Stefano Farris

Wetting of biopolymer coatings: contact angle kinetics and image analysis investigation

The surface wetting of five biopolymers, used as coating materials for a plastic film, was monitored over a span of 8 min by means of the optical contact angle technique. Because most of the total variation was observed to occur during the first 60 s, we decided to focus on this curtailed temporal window. Initial contact angle values (θ(0)) ranged from ∼91° for chitosan to ∼30° for pullulan. However, the water drop profile began to change immediately following drop deposition for all biocoatings, confirming that the concept of water contact angle equilibrium is not applicable to most biopolymers. First, a three-parameter decay equation [θ(t) = θ(0) exp(kt(n))] was fit to the experimental contact angle data to describe the kinetics of the contact angle change for each biocoating. Interestingly, the k constant correlated well with the contact angle evolution rate and the n exponent seemed to be somehow linked to the physicochemical phenomena underlying the overall kinetics process. Second, to achieve a reliable description of droplet evolution, the contact angle (CA) analysis was coupled with image analysis (IA) through a combined geometric/trigonometric approach. Absorption and spreading were the key factors governing the overall mechanism of surface wetting during the 60 s analysis, although the individual quantification of both phenomena demonstrated that spreading provided the largest contribution for all biopolymers, with the only exception of gelatin, which showed two quasi-equivalent and counterbalancing effects. The possible correlation between these two phenomena and the topography of the biopolymer surfaces are then discussed on the basis of atomic force microscopy analyses.

Self-assembled pullulan-silica oxygen barrier hybrid coatings for food packaging applications.

The scope of this study encompassed the evaluation of pullulan as a suitable biopolymer for the development of oxygen barrier coatings to be applied on poly(ethylene terephthalate) (PET), especially for food packaging applications. To enhance the oxygen barrier properties of the organic phase (pullulan) even at high relative humidity values, an inorganic phase (silica), obtained through in situ polymerization, was also utilized to obtain hybrid coatings via the sol-gel technique. Transmission electron microscopy (TEM) images and Fourier transform infrared (FT-IR) spectra showed that mixing the two phases yielded a three-dimensional hybrid network formed by self-assembly and mediated by the occurrence of new hydrogen-bond interactions at the intermolecular level, although the formation of new covalent bonds could not be excluded. The deposition of the hybrid coatings decreased the oxygen transmission rate (OTR) of the plastic substrate by up to 2 orders of magnitude under dry conditions. The best performance throughout the scanned humidity range (0%-80% relative humidity) was obtained for the formulation with the lowest amount of silica (that is, an organic/inorganic ratio equal to 3).

Nanocomposite films and coatings using inorganic nanobuilding blocks (NBB): current applications and future opportunities in the food packaging sector

The aim of this review is to provide an in-depth overview on the use of inorganic nano-sized entities for the generation of nanocomposite materials in the form of films and coatings for food packaging applications. According to recent trends toward “green” strategies, special focus has been dedicated to the development of nanocomposite coatings obtained using biopolymers as the main polymer matrix. After a first introductive part, the discussion has been addressed to the use of inorganic fillers, metals and metal-oxides, zeolites, and graphene. For each class of filler, a first ‘in-depth’ description of the most relevant physicochemical properties for the food packaging sector has been followed by case-by-case references to recent developments and envisaged implementations. The technical aspects that may be crucial in the design and end use of (bio)nanocomposite coatings have been covered in the last part of this work, which also includes an updated list of current applications on nano-sized inorganic fillers in the food packaging field.

Ultrasound-Assisted Pullulan/Montmorillonite Bionanocomposite Coating with High Oxygen Barrier Properties

In this paper, the preparation and characterization of oxygen barrier pullulan sodium montmorillonite (Na(+)-MMT) nanocomposite coatings are presented for the first time. Full exfoliation of platelets during preparation of the coating water dispersions was mediated by ultrasonic treatment, which turned out to be a pivotal factor in the oxygen barrier performance of the final material even at high relative humidity (RH) conditions [oxygen permeability coefficients ~1.43 ± 0.39 and 258.05 ± 13.78 mL·μm·m(-2)·(24 h)(-1)·atm(-1) at 23 °C and 0% RH and 70% RH, respectively]. At the micro- and nanoscale, the reasons are discussed. The final morphology of the coatings revealed that clay lamellae were stacked on top of one another, probably due to the forced confinement of the platelets within the coating thickness after solvent evaporation. This was also confirmed by modeling the experimental oxygen permeability data with the well-known Nielsen and Cussler permeation theoretical models, which suggested a reasonable aspect ratio (α) of ~100. Electron microscopic analyses also disclosed a peculiar cell-like arrangement of the platelets. The stacking of the clay lamellae and the cell-like arrangement create the excellent oxygen barrier properties. Finally, we demonstrated that the slight haze increase in the bionanocomposite coating materials arising from the addition of the clays depends on the clay concentration but not so much on the sonication time, due to the balance of opposite effects after sonication (an increase in the number of scattering centers but a reduction in their size).

