Introduction

Carbohydrates, in the form of starch and sucrose, are the most common organic substances available in nature. From very ancient times, they have been used for food and processed through fermentation. The process of fermentation results in mixture of mono-, di-, and oligo saccharides and, under certain conditions, these degradation processes also lead to the formation of cyclodextrins. The historical development of these cyclic dextrins has been described in detail by Loftsson and Duchene in their review [1].

Cyclodextrins are cyclic oligosaccharides with a hydrophobic cavity and hydrophilic surface. They are also refered to as cyclomaltose, cycloamylases and Schardinger dextrins [2]. Cyclodextrins may contain a large number of glucopyranose units (more than 15 per ring). Cyclodextrins possess a truncated cone shape due to the presence of chair form of glucopyranose units [3, 4]. Generally, they are categorized into naturally occurring and chemically modified cyclodextrins [5]. The most common of naturally occurring forms include α, β and γ cyclodextrins, consisting of 6, 7 and 8 glucopyranose units, linked by α-(1–4) bond [6]. Among these, β-cyclodextrin has been extensively used in the early stages of pharmaceutical applications, owing to its ease of availability and suitable cavity size for a wide range of drugs. Although, its poor aqueous solubility and nephrotoxicity limited its use in parenteral mode of delivery [7]. With the view of extending the inclusion capacity and the physicochemical properties, chemically modified forms of cyclodextrin derivatives have also been prepared and investigated. One of the derivatives of cyclodextrin, hydroxy propylated-β-cyclodextrin is regarded as a relatively safer form, which is also non-irritating to the eye [8]. Derivatization can have a large influence on cyclodextrins capacity to solubilize membrane components, as reported by Motoyama et al. [9]. Dimethylated-α-cyclodextrins have been reported to release proteins form red blood cells in rabbits. Some more modified forms of cyclodextrins include, sulphobutylated-β-cyclodextrin, demethylated-β-cyclodextrin, randomly methylated-β-cyclodextrin [4] branched-β-cyclodextrins (maltosyl- and glucosyl-) and, acetylated and sulphated cyclodextrins. Modifications of cyclodextrins also yield alteration in solubility; for example, methylated form of β-cyclodextrin deputes 50-fold higher aqueous solubility than unsubstituted β-cyclodextrin [10, 11].

The most notable attribute of cyclodextrins is their ability to create inclusion complexes with a wide variety of molecules (solid, liquid and gases) or their portions by molecular complexation [12]. Cyclodextrins act as host and a guest moiety is entrapped within its cavity, resulting in complex formation. During this inclusion phenomena, no covalent bonds are formed or broken [13]. Only non-polar appropriately sized molecules can be held in the lipophilic cavity of cyclodextrin, producing inclusion complexes [14]. The complexes can be formed, both, in solution or crystalline form. During complex formation, the enthalpy-rich water molecules are released from the cyclodextrin cavity and guest moieties set up a dynamic equilibrium with cyclodextrin molecules in the inclusion complex [6].

Cyclodextrin-assisted encapsulation of guest molecule offers numerous advantages namely solubility improvement of poorly soluble molecules, enhancing stability of labile moieties against degradation due to oxidation, light and heat, control of sublimation and volatility, chromatographic separation, physical separation of incompatible compounds, masking unpleasant odors, taste modification and modified release of flavors, fragrances and drugs. These advantages open doors for the application of cyclodextrin complexes in environmental protection [15], agriculture [16], biotechnology [2, 17], packaging, textile industries [18], food [19], pharmaceuticals [20] and cosmetics [21].

Essential oils, extracted from aromatic plants are volatile compounds with strong odour. These secondary plant metabolites are complex mixtures of many hydrocarbons, terpenes, terpenoids and their derivatives. These oils, generally obtained by hydro or steam distillation or by a mechanical process, usually consisting of 20–60 components. Out of these, two or three are the main components, present at reasonably greater concentration (20–70%) as compared to others. It is the major constituents, which are responsible for the biological properties of the essential oils, owing to the presence of varying groups, which are derived from distinct biosynthetic pathways [22]. Terpenoids are a class of natural compounds formed by C-5 building blocks, isopentenyl diphosphate (IPP) and its isomer (DMAPP). Monoterpenes (C-10), sesquiterpenes (C-15) and hemiterpenes (C-5) are the representatives of this class. On the other hand, terpenes carrying oxygen in the form of ether, hydroxyl, ketone, aldehyde or carboxyl groups are termed as terpenoids [23].

Essential oils have found numerous medicinal uses in humans as well as animals [24]. Externally, these are mostly used in the form of gargles, mouthwashes and inhalations. Though rarely used by oral route, these essential oils are designated as safe and placed under GRAS category for ingestion, where in these are administered after dilution with olive oil, milk or soy milk. In addition, these oils are generally considered safe for dermal application, where the oil is present in diluted form in a formulation. However, such an application can lead to skin reactions, especially, in case of citrus oils which are sensitive to UV radiation and may cause darkening of skin or irritation upon exposure to sun [24]. When used in inhalation form, the concentrated vapors of essential oils may lead to irritation in the eyes. It has been found that essential oils undergo rapid metabolism, with their distribution in the body being relatively high. Essential oils afford interesting applications in food, pharmaceutical field and cosmetics owing to their diverse and relavant biological activities. However, due to their unstable nature and fragile, and volatile constituents, they can undergo degradation easily (by heat, light, oxidation, volatilization), if they are not suitably protected from external environment. Figure 1 represents the common challenges encountered with essential oils.

