Aromatic plants have been used in cosmetics and folk medicine for many years; however, plant extracts and essential oils (EO) became attractive for the industry in the last decade since consumers seek sustainable therapeutics agents with possible reduced adverse effects (Winska et al., 2019; Ramsey et al., 2020).

The EOs are composed of volatile metabolites secondary extracted from different parts of plants (flowers, buds, seeds, leaves, twigs, bark, herbs, wood, fruits, and roots) and present anticancer, antimicrobial, antiviral, antioxidant, and antibiotic properties (Aziz et al., 2018; Basavegowda et al., 2020; Ramsey et al., 2020). In dentistry, some EO-containing solutions have shown anxiolytic, antimicrobial, antiplaque, antigingivitis, and anticaries effects against oral pathogens (Zero et al., 2004; Dagli et al., 2015; Yadav et al., 2016; Quintas et al., 2017; Ghaderi and Solhjou, 2020).

Current lifestyle and diet habits may lead to tooth wear in young populations (Lussi et al., 2004; Donovan et al., 2021) and the effects of daily-use oral hygiene products on dental tissues need to be addressed. EO-containing mouthrinses do not require professional prescription or application, and are freely sold in supermarkets, drugstores, convenience stores, and on the internet; therefore, these over-the-counter products represent one of the most rapidly growing sectors of the oral care industry (Jardim et al., 2009; Weijden et al., 2015; Favaro et al., 2020). It is expected that products based on natural compounds such EOs may present limited detrimental effects. Some clinical trials have shown a significant reduction in plaque and gingivitis promoted by daily use of an EO-containing mouthrinse in combination with regular brushing and flossing (Sharma et al., 2004; Araujo et al., 2015; Alshehri, 2018). For a better understanding of the present study, it is worth highlighting that the use of the term essential oil-containing solutions mentioned throughout the text refers to mouthwashes.

This study aimed to determine the pH, TA, and mineral concentrations of EO-containing solutions, as well as their effect on enamel surface loss, morphology, and mineral concentrations.

In the present in vitro study, available essential oil mouthwashes commercially (dōTERRA Cosméticos do Brasil Ltda.), BaCloTea (Basil, Clove e Tea Tree), GeLaTeaPep (Geranium, Lavender, Tea Tree and Peppermint), EucaLem (Eucalyptus and Lemon), Cinnamon, and Spearmint solutions were tested according to the chemical characterization of the mouthwashes in relation to pH, TA; and concentrations of Ca, K and Na by induced plasma optical emission spectrometry (ICP OES). In addition, tests of these rinses were carried out on the surface of bovine tooth enamel by: quantifying C, K, Na and P through ion chromatography; analysis of morphology by scanning electron microscopy (SEM); and loss of enamel surface dental by profilometry. Artificial saliva was used as a positive control group, acid citrus 0.3% as a negative control group. Distilled water and other mouthwashes mouthpieces of OEs were the experimental groups. The sample size was determined according to a previously carried out pilot study. As it is an in vitro study, this research did not follow a guideline.

The EO-containing solutions investigated in this study have low pH, TA, and Ca, K, and Na concentrations. In addition, these solutions released low Ca and P concentrations into the enamel and induced different levels of surface loss without affecting remarkable morphology alterations.

The EO-containing solutions presented pH values below 5.5, which is widely considered critical for enamel dissolution; however, this process also depends on the buffer capacity, TA, and Ca, P, and fluoride (F) concentrations (Dawes, 2003; Saads Carvalho and Lussi, 2020; Harper et al., 2021). Several oral care products such as F-containing mouthrinses have a low pH (Hellwig and Lussi, 2006). The results of this study are in agreement with other investigations on EO- and F-containing mouthrinses (Zero et al., 2004; Delgado et al., 2018; Valdivia-Tapia et al., 2021) and pH values similar to sports drinks (Sato et al., 2021). The EO-containing solutions investigated in this study showed low pH; however, they needed a small amount of base to be neutralized (low TA) (Saads Carvalho and Lussi, 2020), which represents a low erosive potential for tooth tissues (Harper et al., 2021). Delgado et al. (2018) observed some commercially available mouthrinses (Listerine Total Care, Listerine Ultraclean, Listerine Original, and Scope Classic) with pH below 5.5 and high TA; thus, both parameters were significantly correlated to dentin loss and erosive potential. Similar findings were also reported by Cavalcanti et al. (2010) regarding other branded mouthrinses.

