Twin-chain polymer hydrogels based on poly(vinyl alcohol) as new advanced tool for the cleaning of modern and contemporary art

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.

Highly biocidal poly(vinyl alcohol)-hydantoin/starch hybrid gels: A "Trojan Horse" for Bacillus subtilis

HYPOTHESIS

Poly (vinyl alcohol) (PVA) cryogels can be functionalized with n-Halamines to confer biocidal features useful for their application as wound-dressing tools. Their efficacy can be boosted by stably embedding a polymeric bacterial food source (e.g., starch) in the gel matrix. The bioavailability of the food source lures bacteria inside the gel network via chemotactic mechanisms, promoting their contact with the biocidal functionalities and their consequent inactivation.

EXPERIMENTS

The synthesis of a novel hydantoin-functionalized PVA (H-PVA-hyd) is proposed. The newly synthesized H-PVA-hyd polymer was introduced in the formulation of H-PVA-based cryogels. To promote the cryogelation of the systems we exploited phase-separation mechanisms employing either a PVA carrying residual acetate groups (L-PVA) or starch as phase-segregating components. The permanence of the biocidal functionality after swelling was investigated via proton nuclear magnetic resonance (1H NMR) and Fourier transform infrared (FT-IR) microscopy. The activated H-PVA-hyd cryogels have been tested against bacteria with amylolytic activity (Bacillus subtilis) and the outcomes were analyzed by direct observation via confocal laser scanning microscopy (CLSM).

FINDINGS

The cryogels containing starch resulted in being the most effective (up to 90% bacterial killing), despite carrying a lower amount of hydantoin groups than their starch-free counterparts, suggesting that their improved efficacy relies on a "Trojan Horse" type of mechanism.

Tailoring the properties of poly(vinyl alcohol) "twin-chain" gels via sebacic acid decoration

HYPOTHESIS

The development of gels capable to adapt and act at the interface of rough surfaces is a central topic in modern science for Cultural Heritage preservation. To overcome the limitations of solvents or polymer solutions, commonly used in the restoration practice, poly(vinyl alcohol) (PVA) "twin-chain" polymer networks (TC-PNs) have been recently proposed. The properties of this new class of gels, that are the most performing gels available for Cultural Heritage preservation, are mostly unexplored. This paper investigates how chemical modifications affect gels' structure and their rheological behavior, producing new gelled systems with enhanced and tunable properties for challenging applications, not restricted to Cultural Heritage preservation.

EXPERIMENTS

In this study, the PVA-TC-PNs structural and functional properties were changed by functionalization with sebacic acid into a new class of TC-PNs. Functionalization affects the porosity and nanostructure of the network, changing its uptake/release of fluids and favoring the uptake of organic solvents with various polarity, a crucial feature to boost the versatility of TC-PNs in practical applications.

FINDINGS

The functionalized gels exhibited unprecedented performances during the cleaning of contemporary paintings from the Peggy Gugghenheim collection (Venice), whose restoration with traditional solvents and swabs would be difficult to avoid possible disfigurements to the painted layers. These results candidate the functionalized TC-PNs as a new, highly promising class of gels in art preservation.

New sustainable polymers and oligomers for Cultural Heritage conservation

The development of "green" chemistry materials with enhanced properties is a central topic in numerous applicative fields, including the design of polymeric systems for the conservation of works of art. Traditional approaches in art restoration comprise polymer thickeners and viscous dispersions to partially control solvents in the removal of soil or aged varnishes/coatings from artifacts. Alternatively, polymeric gel networks can be specifically designed to grant full control of the cleaning action, yielding safe, time- and cost-effective restorations. The selection of polymers and oligomers in gel design is crucial to tune solvent upload, retention, and controlled release over the sensitive artistic surfaces. Starting from an overview of traditional polymer formulations and state-of-the-art gel systems for cleaning works of art, we provide here the design of a new class of gels, focusing on the selection of oligomers to achieve gels with tailored hydrophilicity/hydrophobicity. We evaluated the oligomers Hydrophilic-Lipophilic Balance (HLB) by developing, for the first time, a novel methodology combining SEC and DOSY NMR analysis, which was tested on a library of "green" oligoesters synthesized by polycondensation and poorly explored in the literature. Oligomers with moderate polydispersity were chosen to validate the new protocol as a robust tool for designing polymeric gels even on industrial scale. The methodology is more time-effective than traditional methods, and gives additional insights on the oligomers physico-chemical nature, evaluating their compatibility with different solvents. Then, we used the selected oligoesters with castor oil to obtain a new class of organogels able to upload solvents with varying polarity, which effectively removed different types of unwanted layers typically found in painting restoration. These results validate the oligomers screening approach and the new class of gels as promising chemical processes/materials in art preservation. The methodology can potentially allow evaluation of HLB also for small molecules (e.g., surfactants), opening for the formulation of polymers solutions/gels beyond Cultural Heritage conservation, as in pharmaceutics, cosmetics, food industry, tissue engineering, agriculture, and others.

Structuring gelatin methacryloyl - dextran hydrogels and microgels under shear

Gelatin methacryloyl (GelMA) is a widely used semi-synthetic polymer for a variety of bioapplications. However, the development of versatile GelMA hydrogels requires tuning of their microstructure. Herein, we report the possibility of preparing hydrogels with various microstructures under shear from an aqueous two-phase system (ATPS) consisting of GelMA and dextran. The influence of an applied preshear on dextran/GelMA droplets and bicontinuous systems is investigated by rheology that allows the application of a constant shear and is immediately followed by in situ UV-curing of the GelMA-rich phase. The microstructure of the resulting hydrogels is examined by confocal laser scanning microscopy (CLSM). The results show that the GelMA string phase and GelMA hydrogels with aligned bands can be formed depending on the concentration of dextran and the applied preshear. The influence of the pH of the ATPS is investigated and demonstrates the formation of multiple emulsions upon decreasing the charge density of GelMA. The preshearing of multiple emulsions, following gelation, leads to the formation of porous GelMA microgels. The diversity of the formed structures highlights the application potential of preshearing ATPS in the development of functional soft materials.

