Synthesis and Characterization of Supramolecular Colloids

Summary

A protocol for the synthesis and characterization of colloids coated with supramolecular moieties is described. These supramolecular colloids undergo self-assembly upon the activation of the hydrogen-bonds between the surface-anchored molecules by UV-light.

Abstract

Control over colloidal assembly is of utmost importance for the development of functional colloidal materials with tailored structural and mechanical properties for applications in photonics, drug delivery and coating technology. Here we present a new family of colloidal building blocks, coined supramolecular colloids, whose self-assembly is controlled through surface-functionalization with a benzene-1,3,5-tricarboxamide (BTA) derived supramolecular moiety. Such BTAs interact via directional, strong, yet reversible hydrogen-bonds with other identical BTAs. Herein, a protocol is presented that describes how to couple these BTAs to colloids and how to quantify the number of coupling sites, which determines the multivalency of the supramolecular colloids. Light scattering measurements show that the refractive index of the colloids is almost matched with that of the solvent, which strongly reduces the van der Waals forces between the colloids. Before photo-activation, the colloids remain well dispersed, as the BTAs are equipped with a photo-labile group that blocks the formation of hydrogen-bonds. Controlled deprotection with UV-light activates the short-range hydrogen-bonds between the BTAs, which triggers the colloidal self-assembly. The evolution from the dispersed state to the clustered state is monitored by confocal microscopy. These results are further quantified by image analysis with simple routines using ImageJ and Matlab. This merger of supramolecular chemistry and colloidal science offers a direct route towards light- and thermo-responsive colloidal assembly encoded in the surface-grafted monolayer.

Introduction

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.

Protocol

1. Synthesis of Core-shell Silica Particles Note: Silica particles are synthesized according to the following procedure, which is based on the Stöber method24,25. Synthesis of fluorescent silica seeds Dissolve 105 mg (0.27 mmol) of fluorescein-isothiocyanate in 5 ml of ethanol. Add 100 µl of (3-aminopropyl) triethoxysilane (APTES, 0.43 mmol) to the previous solution. Sonicate the solution durin…

Representative Results

 Given that the two-step procedure used to synthesize the supramolecular colloids (Figure 1a), couples the BTA- derivatives (Figure 1b) in a second step at room temperature and in mild-reaction conditions, its stability is ensured. Figure 1. Scheme of the synthesis of supramolecular colloids. A) Coupli…

Discussion

When cyclohexane, with a refractive index of 1.426, is used as a solvent to disperse the BTA-colloids, van der Waals interactions are very weak, since the refractive indices of colloids and solvent are nearly the same. Note that the concentration of functionalized colloids used for the SLS experiments in cyclohexane is much higher compared to the bare silica colloids in water. This is necessary to obtain a sufficiently strong scattering due to the low contrast as the refractive indices are almost matched. Trace amounts o…

Materials

APTES	Sigma-Aldrich
FTIC	Sigma-Aldrich
TEOS	Sigma-Aldrich
LUDOX AS-40	Sigma-Aldrich		Silica particles of 13 nm in radius
MilliQ	—	—	18.2 MΩ·cm at 25 °C
Ethanol	SolvaChrom		—
Ammonia (25% in water)	Sigma-Aldrich		—
Chloroform	SolvaChrom		—
Cyclohexane	Sigma-Aldrich		—
Dimethylformamide (DMF)	Sigma-Aldrich		—
Stearyl alcohol	Sigma-Aldrich		—
N,N-Diisopropylethylamine (DIPEA)	Sigma-Aldrich		—
Benzotriazol-1-yl-oxytripyrrolidinophosphonium hexafluorophosphate (PyBOP)	Sigma-Aldrich		—
Succinimidyl 3-(2-pyridyldithio)propionate (SPDP)	Sigma-Aldrich		—
Dithiothreitol (DTT)	Sigma-Aldrich		—
NVOC-C11-OH	Synthesized	—	I. de Feijter, 2014 Responsive materials from adaptive supramolecular constructs, Doctoral thesis, Technical University of Eindhoven, The Netherlands
BTA	Synthesized	—	I. de Feijter, 2014 Responsive materials from adaptive supramolecular constructs, Doctoral thesis, Technical University of Eindhoven, The Netherlands
Centrifuge	Thermo Scientific		Heraeus Megafuge 1.0
Ultrasound bath	VWR		Ultrasonic cleaner
Peristaltic pumps	Harvard Apparatus		PHD Ultra Syringe Pump
UV-oven	Luzchem		LZC-a V UV reactor equipped with 8×8 UVA light bulbs (λmax=354 nm)
Stirrer-heating plate	Heidolph		MR-Hei Standard
Light Scattering	ALV		CGS-3 MD-4 compact goniometer system, equipped with a Multiple Tau digital real time correlator (ALV-7004) and a solid-state laser (λ=532 nm, 40 mW)
UV-Vis spectrophotometer	Thermo Scientific		NanoDrop 1000 Spectrophotometer
Confocal microscope	Nikon		Ti Eclipse with an argon laser with λexcitation=488 nm
Slide spacers	Sigma-Aldrich		Grace BioLabs Secure seal imaging spacer (1 well, diam. × thickness 13 mm × 0.12 mm)
Syringes	BD Plastipak		20 mL syringe
Plastic tubing	SCI	BB31695-PE/5	Ethylene oxide gas sterilizable micro medical tubing
Pulsating vortex mixer	VWR		Electrical: 120V, 50/60Hz, 150W Speed Range: 500–3000 rpm