Insights into the internal structures of nanogels using a versatile asymmetric-flow field-flow fractionation method

Cation-induced morphological transitions and aggregation of thermoresponsive PNIPAM nanogels

Poly(N-isopropylacrylamide) (PNIPAM) nanogels are promising responsive colloidal particles that can be used in pharmaceutical applications as drug carriers. This work investigates the temperature-dependent morphological changes and agglomeration of PNIPAM nanogels in the presence of mono- and multi-valent cationic electrolytes. We described the deswelling, flocculation, thermal reversibility behaviour and aggregated morphology of PNIPAM nanogels over a range of electrolyte concentrations and temperatures revealing the critical transition points from stable suspension to spontaneous agglomeration. We demonstrated that the flocculating ability and the rate of aggregate formation follow the order of deswelling behaviour. Transmission electron microscopy and atomic force microscopy analysis revealed the presence of a shell-like layer with varying density in the multivalent electrolyte solutions when compared to those in aqueous medium. We identified a concentration threshold of the thermally induced reversible aggregation/dispersion for the PNIPAM nanogels in the presence of Na+ and K+ ions at 10 mM, for Mg2+ and Ca2+ ions at 1 mM and for Al3+ ions at 0.1 mM concentrations. Such concentration thresholds indicated the effective destabilization of the electrolyte system with multivalency following the Schulze-Hardy rule. Our findings were supported by applying a Debye screening model that accounts for the shielding effect of multivalent cationic electrolytes on these nanogel systems. Our experiments and the models confirmed the compression of the electric double layer as the valency and ionic strength increased, except for Al3+ at higher concentrations which seemed to disrupt the electrical double layer and cause reversal of zeta potential. Our work highlights the significant impact the presence of multivalent cations can impose on the stability and morphology of nanogels, and this understanding will help in designing responsive nanogel systems based on PNIPAM nanogels.

Biodegradable Covalently Crosslinked Poly[N-(2-Hydroxypropyl) Methacrylamide] Nanogels: Preparation and Physicochemical Properties

Recently, suitably sized polymer-based nanogels containing functional groups for the binding of biologically active substances and ultimately degradable to products that can be removed by glomerular filtration have become extensively studied systems in the field of drug delivery. Herein, we designed and tailored the synthesis of hydrophilic and biodegradable poly[N-(2-hydroxypropyl) methacrylamide-co-N,N'-bis(acryloyl) cystamine-co-6-methacrylamidohexanoyl hydrazine] (PHPMA-BAC-BMH) nanogels. The facile and versatile dispersion polymerization enabled the preparation of nanogels with a diameter below 50 nm, which is the key parameter for efficient and selective passive tumor targeting. The effects of the N,N'-bis(acryloyl) cystamine crosslinker, polymerization composition, and medium including H2O/MetCel and H2O/EtCel on the particle size, particle size distribution, morphology, and polymerization kinetics and copolymer composition were investigated in detail. We demonstrated the formation of a 38 nm colloidally stable PHPMA-BAC-BMH nanogel with a core-shell structure that can be rapidly degraded in the presence of 10 mM glutathione solution under physiologic conditions. The nanogels were stable in an aqueous solution modeling the bloodstream; thus, these nanogels have the potential to become highly important carriers in the drug delivery of various molecules.

Field-Flow Fractionation in Molecular Biology and Biotechnology

Field-flow fractionation (FFF) is a family of single-phase separative techniques exploited to gently separate and characterize nano- and microsystems in suspension. These techniques cover an extremely wide dynamic range and are able to separate analytes in an interval between a few nm to 100 µm size-wise (over 15 orders of magnitude mass-wise). They are flexible in terms of mobile phase and can separate the analytes in native conditions, preserving their original structures/properties as much as possible. Molecular biology is the branch of biology that studies the molecular basis of biological activity, while biotechnology deals with the technological applications of biology. The areas where biotechnologies are required include industrial, agri-food, environmental, and pharmaceutical. Many species of biological interest belong to the operational range of FFF techniques, and their application to the analysis of such samples has steadily grown in the last 30 years. This work aims to summarize the main features, milestones, and results provided by the application of FFF in the field of molecular biology and biotechnology, with a focus on the years from 2000 to 2022. After a theoretical background overview of FFF and its methodologies, the results are reported based on the nature of the samples analyzed.

Understanding the Degradation of Core-Shell Nanogels Using Asymmetrical Flow Field Flow Fractionation

Nanogels are candidates for biomedical applications, and core-shell nanogels offer the potential to tune thermoresponsive behaviour with the capacity for extensive degradation. These properties were achieved by the combination of a core of poly(N-isopropylmethacrylamide) and a shell of poly(N-isopropylacrylamide), both crosslinked with the degradable crosslinker N,N'-bis(acryloyl)cystamine. In this work, the degradation behaviour of these nanogels was characterised using asymmetric flow field flow fractionation coupled with multi-angle and dynamic light scattering. By monitoring the degradation products of the nanogels in real-time, it was possible to identify three distinct stages of degradation: nanogel swelling, nanogel fragmentation, and nanogel fragment degradation. The results indicate that the core-shell nanogels degrade slower than their non-core-shell counterparts, possibly due to a higher degree of self-crosslinking reactions occurring in the shell. The majority of the degradation products had molecule weights below 10 kDa, which suggests that they may be cleared through the kidneys. This study provides important insights into the design and characterisation of degradable nanogels for biomedical applications, highlighting the need for accurate characterisation techniques to measure the potential biological impact of nanogel degradation products.

