New Trends in Clay-Based Nanohybrid Applications: Essential Oil Encapsulation Strategies to Improve Their Biological Activity

Abstract

Essential oils (EOs) are used in medicinal, pharmaceutical, cosmetic, agricultural, and food industries thanks to their key properties and multiple benefits. Several techniques and embedding materials are used to nanoencapsulate EOs, in order to keep them from environmental conditions and boost their bioefficiency by controlled release. In recent years, the interest for clay nanoparticles as nanoencapsulation materials for EOs is increasing owing to their abundance in nature, low cost, inertness, and special structure. Thus, this chapter focuses on highlighting data and contributions dealing with EOs incorporation into nanoclay particles, their current applications and nanohybrid formation benefits on the stability, bioavailability, and sustained release of EOs. An overview about nanoclays used for EOs nanoencapsulation is highlighted in the beginning of this chapter followed by a brief description of EOs’ chemical composition and properties.

1. Introduction

Essential oils (EOs) are secondary metabolites of diverse aromatic plants biosynthesized in different plant organs [1] that can be extracted from leaves, flowers, and fruits by hydrodistillation, solvent-solvent extraction, and liquid CO₂ extraction [2, 3]. The EOs’ chemical composition is too complex. It is a mixture of natural volatile compounds, such as terpenes, phenols, ketones, aldehydes, alcohols, carotenoids, flavonoids, esters, and phenylpropanoid [4]. Thank to these variable bioactive molecules, EOs find uses as gastronomic, nutritional, organoleptic, antiulcer, antiaging, anticancer, antidepressant, antitussive, antipyretic, analgesic, larvicidal, insecticidal, etc. [5, 6]. Owing to their versatile biological properties and the increasing demand, by the consumers, for biobased products, EOs are amply used in pharmaceutics industry, cosmetics, food industry, food packaging, nutraceuticals, and even as agrochemicals [4, 7]. Nonetheless, their practice is constantly facing several barriers comprising the high volatility and high risk of degradation upon direct exposure to heat, humidity, light, and oxygen, intense odor and taste, dose-dependent toxicity, and hydrophobicity [1, 4, 8]. The nanoencapsulation technology has been recommended as an innovative approach to overcome the limitations of the EOs use, by enhancing their bioavailability and bioefficiency and protecting them from extreme conditions [9]. Currently, liposomes, polymeric nanoparticles, metal nanoparticles, and carbon nanotubes are some of the broadly used nanomaterials. Yet, they present some impediments of use, as they are sometimes extremely toxic and/or carcinogenic in nature even in low concentrations, very expensive to acquire, and need very complex preparation processes [10].

Clay nanoparticles represent a promising alternative to nanomaterials mentioned above. In soil science, the term “clay” is related to a material class with a particle size <2 μm in equivalent spherical diameter. “Nanoclays” are included in the clay fraction with particle size <100 nm in diameter. Soil nanoclays are commonly predominated by phyllosilicates and often include metal hydroxides and organic matter. Ultrasonication, ultracentrifugation, and energetic stirring may be used to isolate them from the clay fraction [11]. Their structure may be lamellar, fibrous, or tubular nature with a hydrophilic character. Since ancient time, clay minerals have been widely investigated by humans in many fields including medicine, pharmacy, ceramic, plastic, cracking catalyst industries, food and beverage, coatings industry, agrochemicals. They are also helpful in environmental protection and remediation. All this interest for nanoclay exploitation in various fields is mainly due to their abundance, low cost, ecofriendly nature, nontoxicity as well as their unique and specific structural physicochemical and thermal properties, including large surface area, surface electric charges, immense porosity, low density, inertness, thermal and chemical stability, ion exchange capacity, and high adsorption capacity [12, 13, 14, 15].

In this context, this chapter is dedicated to the illustration and update of clay/EOs nanosystems application in diverse fields. Firstly, the main clays used for EOs encapsulation are highlighted. Then, EOs’ chemical composition and properties are briefly described. Finally, data concerning clay/EOs nanosystems development and valorization in various applications are provided, while emphasizing the benefits associated with nanoencapsulation, namely bioactivity, stability against aggressive conditions, and controlled release.

2. Clay minerals

Clays are natural earth materials resulting from chemical weathering operations on the earth’s crust [16]. These are minerals with very fine grain size smaller than two micrometers [17].

A clay deposit generally comprises of impurities, namely feldspar, quartz, volcanic dust, fossil fragments, heavy minerals, carbonates minerals, etc. [18].

2.1 Clay minerals structure

Clay minerals belong to phyllosilicates family and are constituted by a stacking of sheets. Figure 1 illustrates the terminology used to define the layer silicates structure [19]. So, it can be established:

Figure 1.

A schematic illustration of the general structure of phyllosilicates.

• A simple plane, which is constituted by atoms (like linked O or OH);

• A sheet results from a combination of planes (like a silica tetrahedral sheet);

• A layer results from a combination of sheets;

• The crystal is issued from the stacking of several layers.

Every layer is constituted of two fundamental units:

The tetrahedral sheet, designed as (T), whose basic building unit is a “silica tetrahedron.” In each unit, one silicon atom is surrounded by four oxygens. The resulting silica tetrahedra units are connected horizontally by sharing oxygens anions, via covalent bonding, to produce a sheet of Si₂O₆ (OH)₄ structure.

