Revalorization of coconut husk lignin through ZnO nanoparticles synthesis: antibacterial assay and photocatalytic activities

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Hasnain Mustak, Kazi Suraiya Islam, Md. Shamsul Alam, Md. Minnatul Karim, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4458077/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 21 Oct, 2024 Read the published version in Waste and Biomass Valorization → Version 1 posted 5 You are reading this latest preprint version Abstract Lignin, due to its structural diversity and biodegradability has emerged as a promising alternative to hazardous synthetic materials in multidisciplinary research area. The aim of this paper is to evaluate the lignin potential in value-added applications to be exact zinc oxide nanoparticle synthesis. First, lignin was extracted from coconut husk via sonochemical route in alkaline medium. Afterward, lignin-zinc oxide hybrid nanoparticles (L-ZnO NPs) were synthesized by the reaction of zinc acetate dihydrate salt and lignin, where lignin acts as bioreducing and capping agent. The synthesized lignin and L-ZnO NPs were characterized by UV-Vis spectroscopy, FTIR Spectroscopy, XRD and FESEM analysis. All the identification tests supported the existence of lignin and L-ZnO NPs. The XRD and SEM images disclosed the formation of hexagonal wurtzite shape L-ZnO NPs having mean diameter ≈ 77.76 nm. The antibacterial efficacies of the L-ZnO NPs were assessed against both gram-negative E. coli and gram-positive S. aureus bacteria, employing standard controls and the well diffusion method. The L-ZnO NPs exhibited marvelous bactericidal action against the test microorganisms displaying sufficient zone of inhibition. The photocatalytic performance of the L-ZnO NPs was observed by methylene blue (MB) dye degradation test under UV light irradiation. The UV-Vis absorbance and conductivity tests suggested that the L-ZnO NPs has noticeable photocatalytic efficiency on MB dye degradation. Therefore, the synthesized L-ZnO NPs possess excellent versatile properties and it can be used in medicinal applications and environmental management. Coconut husk lignin Bioreducing agent Lignin-ZnO nanoparticles Antibacterial activity Photocatalytic property Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Introduction In the last few decades, the researches on the production and development of multifunctional biomaterials from lignocellulosic biomass have been received great attention. The lignocellulosic biomass is most plentiful renewable resource in the world; it has potential to be used to produce valuable chemicals and biomaterials. Cellulose, hemicellulose, and lignin are the main components of lignocellulosic biomass amongst which lignin stands as the second most abundant natural polymer globally, following cellulose in prevalence [ 1 – 3 ]. Currently, over 70 million tons of lignin is obtained as a by-product from paper and pulping industries every year [ 4 ]. Almost 98% of lignin undergoes combustion as fuel, which not only harms the environment but also consumes a tremendous portion of this resource [ 5 ]. Chemically, lignin is constructed with phenylpropane monomeric units of monolignols i.e. paracoumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. These monolignols produce three-dimensional building blocks with vast arrangements of functional groups and cross-linkages [ 6 ]. There are numerous sources of lignin, including coconut, jute, cotton, hemp, wood pulp, and others. Depending on the source and extraction technique, their physical and chemical behavior is reported to differ [ 7 – 9 ]. Coconut husk contains a notable amount of lignin (almost 45%) and the rest components are cellulose 44%, pectin 4%, water-soluble parts 5% and ash 2% [ 10 , 11 ]. Lignin has a wide range of prospective uses with added value that might have a big impact on industry. For instance, the derivatives of lignin give rise to functional polymers with multiple applications, including acting as antibiotics, stabilizers in colloidal suspensions, antioxidants, antiviral agents, and dispersants for pesticides. Additionally, their superior properties have made them potential candidates for use as anticarcinogenic substances and more [ 4 , 12 ]. In recent years, lignin owing to nontoxic and biodegradable nature, have paid great attention to be used as bioreducing agents for bioactive nanomaterial synthesis [ 2 , 5 ]. Lignin-mediated metal or metal oxide nanoparticles exhibit excellent properties and application opportunities rather than inorganic nanoparticles alone. In lignin-mediated metal or metal oxide nanoparticles, the lignin and metal or metal oxide nanoparticles maintain their fundamental characteristics on the nano-scale, and they are easily adaptable to align with the overall properties as needed. The lignin served the dual role of reducing and capping agent in the fabrication of hybrid metal or metal oxide nanoparticles namely Ag, Au, Te, Cu 2 O, and ZnO [ 2 , 13 ]. Nonetheless, some research works have presented the possibility of using lignin-metal nanoparticles in various applications namely biomedical, semiconductor nanomaterials etc. In particular, lignin-ZnO hybrid nanoparticle has appeared as one of the most favorable wide-band-gap inorganic materials and shows great promise across various applications, including fluorescence imaging, photocatalytic activities, bacterial activity inhibitor, anticancer agent, etc. [ 14 ]. Liqiang Xiao et al. synthesized hybrid particles known as lignin-zinc oxide (LZn). These particles consist of lignin nanoparticles (LNP) and zinc oxide (ZnO), and were synthesized using a straightforward hydrothermal method. The LZn hybrid nanoparticle displayed excellent antibacterial activity against E. coli and S. aureus [ 15 ]. Klapiszewski et al. produced blow-molded containers from polyethylene, incorporating a hybrid material of ZnO/lignin with varying lignin concentrations. These containers demonstrated notable antimicrobial efficacy, particularly effective against Gram-positive bacteria, including Staphylococcus and Bacillus, in the conducted tests [ 16 ]. Rather than antimicrobial efficacy, the nanoparticles have also been used for hazardous dye degradation [ 14 , 17 ]. Because of the photocatalytic phenomenon of nanoparticles with typical wide band gaps and convenient excitation binding energy at ordinary temperatures can be oxidized dye molecules and produce degraded products with lower toxicity [ 18 ]. S. Venkatesan and coworkers demonstrated the significant photocatalytic capabilities of zinc oxide nanoparticles (ZnO NPs) in decomposing methylene blue dye, achieving an impressive efficiency of 94.07% when exposed to sunlight [ 19 ]. ES Baeissa investigated the photocatalytic efficiency of ZnO nanoparticles when exposed to visible light in the presence of methylene blue dye. His synthesized ZnO NPs could oxidize 100% methylene blue dye within 30 min [ 20 ]. Numerous scientific findings indicate that the photocatalytic and antibacterial activities of lignin-based ZnO NPs depend on their particle size, shape, source of lignin, method of hybrid metal or metal oxide nanoparticles synthesis etc. [ 2 ]. In particular, the morphology, chemical and structural complexity, and variability of lignin from various sources have a stimulatory effect on the properties of nanoparticles. Although much of the earlier research included the development of ZnO NPs using various lignin sources, the coconut husk lignin is still unexplored in literature. Therefore, the objective of our research is to synthesize ZnO nanoparticles using coconut husk lignin, where lignin acts both as a reducing and capping agent. Lignin can be extracted by various processes like hydrothermal, sono-chemical, alkali hydrolysis etc. The depolymerization of lignin molecules is happened at high pressure and temperature during hydrothermal treatment [ 21 , 22 ]. In addition, lignin is readily decomposed in presence of highly alkaline medium [ 23 ]. On the other hand, comparatively better form of lignin attains at sonochemical extraction but the yield percentage of lignin is very low [ 24 ]. Accordingly, the sonochemical methods have been tuned to extract a high percentage of lignin from coconut husk in our investigation. The reduction reaction of lignin to form L-ZnO NPs has also been performed with controlled conditions (pH, temperature, time, and agitation). Subsequently, the antibacterial efficacy of lignin-capped ZnO nanoparticles has been evaluated by the well diffusion method against gram-negative E. coli and gram-positive S. aureus bacteria. Additionally, the photocatalytic activity of the synthesized nanoparticles has been assessed by the degradation of methylene blue dye under UV radiation. Experimental Materials Dried coconut husk was obtained from Kushtia district, Bangladesh. The husk was crashed into fine powder and stored in the desiccator. Demineralized water was obtained from Rahimafrooz, Bangladesh Ltd. Toluene, ethanol, KOH, acetic acid, HCl, polyethylene glycol, zinc acetate dihydrate were purchased from Merck, Germany. Methylene blue dye was manufactured by Sigma-Aldrich. Methods Lignin extraction 10 g of dried coconut husk powder was refluxed with a mixture of toluene and ethanol (toluene: ethanol = 2:1 v/v) at 85°C temperature. The dewaxed husk was then dried in the oven at 60℃ and sonicated with KOH solution at 40 kHz frequency for 30 min using Biobase UC-40A ultrasonicator. The solution was then stirred for 1 h and filtered on a nylon cloth. The pH of the filtrate was adjusted to 5.5 using acetic acid solution. Then the hemicelluloses were precipitated with 3 volumes of 95% ethanol and separated by filtration. The filtrate was concentrated to one-third of its initial volume by evaporating the ethanol. The acid-insoluble lignin was then precipitated from the solution at pH 1.5 adjusted by 6M HCl solution. The precipitated lignin was then washed 3 times with demineralized water and dried at 60°C. The experiment was repeated varying the sonication period for 60 min and 240 min. Lignin- ZnO nanoparticle synthesis 0.1 g lignin was added to 0.1M 100mL of NaOH solution and sonicated for 1 h to obtain a homogeneous solution. The homogenized solution was stirred, maintaining temperature in the range of 80 to 90°C. Then 2 g of zinc acetate dihydrate salt was added to the solution in the duration of 30 min. and again stirred for 1 h. A white precipitate was formed. The precipitate was washed with deionized water and then washed 3 times with absolute ethanol to wash off the remnant Lignin and NaOH. The formed precipitate underwent drying in an oven at 90°C for one hour, followed by calcination in a muffle furnace at 500°C for duration of 3 h. Measurements The isolated alkali soluble lignin from coconut husk as well as L-ZnO NPs were characterized by Shimadzu UV1900i UV-vis. spectrophotometer at a wavelength range of 255 nm to 510 nm and 330 nm to 500 nm respectively. FTIR (ATR) spectra of the lignin, L-ZnO NPs and precursor (zinc acetate dihydrate) were measured using an FTIR-ATR Instrument from SHIMADZU (IRAffinity-1S) with a resolution of 4 cm − 1 in the wavenumber range 400 to 4000 cm − 1 . The surface morphology and particle size of lignin, L-ZnO NPs and zinc acetate dihydrate were examined by ZEISS Sigma 500 VP Field Emission Scanning Electron Microscope operating at 5 kV for 20 thousand and 50 thousand times magnification levels. Crystallographic study of lignin, zinc acetate dihydrate and L-ZnO NPs were examined using Rigaku Ultima IV XRD equipment operating at 40 kV with CuKα(λ = 1.5406Å) radiation. The samples were scanned within the 2θ range of 5°-80°. Antibacterial test The antibacterial properties of the L-ZnO NPs, extracted lignin, commercial ZnO and zinc acetate dihydrate were evaluated using the well diffusion technique. The BP-ZnO NPs was taken from our previous study for comparing the antibacterial properties. Bacterial strains, Escherichia coli O157:H7 and Staphylococcus aureus MTCC 3160 were used in this study which was previously reported as multi-drug resistant strains [ 25 – 27 ]. Bacterial inoculum was prepared by culturing in Luria–Bertani (LB) broth medium for a duration of 18–20 hours to achieve a turbidity equivalent to 0.5 McFarland standard, corresponding to 1.5×10 8 colony-forming units (CFU)/ml. Mueller Hinton agar (MHA) plates with a diameter of 150 mm were formulated, and bacterial inoculum was evenly spread across their surfaces. Wells of 7-mm diameter were generated in the MHA plates using a sterile Aluminum borer. Each well was filled with 70 µl of L-ZnO NPs, extracted lignin, commercial ZnO and zinc acetate dihydrate solution at concentration of 0.1 mg/ml, where streptomycin was used as a control for E. coli and Amoxicillin was for S. aureus respectively. The plates were placed in an incubator at 37°C for 24 h. Measurements of the inhibition zones surrounding the wells were taken in millimeters (mm). The experiment was conducted three times to ensure reliability, and the results are presented as the mean value along with the standard deviation. Photocatalytic activity 70 ml aqueous solution of methylene blue dye of 2 ppm concentration was prepared. 