3D Printable Gelatin Methacryloyl (GelMA)-Dextran Aqueous Two-Phase System with Tunable Pores Structure and Size Enables Physiological Behavior of Embedded Cells In Vitro

The restricted porosity of most hydrogels established for in vitro 3D tissue engineering applications limits embedded cells with regard to their physiological spreading, proliferation, and migration behavior. To overcome these confines, porous hydrogels derived from aqueous two-phase systems (ATPS) are an interesting alternative. However, while developing hydrogels with trapped pores is widespread, the design of bicontinuous hydrogels is still challenging. Herein, an ATPS consisting of photo-crosslinkable gelatin methacryloyl (GelMA) and dextran is presented. The phase behavior, monophasic or biphasic, is tuned via the pH and dextran concentration. This, in turn, allows the formation of hydrogels with three distinct microstructures: homogenous nonporous, regular disconnected-pores, and bicontinuous with interconnected-pores. The pore size of the latter two hydrogels can be tuned from ≈4 to 100 µm. Cytocompatibility of the generated ATPS hydrogels is confirmed by testing the viability of stromal and tumor cells. Their distribution and growth pattern are cell-type specific but are also strongly defined by the microstructure of the hydrogel. Finally, it is demonstrated that the unique porous structure is sustained when processing the bicontinuous system by inkjet and microextrusion techniques. The proposed ATPS hydrogels hold great potential for 3D tissue engineering applications due to their unique tunable interconnected porosity.

Sponge-like Cryogels from Liquid-Liquid Phase Separation: Structure, Porosity, and Diffusional Gel Properties

Macroporous gels find application in several scientific fields, ranging from art restoration to wastewater filtration or cell entrapment. In this work, two-component sponge-like cryogels are challenged to assess their cleaning performances and to investigate how pores size and connectivity affect physico-chemical properties. The gels were obtained through a freeze-thaw process, exploiting a spontaneous polymer-polymer phase-separation occurring in the pre-gel solution. During the freezing step, a highly hydrolyzed polyvinyl alcohol (H-PVA) forms the hydrogel walls. The secondary components, namely a partially hydrolyzed polyvinyl alcohol (L-PVA) or polyvinyl pyrrolidone (PVP), act as modular porogens, being partially extracted during gel washing. H-PVA/L-PVA and H-PVA/PVP mixtures were studied by confocal laser scanning microscopy to unveil sols and gels morphology at the micron-scale, while small angle X-ray scattering was used to get insights about characteristic dimensions at the nanoscale. The gelation mechanism was investigated through rheology measurements, and the characteristic exponents were compared to De Gennes' scaling laws gathered from percolation. In the field of art conservation, these sponge-like gels are ideal systems for the cleaning of artistic painted surfaces. Their interconnected pores allow the diffusion of cleaning fluids at the painted interface, facilitating dirt uptake and/or detachment. This study uncovered a direct relationship linking a gel's cleaning performance to its apparent tortuosity. These findings can pave the way to fine-tuning systems with enhanced cleaning abilities, not restricted to the restoration of irreplaceable priceless works of art, but with possible application in diverse research fields.

Reproducing bronze archaeological patinas through intentional burial: A comparison between short- and long-term interactions with soil

The reproduction of archaeological corrosion patinas is a key issue for the reliable validation of conservation materials before their use on cultural objects. In this study, bronze disks were intentionally buried for 15 years in the soil of the archaeological site of Tharros, both in laboratory and in situ, with the aim of reproducing corrosion patinas typical of archaeological artifacts to be used as representative surfaces for testing novel cleaning gels. The microstructural, microchemical and mineralogical features of the patinas were analyzed by a multianalytical approach, based on optical microscopy (OM), field emission scanning electron microscopy coupled with energy dispersive spectrometry (FE-SEM-EDS) and X-ray diffraction (XRD). The patinas developed in 15 years were compared with an archaeological bronze recovered from the same site after about two thousand years of burial (referred as short-term and long-term interaction, respectively). Results revealed a similar corrosion behavior, especially in terms of chemical composition and corrosion mechanisms. XRD detected the ubiquitous presence of cuprite, copper hydroxychlorides and terrigenous minerals, while OM and FE-SEM-EDS analyses of cross-sections evidenced similar patinas' stratigraphy, identifying decuprification as driving corrosion mechanism. However, some differences related to the type of local environment and to the time spent in soil were evidenced. In particular, patinas developed in situ are more heterogeneous and rougher, while the archaeological one is thicker and presents a major amount of cuprite, terrigenous deposits and uncommon corrosion compounds. Based on our findings, the disks buried in situ were selected and used as disposable substrates to study the cleaning effect of a novel polyvinyl alcohol (PVA)-based gel loaded with a chelating agent (Na2EDTA · 2H2O). Results show that the gel is effective in removing disfiguring degradation compounds and preserving the stable and protective patina. Based on the conservation needs, the time of application can be properly tuned. It is worth noticing that after a few minutes the green corrosion products can be selectively removed. The EDS analysis performed on the gels after cleaning reveals that they are highly selective for the removal of copper(II) compounds rather than Cu(I) oxide or Cu(0) from bronze substrates.

From Nanoparticles to Gels: A Breakthrough in Art Conservation Science

Cultural heritage is a crucial resource to increase our society's resilience. However, degradation processes, enhanced by environmental and anthropic risks, inevitably affect works of art, hindering their accessibility and socioeconomic value. In response, interfacial and colloidal chemistry has proposed valuable solutions over the past decades, overcoming the limitations of traditional restoration materials and granting cost- and time-effective remedial conservation of the endangered artifacts. Ranging from inorganic nanoparticles to hybrid composites and soft condensed matter (gels, microemulsions), a wide palette of colloidal systems has been made available to conservators worldwide, targeting the consolidation, cleaning, and protection of works of art. The effectiveness and versatility of the proposed solutions allow the safe and effective treatment of masterpieces belonging to different cultural and artistic productions, spanning from classic ages to the Renaissance and modern/contemporary art. Despite these advancements, the formulation of materials for the preservation of cultural heritage is still an open, exciting field, where recent requirements include coping with the imperatives of the Green Deal to foster the production of sustainable, low-toxicity, and environmentally friendly systems. This review gives a critical overview starting from pioneering works up to the latest advancements in colloidal systems for art conservation, a challenging topic where effective solutions can be transversal to multiple sectors even beyond cultural heritage preservation, from the pharmaceutical and food industry, to cosmetics, tissue engineering, and detergency.