The octahedral sheet, designed as (O), is built by the horizontal association of numerous octahedra, by covalent bonding. Each unit is composed of Al or Mg ions (and occasionally Fe ion) surrounded by six oxygen atoms or hydroxyl groups. This arrangement allows an eight-sided configuration labeled “octahedron.” When the (O) is dominated by aluminum, two Al³⁺ cations are so needed to maintain the electrical neutrality and the sheet is so designated as “dioctahedral sheet.” When magnesium predominates the (O), three Mg²⁺ cations are required, and the sheet is termed as “trioctahedral sheet” [15, 20, 21, 22].

The multiple associations between (T) and (O) and the numerous chemical substitutions induce different clay minerals with diverse physicochemical characteristics [16]. Consequently, three clay minerals categories are discerned:

• The 1:1clay minerals (or TO type) with the structural formula [SiO₄Al₄O₁₀(OH)₈]. Kaolinite, dickite, halloysite, amesite, and lizardite are included in this category. Each clay layer is formed by one (T) and one (O);

• The 2:1 clay minerals (or TOT type) are built by one (O) interposed between two (T). Smectite, bentonite, montmorillonite, palygorskite, sepiolite, glauconite, mica, vermiculite, and saponite are allied to this category [23, 24];

• The space between two successive 1:1 and 2:1 layers is the “interlayer,” which is empty if the layers are electrostatically neutral. If the layer takes an overabundance charge, defined as “layer charge,” so it is neutralized by diverse interlayer elements, such as cations (Ca, Na, Mg, K), hydrated cations, and hydroxide octahedral groups. The hydroxide interlayer often forms an additional octahedral sheet yielding a 2:1:1 or TOTO layer. This structure is typical of chlorite, chamosite, and donbassite [25].

Clay minerals can be identified according to the assembly method and their shape. It is distinguished by the following:

• Fibrous clay minerals, such as palygorskite (attapulgite) and sepiolite;

• Tubular clay minerals, such as halloysite;

• Lamellar clay minerals, such as smectites and kaolinite [26].

2.2 Main clay minerals used for essential oil nanoencapsulation

2.2.1 Kaolin minerals

Kaolinite (Kaol) is the main element of this class. Its structure results from a combination of one tetrahedral sheet and one dioctahedral sheet. The basal spacing is about 7.1 Å [20]. It is differentiated by the presence of strong bonds between the layers, which opposes to its expansion and swelling [27]. Kaol is also characterized by negligible cation exchange capacity (CEC) and small surface area [28].

Halloysite clay minerals, with an empirical formula Al₂Si₂O₅(OH)_4nH₂O, belong to the Kaolin class, with the same chemical structure as kaolinite [29]. Based on crystalline and geographical conditions, multiple morphologies are defined. The most abundant one in nature is the tubular form resulting from the rolling of the kaolin sheets with the presence of a water molecules layer in the interlayer space [30, 31].

The interlayer water in halloysite nanotubes (HNTs) is one of the principal properties discerning HNTs from Kaol. The dehydration action causes irreversibly the change of the d₀₀₁ spacing from 10 to 7 Å [32].

HNTs are composed of two kinds of hydroxyl groups according to their position. The inner hydroxyl groups are located between layers and the outer hydroxyl groups that are located on the surface of the HNTs. The most ones are inner groups due to the tubular form [33]. HNTs diameter is of 40–70 nm with inner lumen diameter of 10–15 nm and length of 1000–2000 nm [34].

HNTs are characterized by a relatively high specific surface area and a total pore volume that is much higher than that of platy Kaol. This should be attributed to the rich pores in the structure of halloysite [30].

2.2.2 Semectite minerals

Smectites are TOT phyllosilicates type. The association between smectite layers leads to a constant van der Waals break between the layers, known as “interlayer” [35].

Two smectite subgroups are existing:

• Dioctahedral smectites, as montmorillonite, non-tronit, beidellite, etc.

• Trioctahedral smectites, as hectorite, saponite, sauconite, etc. [22].

The smectite clays structure is able to expand and depreciate without losing its crystallinity.

Smectite layers are typified by plentiful isomorphic substitutions in tetrahedral (Si⁴⁺ replaced by Al³⁺) and octahedral (Al³⁺ replaced by Mg²⁺ or Fe²⁺ and Mg²⁺ substituted by Li⁺) positions generating a layer charge that is neutralized by hydrated cations in the interlayer space, named interlayer cations (Na⁺, Ca²⁺, K⁺, etc.) [36].

Montmorillonite (MMT) is the most important smectite clay owing to its commercial value. It emanates from the replacement of the Si⁴⁺ in the silica tetrahedral sheets by Al³⁺ and Al³⁺ in the alumina octahedral sheets by Mg²⁺ leading to a negative charge that is neutralized by the Na⁺ and Ca²⁺ interlayer cations.

These cations can be facilely supplied by other organic or inorganic cations, due to the incomparable hydrophilicity, swelling, adsorption, and fluidity properties of montmorillonite [37].