0.045 g of L-ZnO NPs was added to it and stirred for mixing. The blue solution gradually turned green when subjected to UV radiation with an intensity of 0.7821 mW \({cm}^{-2}\) . After every 15 min, 10 ml solution was withdrawn from the reaction beaker. The UV absorbance of the solutions were measured at 650 nm and 355 nm wavelengths using Shimadzu UV1900i UV-vis. spectrophotometer. The conductivity of the degraded dye solutions were also measured using the HANNA edge HI2003 conductivity meter. Results and Discussion In this study, the sonochemical method has been developed to extract lignin from coconut husk. Figure 1 depicts the yield of extracted lignin with sonication time. The observation indicates that as the sonication time increases, there is a corresponding rise in the percentage of lignin yield. The ultrasound induces mechanical effects on fiber cell walls, leading to enhanced accessibility and extractability of the lignin and cellulose components. The breaking of the ether linkages in-between lignin and celluloses from the cell walls of husk has stronger impact during extraction at alkaline medium by ultrasonic irradiation [ 28 ]. The lignin is identified by UV-vis spectroscopy, FTIR, FE-SEM, and XRD analysis. Morphological and structural characterization of lignin and L-ZnO NPs Figure 2 (a) illustrates the UV absorption spectrum of lignin precipitation isolated from coconut husk. The lignin sample's absorbance was measured within the wavelength range from 255 nm to 510 nm. Clearly, the sample exhibits characteristic absorption peaks featuring two maxima (λ max ) at 320 nm and 280 nm associated with lignin molecule. The absorption peak at 280 nm is attributed to nonconjugated phenolic groups present in the lignin, while the absorption maximum at 320 nm is identified as originating from bound hydroxycinnamic acids like para-coumaric acid and ferulic acids [ 29 ]. The higher peak intensity at 320 nm suggests the lignin sample is rich in para-coumaric acid fractions. Figure 2 (b) represents the UV–visible absorption spectrum of the L-ZnO NPs that has been recorded in the wavelength range of 330–500 nm. From the spectrum it can be observed that the absorption maximum appears at 367 nm which is the characteristic peak for L-ZnO NPs. Similar results have been reported by the previous studies [ 30 ]. Lignin possesses an overall negative surface charge which can be explained by the presence of negatively charged ionic groups such as hydroxyl, carboxyl and phenolics on the surface of lignin. Due to this character, lignin functions as a reducing agent in the fabrication of ZnO nanoparticle composite structure. Phenolic hydroxyl groups are reactive entities susceptible to oxidation, leading to the formation of quinones releasing hydrogen ions and electrons that reduce zinc ions and stabilize ZnO nanoparticles. Because of this electrostatic attraction force between metal oxide and polymer molecules, lignin is able to cap the surface of ZnO nanoparticles and prevent them from agglomeration and thus stabilizes the nanoparticles [ 31 ]. Figure 3 represents the FTIR spectra of lignin, zinc acetate dihydrate and L-ZnO NPs. The absorption bands in lignin such as ‒OH stretching of phenolic compounds (3400–3780 cm − 1 ), C‒H stretching in methylene and methyl groups (2925–2930 cm − 1 ) indicating lignin has abundant side chain structure, unconjugated ketone and carboxyl group stretching (1700–1730 cm − 1 ), conjugated carbonyl stretching in lignin (1600–1660 cm − 1 ), ring associated with C‒O stretching (1220–1240 cm − 1 ), non-esterified phenolic ‒OH group in lignin (1110 cm − 1 ), C‒H deformation (610–660 cm − 1 ) were detected at 3431, 3695, 3770, 2927, 1720, 1620, 1224, 1111, and 651 cm − 1 respectively [ 32 , 33 ]. The absorption bands in zinc acetate dihydrate such as symmetrical stretching vibration of carboxylate group (1420–1450 \({cm}^{-1}\) ), asymmetrical stretching vibrations of the carboxylate group (1555–1565 cm − 1 ) were detected at 1446 cm − 1 and 1558 cm − 1 respectively [ 34 ]. The absorption bands in L-ZnO NPs alike ZnO wurtzite structure (441–665 cm − 1 ), vibrational stretching mode of Zn‒O bounding (470–485 cm − 1 ) is detected at 673 cm − 1 and 478 cm − 1 respectively. The appearance of 3772, 3697, 3435, 1716, 1618, 1111, 673, and 478 cm − 1 absorption bands in the L- ZnO NPs structure indicates the binding of ZnO nanoparticles with lignin without significantly altering the structure of lignin [ 29 ]. The surface morphology of coconut husk lignin can be seen from the Fig. 4 (a) and 4(b) at 20000× and 50000× magnification levels, respectively. From the images it is evident that the sample is composed of spherical particles facilitating a large surface area. The particles appear to be regular in shape, well separated and uniformly dispersed which is reported in previous study [ 35 ]. No presence of additional substances appeared in the images. The average diameter of lignin particles is calculated to be 161.55 nm and the particle size distribution of lignin is represented in Fig. 5 (b). The micrograph of zinc acetate dihydrate shows that the sample exhibits particles with irregular shapes and a distribution that is not uniform. The sample resembles a stratified surface with cracks. The surface morphology of L-ZnO NPs is represented in Fig. 4 (e) and 4(f). The particle size distribution of the L-ZnO NPs is represented in Fig. 5 (a) and the average particle diameter of the composite nanomaterial is calculated to be 77.76 nm. From the micrographs it can be seen that, L-ZnO NPs with a nearly spherical morphology were almost equally dispersed in the sample made by encapsulating ZnO NPs in the voids created by the lignin macromolecular matrix. Nanoparticles are evenly distributed throughout the cavities created by the macromolecule aggregates, which increases their stability and prevents them from aggregating. These characteristics are in accordance with prior research [ 36 ]. The L-ZnO NPs show a porous surface possibly due to the high temperature heat treatment in absence of oxygen that increases surface area for each particle and thus increases the attachment sites on bacterial cell walls. The interfacial adhesion between lignin and ZnO nanoparticles proves the affinity of lignin towards ZnO nanoparticles. Figure 6 represents the XRD pattern of lignin, zinc acetate dihydrate and L-ZnO NPs. The diffractogram shows a wide area under the curve that confirms the amorphous characteristics of coconut husk lignin. The maximum value of 2θ for coconut husk lignin has appeared at 22.64 degrees. Slight variation in the value of 2θ for lignin may occur according to the source of biomass, chemicals used, and method of extraction followed [ 32 ]. From Fig. 6 (b), two characteristic reflections for Zn have been encountered at 2θ values of 36.10 and 44.44 respectively. These reflections are representative of the hexagonal structure of Zn [ 37 ]. Figure 6 (c) shows the presence of ZnO in L-ZnO NPs. The reflections located at 2θ values of 31.67, 34.27, 36.11, 47.39, 56.45, 62.67, 66.33, 67.85, 68.93, 72.55, and 76.95 degrees correspond to (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2), (2 0 1), (0 0 4), and (2 0 2) planes respectively [ 5 , 36 ]. The characteristic reflections at these 2θ values represent hexagonal wurtzite phase of ZnO nanoparticles having highly polycrystalline character [ 38 ]. The crystallite size for L-ZnO NPs is calculated using Scherrer equation [ 39 ], $$D=\frac{K\lambda }{\beta Cos\theta }\dots \dots \dots \dots .\left(1\right)$$ Where, D is the crystallite size in angstrom; Scherrer constant, K = 0.94; Wavelength of X-ray, λ = 1.5406 Å; β, full width at half maximum (FWHM) in radian and θ, half of Bragg’s angle in degrees. The reflections from various planes of the L-ZnO NPs with respect to Bragg’s angle are represented in Table 1 . Table 2 signifies the data of 9 characteristic reflections used to calculate the average crystallite size of L-ZnO NPs. The average crystallite size is found to be 12.23 nm. The intensity of the reflection due to the (1 0 1) plane is notably higher as compared to other planes. This suggests that the lignin stabilized ZnO nanoparticles are enriched in (1 0 1) facets [ 36 ]. The crystallinity index for L-ZnO NPs is calculated to be approximately 93.41%. The crystalline characters are well preserved in the encapsulated product. The diffractogram of L-ZnO NPs also indicates that the synthesized nanoparticles are highly pure. Antibacterial activity evaluation The antibacterial activities in terms of zone of inhibition (mm) of zinc acetate dihydrate, commercial ZnO, lignin and L-ZnO NPs compared to Streptomycin and Amoxicillin against gram-negative E. coli and gram-positive S. aureus bacteria are shown in Fig. 7 and Table 3 . Zinc acetate dihydrate did not create any ZOI against E. coli but showed 14 mm ZOI against S. aureus . Commercial ZnO did not show any ZOI against E. coli but form 14.5 mm ZOI against S. aureus . Coconut husk lignin did not show any ZOI against any of these bacteria. The L-ZnO NPs made 13.5 mm ZOI against E. coli and 22 mm ZOI against S. aureus . Streptomycin showed 18.5 mm ZOI against E. coli and Amoxicillin showed 12 mm ZOI against S. aureus . Consequently, the result clearly indicates that the L-ZnO NPs are sensitive against both gram-negative E. coli and gram-positive S. aureus bacteria, proving its broad-spectrum activity. The antibacterial activity of L-ZnO NPs operates through induced oxidative stress. L-ZnO NPs engage with water, generating various reactive oxygen species (ROS), such as singlet oxygen, superoxide anion ( \(*{O}_{2}^{-}\) ), peroxide anion ( \(*{O}_{2}^{2-}\) ), hydroxyl radicals (*OH) and hydrogen peroxide (H₂O₂) [ 40 , 41 ]. Within the cell body, ZnO nanoparticles infiltrate, with peroxide anions ( \(*{O}_{2}^{2-}\) ) carrying a negative charge remaining on the cell surface. The release of Zn²⁺ is triggered when L-ZnO NPs assemble in the cytoplasm or outer membrane of bacterial cells, potentially leading to cell membrane disintegration, damage to membrane proteins, and genomic instability [ 42 , 43 ]. Oxidative stress ensues within the bacterial cell due to the interaction between Zn⁺ ions and the thiol group of bacterial respiratory enzymes. The abundant production of reactive oxygen species contributes to bacterial cell damage and eventual death. ROS plays a pivotal role in several mechanisms, including localized interaction with L-ZnO NPs causing cell wall damage, increased membrane permeability, NPs internalization due to the loss of proton motive force, and the uptake of toxic dissolved zinc ions. These processes result in weakened mitochondria, intracellular outflow, and alterations in gene expression related to oxidative stress, ultimately leading to inhibited cell growth and cell death. Additionally, the enhanced antibacterial activity can be attributed to abrasive surface texture formed by surface defects on the L-ZnO NPs in certain cases [ 42 ]. Hence, the L-ZnO NPs can inhibit food-borne harmful pathogens (both gram-positive and gram-negative bacteria). Photocatalytic activity evaluation The photocatalytic performance of the L-ZnO NPs was examined in the methylene blue (C 16 H 18 N 3 SCl) dye degradation under UV light irradiation in an aqueous medium. The UV-vis absorption spectra and conductivity of the degraded methylene blue (MB) dye solution were