Synthesis of tailored nanogels by means of two-stage irradiation

On the Mechanism and Kinetics of Synthesizing Polymer Nanogels by Ionizing Radiation-Induced Intramolecular Crosslinking of Macromolecules

Nanogels-internally crosslinked macromolecules-have a growing palette of potential applications, including as drug, gene or radioisotope nanocarriers and as in vivo signaling molecules in modern diagnostics and therapy. This has triggered considerable interest in developing new methods for their synthesis. The procedure based on intramolecular crosslinking of polymer radicals generated by pulses of ionizing radiation has many advantages. The substrates needed are usually simple biocompatible polymers and water. This eliminates the use of monomers, chemical crosslinking agents, initiators, surfactants, etc., thus limiting potential problems with the biocompatibility of products. This review summarizes the basics of this method, providing background information on relevant aspects of polymer solution thermodynamics, radiolysis of aqueous solutions, generation and reactions of polymer radicals, and the non-trivial kinetics and mechanism of crosslinking, focusing on the main factors influencing the outcomes of the radiation synthesis of nanogels: molecular weight of the starting polymer, its concentration, irradiation mode, absorbed dose of ionizing radiation and temperature. The most important techniques used to perform the synthesis, to study the kinetics and mechanism of the involved reactions, and to assess the physicochemical properties of the formed nanogels are presented. Two select important cases, the synthesis of nanogels based on polyvinylpyrrolidone (PVP) and/or poly(acrylic acid) (PAA), are discussed in more detail. Examples of recent application studies on radiation-synthesized PVP and PAA nanogels in transporting drugs across the blood-brain barrier and as targeted radioisotope carriers in nanoradiotherapy are briefly described.

Synthesis and Properties of Targeted Radioisotope Carriers Based on Poly(Acrylic Acid) Nanogels

Radiation crosslinking was employed to obtain nanocarriers based on poly(acrylic acid)—PAA—for targeted delivery of radioactive isotopes. These nanocarriers are internally crosslinked hydrophilic macromolecules—nanogels—bearing carboxylic groups to facilitate functionalization. PAA nanogels were conjugated with an engineered bombesin-derivative—oligopeptide combined with 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate chelating moiety, aimed to provide selective radioligand transport. 4-(4,6-Dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium (DMTMM) toluene-4-sulfonate was used as the coupling agent. After tests on a model amine—p-toluidine—both commercial and home-synthesized DOTA-bombesin were successfully coupled to the nanogels and the obtained products were characterized. The radiolabeling efficiency of nanocarriers with ¹⁷⁷Lu, was chromatographically tested. The results provide a proof of concept for the synthesis of radiation-synthesized nanogel-based radioisotope nanocarriers for theranostic applications.