On the Fractionation and Physicochemical Characterisation of Self-Assembled Chitosan-DNA Polyelectrolyte Complexes

Chitosan is extensively studied as a carrier for gene delivery and is an attractive non-viral gene vector owing to its polycationic, biodegradable, and biocompatible nature. Thus, it is essential to understand the chemistry of self-assembled chitosan-DNA complexation and their structural and functional properties, enabling the formation of an effective non-viral gene delivery system. In this study, two parent chitosans (samples NAS-032 and NAS-075; Mw range ~118-164 kDa) and their depolymerised derivatives (deploy nas-032 and deploy nas-075; Mw range 6-14 kDa) with degrees of acetylation 43.4 and 4.7%, respectively, were used to form polyelectrolyte complexes (PECs) with DNA at varying [-NH₃⁺]/[-PO₄^-] (N/P) molar charge ratios. We investigated the formation of the PECs using ζ-potential, asymmetric flow field-flow fractionation (AF4) coupled with multiangle light scattering (MALS), refractive index (RI), ultraviolet (UV) and dynamic light scattering (DLS) detectors, and TEM imaging. PEC formation was confirmed by ζ-potential measurements that shifted from negative to positive values at N/P ratio ~2. The radius of gyration (R_g) was determined for the eluting fractions by AF4-MALS-RI-UV, while the corresponding hydrodynamic radius (R_h), by the DLS data. We studied the influence of different cross-flow rates on AF4 elution patterns for PECs obtained at N/P ratios 5, 10, and 20. The determined rho shape factor (ρ = R_g/R_h) values for the various PECs corresponded with a sphere morphology (ρ ~0.77-0.85), which was consistent with TEM images. The results of this study represent a further step towards the characterisation of chitosan-DNA PECs by the use of multi-detection AF4 as an important tool to fractionate and infer aspects of their morphology.

Field-flow fractionation for nanoparticle characterization

This review presents field-flow fractionation: The elements of theory enable the link between the retention and the characteristics of the nanometer-sized analytes to be highlighted. In particular, the nature of force and its way of being applied are discussed. Four types of forces which determine four types of techniques were considered: hydrodynamic, sedimentation, thermal, and electrical; this is to show the importance of the choice of technique in relation to the characterization objectives. Then the separation performance is presented and compared with other separation techniques: field-flow fractionation has the greatest intrinsic separation capability. The characterization strategies are presented and discussed; on the one hand with respect to the characteristics needed for the description of nanoparticles; on the other hand in connection with the choice of the nature of the force, and also of the detectors used, online or offline. The discussion is based on a selection of published study examples. Finally, current needs and challenges are addressed, and as response, trends and possible characterization solutions.

Ouzo phase occurrence with alternating lipo/hydrophilic copolymers in water

Selection of monomer couples, ensuring reactivity ratios close to zero, is an effective strategy to induce spontaneous copolymerization into an alternating sequence. In addition, monomer design and customisation of the solvent-monomer interactions open the way to functional copolymers showing molecular self-assembly relevant to their regular amphipathic structure. In this work, we show that the design of comonomers with adequate reactivities and interactions can be used to direct copolymer self-assembly on a mesoscopic scale. We investigate spontaneous formation of nanoparticles through solvent/non-solvent interactions using the so-called "ouzo effect". In this way, an ouzo diagram was built to determine the operation window for the self-assembly, in aqueous suspensions, of alternating copolymers consisting of vinyl phenol and maleimide units carrying long alkyl-pendant groups (C₁₂H₂₅ or C₁₈H₃₇). Also, investigations were pursued to account for the influence of the lateral lipophilic pendant units on the size and structure of the nanoaggregates formed during one-shot water addition. Structure characterisation by light scattering techniques (DLS and SLS), small-angle neutron scattering (SANS) and transmission electron microscopy (cryo-TEM and TEM) confirmed the self-assembly of copolymer chains into nanoparticles (size range: 60-300 nm), the size of which is affected by the lipophilicity of the alternating copolymers, solvent-water affinity and the solvent diffusion in water. Altogether, we present here the spontaneous ouzo effect as a simple method to produce stable alternating copolymer nanoparticles in water without the addition of stabilizing agents.

The effects of cononsolvents on the synthesis of responsive particles via polymerisation-induced thermal self-assembly

Responsive nanogels were synthesised via RAFT-mediated polymerisation-induced thermal self-assembly in cononsolvent mixtures of water and ethanol. The solvent mixture affected the particle size, tacticity and thermal properties.