Gellan Gum Microgels as Effective Agents for a Rapid Cleaning of Paper

Influence of a Solid Surface on PNIPAM Microgel Films

Stimuli-responsive microgels have attracted great interest in recent years as building blocks for fabricating smart surfaces with many technological applications. In particular, PNIPAM microgels are promising candidates for creating thermo-responsive scaffolds to control cell growth and detachment via temperature stimuli. In this framework, understanding the influence of the solid substrate is critical for tailoring microgel coatings to specific applications. The surface modification of the substrate is a winning strategy used to manage microgel-substrate interactions. To control the spreading of microgel particles on a solid surface, glass substrates are coated with a PEI or an APTES layer to improve surface hydrophobicity and add positive charges on the interface. A systematic investigation of PNIPAM microgels spin-coated through a double-step deposition protocol on pristine glass and on functionalised glasses was performed by combining wettability measurements and Atomic Force Microscopy. The greater flattening of microgel particles on less hydrophilic substrates can be explained as a consequence of the reduced shielding of the water-substrate interactions that favors electrostatic interactions between microgels and the substrate. This approach allows the yielding of effective control on microgel coatings that will help to unlock new possibilities for their application in biomedical devices, sensors, or responsive surfaces.

New horizons on advanced nanoscale materials for Cultural Heritage conservation

Nanomaterials have permeated numerous scientific and technological fields, and have gained growing importance over the past decades also in the preservation of Cultural Heritage. After a critical overview of the main nanomaterials adopted in art preservation, we provide new insights into some highly relevant gels, which constitute valuable tools to selectively remove dirt or other unwanted layers from the surface of works of art. In particular, the recent "twin-chain" gels, obtained by phase separation of two different PVAs and freeze-thawing, were considered as the most performing gel systems for the cleaning of Cultural Heritage. Three factors are crucial in determining the final gel properties, i.e., pore size, pore connectivity, and surface roughness, which belong to the micro/nanodomain. The pore size is affected by the molecular weight of the phase-separating PVA polymer, while pore connectivity and tortuosity likely depend on interconnections formed during gelation. Tortuosity greatly impacts on cleaning capability, as the removal of matter at the gel-target interface increases with the uploaded fluid's residence time at the interface (higher tortuosity produces longer residence). The gels' surface roughness, adaptability and stickiness can also be controlled by modulating the porogen amount or adding different polymers to PVA. Finally, PVA can be partially replaced with different biopolymers yielding gels with enhanced sustainability and effective cleaning capability, where the selection of the biopolymer affects the gel porosity and effectiveness. These results shed new light on the effect of micro/nanoscale features on the cleaning performances of "twin-chain" and composite gels, opening new horizons for advanced and "green"/sustainable gel materials that can impact on fields even beyond art preservation, like drug-delivery, detergency, food industry, cosmetics and tissue engineering.

Novel hydrogel poly (GG-co-acrylic acid) for the sorptive removal of the color Rhodamine-B from contaminated water

Textile effluent's treatment is highly desired due to the presence of hazardous, water-soluble and non-biodegradable dyes that not only have harmful effect on the environment but on living beings as well. Treatment of these pollutants by sorption through biosorbents is considered to be a best method of choice due to greener nature of the processes. In this connection hydrogel sorbents might be an intriguing option due to its straightforward application, great efficacy, easy synthesis, rapid turnaround, and potential of recycling. Herein, novel hydrogel was prepared using Gellan Gum and acrylic acid (GG-co-AAc) which were then characterized by instrumental techniques like UV/visible and FTIR spectroscopy, SEM, EDX and XRD. The anionic hydrogel's adsorption capacity, swelling behavior, and sorption potential were determined using Rhodamine-B as potential environmental pollutant. The hydrogel exhibited an impressive adsorption capacity of 1250 mg/g. Swelling experiments were performed in Milli-Q distilled water at different pH levels, reaching maximum swelling of 3230% after 23 h as determined through Fickian diffusion. At pH 7, the anionic hydrogel's sorption potential was thoroughly studied in the subsequent experiments. The adsorption process was found to follow the Langmuir isotherm, indicating a monolayer adsorption mechanism supported by higher R2 values compared to the Freundlich isotherm. Thermodynamic analysis revealed the exothermic nature of the adsorption process, with a negative enthalpy value of -11371 KJmol-1 and negative entropy value of -26.39 Jmol-1K-1, suggesting a less ordered system. These findings provide valuable insights into the adsorption characteristics and potential applications of the synthesized anionic hydrogel.

