Panayiotis Bilalis

Non-covalent functionalization of carbon nanotubes with polymers

Carbon nanotubes have emerged as very promising materials in various research fields spanning from biotechnology to energy storage and transformation. Their poor solubility in aqueous and organic solvents and limited compatibility with polymer matrices are major drawbacks, rendering these materials incapable of achieving their full potential. Covalent or non-covalent functionalization with polymers is considered a major key in circumventing this issue. In this feature article, the non-covalent functionalization through various types of interactions between polymers and carbon nanotubes is highlighted and their potential applications are discussed.

Nanodesigned magnetic polymer containers for dual stimuli actuated drug controlled release and magnetic hyperthermia mediation

Novel magnetic, pH and redox sensitive microcontainers have been prepared using a sacrificial template-directed synthesis procedure, followed by chemical deposition of magnetic nanocrystals via co-precipitation. The fabricated magnetic microcontainers exert operative pH responsiveness, gradual and controlled collapse, once met with highly reducing environment, and efficient magnetic response.

Controlled polymerization of histidine and synthesis of well-defined stimuli responsive polymers. Elucidation of the structure–aggregation relationship of this highly multifunctional material

We present the synthesis of the novel monomer Nim-trityl-protected N-carboxy anhydride of L-histidine (Trt-HIS-NCA) for the synthesis of poly(L-histidine) (PHIS). Kinetic studies of the ring opening polymerization of Trt-HIS-NCA followed first order kinetics, indicating that the polymerization is “living”. The high purity of the synthesized monomer along with the use of high vacuum techniques resulted in the controlled polymerization of histidine in a variety of macromolecular architectures exhibiting high degrees of molecular and compositional homogeneity. The conformation of poly(L-histidine) (PHIS) was studied at different pH values and temperatures by circular dichroism, revealing that it adopts a random coil conformation at low pH and temperatures, a β-sheet conformation at higher pH, and probably a broken β-sheet conformation at higher temperatures. We found that the pKa of the PHIS homopolymer depends on the molecular weight. Addition of hydrophobic amino acids randomly distributed along the PHIS chain hinders the organization of PHIS, resulting in the formation of the random coil conformation even at higher pH. The influence of either leucine (LEU) or γ-benzyl-L-glutamate (BLG) randomly distributed along the PHIS chain on the pKa and degree of protonation in the terpolymers revealed that although the pKa is lower, the protonation of PHIS increases at lower pH values, while it is lower at a higher pH as compared to that obtained for PHIS. The aggregates of PEO-b-P(HIS-co-PLEU(BLG)) in water were found to swell more by decreasing the pH and increasing the hydrophobic amino acids, and eventually become disrupted. Surprisingly, at pH = 7.4, the increase in temperature leads to lower aggregation of the PEO-b-PHIS due to the transition of the secondary structure. The results indicate that it is possible to fine-tune the protonation of PHIS as a function of pH and temperature, and thus to control the conditions where the aggregates will be disrupted, a prerequisite for drug and gene delivery applications.

Development of Multiple Stimuli Responsive Magnetic Polymer Nanocontainers as Efficient Drug Delivery Systems

Magnetic nanodevices based on poly[(methacrylic acid)-co-(N-isopropylacrylamide)] [P(MAA-co-NIPAAm)] are prepared and used as drug delivery systems employing daunorubicin (DNR) as a model drug. The magnetic nanocontainers exploit the pH, temperature, and magnetic response of the polymeric shell constituents and magnetic nanoparticles, respectively, for controlled pH, temperature and alternating magnetic field triggered drug release. The in vitro cytotoxicity of both DNR-loaded and empty nanocontainers is examined on MCF-7 breast cancer cells along with the intracellular distribution of DNR. The results show that the DNR-loaded nanocontainers have an anti-tumor effect comparable to the free drug. The current observations provide important information for potent drug delivery and release systems.

