Preparation of protein-based bionanocomposites and evaluation of their physico-chemical properties

Abstract

The increasing accumulation of non-biodegradable plastics and the growing concerns related to environmental pollution and water contamination have created an urgent need for sustainable and multifunctional materials. In response to these challenges, the present research was undertaken to develop protein-based bionanocomposites (BNCs) with improved physicochemical characteristics for applications in biodegradable food packaging and adsorption-based environmental remediation. The detailedunderstanding of the structural characteristics, preparation approaches, applications, advantages, and existing limitations of bionanocomposite systems, which helped identify key research gaps and establish the foundation for the experimental work. Particular emphasis was placed on the utilization of renewable protein sources in combination with suitable nanofillers to improve the functional performance of biopolymer-based materials. Initially, SPI has emerged as a promising material in recent years because of its availability, sustainability, low cost, and favorable film-forming and processing characteristics. In the present work, novel SPI-based bionanocomposite films were fabricated by incorporating Mg–Al layered double hydroxide (LDH) at varying loadings (0%, 2%, 5%, and 9% w/w) through a straightforward solution-casting technique. The Mg–Al LDH was synthesized using a co-precipitation route with a molar ratio of 2:1. Structural and morphological characterization using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), and thermogravimetric analysis (TGA) confirmed the intercalation and partial exfoliation of Mg–Al LDH layers within the SPI matrix. The extent of LDH particle agglomeration increased as the filler loading was raised from 0% to 9% (w/w). The resulting BNC films exhibited enhanced thermal stability. Their mechanical performance and biodegradation behavior were also evaluated. Tensile strength of the films containing 0%, 2%, 5%, and 9% w/w LDH were measured as 2.12 ± 0.25, 1.60 ± 0.15, 1.64 ± 0.08, and 1.58 ± 0.06 KN m g⁻¹, respectively. Notably, the SPI–Mg/Al LDH film with 5% loading demonstrated optimal mechanical properties; moreover, it was efficiently degraded in non-sterile soil. Building upon these findings, novel wheat gluten (WG)/Cloisite 30B (C30B) organoclay-based bionanocomposite films were fabricated using a solution-casting technique with varying C30B loadings (5%, 10%, and 15%). Structural and morphological analyses using XRD and FESEM confirmed the intercalation and partial exfoliation of C30B layers within the WG matrix. Among the prepared films, the WG–C30B 10% composition exhibited notable improvements in physicochemical properties, including reduced surface roughness, enhanced water barrier performance, and increased surface hydrophobicity. This BNC film demonstrated superior thermal stability. Mechanical testing revealed a significant enhancement in tensile strength, increasing from 0.70 ± 0.02 for neat WG films to 1.11 ± 0.01 for WG–C30B 10% films. Such films effectively inhibited the growth of some bacteria namely, Staphylococcus aureus and Salmonella enterica clearly indicating antibacterial properties. Shelf-life studies on green grapes were conducted under refrigerated (4 °C), ambient, and elevated temperature (42 °C) conditions. It was shown that the WG–C30B 10% film was effective in extending the shelf life up to 18 days under ambient conditions as compared to dip coated with WG-C30B 10% solution. Biodegradation studies demonstrated that over 50% of the BNC films were decomposed in agricultural soil within two weeks, while complete degradation occurred rapidly in sewage sludge soil. The prepared WG–C30B 10% film exhibited promising physicochemical, antibacterial, and biodegradation properties, clearly showing its potential for use in biodegradable food packaging applications. To expand the scope of practical application, protein-based biocomposites were further explored as adsorbent systems for contaminant removal. A sensitive UV–visible spectroscopic technique was successfully employed to detect tartrazine (Tr), an azo dye, in commercially available food products. The study evaluated the adsorption performance of soy protein isolate (SPI)/ZnAl layered double hydroxide (LDH)-based biocomposites for effective Tr removal. ZnAl LDH with a 3:1 molar ratio and SPI/ZnAl LDH biocomposites containing varying SPI loadings (0.25–3 g) were synthesized using a co-precipitation approach. Several techniques namely, XRD, FTIR, SEM, EDS, TGA, DSC, and BET analyses were used in order to characterize the BNC films in a comprehensive manner. The crystallinity index of the 2:1 SPI/ZnAl LDH biocomposite was determined to be 53.8 ± 0.416%. FTIR analysis confirmed the presence of characteristic amide I (C=O stretching at 1621 cm⁻¹) and amide II (N–H bending at 1521 cm⁻¹) functional groups, indicating successful incorporation of SPI. Under optimized conditions i.e. initial dye concentration of 18 mg L⁻¹, pH 2.0, adsorbent dosage of 5.0 mg, and contact time of 60 min, the 2:1 SPI/ZnAl LDH biocomposite remained effective in achieving 99.85% dye removal at ambient temperature. Thermodynamic and adsorption parameters revealed a spontaneous process (ΔG = −2.72 kJ mol⁻¹), with a maximum adsorption capacity (q_max) of 49.01 mg g⁻¹ and an adsorption equilibrium constant of 0.162 g mg⁻¹ min⁻¹. The adsorption behavior was aptly described by both Langmuir (R² = 0.9935) and Freundlich (R² = 0.9959) isotherm models, along with pseudo-second-order kinetics. The biocomposite demonstrated efficient Tr removal from commercial samples, achieving removal efficiencies of 81.43% for Mountain Dew, 79.21% for yellow candy, 77.85% for custard powder, and 75% for food dye wastewater. Moreover, the adsorbent retained approximately 73% efficiency after five regeneration cycles, indicating good reusability. Antimicrobial activity against Listeria species and Acinetobacter calcoaceticus was also observed. The SPI–LDH hybrid system showed potential for the removal of hazardous dyes from food products and wastewater. Overall, the findings of this work demonstrate that the integration of proteins with layered nanofillers offers an effective strategy for developing multifunctional bionanocomposites possessing improved thermal, mechanical, antimicrobial, biodegradable, and adsorption characteristics. These materials show strong potential as sustainable alternatives for food packaging and environmental applications.