Ilja K. Voets

Complex coacervate core micelles

In this review we present an overview of the literature on the co-assembly of neutral-ionic block, graft, and random copolymers with oppositely charged species in aqueous solution. Oppositely charged species include synthetic (co)polymers of various architectures, biopolymers - such as proteins, enzymes and DNA - multivalent ions, metallic nanoparticles, low molecular weight surfactants, polyelectrolyte block copolymer micelles, metallo-supramolecular polymers, equilibrium polymers, etcetera. The resultant structures are termed complex coacervate core/polyion complex/block ionomer complex/interpolyelectrolyte complex micelles (or vesicles); i.e., in short C3Ms (or C3Vs) and PIC, BIC or IPEC micelles (and vesicles). Formation, structure, dynamics, properties, and function will be discussed. We focus on experimental work; theory and modelling will not be discussed. Recent developments in applications and micelles with heterogeneous coronas are emphasized.

Multiresponsive reversible gels based on charge-driven assembly.

Linked in? Coassembly of an ABA triblock copolymer with charged end blocks and an oppositely charged polyelectrolyte yields gels that respond to changes in concentration, temperature, ionic strength, pH value, and charge composition. Above the critical gel concentration, the triblock copolymers bridge micelles, forming a sample-spanning transient network of interconnected micelles

Single chain polymeric nanoparticles as compartmentalised sensors for metal ions

3,3′-Bis(acylamino)-2,2′-bipyridine substituted benzene-1,3,5-tricarboxamide (BiPy-BTA) grafted polynorbornene polymers were prepared via ring-opening metathesis polymerisation using a third generation Grubbs catalyst. The polymers fold intramolecularly via π–π interactions into fluorescent, compartmentalised particles of nanometer-size in mixtures of tetrahydrofuran and methylcyclohexane. Spectroscopic and light scattering techniques show that the compact conformation of the folded polymer is affected by increasing the BiPy-BTA functionalisation degree and by changing the solvent polarity. Changes in the conformation are accompanied by changes in the fluorescence intensity. Due to the affinity of the 2,2′-bipyridine units for metal ions such as copper, the particles obtained are effective sensors for these metals. The compartmentalisation of the binding motifs in SCPNs proves to be advantageous in sensor applications of these particles.

Observation of ice-like water layers at an aqueous protein surface

Significance Antifreeze proteins (AFPs) enable the survival of various organisms in freezing or subfreezing habitats by preventing the macroscopic growth of ice crystals. Understanding how AFPs recognize and bind ice crystals is the most important step to unravel their working mechanism. Using surface-specific sum frequency generation spectroscopy, we were able to directly probe the ice-binding site of the protein and discovered that, already at room temperature and in an aqueous solution, the antifreeze proteins arrange water molecules into an ice-like array, which they then use to bind to ice crystals. We study the properties of water at the surface of an antifreeze protein with femtosecond surface sum frequency generation spectroscopy. We find clear evidence for the presence of ice-like water layers at the ice-binding site of the protein in aqueous solution at temperatures above the freezing point. Decreasing the temperature to the biological working temperature of the protein (0 °C to −2 °C) increases the amount of ice-like water, while a single point mutation in the ice-binding site is observed to completely disrupt the ice-like character and to eliminate antifreeze activity. Our observations indicate that not the protein itself but ordered ice-like water layers are responsible for the recognition and binding to ice.

Spontaneous symmetry breaking: formation of Janus micelles

We describe the preparation and solution properties of Janus micelles, i.e., non-centrosymmetric nanoparticles with compartmentalized shells, viaco-assembly of two fully water-soluble block copolymers. They consist of a mixed core of poly(N-methyl-2-vinyl pyridinium iodide) (P2MVP) and poly(acrylic acid) (PAA), and a shell segregated into two sides, consisting of poly(ethylene oxide) (PEO) or poly(acryl amide) PAAm. These Janus particles form spontaneously and reversibly, i.e., association, dissocation, and reassociation can be carefully controlled via parameters, such as polymer mixing fraction, solution pH, and ionic strength. Dynamic (polarized and depolarized) and static light scattering, cryogenic transmission electron microscopy, small angle neutron and X-ray scattering, and two-dimensional nuclear magnetic resonance spectroscopy are used to monitor the micelle formation and to characterize the micellar structure. The Janus particles were found to be ellipsoidal, with a cigar-like overall shape and a disc-like core. This peculiar morphology is driven by the delicate interplay between two opposing forces: an attraction between the oppositely charged core blocks and a subtle repulsion between the water-soluble, neutral corona blocks.