2.2.3 Palygorskite and sepiolite minerals

Sepiolite (Sep) [Mg₈Si₁₂O₃₀(OH)₄(OH2)_4.n(R₂ + (H₂O)₈] and palygorskite (Pal) [MgAl₃Si₈O₂₀(OH)₃(H₂O)_4.n(R₂ + (H₂O)₄] are 2:1 layer silicates being distinct from other clay minerals owing to the existence of continued two-dimensional tetrahedral sheet and broken octahedral sheets. Their form can be defined as ribbons of 2:1 phyllsilicate structure. Each ribbon is bonded to the next by inversion of SiO₄ tetrahedra along a set of Si-O-Si bonds. Channels between the ribbons are generated due to these structural properties, which are larger in Sep (4 Å x 9.5 Å) in comparison with Pal (4 Å x 6 Å). Pal is dioctahedral and Sep is trioctahedral. Both minerals have elongated habits often forming bundles of lath-like or fibrous crystals. They contain two types of water, structural water coordinated to the octahedral cations and zeolitic water, which is loosely bound in the channels. Due to the existence of channels, Pal and Sep have great microporous volumes. They are also of a big interest and importance due to their large specific surface, exceptional swelling, and good absorbability [25, 38].

2.3 Clay minerals modification

Modification of clay minerals’ native physicochemical and structural characteristics (porosity, CEC, acidity, surface area, etc.) is usually required to adapt and extend their use in different fields of application [20].

Several physical and chemical methods have been investigated to modify clay minerals features, such as acid activation, intercalation or adsorption of organic components, clay pillarization, synthesis of porous clays heterostructures, ultrasonication, and thermal treatment [39, 40, 41, 42].

3. Essential oil: chemical composition and properties

Essential oils (EOs) are volatile liquids extracted from numerous plants. They are hydrophobic and contain volatile aromatic compounds or vegetable essences. These aromatic compounds are volatile organic compounds (VOCs), which result from the plant secondary metabolism [43]. According to the European Pharmacopoeia, an essential oil is defined as an “odorous product, commonly with a complex composition, obtained from a plant raw material by steam distillation, dry distillation, Hydrodistillation or other suitable methods without heating” [44].

Essential oils could be obtained from flowers (Pelargonium Rosat, Lavanduladentata), leaves (Eucalyptus globulus, Thymus vulgaris, Origanum vulgare, melaleuca alternifolia), seeds (Coriandrumsativum, Carumcarvi, Foeniculum vulgare, Pimpinellaanisum), peel (Citrus sinensis, CitrusLimon), rhizomes (Zingiberofficinale), and woods (Cinnamomum Cassia, Santalum album) [45].

Generally, EOs present less than 5% of the vegetal dry matter. Their composition may vary depending on the plant organ utilized, the soil and climatic conditions, and the harvesting season. Essential oils are insoluble in inorganic solvents, while soluble in organic ones (alcohol, ether, and oils). They are colorless, liquid, and volatile at room temperature, having a characteristic odor with a density less than water density [8].

EOs are very interesting natural products with several biological properties, such as antioxidative, anti-inflammatory, cytoprotective, antitumor, antimicrobial, antihypertensive, analgesic, larvicidal, insecticidal, antiparasitic, and other biological activities [5]. Thus, they have been largely used in agriculture, pharmaceutical field, medicine, and cosmetics. Indeed, numerous studies have shown insect-repellant and biocide activities, which are usable in agronomy and food industry [46, 47]. Due to their incredible properties, interest in EOs has massively increased in recent years. They are subsequently useful as complementary medicinal treatments, due to their availability and to synergistic therapeutic effects with conventional medicines, such as antibiotics [48]. Aspects related to the significant antimicrobial effect against multiresistant pathogens of the EOs, and their synergetic effect when one or more oils are mixed used in combination with known drugs [49, 50], are also presented in a recent review [51].

3.1 Chemical composition of essential oils

The number of known compounds present in essential oils has recently increased with the improvement in instrumental analytical chemistry. Nowadays, there are more than 300 components present in pure EOs [52, 53] that can be classified into two categories:

• Volatile compounds: Volatile fraction presents about 90% of total oil weight. It comprises terpenes and their oxygenated derivatives named terpenoids. Aldehydes, esters, and aliphatic alcohols may also be present in volatile fraction.

• Nonvolatile compounds: Nonvolatile residue, which presents less than 10% of total EO in weight, contains hydrocarbons, sterols, waxes, carotenoids, flavonoids, and fatty acids [54].

As mentioned before, the main component of the chemical composition of EOs is a complex mixture of hydrocarbon terpenes and terpenoids. Obviously, chemical compounds contain carbon and hydrogen as their building blocks that are isoprene (C5H8), the basic hydrocarbon unit found in EOs [46]. Moreover, terpenes are a class of natural products represented by general structural formula (C5H8)n. Containing more than 30.000 compounds, these unsaturated hydrocarbons are developed by plants, particularly conifers. The classification of terpenes is based on the number of isoprene units: Two, three, or four isoprene units are joined head to tail form monoterpenes, sesquiterpene, and diterpenes, respectively. Monoterpenes are present with more than 80% of EOs composition with 10 carbons, while sesquiterpenes and diterpenes are composed of 15 and 20 carbons, respectively. However, hemiterpenes (C5), diterpenes (C20), triterpenes (C30), and tetraterpenes (C40) are also observed in nature [55, 56, 57].

The second class of terpenes containing oxygen is called terpenoids. Those oxygenated derivatives of hydrocarbon terpenes can be aldehydes, alcohols, esters, ketones, acids, and phenols. Terpenoids also present many biological properties such as anti-inflammatory, antifungal, antiseptic, bactericidal, antiviral, sedative, and antitumor [58, 59, 60].

