Nanomaterials Based on Cashew Nut Shell Liquid: Comparison
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Please note this is a comparison between Version 3 by Ermelinda Bloise and Version 2 by Lindsay Dong.

Cashew nut shell liquid (CNSL), obtained as a byproduct of the cashew industry, represents an important natural source of phenolic compounds, with important environmental benefits due to the large availability and low cost of the unique renewable starting material, that can be used as an alternative to synthetic substances in many industrial applications. The peculiarity of the functional groups of CNSL components, such as phenolic hydroxyl, the aromatic ring, acid functionality, and unsaturation(s) in the C15 alkyl side chain, permitted the design of interesting nanostructures. Cardanol (CA), anacardic acid (AA), and cardol (CD), opportunely isolated from CNSL, served as building blocks for generating an amazing class of nanomaterials with chemical, physical, and morphological properties that can be tuned in view of their applications, particularly focused on their bioactive properties.

  • CNSL
  • cardanol
  • anacardic acid
  • nanostructures
  • green chemistry
  • renewable materials

1. Introduction

Cashew nut shell liquid (CNSL) is the oily greenish-yellow liquid filling the soft alveolar mesocarp shell of the cashew nut (Figure 1), which is the product of the cashew tree, Anacardium occidentale L., a popular plant in Brazil that is also present in many parts of the world [1].
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Figure 1. Cashew pedicel, fruit, and mesocarp filled with CNSL.
For several years, many investigations have outlined a general overview of the extraction, isolation techniques, chemical composition, and physicochemical properties of CNSL extracts, exploring the different applications of their components and developing environmentally friendly protocols [1][2]. The CNSL classification and its composition, in relation to the extraction conditions, are well-known. It was established that anacardic acid (AA), cardanol (CA), cardol (CD), and 2-methylcardol are the four constituents: organic solvent extraction under mild conditions allows for obtaining a liquid rich in AA, the main component of so-called “natural CNSL”, while a high temperature produces the decarboxylation of AA, making CA the main constituent of the oil, called “technical CNSL”. When CNSL is treated with a high temperature (distillation at 180–200 °C), CA can be obtained at a >90% yield [1][3][4][5].
The four CNSL components differ in their side chain unsaturation, as shown in Figure 2.
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Figure 2. Constituents of CNSL.

2. CNSL-Based Nanomaterials

2.1. Cardanol-Based Nanomaterials

Some examples of CA-based metal nanomaterials have been produced via coordination polymerization of the phenolic compound with metal ions or molecules derived from chemical modifications of CA that have shown very versatile properties (optical, photophysical, electrochemical, etc.) and are used to prepare composite nanomaterials with metal oxides. A promising approach, applied to water purification, is heterogeneous photocatalysis using nanostructured semiconductors. In this context, nanocomposite materials based on ZnO nanostructures, impregnated with CA-derived lipophilic porphyrins (H2Pp-metal-free and CuPp-copper porphyrin), provide an alternative technology to efficiently remove toxic substances from water under environmental conditions [6]. In this way, nanomaterials with a diameter of 55 nm were obtained. Moreover, FTIR studies confirm the noncovalent nature of the interactions between CA-porphyrins and ZnO. The photocatalytic activity was investigated via the degradation of rhodamine B (RhB) in an aqueous solution under visible-light irradiation and natural sunlight. Porphyrins are photosensitizing agents for semiconductors; thus, the composite nanomaterials showed better absorption in the visible region than bare ZnO did. 

An advanced functional material, like a metal–organic framework (MOF), was produced via the microwave-assisted synthesis of a renewable organic ligand CA and nontoxic endogenous cation Mn(II) bivalent salt [7]. The synthesis was carried out by a “solvent-free in situ” approach, and FTIR spectroscopy was used to confirm the structure and verify the curing of the material (MnIIMicCol). The morphological characterization of the nanomaterial investigated through XRD, optical microscopy, SEM, and TEM showed an amorphous and layered morphology and mesoporous (pore diameter of 8.0286 nm) behavior. The thermal behavior measured by the TGA/DTG/DSC techniques confirmed a high inherent thermal stability. Antibacterial activity was tested against Gram-negative (E. coli and K. pneumoniae) and Gram-positive (B. subtilis and S. aureus) bacterial strains, revealing two bactericidal mechanisms: (i) damage to the bacterial cell membrane and (ii) the production of reactive oxygen species (ROS), which may cause oxidative stress on bacteria cells and damage to both DNA and RNA. The excellent chemical–physical properties and the moderate antibacterial activity make this nanostructured MOF usable in thermally stable (up to 230–250 °C) antimicrobial coating materials.