Applying Gel-Supported Liquid Extraction to Tutankhamun's Textiles for the Identification of Ancient Colorants: A Case Study

The identification of the dyes present on a linen fragment from the tomb of Pharaoh Tutankhamun is the objective of the present study. Fiber optic reflectance spectroscopy (FORS) was applied to the archaeological sample for preliminary identification of the dyes and to better choose the extraction methodology for different areas of the sample. The innovative gel-supported micro-extraction with agar gel and the Nanorestore Gel^® High Water Retention (HWR) gel were applied to the archaeological sample after testing of the best concentration for the extraction of the agar gels substrates, performed on laboratory mock-ups by means of UV-Vis transmittance spectroscopy. Immediately after extraction, Ag colloidal pastes were applied on the gel surface and Surface Enhanced Raman Scattering (SERS) analysis was performed directly on them. The combination of information deriving from FORS and SERS spectra resulted in the successful identification of both indigo and madder and, in hypothesis, of their degradation products.

A pH-indicating smart tag based on porous hydrogel as food freshness sensors

HYPOTHESIS

The pH-indicating smart packaging and tags are identified within the general research and pH colorimetric smart tags are effective, non-invasive methods for indicating food freshness on a real-time basis, but their sensitivity is limited.

EXPERIMENTS

In Herin, we developed a porous hydrogel with high sensitivity, water content, modulus, and safety. Hydrogels were prepared with gellan gum, starch, and anthocyanin. The phase separations provide an adjustable porous structure, which can enhance the capture and transformation of gas from food spoilage, hence improving the sensitivity. Hydrogel is physically crosslinked by the entanglement of chains through freeze-thawing cycles, and porosity can be adjusted by the addition of starch, so avoiding the use of toxicity crosslinkers and porogen.

FINDINGS

Our study demonstrates that the gel undergoes an obvious color shift during the spoilage of milk and shrimp, revealing its potential application as a smart tag signaling food freshness.

Nanostructured bio-based castor oil organogels for the cleaning of artworks

HYPOTHESIS

Organic solvents are often used for cleaning highly water-sensitive artifacts in modern/contemporary art. Due to the toxicity of most solvents, confining systems must be formulated to use these fluids in a safe and controlled way. We propose here castor oil (CO) organogels, obtained thorough cost-effective sustainable polyurethane crosslinking. This methodology is complementary to previously demonstrated hydrogels, when conservators opt for organic solvents over aqueous formulations.

EXPERIMENTS

The gels were characterized via Small-angle Neutron Scattering and rheology before and after swelling in two organic solvents commonly adopted in cleaning paintings. The removal of a photo-aged acrylic-ketonic varnish was evaluated under visible and ultraviolet light, and with FTIR 2D imaging.

FINDINGS

The new gels are dry systems that can be easily stored and loaded with solvents before use. Their nanoscale organization, viscoelasticity and cleaning action are controlled changing the amount of crosslinking, the polymeric backbone, and the loaded solvents. The fluids are confined in the nanosized polymeric mesh of the gels, which are highly retentive, granting controlled release over delicate paint layers, and transparent, allowing monitoring of the cleaning process. These features, along with their sustainable synthesis, candidate the CO organogels as feasible solutions for cultural heritage preservation, expanding the palette of advanced tools for conservators over traditional thickeners.

Effect of Composition and Freeze-Thaw on the Network Structure, Porosity and Mechanical Properties of Polyvinyl-Alcohol/Chitosan Hydrogels

We report the synthesis and characterization of poly (vinyl alcohol) (PVA)/Chitosan (CT) cryogels for applications involving the uptake and entrapment of particulate and bacterial colonies. In particular, we systematically investigated the network and pore structures of the gels as a function of CT content and for different freeze-thaw times, combining Small Angle X-Ray Scattering (SAXS), Scanning Electron Microscopy (SEM), and confocal microscopy. The nanoscale analysis obtained from SAXS shows that while the characteristic correlation length of the network is poorly affected by composition and freeze-thaw time, the characteristic size of heterogeneities associated with PVA crystallites decreases with CT content. SEM investigation evidences a transition to a more homogeneous network structure induced by the incorporation of CT that progressively builds a secondary network around the one formed by PVA. A detailed analysis of confocal microscopy image stacks allows to characterize the 3D porosity of the samples, revealing a significantly asymmetric shape of the pores. While the average volume of single pores increases with increasing CT content, the overall porosity remains almost unchanged as a result of the suppression of smaller pores in the PVA network with the progressive incorporation of the more homogeneous CT network. Increasing the freezing time in the FT cycles also results in a decrease of porosity, which can be associated with a growth in the crosslinking of the network due to PVA crystallization. The linear viscoelastic moduli measured by oscillatory rheology show a qualitatively comparable frequency-dependent response in all cases, with a moderate reduction with increasing CT content. This is attributed to changes in the structure of the strands of the PVA network.

Multifunctional and Tunable Coacervate Powders to Enable Rapid Hemostasis and Promote Infected Wound Healing

Hemostatic powders provide an important treatment approach for time-sensitive hemorrhage control. Conventional hemostatic powders are challenged by the lack of tissue adhesiveness, insufficient hemostatic efficacy, limited infection control, and so forth. This study develops a hemostatic powder from tricomponent GTP coacervates consisting of gelatin, tannic acid (TA), and poly(vinyl alcohol) (PVA). The physical cross-linking by TA results in facile preparation, good storage stability, ease of application to wounds, and removal, which provide good potential for clinical translation. When rehydrated, the coacervate powders rapidly form a cohesive layer with interconnected microporous structure, competent flexibility, switchable wet adhesiveness, and antibacterial properties, which facilitate the hemostatic efficacy for treating irregular, noncompressible, or bacteria-infected wounds. Compared to commercial hemostats, GTP treatment results in significantly accelerated hemostasis in a liver puncture model (∼19 s, >30% reduction in the hemostatic time) and in a tail amputation model (∼38 s, >60% reduction in the hemostatic time). In the GTP coacervates, gelatin functioned as the biodegradable scaffold, while PVA introduced the flexible segments to enable shape-adaptability and interfacial interactions. Furthermore, TA contributed to the physical cross-linking, adhesiveness, and antibacterial performance of the coacervates. The study explores the tunability of GTP coacervate powders to enhance their hemostatic and wound healing performances.