Ultrasound-Stimulated PVA Microbubbles as a Green and Handy Tool for the Cleaning of Cellulose-Based Materials

One of the main issues in the cultural heritage field of restoration chemistry is the identification of greener and more effective methods for the wet cleaning of paper artefacts, which serve as witnesses to human history and custodians of cultural values. In this context, we propose a biocompatible method to perform wet cleaning on paper based on the use of 1 MHz ultrasound in combination with water-dispersed polyvinyl alcohol microbubbles (PVAMBs), followed by dabbing with PVA-based hydrogel. This method can be applied to both old and new papers. FTIR spectroscopy, X-ray diffraction, HPLC analysis, pH measurements and tensile tests were performed on paper samples, to assess the efficacy of the cleaning system. According to the results, ultrasound-activated PVAMB application allows for an efficient interaction with rough and porous cellulose paper profiles, promoting the removal of cellulose degradation byproducts, while the following hydrogel dabbing treatment guarantees the removal of cleaning materials residues. Moreover, the results also pointed out that after the treatment no thermal or mechanical damages had affected the paper. In conclusion, the readability of these kinds of artifacts can be improved without causing an alteration of their structural properties, while mitigating the risk of ink diffusion.

Molecular origin of the two-step mechanism of gellan aggregation

Among hydrocolloids, gellan is one of the most studied polysaccharides due to its ability to form mechanically stable gels. Despite its long-standing use, the gellan aggregation mechanism is still not understood because of the lack of atomistic information. Here, we fill this gap by developing a new gellan force field. Our simulations offer the first microscopic overview of gellan aggregation, detecting the coil to single-helix transition at dilute conditions and the formation of higher-order aggregates at high concentration through a two-step process: first, the formation of double helices and then their assembly into superstructures. For both steps, we also assess the role of monovalent and divalent cations, complementing simulations with rheology and atomic force microscopy experiments and highlighting the leading role of divalent cations. These results pave the way for future use of gellan-based systems in a variety of applications, from food science to art restoration.

Progress and opportunities in Gellan gum-based materials: A review of preparation, characterization and emerging applications

Gellan gum, a microbial exopolysaccharide, is biodegradable and has potential to fill several key roles in many fields from food to pharmacy, biomedicine and tissue engineering. In order to improve the physicochemical and biological properties of gellan gum, some researchers take advantage of numerous hydroxyl groups and the free carboxyl present in each repeating unit. As a result, design and development of gellan-based materials have advanced significantly. The goal of this review is to provide a summary of the most recent, high-quality research trends that have used gellan gum as a polymeric component in the design of numerous cutting-edge materials with applications in various fields.

Biohybrid Nanocellulose-Lysozyme Amyloid Aerogels via Electrostatic Complexation

Modern science is increasingly turning to nature for inspiration to design sustainable biomaterials in a smart and effective way. Herein, we describe biohybrid aerogels based on electrostatic complexation between cellulose and proteins-two of the most abundant natural polymers on Earth. The effects of both particle surface charge and particle size are investigated with respect to aerogel properties including the morphology, surface area, stability, and mechanical strength. Specifically, negatively charged nanocellulose (cellulose nanocrystals and cellulose nanofibers) and positively charged lysozyme amyloid fibers (full-length and shortened via sonication) are investigated in the preparation of fibrillar aerogels, whereby the nanocellulose component was found to have the largest effect on the resulting aerogel properties. Although electrostatic interactions between these two classes of charged nanoparticles allow us to avoid the use of any cross-linking agents, the resulting aerogels demonstrate a simple additive performance as compared to their respective single-component aerogels. This lack of synergy indicates that although electrostatic complexation certainly leads to the formation of local aggregates, these interactions alone may not be strong enough to synergistically improve bulk aerogel properties. Nevertheless, the results reported herein represent a critical step toward a broader understanding of biohybrid materials based on cellulose and proteins.