Furthermore, non-terpene components, defined as phenylpropanoids, are also found in EOs providing odor and a particular flavor (Eugenol and Cinnamaldehyde) [55].

3.2 biological properties

EOs have, for a long time, been recognized to possess several biological actions on humans, animals, and plants, namely [61, 62, 63, 64, 65]:

• Antioxidant activity

• Anticancer activity

• Antimicrobial activity

• Antiviral activity

• Antiparasitic activity

• Anti-inflammatory activity

• Insecticidal activity

4. Application of clay-essential oil nanohybrids

Nanostructured materials hailed from clays and EOs are increasingly attractive for diverse applications, due to their eco-friendly nature and hybrid character that affords original features to the implied organic and inorganic components.

4.1 Active packaging and coatings

Active packaging is defined as the packaging technology in which active agents are incorporated to the original packaging system in order to inhibit food contamination and oxidation, maintain the quality and the safety of food longer, and prolong shelf life by preserving food from internal and external environmental factors [66, 67].

Synthetic additives with bioactive properties have been amply used in food industry. However, growing health and ecological concerns due to the use of chemical food ingredients have amplified the consumer interest toward natural bioactive substances such as essential oils generally recognized as safe (GRAS) by the Food and Drug Administration of the United States and thus are promising alternatives to synthetic additives [68].

However, their high volatility, low solubility in water, sensitivity for oxidation, low photothermal resistance, and undesirable flavor restrain their use [69, 70].

In order to overcome these barriers, EOs’ encapsulation and immobilization may be considered as an interesting solution to decrease these drawbacks and to develop new practical antimicrobial packaging materials [71, 72]. Based on their abundance, bio-inert nature, unique layered structure, intercalation, and swelling properties and high retention capacities, loading EOs into clays has been proved to be appropriate for protecting and preserving the efficiency of EOs in storage, providing a controlled release of EOs in polymer matrix. Studies about clay/EOs nanohybrid application in active packaging sector are still emerging.

The encapsulation of thyme essential oil (TO) into halloysite nanotubes, via a vacuum mode, for application in the food packaging, was studied by Lee and Park [73]. TO-loaded HNTs capsules coated with Eudragit polymer (EPO) were also prepared in the purpose to prevent burst release and to extend the release time. The amount of TO released from the HNT/TO and EPO/HNT/TO systems was 61.76% and 45.27% at 24 h, respectively. Also, it was observed that the TO liberation from each type of capsule was maintained up 96 h. Additionally, it was noted that TO/HNT hybrids presented an initial burst release of 47.96% within 12 h, while EPO/HNT/TO hybrids exhibited important retarded TO release. Finally, it was concluded that HNT capsules containing TO exposed good antioxidant activity, compared with Pristine HNTs that showed no antioxidant activity.

In order to increase the loading capacity of HNTs, another research work was investigated by Lee et al. [74] consisting of developing an antimicrobial nanosystem based on TO and modified HNTs (alkali-HNT) by treating pristine HNTs with sodium hydroxide (NaOH, 5 mol/L). It was demonstrated that the encapsulation efficiency passed from 14.5% to 20.5% for raw HNT clays and treated HNT clays, respectively. Moreover, it was found that the release rate of TO in liquid form was very fast than that of TO in hybrid form, which suggests that TO was strongly retained by HNTs in the original and treated state.

The inactivation of E. coli, total mesophilic zerobic bacteria (MAB), and molds and yeasts (MY) on the surface of the cherry tomatoes exposed to alkali-HNT/TO hybrids was assessed. It was deduced that the populations of E. coli, MAB, and MY could be reduced over the storage time of fresh cherry tomatoes after the addition of alkali-HNT as a nanodelivery system for the controlled aerial release of TO.

A third study dealing with the encapsulation of TO in HNTs was conducted by Jang et al. [75]. The TO was loaded in HNTs via vacuum pulling methods, followed by end-capping or a layer-by-layer surface coating process for complete loading. After, the nanohybrids were combined with flexographic ink and covered on a food packaging paper.

Encouraging results were emanated from the study of the EO release and the packaging paper antibacterial activity and emphasized the interest in using such functional packaging material printed with ink containing TO- and HNTs-based nanocapsules.

HNTs were explored by Biddeci et al. [76] as nanodelivery system for peppermint essential oil (PO). The purpose of the study was to develop an antioxidant/antimicrobial biofilm by filling a pectin matrix with modified HNTs loaded with PO. The modification of HNTs was based on the functionalization of clay surface with cucurbit [6] uril (CB [6]) molecules to increase the affinity of the nanoclay toward PO. Pectin-based biofilms containing HNTs/CB [6]/PO hybrid were prepared by casting method under reduced pressure. A considerable inhibition percentage (41%) for biofilms was evidenced using the antioxidant activity test. While in vitro experiments of the antimicrobial properties for E. coli and S. aureus isolated from beef and cow milk displayed more efficiency at higher temperature.

An investigation dealing with the preparation and characterization of green composites based on pectins and nanohybrids clay/EOs was reported by Gorrasi [77]. HNTs were valued as possible nanocontainers for rosemary essential oil (RO). It was observed that the active agent release from the composite containing nanohybrids was much slower than the release of the same molecule simply added to the matrix. Molds formation was detected in pectin films after 2 weeks of storage at room temperature (25–30°C) and environmental humidity (about 60%). On the contrary, it was not noticed in nanohybrid-based films even after 3 months, suggesting so the promising use of clay/RO nanohybrid in the active packaging field.