Fig. 1
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The common challenges of essential oils

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In view of their properties relavant to human use, essential oils have been incorporated in several drug delivery systems, carefully engineered to produce the desired effect and overcoming their drawbacks at the same time. To name a few, liposomes, micro and nanoemulsions, solid lipid nanoparticles, have been developed [22]. Microencapsulation of essential oils with β-cyclodextrins is another technique using the molecular inclusion method and helps in improving the stability and efficacy of essential oils [25]. The chemical structures of active constituents of essential oils complexed with cyclodextrins are depicted in the Fig. 2. In order to further expand the applications of essential oils, inclusion complexes have been widely explored by the research fraternity in literature. The studies undertaken for the entrapment of essential oils or their active moieties in cyclodextrin complexes are reported in Tables 1, 2 and 3.

Fig. 2
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The molecular structure of major components of essential oils investigated as inclusion complexes

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Table 1 Food and cosmetic applications of cyclodexrtin complexes with essential oils or their components
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Table 2 Pharmaceutical applications of cyclodextrin complexes with essential oils or their components
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Table 3 Therapeutic applications of cyclodextrin complexes with essential oils or their major components
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This review article presents an account of the recent developments in the arena of essential oil–cyclodextrin complexes. Preceding this, Marques et al., published an excellent review on this topic in 2010 [65]. To the best of our knowledge based on literature review, there is no updated review publication in this area and our article may prove a timely contribution for the research community. In this review, the preparation techniques and characterization methods of cyclodextrin complexes have been summarized. The advantages and applications of cyclodextrin complexes entrapping essential oils have also been discussed. This review can be felicitous for scientists streaming in this area, to encapsulate essential oils using cyclodextrin inclusion complexes. This article also gives an insight into various factors influencing inclusion complex formation, because an understanding of the same is necessary for proper handling of these versatile materials. However, our paper provides an overview of the work done by various research groups in this arena, although essential-oil loaded cyclodextrin complexes-based delivery systems have been excluded, with the view to limit the scope of the article.

Manufacturing methods for cyclodextrin inclusion complexes

Apart from the ingredients incorporated in the formulation, the preparation technique employed for manufacturing of inclusion complexes plays an indispensible role, affecting the product’s morphometrical properties as well as performance. Literature reports a variety of preparation techniques for encapsulating essential oils in cyclodextrin complexes. Cyclodextrin complexes have been chiefly synthesized using either co-precipitation method and kneading method (Fig. 3).

Fig. 3
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Fabrication of cyclodextrin complexes using co-precipitation technique

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In co-precipitation method, briefly, measured quantities of cyclodextrin and water are added to a flask placed on magnetic stirrer, at a constant temperature (40/50 °C). After stirring for an hour, saturated solution of cyclodextrin is obtained. Ethanolic solution of oil or active moiety may be added dropwise to the saturated cyclodextrin solution, with continuous stirring for an hour, at the same rate and temperature, as mentioned above. Then, this solution is cooled at room temperature, with continuous agitation and, maintained overnight at 4 °C. Finally, the cold, precipitated inclusion complex can be recovered by vacuum filtration. The filtrate is washed with suitable organic solvent, in order to remove free guest components from the surface of cyclodextrin and, finally they are freeze dried [28, 47, 48, 58].

In the kneading or paste method, accurately weighed cyclodextrin is taken in porcelain motor. Adding small amount of purified water, cyclodextrin is mixed with it using pestle, resulting in the form of paste. Then, essential oil or its components can be incorporated into the paste with thorough mixing. The solid obtained should be washed with small amount of suitable organic solvent and dried appropriately, as mentioned in co-precipitation method [66].

Locci and coworkers prepared cyclodextrin inclusion complexes loading carvacrol, thymol and eugenol using super critical carbon dioxide. In this technique, measured amount of cyclodextrin and guest molecule are loaded into the autoclave thermostated and pressurized with carbon dioxide, at constant pressure and contact time. This is followed by a rapid drop in pressure, which allows carbon dioxide vaporization and separation of solvent free solid inclusion complex. Although, poorly explored for inclusion complexes of essential oils, this technique possesses greatest advantage in terms of scale up for commercial application, in comparison to classical methods using water, since it leads to good yield of inclusion complex, an account of higher solubility of guest molecules in carbon dioxide than in water [56, 67].

In another technique, called molecular inclusion or self-assembly aggregation, initially weighed quantity of cyclodextrin is dissolved in specified amount of distilled water and measured quantity of the guest is, then added to produce a desired host:guest molar ratio. The mixture so obtained is then placed in a shaking-incubator, maintained at appropriate rpm and temperature, for desired time period. The obtained samples can be further dried to form solid inclusion product [30].

In solvent evaporation technique, host and guest in specific ratio are dissolved in alcohol or some suitable solvent to get a clear solution. The resulting preparation is allowed to evaporate over night at room temperature. The cyclodextrin complexes are, thus obtained [68].

In slurry method, slurry of weighed amount of β-cyclodextrin is prepared in water. To this slurry, weighed quantity of guest is added and stirred for 36 h by a magnetic stirrer (400 rpm), the resultant product is filtered, and thereafter appropriately dried [57].

Conventionally, complexes are dried using hot air ovens, vacuum ovens, spray dryers and freeze dryers. In case of essential oils, most complexes should be dried below 40 °C to remove water. In addition, optimization of drying temperature and other process conditions should also be done, when highly volatile essential oils or their components are taken as guests.

In literature, freeze drying method has also been reported to have been used to prepare inclusion complexes for essential oils. In these reports, solutions for cyclodextrin and guests were prepared in appropriate molar ratios with agitation for 24 h and freeze dried after filtration [69].

Factors affecting complexation of essential oils with cyclodextrins

In order to fabricate cyclodextrin inclusion complexes for essential oils or their active constituents, with desirable characteristics, it is important to be well versed with the in depth knowledge regarding various factors that may influence process of complex formation. Factors playing vital role in cyclodextrin complexes have been discussed below.