Thermal Behaviour of Microgels Composed of Interpenetrating Polymer Networks of Poly(N-isopropylacrylamide) and Poly(acrylic acid): A Calorimetric Study

Stimuli-responsive microgels have recently attracted great attention in fundamental research as their soft particles can be deformed and compressed at high packing fractions resulting in singular phase behaviours. Moreover, they are also well suited for a wide variety of applications such as drug delivery, tissue engineering, organ-on-chip devices, microlenses fabrication and cultural heritage. Here, thermoresponsive and pH-sensitive cross-linked microgels, composed of interpenetrating polymer networks of poly(N-isopropylacrylamide) (PNIPAM) and poly(acrylic acid) (PAAc), are synthesized by a precipitation polymerization method in water and investigated through differential scanning calorimetry in a temperature range across the volume phase transition temperature of PNIPAM microgels. The phase behaviour is studied as a function of heating/cooling rate, concentration, pH and PAAc content. At low concentrations and PAAc contents, the network interpenetration does not affect the transition temperature typical of PNIPAM microgel in agreement with previous studies; on the contrary, we show that it induces a marked decrease at higher concentrations. DSC analysis also reveals an increase of the overall calorimetric enthalpy with increasing concentration and a decrease with increasing PAAc content. These findings are discussed and explained as related to emerging aggregation processes that can be finely controlled by properly changing concentration, PAAc content an pH. A deep analysis of the thermodynamic parameters allows to draw a temperature–concentration state diagram in the investigated concentration range.

Ultrasound-Stimulated PVA Microbubbles for Adhesive Removal from Cellulose-Based Materials: A Groundbreaking Low-Impact Methodology

In this work, we shed new light on ultrasound contrast agents applied to the field of cultural heritage as an invaluable fine-tune cleaning tool for paper artworks. In this context, one of the primary and challenging issues is the removal of modern adhesives from paper artifacts. Modern adhesives are synthetic polymers whose presence enhances paper degradation and worsens its optical features. A thorough analytical and high-spatial-resolution combined study was successfully performed to test the capability of poly(vinyl alcohol)-based microbubbles stimulated by a proper noninvasive 1 MHz ultrasound field exposure in removing these adhesives from paper surfaces, in the absence of volatile invasive and toxic chemicals and without damaging paper and/or leaving residues. We demonstrate that poly(vinyl alcohol)-shelled microbubbles are suitable for interacting with paper surfaces, targeting and boosting in a few minutes the nondamaging removal of adhesive particles from paper samples thanks to their peculiar shell composition together with their ultrasound dynamics.

Volume fraction determination of microgel composed of interpenetrating polymer networks of PNIPAM and polyacrylic acid

Interpenetrated polymer network microgels, composed of crosslinked networks of poly(N-isopropylacrylamide) and polyacrylic acid (PAAc), have been investigated through rheological measurements at four different amounts of PAAc. Both PAAc content and crosslinking degree modify particle dimensions, mass and softness, thereby strongly affecting the volume fraction and the system viscosity. Here the volume fraction is derived from the flow curves at low concentrations by fitting the zero-shear viscosity with the Einstein-Batchelor equation which provides a parameterkto shift weight concentration to volume fraction. We find that particles with higher PAAc content and crosslinker are characterized by a greater value ofkand therefore by larger volume fractions when compared to softer particles. The packing fractions obtained from rheological measurements are compared with those from static light scattering for two PAAc contents revealing a good agreement. Moreover, the behaviour of the viscosity as a function of packing fraction, at room temperature, has highlighted an Arrhenius dependence for microgels synthesized with low PAAc content and a Vogel-Fulcher-Tammann dependence for the highest investigated PAAc concentration. A comparison with the hard spheres behaviour indicates a steepest increase of the viscosity with decreasing particles softness. Finally, the volume fraction dependence of the viscosity at a fixed PAAc and at two different temperatures, below and above the volume phase transition, shows a quantitative agreement with the structural relaxation time measured through dynamic light scattering indicating that interpenetrated polymer network microgels softness can be tuned with PAAc and temperature and that, depending on particle softness, two different routes are followed.