Polysaccharide-assisted rapid exfoliation of graphite platelets into high quality water-dispersible graphene sheets

Ultrasound exfoliation of graphite with the assistance of three polysaccharides (nonionic pullulan, cationic chitosan, and anionic alginate) was investigated in this work. The effects of polymer type, initial concentration of graphite, and ultrasonication period on the graphene yield and quality were compared. Under a sonotrode-type ultrasonication treatment for 30 min, graphene aqueous dispersions with concentrations of up to 2.3 mg ml−1 in pullulan solutions and 5.5 mg ml−1 in chitosan solutions were achieved. The obtained graphene nanosheets were characterized as low-defect mono-layer, bi-layer, and few-layer (<5), and formed stable dispersions in water for up to 6 months. The adsorption of pullulan and chitosan biopolymers on the graphene surface as determined by TGA technique was approximately 2.5 wt% and 8.5 wt%, respectively, which accounts for the dispersibility and stability of the graphene sheets in water. Findings arising from this work suggest that pullulan and chitosan are more effective in exfoliating graphite into graphene than alginate due to the different surface free energy and thermodynamic affinity. The polysaccharide-assisted aqueous-exfoliation approach enables the production of water-dispersible graphene with high quality and large quantity, thus providing an industrially scalable route for new potential applications of graphene-based nanocomposites, e.g. in the food packaging industry.

Evaluation of borage extracts as potential biostimulant using a phenomic, agronomic, physiological and biochemical approach

Biostimulants are substances able to improve water and nutrient use efficiency and counteract stress factors by enhancing primary and secondary metabolism. Premise of the work was to exploit raw extracts from leaves (LE) or flowers (FE) of Borago officinalis L., to enhance yield and quality of Lactuca sativa ‘Longifolia,’ and to set up a protocol to assess their effects. To this aim, an integrated study on agronomic, physiological and biochemical aspects, including also a phenomic approach, has been adopted. Extracts were diluted to 1 or 10 mL L–1, sprayed onto lettuce plants at the middle of the growing cycle and 1 day before harvest. Control plants were treated with water. Non-destructive analyses were conducted to assess the effect of extracts on biomass with an innovative imaging technique, and on leaf photosynthetic efficiency (chlorophyll a fluorescence and leaf gas exchanges). At harvest, the levels of ethylene, photosynthetic pigments, nitrate, and primary (sucrose and total sugars) and secondary (total phenols and flavonoids) metabolites, including the activity and levels of phenylalanine ammonia lyase (PAL) were assessed. Moreover, a preliminary study of the effects during postharvest was performed. Borage extracts enhanced the primary metabolism by increasing leaf pigments and photosynthetic activity. Plant fresh weight increased upon treatments with 10 mL L–1 doses, as correctly estimated by multi-view angles images. Chlorophyll a fluorescence data showed that FEs were able to increase the number of active reaction centers per cross section; a similar trend was observed for the performance index. Ethylene was three-fold lower in FEs treatments. Nitrate and sugar levels did not change in response to the different treatments. Total flavonoids and phenols, as well as the total protein levels, the in vitro PAL specific activity, and the levels of PAL-like polypeptides were increased by all borage extracts, with particular regard to FEs. FEs also proved efficient in preventing degradation and inducing an increase in photosynthetic pigments during storage. In conclusion, borage extracts, with particular regard to the flower ones, appear to indeed exert biostimulant effects on lettuce; future work will be required to further investigate on their efficacy in different conditions and/or species.

Surface properties and wear performances of siloxane-hydrogel contact lenses.

The low surface roughness of disposable contact lenses made of a new siloxane-hydrogel loaded with hyaluronic acid is reported, as studied by atomic force microscopy (AFM). Before the wear, the surface is characterized by out-of-plane and sharp structures, with maximum height of about 10 nm. After a wear of 8 h, evidence of two typical morphologies is provided and discussed. One morphology (sharp type) has a similar aspect as the unworn lenses with a slight increase in both the height and the number of the sharp peaks. The other morphology (smooth type) is characterized by troughs and bumpy structures. Wettability and clinical performances are also discussed, the latter deduced by the ocular-surface-disease index (OSDI). The main finding arising from this work is the indication of correlation between the change of the OSDI before and after wear and the lens surface characteristics obtained by AFM.