A linear low-density polyethylene (LDPE)-based films incorporated with active nanoclay particles were developed adopting a new method for grafting EOs on nanoclay particles, and this by using Tween 80 as surfactant. It was demonstrated that nanoclays permitted a protective effect for the active substances against evaporation during film manufacturing. In vitro antibacterial activity of the activated films against pathogenic bacteria (Salmonella Typhimurium, E. coli, Listeria monocytogenes, S. aureus, and Bacillus cereus) was marked, while their effectiveness against lactic acid bacteria (Lactobacillus rhamnosus and Lb. casei) was restricted [78].

The formation of new active bilayer films impregnated with attapulgite (ATP) clay nanoparticles loaded with Allium Sativum essential oil (AO) and based on LDPE and polypropylene (PP) was investigated using blown film extrusion method. The preservation performances of these potential active films were tested for large yellow croaker at 4°C of storage. It was able to demonstrate that the lipid oxidation of seafood product could be incessantly stopped. This fact was justified by the controlled release of AO from ATP [79].

A recent study was published aiming to explore the efficiency of sodium montmorillonite (Na-MMT) and organically modified Montmorillonite (Org-MMT) to be nanodelivery systems for the controlled release of EOs in LDPE active films. Thyme, oregano, and basil EOs were chosen for their antioxidant property. It was found that the antioxidant activity of films varied depending on the EO type and content. It was noted also that the use of Na-MMT and Org-MMT as nanocontainers for EOs allowed managing the antioxidant activity of the elaborated films [80].

The formulation of a sustained liberation antibacterial chitosan (CS)-based packaging film, by casting solution method, through filling CS matrix by biological active nanoparticles was presented by Cui et al. [81]. The active nanofillers resulted from the loading of cinnamaldehyde (CIN), the major constituent of cinnamon essential oil, into acid-treated montmorillonite (acid-MMT). The search in the CIN release rate from CS films showed stable rates for CS/CIN and CS/acid-MMT/CIN films equal to 65.01% and 73.20% at 96 h and 168 h, respectively. These outcomes emphasized the promising use of acid-MMT nanoclay as nanoencapsulation materials for slow release of EOs components.

In vitro antibacterial activity of acid-MMT/CIN-based biofilm was tested for the growth of S. aureus and E. coli. Preliminary results displayed a noticeable inhibitory effect.

A new second study was also published by Cui et al. [82] dealing with the development of sodium alginate (SA)-based active package with controlled release of CIN loaded into HNTs. To enhance the uptake capacity of HNTs, the clay nanoparticles were treated with sulfuric acid (acid-HNT). Analog results were found for SA/acid-HNT/CIN film for the slow release behavior of the CIN and in vitro antibacterial activity compared with CS/acid-MMT/CIN film.

Recently, lemon waste natural dye (LD) and EO (LO) were valued in the hybrid form with MMT laminar nanoclays, for a potential application as nanofiller for polyester based matrix. The co-adsorption of LD and LO on MMT nanoparticles was optimized by using statically designed experiments. The polyester-based bionanocomposites were prepared for different nanohybrid loading rates (3, 5, and 7 wt %). The experiments carried out in this study were devoted to assess the effect of nanohybrid (MMT/LD/LO) incorporation on thermomechanical and color properties of the polymer matrix [83].

LDPE/clay nanocomposites films, comprising carvacrol (CRV) with controlled and tunable antimicrobial activity, were conceived. Org-MMT/CRV hybrids were prepared by shear mixing CRV with org-MMT at a weight ratio 2:1 (respectively) followed by ultrasonication at room temperature for 20 min at constant amplitude of 40% (Figure 2) [84].

Figure 2.

A schematic illustration of organoclay galleries modified with carvacrol molecules as achieved by a pre-compounding step in which clay/carvacrol hybrids are produced.

After melt compounding and compression molding, LDPE/org-MMT/CRV films presented higher CRV content (5–6 wt %) in comparison with control LDPE/CRV films, containing only ∼ 3 wt%. As follows, it was concluded that clay platelets acted as nanocontainers for volatile compounds of CRV while improving their thermal stability during high-temperature process.

The antimicrobial activity of the films against E. coli bacteria was assessed. It was found that freshly prepared films reduced E. coli cells to an undetectable rate, affirming the effective bactericidal activity of CRV within the melt-compounded films. But, 1-month-old LDPE/org-MMT/CRV films, stored at room temperature, showed lower antimicrobial potency. However, the LDPE/CRV films totally loosed their biological potency.

An analog comportment of the tested films was noticed against Gram-positive (Listeria innocua) bacteria in a second study conducted by Shemesh et al. [85] where it was also described and discussed the excellent antifungal activity of LDPE/org-MMT/CRV films against the phytopathogenic and clinical fungus A. alternata.

Another study was exposed by Shemesh et al. [86], whose objective was to investigate the use of HNTs as nanocarriers for CRV for its later melt compounding with LDPE. Like it was demonstrated for LDPE/org-MMT/CRV nanocomposites films, HNTs showed their vital role in improving CRV thermal properties during LDPE/HNTs/CRV films manufacturing, as well as controlling and delaying the CRV release.

