Stimuli-Responsive Colloidal Assembly Driven by Surface-Grafted

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Stimuli-Responsive Colloidal Assembly Driven by Surface-Grafted Supramolecular Moieties Isja de Feijter, Lorenzo Albertazzi, Anja R.A. Palmans, and Ilja K. Voets Langmuir, Just Accepted Manuscript • Publication Date (Web): 09 Dec 2014 Downloaded from http://pubs.acs.org on December 10, 2014

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Stimuli-Responsive Colloidal Assembly Driven by Surface-Grafted Supramolecular Moieties Isja de Feijter, Lorenzo Albertazzi, Anja R.A. Palmans, Ilja K. Voets* Institute for Complex Molecular Systems, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands responsive materials, self-assembly, colloid, hydrogen bonding, supramolecular chemistry ABSTRACT A robust method is described for precisely functionalizing silica colloids with short-chain alkanes and self-associating o-nitrobenzyl protected benzene-1,3,5-tricarboxamides (BTAs). Controlled deprotection affords activation of the latent supramolecular moieties by facilitating short range hydrogen-bonding interactions between surface functionalized silica particles. Control of mesoscale assembly of the responsive colloidal suspensions is demonstrated with two different external triggers. Firstly, the amount of active (i.e. deprotected) BTAs is efficiently tuned by varying the exposure time to UV radiation. Controlled activation of the BTAs translates to regulating the valence of the system. After activation, the binding strength of individual BTAs can be modulated with temperature, providing an additional handle with which the assembly behavior is manipulated. This dual-regulation approach is a powerful and sensitive avenue for controlling colloidal assembly processes.

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INTRODUCTION Mesostructured colloidal materials are widely applied in photonics and phononics, drug delivery, paints, coating technology and lithography.1-8 This has sparked great interest in the development of surface forces with a tunable range and strength, since modular colloidal interactions offer ways to control the formation, dynamics, structure, and other properties of the final material.9-14 Particularly well studied in this context are electrostatic interactions15 and depletion interactions,16, 17 which are strongly dependent on the concentration of additives such as salts and polymers. Furthermore, self-organization by application of external magnetic18 and electric fields19 has recently emerged as a highly effective method to drive assembly into a range of ordered structures including rings,20 one-dimensional strings,21 and more exotic structures.22 A third route to engineer complex architectures but without external fields or additives utilizes specific and directional interactions between functional groups tethered to the colloid surface.2335

DNA hybridization of complementary strands grafted onto colloids is by far the most

prominent example of this approach. Its applicability was first demonstrated by functionalization of nanometer-sized gold particles with DNA oligomers.30-33 Various specific interactions have also been introduced to direct the self-organization of larger colloids,23-25 but the window of tunable parameters averting colloidal instability and entrapment in kinetic intermediates is often very limited. For example, both metal ligand interactions23 and DNA hybridization generate relatively strong surface forces, which is ideal for sensory applications, but renders gradual annealing with temperature practically unattainable.34-37 This challenge warrants further research into novel binding motifs that allow the programmable assembly of colloids from nanometer up to micrometer length scales.


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We here propose the use of multiple weak, reversible, and short range hydrogen-bonding interactions which are both temperature and light responsive to precisely control colloidal assembly. For this purpose we select benzene-1,3,5-tricarboxamide (BTA) based supramolecular motifs, that assemble via triple-hydrogen bonding in organic38-40 and aqueous solvents.41-43 Covalent attachment of o-nitrobenzyl protected BTAs onto silica beads allows tuning of the colloidal self-assembly behavior by two independent, external triggers: UV-light and temperature. Controlled activation of the hydrogen-bonding interactions on the surface of the beads is achieved by cleaving off the photo-labile protecting group of the BTA, which initiates clustering of the colloids. In addition, varying the temperature alters the binding strength of the supramolecular motif, which enables to control association at low temperatures and dissociation at high temperatures. This dual-regulation approach allows to efficiently fine-tune both the strength and the number of supramolecular bonds between the colloids aiming to modulate their assembly pathways and promote equilibration into ordered structures.

EXPERIMENTAL SECTION Materials Green fluorescent silica particles (d = 513 nm) were purchased from Corpuscular Inc. Cyclohexane and dichloroethane (SOLVACHROM® quality), n-octadecyl alcohol, chloroform and LUDOX® AS-40 (d = 26 nm) were purchased from Sigma-Aldrich. Synthesis Prior to surface-functionalization of the d = 26 nm beads, the aqueous colloidal suspension was lyophilized.