Exceptional oxygen barrier performance of pullulan nanocomposites with ultra-low loading of graphene oxide

Polymer nanocomposites are increasingly important in food packaging sectors. Biopolymer pullulan is promising in manufacturing packaging films or coatings due to its excellent optical clarity, mechanical strength, and high water-solubility as compared to other biopolymers. This work aims to enhance its oxygen barrier properties and overcome its intrinsic brittleness by utilizing two-dimensional planar graphene oxide (GO) nanoplatelets. It has been found that the addition of only 0.2 wt% of GO enhanced the tensile strength, Youngs modulus, and elongation at break of pullulan films by about 40, 44 and 52%, respectively. The light transmittance at 550 nm of the pullulan/GO films was 92.3% and haze values were within 3.0% threshold, which meets the general requirement for food packaging materials. In particular, the oxygen permeability coefficient of pullulan was reduced from 6337 to 2614 mL μm m(-2) (24 h(-1)) atm(-1) with as low as 0.05 wt% of GO loading and further to 1357 mL μm m(-2) (24 h(-1)) atm(-1) when GO concentration reached 0.3 wt%. The simultaneous improvement of the mechanical and oxygen barrier properties of pullulan was ascribed to the homogeneous distribution and prevalent unidirectional alignment of GO nanosheets, as determined from the characterization and theoretical modelling results. The exceptional oxygen barrier properties of pullulan/GO nanocomposites with enhanced mechanical flexibility and good optical clarity will add new values to high performance food packaging materials.

Self-assembled nanostructured biohybrid coatings by an integrated ‘sol–gel/intercalation’ approach

The combination of sol–gel technology and intercalation was investigated in this study as a strategy to develop bionanocomposite hybrid materials in the form of coatings with specifically intended oxygen barrier properties. To this goal, the exopolysaccharide pullulan was used as the organic phase, whereas tetraethyl orthosilicate (TEOS) and Na+–montmorillonite (MMT) were used as the metal alkoxide precursor and the nanobuilding blocks (NBB) for the sol–gel technology and the intercalation process, respectively. Complementary information from XRD and TEM analyses disclosed a new supramolecular organization arising from the self-assembly of NBB and pullulan, with the latter apparently intercalated between the clay platelets. Although affected by a rise in haze, the hybrid coatings exhibited outstanding oxygen barrier properties, with permeability coefficient values (P′O2) ranging from 0.89 mL μm m−2 (24 h)−1 atm−1 for a filler volume fraction (ϕ) = 0.017 to an impressive 0.15 mL μm m−2 (24 h)−1 atm−1 (ϕ = 0.095) under dry conditions. Modeling of P′O2 suggested a very tight structure under dry conditions, which yielded an apparent clays aspect ratio (α) ∼50, whereas in the hydrated state a more realistic α ∼ 100 was restored. This finding was further supported by SEM analysis, which also highlighted partial embrittlement of the final hybrid coatings.

Cellulose nanofibril core–shell silica coatings and their conversion into thermally stable nanotube aerogels

A facile water-based one-pot reaction protocol for obtaining 20 nm thick uniform silica coatings on cellulose nanofibrils (CNFs) is herein presented for the first time. The fully covering silica shells result in the thermal stability of the CNFs improved by ca. 70 °C and 50 °C under nitrogen and oxygen atmospheres, respectively. Heating of the core–shell hybrid fibres to 400 °C results in complete degradation/removal of the CNF cores, and demonstrates an inexpensive route to large-scale preparation of silica nanotubes with the CNFs used as templates. The key to a uniform condensation of silica (from tetraethyl orthosilicate) to cellulose is a reaction medium that permits in situ nucleation and growth of the silica phase on the fibrils, while simultaneously matching the quantity of the condensed silica with the specific surface area of the CNFs. Most coatings were applied to bundles of 2–3 associated CNFs, which could be discerned from their negative imprint that remained inside the silica nanotubes. Finally, it is demonstrated that the coated nanofibrils can be freeze-dried into highly porous silica/cellulose aerogels with a density of 0.005 g cm−3 and how these hybrid aerogels preserve their shape when extensively exposed to 400 °C in air (>6 h). The resulting material is the first reported silica nanotube aerogel obtained by using cellulose nanofibrils as templates.