The co-encapsulation of CRV and thymol (TYM) into HNTs for developing active food packaging film with synergistic antimicrobial activity was examined by Krepker et al. [87]. Satisfactory results were achieved and the resulting films revealed superior antimicrobial activity against E. coli when compared with LDPE based films including the individual EOs. This was attributed to the synergistic interactions between CRV and TYM.

Rosewood, manuka, oregano, and lavender EOs were valued, by Kinninmonth et al. [88], by their adsorption onto natural and acid-treated bentonites for their controlled release and protection against polymer processing conditions. Promising results were reported.

A smart waterborne paint was designed by incorporating bioactive hybrids in the formulation. The active nanosystems were conceived based on org-MMT, Na-MMT, and citronellol (CIT).

The antimicrobial activity of the synthesized hybrids was assessed against Chaetomium globosum and Alternaria alternate. The outcomes were promising and revealed that org-MMT-based hybrid was more active than Na-MMT-based hybrid.

The bio-resistance tests were conducted, on the acrylic paint indoor formulation containing the org-MMT/CIT nanohybrid and that containing org-MMT and CIT added independently, by exposing the films during 4 weeks to fungal growth in plates. It was observed that the paint including org-MMT/CIT hybrid exposed no growth. By contrast, the paint containing free CIT showed abundant fungal growth. This could be explained by the volatility of CIT when it is added in free form in the paint formulation, and therefore, it would not be available to afford its antifungal activity. Additionally, the integration of CIT in a free form could conduct to reactions with some added ingredients in the paint formulation, which would also affect its bioactivity. On another side, the excellent achievement obtained with coatings comprising the nanohybrid material could be correlated with the sustained and controlled release of CIT from org-MMT [89].

4.2 Bio-derived agrochemicals

Agrochemicals are crucial ingredients needed to reach general food security. About 2.5 million tons of synthetic agrochemicals, comprising fertilizers, pesticides, herbicides, fungicides, insecticides, and others, are used each year [90]. The overuse of these kinds of compounds leads to a major pollution for both soil and water, with a high toxicity toward humans and animals. EOs are an excellent substitute to synthetic agrochemicals as a way to lower negative impacts to human health and the environment. Nevertheless, some of their properties, for example, water insolubility, chemical instability, degradation by temperature and light, may be hindrances to their use as biocontrol agents [91, 92]. Clay nanoparticles have been widely studied as nanocarriers for synthetic agrochemicals over the last decade [93, 94, 95, 96]. Moreover, studies dealing with nanoclay/bio-derived agrochemicals-based formulations are emerging.

An insecticidal powder formulation based on Ocimum gratissimum EO and org-MMT clay was developed by Nguemtchouin et al. [97]. The insecticidal effect of the nanohybrid org-MMT/O-gratissimum was evaluated against the maize weevil Sitophilus zeamais. It was noticed that the mortality of Sitophilus zeamais dropped from 100 to 95%, 87%, and 0% after 7 days, respectively, for org-MMT/O-gratissimum, Na-MMT/O-gratissimum, and crude EO. Moreover, organoclay-based formulation was more stable, since it lost about 60% of its full biocidal capacity after 30 days, whereas unmodified clays-based formulation released entirely its insecticidal activity for the same period of time. Experiment conducted for formulation remnant effect determination indicated that the insecticidal effects of O-gratissimum EO remained for about 7, 45, and 80 days, for free EO, EO included in Na-MMT, and EO included in org-MMT, respectively [97].

The efficiency of aromatized powder based on O-gratissimum EO and organically modified clay, as a bioinsecticide for use in pest control of stored maize, was confirmed in a second study reported also by Nguemtchouin et al. [98].

Xylopia aethiopica EO was investigated too by Nguemtchouin et al. [99] for the development of new bioinsecticides based on kaolinite clay. Ingestion-contact insecticide tests were carried out using maize weevil S. zeamais. As described in the study, the mortality of S. zeamais was proportional to the mass of the powder formulation put together with maize and insects. Insect’s mortality varied from 22% with 2.5% (w/w) of bioinsecticide to 100% with 10% (w/w). The remnant effect evaluation showed that the initial mortality rate of insects induced by the clay-based bioinsecticide was the highest. But, with the time it decreased until reaching 0% at the 8th week. This was explained by the volatility of the EO. To overcome this limit, enhancing the clay structural properties by chemical modification was suggested to raise both the adsorption and the retention capacities.

In relation to X. aethiopica EO and kaolin-based bioinsecticide formulation, a study was reported on the effect of clay particle size and clay treated with hydrogen peroxide (H₂O₂) on its adsorption capacity. It was observed that the amount of EO adsorbed was inversely proportional to the particle size. The treatment of kaolin with H₂O₂ promoted the adsorption rate of X. aethiopica EO components [100].

The adsorption behavior of TYM onto MMT and Kaol clays and their corresponding modified ones (by treatment with aqueous solution of iron polycations) were modelized by Nakhli et al. [101] using a statistical approach to understand this comportment for the future application of these clay samples as adsorbent in biopesticides formulation. Encouraging results were emerged from this study.