Type of cyclodextrins

Type of cyclodextrin may affect encapsulation of essential oils in cyclodextrin inclusion complexes. Amongst the various forms of cyclodextrins, β-form is the most explored by various research groups, owing to its ability of taste masking, improving solubility and ensuring stability. For encapsulation of Salvia sclarea essential oil (SEO), various cyclodextrins like β-cyclodextrin (β-CD), DM-β-CD [(2,6-dimethyl)-β-cyclodextrin], 2-HE-β-CD [mono{2-O-(2-hydroxyethyl)}-β-cyclodextrin] and 2-HP-β-CD [mono{2-O-(2-hydroxypropyl)}-β-cyclodextrin] were investigated and, among these, β-CD showed stronger inclusion interactions with SEO [42]. Recently, in a similar study, β-cyclodextrin DM-β-CD and HP-β-CD were investigated for encapsulation of Folium artemisia argyi essential oil (FAA EO). Fluorescence spectrometry was used to study inclusion interaction for β-cyclodextrin derivatives [50]. This parameter needs to be further explored in furturistic studies involving essential oil cyclodextrin complexes.

Host–guest ratio

Ratio of guest and host can affect inclusion efficiency, an important characteristic feature of inclusion complexes. In addition, for essential oils, maximum retention of volatiles plays a major role and needs to be optimized in prepared inclusion complexes. Most of the literature on essential oil inclusion complexes reports investigations using single ratio of essential oil/essential components and cyclodextrins. Either guest or host amount can be varied for the preparation of cyclodextrin complexes, or their ratio may be optimized (Table 4).

Table 4 Guest host ratio along with preparation techniques of cyclodextrin complex with essential oil or their constituents
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Songkro et al. prepared-cyclodextrin complexes by kneading technique using two weight ratios: 1:1 and 1:2, of citronella oil and β-cyclodextrin. It was observed that different ratios resulted in complexes of different color: yellow (weight ratio 1:1) and white (weight ratio 1:2), due to difference in the amount of β-cyclodextrin used. From scanning electron microscopy (SEM) analysis, increase in size in agglomerated inclusion complexes was seen, on increasing weight ratios from (1:1 to 1:2). In Fourier transform infra red spectroscopy (FTIR) analysis, lower shifts were noted with 1:1 (citronellal and β-cyclodextrin) weight ratio, but higher shifts were obtained with increase in weight ratio up to 1:2. From differential scanning calorimetric (DSC) analysis, the formation of complex was clearly confirmed for the ratio 1:2 due to absence of citronellal peak, unlike 1:1 complex. Based on mosquito repellent action, citronella oil inclusion complex containing lotion (weight ratio 1:1) demonstrated promising results, along with controlled release behavior, as compared to plain citronella oil [53].

All other researchers who investigated variable ratios used co-precipitation method and, investigated their effect on inclusion efficiency and retention of essential oil. Bhandari and coworkers explored, five lemon oil to β-cyclodextrin ratios (3:97, 6:94, 9:91, 12:88, 15:85 w/w), in order to determine their effect on the inclusion efficiency. Retention of lemon oil volatiles reached maximum at the lemon oil to β-cyclodextrin ratio of 6:94. However, maximum inclusion capacity was obtained at ratio 12:88, in which complex contained 9.68% (w/w) lemon oil [26]. In a similar study, Petrovic and his research group fabricated inclusion complexes using co-precipitation technique, with four ratios of cinnamon oil to β-cyclodextrin (5:95, 10:90, 15:85 and 20:80 w/w). The formulation containing 15:85 w/w (oil:β-cyclodextrin) exhibited maximum inclusion efficiency (117.2 mg oil/g of β-cyclodextrin), whereas the formulation with ratio 10:90 (oil to β-cyclodextrin) displayed maximum retention of essential oil volatiles (94.18%) [32]. Haloci and co-workers investigated four ratios of Satureja montana essential oil, (5:95,10:90, 15:85 and 20:80) and maximum retention of essential oil volatiles reached upto 93.15% (at oil to β-cyclodextrin ratio15:85) and maximum inclusion efficiency was achieved at ratio 20:80 w/w (oil to β-cyclodextrin) [58]. However, in the study by Dima et al. both maximum inclusion efficiency and maximum retention of oil (94.23%) was attained with ratio 15: 85 (oil and β-cyclodextrin) [60]. Del Toro-sanchez et al., varied thyme oil concentration and fixed the concentration of β-cyclodextrin and in their study five ratios were investigated (0:100, 4:96, 8:92, 12:88 and 14:86). From the results of recovery of essential oil from inclusion complex, it was found that 8:92 ratio (oil to β-CD) had highest thymol content. These results were also confirmed by gas chromatography mass spectrometry (GC–MS) data [52]. The saturation patterns of garlic oil in β-cyclodextrin inclusion complexes were observed at 12:88 ratio (oil:β-cyclodextrin), whereas for cinnamon oil, no saturation was found up to 14:86 ratio [29]. From the findings of these studies, it may be concluded that saturation of cyclodextrin reaches for guest at a particular ratio and, therefore optimization of the process should be established with respect to the highest recovery of essential oil volatiles.