Polymerization Reactions and Modifications of Polymers by Ionizing Radiation

Ionizing radiation has become the most effective way to modify natural and synthetic polymers through crosslinking, degradation, and graft polymerization. This review will include an in-depth analysis of radiation chemistry mechanisms and the kinetics of the radiation-induced C-centered free radical, anion, and cation polymerization, and grafting. It also presents sections on radiation modifications of synthetic and natural polymers. For decades, low linear energy transfer (LLET) ionizing radiation, such as gamma rays, X-rays, and up to 10 MeV electron beams, has been the primary tool to produce many products through polymerization reactions. Photons and electrons interaction with polymers display various mechanisms. While the interactions of gamma ray and X-ray photons are mainly through the photoelectric effect, Compton scattering, and pair-production, the interactions of the high-energy electrons take place through coulombic interactions. Despite the type of radiation used on materials, photons or high energy electrons, in both cases ions and electrons are produced. The interactions between electrons and monomers takes place within less than a nanosecond. Depending on the dose rate (dose is defined as the absorbed radiation energy per unit mass), the kinetic chain length of the propagation can be controlled, hence allowing for some control over the degree of polymerization. When polymers are submitted to high-energy radiation in the bulk, contrasting behaviors are observed with a dominant effect of cross-linking or chain scission, depending on the chemical nature and physical characteristics of the material. Polymers in solution are subject to indirect effects resulting from the radiolysis of the medium. Likewise, for radiation-induced polymerization, depending on the dose rate, the free radicals generated on polymer chains can undergo various reactions, such as inter/intramolecular combination or inter/intramolecular disproportionation, b-scission. These reactions lead to structural or functional polymer modifications. In the presence of oxygen, playing on irradiation dose-rates, one can favor crosslinking reactions or promotes degradations through oxidations. The competition between the crosslinking reactions of C-centered free radicals and their reactions with oxygen is described through fundamental mechanism formalisms. The fundamentals of polymerization reactions are herein presented to meet industrial needs for various polymer materials produced or degraded by irradiation. Notably, the medical and industrial applications of polymers are endless and thus it is vital to investigate the effects of sterilization dose and dose rate on various polymers and copolymers with different molecular structures and morphologies. The presence or absence of various functional groups, degree of crystallinity, irradiation temperature, etc. all greatly affect the radiation chemistry of the irradiated polymers. Over the past decade, grafting new chemical functionalities on solid polymers by radiation-induced polymerization (also called RIG for Radiation-Induced Grafting) has been widely exploited to develop innovative materials in coherence with actual societal expectations. These novel materials respond not only to health emergencies but also to carbon-free energy needs (e.g., hydrogen fuel cells, piezoelectricity, etc.) and environmental concerns with the development of numerous specific adsorbents of chemical hazards and pollutants. The modification of polymers through RIG is durable as it covalently bonds the functional monomers. As radiation penetration depths can be varied, this technique can be used to modify polymer surface or bulk. The many parameters influencing RIG that control the yield of the grafting process are discussed in this review. These include monomer reactivity, irradiation dose, solvent, presence of inhibitor of homopolymerization, grafting temperature, etc. Today, the general knowledge of RIG can be applied to any solid polymer and may predict, to some extent, the grafting location. A special focus is on how ionizing radiation sources (ion and electron beams, UVs) may be chosen or mixed to combine both solid polymer nanostructuration and RIG. LLET ionizing radiation has also been extensively used to synthesize hydrogel and nanogel for drug delivery systems and other advanced applications. In particular, nanogels can either be produced by radiation-induced polymerization and simultaneous crosslinking of hydrophilic monomers in "nanocompartments", i.e., within the aqueous phase of inverse micelles, or by intramolecular crosslinking of suitable water-soluble polymers. The radiolytically produced oxidizing species from water, •OH radicals, can easily abstract H-atoms from the backbone of the dissolved polymers (or can add to the unsaturated bonds) leading to the formation of C-centered radicals. These C-centered free radicals can undergo two main competitive reactions; intramolecular and intermolecular crosslinking. When produced by electron beam irradiation, higher temperatures, dose rates within the pulse, and pulse repetition rates favour intramolecular crosslinking over intermolecular crosslinking, thus enabling a better control of particle size and size distribution. For other water-soluble biopolymers such as polysaccharides, proteins, DNA and RNA, the abstraction of H atoms or the addition to the unsaturation by •OH can lead to the direct scission of the backbone, double, or single strand breaks of these polymers.

Tailor-made high-performance reverse osmosis membranes by surface fixation of hydrophilic macromolecules for wastewater treatment

Polyamide aromatic (PA) reverse osmosis (RO) membranes are currently the most important materials in the seawater desalination and wastewater treatment industry. This study used hydrophilic macromolecular polyvinylpyrrolidone (PVP) in a PA selective layer to develop a new polyamide thin-film composite (TFC), namely PA-g-PVP RO, which will be used for water treatment. The TFC is prepared via an interfacial polymerisation process, and TFC-based PVP can be transplanted on a PA surface by radiation. PA-g-PVP RO was characterised by ATR-FTIR, SEM, XPS, AFM and contact angle test and then evaluated by determining its permeability, salt retention and antifouling performance, among other properties. Results show that the chemical composition and surface morphology of the polyamide film significantly changed. A PVP brush grafted on an RO membrane surface significantly enhanced the hydrophilicity and antifouling performance of the membrane. When the PVP concentration was increased in an aqueous solution to 2%, the water contact angle of the sacrificial layer of the modified membrane decreased to 24.3°, the fouling recovery ratio to 93.4% and the salt retention increased to 99.5% at a small flux change. This combined technology can also be used for other macromolecules to modify the membrane and study the preparation and modification of ultra-filtration and nano-filtration membranes.