In another research paper, published by Nguemtchouin et al. [102], it was exposed the results of the textural and structural modification of bentonite clay using metallic polycations solutions of Al_x(OH)_y and Fe_x(OH)_y to obtain inorganic bentonite and cetyl and phenyl trimethyl ammonium chlorides solutions to obtain organic bentonite. The attempt to adsorb TYM, chosen like an insecticidal terpenic compound, on the different modified bentonites was carried out. Both ways for clay modification were considered as interesting to promote the adsorption properties of clays utilizable as support for bioinsecticides.

Clausena anisata EO was nanoencapsulated in MMT, and the resulting powdery formulation was evaluated by Ndomo et al. [103] for their insecticidal activity and their effects on progeny production of Acanthoscelides obtectus. It was pointed out that there was a dose-dependent progress in mortality of A. obtectus adults in bean grains treated with clay-EO nanohybrid formulation. It was also highlighted that at the dosage and exposure time, the EO in its nanohybrid form exhibited a higher mortality effects against A. obtectus adults than that caused by pure EO applied directly. Finally, it was stated that although EOs applied alone afford an acceptable level of stored grain pests control, they can cause a persistent odor that can be unpleasant when eating the seeds. So, the use of clay as a support material for such compounds could diminish the adverse effects in addition to the increase of their stability.

Essential oils of Lantana camara L. (Verbenaceae) and Annona senegalensis Pers. (Annonaceae) were valued by Gueye et al. [104] for their insecticidal effect on adults of Caryedon serratus, a pest of groundnut stocks, through using Kaol clay as vehicle for these active agents. Promising outcomes were reported and discussed.

The Kaol clay was also examined by Kéita et al. [105] as a nanocontainer for Ocimum basilicum L. and O. gratissimum L. and their application as an insecticide in a powder form to control Callosobruchus maculates. The effectiveness of the powders formulation was demonstrated and proved by varied bioassays.

A new approach for clay and EO-based bioinsecticide formulation was exposed by Noudem et al. [106]. It considered the modification of MMT by saponins (Sap-MMT) to enhance its adsorption capacity toward O. gratissimum EO compounds. The outcomes of the formulations remnant effect tested against Callosobruchus subinnotatus evidenced the decrease of the insects’ mortality from 96 to 70% and from 96% to 13.12% for Sap-MMT/O. gratissimum nanosystem and Na-MMT/O. gratissimum nanosystem after 42 days of exposure, respectively. This finding was explained by the interaction types existing between the terpenic compounds of the EO and Na-MMT or Sap-MMT. Additionally, the efficiency of Sap-MMT/O. gratissimum nanosystem could be also owing to the high quantity of terpenic compounds adsorbed by Sap-MMT compared with Na-MMT.

Adsorption of EO components of Lavandula angustifolia and TYM on Moroccan-modified bentonite, for a potential use, respectively, as an insecticide and an acaricide, was explored by El Miz et al. [107, 108, 109].

Organically modified palygorskite and beidellite clays were explored by Ghrab et al. [110, 111] as support materials for Eucalyptus globulus EO-active terpenic compounds adsorption for a potential insecticidal application.

The adsorption of Lippia Multiflora EO on two organically modified clays, Kaolinite-rich clay and smectite-rich clay, was explored via a qualitative approach using X-ray diffraction and infrared analyses. It was deduced from this investigation that smectite-rich clay can be considered as a suitable material for producing biopesticides compared with kaolinite-rich clay [112].

TYM was immobilized by Ziyat et al. [113] onto purified Moroccan clay “Rhassoul” and the organically modified one. It was estimated that the adsorption capacities were of 6 mg/g and 16 mg/g for purified Rhassoul and modified one, respectively. Hence, organo-modified Rhassoul was regarded as an attractive material for biofungicides formulation based on EO.

Recently, a study was focused on the evaluation of the antifungal activity of thyme and oregano EOs combined with purified Rhassoul and sulfuric-acid-activated Rhassoul. Penicillium sp. was chosen as a pathogen agent for biological activity. In vitro tests demonstrated that activated-clay-based formulations exhibiter higher inhibition power than purified clay-based formulations. This was attributed, in part, to the adsorbed quantity by activated clay, which was higher than that retained by purified one. Nevertheless, it was considered that EOs in a nanohybrid form with Rhassoul could be applied as an alternative to synthetic fungicides to prevent fungal growth during grain storage [114].

CRV/ATP nanohybrid antibacterial materials were designed by Zhong et al. [115] using a grinding process in the intention to replace the synthetic antibiotic used in animal farming. The antibacterial activity of the hybrid was evaluated against E. coli and S. aureus, and the minimum inhibitory concentration was equal to 2.0 mg/mL for both bacteria model.

A recent study was also published by Zhong et al. [116] reporting the preparation of a series of antibacterial hybrid materials based on Pal and different EOs.

CRV was also valued by Berraaouan et al. [117] by hybridization with purified bentonite for application in pesticide industry and in other fields. The adsorption studies revealed attractive results.

A series of nanohybrid materials were prepared by adsorbing natural active components, such as eugenol, CIN, TYM, allyl isothiocyanate, and diallyl disulfide onto MMT. The biological activity of the nanomaterials was assessed against S. aureus and Aspergillus niger (A. niger). It was observed that the nanoencapsulation enhanced both the antifungal and antibacterial activities of the EOs components. Thereby, the developed nanocapsules may find uses in integrated pest management systems in organic agriculture. The bioactive substances loading into nanoclay may ensure their controlled release in accordance with the needs of vegetation while simultaneously helping to reduce environmental pollution [118].