Preparation techniques

As discussed in “Manufacturing methods for cyclodextrin inclusion complexes” section, the inclusion complexes can be prepared using different techniques. Selection of preparation method directly affects inclusion efficiency of essential oils with cyclodextrins (Table 4). Martins et al. used co-precipitation and kneading technique for encapsulating Mentha × Villosa Hudson oil in β-cyclodextrin cavity. The co-precipitation method showed better results since, in this method the recovery of oil from complex powder was 95.9%, resulting in lesser material loss [28]. Tao and coworkers synthesized thymol and thyme oil β-cyclodextrin inclusion complexes by freeze drying and kneading method. The entrapment efficiency of β-cyclodextrin inclusion complexes prepared by freeze drying was found to be higher than those synthesized by kneading, for both thyme oil and thymol. In kneading method, complexation and drying are carried out in an open container at room temperature, resulting in loss of essential oil, owing to volatilization and difference in entrapment values. Results of antimicrobial activities of prepared complexes carried out in pork meat system revealed that thymol and thyme oil β-cyclodextrin complexes fabricated by freeze drying possessed greater (p < 0.05) anti-microbial activity, when compared to MIC values of free thymol and thyme oil, whereas, complexes prepared by kneading method did not represent such enhancement [59]. In the same year, Menezes et al. fabricated (−)-linalool β-cyclodextrin complexes using paste and slurry methods. Results indicated that total (−)-linalool entrpped in complex cavity was 8.6% by paste method while by slurry method 42.57% (−)-linalool was found to have been entrapped in complex cavity [35].

Effect of drying

Bhandari et al., reported fiffteen min of kneading time as the optimum period for encapsulation of lemon oil in β-cyclodextrin. This group further reported that the flavor inclusion profile in the cyclodextrin complex does not depend on the kneading time. In this study, kneading was followed by drying of paste using, two modes: spray drying and vacuum drying and, the results were compared. Both drying methods were found similar, when extent of encapsulated oil was measured. However, spray drying was described as a simpler and quicker method in comparison to vacuum drying [27]. Further, due to its efficiency at lower drying temperature, spray drying can prove more efficient for encapsulating volatile oils in cyclodextrin complexes.

Characterization aspects of essential oil–cyclodextrin complexes

To study the interaction of cyclodextrins and loaded guest moiety and, to understand the process of their design and fabrication, various analytical techniques are used. The cyclodextrin complexes are characterized by investigating the following studies (Table 5).

Table 5 Characterization of essential oil and their components loaded in cyclodextrin complexes
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Phase solubility studies

In order to investigate the solubility limit, phase solubility studies can be performed. These studies are helpful in determining phase solubility constant and stability constants. Generally, these studies are performed according to the method given by Higuchi and Connors, 1965 [59]. In order to explore molecular association of cyclodextrin with guest, an excess amount of guest moiety can be added to varying concentration of aqueous cyclodextrin solution. The suspension is shaken in screw cap vials using magnetic stirrer for a time period until equilibrium is attained. The samples are centrifuged at 500 rpm to separate insoluble oil/oil components and filtered through a syringe filter. The filtered solution is appropriately diluted and analyzed spectroscopically. Phase solubility diagram can be plotted with guest solubility as a function of cyclodextrin concentration, according to equation given below.

$${\text{K}}=\frac{{{\text{slope}}}}{{{\text{SO}}(1 - {\text{slope}})}}$$
(1)

where SO is the aqueous solubility of oil or intercept of plot (in mol/l) and K is the stability constant. Initially, the solubility of host increases linearly with concentration of cyclodextrin. At one stage, increasing the concentration of cyclodextrin does not enhance solubility of guest, signifying the solubility limits within the cyclodextrin concentration range. The phase solubility profile data alone do not confirm formation of inclusion complexes. These only explain how drug solubility is influenced by increasing cyclodextrin concentration [71].

The value of K from Eq. (1), between 200 and 5000 m−1, is considered most suitable for the enhancement of stability and solubility of poorly soluble drugs [43]. Additionally, phase solubility experiments can be performed to determine thermodynamic properties of the cyclodextrin complex formed. Gibbs free energy, a thermodynamic function can also be calculated. Complexation lowers the thermodynamic potential of the dissolved moiety [65].

The driving forces for cyclodextrin complex formation include Vander-Wall interaction, electrostatic interaction, hydrophobic interaction, hydrogen bonding, charge-transfer interaction and release of enthalpy rich water moieties from the cyclodextrin cavity [14, 72].

Although, phase solubility is an important study for cyclodextrin complexes, only a limited number of researchers have performed it for essential oils or their constituents. Such studies have been carried out for pure α-bisabolol and chamomile essential oils [73], eugenol [45] and thymol and thyme oil [59].

Fourier transform infra-red spectroscopy

Fourier transform Infra-red spectroscopy is the major technique to determine the presence of functional groups. Formation of intra-molecular hydrogen bond/complex formation between the cyclodextrin and host moieties can be confirmed with the aid of this technique. The appearance of new resultant peaks in the FTIR spectrum indicates bond formation between host and guest. Further, due to included part of guest molecule, IR bands are either shifted or their intensities altered. FTIR spectra of the pure oil/oil constituents, cyclodextrin, physical mixture and guest-loaded cyclodextrin complex can be determined and compared.

Songkro et al. carried out FTIR analysis of citronella oil, physical mixture of citronella oil and β-cyclodextrin and inclusion complex of citronella oil. This group observed both physical mixture and inclusion complex weaken some peaks of pure citronella oil such as 1016 and 1617 cm−1. A higher O–H bonding (1645 cm−1) shift was found for physical mixture, 1:1 and 1:2 inclusion complexes. In inclusion complexes, changes in O–H stretching bands for citronella oil (3413 cm−1) were demonstrated, suggesting the interaction between citronella oil and β-cyclodextrin. This group also noted that in 1:1 inclusion complex lower shifts were observed, while higher shifts were seen with 1:2 (citronella oil and β-cyclodextrin inclusion complex) [53].