Hereafter,

AS-40

(43

mg),

N’,N’’-bis((3S)-3,7-dimethyloctyl)-N’’’-

(hydroxyundecyl)-N’’’-(2-nitrobenzyl)benzene-1,3,5-tricarboxamide (cBTA 1) (152 mg; 0.19


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mmol) and n- octadecyl alcohol (208 mg; 0.77 mmol) suspended in 10 ml ethanol were added to a round bottom flask equipped with a stir bar and septum. The flask was purged with argon and under a steady stream of argon, heated to 90 °C for 2 hours, and subsequently to 190 °C for 6 hours. Similar quantities of green fluorescent silica particles were functionalized using the same synthetic procedure. In all cases, the excess of reagents was removed by thorough washing with chloroform and drying in vacuo at 80 °C. To this end, the obtained (cBTA-)ODBs were suspended in 10 ml of chloroform, after which the product was allowed to sediment by centrifugation for 10 minutes at 4000 rpm. The pellet obtained after 6 repetitions of this procedure was dried in vacuo. The surface-functionalized beads were stored as dry powders. Methods Circular dichroism (CD). CD spectra were recorded on a Jasco J-815 spectropolarimeter equipped with a PFD-425S/15 Peltier-type temperature controller. Experiments were performed in a 1 cm Hellma quartz cell. Thermogravimetric analysis (TGA). TGA was performed on a TA instruments TGA Q500 using a temperature ramp from 25-800° C with a speed of 20° C per minute under an air atmosphere. Nuclear magnetic resonance spectroscopy. Magic angle spinning (MAS) nuclear magnetic resonance spectra were recorded on a Varian Unity Inova 500 MHz spectrometer (500 MHz for 1H) equipped with a Varian 500 gHX nano probe. DMSO-d6 was used as a solvent, all 1H chemical shifts are reported in ppm downfield of trimethylsilane. Infrared spectroscopy (IR). IR spectra were acquired on a Perkin-Elmer Spectrum Two equipped with a UATR Two sample stage. Electron microscopy. Scanning electron micrographs were obtained on a FEI Quanta 600F ESEM, using a 10 kV electron beam in low vacuum mode. Centrifugation. Samples were centrifuged in a Thermo Scientific Heraeus Megafuge 1.0 centrifuge with a speed of 4000 rpm.


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Confocal microscopy. Confocal microscopy imaging was carried out on a Nikon Ti Eclipse laser scanning confocal microscope using an argon laser at 488 nm as an excitation source using a 100 × plan fluor oil immersion lens (N.A. = 1.3). Samples were prepared by suspending 0.1 wt % functionalized colloids in optical grade cyclohexane or dichloroethane. After 20 minutes of sonication the samples were illuminated in a Luzchem LZC-4V UV reactor equipped with 8 x 8 Watt UV-A light bulbs (λmax = 354 nm). In approximately 5 minutes the obtained suspensions were pipetted onto a microscopy slide. As soon as the microscope slide is prepared, colloidal assembly is arrested since the beads adhere irreversibly to the glass, which prevents further structural evolution with time. Quantitative image analysis was performed using ImageJ 1.48b based on the area of bright pixels. The procedure consists of three steps. First, a threshold intensity is set to discriminate between noise (I < Ithreshold) and particles (I > Ithreshold). Next, each image is converted to a binary format after which the size of all area larger than 100 pixels is computed. All counted areas between 100 and 600 pixels and with a circularity between 0.6 and 1.0 were assigned to singlets. The average singlet area was used to compute the number of particles per cluster on a picture to picture base. Hereafter, the total number of singlets, ns, doublets, triplets, clusters of 4-10 beads, clusters of 11-50 beads, clusters larger than 50 beads, as well as the total number of particles ntot are determined. Finally, the fraction of singlets, defined as fs = ns / ntot is determined. Static light scattering (SLS). SLS experiments were conducted on an ALV/CGS-3 MD-4 compact goniometer system equipped with a Multiple Tau digital real time correlator (ALV7004) and a solid state laser (λ = 532 nm; 40 mW). Experiments cover scattering angles from 60 to 120°, averaging over 6 x 10 s runs at temperatures between 20 °C and 75 °C. Solutions were prepared in cyclohexane, filtered over 0.45 µm PTFE filters, dried under vacuum and


