Dentin hypersensitivity (DH) is characterized by sharp, short episodes of pain resulting from exposed dentin in vital teeth, triggered by thermal, mechanical, osmotic, or chemical stimuli.^[1] The prevalence of tooth sensitivity has been reported to range from 4% to 57%, with higher rates among patients with periodontal diseases, reaching 60%–98%.^[2] It has been shown that in insensitive dentin, particularly in the cervical region, nearly all dentinal tubules are occluded. Enamel defects in the cervical area and the use of hand instruments on cervical dentin surfaces during periodontal procedures such as scaling, root planing, and surgery are among the most significant factors exposing dentinal tubules to the oral environment and various stimuli.^[2] Additional factors include attrition, abrasion from toothbrushing, chemical erosion, and gingival recession associated with aging or periodontal diseases. Microscopically, the severity of tooth sensitivity is primarily determined by the number and diameter of open dentinal tubules.^[3]

Another increasingly recognized cause of dentinal exposure is the use of bleaching agents in esthetic dentistry. Oxidative compounds such as hydrogen peroxide, commonly found in both in-office and at-home whitening products, can induce morphological and chemical alterations in enamel and dentin, including demineralization and increased dentinal permeability. These changes may facilitate the exposure of dentinal tubules and contribute to DH, particularly when high concentrations or repeated applications are used. Recent systematic reviews have confirmed that bleaching procedures can adversely affect the ultrastructure of dental hard tissues and play a significant role in the etiology of sensitivity symptoms.^[4]

Clinical bleaching procedures often result in temporary tooth sensitivity, affecting a significant proportion of patients. This sensitivity is primarily attributed to structural changes in enamel, including increased surface roughness and porosity, which facilitate the penetration of bleaching agents into the dentin and possibly the pulp. These alterations may compromise enamel microhardness and lead to side effects such as hypersensitivity, gingival irritation, and dehydration of the tooth structure.^[5]

Several theories have been proposed to explain tooth sensitivity, with the hydrodynamic theory being the most widely accepted. According to this theory, hydraulic changes in the intratubular fluid – induced by thermal, mechanical, chemical, bacterial, or evaporative stimuli – lead to either direct stimulation of pulp mechanoreceptors or indirect stimulation of odontoblasts.^[6] In general, desensitizing treatments act through one or both of the following mechanisms: (1) reducing fluid movement within the dentinal tubules, either by decreasing the number of open tubules, narrowing their diameter through coagulation and protein deposition, or forming calcium complexes and (2) reducing dentinal nerve activity by interfering with their electrical conduction via potassium ions.^[6]

The current treatment options range from conservative approaches – such as toothpastes containing strontium salts, potassium nitrate, sodium fluoride, monofluorophosphate or amine fluoride, bioactive glass, hydroxyapatite, or Novamin, as well as mouthwashes and adhesive resins – to more invasive interventions, including restorations and root canal therapy. Given the high prevalence of tooth hypersensitivity and the limited efficacy of current treatments, the search for more effective and efficient therapeutic options remains a priority.^[7]

In recent years, the therapeutic potential of natural substances in dental applications has gained increasing attention. Propolis and Nigella sativa (black cumin seed) have demonstrated promising outcomes in recent studies.^[8-10] Nigella sativa contains thymoquinone and exhibits antibacterial, anti-inflammatory, and anti-cariogenic properties; it also contributes to the prevention of periodontal and gingival diseases.^[11] Propolis, a resinous biomaterial produced by bees, is recognized for its biocompatibility and well-documented benefits to oral health.^[12] Evidence suggests that propolis supports the preservation of periodontal connective tissue, enhances healing and regeneration, and promotes bone biomineralization. Chen et al. (2015) reported that red propolis extract was capable of occluding dentinal tubules.^[13] Propolis possesses antimicrobial, antitumor, anesthetic, anti-inflammatory, antiviral, and regenerative properties. In dentistry, its key applications include alleviating tooth hypersensitivity through tubular occlusion, reducing pulp inflammation, facilitating oral tissue healing, and aiding in the prevention and management of dental caries.^[14]

Nanotechnology has driven significant advancements across the various scientific disciplines. Nanoscale structures, such as nanoparticles, possess a high surface-to-volume ratio, resulting in increased reactivity due to the large number of atoms or molecules relative to their mass. Recently, nanomicelles have attracted attention for their potential in the controlled release of pharmaceuticals. These bipolar structures can simultaneously bind to hydrophilic substances on one side and hydrophobic compounds on the other.^[15]

This in vitro study aimed to compare the dentinal tubule occlusion produced by a nanoemulsion containing Nigella sativa nanoparticles encapsulated in propolis nanomicelles, fluoride varnish, and normal saline under acidic and toothbrush (TB) challenge conditions. The null hypothesis was that there would be no significant difference in occlusion among the treatments, including the nanoemulsion at 15- and 30-min immersion times.

