*Article* **Developing a Highly Stable** *Carlina acaulis* **Essential Oil Nanoemulsion for Managing** *Lobesia botrana*

**Giovanni Benelli 1,\* , Lucia Pavoni <sup>2</sup> , Valeria Zeni <sup>1</sup> , Renato Ricciardi <sup>1</sup> , Francesca Cosci <sup>1</sup> , Gloria Cacopardo <sup>1</sup> , Saverio Gendusa <sup>2</sup> , Eleonora Spinozzi <sup>2</sup> , Riccardo Petrelli <sup>2</sup> , Loredana Cappellacci <sup>2</sup> , Filippo Maggi <sup>2</sup> , Roman Pavela 3,4 , Giulia Bonacucina <sup>2</sup> and Andrea Lucchi <sup>1</sup>**


Received: 20 July 2020; Accepted: 15 September 2020; Published: 18 September 2020

**Abstract:** The growing interest in the development of green pest management strategies is leading to the exploitation of essential oils (EOs) as promising botanical pesticides. In this respect, nanotechnology could efficiently support the use of EOs through their encapsulation into stable nanoformulations, such as nanoemulsions (NEs), to improve their stability and efficacy. This technology assures the improvement of the chemical stability, hydrophilicity, and environmental persistence of EOs, giving an added value for the fabrication of natural insecticides effective against a wide spectrum of insect vectors and pests of public and agronomical importance. *Carlina acaulis* (Asteraceae) root EO has been recently proposed as a promising ingredient of a new generation of botanical insecticides. In the present study, a highly stable *C. acaulis*-based NE was developed. Interestingly, such a nanosystem was able to encapsulate 6% (*w*/*w*) of *C. acaulis* EO, showing a mean diameter of around 140 nm and a SOR (surfactant-to-oil ratio) of 0.6. Its stability was evaluated in a storage period of six months and corroborated by an accelerated stability study. Therefore, the *C. acaulis* EO and *C. acaulis*-based NE were evaluated for their toxicity against 1st instar larvae of the European grapevine moth (EGVM), *Lobesia botrana* (Denis & Schiffermüller, 1775) (Lepidoptera: Tortricidae), a major vineyard pest. The chemical composition of *C. acaulis* EO was investigated by gas chromatography–mass spectrometry (GC–MS) revealing carlina oxide, a polyacetylene, as the main constituent. In toxicity assays, both the *C. acaulis* EO and the *C. acaulis*-based NE were highly toxic to *L*. *botrana* larvae, with LC<sup>50</sup> values of 7.299 and 9.044 µL/mL for *C. acaulis* EO and NE, respectively. The *C. acaulis*-based NE represents a promising option to develop highly stable botanical insecticides for pest management. To date, this study represents the first evidence about the insecticidal toxicity of EOs and EO-based NEs against this major grapevine pest.

**Keywords:** European grapevine moth; green pesticide; insect pest; Integrated Pest Management; Larvicide; nano-insecticide; Tortricidae

#### **1. Introduction**

The European grapevine moth (EGVM), *Lobesia botrana* (Denis & Schiffermüller, 1775) (Lepidoptera: Tortricidae), is a widespread and economically important pest of the grapevine worldwide. EGVM larvae feed on grape bunches (*Vitis vinifera* L.), reducing yield and increasing susceptibility to fungal and bacterial infections (i.e., botrytis and sour rot) [1].

Eco-friendly tools, including mating disruption and biopesticides (BPs) (mainly *Bacillus thuringiensis*), have been available against *L. botrana* for decades [2–5]. However, its control often requires the use of chemicals [6–9]. Finding valid and sustainable alternatives to insecticides is a key challenge for modern agriculture; side effects of insecticide use include environmental pollution, toxicity to non-target insects, and residues on food [10–13]. In this scenario, researchers are looking for new sustainable tools and products. Recently, they have focused on essential oils (EOs) as a new class of BPs to be employed in eco-friendly practices [14].

EOs are mixtures of plant metabolites, mainly monoterpenoids, sesquiterpenoids, and phenylpropanoids [14]; their insecticidal, acaricidal and nematocidal properties make them excellent alternatives to synthetic insecticides [14–16]. EOs are often characterized by two or three main compounds at high concentrations (20–85%) and other molecules at trace levels. A mechanism of action of EOs involves the inhibition of P450 cytochromes (i.e., these cytochromes are responsible for phase I metabolism of xenobiotics); other modes of actions include the neurotoxic activity-modulating octopaminergic system, gamma-aminobutyric acid (GABA) receptors and inhibiting acetylcholinesterase (AChE) [14].

EOs repellence [17,18], larvicidal [19–21], and insecticidal activities are proven on different arthropod pests of economic importance [22–25]. Among them, the EOs' efficacy has also been investigated on some Lepidoptera. Good examples are represented by *Cydia pomonella* (Linneaus) (Lepidoptera: Tortricidae) [26], *Thaumetopoea pityocampa* (Denis & Schiffermüller) (Lepidoptera: Notodontidae) [27], *Cadra cautella* (Walker) (Lepidoptera: Pyralidae) [28] as well as *Spodoptera littoralis* (Boisd.) [29–31] and *Spodoptera litura* (Fabr.) (Lepidoptera: Noctuidae) [32–34]. However, current knowledge about EOs toxicity on *L. botrana* larvae is strictly limited [35].

Even though EOs represent promising BP ingredients, their use in Integrated Pest Management (IPM ) programs is still scarce because their physico-chemical properties (i.e., poor water solubility, scarce stability, high volatility, thermal decomposition, and oxidative degradation) make them difficult to handle in field conditions. A solution to these difficulties is to coat or entrap EOs into a matrix. The encapsulation process enhances physico-chemical stability, prevents degradation of active agents, and improves the bioavailability of EOs [36,37]. Nanotechnology represents a suitable strategy to carry the EOs' active principles, overcoming their physiochemical limitations; the small size of nano-systems increases active ingredients spreading, deposition, permeation, and provides controlled release on the target site. Among nano-delivery systems, nanoemulsions (NEs) represent an efficient, low-priced, and safe way to carry EOs [38].

As defined by Nikam et al. [39], NEs are kinetically stable "biphasic dispersions of two immiscible liquids: either water-in-oil (W/O) or oil-in-water (O/W) droplets stabilized by an amphiphilic surfactant"; in this way, protection from the surrounding environment, suitable spreading, and penetration of the bioactive molecules are guaranteed by the matrix and low surface and interfacial tension [40]. Toxicity of EO-based NEs was tested on several insects of agricultural and medical interest such as aphids [41–43], mosquitoes [44–46], stored-product beetles [47,48], and some Lepidoptera [49–51]. Furthermore, it was also highlighted that the bioactivity of EO-based NEs was often higher compared to the EOs themselves [52–54].

The insecticidal activity of EO-based NEs has been never evaluated against *L. botrana*. Herein, we decided to deepen our knowledge about EO and EO-based NE effectiveness against this harmful insect pest. For this purpose, we selected the EO obtained from the root of *Carlina acaulis* L. (Asteraceae), which has revealed to be promising as an active ingredient of botanical insecticides, highly effective against vectors and stored product insects [55,56].