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resuspended to obtain a concentration of 1 mg/ml. Measurements were performed directly after 20 minutes of sonication, in 5 mm borosilicate cells. Photo-irradiation experiments were performed in a Luzchem LZC-4V UV reactor equipped with 8 x 8 Watt UV-A light bulbs (λmax = 354 nm). The photo-irradiation reactions were performed in borosilicate cells at room temperature. RESULTS Design of supramolecular colloids. To study the utility of supramolecular motifs for colloidal assembly, we compare the behavior of n-octadecyl alcohol coated silica particles (ODB in Figure 1) and ‘supramolecular colloids’ (BTA-ODB in Figure 1) coated with a mixture of caged benzene-1,3,5-tricarboxamide derivatives (cBTA 1 in Figure 1) and n-octadecyl alcohol in cyclohexane and dichloroethane. The molecular structure of cBTA 1 encompasses functional groups for self-assembly, photo activation, surface-functionalization, and to monitor molecular association by circular dichroism (CD) spectroscopy. Specifically, the hydroxyl end group facilitates covalent coupling of cBTA 1 to silica via the van Helden method.44 The two stereogenic centres in cBTA 1 give rise to differential absorption of right and left handed circularly polarized light upon molecular association which is detectable by CD spectroscopy. The UV-labile o-nitrobenzyl protecting group can be cleaved off by illumination at λ = 354 nm to afford the uncaged BTA. Previously, an analogue of cBTA 1 was demonstrated to selfassociate in a temperature-dependent fashion via triple hydrogen bonding between the amides upon uncaging. This BTA was successfully employed to induce UV-controlled folding of single chain polymethacrylate nanoparticles (SNCPs).45 We now extend this work to hydrogen-bond driven colloidal assembly and test if uncaged cBTA 1 can also be used to tune association of


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inorganic particles through the formation of interparticle hydrogen bonds in cyclohexane (CH) and dichloroethane (DCE). The phase behavior of ODBs in CH is well established and can be described by the hardsphere model; i.e., attractive interactions are negligible.46,

47

This is achieved by steric

stabilization in combination with suppression of van der Waals interactions by closely matching the refractive indices of the ODBs and the solvent.48-50 A similar match between the refractive index of BTA-ODBs and CH and DCE is anticipated, therefore, if attractive interactions between BTA-ODBs arise, these originate from the surface-tethered motifs. Since cBTA 1 carries a photolabile group, the non-associating cBTA-ODBs can be ‘activated’ upon UV-illumination which transforms latent cBTA 1 into active BTA 1 and similarly, non-associating cBTA-ODBs into associating BTA-ODBs. The strength of the surface forces between BTA-ODBs should be strongly temperature dependent, since hydrogen bonding between activated BTAs is prominent at low, and marginal at high temperatures. Here we functionalize colloidal silica with cBTA 1 to investigate whether light and temperature can be used to modulate the number and strength of the interparticle bonds. Large (d = 513 nm) green fluorescent particles were selected for real-space visualization of the BTA-ODB behavior by confocal microscopy. Small (d = 26 nm) beads without fluorescent core were chosen for use in light-scattering experiments. Their larger ratio of organic surface coating to inorganic material also facilitates characterization of the surface functionalization. Synthesis of supramolecular colloids. Both d = 26 nm and d = 513 nm silica particles were coated with n-octadecyl alcohol to generate sterically stabilized colloidal silica (ODBs) compatible with organic solvents using the method by van Helden et al.44 To this end, aqueous silica suspensions were first lyophilized, after which the beads were dispersed in a solution of n-