In newer research, sheet-like nano-biocomposites, based on CA thermosetting resin [8], were obtained. The inclusion of cellulose nanofibrils and nanoplatelets of expanded graphite improved in synergy the flammability, thermal, mechanical, and water absorption properties of nano-biocomposites, required for applicability in coating systems and automotive applications, where weight reduction and a reduction in VOCs in the environment are of great importance. The assembly of this composite material allowed for improving the dispersion of nanofiller cellulose nanofibrils with a high specific surface area and a high percentage of exposed atoms on their surfaces. Furthermore, the CA resin showed a stabilizing effect from the expanded graphite in the nanosheets.

The versatile behavior of CA and its derivative small molecules were found to be attractive in functional soft nanomaterials research to generate self-assembled morphologies down to 100 nm dimensions, such as nanotubes, nanofibers, gels, surfactants, and liquid crystals [9][10]. Gels are systems delicately balanced between molecules’ precipitation and solubilization in a solvent that, self-assembling through noncovalent interactions, form a fibrous network that traps the solvent through capillary forces and resists the flow of the medium. Pyrene-coupled coumarin derivatives with varying hydrophobic chains have been synthesized via aldol condensation, starting from CA-aldehyde, obtained through electrophilic aromatic substitution reactions [11]. The formation of transparent fluorescent organogels occurred via supramolecular self-assembly through the π–π stacking of pyrene units and hydrogen bonding. In particular, absorbance and emission and 1HNMR studies showed that hydrogen bonding between carbonyl groups of coumarin coupled pyrene with the hydroxyl group of a solvent, and π–π stacking interactions have driven the self-aggregation and gel formation processes. The presence of saturated and unsaturated hydrophobic tails affects the gelation efficiency tested in different solvents, strongly influencing the optical properties of π-conjugated derivatives. From these results, self-assembly nanoflakes were derived, and in vitro fluorescence imaging reveals that these compounds inhibit the proliferation of PC3 prostate cancer cells, making them potentially applicable in the cell imaging field. Another study reported the synthesis of coumarin-tris-based amphiphiles, which in turn have been derived from CA [12]. This small amphiphilic system showed the ability to form a stable supramolecular hydrogel sensitive to external stimuli such as pH or the presence of the biologically important Fe3+ ion. Optical microscopy and high-resolution transmission electron microscopy (HRTEM) investigations revealed a reversible morphological transition from self-assembled gels at neutral and basic pH levels to vesicles and nanotubes when pH is acidic. 1HNMR and XRD studies suggested that the π–π stacking interactions and hydrogen bonding were the driving forces for the gelation process. 

In recent years, increasing attention has been directed to the field of green chemistry, which has strongly motivated researchers to develop and design new biobased soft materials. The structural uniqueness of the CA molecule allows it to show surface activity in particular conditions, even when not chemically modified. Therefore, some researchers have used CA as a molecular building block for the practical and environmentally friendly batch preparation of nanovesicle classes, in view of their potential applications in the pharmaceutical and biomedical fields. Organic solvent-free synthetic pathways (Figure 3) were developed in which CA and cholesterol (CH) mixtures have been used to embed minor amounts of functional molecules inside nanometric-sized vesicles (from 100 to 300 mn). Specifically, nanovesicles hosting effectively both hydrophobic and hydrophilic bioactive molecules, such as a porphyrin–cardanol hybrid and chlorogenic acids, were prepared [13][14].
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Figure 3. Solvent-free batch method for the preparation of CNSL-based nanovesicles.
Considering the versatility of these CA-based nanomaterials to load molecules of different chemical natures, four various phthalazines derivatives have been embedded in nanovesicles, and their biological activities against different cancer cell lines [15] showed that CA itself confers good antioxidant and moderate cytotoxic properties to unloaded nanovesicles, which have been improved via loading with some phthalazine molecules.
The adaptability of this nanosystem allowed it to incorporate cannabidiol [16], a cannabis extract compound known for its intrinsically low chemical stability, which limits its therapeutic potential. The use of nitrogen gas during the synthesis provided the inert atmosphere required to avoid the thermooxidative degradation of cannabidiol. Stability studies showed that the embedded cannabidiol structure was also preserved because of the antioxidant properties of CA for 30 days when stored at 20 °C.