*Carlina acaulis*, also called "piccolo cardo", is a perennial herb growing on the mountainous soils of central Europe, up to 2000 m of altitude [57]. Being described in several official pharmacopoeias [58–61], this plant has been largely used in the European tradition as a remedy against several diseases [62–64]. Nowadays, its traditional use is still recognised in many European countries as a tonic, diuretic, anti-oedematous, anticancer, and antibiotic agent, and for the treatment of gastritis and cold [65–68]. Along with its various curative applications, *C. acaulis* is also described in the Italian list of botanicals to be used in food supplements [69] and in the BELFRIT (Belgium France Italy) list [70]. The EO obtained from the roots of this plant revealed as a major constituent (>90%) the polyacetylene 2–(3–phenylprop–1–yn–1–yl)–furan, also known as carlina oxide. The biological activities shown by this EO are noteworthy, but its innovative insecticidal potential is attracting the interest of the agrochemical industry.

In this scenario, reminding the importance of botanical EOs for the development of new sustainable pesticides, considering the promising insecticidal activities showed by the *C*. *acaulis* EO [56,71], and the limitations linked with its lipophilicity and volatility as well, herein a highly stable *C*. *acaulis*-based NE was developed. Furthermore, the *C*. *acaulis* EO and *C*. *acaulis*-based NE were evaluated for their toxicity against 1st instar larvae of *L*. *botrana*, a major grape pest worldwide.

#### **2. Materials and Methods**

#### *2.1. Carlina acaulis Oil Extraction and Chemical Characterization*

One kg of dry roots of *C. acaulis* obtained from A. Minardi and Figli S.r.l. (48012 Bagnacavallo RA, Italy), was firstly crushed using a shredder (Albrigi, mod. E0585, Stallavena, Verona, Italy), for then being put into a 10 L round flask with 6 L of distilled water. The roots were then subjected to hydrodistillation using a Clevenger-type apparatus for 8 h and using a heating system consisting of a Falc MA mantle (Falc Instruments, Treviglio, Italy). The EO, which showed a pale orangish colour, was obtained in a 0.4% yield (*w*/*w*). After the hydrodistillation process, the EO was decanted and separated from the aqueous layer, then dehydrated with an hydrous Na2SO4. Finally, it was collected in a vial closed with a polytetrafluoroethylene (PTFE)/silicone cap and kept at <sup>−</sup>20 ◦C until chemical analysis and subsequent biological assays. For the chemical characterization of the *C. acaulis* EO, the analysis was conducted using an Agilent 6890N gas chromatograph furnished of a single quadrupole 5973N mass spectrometer and an auto-sampler 7863 (Agilent, Wilmingotn, DE). The column used for the separation was an HP-5 MS capillary column (30 m length, 0.25 mm i.d., 0.1 µm film thickness; 5% phenylmethylpolysiloxane), supplied by Agilent (Folsom, CA, USA). The column was allowed to reach initially a temperature of 60 ◦C for 5 min, then it was raised up to 200 ◦C at 4 ◦C/min and finally to 280 ◦C at 11 ◦C/min for 15 min. The temperature of the injector and detector was set at 280 ◦C. The mobile phase used was constituted of 99.9% of He, with a flow of 1 mL/min. Before injection, the EO was diluted 1:100 in *n*-hexane, and then 1 µL was injected in split mode (1:50). The peak acquisition was achieved with electron impact (EI, 70 eV) mode in the range 29–400 *m*/*z*. The chromatograms obtained were analysed using the MSD ChemStation software (Agilent, Version G1701DA D.01.00) and the NIST Mass Spectral Search Program for the NIST/EPA/NIH EI and NIST Tandem Mass Spectral Library v. 2.3. The retention index (RI) was calculated using a mix of *n*–alkanes (C7–C30, Sigma-Aldrich, Milan, Italy), using the Vanden Dool and Kratz formula [72].

#### *2.2. Preparation and Characterization of Carlina acaulis Essential Oil (EO) Nanoemulsion*

*Carlina acaulis* EO-based NE was obtained through a high-energy method by using a high-pressure homogenizer. It was prepared according to the procedure reported by Rosi Cappellani et al. [73]. Briefly, 6% (*w*/*w*) of *C. acaulis* EO was added dropwise to a 4% (*w*/*w*) of surfactant (Polysorbate 80, Sigma-Aldrich) aqueous solution under high-speed stirring (Ultraturrax T25 basic, IKA® Werke GmbH and Co.KG, Staufen, Germany) for 5 min at 9500 rpm. The obtained emulsion was then subjected to

homogenization by means of a French Pressure Cell Press (American Instrument Company, AMINCO, Silver Spring, MD, USA) for four cycles at the pressure of 130 MPa.

Visual characterization of NE was performed by a polarizing optical microscope (MT9000, Meiji Techno Co. Ltd., Saitama, Japan) equipped with a 3-megapixel complementary metal oxide semiconductor (CMOS) sensor camera (Invenio 3S, DeltaPix, Smorum, Denmark).

Particle size measurements were carried out through dynamic light scattering (DLS) analyses by using a Zetasizer nanoS (Malvern Instrument, Malvern, UK) equipped with a backscattered light detector working at 173◦ . One mL of the sample was inserted into a disposable cuvette and analysed at 25 ◦C, following a temperature equilibration time (180 s).

#### *2.3. Nanoemulsion Stability Studies*

#### 2.3.1. Long-Term Stability

The sample was stored at room temperature and 12:12 (L:D) h for up to six months. The physico-chemical stability of the samples was evaluated by repeating DLS analysis at different time points: 0 day (t0), 1 month (t1), 3 months (t3), and 6 months (t6).

#### 2.3.2. Accelerated Stability Test

The thermodynamic stability of *C. acaulis* EO NE was evaluated through a three phases (centrifugation, heating/cooling cycles, and freeze/thaw cycles) test, according to the protocol reported by Alkilani et al. [74] with some modifications.


At the end of each phase, the sample was evaluated through visual inspection and DLS analysis.

#### *2.4. Lobesia botrana Mass-Rearing*

*Lobesia botrana* young instars tested in our bioassays were from a laboratory mass-rearing kept at the Entomology lab, University of Pisa. Adults were reared inside a plastic bottle and fed with a liquid diet. Eggs were collected every 2 days and placed into a plastic tray, previously drilled to allow airflow; each tray contained a piece of artificial food medium. Semi-synthetic larval diet is based on Gabel et al. [75] recipe (for 1 kg: deionized water 750 mL, agar-agar 15 g, sucrose 30 g, alfalfa flour 25 g, brewer's yeast 18 g, salts of Wessen 12.5 g, cholesterol 1.25 g, wheat germ 90 g, casein 40 g, sorbic acid 2 g, ascorbic acid 10 g, vitamins wanderzahnt 7.5 g, tetracycline 1.25 g, propionic acid 2.5 g, linoleic acid 1 mL, sunflower oil 2 mL); emerged adults were transferred into a new polyvinyl chloride (PVC) bottle. The rearing was maintained at a temperature of 25 ± 1 ◦C, R.H. 70 ± 10% and 16:8 (L:D) photoperiod.

#### *2.5. Insecticidal Activity on Lobesia botrana*

The insecticidal activity of EO and NE of *C. acaulis* on *L. botrana* was tested adapting the method by Bosch et al. [76] originally developed for insecticide toxicity assessment on *C. pomonella*. A 32 µL-drop of NE or EO formulation was deposited on the surface of a piece of semi-synthetic diet (4 × 4 × 1 cm) using a micropipette. The solution was evenly distributed using a humidified brush and allowed to dry for 2 h. Sixteen 1st instar larvae (L1) of *L. botrana* were deposited on each piece of diet and individualized within a gelatine capsule (00, Fagron, Quarto Inferiore, Bologna, Italy). Each piece of the diet with the larvae was placed in a closed plastic box to avoid desiccation.