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octadecyl alcohol in ethanol. Next, the ethanol was distilled off under a gentle stream of argon and the temperature was raised to 190 °C to allow the formation of silyl ether bonds between the beads and the n-octadecyl alcohol. The excess organic material was removed by sedimentation of the beads after which the supernatant with the free n-octadecyl alcohol can be removed. The obtained beads were dried at 80 °C in vacuo after six washing steps with chloroform. Similarly, mixtures of cBTA 1, synthesized using the method described by Mes et al.,45 and n-octadecyl alcohol with a molar ratio of 1:4 were coupled to both types of beads to obtain the supramolecular colloids denoted as cBTA-ODBs in Figure 1. All functionalized beads formed stable suspensions in cyclohexane and dichloroethane. Characterization of supramolecular colloids. Scanning electron microscopy images of the (c-BTA-)ODBs shown in Figure 2A and B demonstrate that the morphology of the green fluorescent beads (size, shape and surface roughness) remains unchanged upon chemical functionalization. Infrared (IR) spectroscopy experiments were performed to verify whether noctadecyl alcohol and cBTA 1 were successfully coupled to the small and large silica beads (Figure 2C, D). The IR spectra of bare silica beads and solid n-octadecyl alcohol are depicted as a reference. A single pronounced peak at ~1050 cm-1 (SiO stretch) is visible in the spectra of the bare beads, while free n-octadecyl alcohol yields several bands at 2830-2970 cm-1 (CH stretch), ~1470 cm-1 (CH-bend), and ~1050 cm-1 (CO stretch). As expected, the IR spectra of the (cBTA)ODBs show the features of the silica beads and the tethered organic material. A distinct CH stretch vibration is visible at 2830-2970 cm-1, but the concomitant CH bend (~1470 cm-1) and CO stretch (~1050 cm-1) vibrations of the tethered alcohols are invisible due to the strong SiO stretch of the inorganic particles at 1050 cm-1. Careful examination of the CH stretch vibrations,


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enlarged in the insets of Figure 2, reveals more intense peaks for the smaller beads. This is because these have the largest mass fraction of organic material. Magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy experiments were performed on a 1 wt % suspension of ODBs in DMSO-d6 to confirm covalent surfacefunctionalization as indicated by IR spectroscopy. The NMR spectrum depicted in Figure 3 displays triplets at 4.30 ppm (J = 5.2 Hz) and 0.85 ppm, as well as multiplets at 3.36 ppm, 1.39 ppm, and 1.24 ppm. The 4.30 ppm triplet corresponds to the signal of the methylene (A in Figure 3) adjacent to the oxygen of tethered n-octadecyl alcohol. The integral is low compared to the other octadecyl signals because of fast relaxation as it is immobilized at the silica surface. There is no discernible signal at 3.50 ppm, which is where the methylene signal should appear for free octadecyl alcohol. We therefore conclude that all the organic material in the sample is covalently coupled to the surface of the colloids. The amount of n-octadecyl alcohol and BTAs coupled to the small beads with d = 26 nm was determined by thermogravimetric analysis (TGA, Figure 4). Samples dried at 80°C in vacuo show a weight loss of 11 % for ODBs and 13 % for BTA-ODBs. Bare AS-40 also exhibits a weight loss, which was consistently determined to be 4 %. This may be attributed to the removal of ethoxy or hydroxyl groups at high temperatures. Additionally, interstitial water may be released over the full temperature range due to the fast heating rate. Assuming that this contribution is equally large for ODBs and cBTA-ODBs, 4 % is subtracted from the total mass loss during TGA. This results in 7 % organic material for ODBs and 9 % for cBTA-ODBs. Using MOD = 269.28 g mol-1, a silica density of ρsiO2 = 0.89 kg dm-3, and assuming that all organic matter is surface attached (i.e., assuming neither n-octadecyl alcohol nor cBTAs enters into pores in the beads) we obtain a surface coverage of approximately 1 molecule per nm2 for


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ODBs. Functionalizing the beads with a mixture of 20 mol % BTA and 80 mol% n-octadecyl alcohol thus leads to 1 BTA per 5 nm2, assuming that BTAs and the n-octadecyl alcohol have the same reactivity. The molecular weight of cBTA 1 McBTA = 792.58 g mol-1 is almost three times as high as that of the n-octadecyl alcohol; therefore introduction of 20 % BTA instead of noctadecyl alcohol should lead to a 1.4 times larger weight loss for cBTA-ODBs (9.7 %), which is close to the observed value of 9 %. The ratio between organic and inorganic material of the d = 513 nm beads is too low to obtain reliable TGA results. We assume that their surface coverage is similar to that of AS-40. Intermolecular association of tethered supramolecular motifs. To determine whether the tethered BTAs are available for association at the molecular level, so-called Sergeants-andSoldiers experiments51, 52 were performed. Generally, in these experiments a small amount of a chiral seed (the ‘sergeant’) is added to a solution of achiral, optically inactive, molecules (the ‘soldiers’) to induce a non-proportional increase of the optical activity in the mixture. If the chiral seed interacts with the achiral components, its addition induces a non-linear increase in absorption in circular dichroism (CD) spectroscopy as it strongly biases the helicity of the coassembled aggregates. By contrast, if there is no association between the two, the increase in CD signal is linearly proportional to the increase in the concentration of the sergeant and independent of the soldier concentration. A 0.1 wt % suspension of fully activated BTA-ODBs of d = 26 nm– the sergeant – in cyclohexane was added to a 3 · 10-5 M solution of the CD-silent soldier BTA 2 (Figure 5). Although the concentration of the chiral surface-attached sergeant is only 13 % of the total BTA concentration, addition of the sergeant to a solution of the soldiers induces a significant CD effect with maxima at 216 and 240 nm. This is in line with previously reported Sergeants-and-