measured at different durations and are represented in Fig. 8 (a) and 8(b) respectively. Since the MB dye exhibits a strong absorption band at 650 nm (λ max ), the photocatalytic activity of L-ZnO NPs was monitored by the change of absorbance peak at 650 nm on UV–vis spectroscopy. The degraded dye solution shows another absorption band at 355 nm (λ max ) due to the presence of L-ZnO NPs. The photocatalytic degradation behavior has been investigated for 15, 30, 45, 60, 75, and 90 min. It has been observed that the absorption intensity decreased gradually (at 650 nm) with the increment of UV light irradiation time. The evanescence of the methylene blue dye during the decolorization process indicates a gradual reduction in the concentration of methylene blue in the solution. On the other hand, the absorbance at 355 nm is slightly decreased. The previous studies stated that the exposure to light in the presence of the photocatalyst led to the destruction of the azo bonds and aromatic rings within the MB molecules [ 44 ]. The photo degradation of methylene blue (MB) dye with time using L-ZnO NPs has also been assessed by conductivity measurements. Figure 8 (b) shows the change of conductivity of the dye solution with the increases of UV light irradiation time. It has been found that the conductivity of degraded solution is higher than virgin MB solution. With the increase of light projection time, more degraded derivatives or ions were formed [ 19 , 45 ]. Hence, the conductivity of the solution becomes higher. The photocatalytic degradation of methylene blue (MB) dye involves the generation of electron-hole pairs in L-ZnO NPs when exposed to UV light. As the ZnO photocatalyst is illuminated with photons possessing energy equal to or greater than its band-gap energy, electrons are excited from the valence band (VB) to the conduction band (CB), creating electron-hole pairs on the surface of L-ZnO NPs, with holes (h+) remaining in the conduction band. The band gap energy and electronic configuration of L-ZnO NPs are crucial factors influencing the photocatalytic degradation of the MB dye when exposed to UV-radiation. Because of the elevated recombination rate of photoinduced charge carriers (electron-hole (e − - h + ) pairs), there is a significant enhancement in the photocatalytic activity of L-ZnO NPs. The series of reactions occurring during the process of photocatalytic degradation under UV radiation can be expressed by the following chemical reactions [ 46 , 47 ]: L-ZnO NPs + \({h\nu }_{\left(UV\right)}\) → L-ZnO NPs + \({h}_{VB}^{+}\) + \({e}_{CB}^{-}\) \({H}_{2}O\) + \({h}_{VB}^{+}\) → \({OH}^{*}\) + \({H}^{+}\) \({h}_{VB}^{+}\) + \({OH}^{-}\) → \({OH}^{*}\) \({O}_{2}\) + \({e}_{CB}^{-}\) → \({O}_{2}^{*}\) \({O}_{2}^{*}\) + \({H}^{+}\) → \({HO}_{2}^{*}\) 2 \({HO}_{2}^{*}\) → \({H}_{2}{O}_{2}\) + \({O}_{2}\) \({H}_{2}{O}_{2}\) + \({e}_{CB}^{-}\) → \({OH}^{*}\) + \({OH}^{-}\) \({OH}^{*}\) + MB dye → Degraded Product + \({CO}_{2}\) + \({H}_{2}O\) Conclusion In this study, the bioreducing properties of lignin on metal oxide NPs synthesis and its role as stabilizing agent have been discussed. The lignin was extracted by eco-friendly and economically viable bioreduction route eco-friendly and economically viable route coconut husk fiber using sonochemical method. The absorption peaks of lignin appeared at ( \({\lambda }_{max})\) 280 nm and 320 nm in the UV spectrum corresponding to nonconjugated phenolic groups and bound hydroxycinnamic acid, such as para-coumaric acid and ferulic acids. L-ZnO NPs were synthesized from zinc acetate dehydrate salt by the reducing reaction of extracted lignin. The UV-vis. absorption peak at 367 nm confirms the formation of ZnO nanoparticles. The stretching vibration of phenolic compounds, unconjugated ketone and carboxyl group, conjugated carbonyl group and phenolic-OH group indicates the presence of lignin in the composite structure and vibrational stretching of Zn-O bounding indicates the presence of ZnO in the composite structure through FTIR analysis. The average particle diameter of 77.76 nm and perforated pumice-like surface morphology of the composite where lignin particles were agglomerated on ZnO surface were visualized by scanning electron micrograph. The X-ray diffractogram confirms crystalline structure due to the hexagonal phase of ZnO with high intensity of (1 0 1) plane as well as amorphous regions due to the presence of macromolecular structure of lignin in the composite material. The synthesized L-ZnO NPs with prerequisite dimension and crystallinity shows excellent bactericidal action and dye degradation characteristics. The zone of inhibition measurement indicates the marvelous bactericidal effects of the L-ZnO NPs against E. coli and S. aureus bacteria with respect to standard controls proving its broad-spectrum antibacterial activity. The L-ZnO NPs showed tremendous photocatalytic activity in presence of UV light on methylene blue dye degradation. From the discussion it can be concluded that the valuable biopolymer lignin is being utilized very inefficiently and often discarded as an industrial waste product that can be recovered from the biomass and utilized efficiently for noble purposes by industrially optimizing the recovery process. The further studies will investigate the bacterial response to nanoparticle stress, specifically examining bacterial morphology and the association of nanoparticles with bacterial cells. The detail studies would provide valuable insights like the mechanism through which nanoparticles anchor onto the cell surface, penetrate the cell wall, and potentially induce the death of bacterial cells by disrupting their cell walls. Declarations Statement of Novelty Although much of the earlier research included the development of ZnO NPs using various lignin sources, the coconut husk lignin is still unexplored in literature. Therefore, in the present research, ZnO nanoparticles have been synthesized using coconut husk lignin, where lignin acts both as a reducing and capping agent. Lignin can be extracted by various processes like hydrothermal, sono-chemical, alkali hydrolysis etc. The depolymerization of lignin molecules is happened at high pressure and temperature during hydrothermal treatment. Moreover, lignin can be readily decomposed in presence of highly alkaline medium. However, comparatively better form of lignin attains at sonochemical extraction but the yield percentage of lignin is very low. To get high lignin yield percentage, the sonochemical methods have been tuned in our investigation. The reduction reaction of lignin to form L-ZnO NPs has also been performed with controlled conditions (pH, temperature, time, and agitation). Ethics approval and consent to participate Not applicable. Consent for publication Not applicable. Availability of data and materials Not applicable. Competing interests All authors do declare that there are no competing interests. Acknowledgement The authors would like to acknowledge Chairman, department of Biotechnology and Genetic Engineering, Islamic University Bangladesh for instrumental support. Author Contributions Md. Hasnain Mustak: Methodology, Validation, Formal Analysis, Investigation, Data Curation, Writing – Original Draft Preparation. Kazi Suraiya Islam: Conceptualization, Methodology, Validation, Formal Analysis, Investigation, Data Curation, Writing – Original Draft Preparation Md. Shamsul Alam: Technological support, Writing – Review, Visualization Md. Minnatul Karim: Technological support, Writing – Review, Visualization Gazi Md. Arifuzzaman Khan*: Conceptualization, Methodology, Validation, Formal Analysis, Resources, Data Curation, Writing – Review and Editing, Visualization, Supervision, Project Administration Funding This work was financially supported by the special allocation project (Financial Year 2020-2021, G. O. No.: 39.00.0000.009.14.011.20-EAS 406/1742 EAS date 08/12/2020) of the Ministry of Science and Technology, Government of the People’s Republic of Bangladesh. References Zoghlami A, Paës G (2019) Lignocellulosic Biomass: Understanding Recalcitrance and Predicting Hydrolysis. Front Chem 7:. https://doi.org/10.3389/fchem.2019.00874 Morena AG, Tzanov T (2022) Antibacterial lignin-based nanoparticles and their use in composite materials. Nanoscale Adv 4:4447–4469. https://doi.org/10.1039/D2NA00423B Zhong W, Su W, Li P, et al (2024) Preparation and research progress of lignin-based supercapacitor electrode materials. 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Hakeem et al. (ed) Biomass and Bioenergy: Applications. Springer International Publishing Switzerland Garcia Gonzalez MN, Levi M, Turri S, Griffini G (2017) Lignin nanoparticles by ultrasonication and their incorporation in waterborne polymer nanocomposites. J Appl Polym Sci 134:45318. https://doi.org/https://doi.org/10.1002/app.45318 van Dam JEGG, van den Oever MJAA, Keijsers ERPP, et al (2006) Process for production of high density/high performance binderless boards from whole coconut husk. Part 2: Coconut husk morphology, composition and properties. Ind Crops Prod 24:96–104. https://doi.org/https://doi.org/10.1016/j.indcrop.2005.03.003 Khan GMA, Yilmaz ND, Yilmaz K (2016) Okra bast fiber as potential reinforcement element of biocomposites: Can it be the flax of the future? In: Thakur VK (ed) Handbook of Composite from Renewable Materials. Wiley Scrivener, pp 379–406 Vinardell MP, Mitjans M (2017) Lignins and Their Derivatives with Beneficial Effects on Human Health. Int J Mol Sci 18:. https://doi.org/10.3390/ijms18061219 Li S, Li X, Li S, et al (2024) In-situ preparation of lignin/Fe3O4 magnetic spheres as bifunctional material for the efficient removal of metal ions and methylene blue. Int J Biol Macromol 259:128971. https://doi.org/https://doi.org/10.1016/j.ijbiomac.2023.128971 Jose LM, Kuriakose S, Mathew T (2021) Development of photoresponsive zinc oxide nanoparticle - encapsulated lignin functionalized with 2-[(E)-(2-hydroxy naphthalen-1-yl) diazenyl] benzoic acid: A promising photoactive agent for antimicrobial photodynamic therapy. Photodiagnosis Photodyn Ther 36:102479. https://doi.org/https://doi.org/10.1016/j.pdpdt.2021.102479 Xiao L, Yao Z, He Y, et al (2022) Antioxidant and antibacterial PBAT/lignin-ZnO nanocomposite films for active food packaging. Ind Crops Prod 187:115515. https://doi.org/https://doi.org/10.1016/j.indcrop.2022.115515 Klapiszewski Ł, Bula K, Dobrowolska A, et al (2019) A high-density polyethylene container based on ZnO/lignin dual fillers with potential antimicrobial activity. Polym Test 73:51–59. https://doi.org/https://doi.org/10.1016/j.polymertesting.2018.11.018 Malik AQ, Mir T ul G, Kumar D, et al (2023) A review on the green synthesis of nanoparticles, their biological applications, and photocatalytic efficiency against environmental toxins. Environ Sci Pollut Res 30:69796–69823. https://doi.org/10.1007/s11356-023-27437-9 Mahamuni PP, Patil PM, Dhanavade MJ, et al (2019) Synthesis and characterization of zinc oxide nanoparticles by using polyol chemistry for their antimicrobial and antibiofilm activity. Biochem Biophys Reports 17:71–80. https://doi.org/https://doi.org/10.1016/j.bbrep.2018.11.007 Venkatesan S, Suresh S, Ramu P, et al (2022) Methylene blue dye degradation potential of zinc oxide nanoparticles bioreduced using Solanum trilobatum leaf extract. Results Chem 4:100637. https://doi.org/https://doi.org/10.1016/j.rechem.2022.100637 Baeissa E (2016) Photocatalytic degradation of methylene blue dye under visible light irradiation using In/ZnO nanocomposite. Front Nanosci Nanotechnol 2:1–5. https://doi.org/10.15761/fnn.1000134 Martín C, Dixit P, Momayez F, Jönsson LJ (2022) Hydrothermal Pretreatment of Lignocellulosic Feedstocks to Facilitate Biochemical Conversion. Front Bioeng Biotechnol 10:1–17. https://doi.org/10.3389/fbioe.2022.846592 Pińkowska H, Wolak P, Złocińska A (2012) Hydrothermal decomposition of alkali lignin in sub- and supercritical water. Chem Eng J 187:410–414. https://doi.org/10.1016/j.cej.2012.01.092 Shah TA, Khalid S, Nafidi H-A, et al (2023) Sodium Hydroxide Hydrothermal Extraction of Lignin from Rice Straw Residue and Fermentation to Biomethane. Sustainability 15:. https://doi.org/10.3390/su15118755 Subhedar PB, Gogate PR (2014) Alkaline and ultrasound assisted alkaline pretreatment for intensification of