Nanoscale Rings Fabricated Using Self-Assembled Triblock Terpolymer Templates

Although there has been extensive work on the use of self-assembled diblock copolymers for nanolithography, there are few reports of the use of multiblock copolymers, which can form a more diverse range of nanoscale pattern geometries. Pattern transfer from a self-assembled poly(butadiene-b-styrene-b-methyl methacrylate) (PB-b-PS-b-PMMA) triblock terpolymer thin film has been investigated. Polymers of different total molecular weight were synthesized with a predicted morphology consisting of PMMA-core/PS-shell cylinders in a PB matrix. By adjusting the solvent-annealing conditions and the film thickness, thin films with vertically oriented cylinders were formed. The PMMA cylinder cores and the PB matrix were then removed using selective etching to leave an array of PS rings, and the ring pattern was transferred into a silica film by reactive ion etching to form 19 nm high silica rings. This result illustrates the design and use of triblock terpolymers for self-assembled lithography.

Self-assembly of a Model Peptide incorporating a Hexa-Histidine sequence attached to an Oligo-Alanine sequence, and binding to Gold NTA/Nickel nanoparticles

Amyloid fibrils are formed by a model surfactant-like peptide (Ala)10-(His)6 containing a hexa-histidine tag. This peptide undergoes a remarkable two-step self-assembly process with two distinct critical aggregation concentrations (cacs), probed by fluorescence techniques. A micromolar range cac is ascribed to the formation of prefibrillar structures, whereas a millimolar range cac is associated with the formation of well-defined but more compact fibrils. We examine the labeling of these model tagged amyloid fibrils using Ni-NTA functionalized gold nanoparticles (Nanogold). Successful labeling is demonstrated via electron microscopy imaging. The specificity of tagging does not disrupt the β-sheet structure of the peptide fibrils. Binding of fibrils and Nanogold is found to influence the circular dichroism associated with the gold nanoparticle plasmon absorption band. These results highlight a new approach to the fabrication of functionalized amyloid fibrils and the creation of peptide/nanoparticle hybrid materials.

Ring-opening polymerization of ω-pentadecalactone catalyzed by phosphazene superbases

A fast and living ring-opening polymerization (ROP) of ω-pentadecalactone (PDL), a representative monomer of macrolactones, was achieved using a primary alcohol as the initiator and t-BuP4 or t-octP4 as the catalyst. The use of t-BuP2 instead of the t-BuP4 superbase slows down the polymerization rate. The ROP of PDL proceeds to high conversion not only at 80 °C in bulk but also at room temperature and in dilute solution. The synthesized PDL homopolymers and block copolymers with poly(ethylene glycol) were characterized by high-temperature GPC (HT-GPC), 1H NMR and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). Melting temperatures, determined by differential scanning calorimetry, are also reported.

Anionic Polymerization of Styrene and 1,3-Butadiene in the Presence of Phosphazene Superbases

The anionic polymerization of styrene and 1,3-butadiene in the presence of phosphazene bases (t-BuP4, t-BuP2 and t-BuP1), in benzene at room temperature, was studied. When t-BuP1 was used, the polymerization proceeded in a controlled manner, whereas the obtained homopolymers exhibited the desired molecular weights and narrow polydispersity (Ð < 1.05). In the case of t-BuP2, homopolymers with higher than the theoretical molecular weights and relatively low polydispersity were obtained. On the other hand, in the presence of t-BuP4, the polymerization of styrene was uncontrolled due to the high reactivity of the formed carbanion. The kinetic studies from the polymerization of both monomers showed that the reaction rate follows the order of [t-BuP4]/[sec-BuLi] >>> [t-BuP2]/[sec-BuLi] >> [t-BuP1]/[sec-BuLi] > sec-BuLi. Furthermore, the addition of t-BuP2 and t-BuP1 prior the polymerization of 1,3-butadiene allowed the synthesis of polybutadiene with a high 1,2-microstructure (~45 wt %), due to the delocalization of the negative charge. Finally, the one pot synthesis of well-defined polyester-based copolymers [PS-b-PCL and PS-b-PLLA, PS: Polystyrene, PCL: Poly(ε-caprolactone) and PLLA: Poly(L-lactide)], with predictable molecular weights and a narrow molecular weight distribution (Ð < 1.2), was achieved by sequential copolymerization in the presence of t-BuP2 and t-BuP1.

