Green synthesis of AgNPs using lignocellulose nanofibrils as a reducing and supporting agent

Abstract

A novel green approach was developed for producing silver nanoparticles (AgNPs) using lignocellulose nanofibrils (LCNF). This method does not require any additional reducing agent, and the LCNF itself serves both as a reducing agent and a supporting material. A simple autoclave procedure was employed for the synthesis. The synthetic conditions such as concentrations of reactants and reaction time were optimized. Also, the effect of lignin content in LCNF on the formation of AgNPs was evaluated. Three types of cellulose nanofibrils, i.e., HCNF (0% lignin), LCNF-5 (5% lignin), and LCNF-18 (18% lignin), were employed for the preparation of AgNPs. Three types of AgNPs were obtained and thoroughly characterized using UV-vis, Fourier-transform Infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The results suggest that LCNF can be employed as a green source for the reduction and effective stabilization of AgNPs, but an increased content of lignin can have an adverse effect on the yield of AgNPs. However, the presence of lignin greatly influenced the particle size. Therefore, LCNF with small amounts of lignin (5%) is best for producing AgNPs.

Full Article

Green Synthesis of AgNPs Using Lignocellulose Nanofibrils as a Reducing and Supporting Agent

Jun-Kyu Han,^a Alle Madhusudhan,^b Rajkumar Bandi,^b Chan-Woo Park,^d Jin-Chul Kim,^c Yong-Kyu Lee,^a Seung-Hwan Lee,^d,* and Jong-Myoung Won ^a,*

A novel green approach was developed for producing silver nanoparticles (AgNPs) using lignocellulose nanofibrils (LCNF). This method does not require any additional reducing agent, and the LCNF itself serves both as a reducing agent and a supporting material. A simple autoclave procedure was employed for the synthesis. The synthetic conditions such as concentrations of reactants and reaction time were optimized. Also, the effect of lignin content in LCNF on the formation of AgNPs was evaluated. Three types of cellulose nanofibrils, i.e., HCNF (0% lignin), LCNF-5 (5% lignin), and LCNF-18 (18% lignin), were employed for the preparation of AgNPs. Three types of AgNPs were obtained and thoroughly characterized using UV-vis, Fourier-transform Infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The results suggest that LCNF can be employed as a green source for the reduction and effective stabilization of AgNPs, but an increased content of lignin can have an adverse effect on the yield of AgNPs. However, the presence of lignin greatly influenced the particle size. Therefore, LCNF with small amounts of lignin (5%) is best for producing AgNPs.

Keywords: Lignocellulose nanofibril (LCNF); Green synthesis; Silver nanoparticles; Composition of LCNF

Contact information: a: Department of Paper Science and Engineering, College of Forest and Environmental Sciences, Kangwon National University, Chuncheon, 24341, Republic of Korea; b: Institute of Forest Science, Kangwon National University, Chuncheon, 24341, Republic of Korea; c: Department of Medical Biomaterials Engineering, College of Biomedical Science and Institute of Bioscience and Biotechnology, Kangwon National University, Chuncheon 24341, Republic of Korea; d: Department of Forest Biomaterials Engineering, College of Biomedical Science and Institute of Bioscience and Biotechnology, Kangwon National University, Chuncheon 24341 Republic of Korea;

* Corresponding author: lshyhk@kangwon.ac.kr, wjm@kangwon.ac.kr

INTRODUCTION

Nanostructured materials find a wide range of applications in the fields of optoelectronics, photonics, catalysis, and medicine (Khan et al. 2017; Madhusudhan et al. 2019). Noble metal nanoparticles, especially silver, have gained a large amount of attention on account of their optical, electrochemical properties, and strong antimicrobial effects (Abou El-Nour et al. 2010; Natsuki 2015). Abundant research reports have revealed the outstanding antimicrobial, anti-platelet, anti-inflammatory, anticancer and antiviral properties of silver nanoparticles (AgNPs) (Prabhu and Poulose 2012; Ovais et al. 2016; Pugazhendhi et al. 2018). Besides their antimicrobial nature, AgNPs are also well known for their antiplasmodial, larvicidal, leishmanicidal, scolicidal and pupicidal activity (Suman et al. 2013; Arokiyaraj et al. 2015; Saratale et al. 2017). These fascinating properties have driven researchers to explore several synthetic routes for the production of AgNPs.