Polyamide aromatic (PA) reverse osmosis (RO) membranes are currently the most important materials in the seawater desalination and wastewater treatment industry.

Construction of polyporous polymer microspheres with a tailored mesoporous wall

Polymer microspheres with a novel hierarchically porous structure (inner macropores and a mesoporous wall) were fabricated by taking advantage of γ-ray-radiation-initiated dispersion polymerization.

Application of radiation for the synthesis of poly(n-vinyl pyrrolidone) nanogels with controlled sizes from aqueous solutions

Controlling of sizes of nanogels is very important for any biomedical application. In the present study we report a facile and reproducible method of preparing biocompatible nanogels of poly(N-vinyl pyrrolidone) (PVP) which were synthesized by using either electron beam (e-beam) (NGEB) or gamma irradiation (NGG) of dilute aqueous solutions. Nanogels with different hydrodynamic sizes were obtained at the variance of the polymer molecular weight, concentration, type of radiation source hence dose rate and total absorbed dose. For the first time a comparative study of gamma and e-beam irradiation was made on the same polymer with the aim of controlling sizes of nanogels in the range of 30-250 nm. Moreover the stability of radiation-synthesized nanogels was followed up to 2 years in refrigerated solution and found to retain their original sizes and distributions enabling their long-term storage and use. The synthesized nanogels were characterized by using dynamic light scattering (DLS), gel permeation chromatography (GPC), scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques. This work provides a clue to the fundamental question of how to control sizes of nanogels without using any additives which are indispensable with the other techniques. The technique is applicable to any water soluble polymer.

Radiation Engineering of Multifunctional Nanogels

Nanogels combine the favourable properties of hydrogels with those of colloids. They can be soft and conformable, stimuli-responsive and highly permeable, and can expose a large surface with functional groups for conjugation to small and large molecules, and even macromolecules. They are among the very few systems that can be generated and used as aqueous dispersions. Nanogels are emerging materials for targeted drug delivery and bio-imaging, but they have also shown potential for water purification and in catalysis. The possibility of manufacturing nanogels with a simple process and at relatively low cost is a key criterion for their continued development and successful application. This paper highlights the most important structural features of nanogels related to their distinctive properties, and briefly presents the most common manufacturing strategies. It then focuses on synthetic approaches that are based on the irradiation of dilute aqueous polymer solutions using high-energy photons or electron beams. The reactions constituting the basis for nanogel formation and the approaches for controlling particle size and functionality are discussed in the context of a qualitative analysis of the kinetics of the various reactions.

On the origin of functionalization in one-pot radiation synthesis of nanogels from aqueous polymer solutions

E-beam irradiation is a “green”, one-step route for the production of biocompatible nanogels from polymer aqueous solutions. Functional group density is tuned independently from size and molecular weight by a proper choice of irradiation conditions.

Characterization and antimicrobial property of poly(acrylic acid) nanogel containing silver particle prepared by electron beam

In this study, we developed a one step process to synthesize nanogel containing silver nanoparticles involving electron beam irradiation. Water-soluble silver nitrate powder is dissolved in the distilled water and then poly(acrylic acid) (PAAc) and hexane are put into this silver nitrate solution. These samples are irradiated by an electron beam to make the PAAc nanogels containing silver nanoparticles (Ag/PAAc nanogels). The nanoparticles were characterized by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). In addition, the particle size and zeta-potential were confirmed by a particle size analyzer (PSA). The antibacterial properties of the nanogels were evaluated by paper diffusion test. The Ag/PAAc nanogels had an antibacterial effect against Escherichia coli and Staphylococcus aureus. The nanogels also demonstrated a good healing effect against diabetic ulcer. The size of the Ag/PAAc nanogels decreased with increasing irradiation doses, and the absolute value of the zeta potential increased with increasing irradiation doses. Also, the Ag/PAAc nanogels exhibited good antibacterial activity against both Gram-negative and Gram-positive bacteria. In in vivo wound healing, the Ag/PAAc nanogels have a good healing effect.