Flexible Zinc-Air Batteries with Ampere-Hour Capacities and Wide-Temperature Adaptabilities

Flexible Zn-air batteries (FZABs) have significant potentials as efficient energy storage devices for wearable electronics because of their safeties and high energy-to-cost ratios. However, their application is limited by their short cycle lives, low discharge capacities per cycle, and high charge/discharge polarizations. Accordingly, herein, a poly(sodium acrylate)-polyvinyl alcohol (PANa-PVA)-ionic liquid (IL) hydrogel (PANa-PVA-IL) is prepared using a hygroscopic IL, 1-ethyl-3-methylimidazolium chloride, as an additive for twin-chain PANa-PVA. PANa-PVA-IL exhibits a high conductivity of 306.9 mS cm^-1 and a water uptake of 2515 wt% at room temperature. Moreover, a low-cost bifunctional catalyst, namely, Co₉ S₈ nanoparticles anchored on N- and S-co-doped activated carbon black pearls 2000 (Co₉ S₈ -NSABP), is synthesized, which demonstrates a low O₂ reversibility potential gap of 0.629 V. FZABs based on PANa-PVA-IL and Co₉ S₈ -NSABP demonstrate high discharge capacities of 1.67 mAh cm^-2 per cycle and long cycle lives of 330 h. Large-scale flexible rechargeable Zn-air pouch cells exhibit total capacities of 1.03 Ah and energy densities of 246 Wh kg_cell ^-1 . This study provides new information about hydrogels with high ionic conductivities and water uptakes and should facilitate the application of FZABs in wearable electronics.

A Tough Hydrogel Adhesive for the Repair of Archeological Pottery

Pottery is the oldest art and plays a landmark role in human civilization. The repair of ceramic relics often uses acrylic resins and cyanoacrylate adhesives. However, existing adhesives often take hours to get cured, and wet adhesion is not possible. We herein propose a redox initiator-triggered hydrogel adhesive, of which robust (∼700 J m^-2) and wet adhesion with potsherds can be achieved within a few seconds. The high toughness lies in the self-limited delocalized rupture of the porous interface, and the wet adhesion is due to the hydrophilic precursor and its free radical polymerization. The hydrogel adhesive also exhibits high aging resistance for stable preservation of ∼400 annuals. We have applied the adhesive to the restoration of artifacts excavated from Yinxu, Anyang (∼1300 BC) and the Xia Jiao Shan site (∼4000 BC, Neolithic), and the adhesive is expected to be extended to applications beyond archeology.

Evaluation of the Antibacterial Properties of Polyvinyl Alcohol-Pullulan Scaffolds Loaded with Nepeta racemosa Lam. Essential Oil and Perspectives for Possible Applications

Essential oil of Nepeta racemosa Lam. was extracted and characterized to determine its antimicrobial activity and potential use in applications. The essential oil was loaded on polyvinyl alcohol-pullulan films and gels and characterized by optical microscopy, scanning electron microscopy, and UV-Vis spectroscopy before having its antimicrobial capacities assessed. The essential oil extracted from Nepeta racemosa Lam. was characterized using gas chromatography coupled with mass spectroscopy, which indicated that the most abundant component was nepetalic acid (55.5%), followed by eucalyptol (10.7%) and other compounds with concentrations of about 5% or less. The essential oil, as well as the loaded films and gels, exhibited good antibacterial activity on both gram-positive and gram-negative strains, with growth inhibition zones larger in some cases than for gentamicin, indicating excellent premises for using these essential-oil-loaded materials for applications in the food industry or biomedicine.

Construction and Ion Transport-Related Applications of the Hydrogel-Based Membrane with 3D Nanochannels

Hydrogel is a type of crosslinked three-dimensional polymer network structure gel. It can swell and hold a large amount of water but does not dissolve. It is an excellent membrane material for ion transportation. As transport channels, the chemical structure of hydrogel can be regulated by molecular design, and its three-dimensional structure can be controlled according to the degree of crosslinking. In this review, our prime focus has been on ion transport-related applications based on hydrogel materials. We have briefly elaborated the origin and source of hydrogel materials and summarized the crosslinking mechanisms involved in matrix network construction and the different spatial network structures. Hydrogel structure and the remarkable performance features such as microporosity, ion carrying capability, water holding capacity, and responsiveness to stimuli such as pH, light, temperature, electricity, and magnetic field are discussed. Moreover, emphasis has been made on the application of hydrogels in water purification, energy storage, sensing, and salinity gradient energy conversion. Finally, the prospects and challenges related to hydrogel fabrication and applications are summarized.

First assessment of microplastic and artificial microfiber contamination in surface waters of the Amazon Continental Shelf

The composition and distribution of microplastics (MPs) in the Brazilian Amazon Continental Shelf surface waters are described for the first time. The study was conducted during the 2018 rainy and dry seasons, using 57 water samples collected with aluminum buckets and filtered through a 64-μm mesh. The samples were vacuum-filtered in a still-air box, and the content of each filter was measured, counted, and classified. A total of 12,288 floating MPs were retrieved; particles were present at all 57 sampling points. The mean MP abundance was 3593 ± 2264 items·m^-3, with significantly higher values during the rainy season (1500 to 12,967; 4772 ± 2761 items·m^-3) than in the dry season (323 to 5733; 2672 ± 1167 items·m^-3). Polyamides (PA), polyurethane (PU), and acrylonitrile butadiene styrene (ABS) were the most common polymers identified through Fourier Transform Infrared Spectroscopy (FTIR) analysis. Cellulose-based textile fibers were also abundant (~40%). Our results indicate that the Amazon Continental Shelf is contaminated with moderate to high levels of MPs; the highest abundances were recorded at stations near land-based sources such as river mouths and large coastal cities.