Traditionally, physical and chemical methods are widely employed for producing AgNPs. However, the physical process requires a high energy input, and the chemical method is expensive and uses harmful ingredients (Rafique et al. 2017). Alternative methods that ensure sustainability have been developed in recent years. Considerable efforts have been made towards the eco-friendly synthesis with the help of green chemistry principles (Rajoriya et al. 2017). For instance, Saravanan et al. 2018 reported biological synthesis of AgNPs using Bacillus bervis (NCIM 2533) and studied the antibacterial properties against multi-drug resistant pathogens such as Salmonella typhi and Staphylococcus aureus. Oves et al. (2018) used root hair extract of Phoenix dactylifera for the production of AgNPs and studied their antimicrobial and anticancer activity. Similarly, Habibipour et al. 2019 prepared AgNPs using pomegranate peel extract and evaluated their anti-biofilm formation activity against Pseudomonas aeruginosa. The development of biogenic methods generally involves the use of microorganisms and plant extracts (Narayanan and Sakthivel 2010; Ahmed et al. 2016). In comparison to plant extracts, microorganisms present notable disadvantages, such as the time required to obtain them (more than 24 h), health risk, and complications involved in maintaining cultures. Thus, the use of plant extracts in the production of metal nanoparticles is of high interest.

Cellulose is the most abundant fibrous organic biopolymer in nature; it consists of β-1,4-linked anhydro-D-glucose units. It is the chief structural component of plants and is also found in marine animals, algae, fungi, bacteria, invertebrates, and amoeba (Van Rie and Thielemans 2017). Cellulose is renewable, lightweight, and biodegradable. It is present in the form of microfibrils surrounded by hemicellulose and lignin in the cell walls of the plants. Cellulose nanofibril (CNF, also known as nanofibrillated cellulose) is a material composed of nanosized cellulose fibrils with average widths of 5 to 20 nm and has a wide range of lengths (Park et al. 2017a). CNF can be obtained by mechanical treatment of wood with strong shear forces such as a grinder, fluidizer, and homogenizer. On the basis of chemical composition, CNF can be categorized as lignocellulose nanofibrils (LCNF) that contain both lignin and hemicellulose, holocellulose nanofibrils (HCNF) that contain only hemicellulose and no lignin, and pure cellulose nanofibrils (PCNF) that comprise only the cellulose component (Park et al. 2017b).

AgNPs are well known for their antimicrobial properties, but in the colloidal solution state the AgNPs tend to aggregate in physiological solutions (Lee et al. 2007; Li and Lenhart 2012). This aggregation limits the applicability of AgNPs, as smaller AgNPs colloids possess superior antibacterial activity compared to larger AgNPs aggregates (Wang et al. 2014). Various polymers were employed as stabilizing agents to limit the aggregation of AgNPs in physiological media (Wang et al. 2014). It is logical to expect that CNF will best serve this purpose due to its large number of hydroxyl groups. There are some reports on the production of CNF supported AgNPs, but they suffer from drawbacks including the required use of toxic chemicals and long reaction times. For instance, one study prepared a cellulose fiber/AgNPs composite using alkalis (NH₃, NaOH, and Na₂CO₃) as activation agents (Xu et al. 2016). A study in 2018 prepared gold, silver, and nickel nanoparticles anchored in cellulose nanofibers, where NaBH₄ was used for reducing metal ions (Gopiraman et al. 2018). Another study prepared hydrogel, aerogel, and the film of CNF functionalized with AgNPs (Dong et al. 2013). Here, the Ag⁺ ions are spontaneously reduced to AgNPs, but it required a long reaction time of 5 days. Thus, developing a new method to prepare AgNPs functionalized CNF without the use of toxic chemicals and long reaction time is of great need.

In an attempt to address these issues, the authors have explored the possible utilization of LCNF as reducing and stabilizing agent for producing AgNPs. Novelty of the present work lies in the utilization of LCNF with different chemical composition as reducing and stabilizing agent. A simple autoclave procedure was employed for the synthesis and process parameters, such as the concentration of reactants (LCNF and AgNO₃) and reaction time were optimized. Furthermore, the effect of lignin content of LCNF on the formation of AgNPs was evaluated. For this purpose, three types of cellulose nanofibrils, i.e., HCNF (0% lignin), LCNF-5 (5% lignin), and LCNF-18 (18% lignin), were employed for the preparation of AgNPs. Three types of AgNPs were thoroughly characterized using UV-vis, Fourier-transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS).