Consequences of chirality on the dynamics of a water-soluble supramolecular polymer

The rational design of supramolecular polymers in water is imperative for their widespread use, but the design principles for these systems are not well understood. Herein, we employ a multi-scale (spatial and temporal) approach to differentiate two analogous water-soluble supramolecular polymers: one with and one without a stereogenic methyl. Initially aiming simply to understand the molecular behaviour of these systems in water, we find that while the fibres may look identical, the introduction of homochirality imparts a higher level of internal order to the supramolecular polymer. Although this increased order does not seem to affect the basic dimensions of the supramolecular fibres, the equilibrium dynamics of the polymers differ by almost an order of magnitude. This report represents the first observation of a structure/property relationship with regard to equilibrium dynamics in water-soluble supramolecular polymers.

Spatiotemporal control and superselectivity in supramolecular polymers using multivalency.

Multivalency has an important but poorly understood role in molecular self-organization. We present the noncovalent synthesis of a multicomponent supramolecular polymer in which chemically distinct monomers spontaneously coassemble into a dynamic, functional structure. We show that a multivalent recruiter is able to bind selectively to one subset of monomers (receptors) and trigger their clustering along the self-assembled polymer, behavior that mimics raft formation in cell membranes. This phenomenon is reversible and affords spatiotemporal control over the monomer distribution inside the supramolecular polymer by superselective binding of single-strand DNA to positively charged receptors. Our findings reveal the pivotal role of multivalency in enabling structural order and nonlinear recognition in water-soluble supramolecular polymers, and it offers a design principle for functional, structurally defined supramolecular architectures.

Mesoscale Modulation of Supramolecular Ureidopyrimidinone-Based Poly(ethylene glycol) Transient Networks in Water

In natural systems, highly synergistic non-covalent interactions among biomolecular components exert mesoscopic control over hierarchical assemblies. We herein present a multicomponent self-assembly strategy to tune hierarchical supramolecular polymer architectures in water using highly affine and directional ureidopyrimidinone-poly(ethylene glycol)s (UPy-PEG). Using scattering methods and oscillatory rheology, we observe the structural and mechanical regulation of entangled monofunctional UPy-PEG fibrils by cross-linking bifunctional UPy-PEG fibrils. This supramolecular mixing approach opens the door to a range of subtly distinct materials for chemical and biological applications.

On the stability and morphology of complex coacervate core micelles : from spherical to wormlike micelles

We present a systematic study of the stability and morphology of complex coacervate core micelles (C3Ms) formed from poly(acrylic acid) (PAA) and poly(N-methyl-2-vinylpyridinium)-b-poly(ethylene oxide) (PM2VP-b-PEO). We use polarized and depolarized dynamic and static light scattering, combined with small-angle X-ray scattering, to investigate how the polymer chain length and salt concentration affect the stability, size, and shape of these micelles. We show that C3Ms are formed in aqueous solution below a critical salt concentration, which increases considerably with increasing PAA and PM2VP length and levels off for long chains. This trend is in good agreement with a mean-field model of polyelectrolyte complexation based on the Voorn-Overbeek theory. In addition, we find that salt induces morphological changes in C3Ms when the PAA homopolymer is sufficiently short: from spherical micelles with a diameter of several tens of nanometers at low salt concentration to wormlike micelles with a contour length of several hundreds of nanometers just before the critical salt concentration. By contrast, C3Ms of long PAA homopolymers remain spherical upon addition of salt and shrink slightly. A critical review of existing literature on other C3Ms reveals that the transition from spherical to wormlike micelles is probably a general phenomenon, which can be rationalized in terms of a classical packing parameter for amphiphiles.

Temperature responsive complex coacervate core micelles with a PEO and PNIPAAm corona.

In aqueous solutions at room temperature, poly( N-methyl-2-vinyl pyridinium iodide)- block-poly(ethylene oxide), P2MVP 38- b-PEO 211 and poly(acrylic acid)- block-poly(isopropyl acrylamide), PAA 55- b-PNIPAAm 88 spontaneously coassemble into micelles, consisting of a mixed P2MVP/PAA polyelectrolyte core and a PEO/PNIPAAm corona. These so-called complex coacervate core micelles (C3Ms), also known as polyion complex (PIC) micelles, block ionomer complexes (BIC), and interpolyelectrolyte complexes (IPEC), respond to changes in solution pH and ionic strength as their micellization is electrostatically driven. Furthermore, the PNIPAAm segments ensure temperature responsiveness as they exhibit lower critical solution temperature (LCST) behavior. Light scattering, two-dimensional 1H NMR nuclear Overhauser effect spectrometry, and cryogenic transmission electron microscopy experiments were carried out to investigate micellar structure and solution behavior at 1 mM NaNO 3, T = 25, and 60 degrees C, that is, below and above the LCST of approximately 32 degrees C. At T = 25 degrees C, C3Ms were observed for 7 < pH < 12 and NaNO 3 concentrations below approximately 105 mM. The PEO and PNIPAAm chains appear to be (randomly) mixed within the micellar corona. At T = 60 degrees C, onion-like complexes are formed, consisting of a PNIPAAm inner core, a mixed P2MVP/PAA complex coacervate shell, and a PEO corona.