Recurrent caries is one of the causes of long-term restoration failure, leading to their replacement and incurring high costs for patients and healthcare systems [1], [2]. Adhesive resins with added antimicrobial agents have been studied to reduce biofilm formation at the tooth-restoration interface, thus decreasing the incidence of caries, dental tissue loss, and restorative material [3], [4]. The longevity of restorations is also related to the quality of the polymer formed, and the incorporation of inorganic particles into resin matrices reduces hydrolytic degradation and enhances the material's mechanical properties [5]. Therefore, inorganic particles that aid in hybrid layer stability and marginal integrity, besides possessing antimicrobial activity, are considered promising dental restorative materials to aid in reducing patients' susceptibility to recurrent caries.

To reduce bacterial colonization at the tooth/restoration interface, quaternary ammonium compounds (QACs) have shown antimicrobial activity in vitro [3], [4] and in situ [6], [7], [8]. Alkyl trimethyl ammonium bromide (ATAB, C17H38BrN), commercially known as cetrimide, features a long alkyl chain and broad antimicrobial activity [9], [10]. ATAB has been used in endodontic disinfection solutions, demonstrating activity against Enterococcus faecalis in dentinal tubules [11], and has been tested in a commercial resin cement, showing biofilm inhibition [12]. Recently, ATAB was incorporated into an experimental resin cement to confer activity against Enterococcus faecalis and did nit change its physicochemical properties [13]. Also, ATAB has exhibited antibacterial activity against Streptococcus mutans biofilms on dentin when in an aqueous solution at 0.2 % [14].

QACs, including ATAB, exert antimicrobial effects likely due to electrostatic interactions between the positively charged nitrogen atoms of the QAC structure and the negatively charged bacterial cell membrane/wall components. The long alkyl chain of QACs allows them to penetrate microbial membranes, increasing osmotic pressure and causing membrane disruption, leading to cell death [15], [16]. Despite ATAB's antimicrobial effect, it lacks a methacrylate group and does not copolymerize with resin matrices, making it more prone to degradation or loss of efficacy [17]. To overcome the lack of a methacrylate group and allow the delivery of antimicrobial agents, nano- [18] or microcapsules [19] and nanotubes [13], [20], [21] can be used.

Among drug delivery systems, nanotubes made from inorganic materials could provide additional advantages to resins, including enhanced mechanical properties. In this context, titanium dioxide (TiO2) nanotubes are viable options as drug carriers due to their tubular structure [22], potentially offering sustained antimicrobial effects over time and augmenting resin mechanical properties [22]. Moreover, as indicated in previous studies, TiO2 is a bioactive material when incorporated into polymers immersed in simulated body fluid solutions [23], [24]. This biological property could benefit dental tissues by promoting greater dentin remineralization in cases of selective caries tissue removal and facilitating mineral deposition and enamel remineralization at restoration margins. Therefore, in addition to enhancing the physicochemical properties of resin matrices (by increasing mechanical properties and reducing organic matrix), TiO2 nanotubes could be carriers of antimicrobial agents and bioactive additives in adhesive resins.

A previous study incorporated titanium dioxide nanotubes with alkyl trimethyl ammonium bromide (ntTiO2/ATAB) as fillers into an experimental adhesive resin and 1 wt%, 2.5 wt%, or 5 wt% did not impair the dentin bond strength [25]. However, there is no evidence about the chemical properties of the formulated adhesives or their aged antimicrobial efficacy. This study aimed to formulate experimental adhesives with ntTiO2/ATAB and evaluated their physicochemical properties, antimicrobial activity, mineral deposition, and cytotoxicity.