In this in vitro study, the hydroethanolic, hydrophilic extract of Nigella sativa was first produced, enca in bipolar micelle nanoparticles, and placed in a hydrophobic nano-propolis base. This in vitro experimental study was approved by the ethics committee under the ethical code IRKMU. REC.1400.63.

DH is a prevalent clinical condition, and numerous treatment approaches have been proposed. However, despite extensive research, no universally accepted gold standard for DH management has been established.^[1] Under physiological conditions, saliva can contribute to dentinal tubule occlusion by delivering calcium and phosphate ions into the tubules and forming a protective surface layer of salivary glycoproteins combined with these minerals.^[18] Nevertheless, this natural mechanism is relatively slow and provides only temporary relief. Therefore, effective treatment strategies capable of achieving rapid and durable tubule occlusion, even under the challenges of the oral environment, are highly desirable. In this context, the present in vitro study evaluated the efficacy of a nanoemulsion-containing Nigella sativa nanoparticles encapsulated within propolis nanomicelles, compared with fluoride varnish, in achieving dentinal tubule occlusion under acidic and TB challenge conditions. studies.

The null hypothesis of the present study was rejected, as significant differences were observed among the tested substances – normal saline, fluoride varnish, and nanoemulsion (NE) with immersion times of 15 and 30 min – in terms of dentinal tubule occlusion. Both the type of desensitizing agent, the challenge regimen, and their interaction had significant effects on tubule occlusion. Under acidic challenge conditions, NE with a 30-min immersion achieved the highest percentage of occluded dentinal tubules, whereas under TB challenge conditions, the highest occlusion was observed with NE after 15 min of immersion. Overall, the nanoemulsion demonstrated superior performance compared with fluoride varnish in occluding dentinal tubules, with the most pronounced advantage observed under TB and acid challenges following 30 min of immersion.

Kumar et al. demonstrated that Nigella sativa promotes the remineralization of non-cavitated demineralized lesions.^[18] Similarly, Farooq et al. reported an increase in enamel surface microhardness following exposure to thymoquinone, which persisted after immersion and TB challenge tests.^[19] Anthoney et al. further investigated toothpastes containing thymoquinone and nano-oxide/bioactive glass fluoride, showing that their application resulted in uniform dentinal tubule occlusion.^[20] The ability of Nigella sativa to occlude dentinal tubules appears to be linked to the anti-inflammatory, immunomodulatory, and antioxidant properties of thymoquinone, its primary phytochemical component.^[21]

Propolis has also been extensively studied for its anti-inflammatory activity, with bioflavonoids identified as its principal bioactive constituents. These compounds can stimulate dentin formation and reduce dentin permeability.^[22] Based on the hydrodynamic theory, effective management of DH requires closing dentinal tubules to limit fluid movement and nerve stimulation. Accordingly, the nanoemulsion (NE) used in this study may reduce DH by achieving effective tubule occlusion and decreasing dentin permeability.^[6]

Several studies have demonstrated the potential of propolis in managing DH through dentinal tubule occlusion. Using SEM, Midha et al. showed that toothpaste containing propolis effectively occluded dentinal tubules.^[23] Similarly, Gargouri et al. reported that xylitol chewing gum enriched with propolis could achieve comparable effects.^[24] Kripal et al. observed a significant reduction in the number of patent dentinal tubules following the application of propolis varnish.^[25] Arabnejad et al. found that propolis, similar to fluoride varnish, could occlude dentinal tubules, with the resulting deposits demonstrating resistance to acid challenge.^[26]

Other investigations have supported these findings: Davari et al. reported that 40% propolis gel effectively alleviated DH symptoms, suggesting it as a cost-effective treatment option.^[27] Chen et al. observed that propolis extract maintained a high degree of tubule occlusion even after acid exposure.^[13] Madhavan et al. also confirmed the clinical effectiveness of propolis in DH management.^[28] However, Sales-Peres et al. noted that 10% and 30% propolis gels resulted in only partial dentinal tubule occlusion.^[29]