Plastic is in the air: Impact of micro-nanoplastics from airborne pollution on Tillandsia usneoides (L.) L. (Bromeliaceae) as a possible green sensor

Due to the increasing evidence of widespread plastic pollution in the air, the impact on plants of airborne particles of polycarbonate (PC), polyethyleneterephthalate (PET), polyethylene (PE), and polyvinylchloride (PVC) was tested by administering pristine and aged airborne micro-nanoplastics (MNPs) to Tillandsia usneoides for two weeks. Here we showed that exposure to pristine MNPs, significantly reduced plant growth with respect to controls. Particularly, PVC almost halved plant development at the end of the treatment, while the other plastics exerted negative effects on growth only at the beginning of the exposure, with final stages comparable to those of controls. Plants exposed to aged MNPs showed significantly decreased growth at early stages with PC, later in the growth with PE, and even later with PET. Aged PVC did not exert a toxic effect on plants. When present, the plastic-mediated reduction in plant growth was coupled with a decrease in photosynthetic activity and alterations in the plant concentration of macro- and micronutrients. The plastic particles were showed to adhere to the plant surface and, preferentially, on the trichome wings. Our results reported, for the first time, evidence of negative effects of airborne plastic pollution on plant health, thus raising concerns for related environmental risks.

Mapping hierarchical networks of poly(vinyl alcohol)/cellulose nanofiber composite hydrogels via viscoelastic probes

Discriminating the roles of different networks in the multiply cross-linked hydrogels is vital to optimize their overall performance. Poly(vinyl alcohol)/cellulose nanofiber composite hydrogels were used as template for the study. Three types of characteristic networks, including chemical network cross-linked with boronic ester bonds, physical network cross-linked with microcrystallites, and coexistence of these two networks, were constructed in the system, and the viscoelastic responses were used to detect the characteristic relaxation behavior of those networks. The physical network is more sensitive to stress-induced deformation, whereas the chemical network more sensitive to strain-induced one. The former has lower level of viscous dissipation and higher level of elastic storage as compared to the latter, and dominates linear viscoelasticity of hydrogels as the two networks coexist. Their synergistic effect can be well defined by the scaling behavior of hysteretic work. This work proposes an interesting method of probing networks in the multiply cross-linked hydrogels.

Rationally Tuning Phase Separation in Polymeric Membranes toward Optimized All-day Passive Radiative Coolers

The all-day passive radiative cooler has emerged as one of the state-of-the-art energy-saving cooling tool kits but routinely suffers from limited processability, high cost, and complicated fabrication processes, which impede large-scale applications. To address these challenges, this work exploits a polymer-based passive radiative cooler with optimized turbidity, reconfigurability, and recyclability. These cooling membranes are fabricated via selective condensation of octyl side chain-modified polyvinyl alcohol through a non-solvent-induced phase separation method. The rational tuning over spatial organization and distribution of the air-polymer interface renders optimized bright whiteness with solar reflectance at 96%. Meanwhile, the abundant -C-O-C- bonds endow such membranes with infrared thermal emittance over 90%. The optimized membrane realizes a subambient cooling of ∼5.7 °C with an average cooling power of ∼81 W m-2 under a solar intensity of ∼528 W m-2. Furthermore, the supramolecule nature of the developed passive radiative cooling membrane bears enhanced shape malleability and recyclability, substantially enhancing its conformability to the complex geometry and extending its life for an eco-friendly society.

Hazardous contaminants in plastics contained in compost and agricultural soil

Macro-, meso- and microplastic (MAP, MEP, MP) occurrence in compost is an environmental issue whose extent and effects are not yet understood. Here, we studied the occurrence of MAPs, MEPs and MPs in compost samples, and the transfer of hazardous contaminants from plastics to compost and soil. MAPs/MEPs and MPs concentrations in compost were 6.5 g/kg and 6.6 ± 1.5 pieces/kg; from common recommendations for compost application, we estimated ∼4-23 × 10⁷ pieces MPs and 4-29 × 10⁴ g MAPs/MEPs ha^-1 per year ending into agricultural soils fertilized with such compost. Regarding contaminants, bis(ethylhexyl) phthalate, acetyl tributyl citrate, dodecane and nonanal were extracted in higher concentrations from plastics and plastic-contaminated compost than from compost where MAPs/MEPs had been removed prior to extraction and analysis. However, some contaminants were present even after MAPs/MEPs removal, ascribable to short- and long-term release by MAPs/MEPs, and to the presence of MPs. DEHP concentration was higher in soils where compost was applied than in fields where it was not used. These results, along with estimations of plastic load to soil from the use of compost, show that compost application is a source of plastic pollution into agricultural fields, and that plastic might transfer hazardous contaminants to soil.

Multi-Analytical Investigation of the Oil Painting “Il Venditore di Cerini” by Antonio Mancini and Definition of the Best Green Cleaning Treatment

This paper describes the multi-analytical approach implemented for the study of the oil painting Il Venditore di Cerini made by Antonio Mancini in 1878. The research was carried out to characterize both the original stratigraphy and the alleged non-original varnish on the surface. SEM/EDS analysis showed the presence of pigments already detected in other paintings by Antonio Mancini. Multispectral imaging, DinoLite microscope, and FT-IR ATR spectroscopy revealed significant data regarding the invention of the “graticola” method—a technique implemented by Mancini to respect the proportions of the figures—also proving the presence of an aged layer of non-original shellac on the surface. The yellow/brownish tone of the varnish was hiding the real shapes of the figure, requiring a selective removal of the aged coating. The proposed cleaning systems were chosen among the green chemical alternatives present in the market, aiming at promoting a sustainable development in the Cultural Heritage field. The selection was made according to the Fd parameter of the cleaning systems—which defines the energy from dispersion forces between molecules—in relation to what is defined in the literature as the suitable Fd value for the removal of the shellac. The best-performing green cleaning system proved to be the Polar Varnish Rescue GEL—a gelled acetals mixture developed by YOCOCU APS—for its effectiveness in selectively remove the aged shellac while preserving the integrity of the original stratigraphy.