delignification process from sustainable raw-material. Ultrason Sonochem 21:216–225. https://doi.org/https://doi.org/10.1016/j.ultsonch.2013.08.001 Nain Z, Islam MA, Minnatul Karim M (2019) Antibiotic Resistance Profiling and Molecular Phylogeny of Biofilm Forming Bacteria From Clinical and Non-clinical Environment in Southern Part of Bangladesh. Int J Enteric Pathog 7:37–43. https://doi.org/10.15171/ijep.2019.10 Al Masud MA, Shaikh H, Alam MS, et al (2021) Green synthesis of silk sericin-embedded silver nanoparticles and their antibacterial application against multidrug-resistant pathogens. J Genet Eng Biotechnol 19:1–11. https://doi.org/10.1186/s43141-021-00176-5 Hasan MS, Al Foisal J, Khan GMA, et al (2022) Microfibrillated Cellulose-Silver Nanocomposite Based PVA Hydrogels and Their Enhanced Physical, Mechanical and Antibacterial Properties. J Polym Environ. https://doi.org/10.1007/s10924-022-02406-4 Dinh Vu N, Thi Tran H, Bui ND, et al (2017) Lignin and Cellulose Extraction from Vietnam’s Rice Straw Using Ultrasound-Assisted Alkaline Treatment Method. Int J Polym Sci 2017:1063695. https://doi.org/10.1155/2017/1063695 She D, Nie XN, Xu F, et al (2012) Physico-chemical characterization of different alcoholsoluble lignins from rice straw. Cellul Chem Technol 46:207–219 Singh DK, Pandey DK, Yadav RR, Singh D (2012) A study of nanosized zinc oxide and its nanofluid. Pramana 78:759–766. https://doi.org/10.1007/s12043-012-0275-8 Budnyak TM, Slabon A, Sipponen MH (2020) Lignin–Inorganic Interfaces: Chemistry and Applications from Adsorbents to Catalysts and Energy Storage Materials. ChemSusChem 13:4344–4355. https://doi.org/https://doi.org/10.1002/cssc.202000216 Gomide RAC, de Oliveira ACS, Rodrigues DAC, et al (2020) Development and Characterization of Lignin Microparticles for Physical and Antioxidant Enhancement of Biodegradable Polymers. J Polym Environ 28:1326–1334. https://doi.org/10.1007/s10924-020-01685-z Sun R, Tomkinson J (2002) Comparative study of lignins isolated by alkali and ultrasound-assisted alkali extractions from wheat straw. Ultrason Sonochem 9:85–93. https://doi.org/https://doi.org/10.1016/S1350-4177(01)00106-7 Dargatz B, Gonzalez-Julian J, Guillon O (2015) Improved compaction of ZnO nano-powder triggered by the presence of acetate and its effect on sintering. Sci Technol Adv Mater 16:25008. https://doi.org/10.1088/1468-6996/16/2/025008 Wang B, Sun D, Wang H-M, et al (2019) Green and Facile Preparation of Regular Lignin Nanoparticles with High Yield and Their Natural Broad-Spectrum Sunscreens. ACS Sustain Chem Eng 7:2658–2666. https://doi.org/10.1021/acssuschemeng.8b05735 Jose LM, Kuriakose S, Thomas S (2020) Fabrication, Characterization and In Vitro Antifungal Property Evaluation of Biocompatible Lignin-Stabilized Zinc Oxide Nanoparticles Against Selected Pathogenic Fungal Strains. Bionanoscience 10:583–596. https://doi.org/10.1007/s12668-020-00748-8 Olad A, Barati M, Behboudi S (2012) Preparation of PANI/epoxy/Zn nanocomposite using Zn nanoparticles and epoxy resin as additives and investigation of its corrosion protection behavior on iron. Prog Org Coatings 74:221–227. https://doi.org/https://doi.org/10.1016/j.porgcoat.2011.12.012 Talam S, Karumuri SR, Gunnam N (2012) Synthesis, Characterization, and Spectroscopic Properties of ZnO Nanoparticles. ISRN Nanotechnol 2012:372505. https://doi.org/10.5402/2012/372505 Juikar SJ, Vigneshwaran N (2017) Extraction of nanolignin from coconut fibers by controlled microbial hydrolysis. Ind Crops Prod 109:420–425. https://doi.org/https://doi.org/10.1016/j.indcrop.2017.08.067 Arakha M, Saleem M, Mallick BC, Jha S (2015) The effects of interfacial potential on antimicrobial propensity of ZnO nanoparticle. Sci Rep 5:9578. https://doi.org/10.1038/srep09578 Rajesh Kumar, Ahmad Umar, Girish Kumar, Nalwa HS (2017) Antimicrobial properties of ZnO nanomaterials: A review. Ceram Int 43:3940–3961. https://doi.org/https://doi.org/10.1016/j.ceramint.2016.12.062 Sirelkhatim A, Mahmud S, Seeni A, et al (2015) Review on Zinc Oxide Nanoparticles: Antibacterial Activity and Toxicity Mechanism. Nano-Micro Lett 7:219–242. https://doi.org/10.1007/s40820-015-0040-x Jiang S, Lin K, Cai M (2020) ZnO Nanomaterials: Current Advancements in Antibacterial Mechanisms and Applications. Front Chem 8:1–5. https://doi.org/10.3389/fchem.2020.00580 Pudukudy M, Yaakob Z (2015) Facile Synthesis of Quasi Spherical ZnO Nanoparticles with Excellent Photocatalytic Activity. J Clust Sci 26:1187–1201. https://doi.org/10.1007/s10876-014-0806-1 Jain N, Bhargava A, Panwar J (2014) Enhanced photocatalytic degradation of methylene blue using biologically synthesized “protein-capped” ZnO nanoparticles. Chem Eng J 243:549–555. https://doi.org/https://doi.org/10.1016/j.cej.2013.11.085 Alshehri AA, Malik MA (2019) Biogenic fabrication of ZnO nanoparticles using Trigonella foenum-graecum (Fenugreek) for proficient photocatalytic degradation of methylene blue under UV irradiation. J Mater Sci Mater Electron 30:16156–16173. https://doi.org/10.1007/s10854-019-01985-8 Fouda A, Salem SS, Wassel AR, et al (2020) Optimization of green biosynthesized visible light active CuO/ZnO nano-photocatalysts for the degradation of organic methylene blue dye. Heliyon 6:e04896. https://doi.org/10.1016/j.heliyon.2020.e04896 Tables Cite Share Download PDF Status: Published Journal Publication published 21 Oct, 2024 Read the published version in Waste and Biomass Valorization → Version 1 posted Reviewers agreed at journal 16 Jun, 2024 Reviewers invited by journal 16 Jun, 2024 Editor invited by journal 12 Jun, 2024 Editor assigned by journal 22 May, 2024 First submitted to journal 21 May, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4458077","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":315000877,"identity":"50ad7054-0b3b-475e-8d90-7ad2ac528fd2","order_by":0,"name":"Md. Hasnain Mustak","email":"","orcid":"https://orcid.org/0009-0006-4795-7944","institution":"Islamic University Bangladesh","correspondingAuthor":false,"prefix":"","firstName":"Md.","middleName":"Hasnain","lastName":"Mustak","suffix":""},{"id":315000878,"identity":"84877492-35a8-41b8-a250-7518fb0806b2","order_by":1,"name":"Kazi Suraiya Islam","email":"","orcid":"https://orcid.org/0009-0009-1412-0883","institution":"Islamic University Bangladesh","correspondingAuthor":false,"prefix":"","firstName":"Kazi","middleName":"Suraiya","lastName":"Islam","suffix":""},{"id":315000879,"identity":"196f9ac0-d64b-4703-b532-7e5acd9c0fb8","order_by":2,"name":"Md. Shamsul Alam","email":"","orcid":"https://orcid.org/0000-0002-8655-3372","institution":"Islamic University Bangladesh","correspondingAuthor":false,"prefix":"","firstName":"Md.","middleName":"Shamsul","lastName":"Alam","suffix":""},{"id":315000880,"identity":"2d512646-b0a8-4375-a40b-742864b7ef89","order_by":3,"name":"Md. Minnatul Karim","email":"","orcid":"https://orcid.org/0000-0002-5383-6288","institution":"Islamic University Bangladesh","correspondingAuthor":false,"prefix":"","firstName":"Md.","middleName":"Minnatul","lastName":"Karim","suffix":""},{"id":315000881,"identity":"98fd9606-29da-4b95-8136-4cdd65289d91","order_by":4,"name":"Gazi Md. Arifuzzaman Khan","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA+ElEQVRIiWNgGAWjYNACAzkYy4aBQYI4LcYwVhqxWhjgWg4T1iIfdoDxc0WBQeL8/sNPN/zccT6xf3bzwQcMNTbRuLQY3k5gljxjYJDY2HDM7GbvmduJM+4cSzZgOJaW24BLy+wEBskGgz+JzYwNZjd4224nNtzIMZNgbDiMTwvzzwagLW3M7N9u/m07lzifkBZ56QQ2SZCWHjYes9u8bQcSNxDSYgDUYgnUYjyDh6fstmxbsvHGG2nJBgl4/CIPdNjNhj8GsvP7j2+7+bbNTnbejeSDDz7U2OC25QD/BxQBR7DKBBzKwbagm2WPR/EoGAWjYBSMUAAArutdxICb6VoAAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0002-4233-9868","institution":"Islamic University Bangladesh","correspondingAuthor":true,"prefix":"","firstName":"Gazi","middleName":"Md.","lastName":"Arifuzzaman","suffix":"Md."}],"badges":[],"createdAt":"2024-05-22 04:10:37","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4458077/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4458077/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s12649-024-02773-0","type":"published","date":"2024-10-21T15:57:45+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":60343292,"identity":"a87701ad-2099-48b1-a292-dd1920b03768","added_by":"auto","created_at":"2024-07-15 19:15:15","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":11345,"visible":true,"origin":"","legend":"\u003cp\u003e(a) \u0026nbsp;Percentage of lignin yield with sonication time\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-4458077/v1/e2e8873d5e69fea727591d9d.png"},{"id":60343290,"identity":"e477d09a-2590-4682-b8ec-0779a08d6dbc","added_by":"auto","created_at":"2024-07-15 19:15:15","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":142812,"visible":true,"origin":"","legend":"\u003cp\u003eUV absorption spectrum of (a) lignin and (b) L-ZnO NPs\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-4458077/v1/7eed74c232437805e3343ec8.png"},{"id":60343294,"identity":"d6c3814f-1803-4b85-8b5b-e90759fee6b9","added_by":"auto","created_at":"2024-07-15 19:15:16","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":230387,"visible":true,"origin":"","legend":"\u003cp\u003eFTIR spectra of (a) Lignin (b) L-ZnO NPs (c) Zinc acetate dihydrate\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-4458077/v1/676e71bb20ac3907517d1114.png"},{"id":60344526,"identity":"fd56a4ed-1906-49bf-b836-a7ac8233a979","added_by":"auto","created_at":"2024-07-15 19:23:16","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":3767029,"visible":true,"origin":"","legend":"\u003cp\u003eScanning electron micrograph of (a, b) Lignin, (c, d) Zinc acetate dihydrate and (e, f) L-ZnO NPs\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-4458077/v1/756a6de7b621e41175218dad.png"},{"id":60343291,"identity":"dfd0569a-6941-40e3-82a1-e3a66f589c5d","added_by":"auto","created_at":"2024-07-15 19:15:15","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":704127,"visible":true,"origin":"","legend":"\u003cp\u003eParticle size distribution of (a) L-ZnO NPs and (b) Lignin\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-4458077/v1/ae87eb3a525a91eeee917d54.png"},{"id":60343293,"identity":"4372086f-cc03-4b2e-8698-1cd0504be40f","added_by":"auto","created_at":"2024-07-15 19:15:15","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":334406,"visible":true,"origin":"","legend":"\u003cp\u003eXRD pattern of (a) Lignin, (b) Zinc Acetate Dihydrate and (c) L-ZnO NPs\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-4458077/v1/a2291c848052595e5adca717.png"},{"id":60343289,"identity":"963e6921-5737-4ae7-851f-6bc527ea7a88","added_by":"auto","created_at":"2024-07-15 19:15:15","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":1571248,"visible":true,"origin":"","legend":"\u003cp\u003eAntibacterial activities against (a) \u003cem\u003eEschericia coli\u003c/em\u003e and (b) \u003cem\u003eStaphylococcus aureus\u003c/em\u003e bacterial strains for (5) BP-ZnO NPs, (6) Commercial ZnO, (7) Lignin and (8) L-ZnO nanoparticle\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-4458077/v1/051dc717eb3ecd979f67b4ff.png"},{"id":60343288,"identity":"87be92d3-5a78-4769-8b7f-11485f0d3788","added_by":"auto","created_at":"2024-07-15 19:15:15","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":290514,"visible":true,"origin":"","legend":"\u003cp\u003eThe change of (a) UV-vis absorbance and (b) conductivity of MB dye solution with L-ZnO NPs with various photocatalytic degradation times\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-4458077/v1/da721e72662b1f0d116023ae.png"},{"id":67681923,"identity":"c80edce7-14bf-489d-be03-cce54788a1a0","added_by":"auto","created_at":"2024-10-28 16:11:24","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":8995579,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4458077/v1/6665235a-6ccb-46b1-830a-76734d2e6849.pdf"}],"financialInterests":"","formattedTitle":"Revalorization of coconut husk lignin through ZnO nanoparticles synthesis: antibacterial assay and photocatalytic activities","fulltext":[{"header":"Introduction","content":"\u003cp\u003eIn the last few decades, the researches on the production and development of multifunctional biomaterials from lignocellulosic biomass have been received great attention. The lignocellulosic biomass is most plentiful renewable resource in the world; it has potential to be used to produce valuable