Larval mortality was observed 96 h later, gelatine capsules were removed, and the diet was observed under a binocular microscope for larvae inside the diet. A larva was considered dead if it did not respond to a gentle touch with a small brush. Missing larvae were considered escaped and subtracted from the number of treated larvae. Seven concentrations of *C. acaulis* EO (1, 2.5, 6, 7.5, 8, 10, 30 µL/mL) and six concentrations of *C. acaulis* NE (5, 7.5, 8, 10, 30, 60 µL/mL) were tested, water was used as solvent to prepare the dilutions.

To validate the method described above, we also tested positive and negative controls. The positive control was a commercial insecticide, Spinosad (Laser®, Dow) tested at the tab dose (15 mL/hL); the negative control was 0.17% Polysorbate 80 + 99.83% of H2O for NE and H2O + dimethyl sulfoxide (DMSO) at the same concentration of the EO. At least three replicas for each concentration of EO, NE, positive and negative control were performed. For each tested product concentration, four duplicate trials were carried out; replicates were conducted over different days to account for any daily variability. Each concentration was always replicated with a new concentration series prepared for each replicate. All experiments were performed at laboratory conditions of 22 ± 1 ◦C, R.H. 45 ± 5%, and photoperiod 16:8 (L:D).

#### *2.6. Statistical Analysis*

*Lobesia botrana* mortality (%) was arcsine√ transformed before performing an analysis of variance (ANOVA, two factors as fixed effects) followed by Tukey's honestly significant difference (HSD) test (*p* < 0.05). The experimental mortality was corrected with Abbott's formula, if control mortality ranged from 1 to 20%; if control mortality was > 20% experiments were discarded and repeated [77]. LC10, LC30, LC50, and LC<sup>90</sup> with associated 95% confidence interval (CI) and chi-squares, were estimated using probit analysis [78]. JMP9 (SAS) software was used for all analyses, and *p* = 0.05 was selected as a threshold to assess significant differences.

#### **3. Results and Discussion**

#### *3.1. Essential Oil Chemical Composition*

Through gas chromatography–mass spectrometry (GC–MS) analysis, the EO extracted from the roots of *C*. *acaulis* was characterised and the data obtained were in accordance with the work of Benelli et al. [56]. Seven compounds were identified, among which carlina oxide was the predominant EO component, comprising 94.6% of the relative content. Other compounds were identified, such as the aromatic benzaldehyde (3.1%) and the sesquiterpene *ar*-curcumene (0.4%) (Figure 1). Acetophenone, benzyl methyl ketone, camphor, and carvone were detected at trace levels.

#### *3.2. Preparation and Characterization of the Essential Oil Nanoemulsion*

NEs are colloidal systems offering a great advantage to encapsulate a higher amount of oil phase respective to similar nanosystems, i.e., microemulsions [79]. In fact, such a system has allowed to vehiculate 6% (*w*/*w*) of EO, respective to at least 1.5% (*w*/*w*) encapsulated into microemulsions, as reported in previous studies [80,81]. Moreover, NEs require a meager amount of surfactant (4% *w*/*w*), with a surfactant-to-oil ratio (SOR) of around 0.6, respective to that of microemulsions, that is generally higher than 2 (SOR > 2) [82].

However, NEs are energetically disadvantaged nanosystems because they have a higher free energy level respective to that of the two separated phases (water + oil). Thus, to produce a colloidal system, an external energetic input is required to overcome the activation energy barrier separating the two phases. In this respect, one of the most commonly used methods is the homogenization process. It is a high-energy method that consists of a 2-step procedure [83]. The first step gives rise to an emulsion, characterized by oil droplets mainly in the micrometric range, through the high-speed stirring process of the oil and water phases [82]. The second step, the high-pressure homogenization, provides the breakage of oil droplets into small ones by forcing the material to flow through small

nozzles or valves by exerting very high pressures with a piston pump. During the flow, the emulsion is exposed to shear stress able to give rise to nanometric oily droplets [84].

√

**Figure 1.** Gas chromatography–mass spectrometry (GC–MS) chromatogram of the essential oil obtained from the roots of *Carlina acaulis*. The separation of peaks was achieved using a HP-5MS (5% phenylmethylpolysiloxane, 30 m length × 0.25 mm internal diameter, 0.1 µm film thickness).

For the achievement of *C. acaulis* EO-based NE, the sample was subjected to a pressure of 130 MPa four times. The sample showed a monomodal size distribution with a size in the nanometric range. In particular, the droplets' population had a mean diameter centred around 140 nm (Figure 2, black line). DLS analysis recorded Z-average and PDI (polydispersity index) values of 98.85 and 0.33, respectively. The Z-average value or Z-average mean used in DLS is a parameter, also known as the cumulants mean, that can be defined as the "harmonic intensity averaged particle diameter". Assuming that the particle population is a simple Gaussian distribution, the Z-average is the mean, and the PDI is related to the width of this simple distribution. Thus, the smaller the PDI (≤ 0.3), the more monodispersed the system will be [85].

The *C. acaulis* EO NE showed optimal stability at room temperature, evaluated for a storage period of six months. As reported in Figure 2, the size of the oil droplets remained almost unchanged, with a slight shift of mean hydrodynamic diameter from 143.9 nm at t0 to 170.2 nm after 6 months. These results proved the thermodynamic stability of the system. It was also confirmed by the accelerated stability test, generally used to predict the thermodynamic stability of the system for long-term periods. The accelerated stability was evaluated via centrifugation, heating–cooling cycles, and finally, freeze-thaw cycles stress tests. The NE showed a good physical stability at the centrifugal forces (Figure 3B) and remained almost unaltered to the heat–cool cycles. No signs of creaming, phase separation or cracking were detected (Figure 3C). These images were also corroborated by DLS analysis results (Table 1), that revealed the conservation of the internal phase structure, being the Z-average and PDI values almost unchanged with respect to those of the NE at t0. A slight creaming <sup>e</sup>ffect was observed when the NE was frozen at the temperature of −<sup>21</sup> ◦C. However, its homogeneity was recovered upon the thawing phase (Figure 3D). A similar result was reported by Ammar et al. [86], who attributed this transient instability to the low temperature leading to the coagulation of the internal phase. This perturbation of the systems was revealed by the increased value of the Z-average after the

freeze-thaw cycles, as reported in Table 1. However, the size of the oil phase was kept below 200 nm, which is the upper limit generally established by authors for NEs [87]. ≤

**Figure 2.** Dynamic light scattering (DLS) traces of *Carlina acaulis* essential oil-based nanoemulsion, at different time points: 0 day (t0), 1 month (t1), 3 months (t3), 6 months (t6).

**Figure 3.** *Carlina acaulis* essential oil nanoemulsion (EO NE) at t0 (**A**), after the centrifugation (**B**), after the heating–cooling cycles (**C**) and after the freeze-thaw cycles (**D**).

**Table 1.** Thermodynamic stability evaluation, in terms of Z-average, polydispersity index (PDI), creaming, and phase separation, of the *Carlina acaulis* essential oil (EO) nanoemulsion through the accelerated stability test.


<sup>\*</sup> The value is the mean of three measurements. \*\* The creaming phenomenon was observed only after the freezing of the sample. However, at the end of the cycles, after the thawing process, the sample did not more show creaming.