Occurrence and Quantification of Natural and Microplastic Items in Urban Streams: The Case of Mugnone Creek (Florence, Italy)

The terrestrial environment is an important contributor of microplastics (MPs) to the oceans. Urban streams, strictly interwoven in the city network and to the MPs’ terrestrial source, have a relevant impact on the MP budget of large rivers and, in turn, marine areas. We investigated the fluxes (items/day) of MPs and natural fibers of Mugnone Creek, a small stream crossing the highly urbanized landscape of Florence (Italy) and ending in the Arno River (and eventually to the Tyrrhenian Sea). Measurements were done in dry and wet seasons for two years (2019–2020); stream sediments were also collected in 2019. The highest loads of anthropogenic particles were observed in the 2019 wet season (10⁹ items/day) at the creek outlet. The number of items in sediments increased from upstream (500 items/kg) to urban sites (1540 items/kg). Fibers were the dominant shape class; they were mostly cellulosic in composition. Among synthetic items, fragments of butadiene-styrene (SBR), indicative of tire wear, were observed. Domestic wastewater discharge and vehicular traffic are important sources of pollution for Mugnone Creek, especially during rain events. The study of small creeks is of pivotal importance to limit the availability of MPs in the environment.

pH-Responsive Semi-Interpenetrated Polymer Networks of pHEMA/PAA for the Capture of Copper Ions and Corrosion Removal

Bronze artifacts constitute a fundamental portion of Cultural Heritage, but effective methodologies for the removal of corrosion layers, such as those produced by the "bronze disease", are currently missing. We propose the formulation and application of novel poly(2-hydroxyethyl methacrylate) (pHEMA) networks semi-interpenetrated (SIPN) with poly(acrylic acid) (PAA) to achieve enhanced capture of copper ions and removal of corrosion products. The pHEMA/PAA SIPNs were designed to improve previous pHEMA/poly(vinylpyrrolidone) (PVP) networks, taking advantage of the chelating ability of pH-responsive carboxylic groups in PAA. Increasing the pH ionizes carboxyls, increases the porosity in pHEMA/PAA, and leads to the co-presence of enol and enolate forms of vinylpyrrolidone (VP), changing the macroporosity and decreasing the mesh size in pHEMA/PVP. The ion-matrix interaction is stronger in pHEMA/PAA, where the process occurs through an initial diffusion-limited step followed by diffusion in smaller pores or adsorption by less available sites. In pHEMA/PVP, the uptake is probably controlled by adsorption as expected, considering the porogen role of PVP in the network. Upon application of the SIPNs loaded with tetraethylenpentamine (TEPA) onto corroded bronze, copper oxychlorides dissolve and migrate inside the gels, where Cu(II) ions form ternary complexes with TEPA and carboxylates in PAA or carbonyls in PVP. The removal of oxychlorides is more effective and faster for pHEMA/PAA than its /PVP counterpart. The selective action of the gels preserved the cuprite layers that are needed to passivate bronze against corrosion, and the pH-responsive behavior of pHEMA/PAA allows full control of the uptake and release of the Cu(II)-TEPA complex, making these systems appealing in several fields even beyond Cultural Heritage conservation (e.g., drug delivery, wastewater treatment, agricultural industry, and food chemistry).

Cryogenics as an Advanced Method of Cleaning Cultural Heritage: Challenges and Solutions

The conservation and restoration of cultural heritage rely on technology and products designed for other sectors. The incorporation of new equipment requires exhaustive studies to ensure the viability of the new method linked to the safety of the technique, both for the operator and for the artwork. For this purpose, this research presents a preliminary approach to the study of dry ice blasting for its possible incorporation in the field of cultural heritage. This technique is characterized by being harmless for the operator and does not require washing times or subsequent evaporation as a result of solvent retention. It is an efficient and sustainable treatment, widely used in the technological, aerospace and industrial sectors. The article shows a theoretical analysis of the research results obtained by other specialists with the aim of introducing this technique in the eco-sustainable study of innovative technologies for the cleaning of culturally relevant surfaces. It describes the procedure of cryogenics, some cleaning equipment currently available and relevant case studies for both industrial and patrimonial contexts. Through the compilation and processing of documentary sources, we will be able to understand, define and analyze this new technique, specifying some basic aspects for its experimental evaluation. The attempt to incorporate cryogenics in the field of heritage is an improvement towards the reduction of the ecological management derived from the use of chemical waste. It is an innovative resource, full of benefits for the sector, in addition to contributing to five Sustainable Development Goals of the 2030 Agenda. This contribution allows progress towards a safer, greener and more sustainable restoration, reducing the dangers associated with the use of solvents and their irremediable ecological impact.

Microstructural transition of poly(vinyl alcohol)-based aerogels in the presence of interpolymer complexes

Interpolymer interactions play a vital role of determining the microstructure and properties of polymer aerogels.

Paints analysis and conservation treatment of painted sculpture: Jean Dubuffet, Guard Dog II

Abstract

Guard Dog II (1969–1970) by Jean Dubuffet is an artwork that shows the characteristics of the artist's l'hourloupe period (1962–1974). This study revealed the materials and manufacturing techniques used by the artist at the time through material analysis of Guard Dog II, and suggested an appropriate conservation treatment method for the work through a cleaning test. Dust, stains, discoloration, cracks, etc. were observed on the surface of the artwork and stains were particularly severe and discoloration required conservation treatment. Prior to treatment, multiple analytical approaches such as Fourier transform infrared spectroscopy, Raman spectroscopy and gas chromatography/mass spectrometer were applied to identify the materials of the paint layers and confirm the state of the conservation of the artworks. As a result, it was confirmed that polyurethane paint was used in the artwork, and the pigment used for each color was also identified. For the stable conservation treatment of the artwork, the chromaticity, glossiness and workability of the cleaning materials were evaluated, and it was found that isopropyl alcohol and nonionic surfactant were suitable for cleaning. The conservation treatment was performed based on results from the cleaning test, and the artwork was restored cleanly. Since most of the existing research on cleaning methods has been focused on painting, this study contributes to the conservation of sculptures by suggesting cleaning methods suitable for three-dimensional colored objects.

