Rice husks, one of the world’s most abundant agricultural residues, have been transformed into silica nanoparticles that reinforce chitosan films capable of blocking ultraviolet light, slowing bacterial growth, and extending the storage life of cherry tomatoes. The study, published in Discover Nano, describes an environmentally oriented packaging material built almost entirely from waste streams: chitosan derived from chitin and bio-silica extracted from rice husks through a citric acid pretreatment followed by calcination at 600 degrees Celsius. The researchers report that their optimized film formulation preserved the appearance and more than 83 percent of the original weight of packaged cherry tomatoes over 21 days of storage under accelerated ultraviolet exposure and refrigeration.

The work addresses a persistent shortcoming of biopolymer packaging. Pure chitosan films, while biodegradable and naturally antimicrobial, suffer from poor water resistance and weak mechanical properties, which limit their practical use. Earlier studies on chitosan-silica or chitosan-nanocellulose films have reported improvements in mechanical strength, gas barrier performance, and antibacterial activity, but the authors note that few attempts have combined agro-waste-derived bio-silica with and without nanocellulose in a single chitosan matrix, and fewer still have tested whether those laboratory improvements actually protect a real perishable fruit under controlled ultraviolet weathering.

To build the films, the team first extracted two types of bio-silica nanoparticles from rice husks. Silica-based nanocellulose was obtained by suspending rice agro-waste pulp in 64 percent sulfuric acid at 45 degrees Celsius, followed by washing, centrifugation, and roughly a week of dialysis, yielding a suspension with a crystallinity index of 82 percent. Pure silica nanoparticles were separated by pretreating cleaned rice husks with 1 molar citric acid at 120 degrees Celsius for two hours, calcining them at 600 degrees Celsius, dissolving the resulting ash in sodium hydroxide, and precipitating silica gel with hydrochloric acid. The citric acid route was chosen deliberately: unlike strong mineral acids, it chelates metallic impurities without corrosive, non-biodegradable waste, although the resulting silicon content of 44.39 percent is lower than the 91 to 99 percent purity reported for hydrochloric-acid-leached rice husk silica, a trade-off the authors describe as prioritizing a safer, biodegradable process.

The nanoparticles were then incorporated into chitosan films by solution casting. A 2 percent chitosan solution in dilute acetic acid was blended with nanocellulose and silica nanoparticles at loadings ranging from 0.25 to 1.5 percent by weight, with glycerol added as a plasticizer. Silica surfaces were modified with sodium dodecyl sulfate to improve dispersion, and the mixtures were cast onto plates and dried to films roughly 70 micrometers thick. Characterization by scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy, Fourier-transform infrared spectroscopy, and X-ray diffraction confirmed that the nanoparticles were integrated into the polymer matrix through hydrogen bonding, electrostatic attraction, and interfacial siloxane condensation, although the highest-loading hybrid films showed visible particle agglomeration.

The optimized formulation, a chitosan film containing both nanocellulose and 0.75 percent silica nanoparticles, showed marked gains across nearly every measured property. Its char residue at 700 degrees Celsius reached 51 percent, compared with 39 percent for pure chitosan, indicating improved thermal stability. The water contact angle rose to 94.6 degrees, crossing into hydrophobic territory from the moderately hydrophilic 67.9 degrees of neat chitosan, while water vapor permeability fell to 1.146 grams per hour per square meter per atmosphere from 2.107 in the control film. Tensile strength climbed to 25.37 megapascals, elongation at break to 27 percent, and Young’s modulus to 593 megapascals. The reinforcement, however, was concentration dependent: films loaded with 1.5 percent silica showed the lowest tensile strength of all samples, which the authors attribute to particle agglomeration and the hygroscopic nature of the nanoparticles at high loadings.

Ultraviolet protection emerged as one of the film’s strongest attributes. All the films blocked nearly all radiation in the UV-B region and below 280 nanometers, but the hybrid film achieved the lowest transmittance overall in the UV-A range, at less than 4 percent, with an ultraviolet protection factor of 57.8, 3.0 percent UVA transmission, and 1.8 percent UVB transmission. The authors distinguish two mechanisms behind this performance. Chitosan’s amine, hydroxyl, and acetyl groups absorb ultraviolet radiation inherently, giving the neat film a baseline protection factor of 39.7. Silica nanoparticles add physical light scattering, since their refractive index differs from the polymer matrix and their mesoporous structure presents a large internal surface area that lengthens the optical path of incoming photons. The nanocellulose appears to enhance silica dispersion, reducing agglomeration and increasing the effective scattering cross-section per unit mass of silica.

