Lignins from sugarcane bagasse: Renewable source of nanoparticles as Pickering emulsions stabilizers for bioactive compounds encapsulation

Highlights

• Lignins from sugarcane bagasse are renewable templates to obtain colloidal lignin nanoparticles.

• The obtained colloidal lignin nanoparticles stabilize Pickering emulsion systems.

• The lignin nanoparticles properties were determined by the synthesis method applied.

Abstract

Lignin nanoparticles has gained interest in recent years in a wide range of applications due to its unique properties compared to the microsized material. Furthermore, lignin is obtained from lignocellulosic biomass processing and it is still considered a poorly exploited macromolecule due to the heterogeneous nature and low solubility in aqueous medium. This study focus on the comparison between two ways environmentally friendly of obtaining colloidal lignin nanoparticles (LNPs), considering minimal processing steps and employing lignins derived from two sugarcane bagasse pretreatments (alkaline and organosolv). Raw lignins and LNPs were characterized by different techniques such as scanning electron microscopy (SEM), zeta potential, dynamic light scattering (DLS), small-angle X-ray scattering (SAXS) and antioxidant assay in order to evaluate the changes in its morphological, chemical and antioxidant properties. The results showed the formation of spherical-like nanoparticles which sizes were determined by the synthesis method. LNPs obtained from alkaline lignin showed an average diameter varying from 115 to 300 nm, while LNPs obtained from organosolv lignin ranged from 270 to 680 nm, as determined by DLS. All LNPs in aqueous suspension had a zeta potential ranging from −25 to −35 mV, which is considered stable for colloidal systems. The thermal stability properties of micro to nanosized lignins were preserved. The antioxidant capacity against the DPPH (2,2-diphenyl-1-picrylhydrazyl) radical was improved for alkaline-LNPs compared to the raw lignin (IC₃₀ = 12 and 9.9 μg mL⁻¹, respectively), and worsened for organosolv-LNPs compared to the raw lignin (IC₃₀ = 11.4 and 15 μg mL⁻¹, respectively). Furthermore, LNPs were tested as stabilizing agents of Pickering emulsions, used as encapsulation agents of curcumin, a polyphenol with a wide range of pharmacological applications. Organosolv-LNPs were seen to be the most efficient stabilizer, retaining 73% of curcumin in its encapsulated form after 96 h. Therefore, this study demonstrated the potential of nanostructured lignins for bio-based field, and also highlights the influence factors for the choice of methodology and raw lignins over the properties resulted of LNPs.

Introduction

Lignin is one of the main structural components of the plant cell walls and the second most abundant natural macromolecule after cellulose (Ayyachamy et al., 2013; Gordobil et al., 2018). It is a three-dimensional amorphous structure composed of phenylpropane units derived from oxidative coupling reactions involving aromatic alcohols (Rahman et al., 2018). In plants, lignin plays several important roles such as structural support, protection against chemical and biological attacks and the efficient transportation of water in the vascular tissues (Gordobil et al., 2018).

Lignin is considered a renewable material which potential is not fully exploited, despite its unique characteristics and chemical versatility that can be applied to obtain a wide range of value-added products (Beckham et al., 2016; Ragauskas et al., 2014). Moreover, this “green biomolecule” also presents anti-inflammatory, anticarcinogenic and antioxidant properties, lack of toxicity, antimicrobial activity, low market price and it is a platform for aromatic molecules (Doherty et al., 2011; Gordobil et al., 2018). Lignin commercial sources, readily available for use on a large scale, are produced as residual streams from pulp and paper industries and second-generation ethanol mills (Li et al., 2017).

However, its low solubility, high complexity and heterogeneity explains why only 5% of 78 million metric tons of lignin produced at biomass-based industries are employed in commercial products, leaving the remaining 95% to heat and power production (Ayyachamy et al., 2013; Berlin et al., 2014). The expansion of global cellulosic biorefineries activities will considerably increase the amount of lignin streams (Rinaldi et al., 2016). Thus, it is necessary to develop strategies that explore the material to its full potential while improving the economic feasibility of the whole process (Rahman et al., 2018).

The use of natural and biodegradable materials as emulsion stabilizers has recently been explored in replacement of commonly used toxic and non-biodegradable compounds (Calabrese et al., 2018; McClements et al., 2017). Among the possibilities for new emulsions stabilizing materials, nano and microparticles obtained from different types of lignins has been gaining attention (Nypelö et al., 2015; Silmore et al., 2016; Sipponen et al., 2017). Different methods for LNPs synthesis appear in the literature, differing mainly in terms of number of steps, reagents consumption, solvents environmental friendliness, nanoparticles stability and yields (Lievonen et al., 2016; Nypelö et al., 2015). LNPs can be applied in the so-called Pickering emulsions, a specific type of emulsion stabilized by solid particles, very useful for foods, cosmetics, medicines, among other applications (Nypelö et al., 2015).

Due to the high adsorption energy of the particle at oil/water interface, the use of solid particles from natural sources ensures greater biocompatibility and stability to the emulsion system (Xiao et al., 2016). Besides, the formation of a film by the solid particles around the dispersed droplets increases the kinetic stability of the emulsions. For that matter, Pickering emulsions are highly resistant to coalescence. Another important advantage is that Pickering emulsions stabilized by solid particles require a lower energy consumption for their formation when compared to the traditional emulsions, being necessary only a few seconds or minutes of agitation or sonication (low pressure methods), for example (Chevalier et al., 2013).

The number of publications involving LNPs and Pickering emulsions has increased considerably over the last few years (Ago et al., 2016; Lievonen et al., 2016; Silmore et al., 2016; Sipponen et al., 2017). However, there is a lack of studies investigating how the lignins origin and the LNPs preparation methods could affect the nanoparticles properties, as morphology, physical stability and performance as stabilizing agents.

The present work focuses on exploring LNPs preparation from lignins extracted from sugarcane bagasse by alkaline and organosolv approaches during the second-generation ethanol pilot-scale studies (Nakashini et al., 2018). These two lignins were chemically and physically characterized and submitted to different methodologies of nanoparticles synthesis. LNPs were characterized by different techniques and tested as stabilizing agents of Pickering emulsions, which were used as encapsulation agents of curcumin, a polyphenol with a wide range of pharmacological applications. Finally, the stability of the systems overtime was evaluated and correlated to LNP’s chemical and nanoscopic properties.