Synthesis and Characterization of Supramolecular Colloids

Mesostructured colloidal materials find widespread application in science and technology, as model systems for fundamental studies on atomic and molecular materials^1,2, as photonic materials^3,4, as drug delivery systems^5,6, as coatings⁷ and in lithography for surface patterning^8,9. Since lyophobic colloids are metastable materials that eventually aggregate irreversibly due to the omnipresent van der Waals interactions, their manipulation into specific target structures is notoriously difficult. Numerous strategies have been developed to control colloidal self-assembly including the use of additives to tune the electrostatic^10,11 or depletion interactions^12,13, or external triggers such as magnetic¹⁴ or electric¹⁵ fields. A sophisticated alternative strategy to achieve control over the structure, dynamics and mechanics of these systems is their functionalization with molecules interacting through specific and directional forces. Supramolecular chemistry offers a comprehensive toolbox of small molecules exhibiting site-specific, directional and strong yet reversible interactions, which can be modulated in strength by solvent polarity, temperature and light¹⁶. Since their properties have been studied extensively in bulk and in solution, these molecules are attractive candidates to structure soft materials into exotic phases in a predictable manner. Despite the clear potential of such an integrated approach to orchestrate colloidal assembly via supramolecular chemistry, these disciplines have rarely interfaced to tailor the properties of mesostructured colloidal materials^17,18.

A solid platform of supramolecular colloids must fulfill three main requirements. Firstly, coupling of the supramolecular moiety should be done under mild-conditions to prevent degradation. Secondly, surface forces at separations larger than direct contact should be dominated by the tethered motifs, which means that uncoated colloids should nearly exclusively interact via excluded-volume interactions. Therefore, the physico-chemical properties of the colloids should be tailored to suppress other interactions inherent in colloidal systems, such as van der Waals or electrostatic forces. Thirdly, characterization should allow for an unequivocal attribution of the assembly to the presence of the supramolecular moieties. To meet these three prerequisites, a robust two-step synthesis of supramolecular colloids was developed (Figure 1a). In a first step, hydrophobic NVOC-functionalized silica particles are prepared for dispersion in cyclohexane. The NVOC group can be easily cleaved, yielding amine-functionalized particles. The high reactivity of amines enables straightforward post-functionalization with the desired supramolecular moiety using a wide range of mild reaction conditions. Herein, we prepare supramolecular colloids by functionalization of silica beads with stearyl alcohol and a benzene-1,3,5-tricarboxamide (BTA) derivative²⁰. The stearyl alcohol plays several important roles: it makes the colloids organophilic and it introduces short-range steric repulsions which aids to reduce the nonspecific interaction between colloids^21,22. van der Waals forces are further reduced because of the close match between the refractive index of the colloids and the solvent²³. Light-and thermoresponsive short-range attractive surface forces are generated by incorporation of o-nitrobenzyl protected BTAs²⁰. O-nitrobenzyl moiety is a photo-cleavable group that blocks the formation of hydrogen bonds between adjacent BTAs when incorporated on the amides in the discotics (Figure 1b). Upon photocleavage by UV-light, the BTA in solution is able to recognize and interact with identical BTA molecules through a 3-fold hydrogen bond array, with a binding strength that is strongly temperature dependent¹⁷. Since the van der Waals attractions are minimal for stearyl coated silica particles in cyclohexane as well as light- and temperature-independent, the observed stimuli-responsive colloidal assembly must be BTA-mediated.

This detailed video demonstrates how to synthesize and characterize supramolecular colloids and how to study their self-assembly upon UV-irradiation by confocal microscopy. In addition, a simple image analysis protocol to distinguish colloidal singlets from clustered colloids and to determine the amount of colloids per clusters is reported. The versatility of the synthetic strategy allows to readily vary particle size, surface coverage as well as the introduced binding moiety, which opens up new avenues for the development of a large family of colloidal building blocks for mesostructured advanced materials.