Article
Methods
Synthesis of liquid cores
Monodisperse oil droplets^50 are prepared by adding 200 μl of NH 3
(28 wt%) to 100 ml deionized water, followed by the addition of 166 μl
of a 4:1 (v/v) mixture of 3-(trimethoxysilyl)propyl methacrylate (≥98%,
Sigma-Aldrich) and (3-glycidyloxypropyl) trimethoxysilane (≥98%,
Sigma-Aldrich). This mixture is kept under mild magnetic stirring for
2 h at room temperature to allow the oil droplets to nucleate and grow
to a final diameter of approximately 500 nm. If the droplet size deviates
substantially for the target size, the procedure is repeated.
To limit diffusion of TPM into polystyrene during the THF-induced
compression, we reduce the solubility of the TPM in the THF-plasticized
polystyrene by letting the condensation of TPM in ammonia proceed
for a longer period of time, thus obtaining higher-molecular-weight
oligomers. However, as the molecular weight of TPM increases, its
viscosity also increases. If it becomes too viscous, the TPM oil cannot
be extruded to form suitable patches, so this limits the extent to which
we can polymerize the TPM. We find that 2–3 h of condensation gives
the best results.
Next, the emulsion is fluorescently labelled using rhodamine-B iso-
thiocyanate (RITC). The dye is bonded to the oil droplets via siloxane
linkages using 3-aminopropyl trimethoxysilane as a coupling agent
(RITC-APS). Typically, 500 μl of surfactant (Pluronics F-108, 2 wt% in
water) are added to the emulsion, followed by 100 μl of dye solution
(1 mg ml−1 RITC-APS in DMSO). The dyed emulsion is then gently sedi-
mented by centrifugation and resuspended in 40 ml of deionized water.
Synthesis of polystyrene spheres
The polystyrene particles are prepared by surfactant-free emul-
sion polymerization. Typically, 50 ml of styrene monomer (≥99%,
Sigma-Aldrich) is first passed through an activated aluminium oxide
column to remove inhibitor, and then added to 350 ml of deionized
water in a batch reactor equipped with a mechanical stirrer and a reflux
condenser. The reactor is then purged with nitrogen and set to a tem-
perature of 70 °C while keeping the styrene emulsified under vigorous
stirring. The polymerization starts with the addition of the radical initia-
tor to the reactor (400 mg of potassium persulfate dissolved in 15 ml of
deionized water) and continues overnight at 70 °C under mild stirring.
After approximately 12 h, the mixture is brought to room temperature
and the particles set in 400 ml of deionized water via repeated cycles of
centrifugation and resuspension. This procedure yields particles with
a diameter of approximately 550 nm. Larger particles are obtained by
repeated seeded growth. Each growing step follows the polymeriza-
tion protocol described above, except that the initial reaction mixture
contains an additional 40 ml of seed suspension (10 wt%).
Cluster assembly
Tetrahedral clusters are assembled by combining oil droplets and
polystyrene^32. In a typical experiment, two salt solutions (4 ml 500 mM
NaCl) are prepared: one is added to 40 ml of the liquid cores (0.1 wt%)
emulsion and the other to 40 ml of the polystyrene suspension
(10 wt%). The emulsion is then added dropwise to the polystyrene
suspension under continuous magnetic stirring. Once the two sus-
pensions are combined, a surfactant solution (500 μl 2 wt% F-108
solution) is added to the mixture to stabilize the newly assembled
clusters. Tetrahedral clusters are isolated from the excess polystyrene
particles by isopycnic centrifugation. In a typical purification step,
700 parts of the cluster mixture is mixed with 625 parts of 44 wt%
glycerol solution and centrifuged at 2,400g for 90 min. A success-
ful centrifugation results in the sedimentation of the clusters at the
bottom of the centrifuge tube. The sediment is then resuspended in
about 4 ml of deionized water and further purified by density gradient
purification^16. Finally, the purified clusters are washed and resus-
pended in 500 μl of deionized water.
