 min in the
dark were enough for the system to switch back to the
disordered state (Figure 3B, red curves). Several cycles of
crystallization/disassembly were successfully achieved (Fig-
ure 3 B and Movie S4). We also analyzed the fraction of
surface occupied by particles in the center of the assembly and
found that it was oscillating in phase with the light cycle, with
an increase/decrease of the 2D packing density upon irradi-
ation/dark conditions that correlated with the crystallization/
disassembly process (Figure 3 C). Cycles between amorphous
and crystalline structures were obtained using UV light as
well (Figure S9 and Movie S2), but the order-to-disorder
transition after UV irradiation was slightly slower and
required 5 minutes. This shows that the order can be tuned
on demand with the possibility of cycling the system and
adjusting its response time.
In conclusion, we have designed a photoresponsive dis-
sipative self-assembling system composed of anionic micro-
particles and a cationic photosensitive surfactant, where the
structural properties of the colloidal assemblies adsorbed at
the air/water interface can be controlled through stimulation
by light. We exploited the adsorption/desorption dynamics of
the surfactant isomers at/from the water surface to control the
particle–particle potential in a fast and reversible fashion. As
a consequence, cycles between disordered and highly crystal-
line colloidal structures were achieved with a single wave-
length switch, without being hindered by the very slow
thermal relaxation of the excited surfactant isomers. This
system represents a proof of principle of novel light-respon-
sive 2D colloidal systems where specific properties arising
from the collective organization of particles at a fluid inter-
face can be efficiently switched on demand by means of light
stimulation. By evidencing the emergence of a highly organ-
ized self-assembled structure that exists only through con-
tinuous energy consumption, it expands the realm of currently
known dissipative systems, opening routes to man-made
devices closer to biological systems and capable of properties
such as adaptiveness, reconfigurability, and degradability.
Acknowledgements
This work was supported by the Mairie de Paris (Emergence-
(s) 2012), the Institut de France (Subvention Scientifique Del
Duca), the European Commission (FP7-PEOPLE-2013-IEF/
Project 624806 “DIOPTRA”), and the Labex and Equipex
IPGG (ANR-10-LABX-31 and ANR-10-IDEX-0001-02-
PSL).
Conflict of interest
The authors declare no conflict of interest.
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Figure 3. Photoreversible crystallization/disassembly upon light-on/off
cycles. A) Sketch of a disorder–order–disorder transition sequence
driven by a cycle of blue light switched on and off. B) RDFs of the
same sample subjected to cycles of blue light turned on (blue curves)
and off (red curves). Each cycle consisted of irradiation with blue light
(440 nm, 63 Wm
2
) for 1 min and dark conditions for 2 min. C) Frac-
tion of the total area covered by particles in the center of the colloidal
assembly in a box of 218165 mm
2
upon cycles of blue light turned on
(blue background) and off (white background).
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