In 2015, Xi et al. prepared Xiang–Fu–Si–Wu Decoction essential oil (XFSWD) β-cyclodextrin complex. This group has presented FTIR spectra of pure β-cyclodextrin, physical mixture and inclusion complex and reported disappearance or reduction of peak intensities, on formation of inclusion complex [48]. In the following year, Zi-Tao Jing et al. prepared molecular microcapsules of F. artemisia Argyi (FAAEO) using β-cyclodextrin derivatives. This group carried out FTIR analysis of essential oil alone, β-cyclodextrin alone, glucose-β-cyclodextrin, physical mixture and prepared microcapsules. For comparison purpose, the major ratios of FAAEO and β-cyclodextrin were kept constant in physical mixture as well as microcapsules. When all FTIR spectra were compared, it was observed spectra of β-cyclodextrin exhibited major peaks at 3391 cm−1 (OH), 2924 cm−1 (CH), 1649 cm−1 (=O), 1156 cm−1 (–O) and 1028 cm−1 (C–O–C). Very strong absorbance peak at 1739 cm−1 was seen for the C=O (stretching) vibration of FAAEO. The spectrum of physical mixture depicted the peaks of both FAAEO and β-cyclodextrin, but with decreased peak intensity. Additionally, a number of peaks of FAAEO at 3084, 2962, 2725, 1096 and 986 cm−1 were found disappeared representing strong physical interaction of essential oil with β-cyclodextrin in microcapsules, the peaks of FAAEO at 3084, 2962, 1096 and 986 cm−1 were found disappeared as in physical mixture and the peaks of essential oil at 3459, 2930 and 1739 cm−1 were decreased in intensity showing that (C=O stretching) vibration was restricted after formation of the inclusion complex. The C=O with carboxylic functional group of the guest essential oil had been encapsulated in the hydrophobic cavity of the host. Hence, the FTIR analysis confirmed the formation of microcapsules of FAAEO to β-cyclodextrin [50].

Such studies have also been reported for cinnamon leaf and garlic oil [29], Litsea cubeba [44], thyme [52], catfish (Clarias batrachus) oil [49] and essential oil components like geraniol [54] and citronellal [64].

Thermal analysis

Differential scanning calorimetry

Another technique reported to clarify the interaction between guest and host is differential scanning calorimetry (DSC). It is generally observed that the cyclodextrin cavity affects the thermal behavior of inclusion complexes, thus, leading to shift in their endothermic peaks as compared to pure compounds. Shifting, broadening and appearance of new peaks or disappearance of certain peaks in the host may be due to evaporation, oxidation, decomposition, melting or polymorphic transition, suggesting complex formation. Inclusion complex formation generally causes reduction or absence of endothermic peaks of host (at the temperature of its boiling and melting point) [74].

Seo and his research group carried out DSC for eugenol, β-cyclodextrin, their physical mixture and eugenol-loaded inclusion complex. The thermogram of β-CD–eugenol complex showed thermal transitions at 196.8 °C with endothermic peaks. Pure β-CD peaks appear at 187.3–188.7 °C. This may be due to host and guest interaction culminating in increase phase transition temperature. Further, peak corresponding to boiling point of eugenol was found to have disappeared, pointing towards inclusion complex formation between β-CD and eugenol. Partial inclusion reaction was also observed in eugenol physical mixture due to similar transition temperature of eugenol–β-CD inclusion complexes [31]. Similar studies were performed for Allium sativum [41], citronella oil, citronellal and citronellol [53, 64], geraniol [54], β-caryophyllene [47], thymol and thyme oil [59], Xiang–Fu–Si–Wu Decoction essential oil [48], estragole [37] and catfish (C. batrachus) oil [49].

Thermogravimetric analysis

Thermogravimetric analysis (TGA) is employed to understand the thermostability, melting point and crystalline behavior of particles. Data from TGA may be used for confirmation of DSC results [75]. Changes in weight loss provide a supporting evidence for the formation of inclusion complexes. Menenzes and his coworkers analyzed geraniol–β-CD complexes using TGA. Thermogravimetry/differential thermo gravimetry (TG/DTG) curves of the geraniol, β-CD, their physical mixture and inclusion complexes (prepared by paste and slurry method) were analyzed. The mass losses calculated by their data analysis from specific intervals for each one have been reported. Percentage of geraniol evaporated was found upto 230 °C, mass loss due to evaporation of geraniol and water release up to 120 °C from physical mixture and inclusion complex. In addition, percentage of water released from β-CD upto 120 °C was obtained. Further, mass loss was also observed from physical mixture and geraniol inclusion complex, which was attributed to geraniol release (125–270 °C) and thermal decomposition of β-CD, physical mixture and inclusion complex (270–391 °C). Finally, elemental carbon formation from β-CD, physical mixture and geraniol inclusion complexes occurred due to sample carbonization from 365 to 900 °C [54]. Inclusion complexes of A. sativum [41], geraniol [54] and β-caryophyllene [47] have been characterized by TGA as per the literature.

X-ray diffraction analysis

Another simple and useful method for evaluating chemical decomposition and complex formation is powder-ray diffractometry. X-ray diffraction (XRD) pattern of the sample is usually determined as a function of scattering angle [76]. As a result of complex formation, shifting of certain peaks, sharpening of the existing peaks and disappearance or appearance of a few new peaks occurs. This characterization technique is the most beneficial method for the confirmation of inclusion complexes, either essential oils or volatile components, as liquid guest molecules show no diffraction patterns and differences in the diffractograms of uncomplexed cyclodextrin and inclusion complex can be easily established [54, 77, 78]. Menezes and his research group investigated β-cyclodextrin, physical mixture and β-cyclodextrin inclusion complexes (prepared by paste and slurry method) of (−)-linalool by XRD. β-cyclodextrin was found to be crystalline in nature. It was observed that inclusion complex formation led to large shift in the XRD signals of β-cyclodextrin. Also, inclusion complexes represented different XRD signals in comparison to β-cyclodextrin and physical mixture. Inclusion complexes further, showed weak interaction, demonstrating the formation of inclusion complex between (−) linalool and β-cyclodextrin [35].

