###### Abstract  
The aim of the present study was to evaluate the effect of the pre-application of dentin bonding agent and laser application on the prevention of tooth discoloration caused by (MTA) in the presence of blood. Fifty extracted human anterior teeth were prepared by standardizing root lengths to 10 mm and shaping root canals. Specimens were divided into five groups (n = 10) based on the treatment applied to the pulp chamber dentin wall: Group 1/no surface treatment; Group 2/Optibond FL; Group 3/Clearfil SE; Group 4/Optibond Universal adhesive application; Group 5/Nd:YAG laser application. Root canals were filled with fresh human blood below the cement-enamel junction (CEJ), followed by ProRoot MTA, and collagen barrier placement. Color changes were monitored using a spectrophotometer at 0, 7, 30, 90, and 180 days post MTA placement. Color differences (AE) were calculated and analyzed using two-factor mixed-design ANOVA with Sidak adjustment at \(p\) = 0.05. The degree of coloration increased with time within each group (\(p\) < 0.05). No significant differences were observed between Optibond FL and Optibond Universal within each time interval, or between Clearfill SE Bond and the control group (\(p\) > 0.05). When compared to the control group, the Nd YAG group exhibited the least degree of discoloration in all time intervals (\(p\) < 0.05). Although the Nd:YAG laser had promising results, none of the methods can guarantee a 100% prevention of discoloration resulting from tooth discoloration caused by MTA-blood contact. calcium silicate materials; dentin bonding; dentin tubule occlusion; Nd:YAG laser; tooth discoloration +
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light irradiation in an oxygen-free environment [12]. Furthermore, some studies suggest that the color change resulting from a blood clot formation could be linked to the infiltration of blood products like hemoglobin, hematin, and erythrocytes into the dentin tubules [2]. The mechanisms underlying the blood-associated discoloration of Mineral Trioxide Aggregate (MTA) remain unclear. One potential explanation for discoloration resulting from contact between blood and MTA may involve the interaction between erythrocytes and unset MTA. The gradual hydration process of MTA might facilitate the absorption and subsequent hemolysis of erythrocytes from the blood, leading to both material and subsequent tooth discoloration [13]. Notably, discoloration linked to collagen bismuth oxide has been documented as well [14]. Nevertheless, it is important to highlight that MTA without bismuth oxide can also induce discoloration in the presence of blood, potentially due to the porosity of the cement or the hypothesis of heme absorption from hemoglobin [15]. Research indicates that the heme group is a prosthetic group composed of a ferrous (Fe\({}^{2+}\)) ion located at the center of a large heterocyclic organic ring called a porphyrin. This structure is formed by four pyrrolic groups interconnected by methine bridges [16]. Over time, blood in contact with the cement undergoes a natural redox reaction leading to the conversion of Fe\({}^{2+}\) (with a red color) to ferric (Fe\({}^{3+}\)), which imparts a dark brown color. This transformation may contribute to the darkening of the cement and, subsequently, tooth discoloration [13; 17]. Hence, it is advisable to consider alternative treatment approaches, focusing on the pre-treatment occlusion of dentin tubules, to prevent discoloration [7; 18]. The application of one or more layers of dentin bonding agent (DBA) prior to MTA application has demonstrated effectiveness in preventing tooth discoloration [19; 20]. However, this approach may not entirely prevent MTA penetration into the dentin tubules [18; 20]. An alternative strategy for dentin tubule occlusion could be the application of a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser, utilizing a technique known as'melting and re-solidification' [18].