Newly, a study was conducted by Saucedo-Zuñiga et al. [119] dealing with the preparation and the characterization of a multilayer film reservoir including EOs/clay nanohybrids for a potential application as pesticide or attractant for pest control as well as coatings for antimicrobial or fungicidal control. Two kinds of nanoclays were investigated, HNTs and org-MMT, as well as two kinds of EOs, thyme and orange EOs. Promising results were reported and discussed for the convincing use of this multilayer film encapsulated EO/clay composites as aroma-controlled release systems for pesticides and for also active food packaging applications.

More recently, a new green evaporation/adsorption method was exposed by Essifi et al. [120] for the adsorption of EOs on Na-MMT. The aim of the preparation process was to elaborate powdered EO/Na-MMT hybrids for acaricidal, fungicidal, larvicidal, and insecticidal applications. Promising results were reported for the potential use of this kind of nanomaterials.

4.3 Pharmaceutical and cosmetic applications

Hybrid structures based on clay and synthetic drug for biomedical and pharmaceutical applications have been amply investigated over the past decade. Nowadays, reports treating clay/natural bioactive substances nanohybrids are arising [20].

A bio-based antimicrobial mosquito repellent was developed by immobilizing a mixture of Curmuma aromatic and Zanthoxylum limonella EOs onto MMT clays dispersed in methyl ester of the castor oil. The estimation of the mosquitocidal activity and the antibacterial activity of the resulting nanosystems revealed that the formulation efficiency is EOs ratio-dependent [121].

The attempt to develop a gelling viscoelastic film loaded with CRV/clay nanohybrid for infected skin ulcer treatment was exposed by Tenci et al. [122]. MMT, HNTs, and Pal clays were examined for the nanosystems preparation. It was reported that CRV loading capacity was clay structure dependent. The highest amount was obtained for, the fibrous clay, Pal. In vitro assessment of cytocompatibility, and antioxidant and antimicrobial properties was conducted for pure CRV and its corresponding optimal nanohybrid (CRV/ Pal). It was marked that CRV displayed a cytotoxic effect for concentrations >50 μg/mL. However, the corresponding nanohybrid did not present any cytotoxic effect for all the tested concentrations. According to the authors, the incorporation of CRV into Pal permitted to keep safe human fibroblasts against CRV cytotoxicity owing to a controlled release of CRV to cell membrane. The efficiency of CRV in protecting human fibroblasts against oxidative stress was confirmed by the antioxidant properties determination. Even its hybridization with Pal did not affect such activity. The antimicrobial activity experiments tested against S. aureus and E. coli proved a decrease of the minimum inhibitory concentration and the minimum bactericidal concentration for CRV loaded into Pal compared with the pure one, meaning that CRV in the hybrid form manifested a higher antimicrobial activity. This observation was interpreted by the low evaporation of CRV after loading into clay and its slow release from the nanostructure.

Nanohybrid materials based on limonene and lecithins-modified MMT were invented by Nagy et al. [123] to be applied as flavor and fragrance nanodelivery system.

A new academic work was reported looking at the formulation of novel nanodelivery systems with CRV prodrugs and fibrous clays, namely Pal and Sep in the purpose to restrict the chemical conversion of the loaded CRV prodrugs to CRV in the distal small intestine where EOs can execute its maximum antimicrobial activity. Pal was found to have a better affinity to CRV prodrugs than Sep. In vitro release profiles were investigated under conditions simulating the gastric and intestinal transit. It was concluded that tested clays were able to ensure a continuous release of CRV prodrugs that are subjected to delayed conversion into antimicrobial active CRV as demanded to consolidate its pharmacological activity [124].

A nanohybrid based on PAL and ginger EO (GO) was developed by Lei et al. [125]. The EO is known for its pharmacological properties such as antibacterial, antioxidant, and antinociceptive. The antibacterial activity of PAL/GO tested against E. coli and S. aureus revealed minimum inhibitory concentration (MIC) values for GO in PAL/GO hybrid equal to 1.16625 mg/mL and 4.665 mg/mL against S. aureus and E. coli, respectively, while free GO presented MIC values equivalent to 3.5625 mg/mL against S. aureus and 7.125 mg/mL against E. coli. These findings confirmed the high antibacterial activity of GO in hybrid form with PAL compared with its free form. The thermo-stability and acidity and alkalinity-resistance tests were exploited to demonstrate that PAL/GO nanosystems conserve their antibacterial activity in extreme conditions.

Natural MMT and Tween 20-modified MMT were employed as nanocarriers for cinnamic acid delivery systems for oral administration. It was underlined the interest in surfactant using for clay-based nanocarrier preparation, as it ensured the complete release of the cinnamic acid after oral drug administration, in addition to the improvement of the drug loading [126].

5. Conclusion

Overall, the hybrid system obtained from nanoclays with EOs is an efficient approach to protect EOs from light, air, and humidity, which lead to oxidation or volatilization of EOs and a reduction of their biological properties. Moreover, the hybrid system increases the solubility and physicochemical stability of EOs and offers a controlled and sustained release rate of EOs and makes them more available. The hybrid nanoclay/EOs open new perspectives in cosmetic, food, and pharmaceutical industries and could be an economic benefit as it is inexpensive and also fulfills the consumer concern regarding safety as it is environmentally friendly and nontoxic agent.