However, review of the published literature reveals that this technique is poorly explored for essential oils and their constituents. This method has been reported for Mentha × villosa Hudson oil [28], linalool [35], Lippia grata leaf [57], Xiang–Fu–Si–Wu Decoction essential oil [48] and estragole [37].

Microscopic techniques

Microscopic techniques like SEM and transmission electron microscopy (TEM) can be employed for evaluation of particle size and shape and to carry out morphological analysis of cyclodextrin complexes [79]. Another microscopic technique, atomic force microscopy (AFM) may give the information regarding topography of inclusion complexes, complementing well established analytical techniques like X-ray crystallography, nuclear magnetic resonance (NMR), and, light and electron microscopy. AFM, a modern technique, helps to minimise the basic drawbacks of scanning microscopy (only image conducting or semiconducting surfaces).

SEM analysis is useful for visualizing the surface texture of the preparations [80]. In SEM, the developed particle is imparted conductivity under vacuum, using focused electron beam. Siqueira-Lima et al. encapsulated Lippia grata leaf essential oil using β-cyclodextrin. From SEM results, this group reported that there were drastic changes in the original morphology and particle shape of β-cyclodextrin, in inclusion complex. On the other hand, particle morphology and shape of the corresponding physical mixture were found to be similar to β-cyclodextrin [57]. Further, essential oil and β-cyclodextrin complexes appear as agglomerates in microscopic analysis [81]. This technique has been used to study A. sativum [41], Xiang–Fu–Si–Wu decoction [48], thymol and thyme oil [59], carvacrol constituent of Oregano [61], citronellal constituent of Cymbopogon species [64] and citronella oil, citronellal and citronellol [53], eugenol [46] and geraniol [54] cyclodextrin complexes.

Transmission electron microscopy can be used for investigating morphology of particles suspended in liquids. Morphological analysis of TEM have been reported for eugenol [31], thymol and thyme oil [59]. Seo and his research group observed large aggregates of eugenol–β-cyclodextrin complex as an agglomerated form which resulted from self-assembly of cyclodextrins in water [31].

Moisture content determination

Studies for the determination of moisture in cyclodextrin complexes can be performed by Karl Fisher method, vacuum oven drying method or toluene distillation. Moisture determination helps in supporting TG analysis, as TG cannot differentiate between water and oil mass losses from inclusion complexes [82]. Hence, moisture determination can be used for estimating total volatile constituents loss from TG curves. It has been carried out for lemon oil [26], cinnamon leaf and garlic oil [29], cinnamon oil [32], geraniol [54] and linalool [35] cyclodextrin inclusion complexes.

Zeta potential and polydispersity index

Stability of the formed inclusion complexes can be estimated by zeta potential assessment. Generally, at lower zeta potential values, attractive force between particles exceed repulsive forces, leading to aggregation or coalescence of particles [83]. Zeta potential is a measurement of surface charge that affects interaction of formulation with the biological environment. Zeta potential can be estimated by measuring electric potential, i.e., electrophoretic mobility and diffusion coefficient [81]. The pH and electrolyte concentration should be considered while measuring zeta potential [84, 85]. Seo et al. determined zeta potential of eugenol–cyclodextrin complex using Zeta sizer. It was found to be highly stable with zeta potential and PDI of −34.5 ± 1.2 mv (average value) and 0.3, respectively [31].

Polydispersity index (PDI) is a measure of uniform distribution of particle size present in the cyclodextrin complex suspensions, reflecting its tendency to agglomerate [86]. As a result of the self-assembly of cyclodextrin in water, there is strong tendency of cyclodextrin inclusion complexes for agglomeration [31]. Hence, PDI measurements can prove as a useful tool for stability analysis of cyclodextrin complexes.

Nuclear magnetic resonance spectroscopy

Nuclear magnetic resonance (NMR) spectroscopy may provide direct evidence for the entrapment of guest into the cyclodextrin cavity. This technique has been used to verify inclusion of numerous essential oils, or their constituents: thymol and carvacrol [70], eugenol [45], estragole [37] and Thymus catharinae camarda [62]. In cyclodextrins, generally, shift for hydrogen atoms of cyclodextrin in the cyclodextrin guest complex with respect to free cyclodextrin can be measured [67, 70]. If a guest moiety is encapsulated into cyclodextrin cavity, the H-atoms (C–3H) and (C–5H) located in the interior of the cavity will be significantly shielded by the guest and exhibit a considerable up field shift [87, 88]. Whereas, H-atoms (C–2H, C–4H and C–6H) on the outer surface will show a marginal up field shift [89]. H-atoms of the guest moiety depict corresponding downward shift characterized β-cyclodextrin carvacrol inclusion complex using 1H-NMR spectroscopy. Remarkable high yield shift for H3–H5 β-cyclodextrin proton resonance were measured in β-cyclodextrin–carvacrol inclusion complex, which indicated the true formation of this inclusion complex. The inclusion of carvacrol in the hydrophobic cavity of β-cyclodextrin includes this shielding of H3 or H5. Moreover, a strong interaction between the H6 proton (in rim of β-cyclodextrin) and guest was also reported [62].

C13 NMR has been applied for Sardinian Thymus herba-barona Loisel [51], carvacrol, thymol and eugenol [67], inclusion complexes. Using this technique, interaction between the cyclodextrin cavity and guest moiety can be studied from the shifting of carbon atoms.