EXPERIMENTAL

Materials

A wood sample for the preparation of LCNF was obtained from a Korean red pine (Pinus densiflora) tree present in the Kangwon National University’s Experimental Forest. Benzene, ethanol, sodium chlorite, acetic acid, and silver nitrate were procured from Daejung Chemical & Materials Co. (Siheung, Republic of Korea). Other chemicals were obtained from commercial sources and used without further purification. Deionized water was used throughout the experiment.

Preparation of LCNF

Wood powder was prepared by pulverizing pine chips, and alcohol-benzene (1 to 2, volume per volume) extraction was performed to remove the extractives. The extractives content of the sample was measured according to TAPPI T204 cm-97 (2002). The extracted wood powder was delignified with NaClO₂ and CH₃COOH (Cao et al. 2014). The Klason lignin was determined by TAPPI standard method T222 om-06 (2006); the hydrolysis filtrate was further used for the determination of acid-soluble lignin content according to Hyman et al. (2016). The contents of cellulose, hemicellulose, lignin, and extractives of Pinus densiflora wood sample were 35.6%, 21.4%, 38%, and 5% respectively. The lignin content of extracted wood powder was adjusted to 5% and 18% by a delignification reaction. The wood powder partially delignified was subjected to mechanical pre-treatment using a grinder (Super Masscolloider, Masuko Sangyo Co., Ltd., Tokyo, Japan), and then the LCNF was prepared by a high pressure homogenizer (MN400BF, Micronox, Seongnam-si, Korea). The prepared CNF was diluted to a concentration of 0.001% and filtered at a vacuum of 0.8 bar using a membrane filter (pore size: 0.2 μm, diameter: 47 mm). Solvent substitution was performed four times (15 min each) with tert-butanol (C₄H₁₀O, Daejung Chemicals & Metals, Siheung-si, Korea) and then freeze dried for 72 h.

Synthesis of AgNPs

For the synthesis of AgNPs, 1 mL of AgNO₃ solution was added to the 3 mL of LCNF in a 20 mL vial; the reaction mixture was shaken well and treated in an autoclave at 120 °C. Optimal reaction conditions for the formation of AgNPs were identified by systematically varying the parameters such as reaction time, concentration of LCNF (0.005 wt% to 1.25 wt%), and concentration of AgNO₃ (from 0.1 mM to 1.25 mM).

Characterization

A picture was taken with an ultra-high-resolution scanning electron microscope (Hitachi S-4800, Tokyo Japan) to observe the morphology of the LCNF prepared for each condition. UV-vis absorption spectra were recorded on a UV-vis spectrophotometer (Jenway, Stone, UK) within the range of 300 to 700 nm. The morphology and size of the synthesized AgNPs were studied using TEM (LEO-912AB OMEGA, LEO, Jena, Germany) at KBSI (Chuncheon, Korea). An X-ray diffraction apparatus (Cu target; DMAX 2100V, Rigaku, Tokyo, Japan) operating at 40 kV and 40 mA was used to evaluate the crystalline nature of AgNPs. The mechanism of formation of AgNPs was confirmed by using FTIR spectra (FT-3000-Excalibur, Varian Inc., Palo Alto, USA) with a scanning range of 400 to 4000 cm^-1. XPS data for synthesized AgNPs were obtained with an X-ray photoelectron spectrometer (Omicron Nano Technology, Klaus-Weiler, Austria) with a 128-channel collector.

RESULTS AND DISCUSSION

CNF SEM Image

Figure 1 shows the SEM images of HCNF, LCNF-5%, and LCNF-18% under different magnifications.

Fig. 1. SEM images of HCNF, LCNF-5%, and LCNF-18%

The observed fibrillar structure confirms the successful formation of nanofibrils. The fibril diameters of all the samples prepared using the homogenizer were uniform and in the size range of 11 nm to 14 nm. Interestingly, no large difference in fibrillation characteristics was observed between HCNF (0% lignin), LCNF-5%, and LCNF-18% containing lignin.