Nikolay Houbenov

Anisotropic ionic conductivity in fluorinated ionic liquid crystals suitable for optoelectronic applications

In this work, we report a library of thirteen fluorinated ionic liquids consisting of iodide salts of 1-alkyl-3-polyfluoroalkyl-imidazolium cations. By changing the length of the alkyl and polyfluoroalkyl pendants, we discovered that particular combinations of these result in compounds showing a mesophase. The nature and the molecular arrangement of the mesophase are characterised by polarised optical microscopy and powder X-ray diffraction analysis, among others. We demonstrate that, after the addition of I2 to generate the I−/I3− redox couple, anisotropic ionic conductivity takes place along preferential pathways in the lamellar structure of the mesophase. Notably, the addition of I2 does not suppress the mesophase temperature range, contrary to previously reported systems. Furthermore, the tendency of these materials to supercool allows the molecular arrangement in the mesophase to be retained in a solid film at ambient temperatures. Finally, we demonstrate their applicability as a quasi-solid electrolyte by preparing dye-sensitised solar cells with power conversion efficiencies comparable to the previous reports.

Supracolloidal Multivalent Interactions and Wrapping of Dendronized Glycopolymers on Native Cellulose Nanocrystals

Cellulose nanocrystals (CNCs) are high aspect ratio colloidal rods with nanoscale dimensions, attracting considerable interest recently due to their high mechanical properties, chirality, sustainability, and availability. In order to exploit them for advanced functions in new materials, novel supracolloidal concepts are needed to manipulate their self-assemblies. We report on exploring multivalent interactions to CNC surface and show that dendronized polymers (DenPols) with maltose-based sugar groups on the periphery of lysine dendrons and poly(ethylene-alt-maleimide) polymer backbone interact with CNCs. The interactions can be manipulated by the dendron generation suggesting multivalent interactions. The complexation of the third generation DenPol (G3) with CNCs allows aqueous colloidal stability and shows wrapping around CNCs, as directly visualized by cryo high-resolution transmission electron microscopy and electron tomography. More generally, as the dimensions of G3 are in the colloidal range due to their ~6 nm lateral size and mesoscale length, the concept also suggests supracolloidal multivalent interactions between other colloidal objects mediated by sugar-functionalized dendrons giving rise to novel colloidal level assemblies.

Stimuli-responsive command polymer surface for generation of protein gradients.

Mixed polyelectrolyte brushes with a composition gradient were used as a platform for fabrication of stimuli-responsive command surfaces to control the generation of concentration gradients of adsorbed protein molecules. Switching between homogeneously adsorbed protein layers and adsorbed layers with protein concentration gradients was achieved by changing the pH of protein aqueous solutions. Protein adsorption and the direction of the adsorption gradient were tuned and also turned off and on or reversed by tuning the proton concentration in the pH range 4.0–8.6.

Supramolecular amplification of amyloid self-assembly by iodination

Amyloid supramolecular assemblies have found widespread exploitation as ordered nanomaterials in a range of applications from materials science to biotechnology. New strategies are, however, required for understanding and promoting mature fibril formation from simple monomer motifs through easy and scalable processes. Noncovalent interactions are key to forming and holding the amyloid structure together. On the other hand, the halogen bond has never been used purposefully to achieve control over amyloid self-assembly. Here we show that single atom replacement of hydrogen with iodine, a halogen-bond donor, in the human calcitonin-derived amyloidogenic fragment DFNKF results in a super-gelator peptide, which forms a strong and shape-persistent hydrogel at 30-fold lower concentration than the wild-type pentapeptide. This is remarkable for such a modest perturbation in structure. Iodination of aromatic amino acids may thus develop as a general strategy for the design of new hydrogels from unprotected peptides and without using organic solvents.

