## 1 Introduction  
Root canal treatment is an endodontic treatment sequence for removing the infected pulp and protect the fumigated tooth from future bacterial invasion. Mineral trioxide aggregate (MTA), a powder mixture typically consisting of 75% Portland cement, 20% radiopacifier, and 5% gypsum, has been widely used in endodontic treatments like apexification, direct pulp capping, and root-end filling [1; 2; 3]. During application, MTA is mixed with deionized water that can react with Portland cement to produce alkaline calcium hydroxide (CH) and calcium silicate hydrate (CSH) offering hydrated MTA cements unique sealing ability [4], antibacterial properties [5], and preserving pulp vitality [6], whereas the radiopacifier within MTA provide adequate radiopacity to distinguish filling materials from surrounding anatomical structures [7]. Currently, commercially available ProRoot utilizes bismuth oxide (Bi\({}_{2}\)O\({}_{3}\)) as a radiopacifier [8]. Endo CPM Sealer(r) (EGEO SRL, Buenos Aires, Argentina) [9] and EndoSequence(r) root repair material (Brasseler USA, Savannah, GA, USA) [10] use barium sulfate (BaSO\({}_{4}\)) and tantalum oxide (Ta\({}_{2}\)O\({}_{5}\)) respectively to satisfy the radiopacity requirement. Among the reported radiopacifiers for MTA, bismuth oxide (Bi\({}_{2}\)O\({}_{3}\)) exhibited quite a high radiopacity in X-ray photo image contrast for dental clinical uses [11; 12]. Although bismuth oxides have been commonly used as radiopacifier within MTA, their intrinsic toxicity, reduced mechanical strength, setting times, difficult manipulation, and tooth discoloration issues have been investigated. Tooth discoloration that might be attributed to the possible reactions of Bi\({}_{2}\)O\({}_{3}\) with sodium hypochlorite, dentin (collagen), blood contamination (hemoglobin and hematin molecules), or UV-light under anaerobic conditions were extensively addressed [13; 14]. Modifying bismuth oxide by adding secondary oxide (i.e., zirconia) via mechanical milling process [15] and wet chemical methods such as precipitation and sol-gel processes [16; 17] have been attempted. Zirconia-added bismuth oxide exhibited improved radiopacity performance when compared with the pristine counterpart. Using alternative hydration solutions such as calcium lactate gluconate [18] and silk fibroin solution [19] have been attempted to extend the practical applications of MTA. Radiopacity usually refers to a material capable of X-ray attenuation relating to the interactions like coherent scattering, photoelectron effect, and Compton scattering between X-ray photons and traversed matter for diagnostic imaging [20]. The photoelectron effect arising from the interaction of X-ray photos with inner-shell electrons is the dominant factor [21]. Radiopacifiers with high atomic number (Z) improve radiopacity by increasing the mass attenuation coefficient (\(\upmu\)) in accordance with the power law of \(\upmu\infty Z^{3}\)[22]. It shows almost no scattering radiation and results in a high-quality image. Hafnium and its alloys showed superior properties in neutrons absorbing, high melting point, and corrosion-resistant [23]. The perspective of applying hafnium oxide (HfO\({}_{2}\)) in medical science such as neutron detection, bioimplants, biosensors, radiotherapy, and multimodal theranostic imaging [24; 25]. An attempt to modify the radiopacity of Bi\({}_{2}\)O\({}_{3}\) by yttria-stabilized zirconia dopant was reported to show significantly greater radiopacity than that of pure Bi\({}_{2}\)O\({}_{3}\)[26]. Inspiring by the similar structure in shell electrons but a higher atomic number to zirconium, the study aims to optimize the novel radiopacifier by doping hafnium ions to Bi\({}_{2}\)O\({}_{3}\) via the sol-gel process for future dental root canal filling materials.