Demineralization frequently occurs as a result of fixed orthodontic treatment, especially when oral hygiene is insufficient [1]. The acid byproducts generated by bacteria in plaque are crucial in initiating the eventual demineralization of enamel, resulting in the development of white spot lesions (WSL) [2]. Clinically, WSLs can become apparent around orthodontic attachments in as little as 4 weeks following the initiation of treatment [3]. Fluoride treatment stands out as an effective technique for reducing the incidence of WSLs [4]. Contemporary strategies to minimize the occurrence of white spots involve the application of Casein Phosphopeptide-Amorphous Calcium Phosphate (CPP-ACP), bioactive glass, and the incorporation of nano-particles with anti-caries properties within orthodontic adhesives/cement [4,5].

Significant progress has been made in improving the formulation of adhesives in recent years, leading to improved bonding properties [6]. Malfunctions in the bonding mechanism of orthodontic brackets can lead to frequent detachment, causing delays in reaching desired treatment results [7]. Previous research has effectively incorporated nanofillers into adhesives, illustrating that the addition of nanoparticles (NPs) can enhance the micro-tensile bond strength, fracture toughness, bond integrity, and improve antibiofilm action [8]. The higher anti-biofilm action of these particles is related to their nano size, which results in an improved surface area to mass ratio [8,9].

Zinc (Zn) has gained considerable recognition in the field of dentistry because of its firmly established antibacterial characteristics [10]. Zinc-oxide nanoparticles (ZnONPs) show potential in inhibiting the degradation of collagen in enamel. In addition, zinc oxide (ZnO) has been proven to be biocompatible, stable, minimally poisonous, and cost-efficient [11]. The favorable attributes of ZnONPs make them very suitable for incorporation as fillers in adhesives when bonding orthodontic brackets [8]. ZnONPs have the potential to be activated by exposure to visible light, leading to the generation of reactive oxygen species (ROS) that damage bacterial cells and disrupt biofilm formation [12]. In contrast, Curcumin (Cur) has a significant effect in suppressing the growth of Streptococcus Mutans (S.mutans) after being exposed to visible light [13]. Cur has been recognized as a highly effective vehicle in the production of zinc oxide (ZnO) [14]. Visible light activation of Cur results in the production of ROS [15]. These ROS possess the capacity to cause DNA damage, interfere with cellular metabolism, and hinder the creation of biofilms. In essence, this functions as a proactive strategy to mitigate a broad spectrum of infections [14,15].

Prior studies have already shown that the addition of ZnO to adhesive formulations enhances their antibacterial characteristics while maintaining their physical and chemical performance [16,17]. Saffarpour et al. found that the use of ZnONPs improves the antibacterial properties of adhesives without negatively impacting their bond integrity [18]. Nevertheless, there has been no earlier study on photoactivated Cur-doped ZnONPs with visible light in the orthodontic adhesive for bonding of orthodontic brackets.

The main goal of the current study was to incorporate Cur into ZnONPs at varying concentrations (0%, 2.5%, and 5%) and subsequently photoactivate them in the orthodontic adhesive. An extensive investigation was carried out utilizing a range of analytical methods, such as scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDX), Fourier-transform infrared (FTIR) spectroscopy, micro tensile bond strength (μTBS) testing, and evaluation of antibacterial effectiveness. The objective was to acquire a thorough comprehension of the photoactivated Cur-doped ZnONPs on the physical qualities, antibacterial efficacy, degree of conversion, and μTBS bond strength between orthodontic brackets and the enamel surface. The hypothesis proposed that the incorporation of photoactivated Cur-doped ZnONPs in different concentrations (0%, 2.5%, and 5%) into the adhesive for bonding orthodontic brackets will lead to higher μtensile bond strength, increased antibacterial efficacy, and enhanced degree of conversion (DC) values in comparison to adhesives without nanoparticle integration (0%).