Halogen-bonded mesogens direct polymer self-assemblies up to millimetre length scale

Aligning polymeric nanostructures up to macroscale in facile ways remains a challenge in materials science and technology. Here we show polymeric self-assemblies where nanoscale organization guides the macroscopic alignment up to millimetre scale. The concept is shown by halogen bonding mesogenic 1-iodoperfluoroalkanes to a star-shaped ethyleneglycol-based polymer, having chloride end-groups. The mesogens segregate and stack parallel into aligned domains. This leads to layers at ~10 nm periodicity. Combination of directionality of halogen bonding, mesogen parallel stacking and minimization of interfacial curvature translates into an overall alignment in bulk and films up to millimetre scale. Upon heating, novel supramolecular halogen-bonded polymeric liquid crystallinity is also shown. As many polymers present sites capable of receiving halogen bonding, we suggest generic potential of this strategy for aligning polymer self-assemblies.

Gas phase synthesis of non-bundled, small diameter single-walled carbon nanotubes with near-armchair chiralities

We present a novel floating catalyst synthesis route for individual, i.e. non-bundled, small diameter single-walled carbon nanotubes (SWCNTs) with a narrow chiral angle distribution peaking at high chiralities near the armchair species. An ex situ spark discharge generator was used to form iron particles with geometric number mean diameters of 3-4 nm and fed into a laminar flow chemical vapour deposition reactor for the continuous synthesis of long and high-quality SWCNTs from ambient pressure carbon monoxide. The intensity ratio of G/D peaks in Raman spectra up to 48 and mean tube lengths up to 4 microns were observed. The chiral distributions, as directly determined by electron diffraction in the transmission electron microscope, clustered around the (n,m) indices (7,6), (8,6), (8,7) and (9,6), with up to 70% of tubes having chiral angles over 20{\deg}. The mean diameter of SWCNTs was reduced from 1.10 to 1.04 nm by decreasing the growth temperature from 880 to 750 {\deg}C, which simultaneously increased the fraction of semiconducting tubes from 67 to 80%. Limiting the nanotube gas phase number concentration to approx. 100 000 per cubic centimetre successfully prevented nanotube bundle formation that is due to collisions induced by Brownian diffusion. Up to 80 % of 500 as-deposited tubes observed by atomic force and transmission electron microscopy were individual. Transparent conducting films deposited from these SWCNTs exhibited record low sheet resistances of 63 Ohms/sq. at 90 % transparency for 550 nm light.

Modification of surface wettability through adsorption of partly fluorinated statistical and block polyelectrolytes from aqueous medium

The wetting properties of electrostatically charged hydrophilic substrates were modified through adsorption of ultrathin layer of amphiphilic block or statistical polyelectrolyte from aqueous medium. The studied polymers were copolymers of 2-(dimethylamino)ethyl methacrylate (DMAEMA) and 2,2,2-trifluoroethyl methacrylate (TFEMA). They were adsorbed on mica from varying pH conditions, either as dissolved unimers or as kinetically trapped aqueous nanoparticles. The structures (by atomic force microscopy) and wetting properties (by dynamic contact angle measurements) of the obtained surface layers were determined. The majority of the surface layers consisted of polymeric nanoparticles with varying surface coverage. Annealing at 150 °C flattened and spread the particles on the surfaces. The surface wettability was found to be significantly influenced by the morphology and chemical composition of the obtained polymeric surface layer. The surfaces with the most homogeneous and smooth polymer layers exhibited the lowest contact angle hysteresis. The advancing/receding contact angles on the most hydrophilic copolymer layer on mica were 47°/<20°, and on the least hydrophilic layer they were 96°/63°. On unmodified mica surface the water contact angle is ∼0°. When those copolymers that provided the highest contact angles on mica were adsorbed on cellulose fiber substrates and annealed at 120 °C, highly hydrophobic surfaces were obtained, with advancing contact angles around 160°.

