## 1 Introduction  
Due to the unique properties afforded by their size, nanoparticles (NPs) possess a wide range of applications in the industrial, electrical, agricultural, pharmaceutical, and medical fields. For anticancer applications, nanoparticles can be easily modified and conjugated with multiple functionalities, such as targeting molecules, imaging agents, and drugs [1,2]. Nanoparticles are used as a drug carrier for the specific delivery of drugs to cancer cells. In targeted drug delivery, the anticancer drug is delivered specifically to the cancer cells, reducing the toxicity to normal cells.  
Titanium dioxide (TiO\({}_{2}\)) is considered to be a potential semiconductor for biocidal applications due to its photocatalytic properties, which explain its ability to destroy bacteria, viruses, and even cancer cells [34]. In this context, TiO\({}_{2}\) NPs have considerable potential in biomedicines, and a variety of works have been conducted to develop new antibacterial and drug delivery systems based on this nanoparticle [5, 6, 7, 8]. Additionally, TiO\({}_{2}\) has been classified as a biologically inert substance in animals and humans [9, 10], and it has good biocompatibility and no toxicity in vitro or in vivo [11].  
The application of TiO\({}_{2}\) NPs in biosystems may be difficult, due to their poor dispersibility and stability in water or biological sera. To improve the colloidal behavior of NPs in biological systems, it is necessary to study the surface properties and colloidal stability of TiO\({}_{2}\) NPs [11; 12; 13]. Recent studies have focused on covering the surface of TiO\({}_{2}\) NPs with polymeric materials to eliminate aggregation and sedimentation [11; 14], reduce toxicity, and develop biocompatibility [15]. In this context, polyethylene glycol (PEG)--due to its hydrophilic character--permits the modification of the surface of NPs in order to prevent agglomeration, and also renders NPs resistant to protein adsorption and enhances their biocompatibility [15; 16; 17].  
2-Methoxyestradiolol (2 ME) is an endogenous estrogen metabolite with antiangiogenic and anticancer properties, although it has a poor solubility and low bioavailability for in vivo models [18]. With the purpose of maintaining its high anticancer efficacy, different reports are found in the literature about the incorporation of 2 ME in different types of NPs, such as metallic NPs [19; 20], polymers NPs, and other nanostructured materials [21; 22]. These studies confirm that NPs conjugated with 2 ME are more effective from the point of view of cancer cell killing in comparison to 2 ME alone. In this work, based on the suitable properties of TiO\({}_{2}\) and PEG, as well as the potential use of 2 ME in the treatment of many cancer types [18], we formulated 2 ME-loaded TiO\({}_{2}\)-PEG nanoparticles that protect the anticancer agent from early degradation. TiO\({}_{2}\) nanoparticles were modified with PEG to improve their suspension stability, and then 2 ME was loaded on TiO\({}_{2}\)-PEG conjugate (TiO\({}_{2}\)-PEG\(-\)2 ME). This research was performed based on the possibility of the combined application of TiO\({}_{2}\) NPs (as a nanocarrier) with PEG (to improve the colloidal stability) and 2 ME (as an anticancer drug).