chemicals and biomaterials. Cellulose, hemicellulose, and lignin are the main components of lignocellulosic biomass amongst which lignin stands as the second most abundant natural polymer globally, following cellulose in prevalence [\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. Currently, over 70\u0026nbsp;million tons of lignin is obtained as a by-product from paper and pulping industries every year [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. Almost 98% of lignin undergoes combustion as fuel, which not only harms the environment but also consumes a tremendous portion of this resource [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Chemically, lignin is constructed with phenylpropane monomeric units of monolignols i.e. paracoumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. These monolignols produce three-dimensional building blocks with vast arrangements of functional groups and cross-linkages [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. There are numerous sources of lignin, including coconut, jute, cotton, hemp, wood pulp, and others. Depending on the source and extraction technique, their physical and chemical behavior is reported to differ [\u003cspan additionalcitationids=\"CR8\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. Coconut husk contains a notable amount of lignin (almost 45%) and the rest components are cellulose 44%, pectin 4%, water-soluble parts 5% and ash 2% [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. Lignin has a wide range of prospective uses with added value that might have a big impact on industry. For instance, the derivatives of lignin give rise to functional polymers with multiple applications, including acting as antibiotics, stabilizers in colloidal suspensions, antioxidants, antiviral agents, and dispersants for pesticides. Additionally, their superior properties have made them potential candidates for use as anticarcinogenic substances and more [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn recent years, lignin owing to nontoxic and biodegradable nature, have paid great attention to be used as bioreducing agents for bioactive nanomaterial synthesis [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Lignin-mediated metal or metal oxide nanoparticles exhibit excellent properties and application opportunities rather than inorganic nanoparticles alone. In lignin-mediated metal or metal oxide nanoparticles, the lignin and metal or metal oxide nanoparticles maintain their fundamental characteristics on the nano-scale, and they are easily adaptable to align with the overall properties as needed. The lignin served the dual role of reducing and capping agent in the fabrication of hybrid metal or metal oxide nanoparticles namely Ag, Au, Te, Cu\u003csub\u003e2\u003c/sub\u003eO, and ZnO [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Nonetheless, some research works have presented the possibility of using lignin-metal nanoparticles in various applications namely biomedical, semiconductor nanomaterials etc. In particular, lignin-ZnO hybrid nanoparticle has appeared as one of the most favorable wide-band-gap inorganic materials and shows great promise across various applications, including fluorescence imaging, photocatalytic activities, bacterial activity inhibitor, anticancer agent, etc. [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. Liqiang Xiao et al. synthesized hybrid particles known as lignin-zinc oxide (LZn). These particles consist of lignin nanoparticles (LNP) and zinc oxide (ZnO), and were synthesized using a straightforward hydrothermal method. The LZn hybrid nanoparticle displayed excellent antibacterial activity against E. coli and S. aureus [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Klapiszewski et al. produced blow-molded containers from polyethylene, incorporating a hybrid material of ZnO/lignin with varying lignin concentrations. These containers demonstrated notable antimicrobial efficacy, particularly effective against Gram-positive bacteria, including Staphylococcus and Bacillus, in the conducted tests [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. Rather than antimicrobial efficacy, the nanoparticles have also been used for hazardous dye degradation [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Because of the photocatalytic phenomenon of nanoparticles with typical wide band gaps and convenient excitation binding energy at ordinary temperatures can be oxidized dye molecules and produce degraded products with lower toxicity [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. S. Venkatesan and coworkers demonstrated the significant photocatalytic capabilities of zinc oxide nanoparticles (ZnO NPs) in decomposing methylene blue dye, achieving an impressive efficiency of 94.07% when exposed to sunlight [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. ES Baeissa investigated the photocatalytic efficiency of ZnO nanoparticles when exposed to visible light in the presence of methylene blue dye. His synthesized ZnO NPs could oxidize 100% methylene blue dye within 30 min [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eNumerous scientific findings indicate that the photocatalytic and antibacterial activities of lignin-based ZnO NPs depend on their particle size, shape, source of lignin, method of hybrid metal or metal oxide nanoparticles synthesis etc. [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. In particular, the morphology, chemical and structural complexity, and variability of lignin from various sources have a stimulatory effect on the properties of nanoparticles. Although much of the earlier research included the development of ZnO NPs using various lignin sources, the coconut husk lignin is still unexplored in literature. Therefore, the objective of our research is to synthesize ZnO nanoparticles using coconut husk lignin, where lignin acts both as a reducing and capping agent. Lignin can be extracted by various processes like hydrothermal, sono-chemical, alkali hydrolysis etc. The depolymerization of lignin molecules is happened at high pressure and temperature during hydrothermal treatment [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. In addition, lignin is readily decomposed in presence of highly alkaline medium [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. On the other hand, comparatively better form of lignin attains at sonochemical extraction but the yield percentage of lignin is very low [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. Accordingly, the sonochemical methods have been tuned to extract a high percentage of lignin from coconut husk in our investigation. The reduction reaction of lignin to form L-ZnO NPs has also been performed with controlled conditions (pH, temperature, time, and agitation). Subsequently, the antibacterial efficacy of lignin-capped ZnO nanoparticles has been evaluated by the well diffusion method against gram-negative \u003cem\u003eE. coli\u003c/em\u003e and gram-positive \u003cem\u003eS. aureus\u003c/em\u003e bacteria. Additionally, the photocatalytic activity of the synthesized nanoparticles has been assessed by the degradation of methylene blue dye under UV radiation.\u003c/p\u003e"},{"header":"Experimental","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eMaterials\u003c/h2\u003e \u003cp\u003eDried coconut husk was obtained from Kushtia district, Bangladesh. The husk was crashed into fine powder and stored in the desiccator. Demineralized water was obtained from Rahimafrooz, Bangladesh Ltd. Toluene, ethanol, KOH, acetic acid, HCl, polyethylene glycol, zinc acetate dihydrate were purchased from Merck, Germany. Methylene blue dye was manufactured by Sigma-Aldrich.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003eMethods\u003c/h2\u003e \u003cdiv id=\"Sec5\" class=\"Section3\"\u003e \u003ch2\u003eLignin extraction\u003c/h2\u003e \u003cp\u003e10 g of dried coconut husk powder was refluxed with a mixture of toluene and ethanol (toluene: ethanol\u0026thinsp;=\u0026thinsp;2:1 v/v) at 85\u0026deg;C temperature. The dewaxed husk was then dried in the oven at 60℃ and sonicated with KOH solution at 40 kHz frequency for 30 min using Biobase UC-40A ultrasonicator. The solution was then stirred for 1 h and filtered on a nylon cloth. The pH of the filtrate was adjusted to 5.5 using acetic acid solution. Then the hemicelluloses were precipitated with 3 volumes of 95% ethanol and separated by filtration. The filtrate was concentrated to one-third of its initial volume by evaporating the ethanol. The acid-insoluble lignin was then precipitated from the solution at pH 1.5 adjusted by 6M HCl solution. The precipitated lignin was then washed 3 times with demineralized water and dried at 60\u0026deg;C. The experiment was repeated varying the sonication period for 60 min and 240 min.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003eLignin- ZnO nanoparticle synthesis\u003c/h2\u003e \u003cp\u003e0.1 g lignin was added to 0.1M 100mL of NaOH solution and sonicated for 1 h to obtain a homogeneous solution. The homogenized solution was stirred, maintaining temperature in the range of 80 to 90\u0026deg;C. Then 2 g of zinc acetate dihydrate salt was added to the solution in the duration of 30 min. and again stirred for 1 h. A white precipitate was formed. The precipitate was washed with deionized water and then washed 3 times with absolute ethanol to wash off the remnant Lignin and NaOH. The formed precipitate underwent drying in an oven at 90\u0026deg;C for one hour, followed by calcination in a muffle furnace at 500\u0026deg;C for duration of 3 h.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003eMeasurements\u003c/h2\u003e \u003cp\u003eThe isolated alkali soluble lignin from coconut husk as well as L-ZnO NPs were characterized by Shimadzu UV1900i UV-vis. spectrophotometer at a wavelength range of 255 nm to 510 nm and 330 nm to 500 nm respectively.\u003c/p\u003e \u003cp\u003eFTIR (ATR) spectra of the lignin, L-ZnO NPs and precursor (zinc acetate dihydrate) were measured using an FTIR-ATR Instrument from SHIMADZU (IRAffinity-1S) with a resolution of 4 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in the wavenumber range 400 to 4000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe surface morphology and particle size of lignin, L-ZnO NPs and zinc acetate dihydrate were examined by ZEISS Sigma 500 VP Field Emission Scanning Electron Microscope operating at 5 kV for 20 thousand and 50 thousand times magnification levels.\u003c/p\u003e \u003cp\u003eCrystallographic study of lignin, zinc acetate dihydrate and L-ZnO NPs were examined using Rigaku Ultima IV XRD equipment operating at 40 kV with CuKα(λ\u0026thinsp;=\u0026thinsp;1.5406\u0026Aring;) radiation. The samples were scanned within the 2θ range of 5\u0026deg;-80\u0026deg;.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eAntibacterial test\u003c/h2\u003e \u003cp\u003eThe antibacterial properties of the L-ZnO NPs, extracted lignin, commercial ZnO and zinc acetate dihydrate were evaluated using the well diffusion technique. The BP-ZnO NPs was taken from our previous study for comparing the antibacterial properties. Bacterial strains, \u003cem\u003eEscherichia coli\u003c/em\u003e O157:H7 and \u003cem\u003eStaphylococcus aureus\u003c/em\u003e MTCC 3160 were used in this study which was previously reported as multi-drug resistant strains [\u003cspan additionalcitationids=\"CR26\" citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. Bacterial inoculum was prepared by culturing in Luria\u0026ndash;Bertani (LB) broth medium for a duration of 18\u0026ndash;20 hours to achieve a turbidity equivalent to 0.5 McFarland standard, corresponding to 1.5\u0026times;10\u003csup\u003e8\u003c/sup\u003e colony-forming units (CFU)/ml. Mueller Hinton agar (MHA) plates with a diameter of 150 mm were formulated, and bacterial inoculum was evenly spread across their surfaces. Wells of 7-mm diameter were generated in the MHA plates using a sterile Aluminum borer. Each well was filled with 70 \u0026micro;l of L-ZnO NPs, extracted lignin, commercial ZnO and zinc acetate dihydrate solution at concentration of 0.1 mg/ml, where streptomycin was used as a control for \u003cem\u003eE. coli\u003c/em\u003e and Amoxicillin was for \u003cem\u003eS. aureus\u003c/em\u003e respectively. The plates were placed in an incubator at 37\u0026deg;C for 24 h. Measurements of the inhibition zones surrounding the wells were taken in millimeters (mm). The experiment was conducted three times to ensure reliability, and the results are presented as the mean value along with the standard deviation.