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Soldiers experiments on mixtures of achiral BTA 2 and a chiral BTA.38, 52, 53 By contrast, ODBs do not alter the CD-signal of BTA 2, and only a minor change in the CD signal was observed upon mixing cBTA-ODBs with BTA 2. These results clearly demonstrate that surface grafted BTAs retain their ability to bias the helicity of BTAs present in solution, indicating that they remain capable of intermolecular hydrogen-bonding when attached to the surface. Light-induced colloidal assembly. To visualize by confocal microscopy whether intermolecular association between surface-attached BTAs can induce colloidal self-assembly, we use green fluorescent ODBs and BTA-ODBs (d = 513 nm). Exposure to UV-light converts the latent surface-tethered cBTAs into supramolecular moieties that are capable of self-assembling. Before activation, ODBs (Figure 6A), and cBTA-ODBs in cyclohexane (Figure 6B) and dichloroethane (Figure 6C) hardly show interaction; nearly all beads are present as singlets and the fraction of singlets (fs) exceeds 0.8. While there is no discernible effect of illumination on the ODBs (Figure 6D and G), clustering is observed in 0.1 wt % suspensions of BTA-OBDs after cleavage of the onitrobenzyl protecting group. One minute of illumination is sufficient to activate a fraction of the BTAs, which results in the formation of small clusters for the BTA-beads in cyclohexane as well as in dichloroethane (Figure 6E and F). The clusters increase in number and in size until all singlets are present in clusters after an illumination time of 20 minutes (Figure 6H and I). Both these phenomena are indicative of stronger attractive interparticle interactions. Quantitative image analysis was performed to compare light-induced colloidal assembly of BTA-ODBs in cyclohexane and dichloroethane using the number fraction of singlets, fs = ns / ntot in which ns is the number of singlets, and ntot is the total number of particles. Figure 7 shows the measured fraction of singlets in (BTA-)ODB suspensions as a function of illumination time. ODBs without BTAs in cyclohexane remain predominantly dispersed as singlets irrespective of


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the illumination time. By contrast, association of BTA-ODBs in both CH and DCE is strongly light-dependent. In both solvents all particles are present in clusters within 20 minutes of illumination. There is no significant difference observed between suspensions of BTA-ODBs in cyclohexane and dichloroethane. Gratifyingly, these confocal microscopy experiments demonstrate that surface functionalization of silica beads with o-nitrobenzyl protected BTAs transforms beads that interact solely via excluded volume interactions into light responsive selfassembling colloids of which the multivalency, and hence the association strength, is controlled by the degree of activation. Thermo-responsive colloidal assembly. Previous work has demonstrated that o-nitrobenzyl protected BTAs free in solution and grafted to polymer chains are both light- and temperature responsive.45 The confocal microscopy experiments show that covalent attachment of the photoresponsive cBTA 1 transforms organophilic silica beads into light-responsive colloids which cluster upon UV illumination. Static light scattering (SLS) experiments were performed to affirm this result and investigate whether, likewise, the temperature dependence of free BTA selfassembly translates into temperature-dependent association of BTA-ODBs. Microscopy experiments on d = 513 nm beads show that short deprotection times lead to the formation of small clusters, while longer activation results in the formation of larger clusters. For lightscattering experiments, smaller (d = 26 nm), non-fluorescent beads are used. The slower sedimentation of these beads enables longer acquisition times, while the use of non-fluorescent silica beads ensures minimal absorption of the incident laser light. First, we verify if the light-induced clustering of BTA-ODBs observed by microscopy at room temperature can also be achieved for BTA-ODBs with a diameter of 26 nm. Therefore, suspensions of 0.1 wt % non-fluorescent cBTA-ODBs were activated 1 or 20 minutes with UV