2.2. Anacardic Acid-Based Nanomaterials

AA is less popular than CA among researchers because it is obtained from a more expensive treatment of cashew shells, but it is no less attractive. Indeed, it is structurally similar to salicylic acid as a mixture of 2-hydroxy-6-alkylbenzoic acid congeners and has attracted great research interest due to its biological activities [17].
Other researchers, conscious of the well-known potential of AA from both chemical and biological points of view, have focused their studies on the use of AA in the production of nanomaterials. Metallic nanoparticles functionalized with biomolecules have received special attention due to their various biomedical applications. Magnetic iron oxide nanoparticles coated with AA were synthesized and characterized for particle size, magnetic properties, thermal stability, and thermal response in hyperthermia treatments [18]. The AA coating stabilized the nanoparticles by preventing aggregation without losing their magnetization potential. Moreover, the anticancer properties of the AA coating can promote the ability to concentrate at the target area and the stabilization in biological fluids of magnetic nanoparticles, which may be used in a medical application, such as magnetic hyperthermia. CNSL derivatives, such as AA and CD, form micelles above critical micellar concentration, which can act as passivating agents for silver nanoparticles. Bezerra et al. [19] described the preparation, characterization, and in vitro antileishmanial activity of green-based silver nanoparticles with AA and CD. The synthesis was carried out via reduction with sodium borohydride in the presence of AA or CD under microwave irradiation, obtaining silver nanoparticles with a mean hydrodynamic diameter of 167 nm and 260 nm, respectively. I Several studies have exploited the chemical characteristics of AA, rather than its biological properties, to develop nanomaterials with various potential applications. For example, AA was used as a coordinating solvent for the synthesis of metal chalcogenide (Cd and Pb sulfide, selenide, and telluride) nanoparticles via a solution-based technique, obtaining nanoparticles with a well-defined size and morphology [20][21].  An easy way to synthesize biocompatible hybrid silicate nanofibers was proposed by Chacko et al. [22]. Montmorillonite, a self-assembled inorganic–organic silicate hybrid system, was organomodified using cations of aminosilyl derivatives of bio-monomers, such as AA and CA, and showed intercalation attributed to modification via cation exchange and long cis-unsaturated aliphatic chain-induced layer separation. They showed an organization into “arthropodal” branched nanofibers of about 30–60 nm in diameter and 10–20 µm in length, ascribable to rolling into a grain-like morphology stabilized by hydrogen-bonded interactions induced by the carboxyl group and end-to-end assembly of the grains. The absence of any such assembly in the sample modified by cations derived from CA confirmed the carboxyl group effect. Recently, new formulations following the principles of “green chemistry” have been developed for the preparation of nanovesicles, using AA as the only component or mixed with small amounts of CH [23]. The greater polarity of the AA due to the presence of the carboxylic acid group compared to the CA molecule has allowed the development of different formulations. By varying the preparation conditions, it was possible to evaluate the CH amount reduction effect as well as its complete removal from the AA-based nanostructures. The morphological analysis by TEM reveals that the inclusion of CH doubles the mean diameter of the vesicles to approximately 100 nm, and helps to preserve the initial composition over time, as shown by the stability studies. Instead, the formulation with only AA produced smaller nanovesicles (50 nm) that are rather stable for up to 14 days of storage at 20 °C.

3. Conclusions and Perspectives

The increasing use of agro-industry wastes as raw materials has opened a window of opportunity for the development of alternative products to the oil industry. The development of bio-based materials and technologies based on the use of bio-renewable resources represents a strategic approach to offset the economic impact related to the frequent oscillation in petroleum prices. CNSL represents an example of a naturally occurring oil that includes both phenolic and alkenyl structural properties. These characteristic functional groups can undergo selective or simultaneous chemical modifications by selecting the most suitable chemical approach according to their desirable properties. The present review highlights the potential use of cardanol, anacardic acid, and cardol isolated from a more complex mixture known as CNSL, or natural and technical CNSLs themselves, as prospective “evergreen natural resources” suitable as building blocks to obtain different classes of nanosystems. Recent advances in nanomaterials based on CNSL have been discussed, including their potential applications, among which are biomedical materials and templates for the preparation of nanostructured systems.

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