Spectroscopic techniques

Sometimes, a bathochromic shift or/and band broadening takes place on complex formation leading to alteration in the original absorption spectrum (visible or ultra-violet) of the guest moiety. This shift may be due to a partial shielding of the excitable electrons in the cyclodextrin cavity [90]. High electron density in the cyclodextrin cavity may result in mobilization of electrons of the entrapped guest moiety [91]. This phenomenon can be explored for essential oil loaded cyclodextrin complexes.

Raman spectroscopy may prove useful in revealing structural features which are not easily accessible by others spectroscopic techniques. Although, most of the essential oil cyclodextrin complex studies provide IR data as an evidence for inclusion complex formation, but Raman spectroscopy may provide clearer and better evidence of formation of inclusion complex. This technique has been used for validation in curcumin cyclodextrin (1500–1800 cm−1) complex and revealed their complex formation [92]. Hence, this technique can be explored for characterization of essential oil cyclodextrin complexes.

Chromatographic techniques

Although an old technique, thin layer chromatography finds a lot of application in pharmaceutical analysis. It plays a vital role in the early stages for specific quantitative analysis using spot illusion followed by spectrophotometric detection. Another chromatographic technique, gas chromatography is a crucial separation technique for detection of volatile compounds, therefore, it has been widely used for essential oils. It allows accurate quantitative determination due to combination of separation and online detection [93]. Some groups of authors used this technique for analysis of lemon oil [39], Sardinian T. herba-barona Loisel [51], estragole [37] and cyclodextrin complexes.

Gas chromatography is conjugated with mass-spectrometry for quantitative analysis of complex mixtures. In case of essential oils, the GCMS profile helps in identification of major components along with quantities present in oils. Therefore, it plays a vital tool to study composition of essential oil and investigating their bioactivities along with mechanism of action. For essential oil inclusion complexes, this technique have been used for analysis of lemon oil [26], Mentha villosa [28], Litsea cubeba [44], cinnamon oil [32], thyme [52], Mentha piperita [34], (−)-linalool [35], Lippia grata [57] and F. artemisia [50].

Another sophisticated technique, liquid chromatography mass spectroscopy (LCMS) has also been reported for analysis of XSDEO cyclodextrin inclusion complex [48].

Applications of essential oil-inclusion complexes

Food applications

In food science, essential oils have been largely used as a condiment, flavoring agent and food grade preservative, in novel packaging system. Cinnamon oil is widely employed in cooking, as a condiment and flavor for desserts, candies, chocolate, tea and liquor [32]. Other essential oils/ essential oil components employed in food manufacturing include lavender oil [33], lemon oil, peppermint oil (−) linalool, d-limonene, sweet orange, essential oil of Mentha × villosa Hudson and essential oil of S. sclarea L. which are used for their flavoring properties as food additives (Table 1). Flavor is a mixture of aroma of compound or fragrant essential oil, added to enhance the original taste or aroma of a food, without providing aroma or taste of its own [94]. Mostly, the components of flavor are highly volatile liquids, which may undergo volatilization or oxidation on exposure to heat or light. Some essential oils such cinnamon leaf oil and garlic oil, estragole and eugenol have been used as food preservatives. Due to antimicrobial, antifungal, antiviral and insecticidal properties, these essential oils, via (−) linalool, cinnamon leaf oil and garlic oil, estragole have also been approved by FDA (Food Drug and Administration) as GRAS substances to be employed as food additives [95]. Besides these applications, the essential oils also possess a number of medicinal uses, which add to their utility as food ingredients.

The direct utilization of essential oils is limited due to their poor stability (in presence of oxygen, heat and light), solubility and bioavailability. Therefore, these oils have been encapsulated into cyclodextrin complexes. Encapsulation not only enhances solubility, stability and bioavailability but also masks the objectionable taste and odor of volatile oxidation products. In addition, such complexes result in controlled release products. Furthermore, α, β, γ-cyclodextrin have also been approved as additive in the European Union, for food industry, as well as granted GRAS status [34]. Cyclodextrins are known to have adequate thermal and chemical stability [96]. β-amylases and glucoamylases do not hydrolyze cyclodextrins. However, they are more suspectible to α-amylases (salivary and pancreatic amylase) or by hydrolysis. Another important attribute is that cyclodextrins possess negligible cytotoxic effects.

Ciobanu et al. evaluated complexation efficiency of α, β or γ cyclodextrin for 13 volatile flavor compounds: α-pinene, eucalyptol, linalool, limonene, camphene, β-pinene, ρ-cymene, myrcene, menthol, menthone, trans-anethole, camphor and pulegone, and observed β-cyclodextrin to be the most versatile for studing guest molecules [34]. Petrovic et al., encapsulated cinnamon oils with β-cyclodextrin successfully, using co-precipitation method and, reported 15:85 as the optimum ratio of essential oil to β-cyclodextrin for commercial acceptance of this complex. Further, it was reported that encapsulated essential oil maintained its pharmacological properties and organoleptic behavior [32]. Kfoury et al. complexed estragole from basil and tarragon essential oils with cyclodextrin and demonstrated that molecular inclusion allowed the controlled release of the estragole [37]. Ayala-Zavala et al., encapsulated cinnamon leaf and garlic oils using β-cyclodextrin and, the resultant complexes illustrated good anti-fungal activity, which can have application in the food technology [29].

Pharmaceutical and cosmetic applications

Cyclodextrins are equipped with the ability to temporarily modify undesirable physicochemical properties of guest moieties. Therefore, they have been explored for entrapment of essential oils in molecular inclusion complexes [20]. In this context, cyclodextrins have been reported to enhance stability, improve the solubility characteristics and oral bioavailability of essential oils. Table 2 summarizes essential oil–cyclodextrin inclusion complexes reported for pharmaceutical applications.