Propolis is more effective than 5% potassium nitrate in relieving DH. Its immediate tubule-occluding effect is attributed to flavonoid compounds, while its sustained action is related to the stability of the product.^[22] Hussain et al. reported that patients who used propolis after bleaching experienced no hypersensitivity compared to the control group.^[30] The precise mechanism of adhesion of propolis to the tooth surface remains unclear; however, if this adhesion is primarily mechanical, it may be lost over time, leading to diminished effects.^[27]

Propolis is a sticky, lipophilic substance with low water solubility, which increases its contact time with dental tissues and enhances resistance to acid dissolution. Tubule occlusion by propolis has been attributed to interactions among its components, particularly between flavonoids and dentin, resulting in crystal formation that reduces dentinal fluid flow and consequently hypersensitivity, as proposed by Sabir et al.^[31] These effects are further supported by the correlation between high flavonoid content and crystal deposition on the dentin surface.^[29] Additionally, natural resin compounds in propolis confer bonding properties similar to dental adhesives and varnishes, enabling attachment to the tooth surface through diffusion and mechanical interlocking within dentinal microporosities. This process creates a surface coating and occludes the tubules, thereby preventing fluid movement.^[32] Owing to its ability to penetrate deeply into dentinal tubules, propolis can provide long-lasting pain relief.^[33] Flavonoids within propolis may also stimulate reparative dentinogenesis, which further contributes to tubule occlusion.^[31] In the present study, incorporating propolis at the nanoscale alongside Nigella sativa nanoparticles appeared to enhance its occlusive efficacy.

Previous studies have shown that fluoride varnish can effectively occlude dentinal tubules.^[26] Sales-Peres et al. demonstrated that potassium oxalate, fluoride gel, and propolis all reduced dentin permeability through tubule occlusion.^[29] Fluoride is thought to decrease dentin permeability by precipitating insoluble calcium fluoride within the tubules, and its role as a desensitizing agent is further supported by its ability to increase resistance to acid-induced demineralization.^[28]

In the present study, citric acid was applied following the desensitizing agents to simulate the acidic challenge associated with dietary acids from foods and beverages, while TB abrasion was reproduced using a brushing machine. An effective desensitizer should withstand both acid and mechanical abrasion while maintaining its functional properties. Citric acid, an organic hydroxyl acid present in fruits, juices, and soft drinks, was used at a 6% concentration for 2 min, followed by a 1-min rinse with saline.^[34]

The present study demonstrated that nanoemulsion containing Nigella sativa nanoparticles encapsulated in propolis-based nanomicelles achieved higher dentinal tubule occlusion than fluoride varnish, particularly under acid and toothbrushing challenges. Statistical comparisons revealed that extending the immersion time from 15 to 30 min improved acid resistance, while the 15-min immersion group exhibited slightly better resistance to mechanical wear. These findings suggest that the nanoemulsion formulation not only promotes immediate tubule sealing but also maintains a degree of durability under conditions simulating the oral environment. Compared with fluoride varnish, the superior performance of the nanoemulsion may be attributed to the synergistic action of Nigella sativa and propolis, both known for their anti-inflammatory, antimicrobial, and remineralizing effects, as well as the enhanced penetration and surface interaction provided by their nano-scale delivery.

In light of these findings, the use of a nanoemulsion containing Nigella sativa nanoparticles encapsulated in propolis-based nanomicelles offers a promising natural alternative for DH management. The nano-formulation enhances the delivery and penetration of active compounds into the dentinal tubules, improving occlusion efficacy and resistance to acid and mechanical challenges. Unlike conventional desensitizers, this natural biomaterial-based approach combines multiple therapeutic actions in a biocompatible system. Furthermore, the synergistic interaction between Nigella sativa and propolis in nanoform may result in more durable tubule sealing. However, as this was an in vitro study, further clinical trials are required to confirm the long-term effectiveness, safety, and patient-centered benefits of this innovative treatment in real-world dental practice.

Nanoemulsion, particularly with 30-min immersion under acid challenge, achieved the highest dentinal tubule occlusion. Acid and TB challenges reduced occlusive efficacy across all materials, with normal saline outperforming fluoride varnish. Propolis and normal saline show potential as natural agents for DH management, warranting further investigation into optimal application parameters.