Therefore, given the results achieved by the stability study, this *C. acaulis* EO-based NE can be considered a physico-chemically stable nanosystem.

#### *3.3. Insecticidal Activity on Lobesia botrana*

Larval mortality in exposed *L*. *botrana* individuals was directly proportional to *C*. *acaulis* EO and *C*. *acaulis*-based NE concentrations (*F6,23* = 40.47, *p* < 0.0001; *F5,23* = 27.22, *p* < 0.0001, respectively); significant larvicidal activity was observed starting from 2.5 µL/mL of EO and 8.0 µL/mL of NE. Comparable concentrations of *C*. *acaulis* EO showed higher larvicidal activity over the *C*. *acaulis* EO NE. As reported in Table 2, 50% of larval mortality was achieved testing a concentration equal to 7.29 <sup>±</sup> 0.25 <sup>µ</sup>L/mL of *<sup>C</sup>*. *acaulis* EO and 9.04 <sup>±</sup> 0.39 <sup>µ</sup>L/mL of *<sup>C</sup>*. *acaulis* NE. Besides, the LC<sup>90</sup> of *<sup>C</sup>*. *acaulis* EO was lower than that of *<sup>C</sup>*. *acaulis*-based NE (10.92 ± 1.40 µL/mL and 17.70 ± 4.48 µL/mL, respectively); 100% larval mortality was achieved with the positive control represented by a semi-synthetic diet treated with the positive control spinosad (Laser®) at the label dose (i.e., 150 ppm).


**Table 2.** Larvicidal activity of *Carlina acaulis* essential oil (EO) and its 6% nanoemulsion (NE) against 1st instar larvae of *Lobesia botrana*.

 LC = lethal concentration killing 10%(LC10), 30% (LC30), 50%(LC50) or 90% (LC90) of the exposed population; SE = standard error; CI<sup>95</sup> = 95% confidence interval; <sup>4</sup> n.s. = not significant (*p* > 0.05). Positive control spinosad (Laser®) tested at tab dose (150 ppm) achieved 100% mortality.

It is difficult to compare our results with the findings by other authors as, to the best of our knowledge, research on the insecticidal efficacy of plant EOs against *L*. *botrana* is extremely limited. Only one study was retrieved, where Avgin et al. [35] tested 5 essential oils from seeds or aerial parts of aromatic plants such as *Thymus vulgaris* L., *Mentha* x *piperita* L., *Foeniculum vulgare* Mill., *Rosmarinus o*ffi*cinalis* L. and *Carum carvi* L. on field-collected grapes. The authors found that the EO from *C. carvi* was the most effective, since at a concentration of 25 µL on 20 g of grapes it achieved >96% mortality on *L. botrana* larvae. Most research about the EO efficacy on *L. botrana* was undertaken to explore any changes in adults' behaviour in response to EO aroma [88], aimed at using EOs to improve pest control strategies [89]. As far as we know, our study is the first that assesses EO efficacy on the mortality of freshly hatched *L. botrana* larvae as the usual target of insecticide application. Also, the study of EO-based NE efficacy on the larvae of phytophagous lepidopteran species is only beginning, and few papers on NE efficacy on moth pests exist so far [49–51], although EOs have been known to provide very good insecticidal effects on pests including phytophagous moth larvae [26,27]. Moreover, as indicated by previous studies, EO-based NEs actually show very promising effects, often significantly higher if compared to EOs [49,52,53].

*Carlina acaulis* EO was obtained from roots of carline thistle, and its main component is carlina oxide (~94%), one of the oldest known polyacetylenes. A recent study conducted by Benelli et al. [56] proved carlina oxide as a mild acetylcholinesterase (AChE) inhibitor. It has also been documented that polyacetylenes cause phototoxicity in insects [90], and are able to modulate GABA<sup>A</sup> receptors [91]. Recently, the effectiveness of *C. acaulis* EO has been demonstrated on other insect species, showing acute and sub-lethal toxicity on highly important pests and vectors, such as the southern house mosquito, *Culex quinquefasciatus* (Say) (Diptera: Culicidae) (LC<sup>50</sup> = 1.31 µg mL−<sup>1</sup> ) [56] and the common housefly, *Musca domestica* (L.) (Diptera: Muscidae) (LC<sup>50</sup> = 2.74 (♂) and 5.96 (♀) µg fly−<sup>1</sup> ) [71]. Moreover, simulating a small-scale maize conservation environment, the *C. acaulis* EO led to relevant insecticidal

activity against *Prostephanus truncatus* (Horn) (Coleoptera: Bostrychidae), with 500 ppm killing >97% of adult beetles within three days [55]. Either the effectiveness, as well as the availability and low costs of the *C. acaulis* EO, encourage further experimentation on EGVM for green pesticide development.

The comparison of the LC values obtained by testing *C. acaulis* EO and the corresponding NE, showed a comparable larvicidal activity. From the values reported in Table 2, it is possible to observe a higher insecticidal activity of the pure EO (LC<sup>90</sup> = 10.922 <sup>±</sup> 1.40 <sup>µ</sup>L/mL) over the NE (LC<sup>90</sup> <sup>=</sup> 17.706 <sup>±</sup> 4.48 <sup>µ</sup>L/mL). On the other hand, the NE contains 6% of EO, a value 16 times lower respect to pure EO used as reference. This shows that the EO encapsulated in the NE is more active than the pure *C. acaulis* EO, if considered at the same concentration. The increase in the larvicidal activity of pure EO encapsulated into the NE could be attributed to a better interaction between the active substance and the target site. First, the NE, providing a greater dispersion of the lipophilic substance (EO) in the aqueous phase, allows the diffusion of the EO in the *L. botrana* growth environment. Furthermore, the NE is able to increase the concentration of the EO at the interface, leading to a better and direct interaction with the biological components of the target. Moreover, the small size and large surface area of the NE-encapsulated EO droplets allow an increased absorption and cellular penetration into the target site. Therefore, the encapsulated EO can exert its larvicidal activity even at lower concentrations than pure EO. Finally, the NE appears promising in controlling the growth of *L. botrana*, not only for the larvicidal potential but also for the improvement of the physico-chemical properties and stability of the EO [38].

The efficacy of EOs including EO-based NEs may not be necessarily related only to acute or chronic toxicity, but, as already shown, even sub-lethal EO doses or concentrations may reduce the vitality, fertility, and longevity of insects [92,93] including harmful moths [26–28]. This phenomenon was also confirmed for the EO from *C. acaulis*, although for other insect species [56,71] and, therefore, any potential effect of sub-lethal concentrations should be studied for *L. botrana* as well. Similarly, the possibility of enhancing the insecticidal activity of the EO from *C. acaulis* using a suitable synergistic mixture with other EOs or their major constituents should be considered in further studies.

This study opens a new perspective on *L. botrana* management using botanical pesticides. It highlights the potential of Asteraceae EOs as valid alternatives to chemical insecticides, because of their low human toxicity, rapid degradation, low environmental impact, and reduced likelihood to trigger insecticide resistance [14,94,95]. The lack of physico-chemical stability makes EOs difficult to handle in open field conditions. However, the adoption of nano-delivery systems (e.g., NEs) increases EOs stability and solubility, improving their delivery, and establishing a sustained release of the active ingredients [38]. The adoption of nanotechnology in IPM showed it to be useful to overcome EOs' drawbacks and to amend their efficacy as biopesticides.