Stability plays an important role in pharmaceutical formulations, particularly for chemically labile moieties, like essential oils or their bio-actives. It has been well emphasized that cyclodextrins help in improvement of stability of various labile drugs against hydrolysis, dehydration, photodecomposition and oxidation, leading to better shelf life of these drugs [14]. Due to molecular shield, cyclodextrin complexes protect the labile guests against the degradation processes [97]. Tian and his research group demonstrated that S. sclarea L. essential oil–β-cyclodextrin complexes led to protection of this essential oil inside the cyclodextrin cavities, expanding its applications [42].

Essential oils or their components generally possess poor water solubility hampering their bioavailability. This limits their applications in pharmaceutical field. Xiang–Fu–Si–Wu Decoction essential oil (XFSWD) is poorly water soluble, volatile and possesses pungent smell [68, 98, 99]. The oil is also reported to be sensitive to oxygen, humidity, light and high temperature. This essential oil shows decreased bioavailability due to poor water solubility. Additionally, pungent smell is a reason for the poor compliance for oral administration [48]. Similar oral bioavailability enhancement results were obtained with β-caryophyllene/β-cyclodextrin inclusion complexes by Liu et al. [47].

To be pharmacological active, the drugs not only need to possess some aqueous solubility but also, most drugs require to be lipophilic for their permeation through biological membranes. Poorly soluble drugs are classified as class II (poorly soluble/highly permeable) or class IV (poorly soluble/poorly permeable) drugs, as per the Biopharmaceutics Classification System [100]. Hence, formulations of such drugs that increase their apparent water solubility, without affecting lipophilicity, play a major role in enhancing their clinical applications [71]. Cyclodextrins have been reported for apparent drug solubility enhancement through inclusion complexation, as a hydrophilic carrier. The capacity as solubilizer also depends on type of cyclodextrin used. For example, eugenol, major component of clove oil, possesses a wide spectrum of biological activities. However, the limitations such as poor solubility, pungent taste, degradation and irritation characteristics towards the mucosa, make it unsuitable for use. As per Garg et al., inclusion complex of eugenol with hydroxy propyl-cyclodextrin, prepared by lyophilization method retained oxidation potential and resulted in better solubility [45].

In cosmetic industry, essential oils can be employed as fragrance compounds in soaps, perfumes, colognes, shampoos and skin lotions. However, the above-mentioned limitations limit their use in cosmetic products as well. Additionally, due to reactive functionalities such as ketone, aldehyde and terpenes, degradation of fragrance compounds occurs, leading to changes in their sensory features, and eventually forming allergenic products [101,102,103]. Hence, encapsulation as cyclodextrin complexes may be adopted to improve the stability of many fragrance molecules.

Therapeutic applications

As mentioned earlier, essential oils have been known to possess diverse biological activities. They are potential sources of naturally derived compounds with several functions, such as anti-fungal, anti-microbial, antioxidant, anticonvulsant, analgesics, anticancer and antinociceptive effect. Additionally, some bio-actives like citronella and geraniol possess mosquito repellent activity. Encapsulation of these oils and their components improves their physicochemical properties, without affecting the biological efficacy.

As reported in Table 3, essential oil–cyclodextrin complexes have been investigated for therapeutic effectiveness by various research groups. Tao et al. encapsulated thymol and thyme oil in β-cyclodextrin inclusion complexes and investigated their physicochemical characteristics, along with anti-microbial activity against food borne bacteria [59].

Conclusion

Developments in drug design technology over the years have yielded numerous active moieties, whose undesirable properties, such as chemical or physical unstability, irritation, toxicity, poor solubility/dissolution and bioavailability have been a deterrent factor in their clinical utility. Amongst the various strategies to overcome such problems, use of cyclodextrins has been widely investigated and even found promising in alleviating drug delivery issues. This is further supported by the huge number of publications, patents and approved products. Although, carbohydrates in various forms have been used in food products, their degradation products, cyclodextrins, since their inception, opened new avenues not only for the food industry but for pharmaceutical and cosmetic industry, as well. With the use of cyclodextrins and their modified forms, it is possible to entrap active agents at a molecular level in cyclodextrin inclusion complexes.

Essential oils are known to possess properties which make them worthy of application in number of fields, including food, pharmaceuticals and cosmetics. For maximum utilization of the potential benefits of essential oils, cyclodextrin complexes represent a very attractive tool to overcome their limitations and facilitate ease of handling.

Despite the vast literature published in context of essential oil inclusion complexes, there are certain issues which demand the attention of research community and further investigation. The most important objective of fabricating inclusion complexes is to attain maximum inclusion efficiency. In this concern, type of cyclodextrin used, guest–host ratio and method of preparation constitute the most significant parameters, which need to be carefully optimized. Notwithstanding the research carried out by certain investigators, the studies should be more systemically designed, focusing on optimization of the above-mentioned parameters. Moreover, one should not overlook the fact that different essential oils may have different retention and inclusion efficiency in cyclodextrin complexes.

Though variety of techniques have been reported for the preparation of essential oil cyclodextrin complexes, the peculiar physical and chemical nature of essential oils calls for comparative studies, with the view to find the most suitable preparation process. Furthermore, the available literature suggests that β-cyclodextrins are the most explored amongst all the forms of cyclodextrins, while the other forms have received little attention. It is further emphasized that the investigations performed in this domain should be taken a step ahead of formulation development and characterization, so as to validate the claimed activities. With the advent of more sophisticated analytical tools and use of in silico approaches, the understanding of structural features of essential oil inclusion complexes can be further bolstered.