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light at 20 °C. Assuming that intra- and interparticle interference effects are negligible, the average scattering intensity 〈𝐼〉 is proportional to the weight concentration and average mass of

the scatterers in solution. Hence, at a fixed concentration of beads, an increase in

〈𝐼〉 is

indicative of cluster formation. In line with the microscopy results, we find a roughly twofold

increase in 〈𝐼〉 upon 1 minute of activation (Figure 8a), and a close to 150 fold increase in 〈𝐼〉

after 20 minutes. This means that both the small and the large supramolecular colloids exhibit light-dependent assembly. Next, variable temperature experiments were performed on BTA-ODBs activated for 1 and 20 minutes (Figure 8A). Irrespective of illumination time, BTA-ODB suspensions scatter less light at high than at low temperatures. A transition from high to low 〈𝐼〉 occurs at temperatures above 50° C, which indicates a decrease in the amount and/or size of colloidal aggregates evidencing

that the thermoresponsive behavior of the BTAs is transferred successfully onto the colloids. By contrast, ODBs do not exhibit temperature-dependent behavior, which strongly suggests that the observed temperature-dependence of 〈𝐼〉 is due to dissociation of intermolecularly associated BTAs at high temperatures. The transition between a predominantly dispersed state at low temperature and a predominantly clustered state at high temperature occurs in a relatively broad temperature range of approximately 10 °C. This is on a par with optimized DNA functionalized colloids,54, 55 and allows for the formation of rather well-ordered clusters without the need for iterative annealing steps. The association of singlets into clusters and the dissociation clustersinto individual BTA-ODBs is reversible as demonstrated by cycling multiple times between 20 °C and 75 °C (Figure 8B). The SLS experiments also reveal that the o-nitobenzyl protecting group does not fully prevent association of cBTA-ODBs beads prior to illumination. Both latent and activated BTA-ODBs exhibit a similar transition to lower scattering intensities well above room


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temperature. Surprisingly, increasing the illumination time from 1 to 20 minutes, i.e. uncaging virtually all the surface tethered BTAs, does not lead to a sharper transition. We speculate that this is a result of dispersity of both the beads themselves as well as the number of activated BTAs per particle. DISCUSSION Through surface-functionalization of silica beads with n-octadecyl alcohol and o-nitrobenzyl protected BTAs we developed light- and thermoresponsive supramolecular colloids. The attractive supramolecular interactions that are introduced by hydrogen bond formation between activated BTAs is demonstrated by the fact that neither ODBs, without BTAs, nor cBTA-ODBs, with caged BTAs, show clustering. We presume that colloidal assembly is induced by the formation of BTA dimers between BTAs on different beads. With a grafting density of 1 BTA per 5 nm2, a linker of approximately 23 Å between the BTA and the silica surface, and noctadecyl alcohol as a filler, it is highly unlikely that assemblies of more than two BTAs form, or that dimers form between BTAs on the same particle. This means that BTA induced colloidal assembly contrasts with the behavior of BTAs in solution, which occurs via a cooperative mechanism in which dimer formation is unfavorable.56 Previous studies have, however, shown non-cooperative assembly behavior for polymer grafted BTAs, presumably due to the increased entropic penalty that has to be overcome to assemble polymer grafted BTAs.57 Moreover, the cooperative assembly behavior of BTAs in solution is caused by the energetically unfavorable out of plain conformation of the amides to form three hydrogen bonds with adjacent BTAs,58 but other hydrogen bonding arrays have been observed experimentally.59 CONCLUSION


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In conclusion, we developed a new responsive material consisting of silica beads decorated with n-octadecanol and BTAs. The attractive surface forces introduced in this way between colloidal silica are largely driven by hydrogen bond formation. As a consequence, colloidal assembly is pronounced in suspensions of colloids surface-functionalized with light-activated BTAs capable of intermolecular association, while clustering is negligible for n-octadecyl coated beads (ODBs) without BTAs and BTA-functionalized particles prior to UV-illumination (cBTA-ODBs). We have shown that BTA induced colloidal self-assembly is controlled by two independent external triggers: UV light and temperature. The amount of active BTAs on the silica surface, and hence the multivalency of the particles, is simply controlled by the UV illumination time. The ‘strength’ of individual BTA-BTA interactions can be tuned conveniently and reversibly by temperature. This allowed us to produce supramolecular colloids with modular surface forces due to few but strong BTA-BTA interactions after short UV-exposure at low temperature and many but weak interactions upon prolonged UV-exposure at high temperature. This dualregulation approach based on externally addressable multivalent weak interactions offers a new route towards dynamic and responsive colloidal materials. In future work we aspire to engineer a more versatile synthesis route by which we can incorporate a diversity of supramolecular binding motifs and expand the complexity of the self-organized structures formed. FIGURES


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Figure 1. Schematic representation of octadecyl beads (ODB), and the behavior of caged BTA functionalized (cBTA-ODB), and activated (BTA-ODB) octadecyl beads, and structures of caged cBTA 1, with two chiral side chains and one hydroxy-alkane chain and BTA 1, where the o-nitrobenzyl group is cleaved off.