Chemical-Physical Behaviour of Microgels Made of Interpenetrating Polymer Networks of PNIPAM and Poly(acrylic Acid)

Microgels composed of stimuli responsive polymers have attracted worthwhile interest as model colloids for theorethical and experimental studies and for nanotechnological applications. A deep knowledge of their behaviour is fundamental for the design of new materials. Here we report the current understanding of a dual responsive microgel composed of poly(N-isopropylacrylamide) (PNIPAM), a temperature sensitive polymer, and poly(acrylic acid) (PAAc), a pH sensitive polymer, at different temperatures, PAAc contents, concentrations, solvents and pH. The combination of multiple techniques as Dynamic Light Scattering (DLS), Raman spectroscopy, Small Angle Neutron Scattering (SANS), rheology and electrophoretic measurements allow to investigate the hydrodynamic radius behaviour across the typical Volume Phase Transition (VPT), the involved molecular mechanism and the internal particle structure together with the viscoelastic properties and the role of ionic charge in the aggregation phenomena.

Glass and Jamming Rheology in Soft Particles Made of PNIPAM and Polyacrylic Acid

The phase behaviour of soft colloids has attracted great attention due to the large variety of new phenomenologies emerging from their ability to pack at very high volume fractions. Here we report rheological measurements on interpenetrated polymer network microgels composed of poly(N-isopropylacrylamide) (PNIPAM) and polyacrylic acid (PAAc) at fixed PAAc content as a function of weight concentration. We found three different rheological regimes characteristic of three different states: a Newtonian shear-thinning fluid, an attractive glass characterized by a yield stress, and a jamming state. We discuss the possible molecular mechanisms driving the formation of these states.

Molecular insights on poly(N-isopropylacrylamide) coil-to-globule transition induced by pressure

By using extensive all-atom molecular dynamics simulations of an atactic linear polymer chain, we provide microscopic insights into poly(N-isopropylacrylamide) (PNIPAM) coil-to-globule transition addressing the roles played by both temperature and pressure. We detect a coil-to-globule transition up to large pressures, showing a reentrant behavior of the critical temperature with increasing pressure in agreement with experimental observations. Furthermore, again confirming the experimental findings, we report the existence at high pressures of a new kind of globular state. It is characterized by a more structured hydration shell that is closer to PNIPAM hydrophobic domains, as compared to the globular state observed at atmospheric pressure. Our results highlight that temperature and pressure induce a PNIPAM coil-to-globule transition through different molecular mechanisms, opening the way for a systematic use of both thermodynamic variables to tune the location of the transition and the properties of the associated swollen/collapsed states.

Charge affinity and solvent effects in numerical simulations of ionic microgels

Ionic microgel particles are intriguing systems in which the properties of thermo-responsive polymeric colloids are enriched by the presence of charged groups. In order to rationalize their properties and predict the behaviour of microgel suspensions, it is necessary to develop a coarse-graining strategy that starts from the accurate modelling of single particles. Here, we provide a numerical advancement of a recently-introduced model for charged co-polymerized microgels by improving the treatment of ionic groups in the polymer network. We investigate the thermoresponsive properties of the particles, in particular their swelling behaviour and structure, finding that, when charged groups are considered to be hydrophilic at all temperatures, highly charged microgels do not achieve a fully collapsed state, in favorable comparison to experiments. In addition, we explicitly include the solvent in the description and put forward a mapping between the solvophobic potential in the absence of the solvent and the monomer-solvent interactions in its presence, which is found to work very accurately for any charge fraction of the microgel. Our work paves the way for comparing single-particle properties and swelling behaviour of ionic microgels to experiments and to tackle the study of these charged soft particles at a liquid-liquid interface.