Synthesis and Characterization of Supramolecular Colloids

The overall goal of this protocol is to describe how to direct colloidal self-assembly by anchoring supramolecular moieties onto the colloids whose interactions are strong, directional, and reversible. This method can help to answer key questions in the material science field, such as how to accurately control the colloidal self-assembly to create complex and mesostructured materials with interesting properties. The main advantage of this technique is that colloidal association is driven by the supramolecular moieties that remain responsive to light and to temperature upon surface immobilization.

Therefore, colloids self-assemble upon photoactivation of the hydrogen bonds of the molecules by UV light. To synthesize the florescent silica seeds, first mix 2.5 milliliters of dye-functionalized aptes with 25 millileters of 25%ammonia and 250 milliliters of ethanol in a one liter round-bottom flask. To obtain monodisperse silica seeds it is really important to add the tetraethyl orthosilicate under the meniscus of the reaction mixture.

Using a glass pipette, add 10 milileters of TEOS while stirring with a magnetic stirrer. Similarly, after five hours, add another 1.75 milliliters of TEOS and stir the mixture overnight under an argon atmosphere. The following day, wash the seeds as described in the text protocol.

To synthesize the core-shell silica particles, first add ethanol, deionized water, 25%ammonia, and the seed dispersion to a one liter round-bottom flask. Then, fill a plastic syringe with five milliliters of TEOS and 10 milliliters of ethanol. Fill a second plastic syringe with 1.34 milliliters of 25%ammonia, 3.4 milliliters of deionized water, and 10.25 milliliters of ethanol.

Connect both syringes to the round-bottom flask with plastic tubing. Equip the flask with an argon flow. The argon inlet has to be next to the outlet of the second syringe to avoid contact between ammonia gases from the TEOS droplets and prevent secondary nucleation.

Add the content of both syringes at the same time at 1.7 milliliters per hour using peristaltic pumps while stirring the mixture. Ensure to obtain free-falling droplets to avoid colliding on the walls and therefore secondary nucleation. Stop the addition after seven hours to obtain core-shell particles of approximately 300 nanometers in radius before washing the particles as described in the text protocol.

For the synthesis of NVOC-functionalized colloids, disperse 10 milligrams of core-shell silica particles in one milliliter of ethanol together with 12 milligrams of the NVOC-protected linker molecule and 31 milligrams of stearyl alcohol in a 50 milliliter round-bottom flask. Sonicate the mixture for 10 minutes to ensure that all molecules are disolved and the particles are well dispersed. Add a magnetic stir bar to the mixture and evaporate the ethanol with a steady stream of argon at room temperature.

Before proceeding, ensure that there is no ethanol left, otherwise it could react with the silanol groups of the particles. Next, heat the flask up to 180 degrees Celsius for six hours under continuous stirring and under a steady stream of argon, before washing the silica colloids as described in the text protocol. To synthesize the BTA colloids, disperse 10 milligrams of the functionalized particles in three milliliters of chloroform.

To cleave the NVOC group homogeneously, place the sample in the UV oven and stir it gently with a magnetic stirrer while deprotecting. Irradiate the dispersion for one hour to yield the amine functionalized particles. Then, dissolve nine milligrams of BTA, 8.7 microliters of DIPEA, and 5.2 milligrams of PyBOP in one milliliter of chloroform.

Add the solution to the amine functionalized particle dispersion and stir overnight at room temperature and under an argon atmosphere. Following centrifugation of the dispersion, remove the supernatant and add three milliliters of fresh chloroform. Sonicate the new dispersion for three minutes before centrifuging again and removing the supernatant.

Dry the particles at 70 degrees Celsius in vacuo for 48 hours. Disperse 20 milligrams of the small functionalized particles in one milliliter of chloroform and irradiate the dispersion in a UV oven for one hour to cleave the NVOC group. Stir the dispersion gently with a magnetic stir bar while deprotecting.

Spin down the resulting amine functionalized particles and remove the supernatant. Then, dry the particles at 70 degrees Celsius for two hours. Next, dissolve 0.5 milligrams of SPDP in 200 microliters of DMF.

Add the SPDP solution to the 20 milligrams of dried amine functionalized particles and vortex the system for 30 minutes. Wash the particles with one milliliter of DMF, six times. In the last washing step, try to remove as much supernatant as possible.

Then, dissolve 0.53 milligrams of DTT in 50 microliters of DMF. Add the DTT solution to the particles and vortex the dispersion for 30 minutes, within this time, the pyridine 2 thione group is cleaved. Determine the absorbence of the free pyridine 2 thione liberated in the supernatant at a wave length of 233 nanometers with a microvolume UV-Vis spectrophotometer.

To monitor the colloidal assembly by confocal microscopy, first prepare 400 microliters of a dispersion of 0.1 weight percent of BTA functionalized particles in cyclohexane and sonicate the sample for 20 minutes. Then irradiate the sample vial in the UV oven to cleave off the ortho-nitrobenzyl group of the BTA. Take 25 microliter aliquots at different times of irradiation, for example, from zero up to 30 minutes, to monitor the clustering process.

Place the different aliquots on different glass slides with the help of a spacer, and close the chambers with a cover slip. After closing the chamber, turn the cover slip upside-down to let the particles sediment and absorb onto the glass, which facilitates the imaging. Take several images of each sample with the confocal microscope as soon as possible after sample preparation for each irradiation time.

To produce supramolecular colloids, first silica colloids are hydrophobized by functionalization with stearyl alcohol, and the NVOC-protected linker. The NVOC-protective group is then cleaved, and colloids can be post-functionalized with the supramolecular moiety. Static light scattering measurements allow for calculation of the refractive index of the colloids.

By fitting the date, values of 1.391 for the bare colloid and 1.436 for the stearyl alcohol coated colloids were determined. This clearly shows how the functionalization of the colloids has an impact on their refractive index. To assess the amount of active sites per particle, the amount of cleaved pyridine 2 thione groups are directly related to the number of amines per particle.

Here, one amine per 46.4 square nanometers was found. confocal image analysis allows for quantification of the number of singlets as a function of the UV irradiation time. Before UV irradiation to photocleave the ortho-nitrobenzyl moiety, 80%of the colloids are present as singlets.

Irradiation at a maximum wavelength of 354 nanometers initiates colloidal aggregation as the hydrogen bonds of the BTA are activated. While attempting this procedure, it's important to keep the system water-free, as colloids might cluster due to capillary forces. Therefore, water traces should be illiminated from all synthesis steps.

This work demonstrates that bridging the gap between colloidal and supramolecular science paves the way for researchers in the field of material science to create complex materials sensitive to external stimuli. After watching this video, you should have a good understanding on how to direct colloidal self-assembly using hydrogen bonds by anchoring supramolecular motifs on colloidal particles.