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Figure 2. A) Scanning electron microscopy (SEM) image of green fluorescent silica beads prior to functionalization and B) BTA-ODBs. C) Infrared spectra of AS-40 (d = 26 nm) and D) green fluorescent silica beads (d = 513 nm). The non-functionalized beads are depicted in black, ODBs in red, cBTA-ODBs in green and pure octadecyl alcohol in blue.


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Figure 3. Magic angle spinning nuclear magnetic resonance spectrum of silica coupled noctadecyl alcohol, recorded at a spinning speed of 3000 Hz in DMSO-d6.

Figure 4. Thermogravimetric analysis of plain AS-40, ODBs and BTA-ODBS measured from 25 to 800° C with a speed of 20° C per minute under an air atmosphere.


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Figure 5. Schematic representation of the Sergeants-and-Soldiers experiment, BTA 2 forms equal amounts of P and M helical aggregates which forms a CD silent solution. UV activation of the chiral sergeants that are coupled to the bead bias the formation of M helical aggregates and results in a CD effect as shown in the CD spectra of achiral BTA 2 c = 3 · 10-5 M, and spectra of mixtures of achiral BTA 2 with 0.1 wt % ODBs, 0.1 wt % cBTA-ODBs, or 0.1 wt % BTAODBs.


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Figure 6. Confocal fluorescence microscopy images of 0.1 wt %, green fluorescent ODBs (513 nm) in cyclohexane (CH) (A, D and G) and BTA-ODBs in cyclohexane (B, E and H) and dichloroethane (DCE) (C, F and I) after 0, 1 and 20 minutes of exposure to UV light. Scale bar represents 5 µm. The bar plots represent the distribution of beads in singlets, doublets, triplets, clusters of 4-10 beads, clusters of 11-50 beads and clusters larger than 50 beads (ntot > 200).


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Figure 7. Fraction of singlets (fs) versus deprotection time for ODBs in CH in (triangles), BTAODBs in DCE (squares) and BTA-ODBs in CH (circles), fs is the ratio between the number of singlets (ns) an the total number of beads (ntot), which was > 200 for all confocal microscopy images. Lines are drawn to guide the eye.


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Figure 8. A) Polarized static light scattering measured at an angle of 90° as a function of temperature for 26 nm ODBs (blue inverted triangles), latent cBTA-ODBs (red circles), BTAODBs activated for 1 minute (black squares) and activated for 20 minutes (green triangles). Lines are drawn as a guide for the eye. B) Static light scattering data collected at 90°, repeatedly measured at 20 °C and at 75 °C for BTA-ODBs activated for 5 minutes. AUTHOR INFORMATION Ilja K. Voets Email: [email protected] Tel: +31 40 247 3101


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT We gratefully acknowledge E.W. Meijer for scientific discussions, Jolanda A.J.H. Spiering and Patrick J.M. Stals for synthesis of the BTAs, Claudia C.M.M. Schot and Arthur M. de Jong for technical assistance with the microscopy experiments Marko M.L. Nieuwenhuizen for the MAS NMR and Abraham C.H. Pape for the SEM pictures. We acknowledge the Netherlands Organisation for Scientific Research (NWO VENI Grant 700.10.406, ECHO-STIP 717.013.005) (I.K.V.), and the Dutch Ministry of Education, Culture and Science (Gravity program 024.001.035) for financial support. ABBREVIATIONS BTA, benzene-1,3,5-tricarboxamide; cBTA; caged BTA; ODB, octadecyl bead;

TGA,

thermogravimetric analysis; IR, infra-red spectroscopy; MAS-NMR, magic angle spinning nuclear magnetic resonance, CH, cyclohexane; DCE, dichloroethane; CD, circular dichroism; SLS, static light scattering.

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Stimuli-Responsive Colloidal Assembly Driven by Surface-Grafted

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