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003ePhotocatalytic activity\u003c/h2\u003e \u003cp\u003e70 ml aqueous solution of methylene blue dye of 2 ppm concentration was prepared. 0.045 g of L-ZnO NPs was added to it and stirred for mixing. The blue solution gradually turned green when subjected to UV radiation with an intensity of 0.7821 \u003cem\u003emW\u003c/em\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({cm}^{-2}\\)\u003c/span\u003e\u003c/span\u003e. After every 15 min, 10 ml solution was withdrawn from the reaction beaker. The UV absorbance of the solutions were measured at 650 nm and 355 nm wavelengths using Shimadzu UV1900i UV-vis. spectrophotometer. The conductivity of the degraded dye solutions were also measured using the HANNA edge HI2003 conductivity meter.\u003c/p\u003e \u003c/div\u003e"},{"header":"Results and Discussion","content":"\u003cp\u003eIn this study, the sonochemical method has been developed to extract lignin from coconut husk. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e depicts the yield of extracted lignin with sonication time. The observation indicates that as the sonication time increases, there is a corresponding rise in the percentage of lignin yield. The ultrasound induces mechanical effects on fiber cell walls, leading to enhanced accessibility and extractability of the lignin and cellulose components. The breaking of the ether linkages in-between lignin and celluloses from the cell walls of husk has stronger impact during extraction at alkaline medium by ultrasonic irradiation [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. The lignin is identified by UV-vis spectroscopy, FTIR, FE-SEM, and XRD analysis.\u003c/p\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eMorphological and structural characterization of lignin and L-ZnO NPs\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(a) illustrates the UV absorption spectrum of lignin precipitation isolated from coconut husk. The lignin sample's absorbance was measured within the wavelength range from 255 nm to 510 nm. Clearly, the sample exhibits characteristic absorption peaks featuring two maxima (λ\u003csub\u003emax\u003c/sub\u003e) at 320 nm and 280 nm associated with lignin molecule. The absorption peak at 280 nm is attributed to nonconjugated phenolic groups present in the lignin, while the absorption maximum at 320 nm is identified as originating from bound hydroxycinnamic acids like para-coumaric acid and ferulic acids [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. The higher peak intensity at 320 nm suggests the lignin sample is rich in para-coumaric acid fractions.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(b) represents the UV\u0026ndash;visible absorption spectrum of the L-ZnO NPs that has been recorded in the wavelength range of 330\u0026ndash;500 nm. From the spectrum it can be observed that the absorption maximum appears at 367 nm which is the characteristic peak for L-ZnO NPs. Similar results have been reported by the previous studies [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. Lignin possesses an overall negative surface charge which can be explained by the presence of negatively charged ionic groups such as hydroxyl, carboxyl and phenolics on the surface of lignin. Due to this character, lignin functions as a reducing agent in the fabrication of ZnO nanoparticle composite structure. Phenolic hydroxyl groups are reactive entities susceptible to oxidation, leading to the formation of quinones releasing hydrogen ions and electrons that reduce zinc ions and stabilize ZnO nanoparticles. Because of this electrostatic attraction force between metal oxide and polymer molecules, lignin is able to cap the surface of ZnO nanoparticles and prevent them from agglomeration and thus stabilizes the nanoparticles [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e represents the FTIR spectra of lignin, zinc acetate dihydrate and L-ZnO NPs. The absorption bands in lignin such as ‒OH stretching of phenolic compounds (3400\u0026ndash;3780 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), C‒H stretching in methylene and methyl groups (2925\u0026ndash;2930 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) indicating lignin has abundant side chain structure, unconjugated ketone and carboxyl group stretching (1700\u0026ndash;1730 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), conjugated carbonyl stretching in lignin (1600\u0026ndash;1660 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), ring associated with C‒O stretching (1220\u0026ndash;1240 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), non-esterified phenolic ‒OH group in lignin (1110 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), C‒H deformation (610\u0026ndash;660 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) were detected at 3431, 3695, 3770, 2927, 1720, 1620, 1224, 1111, and 651 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e respectively [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe absorption bands in zinc acetate dihydrate such as symmetrical stretching vibration of carboxylate group (1420\u0026ndash;1450\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({cm}^{-1}\\)\u003c/span\u003e\u003c/span\u003e), asymmetrical stretching vibrations of the carboxylate group (1555\u0026ndash;1565 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) were detected at 1446 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 1558 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e respectively [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. The absorption bands in L-ZnO NPs alike ZnO wurtzite structure (441\u0026ndash;665 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), vibrational stretching mode of Zn‒O bounding (470\u0026ndash;485 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) is detected at 673 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 478 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e respectively. The appearance of 3772, 3697, 3435, 1716, 1618, 1111, 673, and 478 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e absorption bands in the L- ZnO NPs structure indicates the binding of ZnO nanoparticles with lignin without significantly altering the structure of lignin [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe surface morphology of coconut husk lignin can be seen from the Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a) and 4(b) at 20000\u0026times; and 50000\u0026times; magnification levels, respectively. From the images it is evident that the sample is composed of spherical particles facilitating a large surface area. The particles appear to be regular in shape, well separated and uniformly dispersed which is reported in previous study [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]. No presence of additional substances appeared in the images. The average diameter of lignin particles is calculated to be 161.55 nm and the particle size distribution of lignin is represented in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(b). The micrograph of zinc acetate dihydrate shows that the sample exhibits particles with irregular shapes and a distribution that is not uniform. The sample resembles a stratified surface with cracks. The surface morphology of L-ZnO NPs is represented in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(e) and 4(f). The particle size distribution of the L-ZnO NPs is represented in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a) and the average particle diameter of the composite nanomaterial is calculated to be 77.76 nm. From the micrographs it can be seen that, L-ZnO NPs with a nearly spherical morphology were almost equally dispersed in the sample made by encapsulating ZnO NPs in the voids created by the lignin macromolecular matrix. Nanoparticles are evenly distributed throughout the cavities created by the macromolecule aggregates, which increases their stability and prevents them from aggregating. These characteristics are in accordance with prior research [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. The L-ZnO NPs show a porous surface possibly due to the high temperature heat treatment in absence of oxygen that increases surface area for each particle and thus increases the attachment sites on bacterial cell walls. The interfacial adhesion between lignin and ZnO nanoparticles proves the affinity of lignin towards ZnO nanoparticles.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e represents the XRD pattern of lignin, zinc acetate dihydrate and L-ZnO NPs. The diffractogram shows a wide area under the curve that confirms the amorphous characteristics of coconut husk lignin. The maximum value of 2θ for coconut husk lignin has appeared at 22.64 degrees. Slight variation in the value of 2θ for lignin may occur according to the source of biomass, chemicals used, and method of extraction followed [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. From Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(b), two characteristic reflections for Zn have been encountered at 2θ values of 36.10 and 44.44 respectively. These reflections are representative of the hexagonal structure of Zn [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(c) shows the presence of ZnO in L-ZnO NPs. The reflections located at 2θ values of 31.67, 34.27, 36.11, 47.39, 56.45, 62.67, 66.33, 67.85, 68.93, 72.55, and 76.95 degrees correspond to (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2), (2 0 1), (0 0 4), and (2 0 2) planes respectively [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. The characteristic reflections at these 2θ values represent hexagonal wurtzite phase of ZnO nanoparticles having highly polycrystalline character [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. The crystallite size for L-ZnO NPs is calculated using Scherrer equation [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e],\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$D=\\frac{K\\lambda }{\\beta Cos\\theta }\\dots \\dots \\dots \\dots .\\left(1\\right)$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eWhere, D is the crystallite size in angstrom; Scherrer constant, K\u0026thinsp;=\u0026thinsp;0.94; Wavelength of X-ray, λ\u0026thinsp;=\u0026thinsp;1.5406 \u0026Aring;; β, full width at half maximum (FWHM) in radian and θ, half of Bragg\u0026rsquo;s angle in degrees. The reflections from various planes of the L-ZnO NPs with respect to Bragg\u0026rsquo;s angle are represented in Table \u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e signifies the data of 9 characteristic reflections used to calculate the average crystallite size of L-ZnO NPs. The average crystallite size is found to be 12.23 nm. The intensity of the reflection due to the (1 0 1) plane is notably higher as compared to other planes. This suggests that the lignin stabilized ZnO nanoparticles are enriched in (1 0 1) facets [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. The crystallinity index for L-ZnO NPs is calculated to be approximately 93.41%. The crystalline characters are well preserved in the encapsulated product. The diffractogram of L-ZnO NPs also indicates that the synthesized nanoparticles are highly pure.