Further studies should be conducted on the larvicidal and adulticidal activity of EOs and EO-based NEs on *L. botrana* to find a valid substance to test in the open field. Moreover, as highlighted by Pavoni et al. [79], it is crucial to consider that a lot of EO-based NEs contain several, non eco-friendly ingredients (i.e., polysorbates). Thus, further research is needed to evaluate the effects of nano-encapsulation on EO toxicological profiles.

As mentioned above, the use of EOs to eliminate insects is an alternative pest control method that minimises any harmful effects on the environment. Since EOs are chemicals commonly found in nature, being contained in almost all vascular plants, and have been shown to be very friendly to non-target organisms, botanical insecticides based on EOs can be considered relatively safe for the environment [14,38,94]. Moreover, as EOs are highly volatile, only minimal problems with their residues are expected when used in soil and aquatic ecosystems [94]. We are aware that further studies on the effects of *C. acaulis* EO on non-target organisms will be needed to confirm environmental safety for this EO. Although solvents are usually added to EO-based formulations [79], NEs used in this study contain no solvents and are based on a surfactant with no effects in terms of eco-toxicity given its high level of biodegradability.

#### **4. Conclusions**

The present study highlighted the promising potential of the *C*. *acaulis* root EO as an effective ingredient for botanical insecticide development. This EO showed high insecticidal efficacy against 1st instar larvae of *L. botrana*, a major pest affecting grape cultivation, causing yearly significant economic damages. Moreover, this research supported the real-world applications of the *C. acaulis* EO through its encapsulation into a nanoformulation. The EO-based NE guarantees the conservation of the insecticidal activity while ensuring dispersibility in the environment as well as its stability along time. Although the results encourage the use of *C. acaulis* EO in the agricultural field, especially in organic farming, further investigations are needed to evaluate its eco-toxicological profile. Similarly, further studies are needed to reveal the effects of lethal and sub-lethal concentrations on fertility, longevity, and behaviour of *L. botrana*.

**Author Contributions:** Conceptualization, G.B. (Giovanni Benelli), L.P., G.B. (Giulia Bonacucina), F.M., and A.L.; methodology, G.B. (Giovanni Benelli), L.P., V.Z., R.R., E.S., S.G., R.P. (Roman Pavela), G.B. (Giulia Bonacucina) and F.M.; software, G.B. (Giovanni Benelli) and R.P. (Roman Pavela); validation, G.B. (Giovanni Benelli), L.P., R.P. (Roman Pavela), R.P. (Riccardo Petrelli) L.C., F.M., G.B. (Giulia Bonacucina) and A.L.; formal analysis, G.B. (Giovanni Benelli), L.P., V.Z., R.R., F.C., G.C., E.S., S.G., R.P. (Roman Pavela), R.P. (Riccardo Petrelli), L.C., F.M., G.B. (Giulia Bonacucina) and A.L.; investigation, G.B. (Giovanni Benelli), L.P., V.Z., R.R., F.C., G.C., E.S., S.G., F.M., and A.L.; resources, G.B. (Giovanni Benelli), G.B. (Giulia Bonacucina), R.P. (Riccardo Petrelli), L.C., and F.M.; data curation, G.B., L.P., R.P. (Roman Pavela), F.M., G.B. (Giulia Bonacucina), V.Z.; writing—original draft preparation, G.B. (Giovanni Benelli), L.P., V.Z., G.B. (Giulia Bonacucina), F.M. and A.L.; writing—review and editing, G.B. (Giovanni Benelli), L.P., V.Z., R.R., F.C., G.C., E.S., S.G., R.P. (Roman Pavela), R.P. (Riccardo Petrelli), L.C., F.M., G.B. (Giulia Bonacucina) and A.L.; visualization, G.B. (Giovanni Benelli), L.P., V.Z., R.R., F.C., G.C., E.S., S.G., R.P. (Roman Pavela), R.P. (Riccardo Petrelli), L.C., F.M., G.B. (Giulia Bonacucina) and A.L.; supervision, G.B. (Giovanni Benelli), G.B. (Giulia Bonacucina), F.M., R.P. (Riccardo Petrelli), R.P. (Roman Pavela), G.B. (Giulia Bonacucina) and A.L.; funding acquisition, G.B. (Giovanni Benelli), R.P. (Roman Pavela), and R.P. (Riccardo Petrelli) All authors have read and agreed to the published version of the manuscript.

**Funding:** Riccardo Petrelli would like to thank the Italian Ministry of Health for the PRIN grant (PRIN 2017,2017CBNCYT\_005) for financial support. Roman Pavela would like to thank the Ministry of Agriculture of the Czech Republic for its financial support concerning botanical pesticide and basic substances research (Project MZE-RO0418).

**Acknowledgments:** We are grateful to Patrizia Mazzarisi and Paolo Giannotti (University of Pisa, Italy) for their technical assistance in *L. botrana* rearing.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

### *Article* **Tunable Terahertz Metamaterial with Electromagnetically Induced Transparency Characteristic for Sensing Application**

**Jitong Zhong, Xiaocan Xu and Yu-Sheng Lin \***

School of Electronics and Information Technology, Sun Yat-Sen University, Guangzhou 510006, China; zhongjt5@mail2.sysu.edu.cn (J.Z.); xuxc5@mail2.sysu.edu.cn (X.X.)

**\*** Correspondence: linyoush@mail.sysu.edu.cn

**Abstract:** We present and demonstrate a MEMS-based tunable terahertz metamaterial (TTM) composed of inner triadius and outer electric split-ring resonator (eSRR) structures. With the aim to explore the electromagnetic responses of TTM device, different geometrical parameters are compared and discussed to optimize the suitable TTM design, including the length, radius, and height of TTM device. The height of triadius structure could be changed by using MEMS technique to perform active tunability. TTM shows the polarization-dependent and electromagnetic induced transparency (EIT) characteristics owing to the eSRR configuration. The electromagnetic responses of TTM exhibit tunable characteristics in resonance, polarization-dependent, and electromagnetically induced transparency (EIT). By properly tailoring the length and height of the inner triadius structure and the radius of the outer eSRR structure, the corresponding resonance tuning range reaches 0.32 THz. In addition to the above optical characteristics of TTM, we further investigate its potential application in a refraction index sensor. TTM is exposed on the surrounding ambient with different refraction indexes. The corresponding key sensing performances, such as figure of merit (FOM), sensitivity (S), and quality factor (Q-factor) values, are calculated and discussed, respectively. The calculated sensitivity of TTM is 0.379 THz/RIU, while the average values of Q-factor and FOM are 66.01 and 63.83, respectively. These characteristics indicate that the presented MEMS-based TTM device could be widely used in tunable filters, perfect absorbers, high-efficient environmental sensors, and optical switches applications for THz-wave optoelectronics.