Poly(Sarcosine)-Based Nano-Objects with Multi-Protease Resistance by Aqueous Photoinitiated Polymerization-Induced Self-Assembly (Photo-PISA)

Poly(sarcosine) (PSar) is a non-ionic hydrophilic polypeptoid with numerous biologically relevant properties, making it an appealing candidate for the development of amphiphilic block copolymer nanostructures. In this work, the fabrication of poly(sarcosine)-based diblock copolymer nano-objects with various morphologies via aqueous reversible addition-fragmentation chain-transfer (RAFT)-mediated photoinitiated polymerization-induced self-assembly (photo-PISA) is reported. Poly(sarcosine) was first synthesized via ring-opening polymerization (ROP) of sarcosine N-carboxyanhydride, using high-vacuum techniques. A small molecule chain transfer agent (CTA) was then coupled to the active ω-amino chain end of the telechelic polymer for the synthesis of a poly(sarcosine)-based macro-CTA. Controlled chain-extensions of a commercially available water-miscible methacrylate monomer (2-hydroxypropyl methacrylate) were achieved via photo-PISA under mild reaction conditions, using PSar macro-CTA. Upon varying the degree of polymerization and concentration of the core-forming monomer, morphologies evolving from spherical micelles to worm-like micelles and vesicles were accessed, as determined by dynamic light scattering and transmission electron microscopy, resulting in the construction of a detailed phase diagram. The resistance of both colloidally stable empty vesicles and enzyme-loaded nanoreactors against degradation by a series of proteases was finally assessed. Overall, our findings underline the potential of poly(sarcosine) as an alternative corona-forming polymer to poly(ethylene glycol)-based analogues of PISA assemblies for use in various pharmaceutical and biomedical applications.

Self-Healing pH- and Enzyme Stimuli-Responsive Hydrogels for Targeted Delivery of Gemcitabine To Treat Pancreatic Cancer

A novel, multifunctional hydrogel that exhibits a unique set of properties for the effective treatment of pancreatic cancer (PC) is presented. The material is composed of a pentablock terpolypeptide of the type PLys- b-(PHIS- co-PBLG)-PLys- b-(PHIS- co-PBLG)- b-PLys, which is a noncytotoxic polypeptide. It can be implanted via the least invasive route and selectively delivers gemcitabine to efficiently treat PC. Simply mixing the novel terpolypeptide with an aqueous solution of gemcitabine within a syringe results in the facile formation of a hydrogel that has the ability to become liquid under the shear rate of the plunger. Upon injection in the vicinity of cancer tissue, it immediately reforms into a hydrogel due to the unique combination of its macromolecular architecture and secondary structure. Because of its pH responsiveness, the hydrogel only melts close to PC; thus, the drug can be delivered directionally toward the cancerous rather than healthy tissues in a targeted, controlled, and sustained manner. The efficacy of the hydrogel was tested in vivo on human to mouse xenografts using the drug gemcitabine. It was found that the efficacy of the hydrogel loaded with only 40% of the drug delivered in one dose was equal to or slightly better than the peritumoral injection of 100% of the free drug delivered in two doses, the typical chemotherapy used in clinics so far. This result suggests that the hydrogel can direct the delivery of the encapsulated drug effectively in the tumor tissue. Enzymes lead to its biodegradation, avoiding removal by resection of the polypeptidic carrier after cargo delivery. The unique properties of the hydrogel formed can be predetermined through its molecular characteristics, rendering it a promising modular material for many biological applications.