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003eAntibacterial activity evaluation\u003c/h2\u003e \u003cp\u003eThe antibacterial activities in terms of zone of inhibition (mm) of zinc acetate dihydrate, commercial ZnO, lignin and L-ZnO NPs compared to Streptomycin and Amoxicillin against gram-negative \u003cem\u003eE. coli\u003c/em\u003e and gram-positive \u003cem\u003eS. aureus\u003c/em\u003e bacteria are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e and Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. Zinc acetate dihydrate did not create any ZOI against \u003cem\u003eE. coli\u003c/em\u003e but showed 14 mm ZOI against \u003cem\u003eS. aureus\u003c/em\u003e. Commercial ZnO did not show any ZOI against \u003cem\u003eE. coli\u003c/em\u003e but form 14.5 mm ZOI against \u003cem\u003eS. aureus\u003c/em\u003e. Coconut husk lignin did not show any ZOI against any of these bacteria. The L-ZnO NPs made 13.5 mm ZOI against \u003cem\u003eE. coli\u003c/em\u003e and 22 mm ZOI against \u003cem\u003eS. aureus\u003c/em\u003e. Streptomycin showed 18.5 mm ZOI against \u003cem\u003eE. coli\u003c/em\u003e and Amoxicillin showed 12 mm ZOI against \u003cem\u003eS. aureus\u003c/em\u003e. Consequently, the result clearly indicates that the L-ZnO NPs are sensitive against both gram-negative \u003cem\u003eE. coli\u003c/em\u003e and gram-positive \u003cem\u003eS. aureus\u003c/em\u003e bacteria, proving its broad-spectrum activity. The antibacterial activity of L-ZnO NPs operates through induced oxidative stress. L-ZnO NPs engage with water, generating various reactive oxygen species (ROS), such as singlet oxygen, superoxide anion (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(*{O}_{2}^{-}\\)\u003c/span\u003e\u003c/span\u003e), peroxide anion (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(*{O}_{2}^{2-}\\)\u003c/span\u003e\u003c/span\u003e), hydroxyl radicals (*OH) and hydrogen peroxide (H₂O₂) [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e, \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]. Within the cell body, ZnO nanoparticles infiltrate, with peroxide anions (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(*{O}_{2}^{2-}\\)\u003c/span\u003e\u003c/span\u003e) carrying a negative charge remaining on the cell surface. The release of Zn\u0026sup2;⁺ is triggered when L-ZnO NPs assemble in the cytoplasm or outer membrane of bacterial cells, potentially leading to cell membrane disintegration, damage to membrane proteins, and genomic instability [\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e, \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]. Oxidative stress ensues within the bacterial cell due to the interaction between Zn⁺ ions and the thiol group of bacterial respiratory enzymes. The abundant production of reactive oxygen species contributes to bacterial cell damage and eventual death. ROS plays a pivotal role in several mechanisms, including localized interaction with L-ZnO NPs causing cell wall damage, increased membrane permeability, NPs internalization due to the loss of proton motive force, and the uptake of toxic dissolved zinc ions. These processes result in weakened mitochondria, intracellular outflow, and alterations in gene expression related to oxidative stress, ultimately leading to inhibited cell growth and cell death. Additionally, the enhanced antibacterial activity can be attributed to abrasive surface texture formed by surface defects on the L-ZnO NPs in certain cases [\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]. Hence, the L-ZnO NPs can inhibit food-borne harmful pathogens (both gram-positive and gram-negative bacteria).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003ePhotocatalytic activity evaluation\u003c/h2\u003e \u003cp\u003eThe photocatalytic performance of the L-ZnO NPs was examined in the methylene blue (C\u003csub\u003e16\u003c/sub\u003eH\u003csub\u003e18\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003eSCl) dye degradation under UV light irradiation in an aqueous medium. The UV-vis absorption spectra and conductivity of the degraded methylene blue (MB) dye solution were measured at different durations and are represented in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(a) and 8(b) respectively. Since the MB dye exhibits a strong absorption band at 650 nm (λ\u003csub\u003emax\u003c/sub\u003e), the photocatalytic activity of L-ZnO NPs was monitored by the change of absorbance peak at 650 nm on UV\u0026ndash;vis spectroscopy. The degraded dye solution shows another absorption band at 355 nm (λ\u003csub\u003emax\u003c/sub\u003e) due to the presence of L-ZnO NPs. The photocatalytic degradation behavior has been investigated for 15, 30, 45, 60, 75, and 90 min. It has been observed that the absorption intensity decreased gradually (at 650 nm) with the increment of UV light irradiation time. The evanescence of the methylene blue dye during the decolorization process indicates a gradual reduction in the concentration of methylene blue in the solution. On the other hand, the absorbance at 355 nm is slightly decreased. The previous studies stated that the exposure to light in the presence of the photocatalyst led to the destruction of the azo bonds and aromatic rings within the MB molecules [\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]. The photo degradation of methylene blue (MB) dye with time using L-ZnO NPs has also been assessed by conductivity measurements. Figure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(b) shows the change of conductivity of the dye solution with the increases of UV light irradiation time. It has been found that the conductivity of degraded solution is higher than virgin MB solution. With the increase of light projection time, more degraded derivatives or ions were formed [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]. Hence, the conductivity of the solution becomes higher.\u003c/p\u003e \u003cp\u003eThe photocatalytic degradation of methylene blue (MB) dye involves the generation of electron-hole pairs in L-ZnO NPs when exposed to UV light. As the ZnO photocatalyst is illuminated with photons possessing energy equal to or greater than its band-gap energy, electrons are excited from the valence band (VB) to the conduction band (CB), creating electron-hole pairs on the surface of L-ZnO NPs, with holes (h+) remaining in the conduction band. The band gap energy and electronic configuration of L-ZnO NPs are crucial factors influencing the photocatalytic degradation of the MB dye when exposed to UV-radiation. Because of the elevated recombination rate of photoinduced charge carriers (electron-hole (e\u003csup\u003e\u0026minus;\u003c/sup\u003e - h\u003csup\u003e+\u003c/sup\u003e) pairs), there is a significant enhancement in the photocatalytic activity of L-ZnO NPs. The series of reactions occurring during the process of photocatalytic degradation under UV radiation can be expressed by the following chemical reactions [\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e, \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e]:\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003eL-ZnO NPs + \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({h\\nu }_{\\left(UV\\right)}\\)\u003c/span\u003e\u003c/span\u003e\u0026rarr; L-ZnO NPs + \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({h}_{VB}^{+}\\)\u003c/span\u003e\u003c/span\u003e+ \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({e}_{CB}^{-}\\)\u003c/span\u003e\u003c/span\u003e\u003c/h2\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\({H}_{2}O\\)\u003c/span\u003e \u003c/span\u003e + \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({h}_{VB}^{+}\\)\u003c/span\u003e\u003c/span\u003e \u0026rarr; \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({OH}^{*}\\)\u003c/span\u003e\u003c/span\u003e + \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({H}^{+}\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\({h}_{VB}^{+}\\)\u003c/span\u003e \u003c/span\u003e + \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({OH}^{-}\\)\u003c/span\u003e\u003c/span\u003e\u0026rarr; \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({OH}^{*}\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\({O}_{2}\\)\u003c/span\u003e \u003c/span\u003e + \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({e}_{CB}^{-}\\)\u003c/span\u003e\u003c/span\u003e \u0026rarr; \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({O}_{2}^{*}\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\({O}_{2}^{*}\\)\u003c/span\u003e \u003c/span\u003e + \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({H}^{+}\\)\u003c/span\u003e\u003c/span\u003e \u0026rarr; \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({HO}_{2}^{*}\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003e2\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({HO}_{2}^{*}\\)\u003c/span\u003e\u003c/span\u003e \u0026rarr; \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({H}_{2}{O}_{2}\\)\u003c/span\u003e\u003c/span\u003e + \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({O}_{2}\\)\u003c/span\u003e\u003c/span\u003e\u003c/h2\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\({H}_{2}{O}_{2}\\)\u003c/span\u003e \u003c/span\u003e + \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({e}_{CB}^{-}\\)\u003c/span\u003e\u003c/span\u003e \u0026rarr; \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({OH}^{*}\\)\u003c/span\u003e\u003c/span\u003e + \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({OH}^{-}\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\({OH}^{*}\\)\u003c/span\u003e \u003c/span\u003e \u003cem\u003e+ MB dye \u0026rarr; Degraded Product +\u003c/em\u003e \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({CO}_{2}\\)\u003c/span\u003e\u003c/span\u003e + \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({H}_{2}O\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusion","content":"\u003cp\u003eIn this study, the bioreducing properties of lignin on metal oxide NPs synthesis and its role as stabilizing agent have been discussed. The lignin was extracted by eco-friendly and economically viable bioreduction route eco-friendly and economically viable route coconut husk fiber using sonochemical method. The absorption peaks of lignin appeared at (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\lambda }_{max})\\)\u003c/span\u003e\u003c/span\u003e 280 nm and 320 nm in the UV spectrum corresponding to nonconjugated phenolic groups and bound hydroxycinnamic acid, such as para-coumaric acid and ferulic acids. L-ZnO NPs were synthesized from zinc acetate dehydrate salt by the reducing reaction of extracted lignin. The UV-vis. absorption peak at 367 nm confirms the formation of ZnO nanoparticles. The stretching vibration of phenolic compounds, unconjugated ketone and carboxyl group, conjugated carbonyl group and phenolic-OH group indicates the presence of lignin in the composite structure and vibrational stretching of Zn-O bounding indicates the presence of ZnO in the composite structure through FTIR analysis. The average particle diameter of 77.76 nm and perforated pumice-like surface morphology of the composite where lignin particles were agglomerated on ZnO surface were visualized by scanning electron micrograph. The X-ray diffractogram confirms crystalline structure due to the hexagonal phase of ZnO with high intensity of (1 0 1) plane as well as amorphous regions due to the presence of macromolecular structure of lignin in the composite material. The synthesized L-ZnO NPs with prerequisite dimension and crystallinity shows excellent bactericidal action and dye degradation characteristics. The zone of inhibition measurement indicates the marvelous bactericidal effects of the L-ZnO NPs against \u003cem\u003eE. coli\u003c/em\u003e and \u003cem\u003eS. aureus\u003c/em\u003e bacteria with respect to standard controls proving its broad-spectrum antibacterial activity. The L-ZnO NPs showed tremendous photocatalytic activity in presence of UV light on methylene blue dye degradation. From the discussion it can be concluded that the valuable biopolymer lignin is being utilized very inefficiently and often discarded as an industrial waste product that can be recovered from the biomass and utilized efficiently for noble purposes by industrially optimizing the recovery process. The further studies will investigate the bacterial response to nanoparticle stress, specifically examining bacterial morphology and the association of nanoparticles with bacterial cells. The detail studies would provide valuable insights like the mechanism through which nanoparticles anchor onto the cell surface, penetrate the cell wall, and potentially induce the death of bacterial cells by disrupting their cell walls.