**Keywords:** electromechanically; tunability; metamaterials; multi-functionalities; terahertz; refraction index sensor

#### **1. Introduction**

Metamaterials are regarded as artificial materials that are remain undiscovered in the natural environment [1–3]. Due to their extraordinary properties, metamaterials are becoming an emerging field in physics, chemical, engineering, and electrics subjects. In the recent years, there have been many investigations and reports in various potential applications of metamaterials, such as cloaking devices, high-sensitive environment sensors, perfect absorbers, security screening, tunable ultrahigh-speed filters, imaging devices, high-efficient light emitters, and non-destructive testing [3–10]. Metamaterials show many unique electromagnetic properties including field enhancement [11,12], negative refraction index [13], artificial magnetism [14], electromagnetically induced transparency (EIT) [15], and so on. By properly tailoring the geometric parameters, metamaterials are able to be easily operated in a wide spectrum range that includes terahertz (THz), infrared (IR), and visible light [16–26]. Among the whole electromagnetic spectra, THz wave is the transition spectrum that usually occupies the spectrum in the frequency range from 0.1 THz to 10 THz, which is between the IR and microwave wavelength. Since that metamaterial has great and ultra-sensitive electromagnetic response in the THz frequency range, THz metamaterial has become an emerging field during the recent years. One of the most used typical configurations of THz metamaterial is a split-ring resonator (SRR), which is commonly a

**Citation:** Zhong, J.; Xu, X.; Lin, Y.-S. Tunable Terahertz Metamaterial with Electromagnetically Induced Transparency Characteristic for Sensing Application. *Nanomaterials* **2021**, *11*, 2175. https://doi.org/ 10.3390/nano11092175

Academic Editor: Giovanni Benelli

Received: 6 July 2021 Accepted: 28 July 2021 Published: 25 August 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

ring with a split. It was theoretically proposed in 1999 for the first time and experimentally verified in 2000. Since then, many derivative designs were investigated and demonstrated based on SRR, such as the complementary SRR (cSRR) [27], V-shaped SRR [28], U-shaped SRR [29], electric SRR (eSRR) [30,31], and three-dimensional SRR (3D SRR) [32,33], etc. However, when the metamaterial structure is fabricated on the traditional solid substrates, the resonant frequencies of metamaterial are usually unable to be tuned, which means that these designs can only absorb or filter certain electromagnetic spectra in a passive manner. Aiming to improve the flexibility and to enhance the electromagnetic response of the THz metamaterial, many literature reports focusing on tuning mechanisms were reported, such as ferroelectric material [34,35], laser pumping [36,37], electrostatic force [38,39], thermal annealing [40,41], liquid crystal [42], semiconductor material [43], and so on. In addition, micro-/nano-electro-mechanical systems (MEMS/NEMS) technologies can easily realize mechanical manipulation in micro-scale or nano-scale and, as a result, can hugely improve flexibility and enhance the electromagnetic response of MEMS-based metamaterial in the THz frequency range. There have been many reports on MEMS devices with different tuning mechanisms, such as the electrothermal actuator, electrostatic actuator, piezoelectric actuator, electromagnetic actuator [44–47], etc.

In this study, we propose and demonstrate a tunable terahertz metamaterial (TTM) based on the MEMS technology in the THz frequency range. This TTM structure is composed of an inner Au layer, which is called a triadius structure, and an outer Au layer, which is called a eSRR structure. The whole structure is fabricated on Si substrate. The inner triadius structures are connected to the MEMS-based electrothermal actuator (ETA). By driving different dc bias voltages, the height between the triadius and eSRR structures can be changed and, therefore, exhibit high flexibility. The geometrical dimensions of the proposed TTM are optimized, including the length and height of inner triadius structure and the radius of outer eSRR structure. The field strengths distributions in this study, including the electric (E) and magnetic (H) fields of the triadius structure, eSRR structure, and TTM structure, will be analyzed and discussed, respectively. In addition, in order to investigate the potential applications of TTM in the environmental sensing application, the key sensing performances of TTM, such as figure of merit (FOM), sensitivity (S), and quality factor (Q-factor), will be calculated and discussed, respectively. Additionally, while exposed on different-refraction-index (*n* value) environment, TTM shows highly linear sensitivity in terms of the *n* values. These unique electromagnetic characteristics indicate that the TTM structure can be widely used in THz-range application fields, such as filters, switches, and high-efficient environment sensors including gas sensors, biosensors, chemical sensors, etc.

#### **2. Design and Method**

The schematic drawings of MEMS-based TTM and TTM unit cell are shown in Figure 1a,b, respectively. TTM is composed of the triadius and eSRR structures. A 300 nm thick Au layer is used in TTM. The inner triadius structures are connected to MEMS-based electrothermal actuator (ETA), which could exhibit high flexibility by driving different dc bias voltages to bend downwards. Figure 1c shows the geometrical denotations of the TTM unit cell, including metal length (*L*), radius of eSRR (*R*), and height between inner triadius and outer eSRR structures (*h*). The metal linewidth and gap of the inner triadius and outer eSRR structures are 5 µm. Figure 1d plots the relationship of driving voltages and displacements of MEMS-based TTM. The inserted images of Figure 1d are the geometrical dimensions of ETA. Due to the different thermal expansion coefficients between different materials, the cantilevers would be upward-bending after the release process in fabrication. Therefore, by driving different dc bias voltages on ETAs, the reconfiguration of MEMS-based TTM was proposed in order to compare and discuss different *h* values. The deformation in the free end of ETA is inversely related to applied voltage. In order to actuate the TTM unit cell for bending downwards, a driving voltage with a maximum value of 0.45 V would be induced on the TTM device. The inserted images

include the inner triadius structure on ETA without and with a driving voltage of 0.45 V, respectively. The initial height is *h* = 2.46 µm when the MEMS-based TTM is released, while *h* value will be bent to 0 µm when the driving voltage increases to 0.45 V. It is clear that the proposed MEMS-based TTM could be actively actuated to tune the resonance by bending the cantilevers downwards. –

bent to 0 μm when the driving voltage increases to 0.45 V. It is clear that the proposed

= 2.46 μm when the MEMS

Solution's finite difference time domain (FDTD) based simulations.

**Figure 1.** Schematic drawing of (**a**) MEMS-based TTM and (**b**) unit cell in detail. (**c**) Top view of TTM unit cell and the corresponding geometrical denotations. (**d**) Relationship of driving voltages and elevating heights (*h*) of TTM with ETA. Inserted images are the ETA simulations.

μm, the TE and TM resonances are 5 μm steps from 47.5 μm to 32.5 μm, both TE and value of 42.5 μm The optical properties of the proposed TTM device are simulated by using Lumerical Solution's finite difference time domain (FDTD) based simulations. Here, we define TE mode when the polarization angle equals to 0 ◦ and TM mode when the polarization angle equals to 90 ◦ . The propagation direction of incident light is set to be perpendicular to the *x-y* plane in the numerical simulations. Periodic boundary conditions are also adopted in the *x*-axis and *y*-axis directions and perfectly matched layer (PML) boundaries conditions are assumed in the *z*-axis direction. The transmission spectra (*T*) are calculated by monitor set on below of device. In these configurations, Si material serves as the substrate with the tailored Au layer atop. The permittivity values of Au and Si materials in the mid-IR wavelength range are calculated according to the Drude–Lorentz model [48,49].

#### **3. Results and Discussion**

The transmission spectra of the triadius structure in TE and TM modes with different *L* values are shown in Figure 2, respectively. This triadius structure shows polarizationdependent characteristics. For example, when *L* = 47.5 µm, the TE and TM resonances are at 0.58 THz. With *L* value decreasing by 5 µm steps from 47.5 µm to 32.5 µm, both TE and TM resonances are increased by 0.16 THz, (from 0.58 THz to 0.74 THz). As plotted in Figure 3, the field strength distributions (E-fields and H-fields) of the triadius structure possess a *L* value of 42.5 µm when monitored at 0.58 THz in TE and TM modes, respectively. The E-field strengths are focused on the end of the triadius structure, while the H-field strengths are concentrated along the contour of the triadius structure.