Extended Self-Assembled Long Periodicity and Zig-Zag Domains from Helix–Helix Diblock Copolymer Poly(γ-benzyl-l-glutamate)-block-poly(O-benzyl-l-hydroxyproline)

We describe the synthesis and self-assembly of particularly high periodicity of diblock copolymers composed of poly(benzyl-l-hydroxyproline) (PBLHyP) and poly(γ-benzyl-l-glutamate) (PBLG), that is, two polypeptide blocks with dissimilar helical structures. The robust helicity of the PBLHyP block is driven by steric constraints of the repeat units, while PBLG forms α-helices driven by hydrogen bonding, allowing defects and deformations. Herein, high-molecular-weight diblock copolypeptides of PBLG-b-PBLHyP with three different volume fractions of the PBLHyP-blocks are discussed. For shorter PBLHyP blocks, hexagonal packing of PBLHyP helices is observed, while by increasing the length of the PBLHyP block, keeping at a similar PBLG block length, the packing is distorted. Zig-zag lamellar structures were obtained due to the mismatch in the packing periodicities of the PBLG and PBLHyP helices. The frustration that takes place at the interface leads the PBLHyP to tilt to match the PBLG periodicity. The zig-zag morphology is reported for the first time for high-molecular-weight helix-helix (rod-rod) copolypeptides, and the self-assembled periodicity is uncommonly large.

Double Smectic Self-Assembly in Block Copolypeptide Complexes

We show double smectic-like self-assemblies in the solid state involving alternating layers of different polypeptide α-helices. We employed rod-coil poly(γ-benzyl l-glutamate)-block-poly(l-lysine) (PBLG-b-PLL) as the polymeric scaffold, where the PLL amino residues were ionically complexed to di-n-butyl phosphate (diC4P), di(2-ethylhexyl) phosphate (diC2/6P), di(2-octyldodecyl) phosphate (diC8/12P), or di-n-dodecyl phosphate (diC12P), forming PBLG-b-PLL(diC4P), PBLG-b-PLL(diC2/6P), PBLG-b-PLL(diC8/12P), and PBLG-b-PLL(diC12P) complexes, respectively. The complexes contain PBLG α-helices of fixed diameter and PLL-surfactant complexes adopting either α-helices of tunable diameters or β-sheets. For PBLG-b-PLL(diC4P), that is, using a surfactant with short n-butyl tails, both blocks were α-helical, of roughly equal diameter and thus with minor packing frustrations, leading to alternating PBLG and PLL(diC4P) smectic layers of approximately perpendicular alignment of both types of α-helices. Surfactants with longer and branched alkyl tails lead to an increased diameter of the PLL-surfactant α-helices. Smectic alternating PBLG and PLL(diC2/6P) layers involve larger packing frustration, which leads to poor overall order and suggests an arrangement of tilted PBLG α-helices. In PBLG-b-PLL(diC8/12P), the PLL(diC8/12P) α-helices are even larger and the overall structure is poor. Using a surfactant with two linear n-dodecyl tails leads to well-ordered β-sheet domains of PLL(diC12P), consisting of alternating PLL and alkyl chain layers. This dominates the whole assembly, and at the block copolypeptide length scale, the PBLG α-helices do not show internal order and have poor organization. Packing frustration becomes an important aspect to design block copolypeptide assemblies, even if frustration could be relieved by conformational imperfections. The results suggest pathways to control hierarchical liquid-crystalline assemblies by competing interactions and by controlling molecular packing frustrations.

Reversible Supracolloidal Self-Assembly of Cobalt Nanoparticles to Hollow Capsids and Their Superstructures

The synthesis and spontaneous, reversible supracolloidal hydrogen bond-driven self-assembly of cobalt nanoparticles (CoNPs) into hollow shell-like capsids and their directed assembly to higher order superstructures is presented. CoNPs and capsids form in one step upon mixing dicobalt octacarbonyl (Co2 CO8 ) and p-aminobenzoic acid (pABA) in 1,2-dichlorobenzene using heating-up synthesis without additional catalysts or stabilizers. This leads to pABA capped CoNPs (core ca. 5 nm) with a narrow size distribution. They spontaneously assemble into tunable spherical capsids (d≈50-200 nm) with a few-layered shells, as driven by inter-nanoparticle hydrogen bonds thus warranting supracolloidal self-assembly. The capsids can be reversibly disassembled and reassembled by controlling the hydrogen bonds upon heating or solvent exchanges. The superparamagnetic nature of CoNPs allows magnetic-field-directed self-assembly of capsids to capsid chains due to an interplay of induced dipoles and inter-capsid hydrogen bonds. Finally, self-assembly on air-water interface furnishes lightweight colloidal framework films.