Antibacterial testing by broth dilution revealed a sharp contrast between formulations. Pure chitosan and chitosan-nanocellulose films showed no inhibition against any of the four tested bacterial strains under the assay conditions, which the authors link to insufficient protonation of chitosan’s amino groups at near-neutral pH. Adding silica nanoparticles changed this: the hybrid film inhibited Escherichia coli by 98.12 percent and Staphylococcus aureus by 61.70 percent. The asymmetry follows the structural differences between bacterial envelopes. E. coli, a Gram-negative species, carries a strongly negatively charged outer membrane of lipopolysaccharide that is readily accessible to cationic chitosan surfaces. S. aureus, which is Gram-positive, buries its negatively charged teichoic acids within a thick peptidoglycan layer, creating steric barriers that slow and weaken the interaction.

The practical test involved wrapping fresh cherry tomatoes in the films and exposing them to a Suntest CPS+ accelerated weathering xenon tester, delivering 30 minutes of high-intensity irradiation per day over 21 days, a regimen the authors calculate as equivalent to roughly 4.5 hours of natural sunlight daily. Samples were then stored at 4 degrees Celsius. By day 21, tomatoes wrapped in the hybrid film had lost 16.12 percent of their initial weight and remained red, plump, and described as marketable, while the other formulations lost between 31.74 and 71 percent, with the control chitosan-wrapped fruit becoming excessively wrinkled, moldy, and partially rotten. The authors attribute the hybrid film’s performance to a combination of moisture barrier function, ultraviolet blocking, and antimicrobial activity, though they note the antimicrobial contribution in the storage trial as probable rather than directly measured.

The authors are explicit about the preliminary nature of the work and the limitations that remain. No comparative screening of the citric acid pretreatment against hydrochloric acid, sulfuric acid, or sodium hydroxide protocols for bio-silica isolation has been performed. The packaging evaluation covered a single fruit type under one storage condition, and the antimicrobial testing should be extended to a broader range of relevant pathogens. Toxicity evaluation and compliance with food-contact material regulations have not been addressed, nor have direct comparisons with commercial packaging materials, biodegradation behavior, or cost-effectiveness. The cherry tomato trials also relied on accelerated ultraviolet exposure rather than ambient storage conditions, and the weight-loss measurements, while recorded weekly, reflect the specific irradiation regimen used.

Even with those caveats, the study offers a concrete demonstration of how agricultural waste can be valorized into functional packaging materials within a circular-economy framework. Rice husk waste is generated at roughly 760 million tons annually, alongside 1.14 billion tons of rice straw, each containing 15 to 20 percent silica, making the biomass a plentiful feedstock for silica extraction. By coupling that feedstock with chitosan, a biopolymer already valued for film formation and inherent antioxidant activity, the researchers produced a material that simultaneously addresses mechanical strength, moisture barrier performance, ultraviolet shielding, and antimicrobial function in a single film. Whether such materials can compete with conventional synthetic packaging on safety, cost, and regulatory grounds remains to be established, but the reported performance under accelerated weathering provides a quantitative baseline for the formulation and optimization work the authors identify as the next step.

Subject of Research: Chitosan nanocomposite films reinforced with rice husk-derived bio-silica for ultraviolet-protective active food packaging

Article Title: Sustainable chitosan nanocomposite films reinforced bio silica for UV protective and active food packaging

Article References: El-Shall, F. N., Kader, A. H. A., Diab, M. A., El-Sabour, M. A., & Adel, A. M. (2026). Sustainable chitosan nanocomposite films reinforced bio silica for UV protective and active food packaging. Discover Nano, 21(1), Article 548. https://doi.org/10.1186/s11671-026-04959-y

Image Credits: AI Generated

DOI: 10.1186/s11671-026-04959-y

Keywords: chitosan, bio-silica, rice husk, food packaging, nanocellulose, UV protection, antibacterial activity, cherry tomatoes, agricultural waste, circular economy, water vapor permeability, Sustainable