In this study, the EO-containing solutions presented ions that are important for the oral environment. Ca is essential for remineralization, while P and Na play a major role in membrane potential and hypersensitivity regulation (Cummins, 2009; González-Cabezas and Fernández, 2018; Li et al., 2020). Moreover, Ca and P are the main components of hydroxyapatite [Ca₁₀(PO₄)₆(OH)₂]. The Spearmint showed the highest pH value, lowest TA, and Ca concentration similar to the other EO-containing solutions. This EO has 58% of carvone (C₁₀H_14O), which is a monoterpene ketone with promising antimicrobial, antispasmodic, anti-inflammatory, antioxidant, antinociceptive, and anticonvulsant activities (Dehsheikh et al., 2020; Pina et al., 2022); in addition, Mentha spicata has demonstrated to retard biofilm formation and can be considered an effective, cheap and safe alternative to improve oral health (Rasooli et al., 2009; Wiwattanarattanabut et al., 2017).

Among the scarce evidence on the effect of EO-containing mouthrinses, their use against cariogenic bacteria (Almeida Freires et al., 2015; Braga et al., 2019; Ma et al., 2020) and to reduce plaque and gingivitis have been suggested (Haas et al., 2016; Richards, 2017; Lynch et al., 2018). Some vegetable oils have been demonstrated to form a lipid-rich pellicle that minimizes enamel and dentin demineralization (Buchalla et al., 2003; Kensche et al., 2013; Ionta et al., 2017). The preventive potential of vegetable oils has been widely studied since they are natural, edible, cheap, and worldwide accessible (Ionta et al., 2017).

In this study, the 14-day protocol of exposure of enamel specimens to EO-containing solutions simulated two daily mouthrinses (morning and afternoon) usually performed by individuals (Vieira-Junior et al., 2019) and evaluated the sole effect of five EO-containing solutions on enamel without other variables such as erosion, wear, or toothbrushing (Zero et al., 2004; Delgado et al., 2018).

Most commercially available EO-containing mouthrinses usually contain 5%–27% of alcohol, which acts as a preservative, germicide, and solubilizer to maintain EO bioavailability (Silverman and Wilder, 2006; Gandini et al., 2012; Lynch et al., 2018; Pelino et al., 2018). The five EO-containing solutions evaluated in this study were prepared without the addition of alcohol by following the manufacturer’s recommendations for at-home use. Some studies have shown reliable antibacterial and antiplaque effects of alcohol-free EO-containing mouthrinses (Quintas et al., 2017; Lynch et al., 2018).

The highest Ca, K, Na, and P concentrations were observed in the enamel specimens exposed to artificial saliva (positive control). Despite the variety of artificial saliva formulations used for in vitro research, this product usually induces greater enamel remineralization than water (Ionta et al., 2014). The chromatography revealed that the EO-containing solutions released smaller amounts of Ca²⁺ and P ions when compared to the distilled water group.

Except for the negative control, the BaCloTea group showed the greatest enamel surface loss, which is in agreement with its lowest pH among the investigated EO-containing solutions. EucaLem, GeLaTeaPep, and Spearmint showed the lowest enamel surface loss mean values and were not significantly different from the distilled water group. However, in the SEM photomicrographs, the enamel surfaces of the specimens exposed to the EO-containing solutions were found similar to the positive control and distilled water groups. Loss of enamel surface was only clearly observed in the negative control group. While SEM is a method for qualitative analysis of the enamel surface microanatomy (Zhang et al., 2000), profilometry is a quantitative method that can accurately determine surface losses greater than 0.4 µm (Attin and Wegehaupt, 2014). Therefore, it can be assumed that profilometry can detect clinically imperceptible losses on the enamel surface that are not visualized through SEM.

Under slightly acidic pH, the Ca²⁺ ions from the tooth surface react with F ions to form a layer of calcium fluoride (CaF₂) or CaF₂-like molecule, which is more acid-resistant (Lussi et al., 2012; Schiffner, 2021). Interestingly, the EO-containing solutions investigated in this study showed chemical parameters that could benefit from the addition of F ions to remineralize the abovementioned enamel surface losses.

The effects of oral hygiene products on tooth structure must be well known, since they are widely used on a daily basis to maintain oral health. Thus, EO-containing solutions can be a viable and non-aggressive alternative to preserve tooth structure. Since this in vitro study did not fully simulate the oral environment, the effect of human saliva on acquired pellicle formation and enamel remineralization was not addressed (Nahsan et al., 2018; Lopes et al., 2020). Although the specimens were stored in artificial saliva, the effect of the EO-containing solutions on the enamel surface may have been more pronounced. Therefore, further in situ studies and clinical trials are encouraged to overcome these limitations.

The EO-containing solutions have low pH, TA, and low concentrations of Ca, Na, and K. Moreover, enamel exposed to these solutions showed low Ca and P concentrations and slight surface loss without morphology alteration.