The transmission spectra with different *R* values of eSRR structure in TE and TM modes are shown in Figure 4a,b, respectively. In Figure 4a, eSRR shows the EIT characteristic at 0.60 THz with *R* = 45 µm. The EIT resonance is shifted to 0.68 THz and the *R* value decreased to 40 µm. The shifting range of EIT resonance is 0.08 THz. This EIT resonance gradually vanishes by continuously decreasing *R* value to 30 µm. By decreasing *R* value in 5 µm steps (gradually from 45 µm to 30 µm), the resonances are blue-shifted with a shifting range of 0.40 THz (from 0.58 THz with *R* = 45 µm to 0.98 THz with *R* = 30 µm). On the other hand, in the TM mode, by decreasing *R* value in 5 µm steps (gradually from 45 µm to 30 µm), the resonances are modified with a modifying range of 0.32 THz (from 0.67 THz with *R* = 45 µm to 0.99 THz with *R* = 30 µm). According to the results mentioned above, eSRR exhibits polarization-dependent characteristics. Figure 5 plots the field strengths distributions (E- and H-fields) of eSRR with a *R* value of 40 µm in TE and TM modes, respectively. In TE mode, the monitors of field distributions are set as 0.67 THz, 0.68 THz, and 0.70 TH, while it is set as 0.74 THz in TM mode. It can be observed in Figure 5a–c that the E-field strengths are focused on the ends of arc-shape of eSRR in order to generate the electric quadrupole, six-polar, and dipolar modes at 0.67 THz, 0.68 THz, and 0.70 THz, respectively. Meanwhile, the E-field strengths of TM resonance are focused on the on the ends of the arc-shape of eSRR, which is the electric quadrupole mode as shown in Figure 5d. The corresponding H-field strengths of eSRR in TE and TM modes are shown in Figure 5e–h for the electric quadrupole, six-polar, and dipolar modes at 0.67 THz, 0.68 THz, and 0.70 THz for TE resonance and electric quadrupole mode at 0.74 THz for TM resonance, respectively.

**Figure 2.** Electromagnetic responses of the triadius structure by changing the *L* parameter in (**a**) TE and (**b**) TM modes.

= 42.5 μm in TE and TM modes. ( **Figure 3.** Field distributions of the triadius structure with *L* = 42.5 µm in TE and TM modes. (**a**) and (**b**) are E-field distributions. (**c**) and (**d**) are H-field distributions.

= 45 μm. The EIT resonance is shifte

= 42.5 μm in TE and TM modes. (

value to 30 μm. By decreasing

value to 30 μm. By decreasing

value of 40 μm in TE and TM

= 45 μm to 0.98 THz with = 30 μm). value in 5 μm steps (gradually from

value of 40 μm in TE and TM

= 45 μm to 0.98 THz with = 30 μm). value in 5 μm steps (gradually from

value in 5 μm steps (gradually from 45 μm to 30 μm), the r

45 μm to 30 μm), the resonances are modified with

= 45 μm. The EIT resonance is shifte value decreased to 40 μm. The shifting range of EIT resonance is 0.08 THz. This EIT reso-

= 45 μm to 0.99 THz with = 30 μm).

value in 5 μm steps (gradually from 45 μm to 30 μm), the r

= 45 μm to 0.99 THz with = 30 μm).

45 μm to 30 μm), the resonances are modified with

–

–

–

**Figure 4.** Electromagnetic responses of eSRR by changing the *R* parameter in (**a**) TE and (**b**) TM modes.

= 40 μm in TE and TM modes. ( – – **Figure 5.** Field distributions of eSRR with *R* = 40 µm in TE and TM modes. (**a**–**d**) are E-field distributions. (**e**–**h**) are H-field distributions.

value of 47.5 μm = 45 μm, the second TE resonance is at 0.66 THz while the second TM resonance value in 5 μm steps (from 45 μm to 30 μm), the second TE resonance is increased = 45 μm to = 30 μm), while the second TM resonance is increased = 45 μm to 1.00 THz for = 30 μm). Meanwhile, 47.5 μm, the first TE and TM resonances influenced by the triadius structure = 45 μm value decreased to 40 μm. The shifting range of EIT resonance is 0.07 THz. This EIT resovalue to 30 μm. value of 47.5 μm and of 40 μm in TE and TM modes are plotted in value of 47.5 μm = 45 μm, the second TE resonance is at 0.66 THz while the second TM resonance value in 5 μm steps (from 45 μm to 30 μm), the second TE resonance is increased = 45 μm to = 30 μm), while the second TM resonance is increased = 45 μm to 1.00 THz for = 30 μm). Meanwhile, 47.5 μm, the first TE and TM resonances influenced by the triadius structure = 45 μm value decreased to 40 μm. The shifting range of EIT resonance is 0.07 THz. This EIT resovalue to 30 μm. value of 47.5 μm and of 40 μm in TE and TM modes are plotted in The transmission spectra of TTM with different *R* values in TE and TM modes with a constant *L* value of 47.5 µm are shown in Figure 6a,b, respectively. The resonances are superimposed from the resonances of the triadius and eSRR structures. For example, when *R* = 45 µm, the second TE resonance is at 0.66 THz while the second TM resonance is at 0.70 THz, respectively. By decreasing the *R* value in 5 µm steps (from 45 µm to 30 µm), the second TE resonance is increased to 0.32 THz (from 0.66 THz with *R* = 45 µm to 0.98 THz with *R* = 30 µm), while the second TM resonance is increased to 0.30 THz (from 0.70 THz for *R* = 45 µm to 1.00 THz for *R* = 30 µm). Meanwhile, since the *L* value is kept as constant at 47.5 µm, the first TE and TM resonances influenced by the triadius structure almost remain unchanged. TTM shows the EIT characteristic at 0.60 THz with *R* = 45 µm in TE mode as shown in Figure 6a. The EIT resonance is shifted to 0.67 THz with the *R* value decreased to 40 µm. The shifting range of EIT resonance is 0.07 THz. This EIT resonance gradually vanishes by continuously decreasing the *R* value to 30 µm. The field strengths distributions (E-fields and H-fields) of TTM with *L* value of 47.5 µm and *R* value of 40 µm in TE and TM modes are plotted in Figure 7, respectively. In TE mode, the monitors of field distributions are set at 0.53 THz, 0.67 THz, and 0.73 THz, while in TM mode they are set at 0.54 THz and 0.78 THz. As observed in Figures 7a–c and 7g–h, the E-field strengths are focused on the end of the triadius structure as well as the ends of the arc-shape of eSRR for both TE and TM resonances, respectively. Meanwhile, plotted in Figures 7d–f and 7i–j are the corresponding H-field distributions of TTM in TE and TM modes (0.53 THz, 0.67 THz, and 0.73 THz for TE resonances; 0.54 THz and 0.78 THz for TM resonances), respectively.

– –

– –

= 47.5 μm. **Figure 6.** Electromagnetic responses of TTM by changing the *R* parameter in (**a**) TE and (**b**) TM modes under the condition of *L* = 47.5 µm.

μm and = 40 μm in TE and TM modes. ( – – **Figure 7.** Field distributions of TTM with *L* = 47.5 µm and *R* = 40 µm in TE and TM modes. (**a**–**c**,**g**,**h**) are E-field distributions. (**d**–**f**,**i**,**j**) are H-field distributions.