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eStatement of Novelty\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAlthough much of the earlier\u0026nbsp;research\u0026nbsp;included the development of ZnO NPs using various lignin sources, the coconut husk lignin is still unexplored in literature. Therefore, in the present research,\u0026nbsp;ZnO nanoparticles have been synthesized using coconut husk lignin, where lignin acts both as a reducing and capping agent. Lignin can be extracted by various processes like hydrothermal, sono-chemical, alkali hydrolysis etc. The depolymerization of lignin molecules is happened at high pressure and temperature during hydrothermal treatment. Moreover, lignin can be readily decomposed\u0026nbsp;in presence of highly\u0026nbsp;alkaline medium. However, comparatively better form of lignin attains at sonochemical extraction but the yield percentage of lignin is very low. To get high lignin yield percentage, the sonochemical methods have been tuned in our investigation. The reduction reaction of lignin to form L-ZnO NPs has also been performed with controlled\u0026nbsp;conditions\u0026nbsp;(pH, temperature, time, and agitation).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthics approval and consent to participate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eConsent for publication\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003cstrong\u003eAvailability of data and materials\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll authors do declare that there are no competing interests.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003cstrong\u003eAcknowledgement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors would like to acknowledge Chairman, department of Biotechnology and Genetic Engineering, Islamic University Bangladesh for instrumental support.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eAuthor Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eMd. Hasnain Mustak: Methodology, Validation, Formal Analysis, Investigation, Data Curation, Writing \u0026ndash; Original Draft Preparation.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eKazi Suraiya Islam: Conceptualization, Methodology, Validation, Formal Analysis, Investigation, Data Curation, Writing \u0026ndash; Original Draft Preparation\u003c/p\u003e\n\u003cp\u003eMd. Shamsul Alam: Technological support, Writing \u0026ndash; Review, Visualization\u003c/p\u003e\n\u003cp\u003eMd. Minnatul Karim: Technological support, Writing \u0026ndash; Review, Visualization\u003c/p\u003e\n\u003cp\u003eGazi Md. Arifuzzaman Khan*: Conceptualization, Methodology, Validation, Formal Analysis, Resources, Data Curation, Writing \u0026ndash; Review and Editing, Visualization, Supervision, Project Administration\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was financially supported by the special allocation project\u0026nbsp;(Financial Year 2020-2021,\u0026nbsp;G. O. No.: 39.00.0000.009.14.011.20-EAS 406/1742\u0026nbsp;EAS date 08/12/2020) of the\u0026nbsp;Ministry of Science and Technology, Government of the People\u0026rsquo;s Republic of Bangladesh.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eZoghlami A, Pa\u0026euml;s G (2019) Lignocellulosic Biomass: Understanding Recalcitrance and Predicting Hydrolysis. 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Front Bioeng Biotechnol 10:1\u0026ndash;17. https://doi.org/10.3389/fbioe.2022.846592\u003c/li\u003e\n\u003cli\u003ePińkowska H, Wolak P, Złocińska A (2012) Hydrothermal decomposition of alkali lignin in sub- and supercritical water. Chem Eng J 187:410\u0026ndash;414. https://doi.org/10.1016/j.cej.2012.01.092\u003c/li\u003e\n\u003cli\u003eShah TA, Khalid S, Nafidi H-A, et al (2023) Sodium Hydroxide Hydrothermal Extraction of Lignin from Rice Straw Residue and Fermentation to Biomethane. Sustainability 15:. https://doi.org/10.3390/su15118755\u003c/li\u003e\n\u003cli\u003eSubhedar PB, Gogate PR (2014) Alkaline and ultrasound assisted alkaline pretreatment for intensification of delignification process from sustainable raw-material. 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J Polym Environ. https://doi.org/10.1007/s10924-022-02406-4\u003c/li\u003e\n\u003cli\u003eDinh Vu N, Thi Tran H, Bui ND, et al (2017) Lignin and Cellulose Extraction from Vietnam\u0026rsquo;s Rice Straw Using Ultrasound-Assisted Alkaline Treatment Method. Int J Polym Sci 2017:1063695. https://doi.org/10.1155/2017/1063695\u003c/li\u003e\n\u003cli\u003eShe D, Nie XN, Xu F, et al (2012) Physico-chemical characterization of different alcoholsoluble lignins from rice straw. Cellul Chem Technol 46:207\u0026ndash;219\u003c/li\u003e\n\u003cli\u003eSingh DK, Pandey DK, Yadav RR, Singh D (2012) A study of nanosized zinc oxide and its nanofluid. Pramana 78:759\u0026ndash;766. https://doi.org/10.1007/s12043-012-0275-8\u003c/li\u003e\n\u003cli\u003eBudnyak TM, Slabon A, Sipponen MH (2020) Lignin\u0026ndash;Inorganic Interfaces: Chemistry and Applications from Adsorbents to Catalysts and Energy Storage Materials. 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Prog Org Coatings 74:221\u0026ndash;227. https://doi.org/https://doi.org/10.1016/j.porgcoat.2011.12.012\u003c/li\u003e\n\u003cli\u003eTalam S, Karumuri SR, Gunnam N (2012) Synthesis, Characterization, and Spectroscopic Properties of ZnO Nanoparticles. ISRN Nanotechnol 2012:372505. https://doi.org/10.5402/2012/372505\u003c/li\u003e\n\u003cli\u003eJuikar SJ, Vigneshwaran N (2017) Extraction of nanolignin from coconut fibers by controlled microbial hydrolysis. Ind Crops Prod 109:420\u0026ndash;425. https://doi.org/https://doi.org/10.1016/j.indcrop.2017.08.067\u003c/li\u003e\n\u003cli\u003eArakha M, Saleem M, Mallick BC, Jha S (2015) The effects of interfacial potential on antimicrobial propensity of ZnO nanoparticle. Sci Rep 5:9578. https://doi.org/10.1038/srep09578\u003c/li\u003e\n\u003cli\u003eRajesh Kumar, Ahmad Umar, Girish Kumar, Nalwa HS (2017) Antimicrobial properties of ZnO nanomaterials: A review. Ceram Int 43:3940\u0026ndash;3961. https://doi.org/https://doi.org/10.1016/j.ceramint.2016.12.062\u003c/li\u003e\n\u003cli\u003eSirelkhatim A, Mahmud S, Seeni A, et al (2015) Review on Zinc Oxide Nanoparticles: Antibacterial Activity and Toxicity Mechanism. Nano-Micro Lett 7:219\u0026ndash;242. https://doi.org/10.1007/s40820-015-0040-x\u003c/li\u003e\n\u003cli\u003eJiang S, Lin K, Cai M (2020) ZnO Nanomaterials: Current Advancements in Antibacterial Mechanisms and Applications. Front Chem 8:1\u0026ndash;5. https://doi.org/10.3389/fchem.2020.00580\u003c/li\u003e\n\u003cli\u003ePudukudy M, Yaakob Z (2015) Facile Synthesis of Quasi Spherical ZnO Nanoparticles with Excellent Photocatalytic Activity. J Clust Sci 26:1187\u0026ndash;1201. https://doi.org/10.1007/s10876-014-0806-1\u003c/li\u003e\n\u003cli\u003eJain N, Bhargava A, Panwar J (2014) Enhanced photocatalytic degradation of methylene blue using biologically synthesized \u0026ldquo;protein-capped\u0026rdquo; ZnO nanoparticles. Chem Eng J 243:549\u0026ndash;555. https://doi.org/https://doi.org/10.1016/j.cej.2013.11.085\u003c/li\u003e\n\u003cli\u003eAlshehri AA, Malik MA (2019) Biogenic fabrication of ZnO nanoparticles using Trigonella foenum-graecum (Fenugreek) for proficient photocatalytic degradation of methylene blue under UV irradiation. J Mater Sci Mater Electron 30:16156\u0026ndash;16173. https://doi.org/10.1007/s10854-019-01985-8\u003c/li\u003e\n\u003cli\u003eFouda A, Salem SS, Wassel AR, et al (2020) Optimization of green biosynthesized visible light active CuO/ZnO nano-photocatalysts for the degradation of organic methylene blue dye. Heliyon 6:e04896. https://doi.org/10.1016/j.heliyon.2020.e04896\u003cstrong\u003e\u003cstrong\u003e\u003cstrong\u003e\u003c/strong\u003e\u003c/strong\u003e\u003c/strong\u003e\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Tables","content":"\u003cp\u003e\u003cimg src=\"https://myfiles.space/user_files/122228_c8a1650c59388082/122228_custom_files/img171992139087.png\"\u003e\u003c/p\u003e\n\u003cp\u003e\u003cimg src=\"https://myfiles.space/user_files/122228_c8a1650c59388082/122228_custom_files/img1719921391.png\"\u003e\u003c/p\u003e\n\u003cp\u003e\u003cimg src=\"https://myfiles.space/user_files/122228_c8a1650c59388082/122228_custom_files/img1719921390.png\"\u003e\u003cbr\u003e\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"waste-and-biomass-valorization","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"wave","sideBox":"Learn more about [Waste and Biomass Valorization](http://link.springer.com/journal/12649)","snPcode":"12649","submissionUrl":"https://submission.nature.com/new-submission/12649/3","title":"Waste and Biomass Valorization","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Coconut husk lignin, Bioreducing agent, Lignin-ZnO nanoparticles, Antibacterial activity, Photocatalytic property","lastPublishedDoi":"10.21203/rs.3.rs-4458077/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4458077/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eLignin, due to its structural diversity and biodegradability has emerged as a promising alternative to hazardous synthetic materials in multidisciplinary research area. The aim of this paper is to evaluate the lignin potential in value-added applications to be exact zinc oxide nanoparticle synthesis. First, lignin was extracted from coconut husk via sonochemical route in alkaline medium. Afterward, lignin-zinc oxide hybrid nanoparticles (L-ZnO NPs) were synthesized by the reaction of zinc acetate dihydrate salt and lignin, where lignin acts as bioreducing and capping agent. The synthesized lignin and L-ZnO NPs were characterized by UV-Vis spectroscopy, FTIR Spectroscopy, XRD and FESEM analysis. All the identification tests supported the existence of lignin and L-ZnO NPs. The XRD and SEM images disclosed the formation of hexagonal wurtzite shape L-ZnO NPs having mean diameter\u0026thinsp;\u0026asymp;\u0026thinsp;77.76 nm. The antibacterial efficacies of the L-ZnO NPs were assessed against both gram-negative \u003cem\u003eE. coli\u003c/em\u003e and gram-positive \u003cem\u003eS. aureus\u003c/em\u003e bacteria, employing standard controls and the well diffusion method. The L-ZnO NPs exhibited marvelous bactericidal action against the test microorganisms displaying sufficient zone of inhibition. The photocatalytic performance of the L-ZnO NPs was observed by methylene blue (MB) dye degradation test under UV light irradiation. The UV-Vis absorbance and conductivity tests suggested that the L-ZnO NPs has noticeable photocatalytic efficiency on MB dye degradation. Therefore, the synthesized L-ZnO NPs possess excellent versatile properties and it can be used in medicinal applications and environmental management.\u003c/p\u003e","manuscriptTitle":"Revalorization of coconut husk lignin through ZnO nanoparticles synthesis: antibacterial assay and photocatalytic activities","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-07-15 19:15:08","doi":"10.21203/rs.3.rs-4458077/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"","date":"2024-06-16T11:46:20+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-06-16T11:33:48+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Waste and Biomass Valorization","date":"2024-06-12T19:51:35+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-05-22T13:32:26+00:00","index":"","fulltext":""},{"type":"submitted","content":"Waste and Biomass Valorization","date":"2024-05-22T00:10:05+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"waste-and-biomass-valorization","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"wave","sideBox":"Learn more about [Waste and Biomass Valorization](http://link.springer.com/journal/12649)","snPcode":"12649","submissionUrl":"https://submission.nature.com/new-submission/12649/3","title":"Waste and Biomass Valorization","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"305b2fbe-8c09-4f9e-bf89-ccff2d0f7a29","owner":[],"postedDate":"July 15th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2024-10-28T16:03:04+00:00","versionOfRecord":{"articleIdentity":"rs-4458077","link":"https://doi.org/10.1007/s12649-024-02773-0","journal":{"identity":"waste-and-biomass-valorization","isVorOnly":false,"title":"Waste and Biomass Valorization"},"publishedOn":"2024-10-21 15:57:45","publishedOnDateReadable":"October 21st, 2024"},"versionCreatedAt":"2024-07-15 19:15:08","video":"","vorDoi":"10.1007/s12649-024-02773-0","vorDoiUrl":"https://doi.org/10.1007/s12649-024-02773-0","workflowStages":[]},"version":"v1","identity":"rs-4458077","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4458077","identity":"rs-4458077","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}