Vladimír Ďorďovič

We present the synthesis and characterization of zwitterionic poly(4-vinylpyridine) nanoparticles modified with phenylboronic acid (QxPVP-PBA). We studied three different degrees of modification, which differed in both physicochemical and saccharide-sensing properties. The pK_a values of QxPVP-PBA, determined by potentiometry and by ¹¹B NMR spectroscopy, increased with the degree of PVP quaternization due to repulsion between charged groups. By dialysis of QxPVP-PBA polymers dissolved in HCl against MOPS buffer, we prepared stable nanoparticles at neutral pH, mostly stabilized by positive charges and by chains likely interconnected through cyclic PBA structures termed boroxines. QxPVP-PBA hydrophobicity increases with the decrease in the degree of PVP quaternization due to hydrophobic uncharged pyridine groups, as indicated by covalent binding to the PBA fluorescent probe alizarin. By potentiometric titrations and zeta potential measurements, we determined the pK_a values of a fructose and glucose complex with the polymer, which were lower than the physiological pH. In addition, the alizarin displacement assay showed that QxPVP-PBA has a higher affinity to fructose than to glucose. Fructose binding leads to an increase in the size of the polymeric nanoparticles because their hydrophobicity increases after zwitterionic complex formation, as evaluated by LS and cryo-TEM. Notably, such fructose-bound particles showed a negative zeta potential. Therefore, QxPVP-PBA has unique charge reversal properties in the presence of saccharides.

Telechelic poly(ethylene oxide) with glucose end-groups interacts with metallacarborane that leads to lamellar nanostructures in water. They can incorporate metallacarborane- and benzoxaborole-based probes via dynamic bonding.

Anionic boron cluster compounds (ABCCs) are intrinsically amphiphilic building blocks suitable for nanochemistry. ABCCs are involved in atypical weak interactions, notably dihydrogen bonding, due to their peculiar polyhedral structure, consisting of negatively charged B–H units. The most striking feature of ABCCs that differentiates them from typical surfactants is the lack of head-and-tail structure. Furthermore, their structure can be described as intrinsically amphiphilic or aquaneutral. Therefore, classical terms established to describe self-assembly of classical amphiphiles are insufficient and need to be reconsidered. The opinions and theories focused on the solution behavior of ABCCs are briefly discussed. Moreover, a comparison between ABCCs with other amphiphilic systems is made focusing on the explanation of enthalpy-driven micellization or relations between hydrophobic and chaotropic effects. Despite the unusual structure, ABCCs still show self- and coassembly properties comparable to classical amphiphiles such as ionic surfactants. They self-assemble into micelles in water according to the closed association model. The most typical features of ABCCs solution behavior is demonstrated on calorimetry, NMR spectroscopy, and tensiometry experiments. Altogether, the unique features of ABCCs makes them a valuable inclusion into the nanochemisty toolbox to develop novel nanostructures both alone and with other molecules.

This is the first experimental evidence that both self-assembly and surface activity is a common feature of all water-soluble boron cluster compounds. Solution behavior of anionic polyhedral boranes (sodium decaborate, sodium dodecaborate, and sodium mercaptododecaborate), carboranes (potassium 1-carba-dodecaborate) and metallacarboranes (sodium [cobalt bis(1,2-dicarbollide)]), was extensively studied, and it is evident that all the anionic boron clusters form multimolecular aggregates in water. However, the mechanism of aggregation is size and polarity dependent. The series of studied clusters spans from small hydrophilic decaborate resembling hydrotrope to bulky hydrophobic cobalt bis(dicarbollide) behaving like a classical surfactant. Despite their pristine structure resembling Platonic solids, nature of anionic boron cluster compounds is inherently amphiphilic – they are stealth amphiphiles.

Alkaline cations (Li⁺, Na⁺ and K⁺) are introduced as agents suitable to control compartmentalization in metallacarborane-rich nanoparticles of double-hydrophilic block copolymer poly(ethylene oxide)-block-poly(2-alkyloxazoline), PEO–POX. Interaction of the metallacarborane (cobalt bis(dicarbollide) anion) with PEO–POX is based mainly on dihydrogen bonding between metallacarborane boron clusters and the polymer backbone resulting in compact nanoparticles. However, the cations are a crucial factor as to whether interaction with PEO or POX segments is preferred. Changes in the bulk concentration of alkaline cations can thus provoke changes in the inner structure of polymeric nanoparticles, which is accompanied by exchange of boron clusters and alkaline cations like Li⁺. Because of the biomedical importance of metallacarboranes, their conjugates and also lithium salts, the hybrid nanoparticles can act as stimuli-responsive systems for drug delivery.

The self‐assembly of metallacarboranes, a peculiar family of compounds exhibiting surface activity and resembling molecular‐scale Pickering stabilizers, has been investigated by comparison to the micellization of sodium dodecylsulfate (SDS). These studies have shown that molecules without classical amphiphilic topology but with an inherent amphiphilic nature can behave similarly to classical surfactants. As shown by NMR techniques, the self‐assembly of both metallacarboranes and SDS obey a closed association model. However, the aggregation of metallacarboranes is found to be enthalpy‐driven, which is very unusual for classical surfactants. Possible explanations of this fact are outlined.

One strategy to control the morphology of hybrid polymeric nanostructures is the proper selection of macromolecule architecture. We prepared metallacarborane-rich nanoparticles by interaction of double-hydrophilic block copolymers consisting of both poly(2-alkyl oxazolines) and poly(ethylene oxide) blocks with cobaltabisdicarbollide anion in physiological saline. The inner structure of the hybrid nanoparticles was studied by cryo-TEM, light scattering, SAXS, NMR, and ITC. Although the thermodynamics of diblock and star-like systems are almost identical, the macromolecular architecture has a great impact on the size and inner morphology of the nanoparticles. While hybrid nanoparticles formed by linear diblock copolymers are homogeneous, resembling gel-like nanospheres, the star-like shape of 4-arm block copolymers with PEO blocks in central parts of macromolecules leads to distinct compartmentalization. Because metallacarboranes are promising species in medicine, the studied nanoparticles are important for targeted drug delivery of boron cluster compounds.

The anion [3,3′‐Co(C₂B₉H₁₁)₂]⁻ ([COSAN]⁻) produces aggregates in water. These aggregates are interpreted to be the result of CH⋅⋅⋅HB interactions. It is possible to generate aggregates even after the incorporation of additional functional groups into the [COSAN]⁻ units. The approach is to join two [COSAN]⁻ anions by a linker that can adapt itself to act as a crown ether. The linker has been chosen to have six oxygen atoms, which is the ideal number for K⁺ selectivity in crown ethers. The linker binds the alkaline metal ions with different affinities; thus showing a distinct degree of selectivity. The highest affinity is shown towards K⁺ from a mixture containing Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺; this can be indicative of pseudo‐crown ether performance of the dumbbell. One interesting possibility is that the [COSAN]⁻ anions at the two ends of the linker can act as a hook‐and‐loop fastener to close the ring. This facet is intriguing and deserves further consideration for possible applications. The distinct affinity towards alkaline metal ions is corroborated by solubility studies and isothermal calorimetry thermograms. Furthermore, cryoTEM micrographs, along with light scattering results, reveal the existence of small self‐assemblies and compact nanostructures ranging from spheres to single‐/multi‐layer vesicles in aqueous solutions. The studies reported herein show that these dumbbells can have different appearances, either as molecules or aggregates, in water or lipophilic phases; this offers a distinct model as drug carriers.