Article highlights

Self-Shaping Microemulsion Gels for Cultural Relic Cleaning

Cleaning is a foundational and essential operation of protection and restoration of cultural relics, which is also the key step of follow-up works. To overcome the problems of uncontrollable diffusion of cleaning solvents and poor coverage of the cleaning solvent carriers on rough surfaces, here, we propose a strategy of using a self-shaping microemulsion gel that is prepared via emulsifying oleophilic solvents into the specific shear-thinning hydrogel structures. The gel can adaptively cover rough surfaces during the cleaning process coupled with avoidance of unnecessary diffusion of the cleaning solvents, and the mechanical reinforcement of in situ polymerized double-network gels enables its easy peeling off from the surfaces without leaving determinable residues. As a representative demonstration, Paraloid B72, a widely used material for the repair and reinforcement of cultural relics, is employed as a model discolored coating, which can be effectively removed from the rough surface of simulated cultural relics after treatment with the resulting gels. Convincingly, the strategy of constructing agarose/polyacrylamide hybrid double-network gels with shear-thinning and self-shaping performances for the cleaning of cultural relics not only improves the convenience and accuracy of operation but also exhibits an efficient cleaning effect, which will greatly expand the application of microemulsion gels in the cleaning of rough surfaces of cultural relics.

Occurrence and characterization of microplastic and mesoplastic pollution in the Migliarino San Rossore, Massaciuccoli Nature Park (Italy)

Microplastics pollution is progressively threatening natural parks across the world. In the framework of monitoring this concerning trend, the present study focuses on the occurrence and identification of mesoplastics (MEPs) and microplastics (MPs) in sand samples collected before and after the summer season from the beach of the Nature Park of Migliarino San Rossore Massaciuccoli (Pisa, Italy). Meso- and microplastics were identified using Fourier transform infrared spectroscopy 2D Imaging, and detected in all samples with average concentrations of 207 ± 30 MPs/kg d.w., and 100 ± 44 MEPs/kg d.w., respectively. Seasonal changes of flow of the Arno River, industrial activities, and urban footprint were considered as the major sources of plastic pollution. Our results showed the occurrence of both natural and synthetic polymers including cellulose, polyethylene, polypropylene, polyamides, polyethylene terephthalate, and acrylonitrile.

Jin Shofu Starch Nanoparticles for the Consolidation of Modern Paintings

Matte, porous, and weakly bound paint layers, typically found in modern/contemporary art, represent an unsolved conservation challenge. Current conservation practice relies on synthetic or natural adhesives that can alter dramatically the optical properties of paints. Alternatively, we propose a novel nanostructured consolidant based on starch, a renewable natural polymer. We synthesized starch nanoparticles (SNPs) to boost their penetration into the porous painted layers; upon solvent evaporation, the particles were expected to adhere to the pigments thanks to their large surface area and abundant -OH groups. The SNPs were formulated through a bottom-up approach, where gluten-removed Jin Shofu wheat starch was gelatinized and then precipitated in a nonsolvent. The low gelatinization temperature of wheat starch is likely key to favor disassembly in alkali and reassembly in the nonsolvent. The synthesis conditions can be tuned to obtain amorphous SNPs of ca. 50 nm with acceptable polydispersity. The particles swell in water to form nanosized gel-like fractal domains (as observed with cryogenic electron microscopy), formed by the organization of smaller units in polymer-rich and -deficient regions. Aqueous and hydroalcoholic particles' dispersions were assessed on aged ultramarine blue mock-ups that mimic degraded modern/contemporary paints. The consolidation effectiveness was evaluated with a specifically designed in-house protocol: the SNPs distribute across the paint section and strongly increase pigments' cohesion while preserving the original optical properties of the painted layer, as opposed to dispersions of bulk starch that simply accumulate on the paint surface, forming superficial glossy films. The Jin Shofu SNPS proved to be a new promising tool for the consolidation of weakened paintings, opening perspectives in the formulation and application of consolidants for modern and contemporary art.

Nanostructured fluids for polymeric coatings removal: Surfactants affect the polymer glass transition temperature

HYPOTHESIS

Nanostructured fluids (NSFs) based on water, organic solvents and surfactants are a valid alternative to the use of neat unconfined organic solvents for polymer coatings removal in art conservation. The physico-chemical processes underpinning their cleaning effectiveness in terms of swelling/dewetting of polymer films were identified as key in this context. The role of surfactants on polymers' dewetting was considered to be mainly restricted to the lowering of interfacial tensions. However, recent experiments evidenced that surfactants have an important role in swelling polymer films.

EXPERIMENTS

Five different amphiphiles were selected, namely: sodium dodecylsulfate, dimethyldodecyl amine oxide, hexaoxyethylene decyl ether (C_9-11E₆), pentadecaoxyethylene dodecyl ether (C₁₂E₁₅), and methyoxypentadecaoxyethylene dodecanoate (C₁₁COE₁₅CH₃). They were combined with a carefully selected organic solvents' mixture (1-butanol/butanone/dimethyl carbonate) to formulate new NSFs, differing for the surfactant only, and used to perform cleaning tests on surfaces coated with Paraloid B72® and Primal AC33®. Here for the first time, polymer swelling induced by surfactants was quantified and correlated with the glass transition temperature of the two polymers by differential scanning calorimetry, before and after the exposure to the fluids. Confocal laser scanning microscopy and small-angle X-ray scattering provided additional insights on the interaction mechanism.

FINDINGS

Nonionics were proven more efficient than zwitterionic/ionic amphiphiles in the polymer swelling, and, overall, methyoxy pentadecaoxyethylene dodecanoate resulted the most effective among the selected surfactants. A direct relation between the effect of surfactants on the polymers' glass transition temperature and cleaning capacity was established. This finding, fundamental to understand the interaction mechanism between NSFs and polymer coatings or paint layers, is key to achieve a selective, effective and complete removal of polymer coatings, as recently shown in the removal of vandalism and over-paintings from street art.