Cluster compression and polymerization
In a typical deformation experiment, 325 μl THF, 675 μl deionized water,
150 μl 1 wt% dodecyltrimethylammonium bromide (DTAB) solution
and 500 μl of cluster solution (the clusters consist of 1.0-μm polysty-
rene spheres and a 500-nm polymerizable TPM oil droplet, 2 wt%) are
mixed together. The amount of THF can be tuned to control the extent
of cluster deformation. The deformed clusters are fixed in a mixture
of 15 ml deionized water, containing 100 μl 1 wt% F-127 and 40 μl 5 wt%
SDS, resulting in a cluster concentration of around 0.06 wt%. The F-127
stabilizes the clusters against aggregation. The SDS controls the wet-
ting between the TPM and the polystyrene spheres. F-127 can also be
used to control the wetting, but using SDS suppresses the amount of
TPM that migrates to the polystyrene surfaces. The size ratio of the
clusters strongly depends on the type and composition of the sur-
factant solution used to fix them (Extended Data Fig. 1). The last step
in the preparation of compressed clusters is the polymerization of the
liquid core, which is carried out at 80 °C in the presence of 5 mg AIBN as
a radical initiator. Polymerized clusters are washed and resuspended
in 1 wt% F-127 solution.DNA functionalization
Single-stranded DNA (5′-DBCO-Cy5-T 50 -TTTACGCGTA-3′) was pur-
chased from Integrated DNA Technologies USA. The DNA strand comes
equipped with a dibenzyl cyclooctane (DBCO) group and a fluores-
cent tag (Cy5, emission maximum 668 nm) and was pre-purified via
high-performance liquid chromatography. The DNA is dissolved in
standard PBS buffer to a concentration of 100 μM and stored at −4 °C.
The compressed clusters (1 ml of suspension at 1 wt% clusters and 1 wt%
F-127) are pre-treated with 10 mg of sodium azide (NaN 3 ) and a cata-
lytic amount of potassium iodide (KI). This mixture is kept at 70 °C for
24 h. The activated particles are then washed and stored in water. To
graft DNA onto the surface of the compressed clusters, 20 μl of clus-
ter suspension is added to 400 μl PBS solution (PBS, 10 mM, pH = 7.4,
500 mM NaCl) containing 0.1 wt% Triton X-100 and 10 μl of the DNA
solution. This mixture is incubated at 55 °C for 2 days before washing
the particles in deionized water. When properly stored at 4 °C in PBS
buffer, our colloidal systems can be stored for several months without
any loss of stability of functionality.Crystallization
Colloidal crystals are assembled in a 50/50 mixture of D 2 O and PBS buffer
solution (PBS, 10 mM, pH = 7.4, 280 mM NaCl, Pluronic F-127 1 wt%). The
suspension is introduced into a glass capillary (100 μm × 2 mm × 5 cm,
VitroCom). The capillary is pretreated with oxygen plasma, exposed
to hexamethyldisilazane (HMDS) vapours, flushed with aqueous 1 wt%
F127 solution and finally dried with compressed nitrogen. Filled capil-
laries are sealed using NOA 68 UV glue and mounted on a custom-made
temperature-controlled microscope stage. The DNA melting tempera-
ture (Tm) is around 40 °C, which varies slightly from batch to batch. Our
thermal stage allows us to set a temperature gradient across the capil-
lary (the temperature difference between the two ends is about 1 °C).
The sample is also tilted at an angle of about 20° to increase the particle
concentration during annealing. The tilting and temperature-gradient
directions are perpendicular to each other. The shaped particles with
interlock result in particle volume fractions around 0.68, not far below
the close-packing limit of 0.74 and well above the 0.34 packing fraction
of a diamond lattice of touching spheres.Fixing and drying of the crystals
8-methoxypsoralen (8-MOP) is chosen as the ultraviolet crosslinker
to fix the colloidal crystals. Before annealing the sample, 8-MOP is
pre-dissolved into the PBS (PBS, 10 mM, pH = 7.4, 280 mM NaCl, Plu-
ronic F-127 1 wt%) solution. After crystallization, the sample is exposed
to utlraviolet light for 6 h. Then, both ends of the glass capillary are