–

The transmission spectra of TTM structure with different *h* values in TE and TM modes with a constant *L* value of 47.5 µm and a constant *R* value of 40 µm are plotted in Figure 8. The resonances are superimposed from the resonances of the triadius and eSRR structures. For example, when *h* = 0 µm, the first TE resonance is at 0.49 THz while the first TM resonance is at 0.51 THz, respectively. By increasing the *h* value from 0 µm to 3 µm, the first TE resonance is increased to 0.09 THz (from 0.49 THz for *h* = 0 µm to 0.58 THz for *h* = 3 µm), while the first TM resonance is increased to 0.07 THz (from 0.51 THz for *h* = 0 µm to 0.58 THz for *h* = 3 µm). Meanwhile, since the height of the eSRR structure is kept constant at 0 µm and the *R* value is kept constant at 40 µm, the second TE and TM resonances influenced by eSRR structure almost remain unchanged. Particularly, in TE mode, TTM with *R* = 40 µm shows the EIT characteristic at 0.67 THz. The field strengths distributions (E-fields and H-fields) of TTM with the *L* value of 47.5 µm, *R* value of 40 µm, and *h* value of 1 µm in TE and TM modes are plotted in Figure 7, respectively. In TE mode, the monitors of field distributions are set at 0.54 THz, 0.67 THz, and 0.73 THz, while in TM mode they are set at 0.54 THz and 0.78 THz. In Figures 9a–c and 9g–h, the E-field strengths are focused on the end of the triadius structure as well as the ends of the arc-shape of eSRR for TE and TM resonances, respectively. Meanwhile, the corresponding H-field distributions of TTM in TE and TM modes are plotted in Figures 9d–f and 9i–j, respectively. μm and = 40 μm in TE and TM modes. ( –

**Figure 8.** Electromagnetic responses of TTM by changing the *h* parameter in (**a**) TE and (**b**) TM modes.

In order to further explore the potential applications of the proposed TTM device in environmental sensing, the key sensing performances of TTM, such as figure of merit (FOM), sensitivity (S), and quality factor (Q-factor), are investigated. In this study, TTM with constant geometrical parameters (*L* = 47.5 µm, *R* = 40 µm, and *h* = 0 µm) is exposed on the surrounding ambient with different refraction indexes (*n* values). Figure 10a,b shows the trends of sensitivities between TE and TM resonances and *n* values, respectively. They are quite linear. Here, we define the corresponding resonances in TE and TM modes and the sensitivities as *ω*1, *ω*2, *ω*3, and *S* = ∆*f*/∆*n*, respectively. The ∆*f* is the shift of resonant frequency and ∆*n* is the change of *n* value. In TE mode, the calculated *S* at *ω*1, *ω*2, and *ω*<sup>3</sup> are 0.138 THz/RIU, 0.21 THz/RIU, and 0.379 THz/RIU, respectively. It is obvious that the third resonance (*ω*3) is more sensitive to the *n* value than the others. In TM mode, the corresponding *S* values at three resonances are 0.15 THz/RIU, 0.223 THz/RIU, and 0.373 THz/RIU, respectively. Obviously, these results indicate that the third resonance is more sensitive to the *n* value as well. The definition of Q-factor is that *Q* = *fr*/FWHM and FOM are defined as FOM = (1 − *Ar*) × *Q* [50], where *f<sup>r</sup>* is the frequency of resonance and *A<sup>r</sup>* is the corresponding transmission amplitudes, respectively. The calculated Q-factors and FOMs values at different TE and TM resonances are plotted in Figure 10c,d, respectively. Table 1 is a summary table of the corresponding Q-factors and FOMs values. Let us take the third TE resonance (TE: *ω*3, green line) as an example, then the maximum, minimum, and

average values of the calculated Q-factors are 72.47, 59.91, and 66.01, respectively, while the corresponding calculated FOMs values are 71.33, 56.49, and 63.83, respectively. The sensing performances of this design are better than those reported in literature reports [9,15,51] as summarized in Table 2. Therefore, the proposed MEMS-based TTM device could be suitably used in environment sensing fields, such as gas sensing, bio-sensing, and chemical sensing, etc.

μm in TE and – – **Figure 9.** Field distributions of TTM with *h* = 1 µm in TE and TM modes. (**a**–**c**,**g**,**h**) are E-field distributions. (**d**–**f**,**i**,**j**) are H-field distributions.

−

*ω*

*ω*

μ μ μ

Δ *ω ω*

*ω*

μ μ μ **Figure 10.** Electromagnetic responses of the TTM exposed on the surrounding ambient with different refraction index (*n*) in (**a**) TE and (**b**) TM modes, where *L*, *R*, and *h* parameters are kept as constants at 47.5 µm, 40 µm, and 0 µm, respectively. (**c**,**d**) are the Q-factors and FOMs of TTM in TE and TM modes, respectively.


**Table 1.** Summaries of Q-factors and FOMs of TTM.

**Table 2.** The comparison of sensing performances in this study and literature reports.


**Table 2.** *Cont.*


#### **4. Conclusions**

In conclusion, a reshaping TTM structure is presented and it is composed of triadius and eSRR structures. By tailoring the geometrical parameters of TTM, such as the length (*L* value) and height (*h* value) of the inner triadius structure and the radius (*R* value) of the outer eSRR structure, the corresponding electromagnetic behavior exhibits polarizationdependent, tunable bandwidth, electro-magnetically induced transparency (EIT), and large resonance-tuning-range characteristics. The variation of the *L* value causes the resonance blue-shift with a frequency range of 0.16 THz. While the variation of the *R* value shows that the transmission bandwidths could be modified to possess EIT characteristics. The variation of the *h* value shows that the resonance could be tuned 0.09 THz. In addition, by changing the surrounding refraction index (*n* value), MEMS-based TTM shows ultrahigh sensitivity to the surrounding environment. In TE mode, the calculated sensitivity value reaches 0.379 THz/RIU at most, the maximum Q-factor is 72.47, and the maximum FOM is 71.33. In TM mode, the calculated sensitivity value reaches 0.373 THz/RIU, the maximum Q-factor is 73.27, and the maximum FOM is 62.33. These results indicate that the presented MEMSbased TTM has great characteristics and great application potential for high-flexibility tunable filter, perfect absorber, imaging device, optical detecting, environment sensor, and switch applications in the THz frequency range.

**Author Contributions:** Conceptualization, Y.-S.L.; methodology, J.Z.; software, J.Z.; validation, J.Z., X.X. and Y.-S.L.; formal analysis, J.Z.; investigation, J.Z.; resources, Y.-S.L.; data curation, J.Z. and X.X.; writing—original draft preparation, J.Z.; writing—review and editing, Y.-S.L.; visualization, J.Z. and Y.-S.L.; supervision, Y.-S.L.; project administration, Y.-S.L.; funding acquisition, Y.-S.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the financial support from the Natural Science Foundation of Basic and Applied Foundation of Guangdong Province (2021A1515012217), National Key Research and Development Program of China (2019YFA0705004), and National Natural Science Foundation of China (11690031).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** No new data were created or analyzed in this study. Data sharing is not applicable to this article.

**Acknowledgments:** The authors acknowledge the financial support from the Natural Science Foundation of Basic and Applied Foundation of Guangdong Province (2021A1515012217), National Key Research and Development Program of China (2019YFA0705004), National Natural Science Foundation of China (11690031), and the State Key Laboratory of Optoelectronic Materials and Technologies of Sun Yat-Sen University for the use of experimental equipment.

**Conflicts of Interest:** The authors declare no conflict of interest.

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