Preparation and Physicochemical Characterization of a Diclofenac Sodium-Dual Layer Polyvinyl Alcohol Patch

The aim of this study is to prepare a dual layer polyvinyl (PVA) patch using a combination of electrospinning techniques and cryogelation (freeze-thaw process) then subsequently to investigate the effect of freeze-thaw cycles, nanofiber thickness, and diclofenac sodium (DS) loading on the physicochemical and mechanical properties and formulation of dual layer PVA patches composed of electrospun PVA nanofibers and PVA cryogel. After the successful preparation of the dual layer PVA patch, the prepared patch was subjected to investigation to assess the effect of freeze-thaw cycles, nanofiber thickness and percentages of DS loading on the morphology, physiochemical and mechanical properties. Various spectroscopic techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR), water contact angle, and tensile tests were used to evaluate the physicochemical and mechanical properties of prepared dual layer PVA patches. The morphological structures of the dual layer PVA patch demonstrated the effectiveness of both techniques. The effect of freeze-thaw cycles, nanofiber thickness, and DS percentage loading on the crystallinity of a dual layer PVA patch was investigated using XRD analysis. The presence of a distinct DS peak in the FTIR spectrum indicates the compatibility of DS in a dual layer PVA patch through in-situ loading. All prepared patches were considered highly hydrophilic because the data obtained was less than 90°. The increasing saturation of DS within the PVA matrix increases the tensile strength of prepared patches, however decreased its elasticity. Evidently, the increasing of electrospun PVA nanofibers thickness, freeze-thaw cycles, and the DS saturation has improved the physicochemical and mechanical properties of the DS medicated dual layer PVA patches, making them a promising biomaterial for transdermal drug delivery applications.

How Science Can Contribute to the Remedial Conservation of Cultural Heritage

Colloid science is contributing solutions to counteract the degradation of artifacts, favoring their transfer to future generations. Advanced materials such as nanoparticles, coatings, gels and microemulsions have been assessed in conservation, spanning from archeological sites to modern and contemporary art. We give an overview of the fundamental milestones and latest innovations in conservation science, targeting solutions and tools for remedial conservation based on green nanomaterials and hybrid systems. Future perspectives and outstanding challenges in this exciting field are then outlined.

Advanced Materials in Cultural Heritage Conservation

Cultural Heritage is a crucial socioeconomic resource; yet, recurring degradation processes endanger its preservation. Serendipitous approaches in restoration practice need to be replaced by systematically addressing conservation issues through the development of advanced materials for the preservation of the artifacts. In the last few decades, materials and colloid science have provided valid solutions to counteract degradation, and we report here the main highlights in the formulation and application of materials and methodologies for the cleaning, protection and consolidation of works of art. Several types of artifacts are addressed, from murals to canvas paintings, metal objects, and paper artworks, comprising both classic and modern/contemporary art. Systems, such as nanoparticles, gels, nanostructured cleaning fluids, composites, and other functional materials, are reviewed. Future perspectives are also commented, outlining open issues and trends in this challenging and exciting field.

Single-Sided Portable NMR Investigation to Assess and Monitor Cleaning Action of PVA-Borax Hydrogel in Travertine and Lecce Stone

In this work, we investigated the potential of PVA-borax hydrogel for cleaning limestones and the dependence of the cleaning on the porosity of the rock and on the action time of the hydrogel treatment. Towards this goal, we used a nuclear magnetic resonance (NMR) spectrometer, developed for non-invasive and non-destructive applications on cultural heritage. T2-NMR parameters were quantified on different samples of Lecce stone and Travertine cut perpendicular (Pe) and parallel (Pa) to the bedding planes under different experimental conditions: untreated samples, treated with Paraloid B72 and cleaned with PVA-PEO-borax hydrogel applied for 4 min and 2 h. The T2 results suggest that the effectiveness of the cleaning strongly depended on the porosity of the stones. In Lecce stone, the hydrogel seemed to eliminate both the paramagnetic impurities (in equal measure with 4 min and 2 h treatment) and Paraloid B72. In Travertine Pe, characterized by a smaller pore size compared to Lecce stone, no significant effects were found regarding both the cleaning and the treatment with Paraloid B72. In Travertine Pa, characterized by a larger pore size than the other two samples, the hydrogel seemed to clean the paramagnetic agents (it worked better if applied for a longer time) but it did not appear to have any effect on Paraloid B72 removal.

Selective removal of over-paintings from "Street Art" using an environmentally friendly nanostructured fluid loaded in highly retentive hydrogels

HYPOTHESIS

The removal of over-paintings or graffiti is a priority for conservators and restorers. This operation is complex, especially when over-paintings lay on painted surfaces that must be preserved, as in the case of vandalism on street art, where the layers are usually chemically similar. Traditional methodologies often do not provide satisfactory results and pose health and eco-compatibility concerns. An alternative methodological approach based on an environmentally friendly nanostructured fluid loaded in a retentive hydrogel is here proposed.

EXPERIMENTS

Six paints (based on vinyl, acrylic and alkyd polymers) were selected and studied by means of attenuated total reflection - Fourier transform infrared spectroscopy. The phase behavior of four alkyl carbonates (green, low-toxicity organic solvents) and a biodegradable nonionic surfactant in water was investigated with Small angle X-ray scattering (SAXS) in order to formulate a novel nanostructured cleaning system. The developed system, which also includes 2-butanol and an alkyl glycoside hydrotrope, was loaded in highly retentive hydrogels and tested in the selective removal of over-paintings from laboratory mockups and from real pieces of street art.

FINDINGS

The selective and controlled removal of modern paints from substrates with similar chemical composition has been achieved using a specifically tailored NSF embedded in a retentive hydrogel. The proposed methodology and cleaning system provided excellent cleaning results, representing a new tool for the conservation of contemporary and, in particular, street art.