## 1 Introduction  
Modern endodontics has two goals: the eradication of microorganisms and the products of their metabolism from the root canal system and the obturation of the prepared root canal space to prevent reinfection [1; 2]. It is impossible to achieve an efficient disinfection without the appropriate irrigation protocol due to the complexity of the endodontic system (irregularities, lateral canals, ramifications, apical delta) [2; 3]. Abundant irrigation with antiseptic and chelating agents plays a crucial complimentary role to the mechanical preparation of the canals as it leads to the removal of debris, necrotic tissues, and the remnants produced during preparation. The most used irrigating solution during root canal treatment is sodium hypochlorite applied in various concentrations (0.5-5.25%). The protocol of irrigation should result in the dissolving of the smear and a considerable reduction of pathogens [4]. The removal of the smear layer is considered as a necessity to allow for the penetration of disinfecting agents into the dentinal tubules, which can contain microorganisms. After the removal of the smear layer, chemical agents such as concentrated sodium hypochlorite should be used to dissolve the organic remnants and bacteria in the tubules [2; 3; 4]. Following the appropriate irrigation protocol ought to assure the optimal interaction between the sealing material and the wall of the root canal, as the sealed closure of the root system is a significant stage of endodontic treatment [2; 5]. Endodontic sealers have important functions for successful canal treatment as they create an impenetrable barrier, which fills in the irregularities of the root canal system and the space between gutta-percha and dentin. After the removal of the smear layer, sealers penetrate into the dentinal tubules, creating resin tags, thus enclosing any microbiota that could have potentially survived the mechanical removal and antiseptic procedures [2; 5]. There has not been any observation of the chemical connection between sealing materials and the dentin yet. Therefore, the sealer that penetrates the dentinal tubules may be considered as a micromechanical barrier that enhances the retention of the sealing material to the walls of the root canal [2; 5; 6]. The depth of the sealer's resins tags in dentinal tubules is inter alia a sign of the efficacy of the removal of smear layer [2; 5; 6; 7]. Different final irrigation protocols may affect the dentinal tubule penetration of the sealer [8]. The depth of the sealer resin tags also relies on the type of solution used in the final stages of the irrigation protocol. The most commonly used liquids for this purpose are saline, sodium hypochlorite, citric acid, alcohols and chlorhexiidine. They are all characterized by various biophysical parameters. The following factors can be mentioned: hydrophilicity, surface tension and contact (wetting) angle [9]. Alcohols (ethanol and isopropyl) are commonly used as solvents for hydrophobic substances and mostly have low surface tension [10; 11; 12]. Various irrigating solutions can also have a positive or negative impact on the depth of the penetration of the hydrophobic epoxy sealer. Confocal laser scanning microscopy (CLSM) is often used to detect and observe the penetration of the sealer into the dentinal tubules. CLSM is a method for imaging but also tracking the location of fluorescent molecules with submicrometer resolution and high sensitivity, commonly used in life sciences including endodontics [13; 14; 15]. Various fluorescent compounds, upon excitation using an appropriate wavelength of light, are able to emit light (fluorescence) that can be collected in a confocal microscope system. If a given material is labeled with a fluorescent compound, CLSM is able to track its location in the nonfluorescent surrounding. Thus, the labeling of the sealer enables the detection and measurement of the depth of resin tags. The recent study carried out by [PERSON] et al.